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Light-Emitting Diodes Based on InGaN/GaN Nanowires on Microsphere-Lithography-Patterned Si Substrates

Light-Emitting Diodes Based on InGaN/GaN Nanowires on Microsphere-Lithography-Patterned Si... nanomaterials Article Light-Emitting Diodes Based on InGaN/GaN Nanowires on Microsphere-Lithography-Patterned Si Substrates 1 1 , 2 2 1 3 Liliia Dvoretckaia , Vladislav Gridchin , Alexey Mozharov , Alina Maksimova , Anna Dragunova , 3 4 4 1 , 4 , 5 , 1 , 2 Ivan Melnichenko , Dmitry Mitin , Alexandr Vinogradov , Ivan Mukhin * and Georgy Cirlin Department of Physics, Alferov University, Khlopina 8/3, 194021 St. Petersburg, Russia; liliyabutler@gmail.com (L.D.); gridchinvo@yandex.ru (V.G.); deer.blackgreen@yandex.ru (A.M.); george.cirlin@mail.ru (G.C.) Institute of Physics, Saint Petersburg State University, Universitetskaya Emb. 7/9, 199034 St. Petersburg, Russia; alex000090@gmail.com Department of Physics, National Research University Higher School of Economics, Kantemirovskaya 3/1 A, 194100 St. Petersburg, Russia; anndra@list.ru (A.D.); imelnichenko@hse.ru (I.M.) Department of Chemistry, ITMO University, Lomonosova 9, 197101 St. Petersburg, Russia; mitindm@mail.ru (D.M.); avv@scamt-itmo.ru (A.V.) Higher School of Engineering Physics, Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia * Correspondence: imukhin@yandex.ru Abstract: The direct integration of epitaxial III-V and III-N heterostructures on Si substrates is a promising platform for the development of optoelectronic devices. Nanowires, due to their unique geometry, allow for the direct synthesis of semiconductor light-emitting diodes (LED) on crystalline lattice-mismatched Si wafers. Here, we present molecular beam epitaxy of regular arrays n-GaN/i- InGaN/p-GaN heterostructured nanowires and tripods on Si/SiO substrates prepatterned with Citation: Dvoretckaia, L.; Gridchin, the use of cost-effective and rapid microsphere optical lithography. This approach provides the V.; Mozharov, A.; Maksimova, A.; selective-area synthesis of the ordered nanowire arrays on large-area Si substrates. We experimentally Dragunova, A.; Melnichenko, I.; show that the n-GaN NWs/n-Si interface demonstrates rectifying behavior and the fabricated n- Mitin, D.; Vinogradov, A.; Mukhin, I.; Cirlin, G. Light-Emitting Diodes GaN/i-InGaN/p-GaN NWs-based LEDs have electroluminescence in the broad spectral range, with Based on InGaN/GaN Nanowires on a maximum near 500 nm, which can be employed for multicolor or white light screen development. Microsphere-Lithography-Patterned Si Substrates. Nanomaterials 2022, 12, Keywords: molecular beam epitaxy; nanowires; III-N; Si; microsphere lithography; light-emitting devices 1993. https://doi.org/10.3390/ nano12121993 Academic Editor: Onofrio M. Maragò 1. Introduction Received: 10 May 2022 The direct growth of III-V and III-N nanostructures on Si substrates is one of the most Accepted: 7 June 2022 promising means for the development of a new generation of optoelectronic devices [1–3]. Published: 10 June 2022 Nanowires (NWs), having quasi-one-dimensional structures, are considered as building Publisher’s Note: MDPI stays neutral blocks for such devices, since these structures can be directly grown on lattice-mismatched with regard to jurisdictional claims in Si substrates and possess high crystal perfection [4,5]. NWs demonstrate high crystal quality published maps and institutional affil- owing to the small footprint and effective mechanical stress relaxation on the developed side iations. surface. Solid alloys of Ga(In, Al)N are often used for the fabrication of light-emitting and light-absorbing devices operating in a broad spectral range [6–12]. Light-emitting diodes (LEDs) based on InGaN/GaN heterostructured NWs have been successfully demonstrated, showing excellent performances in the blue spectral range [13–16]. NWs-based LEDs are Copyright: © 2022 by the authors. considered to be the alternative to conventional organic-based solutions [17]. Moreover, Licensee MDPI, Basel, Switzerland. the epitaxial growth of heterostructured III-N NWs with controllable doping profiles on This article is an open access article relatively cheap Si substrates paves the way for the integration of III–V materials with an distributed under the terms and established complementary metal-oxide-semiconductor (CMOS) technology [18]. conditions of the Creative Commons Using molecular beam epitaxy (MBE), Ga(In)N NWs can be directly synthesized on Si Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ substrates in the form of self-induced disordered arrays [19–21]. Furthermore, within one 4.0/). epitaxial run, MBE enables the simultaneous growth of both vertically-aligned NWs and Nanomaterials 2022, 12, 1993. https://doi.org/10.3390/nano12121993 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 1993 2 of 9 tripods [22] or even nanotube-like [23] structures, which can have different compositions of In. This can be employed for white light screen fabrication. Metalorganic chemical vapor deposition (MOCVD), similar to MBE, was employed to achieve the selective epitaxy of the arrays of heterostructured NWs [24–26]. MOCVD is considered to be a promising epitaxial technique for III-V and III-N mass production, allowing for the time-efficient synthesis of NWs-based heterostructures simultaneously on the set of large-area substrates. However, for the development of NWs-based applications, especially LEDs, the regular arrangement of nanostructures is required. Time-consuming approaches for direct lithography, such as e-beam lithography [27] or focused ion beam milling [28,29], are not fully applicable for the prepatterning of large-area substrates. Among other approaches, microsphere optical lithography, which provides submicro- meter-scale lateral resolution, is one of the most versatile, scalable and cost-effective meth- ods for photoresist patterning [30]. Moreover, the design of the patterning can be easily tuned by the appropriate choice of the diameter of microspheres, while spin-coating enables the covering of the large-area substrates [31,32]. Selective-area epitaxy based on the Si/SiO growth substrates patterning with microsphere lithography allows for the obtention of the ordered arrays of NWs with a narrow distribution in geometrical sizes, which is essential for device processing [33,34]. In this work, we employ microsphere lithography for Si/SiO substrates preparation, allowing further selective-area MBE growth of the regular arrays of n-GaN/i-InGaN/p- GaN heterostructures. We show that the n-GaN NWs/Si substrate interface demonstrates rectifying electrical properties that are appropriate for LED fabrication. The produced NWs-based LEDs have the value of a knee voltage typical for III-N devices and show electroluminescence in a broad spectral range, which can be employed for multicolor or white light screen development. 2. Materials and Methods 2.1. Si/SiO Substrate Patterning For the growth of heterostructured InGaN/GaN NWs, we employed MBE on prepat- terned Si/SiO crystalline substrates. To estimate the electrical properties of the n-GaN/Si interface, we also synthesized an array of n-GaN NWs without InGaN active insertions and p-GaN shell layers on a Si prepatterned substrate. 16 3 To make the NWs’ growth mask, Si substrates (n-doped to the level of 1  10 cm ) were thermally oxidized, that provided the formation of a 60 nm-thin layer of oxide. Then, we employed microsphere lithography and plasma etching to pattern the oxide layer in order to fabricate a growth mask for the selective-area epitaxy of the ordered arrays of the NWs. Microspheres were spin-coated on the layer of the photoresist covering a growth substrate and formed a dense monolayer array. The optimal parameters of microsphere deposition are presented in our previous work [32]. Then, we used ultraviolet (UV) flood exposure with a 365 nm wavelength to illuminate the photoresist. Every microsphere worked as a lens, focusing the UV light into the optical jet underneath [35]. During the development of the exposed photoresist, the spheres were spin off from the substrate; thus, the patterned resist layer served as a mask for the further inductively coupled SF6 etching of SiO . Finally, the resist layer was removed, and we obtained the patterned Si/SiO 2 2 substrates with arrays of the ordered submicron holes in the oxide layer. The workflow of the Si/SiO substrate patterning and the typical scanning electron microscopy (SEM) images of the fabricated substrates are presented in Figure 1. The developed approach allows for the patterning of large-area Si substrates from several cm up to wafers several inches in diameter. Nanomaterials 2022, 12, 1993 3 of 9 Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 9 Figure 1. Workflow of Si/SiO2 substrate patterning: (a) spin-coating of the photoresist, (b) micro- Figure 1. Workflow of Si/SiO substrate patterning: (a) spin-coating of the photoresist, (b) micro- sphere deposition, (d) UV exposure of the photoresist through a monolayer of microspheres, (e) sphere deposition, (d) UV exposure of the photoresist through a monolayer of microspheres, (e) SiO SiO2 layer etching through the patterned photoresist. SEM images of the arrays of microspheres layer etching through the patterned photoresist. SEM images of the arrays of microspheres deposited deposited on the photoresist layer (c) and microholes in the SiO2 mask on the Si substrate (f). on the photoresist layer (c) and microholes in the SiO mask on the Si substrate (f). We used microspheres that were 1.8 µm in diameter, which defined the period of the We used microspheres that were 1.8 m in diameter, which defined the period of the ordered arrays of the NW. This provided the elimination of possible issues, such as the ordered arrays of the NW. This provided the elimination of possible issues, such as the competitive diffusion of the growth material over the substrate at nucleation and the ini- competitive diffusion of the growth material over the substrate at nucleation and the initial tial stages of the NWs’ growth, as well as the shadowing of the NWs with a relatively stages of the NWs’ growth, as well as the shadowing of the NWs with a relatively small small height. These effects can have a negative impact on the epitaxial synthesis of LED height. These effects can have a negative impact on the epitaxial synthesis of LED structures. structures. 2.2. MBE Growth 2.2. MBE Growth The MBE growth of NWs was carried out using Riber Compact 12 equipped with The MBE growth of NWs was carried out using Riber Compact 12 equipped with a a nitrogen plasma source, providing the flux of nitrogen ions. Prior to the growth, the nitrogen plasma source, providing the flux of nitrogen ions. Prior to the growth, the pat- patterned Si/SiO substrates were heated up to a temperature of 915 C and treated for terned Si/SiO2 substrates were heated up to a temperature of 915 °C and treated for 20 20 min, which enabled the removement of a thin native oxide layer. This process was contr min, which olled by ein nabled situ the reflection removement of high-ener a gythin n electra on tive dif oxide fraction. layer. It should This proce be no ss ted was con that - heating trolled b toy this in sitemperatur tu reflection e high enables -ener native gy elec oxide tron d desorption iffraction. It without should b the e no destr teduction that he of at- the ing grto owth this oxide temperat mask. ure en After able that, s nat the ive substrate oxide desorp temperatur tion withou e was t the decrdes eased truct to io830 n of C, the the growt nitrh ogen oxidplasma e mask. Af sour tece r th was at, tignited he substand rate the temperat shutters ure was decrea of the Ga and sed t Sio 830 °C, t cells werh ee simultaneously nitrogen plasma so opened. urce w The as nitr ignogen ited an flow d the and shthe utter power s of the G of the a an nitr d S ogen i cells wer plasma e sim sour uce lta- were 0.4 sccm and 450 W, respectively. The Ga beam equivalent pressure was equal to neously opened. The nitrogen flow and the power of the nitrogen plasma source were 0.4 −7 3  10 Torr. By the end of the n-doped NW cores, the growth the shutter of the Si cell sccm and 450 W, respectively. The Ga beam equivalent pressure was equal to 3 × 10 Torr. was closed to form an undoped part of GaN NWs with an estimated height of 15–20 nm. By the end of the n-doped NW cores, the growth the shutter of the Si cell was closed to This eliminated the emergence of doping atoms in the active InGaN insertions. To grow form an undoped part of GaN NWs with an estimated height of 15-20 nm. This eliminated the active InGaN insertions, the effusion cells were shut and substrate temperature was the emergence of doping atoms in the active InGaN insertions. To grow the active InGaN decreased to 660 C. After that, the shutters of the Ga and In cells were opened. The Ga insertions, the effusion cells were shut and substrate temperature was decreased to 660 and In fluxes were held constant at 1  10 Torr. The estimated height of the grown °C. After that, the shutters of the Ga and In cells were opened. The Ga and In fluxes were active InGaN insertions−7was 30 nm. The growth of the p-type emitters was performed held constant at 1 × 10 Torr. The estimated height of the grown active InGaN insertions with a Mg effusion cell at the same temperature, which provided the formation of p-GaN was 30 nm. The growth of the p-type emitters was performed with a Mg effusion cell at shells, covering the whole length of the NWs. The thickness of the synthesized p-doped the same temperature, which provided the formation of p-GaN shells, covering the whole shells was estimated at 200 nm. Similar to the NW core emitters, the first 15–20 nm of the length of the NWs. The thickness of the synthesized p-doped shells was estimated at 200 shells were grown without Mg doping. It should be noted that the formation of the emitter nm. Similar to the NW core emitters, the first 15–20 nm of the shells were grown without Mg doping. It should be noted that the formation of the emitter covering the entire nan- owires was essentially important for the further post processing of the LED structure. Nanomaterials 2022, 12, 1993 4 of 9 Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 9 covering the entire nanowires was essentially important for the further post processing of the LED structure. Figure 2a,b show schematic views and typical SEM images of the arrays of n-GaN and Figure 2a,b show schematic views and typical SEM images of the arrays of n-GaN n-GaN/i-InGaN/p-GaN NWs grown on n-Si substrates. According to the SEM images, the and n-GaN/i-InGaN/p-GaN NWs grown on n-Si substrates. According to the SEM images, patterned SiO layers enabled the selective MBE growth of NWs in every hole of the mask. the patterned2 SiO2 layers enabled the selective MBE growth of NWs in every hole of the Note that the chosen MBE regime provided the nucleation and growth of both vertically mask. Note that the chosen MBE regime provided the nucleation and growth of both ver- aligned NWs and tripod nanostructures, which can be caused by an insufficiently high tically aligned NWs and tripod nanostructures, which can be caused by an insufficiently growth temperature or by the features of NW nucleation in the holes of the mask [36,37]. high growth temperature or by the features of NW nucleation in the holes of the mask One can also note that the decreased temperature—required for the NW active area and [36,37]. One can also note that the decreased temperature—required for the NW active shell formation—enabled the nucleation of a 2D parasitic layer on the surface of the area and shell formation—enabled the nucleation of a 2D parasitic layer on the surface of SiO mask. the SiO2 mask. Figure 2. SEM images of the arrays of (a) n-GaN and (b) n-GaN/i-InGaN/p-GaN NWs grown on n- Figure 2. SEM images of the arrays of (a) n-GaN and (b) n-GaN/i-InGaN/p-GaN NWs grown on Si substrates. The inserts show the schematic view of the synthesized nanostructures (not in scale) n-Si substrates. The inserts show the schematic view of the synthesized nanostructures (not in scale) and the enlarged SEM images (the scale bar is 1 µm). and the enlarged SEM images (the scale bar is 1 m). Another important peculiarity of the synthesized nanostructures is the changing of Another important peculiarity of the synthesized nanostructures is the changing of the NW facets during the growth from the NW base to the top (see the insert in Figure 2b). the NW facets during the growth from the NW base to the top (see the insert in Figure 2b). This can be governed by two factors: a rotation of the crystal lattice by 30 degrees or a This can be governed by two factors: a rotation of the crystal lattice by 30 degrees or a change in the dominant facet. For more deep analysis, we performed an electron diffrac- change in the dominant facet. For more deep analysis, we performed an electron diffraction tion study near the base and the top of the NWs (see Supplementary Materials for details). study near the base and the top of the NWs (see Supplementary Materials for details). The acquired electron diffraction patterns are the same for both points, which proved the The acquired electron diffraction patterns are the same for both points, which proved the change in the dominant facet. One possible reason for this phenomenon is associated with change in the dominant facet. One possible reason for this phenomenon is associated with the mechanical stress in the NWs that originated from the lattice mismatching between the mechanical stress in the NWs that originated from the lattice mismatching between the the GaN core and the InGaN active area. Another possible reason can be related to the GaN core and the InGaN active area. Another possible reason can be related to the features features of Mg doping of the GaN shell. of Mg doping of the GaN shell. 3. Results and Discussion 3. Results and Discussion 3.1. Optical Properties Study 3.1. Optical Properties Study To evaluate the composition of active InGaN insertions, we performed a photolumi- To evaluate the composition of active InGaN insertions, we performed a photolumines- nescence (PL) study. Figure 3 shows a typical PL spectrum obtained from the cence (PL) study. Figure 3 shows a typical PL spectrum obtained from the GaN/InGaN/GaN GaN/InGaN/GaN NW arrays. One can see that the InGaN insertions demonstrated a PL NW arrays. One can see that the InGaN insertions demonstrated a PL signal in a wide signal in a wide spectral range from visible to near infrared (IR), while the PL maximum spectral range from visible to near infrared (IR), while the PL maximum is located near is located near 500 nm. The relatively broad PL peak can be caused by the decomposition 500 nm. The relatively broad PL peak can be caused by the decomposition of the InGaN of the InGaN insertions on the phases with different contents of In [38] or by different In insertions on the phases with different contents of In [38] or by different In incorpora- incorporations in non-polar and polar wurtzite planes [39]. It also should be mentioned tions in non-polar and polar wurtzite planes [39]. It also should be mentioned that the that the inclined NWs and 2D parasitic layer can contribute to the PL signal, which can inclined NWs and 2D parasitic layer can contribute to the PL signal, which can broaden broaden the spectrum. the spectrum. Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 9 N Nanomaterials anomaterials 2022 2022,, 12 12, x FOR PEER REVIEW , 1993 5 of 5 of 9 9 Figure 3. Normalized to the maximum PL response of the GaN/InGaN/GaN NWs synthesized on Figure 3. Normalized to the maximum PL response of the GaN/InGaN/GaN NWs synthesized on Figure 3. Normalized to the maximum PL response of the GaN/InGaN/GaN NWs synthesized on the Si/SiO2 substrate. the Si/SiO2 substrate. the Si/SiO substrate. 3.2. Device Processing 3.2. De 3.2. Device vice Pr Processing ocessing Next, we carried out the postprocessing of the synthesized structures in order to fab- Next, we carried out the postprocessing of the synthesized structures in order to fab- Next, we carried out the postprocessing of the synthesized structures in order to ricate the functionalized devices. The workflow of processing is shown in Figure 4. To ri fabricate cate the fu thenct functionalized ionalized devi devices. ces. The The workfl workflow ow of processin of processing g is shown is shown in Figure 4. To in Figure 4. fabricate ohmic contacts to Si substrates, their back sides were treated with 10% HF aque- fab To r fabricate icate ohm ohmic ic contcontacts acts to Si to sub Sissubstrates, trates, their b their ack s back ides were sides tre wer ate e d wi treated th 1with 0% H10% F aque HF - aqueous ous solutsolution ion in order in or to der reto mo rv emove e the na the tiv native e oxide oxide layer and layer p and asspassivate ivate the sur thefsurface ace with hy with - ous solution in order to remove the native oxide layer and passivate the surface with hy- hydr drogen. Immediate ogen. Immediately ly after after that, that, the sub the s substrates trates were wer loa edloaded ed into into a vaa cuvacuum um cham chamber ber of a drogen. Immediately after that, the substrates were loaded into a vacuum chamber of a of therm a thermal al evapevaporator orator BocEd BocEdwar wards Ads uto Auto 500 to 500 depo to deposit sit Al con Altcontact act withwith a thic a kn thickness ess of 200 of thermal evaporator BocEdwards Auto 500 to deposit Al contact with a thickness of 200 200 nm. T nm. hen, Then, usinusing g spin-co spin-coating, ating, the sthe ides sides of the of sthe ubstr substrates ates with the with NWs the NWs were co wer ve ered w coveried th nm. Then, using spin-coating, the sides of the substrates with the NWs were covered with with photo-c photo-curing uring epoxy resin epoxy r(SU-8 negativ esin (SU-8 negative e photores photor ist). This provid esist). This pr ed ovided the elec the trical electrical isola- photo-curing epoxy resin (SU-8 negative photoresist). This provided the electrical isola- isolation between the substrate and front contact. The thickness of the SU-8 layer was tion between the substrate and front contact. The thickness of the SU-8 layer was 100-200 tion between the substrate and front contact. The thickness of the SU-8 layer was 100-200 100–200 nm less than the average length of the NWs. To remove the epoxy residue from the nm less than the average length of the NWs. To remove the epoxy residue from the ends nm less than the average length of the NWs. To remove the epoxy residue from the ends ends of the NWs, the substrates were treated in oxygen plasma. In the next technological of the NWs, the substrates were treated in oxygen plasma. In the next technological step, of the NWs, the substrates were treated in oxygen plasma. In the next technological step, step, we formed the mesa front contacts, which required the use of optical lithography, we formed the mesa front contacts, which required the use of optical lithography, con- we formed the mesa front contacts, which required the use of optical lithography, con- conductive layer deposition and the lift-off procedure. In the case of the test n-GaN NWs/n- ductive layer deposition and the lift-off procedure. In the case of the test n-GaN NWs/n- ductive layer deposition and the lift-off procedure. In the case of the test n-GaN NWs/n- Si structures, we evaporated the Al layer, which provided ohmic contact to GaN, while Si structures, we evaporated the Al layer, which provided ohmic contact to GaN, while Si structures, we evaporated the Al layer, which provided ohmic contact to GaN, while for the LED InGaN NWs-based structures, a thin layer of indium tin oxide (ITO) was for the LED InGaN NWs-based structures, a thin layer of indium tin oxide (ITO) was mag- for the LED InGaN NWs-based structures, a thin layer of indium tin oxide (ITO) was mag- magnetron sputtered. The ITO formed a transparent contact to the p-GaN shells of the LED netron sputtered. The ITO formed a transparent contact to the p-GaN shells of the LED netron sputtered. The ITO formed a transparent contact to the p-GaN shells of the LED structures. Note that only the longest NWs were in contact with the ITO, while the others structures. Note that only the longest NWs were in contact with the ITO, while the others structures. Note that only the longest NWs were in contact with the ITO, while the others were buried in the SU-8 layer. were buried in the SU-8 layer. were buried in the SU-8 layer. Figure 4. Workflow of NWs-based LEDs processing: (a) HF treatment, (b) evaporation of Al back Figure 4. Workflow of NWs-based LEDs processing: (a) HF treatment, (b) evaporation of Al back Figure 4. Workflow of NWs-based LEDs processing: (a) HF treatment, (b) evaporation of Al back contact, (c) covering with SU-8, (d) opening of NWs ends, (e) ITO sputtering. contact, (c) covering with SU-8, (d) opening of NWs ends, (e) ITO sputtering. contact, (c) covering with SU-8, (d) opening of NWs ends, (e) ITO sputtering. 3.3. Electrical and Electroluminescent Characterization 3.3. Electrical and Electroluminescent Characterization 3.3. Electrical and Electroluminescent Characterization The current-voltage (I-V) characteristics of the fabricated devices were measured The current-voltage (I-V) characteristics of the fabricated devices were measured The current-voltage (I-V) characteristics of the fabricated devices were measured with with the use of a Keithley 2401 source-meter. The samples were placed on a metallic table with the use of a Keithley 2401 source-meter. The samples were placed on a metallic table the use of a Keithley 2401 source-meter. The samples were placed on a metallic table of a of a probe station with a vacuum clamp. A contact to the face electrode was organized of a probe station with a vacuum clamp. A contact to the face electrode was organized probe station with a vacuum clamp. A contact to the face electrode was organized using Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 9 Nanomaterials 2022, 12, 1993 6 of 9 soft usinCuBe g soft CuBe probes, w probes, which mitigated hich mitig the atemechanical d the mechascratching nical scratcof hing the ostr f th uctur e stru es. ctu Figur res. Fe ig 5- pr ure esents 5 pre the sents the typical typic I-V curves al I-V c for urve the s for test the andtest LED and structur LED struc es. For tures. For all of theall o measur f the meas- ements, the urem positive ents, the input posi of tiv the e inp power ut of th supply e powe was r sup connected ply was to connected the probe. to the probe. Figure 5. I-V characteristics of the (a) test n-GaN NWs-based structure and (b) n-GaN/i-InGaN/GaN Figure 5. I-V characteristics of the (a) test n-GaN NWs-based structure and (b) n-GaN/i-InGaN/GaN NWs-based LED on Si. NWs-based LED on Si. Figure 5a shows a typical I-V curve for one of the test devices, where the NWs con- Figure 5a shows a typical I-V curve for one of the test devices, where the NWs tained only n-GaN cores (without InGaN active insertions and p-GaN shells). One can see contained only n-GaN cores (without InGaN active insertions and p-GaN shells). One can that the curve demonstrates rectifying behavior. Moreover, as shown in the additional see that the curve demonstrates rectifying behavior. Moreover, as shown in the additional experiments (not presented here), for this type of device, the reverse bias current depends experiments (not presented here), for this type of device, the reverse bias current depends on the level of illumination with visible or IR light. Together with the polarity of the I-V on the level of illumination with visible or IR light. Together with the polarity of the I-V curve, these evidence the emergence of a rectifying junction in the Si substrate. We suggest curve, these evidence the emergence of a rectifying junction in the Si substrate. We suggest that this can be caused by a doping to the p-type conductivity of the Si surface layer by that this can be caused by a doping to the p-type conductivity of the Si surface layer by Ga Ga atoms during MBE growth. The device demonstrates a current density up to 30 A/cm atoms during MBE growth. The device demonstrates a current density up to 30 A/cm under an applied positive voltage of 2V and more than 100 A/cm2 under a voltage of 3V under an applied positive voltage of 2V and more than 100 A/cm under a voltage of 3V (see the positive branch of the I-V curve in Figure 5a). Thus, a drop in the voltage on the (see the positive branch of the I-V curve in Figure 5a). Thus, a drop in the voltage on the interface of n-GaN/n-Si should be taken into account while analyzing the I-V characteristic interface of n-GaN/n-Si should be taken into account while analyzing the I-V characteristic of LED structures. of LED structures. Figure 5b presents a typical I-V curve for one of the LED n-GaN/i-InGaN/p-GaN de- Figure 5b presents a typical I-V curve for one of the LED n-GaN/i-InGaN/p-GaN vices. The knee voltage is about 6 V, while the typical current density corresponds to the devices. The knee voltage is about 6 V, while the typical current density corresponds to the 2 2 level of several A/cm . Considering the drop in the voltage on the n-GaN/n-Si interface level of several A/cm . Considering the drop in the voltage on the n-GaN/n-Si interface discussed previously, we can conclude that, for this level of current density, around 1-1.5 discussed previously, we can conclude that, for this level of current density, around 1–1.5 V V drops on the interface and the other voltage drops on the n-GaN/i-InGaN/p-GaN struc- drops on the interface and the other voltage drops on the n-GaN/i-InGaN/p-GaN structure and ture an thed p-GaN/IT the p-GaN/ITO interface O interface appear appe ed. ared Thus, . Thus, the the real real knee knee voltage voltage cancan be be fo found und to be to ar be around ound 4.54.5 V, which corresponds well to V, which corresponds well to the thexpected e expected value fo value for r this type of de this type of device. vice. The The electr electrolumine oluminesce scence nce (EL) (EL) of the L of the LED ED dev devices ices c can an be de be detected tected b by y th the e n naked aked eye eye for for a a curren currentt density densityexceeding exceeding 2 2 A/c A/cm m . However . Howev , e the r, estimation the estimaof tion o the curr f the ent cdensity urrent dens passing ity p thraough ssing the GaN/InGaN/GaN NWs is far from evident, since only part of the NWs is contacted to through the GaN/InGaN/GaN NWs is far from evident, since only part of the NWs is con- the ITO and, therefore, involved in the passing of current. tacted to the ITO and, therefore, involved in the passing of current. Figure 6 shows the obtained EL spectrum for the LED device. Note that only a small Figure 6 shows the obtained EL spectrum for the LED device. Note that only a small part of the NWs (the longest ones) was contacted by the ITO and, thus, could emit light. part of the NWs (the longest ones) was contacted by the ITO and, thus, could emit light. The EL spectrum demonstrates a dominant peak near 500–520 nm (corresponding to the The EL spectrum demonstrates a dominant peak near 500-520 nm (corresponding to the cyan-green color on the optical images) and a wide tail near the IR spectral range. This cyan-green color on the optical images) and a wide tail near the IR spectral range. This result corresponds well to the experimentally measured PL spectra (see Figure 3). The result corresponds well to the experimentally measured PL spectra (see Figure 3). The difference in spectra (the drop in the EL signal in the range of 550–650 nm wavelengths) difference in spectra (the drop in the EL signal in the range of 550-650 nm wavelengths) can be explained by the fact that only the longest (vertically aligned) NWs contribute to can be explained by the fact that only the longest (vertically aligned) NWs contribute to the measured EL signal. According to our experiments, the increase in current density the measured EL signal. According to our experiments, the increase in current density passing through the device led to the growth of the IR tail on the EL spectra, which can passing through the device led to the growth of the IR tail on the EL spectra, which can be be associated with the thermal heating of the NWs. However, the detailed analysis of this associated with the thermal heating of the NWs. However, the detailed analysis of this effect requires additional study. effect requires additional study. Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 9 Nanomaterials 2022, 12, 1993 7 of 9 Figure 6. Normalized to the maximum EL spectrum of the n-GaN/i-InGaN/GaN LED device. The Figure 6. Normalized to the maximum EL spectrum of the n-GaN/i-InGaN/GaN LED device. The insert demonstrates an optical image of the operating NWs-based LED. insert demonstrates an optical image of the operating NWs-based LED. 4. Conclusions 4. Conclusions In this work, we synthesized test n-GaN NWs and LED n-GaN/InGaN/p-GaN NWs In this work, we synthesized test n-GaN NWs and LED n-GaN/InGaN/p-GaN NWs using selective-area MBE growth on Si substrates with prepatterned SiO layers. To achieve using selective-area MBE growth on Si substrates with prepatterned 2 SiO2 layers. To the ordered arrays of the NWs, we employed microsphere lithography, providing the achieve the ordered arrays of the NWs, we employed microsphere lithography, providing handling of the large-area substrates. The PL measurements revealed that the synthesized the handling of the large-area substrates. The PL measurements revealed that the synthe- NWs have a relatively broad optical response, indicating the decomposition of the InGaN sized NWs have a relatively broad optical response, indicating the decomposition of the insertions on the phases with different contents of In. InGaN insertions on the phases with different contents of In. We studied the transport properties of the n-GaN/n-Si interface and demonstrated We studied the transport properties of the n-GaN/n-Si interface and demonstrated rectifying behavior on the I-V curves. The knee voltage of the fabricated n-GaN i-InGaN/p- rectifying behavior on the I-V curves. The knee voltage of the fabricated n-GaN i- GaN structures was found to be around 4.5 V, which is typical for GaN-based LED devices. InGaN/p-GaN structures was found to be around 4.5 V, which is typical for GaN-based The measured EL spectra for the fabricated LED devices demonstrated a peak located LED devices. The measured EL spectra for the fabricated LED devices demonstrated a around 500–520 nm and wide IR tail. The results of the EL measurements are consistent peak located around 500-520 nm and wide IR tail. The results of the EL measurements are with the PL study. consistent with the PL study. Supplementary Materials: The following supporting information can be downloaded at: https:// Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/nano12121993/s1, Figure S1: SEM image of the grown InGaN/GaN www.mdpi.com/xxx/s1, Figure S1: SEM image of the grown InGaN/GaN NWs. The inserts show NWs. The inserts show electron diffraction patterns acquired in the points EBSD11 and EBSD12. electron diffraction patterns acquired in the points EBSD11 and EBSD12. Author Contributions: Conceptualization, A.V., I.M. (Ivan Mukhin) and G.C.; Data curation, A.M. Author Contributions: Conceptualization, A.V., I.M. (Ivan Mukhin) and G.C.; Data curation, A.M. and D.M.; Formal analysis, A.M.; Funding acquisition, A.V., I.M. (Ivan Mukhin) and G.C.; Inves- and D.M.; Formal analysis, A.M.; Funding acquisition, A.V., I.M. (Ivan Mukhin) and G.C.; Inves- tigation, L.D., V.G., A.M. (Alexey Mozharov), A.M. (Alina Maksimova), A.D., I.M. (Ivan Melnichenko) tigation, L.D., V.G., A.M. (Alexey Mozharov), A.M. (Alina Maksimova), A.D., I.M. (Ivan and D.M.; Methodology, L.D.; Project administration, A.V., I.M. (Ivan Mukhin) and G.C.; Writing— Melnichenko) and D.M.; Methodology, L.D.; Project administration, A.V., I.M. (Ivan Mukhin) and original draft, I.M. (Ivan Mukhin) and G.C.; Writing—review & editing, L.D., V.G., A.M. (Alexey G.C.; Writing—original draft, I.M. (Ivan Mukhin) and G.C.; Writing—review & editing, L.D., V.G., Mozharov), A.M. (Alina Maksimova), A.D., I.M., D.M., A.V., I.M. (Ivan Melnichenko) and G.C. All A.M. (Alexey Mozharov), A.M. (Alina Maksimova), A.D., I.M., D.M., A.V., I.M. (Ivan Melnichenko) authors have read and agreed to the published version of the manuscript. and G.C. All authors have read and agreed to the published version of the manuscript. Funding: The samples were grown and then processed under the support of the Ministry of Science Funding: The samples were grown and then processed under the support of the Ministry of Science and Higher Education of the Russian Federation (state task № 0791-2020-0003 and № 0791-2020- and Higher Education of the Russian Federation (state task № 0791-2020-0003 and № 0791-2020- 0005, respectively). D.M. and A.V. thank the Russian Science Foundation (grant 21-79-10202) for 0005, respectively). D.M. and A.V. thank the Russian Science Foundation (grant 21-79-10202) for the the support of the study of growth substrate patterning. A.D. and I.M. (Ivan Melnichenko) thank support of the study of growth substrate patterning. A.D. and I.M. (Ivan Melnichenko) thank the the Basi Basic c Research ResearProg ch Pr ram ogram at the at the National National Resea Resear rch Univer ch University sity Higher Higher School School of Economics ( of Economics HSE Uni- (HSE University) for the support of the study of photoluminescence and electroluminescence. versity) for the support of the study of photoluminescence and electroluminescence. Data Data Availabil Availability ity Statement: Statement: The The data prese data present ned ted in this in this study study ar are available on request e available on request fr from the om the corr corresponding author. The data are esponding author. The data are not not pu publicly blicly available available due duto e to the au the author thor’s readine ’s readiness ss to to pr provide ovide it on request. it on request. 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Light-Emitting Diodes Based on InGaN/GaN Nanowires on Microsphere-Lithography-Patterned Si Substrates

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nanomaterials Article Light-Emitting Diodes Based on InGaN/GaN Nanowires on Microsphere-Lithography-Patterned Si Substrates 1 1 , 2 2 1 3 Liliia Dvoretckaia , Vladislav Gridchin , Alexey Mozharov , Alina Maksimova , Anna Dragunova , 3 4 4 1 , 4 , 5 , 1 , 2 Ivan Melnichenko , Dmitry Mitin , Alexandr Vinogradov , Ivan Mukhin * and Georgy Cirlin Department of Physics, Alferov University, Khlopina 8/3, 194021 St. Petersburg, Russia; liliyabutler@gmail.com (L.D.); gridchinvo@yandex.ru (V.G.); deer.blackgreen@yandex.ru (A.M.); george.cirlin@mail.ru (G.C.) Institute of Physics, Saint Petersburg State University, Universitetskaya Emb. 7/9, 199034 St. Petersburg, Russia; alex000090@gmail.com Department of Physics, National Research University Higher School of Economics, Kantemirovskaya 3/1 A, 194100 St. Petersburg, Russia; anndra@list.ru (A.D.); imelnichenko@hse.ru (I.M.) Department of Chemistry, ITMO University, Lomonosova 9, 197101 St. Petersburg, Russia; mitindm@mail.ru (D.M.); avv@scamt-itmo.ru (A.V.) Higher School of Engineering Physics, Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia * Correspondence: imukhin@yandex.ru Abstract: The direct integration of epitaxial III-V and III-N heterostructures on Si substrates is a promising platform for the development of optoelectronic devices. Nanowires, due to their unique geometry, allow for the direct synthesis of semiconductor light-emitting diodes (LED) on crystalline lattice-mismatched Si wafers. Here, we present molecular beam epitaxy of regular arrays n-GaN/i- InGaN/p-GaN heterostructured nanowires and tripods on Si/SiO substrates prepatterned with Citation: Dvoretckaia, L.; Gridchin, the use of cost-effective and rapid microsphere optical lithography. This approach provides the V.; Mozharov, A.; Maksimova, A.; selective-area synthesis of the ordered nanowire arrays on large-area Si substrates. We experimentally Dragunova, A.; Melnichenko, I.; show that the n-GaN NWs/n-Si interface demonstrates rectifying behavior and the fabricated n- Mitin, D.; Vinogradov, A.; Mukhin, I.; Cirlin, G. Light-Emitting Diodes GaN/i-InGaN/p-GaN NWs-based LEDs have electroluminescence in the broad spectral range, with Based on InGaN/GaN Nanowires on a maximum near 500 nm, which can be employed for multicolor or white light screen development. Microsphere-Lithography-Patterned Si Substrates. Nanomaterials 2022, 12, Keywords: molecular beam epitaxy; nanowires; III-N; Si; microsphere lithography; light-emitting devices 1993. https://doi.org/10.3390/ nano12121993 Academic Editor: Onofrio M. Maragò 1. Introduction Received: 10 May 2022 The direct growth of III-V and III-N nanostructures on Si substrates is one of the most Accepted: 7 June 2022 promising means for the development of a new generation of optoelectronic devices [1–3]. Published: 10 June 2022 Nanowires (NWs), having quasi-one-dimensional structures, are considered as building Publisher’s Note: MDPI stays neutral blocks for such devices, since these structures can be directly grown on lattice-mismatched with regard to jurisdictional claims in Si substrates and possess high crystal perfection [4,5]. NWs demonstrate high crystal quality published maps and institutional affil- owing to the small footprint and effective mechanical stress relaxation on the developed side iations. surface. Solid alloys of Ga(In, Al)N are often used for the fabrication of light-emitting and light-absorbing devices operating in a broad spectral range [6–12]. Light-emitting diodes (LEDs) based on InGaN/GaN heterostructured NWs have been successfully demonstrated, showing excellent performances in the blue spectral range [13–16]. NWs-based LEDs are Copyright: © 2022 by the authors. considered to be the alternative to conventional organic-based solutions [17]. Moreover, Licensee MDPI, Basel, Switzerland. the epitaxial growth of heterostructured III-N NWs with controllable doping profiles on This article is an open access article relatively cheap Si substrates paves the way for the integration of III–V materials with an distributed under the terms and established complementary metal-oxide-semiconductor (CMOS) technology [18]. conditions of the Creative Commons Using molecular beam epitaxy (MBE), Ga(In)N NWs can be directly synthesized on Si Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ substrates in the form of self-induced disordered arrays [19–21]. Furthermore, within one 4.0/). epitaxial run, MBE enables the simultaneous growth of both vertically-aligned NWs and Nanomaterials 2022, 12, 1993. https://doi.org/10.3390/nano12121993 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 1993 2 of 9 tripods [22] or even nanotube-like [23] structures, which can have different compositions of In. This can be employed for white light screen fabrication. Metalorganic chemical vapor deposition (MOCVD), similar to MBE, was employed to achieve the selective epitaxy of the arrays of heterostructured NWs [24–26]. MOCVD is considered to be a promising epitaxial technique for III-V and III-N mass production, allowing for the time-efficient synthesis of NWs-based heterostructures simultaneously on the set of large-area substrates. However, for the development of NWs-based applications, especially LEDs, the regular arrangement of nanostructures is required. Time-consuming approaches for direct lithography, such as e-beam lithography [27] or focused ion beam milling [28,29], are not fully applicable for the prepatterning of large-area substrates. Among other approaches, microsphere optical lithography, which provides submicro- meter-scale lateral resolution, is one of the most versatile, scalable and cost-effective meth- ods for photoresist patterning [30]. Moreover, the design of the patterning can be easily tuned by the appropriate choice of the diameter of microspheres, while spin-coating enables the covering of the large-area substrates [31,32]. Selective-area epitaxy based on the Si/SiO growth substrates patterning with microsphere lithography allows for the obtention of the ordered arrays of NWs with a narrow distribution in geometrical sizes, which is essential for device processing [33,34]. In this work, we employ microsphere lithography for Si/SiO substrates preparation, allowing further selective-area MBE growth of the regular arrays of n-GaN/i-InGaN/p- GaN heterostructures. We show that the n-GaN NWs/Si substrate interface demonstrates rectifying electrical properties that are appropriate for LED fabrication. The produced NWs-based LEDs have the value of a knee voltage typical for III-N devices and show electroluminescence in a broad spectral range, which can be employed for multicolor or white light screen development. 2. Materials and Methods 2.1. Si/SiO Substrate Patterning For the growth of heterostructured InGaN/GaN NWs, we employed MBE on prepat- terned Si/SiO crystalline substrates. To estimate the electrical properties of the n-GaN/Si interface, we also synthesized an array of n-GaN NWs without InGaN active insertions and p-GaN shell layers on a Si prepatterned substrate. 16 3 To make the NWs’ growth mask, Si substrates (n-doped to the level of 1  10 cm ) were thermally oxidized, that provided the formation of a 60 nm-thin layer of oxide. Then, we employed microsphere lithography and plasma etching to pattern the oxide layer in order to fabricate a growth mask for the selective-area epitaxy of the ordered arrays of the NWs. Microspheres were spin-coated on the layer of the photoresist covering a growth substrate and formed a dense monolayer array. The optimal parameters of microsphere deposition are presented in our previous work [32]. Then, we used ultraviolet (UV) flood exposure with a 365 nm wavelength to illuminate the photoresist. Every microsphere worked as a lens, focusing the UV light into the optical jet underneath [35]. During the development of the exposed photoresist, the spheres were spin off from the substrate; thus, the patterned resist layer served as a mask for the further inductively coupled SF6 etching of SiO . Finally, the resist layer was removed, and we obtained the patterned Si/SiO 2 2 substrates with arrays of the ordered submicron holes in the oxide layer. The workflow of the Si/SiO substrate patterning and the typical scanning electron microscopy (SEM) images of the fabricated substrates are presented in Figure 1. The developed approach allows for the patterning of large-area Si substrates from several cm up to wafers several inches in diameter. Nanomaterials 2022, 12, 1993 3 of 9 Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 9 Figure 1. Workflow of Si/SiO2 substrate patterning: (a) spin-coating of the photoresist, (b) micro- Figure 1. Workflow of Si/SiO substrate patterning: (a) spin-coating of the photoresist, (b) micro- sphere deposition, (d) UV exposure of the photoresist through a monolayer of microspheres, (e) sphere deposition, (d) UV exposure of the photoresist through a monolayer of microspheres, (e) SiO SiO2 layer etching through the patterned photoresist. SEM images of the arrays of microspheres layer etching through the patterned photoresist. SEM images of the arrays of microspheres deposited deposited on the photoresist layer (c) and microholes in the SiO2 mask on the Si substrate (f). on the photoresist layer (c) and microholes in the SiO mask on the Si substrate (f). We used microspheres that were 1.8 µm in diameter, which defined the period of the We used microspheres that were 1.8 m in diameter, which defined the period of the ordered arrays of the NW. This provided the elimination of possible issues, such as the ordered arrays of the NW. This provided the elimination of possible issues, such as the competitive diffusion of the growth material over the substrate at nucleation and the ini- competitive diffusion of the growth material over the substrate at nucleation and the initial tial stages of the NWs’ growth, as well as the shadowing of the NWs with a relatively stages of the NWs’ growth, as well as the shadowing of the NWs with a relatively small small height. These effects can have a negative impact on the epitaxial synthesis of LED height. These effects can have a negative impact on the epitaxial synthesis of LED structures. structures. 2.2. MBE Growth 2.2. MBE Growth The MBE growth of NWs was carried out using Riber Compact 12 equipped with The MBE growth of NWs was carried out using Riber Compact 12 equipped with a a nitrogen plasma source, providing the flux of nitrogen ions. Prior to the growth, the nitrogen plasma source, providing the flux of nitrogen ions. Prior to the growth, the pat- patterned Si/SiO substrates were heated up to a temperature of 915 C and treated for terned Si/SiO2 substrates were heated up to a temperature of 915 °C and treated for 20 20 min, which enabled the removement of a thin native oxide layer. This process was contr min, which olled by ein nabled situ the reflection removement of high-ener a gythin n electra on tive dif oxide fraction. layer. It should This proce be no ss ted was con that - heating trolled b toy this in sitemperatur tu reflection e high enables -ener native gy elec oxide tron d desorption iffraction. It without should b the e no destr teduction that he of at- the ing grto owth this oxide temperat mask. ure en After able that, s nat the ive substrate oxide desorp temperatur tion withou e was t the decrdes eased truct to io830 n of C, the the growt nitrh ogen oxidplasma e mask. Af sour tece r th was at, tignited he substand rate the temperat shutters ure was decrea of the Ga and sed t Sio 830 °C, t cells werh ee simultaneously nitrogen plasma so opened. urce w The as nitr ignogen ited an flow d the and shthe utter power s of the G of the a an nitr d S ogen i cells wer plasma e sim sour uce lta- were 0.4 sccm and 450 W, respectively. The Ga beam equivalent pressure was equal to neously opened. The nitrogen flow and the power of the nitrogen plasma source were 0.4 −7 3  10 Torr. By the end of the n-doped NW cores, the growth the shutter of the Si cell sccm and 450 W, respectively. The Ga beam equivalent pressure was equal to 3 × 10 Torr. was closed to form an undoped part of GaN NWs with an estimated height of 15–20 nm. By the end of the n-doped NW cores, the growth the shutter of the Si cell was closed to This eliminated the emergence of doping atoms in the active InGaN insertions. To grow form an undoped part of GaN NWs with an estimated height of 15-20 nm. This eliminated the active InGaN insertions, the effusion cells were shut and substrate temperature was the emergence of doping atoms in the active InGaN insertions. To grow the active InGaN decreased to 660 C. After that, the shutters of the Ga and In cells were opened. The Ga insertions, the effusion cells were shut and substrate temperature was decreased to 660 and In fluxes were held constant at 1  10 Torr. The estimated height of the grown °C. After that, the shutters of the Ga and In cells were opened. The Ga and In fluxes were active InGaN insertions−7was 30 nm. The growth of the p-type emitters was performed held constant at 1 × 10 Torr. The estimated height of the grown active InGaN insertions with a Mg effusion cell at the same temperature, which provided the formation of p-GaN was 30 nm. The growth of the p-type emitters was performed with a Mg effusion cell at shells, covering the whole length of the NWs. The thickness of the synthesized p-doped the same temperature, which provided the formation of p-GaN shells, covering the whole shells was estimated at 200 nm. Similar to the NW core emitters, the first 15–20 nm of the length of the NWs. The thickness of the synthesized p-doped shells was estimated at 200 shells were grown without Mg doping. It should be noted that the formation of the emitter nm. Similar to the NW core emitters, the first 15–20 nm of the shells were grown without Mg doping. It should be noted that the formation of the emitter covering the entire nan- owires was essentially important for the further post processing of the LED structure. Nanomaterials 2022, 12, 1993 4 of 9 Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 9 covering the entire nanowires was essentially important for the further post processing of the LED structure. Figure 2a,b show schematic views and typical SEM images of the arrays of n-GaN and Figure 2a,b show schematic views and typical SEM images of the arrays of n-GaN n-GaN/i-InGaN/p-GaN NWs grown on n-Si substrates. According to the SEM images, the and n-GaN/i-InGaN/p-GaN NWs grown on n-Si substrates. According to the SEM images, patterned SiO layers enabled the selective MBE growth of NWs in every hole of the mask. the patterned2 SiO2 layers enabled the selective MBE growth of NWs in every hole of the Note that the chosen MBE regime provided the nucleation and growth of both vertically mask. Note that the chosen MBE regime provided the nucleation and growth of both ver- aligned NWs and tripod nanostructures, which can be caused by an insufficiently high tically aligned NWs and tripod nanostructures, which can be caused by an insufficiently growth temperature or by the features of NW nucleation in the holes of the mask [36,37]. high growth temperature or by the features of NW nucleation in the holes of the mask One can also note that the decreased temperature—required for the NW active area and [36,37]. One can also note that the decreased temperature—required for the NW active shell formation—enabled the nucleation of a 2D parasitic layer on the surface of the area and shell formation—enabled the nucleation of a 2D parasitic layer on the surface of SiO mask. the SiO2 mask. Figure 2. SEM images of the arrays of (a) n-GaN and (b) n-GaN/i-InGaN/p-GaN NWs grown on n- Figure 2. SEM images of the arrays of (a) n-GaN and (b) n-GaN/i-InGaN/p-GaN NWs grown on Si substrates. The inserts show the schematic view of the synthesized nanostructures (not in scale) n-Si substrates. The inserts show the schematic view of the synthesized nanostructures (not in scale) and the enlarged SEM images (the scale bar is 1 µm). and the enlarged SEM images (the scale bar is 1 m). Another important peculiarity of the synthesized nanostructures is the changing of Another important peculiarity of the synthesized nanostructures is the changing of the NW facets during the growth from the NW base to the top (see the insert in Figure 2b). the NW facets during the growth from the NW base to the top (see the insert in Figure 2b). This can be governed by two factors: a rotation of the crystal lattice by 30 degrees or a This can be governed by two factors: a rotation of the crystal lattice by 30 degrees or a change in the dominant facet. For more deep analysis, we performed an electron diffrac- change in the dominant facet. For more deep analysis, we performed an electron diffraction tion study near the base and the top of the NWs (see Supplementary Materials for details). study near the base and the top of the NWs (see Supplementary Materials for details). The acquired electron diffraction patterns are the same for both points, which proved the The acquired electron diffraction patterns are the same for both points, which proved the change in the dominant facet. One possible reason for this phenomenon is associated with change in the dominant facet. One possible reason for this phenomenon is associated with the mechanical stress in the NWs that originated from the lattice mismatching between the mechanical stress in the NWs that originated from the lattice mismatching between the the GaN core and the InGaN active area. Another possible reason can be related to the GaN core and the InGaN active area. Another possible reason can be related to the features features of Mg doping of the GaN shell. of Mg doping of the GaN shell. 3. Results and Discussion 3. Results and Discussion 3.1. Optical Properties Study 3.1. Optical Properties Study To evaluate the composition of active InGaN insertions, we performed a photolumi- To evaluate the composition of active InGaN insertions, we performed a photolumines- nescence (PL) study. Figure 3 shows a typical PL spectrum obtained from the cence (PL) study. Figure 3 shows a typical PL spectrum obtained from the GaN/InGaN/GaN GaN/InGaN/GaN NW arrays. One can see that the InGaN insertions demonstrated a PL NW arrays. One can see that the InGaN insertions demonstrated a PL signal in a wide signal in a wide spectral range from visible to near infrared (IR), while the PL maximum spectral range from visible to near infrared (IR), while the PL maximum is located near is located near 500 nm. The relatively broad PL peak can be caused by the decomposition 500 nm. The relatively broad PL peak can be caused by the decomposition of the InGaN of the InGaN insertions on the phases with different contents of In [38] or by different In insertions on the phases with different contents of In [38] or by different In incorpora- incorporations in non-polar and polar wurtzite planes [39]. It also should be mentioned tions in non-polar and polar wurtzite planes [39]. It also should be mentioned that the that the inclined NWs and 2D parasitic layer can contribute to the PL signal, which can inclined NWs and 2D parasitic layer can contribute to the PL signal, which can broaden broaden the spectrum. the spectrum. Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 9 N Nanomaterials anomaterials 2022 2022,, 12 12, x FOR PEER REVIEW , 1993 5 of 5 of 9 9 Figure 3. Normalized to the maximum PL response of the GaN/InGaN/GaN NWs synthesized on Figure 3. Normalized to the maximum PL response of the GaN/InGaN/GaN NWs synthesized on Figure 3. Normalized to the maximum PL response of the GaN/InGaN/GaN NWs synthesized on the Si/SiO2 substrate. the Si/SiO2 substrate. the Si/SiO substrate. 3.2. Device Processing 3.2. De 3.2. Device vice Pr Processing ocessing Next, we carried out the postprocessing of the synthesized structures in order to fab- Next, we carried out the postprocessing of the synthesized structures in order to fab- Next, we carried out the postprocessing of the synthesized structures in order to ricate the functionalized devices. The workflow of processing is shown in Figure 4. To ri fabricate cate the fu thenct functionalized ionalized devi devices. ces. The The workfl workflow ow of processin of processing g is shown is shown in Figure 4. To in Figure 4. fabricate ohmic contacts to Si substrates, their back sides were treated with 10% HF aque- fab To r fabricate icate ohm ohmic ic contcontacts acts to Si to sub Sissubstrates, trates, their b their ack s back ides were sides tre wer ate e d wi treated th 1with 0% H10% F aque HF - aqueous ous solutsolution ion in order in or to der reto mo rv emove e the na the tiv native e oxide oxide layer and layer p and asspassivate ivate the sur thefsurface ace with hy with - ous solution in order to remove the native oxide layer and passivate the surface with hy- hydr drogen. Immediate ogen. Immediately ly after after that, that, the sub the s substrates trates were wer loa edloaded ed into into a vaa cuvacuum um cham chamber ber of a drogen. Immediately after that, the substrates were loaded into a vacuum chamber of a of therm a thermal al evapevaporator orator BocEd BocEdwar wards Ads uto Auto 500 to 500 depo to deposit sit Al con Altcontact act withwith a thic a kn thickness ess of 200 of thermal evaporator BocEdwards Auto 500 to deposit Al contact with a thickness of 200 200 nm. T nm. hen, Then, usinusing g spin-co spin-coating, ating, the sthe ides sides of the of sthe ubstr substrates ates with the with NWs the NWs were co wer ve ered w coveried th nm. Then, using spin-coating, the sides of the substrates with the NWs were covered with with photo-c photo-curing uring epoxy resin epoxy r(SU-8 negativ esin (SU-8 negative e photores photor ist). This provid esist). This pr ed ovided the elec the trical electrical isola- photo-curing epoxy resin (SU-8 negative photoresist). This provided the electrical isola- isolation between the substrate and front contact. The thickness of the SU-8 layer was tion between the substrate and front contact. The thickness of the SU-8 layer was 100-200 tion between the substrate and front contact. The thickness of the SU-8 layer was 100-200 100–200 nm less than the average length of the NWs. To remove the epoxy residue from the nm less than the average length of the NWs. To remove the epoxy residue from the ends nm less than the average length of the NWs. To remove the epoxy residue from the ends ends of the NWs, the substrates were treated in oxygen plasma. In the next technological of the NWs, the substrates were treated in oxygen plasma. In the next technological step, of the NWs, the substrates were treated in oxygen plasma. In the next technological step, step, we formed the mesa front contacts, which required the use of optical lithography, we formed the mesa front contacts, which required the use of optical lithography, con- we formed the mesa front contacts, which required the use of optical lithography, con- conductive layer deposition and the lift-off procedure. In the case of the test n-GaN NWs/n- ductive layer deposition and the lift-off procedure. In the case of the test n-GaN NWs/n- ductive layer deposition and the lift-off procedure. In the case of the test n-GaN NWs/n- Si structures, we evaporated the Al layer, which provided ohmic contact to GaN, while Si structures, we evaporated the Al layer, which provided ohmic contact to GaN, while Si structures, we evaporated the Al layer, which provided ohmic contact to GaN, while for the LED InGaN NWs-based structures, a thin layer of indium tin oxide (ITO) was for the LED InGaN NWs-based structures, a thin layer of indium tin oxide (ITO) was mag- for the LED InGaN NWs-based structures, a thin layer of indium tin oxide (ITO) was mag- magnetron sputtered. The ITO formed a transparent contact to the p-GaN shells of the LED netron sputtered. The ITO formed a transparent contact to the p-GaN shells of the LED netron sputtered. The ITO formed a transparent contact to the p-GaN shells of the LED structures. Note that only the longest NWs were in contact with the ITO, while the others structures. Note that only the longest NWs were in contact with the ITO, while the others structures. Note that only the longest NWs were in contact with the ITO, while the others were buried in the SU-8 layer. were buried in the SU-8 layer. were buried in the SU-8 layer. Figure 4. Workflow of NWs-based LEDs processing: (a) HF treatment, (b) evaporation of Al back Figure 4. Workflow of NWs-based LEDs processing: (a) HF treatment, (b) evaporation of Al back Figure 4. Workflow of NWs-based LEDs processing: (a) HF treatment, (b) evaporation of Al back contact, (c) covering with SU-8, (d) opening of NWs ends, (e) ITO sputtering. contact, (c) covering with SU-8, (d) opening of NWs ends, (e) ITO sputtering. contact, (c) covering with SU-8, (d) opening of NWs ends, (e) ITO sputtering. 3.3. Electrical and Electroluminescent Characterization 3.3. Electrical and Electroluminescent Characterization 3.3. Electrical and Electroluminescent Characterization The current-voltage (I-V) characteristics of the fabricated devices were measured The current-voltage (I-V) characteristics of the fabricated devices were measured The current-voltage (I-V) characteristics of the fabricated devices were measured with with the use of a Keithley 2401 source-meter. The samples were placed on a metallic table with the use of a Keithley 2401 source-meter. The samples were placed on a metallic table the use of a Keithley 2401 source-meter. The samples were placed on a metallic table of a of a probe station with a vacuum clamp. A contact to the face electrode was organized of a probe station with a vacuum clamp. A contact to the face electrode was organized probe station with a vacuum clamp. A contact to the face electrode was organized using Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 9 Nanomaterials 2022, 12, 1993 6 of 9 soft usinCuBe g soft CuBe probes, w probes, which mitigated hich mitig the atemechanical d the mechascratching nical scratcof hing the ostr f th uctur e stru es. ctu Figur res. Fe ig 5- pr ure esents 5 pre the sents the typical typic I-V curves al I-V c for urve the s for test the andtest LED and structur LED struc es. For tures. For all of theall o measur f the meas- ements, the urem positive ents, the input posi of tiv the e inp power ut of th supply e powe was r sup connected ply was to connected the probe. to the probe. Figure 5. I-V characteristics of the (a) test n-GaN NWs-based structure and (b) n-GaN/i-InGaN/GaN Figure 5. I-V characteristics of the (a) test n-GaN NWs-based structure and (b) n-GaN/i-InGaN/GaN NWs-based LED on Si. NWs-based LED on Si. Figure 5a shows a typical I-V curve for one of the test devices, where the NWs con- Figure 5a shows a typical I-V curve for one of the test devices, where the NWs tained only n-GaN cores (without InGaN active insertions and p-GaN shells). One can see contained only n-GaN cores (without InGaN active insertions and p-GaN shells). One can that the curve demonstrates rectifying behavior. Moreover, as shown in the additional see that the curve demonstrates rectifying behavior. Moreover, as shown in the additional experiments (not presented here), for this type of device, the reverse bias current depends experiments (not presented here), for this type of device, the reverse bias current depends on the level of illumination with visible or IR light. Together with the polarity of the I-V on the level of illumination with visible or IR light. Together with the polarity of the I-V curve, these evidence the emergence of a rectifying junction in the Si substrate. We suggest curve, these evidence the emergence of a rectifying junction in the Si substrate. We suggest that this can be caused by a doping to the p-type conductivity of the Si surface layer by that this can be caused by a doping to the p-type conductivity of the Si surface layer by Ga Ga atoms during MBE growth. The device demonstrates a current density up to 30 A/cm atoms during MBE growth. The device demonstrates a current density up to 30 A/cm under an applied positive voltage of 2V and more than 100 A/cm2 under a voltage of 3V under an applied positive voltage of 2V and more than 100 A/cm under a voltage of 3V (see the positive branch of the I-V curve in Figure 5a). Thus, a drop in the voltage on the (see the positive branch of the I-V curve in Figure 5a). Thus, a drop in the voltage on the interface of n-GaN/n-Si should be taken into account while analyzing the I-V characteristic interface of n-GaN/n-Si should be taken into account while analyzing the I-V characteristic of LED structures. of LED structures. Figure 5b presents a typical I-V curve for one of the LED n-GaN/i-InGaN/p-GaN de- Figure 5b presents a typical I-V curve for one of the LED n-GaN/i-InGaN/p-GaN vices. The knee voltage is about 6 V, while the typical current density corresponds to the devices. The knee voltage is about 6 V, while the typical current density corresponds to the 2 2 level of several A/cm . Considering the drop in the voltage on the n-GaN/n-Si interface level of several A/cm . Considering the drop in the voltage on the n-GaN/n-Si interface discussed previously, we can conclude that, for this level of current density, around 1-1.5 discussed previously, we can conclude that, for this level of current density, around 1–1.5 V V drops on the interface and the other voltage drops on the n-GaN/i-InGaN/p-GaN struc- drops on the interface and the other voltage drops on the n-GaN/i-InGaN/p-GaN structure and ture an thed p-GaN/IT the p-GaN/ITO interface O interface appear appe ed. ared Thus, . Thus, the the real real knee knee voltage voltage cancan be be fo found und to be to ar be around ound 4.54.5 V, which corresponds well to V, which corresponds well to the thexpected e expected value fo value for r this type of de this type of device. vice. The The electr electrolumine oluminesce scence nce (EL) (EL) of the L of the LED ED dev devices ices c can an be de be detected tected b by y th the e n naked aked eye eye for for a a curren currentt density densityexceeding exceeding 2 2 A/c A/cm m . However . Howev , e the r, estimation the estimaof tion o the curr f the ent cdensity urrent dens passing ity p thraough ssing the GaN/InGaN/GaN NWs is far from evident, since only part of the NWs is contacted to through the GaN/InGaN/GaN NWs is far from evident, since only part of the NWs is con- the ITO and, therefore, involved in the passing of current. tacted to the ITO and, therefore, involved in the passing of current. Figure 6 shows the obtained EL spectrum for the LED device. Note that only a small Figure 6 shows the obtained EL spectrum for the LED device. Note that only a small part of the NWs (the longest ones) was contacted by the ITO and, thus, could emit light. part of the NWs (the longest ones) was contacted by the ITO and, thus, could emit light. The EL spectrum demonstrates a dominant peak near 500–520 nm (corresponding to the The EL spectrum demonstrates a dominant peak near 500-520 nm (corresponding to the cyan-green color on the optical images) and a wide tail near the IR spectral range. This cyan-green color on the optical images) and a wide tail near the IR spectral range. This result corresponds well to the experimentally measured PL spectra (see Figure 3). The result corresponds well to the experimentally measured PL spectra (see Figure 3). The difference in spectra (the drop in the EL signal in the range of 550–650 nm wavelengths) difference in spectra (the drop in the EL signal in the range of 550-650 nm wavelengths) can be explained by the fact that only the longest (vertically aligned) NWs contribute to can be explained by the fact that only the longest (vertically aligned) NWs contribute to the measured EL signal. According to our experiments, the increase in current density the measured EL signal. According to our experiments, the increase in current density passing through the device led to the growth of the IR tail on the EL spectra, which can passing through the device led to the growth of the IR tail on the EL spectra, which can be be associated with the thermal heating of the NWs. However, the detailed analysis of this associated with the thermal heating of the NWs. However, the detailed analysis of this effect requires additional study. effect requires additional study. Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 9 Nanomaterials 2022, 12, 1993 7 of 9 Figure 6. Normalized to the maximum EL spectrum of the n-GaN/i-InGaN/GaN LED device. The Figure 6. Normalized to the maximum EL spectrum of the n-GaN/i-InGaN/GaN LED device. The insert demonstrates an optical image of the operating NWs-based LED. insert demonstrates an optical image of the operating NWs-based LED. 4. Conclusions 4. Conclusions In this work, we synthesized test n-GaN NWs and LED n-GaN/InGaN/p-GaN NWs In this work, we synthesized test n-GaN NWs and LED n-GaN/InGaN/p-GaN NWs using selective-area MBE growth on Si substrates with prepatterned SiO layers. To achieve using selective-area MBE growth on Si substrates with prepatterned 2 SiO2 layers. To the ordered arrays of the NWs, we employed microsphere lithography, providing the achieve the ordered arrays of the NWs, we employed microsphere lithography, providing handling of the large-area substrates. The PL measurements revealed that the synthesized the handling of the large-area substrates. The PL measurements revealed that the synthe- NWs have a relatively broad optical response, indicating the decomposition of the InGaN sized NWs have a relatively broad optical response, indicating the decomposition of the insertions on the phases with different contents of In. InGaN insertions on the phases with different contents of In. We studied the transport properties of the n-GaN/n-Si interface and demonstrated We studied the transport properties of the n-GaN/n-Si interface and demonstrated rectifying behavior on the I-V curves. The knee voltage of the fabricated n-GaN i-InGaN/p- rectifying behavior on the I-V curves. The knee voltage of the fabricated n-GaN i- GaN structures was found to be around 4.5 V, which is typical for GaN-based LED devices. InGaN/p-GaN structures was found to be around 4.5 V, which is typical for GaN-based The measured EL spectra for the fabricated LED devices demonstrated a peak located LED devices. The measured EL spectra for the fabricated LED devices demonstrated a around 500–520 nm and wide IR tail. The results of the EL measurements are consistent peak located around 500-520 nm and wide IR tail. The results of the EL measurements are with the PL study. consistent with the PL study. Supplementary Materials: The following supporting information can be downloaded at: https:// Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/nano12121993/s1, Figure S1: SEM image of the grown InGaN/GaN www.mdpi.com/xxx/s1, Figure S1: SEM image of the grown InGaN/GaN NWs. The inserts show NWs. The inserts show electron diffraction patterns acquired in the points EBSD11 and EBSD12. electron diffraction patterns acquired in the points EBSD11 and EBSD12. Author Contributions: Conceptualization, A.V., I.M. (Ivan Mukhin) and G.C.; Data curation, A.M. Author Contributions: Conceptualization, A.V., I.M. (Ivan Mukhin) and G.C.; Data curation, A.M. and D.M.; Formal analysis, A.M.; Funding acquisition, A.V., I.M. (Ivan Mukhin) and G.C.; Inves- and D.M.; Formal analysis, A.M.; Funding acquisition, A.V., I.M. (Ivan Mukhin) and G.C.; Inves- tigation, L.D., V.G., A.M. (Alexey Mozharov), A.M. (Alina Maksimova), A.D., I.M. (Ivan Melnichenko) tigation, L.D., V.G., A.M. (Alexey Mozharov), A.M. (Alina Maksimova), A.D., I.M. (Ivan and D.M.; Methodology, L.D.; Project administration, A.V., I.M. (Ivan Mukhin) and G.C.; Writing— Melnichenko) and D.M.; Methodology, L.D.; Project administration, A.V., I.M. (Ivan Mukhin) and original draft, I.M. (Ivan Mukhin) and G.C.; Writing—review & editing, L.D., V.G., A.M. (Alexey G.C.; Writing—original draft, I.M. (Ivan Mukhin) and G.C.; Writing—review & editing, L.D., V.G., Mozharov), A.M. (Alina Maksimova), A.D., I.M., D.M., A.V., I.M. (Ivan Melnichenko) and G.C. All A.M. (Alexey Mozharov), A.M. (Alina Maksimova), A.D., I.M., D.M., A.V., I.M. (Ivan Melnichenko) authors have read and agreed to the published version of the manuscript. and G.C. All authors have read and agreed to the published version of the manuscript. Funding: The samples were grown and then processed under the support of the Ministry of Science Funding: The samples were grown and then processed under the support of the Ministry of Science and Higher Education of the Russian Federation (state task № 0791-2020-0003 and № 0791-2020- and Higher Education of the Russian Federation (state task № 0791-2020-0003 and № 0791-2020- 0005, respectively). D.M. and A.V. thank the Russian Science Foundation (grant 21-79-10202) for 0005, respectively). D.M. and A.V. thank the Russian Science Foundation (grant 21-79-10202) for the the support of the study of growth substrate patterning. A.D. and I.M. (Ivan Melnichenko) thank support of the study of growth substrate patterning. A.D. and I.M. (Ivan Melnichenko) thank the the Basi Basic c Research ResearProg ch Pr ram ogram at the at the National National Resea Resear rch Univer ch University sity Higher Higher School School of Economics ( of Economics HSE Uni- (HSE University) for the support of the study of photoluminescence and electroluminescence. versity) for the support of the study of photoluminescence and electroluminescence. Data Data Availabil Availability ity Statement: Statement: The The data prese data present ned ted in this in this study study ar are available on request e available on request fr from the om the corr corresponding author. The data are esponding author. The data are not not pu publicly blicly available available due duto e to the au the author thor’s readine ’s readiness ss to to pr provide ovide it on request. it on request. 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Journal

NanomaterialsMultidisciplinary Digital Publishing Institute

Published: Jun 10, 2022

Keywords: molecular beam epitaxy; nanowires; III-N; Si; microsphere lithography; light-emitting devices

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