Full-Color Single Nanowire Pixels for Projection Displays - Nano

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Full-Color Single Nanowire Pixels for Projection Displays Yong-Ho Ra, Renjie Wang, Steffi Yee-Mei Woo, Mehrdad Djavid, Sharif Md. Sadaf, Jaesoong Lee, Gianluigi A. Botton, and Zetian Mi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01929 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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Full-Color Single Nanowire Pixels for Projection Displays

Yong-Ho Ra†, Renjie Wang†, Steffi Y. Woo‡, Mehrdad Djavid†, Sharif Md. Sadaf†, Jaesoong Lee £, Gianluigi A. Botton‡ and Zetian Mi†*

†

Department of Electrical and Computer Engineering, McGill University 3480 University Street, Montreal, Quebec H3A 0E9, Canada

‡

Department of Materials Science and Engineering, Canadian Centre for Electron Microscopy McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada

£

Nano Electronics Lab, Samsung Advanced Institute of Technology, Suwon-si, Gyeonggi-do, Korea 443-803

*

: E-mail: [email protected]; Phone: +1-514-398-7114

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TABLE OF CONTENTS


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ABSTRACT Multi-color single InGaN/GaN dot-in-nanowire light emitting diodes (LEDs) were fabricated on the same substrate using selective area epitaxy. It is observed that the structural and optical properties of InGaN/GaN quantum dots depend critically on nanowire diameters. Photoluminescence emission of single InGaN/GaN dot-in-nanowire structures exhibits a consistent blueshift with increasing nanowire diameter. This is explained by the significantly enhanced indium (In) incorporation for nanowires with small diameters, due to the more dominant contribution for In incorporation from the lateral diffusion of In adatoms. Single InGaN/GaN nanowire LEDs with emission wavelengths across nearly the entire visible spectral were demonstrated on a single chip by varying the nanowire diameters. Such nanowire LEDs also exhibit superior electrical performance, with a turn-on voltage ~ 2V and negligible leakage current under reverse bias. The monolithic integration of full-color LEDs on a single chip, coupled with the capacity to tune light emission characteristics at the single nanowire level, provides an unprecedented approach to realize ultra-small and efficient projection display, smart lighting, and on-chip spectrometer.

KEYWORDS: nanowire, quantum dot, GaN, light emitting diode, display, selective area growth


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Driven by the need for smaller size, reduced power consumption, and enhanced efficiency and functionality, future ultrahigh resolution display technologies require the development of submicron scale, high efficiency, multi-color light sources monolithically integrated on a single chip.1 The challenges of organic light emitting diodes (OLEDs) for these applications include limited lifetime of organic materials,2 relatively expensive manufacturing process,2 low efficiency and brightness, and poor stability.3-5 Moreover, it has remained difficult to achieve micro or nanoscale devices using organic materials.6, 7 GaN-based quantum well LEDs are bright, stable and efficient, but usually only emit in one color.8-11 It has also remained difficult to achieve efficient deep green and red emission using GaN-based quantum well LEDs. Moreover, there is no established technology to spatially vary In compositions in quantum wells to achieve multi-color emission on the same substrate.12, 13 Recent studies have shown that such critical challenges can be potentially addressed by using InGaN nanowire structures. Nanowire LED heterostructures exhibit low dislocation densities and high light extraction efficiency.14-21 GaN-based nanowire LEDs and lasers operating in the ultraviolet (UV), blue-green, and red wavelength range have been demonstrated.22-33 Furthermore, it has been shown that multi-color emission can be achieved from InGaN nanowire arrays monolithically integrated on a single chip.15, 25, 27, 34-36 It is further envisioned that display technologies based on pixels of single nanowire LED arrays integrated on the same chip represent the ultimate light sources for the emerging three-dimensional (3D) projection display, flexible display, and virtual retinal display (VRD) technologies.37-40 The radiation pattern and emission direction can be well controlled and tailored by the one-dimensional columnar structure of each single nanowire,41-43 which is essential to achieving ultrahigh definition displays. In addition, pixels of single nanowire-based LED arrays can be much more efficient in heat dissipation and can operate at extremely large injection current levels.44-47 Critical to these technology developments is the demonstration of full-color, tunable light sources including LEDs and lasers using single, or a few nanowires on the same chip. This requires a precise tuning of alloy compositions in different nanowire structures and that these compositional 4 ACS Paragon Plus Environment

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variations should be ideally introduced in a single growth/synthesis step. Sekiguchi et al. reported that it was possible to produce multiple colors by varying the diameters of ensemble nanowires in a one-step selective area epitaxy, that is, without changing global growth parameters. 48 This approach takes advantage of the shadowing effect of neighboring nanowires to alter the InGaN composition. To date, however, little is known about the mechanism on how to controllably vary the alloy compositions at the single nanowire level without changing the global growth parameters. The monolithic integration of multi-color, single nanowire LEDs on the same chip has thus remained elusive. In this context, we report on the first demonstration of full-color, single nanowire LEDs monolithically integrated on the same chip, which is achieved by incorporating multiple InGaN/GaN quantum dots in GaN nanowires with various diameters grown in one-step selective area epitaxy. Through detailed scanning transmission electron microscopy (STEM) studies, it is observed that the position, size, and composition of InGaN quantum dots depend critically on the nanowire diameter. For small diameter nanowires, quantum dots with high In content are positioned at the center of nanowires and are vertically aligned along the c-axis. With increasing nanowire diameter, however, the formation of quantum dots with reduced In content becomes more dominant on the semipolar planes. By exploiting such unique diameter-dependent quantum dot formation, we have shown that tunable emission across nearly the entire visible spectral range can be realized from single InGaN/GaN dot-in-nanowire structures grown on the same substrate in a single epitaxy step. Multicolor InGaN/GaN nanowire LEDs are also fabricated on the same chip, which exhibit excellent current-voltage characteristics and strong emission in the blue, green, orange and red spectral ranges. This work offers a new avenue for achieving multi-color photonic devices at the single nanowire level on a single chip for a broad range of applications, including imaging, projection displays, sensing, spectroscopy, and communications. In this work, we have fabricated single InGaN nanowires of various sizes on the same substrate 5 ACS Paragon Plus Environment

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by selective area epitaxy using radio frequency plasma-assisted molecular beam epitaxy (PA-MBE). The epitaxy takes place on an n-type GaN template on sapphire substrate with a thin (10 nm) Ti layer being employed as the growth mask.49, 50 Opening sizes in the range of 80 nm to 1.9 µm were created on the Ti mask by using e-beam lithography and reactive ion etching techniques, which can lead to a precise control of the nanowire diameter. Schematically shown in Figure 1a, each nanowire consists of ~ 0.35 µm GaN, five vertically aligned InGaN/GaN quantum dots and ~ 0.15 µm GaN capping layer. These nanowires were grown using a Veeco GENxplor MBE system. The growth conditions include a substrate temperature of 1030 °C and a gallium (Ga) beam equivalent pressure (BEP) of 3×10-7 torr for the GaN segment. The substrate temperature was reduced to 795 °C, 810 °C and 825

°C for InGaN/GaN quantum dot active regions in Samples I, II, and III, respectively. The growth temperature mentioned here refers to the thermocouple reading near the backside (coated with molybdenum and titanium) of the sapphire substrate. In and Ga BEPs in the ranges of 1.2-1.5×10-7 torr and 5-7×10-9 torr were used for the growth of the quantum dot active regions. Figure 1b shows the field-emission scanning electron microscopy (SEM) image of the single nanowire structures grown with various diameters. The nanowires exhibit near-perfect hexagonal morphology and possess Ga-polarity based on the terminating facets.51 Photoluminescence (PL) emission for single InGaN nanowires was measured using a micro-PL measurement system at room-temperature with a 405 nm laser as the excitation source. Variations of the peak emission wavelengths vs. nanowire diameters are shown in Figure 1c. Nanowires in each group were grown on the same substrate with identical epitaxy conditions, except that their lateral sizes (also referred to as diameters in the subsequent text) were varied in the range of 150 nm to ~ 2 µm. Intriguingly, it is seen that the optical emission shows a consistent blueshift with increasing nanowire diameter under identical epitaxy conditions. Taking nanowires in Sample II as an example, the emission wavelengths can be continuously varied from 640 nm to 465 nm by increasing the nanowire diameters from 150 nm to 2 µm with otherwise identical epitaxy conditions. Similar trend is also observed for nanowires in 6 ACS Paragon Plus Environment

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Sample I and Sample III, though the tuning range is less broad due to variations in the growth conditions. The PL emission spectra for nanowires with different diameters are further shown in Figure 1d. The unique size-dependent optical emission from single nanowire structures has not been measured previously in catalyst-assisted or spontaneously formed InGaN nanowire arrays. The underlying mechanism is directly related to the diameter-dependent incorporation of In and Ga atoms at the nanowire growth front. Illustrated in Figure 1e, compared to the conventional planar epitaxy, the adatom incorporation in single nanowire epitaxy consists of directly impingent adatoms as well as adatoms migrated from the lateral surfaces. Under relatively high growth temperatures, Ga adatoms have much larger diffusion lengths (~ 1 µm) than In adatoms (~ 100 nm), with the latter limited by thermal desorption.48 Since the Ga diffusion length is comparable to, or larger than the nanowire diameters, it is expected that the Ga adatom incorporation shows a small, or negligible dependence on nanowire size. However, significantly reduced In incorporation is expected with increasing nanowire diameter, due the reduced In adatom incorporation from lateral diffusion. As the diameter increases, the directly impingent In adatoms do not lead to any decrease or increase in In content between nanowires, since the In BEP is the same across the wafer. In contrast, with increasing nanowire diameter, the reduced In adatom incorporation from lateral diffusion results in a reduced In content and therefore shorter emission wavelengths, shown in Figures 1c and d. Consequently, in single nanowire epitaxy, the variation of In content is largely determined by diameters of each single nanowire. These results are distinctly different from those of ensemble InGaN nanowires by selective area growth,48,

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which showed a redshift in emission with

increasing nanowire diameter, due to the shadowing effect of neighboring nanowires in arrays with high packing density. To further elucidate the mechanism of wavelength tuning, we have investigated InGaN nanowire arrays with controllably varying spacing amongst nanowires (see Figure S1 and Note S1 in Supporting Information). A consistent redshift with decreasing nanowire spacing is 7 ACS Paragon Plus Environment

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observed due to the reduced Ga incorporation related to the beam shadowing effect.21 Such beam shadowing effect, however, is not present for single nanowire epitaxy. In order to identify the correlation between composition and structure of InGaN/GaN quantum dot active region and the nanowire sizes, we have performed extensive structural characterization by aberration-corrected STEM using an FEI Titan Cubed 80-300 STEM operated at 200 kV. A crosssectional sample of nanowire arrays with different diameters was prepared by focused ion beam (FIB) in a single lift-out, milling at 30 kV using a Zeiss NVision 40 dual-beam system with deposited Pt and C as protection layer, and a final polish at 5 kV. The STEM results further reveal the nature and growth mechanism of InGaN/GaN dot-in-nanowires with different diameters. The high-angle annular dark-field (HAADF) atomic-number contrast images of different diameter nanowires grown on the same substrate are shown in Figure 2a. The high-resolution elemental mapping of Indium using electron energy-loss spectroscopy (EELS)54 of active regions in each nanowire, labelled with A, B, C, and D, are shown in Figure 2b. The In-distribution maps (and subsequent line profiles) have been normalized to the TEM cross-section thickness, which is approximately the same across the different diameter nanowires. In these In-distribution maps, the regions with bright intensity are rich in In, while the regions with dark intensity are In-deficient or In-free (n-GaN region at the bottom and p-GaN at the top of nanowires). InGaN quantum dots present clearly at the center of nanowires, shown in active regions A and B, and are vertically aligned along the c-axis. With increasing nanowire diameter, instead of the formation of InGaN quantum dots at the center of nanowires, InGaN accumulates at the semi-polar planes of nanowires, shown in active regions C and D. Shown in active regions of A, B, C, and D, the larger nanowire diameter, more InGaN is distributed at the semi-polar planes compared to the center of nanowire (the top of n-GaN). These observations clearly indicate that In incorporation at the nanowire top surface shows strong dependence on nanowire diameter. The structural analysis provides unambiguous evidence for the above hypothesis that In incorporation at the nanowire top during the growth of InGaN/GaN active region is hindered with 8 ACS Paragon Plus Environment

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increasing nanowire diameters due to the small diffusion length of In adatoms. Additionally, the accumulation of InGaN in larger diameter nanowires (such as active region C and D) towards the outer diameters on the semi-polar planes further suggests that In incorporation is largely dominated by In-adatom supplied from lateral diffusion. The two-dimensional EELS maps were analyzed along line 1 in active region A, line 2 in active region B, line 3 in active region C, and line 4 in active region D. The line profiles of In content derived from EELS analysis are summarized in Figure 2c. With increasing nanowire diameter from A, B, C, to D, the In content at the center of InGaN quantum dots decreases progressively. This result is consistent with the blue-shift of the PL peak position with increasing diameter of single nanowires, further supporting the proposed growth mechanism of single nanowires by using the selective area epitaxy. 55 To realize full-color (red, green, and blue – RGB) tunable single nanowire LED pixels integrated on the same chip, we have designed single nanowire LED arrays consisting of nanowires with varying diameters. The selective area epitaxy of such full-color single nanowire LED pixels is illustrated in Figures 3a and 3b. The InGaN/GaN dot-in-nanowire LED heterostructures, schematically shown Figure 3b, consist of 0.44 µm n-GaN, six InGaN/GaN quantum dots and 0.15 µm p-GaN. These samples were grown in a Veeco Gen II MBE system. The growth conditions include a substrate temperature of 965 °C and a Ga beam equivalent pressure (BEP) of 3.1×10-7 torr for Ge-doped GaN. The substrate temperature is reduced to 715 °C for the growth of InGaN/GaN quantum dot active regions. The temperature mentioned here refers to the thermocouple reading. The In and Ga beam equivalent pressures used for the growth of the quantum dot active regions are 2.1×10-7 torr and 3.2×10-8 torr, respectively. The growth conditions for the Mg-doped GaN layer included a Ga BEP of 3.1×10-7 torr, Mg BEP of 1.86×10-9 torr, and substrate temperature of 965 °C. It is noticed that the growth parameters are different from those used for single nanowire PL studies, due to the use of a different MBE reactor. Under these growth conditions in the Veeco Gen II MBE system, emission wavelengths across nearly the entire visible spectral range can be realized for 9 ACS Paragon Plus Environment

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nanowires with diameters varying from ~ 200 nm to ~ 600 nm. Figure 3c shows the SEM image of patterned Ti mask with opening hole sizes of ~ 150, 250, 350 and 550 nm. The lateral growth effect was also taken into account in the pattern design. Shown in Figure 3d is a 30° tilted SEM image of InGaN/GaN nanowire arrays grown with four different diameters of ~ 220, 320, 420 and 630 nm. The nanowires have an average height ~ 650 nm, with near-perfect hexagonal morphology and smooth lateral surface, which can contribute to the enhanced light emission from the nanowire top surface. Nanowires with these design parameters and operation wavelengths also exhibit nearly maximum light extraction efficiency (see Figure S2 and Note S2 in Supporting Information). Schematically shown in Figure 4a, multi-color single InGaN/GaN nanowire LEDs were subsequently fabricated on a single chip. First, a polyimide resist layer was spin-coated to fully cover the nanowires, followed by oxygen plasma etching to reveal the top surface of nanowires, shown in Figure 3e. Nanoscale metal electrodes, consisting of Ni (7 nm)/Au (7 nm) metal layers were then deposited on the p-GaN top surface of individual nanowires using e-beam lithography and metallization techniques and then annealed at ~500 °C for 1 min in nitrogen ambient. Subsequently, a 100 nm indium tin oxide (ITO) layer was deposited to serve as a transparent electrode. The complete devices with ITO contacts were annealed at 300 °C for 1 hour in vacuum. Contact metal grids consisting of Ni (20 nm)/Au (100 nm) were then deposited on the ITO to facilitate current injection and device testing. Subsequently, a Ti (20 nm)/Au (100 nm) n-metal contact was deposited on the n-type GaN template and then annealed at ~500 °C for 1 min in nitrogen ambient. Shown in Figure 4b is the optical microscopy image of the single nanowire LED pixel devices after surface passivation and contact metallization. The performance characteristics of single InGaN/GaN dot-in-nanowire nanowire LEDs were measured under continuous wave biasing conditions at room-temperature. Figure 4c shows representative I-V curves of the blue (D ~ 630 nm), green (D ~ 420 nm), orange (D ~ 320 nm) and red (D ~ 220 nm) emitting devices, which exhibit excellent current-voltage (I-V) characteristics. The 10 ACS Paragon Plus Environment

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nanowire LEDs have turn-on voltages ~ 2 V, which is significantly better than previously reported ensemble nanowire LEDs and GaN-based planar devices.44, 56 Current densities as high as 7 kA/cm2 were measured at ~ 3 V. It is also noticed that higher current densities can be achieved in nanowire LEDs with smaller diameters. This is largely due to the significantly enhanced dopant incorporation in smaller diameter nanowires and the resulting efficient current conduction,57, 58 as well as the more efficient heat dissipation.44 The capacity for sustaining higher current densities with decreasing device area has also been reported previously.44,

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optoelectronic and electronic devices can handle unusually large current densities and can deliver extremely high power density compared to conventional planar devices. It is also seen that the leakage current under reverse bias is relatively small, but increases with increasing nanowire diameter, shown in the inset of Figure 4c, which is likely due to the presence of defects in large diameter nanowires and the resulting current leakage. This observation is in reasonable agreement with previous studies.59, 60 Such single nanowire LEDs also exhibit excellent light emission characteristics. The electroluminescence (EL) emission was collected using an optical fiber coupled to a high-resolution spectrometer and detected by a charge coupled device (CCD). Shown in Figure 4d are the EL emission spectra of single nanowire LED sub-pixels with diameters of 220, 320, 420 and 630 nm, which exhibit peak emission wavelengths of 659, 625, 526 and 461 nm, respectively. The spectra were taken at an injection current of approximately 4.5 µA. Light-current (L-I) characteristics of the red, orange, green, and blue single nanowire LED sub-pixels are shown in Figure 4e. It is seen that the light intensity increases near-linearly with injection current for different nanowire LEDs. Stronger light intensity was measured from nanowires with larger diameters under the same injection current density, due to the larger active region area. On the other hand, nanowire LED sub-pixels with smaller diameters can handle higher current density, due to the more efficient current conduction and heat dissipation. Shown in the inset of Figure 4e are the EL spectra of the green11 ACS Paragon Plus Environment

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emitting nanowire LED pixel. There is no significant shift in the emission peak position with increasing injection current, suggesting a small level of quantum-confined Stark-effect, due to the highly efficient strain relaxation of nanowire structures. It is also worthwhile mentioning that, by engineering the nanowire diameter and height, single nanowire LEDs can offer significantly higher light extraction efficiency and more controllable emission pattern, compared to conventional planar LEDs (see Figure S2 and S3 and Note S2 in Supporting Information). Moreover, it is further expected that, with the incorporation of p-AlGaN electron blocking layer and core-shell schemes, the performance of single InGaN/GaN nanowire LEDs can be dramatically improved by reducing nonradiative surface recombination and carrier leakage and overflow.24, 56, 61-63 In summary, we have demonstrated multi-color, single nanowire LED arrays on the same chip by using the special technique of selective area epitaxy. Compared to conventional planar devices, such nanowire LED pixels offer several distinct advantages, including significantly reduced dislocations and polarization fields, enhanced light extraction efficiency, controllable radiation pattern, tunable emission, and extremely efficient current conduction and heat dissipation. Moreover, due to the extremely small size and reduced capacitance, such nanowire devices also promise ultrahigh speed frequency response. This work provides a unique approach for realizing tunable, fullcolor nanoscale optoelectronic devices for a broad range of applications, including ultra-fine imaging and projection display, lighting, communication, sensing, and medical diagnostics on a single chip.

ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada and Samsung Corporation. Part of the work was performed in the McGill Nanotools-Microfab facility. SEM studies were performed in the Facility for Electron Microscopy Research, McGill University. STEM and EELS investigations were performed in the Canadian Centre for Electron Microscopy, a national facility supported by NSERC, the Canada Foundation for Innovation under the MSI 12 ACS Paragon Plus Environment

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program, and McMaster University. The authors wish to thank Mr. David Laleyan and Dr. Songrui Zhao for their help with the operation and maintenance of the MBE systems, and Mr. Travis Casagrande for the FIB sample preparation.

SUPPORTING INFORMATION Supporting Information Available: Additional information on the effect of spacing of ensemble nanowires on the emission wavelengths, the simulation of light extraction efficiency vs nanowire diameter, and light extraction efficiency for different nanowire design parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR CONTRIBUTIONS Y.H.R. and R.W. contributed equally to this work. Y.H.R., Z.M. and J.S.L. designed the experiments. Y.H.R. and R.W. fabricated the selective area pattering mask substrate. R.W. and Y.H.R conducted the MBE growth of nanowires. Y.H.R. performed SEM analysis. R.W. performed micro-PL measurements. Y.H.R. carried out the device fabrication. Y.H.R. and S.M.S contributed to the electrical injection measurements. S.Y.W. and G.A.B conducted the TEM analysis. M.D. performed light extraction efficiency dependent single nanowire simulation. The paper was written by Y.H.R. and Z.M. with contributions from other coauthors.

COMPETING FINANCIAL INTEREST The authors declare no competing financial interests.


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29. Ra, Y. H.; Navamathavan, R.; Park, J. H.; Lee, C. R. ACS Appl. Mater. Interfaces 2013, 5, 21112117. 30. Lu, Y. J.; Wang, C. Y.; Kim, J.; Chen, H. Y.; Lu, M. Y.; Chen, Y. C.; Chang, W. H.; Chen, L. J.; Stockman, M. I.; Shih, C. K.; Gwo, S. Nano Lett. 2014, 14, 4381-4388. 31. Qian, F.; Gradecak, S.; Li, Y.; Wen, C. Y.; Lieber, C. M. Nano Lett. 2005, 5, 2287-2291. 32. Frost, T.; Jahangir, S.; Stark, E.; Deshpande, S.; Hazari, A.; Zhao, C.; Ooi, B. S.; Bhattacharya, P. Nano Lett. 2014, 14, 4535-4541. 33. Zhao, S.; Djavid, M.; Mi, Z. Nano Lett. 2015, 15, 7006-7009. 34. Wright, J. B.; Liu, S.; Wang, G. T.; Li, Q.; Benz, A.; Koleske, D. D.; Lu, P.; Xu, H.; Lester, L.; Luk, T. S.; Brener, I.; Subramania, G. Sci. Rep. 2013, 3, 2982. 35. Albert, S.; Bengoechea-Encabo, A.; Sanchez-Garcia, M. A.; Kong, X.; Trampert, A.; Calleja, E.

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FIGURE CAPTIONS

Figure 1: (a) Schematic of a single InGaN/GaN dot-in-nanowire structure grown on GaN template on sapphire substrate. (b) SEM image of single InGaN/GaN nanowires with various diameters. (c) Variations of the peak emission wavelength of single InGaN/GaN nanowires vs. nanowire diameter with the quantum dot active regions grown at 795 ºC (Sample I), 810 ºC (Sample II), and 825 ºC (Sample III). (d) Normalized PL spectra of InGaN/GaN dot-in-nanowire structures in Sample II with different diameters (D) measured at room-temperature, showing a blueshift in the emission peak with increasing nanowire diameter. (e) Schematic illustrations of the adatom incorporation at the nanowire growth front due to both direct impingement and lateral diffusion (left), the significant contribution of In adatom incorporation due to lateral diffusion for small diameter nanowires (center), and the reduced In incorporation from lateral diffusion for large diameter nanowires (right).

Figure 2: (a) A STEM-HAADF image for InGaN/GaN dot-in-nanowire structures with different diameters grown on GaN template on sapphire substrate along zone-axis. The active region of nanowires with diameters of ~320 nm, ~420 nm, ~500 nm and ~595 nm are labelled as A, B, C, and D, respectively. (b) High-resolution STEM-EELS maps of the In-distribution of active regions A, B, C, and D normalized to the sample thickness. Line profiles were integrated along areas as marked by the dashed red line in each active region. (c) Elemental profiles of relative In content derived from EELS analysis along line 1 in active region A, line 2 in active region B, line 3 in active region C, and line 4 in active region D, showing higher In-content in smaller diameter wires.


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Figure 3: (a) and (b) Schematic illustration for the selective area epitaxy of multi-color single nanowire LED pixel arrays. Single InGaN/GaN dot-in-nanowire LED structures are grown on pattered Ti-mask on GaN template on sapphire substrate. Different emission colors are achieved by varying the nanowire diameters in a single epitaxy step. (c) 30° tilted SEM image of a patterned Ti mask, showing the presence of hexagonal openings with different sizes. (d) SEM image of single InGaN/GaN dot-in-nanowires with different diameters grown by selective area epitaxy. (e) Top-view SEM image of the exposed p-GaN nanowire top-surface after polyimide passivation and dry etching.

Figure 4: (a) Schematic illustration of monolithically integrated multi-color single nanowire LED pixels on a single chip. (b) Top-view optical microscopy image of the single nanowire LED pixel devices after surface passivation and contact metallization. (c) Current-voltage characteristics of single nanowire LEDs with different diameters. Inset: current density vs. voltage in a semilog plot, showing increasing leakage current for nanowire LEDs with larger diameters. (d) Electroluminescence (EL) spectra of single nanowire LEDs with different diameters. (e) Light-current characteristics of single nanowire LED devices with different diameters. The inset shows the EL spectra measured under different injection current densities (1.3 to 6.5 kA/cm2) for the green-emitting nanowire LED device.

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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Full-Color Single Nanowire Pixels for Projection Displays - Nano

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