GaN Nucleation Layer Research Articles

Effect of grain coalescence on dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates

The effect of grain coalescence on the dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates with low-temperature grown GaN and physical vapour deposition AlN nucleation layers is comparably investigated.

Effect of High-Pressure GaN Nucleation Layer on the Performance of AlGaN/GaN HEMTs on Si Substrate.

A high-pressure (HP) GaN nucleation layer (NL) was inserted between AlGaN buffer and an unintentionally doped (UID) GaN layer of an AlGaN/GaN HEMT on Si. The XRD and TEM showed that when the V/III ratio was optimized during the HP-GaN NL growth, the edge dislocation density in the HP-GaN NL layer could be reduced significantly. Experimental results exhibited a lower off-state leakage current, higher maximum ID and Gm (corresponding to 22.5% and 21.7% improvement, respectively), and lower on-state resistance. These results demonstrate that the electrical properties of the AlGaN/GaN HEMT can be improved through the insertion of a HP-GaN NL.

Crack-free 2.2 μm-thick GaN grown on Si with a single-layer AlN buffer for RF device applications

The paramount issues facing commercialization of GaN-on-Si RF devices include the inevitable wafer bowing and large thermal resistance when incorporating the conventional AlN/AlGaN multilayer or superlattice buffer for stress control. The large thermal resistance due to the strong phonon scattering by the AlGaN alloy and multiple interfaces of the AlN/AlGaN multilayer or superlattice buffer must be carefully addressed, which can significantly affect the performance and reliability of GaN-on-Si RF devices. In this letter, we report a successful growth of crack-free 2 μm-thick GaN film on 6 inch high-resistivity (>10 kΩ cm) Si with a single-layer AlN buffer. By virtue of a quasi-2D GaN nucleation on the AlN buffer layer, the as-grown GaN-on-Si wafer presents a tremendous reduction in warpage from 800 to less than 8 μm, while preserving a high crystalline quality. The full width at half maximum of x-ray diffraction of GaN (0002) and (102) planes are 457 and 509 arcsec, respectively, and the residual stress is as low as 0.2 GPa. The underlying physical mechanisms of the quasi-2D GaN nucleation layer involved in stress engineering and dislocation annihilation have been discussed in detail. This work paves the way for the fabrication of low-cost and high-performance GaN RF devices grown on HR-Si.

N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD Part-II: Microstructural investigation

The present work is part-II of a study that addresses in detail the microstructural features of nitrogen-polar gallium nitride (N-polar GaN) deposited on c-plane sapphire wafers under the metalorganic chemical vapor deposition conditions described in the part-I work. The growth process was interrupted at various stages and characterized the depositions using transmission electron microscopy. The plan view diffraction study in combination with the Moiré fringe analysis revealed the presence of rotational domains of 4° in the low-temperature GaN nucleation layer. A crystallographic basis for the occurrence of these rotational domains is proposed based on the arrangement of surface atoms in the underlying sapphire substrate. Pure a dislocations are found to accommodate the twist between the rotational domains. In addition, Ga-polar inversion domains are observed in the matrix of N-polar GaN, and they originate at atomic level steps induced by the initial thermal cleaning and nitridation process of sapphire wafers at 1100°C. The rotational domains, the accommodating a dislocations and the inversion domains are replicated into the N-polar GaN epilayers.

Investigation of wurtzite formation in MOVPE-grown zincblende GaN epilayers on AlxGa1−xN nucleation layers

The influence of AlGaN nucleation layers on zincblende GaN epilayers was studied to investigate the formation of wurtzite phase inclusions in the epilayer. GaN epilayers grown on AlGaN nucleation layers with varying aluminum contents suffer from the increasing presence of wurtzite inclusions as the aluminum content of the nucleation layer increases. High-resolution transmission electron microscopy along with four-dimensional scanning transmission electron microscopy is used to investigate the origin of the wurtzite inclusions in the nucleation layer and at the GaN/AlGaN interface. It was observed that a GaN nucleation layer and an Al0.95Ga0.05N nucleation layer grew in the zincblende and wurtzite phase, respectively. These phases were then adopted by the overgrown GaN epilayers. For a GaN epilayer on an Al0.29Ga0.71N nucleation layer, wurtzite inclusions tend to form at the GaN/ Al0.29Ga0.71N interface due to strong {111}-type faceting observed in the zincblende nucleation layer. This strong faceting is correlated with an enrichment of aluminum in the upper part of the nucleation layer, as observed in energy dispersive x-ray spectroscopy, which may influence the kinetics or thermodynamics controlling the surface morphology.

Long-Range Orbital Hybridization in Remote Epitaxy: The Nucleation Mechanism of GaN on Different Substrates via Single-Layer Graphene.

Remote epitaxy is a very promising technique for the preparation of single-crystal thin films of flexibly transferred III-V group semiconductors. However, the epilayer nucleation mechanism of remote epitaxy and the epilayer-substrate interface interactions on both sides of graphene are not well-understood. In this study, remote homo- and heteroepitaxy of GaN nucleation layers (NLs) were performed by metal organic chemical vapor deposition on GaN, sapphire (Al2O3), and AlN substrates with transferred single-layer graphene, respectively. The results show that the interface damage of SLG/GaN at high temperature is difficult for us to achieve the remote homoepitaxy of GaN. Therefore, we explored the nucleation mechanism of remote heteroepitaxy of GaN on SLG/Al2O3 and SLG/AlN substrates. Nucleation density, surface coverage, diffusion coefficient, and scaled nucleation density were used to quantify the differences in nucleation information of GaN grown on different polar substrates. Using high-resolution X-ray diffraction and high-resolution transmission electron microscopy analysis, we revealed the interfacial orientation relationship and atomic arrangement distribution between the GaN NLs and substrates on both sides of the SLG. The electrostatic potential effect and adsorption ability of the substrates were further investigated by first-principles calculations based on density functional theory, revealing the principle that the substrate polarity affects the atomic nucleation density. The partial density of states shows that there is long-range orbital hybridization of the electronic states of the substrate and adsorbed atoms in remote epitaxy, and the crystal properties of the substrate play an important role in the in-plane orientation relationship of the NL and substrate across the SLG. The abovementioned results reveal the nature of remote epitaxy and broaden the perspective for the rapid and large-area preparation of single-crystal GaN films.

The role of growth temperature on the indium incorporation process for the MOCVD growth of InGaN/GaN heterostructures

Purpose The purpose of this paper is to investigate the effect of growth temperature on the evolution of indium incorporation and the growth process of InGaN/GaN heterostructures. Design/methodology/approach To examine this effect, the InGaN/GaN heterostructures were grown using Taiyo Nippon Sanso Corporation metal-organic chemical vapor deposition (MOCVD) SR4000-HT system. The InGaN/GaN heterostructures were epitaxially grown on 3.4 µm undoped-GaN (ud-GaN) and GaN nucleation layer, respectively, over a commercial 2” c-plane flat sapphire substrate. The InGaN layers were grown at different temperature settings ranging from 860°C to 820°C in a step of 20°C. The details of structural, surface morphology and optical properties were investigated using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), atomic force microscopy and ultraviolet-visible (UV-Vis) spectrophotometer, respectively. Findings InGaN/GaN heterostructure with indium composition up to 10.9% has been successfully grown using the MOCVD technique without any phase separation detected within the sensitivity of the instrument. Indium compositions were estimated through simulation fitting of the XRD curve and calculation of Vegard’s law from UV-Vis measurement. The thickness of the structures was determined using the Swanepoel method and the FE-SEM cross-section image. Originality/value This paper report on the effect of MOCVD growth temperature on the growth process of InGaN/GaN heterostructure, which is of interest in solid-state lighting technology, especially in light-emitting diodes and solar cell application.

Defect structures in (001) zincblende GaN/3C-SiC nucleation layers

The defect structure of zincblende GaN nucleation layers grown by metalorganic vapor-phase epitaxy on 3C-SiC/Si (001) was investigated by high-resolution scanning transmission electron microscopy. Perfect dislocations, partial dislocations, and stacking faults are present in the layers. Perfect dislocations are identified as 60° mixed-type and act as misfit dislocations to relieve the compressive lattice mismatch strain in GaN. Stacking faults are mainly bounded by 30° Shockley partial dislocations and rarely by Lomer–Cottrell partial dislocations, both of which are able to relieve the compressive lattice mismatch strain in the layer. We propose that the stacking faults and their partial dislocations originate from the dissociation of perfect dislocations present in the zincblende GaN layer and by direct nucleation of partial dislocations loops from the surface. These are the two main mechanisms that lead to the final defect structure of the zincblende GaN nucleation layers.

The Surface Morphology Evolution of GaN Nucleation Layer during Annealing and Its Influence on the Crystal Quality of GaN Films

The surface morphology evolution of GaN nucleation layer (NL) after different annealing time has been investigated by atomic force microscope. The surface morphologies of GaN NL after different annealing time are island-like. It is observed that for 0-min annealing time sample nucleation islands (NIs) are high density and small in size which results in high dislocation density GaN films, while the samples with longer annealing time have low density and large size NIs which results in low dislocation density GaN films. The crystal structure of GaN NLs after different annealing time investigated by high-resolution X-ray diffraction indicates GaN NL is polycrystalline. During annealing, GaN nanocrystals misaligned with sapphire gradually disappear and GaN nanocrystals aligned with sapphire survive and grow up. Thus, the GaN NL surface has different NIs’ densities and sizes after different annealing time. The NL annealing time can effectively control the density and size of the NIs and further determine the GaN films crystal quality.

Metalorganic vapour-phase epitaxy of AlGaN/GaN heterostructures on chlorine plasma etched GaN templates without buried conductive layer

In this work we present approach that allows regrowth of AlGaN/GaN heterostructures on plasma etched GaN templates without occurrence of buried conductive layer. Discussion about the influence of RIE (reactive ion etching) process on the properties of GaN surface is followed by presentation of experimental work results focused on reconstruction of GaN template surface after etching in chlorine plasma in order to growth AlGaN/GaN heterostructure without parasitic channel. Analysis of GaN surface treatment after RIE process using 10% aqueus HF solution and low temperature GaN (LT-GaN) nucleation layer is carried out, including SEM (scanning electron microscopy) imaging and SIMS (secondary ion mass spectroscopy) profiling. The AlGaN/GaN heterostructures with the thickness as low as 250 nm deposited on plasma etched GaN templates with sheet resistance under 600 Ω/□ and good uniformity were fabricated using this approach. Presented method can be used in fabrication of current aperture vertical electron transistor (CAVET) structures with low leakage currents.

Investigation of MOVPE-grown zincblende GaN nucleation layers on 3C-SiC/Si substrates

Cubic zincblende (zb-)GaN nucleation layers (NLs) grown by MOVPE on 3C-SiC/Si substrates were studied to determine their optimal thickness for subsequent zb-GaN epilayer growth. The layers were characterised by atomic force microscopy, X-ray diffraction and scanning transmission electron microscopy. The as-grown NLs, with nominal thicknesses varying from 3 nm to 44 nm, consist of small grains which are elongated in the [1 −1 0] direction, and cover the underlying SiC surface almost entirely. Thermal annealing of the NLs by heating in a H2/NH3 atmosphere to the elevated epilayer growth temperature reduces the substrate coverage of the films that are less than 22 nm thick, due to both material desorption and the ripening of islands. The compressive biaxial in-plane strain of the NLs reduces with increasing NL thickness to the value of relaxed GaN for a thickness of 44 nm. Both the as-grown and annealed NLs are crystalline and have high zincblende phase purity, but contain defects including misfit dislocations and stacking faults. The zb-GaN epilayers grown on the thinnest NLs show an enhanced fraction of the wurtzite phase, most likely formed by nucleation on the exposed substrate surface at elevated temperature, thus dictating the minimum NL thickness for phase-pure zb-GaN epilayer growth.

GaN Schottky Contact on the Pattern Sapphire Substrate with Reducted Threading Dislocation

INTRODUCTION Recently, the interest in high quality GaN films has in-creased, GaN and its alloys show good performance at high-power, high-frequency transistors. Because of large mismatches of lattice constant and thermal expansion between GaN epitaxial film and sapphire substrate, however, the high-density dislocation ranging from 108-1010cm-2 does degrade the diode performance profoundly. Thus, growth of GaN with low threading dislocation(TD) has been more and more important. The single-step growth on pattern sapphire substrate(PSS) has been widely used for fabrication of reliable high quality GaN films. To improve the Schottky diode’s performance, high TD density must be considered. Using the PSS technique, we can significantly reduce the TD density. For an ideal Schottky diode the I-V characteristic[1] is given by I=I0exp(qV/nkT) ． [1-exp(-qV/kT)] and I0=SAT2exp(-Фb/kT), In this work, the PSS method developed by ourselves[2] is used to improve the GaN epitaxial film quality, which is then applied to the fabrication of Schottky diode on the GaN film. At last, different pattern dimensions are compared to find an optimum GaN Schottky diode. DEVICE FABRICATION The PSS pattern was defined by Electron-Beam lithography system. The arrayed nanostructure was designed to be arranged on the commercial c-plane(001) sapphire substrate. The dimensions of the arrayed nanostructure are related to the quality of GaN and can be defined as dimension: 600nm, means spacing: 600nm, diameter: 600nm of the fabricated PSS hole array. Next, the reactive ion etching system(RIE) was used to transfer the pattern from photoresist to SiO2 mask, and then the mixed liquid of H2SO4(96%) and H3PO4(85%) was used to etch substrate at 230°C for 60 minutes to make a pyramid shape pattern. Metal-organic-chemical-vapor-deposition(MOCVD) was used to grow GaN thin films. The epitaxial growth began with a 25nm-thick GaN nucleation layer and then an insulating GaN of 4μm thickness as a device buffer layer. The buffer layer has a resistivity greater than 200MΩcm2 to prevent leakage. A 30nm unintentionally doped AlGaN layer and a 2nm GaN cap layer were then grown to complete the modulation doped structure. Schottky devices of both on the planar region and on the pattern region were fabricated on the same substrate for comparison as shown in Fig.1. The process began with mesa iso-selectively etch AlGaN/GaN to mesa etch for electrical isolation. After the dry-etch isolation, Ge/Ti/Al/Ti/Au(10/20/100/55/100nm) were evaporated and annealed at 860°C for 20sec to form source and drain ohmic contacts. THIN FLIM QUALITY ANALYSIS To analyze the quality of GaN thin films, Raman spectrum and EPD(Etching Pit Density) are jointly performed on AlGaN/GaN samples on the planar and pattern regions. The Raman shift and full-width-half-maximum(FWHM) of Raman spectrum were measured to compare the strain and quality of GaN on the planar region and the PSS region. As shown in the Fig.2-1, Raman shifts of the PSS region in different dimension is lower than the planar region and closer to freestanding GaN(567cm-1)[3], which means that the PSS region has less stress accumulation. Fig.2-2 shows that the FWHM trend is the same as Raman shift, which means the quality is improved as the PSS dimension goes down. The narrowest Raman FWHM implies the lowest-stress of GaN and thus the optimum crystal quality, while the widest Raman FWHM of the conventional planar region implies the worst crystal quality of GaN. The EPD experiment used H3PO4 at 230°C for 5 minutes. The dislocation density on the planar region is 3.87x106(cm-2), higher than 1.37x106(cm-2) on the pattern region. This proves that the FWHM of the Raman spectrum corresponds to the reliability of the crystal quality, and that our PSS method does improve the crystal quality. CONTACT RESISTANCE ANALYSIS Schottky contact resistance was measured as shown in Fig.4. As the PSS dimension goes down, the contact resistivity goes down. That is, the proposed GaN thin film on the pattern region has a better quality than that on the planar region. Also, the Schottky contact on the proposed pattern region has a lower resistivity than that on the conventional planar region. Conclusion We have fabricated high quality GaN thin films by MOCVD on the Pattern Sapphire Substrate(PSS). Both Raman spectrum and EPD show that the PSS can magnificently improve the GaN film quality. The defect density has a tendency to correlate with the electrical properties of the component. To reduce defect density and improve electrical performance, different pattern dimensions are designed in the Schottky diodes. By using our PSS technology, the EPD can be reduced from 3.87x106(cm-2) to 1.37x106(cm-2) and the Raman spectrum FWHM can be reduced from 2.46(cm-1) to 2.33(cm-1). Also, the Schottky contact resistivity can be reduced from 0.0657(Ωcm2) to 0.00266(Ωcm2). Figure 1

Effect of strain relaxation on performance of InGaN/GaN green LEDs grown on 4-inch sapphire substrate with sputtered AlN nucleation layer

Here we demonstrate high-brightness InGaN/GaN green light emitting diodes (LEDs) with in-situ low-temperature GaN (LT-GaN) nucleation layer (NL) and ex-situ sputtered AlN NL on 4-inch patterned sapphire substrate. Compared to green LEDs on LT-GaN (19 nm)/sapphire template, green LEDs on sputtered AlN (19 nm)/template has better crystal quality while larger in-plane compressive strain. As a result, the external quantum efficiency (EQE) of green LEDs on sputtered AlN (19 nm)/sapphire template is lower than that of green LEDs on LT-GaN (19 nm)/sapphire template due to strain-induced quantum-confined Stark effect (QCSE). We show that the in-plane compressive strain of green LEDs on sputtered AlN/sapphire templates can be manipulated by changing thickness of the sputtered AlN NL. As the thickness of sputtered AlN NL changes from 19 nm to 40 nm, the green LED on sputtered AlN (33 nm)/sapphire template exhibits the lowest in-plane compressive stress and the highest EQE. At 20 A/cm2, the EQE of 526 nm green LEDs on sputtered AlN (33 nm)/sapphire template is 36.4%, about 6.1% larger than that of the green LED on LT-GaN (19 nm)/sapphire template. Our experimental data suggest that high-efficiency green LEDs can be realized by growing InGaN/GaN multiple quantum wells (MQWs) on sputtered AlN/sapphire template with reduced in-plane compressive strain and improved crystal quality.

Microstructural Evolution of MOVPE Grown GaN by the Carrier Gas

We report the effect of total carrier gas flow of GaN during both GaN nucleation layer and high temperature GaN growth steps on structural, optical, electrical and morphological properties. The formation of dislocations in GaN layer and their effects were investigated in detail as a function of carrier gas flow. It has been found that the more carrier gas requires longer recovery time for transition from 3D (3 dimensional) to 2D growth and results in smaller edge-type dislocation density. The images obtained from the AFM measurements have shown terraces widths varying between 60–150 nm depending on the hydrogen flow rate.

Growth of III-N/graphene heterostructures in single vapor phase epitaxial process

We investigated the growth of III-N/graphene heterostructures as a single process in a MOVPE reactor. Raman spectra revealed that graphene can be successfully deposited on sapphire substrate by propane pyrolysis if temperature exceeds 1060 °C and hydrogen is used as a carrier gas. GaN epitaxial layers and heterostructures on such graphene using both high-temperature AlN buffer layer and low-temperature GaN nucleation layer was demonstrated. Analysis of surface morphology and X-Ray diffraction curves indicate that GaN quality depends on graphene thickness. Use of copper electroplated Ni-based contact layer combined with thermal shock allows exfoliation of large-area III-N LED structures from sapphire.

Vertical and Lateral GaN Power Devices Enabled By Engineered GaN Substrates

GaN has accrued significant research interest as a material platform for next-generation high power devices. This focused research thrust in GaN, fueled by its wide bandgap and high electric field strength, is expected to enable power devices with superior performance compared to the readily available Si power devices. However, the lack of uniform, high quality commercially available GaN substrates has bottlenecked the research progress of GaN high power devices. These substrate issues can be avoided by straying away from native GaN substrates and moving towards cost-effective engineered substrates designated QST™ (Qromis Substrate Technology) [1]. The QST™ substrate is comprised of a core that is thermally matched to GaN, a series of thin films encapsulating the core, and an epi-ready Si (111) surface for GaN epitaxy. Through its coefficient of thermal expansion (CTE) match to GaN, the QST™ substrate enables a material platform for thick, low dislocation density epitaxial layers suitable for high voltage devices. In this work, we present two fabrication processes enabled by QST™ substrates. First, true vertical GaN Schottky barrier diodes (SBDs) were designed from epitaxial GaN layers grown on large area (150 mm) QST™ substrates. The fabrication process is initiated with metal-organic chemical vapor deposition (MOCVD) of epitaxial GaN layers (2 μm GaN nucleation layer, 1.3 μm n+ GaN:Si contact layer with Nd = 5x1018 cm-3, and a 12.3 μm n- GaN drift layer Nd < 1x1016 cm-3) on the engineered QST™ substrate, followed by the deposition of Ni/Au (200 Å/2000 Å) Schottky contacts via a conventional lift-off procedure. The metal contacts and epitaxial GaN were then protected from the remaining etch steps with black wax and loaded metal face down onto a carrier substrate. A backside wet etch was used to selectively remove the QST™ substrate and transition layers. The backside contact was formed with 30 μm of electroplated Cu seeded by Ti/Au (100 Å/1000 Å). Lastly, the black wax was dissolved in xylene, releasing the freestanding vertical GaN device from the carrier substrate. Preliminary current-voltage experiments on true vertical GaN diodes fabricated without the 1.3 μm n+ GaN:Si contact layer (Nd = 5x1018 cm-3), edge termination, or surface passivation show a reverse breakdown voltage of 658 V, corresponding to a 0.58 MV/cm breakdown field. Second, lateral junction field effect transistors (LJFETs) were designed from the following epitaxial GaN layers grown on large area (150 mm) QST™ substrates: 7 μm semi-insulating layer, 1-2 μm n-GaN channel, 0.2 μm p- and 0.2 μm p+ GaN layers, and a 0.2 μm p++ GaN contact cap. To etch the source/drain regions, patterned photoresist on the p++ layer was used as a hard mask for etching approximately 750 nm GaN. An ohmic contact to the n-GaN was formed with Ti/Al/Ni/Au (200 Å/ 1200 Å/ 400 Å/ 800 Å), followed by a 30-second rapid thermal anneal at 850 °C in nitrogen gas. The Pd/Au (200 Å/ 1000 Å) gate metal was deposited on the p-GaN. Preliminary results demonstrate FET modulation. Improvement in the quality of the p layers is expected to improve performance. Moreover, the QST™ substrates provide an economical platform for optimizing epitaxial growth conditions to reach peak device performance. The feasible fabrication processes and promising preliminary results described in this work underline the potential of engineered GaN substrates for producing low cost vertical and lateral power devices. Reference: [1] Travis J. Anderson et al 2017 Appl. Phys. Express 10 126501.

Surface morphology of GaN nucleation layer grown by MOCVD with different carrier gas

GaN nucleation layers were grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD) with different carrier gas. The morphology of nucleation layer which is crucial to GaN crystal quality was systematically studied using nitrogen and hydrogen as carrier gas. The GaN nucleation layer tends to form larger islands under H2 ambient, and forms higher density nucleation islands and thicker nucleation layer under N2 ambient.

High-resistivity unintentionally carbon-doped GaN layers with nitrogen as nucleation layer carrier gas grown by metal-organic chemical vapor deposition

In this letter, high-resistivity unintentionally carbon-doped GaN layers with sheet resistivity greater than 106 Ω/□ have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). We have observed that the growth of GaN nucleation layers (NLs) under N2 ambient leads to a large full width at half maximum (FWHM) of (102) X-ray diffraction (XRD) line in the rocking curve about 1576 arc sec. Unintentional carbon incorporation can be observed in the secondary ion mass spectroscopy (SIMS) measurements. The results demonstrate the self-compensation mechanism is attributed to the increased density of edge-type threading dislocations and carbon impurities. The AlGaN/GaN HEMT grown on the high-resistivity GaN template has also been fabricated, exhibiting a maximum drain current of 478 mA/mm, a peak transconductance of 60.0 mS/mm, an ON/OFF ratio of 0.96×108 and a breakdown voltage of 621 V.

Selective area growth of N-polar GaN nanorods by plasma-assisted MBE on micro-cone-patterned c-sapphire substrates

The site-controlled selective area growth of N-polar GaN nanorods (NR) was developed by plasma-assisted MBE (PA MBE) on micro-cone-patterned sapphire substrates (µ-CPSS) by using a two-stage growth process. A GaN nucleation layer grown by migration enhanced epitaxy provides the best selectivity for nucleation of NRs on the apexes of 3.5-µm-diameter cones, whereas the subsequent growth of 1-μm-high NRs with a constant diameter of about 100nm proceeds by standard high-temperature PA MBE at nitrogen-rich conditions. These results are explained by anisotropy of the surface energy for GaN of different polarity and crystal orientation. The InGaN single quantum wells inserted in the GaN NRs grown on the µ-CPSS demonstrate photoluminescence at 510nm with a spatially periodic variation of its intensity with a period of ∼6µm equal to that of the substrate patterning profile.

Effects of GaN/AlGaN/Sputtered AlN nucleation layers on performance of GaN-based ultraviolet light-emitting diodes

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10–25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm.