Low-temperature GaN Buffer Layer Research Articles

Growth of Ga0.70In0.30N/GaN Quantum-Wells on a ScAlMgO4 (0001) Substrate with an Ex-Situ Sputtered-AlN Buffer Layer.

This study attempted to improve the internal quantum efficiency (IQE) of 580 nm emitting Ga0.70In0.30N/GaN quantum-wells (QWs) through the replacement of a conventional c-sapphire substrate and an in-situ low-temperature GaN (LT-GaN) buffer layer with the ScAlMgO4 (0001) (SCAM) substrate and an ex-situ sputtered-AlN (sp-AlN) buffer layer, simultaneously. To this end, we initially tried to optimize the thickness of the sp-AlN buffer layer by investigating the properties/qualities of an undoped-GaN (u-GaN) template layer grown on the SCAM substrate with the sp-AlN buffer layer in terms of surface morphology, crystallographic orientation, and dislocation type/density. The experimental results showed that the crystallinity of the u-GaN layer grown on the SCAM substrate with the 30 nm thick sp-AlN buffer layer [GaN/sp-AlN(30 nm)/SCAM] was superior to that of the conventional u-GaN template layer grown on the c-sapphire substrate with an LT-GaN buffer layer (GaN/LT-GaN/FSS). Notably, the experimental results showed that the structural properties and crystallinity of GaN/sp-AlN(30 nm)/SCAM were considerably different from those of GaN/LT-GaN/FSS. Specifically, the edge-type dislocation density was approximately two orders of magnitude higher than the screw-/mixed-type dislocation density, i.e., the generation of screw-/mixed-type dislocation was suppressed through the replacement, unlike that of the GaN/LT-GaN/FSS. Next, to investigate the effect of replacement on the subsequent QW active layers, 580 nm emitting Ga0.70In0.30N/GaN QWs were grown on the u-GaN template layers. The IQEs of the samples were measured by means of temperature-dependent photoluminescence efficiency, and the results showed that the replacement improved the IQE at 300 K by approximately 1.8 times. We believe that the samples fabricated and described in the present study can provide a greater insight into future research directions for III-nitride light-emitting devices operating in yellow-red spectral regions.

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

Growth of high-quality nitrogen-polar GaN film by two-step high-temperature method

Normally, nitrogen-polar (N-polar) GaN films with better crystalline quality tend to have poor surface morphology, whereas those with good surface morphology suffer from degraded crystalline quality. In this work, we use a two-step high-temperature (HT) growth method to grow N-polar GaN films on vicinal c-plane sapphire substrates by metalorganic chemical vapor deposition, which can effectively solve the irreconcilability between surface morphology and crystalline quality. The GaN film consists of two GaN layers (HT-GaN 1 and HT-GaN 2), which were grown at different growth parameters, respectively. The 300-nm-thick HT-GaN 1 layer was directly grown on the low-temperature GaN buffer layer under the higher V/III ratio and pressure at 1040 °C to reduce dislocation density. The 1.7-μm-thick HT-GaN 2 layer was grown under the lower V/III ratio and pressure to improve surface morphology. Besides, the growth temperature of HT-GaN 2 layer was ramped up to 1070 °C to suppress the yellow-band luminescence. Finally, high-quality N-polar GaN films with smooth surface morphology and high crystalline quality were obtained, for which the full-width at half-maximum values of (0002) and (101¯2) planes X-ray diffraction rocking curves are 131 and 390 arcsec, respectively, the root mean square roughness over an area of 20 × 20 μm2 is 1.88 nm, and the residual compressive stress is as low as 0.025 GPa. It is reasonably believed that this work provides an effective approach to preparing high-quality N-polar GaN films for the application of N-polar nitride devices.

Macrodefects investigation in a-GaN films

Investigation of the effect of crystal defects on nonpolar a-GaN films grown by metalorganic chemical vapor deposition on r-sapphire substrates is presented. Samples were grown at different temperatures and the same V/III ratio. High-temperature nucleation in combination with low-temperature GaN buffer layer growth was used to achieve a smooth morphology and “mirror” surface. The macrostructures of the defects were investigated using a scanning electron microscope. Defect concentration via dislocation type correlation was analyzed by diffractometry investigation. Structural quality was investigated via x-ray diffraction. The correlation between the densities of mixed threading dislocations and V-defects was determined. It was detected that the origin of V-defects on the a-plane and c-plane was the same despite having completely different geometries and structures. We also investigated the dependence of V-defect density on a-GaN film growth temperature at a constant hydrogen flow through a triethylgallium source, and we also investigated how the V/III ratio influenced the V-defect structure. Optimum growth parameters for defect concentration reduction were determined.

Improving Transport Properties of GaN-Based HEMT on Si (111) by Controlling SiH4 Flow Rate of the SiNx Nano-Mask

The AlGaN/AlN/GaN high electron mobility transistor structures were grown on a Si (111) substrate by metalorganic chemical vapor deposition in combination with the insertion of a SiNx nano-mask into the low-temperature GaN buffer layer. Herein, the impact of SiH4 flow rate on two-dimensional electron gas (2DEG) properties was comprehensively investigated, where an increase in SiH4 flow rate resulted in a decrease in edge-type threading dislocation density during coalescence process and an improvement of 2DEG electronic properties. The study also reveals that controlling the SiH4 flow rate of the SiNx nano-mask grown at low temperatures in a short time is an effective strategy to overcome the surface desorption issue that causes surface roughness degradation. The highest electron mobility of 1970 cm2/V·s and sheet carrier concentration of 6.42 × 1012 cm−2 can be achieved via an optimized SiH4 flow rate of 50 sccm.

Structural and morphological characteristics of GaN-based hybrid heterostructures grown on por-Si

GaN/Si(111) heterostructures were grown by plasma-assisted molecular beam epitaxy on usual Si(111) substrates and compliant por-Si/Si(111) substrates without using AlN buffer layer. The positive influence of the high-temperature nitridation step, as well as the usage of a compliant substrate on crystalline quality, was confirmed. We got a crack-free 850-nm-thick GaN layer at room temperature by using the low-temperature GaN buffer layer with nanocolumnar morphology.

Growth of millimeter wave AlN/GaN heterostructures by MOCVD

The influences of growth temperature on crystalline quality and electrical properties of thin barrier AlN/GaN heterostructures were studied by metal organic chemical vapor deposition (MOCVD). It is found that the high growth temperature (≥1100 °C) is beneficial to improve the crystalline quality of the AlN barrier layer, but increases the defect density in the GaN buffer layer. The AlN/GaN hetero-interface quality was deteriorated when high-temperature AlN barrier layer was grown upon low-temperature GaN buffer layer, resulting in a reduction of electron mobility. In addition, the growth temperature of the GaN cap layer affects the stress evolution of wafer during the cooling process, which influences the wafer bowing and electrical properties. By optimizing the growth temperature of each functional layer, the GaN/AlN/GaN heterostructure with a low defect density and a smooth surface is obtained. The electron mobility at room temperature is 1549 cm2/(V·s) with a sheet resistance value of 504.8 Ω/□, and the root mean square (RMS) of surface roughness is 0.322 nm. Based on the as-grown GaN/AlN/GaN heterostructure, a GaN power MMIC was fabricated, offering an output power of 15 dBm (31.6 mW) and power added efficiency (PAE) of 9.3% at 146 GHz.

Effect of high‐temperature AlN buffer on anisotropy of semi‐polar (11‐22) GaN with two pressure growth stages

The low-temperature GaN (LT-GaN) and high-temperature AlN (HT-AlN) buffer layers were grown on m -plane sapphire by metal–organic chemical vapour deposition. The following semi-polar (11-22) GaN thin films were deposited under high- and low-pressure growth stages. Anisotropy of (11-22) GaN grown on HT-AlN buffer along two in-planar directions ([11-2-3] and [-1100]) were clearly suppressed, the maximum variation of the X-ray rocking curve full width at half maximum of (11-22) GaN on LT-GaN buffer was 0.2498°, that of (11-22) GaN on HT-AlN buffer was 0.0488°. The X-ray diffraction results of on-axis and off-axis both indicated that the crystal quality of the epitaxial GaN layer with HT-AlN buffer was obviously improved, and surface morphology was much smoother.

Improvement of emission efficiency with a sputtered AlN buffer layer in GaInN-based green light-emitting diodes

We examined a sputtered AlN (sp-AlN) buffer layer to improve the emission efficiency of GaInN-based green LEDs. Both the electroluminescence and photoluminescence emission efficiencies are improved in a LED with a sp-AlN buffer layer compared to that with a conventional low-temperature GaN buffer layer. To identify the origin of the improvements, we carried out the analysis including X-ray diffraction, surface morphology, initial nucleation growth, and temperature-dependent PL. Through analysis and consideration, we showed that the improvements are mainly caused by less strain and a lower degree of carrier localization in quantum wells (QWs), that is highly influenced by the grain diameter of the GaN island at initial growth. These results indicate that a sp-AlN buffer layer induces less compositional pulling that results in less strain and alloy fluctuation in QWs, simultaneously.

Enhanced Device Performance of GaInN-Based Green Light-Emitting Diode with Sputtered AlN Buffer Layer

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I–V curves, electroluminescence spectra, L–I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region.

Mechanism for the formation of nitrogen-filled voids after annealing of GaN on a sapphire substrate

This study investigated the formation of nitrogen-filled voids at the interface between a GaN layer grown on a sapphire substrate by metalorganic vapor phase epitaxy. These voids were formed in the sapphire substrate at the interface after annealing and previous research shows that they can induce an inversion domain in AlN, which affects the film quality and the device performance. We investigated the void formation using scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray spectrometry, and electron energy loss spectroscopy. The voids are known to originate from the decomposition of sapphire in the presence of ammonia and hydrogen. Our analysis confirmed that the outgassing reaction between the decomposing sapphire and the low temperature GaN buffer layer resulted in the diffusion of aluminum and oxygen into the GaN buffer layer. During the annealing process, oxygen ions replaced nitrogen ions and created nitrogen-filled voids at the interface between the GaN buffer layer and the sapphire substrate. The presence of molecular nitrogen in the voids was confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy.

Microstructural and optical properties of GaN buffer layers grown on graphene

A conventional two-step growth process is still the state-of-the-art technology for the epitaxy of GaN, even if graphene is used as a substrate. Low-temperature buffer layers play a decisive role in forming a continuous film. In this work, the informative characterization of low-temperature (550 °C) GaN buffer layers grown on graphene by hydride vapor phase epitaxy (HVPE) was performed. High-density and sixfold-symmetry structures of GaN nucleation sites formed the primary morphology of buffer layers. The E2 (high) Raman shift of GaN buffer layers on graphene was 568 cm−1, illustrating that GaN was free of stress. We also captured nonfocusing photoluminescence (PL) spectra and observed the high intensity of near-band-edge emission of wurtzite GaN and the absence of yellow luminescence in the visible band. Thermally induced quantum effects could be found in low-temperature PL. The elementary and detailed characterizations offer referenced values for the growth of GaN films on graphene.

V-shaped pits in HVPE-grown GaN associated with columnar inversion domains originating from foreign particles of α-Si3N4 and graphitic carbon

The v-shaped pits (so-called V-pits) observed in hydride-vapor-phase-epitaxy-grown GaN and associated with the columnar inversion domains originating from foreign particles were investigated. The inversion domains on the front and back surfaces of the test sample were recognized after chemical mechanical polishing. It was found that the V-pits originate from the columnar inversion domains. The inversion domains, in turn, arise from the particles that exist on a low-temperature GaN buffer layer on sapphire substrate. Using transmission electron microscopy, these particles were found to be of α-Si3N4 and graphitic carbon. Such particles are attributable to the components of the reactor and adhere to the low-temperature GaN buffer layer, which has a surface roughness of the order of several nanometers. Thus, an effective way of obtaining HVPE-grown thick GaN layers without the V-pits associated with columnar IDs is to maintain the parts of the HVPE chamber properly to prevent foreign particles from being generated.

High-quality III-nitride films on conductive, transparent (2̅01)-oriented β-Ga2O3 using a GaN buffer layer

We demonstrate the high structural and optical properties of InxGa1−xN epilayers (0 ≤ x ≤ 23) grown on conductive and transparent (01)-oriented β-Ga2O3 substrates using a low-temperature GaN buffer layer rather than AlN buffer layer, which enhances the quality and stability of the crystals compared to those grown on (100)-oriented β-Ga2O3. Raman maps show that the 2″ wafer is relaxed and uniform. Transmission electron microscopy (TEM) reveals that the dislocation density reduces considerably (~4.8 × 107 cm−2) at the grain centers. High-resolution TEM analysis demonstrates that most dislocations emerge at an angle with respect to the c-axis, whereas dislocations of the opposite phase form a loop and annihilate each other. The dislocation behavior is due to irregular (01) β-Ga2O3 surface at the interface and distorted buffer layer, followed by relaxed GaN epilayer. Photoluminescence results confirm high optical quality and time-resolved spectroscopy shows that the recombination is governed by bound excitons. We find that a low root-mean-square average (≤1.5 nm) of InxGa1−xN epilayers can be achieved with high optical quality of InxGa1−xN epilayers. We reveal that (01)-oriented β-Ga2O3 substrate has a strong potential for use in large-scale high-quality vertical light emitting device design.

Polarity control of GaN grown on pulsed-laser-deposited AlN/GaN template by metalorganic vapor phase epitaxy

We report on the polarity control of GaN regrown on pulsed-laser-deposition-grown N-polar AlN on a metalorganic-vapor-phase-epitaxy-grown Ga-polar GaN template. The polarity of the regrown GaN, which was confirmed using aqueous KOH solutions, can be inverted from that of AlN by inserting a low-temperature GaN (LT-GaN) buffer layer. We hypothetically ascribe the Ga-polarity selection of GaN on the LT-GaN buffer layer to the mixed polarity of LT-GaN grains and higher growth rate of the Ga-polar grain, which covers up the N-polar grain during the initial stage of the high-temperature growth. The X-ray rocking curve analysis revealed that the edge-dislocation density in the N-polar regrown GaN is 5 to 8 times smaller than that in the Ga-polar regrown GaN. N-polar GaN grows directly on N-polar AlN at higher temperatures. Therefore, nucleus islands grow larger than those of LT-GaN and the area fraction of coalescence boundaries between islands, where edge dislocations emerge, becomes smaller.

Investigations of in situ reflectance of GaN layers grown by MOVPE on GaAs (0 0 1)

The growth of low temperature GaN (LT-GaN) layers on GaAs (001) substrate was performed by metal organic vapor phase epitaxy (MOVPE) at growth temperature range of 500–800°C. Laser reflectometry (LR) was employed for in situ monitoring of all growth steps. The simulation of experimental time reflectance traces shows that at the first growth stage, the surface roughness increases to reach a limit value depending on growth temperature. Due to surface roughness profile the growth rate time-dependence was found non negligible at the first growth stage. The investigations of in situ reflectance give more precise measurement of growth rates that yields to thermal activation energy close to 0.12eV. The ex-situ analyses by spectral reflectance (SR) and Atomic Force Microscopy (AFM) showed that the better surface morphology was obtained when the GaN buffer layer is grown at lower temperature, while three dimensional (3D) growth mode was observed at higher temperature. A series of high temperature (800°C) GaN (HT-GaN) layers were grown on different thicknesses of low temperature (550°C) GaN buffer layer. The results showed that high density of nucleation sites enhances the initial growth rate and improves the morphological quality of GaN active layer.

A new growth method of semi-insulating GaN layer for HEMT structure by eliminating degenerate layer at GaN/sapphire interface

A new method of growing a semi-insulating GaN layer for high electron mobility transistor (HEMT) structure by eliminating a degenerate layer located at the GaN/sapphire interface is proposed. In the process, during the temperature ramp-up after the growth of a low temperature GaN buffer layer, trimethylgallium (TMGa) was flowed into the reactor together with ammonia, leading to the growth of an additional GaN layer of high carbon concentration. We confirmed that the introduction of TMGa induced high carbon concentration in the degenerate layer. The incorporated carbon formed deep acceptors, CN, compensating donors such as ON, resulting in the elimination of the degenerate layer. A HEMT device made on the sample grown by the new growth process shows a good pinch-off characteristic and high off-state breakdown voltage over 800 V at a gate voltage of −4 V, indicating the new scheme is effective to grow a high quality semi-insulating GaN layer.

Reflectance analysis on the MOCVD growth of AlN on Si(111) by the virtual interface model

AbstractThe reflectance‐time profile of AlN‐on‐Si(111) growth by MOCVD as acquired through an in‐situ optical reflectance monitor is analyzed. It is found that, similar to the case of GaN‐on‐sapphire nucleation and growth through the low temperature GaN or AlN buffer layer techniques, the reflectance profile for AlN‐on‐Si also contains information that relate to the growth mechanism, which can then be correlated to the as‐grown material quality. Based on the equations for the Virtual Interface model, a set of 6 parameters can be used to curve‐fit the reflectance traces of AlN growth as acquired by the in‐situ optical monitor. By analyzing a range of 405 nm AlN reflectance‐time data from single layer AlN growth to multilayer AlN/AlGaN/GaN stack grown on Si(111), the “goodness of fit” statistics are found to correlate with material quality metrics such as the XRD (002) rocking curve FWHM for AlN, microscopic morphological roughness of the AlN surface, as well as the resultant wafer bow and curvature for GaN/AlGaN grown on the AlN/Si(111) substrate. The correlations provide useful insight into the desirable mechanism and growth mode for the high temperature MOCVD growth of AlN and GaN on Si(111). When fully established, the technique has the potential of being used as an in‐situ pass/fail screen for the MOCVD growth of GaN‐on‐Si. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN Film Growth Characteristics Comparison in according to the Type of Buffer Layers on PSS

【GaN is most commonly used to make LED elements. But, due to differences of the thermal expansion coefficient and lattice mismatch with sapphire, dislocations have occurred at about $109{\sim}1010/cm^2$ . Generally, a low temperature GaN buffer layer is used between the GaN layer and the sapphire substrate in order to reduce the dislocation density and improve the characteristics of the thin film, and thus to increase the efficiency of the LED. Further, patterned sapphire substrate (PSS) are applied to improve the light extraction efficiency. In this experiment, using an AlN buffer layer on PSS in place of the GaN buffer layer that is used mainly to improve the properties of the GaN film, light extraction efficiency and overall properties of the thin film are improved at the same time. The AlN buffer layer was deposited by using a sputter and the AlN buffer layer thickness was determined to be 25 nm through XRD analysis after growing the GaN film at $1070^{\circ}C$ on the AlN buffer CPSS (C-plane Patterned Sapphire Substrate, AlN buffer 25 nm, 100 nm, 200 nm, 300 nm). The GaN film layer formed by applying a 2 step epitaxial lateral overgrowth (ELOG) process, and by changing temperatures ( $1020{\sim}1070^{\circ}C$ ) and pressures (85~300 Torr). To confirm the surface morphology, we used SEM, AFM, and optical microscopy. To analyze the properties (dislocation density and crystallinity) of a thin film, we used HR-XRD and Cathodoluminescence.】

Metalorganic vapor phase epitaxy of GaN and lattice-matched InGaN on ScAlMgO4(0001) substrates

ScAlMgO4 (SCAM) (0001) can be used for metalorganic vapor phase epitaxy (MOVPE) of GaN and lattice-matched In0.17Ga0.83N. GaN grown on SCAM(0001) via a low-temperature GaN buffer layer shows excellent structural quality, indicating that the GaN-SCAM interface is stable during MOVPE. For lattice-matched InGaN on SCAM(0001), a lattice-matched InGaN buffer layer grown at a lower temperature effectively improves the surface and luminescence uniformity. The grown InGaN is nearly unstrained and exhibits photoluminescence peaking at 505 nm at room temperature. These achievements indicate that In0.17Ga0.83N/SCAM lattice-matched templates may pave the way toward longer-wavelength light-emitting and -detecting devices using InGaN with higher In contents.