Interfacial engineering for semi-insulating GaN/sapphire template with low dislocation density

Characteristics comparison between GaN epilayers grown on patterned and unpatterned sapphire substrate (0 0 0 1)

GaN epilayers and AlGaN/GaN superlattice structures have been deposited on (0001) ZnO substrates by metalorganic vapor phase epitaxy (MOVPE) using GaN and AlN buffer layers. The growth conditions were first optimized on GaN templates using N2 as carrier gas at relatively low temperature (<800 &deg;C), which is suitable for GaN growth on a ZnO substrate. Experimental results show that high interfacial quality can be achieved in the superlattice by using TMIn as a surfactant. The optimized growth conditions were subsequently transferred to ZnO substrates. The influence of growth temperature on the material quality was studied. A proper growth temperature for both GaN cover layer and AlGaN/GaN superlattice can improve the structural and optical properties of the structures on ZnO. This improvement is verified using x-ray diffraction, atomic force microscopy and photoluminescence characterizations. The growth temperature must be chosen with these two factors in mind, with too low a growth temperature leading to poor quality material and too high a temperature causing reactions at the GaN/ZnO interface that degrade quality. AlN buffer layers on ZnO were also studied to increase subsequent GaN epilayer quality. Effects of buffer layer growth conditions on optical and structural quality were studied.

It is widely believed that the lack of high-quality GaN wafers severely hinders the progress in GaN-based devices, especially for defect-sensitive devices. Here, low-cost AlN buffer layers were sputtered on cone-shaped patterned sapphire substrates (PSSs) to obtain high-quality GaN epilayers. Without any mask or regrowth, facet-controlled epitaxial lateral overgrowth was realized by metal-organic chemical vapor deposition. The uniform coating of the sputtered AlN buffer layer and the optimized multiple modulation guaranteed high growth selectivity and uniformity of the GaN epilayer. As a result, an extremely smooth surface was achieved with an average roughness of 0.17 nm over 3 × 3 μm2. It was found that the sputtered AlN buffer layer could significantly suppress dislocations on the cones. Moreover, the optimized three-dimensional growth process could effectively promote dislocation bending. Therefore, the threading dislocation density (TDD) of the GaN epilayer was reduced to 4.6 × 107 cm-2, which is about an order of magnitude lower than the case of two-step GaN on the PSS. In addition, contamination and crack in the light-emitting diode fabricated on the obtained GaN were also effectively suppressed by using the sputtered AlN buffer layer. All of these advantages led to a high output power of 116 mW at 500 mA with an emission wavelength of 375 nm. This simple, yet effective growth technique is believed to have great application prospects in high-performance TDD-sensitive optoelectronic and electronic devices.

Development of analytical model for strained silicon relaxation on (100) fully relaxed Si 0.8Ge 0.2 pseudo-substrates

Abstract— The structural, electrical, and optical properties of GaN epilayers grown on various ion‐implanted sapphire(0001) substrates by MOCVD were investigated. GaN or AlN buffer layers and pre‐treatment were indispensably introduced before GaN‐epilayer growth. The ion‐implanted substrate's surface had decreased internal free energies during the growth of the ion‐implanted sapphire(0001) substrates. The crystal and optical properties of the GaN epilayers grown in ion‐implanted sapphire(0001) substrates were improved. Also, an excessively roughened and modified surface caused by ions degraded the GaN epilayers. Not only the ionic radius but also the chemical species of implanted sapphire(0001) substrates improved the properties of the GaN epilayers grown by MOCVD. It is obvious that the ion‐implanted pre‐treatment of sapphire(0001) substrates can be an alternative pre‐treatment procedure for GaN deposition and has the potential to improve the properties of the GaN epilayers on sapphire(0001) substrates.

The conditions for preparing normally-off GaN devices incorporating semi-insulating (SI) GaN materials are explored. The properties of SI GaN where carbon behaves as a deep level acceptor are predicted using a Shockley diagram. Metal-oxide-semiconductor (MOS) structures based upon these on SI-GaN layers are designed. The bandgap alignment of these structures is analyzed using Poisson equations. Normally-off operation is shown to be possible in devices featuring a thin n-GaN layer and SI-GaN layer, because of a higher conduction band energy. It is also shown that higher threshold voltage can be achieved by reducing the carrier concentration of the n-GaN channel layer.

Semi-insulating GaN is grown by using a two-step AlN buffer layerby metalorganic chemical vapour deposition. The sheet resistance ofas-grown semi-insulating GaN is dramatically increased to 1013 Ω/sq by usingtwo-step AlN buffer instead of the traditional low-temperature GaNbuffer. The high sheet resistance of as-grown GaN over1013 Ω/sq is due to inserting an insulating buffer layer(two-step AlN buffer) between the high-temperature GaN layer and asapphire substrate which blocks diffusion of oxygen and overcomes theweakness of generating high density carrier near interface of GaN andsapphire when a low-temperature GaN buffer is used. The result suggeststhat the high conductive feature of unintentionally doped GaN ismainly contributed from the highly conductive channel near interfacebetween GaN and the sapphire substrate, which is indirectly manifestedby room-temperature photoluminescence excited by an incident laser beamradiating on growth surface and on the substrate. The functions of thetwo-step AlN buffer layer in reducing screw dislocation and improvingcrystal quality of GaN are also discussed.

High quality GaN films on SiC with low thermal boundary resistance (TBR) are achieved by employing an ultrathin low Al content AlGaN buffer layer. Compared with the conventional thick AlN buffer layer, the ultrathin buffer layer can not only improve the crystal quality of the subsequent GaN layer but also reduce the TBR at the GaN/SiC interface simultaneously. The ultrathin AlGaN buffer layer is introduced by performing a pretreatment of the SiC substrate with trimethylaluminum followed by the growth of GaN with an enhanced lateral growth rate. The enhanced lateral growth rate contributes to the formation of basal plane stacking faults (BSFs) in the GaN layer, where the BSFs can significantly reduce the threading dislocation density. We reveal underling mechanisms of reducing TBR and dislocation density by the ultrathin buffer layer. We propose this work is of great importance toward the performance improvement and cost reduction of higher power GaN-on-SiC electronics.

High-quality GaN epilayers were grown on patterned Si(111) substrates by metalorganic chemical vapor deposition. Crack-free GaN epilayers were obtained by growing the GaN on the plateau of the patterned Si(111) substrate. A high-temperature AlN buffer layer improved the crystalline quality of the GaN epilayer. When the growth temperature of the AlN buffer layer was 1000 °C, the GaN epitaxial layer showed a smooth surface and high crystalline quality. The dislocation density of the subsequently grown GaN layer was 2×109 cm-2, which is comparable to those of conventional GaN epilayers grown on sapphire substrate.

We have grown blue light-emitting diodes (LEDs) with low threading dislocation densities (TDDs) by using SiN interlayers on Si (111) substrates. Our growth technique using SiN layers makes it possible to decrease twist components (edge-type threading dislocation components). The edge-type TDDs are almost the same values as those of LEDs grown on Al₂O₃ (0001) substrates. EQE of LEDs grown on Si (111) substrates increases with decreasing edge-type dislocation in the low-current-density region, and the EQE of the sample with low TDD is almost as high as that of the LED grown on an Al₂O₃ (0001) substrate at room temperature. It is found that the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region, but are less than those of an LED grown on an Al₂O₃ (0001) substrate. Time-resolved photoluminescence (TRPL) shows that the dominant origin of the thermal quenching is edge-type dislocations in our samples, but other defects such as screw-type dislocations also contribute to it. We also found the fluctuated emission patterns consisting of bright and dark areas originated from the difference of Shockley–Read–Hall (SRH) type defect densities in the multi-quantum wells (MQWs) grown on Si (111) substrates. The bright areas spread, and the configurations of the bright areas change into ring-like patterns with reducing edge-type TDDs. We suggest that the internal quantum efficiency (IQE) of dark areas should be promoted to improve the performance of the MQWs grown on Si (111) substrates.

Evolution of threading dislocations in MOCVD-grown GaN films on (1 1 1) Si substrates

Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H2 atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm2/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ∼1 × 1020 cm−3 at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm2 SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

Stress generation and relaxation in AlN nucleation and buffer layers grown on c-Al2O3 substrates by plasma-assisted molecular beam epitaxy at low (LT, TS = 780°C) and high (HT, TS = 850°C) substrate temperatures were studied. Oscillation behavior of the wafer curvature (stress×thickness) was revealed during pulsed deposition of AlN in metal modulated epitaxy and migration enhanced epitaxy modes under the metal-rich conditions, which corresponded to Al bilayer formation and consumption. Minimal average stress ∼ +0.05 GPa in AlN buffer layers was observed at LT growth, which increased up to +0.85 GPa at HT growth. In addition, the LT AlN layers demonstrated lower densities of screw and edge threading dislocations equal to 7.8·108, 5.1·109 cm−2, respectively, while in the case of HT AlN layers those densities were significantly higher.

실리콘 기판 위에 GaN를 성장하기 위해서 AlN 완충층을 사용해 왔다. 그러나 AlN은 아직까지 high doping이 쉽지 않기 때문에, 이로 인해 AlN를 전자소자나 광소자 제작을 위한 완충층으로 이용하는 경우 직렬 저항의 증가라는 문제가 발생할 수 있다. 본 연구에서는 이러한 문제점을 개선하기 위해 AlN 완충층 대신에 금속 완충층을 사용하여 실리콘 기판 위에 GaN 박막 성장실험을 수행하였다. Al, Ti, Cr 그리고 Au 등을 금속 완충층으로 사용하여 실리콘 기판 위에 GaN 층을 성장하였다. 성장된 GaN 박막의 표면 특성을 분석하기 위해 광학현미경과 SEM을 사용하였고, 결정성과 광학적 특성을 평가하기 위하여 PL과 XRD 분석을 실시하였으며 AlN 완충층을 사용한 경우와 금속 완충층을 사용한 경우의 저항 차이를 확인하기 위하여 전류-전압 특성을 측정하였다. AlN buffer layers have been used for the growth of GaN layers on Si substrates. However, the doping of high concentration of carriers into AlN layers is still not easy, therefore it may cause the increase of series resistance when it is used for the electrical or optical devices. In this work, to improve such a problem, the growth of GaN layers on Si substrates were performed using metal buffer layers instead of AlN buffer layer. We tried combinations of Ti, Al, Cr and Au as metal buffer layers for the growth of GaN on Si substrates. Surface morphology was measured by optical microscope and scanning electron microscope (SEM), and optical properties and crystalline quality were measured by photoluminescence (PL) and X-ray diffractometer (XRD), respectively. Electrical resistances for both cases of AlN and metal buffer layer were compared by current-voltage (I-V) measurement.

We demonstrate that GaN-on-diamond technology with an ultra-thin GaN buffer and interface layer offers excellent thermal resistance alongside good electrical performance. Two device sets were investigated, one with 354 nm thick GaN buffer and 17 nm thick interface layers, the other with 700 nm thick GaN buffer and 36 nm thick interface layers. The samples demonstrate excellent thermal resistances of 9 ± 1 K/(W/mm) and 10.0 ± 0.5 K/(W/mm), respectively. Trade-offs between GaN buffer thickness and effective thermal boundary resistance are discussed demonstrating pathways for the advancement of GaN-on-diamond technology. IV measurements show low trapping and reduced thermal non-linearity in devices with ultra-thin GaN layers.