含有Al组分阶变AlGaN过渡层的Si基AlGaN/GaN HEMT

采用一个AlN缓冲层和两个Al组分阶变的AlGaN过渡层作为中间层,在76.2mm Si衬底上外延生长出1.7μm厚无裂纹AlGaN/GaN异质结材料,利用原子力显微镜、X射线衍射、Hall效应测量和CV测量等手段对材料的结构特性和电学性能进行了表征。材料表面平整光滑,晶体质量和电学性能良好,2DEG面密度为1.12×1013cm-2,迁移率为1 208cm2/(V.s)。由该材料研制的栅长为1μm的AlGaN/GaN HEMT器件,电流增益截止频率fT达到10.4GHz,这些结果表明组分阶变AlGaN过渡层技术可用于实现高性能Si基GaN HEMT。     

<Emphasis Type="Italic">In Situ</Emphasis> Stress Measurements During GaN Growth on Ion-Implanted AlN/Si Substrates

In situ wafer curvature measurements were used in combination with postgrowth structural characterization to study the evolution of film stress and microstructure in GaN layers grown by metalorganic chemical vapor deposition on N⁺ ion-implanted AlN/Si (111) substrates. The results were compared with growth on identical unimplanted substrates. In situ stress measurements revealed that, for the unimplanted sample, the GaN initiated growth under compressive stress of −1.41 GPa which arose due to lattice mismatch with the AlN buffer layer. In contrast, GaN growth on the ion-implanted sample began at lower compressive stress of −0.84 GPa, suggesting a reduction in epitaxial stress. In both cases, the compressive growth stress was fully relaxed after ~0.7 μm and minimal tensile stress was generated during growth. During post-growth cooling, tensile stress was introduced in the GaN layer of both samples due to thermal expansion mismatch. Post-growth optical microscopy characterization, however, demonstrated that the ion-implanted sample had lower density of channeling cracks compared with the unimplanted sample. Cross-sectional transmission electron microscopy images of the sample grown on ion-implanted Si with no post-implantation nitrogen annealing revealed the formation of horizontal cracks in the implanted region beneath the AlN buffer layer. The weakened layer acts to decouple the GaN film from the Si substrate and thereby reduces the density of channeling cracks in the film after growth.     

GaN grown by hydride vapor phase epitaxy on p-type 6H-SiC layers

6H-SiC/GaN pn-heterostructures were grown by subsequent epitaxial growth of p-SiC by low temperature liquid phase epitaxy (LTLPE) and n-GaN by hydride vapor phase epitaxy (HVPE). For the first time, p-type epitaxial layers grown on 6H-SiC wafers were used as substrates for GaN HVPE growth. The GaN layers exhibit high crystal quality which was determined by x-ray diffraction. The full width at a half maximum (FWHM) for the ω-scan rocking curve for (0002) GaN reflection was ∼120 arcsec. The photoluminescence spectra for these films were dominated by band-edge emission. The FWHM of the edge PL peak at 361 nm was about 5 nm (80K).     

Comparison of various buffer schemes to grow GaN on large-area Si(111) substrates using metal-organic chemical-vapor deposition

A comparison of gallium-nitride (GaN) films grown on large-area Si(111) using a single aluminum-nitride (AlN) buffer, an AlN/graded-Al_xGa_1−xN buffer, and the introduction of additional low-temperature (LT)-grown AlN interlayers is reported. A graded-AlGaN buffer followed by additional LT-AlN interlayers is shown to completely eliminate cracking in nitride films of thickness >2 μm and also reduce the threading-dislocation density significantly. A partial compensation of GaN-tensile strain by the compressive-lattice strain induced by the AlGaN and AlN layers is responsible for this effect. The surface roughness is increased by the introduction of the LT-AlN buffers.     

Analysis of the growth of GaN epitaxy on silicon

Due to the great potential of GaN based devices,the analysis of the growth of crack-free GaN with high quality has always been a research hotspot.In this paper,two methods for improving the property of the GaN epitaxial layer on Si (111) substrate are researched.Sample A,as a reference,only has an AlN buffer between the Si substrate and the epitaxy.In the following two samples,a GaN transition layer (sample B) and an AlGaN buffer (sample C) are grown on the AlN buffer separately.Both methods improve the quality of GaN.Meanwhile,using the second method,the residual tensile thermal stress decreases.To further study the impact of the two introduced layers,we investigate the stress condition of GaN epitaxial layer by Raman spectrum.According to the Raman spectrum,the calculated residual stress in the GaN epitaxial layer is approximately 0.72 GPa for sample B and 0.42 GPa for sample C.The photoluminescence property of GaN epitaxy is also investigated by room temperature PL spectrum.     

Digitally Alloyed Modulated Precursor Flow Epitaxial Growth of Ternary AlGaN with Binary AlN and GaN Sub-Layers and Observation of Compositional Inhomogeneity

We report the growth of ternary aluminum gallium nitride (AlGaN) layers on AlN/sapphire template/substrates by digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG), which combined an MPEG AlN sub-layer with a conventional metalorganic chemical vapor deposition (MOCVD)-grown GaN sub-layer. The overall composition in DA-MPEG Al _x Ga_1−x N was controlled by adjustment of the growth time (i.e., the thickness) of the GaN sub-layer. As the GaN sub-layer growth time increased, the Al composition in AlGaN decreased to 50%, but the surface morphology of the AlGaN layer became rough, and a three-dimensional structure with islands appeared for the DA-MPEG AlGaN with relatively thick GaN sub-layers, possibly resulting from the Ga adatom surface migration behavior and/or the strain built up from lattice mismatch between AlN and GaN sub-layers with increasing GaN sub-layer growth time. Through strain analysis by high-resolution x-ray diffraction, reciprocal space mapping, and scanning transmission electron microscopy, it was found that there was compositional inhomogeneity in the DA-MPEG AlGaN with AlN and GaN binary sub-layers for the case of the layer with relatively thick GaN sub-layers.     

High-quality AlGaN layers over pulsed atomic-layer epitaxially grown AlN templates for deep ultraviolet light-emitting diodes

In this paper, we report the pulsed atomic-layer epitaxy (PALE) of ultrahigh-quality AlN epilayers over basal-plane sapphire substrates and their use as templates to grow high-quality AlGaN layers with Al content ranging from 0.3 to 1. Symmetric/asymmetric x-ray diffraction (XRD) and room-temperature (RT) photoluminescence (PL) measurements were used to establish the high-structural and optical quality. The XRD (002) and (114) rocking-curve full-width at half-maximum (FWHM) values of the PALE-grown AlN epilayers were less than 60 arcsec and 250 arcsec, respectively. Using these ultrahigh-quality layers as templates, Si-doped AlGaN layers with a large Al content from 30% to 100% were grown and used for milliwatt power sub-280-nm, deepultraviolet (UV) light-emitting diodes (LEDs).     

High quality AIN and GaN grown on compliant Si/SiC substrates by gas source molecular beam epitaxy

Epitaxial layers of AlN and GaN were grown by gas source molecular-beam epitaxy on a composite substrate consisting of a thin (250 nm) layer of silicon (111) bonded to a polycrystalline SiC substrate. Two dimensional growth modes of AlN and GaN were observed. We show that the plastic deformation of the thin Si layer results in initial relaxation of the AlN buffer layer and thus eliminates cracking of the epitaxial layer of GaN. Raman, x-ray diffraction, and cathodoluminescence measurements confirm the wurtzite structure of the GaN epilayer and the c-axis crystal growth orientation. The average stress in the GaN layer is estimated at 320 MPa. This is a factor of two less than the stress reported for HVPE growth on 6H-SiC (0001).     

Growth optimization and doping with Si and Be of high quality GaN on Si(111) by molecular beam epitaxy

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AlN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of aproximately 3 nm lead to the best AlN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2×2) surface reconstruction of the AlN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850°C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AlN layers grown at 850°C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7×10¹⁹ cm⁻³ and mobilities approximately 100 cm²/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be acceptors between 90 and 100 meV was derived from photoluminescence experiments.     

Growth and fabrication of AlGaN/GaN HEMT based on Si(1 1 1) substrates by MOCVD

AlGaN/GaN high electron mobility transistor (HEMT) hetero-structures were grown on the 2-in Si (1 1 1) substrate using metal-organic chemical vapor deposition (MOCVD). Low-temperature (LT) AlN layers were inserted to relieve the tension stress during the growth of GaN epilayers. The grown AlGaN/GaN HEMT samples exhibited a maximum crack-free area of 8 mm×5 mm, XRD GaN (0 0 0 2) full-width at half-maximum (FWHM) of 661 arcsec and surface roughness of 0.377 nm. The device with a gate length of 1.4 μm and a gate width of 60 μm demonstrated maximum drain current density of 304 mA/mm, transconductance of 124 mS/mm and reverse gate leakage current of 0.76 μA/mm at the gate voltage of −10 V.     

利用AlxGa1-xN在Si(111)上生长无裂纹GaN

利用LP-MOCVD技术在Si(111)衬底上,用不同Al组分的AlGaN做缓冲层,再在缓冲层上生长GaN薄膜,采用XRD技术和原子力显微镜(AFM)分析样品知,样品整体的结晶质量良好。通过分析外延层的拉曼谱得出GaN中存在应力,而且是张应力,通过计算得出,应力为0.23 GPa。     

Effect of AlN buffer thickness on GaN epilayer grown on Si(1 1 1)

We studied the influence of high temperature AlN buffer thickness on the property of GaN film on Si (1 1 1) substrate. Samples were grown by metal organic chemical vapor deposition. Optical microscopy, atomic force microscopy and X-ray diffraction were employed to characterize the samples. The results demonstrated that thickness of high temperature AlN buffer prominently influenced the morphology and the crystal quality of GaN epilayer. The optimized thickness of the AlN buffer is found to be about 150 nm. Under the optimized thickness, the largest crack-free range of GaN film is 10 mm×10 mm and the full width at half maximum of GaN (0 0 0 2) rocking curve peak is 621.7 arcsec. Using high temperature AlN/AlGaN multibuffer combined with AlN/GaN superlattices interlayer we have obtained 2 μm crack-free GaN epilayer on 2 in Si (1 1 1) substrates.     

引入GaN过渡层对Si（111）衬底上生长GaN外延的影响

本文研究了在Si(111)衬底上生长GaN外延层的方法。相比于直接在AlN缓冲层上生长GaN外延层,引入GaN过渡层显著地提高了外延层的晶体质量并降低了外延层的裂纹密度。使用X射线双晶衍射仪、光学显微镜以及在位监测曲线分析了GaN过渡层对外延层的晶体质量以及裂纹密度的影响。实验发现,直接在AlN缓冲层上生长外延层,晶体质量较差, X射线(0002)面半高宽最优值为0.686°,引入GaN过渡层后,通过调整生长条件,控制岛的长大与合并的过程,从而控制三维生长到二维生长过渡的过程,外延层的晶体质量明显提高, (0002)面半高宽降低为0.206°,并且裂纹明显减少。研究结果证明,通过生长合适厚度的GaN过渡层,可以得到高质量、无裂纹的GaN外延层。     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs

Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 10⁸ cm^?2. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.     

以金属为缓冲层在Si(111)上分子束外延GaN及其表征

用电子束蒸发方法在Si(111)衬底上蒸发了Au/Cr和Au/Ti/Al/Ti 两种金属缓冲层,然后在金属缓冲层上用气源分子束外延(GSMBE)生长GaN. 两种缓冲层的表面都比较平整和均匀,都是具有Au(111)面择优取向的立方相Au层. 在Au/Cr/Si(111)上MBE生长的GaN,生长结束后出现剥离. 在Au/Ti/Al/Ti/Si(111)上无AlN缓冲层直接生长GaN,得到的是多晶GaN;先在800℃生长一层AlN缓冲层,然后在710℃生长GaN,得到的是沿GaN(0001)面择优取向的六方相GaN. 将Au/Ti/Al/Ti/Si(111)在800℃下退火20min,金属层收缩为网状结构,并且成为多晶,不再具有Au(111)方向择优取向.     

Epitaxial growth of GaN layers on metallic TiN buffer layers

To improve GaN light-emitting diode light emission, we produced metal organic chemical vapor deposition (MOCVD)-grown, continuous, flat GaN layers on metallic TiN buffer layers deposited on sapphire substrates. Three important conditions were found: (a) the sapphire substrate surface plane should be (1120), (b) the TiN layer surface plane should be (111), and (c) the TiN buffer layer nitrogen content should be higher than that of stoichiometric TiN. Reduction of TiN layer thickness reduced TiN buffer layer surface roughness. Threading dislocation density in GaN layers grown on TiN buffer layers was much lower than that in GaN layers grown on AlN.     

Relaxation of InGaN thin layers observed by X-ray and transmission electron microscopy studies

Double-crystal and triple-axis x-ray diffractometry and transmission electron microscopy are used to characterize the microstructure, strain, and composition of InGaN layers grown on GaN by metalorganic chemical vapor deposition (MOCVD). Three different samples with increasing In concentration have been studied, all grown on GaN deposited on sapphire either with GaN or AlN buffer layers. It was found that InGaN layers with nominal 28% and 40% InN content consist of two sub-layers; the first sub-layer is pseudomorphic with the underlying GaN with lower In content than nominal. The top sub-layer is fully relaxed with a high density of planar defects and In content close to the nominal value. This is in contrast to a common assumption applied to InGaN quantum wells that ‘quantum-dot like areas’ are formed with different In content. The sample with the nominal indium concentration of 45% does not exhibit any intermediate strained layer, is fully relaxed and the In concentration (43.8%) agrees well with the nominal value.     

Evolution of Threading Dislocation Density and Stress in GaN Films Grown on (111) Si Substrates by Metalorganic Chemical Vapor Deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga_1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga_1-y N/Al _x Ga_1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga_1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga_1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga_1-y N bilayer prior to growing the compositionally graded Al _x Ga_1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga_1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga_1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.     

Optical and electrical characterization of GaN layers grown on silicon and sapphire substrates

Characterization of the structural, optical and electrical properties of GaN layers grown by two epitaxial techniques (ECR-MBE and MOCVD) using different substrates (vicinal Si111 and sapphire) has been performed. The quality of the samples grown by MOCVD seems to be influenced by the nitrogen source used for the growth. Unintentionally doped MBE samples with n-type concentrations around 10¹⁸ cm⁻³ and Hall mobility of 15 cm² (V s)⁻¹ were studied. GaN films doped with Mg and grown using AlN buffer layers have also been analyzed to study the influence of the thickness of the buffer layer on the optical properties of the GaN epilayer. In the samples with low Mg doping, a thin AlN buffer layer improved the optical quality of the film. In general, all the MBE samples doped with Mg were highly resistive, probably due to a low activation or high ionization energy of the Mg acceptors. Technological issues related to the formation of ohmic contacts on GaN layers are also presented.