Growth of GaN on SiC/Si substrates using AlN buffer layer by hot-mesh CVD

GaN films were grown on SiC/Si (111) substrates by hot-mesh chemical vapor deposition (CVD) using ammonia (NH₃) and trimetylgallium (TMG) under low V/III source gas ratio (NH₃/TMG = 80). The SiC layer was grown by a carbonization process on the Si substrates using propane (C₃H₈). The AlN layer was deposited as a buffer layer using NH₃ and trimetylaluminum (TMA). GaN films were formed and grown by the reaction between NH_x radicals, generated on a tungsten hot mesh, and the TMG molecules. The GaN films with the AlN buffer layer showed better crystallinity and stronger near-band-edge emission compared to those without the AlN layer.     

Synthesis and characterization of heteroepitaxial GaN films on Si(111)

We report crack-free and single-crystalline wurtzite GaN heteroepitaxy layers have been grown on Si (111) substrate by metal-organic chemical vapor deposition(MOCVD). Synthesized GaN epilayer was characterized by X-ray diffraction(XRD), atomic force microscope (AFM) and Raman spectrum. The test results show that the GaN crystal reveals a wurtzite structure with the <0001> crystal orientation and XRD ω-scans showed a full width at half maximum (FWHM) of around 583 arcsec for GaN grown on Si substrate with an HT-AlN buffer layer. In addition, the Raman peaks of E₂^high and A₁(LO) phonon mode in GaN films have an obvious redshit comparing to bulk GaN eigen-frequency, which most likely due to tensile strain in GaN layers. But the AO phonon mode of Si has a blueshit which shows that the Si substrate suffered a compressive strain. And we report that the AlN buffer layer plays a role for releasing the residual stress in GaN films.     

The growth of GaN films by alternate source gas supply hot-mesh CVD method

Gallium nitride (GaN) films and Aluminium nitride (AlN) layers were deposited on SiC/Si (111) substrates by an alternating source gas supply or an intermittent supply of a source gas such as ammonia (NH₃), trimethylgallium (TMG) or trimethylaluminum (TMA) in a hot-mesh chemical vapor deposition (CVD) apparatus. The AlN layer was deposited as a buffer layer using NH₃ and TMA on a SiC layer grown by carbonization on Si substrates using propane (C₃H₈). GaN films were grown on an AlN layer by a reaction between NH_x radicals generated on a ruthenium (Ru) coated tungsten (W)-mesh and TMG molecules. An alternating source gas supply or an intermittent supply of one of the source gases during the film growth are expected to be effective for the suppression of gas phase reactions and for the enhancement of precursor migration on the substrate surface. By the intermittent supply of alkylmetal gas only during the growth of the AlN layer, the defect generation in the GaN films was reduced. GaN film growth by intermittent supply on an AlN buffer layer, however, did not lead to the improvement of the film quality.     

Deposition of GaN films on (1 1 1) Si substrates by alternate supply of TMG and NH3

GaN films were grown on (1 1 1) Si substrates at 1000 °C by separate admittances of trimethylgallium (TMG) and ammonia (NH₃). To achieve high quality GaN films, the optimization in growth temperature and layer thickness of AlN buffer layer between GaN film and Si substrate is required. Cross-sectional transmission electron microscopic observations of the GaN/(1 1 1)Si samples show a nearly parallel orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. Room temperature photoluminescence spectra of high quality GaN films show a strong near band edge emission and a weak yellow luminescence. The achievement of high quality GaN films on (1 1 1) Si substrates is believed to be attributed to enhancement in surface mobilities of the adsorbed surface species and adequate accommodation of lattice mismatch between high temperature AlN buffer layer and Si substrate.     

ZnO/SiC/Si异质结构的特性

用MOCVD方法在p型单晶Si（100）基片上外延SiC层,再用直流溅射在SiC层上生长ZnO薄膜,制备出ZnO／SiC／Si异质结构,用XRD和AFM分析了ZnO／SiC／Si和ZnO／Si异质结构中表层ZnO的结构和形貌的差别,研究了这种异质结构的特性．结果表明,在Si（100）基片上外延生长出的是高取向、高结晶质量的SiC（100）层．这个SiC层缓冲层使在Si基片上外延生长出了高质量ZnO薄膜,因为ZnO与SiC的晶格失配比ZnO与Si的晶格失配更低．     

Graphoepitaxy of High‐Quality GaN Layers on Graphene/6H–SiC

The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self‐heating problems in nitride‐based high‐power electronic and light‐emitting optoelectronic devices. In the present study, high‐quality GaN layers are grown on patterned graphene layers and 6H–SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H–SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O₂ gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al_0.2Ga_0.8N transition layer are deposited. The atomic structures of the interfaces between the 6H–SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al_0.2Ga_0.8N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single‐crystalline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations (≈10⁹ cm⁻²) and inversion domains and the formation of a smooth GaN surface.     

Study of the correlation between GaN material properties and the growth conditions of radio frequency plasma-assisted molecular beam epitaxy

The material properties of GaN thin films grown by radio frequency (RF) nitrogen plasma source molecular beam epitaxy (MBE) on (0001) Al₂O₃ substrates have been correlated to the V/III flux ratio during GaN growth and to the type and thickness of the buffer layer. The most remarkable observation is the change in the sign of the residual strain, from tensile to compressive as the V/III ratio alters from N-rich to stoichiometric (or slightly Ga-rich) conditions for GaN layers with a 17 nm AlN buffer layer. The residual strain was significantly reduced for a thinner 5 nm AlN buffer and it was zero for a 20 nm GaN buffer. A reduction of the rms surface roughness from 20 to 3 nm was achieved by decreasing the V/III ratio. Finally, stacking faults were observed only for significantly N-rich growth conditions.     

Calculation of residual thermal stress in GaN epitaxial layers grown on technologically important substrates

A detailed investigation of residual thermal stress and misfit strain in GaN epitaxial layers grown on technologically important substrates is performed. The thermal stress is low when GaN is grown on AlN, SiC and Si, and relatively higher when Al₂O₃ substrate is used. The stress is compressive for AlN and Al₂O₃ and tensile for Si and SiC substrates. Residual thermal stress analysis was also performed for three layer heterostructures of GaN/AlN/6H-SiC and GaN/AlN/Al₂O₃. The stress remains the same when a sapphire substrate is used with or without an AlN buffer layer but reduces by an order of magnitude when a 6H-SiC substrate is used with an AlN buffer layer.     

The structure and photoluminescence properties of ZnO/SiC multilayer film on Si substrate

ZnO/SiC multilayer film has been fabricated on a Si (111) substrate with a silicon carbide (SiC) buffer layer using the RF (radio frequency)-magnetron technique with targets of a ceramic polycrystalline zinc oxide (ZnO) and a composite target of pure C plate with attached Si chips on the surface. The as-deposited films were annealed at a temperature range of 600–1000°C under nitrogen atmosphere. The structure and photoluminescence (PL) properties of the samples were measured using X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy and PL spectrophotometry. By increasing the annealing temperature to 800°C, it is found that all the ZnO peaks have the strongest intensities, and the crystallinity of ZnO is more consistent on the SiC buffer layer. Further increase of the annealing temperature allows the ZnO and SiC layers to penetrate one another, which makes the interface between ZnO and SiC layer become more and more complicated, thus reduces the crystallinities of ZnO and SiC. The PL properties of a ZnO/SiC multilayer are investigated in detail. It is discovered that the PL intensities of these bands reach their maximum after being annealed at 800°C. The PL peaks shift with an increase in the annealing temperature, which is due to the ZnO and SiC layers penetrating reciprocally. This makes the interface more impacted and complicated, which induces band structure deformation resulting from lattice deformation.     

Si Complies with GaN to Overcome Thermal Mismatches for the Heteroepitaxy of Thick GaN on Si

Heteroepitaxial growth of lattice mismatched materials has advanced through the epitaxy of thin coherently strained layers, the strain sharing in virtual and nanoscale substrates, and the growth of thick films with intermediate strain‐relaxed buffer layers. However, the thermal mismatch is not completely resolved in highly mismatched systems such as in GaN‐on‐Si. Here, geometrical effects and surface faceting to dilate thermal stresses at the surface of selectively grown epitaxial GaN layers on Si are exploited. The growth of thick (19 µm), crack‐free, and pure GaN layers on Si with the lowest threading dislocation density of 1.1 × 10⁷ cm^?2 achieved to date in GaN‐on‐Si is demonstrated. With these advances, the first vertical GaN metal–insulator–semiconductor field‐effect transistors on Si substrates with low leakage currents and high on/off ratios paving the way for a cost‐effective high power device paradigm on an Si CMOS platform are demonstrated     

X-ray absorption and Raman study of GaN films grown on different substrates by different techniques

Raman scattering and polarization-dependent synchrotron radiation X-ray absorption, in combination, have been employed to examine the residual stress of undoped GaN epitaxial layers grown on Si by molecular beam epitaxy and Si-doped n-type GaN layers grown on sapphire by metalorganic chemical vapor deposition. Values of the lattice constant of different GaN films can be deduced from the interatomic distances in the second coordination shell around Ga by polarization-dependent extended X-ray absorption fine structure analysis and the strain of the films can be obtained. This result is further confirmed by Raman scattering spectra in which the phonon modes show a significant shift between different GaN epitaxial layers with different growth conditions.     

The effect on the tensile properties of PIP-processed SiC/SiC composite of a chemical vapor-infiltrated SiC layer overlaid on the pyrocarbon interface layer

A chemical vapor-infiltrated (CVI) SiC layer is often deposited on the pyrocarbon (PyC) fiber–matrix interface layer in SiC fiber-reinforced SiC matrix (SiC/SiC) composites. It is normally applied to protect the PyC layer from reacting with molten Si or sintering aids during manufacturing, and to guard against the effects of high temperature, oxidation and moisture during use. In this study, we investigated the effect of this SiC layer on the tensile properties of a composite. Tensile tests of our composite samples showed the SiC layer to have no noticeable effects on its ultimate load or fracture strain, whereas it decreased the load-to-strain ratio and proportional limit. The test results were analyzed by carrying out element tests on filaments and fiber bundle samples, fracture mirror analysis of pullout fibers, and finite element analysis (FEA) of residual thermal stress around the interface.     

硅基沉积氮化镓,碳化硅,III –V 族及其合金材料研究进展

硅基沉积氮化镓, 碳化硅, III-V 族及其合金材料是近年来的研究热点. 氮化镓, 碳化硅及其III-V 材料在光电子和电子元件领域有着广泛的应用.例如大功率, 高速器件, 大型激光器, 紫外探测器等等. 尽管硅基片具有低成本, 大的尺寸,和极好的电热导性能等优点, 硅基片仍没有成为氮化镓, 碳化硅及III – V 的主要沉积基片, 其原因在于硅基片与氮化镓, 碳化硅及III-V 材料之间的热膨胀系数和晶格常数之间的失配. 自从1998年, IBM 的课题组用分子外延方法在硅基片上沉积氮化镓, 并且成功地制备了氮化镓激光器之后, 硅基氮化镓的研究开始备受关注. 近年来的研究发现, 使用氧化铝和氮化铝镓作为过渡层. 硅基氮化镓的热应力及与硅基片之间的晶格失配可以明显降低.在 6英寸的(111) 取向的硅基片上用化学气相方法可以成功地沉积超过一个微米厚的无裂纹的单晶氮化镓. 德国的AZZURRO 公司成功地制备硅基片氮化镓的大功率的蓝色激光器. 美国的NITRINEX公司也生产了硅基氮化镓大功率电子元件. 超大功率的硅基氮化镓电子元件仍在研究中. 在2007年, 英国政府设立了一个固体照明器件的研究项目. 主要着手研究6英寸的硅基氮化镓激光器. 另一方面, 在过去的40年, 超大规模硅基CMOS 技术已有了长足的发展, 下一代低功耗高速逻辑电路要求低的驱动电流, 小的活门尺寸低于 30 nm 和快速反应性能. 这就要求器件通道材料具有很高的电子(或空穴)迁移率. III-V 材料, 例如InSb, InAs, 和InGaAs 具有电子迁移率高达 80000 cm2/VS. 它们将是下一代低于 30 nm 硅基CMOS 器件最好的候选材料. 在 2007 年美国DARPA/MTO 设立了一个研究项目来发展硅基 III-V材料器件, 着重于发展高速硅基III-V材料CMOS 器件. 第一届”硅基氮化镓,碳化硅,III-V及其合金材料研究进展 ”国际会议也将于3月 24日-28日在旧金山MRS 2008年初春季会议上召开.     

Characterization of GaN epitaxial layers on SiC substrates with AlxGa1−xN buffer layers

High quality GaN epitaxial layers were obtained with Al_xGa_1−xN buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al_xGa_1−xN (0≤x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al_xGa_1−xN buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm² V⁻¹ s⁻¹ and 1.6×10¹⁷ cm⁻³ at 300 K. The splitting diffraction angle between GaN and Al_xGa_1−xN were also analyzed from X-ray diffraction curves.     

掺Si对MOCVD生长的GaN单晶膜的黄带发射及生长速率的影响

对ＭＯＣＶＤ生长ＧａＮ：Ｓｉ薄膜进行了研究,研究表明随ＳｉＨ４／ＴＭＧａ流量比增大,ＧａＮ：Ｓｉ单晶膜的电子浓度增大,迁移率下降,Ｘ射线双晶衍射峰半高宽增加,同时这发射强度得到了大大的提高,并报导了随ＳｉＨ４／ＴＭＧａ流量比增大,ＧａＮ：Ｓｉ的生长速率降低的现象,研究结果还表明,预反应对ＧａＮ：Ｓｉ单晶膜黄带发射影响很大,预反应的减小可以使黄带受到抑制。     

GaN基底上集成介电薄膜材料的生长方法研究

将以极化为特征、具有丰富功能特性的介电氧化物材料通过外延薄膜的方式,在半导体GaN上制备介电氧化物/GaN集成薄膜,其多功能一体化与界面耦合效应可推动电子系统单片集成化的进一步发展。然而,由于2类材料物理、化学性质的巨大差异,在GaN上生长介电薄膜会出现严重的相容性生长问题。采用激光分子束外延技术(LMBE),通过弹性应变的TiO2的缓冲层来减小晶格失配度,降低介电薄膜生长温度,控制界面应变释放而产生的失配位错,提高了介电薄膜外延质量;通过低温外延生长MgO阻挡层,形成稳定的氧化物/GaN界面,阻挡后续高温生长产生的扩散反应;最终采用TiO2/MgO组合缓冲层控制介电/GaN集成薄膜生长取向、界面扩散,降低集成薄膜的界面态密度,保护GaN半导体材料的性能。所建立的界面可控的相容性生长方法,为相关集成器件的研发提供了一条可行的新途径。     

Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

High-resistive GaN (>10⁸ Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and Al_xGa_1?xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.     

Growth and characterisation of SiC films deposited on a-SiO2/Si (100) substrates

Abstract

The growth of polycrystalline SiC films has been carried out by low pressure chemical vapour deposition in a horizontal quartz reaction chamber using tetramethylsilane and H2 as the precursor gas mixture. Silicon (100) wafers were used as substrates. A thin Si O₂ amorphous layer of ~6 nm was formed before SiC deposition to reduce the strain induced by the 8% difference in thermal expansion coefficients between SiC and Si. Samples were. analysed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared reflectivity. The structure of films grown at temperatures between 950 and 1150°C varies from amorphous to polycrystalline SiC. Preferential [111] orientation and columnar growth of polycrystalline films develops with increasing temperature.

MST/3317     

Influence of AlN buffer layer thickness and deposition methods on GaN epitaxial growth

A gallium nitride (GaN) epitaxial layer was grown by metal-organic chemical vapor deposition (MOCVD) on Si (111) substrates with aluminum nitride (AlN) buffer layers at various thicknesses. The AlN buffer layers were deposited by two methods: radio frequency (RF) magnetron sputtering and MOCVD. The effect of the AlN deposition method and layer thickness on the morphological, structural and optical properties of the GaN layers was investigated. Field emission scanning electron microscopy showed that GaN did not coalesce on the sputtered AlN buffer layer. On the other hand, it coalesced with a single domain on the MOCVD-grown AlN buffer layer. Structural and optical analyses indicated that GaN on the MOCVD-grown AlN buffer layer had fewer defects and a better aligned lattice to the a- and c-axes than GaN on the sputtered AlN buffer layer.     

Thermal Oxidation of Silicon Carbide Substrates

Thermal oxidation was used to remove the subsurface damage of silicon carbide (SiC) surfaces. The anisotrow of oxidation and the composition of oxide layers on Si and C faces were analyzed. Regular pits were observed on the surface after the removal of the oxide layers, which were detrimental to the growth of high quality epitaxial layers. The thickness and composition of the oxide layers were characterized by Rutherford backscat-tering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), respectively. Epitaxial growth was performed in a metal organic chemical vapor deposition (MOCVD) system. The substrate surface morphol-ogy after removing the oxide layer and gallium nitride (GaN) epilayer surface were observed by atomic force microscopy (AFM). The results showed that the GaN epilayer grown on the oxidized substrates was superior to that on the unoxidized substrates.