Properties of GaN on different polarity buffer layers by hydride vapour phase epitaxy

This paper reports on N-, mixed-, and Ga-polarity buffer layers are grown by molecular beam epitaxy (MBE) on sapphire (0001) substrates, with the GaN thicker films grown on the buffer layer with different polarity by hydride vapour epitaxy technique (HVPE). The surface morphology, structural and optical properties of these HVPE-GaN epilayers are characterized by wet chemical etching, scanning electron microscope, x-ray diffraction, and photoluminescence spectrum respectively. It finds that the N-polarity film is unstable against the higher growth temperature and wet chemical etching, while that of GaN polarity one is stable. The results indicate that the crystalline quality of HVPE-GaN epilayers depends on the polarity of buffer layers.     

Comparison study of GaN films grown on porous and planar GaN templates

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition(MOCVD)-GaN template by halide vapor phase epitaxy(HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence(PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.     

Progress in bulk GaN growth

Three main technologies for bulk GaN growth,i.e.,hydride vapor phase epitaxy(HVPE),Na-flux method,and ammonothermal method,are discussed.We report our recent work in HVPE growth of GaN substrate,including dislocation reduction,strain control,separation,and doping of GaN film.The growth mechanisms of GaN by Na-flux and ammonothermal methods are compared with those of HVPE.The mechanical behaviors of dislocation in bulk GaN are investigated through nano-indentation and high-space resolution surface photo-voltage spectroscopy.In the last part,the progress in growing some devices on GaN substrate by homo-epitaxy is introduced.     

Characterization of thick HVPE GaN films

The morphology of the top surface for HVPE GaN layers grown on a MOCVD GaN template with a thin LT-AlN interlayer was investigated. This surface is characterized by the formation of numerous hillocks associated with screw dislocations or nanopipes. Their size is large and may reach more than 1 mm. The rocking curves of the 002 and 102 reflections correspond to a relaxed layer. The HREM images of the as-deposited and annealed interlayers show a perfect atomic structure with a very abrupt AlN/HVPE GaN interface. Thus, the deposition of the LT-AlN layer has promoted the growth of an HVPE layer with an excellent crystalline quality.     

Effect of Reactor Pressure on Qualities of GaN Layers Grown by Hydride Vapour Phase Epitaxy

The influence of reactor pressure on GaN layers grown by hydride vapour phase epitaxy （HVPE） is investigated. By decreasing the reactor pressure from0. 7 to 0.5 mm, the GaN layer growth mode changes from the island-like one to the step flow. The improvements in structural and optical properties and surface morphology of GaN layers are observed in the step flow growth mode. The results clearly indicate that the reactor pressure, similarly to the growth temperature, is One of the important parameters to influence the qualities of GaN epilayers grown by HVPE, due to the change of growth mode.     

GaAs(111)衬底上生长的GaN的极性与生长方法和生长条件的关系

研究了在GaAs(111)衬底上生长的六角相GaN的极性的相关关系.在高Ⅴ/Ⅲ比的条件下用MOVPE和MOMBE方法生长的GaN的极性和GaAs衬底的极性一致;在(111)A-Ga表面上的生长层呈现Ga的极性,而在(111)B-As表面上的生长层呈现N的极性.然而,在低的Ⅴ/Ⅲ比,或采用一个AIN中间层的条件下,用HVPE和MOMBE方法在GaAs(111)B表面上生长的GaN呈现出Ga的极性.目前,其原因尚不清楚,但是这些结果表明采用HVPE生长方法或用一高温AlN阻挡层可以得到高质量的GaN.     

Carrier recombination under one-photon and two-photon excitation in GaN epilayers

Luminescence properties of 100-mum thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3ns and diffusion coefficient of 1cm(2)/s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.     

GaN氢化物气相外延生长系统的设计与制作

根据GaN氢化物气相外延生长(HVPE)的原理,设计制作了双温区卧式HVPE系统．根据实际生长中出现的问题和CaN样品的测试情况,对系统进行了逐步的调试和改进．     

氢化物气相外延生长的GaN膜中的应力分析

对在c面蓝宝石上用氢化物气相外延法(HVPE)生长的六方相纤锌矿结构的GaN膜中的应力进行了分析。高分辨X射线衍射(002)面和(102)面摇摆曲线扫描(半高宽数值分别为317和358角秒)表明生长的GaN膜具有较好的晶体质量。利用高分辨X射线衍射技术准确测量了制备的GaN膜的晶格常数,并计算得到GaN膜中面内双轴应变和面外双轴应变分别为3.37×10-4和-8.52×10-4,等静压应变为-7.61×10-5。拉曼光谱和激光光致发光谱测试表明HVPE-GaN外延膜具有较好的光学特性,利用拉曼光谱的E₂模式特征峰和激光光致发光谱中近带边发射峰的频移定量计算了外延膜中的面内双轴压应力和等静压应力。两种方法得到的面内双轴压应力较为相符。     

Surface morphology of GaN nanorods grown by catalyst-free hydride vapor phase epitaxy

GaN nanorods were grown on c-plane sapphire substrates by using catalyst-free hydride vapor phase epitaxy (HVPE). The effects of substrate temperature, Ga boat temperature, and Ga pretreatment on the surface morphology of GaN nanorods were investigated. From the dependence of a radial and axial growth rate on the substrate temperature, the kinetically limited process was found to be a rate determining step in the growth of GaN nanorods in HVPE. In addition, the activation energy of the growth along the both axial and radial directions were estimated. The dependence of a Ga boat temperature and the Ga pretreatment effect revealed that the density of nanorods were dependent on the flux of Ga species on the substrate.     

Influence of AlN buffer layer thickness on structural properties of GaN epilayer grown on Si (111) substrate with AlGaN interlayer

We present the growth of GaN epilayer on Si (111) substrate with a single AlGaN interlayer sandwiched between the GaN epilayer and AlN buffer layer by using the metalorganic chemical vapour deposition. The influence of the AlN buffer layer thickness on structural properties of the GaN epilayer has been investigated by scanning electron microscopy, atomic force microscopy, optical microscopy and high-resolution x-ray diffraction. It is found that an AlN buffer layer with the appropriate thickness plays an important role in increasing compressive strain and improving crystal quality during the growth of AlGaN interlayer, which can introduce a more compressive strain into the subsequent grown GaN layer, and reduce the crack density and threading dislocation density in GaN film.     

Influence of Different Interlayers on Growth Mode and Properties of InN by MOVPE

We grow InN epilayers on different interlayers by metal organic vapour phase epitaxy （MOVPE） method, and investigate the effect of interlayer on the properties and growth mode of InN films. Three InN samples were deposited on nitrided sapphire, low-temperature InN （LT-InN） and high-temperature GaN （HT-GaN）, respectively. The InN layer grown directly on nitrided sapphire owns the narrowest x-ray diffraction rocking curve （XRC） width of 300 aresee among the three samples, and demonstrates a two-dimensional （2D） step-flow-like lateral growth mode, which is much different from the three-dimensional （3D） pillar-like growth mode of LT-InN and HT-GaN buffered samples. It seems that mismatch tensile strain is helpful for the lateral epitaxy of InN film, whereas compressive strain promotes the vertical growth of InN films.     

Effect of thickness on the microstructure of GaN films on Al2O3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire (0001) substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional (3D) island mode to two-dimensional (2D) layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The 110-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.     

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm² of substrate A and on an area of 1×1 cm² of substrate B. Two traps labeled B1 and B2 are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.     

Columnar Structures and Stress Relaxation in Thick GaN Films Grown on Sapphire by HVPE

Thick GaN films with high quality are directly grown on sapphire in a home-built vertical hydride vapour phase epitaxy （HVPE） reactor. The optical and structural properties of large scale columnar domains near the interface are studied using cathodoluminescence and micro-Raman scattering. These columnar domains show a strong emission intensity due to extremely high free carrier concentration up to 2 × 10^19 cm^-3, which are related with impurities trapped in structural defects. The compressive stress in GaN film clearly decreases with increasing distance from interface. The quasi-continuous columnar domains play an important role in the stress relaxation for the upper high quality layer.     

GaN substrate and GaN homo-epitaxy for LEDs:Progress and challenges

After a brief review on the progresses in GaN substrates by ammonothermal method and Na-flux method and hydride vapor phase epitaxy(HVPE) technology,our research results of growing GaN thick layer by a gas flow-modulated HVPE,removing the GaN layer through an efficient self-separation process from sapphire substrate,and modifying the uniformity of multiple wafer growth are presented.The effects of surface morphology and defect behaviors on the GaN homo-epitaxial growth on free standing substrate are also discussed,and followed by the advances of LEDs on GaN substrates and prospects of their applications in solid state lighting.     

GaP layers grown on GaN with and without buffer layers

The growth of GaP layer on GaN with and without buffer layers by metal-organic chemical vapour deposition (MOCVD) has been studied. Results indicate that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature (HT) GaP growth. These sites can promote the two-dimensional (2D) growth of HT GaP and reduce the surface roughness. A GaP single crystal layer grown at 680°C is obtained using a 40-nm thick GaP buffer layer. The full-width at half-maximum (FWHM) of the (111) plane of GaP layer, measured by DCXRD, is 560 arcsec. The GaP layer grown on GaN without low temperature GaP buffer layer shows a rougher surface. However, the FWHM of the (111) plane is 408 arcsec, which is the indication of better crystal quality for the GaP layer grown on GaN without a low temperature buffer layer. Because it provides less nucleation sites grown at high growth temperature, the three-dimensional (3D) growth is prolonged. The crystalline quality of GaP is lightly improved when the surface of GaN substrate is pretreated by PH3 , while it turned to be polycrystalline when the substrate is pretreated by TEGa.     

Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters,such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition,using an in-plane grazing incidence x-ray diffraction technique.The results are analysed and compared with a complementary out-of-plane xray diffraction technique.The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method.The method for directly determining the in-plane lattice parameters of the GaN layers is also presented.Combined with the biaxial strain model,it derives the lattice parameters corresponding to fully relaxed GaN films.The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established,reaching to a maximum level of-0.89 GPa.     

Effect of nanoporous GaN templates with different pore diameters on the subsequent thick GaN layers by HVPE

A set of GaN films were overgrown by hydride vapor phase epitaxy (HVPE) on nanoporous GaN templates with different pore diameters. These samples have various properties as seen from the measurements of X-ray diffraction (XRD) and photoluminescence (PL). Cross-sectional observations under a scanning electron microscopy (SEM) reveal that the overgrowth mechanism and process are strongly related to the dimension of nanopores, indicating that an optimum diameter exists for the properties of subsequent HVPE–GaN layers. When the diameters of nanopores are less than the optimum value, the pores on top of GaN templates can be left, and the properties of HVPE–GaN films show significant improvement. In contrast, the pores are almost stuffed with HVPE–GaN films, which obviously limit the improvement degree of HVPE–GaN films.     

The influence of Ga source and substrate position on the growth of low dimensional GaN wires by chemical vapour deposition

This paper presents the investigation of low dimensional GaN structures synthesized from Ni-catalyzed chemical vapour deposition (CVD) method under two different conditions, i.e. Ga source and substrate position. Comparative studies based on the morphological, structural and optical characteristics of synthesized GaN wires were carried out in this work. The variations of morphological and dimensional aspects of the GaN wires were attributed to the position of Ga precursor and substrates. These factors were found to be able to influence the degree of supersaturation of gaseous reactants, which is essential in the growth of GaN wires by vapour-liquid-solid (VLS) mechanism. The synthesized GaN wires typically were found to have diameters ranging 35-80 nm (nanowires) and 0.4-1.3 μm (microwires), respectively, with length up to several ten of microns. X-ray diffraction (XRD) results indicated that the grown GaN wires were hexagonal wurzite phase. Ultraviolet (UV) and blue emissions were observed from photoluminescence (PL) measurements. Raman spectra displayed asymmetrical and broadened bands which could be ascribed to the size effect, surface disorder and internal strain of the synthesized GaN wires.