Review: GaN growth by ammonia based methods – density functional theory study

Recent results in Density Functional Theory (DFT) simulations of ammonia‐based growth of gallium nitride on GaN (0001) are reviewed. These simulations are important to the following GaN growth methods that use ammonia as active nitrogen source: ammonothermal, MOVPE, HVPE and also ammonia‐source MBE. In the simulations of GaN growth, the two main approaches were discussed: (1) equilibrium, based on chemical potentials of the components, and (2) dynamic, based on consideration of atomistic processes on the surface. These two approaches are unified by the kinetic procedure of determination of the chemical potential levels for nitrogen and hydrogen as a function of partial pressure of ammonia. Here the DFT modeling of GaN(0001) surface employing the technique of the simulation of subsurface electric field is described and employed. The results of DFT modeling include the ammonia and molecular hydrogen adsorption on GaN(0001) surface that allows to determine some basic features of ammonia‐based growth of GaN. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

HVPE生长室气流分布模拟及GaCl载气流量对GaN单晶生长的影响

采用计算流体力学软件Fluent对HVPE反应室进行了数值模拟,研究了GaCl载气流量对HVPE反应室气流分布的影响,发现GaCl载气流量是影响GaCl和NH3在衬底上均匀分布的重要因素.采用HVPE方法在不同GaCl载气流量下生长GaN单晶,研究了GaCl载气流量对GaN单晶质量的影响,得到了与模拟一致的结果.     

Hydride vapor phase epitaxy of GaN boules using high growth rates

The boule-like growth of GaN in a vertical AIXTRON HVPE reactor was studied. Extrinsic factors like properties of the starting substrate and fundamental growth parameters especially the vapor gas composition at the surface have crucial impact on the formation of inverse pyramidal defects. The partial pressure of GaCl strongly affects defect formation, in-plane strain, and crystalline quality. Optimized growth conditions resulted in growth rates of 300–500 μm/h. GaN layers with thicknesses of 2.6 and of 5.8 mm were grown at rates above 300 μm/h. The threading dislocation density reduces with an inverse proportionality to the GaN layer thickness. Thus, it is demonstrated that growth rates above 300 μm/h are promising for GaN boule growth.     

Cross‐sectional and surface Raman mapping of thick GaN layers

Raman scattering spectroscopy was utilized for investigation of the structural properties of thick GaN layers. These layers with thickness ∼ 40 μm have been grown by HVPE technique on the sapphire substrates. The investigations have been focused on the strain distribution in GaN layer cross‐section as a function of distance from an interface sapphire/GaN and mapping of the surface and of the inner layer, near the sapphire/GaN interface. From the observed phonon shifts in the Raman spectra strain differences lower than 6.4×10^–4 corresponding to stress differences of 240 MPa were estimated across the thick GaN epitaxial layer. The measurements exhibit that strain in the layer causes changes in the Raman spectra and allow determining the relaxation process in the crystal. The obtained results confirmed, that the mode frequencies in the measured Raman spectra in both directions (parallel or perpendicular to the growth direction) for layer thicknesses over 30 µm are comparable with typical values for bulk material and match the low strain in the structure due to relaxation processes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Method for HVPE growth of thick crack-free GaN layers

Twenty-five micrometer thick GaN was grown with hydride vapor phase epitaxy (HVPE) on metal-organic chemical vapor deposition (MOCVD) grown templates on sapphire substrates with the gallium treatment step (GTS) technique with varying buffer layer thickness. The samples are studied with atomic force microscopy (AFM), etching and scanning electron microscopy (SEM), photo-luminescence (PL), X-ray diffraction (XRD) and optical microscopy. The results show that the thickness of the buffer layer is not important for the layer quality once the growth in MOCVD starts to make the transition from 3D growth to 2D growth and HVPE continues in the same growth mode. We show that the MOCVD templates with GTS technique make excellent templates for HVPE growth, allowing growth of GaN without cracks in either sapphire or GaN.     

Surface chemistry and transport effects in GaN hydride vapor phase epitaxy

A simple quasi-thermodynamic model of surface chemistry in GaN hydride vapor phase epitaxy (HVPE) is presented. The model is coupled with the detailed 3D simulations of species transport in a horizontal-tube reactor and validated by the comparison with the data on the GaN growth rate obtained by laser reflectometry. Parametric study of the growth rate as a function of temperature and species flow rates has been performed over a wide range of growth conditions. The important role of species transport in an HVPE reactor is demonstrated. In particular, a strong effect of the natural concentration convection resulting in the formation of recirculation zones and in a non-uniform vapor composition is revealed by modeling. The impact of these effects on the GaN growth rate and V/III ratio on the growth surface is discussed in detail.     

Fabrication of a freestanding GaN layer by direct growth on a ZnO template using hydride vapor phase epitaxy

A new hydride vapor phase epitaxy (HVPE)-based approach for the fabrication of freestanding GaN (FS-GaN) substrates was investigated. For the direct formation of low-temperature GaN (LT-GaN) layers, the growth parameters were optimized: the polarity of ZnO, the growth temperature, and the V/III ratio. The FS-GaN layer was achieved by gas etching in an HVPE reactor. A fingerprint of Zn out-diffusion was detected in the photoluminescence measurements, especially for the thin (80 μm) FS-GaN film; however, a thicker film (400 μm) was effectively reduced by optimization of GaN growth.     

Vapor phase growth of GaN using GaN powder sources and thermogravimetric investigations of the evaporating behaviour of the source material

The growth of GaN from the vapor phase is a promising technique for producing both bulk GaN crystals and GaN layers. For establishing a growth method from the vapor phase the source material and reactor setup are of great importance. Highly pure and self synthesized GaN powder was chosen as source material. The evaporation behaviour of the GaN powder was studied by means of thermogravimetry (TG). A vertical growth reactor was set up according to the results of numerical simulations of the temperature distributions and flow patterns. Freely nucleated GaN platelets of some millimetres in length were grown. Furthermore, thin GaN layers were deposited directly on a sapphire substrate. This nucleation layer was successfully overgrown by low pressure solution growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

大尺寸HVPE反应器寄生沉积的数值模拟研究

在氢化物气相外延(HVPE)生长GaN厚膜中,反应腔壁面总会产生大量的寄生沉积,严重影响薄膜生长速率及质量.本文针对自制的大尺寸垂直式HVPE反应器,通过数值模拟与实验对比,研究了反应腔壁面沉积以及GaN生长速率的分布规律,特别是寄生沉积分布与载气流量的关系.研究发现:在基准条件下,顶壁寄生沉积速率由中心向边缘逐渐降低,与实验结果吻合;侧壁沉积出现8个高寄生沉积区域,对应喷头边缘处排布的GaCl管,说明沉积主要取决于GaCl的浓度输运;模拟得出的石墨托表面生长速率低于实验速率,但趋势一致.保持其他条件不变,增大NH3管载气N2流量,顶壁和侧壁的寄生沉积速率及分布区域均随之增大,石墨托表面生长速率随之减小而均匀性却随之提高;增大GaCl管载气N2流量,顶壁和侧壁的寄生沉积速率及分布区域均随之减小,石墨托表面生长速率随之增大而均匀性却随之降低.研究结果为大尺寸HVPE反应器生长GaN的工艺优化提供了理论依据.     

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.     

Bulk growth of gallium nitride: challenges and difficulties

The present status of the GaN bulk growth by High Pressure Solution (HPS) method and combination of HPS and Hydride Vapor Phase Epitaxy (HVPE) methods is reviewed. Up to now the spontaneous high pressure solution growth of GaN results in crystals having habit of hexagonal platelets of surface area of 3 cm² or needles with length up to 1 cm. Recently, the platelets and needles have been used as seeds for the HVPE growth. On the other hand, the LPE technique under pressure with pressure‐grown GaN (hp‐GaN), GaN/sapphire template, patterned GaN/sapphire template and free standing HVPE GaN as seeds has been examined and developed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE生长GaN薄膜的表面吸附和扩散研究

利用量子化学计算方法,对MOVPE生长GaN薄膜的表面反应进行研究.特别针对反应前体GaCH3(简称MMG)在理想、H覆盖和NH2覆盖GaN(0001)面的吸附和扩散进行计算分析.通过建立3×3 超晶胞模型,优化计算了MMG在三种不同覆盖表面的稳定吸附位、吸附能和电子布居,搜寻了MMG在稳定吸附位之间的扩散能垒.计算结果表明:对于三种表面,MMG的稳定吸附位均为T4位和H3位,H3位比T4位略微稳定.MMG在NH2覆盖表面吸附能最大,在H覆盖表面吸附能最小,在理想表面吸附能居中.MMG中的Ga与不同的表面原子形成的化学键的键强的大小顺序为:Ga-N>Ga-Ga>Ga-H.相比于理想表面和H覆盖表面,MMG在NH2覆盖表面的扩散能垒最大,因此表面过量的NH2会抑制MMG的扩散.     

Accelerated surface flattening by alternating Ga flow in hydride vapor phase epitaxy

GaN films were grown on c$c$-plane sapphire substrates by using hydride vapor phase epitaxy (HVPE) with a pulsed flow of HCl over Ga metal. NH₃${\mathrm{NH}}_3$ gas supply was controlled to flow in a constant rate or in a modulated way. The surface morphology dependence of these films on the various flow modulation schemes was investigated. Depending on the duty cycle of NH₃${\mathrm{NH}}_3$ flow, the surface morphology of GaN films was sensitively modified. This sensitive response of surface morphology of GaN films to the flow modulation was attributed to diffusion efficiency variation of Ga species under different gas environment. Under proper modulation conditions, flattened top-surface morphology of nucleated domains was found to be obtained.     

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10⁷ cm⁻²) and a higher edge-dislocation density (8.157×10⁹ cm⁻²) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.     

Enhanced growth rates and reduced parasitic deposition by the substitution of Cl2 for HCl in GaN HVPE

The main limitation in the application of hydride vapor phase epitaxy for the large scale production of thick free-standing GaN substrates is the so-called parasitic deposition, which limits the growth time and wafer thickness by blocking the gallium precursor inlet. By utilizing Cl₂ instead of the usual HCl gas for the production of the gallium chlorine precursor, we found a rapid increase in growth rate from ∼80 to ∼400 μm/h for an equally large flow of 25 sccm. This allowed us to grow, without any additional optimization, 1.2 mm thick high quality GaN wafers, which spontaneously lifted off from their 0.3° mis-oriented GaN on sapphire HCl-based HVPE templates. These layers exhibited clear transparencies, indicating a high purity, dislocation densities in the order of 10⁶ cm⁻², and narrow rocking curve XRD FWHMs of 54 and 166 arcsec in for the 0002 and 101−5 directions, respectively.     

Thick GaN layers on sapphire with various buffer layers

Thick GaN layers deposited in HVPE system on composite substrates made on sapphire substrates in Metalorganic Vapour Phase Epitaxy (MOVPE) system have been investigated. The following substrates were used: (00.1) sapphire substrates with AlN, AlN/GaN and GaN thin layers. The crystallographic structure and the quality of the epitaxial thick GaN layers were determined. Comparison of the three types of thick layers was performed. Significant differences were observed. It was found that thick GaN deposited on the simplest MOVPE‐GaN/sapphire composite substrate has comparable structure's properties as the other, more complicated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bismuth a new dopant for GaN films grown by molecular beam epitaxy—surfactant effects,formation of GaN1−xBix alloys and co-doping with arsenic

We have investigated the influence of an additional bismuth flux during growth on the properties of GaN films prepared by plasma-assisted molecular beam epitaxy (MBE). A wide range of bismuth fluxes have been used, the highest Bi flux was larger than the flux of Ga. We have demonstrated that using a Bi flux during the growth of GaN by MBE at a temperature ∼800°C improves the surface morphology of the films and decreases the deep emission. We have demonstrated for the first time the growth of GaN_1−xBi_x alloys by MBE, the Bi concentration was small and increased with decreasing growth temperature. We have studied the influence of an additional bismuth flux on the growth of As-doped GaN layers and observed an increase of blue emission from the layers at some optimum range of Bi fluxes. All the results allow us to conclude that Bi may be a potential new dopant for the growth of GaN by MBE.     

Synthesis of gallium nitride by ammonia injection into gallium melt

Gallium nitride (GaN) was synthesized by injecting ammonia gas into molten gallium at 900–980°C under atmospheric pressure. A large amount of GaN powder was reproducibly obtained using a simple apparatus. The synthesized powder was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence and energy dispersive X-ray spectroscopy, and was found to consist of fine crystals of hexagonal GaN of good quality. The total of GaN obtained was far more than the amount calculated from expected saturation solubility in the Ga melt at that temperature. We speculate that the GaN crystals were largely formed by direct reaction between Ga and the gaseous N source at the surface of the NH₃ bubbles in the melt. GaN synthesized by this method may be useful as a starting material for bulk growth.     

金属有机物气相外延生长InAlGaN薄膜及其性能表征

在不同的生长温度和载气的条件下,采用低压金属有机物气相外延方法生长了系列的InAlGaN薄膜,通过能量色散谱(EDS),高分辨X射线衍射(HRXRD)和光致发光谱(PL)对样品进行表征与分析,研究了生长工艺对InAlGaN外延层结构和光学性能的影响.发现当以氮气做载气时,样品的发光很弱并且在550nm附近存在一个很宽的深能级发光峰;当采用氮气和氢气的混合气做载气时,样品中的深能级发光峰消失且发光强度明显提高.以混合气做载气,InAlGaN薄膜中铟的组分随生长温度的升高而降低,而薄膜的结构和光学性能却提高.结合PL和HRXRD的测试结果得到了较佳的生长参数:即载气为氢气和氮气的混合气以及生长温度在850℃到870℃.     

CVD金刚石薄膜亚表面层氢杂质对表面活化反应的影响

采用密度泛函理论平面波赝势法研究了氢杂质位于氢终止金刚石薄膜亚表面层中三种不同位点处时金刚石的结构变化,以及氢原子在三种金刚石薄膜表面上的吸附难易程度,并对表面活化反应进行了过渡态搜索以探究化学气相沉积(CVD)过程中金刚石薄膜亚表面层氢杂质对表面活化的影响。对比计算结果发现:生长过程中,亚表面层的氢杂质使其附近的金刚石结构产生了畸变,同时金刚石表面结构会对畸变程度产生影响。氢原子在含有氢杂质的三种金刚石薄膜上的吸附能与理想金刚石薄膜差异很小,但发生萃取反应产生活性位点的能垒比理想金刚石薄膜更低,这与亚表面层中的氢杂质使金刚石薄膜具有P型半导体特征这一现象有关。这一结果说明富氢反应气氛有利于活性位点的产生并以更高速率进行生长。