大尺寸HVPE反应室生长GaN的数值模拟

利用有限元法对单片6英寸（1英寸=2.54 cm）Ga N衬底用氢化物气相外延（HVPE）生长系统的工艺参数进行了数值模拟和优化。通过建立反应室二维几何模型,依次改变HVPE系统中的Ga Cl,NH3和分隔气（N2）流速等主要参数进行数值模拟,研究分析了反应室内各反应物的浓度分布和衬底上Ga N的生长速率变化,同时考虑了涡旋分布以及Ga Cl出口管壁上寄生沉积等对衬底上Ga N生长的影响,并给出了HVPE系统高速率均匀生长Ga N的优化参数。模拟分析还表明,适当降低HVPE反应室内的压强可以改善衬底上Ga N生长的均匀性。

AlN单晶性质与AlGaN外延生长研究

对物理气相传输法(PVT)生长的AlN单晶进行了性质表征,研究了单晶中的杂质和缺陷、发光性质和晶体结构等,结果表明该AlN单晶具有较高的晶体质量。用蓝宝石陪片辅助方法对AlN单晶进行了化学机械抛光,得到了厚度均匀、具有器件质量表面的AlN单晶片。利用MOCVD在AlN单晶片上外延了Al组分约为18.2%的AlGaN薄膜,其生长模式符合StranskiKrastanow模式。利用应变梯度模型解释了AlGaN外延层中应变状态和表面裂纹的形成机制。

AlGaN纳米柱光场强度的计算及其影响机制

使用时域有限差分法（FDTD）研究了AlGaN纳米柱的光学性质。通过仿真分析了铝组分的变化对纳米柱内外光场强度的影响。结果表明，在固定纳米柱尺寸条件下，纳米柱内部场强随铝组分显著变化，最大相差一倍，外部场强同样强烈依赖于铝组分。研究分析了纳米柱直径和高度对内部电场强度的影响。结果表明，纳米柱的内部场强受直径的影响比受高度的影响更为显著，在450nm波长下，直径大于120nm的纳米柱才能形成柱内模式分布。此外，在AlGaN纳米柱结构基础上引入铝金属薄膜，研究了不同波长下铝薄膜厚度对纳米柱电场分布的影响。研究发现，随着铝薄膜厚度的增加，在280和380nm波长下，纳米柱内外电场强度逐渐降低，而在450nm波长下，纳米柱内外场强增强倍数先逐渐减小再缓慢增大。

GaN微盘模式分布与强度研究

使用时域有限差分方法系统地研究了直径从2μm到50μm的GaN基微盘在蓝光波段的模式强度和分布。设计了一种可以比拟微盘内高密度激子分布的偶极子排布方式,使其能够激发盘内所有光学模式。研究了盘内最大激发强度和有效共振频率数量随微盘直径的变化关系,揭示了盘内模式间的竞争关系,并给出了微盘内三种典型的激发后的光场分布。

基于蓝宝石衬底生长GaN的立式HVPE控制系统设计

为了实现氢化物气相外延(HVPE)技术在蓝宝石衬底上外延生长氮化镓(GaN)过程的全自动控制,进一步提高系统的精确性和稳定性,设计了一种新型HVPE控制系统。该控制系统选用可编程逻辑控制器(PLC)为控制器,采用组态王开发系统监控软件。利用模糊推理机制和仿真分析,设计模糊比例积分微分(PID)控制器,提高反应室温区控制的快速性与稳定性。构建大、小质量流量通路,精准控制HVPE系统压力与质量流量,并进行控制系统性能测试。结果表明,该控制系统能较好地实现对GaN生长过程的自动监控,成功在蓝宝石衬底上生长出高质量6英寸(1英寸=2.54 cm)GaN,与现有的HVPE制备技术及监控手段相比,其自动控制程度更高,系统实时响应更快,更具实际生产应用价值。

Effect of the V/Ⅲ ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer

Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction.It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer,and the intensities of the yellow luminescence(YL) and blue luminescence(BL) emissions also increase dramatically.However,the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant,as well as the intensity ratio of YL emission to BL emission.It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions,respectively.

Simulation of a new style vertical HVPE system

In this paper,we simulate a new style vertical HVPE reactor by using computational fluid dynamics program FLUENT.In order to find the best parameter on the growth rate of Gallium nitride(GaN),we change the distance between the inlet and the substrate,GaCl and NH3 inlets,and also we add substrate rotation separately.With the increase of the distance between the substrate and the gas inlet,GaN deposition rate decreases and the uniformity becomes better.The results show that the optimal distance in this new-style vertical hydride vapour phase epitaxy(HVPE) system is 4 cm.Besides,as the distance between the GaCl inlet and the NH3 inlet changes,the uniformity of GaN deposition varies.Our findings indicate that the optimal distance is 3 cm.Furthermore,it is found that substrate rotation also affects the growth rate of GaN.

The temperature dependence of optical properties of InGaN alloys

In our experiments,the PL spectra of several In x Ga 1 x N alloy samples with In contents x=0.1,0.15 and 0.25 were measured as a function of temperature ranging from 10 K to 300 K.S-shaped temperature dependencies were observed in all InGaN samples and a sharp decrease in the full width at half maximum occurred with decreasing temperature in the moderate temperature region.It was found that both phenomena are relative to localization effects.In addition,the degree of localization effects in three samples was investigated using the band-tail model.Our findings are presented in this paper.

A modified simplified coherent potential approximation model of band gap energy of III-V ternary alloys

Based on the modification of the simplified coherent potential approximation(SCPA),a model is developed to calculate the composition dependence of the band gap energy of III-V ternary alloys with the same anion.The derived equation is used to fit the experimental band gap energy of In x Al 1 x N,In x Ga 1 x N and Al x Ga 1 x N with x from 0 to 1.It is found that the fitting results are better than those done by using SCPA.The fitting results are also better than those obtained by using the formula with a small bowing coefficient,especially for In x Al 1 x N.In addition,our model can also be used to describe the composition dependence of band gap energy of other III-V ternary alloys.

Study of buffer and epitaxy technology in two-step growth of aluminium nitride

AlN (aluminium nitride) films were prepared by metalorganic chemical vapor depo- sition (MOCVD) on c-plane sapphire (α-Al2O3) substrates. By means of studying the traces of in-situ optical reflectivity, it was found that the AlN nucleation layer showed a specific (0001) lattice orientation, which differed from the GaN nucleation layer on (0001) sapphire substrates. The AlN buffer suffered the compressive stress at the initial stage of nucleation. And the compressive strain was relaxed gradually along with the thickness of buffer increasing and consequently annealing. Optical transmission spectra revealed that in the process of growing AlN epilayers, higher V/III ratio could improve the crystal quality but reduce the growth rates. In addition, proper doping of silane (SiH4) could improve the surface morphology of AlN film.

The parameters in the band-anticrossing model for In_xGa_(1-x)N_yP_(1-y) before and after annealing

We investigate the parameters in the band-anticrossing (BAC) model for GaNP and InGaPN in this work. The parameters in the BAC model for GaNP and InGaNP are obtained by fitting experimental data. It is found that the band gap bowing of InGaNP is stronger than that of InGaNAs. The effects of annealing on the parameters in the BAC model for InGaNP are also discussed in this work. In addition, the origins for improving luminescence efficiency by annealing are explained. It is relative to the forming of more In-N clusters.

The growth and properties of an m-plane InN epilayer on LiAlO_2 (100) by metal-organic chemical vapor deposition

The m-plane InN (1100) epilayers have been grown on a LiAlO2 (100) substrate by a two-step growth method using a metal-organic chemical vapor deposition (MOCVD) system. The low temperature InN buffer layer (LT-InN) is introduced to overcome the drawbacks of thermal instability of LiAlO2 (LAO) and to relieve the strains due to a large thermal mismatch between LAO and InN. Then the high temperature m-plane InN (1100) epilayers (HT-InN) were grown. The results of X-ray diffraction (XRD) suggest that the m-plane InN (1100) epilayer is a single crystal. The X-ray rocking curves (scans) (XRC) and atomic force microscopy (AFM) indicate that the m-plane InN (1100) epilayer has anisotropic crystallographic properties. The PL studies of the materials reveal a remarkable energy band gap structure around 0.70 eV at 15 K.

The growth temperatures dependence of optical and electrical properties of InN films

InN films grown on sapphire at different substrate temperatures from 550℃ to 700℃ by metalorganic chemical vapor deposition were investigated. The low-temperature GaN nucleation layer with high-temperature annealing (1100℃) was used as a buffer for main InN layer growth. X-ray diffraction and Raman scattering measurements reveal that the quality of InN films can be improved by increasing the growth temperature to 600℃. Further high substrate temperatures may promote the thermal decomposition of InN films and result in poor crystallinity and surface morphology. The photoluminescence and Hall measurements were employed to characterize the optical and electrical properties of InN films,which also indicates strong growth temperature dependence. The InN films grown at temperature of 600℃ show not only a high mobility with low carrier concentration,but also a strong infrared emission band located around 0.7 eV. For a 600 nm thick InN film grown at 600℃,the Hall mobility achieves up to 938 cm2/Vs with electron concen-tration of 3.9×1018 cm-3.

Carrier transport and luminescence properties of n-type GaN

The surface morphology,electrical properties and optical properties of Si doped n-type GaN were investigated. The intentional SiH4 doped GaN films were grown by metal organic chemical vapor deposition with the electron concentration varying from 3×1016 cm-3 to 5.4×1018 cm-3. The surface morphology shows that the roughness and dislocation pits increase as the mass flow rate of SiH4 increases,which indicates that the quality of GaN degrades gradually. The activation energy of Si in GaN with different n concentrations varies from 12 to 22 meV,which may originate from the interactions of donor wave functions. The carrier transport mechanism with increasing temperature from 100 to 420 K was concluded as the complex effect of both impurity scattering and phonon scattering. The position of the near band edge emission peak was determined by both renormalization of the band gap and B-M effect. The intensity variations of the yellow luminescence could be explained by the change of Ga vacancy concentration caused by Si doping.