闪锌矿GaN弹性性质、电子结构和光学性质外压力效应的理论研究

采用基于密度泛函理论(DFT)的第一性原理平面波赝势方法,结合广义梯度近似(GGA)下的RPBE和局域密度近似(LDA)的CA-PZ交换-关联泛函对闪锌矿结构的GaN在高压的性质进行了系统研究. 计算结果表明:弹性常数、体模量、杨氏模量和能隙都具有明显的外压力效应,计算结果与实验值和理论值很好的符合. 同时利用计算的能带结构和态密度系统分析了GaN的介电函数、折射率、反射率、吸收系数和能量损失函数等光学性质及其外压力效应. 分析结果为GaN的设计与应用提供了理论依据. 关键词： 第一性原理计算 电子结构 光学性质 闪锌矿GaN     

GaAs光电阴极激活时Cs的吸附效率研究

从NEA GaAs光电阴极的激活光电流曲线发现,当系统真空度不很高时,在首次Cs激活阶段,表面掺杂浓度较低的阴极材料,其光电流产生需要的时间也较长.同时,随着系统真空度的提高,这种时间上的差异又变得不再明显.该现象表明,Cs原子在阴极表面的吸附效率同表面层掺杂浓度以及系统真空度之间有直接的联系.为定量分析这种关系,本文根据实验数据建立了Cs在阴极表面吸附效率的数学模型,利用该模型仿真的结果同实验现象非常符合.该研究对进一步开展变掺杂阴极结构设计和制备工艺研究具有重要的价值和意义. 关键词： GaAs光电阴极 吸附效率 真空度 表面掺杂浓度     

TiO2分子在GaN(0001)表面吸附的理论研究

采用基于密度泛函理论的平面波超软赝势法,计算了TiO₂分子在GaN(0001)表面的吸附成键过程、吸附能量和吸附位置. 计算结果表明不同初始位置的TiO₂分子吸附后,Ti在fcc或hcp位置,两个O原子分别与表面两个Ga原子成键,Ga-O化学键表现出共价键特征,化学结合能达到7.932-7.943eV,O-O连线与GaN[1120]方向平行,与实验观测(100)[001] TiO₂//(0001)[1120]GaN一致. 通过动力学过程计算分析,TiO₂分子吸附过程经历了物理吸附、化学吸附与稳定态形成的过程,稳定吸附结构和优化结果一致. 关键词： GaN(0001)表面 2分子')" href="#">TiO₂分子 密度泛函理论 吸附     

The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si¹⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si¹⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si⁴⁺ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si₃N₄ by molecular beam epitaxy (MBE).     

Leakage current and sub-bandgap photo-response of oxygen-plasma treated GaN Schottky barrier diodes

The effect of oxygen plasma treatment on the performance of GaN Schottky barrier diodes is studied. The GaN surface is intentionally exposed to oxygen plasma generated in an inductively coupled plasma etching system before Schottky metal deposition. The reverse leakage current of the treated diodes is suppressed in low bias range with enhanced diode ideality factor and series resistance. However, in high bias range the treated diodes exhibit higher reverse leakage current and corresponding lower breakdown voltage. The X-ray photoelectron spectroscopy analysis reveals the growth of a thin GaO_x layer on GaN surface during oxygen plasma treatment. Under sub-bandgap light illumination, the plasma-treated diodes show larger photovoltaic response compared with that of untreated diodes, suggesting that additional defect states at GaN surface are induced by the oxygen plasma treatment.     

The influence of growth temperatures on the characteristics of GaN nanowires

This paper presents the investigation of the properties of GaN nanowires synthesized from Ni-catalyzed chemical vapour deposition method under various growth temperatures. The influence of the growth temperatures on the morphological, structural and optical characteristics of the synthesized GaN nanowires was investigated in this work. Field-emission scanning electron microscopy images revealed that the 950 °C was the optimal growth temperature for synthesizing uniform, straight and smooth morphology of GaN nanowires. X-ray diffraction results demonstrated that the synthesized low dimensional GaN structures have the hexagonal wurtzite structure. Ultraviolet and blue emissions were detected from photoluminescence measurements. In addition, phonon replicas with the energy separation of 90 meV have been observed at the lower energy of the blue emission region in photoluminescence spectra.     

Study of the THGEM detector with a reflective CsI photocathode

The THGEM detector without and with a CsI has been tested successfully. The optimal parameters of THGEM have been determined from eight samples. The UV photoelectric effect of the CsI photocathode is observed. The changing tendency related to the extraction efficiency (ε_extr) versus the extraction electric field is measured, and several electric fields influencing the anode current are adjusted to adapt to the THGEM detector with a reflective CsI photocathode.     

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.     

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.     

Method for modulating the wafer bow of free-standing GaN substrates via inductively coupled plasma etching

The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 0.67 to 0.056 m⁻¹ (i.e. the bowing radius increased from 1.5 to 17.8 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face, and eventually changed the bowing direction from convex to concave. Furthermore, the influences of the bowing curvature on the measured full width at half maximum (FWHM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced, which reduced from 176.8 to 88.8 arcsec with increase in ICP etching time. Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V_Ga–O_N complex defects on removing the GaN layer from N-polar face, which removed large amount of defects, was one of the reasons that improved the bowing of the free-standing GaN substrate. Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching, which released the compressive strain of the free-standing GaN substrate. By doing so, crack-free and extremely flat free-standing GaN substrates with a bowing radius of 17.8 m could be obtained.