SiC/AlN上外延GaN薄膜的黄带发光与晶体缺陷的关系

利用室温光致发光(PL)技术研究了在6HSiC(0001)上用金属有机物化学汽相沉积(MOCVD)外延生长的GaN薄膜“黄带”发光(YL)特点,与扫描电子显微镜(SEM)、X射线衍射(XRD)技术得到的GaN薄膜的表面形貌质量和内部结晶质量的结果相对照,表明“YL”发光强度与GaN薄膜的扩展缺陷多少直接相对应通过二次离子质谱(SIMS)技术获取的GaN薄膜中Ga元素深度分布揭示出镓空位(V_Ga)最可能是“YL”发光的微观来源分析认为,虽然宏观扩展缺陷(丝状缺陷、螺形位错等)和微观点缺陷V_Ga及其与杂质的络合物(complexes)都表现出与“YL”发射密切相关,但V_Ga及其与杂质的络合物更可能是“YL”发射的根本微观来源室温下获得的样品“YL”发射强度和光谱精细结构可用于分析GaN薄膜缺陷和晶体质量.     

Towards controllable optical response of GaN quantum dots in alumina

GaN nanocrystals (in the wurtzite phase) have been synthesized by thermal annealing in reducing atmosphere of Ga⁺+N⁺ sequentially implanted alumina. We show that the reduction of Ga local concentration (by lowering the implantation dose) yields to an overall increase in the intensity of the GaN photoluminescence signal, to a decrease of the PL bandwidth and to the shift of its onset towards higher energies, due to quantum confinement effects in GaN nanocrystals with a narrow size distribution. The interpretation of the optical results is supported by structural analyses. Moreover, by investigating different forming treatments for GaN synthesis, the key role of hydrogen in the annealing atmosphere is evidenced.Received: 20 February 2003, Published online: 24 April 2003PACS: 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 81.05.Ea III-V semiconductors - 78.67.Hc Quantum dots - 85.40.Ry Impurity doping, diffusion and ion implantation technologyC. Maurizio: Present address: INFM, GILDA-CRG ESRF, B.P. 220, 38043 Grenoble, France.E. Borsella: Permanent address: ENEA-UTS FIS, Via E. Fermi 45, 00044 Frascati (Rome), Italy.     

氮化蓝宝石衬底上GaN薄膜的微结构与光学性质

用透射电子显微镜(TEM),X射线衍射(XRD)和光荧光谱(PL)等测量手段研究了GaN薄膜的微结构和光学性质。样品是用光辐射加热MOCVD在蓝宝石衬底上制备的。随着衬底氮化时间的增加,扩展缺陷的密度显著增加。在位错密度增加一个数量级时,XRD摇摆曲线半宽度(FWHM)由11″增加到15″,PL谱的黄光发射从几乎可忽略增加到带边发射强度的100倍。结合生长条件,我们对黄光与微结构的关系作了讨论。     

Effects of ion implantation on yellow luminescence in unintentional doped n-type GaN

The effects of Si, O, C and N ion implantation with different implantation doses on yellow luminescence (YL) of GaN have been investigated. The as-grown GaN samples used in the work were of unintentional doped n-type, and the photoluminescence (PL) spectra of samples had strong YL. The experimental results showed that YL of ion implanted samples exhibited marked reductions compared to samples with no implantation, while the near band edge (NBE) emissions were reduced to a lesser extent. The deep-level centers associated with YL may be produced in GaN films by O and C ion implantation, and identities of these deep-level centers were analyzed. It was also found that the dose dependence of YL was analogous with the one of the intensity ratios of YL to the near band edge (NBE) emission (I _YL /I _NBE ) for ion implanted samples. The possible reason for this comparability has been proposed.      

射频等离子体辅助MBE生长GaN及Mg掺杂的光致发光

采用射频等离子体辅助分子束外延(RF plasma-assisted MBE)系统生长非故意掺杂GaN和p型GaN,并且通过室温和低温光致发光(PL)谱测试研究了材料的发光特性及与杂质态的关系,对于GaN外延层出现的黄带发光进行分析。结果表明,富Ga条件下生长的GaN材料特性要优于富N生长的材料;非故意掺杂的富Ga样品中出现的黄带发光(YL)与GaN中生成能最低的氮空位(V_N)缺陷有关;不同的Mg掺杂浓度对样品的PL特性有较大的影响;结合Hall效应测量结果,认为在Mg重掺杂的样品中出现的黄带发光,与GaN的自补偿效应以及重掺杂导致的晶体质量下降有关。     

GaN厚膜中的质子辐照诱生缺陷研究

本文采用正电子湮没谱研究质子辐照诱生缺陷, 实验发现: 能量为5 MeV的质子辐照在GaN厚膜中主要产生的是Ga单空位, 没有双空位或者空位团形成; 在10 K测试的低温光致发光谱中, 带边峰出现了"蓝移", 辐照后黄光带的发光强度减弱, 说明黄光带的起源与Ga空位(V_Ga)之间不存在必然的联系, 各激子发光峰位置没有改变, 仅强度随质子注量发生变化; 样品(0002)面双晶XRD扫描曲线的半峰宽在辐照后明显增大, 说明质子辐照对晶格的周期性产生了影响, 薄膜晶体质量下降. 关键词： GaN 缺陷 质子 辐照     

Vacancy defects as compensating centers in Mg-doped GaN

We apply positron annihilation spectroscopy to identify V(N)-Mg(Ga) complexes as native defects in Mg-doped GaN. These defects dissociate in postgrowth annealings at 500-800 degrees C. We conclude that V(N)-Mg(Ga) complexes contribute to the electrical compensation of Mg as well as the activation of p-type conductivity in the annealing. The observation of V(N)-Mg(Ga) complexes confirms that vacancy defects in either the N or Ga sublattice are abundant in GaN at any position of the Fermi level during growth, as predicted previously by theoretical calculations.     

Characterization of ZnO:Ga and ZnO:N films prepared by PLD

The thin films of zinc oxide have been produced by the pulse laser deposition method at various levels of gallium and nitrogen doping. To obtain the n-type films we used gallium doping with concentration of gallium from zero up to 5 at %. The dependence of photoluminescence of the epitaxial ZnO:Ga films on the concentration of gallium doping has been studied. An optimum range of the n-type ZnO films doping with gallium has been determined to obtain highly effective films from the viewpoint of realizing p-n transitions. This range, on the one hand, defines the maximal PL amplitude and, on the other hand, specifies the minimal specific resistance that corresponds to an interval of 0.125–1.000 at % Ga. To produce the p-type ZnO:(Ga, N) films, the ZnO targets with the content of GaN from zero up to 2 at % were used. N₂O was used as a buffer gas. A difference is observed in the positions of the peaks of the emission lines of the photoluminescence spectra for the ZnO films, doped with gallium (Ga) and co-doped with gallium and nitrogen (N).     

Effect of synthesis conditions on microstructures and photoluminescence properties of Ga doped ZnO nanorod arrays

Ga doped ZnO nanorod arrays were prepared on silicon substrates in a mixture solution of zinc nitrate hexahydrate, methenamine, and gallium nitrate hydrate. Effect of synthesis conditions on crystal structures, morphologies, surface compositions, and optical properties was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence techniques (PL). Experimental results reveal that Ga doping amount can reach 1.67 at% with the increase of gallium nitrate concentration. Ga doping greatly affects the morphologies of ZnO nanorod arrays. The photoluminescence spectra show a sharp UV emission and a broad visible emission. With Ga doping, UV emission has an apparent broadening effect and its peak shifts from 3.27 eV to 3.31 eV. The intensity ratio of UV emission to visible emission demonstrates that appropriate Ga doping amount is beneficial for the improvement of ZnO crystalline quality.     

Investigation of optical properties of modulation doped GaN/AlGaN MQWS nanostructures

Due to many important applications, the group III-nitride semiconductors have recently attracted remarkable attention among the semiconductor researchers and engineers. In this paper, we report on the impact of the extrinsic and temporal carriers on the screening of the polarization internal fields. The optical efficiency of GaN/AlGaN multiple quantum well (MQW) nanostructures were studied by means of photoluminescence (PL) and time-resolved PL measurements. Extrinsic carriers come from Si doping in the barriers while temporal carriers originate when the samples are excited by the laser beam. The emission peaks of MQWs in PL spectra of the undoped and low-doped samples show a shift towards higher energy levels as excitation intensity increases, while the other samples do not exhibit such a phenomenon due to the dominance of the extrinsic carriers. The transient data confirm the results of the PL measurements.     

Short-gate AlGaN/GaN high-electron mobility transistors with BGaN buffer

Using the semi-insulating property and small lattice constant a of wurtzite BGaN alloy, we propose a BGaN buffer with a B-content of 1% to enhance two-dimensional electron gas(2 DEG) confinement in a short-gate AlGaN/GaN highelectron mobility transistor(HEMT). Based on the two-dimensional TCAD simulation, the direct current(DC) and radio frequency(RF) characteristics of the AlGaN/GaN/B_(0.01)Ga_(0.99)N structure HEMTs are theoretically studied. Our results show that the BGaN buffer device achieves good pinch-off quality and improves RF performance compared with GaN buffer device. The BGaN buffer device can allow a good immunity to shift of threshold voltage for the aspect ratio(LG/d)down to 6, which is much lower than that the GaN buffer device with L_G/d=11 can reach. Furthermore, due to a similar manner of enhancing 2 DEG confinement, the B_(0.01)Ga_(0.99)N buffer device has similar DC and RF characteristics to those the AlGaN buffer device possesses, and its ability to control short-channel effects(SCEs) is comparable to that of an Al_(0.03)Ga_(0.97)N buffer. Therefore, this BGaN buffer with very small B-content promises to be a new method to suppress SCEs in GaN HEMTs.     

单层GaSe表面Fe原子吸附体系电子自旋性质调控

Ⅲ族金属单硫化物因其优越的光电和自旋电子特性而备受关注,实现对其自旋性质的有效调控是发展器件应用的关键.本文采用密度泛函理论系统地研究了GaSe表面Fe原子吸附体系的几何构型及自旋电子特性.Fe/GaSe体系中Fe吸附原子与最近邻Ga,Se原子存在较强的轨道耦合效应,使体系呈现100%自旋极化的半金属性.其自旋极化贡献主要来源于Fe-3d电子的转移及Fe-3d,Se-4p和Ga-4p轨道杂化效应.对于Fe双原子吸附体系,两Fe原子之间的自旋局域导致原本从Fe转移至GaSe的自旋极化电荷量减少,从而费米能级附近的单自旋通道转变为双自旋通道,费米能级处的自旋极化率转变为0.研究结果揭示了Fe_n/GaSe吸附体系自旋极化特性的形成和转变机制,可为未来二维自旋纳米器件的设计与构建提供参考.     

较高掺杂浓度下CdSe/ZnS量子点光纤光致荧光光谱

制备了一种较高浓度掺杂的CdSe/ZnS量子点掺杂光纤.测量了不同掺杂浓度和不同光纤长度下的量子点光纤光致荧光光谱,得剑了荧光峰值增益最大时的量子点掺杂浓度和光纤长度.与低浓度掺杂光纤相比,较高掺杂浓度光纤中的荧光峰值光强明显提高.荧光峰值光强随光纤长度的变化在短距离内(L<1 cm)急剧上升,之后缓慢均匀下降.波长473 nm激励光强随光纤}乏度的变化呈指数形式衰减,消光系数为0.26～1.02 cm-1.在给定激励光强和激励波长的条件下,光纤中可达到最大荧光辐射的晕子点总数为一恒量.光纤中的荧光峰值波长存在红移,红移大小约8～15 nm,红移量与掺杂浓度以及光纤长度有关.这些实验结果可为今后量子点光纤放大器的研制提供参考.     

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs.     

Time- and frequency-domain measurements of carrier lifetimes in GaN epilayers

Results on time-resolved study of GaN photoluminescence (PL) in a power density range from 0.5 mW/cm² under CW excitation by ultraviolet light emitting diode (UV LED) to 1 GW/cm² under pulsed excitation by YAG:Nd laser in the temperature range from 8 to 300 K are presented. Measurements of PL response in the frequency domain by using amplitude-modulated emission of a UV LED as well as time-resolved PL measurements using a streak camera and light-induced transient grating technique have been used in the study. Yellow luminescence (YL) intensity increases with increasing temperature up to 120 K and faster components in YL decay switch to slower components with increasing temperature under UV LED excitation. At low carrier densities, the trapping decreases the carrier lifetime below 250 ps, while the carrier lifetime in the same GaN sample under excitation ensuring saturation of the traps equals 2 ns.     

阶梯AlGaN外延新型Al_(0.25)Ga_(0.75)N/GaNHEMTs击穿特性分析

为了优化AlGaN/GaN HEMTs器件表面电场,提高击穿电压,本文首次提出了一种新型阶梯AlGaN/GaN HEMTs结构.新结构利用AlGaN/GaN异质结形成的2DEG浓度随外延AlGaN层厚度降低而减小的规律,通过减薄靠近栅边缘外延的AlGaN层,使沟道2DEG浓度分区,形成栅边缘低浓度2DEG区,低的2DEG使阶梯AlGaN交界出现新的电场峰,新电场峰的出现有效降低了栅边缘的高峰电场,优化了AlGaN/GaN HEMTs器件的表面电场分布,使器件击穿电压从传统结构的446 V,提高到新结构的640 V.为了获得与实际测试结果一致的击穿曲线,本文在GaN缓冲层中设定了一定浓度的受主型缺陷,通过仿真分析验证了国际上外延GaN缓冲层时掺入受主型离子的原因,并通过仿真分析获得了与实际测试结果一致的击穿曲线.     

Cs/GaN(0001)吸附体系电子结构和光学性质研究

采用基于第一性原理的密度泛函理论平面波超软赝势方法计算了 1/4ML Cs原子吸附 (2 × 2) GaN(0001) 表面的吸附能、能带结构、电子态密度、电荷布居数、功函数和光学性质. 计算发现, 1/4ML Cs 原子在 GaN(0001) 表面最稳定吸附位为 N 桥位, 吸附后表面仍呈现为金属导电特性, Cs原子吸附GaN(0001)表面后主要与表面 Ga 原子发生作用, Cs6s 态电子向最表面 Ga 原子转移, 引起表面功函数下降. 研究光学性质发现, Cs 原子吸附 GaN(0001) 表面后, 介电函数虚部、吸收谱、反射谱向低能方向移动.     

GaN及其Ga空位的电子结构

用团簇埋入自洽计算法对宽禁带半导体GaN的电子结构进行了自旋极化的、全电子、全势场的从头计算,得到了与实验值符合的GaN晶体禁带宽度以及价带中N2p带、N2s带和Ga3d带之间的相对位置.在此基础上Ga空位计算(无晶格畸变)显示,Ga空位周围的费米面显著高于正常GaN晶格的费米面.因此Ga空位周围N原子的处于费米面上的2p电子很易被激发成正常晶格处的传导电子 关键词： GaN 电子结构 团簇埋入自洽计算     

Tunable emission and conductivity enhancement by tellurium doping in CdS nanowires for optoelectronic applications

Improvement of the optical and electrical characteristics is essential to get advanced performance from one dimensional (1D) material. Here, we report the first synthesis of a single crystalline Te-doped CdS nanowires (NWs) by a chemical-vapor-deposition (CVD) method. Room temperature photoluminescence (PL) spectra showed that Te concentration plays an important role in tuning emission color from orange to infrared (IR). Decrease in bandgap and PL intensity with increase in Te concentration was observed as compared to undoped CdS NWs. Red and IR emissions were found at 736.5 and 881 nm for doping concentration >6.06%. To our best knowledge, IR emission band has been observed for the first time in CdS NWs. Red-shift of LO phonon mode and its overtone in Raman spectra, and lifetime of red and IR emissions are longer than bandgap of host indicating the doping effect of CdS NWs. Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) of the Te-doped CdS NWs further confirms the presence of Te in the CdS NWs. Output characteristics confirm enhanced output current I_ds with the increase in doping concentration. A possible growth mechanism was proposed. Doping technique offers to develop high-quality, a very stable, effective, and easily-applicable way to enhance the performance of one dimensional optoelectronic devices and solar cell applications.     

CVD法制备硅基氮化镓薄膜

利用化学气相沉积法(CVD),分别以三氧化二镓(Ga₂O₃)和氨气(NH₃)为镓源和氮源在硅衬底合成了一种由片状微晶构成的氮化镓(GaN)薄膜,实验中没有使用缓冲层。通过场发射扫描电子显微镜(FESEM)、电子能量散射谱(EDS)、X射线衍射(XRD)、高分辨电镜(HRTEM)和光致发光谱(PL)对样品进行分析,生成物为质量较好的富镓的纯氮化镓薄膜。片状氮化镓微晶表面大小约数百纳米,厚度数十纳米,薄膜表面平整、致密,没有裂纹或龟裂现象,与Si衬底结合紧密。氮化镓薄膜的带边峰位于367nm处,同时出现了黄光发射峰。并对此种氮化镓薄膜的生长机理进行了探讨。