INVESTIGATION OF DEFECTS IN Ge-DOPED GaAs CRYSTAL GROWN IN A MAGNETIC FIELD

Ge-doped GaAs single crystals have been grown by liquid-encapsulated Czochralski method in absence and presence of a magnetic field of 4000Gauss. By means of high op-tical efficiency photoluminescence, spectra of the grown crystals at room temperature were obtained, which consist of two emission bands A and B at 1.39-1.42eV and 0.97-1.05eV, respectively. Comparing the photoluminescence mappings with microphotographs of etched wafers, Hall effect results and electron probe microanalyses of the n- and p-type crystals with different Ge concentrations, we considered that the emission bands A and B originate from Ge-related acceptor and donor complexes, respectively. The complexes were formed during crystal growth, mainly due to temperature fluctuations in molten Ge-doped GaAs. The concentrations and homogeneities of the defects can be improved by the application of a magnetic field during crystal growth to suppress the temperature fluctuation.     

Photoluminescence study of native defects in annealed GaAs

Low temperature (6°K) photoluminescence measurements have been performed on GaAs annealed under various conditions, to study defects generated by outdiffusion of the constituent atoms. Several defect-related luminescence peaks have been observed and associated with Ga and As outdiffusion. The outdiffusion of these elements during annealing to 850°C in vacuum and with Ga or As overpressure and SiO₂ coatings is studied by monitoring the intensities of these peaks.     

Photoluminescence properties of GaAs nanowire ensembles with zincblende and wurtzite crystal structure

Self‐standing III–V nanowires (NWs) are promising building blocks for future optoelectronic devices such as LEDs, lasers, photodetectors and solar cells. In this work, we present the results of low temperature photoluminescence (PL) characterization of GaAs NWs grown by Au‐assisted molecular beam epitaxy (MBE), coupled with the transmission electron microscopy (TEM) structural analysis. PL spectra contain exci‐ ton peaks from zincblende (ZB) and wurtzite (WZ) crystal structures of GaAs. The peaks are influenced by the quantum confinement effects. PL bands corresponding to the exciton emission from ZB and WZ crystal phases are identified, relating to the PL peaks at 1.519 eV and 1.478 eV, respectively. The obtained red shift of 41 meV for WZ GaAs should persist in thin NWs as well as in bulk materials. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence of Mn doped epitaxial GaAs

We have measured the near band edge photoluminescence of Mn doped liquid phase epitaxially grown GaAs. The photoluminescence spectra at 2°K shows, at low excitation intensities, a structure of up to eight sharp peaks (widths .2 to 1.0 meV) between 1.517 and 1.512 eV, besides the lower energy bands near 1.41 eV due to the deep Mn acceptor level and the usual donor-acceptor bands around 1.47 eV. Attempts to relate the sharp lines to the Mn electronic states, introduced by doping, were unsuccessful. It is our belief that the presence of this particular impurity in our samples allows for whatever states are responsible for the sharp line structure, to reveal themselves in the emission spectrum. A most unespected result is that near band edge sharp line luminescence is observed for impurity concentration as high as 10¹⁸cm^-3.     

Photoluminescence from chromium diffused GaAs

Low temperature (2dg K) photoluminescence measurements have been performed on (i) Cr-diffused and (ii) thermally converted SI Cr-doped, GaAs substrates. Several defect related luminescence peaks have been observed but the deep levels (present in the original SI samples) associated with Cr impurities are totally absent in the above two substrates, instead a new band peaking at 1.23 eV is observed in the Cr-diffused samples. Possible causes are outlined.     

Temperature dependent photoluminescence studies of ZnSe:I single crystals grown by chemical vapor transport

The single crystal ZnSe:I sample was grown by the chemical vapor transport (CVT) method using iodine as the transporting agent. The iodine incorporates itself effectively as a donor in the lattice. The sample shows a 〈111〉 optical quality surface and has an absorption edge at 2.55 eV due to a deep impurity band nearly 0.15eV below the conduction band. The photoluminescence emission spectra of this crystal have been measured for its temperature dependence as well as for excitation energy dependence. The photoluminescence is in accordance with a donor-acceptor complex formation involving iodine activated donors and self-activated acceptors. The configuration coordinate model has been used to explain the temperature dependent changes in the peak position and the bandwidth of the emission band. The decrease in luminescence efficiency with increasing temperature is explained by using a simple model for thermal quenching. The activation energy at low temperature range (T<200K) is different from that at high temperature range (200K<T<300K).     

利用离子激发发光研究低温条件下ZnO发光行为EI北大核心CSCD

离子激发发光(Ions beam induced luminescence,IBIL)可以实时原位分析不同温度、不同离子辐照条件下材料内部点缺陷的演变行为。本文利用2 MeV H^(+)研究了300,200,100 K温度下ZnO单晶内部点缺陷发光及其随注量的演变行为。实验中发现ZnO深能级发射和近带边发射,结合Voigt分峰与XPS实验结果,确定红光(1.75 eV)与V_(Zn)相关,橙红光(1.95 eV)来自Zn_(i)到O_(i)跃迁;对于与V_(O)相关的绿光(2.10 eV),其红移可能由于温度降低导致更多电子由导带释放到Zn_(i)。峰中心位于3.10 eV和3.20 eV近带边发射分别来自于Zn_(i)到价带的跃迁和激子复合,红移原因分别为Zn_(i)附近局域化能级和带隙收缩。利用单指数公式对发光强度进行拟合,获得的衰减速率常数(f)可以表征缺陷的辐射硬度,对比发现深能级发射峰在200 K时辐射硬度最大,而近带边发射峰在300 K时辐射硬度最大。     

Photoluminescence from gold centre in silicon

A sharp line structure attributable to ?honon assisted radiative emission has been observed in the low temperature photoluminescence spectra from deep centres in bulk samples of gold-doped silicon. The entire luminescence band which peaks near 0.78 eV is attributed to transitions of conduction electrons towards the gold-related donor level.     

硅基硒纳米颗粒的发光特性研究

由于尺寸缩小引起的量子效应, 硒(Se) 材料的低维纳米结构具有更高的光响应和低的阈值激射等特性, 因此成为纳米电子与纳米光电子器件领域一个重要的研究方向. 本文通过对非晶硒薄膜的快速热退火来制备硒纳米颗粒, 退火温度在100–180℃之间时, 结晶后的硒纳米颗粒均为三角晶体结构, 其颗粒尺寸随退火温度的增加而线性增大. 光致发光谱测试发现三个发光峰, 分别位于1.4eV, 1.7eV和1.83eV. 研究发现位于1.4eV处的发光峰来源于非晶硒缺陷发光, 位于1.83eV处的发光峰来源于晶体硒的带带跃迁发光; 而位于1.7eV处的发光峰强度随激发功率增强而指数增大, 且向短波长移动, 该发光峰应该来源于非晶硒与硒纳米颗粒界面处的施主-受主对复合发光. 关键词： 硅基 硒纳米颗粒 光致发光 施主-受主对     

Mechanism of light emission in low energy ion implanted silicon

A silicon wafer implanted with a single low energy (42 keV) silicon ion beam results in strong luminescence at room temperature. The implantation results in the formation of various luminescent defect centers within the crystalline and polymorphous regions of the wafer. The resulting luminescence centers (LC) are mapped using fluorescence lifetime imaging microscopy (FLIM). The emission from the ion-implanted wafer shows multiple PL peaks ranging from the UV to the visible; these emissions originate from bound excitonic states in crystal defects and interfacial states between crystalline/amorphous silicon and impurities within the wafer. The LCs are created from defects and impurities within the wafer and not from nanoparticles.     

Photoluminescence from deep centers in GaAs

Sharp line structure attributable to phonon assisted radiative emission has been observed in the 6 K photoluminescence spectra from deep centers in bulk samples of chromium doped GaAs. Two luminescence bands at 0.56 and 0.8 eV have been observed and both bands exhibit evidence of phonon assisted radiative recombination. An exploration of these luminescence bands in terms of excited state to ground state transitions of Cr³⁺ and Cr²⁺ ions is proposed.     

The photoluminescence of ZnSe bulk single crystals excited by femtosecond pulse

This paper reports on the photoluminescence spectra of ZnSe single crystal with trace chlorine excited by the femtosecond laser pulse. Three emission bands, including second-harmonic-generation, two-photon-excited peak and a broad band at 500--700nm, were detected. The thermal strain induced by femtosecond pulse strongly influences the photoluminescence of ZnSe crystal. The corresponding strain \va in ZnSe crystal is estimated to be about 8.8 \ti10^-3 at room temperature. The zinc-vacancy, as the main point defect induced by femtosecond pulse, is successfully used to interpret the broad emission at 500--700nm. The research shows that self-activated luminescence possesses the recombination mechanism of donor--vacancy pair, and it is also influenced by a few selenium defects and the temperature. The rapid decrease in photoluminescence intensity of two-photon-excited fluorescence and second-harmonic generation emission at lower temperature is attributed to the fact that more point defects result in the thermal activation of the two-photo-absorption energy converting to the stronger recombination emission of chlorine--zinc vacancy in 500--700nm. The experimental results indicate that the femtosecond exciting photoluminescence shows a completely different emission mechanism to that of He--Cd exciting luminescence in ZnSe single crystal. The femtosecond laser exhibits a higher sensitive to the impurity in crystal materials, which can be recommended as an efficient way to estimate the trace impurity in high quality crystals.     

Effects of MBE growth conditions on carbon contamination in GaAs

The effects of growth and pre-growth conditions on the background concentration of carbon in high quality undoped GaAs layers grown by molecular beam epitaxy have been studied. Characterization of the layers by low temperature photoluminescence indicates that a growth temperature of 580°C minimizes carbon contamination, and extended pre-growth outgassing of the substrate under an As₄ over-pressure results in increased carbon concentrations. The carbon incorporation was found to be relatively insensitive to outgassing temperature above 615°C. Contrary to expectations, increasing the As/Ga flux ratio during growth resulted in larger carbon luminescence peaks.     

Measurements of residual stress in semi-insulating GaAs by Cr-related luminescence lines

We have measured systematically the Cr-related zero-phonon lines in the 0.839 eV region in a series of plastically-bent semi-insulating GaAs:Cr with compressive or tensile stress along various bending axes. As a result, it has been found that the residual stress in semi-insulating GaAs:Cr wafers can be sensitively characterized from a splitting and energy shift of the 0.839 eV Cr-related luminescence lines in the low-temperature photoluminescence spectra. Furthermore, we have applied this method to the characterization of the interface stress of OMVPE-grown ZnSe/GaAs:Cr heterostructure and found that anomalous stress exists at the ZnSe/GaAs interface, which is inconsistent with stress predicted by the lattice mismatch of the heterojunctions.     

纳米锗颗粒镶嵌薄膜的荧光特性研究

研究了不同颗粒尺寸的纳米Ge-SiO₂镶嵌薄膜的室温荧光光谱以及不同激发光能量对荧光峰的影响．实验结果表明，沉积态Ge-SiO₂薄膜样品在可见光区域不发光．退火后的样品（平均锗颗粒尺寸为3.2—6.0nm）在380—520nm波长范围内有明显的蓝光发射现象．当用λ=300nm的光激发，观测到中心波长为420nm（2.95eV）的光致荧光峰；而用633nm波长的光激发，谱图上出现中心波长分别为420和470nm的两个荧光峰．随着纳米锗颗粒尺寸的增加，光致荧光峰的相 关键词：     

退火对非故意掺杂4H-SiC外延材料386 nm和388 nm发射峰的影响

10 K条件下,采用光致发光(PL)技术研究了不同退火处理后非故意掺杂4H-SiC外延材料的低温PL特性.结果发现,在370—400 nm范围内出现了三个发射峰,能量较高的峰约为3.26 eV,与4H-SiC材料的室温禁带宽度相当.波长约为386 nm和388 nm的两个发射峰分别位于~3.21 eV和~3.19 eV,与材料中的N杂质有关.当退火时间为30 min时,随退火温度的升高,386 nm和388 nm两个发射峰的PL强度先增加后减小,且退火温度为1573 K时,两个发射峰的PL强度均达到最大. 关键词： 光致发光 退火处理 能级 4H-SiC     

Photoluminescence investigation of Be-doped Npn AlGaAs/GaAs heterojunction bipolar transistor structures

Highly complex Npn AlGaAs/GaAs single heterojunction bipolar transistor (HBT) layers with Be-doped base were investigated by photoluminescence (PL) spectroscopy. Room temperature PL shows only a broad peak of GaAs due to thermalization; 15 K PL shows five peaks. The peak at 1.481 eV is from a p-type GaAs base, that at 1.517 eV is from a low-doped GaAs layer and that at 1.55 eV is from a high-doped GaAs collector. The that at 1.849 eV is due to bound exciton recombination in an AlGaAs emitter, and that at 1.828 eV is due to the acceptor-related transition from the AlGaAs layer. The integrated intensity ratio of these two peaks can be used to investigate the Be outdiffusion behavior, thus optimizing the growth conditions of base. The DC current gain of the HBT structure with different growth conditions was found to be in good agreement with the PL results.     

GaAs纳米线晶体结构及光学特性

采用分子束外延技术在N-型Si (111)衬底上利用自催化生长机制外延砷化镓(GaAs)纳米线,对生长的纳米线进行扫描电子显微镜测试,纳米线垂直度高,长度直径均匀度好.对纳米线进行光致发光(photoluminescence, PL)光谱测试,发现低温10 K下两个发光峰P1和P2分别位于1.493 eV和1.516 eV,推断可能是纤锌矿/闪锌矿(WZ/ZB)混相结构引起的发光以及激子复合引起的发光;随着温度升高,发现两峰出现红移,并通过Varshni公式拟合得到变温变化曲线.对纳米线进行变功率PL光谱测试,发现P1位置的峰位随功率增加而蓝移,而P2位置的峰位不变.通过拟合发现P1峰位与功率1/3次方成线性相关,判断可能是WZ/ZB混相结构引起的Ⅱ型发光;同时,对P2位置的峰位进行拟合,P2为激子复合发光.对纳米线进行拉曼光谱测试,从光谱图中发现GaAs WZ结构特有的E_2声子峰,因此证明生长出的纳米线为WZ/ZB混相结构,并通过高分辨透射电子显微镜更直观地观察到纳米线的混相结构.     

低压-MOCVD方法制备CdS-ZnS应变多量子阱

本文报道了用低压(LP)-MOCVD方法制备CdS-ZnS应变多量子阱结构.通过X-射线衍射谱证实了所制备的样品具有比较好的多层结构,并通过77K下的光致发光(PL)光谱,观测到了大的垒层对电子的限制效应及由大的应变引起的阱层带边变化所导致的发光峰蓝移.     

Strain and optical transitions in InAs quantum dots on (001) GaAs

To investigate the strain characteristics of InAs quantum dots grown on (001) GaAs by solid source molecular beam epitaxy we have compared calculated transition energies with those obtained from photoluminescence measurements. Atomic force microscopy shows the typical lateral size of the quantum dots as 20–22 nm with a height of 10–12 nm, and photoluminescence spectra show strong emission at 1.26 μ m when the sample is capped with a GaAs layer. The luminescence peak wavelength is red-shifted to 1.33 μ m when the dots are capped by an In_0.4Ga_0.6As layer. Excluding the strain it is shown that the theoretical expectation of the ground-state optical transition energy is only 0.566 eV (2.19 μ m), whereas a model with three-dimensionally-distributed strain results in a transition energy of 0.989 eV (1.25 μ m). It has thus been concluded that the InAs quantum dot is spatially strained. The InGaAs capping layer reduces the effective barrier height so that the transition energy becomes red-shifted.