Lateral periodicity in highly-strained (GaIn)As/Ga(PAs) superlattices investigated by X-ray scattering techniques

In this work we investigate the lateral periodicity of symmetrically strained (GaIn)As/GaAs/Ga(PAs)/GaAs superlattices by means of X-ray scattering techniques. The multilayers were grown by metalorganic Vapour phase epitaxy on (001)GaAs substrates, which were intentionally off-oriented towards the [011]-direction. The substrate off-orientation and the strain distribution was found to affect the structural properties of the superlattices inducing the generation of laterally ordered macrosteps. Several high-resolution triple-crystal reciprocal space maps, which were recorded for different azimuth angles in the vicinity of the (004) Bragg diffraction and contour maps of the specular reflected beam collected in the vicinity of the (000) reciprocal lattice point, are reported and discussed. The reciprocal space maps clearly show a two-dimensional periodicity of the X-ray peak intensity distribution which can be ascribed to the superlattice periodicity in the direction of the surface normal and to a lateral periodicity in a crystallographic direction coinciding with the miscut orientation. The distribution and correlation of the vertical as well as of the lateral interface roughness was investigated by specular reflectivity and diffuse scattering measurements. Our results show that the morphology of the roughness is influenced by the off-orientation angle and can be described by a 2-dimensional waviness.

Structural ordering and interface morphology in symmetrically strained (GaIn)As/Ga(PAs) superlattices grown on off-oriented GaAs(100)

In this work we investigate the structural properties of symmetrically strained (GaIn)As/GaAs/Ga(PAs)/GaAs superlattices by means of x-ray diffraction, reciprocal-space mapping, and x-ray reflectivity. The multilayers were grown by metalorganic vapor-phase epitaxy on (001) GaAs substrates intentionally off-oriented towards one of the nearest [110] directions. High-resolution triple-crystal reciprocal-space maps recorded for different azimuth angles in the vicinity of the (004) Bragg diffraction clearly show a double periodicity of the x-ray peak intensity that can be ascribed to a lateral and a vertical periodicity occurring parallel and perpendicular to the growth surface. Moreover, from the intensity modulation of the satellite peaks, a lateral-strain gradient within the epilayer unit cell is found, varying from a tensile to a compressive strain. Thus, the substrate off-orientation promotes a lateral modulation of the layer thickness (ordered interface roughness) and of the lattice strain, giving rise to laterally ordered macrosteps. In this respect, contour maps of the specular reflected beam in the vicinity of the (000) reciprocal lattice point were recorded in order to inspect the vertical and lateral interface roughness correlation, A semiquantitative analysis of our results shows that the interface morphology and roughness is greatly influenced by the off-orientation angle and the lateral strain distribution. Two mean spatial wavelengths can be determined, one corresponding exactly to the macrostep periodicity and the other indicating a further interface waviness along the macrosteps. The same spatial periodicities were found on the surface by atomic-force-microscopy images confirming the x-ray results and revealing a strong vertical correlation of the interfaces up to the outer surface.

Photoluminescence studies of GaAs/AlGaAs single quantum well intermixed by Ga+ ion implantation

Ga(+)ion implantation followed by rapid thermal annealing (RTA) was used to enhance the interdiffusion in GaAs/AlGaAs single Quantum Wells(SQWs). The extent of intermixing was found to be dependent on the well depth, number of implanted ions and annealing time. A very fast interdiffusion process occurs at the initial annealing stage. After that, the enhanced diffusion coefficient goes back to the umimplanted value. We propose a two-step model to explain the diffusion process as a function of the annealing time : a fast diffusion process and a saturated diffusion process. The interdiffusion coefficient of the fast diffusion was found to be of well depth dependence and estimated to be in the range of 5.4x10(-16) similar to 1.5x10(-15)cm(2)s(-1). Copyright (C) 1996 Published by Elsevier Science Ltd

Curie温度附近稀磁半导体(Ga,Mn)As的电学噪声谱性质

建立了自发噪声谱测量系统来研究稀磁半导体(Ga,Mn)As的电学噪声性质.通过测量(Ga,Mn)As材料的自发噪声谱,发现(Ga,Mn)As的自发涨落会随温度升高而逐渐增大,同时,外加磁场会降低(Ga,Mn)As的自发涨落,这来源于外加磁场导致的(Ga,Mn)As磁畴部分有序化.此外,不同频率的噪声随温度的变化规律有很大差异:当频率低于30 kHz的时候,噪声谱和温度的变化关系和热噪声很相似,但数值上明显大于热噪声的值;当频率在30 kHz左右的时候,噪声大小和温度成线性关系;当频率大于30 kHz以后,在相变点附近噪声大小和温度的关系出现了明显的转折,高频高温噪声的大小和热噪声的理论值非常接近.这些结果有助于深入理解(Ga,Mn)As磁性起源的物理机制.

外磁场对(Ga,Mn)As有效g因子的影响

利用时间分辨Kerr旋光技术测量低温下稀磁半导体Ga_(0.937)Mn_(0.063)As中光注入极化载流子的自旋进动信号,并观察到自旋极化载流子的有效g因子值随外磁场的增强而增大的反常现象.这归结于磁场导致局域化空穴转化为非局域化空穴,从而使自发磁化强度增强,有效g因子值增大.基于此物理图像,进一步给出了(Ga,Mn)As的有效g因子与外磁场的关系式.

室温下(Ga,Mn)As中载流子的自旋弛豫特性

运用飞秒时间分辨抽运-探测克尔光谱技术,研究了室温下退火及未退火(Ga,Mn)As的载流子自旋弛豫的激发能量密度依赖性,发现电子自旋弛豫时间随激发能量密度增加而增大,而在同一激发能量密度下,退火样品比未退火样品具有更短的载流子复合时间、电子自旋弛豫时间和更大的克尔转角,显示DP机理是室温下(Ga,Mn)As的电子自旋弛豫的主导机理.退火(Ga,Mn)As的超快克尔增强效应显示其在超高速全光自旋开关方面的潜在应用价值,也为(Ga,Mn)As铁磁性起源的p-d交换机理提供了证据.

(Ga,Mn)As光调制反射光谱

室温下我们研究了稀磁半导体(Ga,Mn)As 的光调制反射(PR)光谱,观测到来自样品的Franz-Keldysh 振荡(FKO)信号.随着Mn原子浓度的增加,PR 线形展宽,但是临界点E_0和E_0+Δ_0没有明显的移动.根据FKO 振荡数据,计算得到样品表面电场强度随Mn原子掺杂浓度的增加而增强.测量到与Mn 原子掺杂相关的杂质带,其能量位置离GaAs价带边～100 meV.根据样品的表面电场强度和表面耗尽层模型,估算样品的空穴浓度为～10~(17) cm~(-3),较低的空穴浓度可能与样品具有较低的居里温度有关,或测量的PR信号来自于样品中外延层的部分耗尽区域.

Effect of Annealing on Structural and Magnetic Properties of a Thick (Ga,Mn)As Layer

We investigate effects of annealing on magnetic properties of a thick (Ga,Mn)As layer, and find a dramatic increase of the Curie temperature from 65 to 115 K by postgrowth annealing for a 500-nm (Ga,Mn)As layer. Auger electron spectroscopy measurements suggest that the increase of the Curie temperature is mainly due to diffusion of Mn interstitial to the free surface. The double-crystal x-ray diffraction patterns show that the lattice constant of (Ga,Mn)As decreases with increasing annealing temperature. As a result, the annealing induced reduction of the lattice constant is mainly attributed to removal of Mn interstitial.

低能离子束制备(Ga,Gd,As)薄膜

室温条件下,用离子束外延设备制备(Ga,Gd,As)样品,X射线衍射(XRD)结果表明除了GaAs衬底峰,没有发现其他新相的衍射峰.俄歇电子能谱(AES)分析了样品中元素随深度的变化,不同样品中元素的分布有着不同的特点.并运用原子力显微镜(AFM)研究了样品表面的形貌特点,表明样品表面的粗糙度与Gd注入过程中在样品表面沉积的多少有关.运用交变梯度磁强计(AGM)对薄膜进行磁性分析,结果表明有的样品在室温条件下出现铁磁性,但金属钆本身具有室温铁磁性,因而需要进一步分析.

离子束外延生长(Ga,Mn,As)的退火研究

利用俄歇电子能谱(AES)和X射线衍射(XRD)分析了室温条件下离子束外延生长Ga、Mn、As样品,在不同的温度条件下进行退火后组分和元素分布的变化.结果表明退火有助于样品内部元素的均匀分布,温度为400 ℃会导致MnO2和Ga5.2Mn的结晶.

Surface Morphology and Photoluminescence of 1.3 μm Wavelength In(Ga)As/GaAs Quantum Dots

Self-organized In_(0.5)Ga_(0.5)As/GaAs quantum island structure emitting at 1. 35 (im at room temperature has been successfully fabricated by molecular beam epitaxy (MBE) via cycled (InAs)_1/( GaAs)_1 monolayer deposition method. Photoluminescence (PL) measurement shows that very narrow PL linewidth of 19.2 meV at 300 K has been reached for the first time, indicating effective suppression of inhomogeneous broadening of optical emission from the In_(0.5)Ga_(0.5)As islands structure. Our results provide important information for optimizing the epitaxial structures of 1.3 μm wavelength quantum dot (QD) devices.

(Ga, Mn, As)/GaAs的发光谱

利用低能双离子束外延技术，在400 ℃条件下生长样品（Ga, Mn, As）/GaAs。样品光致发光谱出现三个峰，即1.5042eV处的GaAs激子峰、1.4875eV处的弱碳峰和低能侧的一宽发光带。宽发光带的中心位置在1.35eV附近，半宽约0.1eV在840 ℃条件下对样品进行退火处理，退火后的谱结构类似退火前，但激子峰和碳杂质峰的峰位分别移至1.5066eV和1.4894eV，同时低能侧的宽发光带的强度大大增加。这一宽发射的来源还不清楚，原因可能是体内杂质和缺陷形成杂质带，生成Mn_2As新相，Mn占Ga位或形成GaMnAs合金。

离子束外延生长（Ga,Mn,As）化合物

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离子损伤对等离子体辅助分子束外延生长的 Ga NAs/ Ga As和Ga In NAs/ Ga As量子阱的影响（英文）

研究了离子损伤对等离子体辅助分子束外延生长的GaNAs/GaAs和GaInNAs/GaAs量子阱的影响。研究表明离子损伤是影响GaNAs和GaInNAs量子阱质量的关键因素。去离子磁场能有效地去除了等离子体活化产生的氮离子。对于使用去离子磁场生长的GaNAs和GaInNAs量子阱样品，X射线衍射测量和PL谱测量都表明样品的质量被显著地提高。GaInAs量子阱的PL强度已经提高到可以和同样条件下生长的GaInAs量子阱相比较，研究也表明使用的磁场强度越强，样品的光学质量提高越明显。