内嵌InAs量子点的δ掺杂GaAs/AlxGa1-xAs二维电子气特性分析

采用分子束外延技术对δ掺杂GaAs/Al_xGa_1-xAs二维电子气(2DEG)样品进行了生长. 在样品生长过程中, 分别改变掺杂浓度(N_d)、空间隔离层厚度(W_d) 和Al_xGa_1-xAs中Al组分(x_Al)的大小, 并在双温(300 K, 78 K)条件下对生长的样品进行了霍尔测量; 结合测试结果, 分别对N_d, W_d及x_Al与GaAs/Al_xGa_1-xAs 2DEG的载流子浓度和迁移率之间的关系规律进行了细致的分析讨论. 生长了包含有低密度InAs量子点层的δ掺杂GaAs/Al_xGa_1-xAs 2DEG 样品, 采用梯度生长法得到了不同密度的InAs量子点. 霍尔测量结果表明, 随着InAs量子点密度的增加, GaAs/Al_xGa_1-xAs 2DEG的迁移率大幅度减小, 实验中获得了密度最低为16×10⁸/cm²的InAs量子点样品. 实验结果为内嵌InAs量子点的δ掺杂GaAs/Al_xGa_1-xAs 2DEG的研究和应用提供了依据和参考. 关键词： 二维电子气 InAs量子点 载流子浓度 迁移率     

Formation of AlxGa1−xAs periodic array of micro-hexagonal pillars and air holes by selective area MOVPE

A two-dimensional (2D) periodic array having air/semiconductor interfaces can be applied to photonic crystals (PCs), which are expected to control spontaneous emission and optical transports in the next-generation devices. In this paper, we report on the selective area metal-organic vapor phase epitaxial (SA-MOVPE) growth of a Al_xGa_1−xAs 2D periodic array on a GaAs (1 1 1)B substrate for application to 2DPCs having GaAs/AlGaAs heterostructures. Al_xGa_1−xAs (x=0, 0.25 and 0.50) growth was carried out on triangular lattice array of hexagonal GaAs openings and hexagonal SiN_x masks. A uniform Al_0.50Ga_0.50As hexagonal pillar array and a GaAs hexagonal air-hole array with a 1 μm-period were successfully obtained. The important growth parameter for uniform 2DPC structure formation by SA-MOVPE was clarified. Furthermore, we describe the successful demonstration of a 400 nm-period pillar array and an air-hole array, which corresponds to the optical communication wavelength λ=1.3–1.55 μm. The results indicate that SA-MOVPE method is very promising for the formation of uniform semiconductor 2DPCs without the occurrence of process-induced damages.     

Interface-related effects on confined excitons in GaAs/AlxGa1−xAs single quantum wells

Confined excitons in non-abrupt GaAs/Al_xGa_1−xAs single quantum wells are studied. The graded interfaces are described taking into account fluctuations in their thickness a and positioning with respect to the abrupt interface picture. Numerical results for confined (0,0),(1,1) and (0,2) excitons in GaAs/Al_0.3Ga_0.7As quantum wells show that while the interfacial fluctuations produce small changes (<0.5 meV) in the exciton binding energies, the confined exciton energies can be red- or blue-shifted as much as 25 meV for wells with mean width of 50 Å and 2 ML wide interfaces.     

Fractional-dimensional approach for excitons in GaAsfilms on AlxGa1-xAs substrates

Binding energies of excitons in GaAs films on Al_xGa_1-xAs substrates are studied theoretically with the fractional-dimensional approach. In this approach, the real anisotropic “exciton+film” semiconductor system is mapped into an effective fractional-dimensional isotropic space. For different aluminum concentrations and substrate thicknesses, the exciton binding energies are obtained as a function of the film thickness. The numerical results show that, for different aluminum concentrations and substrate thicknesses, the exciton binding energies in GaAs films on Al_xGa_1-xAs substrates all exhibit their maxima with increasing film thickness. It is also shown that the binding energies of heavy-hole and light-hole excitons both have their maxima with increasing film thickness.     

Charge trapping in n-AlxGa1-xAs “insulators” and related device instabilities

In some devices based on GaAs/Al_xGa₁-_xAs heterostructures, the Al_xGa₁-_xAs plays the role of a wide band gap “insulator”. These devices are therefore excellent systems for studying charge trapping in Al_xGa₁-_xAs. It is a poorly understood property of Al_xGa₁-_xAs that incorporation of any n-type dopant results in the formation of a deep electron trap, the DX center. Recent experiments on heterostructure devices have probed both thermal and athermal (hot electron) capture and emission by the DX center. By observing the trapping behavior as the composition (Al mole fraction) of the alloy is varied, the relationship between the trap level and the band structure of the host material has been clarified. A remarkable result is the observation of electron trapping at alloy compositions where the trap state is resonant with the conduction band.     

Fractal dimension study of polaron effects in cylindrical GaAs/AlxGa1−xAs core–shell nanowires

Polaron effects in cylindrical GaAs/Al_xGa_1−xAs core–shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/AlxGa1-xAs core–shell nanowires with different core radii and aluminum concentrations are discussed. The polaron binding energy, polaron mass shift, and fractal dimension parameter are numerically determined as functions of shell width. The calculation results reveal that the binding energy and mass shift of the polaron first increase and then decrease as the shell width increases. A maximum value appears at a certain shell width for different aluminum concentrations and a given core radius. By using the fractal dimension method, polaron problems in cylindrical GaAs/Al_xGa_1-xAs core–shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.     

Characterization of alloy composition in Ga1-xAlxAs/GaAs structures: comparison of photovoltage, X-ray, SIMS and RHEED techniques

A comparative study of techniques for the characterization of alloy composition in Ga_1-xAl_xAs/GaAs single and multilayer structures has been carried out. Good agreement (x ± 0.015) is demonstrated between double-crystal X-ray diffraction and photovoltage spectroscopy measurements using the generally accepted calibration of Casey and Panish. A slightly modified form of the band gap versus composition curve is presented which further improves the agreement to x±0.005. Profiling conditions achieving linear SIMS Al profiles are reported, yielding compositions in agreement with PVS and X-ray within x+-0.01 (when calibrated at a single composition). A systematic discrepancy between results from the above techniques and in-situ RHEED oscillation measurements is reported.     

GaAs薄膜的有效量子限制长度及其极化子特性

本文用分数维方法研究Al_xGa_1-xAs衬底上GaAs薄膜中的极化子特性, 提出了确定GaAs薄膜的有效量子限制长度的一个新方法, 解决了原来方法中在衬底势垒处有效量子限制长度发散的困难, 得到了Al_xGa_1-xAs衬底上GaAs薄膜中的极化子的维数和结合能. 关键词： 分数维方法 极化子 GaAs薄膜     

全光固体条纹相机的理论及其静态实验研究

全光固体条纹相机采用空间调制抽运光激发平板波导光偏转器,通过精确控制抽运光和信号光之间的时间延迟,实现对入射到波导芯层信号光的偏转扫描.它能有效解决传统变像管条纹相机因空间电荷效应造成的动态范围降低以及光电阴极材料在红外波段探测受限等问题,且结构简单,系统稳定性高,理论时间分辨率可达皮秒甚至亚皮秒量级.本文围绕全光固体条纹相机的核心部件—–AlxGa1-xAs/GaAs/AlxGa1-xAs平板波导光偏转器,研究了在带填充效应、带隙收缩效应以及自由载流子吸收效应作用下GaAs折射率的变化情况;在GaAs折射率变化达到0.01量级,信号光束斑大小和波导宽度之比p=0.5时,得到系统的理论时间分辨率为2 ps;按照静态实验条件求得的理论空间分辨率为17 lp/mm,实验结果显示其值为9 lp/mm.     

First-principles calculations of structural and electronic properties of Tl_xGa_(1-x)As alloys

The zincblende ternary alloys Tl_xGa_(1-x) As(0 x 1) are studied by numerical analysis based on the plane wave pseudopotential method within the density functional theory and the local density approximation. To model the alloys,16-atom supercells with the 2 × 2 × 2 dimensions are used and the dependency of the lattice parameter, bulk modulus,electronic structure, energy band gap, and optical bowing on the concentration x are analyzed. The results indicate that the ternary Tl_xGa_(1-x) As alloys have an average band gap bowing parameter of 4.48 eV for semiconductor alloys and 2.412 eV for semimetals. It is found that the band gap bowing strongly depends on composition and alloying a small Tl content with GaAs produces important modifications in the band structures of the alloys.     

Electrically-manipulable electron-momentum filter based on antiparallel asymmetric double δ-magnetic-barrier semiconductor microstructure

We theoretically investigate the wave-vector filtering(WVF) effect for electrons in an antiparallel asymmetric doubleδ-magnetic-barrier microstructure under a bias, which can be fabricated experimentally by patterning two asymmetric ferromagnetic(FM) stripes on the top and the bottom of GaAs/Al_xGa_(1-x) As heterostructure, respectively. It is found that an appreciable WVF effect appears because of an essentially two-dimensional(2D) process for electrons across this microstructure. WVF effect is found to be sensitive to the applied bias. WVF efficiency can be tuned by changing bias,which may lead to an electrically-controllable momentum filter for nanoelectronics device applications.     

Electrical properties of LiCr1-xAlxTiO4 (0≤x≤0.4)

The conductivity measurements on polycrystalline samples have been obtained by ac techniques and the ionic transference number has been calculated by charge/discharge d.c. technique. In the LiCr_1-xAl_xTiO₄ (0≤x≤0.4) spinel systems, at low x, the solid solutions are predominantly electronic conductors but on increasing x, the level of electronic conductivity decreases giving rise to materials that are mixed conductors.     

Characteristics of cylindrical surrounding-gate GaAs_xSb_(1-x)/In_yGa_(1-y)As heterojunction tunneling field-effect transistors

A Ⅲ-Ⅴ heterojunction tunneling field-effect transistor(TFET) can enhance the on-state current effectively,and GaAs_xSb_1_x/In_yGa_1_yAs heterojunction exhibits better performance with the adjustable band alignment by modulating the alloy composition.In this paper,the performance of the cylindrical surrounding-gate GaAs_xSb_1_x/In_yGa_1_yAs heterojunction TFET with gate-drain underlap is investigated by numerical simulation.We validate that reducing drain doping concentration and increasing gate-drain underlap could be effective ways to reduce the off-state current and subthreshold swing(SS),while increasing source doping concentration and adjusting the composition of GaAs_xSb_1_xIn_yGa_1_yAs can improve the on-state current.In addition,the resonant TFET based on GaAs_xSb_1_x/In_yGa_1_yAs is also studied,and the result shows that the minimum and average of SS reach 11 mV/decade and 20 mV/decade for five decades of drain current,respectively,and is much superior to the conventional TFET.     

Valuing of the critical layer thickness from the deading time constant of RHEED oscillation in the case of InxGa1−xAs/GaAs heterojunction

In this work, we have looked for the correlation between the observed decay of reflection high-energy electron diffraction intensity oscillation and the critical layer thickness in the case of strained In_xGa_1−xAs/GaAs heterojunctions. The value of deading time constant of oscillation depends on the mismatch and on growth parameters, too. The decay of oscillation was described by two deading time constants which are responsible for the influences of the parameters mentioned above. The critical layer thickness was valued from the deading time constant responsible for the influence of mismatch only. The critical layer thickness determined this way shows good agreement with the theoretical model.     

三元混晶四层系统的表面和界面声子极化激元

运用改进的无规元素等位移模型和玻恩-黄近似,结合电磁场的麦克斯韦方程和边界条件,研究了真空/极性二元晶体薄膜/极性三元混晶薄膜/极性二元晶体衬底四层系统的表面和界面声子极化激元.以AlxGa1-xAs/GaAs和ZnxCd1-xSe/ZnSe为例,获得了表面和界面声子极化激元模的色散关系以及表面模和界面模的频率随混晶组分和薄膜厚度的变化关系.结果表明,三元混晶四层异质结系统中存在七支表面和界面声子极化激元模,且这七支表面模和界面模的频率随混晶组分和薄膜厚度呈非线性变化,三元混晶的"单模"和"双模"性也在色散曲线中得到了很好的体现.     

A spectroscopic ellipsometric investigation of new critical points of Zn1−xMnxS epilayers

Zn_1−xMn_xS epilayers were grown on GaAs (1 0 0) substrates by hot-wall epitaxy. X-ray diffraction (XRD) patterns revealed that all the epilayers have a zincblende structure. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 to 8.5 eV. The obtained data were analyzed for determining the critical points of pseudodielectric function spectra, (E) = ₁(E) + i₂(E), such as E₀, E₀ + Δ₀, and E₁, and three E₂ (Σ, Δ, Γ) structures at a lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The observation of new peaks, as well as the shifting and broadening of the critical points of Zn_1−xMn_xS epilayers, were investigated as a function of Mn composition by ellipsometric measurements for the first time. The characteristics of the peaks changed with increasing Mn composition. In particular, four new peaks were observed between 4.0 and 8.0 eV for Zn_1−xMn_xS epilayers, and their characteristics were investigated in this study.     

BxGa1-xAs合金电子结构与光学性质的第一性原理计算

基于密度泛函理论的第一性原理和特殊准随机近似方法,建立64原子的超胞并且对结构进行几何优化.计算和讨论具有闪锌矿结构的三元合金B_xGa_1-xAs的结构参数、电子结构和光学性质.结果表明：B_xGa_1-xAs的晶格常数与使用Vegard定理计算得到的值有微弱的偏离,键长存在明显的弛豫;计算得到的合金带隙弯曲参数变化较小（2.57 eV-5.01 eV）而且对组分变化的依赖很弱;最后分析硼的并入对GaAs光学参数包括介电函数、反射率、折射率、吸收系数和能量耗散函数的影响.     

Control of symmetric properties of metamorphic In_(0.27)Ga_(0.73)As layers by substrate misorientation

Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic materials with symmetric properties. In this paper, we grew metamorphic In_(0.27)Ga_(0.73) As epilayers with symmetric low threading dislocation density and symmetric strain relaxation in two (110) directions using In Al Ga As buffer layers on 7°misoriented Ga As(001)substrates. To understand the control mechanism of symmetric properties of In_(0.27)Ga_(0.73) As layers by the substrate miscut angles, In_(0.27)Ga_(0.73) As grown on 2°and 15°misoriented substrates were also characterized as reference by atomic force microscopy, transmission electron microscopy, and high resolution triple axis x-ray diffraction. The phase separation and interaction of 60°misfit dislocations were found to be the reasons for asymmetry properties of In_(0.27)Ga_90.73 As grown on 2and 15°substrates, respectively. Photoluminescence results proved that the In_(0.27)Ga_(0.73) As with symmetric properties has better optical properties than the In_(0.27)Ga_(0.73) As with asymmetric properties at room temperature. These results imply that high quality metamorphic In_(0.27)Ga_(0.73) As can be achieved with controllable isotropic electron transport property.     

层数变化对堆叠生长的MoS_(2(1-x))Se_(2x)电子结构的影响

二维过渡金属硫化物因其独特的光电特性在多功能光电器件方面具有广泛的应用前景.为了进一步拓展其在微纳光电子器件方面的应用范围,并提高器件性能,人们开展了通过合金手段改变端组分材料配比实现对二维半导体材料带隙调控的带隙工程以及调控生长条件改变材料形貌和结构的缺陷工程研究.本文利用光学、原子力和扫描电子显微镜等设备以及拉曼和光致发光光谱等手段对由化学气相沉积法生长出来的堆叠状MoS_(2(1-x))Se_(2x)合金的性质进行了研究.不同于大多数单层或少层MoS_(2(1-x))Se_(2x)合金的情况,堆叠生长的阶梯状MoS_(2(1-x))Se_(2x)合金材料在厚度从2.2 nm (约3层)一直增加到5.6 nm (约7层)时都显出了较强的发光特性,甚至在100 nm厚时,样品的发光谱线仍具有两个发光峰.两个激子发光峰分别来源于自旋轨道耦合造成的价带劈裂.随着厚度的增加,两个峰都逐渐红移,显示了合金掺杂时的能带弯曲效应.拉曼光谱给出了类MoS_2和类MoSe_2两套振动模.随着厚度的增加,拉曼峰位几乎不移动,但面内的两个振动模E_(2g(Mo-Se))和E_(2g(Mo-s))逐渐显现并增强.显然缺陷和应力是影响堆叠生长MoS_(2(1-x))Se_(2x)合金样品电子结构的主要因素,这为特殊功能器件的制备和可控缺陷工程的研究提供了有益的参考.     

Hydrogen depth profile measurement in a-Si1-xCx:H films by elastic recoil detection

The hydrogen content and depth profile in a-Si_1-xC_x:H films were measured by the elastic recoil detection (ERD) technique. It is shown that the hydrogen content changes from 15 to 50 at% with increasing carbon content x. For x<0.4 the hydrogen content increases mainly due to the increase of the Si-CH₃ contribution and for x > 0.4 due to C-H bonds. By combining the ERD and IR results, the proportionality between the number of Si-CH₃ bonds and the intensity of IR absorption due to the Si-CH₃ rocking mode vibration is ascertained. The proportionality constant is found to be A^roc_Si-CH3 = 5 × 10¹⁹ cm^-2.