Polaron effects in cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowires

By the fractal dimension method, the polaron properties in cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowire are explored. In this study, the polaron effects in GaAs/Al_xGa_(1-x)As core-shell nanowire at different values of shell width and aluminum concentration are discussed. The polaron binding energy, polaron mass shift and fractal dimension parameter are numerically worked out each as a function of core radius. The calculation results show that the binding energy and mass shift of the polaron first increase and then decrease as the core radius increases, forming their corresponding maximum values for different aluminum concentrations at a given shell width. Polaron problems in the cylindrical GaAs/Al_xGa_(1-x)As core-shell nanowire are solved simply by using the fractal dimension method to avoid complex and lengthy calculations.     

Fractal dimension study of polaron effects in cylindrical GaAs/Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>As core–shell nanowires

Polaron effects in cylindrical GaAs/Al _x Ga_1-xAs core–shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/Al _x Ga_1-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 _x Ga_1-xAs core–shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.     

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     

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.     

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.     

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.     

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.     

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     

Physical characterization of OMVPE-grown AlxGa1-xAs multi-layer films by means of non-destructive optical reflectometry

A novel Fourier transform technique which was developed by us previously, is applied in this work for non-destructive determination of thickness and refractive index steps of experimental multilayer heteroepitaxial structures. No time consuming least-squares curve-fits are required and theoretically there is no limit on the number of layers which can be analyzed. In order to demonstrate the above method experimentally, multilayer structures of Al_xGa_1-xAs containing up to five layers on GaAs substrates, were grown by organometallic vapour phase epitaxy (OMVPE). For calibration purposes, initial experiments involved the growth of Al_xGa_1-xAs single epilayers on GaAs substrates by the same technique. Layers were grown at five different compositions from x = 0.1 to x = 0.5. Both photoluminescence (at 12 K) and double-crystal X-ray diffraction measurements were undertaken on the single layers in order to establish the composition of the material. The results show excellent correspondence with each other. These results were also compared to secondary ion mass spectrometry data. The agreement between the optical technique developed by us and the other analytical methods was excellent.     

分数维方法研究GaAs薄膜中的极化子

本文采用分数维方法, 在讨论Al_0.3Ga_0.7As衬底上GaAs薄膜的分数维基础上, 计算了GaAs薄膜中的极化子结合能和有效质量. 随着薄膜厚度的增加, 极化子结合能和质量变化单调地减小. 当薄膜厚度L_w<70 Å并且衬底厚度L_b<200 Å时, 衬底厚度的变化对薄膜中极化子的结合能和质量变化的影响比较显著, 随着衬底厚度的增加, 薄膜中极化子的结合能和质量变化逐渐变大; 当薄膜厚度L_w>70 Å或者衬底厚度L_b>200 Å时, 衬底厚度的变化对薄膜中极化子的结合能和质量变化的影响不显著. 研究结果为GaAs薄膜电子和光电子器件的研究和应用提供参考. 关键词： 分数维方法 GaAs薄膜 极化子 低维异质结构     

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.     

Novel method to determine effective length of quantum confinement using fractional-dimension space approach

The binding energy and effective mass of a polaron confined in a GaAs film deposited on an Al_xGa_1-x As substrate are investigated, for different film thickness values and aluminum concentrations and within the framework of the fractional-dimensional space approach. Using this scheme, we propose a new method to define the effective length of the quantum confinement. The limitations of the definition of the original effective well width are discussed, and the binding energy and effective mass of a polaron confined in a GaAs film are obtained. The fractional-dimensional theoretical results are shown to be in good agreement with previous, more detailed calculations based on second-order perturbation theory.     

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.     

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.     

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.     

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.     

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.     

空间用GaInP/GaAs/In0.3Ga0.7 As(1 eV)倒装三结太阳电池研制

采用阶变缓冲层技术 (step-graded) 外延生长了具有更优带隙组合的倒装GaInP/GaAs/In_0.3Ga_0.7As(1.0 eV) 三结太阳电池材料, TEM和HRXRD测试表明晶格失配度为2%的In_0.3Ga_0.7As 底电池具有较低的穿透位错密度和较高的晶体质量, 达到太阳电池的制备要求. 通过键合、剥离等工艺制备了太阳电池芯片. 面积为 10.922 cm² 的太阳电池芯片在空间光谱条件下转换效率达到32.64% (AM0, 25 ℃), 比传统晶格匹配的 GaInP/GaAs/Ge(0.67 eV) 三结太阳电池的转换效率提高3个百分点. 关键词： 太阳电池 三结 倒装结构     

Moisture barrier of AlxOy coating on poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(carbonate) substrates

The moisture barrier property of Al_xO_y coated poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN) and poly(carbonate) (PC), have been investigated. The differences in the morphology of the Al_xO_y sputtered grown on these substrate were investigated using atomic force microscopy (AFM). The initial growth of the Al_xO_y followed closely the topology of the substrate and an amplified roughness was observed. In the fully grown Al_xO_y, the comparative roughness followed that of the substrates. It has been found that a single layer Al_xO_y improved the moisture barrier of PET by an order of magnitude, PC by two orders of magnitude while no improvement was observed for PEN. UV-ozone treatment on PC further improved the moisture barrier, while no improvement was observed for PET and PEN. The comparative effects of the substrate surface roughness and surface energy on the moisture barrier are discussed.     

Surface deformation phenomena induced by electron-beam irradiation in strained InGaAs/AlGaAs layers on GaAs (1 0 0) and (3 1 1)B substrates

The strained InGaAs/AlGaAs layer structures have been grown on GaAs ( 10 0) and (3 1 1)B substrates in a horizontal low-pressure metalorganic vapor-phase epitaxy system at a temperature of 800°C. In the surface observation using a high-resolution scanning electron microscope, we have found that surface deformation phenomena induced by electron-beam irradiation in strained In_0.36Ga_0.64As,/Al_0.3Ga_0.7As layers on GaAs (1 0 0) and (3 1 1)B substrates. The change of the surface morphology was observed in real time on the display of SEM with the accelerating voltage of 30 kV and the irradiated time of 60–120 s. The surface deformation through mass transport seems to be the cause of the residual strain relaxation due to electron-beam irradiation.