Quantum-confined Stark effects of InAs/GaAs self-assembled quantum dot

Quantum-confined Stark effects in InAs/GaAs self-assembled quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated in the presence of perpendicular and parallel electric field. In our calculation, the effect of finite offset, valence band mixing, and strain are all taken into account. The results show that the perpendicular electric field weakly affects the electron ground state and hole energy levels. The energy levels are affected strongly by the parallel electric field. For the electron, the energy difference between the ground state and the first excited state decreases as electric field increases. The optical transition energies have clear redshifts in electric field. The theoretical results agree well with the available experimental data. Our calculated results are useful for the application of quantum dots to photoelectric devices. (C) 2000 American Institute of Physics. [S0021-8979(00)11001-7].

Photoluminescence studies on pseudomorphic delta-doped AlGaAs/InGaAs/GaAs quantum wells

Photoluminescence (PL) measurements were performed on several series of single-side Si-doped pseudomorphic high electron mobility transistors (p-HEMTs) quantum well (QW) samples, with different spacer layer widths, well widths and Si delta -doped concentrations , under different temperatures and excitation power densities. The dynamic competitive luminescence mechanism between the radiations of e2-hh1 and e1-hh1 was discussed in detail. The confining potential, subband energies, corresponding envelope functions, subband occupations and transferring efficiency etc., were calculated by self-consistent finite differential method at different temperatures in comparison with the present experiment results. The relative variation of the integrated luminescence intensity of the two transitions (e1-hh1 and e2-hh1) was found to be dependent on the temperature and the structure's properties, e. g. spacer layer width, dopant concentration and well width.

Pressure behaviour of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots with multi-modal distribution in size

The pressure behaviour of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots (QDs) has been studied at 15 K in the pressure range of 0-1.3 GPa. The atomic force microscopy image shows that the QDs have a multi-modal distribution in size. Three emission peaks were observed in the photoluminescence (PL) spectra, corresponding to the different QD families. The measured pressure coefficients are 82, 93 and 98 meV GPa(-1) for QDs with average lateral size of 26, 52 and 62 nm, respectively. The pressure coefficient of small QDs is about 17% smaller than that of bulk In0.55Al0.45As An envelope-function calculation was used to analyse the effect of pressure-induced change of barrier height, effective mass and dot size on the pressure coefficients of QDs. The Gamma-X state mixing was also included in the evaluation of the reduction of the pressure coefficients. The results indicate that both the pressure-induced increase of effective mass and Gamma-X mixing respond to the decrease of pressure coefficients, and the Gamma-X mixing is more important for small dots. The calculated Gamma-X interaction potentials are 15 and 10 meV for QDs with lateral size of 26 and 52 nm, respectively. A type-II alignment for the X conduction band is suggested according to the pressure dependence of the PL intensities. The valence-band offset was then estimated as 0.15 +/- 0.02.

Photocurrent derivative spectra of ZnCdSe-ZnSe double multi-quantum wells

Photocurrent (PC) spectra of ZnCdSe-ZnSe double multi-quantum wells are measured at different temperature. Its corresponding photocurrent derivative (PCD) spectra are obtained by computing, and the PCD spectra have greatly enhanced the sensitivity of the relative weak PC signals. The polarization dependence of the PC spectra shows that the transitions observed in the PC spectra are heavy-hole related, and the transition energy coincide well with the results obtained by envelope function approximation including strain. The temperature dependence of the photocurrent curves indicates that the thermal activation is the dominant transport mechanism of the carriers in our samples. The concept of saturation temperature region is introduced to explain why the PC spectra have different temperature dependence in the samples with different structure parameters. It is found to be very useful in designing photovoltaic devices.

A new interface anisotropic potential of zinc-blende semiconductor interface induced by lattice mismatch

A new interface anisotropic potential, which is proportional to the lattice mismatch of interfaces and has no fitting parameter, has been deduced for (001) zinc-blende semiconductor interfaces. The comparison with other interface models is given for GaAs/AlAs and GaAs/InAs interfaces. The strong influence of the interface anisotropic potential on the inplane optical anisotropy of GaAs/AlGaAs low dimensional structures is demonstrated theoretically within the envelope function approximation.

The effects of electric field on the electronic structure of a semiconductor quantum dot

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field. (C) 1998 American Institute of Physics.

Quantum-confined Stark effects of exciton states in V-shaped GaAs/AlxGa1-xAs quantum wires

Quantum-confined Stark effects are investigated theoretically in GaAs/AlxGa1-xAs quantum wires formed in V-grooved structures. The electronic structures of the V-shaped quantum wires are calculated within the effective mass envelope function theory in the presence of electric field. The binding energies of excitons are also studied by two-dimensional Fourier transformation and variational method. The blue Stark shifts are found when the electric field is applied in the growth direction. A possible mechanism in which the blueshifts of photoluminescence peaks are attributed to two factors, one factor comes from the asymmetric structure of quantum wire along the electric field and another factor arises from the electric-field-induced change of the Coulomb interaction. The numerical results are compared with the recent experiment measurement.

A study of strong photoluminescence of SiOx : H films

We have examined photoluminescence (PL), IR absorption and Raman spectra of a series of hydrogenated amorphous silicon oxide (a-SiOx:H, (0 < x < 2)) films fabricated by plasma enhanced chemical vapor deposition (PECVD). Two strong luminescence bands were observed at room temperature, one is a broad envelope comprising a main peak around 670 nm and a shoulder at 835 nm, and the other, peaked around 850 nm; is found only after being annealed up to 1170 degrees C in N-2 environment. In conjunction with IR and Raman spectra, the origins of the two luminescent bands and their annealing behaviors are discussed on the basis of quantum confinement effects.

Spatially separated excitons in quantum-dot quantum well structures

In the framework of the effective-mass envelope-function theory, the electronic and optical properties of a spherical core-shell quantum-dot quantum well (QDQW) structure with one and two wells have been investigated. The results show that the energies of electron and hole states depend sensitively on the well thickness and core radius of quantum-dot quantum well structure. An interesting spatially separated characteristic of electron and hole in QDQW is found and enhanced significantly in the two-wells case. The normalized oscillator strength for the optical transition between the electron and hole states in QDQW exhibits a deep valley at some special well thickness. The Coulomb interaction between the electron and hole is also taken into account. [S0163-1829(98)02412-6].

Linear and nonlinear intersubband optical properties in a step quantum well with an applied electric field

Within the framework of the effective-mass envelope-function theory, the field-dependent intersubband optical properties of a Al0.4Ga0.6As/Al0.2Ga0.8As/GaAs step quantum well are investigated theoretically based on the periodic boundary condition. A very large Stark shift occurs when the lowest subband electron remains confined to the small well while the higher subband electron confined to the big well. The optical nonlinearity in a step well due to resonant intersubband transition (ISBT) is analyzed using a density-matrix approach. The second-harmonic generation coefficient chi(2 omega)((2)) and nonlinear optical rectification chi(0)((2)) have also been investigated theoretically. The results show that the ISBT in a step well can generate very large second order optical nonlinearities, chi(0)((2)) and chi(2 omega)((2)) can be tuned by the electric field over a wide range.

On detection wavelength and electron-hole wave function overlap of type II InAs/In-x Ga1-x Sb superlattice infrared photodetector

In this paper, the detection wavelength and the electron-hole wave function overlap of InAs/IrxGa1-xSb type II superlattice photodetectors are numerically calculated by using the envelope function and the transfer matrix methods. The band offset is dealt with by employing the model solid theory, which already takes into account the lattice mismatch between InAs and InxGa1-xSb layers. Firstly, the detection wavelength and the wave function overlap are investigated in dependence on the InAs and InxGa1-xSb layer thicknesses, the In mole fraction, and the periodic number. The results indicate that the detection wavelength increases with increasing In mole fraction, InAs and InxGa1-xSb layer thicknesses, respectively. When increasing the periodic number, the detection wavelength first increases distinctly for small periodic numbers then increases very slightly for large period numbers. Secondly, the wave function overlap diminishes with increasing InAs and InxGa1-xSb layer thicknesses, while it enhances with increasing In mole fraction. The dependence of the wave function overlap on the periodic number shows the same trend as that of the detection wavelength on the periodic number. Moreover, for a constant detection wavelength, the wave function overlap becomes greater when the thickness ratio of the InAs over InxGa1-xSb is larger.

LD side-pumped high beam quality passive Q-switched and mode-locked Nd:YAG laser based on SESAM

We report a LD side-pumped fundamental-mode (Mx(2) = 1.35 and My(2) = 1.27) passive Q-switched and mode-locked Nd:YAG laser based on a semiconductor saturable absorber mirror (SESAM). At a pump current of 12.5 A, the average output power of 5.68 W with 80 kHz repetition rate and 2 mu s pulse width of the Q-switched envelope was generated. The repetition rate of the mode-locked pulse within the Q-switched envelope of 88 MHz was achieved.

MAPK在天花粉蛋白抗HIV-1中的作用以及syncytin基因在白血病细胞中的表达

本学位论文主要包括两部分的内容： 一是关于MAPK信号转导在天花粉蛋白（trichosanthin, TCS）抗人类免疫缺陷病毒-1（HIV-1）中的作用的研究。TCS是I型核糖体失活蛋白（RIP），分子量27Kd，可从传统的中期流产和抗绒癌中药栝楼根块茎（天花粉）中提纯获得。该蛋白具有抗HIV-1活性，但其机制尚不清楚。本文用JNK抑制剂CEP-11004，预处理宿主细胞，检测其对TCS抗HIV-1的影响。用以下两种方法检测病毒的复制：一是用ELISA方法检测细胞培养上清中p24抗原的水平，二是检测上清中病毒粒子的逆转录酶（RT）活性。结果显示，TCS剂量依赖性地抑制HIV-1在C8166细胞中的复制。在TCS实验浓度下，HIV-1的复制水平平均为68 ± 4％（p24抗原检测）和52 ± 4％（RT活性检测）。如果用0.4μM CEP-11004对C8166细胞预处理2小时，TCS似乎失去了抗HIV-1活性，HIV-1的复制水平分别恢复为101 ± 4％和101 ± 7％。但无论是p24抗原检测还是RT活性检测，当不含TCS时，CEP-11004预处理病毒宿主细胞，本身并不影响病毒粒子的复制。这说明CEP-11004能够拮抗TCS的抗病毒活性，或者说CEP-11004抑制的信号转导途径的某些信号分子，与TCS的抗HIV-1活性相关。Western Blot方法检测的结果也证明，TCS能够以时间依赖和剂量依赖的方式激活JNK激酶，0.4μM的CEP-11004能有效抑制JNK的磷酸化。因此，TCS与它激活MAPK信号转导途径有关。 二是关于人类内源性病毒HERV-W家族囊膜蛋白基因syncytin在白血病细胞中的表达的研究。该基因在人的胎盘组织中特异性表达，可能与合胞滋养层的形成有关。另外也少量表达于睾丸组织。本论文采用实时定量RT-PCR的方法证明，syncytin能够在白血病细胞系中表达。进一步的检测还表明，syncytin的mRNA也表达于白血病/淋巴瘤患者的外周血细胞，而不表达于作为对照的10名健康志愿者的血细胞。在15名不同类型的白血病/淋巴瘤患者中，有11名有syncytin的表达。细胞系的表达相对稳定，与C8166细胞系的表达量相比较，介于0.5-2.0倍之间；而在白血病患者外周血细胞中的表达则介于0.8-21.7倍不等。上述结果提示，syncytin可能与白血病的形成有关。

EFFECTIVE-MASS THEORY FOR GAAS/GA1-XALXAS QUANTUM WIRES AND CORRUGATED SUPERLATTICES GROWN ON (311)-ORIENTED SUBSTRATES

The electronic structures of GaAs/Ga1-xAlxAs quantum wires (corrugated superlattices) grown on (311)-oriented substrates are studied in the framework of the effective-mass envelope-function method. The electron and hole subband structure and optical transition matrix elements are calculated. When x=1, the results are compared with experiments, and it is found that the direct transition becomes an indirect transition as the widths of well and barrier become smaller.

TUNNELING ESCAPE TIME OF ELECTRONS FROM A QUANTUM-WELL WITH GAMMA-X MIXING EFFECT

By using the envelope function method we calculated the tunneling escape time of electrons from a quantum well. We adopted a simplified interface matrix to describe the GAMMA-X mixing effect, and employed a wave packet method to determine the tunneling escape time. When the GAMMA state in the well was in resonance with the X state in the barrier, the escape time reduced remarkably. However, it was possible that the wave functions in two different channels, i.e., GAMMA-GAMMA-GAMMA and GAMMA-X-GAMMA, could interfere destructively, leading the escape time greater than that of pure GAMMA-GAMMA-GAMMA tunneling.