Nonlinear optical properties of semiconductor quantum wells under intense terahertz radiation

The nonlinear optical properties of semiconductor quantum wells driven by intense in-plane terahertz electric fields are investigated theoretically by employing the extended semiconductor Bloch equations. The dynamical Franz-Keldysh effect of the optical absorption near the band edge is analyzed with Coulomb correlation among the carriers included. The in-plane terahertz field induces a variety of behavior in the absorption spectra, including terahertz replicas of the (dark) 2p exciton and terahertz sidebands of the 1s exciton. The dependence of these interesting features on the intensity, frequency, and phase of the terahertz field is explored in detail.

可生物降解聚合物药物载体及其制备技术

药物释放体系因其具有提高药物的疗效，降低药物的毒副作用，减少药物的服用次数，拓宽给药途径等特点，而成为近几年来人们研究的热点。生物可降解高分子，由于它们在体内可以降解，降解产物可以被机体吸收或代谢，不存在积累在体内的危险，因此成为药物释放体系的载体的首选材料。特别是脂肪族聚酷类高分子，在与聚乙二醇形成嵌段共聚物后，不仅具有生物可降解性，而月＿大大地改善了材料与人体的生物相容性，作为药物载体材料时，延长了药物在体内的循环时间，降低了免疫响应性，引起了人们的极大兴趣。因此本论文主要是以MPEG-PLA两嵌段聚合物为药物的载体材料，详细研究了高分子量的MPEG-PLA两嵌段聚合物对紫杉醇的包裹，研究了MPEG-PLA和PLGA聚合物合金对胰岛素固体粉末的包裹，以及低分子量的MPEG-PLA的紫杉醇前药的合成、表征和由它制备而成的胶束的一些性质，取得了一些有意义的结果：1、采用改进的O／W乳液法，用高分子量的MPEG-PLA嵌段共聚物实现了对紫杉醇的纳米化包裹，并证实了聚合物的分子量对所制备的纳米微球的粒径的影响：分子量越大，粒径越大。同时发现了微球粒径越小，药物的包裹量越低。2、用扫描电镜（SEM）、光电子能谱（XPS）、差热分析（DSC）对纳米微球进行了分析和测定，结果表明，微球的尺寸在30Om-800nm范围，紫杉醇在纳米微球的表面几乎不存在，而是以无定形的状态分布在纳米微球中。3、对纳米微球中紫杉醇体外释放行为进行了侧定。它们显现出了明显的双相行为，即在初期释放速度很快，随后的释放速度变慢。同时，研究了MPEG-PLA的分子量对释放行为的影响：聚合物分子量越大，紫杉醇释放的速度就越慢。4、用固体粉末法和双乳液法对胰岛素进行了包裹，其中固体粉末法采用的是PLGA和MPEG-PLA两聚合物的混合溶液对纳米胰岛素颗粒进行了包裹，包裹率分析表明：固体粉末法对药物的包裹率高于双乳液法。所得的微球都是很好的球形，其尺寸在1-3um左右，它的剖面是核壳结构，胰岛素以晶粒的形式被包裹在微球中间。5、对固体粉末法和双乳液法制备的微球的体外释放行为进行了对比，发现由两种聚合物合金制备的微球的暴释现象得到了缓解，同时发现两种聚合物的配比不一样，其暴释缓解的程度不一样。6、以辛酸亚锡为催化剂成功地合成了低分子量的MPEG-PLA两嵌段聚合物。二经基乙酸配与过量的叔丁醇在DMAP存在下反应，成功制得了二轻基乙酸单叔丁酷。MPEG-PLA的端经基与二经基乙酸单叔丁酷在DCC参与下脱水酷化再将叔丁基去保护，便得到端梭基的MPEG-PLA。7、端基为梭基的MPEG-PLA与紫杉醇的2’-羟基或7-轻基进行了酷化反应，制备出MPEG-PLA-紫杉醇前药。8、制备了四种低分子量的MPEG-PLA-紫杉醇前药，用1H NMR和GPC进行了表征分析。紫杉醇前药中紫杉醇的含量最高可达到20％，依赖于MPEG-PLA中PLA段的长度。9、用荧光探针法考察了MPEG-PLA两嵌段聚合物和MPEG-PLA-紫杉醇前药的胶束化行为，发现前药总比相对应的两嵌段聚合物有更低的临界胶束浓度（CMC）。用透射电镜观察了胶束的形貌和尺寸大小，以及接药前后胶束尺寸的变化。发现都是很好的球状胶束，MPEG-PLA两嵌段聚合物和MPEG-PLA-紫杉醇前药胶束的平均粒径分别为25±3nm和33士Znm，说明聚合物在接药后，随着疏水部分分子量的增加，所形成的胶束粒径也增大。

Abnormal photoabsorption in high resistance GaN epilayer

Unintentionally doped GaN epilayers are grown by the metalorganic chemical vapor deposition (MOCVD). Photovoltaic (PV) spectroscopy shows that there appears an abnormal photoabsorption in some undoped GaN films with high resistance. The peak energy of the absorption spectrum is smaller than the intrinsic energy band gap of GaN. This phenomenon may be related to exciton absorption. Then metal-semiconductor-metal (MSM) Schottky photodetectors are fabricated on these high resistance epilayers. The photo spectrum responses are different when the light individually irradiates each of the two electrodes with the photodetector which are differently biased. When the excitation light irradiates around the reverse biased Schottky junction, the responsivity is almost one order of magnitude larger than that around the forward biased junction. Furthermore, when the excitation light irradiates the reverse biased Schottky junction, the peak energy of the spectrum has a prominent red-shift compared with the peak energy of the spectrum measured with the excitation light irradiating the forward biased Schottky junction. The shift value is about 28 meV, and it is found to be insensitive to temperature. According to the analyses of the distribution of the electric field within the MSM device and the different dependences of the response on the electric field intensity between the free carriers and excitons, a reliable explanation for the different response among various areas is proposed.

HYDROGEN DIFFUSION IN A-SI-H/A-SI STRUCTURE UNDER ELECTRICAL BIAS

By using the technique of elastic recoil detection (ERD), we have measured the hydrogen profiles in a-Si:H/a-Si structure samples annealed at various temperatures with and without electrical bias, and investigated the influence of electrical bias on hydrogen diffusion. The results show that hydrogen diffusion in a-Si is significantly enhanced by the action of electrical bias. The existence of the excess carriers, which are introduced by electrical injection, is considered to be responsible for the enhancement of hydrogen diffusion, and the microprocess of hydrogen transport has been exploited.

SHIFTING PHOTOLUMINESCENCE BANDS IN HIGH-RESISTIVITY LI-COMPENSATED GAAS

It is shown that Li diffusion of GaAs can give rise to semi-insulating samples with electrical resistivity as high as 10(7) OMEGAcm in undoped, n-type, and p-type starting materials. The optical properties of the compensated samples are correlated with the depletion of free carriers caused by the Li diffusion. The radiative recombination of the Li-compensated samples is dominated by emissions with excitation-dependent peak positions that shift to lower energies with increasing compensation. The photoluminescence properties are characteristic of fluctuations of the electrostatic potential in strongly doped, compensated crystals.

TEMPERATURE-DEPENDENCE OF THE LOW-FIELD MOBILITY OF MINIBAND CONDUCTION IN SUPERLATTICES

The existing interpretation of the T-1 temperature dependence of the low-field miniband conduction is derived from certain concepts of conventional band theory for band structures resulting from spatial periodicities commensurable with the dimensionalities of the system. It is pointed out that such concepts do not apply to the case of miniband conduction, where we are dealing with band structures resulting from a one-dimensional periodicity in a three-dimensional system. It is shown that in the case of miniband conduction, the current carriers are distributed continuously over all energies in a sub-band, but only those with energies within the width of the miniband contribute to the current. The T-1 temperature dependence of the low-field mobility is due to the depletion of these current-carrying carriers with the rise of temperature.

PHOTOLUMINESCENCE AND SURFACE PHOTOVOLTAIC SPECTRA OF STRAINED INP ON GAAS

A high energy shift of the band-band recombination has been observed in the photoluminescence (PL) spectra of the strained InP epilayer on GaAs by metalorganic chemical vapor deposit. The strain determined by PL peak is in good agreement with calculated thermal strain. The surface photovoltalic spectra gives the information about energy gap, lattice mismatching, and composition of heteroepilayers, diffusion length, surface, and interface recombination velocity of minority carriers of heteroepitaxy layers.

METASTABLE DEFECT IN AMORPHOUS-SILICON SOLAR-CELLS INVESTIGATED BY JUNCTION RECOVERY TECHNIQUE

The effect of metastable defects caused by light soaking and carrier injection on the transport of carriers in undoped a-Si:H has been investigated by a junction recovery technique. The experiments show that after light soaking or carrier injection the product of mu-p-tau-p decreases, but no detectable change in the distribution of shallow valence band tail states was found.

Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface

We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN-AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented, We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230 +/- 0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect. (C) 1997 American Institute of Physics.

Thermoluminescence of ZnS nanoparticles

The thermoluminescence (TL) of ZnS nanoparticles is reported. The TL intensity increases as the particle size is decreased. The consistency of the size dependence of the TL with that of the surface fluorescence indicates that the TL may be related to the surface states. TL may be caused by the recombination of carriers released from the surface states or defect sites by heating. Smaller particles have higher surface/volume ratio and more surface states, therefore contain more accessible carriers for TL. Besides, the carrier recombination rate increases upon decreasing size due to the increase of the overlap between the electron and hole wave functions. These two effects may make the TL increase upon decreasing size of the particles. The appearance of TL prior to any radiation reveals that trapped carriers have pre-existed. The investigation of TL may provide some useful information about the surface states that may explain the size dependence of the surface fluorescence. (C) 1997 American Institute of Physics.

Carrier relaxation and thermal activation of localized excitons in self-organized InAs multilayers grown on GaAs substrates

We have investigated the temperature dependence of photoluminescence (PL) properties of a number of self-organized InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 to 3 ML. The temperature dependence of InAs exciton emission and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML. The fast redshift of PL energy and an anomalous decrease of linewidth with increasing temperature were observed and attributed to the efficient relaxation process of carriers in multilayer samples, resulting from the spread and penetration of the carrier wave functions in coupled InAs quantum dots. The measured thermal activation energies of different samples demonstrated that the InAs wetting layer may act as a barrier for the thermionic emission of carriers in high-quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to escape thermally from the localized states.

Optical study of heterointerface configuration in narrow GaAs/AlGaAs single quantum wells prepared with growth interruption

Photoluminescence and time-resolved photoluminescence were used to study the heterointerface configuration in GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with growth interruption. Photoluminescence spectra of the growth-interrupted sample are characterized by multiplet structures, with energy separation corresponding to a 0.8 monolayer difference in well width, rather than 1 monolayer as expected from the ''atomically smooth island'' picture. By analyzing the thermal transfer process of the photogenerated carriers and luminescence decay process, we further exploit the exciton localization at the interface microroughness superimposed on the extended growth islands. The lateral size of the microroughness in our sample was estimated to be 5 nm, less than the exciton diameter of 15 nm. Our results strongly support the bimodal roughness model proposed by Warwick et al. [Appl. Phys. Lett. 56, 2666 (1990)]. (C) 1996 American Institute of Physics.

DEVELOPMENT OF CURRENT-BASED MICROSCOPIC DEFECT ANALYSIS-METHODS AND ASSOCIATED OPTICAL FILLING TECHNIQUES FOR THE INVESTIGATION ON HIGHLY IRRADIATED HIGH-RESISTIVITY SILICON DETECTORS

Current based microscopic defect analysis methods such as current deep level transient spectroscopy (I-DLTS) and thermally stimulated current (TSC) have been further developed in accordance with the need for the defect analysis of highly irradiated (Phi(n) > 10(13) n/cm(2)) high resistivity silicon detectors. The new I-DLTS/TSC system has a temperature range of 8 K less than or equal to T less than or equal to 450 K and a high sensitivity that can detect a defect concentration of less than 10(10)/cm(3) (background noise as low as 10 fA). A new filling method using different wavelength laser illumination has been applied, which is more efficient and suitable than the traditional voltage pulse filling. It has been found that the filling of a defect level depends on such factors as the total concentration of free carriers generated or injected, the penetration length of the laser (laser wavelength), the temperature at which the filling is taking place, as well as the decay time after the filling (but before the measurement). The mechanism of the defect filling can be explained by the competition between trapping and detrapping of defect levels, possible capture cross section temperature dependence, and interaction among various defect levels in terms of charge transferring. Optimum defect filling conditions have been suggested for highly irradiated high resistivity silicon detectors.

Transient photocurrent investigation of photo-processes of lattice-matched MQW GaAs/AlxGa1-xAs electrode

Transient photocurrents induced by short light pulses at lattice-matched GaAs/AlxGa1-xAs multiple quantum well (MQW) electrodes were studied as a function of electrode potential. Dual exponential photocurrent decay transients were observed at various potentials. By analysis of the dual exponential decay transients, information on steady state photocurrents (I-s), surface collection of photoexcited minority carriers (G(0)) and lifetimes of surface states (T-s) was obtained. The kinetic behaviors of photoprocesses at illuminated MQW/electrolyte interface were discussed.

Elastic Strain Field Distribution of a Self-Organized Growth Lensed-Shaped Quantum Dot by Finite Element Method

The stress and strain fields in self-organized growth coherent quantum dots (QD) structures are investigated in detail by two-dimension and three-dimension finite element analyses for lensed-shaped QDs. The nonobjective isolate quantum dot system is used. The calculated results can be directly used to evaluate the conductive band and valence band confinement potential and strain introduced by the effective mass of the charge carriers in strain QD.