Avalanche multiplication in graded band gap structures

Properties and peculiarities of avalanche multiplication in the p-i-n structures with the graded i-region are discussed. It is shown that the grading of the band gap of semiconductors in the high field region of infrared avalanche photoheterostructure governs the process of multiplication gain and improves noise characteristics of such photodiodes.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

Luminescence rings in quantum well structures

We review the results of an extensive study of the novel luminescence rings found in GaAs and InGaAs double quantum well structures. We propose that the rings define the edge of a metastable population of carriers between the laser excitation spot and the ring. This population carries wavelike excitations at supersonic speeds. Evidence for and against associating this effect with excitonic superfluidity is reviewed. The effect cannot be easily understood in terms of a Kosterlitz-Thouless transition to a superfluid state. We examine the effects of free carriers in the system and suggest a possible mechanism for the luminescence ring effect.     

Reflection from a composite potential well

Hitherto, the variation of reflectivity as a function of energy for a 1-D potential well has been observed to be either monotonically decreasing or entailing multiple minima. We report that a composite (two-piece) potential well may exhibit an unusual feature, viz., a pronounced single minimum in the reflectivity.     

Polariton dispersion of periodic quantum well structures

We studied the polariton dispersion relations of a periodic quantum-well structure with a period in the vicinity of half the exciton resonance wavelength, i.e., the Bragg structure. We classified polariton modes using an approximation of a large number of quantum wells. The polariton effective masses are found to be very small and equal to 10^?3?10^?4 of the free-electron mass.     

Quasibound states in semiconductor quantum well structures

We present a study on quasibound states in multiple quantum well structures using a finite element model (FEM). The FEM is implemented for solving the effective mass Schrödinger equation in arbitrary layered semiconductor nanostructures with an arbitrary applied potential. The model also includes nonparabolicity effects by using an energy dependent effective mass, where the resulting nonlinear eigenvalue problem was solved using an iterative approach. We focus on quasibound/continuum states above the barrier potential and show that such states can be determined using cyclic boundary conditions. This new method enables the determination of both bound and quasibound states simultaneously, making it more efficient than other methods where different boundary conditions have to be used in extracting the relevant states. Furthermore, the new method lifted the problem of quasibound state divergence commonly seen with many other methods of calculation. Hence enabling accurate determination of dipole matrix elements involving both bound and quasibound states. Such calculations are vital in the design of intersubband optoelectronic devices and reveal the interesting properties of quasibound states above the potential barriers.     

Infrared absorption in parabolic multiquantum well structures

The absorption constant for intraband transitions in parabolic multiquantum well structures is calculated and compared with intraband absorption in a square well superlattice. The parabolic multiple quantum well structure may be used as an Infrared detector with the possibility of lower leakage current compared to one made of square wells.     

Interface phonons in asymmetric quantum well structures

In semiconductor microstructures with many layers, the phonon modes change from their bulk form and split into ‘confined LO phonons’ (LC) and ‘interface phonons’ (IF), the number and variety of which depends on both the number of layers and the number of different materials in the structure. This affects the electron–phonon scattering rates. Because of the current interest in inter-subband THz emitters, we use these LC and IF modes to evaluate the inter-subband electron–phonon scattering rate in THz emitter prototypes that are based on four-subband stepped quantum wells. These scattering rates in turn affect the population inversion predicted for these devices, so we compare the predicted population inversions for the most promising prototypes against those obtained using bulk phonon scattering rates.     

Short pulse physics of quantum well structures

This paper reports some recent results of time-resolved studies of the carrier dynamics in GaAs/GaAlAs quantum well structures with picosecond and subpicosecond time resolution. These experiments have provided insight into carrier trapping, energy relaxation, and carrier recombination processes. Carrier trapping into the quantum well layers is very efficient and determines the decay of the GaAlAs luminescence even for 1 μm thick cladding layers. Carrier recombination is enhanced particularly at low temperatures. This effect has been attributed to the increased overlap of electron and hole (exciton) wavefunctions in the quasi-two-dimensional carrier system.     

有机多层量子阱的能量转移

采用多源有机分子气相沉积系统制备了不同类型的有机多量子阱结构，利用电化学循环伏安法和吸收光谱、荧光光谱研究了量子阱的类型、光致发光的特性.电化学循环伏安法和吸收光谱的测量结果表明，PBD/8-羟基喹啉铝(Alq₃)有机量子阱为Ⅰ型量子阱结构，NPB/Alq₃和BCP/Alq₃有机量子阱为Ⅱ型量子阱结构.荧光光谱的研究结果表明，PBD/Alq₃和BCP/Alq₃量子阱结构可以实现PBD，BCP向Alq₃能量完全转移，而NPB/Alq₃量子阱结构，NPB和Alq₃之间只是部分能量转移.文中对影响能量转移的因素进行了讨论. 关键词： 有机量子阱 能量转移     

有机量子阱的光学性质

采用多源有机分子气相沉积系统(OMBD)制备了Alq₃，PBD/Alq₃，PBD/Alq₃/PBD单层、双层以及量子阱结构，利用电化学循环伏安法和吸收光谱、荧光光谱研究了量子阱的类型和样品的光致发光特性.电化学循环伏安法和吸收光谱的测量结果表明，PBD/Alq₃有机量子阱为Ⅰ型量子阱结构.荧光光谱的研究结果表明，单层Alq₃的光致发光峰不随Alq₃厚度变化而变化；但是双层PBD/Alq₃结构光致发光峰随Alq₃厚度的减小而发生蓝移；同样对于PBD/Alq₃/PBD量子阱结构光致发光峰随Alq₃厚度的减小而发生蓝移.对引起光谱蓝移的原因进行了讨论. 关键词： 有机量子阱 光谱蓝移     

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω _- mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters. Received 2 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: c412-1@aphy.iphy.ac.cn     

Hamiltonian structures for integrable classical theories from graded Kac-Moody algebras

An infinite number of canonical representations for integrable classical field theories of non-ultralocal type are constructed from graded Kac-Moody algebras. The principal chiral field models are shown to be a particular example of this construction.     

Time-dependent escape rate from a potential well

Time-dependent desorption from an interface is studied by obtaining the Green function for the Smoluchowski equation for a one-dimensional model potential barrier and calculating the time-dependence of the number density, particle current, and escape rate. The asymptotic behavior of the system for long times can be described by equations with independent rate constants. For high potential barriers (relative to k_BT) the Kramers expression for the escape rate is recovered, but for low barriers the escape rate can go through a maximum. The steady state Onsager model is related to the transient solution and numerical results are presented for different potential shapes and sizes.     

Noise-activated escape from a sloshing potential well

We treat the noise-activated escape from a one-dimensional potential well of an overdamped particle, to which a periodic force of fixed frequency is applied. Near the well top, the relevant length scales and the boundary layer structure are determined. We show how behavior near the well top generalizes the behavior determined by Kramers, in the case without forcing. Our analysis includes the case when the forcing does not die away in the weak-noise limit. We discuss the relevance of scaling regimes, defined by the relative strengths of the forcing and the noise, to recent optical trap experiments.     

Meshfree eigenstate calculation of arbitrary quantum well structures

A new, explicit, efficient, and meshfree multi-domain spectral method (MDSM) is presented to calculate the eigenstates of arbitrary semiconductor quantum wells (QW). The method is functional expansion based where the quantized energy states are approximated using efficient basis sets and the boundaries are considered using Tau approach. For bounded domains, Chebyshev polynomials and power series are used as the basis sets, while non-orthogonal predefined exponential basis sets are used for the half bounded domains. Many QW structures were studied and the results are compared with published results for validation. The comparisons exhibit the accuracy and efficiency of the presented method. For QWs that can be analyzed analytically, the relative errors in the calculated quantized energy levels are in the order of ¹⁰−12 with very small number of bases in few ms.