Koung-An Chao

WIDELY TUNABLE CONTINUOUS-WAVE THZ LASER

We present experimental results on continuous-wave generation of THz radiation by strained Ge and a theoretical model for population inversion of carriers giving rise to the stimulated THz emission. Resonant acceptor states induced by strain and resonance hole scattering under applied electric field are necessary for the inversion.

Population Inversion Induced by Resonant States in Semiconductors

We present a theoretical prediction of a new mechanism for carrier population inversion in semiconductors under an applied electric field. The mechanism is originated from a coherent capture-emission type inelastic scattering of resonant states. We support our theory with concrete calculations for shallow acceptor resonant states in strained p-Ge where a lasing in THz frequency region has been recently observed.

Spin-current generation and detection in the presence of an ac gate

We predict that in a narrow gap III-V semiconductor quantum well or quantum wire, an observable electron spin current can be generated with a time-dependent gate to modify the Rashba spin-orbit coupling constant. Methods to rectify the so generated ac current are discussed. An all-electric method of spin-current detection is suggested, which measures the voltage on the gate in the vicinity of a two-dimensional electron gas carrying a time-dependent spin current. Both the generation and detection do not involve any optical or magnetic mediator.

Growth of high-quality Ge epitaxial layers on Si(100)

A method of growing high-quality epitaxial Ge layers on a Si(100) substrate is reported. In this method, a 0.8 mum Si0.1Ge0.9 layer was first grown. Due to the large lattice mismatch between this layer and the Si substrate, many dislocations form near the interface and in the lower part of the Si0.1Ge0.9 layer. A 0.8 mum Si0.05Ge0.95 layer and a 1.0 mum top Ge layer were subsequently grown on the Si0.1Ge0.9 layer. The formed interfaces of Si0.05Ge0.95/Si0.1Ge0.9 and Ge/Si0.05Ge0.95 can bend and terminate the upward-propagated dislocations very effectively. The in situ annealing process was also performed for each individual layer. Experimental results show that the dislocation density in the top Ge layer can be greatly reduced, and the surface is very smooth, while the total thickness of the structure is only 2.6 mum. (Less)

COULOMB BLOCKADE THRESHOLD IN INHOMOGENEOUS ONE-DIMENSIONAL ARRAYS OF TUNNEL JUNCTIONS

A general expression is given for the change in free energy when a charge tunnels through a junction in a one-dimensional array of N metallic islands with arbitrary capacitances and arbitrary background charges. This is used to obtain expressions for the (average) threshold voltage of the Coulomb blockade for a few characteristic geometries. We find that including random background charges has a large effect on the N-dependence of the threshold voltage: In an array with identical junction capacitances C and gate capacitances C_g, the threshold voltage, averaged over the background charge, is proportional to N^a, where {a} crosses over from 1/2 to 1 when N becomes larger than 2.5\sqrt{C/C_g}.

Mechanism of terahertz lasing in SiGe/Si quantum wells

Intense terahertz (THz) stimulated emission from boron-doped SiGe/Si quantum well structures with internal strain has been observed recently. We present a theoretical calculation which shows the formation of resonant states, and explains the origin of the observed temperature dependence of the dc conductivity under low bias voltage. Thus, the mechanism of THz lasing is population inversion of the resonant state with respect to the localized impurity states. This is the same mechanism of lasing as in uniaxially stressed p-Ge THz lasers.

Analysis of optothermionic refrigeration based on semiconductor heterojunction

We have examined the theory of optothermionic refrigeration combining the ideas of laser cooling and thermionic cooling [Malshukov and Chao, Phys. Rev. Lett. 86, 5570 (2001)] and its estimation on thermal energy extraction by using self-consistent calculations with the drift-diffusion model in this paper. Both the Auger and the Shockley-Read-Hall dissipation processes are considered. For GaAs/AlGaAs systems with various impurity concentrations and different widths of quantum well, it is found that the optothermionic cooler can extract thermal energy at a rate as much as 10 W/cm(2). The information to perform optothermionic refrigeration in real devices have also been provided. (c) 2006 American Institute of Physics.

Optical transitions in broken gap heterostructures

Department of Physics, Lund University, S¨olvegatan 14A, S-223 62 Lund, Sweden(February 1, 2008)We have used an eight band model to investigate theelectronic structures and to calculate the optical matrix el-ements of InAs-GaSb broken gap semiconductor heterostruc-tures. The unusual hybridization of the conduction bandstates in InAs layers with the valence band states in GaSb lay-ers has been analyzed in details. We have studied the depen-dence of optical matrix elements on the degree of conduction-valence hybridization, the tuning of hybridization by varyingthe width of the GaSb layers and/or InAs layers, and the sen-sitivity of quantized levels to this tuning. Large spin-orbitsplitting in energy bands has been demonstrated. Our calcu-lation can serve as a theoretical modeling for infrared lasersbased on broken gap quantum well heterostructures.73.20.Dx, 78.40.Fy, 78.66.-wI. INTRODUCTION

Donor states in modulation-doped Si/SiGe heterostructures

We present a unified approach for calculating the properties of shallow donors inside or outside heterostructure quantum wells. The method allows us to obtain not only the binding energies of all localized states of any symmetry, but also the energy width of the resonant states which may appear when a localized state becomes degenerate with the continuous quantum well subbands. The approach is nonvariational, and we are therefore also able to evaluate the wave functions. This is used to calculate the optical absorption spectrum, which is strongly nonisotropic due to the selection rules. The results obtained from calculations for Si/Si1-xGex quantum wells allow us to present the general behavior of the impurity states, as the donor position is varied from the center of the well to deep inside the barrier. The influence on the donor ground state from both the central-cell effect and the strain arising from the lattice mismatch is carefully considered. (Less)

Generation of spin current and polarization under dynamic gate control of spin-orbit interaction in low-dimensional semiconductor systems

Based on the Keldysh formalism, the Boltzmann kinetic equation and the drift-diffusion equation have been derived for studying spin-polarization flow and spin accumulation under effect of the time-dependent Rashba spin-orbit interaction in a semiconductor quantum well. The time-dependent Rashba interaction is provided by time-dependent electric gates of appropriate shapes. Several examples of spin manipulation by gates have been considered. Mechanisms and conditions for obtaining the stationary spin density and the induced rectified dc spin current are studied.