Showing papers by "Mark S. Hybertsen published in 1995"

Exciton condensate in semiconductor quantum well structures.

Abstract: We propose that the exciton condensate may form in a well-controlled way in appropriately arranged semiconductor quantum well structures. The mean-field theory of Keldysh and Kopaev, exact in both the high density and low density limits, is solved numerically to illustrate our proposal. The electron-hole pairing gap and the excitation spectrum of the exciton condensate are obtained. The energy scales of the condensate are substantial at higher densities. We discuss how such densities could be achieved experimentally by generating an effective pressure.

Si 2p core-level shifts at the Si(001)-SiO2 interface: A first-principles study.

Abstract: Using a first-principles approach, we calculate core-level shifts at the Si(001)-Si${\mathrm{O}}_{2}$ interface. By fully relaxing interfaces between Si and tridymite, a crystalline form of Si${\mathrm{O}}_{2}$, we obtain interface models with good local structural properties and with no electronic states in the Si gap. Calculated values of Si $2p$ core-level shifts agree well with data from photoemission experiments and show a linear dependence on the number of nearest-neighbor oxygen atoms. Core-hole relaxation accounts for \ensuremath{\sim}50% of the total shifts, in good agreement with Auger experiments.

Analysis of gain in determining T/sub 0/ in 1.3 /spl mu/m semiconductor lasers

Abstract: Rapid decrease of differential gain has been determined to dominate the temperature dependence of threshold current in 1.3-/spl mu/m multiquantum well and bulk active lasers giving rise to low values of T/sub 0/. Extensive experimental characterization of each type of device is described. Results are presented for the dependence of gain on chemical potential and carrier density as a function of temperature. The data indicate the important role of the temperature-insensitive, carrier density dependent chemical potential in determining differential gain. Modeling of the temperature dependence of threshold carrier density in MQW and bulk active lasers based on a detailed band theory calculation is described. The calculated value of T/sub 0/ depends on the structure of the active layer, e.g., multiquantum well versus bulk. However, the calculated values are substantially higher than measured. >

Impedance‐corrected carrier lifetime measurements in semiconductor lasers

Abstract: Differential carrier lifetime as a function of subthreshold bias current in 1.3 m bulk active lasers is obtained by measurement of small‐signal modulation of amplified spontaneous emission together with careful characterization of frequency‐ and current‐dependent device impedance. The strong influence of rapidly varying device impedance upon these measurements is illustrated. In contrast to other studies, neither saturation of differential lifetime at low currents nor linear dependence of spontaneous emission on carrier density is observed. Recombination parameters, fit from current versus carrier density, along with consistent fits of spontaneous emission versus carrier density, are presented.

Analysis of T0 in 1.3 μm multi‐quantum‐well and bulk active lasers

Abstract: Temperature dependence of threshold in 1.3 μm semiconductor lasers is analyzed in terms of contributions due to gain, internal efficiency, internal loss, and nonradiative recombination. Rapid decrease of differential gain and roughly proportional increase in transparency carrier density are determined to dominate temperature dependence of threshold current. Auger recombination is found to play a secondary role in reducing T0 by compounding the effects of rapidly increasing threshold carrier density.

Gain characteristics of 1.55-μm high-speed multiple-quantum-well lasers

Abstract: We describe the important characteristics of high-speed p-doped compressively strained MQW lasers obtained from comprehensive below-threshold DC measurements. Results of gain and differential gain versus wavelength and carrier density are verified by above-threshold resonance measurements. Measurement-derived design curves of gain, differential gain, and linewidth enhancement factor allow device optimization for high speed and low chirp. >

Temperature dependence of the fundamental direct transitions of bulk Ge and two Ge/SiGe multiple-quantum-well structures

Abstract: From a detailed line-shape fit to piezoreflectance spectra, we have evaluated the temperature dependence (19T420 K) of the energies [E(T)] and broadening parameters [\ensuremath{\Gamma}(T)] of the fundamental direct transitions in bulk Ge and two narrow Ge/GeSi multiple-quantum-well structures. The experimental broadening parameters are compared with theoretical expressions. No significant dimensionality dependence of \ensuremath{\Gamma}(T) was observed, in contrast to polar systems. On the other hand, in agreement with polar materials, E(T) of the microstructures was found to be the same as the constituent bulk well material.

Modeling of gain for InGaAsP-based lasers

Abstract: Experimental and theoretical results for gain in bulk and multiquantum well active layer 1.3 micrometers InGaAsP based lasers are reported. Gain, loss, transparency energy, and carrier density have been measured in the subthreshold regime at room temperature and elevated temperatures. Gain has been calculated using an eight band k(DOT)p model for the electronic structure and a conventional density matrix formulation. The calculated and experimental results for the gain spectra, the gain versus density, the chemical potential (quasifermi level separation) versus density, and the gain versus chemical potential are compared at room temperature and 85 C. There is aagreement at several points, but the model substantially underestimates the temperature sensitivity of the gain which has been found in the experiments to be an important factor in the overall temperature sensitivity of threshold current.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Mechanism for light emission from nanoscale silicon

Abstract: Silicon is the basis for the majority of integrated electronic devices. However, due to the indirect band gap in its electronic structure, bulk Si exhibits very weak luminescence. Therefore Si has not been a useful material for the manufacture of active optical devices, e.g. light emitting diodes or laser diodes. Over the past ten years, there has been a rising level of research work focused on the goal of improving the efficiency of light emission from Si based materials through various schemes to produce artificial structures. Examples include special dopants (e.g. Erbium [1]), planar heterostructures (e.g. based on epitaxial growth of Ge containing layers [2]) and most recently etching of Si wafers to produce luminescent porous Si [3]. The common theme is the fundamental alteration of the electronic states by a relatively short range perturbation i.e. the dopant potential, the heterointerface potential or the boundary of a nanoscale crystallite. The bright luminescence from appropriately prepared porous Si has stimulated a burst of activity recently [4]. In particular, it has refocussed attention on fundamental questions concerning light emission from nanoscale Si structures.

Implication of Silicon Nanocrystallites from Combined Absorption and Luminescence Studies of Free-Standing Porous Silicon Films

Abstract: The result of a combined study of absorption and photoluminescence from high optical quality, free-standing, porous Si films is presented. With a scattering loss of less than 5%, these films allow unambiguous, detailed analysis of the optical absorption edge, and, in addition, enable us to correlate the optical absorption and emission behavior on the same sample. The main results of consistent energy position shift in both absorption and photoluminescence, together with the behavior of the emission lifetime versus the energy position shift, support the model that the observed optical processes derive from an ensemble of Si nanocrystallites.