We present experimental verification of type II band alignment in a coherently strained Si 0.7 Ge 0.3 ͞Si͑001͒ quantum well by studying photoluminescence energy shifts under external strains. A recent determination of type I band alignment from a similar experiment is shown to result from band-bending effects due to high excitation. In high quality samples, the type II luminescence can be observed in the absence of external stress by using extremely low excitation. The type II luminescence differs from the well known type I spectrum in a dramatic but as yet unexplained change in the relative intensities of the phonon replicas. [S0031-9007(97)03558-8] PACS numbers: 73.20.Dx, 78.66.Db The growth and properties of strained Si 12x Ge x , and more recently Si 12x2y Ge x C y heterostructures on (001) Si has attracted intense interest, not only because of the technological promise of combining band gap engineering with a materials system compatible with standard Si processing, but also due to the fact that this is the prototypical indirect band gap heterostructure system. Until recently, little was known about the optical properties of such structures, compared to the exhaustively studied direct gap heterostructures, but the discovery of well-resolved neargap photoluminescence (PL) [1,2] led to a speedy adaptation of PL as a standard assessment tool, and to a rapid increase in our understanding of the physical processes involved [2][3][4][5][6][7]. As in bulk, relaxed, Si 12x Ge x alloys [3], the PL of typical samples is dominated by impuritybound excitons (BE) at liquid He temperatures, and "free" excitons (FE) at higher temperatures [2], remembering that these FE move in a random potential due to alloy disorder whose width exceeds kT at low temperatures. Related to this, localized excitons (LE) associated with local band gap minima have been discovered and shown to have much higher PL quantum efficiency and longer lifetimes than BE (dominated by Auger recombination [4,5]). While most descriptions of the now ubiquitous PL spectra of these systems limit discussion to the BE͞FE species, the long carrier lifetimes make it easy to reach high excitation conditions in the quantum well (QW), and the importance of biexcitons and electron-hole plasmas in the PL processes under normal excitation conditions has been demonstrated [6,7].One of the remaining contentious issues regarding the physics and optical properties of these systems involves the band-edge alignments. While it was known early on that the majority of the band gap difference was taken up in the valence-band (VB) discontinuity, the only certainty regarding the conduction band (CB) edge was that the discontinuity was small. Theoretical studies have predicted both type I band alignments [8,9], with electrons ͑e͒, and holes ͑h͒ localized in the Si 12x Ge x , and type II alignments [10,11], with h localized in the Si 12x Ge x and e in the Si. Some PL studies claimed evidence for type I behavior, using line shifts [12] and hydrostatic pressure effects [13], respectiv...

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The objective of this study was to develop and test a method for evaluating simulation models and to illustrate how it can provide insights into a simulation's strengths and weaknesses, especially in terms of identifying areas for improvements. To this end, our resulting evaluative methodology was systematically applied to three Army simulation models that were used in the acquisition of air defense systems. We describe the evaluative framework and the results of our analysis.

Dramatic enhancements of over 300× in the room temperature photoluminescence signal obtained from high purity GaAs epitaxial layers were recorded after a brief heat treatment in tertiarybutylphosphine vapor. Low temperature photoluminescence spectra indicate that, unlike other passivation techniques, the surface layer formed during this simple treatment does not induce any appreciable strain on the underlying epilayer. The increases in photoluminescence intensity are indicative of a reduction in surface recombination brought about by the formation of a very thin GaP layer that protects against surface oxidation.

Low temperature photoluminescence results from high purity epitaxial GaAs are presented, and a new type of transition involving negatively charged donor ions and neutral acceptors is identified. At low temperatures and excitation densities this becomes the dominant radiative process, with a linewidth of only ϳ1 cm 21 . The temperature dependence of this new transition reveals a binding energy of 2.65 6 0.35 cm 21 , consistent with the spectroscopic value of 2.8 6 0.2 cm 21 and with theoretical predictions. This is to our knowledge the first experimental determination of the D 2 binding energy in unperturbed GaAs. [S0031-9007(98)05493-3] PACS numbers: 78.55. -m, 71.55. -i D 2 states in single-valley semiconductors, formed when a shallow neutral hydrogenic donor ͑D 0 ͒ binds a second electron, are the solid state analog of the negatively charged hydrogen ion ͑H 2 ͒. Such systems are of great interest, to a large extent because their small electron effective mass and large dielectric constants allow one, at least in principle, to investigate the plenitude of magnetic bound states predicted for the H 2 ion at much higher (by .10 4 and, thus, inaccessible) fields. While there have been many reports of D 2 centers in III-V semiconductors during the past decade, most are for quantum well (QW) samples, where two-dimensional (2D) effects increase the D 2 binding energy. The handful of studies [1-3] dealing with D 2 in bulk materials all involve significant binding energy enhancements due to magnetic fields. These early far infrared (FIR) investigations, albeit the first to identify D 2 states in bulk GaAs, suffered from a combination of materials and experimental technique-related problems which strictly limited these studies to the observation of relatively broad D 2 bands in a restricted number of compensated samples.As a result of these difficulties, recent effort has been focused on the study of 2D D 2 states in QW geometries, which also offer the possibility of engineering appropriate conditions for the creation of D 2 centers, via selective doping techniques [4][5][6][7]. While this has allowed D 2 centers to be identified in a less restricted variety of materials, the linewidths associated with the D 2 transitions observed in all of the existing FIR studies are still very broad, especially in comparison to the very small binding energy expected for D 2 in bulk GaAs [ϳ3.7 K or 2.6 cm 21 , where the effective Rydberg for neutral donors in GaAs is taken to be R ‫ء‬ 46.09 cm 21 (5.715 meV) [8] ] [9,10]. Further complications arise during the interpretation of QW FIR absorption spectra due to the possibility of confusing D 2 features with those arising from interexcited state neutral impurity transitions (from both well and barrier materials), as well as cyclotron resonance. Hence, identification of D 2 features in a given FIR absorption spectrum is not straightforward, relying on comparisons with quantitative theoretical predictions, and has often been controversial [1,[4][5][6][7]11].Recent advances in the purity of e...