We report on the spectral and temperature dependence of persistent photoconductivity (PPC) in metal-organic chemical vapor deposition grown unintentionally doped n-type GaN. The PPC effect is detectable up to temperatures of at least 352 K, the highest temperature used in this study. At 77 K, the conduction persists at a level 80% higher than the equilibrium dark conduction for over 104 s after removing the excitation. We have determined the spectral dependence for the optical cross section for PPC and obtain an optical ionization energy of ∼2.7 eV. The temperature dependence of the photoconductivity decay and its nonexponential shape are explained by a distribution of capture barriers with a mean capture barrier of 0.2 eV and a width of ∼26 meV.

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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