Three paramagnetic Fe defect complexes have been observed by electron-spin resonance in chemically reduced crystals of SrTi03. These are identified as (1) in Fe + ion substituted on a Ti site with a nearby charge-compensating interstitial impurity, and (2) and (3) two charge states of substitutional Fe with a near-neighbor oxygen v icancy, Fe +-Vp and Fe'+-&p. These three defects are related in the sense that photoexcitation can reversibly switch electrons between the vacancy-associated and interstitial-associatedsites. An analysis of the response to pol prized light, and of the kinetics of charge transfer, shows that in both the forward and reverse directions of optical pumping the mobile charge carrier is a valence-band hole. Four distinct charge states of the Fe-&p system have now been identified, ind the rel itive instability of SrTi03 toward formation of multiple-impurity charge states is briefly discussed.

High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy High magnetic field studies of two-dimensional electron gas in a GaN/GaAlN heterostructure: Mechanisms of parallel conduction Electron mobility exceeding 10 4 cm 2 /Vs in an AlGaN-GaN heterostructure grown on a sapphire substrate

We present results of transport measurements performed on AlGaN/GaN heterostructures grown on silicon substrates. Variable temperature Hall effect measurements revealed that the temperature dependence of the carrier density and mobility were characteristic of a two-dimensional electron gas (2DEG). Carrier densities greater than 1×1013 cm−2 and Hall mobilities in excess of 1500 cm2/V s were measured at room temperature. Variable field Hall measurements at low temperatures, and in magnetic fields up to 6 T, indicated that conduction is dominated by a single carrier type in these samples. Shubnikov–de Haas (SdH) measurements were also performed, but no oscillations were observed in fields up to 8 T and at temperatures as low as 1.2 K. Illuminating some of the samples with a blue (λ=470 nm) light emitting diode (LED) induced a persistent increase in the carrier density. SdH measurements were repeated and again no oscillations were present following illumination. However, exposing the samples to radiation from an UV (λ=395 nm) LED induced well-defined SdH oscillations in fields as low as 4 T. The observation of SdH oscillations confirmed the presence of a 2DEG in these structures. It is hypothesized that small angle scattering suppressed the oscillations before exposure to UV light. This conclusion is supported by the observed increase in the quantum scattering time, τq, with the carrier density and the calculated quantum to transport scattering times ratio, τq/τc. For instance, in one of the samples the τq increased by 32% while the τc changed by only 3% as the carrier density increased; an indication of an increase in the screening of small angle scattering. The absence of SdH oscillations in fields up to 8 T and at temperatures as low as 1.2 K is not unique to AlGaN/GaN on silicon. This behavior was observed in AlGaN/GaN on sapphire and on silicon carbide. SdH oscillations were observed in one AlGaN/GaN on silicon carbide sample following exposure to radiation from an UV LED.

No abstract

Photoluminescence is used to study the behavior of CAs-related and GaAs cation antisite defect centers in GaAs under hydrostatic pressure at 10 K. The pressure coefficients, α, for these transitions were determined. We found α(CAs) to be about 12 meV/kbar which is very close to the band edge pressure dependence. The GaAs center demonstrated only slightly smaller coefficient. This makes the binding energy of CAs and GaAs, at 26 and 77 meV, respectively, almost independent of pressure. At pressures above 30 kbar relatively strong transitions were observed that displayed very small pressure dependence. The deepening of the ΓX-mixed donor levels interacting with these acceptor levels are primarily responsible for small pressure coefficients of the transitions above 30 kbar. A deepening of ∼160 meV is observed for the donors.