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We report the observation of a new trapping center in A103Gao. jAs. The center becomes active under a hydrostatic pressure of -45 kbar, and has an unusually deep emission barrier. It quenches all radiative transitions and causes a hysteresis in the photoluminescence intensity, which we interpret via a lattice relaxation model. It is neither the DX nor the SD center, and is probably related to a donor. Substitutional group-IV and group-VI dopants in Al Ga& -As gives rise to two types of electronic states:' a shallow effective-mass level and a more localized level DX, arising from lattice distortion near the donor. The relative stability of the two states depends on alloy composition. Below x =0. 22, the localized level is a resonance above the conduction-band (CB) minimum. Above x =0. 22, DX becomes a bound state, more stable than the shallow donor state. The application of hydrostatic pressure for x &0.22 can move DX from a resonant to a stable state. This shallow-to-deep transition occurs between 20 and 30 kbar in GaAs. We report the observation of a new localized state in A103Ga07As under hydrostatic pressure, which is in some ways reminiscent of the DX center. It is resonant above the X CB below 40 kbar. At higher pressures it becomes stable and captures all electrons that are photoexcited into the X CB, causing a sharp drop in the intensity of radiative transitions. Upon reducing the pressure, the intensity does not recover despite thermal cycling to room temperature, indicating an unusually deep emission barrier, much deeper than that of DX. The intensity finally recovers at low pressures ( -10-20 kbar). We have repeatedly seen both the decline in intensity and the hysteresis in bulk A103Gao&As ( -2&&10' Si cm ) and in a GaAs/A103Ga07As (40 A/90 A) multiple-quantum-well (MQW) sample. At pressures abovethe I -X crossover, radiative recombination is observed from the X CB of Al"Gai -As barriers to the valence band (VB) of the GaAs wells. ' The initial states of the electron are the same for this staggered transition as in bulk Al"Gal -"As, and, as expected, they show behavior similar to the bulk. Samples were molecular-beam-epitaxy grown on a GaAs substrate. Photoluminescence (PL) was excited at 15 K using principally 5145-A radiation from an Ar laser. Measurements under pressure were made in a diamond anvil cell with argon as the pressure medium and ruby Auorescence as the in situ manometer. The pressure was changed at 300 K, and after an overnight pumpdown, the cell was cooled. Measurements were performed over the next several hours. Alp 3Gao 7As is a direct-gap semiconductor. Under pressure, the energy of the I CB increases while that of the X CB decreases, crossing -13 kbar. At 1 bar a sharp, intense peak due to the neutral donor bound exciton BE", and weaker peaks due to donor-acceptor recombination, DA", are observed. Around 9 kbar, new peaks appear below BE . These peaks exhibit bowing around crossover, indicative of I -L-X mixing, and then decrease in energy with a pressure coeIIicient of -1.6 meV/kba...

Abstract. We present a study of the deep and shallow donor levels under hydrostatic pressure. The shallow levels follow the conduction bands, while the deep levels are strongly sublinear with pressure. The temperature dependence of the intensities and energies is used to obtain an energy level diagram of the deep levels at high pressures.