EVIDENCE FOR PHOTOLUMINESCENCE BAND IN P-TYPE AL0.67GA0.33AS RELATED TO NONEQUILIBRIUM DX(-) CENTERS

The photoluminescence (PL) properties of p-type Al0.67Ga0.33As epitaxi al layers grown by metalorganic vapor phase epitaxy technique and dope d with residual carbon have been studied at 10 K. The PL spectra show two distinct ranges: one dominated by PL lines related to the X band s hallow impurities (acceptor carbon and a background donor, predominant ly Si) and the second including a structural broadband (BB), beginning some 300 meV below the band gap. The ionization energies determined f or the carbon and the Si are approximate to 39 and 52+/-1 meV, respect ively. The BB consists of four poorly resolved peaks at 1.8, 1.74, 1.6 7, and 1.6 eV with a half width >50 meV. Their PL intensity shows very specific temperature dependence with a maximum at temperatures where the persistent photoconductivity reported in n-type AlxGa1-xAs is quen ched. Experiments with selective above band-gap excitation reveal the existence of a threshold for the BB excitation. The excitation density dependence of the BB exhibits some superlinearity for excitation dens ities > 1.5 W/cm(-2). These BB properties gave evidence that its origi n has to be related to a nonequilibrium DX(-) centers, resulting possi bly in a two-step optical recharging process: D+ + e-->D degrees and D degrees + ''hot e''-->DX(-). Two models are accounted for the BB expl anation. One of them assumes radiative transitions (h, DX(-)), while t he second considers internal radiative transitions from the DX degrees state of the distorted configuration to the DX(-) state. (C) 1996 Ame rican Institute of Physics.