Deep states in GaAs grown by molecular beam epitaxy

Abstract

Using deep level transient capacitance spectroscopy (DLTS) we have investigated the growth parameter dependence and electronic properties of deep level centers in GaAs grown by molecular beam epitaxy (MBE), principally in the temperature range 500–650 °C. In both n- and p-type material we find hole traps ascribed to Fe and Cu and electron traps M1, M3 and M4 unique to MBE layers. Concentrations of the ‘‘M’’ levels depend weakly on As:Ga flux ratio but decrease rapidly with increasing growth temperature. Depth profiles show no evidence for bulk annealing and, by comparison with published secondary ion mass spectrometry data, we suggest these centers are impurity-defect complexes. Capture cross-section (σn) measurements show that two different traps give rise to further DLTS peaks at ∼140 K. One trap with σn >4×10−17 cm2 and (Ec −Et) >0.17 eV (at 140 K), which we associate with M2, is observed at a growth temperature of 650 °C and from the influence of the growth conditions we suggest this is As-vacancy related. The other trap, which we label M2′, has σn =1×10−20 cm2 and (Ec −Et) =0.08 eV (at 140 K) and occurs in material grown at temperatures below 600 °C, with similar growth behavior to M1, M3 and M4. For M3 we measured σn =1.1×10−16 cm2 giving (Ec −Et) =0.236 eV at 173 K. Using measurements of the temperature dependence of σn we have calculated the entropy change on emission or capture of an electron at M2′ and M3 and deduce that these levels are associated with bonding (valence band) states. The capture cross sections of M1 and M4 were too large to be measured (>2×10−16 cm2), but we deduce that σn(M4) >σn (M1). Data are given for the signatures of all the electron traps at low electric fields, and these are compared with data for other traps in GaAs. We do not find convincing evidence for identifying the DLTS peak at 200 K in 300 °C grown material with the E3 center in irradiated GaAs. We suggest that the minority carrier lifetime in p-GaAs will be limited by recombination at M4 and M2 for growth temperatures below and above ∼625 °C, respectively.