Temperature-dependent transient capacitance in InGaAs/InAsP diodes

For the past several years, we have been studying lattice-matched In(x)Ga(1-x)As/InAsP double-heterostructures grown on InP substrates. The epistructures under investigation range from the lattice-matched condition (x sim 0.5) to severe mismatch (x sim0.8). We have found that, by including a special step-graded buffer between the substrate and the heterostructure, extremely high-quality lattice-mismatched epistructures can be grown. We would like to understand the physics underlying this important result. Photoexcitation-dependent radiative efficiency measurements suggest that the distribution of defect levels within the bandgap changes dramatically as the epilayers deviate from the lattice-matched condition. In particular, defect-levels appear to be more concentrated near the band edges with increasing lattice mismatch. We are currently using Deep Level Transient Spectroscopy to investigate this phenomenon in detail. We report the discovery of a deep defect level (activation energy = 0.30 +/- 0.02 eV) in the nominally lattice-matched material that does not appear in the more mismatched case.