Effect of composition on deep levels in heteroepitaxial GexSi1?x layers and evidence for dominant intrinsic recombination-Generation in relaxed ge layers on Si

Electron traps, hole traps, and the dominant recombination-generation (R-G) centers have been investigated with deep level transient spectroscopy and current-voltage/temperature measurements in heteroepitaxial Ge_xSi_1-x alloys with x ranging from 0.15 to 1, grown on graded Ge_y.Si_1−y/Si substrates. For all samples with compositions x < 0.85, which retain the Si-like conduction band structure, we detect a dominant electron trap and R-G center whose activation energy is ΔE = 0.5 eV, independent of composition. This energy agrees with that of electron traps previously reported for plastically deformed (PD) Si, suggesting a connection to the Si-like band structure. This 0.5 eV level dominates the reverse leakage current over a wide range of growth and annealing conditions for the 30% Ge samples, indicating that the electronic state at ΔE = 0.5 eV is a very efficient R-G center, as would be expected from its midgap position. Alternatively, for strain relaxed, pure Ge (< 1), we detect electron traps at E_c − 0.42 eV and E_c − 0.28 eV, in agreement with the literature on PD Ge and Ge bicrystals. These energies are significantly different from those observed for x < 0.85, and we conclude that these changes in activation energy are due to changes in the conduction band structure for high Ge content. Moreover, in contrast with the Si-like samples (x < 0.85), the reverse leakage current in the relaxed Ge cap layer is not controlled by deep levels, but is rather dictated by intrinsic, band-to-band generation due to the reduced bandgap of Ge as compared to Si-like alloys. Only for reverse bias magnitudes which incorporate a significant portion of the graded buffer within the depletion region do R-G centers dominate the reverse leakage current. These results confirm the high quality of the strain-relaxed, pure Ge cap region which was grown on a Ge_ySi_1−y/Si step graded heterostructure (where y was increased from 0 to 1) by ultra high vacuum chemical vapor deposition. Finally, we report for the first time, what is apparently the dislocation kink site state at E_c − 0.37 eV, in a Ge_xSi_1−x alloy.