UNIAXIAL-STRESS EFFECTS ON THE ALAS/GAAS DOUBLE-BARRIER HETEROSTRUCTURES

We have measured the current-voltage characteristics of GaAs-AlAs-GaAs-AlAs-GaAs double-barrier heterostructures grown by molecular beam epitaxy on (100) oriented substrates under longitudinal uniaxial stress along <100> (parallel to current) direction and transverse uniaxial stress along <011> (perpendicular to current) direction at 77 K. For longitudinal stress measurement, the peak-to-valley current ratio due to GAMMA-conduction electrons and the peak voltage (the voltage where the current peaks) decrease essentially with longitudinal stress and the negative differential resistance (NDR) disappears at a stress that depends on the dopings and dimensions of the heterostructures. However, it is surprisingly recovered at very high stress and the peak voltage reappears at a lower voltage, which is quite in contrast with the previous report for hydrostatic pressure studies. The observed shifts of the peak voltage, the decrease of the peak current, and hence the disappearance of the NDR are consistent with pressure-induced increase of the effective mass. The recovery of GAMMA-NDR is explained by the more rapid reduction of X(l) (momentum conserving longitudinal X valleys) nonresonant current due to the increase of reflections at GaAs/AlAs interfaces under very high uniaxial stress, where there are potential wells rather than barriers for X(l) current. A distinct NDR near- zero bias (approximately 40 mV) appears when the externally applied uniaxial stress is high enough. This has been attributed by Mendez and Chang [Surf. Sci. 229, 173 (1990)] to the resonant tunneling between two-dimensional electron gases. Our experiment shows that this tunneling is via momentum conserving longitudinal X-valleys (X(l)). For transverse stress measurement, there is no evidence of this feature up to 7 kbar.