Uniaxial Stress Effects on the Electronic Properties of Gallium Arsenide/aluminum Gallium Arsenide Single/double Barrier Heterostructures

Uniaxial stress dependent current-voltage characteristics of GaAs/Al_{rm x}Ga _{rm 1-x}As single and double heterojunction barriers grown on (100) substrates by molecular beam epitaxy with different aluminum compositions and thicknesses have been measured at 77^ circK or 4^circK and compared with theoretical calculations. The stress is compressive and applied longitudinally along the (100) parallel to the (100) current direction or transversely along <011> perpendicular to current direction by means of a mechanical press. It is known that uniaxial stress causes the energy band structure to change, so that the conduction band minima responsible for the tunneling process can be determined. The results of our measurements and calculations are that we are able to demonstrate the usefulness of uniaxial stress in elucidating the transport mechanisms in heterojunction barriers. For the measurements on the single barrier structures, we directly observed the surprising and new result that, for Fowler -Nordheim tunneling, under appropriate conditions, the final state is not the lowest energy valley, but the higher valley favored by smaller effective mass and momentum conservation. For the case of thermionic emission, in which thermally activated electrons are emitted over a small barrier, the lowest energy tends to dominate. Moreover, from the stress dependent barrier height, we can deduce the X-valley shear deformation potential Xi. For the x = 0.8 sample at 4^circK under transverse stress, we observed small oscillations of current with voltage. We attribute this to resonant indirect Fowler -Nordheim tunneling. For the double barrier heterostructures under longitudinal stress, the negative differential resistance (NDR) disappears at a stress that depends on the dopings and dimensions of the heterostructures. However, it is recovered at very high stress and the peak voltage reappears at lower voltage. At high uniaxial stress, we also observe another distinct NDR near ~40mV due to resonant tunneling between the two dimensional electron gases. For the transverse stress measurement of the heterostructure, there is no evidence of this NDR feature at low voltage up to 7kbar. This contrasts with the results of the longitudinal stress measurement.