New resonant tunneling processes between longitudinal and transverse X-states in GaAs/AlAs double barrier structures beyond the type II transition

When a typical double barrier GaAs AlAs structure is pressurised above the type I-type II transition at about 13 kbar, electrons are transferred from the doped GaAs emitter and collector layers into the undoped AlAs layers. This can be verified by capacitance measurements as a function of pressure and bias. In the type II regime, new X-related tunneling resonances were first reported in 1990, for 40 Å barriers at 77 K, but were wrongly attributed to tunneling between longitudinal X-states. In this paper, tunnel current measurements are reported as a function of temperature between 4.2 and 150 K, and AlAs thickness between 30 and 50 Å. From these measurements it can be deduced that the originally reported resonance was between the ground transverse X-states in each AlAs layer, because this resonance is suppressed at 4.2 K for barriers thinner than about 50 Å. Resonant tunneling processes of two types are discussed: X₁ → X_t(n), and X₁(1) → X_t(n) + P with n ≥ 1, where P represents a single or multiple phonon emission process for conservation of parallel momentum. The second type of process is distinguished from the first because it is not suppressed at 4.2 K, even for barriers thinner than 50 Å, consistent with an initial longitudinal state. The suppression of the first type of process for low temperatures and thin barriers indicates that the ordering of the longitudinal and transverse states changes at AlAs thicknesses of about 50 Å, due to competition between quantum confinement and biaxial strain. This process also shows an unusually strong asymmetry between forward and reverse bias for n = 1, even in structures which exhibit highly symmetric resonant tunneling characteristics at ambient pressure. It is suggested that the behaviour is related to differences in the roughness of the normal and inverted interfaces.