X- and Γ-related tunneling resonances in GaAs/AlAs double-barrier structures at high pressure

The paper describes electrical measurements, as a function of pressure and temperature, of multiple resonances that appear above a threshold pressure, for GaAs/AlAs double-barrier resonant-tunneling structures. The threshold pressure is within a few kilobars of the type-I-to-type-II transition, but depends on sample parameters, in particular, the doping of the GaAs emitter layer. Up to six resonances have been observed in the same sample, for each bias direction. The first two resonances occur at low bias and exhibit a strong suppression with decreasing temperature, while three higher-bias resonances are steplike and remain strong at helium temperatures. We have assigned these resonances to Xₜ(1){→}Xₜ(1), Xₜ(1){→}Xₜ(1)+LO, Xₗ(1){→}Xₜ(1)+P₁, Xₗ(1){→}Xₜ(1)+2P₂, and Xₗ(1){→}Xₜ(2)+P₁, where Xₜ(n) and Xₗ(n) are the nth confined subband associated with the four transverse and the two longitudinal X minima, respectively, LO is a zone-center longitudinal optical phonon, and P₁,P₂ are zone-boundary and large-wave-vector phonons, respectively.The temperature dependence of the first two resonances allows us to estimate a separation between Xₜ(1) and Xₗ(1) of approximately 20 meV, for an AlAs layer of thickness 30 {}. This energy is in agreement with calculations that include a 23-meV downshift of Xₜ(1) relative to Xₗ(1) due to biaxial strain. In some cases, the first resonance starts to reduce and to shift to higher bias above a critical pressure. This is explained by the unpinning of the quasi-Fermi levels in the emitter and collector AlAs layers from the corresponding Xₗ(1) states. In all cases, the ambient pressure {Γ} resonance is strongly suppressed with increasing pressure up to the threshold pressure, but then seems to reappear at higher pressure, when it is strongly suppressed below 77 K. For thin-barrier samples (40 {}), the resonance at high pressure is weak and the peak shifts to higher bias with pressure. It is attributed to Xₜ(1){→}{Γ}(1)+P₁. For thicker barrier samples (>40 {}) the resonance is strong, the bias is independent of pressure, and the resonance is assigned to Xₜ(1){→}Xₜ(2).