Pressure Dependence of Negative Differential Resistance in AlGaAs/GaAs Double Barrier Resonant Tunnelling Devices up to 20 Kbar

Abstract

We have investigated the pressure dependence of Negative Differential Resistance (NDR) features observed in the DC current-voltage (I-V) characteristics of Al_xGa_1-xAs/GaAs Double Barrier Resonant Tunnelling Devices (DBRTD’s) with x=0.33 and x=1. For x=0.33, results confirm that both the n=1 and n=2 resonances are determined almost exclusively by tunnelling through the Γ-profile at low pressure and this remains true for the lowest resonance even up to 20 Kbar. Structures grown without undoped spacer layers adjacent to the barriers show an anomalous weakening of the first resonance between about 8 and 11 Kbar, which is not observed in structures grown with spacers. This is attributed to the higher level of incorporation in the barriers of silicon impurities, or of trace impurities associated with the silicon, which give rise to deep levels. For a device with x=1, the pressure dependence of the main n=1 resonance at low pressure appears very sensitive to the height of the Γ(GaAs)-X(AlAs) barrier. The threshold for complete suppression of this feature is related to the pressure induced line up of the AlAs X-minima with the lowest confined state in the Γ-well. Above about 13 Kbar a new resonance feature is observed close to zero bias, which grows in strength up to 20 Kbar. This resonance appears when the AlAs layers change role from “barriers” at low pressure to “wells” at high pressure. We describe the expected band bending in this regime in terms of the pinning of the Fermi level close to the energy of the lowest confined sub-band in the X-profile well, and explain the observed I-V characteristic and its pressure dependence by the sum of two contributions to the current due to tunnelling through the Γ- and X-profiles.