CAPACITANCE STUDIES OF CHARGE REDISTRIBUTION BETWEEN GAMMA-STATE AND X-STATE IN A GAAS ALAS DOUBLE-BARRIER STRUCTURE AT HIGH-PRESSURE/

Capacitance measurements as a function of bias provide evidence for th e redistribution of electrons above the pressure induced type I to typ e II transition in a 43, 72, and 40 angstrom AlAs/GaAs double barrier structure (DBS). Measurements at 77 K for pressures up to 16 kbar allo w the identification of low and high pressure regimes. In the low pres sure regime the DBS exhibits a capacitance whose characteristic length includes both the barriers and the well, and any surrounding depletio n regions. In the high pressure regime, the behavior is like that of a ''quantum capacitor,'' with plates only approximately 70 angstrom apa rt. This causes a marked increase in the zero bias capacitance, which is explained by electron transfer to the X wells in the AlAs regions. The subsequent variation of the capacitance with bias is consistent wi th a band bending model introduced previously, in which the Fermi leve l is pinned to the lowest X level in each well at low bias, but where the collector well is eventually depleted at larger bias.