Magnetotunneling from accumulation layers in AlxGa1-xAs capacitors.

Abstract

Current-voltage (I-V), ac capacitance-voltage (C-V), and conductance-voltage (G-V) measurements have been made of the effect of magnetic fields up to 15 T on tunneling at 1.6 K from accumulation layers on n-type GaAs. The samples used are ${n}^{\mathrm{\ensuremath{-}}}$-GaAs--${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As--n $^{+}$-GaAs capacitors in which the GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As barrier height is low enough, \ensuremath{\sim}0.3 eV, and the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As is thin enough, \ensuremath{\sim}20 nm, that direct tunneling of electrons occurs. Magnetic freezeout of carriers in ${n}^{\mathrm{\ensuremath{-}}}$-GaAs for H\ensuremath{\gtrsim}4 T can be determined from C-V curves. For ${n}^{+}$-GaAs biased positive, ${V}_{G}$>0, a quantized accumulation layer forms at the ${n}^{\mathrm{\ensuremath{-}}}$-GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As interface. Structure is observed in I-V, C-V, and G-V curves due to tunneling from Landau levels in the accumulation layer. Two samples are compared in detail. In one sample the Landau-level structure is relatively simple. In the other, spin splitting is observed in the first three Landau levels. The dependence of the surface concentration of electrons on ${V}_{G}$ is determined from tunneling currents at fixed ${V}_{G}$ and varying magnetic field. A second subband in the accumulation layer begins to be populated when the surface concentration \ensuremath{\gtrsim}7\ifmmode\times\else\texttimes\fi{}${10}^{11}$/${\mathrm{cm}}^{2}$. This is the first example of a system in which it is possible to use tunneling to examine the formation of an accumulation layer in a semiconductor, going from depletion through flat-band into accumulation, by applying a bias voltage.