Magnetotunneling from accumulation layers inAlxGa1−xAs capacitors

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 $n^{\mathrm{-}}$-GaAs–${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As–n ${}_{}{}^{+}{}_{}{}^{}$-GaAs capacitors in which the GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As barrier height is low enough, ∼0.3 eV, and the ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As is thin enough, ∼20 nm, that direct tunneling of electrons occurs. Magnetic freezeout of carriers in $n^{\mathrm{-}}$-GaAs for H≳4 T can be determined from C-V curves. For $n^{+}$-GaAs biased positive, $V_G$>0, a quantized accumulation layer forms at the $n^{\mathrm{-}}$-GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$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 ≳7×${10}^{11}$/${\mathrm{cm}}^2$. 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.