Effect of High Hydrostatic Pressures on the Magneto-Capacitance of Ga0.7Al0.3As/GaAs Heterojunctions

Abstract

High hydrostatic pressure was used to vary the electron sheet density Ns in a Ga0.7Al0.3 As/GaAs heterojunction (HJ) with applied magnetic fields up to 12 T at 4.2 K. The samples used were n+ GaAs, Al0.3Ga0.7As, p-GaAs, n+ GaAs multilayer capacitors. In these samples, the low GaAs/GaAlAs barrier height (0.2eV) and the thin AlGaAs layer (60nm) permit the transfer of electrons between the gate and the two dimensional electron gas (2DEG). Sharp structures are observed in the C vs B curves. Increasing the magnetic field at a given pressure induces deeper and deeper dips in the differential capacitance. These dips correspond to minima in D(EF,B), the Fermti-level density of states (DOS) of the 2DEG, and occur at integer filling factors νi=\\frac{\ ext{h}}{\ ext{e}}·\\frac{\ ext{Ns}}{\ ext{B}} The filling factor, ν, is defined by ν=Ns\\frac{\ ext{h}}{\ ext{eB}}, where h is Planck's constant, e is the electron charge, B is the magnetic induction, and Ns the electron sheet density. The dips have an exact periodicity in 1/B which allows an unambiguous determination of Ns(P) and its pressure sensitivity: \\frac{\\mathrm{d}\ ext{Ns}}{\\mathrm{d}\ ext{P}}=-0.2 1015 m-2kbar-1 A progressive pressure increase up to 10.7 kbar causes a reduction in both Ns and in the magnetic field Bi associated to each integer of νi by the same factor: 2.4. As a result, at pressures greater than 5 kbar, spin splitting is observable for B < 12 T. During the shift towards the lower values of the magnetic field, the amplitude of each capacitance dip decreases as expected from the less reduction of D(EF,B) at mid gaps between the less separated Landau-levels. Tue demonstration that the 2D DOS depends only on the magnetic field B is experimentally justified by the identity of the peaks obtained with two different Ns values but a constant B value. Modelization of the device capacitance is made by assuming that the Fermi-levels of the 2 DEG and of the gate electrode are pinned together. The agreement with experimental results is fairly good.