Hysteretic magnetotransport inp-typeAlxGa1−xAsheterostructures with In/Zn/Au Ohmic contacts

Abstract

The two-terminal magnetoconductance of a hole gas in C-doped AlGaAs/GaAs heterostructures with Ohmic contacts consisting of alloyed In/Zn/Au displays a pronounced hysteresis of the conductance around zero magnetic field. The hysteresis disappears above magnetic fields of around 0.5 T and temperatures above 300 mK. For magnetic fields below 10 mT, we observe a pronounced dip in the magnetoconductance. We tentatively discuss these experimental observations in the light of superconductivity of the Ohmic contacts.Transport measurements in semiconductors require Ohmic contacts between the external metallic leads and the carriers in the semiconductor. At the interface between a semiconductor and a metal, usually a Schottky contact forms giving rise to nonlinear current-voltage ͑I-V͒ characteristics. In order to obtain linear I-V traces, this Schottky barrier has to be overcome, which is usually achieved by highly doping the semiconductor in the contact region. For n-type AlGaAs heterostructures, this is routinely achieved by using Au/ Ge/Ni alloys. For p-type AlGaAs heterostructures, one usually uses layers of AuZn, AuBe, or InZn. 1 Especially at liquid He temperatures and below, the quality of the contacts to hole systems is inferior to the Ohmic contacts on electron gases. As long as the I-V characteristics are linear, high contact resistances can be experimentally overcome by using four-terminal measurements. However, for high impedance devices such as quantum dots, one has to perform voltagebiased measurements, and the contact quality becomes crucial.Here we report on a peculiar hysteresis effect observed in two-terminal magnetoconductance measurements on a twodimensional hole gas contacted by In/Zn/Au alloys. For voltage biased devices, the current signal shows a pronounced dip-like feature around zero magnetic field and hysteretic behavior up to magnetic fields, where Shubnikov-de Haas ͑SdH͒ oscillations become relevant. The feature disappears for high magnetic fields and high temperatures. We tentatively ascribe these observations to the contacts becoming superconducting. This way, we extract an estimate for T c , the superconducting transition temperature, as well as for critical magnetic fields of the In/Zn/Au alloy, which are in tune with values reported in the literature. The purpose of this paper is to show experimental details of unusual Ohmic contacts possibly related to a superconducting phase transition.Our experiments have been performed on AlGaAs heterostructures doped with carbon ͑C͒ acting as an acceptor on ͑100͒ substrates. 2,3 The integer, as well as the fractional quantum Hall effects, have been observed in such samples 4,5 testifying to the electronic quality of these structures. The material quality has strongly improved over the last couple of years 6,7 with mobilities now exceeding 10 6 cm 2 / Vs. 8 Also, single electron transistors, 9 as well as high-quality Aharonov-Bohm rings, 10 have been realized. The effects, which we describe in the following, have been observed in two-terminal measure...

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