A systematic study of non-ideal contacts in integer quantum Hall systems

In the present article we investigate the influence of the contact region on the distribution of the chemical potential in integer quantum Hall samples, as well as the longitudinal and Hall resistance as a function of the magnetic field. First we use a standard quantum Hall sample geometry and analyse the influence of the length of the leads where current enters/leaves the sample and the ratio of the contact width to the width of these leads. Furthermore we investigate potential barriers in the current injecting leads and the measurement arms in order to simulate non-ideal contacts. Second we simulate nonlocal quantum Hall samples with applied gating voltage at the metallic contacts. For such samples it has been found experimentally that both the longitudinal and Hall resistance as a function of the magnetic field can change significantly. Using the nonequilibrium network model we are able to reproduce most qualitative features of the experiments.Comment: 29 pages, 16 Figure

Improving Gold/Gold Microcontact Performance and Reliability Under Low-Frequency AC Through Circuit Loading

This paper investigates the performance and reliability of microcontacts under low-frequency and low-amplitude ac test conditions. Current microcontact theory is based on dc tests adapted to RF applications. To help better apply dc theory to RF applications, frequencies between 100 Hz to 100 kHz were experimentally investigated. Microcontacts designed to conduct performance and reliability measurements were used, which in prior dc testing typically lasted for 100 million cycles or more. Under ac loads, at similar power levels, eight devices were tested under cold-switching conditions, and only one was still operational at 10 million cycles. The effect of external circuitry on dc loaded devices was also considered. The experimental data were presented for dc conditions, which demonstrated that both a parallel capacitance with a microcontact and a series inductance were highly detrimental. For all six tested devices, failure occurred typically in 100 thousand cycles or less. However, utilizing series resistive/capacitive circuits as well as parallel resistor/inductive resulted in improved performance, with only one device of the four tested failing prematurely, but those that lasted showed less variation in measure contact resistance throughout the lifetime of the device. Two devices were tested with passive contact protection using parallel and series resistances, and both devices lasted for the full test duration. Finally, the effects of applying circuit protection to microcontacts and repeating ac test conditions were investigated. Reliability and device lifetime were extended significantly (9.1% success rate without protection was increased to 87% success rate). It was also observed in several instances that devices that failed showed subtle signs of variance during contact closure measurements in the range of 5-30 μ N, indicating a possible means for accurately predicting device failure. For these failed devices, notable physical damage was observed using a scanning electron microscope