Developing an Ar milling process to improve the contact quality to InAs nanowires

Abstract: The aim of this work was to develop a stable and reproducible argon milling process for InAs nanowires to remove the native oxide layer that increases electrical resistance. This was done by identifying a few milling parameters and studying them in relation to the milling rate of silicon dioxide (SiO2). After further experiments with different milling parameters, a set of parameter values was found to give a milling rate of about 6-8 nm/min. The milling rate of Polymethyl methacrylate (PMMA) for the same set of parameter values was 60-80 nm/min which is 10 times more than that for SiO2. PMMA is used during device fabrication and was exposed to the Ar+ beam, thus it’s important to compare it to the milling rate of SiO2. Finally, argon milling was done to a set of nanowires, and the contact resistance to Ti/Al metal was calculated. Two-probe and four-probe measurements were done to the set of InAs nanowires and the data obtained was used to calculate the contact resistance. The contact resistance was found to be around 90 Ω which is relatively small compared to the internal resistance of InAs nanowires which is around 1.5-3 kΩ. For reference, the contact resistance for a set of nanowires that were not treated with argon milling was measured and was found to be around 1 MΩ. Hence argon milling is a critical process that can reduce contact resistance by a significant amount. Finally, the milling rate of InAs nanowires was calculated to be about 25 nm/min for the same set of parameters used for SiO2 and PMMA. This is a relatively high milling rate with respect to the thickness of the oxide layer in InAs nanowires, to get a lower milling rate one would need to lower the acceleration voltage and/or reduce the amount of time of exposure of the sample to the Ar+ beam.