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.

Popular Abstract

A novel way to improve contact quality to nanowires.

All modern technology is based on our ability to manipulate electricity and the ways we transport it to different places. For this, we must have good contacts among wires for more effective and efficient transportation of electric power. A good contact of wires becomes even more important when we go to very small scales such as nanometers which is a billionth of a meter. For comparison, an average human hair is about 60,000 nanometers thick. Wires that are on this very small scale are known as nanowires. Nanowires are made from semiconductors, which are a type of material with properties of both metals and insulators depending on the local temperature of the surrounding environment. These wires are usually grown in special conditions in the laboratory. Due to the nature of semiconductors, an unwanted layer is formed on the outer surface when in contact with air, making nanowires less conductive. Getting rid of this unwanted layer is one of the daunting tasks since the thickness of this layer is enormously small and so are the nanowires.

There has been a lot of research and developed methods employed already in the nanotechnology industry to remove unwanted layers from nanowires. The most employed method is the use of chemical reagents that react with the outer layer leaving behind a relatively cleaner nanowire. This method is sometimes referred to as the wet etching process since it involves the use of chemical reagents that are in a liquid state. The newly developed method that is still in the research phase involves the use of plasma. Ions from the plasma are extracted and accelerated towards the nanowires and directly remove the unwanted layer. This is more like scraping dirt on a bench by shooting bullets at an angle. As you can imagine, this is a very powerful process compared to using wet chemicals. It also means that things can turn the other way if not careful. We must find the right amount of power we want to shoot the ions otherwise we risk destroying the entire nanowire, also the angle and position at which we must fire our ions play an important role in the result. There are even more parameters that can be varied during the entire process of removing the unwanted layer, and all these must be understood in detail before this promising process can be of use.

Scientists are still working on this topic in the hope of revolutionizing the way nanowire-based devices are processed. The result that will be obtained from this project will contribute to the nanotechnology industry and science community in general.

• author: Mkolongo, Chris
• supervisor: Claes Thelander; Markus Aspegren
• organization: Department of Physics; Solid State Physics
• course: FYSK03 20231
• year: 2023
• type: M2 - Bachelor Degree
• subject: Technology and Engineering
• keywords: InAs nanowires, Argon milling
• language: English
• id: 9127937
• date added to LUP: 2023-06-20 17:02:59
• date last changed: 2023-06-21 03:40:56