Design and development of highly textured boron nitride thin films for electronic devices

Abstract

This thesis focuses on the alignment engineering (i.e. the orientation of the basal plane to the substrate surface) and the isotopic engineering (i.e. the modification of the isotopic composition of the compound) of hexagonal boron nitride (h-BN) thin films for its applications on electronic devices for enhancement thermal and electronic performances. Both alignment engineering and isotopic engineering opens up possibilities in the precise manipulation of thermal and electronic properties of the resultant thin films for enhancements in the performances of the eventual devices. Alignment engineering was shown to be achieved via the precise control of the deposition parameters using the high power impulse magnetron sputtering (HIPIMS) technique. In particular, two strategies were used, namely: control of nitrogen gas flow rate ratio and control of mean power supplied to the target. Depositions were conducted at a certain interval gas flow rate ratio and independently, at a lower mean power supplied to the target yield aligned h-BN. Isotopic engineering was demonstrated to be achievable using dual-magnetron HIPIMS technique. Isotopically pure targets were sputtered simultaneously. The resultant h-BN thin films were shown to be isotopically controllable, i.e. the thin films’ isotopic concentrations can be controlled, using the precise manipulation of mean power supplied to each target. Deposited h-BN thin films were applied as encapsulation in InSe-based transistors, with effective encapsulating effect as it prevents the degradation of InSe transistors upon exposure to ambient conditions. InSe-based h-BN encapsulated transistors retain its semiconducting nature and attain a mobility of 6.3 cm2 V-1 s-1 at room temperature. Isotopically enriched h-BN thin films were shown to have an enhancement in thermal conductivities, on an average of over 200%, as compared to the non-isotopic enriched variants. As evaluated by the performance as a thermal self-dissipative layer on AlGaN/GaN high electron mobility transistor (HEMT), the isotopically enriched layer is capable of reducing thermal self-heating effects, although the device may not be as well-functioning as it ought to be due to the ion implantation damage done on the 2-dimensional electron gas. The h-BN thin films were also used as a memory resistive device where oxygen vacancies play a key role in forming the conductive filament in the SET and RESET switching mechanism. The memory resistive mechanism was demonstrated in the h-BN thin films deposited on copper foil and Au/Ti were used as electrodes. The soft-forming step was required, and the breakdown field was measured to be around 10 MV cm-1. Lastly, future work can be done on manipulating the microstructure of isotopically-enriched h-BN thin films. As the dual-magnetron sputtering setup is dynamic and doubles the deposition parameters due to the magnetron, precise microstructure sculpturing may prove to be a challenging and yet rewarding endeavor. Furthermore, alignment engineering using pure boron targets resulting in a dense microstructure remains elusive.