(Invited) Low-Temperature Plasma-ALD of Wide and Ultra-Wide Bandgap Semiconductors: Accomplishments and Hurdles to Overcome

Abstract

III-nitride and III-oxide based wide (GaN, InGaN, AlGaN) and ultra-wide bandgap (Ga2O3, AlN, BN) semiconductors play critical roles in high-power, high-frequency, and harsh-environment electronic components utilized in wireless communications, power grids, homeland security, and space applications. While GaN-based wide bandgap (WBG) devices have reached a certain degree of maturity, the emerging ultra-wide bandgap (UWBG) alloys are still in their early stage of technology development. Conventional chemical vapor deposition (CVD) based crystal growth processes for these materials are carried out at elevated substrate temperatures, reaching over 1000 °C. In an effort to substantially reduce this synthesis temperature, plasma-assisted CVD techniques are of particular interest. Owing to its unique features including pinhole-free synthesis, precision film thickness control, large-area uniformity, and ultimate 3D conformality, plasma-assisted atomic layer deposition (plasma-ALD) offers significant opportunities for electronic device layers at reduced substrate temperatures.Relatively limited number of research groups have targeted this challenging opportunity by using different types of plasma sources integrated into various plasma-ALD reactors. In addition, different metal precursors and plasma gas mixtures have been utilized and the resulting material properties are studied on different substrates. While mostly polycrystalline WBG and UWBG films are reported, recently several groups have reported epitaxial relationship of plasma-ALD grown III-nitride and Ga2O3 films on sapphire and SiC substrates, which hints the potential for success. However, there still exist critical technology barriers for the adoption of plasma-ALD as an alternative low-temperature materials synthesis technique for device layers. Reliable electron mobility values are lacking and atomic layer doping efforts are barely investigated.In this presentation, we would like to provide and updated overview of the field including the latest reports and achievements. In addition, we would like to present our own efforts and recent advances in developing crystalline III-nitride and Ga2O3 films by using the hollow-cathode plasma (HCP) source, which demonstrated its advantages for reducing the oxygen impurities in III-nitride layers. By incorporating a multi-wavelength ellipsometer for in-situ monitoring, unit-cycle growth behavior of WBG and UWBG films were analyzed and correlated with the obtained material properties. Moreover, real-time in-situ monitoring allowed the realization of the saturation curve experiments on a single sample, without the need for breaking the vacuum and using multiple samples necessitating individual ex-situ thickness measurements. The growth parameters studied include the substrate temperature, rf-power applied to the remote HCP source, and plasma gas mixtures (N2/H2/Ar and O2/Ar with different flow ratios for III-nitrides and Ga2O3, respectively). Structural, chemical, optical, and electrical properties of the grown WBG and UWBG films have been characterized and correlated with the growth conditions. One of the main results obtained for these films is the different behavior each binary compound shows for similar synthesis parameter variations. A roadmap towards overcoming the current limitations will be discussed with an outlook for potential device applications of these low-temperature plasma-ALD grown films.