| AVS 64th International Symposium & Exhibition | |
| Vacuum Technology Division | Wednesday Sessions |
| Session VT-WeA |
| Session: | The History and Future of Materials, Surfaces and Interfaces (ALL INVITED SESSION) |
| Presenter: | Virginia Wheeler, U.S. Naval Research Laboratory |
| Authors: | V.D. Wheeler, U.S. Naval Research Laboratory A.C. Kozen, U.S. Naval Research Laboratory B.P. Downey, U.S. Naval Research Laboratory M. Currie, U.S. Naval Research Laboratory N. Nepal, U.S. Naval Research Laboratory L.O. Nyakiti, Texas A&M University D.J. Meyer, U.S. Naval Research Laboratory D.R. Boris, U.S. Naval Research Laboratory S.G. Walton, U.S. Naval Research Laboratory C.R. Eddy, Jr., U.S. Naval Research Laboratory |
| Correspondent: | Click to Email |
Atomic layer deposition (ALD) has emerged as a powerful technique to produce a wide variety of thin film materials including metal oxides, nitrides, and metals for use in numerous applications. This method has become increasingly useful as device dimensions are reduced and complexity is increased often resulting in non-planar architectures. The sequential, self-limiting reactions that define ALD enable excellent conformality on high-aspect ratio structures, angstrom level thickness control, and tunable film compositions. Additionally, ALD is conducted at low growth temperatures (Tg) which allows for integration of dissimilar materials as well as the ability to access new regions of phase diagrams in complex systems (ie. metastable phases, miscibility gaps, etc.). Traditionally, this low Tg yields amorphous films. In many applications, it is becoming increasingly advantageous to incorporate thin, conformal crystalline materials which are currently limited by the low Tg in ALD. To overcome this barrier, many have investigated post-deposition processing or plasma enhanced ALD. In this work, we will explore the advantages and limitations of approaches towards attaining crystalline ALD films through the following case studies: high quality phase transitions in ALD VO2 and phase control of heteroepitaxial Ga2O3.
VO2 is a thermochromic material that undergoes a crystalline phase change at critical temperature (68° C) resulting in drastic changes in optical and electrical properties. While crystallized ALD VO2 films have been shown to have sufficient transitions, they are deposited amorphously. Crystalline films are only obtained through a post-deposition anneal (500-600° C in O2) since the vanadium precursor degrades at elevated temperatures (>150° C). However, this high temperature anneal limits the integration of ALD VO2 films with other materials and without careful consideration of anneal parameters such as temperature, pressure and gas environment can alter the stoichiometry and structure of the initial ALD VO2 film.
A plasma enhanced ALD (PE-ALD) process was used to attain heteroepitaxial Ga2O3 films on c-plane sapphire substrates at 350° C. This is about half of traditional CVD or MBE methods, showing the benefits of PE-ALD. Furthermore, the crystallinity and phase composition of the Ga2O3 film can be control with growth temperature, plasma gas flow, and pressure. For example, reducing the chamber pressure an order of magnitude resulted in a shift from pure β-Ga2O3 to pure α -Ga2O3 at low pressures. Initial results correlating plasma species with phase control will be presented and discussed as a way to overcome the limitations of the low ALD Tg.