| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Wednesday Sessions |
| Session TF+EM+MI-WeM |
| Session: | Thin Film Processes for Electronics and Optics I |
| Presenter: | Virginia Wheeler, U.S. Naval Research Laboratory |
| Authors: | V.D. Wheeler, U.S. Naval Research Laboratory N. Nepal, U.S. Naval Research Laboratory L.O. Nyakiti, Texas A&M University D.R. Boris, U.S. Naval Research Laboratory S.G. Walton, U.S. Naval Research Laboratory B.P. Downey, U.S. Naval Research Laboratory D.J. Meyer, U.S. Naval Research Laboratory C.R. Eddy, Jr., U. S. Naval Research Laboratory |
| Correspondent: | Click to Email |
Ga2O3 has emerged as a promising material for next generation power electronics and UV photodetectors applications due to its large bandgap (4.9 eV) and the availability of affordable native substrates from melt-grown bulk crystals. While β-Ga2O3 (monoclinic) is the most stable and studied of five polymorphs, the slightly less energetically favorable α- and ε-Ga2O3 phases have unique characteristics that can be exploited. The α-Ga2O3 (rhombohedral corundum) has the largest bandgap of 5.3 eV and can be alloyed with α-Al2O3 and α-In2O3 for bandgap engineering. T he ε-Ga2O3 phase (hexagonal wurtzite) is a polar phase, with a calculated polarization strength that is 10 and 3 times larger than that of GaN and AlN, respectively. Like the III-N system, polarization induced charges can lead to higher charge densities and mobilities in two-dimensional electron gases formed at heterojunctions, which would improve the viability of Ga2O3 electronic devices. In this work, we use atomic layer epitaxy (ALEp) to produce high-quality homo- and heteroepitaxial Ga2O3 films and investigate phase selectivity as a function of substrate type and orientation, growth temperature (Tg), plasma gas phase chemistry and gas pressure.
All ALE Ga2O3 films were deposited in a Veeco Fiji G2 reactor equipped with a load lock and turbo pump using trimethygallium and O2 plasma precursors. Initial studies on c-plane sapphire substrates showed that decreasing chamber pressure an order of magnitude during the plasma step resulted in a shift from pure β-Ga2O3 to pure α-Ga2O3. Additionally, at 350°C and 8 mTorr, the phase could be altered by a varying the O2 plasma flow from 5-100 sccm. Optical emission spectroscopy indicate that the ratio of O*/O2 is critical for phase selectivity while the high ion flux to the surface can contribute to the crystallinity at low Tg. By varying Tg from 300 to 500°C at 8 mTorr, films went from mixed β/ε phase at <350°C, to pure α-Ga2O3 at 350°C, to pure β-Ga2O3 at 500°C. Using the optimum growth conditions for α-Ga2O3 on c-sapphire, the influence of substrate was explored using a variety of substrates including AlN, GaN (bulk and epilayers), SiC, diamond, and Si. Deposition on III-N and β-Ga2O3 substrates all resulted in crystalline β- Ga2O3 films, while amorphous films were deposited on both SiC and Si. This suggests that a clean crystalline substrate interface is critical to obtaining high quality films and promoting metastable phases is more dependent on growth parameters than underlying crystal symmetry. Finally, we will discuss simple electrical properties of optimum films of each phase to validate feasibility of the process in device applications.