Hollow-Cathode Plasma- Assisted Atomic Layer Deposition of Wide and Ultrawide Bandgap Semiconductors

In this report the results of three studies conducted on the growth of thin films using plasma-enhanced atomic layer deposition (ALD) are demonstrated.
Firstly, the effect of Ar/N2/H2 plasma power on the growth of AlN films in a compact reactor with a large-diameter hollow-cathode plasma (HCP) source was studied. It was found that rf-power plays a critical role in efficiently removing ligands and improving the film properties by providing the necessary surface energy for crystallization. The trimethylaluminum (TMA) chemisorption highly depended on the rf-power and plasma exposure time. Switching among varying rf-powers had an immediate observable effect on TMA adsorption and ligand removal. Samples grown on Si(100) substrates via 100 – 200 W rf-powers showed hexagonal polycrystalline wurtzite structure, while 50 W resulted in an amorphous film. Extended plasma exposure time at lower rf-powers (25–50 W) improved TMA chemisorption and nitrogen incorporation but not film crystallinity. HR-S/TEM analysis revealed randomly oriented AlN crystallite domains, confirming their polycrystalline nature with a (002) preferred orientation confirmed by x-ray diffraction measurements.
Next, we have demonstrated the first-time achievement of as-grown crystalline β-Ga2O3 films on Si(100) and glass substrates at low substrate temperatures (200 ℃) via HCP-ALD system featuring in situ Ar-plasma annealing. The annealing enhanced surface adatom migration and precursor reactivity, leading to crystalline film growth. In situ ellipsometry was used to monitor the film growth behavior. Higher annealing rf-power values improved the crystallinity of β-Ga2O3 films, whereas as-deposited films exhibited amorphous character, possibly due to ligand re-deposition effect at higher rf-powers leading to elevated GPC, degraded refractive index, and porous structure. We have further demonstrated single-phase epitaxial β-Ga2O3 film growth at 240 ℃ on sapphire using the aforementioned annealing technique, confirming its effectiveness in transforming amorphous wide bandgap oxide semiconductors into epitaxial films at substantially reduced substrate temperatures. Additionally, in situ atomic layer doping of β-Ga2O3 films was studied via both super-cycle and co-dosing methods using Si and Sn as n-type dopants. Preliminary electrical tests showed improvement in electrical conductivity of β-Ga2O3 layers with varying doping cycle-ratios proving PE-ALD as an alternative in situ doping technique to tune the electrical properties of ultrawide bandgap semiconductors.
Lastly, our investigation results on the role of hydrogen radicals in the surface reactions of TMI with nitrogen plasmas are documented. GIXRD analysis revealed that hexagonal InN was achieved with both Ar/N2 and N2-only plasmas, while adding hydrogen to the plasma chemistry resulted in the formation of c-In2O3. Possible reaction mechanisms are provided in accordance with the literature work. This result was attributed to the hydrogen radicals promoting -OH formation on the TMI surface, resulting in oxygen incorporation from residual water vapor. InN films grown at 200 °C with 100W Ar/N2 plasma exhibited pristine hexagonal polycrystalline wurtzite structure.