Ge doping of β-Ga2O3 by MOCVD

We report on the Ge doping of Ga2O3 using metalorganic chemical vapor deposition (MOCVD) epitaxy. The effects of the GeH4/N2 flow rate, substrate temperature, VI/III ratio, type of Ga precursor, and MOCVD reactor geometry on the incorporation efficiency of Ge into Ga2O3 were explored. The Ge concentration incorporated into the films was quantified using Hall and secondary ion mass spectroscopy measurements. The increase in the GeH4/N2 flow rate, decrease in the substrate temperature, and increase in the VI/III ratio increase the amount of Ge incorporated into Ga2O3. The incorporation of Ge into the lattice of Ga2O3 was found to be strongly dependent on the substrate temperature, i.e., lowering the growth temperature leads to a higher doping concentration. Films with a free carrier concentration ranging from ∼2 × 1016 to ∼3 × 1020 cm−3 and corresponding mobilities ranging from ∼140 to ∼38 cm2/Vs were realized. The incorporation of Ge into the films was also found to be strongly dependent on the metalorganic precursor type used for the growth of the Ga2O3 film. We found that it was more challenging to dope Ga2O3 with Ge using trimethylgallium rather than triethylgallium as a source for Ga. Additionally, we found that Ge doping has a strong memory effect dependent on the reactor geometry. The result highlights the challenges in achieving controllable Ge doping for n-type conductivity despite all the positive indicators from theoretical studies that suggest that Ge is a suitable dopant candidate for Ga2O3 similar to Si and Sn.