HCI Flow-Induced Phase Change of alpha-, beta-, and epsilon-Ga2O3 Films Grown by MOCVD

Precise control of the heteroepitaxy on a low-cost foreign substrate is often the key to drive the success of fabricating semiconductor devices in scale when a large low-cost native substrate is not available. Here, we successfully synthesized three different phases of Ga2O3 (alpha, beta, and epsilon) films on c-plane sapphire by only tuning the flow rate of HCl along with other precursors in an MOCVD reactor. A 3-fold increase in the growth rate of pure beta-Ga2O3 was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of beta- and epsilon-Ga2O3 was observed, until the Ga2O3 film transformed completely to a pure e-Ga2O3 with a smooth surface and the highest growth rate (similar to 1 mu m/h) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of a- and epsilon-Ga2O3 with a dominant alpha-Ga2O3, while the growth rate dropped significantly (similar to 0.4 mu m/h). The film became rough as a result of the mixture phases since the growth rate of epsilon-Ga2O3 is much higher than that of alpha-Ga2O3. In this HCl-enhanced MOCVD mode, the Cl impurity concentration was almost identical among the investigated samples. On the basis of our density functional theory calculation, we found that the relative energy between beta-, epsilon-, and alpha-Ga2O3 became smaller, thus inducing the phase change by increasing the HCl flow in the reactor. Thus, it is plausible that the HCl acted as a catalyst during the phase transformation process. Furthermore, we revealed the microstructure and the epitaxial relationship between Ga2O3 with different phases and the c-plane sapphire substrates. Our HCl-enhanced MOCVD approach paves the way to achieving highly controllable heteroepitaxy of Ga2O3 films with different phases for device applications.