Precise control of the hetero-epitaxy 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 (α, β, and ε) films on c-plane sapphire by only tuning the flow rate of HCl along with other precursors in an MOCVD reactor. A threefold increase in the growth rate of pure β-Ga2O3 was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of βand ε-Ga2O3 was observed, until the Ga2O3 film transformed completely to a pure ε-Ga2O3 with a smooth surface and the highest growth rate (~1 µm/hour) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of αand ε-Ga2O3 with a dominant α-Ga2O3, while the growth rate dropped significantly (~0.4 µm/hour). The film became rough as a result of the mixture phases since the growth rate of ε-Ga2O3 is much higher than α-Ga2O3. In this HClenhanced MOCVD mode, the Cl impurity concentration was almost identical among the investigated samples. Based on our density functional theory calculation, we found that the relative energy between β-, ε-, and α-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 2 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 HClenhanced MOCVD approach paves the way to achieving highly controllable hetero-epitaxy of Ga2O3 films with different phases for device applications.

FIG. 7. The band alignment diagram of the b-Ga 2 O 3 /AlN heterojunction, along with that of GaN.

Owing to large bandgaps of BAlN and AlGaN alloys, their heterojunctions have the potential to be used in deep ultraviolet and power electronic device applications. However, the band alignment of such junctions has not been identified. In this work, we investigated the band-offset parameters of a B0.14Al0.86 N/Al0.7Ga0.3N heterojunction grown by metalorganic vapor phase epitaxy. These specific compositions were chosen to ensure a sufficiently large band offset for deep ultraviolet and power electronic applications. High resolution transmission electron microscopy confirmed the high structural quality of the heterojunction with an abrupt interface and uniform element distribution. We employed high resolution X-ray photoemission spectroscopy to measure the core level binding energies of B 1s and Ga 2p3/2 with respect to the valence band maximum of B0.14Al0.86N and Al0.7Ga0.3N layers, respectively. Then, we measured the energy separation between the B 1s and Ga 2p3/2 core levels at the interface of the heterojunction. The valence band offset was determined to be 0.40 ± 0.05 eV. As a consequence, we identified a staggered-gap (type-II) heterojunction with the conduction band offset of 1.10 ± 0.05 eV. The determination of the band alignment of the B0.14Al0.86N/Al0.7Ga0.3N heterojunction facilitates the design of optical and electronic devices based on such junctions.

The spontaneous polarization (SP) and piezoelectric (PZ) constants of BxAl1-xN and BxGa1-xN (0 ≤ x ≤ 1) ternary alloys were calculated with the hexagonal structure as reference. The SP constants show moderate nonlinearity due to the volume deformation and the dipole moment difference between the hexagonal and wurtzite structures. The PZ constants exhibit significant bowing because of the large lattice difference between binary alloys. Furthermore, the PZ constants of BxAl1-xN and BxGa1-xN become zero at boron compositions of ∼87% and ∼74%, respectively, indicating non-piezoelectricity. The large range of SP and PZ constants of BxAl1-xN (BAlN) and BxGa1-xN (BGaN) can be beneficial for the compound semiconductor device development. For instance, zero heterointerface polarization ΔP can be formed for BAlN and BGaN based heterojunctions with proper B compositions, potentially eliminating the quantum-confined Stark effect for c-plane optical devices and thus removing the need of non-polar layers and substrates. Besides, large heterointerface polarization ΔP is available that is desirable for electronic devices.

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