HCl Flow-Induced Phase Change of α‑, β‑, and ε‑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 (α, β, and ε) films on <i>c</i>-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 β-Ga₂O₃ was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of β- and 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/h) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of α- and ε-Ga₂O₃ with a dominant α-Ga₂O₃, while the growth rate dropped significantly (∼0.4 μm/h). The film became rough as a result of the mixture phases since the growth rate of ε-Ga₂O₃ is much higher than that of α-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 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 the phase transformation process. Furthermore, we revealed the microstructure and the epitaxial relationship between Ga₂O₃ with different phases and the <i>c</i>-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

Highly efficient transverse-electric-dominant ultraviolet-c emission employing GaN multiple quantum disks in AlN nanowires matrix

Heavy reliance on extensively studied AlGaN based light emitting diodes (LEDs) to replace environmentally hazardous mercury based ultraviolet (UV) lamps is inevitable. However, external quantum efficiency (EQE) for AlGaN based deep UV emitters remains poor. Dislocation induced nonradiative recombination centers and poor electron-hole wavefunction overlap due to the large polarization field induced quantum confined stark effect (QCSE) in “Al” rich AlGaN are some of the key factors responsible for poor EQE. In addition, the transverse electric polarized light is extremely suppressed in “Al”-rich AlGaN quantum wells (QWs) because of the undesired crossing over among the light hole (LH), heavy hole (HH) and crystal-field split-off (SH) states. Here, optical and structural integrities of dislocation-free ultrathin GaN quantum disk (QDisk) (~ 1.2 nm) embedded in AlN barrier (~ 3 nm) grown employing plasma-assisted molecular beam epitaxy (PAMBE) are investigated considering it as a novel nanostructure to realize highly efficient TE polarized deep UV emitters. The structural and chemical integrities of thus grown QDisks are investigated by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We, particularly, emphasize the polarization dependent photoluminescence (PL) study of the GaN Disks to accomplish almost purely TE polarized UV (~ 260 nm) light. In addition, we observed significantly high internal quantum efficiency (IQE) of ~ 80 %, which is attributed to the enhanced overlap of the electron-hole wavefunction in extremely quantum confined ultrathin GaN QDisks, thereby presenting GaN QDisks embedded in AlN nanowires as a practical pathway towards the efficient deep UV emitters