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Register a new userHVPE growth of corundum-structured $alpha$-Ga$_2$O$_3$ on sapphire substrates with $alpha$-Cr$_2$O$_3$ buffer layer
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Gallium oxide films were grown by HVPE on (0001) sapphire substrates with and without $alpha$-Cr$_2$O$_3$ buffer produced by RF magnetron sputtering. Deposition on bare sapphire substrates resulted in a mixture of $alpha$-Ga$_2$O$_3$ and $epsilon$-Ga$_2$O$_3$ phases with a dislocation density of about $2cdot10^{10}$ cm$^{-2}$. The insertion of $alpha$-Ga$_2$O$_3$ buffer layers resulted in phase-pure $alpha$-Ga$_2$O$_3$ films and a fourfold reduction of the dislocation density to $5 cdot 10^9$ cm$^{-2}$.
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We report on the study of optical properties of mist CVD grown alpha Gallium oxide with the observation of excitonic absorption in spectral responsivity measurements. 163 nm of Gallium oxide was grown on sapphire using Gallium acetylacetonate as the starting solution at a substrate temperature of 450 deg C. The film was found to be crystalline and of alpha phase with an on axis full width at half maximum of 92 arcsec as confirmed from X ray diffraction scans. The Taucs plot extracted from absorption spectroscopy exhibited two transitions in the UV regime at 5.3 eV and 5.6 eV, corresponding to excitonic absorption and direct band to band transition respectively. The binding energy of exciton was extracted to be 114 meV from spectral responsivity measurements. Further, metal semiconductor metal photodetectors with lateral inter digitated geometry were fabricated on the film. A sharp band edge was observed at 230 nm in the spectral response with peak responsivity of around 1 Amperes per Watt at a bias of 20 V. The UV to visible rejection ratio was found to be around 100 while the dark current was measured to be around 0.1 nA.
The suitability of Ti as a band gap modifier for $alpha$-Ga$_2$O$_3$ was investigated, taking advantage of the isostructural {alpha}-phases and high band gap difference between Ti$_2$O$_3$ and Ga$_2$O$_3$. Films of Ti:Ga$_2$O$_3$, with a range of Ti concentrations, synthesized by atomic layer deposition on sapphire substrates, were characterized to determine how crystallinity and band gap vary with composition for this alloy. The deposition of crystalline $alpha$-(Ti$_x$Ga$_{1-x}$)$_2$O$_3$ films with up to x~5.3%, was demonstrated. At greater Ti concentration, the films became amorphous. Modification of the band gap over a range of ~ 270 meV was achieved across the crystalline films and a maximum change in band gap from pure $alpha$-Ga$_2$O$_3$ of ~1.1 eV was observed for the films of greatest Ti fraction (61% Ti relative to Ga). The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a significant modification in band gap shows promise for band gap engineering and the enhancement in versatility of application of $alpha$-Ga$_2$O$_3$ in optoelectronic devices.
$beta$-Ga$_2$O$_3$ is a next-generation ultra wide bandgap semiconductor (E$_g$ = 4.8 eV to 4.9 eV) that can be homoepitaxially grown on commercial substrates, enabling next-generation power electronic devices among other important applications. Analyzing the quality of deposited homoepitaxial layers used in such devices is challenging, in part due to the large probing depth in traditional x-ray diffraction (XRD) and also due to the surface-sensitive nature of atomic force microscopy (AFM). Here, a combination of evanescent grazing-incidence skew asymmetric XRD and AFM are investigated as an approach to effectively characterize the quality of homoepitaxial $beta$-Ga$_2$O$_3$ layers grown by molecular beam epitaxy at a variety of Ga/O flux ratios. Accounting for both structure and morphology, optimal films are achieved at a Ga/O ratio of $sim$1.15, a conclusion that would not be possible to achieve by either XRD or AFM methods alone. Finally, fabricated Schottky barrier diodes with thicker homoepitaxial layers are characterized by $J-V$ and $C-V$ measurements, revealing an unintentional doping density of 4.3 $times$ 10$^{16}$ cm$^{-3}$ - 2 $times$ 10$^{17}$ cm$^{-3}$ in the epilayer. These results demonstrate the importance of complementary measurement methods for improving the quality of the $beta$-Ga$_2$O$_3$ homoepitaxial layers used in power electronic and other devices.
Ultra-wide bandgap semiconductors are ushering in the next generation of high power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here it is discovered that single-crystalline layers of $alpha$-(AlGa)$_2$O$_3$ alloys spanning bandgaps of 5.4 - 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the $alpha$- to the narrower bandgap $beta$-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth front of the m-plane surface of the crystal, the narrower bandgap pathways are eliminated, revealing a route to much wider bandgap materials with structural purity. The resulting energy bandgaps of the epitaxial layers span a range beyond the reach of all other semiconductor families, heralding the successful epitaxial stabilization of the largest bandgap materials family to date.
We determine the anisotropic dielectric functions of rhombohedral $alpha$-Ga$_2$O$_3$ by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for $alpha$-Al$_2$O$_3$, the binary parent compound for the emerging alloy of $alpha$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single mode or more complex mode behaviors.
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