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Register a new userTi alloyed $alpha$-Ga$_2$O$_3$: route towards wide band gap engineering
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Added by
Fabien Massabuau Dr
Publication date
2020
fields
Physics
and research's language is
English
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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.
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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$^{-2}$.
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.
$beta$-Ga$_2$O$_3$ is an ultra-wide bandgap semiconductor and is thus expected to be optically transparent to light of sub-bandgap wavelengths well into the ultraviolet. Contrary to this expectation, it is found here that free electrons in n-doped $beta$-Ga$_2$O$_3$ absorb light from the IR to the UV wavelength range via intra- and inter-conduction band optical transitions. Intra-conduction band absorption occurs via an indirect optical phonon mediated process with a $1/omega^{3}$ dependence in the visible to near-IR wavelength range. This frequency dependence markedly differs from the $1/omega^{2}$ dependence predicted by the Drude model of free-carrier absorption. The inter-conduction band absorption between the lowest conduction band and a higher conduction band occurs via a direct optical process at $lambda sim 349$ nm (3.55 eV). Steady state and ultrafast optical spectroscopy measurements unambiguously identify both these absorption processes and enable quantitative measurements of the inter-conduction band energy, and the frequency dependence of absorption. Whereas the intra-conduction band absorption does not depend on light polarization, inter-conduction band absorption is found to be strongly polarization dependent. The experimental observations, in excellent agreement with recent theoretical predictions for $beta$-Ga$_2$O$_3$, provide important limits of sub-bandgap transparency for optoelectronics in the deep-UV to visible wavelength range, and are also of importance for high electric field transport effects in this emerging semiconductor.
The epitaxial growth of technically-important $beta$-Ga$_2$O$_3$ semiconductor thin films have not been realized on flexible substrates due to limitations by the high-temperature crystallization conditions and the lattice-matching requirements. In this report, for the first time single crystal $beta$-Ga$_2$O$_3$(-201) thin films is epitaxially grown on the flexible CeO2 (001)-buffered hastelloy tape. The results indicate that CeO$_2$ (001) has a small bi-axial lattice mismatch with $beta$-Ga$_2$O$_3$ (-201), thus inducing a simultaneous double-domain epitaxial growth. Flexible photodetectors are fabricated based on the epitaxial $beta$-Ga$_2$O$_3$ coated tapes. Measurements show that the obtained photodetectors have a responsivity of 40 mA/W, with an on/off ratio reaching 1000 under 250 nm incident light and 5 V bias voltage. Such photoelectrical performance is already within the mainstream level of the $beta$-Ga$_2$O$_3$ based photodetectors by using the conventional rigid single crystal substrates; and more importantly remained robust against more than 1000 cycles of bending tests. In addition, the epitaxy technique described in the report also paves the way for the fabrication of a wide range of flexible epitaxial film devices that utilize the materials with lattice parameters similar to $beta$-Ga$_2$O$_3$, including GaN, AlN and SiC.
$beta$-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.
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