Dielectric Function Tensor (1.5 eV to 9.0 eV), Anisotropy, and Band to Band Transitions of Monoclinic \u3cem\u3eβ\u3c/em\u3e-(Al\u3cem\u3e\u3csub\u3ex\u3c/sub\u3e\u3c/em\u3eGa\u3csub\u3e1–\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e (x ≤ 0.21) Films

A set of monoclinic β-(AlxGa1–x)2O3 films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented β-Ga2O3 substrates for compositions x ≤ 0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5 eV–9.0 eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for β-Ga2O3 from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in accordance with the much larger fundamental band to band transition energies of Al2O3 regardless of crystal symmetry. The observed increase in the lowest band to band transition energy is in excellent agreement with recent theoretical predictions. The most important observation is that charge confinement in heterostructures will strongly depend on the growth condition due to the strongly anisotropic properties of the band to band transitions

Role of defects in ultra-high gain in fast planar tin gallium oxide UV-C photodetector by MBE

We report ultra-high responsivity of epitaxial (SnxGa1-x)2O3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (-201) oriented n-type β-Ga2O3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5×104 A/W at -5V applied bias under 250nm illumination with sharp cutoff shorter than 280nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (2.20 eV) not seen in β-Ga2O3 along with enhancement of the blue emission-band (2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β-Ga2O3, VGa–Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga2O3 devices by means of thermionic emission and electron tunneling. Regenerating the VGa–Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity

Plasma-assisted Molecular Beam Epitaxy of Beta-Ga2O3: Growth, Doping, and Heterostructures

As conventional semiconductors reach their materials limits for modern high power switching applications, we must look towards new materials systems. Ultrawide bandgap semiconductors provide opportunities for future efficient high voltage switches due to their ability to withstand high electric fields. In particular, β-Ga2O3 shows promise due to its high critical electric field (6-8 MV/cm), availability of high-quality melt grown bulk substrates, and donor and deep acceptor doping possibilities. This work focuses on growth and doping of β-Ga2O3 and its alloys via plasma-assisted molecular beam epitaxy (PAMBE). Conventional PAMBE shows promise for (010) β-Ga2O3 growth, however other crystallographic orientations have lower growth rates and poor film quality due to significant suboxide desorption during growth. An indium catalyzed growth mechanism using an additional indium flux during PAMBE growth of β-Ga2O3 is demonstrated, allowing for significantly improved growth rates across various crystallographic orientations. This metal oxide catalyzed epitaxy (MOCATAXY) allows for improved film quality, demonstrated by minimal extended defects and smoother surface morphologies, particularly for (001) β-Ga2O3. The supplied In flux during MOCATAXY growth acts as a catalyst, allowing for growth at high growth temperatures and Ga fluxes for which growth would not occur for conventional PAMBE. This In limits suboxide desorption during growth and does not incorporate into the film for sufficiently Ga rich growth conditions. Donor doping is necessary for achieving a variety of device designs, with its use in contacts, channels, modulation doping, and drift regions. Donor doping with Ge, Sn, and Si is demonstrated for β-Ga2O3 grown on various orientations. While Ge doping can be used for a range of concentrations for conventional PAMBE, at higher growth temperatures and Ga fluxes its incorporation decreases, limiting its use for MOCATAXY. Sn shows the ability to achieve high doping concentrations in conventional PAMBE, however surface segregation during growth and a delay in incorporation into the film is observed for lower concentrations. Sn doping during MOCATAXY growth, however, allows for sharp, controllable doping profiles for a variety of Sn concentrations across various orientations. Furthermore, Sn doping of (010) β-Ga2O3 via MOCATAXY demonstrates the highest electron mobility for continuously doped β-Ga2O3 grown via MBE. Sn doping of (001) β-Ga2O3 via MOCATAXY shows significantly higher electron mobility than conventional PAMBE. Si doping is also investigated, showing degradation of (010) β-Ga2O3 film quality, however promising electron mobility and high doping concentrations were achievable for (001) oriented growth via MOCATAXY. Deep acceptor doping allows for realization of potential barriers in β-Ga2O3, as well semi-insulating regions of the device, such as current blocking layers for vertical structures and an intentionally compensated film-substrate interface for lateral devices. Mg is investigated as an intentional dopant in conventional PAMBE growth of (010) β-Ga2O3. While sharp doping profiles and a range of doping concentrations are achievable, annealing at high temperatures (≥ 925 °C) allows for diffusion of Mg, limiting its application to lower growth temperature epitaxial techniques and processing steps. A mechanism of Mg diffusion via the mobile Mg interstitial species is proposed, involving interactions of point defects in the film during annealing. Additionally, Fe incorporation into β-Ga2O3 films grown on Fe doped substrates is shown to be the result of surface segregation, rather than diffusion. This incorporation can be limited using a low temperature Fe trapping buffer layer prior to growth of critical regions of the film structure. Finally, growth of heterostructures with β-(AlxGa1-x)2O3 via MOCATAXY is investigated. Maximum Al contents for (010) β-(AlxGa1-x)2O3 of 22% are achieved with high quality, coherently strained films. (001) β-(AlxGa1-x)2O3 films with Al contents up to 15% are also grown with smooth surface morphology and no evidence of extended defects or relaxation. A relationship between out of plane lattice parameter is derived using the fundamental stiffness tensor and stress and strain expressions for (001) β-(AlxGa1-x)2O3 coherently strained to the β-Ga2O3. Confirmation of the Al content in the films confirms the validity of the derived relationship. This demonstration of high quality (001) β-(AlxGa1-x)2O3 shows promise for future heterostructure based devices in this orientation

Continuous Si doping in (010) and (001) β-Ga2O3 films by plasma-assisted molecular beam epitaxy

We report the continuous Si doping in β-Ga2O3 epitaxial films grown by plasma-assisted molecular beam epitaxy through the use of a valved effusion cell for the Si source. Secondary ion mass spectroscopy results exhibit that the Si doping profiles in β-Ga2O3 are flat and have sharp turn-on/off depth profiles. The Si doping concentration was able to be controlled by either varying the cell temperatures or changing the aperture of the valve of the Si effusion cell. High crystal quality and smooth surface morphologies were confirmed on Si-doped β-Ga2O3 epitaxial films grown on (010) and (001) substrates. The electronic properties of Si-doped (001) β-Ga2O3 epitaxial film showed an electron mobility of 67 cm2/Vs at the Hall concentration of 3 × 1018 cm−3

MOCVD grown epitaxial β-Ga2O3 thin film with an electron mobility of 176 cm2/V s at room temperature

In this work, we report record electron mobility values in unintentionally doped β-Ga2O3 films grown by metal-organic chemical vapor deposition. Using degenerately Sn-doped regrown n+ β-Ga2O3 contact layers, we were able to maintain Ohmic contact to the β-Ga2O3 films down to 40 K, allowing for reliable temperature-dependent Hall measurement. An electron mobility of 176 cm2/V s and 3481 cm2/V s were measured at room temperature and 54 K, respectively. The room and low temperature mobilities are both among the highest reported values in a bulk β-Ga2O3 film. A low net background charge concentration of 7.4 × 1015 cm−3 was confirmed by both temperature dependent Hall measurement and capacitance-voltage measurement. The feasibility of achieving low background impurity concentration and high electron mobility paves the road for the demonstration of high performance power electronics with high breakdown voltages and low on-resistances

Solar blind Schottky photodiode based on an MOCVD-grown homoepitaxial β-Ga2O3 thin film

We report on a high performance Pt/n−Ga2O3/n+Ga2O3 solar blind Schottky photodiode that has been grown by metalorganic chemical vapor deposition. The active area of the photodiode was fabricated using ∼30 Å thick semi-transparent Pt that has up to 90% transparency to UV radiation with wavelengths < 260 nm. The fabricated photodiode exhibited Schottky characteristics with a turn-on voltage of ∼1 V and a rectification ratio of ∼108 at ±2 V and showed deep UV solar blind detection at 0 V. The Schottky photodiode exhibited good device characteristics such as an ideality factor of 1.23 and a breakdown voltage of ∼110 V. The spectral response showed a maximum absolute responsivity of 0.16 A/W at 222 nm at zero bias corresponding to an external quantum efficiency of ∼87.5%. The cutoff wavelength and the out of band rejection ratio of the devices were ∼260 nm and ∼104, respectively, showing a true solar blind operation with an excellent selectivity. The time response is in the millisecond range and has no long-time decay component which is common in photoconductive wide bandgap devices

Strain and Composition Dependencies of the Near-Band-Gap Optical Transitions in Monoclinic (AlxGa1−x)2O3 Alloys with Coherent Biaxial In-Plane Strain on Ga2O3(010)

The bowing of the energy of the three lowest band-to-band transitions in β−(AlxGa1−x)2O3 alloys is resolved using a combined density-functional theory (DFT) and generalized spectroscopic ellipsometry approach. The DFT calculations of the electronic band structure of both β−Ga2O3 and θ−Al2O3 allow the linear portion of the energy shift in the alloys to be extracted, and provide a method for quantifying the role of coherent strain present in the β−(AlxGa1−x)2O3 thin films on (010) β−Ga2O3 substrates. The energies of band-to-band transitions are obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain, which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic β−(AlxGa1−x)2O3 are found