Phonon- and charged-impurity-assisted indirect free-carrier absorption in Ga2O3

Monoclinic β−Ga2O3 has a large band gap of 4.8 eV, and can therefore be used as a contact material that is transparent to visible and UV light. However, indirect free-carrier absorption processes, mediated by either phonons or charged impurities, will set a fundamental limit on transparency. We use first-principles calculations to accurately assess the absorption cross section and to elucidate the microscopic origins of these processes. Phonon-assisted absorption is dominated by the emission of phonons, and is therefore always possible. This indirect absorption is inversely proportional to the cube of the wavelength. The presence of charged impurities, whether intentional or unintentional, leads to additional absorption, but for realistic concentrations, phonon-assisted absorption remains the largest contribution. Direct free-carrier absorption also leads to below-gap absorption, with distinct peaks where optical transitions match energy differences to higher conduction bands. In contrast, indirect absorption uniformly reduces transparency for all sub-band-gap wavelengths

First-principles study of electron-phonon interactions and transport in anatase TiO2

Electron transport in anatase TiO2, which has important applications in oxide electronics and photocatalysis, is still poorly understood. We investigate the electron mobility in anatase TiO2 by performing first-principles calculations of electron and phonon spectra as well as electron-phonon coupling. The formation of large polarons (quasiparticles formed by electrons interacting with phonons in a polar medium) leads to a renormalization of the electronic band structure, which we address using many-body perturbation theory. We correlate the lowering of the mobility of these quasiparticles to the renormalization of band velocities due to the electron-phonon interaction. These results explain why the mobility decreases with increasing temperature, as observed in experiments

Limitations of In2O3 as a transparent conducting oxide

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 115, 082105 (2019); doi: 10.1063/1.5109569 and may be found at https://aip.scitation.org/doi/full/10.1063/1.5109569.Sn-doped In2O3 or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 1020 cm–3. The differences in the photon-energy dependence of the two processes can be explained by band structure effects

Ab initio study of enhanced thermal conductivity in ordered AlGaO3 alloys

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 115, 242103 (2019) and may be found at https://aip.scitation.org/doi/10.1063/1.5131755.We compute the lattice thermal conductivity of monoclinic β-Ga2O3 and the ordered AlGaO3 alloy from the phonon Boltzmann transport equation, with the harmonic and third-order anharmonic force constants calculated from density functional theory. The calculated thermal conductivity of β-Ga2O3 is consistent with experiment. We demonstrate that the lowest-energy structure of an Al0.5Ga0.5 alloy, which is ordered, has a thermal conductivity that is raised by more than 70% compared to β-Ga2O3. We attribute the enhancement to (1) increased group velocities and (2) reduced anharmonic scattering rates due to the reduced weighted phase space. The findings offer an avenue toward improved heat dissipation from Ga2O3 devices. The authors acknowledge Shengying Yue, Jingjing Shi, Samuel Graham, and Yuewei Zhang for fruitful discussions. This work was supported by the GAME MURI of the Air Force Office of Scientific Research (No. FA9550-18-1-0479). Computing resources were provided by the Center for Scientific Computing supported by the California NanoSystems Institute and the Materials Research Science and Engineering Center (MRSEC) at UC Santa Barbara through the National Science Foundation (NSF) (Nos. DMR-1720256 and CNS-1725797) and by the Extreme Science and Engineering Discovery Environment (XSEDE), which was supported by NSF Grant No. ACI-1548562

First-principles surface energies for monoclinic Ga2O3 and Al2O3 and consequences for cracking of (AlxGa1−x)2O3

Crack formation limits the growth of (AlxGa1−x)2O3 epitaxial films on Ga2O3 substrates. We employ first-principles calculations to determine the brittle fracture toughness of such films for three growth orientations of the monoclinic structure: [100], [010], and [001]. Surface energies and elastic constants are computed for the end compounds—monoclinic Ga2O3 and Al2O3—and used to interpolate to (AlxGa1−x)2O3 alloys. The appropriate crack plane for each orientation is determined, and the corresponding critical thicknesses are calculated based on Griffith’s theory, which relies on the balance between elastic energy and surface energy. We obtain lower bounds for the critical thickness, which compare well with available experiments. We also perform an in-depth analysis of surface energies for both relaxed and unrelaxed surfaces, providing important insights into the factors that determine the relative stability of different surfaces. Our study provides physical insights into surface stability, crack planes, and the different degrees of crack formation in (AlxGa1−x)2O3 films for different growth orientations

First-principles calculations of hyperfine interaction, binding energy, and quadrupole coupling for shallow donors in silicon

Spin qubits based on shallow donors in silicon are a promising quantum information technology with enormous potential scalability due to the existence of robust silicon-processing infrastructure. However, the most accurate theories of donor electronic structure lack predictive power because of their reliance on empirical fitting parameters, while predictive ab initio methods have so far been lacking in accuracy due to size of the donor wavefunction compared to typical simulation cells. We show that density functional theory with hybrid and traditional functionals working in tandem can bridge this gap. Our first-principles approach allows remarkable accuracy in binding energies (67 meV for bismuth and 54 meV for arsenic) without the use of empirical fitting. We also obtain reasonable hyperfine parameters (1263 MHz for Bi and 133 MHz for As) and superhyperfine parameters. We demonstrate the importance of a predictive model by showing that hydrostatic strain has much larger effect on the hyperfine structure than predicted by effective mass theory, and by elucidating the underlying mechanisms through symmetry analysis of the shallow donor charge density