We used temperature-resolved cathodoluminescence to determine the characteristics of luminescence bands and carrier dynamics in edge-defined film-fed grown (EFG) beta-Ga2O3 single crystals synthesized by Tamura Corporation. The crystal is nominally undoped and has a (-201) surface orientation. The main impurities are Si, Ir, Al and Fe, with [Fe] ~ 10^17 cm-3 verified by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The CL emission was found to be dominated by a broad UV emission peaked at 3.40 eV, which exhibits strong quenching with increasing temperature; however, its spectral shape and energy position remain virtually unchanged up to 500 K. Depth-resolved analysis reveals the luminescence spectrum is independent of sampling depth. We observed a super-linear increase of CL intensity with excitation density; this kinetics of carrier recombination can be explained in terms of carrier trapping and charge transfer at Fe3+/2+ centers. The temperature-dependent properties of this UV band were found to be consistent with weakly bound electrons in self-trapped excitons with an activation energy of 48 +/- 10 meV. In addition to the self-trapped exciton emission, a blue luminescence (BL) band is shown to be related to a donor-like defect, which increases significantly in concentration after remote hydrogen plasma treatment. The point defect responsible for the BL, likely an oxygen vacancy or a complex, is strongly coupled to the lattice with a Huang-Rhys factor S = 7.3.
Nominally-undoped Ga<sub>2</sub>O<sub>3</sub> layers were deposited on <i>a</i>-, <i>c</i>- and <i>r</i>-plane sapphire substrates using pulsed laser deposition. Conventional x-ray diffraction analysis for films grown on <i>a</i>- and <i>c</i>-plane sapphire showed the layers to be in the β-Ga<sub>2</sub>O<sub>3</sub> phase with preferential orientation of the (-201) axis along the growth direction. Pole figures revealed the film grown on r-plane sapphire to also be in theβ-Ga<sub>2</sub>O<sub>3</sub> phase but with epitaxial offsets of 29.5°, 38.5° and 64° from the growth direction for the (-201) axis. Optical transmission spectroscopy indicated that the bandgap was ~5.2eV, for all the layers and that the transparency was > 80% in the visible wavelength range. Four point collinear resistivity and Van der Pauw based Hall measurements revealed the β-Ga<sub>2</sub>O<sub>3</sub> layer on <i>r</i>-plane sapphire to be 4 orders of magnitude more conducting than layers grown on <i>a</i>- and <i>c</i>-plane sapphire under similar conditions. The absolute values of conductivity, carrier mobility and carrier concentration for the β-Ga<sub>2</sub>O<sub>3</sub> layer on <i>r</i>-sapphire (at 20Ω<sup>-1</sup>.cm<sup>-1</sup>, 6 cm<sup>2</sup>/Vs and 1.7 x 10<sup>19</sup> cm<sup>-3</sup>, respectively) all exceeded values found in the literature for nominally-undoped β-Ga<sub>2</sub>O<sub>3</sub> thin films by at least an order of magnitude. Gas discharge optical emission spectroscopy compositional depth profiling for common shallow donor impurities (Cl, F, Si and Sn) did not indicate any discernable increase in their concentrations compared to background levels in the sapphire substrate. It is proposed that the fundamentally anisotropic conductivity in β-Ga<sub>2</sub>O<sub>3</sub> combined with the epitaxial offset of the (-201) axis observed for the layer grown on <i>r</i>-plane sapphire may explain the much larger carrier concentration, electrical conductivity and mobility compared with layers having the (-201) axis aligned along the growth direction.
We investigated Si nanocrystal samples produced by high dose 600 keV Si+ implantation of fused silica and annealing using cathodoluminescence (CL). CL spectra collected under 5-25 keV electron irradiation show similar features to reported photoluminescence spectra, including the strong near IR peak. The CL intensity distribution is formulated as a linear inverse problem and two methods namely the regularisation method and maximum entropy method can be applied to determine the depth profile without making any assumptions concerning the profile function, i.e. a free form solution. We show using simulated CL data that the maximum entropy method is the most appropriate as it preserves the positivity and additivity of the depth profile. This method is applied to experimental CL data and we have localised the spatial origin of the near IR emission to the near-surface region of the implant, 400 nm from the surface, containing the smallest Si nanocrystals.