This presentation will discuss recent studies on the fabrication and characterization of b-Ga2O3 containing Ge donors or N acceptors. A plasma source was used to dope Ga2O3 nanowires with N by exploiting their nanoscale cross sections, while bulk crystals were uniformly doped with Ge by neutron irradiation. The dopant incorporation was confirmed by chemical analyses. We find defect-related luminescence is strongly enhanced in N-doped Ga2O3, which likely originates from defect compensation effects. With Ge doping, both the UV band due to self-trapped holes (STHs) and defect-related emission increase following neutron irradiation, suggesting STHs being localized close to a defect site.
Monoclinic β-Ga2O3 nanowires were fabricated using chemical vapor deposition and doped with nitrogen using remote plasma. The monoclinic phase and high crystallinity of the nanowires were confirmed by XRD and TEM, while nitrogen incorporation into the nanowires was confirmed by X-ray absorption and Raman spectroscopies. Temperature-resolved cathodoluminescence characterisation revealed a green luminescence band at 2.5 eV due to the N incorporation. The UV emission at 3.4 eV associated with self-trapped holes and its temperature-dependant behaviour were found to be identical for the undoped and N-doped nanowires. The experimental findings will be discussed in context of theoretical calculations for N-doped Ga2O3.
We report the characteristics of luminescence bands in beta-Ga2O3 thin films and single crystals. The dominant UV emission at 3.4 eV exhibits strong thermal quenching but its peak shape remains unchanged. The blue and green bands, attributed to defects, are found to be strongly dependent on growth conditions. Additionally, we observe a distinct red luminescence at 1.9 eV upon hydrogen doping. The emergence of this emission is accompanied by substantially increased electrical conductivity. The red emission is shown to be consistent with shallow donor–deep acceptor pair recombination and will be discussed in the context of defect models.
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 Ga2O3 layers were deposited on a-, c- and r-plane sapphire substrates using pulsed laser deposition. Conventional x-ray diffraction analysis for films grown on a- and c-plane sapphire showed the layers to be in the β-Ga2O3 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-Ga2O3 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-Ga2O3 layer on r-plane sapphire to be 4 orders of magnitude more conducting than layers grown on a- and c-plane sapphire under similar conditions. The absolute values of conductivity, carrier mobility and carrier concentration for the β-Ga2O3 layer on r-sapphire (at 20Ω-1.cm-1, 6 cm2/Vs and 1.7 x 1019 cm-3, respectively) all exceeded values found in the literature for nominally-undoped β-Ga2O3 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-Ga2O3 combined with the epitaxial offset of the (-201) axis observed for the layer grown on r-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.
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