Enhancement of excitonic and defect-related luminescence in neutron transmutation doped β−Ga2O3

Neutron irradiation analysis, inductively coupled plasma mass spectrometry (ICPMS), and cathodoluminescence (CL) spectroscopy are used to investigate the influence of transmuted Ge incorporation on the luminescence properties of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ single crystals. Calculations based on ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-neutron interaction reveal temporal variations of both Ge and Zn concentrations as a function of time during and after neutron irradiation. To produce a concentration of $5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{Ge}\phantom{\rule{0.16em}{0ex}}\mathrm{donors}/\mathrm{c}{\mathrm{m}}^{3}$ from the neutron transmutation of Ga, the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystal was irradiated for 27 h, which was accompanied by the incorporation of ${10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{Zn}\phantom{\rule{0.16em}{0ex}}\mathrm{acceptors}/\mathrm{c}{\mathrm{m}}^{3}$. These calculated dopant concentrations are confirmed by ICPMS. The $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystals exhibit a UV band at 3.40 eV due to self-trapped holes (STHs) and two blue donor-acceptor pair (DAP) peaks at 3.14 eV (BL1) and 2.92 eV (BL2). In addition to the neutron-induced incorporation of substitutional Ge donors and Zn acceptors on Ga sites, Ga vacancies (${V}_{\mathrm{Ga}}$) were created by high-energy neutrons in the flux, which strongly enhanced the BL1 peak. The ${V}_{\mathrm{Ga}}$ acceptors compensate the neutron-induced Ge donors, making the ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystal highly resistive. Concurrent temperature-resolved CL measurements of the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ before and after neutron irradiation reveal a twofold increase in both the STH and BL1 peaks. This result suggests that STHs are preferentially localized at an O site adjacent to ${V}_{\mathrm{Ga}}$, as theoretically predicted by Kananen et al. [Appl. Phys. Lett. 110, 202104 (2017).]. Analysis of the ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ CL temperature dependence reveals that the UV and BL1 bands after the neutron irradiation exhibit an equivalent activation energy of $100\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ due to the presence of a neutron-induced defect that acts as an efficient competitive nonradiative recombination channel. The results also provide evidence that the BL1 and BL2 bands arise from different DAP pairs.