Red luminescence in H-doped $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$

The effects of hydrogen incorporation into $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ thin films have been investigated by chemical, electrical, and optical characterization techniques. Hydrogen incorporation was achieved by remote plasma doping without any structural alterations of the film; however, x-ray photoemission reveals major changes in the oxygen chemical environment. Depth-resolved cathodoluminescence (CL) reveals that the near-surface region of the H-doped $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ film exhibits a distinct red luminescence (RL) band at 1.9 eV. The emergence of the H-related RL band is accompanied by an enhancement in the electrical conductivity of the film by an order of magnitude. Temperature-resolved CL points to the formation of abundant H-related donors with a binding energy of $28\pm 4\mathrm{meV}$. The RL emission is attributed to shallow donor-deep acceptor pair recombination, where the acceptor is a $V_{\mathrm{Ga}}$-H complex and the shallow donor is interstitial H. The binding energy of the $V_{\mathrm{Ga}}$-H complex, based on our experimental considerations, is consistent with the computational results by Varley et al., [J. Phys.: Condens. Matter 23, 334212 (2011)].

Figure 1

XRD $2\theta /\omega$ pattern for the $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ film grown on a $c$-plane sapphire substrate, showing three diffraction peaks corresponding to the (−201), (−402), and (−603) reflections of (−201) oriented $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. Inset: ω rocking curve for the (−201) XRD peak with a FWHM of $0.{21}^{\circ }$.

Figure 2

(a) AFM images of the $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ film before and after the H doping. The rms roughnesses are 6.4 and 6.9 nm for the as-grown and H-doped films, respectively. (b) Sheet resistance vs temperature for the as-grown (undoped) and H-doped $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ films, showing an increase of the electrical conductivity by $\sim 10$ times due to the H doping. (c) O $1s$ XPS spectra for the undoped and H-doped $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ films, showing a shift of the peak position from 532.3 to 533.4 eV resulting from the capture of H by O atoms in the near-surface region. The spectra can be fitted with two Voigt functions denoted as O–Ga and –OH; the dashed curves are the theoretical fits.

Figure 3

(a) Temperature-resolved CL spectra for H-doped $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ showing two emission bands: a UV one at $\sim 3.3\mathrm{eV}$ and a RL one at $\sim 1.9\mathrm{eV}$. The RL is completely quenched at $T>340\mathrm{K}$. (b) Arrhenius analysis of the UV and RL integrated intensities yielding activation energies: $E_{\mathrm{a}}\left(\mathrm{UV}\right)=94\pm 7\mathrm{meV}$ and $E_{\mathrm{a}}\left(\mathrm{RL}\right)=28\pm 4\mathrm{meV}$ for the H-doped $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$, and $E_{\mathrm{a}}\left(\mathrm{UV}\right)=66\pm 4\mathrm{meV}$ for the undoped $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. (c) Dependence of UV and RL intensities on excitation beam current with $V_{\mathrm{b}}=1.5\mathrm{kV}$ and $T=80\mathrm{K}$. A power-law $(I_{\mathrm{CL}}\propto {I_{\mathrm{B}}}^{\mathrm{k}})$ fit reveals a linear dependence of the UV band and sublinear dependence of the RL band with $k\left(\mathrm{RL}\right)=0.6\pm 0.2$.