Ionization Energies of Shallow Donor States in ZnO Created by Reversible Formation and Depletion of H Interstitials

The electronic effects of H atoms at interstitial sites in ZnO have been investigated by high resolution electron energy loss spectroscopy (HREELS). A reversible doping is achieved by exposing single crystalline $(000\overline{1})$-oriented ZnO substrates to atomic hydrogen. At low temperatures, interstitial H atoms form shallow donor states. At sufficiently high temperatures, the electrons are excited into the conduction band. We use EELS to demonstrate the presence of plasmons resulting from this finite density of charge carriers in the conduction band. Above temperatures of 100 K, a strong, plasmon-induced broadening of the quasielastic peak in the HREELS data is observed. The analysis of the temperature dependence yields a donor level ionization energy of $25\pm 5\mathrm{meV}$.

Figure 1

HREEL spectra of A a clean $\mathrm{ZnO}(000\overline{1})$ surface, B the HO-ZnO surface prepared by exposure of the clean surface to 2 L ${\mathrm{H}}_2\mathrm{O}$ at room temperature, and C exposure of the HO-ZnO surface to 1000 L atomic hydrogen at 300 K. All spectra were taken at room temperature in specular geometry with an incidence angle of 55° with respect to the surface normal and with a primary energy of 10 eV.

Figure 2

(a) Thermal desorption spectra for $m/e=2$, representing ${\mathrm{H}}_2$, of the HO-ZnO surface (prepared by exposure to ${\mathrm{H}}_2\mathrm{O}$) before (solid cycles) and after (hollow cycles) exposure to 1000 L atomic H. (b) The same as (a) but for $m/e=18$, representing water. The heating rate was $1.5\mathrm{K}/\mathrm{s}$.

Figure 3

(a) FWHM of the quasielastic peak $\Delta$ in the HREEL spectra (measured at RT) as a function of annealing temperature $T_a$ after exposing the HO-ZnO surface to 1000 L atomic hydrogen at 300 K. (b) $d({\Delta }^2)/dT$ as a function of the annealing temperature. Annealing rate: $1.5\mathrm{K}/\mathrm{s}$; total annealing time: $<2\min $.

Figure 4

The value of ${\Delta }^2$ as a function of different measurement temperatures $T_m$ for the HO-ZnO surface exposed to 1000 L atomic hydrogen at 100 K. The dashed line has been obtained using Eq. (1) assuming a temperature-independent density of carriers $n_c$. The solid line is the result of a nonlinear least squares fit assuming an ionization energy of the H-induced shallow donor of ${\epsilon }_d=25\mathrm{meV}$ (see text).