Oscillatory relaxation of surface photovoltage on a silicon surface

Time-resolved measurement of photoemission spectroscopy was made to trace a change of surface potential after the surface photovoltage effect on a Si(111) $7\times 7$ surface. Two relaxation processes were found with decay times of nanoseconds and hundreds of nanoseconds, which are explained in terms of the tunneling and the thermionic relaxation schemes, respectively. At the high laser power density, the relaxation has become oscillatory with a temporal period of several tens of nanoseconds.

Figure 1

(a) Si $2p$ core-level spectra ($h\nu =253$ eV) before and after the laser irradiation, shown as solid (red) and dashed (blue) lines, respectively. The power density of the pump laser ($h\nu =1.51$ eV) was 2700 $\mu$J cm${}^{-2}$ pulse${}^{-1}$. (b) Energy shift of the Si 2$p$ spectra, measured at different power densities. The delay time between the pump laser and probe synchrotron pulses was set at 1 ns.

Figure 2

Time evolution of the Si $2p$ spectra, taken at different laser power densities per pulse: (a) 3.1, (b) 560, and (c) 2700 $\mu$J cm${}^{-2}$ pulse${}^{-1}$, corresponding to stages I, II, and III, respectively, in Fig. 1b. The vertical axis represents relative binding energy with respect to the Si $2p_{3/2}$ peak position before the photoexcitation. The open circles correspond to data points. The background is a grayscale photoemission diagram whose points in the region between data points are linearly interpolated. The solid line in (c) is the result of fitting to Eq. (1).

Figure 3

Energy shift of the Si $2p_{3/2}$ core-level peak of a Si(111) $7\times 7$ surface with various delay times after laser irradiation. The power densities of the pumping laser are (a) 3.1, (b) 560, and (c) 2700 $\mu$J cm${}^{-2}$ pulse${}^{-1}$, which are indicated in Fig. 1 as I, II, and III, respectively. The experimental data, marked with circles, are parameter fitted with the functions, $V_{\mathrm{tunnel}}\left(t\right)$ in Eq. (4) (black dashed lines) and $V_{\mathrm{thermal}}\left(t\right)$ in Eq. (14) (red solid lines); ${\tau }_{\mathrm{tunnel}}$ and ${\tau }_s$ are the fitted parameters of the time constants of the tunneling and thermionic relaxation models, respectively.