SiO2/Si(001) studied by time-resolved valence band photoemission at MHz repetition rates: Linear and nonlinear excitation of surface photovoltage

Abstract

The authors investigate the fluence and doping dependence of the surface photovoltage (SPV) shifts at SiO2/Si(001) interfaces by time-resolved photoelectron spectroscopy. Charge carriers are excited by pumping photon energies of hνpump=1.2 and 2.4 eV and probed by high-order harmonics of hνprobe=22.6 eV at 0.2 and 0.7 MHz repetition rates. The authors observe SPV shifts of the nonbonding O2p state by 240 meV for SiO2/p-Si and by −140 meV for SiO2/n-Si upon pumping with hνpump=1.2 eV, and their decay rate is estimated from time-resolved measurements. Moreover, the authors observe a striking pumping fluence dependence of SPV at these interfaces, which indicates charge carrier generation by both linear and nonlinear optical excitations.

Carrier Dynamics in the Space Charge Layer of MoS2 Flakes Studied by Time-Resolved μ-Surface Photovoltage

“…Under strong excitation, the SPV value can saturate as more electrons and holes are transported, which results in the flattening of the energy bands. The saturated SPV value is determined by the degree of band bending, and the dependence of the SPV on pump fluence can be fitted using the following expression where k is the Boltzmann constant, T is the absolute temperature, I represents the pump fluence, γ is the fitting coefficient, and the prefactor α is related to the material properties. , High concentration of trapping centers for minority and recombination centers in the SCL can raise the α value to larger than 1 or even larger than 2 Figure b shows the fitting results at different temperatures, where the α value is 1.71 ± 0.4 (140 K), 1.86 ± 0.37 (315 K), and 1.81 ± 0.32 (425 K).…”

“…After the plateau period, all the SPV dynamic curves measured at the various pump fluences converge to the same decay process at the same sample temperature. According to the literature, , the derivative of the SPV with respect to time t can be expressed as where t ∞ is the dark carrier lifetime (that is, the carrier lifetime for SPV = 0); α has the same meaning as in eq ; α′ should be understood as an ideality factor related to the diode theory, and it also accounts for carrier recombination as the charge carriers cross the SCL. Based on the assumption α kT ≪ SPV, eq can be solved as where SPV 0 represents the initial SPV value.…”

“…After the plateau period, all the SPV dynamic curves measured at the various pump fluences converge to the same decay process at the same sample temperature. According to the literature, 9,27 the derivative of the SPV with respect to time t can be expressed as…”

“…(1) where k is the Boltzmann constant, T is the absolute temperature, I represents the pump fluence, γ is the fitting coefficient, and the prefactor α is related to the material properties. 9,27 High concentration of trapping centers for minority and recombination centers in the SCL can raise the α value to larger than 1 or even larger than 2. 28 Figure 1b shows the fitting results at different temperatures, where the α value is 1.71 ± 0.4 (140 K), 1.86 ± 0.37 (315 K), and 1.81 ± 0.32 (425 K).…”