Semiconductor interfacial carrier dynamics via photoinduced electric fields by Ye Yang, Jing Gu, James L. Young, Elisa M. Miller, John A. Turner, Nathan R. Neale, and Matthew C. Beard Science Volume 350(6264):1061-1065 November 27, 2015 Copyright © 2015, American Association for the Advancement of Science

Fig. 1 Schematic illustration of TPR spectroscopy applied to p-GaInP2. Schematic illustration of TPR spectroscopy applied to p-GaInP2. A broadband probe pulse spanning the semiconductor bandgap is reflected from an interface of interest. An above-bandgap monochromatic pump pulse modulates the reflectance, either via band filling due to the presence of free carriers or via surface field due to charge separation across the interface. For this experiment, the pump pulse frequency was tuned to ensure an optical penetration depth shallower than that of the depletion region so that all of the photoinduced carriers were separated. Ye Yang et al. Science 2015;350:1061-1065 Copyright © 2015, American Association for the Advancement of Science

Fig. 2 TPR spectra. TPR spectra. Pseudocolor image and spectral snapshot of TPR spectra for (A and B) p-GaInP2, (C and D) p-GaInP2/Pt, and (E and F) p-GaInP2/TiO2. Intensities of red and blue in pseudocolor images represent the magnitude of the reduced and increased reflectance, respectively. The blue and red spectra in (B), (D), and (F) are snapshots from the image at 2 ps and 1 ns delays, indicated by the dashed blue and red lines in (A), (C), and (E). For the p-GaInP2 sample, the spectra evolve over time owing to diffusion and surface trapping effects. The oscillations at 2.3 eV [red dash-line boxes in (D) and (F)] are assigned to the transition from the valence band edge to the upper conduction band (fig. S3). The black-dash traces are simulations discussed in the text and detailed in the supplementary material (SM section 3). Ye Yang et al. Science 2015;350:1061-1065 Copyright © 2015, American Association for the Advancement of Science

Fig. 3 Carrier density dependence and TPR kinetics. Carrier density dependence and TPR kinetics. (A) Band-filling–induced and low-field–modulated TPR signal as a function of carrier density for p-GaInP2. (B) FKO amplitude as a function of carrier density plotted on a logarithmic scale. For each pump intensity, the data points represent the peak-to-peak amplitude between the first positive and negative peaks in the spectra at 50 ps delay time. The black dashed lines show a linear dependence on log(N). (C) Kinetics of the band-filling–induced TPR in p-GaInP2 (recorded at 2.2 eV), which represent the dynamics of the surface carrier density. The black dashed trace is a diffusion model discussed in the text. (D) Kinetics of FKO in p-GaInP2/Pt and p-GaInP2/TiO2 (recorded at 1.82 eV), which represent the dynamics of the transient field. The black dashed lines represent a model discussed in the text. All of the kinetic traces are scaled by normalizing the maximum to 1. In (C) and (D), the data are plotted on a linear scale for delays less than 10 ps and on a logarithmic scale from 10 ps to 4 ns (indicated by the vertical line). Ye Yang et al. Science 2015;350:1061-1065 Copyright © 2015, American Association for the Advancement of Science

Fig. 4 Energy band and charge flow diagram. Energy band and charge flow diagram. Band bending and carrier dynamics at the surface or interface for (A) p-GaInP2, (B) p-GaInP2/Pt, and (C) p-GaInP2/TiO2. The energy band positions are determined by XPS and UPS characterization and the optical bandgaps. The energy scale bar (left) is referenced to the p-GaInP2 vacuum level. (A) For p-GaInP2, the photocarriers are initially generated near the surface and are primarily depopulated by diffusion to the bulk and surface trapping. The trapped electron and the remaining hole form a weak transient electric field at the surface, which modulates the reflectance at longer delay times. In contrast, (B) the Schottky (p-GaInP2/Pt) and (C) p-n (p-GaInP2/TiO2) junctions establish a depletion region width larger than the pump penetration depth. These built-in fields accelerate electrons and holes toward Pt/TiO2 and the bulk, respectively. The electron transfers from the depletion region to the interfacial layer within the time resolution (~150 fs) of the experiment, whereas the hole requires several picoseconds to drift out of the depletion region. The charge separation screens the built-in field and forms the FKOs in TPR spectra. Though the charge separation process is similar, the recovery of ΔF arising from charge recombination at the Schottky (p-GaInP2/Pt) junction is faster (by about one order of magnitude) than that at the p-n (p-GaInP2/TiO2) junction. Ye Yang et al. Science 2015;350:1061-1065 Copyright © 2015, American Association for the Advancement of Science