Abstract
The addition of a metal overlayer to a semiconductor photocatalyst is a frequently used synthetic route to passivate the surface and, via the formation of a Schottky barrier, to enhance catalytic activity of the photocatalyst material. While it is known that Schottky junctions decrease recombination by charge separation, measurements of the depletion region dynamics have remained elusive. Here, we use ultrafast pump-probe transient photoelectron spectroscopy to measure material-specific dynamics of the Zn/n-GaP(100) system. Through photoemission measurements the Schottky barrier height is determined to be 2.1 ± 0.1 eV at 10 monolayers of total Zn deposition. Transient photoemission measurements utilizing a 400 nm pump pulse show that, after excitation, holes are transferred from n-GaP(100) to the Zn overlayer within a few ps, as evidenced by shifts of the Zn 3d and Ga 3d core levels to higher binding energies. Within the timescale of the experiment (130 ps) no carrier recombination is observed in the junction. Furthermore, a long-lived surface photovoltage signal is observed at times >1 ms after photoexcitation. This work further exemplifies the potential of transient extreme ultraviolet photoelectron spectroscopy as a material-specific technique for the study of heterojunctions.
Highlights
Gallium phosphide, a semiconducting material with an indirect band gap of 2.26 eV, has received much attention for its potential applications in optics, electronics, and photocatalysis.1 Of particular importance for photocatalysis is the ability of GaP to retain its original surface structure and electronic properties while operating in an aqueous solution
Transient photoemission measurements utilizing a 400 nm pump pulse show that, after excitation, holes are transferred from n-GaP(100) to the Zn overlayer within a few ps, as evidenced by shifts of the Zn 3d and Ga 3d core levels to higher binding energies
XUV-PES spectra reveal that deposition of Zn results in an additional 0.5 eV of band bending from the clean n-GaP surface, giving a Schottky barrier height of 2.1 eV and showed a 1.5 eV surface barrier height in clean n-GaP
Summary
A semiconducting material with an indirect band gap of 2.26 eV, has received much attention for its potential applications in optics, electronics, and photocatalysis. Of particular importance for photocatalysis is the ability of GaP to retain its original surface structure and electronic properties while operating in an aqueous solution. Incorporation of a time-delayed UV/Vis laser pulse in conjunction with the XUV probe pulse This technique has been previously used to observe the dynamics of electron transport in defect rich and defect poor TiO2 films on p-Si(100) as well as Zn layers on p-Si(100).. Through the use of transient XUV-PES, material-specific changes in the surface photovoltage of the Zn/nGaP(100) system are observed in the overlayer (substrate) through energy shifts in the position of the Zn 3d (Ga 3d) The electronic properties, such as Fermi level pinning and carrier transport, which are observed and discussed in the transient XUV-PES measurements, serve to further the understanding of carrier dynamics within the depletion region of metal-semiconductor heterojunction photocatalytic systems
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