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Isolating spectral signatures and/or the carrier dynamics that are specific to semiconductor junctions and not just the interface or bulk is challenging. Junctions that form between a semiconductor surface and a contacting layer are the key to their function. Equilibration of chemical potential at such junctions creates an internal electric field and establishes a region where mobile charges are driven away (depletion region). Absorption of light produces electrons and holes within the depletion region where the charges are separated. We developed transient photoreflectance (TPR) as an innovative time-resolved spectroscopic probe that can directly monitor carrier dynamics within and across such junctions. In the TPR method, the change in reflectance (ΔR) of a broadband probe from a specific interface is monitored as a function of pump-probe delay. The spectral nature of the reflected beam provides quantitative information about the built-in field; thus, TPR is a non-contact probe of the electric field at that interface. We applied TPR to study charge transfer at p-type gallium-indium phosphide (p-GaInP2) and n-type gallium-arsenide (n-GaAs) interfaces. We monitored the formation and decay of transient electric fields that form upon photoexcitation within bare p-GaInP2, p-GaInP2/platinum (Pt), and p-GaInP2/amorphous titania (TiO2) interfaces. A field at both the p-GaInP2/Pt and p-GaInP2/TiO2 interfaces forms that drives charge separation, however, recombination at the p-GaInP2/TiO2 interface is significantly reduced compared the p-GaInP2/Pt interface. On the other hand, n-GaAs forms an ohmic contact with TiO2 while only a small field forms at the n- GaAs/NiO interface that promotes hole transfer to nickel oxide (NiO).
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