Ultrafast Probes of Semiconductor Junctions

Built-in electric fields at semiconductor junctions are vital for optoelectronic and photocatalytic applications since they govern the movement of photogenerated charge carriers near critical surfaces and interfaces. I will discuss the use of transient photoreflectance (TPR) spectroscopy to probe the dynamical evolution of carriers near the junctions of n-GaAs, p,n-GaInP and Si surfaces upon photoexcitation. In the case of n-GaAs we measured the transient electric fields and developed models in order to quantitatively describe the surface carrier dynamics that influence those fields. The photoinduced surface field at different types of junctions between n-GaAs and n-TiO₂, Pt, electrolyte and p-NiO are examined, and the results reveal that surface Fermi-level pinning, ubiquitous for many GaAs surfaces, can have beneficial consequences that impact photoelectrochemical applications. Fermi-level pinning results in the primary surface carrier dynamics being invariant to the contacting layer and promotes beneficial carrier separation. In contrast, when the Fermi-level is un-pinned via molecular surface functionalization on p-GaAs, the carriers undergo surface recombination faster due to a smaller built-in field, thus potentially degrading their photochemical performance. We studied the carrier dynamics at the interface of Si and Pt. We find that the charge transfer velocity from Si to Pt reaches 10⁷ cm×S^-1, which surprisingly is the same order of magnitude as the carrier thermal velocity in Si. The average temperature for the extracted carriers is estimated to be ~600 K, two times higher than the lattice temperature, and carrier kinetic energy loss takes place primarily during diffusion toward the interface. We also show that the charge transfer velocity can be controllably reduced and eventually brought to zero by inserting silica spacing layers between the Si and Pt interface