Photoassisted scanning tunneling microscopy

Abstract

The combination of scanning tunneling microscopy (STM) with optical excitation adds new information to STM. A review is presented covering the work done on light-induced effects in STM during the past 15 years. Effects discussed include thermal effects, nonlinear effects, field enhancement at the STM tip, various effects on semiconductor surfaces, excitation of surface plasmons, detection of photoelectrons, spin-polarized tunneling, as well as light-induced nanomodifications, local optical spectroscopy, the use of ultrashort laser pulses for time-resolved STM, and the combination of STM and scanning near-field optical microscopy.

Real-space imaging of transient carrier dynamics by nanoscale pump–probe microscopy

“…In STM on a semiconductor, a nanoscale metal-insulator-semiconductor (MIS) junction is formed by the STM tip, tunneling gap, and sample. Thus, when a reverse bias voltage is applied to the junction, tip-induced band bending (TIBB) occurs in the surface region owing to the leakage of the electric field into the sample [9,18,21]. With optical illumination, the redistribution of photocarriers reduces the electric field and changes the surface potential, which is called SPV, and increases the effective bias voltage applied to the tunnel junction.…”

“…Hamers et al determined the recombination lifetime of photoexcited carriers in Si, in their pioneering work, by analyzing the observed surface photovoltage (SPV) by simulation [10], and a method that involves the combined use of an optical pulse pair and STM was proposed as photoconductive-gate STM (PG-STM) [11][12][13][14][15][16][17][18]. However, the spatial and temporal resolutions of the former method were limited to the 1 µm scale and laser-pulse repetition rate, respectively, and the PG-STM probes dI/dV or the quantity mediating the signal rather than the transient effect itself [18].…”

“…However, the spatial and temporal resolutions of the former method were limited to the 1 µm scale and laser-pulse repetition rate, respectively, and the PG-STM probes dI/dV or the quantity mediating the signal rather than the transient effect itself [18]. With the shaken-pulse-pair-excited STM (SPPX-STM) [19,20], which was designed to detect a weak tunneling current of the transient signal under optical excitation, the time-resolved tunneling current in the subpicosecond range was successfully probed; however, its temporal range was very narrow and it was still difficult to reliably measure a very weak time-resolved STM signal in a short period of time, preventing the microscopy technique from revealing transient carrier dynamics in nanostructures consisting of composite materials with a wide variety of lifetimes.…”

Probing nanoscale potential modulation by defect-induced gap states on GaAs(110) with light-modulated scanning tunneling spectroscopy

“…This change in the surface potential of a semiconductor induced by super-band-gap illumination is the SPV, which provides information about the polarity and magnitude of surface band bending under dark condition. 9,10 If a flat band condition is achieved with a sufficient laser intensity, a spatially resolved SPV directly exhibits a local electrostatic potential variation. However, the penetration of the bias voltage applied between a sample and a tip induces band bending ͓tip-induced band bending ͑TIBB͔͒ below a STM tip.…”

Laser and voltage manipulation of bistable Si dopants in the GaAs (110) surface

“…19 If the laser is tuned above the band gap of GaAs, electrons and holes are created that result in a photocurrent and a photovoltage. The photocurrent is in the same direction as the tunneling current between tip and sample, so the STM feedback in constant current mode retracts the tip slightly for compensation.…”