Very low thermally induced tip expansion by vacuum ultraviolet irradiation in a scanning tunneling microscope junction

D. Riedel, C. Delacour, A. J. Mayne, and G. Dujardin

Phys. Rev. B 80, 155451 – Published 28 October 2009

Abstract

The thermal and photoelectronic processes induced when a vacuum ultraviolet (VUV) laser irradiates the junction of a scanning tunneling microscope (STM) are studied. This is performed by synchronizing the VUV laser shots with the STM scan signal. Compared to other wavelengths, the photoinduced thermal STM-tip expansion is not observed when the VUV radiation is freed from spurious emissions. Furthermore, we demonstrate that the purified VUV photoinduced transient signal detected in the tunnel current is entirely due to photoelectronic emission and not combined with thermionic processes. The ensuing photoelectron emission is shown to be independent of the tip-surface distance while varying linearly with the pure VUV laser intensity. These results illustrate a strong decoupling between phonons and photoelectrons which allows a very weak STM-tip expansion.

Received 16 June 2009

DOI:https://doi.org/10.1103/PhysRevB.80.155451

Authors & Affiliations

D. Riedel^*, C. Delacour, A. J. Mayne, and G. Dujardin

Laboratoire de Photophysique Moléculaire, CNRS-UPR 3361, Bâtiment. 210, Université Paris Sud, 91405 Orsay, France

^*Corresponding author; damien.riedel@u-psud.fr

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Vol. 80, Iss. 15 — 15 October 2009

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Images

Figure 1
(Color online) (a) Schematic view of the scanning tunneling microscope irradiation conditions showing the collimated VUV beam waist of diameter $d \sim 1 mm$ irradiating the STM junction with a an incident angle of $62 °$ and a polarization orientated along the STM-tip axis. (b) Chronograph of the horizontal STM scanning signal $V_{x} (t)$ and the corresponding fabricated triggering signal $s (t)$ . (c)–(e) are $50 \times 50 {nm}^{2}$ STM topographies $(V_{s} = - 1.5 V, I = 200 pA)$ of the Si(100):H surface before irradiation, during the laser irradiation and after irradiation, respectively (the arrows indicate landmark positions). (f) $30 \times 30 {nm}^{2}$ STM topography comprising the upper part of (d) and the lower part of (e), respectively. (g) Averaged profile derived from the dashed lines [see (d)] as a function of the lateral distance. Note that the feedback signal, originally measured in volts, is converted in nm after a precise calibration of the STM-tip $z$ -piezo actuator on a Si(100):H surface step edge.Reuse & Permissions
Figure 2
(Color online) (a) Visible-IR spectrum of the spurious excimer cavity emission before separation by a Pellin-Broca prism (see the inset). (b)–(d) $50 \times 50 {nm}^{2}$ STM topographies $(V_{s} = - 1.5 V, I = 200 pA)$ of the Si(100):H surface before, during, and after the STM junction irradiation with the IR and visible light radiations, respectively. During (c), the surface voltage has been changed shortly (a few line scans) from $- 1.5$ to $- 3.5 V$ (and reverse) resulting in the observable hydrogen desorption variation as indicated by arrows A and B in (d). (e) Recorded feedback signal following the laser shots, taken from the profile of (c) and the corresponding tunnel current during a corresponding line scan. Note that the red curves in (e) correspond to numerical fits of the feedback and current signals with a single exponential decay function.Reuse & Permissions
Figure 3
(Color online) (a) Transient variation in the STM current signal and the corresponding feedback response $(V_{s} = 0)$ as functions of time following a pure VUV laser pulse. The red curve is the response of the current measurement chain when excited by a 10 ns voltage pulse (100 mV). (b) $12 \times 12 {nm}^{2}$ STM topographies on Si(100):H surface, during (left) and after (right) pure VUV laser shots synchronized on a line $(V_{s} = - 2.5 V, I = 200 pA)$ . (c) Variation of the integrated photoelectrons signal [black curve of (a)] as a function of the STM junction surface voltage for two different tip-surface distances (upper left inset) and for three different VUV laser intensities $I_{1}$ , $I_{2} = 2 I_{1}$ , and $I_{3} = 2.6 I_{1}$ (lower right inset). A and B arrows indicate the equivalent open circuit voltage and the short-circuit current, respectively.Reuse & Permissions

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