We investigated carrier recombination dynamics in a low-temperature-grown GaAs (LT-GaAs)/AlGaAs/GaAs heterostructure by laser-combined scanning tunneling microscopy, shaken-pulse-pair-excited STM (SPPX-STM). With the AlGaAs interlayer as a barrier against the flow of photocarriers, recombination lifetimes in LT-GaAs of 4.0？ps and GaAs of 4.8？ns were successfully observed separately. We directly demonstrated the high temporal resolution of SPPX-STM by showing the recombination lifetime of carriers in LT-GaAs (4.0？ps) in the range of subpicosecond temporal resolution. In the carrier-lifetime-mapping measurement, a blurring of recombination lifetime up to 50？nm was observed at the LT-GaAs/AlGaAs boundary, which was discussed in consideration of the screening length of the electric field from the STM probe. The effect of the built-in potential on the signal, caused by the existence of LT-GaAs/AlGaAs/GaAs boundaries, was discussed in detail. 1. Introduction The development of semiconductor physics and devices has been progressing with the innovation of atomically controlled fabrication technologies [1–3]. However, the carrier dynamics in materials have been analyzed only by evaluating macroscopic functions, or with techniques that provide spatially and/or temporally averaged information. Microscopic carrier dynamics has not been directly observed, although it is fundamental for the device functions. Therefore, there has been an increasing desire for developing a microscopic technique that allows the investigation of carrier dynamics at high spatial and temporal resolutions. Laser-combined scanning tunneling microscopy (STM) is a promising candidate, because the tunneling junction itself responds on a time scale of a few femtoseconds and thereby ultrafast dynamics can principally be studied by STM with ultrashort pulse laser techniques [4–12]. Recently, we have reported a novel microscopy technique that enables the visualization of subpicosecond carrier dynamics in nanometer-scale structures [13], that is, shaken-pulse-pair-excited scanning tunneling microscopy (SPPX-STM) with a novel delay-time modulation method based on a pulse-picking technique [14–21]. A nonequilibrium carrier distribution is generated with ultrashort laser pulses and its relaxation processes are probed using STM by a pump-probe technique. We proposed a model clarifying the physical origin of the signal for semiconductors and verified it experimentally. The SPPX-STM signal consists of two components, that is, a faster one reflecting the decay process of bulk-side photocarriers and a
References
[1]
T. Shinada, S. Okamoto, T. Kobayashi, and I. Ohdomari, “Enhancing semiconductor device performance using ordered dopant arrays,” Nature, vol. 437, no. 7062, pp. 1128–1131, 2005.
[2]
B. Cheng, S. Roy, G. Roy, F. Adamu-Lema, and A. Asenov, “Impact of intrinsic parameter fluctuations in decanano MOSFETs on yield and functionality of SRAM cells,” Solid-State Electronics, vol. 49, no. 5, pp. 740–746, 2005.
[3]
S. I. Takagi, J. L. Hoyt, J. J. Welser, and J. F. Gibbons, “Comparative study of phonon-limited mobility of two-dimensional electrons in strained and unstrained Si metal-oxide-semiconductor field-effect transistors,” Journal of Applied Physics, vol. 80, no. 3, pp. 1567–1577, 1996.
[4]
R. J. Hamers and D. G. Cahill, “Ultrafast time resolution in scanned probe microscopies,” Applied Physics Letters, vol. 57, no. 19, pp. 2031–2033, 1990.
[5]
G. Nunes Jr. and M. R. Freeman, “Picosecond resolution in scanning tunneling microscopy,” Science, vol. 262, no. 5136, pp. 1029–1032, 1993.
[6]
G. M. Sleeves, A. Y. Elezzabi, and M. R. Freeman, “Advances in picosecond scanning tunneling microscopy via junction mixing,” Applied Physics Letters, vol. 70, no. 14, pp. 1909–1911, 1997.
[7]
S. Grafstr？m, P. Schuller, J. Kowalski, and R. Nuemann, “Thermal expansion of scanning tunneling microscopy tips under laser illumination,” Journal of Applied Physics, vol. 83, pp. 3453–3460, 1998.
[8]
J. Jersch, F. Demming, I. Fedotov, and K. Dickmann, “Time-resolved current response of a nanosecond laser pulse illuminated STM tip,” Applied Physics A, vol. 68, no. 6, pp. 637–641, 1999.
[9]
V. Gerstner, A. Knoll, W. Pfeiffer, A. Thon, and G. Gerber, “Femtosecond laser assisted scanning tunneling microscopy,” Journal of Applied Physics, vol. 88, no. 8, pp. 4851–4859, 2000.
[10]
N. N. Khusnatdinov, T. J. Nagle, and G. Nunes, “Ultrafast scanning tunneling microscopy with 1 nm resolution,” Applied Physics Letters, vol. 77, no. 26, pp. 4434–4436, 2000.
[11]
S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, and A. J. Heinrich, “Measurement of fast electron spin relaxation times with atomic resolution,” Science, vol. 329, no. 5999, pp. 1628–1630, 2010.
[12]
S. W. Wu and W. Ho, “Two-photon-induced hot-electron transfer to a single molecule in a scanning tunneling microscope,” Physical Review B, vol. 82, Article ID 085444, 8 pages, 2010.
[13]
Y. Terada, S. Yoshida, O. Takeuchi, and H. Shigekawa, “Real-space imaging of transient carrier dynamics by nanoscale pumpg-probe microscopy,” Nature Photonics, vol. 4, no. 12, pp. 869–874, 2010.
[14]
O. Takeuchi, R. Morita, M. Yamashita, and H. Shigekawa, “Development of time-resolved scanning tunneling microscopy in femtosecond range,” Japanese Journal of Applied Physics, vol. 41, no. 7B, pp. 4994–4997, 2002.
[15]
O. Takeuchi, M. Aoyama, R. Oshima et al., “Probing subpicosecond dynamics using pulsed laser combined scanning tunneling microscopy,” Applied Physics Letters, vol. 85, no. 15, p. 3268, 2004.
[16]
O. Takeuchi, M. Aoyama, and H. Shigekawa, “Analysis of time-resolved tunnel current signal in sub-picosecond range observed by shaken-pulse-pair-excited scanning tunneling miscroscopy,” Japanese Journal of Applied Physics, vol. 44, no. 7B, pp. 5354–5357, 2005.
[17]
O. Takeuchi, M. Aoyama, H. Kondo, A. Taninaka, Y. Terada, and H. Shigekawa, “Nonlinear dependences in pulse-pair-excited scanning tunneling microscopy,” Japanese Journal of Applied Physics, vol. 45, no. 3B, pp. 1926–1930, 2006.
[18]
Y. Terada, M. Aoyama, H. Kondo, A. Taninaka, O. Takeuchi, and H. Shigekawa, “Ultrafast photoinduced carrier dynamics in GaNAs probed using femtosecond time-resolved scanning tunnelling microscopy,” Nanotechnology, vol. 18, no. 4, Article ID 044028, 2007.
[19]
H. Shigekawa, S. Yoshida, O. Takeuchi et al., “Nanoscale dynamics probed by laser-combined scanning tunneling microscopy,” Thin Solid Films, vol. 516, no. 9, pp. 2348–2357, 2008.
[20]
Y. Terada, S. Yoshida, O. Takeuchi, and H. Shigekawa, “Laser-combined scanning tunnelling microscopy for probing ultrafast transient dynamics,” Journal of Physics: Condensed Matter, vol. 22, no. 26, Article ID 264008, 2010.
[21]
H. Shigekawa, O. Takeuchi, Y. Terada, and S. Yoshida, “STM based techniques combined with optics,” in Handbook of Nanophysics, K. D. Sattler, Ed., vol. 6 of Principles and Methods, Taylor & Francis, 2010.
[22]
O. Takeuchi, S. Yoshida, and H. Shigekawa, “Light-modulated scanning tunneling spectroscopy for nanoscale imaging of surface photovoltage,” Applied Physics Letters, vol. 84, no. 18, pp. 3645–3647, 2004.
[23]
S. Yoshida, Y. Kanitani, R. Oshima, Y. Okada, O. Takeuchi, and H. Shigekawa, “Microscopic basis for the mechanism of carrier dynamics in an operating p-n junction examined by using light-modulated scanning tunneling spectroscopy,” Physical Review Letters, vol. 98, no. 2, Article ID 026802, 2007.
[24]
S. Yoshida, Y. Kanitani, O. Takeuchi, and H. Shigekawa, “Probing nanoscale potential modulation by defect-induced gap states on GaAs(110) with light-modulated scanning tunneling spectroscopy,” Applied Physics Letters, vol. 92, no. 10, Article ID 102105, 2008.
[25]
S. Yoshida, Y. Kanitani, R. Oshima, Y. Okada, O. Takeuchi, and H. Shigekawa, “Nanoscale mapping of built-in potential in GaAs p-n junction using light-modulated scanning tunneling microscopy,” Japanese Journal of Applied Physics, vol. 47, no. 7, pp. 6117–6120, 2008.
[26]
M. McEllistrem, G. Haase, D. Chen, and R. J. Hamers, “Electrostatic sample-tip interactions in the scanning tunneling microscope,” Physical Review Letters, vol. 70, no. 16, pp. 2471–2474, 1993.
[27]
S. Grafstr？m, “Photoassisted scanning tunneling microscopy,” Journal of Applied Physics, vol. 91, no. 4, pp. 1717–1753, 2002.
[28]
L. Kronik and Y. Shapira, “Surface photovoltage phenomena: theory, experiment, and applications,” Surface Science Reports, vol. 37, no. 1, pp. 1–206, 1999.
[29]
D. G. Cahill and R. J. Hamers, “Scanning tunneling microscopy of photoexcited carriers at the Si(001) surface,” Journal of Vacuum Science & Technology B, vol. 9, no. 2, pp. 564–568, 1991.
[30]
Ch. Sommerhalter, Th. W. Matthes, J. Boneberg, P. Leiderer, and M. C. Lux-Steiner, “Tunneling spectroscopy on semiconductors with a low surface state density,” Journal of Vacuum Science & Technology B, vol. 15, no. 6, pp. 1876–1883, 1997.
[31]
J. Shah, Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, Springer, Berlin, Germany, 1999.
[32]
A. Othonos, “Probing ultrafast carrier and phonon dynamics in semiconductors,” Journal of Applied Physics, vol. 83, no. 4, pp. 1789–1830, 1998.
[33]
S. Gupta, M. Y. Frankel, J. A. Valdmanis et al., “Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures,” Applied Physics Letters, vol. 59, no. 25, pp. 3276–3278, 1991.
[34]
R. Yano, Y. Hirayama, S. Miyashita, H. Sasabu, N. Uesugi, and S. Uehara, “Pump-probe spectroscopy of low-temperature grown gaAs for carrier lifetime estimation: arsenic pressure dependence of carrier lifetime during MBE crystal growth,” Physics Letters A, vol. 289, no. 1-2, pp. 93–98, 2001.
