Study on (NH4)2S-treated GaAs(001) Surface
by Time-resolved Scanning Tunneling Microscopy
Haruhiro Oigawa, Munenori Yokota, Toshihiko Kishizawa, Shoji Yoshida, Osamu Takeuchi
and Hidemi Shigekawa
Institute of Applied Physics, University of Tsukuba,
Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan.
http://dora.bk.tsukuba.ac.jp/7u t㍗

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High-density defects on semiconductor surfaces deteriorate the performance of photoelectronic devices.
In order to solve the significant problem, an excellent method, surface treatment with (NH4)2S solution,
was developed. According to the previous report, (NH4)2S treatment effectively removes surface oxide andcovers the pristine surface with sulfur atoms.1) In this study, we have investigated the effectiveness of thetreatment by measuring minority carrier lifetime on a (NH4)2S-treated n-GaAs(001) surface using thetime-resolved scanning tunneling microscopy (STM) which has high resolutions of time and space.2-3)
In our time-resolved STM, the tunnel gap of STM is illuminated by a sequence of paired pulses and thecorresponding change in tunneling current ΔI is measured as a function of delay time between the paired
pulses. The decay time of photocarriers can be estimated from the delay-time dependence of ΔI. 2-3) Often,
the decay process provides two different decay times, one for the bulk-side decay process that is ordinary
recombination of the minority carriers with the majority carrier and the other for the surface-side decay
process where the majority carrier is depleted thus the minority carriers decay with thermal ionization fromthe surface-side to the bulk-side. When the sample is under STM observation, the surface-side decay also
involves the recombination of the minority carriers with the injected tunnel carriers, which often accelerates
the recombination especially when there are high density of surface defects. Thus, the density of surface
defects can be estimated with this method.
Figure 1 shows the result. Decay time of surface-side carrier density was obtained by exponential fitting.
The long carrier decay time (361 ns) for the surface-side recombination is almost the same as thedetect-free GaAs surface that is cleaved in UHV. 2) We interpret this result that surface defects of GaAs are
successfully reduced by the (NH4)2S treatment. The interpretation was supported by measuring
tunneling-current dependence of carrier decay time. As shown in Fig. 2, the decay constant is independentof tunnel current, which suggest the surface defects does not accelerates the recombination of the minority
carriers with the tunnel carriers at least around the measured point. We are now planning to measuring thedensity and distribution of the remaining surface detects on the (NH4)2S-treated GaAs surface.

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Fig. 1. ΔI vs. delay time curve obtainedfor a (NH4)2S-treated surface.

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Fig. 2. Tunneling-current dependence of decay-time
constant on (NH4)2S-treated GaAs(001) surfaces.

References(1) Oigawa, H.; Fan, J.; Nannichi, Y.; Sugahara, H.; Oshima, M.; Japanese Journal of Applied Physics
1991, 30, L322-L324.
(2) Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H.; Nature Photonics 2010, 4, 12, 869.
(3) Takeuchi, O.; Aoyama, M.; Oshima, R.; Okada, Y.; Sano, N.; Shigekawa, H.; Morita, R.; Yamashita,M.; Applied Physics Letters 2004, 85, 3268-3270.