Carrier capture dynamics at localized gap states of Co nanoparticles
grown on GaAs(110) probed by time-resolved STM
M. Yokota, Y. Terada, S. Yoshida, A. Okubo, T. Ishikawa,O. Takeuchi, and H. Shigekawa
Inst. of Appl. Phys. Univ. of Tsukuba
http://dora.bk.tsukuba.ac.jp/

Localized gap states in semiconductors, which originate from defects, impurities,and adsorbates, affect the carrier dynamics dramatically through capture, emission,and recombination processes. Considering control of carrier dynamics, the localized
gap states are sometimes problem to be avoided and sometimes useful to design
ultra-fast semiconductor devices. With the recent development of semiconductor
nanotechnology, it became important to probe and understand the carrier dynamics
around localized gap states with high spatial resolution. In this study, we havevisualized the carrier capture dynamics around gap states artificially formed on GaAs,
using shaken pulse-paired excited scanning tunneling microscopy (SPPX-STM) [1].
In an SPPX-STM measurement, the tunnel gap of STM is illuminated by a sequence ofpaired pulses and the corresponding change in tunneling current ΔI is measured as afunction of delay time between the paired pulses (td). The decay time of thephotocarriers due to the surface recombination can be estimated from the dependence of
the ΔI against delay time. Cobalt nanoparticles were grown on GaAs(110) (n-type,Si-doped 1x1017cm-3) surface to generate localized gap states deep in the GaAs bandgap.
STM images on Co/GaAs reveal that the Co nanoparticles are well isolated and almost
the same in size (Fig. 2). Figure 3 shows ΔI vs. delay time curves obtained (a) above bare
GaAs surface and (b) above a single Co nanoparticle that associates with the gap state.
As expected, the decay constant above Co, 2.3 ns, is shorter than that above the bare
GaAs surface, 220 ns due to the high recombination rate at the gap state. We found,Fig. 1 Schematic of SPPX-STMmeasurement.

Fig. 2 STM topography Co/GaAs

however, that the apparent lifetime of the surface photocarriers are dependent on themagnitude of the tunnel current. We interpreted the fact as following. The minorityphotocarriers (holes in this case) that is excited in the depletion layer drift towards the surface
under the bias voltage. There, the electrons (counter carriers) are depleted and, therefore,
regardless of the existence of the gap states, no recombination occurs without tunnel current. In
our experiment, the counter carriers are provided from the STM tip via tunneling as illustrated
in Fig. 4. Above a Co particle, accumulated holes are trapped by the gap state, and recombinewith the electrons tunneling from the STM tip (Fig. 4(b)). Assuming that electrons and holes
encountered at the gap state recombine immediately, carrier recombination rate at Co ananoparticle is determined by the relationship between the injection rate of tunneling
electron to a gap state (Je) and the capture rate of minority holes (Jh). Based on thisphysical model, we derive the hole capture rate (Jh) of single Co particles and discuss its

relationship to the particle size.
(a)(b)
00-200-200-400

Ga As-400

Co
2.3ns-600-600

0500

01000

510

Delay time [ns]Delay time [ns]Fig. 3 ΔI vs. delay-time curve obtained above (a) bare GaAs (b) Co/GaAs(a)(b)Fig. 4 1-D energy band diagram of tunnel junction(a) bare GaAs (b) Co/GaAsReferences[1] Y. Terada, M. Aoyama, H. Kondo, A. Taninaka, O. Takeuchi and H. Shigekawa,
Nanotechnology 18, 44028 (2007).