APPLIED PHYSICS LETTERS 92, 102105 共2008兲

Probing nanoscale potential modulation by defect-induced gap states
on GaAs„110… with light-modulated scanning tunneling spectroscopy
Shoji Yoshida, Yuya Kanitani, Osamu Takeuchi, and Hidemi Shigekawaa兲
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba 305-8573, Japan

共Received 6 October 2007; accepted 5 February 2008; published online 12 March 2008兲
We investigated charged defects on an n-GaAs共110兲 surface using light-modulated scanningtunneling spectroscopy. Tunneling via a single defect-induced gap state under photoillumination wasobserved for the isolated atomic defects. Screened Coulomb potentials induced around a charged Ga
vacancy and a step edge were visualized, for the first time, with a nanometer spatial resolution.
Furthermore, the charge states of the individual defects were determined on the atomic level.
© 2008 American Institute of Physics. 关DOI: 10.1063/1.2888733兴
In the history of the development of functional materials
and devices, defects, which disturb the electronic and opticalproperties, were considered a problem to be avoided. However, recent progress in nanoscale science and technology
has drastically changed the entire situation through the introduction of a new concept; “defect engineering”—defects areactively designed and controlled to realize desired functions.
The current reduction in the size of semiconductor devices
has intensified the urgency of the requirement and made theatomic scale characterization of defects and impurities unprecedentedly important.1
Scanning probe microscopy is a powerful technique because of its high spatial resolution. Since the development ofscanning tunneling microscopy 共STM兲, various techniques
based on STM have been employed to analyze the atomicstructures and electronic states of defects and impurities on
semiconductor surfaces.2–8 Defects in semiconductors areelectrically charged in general and affect the electronic properties of semiconductors through, for example, the Fermi
level pinning and compensation of dopants.3,4 Therefore,
considerable efforts have been focused on determining thecharge states of various types of defects by using STM.5,6
The sign of the electronic charge of a defect can be
deduced from the topographic contrast of a STM image,which depends on the relationship between the charge of thedefect and the polarity of the tunneling bias voltage.7 The
spatial distribution of the topographic contrast originatesfrom the screened Coulomb potential 共SCP兲, which is comparable to the Debye screening length of a sample. However,since an exact electrostatic potential cannot be obtained using STM, determining the exact charge state of a defect using STM is difficult. Therefore, indirect methods, such asmeasuring the average distance between defects that reflect
Coulomb repulsion and measuring the number of charged
defects that are coupled with the predetermined dopants,
have been employed in pioneering works to determine thecharge states of defects.3–6 For further advancement infunctional materials and devices based on nanoscale scienceand technology, development of a method that enables thedirect characterization of the surveyed individual defects isrequired.
In this study, we demonstrated a method that enables us
to visualize the nanoscale electrostatic potential distributiona兲

URL: http://dora.bk.tsukuba.ac.jp.

induced by individual defects and to directly determine thecharge states of these defects. We determined the charge
states of Ga vacancies and 关110兴 atomic steps from the results of the analysis of the SCPs visualized using the analysisof spatially resolved surface photovoltage 共SPV兲. Tunnelingunder photoillumination via defect-induced gap states
共DIGSs兲 is also discussed.
Around a charged defect, band bending is induced by thescreening of the charge. The profile of the potential distribution shows the electronic charge trapped in the defect. Under
laser illumination, photogenerated electron-hole pairs areseparated by electric fields and redistributed to reduce bandbending. 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 magnitudeof surface band bending under dark condition.9,10 If a flatband condition is achieved with a sufficient laser intensity, aspatially 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 关tip-induced band bending 共TIBB兲兴 below a STM tip.
TIBB strongly depends on various parameters, such as tunneling bias voltage, tip-sample distance, and the doping levelof the sample, resulting in difficulty in SPV measurement.
To accurately estimate the SPV, we employed lightmodulated scanning tunneling spectroscopy 共LM-STS兲,which enables us to measure the tunneling current–bias voltage 共I-V兲 spectra at a fixed tip-sample distance under bothdark and light conditions simultaneously.11,12 The SPV is calculated from the shift of the two I-V spectra.
As a sample, an n-GaAs共110兲 surface 共Si doped,2 ⫻ 1018 cm−3兲 was prepared by cleavage in a vacuum
共⬍1 ⫻ 10−8 Pa兲. Mechanically chopped illumination from alaser diode 共635 nm, 100 Hz兲, 60° off normal to the surface,
was focused onto the tunnel junction area with a spot diameter of 0.02 mm. For a STM tip, an electrochemically etchedtungsten tip was used.
Figure 1 shows the filled state topographic images of 共a兲
a Ga vacancy and 共b兲 a 关110兴 step on a GaAs共110兲 surface. InFig. 1共a兲, the Ga vacancy exhibits a contrast of a bright elevation at the center of the defect location, which is surrounded by a dark depression region. This is the signature ofa negatively charged defect;7 a dangling bond of the Ga vacancy appears bright in the filled state image. The 关110兴 step

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102105-2

Yoshida et al.

FIG. 1. 共Color online兲 Filled state topographic images of 共a兲 Ga vacancyand 共b兲 关110兴 step on GaAs共110兲 surface.

shows a similar image consisting of a depression region
along the bright step line.
Figure 2 shows the obtained tunneling current–bias voltage 共I-V兲 spectra for 共a兲 A and 共b兲 B 共Ga vacancy兲 in Fig.
1共a兲 and 共c兲 C in Fig. 1共b兲 共关110兴 step兲. The tunneling junction has a metal-insulator-semiconductor 共MIS兲 structure, as
schematically illustrated in Fig. 2共d兲. Since the GaAs共110兲
surface has no surface state within the bulk band gap, TIBB
easily occurs at positive sample bias voltages, while it
scarcely occurs at negative bias voltages.12
A negative sample bias voltage of −1.7 V, at which nophotocurrent was detected, was chosen as a set point for
LM-STS measurements. In the positive-sample-bias-voltage
region, the tunneling current oscillates owing to the illumination with a chopped laser light, and the two virtual I-V
curves, which correspond to those under dark 共blue兲 and under light 共red兲 conditions, are simultaneously obtained. The
SPV spectra 共green circles兲 are obtained by calculating thelateral shift of the two I-V curves with respect to the bias
voltage for the I-V curve under dark condition. The central
part of the SPV spectra with a tunneling current below 3 pAAppl. Phys. Lett. 92, 102105 共2008兲

is missing due to the difficulty in calculating the shift of thetwo I-V curves in that region.
As shown in Fig. 2共b兲, the I-V curve measured in a Gavacancy defect 关B in Fig. 1共a兲兴 has a hump at around 0.7 V,
which is completely different from that in Fig. 2共a兲 and that
under dark condition in Fig. 2共b兲. The I-V curve is divided
into the following three regions: 共1兲 rapid increase 共Vs⬍ 0.7 V兲, 共2兲 plateau 共0.7 V ⬍ Vs ⬍ 1.2 V兲, and 共3兲 increase
with a slope similar to that for the I-V curve under dark
condition 共1.2 V ⬍ Vs兲. What causes the difference?
SPV analysis is, in general, based on the assumption thatthe lateral shift of I-V curves obtained under dark and light
conditions is equal to the difference in the surface potential
under the two conditions. This assumption is valid when atunneling current is produced by majority carriers, corresponding to direct tunneling from the STM tip into the conduction band 关Fig. 2共a兲兴. This condition was valid even
above the atom next to the defect. However, the situation
significantly changed when the I-V curve was measured immediately above a Ga-vacancy defect with a DIGS.10,13
Figure 2共d兲 shows schematics for explaining the mechanism of the DIGS-mediated tunneling process.14 Under thedark condition, since the Fermi level is pinned by DIGS,
which results in the formation of a Schottky barrier, the carrier flow is suppressed in the low-positive-sample-biasvoltage region. In contrast, under the light condition, the pinning is reduced and the minority carrier flow via DIGS 共ID兲occurs and produces tunneling current 共It兲. The tunnelingcurrent via DIGS is produced through a thermoionic process
共Is兲 and the recombination of photoinduced holes 共I p兲 withthe electrons from the STM tip. Therefore, total tunnel current is represented as It = ID + IC 共ID = I p + Is兲, where IC is thedirect tunnel current from the STM tip to the sample conduction band. In this case, Is can be neglected and ID = I p.
With an increase in bias voltage, the tunneling current
via DIGS increases depending on the relationship between
the energy level of DIGS and the Fermi level of the tip; thefirst increase in It 共=ID兲 depends on Vs 共Vs ⬍ 0.7 V兲. At acertain bias voltage, I p reaches a maximum that is limited by
the amount of photoinduced carriers, yielding a plateau
共0.7 V ⬍ Vs ⬍ 1.2 V兲. For higher bias voltages 共1.2 V ⬍ Vs兲,direct tunneling from the STM tip to the sample conduction
band occurs, which dominantly increases It. This is the first
demonstration of the DIGS-mediated tunneling process under photoillumination observed for the isolated atomic defect.
Next, we focus on the spatial variation of SPV. Figures
3共a兲 and 3共b兲 show SPV images taken when the topographic
images shown in Fig. 1 共Vs = + 1.7 V兲 are obtained. The SPV
images are obtained from the I-V curves acquired at equally
spaced points 关共a兲 17⫻ 17 points, 共b兲 40⫻ 20 points兴 during
the topographic imaging. Figures 3共c兲 and 3共d兲 respectively
represent the cross-sectional profiles of the SPV images
along line I in Fig. 3共a兲 and line II in Fig. 3共b兲, obtained
under various bias voltages. Since SPV is accompanied bybias-dependent TIBB,11–14 the bias-dependent component is
considered to be the change in TIBB. To clarify this point,
the SPV profiles after offset adjustment of those in Figs. 3共c兲and 3共d兲 are shown in Figs. 3共e兲 and 3共f兲, respectively. The
magnitude of SPV increases with bias voltage; however, thespatial variation of the cross-sectional profile remains unchanged. Therefore, the observed bias-independent profile isFIG. 2. 共Color online兲 I-V spectra obtained by LM-STS for 共a兲 A and 共b兲 B共Ga vacancy兲 in Fig. 1共a兲 and 1共c兲 C in Fig. 1共b兲 共关110兴 step兲. Red and blue
curves correspond to the I-V curves with and without illumination, respectively. Oscillation between the two curves is due to on and off switching of
the illumination. Green dots indicate SPV determined from the shift of thetwo I-V curves as related to the bias voltage under dark condition. 共d兲Schematic models for DIGS-mediated tunneling processes: Is, current produced by thermoionic process; I p, current of photoinduced holes; IG, tunneling mediated by DIGS from STM tip; ID, direct tunnel current from STM tip
to sample conduction band.
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102105-3

Appl. Phys. Lett. 92, 102105 共2008兲

Yoshida et al.

cancy pairs, the charge state at a surface is −1e.3 To determine the charge state of a Ga vacancy, the SCP theoreticalprofiles obtained for the charges of −1e to −3e are shown inFig. 3共e兲. Taking account of the effect of TIBB, the value
obtained by averaging the TIBB measured at several places
free from defects, 0.25 V under Vs = + 1.7 V, was added as
an offset to the theoretical values. Although there is a deviation near the Ga vacancy due to the saturation of band bending in real experimental measurements, the best fitting looks
achieved for the −3e state rather than the −1e state. For
further advance, structure-dependent calculations are needed.
The charge distribution for the step edge 关Fig. 3共d兲兴 iswider than that for the Ga vacancy 关Fig. 3共c兲兴, which is attributed to the fact that the step edge consists of continuously
ordered point defects equally spaced along the step. The potential distributions, calculated by the superposition of thealigned point charges for −1e per double lattice spacing 共represented by −0.5e兲 and −1e and −2e per lattice spacing,6 areshown in Fig. 3共d兲. The SPV profile is close to the distribution calculated for the −1e state with this simple model.
In conclusion, SCPs induced around a charged Ga vacancy and a step edge were visualized on the nanometerlevel. Furthermore, the charge states of the individual defects
were determined at the atomic level.
FIG. 3. 共Color online兲 关共a兲 and 共b兲兴 SPV images simultaneously obtainedwith the topographic images shown in Fig. 1 关Vs = + 1.7 V; 共a兲 17⫻ 17points, 共b兲 40⫻ 20 points兴. 关共c兲 and 共d兲兴 Cross sections of SPV mappings,
obtained under various bias voltages, along line I in 共a兲 and line II in 共b兲,respectively. 关共e兲 and 共f兲兴 SPV profiles in 共c兲 and 共d兲 after offset adjustment.
Potential profiles obtained from theoretical calculations for different charges
are shown together.

attributed to the intrinsic electrostatic potential variations
around the defects; this the first result that directly shows thecharge state of an isolated Ga vacancy.
An electrostatic potential variation around a point charge
on a semiconductor surface is described with a SCP as

␾共r兲 =

冉 冊

2ner,
exp −4 ␲ ␧ 0␧ srRb

where r is the lateral distance from the center of the defectand n is the effective charge of the defect. Rb is the Debye
screening length of bulk n-GaAs. In this form, the effective
charge is double counted compared with the bulk SCP because of reduction in the screening efficiency by half on thesurface, where the relative dielectric constant of bulk GaAs
is used.15 The Debye screening length Rb due to the conduction of electrons is calculated to be 4.4 nm for the sample
with a doping concentration of 2.0⫻ 1018 cm−3.16 Despite
considerable theoretical and experimental studies, the charge
state of a Ga vacancy is still unclarified; thus, it is an issue inthis case. Theoretical calculations17 and a recent positron annihilation experiment18 predict the charge of a Ga vacancy as−3e, while diffusion experiments support the values forthe −2e 共Ref. 19兲 and −1e 共Ref. 20兲 cases 共e, elementarycharge兲. According to the measurements of Si donor–Ga va-

We thank Ms. Rie Yamashita, a member of our group,for her help in preparing this paper.
E. G. Seebauer and M. C. Kratzer, Mater. Sci. Eng., R. 55, 57 共2006兲.
Ph. Ebert, Surf. Sci. Rep. 33, 121 共1999兲.
3C. Domke, Ph. Ebert, M. Heinrich, and K. Urban, Phys. Rev. B 54, 10288
共1996兲.
4J. Gebauer, E. R. Weber, N. D. Jäger, K. Urban, and Ph. Ebert, Appl.
Phys. Lett. 82, 2059 共2003兲.
5
Ph. Ebert, X. Chen, M. Heinrich, M. Simon, K. Urban, and M. G. Lagally,Phys. Rev. Lett. 76, 2089 共1996兲.
6
M. Heinrich, C. Domke, Ph. Ebert, and K. Urban, Phys. Rev. B 53, 10894
共1996兲.
7C. Domke, Ph. Ebert, M. Heinrich, and K. Urban, J. Vac. Sci. Technol. B16, 2825 共1998兲.
8L. Kronik and Y. Shapira, Surf. Sci. Rep. 37, 1 共1999兲.
9
M. Berthe, S. Yoshida, Y. Ebine, K. Kanazawa, A. Okada, A. Taninaka, OTakeuchi, N. Fukui, H. Shinohara, S. Suzuki, K. Sumitomo, Y. Kobayashi,
B. Grandidier, D. Stievenard, and H. Shigekawa, Nano Lett. 7, 3623
共2007兲.
10
S. Grafstrom, J. Appl. Phys. 91, 1717 共2002兲.
11O. Takeuchi, S. Yoshida, and H. Shigekawa, Appl. Phys. Lett. 84, 3645
共2004兲.
12
S. Yoshida, Y. Kanitani, R. Oshima, Y. Okada, O. Takeuchi, and H.
Shigekawa, Phys. Rev. Lett. 98, 026802 共2007兲.
13
M. W. J. Prins, R. Jansen, R. Groeneveld, A. Gelder, and H. Kempen,Phys. Rev. B 53, 8090 共1996兲.
14
M. McEllistrem, G. Haase, D. Chen, and R. J. Hamers, Phys. Rev. B 70,2471 共1993兲.
15
M. Kremar, W. M. Saslow, and M. B. Weimer, Phys. Rev. B 61, 13821
共2000兲.
16
R. B. Dingle, Philos. Mag. 46, 831 共1955兲.
17
S. B. Zhang and J. E. Northrup, Phys. Rev. Lett. 67, 2339 共1991兲.
18J. Gebauer, M. Lausmann, F. Redmann, R. Krause-Rehberg, H. S. Leipner,E. R. Weber, and Ph. Ebert, Phys. Rev. B 67, 235207 共2003兲.
19
H. Bracht, M. Norseng, E. E. Haller, K. Eberl, and M. Cardona, Solid
State Commun. 112, 301 共1999兲.
20
R. M. Cohen, Mater. Sci. Eng., R. 20, 167 共1997兲.
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