Japanese Journal of Applied Physics
Vol. 47, No. 7, 2008, pp. 6117–6120#2008 The Japan Society of Applied Physics

Nanoscale Mapping of Built-in Potential in GaAs p–n Junction
Using Light-Modulated Scanning Tunneling Microscopy
Shoji Y OSHIDA, Yuya K ANITANI, Ryuji O SHIMA, Yoshitaka O KADA,Osamu T AKEUCHI, and Hidemi S HIGEKAWA
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan(Received February 7, 2008; revised March 21, 2008; accepted April 17, 2008; published online July 18, 2008)

Surface photovoltage (SPV) was visualized over the interface of a GaAs p–n junction using light modulated scanningtunneling spectroscopy. Spatially resolved SPV includes information about the built-in potential of the p–n junction as well as
the photo-induced relaxation of tip-induced band bending. These two components were separately evaluated, and mapping of
the built-in potential was accomplished on the nanoscale. [DOI: 10.1143/JJAP.47.6117]
KEYWORDS: scanning tunneling microscopy, surface photovoltage, semiconductor, GaAs, p–n junction

1.

Introduction(a)With the development of nanoscale science and technology, we are now facing the limits of fundamental issues.
Through the miniaturization of semiconductor devices down
to less than 50 nm, for example, ﬂuctuation in the distribution of atomic defects or dopant materials on the nanoscale
now directly aﬀects the macroscopic functions.1) Fluctuationof component materials or irregularity of ordering at ahetero-semiconductor interface also results in the variationof the local potential.2) To achieve ﬁne control of these basic
elements, it is desired to establish techniques that enable
us to characterize the physical properties of materials anddevices in relation to the nanoscale structures.
Scanning probe microscopy (SPM) is one of the mostprominent candidates for this purpose. Various techniques
based on SPM have thus far been developed.3–13) Amongthem, light modulated scanning tunneling spectroscopy(LM-STS) enables us to evaluate the local potential insemiconductor devices on the nanoscale, through themeasurement of surface photovoltage (SPV).5,6) In scanningtunneling microscopy (STM) on a semiconductor sample,
band bending is induced in the surface region by theleakage of the electric ﬁeld produced by the bias voltage
applied between the STM tip and the sample;5,6,12) thisphenomenon is called tip induced band bending (TIBB).
When the sample surface below the STM tip is photoilluminated, carriers are excited and redistributed to reduceTIBB,12–14) where the shift of the current–voltage (I–V)
curves between dark and photoilluniated conditions provides SPV. Using LM-STM, SPV can be visualized on thenanoscale.
Since TIBB depends on the local carrier density,
information about the dopant proﬁle and excess minority
carriers injected into a p–n junction can be obtained byLM-STM.5) For a p–n junction, the intrinsic band bending
produced by the equalization of the Fermi level on bothsides, built-in potential, which is an important factor inhetero-semiconductor devices, is also modiﬁed by photoillumination, contributing to the SPV detected by LM-STM.
Therefore, built-in potential can be probed by LM-STM.
However, since TIBB is also included in SPV, how to
distinguish the two components is the essential point. In thisstudy, we demonstrate a method for probing the built-inpotential, free from the eﬀect of TIBB, and visualize it onthe nanoscale.

(b)
ser1

La2VSVS(c)SPVBIP
ECEV(d) i : n - dark
VS> 0ii : p - dark

VS< 0VS> 0V S< 0iii : n - illuminatediv : p - illuminated

VS> 0

VS> 0VS< 0V S< 0Fig. 1. (Color online) (a, b) Schematic illustrations of experimental setupfor three diﬀerent conditions. (c) Potential of p–n junction. EC , EV , andSPBBIP represent the conduction band minimum, valence band maximum
and surface photovoltage due to the reduction of built-in potential,respectively. (d) 1-D band structures of metal–insulator–semiconductor(MIS) conﬁguration of STM measurement. i, ii: Dark condition. iii, iv:
Under photoillumination. The change in the built-in potential [SPV in (c)]observed for the open-circuit setup is also shown in iv by red lines.

2.

Experimental Methods

Experiments were carried out under UHV (<1  108 Pa)
conditions at room temperature. A p–n junction wasprepared by growing n-type (Si-doped, 2:0  1018 cm3 ,500 nm) and p-type (Be-doped, 2:0  1018 cm3 , 500 nm)
GaAs layers on an n-GaAs(001) substrate. Both ends of thesample were connected to electrodes [Fig. 1(a)], or the endof the p-type side was opened [Fig. 1(b)], depending on the6117

(a)

(b)
Tunneling Current (nA)

S. YOSHIDA et al.

Tunneling Current (nA)

Jpn. J. Appl. Phys., Vol. 47, No. 7 (2008)0.2
0.1
0.0
IlluminatedDark-0.1-30.0-0.2-0.4

Illuminated
Dark

-0.6

2-3(d)-2 -1 01
Bias Voltage (V)

20.6

0.5

SPV(V)SPV (V)(c)-2 -1 01
Bias Voltage (V)

0.2

0.0
n-typep-type-0.5-3-2 -1 0
1Bias Voltage (V)0.4
0.2ExpFit

0.0

2-5-4-3101010
Light Intensity (W)Fig. 2. (Color online) (a, b) I–V curves obtained for the condition shown in Fig. 1(b)-1. (c) Sample-bias voltage-dependent SPV.
Blue and red lines are derived from the I–V curves in (a) and (b). (d) Light intensity dependence of SPV.

experiment. Mechanically chopped illumination from a laser
diode (635 nm, 100 Hz), 60 oﬀ-normal to the surface, wasfocused onto the sample with a spot diameter of 0.02 mm. In
the case of the open circuit [Fig. 1(b)], light-spot position 1or 2 was adopted depending on the experiment. An electrochemically etched tungsten tip was used.
3.

Results and Discussion

Figure 1(c) shows a schematic illustration of the potentialin a p–n junction. Band bending due to the charge transfer toequalize the Fermi energy on both sides of n- and p-type
region corresponds to built-in potential. If both electrodes
are closed [Fig. 1(a)], SPV produced by the reduction ofTIBB, SPVTIBB , is large (small) at positive (negative)
sample bias voltages for the n-type (p-type) region, due tothe positional relation between the conduction (valence)band edge and the Fermi energy [Fig. 1(d), black lines].
However, when the end of the p-type side is open
[Fig. 1(b)], large SPV is expected to appear at positive bias
voltages for the p-type region, due to the reduction of thebuilt-in potential under photoillumination, surface photovoltage SPVBIP [Figs. 1(c) and 1(d)-iv, red lines].
Figures 2(a) and 2(b) show typical I–V curves obtained
for n- and p-type regions. For a positive-sample-bias-voltage
condition shown in Fig. 2(a), tunneling current oscillates dueto the change in SPVTIBB caused by the chopped-laser
illumination, and the two virtual I–V curves, whichcorrespond to those under dark (blue dotted line) andilluminated (red solid line) conditions, can be simultaneously obtained. The SPV spectrum [blue dots in Fig. 2(c)] isobtained by calculating the lateral shift of the two I–Vcurves with respect to the bias voltage for the I–V curveunder the dark condition. The central part of the spectrumwith the tunneling current below 3 pA is missing due to thediﬃculty in calculating the shift of the two I–V curves inthat region. SPVTIBB is small for the negative sample bias
region as expected.
In contrast, in the case of the p-type region [Fig. 2(b)],tunneling current oscillates for both positive and negative
bias voltages, and a notable point is that the SPV observed in
the positive sample-bias voltage region is bias-independent,
0:3 V, as shown by the red dots in Fig. 2(c). Since SPVTIBB
is bias-dependent and appears only in the p-type layer for
negative sample-bias voltage, this SPV observed for thep-type layer is attributed to the reduction of the built-inpotential under photoillumination, SPVBIP .
From the theory of superband-gap illumination, SPV has alogarithmic dependence on the light intensity I as SPV ¼
C lnðI=I0 Þ,14,15) where I0 is an arbitrary light intensity used
for normalization, and C is a constant needed for unit
conversion. Figure 2(d) shows the relationship between I
and SPV (¼ SPVBIP ) obtained by averaging the values
measured over the p-type area (300  400 nm2 ) far from theinterface of the p–n junction with a positive sample biasvoltage (+2.5 V). The agreement between the theory and theexperimental results indicates that our method can quantitatively provide SPVBIP in a semiconductor device regardless of the TIBB eﬀect.
As has been seen, with the open-circuit condition shownin Fig. 1(b)-1, we conﬁrmed that there are two components

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Fig. 3. (Color online) SPV-mapping images obtained under the three diﬀerent conditions (a–b, c–d, e–f) corresponding to those inFig. 1(a), Fig. 1(b)-1, and Fig. 1(b)-2, respectively.

(a)(b) positive VS

0.6

Ec(c)
(a)
(d)

0.4
SPV (V)STM tip

0.2
0.0
2.5V (a)-2.7V (b)2.5V (c)-2.7V (d)

-0.2
-0.4

negative VS

SCR

-0.6
0100

Ev

200

300

(b)
400

Ec500

Line-profile (nm)

EvFig. 4. (Color online) (a) Cross sections of mapping images in Figs. 3(c) to 3(f), along the blue line in Fig. 3(c). (b) Apparent changes
in the potential of p–n junction with positive (upper) and negative (lower) bias voltages.

in SPV, namely, SPVTIBB and SPVBIP , and succeeded indistinguishing SPVBIP from SPVTIBB . Next, we show theresults of SPVBIP mapping over the p–n junction. If thelaser spot is focused below the STM tip [Fig. 1(b)-1],
both SPVTIBB and SPVBIP are included in SPV, which is
unproﬁtable for the mapping of SPV. One way to be freefrom the eﬀect of TIBB is to use the fact that TIBB is a local
eﬀect appearing just below the STM tip. Namely, we canremove the SPVTIBB component by illuminating only thep-type layer, away from the tunnel junction [Fig. 1(b)-2].
Figures 3(a) to 3(f) show SPV-mapping images obtained
under the conditions shown in Figs. 1(a), 1(b)-1, and 1(b)-2.
Since SPVTIBB is dominant with the closed-circuit condition[Figs. 1(a)], a large SPV (¼ SPVTIBB ) is observed over then-type (p-type) region for positive (negative) sample bias
voltages, as explained before using the 1D-band structuresin Fig. 1(d). The intermediate region (marked by the greenrectangle) where SPV is small under both bias polarities is
the space-charge region (SCR) produced in a p–n junction
interface due to charge transfer. In SCR, the concentration ofphoto-generated carriers rapidly decreases due to the driftcaused by the lateral electric ﬁeld existing in this region.
Therefore, photogenerated carriers do not relax the TIBB,
and SPVTIBB is suppressed in this region. As reported in
our previous paper, these SPV images reﬂect local doping
proﬁles.5)
In contrast, SPVBIP is included for the open-circuitcondition [Fig. 1(b)], where the end of the n-type region isgrounded. When the sample surface below the STM tip is
photoilluminated [Fig. 1(b)-1], both eﬀects appear in SPV(¼ SPVTIBB þ SPVBIP ). Therefore, SPV is observed even inthe p-type region at +2.5 V. The cross sections of Figs. 3(c)and 3(d), along the blue line in Fig. 3(c), are shown in
Fig. 4(a).

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Furthermore, when the light spot is moved away from theSTM tunneling junction to the end of the p-type region[Fig. 1(b)-2], the eﬀect of SPVTIBB is eliminated, and only
the SPV induced by the change in built-in potential(SPV  SPVBIP ) can be visualized.
In fact, the SPVs obtained at +2.5 and 2:7 V for thep-type region are close to each other. The small SPV(0:1 V) observed here for the n-type region at +2.5 V[yellow line (c) in Fig. 4(a)] is due to the existence ofSPVTIBB caused by the weak illumination leaking into thetunnel junction. In the p-type region, the SPV under negative
bias voltages [green line (d)] is lower than that obtained for
positive bias voltages [yellow line (c)], which is caused bythe SPVTIBB (0:2 V) induced under the negative biasvoltage condition.
Although SPVBIP should increase from the SCR region,
SPVBIP appears even in the n-type region under positive bias
voltages as shown by the yellow line (c). To understand thisresult, let us see how TIBB aﬀects the potential of the p–njunction. Figure 4(b) shows a one-dimensional (1-D) band
diagram of the p–n junction with the eﬀect of TIBB underpositive (top) and negative (bottom) bias voltages. Under
positive bias voltage, the STM tip locally induces upward
band bending, resulting in the apparent positional shift of thejunction interface toward the n-type region.16) The opposite
directional eﬀect occurs under negative sample bias voltage[Fig. 4(b) bottom]: the interface position apparently shifts
toward the p-type region, [green line (d) in Fig. 4(a)]. The
observed gradual slope of SPVBIP extending over 50 nm(yellow line) reﬂects the size of TIBB at this bias voltage.
4.

Conclusions

In conclusion, we demonstrated a method of nanoscalemapping of the built-in potential of a GaAs p–n junction
using LM-STS. The spatially resolved SPV obtained by
LM-STM includes information about the built-in potential of
the p–n junction as well as the photo-induced relaxation of
tip induced band bending. These two components wereseparately evaluated, and mapping of the built-in potential

was accomplished on the nanoscale. These results indicate
the potential of LM-STS as a promising method of probing
the nanoscale characteristics of advanced semiconductor
devices of the future.
Acknowledgements
This work was supported in part by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education, Culture,Sports, Science, and Technology of Japan. We thank Ms. Rie
Yamashita, in our group at the University of Tsukuba, for
her help in preparing this paper.

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