Surface Science 479 (2001) 33±42

www.elsevier.nl/locate/susc

STM, STS, and local work function study of Cs/p-GaAs(1 1 0)
T. Yamada, J. Fujii, T. Mizoguchi *Faculty of Science, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171-8588, Japan
Received 7 December 2000; accepted for publication 24 January 2001

Abstract
Various surface con®gurations of Cs/p-GaAs(1 1 0) have been studied by scanning tunneling microscopy and scanningtunneling spectroscopy with an interest in relation to the photoelectron emission. The local work function is found to be
4.8, 3.3, and 3.3 eV from clean p-GaAs(1 1 0), Cs one-dimensional (1D) lines (Cs coverage of 0.23 ML), and partiallyordered Cs polygons (Cs coverage of 0.5 ML), respectively. It is understood that only a coherently c(4  4)-ordered Cspolygon surface (Cs coverage of 0.6 and 0.7 ML) can emit photoelectrons due to a sucient reduction of the local work
function down to 1.3 eV to get the negative electron anity state. The local work function image shows that the boundaryof Cs atoms has a lower local work function than at the top of Cs atoms. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: Photoemission (total yield); Scanning tunneling microscopy; Scanning tunneling spectroscopies; Work function
measurements; Photoelectron emission; Alkali metals; Gallium arsenide; Low index single crystal surfaces

1. Introduction
Spin-polarized electron beam experiments belong to one of the most important experimental
methods for understanding spin-dependent phenomena in solid state physics and also high energy
physics. p-GaAs is now practically used as a spinpolarized photoelectron source [1]. Circularly polarized GaAlAs laser light of 1.58 eV creates a spin
dependent excitation of P3=2 -state valence band
electrons to the conduction band at the C point.
The most crucial point of this method is the surface treatment of the p-GaAs source to get anegative electron anity (NEA) state in order to*Corresponding author. Tel.: +81-3-3986-0225; fax: +81-35992-1029.
E-mail address: tadashi.mizoguchi@gakushuin.ac.jp (T.
Mizoguchi).

emit spin-polarized conduction electrons to thevacuum.
Alkali metals on semiconductor surfaces havebeen studied for a long time in relation to the metal±
semiconductor interface electronic state [2]. In order
to investigate the NEA surface, many experimental
studies have been performed, including low energy
electron diraction (LEED), Auger electron spectroscopy (AES), and photoelectron spectroscopy(PES) [3±15]. A Cs2 O overlayer, which has thecharacter of an n-type semiconductor (band gap of 2eV), is considered to be necessary for the NEA state.
There is a recipe, the so-called yo±yo treatment, topromote photoelectron emission by alternate deposition of Cs and O2 on a p-GaAs(0 0 1) surface;
however, the mechanism of this procedure is not yet
understood. Bands in GaAs bend down approximately 0.4 eV at the interface with Cs2 O, and thework function is reduced from 4.5 to 1.2 eV. In thisway, the NEA condition is achieved. Although0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 8 9 4 - 9

34

T. Yamada et al. / Surface Science 479 (2001) 33±42

Cs2 O/p-GaAs(0 0 1) has been used for a practicalpolarized photoelectron source, no atomic scale information about this surface has been reported. We
tried to observe the p-GaAs(0 0 1) surface after various treatments according to the earlier describedrecipe for a practical photoelectron source. We did
not, however, succeed in getting atomic scale images
with STM for the (0 0 1) surface as also reported byB
urgler et al. [16].
On the other hand, it was shown by Fujii et al.
[10] that photoelectrons could be emitted from Cs/
p-GaAs(1 1 0) without oxygen deposition although
the emission eciency is a little bit lower than ofthe (0 0 1) surface of p-GaAs.
While scanning tunneling microscopy (STM)
has been previously used to observe the atomic
scale arrangement of Cs atoms on p-GaAs(1 1 0)[17], no study has been reported in relation to thephotoemission. The purpose of this study is to®nd the relation between photoelectron emission and various surface con®gurations of Cs/pGaAs(1 1 0). The most important parameter in thisperspective is the local work function which has
been measured in this study by STM and scanningtunneling spectroscopy (STS) for various Cs coverages on p-GaAs(1 1 0).
2. Experimental
The UHV-STM system comprises an STM head
(JEOL-TM-51020STMS) whose limit of measurement is 0.01 nm, an STM controller (JEOLTM93011000) supplied by JEOL Ltd., an STM
chamber with a laser system and a Cs evaporation
system, and a preparation chamber with a Au
evaporator and a cleaving system, which was constructed in our laboratory. The base pressure of the
STM and the preparation chambers were maintained at less than 6  10 11 and 2  10 10 mbar,
respectively. We succeeded to reduce the vibrationlevel of the STM stage to less than 0.01 nm withviton rings, which can damp vibrations above 40
Hz and correctly arranged ring mounts (RM-120-3,
Kurashiki Kako Ltd.), which are eective to damp
a 25 Hz vibration of the ¯oor.
A p-GaAs(0 0 1) wafer doped with Zn at aconcentration of 1019 cm 3 was cleaved in the

UHV chamber to get a clean and ¯at (1 1 0) surface. Cs atoms were adsorbed on the (1 1 0) surface
from a SAES Getters chromate dispenser after asucient outgas procedure with a low deposition
rate of about 0.08±0.2 ML/min at room temperature in UHV (5  10 10 mbar during the deposition). Here, 1 ML (monolayer, ML) is de®ned as
one Cs atom per As atom on the (1 1 0) surface.
The Cs coverage was estimated from the STM
images and the deposition time. Each Cs polygon
was assumed to contain ®ve Cs atoms.
In order to observe photoelectron emission, a
3:0  0:28 mm2 area of the Cs/p-GaAs(1 1 0) surface, which was biased at 9.2 V, was illuminatedwith linearly polarized GaAlAs laser light (k  780
nm, 2 mW/mm2 ), incident with an angle of 19°
relative to the surface normal. The photocurrent wasmeasured with a picoampere meter.
Most STM images were obtained at room temperature in constant current mode with typical set
points of 0.130 nA and 1.580 V. Electrochemically etched tungsten tips, which were dipped in
2% HF solution for 10 s to remove the oxide layer[18], were frequently conditioned by high-voltage
pulses during tunneling in UHV.
STS was utilized to study the electronic structure of the cleaved, clean p-GaAs(1 1 0) surface
and various arrangements of Cs on p-GaAs(1 1 0).
I±V spectra were taken by cutting o the feed-backloop at a selected position of the image while thesample bias voltage was varied between 2 V to
2 V. In order to get information about the local
work function, the tunneling current was measured as a function of the separation between the
tip and a selected point of the sample surface [19±22]. The local work function images were takenwith a lock-in ampli®er by modulating the tip at 7.47 kHz, whichsample separation by 0.5 A
was higher than the cut-o frequency (1 kHz) of
the feed-back loop for normal constant mode
imaging but within the range of the response frequency ( 6 8 kHz) of the current ampli®er.
3. Results and discussion
Atomic images of Ga and As in the cleaved(1 1 0) surface of p-GaAs from the (0 0 1) wafer are

T. Yamada et al. / Surface Science 479 (2001) 33±42

35clearly observed at positive bias voltage, 1.795 V,
and negative bias voltage, 1.580 V, respectively.
The same result has been reported by Feenstra
et al. [23].
STM topographic images of Cs atoms on pGaAs(1 1 0) at various stages of coverage were obtained. Fig. 1(a) shows the cleaved p-GaAs(1 1 0)
surface, from which no photocurrent is detected.
Then, 0.1 ML of Cs atoms is adsorbed as shown in
Fig. 1(b). Cs atoms adsorbed on Ga atoms form a
zig-zag one-dimensional (1D) line along the [1 1 0]
direction. The corrugation of the Cs line is about 1
 from the As surface, and the corrugation in theA No photoemission is obline is about 0.2±0.3 A.
served at this stage.
Fig. 1(c) shows a surface covered by 0.23 ML
Cs. One more Cs line starts beside the zig-zag line
making a line of three rows of Cs atoms along the[1 1 0] direction on the (1 1 0) surface. The arrangement of Cs atoms is c(2  2) for the substrate
As lattice, where the central Cs atoms are observed
 higher than the Cs atoms in the side
to be 0.4 A
rows in the constant current imaging mode asshown in Fig. 2. This height dierence is considered to be apparent because the tunneling current
will ¯ow more easily to the Cs atom in the central
row if the tip±sample separation is kept constant.
The tunneling current is estimated to increase by afactor of 2.1 from the observed decay rate, 1 for a Cs 1D line. No photoemissionj  1:85 A
is observed at this stage.

Fig. 1. STM topographic images of Cs/p-GaAs(1 1 0) for different coverages of Cs atoms (sample bias: 1.580 V, tunnelingcurrent: 0.130 nA.) The crystallographic direction [1 1 0] is oriented 45° counterclockwise from the horizontal. (a) Cleaved
p-GaAs(1 1 0) (3  3 nm2 ), (b) 0.1 ML Cs (11  11 nm2 ), 1D
zig-zag line, (c) 0.23 ML Cs (4  4 nm2 ), 1D triple line, (d) 0.35
ML Cs (28  28 nm2 ), Cs polygons are observed. No photocurrent is detected for cases (a)±(d). (e) 0.5 ML Cs (59  59
nm2 ), partially ordered Cs polygons. A photocurrent of 0.18
nA/mm2 is detected. (f) 0.6 ML Cs (22  22 nm2 ), coherentlyc(4  4)-ordered Cs polygons. A photocurrent of 240 nA/mm2
is detected. (g) 0.7 ML Cs (65  65 nm2 ), layer-by-layer growthof ordered c(4  4) Cs polygons. A photocurrent of 360 nA/
mm2 is detected. (h) Rough surface (40  40 nm2 ) of Cs grainsof 5 nm in size is observed when Cs is deposited at a high rate.
No photocurrent is detected.

Fig. 2. The observed height of the triple row Cs line in a constant current image (0.130 nA, sample bias 1.580 V). (A) STM
topographic image (2:5  3:0 nm2 ) of a Cs 1D triple line. (B)The line pro®les corresponding to the white lines in (A).

36

T. Yamada et al. / Surface Science 479 (2001) 33±42

Increasing the coverage of Cs, the Cs lines becomes longer and the number of lines increases.
Above a coverage of 0.25 ML, the Cs lines start tocontact each other, and the lines transform into Cs
polygons. At 0.35 ML Cs coverage, the 1D Cs lines
disappear, and Cs polygons cover the (1 1 0) surface as shown in Fig. 1(d). A Fourier-transformSTM image suggests that Cs polygons start tomake ordering. No photoemission is observed at
this stage.
At about 0.5 ML Cs on the (1 1 0) surface, theCs polygons adopt partial ordering as shown in
Fig. 1(e). At this stage, a weak photocurrent (0.18
nA/mm2 ) is detected. Fig. 1(f) shows a surface
covered with about 0.6 ML Cs. A coherent c(4 4) two-dimensional (2D) long range ordering of Cs
polygons covers the (1 1 0) surface.
In Fig. 3(A) an enlarged image of the ordered Cs
polygons is shown. Whitman et al. [17] proposedthat each polygon contains ®ve Cs atoms, which
are arranged in a unit cell as shown in Fig. 3(C).
Although Cs atoms inside the polygon cannot be
separated in our observation, it seems to us moreplausible, from the observed total shape of eachpolygon, that the Cs atoms are arranged as shownin Fig. 3(B). This con®guration can be understoodas the alternative arrangements of the half ®lledand 3=4 ®lled 1D Cs lines with respect to the As
lattice along the [1 1 0] direction.
The photocurrent drastically increases up to 240
nA/mm2 for this c(4  4)-ordered Cs-polygon

Fig. 3. (A) STM topographic image (7:5  7:5 nm2 ) of 2Dc(4  4)-ordered Cs polygons (sample bias: 1.580 V, tunnelingcurrent: 0.130 nA). (B) Although Cs atoms inside the polygon
cannot be separated in our observation, it seems to us moreplausible from the observed total shape of each polygon. Cs
atoms (white circles) are arranged in (A) with respect to substrate As atoms (small black spots). (C) The reported arrangement of Cs atoms in a unit cell of Cs polygons by
Whitman et al. [17].

surface. The photocurrent decays by 1=3 with irradiation of laser light for 1 min, but becomes
more stable afterwards.
When the (1 1 0) surface was covered with about
0.7 ML Cs, we observed layer-by-layer growth ofc(4  4) 2D-ordered Cs polygons as shown in Fig.
 which1(g). The height of the layer is 3.2 and 7 A,
is roughly equal to an atomic diameter of the Cs
atom and twice that, respectively. The photocurrent reaches 360 nA/mm2 at this stage. A c(4  4)
2D-ordered Cs polygon-growth on the stepped
p-GaAs(1 1 0) surface was also observed, for which
 ofthe step height was equal to the step height (2 A)
p-GaAs(1 1 0).
When Cs atoms are deposited with higher deposition rates, they do not grow layer-by-layer andthe surface becomes very rough with Cs grains of5 nm in size as shown in Fig. 1(h). No photocurrent is observed for this thick Cs surface.
All topographic images in Fig. 1 were taken
without laser light illumination. Although no
photocurrent by electron emission is detected fromthe Cs 1D lines, there is a clear enhancement of the
tip±sample distance in the constant current mode(roughly 0.1 nm) at Cs lines as shown in Fig. 4
when 0.5 mW/mm2 laser light (k  780 nm) illumination is turned on. The tunneling current to theCs 1D line with photoexcitation is estimated toincrease by a factor of 6 from the observed decay
 1 for a Cs 1D line.
rate, j  1:85 A
In Fig. 5, the results of I±V spectra measured
at dierent surface con®gurations of Cs/pGaAs(1 1 0) are shown. In bulk p-GaAs, the band
gap at the C point is 1.43 eV at room temperature, and the Fermi level is very close to the topof the valence band. The I±V spectrum obtainedon the cleaved p-GaAs(1 1 0) surface indicatesthat the bottom of the conduction band is about
1.4 eV above the Fermi level. The top of thevalence band is about 0.05 eV below the Fermi
level, which suggests that the band bending forthe p-GaAs(1 1 0) surface is quite small. At theposition of the Cs line or Cs polygon in Fig.
1(b)±(g), the band gap is reduced to 0.9±1.2 eV.
The Fermi level is 0.45±0.6 eV below the bottomof the conduction band, i.e., at the center of theband gap. These adsorbed Cs overlayers havecharacteristics of semiconductors without any

T. Yamada et al. / Surface Science 479 (2001) 33±42Fig. 4. Enhancement of the tip±sample distance (B) on the Cs lines (A) in the constant current mode (0.130 nA, sample bias:
shown when the laser illumination (0.5 mW/mm2 ) is turned on.

explicit dierences. Whitman et al. [17] reported
that the band gaps of Cs 1D zig-zag lines on nGaAs(1 1 0), Cs 1D-triple lines on p-GaAs(1 1 0),
and a c(4  4) 2D-ordered Cs-polygon overlayer
were 1.10, 0.65 and 0.6 eV, respectively. The thickCs surface with Cs grains of 5 nm in size (Fig.
1(h)) has a metallic characteristic without a band
gap.

37

1.580 V)In order to get information on the local surface
work function, the tunneling current is measured
with varying tip±sample separation S [19,24±29].
The tunneling current decays with S asI  I0 exp jS

1

The decay rate j is related to an eective local
potential barrier height /A via,

Fig. 5. The tunneling current versus bias voltage (I±V spectra) measured at (a) p-GaAs(1 1 0), (c) Cs 1D triple line, (e) partially ordered
Cs polygons, (f) c(4  4) 2D-ordered Cs polygons, (g) ordered Cs-polygon layer, and (h) Cs metallic thick surface. (a), (c), (e), (f), (g),and (h) correspond with those in Fig. 1, respectively.

38p
j  32p2 m/A =h

T. Yamada et al. / Surface Science 479 (2001) 33±422where m is the electron mass and h is the Plank
constant. Here /A is the averaged value of thework function of the sample and the tungsten tip,i.e.,
/A  /tip  /sample =23In order to calibrate the work function of ourtungsten tip, we prepared clean a Au ®lm of 100
nm thick on p-GaAs(1 1 0) and Si(1 1 1), and annealed at over 1300 K to get an ordered (1 1 1)

surface. The observed barrier height between
Au(1 1 1) and our tungsten tip was 4:3  0:3 eV.
Using the reported value of the work function ofAu(1 1 1) surface in the literature, 5.38 eV [30], thework function of our tungsten tips was deduced tobe 3.2 eV.
In Fig. 6, the tunneling current, in logarithmic
scale, was measured as a function of DS at all theobserved Cs/p-GaAs surfaces. Here, DS is the shiftof tip±sample distance from a tip-approachingposition with tunneling current 0.130 nA andsample bias 1.580 V, i.e., I is 0.130 nA at DS  0,

Fig. 6. The tunneling current (I) in logarithmic scale versus DS from (a) p-GaAs(1 1 0) ( ), (c) Cs triple line ( ), and (f) c(4  4) 2Dordered Cs polygons ( ). (a), (c), and (f) correspond with those in Fig. 1, respectively. Here, DS is the shift of tip±sample distance froma tip-approaching position with tunneling current 0.130 nA and sample bias 1.580 V, i.e., I is 0.130 nA at DS  0, and DS < 0 meansthat the tip approaches to the sample. The decay rate j is obtained from these plots, and then the local work function /sample is derived
using Eqs. (2) and (3).

T. Yamada et al. / Surface Science 479 (2001) 33±42

and DS < 0 means that the tip approaches to thesample. By way of illustration, three typical spectra are shown in Fig. 6 for a cleaved clean p-GaAs(1 1 0) (Fig. 1(a)), the Cs triple line (Fig. 1(c)), andthe c(4  4)-ordered Cs polygons (Fig. 1(f)).
The decay rate j is deduced from the slope of 1 forthe log I versus DS plot to be 2:05  0:07 A
p-GaAs(1 1 0). The j for the Cs zig-zag line, the Cs
triple line, the Cs polygons, and the partially ordered Cs polygons coincide within the experi 1.
mental error to the same value of 1:85  0:07 A
The j for the c(4  4)-ordered Cs polygons is 1 . The local work
found to be 1:54  0:07 A
function of each con®guration is calculated by
Eqs. (2) and (3). It is deduced to be 4:8  0:27 eV
for the clean p-GaAs(1 1 0) (Fig. 1(a)). The local
work functions at the position of the Cs line or Cs
polygon in Fig. 1(b)±(e) have the same value of
3:3  0:25 eV. Only the local work function ofc(4  4) 2D-ordered Cs polygons on the (1 1 0)
surface (0.6 ML Cs) (Fig. 1(f)) is found to be remarkably reduced down to 1:3  0:21 eV, which isless than the band gap of bulk p-GaAs, i.e., 1.43
eV.
In Fig. 7, the local work function images ((A-2)
and (B-2)) are shown together with normal topographic images for the Cs 1D line (A-1) andthe coherently c(4  4)-ordered Cs polygons (B-1)
on p-GaAs(1 1 0). The region of Cs 1D lines,
which are bright in the topographic image (A-1),
clearly corresponds to the dark region in (A-2)
where the local work function is lower. The noiseof the local work function image is reduced by
the lock-in technique, and the atomic image becomes clearer than in the normal topographic
imaging mode.
It is quite interesting to point out that the top of
the Cs atoms is brighter in the dark region of theCs lines in the local work function image (Fig.
7(A-2)). For the coherently c(4  4)-ordered Cs
polygons, the boundary between the Cs polygons
is darker than the top of the polygons in Fig. 7(B2), where white particles in the polygons would
correspond to individual Cs atoms although theresolution and the signal to noise ratio is critical. It
should be noted that photoelectrons emerge fromthe boundary of Cs atoms where the local work
function is lower than that at the top of Cs atoms.

39

Fig. 7. Topographic atomic image (A-1) and the local work
function image (A-2) for a Cs 1D-line (19  19 nm2 ) surface.
Topographic atomic image (B-1) and the local work function
image (B-2) for a c(4  4) 2D-ordered Cs-polygon surface(40  40 nm2 ). In the local work function images, the darker
part has a lower work function. In (A-2) and (B-2), one pixel
corresponds with 0.156 and 0.312 nm, respectively.

A phenomenological explanation why theboundary of Cs atoms shows a lower work function may be as follows. In constant current mode,the path of the tip follows the corrugation of Cs
atoms more sensitively for a high constant tunneling current than for a low tunneling current for
which the path becomes smooth and ¯at because
the tip is further away from the surface. Thismeans that the derivative (D ln I/DS), i.e., j becomes lower at the boundary of Cs atoms, where
the path is concave, than at the top of Cs atoms asshown in Fig. 8.
In Fig. 9, the results of the I±V and I±S experiments are summarized in an energy band
scheme together with STM images for the cleaved
p-GaAs(1 1 0), the Cs 1D zig-zag line, and the coherently c(4  4)-ordered Cs-polygon surface. It is
understood that the photoexcited conduction
electrons in p-GaAs can be emitted from the coherently c(4  4)-ordered Cs-polygon surface only
thanks to the remarkable reduction of the local

40

T. Yamada et al. / Surface Science 479 (2001) 33±42Fig. 8. A schematic illustration of the local work function images (Fig. 7). The dash-dotted line reveals the path of the tip forI0  0:130 nA. The two solid lines reveal paths of the modu The upper and lower dotted lines related tip (DS  0:5 A).
veal paths of the tip for ln I1  ln I0 0:925 and ln I2  ln I0 
0:925, respectively. The separation between the dotted lines isinversely proportional to the derivative d ln I=dS.

Fig. 9. The energy band diagrams for the photoemission-process at p-GaAs(1 1 0), the Cs 1D line, and the c(4  4) 2D-ordered Cs-polygon surface. The band gap in the p-GaAs is 1.43
eV at room temperature [1]. Square marks represent our STS
results.

work function down to 1:3  0:21 eV, which isless than the bottom of the conduction band in
p-GaAs.
In Fig. 10, the local work function, the top of
the valence band, the bottom of the conduction

band, and the observed photocurrent are plottedversus the coverage of Cs on p-GaAs. The quantum eciency (photoelectrons/photons) is so low,of the order of 10 3 in our observation, that thephotocurrent indicates only the onset of thephotoemission. It should be noted that the local
work function of Cs/p-GaAs(1 1 0), which is measured at the position of the Cs line or Cs polygon,
must be distinguished from the averaged work
function, which is usually treated as a function of
the coverage of Cs, h, on the surface.
There is a general tendency that the averaged
work function of the surface adsorbed by alkali
atoms decreases linearly with increasing coverageof alkali atoms on the surface. The local work
functions at the Cs con®gurations of Fig. 1(b)±(e)are observed to reduce to 3.3 eV, which gives theaveraged work function 4.8 1 h  3:3h (eV).
Further reduction of the local work function occurs down to 1.3 eV for the coherently c(4  4)ordered Cs-polygon surface whose coverage is 0.6
ML.

Fig. 10. The local work function ( ), the top of the valenceband (), the bottom of the conduction band (j), and theobserved photocurrent ( ) are plotted versus the coverage of Cs
on p-GaAs. a±h correspond with those in Fig. 1, respectively.
The local work function, the top of the valence band, and thebottom of the conduction band of Cs/p-GaAs(1 1 0) are measured at (a) the (1 1 0) surface, (b±g) the position of Cs line or Cs
polygon, and (h) the Cs metallic surface. Although we did notmeasure the local work function at the Cs thick surface, thework function of Cs bulk [2] is shown at the thick Cs coverage.

T. Yamada et al. / Surface Science 479 (2001) 33±42

The averaged work function of a metal has beeninterpreted by a simple jellium model [31,32]. The
reported calculation of electronic states of adsorbed Na atoms on jellium [33] shows that thedensity of electrons is reduced in the region of thevacuum side of the adsorbed atoms and increases
in the intermediate region between the adsorbed
atoms and the surface of jellium. This chargetransfer, due to the bonding mixture between
electron orbitals of alkali atoms and those of
substrate atoms, is thought to reduce the averaged
work function.
At full coverage, the valence electrons of adsorbed atoms are thought to form energy bands
through direct mixing of bonding orbitals. The
local situation of Cs polygons or even Cs lines
where Cs atoms are lined closely may correspondto the full coverage in the calculation of the uniform square lattice where the nearest neighbordistance between Na atoms is de®ned to be 8 a.u.
 at full coverage. It is interesting to note
(4.24 A)
that the density of electrons is reduced rather between alkali atoms than at the top of them in theregion of the vacuum side at full coverage. Thismeans that the local work function is lower between Cs atoms rather than at the top of them,which may be an additional explanation of ourobservation that the boundary of Cs atoms seems
darker in the local work function images (Fig. 7(A2) and Fig. 7(B-2)).
The calculated dipole moment per Na adatom
decreases with coverage and the averaged work
function reaches a minimum at half coverage andincreases with coverage beyond the minimum. Thistendency suggests that the local work function
should increase with a coverage of more than half.
The observed local work function, however, decreases drastically at a coverage of 0.6 ML, which
may correspond to the full coverage in the squarelattice calculation. This remarkable reduction of
the local work function must be related to thecoherent ordering of Cs polygons. There must be
some mechanism to enhance the reduction of thelocal work function induced by the coherent ordering of Cs polygons.
The ordered polygons induce a perturbation
potential, which opens up the additional band gap
at the zone boundary of new the Brillouin zone,

41

which corresponds to a c(4  4) super-lattice in the
2D band of surface electrons. There may be apossibility with this mechanism that the chargetransfer and then the reduction of the local work
function are enhanced so as to reduce the totalenergy of the electronic system.
4. Conclusion
The local work function for various surface
con®gurations of Cs/p-GaAs(1 1 0) was found to
be 4.8, 3.3, 3.3 eV for the clean p-GaAs(1 1 0), Cs
1D lines (Cs coverage of 0.1 and 0.23 ML), andpartially-ordered Cs polygons (Cs coverage of 0.5ML), respectively. It was understood that only thecoherently c(4  4)-ordered Cs-polygon surface(Cs coverage of 0.6 and 0.7 ML) can emit photoelectrons due to a sucient reduction of the local
work function down to 1.3 eV to get the NEA
state.
Acknowledgements
We would like to express our sincere thanks forvaluable discussions to Prof. Dr. Herman van
Kempen, M.M.J. Bischo, and R. de Kort, and to
Dr. M. Tokii and Dr. A. Keen for the preparationof the manuscript. We appreciate the support by
the High Tech Research Center from the Ministryof Education Science, Sports and Culture.
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