Photoemission studies of the GaAs(100)-4xl surface

Journal of Electron Spectroscopy and Related Phenomena, 52 (1990) 133-138

133

Elsevier Science PublishersB.V., Amsterdam- Printedin The Netherlands

PHOTOEMISSION

STUDIES OF THE GaAs(100)-4xl SURFACE

J. KANSKIl, QU HUAl, P.O. NILSSONl, and U.O. KARLSSON* 1Department of Physics, Chalmers University of Technology, 5412 96 Goteborg (Sweden) *MAX-laboratory, University of Lund, Box 118, S-221 00 Lund (Sweden) SUMMARY The electron structure and surface composition of the MBE-grown GaAs(lOO)-4x1 surface has been studied with photoelectron spectroscopy. The core level lineshape analysis reveals two surface shifted components for Ga, only one of which is associated with the first atomic layer. This, together with the fact that surface states are found in both [IlOJ and [llO] azimuths with the periodicity of the bulk, suggests that the 4x1 reconstruction is not associated with dimerization. A model involving a ridge As-Ga structure beyond the last Ga plane is considered as one possible arrangement consistent with our observations. 1.

INTRODUCTION

The polar surfaces of compound semiconductors are known to be electrostatically unstable. This instability is relaxed by charge redistribution, associated with rehybridization and surface reconstruction.

Despite this fundamentally

interesting,

and poten-

tially technologically important interplay between geometric and electronic structure, detailed characterization of such surfaces is still lacking but for a few cases. So, out of the many reconstructions

reported for the GaAs(lOO) surface, only the As-stabilized

2x4 and ~(4x4) are well understood (1,2). As part of a project aiming at characterizing the different surface reconstructions of GaAs(lOO), we report here a study of the Gastabilized 4x1 surface. Previous studies of this surface are very limited (3,4). EXPERIMENT

2.

Our experimental setup at the MAX synchrotron radiation laboratory in Lund, Sweden, consists of a molecular beam epitaxy (MBE) system from KRYOVAK, equipped with six effusion cells and RHEED and a surface analysis system from VSW containing a hemispherical electron energy analyzer and LEED. The two chambers are interlocked

with a small UHV vessel, which can be loaded with six samples and

through which sample transfer between the MBE and the spectrometer systems can be made

in UHV.

monochromator

0368-2048/90/$03.50

This

three

chamber

configuration

at beamline 41. 0 1999 ElsevierScience PublishersB.V.

is connected

to the TGM

134

The GaAs wafers were chemically polished and mounted with In to MO holders. During the MBE growth the substrate was kept at 800 K and a 2x4 reconstruction was observed in RHEED. The 4x1 reconstruction was obtained by annealing the substrate at about 900 K after shuttering the Ga and As4 sources. In the background As pressure of about 3x10-7 ton this surface rapidly converted to a 2x6 reconstruction. After the As pressure had beed reduced to about 1x10-9 torr, it was possible to maintain the 4x1 surface for several minutes. When transfered to the electron analysis chamber the 4x1 geometry remained stable throughout the following photoemission

experiments as

confirmed by LEED. The angle resolved photoemission data presented here were obtained with the light (predominantly p-polarized) incident at 450 relative to the surface normal. The photoelectrons were detected in the plane of incidence. 3.

RESULTS

3.1 Core level lineshapes As a first step in our examination of the 4x1 reconstructed surface we discuss the Ga(3d) and As6d)

core level line shapes. Previous core level studies of other GaAs

surfaces have proven to provide useful information

concerning

the surface stoi-

chiometry and chemical state (15). The present results are shown in figure 1.

normalemwon

32

33

34

35

36

37

38

39 32

Kineticenergy (eV)

33

34

35

36

37

36

39

Kineticenergy (eV)

Figure 1. Spectral decomposition of the Ga(3d) and As(3df emission. The bottom lines show the difference between the measured and the fitted spectra. For the decomposition of the measured spectra all components were assumed to be identical, except for relative intensities and energy positions. In the fitting procedure we used the following

parameters

for Ga (As): spin orbit splitting 0.45 eV

(0.69 eV1, branching ratio 1.55 (1.53) and lifetime width 0.18 eV (0.22 eV). The total instrumental broadening was taken to be 0.35 eV. We see in figure 1 that the peak fitting analysis results in three components for both Ga and As. (The small peak D is believed to originate from traces of In which has

135

diffused from the edge of the wafer.) The three components

are associated with

emission from the bulk (B) and from two surface components for both Ga and As (A and C). The occurrence of two surface Ga(3d) components has been observed previously for the likewise Ga- stabilized 4x6 reconstruction (5). The surface shifts were in that case similar to those observed here, but the relative weights of these components were essentially smaller. 3.2 Valence band swctra Valence band spectra contain generally information about surface as well as bulk electron states. The surface state features can be identified by several methods, e.g. by their sensitivity to surface contamination. With the access to synchrotron radiation one can exploit another property of surface states, namely their lack of energy dispersion as a function of electron momentum normal to the surface. Figure 2a shows a series of normal emission spectra from the GaAs(lOO)- 4x1 surface. As the photon

..,,,._.,.....,..___.. , hv

GaAs(100)-4x I normal emission

(eV) -

-8

-6

-4

-2

Inltialenergy(eV)

0

l-

A Wavevector

X

Figure 2. a) Normal emission valence band spectra from GaAs(100)-4xl and b) the corresponding structure plot (crosses). The full lines show calculated valence bands from ref. 6. energy is raised we find two peaks which disperse towards larger binding energies and two structures which appear stationary. In figure 2b we have condensed these data into a structure plot assuming an inner potential (relative vacuum) of 13.4 eV and free electron final states. The full lines in figure 2b show the second and third (degenerate) bands from a semi-empirical pseudopotential band calculation (6). Obviously, the two dispersive peaks can be directly interpreted as bulk interband transitions. Non-dispersive structures can be due to either surface states or high density of bulk states. Examination of the calculated band structure shows that there is indeed a high density of states around 1.3 eV in the region of the L-point. However, since such features are generally relatively weak, we may suspect that the 1.3 eV structure reflects a surface state. Also the shoulder-like

structure near the valence

136

band maximum (VBM) is likely to be surface related, since the bulk density of state in this energy region is quite low. Some further insight into the nature of the surface states can be gained by studying the angular dependence of the valence band emission. In figure 3 we show two sets of spectra excited with 21 eV photons, measured perpendicularly and parallel to the direction of the fourfold reconstruction (i.e. in the blol and Ill01

azimuths). To

identify

the surface related

features,

we have indicated

the expected positions for bulk transitions (dashed lines).

The two

stationary

features

are interpreted

emission

from high density

as

of states regions at ZClr;J” and X6,, . Without proper interpolation of the band structure, the dispersion

of the

above noted interband transitions can only be estimated qualitatively

from the calcu-

lated band structure. As the detection angle is increased from

normal

emission

to-

wards either @IO1 or [llO], the lowest band (“2”) is expected to disperse to lower energies. The degeneracy of the upper bands (“3” and “4”) is lifted. The two bands disperse down in energy and a band gap opens in the projected density of states (4). Ener~rel.VBM Figure 3. The polar valence band emission arrows, which indicate at the same energies in

CeV)

angle dependence of the from GaAs(l00)-4xl. The a surface state, are drawn the two panels.

The shoulder

structure

ob-

served around 35’ emission angle falls in this band gap and is then definitely due to a surface state.

Having established this, we conclude that just as in the case of As-terminated reconstructions,

the upper part of the valence band spectrum is dominated by surface

state emission. Photoemission data indicate two such states, but only one of them has

137

sufficiently high intensity to be traced as the polar angle is varied. The dispersion of this state (indicated with arrows) is plotted in figure 4.

We note that

within the accuracy

of our mea-

surements

we cannot distinguish

the dispersions imuths

and

in the two az-

that

the

geometric

fourfold surface periodicity

is not

reflected by the surface state dispersion. A similar result has been re-

r

J Surface

IX1

T

wave vector

Figure 4. Structure plot showing the dispersion of the surface state found in figure 3. 4.

ported for the ~(4x4) surface,

in

which case substrate periodicity was observed for a dangling bond state in the direction orthogonal

to the

As bridge bonds (4).

DISCUSSION

To our knowledge there are no detailed theoretical studies of the surface electron structure of the Ga-terminated surfaces. From ab initio calculations of the surface energies (7) the tendency to dimerization is inferred for both Ga and As termination. It is, however, also pointed out that for the more complex reconstructions further rehybridization may produce more stable surfaces. Thus, the discussion of our results can only be very qualitative. Without specifying

the detailed mechanisms responsible for the measured sur-

face core level shifts, the presence of two surface components obviously reveals two inequivalent sites in the surface region. Such inequivalent sites could for instance be those in an asymmetric surface dimer. Assuming that the crystal consists of perfectly alternating atomic Ga and As layers, one can estimate the fractional Ga coverage from the relative intensity of the surface related emission (8). Such analysis shows that a surface dimer situation, i.e. that both Ga sites are in the first atomic layer is unlikely: with an escape length of about 5.5

A at 35 eV kinetic energy we obtain a coverage of

about 1.8 monolayer. If we instead assume that only the “C” emission comes from the first layer (the other surface shifted doublet may then originate from the next Ga layer), a more realistic fractional coverage of about 0.8 is obtained. Another reason for excluding

asymmetric dimers as the cause for the two surface shifts is the fact that the

relative intensity of the two components is not constant for different samples. Symmetric dimers are obviously not contradicted by these results. The observation of similar surface state dispersion in the two orthogonal azimuths strongly suggests that also symmetric dlmers can be ruled out. The bulk periodicity can then be explained by strong coupling of the dangling bond (sp,) state to the substrate and weak coupling between the overlayer Ga atoms. The situation would then be rather different from that on the As-terminated

2x4 surface, in which case

138

surface dimerizaticn

is the main mechanism behind the reconstruction.

Due to the

polar nature of GaAs, the energies of surface states on the Ga-terminated

surfaces

should be at larger than on the As-terminated ones. One might therefore speculate that the occupancy of the bridge bond state on the Ga-terminated surface should be lower than on the As-terminated

one, and consequently

the dimerization less pro-

nounced with Ga termination. Another driving mechanism for surface reconstruction is the tendency to eliminate charge accumulation

at the surface (9). One simple Ga-terminated atomic arrangement which satisfies the requirement of zero average electrostatic potential is shown in figure 5. Without claiming that this is the actual geometry of the present surface, we note that the model has the observed 4x1 surface symmetry and is consistent with our observation of several surface shifted core lines. It also appears that the overlayer Ga atoms should interact primarily with the substrate, while the interaction

within

the

overlayer

8

0

should be weak. This explains our

Ga

As

observation

A possible model for the GaFigure 5. terminated 4x1 surface (from ref. 9)

of surface state disper-

sion with bulk periodicity. larly this last argumentation

Particushould

however be born out in numerical calculations.

4.

ACKNOWLEDGEMENTS We wish to thank Semitronic AB for supplying us with GaAs wafers and the

technical staff at MAX-lab for all effort dedicated to our project. The financial support from the Swedish Natural Science Research Council is gratefully acknowledged. REFERENCES

2 3 4 5 6 7 8 9

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