Surface anisotropy of III–V compounds

524

Surface Scrence 211,‘212 (1989) 5244533 North-Holland. Amsterdam

SURFACE

ANISOTROPY

OF 111-V COMPOUNDS

Yu CHEN,

J. ANDERSON

and G.J. LAPEYRE

Deppurtment of Ph_wws, Montana State Unioersrty, Boxw~an. MT 59715, USA Received

6 July 1988; accepted

for publication

24 October

198X

The azimuthal dependence of the HREELS spectra of the {I 10) surfaces of GaAs and GaP was studied in the region of the electronic excitations, starting from the energy gap up to 7 eV. A clear azimuthal dependence of the loss spectra was found for both samples, demonstrating - tn WI uh.dute w0.v - the anisotropic nature of the electronic properties of these surfaces. In both compounds, a higher and more structured loss spectrum was obtained when the plane of tncidencr was parallel to the [liO] direction, i.e. the direction of the atomic chains of the (I 10) surface, while a less intense and less structured spectrum was measured with the incidence plane normal to the chain direction. The effect of the exposure to hydrogen was investigated. The degree of anisotropy observed in loss spectra is reduced by hydrogen exposure. revealing the surface nature of the electronic states involved in the scattering processes.

1. Introduction This paper deals with anisotropic effects in the surface electronic states of GaAs(ll0) and of GaP(llO) investigated by high resolution electron energy loss spectroscopy (HREELS). This technique is very useful for this kind of studies because of its surface sensitivity, wide spectral range, absoluteness of results, capability of giving information on filled and empty electronic states, and angular resolution. The (110) surfaces of III-V compounds, either in their ideal or relaxed configurations, are characterized by the occurrence of zig-zag chains of alternate anion and cation atoms in the [llO] direction, corresponding to TX direction in the surface Brillouin zone [l-.5]. This situation presents strong analogies with the (2 x 1) reconstruction on Si( 111) and Geflll) [6], for which a model introduced by Pandey [7] suggests the presence on the first layer of 0039~6028/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

S. Nannarone

et al. / SurJuce anisotropy

of III-

V compounds

525

chains of bonded atoms oriented along a specific crystallographic direction: the consequent lowering of the original square symmetry has been verified by a number of experiments, all of them making use of “polarized” probes, like polarized surface differential reflectivity [8], photothermal displacement spectroscopy [9] and HREELS [lo]. In HREELS experiments, the surface lattice was rotated around the normal to the surface, thus obtaining the azimuthal dependence of the surface electron energy loss structures: intensity maxima were observed when the incidence plane, defined by the incidence direction and the normal to the surface (see fig. 1) was parallel to the atomic chain direction, and the minima when it was perpendicular to the chain direction. We extended this kind of analysis to cleavage surfaces of 111-V compounds and, in particular, to GaAs(l10) and to GaP(110); we show here results on clean and hydrogen-exposed surfaces, in the loss energy region between the bulk gap and 7 eV. In both compounds the losses of the clean surfaces show a marked azimuthal dependence. The anisotropy tends to disappear at increasing coverages, revealing a surface nature of the electronic states involved in this behaviour. The results are compared in the discussion section with optical [ll] and angle-integrated EELS measurements reported in the literature [12].

2. Experimental Measurements were performed by the Leybold-Heraeus model ELS-22 spectrometer. The system, in UHV environment (base pressure was less than 5 x lo-” Torr) and containing also LEED, ESCA and UPS equipments, is located at the CRISS lab in the Physics Department of the University of Montana. Clean (110) surfaces of GaAs and GaP were obtained by cleaving the single crystal samples. The surfaces were exposed at different atomic hydrogen dosages by admitting the molecular gas, subsequently dissociated by a hot (T = 2200 K) W wire, positioned in front of the sample. The different exposures were recorded as molecular dosages, since the convertion rate during this dissociation procedure is unknown. A 10 eV monochromatized primary beam was used; the incidence angle was 45” and the analyzer axis was aligned along the specular direction. The FWHM of the elastic peak was about 40 meV. In fig. la a sketch of the scattering geometry is shown. Spectra were taken for different values of the azimuthal angle + between the incidence plane and the [liO] direction, corresponding to IX in the surface Brillouin zone (see fig. lb). The azimuthal directions and the surface structure were checked by LEED.

526

Fig. 1. (a) Scattering geometry of a HREELS dIrection

of incident

respect lo

the [IjO]

experiment:

electron and the normal

direction,

or TX

direction

in the surface Brillouin

azimuthal

the plane of incidence, defined by the 10 the surface forms

each rate

variation

run.

Slight

corrections

of the azimuthal

on the elastic

iwne with

angle $I in evidence.

To get comparable spectra taken at different orientations. electron gun and of the analyzer have been fixed once and experimental

an angle I#I with

zone. (b) Surface Brlllouin

were

angle

made

in order

the optics of the for all within each

to the geometrical to obtain

set-up

the maximum

after count

peak.

3. Results and discussion The as-recorded direct comparison tion to the elastic

experimental of the spectra peak intensity

data are presented in figs. to be made. No manipulation has been carried out.

2-5. allowing a or normaliza-

In fig. 2 we report the spectra for the clean surface of GaAs(ll0) taken at + = 0” (corresponding to the [liO] or rX direction) and at + = 90” ([OOl] or rX’ direction), respectively. A marked difference in the energy region between 1.5 and 5 eV is clearly broad

structure

with

observable. a maximum

In fact, positioned

the spectrum

for C#I= 0”

at 2.9 eV. while

presents

for + = 90”

a the

band maximum, less intense, is at 4 eV. For E 2 5 eV, the two spectra have the same behaviour and they are equal in intensity. In fig. 3 we present the two spectra obtained after an exposure of 1000 L. (1 langmuir = 1 x lO--’ Torr . s) of hydrogen; anisotropy effects have almost completely disappeared. In figs. 4 and 5 spectra of GaP( 110) taken following the same procedure used for GaAs( 110) are shown. Also in this case for the clean surface (fig. 4)

S. Nannurone

er (11./ Surfuce onrsotropy of III-

500

V

compounds

521

I

GAAS(llO)CLEAN

0

4

2

6

ENERGY LOSS (eV)

Fig. 2. HREEL

spectra

on clean GaAs(ll0)

for C#J = O” (continuous line).

line) and

$I = 90 o (dotted

differences in the energy loss features are evident. In particular, for + = 0” there are two pronounced broad structures with their maxima at 3.5 and 5.6 eV, respectively, while for $ = 90” there is only a gradual onset from 2 up to 4.5 eV (the energy position of the band maximum), and a structure between 6 and 7 eV. Up to 6 eV the intensity of the first spectrum is greater than the second. They are approximately the same above 6 eV. The differences that we remark for the clean surface disappear after 1000 L of hydrogen exposure (see fig. 6) but the continuous background remains

5oo iv,,,,,,,

2

6

4 ENERGY LOSS (eV)

Fig. 3. Spectra

for GaAs(ll0)

taken after 1000 L of hydrogen incidence plane.

exposure

in the two directions

of

i ,

a

2

8

‘

4 ENERGY

LOSS

.1

ii

(cV)

Fig. 4. Spcc~~ on clean surface of CsP(

i 10).

higher for C$= 0”. This observation could also be related to the fact that for GaP the measured FWHM values of the elastic peaks for any exposure are always greater for (p = 0 O. We did not obtain this evidence for GaAs. However the residual differences between the two directions in both compounds can be reasonably ascribed to some persistency of the surface state contribution to the energy loss features. These results can be compared with optical data obtained by d~ffere~tial reflectivity techniques with polarized light. In fact it is known. for instance in the cases of ZnO [13] and Si(111)2 X 1 [lo], that the plane of incidence for HREELS plays essentially the same role of the polarization direction of light 250

_.-..-..-. r--T

~l__---__ 1OOOC tICaP(110)

i /

S. Nannarone

et al. / Surface anisotropy of III-V

compounds

529

for differential reflectivity. In this way when the plane of incidence in EELS is parallel to the [liO] direction the spectroscopic information is directly comparable with what is measured when the electric field vector is parallel to the same direction. In any case, if one goes deeper into the details of the EELS and optical spectroscopies, a number of remarks and warnings should be hold in the due consideration when comparing optical with EELS data. In fact both spectroscopical responces are written in terms of a dielectric function which depends in general on the exchanged wavevector q, besides the energy. Moreover this q-dependent function is rigorously different for photons (transverse probe, q nearly zero) and electrons (longitudinal probe). However longitudinal and transverse dielectric functions coincide in cubic crystals in the zero q limit [14]; the small angle characteristic of the scattering processes allows one to assume that in qualitative comparisons the differences between optical and energy loss dielectric function are negligible. The q’s exchanged in the spectra presented in this paper are of the order of some tenths of A-’ which corresponds to a regime where optics can be compared with EELS [15]. Moreover possible influences of the presence of the surface-vacuum and surface-bulk transition region could make a quantitative comparison between EELS and optics more delicate to be carried on in a scheme similar to that indicated in refs [16-181. It seems then reasonable and worthy to make a comparison, though qualitative, between the results of surface optical spectroscopy and our HREELS data. Available surface optical data come from differential reflectivity experiments for GaAs(ll0) [ll] and GaP(llO) [II] surfaces and from polarization modulated reflectivity for GaAs(ll0) [19]. With both optical techniques anisotropies have been observed in a spectral region which is included in the energy loss range of our experiments. What was observed in optics shows strong analogies with the findings we present here; in particular the surface reflectivity is higher when the light polarization direction is parallel to the [liO] direction. In the case of GaP(llO) it is natural to associate the maximum difference between the two polarization directions observed in optics at about 3.6 eV with the maximum difference observed in HREELS at about the same energy; the smaller structure at about 2.8 eV can be associated with the shoulder present in our spectrum at about 3 eV. For GaAs(ll0) we have a similar situation; the maximum in optical anisotropy observed in differential reflectivity [ll] occurred at about 3 eV and this results can be directly related to the maximum anisotropy obtained in HREELS nearly at the same energy. Also the Sl and S2 structures observed in modulated differential spectroscopy [19], and interpreted as due to a surface states contribution to the optical anisotropy have a correspondence with the HREELS results of present paper.

The authors of the papers [ll] and [19] interpretated the anisotropic structures observed at about 3 eV as optical transitions between bulk states affected by the presence of the space charge electric field. At present we do not have evidence of such kind of contributions in our spectra. However more work should be needed in order to clarify ihis point. On the other side, as discussed below, theoretical calculations show that at about the same energy there is a maximum contribution to surface differential reflectivity coming from bulk electronic states affected only by the presence of the surface, without taking into account any effect from space charge. The calculation of the surface opticaf reflectivity carried out by Manghi et al. for GaP [20] and GaAs [21] allows one to single out the electronic states contributing to the anisotropic surface differential reflectivity. In fact they showed for GaP that anisotropic contributions at 2.5, 2.9, 3.2 and 3.4 eV are related to transitions occurring, in the order: between the states A5 and C3 near X, between A5 and C3 at %%,between the two M, critical points at T and eventually between the states A4, A5 and C3 along the MX’ direction. Then, under the same approximations used to compare optical results and present data, it is possible to assume that the same electronic surface states are involved in the EELS anisotropy. A similar analysis has been carried out by the same authors for GaAs(l10) 1211. They found surface states contributing in the region between 2 and 2.5 eV while bulk-like states (i.e. extended states “feeling” the crystal termination) give a contribution two times higher in the 2.5.-3.0 eV range. Very recently experiments similar to the one presented in this paper were carried out on GaAs(ll0) by de1 Pennino et al. [22]. They reported the observation of an azimuthal dependence in their HREEL data. However their spectra appear less structured showing slight differences, and substantially only in the loss intensity, between the + = 0” and the + = 90° directions. Finally it is worthwhile to make a comparison between our results and the angle integrated EEL spectra of clean surfaces obtained in the second derivative mode. We will refer to the data obtained by van Laar, Huyser and van Rooy (VLHVR in this paper) [12]. To this purpose we made a numerical second derivative of our spectra taken at different angles. The results are plotted on figs. 6a and 6b for GaAs and for GaP, respectively, toghether with the VLHVR data. For both systems, the spectra taken at + = 0” are closer. in shape and energy position of the losses, to the VLHVR ones than the + = 90 o spectra. In the GaAs case, the agreement is satisfactory between 1 and 4.5 eV. poorer in the 6 eV region where the 6 eV structure is replaced in the calculations by two weak structures at 5 and 6.5 eV, respectively. In the GaP case the numerical second derivative spectrum is very close from the point of view of energy position and relative intensity of the structures to the VLHVR spectrum; the agreement is further improved by

S. Nannarone

et al. / Surface amsotropy

of III-

GAAS(l10)

a

531

V compounds

GAPC 1101

b:

1

-

R

o-

Y

I

,.

.,

ENERGY

I,

I

6

4

2

LOSS

(eV)

11.

1.

ENERGY

:

6

4

2

LOSS

(eV)

Fig. 6. (a) Angle integrated energy loss spectrum on clean GaAs(ll0) shown in ref. [12] (VLHVR). spectra for + = 0 o and C#B = 90 o after a numerical second-derivative evaluation. (b) Same as (a) for clean GaP(ll0). The lowest curve is obtained by adding up the two azimuthal contributions.

adding up the two contributions, as shown in fig. 6b. The agreement being improved by a suitable superposition of the two spectra, it is reasonable to ascribe the main origin of the differences to different azimuthal contributions. Scattering

geometry

causes. A last comment experiment

and collection concerns

angle could be in any case other possible

the primary

and 70 eV in VLHVR’s

beam energy which was 10 eV in our

one.

Possible

differences

arising

from

different q involved in the scattering have been mentioned above; moreover also a different surface to volume contribution ratio could be in principle at the origin

of the differences

derivatives.

The rather satisfactory

entiation

of the 10 eV spectra

above mentioned

effects

between

numerical

agreement

and

experimental

found between

and the results

of VLHVR

numerical indicates

second differthat the

are not dominant.

4. Conclusions In this work measured surface anisotropy effects in HREELS on GaAs (110) and on GaP(110) have been presented. We studied the energy loss range starting from the bulk gap up to 7 eV in the two azimuths [liO] and [OOl] which correspond to the directions parallel and normal, respectively, to the “zig-zag” atomic chains on the (110) surfaces of III-V compounds. Spectra on

532

S. Nunnarone et al. / Surface anrsorropy of III- V compounds

clean surfaces show differences in the 227 eV energy loss range for two different azimuths of the incidence plane. From the point of view of the experimental technique, we showed that a 10 eV primary beam is suitable to observe surface anisotropies by HREELS on semiconductor surfaces in this energy loss range. The observed differences disappear almost completely after hydrogen exposures, so that we could conclude that they are induced by anisotropy in surface electronic states. HREEL data have been compared with surface optical spectroscopy results. The two sets of data show a satisfactory agreement and complement each other resulting in the whole as a complete description of the dielectric properties of these surfaces. Finally the spectra were numerically differentiated to be compared with the second derivative EELS data. The energy positions of the losses and their relative intensities were found in agreement with the experimental second derivative EELS data. The agreement is improved by adding up the two azimuthal contributions.

References [I] A.R. Lubinsky, C.B. Duke, B.W. Lee and P. Mark, Phys. Rev. Letters 36 (1976) 1058. [2] S.Y. Tong, A.R. Lubinsky, B.J. Mrstik and M.A. Van Hove, Phys. Rev. B 17 (1978) 3303. [3] C.B. Duke, R.J. Meyer. A. Paton, P. Mark, A. Kahn. E. So and J.L. Yeh, J. Vacuum Sci. Technol. 16 (1979) 1252. [4] C.B. Duke, A. Paton, W.K. Ford, A. Kahn and J. Carelli. Phys. Rev. B 24 (1981) 562. [5] A. Kahn, Surface Sci. Rept. 3 (1983) 193. (61 For a review on this subject, see: M.A. Olmstead, Surface Sci. Rept. 6 (1986) 159. [7] K.C. Pandey, Phys. Rev. Letters 47 (1981) 1913. (81 P. Chiaradia, A. Cricenti, S. Selci and G. Chiarotti. Phys. Rev. Letters 52 (1984) 1148: S. Selci, P. Chiaradia, F. Ciccacci, A. Cricenti, N. Sparvieri. G. Chiarotti, Phys. Rev. B 31 (1985) 4096. [9] M.A. Olmstead and N. Amer. Phys. Rev. Letters 52 (1984) 1145; Phys. Rev. B 29 (1984) 7048. [lo] U. del Pennino, M.G. Betti. C.M. Bertoni, S. Nannarone, I. Abbati, L. Braicovich and A. Rizzi, Solid State Commun. 60 (1986) 337. 1111 S. Selci. A.Cricenti, F. Ciccacci, A.C. Felici. C. Goletti, Zhu Yong and G. Chiarotti, Surface Sci. 1X9/190 (1987) 1023. 1121 J. van Laar, A. Huyser, T.L. van Rooy. J. Vacuum Sci. Technol. 14 (1977) 894. (131 H. Froitzheim and H. Ibach, Z. Phys. 269 (1974) 17. [14] F. Wooten, Optical Properties of Solids (Academic Press, New York. 1972) p, 34. [15] See for instance: A.E.M. Meixner, R.E. Dietz, C.S. Brown and P.M. Platzman, Solid State Commun. 27 (1978) 1255; J. Daniels. C. van Fenstenberg, H. Raether and K. Zeppenfeld in: Springer Tracts in Modern Physics, Vol. 38 (Springer. Berlin, 1965). [lh] S. Nannarone and S. Selci, Phys. Rev. B 2X (1983) 5930. [17] S. D’Addato. S. Nannarone, M. De Crescenzi, Surface Sci. 162 (1985) 175.

S. Nannarone et al. / Surface anrsotropy of III- V compounds [18] A. Selloni and [19] V.L. Berkovits, (1985) 449; V.L. Berkovits, 1201 F. Manghi, E. (211 F. Manghi, R. [22] U. de1 Pennino,

R. de1 Sole, Surface Sci. 168 (1986) 35. I.V. Makarenco, T.A. Minashvili and V.I. Safarov,

Solid State Commun.

V.A. Kiselev and V.I. Safarov, Surface Sci. 211/212 (1989) 489. Molinari, R. de1 Sole and A. Selloni, Surface Sci. 189/190 (1987) 1028. de1 Sole, E. Molinari and A. Selloni. Surface Sci. 211/212 (1989) 518. M.G. Betti, C. Mariani and 1. Abbati, Surface Sci. 207 (1988) 133.

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