Comparative study of hydrogen adsorption on Ge(100) and Ge(111) surfaces

40

Surface Science 138 (1984) 40-50 North-Holland. Amsterdam

COMPARATIVE STUDY AND Ge( 111) SURFACES

L. SURNEV Institute Received

OF HYDROGEN

ADSORPTION

ON

Ge(100)

and M. TIKHOV

of General and Inorganic Chemistry, 3 August

1983; accepted

Bulgarian Academy

for publication

15 November

of Saences,

1040 Sofia, Bulgarm

1983

In this study, the work function changes, A+, surface conductivity, Aa, and electron energy loss spectra (ELS) have been obtained as a function of relative hydrogen coverage, 0, determined by thermal desorption (TD) measurements. A comparative measurement has been performed. The A+ and do data favour the ionic type models of a Ge(lOO) surface and the covalent type models of a Ge(l11) surface. However, the adsorption kinetics and ELS data are almost the same for both the Ge(l11) and Ge(100) surfaces.

Introduction

Extensive experimental [l-12] and theoretical [12-181 studies of hydrogen adsorption on Si and Ge surfaces are usually associated with the problems of the clean surface atomic structure. According to the buckled model [19] for a Si(100) 2 X 1 surface, asymmetric dimers are formed. A Jahn-Teller charge transfer resulting in an increase of the work function of the reconstructed surface is associated with each asymmetric dimer pair. Fernandez et al. [20] discussed a similar asymmetric dimer model for the Ge(lOO) 2 x 1 surface. The cluster calculations of Verwoerd [16] predicted removal of the asymmetry when one hydrogen atom is bonded to each Si atom (monohydride phase). This results in a A+ decrease by - 0.37 eV. After further H adsorption, the dimer bond is broken and a dihydride phase is formed (two hydrogen atoms are attached to each surface Si atom). A A+ decrease by - 0.34 eV with respect to the clean surface is expected in this case [16]. The experimental result of Koke and Miinch [4] is in satisfactory agreement with the theoretical value predicted above. The high-resolution ELS data concerning hydrogen adsorption on Si(ll1) 7 X 7 [5] and Si(ll1) 2 X 1 [9] surfaces show the simultaneous formation of SiH and SiH, complexes in the early stages of H adsorption. On the other hand, Appelbaum et al. [12] gave evidence that for a Ge(lOO) surface the LEED pattern remains 2 X 1 till saturation with hydrogen, i.e. it is not possible to observe a dihydride phase on a Ge(lOO) surface at 300 K. This finding made it possible to associate A+ upon H adsorption on a Ge(lOO 2 X 1 surface with 0 Elsevier Science Publishers B.V. Physics Publishing Division)

0039-6028/84/$03.00

(North-Holland

L. Surnev, M. Tikhov / Hydrogen on Ge(lO0) and Ge(lll)

41

symmetrization of the surface dimers alone, assuming that the model of Verwoerd [16] is also valid for this surface. Recently Chadi and Chiang [21] assumed the Ge(lll) 2 X 8 surface to be consisting of raised and lowered surface atoms. If hydrogen adsorption removes the Jahn-Teller distortion, a decrease of A+ should also be expected with a Ge(ll1) 2 x 8 surface. Since only atomic hydrogen can be adsorbed on Ge and Si surfaces, molecular hydrogen exposure is not especially significant. Nevertheless, only in a few papers [8,22-241 the measured quantities were presented as a function of hydrogen coverage (relative or absolute). That is why in this study A& Au and ELS are given as a function of relative hydrogen coverage, 8 (as determined by TD measurements).

2. Experimental The experiments were performed in a conventional stainless-steel UHV system equipped with a single-pass cylindrical mirror analyzer for AES and ELS, a vibrating Kelvin probe and a low-energy electron gun for A$ measurements, as well as a quadrupole mass spectrometer (Riber) for TDS. The system was pumped out by a 200 l/s noble-gas ion pump and a LN-cooled Ti sublimation pump (base pressure of - 1 x 10-i’ Torr). Two Ge samples with (111) and (100) orientation (accuracy of 0.5’) were mounted simultaneously on a precision manipulator by means of tantalum clips. The crystals were n-type, with a specific resistance of - 30 D cm and the same dimensions (1.50 x 0.5 x 0.03 cm3). Since only the front planes of the samples can be cleaned by Ar+ bombardment and annealing (up to 900 K), these samples were not suitable for Au measurements. For these measurements, another pair of Ge samples were mounted on two flanges situated at both sides of the QMS. The crystals had the same dimensions and orientations as those used for the A@ measurements. Cylindrical grids (diameter, 2.5 cm) were fixed axially around the samples. Outside the grids, in the largest planes of the crystals, two tungsten cathodes were mounted for ionization of Ar. In this manner, both sides of the samples could be cleaned. For the surface conductivity measurements, a four-probe method was used under thermostatic conditions. For TD measurements, a sister sample was switched in series with the investigated one. The sister sample, situated in an evacuated glass tube, was cut from the same crystal and had the same dimensions, orientation and mounting geometry as the crystal under investigation. Thin (0.1 mm in diameter) Pt/‘Pt-lO%Rh thermocouples were welded to the sister samples. A linear heating rate was achieved by feed back from the thermocouple. Intrinsic conductivity measurements showed that the temperature difference between the sample under investigation and the corresponding sister sample was less than f10 K.

L. Surneu. M. Tlkhou / Hydrogen on Ge(lO0) and Geclll)

42

heatmg h

rate 15Ksti'

Ge(ll1) g

500

UN

T(K)

700

Fig. 1. H, thermal desorption spectra following different H, exposures 0.13; (b) 0.4; (c) 0.5; (d) 1.0; (e) 1.8; (f) 2.5; (g) 6; (h) 30.

(in Torr min X 10’): (a)

For exposure to atomic hydrogen, the samples mounted on a manipulator were positioned at - 5 cm in front of a tungsten ribbon which was heated up to 1900 K. In the case of Au measurements, two W filaments were placed symmetrically at both sides of the sample. All H exposures were carried out at PH, = 5 X lo-’ Torr. The hydrogen purified by diffusion through a Pd membrane was introduced into the system by means of a leak valve.

3. Experimental results 3.1. Adsorption

and desorption of hydrogen

Fig. 1 presents a series of hydrogen TD spectra recorded following adsorption on Ge(ll1) at 300 K. Only one peak is visible in the spectra. With increasing coverage 8, the peak maxima shift to lower temperatures, thus

0

0

10

20 H, DOSE

.’

30

Q,(ML)

‘.

LO

x10-

Torrmin

Fig. 2. Hydrogen coverage, 0, normalized to saturation relative sticking coefficient S/S, against 19.

versus

exposure.

The insert

shows

the

L.. Surneu, M. Tikhou / Hydrogen an Ge(100) and Ge(lIl)

43

indicating second order desorption kinetics. Since the thermocouple is not directly attached to the sample under investigation, no detailed analysis of the desorption energy E(B) and the frequency factor v(8) can be made. Assuming coverage-independent E and Y values, evaluation based on the ln(8-*dt9/dt) versus l/RT plots yields E = 145 f 10 kJ/mol with both Ge surfaces. Fig. 2 illustrates the relative coverage, 8, determined from the area under the desorption curves as a function of molecular hydrogen exposure. The adsorption kinetics as well as the saturated hydrogen population are identical for both Ge(lll) and Ge(lOO) surfaces. Further, we shall postulate that the saturation area under the TD curve corresponds to 1 monolayer, i.e. 1 H atom corresponds to each Ge surface atom. The insert in fig. 2 shows the relative sticking coefficient S/S, (determined by graphical differentiation of the 8 versus exposure curve) as a function of 8. As can be seen S/S, is a linear function of (1 - e), which changes its slope at 6 - 0.5. 33.2. Work function

and surface conductivity

Fig. 3 shows A+ against the hydrogen coverage plots. It can be seen that the curve observed for the Ge(ll1) surface significantly differs from those obtained for a (100) plane. As fig. 4 shows, a similar difference is observed for the Au versus 8 plots (the solid lines) obtained on Ge(ll1) and Ge(lOO) surfaces. The Au values are given with respect to an arbitrary zero: the Au of the clean surface. The surface conductivities of the clean Ge(ll1) and Ge(lOO) surfaces are 95 X 1O-6 and 230 x lop6 A/V, respectively, as obtained after admission of atmospheric air to the vacuum system. For both surfaces under investigation we observed a Au minimum.

I

0

.2

.L COVERAGE,8

.6

.8

1.

(ML)

Fig. 3. Work function changes, A+, versus hydrogen coverage 8.

_

L. Surneu, M. Tikhov / Hydrogen on Ge(lO0) and Ge(ll1)

44

IV,bVl

200

1

035

4 0.30

1

025

tl0

I

I

/

I

I

/

2

i,

6

8

1.

COVERAGE.9 Fig. 4. Surface conductivity, present the band bending, respectively.

The work function follows:

(ML)

Au, versus 8 plots (the solid lines). The dashed and dotted curves v,, against 0 plots obtained for Ge(ll1) and Ge(100) surfaces,

changes,

A+, upon

H adsorption

can be expressed

as

where AVs is the band bending change, AI, are the ionization energy UE) changes due to the reconstruction, and AZ,,_, the IE changes associated with H-Ge dipoles, To calculate AV, from the Au data, the theoretical Au, versus V, curve was used [25]. However, we do not know in detail the current carrier scattering mechanism on a semiconductor surface. That is why the Aa, values were calculated as an arithmetic mean of two Au values. One was obtained using the bulk values of current carrier mobilities, pp.” (specular reflection) and the other on the basis of corrected pLp,nvalues according to a diffusion surface scattering mechanism [26]. It is evident that this procedure is associated with some uncertainty of the calculated V, values. The dashed and dotted curves in fig. 4 represent the V, obtained for Ge(ll1) and Ge(lOO) surfaces, respectively. As can be seen, with a Ge(lOO) surface, the contribution of V, to the decreasing part of A$ changes is very small. However, with a Ge(ll1) plane, A+ is determined almost entirely by the band bending. With both Ge surfaces, A+ and V, are practically coverage-independent at 8 > 0.5. It is easy to realize that this constancy of A+ is due to the fact that ZR and Zoe_n do not change with 0 at coverages higher than 0.5 rather than to the compensating effect. Indeed, since at 0 < 0.5 the work function changes on a Ge(lOO) surface have a sign opposite to that on a Ge(ll1) surface, AZ,,_,, could compensate the I,

L. Sumeu, h4. Tikhou / Hydrogen on Ge(100) and Ge(ll I)

45

changes at 8 > 0.5 only for one of the Ge planes under consideration. Hence, the contribution of the adsorbed hydrogen layer (AZ,,_,) to A+ is extremely small, which suggests a negligible polarity of the Ge-H bond. This result is to be expected since the electronegativity of Ge (2.02) is very close to that of H (2.10). The initial A+ and Au increases upon H adsorption on a Ge(lOO) surface is very puzzling. One may expect this effect to be associated with adsorption at step edge atoms. Schulze and Henzler [27] have shown that the step atom density at an argon-bombarded Ge(lOO) surface depends on the annealing temperature. By increasing this temperature, the step atom density decreased as determined by LEED intensity profiles analyses. This finding gave us the opportunity to check whether the A+ and Au sharp increase observed at low 8 in figs. 3 and 4 was due to hydrogen adsorption at step atoms. Fig. 5 presents the initial A+ and Au as functions of molecular hydrogen exposure. The curves are obtained with Ge(lOO) surfaces after Ar+ bombardment (10 min, 2 X 10e6 A, 450 eV) followed by 20 min annealing at gradually increasing temperatures. As can be seen, the initial A+ and Au changes increase with increasing annealing temperature, i.e. an effect opposite to the expected one is observed. 3.3. EL spectroscopy Figs. 6a and 6b show series of second derivative EL spectra recorded at gradually increasing ti with Ge(ll1) and Ge(lOO) surfaces, respectively. The

I

I

I

0

2

L

JII

I

”0 H, DOSE Torrmin

2

1

I

LXl[T’

Fig. 5. The initial PO (a) and A+ (b) changes as a function of molecular hydrogen exposure obtained on a Ge(100) surface after Ar+ bombardment followed by 20 min annealing at 600,700, 800 and 900 K, respectively.

30 20 ENERGY

LOSS$‘,

Fig. 6. Second derivative electron surfaces; (bf Ge(lll) surfaces.

loss spectra

recorded

at gradually

increasing

6 with: (a) Ge(100

bottom curves illustrate the corresponding clean surface spectra. The labeiling and energy positions of the EL features have been given elsewhere [28,29]. Since the elastic peak and all bulk related features grow in magnitude with increasing 8, the curves in fig. 6 are normalized with respect to the corresponding elastic peak heights. As can be seen the peaks d, (due to transitions from the Ge 3d core level to the empty dangling band surface state) gradually decrease with increasing 8 and vanish at B - 0.4-0.5 for both Ge surfaces under consideration. The hydrogen adsorption is also accompanied by the disappearance of the peak S, (associated with a transition from the back bond surface state) and with the emergency of new hydrogen-related features at 8.5 and 8.2 eV for Ge(lll) and Ge(100) surfaces, respectively. The structures at - 11 eV which are observed before and after H adsorption, might be attributed to the surface plasmon excitation or to another H related EL feature. The intensities of the structures d,, A$, and those of the elastic peaks (EP) are shown in figs. 7a and 7b for Ge(lll) and Ge(lO0) surfaces, respectively, as functions of 8. It is evident that with both Ge surfaces the increase of the intensities of the EP and fiwp occur in the same 8 range (up to 0.5) in which peak d, decreases in magnitude.

L Szmiev, M. Tikhov /

COVERAGE,@

Hydrogen on G&l 00) and Ge(l I I)

47

(ML)

Fig. 7. The intensities of the loss structures against their maximum values as a function surfaces.

d,, hwp, and those of the elastic peaks normalized of 9 obtained on: (a) Ge(100) surfaces; (b) Ge(ll1)

After H adsorption on a Si(lO0) 1 X 2 surface, Madden [6] also observed enhancement in the bulk plasmon intensity. He suggested that this effect might be a result of the appearance of a new feature associated with the Si-H bond or of a hydrogen-induced increase of the Si bulk plasmon oscillation strength. However, the simultaneous intensity enhancement of the &or structures and the elastic peaks (fig. 7) shows that this effect is more probably due to changes of the electron scattering properties of the Ge surface upon H adsorption.

2. Discussion 4. I. Gefi 00) surface The extremely small contributions of the AlGe_H and the H induced band bending, AK, to the decreasing part of the work function changes, A+, on the Ge(tO0) surface makes it possible to associate the A@ only with the changes in ionization energy, Ai, (due to the surface atoms re~rangement}. The steep initial A+ increase (observed also with oxygen adsorption on Ge(lOO) [30]), is very puzzling. It is not accompanied by LEED pattern changes [12], which indicates H adsorption on randomly distributed sites with very low concentrations. The primary focus of this study is on the decreasing part of the A# versus 8 curve. It is reasonable to relate the A+ decrease to the removal of the asymmetry of the surface dimers, which is in agreement with the models for reconstructed Si(lO0) [19] and Ge(lOO) [20] surfaces.

48

L. Surneo, M. Tikhov / Hydrogen on Ge(100) and Ge(l1 I)

Observation of the 2 x 1 LEED pattern up to 0 = 1 [12] implies that the symmetrization of the surface dimers upon hydrogenation does not result in breaking of the pair bonding. This is in accordance with predictions of the cluster calculation of Verwoerd [16]. A very important new result obtained in this paper is that only 0.5 hydrogen monolayer is sufficient to remove the asymmetry of surface dimers. As our ELS data show, at half monolayer the empty dangling band state also vanishes. According to Haneman’s idea [31] this state is associated with the lowered surface atoms, to which a p, orbital corresponds. These orbitals are expected to be more reactive than the s orbitals associated with raised surface atoms. Indeed, the change in slope of the S/S, versus 8 curve at 8 - 0.5 shows that after filling of the sites corresponding to the empty dangling bond states, the adsorption properties of the Ge(100) surface sharply change. The initial steep A+ increases makes it difficult to evaluate the decrease of the ionization energy, Al,, due to the dimer symmetrization. One might assume that the A+ increase is associated with H adsorption at defect sites with a very low concentration and with a large effect on A+ Thus, AZ, can be obtained with a moderate accuracy by extrapolation of the decreasing part of the A+ versus 8 curve to 13= 0. The value for Al, is 0.21 + 0.02 eV as determined from 10 adsorption runs. This value can be used to calculate the elementary dipole moment Ap corresponding to each asymmetric dimer. Since each pair of dimers are bonded to common second layer neighbouring atoms, as pointed out by Verwoerd [16], a short range interaction arises, which almost completely compensates the electrostatic long-range depolarization effect. That is why the relation between Ap and A+ is given by

where cs is the surface permittivity and N is the density of the dipoles. Using eq. (I) with E, = 15.8 (the bulk Ge value), we calculated the values for Ap = -(2.8 f 0.2) D. This value may be used to estimate the buckling of the Ge(lOO) surface if the Jahn-Teller charge transfer is known and vice versa: if we know the buckling, the charge transfer may be determined. Fernandez et al. [20] considered three buckled dimer models for a Ge(lOO) surface, none of which explained the LEED data quite satisfactorily. To author’s knowledge there is no publication concerning the Jahn-Teller charge transfer calculation for a Ge(lOO) surface. 4.2. Ge(1 I I) surface The surface potential changes, Av,, which are due to the re-distribution of the density of surface states upon H adsorption, have not been observed beyond the value of 0 - 0.5. It is not clear whether these V, changes may be accompanied or not by a surface reconstruction. Bringans and Hochst [7]

L. Surnev, M. Tikhov / Hydrogen on Ge(100) and Ge(lll)

49

argued that upon H adsorption the reconstructed Ge(ll1) surfaces are transformed to a Ge(ll1) 1 x 1 structure. Up to now there are no data on the hydrogen coverage value at which this structure conversion takes place (if it occurs at all). As figs. 3 and 4 show, the V, changes almost completely follow those of A+, implying that there is no significant contribution of Al, to A+, i.e. the ionization energy changes associated with eventual removal of the buckling of the Ge(ll1) 8 x 2 surface are extremely small. Thus, our A+ and Au data favour the covalent models for the reconstructed Ge(ll1) 2 X 8 structure. Recently Pandey [32] proposed a new T-bonded chain model for the reconstructed Si(ll1) 2 x 1 surface. According to this model, only covalent bonds between the surface Si atoms exist, i.e. no charge transfer between surface atoms is predicted. More recently Pandey [33] and Chadi [34] suggested that a similar model is also valid for the Si(lll) 7 X 7 surface. Contrary to the ionic buckled model in which the empty and filled surface states are located at lowered and raised surface atoms, respectively, with the chain model they are bonding and antibonding states. Assuming that on the Ge(lOO) surface the H atoms adsorb first at more reactive lowered surface atoms, we have been successful in associating the vanishing of the empty dangling bond state with the break point in the S/S,, versus 8 curve at 6J- 0.5. However, a very similar observation upon H adsorption on the Ge(ll1) 2 X 8 surface should be explained quite differently on the basis of the r-bonded chain model.

5. Summary The A+ and Au data favour the buckled model for the Ge(lOO) surface and the covalent type models for the Ge(ll1) surface, respectively. The observed decrease in A+ up to 8 - 0.5 and the vanishing of the empty dangling bond surface state at the same 0 value imply that a H atom bonding at one (the lowered) dimer atom alone is sufficient to remove the asymmetry on the Ge(lOO) surface. The identical ways of behaviour of the EL spectra and the H adsorption kinetics observed with the two Ge surfaces under consideration are difficult to explain, suggesting quite different chemical nature of the Ge(ll1) and Ge( 100) surfaces.

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50

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L. Surneu, M. Tikhou / Hydrogen on Ge(100) and Ge(lll)

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