Diamond (111) studied by electron energy loss spectroscopy in the characteristic loss region

Surface Science 123 (1982)47-60 North-Holland Publishing Company

47

DIAMOND (111) STUDIED BY ELECTRON ENERGY SPECTROSCOPY IN THE CHARACTERISTIC LOSS Stephen Lewis USA

V. PEPPER

Research

Received

LOSS REGION

Center, National

16 August

1982; accepted

Aeronautics

and Space Administration,

for publication

14 September

Cleveland,

Ohm 44135.

1982

Unoccupied surface states on diamond (111)annealed > 900°C are studied by electron energy loss spectroscopy with valence band excitation. A feature found at 2.1 eV loss energy is attributed to an excitation from occupied surface states into unoccupied surface states of energy within the This result supports a previous bulk band gap. A surface band gap of - I eV is estimated. suggestion for unoccupied band gap states based on core level energy loss spectroscopy. Using the valence band excitation energy loss spectroscopy, it is also suggested that hydrogen is removed from the as-polished diamond surface by a Menzel-Gomer-Redhead mechanism.

1. Introduction There has recently been significant progress in understanding the effect of vacuum annealing on the electronic structure of the diamond surface. The polished (111) surface annealed < 900°C exhibits a (1 X 1) LEED pattern and no occupied surface states are observed by photoemission [ 11. After annealing > 900°C, the LEED pattern reconstructs to (2 X 2)/(2 X 1) and occupied surface states are observed near the top of the valence band by photoemission [2,3]. It has been speculated [l], and recently verified [4] that the polished surface, devoid of surface states, is covered with chemisorbed hydrogen, whereas hydrogen is absent from the surface annealed > 9OO“C. The transformation of the diamond surface has also been observed by the author using the technique of core level electron energy loss spectroscopy (ELS) to probe unoccupied states near the surface [5]. The features in the ELS spectra were interpreted in terms of maxima in the conduction band density of states. The band gap region in the spectrum was located with the aid of a band structure calculation. The spectrum from the as-polished surface was devoid of structure in the band gap region indicating that this surface contained no unoccupied states with energy in the gap. Annealing to > 9OO’C produced a feature with energy in the band gap spectral region which was attributed to the creation of an unoccupied state on the surface. The surface nature of this state 0039-6028/82/0000-0000/$02.75

0 1982 North-Holland

was verified by its disap~ear~~~~ when the (1 fO) surface was exposed to excited hydrogen [6J. A~t~~~~~ there is no doubt that an ~~~c~~~~d surface state was ~r~d~c~~ by a~~~~~~g, a closer consideration of its energy is warranted_ The electranic structure of the surface that is relevant to band structure calculations, Schattky barrier heights and adhesion with metals is the ground state of the solid. Eiectron emission sgec~roscopics, hmvevm, measwe excited states of the sofid. Because of the attractive interaction between an excited electron and a hole ~~xcito~~c ~~teraet~on~ the excitation energy measured in an energy Loss experiment wiH ~e~~raI~y be smalfer t.han the energy difference between ~~occup~cd and occupied states of the system in its ground state. In addition, the su$&+e ~xcita~~c energy may be an order of magnitude higher thas that for a tra~sit~o~ in the bulk f7J. This means that the energy of the unoccupied surface state on diamond (found by core Ievel (ELS)] may in fact he in the conduction band and onty appear in the gap region of the core level energy loss s~ect~~rn by virtue of the large excitonic binding energy. This was first noticed for the core Xevelexcitons on GaAs [‘?I_Thus addis~ona~ evidence of the ~~~c~g~of the ~~~cu~~cd surface state of diamond is necessary to verify the assi~nm~~t OF the state to the gap. In this paps- the energy of the ~~~~~~~~~ surface state is assessed in 8n energy iaJss ~~~er~rnent by exciting electrons into this state From the vahnei: band instead oF from the core &et. ff the surface exeitonic intera&on is s~~~~~cje~t~~ smatt, then the tra~sit~~~~energy will accurately give the energy between initial and final states in the ground state. Although a n~~~~~~ca~ vaXue of the surface vafence band exciton binding energy cannot be given as yet, it is expected to he smalfer than the surface core levei exciton because the re’tativeIy mabiie valence electrons may more effectively screen the hole fgf, In any case, since the axcitonic energies should be different. the observation by valence band ELS of transitions into a gap state would provide a col~s~ste~t picture with core levet EL5 and support the original ass~~~rn~nt. Evidence for unoccupied gap states from valence band ELS is sought as. follows: it is known from ~~ot~miss~~~ that there are no occupied states, surface or b&k, with energies higher than the bulk valence band rna~~mum, The energy difference between the highest occupied and the lowest ~~~ccu~ied bulk state is the 53 eV band gap energy and is the smaflest transitives energy in the bulk band picture. A feature with energy loss c 5.5 eV must thus be assigned to transitions from occupied states near the vafence band rnax~mu~~ into ~~~~~~ed states within the bararf gap: it is these features that are sought on the annealed surface. Although the initial state energy is not as sharply defined, a bound may still be put on the lowest energy of the unoccupied surface state and it witl be shown to lie in the Lower half of the band gap. fn addition to the determination of surface state energy, it is aku possible tO take advantage of this experimental tech~~~u~ to investigate the mechanism of

S. V. Pepper / Diamond (I 1 I) srudied by EELS

49

electron stimulated desorption (ESD) of hydrogen from diamond. It has been shown by core level ELS that the unoccupied surface states produced by vacuum annealing < 900°C also appear after sufficient 500 eV electron bombardment of the as-polished surface [.5]. Evidently the unoccupied surface states appear in the absence of hydrogen which can be removed thermally or by electron bombardment. There are two principle mechanisms for ESD of surface species - the Menzel-Gomer-Redhead (MGR) mechanism which requires excitation of a bonding electron to an antibonding state of the chemisorbed system [9] and the Knotek.-Feibelman [IO] mechanism which requires a core hole as the initial step in the Auger process that leads to two holes in the bonding orbitals and subsequent chemisorption bond breaking. Since the valence band ELS used here can be done at incident beam energies less than the carbon 1s binding energy, the importance of a core hole for hydrogen ESD can be assessed by comparing the cross section for generation of the surface state by bombardment with electrons of energy both below and above the 1s ionization energy (285 eV). it is found that the MGR mechanism is the principal one involved here in ESD of hydrogen from diamond (111). The results will be presented in the form of valence band energy loss spectra from the diamond surface in four states - the as-polished surface, this surface annealed > 9OO*C, this annealed surface exposed to excited hydrogen and finally the as-polished surface subjected to intense electron bombardment. In addition, core level energy loss spectra from these surfaces will be presented to make contact with our previous work. These results will then first be discussed in terms of earlier work by Lurie and Wilson (LW) f 1 l]_ The implications for the energy of unoccupied surface states will then be considered and a possible density of states diagram proposed. Finally some comments will be made on the mechanism of ESD of hydrogen from diamond.

2. Experimental Two insulating type IIa and one semiconducting type IIb specimens, cut and polished to within 1o of the (111) plane were used with similar results [ 121. The spectra were obtained by electron bombardment at normal incidence with the coaxial electron gun of the cylindrical mirror analyzer. N(E) spectra were obtained by measuring the current incident on the first dynode of the electron multiplier with a picoammeter. The energy loss of the features in the derivative spectra are taken at the mean of the maximum and minimum whereas the energy loss of the features in the N(E) spectrum is taken at the maximum of intensity. The valence band energy loss spectra were obtained with electron beam fluxes low enough to avoid noticeable time dependence to the spectra. When it was desired to observe the effect of the electron beam, the beam current was

raised for a given time and then reduced to acquire the spectrum. Exposure to excited hydrogen was done by raising the chamber pressure to 5 x IO-* Torr of hydrogen for 10 min in the presence of the hot filament of the ionization gauge [6]. Further experimental details may be found in prior publications [WI.

3. Results 3.1. The as-polished surface Tk energy Ioss spectrum due to core Ievel excitation is presented in fig. la. The effect of the electron beam has been eliminated as reported [S]. The large feature (labeled K,) is due to a transition into the lowest maximum in the conduction band density of states. The top of the 5.5 eV band gap has been positioned at the bottom of the conduction band which was located by taking a

Ep = 500 ev lP = .5pA

-I aAND GAP

H

BAND CAP

H BAND GAP

:a)

___-I

3cQ

280

fh)

300

-_-I

_--I

280

280

ENERGY LOSS, eV Fig. I. Energy loss spectrum by excitation

porticln of the spectrum

due to excitation

of carbon

Is core tcvei by 500 eV electrons. OnIy the

into the Iuwer

part of the conducti0n

is from the as-polished surface, (b) from the surface annealed surface exposed to excited hydrogen.

> 900°C

band is shown: (a)

and (c) from the annealed

S. V. Pepper /

Diamond (I I I) studied by EELS

51

3.6 eV energy difference between the lowest maximum (K,) and the bottom of the conduction band from a band structure calculation [ 131. The band gap region of this core level ELS contains no strong features, indicating that the as-polished surface contains no unoccupied states in the band gap. The d( EN( E))/dE energy loss spectrum due to valence band excitation is presented in fig. 2a for the characteristic loss region Q 50 eV. The EN(E) spectrum in the band gap region < 5.5 eV is presented in fig. 3a. It is this portion of the spectrum that is expected to contain features due to transitions from the valence band into unoccupied states in the band gap. In figs. 2a and 3a this region is featureless, in agreement with core level ELS that the as-polished surface exhibits no unoccupied gap states. The characteristic loss spectrum of fig. 2 has been previously discussed by LW [ 1 l] who assigned the 33 eV feature to a volume plasmon, the 23 eV

Ep - 125eV Ip - 15 nA

d[EN(Ejl dE

50

40 20 30 10 ENERGYLOSS, eV

0

Fig. 2. Energy loss derivative spectrum by valence band excitation: characteristic loss region 4 50 eV; (a) is from the as-polished surface, (b) from the surface annealed > 9OO’C and (c) from the annealed surface exposed to excited hydrogen.

S. V. Pepper / Diamond (I I I) studied by EELS

Ep - 80 eV Ip-7nA

EN(E)

5.5

5

4

3

0 EN&

LOS&I

Fig. 3. Energy loss N(E) spectrum by valence band excitations: band gap region < 5.5. eV: (a) is from the as-polished surface, (b) is from the surface annealed > 900°C, (c) is from the annealed surface exposed to excited hydrogen, and (d) is the difference spectrum b-c.

feature to a surface plasmon or interband transition and the 15 eV feature to an interband transition. Although the main interest in this paper is in the band gap region, it is also found that all these prominent features are sensitive to the state of the surface. A comparison with LW will be made after presentation of the spectra from the surfaces subjected to annealing, exposure to excited hydrogen and electron bombardment. 3.2. The annealed surface The ELS for the surface annealed > 900°C is presented in figs. lb, 2b and 3b. The core level ELS (fig. lb), now exhibits the feature, labeled K,, in the band gap that has been attributed to the surface state [5]. Note that the magnitude of K, has been reduced as well. This result is different from that for the (110) surface for which little decrease of K, upon generation of the surface state was observed [6]. The valence band ELS for the annealed surface (figs. 2b and 3b), now contains a feature, labeled C,, that lies in the band gap region. This is the feature that was sought as evidence for unoccupied states in the band gap. In

S. V. Pepper / Lhmond

(Ii I) studied h,v EELS

53

fig. 3b, C, exhibits a maximum in intensity at 2.1 eV and is not completely resolved from the elastic peak. The peak-to-peak height of C, in the d( EN( E))/d E spectrum together with of annealing temperature for 2 min anneals at that of K,, as a function successively higher temperatures is presented in fig. 4. It is seen that Co and K, increase together, the maximum being achieved by - 950°C. This same transformation temperature is consistent with the appearance of the same final state for both transitions. The transformation temperature is - 50°C higher than the value previously observed for the (110) surface 16,141. In addition to the appearance of C,, the valence band ELS (fig. 2), has changed in other ways. The feature at 33 eV in fig. 2a, now appears at a smaller energy of 31 eV with about the same intensity. The feature at 23 eV is greatly attenuated while the feature at 15 eV has disappeared. The feature at 5.5 eV in fig. 2a is also shifted due to the influence of the much larger adjacent C,. Thus all the features in the valence band ELS are modified by the surface transformation. 3.3. Exposure to excited hydrogen As noted in the introduction, exposing the annealed (110) surface to excited hydrogen completely removed the K, feature in the core level ELS. The results

1.0

.8

0

0

cl

0

0

f3

: c,

q : ‘$,

•J

0 0

0

8%

RI

a75

I 0 1 950 900 924 ANNEAL TEMPERATURE, o C

I

lal

975

1000

Fig. 4. Magnitude of band gap features K, (from core level excitation) and CO (from valence band excitation) from the as-polished surface subjected to 2 min anneals at successively higher temperature.

of exposing the annealed (111) surface to excited hydrogen are presented in figs. Ic, 2c and 3~. The band gap feature K,, is reduced by - 50% and is not completely removed as it is on the (110) surface. On the other hand the C, feature is completely removed from the valence band ELS, although the shape of the spectrum is somewhat different from that of the as-polished surface. Also note that the 15 eV feature has reappeared at its original intensity and the 23 eV feature has also grown, but not to its original intensity. Thus the exposure tends to return the spectral features from the annealed surface to those of the as-polished surface, although this $‘regeneration” is not complete. The appearance of surface states has been associated with the loss of chemisorbed hydrogen [2,3,6]. Evidently exposing the annealed surface to excited hydrogen allows readsorption to occur, although the resufting (111) surface is not the same as the original surface. The rehydrogenated surface can be used to obtain a value for the minimum energy loss at which the transition represented by C, takes place, This is the energy between the highest occupied and lowest unoccupied state participating in the transition and is considered to be the surface band gap energy. Since this energy cannot cimpfy be taken from fig. 3b because of the lack of complete resotution of C, from the elastic peak, a subtraction of spectra with and without the C, feature is employed. The spectrum (fig. 3c) of the rehydrogenated surface is subtracted from the spectrum (fig. 3b) of the annealed surface to yield the difference spectrum (fig. 3d). This is the procedure used by Himpsel et al. [15] to reveai the occupied surface states on the closely related In the difference spectrum the Si( 11 I)-(2 x 1) surface by photoemission. minimum transition energy appears to be - 1.0 eV, which is identified here as the surface band gap energy. Since this subtraction technique is rather crude this value should be taken provisionally and a more. accurate determination made with a high resolution energy loss spectrometer 1161. The full width at half maximum of C, appears to be - 1.6 eV. Taking the 0.63 eV energy resofution of the system into account yieids an intrinsic width of - 1.5 cV.

The unoccupied surface states were originahy found in the core level ELS ]5] by excessive bombardment of the polished surface with 500 eV electrons. Bombarding the polished surface with 125 eV electrons at 1 PA for 2 min also generates the C, feature and the valence band ELS is presented in fig. 5. Aside from the appearance of C,, the 15 eV feature is absent and the 23 eV feature is reduced in magnitude, effects similar to those produced by annealing. Having estabhshed that the C, feature can be produced by sufficient electron bombardment at energy less than the carbon Is ionization energy, it is of interest to determine whether the core hole creation furnishes an important channel for gap state production. To this end the growth of C, (evaluated at

55

S. V. Pepper / Diamond (I I I) studied by EELS

Ep = I25 eV

d[iN(Ejl dE

BAND

50

40

30 20 ENERGY LOSS, eV

10

0

Fig. 5. Energy loss derivative spectrum by valence band excitation. electron bombardment for 2 min at 1 J.LA with 125 eV electrons.

Polished

surface

subjected

to

E,,= 125 eV and at an incident flux low enough to preclude electron beam effects) as a function of electron bombardment time was used to obtain a production cross section at bombardment energies both below and above the 1s ionization energy of 285 eV. The cross section was obtained from M = M,( 1 value of C, in the d N/d E spectrum, M,, -e -““) where M is the peak-to-peak

FROM K, 2

I

0, cm2 ClS

I

,

300 PRIMARY

400 ELECTRON ENERGY, eV

Fig. 6. Cross section for generations of band gap features C, and K, as a function of primary electron beam energy. The C, feature was obtained under the conditions given in fig. 2.

56

S. V. Pepper / Uiatmnd

(I II)

studred by EELS

is the maximum value it attains, f is the incident electron flux, u( Eh) is the cross section at bombardment energy E, and t is the bombardment time. The values obtained for u are plotted in fig. 6 in which it is seen that there is no evidence for enhanced C, production above the carbon 1s ionization energy. Instead, the cross section appears to decrease and is consistent with the previous determination at E, = 500 eV using K, [5]. These results will be discussed below in terms of the mechanism of ESD of hydrogen from the polished diamond surface.

4. Discussion 4.1. Comparison

with brie

and Wilson

LW [ 111 have previously published characteristic loss spectra for diamond. Although their experimental procedure employed annealing > 900°C electron bombardment and exposure to hydrogen, the effect of these treatments on the surface was not recognized by them. Difference between their spectra and the present ones are noted here. First, the important C, feature in the spectrum from the annealed (1 II) surface was not reported by LW. This is the Iargest feature in the valence band ELS and indicates the degree of transformation of the surface. The feature was present in the spectra from all three (111) specimens used here and, since LW annealed their (111) specimens > 900°C, should have been present in their spectra as well. The energy resolution of their spectrometer may not have been sufficient to allow C, to be seen next to the elastic peak. Second, although LW made no distinction between spectra from different crystallographic planes, differences do exist. It has been pointed out above that annealing > 900°C results in appreciable reduction in the K, feature of the core level ELS for the (111) plane, but not for the (110) plane. Also, exposing the annealed surfaces to excited hydrogen completely eliminated the K, band gap feature on the (1 IO) plane but not on the (111) plane. In this sense the surface transformation is reversible on (110) but not on (ill). Doubtless further differences in the electronic properties of different planes will be revealed in the future. Finally, the interpretation offered by LW for the other loss features can be considered in terms of the effect of the surface treatment on these features. The large feature at 33 eV was ascribed to excitation of the bulk plasmon in diamond. This feature is least affected by the surface alterations, consistent with a bulk excitation. The 23 and 15 eV features have been reported by LW and it is seen by comparing the spectra in figs. 2 and 5 that they are strongly surface sensitive. LW have ascribed them to interband transitions originating on different maxima in the valence band and ending in the lowest maximum of

S. K Pepper / Diamond (I I I) studied by EELS

57

the conduction band as represented by K, in the core level ELS. The surface sensitivity may thus be due to that of the final state, since K, varies with surface treatment in the same manner as the strength of the 23 and 15 eV features. Although LW’s assignments find some support in the present work, further work is desired to clarify the nature of these excitations, 4.2. Interpretation

of C,

Following the arguments presented in the introduction, it is concluded that the loss feature C, at 2.1 eV is due to an electron excited into unoccupied surface states with energy in the band gap. Thus both core level and valence band ELS agree that the annealed surface contains unoccupied gap states. Since the excitonic effects are expected to be different for core and valence band excitations, this agreement implies that excitonic effects are not dominant in assigning the final state to the gap. The energy of the unoccupied state in the band gap may now be considered with the aid of the schematic density of states diagram of fig. 7. The occupied surface states have been taken from Pate et al. [2]. The initial state for the C, transition may be either the bulk valence band or the occupied surface states. A choice can be made based on the changes in C, and K, due to exposing the annealed surface to excited hydrogen. While C, is completely removed from the spectrum, K, is only reduced by - 50%. Since the initial state for K, is the carbon 1s level whose existence is not affected by surface hydrogen, the persistence of K, must be due to incomplete removal of the unoccupied surface states by hydrogen readsorption. The complete disappearance of C, must thus be due to the removal of its initial state. The bulk valence band is not changed by surface hydrogen and is thus eliminated as a candidate for the initial state. It is thus concluded that C, originates from occupied surface states and that these occupied surface states can be completely removed by adsorption of hydrogen.

BULK VALENCE BAND \

Fig. 7. Density of states diagram for diamond (111) annealed

> 9OOT

The fowest energy at which unoccupied states exist in the band gap can be estimated from the threshold loss energy of - I eV for the C, feature (fig. 3d). The bottom of the unoccupied surface band will thus be - 1 eV above the top of the occupied surface band, with the density of unoccupied states possibly increasing higher up in the gap. Note that even if this threshold loss energy is not welt determined, the maximum intensity of C, at 2.1 eV places the lowest unoccupied surface states in the middle of the gap and not near the bulk conduction band minimum. Thus the surface band gap is certainly not greater than 2. t eV and may be as small as 1 eV, These results find theoretical support in a recent band structure calculation of the diamond f f 1 I)-(2 x t) surface by Pandey [I7], He showed that the photoemission results f3] for the occupied surface states may be understood within a v-bonding chain mode! for the surface atoms. His model also shows that unoccupied surface states may exist rather deep in the band gap in accord with the present experiment. Further test requires comparing the theoretical joint density of surface states with the proper experimental surface optical constants from an analysis of ELS [ 141. We finally note that the results presented here for diamond are entirely similar to those found for Sif I I Ij-2x t and Ge(l I I)-2X t in finding unoccupied surface states in the band gap by ELS ft6]. However, even for the welt-studied silicon surface, the density of un~cup~ed surface states has not been completely determined. Perhaps the recent development of inverse-photoelectron spectroscopy (IPES) t18] can be used to yield directly the density of unoccupied surface states within the band gap of diamond. 4.3. Effect of electron bombardment It is indicated

in fig. 6 that there is no increase in the rate of C, production Is ionization threshdd. Since the presence of surface states is correlated to #he absence of surface hydrogen, this cross section is considered to be that for electron stimuiated desorption of hydrogen from diamond. We note here that the insensitivity of the surface deso~tion to core level excitations cannot be due to the primary electrons penetrating too deeply into the solid before energy Toss scattering takes place - the surface sensitivity of the core ievel ELS (fig. 1) clearly shows that core Ievel excitations take @ace in the top carbon atoms. The lack of the above threshold indicates that the Auger decay which leaves two holes in the carbon valence band is not an important mechanism leading to ESD. Although this K-F mechanism [IO] daes not seem important here, it must be stressed that what is observed here is the possibility for ESD of hydrogen in any state. neutral or ionized. The K-F mechanism may be effective in ESD of H’ but not H e. ff a strong threshold is found for is smail ESD of H+, then the present results imply that this probabiiity compared to ESD of HO. The present results are thus consistent with the

at the carbon

Menzel-Comer-Redhead [9] mechanism alone is sufficient to promote desorption.

in which the valence

band excitation

5, Conclusions The characteristic energy loss spectrum from diamond f 11I> was studied with special attention paid to the preparation of the surface. It was found that ati the features in the O-50 eV toss spectrum were sensitive to the state of the surface. In particular, the band gap region < 5.5 eV contained a loss feature at 2.1 eV for the surface annealed r 9OO’C that was absent for the as-polished surface. This feature could be removed from the spectrum by exposing the annealed surface to excited hydrogen. The feature was interpreted as due to a transition from occupied surface states with energies within the valence band into unoccupied surface states with energies within the band gap, supporting a previous claim based on core level ELS. The unoccupied surface states are present when hydrogen is absent from the surface. The removal of hydrogen by electron bombardment exhibits no threshold at the carbon Is ionization energy and thus appears to proceed by a Men~el-Gomer-Redhead valence electron excitation mechanism.

Note added in proof A paper 1191 published after this work was submitted shaws that the occupied surface states are indeed removed by exposure to atomic hydrogen as predicted in section 4.2. In addition, they find a strong threshold at the carbon 1s ionization energy for photon induced desorption of H+. It would be interesting to repeat their experiment using electron excitation.

[l] F.J. Himpsel, J.A. Kanpp, J.A. Van Vechten and DE Eastman, Phys. Rev. B20 (tY79) 624: B.B. Pate, W.E. Spicer, T. Ohta and I. Lindau, J. Vacuum Sci. Technol. 17 (1980) 1087, [2] B.B. Pate, P.M. Stefan. C. Binns, P.J. Jupiter, M.L. Shek, I. Lindau and W.E. Spicer, 3. Vacuum Sci. Technol. 19 (1981) 349. 131 F.J. Himpsel, D.E. Eastman, P. Heimann and F.J. van der Veen, Phys. Rev. 824 (1981) 7270. [4] B.J. Waclawski, D.T. Pierce, N. Swansan and R.J. Cellota, in: Proc. 9th Annual Conf. on the Physics of Compound Semiconductor Interfaces [J. Vacuum Sci. Technol. 21 (1982) 3681. [5] S.V. Pepper, Appl. Phys. Letters 38 (1981) 344. [6] S-V. Pepper, J. Vacuum S-5. Technol. 20 (1982) 213. [73 W. Gudat, D.E. Eastman and J.L. Freeouf, J. Vacuum Sci. Tecfinol. 13 (1976) 250: W. Gudat and D.E. Eastman, J. Vacuum Sci. Technol. 13 fi976) 831. [8] J.C. ~cMenamin and R-S. Barter, f. Vacuum Sci. Technoi. $5 (1978) 1262, [9] D. Menzef and R. Gsmer, J. Chem. Phys. 41 (1954) 331 f; P.A. Redhead. Can. J. Phys. 42 (1964) 886,

60

S. V. Pepper / Dmmond (I I I) studied by EELS

[IO] M.L. Knotek and P.J. Feibelman, Phys. Rev. Letters 40 (1978) 964; Phys. Rev. B18 (1978) 6531. [ 1 l] P.G. Lurie and J.M. Wilson, Surface Sci. 65 (1977) 476. [ 12) Diamond windows supplied by D. Drukker, Amsterdam, The Netherlands. [13] G.S. Painter, D.E. Ellis and A.R. Lubinsky, Phys. Rev. B4 (1971) 3610. [ 141 S.V. Pepper, J. Vacuum Sci. Technol. 20 (1982) 643. [15] F.J. Himpsel, P. Heimann and D.E. Eastman, Phys. Rev. B24 (1981) 2003. 1161 H. Froitzheim, H. lbach and D.L. Mills, Phys. Rev. Bl I (1975) 4980; see also H. Froitzheim, in: Electron Spectroscopy for Surface Analysis, Ed. H. Ibach (Springer, Berlin, 1977) p. 205, for review of high resolution ELS. (171 K.C. Pandey, Phys. Rev. B25 (1982) 4338. [ 181 V. Dose, H.-J. Gossman and D. Straub, Phys. Rev. Letters 47 (198 I) 608. [ 191 B.B. Pate, M.H. Hecht, C. Binns, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. 2 I (1982) 364.