The βSiC(100) surface studied by low energy electron diffraction, Auger electronspectroscopy, and electron energy loss spectraMoshe Dayan Citation: Journal of Vacuum Science & Technology A 4, 38 (1986); doi: 10.1116/1.573495 View online: http://dx.doi.org/10.1116/1.573495 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/4/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in The effect of Sr and Bi on the Si(100) surface oxidation: Auger electron spectroscopy, low energy electrondiffraction, and xray photoelectron spectroscopy study J. Vac. Sci. Technol. A 8, 4017 (1990); 10.1116/1.576438 Auger line shape and electron energy loss spectroscopy analysis of amorphous, microcrystalline, and βSiC J. Vac. Sci. Technol. A 8, 1538 (1990); 10.1116/1.576761 Differences in Auger electron spectroscopy and xray photoelectron spectroscopy results on the bonding states ofoxygen with βSiC(100) surfaces J. Appl. Phys. 67, 264 (1990); 10.1063/1.345301 Auger electron spectroscopy–electron energyloss spectroscopy–lowenergy electron diffraction study of aV6C5(100) surface J. Vac. Sci. Technol. A 5, 985 (1987); 10.1116/1.574307 Oxidation of the Si(111) (7×7) surface: Electron energy loss spectroscopy, lowenergy electron diffraction, andAuger electron spectroscopy studies J. Chem. Phys. 83, 428 (1985); 10.1063/1.449787

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The P-SiC(100) surface studied by low energy electron diffraction, Auger electron spectroscopy, and electron energy loss spectra

Moshe Dayan8)

National Aeronautics and Space Administration. Lewis Research Center. Cleveland. Ohio 44135

(Received 15 July 1985; accepted 22 September 1985)

The P-SiC( 1(0) surface has been studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), high resolution electron energy loss spectra (HREELS), and core level excitation EELS. Two new Si-terminated phases have been discovered, one with (3 X 2) symmetry, and the other with (2 Xl) symmetry. Models are presented to describe these phases. New results, for the C-rich surface, are presented and discussed. In addition, core level excitation EELS results are given and compared with theory.

I. INTRODUCTION

The present work is a continuation of a recent surface study on cubic SiC. I Its main motivation, in the beginning, was to gain more understanding about the C-rich surface, and the oxygenation and early oxidation characteristics of the var­ious surface phases. Although the achievement of these goals remains limited, we have discovered more about the Si sur­face, namely, an additional two Si-terminated phases.

Throughout this study, we have used mainly AES and LEED in the same manner as in Ref. 1. The present work, however, is more extensive in that more tests have been con­ducted on more samples with different characteristics. High resolution EELS, SEM, and core level excitation EELS have also been used, but to a smaller extent. The analysis is mainly qualitative. In some cases, gross semiquantitative analysis is used to support or exclude models under consideration.

The structure of this paper is the following: In Sec. II, a brief outline of the experimental procedure is given. The re­sults are reported in Sec. III without interpretation. A dis­cussion of the results, and a partial interpretation are given in Sec. IV.

II. EXPERIMENTAL

The experimental system, the procedures for sample mounting and handling, and the experimental methods, are described in Refs. 1 and 2. As stated before, the present work also includes core level excitation EELS measurements which were carried out with the same single stage CMA used for the AES measurements. One C-rich surface was also ex­amined by field emission SEM.

Together with the three samples reported in Ref. 1, we have tested a total of nine samples. These samples have been produced over a period longer than 1 year.3 During this peri­od, minor adjustments to the CVD growth system were found to be necessary, to compensate for a decline in the quality of the samples used in Ref. 1. The free carrier concen­trations ofthe nine samples tested ranged over 1.5 orders of magnitude, from the upper 1016 cm- 3 range to the lower 1018 cm- 3 range.

Two different surface treatments were used. The first is a heat treatment (usually for 3 min) in a base vacuum of 2-3 X 10 - 10 Torr, and in the temperature range 700--1285 .c.

38 J. Vac. Sci. Technol. A 4 (1), Jan/Feb 1988

The second is an oxygen treatment with various doses of oxygen, at temperatures ranging from room temperature to 900 ·C. Samples which underwent such oxygen treatment are termed oxygen-treated, in contrast to the as-received samples.

The temperature was measured with an optical pyro­meter. Unlike the work reported in Ref. 1, corrections were made to account for the nonunit emissivity of our samples. For a normal incidence oflight of 0.65 J1- wavelength, using a refractive index of2.6,4 about 20% of the intensity is reflect­ed. About 2.5% of the transmitted part escapes the material after reflections from the SiC-Si substrate interface, and the rest is absorbed (mostly in the Si substrate). Thus, we have assumed a spectral emissivity ofO. 78 and corrected the mea­sured temperature accordingly. This correction. which should also be applied to Ref. 1, is a temperature increase of 13 ·C at 800 ·C, 19 ·C at 1000 ·C, and 25 ·C at 1200 .c. Con­sequently, the oxygen desorption temperature of the as-re­ceived samples in Ref. 1 becomes 1080 ·C, and the highest temperature employed becomes 1225 .c.

III. RESULTS

A. AES and LEED on the (3 X 2), (2 X 1), and c(2 X 2) surfaces

Most of the tested as-received samples exhibited the

c(2 X 2) Si surface [or (J2 X J2) R45· in another notation], when the oxygen was desorbed. I Three as-received samples. however, exhibited a (3 X 2) LEED pattern when the oxy­gen was desorbed. These samples are also different in other features: (1 ) their oxygen desorption temperature is 1010 ·C, about 70·C lower, (2) their Si Auger signal is dif­ferent in shape, and its peak-to-peak intensity in the deriva­tive mode is larger. These surfaces transform to (2 X I) at Tm = 1105 ·C, and to c(2X2) at Tm = 1155 ·C, where Tm is the maximum temperature at which the sample has been treated for 3 min.

The LEED patterns of the three phases of these samples are shown consecutively in Fig. 1. The patterns in Fig. 1 are superpositions of patterns originated by domains whose sur­face bonds are either along the (1,1) or the (1,1) directions, resulting in patterns with an apparent symmetry to 90· rota­tion about an axis normal to the surface. In Fig. l(a), as well

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39 Moshe Dayan: The P-SIC(100) surface

a

b

c FIG. I. LEED patterns of the three Si-terrninated phases. The heat treat­ment temperature and the beam energy are the following: (a) 1015 "C, 109 V, (b) 1120 'C, 111 eV, and (c) 1200 'C, 108 eV.

as in other (3 X 2) LEED patterns taken under different conditions, the intensities of the spots at the multiples of half-indices are considerably weaker than multiples of third indices. For two out of the three samples, thec(2X2) phase exhibited weak and different traces of superimposed (2 Xl) patterns. In general, the LEED patterns of the three phases under discussion are intense, sharp, and clear.

AES plots taken on one of the discussed three samples are presented in Fig. 2. The spectra shown in Fig. 2 correspond to consecutively higher T m from top to bottom. These spec­tra are part of a larger group of spectra (taken with T m

intervals of 10 ·C) on which our qualitative and semiquanti­tative analysis is based. The C Auger signal is constant in shape, for all T m 's well below the transition to the C-rich phase. Around T m 's in which the surface transforms, the peak-to-peak intensity of the carbon signal increases with T m . The Si-Auger signal decreases around these phase trans­formations. Some changes in the line shape are noticeable. One such change is the broadening of the large negative peak

J. Vac. Sci. Technol. A, Vol. 4, No.1, Jan/Feb 1986

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1105 Oc

~ (e) 1135 Oc

Z ':!;! -c

1155 0C

1190 0C

50 100 150 200 250 ENERGY. eV

FIG. 2. AES spectra taken on one sample for the three Si-terrninated phases. The plots were taken under the same conditions. The heat treatment tem­perature T m is indicated on each plot.

around 95 eV, while the other is the background line at low energies, 3{}-80 e V. The higher the T m , the larger the curva­ture of the low energy background line. A measure for this effect may be the deviation of the curve, at 30 eV, from the extrapolation of the straight line in the range 11{}-200 eV to low energies. All the large group of spectra, whose part is presented in Fig. 2 , were taken under the same conditions. These conditions include the sample position, the primary beam parameters, the ac modulation, the sensitivity scale, the time constant, and the scanning speed. This enables a comparison between the Auger signals of the same element in different plots, with different T m 'so

Two interesting features, regarding the oxygenated sur­face state and the oxygen desorption of the discussed sam­ples, are worth pointing out. One is that its LEED pattern, for T m = 960 ·C, exhibits sharp spots corresponding to a (3 Xl) structure. Although the spots appear to be sharp, they are partially masked by a bright background. For T m = 850 ·C, the LEED pattern is a sharp (1 Xl) with a bright background. In no case do the discussed patterns re­semble the one depicted in Figs. 4(a) and 4(b) of Ref. 1. The other interesting feature is related to the small effect the oxy­gen desorption has on the intensity of the C-Auger signal. Auger spectra were carefully taken under the same condi­tions, starting from T m = 710 ·C and increasing it until T m

is slightly above the oxygen desorption temperature. During this desorption process, the C-Auger signal remained con­stant in shape, as happened for all the other samples. A fea­ture unique to these three samples, however, is the very small increase of the signal intensity. In samples which exhibited a c(2X2) structure on oxygen desorption, the carbon Auger signal increased 50%-80% (see Fig. 1 in Ref. 1). In the

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40 Moshe Dayan: Thep-SIC(100) surface

other type of samples, the equivalent increase is only 3%-5%, comparable with the experimental reproducibility. A set of Auger spectra measured on a sample of the latter kind is shown in Fig. 3. All three spectra were taken under the same conditions, so that intensities of the carbon signal could be compared.

The factors determining whether a sample would exhibit a (3 X 2) or a e(2 X 2) surface structure on oxygen desorption are not clear. These factors are related to the sample growth conditions. Variations in growth conditions which might have taken place and might have affected the surface proper­ties have not been identified.

B. The C-rlch surface

A major limitation of the results reported in Ref. 1, on the C-rich surface, is the maximum T m of 1225 'C, which is too low to fully establish this kind of surface. This limitation has been overcome by testing a sample whose combined sub­strate-film resistance, at high temperature, enables reaching a higher maximum temperature with the given maximum current. The maximum temperature achieved with this sam­ple is 1285 ·C.

Two Auger spectra of the discussed sample are shown in Fig. 4. Figures 4(a) and 4(b) correspond to heat treatment at 1285 'C for 1.5 and 3.5 min, respectively. In Fig. 4 the two intensity scales are different (by an unknown factor). The plots in Fig. 4 should be viewed as a continuation of the sequence in Fig. 5 of Ref. 1. The combined sequence shows the following trends: (1) The Si intensity is vanishing rela­tive to the C intensity. (2) The Coline is converting from a SiC-like shape to a shape resembling either that of graphite or that of hard disordered carbon films. 5

•6 (3) The Si line

(e) -

550

FIG. 3. AES spectra showing the oxygen desorption from a sample which exhibited (3 X 2) surface symmetry. All plots were obtained under identical conditions. The indicated temperatures are the corresponding T", ·s.

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o ENERGY, eV

FIG. 4. AES spectra of the C-rich surface which was treated at 1285 ·C; (a) for 1.5 min, and (b) for 3.5 min. The intensity scales of the two plots are different.

shape is also changing, mainly by broadening of sharp fea­tures. For the discussed T m of 1285 'C, no LEED pattern is observable, unlike for Tm = 1225 'C, for which the pattern is depicted in Figs. 4(e) and 4(f) of Ref. 1. The C-rich sur­face is, therefore, disordered.

High resolution EELS measurements in the phonon ener­gy range have been performed on this C-rich surface. Figure 5 depicts two HREELS spectra for Tm = 1285 ·C. Figures

la)

-100 0 100 200 ENERGY, meV

FIG. 5. High resolution EELS on the C·rich surface which was treated at 1285 ·C; (a) for 1.S min, and (b) for 3.5 min.

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41 Moshe Dayan: The P-SiC(100) surface

5(a) and 5(b) correspond to heat treatment for 1.5 and 3.5 min, respectively. While the energy of the loss peak is the same as in Ref. 2, its intensity, normalized by the elastic peak, 11/10, diminished with heat treatment. To evaluate the normalized intensities in Fig. 5, one needs to calculate the background around the energy of the loss peak. A rough estimation for this background results in close values for the normalized intensities of Figs. 5(a) and 5(b). An apparent difference between these figures is the broadening of the elas­tic peak, from FWHM of 44 meV in Fig. 5 (a) to 49 meV in Fig. 5(b). The corresponding value, for the sample under consideration and Tm = 1225 °C, is about 30 meV. The in­strumental broadening is negligible.

The discussed C-rich surface, in its final state ( T m = 1285 °C); has been observed by a field emission SEM, and found to exhibit large bright spots, which occupy a substantial fraction of its area. A picture with magnifica­tion of21O and acceleration voltage of 15000 V, is shown in Fig. 6. When a negative bias of several e V is applied to the detector, the surface appears uniform in brightness. This is an indication that the origin of the bright spots is related to low energy secondary electrons, which in turn stem from material differences rather than surface morphology.

C. Oxygen treatment

The term "oxygen treatment" is used, rather than oxida­tion, because we limit the treatment to give an oxygen cover­age ranging between submonolayer to a couple of layers. Oxygen treatment was done on many samples under various conditions and for different phases. Our study of oxygen treatment, however, has not been intensive and systematic enough for any quantitative examination. This report is, therefore, partial and qualitative.

Two samples, which had previously been transformed to be C-rich by heat treatment at 1225 °C, were treated with 109

L of molecular oxygen. One of the samples was held at 750°C, and the other at 800 °C, during the oxygen treat­ment. The Auger results for both cases were quite similar to

FIG. 6. Field emission SEM image on theC-richsurface (Tm = 1285 'C; 3.5 min).

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the spectra depicted in Fig. 2 of Ref. 1. This indicates that the oxygen on the surface is bound to Si, while the carbon signal is SiC-like, which implies no C-O bonding. When the oxy­gen is desorbed from these surfaces, one exhibits a (2 XI) LEED pattern where the others exhibit mostly c (2 X 2) with superimposed weak traces of (2 XI). One of the discussed samples had been transformed to have a C-rich surface 11 months before it was oxygen treated at high temperature. During this long period, it was stored at room temperature in ambient conditions. Auger measurements, taken before the high temperature oxygen treatment, and after heat treat­ment at T m = 710 °C, revealed a very weak oxygen signal. This proves that either the C-rich surface does not react with molecular oxygen at room temperature or, ifit does, the rate is very low and the carbon oxide products are easily de­sorbed.

Oxygen treatment of Si-rich phases may be divided into two groups: (1) Treatments at room temperature by atmos­pheric air. (2) Treatments by various doses of dry O2, rang­ing from 3 X 104 to 1010 L, while the sample was held at temperatures higher than 700 °C. The treatment at high tem­peratures resulted in a range of oxygen coverage whose de­sorption temperatures ranged between 990 and 1040 °C. Usually the higher the oxygenation, the higher the desorp­tion temperature. We have not been able to produce states similar to the as-received states reported in Ref. 1, with their high desorption temperature of about 1080 0c. Oxygen treatment at high temperature followed by oxygen desorp­tion, result in Si-rich surfaces of (2 Xl) symmetry, or a mix­ture of regions of c (2 X 2) symmetry with regions of (2 Xl). The Si to C ratios of the peak-to-peak Auger intensities are in the ranges 1.8-3.5, for (2Xl) surfaces, 1.38-1.75, for c (2 X 2) surfaces, regardless of how the particular surfaces were obtained. The ratio is in the range 1.75-2.2 for mixed (2X 1) and c(2X2) surfaces.

Oxygenation by atmospheric air at room temperature was applied to one C-rich surface, and three Si-rich surfaces, two of the latter arec(2X 2) and one is (3 X2). The twoc(2X 2) surfaces were treated at ambient conditions, one for 20 hand the other for 44 h. The resulting oxygenation states, as ob­served by Auger measurements, exhibit oxygen signals whose peak-to-peak intensities are roughly 10% of the C or Si signals. The (3 X 2) Si-rich surface had been treated for 65 h and resulted in an oxygen to carbon peak-to-peak ratio of 0.53, comparable to one 24 h old as-received sample we test­ed.

D. Core level excitation EELS measurements

EELS measurements, in the loss range corresponding to the excitation of the L 2,3 core level, have been carried out on the three Si-rich phases, using a primary beam of 250 eV. The analyzer used is a CMA with an integral gun (Varian 981-2607). The results, in the derivative mode, for the (3 X 2), (2X 1), and c(2X2) phases are shown in Figs. 7(a), 7(b), and 7(c), respectively. The features in the energy range 125-155 e V are due to exciting a hole in the Si core level L 2,3 and an electron in the conduction band. The three spectra are quite similar in the range 125-145 eV, but somewhat different in the range 145-155 eV. The latter range corresponds to exci-

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42 Moshe Dayan: The P-SIC(100) surface

dN/dE~

120 130 140 150 160 170 Energy. eV

FIG. 7. Core level excitation EELS spectra (in the derivative mode) on (a) the (3X2) surface; (b) the (2X I) surface; and (c) the c(2X2) surface. The primary beam energy is 250 eV. In the insets are the results of Hem­street and Fong and the loss spectrum calculated from it.

tations in the bottom of the conduction band and the energy gap of the bulk, a region most likely to be affected by surface states. The poor resolution of our measurements, however, does not permit a conclusive determination of the density of states in the gap.

IV. DISCUSSION

A. The (3 X 2), (2 X 1), and c{2 X 2) Si surfaces

It has been suggested that thec(2x2) surface is terminat­ed by one layer ofSi, whose atomic constituents shift to form dimers in such a manner that dimers of adjacent rows are opposite in phase. 1 It is instructive to compare this surface to the (2 Xl) and (3 X 2) surfaces in terms of their Si content. This is not as straightforward as it might seem from a com­parison of the Si to C Auger peak-to-peak ratios. This is so because, to date, there is no general way to compare quanti­tatively the concentrations of a given element in different material matrices. Besides, if one attempts a semiquantita­tive analysis of Auger data, the use of peak-to-peak values of derivatives, would certainly be worse than the use of other obtainable quantities. One step better would be the use of the amplitude of the integrated signal (after subtracting the background). This would eliminate the misinterpretation caused by changes of the derivative intensity whose only ori­gin is the broadening of some features in the Auger signal.

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Such a broadening, of the large negative peak of the Si signal, is apparent in the transition from the (2X 1) surface to the c (2 X 2) surface. The use of integrated signals, although bet­ter than derivatives, still would not take into account the reduction of the Auger signal by inelastic scatterings, which is dependent on the matrix. The increasing curvature of the low energy background line, with increasing T m' suggests that the number oflow energy secondary electrons is depen­dent on Tm. This number, in turn, is a function of the prob­ability of inelastic scattering. This suggests that inelastic scatterings may reduce the integrated intensity of higher T m

stronger than that of lower T m' It would be advantageous (for quantitative analysis) to deconvolute the integrated sig­nal from its multiple loss function, to obtain the intrinsic Auger peak, and take the integral of this peak to be propor­tional to the number of Auger electrons. This sort of decon­volution study has been developed and used mainly to study Auger line shapes.7

•8 It could be adapted to deal with the

quantitative analysis under consideration. To date, however, the method has not resulted in a good agreement with the­ory.9

In the present work, deconvolution of Auger signals from their loss is not attempted. By using the integrated intensities of both Si and C signals of the discussed three surfaces, it is possible to find which surface contains more or less Si but not how much. To obtain these "integrated intensities," it is sufficient to integrate over the large negative peak after sub­tracting it from the straight line extrapolated from higher energies. The results I of these integrations for the Si and C signals are plotted versus T m in Fig. 8. The FWHM of the large negative peak of the derivative of the Si signal!l. is also shown in the same figure by the squares and the dashed line. All the data of Fig. 8 resulted from a series of Auger spectra which were obtained under the same conditions, as discussed in the last section.

It is clear from Fig. 8 that the integrated Si signal de­creases when the surface transforms from (3 X 2) to (2 Xl) symmetry, and also from the latter to c(2X2) symmetry. This decrease could not be entirely attributed to an increase in the rate of inelastic scatterings, because if it could be, the C signal should decrease too. The intensity of the C signal in-

800

700

V'I 600 ....

Z => >- 500 '" <C

'" .... 400 co '" <C

300 --200

100 1000

Si

..----D---G----{]

I Si /

-...[]..------------o------~"LT#..[J--o-a

c

-~- 3x2 ~~- -2xl- --c12x21-

1050 1100 Tm·Dc

5~

<i 4

FIG. 8. The integrated Auger intensities I (solid lines) and the width of the large negative peak of the Si signal 1::. (dashed line). The results were calcu­lated from a series of plots taken under identical conditions.

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43 Moshe Dayan: The /l-SIC(100) surface

creases through the two transitions, while its shape remains unchanged. This may be explained by the loss of Si atoms at the surface in each of the transitions. These losses occur without significant changes in the electronic bonding in the immediate vicinity of the C atoms. Consequently, we are led to conclude that a full Si layer is present next to the C in all three Si-terminated surfaces, and that the (3 X 2) surface is richer in Si than the (2 XI) surface, which in tum, is richer than the e(2 X 2) surface.

Based on the results and physical feasibility, we suggest the following models for the (2 Xl) and (3 X 2) surfaces. In the (2 Xl) surface, pairs of adjacent rows of Si atoms move closer to each other to form dimers by interbonding (in phase), as suggested for Si, Ge, 10.11 and diamond. 12 If each dimer is bound by one bond, and only one layer ofSi exists on the surface, then the space between two adjacent rows of dimers is wide and bound by two rows of electrons not parti­cipating in any bonding. This would result in a reactive sur­face with a high surface energy. It is conceivable, therefore, that extra Si atoms bridge between each pair of such elec­trons and bond to them. The result is an extra halflayer ofSi made of rows whose interspacing is twice as large as the bulk value. The extra Si atoms within one row are spaced 3.08 A apart, a distance too large to accommodate strong Si-Si bonds. If weak interactions exist along the row direction, one would be puzzled by the lack of dimerization along that same direction. This may be solved by assuming an elec­tronic configuration, which does not allow any interaction between these extra Si atoms. This is the configuration with­out any hybridazation, where the bonding to the full Si layer is done by the p electrons, and the pair of s electrons remains as a lone pair. The 90· angle between the p orbitals is close to the angle between the orbitals they are bound to. Thus, only a small bond bending is necessary. Finally, it would be inter­esting to check the validity of the discussed model to the Si( 100)-(2 Xl) surface, and to the equivalent surfaces of Ge and diamond.

On the e(2 X 2) surface, any Si interbonds, apart from those which make the dimers, ought to be very weak because of the distance involved. Consequently, it is likely that the dimers are bound by double bonds. This could be achieved by the planar Sp2 hybridization plus a p orbital. If this is the prevailing configuration, the dimers are bound by one (7

bond of the p orbital and one 1T bond of the hybridized orbi­tal. The other two hybridized orbitals are directed adequate­ly to make strong (7 bonds with the underlaying C atoms (120·, which is close to the tetrahedral angle of 109S).

The model for the (3 X 2) surface also assumes an extra sub layer of Si atoms on top of the full layer of Si. This sub-

SIDE VIEW ( OF THE (olll PLANE).

FrG 9. Model for the (3 X 2) surface.

• Si ATOMS o C ATOMS

J. Vac. Sci. Technol. A, Vol. 4, No.1, Jan/Feb 1986

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layer contains two rows of Si atoms for every three rows of bulk spacings. These two rows are bound with three rows of the underlaying Si layer, as depicted in Fig. 9. For every three rows of atoms in this full layer, there is one row of intrabonds, which causes some atoms to shift and rotate due to the torque exerted by the relative stiffness of covalent bonding. Atoms in the top sublayer interact, in the direction normal to the plane of Fig. 9, to form dimers. These dimers can be either in phase or out of phase, and still result in a (3 X 2). symmetry.

This last model is consistent with the (3 X 2) symmetry, with the results indicating an excess of Si over the (2 XI) surface, and is also consistent with the oxygen desorption results presented in Fig. 3. The oxygen desorption has only a small effect on the C Auger signal, because the latter exits the surface mostly from the rows which lack the extra top Si. The C Auger signal which originated at the other two rows, undergoes an excessive attenuation by the extra top Si atoms and by the oxygen. The oxygen desorption enhances only the latter part of the C Auger signal, while the former part (which produces most of the intensity) remains unaffected.

The (3 XI) LEED pattern, observed for the (3 X 2) surface before its oxygen desorption, proves that the tripling of the unit cell in one direction existed even when the oxygen was bound to the surface. This component of the reconstruction, therefore, is not caused by the orbitals which participate in bonding to oxygen, a feature consistent with our model.

B. The C-rich surface

The recovery of disordered C-rich surfaces to Si surfaces, discards one of the three alternative models suggested in Ref. 1, for the C-rich surface. This is the model which assumes the termination of the surface by one layer of C atoms, and the deterioration of its morphology. This model is discarded be­cause it is unlikely that the oxygen treatment applied to the discussed surface would improve its morphology. Also, this model would not explain the larger number of low energy secondary electrons, as observed in the present work. Al­though the model is discarded, the attribution of the origin of the C-rich surface to Si desorption may still be valid. It is possible that, at the Tm range of 1225-1285 ·C, Si atoms desorb at a higher rate than C atoms, because the latter inter­act very strongly. This results in regions covered by disor­dered C films. The microstructure of the C films in not clear. The carbon Auger line shape of Fig. 4(b) is quite similar to that of graphite, but it is just as similar to the line shape of hard carbon films. s.6 Van Bommel eta!.13 observed a gradual transformation, of the a-Si(ooo 1) surface to the graphite (0001) surface, by heat treatment. The positive identification of graphite on their surface should not necessarily imply the graphitization of our surface, although it suggests its feasi­bility. The main differences between the two cases are the bulk crystalline structure, and the amount of structural or­der of the final surface product. While the C-rich surface of their a-SiC is crystalline, ours seems to lack any long range order. It is difficult to determine whether the short range structure of our C-rich films is diamondlike, graphitelike, or both. The same difficulty exists for the hard carbon films produced by various deposition methods. The plasmon ener-

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44 Moshe Dayan: The {J -SiC(1 00) surface

gy of these films, for example, covers a wide range which includes the plasmon energy measured on our C-rich surface 27 eV. 14

- 16

The line shape of the Si signal in Fig. 4(b) lacks the sharp feature of its counterpart in ordered SiC surfaces. The broad featureless shape is an indication that the Si is embedded in a disordered matrix, possibly (but not necessarily) in C-rich films. The Si signal diminishes with increasing T m , a process whose saturation has not yet been reached in the state depict­ed in Fig. 4(b). The transition to the C-rich surface is not a sharp one, neither in T m nor in heat treatment time.

An important relevant quantity, yet to be found, is the thickness distribution of the carbon film. For this purpose, the high resolution EELS results are useful. It is well known that this technique, when applied on polarizable bulk mate­rials in the specular direction, probes long wavelength sur­face phonons.2

•17

•18 The probing depth, as well as the decay

length of fields (in the vacuum) produced by the discussed phonons, are typically 200-300 A. Material changes that might occur on the surface, over a few A thickness, have a very small effect on the intensity and energy of the loss peak. 2

,17.18 The reduction of the intensity of the HREELS loss peak suggests that the SiC structure deteriorates within an average depth of 200 A. A deterioration over such a large depth occurs even in surfaces treated at 1225 °C. The bright spots observed on the latter kind of surfaces by SEM, how­ever, occupy an insignificant total area (much smaller than the total bright area in Fig. 6). This indicates that the surface phonon, and the field intensity produced by it are sensitive to even a small amount of structural deterioration. The bright images on the SEM screen, however, are not apparent before the (local) carbon concentration riches its maximum.

C. Oxygen treatment

The recovery of the C-rich surfaces by oxygen treatment implies that carbon is removed from the surface as CO or CO2, A similar process may occur, to a smaller extent, when Si-terminated surfaces are treated by oxygen at high tem­peratures. This is suggested by the Si enrichment found on the latter surfaces on oxygen desorption, as evident from Auger measurements and from the conversion from c(2 X 2) to (2X 1) symmetry. The desorption of carbon (as carbon oxide) during oxygen treatments at high temperature, may explain why the oxide grown on SiC is similar to that grown on Si.

D. The core level excitation EELS

The theoretical equivalent of the spectra measured in our core level excitation EELS experiment are not available in the literature. Related functions, which are usually avail­able, are the bulk density of states and electronic dispersion relations along symmetry lines in the Brillouin zone. Even if one makes the incorrect assumption that the matrix element for the process under consideration is a constant, none of the above-mentioned functions is comparable with our spectra. This is so because instrumental characteristics are not incor­porated into the theoretical analyses. For the energy range under consideration, 125-155 eV, and for the CMA angle of acceptance, only a few contours in the 2D reduced Brillouin

J. Vac. Sci. Technol. A, Vol. 4, No.1, Jan/Feb 1986

44

zone contribute to the measured spectrum. This could be verified by viewing a diffraction pattern taken with a pri­mary beam energy in the range 125-155 eV. The circle on the LEED screen, which corresponds to the CMA angle of ac­ceptance, consists of only a few contours in the reduced Bril­louin zone. Increasing the primary energy results in a more uniform averaging over the Brillouin zone, but reduces the energy resolution. Increasing the primary energy may also result in undesirable mixing of the discussed spectrum with other signals, such as the C Auger signal and the C core level EELS.

There are several calculations of the electronic band struc­ture ofbulk,B-SiC. 19-22 In some of these reports, only a limit­ed low energy portion of the conduction band is given. The results by Hemstreet and Fong '9 cover the range up to 20 eV above the maximum of the valence band. Dispersion rela­tions are given along straight lines between certain points in the 3D Brillouin zone, but the density of states of the con­duction band is not given. The latter, together with disper­sion relations, are given by Lubinsky et al. 22 This density of states, after differentiating with respect to E and broadening by the experimental resolution, gave results which were in complete disagreement with our spectra. A second attempt to compare our results with theoretical calculations has been made with the results of Hemstreet and Fong. '9 Since these authors do not provide the conduction band density of states, we calculated the "linear" density of states along the lines for which their dispersion relations are given (in Fig. 2 of Ref. 19). The result has been differentiated with respect to E, and broadened by the experimental resolution. The re­sulting function dN IdE together with the dispersion func­tions used to calculate it, are shown in Fig. 7 (as insets). A qualitative agreement with the experimental curves is appar­ent. There is a one-to-one correspondence between features in the theoretical function and in the measured curves. Cor­responding features are close in energy, but differing in shape and relative intensity. Given the difference between those regions of the Brillouin zone probed by the CMA, those which are accentuated by the matrix element and those presented in Ref. 19, the agreement is surprisingly good.

ACKNOWLEDGMENTS

The author thanks P. Aron for carrying out the SEM mea­surements; and M.J. Zehe, D.R. Wheeler, and J. Ferrante for their critical review of the manuscript.

8' NASA Resident Research Associate. 1M. Dayan, J. Yac. Sci. Techno!. A 3(2),361 (1985). 2M. Dayan, Surf. Sci. 149, L33 (1985). 3S. Nishino, J. A. Powell. and H. A. Will, App!. Phys. Lett. 42, 460 (1983). 'w. J. Choyke and L. Patrick, Silicon Carbide-l 973, edited by R.C Mar­shall, J.W. Faust, Jr., and C.E. Ryan (University of South Carolina, Co-lumbia, SC, 1974), p. 261.

SA. K. Green and Y. Rehn, J. Yac. Sci. Techno!. A 1,1877 (1983). "T. J. Moravec and T. W. Orent, J. Yac. Sci. Techno!. 18(2), 226 (1981). 7R. Weissmann and K. Miiller, Surf. Sci. Rep. I, 251 (1981). RH. H. Madden and J. E. Houston, J. Appl. Phys. 47, 3071 (1976). 9M. Dayan and S.Y. Pepper, Surf. Sci. 138,549 (1984). lOR. E. Schlier and H. E. Farnsworth, Semiconductor Surface Physics, edit­

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45 Moshe Dayan: The P-SiC(100) surface

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18L. H. Dubois and G. P. Schwartz, Phys. Rev. B 26, 794 (1982). '9L. A. Hemstreet and C. Y. Fong, Silicon Carbide-1973, edited by R.C.

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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.24.51.181 On: Sun, 23 Nov 2014 00:15:40