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Hydrogen adsorption and surface structures of silicon
SURFACE
SCIENCE 43 (1974) 481-492 Q North-Holland
Publishing Co.
HYDROGEN ADSORPTION AND SURFACE STRUCTURES OF SILICON I-I. IBACH*
and J. E. ROWE
Bell Laboratories, Murray Hill, New .krsey 01974, U.S.A.
Received 23 November 1974 The adsorption of atomic hydrogen on silicon (111) 2 x 1 cleaved, (111) 7 x 7, and (100) 2 x I surfaces has been studied by using electron energy loss spectrscopy (ELS) and photoemission spectroscopy (UPS). On ail surfaces the hydrogen removes the “dangling bond” surface state and a new peak in the density of states at lower energies corresponding to the Si-H bond is found. The LEED pattern of the equilibrium surfaces (I 11) 7 x 7 and (100) 2 x I is not altered by hydrogen adsorption, while on the cleaved (111) 2 x 1 surface the fractional order spots are extinguished. The Haneman surface-buckling model therefore provides an explanation for the surface reconstruction of the cleaved (111) 2 x 1 surfaces. For the equilibrium surfaces, (Ill) 7 x 7 and the (100) 2 x I, the data are consistent with the Lander-Phillips model.
1. Introduction It has been recognized for a long time that unsaturated “dangling” bonds play an important role in the electronic and structural properties of semiconductor surfaces. These properties may be studied not only on clean surfaces but also by observing the differences between clean surfaces and surfaces where the dangling bonds are saturated by adsorbed atoms. The adsorption of atomic hydrogen is expected to be the most simple adsorption system. Nevertheless this system has not been studied systematically on clean, low-index single-crystal silicon surfaces. Becker and Gobelil) observed infrared vibrational spectra of adsorbed atomic hydrogen. The frequency was close to the Si-H stretching vibration of SiH+. This suggests that adsorbed atomic hydrogen sits on top of the silicon atoms bonded by a single Si-H bond. Law a) studied the adsorption of hydrogen on thin evaporated silicon films and concluded that atomic hydrogen is adsorbed up to a monolayer coverage. Contrary to atomic hydrogen, molecular hydrogen is adsorbed only in small quantities2-4) (N 3%) and different binding states are observed. It is possible that this adsorption is connected to surface irregularitiess,@). * On leave of absence from 2. Physikalisches Germany. 481
Institut, Technische Hochschule,
Aachen,
482
H.~BA~HAND~.E.RO~~~
In this paper we have studied the adsorption of atomic hydrogen on the (1 i I) 2 x 1 cleaved, the (1 i I) 7 x 7, and the (100) 2 x I annealed silicon surfaces. Photoemission as well as energy loss spectra show that the electronic energy levels of hydrogen adsorbed on the different faces are very similar and can be interpreted in terms of a formation of an Si-H surface bond. The presence of unsaturated “dangling bonds” on the clean silicon surfaces has been considered to be related to the surface lattice reconstruction. LEED patterns of the hydrogen covered silicon surfaces however show, that this model obviously does not apply to equilibrium structures prepared by annealing. 2. Experimental techniques The vacuum system consisted of an ion pumped stainless steel chamber with various ports for the electron spectrometer and the LEED system. The base pressure with the liquid nitrogen trap being cool was N 5 x 10-‘r Torr. A single-pas PHI l&2346 Auger analyzer with a coaxial electron gun was used for Auger electron spectroscopy (AES) and for electron energy loss spectroscopy (ELS). ELS spectra were taken as second derivative loss spectra by modulating the deflection voltage (0.8 V,_,) and observing the second harmonic. A PHI 15-250 double pass analyzer operated in the retarded mode was used to analyze the photoelectrons. At 100 eV pass energy the resolution was -0.5 eV. The resolution was measured by observing 3p photoelectrons from gas phase argon. The light source was a resonance lamp. Details of the microwave lamp and the differential pumping have been described dsewhere 7). The lamp was operated on the He I 21.2 eV line at a pressure of 1.5 Torr. Spectra were recorded in a single scan in one second per point and with 0.1 eV per point separation, The data were stored in digital form in a PDP-8/Lminicomputertofacilitatefurtherdata processing such as smoothing or computing the difference between two curves. In order to allow an easier comparison of energy levels to those of free molecules all electron energies in this paper refer to the vacuum level E,,,. The point E= -ho was determined from the photoemission spectra by a linear extrapolation of the low energy side of the slow peak. To insure that the slowest electrons were not prohibited from entering the analyzer by a contact potential a small negative bias (O-1 eV> was applied to the sample. The necessary amount of compensation could be estimated by measuring the energy differences of the onsets of the sample secondary electron peak and the secondaries from the first grid of the analyzer at a higher bias. The error in the determination of the vacuum level by this method is estimated to be less than _tO.2 eV. Clean silicon surfaces were prepared by argon bombardment and annealing and by cleaving. The cleavage samples were cut from B-doped material
HYDROG~ADSORPTIONAND~LJRF.~~ESTRU~TU~~~F
(- 1Or4 cmm3 ). The samples
to be cleaned
Si
by bombardment
483
(As-doped,
5 x 10’ 7 cmm3) were cut to thin plates of 18 x 18 mm size and 0.25 mm thickness. They were mounted on a manipulator arm so that they could be rotated on a - 11.5 cm radius and positioned in front of either the ion gun, the analyzers, or the LEED system. The samples were heated by a Al,O, covered tungsten spiral. Occasionally two samples of different surface orientations were mounted on the same sample holder to allow a direct comparison. Cleaved surfaces were prepared with a multiple cleavage tool (see fig. 1). The silicon crystal was cut into a bar of 5 x 15 x 70 mm with - 18 slots of 0.6 mm width and 1.5 mm depth across the upper and lower sides of the bar to mark the intended cleavage plane. Cleavage was performed by pressing a 20” wedge into one of the upper notches while the crystal was resting with the lower notch on a 120” wedge. The cleavage apparatus was mounted on a 2.75 in. O.D. flange 11.5 cm off the center of the chamber. With the same tool the crystal bar could also be pulled out farther after loosening a clamp on the manipulator. The bar could be heated to - 700 “C by direct current heating. Electrial contacts were made by rhodium plating. The LEED pattern of the cleaved crystals exhibited in general a clear one
Fig. I.
Multiple cleavage sample holder in the “ready to cleave” position.
484
H. IBACH
AND J. E. ROWE
domain
2 x 1 pattern
with no indication
steps8).
Occasionally,
however,
of spot splitting
on poor cleavages
due to cleavage
high step densities
were
observed as well. The argon bombarded and annealed surfaces showed a clear 7 x 7 and a two-domain 2 x 1 pattern for the (111) and (100) surfaces respectively. Exposure to atomic hydrogen was made by placing the sample in front of the filament of a carefully outgassed ion gun, with no voltage applied to the electrodes. The exposures were controlled by measuring the partial pressure of molecular hydrogen introduced to the UHV system from a high purity flask source. The percentage of atomic hydrogen at the position of the sample however was not established. The hydrogen peaks in the UPS and ELS spectra saturated on all surfaces around an exposure of lo- 5 Torrmin of molecular hydrogen. Based on the work of Lawr) we shall call the corresponding coverage a monolayer coverage in the following. It was found that the UPS and ELS data are consistent with this interpretation. No change in the UPS and ELS spectrum was observed after exposure to molecular hydrogen alone, with all filaments in the chamber turned off.
3. Experimental results The photoemission spectrum for the clean cleaved silicon (111) 2 x 1 surface and for the same surface covered with a monolayer of hydrogen is shown in fig. 2. The peak on the shoulder of the valence band photoemission of the clean surface, near - 5 eV has been identified as due to a band of surface statess-ll). Details of the surface state band have been found to be sensitive to the quality of the cleavagela) and will not be considered in the following. The spectrum in fig. 2 corresponds to a low step density concentration. In order to distinguish the surface state band in fig. 2 from the lower lying surface states associated with the silicon back bondsrs) we shall refer to these calculasurface states as the “dangling bond” surface states. Theoretical tionsla-1s) have shown that a surface state with a charge concentration around the direction of the unsaturated bond on the (111) surfaces is expected in the same energy range where the surface state peak in the photoemission spectrum occurs. After hydrogen adsorption the surface state peak has completely disappeared and a new peak at Ez - I1 eV is observed together with an increase in the background of secondary electrons. The valence band photoemission threshold decreased by approximately 0.2 eV. In fig. 3 the difference in the spectra in fig. 2 between the hydrogen covered and the clean surface is shown together with similar results for the (111) 7 x 7 and the (100) 2 x 1 surfaces. The positions of the peaks from the photoemission of the dangling bond surface states are marked by arrows for all these surfaces. The arrows coincide within the limits of error with the minima
HYDROGEN
ADSORPTION
AND SURFACE
CLEAN
STRUCTURES
OF
si
485
SI (111) 2x1 CLEAVED SURFACE
hw = 21.2 eV
COVERED
I
WITH
HYDROGEN
I
I
-20
I
-10 ENERGY
I
0 (eV)
Fig. 2. Photoemission spectra of a clean cleaved (111) 2 x 1 silicon surface and the same surface covered with approximately a monolayer of atomic hydrogen. The bump on the high energy side of the valence band emission has been identified as due to the “dangling
bond” surface state9*10). in the difference
spectra.
It is noteworthy
however
that on the (100) 2 x 1
surface the “dangling bond” surface state is only partly removed by the hydrogen adsorption. The (100) surface saturated with hydrogen still exhibits a peak at - 5.8 eV of approximately half the size of the clean surface. Electron energy loss spectra of clean and hydrogen covered surfaces are shown in fig. 4. The origin of the losses on the clean surfaces have been discussed earlier13). The transitions E, and E, are bulk interband transitions, tto, and ttw, surface and bulk plasmon excitations respectively and S,, SZ, and S, are transitions from surface states. The initial state for the S1 transition is the “dangling bond” surface state (fig. 2)13). The initial states for S, and S3 are deeper lying surface states of the silicon surface atom back bondsl4). After hydrogen exposure a new peak at -8.5 eV is observed on all surfaces. At the same time the surface state transitions S1, SZ, and S3
486
H. IBACH
AND 1, E. ROWE
(100)
\
2x1
l_r --5.8 i
I -20
I
-15 ENERGY
I
I
-10 (ev)
-5
1
Fig. 3. Photoemission difference spectra between hydrogen covered and clean surfaces for a photon energy of f2~0= 21.2 eV. The minima occur within the limit of error at the positions of the peak in the surface state photoemission at ~ 5.4, - 4.9, and - 5.8 eV, respectively. In connection with other results this suggests that atomic hydrogen is bonded to a single silicon surface atom.
disappear. With increasing exposure the hydrogen peak is shifted towards lower energies. This shift is similar to that observed for oxygen17) and probably therefore also due to a change in the dielectric function E(O) of the surface. The true transition energy could be estimated if one extrapolated the observed peak position to zero coverage17). Since an independent determination of the coverage was not available here this procedure was not possible. We estimate, however, the transition energy to be approximately 8.5 f 1.0 eV on all surfaces. No attention should be paid to the different magnitudes of the peaks on different surfaces, since these are also influenced by the width of the transitions and neighboring peaks. The results are summarized in table 1. Since only a single UPS peak and a single transition is observed for the hydrogen covered surfaces, the energy of the final state in ELS can be estimated. With the exception of the clean
Si
H~ROGENAD~~RPTI~NAND~~RFACE~TRUCT~RESOF
Si(lll)
2X1
481
tH
CLEAN
1.5X lob5 torr min
hWP (111) 7X7
tH
CLEAN
10e5 torr min
3X10m5 torr
Si(lO0)
2X1
min
tH
CLEAN
10-6 torr min
3X10-5
L
I
0
Fig. 4. Negative second faces covered with atomic cular hydrogen. The shift to the dielectric
I
8
I 16 ENERGY
LOSS
torr
min
I 24 (ev)
derivative of energy loss spectra hydrogen. The exposures refer in the positions of the hydrogen coupling of single particle and
for silicon (111) and (100) surto the partial pressure of molepeak at higher coverages is due collective excitations.
488
H. IBACH
AND J. E. ROWE
TABLE
1
Energies of surface states and transition energies for clean and hydrogen covered surfaces
Surface
(111)2x (111)2x (111)7x7 (111)7 x (100) 2 x (100)2x
1 1 7 I I
Adsorbate
_ H H H
Surface state energies, UPS ~
5.4 10.7 ~~ 4.9 11.3 ~ 5.8 ~ II.1
Transition energies, ELS 2.4 8.5 * I .o 2.0 8.5 I_ 1.0 1.7 8.5 * 1.0
Final state
~ 2.9 2.2* 1.0 ~~2.9 2.8 * I .o ~~4.1 - 2.6 & I .O
(100) 2 x 1 surface the energies of the final states fall in the same energy range. Contrary to the obvious similarity in the electron states of the adsorbed hydrogen atoms, the influence of the hydrogen adsorption on the LEED pattern is very different for the cleaved surfaces and the annealed surfaces. On the (111) 7 x 7 and on the (100) 2 x 1 surfaces no changes in the surface structure are observed (see fig. 5). Photographs of the LEED pattern before and after exposure to N IO- 5 Torr min of hydrogen (H,+ H) are indistinguishable. On the (I 11) 2 x 1 cleaved surface, however, the half order spots bebecome weak during exposure and are completely extinguished at saturation coverage
(see fig. 5). 4. Discussion
The photoemission and energy loss spectra show that on all surfaces the adsorbed atomic hydrogen removes (at least partially) the “dangling bond” surface state and a new peak at Ecz - 11 eV occurs. This energy is close to the ionization energy of the T, orbital in gaseous SiH, (- 12.7 eV)r*). Simultaneously with the disappearing of “dangling bond” surface state and of the transition from this state S,, the transitions from the lower surface states S, and S, also disappear. According to Appelbaum and Hamann14) the charge density on a clean surface is redistributed towards the back bonds of the silicon surface atoms. This strengthening of the back bonds reduces the lattice spacing between the first and second layer and gives rise to deep lying surface states at the bottom of the s and p-like valence bands respectively. The transitions S, and S, have been associated with these surface states. When we interpret our data within this scheme we are lead to the conclusion that on all surfaces hydrogen is bonded by the “dangling bond” surface state and the peak at - 11 eV corresponds to the energy of the electron in the Si-H bond. The presence of this bond redistributes the charge density
HYDROGEN
ADSORPTION
AND SURFACE
STRUCTURES
OF
si
489
(4
(4
03
Fig. 5. LEED pattern of clean and hydrogen covered silicon surfaces. The (111) 7 x 7 and the (100) 2 x 1 surface structures do not change during hydrogen adsorption. On the (111) 2 x 1 cleaved surface, however, the fractional order spots disappear. (a) (111)2x 1 cleaved,clean; (c) (lll)7x7clean; (e) (111)2X1 clean;
(b) (lll)cleaved+H; (d) (lll)7x7+H; (f) (100) 2 x 1 + H.
490
towards relaxation
H.IBACH
a distribution
similar
and the corresponding
AND
J. E. ROWE
to the bulk
silicon.
Therefore
the surface
surface states disappear.
On the (100) surface the adsorption half of the surface state photoemission
saturates when approximately only is extinguished. This could be inter-
preted by assuming that the adsorption on the (100) surface saturates or at least the sticking coefficient is considerably reduced when only one of the two dangling bonds per surface atom on the (100) surface is filled. On the (111) 7 x 7 surface the photoemission due to dangling bond surface states appears only as a weak shoulderll) on the valence band emission. From the photo-emission data alone one cannot determine whether all dangling bonds are occupied at saturation coverage. The loss spectrum (fig. 4) however, indicates that this is indeed the case. The same holds for the (111) 2 x 1 cleaved surface (figs. 2 and 4). We conclude that on all surfaces the coverage with atomic hydrogen saturates at a monolayer coverage. The stability of the (111) 7 x 7 and the (100) 2 x 1 superstructures with respect to hydrogen adsorption seems most surprising. At least for the (111) 7 x 7 where all dangling bonds are saturated by hydrogen it clearly demonstrates that the surface reconstruction is not just due to the presence of dangling bonds. Several models for the surface reconstruction have been discussed. One of the earliest is the rumpled surface or H-model proposed by Hanemanrs). According to this model at the (111) surface the sp3-hybridization of the orbitals will be partly reversed due to the half-filled dangling-bond orbitals. This leads to a kind of surface Jahn-Teller effect where one-half of the surface atoms are raised above the average position on the surface and the other half are depressed because of more s-like or p-like bonds respectively. Since adsorbed hydrogen would restore the bulk sp3-hybridization of the silicon surface atoms this model would predict that the fractional order LEED spots would be extinguished during adsorption, and the backbond surface states S, and S, would disappear. This is an accurate description of what has been actually observed on the (I 11) 2 x I cleaved surface but it seems not to apply to the other surfaces. Based on an earlier discussion of Schlier and Farnsworth20), Levine has recently suggested that on the (100) 2 x 1 surface nearest neighbor surface atoms would form double rowssl). If the silicon surface atoms actually had a lateral displacement from the bulk positions on the clean surface one would expect this displacement to vary when the dangling bonds are saturated by hydrogen adsorption. However, no change in the LEED pattern is observed after hydrogen adsorption as shown in fig. 5. One is therefore inclined to say that the results presented here do not favor the Levine model. Another explanation of the different equilibrium surface structures based on a model of surface vacancies and molecular reconstruction has been given
HYDROGEN~~~RFTI~NA~~D~~~~ACESTR~~~RE~~F
Si
491
by Landeraa). According to this model on the (111) 7 x 7 surface the silicon atoms of the first and second surface layer would form “warped benzene rings”. Phillipsas) has recently expanded Landers arguments by thermodynamic considerations of metallic and covalent contributions to binding. The results of hydrogen adsorption are consistent with this Lander-Phillips model, since large distortions such as surface vacancies could certainly not be removed by hydrogen. The suggestions of Phillipszs) that the surface transitions labeled S, and S, might be associated with orbitals sticking out in the surface vacancies, however, is not confirmed, since these transitions disappear simultaneously with Sr. The recently proposed soft mode reconstruction mechanisma4) is not supported by the data for the equilibrium surfaces. The effective ionic charge of the silicon surface atoms responsible for the occurance of a soft mode should be considerably reduced by hydrogen adsorption and thus a 1 x 1 pattern would be expected. 5. Conclusion The analysis of the adsorption data for atomic hydrogen on several silicon surfaces suggests that the different surface structures are not caused by a single reconstruction mechanism. For the equilibrium structures the data are consistent with the Lander-Phillips vacancy model. The superstructure of the cleaved surface is probably a hybridization or soft mode effect, which are alternate descriptions of the fact that an up and down displacement of every second surface atom perpendicular to the surface lowers the free energy compared to an unreconstructed surface. On all surfaces a dangling bond surface state at approximately the same energy is found. The data indicate that the hydrogen atoms are bound by the electrons in the dangling bonds and their electron energy is lowered by N 6 eV. Thus the hydrogen on silicon provides a fairly simple adsorption system that might be a useful test case for theoretical calculations. Acknowiedgements It is a pleasure to acknowledge a number of helpful discussions with J. A. Appelbaum, G. E. Becker, H. D. Hagstrum, and J. C. Phillips. The technical assistance of S. B. Christman and of E. E. Chaban is also greatly appreciated.
References 1) G. E. Becker and G. W. Gobeli, J. Chem. Phys. 38 (1963) 2942. 2) J. T. Law, J. Chem. Phys. 30 (1959) 1568. 3) M. F. Chung and D. Haneman, J. Appl. Phys. 37 (1966) 1879.
492
H. IBACH
AND J. E. ROWE
4) B. A. Joyce and J. H. Neave, Surface Sci. 34 (1973) 401.
5) 6) 7) 8) 9) IO)
I I) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24)
H. Ibach, K. Horn, R. Dorn and H. Liith, Surface Sci. 38 (1973) 433. B. Lang, R. W. Joyner and G. A. Somorjai, Surface Sci. 30 (1972) 454. J. E. Rowe, S. B. Christman and E. E. Chaban, Rev. Sci. Instr. 44 (1973) 1675. M. Henzler, Surface Sci. 36 (1973) 109. D. E. Eastman and W. D. Grobman, Phys. Rev. Letters 28 (1972) 1378. L. F. Wagner and W. E. Spicer, Phys. Rev. Letters 28 (1972) 1381. J. E. Rowe and H. Ibach, Phys. Rev. Letters 32 (1974) 421. J. E. Rowe and H. Ibach, to be published. J. E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. J. A. Appelbaum and D. R. Hamann, Phys. Rev. Letters 31 (1973) 106. K. C. Pandey and J. C. Phillips, Solid State Commun. to be published. F. Yndurain and M. Elites, Surface Sci. 29 (1972) 540. H. Ibach and J. E. Rowe, to be published. R. G. Cavell, S. P. Kowalczyk, L. Ley, R. A. Pollak, B. Mills, D. A. Shirley and W. Perry, Phys. Rev. B 7 (1973) 53 13. D. Haneman, Phys. Rev. 121 (1961) 1093. R. E. Schlier and H. E. Farnsworth, J. Chem. Phys. 30 (1959) 917. J. D. Levine, Surface Sci. 34 (1973) 90. J. J. Lander, in: Progress irr So/id State Chenzisfry, Vol. 2, Ed. H. Reiss (Pergamon Press, Oxford 1965) p. 26. J. C. Phillips, Surface Sci. 40 (1973) 459. S. E. Trullinger and S. Cunningham, Phys. Rev. Letters 30 (1973) 913.
SCIENCE 43 (1974) 481-492 Q North-Holland
Publishing Co.
HYDROGEN ADSORPTION AND SURFACE STRUCTURES OF SILICON I-I. IBACH*
and J. E. ROWE
Bell Laboratories, Murray Hill, New .krsey 01974, U.S.A.
Received 23 November 1974 The adsorption of atomic hydrogen on silicon (111) 2 x 1 cleaved, (111) 7 x 7, and (100) 2 x I surfaces has been studied by using electron energy loss spectrscopy (ELS) and photoemission spectroscopy (UPS). On ail surfaces the hydrogen removes the “dangling bond” surface state and a new peak in the density of states at lower energies corresponding to the Si-H bond is found. The LEED pattern of the equilibrium surfaces (I 11) 7 x 7 and (100) 2 x I is not altered by hydrogen adsorption, while on the cleaved (111) 2 x 1 surface the fractional order spots are extinguished. The Haneman surface-buckling model therefore provides an explanation for the surface reconstruction of the cleaved (111) 2 x 1 surfaces. For the equilibrium surfaces, (Ill) 7 x 7 and the (100) 2 x I, the data are consistent with the Lander-Phillips model.
1. Introduction It has been recognized for a long time that unsaturated “dangling” bonds play an important role in the electronic and structural properties of semiconductor surfaces. These properties may be studied not only on clean surfaces but also by observing the differences between clean surfaces and surfaces where the dangling bonds are saturated by adsorbed atoms. The adsorption of atomic hydrogen is expected to be the most simple adsorption system. Nevertheless this system has not been studied systematically on clean, low-index single-crystal silicon surfaces. Becker and Gobelil) observed infrared vibrational spectra of adsorbed atomic hydrogen. The frequency was close to the Si-H stretching vibration of SiH+. This suggests that adsorbed atomic hydrogen sits on top of the silicon atoms bonded by a single Si-H bond. Law a) studied the adsorption of hydrogen on thin evaporated silicon films and concluded that atomic hydrogen is adsorbed up to a monolayer coverage. Contrary to atomic hydrogen, molecular hydrogen is adsorbed only in small quantities2-4) (N 3%) and different binding states are observed. It is possible that this adsorption is connected to surface irregularitiess,@). * On leave of absence from 2. Physikalisches Germany. 481
Institut, Technische Hochschule,
Aachen,
482
H.~BA~HAND~.E.RO~~~
In this paper we have studied the adsorption of atomic hydrogen on the (1 i I) 2 x 1 cleaved, the (1 i I) 7 x 7, and the (100) 2 x I annealed silicon surfaces. Photoemission as well as energy loss spectra show that the electronic energy levels of hydrogen adsorbed on the different faces are very similar and can be interpreted in terms of a formation of an Si-H surface bond. The presence of unsaturated “dangling bonds” on the clean silicon surfaces has been considered to be related to the surface lattice reconstruction. LEED patterns of the hydrogen covered silicon surfaces however show, that this model obviously does not apply to equilibrium structures prepared by annealing. 2. Experimental techniques The vacuum system consisted of an ion pumped stainless steel chamber with various ports for the electron spectrometer and the LEED system. The base pressure with the liquid nitrogen trap being cool was N 5 x 10-‘r Torr. A single-pas PHI l&2346 Auger analyzer with a coaxial electron gun was used for Auger electron spectroscopy (AES) and for electron energy loss spectroscopy (ELS). ELS spectra were taken as second derivative loss spectra by modulating the deflection voltage (0.8 V,_,) and observing the second harmonic. A PHI 15-250 double pass analyzer operated in the retarded mode was used to analyze the photoelectrons. At 100 eV pass energy the resolution was -0.5 eV. The resolution was measured by observing 3p photoelectrons from gas phase argon. The light source was a resonance lamp. Details of the microwave lamp and the differential pumping have been described dsewhere 7). The lamp was operated on the He I 21.2 eV line at a pressure of 1.5 Torr. Spectra were recorded in a single scan in one second per point and with 0.1 eV per point separation, The data were stored in digital form in a PDP-8/Lminicomputertofacilitatefurtherdata processing such as smoothing or computing the difference between two curves. In order to allow an easier comparison of energy levels to those of free molecules all electron energies in this paper refer to the vacuum level E,,,. The point E= -ho was determined from the photoemission spectra by a linear extrapolation of the low energy side of the slow peak. To insure that the slowest electrons were not prohibited from entering the analyzer by a contact potential a small negative bias (O-1 eV> was applied to the sample. The necessary amount of compensation could be estimated by measuring the energy differences of the onsets of the sample secondary electron peak and the secondaries from the first grid of the analyzer at a higher bias. The error in the determination of the vacuum level by this method is estimated to be less than _tO.2 eV. Clean silicon surfaces were prepared by argon bombardment and annealing and by cleaving. The cleavage samples were cut from B-doped material
HYDROG~ADSORPTIONAND~LJRF.~~ESTRU~TU~~~F
(- 1Or4 cmm3 ). The samples
to be cleaned
Si
by bombardment
483
(As-doped,
5 x 10’ 7 cmm3) were cut to thin plates of 18 x 18 mm size and 0.25 mm thickness. They were mounted on a manipulator arm so that they could be rotated on a - 11.5 cm radius and positioned in front of either the ion gun, the analyzers, or the LEED system. The samples were heated by a Al,O, covered tungsten spiral. Occasionally two samples of different surface orientations were mounted on the same sample holder to allow a direct comparison. Cleaved surfaces were prepared with a multiple cleavage tool (see fig. 1). The silicon crystal was cut into a bar of 5 x 15 x 70 mm with - 18 slots of 0.6 mm width and 1.5 mm depth across the upper and lower sides of the bar to mark the intended cleavage plane. Cleavage was performed by pressing a 20” wedge into one of the upper notches while the crystal was resting with the lower notch on a 120” wedge. The cleavage apparatus was mounted on a 2.75 in. O.D. flange 11.5 cm off the center of the chamber. With the same tool the crystal bar could also be pulled out farther after loosening a clamp on the manipulator. The bar could be heated to - 700 “C by direct current heating. Electrial contacts were made by rhodium plating. The LEED pattern of the cleaved crystals exhibited in general a clear one
Fig. I.
Multiple cleavage sample holder in the “ready to cleave” position.
484
H. IBACH
AND J. E. ROWE
domain
2 x 1 pattern
with no indication
steps8).
Occasionally,
however,
of spot splitting
on poor cleavages
due to cleavage
high step densities
were
observed as well. The argon bombarded and annealed surfaces showed a clear 7 x 7 and a two-domain 2 x 1 pattern for the (111) and (100) surfaces respectively. Exposure to atomic hydrogen was made by placing the sample in front of the filament of a carefully outgassed ion gun, with no voltage applied to the electrodes. The exposures were controlled by measuring the partial pressure of molecular hydrogen introduced to the UHV system from a high purity flask source. The percentage of atomic hydrogen at the position of the sample however was not established. The hydrogen peaks in the UPS and ELS spectra saturated on all surfaces around an exposure of lo- 5 Torrmin of molecular hydrogen. Based on the work of Lawr) we shall call the corresponding coverage a monolayer coverage in the following. It was found that the UPS and ELS data are consistent with this interpretation. No change in the UPS and ELS spectrum was observed after exposure to molecular hydrogen alone, with all filaments in the chamber turned off.
3. Experimental results The photoemission spectrum for the clean cleaved silicon (111) 2 x 1 surface and for the same surface covered with a monolayer of hydrogen is shown in fig. 2. The peak on the shoulder of the valence band photoemission of the clean surface, near - 5 eV has been identified as due to a band of surface statess-ll). Details of the surface state band have been found to be sensitive to the quality of the cleavagela) and will not be considered in the following. The spectrum in fig. 2 corresponds to a low step density concentration. In order to distinguish the surface state band in fig. 2 from the lower lying surface states associated with the silicon back bondsrs) we shall refer to these calculasurface states as the “dangling bond” surface states. Theoretical tionsla-1s) have shown that a surface state with a charge concentration around the direction of the unsaturated bond on the (111) surfaces is expected in the same energy range where the surface state peak in the photoemission spectrum occurs. After hydrogen adsorption the surface state peak has completely disappeared and a new peak at Ez - I1 eV is observed together with an increase in the background of secondary electrons. The valence band photoemission threshold decreased by approximately 0.2 eV. In fig. 3 the difference in the spectra in fig. 2 between the hydrogen covered and the clean surface is shown together with similar results for the (111) 7 x 7 and the (100) 2 x 1 surfaces. The positions of the peaks from the photoemission of the dangling bond surface states are marked by arrows for all these surfaces. The arrows coincide within the limits of error with the minima
HYDROGEN
ADSORPTION
AND SURFACE
CLEAN
STRUCTURES
OF
si
485
SI (111) 2x1 CLEAVED SURFACE
hw = 21.2 eV
COVERED
I
WITH
HYDROGEN
I
I
-20
I
-10 ENERGY
I
0 (eV)
Fig. 2. Photoemission spectra of a clean cleaved (111) 2 x 1 silicon surface and the same surface covered with approximately a monolayer of atomic hydrogen. The bump on the high energy side of the valence band emission has been identified as due to the “dangling
bond” surface state9*10). in the difference
spectra.
It is noteworthy
however
that on the (100) 2 x 1
surface the “dangling bond” surface state is only partly removed by the hydrogen adsorption. The (100) surface saturated with hydrogen still exhibits a peak at - 5.8 eV of approximately half the size of the clean surface. Electron energy loss spectra of clean and hydrogen covered surfaces are shown in fig. 4. The origin of the losses on the clean surfaces have been discussed earlier13). The transitions E, and E, are bulk interband transitions, tto, and ttw, surface and bulk plasmon excitations respectively and S,, SZ, and S, are transitions from surface states. The initial state for the S1 transition is the “dangling bond” surface state (fig. 2)13). The initial states for S, and S3 are deeper lying surface states of the silicon surface atom back bondsl4). After hydrogen exposure a new peak at -8.5 eV is observed on all surfaces. At the same time the surface state transitions S1, SZ, and S3
486
H. IBACH
AND 1, E. ROWE
(100)
\
2x1
l_r --5.8 i
I -20
I
-15 ENERGY
I
I
-10 (ev)
-5
1
Fig. 3. Photoemission difference spectra between hydrogen covered and clean surfaces for a photon energy of f2~0= 21.2 eV. The minima occur within the limit of error at the positions of the peak in the surface state photoemission at ~ 5.4, - 4.9, and - 5.8 eV, respectively. In connection with other results this suggests that atomic hydrogen is bonded to a single silicon surface atom.
disappear. With increasing exposure the hydrogen peak is shifted towards lower energies. This shift is similar to that observed for oxygen17) and probably therefore also due to a change in the dielectric function E(O) of the surface. The true transition energy could be estimated if one extrapolated the observed peak position to zero coverage17). Since an independent determination of the coverage was not available here this procedure was not possible. We estimate, however, the transition energy to be approximately 8.5 f 1.0 eV on all surfaces. No attention should be paid to the different magnitudes of the peaks on different surfaces, since these are also influenced by the width of the transitions and neighboring peaks. The results are summarized in table 1. Since only a single UPS peak and a single transition is observed for the hydrogen covered surfaces, the energy of the final state in ELS can be estimated. With the exception of the clean
Si
H~ROGENAD~~RPTI~NAND~~RFACE~TRUCT~RESOF
Si(lll)
2X1
481
tH
CLEAN
1.5X lob5 torr min
hWP (111) 7X7
tH
CLEAN
10e5 torr min
3X10m5 torr
Si(lO0)
2X1
min
tH
CLEAN
10-6 torr min
3X10-5
L
I
0
Fig. 4. Negative second faces covered with atomic cular hydrogen. The shift to the dielectric
I
8
I 16 ENERGY
LOSS
torr
min
I 24 (ev)
derivative of energy loss spectra hydrogen. The exposures refer in the positions of the hydrogen coupling of single particle and
for silicon (111) and (100) surto the partial pressure of molepeak at higher coverages is due collective excitations.
488
H. IBACH
AND J. E. ROWE
TABLE
1
Energies of surface states and transition energies for clean and hydrogen covered surfaces
Surface
(111)2x (111)2x (111)7x7 (111)7 x (100) 2 x (100)2x
1 1 7 I I
Adsorbate
_ H H H
Surface state energies, UPS ~
5.4 10.7 ~~ 4.9 11.3 ~ 5.8 ~ II.1
Transition energies, ELS 2.4 8.5 * I .o 2.0 8.5 I_ 1.0 1.7 8.5 * 1.0
Final state
~ 2.9 2.2* 1.0 ~~2.9 2.8 * I .o ~~4.1 - 2.6 & I .O
(100) 2 x 1 surface the energies of the final states fall in the same energy range. Contrary to the obvious similarity in the electron states of the adsorbed hydrogen atoms, the influence of the hydrogen adsorption on the LEED pattern is very different for the cleaved surfaces and the annealed surfaces. On the (111) 7 x 7 and on the (100) 2 x 1 surfaces no changes in the surface structure are observed (see fig. 5). Photographs of the LEED pattern before and after exposure to N IO- 5 Torr min of hydrogen (H,+ H) are indistinguishable. On the (I 11) 2 x 1 cleaved surface, however, the half order spots bebecome weak during exposure and are completely extinguished at saturation coverage
(see fig. 5). 4. Discussion
The photoemission and energy loss spectra show that on all surfaces the adsorbed atomic hydrogen removes (at least partially) the “dangling bond” surface state and a new peak at Ecz - 11 eV occurs. This energy is close to the ionization energy of the T, orbital in gaseous SiH, (- 12.7 eV)r*). Simultaneously with the disappearing of “dangling bond” surface state and of the transition from this state S,, the transitions from the lower surface states S, and S, also disappear. According to Appelbaum and Hamann14) the charge density on a clean surface is redistributed towards the back bonds of the silicon surface atoms. This strengthening of the back bonds reduces the lattice spacing between the first and second layer and gives rise to deep lying surface states at the bottom of the s and p-like valence bands respectively. The transitions S, and S, have been associated with these surface states. When we interpret our data within this scheme we are lead to the conclusion that on all surfaces hydrogen is bonded by the “dangling bond” surface state and the peak at - 11 eV corresponds to the energy of the electron in the Si-H bond. The presence of this bond redistributes the charge density
HYDROGEN
ADSORPTION
AND SURFACE
STRUCTURES
OF
si
489
(4
(4
03
Fig. 5. LEED pattern of clean and hydrogen covered silicon surfaces. The (111) 7 x 7 and the (100) 2 x 1 surface structures do not change during hydrogen adsorption. On the (111) 2 x 1 cleaved surface, however, the fractional order spots disappear. (a) (111)2x 1 cleaved,clean; (c) (lll)7x7clean; (e) (111)2X1 clean;
(b) (lll)cleaved+H; (d) (lll)7x7+H; (f) (100) 2 x 1 + H.
490
towards relaxation
H.IBACH
a distribution
similar
and the corresponding
AND
J. E. ROWE
to the bulk
silicon.
Therefore
the surface
surface states disappear.
On the (100) surface the adsorption half of the surface state photoemission
saturates when approximately only is extinguished. This could be inter-
preted by assuming that the adsorption on the (100) surface saturates or at least the sticking coefficient is considerably reduced when only one of the two dangling bonds per surface atom on the (100) surface is filled. On the (111) 7 x 7 surface the photoemission due to dangling bond surface states appears only as a weak shoulderll) on the valence band emission. From the photo-emission data alone one cannot determine whether all dangling bonds are occupied at saturation coverage. The loss spectrum (fig. 4) however, indicates that this is indeed the case. The same holds for the (111) 2 x 1 cleaved surface (figs. 2 and 4). We conclude that on all surfaces the coverage with atomic hydrogen saturates at a monolayer coverage. The stability of the (111) 7 x 7 and the (100) 2 x 1 superstructures with respect to hydrogen adsorption seems most surprising. At least for the (111) 7 x 7 where all dangling bonds are saturated by hydrogen it clearly demonstrates that the surface reconstruction is not just due to the presence of dangling bonds. Several models for the surface reconstruction have been discussed. One of the earliest is the rumpled surface or H-model proposed by Hanemanrs). According to this model at the (111) surface the sp3-hybridization of the orbitals will be partly reversed due to the half-filled dangling-bond orbitals. This leads to a kind of surface Jahn-Teller effect where one-half of the surface atoms are raised above the average position on the surface and the other half are depressed because of more s-like or p-like bonds respectively. Since adsorbed hydrogen would restore the bulk sp3-hybridization of the silicon surface atoms this model would predict that the fractional order LEED spots would be extinguished during adsorption, and the backbond surface states S, and S, would disappear. This is an accurate description of what has been actually observed on the (I 11) 2 x I cleaved surface but it seems not to apply to the other surfaces. Based on an earlier discussion of Schlier and Farnsworth20), Levine has recently suggested that on the (100) 2 x 1 surface nearest neighbor surface atoms would form double rowssl). If the silicon surface atoms actually had a lateral displacement from the bulk positions on the clean surface one would expect this displacement to vary when the dangling bonds are saturated by hydrogen adsorption. However, no change in the LEED pattern is observed after hydrogen adsorption as shown in fig. 5. One is therefore inclined to say that the results presented here do not favor the Levine model. Another explanation of the different equilibrium surface structures based on a model of surface vacancies and molecular reconstruction has been given
HYDROGEN~~~RFTI~NA~~D~~~~ACESTR~~~RE~~F
Si
491
by Landeraa). According to this model on the (111) 7 x 7 surface the silicon atoms of the first and second surface layer would form “warped benzene rings”. Phillipsas) has recently expanded Landers arguments by thermodynamic considerations of metallic and covalent contributions to binding. The results of hydrogen adsorption are consistent with this Lander-Phillips model, since large distortions such as surface vacancies could certainly not be removed by hydrogen. The suggestions of Phillipszs) that the surface transitions labeled S, and S, might be associated with orbitals sticking out in the surface vacancies, however, is not confirmed, since these transitions disappear simultaneously with Sr. The recently proposed soft mode reconstruction mechanisma4) is not supported by the data for the equilibrium surfaces. The effective ionic charge of the silicon surface atoms responsible for the occurance of a soft mode should be considerably reduced by hydrogen adsorption and thus a 1 x 1 pattern would be expected. 5. Conclusion The analysis of the adsorption data for atomic hydrogen on several silicon surfaces suggests that the different surface structures are not caused by a single reconstruction mechanism. For the equilibrium structures the data are consistent with the Lander-Phillips vacancy model. The superstructure of the cleaved surface is probably a hybridization or soft mode effect, which are alternate descriptions of the fact that an up and down displacement of every second surface atom perpendicular to the surface lowers the free energy compared to an unreconstructed surface. On all surfaces a dangling bond surface state at approximately the same energy is found. The data indicate that the hydrogen atoms are bound by the electrons in the dangling bonds and their electron energy is lowered by N 6 eV. Thus the hydrogen on silicon provides a fairly simple adsorption system that might be a useful test case for theoretical calculations. Acknowiedgements It is a pleasure to acknowledge a number of helpful discussions with J. A. Appelbaum, G. E. Becker, H. D. Hagstrum, and J. C. Phillips. The technical assistance of S. B. Christman and of E. E. Chaban is also greatly appreciated.
References 1) G. E. Becker and G. W. Gobeli, J. Chem. Phys. 38 (1963) 2942. 2) J. T. Law, J. Chem. Phys. 30 (1959) 1568. 3) M. F. Chung and D. Haneman, J. Appl. Phys. 37 (1966) 1879.
492
H. IBACH
AND J. E. ROWE
4) B. A. Joyce and J. H. Neave, Surface Sci. 34 (1973) 401.
5) 6) 7) 8) 9) IO)
I I) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24)
H. Ibach, K. Horn, R. Dorn and H. Liith, Surface Sci. 38 (1973) 433. B. Lang, R. W. Joyner and G. A. Somorjai, Surface Sci. 30 (1972) 454. J. E. Rowe, S. B. Christman and E. E. Chaban, Rev. Sci. Instr. 44 (1973) 1675. M. Henzler, Surface Sci. 36 (1973) 109. D. E. Eastman and W. D. Grobman, Phys. Rev. Letters 28 (1972) 1378. L. F. Wagner and W. E. Spicer, Phys. Rev. Letters 28 (1972) 1381. J. E. Rowe and H. Ibach, Phys. Rev. Letters 32 (1974) 421. J. E. Rowe and H. Ibach, to be published. J. E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. J. A. Appelbaum and D. R. Hamann, Phys. Rev. Letters 31 (1973) 106. K. C. Pandey and J. C. Phillips, Solid State Commun. to be published. F. Yndurain and M. Elites, Surface Sci. 29 (1972) 540. H. Ibach and J. E. Rowe, to be published. R. G. Cavell, S. P. Kowalczyk, L. Ley, R. A. Pollak, B. Mills, D. A. Shirley and W. Perry, Phys. Rev. B 7 (1973) 53 13. D. Haneman, Phys. Rev. 121 (1961) 1093. R. E. Schlier and H. E. Farnsworth, J. Chem. Phys. 30 (1959) 917. J. D. Levine, Surface Sci. 34 (1973) 90. J. J. Lander, in: Progress irr So/id State Chenzisfry, Vol. 2, Ed. H. Reiss (Pergamon Press, Oxford 1965) p. 26. J. C. Phillips, Surface Sci. 40 (1973) 459. S. E. Trullinger and S. Cunningham, Phys. Rev. Letters 30 (1973) 913.