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Simulation of thin film growth and in situ characterization by RHEED and photoemission
Vacuum/volume
46/numbers %lO/pages 931 to 934/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
0042-207x/95
$9.50+.00
0042-207X(95)00075-5
Simulation of thin film growth and in situ characterization by RHEED and photoemission M Djafari Rouhani”, N Fazouan, A M Gue and D Estke, CNRS 7Ave Colonel Roche, 31077 Toulouse, France
Laboratoire
d’Ana/yse et d/Architecture
des Systkmes,
Simulations of epitaxial growth of GaAs associated with RHEED intensity and photoemission current, as in situ characterization techniques, have been performed. The nature of incoming species which can be of atomic or molecular form is taken into account, The simulations show in phase RHEED and photoemission oscillations, in agreement with experimental results.
1. Introduction
2. Experimental observations and continuum models
In situ characterization techniques are the keypoints for the deposition of high quality epitaxial films, since they allow one to monitor the deposition process by adjusting continuously the experimental conditions. Reflection high energy electron diffraction (RHEED) developed two decades ago’ is now regularly implemented in commercial setups and is widely used in experimental runs. RHEED patterns provide information on the morphology of the growing surface, but more important are the RHEED oscillations which are related to monolayer deposition. The number of oscillations is a measure of the thickness of the growing film and their damping a measure of the roughness of the surface. Therefore, the quality of the oscillations reflects directly the quality of the deposited layer. More recently, similar results were found using the photoemission technique during the epitaxial growth of GaAs, with photon energies at the emission threshold’-‘. The difference with the RHEED technique is an initial rise of the photoemission current at the opening of the Ga shutter, when starting with an As stabilized substrate surface. The advantage of optical characterization techniques such as photoemission and reflectance difference spectroscopy over RHEED is their possible use in relatively high pressure environments encountered in usual MOCVD experiments. Therefore, optical techniques can be efficiently used as complementary to RHEED intensity measurements. Our aim is to show that the main experimental features can be obtained using a semi-empirical model and that the results can be compared to calculated RHEED intensities. After a brief survey, in Section 2, of the experimental data and previous theories based on continuum models, the atomic scale simulations are described in Section 3. Results of these simulations are reported and discussed in Section 4.
Figure 1 shows the main features of experimental results by comparing the variations of the photoemission current and the RHEED intensity during the epitaxial growth of GaAs’. In this figure, the total photoemitted current using an incident beam in the 190-220 nm range is plotted. This range corresponds to the near threshold photoemission of GaAs. A rise of photoemission current upon the opening of the Ga shutter, followed by regular
PE
t-
Ga SHUTTER
OPEN 50
0 TIME IS.1
*Also at Laboratoire de Physique des Solides, UniversitC Paul Sabatier, 118 Route de Narbonne, 3 1062 Toulouse Cedex, France.
Figure 1. Photoemission current oscillations during the epitaxial growth of GaAs, after ref 2. 931
M Djafari Rouhani et a/: Simulation of thin film growth oscillations is observed. These oscillations are of small amplitude and rapidly damped but are in phase with RHEED oscillations. Further experiment? have led to improvements in the quality of the oscillations, but have also shown the dependence of these oscillations on the As/Ga flux ratio. It is observed that the initial photoemission current rise and the oscillation amplitude decrease with increasing As/Ga flux ratio. Use of different surface phases of As stabilized GaAs (100) substrate leads to a shift of photoemission oscillations with respect to RHEED oscillations4,‘. On the theoretical side, we have already shown6 that a model based on the kinetic rate theory can conveniently describe the photoemission current variations provided the growth mode is layer by layer. In this model, the photoemission was assumed to be due to Ga atoms not incorporated in clusters. The model was therefore based on the probability of formation of Ga clusters, the treatment being independent of the position of the cluster. An infinite As/Ga flux ratio was assumed which leads to an immediate coverage of Ga clusters by As atoms. This assumption is qualitatively justified since, in the MBE experimental conditions, a high As pressure is used to maintain the stability of the growing surface. A quantitative analysis should take into account not only a finite As/Ga flux ratio, but also the migration of atoms on the surface leading to a position dependent treatment. Furthermore, in the kinetic rate theory, a layer by layer growth mode has only been ensured by assuming an arbitrary terrace width which is taken as an adjustable parameter. These show the necessity of developing an atomic scale model where the As/Ga flux ratio, the atomic migrations and the individual terrace widths arise naturally in the simulations.
3. Atomic scale model Our atomic scale simulations associate a Monte Carlo epitaxial growth model and a local photoemission model. The growth simulation starts with an As stabilized (100) GaAs surface on which several types of events are allowed to occur. These events are : the adsorption of Ga and As atoms from the gas phase, the intralayer and the interlayer migrations of atoms on the surface and the evaporation of atoms into the gas phase. The probabilities 1 of occurrence of migrations and evaporation are calculated using the Arrhenius law, 1 = v exp (- AE/kT) where v is the attempt frequency set equal to 10” s-l, k the Boltzmann constant, T the temperature and AE the activation energy for the particular event. The activation energies are calculated by taking into account up to second nearest neighbour interactions. We have used the values given in the literature7xs, namely EGa_As = 0.8 eV for the nearest neighbour interactions, EGa_Ga = 0.17 eV and EA_+ = 0.2 eV for the second nearest neighbour interactions. The adsorption of atoms from the gas phase onto the surface occurs at random positions and times. We have developed a second version of the software package which can also deal with species in the molecular form. As is known from the literature’*“, the arsenic is supplied in the As, molecular form during the MBE of GaAs. This As4 molecule, in contact with GaAs surface, is further decomposed into two As, molecules, one of them being incorporated into the surface and the other evaporated into the gas phase. To simulate the incorporation of the As, molecules, we have followed the ideas of the ‘configuration dependent reactive incorporation’ (CDRI) model developed by Madhukar and Ghaisas”,“. In the CDRI model, the As* dissociation probability is assumed to depend on the local configuration. Five illustrative 932
examples of such configurations are represented in Figure 2, where the highest incorporation probabilities correspond to the cases where three or four neighbouring Ga atoms are present. With the above assumptions, the simulation proceeds as follows. At a given time, all possible events on all surface atoms are listed and a time t is attributed to each of them according to t = - l/n In z where z is a random number in the (0,l) interval. This timetable is then mixed up with the incoming atoms (or molecules) timetable, the event corresponding to the minimum time is assumed to occur and the whole process is repeated. A semi-empirical model for the photoemission current has been used on the basis of the experimental rise of the photoemission current upon the opening of the Ga shutter. We conclude that the rise is due to uncovered Ga atoms which appear on the surface. With continued arrival of Ga atoms, more probable configurations for the incorporation of As atoms, as indicated on Figure 2, appear and lead to a decrease of the photoemission current. In this model, the rise of the photoemission current with respect to IAs, referring to an As stabilized surface, is assumed to be proportional to the number noa of uncovered Ga atoms present on the top layer of the surface. Therefore, the photoemission current is written as I = IAs+ ioa.noa, iGa is the photoemission current rise due to a single Ga atom on the surface resulting from localized surface dipoles and a local modification of surface band bending. This semi-empirical model is justified by the fact that the presence of Ga atoms on the surface lowers the surface barrier, and by the absence of any type of symmetry on the growing surface which prohibits the calculation of uniform surface dipoles and global band bending. A more thorough discussion of this model is given in ref 13. In order to compare photoemission and RHEED oscillations, the RHEED intensity is also calculated using a kinematic theory, which is acceptable for incidence angles leading to destructive interference between adjacent (100) molecular planes. Reference 15 gives a detailed review of the RHEED theory. Within the kinematic theory, all atoms are assumed to diffuse the electrons with the same intensity such that the total diffracted intensity is given asi I(q) = 1xexp (iq.RJ ’ where R, are the atomic positions and q the difference
As;
Rt
As;
R2,
\
As;
z
lGa/
As;
R4,
‘(J
AS\Ga/
\&/
As\
Ga/As\Ga/As
AS\Ga/
As\Gi
Ga/
Figure 2. Schematic representation of surface configurations where As atoms can be incorporated into the growing layer.
M Djafari Roohani et al: Simulation
of thin film growth
between the incident and diffracted beams wave vectors. In our case of specular reflected beam observations, q is perpendicular to the substrate surface, with qR, = TCfor one molecular plane (i.e. two atomic plane) spacing. The sum extends over the atoms on the top surface layer, since the small incidence angle prohibits the beam penetration inside the bulk. ‘338
4. Results and discussion We have performed simulations of the growth of GaAs on GaAs (100) substrates and of its characterization by RHEED and photoemission. Substrates of 50 x 50 atoms with periodic boundary conditions have been used and simulations of the deposition of up to 10 atomic layers have been carried out. Substrate surfaces are assumed perfect and flat. RHEED oscillations are well observed but the photoemission oscillations are, like experimental results, weak, and show large statistical fluctuations. In order to observe well defined oscillations, simulations have been performed using several series of random numbers, and averaging the photoemission current over the different sets. The results reported in the following are averages over three sets of random numbers, the growth temperature has been set to 570°C and the As/Ga flux ratio to 20. Figure 3 shows the photoemission current and the RHEED intensity versus time in the case where arsenic is supplied in atomic form. It is seen that the two oscillations are in phase with each other. Indeed, the high flux ratio ensures a layer by layer growth mode. The average terrace width is also calculated and found to be in the order of 8-10 interatomic spacings, which corresponds to the values previously used in the kinetic rate models6%i3 on an empirical basis. Figure 4 shows similar results as in Figure 3 for the case where arsenic is supplied in As, molecular form. The photoemission current and the RHEED intensity oscillations are still in phase showing that the nature of the supplied species (atomic or molecular) is not the main parameter governing the experimental
0.04
U
2
ii 3 g 3 .* 8 $ ?j !z
4
Time Figure 4. RHEED intensity and photoemission current versus time in the case where As is supplied in As, molecular form. The As/Ga flux ratio is 20 and the growth temperature 570°C.
features. However, we observe a slight decrease in the RHEED intensity and a noticeable increase in the photoemission current, with respect to the case where arsenic was supplied in atomic form. This shows that the surface is more rough and more rich in Ga. Both these features are due to the more complex configurations necessary for the incorporation of As, molecules. For the same reason, the average terrace width decreases to 5-6 atomic spacings.
5. Conclusions 0,os
2
4
Time Figure 3. RHEED intensity and photoemission current versus time in the case where As is supplied in atomic form. The As/Ga flux ratio is 20 and the growth temperature 570°C.
We have associated a growth model based on the Monte Carlo technique with a semi-empirical model for the photoemission from surfaces showing no symmetry and with the kinematic theory of RHEED, to perform simulation of epitaxial growth of GaAs with in situ characterization techniques as a test for the validity of the model, we have shown that the experimental features can be obtained by simulation. This model has been extended to take the atomic or molecular nature of incoming species into account. It is shown that the most important parameter governing the photoemission current oscillations is not the nature of these species, but the layer by layer growth mode. The advantage of associating characterization techniques with the growth simulation is to produce computed results directly comparable to experimental data, avoiding the use of intermediate surface properties, hard to access experimentally. Further, the simulation results can be used to optimize experimental conditions for obtaining higher quality deposited films.
References ‘A Y Cho, J Vat Sci Technol, 13,241 (1971). ‘J N Eckstein, C Webb, S L Weng and K A Bertness, Appl Phys Lett, 51, 1833 (1987).
933
A4 Djafari Rouhani et al: Simulation
of thin film growth
3J N Eckstein, C Webb, S L Weng and K A Bertness, J Vuc Sci Technol, B26, 736 (1988). “H Tsuda and T Mizutani, ApplPhys Lett, 60, 1570(1992). ‘H Tsuda, J Appl Phys, 73, 1534 (1993). 6A Laamouri, M Djafari Rouhani, A M GuC, N Fazouan and D Esteve, Surf Znterf Anal, 18, 505 (1992). 7K K Bajaj, J Vat Sci Technol, B2,576 (1984). *K K Bajaj and J Singh, J Vat Sci Technol, B3, 520 (1985). 9J R Arthur, Surf Sci, 43,499 (1974).
934
“‘C T Foxon and B A Joyce, Surf Sci, 64,293 (1977). “S V Ghaisas and A Madhukar, Phys Rev Lett, 56, 1066 (1986). “A Madhukar and S V Ghaisas, Appl Phys Lett, 47,247 (1985). 13M Djafari Rouhani, N Fazouan, A M GuC and D Esteve, Appl Surf Sci, 63,273 (1993). 14P A Maksym and J L Beeby, Surf Sci, 110,423 (1981). “J M Van Hove C S Lent, P R Pukite and P I Cohen, J Vat Sci Technol, Bl, 741 (1983). ’
46/numbers %lO/pages 931 to 934/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
0042-207x/95
$9.50+.00
0042-207X(95)00075-5
Simulation of thin film growth and in situ characterization by RHEED and photoemission M Djafari Rouhani”, N Fazouan, A M Gue and D Estke, CNRS 7Ave Colonel Roche, 31077 Toulouse, France
Laboratoire
d’Ana/yse et d/Architecture
des Systkmes,
Simulations of epitaxial growth of GaAs associated with RHEED intensity and photoemission current, as in situ characterization techniques, have been performed. The nature of incoming species which can be of atomic or molecular form is taken into account, The simulations show in phase RHEED and photoemission oscillations, in agreement with experimental results.
1. Introduction
2. Experimental observations and continuum models
In situ characterization techniques are the keypoints for the deposition of high quality epitaxial films, since they allow one to monitor the deposition process by adjusting continuously the experimental conditions. Reflection high energy electron diffraction (RHEED) developed two decades ago’ is now regularly implemented in commercial setups and is widely used in experimental runs. RHEED patterns provide information on the morphology of the growing surface, but more important are the RHEED oscillations which are related to monolayer deposition. The number of oscillations is a measure of the thickness of the growing film and their damping a measure of the roughness of the surface. Therefore, the quality of the oscillations reflects directly the quality of the deposited layer. More recently, similar results were found using the photoemission technique during the epitaxial growth of GaAs, with photon energies at the emission threshold’-‘. The difference with the RHEED technique is an initial rise of the photoemission current at the opening of the Ga shutter, when starting with an As stabilized substrate surface. The advantage of optical characterization techniques such as photoemission and reflectance difference spectroscopy over RHEED is their possible use in relatively high pressure environments encountered in usual MOCVD experiments. Therefore, optical techniques can be efficiently used as complementary to RHEED intensity measurements. Our aim is to show that the main experimental features can be obtained using a semi-empirical model and that the results can be compared to calculated RHEED intensities. After a brief survey, in Section 2, of the experimental data and previous theories based on continuum models, the atomic scale simulations are described in Section 3. Results of these simulations are reported and discussed in Section 4.
Figure 1 shows the main features of experimental results by comparing the variations of the photoemission current and the RHEED intensity during the epitaxial growth of GaAs’. In this figure, the total photoemitted current using an incident beam in the 190-220 nm range is plotted. This range corresponds to the near threshold photoemission of GaAs. A rise of photoemission current upon the opening of the Ga shutter, followed by regular
PE
t-
Ga SHUTTER
OPEN 50
0 TIME IS.1
*Also at Laboratoire de Physique des Solides, UniversitC Paul Sabatier, 118 Route de Narbonne, 3 1062 Toulouse Cedex, France.
Figure 1. Photoemission current oscillations during the epitaxial growth of GaAs, after ref 2. 931
M Djafari Rouhani et a/: Simulation of thin film growth oscillations is observed. These oscillations are of small amplitude and rapidly damped but are in phase with RHEED oscillations. Further experiment? have led to improvements in the quality of the oscillations, but have also shown the dependence of these oscillations on the As/Ga flux ratio. It is observed that the initial photoemission current rise and the oscillation amplitude decrease with increasing As/Ga flux ratio. Use of different surface phases of As stabilized GaAs (100) substrate leads to a shift of photoemission oscillations with respect to RHEED oscillations4,‘. On the theoretical side, we have already shown6 that a model based on the kinetic rate theory can conveniently describe the photoemission current variations provided the growth mode is layer by layer. In this model, the photoemission was assumed to be due to Ga atoms not incorporated in clusters. The model was therefore based on the probability of formation of Ga clusters, the treatment being independent of the position of the cluster. An infinite As/Ga flux ratio was assumed which leads to an immediate coverage of Ga clusters by As atoms. This assumption is qualitatively justified since, in the MBE experimental conditions, a high As pressure is used to maintain the stability of the growing surface. A quantitative analysis should take into account not only a finite As/Ga flux ratio, but also the migration of atoms on the surface leading to a position dependent treatment. Furthermore, in the kinetic rate theory, a layer by layer growth mode has only been ensured by assuming an arbitrary terrace width which is taken as an adjustable parameter. These show the necessity of developing an atomic scale model where the As/Ga flux ratio, the atomic migrations and the individual terrace widths arise naturally in the simulations.
3. Atomic scale model Our atomic scale simulations associate a Monte Carlo epitaxial growth model and a local photoemission model. The growth simulation starts with an As stabilized (100) GaAs surface on which several types of events are allowed to occur. These events are : the adsorption of Ga and As atoms from the gas phase, the intralayer and the interlayer migrations of atoms on the surface and the evaporation of atoms into the gas phase. The probabilities 1 of occurrence of migrations and evaporation are calculated using the Arrhenius law, 1 = v exp (- AE/kT) where v is the attempt frequency set equal to 10” s-l, k the Boltzmann constant, T the temperature and AE the activation energy for the particular event. The activation energies are calculated by taking into account up to second nearest neighbour interactions. We have used the values given in the literature7xs, namely EGa_As = 0.8 eV for the nearest neighbour interactions, EGa_Ga = 0.17 eV and EA_+ = 0.2 eV for the second nearest neighbour interactions. The adsorption of atoms from the gas phase onto the surface occurs at random positions and times. We have developed a second version of the software package which can also deal with species in the molecular form. As is known from the literature’*“, the arsenic is supplied in the As, molecular form during the MBE of GaAs. This As4 molecule, in contact with GaAs surface, is further decomposed into two As, molecules, one of them being incorporated into the surface and the other evaporated into the gas phase. To simulate the incorporation of the As, molecules, we have followed the ideas of the ‘configuration dependent reactive incorporation’ (CDRI) model developed by Madhukar and Ghaisas”,“. In the CDRI model, the As* dissociation probability is assumed to depend on the local configuration. Five illustrative 932
examples of such configurations are represented in Figure 2, where the highest incorporation probabilities correspond to the cases where three or four neighbouring Ga atoms are present. With the above assumptions, the simulation proceeds as follows. At a given time, all possible events on all surface atoms are listed and a time t is attributed to each of them according to t = - l/n In z where z is a random number in the (0,l) interval. This timetable is then mixed up with the incoming atoms (or molecules) timetable, the event corresponding to the minimum time is assumed to occur and the whole process is repeated. A semi-empirical model for the photoemission current has been used on the basis of the experimental rise of the photoemission current upon the opening of the Ga shutter. We conclude that the rise is due to uncovered Ga atoms which appear on the surface. With continued arrival of Ga atoms, more probable configurations for the incorporation of As atoms, as indicated on Figure 2, appear and lead to a decrease of the photoemission current. In this model, the rise of the photoemission current with respect to IAs, referring to an As stabilized surface, is assumed to be proportional to the number noa of uncovered Ga atoms present on the top layer of the surface. Therefore, the photoemission current is written as I = IAs+ ioa.noa, iGa is the photoemission current rise due to a single Ga atom on the surface resulting from localized surface dipoles and a local modification of surface band bending. This semi-empirical model is justified by the fact that the presence of Ga atoms on the surface lowers the surface barrier, and by the absence of any type of symmetry on the growing surface which prohibits the calculation of uniform surface dipoles and global band bending. A more thorough discussion of this model is given in ref 13. In order to compare photoemission and RHEED oscillations, the RHEED intensity is also calculated using a kinematic theory, which is acceptable for incidence angles leading to destructive interference between adjacent (100) molecular planes. Reference 15 gives a detailed review of the RHEED theory. Within the kinematic theory, all atoms are assumed to diffuse the electrons with the same intensity such that the total diffracted intensity is given asi I(q) = 1xexp (iq.RJ ’ where R, are the atomic positions and q the difference
As;
Rt
As;
R2,
\
As;
z
lGa/
As;
R4,
‘(J
AS\Ga/
\&/
As\
Ga/As\Ga/As
AS\Ga/
As\Gi
Ga/
Figure 2. Schematic representation of surface configurations where As atoms can be incorporated into the growing layer.
M Djafari Roohani et al: Simulation
of thin film growth
between the incident and diffracted beams wave vectors. In our case of specular reflected beam observations, q is perpendicular to the substrate surface, with qR, = TCfor one molecular plane (i.e. two atomic plane) spacing. The sum extends over the atoms on the top surface layer, since the small incidence angle prohibits the beam penetration inside the bulk. ‘338
4. Results and discussion We have performed simulations of the growth of GaAs on GaAs (100) substrates and of its characterization by RHEED and photoemission. Substrates of 50 x 50 atoms with periodic boundary conditions have been used and simulations of the deposition of up to 10 atomic layers have been carried out. Substrate surfaces are assumed perfect and flat. RHEED oscillations are well observed but the photoemission oscillations are, like experimental results, weak, and show large statistical fluctuations. In order to observe well defined oscillations, simulations have been performed using several series of random numbers, and averaging the photoemission current over the different sets. The results reported in the following are averages over three sets of random numbers, the growth temperature has been set to 570°C and the As/Ga flux ratio to 20. Figure 3 shows the photoemission current and the RHEED intensity versus time in the case where arsenic is supplied in atomic form. It is seen that the two oscillations are in phase with each other. Indeed, the high flux ratio ensures a layer by layer growth mode. The average terrace width is also calculated and found to be in the order of 8-10 interatomic spacings, which corresponds to the values previously used in the kinetic rate models6%i3 on an empirical basis. Figure 4 shows similar results as in Figure 3 for the case where arsenic is supplied in As, molecular form. The photoemission current and the RHEED intensity oscillations are still in phase showing that the nature of the supplied species (atomic or molecular) is not the main parameter governing the experimental
0.04
U
2
ii 3 g 3 .* 8 $ ?j !z
4
Time Figure 4. RHEED intensity and photoemission current versus time in the case where As is supplied in As, molecular form. The As/Ga flux ratio is 20 and the growth temperature 570°C.
features. However, we observe a slight decrease in the RHEED intensity and a noticeable increase in the photoemission current, with respect to the case where arsenic was supplied in atomic form. This shows that the surface is more rough and more rich in Ga. Both these features are due to the more complex configurations necessary for the incorporation of As, molecules. For the same reason, the average terrace width decreases to 5-6 atomic spacings.
5. Conclusions 0,os
2
4
Time Figure 3. RHEED intensity and photoemission current versus time in the case where As is supplied in atomic form. The As/Ga flux ratio is 20 and the growth temperature 570°C.
We have associated a growth model based on the Monte Carlo technique with a semi-empirical model for the photoemission from surfaces showing no symmetry and with the kinematic theory of RHEED, to perform simulation of epitaxial growth of GaAs with in situ characterization techniques as a test for the validity of the model, we have shown that the experimental features can be obtained by simulation. This model has been extended to take the atomic or molecular nature of incoming species into account. It is shown that the most important parameter governing the photoemission current oscillations is not the nature of these species, but the layer by layer growth mode. The advantage of associating characterization techniques with the growth simulation is to produce computed results directly comparable to experimental data, avoiding the use of intermediate surface properties, hard to access experimentally. Further, the simulation results can be used to optimize experimental conditions for obtaining higher quality deposited films.
References ‘A Y Cho, J Vat Sci Technol, 13,241 (1971). ‘J N Eckstein, C Webb, S L Weng and K A Bertness, Appl Phys Lett, 51, 1833 (1987).
933
A4 Djafari Rouhani et al: Simulation
of thin film growth
3J N Eckstein, C Webb, S L Weng and K A Bertness, J Vuc Sci Technol, B26, 736 (1988). “H Tsuda and T Mizutani, ApplPhys Lett, 60, 1570(1992). ‘H Tsuda, J Appl Phys, 73, 1534 (1993). 6A Laamouri, M Djafari Rouhani, A M GuC, N Fazouan and D Esteve, Surf Znterf Anal, 18, 505 (1992). 7K K Bajaj, J Vat Sci Technol, B2,576 (1984). *K K Bajaj and J Singh, J Vat Sci Technol, B3, 520 (1985). 9J R Arthur, Surf Sci, 43,499 (1974).
934
“‘C T Foxon and B A Joyce, Surf Sci, 64,293 (1977). “S V Ghaisas and A Madhukar, Phys Rev Lett, 56, 1066 (1986). “A Madhukar and S V Ghaisas, Appl Phys Lett, 47,247 (1985). 13M Djafari Rouhani, N Fazouan, A M GuC and D Esteve, Appl Surf Sci, 63,273 (1993). 14P A Maksym and J L Beeby, Surf Sci, 110,423 (1981). “J M Van Hove C S Lent, P R Pukite and P I Cohen, J Vat Sci Technol, Bl, 741 (1983). ’