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The adsorption of water vapor on clean cleaved germanium
SURFACE
SCIENCE 41 (1974) 377-386 0 North-Holland
THE ADSORPTION
Publishing Co.
OF WATER VAPOR ON
CLEAN CLEAVED II. ELECTRICAL
GERMANIUM PROPERTIES
J. Tt)PLER * and M. HENZLER ** 2. Physikalisches Institut der Technischen Hochschule, Aachen, Germany Received 12 July 1973; revised manuscript received 30 August 1973 Clean germanium surfaces obtained by cleavage in ultrahigh vacuum were exposed to water vapor in the range from 1OV up to 1 Torr min. Surface state properties have been derived from measurements of surface conductivity and field effect mobility. At coverages close to a monolayer a steplike change in surface state density indicates a change of the adsorption state similar to a phase transition. The results are described quantitatively by assuming that each structure has its characteristic surface state distribution.
1. Introduction The electrical properties of real semiconductor surfaces are strongly affected by water vapor, as has been shown for many semiconductor devices. Also clean surfaces show drastic changes in electrical characteristics after gas adsorption (reviewed in refs. l-3). In most cases only the exposure or the equilibrium pressure is known, the adsorbed amount, however, and the structure of the surface is not known simultaneously. For adsorption of water vapor on cleaved germanium surfaces both adsorbed amount and atom arrangement have been measured by Auger electron spectroscopy and LEED respectively, as reported in part 14). In the present investigation surface conductivity and field mobility have been measured for identical prepared samples and identical exposure procedures. In this way surface state properties with respect to surface structure and surface composition could be derived.
2. Experimental The germanium samples have been cut from a 20 ohm cm p-type single crystal in a geometry as shown in fig. 1 of ref. 4. The electrical contacts have l
* Present address: lnstitut fur Festkorperforschung der KFA, Jtilich, Germany. * Present address: Physikalisches lnstitut der Techn. Universitat Clausthal, Germany.
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J. T6PLER
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been produced by alloyed Al films and thermocompression bonded goldwires. The surfaces of the sample have been covered before cleavage with an evaporated layer of SiO, so that the sample conductance of the uncleaved faces was insensitive to water vapor adsorption. The crystal was cleaved in an ultra high vacuum of better than lo- lo torr by pressing the wedges into the pre-cut notches, as described earliers~s). For the field effect experiment the two parts of the crystal could be used as field electrodes mutually. The distance between the two parts could be varied and accurately measured from 0.2 to 5 mm. The procedure for measurement and evaluation was the same as reported earliersys). The water vapor was admitted with a variable leak valve from a Pyrex tube filled with destilled water. Exposures have been limited to one minute at constant pressure with following reevacuation to less than lo-* Torr. Thus only the irreversible effects of the adsorbed water vapor on the germanium surface have been recorded. For half of the runs the measurements were performed without any treatment of the sample after cleavage, so that the influence of water vapor on the electrical properties of a germanium (111) surface with a 2 x 1 superstructure could be investigated. For the other runs the surface structure has been converted into an &superstructure by heating the sample at 250°C.
3. Experimental
check and calibration
By measuring the conductance of the sample only changes of the surface conductivity dAa may be determined. The absolute value of do can be found by changing the band bending from p- to n-type or vice versa, so that the absolute minimum of 80 is obtainedlls). Since this value may be derived theoretically using only bulk parameters Ao is determined quantitatively for all sample conductances. The clean germanium surface is p-type. After adsorption of water vapor or oxygen at room temperature, this minimum could not be obtained. However, after admitting oxygen up to a pressure of several torr and simultaneously heating the crystal up to about 100 “C the surface turned n-type. By measurement of the conductivity minimum between p- and n-type surfaces absolute values of the surface conductivity could be determined in our experiments for all band bendings. Although the uncleaved surfaces of the sample have been protected with an evaporated layer of SiO, the influence of water vapor on these surfaces could not be eliminated completely. In order to get just the contributions of the cleaved faces, all treatments and conductivity measurements have first been made with the uncleaved sample. The conductances measured after cleavage have been corrected with the changes which have been
ADSORPTIONOFWATERVAPORON
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observed before cleavage with the same sample at identical exposures. So the influence of the uncleaved surfaces on surface conductance, which was in all cases smaller than 20x, is eliminated in the reported results. 4. Results The result for clean cleaved surface (2 x 1 superstructure) is shown in fig. 1 as a function of the exposure to water vapor in the range between 10e9 and 10-r torr min. The field effect mobility and the change of surface conductivity are shown in the upper and center plot respectively. Aa, is the value obtained immediately after cleavage. The change of conductivity before admission of water vapor has been caused by a small temperature increase during the annealing of the opposite half of the crystal. For a comparison the coverage 0 with absorbed H,O-molecules and the coverage with 2 x 1 superstructure are plotted as a function of the same exposure range in the lower part of the figure (as obtained by iden-
.g 300 c 0 0 = 200 ii = w 3 .z u_
100 0
__ E t$A
(1
1 ~
A
0
:
: z
Id*
Water
166
Vapor
10-b
Exposure
0 10-2
1
llorr min)
Fig. 1. Field effect mobility (upper) and change of surface conductivity (center) after increasing exposures of cleaved germanium surface with 2 x 1 superstructure to water vapor. The measurements have been made after exposures of 1 min and reevacuation. The lower plot is taken from part I (ref. 4).
380
Fig. 2.
J. TdPLER
AND M. HENZLER
Same as fig. 1 except that the germanium surface has been converted 8-structure by heat treatment to 250°C prior to the first exposure.
to the
tical treatment of a different sample from ref. 4). In the range up to lo-’ torr min no change of surface properties could be observed. Between lo- ’ and lo- 3 torr min electrical and structural properties vary simultaneously with increasing exposure, while in the range of more than lo- 3 torr min only electrical properties but not the coverages undergo a remarkable change. The results with samples, which have been structurally converted to the 8 structure by heat treatment before the first gas admission, are shown in fig. 2. The plots and the exposure range are the same as in fig. 1 (the coverage a with superstructure refers in this case to the 8-structure of the annealed surface). The surface conductivity decreases irreversibly due to the annealing while field effect mobility remains constant. With increasing water vapor exposure the electrical properties do not change up to an exposure of lo- 5 torr min, although the superstructure disappears. It is remarkable that in the range of more than lo- 3 torr min only the electrical properties change, just as in the case of the not annealed surface.
ADSORPTIONOFWATERVAPORON
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381
$:::m! z
Water Vapor Exposure (Torr mln)
AS U
Fig. 3. Calculated field effect mobility and surface conductivity (solid line) for adsorption on the 2 x 1 structure. The following parameters for effective surface state densities and neutral levels have been used: Nl= 1.5 x 1Or4 states/(cma eV), EI = - 10.5kT (below intrinsic level Ei of the gap) for the 2 x 1 structure); NZ = 3 x 1Orastates/(cma eV), Et=-9kT for the 1 x 1 structure; Na =7.5 x 1012 states/(cm2 eV), ES =-7kT for the HzO-covered surface.
Fig. 4. Calculated field effect mobility and surface conductivity (solidline) for adsorption on the g-structure. The following parameters for effective surface state densities and neutral levels have been used: Nr = 3.3 x 10’3 states/(cm” eV), El = - 8 kT(below intrinsic level Ei of the gap) for 8 and 1 x 1 structure; Na = 4 x 1Or3states/(cma eV), E3 = - 7 kT for HeO-covered surface.
382
J.TijPLER
AND M.HENZLER
The surface conductivity of all crystals, annealed or not annealed shows a characteristic minimum approximately at 10-4-10-3 torr min. At this point the surface is p-type and the conductivity has been calibrated (by heating in oxygen) to Aa= 1 x lo- ‘A/V. The same value has been adopted also for those samples, where the direct calibration procedure has not been followed up. The error has to be less than 10-5A/V, otherwise unreasonable values for the effective surface state density would have to be admitted. All values of Aa (including the minima) in figs. l-4 correspond therefore to a p-type surface. Thus the surface conductivity of the clean surface is 10 x lo- ‘A/V (+2 x lo- 5), corresponding to a surface potential of u,= 10.5kO.5 and, after heat treatment, (25 1) x lo-‘A/V with u,=7+ 1; which is in good agreement with earlier resultse). 5. Discussion 5.1. EVALUATION OF SURFACE STATE PROPERTIES Several surface state parameters are derived from surface conductivity and field effect mobility. First the charge in the surface states is equal (with opposite sign) to the charge in the space layer. Second an effective surface state density w may be derived which is a weighted average of the surface states close to the Fermi energye). If the surface states form a broad band with a constant density (versus energy) then the effective density mequals the number of states per area within an energy range of one kT 6). The effective density m is defined, however, for any surface state distribution and may depend on Fermi level position and temperature. To reduce the number of open parameters broad bands of constant density are assumed. To account for the observed changes in the electrical properties the surface state distribution has to change with adsorption. As discussed in part 14), both the structure and the coverage change with exposure to water vapor. As has been shown before, superstructure affects surface states drasticallye). Therefore different surface state distributions are adopted for the different structures of the clean surface and for the water vapor covered portions. Each distribution is characterized by an effective surface state density, and a neutral energy level, which indicates charge neutrality of the surface state band, if that level coincides with the Fermi energy. If N,, N, and N3 are the effective surface state densities for a clean surface with 2 x 1 superstructure, a clean surface with 1 x 1 structure and a surface with monolayer coverage of water vapor, respectively, then an effective surface state density for any state during adsorption is derived by N = N,x, + N,x, + N,x,,
ADSORPTION~FWATER
VAPOR
oNGe.11
383
with x1, x1 and xJ being the relative coverages of the surface with the respective surface structure. In the results in part I these coverages have been derived as ~1,(1 - c(-O), and 0 respectively. In the same way the neutral level of a mixed surface state distribution is derived as E = (E,N,x,
+ E,N,x,
+ E,N,x,)/fl,
with E,, E, and E, being the neutral levels of the respective surface state distributions. In this way Aa and life are calculated for any c( and 6 if assumptions for the parameters N,, E,, N,, E,, N,, E, are made. The values for c( and 0 may be taken from the results in part I. Therefore each model may be checked with experimental results at any exposure. It should be pointed out, that all parameters concerning the coverages with the different structures are fully determined in part I and are therefore not open for adjustment. The adjustable parameters refer only to the respective surface state densities, that is to the vertical displacement in figs. 3 and 4, whereas the horizontal position is fixed and not adjustable. In spite of the high number of open parameters the model therefore can give valuable information on the validity of a surface state model by fitting to two curves of the with many experimental points, since it reveals the importance different structural states. 5.2. THE HOMOGENEITY OF THE CLEAVED FACES The above model includes the assumption that the cleaved face is more or less homogeneously covered with the 2 x 1 structure. It has been shown7), however, that surface roughness present on cleaved faces has some influence on superstructure. Additionally surface states due to roughness have been found on cleaved silicon facess). In the present investigation surface conductivity and field effect mobility have been measured which give an average of the whole face. So only variations of the average could enter the results. The observed scattering of the values for different samples may be due to roughness of the surface. A direct proof has not been given so far. Another requirement for the validity of the model is a homogeneous distribution of the adsorbate. For oxygen on silicon a strong variation in sticking coefficient depending on roughness has been reporteds). Preliminary measurements of water adsorption on silicons), however, show a homogeneous adsorption independent of roughness. The Auger electron spectroscopy measurements (as reported in part I) did not show inhomogeneities, although the experiments have not been designed to check for such effects. It is therefore assumed that water vapor on germanium is also adsorbed homogeneously.
384
J. TiiPLER
AND M. HENZLER
5.3. MODEL CALCULATIONFOREXPOSURES UPTO 10-3~~~~~~~ The experimental results of figs. 1 and 2 have been compared with model calculations in the following way. The parameters of the initial surface state distribution N,, E, (for 2 x 1 or S-structure) have been chosen to fit the experimental points before the first admission. The parameters N3, E, are selected for the water vapor monolayer coverage at lop4 to 10m3 torr min exposure so that the step like change at higher exposure is not included. For the intermediate 1 x 1 structure which does not cover the whole surface for any exposure the parameters are chosen to fit the experimental points at lo- 5 torr min exposure together with the appropriate percentage of the other distributions N1 and N,. The model calculation for adsorption on a sample with 2 x 1 structure is shown in fig. 3. It should be pointed out that the coverages are used as derived in part 14). It is seen that the experimental results are well described within experimental error up to lo- 3 torr min exposure. It is necessary to have three different surface state distributions. If the distribution of the intermediate structure is set equal to either the initial or the final distribution no satisfying fit is obtained. On the other hand it is not required to introduce surface states due to domain boundaries or those depending on the average distance of the adsorbed molecules. The corresponding calculations for adsorption on the &structure are shown in fig. 4. Here the parameters for the &structure and the 1 x 1 structure are taken identical. It is therefore concluded, that the reconstruction which forms the S-structure changes the surface state distribution only very weakly, whereas the surface states due to the 2 x 1 structure are substantially different as shown in the preceding paragraph and in ref. 6. This may be due to fairly small atomic displacements during forming the &structure out of a 1 x 1 structure, so that the changes in electronic state distributions is small. As pointed out in fig. 5 of ref. 3 the band splitting occurs due to the large 8 x 8 unit mesh at many k-values and may be dependent on both k,, components so that the changes may be negligible on energy scale. In antoher explanation the 1 x 1 structure may be considered similar to the &structure in nearest neighbor configuration, only different in long range order. A distinction, however, is not possible so far. In figs. 3 and 4 the surface state distribution for the intermediate 1 x 1 structure is the same within experimental accuracy. The distributions, however, after adsorption of a monolayer differ substantially. So far it has been assumed that the structure change by heat treatment affects only the superstructure, that is the atomic arrangement within an atomic distance. As a consequence there should be no difference left after an adsorption
ADSORPTIONOFWATERVAPORON
which removes
superstructure.
385
Ge. II
If no other changes
besides
superstructure
(like surface segregration of an impurity or surface migration over many atomic distances after heat treatment to only 250°C) have been induced by heat treatment, the adsorption process itself should be different so that the bonding state or the atomic arrangement of the adsorbed molecule is different for adsorption on the two superstructures. A similar observation has been reported by Ertl and Giovanelli12): the activation energy for catalytic dissociation of water vapor on clean germanium is quite different if the 2 x 1 or the 8-superstructure of the germanium is chosen to start with. 5.4. MODEL
FOR HIGH WATER VAPOR EXPOSURES(PHASE TRANSITION)
The steplike increase in field effect mobility after an exposure to lo- 3lo- ’ torr min is not accompanied by any similar change in the observed structural data (see figs. 1 and 2). It is therefore not possible to describe this change with the model given in section 5.1. It has to be a drastic change without appreciable change in adsorbed amount. It is therefore suggested that a change similar to a phase transition of the adsorbate occurs at that exposure: by some interaction of the adsorbed molecules the atomic arrangement and the electronic structure is changed in a steplike behaviour. If for example the density of surface states is reduced by the transition (without changing the neutral level) then the field effect mobility should change in the observed way (with nearly constant surface conductivity). The change should be seen in other parameters too like workfunction and LEED intensities. Such experiments have not yet been done for this system. It should be noted that the transition occurs for the heated samples at higher exposures, whereas no difference in the sticking coefficient has been found [see part 14)]. It should be noted, that so far little is known on this “phase transition”: for example it cannot be decided on, if the first state is obtainable as an equilibrium state or only during build up of the layer. Surface phase transitions have been reported for other systems like carbon on nickello) or sulfur on goldil). A phase transition in connection with adsorption from the gas phase has not been reported so far to our knowledge.
6. Conclusion The experiments show that surface states are very sensitive to many surface parameters like atomic arrangement of substrate, amount and arrangement of adsorbate. It has been demonstrated that the &structure is very similar to the 1 x 1 structure of the clean surface. This may be an encouragement for theoreticans to calculate the surface distribution of the (111) surface without atomic rearrangement in a superstructure, since comparison with results for
386
I. T6PLER
AND
M. HENZLER
the 8-structure seems now reasonable, at least with respect to neutraf level and effective surface state density. A special feature of the reported adsorption system is the phase transition at high exposures. It is expected that with further investigation of phase transition parameters more information on binding states and on possible mechanisms for adsorption may be available. Acknowledgements The authors thank Prof. Dr. G. Heiland for his steady interest and for reading the manuscript. A discussion with Prof. Dr. V. G. Litovchenko has been very helpful. The support by Stiftung Volkswagenwerk and by the ~onderforschungsbereich 56 “Festk~rperelektron~k” at the Technische Hochschule Aachen is acknowledged. References 1) A. Many, Y. Goldstein and N. B. Grover, Semiconductor Surfaces (North-Holland, Amsterdam, 1965). 2) D. R. Frankl, Electrical Properties of Semiconductor Surfaces (Pergamon, Oxford, 1967). 3) M. Henzler, Surface Sci. 25 (1971) 650. 4) M. Henzler and J. Topler, Surface Sci. 40 (1973) 388. 5) M. Henzler, Phys. Status Solidi 19 (1967) 833. 6) M. Henzler, J. Appt. Pbys. 40 (1969) 3758. 7) M. Henzler, Surface Sci. 36 (1973) 109. 8) J. Clabes and M. Henzler, to be published. 9) H. lbach, K. Horn, R. Dorn and H. Ltith, Surface Sci. 38 (1973) 433. 10) J. C. Shelton and J. M. Blakely, 33rd Physical Electronic Conference, Berkeley, Cal., March 1973. 11) M. Kostelitz, J. L. Domange and J. Oudar, 7th LEED-Seminar, San Diego, Cal., March 73. 12) G. Ertl and T. Giovanelli, Ber. Bunsenges. Physik. Chem. 72 (1968) 74.
SCIENCE 41 (1974) 377-386 0 North-Holland
THE ADSORPTION
Publishing Co.
OF WATER VAPOR ON
CLEAN CLEAVED II. ELECTRICAL
GERMANIUM PROPERTIES
J. Tt)PLER * and M. HENZLER ** 2. Physikalisches Institut der Technischen Hochschule, Aachen, Germany Received 12 July 1973; revised manuscript received 30 August 1973 Clean germanium surfaces obtained by cleavage in ultrahigh vacuum were exposed to water vapor in the range from 1OV up to 1 Torr min. Surface state properties have been derived from measurements of surface conductivity and field effect mobility. At coverages close to a monolayer a steplike change in surface state density indicates a change of the adsorption state similar to a phase transition. The results are described quantitatively by assuming that each structure has its characteristic surface state distribution.
1. Introduction The electrical properties of real semiconductor surfaces are strongly affected by water vapor, as has been shown for many semiconductor devices. Also clean surfaces show drastic changes in electrical characteristics after gas adsorption (reviewed in refs. l-3). In most cases only the exposure or the equilibrium pressure is known, the adsorbed amount, however, and the structure of the surface is not known simultaneously. For adsorption of water vapor on cleaved germanium surfaces both adsorbed amount and atom arrangement have been measured by Auger electron spectroscopy and LEED respectively, as reported in part 14). In the present investigation surface conductivity and field mobility have been measured for identical prepared samples and identical exposure procedures. In this way surface state properties with respect to surface structure and surface composition could be derived.
2. Experimental The germanium samples have been cut from a 20 ohm cm p-type single crystal in a geometry as shown in fig. 1 of ref. 4. The electrical contacts have l
* Present address: lnstitut fur Festkorperforschung der KFA, Jtilich, Germany. * Present address: Physikalisches lnstitut der Techn. Universitat Clausthal, Germany.
378
J. T6PLER
AND M. HENZLER
been produced by alloyed Al films and thermocompression bonded goldwires. The surfaces of the sample have been covered before cleavage with an evaporated layer of SiO, so that the sample conductance of the uncleaved faces was insensitive to water vapor adsorption. The crystal was cleaved in an ultra high vacuum of better than lo- lo torr by pressing the wedges into the pre-cut notches, as described earliers~s). For the field effect experiment the two parts of the crystal could be used as field electrodes mutually. The distance between the two parts could be varied and accurately measured from 0.2 to 5 mm. The procedure for measurement and evaluation was the same as reported earliersys). The water vapor was admitted with a variable leak valve from a Pyrex tube filled with destilled water. Exposures have been limited to one minute at constant pressure with following reevacuation to less than lo-* Torr. Thus only the irreversible effects of the adsorbed water vapor on the germanium surface have been recorded. For half of the runs the measurements were performed without any treatment of the sample after cleavage, so that the influence of water vapor on the electrical properties of a germanium (111) surface with a 2 x 1 superstructure could be investigated. For the other runs the surface structure has been converted into an &superstructure by heating the sample at 250°C.
3. Experimental
check and calibration
By measuring the conductance of the sample only changes of the surface conductivity dAa may be determined. The absolute value of do can be found by changing the band bending from p- to n-type or vice versa, so that the absolute minimum of 80 is obtainedlls). Since this value may be derived theoretically using only bulk parameters Ao is determined quantitatively for all sample conductances. The clean germanium surface is p-type. After adsorption of water vapor or oxygen at room temperature, this minimum could not be obtained. However, after admitting oxygen up to a pressure of several torr and simultaneously heating the crystal up to about 100 “C the surface turned n-type. By measurement of the conductivity minimum between p- and n-type surfaces absolute values of the surface conductivity could be determined in our experiments for all band bendings. Although the uncleaved surfaces of the sample have been protected with an evaporated layer of SiO, the influence of water vapor on these surfaces could not be eliminated completely. In order to get just the contributions of the cleaved faces, all treatments and conductivity measurements have first been made with the uncleaved sample. The conductances measured after cleavage have been corrected with the changes which have been
ADSORPTIONOFWATERVAPORON
319
Ge.11
observed before cleavage with the same sample at identical exposures. So the influence of the uncleaved surfaces on surface conductance, which was in all cases smaller than 20x, is eliminated in the reported results. 4. Results The result for clean cleaved surface (2 x 1 superstructure) is shown in fig. 1 as a function of the exposure to water vapor in the range between 10e9 and 10-r torr min. The field effect mobility and the change of surface conductivity are shown in the upper and center plot respectively. Aa, is the value obtained immediately after cleavage. The change of conductivity before admission of water vapor has been caused by a small temperature increase during the annealing of the opposite half of the crystal. For a comparison the coverage 0 with absorbed H,O-molecules and the coverage with 2 x 1 superstructure are plotted as a function of the same exposure range in the lower part of the figure (as obtained by iden-
.g 300 c 0 0 = 200 ii = w 3 .z u_
100 0
__ E t$A
(1
1 ~
A
0
:
: z
Id*
Water
166
Vapor
10-b
Exposure
0 10-2
1
llorr min)
Fig. 1. Field effect mobility (upper) and change of surface conductivity (center) after increasing exposures of cleaved germanium surface with 2 x 1 superstructure to water vapor. The measurements have been made after exposures of 1 min and reevacuation. The lower plot is taken from part I (ref. 4).
380
Fig. 2.
J. TdPLER
AND M. HENZLER
Same as fig. 1 except that the germanium surface has been converted 8-structure by heat treatment to 250°C prior to the first exposure.
to the
tical treatment of a different sample from ref. 4). In the range up to lo-’ torr min no change of surface properties could be observed. Between lo- ’ and lo- 3 torr min electrical and structural properties vary simultaneously with increasing exposure, while in the range of more than lo- 3 torr min only electrical properties but not the coverages undergo a remarkable change. The results with samples, which have been structurally converted to the 8 structure by heat treatment before the first gas admission, are shown in fig. 2. The plots and the exposure range are the same as in fig. 1 (the coverage a with superstructure refers in this case to the 8-structure of the annealed surface). The surface conductivity decreases irreversibly due to the annealing while field effect mobility remains constant. With increasing water vapor exposure the electrical properties do not change up to an exposure of lo- 5 torr min, although the superstructure disappears. It is remarkable that in the range of more than lo- 3 torr min only the electrical properties change, just as in the case of the not annealed surface.
ADSORPTIONOFWATERVAPORON
Ge.11
381
$:::m! z
Water Vapor Exposure (Torr mln)
AS U
Fig. 3. Calculated field effect mobility and surface conductivity (solid line) for adsorption on the 2 x 1 structure. The following parameters for effective surface state densities and neutral levels have been used: Nl= 1.5 x 1Or4 states/(cma eV), EI = - 10.5kT (below intrinsic level Ei of the gap) for the 2 x 1 structure); NZ = 3 x 1Orastates/(cma eV), Et=-9kT for the 1 x 1 structure; Na =7.5 x 1012 states/(cm2 eV), ES =-7kT for the HzO-covered surface.
Fig. 4. Calculated field effect mobility and surface conductivity (solidline) for adsorption on the g-structure. The following parameters for effective surface state densities and neutral levels have been used: Nr = 3.3 x 10’3 states/(cm” eV), El = - 8 kT(below intrinsic level Ei of the gap) for 8 and 1 x 1 structure; Na = 4 x 1Or3states/(cma eV), E3 = - 7 kT for HeO-covered surface.
382
J.TijPLER
AND M.HENZLER
The surface conductivity of all crystals, annealed or not annealed shows a characteristic minimum approximately at 10-4-10-3 torr min. At this point the surface is p-type and the conductivity has been calibrated (by heating in oxygen) to Aa= 1 x lo- ‘A/V. The same value has been adopted also for those samples, where the direct calibration procedure has not been followed up. The error has to be less than 10-5A/V, otherwise unreasonable values for the effective surface state density would have to be admitted. All values of Aa (including the minima) in figs. l-4 correspond therefore to a p-type surface. Thus the surface conductivity of the clean surface is 10 x lo- ‘A/V (+2 x lo- 5), corresponding to a surface potential of u,= 10.5kO.5 and, after heat treatment, (25 1) x lo-‘A/V with u,=7+ 1; which is in good agreement with earlier resultse). 5. Discussion 5.1. EVALUATION OF SURFACE STATE PROPERTIES Several surface state parameters are derived from surface conductivity and field effect mobility. First the charge in the surface states is equal (with opposite sign) to the charge in the space layer. Second an effective surface state density w may be derived which is a weighted average of the surface states close to the Fermi energye). If the surface states form a broad band with a constant density (versus energy) then the effective density mequals the number of states per area within an energy range of one kT 6). The effective density m is defined, however, for any surface state distribution and may depend on Fermi level position and temperature. To reduce the number of open parameters broad bands of constant density are assumed. To account for the observed changes in the electrical properties the surface state distribution has to change with adsorption. As discussed in part 14), both the structure and the coverage change with exposure to water vapor. As has been shown before, superstructure affects surface states drasticallye). Therefore different surface state distributions are adopted for the different structures of the clean surface and for the water vapor covered portions. Each distribution is characterized by an effective surface state density, and a neutral energy level, which indicates charge neutrality of the surface state band, if that level coincides with the Fermi energy. If N,, N, and N3 are the effective surface state densities for a clean surface with 2 x 1 superstructure, a clean surface with 1 x 1 structure and a surface with monolayer coverage of water vapor, respectively, then an effective surface state density for any state during adsorption is derived by N = N,x, + N,x, + N,x,,
ADSORPTION~FWATER
VAPOR
oNGe.11
383
with x1, x1 and xJ being the relative coverages of the surface with the respective surface structure. In the results in part I these coverages have been derived as ~1,(1 - c(-O), and 0 respectively. In the same way the neutral level of a mixed surface state distribution is derived as E = (E,N,x,
+ E,N,x,
+ E,N,x,)/fl,
with E,, E, and E, being the neutral levels of the respective surface state distributions. In this way Aa and life are calculated for any c( and 6 if assumptions for the parameters N,, E,, N,, E,, N,, E, are made. The values for c( and 0 may be taken from the results in part I. Therefore each model may be checked with experimental results at any exposure. It should be pointed out, that all parameters concerning the coverages with the different structures are fully determined in part I and are therefore not open for adjustment. The adjustable parameters refer only to the respective surface state densities, that is to the vertical displacement in figs. 3 and 4, whereas the horizontal position is fixed and not adjustable. In spite of the high number of open parameters the model therefore can give valuable information on the validity of a surface state model by fitting to two curves of the with many experimental points, since it reveals the importance different structural states. 5.2. THE HOMOGENEITY OF THE CLEAVED FACES The above model includes the assumption that the cleaved face is more or less homogeneously covered with the 2 x 1 structure. It has been shown7), however, that surface roughness present on cleaved faces has some influence on superstructure. Additionally surface states due to roughness have been found on cleaved silicon facess). In the present investigation surface conductivity and field effect mobility have been measured which give an average of the whole face. So only variations of the average could enter the results. The observed scattering of the values for different samples may be due to roughness of the surface. A direct proof has not been given so far. Another requirement for the validity of the model is a homogeneous distribution of the adsorbate. For oxygen on silicon a strong variation in sticking coefficient depending on roughness has been reporteds). Preliminary measurements of water adsorption on silicons), however, show a homogeneous adsorption independent of roughness. The Auger electron spectroscopy measurements (as reported in part I) did not show inhomogeneities, although the experiments have not been designed to check for such effects. It is therefore assumed that water vapor on germanium is also adsorbed homogeneously.
384
J. TiiPLER
AND M. HENZLER
5.3. MODEL CALCULATIONFOREXPOSURES UPTO 10-3~~~~~~~ The experimental results of figs. 1 and 2 have been compared with model calculations in the following way. The parameters of the initial surface state distribution N,, E, (for 2 x 1 or S-structure) have been chosen to fit the experimental points before the first admission. The parameters N3, E, are selected for the water vapor monolayer coverage at lop4 to 10m3 torr min exposure so that the step like change at higher exposure is not included. For the intermediate 1 x 1 structure which does not cover the whole surface for any exposure the parameters are chosen to fit the experimental points at lo- 5 torr min exposure together with the appropriate percentage of the other distributions N1 and N,. The model calculation for adsorption on a sample with 2 x 1 structure is shown in fig. 3. It should be pointed out that the coverages are used as derived in part 14). It is seen that the experimental results are well described within experimental error up to lo- 3 torr min exposure. It is necessary to have three different surface state distributions. If the distribution of the intermediate structure is set equal to either the initial or the final distribution no satisfying fit is obtained. On the other hand it is not required to introduce surface states due to domain boundaries or those depending on the average distance of the adsorbed molecules. The corresponding calculations for adsorption on the &structure are shown in fig. 4. Here the parameters for the &structure and the 1 x 1 structure are taken identical. It is therefore concluded, that the reconstruction which forms the S-structure changes the surface state distribution only very weakly, whereas the surface states due to the 2 x 1 structure are substantially different as shown in the preceding paragraph and in ref. 6. This may be due to fairly small atomic displacements during forming the &structure out of a 1 x 1 structure, so that the changes in electronic state distributions is small. As pointed out in fig. 5 of ref. 3 the band splitting occurs due to the large 8 x 8 unit mesh at many k-values and may be dependent on both k,, components so that the changes may be negligible on energy scale. In antoher explanation the 1 x 1 structure may be considered similar to the &structure in nearest neighbor configuration, only different in long range order. A distinction, however, is not possible so far. In figs. 3 and 4 the surface state distribution for the intermediate 1 x 1 structure is the same within experimental accuracy. The distributions, however, after adsorption of a monolayer differ substantially. So far it has been assumed that the structure change by heat treatment affects only the superstructure, that is the atomic arrangement within an atomic distance. As a consequence there should be no difference left after an adsorption
ADSORPTIONOFWATERVAPORON
which removes
superstructure.
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Ge. II
If no other changes
besides
superstructure
(like surface segregration of an impurity or surface migration over many atomic distances after heat treatment to only 250°C) have been induced by heat treatment, the adsorption process itself should be different so that the bonding state or the atomic arrangement of the adsorbed molecule is different for adsorption on the two superstructures. A similar observation has been reported by Ertl and Giovanelli12): the activation energy for catalytic dissociation of water vapor on clean germanium is quite different if the 2 x 1 or the 8-superstructure of the germanium is chosen to start with. 5.4. MODEL
FOR HIGH WATER VAPOR EXPOSURES(PHASE TRANSITION)
The steplike increase in field effect mobility after an exposure to lo- 3lo- ’ torr min is not accompanied by any similar change in the observed structural data (see figs. 1 and 2). It is therefore not possible to describe this change with the model given in section 5.1. It has to be a drastic change without appreciable change in adsorbed amount. It is therefore suggested that a change similar to a phase transition of the adsorbate occurs at that exposure: by some interaction of the adsorbed molecules the atomic arrangement and the electronic structure is changed in a steplike behaviour. If for example the density of surface states is reduced by the transition (without changing the neutral level) then the field effect mobility should change in the observed way (with nearly constant surface conductivity). The change should be seen in other parameters too like workfunction and LEED intensities. Such experiments have not yet been done for this system. It should be noted that the transition occurs for the heated samples at higher exposures, whereas no difference in the sticking coefficient has been found [see part 14)]. It should be noted, that so far little is known on this “phase transition”: for example it cannot be decided on, if the first state is obtainable as an equilibrium state or only during build up of the layer. Surface phase transitions have been reported for other systems like carbon on nickello) or sulfur on goldil). A phase transition in connection with adsorption from the gas phase has not been reported so far to our knowledge.
6. Conclusion The experiments show that surface states are very sensitive to many surface parameters like atomic arrangement of substrate, amount and arrangement of adsorbate. It has been demonstrated that the &structure is very similar to the 1 x 1 structure of the clean surface. This may be an encouragement for theoreticans to calculate the surface distribution of the (111) surface without atomic rearrangement in a superstructure, since comparison with results for
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I. T6PLER
AND
M. HENZLER
the 8-structure seems now reasonable, at least with respect to neutraf level and effective surface state density. A special feature of the reported adsorption system is the phase transition at high exposures. It is expected that with further investigation of phase transition parameters more information on binding states and on possible mechanisms for adsorption may be available. Acknowledgements The authors thank Prof. Dr. G. Heiland for his steady interest and for reading the manuscript. A discussion with Prof. Dr. V. G. Litovchenko has been very helpful. The support by Stiftung Volkswagenwerk and by the ~onderforschungsbereich 56 “Festk~rperelektron~k” at the Technische Hochschule Aachen is acknowledged. References 1) A. Many, Y. Goldstein and N. B. Grover, Semiconductor Surfaces (North-Holland, Amsterdam, 1965). 2) D. R. Frankl, Electrical Properties of Semiconductor Surfaces (Pergamon, Oxford, 1967). 3) M. Henzler, Surface Sci. 25 (1971) 650. 4) M. Henzler and J. Topler, Surface Sci. 40 (1973) 388. 5) M. Henzler, Phys. Status Solidi 19 (1967) 833. 6) M. Henzler, J. Appt. Pbys. 40 (1969) 3758. 7) M. Henzler, Surface Sci. 36 (1973) 109. 8) J. Clabes and M. Henzler, to be published. 9) H. lbach, K. Horn, R. Dorn and H. Ltith, Surface Sci. 38 (1973) 433. 10) J. C. Shelton and J. M. Blakely, 33rd Physical Electronic Conference, Berkeley, Cal., March 1973. 11) M. Kostelitz, J. L. Domange and J. Oudar, 7th LEED-Seminar, San Diego, Cal., March 73. 12) G. Ertl and T. Giovanelli, Ber. Bunsenges. Physik. Chem. 72 (1968) 74.