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The influence of surface structure on the chemisorption of oxygen by CdS single crystals
SURFACE
SCIENCE 29 (1972) 165-172 0 North-Holland
THE INFLUENCE CHEMISORPTION
OF OXYGEN
S. BAIDYAROY Department
OF SURFACE
Publishing Co.
STRUCTURE
BY CdS SINGLE
**, W. R. BOTTOMS and PETER
of Electrical Engineering, Princeton Princeton, New Jersey 08540, U.S.A.
Received 9 August 1971; revised manuscript
ON THE CRYSTALS* MARK
University,
received 29 September
1971
Atomically ordered surfaces of CdS single crystals are found to be relatively insensitive to oxygen chemisorption. For such surfaces prepared by argon bombardment and high vacuum annealing, the effect of ambient oxygen on the equilibrium conductivity is drastically reduced. This indicates that active sites associated with surface imperfections may be necessary to achieve chemisorption.
1. Introduction The effects of oxygen chemisorption on the equilibrium conductivity of CdS single crystals have been investigated extensively by Goodwin and Markl). Usually, there is a gradual reduction in the conductivity coupled with a gradual increase in the activation energy with increasing oxygen pressure. For thin and insulating samples the change in the equilibrium conductivity may be as large as 7 decades. It has recently been suggested that atomically ordered surfaces of compound semiconductors are essentially inert to chemisorption2). From low energy electron diffraction (LEED) studies of the principal surfaces of ZnO single crystals, it was found that surfaces cleaned by argon bombardment and subsequent high vacuum annealing retained their original diffraction spot brightness and sharpness even after exposure to room ambient conditions for as long as three months. The inert character of these atomically ordered surfaces was also demonstrated by their inability to sustain a charge deposited by a corona. These surfaces were, however, chargeable before “clean up”. The failure of the atomically ordered surfaces to sustain a corona charge was attributed to the absence of chemisorption sites. This paper discusses changes induced in the ambient-sensitivity of single crystals of CdS when the surfaces are rendered atomically clean and ordered
* Supported by the Office of Naval Research and by the National Science Foundation. ** Part of Ph.D. dissertation. 165
166
S. BAIDYAROY,
W.R.
BOTTOMS
AND
P. MARK
by ion bombardment and high vacuum annealing. investigated was (IIZO) and the ambient-sensitivity equilibrium conductivity measurements.
The principal surface was inferred from
2. Experimental The samples were vapor phase grown CdS platelets of approximate size: 3 mm x 1 mm x 20 pm (courtesy: Dr. D. C. Reynolds, Wright-Patterson Air Force Base, Dayton, Ohio). The crystals were mounted on sapphire disks and were electroded with indium such that current flowed parallel to the large area (1120) surfaces. The specimens were mounted on a thermal finger housed inside a stainless steel chamber which had a vacuum capability of lo-* torr without a bake-out. The chamber was equipped with a variable leak valve for controlling ambient gas conditions, and with electrical feed-throughs and optical view ports. The vacuum system is described more fully elsewherel). Two sets of conductivity-temperature data, with ambient oxygen pressure as a parameter, were recorded. The first was taken on the virgin crystal (before surface clean up) and the second after clean up. The samples were cleaned by the following steps: (i) Place the sample inside the LEED system and pump down to a vacuum better than lop9 torr. (ii) Heat the sample at 125 “C for several hours in order to desorb some adsorbed species thermally (heating was limited to 125°C because of In-electrodes). (iii) Next, bombard the crystal surface with argon ions (ion current ~2 pA/cm2, beam voltage = 365 V, argon pressure = lo-’ torr) for about 2 hr. (iv) In order to remove embedded argon ions and to anneal any lattice damage created by argon bombardment, anneal the sample at 125°C for about 6 hr. Such a sequence gives sharp and bright low energy electron diffraction spots characteristic of an atomically ordered and clean (I 120) surface3).
3. Results and discussion The virgin crystal gave no diffraction pattern either after exposure to oxygen or upon isothermal photodesorption, characteristic of an atomically disordered and possibly contaminated surface. The typical oxygen ambient semiconductivity of the virgin crystal is demonstrated by curves A (10e8 torr) and B (10 torr) of fig. 1 (before taking the data of curve A the sample was photodesorbed). Presumably, the curve A corresponds to the ‘“adsorbate free” surface and is characterized by a slope equal to the bulk donor level; consequently, curve A exhibits a bulk-controlled regime 1$4, 5). On the other hand, the slope of the curve B is determined, among other parameters, by
INFLUENCE
Fig. 1.
Effects of ion bombardment conductivity
OF SURFACE
STRUCTURE
and high vacuum annealing of CdS single crystal.
167
on the equilibrium
the surface state energy level(s) of chemisorbed oxygen. Hence, curve B represents a surface-controlled regime lp 4v5). The curves C (lo-’ torr) and D (10 torr) of fig. 1 illustrate the ambientsensitive semiconductivity of the atomically ordered and clean surface obtained by argon bombardment and annealing. Notice that, as a result of this surface treatment: (i) The IO-’ torr curve (C) is raised by more than a decade, having the same slope as that of A. (ii) The lo-’ torr curve (C) and the 10 torr curve (D) are close to each other, and have the same slope. The increase in the current level can be attributed to further oxygen desorption by the cleaning procedure. This implies that prior to the bombard-anneal treatment photodesorption had not removed all the adsorbed species; therefore, the curve A in fig. 1 is not the actual adsorbate free curve although it belongs to the bulk-controlled regime. The curve C is closer to the “true” adsorbate free curve and hence it lies above A. The close spacing and identical slope of curves C and D (both belong to the bulk-controlled regime) indicate that D corresponds to a very small surface state concentration. This shows that an atomically ordered and clean surface is virtually insensitive to oxygen exposure. LEED and Auger
168
S. BAIDYAROY,
W. R, BOTTOMS
AND
P. MARK
electron spectroscopy measurements with other CdS crystals have also revealed that an atomically ordered surface does not chemisorb oxygen even after exposure to room ambient conditions for several months while a chlorine contaminated, disordered surface readily chemisorbs oxygen after exposure for several seconds at 10P5 torr oxygen pressure. Similar results have also been obtained with ZnO 2). The inert character of atomically ordered surfaces is discussed further in the Appendix. The experimental curves of fig. 1 can be interpreted in terms of fig. 2, which is a computed plot of the average free electron density (n) in the conduction band of a compensated n-type semiconductor as a function of reciprocal temperature with surface state concentration (N, cm-“) as a parameter. If the temperature and pressure dependence of mobility is ignored, (n) is proportional to the measured conductivity. For the curves of fig. 2, the bulk donor and acceptor levels are assumed discrete, the donor density ND being greater than the acceptor density NA for an n-type material. For convenience, in this example, the surface states are also assumed to have a discrete energy level, EC, below the bottom of conduction band. The details of the calculations are not shown here as they are described elsewhere1~4~5). The following material parameters were used for the curve
Fig. 2. Computed curves of log [(PI>] versus reciprocal temperature with surface state concentration as a parameter (ND = 1Ol6crnm3, ND - NA = 2 x 1Ol2cm-3, ECD = 0.2 eV. Ecs = 0.9 eV, W= 0.001 cm and K = 10).
INFLUENCE
OF SURFACE
169
STRUCTURE
NA =2 x 1012 cme3, Ec,=O.2 eV, the of fig. 2: Nn=10’6cm-3, NA=N,depth of the bulk donor level below the bottom of conduction band, dielectric constant K = 10 and half width of the crystal W=O.OOl cm. These values are typical of the CdS crystal used here. EC, was taken to be 0.9 eV, representing the oxygen chemisorption level on CdS 6). When the surface state concentration is less than WNL, the total number of available electrons in the conduction band, the slope of the semiconductivity curves (A, B, C) is EC,. This is called the bulk-controlled regime 1*4,5). On the other hand, when N, is greater than WNL, the slope is EC,- V, (=0.72 eV, in this case), where V, (= eN6 W2/2~s,) is the maximum height of the surface barrier. This is called the surface controlled regime (curve D)1941 5). Now, the experimental data can be interpreted clearly by assuming that the curves A, B, C and D of fig. 1 correspond to the curves C, D, A and B of fig. 2, respectively. In practice, the oxygen chemisorption states of CdS are distributed in energy41 5). Nevertheless, the above correspondence is still valid for distributed surface states, subject to only minor quantitative changes49 5). Finally, it should be noted that only one large face of the crystal was cleaned by ion bombardment and photodesorption. The other large surface has, therefore, a relatively higher concentration of chemisorbed species. Hence, the unprocesssed side is more resistive and does not effect the measured conductivity significantly. 4. Conclusion Our electrical equilibrium conductivity measurements on single crystals of CdS indicate that atomically ordered and clean surfaces are inert to chemisorption. It appears that the oxygen surface states responsible for large observed equilibrium conductive changes are associated with surface impurities and/or surface imperfections. This is in agreement with similar observations on ZnOs). Appendix A semiquantitative criterion based on a semiclassical ionic bonding picture is presented here with which the stability of atomically ordered and smooth non-polar surfaces against chemisorption may be illustrated. It has been shown that the energy AE, relative to the conduction band edge of an acceptor-like adsorbate on an atomically flat, non-polar, binary (MX) ionic adsorbent is7-9) AE = Ep 1 a
2 [
(ZJZ) (2/A.T 1-P
I-
=a>
170
S.BAIDYAROY,
W.R.BOTTOMS
AND
P.MARK
Here E, is the adsorbent band gap, Z and Z, are the lattice valence and the adsorbate valence, respectively (both may be non-integral to accommodate a covalent contribution to the bonding),
(Z&l- Ax) a0
’ = 2ZCe [l tzM ”
=
2Z,Ce[l
- (R/a,)] -
AA)
(2) ’
aO
- (R/a,)]’
(3)
and rQ = CJC)
(4)
where -Zhl is the ionization potential of the cationic lattice constituent, -A, is the electron affinity of the anionic lattice constituent, --A, is the electron affinity of the adsorbate, a, is the nearest neighbor lattice constant, R is the range of the quantum mechanical repulsive forces between nearest neighbors, e is the electronic charge, C is the bulk (infinite lattice) Madelung constant and C, is the adsorption Madelung constant 7, is), computed at a virtual lattice site one atomic layer outside an atomically flat non-polar adsorbent surface. Values of C, for various surfaces have been computed; for low index surfaces, 0.04’7 ra? 0.25, usually 7*lo). For a given lattice type, the value of ra depends solely on the surface index. The - ( +) sign in (1) corresponds to adsorption at a virtual anionic (cationic) site. Adsorption at an intermediate site would lead to a surface state in the energy interval delineated by these extremes as illustrated by the shaded triangle in fig. 3. Note that AE, > O(AE, < 0) corresponds to the adsorption state situated below (above) the conduction band edge. If the various parameters in (1) are such that AE,
(5)
flat, non-polar ionic surface where To = 2~~ - (Z/Z=), and an atomically will be stable against chemisorption when condition (5) is satisfied, a condition that is favored by low index surfaces which display the smallest r,-values. Stability is also favored by small Z-values (large ionicity in the chemisorption band). The latter point is illustrated by table 1 which shows r,-values for the CdS (1120) surface oxygen adsorbate combination for
INFLUENCE
OF SURFACE
171
STRUCTURE
Fig. 3. Energy band diagram of an ionic surface which is expected chemisorption. The surface state band lies in the conduction band triangular area shown developing from the isolated energy -AA as proaches the surface. Also shown is the semiclassical development structure from isolated atomic energies -ZM and -AX as the lattice to its equilibrium value.
to be inert against and is the shaded the adsorbate apof the bulk band constant decrease
three values of 2 and 2, [r, =0.082 for this surfacelo)]. The checked values, for which ra I ro, indicate inert configurations. As the experiments indicate that the atomically flat surface is inert, some combination of valences corresponding to checked values is appropriate, most probably Z=O.5 (62.5% ionicity) and Z, = 1.07~9). A structural imperfection can generate an adsorption site by increasing the number of nearest neighbors, the consequence of which is to increase C, and hence to broaden the allowed range of adsorption state energies over that of an atomically flat surface by raising the upper limit and by lowering TABLE
1
for the (1120) CdS surface (Ta = 0.082) and oxygen adsorbate for different values of Z and Z,
Values
of TO (=
2p,
-
Z/Z,)
Lattice valence Z
Adsorbate valence Zs
0.5
1.0
2.0
0.5 1.0 2.0
-0.7 0.12* 0.95 *
-1.7 - 0.38 0.7 *
-3.7 - 1.38 0.2*
* The checked values (i.e., Ta 2
l-00)
indicate inert configurations.
172
S. BAIDYAROY,
W. R. BOTTOMS
AND
P. MARK
the lower limit into the bandgap. A similar lowering of the adsorption surface state could be achieved with the aid of a surface impurity. There is evidence that chlorine impurities accomplish this on ZnO surfaces2). However, the present arguments are inapplicable to this process. References T. A. Goodwin and P. Mark, Prog. Surface Sci. 1 (1971) 1. J. D. Levine, A. Willis, W. R. Bottoms and P. Mark, Surface Sci. 29 (1972) 144. B. D. Campbell and H. E. Farnsworth, Surface Sci 10 (1968) 197. P. Mark, Surface Sci. 25 (1971) 192. S. Baidyaroy and P. Mark, Surface Sci. in press. P. Mark, J. Phys. Chem. Solids 26 (1965) 1767. P. Mark, Conductivity Changes in Semi-Insulating, Thin CdS Layers Induced by Chemisorption, in: Optical Properties of Dielectric Films, Ed. N. N. Axelrod (The Electrochemical Society, New York, 1968). 8) P. Mark, The Electronic Surface State of Finite Lattices, in: Clean Surfaces, Ed. G. Goldfinger (Marcel Dekker, New York, 1970) ch. 16. 9) J. D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 10) R. W. Nosker and P. Mark, Surface Sci. 20 (1970) 421. 1) 2) 3) 4) 5) 6) 7)
SCIENCE 29 (1972) 165-172 0 North-Holland
THE INFLUENCE CHEMISORPTION
OF OXYGEN
S. BAIDYAROY Department
OF SURFACE
Publishing Co.
STRUCTURE
BY CdS SINGLE
**, W. R. BOTTOMS and PETER
of Electrical Engineering, Princeton Princeton, New Jersey 08540, U.S.A.
Received 9 August 1971; revised manuscript
ON THE CRYSTALS* MARK
University,
received 29 September
1971
Atomically ordered surfaces of CdS single crystals are found to be relatively insensitive to oxygen chemisorption. For such surfaces prepared by argon bombardment and high vacuum annealing, the effect of ambient oxygen on the equilibrium conductivity is drastically reduced. This indicates that active sites associated with surface imperfections may be necessary to achieve chemisorption.
1. Introduction The effects of oxygen chemisorption on the equilibrium conductivity of CdS single crystals have been investigated extensively by Goodwin and Markl). Usually, there is a gradual reduction in the conductivity coupled with a gradual increase in the activation energy with increasing oxygen pressure. For thin and insulating samples the change in the equilibrium conductivity may be as large as 7 decades. It has recently been suggested that atomically ordered surfaces of compound semiconductors are essentially inert to chemisorption2). From low energy electron diffraction (LEED) studies of the principal surfaces of ZnO single crystals, it was found that surfaces cleaned by argon bombardment and subsequent high vacuum annealing retained their original diffraction spot brightness and sharpness even after exposure to room ambient conditions for as long as three months. The inert character of these atomically ordered surfaces was also demonstrated by their inability to sustain a charge deposited by a corona. These surfaces were, however, chargeable before “clean up”. The failure of the atomically ordered surfaces to sustain a corona charge was attributed to the absence of chemisorption sites. This paper discusses changes induced in the ambient-sensitivity of single crystals of CdS when the surfaces are rendered atomically clean and ordered
* Supported by the Office of Naval Research and by the National Science Foundation. ** Part of Ph.D. dissertation. 165
166
S. BAIDYAROY,
W.R.
BOTTOMS
AND
P. MARK
by ion bombardment and high vacuum annealing. investigated was (IIZO) and the ambient-sensitivity equilibrium conductivity measurements.
The principal surface was inferred from
2. Experimental The samples were vapor phase grown CdS platelets of approximate size: 3 mm x 1 mm x 20 pm (courtesy: Dr. D. C. Reynolds, Wright-Patterson Air Force Base, Dayton, Ohio). The crystals were mounted on sapphire disks and were electroded with indium such that current flowed parallel to the large area (1120) surfaces. The specimens were mounted on a thermal finger housed inside a stainless steel chamber which had a vacuum capability of lo-* torr without a bake-out. The chamber was equipped with a variable leak valve for controlling ambient gas conditions, and with electrical feed-throughs and optical view ports. The vacuum system is described more fully elsewherel). Two sets of conductivity-temperature data, with ambient oxygen pressure as a parameter, were recorded. The first was taken on the virgin crystal (before surface clean up) and the second after clean up. The samples were cleaned by the following steps: (i) Place the sample inside the LEED system and pump down to a vacuum better than lop9 torr. (ii) Heat the sample at 125 “C for several hours in order to desorb some adsorbed species thermally (heating was limited to 125°C because of In-electrodes). (iii) Next, bombard the crystal surface with argon ions (ion current ~2 pA/cm2, beam voltage = 365 V, argon pressure = lo-’ torr) for about 2 hr. (iv) In order to remove embedded argon ions and to anneal any lattice damage created by argon bombardment, anneal the sample at 125°C for about 6 hr. Such a sequence gives sharp and bright low energy electron diffraction spots characteristic of an atomically ordered and clean (I 120) surface3).
3. Results and discussion The virgin crystal gave no diffraction pattern either after exposure to oxygen or upon isothermal photodesorption, characteristic of an atomically disordered and possibly contaminated surface. The typical oxygen ambient semiconductivity of the virgin crystal is demonstrated by curves A (10e8 torr) and B (10 torr) of fig. 1 (before taking the data of curve A the sample was photodesorbed). Presumably, the curve A corresponds to the ‘“adsorbate free” surface and is characterized by a slope equal to the bulk donor level; consequently, curve A exhibits a bulk-controlled regime 1$4, 5). On the other hand, the slope of the curve B is determined, among other parameters, by
INFLUENCE
Fig. 1.
Effects of ion bombardment conductivity
OF SURFACE
STRUCTURE
and high vacuum annealing of CdS single crystal.
167
on the equilibrium
the surface state energy level(s) of chemisorbed oxygen. Hence, curve B represents a surface-controlled regime lp 4v5). The curves C (lo-’ torr) and D (10 torr) of fig. 1 illustrate the ambientsensitive semiconductivity of the atomically ordered and clean surface obtained by argon bombardment and annealing. Notice that, as a result of this surface treatment: (i) The IO-’ torr curve (C) is raised by more than a decade, having the same slope as that of A. (ii) The lo-’ torr curve (C) and the 10 torr curve (D) are close to each other, and have the same slope. The increase in the current level can be attributed to further oxygen desorption by the cleaning procedure. This implies that prior to the bombard-anneal treatment photodesorption had not removed all the adsorbed species; therefore, the curve A in fig. 1 is not the actual adsorbate free curve although it belongs to the bulk-controlled regime. The curve C is closer to the “true” adsorbate free curve and hence it lies above A. The close spacing and identical slope of curves C and D (both belong to the bulk-controlled regime) indicate that D corresponds to a very small surface state concentration. This shows that an atomically ordered and clean surface is virtually insensitive to oxygen exposure. LEED and Auger
168
S. BAIDYAROY,
W. R, BOTTOMS
AND
P. MARK
electron spectroscopy measurements with other CdS crystals have also revealed that an atomically ordered surface does not chemisorb oxygen even after exposure to room ambient conditions for several months while a chlorine contaminated, disordered surface readily chemisorbs oxygen after exposure for several seconds at 10P5 torr oxygen pressure. Similar results have also been obtained with ZnO 2). The inert character of atomically ordered surfaces is discussed further in the Appendix. The experimental curves of fig. 1 can be interpreted in terms of fig. 2, which is a computed plot of the average free electron density (n) in the conduction band of a compensated n-type semiconductor as a function of reciprocal temperature with surface state concentration (N, cm-“) as a parameter. If the temperature and pressure dependence of mobility is ignored, (n) is proportional to the measured conductivity. For the curves of fig. 2, the bulk donor and acceptor levels are assumed discrete, the donor density ND being greater than the acceptor density NA for an n-type material. For convenience, in this example, the surface states are also assumed to have a discrete energy level, EC, below the bottom of conduction band. The details of the calculations are not shown here as they are described elsewhere1~4~5). The following material parameters were used for the curve
Fig. 2. Computed curves of log [(PI>] versus reciprocal temperature with surface state concentration as a parameter (ND = 1Ol6crnm3, ND - NA = 2 x 1Ol2cm-3, ECD = 0.2 eV. Ecs = 0.9 eV, W= 0.001 cm and K = 10).
INFLUENCE
OF SURFACE
169
STRUCTURE
NA =2 x 1012 cme3, Ec,=O.2 eV, the of fig. 2: Nn=10’6cm-3, NA=N,depth of the bulk donor level below the bottom of conduction band, dielectric constant K = 10 and half width of the crystal W=O.OOl cm. These values are typical of the CdS crystal used here. EC, was taken to be 0.9 eV, representing the oxygen chemisorption level on CdS 6). When the surface state concentration is less than WNL, the total number of available electrons in the conduction band, the slope of the semiconductivity curves (A, B, C) is EC,. This is called the bulk-controlled regime 1*4,5). On the other hand, when N, is greater than WNL, the slope is EC,- V, (=0.72 eV, in this case), where V, (= eN6 W2/2~s,) is the maximum height of the surface barrier. This is called the surface controlled regime (curve D)1941 5). Now, the experimental data can be interpreted clearly by assuming that the curves A, B, C and D of fig. 1 correspond to the curves C, D, A and B of fig. 2, respectively. In practice, the oxygen chemisorption states of CdS are distributed in energy41 5). Nevertheless, the above correspondence is still valid for distributed surface states, subject to only minor quantitative changes49 5). Finally, it should be noted that only one large face of the crystal was cleaned by ion bombardment and photodesorption. The other large surface has, therefore, a relatively higher concentration of chemisorbed species. Hence, the unprocesssed side is more resistive and does not effect the measured conductivity significantly. 4. Conclusion Our electrical equilibrium conductivity measurements on single crystals of CdS indicate that atomically ordered and clean surfaces are inert to chemisorption. It appears that the oxygen surface states responsible for large observed equilibrium conductive changes are associated with surface impurities and/or surface imperfections. This is in agreement with similar observations on ZnOs). Appendix A semiquantitative criterion based on a semiclassical ionic bonding picture is presented here with which the stability of atomically ordered and smooth non-polar surfaces against chemisorption may be illustrated. It has been shown that the energy AE, relative to the conduction band edge of an acceptor-like adsorbate on an atomically flat, non-polar, binary (MX) ionic adsorbent is7-9) AE = Ep 1 a
2 [
(ZJZ) (2/A.T 1-P
I-
=a>
170
S.BAIDYAROY,
W.R.BOTTOMS
AND
P.MARK
Here E, is the adsorbent band gap, Z and Z, are the lattice valence and the adsorbate valence, respectively (both may be non-integral to accommodate a covalent contribution to the bonding),
(Z&l- Ax) a0
’ = 2ZCe [l tzM ”
=
2Z,Ce[l
- (R/a,)] -
AA)
(2) ’
aO
- (R/a,)]’
(3)
and rQ = CJC)
(4)
where -Zhl is the ionization potential of the cationic lattice constituent, -A, is the electron affinity of the anionic lattice constituent, --A, is the electron affinity of the adsorbate, a, is the nearest neighbor lattice constant, R is the range of the quantum mechanical repulsive forces between nearest neighbors, e is the electronic charge, C is the bulk (infinite lattice) Madelung constant and C, is the adsorption Madelung constant 7, is), computed at a virtual lattice site one atomic layer outside an atomically flat non-polar adsorbent surface. Values of C, for various surfaces have been computed; for low index surfaces, 0.04’7 ra? 0.25, usually 7*lo). For a given lattice type, the value of ra depends solely on the surface index. The - ( +) sign in (1) corresponds to adsorption at a virtual anionic (cationic) site. Adsorption at an intermediate site would lead to a surface state in the energy interval delineated by these extremes as illustrated by the shaded triangle in fig. 3. Note that AE, > O(AE, < 0) corresponds to the adsorption state situated below (above) the conduction band edge. If the various parameters in (1) are such that AE,
(5)
flat, non-polar ionic surface where To = 2~~ - (Z/Z=), and an atomically will be stable against chemisorption when condition (5) is satisfied, a condition that is favored by low index surfaces which display the smallest r,-values. Stability is also favored by small Z-values (large ionicity in the chemisorption band). The latter point is illustrated by table 1 which shows r,-values for the CdS (1120) surface oxygen adsorbate combination for
INFLUENCE
OF SURFACE
171
STRUCTURE
Fig. 3. Energy band diagram of an ionic surface which is expected chemisorption. The surface state band lies in the conduction band triangular area shown developing from the isolated energy -AA as proaches the surface. Also shown is the semiclassical development structure from isolated atomic energies -ZM and -AX as the lattice to its equilibrium value.
to be inert against and is the shaded the adsorbate apof the bulk band constant decrease
three values of 2 and 2, [r, =0.082 for this surfacelo)]. The checked values, for which ra I ro, indicate inert configurations. As the experiments indicate that the atomically flat surface is inert, some combination of valences corresponding to checked values is appropriate, most probably Z=O.5 (62.5% ionicity) and Z, = 1.07~9). A structural imperfection can generate an adsorption site by increasing the number of nearest neighbors, the consequence of which is to increase C, and hence to broaden the allowed range of adsorption state energies over that of an atomically flat surface by raising the upper limit and by lowering TABLE
1
for the (1120) CdS surface (Ta = 0.082) and oxygen adsorbate for different values of Z and Z,
Values
of TO (=
2p,
-
Z/Z,)
Lattice valence Z
Adsorbate valence Zs
0.5
1.0
2.0
0.5 1.0 2.0
-0.7 0.12* 0.95 *
-1.7 - 0.38 0.7 *
-3.7 - 1.38 0.2*
* The checked values (i.e., Ta 2
l-00)
indicate inert configurations.
172
S. BAIDYAROY,
W. R. BOTTOMS
AND
P. MARK
the lower limit into the bandgap. A similar lowering of the adsorption surface state could be achieved with the aid of a surface impurity. There is evidence that chlorine impurities accomplish this on ZnO surfaces2). However, the present arguments are inapplicable to this process. References T. A. Goodwin and P. Mark, Prog. Surface Sci. 1 (1971) 1. J. D. Levine, A. Willis, W. R. Bottoms and P. Mark, Surface Sci. 29 (1972) 144. B. D. Campbell and H. E. Farnsworth, Surface Sci 10 (1968) 197. P. Mark, Surface Sci. 25 (1971) 192. S. Baidyaroy and P. Mark, Surface Sci. in press. P. Mark, J. Phys. Chem. Solids 26 (1965) 1767. P. Mark, Conductivity Changes in Semi-Insulating, Thin CdS Layers Induced by Chemisorption, in: Optical Properties of Dielectric Films, Ed. N. N. Axelrod (The Electrochemical Society, New York, 1968). 8) P. Mark, The Electronic Surface State of Finite Lattices, in: Clean Surfaces, Ed. G. Goldfinger (Marcel Dekker, New York, 1970) ch. 16. 9) J. D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 10) R. W. Nosker and P. Mark, Surface Sci. 20 (1970) 421. 1) 2) 3) 4) 5) 6) 7)