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Temperature programmed desorption studies of CO on CdTe(110)
Surface Science 195 (1988) L193-L198 North-Holland, Amsterdam
L193
SURFACE SCIENCE LETTERS TEMPERATURE PROGRAMMED DESORPTION STUDIES OF CO ON CdTe(ll0) H.S. TAN, A. MORAWSKI * and W.E. JONES Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B31t 4J3
Received 22 July 1987; accepted for publication 12 November 1987
The thermal desorption of CO from the CdTe(ll0) surface was studied. The initial rate of desorption exhibits zero order kinetics with an activation energy of 2.8 kcal/mol at low surface coverage rising to 4.4 kcal/mol at high surfacecoverage. The results may he explainedin terms of the mechanism of island formation on the semiconductor surface.
The adsorption and desorption of gases on semiconductors such as CdTe has received very little attention. A search of the literature has failed to identify any study in this area. This is surprising in view of the importance of such materials in the preparation of infrared detectors, photocells and other electroluminescent devices and the fact timt impurities and concentration ch='~ges at the surface or thin overlayers of different composition may affect the electrical properties of these materials. In this Letter we report the first temperature programmed desorption spectra of CO from CdTe(110) surface. The experimental apparatus is shown in fig. 1. The stainless steel ultra-high vacuum (UHV) chamber contains the sample crystal, a sputtering gun for cleaning the surface of the crystal, an X-ray photoelectron spectrometer for surface analysis, an ionization gauge for monitoring the chamber pressure, and a quadrupole mass spectrometer for monitoring the desorption flux. A collimating cone having a 0.4 cm diameter aperture, when placed close ( - 1 nun) to the crystal surface, provides desorbed molecules with line of sight passage to the ion source of the mass spectrometer. The main chamber was pumped with a liquid nitrogen-trapped polyphenyl ether diffusion pump and titanium sublimation pump. 3-he coiiimating cone and mass spectrometer are evacuated by a turbo-molecular pump. Two additional liquid rfitroge~a traps, one attached to the sample support and the other located between the mass spectrometer and the turbo pump assist in further reducing the pressure of the system. Background pressures of the order * Present address: Institute of Physics, Av. Lomikow 32, 02-668 Warsaw, Poland. 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
L194
H.S. Tan et al. / TPD studies of CO on CdTe(llO)
Liquid Nitrogen Thermocouple r~ . . Leads-e..--~[ l ~=.~-..Heater i.eaos Li u id , o Ni(~rogen"~ ~'..360 Rotation Turbo Pum~l ~11' ..~ X-Ray Photoelectron ~ ~ ~//~-~--- Spectrometer
~,
_.L_ ~ Mass ..~. ~ ~ . . ~ Spectrometerl
H f/ ',
l Sputter _ . ~ ' - I o n G un _L~_ Ionization
Fig. 1. Schematic view of experimental apparatus.
of 1 × 10 -1° Torr were routinely achieved, with H2, CO and H20 being the major detectable residuals. During the resorption experiment, large pumping speeds were maintained ensuring that the rate of desorption is directly proportional to the partial pressure of CO, /'co. The sample is a thin (1.0 ram) single crystal of CdTe and the exposed surface is of (110) orientation. The CdTe sample was polished with alumina grains and etched with 5% bromine solution in methanol. The back face of the crystal was soldered onto a Ni wire support using indium as a soldering medium. Similarly, a copper-constantan thermocouple was soldered to the back face of the crystal. The output of the thermocouple was compared with a second identical junction maintained at ice point. The sample temperature could be raised linearly with time by resistively heating the support wires. The thermocouples and power supply were connected to a microcomputer which runs the temperature ramp program. Temperature-time ds~.a were stored and the heating rate could readily be determined by running a linear regression program. The typical experimental procedure is as follows: the crystal surface is cleaned by argon ion sputtering for 20 rain at a partial pressure of argon of 5 x 10 -5 Tort. Ion bombardment with Ar + was found to be an effective cleaning treatment for GaAs [1]. The sample is then heated to 300 K to desorb any gas remaining on the surface. After sputtering and heating, XPS shows no impurities on the surface. Adsorption was carried out by allowing CO to adsorb onto the sample crystal for a specific time, while the crystal is held at - 85 K. Following the desired exposure, a temperature programmed desorption spectrum is obtained by resistively heating the sample. The partial
H.S. Tan et aL / TPD studies of CO on CdTe(llO)
L195
pressure data were transmitted from the mass spectrometer, through an IEEE interface, to a microcomputer for storage. The desorption spectrum, as a function of time or temperature, could readily be shown on the computer screen or graphed on a printer-plotter by running an appropriate program. Examples of the thermal desorption spectra (TDS) at a heating rate of 2.0 K / s for various initial CO coverages are presented in fig. 2. The relative coverages, 0, are indicated for each curve. These were obtained from graphical integration of /'co versus time curves and then divided by the saturation coverage. Since an accurate value of the pumping speed was not determined, the absolute coverage could not be obtained. The spectra show a common leading edge and the peak temperature increases with increase in surface coverage, from 142 K at the lowest coverage to 152 K at the saturation coverage. The common leading edge indicates a zero order desorption, and the shifting of the peak maximum to higher temperature is characteristic of a fractional order resorption mechanism. The high temperature side of the peak did not drop back to the base line which may indicate thermal desorption from the crystal support assembly at higher temperatures. The desorption data were analyzed using the complete lineshape analysis method described by King [2]. This procedure was subject to some error since the high-temperature portion of the desorption peak did not return to the base line. However, the error is small when only the data of the low-temperature region are analyzed. Thus in the following analysis, only the low-temperature region of the desorption data is used. This corresponds to the high coverage region of a single desorption curve. Arrhenius plots of In Pco versus 1/T, 1.81
A
O
~1.00
1.4
¢.
I
V
mo 1.(] X 0 aPo.s i
w
~
0.2-! -~, 100
11o
'
' 140
160 ' Temperature (K) '
1~o
:oo
Fig. 2. Thermal desorption spectra of CO from CdTe(ll0) at various initial coverages. Heating rate: 2.0 K / s . Relative coverage, 0, is indicated for each curve.
L196
H.S. Tan et al. / TPD studies of CO on CdTe(11(7)
3.0
\ \
2.5
\\0 '~V, O0
V
"~
2.0"
:o v
5 l.s \ 1.0 6.4
i
\
1
6.7 7.0 1. x l o 3
(K-1)
7.3
Fig. 3. Arrhenius plots of CO desorption from CdTe(ll0). Fractional coverage: (zx) 0.09, (13) 0.12, ( ~ ) 0.14, ( + ) 0.16, (~z) 0.18, ( x ) 0.21, (E) 0.24, (@) 0.43, (A) 0.51, ( O ) 0.55, (o) 0.61. ( - - - - - - ) Regression line using all data (E a = 3.61 kcal/mol).
where T is the measured surface temperature, at various surface coverages, are given in fig. 3. The activation energies, Ed, estimated using a linear least-squares method are as presented in table 1. Although there may be considerable error in these estimates because of the few data points used for each coverage it is evident that the estimated activation energy increases with surface coverage. Table 1 Activation energy of desorption of CO from CdTe(ll0) Fractional coverage, O
Activation energy (kcal/mol)
Fractional coverage, 0
Activation energy (kcal/mol)
0.09 0.12 0.14 0.16 0.18 0.21
2.75 2.86 3.03 3.25 3.11 3.27
0.24 0.43 0.51 0.55 0.61
3.45 3.51 4.39 3.86 4.33
L197
H.S. Tan et aL / TPD studies of CO on CdTe(llO)
The reaction order of desorption was determined from plots of In Pco versus In 0 at constant temperature. These plots are presented in fig. 4. In the low-temperature high-coverage region the slopes of these plots indicate a zero order desorption. Because of the error associated with the trailing edge of the desorption peak, the graphs for the high-temperature low-coverage regions contain too much scatter to give a reliable value for the desorption order. Nevertheless, a rough calculation (fig. 4) indicates that the kinetic order is neither zero nor unity but between 0.4 and 0.5. The most distinctive characteristic of the CO/CdTe TDS is the zero order kinetics with a coverage dependent activation energy of 2.8 to 4.4 kcal/mol. Zero order desorption has been reported for several systems involving the adsorption of atoms, e.g., Zn on GaAs [1], Xe on graphite (1000) [3], Hg on W(100) [4], formic acid on graphitized Ni(110) [5], Cu on Ru(0001) [6], and Xe on W(110) [7]. Several mechanisms have been proposed in the literature to describe the zero order desorption, with the island mechanism being most common [1,4.5,7,8]. Generally, in this mechanism, the surface is conceived to contain two adsorbed phases: (1) the condensed phase which consists of islands of adsorbate, and (2) the rarefied phase which consi.qts of weakly-bound
--V?
-- X--)(
X-X--
V V
v
X¢ ~
"-o
2.0"
¢ ....
00-
06
0
X ,0 X X D
v
-J 1.5
ox v
-,~
X XV
?
1.0
I
,~-l-..-a
--+--
I
-2 Ln
++++~-+-- ++---+
I
-4
~a--~.
I
"I
0
e
Fig. 4. Plots of in Pco versus In 0. Temperature: ( + ) 134.5°C, (zx) 138.2°C, (121)141.4°C, (O) 144.1° C, ( x ) 148.6° C, (v) 151.5° C.
L198
H.S. Tan et al. / TPD studies of CO on CdTe(l lO)
mobile adatoms. It is often suggested that the islands of the adsorbed atoms are formed because the adsorbate-substrate interactions are weak compared to the adsorbate-adsorbate interaction. McCarty and Madix [5] have indicated that the similar behavior of island formation can arise following the adsorption of molecular species, provided the adsorbate-adsorbate interactions are relatively strong. Using the steady state assumption for weakly-bound mobile adatoms, Arthur [2] derived the following equation for the rate of desorption,
R -- kkaOl/2/(k
+ knot~2),
where k is the desorption rate constant, k a is the rate constant for incorporation of adatoms into the condensed phase, and k a is the rate constant for dissociation from the island edge into the rarefied phase. When kaO1/2 >> k (large coverage), then R ffi k k J k a , the desorption is zero order; when ka01/2 << k (small coverage), then R ffi kdO1/2, the desorption rate is half-order with respect to surface coverage. It appears that our results of CO desorption from CdTe(ll0) could be described by the island mechanism. Analysis of the desorption data indicates a zero order desorption at high coverage, and gives a fractional order at low coverage. This is in qualitative agreement with Arthur's model. The increase of the desorption activation energy with coverage also supports the mechanism of island formation. This behavior has been observed for the desorption of Cu and Au from the basal plane of graphite [8]. Further studies of the desorption of other gases from CdTe and other semiconductor materials are in progress in an attempt to provide a more detailed understanding of adsorption on semiconductors. We thank the Natural Science and Engineering Research Council of Canada for support in the form of a Strategic Grant to W.E.J. We are also indebted to C.I.L. for financial support and to Cominco for semiconductor samples. We also thank M. Grunze, Institute for Surface Science and Technology, University of Maine, Orono and H.J. Kreuzer, Department of Physics, Dalhousie University for many helpful comments.
References [1] J.R. Arthur, Surface Sci. 38 (1973) 394. [2] D.A. King, Surface Sci. 47 (1975) 384. [3] M. Bienfait and J.A. Venables, Surface Sci. 64 (1977) 425. [4] R.G. Jones and D.L. Perry, Surface Sci. 71 (1978) 48. [5] J.G. McCarty and J.R. Madix, Surface Sci. 54 (1976) 210. [6] K. Christman, G. Ertl and H. Shimizu, J. Catalysis 61 (1980) 397. [71 R. Opila and R. Gomer, Surface Sci. 112 (1981) 1. [8] J.R. Arthur and A.Y. Cho, Surface Sci. 36 (1973) 641.
L193
SURFACE SCIENCE LETTERS TEMPERATURE PROGRAMMED DESORPTION STUDIES OF CO ON CdTe(ll0) H.S. TAN, A. MORAWSKI * and W.E. JONES Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B31t 4J3
Received 22 July 1987; accepted for publication 12 November 1987
The thermal desorption of CO from the CdTe(ll0) surface was studied. The initial rate of desorption exhibits zero order kinetics with an activation energy of 2.8 kcal/mol at low surface coverage rising to 4.4 kcal/mol at high surfacecoverage. The results may he explainedin terms of the mechanism of island formation on the semiconductor surface.
The adsorption and desorption of gases on semiconductors such as CdTe has received very little attention. A search of the literature has failed to identify any study in this area. This is surprising in view of the importance of such materials in the preparation of infrared detectors, photocells and other electroluminescent devices and the fact timt impurities and concentration ch='~ges at the surface or thin overlayers of different composition may affect the electrical properties of these materials. In this Letter we report the first temperature programmed desorption spectra of CO from CdTe(110) surface. The experimental apparatus is shown in fig. 1. The stainless steel ultra-high vacuum (UHV) chamber contains the sample crystal, a sputtering gun for cleaning the surface of the crystal, an X-ray photoelectron spectrometer for surface analysis, an ionization gauge for monitoring the chamber pressure, and a quadrupole mass spectrometer for monitoring the desorption flux. A collimating cone having a 0.4 cm diameter aperture, when placed close ( - 1 nun) to the crystal surface, provides desorbed molecules with line of sight passage to the ion source of the mass spectrometer. The main chamber was pumped with a liquid nitrogen-trapped polyphenyl ether diffusion pump and titanium sublimation pump. 3-he coiiimating cone and mass spectrometer are evacuated by a turbo-molecular pump. Two additional liquid rfitroge~a traps, one attached to the sample support and the other located between the mass spectrometer and the turbo pump assist in further reducing the pressure of the system. Background pressures of the order * Present address: Institute of Physics, Av. Lomikow 32, 02-668 Warsaw, Poland. 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
L194
H.S. Tan et al. / TPD studies of CO on CdTe(llO)
Liquid Nitrogen Thermocouple r~ . . Leads-e..--~[ l ~=.~-..Heater i.eaos Li u id , o Ni(~rogen"~ ~'..360 Rotation Turbo Pum~l ~11' ..~ X-Ray Photoelectron ~ ~ ~//~-~--- Spectrometer
~,
_.L_ ~ Mass ..~. ~ ~ . . ~ Spectrometerl
H f/ ',
l Sputter _ . ~ ' - I o n G un _L~_ Ionization
Fig. 1. Schematic view of experimental apparatus.
of 1 × 10 -1° Torr were routinely achieved, with H2, CO and H20 being the major detectable residuals. During the resorption experiment, large pumping speeds were maintained ensuring that the rate of desorption is directly proportional to the partial pressure of CO, /'co. The sample is a thin (1.0 ram) single crystal of CdTe and the exposed surface is of (110) orientation. The CdTe sample was polished with alumina grains and etched with 5% bromine solution in methanol. The back face of the crystal was soldered onto a Ni wire support using indium as a soldering medium. Similarly, a copper-constantan thermocouple was soldered to the back face of the crystal. The output of the thermocouple was compared with a second identical junction maintained at ice point. The sample temperature could be raised linearly with time by resistively heating the support wires. The thermocouples and power supply were connected to a microcomputer which runs the temperature ramp program. Temperature-time ds~.a were stored and the heating rate could readily be determined by running a linear regression program. The typical experimental procedure is as follows: the crystal surface is cleaned by argon ion sputtering for 20 rain at a partial pressure of argon of 5 x 10 -5 Tort. Ion bombardment with Ar + was found to be an effective cleaning treatment for GaAs [1]. The sample is then heated to 300 K to desorb any gas remaining on the surface. After sputtering and heating, XPS shows no impurities on the surface. Adsorption was carried out by allowing CO to adsorb onto the sample crystal for a specific time, while the crystal is held at - 85 K. Following the desired exposure, a temperature programmed desorption spectrum is obtained by resistively heating the sample. The partial
H.S. Tan et aL / TPD studies of CO on CdTe(llO)
L195
pressure data were transmitted from the mass spectrometer, through an IEEE interface, to a microcomputer for storage. The desorption spectrum, as a function of time or temperature, could readily be shown on the computer screen or graphed on a printer-plotter by running an appropriate program. Examples of the thermal desorption spectra (TDS) at a heating rate of 2.0 K / s for various initial CO coverages are presented in fig. 2. The relative coverages, 0, are indicated for each curve. These were obtained from graphical integration of /'co versus time curves and then divided by the saturation coverage. Since an accurate value of the pumping speed was not determined, the absolute coverage could not be obtained. The spectra show a common leading edge and the peak temperature increases with increase in surface coverage, from 142 K at the lowest coverage to 152 K at the saturation coverage. The common leading edge indicates a zero order desorption, and the shifting of the peak maximum to higher temperature is characteristic of a fractional order resorption mechanism. The high temperature side of the peak did not drop back to the base line which may indicate thermal desorption from the crystal support assembly at higher temperatures. The desorption data were analyzed using the complete lineshape analysis method described by King [2]. This procedure was subject to some error since the high-temperature portion of the desorption peak did not return to the base line. However, the error is small when only the data of the low-temperature region are analyzed. Thus in the following analysis, only the low-temperature region of the desorption data is used. This corresponds to the high coverage region of a single desorption curve. Arrhenius plots of In Pco versus 1/T, 1.81
A
O
~1.00
1.4
¢.
I
V
mo 1.(] X 0 aPo.s i
w
~
0.2-! -~, 100
11o
'
' 140
160 ' Temperature (K) '
1~o
:oo
Fig. 2. Thermal desorption spectra of CO from CdTe(ll0) at various initial coverages. Heating rate: 2.0 K / s . Relative coverage, 0, is indicated for each curve.
L196
H.S. Tan et al. / TPD studies of CO on CdTe(11(7)
3.0
\ \
2.5
\\0 '~V, O0
V
"~
2.0"
:o v
5 l.s \ 1.0 6.4
i
\
1
6.7 7.0 1. x l o 3
(K-1)
7.3
Fig. 3. Arrhenius plots of CO desorption from CdTe(ll0). Fractional coverage: (zx) 0.09, (13) 0.12, ( ~ ) 0.14, ( + ) 0.16, (~z) 0.18, ( x ) 0.21, (E) 0.24, (@) 0.43, (A) 0.51, ( O ) 0.55, (o) 0.61. ( - - - - - - ) Regression line using all data (E a = 3.61 kcal/mol).
where T is the measured surface temperature, at various surface coverages, are given in fig. 3. The activation energies, Ed, estimated using a linear least-squares method are as presented in table 1. Although there may be considerable error in these estimates because of the few data points used for each coverage it is evident that the estimated activation energy increases with surface coverage. Table 1 Activation energy of desorption of CO from CdTe(ll0) Fractional coverage, O
Activation energy (kcal/mol)
Fractional coverage, 0
Activation energy (kcal/mol)
0.09 0.12 0.14 0.16 0.18 0.21
2.75 2.86 3.03 3.25 3.11 3.27
0.24 0.43 0.51 0.55 0.61
3.45 3.51 4.39 3.86 4.33
L197
H.S. Tan et aL / TPD studies of CO on CdTe(llO)
The reaction order of desorption was determined from plots of In Pco versus In 0 at constant temperature. These plots are presented in fig. 4. In the low-temperature high-coverage region the slopes of these plots indicate a zero order desorption. Because of the error associated with the trailing edge of the desorption peak, the graphs for the high-temperature low-coverage regions contain too much scatter to give a reliable value for the desorption order. Nevertheless, a rough calculation (fig. 4) indicates that the kinetic order is neither zero nor unity but between 0.4 and 0.5. The most distinctive characteristic of the CO/CdTe TDS is the zero order kinetics with a coverage dependent activation energy of 2.8 to 4.4 kcal/mol. Zero order desorption has been reported for several systems involving the adsorption of atoms, e.g., Zn on GaAs [1], Xe on graphite (1000) [3], Hg on W(100) [4], formic acid on graphitized Ni(110) [5], Cu on Ru(0001) [6], and Xe on W(110) [7]. Several mechanisms have been proposed in the literature to describe the zero order desorption, with the island mechanism being most common [1,4.5,7,8]. Generally, in this mechanism, the surface is conceived to contain two adsorbed phases: (1) the condensed phase which consists of islands of adsorbate, and (2) the rarefied phase which consi.qts of weakly-bound
--V?
-- X--)(
X-X--
V V
v
X¢ ~
"-o
2.0"
¢ ....
00-
06
0
X ,0 X X D
v
-J 1.5
ox v
-,~
X XV
?
1.0
I
,~-l-..-a
--+--
I
-2 Ln
++++~-+-- ++---+
I
-4
~a--~.
I
"I
0
e
Fig. 4. Plots of in Pco versus In 0. Temperature: ( + ) 134.5°C, (zx) 138.2°C, (121)141.4°C, (O) 144.1° C, ( x ) 148.6° C, (v) 151.5° C.
L198
H.S. Tan et al. / TPD studies of CO on CdTe(l lO)
mobile adatoms. It is often suggested that the islands of the adsorbed atoms are formed because the adsorbate-substrate interactions are weak compared to the adsorbate-adsorbate interaction. McCarty and Madix [5] have indicated that the similar behavior of island formation can arise following the adsorption of molecular species, provided the adsorbate-adsorbate interactions are relatively strong. Using the steady state assumption for weakly-bound mobile adatoms, Arthur [2] derived the following equation for the rate of desorption,
R -- kkaOl/2/(k
+ knot~2),
where k is the desorption rate constant, k a is the rate constant for incorporation of adatoms into the condensed phase, and k a is the rate constant for dissociation from the island edge into the rarefied phase. When kaO1/2 >> k (large coverage), then R ffi k k J k a , the desorption is zero order; when ka01/2 << k (small coverage), then R ffi kdO1/2, the desorption rate is half-order with respect to surface coverage. It appears that our results of CO desorption from CdTe(ll0) could be described by the island mechanism. Analysis of the desorption data indicates a zero order desorption at high coverage, and gives a fractional order at low coverage. This is in qualitative agreement with Arthur's model. The increase of the desorption activation energy with coverage also supports the mechanism of island formation. This behavior has been observed for the desorption of Cu and Au from the basal plane of graphite [8]. Further studies of the desorption of other gases from CdTe and other semiconductor materials are in progress in an attempt to provide a more detailed understanding of adsorption on semiconductors. We thank the Natural Science and Engineering Research Council of Canada for support in the form of a Strategic Grant to W.E.J. We are also indebted to C.I.L. for financial support and to Cominco for semiconductor samples. We also thank M. Grunze, Institute for Surface Science and Technology, University of Maine, Orono and H.J. Kreuzer, Department of Physics, Dalhousie University for many helpful comments.
References [1] J.R. Arthur, Surface Sci. 38 (1973) 394. [2] D.A. King, Surface Sci. 47 (1975) 384. [3] M. Bienfait and J.A. Venables, Surface Sci. 64 (1977) 425. [4] R.G. Jones and D.L. Perry, Surface Sci. 71 (1978) 48. [5] J.G. McCarty and J.R. Madix, Surface Sci. 54 (1976) 210. [6] K. Christman, G. Ertl and H. Shimizu, J. Catalysis 61 (1980) 397. [71 R. Opila and R. Gomer, Surface Sci. 112 (1981) 1. [8] J.R. Arthur and A.Y. Cho, Surface Sci. 36 (1973) 641.