- Email: [email protected]
Oxygen adsorption and electrical behaviour of thin, very-low-pressure chemical-vapour-deposited polysilicon films
71
Sensors and Acruators A, 33 (1992) 71-75
Oxygen adsorption and electrical behaviour chemical-vapour-deposited polysilicon films B. Fortin,
D. Mostefa, F. Raoult,
H. Lhermite
of thin, very-low-pressure
and M. Sarret
Groupe de MicrotYectronique, LJniversit6de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex (France)
Abstract We report here the results of experiments on the influence of oxygen adsorption on the conductivity of very-low-pressure chemically-deposited polysilicon thin films of various thicknesses obtained by plasma reactive etching a 600 nm initial film. It is observed that oxygen acts as an acceptor on the unintentionally-doped as on the lightly-doped polysilicon films, provided they are adequately annealed. Oxygen adsorptions made over 2 min at atmospheric pressure increase the film resistance variation by up to 2 decades; this variation depends on adsorption temperature and film thickness. Temperature-programmed desorptions at 3 K/min and 0.1 mPa up to 650 K always restore the films to their initial resistivity value. The AR-TPD curves are nearly independent of adsorption temperature. Deduced from a simple model, the desorption energy values are 0.31 + 0.05 eV for unetched films and 0.64 * 0.05 eV for etched films of thickness below 500 nm. Associated frequency factor values confirm that no isothermal oxygen desorption is observed even at 433 K.
1. Introduction
The influence of surface contamination on the electrical conductivity of thin semiconducting films is fundamental if the film surface is not passivated. Polycrystalline silicon being used in a variety of sensor applications [ 11,we have chosen to study the influence of oxygen on its electrical behaviour. Referring to our experience with other polycrystalline thin films (CdSe [2], Zn,GeO,N, [ 35]), we used a rigorous experimental procedure, involving previous outgassing, isothermal adsorption, isothermal desorption, and thermally-stimulated desorption. We have tested two sets of unintentionally-doped films: all were deposited on glass by the same VLPCVD technique but in two different furnaces: the first set was deposited in a furnace also used for in situ doped film deposition (A samples). For these samples, the thickness of the film is varied by etching an initial 600 nm-thick layer. The second set (B samples; thickness: 200 nm) was deposited in a furnace only used for undoped polysilicon deposition with a view to studying the influence of the nature of the glass substrate (Corning and Hoya) and of post-deposition annealing. We have also varied the adsorption temperature to give a set of 0924-4247/92/$5.00
experimental results large enough to verify the validity of a model, almost identical with those proposed for above-mentioned semiconductors.
2. Film preparation and measurement
conditions
2.1. Film deposition and annealing Films (A and B) were deposited at 823 K; the total pressure was 10 Pa and the silane flow was controlled to 50 seem (standard cubic centimeter per minute). This set temperature-pressure ensures the deposition of an amorphous layer at a 1.5 nm s-l rate. This kinetic is in accordance with the results of Wilke et al. [6]. Films were annealed in the same furnace for 12 h at 873 K to ensure film crystallisation [ 71. The various thicknesses were obtained by plasma reactive etching of the initial 600 nm-thick A film. The accuracy of the thickness measurements, made with a Talystep, is better than 10 nm. Parallel aluminium contacts (with 1.5% silicon) were vacuum deposited on the surface, then annealed for 30 min at 720 K in forming gas. The resistance of the contacts was verified by Z(V) plots at every temperature. The film area was 1.2 cm*. @I 1992 -
Elsevier Sequoia. All rights reserved
72
TEMPERATURE
(f) the same sequence is used for the further adsorption/desorption runs.
(“c)
#~,
3. Theoretical 3. I. Ionosorption and conductivity
104 1.5
2.0
2.5 lO'/T
3.0
3.5
(K.')
Fig. 1. Experimental resistance vs. temperature plots: (a) reference; (b) isothermal adsorption; (c) temperature stimulated desorption. Sample A, 500 nm, T, = 373 K.
2.2. Electrical measurements Samples were tested in a stainless steel chamber. A high-quality vacuum was obtained with a turbomolecular pump. Pure oxygen was introduced via a calibrated valve. The following sequence summarises the electrical measurements (see Fig. 1): and outgassing at 0.1 mPa (a) vacuum TM = 653 K in the dark, until the electrical resistance, R(T,), is constant; (b) after cooling, plotting R(T) during a linear temperature increase at a constant rate, b. After cooling again, a new plot of R(T) proves, if it is identical to the previous one, that the curve is a reference resistance/temperature characteristic curve; it is then called R,(T) ; (c) oxygen isothermal adsorptions are made at a constant temperature, T,, for 2 min (atmospheric pressure is reached). Oxygen is then evacuated by pumping. In less than 15 min (standard time), the residual pressure reaches 0.1 mPa; (d) after slight cooling, the film is submitted to a linear temperature increase at the same rate, b (T = TO+ bt). A AR - TPD curve (resistance variation during a temperature-programmed desorption) is obtained. If this curve becomes identical with the reference, R,(T), before T reaches TM, the adsorption/desorption process is apparently electrically reversible; by contrast, the process is not reversible in the experimental temperature range; (e) a new plot of R,(T) confirms the above assertion;
variations Whatever the structure of the semiconducting layer (monocrystalline, polycrystalline or amorphous), the surface offers unsatisfied bonds. Some of the dangling bonds or other surface defects can chemisorb gaseous species, some of them being able to chemisorb gaseous species strongly and to trap free carriers of the semiconducting layer. This process is called ionosorption [8]. The influence of the surface concentration of ionosorbed particles on the conductivity of an n-type monocrystalline semiconductor results from the existence of a surface space charge. If chemisorbed gas acts as an acceptor, the semiconductor surface is depleted, and the film conductance decreases [2,8-l 11. If we suppose that the carrier mobility is not particularly affected by the space-charge presence, the relative conductivity variations are given by: G-Go
o-o0 =-=-=
Go
00
Aa
_-- 1 N-
CO
xo ND
(1)
where: Go is the film conductance under vacuum without ionosorbed species; G the film conductance under vacuum after ionosorption; x0 the film thickness; N- the surface concentration of the chemisorbed species; ND the volumic concentration of ionized donors. This relation may be used for n-type polycrystalline semiconductors, provided that the above assumptions are satisfied, that there is a large number of grains, and that the film is homogeneous. 3.2. Evolution of the surface ionosorbed oxygen concentration during a desorption If the thermodynamic equilibrium condition is satisfied, the surface concentration of the species remaining ionosorbed at the film surface under vacuum obeys the following relation, for monoenergetical ionosorption sites:
dN dt
= -N-v
exp
73
where W is the desorption energy and v the frequency factor. Assuming W and v are independent of time and temperature during experiments, the variations of the ionosorbed species are: N-=NO-exp[-vrexp(-g)] for an isothermal
desorption
(3) at T,,
W=N0-exp{-X$[T2exp(-g)
- G’exp(
-g)]}
for a thermally stimulated desorption starting at T,, (T = To + bT) where N,,- is the initial concentration of ionosorbed species and b is the temperature rate. Consequently, the conductivity variation rate may be written:
for an isothermal
desorption
at T,,
Fig. 2. Cross-section micrograph of a 500 nm A sample. The grains arc large and randomly distributed within the layer; the first 100 nm above-glass substrate consist of smaller grains.
4.2. Reference curves The reference curves (Fig. 1) approximately follow an Arrhenius plot in the 450-650 K temperature range. Samples offer a variety of mean activation energies (E), depending on their thickness: E = 0.32 + 0.05 eV for samples of thickness greater than 300 nm (etched or not) E = 0.53 f 0.05 eV for samples of thickness less than 200 nm (etched (A) or unetched (B))
for a thermally-stimulated desorption starting at To. [AcJ/cJ~]~~~is the adsorption conductivity rate. The parameters b and To are experimentally controlled, while [Acr/crOladsis a measured quantity. The values of W and v are deduced from the fit of experimental and theoretical curves of
[~~/~ol/[~~/~olacis. 4. Experimental results and discussion 4.1. Film structure The cross section micrograph of an unetched A sample (Fig. 2) shows that the grains are large and randomly distributed inside the layer; the first 100 nm of the above-glass substrate consist of smaller grains.
The reference resistivity ( lo4 fl cm for the unetched A sample) and the slopes of In R,( l/T) indicate clearly that the films are slightly n-doped by the residual doping gas (phosphine) in the furnace (also used for in situ dopings) or in the gas feed pipes. Referring to Lu et al. [ 121, we estimate N o w 10’6cln3. 4.3. Isothermal adsorptions and desorptions For all the samples we have studied that were subjected to oxygen adsorption under standard conditions (2 min, atmospheric pressure, temperature fluctuations less than +2 K), the resistance increase exactly follows the pressure variation and is almost instantaneous. No resistance variation was observed during the 15 min pumping following adsorption and before thermally-stimulated desorption; a correlation with energy and pre-exponential factors will confin. these observations.
TEMPERATURE
o TA +
TA
I
373 K
= 433
SAMPLE
SAMPLE A x0 -500 nm
K
V
A
. 100 200 300 400 500 600 ;
THICKNESS Fig. 3. Plot of [Au/u,Jads+,
(“C)
( am
= N-/N;,,
-44 1.5
)
The plot [A~/G,]~~~x~= N-/N& =f(x,,) (Fig. 3) shows a very good linear dependence with film thickness. It may be concluded that the surface density of ionosorbed species is proportional to the layer volume, then to the concentration of free electrons (since A films are slightly n-type). This limitation of ionosorption capability by the number of free electronic carriers is a plausible hypothesis: (i) assuming n w ND z lOI cmP3 we can estimate the order of magnitude in the surface concentration of ionosorbed species: No- x 10” cme2 (sample A, 600 nm, TA = 373 K), that is probably lower than the density of available ionosorption sites [9]; (ii) this limitation does not exist for much higher free-electron densities, as we have shown for CdSe where n = 10” cmP3 [3]. 4.4. Temperature programmed desorptions The typical shape of a AR - TPD curve is given in Fig. 1 for a temperature rate b = 3 f 0.2 K/min. All AR-TPD curves join the reference curve (R,(T)) at a temperature less than TM, indicating that the adsorption/desorption process is electrically reversible. From these curves, parameters W and v were deduced by fitting the theoretical and experimental plots of ln{[Aa/ao]/[A~/oolads) = ln{N-IN.&,) versus reciprocal temperature. There is no ambiguity in the determination of these parameters, since W determines, essentially, the curvature radius of the plot, while v introduces a horizontal shift. Figure 4 reproduces the experimental and theoretical plots of ln(N-IN&} =f(l/T) and the AR - TPD curve for one A sample. The good
K
w = .58
e”
”
2.0
vs. thickness.
2.5
103/T Fig. (-)
T., ~373 =
160
lo1 S‘
3.0
i
IO4
3;
5.
(K”)
4. Comparison betweenexperimental (. ‘) and theoretical plots of In{N-IN,,) =f(l/r) (a) and AR - TP (b).
agreement between theory and experiment observed in this Figure also exists for all the other samples and adsorption conditions. The existence of a single type of ionosorption site is the main conclusion of this study. It is also true for adsorptions that we have made at ambient or higher temperatures. Table 1 displays the W and v values according to adsorption temperature and film thickness. Oxygen desorption energy is nearly constant and equals about 0.6 eV for etched samples of thickness below 500 nm, while its value is 0.3 eV for unetched samples (A and B) at TA = 373 K. The lightly-etched A sample (580 nm thick) gives the same value, 0.3 eV. Desorption energies increase very slightly with adsorption temperature. A simple numeric calculation with eqn. (3) and with Wand v values given in Table 1 demonstrates TABLE factor
1. Fitting
values of desorption
Sample thickness (nm)
energy and pre-exponential
600
580
500
300
200
100
50
0.3 0.33
0.3 0.3
0.58 0.58
0.64 0.69
0.6 0.62
0.62 0.65
0.59 0.6
0.45 0.45
0.2 0.2
160 160
1000 3000
250 370
250 600
60 60
Sample A:
Desorption energy (eV) T, = 373 K TA = 433 K Frequency factor (s-l) T, = 373 K TA = 433 K Samples Bl, B2: Desorption energy (eV) Frequency factor (s-l) T, = 373K
0.29 0.17
75
that an isothermal desorption at 373 K needs between 6 and 400 h to release 90% of the initial ionosorbed species. These values corroborate the resistance constancy observed during the 15 min pumping after adsorption. It contributes to the validation of the modeling.
Martinez of CELAR, Bruz, for the scanning electron micrographs, and to Professors Colin, Coulouarn and Bourgeois of I.U.T., Rennes for valuable discussions concerning the manuscript.
References
5. Conclusions The VLPCVD polysilicon films deposited on unintentionallyor lightly-doped glass substrate show a single behaviour for the adsorption of pure molecular oxygen: (i) quick variation of electrical resistance; (ii) no isothermal desorption over a time of a few minutes; (iii) single AR - TPD curve, whatever the adsorption temperature; (iv) fully electrical reversibility of the adsorption/temperature programmed desorption, provided the film is sufliciently annealed. The simple analytical modeling we have developed previously for other thin semiconducting films still applies (CdSe, Zn, GeO,N,). Oxygen ionosorption sites existing on the film surface seem films, with a W= unique for as-deposited 0.31 k 0.05 eV desorption energy, while etched films offer nearly single sites but of higher desorption energy (W = 0.64 A 0.05 eV). The set of values W, v justify that there is no isothermal desorption in the 273-473 K temperature range and that a thermally-stimulated desorption allows a full outgassing of the film.
Acknowledgements The authors are grateful to Dr Loisel of CNET Lannion B who supplied the B samples, to M.
1 T. I. Kamins, Design properties of polycrystalline silicon, Sensors and Actuators, A21-A23 (1990) 817-824. 2 F. Raoult, B. Fortin and Y. Colin, Standardization and stabilization of the resistivity-temperature characteristics of CdSe thin films by vacuum annealing, Thin Solid Films, 182 (1989) l-14. 3 B. Fortin, F. Raoult, G. Rosse, Y. Cohn and M. El Khadir, Analysis of the polycrystalline film electrical resistance variation due to isothermal desorption and temperature stimulated desorption of oxygen, in J. MBller, H. Strunk and J. Werner (eds.), Polycrystalline Semiconductors, Springer, Berlin, 1990, pp. 139145. 4 M. Ghers, G. Rosse, J. Guyader, Y. Laurent and Y. Colin, Oxygen ionosorption on compressed semiconducting powders of zinc and germanium oxynitride, Sensors and Actuators, 13 (1988) 263-213. 5 G. Rosse, M. Ghers, J. Lebigot, J. Guyader, Y. Laurent and Y. Colin, Selective detection of ammonia by semiconducting pellets of zinc and germanium oxynitride, Sensors and Actuators, 24 (1988) 133-141. 6 T. E. Wilke, K. A. Turner and C. J. Takoudis, Chemical vapor deposition of silicon under reduced pressure in hot-wall reactors, Chem. Eng. Sci., 41 (1986) 643-650. 7 P. Joubert, B. Loisel, Y. Chouan and L. Haji, The effect of low pressure on the structure of LPCYD polycrystalline silicon films, J. Elecrrochem. Sot., 134 (1987) 2541-2545. 8 S. R. Morrison, The Chemical Physics of Surfaces, Chs. 2 and 7, Plenum, New York, 1977. 9 A. Many, Relation between physical and chemical processes on semiconductor surfaces, C.R.C. Critical Review in Solid State Science, May, 1974, pp. 515-539. 10 E. Guesne, C. Sebenne and L. Balkanski, Electronic surface properties of CdSe single crystals under vacuum or oxygen, Surf. Sci., 24 (1971) 18-30. 11 W. Gbpel, Chemisorption and charge transfer at ionic semiconductor surfaces: implications in designing gas sensors, Prog. Surf. Sci., 20(l) (1985) 9-103. 12 N. C. C. Lu, L. Gerzberg, C. Y. Lu and J. D. Meindl, Modeling and optimization of monolithic polycrystalline silicon resistor, IEEE Trans. Electron Devices, ED-28 (1981) 8188830.
Sensors and Acruators A, 33 (1992) 71-75
Oxygen adsorption and electrical behaviour chemical-vapour-deposited polysilicon films B. Fortin,
D. Mostefa, F. Raoult,
H. Lhermite
of thin, very-low-pressure
and M. Sarret
Groupe de MicrotYectronique, LJniversit6de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex (France)
Abstract We report here the results of experiments on the influence of oxygen adsorption on the conductivity of very-low-pressure chemically-deposited polysilicon thin films of various thicknesses obtained by plasma reactive etching a 600 nm initial film. It is observed that oxygen acts as an acceptor on the unintentionally-doped as on the lightly-doped polysilicon films, provided they are adequately annealed. Oxygen adsorptions made over 2 min at atmospheric pressure increase the film resistance variation by up to 2 decades; this variation depends on adsorption temperature and film thickness. Temperature-programmed desorptions at 3 K/min and 0.1 mPa up to 650 K always restore the films to their initial resistivity value. The AR-TPD curves are nearly independent of adsorption temperature. Deduced from a simple model, the desorption energy values are 0.31 + 0.05 eV for unetched films and 0.64 * 0.05 eV for etched films of thickness below 500 nm. Associated frequency factor values confirm that no isothermal oxygen desorption is observed even at 433 K.
1. Introduction
The influence of surface contamination on the electrical conductivity of thin semiconducting films is fundamental if the film surface is not passivated. Polycrystalline silicon being used in a variety of sensor applications [ 11,we have chosen to study the influence of oxygen on its electrical behaviour. Referring to our experience with other polycrystalline thin films (CdSe [2], Zn,GeO,N, [ 35]), we used a rigorous experimental procedure, involving previous outgassing, isothermal adsorption, isothermal desorption, and thermally-stimulated desorption. We have tested two sets of unintentionally-doped films: all were deposited on glass by the same VLPCVD technique but in two different furnaces: the first set was deposited in a furnace also used for in situ doped film deposition (A samples). For these samples, the thickness of the film is varied by etching an initial 600 nm-thick layer. The second set (B samples; thickness: 200 nm) was deposited in a furnace only used for undoped polysilicon deposition with a view to studying the influence of the nature of the glass substrate (Corning and Hoya) and of post-deposition annealing. We have also varied the adsorption temperature to give a set of 0924-4247/92/$5.00
experimental results large enough to verify the validity of a model, almost identical with those proposed for above-mentioned semiconductors.
2. Film preparation and measurement
conditions
2.1. Film deposition and annealing Films (A and B) were deposited at 823 K; the total pressure was 10 Pa and the silane flow was controlled to 50 seem (standard cubic centimeter per minute). This set temperature-pressure ensures the deposition of an amorphous layer at a 1.5 nm s-l rate. This kinetic is in accordance with the results of Wilke et al. [6]. Films were annealed in the same furnace for 12 h at 873 K to ensure film crystallisation [ 71. The various thicknesses were obtained by plasma reactive etching of the initial 600 nm-thick A film. The accuracy of the thickness measurements, made with a Talystep, is better than 10 nm. Parallel aluminium contacts (with 1.5% silicon) were vacuum deposited on the surface, then annealed for 30 min at 720 K in forming gas. The resistance of the contacts was verified by Z(V) plots at every temperature. The film area was 1.2 cm*. @I 1992 -
Elsevier Sequoia. All rights reserved
72
TEMPERATURE
(f) the same sequence is used for the further adsorption/desorption runs.
(“c)
#~,
3. Theoretical 3. I. Ionosorption and conductivity
104 1.5
2.0
2.5 lO'/T
3.0
3.5
(K.')
Fig. 1. Experimental resistance vs. temperature plots: (a) reference; (b) isothermal adsorption; (c) temperature stimulated desorption. Sample A, 500 nm, T, = 373 K.
2.2. Electrical measurements Samples were tested in a stainless steel chamber. A high-quality vacuum was obtained with a turbomolecular pump. Pure oxygen was introduced via a calibrated valve. The following sequence summarises the electrical measurements (see Fig. 1): and outgassing at 0.1 mPa (a) vacuum TM = 653 K in the dark, until the electrical resistance, R(T,), is constant; (b) after cooling, plotting R(T) during a linear temperature increase at a constant rate, b. After cooling again, a new plot of R(T) proves, if it is identical to the previous one, that the curve is a reference resistance/temperature characteristic curve; it is then called R,(T) ; (c) oxygen isothermal adsorptions are made at a constant temperature, T,, for 2 min (atmospheric pressure is reached). Oxygen is then evacuated by pumping. In less than 15 min (standard time), the residual pressure reaches 0.1 mPa; (d) after slight cooling, the film is submitted to a linear temperature increase at the same rate, b (T = TO+ bt). A AR - TPD curve (resistance variation during a temperature-programmed desorption) is obtained. If this curve becomes identical with the reference, R,(T), before T reaches TM, the adsorption/desorption process is apparently electrically reversible; by contrast, the process is not reversible in the experimental temperature range; (e) a new plot of R,(T) confirms the above assertion;
variations Whatever the structure of the semiconducting layer (monocrystalline, polycrystalline or amorphous), the surface offers unsatisfied bonds. Some of the dangling bonds or other surface defects can chemisorb gaseous species, some of them being able to chemisorb gaseous species strongly and to trap free carriers of the semiconducting layer. This process is called ionosorption [8]. The influence of the surface concentration of ionosorbed particles on the conductivity of an n-type monocrystalline semiconductor results from the existence of a surface space charge. If chemisorbed gas acts as an acceptor, the semiconductor surface is depleted, and the film conductance decreases [2,8-l 11. If we suppose that the carrier mobility is not particularly affected by the space-charge presence, the relative conductivity variations are given by: G-Go
o-o0 =-=-=
Go
00
Aa
_-- 1 N-
CO
xo ND
(1)
where: Go is the film conductance under vacuum without ionosorbed species; G the film conductance under vacuum after ionosorption; x0 the film thickness; N- the surface concentration of the chemisorbed species; ND the volumic concentration of ionized donors. This relation may be used for n-type polycrystalline semiconductors, provided that the above assumptions are satisfied, that there is a large number of grains, and that the film is homogeneous. 3.2. Evolution of the surface ionosorbed oxygen concentration during a desorption If the thermodynamic equilibrium condition is satisfied, the surface concentration of the species remaining ionosorbed at the film surface under vacuum obeys the following relation, for monoenergetical ionosorption sites:
dN dt
= -N-v
exp
73
where W is the desorption energy and v the frequency factor. Assuming W and v are independent of time and temperature during experiments, the variations of the ionosorbed species are: N-=NO-exp[-vrexp(-g)] for an isothermal
desorption
(3) at T,,
W=N0-exp{-X$[T2exp(-g)
- G’exp(
-g)]}
for a thermally stimulated desorption starting at T,, (T = To + bT) where N,,- is the initial concentration of ionosorbed species and b is the temperature rate. Consequently, the conductivity variation rate may be written:
for an isothermal
desorption
at T,,
Fig. 2. Cross-section micrograph of a 500 nm A sample. The grains arc large and randomly distributed within the layer; the first 100 nm above-glass substrate consist of smaller grains.
4.2. Reference curves The reference curves (Fig. 1) approximately follow an Arrhenius plot in the 450-650 K temperature range. Samples offer a variety of mean activation energies (E), depending on their thickness: E = 0.32 + 0.05 eV for samples of thickness greater than 300 nm (etched or not) E = 0.53 f 0.05 eV for samples of thickness less than 200 nm (etched (A) or unetched (B))
for a thermally-stimulated desorption starting at To. [AcJ/cJ~]~~~is the adsorption conductivity rate. The parameters b and To are experimentally controlled, while [Acr/crOladsis a measured quantity. The values of W and v are deduced from the fit of experimental and theoretical curves of
[~~/~ol/[~~/~olacis. 4. Experimental results and discussion 4.1. Film structure The cross section micrograph of an unetched A sample (Fig. 2) shows that the grains are large and randomly distributed inside the layer; the first 100 nm of the above-glass substrate consist of smaller grains.
The reference resistivity ( lo4 fl cm for the unetched A sample) and the slopes of In R,( l/T) indicate clearly that the films are slightly n-doped by the residual doping gas (phosphine) in the furnace (also used for in situ dopings) or in the gas feed pipes. Referring to Lu et al. [ 121, we estimate N o w 10’6cln3. 4.3. Isothermal adsorptions and desorptions For all the samples we have studied that were subjected to oxygen adsorption under standard conditions (2 min, atmospheric pressure, temperature fluctuations less than +2 K), the resistance increase exactly follows the pressure variation and is almost instantaneous. No resistance variation was observed during the 15 min pumping following adsorption and before thermally-stimulated desorption; a correlation with energy and pre-exponential factors will confin. these observations.
TEMPERATURE
o TA +
TA
I
373 K
= 433
SAMPLE
SAMPLE A x0 -500 nm
K
V
A
. 100 200 300 400 500 600 ;
THICKNESS Fig. 3. Plot of [Au/u,Jads+,
(“C)
( am
= N-/N;,,
-44 1.5
)
The plot [A~/G,]~~~x~= N-/N& =f(x,,) (Fig. 3) shows a very good linear dependence with film thickness. It may be concluded that the surface density of ionosorbed species is proportional to the layer volume, then to the concentration of free electrons (since A films are slightly n-type). This limitation of ionosorption capability by the number of free electronic carriers is a plausible hypothesis: (i) assuming n w ND z lOI cmP3 we can estimate the order of magnitude in the surface concentration of ionosorbed species: No- x 10” cme2 (sample A, 600 nm, TA = 373 K), that is probably lower than the density of available ionosorption sites [9]; (ii) this limitation does not exist for much higher free-electron densities, as we have shown for CdSe where n = 10” cmP3 [3]. 4.4. Temperature programmed desorptions The typical shape of a AR - TPD curve is given in Fig. 1 for a temperature rate b = 3 f 0.2 K/min. All AR-TPD curves join the reference curve (R,(T)) at a temperature less than TM, indicating that the adsorption/desorption process is electrically reversible. From these curves, parameters W and v were deduced by fitting the theoretical and experimental plots of ln{[Aa/ao]/[A~/oolads) = ln{N-IN.&,) versus reciprocal temperature. There is no ambiguity in the determination of these parameters, since W determines, essentially, the curvature radius of the plot, while v introduces a horizontal shift. Figure 4 reproduces the experimental and theoretical plots of ln(N-IN&} =f(l/T) and the AR - TPD curve for one A sample. The good
K
w = .58
e”
”
2.0
vs. thickness.
2.5
103/T Fig. (-)
T., ~373 =
160
lo1 S‘
3.0
i
IO4
3;
5.
(K”)
4. Comparison betweenexperimental (. ‘) and theoretical plots of In{N-IN,,) =f(l/r) (a) and AR - TP (b).
agreement between theory and experiment observed in this Figure also exists for all the other samples and adsorption conditions. The existence of a single type of ionosorption site is the main conclusion of this study. It is also true for adsorptions that we have made at ambient or higher temperatures. Table 1 displays the W and v values according to adsorption temperature and film thickness. Oxygen desorption energy is nearly constant and equals about 0.6 eV for etched samples of thickness below 500 nm, while its value is 0.3 eV for unetched samples (A and B) at TA = 373 K. The lightly-etched A sample (580 nm thick) gives the same value, 0.3 eV. Desorption energies increase very slightly with adsorption temperature. A simple numeric calculation with eqn. (3) and with Wand v values given in Table 1 demonstrates TABLE factor
1. Fitting
values of desorption
Sample thickness (nm)
energy and pre-exponential
600
580
500
300
200
100
50
0.3 0.33
0.3 0.3
0.58 0.58
0.64 0.69
0.6 0.62
0.62 0.65
0.59 0.6
0.45 0.45
0.2 0.2
160 160
1000 3000
250 370
250 600
60 60
Sample A:
Desorption energy (eV) T, = 373 K TA = 433 K Frequency factor (s-l) T, = 373 K TA = 433 K Samples Bl, B2: Desorption energy (eV) Frequency factor (s-l) T, = 373K
0.29 0.17
75
that an isothermal desorption at 373 K needs between 6 and 400 h to release 90% of the initial ionosorbed species. These values corroborate the resistance constancy observed during the 15 min pumping after adsorption. It contributes to the validation of the modeling.
Martinez of CELAR, Bruz, for the scanning electron micrographs, and to Professors Colin, Coulouarn and Bourgeois of I.U.T., Rennes for valuable discussions concerning the manuscript.
References
5. Conclusions The VLPCVD polysilicon films deposited on unintentionallyor lightly-doped glass substrate show a single behaviour for the adsorption of pure molecular oxygen: (i) quick variation of electrical resistance; (ii) no isothermal desorption over a time of a few minutes; (iii) single AR - TPD curve, whatever the adsorption temperature; (iv) fully electrical reversibility of the adsorption/temperature programmed desorption, provided the film is sufliciently annealed. The simple analytical modeling we have developed previously for other thin semiconducting films still applies (CdSe, Zn, GeO,N,). Oxygen ionosorption sites existing on the film surface seem films, with a W= unique for as-deposited 0.31 k 0.05 eV desorption energy, while etched films offer nearly single sites but of higher desorption energy (W = 0.64 A 0.05 eV). The set of values W, v justify that there is no isothermal desorption in the 273-473 K temperature range and that a thermally-stimulated desorption allows a full outgassing of the film.
Acknowledgements The authors are grateful to Dr Loisel of CNET Lannion B who supplied the B samples, to M.
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