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Mo structures
Solid-State Electronics Vol. 36, No. 6, pp. 921-932, 1993 Printed in Great Britain. All rights reserved
copyright
0038-I m/93 $5.00 + 0.00 0 1993 Pcrgamon Pms Ltd
INFLUENCE OF SULFUR CONCENTRATION ON THE CURRENT-VOLTAGE CHARACTERISTICS OF Au/GaAs,, _ ,..S, /MO STRUCTURES B~KEYI SCKXIYOU,FR~DBRIC LALANDEand HERV~ CARCHANO Electronique et Physicochimie des Couches Minces, Facultts des Sciences et Techniques de St Jkrbme, Service A62, 13397 Marseille Cedex 13, France (Received 29 January 1992; in revised form 14 July 1992) Abstract-We present the results of a study of current-voltage characteristics of Au/GaAs(, _&/MO test structures. The GaAs(, _XjS, semiconductor film is deposited by radio frequency sputtering in a mixed (Ar, H,S) gas. We observe three types of characteristics whose shape can be explained by the grain boundaries and the metal-semiconductor interface states. For x values exceeding 0.16, the characteristics are of fully rectifying type. The increase in sulfur concentration induces a decrease in grain boundary activity and an improvement of the gold/semiconductor interface.
NOTATION
4
2 Et
Es E”
:: NS 4
T Rd Ri Ro R, V,
modified Richardson constant (4.4 A/K2 cm* for GaAs) interface index Boltzmann constant: k = 0.864 10e4 eV/K Fermi-level energy (eV) band-gap energy (ev) valence band edge energy (eV) ideality factor grain boundaries number interface states density (cme2 . eV-‘) electron charge (q = 1.6. 1O-‘9C) temperature (K) dynamic resistance [slope of current-voltage curve Z(V) at +0.35v] dynamic resistance [slope of Z(V) curve at -0.35 v] dynamic resistance [slope of Z(V) curve at 0 v] series resistance potential drop, for a given Z, between the log(Z) carve and its linear extrapolation molecular fraction interface film thickness interface film permittivity barrier height metal work function Fermi level position measured from the valence band semiconductor electron affinity
INTRODUCTION Devices based on thin gallium arsenide films are efficient for electronic applications, especially for photovoltaic conversion[ 11. Fabricating these devices from thin polycrystalline films of gallium arsenide a few micrometers in thickness is economically interesting. But we are confronted with many problems, obtaining a stoichiometric composition[2,3], and a good crystallization reproducible under industrial conditions is one of the major problems. By incorporating sulfur in the films by radio frequency sputtering, during their elaboration, we obtain a compound semiconductor material: GaAs(, _ 3 S, (0 < x < 0.26) SSE 36/E-1
with a stable
concentration of gallium equal to 50%[4]. The sulfur concentration x can be controlled by the H2S partial pressure for the given deposition conditions. In this paper, we study the influence of sulfur concentration on the Z(V) characteristics of Au/ GaAs(, _ J)$/MO devices. These characteristics can be symmetric, semi-rectifying or of a Schottky type., depending on the sulfur concentration. EXPERIMENTAL METHOD
The films are prepared by R.F. sputtering of a polycrystalline GaAs target. Sulfur is incorporated in the films by adding hydrogen sulphide H,S (99.995%) to the sputtering gas. The experimental chamber is initially pumped down to a 10e6 torr pressure, then H2S is introduced up to a tixed pressure and, finally, ultra high purity argon (99.9997%), used as sputtering gas, is introduced up to working pressure. The hydrogen sulphide and argon flows are separately controlled by a massic flowmeter MKS 247C. During the experiments, the total working pressure of the mixed gas (Ar, H,S) in the bell-jar is kept constant and equal to 30mtorr. This total pressure and the H,S partial pressure are measured with a MKS Baratron capacitive gauge. This gauge is indifferent to the nature of the gas and does not require correcting coefficients. The electrical power applied to the target ranges between 95 and 100 W so that the d.c. cathode bias reaches a value of about 2 kV. The substrate-target distance is about 30 mm. The substrate is a molybdene foil mechanically polished with a diamond compound and chemically cleaned. During the deposition process, the substrate temperature is kept constant at about 650°C. The atomic concentration of sulfur, x, is determined by EDAX analysis. When the H,S partial 927
BBKEYISOGOYOUet al.
928 IW) 2 1
2 x = 0.11
0
-2 -1 E
I
Fig. 1. Plots of current-voltage I(V) characteristics of Au/GaAs,, x,S, /MO test structures. (I) Symmetrical I(V) characteristic (x = 0.02). (2) Dissymmetrical I(V) characteristic without reverse current saturation (x = 0.11). (3) Dissymmetrical I(V) characteristic with reverse current saturation (x = 0.18).
pressure increases from 0 to 1.1 mtorr, x increases from 0 to 0.26. Electrical characterization is made with a metalsemiconductor-metal structure. For this purpose, circular gold electrodes 4 mm in diameter are evaporated onto GaAs,, _ _XI S, film. Since this evaporation is done in another vacuum chamber, the film surface is contaminated by room atmosphere. We have observed that this contamination is independent of the time of exposure to the room atmosphere. With different exposure times, no change can be noted in electrical characteristics. A Hewlett Packard 4140B pAmeter-d.c. voltage source, connected to a microcomputer, is used for dark current-voltage Z(V) measurements.
EXPERIMENTAL
RESULTS
Z(V) plot shows three types of characteristics (Fig. 1): symmetrical (l), dissymmetrical without reverse current saturation (2) and dissymmetrical with reverse current saturation (3). Each I(V) curve shape can be associated to a sulfur concentration range and a particular film texture as shown on Fig. 2-(l), 2-(2) 3-(3), respectively. To analyse the shape of Z(V) characteristics, we computed three different dynamic resistances, R,, R,, and R,, corresponding to the slope values of -0.35, 0 and +0.35 V, respectively. These resistance values depend on the bulk resistance and the contact resistance values. We must note that R, is not always equal
Fig. 2. (a) Caption on facing page.
Influence of sulfur on Au/GaAs(, _ YjS,/Mo structures
929
Fig. 2. Electron scanning microscopy photographs (a+) of GaAs,, _ dSV films surface textures. (a) x = 0.02; (b) x = 0.11; (c) x = 0.24.
to the series resistance value, R,,because for several samples the characteristic is not yet linear at about 0.35 V; consequently R, can be of the same order as or higher than R,. In Fig. 3, we plotted the variation of these three resistances as a function of the atomic concentration of sulfur in the deposited film and we noted the order of magnitude of film thickness. These resistances
increase with sulfur concentration. We note that the high resistance values correspond to thin films; so, it can be supposed that this increase is not due to thickness. It is not possible to separate the bulk effect from the contact effect. Moreover, as previously noted, r,, is not the series resistance. It is difficult to attribute a physical signification to this resistance increase. Nevertheless, we could use these curves
B&KEYISOGOYOUet al.
930
0.2
0.1
1 (A)
0 0
4.0 10-h
8.0 10-6
1.2 10-5
Fig. 5. Plot of V, vs I.
10'
Fig. 3. Variations of dynamic resistances Ri, & and R, (corresponding to the Z(V) slope values at -0.35, 0 and f0.35 V respectively) with X. (Fig. 3) to characterize the Z(V) shape by comparing the Ri, R,, and Z& values for a given sample. We can then distinguish three cases: (1) x < 0.04: the Z(V) characteristics are exclusively symmetrical and we find Ri = R,, c R,,. (2) 0.04 < x < 0.16: the characteristics are dissymmetric without reverse current saturation. In the direct part of the characteristic, the current is higher than in the reverse part. Thus we have: R,, $ R, > Rd. The rectifying effect starts and increases with the sulfur concentration. (3) x > 0.16: in this case, the structures are entirely of the rectifying type and & = 4 g Rd. For x higher than 0.11, the characteristics are rectifying enough to allow the use of log(Z) vs I/ curves to estimate the value of the reverse saturation current Z, and the value of TV, the ideality factor, from the eqn (1): Z = Z,[exp(qV/@T) - l] (1) where q is the electron charge (q = 1.6 . lo-” C) and k the Boltzmann constant (k = 1.38. 10Tz3J/K). In Fig. 4, we can see that at high voltages, for a given Z, there is a horizontal shift, V,, between the experimental curve and the extrapolation of the linear
region. This displacement is due to the series resistance R, effect[S]; R, can be determined by plotting L’, against Z, as shown in Fig. 5. In the more unfavourable case, sample with x = 0.26, we found & equal to about 39 k.Q. This shows that the effect of R, greatly influences the electrical conduction when x is high. So, for the graphical determination of parameters such as Z,, n and &,, we use exclusively the range 100-200 mV in the log(Z) curves, where V, is negligible compared to the external applied tension. In this case, eqn (1) gives a good description of the electrical conduction phenomenon. In the hypothesis of thermoionic emission model, the barrier height & of the metal-semiconductor junction can be calculated from the Z, value: Z, = SA, TZ exp( - &kT),
(2)
where T is the absolute temperature, S the junction surface (cm’) and A, the semiconductor effective Richardson constant. The variations of the ideality factor rl and of the barrier height &, as a function of the sulfur concentration x are plotted in Fig. 6(A) and (B). The TV values higher than 2 are generally attributed to Au/GaAs-polycrystalline junctions which do not correspond to the thermoionic emission model. In the case, the calculated barrier height takes into account the non thermoionic effects. It particulary takes into account the effects due to interface states of the junction. Therefore, we can suppose that, as n decreases and +,, rises linearly with the increasing concentration of sulfur [Fig. 6(A, B)], there is an improvement of the Au/GaAs(, _ X1SX junction. DISCUSSION
(a) Symmetrical characteristics
0
0.2
0.4
0.6
Fig. 4. Plot of log(Z) vs V.
0.8 V(V)
When the sulfur concentration is below 2% (x < 0.04), the structure has symmetrical characteristics similar to those of the Schottky’s symmetrical barrier (S.S.B.). The corresponding model has been developed by Tamg[6] and applied to polycrystalline gallium arsenide by Hwang et al.[7j.
931
Influence of sulfur on Au/GaAs(, _X,S,/Mo structures @b (eV)
8
“‘1’1’ 6
1-
IIIl”I
n “0.1
0.2
6
X
0.1
‘I”“’ 0.2
X
Fig. 6. Plots of the ideality factor rl(A) and the barrier height $,(B) vs X. The small values of R,, R,, and & suggest that there is no barrier at the metal/semiconductor junction. This may be attributed to the distortion of the energy bands due to the accumulation of electrical charges in the film interface[8]. In the S.S.B. model, the current Z, flowing through a film with N identical grain boundaries, is given by the following relation: Z = 2A, T2 exp[ -q$,/kT]sinh[qV/2NklJ].
(3)
In Fig. 7, we can see the coincidence of the experimental plot with the plot calculated from relation (3), with N = 4 and 4,, = 0.45 eV. The experimental curve was plotted with a semiconductor layer 5.5 pm thick and composed of crystallites 1 micrometer in average size. So, it can be reasonably supposed that this layer, with columnar tendency structure, has 5 grains between both electrodes. The conformity of Z(V) characteristics with the Schottky symmetrical barrier model for x < 0.04 indicates that conduction processes are essentially governed by grain boundaries effects. The dissymmetrical shape appears and the test structure becomes more and more rectifying when the
x value exceeds 0.04. The grain size and consequently the grain boundaries density do not significantly change with the sulfur concentration. Therefore, we may associate the decrease in grain boundaries electrical activity with the increase in sulfur concentration. (13)Rectifying type characteristics In order to explain the variation of the barrier height & observed for the rectifying type characteristics [Fig. 5(B)], we must take into account the interface states effects. According to Crowel and Sze[8], in the presence of interface states, the barrier height is approximately given by: &(eV) = C2(cP, - x,) + (1 - CW,
- +J
(4)
the metal work function (for Au, with 4,. 4, = 5.1 eV,[9,10]); x., the semiconductor electron affinity; q&, the position of Fermi level if the metal were absent, measured from the valence band; & = Er - E,, where E, and E, are the Fermi level energy and the valence band energy, respectively; Es, the semiconductor gap (Es = 1.43 eV for GaAs); C,, the interface index at the metal-semiconductor junction, given by the following relation: C, = ci/[ci + q2SN$]
(5)
where ci is the interface film permittivity, 6 the film thickness, and N, the interface states density. For an ideal Schottky junction, the interface states density is zero (N, = 0), C, takes the value one and we have: &=dL-&,
-0.4
-0.2
0
0.2
0.4 v (V)
Fig. 7. Symmetrical current-voltage characteristic of an Au/GaAs,, _ .,%/M• test structure with x = 0.026. Theoreti&l %o]nts are calculated from: Z = 2A, T2expI-&lkTlsinhlaV/2NkTl: with: N = 4: T=298K: ;b, :&45kV, gold electr&ie diameter: 4 mm.’0: experimen: tal plot; x : theoretical plot.
which is the expression of the classical barrier height. In the case of a structure with high interface states, N, is very high and C, is equal to zero so that the barrier height is independent of the used metal. Spicer[ 111places the GaAs (n doped) Fermi level at about 0.75 eV from the top of the valence band. As mentioned in the experimental method paragraph, the Au/GaASt, _X$Xinterface is contaminated, so we can assume that the interface states density N,
BEKEYI SOGOYOU
932
et
al. CONCLUSION
0.1
0
0
0.1
Fig. 8. Au/GaAs(,
_,,S, interface
0.2
index C, variation
0.3
x
with x.
remains always high enough for the Fermi level pinning at 0.75eV. With this hypothesis, we have: Eg - qb,,= 0.68 eV.
(6)
The electron affinities of GaAs and GaS are very close (~,=4.07eV for GaAs, xs = 4.0eV for GaS[12]); As xs is assumed to vary linearly in the compounds[ 131,for low values of x(0 < x < 0.26) we keep for GaAsu _XjSXelectron affinity, the same value as GaAs in the following relationship of C,: C* = [db - (& - &)1/K& - X,) - (E, - &Jl,
(7)
with O
(8)
C, =2.3x - 0.28,
(9)
Then: withO
The electrical characterization of Au/GaAs(, _rjSx/ MO structures shows the presence of a threshold at x = 0.12 beyond which the interface index stops being equal to zero. In the used experimental conditions, it corresponds to the evolution of a bad quality polycrystalline GaAs(, _ ,rISXfilm leading to symmetrical I(V) characteristics (corresponding to the Schottky symmetrical barrier model) into a polycrystalline GaAs(, _ xIS, film with a low grain boundary activity and a good Au/GaAs+,,S, Schottky junction with a significant barrier height. This work shows that the continuous increase in the film amount of sulfur leads to a progressive amelioration of the Au/GaAS(, _ .,S, junction. This improvement may be compared with the results observed in the GaAs surface passivation by sulfur treatment[l4-161. Acknowledgements-This work was supported by (Electricite de France) and PIRSEM (CNRS-France).
EDF
REFERENCES
1. M. Rodot, M. Barbe and Dixier, Revue Phys. Appl. 12, 1223 (1977). 2. H. Carchano, F. Lalande and R. Loussier, Thin Solid Films 135, 107 (1985). 3. H. Carchano, F. Lalande and R. Loussier, Thin Solid Films 120, 47 (1984). 4. H. Carchano, F. Lalande and N. M’Barek, Proc. Int. Symp. on Trends and New Applications in Thin Films, Strasbourg, Vol. 1, p. 233. S.F.V., Paris (1987). 5. E. H. Rhoderick, Metal-Semiconductor Contacts, p. 121. Oxford Univ. Press (1978). 6. M. L. Tamg, J. appl. Phys. 49, 4069 (1978). 7. W. H. Wang, H. C. Card and E. S. Yang, Appl. Phys. ht. 36, 315 (1980). 8. C. R. Crowley and S. M. Sze, J. appl. Phys. 37, 2685 (1966). 9. H. B. Michaelson, J. appl. Phys. 48, 1589 (1977). 10. H. C. Card and E. H. Rhoderick, J. Phys. D4, 1589 (1971). 11. W. E. Spicer, T. Lindau, P. S. Keat and C. Y. Yu, J. Vat. Sci. Technol. 17, 1019 (1980). 12. R. H. Williams and A. J. McEvoy, Physica status solidi (a) 12, 277 (1972). 13. S. Adachi, J. appl. Phys. 58, Rl(1985). 14. J. R. Waldrop, Appl. Phys. Left. 47, 1301 (1985). 15. J. S. Herman and F. L. Terry, Appl. Phys. Lerr. 60, 716 (1991). 16. X. Y. Hou, W. 2. Cai, Z. Q. He, P. H. Hao, Z. S. Li, X. M. Durgant and X. Wang, Appl. Phys. Lett. l&2552 (1992).
copyright
0038-I m/93 $5.00 + 0.00 0 1993 Pcrgamon Pms Ltd
INFLUENCE OF SULFUR CONCENTRATION ON THE CURRENT-VOLTAGE CHARACTERISTICS OF Au/GaAs,, _ ,..S, /MO STRUCTURES B~KEYI SCKXIYOU,FR~DBRIC LALANDEand HERV~ CARCHANO Electronique et Physicochimie des Couches Minces, Facultts des Sciences et Techniques de St Jkrbme, Service A62, 13397 Marseille Cedex 13, France (Received 29 January 1992; in revised form 14 July 1992) Abstract-We present the results of a study of current-voltage characteristics of Au/GaAs(, _&/MO test structures. The GaAs(, _XjS, semiconductor film is deposited by radio frequency sputtering in a mixed (Ar, H,S) gas. We observe three types of characteristics whose shape can be explained by the grain boundaries and the metal-semiconductor interface states. For x values exceeding 0.16, the characteristics are of fully rectifying type. The increase in sulfur concentration induces a decrease in grain boundary activity and an improvement of the gold/semiconductor interface.
NOTATION
4
2 Et
Es E”
:: NS 4
T Rd Ri Ro R, V,
modified Richardson constant (4.4 A/K2 cm* for GaAs) interface index Boltzmann constant: k = 0.864 10e4 eV/K Fermi-level energy (eV) band-gap energy (ev) valence band edge energy (eV) ideality factor grain boundaries number interface states density (cme2 . eV-‘) electron charge (q = 1.6. 1O-‘9C) temperature (K) dynamic resistance [slope of current-voltage curve Z(V) at +0.35v] dynamic resistance [slope of Z(V) curve at -0.35 v] dynamic resistance [slope of Z(V) curve at 0 v] series resistance potential drop, for a given Z, between the log(Z) carve and its linear extrapolation molecular fraction interface film thickness interface film permittivity barrier height metal work function Fermi level position measured from the valence band semiconductor electron affinity
INTRODUCTION Devices based on thin gallium arsenide films are efficient for electronic applications, especially for photovoltaic conversion[ 11. Fabricating these devices from thin polycrystalline films of gallium arsenide a few micrometers in thickness is economically interesting. But we are confronted with many problems, obtaining a stoichiometric composition[2,3], and a good crystallization reproducible under industrial conditions is one of the major problems. By incorporating sulfur in the films by radio frequency sputtering, during their elaboration, we obtain a compound semiconductor material: GaAs(, _ 3 S, (0 < x < 0.26) SSE 36/E-1
with a stable
concentration of gallium equal to 50%[4]. The sulfur concentration x can be controlled by the H2S partial pressure for the given deposition conditions. In this paper, we study the influence of sulfur concentration on the Z(V) characteristics of Au/ GaAs(, _ J)$/MO devices. These characteristics can be symmetric, semi-rectifying or of a Schottky type., depending on the sulfur concentration. EXPERIMENTAL METHOD
The films are prepared by R.F. sputtering of a polycrystalline GaAs target. Sulfur is incorporated in the films by adding hydrogen sulphide H,S (99.995%) to the sputtering gas. The experimental chamber is initially pumped down to a 10e6 torr pressure, then H2S is introduced up to a tixed pressure and, finally, ultra high purity argon (99.9997%), used as sputtering gas, is introduced up to working pressure. The hydrogen sulphide and argon flows are separately controlled by a massic flowmeter MKS 247C. During the experiments, the total working pressure of the mixed gas (Ar, H,S) in the bell-jar is kept constant and equal to 30mtorr. This total pressure and the H,S partial pressure are measured with a MKS Baratron capacitive gauge. This gauge is indifferent to the nature of the gas and does not require correcting coefficients. The electrical power applied to the target ranges between 95 and 100 W so that the d.c. cathode bias reaches a value of about 2 kV. The substrate-target distance is about 30 mm. The substrate is a molybdene foil mechanically polished with a diamond compound and chemically cleaned. During the deposition process, the substrate temperature is kept constant at about 650°C. The atomic concentration of sulfur, x, is determined by EDAX analysis. When the H,S partial 927
BBKEYISOGOYOUet al.
928 IW) 2 1
2 x = 0.11
0
-2 -1 E
I
Fig. 1. Plots of current-voltage I(V) characteristics of Au/GaAs,, x,S, /MO test structures. (I) Symmetrical I(V) characteristic (x = 0.02). (2) Dissymmetrical I(V) characteristic without reverse current saturation (x = 0.11). (3) Dissymmetrical I(V) characteristic with reverse current saturation (x = 0.18).
pressure increases from 0 to 1.1 mtorr, x increases from 0 to 0.26. Electrical characterization is made with a metalsemiconductor-metal structure. For this purpose, circular gold electrodes 4 mm in diameter are evaporated onto GaAs,, _ _XI S, film. Since this evaporation is done in another vacuum chamber, the film surface is contaminated by room atmosphere. We have observed that this contamination is independent of the time of exposure to the room atmosphere. With different exposure times, no change can be noted in electrical characteristics. A Hewlett Packard 4140B pAmeter-d.c. voltage source, connected to a microcomputer, is used for dark current-voltage Z(V) measurements.
EXPERIMENTAL
RESULTS
Z(V) plot shows three types of characteristics (Fig. 1): symmetrical (l), dissymmetrical without reverse current saturation (2) and dissymmetrical with reverse current saturation (3). Each I(V) curve shape can be associated to a sulfur concentration range and a particular film texture as shown on Fig. 2-(l), 2-(2) 3-(3), respectively. To analyse the shape of Z(V) characteristics, we computed three different dynamic resistances, R,, R,, and R,, corresponding to the slope values of -0.35, 0 and +0.35 V, respectively. These resistance values depend on the bulk resistance and the contact resistance values. We must note that R, is not always equal
Fig. 2. (a) Caption on facing page.
Influence of sulfur on Au/GaAs(, _ YjS,/Mo structures
929
Fig. 2. Electron scanning microscopy photographs (a+) of GaAs,, _ dSV films surface textures. (a) x = 0.02; (b) x = 0.11; (c) x = 0.24.
to the series resistance value, R,,because for several samples the characteristic is not yet linear at about 0.35 V; consequently R, can be of the same order as or higher than R,. In Fig. 3, we plotted the variation of these three resistances as a function of the atomic concentration of sulfur in the deposited film and we noted the order of magnitude of film thickness. These resistances
increase with sulfur concentration. We note that the high resistance values correspond to thin films; so, it can be supposed that this increase is not due to thickness. It is not possible to separate the bulk effect from the contact effect. Moreover, as previously noted, r,, is not the series resistance. It is difficult to attribute a physical signification to this resistance increase. Nevertheless, we could use these curves
B&KEYISOGOYOUet al.
930
0.2
0.1
1 (A)
0 0
4.0 10-h
8.0 10-6
1.2 10-5
Fig. 5. Plot of V, vs I.
10'
Fig. 3. Variations of dynamic resistances Ri, & and R, (corresponding to the Z(V) slope values at -0.35, 0 and f0.35 V respectively) with X. (Fig. 3) to characterize the Z(V) shape by comparing the Ri, R,, and Z& values for a given sample. We can then distinguish three cases: (1) x < 0.04: the Z(V) characteristics are exclusively symmetrical and we find Ri = R,, c R,,. (2) 0.04 < x < 0.16: the characteristics are dissymmetric without reverse current saturation. In the direct part of the characteristic, the current is higher than in the reverse part. Thus we have: R,, $ R, > Rd. The rectifying effect starts and increases with the sulfur concentration. (3) x > 0.16: in this case, the structures are entirely of the rectifying type and & = 4 g Rd. For x higher than 0.11, the characteristics are rectifying enough to allow the use of log(Z) vs I/ curves to estimate the value of the reverse saturation current Z, and the value of TV, the ideality factor, from the eqn (1): Z = Z,[exp(qV/@T) - l] (1) where q is the electron charge (q = 1.6 . lo-” C) and k the Boltzmann constant (k = 1.38. 10Tz3J/K). In Fig. 4, we can see that at high voltages, for a given Z, there is a horizontal shift, V,, between the experimental curve and the extrapolation of the linear
region. This displacement is due to the series resistance R, effect[S]; R, can be determined by plotting L’, against Z, as shown in Fig. 5. In the more unfavourable case, sample with x = 0.26, we found & equal to about 39 k.Q. This shows that the effect of R, greatly influences the electrical conduction when x is high. So, for the graphical determination of parameters such as Z,, n and &,, we use exclusively the range 100-200 mV in the log(Z) curves, where V, is negligible compared to the external applied tension. In this case, eqn (1) gives a good description of the electrical conduction phenomenon. In the hypothesis of thermoionic emission model, the barrier height & of the metal-semiconductor junction can be calculated from the Z, value: Z, = SA, TZ exp( - &kT),
(2)
where T is the absolute temperature, S the junction surface (cm’) and A, the semiconductor effective Richardson constant. The variations of the ideality factor rl and of the barrier height &, as a function of the sulfur concentration x are plotted in Fig. 6(A) and (B). The TV values higher than 2 are generally attributed to Au/GaAs-polycrystalline junctions which do not correspond to the thermoionic emission model. In the case, the calculated barrier height takes into account the non thermoionic effects. It particulary takes into account the effects due to interface states of the junction. Therefore, we can suppose that, as n decreases and +,, rises linearly with the increasing concentration of sulfur [Fig. 6(A, B)], there is an improvement of the Au/GaAs(, _ X1SX junction. DISCUSSION
(a) Symmetrical characteristics
0
0.2
0.4
0.6
Fig. 4. Plot of log(Z) vs V.
0.8 V(V)
When the sulfur concentration is below 2% (x < 0.04), the structure has symmetrical characteristics similar to those of the Schottky’s symmetrical barrier (S.S.B.). The corresponding model has been developed by Tamg[6] and applied to polycrystalline gallium arsenide by Hwang et al.[7j.
931
Influence of sulfur on Au/GaAs(, _X,S,/Mo structures @b (eV)
8
“‘1’1’ 6
1-
IIIl”I
n “0.1
0.2
6
X
0.1
‘I”“’ 0.2
X
Fig. 6. Plots of the ideality factor rl(A) and the barrier height $,(B) vs X. The small values of R,, R,, and & suggest that there is no barrier at the metal/semiconductor junction. This may be attributed to the distortion of the energy bands due to the accumulation of electrical charges in the film interface[8]. In the S.S.B. model, the current Z, flowing through a film with N identical grain boundaries, is given by the following relation: Z = 2A, T2 exp[ -q$,/kT]sinh[qV/2NklJ].
(3)
In Fig. 7, we can see the coincidence of the experimental plot with the plot calculated from relation (3), with N = 4 and 4,, = 0.45 eV. The experimental curve was plotted with a semiconductor layer 5.5 pm thick and composed of crystallites 1 micrometer in average size. So, it can be reasonably supposed that this layer, with columnar tendency structure, has 5 grains between both electrodes. The conformity of Z(V) characteristics with the Schottky symmetrical barrier model for x < 0.04 indicates that conduction processes are essentially governed by grain boundaries effects. The dissymmetrical shape appears and the test structure becomes more and more rectifying when the
x value exceeds 0.04. The grain size and consequently the grain boundaries density do not significantly change with the sulfur concentration. Therefore, we may associate the decrease in grain boundaries electrical activity with the increase in sulfur concentration. (13)Rectifying type characteristics In order to explain the variation of the barrier height & observed for the rectifying type characteristics [Fig. 5(B)], we must take into account the interface states effects. According to Crowel and Sze[8], in the presence of interface states, the barrier height is approximately given by: &(eV) = C2(cP, - x,) + (1 - CW,
- +J
(4)
the metal work function (for Au, with 4,. 4, = 5.1 eV,[9,10]); x., the semiconductor electron affinity; q&, the position of Fermi level if the metal were absent, measured from the valence band; & = Er - E,, where E, and E, are the Fermi level energy and the valence band energy, respectively; Es, the semiconductor gap (Es = 1.43 eV for GaAs); C,, the interface index at the metal-semiconductor junction, given by the following relation: C, = ci/[ci + q2SN$]
(5)
where ci is the interface film permittivity, 6 the film thickness, and N, the interface states density. For an ideal Schottky junction, the interface states density is zero (N, = 0), C, takes the value one and we have: &=dL-&,
-0.4
-0.2
0
0.2
0.4 v (V)
Fig. 7. Symmetrical current-voltage characteristic of an Au/GaAs,, _ .,%/M• test structure with x = 0.026. Theoreti&l %o]nts are calculated from: Z = 2A, T2expI-&lkTlsinhlaV/2NkTl: with: N = 4: T=298K: ;b, :&45kV, gold electr&ie diameter: 4 mm.’0: experimen: tal plot; x : theoretical plot.
which is the expression of the classical barrier height. In the case of a structure with high interface states, N, is very high and C, is equal to zero so that the barrier height is independent of the used metal. Spicer[ 111places the GaAs (n doped) Fermi level at about 0.75 eV from the top of the valence band. As mentioned in the experimental method paragraph, the Au/GaASt, _X$Xinterface is contaminated, so we can assume that the interface states density N,
BEKEYI SOGOYOU
932
et
al. CONCLUSION
0.1
0
0
0.1
Fig. 8. Au/GaAs(,
_,,S, interface
0.2
index C, variation
0.3
x
with x.
remains always high enough for the Fermi level pinning at 0.75eV. With this hypothesis, we have: Eg - qb,,= 0.68 eV.
(6)
The electron affinities of GaAs and GaS are very close (~,=4.07eV for GaAs, xs = 4.0eV for GaS[12]); As xs is assumed to vary linearly in the compounds[ 131,for low values of x(0 < x < 0.26) we keep for GaAsu _XjSXelectron affinity, the same value as GaAs in the following relationship of C,: C* = [db - (& - &)1/K& - X,) - (E, - &Jl,
(7)
with O
(8)
C, =2.3x - 0.28,
(9)
Then: withO
The electrical characterization of Au/GaAs(, _rjSx/ MO structures shows the presence of a threshold at x = 0.12 beyond which the interface index stops being equal to zero. In the used experimental conditions, it corresponds to the evolution of a bad quality polycrystalline GaAs(, _ ,rISXfilm leading to symmetrical I(V) characteristics (corresponding to the Schottky symmetrical barrier model) into a polycrystalline GaAs(, _ xIS, film with a low grain boundary activity and a good Au/GaAs+,,S, Schottky junction with a significant barrier height. This work shows that the continuous increase in the film amount of sulfur leads to a progressive amelioration of the Au/GaAS(, _ .,S, junction. This improvement may be compared with the results observed in the GaAs surface passivation by sulfur treatment[l4-161. Acknowledgements-This work was supported by (Electricite de France) and PIRSEM (CNRS-France).
EDF
REFERENCES
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