- Email: [email protected]
Thin oxide interface layers in a-Si:H MIS structures
Journal of Non-Crystalline Solids 227–230 Ž1998. 1230–1234
Thin oxide interface layers in a-Si:H MIS structures Elvira Fortunato ) , Alexander Malik, Rodrigo Martins Department of Materials Science, FCT-UNL and Centre of Excellence for Microelectronic and Optoelectronic Processes, UNINOVA, Quinta da Torre, P-2825 Monte de Caparica, Portugal
Abstract Pd-metalrinsulatorrsemiconductor based on hydrogenated amorphous silicon were produced by plasma enhanced chemical vapour deposition with two different oxidised surfaces: thermal in ambient air and chemical with hydrogen peroxide. The diode characteristics have been investigated using dark and light current as f Ž Õ . measurements in the temperature range from 300 K to 380 K, from which it was possible to infer the electron barrier height. The data obtained show that the incorporation of a thin insulator layer between the semiconductor and the metal improves the performances of the devices by preventing the formation of silicides at the interface. Apart from that we also show that the MIS structures with the thermal oxide presents ‘better’ performances than the ones with the chemical oxide due to the type of interface states and of the oxide charges associated with the interface between the insulator and the semiconductor. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-Si:H; Oxide interface layers; MIS structures
1. Introduction The metal-insulator-semiconductor ŽMIS. structure is a useful device with which to study the semiconductor surfaces and interfaces. In applications surface and interface state determine the final performances of the devices w1x. In the particular case of a MIS structure the incorporation of a thin oxide layer can modify the electrical properties of such interfaces. To understand the kinetics of formation of interface oxides and its effects on the final device performances, a study was performed before and after
oxidation, using two different oxidation techniques: thermal Žin air. and chemical Žin hydrogen peroxide, H 2 O 2 .. The devices used in this work are based on glassrCrra-SiH Žnq .ra-Si:H Ži.rSiO 2 rPd structures, where the a-Si:H was deposited in a plasma enhanced chemical vapour deposition ŽPECVD. system. The electrical properties of these structures were investigated through their diode current–voltage Ž I–V . properties as a function of temperature and the determination of how oxygen ŽO. was incorporated by means of infra-red ŽIR. spectroscopy.
2. Experimental details )
Corresponding author. Tel.: q351-1 294 8564; fax: q351-1 295 7810; e-mail: [email protected].
The a-Si:H Pd-MIS structures were prepared by PECVD. First, a thin Ž; 40 nm. n type layer was
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 5 0 - 6
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
1231
Table 1 Deposition parameters of n-type doped and intrinsic a-Si:H layers a-Si:H
Substrate temperature ŽK.
RF power ŽW.
Total gas pressure ŽPa.
Gas composition
n-type intrinsic
473 483
10 5
71.78 71.78
1%PH 3r2.8%H 2 r30% SiH 4 in He SiH 4
deposited on Cr-coated Ž100 nm. corning 7059 glass substrates. The intrinsic a-Si:H layer of 1 m m thickness were then deposited in a separate chamber to prevent cross contamination of the doping gas. The deposition conditions for the doped and intrinsic layers are listed in Table 1. Prior to the oxidation of the a-Si:H surfaces, the native oxides were removed through a HF solution. The oxide layers were grown either by thermal oxidation in ambient air at 473 K for 10 min, or by chemical oxidation, in H 2 O 2 at 333 K, for the same time. The thickness of the oxide, inferred from ellipsometry measurements, was ; 3 nm. Finally, semitransparent Pd dots Ž2 mm2 and 20 nm thick. were deposited by electron gun evaporation using a stainless-steel shadow mask. Prior to measurements all samples were annealed for 1 h at 373 K, in vacuum. The dark forward and reverse I–V properties were measured from 300 K to 380 K in 20 K increments. The measurements were performed in vacuum. The temperature was measured and controlled by a thermocouple mounted on the heating stage close to the sample. The current as a function
Fig. 1. Time dependence of oxide thickness on the surface of a-Si:H samples, determined from ellipsometry measurements, for the two types of oxides.
of applied voltage was measured by a voltmeter ŽKeithley 238.. All of the experiments were controlled by a PC. The illuminated I–V properties have been measured at room temperature, using the same system as used for the dark measurements. Ellipsometry experiments have been performed using an ellipsometer ŽRudolph. at the wavelength of 632.8 nm and with a 708 angle of incidence. The IR spectroscopy have been carried out in a FITR spectrometer ŽATI Mattson. ŽGenesis., in the wavenumber between 400 cmy1 and 4000 cmy1 .
3. Results 3.1. Oxidation of the a-Si:H surface Fig. 1 shows the oxide thicknesses on a-Si:H Žas deduced from ellipsometry measurements. for the two types of oxidations performed. Both oxidation rates follow a polynomial dependence with time, leading to growth rates of about 0.37 nmrmin and 0.25 nmrmin for the chemical and thermal oxide
Fig. 2. Infra-red absorbance spectra of a typical a-Si:H sample, as deposited Žfull curve. and after oxidation in ambient air Ždashed curve.. The arrows indicate the peak position for the a-SiO 2 vibrating modes.
1232
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
Fig. 3. Room temperature for a-Si:H MIS structures formed without oxide Ždashed line.; with chemical oxide Žfull line. and with thermal oxide Žpointed line..
layers, respectively. That is, we observed a faster oxidation rate for samples oxidised on H 2 O 2 , than in ambient air, attributed to the surface reactivity of the a-Si:H surface to H 2 O 2 . In Fig. 2 we present the IR absorbance spectra of a typical as-deposited a-Si:H Žfull curve. and for the
Fig. 5. A plot of prefactor J0 for a-Si:H MIS structures with different oxides, as indicated inside the graph. The data is fitted to functions lmy J0,i s A i t Bi r T where i designates the oxidation state. The correlation factors are R th.ox s 0.998, R chem.ox s 0.98, and R w.o.s 0.98.
same sample after exposure to ambient air at 473 K for 20 min Žbroken curve.. 3.2. I–V Characteristics Forward and reverse dark I–V properties for a-Si:H MIS structures formed on thermal and chemi-
Fig. 4. Forward and reverse I–V characteristics as a function of temperature, for a-Si:H MIS structures: Ža. formed without oxide; Žb. with chemical oxide; Žc. with thermal oxide. The temperature range used are reported in the graphs.
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
1233
the microstructure parameter w2x, 0.53 before oxidation and 0.71 after oxidation., which proves the incorporation of O in the film. The same trend was also observed in the a-Si:H films chemically oxidised, but now is clearly seen a broad band related to the Si:O:H modes, that appear around the 980–1020 cmy1 wavenumber. These results agree with the ones presented by Drevillon and Vaillant w3x, due to the oxidation of a-Si:H films. 4.2. I–V Properties Fig. 6. Room temperature illuminated I – V characteristics of a-Si:H MIS structures with chemical oxide Ždashed line. and thermal oxide Žfull line..
cal oxidised a-Si:H surfaces are shown in Fig. 3, together with a Schottky barrier prepared in the same conditions as the a-Si:H MIS structures, for comparative purposes. Fig. 4a, b and c present the dark I–V properties in the temperature range between 300 K and 380 K, for the three structures under analysis. In Fig. 5 it is presented a plot of the prefactor J0 Žsee Eq. Ž1a.. for the a-Si:H MIS structures with different oxides while in Fig. 6 we present the I–V curves under illumination of the a-Si:H MIS structures under analysis.
4. Discussion 4.1. Oxidation of the a-Si:H surface To effect of the surface oxidation on the a-Si:H samples was investigated by IR spectroscopy measurements, as shown in Fig. 2. The spectra of the as-deposited a-Si:H sample do not show the characteristics modes of a-SiO 2 Žnear 450 cmy1 and 1000 cmy1 . but mainly the SiH and SiH 2 dominant modes ascribed to intrinsic a-Si:H films: 630 cmy1 ; 2000 cmy1 and 2100 cmy1 . After oxidation the characteristics a-SiO 2 modes are clearly seen as well as a small increase associated with the peak around 2100 cmy1 , meaning that this material presents some degree of porosity Žalso revealed by the magnitude of
From Fig. 3 the trend of the forward current is the same for all the structures, showing a ratio between the forward and the reverse current at < V < s 1 V of about five orders of magnitude. At far forward bias the current is limited at first by the series resistance of the film and then by the accumulation of free-carrier space-charge w4x. For the reverse regime the dark current for the structure having the chemical oxide is lower than the one having the thermal oxide. This could be related to the thickness of the oxides Žsee Fig. 1. and to the nature and type of charges existing in the oxidersemiconductor interface w5x. Usually, in MIS devices the reverse current is a function of the nature and type of the barrier. For low temperatures the reverse current has a pronounced bias dependence due to effects of parallel shunt paths through the thin a-Si:H film. As these shunt currents have a small temperature dependence w1x, we have performed dark I–V measurements as a function of temperature, since these currents become insignificant and the bias dependence diminishes. By taking into account the experimental data shown in Fig. 4a, b and c and assuming that the current through the a-Si:H MIS structures can be described by the diode equation w6x: J s J0 exp
ž
qV
h kT
y1
/
Ž 1a .
where J0 is the current prefactor, q is the electronic charge, V the applied bias, T the device temperature, k the Boltzmann constant and h the diode quality factor and that:
ž
J0 s C exp y
fB kT
/
Ž 1b .
1234
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
where C is a constant Ždependent on the type of transport across the barrier. and f B the electron barrier height, the values of J0 and f B can be determined. First, from the forward J–V curves for the different temperatures plotted in a semilog scale a linear relation is obtained where the y intercept is J0 and the slope is qrh kT. Then, by plotting in a semilog scale the dependence of J0 vs. 1rT the value of f B is obtained, as shown in Fig. 5, leading to: Schottky barrier f B s 0.58 eV; a-Si:H MIS structure with thermal oxide f B s 1.0 eV and a-Si:H MIS structure with chemical oxide f B s 0.96 eV. These results agree with those of other authors w7x. That is, the thin oxide layers formed before Pd deposition, effectively passivates defects, preventing the interaction between Si and Pd and the silicide formation. The a-Si:H MIS structures with thermal oxide has the highest f B , attributed to the type of charges in the oxide and on the resulting interface states at the semiconductorroxide interface Žmore compact and with less induced charged or neutral defects than the chemical oxide.. These results were confirmed by the illuminated I–V curves shown in Fig. 6. There, the open circuit voltage Ž Voc . and the short circuit current Ž Isc . present the lowest values for the structure with the chemical oxide. The reduction of Voc is a direct consequence of the low f B of the MIS structure with the chemical oxide. The reduction of Isc is also a further indication of the barrier-lowering effect and the type of defects of the interface, since the reduced electric field of the junction lowers the collection efficiency of photogenerated carriers.
5. Conclusions In the present work we prove that by incorporating a thin insulator layer between the semiconductor and the metal, the performances of the devices are
improved by preventing the formation of silicides at the interface. During the production of the different MIS devices we see that the oxidation rate at ambient air is less than that obtained with hydrogen peroxide, as expected from the case of c-Si w8x. The IR measurements have demonstrated the incorporation of O at the internal surfaces of the void network near the surface, mainly because the a-Si:H layers present a microstructure parameter, as indicated by the band at 2100 cmy1 . From the two types of oxidations performed, it seems that the thermal oxidation is preferable since the electron barrier height is improved and because it leads to a formation of a more less defect free Žcharged or neutral. interface. Acknowledgements The authors would like to thank to A. Mac¸arico for producing of the a-Si:H layers. This work was supported by JNICT through ‘Financiamentos Plurianuais’ of CENIMAT and through the projects PRAXISr3r3.1rMMAr1788r95 and NATO SfS PO-THINFILM. References w1x D.E. Heller, R.M. Dawson, C.T. Malone, S. Nag, C.R. Wronski, J. Appl. Phys. 76 Ž1992. 2377. w2x E. Fortunato, PhD Thesis, Lisbon, 1995. w3x B. Drevillon, F. Vaillant, Thin Solid Films 124 Ž1985. 217. w4x R.L. Weisfield, J. Appl. Phys. 54 Ž1983. 6401. w5x E.H. Rhoderick, R.H. Williams, Metal–Semiconductor Contacts, 2nd edn., Oxford Univ. Press, New York, 1988. w6x S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, p. 284. w7x A. D’Amico, G. Fortunato, in: J.I. Pankov ŽEd.., Semiconductors and Semimetals, Vol. 21, Part D, Academic Press, New York, 1984, p. 209. w8x P.J. Ponpon, B. Bourdon, Solid State Electron. 25 Ž1982. 875.
Thin oxide interface layers in a-Si:H MIS structures Elvira Fortunato ) , Alexander Malik, Rodrigo Martins Department of Materials Science, FCT-UNL and Centre of Excellence for Microelectronic and Optoelectronic Processes, UNINOVA, Quinta da Torre, P-2825 Monte de Caparica, Portugal
Abstract Pd-metalrinsulatorrsemiconductor based on hydrogenated amorphous silicon were produced by plasma enhanced chemical vapour deposition with two different oxidised surfaces: thermal in ambient air and chemical with hydrogen peroxide. The diode characteristics have been investigated using dark and light current as f Ž Õ . measurements in the temperature range from 300 K to 380 K, from which it was possible to infer the electron barrier height. The data obtained show that the incorporation of a thin insulator layer between the semiconductor and the metal improves the performances of the devices by preventing the formation of silicides at the interface. Apart from that we also show that the MIS structures with the thermal oxide presents ‘better’ performances than the ones with the chemical oxide due to the type of interface states and of the oxide charges associated with the interface between the insulator and the semiconductor. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-Si:H; Oxide interface layers; MIS structures
1. Introduction The metal-insulator-semiconductor ŽMIS. structure is a useful device with which to study the semiconductor surfaces and interfaces. In applications surface and interface state determine the final performances of the devices w1x. In the particular case of a MIS structure the incorporation of a thin oxide layer can modify the electrical properties of such interfaces. To understand the kinetics of formation of interface oxides and its effects on the final device performances, a study was performed before and after
oxidation, using two different oxidation techniques: thermal Žin air. and chemical Žin hydrogen peroxide, H 2 O 2 .. The devices used in this work are based on glassrCrra-SiH Žnq .ra-Si:H Ži.rSiO 2 rPd structures, where the a-Si:H was deposited in a plasma enhanced chemical vapour deposition ŽPECVD. system. The electrical properties of these structures were investigated through their diode current–voltage Ž I–V . properties as a function of temperature and the determination of how oxygen ŽO. was incorporated by means of infra-red ŽIR. spectroscopy.
2. Experimental details )
Corresponding author. Tel.: q351-1 294 8564; fax: q351-1 295 7810; e-mail: [email protected].
The a-Si:H Pd-MIS structures were prepared by PECVD. First, a thin Ž; 40 nm. n type layer was
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 5 0 - 6
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
1231
Table 1 Deposition parameters of n-type doped and intrinsic a-Si:H layers a-Si:H
Substrate temperature ŽK.
RF power ŽW.
Total gas pressure ŽPa.
Gas composition
n-type intrinsic
473 483
10 5
71.78 71.78
1%PH 3r2.8%H 2 r30% SiH 4 in He SiH 4
deposited on Cr-coated Ž100 nm. corning 7059 glass substrates. The intrinsic a-Si:H layer of 1 m m thickness were then deposited in a separate chamber to prevent cross contamination of the doping gas. The deposition conditions for the doped and intrinsic layers are listed in Table 1. Prior to the oxidation of the a-Si:H surfaces, the native oxides were removed through a HF solution. The oxide layers were grown either by thermal oxidation in ambient air at 473 K for 10 min, or by chemical oxidation, in H 2 O 2 at 333 K, for the same time. The thickness of the oxide, inferred from ellipsometry measurements, was ; 3 nm. Finally, semitransparent Pd dots Ž2 mm2 and 20 nm thick. were deposited by electron gun evaporation using a stainless-steel shadow mask. Prior to measurements all samples were annealed for 1 h at 373 K, in vacuum. The dark forward and reverse I–V properties were measured from 300 K to 380 K in 20 K increments. The measurements were performed in vacuum. The temperature was measured and controlled by a thermocouple mounted on the heating stage close to the sample. The current as a function
Fig. 1. Time dependence of oxide thickness on the surface of a-Si:H samples, determined from ellipsometry measurements, for the two types of oxides.
of applied voltage was measured by a voltmeter ŽKeithley 238.. All of the experiments were controlled by a PC. The illuminated I–V properties have been measured at room temperature, using the same system as used for the dark measurements. Ellipsometry experiments have been performed using an ellipsometer ŽRudolph. at the wavelength of 632.8 nm and with a 708 angle of incidence. The IR spectroscopy have been carried out in a FITR spectrometer ŽATI Mattson. ŽGenesis., in the wavenumber between 400 cmy1 and 4000 cmy1 .
3. Results 3.1. Oxidation of the a-Si:H surface Fig. 1 shows the oxide thicknesses on a-Si:H Žas deduced from ellipsometry measurements. for the two types of oxidations performed. Both oxidation rates follow a polynomial dependence with time, leading to growth rates of about 0.37 nmrmin and 0.25 nmrmin for the chemical and thermal oxide
Fig. 2. Infra-red absorbance spectra of a typical a-Si:H sample, as deposited Žfull curve. and after oxidation in ambient air Ždashed curve.. The arrows indicate the peak position for the a-SiO 2 vibrating modes.
1232
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
Fig. 3. Room temperature for a-Si:H MIS structures formed without oxide Ždashed line.; with chemical oxide Žfull line. and with thermal oxide Žpointed line..
layers, respectively. That is, we observed a faster oxidation rate for samples oxidised on H 2 O 2 , than in ambient air, attributed to the surface reactivity of the a-Si:H surface to H 2 O 2 . In Fig. 2 we present the IR absorbance spectra of a typical as-deposited a-Si:H Žfull curve. and for the
Fig. 5. A plot of prefactor J0 for a-Si:H MIS structures with different oxides, as indicated inside the graph. The data is fitted to functions lmy J0,i s A i t Bi r T where i designates the oxidation state. The correlation factors are R th.ox s 0.998, R chem.ox s 0.98, and R w.o.s 0.98.
same sample after exposure to ambient air at 473 K for 20 min Žbroken curve.. 3.2. I–V Characteristics Forward and reverse dark I–V properties for a-Si:H MIS structures formed on thermal and chemi-
Fig. 4. Forward and reverse I–V characteristics as a function of temperature, for a-Si:H MIS structures: Ža. formed without oxide; Žb. with chemical oxide; Žc. with thermal oxide. The temperature range used are reported in the graphs.
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
1233
the microstructure parameter w2x, 0.53 before oxidation and 0.71 after oxidation., which proves the incorporation of O in the film. The same trend was also observed in the a-Si:H films chemically oxidised, but now is clearly seen a broad band related to the Si:O:H modes, that appear around the 980–1020 cmy1 wavenumber. These results agree with the ones presented by Drevillon and Vaillant w3x, due to the oxidation of a-Si:H films. 4.2. I–V Properties Fig. 6. Room temperature illuminated I – V characteristics of a-Si:H MIS structures with chemical oxide Ždashed line. and thermal oxide Žfull line..
cal oxidised a-Si:H surfaces are shown in Fig. 3, together with a Schottky barrier prepared in the same conditions as the a-Si:H MIS structures, for comparative purposes. Fig. 4a, b and c present the dark I–V properties in the temperature range between 300 K and 380 K, for the three structures under analysis. In Fig. 5 it is presented a plot of the prefactor J0 Žsee Eq. Ž1a.. for the a-Si:H MIS structures with different oxides while in Fig. 6 we present the I–V curves under illumination of the a-Si:H MIS structures under analysis.
4. Discussion 4.1. Oxidation of the a-Si:H surface To effect of the surface oxidation on the a-Si:H samples was investigated by IR spectroscopy measurements, as shown in Fig. 2. The spectra of the as-deposited a-Si:H sample do not show the characteristics modes of a-SiO 2 Žnear 450 cmy1 and 1000 cmy1 . but mainly the SiH and SiH 2 dominant modes ascribed to intrinsic a-Si:H films: 630 cmy1 ; 2000 cmy1 and 2100 cmy1 . After oxidation the characteristics a-SiO 2 modes are clearly seen as well as a small increase associated with the peak around 2100 cmy1 , meaning that this material presents some degree of porosity Žalso revealed by the magnitude of
From Fig. 3 the trend of the forward current is the same for all the structures, showing a ratio between the forward and the reverse current at < V < s 1 V of about five orders of magnitude. At far forward bias the current is limited at first by the series resistance of the film and then by the accumulation of free-carrier space-charge w4x. For the reverse regime the dark current for the structure having the chemical oxide is lower than the one having the thermal oxide. This could be related to the thickness of the oxides Žsee Fig. 1. and to the nature and type of charges existing in the oxidersemiconductor interface w5x. Usually, in MIS devices the reverse current is a function of the nature and type of the barrier. For low temperatures the reverse current has a pronounced bias dependence due to effects of parallel shunt paths through the thin a-Si:H film. As these shunt currents have a small temperature dependence w1x, we have performed dark I–V measurements as a function of temperature, since these currents become insignificant and the bias dependence diminishes. By taking into account the experimental data shown in Fig. 4a, b and c and assuming that the current through the a-Si:H MIS structures can be described by the diode equation w6x: J s J0 exp
ž
qV
h kT
y1
/
Ž 1a .
where J0 is the current prefactor, q is the electronic charge, V the applied bias, T the device temperature, k the Boltzmann constant and h the diode quality factor and that:
ž
J0 s C exp y
fB kT
/
Ž 1b .
1234
E. Fortunato et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1230–1234
where C is a constant Ždependent on the type of transport across the barrier. and f B the electron barrier height, the values of J0 and f B can be determined. First, from the forward J–V curves for the different temperatures plotted in a semilog scale a linear relation is obtained where the y intercept is J0 and the slope is qrh kT. Then, by plotting in a semilog scale the dependence of J0 vs. 1rT the value of f B is obtained, as shown in Fig. 5, leading to: Schottky barrier f B s 0.58 eV; a-Si:H MIS structure with thermal oxide f B s 1.0 eV and a-Si:H MIS structure with chemical oxide f B s 0.96 eV. These results agree with those of other authors w7x. That is, the thin oxide layers formed before Pd deposition, effectively passivates defects, preventing the interaction between Si and Pd and the silicide formation. The a-Si:H MIS structures with thermal oxide has the highest f B , attributed to the type of charges in the oxide and on the resulting interface states at the semiconductorroxide interface Žmore compact and with less induced charged or neutral defects than the chemical oxide.. These results were confirmed by the illuminated I–V curves shown in Fig. 6. There, the open circuit voltage Ž Voc . and the short circuit current Ž Isc . present the lowest values for the structure with the chemical oxide. The reduction of Voc is a direct consequence of the low f B of the MIS structure with the chemical oxide. The reduction of Isc is also a further indication of the barrier-lowering effect and the type of defects of the interface, since the reduced electric field of the junction lowers the collection efficiency of photogenerated carriers.
5. Conclusions In the present work we prove that by incorporating a thin insulator layer between the semiconductor and the metal, the performances of the devices are
improved by preventing the formation of silicides at the interface. During the production of the different MIS devices we see that the oxidation rate at ambient air is less than that obtained with hydrogen peroxide, as expected from the case of c-Si w8x. The IR measurements have demonstrated the incorporation of O at the internal surfaces of the void network near the surface, mainly because the a-Si:H layers present a microstructure parameter, as indicated by the band at 2100 cmy1 . From the two types of oxidations performed, it seems that the thermal oxidation is preferable since the electron barrier height is improved and because it leads to a formation of a more less defect free Žcharged or neutral. interface. Acknowledgements The authors would like to thank to A. Mac¸arico for producing of the a-Si:H layers. This work was supported by JNICT through ‘Financiamentos Plurianuais’ of CENIMAT and through the projects PRAXISr3r3.1rMMAr1788r95 and NATO SfS PO-THINFILM. References w1x D.E. Heller, R.M. Dawson, C.T. Malone, S. Nag, C.R. Wronski, J. Appl. Phys. 76 Ž1992. 2377. w2x E. Fortunato, PhD Thesis, Lisbon, 1995. w3x B. Drevillon, F. Vaillant, Thin Solid Films 124 Ž1985. 217. w4x R.L. Weisfield, J. Appl. Phys. 54 Ž1983. 6401. w5x E.H. Rhoderick, R.H. Williams, Metal–Semiconductor Contacts, 2nd edn., Oxford Univ. Press, New York, 1988. w6x S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, p. 284. w7x A. D’Amico, G. Fortunato, in: J.I. Pankov ŽEd.., Semiconductors and Semimetals, Vol. 21, Part D, Academic Press, New York, 1984, p. 209. w8x P.J. Ponpon, B. Bourdon, Solid State Electron. 25 Ž1982. 875.