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Amorphous silicon thin films applied to photochemical sensors
PERGAMON
Vacuum 52 (1999) 41±44
Amorphous silicon thin ®lms applied to photochemical sensors E. Fortunato *, A. Malik, R. 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 The present paper describes the properties of a photochemical sensor based on amorphous silicon MIS (Metal-InsulatorSemiconductor) diodes. The structure of the sensors used in this work are based on glass/Cr/a-SiH(n + )/a-Si:H(i)SiOx/Pd, where the amorphous silicon layers have been deposited by conventional plasma r.f. techniques. The proposed photochemical sensors present a 2±3 orders of magnitude change in the saturation current and a decrease of up to 40% on the open circuit voltage when in the presence of 400 ppm of hydrogen. The overall performance of these sensors, associated with the low cost fabrication technology, suggests that, in the near future, it will be possible to use them in several industrial applications. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction The sensitivity of Pd in Metal/Insulator/ Semiconductor (Pd±MIS) devices (transistors, capacitors and diodes) to the presence of hydrogen has created considerable interest in their use as hydrogen sensors [1]. The main interest in these devices is related to the fact that their electrical characteristics are strongly dependent on the hydrogen partial pressure. The sensitivity to hydrogen comes from the dissociation of hydrogen at the Pd surface, resulting in an interface dipole layer that aects the barrier height and, consequently, its electrical characteristics, namely the dark saturation current and the built-in potential [1]. This process is reversible in the presence of oxygen [2]. In this paper, we present results on the photochemical performances of Pd±MIS sensors that are simultaneously very sensitive to light and to hydrogen. The sensors used in this work are based on an MIS structure [glass/Cr/a-Si:H(n + )/a-Si:H(i)/SiOx/Pd] where the dierent amorphous silicon (a-Si:H) ®lms were deposited by a conventional Plasma Enhanced Chemical Vapour Deposition (PECVD) system [3]. The devices have been characterised through dark and illuminated current±voltage (I±V) characteristics, in air and in the * Corresponding author.
presence of hydrogen. In order to evaluate the optical eciency of the sensors, spectral response measurements have also been performed.
2. Device fabrication The Pd±MIS structures were based on a-Si:H layers deposited onto Cr-coated corning glass substrates. The deposition conditions for the n + and i layers are listed in Table 1. The thicknesses of the i and n + layers were 1 mm and 40 nm, respectively. Before surface oxidation, the native oxide was removed using an HF:H2O (1:10) solution and then rinsed with deionized water. The oxide layer was grown by thermal oxidation in ambient air at 2008C for 10 min. The thickness of the oxide was inferred from ellipsometry measurements, leading to values between 20 AÊ and 30 AÊ. Finally, semitransparent Pd contacts (with an area of 2 mm2) were deposited by e-gun evaporation using a stainless-steel shadow mask. Prior to measurements, all samples were annealed for 1 h at 373 K in vacuum. In Fig. 1, we show a schematic diagram of the type of structure fabricated. The devices were tested in a stainless-steel chamber specially designed for these measurements. The steady hydrogen partial pressure inside the chamber was
0042-207X/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 2 1 9 - X
42
E. Fortunato et al. / Vacuum 52 (1999) 41±44
Table 1. Deposition parameters of n + and i (a-Si:H) layers a-Si:H n+ i
Substrate temperature (8C)
RF power (W)
Total gas pressure (mTorr)
Gases
200 210
10 5
700 700
1%PH3/2.8%H2/30%SiH4 in He SiH4
wavelength range from 400 to 750 nm, using a Jobin Yvon monochromator (H10 IR) and a Keithley 236, both controlled by a PC. The absolute value of the spectral response was inferred through a calibrated cSi photodetector. The experimental errors of the measurements have been taken into account within the software used to collect and to display the obtained data. Fig. 1. Schematic diagram of the sensor structure used in this work.
measured by a specially designed piranni and controlled by an inlet millimetre valve, connected to a rotameter and by an output throttling valve, connected to a rotary pump to exhaust the gases from the chamber. The experimental set-up used to perform the electrical and optical measurements is shown in Fig. 2. The dark and illuminated forward and reverse I±V characteristics have been measured at room temperature, in air and in the presence of hydrogen, within the testing chamber, using a white lamp (intensity power 110 mW/cm2). The current, as a function of applied voltage, was measured by a Keithley 238, controlled by a PC. The spectral response was measured in the
3. Results and discussion Fig. 3 shows the forward and reverse I±V characteristics for the a-Si:H MIS structure produced in air (dark symbols) and in the presence of 400 ppm of H2 (open symbols). From the ®gure, it is possible to observe that the sensor is very sensitive to the presence of H2, leading to large variations of the measured reverse dark current density. The reverse dark current density increases from 4 10 ÿ 9 to 1 10 ÿ 6 A/cm2, while the diode recti®cation ratio (for an absolute voltage value of 1 V) decreases from 5 104 to 2 102. This behavior is monotonically dependent on the amount of H2 present in the chamber.
Fig. 2. Experimental set-up used for the electrical and optical measurements in air and in the presence of hydrogen.
E. Fortunato et al. / Vacuum 52 (1999) 41±44
43
Fig. 4. Room temperature illuminated I±V characteristics for an a-Si:H MIS diode in air and in the presence of H2 (400 ppm).
Fig. 3. Room temperature forward and reverse I±V characteristics for an a-Si:H MIS structure, in air (dark symbols) and in the presence of H2 (open symbols), with a concentration of 400 ppm.
Fig. 4 shows the illuminated I±V characteristics for the same structure as that presented in Fig. 3, in air and in the presence of 400 ppm of H2. The data reveal that the open circuit voltage (Voc) decreases from 0.33 V to 0.23 V when 400 ppm of H2 is present. This
is another con®rmation of the eect of the hydrogen on the decrease of the barrier height. With regard to the short circuit current density, we did not observe any signi®cative change, as happens with crystalline silicon [4]. Here, it is important to notice that changes in the barrier height are directly dependent upon the amount of H2 used and that the rectifying behaviour of the device is kept. Even by using H2 quantities larger than those referred to in Fig. 4, the behaviour observed is the opposite to what happens with c-Si based devices [4]. Fig. 5 shows a typical spectral response (measured at zero bias) of an a-Si:H MIS structure. The Figure also shows the transmittance of the Pd ®lm deposited on glass, having the same thickness (20 nm) as the contacts deposited on the MIS structures. As we can observe, the total transmittance associated with the maximum of the spectral response, near 500 nm, is
Fig. 5. Spectral response for a-Si:H MIS structure. In the ®gure is also plotted the total transmittance of 20 nm Pd deposited on glass.
44
E. Fortunato et al. / Vacuum 52 (1999) 41±44
112%, which explains the low value (0.053 A/W) observed for the spectral response. Taking into account the value of the spectral response, corrected by the Pd transmittance, the device's quantum eciency, normalized to the power used, is near 0.95.
and we are making eorts to improve upon the measurement conditions as well as on the sensor design.
Acknowledgements 4. Conclusions High steady-state sensitivity photochemical sensors based on a-Si:H Pd±MIS structures have been produced and can operate as a gas sensor, either in the dark (by changing its reverse current), or in the presence of light (by changing the open circuit voltage). They are able to be integrated into self-sustained circuits. However, it is important to notice that the a-Si:H sensors presented in this study have not yet been optimised, mainly with regard to to the response time. The response time of these sensors is still quite low (of the order of several minutes, at room temperature) which is directly related to a slow surface process due to surface contamination, fabrication procedures, storage conditions, etc. These problems are under investigation
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 PRAXIS/3/3.1/MMA/1788/95 and NATO SfS POTHINFILM.
References [1] Ruths PF, Ashok S, Fonasch SJ, Ruths JM. IEEE Trans. Electron Devices 1981;ED-28:1003. [2] LundstroÈm I. Sensors & Actuators 1981;1:403. [3] Martins R, Mac° arico A, Vieira M, Ferreira I, Fortunato E. Philosophical Magazine B 1997;76:249. [4] Fortunato E, Malik A, SeÃco A, Ferreira I, Martins R. J. NonCryst. Solids, 1997 (in print).
Vacuum 52 (1999) 41±44
Amorphous silicon thin ®lms applied to photochemical sensors E. Fortunato *, A. Malik, R. 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 The present paper describes the properties of a photochemical sensor based on amorphous silicon MIS (Metal-InsulatorSemiconductor) diodes. The structure of the sensors used in this work are based on glass/Cr/a-SiH(n + )/a-Si:H(i)SiOx/Pd, where the amorphous silicon layers have been deposited by conventional plasma r.f. techniques. The proposed photochemical sensors present a 2±3 orders of magnitude change in the saturation current and a decrease of up to 40% on the open circuit voltage when in the presence of 400 ppm of hydrogen. The overall performance of these sensors, associated with the low cost fabrication technology, suggests that, in the near future, it will be possible to use them in several industrial applications. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction The sensitivity of Pd in Metal/Insulator/ Semiconductor (Pd±MIS) devices (transistors, capacitors and diodes) to the presence of hydrogen has created considerable interest in their use as hydrogen sensors [1]. The main interest in these devices is related to the fact that their electrical characteristics are strongly dependent on the hydrogen partial pressure. The sensitivity to hydrogen comes from the dissociation of hydrogen at the Pd surface, resulting in an interface dipole layer that aects the barrier height and, consequently, its electrical characteristics, namely the dark saturation current and the built-in potential [1]. This process is reversible in the presence of oxygen [2]. In this paper, we present results on the photochemical performances of Pd±MIS sensors that are simultaneously very sensitive to light and to hydrogen. The sensors used in this work are based on an MIS structure [glass/Cr/a-Si:H(n + )/a-Si:H(i)/SiOx/Pd] where the dierent amorphous silicon (a-Si:H) ®lms were deposited by a conventional Plasma Enhanced Chemical Vapour Deposition (PECVD) system [3]. The devices have been characterised through dark and illuminated current±voltage (I±V) characteristics, in air and in the * Corresponding author.
presence of hydrogen. In order to evaluate the optical eciency of the sensors, spectral response measurements have also been performed.
2. Device fabrication The Pd±MIS structures were based on a-Si:H layers deposited onto Cr-coated corning glass substrates. The deposition conditions for the n + and i layers are listed in Table 1. The thicknesses of the i and n + layers were 1 mm and 40 nm, respectively. Before surface oxidation, the native oxide was removed using an HF:H2O (1:10) solution and then rinsed with deionized water. The oxide layer was grown by thermal oxidation in ambient air at 2008C for 10 min. The thickness of the oxide was inferred from ellipsometry measurements, leading to values between 20 AÊ and 30 AÊ. Finally, semitransparent Pd contacts (with an area of 2 mm2) were deposited by e-gun evaporation using a stainless-steel shadow mask. Prior to measurements, all samples were annealed for 1 h at 373 K in vacuum. In Fig. 1, we show a schematic diagram of the type of structure fabricated. The devices were tested in a stainless-steel chamber specially designed for these measurements. The steady hydrogen partial pressure inside the chamber was
0042-207X/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 2 1 9 - X
42
E. Fortunato et al. / Vacuum 52 (1999) 41±44
Table 1. Deposition parameters of n + and i (a-Si:H) layers a-Si:H n+ i
Substrate temperature (8C)
RF power (W)
Total gas pressure (mTorr)
Gases
200 210
10 5
700 700
1%PH3/2.8%H2/30%SiH4 in He SiH4
wavelength range from 400 to 750 nm, using a Jobin Yvon monochromator (H10 IR) and a Keithley 236, both controlled by a PC. The absolute value of the spectral response was inferred through a calibrated cSi photodetector. The experimental errors of the measurements have been taken into account within the software used to collect and to display the obtained data. Fig. 1. Schematic diagram of the sensor structure used in this work.
measured by a specially designed piranni and controlled by an inlet millimetre valve, connected to a rotameter and by an output throttling valve, connected to a rotary pump to exhaust the gases from the chamber. The experimental set-up used to perform the electrical and optical measurements is shown in Fig. 2. The dark and illuminated forward and reverse I±V characteristics have been measured at room temperature, in air and in the presence of hydrogen, within the testing chamber, using a white lamp (intensity power 110 mW/cm2). The current, as a function of applied voltage, was measured by a Keithley 238, controlled by a PC. The spectral response was measured in the
3. Results and discussion Fig. 3 shows the forward and reverse I±V characteristics for the a-Si:H MIS structure produced in air (dark symbols) and in the presence of 400 ppm of H2 (open symbols). From the ®gure, it is possible to observe that the sensor is very sensitive to the presence of H2, leading to large variations of the measured reverse dark current density. The reverse dark current density increases from 4 10 ÿ 9 to 1 10 ÿ 6 A/cm2, while the diode recti®cation ratio (for an absolute voltage value of 1 V) decreases from 5 104 to 2 102. This behavior is monotonically dependent on the amount of H2 present in the chamber.
Fig. 2. Experimental set-up used for the electrical and optical measurements in air and in the presence of hydrogen.
E. Fortunato et al. / Vacuum 52 (1999) 41±44
43
Fig. 4. Room temperature illuminated I±V characteristics for an a-Si:H MIS diode in air and in the presence of H2 (400 ppm).
Fig. 3. Room temperature forward and reverse I±V characteristics for an a-Si:H MIS structure, in air (dark symbols) and in the presence of H2 (open symbols), with a concentration of 400 ppm.
Fig. 4 shows the illuminated I±V characteristics for the same structure as that presented in Fig. 3, in air and in the presence of 400 ppm of H2. The data reveal that the open circuit voltage (Voc) decreases from 0.33 V to 0.23 V when 400 ppm of H2 is present. This
is another con®rmation of the eect of the hydrogen on the decrease of the barrier height. With regard to the short circuit current density, we did not observe any signi®cative change, as happens with crystalline silicon [4]. Here, it is important to notice that changes in the barrier height are directly dependent upon the amount of H2 used and that the rectifying behaviour of the device is kept. Even by using H2 quantities larger than those referred to in Fig. 4, the behaviour observed is the opposite to what happens with c-Si based devices [4]. Fig. 5 shows a typical spectral response (measured at zero bias) of an a-Si:H MIS structure. The Figure also shows the transmittance of the Pd ®lm deposited on glass, having the same thickness (20 nm) as the contacts deposited on the MIS structures. As we can observe, the total transmittance associated with the maximum of the spectral response, near 500 nm, is
Fig. 5. Spectral response for a-Si:H MIS structure. In the ®gure is also plotted the total transmittance of 20 nm Pd deposited on glass.
44
E. Fortunato et al. / Vacuum 52 (1999) 41±44
112%, which explains the low value (0.053 A/W) observed for the spectral response. Taking into account the value of the spectral response, corrected by the Pd transmittance, the device's quantum eciency, normalized to the power used, is near 0.95.
and we are making eorts to improve upon the measurement conditions as well as on the sensor design.
Acknowledgements 4. Conclusions High steady-state sensitivity photochemical sensors based on a-Si:H Pd±MIS structures have been produced and can operate as a gas sensor, either in the dark (by changing its reverse current), or in the presence of light (by changing the open circuit voltage). They are able to be integrated into self-sustained circuits. However, it is important to notice that the a-Si:H sensors presented in this study have not yet been optimised, mainly with regard to to the response time. The response time of these sensors is still quite low (of the order of several minutes, at room temperature) which is directly related to a slow surface process due to surface contamination, fabrication procedures, storage conditions, etc. These problems are under investigation
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 PRAXIS/3/3.1/MMA/1788/95 and NATO SfS POTHINFILM.
References [1] Ruths PF, Ashok S, Fonasch SJ, Ruths JM. IEEE Trans. Electron Devices 1981;ED-28:1003. [2] LundstroÈm I. Sensors & Actuators 1981;1:403. [3] Martins R, Mac° arico A, Vieira M, Ferreira I, Fortunato E. Philosophical Magazine B 1997;76:249. [4] Fortunato E, Malik A, SeÃco A, Ferreira I, Martins R. J. NonCryst. Solids, 1997 (in print).