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MIS hydrogen sensors containing LB film insulator layers
51
Sensors and Actuators B, 2 (1990) 51-62
MIS Hydrogen Sensors Containing LB Film Insulator Layers R. SHANLEY*, B. O’BEIRN, V. CASEY and J. B. McMONAGLE Chemical Sensor Research Laboratory, Limerick (Ireland)
College of Engineering and Science,
University of Limerick, Plassey Technological Park,
(Received March 3, 1989; in revised form July 27,1989; accepted August 31,1989)
Abstract Langmuir-Blodgett (LB) solid ultrathin films have been incorporated as the insulator layer in Pd/ insulator/n-Si Schottky barrier diode metal-insulator-semiconductor (MIS) hydrogen sensors. The effects on device response and recovery of M- and Ilayer thickness, temperature, hydrogen concentration and ambient atmosphere are examined. Optimized devices, containing organic LB films, show better responses than conventional oxide-containing MIS sensors and are capable of withstanding extended exposure to temperatures of up to 180 “C. Sensitivities of 0.5 ppm or better are achieveable with good long-term operating stability.
1. Introduction The conventional Schottky barrier diode is a metal-semiconductor (MS) structure. Metal-insulator-semiconductor (MIS) Schottky diodes are also possible, however, provided that the insulator layer is ultrathin (<5 nm). When the metal is capable of reversibly interacting with gases via dissociation and/ or absorption mechanisms, the subsequent modification of the electrical properties of the interface region(s) can result in the device being gas sensitive. Much work has been done in this area since the first reported successful fabrication of such a device and a number of recent reviews are available [ l-41. The development of palladium-silicon MS sensors that are compatible with planar silicon technology has been prevented due to the formation of a gasinsensitive palladium silicide layer at the metalsemiconductor interface [5]. This problem has been circumvented to some extent by the fabrication of palladium-silicon dioxide-silicon MIS devices, which have been shown to be gas sensitive [6,7]. Accurate control of thickness and continuity of the ultrathin oxide layer is of critical importance in such MIS structures, and the nature of the thermal oxidation *Present address: Plessey Semiconductors, Plymouth, Devon, U.K.
09254005/90/$3.50
Roborough,
processing step is such that good device reproducibility may be difficult to achieve on a batch to batch basis. The use of field-effect MOS devices containing thick (z 100 nm) oxide layers is an established method [2] of circumventing such difficulties. Such devices, however, show lower inherent sensitivities to hydrogen. An alternative approach to the formation of the ultrathin insulating layer is used in the present work, namely the use of Langmuir-Blodgett (LB) monomolecular solid film deposition techniques [8]. This method allows for the fabrication of ultrathin organic insulator layers of good uniformity and accurately controlled thickness (by choice of appropriate organic molecule size and number of deposited monomolecular layers). LB films have previously been used as insulator layers both in Si-based sensing devices [9] and in non-sensing devices based on semiconductors such as InP [lo] and GaP [ll], both of which are incapable of forming electronics quality native oxide insulating layers. In this paper we report details of the fabrication and performance of Pd/LB film/r&i Schottky barrier diode hydrogen sensors.
2. Experimental The MIS diodes used in this study were fabricated on Monsanto (100) n-type silicon with a resistivity of about 0.8 $2 cm. The wafers were subjected to a standard degreasing/HF etching/cleaning procedure prior to use, and were then cut into slices (3 cm X 1 cm). An oxide step (1 cm wide X 100 nm thick) was then formed on one end of each slice by first growing oxide on the entire slice using a dry oxidation process and then re-etching. The step (see Fig. 1) serves as a barrier to probe penetration to the underlying silicon while contacting the palladium top electrode. The LB film deposition step was carried out immediately after the etch process. LB films were of stearic acid (Aldrich Chemicals, Gold Label grade). LB films were deposited using a conventional Nima LB trough, which was situated in 0 Elsevier Sequoia/Printed
in The Netherlands
58 gas mixture compositions could be made up by further dilution of these mixtures using a Geo/National Gas Technology Dyna-Blender model 8210. The total gas flow rate through the test system was maintained in the range 200-300 ml mm-‘.
Fig. 1. Diagram of MIS hydrogen sensor containing an LB film insulator layer. M
I
s
Fig. 2. Band energy diagram for MIS Schottky barrier diode device. ‘$B= Schottky barrier height, Ef = Fermi level energy, E, and E, are the conduction and valence energy levels respectively.
a class 1000 clean room. Themal evaporations of both the palladium (Schottky) and aluminium (ohmic) metal contacts were performed in an Edwards E306A system equipped with a FTMS film thickness monitor. Palladium and aluminium wires were obtained from Johnson Matthey (grade 1) and Hopkin and Williams (Analar grade) respectively. Four palladium contacts were evaporated on each silicon slice, hence giving four identical devices in each case. In general, measurements were taken on more than one of the devices on a given slice in order to ensure reproducibility. The identical devices on each slice will be referred to collectively as Dn, where n is a number indicating that particular silicon slice. Gas-induced modifications in Schottky diode electrical properties are quantified by measuring changes in the Schottky barrier height, #n, shown in the band energy diagram (Fig. 2). Changes in device Schottky barrier height, A$,, were determined by monitoring capacitance-voltage characteristics [7] using a Hewlett-Packard 4276A LCZ meter operating at 10 kHz. All of the reported A@n values were obtained at steady-state conditions unless otherwise indicated. Electrical contacts with each sensor were achieved via gold-coated probes. Devices were tested in a Pyrex glass environmental chamber located inside a constant temperature enclosure. The test chamber (internal volume = 550 ml) was constructed in such a way as to allow sensor exposure to test gases or vacuum and the sensor was held in place using a micromanipulator. The test gases used were nitrogen (OFN grade), air (zero grade), 120 ppm and 1000 ppm hydrogen in nitrogen, and 2% hydrogen in air (all certified mixtures). All of the gases were obtained from Irish Industrial Gases. Other
3. Results and Discussion 3.1. Control Devices Three non-sensing control devices were fabricated in order to ensure that the processing conditions used for making the MIS sensors were necessary and appropriate. A Pd/Si MS device was constructed and tested for response to 120 ppm hydrogen in nitrogen, with no observable barrier height change. This verified the earlier work [5] that showed the necessity of incorporating an ultrathin insulator layer in order to prevent palladium silicide formation. A stepped metal-insulator-metal (MIM) device, consisting of various thicknesses of stearic acid LB films sandwiched between aluminium electrodes, was constructed. Capacitance versus thickness measurements on this device gave values for the dielectric constant of stearic acid in agreement with the literature [12], thus indicating that the LB deposition technique produced ultrathin films of good quality and reproducibility. Finally a nickel/ stearic acid LB film (5.16 nm thick)/silicon MIS device was fabricated in order to assess the effects, if any, of the metal evaporation process on the integrity of the LB film (nickel evaporates in vacuum under similar conditions of temperature as palladium). The device barrier height, @n, was found to be 0.797 eV, in reasonable agreement with a value of 0.712 eV, as predicted by theory [13] for this type of device, again indicating the organic LB film ‘insulating properties to be unaffected by the metal deposition process. 3.2. Pd/LB Film/n-,% Sensing Devices Figure 3 shows the capacitance-voltage profile obtained from a MIS diode (Dl) having two LB layers of stearic acid as the I-layer (each LB layer was of monomolecular thickness, giving a total thickness of approximately 5.0 nm). The nominal thickness of the Pd layer was 13 nm. The device was tested in a variety of gaseous environments, showing the same C-F profile for air, vacuum and nitrogen, and a changed profile on exposure to 120 ppm hydrogen in nitrogen. The observed change in Schottky barrier height, A$n, on exposure to the Hz/N2 mixture was 0.49 eV, a value that is comparable with the result of 0.21 eV reported [7] for a similar, conventional MIS sensor. This improvement in steady-state response could be due to one or more of a number of factors, including differences in metal and/or insulator layer morphology.
59
Fig. 3. Capacitance-voltage profiles for Dl in air (0) and in 120 ppm hydrogen
in nitrogen
0.80
T > 0.40 aJ
l
*"
a
0.20
4.0
8.0
0
Thickness (nmlFig. 4. Effect of insulator layer thickness on A@, for D2-DS on exposure to 120 ppm hydrogen in nitrogen.
Having shown that this type of device can act as a sensor, a number of experiments were carried out in order to investigate the effects of sensor design on hydrogen response. In general it was found that individual, identical diodes gave A@n values within kO.07 eV when exposed to saturation levels of hydrogen in nitrogen at room temperature (more than 20 such devices were tested). The effect of insulator layer thickness on device response to 120 ppm hydrogen in nitrogen is shown in Fig. 4. All of these devices had stearic acid Ilayers. The trend of decreasing A@n with I-layer thickness is in agreement with established theory [ 141, which suggests that beyond the upper boundary of 5 nm thickness, the insulator is no longer
transparent to charge transport by the direct tunnelling mechanism. Table 1 (D6-8) shows the effects of varying palladium layer thickness on device performance. Although there is no major change in A#, with increasing Pd thickness, there is a definite increase in the time required for the device to attain a steady-state response (C-V measurement system limitations introduced an uncertainty of +l minute in response time measurements). It should be noted, of course, that due to the relatively large volume of the environmental chamber, a significant proportion of the time required to attain steady-state response was due to gas exchange in the chamber. Nevertheless, assuming no change in gas exchange rate at
60 TABLE 1. Effects of palladium layer thickness and operating temperature on device performance Diodea
Palladium Layer thickness (nm)
Operating temperature ec,
A@ (eV)
Steady-state response time (min)
D6 D7 D8
5.2 19.7 30.2
20 20 21
0.49 0.50 0.44
18 22 24
D9 D9 D9
8.2 8.2 8.2
20 47 88
0.58 0.53 0.50
22 18 12
aL-B insulator layer thickness was 2.5 run in all cases.
constant gas flow rate, it is reasonable to infer that the device response time does increase with increasing Pd thickness. Since it is known that the diffusion of hydrogen through the ultrathin Pd layer is extremely rapid, the aforementioned effect must be ascribed to changes in the rates of the interfacial processes at either side of the Pd film, presumably as a result of variations in film morphology. Another factor that would be expected to influence the rate of dissociation and diffusion of hydrogen is the operating temperature: diode D9 (Table 1) exhibited a decrease in both response time and A#n with increasing operating temperature (for a hydrogen in nitrogen concentration of 120 ppm). Whilst the former change can be ascribed to the increased rates of hydrogen dissociation and diffusion in the Pd layer, the latter is indicative of a lower maximum device response (sensor response showed saturation behaviour at hydrogen concentrations above 25 ppm). This effect has also been
observed in conventional MIS hydrogen sensors [4, 151 and can be explained by using Le Chatelier’s principle to consider the effect of increased temperature on the reversible exothermic adsorption/ dissociation of hydrogen at the palladium surface: Hz(g) *
2H(ads) + heat
On increasing the temperature at which this reaction takes place, the value of the equilibrium constant decreases and hence the amount of adsorbed hydrogen present under equilibrium conditions also decreases. This in turn limits the extent of interfacial dipole layer formation and the net result is a smaller change in barrier height. Device response (at 20 “C and 100 “C) as a function of concentration of hydrogen in nitrogen is shown in Fig. 5 for diode DlO. Hydrogen concentration is denoted on a logarithmic scale due to the wide effective sensing range observed (0.05-25 ppm). Although the maximum detectable hydrogen concentration does not appear to be much affected by the sensor operating temperature, measurable values of A@, are obtained for hydrogen concentrations as low as 0.05 ppm, whereas the lower limit at room temperature is around 0.2 ppm. It should be noted that results obtained at hydrogen concentrations below -1 ppm must be treated with some caution, since this represents the normal contamination level in experimental systems of this kind. Nevertheless, the present results are considered reasonably valid since these devices showed no change in &, whether exposed to nitrogen gas or to vacuum, the latter situation representing a much reduced level of contaminant hydrogen. The device gain (rate of change of A& per unit change in hydrogen concentration) is smaller at the higher operating temperature.
0.450
[H,l Fig. 5. Effect of hydrogen concentration
(ppm) (in nitrogen) on A@JBfor DlO at 20 “C (A) and at 100 “C (A)).
61
semiconductor in such a way that disordering/ dissociation of this two-dimensional solid layer does not occur until temperatures well in excess of the melting point of the normal three-dimensional ‘bulk’ solid. This is an important finding for all electronic devices that contain organic LB films, in particular those that perform best at elevated temperatures, since it indicates that the effective temperature stabilities of such devices may not be limited by the melting point of the organic material. In the present case these results mean that it is possible to operate the device at elevated temperatures in order to take advantage of the aforementioned enhanced response time and detection range observed under such conditions. Conventional Pd-SiOz-Si Schottky barrier sensors show a gradual decrease in response (to a given hydrogen concentration) with operating time, usually over a period of minutes [7]. This effect is particularly evident at higher operating temperatures and has been ascribed to slow diffusion of atomic hydrogen beyond the metal-insulator interface, with a subsequent reduction in oxide defect states. In the present case it was evident that there was no such response stability problem in the LB film-containing devices: even at elevated operating temperatures diode DlO showed no change in A#B over a period of six hours’ exposure to hydrogen in nitrogen at 100 “C. Some possible explanations for this behaviour are:
Sensors were found to recover their original C--l/ characteristic much more rapidly when exposed to air as compared to nitrogen. The influence of the relatively fast catalytic oxidation of hydrogen at the palladium surface is a major parameter that speeds up the removal of hydrogen from the device when air is used. It is also interesting to note that device recovery occurred at much the same rate whether nitrogen, argon or vacuum was used in the recovery process. Increasing temperature was found to increase the rates of recovery in all cases. One device (Dl 1) was tested for response to 2.03% hydrogen in air. Results indicated that although the A&, value was the same as that obtained from the same device on exposure to 120 ppm hydrogen in nitrogen (Ar$, = 0.5 eV), the response time was much faster (a steady-state response time of 11 minutes compared to 22 minutes for the latter). The similar A$J, values are most likely due to hydrogen saturation of the device in both cases. A commonly cited argument against the use of LB films as components in electronic devices is the fact that they are organic in nature and hence susceptible to melting and/or chemical degradation on exposure to elevated temperatures. The temperature stabilities of the present devices were tested by measuring the change in response (to 120 ppm hydrogen in nitrogen at room temperature) for device D12 as a function of exposure to progressively increasing pretreatment temperatures (one hour in air). Results, shown in Fig. 6, indicate that D12 (which contains one layer of stearic acid, melting point 70 “C) was capable of withstanding temperatures of up to 180 “C without any detrimental effects on device response. This behaviour suggests that the stearic acid LB film is strongly held between the metal and
(i) the LB film does not contain defect states that are reactive with hydrogen; (ii) the closely packed structure of the LB film prevents diffusion of the hydrogen beyond the metal-insulator interface; and
1
0.50
. l
ti
.
0.20
&
a
.
0.10
0.
t
005b
100
150
Pretreatment Fig. 6. Device
200
250
30 0
temp (“Cl -
temperature stability: effect of pretreatment temperature (one hour in air) on device response to 120 ppm hydrogen in nitrogen at 20 ‘C for D12.
62 (iii) the LB film passivates sites on the Si surface that would have been reactive to hydrogen (this assumes diffusion of hydrogen through the metal and insulator layers). It could, of course, be argued that these processes are fast and reversible, and hence not amenable to the type of experiments performed in the present work. If this were the case, however, it would be expected that the present devices would show little improvement in response over conventional diodes that contain silicon dioxide insulator layers. The results of this work, as previously mentioned, show significant improvements with the LB tilm-containing devices.
9 N. J. Evans, G. G. Roberts
and M. C. Petty, Effects of hydrogen gas on palladium/LB film/silicon MIS devices, SensorsandActuators,
16 (1989)
255-261.
Acknowledgements
10 G. G. Roberts, K. P. Pande and W. A. Barlow, InP/ Lanamuir-film m.i.s.f.e.t.. IEEE J. Solid State Electron De&es, 2 (1978) 169-175. 11 C. S. Winter, R. H. Tredgold, P. Hodge and E. Khoshdel, Dependence of Schottky barrier height of Gap/polymer/ Au diodes on the area per carboxylic acid group, IEE Proc., 131 (1984) 125-128. 12 B. Mann and H. Kuhn, Tunneling through fatty acid monolayers, J. Appl. Phys., 42 (1971) 4398-4405. 13 R. H. Tredgold and R. Jones, Schottky barrier diodes incorporating Langmuir-film interfacial monolayers, IEEProc., 128 (1981) 202-206. Schottky 14 B. L. Sharma (ed.), Metal-Semiconductor Barrier Junctions and their Applications, Plenum Press, New York, 1984. 15 I. Lundstrom, M. S. Shivaraman and C. Svensson, Hydrogen sensitive MOS structures, Vacuum, 27 (1977) 245247.
The authors would like to thank the Irish Science and Technology Agency, EOLAS, for funding of this work through their Strategic Research Programme.
Biographies
References I. Lundstrom, Hydrogen sensitive MOS structures. Part 1: Principles and applications, Sensors and Actuators, I (1981) 403-426. I. Lundstrom and D. Soderberg, Hydrogen sensitive MOS structures. Part 2: Characterization, Sensors and Acruators, 2 (1982) 105-138. I. Lundstrom and C. Svensson, Gas sensitive metal gate semiconductor devices, in J. Janata and R. J. Huber (eds.), Solid State Chemical Sensors, Academic Press, London, 1985, pp. l-63. S. J. Fonash and 2. Li, Schottky-barrier diode and metal-oxide-semiconductor capacitor gas sensors, in D. Schuetzle, R. Hammerle and J. W. Butler (eds.), Fundamentals and Applications of Chemical Sensors, American Chemical Society, Washington, DC, 1986, pp. 177202. L. L. Tongson, B. E. Knox, T. E. Sullivan and S. J. Fonash, Comparative study of the chemical and polarization characteristics of Pd/Si and Pd/SiOa/Si Schottkybarrier-type devices, J. Appl. Phys., 50 (1979) 1535 1537. M. S. Shivaraman, I. Lundstrom, C. Svensson and H. Hammarasten, Hydrogen sensitivity of palladium-thinoxide-silicon Schottky barriers, Electron. Lett., 12 (1976) 483-484. P. F. Ruths, S. Ashok, S. J. Fonash and J. M. Ruths, A study of Pd/Si MIS Schottky barrier diode hydrogen detector, IEEE Trans. Electron Devices, ED-28 (1981) 1003-1009. M. Sugi, Langmuir-Blodgett films - a course towards molecular electronics,J. Mol. Electron., I (1985) 3-17.
James B. McMonugle received the B.Sc. degree in chemistry in 1978 and the D.Phil. in 1983 from the University of Ulster at Coleraine. He is at present lecturing in industrial chemistry at the University of Limerick and has research interests in chemical gas sensing, adsorption systems and heterogeneous catalysis. Dr McMonagle is a fellow of the Institute of Chemists of Ireland. Vincent Casey received the B.Sc. degree in applied science in 1981 from Trinity College Dublin and the M.Sc. in applied geophysics in 1982 from the National University of Ireland. He is at present lecturing in electronic engineering at the University of Limerick and has research interests in thin-film technology and gas-sensing devices. He is a member of the Institute of Physics. Richard Shanley received the Diploma in applied physics in 1986 from Kevin St. College of Technology, Dublin, and the M.Eng. degree in electronic engineering from the University of Limerick in 1988. He is currently employed by Plessey Semiconductors, Devon, U.K. Brendan U’Beirn received the Diploma in applied physics in 1987 from Kevin St. College of Technology, Dublin, and is currently doing postgraduate research at the University of Limerick.
Sensors and Actuators B, 2 (1990) 51-62
MIS Hydrogen Sensors Containing LB Film Insulator Layers R. SHANLEY*, B. O’BEIRN, V. CASEY and J. B. McMONAGLE Chemical Sensor Research Laboratory, Limerick (Ireland)
College of Engineering and Science,
University of Limerick, Plassey Technological Park,
(Received March 3, 1989; in revised form July 27,1989; accepted August 31,1989)
Abstract Langmuir-Blodgett (LB) solid ultrathin films have been incorporated as the insulator layer in Pd/ insulator/n-Si Schottky barrier diode metal-insulator-semiconductor (MIS) hydrogen sensors. The effects on device response and recovery of M- and Ilayer thickness, temperature, hydrogen concentration and ambient atmosphere are examined. Optimized devices, containing organic LB films, show better responses than conventional oxide-containing MIS sensors and are capable of withstanding extended exposure to temperatures of up to 180 “C. Sensitivities of 0.5 ppm or better are achieveable with good long-term operating stability.
1. Introduction The conventional Schottky barrier diode is a metal-semiconductor (MS) structure. Metal-insulator-semiconductor (MIS) Schottky diodes are also possible, however, provided that the insulator layer is ultrathin (<5 nm). When the metal is capable of reversibly interacting with gases via dissociation and/ or absorption mechanisms, the subsequent modification of the electrical properties of the interface region(s) can result in the device being gas sensitive. Much work has been done in this area since the first reported successful fabrication of such a device and a number of recent reviews are available [ l-41. The development of palladium-silicon MS sensors that are compatible with planar silicon technology has been prevented due to the formation of a gasinsensitive palladium silicide layer at the metalsemiconductor interface [5]. This problem has been circumvented to some extent by the fabrication of palladium-silicon dioxide-silicon MIS devices, which have been shown to be gas sensitive [6,7]. Accurate control of thickness and continuity of the ultrathin oxide layer is of critical importance in such MIS structures, and the nature of the thermal oxidation *Present address: Plessey Semiconductors, Plymouth, Devon, U.K.
09254005/90/$3.50
Roborough,
processing step is such that good device reproducibility may be difficult to achieve on a batch to batch basis. The use of field-effect MOS devices containing thick (z 100 nm) oxide layers is an established method [2] of circumventing such difficulties. Such devices, however, show lower inherent sensitivities to hydrogen. An alternative approach to the formation of the ultrathin insulating layer is used in the present work, namely the use of Langmuir-Blodgett (LB) monomolecular solid film deposition techniques [8]. This method allows for the fabrication of ultrathin organic insulator layers of good uniformity and accurately controlled thickness (by choice of appropriate organic molecule size and number of deposited monomolecular layers). LB films have previously been used as insulator layers both in Si-based sensing devices [9] and in non-sensing devices based on semiconductors such as InP [lo] and GaP [ll], both of which are incapable of forming electronics quality native oxide insulating layers. In this paper we report details of the fabrication and performance of Pd/LB film/r&i Schottky barrier diode hydrogen sensors.
2. Experimental The MIS diodes used in this study were fabricated on Monsanto (100) n-type silicon with a resistivity of about 0.8 $2 cm. The wafers were subjected to a standard degreasing/HF etching/cleaning procedure prior to use, and were then cut into slices (3 cm X 1 cm). An oxide step (1 cm wide X 100 nm thick) was then formed on one end of each slice by first growing oxide on the entire slice using a dry oxidation process and then re-etching. The step (see Fig. 1) serves as a barrier to probe penetration to the underlying silicon while contacting the palladium top electrode. The LB film deposition step was carried out immediately after the etch process. LB films were of stearic acid (Aldrich Chemicals, Gold Label grade). LB films were deposited using a conventional Nima LB trough, which was situated in 0 Elsevier Sequoia/Printed
in The Netherlands
58 gas mixture compositions could be made up by further dilution of these mixtures using a Geo/National Gas Technology Dyna-Blender model 8210. The total gas flow rate through the test system was maintained in the range 200-300 ml mm-‘.
Fig. 1. Diagram of MIS hydrogen sensor containing an LB film insulator layer. M
I
s
Fig. 2. Band energy diagram for MIS Schottky barrier diode device. ‘$B= Schottky barrier height, Ef = Fermi level energy, E, and E, are the conduction and valence energy levels respectively.
a class 1000 clean room. Themal evaporations of both the palladium (Schottky) and aluminium (ohmic) metal contacts were performed in an Edwards E306A system equipped with a FTMS film thickness monitor. Palladium and aluminium wires were obtained from Johnson Matthey (grade 1) and Hopkin and Williams (Analar grade) respectively. Four palladium contacts were evaporated on each silicon slice, hence giving four identical devices in each case. In general, measurements were taken on more than one of the devices on a given slice in order to ensure reproducibility. The identical devices on each slice will be referred to collectively as Dn, where n is a number indicating that particular silicon slice. Gas-induced modifications in Schottky diode electrical properties are quantified by measuring changes in the Schottky barrier height, #n, shown in the band energy diagram (Fig. 2). Changes in device Schottky barrier height, A$,, were determined by monitoring capacitance-voltage characteristics [7] using a Hewlett-Packard 4276A LCZ meter operating at 10 kHz. All of the reported A@n values were obtained at steady-state conditions unless otherwise indicated. Electrical contacts with each sensor were achieved via gold-coated probes. Devices were tested in a Pyrex glass environmental chamber located inside a constant temperature enclosure. The test chamber (internal volume = 550 ml) was constructed in such a way as to allow sensor exposure to test gases or vacuum and the sensor was held in place using a micromanipulator. The test gases used were nitrogen (OFN grade), air (zero grade), 120 ppm and 1000 ppm hydrogen in nitrogen, and 2% hydrogen in air (all certified mixtures). All of the gases were obtained from Irish Industrial Gases. Other
3. Results and Discussion 3.1. Control Devices Three non-sensing control devices were fabricated in order to ensure that the processing conditions used for making the MIS sensors were necessary and appropriate. A Pd/Si MS device was constructed and tested for response to 120 ppm hydrogen in nitrogen, with no observable barrier height change. This verified the earlier work [5] that showed the necessity of incorporating an ultrathin insulator layer in order to prevent palladium silicide formation. A stepped metal-insulator-metal (MIM) device, consisting of various thicknesses of stearic acid LB films sandwiched between aluminium electrodes, was constructed. Capacitance versus thickness measurements on this device gave values for the dielectric constant of stearic acid in agreement with the literature [12], thus indicating that the LB deposition technique produced ultrathin films of good quality and reproducibility. Finally a nickel/ stearic acid LB film (5.16 nm thick)/silicon MIS device was fabricated in order to assess the effects, if any, of the metal evaporation process on the integrity of the LB film (nickel evaporates in vacuum under similar conditions of temperature as palladium). The device barrier height, @n, was found to be 0.797 eV, in reasonable agreement with a value of 0.712 eV, as predicted by theory [13] for this type of device, again indicating the organic LB film ‘insulating properties to be unaffected by the metal deposition process. 3.2. Pd/LB Film/n-,% Sensing Devices Figure 3 shows the capacitance-voltage profile obtained from a MIS diode (Dl) having two LB layers of stearic acid as the I-layer (each LB layer was of monomolecular thickness, giving a total thickness of approximately 5.0 nm). The nominal thickness of the Pd layer was 13 nm. The device was tested in a variety of gaseous environments, showing the same C-F profile for air, vacuum and nitrogen, and a changed profile on exposure to 120 ppm hydrogen in nitrogen. The observed change in Schottky barrier height, A$n, on exposure to the Hz/N2 mixture was 0.49 eV, a value that is comparable with the result of 0.21 eV reported [7] for a similar, conventional MIS sensor. This improvement in steady-state response could be due to one or more of a number of factors, including differences in metal and/or insulator layer morphology.
59
Fig. 3. Capacitance-voltage profiles for Dl in air (0) and in 120 ppm hydrogen
in nitrogen
0.80
T > 0.40 aJ
l
*"
a
0.20
4.0
8.0
0
Thickness (nmlFig. 4. Effect of insulator layer thickness on A@, for D2-DS on exposure to 120 ppm hydrogen in nitrogen.
Having shown that this type of device can act as a sensor, a number of experiments were carried out in order to investigate the effects of sensor design on hydrogen response. In general it was found that individual, identical diodes gave A@n values within kO.07 eV when exposed to saturation levels of hydrogen in nitrogen at room temperature (more than 20 such devices were tested). The effect of insulator layer thickness on device response to 120 ppm hydrogen in nitrogen is shown in Fig. 4. All of these devices had stearic acid Ilayers. The trend of decreasing A@n with I-layer thickness is in agreement with established theory [ 141, which suggests that beyond the upper boundary of 5 nm thickness, the insulator is no longer
transparent to charge transport by the direct tunnelling mechanism. Table 1 (D6-8) shows the effects of varying palladium layer thickness on device performance. Although there is no major change in A#, with increasing Pd thickness, there is a definite increase in the time required for the device to attain a steady-state response (C-V measurement system limitations introduced an uncertainty of +l minute in response time measurements). It should be noted, of course, that due to the relatively large volume of the environmental chamber, a significant proportion of the time required to attain steady-state response was due to gas exchange in the chamber. Nevertheless, assuming no change in gas exchange rate at
60 TABLE 1. Effects of palladium layer thickness and operating temperature on device performance Diodea
Palladium Layer thickness (nm)
Operating temperature ec,
A@ (eV)
Steady-state response time (min)
D6 D7 D8
5.2 19.7 30.2
20 20 21
0.49 0.50 0.44
18 22 24
D9 D9 D9
8.2 8.2 8.2
20 47 88
0.58 0.53 0.50
22 18 12
aL-B insulator layer thickness was 2.5 run in all cases.
constant gas flow rate, it is reasonable to infer that the device response time does increase with increasing Pd thickness. Since it is known that the diffusion of hydrogen through the ultrathin Pd layer is extremely rapid, the aforementioned effect must be ascribed to changes in the rates of the interfacial processes at either side of the Pd film, presumably as a result of variations in film morphology. Another factor that would be expected to influence the rate of dissociation and diffusion of hydrogen is the operating temperature: diode D9 (Table 1) exhibited a decrease in both response time and A#n with increasing operating temperature (for a hydrogen in nitrogen concentration of 120 ppm). Whilst the former change can be ascribed to the increased rates of hydrogen dissociation and diffusion in the Pd layer, the latter is indicative of a lower maximum device response (sensor response showed saturation behaviour at hydrogen concentrations above 25 ppm). This effect has also been
observed in conventional MIS hydrogen sensors [4, 151 and can be explained by using Le Chatelier’s principle to consider the effect of increased temperature on the reversible exothermic adsorption/ dissociation of hydrogen at the palladium surface: Hz(g) *
2H(ads) + heat
On increasing the temperature at which this reaction takes place, the value of the equilibrium constant decreases and hence the amount of adsorbed hydrogen present under equilibrium conditions also decreases. This in turn limits the extent of interfacial dipole layer formation and the net result is a smaller change in barrier height. Device response (at 20 “C and 100 “C) as a function of concentration of hydrogen in nitrogen is shown in Fig. 5 for diode DlO. Hydrogen concentration is denoted on a logarithmic scale due to the wide effective sensing range observed (0.05-25 ppm). Although the maximum detectable hydrogen concentration does not appear to be much affected by the sensor operating temperature, measurable values of A@, are obtained for hydrogen concentrations as low as 0.05 ppm, whereas the lower limit at room temperature is around 0.2 ppm. It should be noted that results obtained at hydrogen concentrations below -1 ppm must be treated with some caution, since this represents the normal contamination level in experimental systems of this kind. Nevertheless, the present results are considered reasonably valid since these devices showed no change in &, whether exposed to nitrogen gas or to vacuum, the latter situation representing a much reduced level of contaminant hydrogen. The device gain (rate of change of A& per unit change in hydrogen concentration) is smaller at the higher operating temperature.
0.450
[H,l Fig. 5. Effect of hydrogen concentration
(ppm) (in nitrogen) on A@JBfor DlO at 20 “C (A) and at 100 “C (A)).
61
semiconductor in such a way that disordering/ dissociation of this two-dimensional solid layer does not occur until temperatures well in excess of the melting point of the normal three-dimensional ‘bulk’ solid. This is an important finding for all electronic devices that contain organic LB films, in particular those that perform best at elevated temperatures, since it indicates that the effective temperature stabilities of such devices may not be limited by the melting point of the organic material. In the present case these results mean that it is possible to operate the device at elevated temperatures in order to take advantage of the aforementioned enhanced response time and detection range observed under such conditions. Conventional Pd-SiOz-Si Schottky barrier sensors show a gradual decrease in response (to a given hydrogen concentration) with operating time, usually over a period of minutes [7]. This effect is particularly evident at higher operating temperatures and has been ascribed to slow diffusion of atomic hydrogen beyond the metal-insulator interface, with a subsequent reduction in oxide defect states. In the present case it was evident that there was no such response stability problem in the LB film-containing devices: even at elevated operating temperatures diode DlO showed no change in A#B over a period of six hours’ exposure to hydrogen in nitrogen at 100 “C. Some possible explanations for this behaviour are:
Sensors were found to recover their original C--l/ characteristic much more rapidly when exposed to air as compared to nitrogen. The influence of the relatively fast catalytic oxidation of hydrogen at the palladium surface is a major parameter that speeds up the removal of hydrogen from the device when air is used. It is also interesting to note that device recovery occurred at much the same rate whether nitrogen, argon or vacuum was used in the recovery process. Increasing temperature was found to increase the rates of recovery in all cases. One device (Dl 1) was tested for response to 2.03% hydrogen in air. Results indicated that although the A&, value was the same as that obtained from the same device on exposure to 120 ppm hydrogen in nitrogen (Ar$, = 0.5 eV), the response time was much faster (a steady-state response time of 11 minutes compared to 22 minutes for the latter). The similar A$J, values are most likely due to hydrogen saturation of the device in both cases. A commonly cited argument against the use of LB films as components in electronic devices is the fact that they are organic in nature and hence susceptible to melting and/or chemical degradation on exposure to elevated temperatures. The temperature stabilities of the present devices were tested by measuring the change in response (to 120 ppm hydrogen in nitrogen at room temperature) for device D12 as a function of exposure to progressively increasing pretreatment temperatures (one hour in air). Results, shown in Fig. 6, indicate that D12 (which contains one layer of stearic acid, melting point 70 “C) was capable of withstanding temperatures of up to 180 “C without any detrimental effects on device response. This behaviour suggests that the stearic acid LB film is strongly held between the metal and
(i) the LB film does not contain defect states that are reactive with hydrogen; (ii) the closely packed structure of the LB film prevents diffusion of the hydrogen beyond the metal-insulator interface; and
1
0.50
. l
ti
.
0.20
&
a
.
0.10
0.
t
005b
100
150
Pretreatment Fig. 6. Device
200
250
30 0
temp (“Cl -
temperature stability: effect of pretreatment temperature (one hour in air) on device response to 120 ppm hydrogen in nitrogen at 20 ‘C for D12.
62 (iii) the LB film passivates sites on the Si surface that would have been reactive to hydrogen (this assumes diffusion of hydrogen through the metal and insulator layers). It could, of course, be argued that these processes are fast and reversible, and hence not amenable to the type of experiments performed in the present work. If this were the case, however, it would be expected that the present devices would show little improvement in response over conventional diodes that contain silicon dioxide insulator layers. The results of this work, as previously mentioned, show significant improvements with the LB tilm-containing devices.
9 N. J. Evans, G. G. Roberts
and M. C. Petty, Effects of hydrogen gas on palladium/LB film/silicon MIS devices, SensorsandActuators,
16 (1989)
255-261.
Acknowledgements
10 G. G. Roberts, K. P. Pande and W. A. Barlow, InP/ Lanamuir-film m.i.s.f.e.t.. IEEE J. Solid State Electron De&es, 2 (1978) 169-175. 11 C. S. Winter, R. H. Tredgold, P. Hodge and E. Khoshdel, Dependence of Schottky barrier height of Gap/polymer/ Au diodes on the area per carboxylic acid group, IEE Proc., 131 (1984) 125-128. 12 B. Mann and H. Kuhn, Tunneling through fatty acid monolayers, J. Appl. Phys., 42 (1971) 4398-4405. 13 R. H. Tredgold and R. Jones, Schottky barrier diodes incorporating Langmuir-film interfacial monolayers, IEEProc., 128 (1981) 202-206. Schottky 14 B. L. Sharma (ed.), Metal-Semiconductor Barrier Junctions and their Applications, Plenum Press, New York, 1984. 15 I. Lundstrom, M. S. Shivaraman and C. Svensson, Hydrogen sensitive MOS structures, Vacuum, 27 (1977) 245247.
The authors would like to thank the Irish Science and Technology Agency, EOLAS, for funding of this work through their Strategic Research Programme.
Biographies
References I. Lundstrom, Hydrogen sensitive MOS structures. Part 1: Principles and applications, Sensors and Actuators, I (1981) 403-426. I. Lundstrom and D. Soderberg, Hydrogen sensitive MOS structures. Part 2: Characterization, Sensors and Acruators, 2 (1982) 105-138. I. Lundstrom and C. Svensson, Gas sensitive metal gate semiconductor devices, in J. Janata and R. J. Huber (eds.), Solid State Chemical Sensors, Academic Press, London, 1985, pp. l-63. S. J. Fonash and 2. Li, Schottky-barrier diode and metal-oxide-semiconductor capacitor gas sensors, in D. Schuetzle, R. Hammerle and J. W. Butler (eds.), Fundamentals and Applications of Chemical Sensors, American Chemical Society, Washington, DC, 1986, pp. 177202. L. L. Tongson, B. E. Knox, T. E. Sullivan and S. J. Fonash, Comparative study of the chemical and polarization characteristics of Pd/Si and Pd/SiOa/Si Schottkybarrier-type devices, J. Appl. Phys., 50 (1979) 1535 1537. M. S. Shivaraman, I. Lundstrom, C. Svensson and H. Hammarasten, Hydrogen sensitivity of palladium-thinoxide-silicon Schottky barriers, Electron. Lett., 12 (1976) 483-484. P. F. Ruths, S. Ashok, S. J. Fonash and J. M. Ruths, A study of Pd/Si MIS Schottky barrier diode hydrogen detector, IEEE Trans. Electron Devices, ED-28 (1981) 1003-1009. M. Sugi, Langmuir-Blodgett films - a course towards molecular electronics,J. Mol. Electron., I (1985) 3-17.
James B. McMonugle received the B.Sc. degree in chemistry in 1978 and the D.Phil. in 1983 from the University of Ulster at Coleraine. He is at present lecturing in industrial chemistry at the University of Limerick and has research interests in chemical gas sensing, adsorption systems and heterogeneous catalysis. Dr McMonagle is a fellow of the Institute of Chemists of Ireland. Vincent Casey received the B.Sc. degree in applied science in 1981 from Trinity College Dublin and the M.Sc. in applied geophysics in 1982 from the National University of Ireland. He is at present lecturing in electronic engineering at the University of Limerick and has research interests in thin-film technology and gas-sensing devices. He is a member of the Institute of Physics. Richard Shanley received the Diploma in applied physics in 1986 from Kevin St. College of Technology, Dublin, and the M.Eng. degree in electronic engineering from the University of Limerick in 1988. He is currently employed by Plessey Semiconductors, Devon, U.K. Brendan U’Beirn received the Diploma in applied physics in 1987 from Kevin St. College of Technology, Dublin, and is currently doing postgraduate research at the University of Limerick.