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Minority-carrier MIS tunnel diode hydrogen sensors
ELSEVIER
Sensors and Actuators B 30 (1996) 233-240
Minority-carrier MIS tunnel diode hydrogen sensors V. Casey, J. B. McMonagle, B. O'Beim * Advanced Sensors Research Unit, University of Limerick, Plassey Technological Park, Limerick, Ireland Received 8 November 1994; in revised form 13 September 1995; accepted 14 September 1995
Abstract
The electrical characteristics of Pd/LB film/n-Si tunnel diode hydrogen sensors containing one and three cadmium stearate LangmuirBlodgett (LB) films are reported. The devices exhibit good rectification behaviour. However, the/V characteristics are complex and deviate considerably from ideal Schottky-barrier theory. The semiconductor surface appears to be inverted at zero bias. Saturation of the forward current is attributed to tunnel-limited transport in the case of single LB film devices and to insulator-limited conduction in the case of devices containing three LB films. Exposure to hydrogen causes pronounced degradation of the diode behaviour. In particular, the forward IV characteristic shows an lcx V 2 behaviour that cannot be satisfactorily explained. A plausible explanation, however, is offered for much of the observed data based upon a minority-carrier metal-insulator-semiconductor (MIS) tunnel diode model. However, the full range of behaviour observed cannot be treated adequately by any one theoretical model. Keywords: MIS tunnel diodes; Hydrogen sensors
1. Introduction
Hydrogen gas has become an increasingly important raw material in the chemical, food, metallurgical and electronics industries. Hydrogen forms an explosive mixture with air and so it is desirable to have highly sensitive hydrogen detectors in order to allow early leak detection. Useful metal-insulatorsemiconductor (MIS) Schottky diode hydrogen sensors have been fabricated on silicon using a catalytic metal such as palladium as the top electrode and a very thin ( < 5 nm) insulating layer of SiOz [ 1-6]. Both/V and CV characteristics of such structures change on exposure to hydrogen. The difficulty of producing such thin films of SiOe has prompted this group to use an alternative approach, the LangmuirBlodgett (LB) technique, to incorporate ultrathin organic insulating films in MIS tunnel diode hydrogen sensors. This technique has been used extensively in the fabrication of MIS diode structures for photovoltaic applications [7], as well as in the fabrication of MIS capacitor-type hydrogen detectors [8]. MIS tunnel junctions incorporating thin insulating layers on silicon substrates can demonstrate remarkably diverse electrical properties as evidenced by zero-bias semiconductor surface conditions ranging from strong inversion to accu* Present address: European Commission Joint Research Centre, Institute of Advanced Materials, TP201-I-21020, Ispra, Italy. 0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10925 - 4 0 0 5 ( 95 ) 0 1 7 9 9 -5
mulation [ 9-11]. A number of models have been developed to describe the current flow in MIS tunnel junctions. In the model developed by Green et al. [ 11 ], as the insulator thickness is reduced, a point is reached, typically around 3.0 nm, where the tunnel current is so large that it causes the semiconductor surface to depart from thermal equilibrium and 'non-equilibrium' MIS tunnel diodes are formed [ 12]. The use of a contact metal with a large work function, such as palladium, on n-type silicon will ensure that the semiconductor surface is inverted at zero bias. In this case the minority carrier quasi-Fermi level in the semiconductor is pinned to its respective minority carrier band edge over a limited bias range (reverse and moderate forward bias). This means that the carrier concentration at the surface is fixed and the surface layer behaves as though it were doped by a diffusion process [ 11 ]. The device can be thought of as a p + n junction diode, the p + region being the induced inversion layer under the contact. Current flowing through the diode at low forward bias will be limited by drift and diffusion processes in the p + region, i.e., semiconductor limited, with the tunnel current acting as an 'ohmic contact'. High forward bias, on the other hand, will take the diode out of inversion and into depletion. Normal Schottky diode behaviour should commence at this stage, i.e., majority carrier current should flow. Eventually, as the bias is increased, the semiconductor current will exceed that which can be supported by the tunnelling process and the current becomes tunnel limited. Increasing the insulator
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3. Results
thickness is expected to displace the tunnel-limited region to lower currents or eliminate tunnelling completely if the thickness increases beyond about 5 nm. Straightforward calculations using numerical values appropriate to the Pd/LB film/n-Si devices used in this work indicate that such systems should correspond closely to the 'non-equilibrium' minority-carrier devices described above, i.e., should be strongly inverted at zero bias and have behaviour similar to that of a p+n junction diode. In this paper,/V and C V data for the Pd/LB film/n-Si diode system are presented. While the behaviour of the diodes cannot be uniquely described using MIS thermionic emission theory or pn junction theory, it is possible to explain many aspects of device performance in terms of minority-carrier-dominated processes.
In general, the MIS diode structures exhibited good diode behaviour with typical rectification ratios of 103 to 104 at 0.5 V bias for a device with a single LB film (insulator thickness, d = 2.5 nm) dropping to about 102 for devices incorporating three layers of LB film (d--7.5 nm). Typical/V characteristics for one- and three-layered devices at 293 K and under dry nitrogen are shown in Fig. 1. In the low forward-bias region (0-0.3 V) the current increases rapidly. The current tends to saturate above 0.3 V. Reverse characteristics were variable but generally showed some bias dependence. Increasing the number of LB films from one to three has no discernible effect on the reverse characteristics. However, the increased insulator thickness displaces the entire forward characteristic to lower currents. Diode forward/V responses were analysed using the ideal diode law
2. ExLperimental
I= Io[exp(qV/nkT) - 1]
( 1)
where Io is given by The fabrication and hydrogen response of the Pd/LB film/ n-Si diodes has been described previously [ 13,14]. Monsanto (100) n-type silicon substrates with nominal resistivity of about 0.8 ['1 cm were used. A NIMA trough was used to deposit insulating cadmium stearate LB films. Diodes were fabricated with either one or three LB films./V characteristics were obtained in the dark using a Keithley 485 picoammeter and 169 voltmeter. A Hewlett-Packard LCZ4276A meter was used to obtain the C V data. A 10 kHz, 50 mV(r.m.s.) test signal was used. The device under test was mounted in a specially constructed glass test chamber [ 14]. More than 30 devices were fabricated and tested in this study.
Io = A*BT2exp ( - q ~no/ kT)
(2)
in the low bias region, i.e., below 0.3 V. In the analysis a Richardson's constant A ° = 1 2 0 A cm -2 K -2 [15] was assumed and the contact area, B, was taken as 3 × 10- 2cm- 2. Ideality factors, n, were in the range 1.2-1.8 for the singlelayer devices, increasing to 2.0-3.0 for devices containing three LB layers. The values for single-layer devices are similar to values obtained by other groups using thin SiO2 insulating layers [ 5,6]. Zero-bias barrier heights, ( qbeo,evaluated using Eq. (2) were in the region of 0.75 +0.05 eV for both single-layer and three-layer devices. In view of the marked
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Fig. 1: Forward (open symbols) and reverse (solid symbols)/V characteristics for Pd/LB film/n-Si diode structures with one monolayer (circles) and three monolayers (squares) of cadmium stearate LB film. The data were obtained with the device in nitrogen, in the dark, at 293 K.
235
V. Casey et al. / Sensors and Actuators B 30 (1996) 233-240 9.01E-010
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Fig. 2. CV (open triangle)and C-2V (solidtriangle)plotsfor a single-monolayerdevicein the dark,undernitrogen,at 293K. curvature of the/V characteristic in the case of the singlelayer devices at low forward bias (Fig. 1) and because of the high ideality factors for all devices, these barrier heights must only be considered as rough approximations of the true band bending in the semiconductor. Nevertheless, the values obtained correspond closely to those reported for Pd/SiO2/ n-Si systems by other workers [2,5]. Diffusion potentials obtained from C -2 versus Vplots (Fig. 2) were within the range 0.70 + 0.03 eV. Values for the dopant density, ND, evaluated from the slopes of these plots were typically of the order of 5 × 1015 c m - 3. This is in reasonable agreement with the value obtained from the resistivity measurement, N D 1015c m -3. /V data were obtained for a single monolayer diode under different ambients; air, nitrogen and vacuum (1.52× 10 -3 torr) at 291 K. Contrary to some previous results [6,16], the effect of these various environments on the electrical characteristics of the device was negligible. Even prolonged operation in vacuum did not change the diode characteristics. The operation of these Pd/LB film/n-Si devices as hydrogen sensors has been described previously [ 13,14 ]. A typical W response for a device with a single LB film to a 120 ppm hydrogen in nitrogen ambient is shown in Fig. 3(a) along with the corresponding nitrogen characteristic. The low forward bias and reverse bias currents increase in the presence of hydrogen. These increases in current might simply be attributed to hydrogen-induced barrier-height reductions. However, the poor correlation of the data with Eq. (1) indicates strong degradation of the diode-like behaviour of the device. Indeed device behaviour may no longer be determined by the Schottky barrier but rather is dominated by LB film-related transport processes (see Section 4). For the device shown in Fig. 3, the apparent zero-bias barrier height =
drops from 0.75 to 0.63 eV on exposure to hydrogen. This is accompanied by an ideality factor increase from 1.3 to 2.5. These results were typical for all single-film devices tested, i.e., barrier height reductions of about 0.12--0.17 eV accompanied by an approximate doubling of the ideality factor. The corresponding barrier-height reductions determined from C V measurements (Fig. 3 (b)) were typically double those determined by the/V technique, i.e., = 0.30 eV. The increase in current due to hydrogen exposure is far more pronounced under reverse bias, increasing by over two orders of magnitude. Hydrogen-induced barrier-height changes for the devices with three LB films were similar to those obtained for the single-film devices.
4. Discussion
The electrical behaviour of these MIS diodes is clearly non-ideal. The large barrier heights obtained for the devices under nitrogen, using thermionic emission theory, indicate that the semiconductor surface is inverted at zero bias. In such a case it might be more appropriate to use diffusion theory, as supported by the work of Green et al. [ 11 ], to interpret our results. Unambiguous interpretation of the forward characteristic is difficult, however, since the current flow does not appear to be dominated by one particular transport process as evidenced by the range of ideality factors obtained. In this work, it must be remembered, therefore, that where thermionic emission theory is used it can at best provide a qualitative insight into device behaviour. The saturation of the forward current is unlikely to be due to the bulk resistance of the semiconductor, considering its high conductivity. Back-contact resistance and tunnel- or
236
V. Casey et al. /Sensors and Actuators B 30 (1996) 233-240
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Fig. 3. (a) Forward (open symbols) and reverse (solid symbols)/V characteristicsfor a single-monolayerdeviceunderdry nitrogen (squares) and under 120 ppm hydrogen in nitrogen (circles) at 293 K. (b) Reversebias C-2Vp!ots for the diode shown in (a) under nitrogen (open triangle) and under 120 ppm hydrogenin nitrogen (solid triangle) at 293 K.
insulator-limited conduction may be producing this effect. The Ict Vdependence of the characteristic at forward voltages greater than 0.3 V for single-layer devices under nitrogen (Fig. 1) favours tunnel-limited current. The presence of a high density of carders at the semiconductor surface in the form of an inversion layer would facilitate this. The forwardbias diode characteristic for single-monolayer hydrogen-
exposed devices gave an excellent fit to an l c x V 2 law (Fig. 4 ( a ) ) , indicating possible space-charge-limited injection of carriers into the LB film [ 15,17]. However, estimates of the insulator thickness from the graphical data yield a value for d, the LB film thickness, which is many orders of magnitude too big. We suspect that the interpretation of the data in the case of single-layer devices is complicated by the pres-
V. Case), et al. / Sensors and Actuators B 30 (1996) 233-240
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Fig. 4. ( a ) Log-log plot of the forward/V dam for a single-monolayer device obtained under 120 ppm hydrogen in nitrogen, in the dark, at 293 K. ( b ) Plot of log (forward current) vs. V I/2 for three-monolayer device under nitrogen (solid triangle ) and under 120 ppm hydrogen in nitrogen (open triangle) in the dark at 293 K.
ence of a combination of barrier-related transport processes such as space-charge-limited conduction and tunnel and field emission. It was possible to fit the forward characteristics of threelayered devices under hydrogen to an expression of the form Ln Icx V 1/2; similar devices exposed to nitrogen followed this expression at high forward bias (Fig. 4(b) ). This behaviour is similar to that obtained by Petty [6] for Pd/SiO2/nSi Schottky diodes and is consistent with Poole-Frenkel or Schottky conduction through the LB films, i.e., insulatorlimited conduction where Ln l(x/3V 1/2. Calculations of the
slope,/3, using appropriate values of dielectric constant and dielectric thickness, i.e., • = 2.7 and d = 2.5 rim, for cadmium stearate yielded a value for Schottky conduction, Bs [ 17], of 10.5 + 0.5; the predicted slope for Poole-Frenkel conduction, ~ F is twice this value. Experimentally determined slopes for three-layered devices exposed to nitrogen and hydrogen were 14 + 1 and 10 + 1, respectively (Fig. 4(b) ). Thus Schottky conduction through the insulating layers would appear to be the dominant conduction mechanism for three-layered devices exposed to hydrogen, while this mechanism accompanied by an element
238
V. Casey et al. ~Sensors and Actuators B 30 (1996) 233-240 0.50
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Fig. 5. Plot of barrier-height change A Oso vs. hydrogen concentration (ppm in nitrogen) for a single-monolayer device determined from/V (open circle) and CV (solid circle) data at 293 K.
of Poole-Frenkel conduction may be occurring in three-layered devices in nitrogen. It may be concluded, therefore, that the saturation of the forward current at high bias in devices containing three LB films is due to insulator-related conduction processes. Furthermore, the large reduction in current under forward bias which accompanies an increase from one to three in the number of LB films incorporated in these devices arises because the tunnelling limit of about 5 nm has been exceeded. The reasons for the variability of reverse characteristics are well documented [ 15]. Guard rings were not used in the construction of these devices and so it is possible that edge effects and generation-recombination currents may be contributing to the reverse current. These effects can result in an unusually strong (fictitious) dependence of barrier height on voltage if not eliminated or accounted for [ 18]. However, because of the presence of an inversion layer, minority-carrier transport in the semiconductor must also be considered, as must lateral surface effec~ [6]. In a minority-carrier device, the reverse current is likely to be limited, not by tunnelling, but by the rate at which holes (minority carders) can drift/ diffuse through the depletion layer between the n-type bulk and the p+ inversion layer. This would account for the observed lack of sensitivity of the reverse current to insulator thickness. The apparent lack of sensitivity of these devices to vacuum exposure indicates that surface states may n0tbe important in determining the overall behaviour of these devices. Such a result is expected for devices that involve metals with large work functions [ 11 ]. The large increases in current that occur under reverse bias for hydr0gen-exposed devices would not be expected for a
minority-carrier device. However, this result can be explained if it is assumed that the barrier-height change on exposure to hydrogen reduces the band bending in the semiconductor, thereby removing the inversion. Normal Schottky diode behaviour will then be established and the reduced barrier height will give rise to a relatively large majority-carrier current. It has been pointed out above that hydrogen-induced barrier-height changes determined from CV measurements were larger than the corresponding values obtained from/Vmeasurements. An interesting aspect of this effect is illustrated in Fig. 5, where the change in barrier height measured by each technique is recorded as a function of hydrogen concentration for a typical single-film device. At very low hydrogen concentrations (0.66 ppm) there is a measurable change in the device barrier height as recorded by CV measurements. For /V measurements, on the other hand, there is no apparent change in barrier height until the ambient hydrogen concentration is at least greater than 1 ppm, giving a measurable response at 5 ppm. Thus, the change in barrier height at 0.66 ppm for CV measurements is roughly equivalent to that obtained at 5 ppm for/V measurements. The difference in the ultimate sensitivities of the two measurement techniques to hydrogen may be explained in terms of a minority-carrier model as follows. The most commonly cited mechanism for hydrogen sensitivity proposed by Luridstrum [ 19] has hydrogen introducing a dipole layer at the Pal-insulator interface, thereby reducing the work-function difference between the metal and the semiconductor. In nitrogen the diode beha~,,esas a minority-carrier device at zero and reverse bias. For a:minority-carrier device it is expected that the reverse current will be relatively insensitive to a change
I/. Casey et al. / Sensors and Actuators B 30 (1996) 233-240
in the metal work function. This is because the minoritycarrier current (hole current) is controlled by diffusion in the depletion region and is not dependent on the barrier height, unlike the majority-carrier current (electrons). The transition from minority- to majority-carrier device can only occur when the inversion layer at the semiconductor surface disappears [4]. Since the Fermi level is pinned at the valence band edge, a finite change in work function will be required in order to unpin it. While the Fermi level remains pinned, changes in the work function will appear mainly as a change in the width of the depletion region behind the interracial inversion layer. Thus, while t h e / V characteristic will not change significantly while unpinning is taking place, the CV characteristic will. A small decrease in the work function on hydrogen exposure will show up as a reduction in the width of the depletion layer, which produces an increase in the device capacitance. It would appear that a work-function change of about 0.10 to 0.15 eV is required in order to unpin the Fermi level. This is equivalent to a hydrogen concentration of about 5 ppm.
5. Conclusions The electrical characteristics of Pd/LB film/n-Si hydrogen sensors have been reported. The devices show non-ideal behaviour. Estimates of Schottky barrier heights obtained by applying thermionic emission theory to the/I/data for these Pd/LB film/n-Si diodes indicate that the semiconductor surface is inverted at zero bias. This result is confirmed by the CV data. The forward current tends to saturate at voltages greater than 0.3 V for nitrogen-exposed devices. This saturation effect is attributed to tunnel limiting of the current in the case of single LB-film devices and to insulator-limited conduction in three-layered devices. Exposure to hydrogen degrades the diode forward-bias behaviour in both singleand three-layer devices. In single-layer devices the forward characteristic shows an lot V 2 behaviour when exposed to hydrogen, which is difficult to explain but which may be due to a combination of conduction processes such as spacecharge-limited conduction and field or tunnel emission. Exposure of three-layered devices to hydrogen results in the entire forward characteristic being dominated by Schottky conduction through the LB films. Some aspects of device performance are satisfactorily explained in terms of a minority-carrier model. In particular, it is possible to account for the large increases in reverse current on hydrogen exposure and the lack of sensitivity of the reverse current to insulator thickness. It is suggested that exposure of the devices to hydrogen at a concentration greater than about 5 ppm converts the device from a minority-carrier device (pn junction type behaviour) to a majority-carrier device (Schottky diode type behaviour). This idea may be used to account for the difference in the ultimate sensitivities of the devices to hydrogen when determined using/V and CV measurements.
239
Acknowledgements Partial support for this work was provided by EOLAS under its Strategic Research Programme.
References [1] B. Keramati and J.N. Zemel, Pd-thin-SiO2-Si diode. Isothermal variation of H2-inducod inteffacial trapping states, J. Appl. Phys., 53 (1982) 1091-1099. [2] A. Diligenti, M. Stagi and V. Ciuti, Pd-Si Schottky diodes as hydrogen sensing devices: capacitance-voltage characteristics, Solid State Comraun., 45 (1983) 347-350. [3] S.J. Fonash, H. Huston and S. Ashok, Conducting MIS diode gas detectors: the Pd/SiOx/Si hydrogen sensor, Sensors and Actuators, 2 (1982) 363-369. [4] M.S. Shivaraman, I. LundstrOm, C. Svensson and H. Hammarshin, Hydrogen sensitivity of palladium-thin-oxide-silicon Schottky barriers, Electon. Lett., 12 (1976) 483--484. [5] 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. [6] M.C. Petty, Conduction mechanisms in Pd/SiO2/n-Si Schottky diode hydrogen detectors, Solid-State Electrott, 29 (1986) 89-97. [7] P.S. Vincett and G.G. Roberts, Electrical and photoelectrical transport properties of Langmuir-Blodgett films and a discussion of possible applications, Thin Solid Films, 68 (1980) 135-171. [8] N.J. Evans, G.G. Roberts and M.C. Petty, Effects of hydrogen gas on palladium/LB Film/silicon MIS devices, Sensors and Actuators, 16 (1989) 255-261. [9] N.G. Tan', D.L. Pulfrey and D.S. Camporese, An analytic model for the MIS tunnel junction, IEEE Trans. Electron Devices, ED-30 ( 1983) 1760-1983. [ 10] J. Shewchun, M.A. Green and F.D. King, Minority carder MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion. II. Experiment, Solid-State Electron., 17 (1974) 563572. [ 11] M.A. Green, F.D. King and J. Shewchun, Minority carrier MIS tunnel diodes arid their application to electron- and photo-voltaic energy conversion. I. Theory, Solid-State Electron., 17 (1974) 551-561. [ 12] J. Shewchun, D. Burk and M.B. Spitzer, MIS and SIS solar cells, IEEE Trans. Electron Devices, ED-27 (1980) 705-724. [ 13] R. Shanley, B. O'Beirn, V. Casey and J.B. McMonagle, MIS hydrogen sensors containing LB film insulator layers, Sensors and Actuators B, 2 (1990) 57--62. [ 14] J.B. McMonagle, V. Casey and B. O'Beirn, Application of thin-film materials in solid state gas sensors, Analyst, 118 (1993) 389-393. [ 15] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [ 16] N. Yamamoto, S. Tonomura, T. Matsuoka and H. Tsubomura, A study on a palladium-titanium oxide Schottky diode as a detector for gaseous components, Surface Sci., 92 (1980) 400-406. [17] N.J. Geddes, J.R. Sambles, W.G. Parker, N.R. Couch and D.J. Jarvis, Electrical characterisation of MIM structures incorporating thin layers of 22-tricosenoic acid deposited on noble metal base electrodes, J. Phys. D: Appl. Phys.,23 (1990) 95--102. [ 18] S.S. Cohen and G.S. Gildenblat, Metal-semiconductor contacts and devices, in D.N.G. Einspruch ( exl,), VLSI Electronics Microstructure Science, Vol. 13, Academic Press, New York, 1986. [19] I. Lundstr~m, Hydrogen sensitive MOS-struetures. Part 1: principles and applications, Sensors and Actuators, 1 ( 1981 ) 403---426.
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Biographies 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 was awarded a Ph.D. by the University of Limerick in 1989. He lectures in physics at the University of Limerick and has research interests in thin-film technology and sensors. He is a member of the Institute of Physics. James B. McMonagle received the B.Sc. degree in chemistry in 1978 and the D.Phil. in 1983 from the University of
Ulster at Coleralne. 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 MeMonagle is a fellow of the Institute of Chemists of Ireland. Brendan O'Beirn received the B.Sc. in applied science from Trinity College Dublin in 1987. He was awarded a Ph.D. by the University of Limerick in 1993. He is currently working as a post-doctoral research fellow in the Institute of Advanced Materials, Ispra, Italy.
Sensors and Actuators B 30 (1996) 233-240
Minority-carrier MIS tunnel diode hydrogen sensors V. Casey, J. B. McMonagle, B. O'Beim * Advanced Sensors Research Unit, University of Limerick, Plassey Technological Park, Limerick, Ireland Received 8 November 1994; in revised form 13 September 1995; accepted 14 September 1995
Abstract
The electrical characteristics of Pd/LB film/n-Si tunnel diode hydrogen sensors containing one and three cadmium stearate LangmuirBlodgett (LB) films are reported. The devices exhibit good rectification behaviour. However, the/V characteristics are complex and deviate considerably from ideal Schottky-barrier theory. The semiconductor surface appears to be inverted at zero bias. Saturation of the forward current is attributed to tunnel-limited transport in the case of single LB film devices and to insulator-limited conduction in the case of devices containing three LB films. Exposure to hydrogen causes pronounced degradation of the diode behaviour. In particular, the forward IV characteristic shows an lcx V 2 behaviour that cannot be satisfactorily explained. A plausible explanation, however, is offered for much of the observed data based upon a minority-carrier metal-insulator-semiconductor (MIS) tunnel diode model. However, the full range of behaviour observed cannot be treated adequately by any one theoretical model. Keywords: MIS tunnel diodes; Hydrogen sensors
1. Introduction
Hydrogen gas has become an increasingly important raw material in the chemical, food, metallurgical and electronics industries. Hydrogen forms an explosive mixture with air and so it is desirable to have highly sensitive hydrogen detectors in order to allow early leak detection. Useful metal-insulatorsemiconductor (MIS) Schottky diode hydrogen sensors have been fabricated on silicon using a catalytic metal such as palladium as the top electrode and a very thin ( < 5 nm) insulating layer of SiOz [ 1-6]. Both/V and CV characteristics of such structures change on exposure to hydrogen. The difficulty of producing such thin films of SiOe has prompted this group to use an alternative approach, the LangmuirBlodgett (LB) technique, to incorporate ultrathin organic insulating films in MIS tunnel diode hydrogen sensors. This technique has been used extensively in the fabrication of MIS diode structures for photovoltaic applications [7], as well as in the fabrication of MIS capacitor-type hydrogen detectors [8]. MIS tunnel junctions incorporating thin insulating layers on silicon substrates can demonstrate remarkably diverse electrical properties as evidenced by zero-bias semiconductor surface conditions ranging from strong inversion to accu* Present address: European Commission Joint Research Centre, Institute of Advanced Materials, TP201-I-21020, Ispra, Italy. 0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10925 - 4 0 0 5 ( 95 ) 0 1 7 9 9 -5
mulation [ 9-11]. A number of models have been developed to describe the current flow in MIS tunnel junctions. In the model developed by Green et al. [ 11 ], as the insulator thickness is reduced, a point is reached, typically around 3.0 nm, where the tunnel current is so large that it causes the semiconductor surface to depart from thermal equilibrium and 'non-equilibrium' MIS tunnel diodes are formed [ 12]. The use of a contact metal with a large work function, such as palladium, on n-type silicon will ensure that the semiconductor surface is inverted at zero bias. In this case the minority carrier quasi-Fermi level in the semiconductor is pinned to its respective minority carrier band edge over a limited bias range (reverse and moderate forward bias). This means that the carrier concentration at the surface is fixed and the surface layer behaves as though it were doped by a diffusion process [ 11 ]. The device can be thought of as a p + n junction diode, the p + region being the induced inversion layer under the contact. Current flowing through the diode at low forward bias will be limited by drift and diffusion processes in the p + region, i.e., semiconductor limited, with the tunnel current acting as an 'ohmic contact'. High forward bias, on the other hand, will take the diode out of inversion and into depletion. Normal Schottky diode behaviour should commence at this stage, i.e., majority carrier current should flow. Eventually, as the bias is increased, the semiconductor current will exceed that which can be supported by the tunnelling process and the current becomes tunnel limited. Increasing the insulator
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V. Casey et al. / Sensors and Actuators B 30 (1996) 233-240
3. Results
thickness is expected to displace the tunnel-limited region to lower currents or eliminate tunnelling completely if the thickness increases beyond about 5 nm. Straightforward calculations using numerical values appropriate to the Pd/LB film/n-Si devices used in this work indicate that such systems should correspond closely to the 'non-equilibrium' minority-carrier devices described above, i.e., should be strongly inverted at zero bias and have behaviour similar to that of a p+n junction diode. In this paper,/V and C V data for the Pd/LB film/n-Si diode system are presented. While the behaviour of the diodes cannot be uniquely described using MIS thermionic emission theory or pn junction theory, it is possible to explain many aspects of device performance in terms of minority-carrier-dominated processes.
In general, the MIS diode structures exhibited good diode behaviour with typical rectification ratios of 103 to 104 at 0.5 V bias for a device with a single LB film (insulator thickness, d = 2.5 nm) dropping to about 102 for devices incorporating three layers of LB film (d--7.5 nm). Typical/V characteristics for one- and three-layered devices at 293 K and under dry nitrogen are shown in Fig. 1. In the low forward-bias region (0-0.3 V) the current increases rapidly. The current tends to saturate above 0.3 V. Reverse characteristics were variable but generally showed some bias dependence. Increasing the number of LB films from one to three has no discernible effect on the reverse characteristics. However, the increased insulator thickness displaces the entire forward characteristic to lower currents. Diode forward/V responses were analysed using the ideal diode law
2. ExLperimental
I= Io[exp(qV/nkT) - 1]
( 1)
where Io is given by The fabrication and hydrogen response of the Pd/LB film/ n-Si diodes has been described previously [ 13,14]. Monsanto (100) n-type silicon substrates with nominal resistivity of about 0.8 ['1 cm were used. A NIMA trough was used to deposit insulating cadmium stearate LB films. Diodes were fabricated with either one or three LB films./V characteristics were obtained in the dark using a Keithley 485 picoammeter and 169 voltmeter. A Hewlett-Packard LCZ4276A meter was used to obtain the C V data. A 10 kHz, 50 mV(r.m.s.) test signal was used. The device under test was mounted in a specially constructed glass test chamber [ 14]. More than 30 devices were fabricated and tested in this study.
Io = A*BT2exp ( - q ~no/ kT)
(2)
in the low bias region, i.e., below 0.3 V. In the analysis a Richardson's constant A ° = 1 2 0 A cm -2 K -2 [15] was assumed and the contact area, B, was taken as 3 × 10- 2cm- 2. Ideality factors, n, were in the range 1.2-1.8 for the singlelayer devices, increasing to 2.0-3.0 for devices containing three LB layers. The values for single-layer devices are similar to values obtained by other groups using thin SiO2 insulating layers [ 5,6]. Zero-bias barrier heights, ( qbeo,evaluated using Eq. (2) were in the region of 0.75 +0.05 eV for both single-layer and three-layer devices. In view of the marked
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I .OE-O04
~14
~2.3
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L
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0.20
30
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I
0.40
v ~
,
I
0.50
J
0.60
I
0.70
I
0.80
I
0.90
v(v)
Fig. 1: Forward (open symbols) and reverse (solid symbols)/V characteristics for Pd/LB film/n-Si diode structures with one monolayer (circles) and three monolayers (squares) of cadmium stearate LB film. The data were obtained with the device in nitrogen, in the dark, at 293 K.
235
V. Casey et al. / Sensors and Actuators B 30 (1996) 233-240 9.01E-010
5.0E+018
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8
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,
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........
O.OE+O00 1.00
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Fig. 2. CV (open triangle)and C-2V (solidtriangle)plotsfor a single-monolayerdevicein the dark,undernitrogen,at 293K. curvature of the/V characteristic in the case of the singlelayer devices at low forward bias (Fig. 1) and because of the high ideality factors for all devices, these barrier heights must only be considered as rough approximations of the true band bending in the semiconductor. Nevertheless, the values obtained correspond closely to those reported for Pd/SiO2/ n-Si systems by other workers [2,5]. Diffusion potentials obtained from C -2 versus Vplots (Fig. 2) were within the range 0.70 + 0.03 eV. Values for the dopant density, ND, evaluated from the slopes of these plots were typically of the order of 5 × 1015 c m - 3. This is in reasonable agreement with the value obtained from the resistivity measurement, N D 1015c m -3. /V data were obtained for a single monolayer diode under different ambients; air, nitrogen and vacuum (1.52× 10 -3 torr) at 291 K. Contrary to some previous results [6,16], the effect of these various environments on the electrical characteristics of the device was negligible. Even prolonged operation in vacuum did not change the diode characteristics. The operation of these Pd/LB film/n-Si devices as hydrogen sensors has been described previously [ 13,14 ]. A typical W response for a device with a single LB film to a 120 ppm hydrogen in nitrogen ambient is shown in Fig. 3(a) along with the corresponding nitrogen characteristic. The low forward bias and reverse bias currents increase in the presence of hydrogen. These increases in current might simply be attributed to hydrogen-induced barrier-height reductions. However, the poor correlation of the data with Eq. (1) indicates strong degradation of the diode-like behaviour of the device. Indeed device behaviour may no longer be determined by the Schottky barrier but rather is dominated by LB film-related transport processes (see Section 4). For the device shown in Fig. 3, the apparent zero-bias barrier height =
drops from 0.75 to 0.63 eV on exposure to hydrogen. This is accompanied by an ideality factor increase from 1.3 to 2.5. These results were typical for all single-film devices tested, i.e., barrier height reductions of about 0.12--0.17 eV accompanied by an approximate doubling of the ideality factor. The corresponding barrier-height reductions determined from C V measurements (Fig. 3 (b)) were typically double those determined by the/V technique, i.e., = 0.30 eV. The increase in current due to hydrogen exposure is far more pronounced under reverse bias, increasing by over two orders of magnitude. Hydrogen-induced barrier-height changes for the devices with three LB films were similar to those obtained for the single-film devices.
4. Discussion
The electrical behaviour of these MIS diodes is clearly non-ideal. The large barrier heights obtained for the devices under nitrogen, using thermionic emission theory, indicate that the semiconductor surface is inverted at zero bias. In such a case it might be more appropriate to use diffusion theory, as supported by the work of Green et al. [ 11 ], to interpret our results. Unambiguous interpretation of the forward characteristic is difficult, however, since the current flow does not appear to be dominated by one particular transport process as evidenced by the range of ideality factors obtained. In this work, it must be remembered, therefore, that where thermionic emission theory is used it can at best provide a qualitative insight into device behaviour. The saturation of the forward current is unlikely to be due to the bulk resistance of the semiconductor, considering its high conductivity. Back-contact resistance and tunnel- or
236
V. Casey et al. /Sensors and Actuators B 30 (1996) 233-240
(a) 1.0E-003
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Fig. 3. (a) Forward (open symbols) and reverse (solid symbols)/V characteristicsfor a single-monolayerdeviceunderdry nitrogen (squares) and under 120 ppm hydrogen in nitrogen (circles) at 293 K. (b) Reversebias C-2Vp!ots for the diode shown in (a) under nitrogen (open triangle) and under 120 ppm hydrogenin nitrogen (solid triangle) at 293 K.
insulator-limited conduction may be producing this effect. The Ict Vdependence of the characteristic at forward voltages greater than 0.3 V for single-layer devices under nitrogen (Fig. 1) favours tunnel-limited current. The presence of a high density of carders at the semiconductor surface in the form of an inversion layer would facilitate this. The forwardbias diode characteristic for single-monolayer hydrogen-
exposed devices gave an excellent fit to an l c x V 2 law (Fig. 4 ( a ) ) , indicating possible space-charge-limited injection of carriers into the LB film [ 15,17]. However, estimates of the insulator thickness from the graphical data yield a value for d, the LB film thickness, which is many orders of magnitude too big. We suspect that the interpretation of the data in the case of single-layer devices is complicated by the pres-
V. Case), et al. / Sensors and Actuators B 30 (1996) 233-240
0.01
237
(a)
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I 0.40
I 0.60
I 0.80
I 1.00
Fig. 4. ( a ) Log-log plot of the forward/V dam for a single-monolayer device obtained under 120 ppm hydrogen in nitrogen, in the dark, at 293 K. ( b ) Plot of log (forward current) vs. V I/2 for three-monolayer device under nitrogen (solid triangle ) and under 120 ppm hydrogen in nitrogen (open triangle) in the dark at 293 K.
ence of a combination of barrier-related transport processes such as space-charge-limited conduction and tunnel and field emission. It was possible to fit the forward characteristics of threelayered devices under hydrogen to an expression of the form Ln Icx V 1/2; similar devices exposed to nitrogen followed this expression at high forward bias (Fig. 4(b) ). This behaviour is similar to that obtained by Petty [6] for Pd/SiO2/nSi Schottky diodes and is consistent with Poole-Frenkel or Schottky conduction through the LB films, i.e., insulatorlimited conduction where Ln l(x/3V 1/2. Calculations of the
slope,/3, using appropriate values of dielectric constant and dielectric thickness, i.e., • = 2.7 and d = 2.5 rim, for cadmium stearate yielded a value for Schottky conduction, Bs [ 17], of 10.5 + 0.5; the predicted slope for Poole-Frenkel conduction, ~ F is twice this value. Experimentally determined slopes for three-layered devices exposed to nitrogen and hydrogen were 14 + 1 and 10 + 1, respectively (Fig. 4(b) ). Thus Schottky conduction through the insulating layers would appear to be the dominant conduction mechanism for three-layered devices exposed to hydrogen, while this mechanism accompanied by an element
238
V. Casey et al. ~Sensors and Actuators B 30 (1996) 233-240 0.50
0.40¸
0.3
0.2(
0.1(
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t
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,
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~ , , ,
~ ,,~1
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,
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~
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,,~,1
, 100.0
,
,
, ,,,,I 1000.0
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Fig. 5. Plot of barrier-height change A Oso vs. hydrogen concentration (ppm in nitrogen) for a single-monolayer device determined from/V (open circle) and CV (solid circle) data at 293 K.
of Poole-Frenkel conduction may be occurring in three-layered devices in nitrogen. It may be concluded, therefore, that the saturation of the forward current at high bias in devices containing three LB films is due to insulator-related conduction processes. Furthermore, the large reduction in current under forward bias which accompanies an increase from one to three in the number of LB films incorporated in these devices arises because the tunnelling limit of about 5 nm has been exceeded. The reasons for the variability of reverse characteristics are well documented [ 15]. Guard rings were not used in the construction of these devices and so it is possible that edge effects and generation-recombination currents may be contributing to the reverse current. These effects can result in an unusually strong (fictitious) dependence of barrier height on voltage if not eliminated or accounted for [ 18]. However, because of the presence of an inversion layer, minority-carrier transport in the semiconductor must also be considered, as must lateral surface effec~ [6]. In a minority-carrier device, the reverse current is likely to be limited, not by tunnelling, but by the rate at which holes (minority carders) can drift/ diffuse through the depletion layer between the n-type bulk and the p+ inversion layer. This would account for the observed lack of sensitivity of the reverse current to insulator thickness. The apparent lack of sensitivity of these devices to vacuum exposure indicates that surface states may n0tbe important in determining the overall behaviour of these devices. Such a result is expected for devices that involve metals with large work functions [ 11 ]. The large increases in current that occur under reverse bias for hydr0gen-exposed devices would not be expected for a
minority-carrier device. However, this result can be explained if it is assumed that the barrier-height change on exposure to hydrogen reduces the band bending in the semiconductor, thereby removing the inversion. Normal Schottky diode behaviour will then be established and the reduced barrier height will give rise to a relatively large majority-carrier current. It has been pointed out above that hydrogen-induced barrier-height changes determined from CV measurements were larger than the corresponding values obtained from/Vmeasurements. An interesting aspect of this effect is illustrated in Fig. 5, where the change in barrier height measured by each technique is recorded as a function of hydrogen concentration for a typical single-film device. At very low hydrogen concentrations (0.66 ppm) there is a measurable change in the device barrier height as recorded by CV measurements. For /V measurements, on the other hand, there is no apparent change in barrier height until the ambient hydrogen concentration is at least greater than 1 ppm, giving a measurable response at 5 ppm. Thus, the change in barrier height at 0.66 ppm for CV measurements is roughly equivalent to that obtained at 5 ppm for/V measurements. The difference in the ultimate sensitivities of the two measurement techniques to hydrogen may be explained in terms of a minority-carrier model as follows. The most commonly cited mechanism for hydrogen sensitivity proposed by Luridstrum [ 19] has hydrogen introducing a dipole layer at the Pal-insulator interface, thereby reducing the work-function difference between the metal and the semiconductor. In nitrogen the diode beha~,,esas a minority-carrier device at zero and reverse bias. For a:minority-carrier device it is expected that the reverse current will be relatively insensitive to a change
I/. Casey et al. / Sensors and Actuators B 30 (1996) 233-240
in the metal work function. This is because the minoritycarrier current (hole current) is controlled by diffusion in the depletion region and is not dependent on the barrier height, unlike the majority-carrier current (electrons). The transition from minority- to majority-carrier device can only occur when the inversion layer at the semiconductor surface disappears [4]. Since the Fermi level is pinned at the valence band edge, a finite change in work function will be required in order to unpin it. While the Fermi level remains pinned, changes in the work function will appear mainly as a change in the width of the depletion region behind the interracial inversion layer. Thus, while t h e / V characteristic will not change significantly while unpinning is taking place, the CV characteristic will. A small decrease in the work function on hydrogen exposure will show up as a reduction in the width of the depletion layer, which produces an increase in the device capacitance. It would appear that a work-function change of about 0.10 to 0.15 eV is required in order to unpin the Fermi level. This is equivalent to a hydrogen concentration of about 5 ppm.
5. Conclusions The electrical characteristics of Pd/LB film/n-Si hydrogen sensors have been reported. The devices show non-ideal behaviour. Estimates of Schottky barrier heights obtained by applying thermionic emission theory to the/I/data for these Pd/LB film/n-Si diodes indicate that the semiconductor surface is inverted at zero bias. This result is confirmed by the CV data. The forward current tends to saturate at voltages greater than 0.3 V for nitrogen-exposed devices. This saturation effect is attributed to tunnel limiting of the current in the case of single LB-film devices and to insulator-limited conduction in three-layered devices. Exposure to hydrogen degrades the diode forward-bias behaviour in both singleand three-layer devices. In single-layer devices the forward characteristic shows an lot V 2 behaviour when exposed to hydrogen, which is difficult to explain but which may be due to a combination of conduction processes such as spacecharge-limited conduction and field or tunnel emission. Exposure of three-layered devices to hydrogen results in the entire forward characteristic being dominated by Schottky conduction through the LB films. Some aspects of device performance are satisfactorily explained in terms of a minority-carrier model. In particular, it is possible to account for the large increases in reverse current on hydrogen exposure and the lack of sensitivity of the reverse current to insulator thickness. It is suggested that exposure of the devices to hydrogen at a concentration greater than about 5 ppm converts the device from a minority-carrier device (pn junction type behaviour) to a majority-carrier device (Schottky diode type behaviour). This idea may be used to account for the difference in the ultimate sensitivities of the devices to hydrogen when determined using/V and CV measurements.
239
Acknowledgements Partial support for this work was provided by EOLAS under its Strategic Research Programme.
References [1] B. Keramati and J.N. Zemel, Pd-thin-SiO2-Si diode. Isothermal variation of H2-inducod inteffacial trapping states, J. Appl. Phys., 53 (1982) 1091-1099. [2] A. Diligenti, M. Stagi and V. Ciuti, Pd-Si Schottky diodes as hydrogen sensing devices: capacitance-voltage characteristics, Solid State Comraun., 45 (1983) 347-350. [3] S.J. Fonash, H. Huston and S. Ashok, Conducting MIS diode gas detectors: the Pd/SiOx/Si hydrogen sensor, Sensors and Actuators, 2 (1982) 363-369. [4] M.S. Shivaraman, I. LundstrOm, C. Svensson and H. Hammarshin, Hydrogen sensitivity of palladium-thin-oxide-silicon Schottky barriers, Electon. Lett., 12 (1976) 483--484. [5] 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. [6] M.C. Petty, Conduction mechanisms in Pd/SiO2/n-Si Schottky diode hydrogen detectors, Solid-State Electrott, 29 (1986) 89-97. [7] P.S. Vincett and G.G. Roberts, Electrical and photoelectrical transport properties of Langmuir-Blodgett films and a discussion of possible applications, Thin Solid Films, 68 (1980) 135-171. [8] N.J. Evans, G.G. Roberts and M.C. Petty, Effects of hydrogen gas on palladium/LB Film/silicon MIS devices, Sensors and Actuators, 16 (1989) 255-261. [9] N.G. Tan', D.L. Pulfrey and D.S. Camporese, An analytic model for the MIS tunnel junction, IEEE Trans. Electron Devices, ED-30 ( 1983) 1760-1983. [ 10] J. Shewchun, M.A. Green and F.D. King, Minority carder MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion. II. Experiment, Solid-State Electron., 17 (1974) 563572. [ 11] M.A. Green, F.D. King and J. Shewchun, Minority carrier MIS tunnel diodes arid their application to electron- and photo-voltaic energy conversion. I. Theory, Solid-State Electron., 17 (1974) 551-561. [ 12] J. Shewchun, D. Burk and M.B. Spitzer, MIS and SIS solar cells, IEEE Trans. Electron Devices, ED-27 (1980) 705-724. [ 13] R. Shanley, B. O'Beirn, V. Casey and J.B. McMonagle, MIS hydrogen sensors containing LB film insulator layers, Sensors and Actuators B, 2 (1990) 57--62. [ 14] J.B. McMonagle, V. Casey and B. O'Beirn, Application of thin-film materials in solid state gas sensors, Analyst, 118 (1993) 389-393. [ 15] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [ 16] N. Yamamoto, S. Tonomura, T. Matsuoka and H. Tsubomura, A study on a palladium-titanium oxide Schottky diode as a detector for gaseous components, Surface Sci., 92 (1980) 400-406. [17] N.J. Geddes, J.R. Sambles, W.G. Parker, N.R. Couch and D.J. Jarvis, Electrical characterisation of MIM structures incorporating thin layers of 22-tricosenoic acid deposited on noble metal base electrodes, J. Phys. D: Appl. Phys.,23 (1990) 95--102. [ 18] S.S. Cohen and G.S. Gildenblat, Metal-semiconductor contacts and devices, in D.N.G. Einspruch ( exl,), VLSI Electronics Microstructure Science, Vol. 13, Academic Press, New York, 1986. [19] I. Lundstr~m, Hydrogen sensitive MOS-struetures. Part 1: principles and applications, Sensors and Actuators, 1 ( 1981 ) 403---426.
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Biographies 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 was awarded a Ph.D. by the University of Limerick in 1989. He lectures in physics at the University of Limerick and has research interests in thin-film technology and sensors. He is a member of the Institute of Physics. James B. McMonagle received the B.Sc. degree in chemistry in 1978 and the D.Phil. in 1983 from the University of
Ulster at Coleralne. 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 MeMonagle is a fellow of the Institute of Chemists of Ireland. Brendan O'Beirn received the B.Sc. in applied science from Trinity College Dublin in 1987. He was awarded a Ph.D. by the University of Limerick in 1993. He is currently working as a post-doctoral research fellow in the Institute of Advanced Materials, Ispra, Italy.