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n-Si Schottky diode hydrogen detectors
SohbSrrtre Elecrronrrs Vol. 29. No. 1. pp. X9-97. Printed in Great Britain.
1986
0038-1101/X6 $3.00+ 00 b 1986 Pergamon Press Ltd.
CONDUCTION MECHANISMS IN Pd/SiO,/n-Si SCHOTTKY DIODE HYDROGEN DETECTORS M. C.
PETTY
Department of Applied Physics and Electronics, University of Durham, South Road, Durham DHl 3LE, U.K. (Received
28 Much
1984; in revisedform
25 May 1984)
Abstract-The electrical characteristics of Pd/Si02/n-Si Schottky barrier type structures are reported over a range of temperatures and after exposure to differem ambient conditions. A number of conduction mechanisms through the diodes have been identified. Exposure to hydrogen gas is shown to produce an increase in the conductivity of the diodes and a voltage shift in the capacitance-bias characteristics; this is attributed to both an increase in the interface state density and also to a reduction in the work function of the palladium. It is suggested that the nature of the interfacial oxide layer is of paramount determining the dominant conduction process through the devices.
1. INTRODUCTION
importance
in
on the Pd surface. The chemical bond between the Pd and these oxygen anions is though to form an electrical double layer at the Pd surface and thus affect the work function of the metal. The decrease in work function on exposure to hydrogen is attributed to the reaction between hydrogen and the adsorbed anions. A completely different model is offered by Keramati and Zemel[l] to account for the effects of hydrogen on the electrical characteristics of their Pd/SiO,/n-Si Schottky diodes. These authors have suggested that H, can penetrate the thin oxide and introduce traps at the Si/SiO, interface. Theoretical modelling has shown that at least two hydrogen induced surface states are required in order to explain the admittance characteristics of these diodes [14]. Some evidence to support this surface state model has been provided by Diligenti ef al. [4] on the basis of capacitance measurements, and by Petty[S] using deep level transient spectroscopy (DLTS). However, it is not clear how these ideas fit in with the accepted effect of hydrogen on the Si/SiO, interface, namely that annealing in H, gas decreases the interface state density[l5]. In this paper we report on a detailed investigation into the electrical characteristics of Pd/SiO,/n-Si Schottky barrier devices. Measurements are reported over a range of temperatures and under different ambients. The results of these experiments reveal that, under certain conditions, interface states can play a dominant role in determining the electrical characteristics of the diodes.
Interest in the development of solid-state chemical sensors has increased substantially over the past few years. If integrated with silicon microelectronic circuitry such transducers could form the basis for a new generation of sensing devices. One particular structure that has received considerable attention is a hydrogen detector based on a palladium/n-type semiconductor Schottky diode. Devices have now been fabricated on n-type Si[l-51, CdS[6], ZnO[7], TiO, [8], InP[9], GaAs[lO], GaP[ll] and hydrogenated amorphous silicon[12]. It should be noted that most of these Schottky barrier devices are, in practice, metal-insulator-semiconductor (MIS) structures. In fact, in the case of devices fabricated on silicon, a thin (< 5 nm) oxide layer is deliberately introduced between the metal and semiconductor in order to prevent the formation of palladium silicide[l]. Exposure to hydrogen gas invariably results in changes in the current-voltage and capacitance-voltage characteristics of these diodes. Unfortunately, despite the existence of a considerable quantity of experimental data, there is still some debate as to the origin of the hydrogen sensitivity. Shivaraman et al. [3] and Ruths et al.[2] have attributed changes in the characteristics of devices fabricated on Si as due to a work function decrease of the Pd on exposure to H,. Lundstrom[l3] has explained this process in more detail: hydrogen molecules are first dissociated on the catalytic metal surface and then the atoms are adsorbed onto the metal. Subsequently, some of the hydrogen atoms diffuse through the palladium film and are adsorbed at the metal-insulator interface; these atoms become polarized and give rise to a dipole layer which effectively changes the work function of the palladium. An alternative explanation is given by Yamamoto et al. [8] to account for hydrogen effects on Pd/TiO, diodes. In this case it is proposed that O2 is chemisorbed in the form of anions (e.g. O-, 0; and O’-)
2.
EXPERIMENTAL
PROCEDURE
The fabrication of the Pd/SiO,/n-Si diodes has been described previously[S]. The resulting devices were mounted in an Oxford Instruments DN704 He exchange gas cryostat and electrical contacts were made to the Pd and Au/Sb using air drying Ag paste. The temperature of the sample could be varied 89
M. C. PETTI
90
using an Oxford Instruments DTC2 temperature controller. Current-voltage characteristics were measured using a Time Electronics voltage calibrator and a Keithley 410A picoammeter. The capacitance of the devices, at 1 MHz, was measured with a Boonton 72 BD capacitance meter. A Signal Instruments gas blender was used to produce various concentrations of hydrogen in nitrogen. 3. RESULTS
AND DISCUSSION
3.1 Diode Characteristics Most of the devices exhibited good diode behaviour with typical current rectification ratios of 104-10’ at a bias of 0.5 V. Figure 1 shows the room temperature current-voltage characteristics of a typical device; the inset shows a plot of reciprocal capacitance squared versus applied voltage for the same device. In order to elucidate the conduction mechanisms in our diodes, the current-voltage characteristics were measured as a function of temperature. These are shown in Fig. 2(a) (forward bias) and Fig. 2(b) (reverse bias). The forward current curves reveal three distinct features: a linear region below 0.4 V, an excess current or “hump” at - 0.5 V, and a deviation from the log (current) versus voltage relationship above 0.6 V. The physical mechanisms that could be operating in these bias regimes are now discussed.
0
0.2
04
0.6
0.8
BIAS IV) Fig. 1. Current-voltage characteristics for a Pd/SiO,/n-Si Schottky barrier structure. The data were obtained with the device in He, in the dark, at room temperature and after exposure to the atmosphere for some time. The inset shows the reciprocal capacitance squared versus applied voltage for the same device.
(a) v< 0.4 V: The forward current may be described by the equation J = J,,exp
i
-e&
1
up to 0.4 V
(1)
where V is the voltage, J is the current density, J, is a constant and n is the ideality factor of the device. Most of the diodes possessed ideality factors in the range 1.3-2.0 at room temperature. This is between values reported for similar devices by Ruths et al. ([2]-ideality factors “very close to unity”) and those reported by Keramati and Zemel ([l]-ideality factors - 5). The J, value in eq (1) may be related to the diode barrier height, es, using the thermionic emission theory[16]. From the intercept on the voltage axis in Fig. 1 and using a value of 110 A cm-*K-* for the effective Richardson constant[l7], a barrier height of 0.83 eV is obtained: this contrasts with a figure of 1.0 eV from the capacitance experiment. These values of barrier height must be considered to be only approximate in view of the nonideality of the diodes, Card and Rhoderick [IS], Cowley[19] and, more recently, Fonash[20] have shown that barrier heights obtained from current-voltage and capacitance-voltage data do not necessarily reflect the true bandbending in the device, especially when an interfacial layer is present between the metal and semiconductor. Over the temperature range investigated the slopes of all the forward characteristics in Fig. 2(a) are similar, which indicates from eq (1) that the ideality factor is inversely proportional to temperature. Over the voltage range O-O.4 V, the forward current exhibits an activation energy of 0.44 eV, less than that expected from simple thermionic emission theory. This contrasts with the work of Keramati and Zemel[l] who have reported no significant temperature dependence of the forward current for their Pd/SiO,/n-Si devices. Above 250 K the reverse current in Fig. 2(b) also shows an activation energy of 0.44 eV; below this value the current tends to saturate and the current-voltage characteristics become softer. Yu and Snow[21] have demonstrated that the semiconductor surface immediately surrounding a Schottky barrier can have a marked effect on the temperature dependence of the forward current; they have shown clearly that if this surface were accumulated then the forward current would be less temperature dependent than for the case of the surface under flat band conditions. This effect can be explained by the presence of excess tunnelling currents associated with the reduced space charge region at the edge of the device. Such currents could also account for the nonsaturating nature of our reverse bias curves [Fig. 2(b)]. A guard ring is usually required to completely eliminate these effects [22]. (b) V = 0.5 V: At - 0.5 V the forward characteristics in Fig. 2(a) exhibit a “hump.” The voltage corresponding to this excess current maximum increases with decreasing temperature. Kar and
Conduction
in Schottky diode hydrogen detectors
mechanisms
91
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2. Current-voltage characteristics measured over a range of temperatures for a Pd/SiO,/n-Si structure. Same conditions as for Fig. 1. (a) Forward bias. (b) Reverse bias.
Dahlke [23] have shown that “humps” in the forward current-voltage characteristics of MIS diodes originate from carrier generation-recombination at interface states and subsequent tunnelling through the oxide to the metal. The density of this excess current is proportional to the surface trap density, N,,, and to the difference in the probability of occupancy of the metal and trap states[24]. Its magnitude, Jss, may be calculated from the equation
where rr is a tunnelling time constant and f,, f, are the bulk silicon and metal state occupancy functions, respectively. The difference in occupancy of the states is established by the bias voltage on the metal. For an n-type semiconductor with a trap located above the Fermi energy at zero bias, Js, will increase exponentially with an increasing positive voltage on the metal (i.e. forward bias) until the Fermi level of the semiconductor crosses the trap energy; at this point the current will begin to saturate. If the band bending in the semiconductor is increased for example, by reducing the temperature, then the excess current maximum will be shifted to larger bias voltages, as an increase in the applied voltage will be necessary to achieve the same surface potential. Using low frequency capacitance data, Kar and Dahlke [23] have obtained the relationship between surface potential and applied voltage for their devices. Sub-
sequently they were able to determine the energy position of the trapping levels associated with the excess currents. From our data we can only estimate the surface trap energy; probably 0.2-0.4 eV below the conduction band edge: (c) V > 0.6 V: Above 0.6 V our forward current-voltage curves [Fig. 2(a)] deviate from the relationship given by eqn (1). The current through the diode appears to become limited by another process. The absolute current values reveal that this limitation is not due to the series resistance of the bulk silicon or to the ohmic back contact. The most likely explanation is that, above 0.6 V, the current is determined by transport across the interfacial oxide layer. Possible conduction processes include quantum mechanical tunnelling and Poole-Frenkel conduction via impurity states. The temperature dependence of the current would seem to favour the latter mechanism. Effects of Ambient 3.2 All the diode characteristics reported in Section 3.1 were measured in He after the device had been exposed to air for several weeks. During the course of this work it was discovered that large variations in the electrical characteristics could be produced by altering the ambient conditions. For example, increases in the conductivity of the diodes were produced by exposing them to hydrogen gas; this effect is now well documented. However changes in the electrical characteristics could also be produced by
M. C. PETTY
mixture was pumped away and replaced by the He exchange gas after approximately 30 min). The corresponding reverse bias capaci:ance date, plotted in the form of reciprocal capacitance squared versus voltage, are shown in Fig. 4. All the curves in Figs. 3 and 4 were stable over several days, but if the devices were left in the dark and in He for several weeks the characteristics approached those of the devices exposed to air (curve A). This was probably due to the small amount of oxygen or water vapour present in the He exchange gas. The electrical characteristics will now be considered in detail in the following sections. 3.2.1 Devices stored in vacuum. Figure 3 reveals a large increase in forward current in the low bias (i.e. < 0.4 V) regime. The capacitance data in Fig. 4 show a slight decrease in the voltage intercept of the Cm2 versus V curve for the vacuum-stored device, implying a small reduction in the diffusion voltage and hence in the Schottky barrier height. This would seem to be confirmed by the slight decrease, on vacuum storage, in the voltage of the excess current peak (0.55 V-O.50 V). The forward and reverse current-voltage characteristics for the vacuum-stored devices were investigated over a range of temperatures. The shape of the forward characteristic was retained down to 210 K. The reverse characteristics were slightly “softer” than for the air treated structures and, in contrast, did not become temperature independent as the temperature was reduced. In fact, over the voltage range +0.3 V (forward bias) to -0.9 V (reverse bias) the current exhibited a thermal activation energy of approximately 0.24 eV. For values of applied voltage less than 0.3 V the reverse current was slightly greater than the forward current. From the ideas outlined in the previous section, the “hump” in the forward current-voltage curve at 0.2 V implies that the vacuum treatment has produced a second set of interface traps in the device. In order to explain
A
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Fig. 3. Forward current-voltage characteristics for a Pd/SiOJn-Si diode. The data were obtained with the device in He, in the dark, and at room temperature. A-Device previously exposed to air for one week. B-
device previously evacuated to - 10e4 torr for one week. C-Device previously exposed to 4000 ppm H, in N,.
keeping the devices under a vacuum ( - 10m4 torr) for some time. Figure 3 shows the forward current-voltage characteristics for the same diode measured at room temperatures and in He, after exposure to air for one week (curve A), after being evacuated to a pressure of - 10m4 torr for one week (curve B) and after exposure to a mixture of 4000 ppm H? in N, (curve C) (in this case the Hz/N,
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Conduction mechanisms in Schottky diode hydrogen detectors the larger currents in the reverse direction, these states must be positioned so that most of them are located below the silicon Fermi level at zero bias [23]. Taking into account the uncertainty in the barrier height of our devices, the position of these states is estimated to be approximately 0.6-0.8 eV below the silicon conduction band edge. Figure 3 reveals that the forward currents in our air-treated and vacuum-stored devices are almost identical above 0.6 V; this is consistent with the device conductivity being dominated by the insulating oxide layer, rather than by the semiconductor. In this voltage region the logarithm of the current density was found to be proportional to the square root of the voltage, and the thermal activation energy was found to decrease with increasing bias. Such behaviour is reminiscent of a Poole-Frenkel conductivity mechanism for which the current density-voltage relationship is given by the expression[25].
5 tr”
2
1cY5-
loq6_
llP7 0 where F is the electric field and y is the Poole-Frenkel coefficient; A+ is a constant equal to the energy of the trapping level from the insulator “band edge.” Direct electron tunnelling through the insulator usually exhibits a weak dependence of current upon temperature[25]. 3.2.2 Devices exposed to hydrogen. A simple explanation for the data shown in Figs. 3 and 4 is that exposure to the hydrogen gas produces a decrease in the barrier height of the device. However, the resulting forward current-voltage curve does not follow the relationship given by eqn (l), but instead shows an excellent fit, over several orders of magnitude of current, to the expression 10gpJ a V2. is seen in Fig. 5. Such behaviour is, of course, expected for Poole-Frenkel conduction. The slope of the line in Fig. 5 may be used to evaluate the constant y in eqn (3). For the specific case where the impurity centres are located below the Fermi level in the insulator (and thus can be regarded as neutral traps) y is related to the relative permittivity of the insulator, e, by eqn (5)
This
i-1 e3
y=
l/2
4nee,
where e is the electronic charge. Assuming that the native oxide layer is SiO, with a relative permittivity of 3.9, a thickness of 1.9 nm is then obtained for this layer; a reasonable value for an oxide formed at room temperature. The effect of temperature on the forward current for our device exposed to 4000 ppm H, is shown in Fig. 6. It is interesting to note that for biases above
93
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1
0.2
0.6
0.8
BtA:; MY1 Fig.
5. Log (forward current) versus bias’.’ for a Pd/SiOJn-Si diode. The data were obtained at room temperature and with the device in He; the diode had previously been exposed to 4000 ppm H, in N,
0.4 V the activation energy becomes bias independent and equal to approximately 0.12 eV. The overall conductivity process through the insulator is, therefore, possibly more complicated than simple Poole-Frenkel conduction. However, from the forgoing discussion it seems probably that, for the devices exposed to 4000 ppm of Hz, the forward current is no longer limited by a depletion layer in the semiconductor. Further evidence for this is provided by the large zero-bias capacitance shown in Fig. 4; this is now dominated by the capacitance of the insulating layer rather than by the semiconductor. At zero bias the silicon surface is, therefore, under flat-band conditions, or even accumulated. A number of experiments were performed in order to investigate how the Pd/SiO,/n-Si diodes would behave when exposed to concentrations of H, lower than 4OOO ppm. Figure 7 shows the forward characteristics for one device after it had been exposed to 300, 400 and 500 ppm of H, in N2. All these data were taken at room temperature, and with the device in the H,/N, mixture. For the two higher hydrogen concentrations the current-voltage characteristics are very similar, indicating that the conduction processes are probably the same. In fact, when plotted in the form of eqn (4) two straight lines are obtained with slopes identical to that shown in Fig. 5. The curve for the device exposed to 300 ppm H, is clearly different to those for the other two gas concentrations, exhibiting a distinct “hump” at 0.2 V: it is similar to that obtained when the devices were stored
M. c.
PElTY
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Fig. 6. Forward current versus reciprocal temperature, at various biases, for the device whose current-voltage characteristics are shown in Fig. 5.
in a vacuum (Fig. 3). We therefore exposure to hydrogen can produce effects in our devices. 3.3
conclude surface
1
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0.2
0-L
*
BAS (VI Fig. 7. Forward current-voltage characteristics for a Pd/SiO,/n-Si diode after exposure to different concentrations of H, in N,. The data were recorded in the dark, at room temperature, and with the devices in the H,/N, mixture.
that state
Band Diagrams The results discussed in the previous sections can probably best be summarized in the form of potential energy diagrams for the devices under different ambient conditions. The proposed band diagrams are presented in Fig. 8; all the devices are shown at a small (- 0.2 V) forward bias. Figure 8(a) is the structure for the devices exposed to air, Fig. 8(b) for the devices stored in a vacuum or exposed to low (i.e. 100’s ppm) hydrogen concentrations, and Fig. 8(c) for the devices treated in higher concentrations (1000’s ppm) of hydrogen gas. The dominant conduction mechanisms are shown as JT, Jx, J, and J,: J, represents thermionic emission over the top of the barrier; J, and J, are currents which result from generation-recombination/tunnelling at the interface traps X and Y, respectively; J, represents the current (possibly a Poole-Frenkel process) through the oxide layer. Figure 8(a) shows one set of interface traps, X, positioned above the Fermi level of the zero biased device. As discussed in Section 3.2.1, these states will contribute an excess current as the device is forward biased and they become populated, The main conduction mechanism at low forward voltages is thought to be thermionic emission over the barrier, JT. On exposure to low hydrogen concentrations, or after storage in a vacuum, Fig. 8(b) reveals that a
second set of interface states, Y, is formed. At zero bias most of these states are located close to the Fermi energy, explaining the increased reverse current in the diodes. Assuming that the states are donor-like, the fraction of them that are located above the Fermi energy will contribute an additional positive charge to the system: this will decrease the band bending in the semiconductor, as observed. On exposure to higher concentrations of hydrogen gas, one of two processes could explain our observed effective decrease in the Schottky barrier heights, $B~. Neglecting image force effects, as zero bias the barrier height is related to the work function of the palladium, +,,, , and to the electron affinity of the silicon, xs, by the expression +B=&,-x.7-A
(6)
where A is the voltage drop across the oxide layer. This may be seen from Fig. 8(a). A decrease in (pB on hydrogen exposure can therefore be produced by either a decrease in +,,,, or by a increase in A. The former process is that postulated by LundstrBm[l3]. The increase in the voltage across the insulator can be accomplished by the introduction of a positive charge (interface traps) at the oxide-semiconductor interface: this idea has received much attention from Keramati and Zemel[l,14]. However, in the case of the latter mechanism, applying a forward bias to the diode would result in a decrease in the electric field
Conduction
mechanisms
in Schottky
(cl Fig. g. Proposed band structure for our Pd;SiO,/n-Si diodes. The diagrams all show the devices with a small (- 0.2 V) forward bias applied. (a) Device after exposure to air. (b) Device after treatment in a vacuum or exposure to low (loo’s ppm) concentrations of H,. (c) Device after exposure to4OOOppmH, inN,.
in the oxide
layer. This would very much complicate the interpretation, in terms of a Poole-Frenkel mechanism, of the current-voltage characteristics for our devices exposed to high concentrations of H,. For this reason we believe that the most significant hydrogen effect in these devices is a reduction in the metal work function; we therefore favour the band diagram shown in Fig. 8(c). At zero bias the silicon surface is almost accumulated and the oxide field is either zero, or in the opposite sense to that shown in Fig. 8(a) and (b). An applied forward bias will increase this electric field, accounting for the bias dependent activation energy that is observed for the resulting current. 3.4 Mechanism /or Gas Sensitivity Before discussing the possible mechanisms that might account for our experimental results, it is useful to review that many reports of hydrogen sensi-
diode hydrogen
detectors
95
tivity in Pd/semiconductor structures that are now in the literature. These may be conveniently summarized as follows. 1. Hydrogen sensitivity has been reported for Schottky barrier structures, for MIS diodes, for MIS structures in which the insulator is relatively thick (- 100 nm) and for field effect transistors (FETs). The phenomenon does not appear to be restricted to any particular semiconductor or to any particular semiconductor-insulator combination. The only common factor in all these devices is the palladium metal. 2. In the case of thick insulator MIS structures and FETs, the only reported hydrogen effects are shifts in the flat-band voltage of the MIS structures and in the threshold voltage of the FETs [26-311. Furthermore Poteat and Lalevic[26] have reported on the interface state density for Pd/SiO,/Si and Pd/Si,N,/Si “ thick” insulator MIS devices befcre and after exposure to hydrogen. These data, which were derived from low frequency capacitance experiments, revealed no significant change in either the density of interface states or in their distribution across the bandgap after hydrogen exposure. 3. Reports on the effects of H, on MIS Schottky barrier type diodes fall into two distinct categories; one in which only a simple work function change is observed [2,3,7,8,11], and the other in which hydrogen induced surface states are thought to play a significant role [l, 4,5,9, lo]. It is interesting to note that devices in the former category show almost ideal diode characteristics (e.g. [2]) whereas those in the latter category (including this work) exhibit diode ideality factors that are greater than unity. All the reports of surface state effects on Pd/t;iO,/Si diodes [l, 4,5] appear to have been based on devices in which the oxide was grown in the atmosphere. Two independent mechanisms are necessary in order to explain fully all of the above experimental observations. We propose that for thick insulator MIS structures, FETs and for ideal MIS diodes the sensitivity to hydrogen is simply the result of a work function change of the palladium metal. Possible mechanisms for this are the dipole layer theory of Lundstriim[l3] or the electrical double layer idea of Yamamoto et af.[8]. In the case of nonideal MIS diodes, we postulate that interface state effects can become important, or even dominant. These interface states are very likely associated with the nature of the insulating layer. For Si/SiO, devices in which the oxide was grown in the atmosphere, it is probable that this layer will be highly imperfect; specific defects that could exist include trivalent silicon (excess Si) and nonbridging oxygen (excess 0) [15]. It is certainly possible that defects in the SiO, layer and located close to the Si/SiO, interface can act as trapping centres[32], accounting for the effects seen in our devices and for the frequency dependence of admittance reported by other workers[l]. It is also possible that such centres are responsible for the DLTS signal reported previously by us[5]. Expo-
M. C.
96
sure to the atmosphere results in the elimination of the trapping centres. For example, Ponpon and Siffert[33] have suggested that oxygen exposure can saturate free silicon bonds in a native SiO, layer and thus explain the increase in the barrier heights of their metal-silicon Schottky diodes. Another possibility is that water removes the trivalent silicon defects according to the reacticn [34] -Si
Si-
+ H,O -_) +jiOH
+ -SiH
where &!ii designates a Si atom bound to 3 bridging 0 atoms, and the dots represent electrons that do not participate in a bond but are still attached to an atom. The effect of hydrogen is twofold: in the first instance it is adsorbed onto the Pd metal, reducing its work function; secondly, it is effective at removing 0 and/or OH from the oxide, thereby increasing the number of interfacial traps by the reverse of one of the processes discussed above. For any particular hydrogen concentration, the effect which dominates the electrical characteristics of the diode will clearly depend on the number of defects and hence on the nature of the oxide layer. For the devices reported in this work it would appear that the work function change is the most important effect at high (i.e. 1000’s ppm) H, concentrations. However, at lower concentrations the H, has a similar effect on our devices as storage in a vacuum, indicating that interface states are significant in determining the device characteristics. The fact that hydrogen produces interface states in our devices but, in other work, is usually associated with a decrease in the number of surface traps is probably due to the very different nature of a native SiO, layer compared with that of a layer grown at elevated temperatures in oxygen. Keramati and Zemel[l4] have identified two hydrogen induced interface traps in their Pd/SiO,/n-Si devices, with approximate energies of 0.65 and 0.4 eV below the Si conduction band edge. These authors experiments were performed with H, concentrations in the 100’s ppm range. It is interesting to note that the two traps (or sets of traps) identified from this work could be located at similar energies. However, our experiments have revealed that only one of these traps appears to be affected by different ambient conditions.
4. CONCLUSIONS
The electrical characteristics of PdSiO,/n-Si Schottky barrier type devices at different temperatures and under different ambient conditions have been reported. The forward conductivity of the devices is dominated at different bias voltages by one of the following processes: (i) emission over the Schottky barrier; (ii) generation-recombination/tunnelling processes at interface states; or (iii) conduction through the oxide layer.
PETTY
Two sets of interface states have been identified; one of these can be effectively eliminated by exposing the devices to air. These surface states are thought to originate from an imperfect oxide layer. It is suggested that the different effects seen by workers on Pd/SiO,/Si structures are related to the nature of the oxide layer produced in different iaboratoties. Exposure to hydrogen gas is thought to both increase the interface state density in the devices and also to lower the work function of the palladium metal. The work reported here has concentrated on studying the response of Pd/SiO,/n-Si diodes to Hz/N, mixtures. Clearly, if these devices are to be used commericially, conduction
H,O)
is also
a
detailed
mechanisms
understanding
in other
ambients
of
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desirable.
Acknowledgements--The author wishes to thank Prof. G. G. Roberts for many useful discussions and for critically reading this manuscript.
REFERENCES 1. B. Keramati and J. N. Zemel, J. Appl. Ph_vs. 53. 1091 (1982). 2. P. F. Ruths, S. Ashok, S. J. Fonash and J. M. Ruths, IEEE Trans. Hectron Deo. ED-28, 1003 (1981). 3. M. S. Shivaraman, I. Lundstrom, C. Svensson and H. Hammarsten, Electron Len., 12, 483 (1976). 4. A. Diligenti, M. Stagi and V. Ciuti, Solid-St. Commun. 45, 347 (1983). 5. M. C. Petty, Electron Left. 18, 314 (1982). 6. M. C. Steele and B. A. Maclver, Appl. Phys. L&t. 2% 687 (1976). 7. K. Ito, Suquce Sci. 86, 345, (1979). S. Tonomura, T. Matsuoka and H. 8. N. Yamamoto, Tsubomura, Surfuce Sci. 92, 400 (1980). 9. M. Yousuf, B. Kuhyev, B. Lalevic and T. L. Poteat, Solid-St. Electron. 25, 753 (1982). 10. T. L. Poteat, B. Lalevic, B. Kuliyev, M. Yousuf and M Chen., J. Electron. Mat. 12, 181 (1983). S. Tonomura, T. Matsuoka and H. 11. N. Yamamoto, Tsubomura, J. Appl. Phys. 52, 6227 (1981). G. Fortunata, G. Petrocco and C. 12. A. D’Amico, Coluzza, Appl. Phys. ht. 42, 964 (1983). Sensors and Actuators 1, 403 (1981). 13. I. Lundstrom, and J. N. Zemel, J. Appl. Ph_vs. 53, 1100 14. B. Keramati (1982). and J. R. Brews, MOS Physics und 15. E. H. Nicollian Technology, Wiley, New York, 814 (1982). Metal-Semiconductor Contucts. 16. E. H. Rhoderick, Clarendon. Oxford (1978). 17. S. M. Sze. Phvsics of Semiconductor Deuices, Wiley, New York’(1969). . J. Phvs. D.: Appl. 18. H. C. Card and E. H. Rhoderick, Phvs. 4, 1589 (1971). 19. A. M. Cowley, J. Appt. Phvs. 31, 3024 (1966). 20. S. J. Fonash, J. Appl. Phys. 54, 1966 (1983). 21. A. Y. C. Yu and E. H. Snow, J. Appi. Phvs. 39, 3008 (1968). 22. A. N. Saxena, Surface Sci. 13, 151 (1969). 23. S. Kar and W. E. Dahlke, Solid-St. Electron. 15, 869 (1972). 24. L. B. Freeman and W. E. Dahlke, Solid-St. Electron. 13, 1483 (1970). 25. G. G. Roberts, N. Apsley and R. W. Munn. Phvs. Rep. 60, 61 (1980). 26. T. L. Poteat and B. Lalevic, IEEE Trtms. Electron Deu. ED-29, 123 (1982).
Conduction
mechanisms
in Schottky
27. M. C. Steele, J. W. Hile and B. A. Maclver. J. Appl. Ph_vs. 47, 2537 (1976). 28. P. J. Martin, Ph.D. thesis, Univ. Durham, U.K. (1980). 29. I. Lundstrcm, S. Shivaraman, C. Svensson and L. Lundkvist, Appl. Phys. L.&t. 26, 55 (1975). 30. K. I. Lundstriim, M. S. Shivamman and C. M. Svensson, J. Appl. Phys. 46. 3876 (1975).
diode hydrogen
detectors
97
31. M. Armgarth, Ph.D. thesis, Univ. Linkaping, Sweden (1983). 32. F. P. Heiman and G. Warfield. IEEE Trans. Electron Dev. ED-12, 167 (1965). 33. J. P. Ponpon and P. Siffert, J. Appl. Phy. 49, 6004 (1978). 34. A. G. Revesz, J. Electrochrm. Sot. 126, 122 (1979).
1986
0038-1101/X6 $3.00+ 00 b 1986 Pergamon Press Ltd.
CONDUCTION MECHANISMS IN Pd/SiO,/n-Si SCHOTTKY DIODE HYDROGEN DETECTORS M. C.
PETTY
Department of Applied Physics and Electronics, University of Durham, South Road, Durham DHl 3LE, U.K. (Received
28 Much
1984; in revisedform
25 May 1984)
Abstract-The electrical characteristics of Pd/Si02/n-Si Schottky barrier type structures are reported over a range of temperatures and after exposure to differem ambient conditions. A number of conduction mechanisms through the diodes have been identified. Exposure to hydrogen gas is shown to produce an increase in the conductivity of the diodes and a voltage shift in the capacitance-bias characteristics; this is attributed to both an increase in the interface state density and also to a reduction in the work function of the palladium. It is suggested that the nature of the interfacial oxide layer is of paramount determining the dominant conduction process through the devices.
1. INTRODUCTION
importance
in
on the Pd surface. The chemical bond between the Pd and these oxygen anions is though to form an electrical double layer at the Pd surface and thus affect the work function of the metal. The decrease in work function on exposure to hydrogen is attributed to the reaction between hydrogen and the adsorbed anions. A completely different model is offered by Keramati and Zemel[l] to account for the effects of hydrogen on the electrical characteristics of their Pd/SiO,/n-Si Schottky diodes. These authors have suggested that H, can penetrate the thin oxide and introduce traps at the Si/SiO, interface. Theoretical modelling has shown that at least two hydrogen induced surface states are required in order to explain the admittance characteristics of these diodes [14]. Some evidence to support this surface state model has been provided by Diligenti ef al. [4] on the basis of capacitance measurements, and by Petty[S] using deep level transient spectroscopy (DLTS). However, it is not clear how these ideas fit in with the accepted effect of hydrogen on the Si/SiO, interface, namely that annealing in H, gas decreases the interface state density[l5]. In this paper we report on a detailed investigation into the electrical characteristics of Pd/SiO,/n-Si Schottky barrier devices. Measurements are reported over a range of temperatures and under different ambients. The results of these experiments reveal that, under certain conditions, interface states can play a dominant role in determining the electrical characteristics of the diodes.
Interest in the development of solid-state chemical sensors has increased substantially over the past few years. If integrated with silicon microelectronic circuitry such transducers could form the basis for a new generation of sensing devices. One particular structure that has received considerable attention is a hydrogen detector based on a palladium/n-type semiconductor Schottky diode. Devices have now been fabricated on n-type Si[l-51, CdS[6], ZnO[7], TiO, [8], InP[9], GaAs[lO], GaP[ll] and hydrogenated amorphous silicon[12]. It should be noted that most of these Schottky barrier devices are, in practice, metal-insulator-semiconductor (MIS) structures. In fact, in the case of devices fabricated on silicon, a thin (< 5 nm) oxide layer is deliberately introduced between the metal and semiconductor in order to prevent the formation of palladium silicide[l]. Exposure to hydrogen gas invariably results in changes in the current-voltage and capacitance-voltage characteristics of these diodes. Unfortunately, despite the existence of a considerable quantity of experimental data, there is still some debate as to the origin of the hydrogen sensitivity. Shivaraman et al. [3] and Ruths et al.[2] have attributed changes in the characteristics of devices fabricated on Si as due to a work function decrease of the Pd on exposure to H,. Lundstrom[l3] has explained this process in more detail: hydrogen molecules are first dissociated on the catalytic metal surface and then the atoms are adsorbed onto the metal. Subsequently, some of the hydrogen atoms diffuse through the palladium film and are adsorbed at the metal-insulator interface; these atoms become polarized and give rise to a dipole layer which effectively changes the work function of the palladium. An alternative explanation is given by Yamamoto et al. [8] to account for hydrogen effects on Pd/TiO, diodes. In this case it is proposed that O2 is chemisorbed in the form of anions (e.g. O-, 0; and O’-)
2.
EXPERIMENTAL
PROCEDURE
The fabrication of the Pd/SiO,/n-Si diodes has been described previously[S]. The resulting devices were mounted in an Oxford Instruments DN704 He exchange gas cryostat and electrical contacts were made to the Pd and Au/Sb using air drying Ag paste. The temperature of the sample could be varied 89
M. C. PETTI
90
using an Oxford Instruments DTC2 temperature controller. Current-voltage characteristics were measured using a Time Electronics voltage calibrator and a Keithley 410A picoammeter. The capacitance of the devices, at 1 MHz, was measured with a Boonton 72 BD capacitance meter. A Signal Instruments gas blender was used to produce various concentrations of hydrogen in nitrogen. 3. RESULTS
AND DISCUSSION
3.1 Diode Characteristics Most of the devices exhibited good diode behaviour with typical current rectification ratios of 104-10’ at a bias of 0.5 V. Figure 1 shows the room temperature current-voltage characteristics of a typical device; the inset shows a plot of reciprocal capacitance squared versus applied voltage for the same device. In order to elucidate the conduction mechanisms in our diodes, the current-voltage characteristics were measured as a function of temperature. These are shown in Fig. 2(a) (forward bias) and Fig. 2(b) (reverse bias). The forward current curves reveal three distinct features: a linear region below 0.4 V, an excess current or “hump” at - 0.5 V, and a deviation from the log (current) versus voltage relationship above 0.6 V. The physical mechanisms that could be operating in these bias regimes are now discussed.
0
0.2
04
0.6
0.8
BIAS IV) Fig. 1. Current-voltage characteristics for a Pd/SiO,/n-Si Schottky barrier structure. The data were obtained with the device in He, in the dark, at room temperature and after exposure to the atmosphere for some time. The inset shows the reciprocal capacitance squared versus applied voltage for the same device.
(a) v< 0.4 V: The forward current may be described by the equation J = J,,exp
i
-e&
1
up to 0.4 V
(1)
where V is the voltage, J is the current density, J, is a constant and n is the ideality factor of the device. Most of the diodes possessed ideality factors in the range 1.3-2.0 at room temperature. This is between values reported for similar devices by Ruths et al. ([2]-ideality factors “very close to unity”) and those reported by Keramati and Zemel ([l]-ideality factors - 5). The J, value in eq (1) may be related to the diode barrier height, es, using the thermionic emission theory[16]. From the intercept on the voltage axis in Fig. 1 and using a value of 110 A cm-*K-* for the effective Richardson constant[l7], a barrier height of 0.83 eV is obtained: this contrasts with a figure of 1.0 eV from the capacitance experiment. These values of barrier height must be considered to be only approximate in view of the nonideality of the diodes, Card and Rhoderick [IS], Cowley[19] and, more recently, Fonash[20] have shown that barrier heights obtained from current-voltage and capacitance-voltage data do not necessarily reflect the true bandbending in the device, especially when an interfacial layer is present between the metal and semiconductor. Over the temperature range investigated the slopes of all the forward characteristics in Fig. 2(a) are similar, which indicates from eq (1) that the ideality factor is inversely proportional to temperature. Over the voltage range O-O.4 V, the forward current exhibits an activation energy of 0.44 eV, less than that expected from simple thermionic emission theory. This contrasts with the work of Keramati and Zemel[l] who have reported no significant temperature dependence of the forward current for their Pd/SiO,/n-Si devices. Above 250 K the reverse current in Fig. 2(b) also shows an activation energy of 0.44 eV; below this value the current tends to saturate and the current-voltage characteristics become softer. Yu and Snow[21] have demonstrated that the semiconductor surface immediately surrounding a Schottky barrier can have a marked effect on the temperature dependence of the forward current; they have shown clearly that if this surface were accumulated then the forward current would be less temperature dependent than for the case of the surface under flat band conditions. This effect can be explained by the presence of excess tunnelling currents associated with the reduced space charge region at the edge of the device. Such currents could also account for the nonsaturating nature of our reverse bias curves [Fig. 2(b)]. A guard ring is usually required to completely eliminate these effects [22]. (b) V = 0.5 V: At - 0.5 V the forward characteristics in Fig. 2(a) exhibit a “hump.” The voltage corresponding to this excess current maximum increases with decreasing temperature. Kar and
Conduction
in Schottky diode hydrogen detectors
mechanisms
91
-3
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:
-7
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9
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BIAS IV)
(a)
(b)
2. Current-voltage characteristics measured over a range of temperatures for a Pd/SiO,/n-Si structure. Same conditions as for Fig. 1. (a) Forward bias. (b) Reverse bias.
Dahlke [23] have shown that “humps” in the forward current-voltage characteristics of MIS diodes originate from carrier generation-recombination at interface states and subsequent tunnelling through the oxide to the metal. The density of this excess current is proportional to the surface trap density, N,,, and to the difference in the probability of occupancy of the metal and trap states[24]. Its magnitude, Jss, may be calculated from the equation
where rr is a tunnelling time constant and f,, f, are the bulk silicon and metal state occupancy functions, respectively. The difference in occupancy of the states is established by the bias voltage on the metal. For an n-type semiconductor with a trap located above the Fermi energy at zero bias, Js, will increase exponentially with an increasing positive voltage on the metal (i.e. forward bias) until the Fermi level of the semiconductor crosses the trap energy; at this point the current will begin to saturate. If the band bending in the semiconductor is increased for example, by reducing the temperature, then the excess current maximum will be shifted to larger bias voltages, as an increase in the applied voltage will be necessary to achieve the same surface potential. Using low frequency capacitance data, Kar and Dahlke [23] have obtained the relationship between surface potential and applied voltage for their devices. Sub-
sequently they were able to determine the energy position of the trapping levels associated with the excess currents. From our data we can only estimate the surface trap energy; probably 0.2-0.4 eV below the conduction band edge: (c) V > 0.6 V: Above 0.6 V our forward current-voltage curves [Fig. 2(a)] deviate from the relationship given by eqn (1). The current through the diode appears to become limited by another process. The absolute current values reveal that this limitation is not due to the series resistance of the bulk silicon or to the ohmic back contact. The most likely explanation is that, above 0.6 V, the current is determined by transport across the interfacial oxide layer. Possible conduction processes include quantum mechanical tunnelling and Poole-Frenkel conduction via impurity states. The temperature dependence of the current would seem to favour the latter mechanism. Effects of Ambient 3.2 All the diode characteristics reported in Section 3.1 were measured in He after the device had been exposed to air for several weeks. During the course of this work it was discovered that large variations in the electrical characteristics could be produced by altering the ambient conditions. For example, increases in the conductivity of the diodes were produced by exposing them to hydrogen gas; this effect is now well documented. However changes in the electrical characteristics could also be produced by
M. C. PETTY
mixture was pumped away and replaced by the He exchange gas after approximately 30 min). The corresponding reverse bias capaci:ance date, plotted in the form of reciprocal capacitance squared versus voltage, are shown in Fig. 4. All the curves in Figs. 3 and 4 were stable over several days, but if the devices were left in the dark and in He for several weeks the characteristics approached those of the devices exposed to air (curve A). This was probably due to the small amount of oxygen or water vapour present in the He exchange gas. The electrical characteristics will now be considered in detail in the following sections. 3.2.1 Devices stored in vacuum. Figure 3 reveals a large increase in forward current in the low bias (i.e. < 0.4 V) regime. The capacitance data in Fig. 4 show a slight decrease in the voltage intercept of the Cm2 versus V curve for the vacuum-stored device, implying a small reduction in the diffusion voltage and hence in the Schottky barrier height. This would seem to be confirmed by the slight decrease, on vacuum storage, in the voltage of the excess current peak (0.55 V-O.50 V). The forward and reverse current-voltage characteristics for the vacuum-stored devices were investigated over a range of temperatures. The shape of the forward characteristic was retained down to 210 K. The reverse characteristics were slightly “softer” than for the air treated structures and, in contrast, did not become temperature independent as the temperature was reduced. In fact, over the voltage range +0.3 V (forward bias) to -0.9 V (reverse bias) the current exhibited a thermal activation energy of approximately 0.24 eV. For values of applied voltage less than 0.3 V the reverse current was slightly greater than the forward current. From the ideas outlined in the previous section, the “hump” in the forward current-voltage curve at 0.2 V implies that the vacuum treatment has produced a second set of interface traps in the device. In order to explain
A
/, 04
0.2
BIAS
,
,
0.6
08
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Fig. 3. Forward current-voltage characteristics for a Pd/SiOJn-Si diode. The data were obtained with the device in He, in the dark, and at room temperature. A-Device previously exposed to air for one week. B-
device previously evacuated to - 10e4 torr for one week. C-Device previously exposed to 4000 ppm H, in N,.
keeping the devices under a vacuum ( - 10m4 torr) for some time. Figure 3 shows the forward current-voltage characteristics for the same diode measured at room temperatures and in He, after exposure to air for one week (curve A), after being evacuated to a pressure of - 10m4 torr for one week (curve B) and after exposure to a mixture of 4000 ppm H? in N, (curve C) (in this case the Hz/N,
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Conduction mechanisms in Schottky diode hydrogen detectors the larger currents in the reverse direction, these states must be positioned so that most of them are located below the silicon Fermi level at zero bias [23]. Taking into account the uncertainty in the barrier height of our devices, the position of these states is estimated to be approximately 0.6-0.8 eV below the silicon conduction band edge. Figure 3 reveals that the forward currents in our air-treated and vacuum-stored devices are almost identical above 0.6 V; this is consistent with the device conductivity being dominated by the insulating oxide layer, rather than by the semiconductor. In this voltage region the logarithm of the current density was found to be proportional to the square root of the voltage, and the thermal activation energy was found to decrease with increasing bias. Such behaviour is reminiscent of a Poole-Frenkel conductivity mechanism for which the current density-voltage relationship is given by the expression[25].
5 tr”
2
1cY5-
loq6_
llP7 0 where F is the electric field and y is the Poole-Frenkel coefficient; A+ is a constant equal to the energy of the trapping level from the insulator “band edge.” Direct electron tunnelling through the insulator usually exhibits a weak dependence of current upon temperature[25]. 3.2.2 Devices exposed to hydrogen. A simple explanation for the data shown in Figs. 3 and 4 is that exposure to the hydrogen gas produces a decrease in the barrier height of the device. However, the resulting forward current-voltage curve does not follow the relationship given by eqn (l), but instead shows an excellent fit, over several orders of magnitude of current, to the expression 10gpJ a V2. is seen in Fig. 5. Such behaviour is, of course, expected for Poole-Frenkel conduction. The slope of the line in Fig. 5 may be used to evaluate the constant y in eqn (3). For the specific case where the impurity centres are located below the Fermi level in the insulator (and thus can be regarded as neutral traps) y is related to the relative permittivity of the insulator, e, by eqn (5)
This
i-1 e3
y=
l/2
4nee,
where e is the electronic charge. Assuming that the native oxide layer is SiO, with a relative permittivity of 3.9, a thickness of 1.9 nm is then obtained for this layer; a reasonable value for an oxide formed at room temperature. The effect of temperature on the forward current for our device exposed to 4000 ppm H, is shown in Fig. 6. It is interesting to note that for biases above
93
/
1
0.2
0.6
0.8
BtA:; MY1 Fig.
5. Log (forward current) versus bias’.’ for a Pd/SiOJn-Si diode. The data were obtained at room temperature and with the device in He; the diode had previously been exposed to 4000 ppm H, in N,
0.4 V the activation energy becomes bias independent and equal to approximately 0.12 eV. The overall conductivity process through the insulator is, therefore, possibly more complicated than simple Poole-Frenkel conduction. However, from the forgoing discussion it seems probably that, for the devices exposed to 4000 ppm of Hz, the forward current is no longer limited by a depletion layer in the semiconductor. Further evidence for this is provided by the large zero-bias capacitance shown in Fig. 4; this is now dominated by the capacitance of the insulating layer rather than by the semiconductor. At zero bias the silicon surface is, therefore, under flat-band conditions, or even accumulated. A number of experiments were performed in order to investigate how the Pd/SiO,/n-Si diodes would behave when exposed to concentrations of H, lower than 4OOO ppm. Figure 7 shows the forward characteristics for one device after it had been exposed to 300, 400 and 500 ppm of H, in N2. All these data were taken at room temperature, and with the device in the H,/N, mixture. For the two higher hydrogen concentrations the current-voltage characteristics are very similar, indicating that the conduction processes are probably the same. In fact, when plotted in the form of eqn (4) two straight lines are obtained with slopes identical to that shown in Fig. 5. The curve for the device exposed to 300 ppm H, is clearly different to those for the other two gas concentrations, exhibiting a distinct “hump” at 0.2 V: it is similar to that obtained when the devices were stored
M. c.
PElTY
.2 I-
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Fig. 6. Forward current versus reciprocal temperature, at various biases, for the device whose current-voltage characteristics are shown in Fig. 5.
in a vacuum (Fig. 3). We therefore exposure to hydrogen can produce effects in our devices. 3.3
conclude surface
1
k
0.2
0-L
*
BAS (VI Fig. 7. Forward current-voltage characteristics for a Pd/SiO,/n-Si diode after exposure to different concentrations of H, in N,. The data were recorded in the dark, at room temperature, and with the devices in the H,/N, mixture.
that state
Band Diagrams The results discussed in the previous sections can probably best be summarized in the form of potential energy diagrams for the devices under different ambient conditions. The proposed band diagrams are presented in Fig. 8; all the devices are shown at a small (- 0.2 V) forward bias. Figure 8(a) is the structure for the devices exposed to air, Fig. 8(b) for the devices stored in a vacuum or exposed to low (i.e. 100’s ppm) hydrogen concentrations, and Fig. 8(c) for the devices treated in higher concentrations (1000’s ppm) of hydrogen gas. The dominant conduction mechanisms are shown as JT, Jx, J, and J,: J, represents thermionic emission over the top of the barrier; J, and J, are currents which result from generation-recombination/tunnelling at the interface traps X and Y, respectively; J, represents the current (possibly a Poole-Frenkel process) through the oxide layer. Figure 8(a) shows one set of interface traps, X, positioned above the Fermi level of the zero biased device. As discussed in Section 3.2.1, these states will contribute an excess current as the device is forward biased and they become populated, The main conduction mechanism at low forward voltages is thought to be thermionic emission over the barrier, JT. On exposure to low hydrogen concentrations, or after storage in a vacuum, Fig. 8(b) reveals that a
second set of interface states, Y, is formed. At zero bias most of these states are located close to the Fermi energy, explaining the increased reverse current in the diodes. Assuming that the states are donor-like, the fraction of them that are located above the Fermi energy will contribute an additional positive charge to the system: this will decrease the band bending in the semiconductor, as observed. On exposure to higher concentrations of hydrogen gas, one of two processes could explain our observed effective decrease in the Schottky barrier heights, $B~. Neglecting image force effects, as zero bias the barrier height is related to the work function of the palladium, +,,, , and to the electron affinity of the silicon, xs, by the expression +B=&,-x.7-A
(6)
where A is the voltage drop across the oxide layer. This may be seen from Fig. 8(a). A decrease in (pB on hydrogen exposure can therefore be produced by either a decrease in +,,,, or by a increase in A. The former process is that postulated by LundstrBm[l3]. The increase in the voltage across the insulator can be accomplished by the introduction of a positive charge (interface traps) at the oxide-semiconductor interface: this idea has received much attention from Keramati and Zemel[l,14]. However, in the case of the latter mechanism, applying a forward bias to the diode would result in a decrease in the electric field
Conduction
mechanisms
in Schottky
(cl Fig. g. Proposed band structure for our Pd;SiO,/n-Si diodes. The diagrams all show the devices with a small (- 0.2 V) forward bias applied. (a) Device after exposure to air. (b) Device after treatment in a vacuum or exposure to low (loo’s ppm) concentrations of H,. (c) Device after exposure to4OOOppmH, inN,.
in the oxide
layer. This would very much complicate the interpretation, in terms of a Poole-Frenkel mechanism, of the current-voltage characteristics for our devices exposed to high concentrations of H,. For this reason we believe that the most significant hydrogen effect in these devices is a reduction in the metal work function; we therefore favour the band diagram shown in Fig. 8(c). At zero bias the silicon surface is almost accumulated and the oxide field is either zero, or in the opposite sense to that shown in Fig. 8(a) and (b). An applied forward bias will increase this electric field, accounting for the bias dependent activation energy that is observed for the resulting current. 3.4 Mechanism /or Gas Sensitivity Before discussing the possible mechanisms that might account for our experimental results, it is useful to review that many reports of hydrogen sensi-
diode hydrogen
detectors
95
tivity in Pd/semiconductor structures that are now in the literature. These may be conveniently summarized as follows. 1. Hydrogen sensitivity has been reported for Schottky barrier structures, for MIS diodes, for MIS structures in which the insulator is relatively thick (- 100 nm) and for field effect transistors (FETs). The phenomenon does not appear to be restricted to any particular semiconductor or to any particular semiconductor-insulator combination. The only common factor in all these devices is the palladium metal. 2. In the case of thick insulator MIS structures and FETs, the only reported hydrogen effects are shifts in the flat-band voltage of the MIS structures and in the threshold voltage of the FETs [26-311. Furthermore Poteat and Lalevic[26] have reported on the interface state density for Pd/SiO,/Si and Pd/Si,N,/Si “ thick” insulator MIS devices befcre and after exposure to hydrogen. These data, which were derived from low frequency capacitance experiments, revealed no significant change in either the density of interface states or in their distribution across the bandgap after hydrogen exposure. 3. Reports on the effects of H, on MIS Schottky barrier type diodes fall into two distinct categories; one in which only a simple work function change is observed [2,3,7,8,11], and the other in which hydrogen induced surface states are thought to play a significant role [l, 4,5,9, lo]. It is interesting to note that devices in the former category show almost ideal diode characteristics (e.g. [2]) whereas those in the latter category (including this work) exhibit diode ideality factors that are greater than unity. All the reports of surface state effects on Pd/t;iO,/Si diodes [l, 4,5] appear to have been based on devices in which the oxide was grown in the atmosphere. Two independent mechanisms are necessary in order to explain fully all of the above experimental observations. We propose that for thick insulator MIS structures, FETs and for ideal MIS diodes the sensitivity to hydrogen is simply the result of a work function change of the palladium metal. Possible mechanisms for this are the dipole layer theory of Lundstriim[l3] or the electrical double layer idea of Yamamoto et af.[8]. In the case of nonideal MIS diodes, we postulate that interface state effects can become important, or even dominant. These interface states are very likely associated with the nature of the insulating layer. For Si/SiO, devices in which the oxide was grown in the atmosphere, it is probable that this layer will be highly imperfect; specific defects that could exist include trivalent silicon (excess Si) and nonbridging oxygen (excess 0) [15]. It is certainly possible that defects in the SiO, layer and located close to the Si/SiO, interface can act as trapping centres[32], accounting for the effects seen in our devices and for the frequency dependence of admittance reported by other workers[l]. It is also possible that such centres are responsible for the DLTS signal reported previously by us[5]. Expo-
M. C.
96
sure to the atmosphere results in the elimination of the trapping centres. For example, Ponpon and Siffert[33] have suggested that oxygen exposure can saturate free silicon bonds in a native SiO, layer and thus explain the increase in the barrier heights of their metal-silicon Schottky diodes. Another possibility is that water removes the trivalent silicon defects according to the reacticn [34] -Si
Si-
+ H,O -_) +jiOH
+ -SiH
where &!ii designates a Si atom bound to 3 bridging 0 atoms, and the dots represent electrons that do not participate in a bond but are still attached to an atom. The effect of hydrogen is twofold: in the first instance it is adsorbed onto the Pd metal, reducing its work function; secondly, it is effective at removing 0 and/or OH from the oxide, thereby increasing the number of interfacial traps by the reverse of one of the processes discussed above. For any particular hydrogen concentration, the effect which dominates the electrical characteristics of the diode will clearly depend on the number of defects and hence on the nature of the oxide layer. For the devices reported in this work it would appear that the work function change is the most important effect at high (i.e. 1000’s ppm) H, concentrations. However, at lower concentrations the H, has a similar effect on our devices as storage in a vacuum, indicating that interface states are significant in determining the device characteristics. The fact that hydrogen produces interface states in our devices but, in other work, is usually associated with a decrease in the number of surface traps is probably due to the very different nature of a native SiO, layer compared with that of a layer grown at elevated temperatures in oxygen. Keramati and Zemel[l4] have identified two hydrogen induced interface traps in their Pd/SiO,/n-Si devices, with approximate energies of 0.65 and 0.4 eV below the Si conduction band edge. These authors experiments were performed with H, concentrations in the 100’s ppm range. It is interesting to note that the two traps (or sets of traps) identified from this work could be located at similar energies. However, our experiments have revealed that only one of these traps appears to be affected by different ambient conditions.
4. CONCLUSIONS
The electrical characteristics of PdSiO,/n-Si Schottky barrier type devices at different temperatures and under different ambient conditions have been reported. The forward conductivity of the devices is dominated at different bias voltages by one of the following processes: (i) emission over the Schottky barrier; (ii) generation-recombination/tunnelling processes at interface states; or (iii) conduction through the oxide layer.
PETTY
Two sets of interface states have been identified; one of these can be effectively eliminated by exposing the devices to air. These surface states are thought to originate from an imperfect oxide layer. It is suggested that the different effects seen by workers on Pd/SiO,/Si structures are related to the nature of the oxide layer produced in different iaboratoties. Exposure to hydrogen gas is thought to both increase the interface state density in the devices and also to lower the work function of the palladium metal. The work reported here has concentrated on studying the response of Pd/SiO,/n-Si diodes to Hz/N, mixtures. Clearly, if these devices are to be used commericially, conduction
H,O)
is also
a
detailed
mechanisms
understanding
in other
ambients
of
the
(e.g.
0,,
desirable.
Acknowledgements--The author wishes to thank Prof. G. G. Roberts for many useful discussions and for critically reading this manuscript.
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diode hydrogen
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97
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