- Email: [email protected]
Changes in tunnel barrier parameters on incorporation of reactive species
Thin Solid Films, 198 (1991)8~92 ELECTRONICSAND OPTICS
85
C H A N G E S IN T U N N E L B A R R I E R P A R A M E T E R S ON I N C O R P O R A T I O N O F REACTIVE SPECIES J. R. BELLINGHAM,C. J. ADKINS AND W. A. PHILLIPS Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE ( U.K.)
(ReceivedJune 18, 1990;acceptedOctober 1, 1990)
Simultaneous monitoring of both capacitance and resistance during the exposure of tunnel junctions to water vapour is shown to be a powerful technique for studying the infusion doping process. Results depend critically on the metal used for the top electrode and show capacitance increases on exposure to water vapour. Observed changes in both capacitance and resistance can be explained in terms of current ideas about tunnel junction structure.
1. INTRODUCTION It is now over 10 years since Jaklevic and Gaerttner t showed that small molecules can enter a metal-insulator-metal tunnelling structure through the thin metal film which forms the top electrode of the device. This technique, known as infusion doping, has been shown to yield inelastic electron tunnelling (IET) spectra which are similar to those obtained when the molecules are incorporated by the more conventional route of exposing the insulating oxide layer to a vapour of the required species prior to deposition of the top electrode. The early papers 1"2 stressed the importance of the presence of water vapour together with the species of interest if good IET spectra were to be obtained via infusion doping. Other than this, these early papers made no attempt to investigate the processes occurring during the infusion, concentrating instead on the spectral measurements. Nelson et al.3 studied changes in resistance as a function of time for junctions as they were exposed to water vapour. They showed that with thicker lead top electrodes the resistance increased more slowly with time, and they also suggested that different rates of increase in tunnelling resistance resulted from changes in the microstructure of the lead film. Their basic model was that the lead electrode was thin enough for channels to exist in it through which the water penetrated. The rise in resistance was attributed to thickening o f the insulating layer due to the penetration of water molecules between the oxide and top electrode. No capacitance measurements were taken in order to verify this. The only other attempt to understand the mechanics of the process is that of 0040-6090/91/$3.50
© ElsevierSequoia/Printedin The Netherlands
86
J . R . BELLINGHAM, C. J. ADK1NS, W. A. PHILLIPS
Mallik et al. 4 These workers proposed a model involving growth of successive monolayers of the infusing species under the top electrode. Changes in the junction resistance were assumed to result from the consequent thickening of the barrier layer. They emphasize the observation of changes in gradient in the conductance-time plot which they attribute to the start of the growth of successive monolayers. It is stated that capacitance measurements support this interpretation, but no figures are given. In this paper we are concerned not with molecular spectroscopy but with extending the work discussed above by simultaneous measurement of conductance and capacitance during exposure of completed junctions to water vapour. We also extend measurements to a greater range of systems by examining samples with gold and aluminium top electrodes as well as lead. This allows a more thorough test of the ideas which have been developed to explain the infusion doping process. 2.
EXPERIMENTAL DETAILS
2.1. Sample preparation and doping Al/alumina/metal samples were prepared in a conventionai diffusion pumped evaporation system. The aluminium layer was first deposited from a thermal source, followed by exposure to an oxygen glow discharge in order to grow the insulating oxide layer. The junction was completed by deposition of the top metal electrode from a further thermal source. The substrates were mounted in the evaporator in such a way that the electrodes were deposited onto contact pads which were connected to an electrical feed-through. In this way four-terminal electrical measurements could be made as soon as the junctions were completed. The effects of water infusion were observed by placing a beaker of distilled water in the same chamber as the samples. This is essentially the same technique that has been used in all previous studies of these phenomena. For this purpose the evaporation chamber itself was used as it was convenient to terminate the exposure by removing the beaker and pumping down to high vacuum. 2.2. Electrical measurements The circuit used for the electrical measurements is shown in Fig. I. The tunnel structure itself is shown as R and C in parallel and r represents the lead and electrode resistances. The oscillator, operational amplifiers and feedback network together maintain a constant a.c. voltage across the sample itself, independent of the lead resistances. The resulting current through the sample also passes through the standard resistor R1 and the corresponding potential difference is measured by the two-phase lock-in amplifier. This active system totally eliminates effects of lead and electrode resistances so that the in-phase and quadrature outputs of the phasesensitive detector are proportional to the conductance 1/R and the capacitance C respectively. The values of the two outputs were recorded directly by a microcomputer via a digital interface. As the two channels of the lock-in amplifier are constrained to operate at the same sensitivity, the best simultaneous resolution of both conductance and capacitance is achieved by adjusting the frequency of the voltage source such that o~CR ~ 1. The samples used in this work varied in
TUNNEL BARRIER PARAMETERS AND REACTIVE SPECIES
•
I
87
]
Typically 20 mV RMS
R1
Fig. 1. Schematicdiagram of the circuit used to make the measurements (LIA, lock-in amplifier).
resistance from 300 ~ to 70 kf~ with the result that a range of frequencies from 80 Hz to 15 kHz were used at various stages. The capacitances were always around 15 nF, corresponding to tunnel junction areas of about 0.8 m m 2 and barrier thicknesses of about 2.5 nm. Care was taken throughout the experimental runs that the d.c. sample resistance was equal to that obtained using a.c. methods. They were found to be equal to within a few per cent for all the samples for which results are reported here. Similarly, measurements were frequently made of the resistance of the sample electrodes during the experiments, but no changes were observed. 3.
RESULTS AND DISCUSSION
The results obtained when samples with three different top electrodes were exposed to water are shown in Figs. 2, 3 and 4. The thicknesses of the top electrodes are shown with the diagrams. Similar experiments were performed with a junction with a thicker lead top electrode, in which case slower, but essentially similar, effects were observed. Brief investigations were performed to eliminate the possibility that the air, to which the junction is also inevitably exposed when the water is admitted, is responsible for these effects. Exposure to pure nitrogen produced no effect on a scale comparable with that of the effects discussed here. Exposure to oxygen produced a resistance increase on a much shorter time scale than that obtained when water was present, accompanied by a large drop in capacitance. The effect of oxygen is clearly different from that of water, and it would seem that in the presence of water the processes involving oxygen are severely inhibited. The most striking result is the increase in capacitance observed on exposure to water vapour for junctions with lead or gold top electrodes. This result is not
88
J.R. BELLINGHAM, C. J. ADKINS,W.A. PHILLIPS
Waterout $
Lead
15 Water.~in
~
~
10
1.3 12
C
] .{I
Time/s
Fig. 2.
R and C results for a lead sample with top electrode thickness 80 nm. The water was introduced at 2550 s and removedat 13900 s. consistent with a thickening of the barrier region due to insertion of a molecular layer between the top electrode and the alumina or to growth of the oxide layer. Any increase in barrier thickness must decrease the capacitance, whatever the permittivity of the extra material, as is obvious from the expression for capacitance with two insulating layers:
1
1 (d,
d2)
C- oA 71+7
(i)
where A is the area and dl and d2 are the thicknesses of the two dielectric layers of relative permittivity el and e2. An increase in capacitance is possible, however, if the water molecules are filling previously empty space between the top electrode and the alumina layer. If the alumina layer has thickness dl and the initially empty layer has thickness d2 then this situation is modelled by hold_ing all parameters except e2 constant in eqn. (1),
TUNNEL BARRIER PARAMETERSAND REACTIVE SPECIES
89
Gold
1.4
Water out
+ 1.3
Water in
1.2
1.1 1.0 1.20
1.15
C r.,.) 1.1o 1.05
/
/
~
~
-
'
~
-
1.(10 ~ - 5000
10000 Time/s
15000
Fig. 3. R and C results for a good sample with top electrode thickness 80 nm. The water was introduced 3800 s and removed at 10 100 s.
at
while e2 increases from 1 to the effective relative permittivity of the molecular layer. If the molecular layer is water, and if the water is free to rotate, then this value will be 80. For a junction with a barrier of total thickness 2.5 nm, consideration of eqn. (1) shows that the empty space need only be of thickness 0.1 nm in order to account for the size of capacitance change observed. Detailed analysis of IET spectra by Sleigh e t al.5"6 taken from junctions with different top electrode metals has shown that certain metals, including lead and gold, do not form intimate contact with the underlying alumina. The width of the resulting gap extracted from the spectral analysis, 0.2 nm, is similar to the gap required to explain the capacitance measurements. The failure to observe any capacitance increases in junctions with aluminium is also consistent with the work of Sleigh e t al. 6 who found that aluminium makes close contact with the underlying alumina, leaving no space for the molecules to fill. It is clear then that the observed capacitance changes can be readily explained using
90
J.R.
B E L L I N G H A M , C. J. A D K I N S , W . A. P H I L L I P S
Aluminium 1.4 i
. . . .
I
W a t e r out
$ 1.3 W a t e r in
$ r~
1.2
1.1
1.0 1.15
1.10
C ~..
1.05
1.00
0.95
,
,
,
,
I , 50O0
,
,
,
I . . . . 10000
I , 15000
,
,
,
Time/s Fig. 4. R a n d C results f o r a n a l u m i n i u m s a m p l e w i t h t o p e l e c t r o d e t h i c k n e s s 50 n m . T h e w a t e r w a s i n t r o d u c e d a t 4 8 0 s a n d r e m o v e d a t 17 250 s.
the ideas which have already been developed about lET junctions and that indeed capacitance increases rather than decreases are what should be expected. The picture becomes slightly more complicated when resistance changes are considered, together with what happens when the water is removed and the chamber returned to vacuum. Here, different behaviours are observed for lead and gold. To consider gold first, the resistance has a similar time dependence to the capacitance, suggesting that the two processes are very closely linked. A consistent model is obtained if one supposes that the tunnel barrier height is increased as the water fills in the previously unfilled space under the top electrode, resulting in an increase in both capacitance and resistance. The total resistance change suggests an average barrier height increase of about 5~o averaged across the whole barrier. As the water fills only a small part of the total thickness, the local barrier would have approximately to double to account for the change. The model of a thin high
TUNNEL BARRIER PARAMETERS AND REACTIVE SPECIES
91
tunnelling barrier being formed by a molecular layer is consistent with the work of Walmsley e t al. 7 who used such a model to explain conductance results for their tunnel junctions. On re-evacuating the chamber, the most likely explanation for the failure of the junction to recover its original parameters is that some of the molecules do not emerge, presumably because they have been absorbed on the alumina surface. Turning to lead, it can be seen that the resistance changes are far more dramatic than in the case of gold and that they are also far greater than the simultaneous capacitance changes. A further point to note is that on re-evacuating the chamber the junction resistance remains several times higher than its starting value. To account for such a large increase in R it is necessary to postulate that the barrier is becoming thicker. There are two obvious possible explanations as to why this should occur for lead and not for gold. First, the fact that lead is malleable at room temperature make it easier for the lead to be forced away from the alumina by the water molecules as they penetrate underneath. This model is broadly similar to that proposed by Mallik e t al. 4 The second possibility is that lead, being more reactive than gold, oxidizes to form a layer of insulator, possibly lead oxide, or lead hydroxide, as has been suggested by Nelson e t al. 3 This would increase the total thickness of the insulating region. On evacuating the chamber, some of or all the unreacted water emerges from the junction, allowing the lead to move back towards its original position. The fact that the resistance does not return to its original value suggests that the barrier remains thicker than it originally was, presumably as a result of the growth of an insulating lead compound or possibly because some of the water molecules remain trapped in the junction. It is important to realize that careful consideration of eqn. (1) shows that barrier thickening such as proposed here for the case of lead is completely consistent with the increases in capacitance discussed earlier. The important point is that for both the possible models discussed above for barrier thickening, the extra layer of barrier will consist of a material of very high relative permittivity (water has a value of up to 80, lead oxide has 24). Thus the growth of this extra layer will have little effect on the capacitance compared with the filling by molecules of previously empty space. Lack of precise knowledge of the relative permittivities and thicknesses of the various layers involved makes it impossible to analyse this model in more detail to tie together the capacitance and resistance changes quantitatively. The picture presented here is however, entirely self-consistent. Finally, the results for aluminium must be considered. In this case both the resistance and the capacitance changes are irreversible, and the response is slower than for either of the other two metals. (It should be mentioned that the irreversibility was observed over a longer time scale than is shown in Fig. 4.) The sense of the changes is consistent with growth of the insulating layer. Quantitatively, this model can be tested, if the barrier height and relative permittivity are assumed not to change as the barrier grows. Under these assumptions, the changes in resistance and capacitance are linked by the following expression. In{ ( R + A R ) / R } = - 2 ~ d A C / C
(2)
92
J . R . BELLINGHAM, C. J. ADKINS, W. A. PHILLIPS
In this expression dis the total thickness of the barrier and a is the decay constant of the evanescent tunnelling wave function. For the aluminium results shown here a value of 26 is found for 2~td, which is believed to be typical of those for tunnel junctions of this type. It therefore appears that barrier growth is occurring in this experiment. The slow speed of the process is most probably a consequence of the relative impenetrability of the aluminium film, which is known to have a less open structure than lead or gold. 4. CONCLUSIONS Several important points have emerged from the work reported here. First we have shown that increases in capacitance can accompany increases in resistance when tunnel junctions are exposed to water vapour. We have shown that this is entirely consistent with established ideas about the structure of tunnel junctions. Secondly, the sense and magnitude of the changes are critically dependent on the choice of top electrode metal. Lead and gold both allow water to fill space underneath the top electrode and to increase the capacitance, but only lead reacts to grow an extra insulating layer, with an irreversible change in resistance as the consequence. There is no space under an aluminium electrode and molecules penetrate only slowly, giving rise to the observed slow barrier growth. We have only obtained these results because we have used a combination of simultaneous resistance and capacitance measurements. Such measurements are straightforward and the associated circuitry is hardly more complicated than that required for measurements of resistance alone. The extra information obtained, however, can be crucial in obtaining a proper understanding of the processes occurring. ACKNOWLEDGMENT
We would like to thank the Science and Engineering Research Council for the research grant which supported this work. REFERENCES 1 2 3 4 5 6 7
R.C. Jaklevic and M. R. Gaerttner, Appl. Phys. Lett., 30 (1977) 646. R.C. Jaklevic and M. R. Gaerttner, Appl. Surf Sci., 1 (1978) 479. W.J. Nelson, D.G. WalmsleyandJ. M. Bell, ThinSolidFilms, 79(1981) 229. R . R . Mallik, R. G. Pritchard, D. P. Oxley, C. C. Horley and J. Comyn, Thin Solid Films, 112 (1984) 193. A . K . Sleigh, W. A. Phillips, C. J. Adkins and M. E. Taylor, J. Phys. C, 19 (1986) 6645. A . K . Sleigh, M. E. Taylor, C. J. Adkins and W. A. Phillips, J. Phys.: Condens. Matter, 1 (1989) 1107. D . G . Walmsley, R. B. Floyd and W. E. Timms, Solid State Commun., 22 (1977) 497.
85
C H A N G E S IN T U N N E L B A R R I E R P A R A M E T E R S ON I N C O R P O R A T I O N O F REACTIVE SPECIES J. R. BELLINGHAM,C. J. ADKINS AND W. A. PHILLIPS Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE ( U.K.)
(ReceivedJune 18, 1990;acceptedOctober 1, 1990)
Simultaneous monitoring of both capacitance and resistance during the exposure of tunnel junctions to water vapour is shown to be a powerful technique for studying the infusion doping process. Results depend critically on the metal used for the top electrode and show capacitance increases on exposure to water vapour. Observed changes in both capacitance and resistance can be explained in terms of current ideas about tunnel junction structure.
1. INTRODUCTION It is now over 10 years since Jaklevic and Gaerttner t showed that small molecules can enter a metal-insulator-metal tunnelling structure through the thin metal film which forms the top electrode of the device. This technique, known as infusion doping, has been shown to yield inelastic electron tunnelling (IET) spectra which are similar to those obtained when the molecules are incorporated by the more conventional route of exposing the insulating oxide layer to a vapour of the required species prior to deposition of the top electrode. The early papers 1"2 stressed the importance of the presence of water vapour together with the species of interest if good IET spectra were to be obtained via infusion doping. Other than this, these early papers made no attempt to investigate the processes occurring during the infusion, concentrating instead on the spectral measurements. Nelson et al.3 studied changes in resistance as a function of time for junctions as they were exposed to water vapour. They showed that with thicker lead top electrodes the resistance increased more slowly with time, and they also suggested that different rates of increase in tunnelling resistance resulted from changes in the microstructure of the lead film. Their basic model was that the lead electrode was thin enough for channels to exist in it through which the water penetrated. The rise in resistance was attributed to thickening o f the insulating layer due to the penetration of water molecules between the oxide and top electrode. No capacitance measurements were taken in order to verify this. The only other attempt to understand the mechanics of the process is that of 0040-6090/91/$3.50
© ElsevierSequoia/Printedin The Netherlands
86
J . R . BELLINGHAM, C. J. ADK1NS, W. A. PHILLIPS
Mallik et al. 4 These workers proposed a model involving growth of successive monolayers of the infusing species under the top electrode. Changes in the junction resistance were assumed to result from the consequent thickening of the barrier layer. They emphasize the observation of changes in gradient in the conductance-time plot which they attribute to the start of the growth of successive monolayers. It is stated that capacitance measurements support this interpretation, but no figures are given. In this paper we are concerned not with molecular spectroscopy but with extending the work discussed above by simultaneous measurement of conductance and capacitance during exposure of completed junctions to water vapour. We also extend measurements to a greater range of systems by examining samples with gold and aluminium top electrodes as well as lead. This allows a more thorough test of the ideas which have been developed to explain the infusion doping process. 2.
EXPERIMENTAL DETAILS
2.1. Sample preparation and doping Al/alumina/metal samples were prepared in a conventionai diffusion pumped evaporation system. The aluminium layer was first deposited from a thermal source, followed by exposure to an oxygen glow discharge in order to grow the insulating oxide layer. The junction was completed by deposition of the top metal electrode from a further thermal source. The substrates were mounted in the evaporator in such a way that the electrodes were deposited onto contact pads which were connected to an electrical feed-through. In this way four-terminal electrical measurements could be made as soon as the junctions were completed. The effects of water infusion were observed by placing a beaker of distilled water in the same chamber as the samples. This is essentially the same technique that has been used in all previous studies of these phenomena. For this purpose the evaporation chamber itself was used as it was convenient to terminate the exposure by removing the beaker and pumping down to high vacuum. 2.2. Electrical measurements The circuit used for the electrical measurements is shown in Fig. I. The tunnel structure itself is shown as R and C in parallel and r represents the lead and electrode resistances. The oscillator, operational amplifiers and feedback network together maintain a constant a.c. voltage across the sample itself, independent of the lead resistances. The resulting current through the sample also passes through the standard resistor R1 and the corresponding potential difference is measured by the two-phase lock-in amplifier. This active system totally eliminates effects of lead and electrode resistances so that the in-phase and quadrature outputs of the phasesensitive detector are proportional to the conductance 1/R and the capacitance C respectively. The values of the two outputs were recorded directly by a microcomputer via a digital interface. As the two channels of the lock-in amplifier are constrained to operate at the same sensitivity, the best simultaneous resolution of both conductance and capacitance is achieved by adjusting the frequency of the voltage source such that o~CR ~ 1. The samples used in this work varied in
TUNNEL BARRIER PARAMETERS AND REACTIVE SPECIES
•
I
87
]
Typically 20 mV RMS
R1
Fig. 1. Schematicdiagram of the circuit used to make the measurements (LIA, lock-in amplifier).
resistance from 300 ~ to 70 kf~ with the result that a range of frequencies from 80 Hz to 15 kHz were used at various stages. The capacitances were always around 15 nF, corresponding to tunnel junction areas of about 0.8 m m 2 and barrier thicknesses of about 2.5 nm. Care was taken throughout the experimental runs that the d.c. sample resistance was equal to that obtained using a.c. methods. They were found to be equal to within a few per cent for all the samples for which results are reported here. Similarly, measurements were frequently made of the resistance of the sample electrodes during the experiments, but no changes were observed. 3.
RESULTS AND DISCUSSION
The results obtained when samples with three different top electrodes were exposed to water are shown in Figs. 2, 3 and 4. The thicknesses of the top electrodes are shown with the diagrams. Similar experiments were performed with a junction with a thicker lead top electrode, in which case slower, but essentially similar, effects were observed. Brief investigations were performed to eliminate the possibility that the air, to which the junction is also inevitably exposed when the water is admitted, is responsible for these effects. Exposure to pure nitrogen produced no effect on a scale comparable with that of the effects discussed here. Exposure to oxygen produced a resistance increase on a much shorter time scale than that obtained when water was present, accompanied by a large drop in capacitance. The effect of oxygen is clearly different from that of water, and it would seem that in the presence of water the processes involving oxygen are severely inhibited. The most striking result is the increase in capacitance observed on exposure to water vapour for junctions with lead or gold top electrodes. This result is not
88
J.R. BELLINGHAM, C. J. ADKINS,W.A. PHILLIPS
Waterout $
Lead
15 Water.~in
~
~
10
1.3 12
C
] .{I
Time/s
Fig. 2.
R and C results for a lead sample with top electrode thickness 80 nm. The water was introduced at 2550 s and removedat 13900 s. consistent with a thickening of the barrier region due to insertion of a molecular layer between the top electrode and the alumina or to growth of the oxide layer. Any increase in barrier thickness must decrease the capacitance, whatever the permittivity of the extra material, as is obvious from the expression for capacitance with two insulating layers:
1
1 (d,
d2)
C- oA 71+7
(i)
where A is the area and dl and d2 are the thicknesses of the two dielectric layers of relative permittivity el and e2. An increase in capacitance is possible, however, if the water molecules are filling previously empty space between the top electrode and the alumina layer. If the alumina layer has thickness dl and the initially empty layer has thickness d2 then this situation is modelled by hold_ing all parameters except e2 constant in eqn. (1),
TUNNEL BARRIER PARAMETERSAND REACTIVE SPECIES
89
Gold
1.4
Water out
+ 1.3
Water in
1.2
1.1 1.0 1.20
1.15
C r.,.) 1.1o 1.05
/
/
~
~
-
'
~
-
1.(10 ~ - 5000
10000 Time/s
15000
Fig. 3. R and C results for a good sample with top electrode thickness 80 nm. The water was introduced 3800 s and removed at 10 100 s.
at
while e2 increases from 1 to the effective relative permittivity of the molecular layer. If the molecular layer is water, and if the water is free to rotate, then this value will be 80. For a junction with a barrier of total thickness 2.5 nm, consideration of eqn. (1) shows that the empty space need only be of thickness 0.1 nm in order to account for the size of capacitance change observed. Detailed analysis of IET spectra by Sleigh e t al.5"6 taken from junctions with different top electrode metals has shown that certain metals, including lead and gold, do not form intimate contact with the underlying alumina. The width of the resulting gap extracted from the spectral analysis, 0.2 nm, is similar to the gap required to explain the capacitance measurements. The failure to observe any capacitance increases in junctions with aluminium is also consistent with the work of Sleigh e t al. 6 who found that aluminium makes close contact with the underlying alumina, leaving no space for the molecules to fill. It is clear then that the observed capacitance changes can be readily explained using
90
J.R.
B E L L I N G H A M , C. J. A D K I N S , W . A. P H I L L I P S
Aluminium 1.4 i
. . . .
I
W a t e r out
$ 1.3 W a t e r in
$ r~
1.2
1.1
1.0 1.15
1.10
C ~..
1.05
1.00
0.95
,
,
,
,
I , 50O0
,
,
,
I . . . . 10000
I , 15000
,
,
,
Time/s Fig. 4. R a n d C results f o r a n a l u m i n i u m s a m p l e w i t h t o p e l e c t r o d e t h i c k n e s s 50 n m . T h e w a t e r w a s i n t r o d u c e d a t 4 8 0 s a n d r e m o v e d a t 17 250 s.
the ideas which have already been developed about lET junctions and that indeed capacitance increases rather than decreases are what should be expected. The picture becomes slightly more complicated when resistance changes are considered, together with what happens when the water is removed and the chamber returned to vacuum. Here, different behaviours are observed for lead and gold. To consider gold first, the resistance has a similar time dependence to the capacitance, suggesting that the two processes are very closely linked. A consistent model is obtained if one supposes that the tunnel barrier height is increased as the water fills in the previously unfilled space under the top electrode, resulting in an increase in both capacitance and resistance. The total resistance change suggests an average barrier height increase of about 5~o averaged across the whole barrier. As the water fills only a small part of the total thickness, the local barrier would have approximately to double to account for the change. The model of a thin high
TUNNEL BARRIER PARAMETERS AND REACTIVE SPECIES
91
tunnelling barrier being formed by a molecular layer is consistent with the work of Walmsley e t al. 7 who used such a model to explain conductance results for their tunnel junctions. On re-evacuating the chamber, the most likely explanation for the failure of the junction to recover its original parameters is that some of the molecules do not emerge, presumably because they have been absorbed on the alumina surface. Turning to lead, it can be seen that the resistance changes are far more dramatic than in the case of gold and that they are also far greater than the simultaneous capacitance changes. A further point to note is that on re-evacuating the chamber the junction resistance remains several times higher than its starting value. To account for such a large increase in R it is necessary to postulate that the barrier is becoming thicker. There are two obvious possible explanations as to why this should occur for lead and not for gold. First, the fact that lead is malleable at room temperature make it easier for the lead to be forced away from the alumina by the water molecules as they penetrate underneath. This model is broadly similar to that proposed by Mallik e t al. 4 The second possibility is that lead, being more reactive than gold, oxidizes to form a layer of insulator, possibly lead oxide, or lead hydroxide, as has been suggested by Nelson e t al. 3 This would increase the total thickness of the insulating region. On evacuating the chamber, some of or all the unreacted water emerges from the junction, allowing the lead to move back towards its original position. The fact that the resistance does not return to its original value suggests that the barrier remains thicker than it originally was, presumably as a result of the growth of an insulating lead compound or possibly because some of the water molecules remain trapped in the junction. It is important to realize that careful consideration of eqn. (1) shows that barrier thickening such as proposed here for the case of lead is completely consistent with the increases in capacitance discussed earlier. The important point is that for both the possible models discussed above for barrier thickening, the extra layer of barrier will consist of a material of very high relative permittivity (water has a value of up to 80, lead oxide has 24). Thus the growth of this extra layer will have little effect on the capacitance compared with the filling by molecules of previously empty space. Lack of precise knowledge of the relative permittivities and thicknesses of the various layers involved makes it impossible to analyse this model in more detail to tie together the capacitance and resistance changes quantitatively. The picture presented here is however, entirely self-consistent. Finally, the results for aluminium must be considered. In this case both the resistance and the capacitance changes are irreversible, and the response is slower than for either of the other two metals. (It should be mentioned that the irreversibility was observed over a longer time scale than is shown in Fig. 4.) The sense of the changes is consistent with growth of the insulating layer. Quantitatively, this model can be tested, if the barrier height and relative permittivity are assumed not to change as the barrier grows. Under these assumptions, the changes in resistance and capacitance are linked by the following expression. In{ ( R + A R ) / R } = - 2 ~ d A C / C
(2)
92
J . R . BELLINGHAM, C. J. ADKINS, W. A. PHILLIPS
In this expression dis the total thickness of the barrier and a is the decay constant of the evanescent tunnelling wave function. For the aluminium results shown here a value of 26 is found for 2~td, which is believed to be typical of those for tunnel junctions of this type. It therefore appears that barrier growth is occurring in this experiment. The slow speed of the process is most probably a consequence of the relative impenetrability of the aluminium film, which is known to have a less open structure than lead or gold. 4. CONCLUSIONS Several important points have emerged from the work reported here. First we have shown that increases in capacitance can accompany increases in resistance when tunnel junctions are exposed to water vapour. We have shown that this is entirely consistent with established ideas about the structure of tunnel junctions. Secondly, the sense and magnitude of the changes are critically dependent on the choice of top electrode metal. Lead and gold both allow water to fill space underneath the top electrode and to increase the capacitance, but only lead reacts to grow an extra insulating layer, with an irreversible change in resistance as the consequence. There is no space under an aluminium electrode and molecules penetrate only slowly, giving rise to the observed slow barrier growth. We have only obtained these results because we have used a combination of simultaneous resistance and capacitance measurements. Such measurements are straightforward and the associated circuitry is hardly more complicated than that required for measurements of resistance alone. The extra information obtained, however, can be crucial in obtaining a proper understanding of the processes occurring. ACKNOWLEDGMENT
We would like to thank the Science and Engineering Research Council for the research grant which supported this work. REFERENCES 1 2 3 4 5 6 7
R.C. Jaklevic and M. R. Gaerttner, Appl. Phys. Lett., 30 (1977) 646. R.C. Jaklevic and M. R. Gaerttner, Appl. Surf Sci., 1 (1978) 479. W.J. Nelson, D.G. WalmsleyandJ. M. Bell, ThinSolidFilms, 79(1981) 229. R . R . Mallik, R. G. Pritchard, D. P. Oxley, C. C. Horley and J. Comyn, Thin Solid Films, 112 (1984) 193. A . K . Sleigh, W. A. Phillips, C. J. Adkins and M. E. Taylor, J. Phys. C, 19 (1986) 6645. A . K . Sleigh, M. E. Taylor, C. J. Adkins and W. A. Phillips, J. Phys.: Condens. Matter, 1 (1989) 1107. D . G . Walmsley, R. B. Floyd and W. E. Timms, Solid State Commun., 22 (1977) 497.