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Electrical conduction in niobium-niobium oxide-gold diodes
Thin Solid Films ~ Elsevier Sequoia S.A., L a u s a n n e - Printed in the N e t h e r l a n d s
E L E C T R I C A L C O N D U C T I O N IN N I O B I U M - N I O B I U M O X I D E - G O L D DIODES R. J. SCHWARTZ, Y. L. CHIOU* AND H. W. THOMPSON, JR.
School o f Electrical Engineering, Purdue University, Lafayette, Ind. ( U. S.A. ) (Received M a y 16, 1970)
SUMMARY
It is shown that for voltages between 0.2 V and 1 V ( N b - ) a linear relationship is obtained between In J and ~/V for diodes with relatively thick oxides (1700 A). The effects of heat treatment under vacuum conditions upon the conduction characteristics is reported. It is suggested that oxygen vacancies may be responsible for many of the electrical characteristics observed in thin Nb205 films.
INTRODUCTION
The electrical and optical properties of niobium-niobium oxide-metal diodes have been extensively studied using a variety of metals for the counterelectrode 1-8. The most extensive studies to date are those reported by Hickmott 3,4, 7.9, in which he has reported on the conduction properties, photo effects, bistable switching effects and electroluminescence of these diodes. It is the purpose of this paper to report on our observations on niobium-niobium oxide-gold diodes, and to comment on the conduction model as proposed by Hickmott 7. In particular, it will be shown from our data and from a replot of Hickmott's data that a linear relationship is obtained between In J and x/V in the voltage region between 0.2 V and 1 V for diodes with thick oxides indicating a PooleFrenkel type conduction mechanism. Also, it is suggested that a possible source for the "impurity band" which has been proposed by Hickmott is that of oxygen vacancies. The latter conclusion is based on the similarity between the results of electron ionization energy studies on near stoichiometric ~-Nb20 s niobium oxide and the energy of the electroluminescent radiation observed on amorphous Nb2Os. Supporting evidence for this supposition is found by studying the effects * Present address: University o f Illinois, Circle C a m p u s , Chicago, I11., U.S.A.
Thin Solid Films, 6 (1970) 81-89
82
R.J. SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON,JR.
of heat treatment under vacuum conditions on the conduction properties of niobium-niobium oxide-gold diodes.
FABRICATION In order to reduce the possibility of contamination, the source of niobium used for these experiments was triple-pass electron beam zone refined 3/8 in. diam. polycrystalline rod. Wafers 0.080 in. thick were cut from this rod. The wafers were then alternately mechanically polished and etched in 35 HF, 65 HNO3 (parts by volume), until a polished surface was obtained. The niobium oxide was grown by anodization in a saturated boric acid solution at a current density of 0.5 mA/cm 2. All samples reported below were anodized at constant current to a voltage of 72 V. Electrodes were obtained by the vacuum evaporation of gold from an alumina crucible at a pressure of 10-6 torr. The spacing between the substrate and the gold evaporation source was approximately 6 in. This relatively close spacing resulted in appreciable heating of the substrate during the evaporation of the gold. Electrode geometry was defined by the use of a metal mask during the deposition. The gold electrodes ranged in size from 0.005 in. to 0.010 in. diam. N o significant differences were observed in the electrical behavior as a result of using different electrode diameters.
MEASUREMENT TECHNIQUES All measurements performed at voltages above 10 V were made using a pulse technique, in which a single triangular-shaped voltage pulse or a pulse train was applied to the diode. The current voltage characteristic was displayed on a Tektronix 564 storage oscilloscope. All current-voltage measurements were made with the diodes in the dark. For voltages between 0.2 V and 1 V, d.c. measurements were made using Keithley 602 and Keithley 610B electrometers. Both d.c. and pulse techniques were utilized in the 1-10 V range. The results in this range were independent of the measurement technique. The reason for using a pulse technique was that above 20 V and below 0.2 V, the electrical properties of the diode were found to be a function of the previous electrical history of the diode.
RESULTS Immediately after deposition of the gold counterelectrodes, the current voltage characteristics were as shown in Fig. 1, where it may be seen that there was relatively little difference between the two bias polarities; that is, there was relaThin Solid Films, 6 (1970) 81-89
ELECTRICALCONDUCTIONIN Nb--Nb2Os-Au DIODES
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Fig. 1. Current-voltage characteristics immediately after evaporation of the gold electrode. Fig. 2. Drift observed in fresh diodes. tively little rectification exhibited by the fresh diodes. However, if the diodes were exposed to voltages in excess of 10 V, a pronounced shift was observed in the 1I-1 characteristics, particularly in the (Nb +) direction (see Fig. 2). The direction of the drift was a function of the polarity of the applied bias. Voltage pulses with Nb ÷ would cause the current to decrease, voltage pulses with N b - would cause a reversal of the drift direction. If the pulses (with Nb ÷) were continued for a long period of time the drift would eventually cease. The resultant 1I-1 curves exhibited a large rectification ratio, i.e. relatively easy current flow with N b - and relatively little current flow with Nb +. A similar high rectification state could be obtained by exposing the diode to the atmosphere for a few days. Hickmott has previously reported drift phenomena associated with the presence of humidity. However, the effect described above does not seem to be dependent upon the humidity, as it was observed to occur for diodes stored in low humidity environments as well as diodes exposed to normal laboratory humidity conditions. After a period of three days of exposure to the atmosphere, the current-voltage characteristics were those shown in Fig. 3. The characteristics shown in Fig. 3 are similar to the final characteristics which were obtained from a fresh diode after sustained exposure to Nb + voltage pulses. For reasons that will be explained later, it was felt that the conduction process was strongly influenced by oxygen vacancies within the N b 2 0 s. As a test of this hypothesis, a diode which had previously been exposed to the atmosphere until it had a large rectification ratio was exposed to a vacuum of 4 x 10- 7 torr for 15 hours. The V - I characteristic was measured before and after the prolonged exposure to vacuum conditions. As can be seen in Fig. 4, only a slight change was observed in the forward ( N b - ) II-1 characteristic*. A measure of the 1I-1 curves * The reader is cautioned to note the change in plotting technique which occurs at 1.2 V. Below 1.2 V, the abscissa is plotted as VL Above 1 V, a logarithmic scale is used. This procedure has been followed in all subsequent plots in order to emphasize the linear character of such curves in the two regions. Thin Solid Films, 6 (1970) 81-89
84
R . J . SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON, JR,
I0"~~
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Fig. 3. Stable characteristics observed after exposure to the atmosphere. Fig. 4. Current-voltage characteristics under atmospheric and vacuum ambient conditions. Note scale change of the abscissa.
with N b ÷, however, resulted in the observation of a large increase in the current. Before exposure to the vacuum, for instance, the measured current at 25 V was on the order of 10 -12 A. After exposure to the vacuum this value had increased to 2 × 10-4 A. Forward and reverse characteristics under vacuum conditions are shown in Fig. 5. Because of the relatively small spacing between the gold evaporation source and the diode and the relatively long evaporation time, the diode was subjected to considerable heating during fabrication. In order to observe the possible effects of a vacuum heat treatment during fabrication, a diode was radiantly heated while at 3 x 10- v torr. When the diode had returned to room temperature, the V--I characteristics were again measured. Figure 6 shows the results of measurements before exposure to vacuum heating (Curve A), immediately after vacuum heating (Curve B), after a second vacuum heating (Curve C), and after continued exposure to vacuum for 12 hours following termination of the heat treatment (Curve D). As can be seen in Fig. 6, the application of heat under vacuum conditions produced a significant increase in the current level as well as a decrease in the slope. Twelve hours after termination of the heat treatment the current had partially returned to the preheated condition. Thin Solid Films, 6 (1970) 81-89
ELECTRICAL CONDUCTION IN
Nb-Nb2Os-Au
DIODES
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Fig. 5. Current-voltage characteristics after exposure to a vacuum for 15 hours. Fig. 6. Effects of vacuum heating on the current-voltage characteristics. A, before heating; B, immediately after heating; C, immediately after a second heating; D, after continued exposure to vacuum conditions for 12 hours after termination of heating.
In all cases, if the voltage applied to the diodes was maintained within the range from 0.2 to 5.0 V (Nb-), the drift observed in the V-1 curves was less than 5 % as the voltage was cycled within this range. However, for voltages less than 0.2 V ( N b - ) and for all reverse bias values (Nb+), the current decreased with time at constant bias. In addition, at reverse biases greater than 5 V (Nb÷), the current was found to be quite noisy. These effects prevented obtaining plots of the type shown in Figs. 4 and 6 for voltages less than 0.2 V and for all reverse voltages (Nb+).
DISCUSSION
A current-voltage characteristic in which In J is proportional to V~', as is observed below 1 V in Figs. 4 and 6, is usually interpreted as being due to the Poole-Frenkel emission of carriers from traps or due to the electric field lowering of a potential barrier such as that which occurs at the niobium oxide-gold interface, For the Poole-Frenkel effect, one finds J oc exp 2 fl V~ and for the Schottky barrier lowering effect J oc exp/~ V½ Thin Solid Films, 6 (1970) 81-89
86
R . J . SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON, JR.
where (1)
fl = k T \ 4 n % e r d ]
and where d is the insulator thickness. The two effects are usually distinguished from one another by a measurement of the slope of a log J versus V ~ plot such as Figs. 4 and 6. The Poole-Frenkel effect is expected to have twice the slope as that of the Schottky barrier lowering effect. However, as has been pointed out by Jonscher 1°, this is not always a reliable technique for separating the two effects. The value of e, to be used in eqn. (1) is the high-frequency dielectric constant which may be obtained from the square of the refraction index. Y o u n g I 1 has reported a value of the refraction index of 2.4 at high frequencies. If a value of e, = 5.76 and a thickness of d = 1700/k are used to compute fl, the predicted slopes of the Poole-Frenkel effect and the Schottky effect are 3.0 and 1.5 respectively. The experimental values shown in Figs. 4 and 6 are found to range from 10.7 to 13. However, if we refer to the energy band model proposed by Hickmott (Fig. 7), we observe that the field is not uniformly distributed across the niobium oxide, but is concentrated in the vicinity of the gold electrode. Hickmott has estimated this region to be in the order of 100 A thick. In fact, assuming that the conduction is dominated by Poole-Frenkel processes, we find that a value of d ranging from 90 A to 125 A is required in eqn. (1) to predict the slopes of Figs. 4 and 6. Reference to Fig. 7 shows that if the diode is under forward bias (Nb-), one would not expect Schottky barrier lowering at the oxide-gold interface as this bias polarity corresponds to forward bias conditions in a standard metal-semiconductor junction. For this reason the observed linear relationship between In J and x/V is thought to be controlled by a Poole-Frenkel mechanism. However, even in the case of the Poole-Frenkel effect the interpretation is complicated by the fact that if we use the model of Fig. 7 the application of a forward bias ( N b - ) will reduce the field strength in region II, thus increasing Nb
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ELECTRICAL CONDUCTION IN Nb--Nb2Os-Au DIODES
A IO-j
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Fig. 8. Replot of data presented by Hickmott 3.
the trapped charge population, rather than enhancing the emission of trapped charges as in the usual Poole-Frenkel effect. Hickmott has divided the current flow below 1 V into an ohmic region and a region for which J oc exp ( e V / k T ) . However, if we replot the data as given in Fig. 4 of ref. 3, we observe (Fig. 8) that it is linear in a log J vs. I1"~ plot below 1 V just as are the diodes reported above. This linear relationship between In J and ~/V is not found for diodes with thin oxides (50-100 A). Reference to Fig. 6 shows that the effect of heating under vacuum conditions was to increase the current in all voltage ranges. However, the increased current flow was accompanied by a decrease in the slope of the log J vs. V ~ portion of the curve. This indicates that the effective width of the region in which the electric field is concentrated is increased by the heat treatment. However, since the current is simultaneously increased as fl decreases, this indicates that the number of traps in region 1I has increased or the barrier height has decreased. There has been considerable study of the conduction mechanisms in both single crystal and polycrystalline Nb2Os, in which it has been established that the conduction is controlled by the concentration of oxygen vacancies ~2. Greener and Hirthe 13 have measured the electronic activation energy of these oxygen vacancies and found them to have levels which are 0.22 eV and 1.1 eV below the conduction band. Although we are dealing with amorphous niobium oxide, it is interesting to note the similarity between the crystalline and amorphous systems. If we interpret Hickmott's "impurity band" as possibly being caused by oxygen vacancies, then the material in region I would behave as an n-type semiconductor due to the presence of a large concentration of oxygen vacancies. The rise in potenThin Solid Films, 6 (1970) 81-89
88
R.J. SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON,JR.
tial as the gold electron is approached would indicate a reduction in the vacancy content. The behavior upon heat treatment can then be interpreted as an increase in the width of region II due to a transfer of vacancies from the surface or from region I into region II. A transfer of vacancies from region I would result in an increase in d and an increase in the vacancy concentration in region II. Increasing the vacancy concentration in region II would then lower the barrier height and increase the current flow while simultaneously decreasing the slope of the currentvoltage curves due to the increasing width of region II. There is an interesting similarity between the electronic energy levels measured by Greener and Hirthe and the energy of the electroluminescence radiation measured by Hickmott 7. If allowances are made for the probable energy broadening effects in amorphous materials, the 1.1 eV measured for a doubly ionized oxygen vacancy level in polycrystalline Nb205 corresponds quite well with the measured peak in electroluminiscence radiation at 1.2 eV. In addition, Hickmott 7 has speculated on the existence of a donor level lying 0.3 eV below the conduction band, which again is in the vicinity of the 0.22 eV level reported by Greener and Hirthe for singly ionized oxygen vacancies. The similarity of the drift behavior of the diodes either due to the application of strong electric fields or due to the exposure to the atmosphere leads one to speculate that both effects are caused by a variation in the oxygen vacancy content in the region of the film nearest to the gold electrode. In one case the driving force would be electronic, in the other case chemical or thermodynamic. Also, if one speculates on the existence of a mobile species within the film (that species being either niobium or oxygen vacancies), then it is possible that the switching mechanism which has been observed* is due to a transfer of either niobium ions or oxygen vacancies into or out of region II. When vacancies are transferred into region II, a lowering of the barrier at the niobium oxide-gold interface would occur, and one would expect switching into a state with a much higher conductivity than the original state of the diode. In addition to this, if the mobile species could be reversibly transferred into and out of region II by the application of electric field or by a combination of electric field and heating due to localized conduction, then it would be expected that one could switch the conduction states between a relatively high conduction state (one with a low barrier height occurring at the gold-niobium oxide interface) and a low conduction state (one in which a moderate barrier existed at the niobium oxide gold-interface).
CONCLUSIONS The electrical conduction which occurs in Nb--Nb2Os-Au diodes when the oxide is thick ( ~ 1700 A) is controlled by a narrow region of the oxide lying adjacent to the Au contact for applied voltages lying between 0.2 V and 1 V. The Thin Solid Films, 6 (1970) 81-89
ELECTRICAL CONDUCTION IN
Nb-Nb2Os-Au DIODES
89
observed electrical properties in this voltage range indicated that the controlling conduction mechanism is that of the Poole-Frenkel effect. It also appears that oxygen vacancies occurring within the Nb205 can account for the "impurity band" proposed by Hickmott. The movement of these vacancies, or other mobile species such as Nb atoms, can be employed to explain observed changes in the conduction due to the application of strong electric fields or heat under vacuum conditions.
ACKNOWLEDGEMENT
This work was supported by the Advanced Research Projects Agency of the Department of Defense.
REFERENCES 1 W. R. BEAMA N D A. L. ARMSTRONG,Proc. IEEE, 52 (1964) 300. 2 K. L. CHOPRA, J. AppL Phys., 36 (1965) 184. 3 T. W. HICKMOTT, J. Appl. Phys., 37 (1966) 4380. 4 W. R. HIATT AND T. W. HICKMOTT, AppL Phys. Letters, 6 (1965) 106. 5 K. L. CHOPRA,Proc. IEEE, 51 (1963) 941. 6 D. V. GEPPERT, Proc. IRE, 51 (1963) 223. 7 T. W. HICKMOTT, Thin Solid Films, 3 (1969) 85. 8 K. L. CHOPRAAND L. C. Bonn, Proc. 1EEE, 51 (1963) 1784. 9 T. W. HICKMO'I% J. AppL Phys., 37 (1966) 4588. 10 A. K. JONSCHER, Thin Solid Films, 1 (1967) 213. 11 L. YOUNG, Can. J. Chem., 38 (1960) 1141. 12 E. H. GREENER, D. H. WHXTMOREAND M. E. FINE, J. Chem. Phys., 38 (1963) 133. 13 E. H. GREENERAND W. M. HIRTHE, J. Electrochem. Soc., 109 (1962) 600.
Thin Solid Films, 6 (1970) 81-89
E L E C T R I C A L C O N D U C T I O N IN N I O B I U M - N I O B I U M O X I D E - G O L D DIODES R. J. SCHWARTZ, Y. L. CHIOU* AND H. W. THOMPSON, JR.
School o f Electrical Engineering, Purdue University, Lafayette, Ind. ( U. S.A. ) (Received M a y 16, 1970)
SUMMARY
It is shown that for voltages between 0.2 V and 1 V ( N b - ) a linear relationship is obtained between In J and ~/V for diodes with relatively thick oxides (1700 A). The effects of heat treatment under vacuum conditions upon the conduction characteristics is reported. It is suggested that oxygen vacancies may be responsible for many of the electrical characteristics observed in thin Nb205 films.
INTRODUCTION
The electrical and optical properties of niobium-niobium oxide-metal diodes have been extensively studied using a variety of metals for the counterelectrode 1-8. The most extensive studies to date are those reported by Hickmott 3,4, 7.9, in which he has reported on the conduction properties, photo effects, bistable switching effects and electroluminescence of these diodes. It is the purpose of this paper to report on our observations on niobium-niobium oxide-gold diodes, and to comment on the conduction model as proposed by Hickmott 7. In particular, it will be shown from our data and from a replot of Hickmott's data that a linear relationship is obtained between In J and x/V in the voltage region between 0.2 V and 1 V for diodes with thick oxides indicating a PooleFrenkel type conduction mechanism. Also, it is suggested that a possible source for the "impurity band" which has been proposed by Hickmott is that of oxygen vacancies. The latter conclusion is based on the similarity between the results of electron ionization energy studies on near stoichiometric ~-Nb20 s niobium oxide and the energy of the electroluminescent radiation observed on amorphous Nb2Os. Supporting evidence for this supposition is found by studying the effects * Present address: University o f Illinois, Circle C a m p u s , Chicago, I11., U.S.A.
Thin Solid Films, 6 (1970) 81-89
82
R.J. SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON,JR.
of heat treatment under vacuum conditions on the conduction properties of niobium-niobium oxide-gold diodes.
FABRICATION In order to reduce the possibility of contamination, the source of niobium used for these experiments was triple-pass electron beam zone refined 3/8 in. diam. polycrystalline rod. Wafers 0.080 in. thick were cut from this rod. The wafers were then alternately mechanically polished and etched in 35 HF, 65 HNO3 (parts by volume), until a polished surface was obtained. The niobium oxide was grown by anodization in a saturated boric acid solution at a current density of 0.5 mA/cm 2. All samples reported below were anodized at constant current to a voltage of 72 V. Electrodes were obtained by the vacuum evaporation of gold from an alumina crucible at a pressure of 10-6 torr. The spacing between the substrate and the gold evaporation source was approximately 6 in. This relatively close spacing resulted in appreciable heating of the substrate during the evaporation of the gold. Electrode geometry was defined by the use of a metal mask during the deposition. The gold electrodes ranged in size from 0.005 in. to 0.010 in. diam. N o significant differences were observed in the electrical behavior as a result of using different electrode diameters.
MEASUREMENT TECHNIQUES All measurements performed at voltages above 10 V were made using a pulse technique, in which a single triangular-shaped voltage pulse or a pulse train was applied to the diode. The current voltage characteristic was displayed on a Tektronix 564 storage oscilloscope. All current-voltage measurements were made with the diodes in the dark. For voltages between 0.2 V and 1 V, d.c. measurements were made using Keithley 602 and Keithley 610B electrometers. Both d.c. and pulse techniques were utilized in the 1-10 V range. The results in this range were independent of the measurement technique. The reason for using a pulse technique was that above 20 V and below 0.2 V, the electrical properties of the diode were found to be a function of the previous electrical history of the diode.
RESULTS Immediately after deposition of the gold counterelectrodes, the current voltage characteristics were as shown in Fig. 1, where it may be seen that there was relatively little difference between the two bias polarities; that is, there was relaThin Solid Films, 6 (1970) 81-89
ELECTRICALCONDUCTIONIN Nb--Nb2Os-Au DIODES
83
1.2
12C
5
0
..-- ~b"
~ ec
~ o.G t~
I-
~
~ 40 o 0 o
i
4
~
,
i
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B
0.4
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t Nb÷
i
I0 20 DIODE VOLTAGE(VOLTS)
Fig. 1. Current-voltage characteristics immediately after evaporation of the gold electrode. Fig. 2. Drift observed in fresh diodes. tively little rectification exhibited by the fresh diodes. However, if the diodes were exposed to voltages in excess of 10 V, a pronounced shift was observed in the 1I-1 characteristics, particularly in the (Nb +) direction (see Fig. 2). The direction of the drift was a function of the polarity of the applied bias. Voltage pulses with Nb ÷ would cause the current to decrease, voltage pulses with N b - would cause a reversal of the drift direction. If the pulses (with Nb ÷) were continued for a long period of time the drift would eventually cease. The resultant 1I-1 curves exhibited a large rectification ratio, i.e. relatively easy current flow with N b - and relatively little current flow with Nb +. A similar high rectification state could be obtained by exposing the diode to the atmosphere for a few days. Hickmott has previously reported drift phenomena associated with the presence of humidity. However, the effect described above does not seem to be dependent upon the humidity, as it was observed to occur for diodes stored in low humidity environments as well as diodes exposed to normal laboratory humidity conditions. After a period of three days of exposure to the atmosphere, the current-voltage characteristics were those shown in Fig. 3. The characteristics shown in Fig. 3 are similar to the final characteristics which were obtained from a fresh diode after sustained exposure to Nb + voltage pulses. For reasons that will be explained later, it was felt that the conduction process was strongly influenced by oxygen vacancies within the N b 2 0 s. As a test of this hypothesis, a diode which had previously been exposed to the atmosphere until it had a large rectification ratio was exposed to a vacuum of 4 x 10- 7 torr for 15 hours. The V - I characteristic was measured before and after the prolonged exposure to vacuum conditions. As can be seen in Fig. 4, only a slight change was observed in the forward ( N b - ) II-1 characteristic*. A measure of the 1I-1 curves * The reader is cautioned to note the change in plotting technique which occurs at 1.2 V. Below 1.2 V, the abscissa is plotted as VL Above 1 V, a logarithmic scale is used. This procedure has been followed in all subsequent plots in order to emphasize the linear character of such curves in the two regions. Thin Solid Films, 6 (1970) 81-89
84
R . J . SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON, JR,
I0"~~
/
lO-S Vn
i iO.r I--Z lO_S t)
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0
~
x Measured Before Exposure to Vacuum
/
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I0 20 DIODE VOLTAGE (VOLTS)
,o
0
J
l
J
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,
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Fig. 3. Stable characteristics observed after exposure to the atmosphere. Fig. 4. Current-voltage characteristics under atmospheric and vacuum ambient conditions. Note scale change of the abscissa.
with N b ÷, however, resulted in the observation of a large increase in the current. Before exposure to the vacuum, for instance, the measured current at 25 V was on the order of 10 -12 A. After exposure to the vacuum this value had increased to 2 × 10-4 A. Forward and reverse characteristics under vacuum conditions are shown in Fig. 5. Because of the relatively small spacing between the gold evaporation source and the diode and the relatively long evaporation time, the diode was subjected to considerable heating during fabrication. In order to observe the possible effects of a vacuum heat treatment during fabrication, a diode was radiantly heated while at 3 x 10- v torr. When the diode had returned to room temperature, the V--I characteristics were again measured. Figure 6 shows the results of measurements before exposure to vacuum heating (Curve A), immediately after vacuum heating (Curve B), after a second vacuum heating (Curve C), and after continued exposure to vacuum for 12 hours following termination of the heat treatment (Curve D). As can be seen in Fig. 6, the application of heat under vacuum conditions produced a significant increase in the current level as well as a decrease in the slope. Twelve hours after termination of the heat treatment the current had partially returned to the preheated condition. Thin Solid Films, 6 (1970) 81-89
ELECTRICAL CONDUCTION IN
Nb-Nb2Os-Au
DIODES
85
10-7
lO.i
,o-'
82oo 80 ~ , 40 06
'
[
,°-,, °/-°
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3'0
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DIODE VOLTAGE (VOLTS)
.
.
oz5 d~ . .
.
,'.o" ....
DIODE VOLTAGE (VOLTS)
Fig. 5. Current-voltage characteristics after exposure to a vacuum for 15 hours. Fig. 6. Effects of vacuum heating on the current-voltage characteristics. A, before heating; B, immediately after heating; C, immediately after a second heating; D, after continued exposure to vacuum conditions for 12 hours after termination of heating.
In all cases, if the voltage applied to the diodes was maintained within the range from 0.2 to 5.0 V (Nb-), the drift observed in the V-1 curves was less than 5 % as the voltage was cycled within this range. However, for voltages less than 0.2 V ( N b - ) and for all reverse bias values (Nb+), the current decreased with time at constant bias. In addition, at reverse biases greater than 5 V (Nb÷), the current was found to be quite noisy. These effects prevented obtaining plots of the type shown in Figs. 4 and 6 for voltages less than 0.2 V and for all reverse voltages (Nb+).
DISCUSSION
A current-voltage characteristic in which In J is proportional to V~', as is observed below 1 V in Figs. 4 and 6, is usually interpreted as being due to the Poole-Frenkel emission of carriers from traps or due to the electric field lowering of a potential barrier such as that which occurs at the niobium oxide-gold interface, For the Poole-Frenkel effect, one finds J oc exp 2 fl V~ and for the Schottky barrier lowering effect J oc exp/~ V½ Thin Solid Films, 6 (1970) 81-89
86
R . J . SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON, JR.
where (1)
fl = k T \ 4 n % e r d ]
and where d is the insulator thickness. The two effects are usually distinguished from one another by a measurement of the slope of a log J versus V ~ plot such as Figs. 4 and 6. The Poole-Frenkel effect is expected to have twice the slope as that of the Schottky barrier lowering effect. However, as has been pointed out by Jonscher 1°, this is not always a reliable technique for separating the two effects. The value of e, to be used in eqn. (1) is the high-frequency dielectric constant which may be obtained from the square of the refraction index. Y o u n g I 1 has reported a value of the refraction index of 2.4 at high frequencies. If a value of e, = 5.76 and a thickness of d = 1700/k are used to compute fl, the predicted slopes of the Poole-Frenkel effect and the Schottky effect are 3.0 and 1.5 respectively. The experimental values shown in Figs. 4 and 6 are found to range from 10.7 to 13. However, if we refer to the energy band model proposed by Hickmott (Fig. 7), we observe that the field is not uniformly distributed across the niobium oxide, but is concentrated in the vicinity of the gold electrode. Hickmott has estimated this region to be in the order of 100 A thick. In fact, assuming that the conduction is dominated by Poole-Frenkel processes, we find that a value of d ranging from 90 A to 125 A is required in eqn. (1) to predict the slopes of Figs. 4 and 6. Reference to Fig. 7 shows that if the diode is under forward bias (Nb-), one would not expect Schottky barrier lowering at the oxide-gold interface as this bias polarity corresponds to forward bias conditions in a standard metal-semiconductor junction. For this reason the observed linear relationship between In J and x/V is thought to be controlled by a Poole-Frenkel mechanism. However, even in the case of the Poole-Frenkel effect the interpretation is complicated by the fact that if we use the model of Fig. 7 the application of a forward bias ( N b - ) will reduce the field strength in region II, thus increasing Nb
-
Nb205
I
Au
.eo,oo, i"=ol v-Conduction Band [ I'll Edge [-~"
I
....
l
-
[
I I
~
-
I
~--Oxygen Vacancy
Fe.~i
t.d~ d ~ I
-' LValence Bond Edge
Fig. 7. Electronic energy bands in Nb2Os. Thin Solid Films, 6 (1970) 81-89
ELECTRICAL CONDUCTION IN Nb--Nb2Os-Au DIODES
A IO-j
•
87
•
N I0-*
295
oK
g r~ iO.C I-z D O
[O-I°O
O.I
05
OIOOE VOLTAGE(VOLTS)
1.0
Fig. 8. Replot of data presented by Hickmott 3.
the trapped charge population, rather than enhancing the emission of trapped charges as in the usual Poole-Frenkel effect. Hickmott has divided the current flow below 1 V into an ohmic region and a region for which J oc exp ( e V / k T ) . However, if we replot the data as given in Fig. 4 of ref. 3, we observe (Fig. 8) that it is linear in a log J vs. I1"~ plot below 1 V just as are the diodes reported above. This linear relationship between In J and ~/V is not found for diodes with thin oxides (50-100 A). Reference to Fig. 6 shows that the effect of heating under vacuum conditions was to increase the current in all voltage ranges. However, the increased current flow was accompanied by a decrease in the slope of the log J vs. V ~ portion of the curve. This indicates that the effective width of the region in which the electric field is concentrated is increased by the heat treatment. However, since the current is simultaneously increased as fl decreases, this indicates that the number of traps in region 1I has increased or the barrier height has decreased. There has been considerable study of the conduction mechanisms in both single crystal and polycrystalline Nb2Os, in which it has been established that the conduction is controlled by the concentration of oxygen vacancies ~2. Greener and Hirthe 13 have measured the electronic activation energy of these oxygen vacancies and found them to have levels which are 0.22 eV and 1.1 eV below the conduction band. Although we are dealing with amorphous niobium oxide, it is interesting to note the similarity between the crystalline and amorphous systems. If we interpret Hickmott's "impurity band" as possibly being caused by oxygen vacancies, then the material in region I would behave as an n-type semiconductor due to the presence of a large concentration of oxygen vacancies. The rise in potenThin Solid Films, 6 (1970) 81-89
88
R.J. SCHWARTZ, Y. L. CHIOU, H. W. THOMPSON,JR.
tial as the gold electron is approached would indicate a reduction in the vacancy content. The behavior upon heat treatment can then be interpreted as an increase in the width of region II due to a transfer of vacancies from the surface or from region I into region II. A transfer of vacancies from region I would result in an increase in d and an increase in the vacancy concentration in region II. Increasing the vacancy concentration in region II would then lower the barrier height and increase the current flow while simultaneously decreasing the slope of the currentvoltage curves due to the increasing width of region II. There is an interesting similarity between the electronic energy levels measured by Greener and Hirthe and the energy of the electroluminescence radiation measured by Hickmott 7. If allowances are made for the probable energy broadening effects in amorphous materials, the 1.1 eV measured for a doubly ionized oxygen vacancy level in polycrystalline Nb205 corresponds quite well with the measured peak in electroluminiscence radiation at 1.2 eV. In addition, Hickmott 7 has speculated on the existence of a donor level lying 0.3 eV below the conduction band, which again is in the vicinity of the 0.22 eV level reported by Greener and Hirthe for singly ionized oxygen vacancies. The similarity of the drift behavior of the diodes either due to the application of strong electric fields or due to the exposure to the atmosphere leads one to speculate that both effects are caused by a variation in the oxygen vacancy content in the region of the film nearest to the gold electrode. In one case the driving force would be electronic, in the other case chemical or thermodynamic. Also, if one speculates on the existence of a mobile species within the film (that species being either niobium or oxygen vacancies), then it is possible that the switching mechanism which has been observed* is due to a transfer of either niobium ions or oxygen vacancies into or out of region II. When vacancies are transferred into region II, a lowering of the barrier at the niobium oxide-gold interface would occur, and one would expect switching into a state with a much higher conductivity than the original state of the diode. In addition to this, if the mobile species could be reversibly transferred into and out of region II by the application of electric field or by a combination of electric field and heating due to localized conduction, then it would be expected that one could switch the conduction states between a relatively high conduction state (one with a low barrier height occurring at the gold-niobium oxide interface) and a low conduction state (one in which a moderate barrier existed at the niobium oxide gold-interface).
CONCLUSIONS The electrical conduction which occurs in Nb--Nb2Os-Au diodes when the oxide is thick ( ~ 1700 A) is controlled by a narrow region of the oxide lying adjacent to the Au contact for applied voltages lying between 0.2 V and 1 V. The Thin Solid Films, 6 (1970) 81-89
ELECTRICAL CONDUCTION IN
Nb-Nb2Os-Au DIODES
89
observed electrical properties in this voltage range indicated that the controlling conduction mechanism is that of the Poole-Frenkel effect. It also appears that oxygen vacancies occurring within the Nb205 can account for the "impurity band" proposed by Hickmott. The movement of these vacancies, or other mobile species such as Nb atoms, can be employed to explain observed changes in the conduction due to the application of strong electric fields or heat under vacuum conditions.
ACKNOWLEDGEMENT
This work was supported by the Advanced Research Projects Agency of the Department of Defense.
REFERENCES 1 W. R. BEAMA N D A. L. ARMSTRONG,Proc. IEEE, 52 (1964) 300. 2 K. L. CHOPRA, J. AppL Phys., 36 (1965) 184. 3 T. W. HICKMOTT, J. Appl. Phys., 37 (1966) 4380. 4 W. R. HIATT AND T. W. HICKMOTT, AppL Phys. Letters, 6 (1965) 106. 5 K. L. CHOPRA,Proc. IEEE, 51 (1963) 941. 6 D. V. GEPPERT, Proc. IRE, 51 (1963) 223. 7 T. W. HICKMOTT, Thin Solid Films, 3 (1969) 85. 8 K. L. CHOPRAAND L. C. Bonn, Proc. 1EEE, 51 (1963) 1784. 9 T. W. HICKMO'I% J. AppL Phys., 37 (1966) 4588. 10 A. K. JONSCHER, Thin Solid Films, 1 (1967) 213. 11 L. YOUNG, Can. J. Chem., 38 (1960) 1141. 12 E. H. GREENER, D. H. WHXTMOREAND M. E. FINE, J. Chem. Phys., 38 (1963) 133. 13 E. H. GREENERAND W. M. HIRTHE, J. Electrochem. Soc., 109 (1962) 600.
Thin Solid Films, 6 (1970) 81-89