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Observation on phenomena associated with a slowly varying surface barrier at niobium oxide and aluminum interface
Solid-Slarr
Electrorrics.
1972,Vol. 15,pp.999-1001. PergamonPress.
PrintedinGreat
Britain
OBSERVATION ON PHENOMENA ASSOCIATED WITH A SLOWLY VARYING SURFACE BARRIER AT NIOBIUM OXIDE AND ALUMINUM INTERFACE Y. L. CHIOU Department of Information Engineering, University of Illinois, Chicago, Illinois 60680, U .S .A. (Received
17 January
1972; in revisedform
3 April 1972)
Abstract-The V-l characteristics of a diode, which consists of a silver contaminated niobium oxide sandwiched between niobium and aluminum, has been studied. It is observed that the diode can be either rectifying or non-rectifying depending upon the duration and repetition rate of the pulse excitation. This behavior is attributed to the drift of the silver ions that slowly changes the surface barrier at the interface of the niobium oxide-aluminum. The V-Z characteristics of the diode are linear in a log V vs. log I plot. This linearity can be explained as due to space charge limited current flow. 1. INTRODUCTION
performed in a saturated boric acid solution with a constant current density of 0.5 mA/cm*. Subsequently the oxide was heat-treated in a trichloroethylene solution in which a small amount of silver has been suspended. Aluminum films were then deposited on the oxide films by vacuum evaporation at a pressure of 10e6 Torr. The aluminum electrode geometry was defined by the use of a metal mask during deposition. The diameter of the electrode was about 5 mils.
THE
voltage-current characteristics of niobiumniobium oxide-aluminum diodes have been studied by Chopra [ 11. It was reported that the rectification was poor. This was attributed to the diffusion of aluminum in the oxide, thus modifying the potential barrier. We would like to report our observation on the electrical properties of the same sort of structure except that the oxide has been contaminated by silver. This type of diode has an interesting behavior; with a proper choice of pulse excitation, the V-I characteristic of the diode can stay in either a rectifying or non-rectifying state. The mechanism responsible for this behavior is attributed to the impurities which under the presence of an applied electric field drift from their stationary sites and modify the surface barrier. This surface barrier is located at the niobium oxide-aluminum interface and has a slow time response to the change of applied voltage. The functional dependence of voltage and current can be interpreted in terms of one-canier space charge limited flow[2].
3. RESULTS
In order to minimize thermal effects in the oxide, a triangular-shaped voltage pulse or a pulse train was chosen as the voltage source for the V-Z characteristic measurements for the diode. The V-Z curve was displayed on a Tektronix 564 memory oscilloscope. It was found that the V-Z curves were stable when the duration of the pulse was in the range from 10 to 20 msec. Hereafter, the duration of the pulse is to be considered 20 msec unless specifically stated. A typical pulse wave form is shown in Fig. l(a). In the following measurements, the diode will be considered forward-biased if the applied voltage is negative at the niobium electrode, and reversebiased if the applied voltage is positive. The thickness of the niobium oxide film was approximately 3600 A. It was observed that the diode was rectifying when the voltage pulse was initially applied in the reverse direction and subsequently applied
2. FABRICATION In this study the source of niobium was triplepass, electron beam zone refined, 318 in. diameter, polycrystalline rod. Wafers 80 mils thick were cut from this rod. The wafers were then alternately mechanically polished and etched in 35 HF, 65 HNO, (parts by volume), until a polished surface was obtained. All the anodization was 999
Y. L. CHIOU
1000
9
drift either downward or upward by applying a pulse in the opposite direction. By varying the magnitude of the added pulse, various stable V-Z curves were obtained. They are expressed in a log Z vs. log V plot as shown in Figs. 2 and 3.
20msec
I
I 03
(0)
Area
Vertical ’
Horizontal
20~
=I 27x
10-4cm’
A/cm
SV/cm
(b)
Fig.
1. (a) Pulse waveform. (b) Transition rectifying and non-rectifying.
state between
in the forward direction. An interesting behavior of this type of diode is shown in Fig. l(b). In this measurement, the voltage pulse was initially applied in the forward direction. The lowest curve in the reverse direction was obtained immediately after the polarity of the pulse is reversed. By continuously applying the pulses to the diode in the reverse direction, the other two curves were obtained. The spike in the reverse direction is reproducible from measurement to measurement. The necessary condition for producing the spike is that the voltage pulse must be applied in the forward direction first and then in the reverse direction. The time interval between the pulses is 1.6 sec. If we increase the repetition rate and decrease the time interval of the pulses the diode can be in non-rectifying state i.e. the V-Z curve is antisymmetric with respect to the current axis. The stability of the V-Z characteristic of the diode was studied. When the pulses were applied with the niobium electrode negative, to a freshly prepared diode, the V-I curves drifted upward continuously and eventually reached a steady state. If a pulse with opposite polarity was added to the pulse, the V-Z curve could be forced downward by increasing the magnitude of the added pulse. Similarly the reverse characteristic could be forced to drift either downward or upward with respect to the voltage axis. As was mentioned above, the V-Z characteristic of the diode could
Fig. 2. Downward
drift of the V-l characteristics.
V,
Fig. 3. Upward
”
drift of the V-l characteristics.
NIOBIUM
OXIDE
AND
Figure 2 shows the downward drift of the V-Z curves and Fig. 3 the upward drift. As can be seen from the figures, the V-I curves are linear in a log I vs. log V plot. The slopes increase as the V-I curve drift downward and decrease for upward drift. 4. CONCLUSIONS
All of the above observed phenomena can be explained in terms of the formation of a slowly varying surface barrier at the niobium oxidealuminum electrode interface. The change in the surface barrier is attributed to the movement of the impurity ions which came from the heat treatment in the trichloroethylene solution containing silver. When an applied electric field is present in the diode, the impurities will be displaced from their stationary sites, hence modulate the barrier height at the interface between the niobium oxide and aluminum electrode. Under forward bias, the surface barrier height decreases from that of the zero bias value, while it increases under reverse bias. When the polarity of the applied voltage is
SSEVol.
I5 No.9-D
ALUMINUM
INTERFACE
1001
switched from forward to reverse direction, the change in the surface barrier height cannot respond as rapidly as to the change in the applied voltage. As a result of this delay in the response to the applied voltage, the spike and drift- in the V-I characteristic of the diode are observed. The V-I characteristic of the diode can be explained by the model of one-carrier space charge limited flow with a variable trap distribution[2]. The relationship between the current and voltage is I lx V”
(1)
where n depends upon the distribution of the traps in the forbidden gap. When an electric field is applied to the diode, the silver ions are displaced from their stationary sites. This disturbs the trap distribution and accordingly the slopes of log I vs. log V plots as shown in Figs. 2 and 3. REFERENCES 1. K. L. Chopra, Solid-St. Electron. 8,715(1965). 2. A. Rose,Phys. Rev. 97, 1538(1955).
Electrorrics.
1972,Vol. 15,pp.999-1001. PergamonPress.
PrintedinGreat
Britain
OBSERVATION ON PHENOMENA ASSOCIATED WITH A SLOWLY VARYING SURFACE BARRIER AT NIOBIUM OXIDE AND ALUMINUM INTERFACE Y. L. CHIOU Department of Information Engineering, University of Illinois, Chicago, Illinois 60680, U .S .A. (Received
17 January
1972; in revisedform
3 April 1972)
Abstract-The V-l characteristics of a diode, which consists of a silver contaminated niobium oxide sandwiched between niobium and aluminum, has been studied. It is observed that the diode can be either rectifying or non-rectifying depending upon the duration and repetition rate of the pulse excitation. This behavior is attributed to the drift of the silver ions that slowly changes the surface barrier at the interface of the niobium oxide-aluminum. The V-Z characteristics of the diode are linear in a log V vs. log I plot. This linearity can be explained as due to space charge limited current flow. 1. INTRODUCTION
performed in a saturated boric acid solution with a constant current density of 0.5 mA/cm*. Subsequently the oxide was heat-treated in a trichloroethylene solution in which a small amount of silver has been suspended. Aluminum films were then deposited on the oxide films by vacuum evaporation at a pressure of 10e6 Torr. The aluminum electrode geometry was defined by the use of a metal mask during deposition. The diameter of the electrode was about 5 mils.
THE
voltage-current characteristics of niobiumniobium oxide-aluminum diodes have been studied by Chopra [ 11. It was reported that the rectification was poor. This was attributed to the diffusion of aluminum in the oxide, thus modifying the potential barrier. We would like to report our observation on the electrical properties of the same sort of structure except that the oxide has been contaminated by silver. This type of diode has an interesting behavior; with a proper choice of pulse excitation, the V-I characteristic of the diode can stay in either a rectifying or non-rectifying state. The mechanism responsible for this behavior is attributed to the impurities which under the presence of an applied electric field drift from their stationary sites and modify the surface barrier. This surface barrier is located at the niobium oxide-aluminum interface and has a slow time response to the change of applied voltage. The functional dependence of voltage and current can be interpreted in terms of one-canier space charge limited flow[2].
3. RESULTS
In order to minimize thermal effects in the oxide, a triangular-shaped voltage pulse or a pulse train was chosen as the voltage source for the V-Z characteristic measurements for the diode. The V-Z curve was displayed on a Tektronix 564 memory oscilloscope. It was found that the V-Z curves were stable when the duration of the pulse was in the range from 10 to 20 msec. Hereafter, the duration of the pulse is to be considered 20 msec unless specifically stated. A typical pulse wave form is shown in Fig. l(a). In the following measurements, the diode will be considered forward-biased if the applied voltage is negative at the niobium electrode, and reversebiased if the applied voltage is positive. The thickness of the niobium oxide film was approximately 3600 A. It was observed that the diode was rectifying when the voltage pulse was initially applied in the reverse direction and subsequently applied
2. FABRICATION In this study the source of niobium was triplepass, electron beam zone refined, 318 in. diameter, polycrystalline rod. Wafers 80 mils thick were cut from this rod. The wafers were then alternately mechanically polished and etched in 35 HF, 65 HNO, (parts by volume), until a polished surface was obtained. All the anodization was 999
Y. L. CHIOU
1000
9
drift either downward or upward by applying a pulse in the opposite direction. By varying the magnitude of the added pulse, various stable V-Z curves were obtained. They are expressed in a log Z vs. log V plot as shown in Figs. 2 and 3.
20msec
I
I 03
(0)
Area
Vertical ’
Horizontal
20~
=I 27x
10-4cm’
A/cm
SV/cm
(b)
Fig.
1. (a) Pulse waveform. (b) Transition rectifying and non-rectifying.
state between
in the forward direction. An interesting behavior of this type of diode is shown in Fig. l(b). In this measurement, the voltage pulse was initially applied in the forward direction. The lowest curve in the reverse direction was obtained immediately after the polarity of the pulse is reversed. By continuously applying the pulses to the diode in the reverse direction, the other two curves were obtained. The spike in the reverse direction is reproducible from measurement to measurement. The necessary condition for producing the spike is that the voltage pulse must be applied in the forward direction first and then in the reverse direction. The time interval between the pulses is 1.6 sec. If we increase the repetition rate and decrease the time interval of the pulses the diode can be in non-rectifying state i.e. the V-Z curve is antisymmetric with respect to the current axis. The stability of the V-Z characteristic of the diode was studied. When the pulses were applied with the niobium electrode negative, to a freshly prepared diode, the V-I curves drifted upward continuously and eventually reached a steady state. If a pulse with opposite polarity was added to the pulse, the V-Z curve could be forced downward by increasing the magnitude of the added pulse. Similarly the reverse characteristic could be forced to drift either downward or upward with respect to the voltage axis. As was mentioned above, the V-Z characteristic of the diode could
Fig. 2. Downward
drift of the V-l characteristics.
V,
Fig. 3. Upward
”
drift of the V-l characteristics.
NIOBIUM
OXIDE
AND
Figure 2 shows the downward drift of the V-Z curves and Fig. 3 the upward drift. As can be seen from the figures, the V-I curves are linear in a log I vs. log V plot. The slopes increase as the V-I curve drift downward and decrease for upward drift. 4. CONCLUSIONS
All of the above observed phenomena can be explained in terms of the formation of a slowly varying surface barrier at the niobium oxidealuminum electrode interface. The change in the surface barrier is attributed to the movement of the impurity ions which came from the heat treatment in the trichloroethylene solution containing silver. When an applied electric field is present in the diode, the impurities will be displaced from their stationary sites, hence modulate the barrier height at the interface between the niobium oxide and aluminum electrode. Under forward bias, the surface barrier height decreases from that of the zero bias value, while it increases under reverse bias. When the polarity of the applied voltage is
SSEVol.
I5 No.9-D
ALUMINUM
INTERFACE
1001
switched from forward to reverse direction, the change in the surface barrier height cannot respond as rapidly as to the change in the applied voltage. As a result of this delay in the response to the applied voltage, the spike and drift- in the V-I characteristic of the diode are observed. The V-I characteristic of the diode can be explained by the model of one-carrier space charge limited flow with a variable trap distribution[2]. The relationship between the current and voltage is I lx V”
(1)
where n depends upon the distribution of the traps in the forbidden gap. When an electric field is applied to the diode, the silver ions are displaced from their stationary sites. This disturbs the trap distribution and accordingly the slopes of log I vs. log V plots as shown in Figs. 2 and 3. REFERENCES 1. K. L. Chopra, Solid-St. Electron. 8,715(1965). 2. A. Rose,Phys. Rev. 97, 1538(1955).