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Electrode effects and bistable switching of amorphous Nb2O5 diodes
Solid-State
Electronics
Pergamon
ELECTRODE
Press
1970.
Vol.
EFFECTS
13, pp. 1033-1047.
AND
OF AMORPHOUS
Printed
BISTABLE
Nb,O,
in Great
Britain
SWITCHING
DIODES*
T. W. HICKMOTT IBM
Components
Division,
East
Fishkill
Laboratory,
Hopewell
Junction,
N.Y.
12533,
U.S.A.
W. R. HIATT Electronics
Laboratory,
General
Electric
Company,
(Received 26 September 1969; is revised
Syracuse,
N.Y.
13088,
U.S.A.
form 15 December 1969)
Abstract-Nb-Nb,O,-metal diodes, after breakdown, exhibit switching between two stable conduction states and are potentially usable in an electrically alterable memory. Detailed procedures for oxide formation and electrical breakdown to produce Nb-NbaOs-Bi diodes with good switching characteristics are described. The ratio of conductivities in the two stable states varies between 30 and 100. Switching from low to high conductivity can be done with 0.5-1.0 +ec pulses; switching from a high- to low-conductivity state requires 1.0-10 psec pulses. Switching voltages are less than 1 V, switching powers are less than 100 pW, and the direction of switching is controlled by the polarity of the applied voltage. The dependence of device stability on temperature, storage, and bias are reported. When low-work function metals such as In, Cd, Zn, or Pb are used as counterelectrodes, a voltage-controlled negative resistance inflection occurs in the high-conductivity state. Diodes switch at 4.2”K. Switching appears to be an electronic phenomenon, occurring at a small number of regions in the oxide. R&sum&-Les diodes Nb-Nb&-m&al, apr&s fiasco, exhibent une commutation entre deux Ctats de conduction stables et sent potentiellement utilisables dans une mCmoire Clectriquement changeante. On dCcrit en d&ail les processus de formation d’osyde et de fiasco Clectrique pour produire des diodes Nb-Nb&-Bi ayant de bonnes caractiristiques de commutation. Le rapport de conductivitCs dans les deus Ctats stables varie entre 30 et 100. La commutation d’une conductivitC faible g une conductivitC ClevCe necessite des imp&ions de 0,s & 1,0 psec; la commutation d’une conductivitC ClevCe g une conductivitC faible demande des impulsions de 1,0 & 10 psec. Les tensions de commutation sont infirieures & 1 volt, les puissances de commutation sont infirieures i 100 pwatt, et la direction de commutation est contr81Ce par la polarit& de la tension appliquee. On indique comment la stabilitC du dispositif est fonction de la temperature, de l’emmagasinage et de la polarisation. Lorsque des m&aux B fonction de travail faible sont utilisCs comme contre-klectrodes, tels que In, Cd, Zn ou Pb, une inflection de resistance nCgative contr81Ce par tension a lieu dans 1’Ctat de conductivitC ClevCe. Les diodes commutent g 4,2”K. La commutation semble ctre un phCnom&ne Clectronique prenant place dans un nombre limit6 de rigions dans l’oxyde.
Zusammenfassung-Nb-lb20G-Metalldioden
weisen nach dem Durchbruch eine Schaltcharakteristik auf mit zwei stabilen Leitftihigkeitszustznden und sind miiglicherweise fiir ein elektrisch verPnderbares Speicherelement brauchbar. Verfahren zum Erzielen des Oxides und der Nb-Nbz05Bi-Diode durch Formieren im Durchbruchsbereich werden detailliert beschrieben. Das Verhiltnis der LeitfPhigkeiten in den beiden stabilen Zustanden variiert zwischen 30 und 100. Zum Schalten vom Zustand niedriger zu hoher Leitfiihigkeit werden 0.5 psec. bis 1.0 psec-Impulse ben6tigt. Das * Work done at General Electric Research and Development was sponsored by the Air Force Avionics Laboratory, Research Wright-Patterson Air Force Base, Ohio under Contract No. AF 1033
Center, Schenectady, New and Technology Division, Air 33 (615) (1218).
York. Force
The work Command,
1034
T.
W.
HICKRIOTT
and
‘vv. Ii.
HIATT
SChaltell in umgekehrter Richtung erfordert Impulsllngen zwischen 1.0 und 10 psec. Die Schaltspannungen liegen unter 1 V, die erforderlichen Leistungen sind unter 100 p\V und die Schaltrichtung wird durch die Polaritat der angelegten Spannung bestimmt. Die .4bhangigkeit der Stabilitat des Bauelementes van der Temperatur, der Speicherung und Vorspannung werden beschrieben. Werden Metalle mit kleiner Austrittsarbeit, wie In, Cd, Zn oder Pb als Gegenelektrode verwendet, dann tritt eine spannungsgesteuerte Verbiegung des negativen Widerstand& im Bereich der hohen Lcitfahigkeit auf. Die Diode” schalten bei 4.2 K, der Mechanismus beruht auf einem elektronischen Phanomen, das sich in einigen wenigcn Bereichen des Oxids abspielt.
INTRODUCTION REVERSIBLE
electrical switching between a stable high-impedance state and a stable low-impedance state has been reported for many thin film or glassy materials, including Nb,0,,‘1~2) X0,(3) chalcogenide glasses, (4-6) GaAs,‘7) Ti0,,‘8’ transition-metal doped glasses,@) and a variety of other insulators.‘10-13) For all except VO,,‘lo*ll) the devices as prepared are in a low-conductivity state and require a breakdown, or forming process, to develop the high-conductivity state. A basic problem for each system is accounting for the stability of the high-conductivity state when bias is removed from the devices, and its destruction on switching. For XiO, metallic bridges between electrodes have been detected.‘3’ For chalcogenide glasses, a phase change in glasses that exhibit memory has been suggested as the origin for a stable high-conductivity state.(14) Switching in transition-metal doped phosphate glasses has been attributed to electronic effects due to the presence of mixed valency ions such as Cu’ or CUI’.‘~’ Switching in \:O, films is thermal and related to the well known metal-insulator transition which occurs at 68”C.‘10*11) Thus bistable switching in amorphous materials and thin films is due to a variety of causes, both thermal and electronic. No general mechanism is applicable in every case, although some type of filamentary conduction is involved for all, including VO,.‘ll’ Among insulators that show bistable switching and memory, thin film NbaO, diodes are unique because of the low voltages (< 1 V) at which switching occurs and because of the independence of switching voltage on insulator thickness. In chalcogenide glasses and transition-metal doped phosphate glasses, switching from high to low conductivity requires a very high current pulse, consistent with the occurrence of a phase change or thermal transformation. For Xb,O,, and also for TiO,, polarity reversal of the voltage will
switch the diode from its high-conductivity state. For Nb-NbaOs-_\u diodes, formation of the high-conductivity state occurs at a very small number of singularities in the oxide.“’ However, the occurrence of electroluminescence in switching diodes, the similarity of the electroluminescent spectra to electroluminescence before breakdown,‘15) the speed of switching, and the low voltage and power required for switching suggest that an electronic mechanism rather than melting or a phase change is responsible for bistable switching. In this paper, switching and stability of NbNbaOs-Bi diodes will be discussed in detail. Some observations of switching at 42°K in Nb-Nb,O,-Bi, Nb-Nb,O,-In, and Nb-Nb,O,Pb diodes will be presented. The important role of the counterelectrode in determining currentvoltage characteristics of Nb,O, diodes will be reemphasized.(‘) EXPERIMENTAL
The preparation of Nb films and of Xb-Xb,O,metal diodes has been described.(l*16’ A variety of methods to make Sb,O, films was tried. The final process to produce diodes that switched well used an electrolyte prepared by dissolving 9 g of ammonium pcntaboratc in 100 cm3 of ethylene glycol. Nb films were anodized at a constant current density of 1 mA/cm2 until the cell voltage reached 60 V; the oxide film was then held at 60 V for 5 min while the current decreased, forming an oxide about 1500 A thick. The electrolyte was maintained at 110°C during anodization, and a stainless steel sheet was used as cathode. The oxide films which were used to study the effect of metal counterelectrode were anodized in saturated boric acid electrolyte. Switching was observed in diodes with Nb,O, film thicknesses between about 500 A and 32OOA. These limits
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
are not necessarily inclusive. Metal counterelectrodes were evaporated through a mask to form diodes of 1 mm2 area.
Development
of switching in Nb-Nb,O,-Bi
diodes
Freshly prepared Nb-Nb,O,-Bi diodes are rectifying; the direction of easy current flow is with Bi positive. The procedure for development of bistable switching can be either simple or complex. If the voltage across an Nb-Nb,Os-Bi diode is slowly raised with Bi positive, using a constant-current generator, a sudden breakdown to a high-conductivity state occurs. In fact, as discussed before,(l) several high-conductivity
OF AMORPHOUS
Nbz05
DIODES
states can occur and switching between these states affected with pulsed voltages. However, when such a simple breakdown procedure is used, switching characteristics that result are erratic and degrade rapidly. A more complex breakdown procedure using and current-limited electrical both a pulsedbreakdown to establish reproducible switching characteristics was developed by trial and error. Detailed reasons for each step in the procedure are not clear but the final device characteristics were the most stable found. The breakdown procedure is described in detail because it illustrates the complexities of the process and may serve as starting point for better procedures.
A ima 2.0-
1.0-
I IS.0
b
E (VOLTS1 BISMUTH ELECTRODE POSITIVE
(a)
ima I.0
f
0.5-
b
(1. CHARACTERISTIC
PRODUCED
BY 1st BREAKDOWN
b.
PRODUCED
BY 2nd
CHARACTERISTIC
C. PORTION
OF FINAL
1035
E (VOLTS) BISMUTH ELECTRODE POSITIVE
BREAKDOWN
CHARACTERISTIC
(b) FIG. 1. (a) I-V characteristic of Nb-Nb&-Bi diode before breakdown. (b) Stages in breakdown of Nb-NbaOs-Bi diode to high-conductivity state.
1036
T.
M-. HICKMOTT
The first step in the electrical breakdown procedure was to apply a single high-voltage pulse driving the bismuth electrode positive. A single 1 psec pulse of 50 V amplitude, corresponding to a field of about 3.5 x lo6 V/cm, was applied to the Bi diode through a 1500 fi series resistor. Application of such a pulse did not significantly alter the current-voltage characteristic of the device. It was found to be essential if the final device characteristics were to be stable; deletion of this step resulted in devices whose final characteristics were subject to spontaneous, erratic fluctuations. The second step in the breakdown was to apply a continuous voltage to the Bi electrode from a high-impedance source for as long as was necessary to cause the device to increase its conductivit! abruptly and irreversibly by a large amount. For example, a variable voltage source with a series resistor of at least 40,000 fi was connected to the Nb-Nb,O,-Bi diode. The voltage was then adjusted to produce a potential drop across the device of 12 to 13 V. Figure la shows a typical I-T’ characteristic at this stage of breakdown. After a time which was found to vary between 20 and 300 set, a sudden increase in the electrical conductivity of the device occurred. The diode voltage would suddenly drop from the initial value of 12-13 \:. The I-17 characteristics of the device at this stage of the breakdown process was typically that of (a) in Fig. lb, or in some cases (b). It was important at this point not to apply a negative voltage to the Bi electrode. If negative voltage were applied at this stage of the process, it would increase the average conductivity of the low conductivity state of the final device. The third step in the electrical breakdown was to increase the voltage across the device slowly, using the same voltage source as in the previous step, with the series resistance reduced to about 3500 ,O_. The second breakdown or third breakdown, depending on whether the device had been brought to state (a) or (b) of Fig. lb, would occur once the appropriate voltage threshold was reached. The second breakdown transition was abrupt, much like the first. The third breakdown occurred at a rate determined by how rapidly the output of the voltage source was increased. The transition was usually accompanied by oscillations. The average conductivity of the final I-V characteristic depended on the peak current reached during
and
W.
R.
HIATT
the last breakdown. If device current during breakdown exceeded 0.8 mA, the low-conductivity characteristic of the final device would be increased in average conductivity. If this final step were carried out, or repeated, with the device at a temperature in excess of 60°C the final characteristics were less sensitive to changes in ambient temperature than was the case if final breakdown had been done at room temperature. Typical final device characteristics are shown in Fig. 2a. The device had two distinct conductivity states, a low state (a) and a high state (b). State (b) of Fig. 2a corresponds to state (c) of Fig. la. Typical ratios of high/low conductivity were 50/l. It could be reversibly driven from either of these states to the other. Switching from low conductivity to high conductivity required application of a positive voltage in excess of the switching threshold T2. The opposite transition required application of a voltage more negative than T,. The state of the Eb-Nb,O,-Bi diode at any time could be determined, without altering the state, by applying a voltage smaller than the threshold voltage and measuring the resulting current. The currentcontrolled negative resistance (CCNR) inflection at about 0.2 1’ was characteristic of the highconductivity state when Bi counterelectrodes were used. Figure 2a is for a diode with 1500 4 Kb,O,; nearlv identical characteristics were observed for all oxide thicknesses. Temperature sensitivity of switclhg OJ‘Nb-Sb,O,jBi diodes Temperature tests above room temprraturc were made in a heated metal desiccator. Corrosion of Bi electrodes was minimized by spraying varnish over the entire slide. This had no apparent effect on the electrical behavior of devices. The maximum temperature that could be used was limited to 110°C by melting of the alloy formed when leads were attached to Bi electrodes by indium solder. At temperatures below 100°C there was visible evidence of diffusion of indium along the Bi strip. This had no effect on electrical properties of the device unless it extended to the active diode region where Bi was in contact with Nb,O,. The first time a device was tested at elevated temperature, an abrupt change occurred in both positive- and negative-switching threshold voltages at a temperature between 60°C and 7OC. To
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
Nb,O,
OF AMORPHOUS
DIODES
1037
jrno 0.4
. . .. 0.2
i
‘.
.\
15ooII t.
/
)
0.
LOW CONDUCTIVITY
CHARACTERISTIC
b.
HIGH CONDUCTIVITY
CHARACTERISTIC
t
E(VOLT9 BISMUTH POSITIVE
ima
l.O-
b
SLOW PATH
E (VOLTS) BISMUTH POSITIVE
HIGH-TO-LOW SWITCHING 16001-z SOURCE
ASSUMED PATH OF FAST HIGH-TO-LOW SWITCHING (NOT INCLUDING DISPLACEMENT CURRENTS)
@I
FIG. 2. Bistable switching in Nb-NbzOs-Bi diodes. (a) Switching characteristic for slow voltage changes. (b) Switching characteristic for fast pulse switching.
produce switching after this required a re-breakdown of the device. Following such re-breakdown, switching characteristics were not greatly different from room temperature characteristics up to the highest temperature used. The characteristics of
the device, when returned to room temperature were improved. Conductivity in both high- and low-conductivity states was slightly lower. On repeating temperature tests, changes in device characteristics with temperature were smaller
T.
1038
W;. HICKMOTT
than in the initial run. The peculiar behavior in the 60-70°C range did not recur on subsequent temperature runs. Table 1 shows least-squares Table 1. Temperature sensitivity of deviceparameters Value Parameter
at 20°C
Rate of change with temperature -0.011
n:s2i0c
Low State Resistance (secant resistance at 0.4 V)
259 kfi
High State Resistance (secant resistance at 0.2 V)
1.92 kc2
-0.014
Absolute Value of Negative Threshold Voltage
0.18 V
-0.0028
Positive Threshold Voltage
0.49 1‘
0.014 v/v/:c
n/WC
v/v/;c
of the temperature sensitivity, AVjV and AR/R, of device parameters relative to their value at 20°C. Data were fitted over the range 20-90°C.
fit
Switching characteristics of Kb-Nb,O,-Bi
diodes
1. Low speed repetitive switching. Nb-Nb,O,-Bi devices were tested at low frequencies by applying a d.c.-biased sinusoidal voltage which had an excursion between the same limits as required for tracing the d.c. characteristics of the device. By monitoring the I-V Lissajous pattern on an oscilloscope, switching was monitored for frequencies up to about 10 kHz. At higher frequencies, displacement currents obscured switching, and discontinuities in the voltage waveform across the device showed the occurrence of switching. Figure 3 shows a scope tracing of switching at 20 Hz. At frequencies up to 100 Hz, adjustment of bias voltage and sinusoidal amplitude could be found which produced reliable switching, without extensive change in device characteristics. Such low-speed repetitive switching was continued for up to 6 hr with only minor changes in device characteristics. Representative I-V characteristics, before and after low-speed repetitive switching, are shown in Fig. 4a and b. The current-controlled negative resistance inflection at 0.2 V on the positive arch of the high conductivity characteristic
and
11’. R.
HIATT
typically became less pronounced, and the average conductivity of both states increased. Raising the frequency to 1 kHz required that the amplitude of the driving voltage be increased to obtain consistent switching. Such switching produced permanent degradation of characteristics as shown in Fig. 4c. 2. Pulse and high-frequency
switching. Fast
switching
to
from
the
low-
pulse
high-conductivity
state, along the d.c. load line, was easily done. The most dependable means with single pulses was to use a pulse of 0.7 V in series with a resistor of 1600 Q by-passed by a ‘speed-up’ capacitor of 1.5 nF. The minimum pulse width was 0.5 psec. The actual transition could be seen on the scope trace and required less than 0.1 psec. The reverse transition along the d.c. I-V characteristic, as in Fig. 2a, was slow, frequently rcquiring milliseconds. At higher voltages with Bi negative, a current-controlled negative resistance region occurs in the low-conductivitv state as shown in Fig. 2b. Rapid switching into this state was possible. A -2.6 V pulse through a series resistance of 30,000 0 was used. The minimum pulse duration required for switching was more variable from device to device, ranging from 1.0 to 9.0 psec. Repeated cycling with single pulses did not degrade the d.c. device characteristics for switching in either direction. For some devices, when a long duration pulse was applied with Hi negative, 500 kHz relaxation oscillations between states were obscrved.‘17-1”) During oscillation, the transition from low to high conductivit! occurred in tenths of microseconds while the transition difficult highto low-conductivit!. required 2 psec. Progressive and permanent degradation of device characteristics occurred in seconds for sinusoidal drive at 100 kHz or higher. For highfrequencv repetitive pulse switching, alternate polarity pulse trains were generated by using two pulse generators, one of which was synchronized by a delaved trigger from the other, which \vas free running at the chosen frequency. Such an arrangement allowed freedom of choice of pulse widths. pulse separation, and amplitudes for a train of pulses of alternate polarity, No adjustment could be found which did not result in sporadic switching and permanent degradation of device
.FIG. 3. Oscilloscope photograph of bistable switching of Nb-NbaOs-Bi diode at 20 Hz. x = 0.2 V/cm, y = 0.5 mA/cm, oxide thickness = 1250 A.
[facing p. 1038
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
OF AMORPHOl2S
a. INITIAL
Sb,O,
DIODES
1039
CHARACTERISTICS
EWOLTS) W BISMUTH POSlTiVE
b. CHARACTERISTICS CYCLES AT SOCFS
AFTER
2x105
C. CHARACTERISTICS AFYER AN ADDITIONAL ZXIOE CYCLES AT
.I:C
0.2
FIG. 4. Degradation
of I-V
characteristic of Nb-NbpO,-Bi switching.
characteristics. Various drive circuit arrangements were tried which allowed control of source impedance, rate of rise of voltage waveform, and rate of rise of current waveform. In all cases only brief, erratic switching was achieved at high pulse rates. Degradation of device characteristics was somewhat greater than in Fig. 4.
Information retentivity and storage tests The long term stability of the information content of a set of bistable Nb,O, devices was determined under three conditions: (1) the devices were externally open-circuited during intervals between observations; (2) the devices were shortcircuited during storage; (3) the length of time that a device would remain in its initial state while jSSE
diode after repeated
connected to an active external network was observed under the four possible combinations of device state and electrical excitation. For open circuit storage, the d.c. I-V characteristics of a set of twenty devices was measured and the devices were stored in a glass desiccator for 24 days. Seven devices changed state, three making the transition from low to high conductivity and four making the opposite transition. The original characteristic could be recovered in all cases. The test was reinitiated. No observable change in devices occurred in the following week. After an additional ten days, five devices had changed state; none of these were devices that had changed state during the first 24 day period. After storage for an additional 34 days, five failures were found. Three were first failures, two were devices that
1040
T.
n-.
HICKMOTT
had failed in preceding intervals. Thus 15 out of 20 devices changed state on open circuit storage for 80 days. Devices which were stored in a desiccator while shortcircuited showed a steady drift of characteristics with time. Based on a small amount of data, the transition between states would occur in about 50 days. The original device characteristics could be recovered by completing switching at any stage of storage. Tests with bistable Nb,O, devices attached to an active external network were made to determine if the device would be stable enough for repeated non-destructive read cycles. A device with the d.c. characteristics shown in Fig. 2a was used, after it had been stabilized bv second breakdown at 84°C. The device would switch reliably with a one psec pulse of 0.8 1: amplitude and 1600 0 source impedance. The diode was set in its lowconductivity state and a continuous train of 0.4 V, 1 psec pulses with a repetition rate of 100,000 pps applied. The test was continued for five hours, representing some lo9 read operations. The state of the device and its d.c. characteristic were unaltered. For the low conductivity, positive bias test with steady potentials instead of repetitive pulses, the switching transition would occur abruptly after an induction period varying from less than a second to several minutes if voltages greater than 75 per cent of the nominal switching threshold were applied. The waiting period was not reproducible. It may depend on how far the device was driven negative when last switched to the low-conductivity state, the time which had elapsed between switching to the low-conductivity state and the application of the positive ‘read’ voltage, to the rate of application of the read voltage or other factors. Attempts to control these factors did not yield reproducible data on time-to-switch vs. applied potential. The use of well-filtered power supplies and shielded interconnections and the enclosure of the device under test in a metal container were standard procedure. These precautions should eliminate the possibility that line noise or instrument transients were triggering the switching. Whatever device property, past history, or external agency caused the variability of time to switch from low to high conductivity under steady positive bias is not known.
and
li..
K.
HIATl
The stability of the high-conductivity state under positive bias was tested with a 1600 Q voltage source adjusted to produce a 0.2 V drop across the device. The device abruptly switched to the lowconductivity characteristic (reverse switching), after bias had been applied for 45-50 min. The original characteristics were then recovered I~!applying a voltage approximately twice the normal low-to-high switching threshold voltage. LVhen the process was repeated, the time required for reverse switching varied between 2-3 hr. A third attempt was made with bias applied for 6:.,hr. No switching occurred. The number of devices tested, on only one set of Bi diodes, was inadequate to establish the generality of the results. n-0 systematic data were obtained for the stability of the high-conductivity state with the bismuth electrode biased negatively. In general, this region is quite stable, and the negative threshold is more clearly defined than the positive threshold. No transition from low to high conductivity was observed for diodes in the low-conductivity state under negative bias between 0 and -0.3 \*, the region of interest for d.c. characteristics. Continuous bias in this range applied for several hours had no effect on device characteristics. Pulsed negative voltages that produced potentials across the device in the range -0.3 to -0.6 V would consistently switch the devices from low to high conductivity, in fractions of a psec. Continuousl> applied voltages in this voltage range would produce sporadic transitions to the high-conductivity characteristics followed by relaxation back to the low-conductivity characteristic. For to -0.8 V, bistable greater negative bias, -0.6 diodes exhibited a region of current-controlled negative resistance as shown in Fig. 2b. Howclrer large the source impedance, RC relaxation oscillations occurred. If these oscillations were escited bv a continuously applied bias, they persisted for minutes to hours, then slowly decayed in amplitude, became sporadic and finally disappeared altogether. Devices subjected to such treatment became useless. The absence of long-term stability is a serious shortcoming of bistable Nb-Nb,O,-Bi diodes. Because of instabilities in information retention and those associated with pulsed switching, the device is not feasible in its present form for
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
OF
AMORPHOUS
NbzOs
DIODES
Nb-Nb,O,-Cd
DIODE
VOLTAGE
FIG. 5. Voltage controlled negative resistance characteristic state of Nb-Nb205-metal diodes with low work function
Ilma
0
1
6()--
b
Nb-Nb,O,-Zn
,- Iha)
6C
413--
4c
2(I--
,-,
2CI-
i
I
EIVOLTS)
7-
a;
in high-conductivity counterelectrodes.
012
t
m 02
POZS:TIVE
40 d
60
60
I
04
0.6
Zn POSITIVE
FIG. 6. Bistable switching in Nb-Nbz05-Zn diode. (a) Symmetrical I-V characteristic. (b) Switching to low-conductivity instead of tracing VCNR with Zn negative.
1041
1042
T.
W.
HICKMOTT
and
n-.
R.
HIATT
Elecf rode efjrectsin bisf able switching A characteristic and striking feature of conduction in the YJb-Nb,O,-metal system is its dependence on the counterelectrode metal. Conduction before brcakdown,(15J6) conduction after breakdown to a bistable state,(ls2) and destructive
practical application in electrically alterable memor!- systems. The instabilities also hinder efforts to characterize the device unequivocally. Process improvements in forming the oxide and developing conductivity, and improved switching networks could result in improved device stability.
, , 51 Iimoi I
/
i=300"1(
E (VOLTS1 I IO
o;e
06
I 08
04 91 ?OSITIVE 0.5 t
FIG. 7. Bistable
switching
in an Nb-Nb20s-Bi
diode
at 300°K
and 77°K.
I I.0
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
dielectric breakdown vary with counterelectrode.(2) For bistable switching, the magnitude of conductivity in the high-conductivity state, the polarity to switch from high to low conductivity,
NbzOs DIODES
t
4 2 "K
60 t
FIG. 8. Bistable
0
switching
in an Nb-NbzOs-In
Nb-Nb,O,-In
b
diode at 4.2”K.
Nb-Nb,O,-Bi
4.2'K /
DIODE
FIG. 9. Temperature
VOLTAGE
1043
and inflections observed in the high-conductivity state depend on counterelectrode. Figures 2 and 3 show the current-controlled negative resistance region which usually occurs in
60
Nb- Nb, 0,. In
OF AMORPHOUS
(VOLTS1
dependence of high conductivity and Nb-NbzOs-In diodes.
state of Nb-NbsOs-Bi
1044
T.
W.
HICKMOTT
the high-conductivity state of Nb-Nb,O,-Bi diodes. When low-work-function metals are used as counterelectrodes the procedure to establish bistable switching is similar to that for Bi diodes. 1’oltage-controlled negative resistance (VCYR) occurs in the high-conductivity I-1’ state at 0.1 to 0.2 V. This is shown for diodes with different counterelectrodes in Fig. 5. In each case, the oxide thickness was 1450A. A similar inflection at 0.15 was also observed for Sn diodes. Neither VCNR nor CCNR inflections were observed in the high-conductivity state with Au, Ag, Al, Sb, or Hg counterelectrodes. Figures 2 and 5 also show the wide range in resistance of the highconductivity state, with Zn diodes having the lowest resistance and Bi diodes the highest. The VCNR inflection is related to switching from high to low conductivity when counterelectrode is negative. For Nb-NbsO,-Zn diodes, 180
and
TV. R.
HIATT
a symmetrical I-V characteristic with VCNR at 0.2 V was traced out for both polarities as shown in Fig. 6a. On the following tracing of the I-V characteristic, shown in Fig. 6b, a typical bistable loop was observed. VCNR was observed with Zn positive but the diode switched to low conductivity instead of tracing out VCNR with Zn negative. In Fig. 6a there was a tendency to switch to low conductivity with Zn negative. Similar effects have been observed with In, Cd, and Pb diodes where VCNR was occasionally observed in the third quadrant at the voltage where switching to low conductivity usually occurred. The ratios of high/low conductivity varied widely for different counterelectrodes. Typical values for diodes that had oxides 1450A thick were Bi, 50/l; Cu, 20/l; Au, 500/l; In, 100/l ; Sn, 30,000/l; Zn, 800/l; and Pb 800/l. All these
t
Nb-Nb205 LOW
I 100
1
1
150
-Bi
CONDUCTIVITY STATE
-.-_
~
TEMPERATURE
I
250
200 ["K)
FIG. 10. Temperature dependence of low conductivity state of two different Nb-NbzOj-Bi diodes.
300
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
values are representative. Repeated switching of diodes, or overdriving of current, could change the observed values drastically. If diodes were overdriven too much, bistable switching disappeared completely. Bistable switching occurs at both 77°K and 4.2”K for Bi, In, and Pb diodes, and at 77°K for Zn diodes. Diodes with other counterelectrodes were not tried at low temperatures. For each sample, conductivity was developed at 300°K. After a good bistable characteristic developed, the temperature was lowered and diode switching measured at the lower temperature. Figure 7 shows switching of a Bi diode at 300°K and at 77°K. Figure 8 shows switching of an In diode at 4.2”K. For the Bi diode, the CCNR inflection was shifted about 0.12 V higher and the voltage to switch from high to low conductivity was about 0.2 V higher. Otherwise the characteristics were remarkably similar. At 4.2”K, the VCNR inflection of the In diode was at 0.075 V for both polarities compared to 0.12 V at room temperature. Switching from high to low conductivity was at a slightly higher voltage. In both cases, switching between states was more sluggish at low temperatures. The difference between Bi and In diodes is particularly well illustrated by the temperature dependence of the high-conductivity state, illustrated in Fig. 9. For Fig. 9, diodes were established in the high-conductivity state and conductivity at lower temperatures was measured only at low voltages to avoid any possibility of switching during cooling down of samples. Conductivity of Bi diodes is either temperature independent or else the resistance increases by about 20 per cent between 300°K and 77°K. Nb-Nb,O,-In diodes are metallic in the high-conductivitv state. The resistance decreases as temperature is lowered. The low-conductivity state of Bi diodes is much more temperature dependent. Resistance increases as temperature decreases, as shown in Fig. 10 for two Bi diodes that had different initial resistances. (The discontinuities at 210°K and 130°K for diode a are an instrumental artifact.) Nb-Nb,O,Pb diodes at 4.2”K and 2.8”K behaved similarly to In diodes. They were metallic in the highconductivity state, VCNR was observed for both diode polarities, and resistance decreased as temperature was lowered.
OF AMORPHOUS
Nb205 DIODES
1045
DISCUSSION
As is the case for other systems in which switching occurs, a critical problem is to understand the nature of conduction in the high-conductivity state of bistable NbaO, diodes. Certain mechanisms can be rejected on the basis of the present results and earlier work. It has been shown(2) by photographing NbNb,O,-Au diodes that the development of conductivity in that system occurs at one, or a very small number, of singular regions, less than 10e3 mm2 in area, that are modified by the breakdown procedure so that switching can occur. It is probable that switching with other counterelectrodes also involves very small regions of the oxide. The results of Beam and Armstrong are in agreement with this.(lg) One possible mechanism to establish conductivity is the formation of a metallic conducting bridge, as with Ni0.c3) This mechanism has been suggested for several systems. In each case the switching voltages are greater than 10 V, and in many materials several hundred volts are needed for bistable switching. In contrast, switching in Nb,Os diodes requires less than one volt for diodes whose conductivity in the high-conductivity state varies as much as that of Au and Bi diodes. Bistable switching in Nb-Nb,O,-Pb diodes is very similar at 300°K and at 4.2”K although both Nb and Pb are superconducting at 4.2”K. There is no experimental evidence for superconducting bridges in the temperature range where a metallic short would be superconducting; switching does not depend on metallic filaments. Bistable switching in chalcogenide glasses has been associated with a phase change in the glass system.(14) Chalcogenide glasses melt at much lower temperatures than Nb,O,, which is a very refractory oxide. The power input in switching Nb,O, diodes is minute compared to the power needed to switch chalcogenide glass devices. In addition, switching is polarity dependent while switching in chalcogenide glass devices requires a high-current pulse of either polarity to change to the low-conductivity state. Switching at 4.2”K in Bi diodes requires about as much power as at 300°K. If a phase change occurred at high temperatures, appreciably more power would be needed to raise the diode to the temperature of the phase change. A phase change in the insulator
1046
T.
W.
HICKMOTT
is not a probable mechanism for switching in NbzO,. Ionic conduction in insulators is generally field dependent and will carry only limited currents. In Nb,Os diodes, the switching voltages are independent of oxide thickness, at least between 5OOA and 3200& and possibly over a greater thickness range. Switching is voltuge dependent, not field dependent. In addition, the rapid switching, high-frequency oscillations, and electroluminescence before breakdown and during switching, all are in accord with an electronic conduction and switching mechanism, rather than an ionic mechanism. In the high-conductivity state, the VCNR inflection observed with low-work-function metals is very similar to the characteristic of a tunnel diode. The inflection voltage depends only slightly on temperature, and the resistance of the highconductivity state is either metallic or temperature independent down to 4.2”K. Both these effects suggest either impurity conduction or tunneling as the conduction mechanism. The oxide films are too thick for tunneling between electrodes. However, tunneling between energy levels in the insulator is possible. The development of bistable switching in NbsO, diodes results from the modification by breakdown of a very small region of the oxide, less than 10e3 mm2.(2) After breakdown, electroluminescence from Nb-Nb,O,-Au diodes has a spectrum similar to that before breakdown.c2) A band model for conduction in amorphous Nb-Nb,O,-Au diodes before breakdown was proposed whose novel feature was the presence of an impurity (or intrinsic) band, E,, in the forbidden gap about 1 eV below the conduction band.(15*16) A similar band model is appropriate after breakdown, but it is possible that an additional energy level, E,, is required between conduction band and impurity band. Before breakdown, the occupation of level ,E, depends on the work function of the counterelectrode. Establishment of high conductivity and switching involves ionization of traps in the level E,, while switching to low conductivity involves their neutralization. Such a model is speculative. It may, however, provide a starting point to understand some of the striking electronic conduction phenomena in NbaO, diodes. More detailed examination of
and
W. R. HIATT
current-voltage characteristics at low temperature, both before and after breakdown to a switching state, may show whether the model is correct. Regardless of the mechanism of switching, bistable switching can be developed readily in many Nb-Nb,O,-metal diodes. In all cases, switching occurs at low diode voltages, characteristic of electronic effects. The most satisfactory and reproducible switching characteristics occur in Nb-Nb,O,-Bi diodes. The ratio of d.c. conductivities in the two stable states ranges between 50 and 100. Switching from low to high conductivity can be accomplished with 0.5 to 1.0 psec pulses; switching from a high to a low-conductivity state requires 1.0 to 10 psec pulses. Power input at switching is in the 1OOpW range, and switching voltages are less than 1 V. The environmental stability of switching Nb,O, diodes is good. Although construction of an electrically adjustable computer memory from bistable oxide films may eventually be feasible, there are several major problems at the present time. First, the electrical breakdown procedure required to develop the conductivity of every diode is variable. Detailed procedures for anodizing and breakdown need further development since final device characteristics are critically dependent on breakdown. Second, irreversible changes occur in the electrical characteristics of bistable Nb-Nb20,-Bi diodes under repeated switching, particularly when the rate exceeds lo5 kHz or when pulsed voltages are used. Third, there is a lack of diode stability with sustained d.c. voltage bias below switching thresholds. Since the nature of the breakdown and the conduction mechanism after breakdown are not well understood, it is not known if these are inherent limitations in the utilization of bistable Nb,O, diodes for an electricallv alterable memory.
REFERENCES 1. W.
R. HIATT and T.
W.
HICKMOTT, Appt.
Letters 6, 106 (1965). 2. T. W. HICKhIOTT, J. Vnc~zrm Sci.
@
Phys-
Tech&.
6,
828 (1969). 3. J.
F. GIBBONS and W. E. BEADLE, Solid-St.
Electron.
7, 785 (1964). 4. A. D. PEARSON, W. I<. NORTHOVER, J. F. DEWALD and W. F. PECK, Advances in Glass Technology, p. 357. Plenum Press, New York (1963). 5. S. R. OVSHINSKY, Phys. Rev. Lett. 21, 1450 (1968).
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
6. A. D. PEARSONand C. E. MILLER, Appl. Phys. Lett. 14, 280 (1969). 7. J. RICHARDSON,General Electric Co., Schenectady, N.Y., AD666445 (1968). 8. F. ARGALL, Solid-St. Electron. 11, 535 (1968). 9. C. F. DRAKE, I. F. SCANLAN and A. ENGEL, Phys. Status Solidi 32, 193 (1969). 10. K. VAN STEENSEL, F. VAN DE BURG and C. KOOY, Philips Res. Rep. 22, 170 (1967). 11. C. N. BERGLUND, IE.EE Trans.’ Electron Devices, ED16,432 (1969).
OF AMORPHOUS
Nb,O,
DIODES
1047
12. A. SZYMANSKI, D. C. LARSON and M. M. LABES, Appl. Phys. Lett. 14, 88 (1969). 13. R. CHAPMAN,Electron. Letters 5, 246 (1969). 14. H. FRITZSCHE, IBMJ. Res. Dev. 13, 515 (1969). 15. T. W. HICKMOTT, Thin Solid Films 3, 85 (1969). 16. T. W. HICKMOTT, J. appl. Phys. 37, 4380 (1966). 17. D. V. GEPPERT, Proc. IEEE 51,233 (1963). 18. K. L. CHOPRA,J. appl. Phys. 36,184 (1965). 19. W. R. BEAM and A. L. ARMSTRONG,Proc. IEEE 52, 300 (1964).
Electronics
Pergamon
ELECTRODE
Press
1970.
Vol.
EFFECTS
13, pp. 1033-1047.
AND
OF AMORPHOUS
Printed
BISTABLE
Nb,O,
in Great
Britain
SWITCHING
DIODES*
T. W. HICKMOTT IBM
Components
Division,
East
Fishkill
Laboratory,
Hopewell
Junction,
N.Y.
12533,
U.S.A.
W. R. HIATT Electronics
Laboratory,
General
Electric
Company,
(Received 26 September 1969; is revised
Syracuse,
N.Y.
13088,
U.S.A.
form 15 December 1969)
Abstract-Nb-Nb,O,-metal diodes, after breakdown, exhibit switching between two stable conduction states and are potentially usable in an electrically alterable memory. Detailed procedures for oxide formation and electrical breakdown to produce Nb-NbaOs-Bi diodes with good switching characteristics are described. The ratio of conductivities in the two stable states varies between 30 and 100. Switching from low to high conductivity can be done with 0.5-1.0 +ec pulses; switching from a high- to low-conductivity state requires 1.0-10 psec pulses. Switching voltages are less than 1 V, switching powers are less than 100 pW, and the direction of switching is controlled by the polarity of the applied voltage. The dependence of device stability on temperature, storage, and bias are reported. When low-work function metals such as In, Cd, Zn, or Pb are used as counterelectrodes, a voltage-controlled negative resistance inflection occurs in the high-conductivity state. Diodes switch at 4.2”K. Switching appears to be an electronic phenomenon, occurring at a small number of regions in the oxide. R&sum&-Les diodes Nb-Nb&-m&al, apr&s fiasco, exhibent une commutation entre deux Ctats de conduction stables et sent potentiellement utilisables dans une mCmoire Clectriquement changeante. On dCcrit en d&ail les processus de formation d’osyde et de fiasco Clectrique pour produire des diodes Nb-Nb&-Bi ayant de bonnes caractiristiques de commutation. Le rapport de conductivitCs dans les deus Ctats stables varie entre 30 et 100. La commutation d’une conductivitC faible g une conductivitC ClevCe necessite des imp&ions de 0,s & 1,0 psec; la commutation d’une conductivitC ClevCe g une conductivitC faible demande des impulsions de 1,0 & 10 psec. Les tensions de commutation sont infirieures & 1 volt, les puissances de commutation sont infirieures i 100 pwatt, et la direction de commutation est contr81Ce par la polarit& de la tension appliquee. On indique comment la stabilitC du dispositif est fonction de la temperature, de l’emmagasinage et de la polarisation. Lorsque des m&aux B fonction de travail faible sont utilisCs comme contre-klectrodes, tels que In, Cd, Zn ou Pb, une inflection de resistance nCgative contr81Ce par tension a lieu dans 1’Ctat de conductivitC ClevCe. Les diodes commutent g 4,2”K. La commutation semble ctre un phCnom&ne Clectronique prenant place dans un nombre limit6 de rigions dans l’oxyde.
Zusammenfassung-Nb-lb20G-Metalldioden
weisen nach dem Durchbruch eine Schaltcharakteristik auf mit zwei stabilen Leitftihigkeitszustznden und sind miiglicherweise fiir ein elektrisch verPnderbares Speicherelement brauchbar. Verfahren zum Erzielen des Oxides und der Nb-Nbz05Bi-Diode durch Formieren im Durchbruchsbereich werden detailliert beschrieben. Das Verhiltnis der LeitfPhigkeiten in den beiden stabilen Zustanden variiert zwischen 30 und 100. Zum Schalten vom Zustand niedriger zu hoher Leitfiihigkeit werden 0.5 psec. bis 1.0 psec-Impulse ben6tigt. Das * Work done at General Electric Research and Development was sponsored by the Air Force Avionics Laboratory, Research Wright-Patterson Air Force Base, Ohio under Contract No. AF 1033
Center, Schenectady, New and Technology Division, Air 33 (615) (1218).
York. Force
The work Command,
1034
T.
W.
HICKRIOTT
and
‘vv. Ii.
HIATT
SChaltell in umgekehrter Richtung erfordert Impulsllngen zwischen 1.0 und 10 psec. Die Schaltspannungen liegen unter 1 V, die erforderlichen Leistungen sind unter 100 p\V und die Schaltrichtung wird durch die Polaritat der angelegten Spannung bestimmt. Die .4bhangigkeit der Stabilitat des Bauelementes van der Temperatur, der Speicherung und Vorspannung werden beschrieben. Werden Metalle mit kleiner Austrittsarbeit, wie In, Cd, Zn oder Pb als Gegenelektrode verwendet, dann tritt eine spannungsgesteuerte Verbiegung des negativen Widerstand& im Bereich der hohen Lcitfahigkeit auf. Die Diode” schalten bei 4.2 K, der Mechanismus beruht auf einem elektronischen Phanomen, das sich in einigen wenigcn Bereichen des Oxids abspielt.
INTRODUCTION REVERSIBLE
electrical switching between a stable high-impedance state and a stable low-impedance state has been reported for many thin film or glassy materials, including Nb,0,,‘1~2) X0,(3) chalcogenide glasses, (4-6) GaAs,‘7) Ti0,,‘8’ transition-metal doped glasses,@) and a variety of other insulators.‘10-13) For all except VO,,‘lo*ll) the devices as prepared are in a low-conductivity state and require a breakdown, or forming process, to develop the high-conductivity state. A basic problem for each system is accounting for the stability of the high-conductivity state when bias is removed from the devices, and its destruction on switching. For XiO, metallic bridges between electrodes have been detected.‘3’ For chalcogenide glasses, a phase change in glasses that exhibit memory has been suggested as the origin for a stable high-conductivity state.(14) Switching in transition-metal doped phosphate glasses has been attributed to electronic effects due to the presence of mixed valency ions such as Cu’ or CUI’.‘~’ Switching in \:O, films is thermal and related to the well known metal-insulator transition which occurs at 68”C.‘10*11) Thus bistable switching in amorphous materials and thin films is due to a variety of causes, both thermal and electronic. No general mechanism is applicable in every case, although some type of filamentary conduction is involved for all, including VO,.‘ll’ Among insulators that show bistable switching and memory, thin film NbaO, diodes are unique because of the low voltages (< 1 V) at which switching occurs and because of the independence of switching voltage on insulator thickness. In chalcogenide glasses and transition-metal doped phosphate glasses, switching from high to low conductivity requires a very high current pulse, consistent with the occurrence of a phase change or thermal transformation. For Xb,O,, and also for TiO,, polarity reversal of the voltage will
switch the diode from its high-conductivity state. For Nb-NbaOs-_\u diodes, formation of the high-conductivity state occurs at a very small number of singularities in the oxide.“’ However, the occurrence of electroluminescence in switching diodes, the similarity of the electroluminescent spectra to electroluminescence before breakdown,‘15) the speed of switching, and the low voltage and power required for switching suggest that an electronic mechanism rather than melting or a phase change is responsible for bistable switching. In this paper, switching and stability of NbNbaOs-Bi diodes will be discussed in detail. Some observations of switching at 42°K in Nb-Nb,O,-Bi, Nb-Nb,O,-In, and Nb-Nb,O,Pb diodes will be presented. The important role of the counterelectrode in determining currentvoltage characteristics of Nb,O, diodes will be reemphasized.(‘) EXPERIMENTAL
The preparation of Nb films and of Xb-Xb,O,metal diodes has been described.(l*16’ A variety of methods to make Sb,O, films was tried. The final process to produce diodes that switched well used an electrolyte prepared by dissolving 9 g of ammonium pcntaboratc in 100 cm3 of ethylene glycol. Nb films were anodized at a constant current density of 1 mA/cm2 until the cell voltage reached 60 V; the oxide film was then held at 60 V for 5 min while the current decreased, forming an oxide about 1500 A thick. The electrolyte was maintained at 110°C during anodization, and a stainless steel sheet was used as cathode. The oxide films which were used to study the effect of metal counterelectrode were anodized in saturated boric acid electrolyte. Switching was observed in diodes with Nb,O, film thicknesses between about 500 A and 32OOA. These limits
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
are not necessarily inclusive. Metal counterelectrodes were evaporated through a mask to form diodes of 1 mm2 area.
Development
of switching in Nb-Nb,O,-Bi
diodes
Freshly prepared Nb-Nb,O,-Bi diodes are rectifying; the direction of easy current flow is with Bi positive. The procedure for development of bistable switching can be either simple or complex. If the voltage across an Nb-Nb,Os-Bi diode is slowly raised with Bi positive, using a constant-current generator, a sudden breakdown to a high-conductivity state occurs. In fact, as discussed before,(l) several high-conductivity
OF AMORPHOUS
Nbz05
DIODES
states can occur and switching between these states affected with pulsed voltages. However, when such a simple breakdown procedure is used, switching characteristics that result are erratic and degrade rapidly. A more complex breakdown procedure using and current-limited electrical both a pulsedbreakdown to establish reproducible switching characteristics was developed by trial and error. Detailed reasons for each step in the procedure are not clear but the final device characteristics were the most stable found. The breakdown procedure is described in detail because it illustrates the complexities of the process and may serve as starting point for better procedures.
A ima 2.0-
1.0-
I IS.0
b
E (VOLTS1 BISMUTH ELECTRODE POSITIVE
(a)
ima I.0
f
0.5-
b
(1. CHARACTERISTIC
PRODUCED
BY 1st BREAKDOWN
b.
PRODUCED
BY 2nd
CHARACTERISTIC
C. PORTION
OF FINAL
1035
E (VOLTS) BISMUTH ELECTRODE POSITIVE
BREAKDOWN
CHARACTERISTIC
(b) FIG. 1. (a) I-V characteristic of Nb-Nb&-Bi diode before breakdown. (b) Stages in breakdown of Nb-NbaOs-Bi diode to high-conductivity state.
1036
T.
M-. HICKMOTT
The first step in the electrical breakdown procedure was to apply a single high-voltage pulse driving the bismuth electrode positive. A single 1 psec pulse of 50 V amplitude, corresponding to a field of about 3.5 x lo6 V/cm, was applied to the Bi diode through a 1500 fi series resistor. Application of such a pulse did not significantly alter the current-voltage characteristic of the device. It was found to be essential if the final device characteristics were to be stable; deletion of this step resulted in devices whose final characteristics were subject to spontaneous, erratic fluctuations. The second step in the breakdown was to apply a continuous voltage to the Bi electrode from a high-impedance source for as long as was necessary to cause the device to increase its conductivit! abruptly and irreversibly by a large amount. For example, a variable voltage source with a series resistor of at least 40,000 fi was connected to the Nb-Nb,O,-Bi diode. The voltage was then adjusted to produce a potential drop across the device of 12 to 13 V. Figure la shows a typical I-T’ characteristic at this stage of breakdown. After a time which was found to vary between 20 and 300 set, a sudden increase in the electrical conductivity of the device occurred. The diode voltage would suddenly drop from the initial value of 12-13 \:. The I-17 characteristics of the device at this stage of the breakdown process was typically that of (a) in Fig. lb, or in some cases (b). It was important at this point not to apply a negative voltage to the Bi electrode. If negative voltage were applied at this stage of the process, it would increase the average conductivity of the low conductivity state of the final device. The third step in the electrical breakdown was to increase the voltage across the device slowly, using the same voltage source as in the previous step, with the series resistance reduced to about 3500 ,O_. The second breakdown or third breakdown, depending on whether the device had been brought to state (a) or (b) of Fig. lb, would occur once the appropriate voltage threshold was reached. The second breakdown transition was abrupt, much like the first. The third breakdown occurred at a rate determined by how rapidly the output of the voltage source was increased. The transition was usually accompanied by oscillations. The average conductivity of the final I-V characteristic depended on the peak current reached during
and
W.
R.
HIATT
the last breakdown. If device current during breakdown exceeded 0.8 mA, the low-conductivity characteristic of the final device would be increased in average conductivity. If this final step were carried out, or repeated, with the device at a temperature in excess of 60°C the final characteristics were less sensitive to changes in ambient temperature than was the case if final breakdown had been done at room temperature. Typical final device characteristics are shown in Fig. 2a. The device had two distinct conductivity states, a low state (a) and a high state (b). State (b) of Fig. 2a corresponds to state (c) of Fig. la. Typical ratios of high/low conductivity were 50/l. It could be reversibly driven from either of these states to the other. Switching from low conductivity to high conductivity required application of a positive voltage in excess of the switching threshold T2. The opposite transition required application of a voltage more negative than T,. The state of the Eb-Nb,O,-Bi diode at any time could be determined, without altering the state, by applying a voltage smaller than the threshold voltage and measuring the resulting current. The currentcontrolled negative resistance (CCNR) inflection at about 0.2 1’ was characteristic of the highconductivity state when Bi counterelectrodes were used. Figure 2a is for a diode with 1500 4 Kb,O,; nearlv identical characteristics were observed for all oxide thicknesses. Temperature sensitivity of switclhg OJ‘Nb-Sb,O,jBi diodes Temperature tests above room temprraturc were made in a heated metal desiccator. Corrosion of Bi electrodes was minimized by spraying varnish over the entire slide. This had no apparent effect on the electrical behavior of devices. The maximum temperature that could be used was limited to 110°C by melting of the alloy formed when leads were attached to Bi electrodes by indium solder. At temperatures below 100°C there was visible evidence of diffusion of indium along the Bi strip. This had no effect on electrical properties of the device unless it extended to the active diode region where Bi was in contact with Nb,O,. The first time a device was tested at elevated temperature, an abrupt change occurred in both positive- and negative-switching threshold voltages at a temperature between 60°C and 7OC. To
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
Nb,O,
OF AMORPHOUS
DIODES
1037
jrno 0.4
. . .. 0.2
i
‘.
.\
15ooII t.
/
)
0.
LOW CONDUCTIVITY
CHARACTERISTIC
b.
HIGH CONDUCTIVITY
CHARACTERISTIC
t
E(VOLT9 BISMUTH POSITIVE
ima
l.O-
b
SLOW PATH
E (VOLTS) BISMUTH POSITIVE
HIGH-TO-LOW SWITCHING 16001-z SOURCE
ASSUMED PATH OF FAST HIGH-TO-LOW SWITCHING (NOT INCLUDING DISPLACEMENT CURRENTS)
@I
FIG. 2. Bistable switching in Nb-NbzOs-Bi diodes. (a) Switching characteristic for slow voltage changes. (b) Switching characteristic for fast pulse switching.
produce switching after this required a re-breakdown of the device. Following such re-breakdown, switching characteristics were not greatly different from room temperature characteristics up to the highest temperature used. The characteristics of
the device, when returned to room temperature were improved. Conductivity in both high- and low-conductivity states was slightly lower. On repeating temperature tests, changes in device characteristics with temperature were smaller
T.
1038
W;. HICKMOTT
than in the initial run. The peculiar behavior in the 60-70°C range did not recur on subsequent temperature runs. Table 1 shows least-squares Table 1. Temperature sensitivity of deviceparameters Value Parameter
at 20°C
Rate of change with temperature -0.011
n:s2i0c
Low State Resistance (secant resistance at 0.4 V)
259 kfi
High State Resistance (secant resistance at 0.2 V)
1.92 kc2
-0.014
Absolute Value of Negative Threshold Voltage
0.18 V
-0.0028
Positive Threshold Voltage
0.49 1‘
0.014 v/v/:c
n/WC
v/v/;c
of the temperature sensitivity, AVjV and AR/R, of device parameters relative to their value at 20°C. Data were fitted over the range 20-90°C.
fit
Switching characteristics of Kb-Nb,O,-Bi
diodes
1. Low speed repetitive switching. Nb-Nb,O,-Bi devices were tested at low frequencies by applying a d.c.-biased sinusoidal voltage which had an excursion between the same limits as required for tracing the d.c. characteristics of the device. By monitoring the I-V Lissajous pattern on an oscilloscope, switching was monitored for frequencies up to about 10 kHz. At higher frequencies, displacement currents obscured switching, and discontinuities in the voltage waveform across the device showed the occurrence of switching. Figure 3 shows a scope tracing of switching at 20 Hz. At frequencies up to 100 Hz, adjustment of bias voltage and sinusoidal amplitude could be found which produced reliable switching, without extensive change in device characteristics. Such low-speed repetitive switching was continued for up to 6 hr with only minor changes in device characteristics. Representative I-V characteristics, before and after low-speed repetitive switching, are shown in Fig. 4a and b. The current-controlled negative resistance inflection at 0.2 V on the positive arch of the high conductivity characteristic
and
11’. R.
HIATT
typically became less pronounced, and the average conductivity of both states increased. Raising the frequency to 1 kHz required that the amplitude of the driving voltage be increased to obtain consistent switching. Such switching produced permanent degradation of characteristics as shown in Fig. 4c. 2. Pulse and high-frequency
switching. Fast
switching
to
from
the
low-
pulse
high-conductivity
state, along the d.c. load line, was easily done. The most dependable means with single pulses was to use a pulse of 0.7 V in series with a resistor of 1600 Q by-passed by a ‘speed-up’ capacitor of 1.5 nF. The minimum pulse width was 0.5 psec. The actual transition could be seen on the scope trace and required less than 0.1 psec. The reverse transition along the d.c. I-V characteristic, as in Fig. 2a, was slow, frequently rcquiring milliseconds. At higher voltages with Bi negative, a current-controlled negative resistance region occurs in the low-conductivitv state as shown in Fig. 2b. Rapid switching into this state was possible. A -2.6 V pulse through a series resistance of 30,000 0 was used. The minimum pulse duration required for switching was more variable from device to device, ranging from 1.0 to 9.0 psec. Repeated cycling with single pulses did not degrade the d.c. device characteristics for switching in either direction. For some devices, when a long duration pulse was applied with Hi negative, 500 kHz relaxation oscillations between states were obscrved.‘17-1”) During oscillation, the transition from low to high conductivit! occurred in tenths of microseconds while the transition difficult highto low-conductivit!. required 2 psec. Progressive and permanent degradation of device characteristics occurred in seconds for sinusoidal drive at 100 kHz or higher. For highfrequencv repetitive pulse switching, alternate polarity pulse trains were generated by using two pulse generators, one of which was synchronized by a delaved trigger from the other, which \vas free running at the chosen frequency. Such an arrangement allowed freedom of choice of pulse widths. pulse separation, and amplitudes for a train of pulses of alternate polarity, No adjustment could be found which did not result in sporadic switching and permanent degradation of device
.FIG. 3. Oscilloscope photograph of bistable switching of Nb-NbaOs-Bi diode at 20 Hz. x = 0.2 V/cm, y = 0.5 mA/cm, oxide thickness = 1250 A.
[facing p. 1038
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
OF AMORPHOl2S
a. INITIAL
Sb,O,
DIODES
1039
CHARACTERISTICS
EWOLTS) W BISMUTH POSlTiVE
b. CHARACTERISTICS CYCLES AT SOCFS
AFTER
2x105
C. CHARACTERISTICS AFYER AN ADDITIONAL ZXIOE CYCLES AT
.I:C
0.2
FIG. 4. Degradation
of I-V
characteristic of Nb-NbpO,-Bi switching.
characteristics. Various drive circuit arrangements were tried which allowed control of source impedance, rate of rise of voltage waveform, and rate of rise of current waveform. In all cases only brief, erratic switching was achieved at high pulse rates. Degradation of device characteristics was somewhat greater than in Fig. 4.
Information retentivity and storage tests The long term stability of the information content of a set of bistable Nb,O, devices was determined under three conditions: (1) the devices were externally open-circuited during intervals between observations; (2) the devices were shortcircuited during storage; (3) the length of time that a device would remain in its initial state while jSSE
diode after repeated
connected to an active external network was observed under the four possible combinations of device state and electrical excitation. For open circuit storage, the d.c. I-V characteristics of a set of twenty devices was measured and the devices were stored in a glass desiccator for 24 days. Seven devices changed state, three making the transition from low to high conductivity and four making the opposite transition. The original characteristic could be recovered in all cases. The test was reinitiated. No observable change in devices occurred in the following week. After an additional ten days, five devices had changed state; none of these were devices that had changed state during the first 24 day period. After storage for an additional 34 days, five failures were found. Three were first failures, two were devices that
1040
T.
n-.
HICKMOTT
had failed in preceding intervals. Thus 15 out of 20 devices changed state on open circuit storage for 80 days. Devices which were stored in a desiccator while shortcircuited showed a steady drift of characteristics with time. Based on a small amount of data, the transition between states would occur in about 50 days. The original device characteristics could be recovered by completing switching at any stage of storage. Tests with bistable Nb,O, devices attached to an active external network were made to determine if the device would be stable enough for repeated non-destructive read cycles. A device with the d.c. characteristics shown in Fig. 2a was used, after it had been stabilized bv second breakdown at 84°C. The device would switch reliably with a one psec pulse of 0.8 1: amplitude and 1600 0 source impedance. The diode was set in its lowconductivity state and a continuous train of 0.4 V, 1 psec pulses with a repetition rate of 100,000 pps applied. The test was continued for five hours, representing some lo9 read operations. The state of the device and its d.c. characteristic were unaltered. For the low conductivity, positive bias test with steady potentials instead of repetitive pulses, the switching transition would occur abruptly after an induction period varying from less than a second to several minutes if voltages greater than 75 per cent of the nominal switching threshold were applied. The waiting period was not reproducible. It may depend on how far the device was driven negative when last switched to the low-conductivity state, the time which had elapsed between switching to the low-conductivity state and the application of the positive ‘read’ voltage, to the rate of application of the read voltage or other factors. Attempts to control these factors did not yield reproducible data on time-to-switch vs. applied potential. The use of well-filtered power supplies and shielded interconnections and the enclosure of the device under test in a metal container were standard procedure. These precautions should eliminate the possibility that line noise or instrument transients were triggering the switching. Whatever device property, past history, or external agency caused the variability of time to switch from low to high conductivity under steady positive bias is not known.
and
li..
K.
HIATl
The stability of the high-conductivity state under positive bias was tested with a 1600 Q voltage source adjusted to produce a 0.2 V drop across the device. The device abruptly switched to the lowconductivity characteristic (reverse switching), after bias had been applied for 45-50 min. The original characteristics were then recovered I~!applying a voltage approximately twice the normal low-to-high switching threshold voltage. LVhen the process was repeated, the time required for reverse switching varied between 2-3 hr. A third attempt was made with bias applied for 6:.,hr. No switching occurred. The number of devices tested, on only one set of Bi diodes, was inadequate to establish the generality of the results. n-0 systematic data were obtained for the stability of the high-conductivity state with the bismuth electrode biased negatively. In general, this region is quite stable, and the negative threshold is more clearly defined than the positive threshold. No transition from low to high conductivity was observed for diodes in the low-conductivity state under negative bias between 0 and -0.3 \*, the region of interest for d.c. characteristics. Continuous bias in this range applied for several hours had no effect on device characteristics. Pulsed negative voltages that produced potentials across the device in the range -0.3 to -0.6 V would consistently switch the devices from low to high conductivity, in fractions of a psec. Continuousl> applied voltages in this voltage range would produce sporadic transitions to the high-conductivity characteristics followed by relaxation back to the low-conductivity characteristic. For to -0.8 V, bistable greater negative bias, -0.6 diodes exhibited a region of current-controlled negative resistance as shown in Fig. 2b. Howclrer large the source impedance, RC relaxation oscillations occurred. If these oscillations were escited bv a continuously applied bias, they persisted for minutes to hours, then slowly decayed in amplitude, became sporadic and finally disappeared altogether. Devices subjected to such treatment became useless. The absence of long-term stability is a serious shortcoming of bistable Nb-Nb,O,-Bi diodes. Because of instabilities in information retention and those associated with pulsed switching, the device is not feasible in its present form for
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
OF
AMORPHOUS
NbzOs
DIODES
Nb-Nb,O,-Cd
DIODE
VOLTAGE
FIG. 5. Voltage controlled negative resistance characteristic state of Nb-Nb205-metal diodes with low work function
Ilma
0
1
6()--
b
Nb-Nb,O,-Zn
,- Iha)
6C
413--
4c
2(I--
,-,
2CI-
i
I
EIVOLTS)
7-
a;
in high-conductivity counterelectrodes.
012
t
m 02
POZS:TIVE
40 d
60
60
I
04
0.6
Zn POSITIVE
FIG. 6. Bistable switching in Nb-Nbz05-Zn diode. (a) Symmetrical I-V characteristic. (b) Switching to low-conductivity instead of tracing VCNR with Zn negative.
1041
1042
T.
W.
HICKMOTT
and
n-.
R.
HIATT
Elecf rode efjrectsin bisf able switching A characteristic and striking feature of conduction in the YJb-Nb,O,-metal system is its dependence on the counterelectrode metal. Conduction before brcakdown,(15J6) conduction after breakdown to a bistable state,(ls2) and destructive
practical application in electrically alterable memor!- systems. The instabilities also hinder efforts to characterize the device unequivocally. Process improvements in forming the oxide and developing conductivity, and improved switching networks could result in improved device stability.
, , 51 Iimoi I
/
i=300"1(
E (VOLTS1 I IO
o;e
06
I 08
04 91 ?OSITIVE 0.5 t
FIG. 7. Bistable
switching
in an Nb-Nb20s-Bi
diode
at 300°K
and 77°K.
I I.0
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
dielectric breakdown vary with counterelectrode.(2) For bistable switching, the magnitude of conductivity in the high-conductivity state, the polarity to switch from high to low conductivity,
NbzOs DIODES
t
4 2 "K
60 t
FIG. 8. Bistable
0
switching
in an Nb-NbzOs-In
Nb-Nb,O,-In
b
diode at 4.2”K.
Nb-Nb,O,-Bi
4.2'K /
DIODE
FIG. 9. Temperature
VOLTAGE
1043
and inflections observed in the high-conductivity state depend on counterelectrode. Figures 2 and 3 show the current-controlled negative resistance region which usually occurs in
60
Nb- Nb, 0,. In
OF AMORPHOUS
(VOLTS1
dependence of high conductivity and Nb-NbzOs-In diodes.
state of Nb-NbsOs-Bi
1044
T.
W.
HICKMOTT
the high-conductivity state of Nb-Nb,O,-Bi diodes. When low-work-function metals are used as counterelectrodes the procedure to establish bistable switching is similar to that for Bi diodes. 1’oltage-controlled negative resistance (VCYR) occurs in the high-conductivity I-1’ state at 0.1 to 0.2 V. This is shown for diodes with different counterelectrodes in Fig. 5. In each case, the oxide thickness was 1450A. A similar inflection at 0.15 was also observed for Sn diodes. Neither VCNR nor CCNR inflections were observed in the high-conductivity state with Au, Ag, Al, Sb, or Hg counterelectrodes. Figures 2 and 5 also show the wide range in resistance of the highconductivity state, with Zn diodes having the lowest resistance and Bi diodes the highest. The VCNR inflection is related to switching from high to low conductivity when counterelectrode is negative. For Nb-NbsO,-Zn diodes, 180
and
TV. R.
HIATT
a symmetrical I-V characteristic with VCNR at 0.2 V was traced out for both polarities as shown in Fig. 6a. On the following tracing of the I-V characteristic, shown in Fig. 6b, a typical bistable loop was observed. VCNR was observed with Zn positive but the diode switched to low conductivity instead of tracing out VCNR with Zn negative. In Fig. 6a there was a tendency to switch to low conductivity with Zn negative. Similar effects have been observed with In, Cd, and Pb diodes where VCNR was occasionally observed in the third quadrant at the voltage where switching to low conductivity usually occurred. The ratios of high/low conductivity varied widely for different counterelectrodes. Typical values for diodes that had oxides 1450A thick were Bi, 50/l; Cu, 20/l; Au, 500/l; In, 100/l ; Sn, 30,000/l; Zn, 800/l; and Pb 800/l. All these
t
Nb-Nb205 LOW
I 100
1
1
150
-Bi
CONDUCTIVITY STATE
-.-_
~
TEMPERATURE
I
250
200 ["K)
FIG. 10. Temperature dependence of low conductivity state of two different Nb-NbzOj-Bi diodes.
300
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
values are representative. Repeated switching of diodes, or overdriving of current, could change the observed values drastically. If diodes were overdriven too much, bistable switching disappeared completely. Bistable switching occurs at both 77°K and 4.2”K for Bi, In, and Pb diodes, and at 77°K for Zn diodes. Diodes with other counterelectrodes were not tried at low temperatures. For each sample, conductivity was developed at 300°K. After a good bistable characteristic developed, the temperature was lowered and diode switching measured at the lower temperature. Figure 7 shows switching of a Bi diode at 300°K and at 77°K. Figure 8 shows switching of an In diode at 4.2”K. For the Bi diode, the CCNR inflection was shifted about 0.12 V higher and the voltage to switch from high to low conductivity was about 0.2 V higher. Otherwise the characteristics were remarkably similar. At 4.2”K, the VCNR inflection of the In diode was at 0.075 V for both polarities compared to 0.12 V at room temperature. Switching from high to low conductivity was at a slightly higher voltage. In both cases, switching between states was more sluggish at low temperatures. The difference between Bi and In diodes is particularly well illustrated by the temperature dependence of the high-conductivity state, illustrated in Fig. 9. For Fig. 9, diodes were established in the high-conductivity state and conductivity at lower temperatures was measured only at low voltages to avoid any possibility of switching during cooling down of samples. Conductivity of Bi diodes is either temperature independent or else the resistance increases by about 20 per cent between 300°K and 77°K. Nb-Nb,O,-In diodes are metallic in the high-conductivitv state. The resistance decreases as temperature is lowered. The low-conductivity state of Bi diodes is much more temperature dependent. Resistance increases as temperature decreases, as shown in Fig. 10 for two Bi diodes that had different initial resistances. (The discontinuities at 210°K and 130°K for diode a are an instrumental artifact.) Nb-Nb,O,Pb diodes at 4.2”K and 2.8”K behaved similarly to In diodes. They were metallic in the highconductivity state, VCNR was observed for both diode polarities, and resistance decreased as temperature was lowered.
OF AMORPHOUS
Nb205 DIODES
1045
DISCUSSION
As is the case for other systems in which switching occurs, a critical problem is to understand the nature of conduction in the high-conductivity state of bistable NbaO, diodes. Certain mechanisms can be rejected on the basis of the present results and earlier work. It has been shown(2) by photographing NbNb,O,-Au diodes that the development of conductivity in that system occurs at one, or a very small number, of singular regions, less than 10e3 mm2 in area, that are modified by the breakdown procedure so that switching can occur. It is probable that switching with other counterelectrodes also involves very small regions of the oxide. The results of Beam and Armstrong are in agreement with this.(lg) One possible mechanism to establish conductivity is the formation of a metallic conducting bridge, as with Ni0.c3) This mechanism has been suggested for several systems. In each case the switching voltages are greater than 10 V, and in many materials several hundred volts are needed for bistable switching. In contrast, switching in Nb,Os diodes requires less than one volt for diodes whose conductivity in the high-conductivity state varies as much as that of Au and Bi diodes. Bistable switching in Nb-Nb,O,-Pb diodes is very similar at 300°K and at 4.2”K although both Nb and Pb are superconducting at 4.2”K. There is no experimental evidence for superconducting bridges in the temperature range where a metallic short would be superconducting; switching does not depend on metallic filaments. Bistable switching in chalcogenide glasses has been associated with a phase change in the glass system.(14) Chalcogenide glasses melt at much lower temperatures than Nb,O,, which is a very refractory oxide. The power input in switching Nb,O, diodes is minute compared to the power needed to switch chalcogenide glass devices. In addition, switching is polarity dependent while switching in chalcogenide glass devices requires a high-current pulse of either polarity to change to the low-conductivity state. Switching at 4.2”K in Bi diodes requires about as much power as at 300°K. If a phase change occurred at high temperatures, appreciably more power would be needed to raise the diode to the temperature of the phase change. A phase change in the insulator
1046
T.
W.
HICKMOTT
is not a probable mechanism for switching in NbzO,. Ionic conduction in insulators is generally field dependent and will carry only limited currents. In Nb,Os diodes, the switching voltages are independent of oxide thickness, at least between 5OOA and 3200& and possibly over a greater thickness range. Switching is voltuge dependent, not field dependent. In addition, the rapid switching, high-frequency oscillations, and electroluminescence before breakdown and during switching, all are in accord with an electronic conduction and switching mechanism, rather than an ionic mechanism. In the high-conductivity state, the VCNR inflection observed with low-work-function metals is very similar to the characteristic of a tunnel diode. The inflection voltage depends only slightly on temperature, and the resistance of the highconductivity state is either metallic or temperature independent down to 4.2”K. Both these effects suggest either impurity conduction or tunneling as the conduction mechanism. The oxide films are too thick for tunneling between electrodes. However, tunneling between energy levels in the insulator is possible. The development of bistable switching in NbsO, diodes results from the modification by breakdown of a very small region of the oxide, less than 10e3 mm2.(2) After breakdown, electroluminescence from Nb-Nb,O,-Au diodes has a spectrum similar to that before breakdown.c2) A band model for conduction in amorphous Nb-Nb,O,-Au diodes before breakdown was proposed whose novel feature was the presence of an impurity (or intrinsic) band, E,, in the forbidden gap about 1 eV below the conduction band.(15*16) A similar band model is appropriate after breakdown, but it is possible that an additional energy level, E,, is required between conduction band and impurity band. Before breakdown, the occupation of level ,E, depends on the work function of the counterelectrode. Establishment of high conductivity and switching involves ionization of traps in the level E,, while switching to low conductivity involves their neutralization. Such a model is speculative. It may, however, provide a starting point to understand some of the striking electronic conduction phenomena in NbaO, diodes. More detailed examination of
and
W. R. HIATT
current-voltage characteristics at low temperature, both before and after breakdown to a switching state, may show whether the model is correct. Regardless of the mechanism of switching, bistable switching can be developed readily in many Nb-Nb,O,-metal diodes. In all cases, switching occurs at low diode voltages, characteristic of electronic effects. The most satisfactory and reproducible switching characteristics occur in Nb-Nb,O,-Bi diodes. The ratio of d.c. conductivities in the two stable states ranges between 50 and 100. Switching from low to high conductivity can be accomplished with 0.5 to 1.0 psec pulses; switching from a high to a low-conductivity state requires 1.0 to 10 psec pulses. Power input at switching is in the 1OOpW range, and switching voltages are less than 1 V. The environmental stability of switching Nb,O, diodes is good. Although construction of an electrically adjustable computer memory from bistable oxide films may eventually be feasible, there are several major problems at the present time. First, the electrical breakdown procedure required to develop the conductivity of every diode is variable. Detailed procedures for anodizing and breakdown need further development since final device characteristics are critically dependent on breakdown. Second, irreversible changes occur in the electrical characteristics of bistable Nb-Nb20,-Bi diodes under repeated switching, particularly when the rate exceeds lo5 kHz or when pulsed voltages are used. Third, there is a lack of diode stability with sustained d.c. voltage bias below switching thresholds. Since the nature of the breakdown and the conduction mechanism after breakdown are not well understood, it is not known if these are inherent limitations in the utilization of bistable Nb,O, diodes for an electricallv alterable memory.
REFERENCES 1. W.
R. HIATT and T.
W.
HICKMOTT, Appt.
Letters 6, 106 (1965). 2. T. W. HICKhIOTT, J. Vnc~zrm Sci.
@
Phys-
Tech&.
6,
828 (1969). 3. J.
F. GIBBONS and W. E. BEADLE, Solid-St.
Electron.
7, 785 (1964). 4. A. D. PEARSON, W. I<. NORTHOVER, J. F. DEWALD and W. F. PECK, Advances in Glass Technology, p. 357. Plenum Press, New York (1963). 5. S. R. OVSHINSKY, Phys. Rev. Lett. 21, 1450 (1968).
ELECTRODE
EFFECTS
AND
BISTABLE
SWITCHING
6. A. D. PEARSONand C. E. MILLER, Appl. Phys. Lett. 14, 280 (1969). 7. J. RICHARDSON,General Electric Co., Schenectady, N.Y., AD666445 (1968). 8. F. ARGALL, Solid-St. Electron. 11, 535 (1968). 9. C. F. DRAKE, I. F. SCANLAN and A. ENGEL, Phys. Status Solidi 32, 193 (1969). 10. K. VAN STEENSEL, F. VAN DE BURG and C. KOOY, Philips Res. Rep. 22, 170 (1967). 11. C. N. BERGLUND, IE.EE Trans.’ Electron Devices, ED16,432 (1969).
OF AMORPHOUS
Nb,O,
DIODES
1047
12. A. SZYMANSKI, D. C. LARSON and M. M. LABES, Appl. Phys. Lett. 14, 88 (1969). 13. R. CHAPMAN,Electron. Letters 5, 246 (1969). 14. H. FRITZSCHE, IBMJ. Res. Dev. 13, 515 (1969). 15. T. W. HICKMOTT, Thin Solid Films 3, 85 (1969). 16. T. W. HICKMOTT, J. appl. Phys. 37, 4380 (1966). 17. D. V. GEPPERT, Proc. IEEE 51,233 (1963). 18. K. L. CHOPRA,J. appl. Phys. 36,184 (1965). 19. W. R. BEAM and A. L. ARMSTRONG,Proc. IEEE 52, 300 (1964).