metal capacitors

Thm Sohd Fdms, 94 (1982) 101-109

101

ELECTRONICS AND OPTICS

TEMPERATURE DEPENDENCE OF THIN FILM M E T A L / M o O 3 / M E T A L CAPACITORS G. S. NADKARNI* AND V. S. SHIRODKAR

Sohd State Electromcs Laboratory, Department of Phyatcs, The Institute of Sctence, Bombay 400032 ( lndta ) J. G. SIMMONS

School of Electrlcal and Electromc Engmeermg, Untverstty of Bradford, Bradford (Gt Brttam) (Recezved October 20. 1981, accepted January 8, 1982)

Thin film capacitors of M o O 3 prepared in a vacuum of the order of 10-6 Torr are found to exhibit maxima in their capacitance v e r s u s temperature characteristics and a monotonic rise in the conductance v e r s u s temperature characteristics. These results are compared with those previously reported for oxygen-rich MoO3 capacitors. In the case of the oxygen-rich samples the experimental results were analysed using an equivalent circuit consisting of impedances representing the metal-insulator interfaces and the interior bulk of the metal/insulator/metal capacitors. Using the same model, but taking into account the oxygen-poor condition of the present films, the experimental results are shown to be consistent with the theoretical results. 1. INTRODUCTION

It is difficult to prepare stoichlometrlc films of compound insulators by vacuum deposition because of dissociation and the preferential evaporation of atoms of lower vapour pressure. For metal oxide insulator films this generally gives rise to an excess of metal atoms which act as donor centres in the insulator. In fact, dissociation of only one molecule per l06 gives rise to a donor density of the order of 1017 c m - 3 .

In particular, M o O 3 is known to decompose into lower oxides as well as to dissociate when heated or vacuum deposited I 3. In recent papers *-6 it has been shown that, because of this autodoping effect, the energy band diagram of metal/MoO3/metal capacitors can be represented as shown in Fig. 1. The observed a.c. 4'5, d.c. 5 and dielectric relaxation current (DRC) 6 characteristics were interpreted on the basis of such an energy band diagram, and excellent correlation between the theoretical and experimental characteristics was achieved. The same energy band diagram has also been used to interpret the electrical properties of the other metal oxide capacitors 7. In the previous study of MOO3, however, the dissociation and hence the autodoping of the deposited films were controlled by evaporating the M o O 3 in an oxidizing ambient. In the present paper we report the effects of uncontrolled * Present address Research and Development Division, Elpro International Ltd, Chmchwad Gaon, Poona 411033, In&a 0040-6090/82/0000-0000/$02 75

~) Elsevier Sequoia/Printed m The Netherlands

102

c; s NADKARNI et al

a u t o d o p m g on the e l e c m c a l p r o p e r t i e s of AI/MoO3/A1 thin film c a p a c i t o r s for whmh the M o O 3 films were p r e p a r e d m a g o o d v a c u u m of the o r d e r of 10 ~ T o r t . F u r t h e r m o r e , the o b s e r v e d electrical characteristics of these c a p a c i t o r s are a n a l y s e d on the basis of the same energy b a n d m o d e l but the higher d o p i n g density is t a k e n into a c c o u n t The b e h a w o u r s of the oxygen-rich a n d o x y g e n - p o o r M o O 3 films are c o m p a r e d , a n d theoretmal c h a r a c t e r l s n c s for the o x y g e n - p o o r samples are presented. "~o.j~

Inter,or

? "1" De0,e,,on

"~o]

j

"l

Vacuum

level

,.0.o0


Donor 8and ~ Fermi Level

--

~Energy Gap Insulator"

Electrode

E lect r.ode

Fig 1 Energy'band diagram for an MIM system with a doped insulator EXPFR1MFN FAL I)FTAI1 S A I / M o O 3 / A I samples were p r e p a r e d on a glass s u b s t r a t e by the sequential v a c u u m d e p o s i t i o n of a l u m l m u m as the b o t t o m electrode, M o O 3 as the i n s u l a t o r a n d a l u m i n l u m as the c o u n t e r e l e c t r o d e . F i g u r e 2 shows the c o n f i g u r a t i o n of the m e t a l / I n s u l a t o r / m e t a l ( M I M ) c a p a c i t o r s fabricated for the present study. The a r e a of the c a p a c i t o r s was 0 2 cm z a n d the thickness of the M o O 3 r a n g e d from 1200 to 3500/~. A l t h o u g h the f a b n c a n o n technique was essentially the same as that described before 4, there were differences in the a m b ] e n t c o n d i t i o n s d u r i n g the d e p o s m o n of the M o O 3 Insulator. In the previous study, after the system h a d been e v a c u a t e d initially to a pressure of a b o u t 10 6 T o r t , oxygen was leaked into the system such that an a m b m n t oxygen pressure of the o r d e r of 10 4 T o r r was m a i n t a i n e d d u r i n g the d e p o s m o n of M o O 3 4 In this investigation, however, the e v a p o r a t i o n of M o O 3 was carried out at a lower pressure w i t h o u t l e a k i n g oxygen into the system The actual pressure d u r i n g the d e p o s i n o n of M o O 3 was of the o r d e r of 10 ~' T o r r

1" ~'

~ ] ' e - - - t - - - Bat t om

, I/~/,Y/l'~ "-TnsulatOr

~

.~:~ ~

Electrode MoO 3

~-U pper

Top Electrode

Glass Substrote

MoO3 (a~

Interface

Lower Electrode

(b)

Fig 2 (a) Schemanc vmw of the metal/MoO3..'metal capacltop,. {b) cross secuon of the device,,

TEMPERATURE DEPENDENCE OF M E T A L / M o O 3 / M E T A L CAPACITORS

103

Samples were then transferred to a cryostat which permitted the variation in the sample temperature from that of liquid nitrogen up to 400K during the investigation of the electrical properties. A chromel-alumel thermocouple was attached to the substrate close to the MIM capacitor under test, so as to measure the device temperature accurately. The capacitance and the conductance of the devices were measured using a Wayne-Kerr B641 autobalance bridge (internal test frequency f = 1592Hz; 09 = 104 rad s-1). The conductance measurements were performed continuously and automatically by feeding the analogue voltage output (proportional to the conductance) of the bridge and the thermocouple voltage output to an x-y recorder. However, automatic plotting was not used for capacitance measurements because the "structure" in the capacitance-temperature (C-T) curves required further elucidation. Therefore the capacitance was manually monitored at the highest sensitivity of the bridge to improve the accuracy of the measurements. 3. RESULTS 3.1. Capacitance versus temperature The C - T characteristics of these samples, prepared in a non-oxidizing ambient,

were markedly different from those of samples prepared in an oxidizing atmosphere. It was extremely difficult to measure the capacitance of the samples at room temperature as they were found to be highly conductive. However, when the samples were cooled to liquid nitrogen temperature, the conductivity decreased markedly and capacitances of all the samples could be measured.

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I

100

140

t80

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220

I

260

Temperature (K) Fig 3 C - T characteristics of oxygen-rich metal/MoOa/metal capacitors with various oxide thicknesses. El, 1200/~, e , 1900/~, A, 2200/~, i , 2900/~, O, 3500/It, - - - , theoreUcal characteristic for a sample 1200 /~ thick (see text) The inset shows the C - T characteristics for oxygen-rich samples 4" - experimental, - - -, theoretical

104

c. s NADKARNIetal.

Figure 3 shows the variation m the capacitance with the temperature The inset shows the characteristics of oxygen-rich samples for comparison (see ref. 4 for details). It is clearly seen that initially the capacitance increases monotonically and reaches a maximum value at a temperature To. However, as the temperature is further increased, it is observed that the sample capacitance tends to decrease This behaviour is markedly different from that of the oxygen-rich samples for which the capacitance tends to saturate at h~gher temperature Furthermore, it should also be noted that the maximum m the capacitance moves to lower values and occurs at higher temperatures as the sample thickness is increased The plot m Fig. 4 shows the variation in the capacitance with the reverse thickness of the insulator at liquid nitrogen temperature. Th~s plot is seen to be a straight hne as expected, and thus the capacitance of the sample at this temperature may be assumed to be the geometric capacitance of the sample. The dielectric constant calculated from this curve was found to be 16 2, which Is close to that calculated before for the oxygen-rich samples "~ 3 2. C o n d u c t a n c e rer,sus t e m p e r a t m e

Figure 5 shows typical conductance ~,ersus temperature characteristics of the oxygen-poor devices It was observed that the a.c conductance G of all the samples Increased monotonically with temperature and hence a representative curve ~s shown here. Whereas maxima were observed (reset, curve for 0 V) for the oxygenrich samples 5 no maxima were observed m this case. For comparison, the G T characteristics of a typical oxygen-rich sample (2900 ~ thick) are shown in the Inset. The characteristic peak observed in the curves at low voltages is not observed m the oxygen-poor samples

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Fig 5 E x p e r i m e n t a l ( - - ) and t h e o r e t l c a l l - -I C T characteristics of the c a p a c i t o r s The inset shows the C T characteristics of the oxygen-rich samples under various voltage b~as c o n d i t i o n s 5

TEMPERATURE DEPENDENCE OF METAL/MoO3/METAL CAPACITORS

105

4. DISCUSSION It was shown in the previous studies of oxygen-rich M o O 3 that autodoping of due to decomposition during vacuum deposition leads to the formation of Schottky barriers at the metal-insulator interface 4. It was also shown experimentally that an tmpurity level energy band exists in the forbidden gap of the insulator at 0.28 eV below the conduction band 4"6. The Schottky barrier width under these conditions was estimated to be approximately 160 ~. The energy band diagram and its equivalent circuit are represented in Fig. 1 and Fig. 6 respectively. Furthermore, for the oxygen-rich samples it was found that the resistance of the Schottky barrier could be neglected, being too high in comparison with the bulk resistance at all temperatures. In the present samples, however, since the oxide is deposited in a nonoxidizing atmosphere, it is believed that the doping density is higher than that of the oxygen-rich samples because of the lack of oxygen, resulting in narrower barrier widths at the metal-insulator interface. In fact, we shall assume that the barrier is sufficiently thin that electrons in the electrode penetrate the barrier by the tunnel effect (Fig. 7). The resistance of these "leaky" barriers must therefore be considered in the a.c. model. MoO 3

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Interior

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(a) (b) Fig. 6 Eqmvalentclrcmtfor Fig 1 (a) general representation;(b) simplifiedclrcmt

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(a) (b) Fig. 7 (a) Tunnelhngand (b) tunnel hoppingprocessesm a leakymetal-insulator(doped)contact5.

4.1. Analysis of the equivalent a.c. circuit The parallel equivalent capacitance C and parallel equivalent conductance G of the circuit shown in Fig. 6 are given by the equations s C o - C, C = Cg-~ 1 +(wT¢) 2

(1)

Rd¢ - - R 1 R =

R 1 -~ 1 +(~oTR)2

(2)

106

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NADKARN[

el a/

the conductance G is given b~y G=

(3)

1/R

where in eqn. ( 1)

C,Cb Cg - C , + C b

(4)

C~R, 2 + C b R b 2 CO =

Te - -

(5t

(R, + Rb) 2

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(6)

( C , q- CD)

and in eqn. (2), R1 is given by R,Rb{C~+Cb) 2

R1

=

t7)

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( R,Rb

~

/ 1'2

TR = ~ R , + R b ( C , ' R , + C b 2 R b )

(8)

d

and the d c resistance Ra, is given by (9)

Rd¢ = R, + R b The temperature variation in the barrier resistance R, is expressed as

where ~b, is the activation energy measured from the b o t t o m of the impurity band to the b o t t o m of the conduction b a n d R, is the leakage resistance of the barriers arising from the tunnel hopping process shown in Fig. 7 Similarly, the temperature variation in the bulk resistance Rh can be written as Ru=Roexp

kT

where ~bb is the activation energy of the d o n o r centres measured from the Insulator Fermi level to the b o t t o m of the conduction band. (It should be noted that 4, is greater than ~bb by an a m o u n t equal to the width of the impurity band.) The values of R,o and Ro are the characteristic properties of the barrier at the interface and the bulk material

4 2 Sele~tlon o/phystcalparameters The above equations were used for analysing the equivalent circuit shown in Fig. 6. The desired parameters were chosen in the following manner. Since the capacitance of all the samples was found to be inversely proportional to the thickness of the insulator at liquid nitrogen temperature, it was assumed that Cb is approximately equal to the geometric capacitance, t e the capacitance measured at liquid nitrogen temperature As it was difficult in this investigation to obtain the value of the barrier capacitance by studying the high temperature saturation value

TEMPERATURE DEPENDENCE OF METAL/MoO3/METAL CAPACITORS

107

(as was possible for the oxygen-rich samples) it was assumed that the doping density in this case is higher than that for the oxygen-rich samples, thus leading to thinner barriers. Therefore the capacitance Cs was assumed to be higher than that for the oxygen-rich samples. Similarly, the values of Rso and Ro were also chosen arbitrarily during computation. It was observed that when Cs was increased the height of the peak increased. Similarly, slight changes in Rso and R o also shifted the temperature TOat which the peak occurs. However, the interrelation between Cs, Rso and R o was found to be more complicated. For computing the theoretical characteristics, the following parameters were chosen such that the theoretical characteristics matched the experimental curves (see also Section 4.4). The bulk capacitance Cb was obtained from the capacitance of the devices measured at liquid nitrogen temperature. (Here C s >> Cb and since both of the capacitors are in series the assumption that C b ~ Cg (geometric capacitance) is acceptable.) The barrier capacitance Cs was assumed to be 380 nF; Cb = 23 nF; 4~ -~b b was chosen to be 0.07 eV; R~o was taken as 2.5 x 10 - 7 f~ while R o, which also depends on the sample thickness, was chosen to be 2.25 x 10- 5 f~ for a sample of thickness 1200/~. 4.3. Variation in the capacitance with temperature

With the above choice of the parameters the theoretical curve for the sample of thickness 1200/~ was computed as shown by the broken curve in Fig. 3. It is seen that the qualitative agreement between the theoretical and the experimental curve is excellent. The general theoretical C - T characteristics for various oxide thicknesses are shown in Fig. 8; a marked similarity between the experimental curves (Fig. 3) and the theoretical curves (Fig. 8) is observed. Furthermore, it is also observed that the temperature at which the maximum occurs depends on the sample thickness and shifts towards higher temperature as the thickness of the insulator increases (Fig. 8, inset), m agreement with the experimental results. The slight intermixing of the experimental curves compared with the complete separation of the theoretical curves in Fig. 8 is explained as follows. As the parameters during the fabrication of the samples could not be precisely controlled, it is probable that R o may not be proportional to the thickness of the oxide layer as assumed in computing the theoretical curves. The dependence of R o on T was also neglected in computing the theoretical characteristics. Furthermore, Rso is assumed to be constant for all thicknesses m the theoretical computations. This implies that variations in the doping density due to the deposition conditions were also neglected. 4.4. Vartatton in the conductance with temperature

Figure 5 shows the theoretical conductance versus temperature curve along with the experimental characteristic of these samples. The similarity between the two curves is obvious and thus the validity of our model need not be stressed again. However, to emphasize the point that the barriers are leaky and the conduction process is due to tunnel hopping, we have reproduced the curves for oxygen-rich samples in the inset of Fig. 5. It was pointed out in the previous paper 5 that when high voltage biases (high fields) are applied to the samples the barriers tend to be leaky because the conduction mechanism is then governed by the tunnel hopping process (Fig. 7). The full curve marked 1.5 V in the inset is the conductance versus

108

G

S NADKARNI

et

al.

temperature characteristic of oxygen-rich samples. This is obviously slmdar to the characteristics of the present samples, since it is equivalent to the leaky barrier case

-

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2 0 0 0 A°

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5000A °

50 ,

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190

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2500

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J

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l I00

I 140

I

I

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220

260

T(K)

Fig

8

Theoretical

temperatures

5.

C-T characteristics

The

reset s h o w s

t h e shift m t h e m a x i m a

towards

higher

as t h e t h i c k n e s s o f t h e o x i d e i n c r e a s e s ( / = 1 5 9 2 H z )

CONCLUSION

It is shown in this paper that in the case of M I M systems where leaky barriers exist at the metal-insulator interface the capacitance shows a maximum when plotted as a function of temperature. The a.c. conductance, however, increases m o n o t o m c a l l y in this case. The agreement between the experimental and theoretical curves provides further evidence for the validity of the model and the assumptions made in computing the theoretical characteristics. The donor density calculated from the curve fitting data (Cs) was found to be about three times higher in the oxygen-poor samples than that in the oxygen-rich samples 4. ACKNOWLEDGMENT

This work was supported in part by the Umverslty Grants Commlsston, N e w Delhi (Grant Sanction F-25- l(11005)/80).

TEMPERATURE DEPENDENCE OF METAL/MoO3/METAL CAPACITORS

REFERENCES 1 2 3 4 5 6 7 8

L Klhlborg, Acta Chem. Scand, 13 (1959) 954 P E. Blackburn, M Hoch and H L. Johnston, J. Phys. Chem, 62 (1958) 769 L.A. Burslll, Proc. R. Soc. London, Ser A, 311 (1969) 267. G S. Nadkarnl and J. G. Simmons, J. Appl. Phys., 41 (1970) 545 G. S Nadkarni and J G Simmons, J Appl. Phys., 43 (1972) 3741 G S Nadkarnl and J G. Simmons, J. Appl. Phys., 43 (1972) 3650 B. Lalevlc, M. Gvlslu and M Shoga, Phys Status Sob& A, 56 (1979) 379. G. S Nadkarm and J G. Simmons, J. ,4ppl. Phys., 47 (1976) 114.

109