Electrical conduction in silicon monoxide films

Thin Solid Films - Elsevier Sequoia S.A., L a u s a n n e - Printed in the N e t h e r l a n d s

E L E C T R I C A L C O N D U C T I O N IN SILICON M O N O X I D E FILMS A. E. HILL, A. M. PHAHLE A N D J. H. CALDERWOOD University o f SalJbrd, Saltbrd 5, Lancs. ( Gt. Britain) (Received J a n u a r y 13, 1970; in revised f o r m M a r c h 2, 1970)

SUMMARY

The permittivity and loss tangent of vacuum-deposited A1-SiO-AI capacitors have been measured in the frequency range 102-106 Hz at temperatures in the range 77-573 °K together with d.c. characteristics in the range 77-293 °K. The measurements were made in the original vacuum in which the films were deposited in order to determine the effect of atmospheric exposure. The substrate temperature during deposition was varied in the range 320-573 °K. A d.c. activation energy of 0.38 eV was found, while at high frequencies the activation energy was 0.004 eV. The results are, in general, in good agreement with those previously reported for specimens exposed to the atmosphere before investigation.

IN TRODUCTION

Silicon monoxide is a material which is commonly used as a dielectric medium in thin film capacitors and as an insulating layer in various vacuumevaporated devices, such as thin film transistors. It has been shown in a previous publication ~ that the most important parameter in the preparation of this material is the ratio of the number of silicon monoxide molecules arriving at the substrate in unit time compared with the corresponding number of residual oxygen or water vapour molecules. This ratio affects the degree of oxidation of the material during deposition and this in turn affects the physical properties, e.g. mechanical stress, porosity and susceptibility to further oxidation. This is shown in Fig. 1. The horizontal axis is calibrated in units of the arrival ratio N (i.e. the rate of arrival of SiO/gas molecules, 02 in this case). The fraction of incident light transmitted gives a measure of the degree of film oxidation, since at the wavelength used (3,500 A) the transmission of SiO is very much less than that of SiO2. It can be seen from the vacuum value curve that the film tends to SiO2 as N is decreased (either by decreasing the evaporation rate or by increasing the gas pressure). Thin Solid Films, 5 (1970) 287-295

288

A . E . HILL, A. M. PHAHLE, J. H. CALDERWOOD 1.0

0.8 L_

I=: 0.6 e

0,4

c

.o_ d w

0.2

0 10-2

I

I

I

I

lO -i

I

I

i

N(SiO / \02

i

i

1

'

,

,

,I

lO

/

Fig. 1. T h e variation o f the t r a n s m i s s i o n characteristic as a function o f the arrival ratio N a n d o f the a t m o s p h e r i c exposure time.

Typically, for a residual atmosphere of oxygen, N = 1 when the rate of deposition is 10 /~/sec and the pressure is 8 x 10 -6 torr. It can be seen that exposing the film to the atmosphere causes an increase in the oxidation in every case. This increase is particularly strong for low N films which are very susceptible to the atmosphere, both the oxygen and the water vapour contents of the atmosphere having a strong effect. For this reason it was decided to perform all tests on the films in the original vacuum to ensure that the effects of oxidation were minimised. EXPERIMENTAL

The apparatus is shown in Fig. 2 and combines the functions of a vacuum evaporator (working pressure approximately 3 x 10-6 torr) and a variable temperature, vacuum test platform. The glass substrates are clipped to the underside of the chamber, which may be raised or lowered by the bellows-sealed height adjusting unit. The chamber, which is open to the atmosphere, may be cooled by filling with water or liquid nitrogen or heated to 300 °C by a heater in the baseplate. The specimens tested were simple A1-SiO-A1 capacitors with an electrode area of 1 c m 2. The capacitors were deposited onto glass substrates by exposure to aluminium and silicon monoxide sources through a series of masks held on a rotating vane. The substrates carried predeposited contacts and connecting wires bakeable to 300 °C. The deposit thickness was controlled by a quartz crystal monitor. The silicon monoxide furnace (which operated at 1400 °C) was specially designed to minimise large temperature gradients and hence to avoid overheating the charge. This completely eliminated spitting and the associated pin-hole problem. After deposition, the specimen was lifted clear of the vane to avoid short circuiting the connections. Measurements were taken of the direct current/voltage relationship by plotting the curves on a sensitive plotting table. Thin Solid Films, 5 (1970) 287 295

289

ELECTRICAL CONDUCTION IN S i O FILMS

4.

Substrate height odiustina unit

J connections

.Thermocouple •eater

Heater ermocouple Substrate SubstrotemoSks

j

Ix\\\\\

[\\\'1

I

k\\\\"

\\x

Silicon monoxide furnace

x\'q

Fig. 2. The deposition and test apparatus.

This minimised the effect of drift in the characteristics. Alternating current measurements were made using two transformer bridges covering 40 Hz to 20 kHz with an accuracy of 0.1 ~ and 40 kHz to 1 M H z with an accuracy of 1.0 ~ .

DIRECT CURRENT MEASUREMENTS

Typical d.c. characteristics are shown in Fig. 3, which is a Schottky plot at room temperature of log current v e r s u s the square root of the applied potential. These are linear in the range shown although some deviation occurs for applied potentials below l V, which is probably due to contact potential effects. It can be seen that the leakage current increases with N (i.e. as the degree of oxidation is reduced). If the Schottky equation is applied, the displacement between the two plots can be shown to be equivalent to a difference in the barrier height of 0.118 eV. I f the effective value of Richardson's constant is assumed to be 120 A/cm2/ Thin Solid Films, 5 (1970) 287-295

290

A.E. HILL, A. M. PHAHLE, J. H. CALDERWOOD

102

/N=I.Q T=30OOK

lO

~

/

t

.x"

~ "

N=0.1 T = 30Q°K

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v

IO-1

} 10-2

0

1.0

2.0 3.0 4.0 5.0 ~/Applied potentiQI (V 1/2)

J

(5.0

Fig. 3. Schottky plots for two values of N at room temperature and for N = 1 at liquid nitrogen temperature. °K 2 the absolute b a r r i e r height governing the c o n d u c t i o n process is 0.85 eV for N = 1.0 a n d 0.966 eV for N = 0.1. It has been suggested by S i m m o n s 2 that the c o n d u c t i o n process for silicon m o n o x i d e u n d e r these c o n d i t i o n s is P o o l e - F r e n k e l emission f r o m d o n o r levels within the f o r b i d d e n b a n d of the silicon m o n o x i d e r a t h e r t h a n true S c h o t t k y emission f r o m the electrodes. B o t h processes could explain the characteristics o f Fig. 3, a n d the calculated value o f the b a r r i e r heights tends to s u p p o r t S i m m o n s ' hypothesis. S c h o t t k y plots at 77 °K are still linear a l t h o u g h the current is r e d u c e d by a factor o f 104. A plot o f log I / T 2 v e r s u s recip r o c a l t e m p e r a t u r e is shown in Fig. 4 a n d is very similar to that of E m t a g e and

10-9 I

10-1C

10-1

el

'

~°-'~.o 21~ ~

2J6 ~5 ~.b ~.~ ~.~

103/T (°K~1)

Fig. 4. Plot of l I T 2 against reciprocal temperature. Thin Solid Films, 5 (1970) 287-295

291

ELECTRICAL CONDUCTION IN S i O FILMS 400

0,48

c°--.o.. 0.44

/I

~

J12ooo

. 432OK

o.4c

77°K

\

0.3;

/:/

1200 o

o.2~

8

~

8 U

0,24 0.2C 0.1~

.

~

~:...' '~ . . . . 102

_

103

. ,.

_

104 Frequency (Hz)

O

105

Fig. 5. The variation of capacitance and conductance with frequency and temperature.

9,0 8.0

I7

°K

ZC 6.C

g 3.C 2.C 1.C 10

102

103 104 Frequency (Hz)

10 5

106

Fig. 6. The variation of loss tangent with frequency and temperature.

Tantraporn 3. Emtage attributes his deviation from linear at low temperatures to the dominance of tunnel emission in his 100 A films, but in our case this is not likely because the film thickness was 1,500 A. The value of activation energy for the conduction process appears to be 0.38 eV, which is in excellent agreement with Stuart's 4 value of 0.4 eV. The films tended to become unstable when subjected to fields approaching 10 6 V / c m and at r o o m temperature invariably broke down for fields greater than this. However, this breakdown was probably in part due to the heating effect of the resulting currents in excess of 100 #A, because at Thin Solid Films, 5 (1970) 287-295

292

A . E . HILL~ A. M. PHAHLE, J. H. CALDERWOOD

77 °K with very low leakage currents the films would withstand fields greater than 4 x 10 6 V / c m without failure.

ALTERNATING CURRENT MEASUREMENTS

Typical curves of the variation of capacitance and conductance with frequency are shown in Fig. 5, and the corresponding values of tan 6 are shown in Fig. 6. The loss factor and capacitance rise rapidly at low frequencies, but the peak appears to occur at a frequency lower than 40 Hz, the minimum working value of the apparatus used. The specimens exhibit a strong temperature dependence which moves the dispersion peak towards a higher frequency, but even at 300 °C the peak is still below 40 Hz. This dispersion region is almost certainly the same as that reported by Argall and Jonscher 5, who attributed the effect to the relaxation of impurity ions (probably hydrogen or silicon) in a glass-like network. They also noted that the magnitude and frequency of the peak in tan 6 at a given temperature increased with the deposition rate (i.e. with N for a constant evaporation pressure). The predicted magnitude of the peak in Fig. 6 is at least five times greater than that reported by Argall 5, which is compatible with the use of a greater value of N in the present case. The predicted peak frequency is less than Argall's 5 but an increase in peak frequency with N does occur, as shown in Fig. 7 which is a plot of the loss component e" versus frequency at two values of N. If it is assumed that the peaks follow the troughs in moving to a higher frequency with N then Argall's 5 observation is confirmed. The difference in the value of peak frequency could be due to some other experimental parameter such as the design and effective temperature of the source. Figure 7 also indicates a significant increase in loss with N. This is confirmed by Fig. 8, a plot of dielectric constant and tan & at 1 kHz as a function of N. This shows the effect of oxidation towards silicon dioxide at low values of N which reduces the dielectric constant from 5.2 to approximately 3.6 (the value quoted for silicon 10

high N (u

lO-ll

10-210

1(~2

10/ 10 / Frequency (Hz)

1015

1016

Fig. 7. The variation of dielectric loss with frequency for two values of N at r o o m temperature.

Thin Solid Films, 5 (1970) 287-295

EI_ECTR1CAL CONDUCTION IN

SiO

293

FILMS

- 5.4

x x ~ x ' ~

~

5.0

-- x,-.,-,4.6 4.2 -t-,

l o -~

x

tan 5

3.8 o

oo

u 3.4"E

r~

10-10_ 2 2

I

i

i

I I 10 -1

i

,

,

NfSiO

,

I

1

,

,

,

,

I 10

t2~B

k-~-2 J

Fig. 8. The dependence o f dielectric constant and loss tangent on N at r o o m temperature.

573°K

1°~ 4 2 % 1C C"

293°K 77"K

10

1°-10

10I 2

t~l 10I 3 1,J 4 Frequency(Hz)

10I 5

10I 6

Fig. 9. The variation o f dielectric loss with frequency and temperature.

dioxide is approximately 3.7) and correspondingly reduces the loss tangent by a factor of 10. Unfortunately, this improved loss factor cannot normally be utilised because of the previously mentioned instability associated with low N films. Figure 9 shows the variation of e" with frequency in a film deposited at a substrate temperature of 150 °C and measured over a range of temperatures from - 1 9 5 °C to + 300 °C. These plots also rise towards a peak at low frequency and the shift in the troughs towards a higher frequency with increasing temperature indicates a similar shift in the peaks. The corresponding curves for a film deposited at 300 °C are similar to the previous set except that the troughs (and so presumably the peaks) are displaced towards a lower frequency. This would Thin Solid Films, 5 (1970) 287-295

294

A. E. HILL, A. M. PHAHLE, J. H. CALDERWOOD

indicate an effect equivalent to a reduction in the value of N, which may be explained by the increased probability of oxidation occurring at the higher temperature. In practice it was noted that the films deposited at high substrate temperatures were more stable in their electrical properties than those deposited at lower temperatures. Thus room temperature substrate specimens were not able to withstand temperatures in excess of 150 °C without suffering rapid, permanent drifts in their characteristics. This drifting was much less marked in specimens prepared at 300 °C. Figure 10 shows a typical change in capacitance in a specimen prepared and measured at 150 °C. However, at room temperature in vacuum such films became very stable with drifts of less than 0.01% over several days. A plot of log conductance v e r s u s frequency for a typical film is shown in Fig. 11. This again precisely confirms the findings of Argall s with a temperaturesensitive region at low frequencies and a temperature-insensitive region with a square-law dependence of conductivity on frequency at high frequencies. A plot 20

40

i

i

Time after deposition (rain) 60 80 100 120 140 i

i

i

i

i

160 180 i

i

O

0.4 Y, o.~

13-

._c o.,~ g -~ 1.C 1.2 1.4

Fig. 10. T h e decay o f capacitance with time at 150 °C after deposition. 10~

lo" o 1©~ E

10~

8

4

g lc 'd

2

3

°

~

29~ 77"K

10lo-

i i , lO 2 103 104 Frequency ( H z)

, 105

j 1 6

Fig. 11. T h e variation o f a.c. c o n d u c t a n c e with frequency a n d temperature.

Thin Solid Films, 5 (1970) 287-295

ELECTRICAL CONDUCTION IN

SiO

FILMS

295

10 ~

"~ 1£ ~:

5kHz .x----

lk Hz 9 g

~---.......~

0 0

I 2

I 4

I 6

I 8 V

I 10

Hz I 12

/ 14

(oK-I)

Fig. 12. T h e variation o f a.c. c o n d u c t a n c e with reciprocal t e m p e r a t u r e at different frequencies.

of conductance against reciprocal temperature at various frequencies is shown in Fig. 12, which suggests an activation energy for the conduction process of 0.004 eV. Argall 5, who found an activation energy of 0.01 eV, attributes these results to an electron hopping process similar to that described in semiconductors by Pollak and Geballe 6. This process predicts a strong frequency dependence tending to a square law at high frequencies, a low activation energy and an absence of dispersions at high frequencies, all of which have been observed. Also, the hopping mechanism is highly compatible with the amorphous nature of silicon monoxide which is known to exist.

CONCLUSIONS

It has been found that the properties of silicon monoxide films measured under the original vacuum conditions are in good agreement with those found by other workers who exposed their specimens to the atmosphere before taking measurements. Similarly, the measured characteristics of the films did not vary greatly when exposed to the atmosphere provided that they had first returned to room temperature and that the value of N during deposition was not so low as to introduce destructive stresses into the film. REFERENCES 1 2 3 4 5 6

A . E . HILL AND G. R. HOFFMAN, Brit. J. Appl. Phys., 18 (1967) 13-22. J . G . SIMMONS, Phys. Rev., 155 (1967) 657-60. P . R . EMTAGE AND W. TANTRAPORN, Phys. Rev. Letters, 8 (1962) 267-68. M. STUART, Brit. J. Appl. Phys., 18 (1967) 1637-40. F. ARGALL AND A. K. JONSCHER, Thin Solid Films, 2 (1968) 185-210. M. POLLAK AND W. GEBALLE, Phys. Rev., 122 (1961) 1742-53.

Thin Solid Films, 5 (1970) 287-295