Electrical conduction through thin amorphous SiC films

ELECTRICAL

CONDUCTION

THROUGH

THIN

AMORPHOUS

Institute

of Technology,

Sic

FILMS* T. E. HARTMAN, 3. C. BLAIR AND C. A. MEAD Texas Instruments Incorporated, Pasadena, California (U.S.A.)

Dallas,

Texas and California

(Received January 22, 1968)

SUMMARY

The current density through amorphous Sic films 80 to 800 A thick deposited pyrolytically on refractory metal substrates is described by j = K(T’,d)Y”“‘~d’ where T is the absolute temperature and d the film thickness. This type of electrical characteristic is similar to that obtained for compressed granulated Sic pellets or varistors and also for amorphous thin films of semiconductors and insulators.

INTRODUCTION

Very thin Sic films can be deposited on refractory metal strips by pyrolytic techniques’. The resultant film is relatively uniform over the center half of the strip, continuous and amorphous as shown by electron diffraction. When very small go?d dots are evaporated on the Sic fiIm met&Sic-Au sandwiches are formed. In this way electrical conduction through amorphous Sic films between 80 and 800 A thick can be investigated. Thicker films can be made easily, but it is difficult to obtain suitable films which are thinner. The electrical conduction through thin amorphous Sic films has many of the characteristics of conduction in a symmetrical varistor’, formed from a pellet of granulated Sic in a suitable binder. The main difference lies in the effect of physical size upon the characteristics. The current density through the amorphous silicon carbide films is proportional to the voltage raised to the nth power. Similar electrical characteristics have been observed for amorphous films of selenium3, AS&~, Ge5, Si5, boron’, zirconium oxide6, and titanium oxide6. When considering a power-law dependence of current upon voltage in the context of a dielectric film, it is tempting to suspect a space-charge-limited me* Presented in part at the Cleveland meeting of the Electrochemical Thin Solid Films, 2 (1968) 79-93-Elsevier,

Lausanne-Printed

Society, May 1966.

in the Netherlands

80

T . E . ItARTMAN, J. C. BLAIR, C. A. MEAD

chanism. The conduction through selenium films has been attributed to spacecharge-limited currents 3. In a recent review of electronic conduction in amorphous dielectric films Jonscher 7 has suggested that the space-charge-limited model should be tested against the thickness dependence and the transient response. Both of these tests were performed on SiC films and are reported below. The results tend to eliminate space-charge-limited currents as an explanation of conduction in SiC flms. The current density through SiC films increases with film thickness at a constant applied electric field. The conduction through somewhat thicker boron films 8 has a thickness dependence which is very similar to that of SiC films. The mechanism which has been proposed for the conduction in amorphous boron films is impact ionization of acceptor states 5. Analysis of conduction through SiC films is complicated by a large amount of scatter in the data (which is characteristic of the boron data as wellS'8). Empirical equations for the electrical properties of SiC films have been derived but, due to the scatter in the data, these equations are regarded more as correlative than descriptive. The formalism is also applicable to the data on boron films, but leads to equations which are inconsistent with those previously derived for impact ionization of acceptor states s. As a result of the experimental evidence at hand, it is not possible to establish the mechanism which is responsible for current flow in SiC films. The electrical properties of SiC varistors are believed to be associated with the potential barriers to electron flow which are formed at the contacts between silicon carbide grains 9. Whether the barrier is of an electrical Mott type 10 or due to a physical amorphous layer ~t is not certain. The similar electrical properties of Ge and Si films disappear when the structure changes from amorphous to polycrystalline 5. The results of this study on amorphous SiC films suggests that a single physical barrier of the type found between the grains in SiC varistors may be involved.

MEASUREMENTS

Technique The metal-SiC-Au sandwiches described above are made by depositing an array of circular gold dots on top of the SiC film, each having a surface area of 1.27 × l0 -4 cm 2 and spaced 0.05 cm apart. Electrical contact to the gold dots at room temperature is made mechanically with a fine gold wire attached to a vernier movement. Measurements as a function of temperature are made by attaching the gold wire to the dot with indium solder. The thickness of the SiC films was computed from the capacitance and Thin Solid Films, 2 (1968) 79-93

ELECTRICAL CONDUCTION THROUGH THIN AMORPHOUS

SiC

FILMS

81

surface area of the gold dots using a dielectric constant 12 of 10.2. The capacitance and conductance are measured at 100 kHz on a Boonton Electronics Capacitance Bridge 74C-$8 with an applied voltage of less than 50 mV rms. The temperature-controlling apparatus has been described elsewhere 13. The apparatus is designed to maintain the sample in a temperature-controlled lighttight chamber at any temperature between 80 and 300 °K. Four types of volt--current characteristics are measured. The d.c. current is measured with a Hewlett-Packard 425A Microvolt Ammeter and the voltage is measured with a Keithley 200B Electrometer. The characteristics are reproducible and reversible. No transient effects indicative of space-charge-limited current in the presence of traps ~4 are observed. The a.c. volt-current characteristics are measured with a Hewlett-Packard 200CD Audio-Oscillator and a Tektronix 502 Differential-Input Oscilloscope. The pulsed volt--current characteristics are measured point by point on the oscilloscope with a duty cycle of about 0.1 ~o. Swept volt-current characteristics are displayed on the oscilloscope with a relatively slowly varying current ramp using a circuit modification to the oscilloscope described by Mead 1s. The average temperature rise of the gold dot was measured with the sample at ambient room temperature using an iron-constantan thermocouple made of wire .001 in. diameter. Temperature rises of as much as 100 °C were observed using the current-ramp technique of volt-current measurement. As a result the a.c. and current-ramp measurements are subject to heating effects which are not evident in the pulsed measurements. The d.c. measurements are restricted to power dissipation levels such that heating effects are negligible. Finally the vector impedance as a function of frequency was measured with a Boonton Radio 4800A Vector Impedance Meter. Results The most characteristic electrical property of metal-SiC-Au sandwiches is shown in Fig. 1 which is a typical volt-current oscilloscope trace. Figure 1 is a current-ramp characteristic with the current displayed vertically. Reversal of the voltage polarity results in the same trace in the opposite direction, indicating that the curves are completely symmetrical about the origin. This effect is evident in the low frequency a.c. volt-current characteristic shown in Fig. 10(a) where the current is displayed horizontally. The functional form of the volt-current characteristic is independent of the metal used for the electrodes but the electric field corresponding to electrical breakdown is not. Permanent irreversible changes in the characteristics occur at lower fields when the polarity of the applied voltage is such that the Au dot is negative. The effect of excessive fields applied as pulses for time intervals too short to cause disruptive breakdown is shown in Fig. 2 for the Au biased negatively. Thin Solid Films, 2 (1968) 79-93

82

T.E. HARTMAN~ J. C. BLAIR, C. A. MEAD

At current densities in excess of 100 A/cm z a blue-green light is emitted from the specimen for either field polarity. Light emitted from the periphery of a single gold dot viewed through a microscope is shown in Fig. 2(a). After several negative pulses are applied to the gold dot the surface of the illuminated gold dot is as

Fig. 1. Oscilloscope trace showing the volt-current relationship for a 473 ,~ thick SiC film. The trace was obtained with a current-ramp. Scales: vertical, 2 mA/div.; horizontal, 2 V/div.

shown in Fig. 2(b), The blistering of the gold shown in Fig. 2(b) was not present before applying the negative pulses. The irregular dark lines are grain boundaries in the refractory metal substrate, The dark area on the lower portion of the dot is the movable gold wire contact.

Fig. 2. A gold dot viewed through a high-power microscope. (a) Light emitted from the periphery of the dot at current densities in excess of 100 A/cm ~. (b) Blistering of the dot caused by the pulsed application of excessive negative voltages.

The metal-SiC-Au sandwiches formed by gold dots which are above a single crystaUite of the substrate (i.e., do not cross a grain boundary) have larger breakdown fields for the gold biased positively than do those which contain grain Thin Solid Films, 2 (1968) 79-93

ELECTRICAL CONDUCTION THROUGH THIN AMORPHOUS

SiC

FILMS

83

boundaries. Breakdown fields in excess of 10 7 V/cm have been observed. On the other hand for the Au biased negatively, deleterious effects usually occur for fields in excess of 5 x 106 V/cm. The following empirical equation is representative of the observed characteristics: j = K(VlVo)";

v.,,. < v <

(I)

Vm~x

where: n

=

n(d, 7");

K

K(d, T)

=

j is the current density, V the applied voltage, Vmin is the threshold voltage, Vmax is the breakdown voltage, d is the thickness of the SiC film, T is the absolute temperature and Vo is a constant with the dimensions of voltage. Note that the expression is valid only for a particular range of voltages. In the region of small voltages ohmic behavior prevails. As shown in Fig. 3 the log I vs. log V plot for a 482 A film is ohmic for voltages less than 8 x 10 -2 V. The volt-current characteristic gradually approaches a I oc V" at V,,i. approximately equal 5 V. At voltages in excess of Vma~which correspond to fields in excess 10-s

ro-1 F I

]

J

T

10-2~

¢#

t0-r

/?

,\, \

I

I

I

--

\ .~

°,Oo

10-8

~o %

G(m ~0)

/"

I(A)IGT[ (o) 300 °K .i"" i 0-8]__

\

:i

./

o

10.9

/*/

,,,'" . /

\

\

!

rOI0]~_

!

j

%

\

/" /"

~6'3[__~Z__ I0~2

"o

x PULSE "de

/"

,~2 I_

°, o

/,/" (b) 93 °K

fCI I

I __ 10 0

v(v)

ro I

ro 2

t6'

I

I

I

I

I

2

4

6

8

I0

103/T

12

{*K -I)

Fig. 3. Current-voltage plot of a M-SiC-Au sandwich with a 482 A SiC film at (a) 300 °K and (b) 93 °K. Fig. 4. The zero applied voltage d.c. conductance G as a function of temperature. Thin Solid Films, 2 (1968) 79-93

84

T . E . H A R T M A N , J. C. BLAIR, C. A. MEAD

of 5 x 106 V/cm, disruptive breakdown occurs. Neither the low nor high voltage (i.e., V >_ Vmax)have been explored in detail. The high-voltage range is discussed above briefly. The range of greatest interest is Vml. < V < Vma x which will be covered in detail. First, however, a brief discussion of the low-voltage properties will be given. The low-voltage d.c. conductance is given by the initial slope of the curves shown in Fig. 3. Only two of a family of curves at various temperatures are shown. The variation of the initial slopes with temperature is shown in Fig. 4. The hightemperature slope corresponds to A E ,.~ 0.28 eV while the low-temperature slope corresponds to A E ~ 0.06 eV. Figure 5 shows the low-voltage conductance and capacitance measured at 100 kHz. The slope of the conductance curve shown in Fig. 5(a) corresponds to A E "~ 0.027 eV, a value considerably less than those obtained at d.c. The capacitance is a unique function of the conductance as shown in Fig. 5(b).

I031T (*K-I) 4XlO-S

5

'

15

I

(o)

",.

Glmho)

ixlo-~

__

"'.

"°.

%%%

r-

4XIO-7

]

(b) t . f ." S 20

.+O #+" Q

C(pF)

--

0

I

I

I0

0

__

~°

,,"

I 5

I I0

15

G (Frnho) Fig. 5. T h e s m a l l s i g n a l a.c. (a) c o n d u c t a n c e a t 100 k H z as a f u n c t i o n o f t e m p e r a t u r e a n d (b) corresponding capacitance.

Because of the relatively large difference in the conductance between d.c. and 100 kHz the vector or polar impedance of a 790 A film was measured as a function of frequency. Assuming the equivalent circuit at low applied voltages is a resistance R in parallel with a capacitance C, the results are shown in Fig. 6. Thin Solid Films, 2 (1968) 79-93

ELECTRICAL CONDUCTION THROUGH THIN AMORPHOUS S i C FILMS

85

No anomaly in the conductance is observed. Very little tendency toward increased conductance with increasing frequency is observed, certainly nothing like as large a change as observed between d.c. and 100 Hz measurements. The values measured on the 100 kHz bridge are in good agreement as shown in Fig. 6. Therefore, the large change in conductance must occur at a frequency of less than 10 Hz. T(°K)

100

r

200

I

6

""~'°~°~

300

q--

°

I

~°~0~o

n

5 --

tQ)

-

~o~..

4

J

I

--

I (b)

/ °/° io-4

_

/

/

/ io 5 -

-

T

r

~

,

i

o--o--o-o--o--o-~oc?~-o-o-o.o oo~.o.aOo~ Io - ~

104

_

/o

R

C(pF) IO3

/

°\

c

/

"

°"\o~.//

o

° -o-o.Oo.o o"

I0

JO

ro e

=O3

=04 f(Hz)

rO~

=

JO6

¢o- e

]

~oo

I

zoo T(*K)

I

300

Fig. 6. The small signal impedance as a function o f frequency where the equivalent circuit is assumed to be a resistance, R in parallel with a capacitance, C. Fig, 7. A plot o f the parameters (a) n and (b) K as functions o f temperature for a 482 A film,

The intermediate voltage range (i.e., Vmi,~ < V < V,~a~) is the region of greatest interest. Most of the effort in these studies was concentrated on this region. With a few notable exceptions, the results have one for one analogs in the characteristics of granulated bulk SiC varistors as summarized by Ashworth, Needham and Sillars 16. Unless specifically stated otherwise it may be assumed that the characteristics described below for M-SiC-Au sandwiches do have their analog in the varistor characteristics. It is important to note that in Fig. 3 the d.c. data and the short pulse data overlap and are consistent. If the conduction mechanism was space-chargelimited the traps might not respond to the short pulse and the pulsed characteristics Thin Solid Films, 2 (1968) 79-93

86

r . E . HARTMAN, J. C. BLAIR, C. A. MEAD

would tend to follow a trap-free space-charge-limited law, while the d.c. characteristic might be affected by traps 7. No effect of this sort was observed. As mentioned previously with reference to Fig. 3, in the intermediate voltage range the volt-current characteristics are relatively temperature-insensitive. The slopes of the family of characteristics, two of which are shown in Fig. 3, are denoted by n in eqn. (1) which decreases linearly with increasing temperature as shown in Fig. 7(a). The effect of this gradual decrease in n on the V - I characteristic is largely offset by a corresponding exponential increase in the parameter K in eqn. (l) as shown in Fig. 7(b). The combined result of the temperature variations of n and K is a volt-current characteristic which is relatively insensitive to temperature compared for example to the low-voltage conductance which varies greatly with temperature as shown in Fig. 4. The result of this temperature insensitivity is manifest in the small change in the volt-current characteristic even when the electrical power dissipation results in severe heating of the sample under test. This effect is shown in Fig. 8 for a 480 A SiC film where the upper curve was measured by the current-ramp technique i(

l

i0-Z

__

F

T

##-

PULSE • dc. o CURRENTRAMP

i0-3

fO- 4 _

10~

--

o//

I(A)t0-' -

--

o/O d /o

/

-

/

o

I0-7 _

/o~"

#/ **,

/° tO-e ~ . o / °

l

io9l --

[010~_ Ic?q i(] 2

_

.,

_

, ./'/"

_

° ,~.o.~,oo.Oi "

]_

I

]

i0-I

i0 °

i0 I

v(v) Fig. 8. Current-voltage plot for a 780 A SiC film. The upper curve (a) is measured by the currentramp technique. The lower curve (b) is measured by the pulsed technique at high fields and d.c. at low fields.

and the lower curve was obtained by pulsed and d.c. techniques. An average temperature rise of 40 °C was measured during the current-ramp testing while negligible heating occurs during pulsed (and low-voltage d.c.) measurements. Thin Solid Films, 2 (1968) 79-93

ELECTRICAL CONDUCTION THROUGH THIN AMORPHOUS

SiC

FILMS

87

The increased conductance at low voltages during r a m p testing as compared to d.c. testing is not due to heating but rather to a.c. frequency phase shift effect. As previously observed by Suchet 17 for varistors and shown in Fig. 9 for SiC films, for a given voltage (plotted along the ordinate axis) the alternating current I0

1

I

--~'"°'° /

{0Hz

V(Vi'O° --50Hz _o'~S iKH2 / ,'~500 H z !

10_{ ~ , K H z / 0-6

10-5

"---50KHz I0 KHz I

] (A)

10-4

" 1°- 3

Fig. 9. The a.c. volt-current plot as a function o f frequency for a 173 A film.

is generally greater than the direct current and difference increases with frequency but decreases with increasing voltage. The wave forms which attend this observation are shown in Fig. 10. Figure 10(a) was taken at 50 Hz while 10(b) was taken at 10 kHz. Because this effect tends to diminish with increasing voltage it is unlikely to be a heating phenomenon. The resultant phase shift continues to increase with frequency until a very open pattern results as shown in Fig. 10(c) at 30 kHz. Although over 800 samples were involved in these experiments and data on over 50 samples is recorded, the data involved in any particular curve thus far presented was obtained on a particular specimen. Even so, each of the curves is qualitatively characteristic of all the films on which observations were made. Below data will be presented as a function of SiC film thickness. A given curve necessarily involves data obtained from several samples. Analysis of this data is complicated by an inordinate amount of scatter. The origin of the scatter is not known but is apparently a characteristic of this type of amorphous system inasmuch as it has been a problem with the boron system as well 8. Most of the manifestations of the scatter are believed to be secondary effects of the scatter in the values of the parameter n. H o w this comes about is seen in Fig. l l where the room temperature volt-current characteristics for films of various thickness are shown. The thicker films tend to correspond to the upper curves and the thinner films to the lower ones. It should be noted that the slopes or n-values are not uniformly ordered, thus some crossing of the characteristics occurs. As a result the voltage required to produce a given current may be less Thin Solid Films, 2 (1968) 79-93

88

T . E . HARTMAN, J. C. BLAIR, C. A. MEAD

i

Fig. I 0. Oscilloscope traces showing the phase relationship between the voltage and the current at (a) 50 Hz, (b) l0 kHz and (c) 30 kHz. Vertical, 2 V/div. ; horizontal, 5 mA/div. 2

I

Io

I

/

I 32o~-,

" ,//'~292 A

I v (v) , o '

l0 ° L j ~ 10 - 4

~388 / ~44d

, ~

~

/

/

1

~

5od -

1

10-3

~473A

I0 - z

~ 91A I0 - I

I (A) Fig. 11. Volt-current characteristics for SiC films of various thicknesses Between 80 and 800 A.

for a particular film than it is for s o m e other thicker film depending on the corresponding value of n associated with each film. This effect is shown in Fig. 12 which is a section through characteristics of the sort shown in Fig. I l at I = l0 m A Thin Solid Films, 2 (1968) 79-93

89

ELECTRICAL CONDUCTION THROUGH THIN AMORPHOUS S i C FILMS

with the corresponding voltage plotted against the film thickness (or the inverse of the capacitance). In spite of the scatter it is clear from Fig. 12 that the current density in the intermediate voltage range is a function of the applied electric field E where E = V/d. The parameters n and K, for the volt-current characteristics shown in Fig. 1 l, are plotted as functions of film thickness in Figs. 13(a) and 13(b) respectively. The straight lines are the simplest empirical relationships which are consistent with the data in view of the scatter involved.

'1 I

41 20

I00 200 300 400 500 600 700 800 I I I r I I /

/

oo~ o

15

Jo-'

oO O;7o

of,:

v(v)lo

kl°2 I

I\

I=I0 ,,'hA

o 6,/°oO

o/ 10-4t

I0

20

I

I

f

r

30 4O 60 60 70

103/C (pF -I)

'l

0

1

JO0 200

300 400 500 600

700

d (i)

Fig. 12, The voltage required to produce a I0 mA current through SiC films of various thicknesses. Fig. 13, A plot of the parameters (a) n and (b) K as functions of film thickness.

DISCUSSION

The empirical equation given as eqn. (1) is characteristic of the data presented. The functional form of the parameters n and K can be obtained empirically from the plots in Figs. 7 and 13 and given as K = Ko exp (~/T-2d),

(2a)

n = no--TT+fld,

(2b)

Thin'Solid Films, 2 0968) 79-93

90

T.E. HARTMAN, J. C. BLAIR, C. A. MEAD

with the following numerical values of the associated constants )1o = 1 v ,

(2c)

K o = 2.5 x 10 -3 A c m - 2 ,

(2d)

no = 5.6,

(2e)

2

= 1.7x106cm-1,

(20

fl

= 5.0x l0 s c m - t ,

(2g)

r/

= 2 . 0 x 1 0 -3 ° K - i ,

(2h)

y

= 5 . 9 x 1 0 -3 ° K - X .

(2i)

Within the limitations imposed by the scatter in the data, these values appear to be valid over the range of thickness and temperature investigated. The internal consistency of the empirical equations and the parametric values given in eqns. (1) and (2) can be verified by considering the current produced by a constant applied voltage as a function of temperature. Substituting eqns. (2a) and 2(b) into eqn. (1) and rearranging yields In j = [(In Ko) - 2d + (no - fld) In (V/Vo)] + [ t / - 7 In (V/Vo)] T.

(3)

Equation (3) predicts that the logarithm of the current should be a linear function of the absolute temperature, where the magnitude of the zero temperature intercept and the slope are predetermined by the values of the parameters given in eqns. (2c)-(2i). The results are shown in Fig. 14 where the data points were taken with a 478 A SiC film at 12 V. The solid line is the calculated response from eqn. (3), using the numerical values set forth in eqns. (2c)-(2i). The scatter in the experimental data is small due to the fact that measurements on only one film are involved. Nevertheless the agreement is quite good and tends to justify the use of the empirical equations given above. Initial results reported on the conduction through boron films were characterized by a large amount of scatter s which complicated analysis of the data. Subsequently 8 it was reported that a clue to an understanding of the phenomena lies in the thickness dependence of the current which was described by J = Jo exp (~d),

(4)

where Jo = jo(T);

~z = ~(E, V).

A plot of the In j at a constant applied electric field E = V/d as a function of thickness, therefore resulting in a straight line of "positive" slope. The form of eqn. (4) is similar to that found in avalanche multiplication and a(E) was interThin Solid Films, 2 (1968) 79-93

SiC

ELECTRICAL C O N D U C T I O N T H R O U G H T H I N AMORPHOUS

91

FILMS

preted as an ionization coefficient similar to Townsend's first coefficient in a gaseous discharge 18. The empirical equations for SiC given in eqns. (1) and (2) can be rearranged into the form of eqn. (4); unfortunately the ~ derived in this way is a function of both E and d. This result means that eqn. (4) is nonlinear. The effect is more pronounced at small d and arises primarily from the nonzero intercept shown in Fig. 13(a) and given as n o in eqn. (2b). tO°

I

I

I

I

I o o

J0-*

10-; 162 I

I

o oo~. _ dp=0o

I (A)

.as" Qooo°"

t5 ~

ooo

I (A)

o

E =2XI06V/cm

V=12V

16 ¸¸

I

P

100

200 T (~K)

300

16

I

100

I

200

I

300

I

400

I

500

600

d (A)

Fig. 14. The current produced in a 482 A film by 12 V applied as a function of temperature. The points are measured data and the solid linear plot is calculated from eqns. (1) and (2). Fig. 15. The current produced by an applied electric field of 2 × 108 V/cm as a function of film thickness. The points are measured data and the solid curve is calculated from eqns. (1) and (2).

The boron data, previously reported s, can be analyzed in the same manner as the SiC data shown in Fig. 13(a), and results in a nonzero no as well. This observation as well as several other pecularities in the analysis suggest that considerable more work is required before a satisfactory understanding of the phenomenon will be attained. In spite of this inconsistency, one of the more important conclusions which arises from the work on boron films is still valid even for the SiC films. The current through the film at a constant applied electric field increases with increasing film thickness, thus tending to eliminate as an explanation of the phenomenon, space-charge-limited currents as described by Landon and Spear 7 and Thin Solid Films, 2 (1968) 79-93

92

T . E . HARTMAN, J. C. BLAIR, C. A. MEAD

Hartke 7 for amorphous selenium films. This effect is shown for SiC films in Fig. 15. The nonlinearity associated with eqns. (1) and (2) is shown by the solid calculated curve. Considering the characteristic scatter in the data, the fit is best regarded as not in disagreement with the data. It is clear from Fig. 15 that the current tends to increase with film thickness at a constant applied electric field. In conclusion it should be pointed out that the phenomenon probably does arise from an impact ionization process but possibly not of the simplest conceptual type. The increasing current with thickness at constant applied field, the light emission, the relative independence of current on temperature but strong dependence on voltage, all suggest an impact ionization mechanism. Previous results on bulk SiC crystals using surface ~9 and point contact 2° p n junctions are interpreted in terms of space-charge-limited currents and impact ionization respectively.

CONCLUSIONS

Empirical equations describing the electrical conduction through amorphous silicon carbide films in the high electric field region have been presented along with the numerical values of the associated constants which are valid in the thickness and temperature range investigated. The electrical characteristics of M - S i C - A u sandwiches show that they are the thin film analog of pellets of granulated bulk SiC crystals commonly called varistors. The properties may arise from an amorphous surface layer of SiC in both cases. The characteristics of the electrical conduction through thin amorphous silicon carbide films are quite similar to those previously reported for amorphous boron films except for the magnitude of certain parameters. The similarity suggests that the same mechanism may be responsible for the properties cf both systems and possibly in amorphous films of other semiconductors.

ACKNOWLEDGEMENTS

The authors acknowledge with thanks the helpful discussions with R. Stratton, the electron diffraction studies by G. G. Sumner and assistance in the preparation of samples by R. C. Bracken. REFERENCES

W. BRENNER, A chemical approach to the synthesis of silicon carbide, in J. R. O'CONNOR AND J. SM1LTENS (Eds.), Silicon Carbide a High Temperature Semiconductor, Pergamon, London, 1960, p. If0. Thin Solid Films, 2 (1968) 79-93

ELECTRICAL CONDUCTION THROUGH THIN AMORPHOUS S i C FILMS

93

2 F . R . STANSEL,Proc. IRE, 39 (1951) 342; H. F. DIENEL, Bell Lab. Record, 34 (1956) 407. 3 P . K . WEIMER,Phys. Rev., 79 (1965) 171 ; P. D. LANDONAND W. E. SPEAR,Proc. Phys. Soc. (London), 77 (1961) 1157; J. L. HARTKE, Phys. Rev., 125 (1962) 1177. 4 C. BOULT, Proc. Phys. Soc. (London), 80 (1962) 810. 5 C. FELOMAN, Nature, 203 (1964) 964. 6 F. ARGALL, to be published. 7 A . K . JONSCHER, Thin Solid Films, 1 (1967) 213. 8 C. EELDMAN,E. ORDWAY, W. ZIMMERMANAND K. MOORJANI,Vacuum deposited amorphous boron films, in G. K. GAUL~ (Ed.), Boron, Vol. 2--Preparation, Properties and Applications, Plenum Press, New York, 1965, p. 235. 9 E . W . J . MTTCHELLAND R. W. SILLARS,Proc. Phys. Soc. (London), B57 (1949) 509. 10 R. STRATTON, Proc. Phys. Soc. (London), B69 (1956) 513. 11 R. GOFFAUX, Electrical properties of silicon carbide varistors, in J. R. O'CONNOR AND J. SM1LTENS (Eds.), Silicon Carbide a High Temperature Semiconductor, Pergamon, London, 1960, p. 462. 12 D. HOFMAN, J. LELY AND J. VOGLER,Physica, 23 (1957) 236. 13 T . E . HARTMANAND J. S. CHIVIAN, Phys. Rev., 134 (1964) A1094. 14 R . W . SMITFtAND A. ROSE, Phys. Rev., 97 (1955) 1531. 15 C . A . MEAD, Rev. Sci. Instr., 33 (1962) 376, 16 F. ASHWORTH, W. NEEDHAMAND R. W. SILLARS,J. lEE, 93 (1946) 385. 17 J. SUCHET, Les Varistances, Chiron, Paris, 1955. 18 K . L . CHOPgA, Proc. 1EEE, 51 (1963) 941. 19 L. PATRICK, J. AppL Phys., 28 (1957) 765. 20 I . K . VERESHCHAGINAND A. S. KIRICHUK,Phys. Status Soh'di, 9 (1965) 873.

Thin Solid Films, 2 (1968) 79-93