Some studies of thin film diodes utilizing cadmium sulphide and sulphur

Thin Solid Films, 26 (1975) 213-220 © ElsevierSequoia S.A., Lausanne--Printed in Switzerland

213

SOME STUDIES O F T H I N F I L M D I O D E S UTILIZING CADMIUM SULPHIDE AND SULPHUR

c. K. CAMPBELLAND C. H. MORGAN* Electrical Engineering Department, McMaster University, Hamilton, Ontario (Canada)

(ReceivedJuly 9, 1974;accepted July 22, 1974)

The fabrication and electrical characteristics o f vacuum-evaporated thin film A1--CdS-A1, A1--CdS-Au, Au-CdS-A1 and A u - C d S - A u Schottky barrier diodes are described; the CdS films were produced by evaporating a mixture o f CdS and $2 powders from a single source onto a heated substrate. Particularly good diode action is observed for AI-CdS barriers, despite the fact that these are normally ohmic. Diode action is observed in each case between the underlying metal and the semiconductor, and this is attributed to an excess o f semiconductor acceptor surface states at such interfaces. Results are exemplified for the four configurations at 20 °C, which exhibit (a) reverse-forward resistance ratios o f about 105, (b) reverse voltage capabilities o f 8-10 V, (c) space-chargelimited current behaviour at forward voltages above about 1 V, and (d) voltage dependences as high as Vs at lower voltages. Measurements on the A1-CdS barriers yielded a barrier height ~bB, = "-~0.7 eV.

INTRODUCTION Investigations have been reported in recent years 1-4 of thin film Schottky barrier diodes o f the type metal 1--semiconductor-metal 2 (M1-S-M2) which have a blocking contact at one interface and an ohmic contact at the other. (In the notation used here, M1 represents the first-deposited underlying metal film.) Particular attention has been given to such diodes incorporating n-type polycrystalline cadmium sulphide. Unless special precautions are taken, dissociation o f the CdS during vacuum evaporation ordinarily causes a cadmiumrich layer to be formed initially, which yields an ohmic contact 5 with the underlying metal (M1-S). Diode action is then obtained by the formation o f a top blocking contact S-M2, using a metal M2 with a suitable work function. Ohmic contacts M1-S have been reported using aluminium, chromium, indium and gold 1, while blocking contacts S-M2 have been obtained using large work function metals such as palladium, gold or platinum 3. Present address: Electrical Engineering Department, Mohawk College of Applied Arts and Technology, Hamilton, Ontario, Canada.

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It should be noted, however, that this use of a large work function metal is not essential for the establishment o f a blocking contact. It may be shown 6 that in the presence o f a large density Ds c m - 2 eV- 1 of semiconductor acceptor surface states the barrier height ~ban at the metal-semiconductor interface will tend to be independent o f the metal work function; ~ban will be determined instead by the doping and surface properties o f the semiconductor. As will be indicated, the successful operation o f the various diode configurations described below is attributed to such influences. Vacuum evaporation methods that have been employed to control the stoichiometry o f the deposited CdS film include multiple direct and indirect evaporation techniques in which (a) CdS and $2, or (b) Cd and $2, are evaporated from separately controllable sources 7-~°. The stoichiometry and resistivity o f the deposited CdS film will still be significantly affected, however, by the substrate temperature governing the temperature-sensitive sticking coefficients 1~ o f Cd and $2. de Klerk ~2 has observed that sulphur, vacuum evaporated by itself, will not stick to the substrate if the substrate temperature Ts> 50 °C. On the other hand, cadmium is found to stick preferentially if T~>200 °C. If both species are present, a CdS film will be formed if Ts is in the range 50°-200 °C. This temperature sensitivity has prompted the use o f another method for controlling the resultant CdS film stoichiometry, in which free sulphur is vacuum deposited onto a cooled substrate prior to the evaporation o f CdS. Barrier contacts M1-S with gold were reported with this method 3. In this paper, the authors report on the fabrication and electrical characteristics o f various CdS-based thin film Schottky barrier diodes, using a variation o f the above-described processes. Here, we evaporated a mixture o f CdS and $2 powders from a single source onto a heated substrate to establish a blocking contact at interface M 1-S and an ohmic contact at interface S-M2. For comparative purposes, various diode combinations of the type A1-CdS-A1, AI-CdS-Au, Au-CdS-A1 and A u - C d S - A u were prepared in the same evaporation cycle using an Edwards-type sequential mask-changing apparatus ~3. The blocking contact was attributed to the M 1-S interface in each case---even with aluminium, which is normally used as an ohmic contact 14, ~s. The high quality blocking contacts thus obtained were attributed to the influence o f a large density of sulphur-induced semiconductor acceptor states at the M1-S interface. Results are exemplified for A1-CdS-A1, AI-CdS-Au, Au-CdS-AI and A u - C d S - A u diodes. At 20 °C, typical forward resistances of 100 f~ were obtained with reverse-forward resistance ratios o f 105 and reverse voltage capabilities of 8-10 V. Current-voltage dependences as high as V 8 were observed at low forward bias voltages, changing to space-charge-limited current behaviour typical o f one-carrier injection at voltages above about 1 V. EXPERIMENTAL METHOD

Flame-polished cover glass slides o f dimensions 5 cm x 5 cm x 0.1 cm were employed as substrates. Before their use, they were outgassed at 200 °C in a pres-

THIN FILM DIODES USING C d S A N D S

215

sure of 1 torr for 1-2 h. This was followed by successive cleaning with (a) a commercial cleanser, (b) distilled water and (c) methyl alcohol. The vacuum coating unit used was an Edwards 19E2 type and sequential mask-changer with six filament, six mask and six substrate positions 13. Evaporation masks of 0.005 in. stainless steel or brass foils were used, and mask filament tolerances were held to 0.001 in. With this system A1-CdS-A1, A1-CdS-Au, Au--CdS-A1 and Au--CdS-Au diodes and a CdS resistance monitor track could be vacuum deposited in a single evaporation cycle without breaking vacuum. A sketch o f the configuration is shown in Fig. 1. Working pressures o f 10-s-10 -6 torr were employed. In the successive deposition procedure the substrate temperature was first raised to 200 °C, and the lower aluminium and gold electrodes M1 were then deposited through respective masks to a thickness o f 600 A. Both metals were evaporated at rates o f about 10/~ s -1 at 10 - 6 torr. Upper

Electrodes

/ \,

< lower

I

J

Electrodes.

N

I

/ CdS Resistonce Yrock

Fig. 1. Sketch of A I - C d S - A u , AI-CdS-AI, A u - C d S - A I , A u - C d S - A u diodes and a CdS monitor track. Strip lengths 22 m m × 0.8 ram. Junction area 0.64 m m 2.

The cadmium sulphide and sulphur mixture next evaporated was prepared using RCA electronic grade CdS powder. In the course o f evaporating about 100 diode configurations it was observed that a mixture o f 1 part sulphur to 30 parts CdS by weight yielded the best diode characteristics. The single evaporation source used for the mixture was an alumina crucible, heated from above by a tungsten filament. The temperature o f the CdS-S2 source was raised very slowly and an outgas time o f at least 45 rain was employed, during which time the pressure was held to below 10-* torr. With the substrate temperature held at 200°C a film o f 15000 A was deposited at a rate o f 8 A s -1 at 10 -5 torr. The resistance o f the sulphur-affected CdS monitor track (17 m m x 1 mm) was measured throughout this deposition. While this resistance could not be directly related to diode resistance, a 5 Mf~ final value was usually indicative o f a good diode. Following the CdS film deposition, the substrate was allowed to cool to room temperature in vacuum over a period o f several hours. The upper electrodes were then deposited under the same conditions as for the lower electrodes except that the substrate temperature was now held at 20 °C.

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CAMPBELL, C. H. MORGAN

RESULTS

Typical I - Vcharacteristics are shown in Fig. 2 for the A1--CdS-A1, A1--CdS-Au, Au-CdS-A1 and A u - C d S - A u diode combinations described. These diodes were measured at atmospheric pressure in a dark enclosure at 20 °C. In each case the diode became forward biased with a positive voltage applied to the lower electrode M1. Diodes with lower electrodes o f aluminium, as in Fig. 2 (a) and (d), displayed similar characteristics, in which the knee o f the I - V curve occurred at about 1.4 V. F o r the diodes with a lower gold electrode, as in Fig. 2 (b) and (c), similar I - V curves were again obtained, with a knee at about 1.8 V. No similarity was noted between Fig. 2 (a) and (b) with a c o m m o n aluminium upper electrode, or between Fig. 2(c) and (d) with a c o m m o n gold upper electrode. This would indicate, therefore, that the rectifying barrier is formed at M 1-S in each instance. (a)

(b)

6 mA 4-

AI-CdS-AI 2

l

Au-CdS-AI

I--V

i

-I

(d) AI-CdS-Au

mA

(c)

6- mA

41

Au-CdS-Au

4

2-

,i

i

-8

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6t°,

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-I Fig. 2. I - V characteristics at 20°C o f (a) A1-CdS-A1, (b) Au--CdS-A1, (c) A u - C d S - A u a n d (d) A I - C d S - A u thin film diodes.

Following an argument by Neugebauer et aL 11, one may postulate that the ratio o f Cd to $2 in the resulting CdS film will be a function o f the adsorption energy Fad o f Cd and $2 on the substrate. If ~ d ( C d ) > F~d(S2) then the excess o f cadmium atoms and donor states will become smaller as the substrate temperature is increased. Indeed, we may additionally postulate that in our depositions we actually have a non-stoichiometric M 1-S interface layer due to an excess o f sulphurinduced semiconductor surface acceptor states. This could be due to the combined effects o f the substrate temperature o f 200 °C employed here, and an initial excess of sulphur deposited from the CdS-S2 source. In support of this argument, it was noted that the knee of the I - V curves was reduced to about 0.2 V forward bias when the free sulphur was not added to the CdS source, while the maximum reverse voltage capability o f such diodes was also reduced to about 2 V. A lower forward voltage knee indicates a lower barrier height, which is consistent with the postulated surface state control influence. Further, ohmic contacts formed

THIN FILM DIODES USING C d S AND S

217

with the upper conductors can be attributed to the suggested existence of a heavily doped n-type CdS upper surface, which could have resulted at the end of the CdS-S2 evaporation when the excess sulphur was driven off, leaving the upper CdS film surface in a complementary cadmium-rich non-stoichiometric state. The Schottky barrier performance of the diodes was examined with the aid of the low frequency dynamic capacitance C - V relation for reverse-biased diodes with a uniform impurity content, namely C - 2 = 2 (VB'-~ V D ) / ~ E Nd

(1)

in m.k.s, units where C is the capacitance per unit area, VB is the reverse bias voltage, VD the diffusion potential, e = 80er is the total dielectric permittivity and N d the density of donor electrons. A plot of C-2 versus V as shown in Fig. 3 confirms the Schottky barrier relationship. The non-linearity around 0 V, however, is large compared with results given by other workers for uniform doping profiles. If the more general C- V relationship I 6 d (C- 2) - 2 d VB q e N0,)

(2)

is considered, where N(2) is now a spatially dependent donor density parameter expressed as a function of an equivalent barrier width 2, it may be postulated that the slope variations in Fig. 3 are consistent with the arguments favouring acceptor surface state contributions. An estimate of the effective barrier height of the A1-CdS diode junctions at 20 °C can be made by recourse to the log I - Vplot of Fig. 4. The thermal diffusion relationship I/C t

\

5XlO Iv

\ 5

AI-C :IS-A

2

-I

-,5

0 VOLTS

.5

I

Fig. 3. C -2 vs. Vfor an A1-CdS-Au diode.

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C. K. CAMPBELL, C. H. MORGAN

I = Jo A {exp ( q V / n k T ) - 1}

(3)

where Jo is the saturation current density and A the junction area (0.64 ram2), as well as 47tm~ q k 2 T 2 exp (-qdpsn/kT) J0 = ha (4) are used in conjunction with the extrapolated value of Jo from Fig. 4 to obtain an estimated A1-CdS barrier height for these specimens of 4) s , = 0 . 7 eV, using an effective mass ratio m , * / m = 0.27. Similar barrier height values were obtained for the Au-CdS diodes, in good agreement with results obtained by other workers for such junctions aT. The constant n in eqn. (3) was evaluated as n -- 5.2. Measurements on other diodes, however, showed n varying from 5 to 8. These values of n are about a factor o f 2 larger than those found for single-crystal diodes la. The zero voltage depletion layer thickness was estimated to be 230 A for the A1--CdS diodes. 1000

/ /

l

I00

IO

t.)

1.0

O.I

0.01

0

.2

.4

.6

.8

1.0

1.2

1.4

0,1

1.0

.... i V I0

Volts

Fig. 4. Log I - V characteristic for a forwardbiased A 1 - C d S - A u diode.

Fig. 5. I - V characteristic for an A u - C d S - A I diode showing space charge current limiting.

THIN FILM DIODES USING C d S AND S

219

F r o m eqn. (1) a n a p p r o x i m a t e value o f d o n o r c o n c e n t r a t i o n N d , ~ 6 x 1017 c m - 3 at 20 °C m a y be o b t a i n e d f r o m a l i n e a r i n t e r p o l a t i o n in Fig. 3. T h e m e a s u r e d specific c o n t a c t resistance 19 /~J~- 1 Re = \~--V,]o=o = 6 x 10 a f~ c m 2 for the AI--CdS j u n c t i o n s was consistent with p r e d i c t e d values 19 using these .xm a g n i t u d e s o f N d a n d m n. C u r r e n t - v o l t a g e d e p e n d e n c e s as h i g h as I/8 were o b t a i n e d at l o w v o l t a g e s ( < 1 V) for m a n y o f these d i o d e s , w i t h s p a c e - c h a r g e - l i m i t e d b e h a v i o u r t y p i c a l o f o n e - c a r r i e r i n j e c t i o n a t h i g h e r voltages. F i g u r e 5 d e m o n s t r a t e s one t y p i c a l result for a n A u - - C d S - A 1 d i o d e . T y p i c a l r e v e r s e - f o r w a r d resistance r a t i o s o f 10 5 were also o b t a i n e d for the d i o d e s e x a m i n e d , with reverse voltage c a p a b i l i t i e s o f 8-10 V at 20 °C. CONCLUSIONS Studies have been c o n d u c t e d o n v a r i o u s thin film S c h o t t k y b a r r i e r d i o d e c o n f i g u r a t i o n s f a b r i c a t e d f r o m a C d S - S 2 single e v a p o r a t i o n source. D i o d e b l o c k i n g a c t i o n was e s t a b l i s h e d as being b e t w e e n the u n d e r l y i n g m e t a l a n d the s e m i c o n d u c t o r in e a c h c a s e - - e v e n with a l u m i n i u m , w h i c h is n o r m a l l y used as a n o h m i c c o n t a c t . B l o c k i n g c o n t a c t s t h u s e s t a b l i s h e d a r e a t t r i b u t e d to a large d e n s i t y o f s u l p h u r - i n d u c e d s e m i c o n d u c t o r a c c e p t o r surface states a t such interfaces. ACKNOWLEDGMENTS This w o r k was s u p p o r t e d in p a r t b y the N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a to w h o m a p p r e c i a t i o n is expressed. REFERENCES 1 P.K. Weimer, in L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGrawHill, New York, 1970, p. 20-7. 2 R. Zuleeg, Solid State Electron., 6 (1963) 193. 3 J.A. Scott-Monck and A. J. Learn, Proc. IEEE, 56 (1968) 68. 4 A.S. Volkov, Yu. A. Gold'Berg, D. N. Nasledov, N. G. Neronova and Z. A. Saimkulov, Soy. Phys. Semiconductors, 6 (1973) 1987. 5 P.K.C. Pillai and S. K. Arya, Solid State Electron., 14 (1971) 1299. 6 S.M. Sze, Physics o f Semiconductor Devices, Wiley-Interscience, New York, 1969, p. 375. 7 N.F. Foster, in L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGrawHill, New York, 1970, p. 15-5. 8 G. Le Floch, Solid State Electron., 15 (1972) 753. 9 F.V. Shallcross, RCA Rev., 28 (1967) 569. 10 F.A. Pizzarello, J. Appl. Phys., 35 (1964) 2730. 11 C. A. Neugebauer, D. C. Miller and J. W. Hall, Thin Solid Films, 2 (1968) 58. 12 J. de Klerk, in W. P. Mason (ed.), Physical Acoustics, Vol. 4A, Academic Press, New York, 1966, p. 195. 13 L. Holland, in L. Holland (ed.), Thin Film Microelectronics, Chapman and Hall, London, 1965, p. 168.

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14 D . D . M . Allan, A. J. Hay and M. A. Reid, Solid State Electron., 16 (1973) 951. 15 D . N . Nasledov, Yu. V. Protasov and A. P. Rumyantsev, Soy. Phys. Semiconductors, 3 (1970) 968. 16 H. K. Heniseh, Rectifying Semiconductor Contacts, Oxford Univ. Press: Clarendon Press, London, 1957, pp. 214-216. 17 R. Kohler and L. Wauer, Solid State Electron., 14 (1971) 581. 18 P . M . Webb and G. T. Wright, Brit. J. Appl. Phys., 15 (1964) 385. 19 C.Y. Chang, Y. K. Fang and S. M. Sze, Solid State Electron., 14 (1971) 541.