Dielectric properties of thin germanium monosulphide films

Thin Solid Films, 29 (1975) L13 - L16 © Elsevier S e q u o i a S.A., Lausanne - - Printed in

L13 Switzerland

Letter

Dielectric properties o f thin germanium monosulphide films A. STANCHEV and C. VODENICHAROV Department of Physics, Institute of Chemical Technology, Sofia 56 (Bulgaria) (Received July 2, 1975; accepted July 16, 1975)

The knowledge of the dielectric permittivity of a substance, as well as of the equivalent circuit of thin film systems formed with t h a t substance, is important to the solution of a number of problems in the field of electron emission. The purpose of this letter is to report the investigation of the relative dielectric permittivity and the dielectric loss of the thin film system M - G e S - M at different temperatures and frequencies. The semiconductor films were obtained by deposition of germanium monosulphide at an evaporation temperature of 725 K and with a substrate temperature of 300 K in a vacuum of 10 - 5 torr*. The film thicknesses varied from 150 to 600 nm and were measured by means of an interferometer (MII-10) with an accuracy to -+ 10 nm. Vacuum-deposited metal electrodes of AI (6N), Ag (5N), Bi (5N), Cu (4N) and Zn (5N) were used to form the thin film capacitors. The samples had an area of I X 10 - 7 , 5 X 10 - 7 and 1 X 10 - e m 2. The temperature dependences of the capacitance and dielectric loss were studied in the range 150 - 400 K. The dependence of these parameters on frequency was investigated in the range 400 - 4 × 107 Hz. The relative permittivity e and dielectric loss tan 6 of the film were determined by measuring the capacitance of the systems. The values e = 11.7 -+ 0.3 and tan 6 = 0.022 + 0.002 were obtained for freshly prepared samples at 293 K and 1 kHz. After 4000 h in air these values became 10.0 +0.3 and 0.018 + 0.002, respectively, with no further change. The process of aging is accelerated significantly when the samples are heated at 390 - 400 K for 2 h. The present investigations of M - G e S - M systems were performed on thermally treated samples. Figure I shows the dependence on frequency of the dielectric loss at different temperatures with the relative permittivity e normalized to its value at 293 K and 1 kHz. In the temperature range studied, 153 - 393 K, the relative permittivity variation is insignificant (e/% is between 0.95 and 1.05), but it shows a slight tendency to increase as the temperature increases *The c o n d i t i o n s m e n t i o n e d basis o f the results in ref. 1.

for vacuum deposition of GeS were s e l e c t e d o n the

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LETTERS

1 r~ o :~ 1,0

Gt

Gt+

~

~

o.

o 0,9 10 3

frequency [HzJ

I0 ~

Fig. 1. Frequency dependence o f e / e s ( - - - - - - ) and tan 5 ( ) of the M-GeS-M system (e s is the relative dielectric permittivity at 293 K and 1 kHz) at different temperatures (K): a, 153; b, 183; c, 213; d, 243; e, 273; f, 293; g, 323; h, 353; i, 383. Fig. 2. Equivalent circuit of the AI-GeS-A1 system; r is the resistance of the metal electrodes; Gb, G I are the conductivities and Cb, C1 are the capacitances of the semiconductive film and the interfacial regions respectively.

160

% /

~

~°

I0 ~

10 s

I0 ~

2 ~

10 7

rHzn Fig. 3. Juxtaposition o f the theoretical curves o f C and tan 6 calculated from eqns. (1), (2), (3) and (4) with the respective experimental data for the A1-GeS- Al system at 293 K (the thickness o f the film is 300 nm and its plate area is 5 x 10 - 7 m2). frequency

(0.046 - 0.058%/K-1). The dielectric permittivity retains the value of 10.0 -+ 0.3 in the frequency interval from 400 to 10 000 Hz. As seen in Fig. 1, the dielectric loss hardly alters with frequency and temperature changes. The greatest increase in tan 5 is observed between 323 and 383 K, especially at lower frequencies where the loss exhibits an increase of up to 500% with respect to that at 323 K. The frequency dependence of the capacitance and dielectric loss o f the systems could be described satisfactorily over a wide interval up to 107 Hz by the equivalent circuit shown in Fig. 2. A similar but more complicated equivalent circuit for the interpretation of the frequency dependence up to 105 Hz o f the capacitance of the thin film system M - G e S e - M was used by StStzel and Kottwitz 2. The proposed simplified equivalent circuit in Fig. 2 takes into account the conductivity G b and capacitance Cb of the semiconductive film. The

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penetration of metal ions into the film and the availabilityof surface states justify the assumption of the existence of two regions in the vicinity of the metal electrodes which possess conductivity and capacitance different from those of the semiconductor film. The parameters of these two regions are equal, provided the system is symmetric, and are labelled in Fig. 2. The resistance of the metal electrodes was taken into account, since it influences in particular the high frequency characteristics of the systems. Figure 3 shows the experimental data for the capacitance and dielectric loss of the AI-GeS-AI system with a semiconductor film thickness of 300 nm and an area of 5 × 1 0 - 7 m 2 . These data are interpreted in terms of the theoretical curves corresponding to the assumed equivalent circuit. As seen in Fig. 3, a variation in capacitance of the system occurs only in the range 104 - 105 Hz. The capacitance C and tan 8 of the system in the frequency v interval 104 - 105 Hz can be expressed according to the equivalent circuit (and neglecting the resistance r) by the following equations: C = Cb ÷

tan 8 -

(Gb/G)2CI(1 + Gb

2~PC b

(v/vi)2) - I

G1/G + (v/vi) 2

(1)

(2)

1 + (Gb/G)2CI/Cb + (V/Vi) 2

where G = Gb ÷ G~ and vi is the frequency at which C(vi) --Cb = ~ C / 2 . . Of special interest are the variations in C and tan 8, since the parameters of the semiconductive film, as well as those of the interfacial regions, can be determined by using eqns. (1) and (2). For example, for the above-mentioned system, with film thickness 300 nm and area 5 × 10 -7 m 2, the following values of the parameters for the semieonductive film and interfacial regions were calculated: Gb = 8 )( 10 -7 fi--1., Cb = 144 pF, GI = 1 × 10 -7 fi-1 and C~ = 12 pF. The small value of C1, the observed independence of the capacitance with bias voltage, and the commensurate values of Gb and Gl suggest t h a t the electric field is uniformly distributed between both electrodes and t h a t the penetration of metal ions in the film is insignificant. From the capacitance of the contact region, the ion concentration was estimated to be 1021 m -3. The metals Ag, Bi and Cu are exceptions because they evidently possess an increased chemi-diffusion in the semiconductive material. The system formed with these metals showed defects after several hours. In the frequency range 10 5 - 10 7 Hz, C and tan 8 exhibit constant experimental values which are well described by a simplified equivalent circuit including only the parallel conductivity Gb and capacitance Cl of the semiconductive film. At high frequencies above 10 7 Hz, a decrease in C was observed together with a sharp increase in tan 8. These changes are due to the increased influence of the resistance r. The equivalent circuit is composed of the parallel group G b and Cb, and series resistance r. The following expressions are valid for C and tan 6 : *This corresponds to the value obtained by Jabumoto 3 for the specific resistance of thick GeS films.

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LET'rERS C = Cb/(1 +

4u2vZr2C2b)

tan ~ = Gb /2~VCb + 2nVrCb

(3) (4)

T h e value o f t h e resistance r was d e t e r m i n e d f r o m o t h e r e x p r e s s i o n s . F o r t h e s a m p l e e x a m i n e d t h e value o f r was 5 ~ . F o r d i f f e r e n t s a m p l e s t h e value o f r was b e t w e e n 5 a n d 1 0 ~ d e p e n d i n g on t h e m e t a l e l e c t r o d e thickness. 1 A. Stanchev and H. Vodenicharov, Phys. Status Solidi, 27 (1975) 615. 2 H. St~itzel and A. Koltwitz, Conf. on Amorphous Liquid and Glass Semiconductors, Sofia, 1972, p. 107. 3 T. Jabumoto, J. Phys. Soc. Jpn, 13 (1958) 599.