Electron drift mobility in DLC thin films

Thin Solid Films, 235 (1993) 13-14

13

LeRer

Electron drift mobility in DLC thin films W. Mycielski, E. Staryga and A. Lipifiski Technical University of Ldd~, Institute of Physics, W61czahska 219/223, 93-005 Lbd~ (Poland) (Received June 4, 1993; accepted July 23, 1993)

The carder drift mobility is the most important parameter characterizing the electronic transport properties of insulators and high-ohmic semiconductors. It is surprising that there should be no information on the direct measured mobility in relation to the diamondlike carbon (DLC) films, although the other electrical properties of the DLC layers, such as d.c. and a.c. conductivity, have been intensively studied in recent years (see, for example, refs. 1-11). This letter presents our first results for electron drift mobility in thin DLC films. The films investigated were prepared by r.f. glow discharge from methane using technology described elsewhere [12, 13]. The films (thickness, 0.06-0.60 mm) were deposited on platinum, gold or low-resistivity silicon substrates. The applied autopolarization voltage was from 80 to 250 V. For the drift mobility measurements the so-called xerographic or open-circuit technique was applied.This method was first proposed and described by Batra and Kanazawa [14] and its equivalence with the conventional time-of-flight technique was presented by Enck and Abkowitz [15]. In this technique the free surface of the sample is initially charged to voltage Vo. This voltage then decreases owing to the transport of charge carriers. According to the theory for negative charging [14] the V = f ( t ) and d V / d t = f ( t ) dependencies are given by: V( t) = Vo( 1 - t /2tt,) d V / d t = - Vo/Zttr

'~1

i

i x\ 1.6

i

i

i

(2)

V(t) = - (D 2/2/~)( 1/t)

(3)

d V / d t = ( D 2/2/0( 1It2)

(4)

for t > ttr, where ttr is the transit time of the leading charge front, D is the thickness of the sample and Iz = D2/ttrVo is the electron drift mobility. The transit

i

i

i

i

i

r

i

T

I

I

~

i

I

i

t

~

i

i

r

I

\

...... ~,. . . . . . . . . . . . . . . . ~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

\::

(1)

for t < tt,, and

0040-6090[93/$6.00

time may be determined from eqn. (1) (ttr is the time when the voltage has decayed to half its initial value) or from eqn. (2) using the value of d V / d t , initially independent of time. In our experiment the samples were placed in vacuum and charged by a low-energetic ( ~ 1 keV) electron beam (in refs. 14 and 15 a corona charging devices were used). The decaying surface voltage V(t) was measured using an FET preamplifier connected to a 20 MHz digitizing transient recorder MC 101 (MesComp, Poland). The transient recorder was interfaced with AT computer for registration of the V(t) dependencies and their further numerical analysis. The apparatus was tested using amorphous layers of selenium, which is very convenient for xerographic measurements. The obtained electron drift mobility 7.0 x 10 - 3 c m 2 V - 1 s - ] at room temperature is in very good agreement with that reported by other authors ( ,~ 5 x 10 -3 cm 2 V -] s -l) [16-18]. Figure 1 displays a typical discharge curve V(t) for DLC films (D = 0.6 mm, Ve = 250 V) and Fig. 2 shows a derivative d V / d t for the same sample. As can be seen, the shape of these dependencies is in good agreement with the theoretical predictions, i.e. the derivative d V / d t for t < ttr is practically time independent (eqn. (2)). Numerical calculations for t > ttr have shown that V(t) ,~ lit and d V / d t ~ lit 2 according to eqns. (3) and (4) with a correlation coefficient greater than 0.99 and 0.95, respectively. Table 1 presents the mean values of the electron drift mobility kt and the dark conductivity s for the DLC

\

o.e

...............

~. ................ \

\ 4

]

6

8

10

12 (X tO0)

Fig. I. The time dependence o f surface voltage V(t) f o r diamond-like film at r o o m temperature (D = 0.6 m m , V¢ = 250 V).

© 1993 - - Elsevier Sequoia. All rights reserved

Letter

14 (X

~.£'-41

discussed in further publications after more detailed investigation.

Acknowledgments The authors wish to thank Prof. A. Sokotowska and Dr S. Mitura for many useful and interesting discussions. The work was partly supported by Grant No. 3 3602 91 02 of the Polish State Committee for Scientific Research.

-3, , N

....................

0

t0

4

.t ×1i200)

References

t [~sI Fig. 2. The time dependence of the derivative d V[dt for the curve in Fig. 1.

TABLE 1. The electron drift mobility and conductivity in DLC films Autopolarization voltage lie (V)

Electron drift mobility /z (cm2V -1 s -l)

Conductivity a (~-1 cm-1)

80 150 250

1.6 x 10 -6 2.3 x 10 -6 8.0 x 10 -6

4 x 10 -t3 7 × 10 -I° 6 × 10 -9

films prepared at different autopolarization voltages lie. The values of s are similar to those previously reported (e.g. [2, 4, 5]), All measurements were carried out at room temperature. N o influence of the substrate material on the experimental results was observed. The electron drift mobility values obtained in our experiment are from 1 x 10- 6 to 8 x 10- 6 cm 2 V - 1 s - 1. These mobilities in D L C films are very low in comparison with those reported by other authors for diamond films (DF) ( ,~ 10/50 cm 2 V-1 s - i ) [19-21]. It is evident that this difference is a result of the quite different structure of diamond-like and diamond films. The values of mobility of the order of 10-7 x 10--6 cnl2 V-- 1 S-- ! are rather typical for polymers [22-24]. This fact leads to the conclusion that the structure and transport mechanism in the D L C films obtained at an autopolarization voltage of 80-250 V and diamond films (DF) are quite different. This problem will be

1 J. D. Lamb and J. A. Woollam, J. Appl. Phys., 57(1985) 5420. 2 Z. Ha~, S. Mitura, M. Clapa and J. Szmidt, Thin Solid Films, 136 (1986) 161. 3 E. Staryga, A. Lipifiski, S. Mitura and Z. Ha~, Thin Solid Films, 145 (1986) 17. 4 C. V. Deshpandey and R. F. Bunshah, J. Vac. Sci. Technol..4, 7 (1989) 2294. 5 K. W. Whang and H. S. Tae, Thin Solid Films, 204 (1991) 49. 6 J. Robertson, Prog. Solid St. Chem., 21 (1991) 199. 7 B. Bhushan, A. J. Kellock, N. H. Cho and J. W. Ager III, J. Mater. Res., 7 (1992) 404. 8 J. Seth, M. I. Chaudhry and S. V. Babu, J~ Vac. Sci. Technol. A, 10 (1992) 3125. 9 K. K. Chan, S. R. P. Silva and G. A. J. Amaratunga, Thin Solid Films, 212 (1992) 232. 10 O. Stenzel, G. Schaarschmidt, M. Vogel, R. Petrich, F. Wolf, T. Wallendorf, F. Scholze and W. Scharff, Diamond Related Mater., 1 (1992) 434. 11 T. Sugino, Y. Muto, J. Shirafuji and K. Kobashi, Diamond and Related Materials, 2 (1993) 797 . 12 Z. Ha~, S. Mitura and B. Wendler, Proc. Int. Ion Eng. Congress ISIAT-IPAT '83, Kyoto, 1983, p.1143. 13 Z. Ha~ and S. Mitura, Thin Solid Films, 128 (1985) 353. 14 I. P. Batra and K. Keiji Kanazawa, J. Appl. Phys., 41 (1970) 3416. 15 R. C. Enek and M. Abkowitz, J. Non-Cryst. Solids, 66(1984) 255. 16 W. E. Spear, Proc. Phys. Soc., 76 (1960) 826. 17 W. E. Spear, J. Non-Cryst. Solids, 1 (1969) 197. 18 J. I. Hartke, Phys. Rev., 125(1962) 1177. 19 K. Okano, H. Kiyota, T. Iwasaki, Y. Nakamura, Y. Akiba, T. Kurosu, M. Iida and T. Nakamura, `4ppl. Phys. A, 51 (1990) 344. 20 C. A. Hewett and J. R. Zeidler, Diamond and Related Mater., 1 (1992) 688. 21 A. T. Collins, Mater. Sci. Eng. B, 11 (1992) 257. 22 Y. Kanemitsu and J. Einami, `4ppl. Phys. Lett., 57 (1990) 673. 23 L. B. Schein, A. Peled and D. Glatz, J. `4ppl. Phys., 66(1989) 686. 24 M. Stolka and M. A. Abkowitz, J. Non-Cryst. Solids, 97&98 (1987) l l l l .