Thin Solid Films, 156 (1988) 345-350

GENERAL FILM BEHAVIOUR 345

STUDY OF C R Y S T A L L I Z A T I O N IN A M O R P H O U S T E L L U R I U M FILMS USING RESISTIVITY MEASUREMENTS

K. OKUYAMA AND Y. KUMAGAI

Department Of Electrical Engineering, Faculty of Engineering, Yamagata University, 992 Yonezawa (Japan) (Received September 30, 1986; revised July 24, 1987; accepted September 29, 1987)

A rapid and drastic decrease in resistivity as a result of the crystallization of amorphous tellurium films deposited onto a glass substrate at 10-40°C was measured. On combining the above results with a model proposed for the change in resistivity of the crystallizing film, the activation energy for crystallization was evaluated to be 0.86 eV for pure tellurium films. A higher value of the activation energy was obtained for gold-nucleated tellurium films.

1, INTRODUCTION

A semiconducting tellurium film evaporated onto a substrate held at temperatures below about 50°C is amorphous at least in the early stage of deposition. The phase transition from the amorphous to the crystalline state takes place during deposition or immediately after the completion of deposition ~. It is known that small islands of metals such as gold 2-5, silver and tin 6 predeposited onto a substrate enhance markedly the final grain growth of tellurium films deposited onto them at substrate temperatures below about 50 cC. This fact suggests that such island materials affect the crystallization process of amorphous tellurium films. One possibility might be to reduce the activation energy of crystallization.

The purpose of this paper is to evaluate the activation energy necessary for the crystallization of amorphous tellurium films evaporated onto a glass substrate with and without predcposited gold islands. A decrease in the resistivity as a result of the crystallization is measured as a function of time at a constant substrate temperature between 10 and 40 °C. On the basis of a simple model proposed for the change in resistivity caused by crystallization, values of the activation energy are obtained for tellurium films both with and without gold islands.

2. MODEL

We consider an amorphous film in which are embedded crystallites in the shape of a disk whose radius is growing laterally at a rate u. For simplicity, we assume that

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346 K. O K U Y A M A , Y. K U M A G A I

the resistivity of the film during crystallization is given by

P = P a 1 + p o - - - - ~

nNu 2 = pa+ - V ~ ( p ~ - p a ) t 2 (1)

where p, is the resistivity of the amorphous film, Pc the resistivity of the crystallites, N the number of the crystallites, t the time since the start of the crystallization and S the total area of the film surface. Equation (1) gives p = Pa at t = 0 and p = po at the time when the whole of the film is crystallized, i.e. at t = (1/u)(S/xN) ~/2. The number of the crystallites is assumed to be constant before the coalescence of crystallites occurs. In the coalescence stage, the number of the crystallites is no longer constant but decreases with time. However, this does not affect eqn. (1) because the dominant factor in this equation is the total area of the crystallized region; the decrease in the number ofcrystallites as a result of coalescence is compensated by the increase in the area of crystallites. From eqn. (1), a p vs. t 2 plot is expected to yield a straight line and from its slope we obtain the value (rtNu 2/S)(p~ - Pa). The rate of lateral growth of the crystallites may be expressed as 7

u = A exp kT (2)

where A is a constant, Ea is the activation energy for crystallization, k the Boltzmann constant and T the absolute temperature. Thus we have

rtNu 2 nN ( ~T j2Ea'] T ( p c - - P a ) ~-- - T ( p c - - P a ) A 2 exp -- (3)

If a logarithmic plot is made of ( 1 [ N u 2 / S ) [ P c - - P a l against the reciprocal tempera- ture, the activation energy will be obtained from the slope.

3. E X P E R I M E N T A L P R O C E D U R E

The vacuum system consisted of an oil diffusion pump with a liquid nitrogen trap which gave a vacuum of 3 × 10-4 Pa during deposition. Tellurium (99.9999~o in purity) was evaporated from a BeO crucible heated by a tungsten conical basket heater. Two glass substrates, one predeposited with gold islands and the other without them, were mounted side by side on a copper plate with Apiezon grease. The surface coverage of the gold islands was about 5/o.°/ The substrate was kept at a constant temperature in the range 10-40 °C using a tungsten heater set above the copper plate and a liquid-nitrogen-cooled pipe soldered to the plate. The substrate temperature was measured at the back face of the substrate with a chromel-alumel thermoeouple which was inserted between the substrate and the copper plate. The deposition rate and thickness of the tellurium films were in the ranges 40-120 nm rain-1 and 75-165 nm respectively. To measure the resistivity of the films during and after deposition, electrodes of gold, which is known to yield an ohmic contact to tellurium films 8, were predeposited. The change in resistivity of the

CRYSTALLIZATION OF a-Te FILMS 347

film was evaluated by measuring the film current, assuming that the film thickness increased linearly with deposition time. The film current was limited to 50 ~tA by a series resistor.

4. RESULTS

The changes in resistivity of tellurium films (of final thickness 75 nm) deposited simultaneously onto glass substrates with and without predeposited gold islands are shown in Fig. 1. The substrate temperature during deposition is 23 °C. As seen in Fig. 1, an abrupt decrease in the resistivity occurs at 42 s and 35 s after the start of the deposition for the films with and without gold islands respectively. This large decrease in the resistivity is believed to be due to the phase transition from the amorphous to the crystalline structure 1. The change in resistivity of the above samples is replotted against ( t - t o ) 2 in Fig. 2, where t o is the time at which the sudden decrease in resistivity takes place. Similar plots are shown for tellurium films deposited at 11 °C, 28 °C and 40 °C in Figs. 3, 4 and 5 respectively. The slopes of the p VS. (t to) 2 plots, which correspond t o ( l~Nu2/S) (pc - -Pa) , are depicted as a function of the reciprocal temperature in Fig. 6. For pure tellurium films, the experimental data fall nearly on a straight line and so we obtain an activation energy of 0.86 eV from its slope. In contrast, for the film with gold islands the scatter of the data is relatively large. Hence the activation energy of 1.1 eV evaluated from the assumed curve is less reliable.

1500

E C9

1000

500

i mmm t o •

A

t o

a 1500

1000

500

\ \:

10 20 30

( t - to) 2 (.S 2)

• zx

O, I 1 % 0 I ~ 0 30 35 40 45 50

t (s) 4o

Fig. 1. Res i s t i v i tyo fpu re ( . ) andgo ld -nuc lea t ed (A) tellurium films during deposition (deposition rate, 75 nm min 1; final film thickness, 75 nm; substrate temperature, 23 :'C). t o is the time when crystallization is assumed to start.

Fig. 2. Dependence of the resistivity on ( t - to ) 2 for pure ( I ) and gold-nucleated ( ~ ) tellurium films deposited at 23 °C (replotted from data in Fig. I).

348 K. O K U Y A M A , Y. K U M A G A I

1500

L~

~.1001 I ~

1000

k)

~.~

5oc

o I o 5o00 10000 30

( t - t o ) 2 (S 2 ) (S 2)

~ A A

10 20 ( t - t ~ ) 2

4 0

Fig. 3. Dependence of the resistivity on ( t - t o ) 2 for pure ( i ) and gold-nucleated (/X) tellurium films deposited at 11 °C (deposition rate, 60 nm min - ~ ; film thickness, 132 nm).

Fig. 4. Dependence of the resistivity on (t to) 2 for pure ( i ) and gold-nucleated (A) tellurium films deposited at 28 C (deposition rate, 102 nm m i n - ~; film thickness, 165 nm).

500 !

400

300

~- 200

100 ~

0 0

B

10

5

,~ 2 03

E 102 (J

~2 2

@ lo

5

1

2 zx

i lo -~ I I I I 5 10 3.1 3.9 3-3 3.4 3.5 3.6

( t - to) 2 (s 2 ) 1 / ~ CK -~ ) xlO -3

Fig. 5. Dependence of the resistivity on ( t - t o ) 2 for pure ( i ) and gold-nucleated (A) tellurium films deposited at 40 °C (deposition rate, 126 nm min - t ; film thickness, 127 nm).

Fig. 6. Temperature dependence of(nNu2/S)lpc Pal: el, pure tellurium; A, gold-nucleated tellurium.

CRYSTALLIZATION OF a-Te FILMS 349

5. DISCUSSION

As seen in Figs. 2-5, the p vs. ( t - to) 2 relation can be approximated by a straight line in the early stage of crystallization. In the later stage, however, the experimental data deviate strongly from a straight line. This deviation may be due to the oversimplified resistivity model in which we assumed a single activated process. Hence the interpretation and subsequent conclusions concern the early stage of crystallization, where the experimental data fit the expected straight line relatively well.

From the curves assumed in Fig. 6, values of the activation energy for the crystallization of pure and gold-nucleated tellurium films were obtained and are 0.86 eV and 1.1 eV respectively. These values are comparable with the values of 0.4-1.1 eV obtained for antimony films under similar conditions 9. Comparison of the activation energy for tellurium film with values in the literature cannot be done because, to our knowledge, literature data are lacking. Kinbara e t al. ~ investigated the relation between the thickness of antimony films and the critical temperature at which the crystallization occurs, and they obtained a value of 0.5 eV for the activation energy. For antimony films the critical thickness below which the films remain amorphous at room temperature has been reported to be 10-20 nm I°. As for tellurium films, however, no evidence was obtained that suggests the amorphous state persists at room temperature even at thicknesses below 5 nm.

Dutton and Muller 2'3 found that gold islands predeposited onto a glass substrate could considerably enhance the grain growth of a tellurium films deposited onto them at room temperature. The grain diameters of the gold- nucleated tellurium films exceeded 5 ~m at a thickness of 40 nm. These values are one order of magnitude larger than those for films deposited onto clean glass. Okuyama et al. 4 showed that there is an optimum condition in the coverage of gold islands which is most effective in promoting tellurium grain growth and that gold islands (5-10 nm in diameter) make an alloy with tellurium. Islands of tin and silver were also found to stimulate tellurium grain growth when the island materials were not exposed to air 6. Alloys were formed between the island materials and tellurium. When such islands were exposed to air before the evaporation of tellurium, neither alloy formation nor any promotion of tellurium grain growth occurred. Since the gold islands never enhanced tellurium grain growth when the substrate temperature during tellurium deposition was raised above 60 °C (tellurium vapour condenses as a crystalline solid on the substrate under this condition), gold islands or Au-Te alloy are only effective in the crystallizing process from the amorphous state. Taking these facts into consideration, a decrease in the activation energy for gold-nucleated tellurium films was expected. Contrary to the expectation, however, the activation energy obtained for gold-nucleated films appears to be higher or at least no lower than that for pure tellurium films. Consequently, the reason why gold islands promote the grain growth of a tellurium overlayer remains unclear.

6. CONCLUSIONS

A simple model was proposed for the change in resistivity of an amorphous film

350 K. OKUYAMA, Y. KUMAGAI

caused by crystallization. On the basis of the above model and of resistivity measurements at various temperatures, the activation energy for crystallization was evaluated for pure and gold-nucleated tellurium films to be 0.86eV and 1.1 eV respectively.

ACKNOWLEDGMENTS

The authors wish to thank Y. Kobayashi and H. Nakamura for their assistance with the experimental work.

REFERENCES

1 K. Okuyama, M. Chiba and Y. Kumagai, Jpn. J. Appl. Phys., 18 (1979) 507. 2 R .W. Dut tonandR. S. Muller, Proc. lEEE, 59(1971)1511. 3 R.W. Dutton and R. S. Muller, Thin Solid Films, 11 (1972) 229. 4 K. Okuyama, H. Yamamoto and Y.Kumagai, J. Appl. Phys., 46 (1975) 105. 5 K. Okuyama and Y. KumagaL J. Appl. Phys., 46 (1975) 1473. 6 K. Okuyama and Y. Watanabe, Jpn. J. Appl. Phys., 15 (1976) 1881. 7 A. Kinbara, M. Ohmura and A. Kikuchi, Thin Solid Films, 34 (1976) 37. 8 K. Ok uyama, J. Tsuhako and Y. Kumagai, Thin Solid Films, 30 (1975) 119.

9 A. G6tzberger, Z. Phys., 142 (1955) 182. l0 K. Maki, Proc. 6th Int. Vacuum Congr., 1974, in Jpn. J. Appl. Phys., Suppl. 2, Part 1, (1974) 649.