Thin Solid Films, 141 (1986) 71-76

PREPARATION AND CHARACTERIZATION 71

T H I N T E L L U R I U M FILMS P R E P A R E D BY PARTIALLY I O N I Z E D V A P O U R D E P O S I T I O N

K. OKUYAMA AND Y. KUMAGAI

Department of Electronic Engineering, Faculty of Engineering, Yamagata University, Yonezawa 992 (Japan)

(Received January 25, 1985; revised November l, 1985; accepted December 6, 1985)

The effects of partially ionized vapour deposition on the morphology of tellurium films on KBr substrates were investigated. Compared with films formed by conventional neutral vapour deposition, films prepared by partially ionized vapour deposition show several pronounced structural features: (1)enhanced surface coverage on the substrate; (2) disappearance of the separate tips in the shape of a swallow's tail which grow on the end of the crystallites formed by neutral vapour deposition; (3) almost no decoration effect along the step line of the cleavage face of KBr substrates. Enhanced surface coverage of the tellurium film also occurred on glass and mica substrates.

1. INTRODUCTION

It is well known that the presence of ionized particles in the evaporated vapour has a notable effect on the condensation process of thin films 1. In a previous paper 2 we reported that both the sticking probability and the epitaxial quality were enhanced for very thin semiconducting tellurium films formed on a cleaved (001) surface of KBr by the use of a partially ionized vapour. The purpose of this paper is to present additional experimental data concerning the effects that the ionized particles in the evaporated vapour have on the crystallographic properties of tellurium films in the early stages of deposition. Surface morphologies of films prepared with and without ions are compared using transmission electron microscopy.

2. EXPERIMENTAL PROCEDURE

The partially ionized vapour deposition (PIVD) system is shown in Fig. 1. The vacuum system consisted of an ion pump and a titanium sublimation pump. Tellurium of purity 99.9999~o was evaporated from a BeO crucible heated by a tungsten conical basket heater at a background pressure of 4 x 10- 5 Pa. A small part of the vapour was ionized by bombardment with electrons with an acceleration voltage of 175 V. The thermionic emission current was adjusted in the range

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72 K. OKUYAMA, Y. KUMAGAI

substrato 1 suhstrate 2 , I° °1-~-1 ° ° / ° °~hoater

,

,shutter ~-----ion current

monitor electron - . . . . .~- ~ o'T---~ electron emitter accelerator

BeO ~ heater crucible o . ~ Te

Fig. 1. Schematic diagram of the PIVD of tellurium films.

20 -30 m A to ob ta in an ion conten t of abou t 0.1 ~o. The ion conten t was es t imated by measur ing the ion current and the depos i t ion rate of tel lurium. A quar tz osc i l la tor was used to m o n i t o r the film thickness. A K B r crysta l was cleaved in air, and the two halves were m o u n t e d side by side and loaded into a vacuum depos i t ion chamber . A te l lur ium thin film was formed on one of the subs t ra tes by expos ing it to par t ia l ly ionized v a p o u r for a prescr ibed t ime interval . The o ther subs t ra te was exposed to neu t ra l v a p o u r for the same t ime interval by the ope ra t ion of a shut ter and the in te r rup t ion of e lect ron b o m b a r d m e n t . The aim of this p rocedure is to ma in ta in a cons tan t evapo ra t i on rate for bo th cases. The subs t ra te t empera tu re was ma in ta ined at 150 °C dur ing depos i t ion by a tungsten heater m o u n t e d above the substrates .

3. RESULTS AND DISCUSSION

Repl ica e lect ron mic rog raphs of te l lur ium films p repa red on a cleaved (001) surface of KBr with and wi thou t ions are shown in Figs. 2(a) and 2(b) respectively. N o electric field was app l ied to accelera te the ions. The nomina l thickness of the film es t imated from a quar tz osc i l la tor was 18 nm. As descr ibed in previous papers 2'3, rod- l ike crystal l i tes of hexagona l te l lur ium with the longest d imens ion in the

(a) (b) 1 gm Fig. 2. Replica electron micrographs of tellurium films formed on KBr by (a) PIVD and (b) NVD (substrate temperature ~ = 150 °C).

Te FILMS PREPARED BY PARTIALLY IONIZED VAPOUR DEPOSITION 73

direction of the c axis were lying with the c axis parallel to the [110] and [150] directions of the KBr for both films. The preferred orientation of tellurium crystallites on the substrate results from the large difference in surface energy between the basal and prismatic plane 4. The (1050) surface of tellurium was in the plane of the KBr substrate as previously observed using X-ray and electron diffraction techniques 2'3. It is clear from Fig. 2 that the surface coverage of the film formed by PIVD was much higher than that of the film prepared by neutral vapour deposition (NVD). Th6 higher surface coverage obtained with PIVD means that a small amount of ionized atoms (0.1~) contained in the vapour flux enhanced the sticking coefficient of the vapour and the coalescence of nuclei. Figure 3 shows replica electron micrographs of tellurium films prepared on a glass substrate by PIVD and NVD. As seen from Fig. 3(a), the film made by PIVD was continuous with the c axis oriented in a random direction. No continuous film was formed by conventional NVD (Fig. 3(b)), although the amount of the incident vapour flux was nearly identical with that of the PIVD film. A similar result was obtained for the combination of a mica substrate and tellurium vapour.

/

I I (a) (b) 1 rtm

Fig. 3. Replica electron micrographs of tellurium films formed on a glass substrate by (a) PIVD and (b) NVD (T~ = 150 °C).

For the crystallites formed by NVD (Fig. 2(b)) separate tips in the form of a swallow's tail were observed on the end of the crystallites. For the PIVD film (Fig. 2(a)), however, the ends of the crystallites were somewhat rounded and had no separate tips. Saito and Shiojiri s have observed several types of tips in the form of a swallow's tail on the ends of tellurium crystallites evaporated conventionally onto a heated NaC1 substrate. They have given the following explanation for the appearance of such separate tips. Formation of re-entrant tips could be deduced from the growth of skeletal crystal which occurred in a solution under a supersaturation high enough to form a two-dimensional nucleus but not sufficient for the crystal to grow dendritically. The surface mobility of tellurium on a hot substrate might be so high that the two-dimensional nuclei could be formed on the outside near the tips of the rod-shaped crystal. With a further supply of evaporated atoms, the two-dimensional layer extended to form atomic chains along the c axis, and thus the slender crystal with separate tips was formed. On the basis of this

74 K. OKUYAMA, Y. KUMAGAI

model, a reduced growth of separate tips by PIVD means that the effective supersaturation is much higher for partially ionized tellurium vapour than for neutral vapour. Nishida and Kimoto 6 evaporated tellurium in argon gas at a pressure of 800 Pa and obtained fine tellurium crystals with various shapes. It is interesting that both rod-shaped crystallites with separate tips on the ends and also crystallites with round ends were obtained, presumably depending on the conditions under which they were formed. Unfortunately, however, no conditions were given for the formation of crystallites with and without separate tips.

Figures 4(a) and 4(b) show surface replicas of a tellurium film deposited onto steps of cleaved KBr substrates with and without ions respectively. For the NVD film (Fig. 4(b)) a marked disorder in the crystallite orientation was observed along the step line. In contrast, the disorder of the crystallites on the step line was greatly reduced for the PIVD film (Fig. 4(a)). This phenomenon was repeatedly observed and was quite reproducible. In Fig. 5 tellurium crystallites at an earlier stage of PIVD are shown. It is obvious that steps on the substrate had almost no influence on the nucleation and growth of tellurium crystallites. Steps on the cleavage surface of crystalline substrates are known to act as preferential sites for the nucleation of vapour condensation. Experimental results obtained here, however, suggest that the nucleation process of partially ionized tellurium vapour was homogeneous, and cleavage steps rarely acted as preferential sites for nucleation. Shorshorov e t al. 7

have reported that there was no decoration effect on the cleavage face of KC1 by the deposition of partially ionized aluminium vapour which was accelerated by

(a) (b) I J 1 lain

Fig. 4. Replica electron micrographs of tellurium films demonstrat ing the influence of step lines of KBr substrate on the orientation of crystallites (T~ = 150°C): (a) PIVD; (b) NVD. Arrows indicate the direction of step lines.

I -J 1 ~ m

Fig. 5. Replica electron micrograph of tellurium crystallites prepared on KBr by PIVD. The crystallite growth appears not to be influenced by step lines of the KBr.

Te FILMS P R E P A R E D BY P A R T I A L L Y I O N I Z E D V A P O U R DEPOSITION 75

- 1.5 kV. The reason for the absence of decoration was that a large number of nucleation centres were created on the substrate by ion bombardment . The results obtained for P IVD tellurium films are noteworthy, because when non-accelerated ions were used no defects were created on the face of the substrate and, even so, no decoration effect occurred on the steps. The distinctive features of P IVD tellurium films described so far can be understood in terms of an explanation given by Chopra s - l ° for metal films formed by a vapour that includes ions. Chopra showed that the presence of charges resulted in increased nucleation and an enlarged surface area as a result of the deformation of the sphericity of islands by the additional surface energy. The presence of a charge q on a spherical drop of radius r increases its Gibbs free energy of formation by q2/8r~eor, where e0 is the permittivity. The increase in surface energy would be accommodated by an increase in surface area, i.e. the sphere would change into an oblate spheroid. The deformation corresponds to a flattening of three-dimensional islands, bringing the islands closer together and thereby enhancing the surface diffusion between the islands. In addition, if image forces are taken into consideration, attractive image forces dominate between the charged islands, irrespective of the sign of the charges, if either charge is much larger than the other or if the radii of the islands are nearly equal to the separation distance. Such image forces will also promote the coalescence of small islands.

The above argument is based on the physical effect of electrostatic charges attached to the islands. Matsunaga et al. 11 have found that the doping yield of zinc impurity into GaA11 _xAsx layers being grown by molecular beam deposition was greatly increased by ionizing the zinc atoms. Since zinc ions are directly in- corporated into the growing layer of GaA11 _xAsx it is reasonable to assume that the dopant atoms are chemically activated, and thus they react and bond with the surface atoms more easily than do neutral atoms 11. This might be another possible cause for the observed phenomena in P IVD tellurium films.

4. CONCLUSIONS

Compared with N V D tellurium films, P IVD tellurium films on a KBr substrate show several distinctive features in their surface morphology, as follows.

(1) The surface coverage of the film is enhanced and thus very thin continuous films are obtainable on KBr as well as glass and mica.

(2) Tips in the form o fa swallow's tail on the end of crystallites disappear. (3) The decoration and orientation effect at the cleavage steps of the substrate

is greatly reduced. The above results were obtained without acceleration of the ionized particles

and can be explained as due to the enhanced nucleation and coalescence associated with the physical effect of electrostatic charges on the islands and/or the chemical effect of the enhanced activity of ions.

A C K N O W L E D G M E N T S

The authors are very grateful to Y. Seki and S. Numazawa for their assistance in the course of the experimental work and to Y. Takahashi for helpful discussion.

76 K. OKUYAMA, Y. KUMAGAI

REFERENCES

1 T. Takagi, Thin Solid Films, 92 (1982) 1. 2 K. Okuyama and Y. Kumagai, Jpn. J. Appl. Phys., 21 (1982) 1384. 3 K. Okuyama and Y. Kumagai, Thin Solid Films, 98 (1982) 203. 4 E.J. Weidmann and J. C. Anderson, Thin Solid Films, 7 (1971) 265. 5 Y. Saito and M. Shiojiri, Jpn. J. Appl. Phys., 19 (1980) 427. 6 I. Nishida and K. Kimoto, Jpn. J. Appl. Phys., 14 (1975) 1425. 7 M. Kh. Shorshorov, O. V. Gusev, T. L. Roslyakova and M. V. Grusin, Thin Solid Fihns, 54 (1978)

323. 8 K.L. Chopra, Appl. Phys. Lett., 7(1965) 140. 9 K.L. Chopra, J. Appl. Phys., 37 (1966) 2249.

10 K.L. Chopra, J. Appl. Phys., 37 (1966) 3405. 11 N. Matsunaga, T. Suzuki and K. Takahashi, J. Appl. Phys., 49 (1978) 5710.