Structure and growth of oriented tellurium thin films

Thin Solid Films

STRUCTURE THIN

Elsevier Sequoia

AND

S.A., Lausanne

GROWTH

- Printed

in Switzerland

OF ORIENTED

TELLURIUM

FILMS

E. J. WEIDMANN

AND J. C. ANDERSON

Department qf Electrical Engineering, Imperial College of Science and Technology. London, 5’. M’.7 (Gt. Britain)

(Received

October

22. 1970; in revised form December

10. 1970)

SUMMARY

The growth of evaporated Te films on rock-salt, mica and TGS has been studied. Te adatoms have been demonstrated to have very high mobilities on heated substrates allowing the structure to be governed by the surface free energies of the crystallites. This leads to needle-like crystallites, having the c-axis parallel to the substrate, which on mica substrates readily form twins along the (1012) plane in Te. Epitaxial growth with the c-axis perpendicular occur on rock-salt at room temperature.

to the substrate

may

INTRODUCTION

effects in tellurium thin Several papers ‘3’ have reported on orientation films grown on a variety of substrate materials, but no investigations of the detailed film structure

by electron

microscopy

to verify the results of diffraction

measure-

ments appear to have been reported. Te films deposited in vacuum by evaporation at pressures below 10m6 torr have been examined by electron microscopy and have been found to exhibit unusual structure and growth behaviour from the earliest stages. It is proposed that the structure derives from the anisotropy of the surface free energy of the crystallites and the high mobility of tellurium adatoms on the substrate before nucleation. STRUCTURE

OF TELLURIUM

In order to understand the observed behaviour of Te films it is necessary to consider the structure of the trigonal form of the element. Tellurium is in Group VIb of the periodic table and has six outer electrons s2p4. Two of these electrons are paired in the s-orbital, two in one of the three p-orbitals and the remaining

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two are available for covalent bonding in half filled F-orbitals. The structurc o1' tellurium was first determined by Bradley 3 and is shown in Fig. 1. It consists of spiral chains of atoms with three atoms per turn and corresponding atoms in each chain forming a hexagonal network, although the highest symmetry axis is threefold and is chosen as the c-axis. The bonds between atoms on the same chain arc covalent, whereas between chains they are thought to be a mixture o1" electronic and van der Waals 4. Every atom of Te touches two atoms in the same spiral and fonr atoms in adjacent spirals. In the former case the contact is much closer, the distance from centre to centre of adjacent atoms being 2.86A. The distance between atoms in different spirals is 3.74A s. The difference between these two radii of combination is very large and indicates a much greater cohesion between atoms in the same spiral than between those in different spirals. (This would be expected fl'om the nature of the respective chemical bondst. It was suggested by Bradley I'or the similar case of selenium that this would cause a tendencv for crystals to grox~ in the direction of the trigonal axis more readily than in other directions. The mechanical properties of tellurium crystals ~' are m accordance with this model of the bond strengths. For example, tellurium cleaves most readily along the (10]0) prismatic plane and not along the (0001) basal plane like most hexagonal structures. The response of telhlrium to applied stress is markedly different parallel and perpendicular to the c-axis. Under uniform pressure the spirals extend and at the same time bunch more closely together i.e. the compressibility is negative along the c-axis and this behaviour is similar to the effect ot'a uniaxial tension along the c-axis. The value of Young's modulus along the ~:-axis is - 4 x l 0 '~ dynes cm 2 and the response is purely elastic up to the point of brittle fracture. In contrast it is very weak and ductile when forces are applied perpendicular to the c-axis. The crystals cleave in the {10]'01 planes and slip occurs along the

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1. P e r s p e c t i v e v i e w o1" t h e T e l l u r i u m l a u i c c . ( F r o m . I . S . B l a k e m o r e .S},//d/'i/m~.

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S T R U C T U R E A N D G R O W T H OF T H I N FILMS

267

[1210] directions which lie in the cleavage planes and perpendicular to the c-axis. Slip occurs for resolved shear stresses as low as 2 × 107 dynes cm -2 at room temperature. The anisotropy in the mechanical properties is strong evidence that the bond strengths along the c-axis, and hence the surface free energy of the {0001 } surfaces is between two and three times greater than the corresponding values for the prismatic {1010} surfaces as shown by the values of the elastic stiffness constants given by Stuke 7. This large difference in surface free energy between the basal and prismatic planes results in a strong tendency for the crystals to grow in the form of extended hexagonal prisms or dendrites parallel to the substrate surface, in accordance with the theory of Gibbs 8 that the shape assumed by a growing crystal is that which has minimum surface free energy. W u l f f 9 also showed that the equilibrium shape is related to the free energies of the faces, and suggested that the crystal faces would grow at rates proportional to their respective surface energies. S U R F A C E M O B I L I T Y OF T E L L U R I U M A D A T O M S

The mobility of a species of adatom on a substrate surface can be expressed in terms of the ratio between the adsorption energy, Ea, of a single adatom to the substrate and the surface diffusion energy, Ed, which is the energy required for an adatom to move between adjacent adsorption sites. The binding energy per atom, E~, of atoms in a nucleus of i atoms, will also influence the lifetime of a free adatom on the surface, which is dependent on the size of nucleus and, if crystallization has commenced, on the free energy of the different faces, as discussed. The high mobility of tellurium adatoms by comparison with, for example, gold on a substrate of single crystal rock-salt can be demonstrated by a method due to Lewis and Campbell l°, and shown in Fig. 2. By choosing an area of substrate containing an array of parallel cleavage steps of variable separation and arranging that, as far as possible, all the evaporation conditions for the two species are identical, the relative density of nuclei between the steps for the two different types of adatom give an indication of the relative mobilities of the adatoms. The steps are preferred adsorption sites and will collect adatoms which land within the diffusion distance " d " , which is related to the diffusion energy Ed. When the separation between steps becomes less than 2d the probability of nucleation occurring between steps approaches zero. Typical values of " d " for gold and tellurium are 200 A and 360 A respectively. EXPERIMENTAL METHOD

The films were grown by evaporation from a resistance heated silica crucible in a standard cold-trapped vacuum system capable of pressures in the 10-v torr range. Substrate temperatures could be varied from room temperature to 200 °C and were measured by thermocouple or thermistor in contact with the substrate. Thin Solid Films, 7 ( 1971 ) 2 6 5 - 2 7 6

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Evaporation rates and film thickness were measured with a quarlz crystal rate meter calibrated againsl a multiple beam interferometcr. After evaporation a backing film of amorphous carbon or platinum/carbon was deposited from a carbon arc. The composite film was removed from lhe substrate by immersion in distilled water or dilute nitric acid for examination in the elcctron microscope. The substrate materials used were mica, rock-salt (Harshaw Company single crystals) and triglycine sulphate (Imperial College Crystal Growth Lab.). It was found that re-evaporation occurred at the deposition rates used (between 0.1 and 5 ~/secL if the substrates were heated above aboul 150 'C. lDIJl.S~l/U I /Inl~. - ( 1 9 7 1 1 2 ( 4

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STRUCTURE AND G R O W T H OF THIN FILMS

No significant difference was observed with films above ~ 25 A thick, if the substrates were cleaved in vacuo or in air immediately before evacuation. RESULTS

(a) Te on rock salt

Figure 3(a) and (b) show two cases of Te deposited on freshly-cleaved rocksalt at room temperature. Figure 3(a) represents the normal case in which the film is polycrystalline and is becoming continuous without converting to dendritic form. Figure 3(b) is exceptional for Te, which seldom forms good single crystal films on unheated substrates, having grown with the c-axis perpendicular to the substrate surface forming crystallites of average diameter ~ 1.5 X 1 0 3 / ~ . The selected area diffraction pattern shows splitting of the spots due to the differing orientation of crystallites. On raising the substrate temperature at which deposition is carried out dendritic growth occurs, with the [0001] direction parallel to the substrate surface, as shown in Fig. 4(a) and (b). In Fig. 4(a) the substrate temperature is 90 °C and the rate 1 A/sec and the dendrites, although themselves single crystal, show no preferred orientation on the substrate. At 110 °C and a rate of 3.5 A/sec Fig. 4(b) shows some azimuthal orientation of the dendrites which have a tendency to grow

(a)

(b)

Fig. 3. Tellurium on air-cleaved rock-salt at r o o m temperature: (a) rate 100 A, ( × 144,000); (b) rate ~ 1 A/sec, thickness 550 A. ( x 30,000) lhil~ Solid Films, 7 ( 1971 ) 265 276

~ 1 A./sec, thickness

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tqg. 4. l d l u r i u m on a i r - c l e a v e d r o c k - s a l t : (at) rate ~ I A sou, t h i c k n e s s 55(1A. subst~atc t e m p e r a t u r e 9() (', { ×22,500): (bl rate - 3 . S A , s c c . t h i c k n e s s 3 5 0 A , s u b s t r a t c t e m p e r a t u r e I l l ) ( ' . { "~'20.00(l)

along the [100] and [010] directions in the (100) NaC1 surface, as is evidenced by the thickening of the diffraction rings at the four corners of a square corresponding to the < 100> direction in the NaC1 lattice. There is some tendency also to growth in the [110] direction as shown by the thickening of the outermost ring of the diffraction pattern. ( h ) T e on nlica

Dendritic growth is also obtained on mica substrates at elevated temperatures and orientation is obtained as, for example, in Fig. 5 for a 400 A thick film deposited onto mica heated to 78 ~C. The diffraction pattern was obtained from tile whole area of the film shown and can be indexed as two single crystal patterns inclined at an angle of 77.5 '> and with a common < 1 0 1 2 > zone axis, i.e., the pattern represents twinning along the (i-012) plane and c o m m o n azimuthal orientation on the substrate of all crystallites in the film. This is not immediately obvious from the micrograph, but can be seen more clearly in Fig. 6 which shows a carbon replica of a 2,200 A film grown on mica at a temperature of 154 °C. Actual measurement of the angle between crystallites gives values varying between 74 and 8 2 . The diffraction pattern also shows that the electron beam is normal to tile (01 i0) plane of the crystal, which therefore lies in the plane of the subst,ate. 11:.: s,,/:d / ~/m,. - < 1 ' : 1 )

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STRUCTURE AND GROWTH OF THIN FILMS

271

Fig. 5. Tellurium on mica: rate 3.5 A/sec, thickness 400 A, substrate temperature 78" C. ( x 100,000)

The best fit between the tellurium lattice and mica surface is given by the [1120] direction in the quasi-hexagonal oxygen lattice in the mica with 4.51 A spacing between atoms and the 4.44 A tellurium atom spacing in the (01 TO) planes, although this is in disagreement with Semiletov2. Fig. 6 shows that the (1010) faces of the prisms have developed as the film grows to higher thicknesses.

Fig. 6. Carbon replica of 2200 A film of Te on mica at a substrate temperature of 154 C. ( x6,000t Thin Solid Films, 7 (1971) 265-276

272

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( c ) Tu on TGS

Figure 7(a) shows a 22 A thick film evaporated at a rate of 0.1 A/sec onto TGS at room temperature. The substrate had been subjected to electron bombardment before evaporation, which would neutralise positively charged domains. A number of features characteristic of the growth of tellurium films can be observed: (i) Nucleation commences by the formation of round islands, which reach a size of about 150A diameter and which diffraction patterns show to be amorphous. (ii) These islands are mobile on the substrate surface and frequently coalesce. At the places marked A in Fig. 7(a) there are examples of cases where

(a)

Ib) F i g 7 le on F ( I S al r o o m t e m p e r a t u r e : rate 0 . 1 A , s e c , n o m i n a l t h i c k n e s s 2 2 A : (a) on tile subsll-[lle plane, i ×75,000): (b) e d g e - o n vie\~ o f g r o w t h o n a c l e a v a g e step. ( x 150.000) Thin >,ohd l'iln~,

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STRUCTURE AND GROWTH OF THIN FILMS

(a) 10-15,~

(b) ~ 2 5 A

(c) ~ I 2 A

(d) 100A

(e) 200 A

(f) ~ 500 h

273

Fig. 8. Growth sequence of T¢ on TGS for substrate temperatures between 55 C and 80°C (x30,000). Some re-evaporation has probably occurred.

adjacent Te islands have coalesced during carbon evaporation, after cessation of the Te evaporation. These are not due to damage to the film during stripping since the area from which the mobile island has moved has, in several cases, 77fi,'I 5"olid Films. 2 (1971) 265~-276

274

I~. J.

WEll)MANN,

J.

('.

ANDERSON

received a subsequent deposit of carbon. This is evidenced by the varying density (from white to grey) of the photographic image of the blank spot left behind by the island. Furthermore the darker appearance of the island adjacent to the blank area indicates that coalescence of two islands has taken p l a c e - a process unlikely to occur during stripping. (iii) Crystallization commences at the critical size of - 1 5 0 A when the islands adopt a hexagonal habit (arrowed at B in Fig. 7(a)) and begin to grow rapidly along the ('-axis direction. This is usually accompanied by a decrease in the thickness of the island, as material is removed to the growing extremities. Growth is most rapid at the outer edges of the advancing (0001) faces, which leads to the characteristic wedge shapes seen. (iv) The growth edges have no well defined orientation and are probably rough, serving only the purpose of advancing the (11!0) face along the substrate. The transformation from amorphous island to crystalline prism can be particularly well seen on the photograph of Fig. 7(b) which shows both species edge-on, growing on cleavage steps in the substrate surface. Figure 8 is a composite photograph to show the stages in the growth of a typical tellurium film on TGS at temperatures between 5 5 and 80 C. The initially formed round islands have liquid-like behaviour and are highly mobile. They coalesce with each other up to a critical size ( 4 150A) when crystallization begins, and they also coalesce with crystallites already formed. Eventually all the islands are absorbed by crystallites and fresh nucleation only occurs onto crystallites which are in the form of needle-like prisms. The prisms grow in length until they are contiguous when further growth along the (-axis is prevented. Fresh nucleation then occurs in the interstices until the film eventually becomes continuous. SUMMARY

OF RESULTS

a l R o c k salt

Films grown on substrates at room temperature were generally polycrystalline (with c-axis normal to substrate) when thin (100A) but occasionally nearly single crystal (mosaic) films occurred when thicker. (Possibly this depends on evaporation conditions.) At higher temperatures ( > 7 0 " C ) texture type films were produced with crystallites in the form of extended hexagonal prisms, with varying degrees of orientation with the substrate. Three cases can occur, as shown by Semiletov2: (1010)i~. II (lO0)N~,C~and IO001]f ~ LLI l(lO]n~,c~ (lOfO).,.~ II (lO0)N.o and [O()OI]T c II [l lO]NaC I (11_~O)T~ II (IO0)N.Ct and 10001]r~ k] [llO]N~ct Ibm

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276

STRUCTURE AND GROWTH OF THIN FILMS

275

(b) Mica Films grown on mica at room temperature were polycrystalline. On mica heated above ~ 7 0 °C they were dendritic with random orientation of the c-axis in the plane of the substrate, but generally with the (1010)a-e face parallel to the substrate plane. For substrate temperatures above ~ 1 0 0 ° C the films reevaporated unless the incidence rate was in excess of 3A/sec. Where some re-evaporation had taken place very good orientation was frequently obtained, with the (11,~0)xe face parallel to the substrate and the [0001]T . direction parallel to the [1120] equivalent directions in the mica surface (mica cleavage surface is (001) plane and atoms in this plane have pseudo-hexagonal symmetry). (c) TGS This is not a typical substrate material insofar as it is ferro-electric and therefore not electrically neutral below its Curie temperature (49°C). At temperatures above this, however, the surface charge disappears and results are comparable with those obtained on other single crystal surfaces. DISCUSSION AND CONCLUSIONS

Te adatoms have a high mobility on the crystalline substrates used and, at elevated temperatures, even whole islands are mobile. Initially the islands are hemispherical and show every appearance of being liquid-like. At a critical size ( ~ 150 A) they develop a crystalline hexagonal form which, for elevated substrate temperatures, is quickly supplanted, possibly by the addition of just one or two atoms, by a dendritic form in which the dendrites ultimately form extended hexagonal prisms. This growth habit is consistent with the free energy of the {0001 } surfaces, which are the largest areas in the hexagonal form, being much greater than the free energy of the { 1010} surfaces which have the greatest area in the dendritic form. Epitaxial growth occurs, in the case of mica, at temperatures above 70 °C but the films exhibit a strong tendency to twinning along the (1012) plane. On rock salt epitaxial growth may occur exceptionally at room temperature with the c-axis perpendicular to the surface. This suggests that, at room temperature, the mobility of the Te adatoms is not sufficiently high to permit the change to the lower surface free energy form of crystallite. At higher substrate temperatures oriented films with the dendritic habit are obtained. REFERENCES 1 2 3 4

R . W . DUTTON AND R. S. MULLER, Solid State Electronics, 12 (1969) 136. S . A . SEMmETOV, Trud. Inst. Krist. Akad. Nauk. SSSR., 11 (1955) 115, 121 (in Russian). A . J . BRADt,EV, Phil. Mag., 48 (1924) 477. A. VON HIPPEL, J. Chem. Phys., 16 (1948) 372.

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