The growth of rutile (TiO2) single crystals by chemical transport with TeCl4

Journal of Crystal Growth 1 (1967) 177—182 © North-Holland Publishing Co., Amsterdam

THE GROWTH OF RUTILE (TiO2) SINGLE CRYSTALS BY CHEMICAL TRANSPORT WITH TeC14

T. NIEMYSKI and W. PIEKARCZYK Department of Semiconductor Technology, Institute of Electron Technology, Polish Academy of Sciences, Warsaw, Poland

Received 15 February 1967

tellurium Rutile (Ti02) tetrachioride crystals were as a grown transport by chemical agent. Isometric transportcrystals using as well as rhombohedral-shaped plates were obtained, The crystal size exceeded 4 mm; the average dislocation density

1. Introduction

2. Experimental conditions, rate of transport, about was morphology 4>< 1O~cmand quality of crystals are described. The choice of the transport agent is discussed.

About 1.5 g of Ti0 2 powder was introduced into the ampoule together with a quantity of TeCh, calculated

Single crystals of rutile may be prepared in several ways. Best known is the Verneuil technique’); other for a total pressure in the ampoule during the transport process of 4 atm (5.2 mg of TeCh, for each ml of the means are solution growth from molten salts (e.g., capacity of the ampoule). borates), the hydrothermal method, and vapour phase 2—4). All except The ampoule containing TeCh, and the initial Ti0 growth by chemical transport reactions 2 powder was heated in a horizontal tubular resistance the Verneuil technique have not been widely studied as furnace, with two independent heaters. Both ends of yet, though they yield crystals of good quality. Par- the ampoule were surrounded by heat-resistant steel ticularly scarce are the data on vapour phase crystalli- blocks, which levelled out the temperature at the ends zation. of the ampoule and increased the temperature gradient The present investigation reports vapour phase crysin the centre of the ampoule. tal growth of rutile using the chemical transport process Before the transport process was started, the walls of and choosing the following endothermal, reversible the crystallization zone were purified from absorbed reaction: fine particles of Ti02. For this purpose the crystallizaTi02(s)+TeCl4(g) = TiCl4(g)+Te02(g). (1) tion zone was heated to 1100°Cand the source zone with Ti02 powder to 900 °Cfor about 10 hours. The reaction was carried out in a sealed ampoule. The Then, the temperature gradient was reversed. Once temperature gradient was between 1100 °Cand 900 °C. the proper temperatures had been established, transport of the Ti02 began according to the endothermal, 2. Experimental reversible reaction (1) from the higher (1100 °C)to the The experiments were carried out in ampoules of lower (900 °C) temperature. The transported Ti02 transparent quartz glass, having an internal diameter crystallized on the walls of the ampoule in the crysof about 12 mm and a total length of about 100 mm. tallization zone, The duration of the process was around The procedure of filling the ampoules wassilicon, basically 90 temperatures of both levelled 5) for in hours. order The to stop the process andzones the were furnace was theTellurium same as that described by Schafer tetrachloride (TeCh,) was prepared by shut off. chlorination by condensed chlorine of a weighed porAfter cooling the furnace the ampoule was taken out tion of powdered tellurium in a thin-walled glass cap- and opened. Crystals were removed from the walls of illary tube, air-tight because of the highly hygroscopic the ampoule and etched in hot, diluted HCI in order material, to dissolve adsorbed TeCh,. 177

178

T. NIEMYSKI

AND

3. Results Typical experimental conditions are shown in tab!e I

Am-

No.

poule volume (ml)

I 2

12.4 10.7

TeCl

4 weight

parallel growth is observed among plates (fig. 5). As in the first case, the growth-plane is mostly (031) and rarely (Oil). The growth-plane is distinctly visible (see

I

TABLE

Run

W. PIEKARCZYK

Crystal weight

(mg)

Transport time (h)

65.2 57.4

88 91

403 397

(mg)

Trans- Temperport ature rate gradici (mg/h) C) 4.58 4.36

1100/900 1100/900

Transport of Ti02 by TeCh, yielded transparent. light-yellow crystals of rutile, up to 4 mm in size. Two kinds of crystal habit were observed: rhombohedralshaped plates. up to 1.5mm thick (about 42%, fig. I)

$ t

,

Fig. 2.

crystals, rathercrystals an ingrowth of two individuals 11g. 5). have so but that are undoubtedly not single which an such identical orientation. The best developed plane on the plates is the (100)

_______ ~

~

face. Other faces, belonging to the families (011), (110) and (Ill) are much smaller, but distinctly marked. Isometric crystals are in general not so well developed as the plates. They have the same faces as the

. .

~

l-i~. I.

Rutile crystals in the form of plates.

58%, fig. 2). Most of the plates are single twins (fig. 3). Sometimes also multiple twins (fig. 4) are observed. The twin plane is mostly a (031) plane, rarely a (OIl) plane. In multiple twins these planes occur alternatively. Beside twins also and isometric crystals (about

konietrie rutile crystals.

plates, belonging to the families (100), (110), (011) and (Ill). hut more uniformly developed. Most of the isonictric crystals show no twinning, being probably

single crystals. The rest of the crystals are twins. The (Oil) plane is observed in the isometric crystals beside the twin plane (031) more often than in the plates. The directions of the main crystallographic axes were found by means of the Fedorov polarized light technique and the Laue X-ray technique. The tetragonal unit

THE GROWTH OF RIJTILE

(Ti02)

SINGLE CRYSTALS BY CHEMICAL TRANSPORT WITH

cell constants determined by the powder technique are: a = 4.60 ±0.02 A; = 2.96 + 0.02 A —

—

which is in agreement with the published data for rutile. Miller indices of the crystal faces were determmed by means of goniometer measurements.

_

Fig. 3.

‘p

TeCh,

179

Etch pits were revealed by etching in boiling concentrated sulphuric acid for 0.5 to 2 hours: Etch patterns on the (100) face were elongated ellipses, their longitudinal axes directed along the <010> direction. Dislocations are distributed not uniformly, being concentrated near the mentioned defects, where their densi-

Single twin (between not quite crossed nicolsl.

Fig. 4.

Multiple tss in )hetssecn crossed nicols).

Serious damages of the crystal structure visible by the nakedtoeye rule in theUnder part ofmicroscopic the crystal adjacent theappear wall ofas thea ampoule.

ties were about ten times higher than elsewhere. 2. The average dislocation density was about 4 x of l0~ cm In order to estimate the redistribution impurities

observation these places appear as cracks with conchoidal fractures. Such defects occur in other parts of the crystals too, being however more common in the isometric crystals than in the plates (see figs. I and 2). Similar defects, but of a parallel configuration, occur sometimes near the twin planes and are triangular in shape, limited by the faces (031), (010) and (011) (fig. 4).

during transport, a spectral analysis of starting materials and resulting crystals was performed. The results are shown in table 2.

Microscopic observation reveals also cavities in form

of thin capillaries (spliting into small bubbles) arranged parallel to the external faces (fig 5)

TABLE

2 -

Material Tellurium

Si l0~

-

Fe 10°

Ti0 2 powder Ruttle crystals

5 10° 10

I0 10

—

Impurities (~) Mg Sn Cd Ca I0~ — I0~ 10

-

V —

I0~

10

—

—

?

10

—

—

—

—

180

T. NIEMYSKI

AND W. PIEKARCZYK

No presence of tellurium in rutile crystals was shown by the spectral analysis. The sensitivity of the spectral tc!vi.i~c i:~t~.c ‘~fTe is 10 to 100 times lower

I E

°°~

________________________________________________

Fig. 5.

~

account of the danger of the possible formation of anatase2). (When chlorine was used as the transport agent for TiO 2, plates of anatase were found at temperatures up to 1000 ~C. No anatase crystals were observed when using TeCh, at 900 °C.) Among all titanium compounds only the halides are volatile. Hence, only they may take part in the gas phase chemical transport of Ti02. So, as transport agents for TiO2 may be used halogens or volatile halides of such elements, which form oxides volatile at about 1000 °C,but no fixed compounds with titanium under the given conditions of transport. Fluorine and fluorides have been excluded beforehand on account of their reactivity towards the quartz glass. The approximate evaluation of substances theoretically suitable for transport was based on the Schafer 7). This required the knowledge of values for the principle enthalpy 4H° and for the entropy AS° of the reaction or the knowledge of the temperature dependence of the equilibrium constant of the reaction. The precise calculation of the transport rate is possible only with the knowledge of the equilibrium partial pressures of one of the gases involved in the transport and the mechanism of transport (diffusion, convection). The equilibrium constant Ka of the reaction is calculated from the standard relation: 4G~.= —RTln Ka,

Parallel gros~th(bet~-eencrossed nicols).

than in the case of the other revealed elements (Si, Fe, Mg), so that the content of Te may be assumed as lower than 1O~—l0~%. 4. Discussion

where LIG°T is the change of the free energy for the reaction: bB+cC+...

and anatase (unstable at high temperatures) are transformed into Prolonged rutile at 700—800°Cand 830—950°Crespectively6). heating of the quartz glass may be carried out up to 1000—1100 °C. Therefore, the 1100—900 °Cwas choosen for the growth experiments. Lower temperatures were rather undesirable on

range

=

rR+sS+....

preliminary calculated from the approximate formula: LIG°T is

The temperature range for growing rutile crystals by chemical transport is limited by two factors: the lower limit is the lowest temperature where no crystal modification of Ti02 other than rutile exists, and the upper limit is the thermal stability of the ampoules. Brookite

(2)

LIGT

L1H 0 2 98

=

—

T4S~°98.

(3)

.

In some cases besides 4H298 and z1S298 (or AG298) there are known also the temperature dependences of o CF for the substances B, C, R, S, zIGT is calculated from the formula; 2—*4cT3+ AG~= zlHg—zlaTln T—~AbT ...~dT_i +JT, (4) . . .,

where 4H

. . .,

.

0 is found from the equation: 2+-~cT3—dT’ (5) H°T= H~±aT+-~-bT .

(Ti02)

THE GROWTH OF RUT1LE

SINGLE CRYSTALS BY CHEMICAL TRANSPORT WITI-I

When the changes of the free energy of formation (AG~)T of the compounds B, C R, S, from simple substances are known, AG~-is calculated from the formula: ...

z1G~=


.

TeCh,

181

Thermodynamic data reported in refs. 8—11 were used for the calculation of Ka. Some of these data are shown in table 3. The approximate value for the entropy of gaseous tellurium tetrachloride S~98= 85 cal/mole 8). Curves of log Kabyversus l/T were calculated for the deg is calculated the Kireev method reactions of one mole of Ti0 2 with all the substances .

—
(6)

.>.

TABLE 3

2+dT2 ~/cal Substances

State

4H°298 /kcal \ ~mole)

JG°298 kcal ‘calS°2~ (mole)_~mole.deg)

—218.0 —49.4 —180.5 —12.4 +41.563 0 0

—203.8

=

deg)\

Temperature

a-)-bT-t-cT

a

10°

Ref.

interval -

cxl0~

Ti0 2(rutile) TeCl4 TiCl4 Te02 TeO Cl2 02

s g g g g g g

-

______

—

—171.94 —13.731 +35.252 0 0

9OO~C

1100°c

_____

—

7~

a

-I__

-5 -6

__________ _______

~

________

-7

—11

______ _______

-~

——

—

—

~

HJ SbCI3 Br2

~

~

2.36

—

—

—

—

—

0.06 0.594

7000

‘700

Ti02(s)+2X2(g)

—

—0.68 —2.972

— —

~TiCi4

298—3000 600—3000

was calculated for the reaction of

the activity of TiO2 and activity coefficients of the gaseous substances are assumed as equal to unity: ~ YTeCi~’ YT1Ci4’ YTeO2’ ... = I. Then Ka is expressed by the formula:

7200 1300 1400 1500

O~

4 -

0

MeX,,(g)

=

=

TiX4(g)-f-05(g) TiX4(g) +

=

Ka

=

KNP

or Ka

2 -

~ ~

=

Kp

~ =

Me20~(g)

——-----,

pbpc B C~

(7)

/1

where N, molar fraction, P, =

—

8 10, II 8, 9 10 9, 10 8 8

Ti02 with the above substances and HCI. ~TiCi. was calculated by the solution of equations, determining the equilibrium constants of all simultaneous reactions. The dissociation reactions, which

Logarithms of the equilibrum constants Ka of the reaction:

(where X

298—1800 665—2500 298—2000

have into account. very small Asequilibrium the pressures constants in the system were not aretaken low,

______

Ti02(s) +

—

—

tetrachloride

~

_________ ______

or

—

For aof ence more the precise equilibrium evaluation partial thepressures temperature of titanium depend-

-13

Fig. 6.

—

ABCI~

________

~O0

—4.35

—

C,2

~

-12

0.28 3.5 0.24

-

_____

_—

—

—

_____

_—~~

~ -9 -10

________ ______

17.97 18.5 25.45 14.5 8.9 8.82 8.186

2’’2) Cl PCI5, 13’’4)PbCl2 have shown in fig. 6 and additionally withand PCI3, and Cd4. So, far, only HC1 2 been used for gas transport of Ti02. Within the tern2). perature range 900—1300 °KHC1 gave better resultsi Our calculations show SeCI 4, TeCh, and Cl2 to be the most suitable transport agents for TiO2 (see fig. 6).

SeCI~ TeCI 4

—

-2 -3

12.01 85.0 84.4 65.3 57.5 53.286 49.003

Cl, Br, I, and Me = metal or H), as a function of the reciprocal of the absolute temperature.

mospheres. and

An

=

=

partial pressure in at-

(r+s± . . .)—(b±c± . .

182

r.

A plot of

NIEMYSKi

ANI) W. PIEKARCZYK

PT,CI

4 versus T (for the total pressure of I atm) is shown in fig. 7.

aim PTa:,4

—

—

—

~— ~

000

crystals with only trace impurities and a relatively low dislocation density. The crystals are sufficiently large for various physical measurements. The method is very on a foreign substrate. Rutile is a crystal with very promising in view of depositing monocrystalline layers interesting electrical properties. Depending upon its stoichiometry (or its impurity content) it behave,s as a

ci2

—~

003 005

port seems Therefore, conductor. to have great the perspectives method of chemical in the field transof modern microelectronics.

/

--

-

It should noticed that transport preliminary die!ertric show the TeCl4 withbea as high a possible dielectric constant agent orcalculations asfora growsemiing e.g., otherSc203, refractory oxides by chemical transport such as, Ga203. Fe203, Cr2O3 etc.

002

—

900

1000

SeC!4

—-

1100

1200

1300

1400

i~oo’i<

Fig. 7.

Equilibrium partial pressures of TiCI4 (where the total pressure P I atm), as a function of the temperature.

The largest differences in partial pressures P.1.,~,4at two different temperatures above 900 °Coccur for the reactions: TiO2(s)±TeCl4(g)= TiCl4(g)+TeO2(g)

(I)

TiO2(s)±2C12(g)= TiCl4(g)+O2(g).

(8)

and T1CI4 is slightly more advantageous than Cl2. SeCI4, despite its highest value of log Ka, is not very active due to its nearly complete dissociation. Other substances situated below Cl2 in fig. 6 are less active. Moreover, the hydrogen halides act as reducers. Their reducing properties increase from HCI to HI, which in the case of HBr and HI makes the preparation of stoichiometric rutile crystals impossible. The reducing agent is here the hydrogen resulting from the thermal dissociation of the hydrogen halides. All these reasons led us to choose the chemical reaction (I) as the more promising from the two suitable reactions (I) and (8). 5. Conclusions The method of chemical transport by TeCI4 as transport agent makes possible the preparation

of rutile

Acknowledgments

The authors would like to thank Mr. M. Chabiera for technical assistance with experiments. Mrs. S. Skalska for carrying out the spectral analyses, Mr. R. Kowalczyk for making the computer calculations and Mr. A. Badzian for the X-ray examination of crystals.

References I) C. H. Moore, Trans. Ani. Inst. Mm. Met. Eng. 184 (1949) 194. 2) I. N. Anikin, I. I. Naumova and G. V. Rumjanceva, KristalIografiya 10 (1965) 230. 3) V. A. Kuznecov and V. V. Panteleev, Kristallograflya 10 (1965) 445.

4) J. S. Berkes, W. B. White and R. Roy, J. App!. Phys. 36

(1965) 3276. 5) H. Schafer and B. Morcher, Z. Anorg. AlIgem. Chem. 290 (1959) 279.

6) Go-ic/ins Handbuch der anorganisehen Cheat/c, Syst.—N r. 41,

Titan, 8. Auflage (Verlag Chemie, GmbU, Weinheim/Bergstrasse, 1951). 7) H. Schafer, Chernisehe Transportreaktionen (Verlag Chemie, GmbH, Weinheim/Bergstrasse, 1962).

8) M. Ch. Karapet’janc, ChimiCes/~ajaterinoclinamika (Goskhimizdat, Moscow-Leningrad, 1953). 9) Titan iego splare, Vyp. V (lzd. Akad. Nauk SSSR, Moscow, 1961) pp. 211—219. 10) Tennodina,niCeskie .svoj.s-tra ,ieorganiCeskik/, ve6~’estr(Atomizdat, Moscow, 1965). II) Ter,ni7eskie konstantv ve!~estv, Vyp. II (lzd. Akad. Nauk SSSR, Moscow, 1966). 12) M. Farber and A. J. Darnell, J. Chem. Phys. 23(1955)1460. 13) W. Kongoro and R. Jahn, Z. Anorg. Chcm. 210 (1933) 325. 14) P. Galmische, Ann. Chim. 3 (1948) 243.