Technical Physics, Vol. 46, No. 4, 2001, pp. 495–497. Translated from Zhurnal Tekhnichesko

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Fiziki, Vol. 71, No. 4, 2001, pp. 133–135.Original Russian Text Copyright © 2001 by Belyaev, Rubets, Kalinkin.

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Formation of Oriented Cadmium Telluride Films on an Amorphous Substrate under Extremely Nonequilibrium

ConditionsA. P. Belyaev, V. P. Rubets, and I. P. Kalinkin

St. Petersburg State Institute of Technology, Zagorodnyœ pr. 49, St. Petersburg, 198013 Russiae-mail: belyaev@tu.spb.ru

Received June 20, 2000

Abstract—The results of the first experiments related to oriented CdTe film growth on a nonorienting substrate(glass) cooled to negative Celsius temperatures under extremely nonequilibrium conditions are reported. Tech-nological, electron-diffraction, and X-ray investigation results are presented. A condensation diagram charac-terized by two regions within which the growth rate of films is anomalously low is obtained. The films grownat these rates are shown to possess a nearly perfect crystalline texture. The formation processes of the orientedfilms on an amorphous substrate under the above conditions are adequately interpreted in the context of a het-eroepitaxy soliton model. © 2001 MAIK “Nauka/Interperiodica”.

INTRODUCTION

As a rule, oriented films are obtained at elevatedtemperatures (in order to achieve the required mobilityof particles) under conditions that are close to equilib-rium. However, recently, exceptions to this rule havebeen observed. In particular, we unexpectedly obtainedhighly oriented films during condensation of cadmiumtelluride on a cooled mica substrate under extremelynonequilibrium conditions [1, 2]. Afterwards, we man-aged to obtain fairly well oriented films by the samemethod even on an amorphous substrate. In what fol-lows, the first results of studying this phenomenon arereported.

EXPERIMENTAL TECHNIQUE

The specimens under investigation were prepared ina vacuum chamber (with residual pressure of 10–3 Pa)by the quasiclosed volume method on a glass substratecooled by liquid nitrogen [1]. The reactor-evaporatortemperature was equal to 900 K. The substrate temper-ature was measured with a copper–constantan thermo-couple. To control overheating of the substrate surface,we applied the technique from [2]. The deposition ratewas evaluated from experimental data on film thicknessand growth time.

Thickness measurements were performed using anMII-4 interferometer. To investigate the structure, weused an ÉMR-100 electron-diffraction camera and aDRON-4 X-ray diffractometer using CuKα radiation atroom temperature. Surface morphology investigationswere carried out using a PÉM-100 electron micro-scope.

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EXPERIMENTAL RESULTS

We studied the formation of CdTe films underextremely nonequilibrium conditions during CdTe con-densation on a glass substrate. The investigationsinvolved technological experiments, surface morphol-ogy studies with an electron microscope, and X-ray andelectron diffraction studies. The main results are pre-sented in Figs. 1–4.

A typical film surface in the final growth stage isshown in Fig. 1, which demonstrates layer-by-layer andnormal formation. One can distinctly see nuclei (dis-perse particles of a new layer) on the surface of analready formed continuous layer of the film.

Fig. 1. A fragment of a typical CdTe-film surface in the finalgrowth stage. The substrate temperature Ts = 210 K. Magni-fication is ×70 000.

001 MAIK “Nauka/Interperiodica”

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BELYAEV

et al

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10–1

10–2

10–3

300 200 100

I II

2 4 6 8 10

T, Kν,

µm

/s

10–3/T, K–1

Fig. 2. The condensation diagram of CdTe films on a glasssubstrate under extremely nonequilibrium conditions.

(‡)

(b)

(c)

Fig. 3. Electron diffraction patterns of CdTe h-thick filmsgrown under extremely nonequilibrium conditions on aglass substrate at the temperatures (a, b) Ts = 210 and(c) 250 K; h = (a, c) 0.8 and (b) 0.1 µm.

Figure 2 is a condensation diagram that shows tworegions of anomalously low condensation rates. Thefilm growth rate decreased by several orders of magni-tude in these regions. The thick films (with a thicknessgreater than 0.8 µm) grown under these conditions weredistinguished by good crystal quality. This can be seenfrom Fig. 3a, where a typical electron diffraction pat-tern of a 0.8-µm-thick film is shown. In the earliergrowth stage, the same film had lower crystal perfec-tion. In Fig. 3b, this is confirmed by the typical elec-tron-diffraction pattern of a 0.1-µm-thick film. In addi-tion, the earlier stage of film growth was characterizedby a very high rate of layer formation. We failed toobtain an island film directly on a glass substrate underall conditions covered by the condensation diagram inFig. 2.

We note that the films grown outside the modes ofanomalous condensation rate had polycrystalline struc-ture even if these films were thick. This is evident fromFig. 3c, in which a typical electron-diffraction patternof a 0.8-µm-thick film grown at the substrate tempera-ture of Ts = 250 K is shown.

The structural distinction between oriented filmsgrown in different regions of the anomalous condensa-tion rate is illustrated in Fig. 4. The portions of typicalX-ray diffraction patterns from the films grown at thesubstrate temperatures of Ts = 140 and 210 K are shownin Figs. 4a and 4b, respectively. Their comparison indi-cates that the films have different crystallographicgrowth directions.

DISCUSSION

Formation of an oriented layer on an orienting sub-strate under highly nonequilibrium conditions consistsof nucleation, condensation, incorporation, coales-cence, and merging into a continuous layer. Nucleationand incorporation are the characteristic features of theabove process [1, 2]. For such supersaturation levels,the former occurs in a vapor phase, whereas the latterresults from the motion of disperse particles of a newphase due to a soliton mechanism of mass transport atlow temperatures [3]. The combined effect of these fac-tors leads to the correlated orientation of disperse par-ticles and to a low rate of layer formation.

Emergence of solitons is caused by a certain relationbetween lattice constants of a disperse particle a(Tr)and a substrate b(Ts). In the case of their closeness, theneeded relation between them can be attained by vary-ing the substrate temperature Ts in view of its distinc-tion from the disperse particle temperature Tr .

Theoretically, the necessary condition for the origi-nation of solitons was derived only for a one-dimen-sional model. The corresponding specific expression

TECHNICAL PHYSICS Vol. 46 No. 4 2001

FORMATION OF ORIENTED CADMIUM TELLURIDE FILMS 497

with allowance for the temperature dependence of thelattice parameters a(Tr) and b(Ts) is written as [2]

(1)

where f and λ are the parameters characterizing theinteraction forces between atoms of a disperse particleand a substrate and those between the substrate atoms,respectively. At the onset of film formation on an amor-phous substrate, the origination of solitons is impossi-ble due to the absence of a regular crystal lattice on thesubstrate surface. However, in the case of normal layer-by-layer growth, all film layers (except for the firstlayer) are already formed on even highly defectivecrystalline substrates (we recall that, under the aboveconditions, layers are produced by the condensation ofdisperse particles rather than individual atoms). Then,at certain substrate temperatures Ts satisfying the con-ditions (1), the origination of solitons is quite possible.In view of their features [4], solitons enhance themobility of disperse particles and delay the dissipationof their excess energy for a finite period of time, whichresults in a smaller accommodation coefficient and inthe formation of oriented layers.

In our case, the experimental conditions completelycorrespond to the aforesaid. The oriented films weregrown only at the strictly specified substrate tempera-

a Tr( ) b Ts( )–b Ts( )

-------------------------------- 2/π( )3/2 f /λa Tr( )…,>

~ ~

I, a

rb. u

nits

41 39 37 35 25 23 21 19 17 15 13 11 9

~ ~

θ, deg

CdTek(220)

CdTek(111)

CdTek(111)CdTek(511)

(a)

(b)

Team

Fig. 4. A portion of an electron diffraction pattern of CdTe1-µm-thick films grown at the substrate temperatures of Ts =(a) 140 and (b) 210 K.

TECHNICAL PHYSICS Vol. 46 No. 4 2001

tures (when conditions (1) were satisfied). When theoriented layers were formed, the integral growth rateabruptly dropped. In Fig. 2, the soliton energy wasslowly dissipated and the temperature of disperse parti-cles remained high for a finite time. It follows that theaccommodation coefficient was small. The crystal per-fection of a film became better as its thicknessincreased. In Fig. 3, the film grew layer-by-layer, andthe healing of preceding layer defects occurred witheach new layer. The initial stage of film growth wascharacterized by a high rate (when solitons could notyet emerge). In two temperature ranges (I and II) of theoriented growth, our films had two different crystallo-graphic directions of the layer growth: in Figs. 2 and 4,the constants a(Tr) and b(Ts) were different in the con-tact plane of lattices in diverse temperature ranges.

CONCLUSIONS

(1) Under extremely nonequilibrium conditions ofthe condensation of films on an amorphous cooled sub-strate, there are regimes that give rise to the formationof oriented layers.

(2) Formation of oriented films on an amorphoussubstrate under the above conditions is well describedin the framework of a heteroepitaxy soliton model.

ACKNOWLEDGMENTS

This study was supported by the Russian Founda-tion for Basic Research, project no. 99-03-32676.

REFERENCES

1. A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin, Fiz. Tverd.Tela (St. Petersburg) 39 (2), 383 (1997) [Phys. SolidState 39, 333 (1997)].

2. A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin, Neorg.Mater. 34 (3), 281 (1998).

3. S. A. Kukushkin and A. V. Osipov, Fiz. Tverd. Tela (St.Petersburg) 36 (5), 1461 (1994) [Phys. Solid State 36,799 (1994)].

4. R. K. Dodd, J. C. Eilbeck, J. Gibbon, and H. C. Morris,Solitons and Nonlinear Wave Equations (Academic,New York, 1982; Mir, Moscow, 1988).

Translated by Yu. Vishnyakov