Indian Journal of Pure & Applied PhysicsVol. 46, September 2008, pp. 646-650

Growth of ZnSe single crystals by CVT technique and study micromorphology

M J Tafreshi" & M FazIib

"Physics Department, bChemistry Department, Faculty of Science, Semnan University, Semnan, IranE-mail: mjtafreshi@yahoo.com

Received 12 December 2007; revised 24 June 2008; accepted I July 2008

The vapour phase chemical transport in a closed tube has been investigated for a ZnSe-I2 system. Based on a proposedthermodynamic model, the optimum condition for the growth of ZnSe single crystals is determined. A good agreement hasbeen observed between the predicted optimum growth parameters and the experimental results. ZnSe single crystalsare grown at different temperatures including optimum temperature for 2.5 mg/crrr' of iodine concentration. ZnSe singlecrystals grown at optimum temperature show better structural and morphological properties as compared to othertemperature range.

Keywords: Semiconductors, Crystal growth, Optical microscopy, Crystal Structure

1 IntroductionZnSe is a wide-band-gap semiconductor and an

important contender for the fabrication of blue light-diodes and non-linear opto-electronic components. Inthe process of opto-electronic devices, GaAs hasusually been employed as the substrate, but it ispreferable to use bulk ZnSe single crystal as thesubstrate because homo epitaxial - type componentswith a simple structure and high performance lightemitting can be achieved. For this, efforts have beenmade to grow ZnSe single crystals with low twin anddislocation densities'". The advantages of chemicalvapour transport (CVT) are applicability to the growthof various materials, lower growth temperatures thanthe melting and sublimation points of materials, noloss of source materials or leak of toxic gas comparedwith chemical vapour deposition (CVD) and technicalsimplicity in process control, especially in growingmaterials in space. However, CVT has somedisadvantages such as the problem of establishingoptimum growth condition. The exploratory workneeded to achieve the desired results, when carriedout in an empirical, trial and error fashion, will oftenbe ineffective and time-consuming. The slowness ofthe empirical approach arises from the manyexperimental variables which can affect the results,coupled with the time needed to set-up and run anexperiment before it can be evaluated'": In the presentstudy, a simple thermodynamic model has beenintroduced for the ZnSe-12 system. The system is used

to calculate the optimum conditions for the growthofZnSe single crystals by CVT technique. The optimumvalues have been compared with experimental results.Different growth runs have been carried out inpredicted optimum condition and in conditionsdifferent from optimum values. Structural and micro-morphological properties of all the grown crystalshave been compared.

2 Experimental Details2.1 Estimation of optimum growth condition

A suitable thermodynamic model to describe thevapour phase of chemical transport of ZnSe withiodine is a triple reaction system":

. .. (I)

... (2)

ZnSe (s) + 21 (g) ~Zn12 (g) + 0.5 Se2 (g) ... (3)

Neglecting the dissociation of the solid ZnSe via

ZnSe (s) + 12(g) ~ZnI2 (g) + 0.5 Se2 (g)

ZnSe (s) + h (g) ~ Zn (g) + 21 (g) + 0.5 Se2 (g)

ZnSe (s) ~ Zn (g) + 0.5 Se2 (g) ... (4)

and the dissociation of Znl, vapour via:

Znl2 (g) ~ Zn (g) + 12(g) . .. (5)

the thermodynamic model for Znxe-I, system canbedescribed in the simplest form to a first approxi-mation, assuming the vapour phase contains onlythefour components I, h, Se2 and Zn12' by reaction (I)along with the well-known reaction:

h (g)

Candsourresp

iodicorr

Tare 1

TZns'turenumtemjtramthe t

Fi

PI '2

temj

The

cornZnI2phasvariaconcsho~a furvalur

IS nc

soun

appn

TAFRESHI & FAZLI: GROWTH OF ZnSe SINGLE CRYSTALS 647

12 (g) f-+ 21 (g) ... (6) 0.9

Conservation of iodine inside the ZnSe-12 system 0.8

andassuming the exact stoichiometry of ZnSe in0.7r Pl2source material give the following equations

Irespectively,where n~z is the mole numbers of initial0.6

:y En;iodineand the other n's are the mole numbers of the

';;;' 0.5

"'"correspondingcomponents. ~ 0.4n,

n~2= nl2 + O.Snl + nZn/z ... (7) 0.3

0.2

PZII/2 = 2PSez ... (8)0.1

The equilibrium constants for reactions (1) and (6)aregiven as:

PZlllz

(PSez

)0.5

r,z

K(I) ... (9)

p/K(6) =}J

12

... (10)

:h ofThe partial pressures of components inside the

ZnSe-12may be calculated6 as a function of tempera-ture(1) and the initial concentration of iodine (C) bynumerically solving the Eqs (7) to (10) in thetemperaturerange 400-1200°C in steps of SO°C, withtransporter concentrations of 0.2S-10 mg/cnr' usingthethermodynamic data",

Fig. 1 shows the calculated partial pressures of PI,P12, PSez and PZlllz for C=2.S mg/cm' as a function of

temperature.

Thevalue of a = Pz,~z ,wherePI

2

numults.t in.ions.cro-stals

thewith

(1)

(2)

(3)

PIG = PI + a.5PI + PZn!2 Z Z... (11)

correlates the ratio between vapour pressure of theZnl, molecules and iodine molecules in the gaseousphase, inside the ZrrSe-L system. Fig. 2 shows thevariationof a as a function of temperature for variousconcentrations oftransporter for T2- TI = SO°C. Fig. 3showsthe L\a = a(T2) - a(TI) where, T2- TI = SO°C asa function of deposition temperature (T!) for variousvaluesof concentration of the transporter. Since thereis not much difference between the values of P/ in

2

sourceand deposition zones for L\T = SO°C, L\a can beL\P

approximated as ~. In addition, it has been~2

(4)

(S)

nbe'OXI-

, theI (1)

PZnl2

PSe2

500 600 700 800 900 1000 1100 1200Temperalure (oC)

Fig. I-Partial pressures PJ, Plz, PSez and PZlllz as a function of

temperature for 2.5 mg/crrr' iodine

0.35r /0.3

0.25

~ 0.2

0.15

0.1

0.05

o~ I!' ! I I , "

500 600 700 800 900 1000 1100 1200Temperalure (oC)

Fig. 2 - Variation of a with temperature for various concen-trations of iodine

shown that the change in transport rate is proportionalto theLViIl/

2' i.e., the difference in partial pressures of

Znh in source and growth zones 10. Withoutconsidering the type of migration along the tube, itcan be assumed that the transport rate is proportionalto L\a, i.e., nz-s« is proportional to L\aX t, where nZIlSe isthe mole numbers of ZnSe transported in time t. It canbe observed from Fig. 3, that as the concentration oftransporting agent increases, the curves broaden andthe position of the peaks are shifted to highertemperatures. For all the curves, L\a changes slowly atthe temperatures close to the peaks. This in fact showsthat for a particular amount of concentration oftransporter, there is an optimum growth temperature.

metrotatheopti

648 INDIAN J PURE & APPL PHYS, VOL 46, SEPTEMBER 2008

When crystal is growing at this temperature, the flowof material has more stability along the ampoule. Forgrowth temperatures near each peak, the sensitivity oftransport rate to the temperature fluctuations is less.The flow of material is very much temperature-dependent and the present experimental work hasshown that the control of temperature at the growthinterface is always poor than the accuracy shown bytemperature controllers. Table 1 shows the conditionsused by different researchers to grow proper ZnSesingle crystals by CVT technique. It also demons-trates that ZnSe single crystals have been grown usingdifferent concentrations of transporting agent in therange 0.3 up to 10 mg/crrr'.

The predicted growth temperatures according topresent theoretical calculations are also given inTable 1. A comparison between both values shows

x 10.3

11 682°C

10

9

8

7

tl 6<l

5

4

812°C

,842°C

600 700 800 900 1000 1100 1200Deposition temperature (, C)

Fig. 3 - Variation of f:. a with deposition temperature (TJ) forvarious concentration of iodine

that ZnSe single crystals with proper size and qualityhave been grown in conditions close to predictedvalues of this study.

2.2 Crystal growthAmpoule with the length of ISO mm and inner

diameter of 12 mm was filled with 2 gm of heat-treated ZnSe polycrystalline powder along with iodineat a concentration of 2.S rug/em' of the empty spaceof the ampoule. Both compounds were purchasedfrom Fluka with 99.999% purity. The ampoule,cooled by ice, was evacuated to 2X 10.6 torr andsealed off. The capsule was placed into a three-zonehorizontal furnace controlled by Eurothermcontrollers with the accuracy of ±O.I°C. A reversetemperature profile was developed across the ampoulewith growth zone at a higher temperature for 24 h toremove the sticking powders from deposition zone ofthe ampoule and diminish the active sites. Differentgrowth runs were carried out for the same amount ofundercooling (t.T=SO°C), starting material andampoule geometry but with various growthtemperatures. The growth temperature (TJ) wasselected as the optimum value, i.e., 777°C (corres-ponding to 2.S mg/crrr' concentration of iodine) andvalues which are IS0°C higher and lower than theoptimum value. Each growth run was repeated twiceand carried out for 2 weeks. At the end of eachgrowth process, the furnace was slowly cooled toroom temperature at a cooling rate of SO°CIh to

f

prevent thermal strains. The compositions of growncrystals were determined by Atomic AbsorptionSpectroscopy (AAS) instrument. The structure andmorphology of the grown crystals were studied byusing computer aided single crystal X-ray diffracto-

Fig.777c

3R.T

expeTablwithstrucruns.are twith

Tla go.All t

Table 1- Experimental\y applied and theoretical\y predicted optimum conditions for the growth of ZnSe single crystals

Growth conditions (Experimental values) Predicted growth temperatures f:.T= TExp. - TPredictedRef. no C mg/crrr' T2 ("e) TJ ("e) (0e) (0e)

10 2 80(l 780-790 767 182 3.6 900 850 792 587 4-6 765-825 750-800 802-822 -37II 5 1050 800 812 -1212 5 850 840 812 +2813 5 850 840 812 +2814 5 810 790 812 -22I 5 850-860 840-850 812 +33

85015 5.4 900 840 812 +28

107U4 0.3-10 800-900 700-850 687-842 +10.5

cone:valuecrystsmoostruct

alitycted

Table 2 - The growth conditions and properties of ZnSe crystals

TAFRESHI & FAZLI: GROWTH OF ZnSe SINGLE CRYSTALS 649

Growth parameters Crystal habit, shape and dimensions Observed microstructuresCmg/cm3 t.T(°C) T/ (0C) (mrrr') (shown in)

Cubic, platelets and pyramidal Microsteps2.5 50 777 2 x 3 x I (Fig. 4)

Cubic, tetragonal and hexagonal platelets Microsteps + kinks2.5 50 927 2 x 1.5 x I (Figs. 5,6)

Thin platelets Kinks2.5 50 627 2 x 3 x 0.2 (Fig. 7)

nnerreat-.dinepace.asedoule,and

zone.iermterseJouleh to

ne oferentnt ofand

owthwas

irres-) and11 thetwiceeach

ed toIh torownption: andxl by.acto-

Fig. 4-Microsteps on the surface of ZnSe crystals grown at177°C

meter(make: Enraf-Nonius CAD4-F) attached withrotatinggoniometer. The microscopic observation oftheas-grown crystals surfaces was carried out usingopticalmicroscope and SEM.

3 Results and DiscussionThe results of growth runs carried out in different

experimental conditions have been presented inTable2. In most of growth runs ZnSe single crystalswith cubic structures were obtained. Hexagonalstructureswere also observed in some of the growthruns.X-ray analysis showed that hexagonal crystalsarethe combination of (111), (001) and (011) faceswitha more developed (001) face.

The results of growth experiment are found to havea good agreement with the theoretical calculations.Allthe crystals were grown for the same amount ofconcentration of transporting agent and undercoolingvalue, but at different growth temperatures. Thecrystalswhich have been grown at 777°C showedsmooth, flat and complete faces without micro-structures under a SEM magnification of 3000X.

Cled

Fig. 5 - Microscopes on the surface of ZnSe crystals grown at927°C

Microsteps in the form of parallel and straight lineswith the thickness of the order of micrometer havebeen also observed on the surfaces of the "cfystallinepieces. Fig. 4 shows these microsteps, which aredistributed uniformly on the surface. This quality ofthe crystal surface is due to the stability in flow of thematerials which are transported from source zone tothe growth zone at theoretically predicted optimumgrowth temperature. The crystals which have beengrown at 927°C were found to have uncompletedfaces. This deficiency in quality can be due to theinconsistency of the transport rate caused by smallthermal fluctuation in temperature of the depositionzone. Fig. 5 shows micro steps on the surface of thesecrystals. The steps are not distributed uniformly onthe surface and appeared to be denser and bunched insome portions of the surface. This can be due to thenon-uniformity in the supersaturation on the surfaceof the crystals during growth process. Kinks withresolved structures were also observed on the surfaceof the crystals grown at 927°C (Fig. 6).

650 INDIAN J PURE & APPL PHYS, VOL 46, SEPTEMBER 2008

Fig. 6 - Kinks with well-resolved structures on the surface ofZnSe crystals grown at 927°C

Fig. 7 - Kinks with irregular shapes on the surface of ZnSecrystals grown at 627°C

The crystals which have been grown at 627°C dueto higher partial supersaturation showed irregularhabits. These crystals showed the poorest qualityamong all the grown crystals. Fig. 7 shows the kinkswith irregular shapes on the surface of these growncrystals.

4 ConclusionsBased on a thermodynamic model, the optimum

temperature for the growth of ZnSe single crystal byCVT technique for different concentrations oftransporting agent was predicted and compared withexperimental results. For 2.5 rug/em' concentration ofiodine, ZnSe single crystals were grown in differenttemperatures. Structural and micromorphologicalstudies showed that ZnSe crystals grown at 777°C(optimum predicted temperature) have a better qualityin comparison with crystals grown at 927 and 627°C.

AcknowledgementThe authors are thankful to Dr Babu Varghese,

RSIC, lIT, Chennai, India for his help in X-rayanalysis.

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