Solid state growth of some I-III-VI2 chalcopyrite crystals

Journal of Crystal Growth 99 (1990) 747-751 North-Holland

747

SOLID STATE G R O W T H OF S O M E I - I t I - V I 2 C H A L C O P Y R I T E CRYSTALS

Nobuyuki YAMAMOTO, Kazumi YOKOTA and Hiromichi H O R I N A K A College of Engineering, University of Osaka Prefecture, Mozu, Sakai, Osaka 591, Japan

Solid state growth of some I - I I I - V I 2 chalcopyrite crystals has been carried out using the respective I2VI and III2VI 3 binary compounds as the starting materials. In the cases of CuInS2, CuInS% and CulnTe 2, highly densified ( > 90% of theoretical density) and oriented crystals can be obtained by preparing a highly condensed pressed compact of a finely powdered stoichiometric mixture and utilizing a closely fitted carbon crucible during growth. In the cases of CuGaS2, AgGaS2 and AgGaSe 2, however, highly densified but disoriented polycrystals are obtained. The result is discussed with the aid of respective pseudo-binary phase diagrams.

1. Introduction

I - I I I - V I 2 chalcopyrite compound consists of a low melting point group III element (A1, Ga and In), a volatile high vapour pressure group VI element (S, Se and Te) and a group I noble metal having a high migrating activity in the solid. Therefore, group I vacancies or interstitials and group VI vacancies may be the most prominent intrinsic defects in the chalcopyrite lattice. Control of stoichiometric composition during the melt-growth from the constituent elements of the chalcopyrite compound is thus achieved only with considerable difficulty. In addition, there is a wide liquid immiscible region in all the I - V I and I I I - V I binary sections of the I - I I I - V I ternary phase diagram and, as was experimentally discovered by Fearheiley in the case of CuInSe2 [1], this liquid immiscible region extends widely into the ternary triangle composition diagram. Inhomogeneous compositions of melting mixtures are thus plausible when the constituent elements are selected as the starting materials. In order to obtain a stoichiometric and homogeneous chalcopyrite compound, we selected two binary compounds, I2VI and III2VI 3 compounds (e.g., Cu2S and In2S 3 in the case of CulnS2), as the starting materials. These binary compounds are the most stable compounds respectively in the I - V I and I I I - V I binary phase diagrams, and also have high enough melting points comparable with 0022-0248/90/$03.50 (North-Holland)

© Elsevier Science Publishers B.V.

those of the respective chalcopyrite compounds. In this paper, we describe the experiment and results of solid state growth of some I - I I I - V I 2 compounds (CuInS2, CuInSe 2, CuInTe 2, CuGaS 2, AgGaS 2 and AgGaSe2) by sintering the two binary compounds below the melting point of the respective I - I I I - V I 2 compound.

2. Growth experiment A stoichiometric mixture of the respective binary compounds ( C u 2 S : In2S 3 = 1 : 1 in the case of CuInS2) was crushed manually into a fine powder with an agate mortar and pestle. This made an average powder grain size of 1 / t m within about 10 h of manual crushing [2]. For solid state growth by sintering, both large active surface area and enough homogeneity of mixture are desirable. After a sufficient drying time in the desiccator the fine powder mixture was pressed into a disc-shaped dense compact of 8 mm in diameter and 1-2 mm in thickness under a pressure of up to 7 t o n / c m 2. The compact was then inserted into a closely fitted carbon cylinder with a capped carbon rod of 20-50 mm in length. The assembly was sealed into a silica ampoule under evacuation of about 2 × 10 -5 Torr, as is shown in fig. 1. The sintering process was carried out in a box type fire brick furnace which provides a minimal vertical temperature fluctuation of the order,

748

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N. Yamamoto et al.

,~

silica ampoule

J

carbon

Solid state growth o f some I - I I I - V I 2 chalcol~yrite c ~ s t a l s

rod~

carbon

cylinder ~

1

. compact

carbon rod

compact

lOmm

Fig. 1. Setting of two pressed compacts in closely fitted carbon cylinder with capped carbon rods.

+_5°C at 1000°C. A typical sintering process was: (1) constant rate ( 2 ° C / r a i n ) heating up to each selected sintering temperature, ~, (2) 60 h holding at the temperature, and (3) slow cooling at a rate of about 0 . 2 ° C / m i n down to a selected temperature depending on the compound to be

o¢ t y p e - B

oc t y p e - A 100( CuInSe2

AgGo S

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~'k

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i

100 /

Cu2Se °c

:.

30

,

i

100(

60C ',',

50 mot, % 701n 2S3

type-C iglhf~ . . . . . .

3. Results and discussion

20C AgzS

°c

30

1100

700

I OOC

-:J~'e-O, '""

400

/

.

[:

90C Ag2Te

50 70 Gozs3 mol. °/o ,

CuGaSe~

1000 -

lOO~..

grown. Selection of the sintering temperature, Ts was made with the aid of the known pseudo-binary cross-section phase diagrams between I2VI and IIIzVI 3 binary compounds. In the first approximation, I - I I I - V I 2 compounds are classified into four typical types depending on the respective IzVI-III2VI 3 pseudo-binary phase diagrams. Fig. 2 shows the four representative phase diagrams for CuInSe 2 (type-A) [3], AgGaS 2 (type-B) [4], AgInTe 2 (type-C) [3], and CuGaSe 2 (type-D) [5]. Type-A compounds have a nearly congruent melting point and the zincblende (6) to the chalcopyrite (7) solid state phase transition occurs below the melting point. CuInS 2, CuInSe 2 and CuInTe 2 belong to this type and CuInS 2 has an additional solid state phase transition between the wurtzite-like (~) and the zincblende (6) structures. Type-B compounds have also congruent melting points but crystallize directly into the chalcopyrite phase without the solid state phase transition. AgGaSe 2 and AgGaTe 2 belong to this type. Type-C and type-D compounds have an incongruent melting point, with the zincblende (6) to the chalcopyrite (y) solid state phase transition in type-C compound and without any solid state phase transition in type-D compound. AgInSe 2 and CuGaS 2 may belong to the type-C and type-D, respectively. In the following, we use a notation Ts°C (6), which represents sintering within the (6) phase temperature range.

',i

All the compounds belonging to type-A, CuInS:, CuInSe 2 and CuInTe 2 were selected to be grown by the solid state growth method. For comparison, AgGaS 2 and AgGaSe 2 were selected as representative of type-B whereas CuGaS z is representative of type-D. Omitted was a type-C compound which is considered to be the same as type-A for the purposes of solid state growth below the melting point.

!!

30 50 70 In,T% real, %

Cu25 e

50

70

Ga2 Ses

mol, %

Fig. 2. Four representative pseudo-binary phase diagrams in the cases of CuInSe 2 (type-A), AgGaS 2 (type-B), AgInTe 2 (type-C) and CuGaSe 2 (type D) compounds.

3.1. CulnS~

As was described in the preliminary report [2], the compact of a stoichiometric mixture of Cu2S

N. Yamamoto et a L / Solid state growth of some I - I I I - V1z chalcopyrite crystals

749

to be (6-30) × 1 0 4 ~2 cm in 950°C (y), (4-30) × 104 f2 cm in 1000°C (8) and (3-10) × 1 0 4 ~ cm in 1050°C (4) samples with an ambiguous increasing tendency in achieved density. Electrical conduction type was found to be all p-type by the thermal probe measurement. However, in the single crystal fragment obtained from the sample sintered at 1050°C (~), the measured electrical resistivity was found to be only 10-60 $2 cm, about 3 orders of magnitude lower than that of the whole tablet. Strangely, the conduction type of the single crystal was measured to be n-type with an electron mobility of about 30 cm2/V • s. Therefore, electrical conduction in the whole tablet is considered to be dominated by grain boundaries. Fig. 3. Parallel striped as grown surface of CuInS 2 crystal grown at 1050 ° C (8).

3.2. C u l n S e 2

and In2S 3 was sintered at 950°C (y), 1000°C (8) and 1050°C (4). The sample sintered at 950°C (y) was found to be polycrystalline consisting of many disoriented small single crystal grains, whereas the 1000 ° C (6) sample is polycrystalline with some large single crystal grains which form disoriented domains but have the same (112) asgrown surface. The density of these samples was measured to be 90-98% of the theoretical density of CuInS 2. In the sample sintered at 1050 ° C (~), there appeared frequently a parallel striped pattern throughout the whole tablet as is shown in fig. 3. The as-grown surface of the sample was found to be (112), and also, the parallel stripes were found to be oriented grain boundaries between (112) surfaces. The density of this tablet reached 100% of the theoretical density within the experimental error. Small rhombohedral column crystals with (112) surfaces could be easily cleaved off from the tablet along the parallel striped grain boundaries. Each small crystal thus obtained is considered to be a single crystal fragment. This result suggests that orientating of the single crystal domains can occur in passing through the solid state phase transition temperature during slow cooling. Electrical resistivity of the samples gradually varied as a function of the sintering temperature

For this compound the sintering temperatures, T s were selected to be 780 ° C (~,), 860 ° C (8) and 900 ° C (8), the phase being related to its pseudobinary phase diagram (fig. 2). Fig. 4 shows the surface photograph of the as-grown tablets and X-ray diffraction line patterns of the respective surfaces. The tablet sintered at 780 °C (~,) is disoriented and its diffraction pattern is not so different from that of a powder diffraction pattern. However, the 860°C (8) sample appeared to be almost a bicrystal considering both its surface photograph and the diffraction pattern. At 900 °C (8), single-crystal-like orientation is observed both in surface photographs and the diffraction pattern. This result suggests that orientation of the crystal grains occurs at the solid state phase tansition temperature, consistent with the result on CuInS 2. The result of density and Hall effect measurements (resistivity p, mobility/~ and electron concentration n) made on these tablets at room temperature is shown in table 1. As the sintering temperature increases the achieved density increases. Electrical conduction type is p-type when the density is low whereas n-type conduction occurs as the density becomes high. Electron mobility of the sample at 900°C (8) is as high as 340 cmZ/V- s, which is high enough and comparable with that of single crystals grown from the melt.

N. Yamamoto et al. / Solid state growth of some I - I I I - VI: chalcopyrite crystals

750

100 -

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I

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g

'= 5 0 " 0 ¢M

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10

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Fig. 4. Surface photograph of as grown CulnS% crystals and their X-ray diffraction line patterns grown at (a) 780 o C (,/), (b) 860 o C (3) and (c) 900 °C (8). G o m b i a et al. [6] have r e p o r t e d the grain g r o w t h in polycrystalline samples b y sintering o n l y at 780 o C ( 7 ) using the pressed c o m p a c t of p r e - g r o w n C u l n S e 2 p o w d e r as starting m a t e r i a l w i t h o u t a c a r b o n crucible. A c h i e v e d d e n s i t y of their s a m p l e

Table 1 Room temperature density and electrical properties of CulnSe2 grown at 780°C (%,), 860°C (8) and 900°C (8) Ts (°C)

Density (%)

Type

p (I2 cm)

/~ (cmZ/V-s)

n (cm -3)

900 860 780

90 88 78

n n p

4 1800 60

350 45 3

4 X 1015 8 X 1013 4 X 1016

was at m o s t 90% of theoretical d e n s i t y a n d no o r i e n t a t i o n of g r a i n d o m a i n s was reported. 3.3. C u l n T e 2

A successful result was o b t a i n e d for this c o m p o u n d g r o w n at a sintering t e m p e r a t u r e of 700 ° C (8). T h e achieved d e n s i t y was a b o u t 93% of the theoretical o n e a n d p - t y p e c o n d u c t i o n with 15 c m 2 / V • s in hole m o b i l i t y a n d 5 × 1019 c m - 3 in hole c o n c e n t r a t i o n was observed. 3.4. A g G a S e , A g G a S e e a n d CuGaS2

A g G a S 2 a n d A g G a S e 2 are selected as representative of type-B c o m p o u n d s whereas C u G a S 2

N. Yamamoto et al. / Solid state growth of sorne 1 - I l l - V I 2 chalcopyrite crystals"

is type-C. Although the attained density could be made as high as 90% of the theoretical one, no orientation of grains was observed for all sintering temperatures examined.

4. Conclusion Solid state growth of some I - I I I - V I 2 compounds was carried out by sintering the pressed compact of the finely powdered mixtures of the respective binary compounds. Successful results were obtained on CuInS2, CuInS% and CuInTe2. Highly densified (almost 100% of theoretical density in the case of CuInS2) and oriented single crystal like tablets were grown by slow cooling through the solid state phase transition tempera-

751

ture after long enough sintering times. Electrical properties of these samples were reasonably good when compared with single crystals grown from the melt.

References [1] M.L. Fearheiley, Solar Cells 16 (1986) 91. [2] T. Miyauchi, N. Yamamoto and H. Higuchi, Japan J. Appl. Phys. 27 (1988) Ll178. [3] L.S. Palatnik and E.I. Rogacheva, Soviet Phys.-Dokl. 12 (1967) 503. [4] G. Brandt and V. Kramer, Mater. Res. Bull. 11 (1976) 1381. [5] L.S. Palatnik and E.K. Belova, Inorg. Mater. 3 (1967) 1914. [6] E. Gombia, F. Leccabue, G . Salviati and D. Seuret, J. Crystal Growth 65 (1983) 270.