CdTe and CdTe: Cl vapour growth in a semi-closed system

ELSEVIER

Journal of Crystal Growth 184/185 (1998) 1005-1009

CdTe and CdTe

:

Cl vapour growth in a semi-closed system

T. Kunz*,

M. Laasch, J. Meinhardt,

K.W. Benz

Kristallographisches Institut, UniversitiitFreiburg, Hebelstr. 25, D-79104 Freiburg, Germany

Abstract Undoped and chlorine-doped CdTe single crystals of 2.4 cm diameter were grown from the vapour phase by a modified Markov method. Boundary conditions for the control of partial pressures were realized applying a heat sink at different temperatures. The influence of the sink on dopant incorporation was investigated by photoluminescence. Transitions of free and bound excitons dominate the PL spectra in cases of undoped CdTe. The characteristic luminescence of the A-centre was identified in CdTe : Cl. Alpha measurements yielded a product of carrier mobility and lifetime of 2 x 10m4 cm*/V. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: CdTe; Physical vapour deposition;

Semi-closed

system; Photoluminescence;

1. Introduction CdTe, (Cd,Zn)Te and Cd(Te,Se) crystals are used for X- and y-ray detectors [ 1,2]. Particularly, chlorine doping has been used successfully to produce detector material with high resistivity [1,2]. However, the role of chlorine with respect to resistivity is still unclear [3,4]. Commonly, the crystals are grown by the Bridgman method from the melt but the low fraction of useful material grown by this method is a severe problem. Therefore, vapour growth in the semi-

*Corresponding

author. Fax: + 49 761 203 6434; e-mail:

[email protected]. 0022-0248/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00748-3

Chlorine

doping; Compensation

closed system with a potentially excellent reproducibility of the nutrient’s phase composition could be a promising alternative. The growth of twin-free and homogeneous crystals up to 1 in in diameter in such a system has been demonstrated previously [S]. The incorporation of dopants such as Zn, Cu, Ag and Pb in semi-closed vapour growth of CdS was studied by Lauck et al. using the perforated ampoule method [6]. They showed that a low segregation coefficient of a dopant results in an overall low concentration in the grown crystal, whereas in cases of high segregation coefficients, the concentration remains approximately the same as in the feed material. To our knowledge so far there has not been any study on Cl doping in semi-closed CdTe vapour growth.

T. Kunz et al. /Journal

1006

2. Experimental

ofCptal

procedure

7N Cd and Te (Japan Energy) served as starting materials for the synthesis. In cases of Cl-doping, the ampoules were filled with CIZ gas (98%) before synthesis. The synthesized CdTe is granulated and filled into the growth ampoule. The growth setup (cf. Refs. [5,7]) consists of a feed container and a growth chamber which is connected to a mass sink (temperature sink) at a low temperature by an annular slit as shown in Fig. 1. The seed is supported by a plate of either fused silica or glassy carbon. The plate diameter determines the diameter of the crystal during growth; therefore, the crystal grows without wall contact. Calculations have confirmed the relevance of the temperature sink: only in case of a sink temperature below 3Oo”C, a pressure ratio pCd/ pTe2 of about 2 is obtained despite a small excess of either Cd or Te which is typically introduced into the feed material during preparation [S]. In order to establish a diffusive flow regime, a mixture of Ar and H2 (lo/10 hPa at 20°C) was filled into the growth ampoule. Further details of

Growth 184/185 (1998) 1005-1009

the growth set-up, of the ampoule preparation, evacuation and annealing have been published elsewhere [S].

3. Crystal characterisation Hall and Van der Pauw measurements were made at Cl-doped and undoped CdTe crystals. The results are given in Table 1. From the chlorinedoped sample, detector spectra were taken using an 241Am alpha source [l]. A Schottky diod was prepared. A charge collection efficiency of 80% was obtained for a wafer thickness of 0.8 mm applying a bias of 74 V. This yielded a mobility lifetime product of ~7 = 2 x 1O-4 cm2/V. The peak resolution was 10%. Photoluminescence spectra were taken from the chlorine-doped sample, from the undoped sample and for comparison from the (undoped) feed material after growth. Measurement temperature was 4-6 K. The sample was excited by the 488 nm line of an argonion laser with a power density up to 10 W/cm2. The

T/“C

Temperature

feed

:

permeable silica disc

feed seed

crystal platelet

condensed material sink

Fig. 1. Growth

ar

ent and temperature

profile.

T. Kunz et al. J Journal

Table 1 Results of electrical

characterisation

Dopant contenta (cm-3) Contact material Conduction type Resistivity (a cm) Carrier concentration (cm-‘) Carrier mobility (cm2/V s) “Nominal,

added

of CrystalGrowth

CdTe : Cl

CdTe

5 x 10’8 Pt

Au

P 5 x lo5 4 x 10” 30

P 1 x103 8 x lOI 81

1841185 (1998) 1005-1009

1007

30

(a) feed material CdTe undoped Ts=4Oo”C

3.0

(b) feed material Cdle undoped

to the synthesis.

luminescence was analysed using a monochromator SPECTRA PRO 750 mm (ACTON RESEARCH) and a Ge detector (NZ cooled; NORTH COAST). 3. I. PL of undoped crystals The excitonic region of the PL spectra of undoped CdTe is dominated by the luminescence of bound excitons at 1.589 eV (A’, X) and 1.592 eV (Do, X) which can be attributed to acceptor impurities like Na and K or donor impurities such as Cl or F [8,9]. Material that was not transported via the growth chamber shows an additional deep level at 1.473 eV which could be attributed to the “D line” [3]. A broad luminescence band at 1.06 eV is observed only if the sink temperature is high (400°C cf. Fig. 2a and Fig. 2b). The latter has been reported for melt growth; its intensity was reduced by vapour pressure control in a vertical Bridgman arrangement applying a temperature-controlled Cd reservoir [lo]. Consequently, it is interpreted as a deep intrinsic defect not yet known. In contrast, if efficient vapour pressure control was facilitated by a sufficiently low sink temperature (about 50°C) and if the material was transported via the growth chamber, no deep levels could be detected. All the peaks can be attributed either to bound excitons, to the free exciton transition (X) at 1.595 eV or to related transitions implying the creation of LO phonons of about 21 meV. The peaks related to the free exciton exhibit a characteristic asymmetric broadening towards higher energies (cf. Fig. 3). The appearance of free exciton luminescence is taken as an indica-

4

2.0-

F

P .D

3 a

l.O-

0.0 -

J 0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Energy IeV

Fig. 2. Photoluminescence terial after growth using (b) 5O’C.

spectra of undoped CdTe feed maa temperature sink T, of (a)400”C,

tion for low defect concentration and excellent structural perfection of the crystal. It has also been reported for high-purity, vapour-grown crystals and MBE layers [11-131. 3.2. PL of chlorine doped crystals A characteristic deep level was found in cases of Cl doping in the region of 1.3551.48 eV. The dominant lines could be attributed to a zero phonon line (ZPL) at 1.475 eV followed by serveral phonon replicas shifted by 21.5 eV to one another. This is confirmed by a fit assuming a Poisson distribution of Gaussian lines (cf. Refs. [3,4]) which yields a Huang-Rhys coupling parameter of 2.78 (Fig. 4). Thus, the origin of the luminescence was identified as the A-centre involving chlorine (VcdCITe). Additionally, transitions at 1.475 eV (“D line”) and 1.490 eV ((e, A”), e.g. Ag) with relatively weak luminescence have been found.

1008

T. Kunz et al. /Journal

of Crystal Growth 1841185 (1998) 1005-1009

to introduce a shallow donor with an activation energy of E = 14 meV as well as an acceptor with E = 120 meV (e.g. Refs. [3,4]). However, in order to explain the reproducible high resistivity found in crystals grown from the melt or in closed vapour growth systems, additional deep levels have to be postulated [14]. Those defects are assumed to be reduced in our crystals due to the growth conditions, i.e. vapour growth at a partial pressure ratio

vapour grown CdTe undoped Ts=WC

I

PCdIPTe2 of 2. I,.

h,,

1.45

1.50

8,.

1.55

a,

“,

1.60

I

1.65

Energy IeV

5. Conclusions

Fig. 3. Photoluminescence spectrum of undoped CdTe vapour grown using a sink temperature T, of 50°C. The theoretical curve was obtained using a ZPL energy of 1.595 eV, a phonon energy of 20.3 meV and a Huang-Rhys parameter of 0.7.

100

vapour grown Cd1e.C

(A”. XI. (Do. XJ

Ts=28O’=C 80

Energy /eV Fig. 4. Photoluminescence spectrum of chlorine-doped CdTe vapour grown using a sink temperature T, of 280°C. The theoretical curve was obtained using a ZPL energy of 1.475eV, a phonon energy of 21 meV and a Huang-Rhys parameter of

4. Discussion The PL spectra of undoped CdTe crystals show a reduction of extrinsic and intrinsic deep defect levels in cases of semi-closed vapour growth, especially if a low sink temperature is applied, compared to the feed material. Chlorine doping typically increases the resistivity upto lo9 fl cm and above in other CdTe bulk growth methods [l]. In our growth system, it reaches a value of about 5 x 10’ Sz cm. Cl is known

Undoped and Cl-doped single crystals with 2.4 cm in diameter and about 1 cm height were grown from the vapour phase in the semi-closed system. The photoluminescence of the free excitons confirms the good quality of the undoped samples. Additionally, a reduction of deep levels compared to the feed material is revealed. This explains the relatively low resistivity of about 5 x lo5 R cm of the Cl-doped samples. For detector application the results are promising as far as homogeneity and the pr product is concerned. A Schottky diode detector was prepared which resulted in a charge collection efficiency of 80%. However, for the use as a photoresistance detector, instead of a Schottky diode detector, the obtained resistivity would be too low. Further studies are necessary to elucidate the compensation mechanism.

Acknowledgements The authors wish to thank Mrs. L. Rees-Isele for ampoule preparation, Mr. M. Kranz-Probst for technical support and Mrs. I. Koch, Mrs. S. Stoll and Mrs. N. Kaiser for wafer preparation. Financial support from the European Community under Brite EuRam contract BRE2.CT94.0609 is acknowledged.

References [l]

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of CrystalGrowth

[Z] P. Doty, B.A. Apotovsky, J.F. Butler, P.L. Hink, Proc. 9th Int. Workshop on Room Temperature Semiconductor Xray Detectors, Grenoble, 1995. [3] W. Stadler, D.M. Hofmann, H.C. Alt, T. Muschik, B.K. Meyer, E. Weigel, G. Miiller-Vogt, M. Salk, E. Rupp, K.W. Benz, Phys. Rev. B 51 (16) (1995) 10619. [4] D.M. Hofmann, P. Omling, H.G. Grimmeis, B.K. Meyer, K.W. Benz, D. Sinerius, Phys. Rev. B 45 (11) (1991) 6247. [S] M. Laasch, T. Kunz, C. Eiche, M. Fiederle, W. Joerger, G. Kloess, K.W. Benz, J. Crystal Growth 174 (1997) 696. [6] R. Lauck, G. Mtiller-Vogt, J. Crystal Growth 74 (1986) 513. [7] E.V. Markov, A.A. Davydov, Neorg. Mater. 4 (1971) 575.

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