Both dislocations with Burgers vector b = a/6 (1 12) and b = a/2 (1 ... method used (DUROSE, RUSSELL; THOMAS et al.; TRUKHANOV et al.; WILLIAMS, VERE).
Cryst. Res. Technol.
26
1991
8
961- 912
Original Papers
I. V. SABININA, A. K. GUTAKOVSKI, T. I. MILENOV, N. N. LYAKH, Y. G. SIDOROV, M. M. GOSPODINOV Institute of Semiconductor Physics, Academy of Sciencesof the USSR (Siberian Branch), Novosibirsk, USSR, and Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria
Melt Growth of CdTe Crystals and Transmission Electron Microscopic Investigations of their Grain Boundaries
Transmission electron microscopy investigations are carried out on CdTe crystals grown in quartz ampoules in a temperature region (1020- 1091 "C) near to the melting point of 1092 "C, by travelling heater method in quasi-closed and in sealed (at 0.135 Pa) volume, and by the Bridgman method from nearly stoichiometric melts. An original method for preparation of CdTe thin foil is reported. Two types of grain boundaries are observed: high-angle misoriented grain boundaries (more than ten degrees misorientation between adjacent grains) and low-angle misoriented grain boundaries (less than one degree misorientation between adjacent sub-grain). Both dislocations with Burgers vector b = a/6(1 12) and b = a/2 (1 10) are present.
Introduction It is known that bulk CdTe crystals exhibit grain structure regardless of the preparation method used (DUROSE,RUSSELL;THOMAS et al.; TRUKHANOV et al.; WILLIAMS, VERE). DUROSEand VERE,and other previously published papers referenced in the first one, have demonstrated that the principal defects in the bulk CdTe are twins, sub-grain boundaries (low-angle misorientations between adjacent grains) and precipitates. These defects have been characterized by means of transmission electron microscopy (TEM) (DUROSE, RUSSELL), scanning electron microscopy (SEM) (IWANAGA et al.), electron beam-induced current/cathod luminescence (EBIC/CL) (CHIN),chemical etching (CHIN;DUROSE,RusSELL; IWANAGA et al. ; WILLIAMS, VERE),optical microscopy and X-ray diffraction (HOLT; TRUKHANOV et al.). DUROSE and RUSSELL show data about sub-grain boundaries microstructure in CdTe crystals grown by two different vapour phase techniques. The present paper report results of experimental study of grain-boundary structure in melt grown CdTe crystals obtained by means of TEM.
Experiments Crystals of CdTe have been obtained in quartz ampoules with coatted walls at temperatures close to the melting point of CdTe (1020-1091 "C) by the following methods:
1.1. Unseeded travelling heater method (THM) in quasi-closed volume In the processes of crystal growth open quartz ampoules have been used with inner diameter of 28 mm and boro-silicate glass coated walls. The source materials used, Cd and Te, had minimum purity of 63'
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SABININA et al.: Melt Growth of CdTe Crystals
99.9999 at.% and the tellurium excess was about 2-6 w.% above stoichiometry. The quartz ampoule was situated in a hermetically closed quartz container, filled with pure ( l o T 6vol.% total quantity of impurities) hydrogen to 1.62 x lo5 Pa. Crystal growth was carried out in a vertical furnace (Fig. I), and the container pulling rates were in the interval 1- 1.6 mm/h for the different experiments.
1.2. Unseeded THM in sealed ampoules The difference between the former method consists in the use of evacuated to 0.135 Pa and sealed quartz ampoules that had coated internal walls with pyrolitic carbon and the container was open (Fig. 1). The other parameters of the process have been kept unchanged.
Ka 2
1
3
40
30
m
10
goo
moo -1. - c
Fig. 1. Sketch of the experimental installation for crystal growth by THM in quasi-closed and in sealed (at 0.135 Pa) volume, 1 three zone furnace, 2 quartz container, 3 quartz ampoule
Fig. 2. Sketch of the experimental installation for crystal growth by Bridgman method. 1 two-zone furnace, 2 quartz container, 3 quartz ampoule
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2. Growth by the Bridgman method Crystal growth has been carried out in evacuated (to 0.135 Pa) and sealed quartz ampoules (internal diameter 28 mm), placed in open quartz containers. Inner walls of the ampoules have been coated with pyrolitic carbon. The source materials used had minimum purity of 99.9999 at.% and ratios from stoichiometry to 2 at.% cadmium excess. The growth had been carried out in vertical furnace (Fig. 2). The temperature gradient was in the range of 3 - 10 K/cm and the container pulling rates were in the interval of 1- 1.6 mm/h for different experiments. The crystals grown by the methods described here contain less than three grains on a cross section, with misorientations of a few tens of degrees between the grains. The X-ray topograms show the presence of low-angle (less than one degree) misoriented grains, as well as highly misoriented grains (Fig. 3).
Fig. 3. Typical X-ray topography of melt-grown CdTe crystal. a high-angle misoriented grain boundaries, b low-angle misoriented sub-grain boundaries
3. TEM investigation In studying grain boundaries by TEM, the specimen preparation technique should enable one to obtain sheet material 50- 100 mm thick from areas of interest, and in the process of local thinning selective etching, due to local strains, should be avoided. TEM specimens (3 x 0.1 mm) were thinned locally to 50- 100 nm on the apparatus as shown in Figure 4 and examinated on a EM 200 TEM, operating at 200 kV acceleration voltage. Microdiffraction mode was used to determine the orientation of adjacent grains. Conjugation plane was
1 3
Fig. 4. Sketch of the apparatus for chemico-mechanical local polishing. I TEM specimen, 2 horizontally rotating table. 3 soft vertically rotating ring, soaked in 2 vol.nfO Br methanol solution
SABININAet al.: Melt Growth of CdTe Crystals
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determined from analysis of stereo pairs, and the dislocation structure was analyzed by the standard method of the dark-field diffraction-contrast image. The results from this study show that all investigated boundaries between grains with large misorientation are either of first or of second order twin nature and have appeared as a result of crystal twinning in the (11 l} plane. Figure 5a shows a microdiffraction pattern of a crystal area containing highly misoriented grain boundary. As far as the selector diaphragm encompasses two adjacent grains, the pattern in Figure 5 a should be regarded as a superposition of two diffraction patterns coresponding to two reciprocal CdTe lattice sections: (110) and (114) (Fig. 5b), i.e. the (110) plane of one grain is parallel to the (114) plane of the second one. This configuration occurs uniquely in the crystal twinning of the zinc blende structure crystals in the (111) plane. By analogy, this can be inferred by the following speculations: the basic matrix Miller indices ( H K L ) are related to their parallel indices (H’K’L’) in a cubic twinned lattice by the following equations:
+ 2QK + 2RL) - H ( Q 2 + R 2 ) ] / ( P 2+ Q2 + R 2 ) , K’ = [Q(ZPH + Q K + 2RL) - K ( P 2 + R 2 ) ] / ( P 2+ Q2 + R 2 ) . L’ = [R(2PH + 2QK + RL) - L(P2 + Q 2 ) ] / ( P 2+ Q2 + R ’ ) , H’ = [P(PH
(1) (2) (3)
where (PQR) are twin plane Miller indices. If ( H K L ) = (110) and ( P Q R ) = (111) according to eqs 1, 2, 3, (H’K’L’)= (114), which coincides with the experimental findings.
Fig. 5a) Microdiffraction pattern of a high-angle grain boundary, b) Section of CdTe reciprocal lattice near the grain boundary
Fig. 6a) and b) TEM micrographs showing lateral twin boundary by different diffractrion vectors. Marker represents 1 pm
Cryst. Res. Technol. 26 (1991) 8
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Fig. 7. TEM micrograph of network of dislocations between low-angle misoriented sub-grains. Marker represents 1 pm
The TEM image of a second order lateral twin boundary area is shown Figure 6a and b. Two areas along this boundary can be delineated: a) - along [lil] and b) - along [ill]. From stereo image analysis it was established that the angle between plane a) and (110) is 90 degrees and between plane b) and (110) is 60 degrees. From this it can be concluded that the grain conjugation plane at area a) is (712) and at area b) it is (011). Dislocation contrast analysis of a second order lateral twin boundary, comparing Figure 6 a and 6b, shows that both dislocations with Burgers vector b = a/6
