Journal of Crystal Growth 55 (1981) 465—469 North-Holland Publishing Company
THE GROWTH OF SINGLE CRYSTALS OF GaSe M. Khalid ANIS Centre for Solid State Physics, University of the Punfab, New Campus, Lahore, Pakistan Received 3 March 1981; manuscript received in final form 12 May 1981
Single crystal ingots with diameter of nearly 10 mm and 20 to 50 mm in length were obtained from the melt using the Bridgman—Stockbarger method. Important physical parameters involved for such a growth process have been studied in detail, particularly the relationship between the lowering rate of the ampoule and the temperature gradient. Thin plate-like GaSe single crystals 2 area and of thickness up to 300 ~m were obtained by chemical vapour transport technique. up to 10 mm
1. Introduction In layer compound crystals, screw dislocations exist with Burgers vectors perpendicular to the plane of the layers, which give rise to growth spirals [1,2]. Gallium selenide crystallizes in a layer structure with perfect cleavage planes along 0001 , due to the weak coupling between the layers. In the system Ga-Se, selenium forms two stable compounds with gallium, GaSe (MP 960 ±10°C) which has four modifications (f3, c, ‘y and ö) and Ga 2Se3 (MP 1020 ±10°C) with a eutectic point present at an approximate composition of 55% Se [4—7].The formation of the two further compounds, Ga 3Se2 and Ga2Se, has been reported by Terhell and Lieth  and Klemm and von Vogel , respectively. However, these researchers concluded that neither of these two compounds could be obtained by direct synthesis of the elements, the only possibility to grow these compounds (Ga3Se2 Ga2Se) is the sublimation method, By surveying the literature on growth of GaSe layer compound single crystals, two suitable techniques were chosen, from the vapour phase by the chemical vapour transport method (in which iodine is used as the transport agent) and from the melt by the Bridgman—Stockbarger (B—S) method. In this paper a detailed study of important growth parameters involved in these techniques have been made, particularly for B—S technique the relation between 0022-0248/81 /0000—0000/$02 .50 © 1981 North-Holland
the lowering rate and the temperature gradient which has not yet been reported in detail.
2. Experimental techniques Silica ampoules, 205 mm long and with 15.7 mm inside diameter, were cleaned with detergent “Decon 90” in an ultrasonic bath, rinsed with distilled water, left for half an hour in isopropyl alcohol refluxer, then heated under vacuum to evaporate water or any other impurity. Stoichiometric quantities of SN gallium and 5N 3 ampoule selenium with nearly 1 mg of iodine per cm volume were loaded in the ampoule. The ampoules were evacuated and sealed at a pressure of i0~Torr, with the reagents maintained at liquid nitrogen ternperature. Crystal growth took place in a two-zone horizontal furnace. The temperature gradient between the two zones is shown in fig. 1. Ampoules were placed in the centre of the furnace with the charge towards the higher temperature zone of 870°C,the lower temperature zone of 720°Cand a growth time of nearly 140 h. On removal from the furnace the empty end of the ampoule was water cooled, which condenses the iodine away from the crystalline material. Two different vertical tubular furnaces were designed to grow GaSe single crystals using the B—S technique. One with two zones, the other containing
M.K. Anis / Growth of single crystals of GaSe
700.~iSe single crystal
600 0__________________________________________________ 20 40 60 Distance cn,
Fig. 1. Temperature profile for the compound GaSe using a two-zone horizontal static furnace.
three zones, which was designed to study the effect of different temperature gradients with respect to the lowering rate. Temperature profiles for the compound GaSe, using a two-zone and a three-zone furnace are shown in figs. 2 and 3 respectively. Two types of silica ampoule of 140 to 160mm in length, 10 mm inside dian~eter,with pointed bottom were used to facilitate the nucleation. One with a straight sharp pointed edge which helps the crystal to grow with its layers along the axis of ampoule and is shown in fig 4a The other was bent at the bottom at 35°from the axis of the ampoule with a sharp end and is shown in fig 4b By bending the tip it is pos
Fig. 3. Temperature profile for compound GaSe using a three-zone B—S furnace. Top zone = 1000°C.
sible to grow crystals where the layers form an angle with the axis of the ampoule which helps the easy cleaving of thicker samples.
Fig. 4. Two types of ampoule used for the Bridgman—
Fig..~2.Temperature profile for the compound GaSe using a two-zone B—S furnace. Top zone 1050°C.
Stockbarger method: (a) straight sharp pointed edge; (b) bent at the bottom at 35°off axis of the ampoule with the sharp end.
/ Growth of single crystals of GaSe
Ampoules were cleaned in a similar way as used for chemical vapour transport technique, and sealed off with the initial stoichiometric total charge of gallium + selenium not exceeding 6 g, under an argon pressure of l0~ to 10~Torr. The charge was slowly heated up to 1050°Cand kept at this temperature for 24 h in order to complete the reaction. The GaSe compound so obtained was then allowed to crystallize by moving the ampoule through a temperature gradient ranging from 30 to 80°C/cm,with the lower ing rates ranging from 0.1 to 5 mm/h. Most of the GaSe single crystals were grown at a temperature of the top zone of 1050°Cand of the bottom zone of 725°Cwith a lowering rate of 2 mm/h in case of the two zone furnace. For the three-zone furnace the top zone was at 1000°C,the middle zone at 850°C,and the bottom zone at 720°Cwith a lowering rate of 3 mm/h.
3. Results and discussion Thin plate-like GaSe single crystals up to 10mm2 area and a thickness of up to 300 1um were obtained, as shown in fig. 5. The hexagonal c-axis is perpendicular to the plane of the plates and the (0001) is the basal plane [81,on which numerous growth spirals in the form of equilateral triangles have already been reported . Good GaSe transparent single crystal ingots with
10mm —I Fig. 5. Chemical vapour grown GaSe single crystals.
Table 1 Temperature gradient and lowering rate for satisfactory growth of GaSe single crystals Lowering rate Temperature gradient (mm/h) (°C/cm) 4.0—6
~ 6085 -______________________ ________________________
no cracks having a diameter of nearly 10 mm and 20 up to 50 mm in length were obtained. The crystals were optically clear with dark red colour and could be cleaved easily, figs. 6 and 7. The crystals have the c-axis perpendicular to the ampoule axis for straight bottom ampoules and at an angle nearly 35°off the axis of screw ended ampoules. X-ray and electron microscope analysis of the grown crystals have shown an excellent crystallographic symmetry of the hexagonal structure which has already been reported , the growth of single crystals of GaSe with natural facets at large angle to the layer by using B—S technique has been reported by Anis and Piercy , and the study of its structure by Anis . Suitable values obtained for satisfactory growth of GaSe single crystals for B—S technique are listed in table 1. In some cases contraction of the GaSe
M.K. Anis/ Growth of single crystals of GaSe
/ Growth of single crystals of GaSe
crystal during cooling causes it to pull away from the silica, breaking the ampoule. This occurred somewhere between 700 and 800°C when the bottom zone was set lower than 700°C.This was avoided by setting the bottom zone at 720°Cwhich helped to anneal the crystal. The effect of other parameters. found to be involved in B—S technique are as follows: (a) Usually a small amount of impurities is always found at the top of the ingot, and due to these impurities, polycrystalline material grows near the top surface of the ingot. This shows that the purity of the starting material has a considerable effect on growth of single crystals. (b) Nonstabiity of the ampoule during growth can result for the main ingot containing the mixture of more than one crystal or in polycrystalline form. (c) The ampoule size and its shape, particularly the shape and sharpness of the pointed end, are very important to form initially only one nucleus. (d) The argon gas pressure around 5 X l0~Torr in the ampoule reduces the possibility of formation of voids in the ingot.
Acknowledgements This work was carried out in the crystal growth laboratory, Applied Physics Department, Brighton
Polytechnic, Brighton, England. Thanks are due to Dr. A.R. Piercy for encouragement and helpful discussion during these investigations.
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