3. GROWTH OF SAMPLE CRYSTALS

Transcription

1 3. GROWTH OF SAMPLE CRYSTALS In this chapter, we present a brief description of the melt method for growing single crystals together with providing briefly the details of growing crystals and determination of density, refractive index and composition of the grown crystals. The results are also reported and discussed The Melt Method Melt growth is the best method for growing large single crystals of high perfection relatively rapidly. This method is widely employed for materials which melt congruently and has a manageable vapour pressure at its melting point. Different aspects of melt growth technique have been discussed by Gilman [2], Pamplin [5] and Santhana Raghavan and Raniasamy [8]. Some of the commonly used melt growth techniques are mentioned below Bridgman and related techniques In this method the material to be grown is taken in a vertical cylindrical container, tapered conically with apoint bottom and made to melt using a suitable furnace. The container is lowered slowly from the hot zone of the furnace into the cold zone, which is below the melting point so that freezing started at the lowest point in the crucible and the solidification face moves slowly up the crucible. The requirement that the freezing isotherm should move systematically through the molten charge can be satisfied by moving the crucible on the furnace or by changing the furnace temperature. This method is more suitable for growing crystals like GaAs, silver halides, etc.

2 Kyropoulos technique This method is similar to Czochralski technique but growth onto a rotating seed is achieved by slowly lowering the melt temperature instead of withdrawing the seed. Shallow, large diameter crucibles are required but little control of crystal shape is possible. The quality of the crystal is strongly dependent upon the control of the cooling. This technique has been used mainly to grow halide and oxide crystals Zone melting techniques The birth of zone melting and the discovery of its power of impurity manipulation should be attributed to the paper published by Hann in Zone melting is a generic title given to a large family of techniques (float-zone, traveling solvent zone, zone-pass, etc) which have in common the following feature: "A liquid zone is created by melting a small amount of material in relatively large or long solid charge or ingot. It is then made to traverse through a part or the whole of the charge". A seed crystal can be introduced at the starting end to grow single crystals. In float-zone technique, invented by Therever, a vertical (silicon) rod is held by end clamps and a short molten zone is produced by induction heating and moved along the rod. This method has got the greatest advantage of being crucibleless. Maintenance of the stable zone is due to surface tension and it is suitable for materials with high surface tension and low density.

3 Vernueil technique In this method, chemically pure fine powder of size 1-20 microns emerges through an oxy-hydrogen flame and falls onto the fused end of an oriented single crystal seed fixed to a lowering mechanism. The powder charge is fed from a bunker by means of a special tapping mechanism. Coordinating the consumption of the charge, hydrogen and oxygen with the rate of descent of the seed ensures crystallization at a prescribed level of the apparatus Czochralzki growth (Crystal piilhng) Historically crystal pulling owes its origin to the work of Czochralski (1918) although a vast amount of research and development work by many authors particularly those working in the field of electronics materials has developed the simple Czochralski pulling to a sophisticated technology. Of many crystal growth methods in use today, crystal pulling technique is the only method which can produce crystals weighing from several grams to many kilograms. The Czochralski growth geometry is shown schematically [8] in Figure 4. The material to be grown is melted by induction or resistance heating under a controlled atmosphere in a suitable non-reacting container. The melt temperature is then adjusted to be slightly above the melting point and a seed crystal is lowered to the melt surface. After thermal equilibrium the seed is contacted with the melt and the crystal growth or pulling process begins by slowly withdrawing the seed. With proper temperature control of the liquid, crystallization on the seed crystal can be started as the seed is withdrawn from the melt. Further adjustments of the

4 'cd - QrOove.Ng slot I 1 1 WoI r l r.f coil MCI Plotirium c rcsbce -mac au p1 e DhI Fig.4: Schematic diagram of Czochralski growth geometry

5 49 liquid temperature during the pulling process provide control of the crystal diameter. When the desired length has been reached, the crystal is quickly raised from the liquid surface or the liquid temperature is slowly increased to reduce the diameter. When the crystal is free from the liquid, the temperature is lowered to room temperature and the crystal withdrawn from the growth apparatus [S]. When considering crystal growth using a crystal pulling technique, one has to think about the restrictions placed on the material to be grown, selection of a crucible material and the availability of a heat source. Restrictions placed on the material are: i) the material should melt congruently; ii) iii) it should have a relatively low vapour pressure; it should not have any first order solid-solid phase transition or reconstructive phase transition; and v) there must be a crucible material which is non-reactive with the material above its melting point. Selection of a crucible material is based upon the following facts: i) compatibility with the melt; melting point of the crucible material versus that of the compound; iii) type of heating; iv) chemical stability; and v) mechanical properties.

6 50 For semiconductor materials like germanium, GaAs, GaP, etc the most commonly used crucible materials is silica. For oxide crystals like Al203, LiNb0 3, LiTa0 3, etc. which have high melting point refractory metals or the noble metals are used. For the growth of semiconductor materials and a few of the low melting oxides for which the melting point is less than 1500 C resistance heated furnaces are used as the heat source. For higher melting oxides inductive heating is generally employed. For inductive heating the crucible should be conducting since radio frequency (r.f.) heating induces a current flown in the crucible. Growth Rate: The art of growing crystals by the pulling technique is the manipulation of the pulling rate, rotation rate, thermal geometry and atmosphere to minimize crystal imperfections. Growth rate of a crystal is not the pull rate or the rate at which the seed is withdrawn from the melt. The instantaneous growth rate of any crystal is subjected to wide variations which are due to temperature oscillations in the liquid or rotation about an axis other than the thermal axis. The actual growth rate is therefore the time average of the instanteons growth rate. The rate of pulling is an important factor, which should be considered from the practical point of view. If the rate of shaft withdrawal is f1 and the crystal of radius r, is growing from a crucible of radius r then because the material is conserved, the average growth rate f can be obtained by equating the mass of the solid that has formed and the effective decrease in mass of the liquid [8]. If d 1 and d are densities of the liquid and solid, respectively, then,

7 51 it r2df f (rd 1 - r2d) f = it rd 1f - it rd1f = i2fd1 fdir2 d ir., 2 - dr2 If the densities of the liquid and solid are same then average growth rate is given by fprc 2 f = rcr 2 Thus the average growth rate of a crystal is determined by the pulling rate and the radii of the crucible and the crystal. The radius of the crystal is however controlled by the melt temperature and the temperature gradient at the growth interface Growth of (NaCI).(KCI)y-.(KBr)j L -y Single Crystals Some alkali halide crystals can be grown from solution and all of them from melt. Although some alkali halide mixed crystals can be grown by the solution method, melt method has been preferentially used for the growth of alkali halide mixed crystals. According to a simple model by Toboisky [24] a pair of alkali halides should show complete miscibility if they satisfy the equation Te 4.5S2 where S is the percentage difference in the lattice parameter of the two pure alkali halide crystals. Considering the lattice constants of NaCl (5.6402A), KCI ( A) and KBr ( A) there should be complete

8 52 miscibility at near ambient temperatures for (KCl)(KBr) i system but not for (NaC1),((KCI)I..X, (NaCl),(KBr) 1 and (NaCl)(KC1)(KBr) 1.. systems. So, the melt method (Czochralskj method) was preferred to be used in the present study. Sample crystals needed for the present study were grown by the Czochralski method. A total of 26 crystals [the three end member crystals (NaCl, KC1 and KBr); three binary mixed crystals, viz. (NaCI)05(KCI)05, (NaC1)03(KBr)05 and (KCI)05(KBr)05; and twenty ternary mixed crystals, viz. (NaCl)(KCl)(KBr) 1. with y = 0.2, 0.4, 0.5, 0.6 and 0.8 and x ranging from 0.1 to 0.7 in steps of 0.1] were grown under identical conditions. AnalaR grade samples of NaCl, KCI and KBr were used as the starting materials for the growth. In the case of ternary mixed crystals, mass of each substance, according to the molecular ratio, was calculated using the following relation m = P[x xm 1 + (y x)m 2 + (l -y) m31 where m is the total mass of the mixed substance (in the present work, it was fixed as loog); m 1, m 2 and m 3 the molecular weights of NaCl, KC1 and KBr respectively; x, (y-x) and (l-y) the molecular ratios of NaCl, KCL and KBr respectively, and p a constant to be evaluated by substituting all other quantities. Weight of NaCl = p x x x Weight of KC1 = p x (y-x) x Weight of KBr = p x 0-y) x

9 53 In the case of binary mixed crystals, m = p [x x m 1 + (1-x) x m21 where the symbols have similar meanings as above. NaCl, KC1 and KBr needed for the growth of each sample was weighed according to the above relation and was thoroughly mixed using an agate mortar. This mixed substance was transferred into a silica crucible of 5.0 cm diameter and 5.5 cm height and was kept inside a muffle furnace capable of heating upto 1200 C and controlled by a temperature controller of accuracy ±2 C. The mixture was allowed to melt by supplying power to the furnace. The mixture was heated till the temperature became 10 C higher than the melting point of the substance which has the highest melting point. The system was allowed to remain at this temperature for about 45 minutes so that homogeneous mixing would take place due to convection. The normal procedure followed in crystal pulling technique is to bring a seed crystal in contact with the melt and the crystallization on the seed crystal will begin as it is slowly withdrawn from the melt. In the present study, due to the non-availability of the seed crystal of desired composition, the following technique was employed to grow crystals. The seed rod was brought in contact with the melt and by the suitable temperature control of the melt some polycrystalline material was made to stick into the seed rod. This was used as the seed crystal. A common pulling rate of I.2cm/hour was used to grow all the crystals. All the twenty six crystals were grown by the above method and under identical conditions. A photograph of the puller with the furnace set up is shown in Figure 5.

10 A ft Fig.5: Photograph showing the crystal growth set up

11 Density and Refractive Index Density measurement The technique most commonly used for determining the density of crystals is floatation in a mixture of two liquids, one lighter than the crystal and the other heavier than the crystal, whose proportions are adjusted until the crystal remains suspended in the medium [75, ]. The density of this liquid determined by weighing a sample of known volume is the density of the crystal. In the present study also, this technique was used to determine the density of all the grown crystals. Carbon tetrachloride (CC1 1) of density glee and bromoform of density glee were used as the lower and higher density liquids respectively. About 20m1 of bromoform was taken in a test tube and the crystal for which the density had to be determined was dropped into it. The crystal was floating. CC4 was then gradually added until the crystal was brought to a suspended form. Then the density of the solution was the density of the crystal. Density of the solution was determined by finding the mass of 20m1 of the solution and using the relation d = MN where M is the mass of the solution and V the volume of the solution. Densities of all the grown crystals were determined by this method Refractive index measurement An investigation of the optical constants of a series of chemical salts such as an isomorphous series or other definitely related set of compounds is not complete unless it includes the calculation of molecular refraction and dispersion, in which besides refractive index, density of crystals is also taken into consideration as well as molecular weight of the

12 55 substance. Since all the grown crystals in the present study are not perfectly transparent, direct determination of refractive index was found to be difficult and hence Gladstone's rule [114] was used. Specific refraction according to Gladstone and Dale [114] is refractive index (n) minus unity divided by density (d), i.e., (n - 1) / d. Molecular refraction is this expression multiplied by molecular weight (m), i.e., [(n- 1)/ d]m. The quantity (n - 1) was defined by Gladstone and Dale as the refractive energy and (n - 1) / d the specific refractive energy. In the year 1868, Gladstone [115] advanced the generalization that refraction equivalent of a solution is the sum of refraction equivalents of the solvent and the substance dissolved (solute). It may be expressed by the following formula in which letter P represents the total amount (percentage) of solvent and solute dissolved and P j and P2 are the respective percentages of solute and solvent. 1n_ n P I I = p1 + P2 dj [dij [n2d2 where (n - 1)! d is the specific refraction of solution, (n 1-1)! d 1 the unknown required specific refraction of crystalline substance (solute) in the dissolved condition and (n 2-1) / d2 the specific refraction of distilled water (solvent). About ig of the crystal (weighed accurately) was dissolved in 10cc (log) of distilled water to form an undersaturated solution. Density of the solution was determined by specific gravity bottle method and refractive index by using an Abbe refractometer which can read upto an accuracy of Hence the specific refraction of the solution was calculated.

13 56 Knowing the specific refraction of the solvent (distilled water), required specific refraction of the crystal was calculated. Refractive index of the crystal was determined using the measured refractive equivalent and density Composition determination When a mixed crystal is grown from the melt, in general, composition of the mixed crystal may not be the same as the starting material. This may be due to various factors like the evaporation rate of the end members which may be different for different members. Hence, it is necessary to find the composition of the grown mixed crystals. Several methods have been proposed in the past for estimating the composition of the mixed crystals (mostly binary ones) of alkali halides (see chapter 2, section 6 in this thesis). It has been found that the density and refractive index values form linear relationship with composition for the binary mixed crystals [11]. Assuming that these values form linear relationship with composition for the ternary mixed crystals also, the following relations may be written: d xd1 + (y-x)d2 + ( I -y)d3 n = xn1 + (y - x)n2 + ( 1 - y)n3 Here, d, d 1, d 2 and d 3 represent the densities of mixed crystal, NaCl, KC1 and KBr respectively. n, n 1, n 2 and n3 represent the refractive indices of mixed crystal, NaCl, KC1 and KBr respectively. Composition of all the twenty grown ternary mixed crystals were estimated by solving the above two equations for x and y values. In the case of binary mixed crystal, as it contains only one unknown, composition of the end members present was determined separately by using the linear relations for density and refractive index and the average was taken.

14 Results and Discussion The ternary mixed crystals grown are found to be more hard and stable and less transparent when compared to the end member crystals. Good transparency was exhibited by all the twenty six grown crystals at the time of pulling (at the pulling temperature) but as the crystal was cooled down to room temperature transparency was found to be reduced which may be due to the introduction of thermal defects. Increase in KC1 content also reduces the transparency [NaCl and KBr are colourless while KC1 is white in colour (reported in the literature [137]]. Mixed crystals with high KBr content is found to be hygroscopic. All the crystals grown exhibit cleavage property which shows that the grown crystals are single crystals. Crystals upto a size of 3.5 cm could be grown. Photographs of some sample crystals are shown in Figure 6. The necks formed are due to the temperature flucuations while pulling the crystal (the temperature was controlled to an accuracy of +2 C only). The observed specific refractive energy, density and refractive index of all the mixed as well as pure (end member) crystals are given in table 13. Observed density and refractive index of the end members compare well with those reported in the literature (reported values are given in brackets). Composition of the starting material and estimated composition of the grown crystals are also provided in table 13.

15 r"'. (KCI)03(KBr)03 (NaCI)1(KC1)1(KBr)1 with 1-y0.4 (NaC1)02(KCI)0.4(KBr)04 Fig.6: Photograph showing some of the grown crystals

16 Table 13. Specific refractive energy, density and refractive index values together with the initial and final compositions. Values reported in the literature are given in brackets. System (with Specific Density, Refractive Estimated composition taken refractive d (g/cc) index, composition in for crystallisation) energy, n the crystal (n-1)/d 58 NaCI (2.161) (1.5443) KCI (1.988) (1.4904) KBr (2.751) (1.5594) NaClO5KCl45 NaCI05KBr0 KCI05KBr05 NaCI01KCJ07KBr02 NaCIO2KCI06KBr02 NaCI03KCI05KBr02 NaC1o4KC4KBr02 NaCI05KCI03KBr02 NaCI06KCIO2KJ3r02 NaCI07KC10 1KBr02 NaCI01 KC105KJ3r04 NaC102KC104KBr04 NaCI03KCI03KJ3r04 NaCI04KC102KBr04 NaCI05 KCI01 KBr04 NaCI01 KCI04KBr05 NaCIO2KCI03KBr05 NaCI03KCIO2KBr05 NaCIO.4KCIO 1KHr05 NaCl0 1KC103KBr06 NaCI0 2 KC10 2KBr06 NaCk3KCI0 1KBr06 NaC101 KCI0 K Br NaCI0 296KC10704 NaCI0 690KBr0310 KCI0 499KBr0501 NaC10078KC10724KBr0198 NaCl0159 KCL 641 KBr0200 NaC6282KC10524KBr0194 NaC10389 KCI041 8KBr0193 NaC10479 KC1031 9KBrO202 NaC10595 KQ 21 3K113r0187 NaC1o 704 KCL 091 K8r0205 NaCJO063KCI054IKBr0396 NaCI0 159KC10453KBr0388 NaC10292KC10290K NaC10361 KC10212KB NaCI0 505 KC10 039KBr0457 NaC10133 KC10 363KBro5c NaC10230 KC10 274KBr0495 NaCL 261 KC10 231K13r0508 NaCI0 389 KC10 075KBr0536 NaCI0 110KCIo,93KI3ros97 NaCI0 240 KC10 159KBrO.602 NaCI0 277KC10 103KBr0625 NaCI0 104KC100791(Br0817