DE1251294B - Ernest Denicke KoIb New Providence, N J, Robert Alfred Iaudise Berkeley Heights N] (V St A) I Method for growing zinc oxide single crystals - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN S / fflWW> PATENT OFFICE

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COlg

COiG -

12 η -9/02

W36299IV a / 12 η
February 29, 1964
5th October 1967

The invention relates to a method for growing zinc oxide single crystals from a hydrothermal Solution.

In addition to a strong piezoelectric effect, zinc oxide has a considerably higher coupling coefficient than quartz. This discovery led to great efforts to develop breeding methods, with the help of which zinc oxide crystals of sufficient size and quality can be fabricated to convert them into devices comprising a variety of piezoelectric transducer elements to use.

One method of growing zinc oxide crystals from hydrothermal solution is in the journal "Journal of Physical Chemistry", Vol. 64, pp. 688 to 691, May 1960, described. This procedure proved In principle, it was found to be useful, but it often produced heavily contaminated crystals and their structure was faulty or severely disturbed. The faulty structure is the result of deficiencies in the this process occurring growth mechanism are peculiar. In the case of zinc oxide crystals, in Direction of the e-axis the greatest growth speed. During crystal growth the plane of growth, the (OOOl) plane, becomes rough or irregular, as a result of the growth new atomic layers before the original network layer is completed. As a result of the existing gradient The raised areas of the uneven growth surface then begin to over-saturate more quickly to grow as their valleys. Eventually the crystal growth goes into dendritic growth above.

The invention is therefore based on the object of the formation of additional growth stages during to hinder or suppress a growth occurring in crystal planes, so that crystal growth becomes uniform, d. H. that crystal growth under a substantially parallel to shifting of the crystal growth planes taking place.

To solve this problem, the starting point is a method for growing zinc oxide single crystals from a hydrothermal, strongly basic solution containing a metal hydroxide by immersing a zinc oxide crystal seed at a pressure of at least 225 kg / sq.cm and a temperature of at least 300 ° C, with between the In the vicinity of the crystal nucleus and that of the zinc oxide sediment, a temperature difference between 5 and 100 ° C is maintained; the solution according to the invention is that the hydro-method of growing
of zinc oxide single crystals

Applicant:

Western Electric Company Incorporated,

New York, N.Y. (V. St. A.)

Representative:

Dipl.-Ing. H. Fecht, patent attorney,

Wiesbaden, Hohenlohestr. 21

Named as inventor:

Anthony Joseph Caporaso, Summit, NJ;
Ernest Denicke KoIb, New Providence, NJ;
Robert Alfred Laudise, Berkeley Heights, NJ
(V. St. A.)

Claimed priority:

V. St. v. America March 27, 1963 (268 326)

thermal, OH ions in a 2 to 8 molar concentration solution containing lithium ions in a molar concentration between 0.1 and 4.0 be admitted.

The addition of lithium ions is expediently carried out by adding LiF, LiOH or LiB ₄ O ₇ .

In the following the invention is based on an in

the drawing shown in perspective and partially in section, in a similar manner already known Device for performing the method described in detail.

The apparatus shown is an autoclave well known in the art. Its housing 10 is made of high tensile steel. The housing 10 is closed by a cover 11. In the lid 11 a plunger is arranged. A lining 13 is provided in the interior of the housing. The lining is closed by a sealing disk 14 and by the lower part of the plunger 12. The lining consists of a precious metal. In particular, silver is suitable for this because it is hot compared to the alkaline hydrothermal solution is corrosion resistant.

709 650/270

3 4

Lithium acetate, lithium tetraborate and lithivon are suitable for lining the inside of the autoclave Unevenness and machining marks removed. citrate, lithium format, lithium hydroxide, lithium der The bottom of the cavity is processed so that nitrate, lithium oxalate, lithium sulfate and lithium has a radius of curvature of 0.317 cm halogen compounds.

and it is smooth, especially at the transition point 5 The bottom of the autoclave is used as a nutrient from the floor to the wall. A 0.0794 cm thick drainage filled with zinc oxide particles. Entering Hole (not shown) is in the bottom of the Ge the zinc oxide grain size on the growth rate incorporated. The lining is made out of strength and the perfection of the bred Kri-1,14 mm anode silver 'and with a stalle can mean surface size of the particles Sterling silver existing 1.91 mm thick flange io and convection flow in the vessel interpreted who-15 Mistake. The lining is under 72 hours. the. A small grain size creates a dense one At 400 ° C in a helium atmosphere, there is an intermediate packing in the autoclave and prevents a circulating ash glow deep drawn. During the drawing process, the solution passes through the nutrient. the Occurring material stresses and grain wax is effectively presented for a dissolution of the surface By using 15 in this borderline case, tumbling processes are equal to the cross-section of the car lubricant and by slowly pulling on a klavs. Under these conditions the crystal minimum becomes reduced. The flange 15 is attached to the outgrowth due to the size of the dissolving angle determined as the slowest process by means of helium shielding gas arc sweat and welded on. The basis of the invention will be very small. Also, the little ones lying investigations are two autoclave nutrient particles by way of the convex sizes that occur have been used, namely, firstly, the induction currents are swept up and then act in called short autoclaves, the inner dimensions of which the growth zone as additional centers more spontaneous germ formation after lining about 2.22 cm in diameter and as the cause of an irregular and 6.98 cm in height, and secondly, the subsequent growth of the "target" germs. With additional names long autoclave, the dimensions of which are about 25 mender grain size of the nutrient, the circular 3.02 cm in diameter by 17.5 cm in height. tion facilitated by the same, whereby the effec- die Wall thickness was in both cases about tive dissolution surface is larger, so that the 2.22 cm. Dissolution process no longer determines growth

For the hydrothermal growth suitable germs, with the consequence that the growth rate can with the help of the 30 taking place from the melt increases. Likewise, the larger nutrient-breeding methods are used be obtained, which is no longer so easily flooded up in the particles, and journal "Journal of Physical Chemistry", Vol. 64, the possibilities of spontaneous nucleation and 1960, p. 688. The germs are on- irregular growth will be less. but approximately 0.3 mm thick and about 10 X 10 mm in size. very coarse-grained nutrients, on the other hand, only offer one The components of the solution should represent at least the 35 small effective surface area, with the result have the required purity for chemical reagents. that the dissolution process in turn slows growth Furnace and the control devices (not shown, so that the growth rate is set) must be designed in such a way that sufficient speed is again very small. But regardless of this Temperature control can be maintained general considerations is the actual nutrient and the temperature fluctuations within + 3 ° C 40 fabric grain size are not particularly critical. The best lie. The temperatures are obtained by thermal results if the grain size is between elements measured, which is at different points of the 0.2 and 0.635 cm.

Are attached to the autoclave. The inner part of the car As mentioned above, the seed crystals can klavs shows, as shown, the arrangement of the grown from the melt. However, you can Growth germs 16, with regard to the amount of nutrients 45, also germs that can be used by means of the invented 17. A diaphragm 18 separates the growth zone according to the method. In of the nutrient zone and serves to maintain it- in any case it has been considered beneficial and important a temperature gradient in between. That found that the surface properties of the Kri diaphragm but should a flow of zinc oxide-rich stall germs essential for the quality of the resulting ge solution allow in the growth zone. The dia- 50 are grown crystals. A previous etching phragma has for this purpose passage openings, whose the crystal nuclei, with the help of which surface cross-section preferably 2 to 20% of the total damage and surface irregularities cross section is. An advantageous diaphragm can be removed is particularly effective, structure has 5% passage cross-section, with order practically a useful growth To achieve a half through a central opening and the 55 speed it is necessary to change the car Half by the open clav distributed around the circumference at least to 60 per cent of its total volume is formed. The seed crystals will replenish at room temperature. With increasing suspended from a silver wire 19. The degree of filling increases the rate of growth

The hydrothermal solution 20, which the autoclave too. Advantageously, one degree of filling is used

Fills at the operating temperature, consists of a 60 worked from 70 to 9O%.

Solution of 2 to 8 molar alkali or alkaline earth For the correctly occurring wax metal hydroxide and lithium ions, which in a 0.1 turn it is essential, a temperature gradient of up to 4.0 molar concentration is present. Suitable alkali and alkaline earth compounds between the nutrient zone and the crystal wax are to be maintained within the solution. NaOH, KOH, CsOH, RbOH, Sr (OH), Ba (OH) ₂ and 65. This factor is essential for the mixtures of these. Sufficiently soluble lithium growth rate. If the temperature compounds are too small, they are suitable for generating the desired temperature gradient, which leads to uneconomically low wax / lithium ion concentrations. Especially fantastic speeds. Becomes the temperature gradient

increases, the rate of growth increases, but at the same time the probability of an undesired occurrence on the walls of the autoclave increases spontaneous nucleation. In addition, irregular crystal growth also begins here. Likewise, at very high growth rates, there is a tendency for crystal growth turns into undesired dendritic growth.

It is also useful to detect large temperature differences between the nutrient zone and the walls of the Avoid using an autoclave. This is achieved by slowly increasing the temperature on the operating temperature. It generally takes from a few hours to a few days to complete the To maintain operating conditions. The best results are obtained when the autoclave is on a Temperature is heated, which approximates the operating state with a small temperature difference and which generates the desired temperature difference at or in the vicinity of the operating temperature. It is considered to be useful, the temperature gradient during the heating period below 25 ° C to keep.

Usable growth rates and good crystal qualities are obtained when temperature gradients between 5 and 100 ° C are used. Particularly good results are obtained with temperature gradients between 5 and 25 ° C. This difference relates to the operating temperature is advantageously maintained between 300 and 400 ⁰ C the growth zone. The extreme values of this range are determined by practically unusable growth rates on the cooler side, while temperatures above 400 ° C result in excessively high pressures (when using the above-mentioned degree of filling), which are useless since there is no thermodynamic maximum in the reaction .

The pressure prevailing inside the autoclave depends on the filling level selected and the prevailing pressure Temperature determined. An advantageous pressure range is between 225 and 562 kg / qcm lying pressure viewed. The upper limit is determined, among other things, by the maximum permissible pressure load of the autoclave that was used in the course of the investigations carried out is. However, higher pressures can be used in improved autoclave designs. the lower pressure limit essentially coincides with the critical pressure of the solution, which is known should be exceeded.

The following is the zinc oxide crystal growth performed in accordance with the method of the present invention described using a few examples.

Example I.

The autoclave was charged with a 6 molar KOH and O.lmolar LiF solution. The amount the solution was sufficient to save the total volume of the autoclave at room temperature the volume of the seed crystal and nutrient to make up 83%. The crystal seeds were 0.05 mm thick and had (0001) and (OOOT) crystal faces on. The crystal seeds were attached to a silver wire in the upper area of the autoclave, as shown hung up. Sintered recrystallized zinc oxide was used as the nutrient material. The nutrient The grain size was larger than 0.2 cm but smaller than 0.635 cm. The autoclave was closed and slowly warmed to 353 ° C. The diaphragm used had a passage cross section of 5% and enabled a temperature gradient to exist between the growth and nutrient zones of 15 ° C. The crystal growth was carried out for 30 days. At the end

ίο the growth period crystals were obtained, whose quality was extremely good and whose size averaged 0.797 cm. From this it follows a growth rate of 0.264 mm per day. These crystals are for one application suitable in piezoelectric transducers.

Example II

Example I was repeated, but with a lower lithium ion concentration. The autoclave was charged with a 6.4 molar KOH and 0.01 molar lithium fluoride solution and Filled up to 83% of the capacity. The autoclave was closed and the temperature slowly increased to 353 ° C. A temperature gradient of 15 ° C was created between the growth and nutrient zones maintain. Under these conditions the growth rate was quite fast, and the crystals thus obtained showed a highly disturbed structure and were similar to those of Example I inferior. Since practically only one variable was changed in Example II, these results are the consequence of a smaller and insufficient lithium ion concentration present in the growth solution. Therefore, it is considered essential that the lithium ion concentration under working conditions is at least 0.1 molar.

Example III

Example I was repeated using a temperature gradient of 14 ⁰ C and a hydrothermal solution of KOH and 6,4molarem 0,3molarem LiF. The observed growth rate was 0.228 mm per day and the crystal quality remained excellent.

Example IV

The cultivation conditions of Example III were carried out using a 6.4 molar KOH and 0.2 molar LiOH solution complied with. The results were practically the same.

Example V

In this example, the autoclave was filled to 85% of its capacity with a 6.4 molar KOH and 0.3 molar LiOH solution. The growth ^{temperature was 340 °} C. and the temperature difference was 8 ^° C. The growth rate observed under these conditions was 0.400 mm per day, and the crystals obtained were again of high quality.

Example VI

Example V was repeated omitting the addition of lithium. The crystals thus obtained were severely disturbed and of no practical use.

Example VII

In this example, the growth conditions of Example V were again with the exception chosen that the lithium additive was added in the form of 0.9 molar LiOH. The Crystals were of high quality, but the observed rate of growth was slower.

Example VIII

At 85% filling, a solution of 6.4 molar KOH and 0.2 molar Li ₂ B ₄ O _{7 was} used. The growth temperature was 340 ⁰ C and the nutrient temperature 327 ° C. Again, had the thus-grown crystals excellent quality. The rate of growth was 0.233 mm per day.

Example IX

Example I was repeated. But this time NaOH was used instead of KOH. There were in obtained essentially the same results.

Example X

An experiment was carried out to determine how much lithium ions are compatible with the desired result carried out according to the conditions of Example V, with the exception that lithium in 4 molar concentration. Essentially the same crystals as they have been obtained in Example VII. However, the growth rate was somewhat slower.

Claims (2)

Patent claims: 1. A method for growing zinc oxide single crystals from a metal hydroxide containing one hydrothermal, strongly basic solution ίο immersion of a zinc oxide crystal nucleus at a pressure of at least 225 kg / qcm and a temperature of at least 300 ° C, with a temperature difference between 5 and 10O ⁰ C being maintained between the environment of the crystal nucleus and that of the zinc oxide sediment, characterized in that the hydrothermal, OH ions in a solution containing 2 to 8 molar concentration, lithium ions are added in a molar concentration between 0.1 and 4.0. 2. The method according to claim 1, characterized in that LiF, LiOH or LiB ₄ O _{7 is} added. as Considered publications: Journal of Physical Chemistry, Vol 64 (1960). Vol. 1, p. 689. 1 sheet of drawings