CA1214380A - Method of preparing a plurality of castings having a predetermined composition - Google Patents

Abstract

ABSTRACT:

A method of simultaneously preparing a plurality of castings of a solution of an element or compound A in a solvent B which is an element and/or a compound, which solution is saturated at a temperature Ts. A problem in liquid epitaxy growth processes is the simple preparation of growth charges which are of sufficiently reproducible composition and size to produce epitaxial layers of repro-ducible composition and thickness. A melt 12 consisting of B, or of B together with at least one of the constituents of A, or of a solution of A in B which is unsaturated at Ts is prepared in a trough 5 of a boat body of a liquid-tight boat assembly 1. An equilibrium is established at Ts between the melt 12 and a solid body 13 which is an excess of any constituent in which the melt 12 is deficient with respect to the saturated solution, so as to form a saturated solution.
The saturated solution is transferred into a plurality of moulds 9 in which this solution is allowed to solidify to form the castings. The solid body 13 is contained in a solid-retaining portion 6 of the trough 5, which portion 6 is separated from the remainder of the trough 5 by an aper-tured partition 8.

Description

P~ 32886 ~ 3~ 9-5-1983 "Method of preparing a plurality of castings having a pre-determined composition".

The invention relates to a method of preparing a plurality of castings having a predetermined composition, and particularly to a method of preparing castings which consists of charges of a solid s~lution which are to be used in liquid phase epitaxy growth processes.
A problem in the growth of layers of ternary of quaternary compositions by liquid phase epitaxy processes is to produce layers of a reproducible composition and thickness. This problem is particularly significant in processes using a sliding boat method due to the small quaniity of liquid used for growing a layer, which makes composition control difficult~ and also when growing terna~
r-~ materials having a general formula iIl which the solid deposited from a liquid solution has a very different composition from that prevailing in the liquid. Many of the III-V compounds show this effect, as does cadmium mercury tellu~.ide. ~1 a sliding boat meth.od~ a substrate is contacted with a solution of the layer material~
which solution must at some stage of the contact time be supersaturated in order to allow growth to occur. Since for most solvents used in liquid phase epitaxy9 the solu-bility of the layer material in the solvent increases with increasing temperature, it is possible to define a ~aturation temperature Ts below which the solution is supersaturated with respect to the layer material~
I tmay be difficult to prepare a growth solution having a predetermined composition by weighing each of the constituents of the solution sufficiently accurately9 par~
ticularly when one of these constituents forms a small proportion (say less than 5% by weight) of the solution, and it would be necessary in such a case to add this constituent in the form of very fine particles, thus having a large specific surface and being prone to contaminationO

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P B 328~ -2- 9_5-1~83 It is desirable :in a manufacturing process of liquid phase epitaYy growth that each charge of solution used should h~e an almost identical Ts value. Preferably each charge should consis-t of essentially the same quanti-ty of material in order that both the composition andthickness of the gro~n epitaxial layers should be con-sistentO
The invention provides a method of simultaneously preparing a plurality of castings of a solution of A which is an element or a compound in a solvent B which is an ~ement and/or a compound, which solution is saturated with A at a temperature Ts9 the method comprising the steps of preparing a melt consisting o~ B, or of B togethcr with at least one of the constituent elements of A, or of a solution of A in B which is unsaturated with respect to A
at Ts in a trough of a boat body of a liquid-t-rght boat assembly, establishing an equilibrium at the temperature Ts between an excess of a solid body and the~elt so that part of the solid body dissolves in the melt so as to form the saturated solution, which body is solid at Ts and consists of A or a~least of any constituent of A in which the initial melt composition is deficient wi-th res~ect to the saturated solution composition, pouring the saturated solution into a plurality of moulds so as to fill the moulds which open directly or indirectly into the trough~
and then allowing the saturated solution in the moulds to solidify to form the castings, wherein the solid body is con-tained in a solid-retaining portion of the trough separated by an apertured partition from the remainder of the trough~
and the equilibrium is established by repeatedly rocking the trough so that the melt cyclically flows into and out of the solid-retaining por-tion of the trough and progressive-ly dissolves part of -the solid bod~.
A method according to the invention provides a simple process for reproducibly ma~ing castings of a solid solution having a predetermined composition. It is pre-ferred that the melt which is equilibrated with the solid body should have the composition of a solution of A in B

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PIIB 32886 -3 9-~ 1983 which is nearly saturated at Ts, in order that the time tal~en to reach an equilibri~m should not be unduly long.
The castings made by a method according to the invention may be used~ for example, as charges for a liquid epitaxy growth process performed in a sliding boat apparatus. A method according to the invention may be used to produce pellets of a material containing a predetermined quantity of a dopant~ Such pellets ca~ subsequently be used9 for example~ to provide a small concentration such as 0-1% Of a dopant in a s~lutio~ used in a sliquid phase epitaYy growth process; or for introducing a smallamount of a dopant into a melt used for growing a single crystal by a Bridgman process.
The saturated solution m~y be a solution of a III-V compound, for example a solution of gallium indium phos-phide in indium~ prepared by .bringing a melt consisting of indium and indium phosphide, either with or without gallium phosphide into equilibrium with a solid gallium phosphide body at Ts.
Tha saturated solution may be a solution of a II-~I compound, for example 9 cadmium mercury telluride in tellurium~ made by bringing a melt consisting of mercury telluride and tellurium~ either with or without cada.nium telluride, into equilibrium with a so~d cadmium telluride body at Ts in a hydrog~n atmosphere. Preferably this melt is contained in a carbon boat in order to improve the freedom from surface oxidation of the charges~ By using a carbon boat in the presence of hydrogen, a much cleaner product is obtained, since the water produced by reducing the oxide with hydrogen is reduced by the oarbon to hydrogen and carbon monoxide. Thus much more oxide from the surfaces of the melt components can be reduced with a given quantity of hydrogen in the presence of carbon than is possible when the same quantity of hydrogen is used in the absence of carbon.
It was found that a method according to the in-vention made it possible to prepare saturated cadmium mercury telluride solutions in an acceptable -time and to P~B 32886 -4- 9-5-1983 grow epitaxial films having reproducible compositions 7 as will be evident :E'rom Example 1 described below.
The composition of a saturated solution may be ascertained from a phase diagram of -the relevant system, for example, see the article "Liquidus isotherms, solidus lines and LPE growth in the Te-rich corner of the Hg-Cd-Te system" by T.C. Harman in the Journal of Electronic Materials, ~ol. 9 No. 6 (1980),-pages 9~5-961. In order to reduce the time needed to establish the equilibrium, it is preferred to use a melt containing all the constituents of the composition~ and then the equilibrating process is effectively a process in which the melt composition is adjusted to the saturated solution composition, rather than a process in which a major change of composition is effected.
An apparatus used in a method according to the invention may comprise a liquid-tight boat assembly com prising a boat body having a trough provided with a solid-retaining portion separated from a melt-containing portion by partition means which allow the melt to f~ow between the two portions, a plurality of moulds within the boat assembly and which open direotly or indirec-tly into the trough, and a lid, rocking means which rock the boat assembly so as to cause the melt to cyclically flow into and out of the solid-retaining por-tion of the -trougll~ and rotation means which rotate the boat assembly so as to transfer the saturated solution from the trough into each of the moulds. The lid of the boat assembly may define the bottom of each mould. Preferably each mould is tapered.
The boat assembly may be made, for example, of graphite or of a machinable ceramic.
Tlle starting materials used to prepare the sa--turated solution can be in the form of relatively large pieces~ thus having relatively small specific surfaces, so that con-tamination of the solution from contaminants present on the surfaces of the starting materials is reduced. The composition of the charges is automatically adjusted to the composition of the saturated solution at the equilibration 6~3 temperature, lt is thus not necessary to know the liquidus temperature accurately. 1~hen the char~es are used in a liquid phase epitaxy growth processa growth is commenced at the equilibration temperature.
In a method according to the invention, the sa-tur~ted solution is prepared in a scaled boat assembly in which the pellets are cast, and this provides advantages of eliminating contamination (due to changing vessels), o,f ef~iciency ~perations saved), and o~ achieving improved compositional uniformity~ both within a batch of castings and between di~ferent batches.
Some embodiments o~ the invention will now be described with reference to the following examples and to the accompanying drawings, in which--Figure 1 is a schematic side-sectional elevation of a sealed "PYREX" (Trade Mark) ampoule containing a boat assembly used to perform a method according~ to the inventio~
Figure 2 is a plan view of a mould-de,fining member 3 of the boat assembly shown in Figure 1, Figure 3 is a plan view o~ a boat body 2 o~ the boat assembly shown in Figure 1, and Figure 4 is a phase diagram of part of the Al-Te system.
Referring to Figures 1 to 3, a liquid-tight boat assembly 1 made o~ graphite consisted of three main components, namely a boat body 2, a mould-de,fining member 3 and a lid 4. The boat body 2 contained a trough 5 having a solid-retaining portion 6 separated ~rom a melt-contain-ing portion by partition means in the ~orm o,f vertically extending graphite pins 8 which allowed rnolten material free access to a solid body located in the solid-retaining portion 6 o~ the trough 5. The mould-de~ining member 3 was sandwiched between the boat body 2 and thelid IL~ and was provided with five pairs of tapered through bores 9, each bore 9 being closed by the lid 4~hich served as the bottom of each of the moulds formed by the bores 9. The main components 2~ 3 and 4 of the boat assembly were locat-ed by means of clamping members 10 which fitted over taper-~2~
Plm 3288~ -6- 9-5-1983 ed end portions of the main components 2, 3 and 4D Graphite pins 11 passed through holes in the clamping members 10 and the main components 2, 3 and 4.
E~ IPLE 1 '--~a.+o ~$~ derived from Journal of Electronic Materials, Vol, 9 No. 6 (1980) 945-61~ and Journal of Crystal Growth, Vol. 13-14 (1972) 668, indicate that when growing CdX~gl xTe at 500C from a solution having the composition (CdzHgl z)l yTey~ when x = 0.27 z should be approximately 0.054 and y should be approximately o.806. These values of z and y correspond to 0..3931 g CdTe, 9.3943 g ~gTe and 12.1820 g Te in order to provide a quantity of solution which was sufficient to fill the ten mould cavities and to provide an excess o~ 3% by volume.
In order to reduce the time required to reach equilibrium, a melt 12 was prepared using the above-spe-cified quantities of HgTe and Te together with 0.275g of CdTe. A polycrystalline 30urce crystal 13 of CdTe weighing 0.5g was placed in the solid-retaining portion 69and the boat assembly was fitted together and placed in a "PYREX"
ampoule 14. The ampoule 14 was evacuated, filled with 3 x 10 Pa of hydrogen and was sealed. The sealed ampoule was placed in a rocking furnace (not shown) having a temperature uniformity of + 0 D 1 C over a length of 200 mm and a tem-perature stability of ~ 0.1C over a period of 2~ hours~With the charge tipped away from the CdTe source crystal 13~ the furnace temperature was increased to 500.0C as indic~ted by a chromel-alumel thermocouple which has been calibrated to ~ 001C against a similar thermocouple used for the LPE growth systemO After l hour at this temperature~
th~ampoule 14 was rocked (so as to wash the CdTe crystal 13 with melt 12) through an angle ~ 15 about an aYis transverse to its longitudinal axis with a period of 3 minutes for each rocking cycleO Rocking was continued for a further 5 hours. The ampoule was brought to the horizon-tal position and was then rotated 180 about its longitu-dinal axis, and was then rocked 5 times to help distribute the molten charge in the ten mould cavities 9~ The ampoule ~ 3~f~
P~ 32886 -7- 9-5-1983 was then left with a 5 tilt so that the excess liquid was in contact with the CdTe source1 and the ampoule was cooled to room temperature n This procedure was repeated for rock-ing timcs of 10 hours and 15 hours.
The composition of the charge after 5 hours was, in weight %~ Cd 007; Hg 26.2, Te 73~1. After 10 hours the com-position was Cd 0.819 Hg 26.2, Te 73l0 and after 15 hours the composition was Cd 0.83~ Hg 26.3, Te 72.9. The error on the Cd composition is ~ 0.02. The composition expected l from published data is Cd 0~8L~y Hg 26~14~ Te 73.03. Con-verting these compositions to liquiclus temperature indi-cates that for 10-15 hours equilibration, the liquidus temperature of the prepared charges were within 2C of the literature values. Such an error was probably due to tolerances in the thermocouple which are difficult to allow for but by calibrating the LPE growth couple with the charge preparation couple, absolute temperatures are not important. The weights of the charges in a series of runs were constant to ~ lmg (that i.s to ~ 0.5%). This is ne-cessary in order to achieve the desired reproducibilityof layer thicknesses.
The source crystal of CdTe and excess charge was sectioned and polished. T~e first solid to form on -the CdTe source should be the same composition as the LPE layers gro~n from the charge. Analysis of the thin CMT layer on the source gave x - 0.23 measured by energy dispersive analysis of X-rays to an accuracy of 5%. LPE layers were grown from these charges by a sliding boat method similar to that used for the growth of III-V compound layers by I,PE, see, for example "1976 Crystal ~rown and Materials", edited by E. Kaldis and ~I.J. Scheele (North-Holland Publishing Company, 1977), pages 578 to 580, with the refinement of maintaining a partial pressure of l.l x 104 Pa of mercury inside the furnace. The compositions o~ the CdX~Igl xTe layers grown using the$e charges were determined by X-ray fluoroscopy and were found to have x-values in the r~nge from 0~225 to 0.231, the accuracy of the measure-ments being + 1% (-that is ~ 0.002 in terms of the value o~

x). I~hen the oomposi-tions of ~ilrns produc0d with different batches of charges were compared~ it was found that mean x values for sets of films varied by less than ~ o.oo6 from this mean value. It can be seen that this method of charge preparation provides both liquidus and solidus data, as well as making it possible to prepare reproducibly charges having accurat~y controlled compositions.
EXA~IPLE 2 o.3In0.7ASoOL~Po 6 is one of a set of compoundS
which are lattice matched for growth by liquid phase epitaxy on InP substrates. It is shown, for example, by A.T. Gorelenok et al in J. Crystal Growth 60 (1982), p.355, that the growth of t~is material from an indium solution at 650 C requires a solution having the following com-position (expressed in % by weight):
Ga 0~51, As 2.32, P 0.08 and In 97.08. If chargesof two gram mass are to be prepared, each charge will con--tain about 2 mg of phosphorus, and when indium phosphide is used as the phosphorus source, about 7.5 g of InP will be needed for each chargeO Even when making a charge of about 20 g from which ten pellets could be cast, it would be necessary to weigh 75.3 mg of InP.
I~hen using a method according to the invention, a melt was made using 19.3L~ g of indium, 103 mg of gallium 25 and 465 mg of As. This melt was equilibrated at 650 C with a 0.5g piece of InP using a method similar to that described in Example l, pellets being cast after 15 hours rocking. The pellets obtained weighed 1.9 ~ O.lg and had the following composition (expressed in atom /0): Ga o.83 30 In 95.37 7 AS 3 ~ 50 and P 003~. If the above-mentioned gallium aluminium arsenide phosphide formula is written as GayInl yAsxPl x' the values of x and y achie~ed differ from the target values by within + 0.005.
EX~IPLE ~
3s A method similar to that described in Example 1 was used to produce pellets of a tellurium-aluminium com-position containing 2 atom % of aluminium. These pellets were subsequently used to introduce 0.1 atom ~/o of P~ 32886 -9- g-5-1983 aluminium into charges used for t~e growth of n-type cadmium telluride layers 13y a liquid phase epitaxy process.
In this me-thod an unsaturated solution of tellurium in a~uminium telluride was equilibrated at 444C with a pure~
solid tellurium body in order to satura-te the s31ution with tellurium. It will be seen from Figure 4 that the saturated solution contains 98 atom % of tellurium. The saturated solution was then cast to ~orm solid pellets which contained

2 atom % of aluminium.
A melt was prepared consisting of 88mg of aluminium and 20 J 2 g o~ pure tellurium (at 444 C a saturated solution of tellurium in alurninium telluride containing 88mg of aluminium contains 20.37 ~ of tellurium). This melt was then equilibrated at 444C with a solid body consisting of 0.5 g of pure telluriurn. The equilibrated liquid was used to cast ten ;3ellets each weighing lo8 g and which contained 0.43 ~ 0~01% by weight of aluminium, which is within 2~5% of the desired aluminium atom fraction of 0.02.
EXAMPJ,_ The aluminium-tellurium pellets produced by the method described in Example 3 were used to produce charges of an aluminium-doped cadmium telluride solution in tel-lurium, which solution was saturated at 500C with cadmium telluride. These charges were used subsequently ~or the growth of n-type cadmium telluride layers by liquid phase epitaxyO
The phase diagram of the Cd-Te system shows that this cadmium telluride solution contains 2.5 atom % Cd, or 4.720/o by weight of cadmiumO A ~elt was prepared consisting of one of the aluminium-tellurium pellets produced by the me-thod described in ~xample 39 34.2g of pure tellurium and 1.5g o~ cadmiurn telluride~ This melt was equilibrated at 500 C with a lg single crystal of cadmium telluride, and eigh-teen pellets ware cast from the equilibrated liquid, using a method similar to that described in Example l. The aluminium atorn fraction of the equilibrated liquid was O . 001 .

~L2~ ir3 PHB 32886 -lO- 9-5-1983 These cadmium te:Lluride-containing pellets were subseq~ently used to grow n~type cadmium telluride layers by liquid phase epitaxy~ and the grown layers had carrier concentrations of about 2 x 10 5/cm3 at 77Ko EXAMPLE_~
.
A solution was prepared for the growth of GaAsO 3oPo 70 by liquid phase epitaxy at 1000C. In order to grow this material~ a melt was required consisting o~
5.413% by weight As, o~890~0 by weight P~ and 96.690/o by weight Ga~ It is convenient to prepare the melt using GaAs and GaP as the respective sources for the As and P~ pro-ducing a solution consisting of 10.450% by weight GaAs~
2.9060/o by weight GaP and 86~6440/o by weight Ga. A melt was prepared consisting of 1.88g of GaAs, 15.60g o~ Ga and 0.51g of GaP. This melt was equilibrated at 1000C with a solid GaP body and 10 pellets were cast from the equili-brated melt, using a method similar to that described in Exampla l.

Claims (4)

The embodiments of the invention in wich an exclusive property or privilege iz claimed are defined as follows:

1. A method of simultaneously preparing a plurality of castings of a solution of A which is an element or a compound in a solvent B which is an element and/or a com-pound, which solution is saturated with A at a temperature Ts, the method comprising the steps of preparing a melt consisting of B, or of B together with at least one of the constituent elements of A, or of a solution of A in B Which is unsaturated With respect to A at Ts in a trough of a boat body of a liquid-tight boat assembly, establishing an equilibrium at the temperature Ts between an excess of a solid body and the melt so that part of the solid body dissolves in the melt so as to form the saturated solution, which body is solid at Ts and consists of A or at least of any constituent of A in which the initial melt composition is deficient with respect to the saturated solution com-position , pouring the saturated solution into a plurality of moulds so as to fill the moulds which open directly or indirectly into the trough, and then allowing the saturated solution in the moulds to solidify to form the castings, wherein the solid body is contained in a solid-retaining portion of the trough separated by an apertured partition from the remainder of the trough, and the equilibrium is established by repeatedly rocking the trough so the melt cyclically flows into and out of the solid-retaining portion of the trough and progressively dissolves part of the solid body.

2. A method as claimed in Claim 1, wherein the pel-lets consist of charges to be used in a liquid phase epitaxy growth process.

3. A method as claimed in Claim 2, wherein the pellets consist of a solid solution of a II-VI compound.

4. A method as claimed in Claim 2, wherein the pellets consist of a solid solution of a III-V compound.