CA1087964A

CA1087964A - Crystal growing furnace

Info

Publication number: CA1087964A
Application number: CA266,569A
Authority: CA
Inventors: Alain Brunet-Jailly; Jean Gallet; Bernard Pelliciari
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1975-11-25
Filing date: 1976-11-25
Publication date: 1980-10-21
Also published as: NL184526B; JPS5265777A; NL7612969A; FR2332799B1; GB1545966A; FR2332799A1; DE2653414C2; NL184526C; JPS5946917B2; IT1064616B; DE2653414A1

Abstract

Abstract of the Disclosure The furnace to the production of monocrystals according to the so-called solution depletion method, according to which a solution comprising a solute dissolved in a solvent is displaced in such a way as to crystallise the.
said solute by cooling the solution of the solute in the solvent.
The crystal growing furnace according to the invention is applicable to all crystallisation processes which start with a product (solute and solvent) in the liquid state contained in a substantially cylindrical vertical body which is moved downwards in a thermal profile in such a way that in the lower part of said body the solute crystallises slowly from bottom to top. The crystallisation furnace permits the.
continuous adaptation as a function of the displacement speed of the body and the crystallisation characteristics of the solute in the solvent of the solid-liquid interface position in such a way that the latter remains fixed over a period of time relative to the crystallisation furnace.

Description

1~87J9~4 The present invention relates to a crystal growirlg Eurnace which is used or pLoducing monocrystals.
The furnace accordin~ to the invention- applies more-partlcula-rly-to the production of monocrystals according to the so-called solution depletion method, according to which a solution comprising a solute dissolved in a solvent is displaced in such a way as to crystallise the said solute by cooling the solution of the solute in the solvent. This crystallisation method in which the solute to be crystallised is Iocated in a solvent ge~erally has the maill advantage of crystallisation at a lower temperature than in the case of crystallisation of the solute alone, leading to a crystal with a better crystalographic quality. However, the crystal growing furnace according to the invention can also apply to other monocrystal production procedures, particularly puriflcation by liquefaction and recrystallisation, The cxystal growing furnace according to the invenhon is applicable to all crystallisation processes which start with a product (solute and solvellt) in the liquid state contained in a substantially cylindrical vertical body which is moved downwards in a 1~hermal profile in such a way that in the lower part of said body the solute crystallises 910wly rom bottom to top. The qualities of the monocrystal are greatly dependent on the precision and stability of the thermal profile in which is moved the body containing the substance to be crystallised.
In such crystal growing furnaces the body generally has a vertical movement. The conventionally used furnaces cornprise heating .

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means which make it possible to p~oduce an axial thermal profile within the furnace and means for the relevant linear clisplacement of the body containing the substa~ce to be crystallised and the furnace, Such types of furnace do not always make it possible to obtain good quality crystals by the solutian depletion method" In actual fact, the crystal production conditions change over a period of time whereby at the start of crystallisation the operating condltions (heating the furnace to create the axial thermal profile and the displacement speed of the body) are chosen in such a way that crystallisation takes place in a furnace region where the gradients following the body axis are at a maximum and the transverse gradient is zero in such a way as to create a horizontal flat crystallisation interface. During crystallisation there is a development of the solvent concentration in the solute (i. e. as only the solute crystallises the solute content of the solvent decreases)O This leads to a variation o the crystallisation temperature of the solute in the solvent and consequently a relative displacement of the interface in the axial temperature fixed profile created in the furnace. This phenolnenon more particularly causes a deformation of the interface because crystallisation then takes place at a point where the temperature distribution is les i precisely controlled, i. e. where the transverse gra~ients are no longer zeroO
~oreover, as the crystallisation interface is displaced in the temperature profile, the crystallisation conditions cannot generally be controlled in optimum manner.
In the most frequently encountered case where the body containing ' .. . .. , .. . . . . .. . .. . , . .. ., . ....... _ ..... . . .. . . . .. . .

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' 379~4 the solute and solvent moves relative to the furnace,~the crystallisation furnace.according~to the invention permits the continuous adaptation as a function of the displacement speed of the body and the crystallisation characteristics of the solute in the solvent of the solid--liquid interface position in such a way that the latter remains fixed over a period of time relative to the crystallisation furnacel Thus" the crystallisation furnace is always located at the same point of the furnace in a maxirnum axial temperature gradient and a zero transverse gradient, In this way, as the position of the interface remains unchanged relative to the heating means of the crystallisation furnace, the crystallisation speed is equal to the displacement speed of the body, In the prior art devices, when the crystallisation interface is moved relative to the furnace heating means the crystallisation speed is not equal to the speed of the body but to the difference between the displacement speed of the interface and the speed of the body which, as stated herein-before, does not make it possible to accurately control the operating conditions, Thus, the crystallisation furnace used according to the invention comprises in conventional manner a body filled with a solute mixture to be crystallised in a solvent located in a heating chamber, heating means for establishing a temperature profile according to the axis of the said chamber and means for displacing the said body according to said axis~
The crystallisation furnace according to the invention also comprises:
means A for controlling the instantaneous displacement speed of the body, for e~ample, a tachymetric device whose reference point ., .
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7S~ii4 is controlled by C and which controls the rota.tion speed o:E the motor;
- means B for measuring as a function of the time t the temperature Tma~ (t) within the chamber at a point where said temperature is substantially maximum, for example, an arnplifiex whlch amplifies the voltage supplied by a thermocouple;
means C connected to the means A for calculating as a function of the said displacement speed of the said body the solute conceIltrat;on of the solvent and the crystalllsation temperature T(t) of the solute corresponding to this concentration in the solvent as a function of tha time t, whereby C is for example a mini-computer;
means D for controlling the heating means in such a way that the temperature Tmax (t) is e~lual to the sum of temp~rature T(t) and a temperature increase L~T of constant value, whereby said means .
D are controlled by said means C, Means D comprise, or example, an ampllfier assembly controlling n power stages, fox example having thyristors,- followed by n auto-transormsrs per~nittin~ the distribution of the total power in the . . :
various windings o:~ the fuxnaceO
The accurate knowledge of the concentratiorl of the: 901u-te in the solvent mal~es it possible to calculate the crystallisation temperature o the solute in the solvent at any time and thus adjust the maximum temperatuxe o:~ the chamber above the crystallisation interface in such `. a way that the distance between the maximum tempexature level and the crystallisatlon level remains constant, and the crystallisation temperature T(t) is obtalned at th~ sarne level of ~he chamber.

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- . . - - -,, , : ~ -The chamber ln which the body is located can be scavenged by an inert, reducing or oxidising gas, or a yas containing a gaseous doping product which, for example, circulates from the bottom to the top. In thiscase the furnace operates with an open chamber. It is also possible to operate with a closed chamber by ~illing the complete chamber with a given gas and then closing the same, followed by the commence-ment of the crystallisation operations. In both cases an open body must be used. According to a variant, an open chamber and a closed body are used to which has optionally been added a neutral reducing oxidising gas or a doping agent.
Other characteristics and advantages of the invention can be gathered from the following description relative to a non-limitative embodiment with reference to the attached ; 15 drawings, wherein show:
Figure 1, a general diagram of the crystal growing furnace according to the invention;
Figure 2, an explanatory diagram showing the develop-ment of the crystallisation temperature as a function of concentrations and therefore of time.
In Fig. 1 can be seen chamber 2 equipped with heating -means comprising a plurality of heating rings such as 4 which are heated by resistance and are coaxial to the axis 6 of the chamber. It also shows a body 8 containing the solution 10 o~ the solute in the liquid solvent. The body moves downwards in accordance with arrow 12, being controlled by a motor 14 which also makes it possible to rotate arm 16 integral with body 8 in accordance with arrow 18. As a result of a power , .
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87S`~;4 supply D ~e heating rings 4 make it posslble to produce a thermal proEile within the chamber which is shown in greater detail in fig, 2~ There is an independent power supply for each resistance heating ring such as 4.
In the apparatus shown in fig. 1, a gas supply pipe 20 from a source of supply 22 is shown, whereby the gas which arrives at the bottom is discharged at the top at 23 by, for example, pump 2 ~ This gas is an inert reducing oxidising gas or contains gaseous doping productsO
The upper part 11 of the body is open. The crystallisation furnace also comprises a thermacouple 26 connected to means B for measuring the temperature TmaX (t) within the chamber at level N2J i. e. ordina~e level Z2 where there is a maxlmum temperature in the chamber. The crystallisation interface 30 is located at leyel Nl, i, e. ordinate level 1- Below level Nl at 32 the solute in the solute-solvent mixture 10 is crystallised, The furnace according to the inventlon makes it posslble to maintain the crystallisation interface 30 at a constant level Zl relative to the chamber, whereby the temperature difference between levels Nl and N2 o respective ordinates Zl and Z2 ( S i = Z2 Zl) being constant.
Chamber 2 is, for example, made from refractory alumina and body 8 from quartzO The temperature measuring means B associated with thermocouple 26 ensure that this temperature T (t) has the value controlled by power supply D, As a function o the downward displacement speed of the body, means C malce it possible to calculate the solute concentration of the solvent and conse9uently the crystallisation temperature T(t) of the solute in solvent 10. The linear displacement motor 14 is controlled by means Ao Means C are connected to means A for the purpose of supplying on wire 40 instruchons for the temperature 1 ~7--` . . ~

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programming oE the Eurnace determined by means D. Means C can be, for example, a mini-computer or any other system making it possible to form l~e operations described in greater detail with regard to Eig, 2,~
The crystal growing furnace according to the invention functions in the following way. The body 8 is filled with a solute-solvent mixture, The linear displacement speed oE the body can be constant but is chosen below a critical speed at which the constitutional super-cooling phenomenon appears or if desired is chosen in a continually decreasing manner.
This speed information is despatched in means C where the displacement speed is integrated to determine the solute concentration af the solvent 10 as a function o time, As a function of this concentration, means D
controlled by means C adjust the temperature of le~rel N2 of ordinate Z2 in such a way that the temperature variation between levels Nl and N2 is constant, the temperature at level Nl being such that it corresponds to the crystallisation temperature of the solute In the solvent at a given concentration, Means B associated with thermocouple 26 make it possible to check the maximum temperature value at level N2, The diagram of ~ig. 2 makes it possible to obtain a better understanding of the operation of the crystal growing furnace according to the invention. Body 8 is shown in a first position at 50, representing the start of crystallisation position and at 52 in a second position after a time t, during which time the solute substance 3Z has crystallised, Levels Nl and N2 of ordinate Zl and Z2 are marked relative to axis Oz of the furnace chamber. The temperature axis is shown on $he abscissa whereby curves 54 and 56 representing the temperature gra~ients in the , ' ' .

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~ - ~ j ., .' ., . ~' -' ,' , ;' ' ' '- ' . , ...... ... ,~ - . , . . : , ~ . ., - -~ ~V87~4 chamber develop as a function o timeD Curve 54 corresponds to the initial temperature profile and curve 56 to the temperature profile at time t. The variat ion ~T between the temperatures at levels N2 and Nl at the initial time determine the temp0rature gradient, G T (t) - T(t) 3i The maximum initial temperature is T (O) and higher by ~T than temperature T(O) of the interface. In curves 54 and 55 representing the temperature profile measured in accordance with O for two values o time t, it can be seen that the samé variation ,~,T between tke temperatures TmaX(t) and T(t) is kept constant~, The ~aximum temperatures in the furnace at level N2, T (t) are calculated by means o curve 58, representing the crystallisation temperature T(t) of the solute in the solvent as a function of the solute concentration x in the solvent. It can be seen that as from an initial concentration x( O) at the initial time corresponding to the solute crystallisation temperature T(O), concentration x(t) decreases with time in the same way as the corresponding c:rystallisation temperatu~e T(t). Curve 58 is a curve plotted as a fu~ction o the phase diagram of the solute in the solvent.
W-ith reference to flg. 1, as the lowering speeds are determined by means A, the integration of these means makes it possible at C to determine the quantity of solute crystallised and consequently the solute concentration value x(t) o the solvent. The programmer in C
then determines the crystallisation temperature value T(t) and despat~hes by heating means D the temperature value T (t) = T(t) + l~T to be , ~

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9~4 applied at level N2, whilst the value applicd at level Nl is T(t)~
The furnace according to the invention malces it possible to produce monocrystals particularly for semi-conductors of the type II~VI and IV-VI such as for example CdTe, ZnTe, HgTe, PbTe, PbSe~
SnTe, MgTe, CdHgTe, PbSnTe, PbSnSe, CdZnTe, ZnMgTe, CdMgTe7 the solvent being tellurium.
The compositions obtained are o excellent quality because the crystallisation speed is the true displacement speed of the body, making it possible to maintain the crystallisation interface in the regions of optimum gradients.
Example s The crystal growing furnace has made it possible to produce the following crystals:
ZnTe: rod: length 250 mm diameter 4S mm low impurity concentration dislocations below 1000/cm suitable for electroluminescence ZnMgTe: rod: length 250 mm diameter 4S mm i'' '.
low impurity concentration dislocations below 1000/cm suitable for electroluminescence CdTe: rod: leng~ 2S0 mm CdMgTe: diameter 45 mm .

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. ~L(187~;4 low im purity concentra~on dislocahons below 1000 / cm suitable for nuclear detection ' '. ~ ~ ' ' ' .

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Example 1 Constant crystallisation speed.
Body 8 is filled with a mixture formed from 620 g of cadmium and 1055 g of tellurium corresponding to a molar tellurium fraction equal to 0. 6 and an i~itial crystallisation temperature of 967 C.
In this Example a constant displacement speed for the body is selected~ being equal to 300 microns per hour. Under t~iese conditIons the furnace regulating temperature given by means D is a parabolic time function given in column 6 of Table I hereinafter~ This unction is programmed in means C whlch control means D.
On the basis of these constituents a CdTe crystal of diameter 45 mm and length 148 mm is produced.
The development of the system is summarised ln Table I.
Example 2 Varizble crystallisation spe=d, The body 8 is filled with a mixture formed rom 620 g of .
'~ - cadmium and 105S g of tellurium corresponding to a molar tellurium fraction equal to 0. 6 and an initial crystallisation temperature of 967C.
In this Example the furnace temperature.variations are linear t ` a~ a function o~ time (column 6 of Table II hereina:Eter) and the speed controlled by means A is continually variable. This speed decrea :es - in linear manner as a function of the crystallised length (column 2). It is programmed in means C; which control means k.
On the basis of these constituents, a CdTe crystal of diameter 4S mm and length 148 mm was produced, ~ .

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The development of the system is summarised in Table II, Table I
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Crystal- Speed in Time in Te concen- Interface Temperature lised mm/hhours tration temper- measured in length ature . 26 y mm y'mm/h t h x Te Ti C Ti + 30C

O 0~,30 0 0. 600 967C 997C
~4 113 0. 617 9S0 980 56 187 0. 838 930 960 76 2S3 0. 662 910 940 89 297 0. 686 890 920 -100 333 0. 711 870 900 109 363 0. 737 850 ~ 880 116 387 0. 762 830 860 120 400 0. 786 810 840 126 420 0. ~09 790 820 130 433 0, 830 770 800 133 4g3 0. 849 750 780 138 . 460 0. 890 700 730 148 g93 0.990 446 . ` :
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Table II
Example CdTe at variable speed Crystal- Speed in Time i~ Te concen- Interface Tem~erature lised mm/h hours tration temper- measured in length ~ture 26 y mm y'mm/ht h x Te T. C T 26 =
Ti ~ 30C

0 0.92 0 0. 600 967 997 34 0,72 42 0. 617 950 980 ~ 0.59 75 0. 638 930 960 76 0~4~3 112 0,662 910 . 940 89 0~ 40 142 0. 686 890 920 100 0.34 171 0. 711 870 900 109 0.29 200 0.737 8S0 880 116 0.25 226 0.762 830 860 120 0.22 243 0.786 810 840 126 0. 19 272 0. 809 790 820 130 0. 17 295 ~. 830 770 80~
133 0.15 314 Ø 849 750 780 138 0. 12 351 0. 890 700 730 148 0.06 464 0.990 446 .. .. ... . . ...... .. ......... .. .. ..... ... . .. .. . .. ...

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Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. Crystal growing furnace comprising: a body filled with a solution constituted by solute to be crystallised in a solvent located in a heating chamber having a cylindrical form;
heating means for producing a temperature profile in accordance with the axis of said chamber; and means for displacing said body in accordance with said axis, characterized in that it also comprises:
means A for controlling the instantaneous displacement speed of the body;
means B for measuring as a function of time the temperature Tmax(t) within the chamber at a level where the said temperature is substantially at a maximum;
means C connected to means A for calculating as a function of the displacement speed of the body the solute concentration of the solution and the crystallisation temperature T(t) of the liquid corresponding to this concentration as a function of time t;
means D for controlling the heating means in such a way that for all time values t the maximum temperature Tmax(t) is equal to the sum of the temperature T(t) and a temperature increase .DELTA.T of constant value.

2. Crystal growing furnace according to Claim 1, characterized in that the said chamber is open and in that the said body is closed.

3. Crystal growing furnace according to Claim 1, characterized in that the said chamber is open and in that the said furnace also comprises a supply pipe located in the lower area of the chamber and a discharge pipe located in the upper area, as well as a supply source for feeding a gas into said supply pipe.

4. Crystal growing furnace according to Claim 1, characterized in that the said chamber is closed and in that it is filled with a gas.

5. Crystal growing furnace according to Claim 3, characterized in that the said gas is an inert, reducing or oxidising gas, or a gas containing a gaseous doping product.

6. Crystal growing furnace according to Claim 1, characterized in that the heating means for producing a temperature profile within the furnace chamber comprises a series of resistance heating rings surrounding the said chamber, whereby the temperature in the vicinity of each ring is regulated by means D.