FR2633943A1 - PROCESS FOR THE MANUFACTURE OF SILICON-GERMANIUM ALLOYS - Google Patents

Abstract

A process for the manufacture of silicon-germanium alloys, characterized in that initially an SiH4 gas, a GeCl4 gas and a conductivity type dopant gas are introduced into a reaction vessel, in that a silicon-germanium alloy element is deposited on a substrate heated to a temperature not lower than 750 degree (s) C in the reaction chamber, in that a gas of SiH4 a gas of GeCl4 and another gas dopant of the type with opposite conductivity to that of the first dopant gas are then introduced on said first silicon-germanium alloy element, and in that another (second) silicon-germanium alloy element of the opposite conductivity type to that of the first silicon-germanium alloy element is deposited on said first silicon-germanium alloy element, and in that this continuous operation in series is repeated two or more times.

Description

The present invention relates to a method for manufacturing materials

  with directly convertible thermal and electrical energy, that is to say of thermoelectric materials, and in particular of silicon-germanium alloys, and a process suitable for the

  treatment of an element using such alloys.

  As a method for manufacturing a silicon germanium alloy in the form of a thermoelectric material, there can be mentioned a method of

  powder sintering as described in R.A. Lefever, G.L.

  McVay and R.J. Baughman: "Preparation of Hot-Pressed Silicon-Germanium Ingot: Part III-Vacuum Hot Pressing", Materials Research Bulletin 9 863 (1974), and his collection (Part I and Part II). According to this literature, the powder sintering process comprises the following operations: (1) an operation for melting metallic silicon, metallic germanium and doping material, (2) an operation for cooling the fusion obtained during the operation (1), (3) an operation for grinding a silicon-germanium alloy obtained during operation (2) into particles of approximately 10 mesh, that is to say passing through

  the mesh of a 0.740 mm opening sieve.

  (4) an operation of reduction by molding of the silicon-germanium alloy particles obtained during operation (3) into finer particles, and (5) an operation of hot compression of the particles of silicon alloy -germanium obtained during operation (4) in a vacuum enclosure not exceeding 10-5 Torr at around 1300 C and under a

  high pressure of approximately 2000 kg / cm2.

  Furthermore, the Japanese patent Kokai (description

  initial) no 190077/83 describes an invention relating to a thermoelectric material composed of a non-simple crystalline solid solution -2- comprising a plurality of elements and an example in this describes a process for the production of SixGeyBz (x + y + z = 1). In accordance with said process, gaseous materials of SiH4, GeH4 and B2H6 are introduced into a vacuum enclosure with a carrier gas H2 and are decomposed in the enclosure in order to thus obtain a ternary amorphous crystal consisting of

  Si, Ge and B at the growth rate of 50 A / min.

  Thus, it can also be mentioned a process in which a gaseous compound is used as a material and is decomposed in the vapor phase in an enclosure & vacuum

  in order to obtain a thermoelectric material.

  In addition, the briefs of US Pat. Nos. 2,552,626 and 4,442,449 describe methods for the manufacture of Si-Ge alloys in accordance with chemical vapor deposition for thin film but the growth rate of thin films in accordance to these processes

is less than 500 A / min.

  When the thermoelectric materials thus prepared are used in the form of unicouples-PN, silicon-germanium of type P and silicon-germanium of type N are processed in a conventional manner and melted to cope with their use and the product has been bonded.

  by means of hot skates and cold skates.

  The powder sintering process, one of the previously mentioned processes for the manufacture of thermoelectric materials, involves complicated operations as described above and requires special manufacturing conditions such as high temperature not lower than 1400 C during the operation. of material melting, high temperature around 1300 C, high pressure of about 2000 kg / cm2 and high vacuum level not higher than 10-5 Torr during the compression stage & hot or high technical standards to achieve

These conditions.

  -3- Furthermore, in the growth process in accordance with the example of the Kokai patent n 190077/83 o SiH4, GeH4, and B2H6 are materials and are introduced into an empty chamber, the growth rate is as slow as 50 A / mn and such a speed does not respond

not industry goals.

  As described above, even in the technique of said two US patent memories, the growth speed is not greater than 500 k / min, Si-Ge alloys of homogeneous composition in large volumes cannot be obtained, and therefore a desired morphology cannot be expected for the growth layer. Furthermore, when the silicon-germanium alloys thus prepared are used as thermoelectric elements, these must be treated, for example, in the structure as shown in FIG. 6, but a series connection of the Si-Ge branches of the "n type" with Si-Ge branches of the "p type" by means of hot & cold pads and generally must be carried out by a metallization operation and this which is called a bonding operation such as welding. However, the connection operations are complicated and it is difficult to choose a suitable solder for the connection in view of the point of the

  fusion and link quality.

  The present invention creates a method for solving the problems specific to said known methods, namely specific manufacturing conditions such as high temperature, high pressure and high degree of vacuum, and a new method for molding a thermoelectric element which offers a means for simple industrialization, allows the element to be manufactured at a high yield and with a high growth speed and which does not require the separate operations -4 for the series connection of the Si-Ge branches of "type n "with Si-Ge branches of the" p type "by means of hot shoes and cold shoes for molding the element, said operations having been necessary in known methods. That is to say, the invention is characterized by repeating two or more times a continuous operation in series in which a gas of SiH4, a gas of GeC14 and a doping gas of the conductivity type are first introduced into a reaction chamber, a first silicon-germanium alloy is deposited on a substrate heated to a temperature not lower than 750 C in the reaction chamber and a gas of SiH4, a gas of GeC14 and a doping gas of the type with opposite conductivity to that for the above doping gas are introduced on said first silicon-germanium alloy when another (second) silicon-germanium alloy is deposited on said first alloy in such a way that the first silicon alloy- germanium and the second silicon-germanium alloy can be of opposite conductivity types. According to the invention, it is also possible to manufacture a thermoelectric element by a continuous process in series similar to the above process, in which the substrate is composed from the start of a silicon-germanium alloy of the type & conductivity, and in which a silicon-germanium alloy of the type of conductivity opposite to that for the substrate is deposited

on said substrate.

  The invention will now be described in more detail with reference to the accompanying drawings, in which: FIG. 1 is a schematic view of the device

  for carrying out the present invention.

  Fig. 2 shows silicon-germanium alloys produced in accordance with the invention and

unicouples of these.

  Fig. 3 is a thermoelectric module of silicon-germanium alloys produced in accordance with the invention. Fig. 4 shows other silicon-germanium alloys produced in accordance with the invention and

multicouples of these.

  Fig. 5.is a thermoelectric module of said other silicon-germanium alloys manufactured in accordance with the invention, and FIG. 6 is a thermoelectric module of known elements.

  The invention will be described with reference to FIG. 1.

  As will be seen from FIG. 1, the N2 gas, the SiH4 gas, the doping gas and the H2 gas of the gases used in the invention are supplied through pipes 1, 2, 3 and 4. The reference numerals 11, 12

  and 13 are the respective gas flow controllers.

  The hydrogen supplied through line 4 is used for gas substitution in the device and under the

  form of a gas carrying GeC14 stored in a tank 40.

  First, a substrate 31 is placed in a reaction chamber 32. Before the growth of an alloy, the interior of the reaction chamber is emptied by means of a vacuum pump 23 and is converted by H2 , and then

  a predetermined amount of H2 is caused to flow.

  The H2 is placed under a predetermined pressure by means of a constant pressure inlet valve 24 and is then discharged. At the same time, during this operation, the SiH4 and a doping gas of the conductivity type in the same amount as under the initial growth conditions, and in addition to the GeCl4 causing the gez carrying H2, are caused to flow from from a pipe 50 into an adsorption device 27. The substrate is supplied with electricity from a power source 35 and is raised to a desired temperature. The substrate 31 is controlled at a constant temperature by means of a pyrometer 34 for temperature control through a sighting hole 33. The growth is carried out by stopping the flow of H2 and by introducing into the enclosure & reaction gaseous materials flowing in the pipe 50. The subsequent deposition of a silicon-germanium alloy of the opposite conductivity type is carried out in such a way that the gaseous materials are immediately stopped when the initial deposition is completed, H2 is brought to flow to evacuate said gases, and then said silicon-germanium alloy of the opposite conductivity type is deposited in accordance with the operation to which it has been

alluded previously.

  In another method, GeC14 and SiH4 are caused to flow continuously as they are while only one doping gas is stopped and, after a short time, a doping gas of the opposite conductivity type is sent

for depositing alloys.

  In addition, the repetition of these operations makes it possible to have silicon-germanium alloys composed of multiple layers constituted by deposits in

  alternation of alloys of the opposite conductivity type.

  After the completion of growth, the flow of gaseous materials is stopped and only H2, in the form of a carrier gas, is caused to flow. After the lapse of

  desired time, 1 substrate temperature is lowered.

  After the temperature of the substrate has been completely lowered, N2 is caused to flow into the reaction chamber 32 so as to replace the interior of the chamber with N2. To ensure a complete replacement, it is carried out by means of a

  empty when the product is discharged.

  According to the process for the manufacture 7- of silicon-germanium alloys of the invention, in the form of a thermoelectric material, it is possible to obtain silicon-germanium alloys of a

  homogeneous composition through the following actions.

  That is to say that the gases inside the enclosure & reaction 32 become homogeneous in that a gaseous compound in the form of a material is used, and in that the material is sufficiently agitated, by relying on natural convection inside the reaction chamber. Then, the materials reach the surface of the substrate where said gases are subjected to a thermal energy presented by the substrate in order to be decomposed, when a desired silicon-germanium alloy is deposited successively on the surface of the substrate. Because the silicon-germanium alloy goes through a molten state at this time, it is not subjected to separation and it follows that it is possible to obtain a silicon-germanium alloy of composition homogeneous. In the case where the pressure inside the reaction vessel 32 is particularly higher than atmospheric pressure, natural convection occurs easily in the vessel and, as a result, the gaseous materials effectively deplete and a speed of

  high growth and high yield are achieved.

  When a semi-polycrystalline silicon

  conductor is manufactured by relying on a

  thermal composition of SiH4, it is considered that the suitable temperature for the surface of the substrate is in the range of 750 C & 850 C. This is due to the fact that, due to the uniformity (which is designated ci -after by morphology) of the surface, the substrate layer deteriorates as the temperature rises. It is therefore not allowed to raise the temperature of the substrate at random taking into account the 8 - problem such as the yield at the time of the contour treatment. With the present process for the preparation of a silicon-germanium alloy, however, it has been confirmed by test that using a gas of GeCl4, the morphology is very good even in the case of a temperature higher than that which is found in the surface of the substrate at the time of the only thermal decomposition of SiH4. The rise in the temperature of the substrate can by

  further accelerate the growth speed.

ExampleA

  A graphite 60 substrate of the dimension 23 x 3 x 940 mm was directly supplied with electricity for heating in a reaction chamber of approximately mm in internal diameter and 1300 mm in height, and said substrate was maintained & 870 C. Phosphine (PH3) and diborane (B2H6) were chosen as doping gases. Initially, monosilane (SiH4), germanium tetrachloride (GeC14) and B2H6 were introduced into the reaction vessel in the proportion of 210 Nml / min, 620 mg / min and 0.10 Nml / min, respectively. Because GeCl4 is a liquid at room temperature, its flow was controlled by the flow of carrier gas (H2) and by the vapor pressure of GeC14. Then, in the form of silicon-germanium deposits, the flows of SiH4, GeCl4 and B2H6 were increased in the proportion of the surface area of the silicon-germanium alloy that was grown. B2H6 was stopped from its supply in 18 hours after the start of the introduction of the gas into the reaction vessel and the supply of PH3 was started. PH3 was sent in the proportion of 1.8 times B2H6, period during which the interior of the reaction vessel 32 was kept under 1.4 times atmospheric pressure. The total reaction time was established at 36 hours. It has been observed

  that the growth rate was 2.4 mm / min.

  The alloy obtained was such that its inner layer was Si0.83 Geo, 17 of the P type (B: 5.2 x 1019 atoms / cc) and that its outer layer was of Si0.83 Ge0.17 of the N type ( P: 4.8 x 1019 atoms / cc). A unicouple of elements of type P and N was cut from the alloy obtained in the process shown in fig. 2 and, as shown in FIG. 3, 17 pairs of said unicouples were connected on the & low temperature side to prepare a test module 83. The performance tested of said module showed that the temperature difference was 600 C and the output of 2.4 W. Bigmple 2 A graphite substrate 61 of the dimension 23 z 3 z 940 mm was directly supplied with electricity for heating in a reaction chamber of approximately mm of internal diameter and 1300 mm in height and said substrate was maintained at 870 C. Phosphine (PH3) and diborane (B2H6) were chosen as the gas

doping.

  Initially, monosilane (SiH4), germanium tetrachloride (GeC14) and B2H6 were introduced into the reaction vessel in the proportion of 210 Nml / min, 620 mg / min and 0.10 Nml / min, respectively. Because GeC14 is a liquid at room temperature, its flow was controlled by the flow of carrier gas (H2) and by the vapor pressure of GeC14. Then, in the form of silicon-germanium deposits, the flows of 8iH4, GeC14 and B2H6 were increased in the same

  proportion that the surface of the silicon alloy-

  germanium brought to grow. B2H6 was stopped from its supply in 18 hours after the start of the introduction of the gas into the reaction vessel, and the supply of PH3 was started. PH3 has been sent to

  the proportion of 1.8 times B2H6 for 18 hours.

- 10 -

  By repeating said operations in series, elements of silicon-germanium alloy of the opposite conductivity types were deposited alternately, thus making it possible to obtain multicouples of structure with multiple layers. The total reaction time was established at 144 hours during which the reaction vessel was maintained internally under 1.4 times atmospheric pressure. It has been observed that the

  growth rate was 2.4 µm / min.

  The alloy obtained was such that the P-type layer was SiO, 83 Ge0.17 (B: 5.2 x 1019 atoms / cc) and that the N-type layer was Si0.83 Ge0, 17 (P: 4.8 x

1019 atoms / cc).

  A multicouple of elements of type P and N has been cut from the alloys obtained in the process shown in FIG. 4, and a test module 846 (provided with cold pads and a conductive film at the level of the multicouple of the Pet N type of FIG. 4) shown in FIG. 5 has been prepared. The tested efficiency of said module showed that the temperature difference was 6000C and the output of

540 mW.

  The process for the production of silicon-germanium alloys in the form of a thermoelectric material in accordance with the invention results in the following:

effects mentioned below.

  1) Because the alloys grow in the vapor phase, the type of conductivity can easily be

  converted by changing the dopant gas to be supplied.

  This eliminates the metallization and bonding operations of the electrodes on the high temperature side for what is called bridging between the P type alloy and the N type alloy when the two elements are treated. Thus, the elements can be formed easily and they are not contaminated by an adhesive

or the like.

- he -

  2) Not being subjected to separation, it is possible to obtain a silicon-germanium alloy of

very homogeneous composition.

  3) Because the process does not involve a spraying or molding operation, any more than a melting operation, contamination caused by washing or by bagging for spraying and molding can be avoided. 4) The process does not require any particular condition such as high temperature, high pressure and high degree of vacuum, nor advanced technique to achieve this, so that the manufacturing equipment is small in size and that consumption elements lesser

can be used.

  5) As the manufacturing operations become simpler, the process is capable of reducing the control of the articles during each operation and the administrative posts in the

manufacturing conditions.

  6) Particularly under pressures not lower than atmospheric pressure, natural convection can be effectively used and, therefore, growth rate and high yield can be ensured. The present process is therefore useful, particularly as a process for the manufacture of silicon-germanium alloys for production

  thermoelectric which uses large quantities.

  7) Bn monitoring the evacuation of gas by gas chromatography or the like during the manufacturing operation, it is easily possible to manage the

  conditions for manufacturing a silicon alloy-

germanium.

- 12 -

REYEN1CATION

  1. A process for the manufacture of silicon-germanium alloys, characterized in that, at the start, a gas of SiH4, a gas of GeC14 and a doping gas of the conductivity type are introduced into a chamber with

  reaction, in that a silicon alloy element-

  germanium is deposited on a substrate heated to a temperature not lower than 750 C in the reaction enclosure, in that a gas of SiH4, a gas of GeC14 and another doping gas of the type with conductivity opposite to that of the first gas dopant are then introduced on said first element of silicon-germanium alloy, and in this

  than another (second) element of silicon alloy-

  germanium of the type with conductivity opposite to that of the first element of silicon-germanium alloy is

  deposited on said first silicon alloy element-

  germanium, and in that this operation continues in series

is repeated two or more times.

  2. A process for the manufacture of silicon-germanium alloys as described in claim 1, characterized in that the material of said substrate is a silicon-germanium alloy of the conductivity type, and in that the element silicon-germanium alloy deposited first on said substrate is of the type with conductivity opposite to that of the first alloying element.