GB2221923A

GB2221923A - Process for manufacturing silicon-germanium alloys

Info

Publication number: GB2221923A
Application number: GB8912077A
Authority: GB
Inventors: Shinji Maruya; Toru Takahashi; Junji Izawa; Yoshifumi Yatsurugi
Original assignee: Komatsu Electronic Metals Co Ltd
Current assignee: Sumco Techxiv Corp
Priority date: 1988-07-08
Filing date: 1989-05-25
Publication date: 1990-02-21
Also published as: FR2633943A1; FR2633943B1; JPH0219467A; GB8912077D0; DE3922270A1; JPH0628246B2

Description

PROCESS FOR MANUFACTURING SMCON-GERMANIUM ALLOYS 2221923 This invention

relates to a process for manufacturing materials of directly converting heat energy and electric energy, that is thermoelectric materials, and in particular silicon-germanium alloys and a process suitable for processing an element using such alloys.

As a process for manufacturing a silicon-germanium alloy as a thermoelectric material there can be mentioned a powder sintering process as disclosed 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 its series (Part I and Part II). According to this literature the powder sintering process comprises is the following steps:

(1) a step of melting metallic silicon, metallic germanium and doping material, (2) a step of cooling the melt obtained in step (1), (3) a step of crushing a silicon-germanium alloy obtained in step (2) to particles of about 10 mesh, (4) a step of grinding down the silicon-germanium alloy particles obtained in step (3) into further fine particles, and (5) a step of hot pressing the silicon-germanium alloy particles obtained in step (4), in a vacuum vessel of not higher than 10-5 torr at about 13000C and under a high pressure of about 2000 kg/cm2.

On the other hand, Japanese Patent Kokai (Early Disclosure) No. 190077183 discloses an invention relating to a thermoelectric material consisting of a nonsingle crystalline solid solution comprising a plurality of elements, and an example therein describes a process for producing SixGeyBz (x+y+z = 1). According to said process, material gases of SiH4, GeH4, and B2H6 are introduced into a vacuum vessel with H2 carrier gas, and they are decomposed in the vessel thereby to obtain a ternary amorphous crystal consisting of Si, Ge and B at the growth rate of 50 AO/min. Thus, there may also be mentioned a process in which a gaseous compound is used as material and it is decomposed in a vapor phase in a vacuum vessel to obtain a - thermoelectric material.

Further, U.S. Patent Specification Nos. 2,552,626 and 4,442,449 disclose processes for manufacturing Si-Ge alloys according to a chemical vapor deposition for thin film, but the growth rate of thin films according to these processes is lower than 500 A0/min.

1 - 3 When the thermoelectric materials thus prepared are used as PN-unicouples, P-type silicon-germanium and N-type silicon-germanium were conventionally processed and molded to meet their use, and the product was adhered with hot shoes and cold shoes.

The powder sintering process, one of the abovementioned processes for manufacturing thermoelectric materials, takes complicated steps as described above, and it requires particular manufacturing conditions such as high temperature not lower than 14001C in the melting stem of material, high temperature near 1300'C, high pressure of about 2000 kg/cm2 and high vacuum degree not higher than 10-5 torr in the hot pressing step or high technical standards for achieving these conditions.

On the other hand, in the growth method according to the example of Patent Kokai No. 190077/83 where SiH4, GeH4, and B2H6 are materials and they are introduced in a vacuum vessel, the-growth rate is so slow as 50 1/min and such rate does not answer the industrial purposes.

As described above, even in the technics of said two U.S. Patent Specifications the growth rate is not higher than 500 A0/min, homogeneous composition Si-Ge alloys in the bulky state cannot be obtained, and therefore desired morphology cannot be expected for the growth layer.

Further, when the silicon-germanium alloys thus prepared are used as thermoelectric elements they must be processed, for example, in the structure as shown in Fig. 6, but connecting in series "n-type" Si-Ge legs with "p-type" Si-Ge legs by hot shoes and cold shoes must be carried out generally by a metallizing step and a so-called bonding step such as soldering.

However, in the connecting steps are complicated and it is difficult to select a suitable soldering agent for bonding from the view points of melting point and bonding property.

The present invention provides a process for manufacturing silicongermanium alloys, characterized in that in the beginning, S94 gas, GeC14 gas and one conductivity type doping gas are introduced into a reaction vessel, a silicon-germanium alloy is deposited on a substrate heated at a temperature not lower than 750C in the reaction vessel., 594 gas, GeC14 gas and another doping gas of the conductivity type opposite to that of the first doping gas are then introduced onto said first silicon-gemanium alloy, and another (second) silicongermanium alloy of the conductivity type opposite to that of the first silicon-germanium alloy is deposited onto said first silicon-germanium alloy, and this continuous operation in series is repeated twice or more.

Preferably the material of said substrate is one conductivity type silicon-germanium alloy, and the silicon-germanium alloy first deposited onto said substrate is of the conductivity type opposite to that of the first alloy.

- 5 The invention is characterized by repeating twice or more a continuous step in series in which SiH4 gas, GeC14 gas and a conductivity type doping gas are first introduced in a reaction vessel, a first silicon-germanium alloy is deposited on a substrate heated at a temperature not lower than 7500C within the reaction vessel, and SiH4 gas, GeC14 gas and a doping gas of the conductivity type opposite to that for the above doping gas are introduced upon said first silicon-germanium alloy when another (second) silicon- germanium alloy is deposited on said first alloy in such a manner that the first silicon-germanium alloy and the second silicon-germanium alloy may be opposite conductivity types.

According to preferred embodiments of the present 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 constituted by a conductivity type silicon- germanium alloy from the beginning, and a silicon-germanium alloy of the conductivity type opposite to that for the substrate is deposited upon said substrate.

Preferred embodiments of the invention will now be described in more detail by way of example only with reference to the accompanying drawings, in which:- - 6 Fig. 1 is a schematic view of the apparatus for carrying out-the present invention; Fig. 2 are silicon-germanium alloys manufactured according to the invention and unicouples thereof; Fig. 3 is a thermoelectric module of the silicon germanium alloys manufactured according to the invention; Fig. 4 are other silicon-germanium alloys manufactured according to the invention and multicouples thereof; Fig. 5 is a thermoelectric module of said other silicon-germanium alloys manufactured according to 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, SiH4 gas, doping gas and H2 gas of the gases used in the invention are supplied through pipes 1, 2, 3 and 4.

Reference numerals 11, 12 and 13 are the flow controllers of the respective gases. The hydrogen supplied through the pipe 4 is used for the substitution of gases in the apparatus and as a carrier gas of GeC14 stored in a vessel 40.

Firstly, a substrate 31 is set in a reaction vessel 32. Before growing of an alloy, the interior h of the reaction vessel is made vacuum by a vacuum pump 23, and it is converted by H2, and thereafter a predetermined amount of H2 is flown. The H2 is set to a predetermined pressure by a constant pressure inlet valve 24 and then discharged. At the same time during this operation, the SiH4 and one conductivity type doping gas in the same amount as in the initial growth conditions, and in addition GeC14 carrying with the H2 carrier gas are flown from a purge line 50 into an adsorption apparatus 27. The substrate is supplied with electricity from a power source 35 and raised to a desired temperature. The substrate 31 is controlled at a constant temperature by means of a pyrometer 34 for temperature control through is a sight hole 33. Growth is effected by stopping the H2 flow and introducing into the reaction vessel the material gases flowing in the purge line 50. The subsequent deposition of an opposite conductivity type. silicon-germanium alloy is effected in such a manner that the material gases are once stopped at the time when the initial deposition has finished, H2 is flown for purging said gases, and thereafter, said oppositive conductivity type silicon-germanium alloy is deposited according to the operation referred to above.

In another way, GeC14 and SiH4 are continuously flown as they are while one doping gas only is stopped, and after a little period of time an opposite conductivity type doping gas is passed for deposition of alloys.

Further, repeating of these operations will present silicon-germanium alloys consisting of multilayer constituted by alternative depositions of opposite conductivity type alloys.

After the finishing of the growth, the flow of the material gases is stopped and only H2 as the carrier gas is flown. After the lapse of a desired time the temperature of the substrate is lowered. After the complete lowering of the temperature of the substrate, N2 is flown into the reaction vessel 32 thereby to replace the interior of the vessel by N2. To ensure a complete replacement it is carried out by means of a vacuum pump when the product is discharged.

According to the process for manufacturing silicon-germanium alloys of the invention as thermoelectric material it is possible to obtain homogeneous composition silicon-germanium alloys through the following actions.

That is, the gases within the reaction vessel 32 become homegeneous in that a gaseous compound as material is used and the material is suficiently stirred relying on the natural convection within the reaction vessel. Then, the material gases reach the surface of the substrate, where said gases are subject to thermal energy possessed by the substrate thereby to be decomposed, when an aimed silicon-germanium alloy successively deposits on the surface of the substrate. Since the silicon-germanium alloy passes through a molten state at that time it is not subject to segregation, and as the result it is possible to obtain a homogeneous composition silicon-germanium alloy. In case the pressure within the reaction vessel 32 is particularly higher than the atmospheric pressure, the natural convection easily generates in the vessel, and therefore, the material gases effectively consume, and high growth rate and high yielding are achieved.

When a polycrystalline silicon for semiconductors is manufactured relying on thermal decomposition of SiH4 it is considered that the suitable temperature for the surface of the substrate is in the range from 7500C to 8500C. This is because that the smoothness (hereinafter called morphology) of the surface the substrate layer is deteriorated as the temperature rises. It is therefore not allowed to raise the temperature of the substrate at random in consideration of the problem such as yield at the time of contour working.

1 Under the present process for preparing a silicongermanium alloy, however, it has been confirmed by tests that by using GeC14 gas the morphology is very good even in the case of temperature higher than that in the substrate surface at the thermal decomposition of SiH4 alone. Raising of the temperature of the substrate could therefore accelerate the growth rate. Example 1 A graphite substrate 60 of the dimension 23 x 3 x 940 mm was directly passed with electricity for heating in a reaction vessel of about 200 mm in inside diameter and 1300 mm in height, and said substrate was retained at 870'C. Phosphine (PH3) and diborane (B2H6) were selected as the doping gases.

In the beginning, 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. Since GeC14 is a liquid at room temperature its flow was controlled by the flow of the carrier gas (H2) and the vapor pressure of GeC14. Then, as silicon-germanium deposits, the flows of the SiH4, GeC14 and B2H6 were increased in the same proportion of the surface area of the grown silicon-germanium alloy. B2H6 was stopped from its feeding in 18 hours after the starting of gas 1 introduction into the reaction vessel, and the feeding of PH3 was started. PH3 was fed at the ratio of 1.8 times B2H6, during which period the interior of the reaction vessel 32 was retained under 1.4 atmospheric 5 pressure. The total reaction time was set to 36 hours. It was observed that the growth rate was 2.4 pm/min. The resulting alloy was such that its inner layer is of P-type (B: 5.2 x 1019 atoms/cc) Si Ge o.83 o.17 and its outer layer was of N-type (P: 4.8 x 1019 atoms / cc) S i o. 83 Ge.. 17 - A unicouple of P and N type elements were cut off from the resulting alloy in the process shown in Fig. 2, and as shown in Fig. 3, 17 pairs of said unicouples were connected at the low temperature side to prepare a test module 83. The tested performance of said module showed that the temperature difference was 6000C and the output 2.4 W.

Example 2

A graphite substrate 61 of the dimention 23 x 3 x 940 mm was directly passed with electricity for heating in a reaction vessel of about 200 mm in inside diameter and 1300 mm in height, and said substrate was maintained at 8701C. Phosphine (PH3) and diborane (B2H6) were selected as doping gases.

In the beginning, monosilane (SiH4), germanium 1 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. Since GeC14 is a liquid at room temperature its flow was controlled by the flow of the carrier gas (H2) and the vapor pressure of GeC14. Then, as silicon-germanium deposits, the flows of the SiH4, GeC14 and B2H6 were increased in the same proportion of the surface area of the grown silicon-germanium alloy. B2H6 was stopped from its feeding in 18 hours after the starting of gas introduction into the reaction vessel, and the feeding of PH3 was started. PH3 was fed at the ratio of 1.8 times B2H6 for 18 hours.

By repeating said steps in series, silicon- germanium alloy elements of opposite conductivity types were deposited alternately thereby obtaining muticouples of multilayer structure. The total reaction time was set to 144 hours, during which the reaction vessel was internally retained under 1.4 atmospheric pressure. It was observed that the growth rate was 2.4 pm/min.

The resulting alloy was such that the P-type layer is (B: 5.2 x 1019 atoms/cc) Sio.83 Ge o. 17 and the N-type layer is (P: 4.8 x 1019 atoms/cc) si o.83 Ge o.17 - 13 - A multicouple of P and N type elements were cut off from the resulting alloys in the process shown in F19.4, and a test module 84 (being provided with cold shoes and conductive film at one surface of the P and N type multicouple of Fig. 4) shown in Fig.

was prepared. The tested performance of said module showed that the temperature difference was 6001C and the output 540 mW.

The process for manufacturing silicon-germanium alloys as thermoelectric material according to the invention brings about the undermentioned effects.

1) Since the alloys grow in vapor phase, conductivity type can be easily converted by changing the doping gas to be supplied. This eleminates the metallizing and bonding steps of the electrodes at high temperature side for the so-called bridging between the P-type alloy and the N-type alloy when processing to elements. Thus, elements can be made easily and they are not contaminated by adhesive or the like.

2) Being not subject to segregation it is possible to obtain a very homogeneous composition silicon-germanium alloy.

3) Since the process has no pulverizing or grinding step or no melting step,-contamination caused by jig or ladle for pulverizing and grinding can be _A prevented.

4) The process does not require particular conditions such as high temperature, high pressure and high vacuum degree, and advanced techniques for attaining them, so that manufacturing equipment is of small scale and less consuming members will do.

5) Since the manufacturing steps become simple it is capable of decreasing inspection items in each step and administrative items in manufacturing conditions.

6) Particularly under pressures not lower than the atmospheric pressure natural-convection can effectively be utilized, and therefore high 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 thermoelectric generation which uses bulk.

7) By monitoring exhaust gas by gas chromatograph or the like during the manufacturing operation it is possible easily to administer the manufacturing conditions of a silicon-germanium alloy.

The preferred embodiments of the present Invention can provide a method of resolvIng the problems possessed by known processes. namely the specific manufacturing conditions such as high temperature, high pressure and high vacuum degree, and a novel process for molding a thermoelectric element, which presents means for easy Industrialization, allows the element to be manufactured In high yield and at high growth rate. and does not require the separate steps for connecting In series "n-type" SI- Ge legs with of p-type" SI-Ge legs by hot shoes and cold shoes for the molding of the element, said steps having been necessary In the known processes.

K 1 1

Claims

CLAIMS:

- 1. A process for manufacturing silicon-gemanium alloys, characterized in that in the beginning, 594 gas, GeC14 gas and one conductivity type doping gas are introduced into a reaction vessel, a silicon-germanium alloy is deposited on a substrate heated at a temperature not lower than 7500C in the reaction vessel, SiH4 gas, GeC14 gas and another doping gas of the conductivity type opposite to that of the first doping gas are then introduced onto said first silicon-germanium alloy, and another (second) silicon-germanium alloy of the conductivity type opposite to that of the first silicongermanium alloy is deposited onto said first silicon-germanium alloy, and this continuous operation in series is repeated twice or more.
2. A process for manufacturing silicon-germanium alloys as described in Claim 1 wherein the material of said substrate is one conductivity type silicon-germanium alloy, and the silicon-germanium alloy first deposited onto said substrate is of the conductivity type opposite to that of the first alloy.
3. A process for manufacturing silicon-gemanium alloys substantially as hereinbefore described with reference to Figs. 1 to 5 of the accompanying drawings.

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