CA2326056C - Direct synthesis process of indium phosphide - Google Patents

Abstract

Direct synthesis process of indium phosphide starting from indium and phosphorous characterized in that the synthesis takes place in a completely closed reaction system using a reactor in which at least two containers are used, one in-side the other(s), the temperature being brought to a maxi-mum value ranging from 1070 to 1250°C, preferably from 1100 to 1200°C, and the pressure to a maximum ranging from 1850 to 2000 bars with a constant temperature increase with re-spect to the time, according to the following formula y = kx wherein y is the temperature in °C, x is the time in min-utes and k is a constant which has a value ranging from 5°C/minute to 20°C/minute.

Description

DIRECT SYNTHESIS PROCESS OF INDIUM PHOSPHIDE

The present invention relates to a direct synthesis process of indium phosphide.

There is a growing interest in indium phosphide for the production of optoelectronic (Lasers, Photodetectors) and microelectronic (HEMTs, HBT and JFETs) instruments.

Recent developments relate to an improvement in the purity of polycrystalline and monocrystalline material and a reduction in the dislocation density EPD < 10' cm-2 (Etch Pits Density) by controlling the growth conditions (growth of phosphorous under controlled pressure and reduction of the thermal gradients).

At present, there is a particular interest in InP
semi-insulating wafers. Although the use of InP substrates is dominant in the field of optoelectronics, semi-insulating InP is becoming more and more important in elec-tronics as a material for high power and high frequency in-struments, which support transfer service systems provided in telecommunications, markets in continuous expansion. Un-fortunately, however, its technology is still not suffi-ciently mature to justify large volume production.

The LEC technique, which involves doping with Fe, is used at present for the preparation of semi-insulating Inp, on an industrial scale. Commercially, the polycrystalline or pre-stretched material generally obtainable, is semi-conductive of the n-type; it is therefore indispensable to use acceptor doping agents: metallic iron having a high pu-rity (99.999%). The quantity of doping agent necessary for obtaining a semi-insulating material is directly propor-tional to the quantity of residual impurities present; as InP S.I. wafers with high concentrations of Fe (high den-sity of precipitates) negatively influence the performance of the apparatus, it is necessary to use high purity raw materials. One of the essential requisites for obtaining monocrystals having a high crystalline quality and suitable electric properties is to have high purity polycrystalline Inp and a controlled stoichiometry. Particular attention must be paid to controlling the purity of the starting ma-.20 terials and synthesis products, the stoichiometry and pro-duction costs.

The specific conditions for preparing stoichiometric and ultra-pure InP are as follows:

- the TIL (total impurity level) of the starting materi-als, In and P, used must be lower than 1 ppm;

- the synthesis process should be carried out in an in-ert atmosphere (Ar, N2) and in non-reactive and non-contaminating crucibles;

- the reaction systems must be closed or pressurized to prevent evaporation of the phosphorous.

The synthesis of InP is an extremely critical process due to the high vapor pressure of the liquid P and high flammability of the yellow phosphorous present in small quantities in the starting red phosphorous or formed during the synthesis. The traditional synthesis methods for the production of polycrystalline indium phosphide are:

- High pressure Bridgman (HB) - Solute Diffusion (SSD) - Phosphorous injection method In the horizontal high pressure Bridgman method (Adam-ski, J.A., Synthesis of Indium Phosphide, J. Crystal Growth (1983) 64, 1-9; Bonner K.A., and Temkin, H., Preparation and Characterization of high purity bulk InP, J. Crystal Growth (1983) 64, 10-14), the InP synthesis is carried out with high pressure ovens to prevent the ampoule from ex-ploding. The indium is contained in a graphite tube closed with plugs made of the same material and supported by a quartz tube. On the outside, there is another quartz tube in which the phosphorous is placed in pieces and a quartz wool disk which separates the P from the graphite con-tainer.

The quartz ampoule is inserted in a steel jacket and pressurized at 20-30 atm. The system essentially consists of a three-zone oven.

During the process, the quartz tube is moved by means of the coil at a rate of 6 cm/h.

The main drawback of this procedure lies in the impu-rities deriving from the graphite container. It might be possible to avoid contamination of the product by using pBN

(pyrolytic boron nitride) crucibles; this however could cause sticking phenomena as the indium does not completely react and therefore sticks to the walls of the container.
In order to avoid the use of graphite containers, a bal-anced pressure system was subsequently developed. In this case the reaction is carried out in quartz containers (boats and crucibles). The polycrystalline InP synthesis takes place in an oven with a horizontal cooling gradient situated in a high pressure autoclave.

The pressure of the P inside the quartz ampoule is balanced by the pressure of an inert gas in the autoclave in order to obtain a differential pressure between the two chambers close to zero. The system has a transducer which detects the differential pressure and transfers it to a servomechanism which makes the necessary pressure correc-tions in the autoclave; during the reaction phase the pres-sure in the system reaches about 30 atm.

The SSD (Synthesis Solute Diffusion) technique (Ku-bota, E., and Sugii, K., Preparation of High purity InP by the Synthesis, Solute Diffusion Technique., J. Appl. Phys.

(1981) 52 2983-2986) is one of the growth methods from so-lution which can be used for the preparation of polycrys-talline InP.

Red phosphorous is put onto the bottom of a quartz am-poule; the crucible containing indium is placed inside at a certain height from the base.

The indium is distilled under vacuum for a few hours to remove the indium oxides from the surface, using the same synthesis oven and an appropriate temperature profile.

The ampoule is subsequently evacuated at 10-6 Torr and sealed.

Ingots consisting of small grained aggregates (2-10 mm2) are produced, at an average synthesis temperature of 900 C with a thermal gradient of the molten indium of 20 C
per cm, and a solidification rate of 3-4 mm a day.

The SSD method is simple and inexpensive, but very long times are required and it cannot therefore be applied industrially.

A high pressure reactor is used for the injection method (Farges, J.P. A method for the "in-situ" Synthesis and Growth of Indium Phosphide in a Czochralski Puller. J.

Crystal Growth (1982) 59, 665-668; Hyder, S.B. and Holloway C.J. Jr. In-situ synthesis and growth of Indium Phosphide.
J. Electron. Mater. (1983) 12, 575-585). The phosphorous is contained in an ampoule, separately from the indium. The P

vapors come into contact with the molten indium through a layer of molten B203. The reaction chamber is pressurized at 30-60 atm with Ar or N2. After the In and B203 have melted, the crucible is vertically moved until the end part of the quartz ampoule appendix is immersed in the molten indium. The temperature of the P in the ampoule is between 520 C and 570 C.

With this procedure, 1-2 kg of polycrystalline InP can be obtained with a slight excess of In on the outside of the crystals and in the final solidified part. A consistent pollution by Si may occur if the ampoule is not coated with PBN.

The process was initially developed with two main ob-jectives:

- the potentiality of the method to produce InP rich in P and the consequent hope of thus obtaining non-doped S.I. material.

- the thermal potentiality of synthesizing the poly ma-terial and stretching the crystal in a single process and in the same reactor, as occurs for GaAs.

Neither of these objectives were fully reached and at present, it seems that only a few operators (of which only one commercial) use this method to prepare a polycrystal-line product deriving from synthesis and subsequent growth to limit silicon pollution. This polycrystalline material is then stretched to become monocrystalline using the tra-ditional technique.

A non-traditional direct synthesis method in an ultra high pressure autoclave (HPDS) (27000 p.s.i.) is described in literature (Savage, R.O., Anthony, J.E., AuCoin, T.R., Ross, R.L., Harsh, W., and Cantwell, H.E. High Pressure Di-rect Synthesis of Bulk Indium Phosphide. In "Semi-insulating III-V Materials" (D.C. Look, and J.S. Blakemore, eds.), (1984) 171-174. Shiva Publishing Limited).

The authors use a pressure-temperature cycle which comprises venting operations of the gas (Argon and Phospho-rous) present in the reaction environment at process tem-peratures of over 750 C, indispensable for not exceeding the maximum operating pressure of the apparatus. The reac-tion system can therefore be considered as being open. This process does not allow an accurate control of the stoichi-ometric ratio, owing to the release of phosphorous vapors during the venting phases and increases problems relating to safety. Furthermore a single non-sealed container is used (crucible and relative lid).

We have now found an improved process with respect to the current industrial process, i.e. the horizontal Bridg-man process, which uses a completely closed reaction system and there is consequently no venting phase as the maximum operating pressure of the reactor is never reached.

The direct synthesis process of indium phosphide starting from indium and phosphorous, object of the invention, is characterized in that the synthesis is carried out in a completely closed reaction system by means of a reactor in which at least two containers are used, one inside the other(s), to reduce the quantity of phosphorous which is released into the reaction environment, the temperature be-ing brought to a maximum value ranging from 1070 to 1250 C, preferably from 1100 to 12000C, and the pressure to a maxi-mum value ranging from 1850 to 2000 bars with a constant temperature increase with respect to the time, according to the following formula y=kx wherein y is the temperature in C, x is the time in min-utes and k is a constant which has a value ranging from 5 C/minute to 20 C/minute.

One of the main problems from the point of view of production, is the necessity of carrying out the synthesis process and stretching of the monocrystal with two differ-ent apparatuses. In fact, direct reaction in the liquid state in the same reactor where the monocrystal is stretched, is not possible with standard equipment, due to the high pressure developed by the phosphorous at temperatures close to the melting point of indium phosphide. For this reason the synthesis process according to the invention requires the use of a reactor capable of sustaining very high pressures obtained with an inert gas (argon or nitrogen at 2000 bars).
Pressures with such high values and a suitable geometry of the containers are necessary for limiting the phosphorous leakages which inevitably occur during the process.
In the accompanying drawings:
Figure 1 a is a graph showing the temperature-pressure cycle with the closed system according to the invention; and Figure 1 b used as a comparison is a graph showing the open system synthesis cycle adopted by the authors Savage et al., mentioned above.
From the graphs, it can be observed that the technique according to the invention does not comprise any venting phase as the maximum operating pressure of the equipment is never reached, whereas from the graph of figure 1 b, it can be seen that over 750 C, it is necessary to reduce the internal pressure with the consequent discharge, through the venting line, of a mixture of argon and phosphorous vapors. The elimination of this operation allows a better control of the stoichiometric value and fewer problems relating to environmental impact.

The advantages of the process according to the inven-tion are indicated hereunder:

- the possibility of obtaining ingots having dimensions which enable them to be introduced as such into the 5"
crucible for the production of 211 InP monocrystals.

This has considerable advantages with respect to the Bridgman method used industrially: elimination, with a reduction in possible contamination, of two handling phases of the material; improvement in the stoichi-ometric characteristics of the product as the use of one or more disks made of polycrystalline material with a suitable diameter reduces phosphorous leakages during the heating phase in the LEC growth process.

- the possibility of reaching charge carrier concentra-tion values lower than 1015 atoms cm-3. This can be ob-tained by eliminating all the quartz parts. In the case of the HB technique, attempts made by various op-erators in the past to substitute the quartz ampoule with other materials in order to eliminate contamina-tion by Si (which makes it necessary to pre-stretch the polycrystalline material obtained), have come up against difficulties which cannot be solved with the materials available. This problem has been completely overcome by the process claimed herein which directly produces material with low levels of Silicon.

An example is provided which should not be considered as limiting the scope of the present invention.

EXAMPLE
The raw materials, Indium 6N and red Phosphorous 6N, are charged in a controlled atmosphere (class 100) into quartz crucibles equipped with a lid, previously cleaned with aqua regia for at least two hours and rinsed with ul-tra-pure water 18 MegaOhm. The crucible is placed in a graphite crucible-holder closed in turn by a lid and put into the reactor.

The system is pressurized to 50 MPa and is heated to 1150 C (See figure la).

The polycrystalline InP product obtained is treated with ethyl alcohol to remove any possible phosphorous resi-dues present on the surface and subsequently with HCl-HNO3 1:1 to remove the possible excess of indium.

Claims (2)

1) A direct synthesis process of indium phosphide start-ing from indium and phosphorous characterized in that the synthesis takes place in a completely closed reac-tion system using a reactor in which at least two con-tainers are used, one inside the other(s), the tem-perature being brought to a maximum value ranging from 1070 to 1250°C and the pressure to a maximum value ranging from 1850 to 2000 bars with a constant tem-perature increase with respect to the time, according to the following formula y = kx wherein y is the temperature in °C, x is the time in minutes and k is a constant which has a value ranging from 5°C/minute to 20°C/minute.

2) The process according to claim 1, wherein the maximum temperature ranges from 1100 to 1200°C.