WO1997031140A1

WO1997031140A1 - Method of epitaxial growth of monocrystalline '3a' group metal nitrides

Info

Publication number: WO1997031140A1
Application number: PCT/RU1997/000031
Authority: WO
Inventors: Jury Alexandrovich Vodakov; Evgeny Nikolaevich Mokhov; Mark Grigorievich Ramm; Alexandr Dmitrievich Roenkov; Jury Nikolaevich Makarov; Sergei Jurievich Karpov; Mark Spiridonovich Ramm
Original assignee: Jury Alexandrovich Vodakov; Evgeny Nikolaevich Mokhov; Mark Grigorievich Ramm; Alexandr Dmitrievich Roenkov; Jury Nikolaevich Makarov; Sergei Jurievich Karpov; Mark Spiridonovich Ramm
Priority date: 1996-02-22
Filing date: 1997-02-11
Publication date: 1997-08-28
Also published as: RU2097452C1

Abstract

The method of vapour-phase epitaxial growth of monocrystalline nitride of at least one metal belonging to subgroup 'A' of the third group of chemical elements comprises arrangement in parallel, opposite each other, of the evaporating surface of a source (2) of the metal specified in the composition of the single crystal grown and the growing surface of the substrate (3), defining the growth zone (4), generation in the growth zone (4) of an ammonia flux (e.g. 11, 12), heating of the source (2) and the substrate (3) up to temperatures providing the growth of the single crystal on the substrate (3), while maintaining the temperature of the source (2) above the temperature of the substrate (3). The material of the source (2), according to the invention, is a mixture containing a metallic component including at least one free metal specified in the composition of the single crystal grown, and a nitride component including at least one nitride of at least one metal specified in the composition of the single crystal grown.

Description

METHOD OF EPITAXIAL GROWTH OF MONOCRYSTALLINE "3A" GROUP METAL NITRIDES

Field of the Invention The proposed invention relates to methods of epitaxial growth of mono- crystalline semiconductors employed in electronic industry. Nitrides of metals belonging to A subgroup of the third group (i.e. group 3A) of chemical elements are very promising semiconductor materials for electronic industry. Of particular interest among such compounds is gallium nitride (GaN), as well as variable- composition nitrides based on gallium and indium (Ga-ln-N), gallium and aluminium (Ga-AI-N), which show considerable advantages over other semi¬ conductor materials, as far as the efficiency of optoelectronic devices based thereon, such as light-emitting diodes and lasers, is concerned.

Background of the Invention Investigations made in this field are mostly devoted to the development of gallium nitride epitaxial growth methods.

Known in the art are a number of methods of epitaxial growth of high- purity monocrystalline gallium nitride films suitable for using in electronic industry applications.

In a method of epitaxial liquid-phase growth of GaN as described in R.F. Logan and CD. Thurmond, J. Electrochem. Soc. 1 19 ( 1972) 1727, GaN is precipitated out from a solution in liquid gallium and caused to be deposited on a sapphire substrate submerged in gallium. In order to enhance the solubility of GaN in Ga, this method requires the monocrystalline layer growth process to be carried on at high pressures of 8 kbar and at a temperature of about 1500° C. Also known is a method of epitaxial growth of monocrystalline GaN layers by a chemical vapour-phase deposition (e.g. S.S. Lin, D.A. Stevenson, J. Electrochem. Soc. 125 ( 1978) 1 161 ). In this method, GaN is formed in the immediate vicinity of the substrate, as a result of interaction between gallium chloride and ammonia. Deposition of the resulting GaN is done at a temperature of 950° C and a nearatmospheric pressure.

Further, there is a method of epitaxial growth of monocrystalline layers of GaN produced as a result of a reaction occurring in the intermixing molecular fluxes of gallium and ammonia vapours in vacuum (cf. H.Cotoh, T.Suga, H. Suzuki, M. Kimata, Japan J. Appl. Phys. 20 ( 1981 ) L545).

The above methods all suffer from a serious drawback residing in a low epitaxial growth rate which does not exceed a few dozens of microns per hour. The maximum single-crystal growth rate, as compared to the aforementioned methods, namely, 70 microns per hour, was obtained when epitaxially growing monocrystalline GaN layers by a sublimation technique (cf. Yu.A. Vodakov, M.I. Karklina, E.N. Mokhov, A.D. Roenkov, Proc. USSR Academy of Sciences, Inorganic materials, vol.16, ( 1980) p.537). This method is distinguished by the substrate lying in close vicinity to and above the Ga source. A polycrystalline GaN powder served as the Ga source. The vapour-phase deposition of GaN layers onto the substrate was carried out in a flux of ammonia introduced between the source and the substrate, at temperatures ranging from 1 100 to 1200° C. The insufficient single-crystal growth rate prevents all these methods from being applied to growing bulk single crystals (several mm thick or more).

The nearest analogy to the proposed invention is a method described in C.

Wetzel, D. Volm, B.K. Meyer, K.Pressel, S. Nilsson, E.N. Mokhov and P.G.

Baranov, Appl. Phys. Lett., vol.65, No.8 ( 1994) 1033. In this method, similarly to the lastmentioned one, the narrow growth zone was defined by the evaporating surface of the Ga source and the substrate lying above it in a parallel spaced relationship, the spacing being 5mm. In this method, though, unlike the previous one, the Ga source material was free gallium, rather than gallium nitride. The monocrystalline layer deposition process was run at temperatures ranging from 1 170 to 1270° C that were reached, in the growth zone, by heating the source by means of a heater being in thermal contact therewith, the substrate heated by virtue of its proximity to the source having a lower temperature than the source. Gallium nitride in the growth zone was formed as a result of gallium vapours interacting with ammonia, the flux of which was introduced between the source and the substrate. The monocrystalline layer growth rate was several times the values achieved when using the aforementioned methods. This method may be used for epitaxial growth of single crystals not only of gallium nitride, but also nitrides of other metals belonging to group 3A of chemical elements, owing to the similarity of their chemical properties, as well as for growing single crystals of variable-composition nitrides based on these metals.

Despite the high growth rate attained, this method has enabled only thin monocrystalline epitaxial layers to be produced, which is due to an inherent difficulty in an attempt to stabilize the growth process, arising from a rapid evaporation of the gallium of the source, which is started long before the operating growth temperatures have been reached. It involves a great loss of the source material and results in its being quickly exhausted. Another disadvantage of the method is a nonuniform growth of the epitaxial layer across the area of the substrate, its peripheral thickness exceeding that in the central portion. Thus, this method fails to provide the growth of bulk sngle crystals whose production is indispensable for an extensive use of the semiconductor material in electronic industry.

Disclosure of the Invention It is the object of the present invention to provide a method of growing monocrystalline nitride of at least one metal belonging to group 3A of chemical elements, such that stable parameters are achieved during a specified period of the single-crystal growing process, while maintaining a good quality and a high growth rate, thereby permitting a commercial production of bulk monocrystalline nitrides of metals belonging to group 3A of chemical elements, as well as variable composition nitrides based thereon.

With this object in view, in a method of vapour-phase epitaxial growth of a monocrystalline nitride of at least one metal belonging to subgroup A of the 3rd group of chemical elements, comprising arrangement in parallel, opposite each other, of the evaporating surface of the source of the metal specified in the composition of the single crystal grown and the growing surface of the substrate defining the growth zone, generation, within the growth zone, of an ammonia flux, heating both the source and the substrate up to temperatures providing the growth of the single crystal on the substrate, with the temperature of the source maintained at a level above that of the substrate, according to the invention, the material of the source is a mixture containing a metallic component including at least one free metal specified in the composition of the single crystal grown and a nitride component including at least one nitride of at least one metal specified in the composition of the single crystal grown.

The authors have found, by experiment, that the source constituting a mixture of a metallic and a nitride components has great advantages over a source largely composed either of free metals or of nitrides of metals contained in the grown single crystal. The use of said mixture enables the loss of the source material to be minimized, while maintaining a consistently good quality in the process of growth, with but a slightly reduced growth rate. The proportion of the metallic component should be preferably specified in the range of 8 to 30% of the source material weight. It has been established by the authors that with this component ratio in the source material, the above merits show to the best advantage.

It is preferred that when growing bulk single crystals, the source be replenished, as the crystal grows, since it is technically more feasible to have the source of a smaller size than the crystal to be grown. Now, in case of growing a single crystal with a uniform specified composition along the line normal to the substrate, the source is replenished with a material of the same composition as the original one. In case a single crystal is grown which consists of layers of different specified compositions, the source is replenished with a material of a composition that varies in accordance with the growing pattern of said layers.

It is preferable that introduced into the growth zone be an ammonia flux directed along the path between the substrate and the source and an ammonia flux directed towards the substrate through the source material. This contributes to a more uniform epitaxial growth rate on different portions of the substrate.

It is preferable that the source and the substrate be heated separately. The separate heating allows an increased spacing between the substrate and the source, in comparison to the prior art method, thereby providing a better uniformity of the epitaxial layer growth rate on different portions of the substrate. The separate heating also permits a higher, as compared to the prior art, specified temperature of the source, for the given optimum temperature of the substrate, whereby a certain decrease of the growth rate, against that provided by the source material of the prior art, can be compensated for, while maintaining a superior quality of the grown single crystal. The material used as the source in growing gallium nitride single crystals is a mixture of gallium melt and polycrystalline gallium nitride powder. Gallium nitride is one of the most promising semiconductor materials. Of particular interest for electronic industry among the variable-composition nitrides are Ga-AI-N and Ga-ln-N compounds. For growing gallium and aluminium nitride single crystals, Ga-AI-N, according to the invention, a mixture of gallium melt and polycrystalline aluminium nitride powder is used as the source material.

For growing gallium and indium nitride single crystals, Ga-ln-N, a mixture of gallium melt and polycrystalline indium nitride powder is used as the source material. Doping additives are introduced into the source, according to the invention, in order to produce monocrystalline semiconductors of a given conductivity type and value based on nitrides of metals belonging to the 3A group of chemical elements.

Brief Description of the Drawing The invention is further illustrated by a description of its embodiments, with reference to Fig 1 of the accompanying drawing which represents a reactor with a growth zone enclosed therein. Preferred Embodiments of the Invention

The single crystals are grown in a reactor (Fig.1 ) comprising a quartz frame 1 capable of being evacuated. A source 2 of the metal specified in the composition of the single crystal to be grown and a substrate 3 are positioned inside the frame 1 so that the evaporating surface of the source 2 and the growing surface of the substrate 3 are arranged in parallel, opposite each other, to define a growth zone 4.

The configuration and the size of the evaporating surface of the source 2 are determined by a container 5 enclosing the material of the source 2. In order to provide the conditions allowing a uniform growth of the single crystal throughout the surface area of the substrate, its shape and size are preferably made to coincide with those of the evaporating surface of the source 2. It is preferred that both the source 2 and the substrate 3 be of a circular shape. The container 5 made of a porous electrical conducting material, such as graphite, is mounted on a lock 6 representing a quartz tube locked in position in a through hole in the centre of the bottom of the container 5. Rotatably mounted inside said quartz tube is a screw conveyer 7 forming part of the mechanism for replenishment of the source 2 and allowing the material of the source 2 to be fed into the growth zone 4 from the hopper (not shown in the drawing) located inside the reactor, in a region where during the entire period of crystal growth, the temperature does not exceed 400° C.

A holder 8 of the substrate 3 composed, e.g. of graphite with a silicon carbide coating on the outside, is secured in a lock 9 formed by a quartz tube, such that the axis of the lock 9 passes through the centre of the substrate 3, perpendicular to its growing surface. The lock 9 is capable of being rotated about its axis and displaced therealong.

The frame 1 is provided with a branch pipe 10 designed to introduce an ammonia flux II into the growth zone 4 between the source 2 and the substrate 3, and the branch pipe (not shown) for introducing an ammonia flux 12 into the growth zone 4 through the material of the source 2, and another branch pipe (not shown) serving to evacuate the air prior to starting the single crystal growth process, and to remove gaseous reaction products formed during the growth process out of the reactor's inner space.

The frame 1 is provided with induction heaters 13 intended for separate heating of the substrate 3 and the source 2. The temperature monitoring of the source 2 and the substrate 3 is conducted by thermocouples 14 mounted on the holder 8 of the substrate 3 and the container 5 of the source 2, respectively. When growing gallium nitride single crystals and single crystals of variable- composition nitrides, such as (Ga-ln-N), (Ga-AI-N), monocrystalline SiC is used as the substrate material, as the crystal lattice of SiC exhibits a better coincidence with the crystal lattices of said nitrides, than does the lattice of other materials such as sapphire, thus minimizing the stresses developed in the growing crystal. According to the author's findings, the best results are obtained when surfaces of {0001 } silicon carbide crystals not over 0.15 mm thick are used as the growing surfaces of the substrates.

Implementation of the proposed method will be now discussed in more detail, as exemplified by epitaxial growth of monocrystalline gallium nitride. The growing of single crystals of nitrides of other metals belonging to group 3A of chemical elements, as well as variable-composition nitrides based on 3A group metals, is accomplished in a similar manner. According to the invention, a mixture of polycrystalline gallium nitride and gallium melt is used as the material of the source 2 for growing monocrystalline gallium nitride. The mixture prepared in advance is loaded into the hopper of the reactor, and the container 5 is filled with the mixture, using the screw conveyer 7. In case the growing single crystal is required to be doped, for instance, to produce a monocrystalline semiconductor material of a specified conductivity type, dopants such as magnesium are added to said mixture, as it is prepared, these additives being uniformly distributed in the material prepared through its agitation in process of preparation.

When growing GaN crystals, the free gallium content of the material of the source 2 is preferably selected to be within the range of 10-20% by weight.

When growing Ga-ln-N single crystals, the free gallium content of the material of the source 2 is preferably to be within the range of 8-15% by weight.

When growing Ga-AI-N single crystals, the free gallium content of the material of the source 2 is preferably to be within the range of 20-30% by weight.

The growing surface of the substrate 3 secured to the holder 8 is positioned at a distance of the 5 to 10 mm from the evaporating surface of the source 2, by moving the lock 9 along its axis.

The reactor is then closed vacuum -tightly, and the source 2 and the substrate 3 are heated up to a temperature of about 500° C by means of the induction heaters 13 inducing the electric current in the container 5 and the holder 8 made of a conducting material. The reactor is evacuated to remove the air contained therein, ammonia is introduced, so as to bring the internal pressure to near-atmospheric level, and the ammonia fluxes 1 1 and 12 are generated in the growth zone 4, one of which (1 1 ) is directed between the substrate 3 and the source 2, and the other ( 12), through the material of the source 2 and to the substrate 3. In order that the flux 11 be generated, the ammonia flow rate through the branch pipe 10 is specified to range from 25 to 50 litres per hour. The ammonia flow rate serving to generate the flux 12 is set to be within the range of 30 to 50% of that used for generation of the flux 1 1 . The single crystal growth starts after the temperature of the source 2 has reached the operating value specified, in the case of GaN crystal growth, to be within 1250-1350° C.

When growing Ga-ln-N crystals, the operating temperature of the source 2 is specified to be 1000-1100° C.

When growing Ga-AI-N crystals, the operating temperature of the source 2 is specified to be 1300-1400° C. The temperature of the substrate 3 is set at a level some 50 to 70° C below the temperature of the source 2. The temperatures of the source 2 and the substrate 3 are both monitored by the thermocouples 14.

The porous structure of the container 5 enables the ammonia flux 12 directed to the bottom of the container 5 to pass through the pores of the material of the container 5 and further, along the ducts between the molten gallium -wetted grains of the nitride of the metal specified in the composition of the crystal grown, into the growth zone 4. The ammonia penetrating the source material prevents the nitride of said metal from being decomposed, thus stabilizing the single crystal growth process and minimizing the material loss in the source 2. In addition, the ammonia flux 12, by virtue of its being directed towards the substrate 3, aids in the directional mass transfer from the source 2 to the substrate 3, again reducing the loss of material from the source 2 and providing a uniform crystal growth rate over the entire area of the substrate 3. As the single crystal is grown on the substrate 3, the substrate is rotated, at a constant speed, by means of the lock 9, about an axis extending through the centre of the growing surface of the substrate and perpendicular thereto This rotation causes the crystal growth rate to be further equalized across the surface area of the substrate 3.

As the single crystal grows, the source 2 is continuously replenished by using the screw conveyer 7 to feed the material of the source 2 from the hopper to the container 5, so as to maintain a fixed position of the evaporating surface of the source 2 within the space of the reactor

With the proposed method, the achievable growth rate for a single crystal of the specified composition proves to be as high as 1 mm/hr, in terms of thickness. In the case of growing single crystals over 1 mm thick, the substrate 3 is moved away from the evaporating surface of the source 2 at the crystal growth rate, by advancing the supporting lock 9 along its axis, and the temperature of the holder 8 of the substrate 3 is simultaneously increased, accounting for the known thermal conductivity of the crystal grown, in order to maintain the initial temperature value of the growing surface. Thus, the stability of temperature and the spacing between the growing surface of the single crystal being grown and the evaporating surface of the source 2 is achieved throughout the growth process

It has been experimentally found by the authors that all the crystal growth parameter values listed hereinabove ensure the optimum conditions of the process flow. What is implied thereby is the possibility to maintain, for a long time, the high rate of monocrystalline growth, the perfect structure of the single crystal grown, and the small loss of the source material With a growth rate of 1 mm/hr, this loss does not exceed 50%. The proposed method may also be used, if necessary, to grow indium nitride, aluminium nitride crystals, as well as variable-composition nitrides based thereon. Described hereinabove are specific embodiments of the invention allowing different modifications and additions which are apparent to those skilled in the art. Therefore, the present invention is not restricted to the examples described or individual elements thereof, and appropriate modifications and additions may be made which do not go beyond the scope and spirit of the invention, as defined by the claims.

Industrial Application The proposed method of epitaxial growth of monocrystalline nitrides of 3A group metals can be applied to the manufacture of semiconductor materials employed in electronic industry. Of particular interest are gallium nitride (GaN) and variable-composition nitrides, such as those based on gallium and indium (Ga-ln-N), gallium and aluminium (Ga-AI-N), which show considerable advantages over other semiconductor materials to make them suitable for the manufacture of such optoelectronic devices as light-emitting diodes and lasers.

Claims

Claims A method of vapour-phase epitaxial growth of monocrystalline nitride of at least one metal belonging to subgroup "A" of the third group of chemical elements, comprising arrangement in parallel, opposite each other, of the evaporating surface of the source (2) of the metal specified in the composition of the single crystal grown and the growing surface of the substrate (3), defining the growth zone (4), generation, within the growth zone (4), of an ammonia flux, heating both the source (2) and the substrate (3) up to temperatures providing the growth of the single crystal on the substrate (3), with the temperature of the source (2) maintained at a level above that of the substrate (3), characterized in that the material of the source (2) is a mixture containing a metallic component including at least one free metal specified in the composition of the single crystal grown and a nitride component including at least one nitride of at least one metal specified in the composition of the single crystal grown.

2. A method of Claim 1 characterized in that the proportion of the metallic component ranges from 8 to 30% of the source material weight.

3. A method of Claims 1 ,2 characterized in that as the single crystal grows, the source (2) is replenished.

4. A method of Claims 1 to 3 characterized in that an ammonia flux (11 ) directed between the substrate (3) and the source (2) and an ammonia flux (12) directed towards the substrate through the source material are introduced into the growth zone (4).

5. A method of Claims 1 to 4 characterized in that the source (2) and the substrate (3) are heated separately.

6. A method of Claims 1 to 5 characterized in that a mixture of gallium melt and polycrystalline gallium nitride powder is used as the source material.

7 A method of Claims 1 to 5 characterized in that a mixture of gallium melt and polycrystalline aluminium nitride powder is used as the material source

8. A method of Claims 1 to 5 characterized in that a mixture of gallium melt and polycrystalline indium nitride powder is used as the source material

9. A method of Claims 1 to 8 characterized in that dopants are additionally introduced into the composition of the source.