Method of and apparatus for obtaining voluminous, gallium containing, monocrystalline nitride

Description of the invention

The present invention relates to a method for obtaining bulky monocrystalline gallium nitride by crystallizing it from a supercritical solution on a nucleus, and a device for obtaining a bulky monocrystalline gallium nitride, in particular enabling the production of bulk monocrystalline gallium nitride by supercritical NH3 technology.

Nitride-based optoelectronic devices are usually produced on sapphire or silicon carbide substrates, different from deposited nitride layers (so-called heteroepitaxy). In the most commonly used MOCVD method, GaN deposition is carried out from ammonia and organometallic compounds from the gas phase, and the achieved growth rates make it impossible to obtain a bulk layer. The use of a buffer layer reduces the surface dislocation density, but not more than to approx. 10⁸/ cm². Another method has been proposed for the production of monocrystalline bulky gallium nitride, which is epitaxial deposition using gas phase halides (HVPE) ["Optical pattering of GaN films" by MK Kelly, O. Ambacher, Appl. Phys. Lett. 69 (12) (1996 ) and "Fabrication of thin-film InGaN light-emitting diode membranes" by WS Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)]. This method allows for the production of GaN substrates with a diameter of two inches, the quality of which, however, is not sufficient for laser diodes, because the surface density of defects is still from approx. 10⁷ up to approx. 10⁹/ cm². Recently, the method of lateral epitaxial rise (ELOG) has been used to lower the defect density. In this method, a GaN layer is first created on the sapphire substrate, on which the SiO2 layer is deposited in the form of stripes or mesh. On such a prepared substrate, lateral GaN build-up can be carried out, leading to a reduction of the defect density to approx. 10⁷/ cm². The growth of bulk crystals of gallium nitride and other Group XIII metals (IUPAC, 1989) is extremely difficult. Standard melt crystallization and sublimation methods are not applicable due to the decomposition of metal nitrides and N2. In the HNP method ["Prospects for high-pressure crystal growth of III-V nitrides" S. Porowski et al, Inst. Phys. Conf Series, 137, 369 (1998)] this decomposition is inhibited by the use of a nitrogen atmosphere at high pressure. The crystal growth is carried out in molten gallium, and therefore in the liquid phase, and makes it possible to obtain GaN wafers with dimensions in the order of 10 mm. To achieve a sufficient solubility of nitrogen in gallium it is necessary to use temperatures of 1500 ° C and nitrogen pressures of 15 x 10⁸ Pa (15 kbar).

In another known method to reduce the temperature and pressure of the growth process, the use of supercritical ammonia has been proposed. In particular, it has been shown that it is possible to obtain crystalline gallium nitride by synthesis from gallium and ammonia, as long as alkali metal amides (KNH2 or LiNH2) are introduced into the latter. The processes were carried out at temperatures up to 550 ° C and pressures up to 5 x 10⁸ Pa (5 kbar), and the obtained crystals had a size of the order of 5 μm ["AMMONO method of BN, AIN, and GaN synthesis and crystal growth" R. Dwiliński et al. Proc. EGW-3, Warsaw, June 22-24, 1998, MRS Internet Journal of Nitride Semiconductor Research, http://nsr.mii.mrs.org/3/25]. Using supercritical ammonia also succeeded in achieving recrystallization of gallium nitride in the region of the source material, which was fine crystalline GaN ["Crystal Growth of gallium nitride in supercritical ammonia" JW Kolis et at., J. Cryst. Growth 222,431-434 (2001)]. The main factor enabling this recrystallization was the introduction into supercritical ammonia of amide (KNH2) and at the same time a small amount of halide (KI). In processes carried out at a temperature of 40 ° C and a pressure of 3.4 x 10⁸ Pa (3.4 kbar), GaN crystals with dimensions in the order of 0.5 mm were obtained. However, it was not possible to observe chemical transport in the supercritical solution, and in particular, growth on the embryos.

The service life of optical semiconductor devices essentially depends on the crystal quality of the optically active layers, and in particular on the surface dislocation density. For GaN-based laser diodes, it is preferable to lower the dislocation density in the GaN substrate layer to below 10⁶/ cm²which is extremely difficult with the methods currently used. Hence, the main motivation of the present invention is to obtain nitride bulk crystals of a quality that allows their use as substrates for optoelectronics.

This aim was achieved by developing an autoclave autoclave process for the preparation of volumetric monocrystalline nitride containing gallium under supercritical conditions, which according to the invention is carried out in an ammonium-basic solvent medium containing ammonia and / or its inorganic derivatives and alkali metal ions at a molar ratio

The preparation of alkali metal ions to the remaining components in a supercritical solvent not exceeding 1: 2 and includes dissolving the gallium-containing source material, forming a supercritical solution, and crystallizing the gallium-containing nitride from this solution on the surface of the seed at a temperature higher than for dissolution.

The method of the invention may include the step of dissolving the source material, followed by the separate step of moving the supercritical solution to a higher temperature and / or lower pressure.

Alternatively, the method of the invention comprises the step of simultaneously autoclaving at least two zones of differing temperature, wherein the gallium-containing source material is placed in the lower temperature dissolution zone and the seed is placed in the higher temperature crystallization zone.

In the method of the invention, the temperature difference between the dissolution zone and the crystallization zone is controlled to ensure chemical transport in the supercritical solution. Chemical transport in the supercritical solution occurs by convection, and the temperature difference between the dissolution zone and the crystallization zone is greater than 1 ° C.

The gallium-containing nitride of the form is obtained by the process of the invention

Al_xGa_1-xyIn_yN, where 0 <x <1, 0 <y <1, 0 <x + y <1. The gallium-containing nitride may contain impurities of the donor and / or acceptor and / or magnetic type.

In the process of the invention, the supercritical solvent comprises at least potassium ions.

In the process of the invention, the source material consists essentially of gallium-containing nitride and / or its precursors. The precursors are selected from the group consisting of azides, imides, amidoimides, amides, hydrides, metallic compounds and alloys containing gallium, as well as gallium metallic.

In the process according to the invention, the seed has at least a crystalline layer of nitride containing gallium and / or other group XIII elements (according to IUPAC, 1989).

In the process according to the invention, the seed has a gallium-containing nitride crystalline layer with an surface defect density below 10⁶/ cm².

In the process according to the invention, the crystallization of the gallium-containing nitride is carried out at temperatures of 100 to 800 ° C. In the process according to the invention, the crystallization of the gallium-containing nitride is carried out at pressures from 100 x 10⁸ Pa up to 1000 x 10⁸ Pa (from 100 to 1000 kbar).

In the process of the invention, the alkali metal ion content of the supercritical solvent is adjusted to ensure the correct solubilities of the source material and the gallium-containing nitride.

In the process according to the invention, the molar ratio of alkali metal ions to the remaining components in the supercritical solvent is controlled in the range from 1: 200 to 1: 2.

The apparatus for obtaining monocrystalline nitride containing gallium according to the invention has an autoclave for the production of a supercritical solvent, equipped with a convection flow setting device embedded inside a furnace or furnace assembly, equipped or equipped with heating devices and / or cooling devices, the furnace or furnace set has a high temperature zone, coinciding with the autoclave crystallization zone, having heating devices and a low temperature zone, coinciding with the autoclave dissolution zone, with heating and / or cooling devices.

In the device according to the invention, the furnace or set of ovens has a high temperature zone coinciding with the crystallization zone of the autoclave having heating and / or cooling devices, and a low temperature zone coinciding with the dissolution zone of the autoclave with heating and / or cooling devices.

In the device according to the invention, the installation is made in the form of a horizontal partition or horizontal partitions with central and / or circumferential openings, separating the crystallization zone and the dissolution zone.

The device according to the invention is characterized in that in the autoclave the source material is placed in the dissolution zone and the seed is placed in the crystallization zone, and the flow of supercritical solution between these zones is determined by the installation.

The device according to the invention is characterized in that in the autoclave the dissolution zone is above the horizontal partition or the horizontal partitions, while the crystallization zone is below the horizontal partition or the horizontal partitions.

PL 207 400 B1

The conducted research showed that the best obtained volumetric monocrystalline gallium nitride has a defect density close to 10⁴/ cm² while the half-width of the X-ray reflection from the plane (0002) is below 60 arcsec, which guarantees the appropriate quality and lifetime of optical semiconductor devices manufactured with its use.

The subject of the invention is illustrated in the drawing in which Fig. 1 shows a graph of the relationship between GaN solubility in supercritical ammonia containing potassium amides (with KNH2: NH3 = 0.07) on the pressure for T = 400 ° C and T = 500 ° C, Fig. 2 shows a graph of the change in temperature over time in an autoclave with p = const. for example 1, fig. 3 shows the change in pressure in the autoclave over time with T = const. for example 2, fig. 4 is a plot of autoclave temperature change over time at constant volume for example 3, fig. 5 is a plot of autoclave temperature change over time for the description of example 4, fig. 6 is a plot of autoclave temperature variation over time for the description of the example 5, Fig. 7 is a graph of the temperature variation in the autoclave over time for the description of Example 6, Fig. 8 is a graph of the temperature variation of the autoclave over time for Example 7. And Fig. 9 is an axial sectional view of an autoclave arranged in a set of ovens as described in Examples 4, 5, 6 and 7;

GaN shows good solubility in supercritical NH3 as long as alkali metals or their compounds such as KNH2 are incorporated into it. The graph of Fig. 1 shows the solubility of GaN in a supercritical solvent as a function of pressure for temperatures of 400 and 500 ° C with the solubility being defined by the mole percent: Sm = GaN^solution (KNH2 + NH3) 100%. In the presented case, the solvent is a solution of KNH2 in supercritical ammonia with a molar ratio x = KNH2: NH3 equal to 0.07. The graph shows that solubility is an increasing function of pressure and a decreasing function of temperature. These relationships make it possible to obtain volumetric monocrystalline nitride containing gallium by dissolving it under conditions of higher solubility, and crystallization under conditions of lower solubility. In particular, a negative temperature coefficient means that, in the presence of a temperature gradient, chemical transport of the gallium-containing nitride will occur from the lower temperature dissolution zone to the higher temperature crystallization zone. It turned out that also other gallium compounds, and even metallic gallium, can be the source of ammonium gallium complexes. For example, gallium complexes can be introduced into a solvent of the above composition starting from the simplest substrate, gallium metal. Thereafter, by appropriately changing the conditions (e.g. increasing the temperature), a solution supersaturated with gallium-containing nitride is obtained and seed crystallization is achieved. The process according to the invention allows the growth of the gallium-containing bulk monocrystalline nitride on the seed and leads in particular to the production of a stoichiometric gallium nitride obtained as a monocrystalline volume layer on a gallium nitride seed crystal. Due to the fact that such a single crystal is obtained in a supercritical solution containing alkali metal ions, it also contains alkali metals in an amount higher than 0.1 ppm. However, in order to maintain the purely basic character of the supercritical solution, in the first place to avoid corrosion of the apparatus, halides are deliberately not introduced into the solvent. The method also allows to obtain volumetric monocrystalline gallium nitride in which from 0.05 to 0.5 Ga may be replaced by Al and / or In. The possibility of a smooth change of composition means the possibility of adjusting the constant network of the obtained nitride. Moreover, the bulk monocrystalline gallium nitride can be doped with dopants of the donor (e.g. Si, O) and / or acceptor (e.g. Mg, Zn) and / or magnetic (e.g. Mn, Cr) type in concentrations from 10¹⁷ up to 10²¹/ cm³. These admixtures alter the optical, electrical and magnetic properties of the gallium-containing nitride. As for other physical properties, the resulting bulk monocrystalline gallium nitride has a surface defect density below 10⁶/ cm², more preferably less than 10⁵/ cm²and most preferably less than 10⁴/ cm². Moreover, its X-ray reflection half width from plane (0002) is less than 600 arcsec, more preferably less than 300 arcsec, and most preferably less than 60 arcsec. The best bulk monocrystalline gallium nitride obtained may have a defect density lower than 10⁴/ cm² and at the same time the half-width of the X-ray reflection from plane (0002) is less than 60 arcsec.

Example 1 For a 10.9 cm high pressure autoclave³, made on the basis of a well-known design [H. Jacobs, D. Schmidt, Current Topics in Materials Science, vol. 8, ed. E. Kaldis (North-Holland, Amsterdam, 1981), 381], two crucibles were introduced. One of them contains 0.4 g of gallium nitride source material in the form of plates with a thickness of 0.1 mm,

In the second one there was placed a twice thicker embryo weighing 0.1 g, also obtained by the HVPE method. 0.72 g of metallic potassium with a purity of 4N was also introduced into the autoclave. The autoclave was filled with ammonia weighing 4.81 g and closed. The autoclave was placed in an oven and heated to a temperature of 400 ° C. The pressure inside the autoclave was 2 x 10⁸ Pa (2 kbar). After 8 days, the temperature was increased to 500 ° C and the pressure was maintained at 2 x 10⁸Pa (2 kbar) and under these conditions the autoclave was maintained for a further 8 days (diagram of FIG. 2). The process resulted in complete dissolution of the source material and recrystallization of the gallium nitride layer on the partially dissolved nucleus.

Example II. For 10.9 cm high pressure autoclave³ Two crucibles were introduced, one of which contained 0.44 g of gallium nitride source material in the form of 0.1 mm thick plates, produced by the HVPE method, and the second one placed a twice as thick embryo weighing 0.1 g, obtained also by the HVPE method. 0.82 g of metallic potassium with a purity of 4N was also introduced into the autoclave. The autoclave was filled with ammonia weighing 5.43 g and closed. The autoclave was placed in an oven and heated to a temperature of 500 ° C. The pressure inside the autoclave was 3.5 x 10⁸ Pa (3.5 kbar). After two days, the pressure was reduced to 2x10⁸ Pa (2 kbar) and the temperature was kept at 500 ° C and the autoclave was kept under these conditions for a further 4 days (diagram Fig. 3). The process resulted in complete dissolution of the source material and recrystallization of the gallium nitride layer on the partially dissolved nucleus.

Example III. For 10.9 cm high pressure autoclave³ Two crucibles were introduced, one of which contained 0.3 g of the source material in the form of metallic gallium with a purity of 6N, and the other containing a 0.1 g embryo obtained by the HVPE method. 0.6 g of metallic potassium with a purity of 4N was also introduced into the autoclave. The autoclave was filled with ammonia weighing 4 g and closed. The autoclave was placed in an oven and heated to a temperature of 200 ° C. After two days, the temperature was increased to 500 ° C, the pressure being 2 x 10⁸ Pa (2 kbar). The autoclave was kept under these conditions for a further 4 days (diagram Fig. 4). The process resulted in complete dissolution of the source material and recrystallization of the gallium nitride layer on the partially dissolved nucleus.

Example IV. In a high pressure autoclave 1 with a volume of 35.6 cm³ (Fig. 9) 3.0 g of gallium nitride produced by the HVPE method, which has been distributed in equal amounts in the dissolution zones 13 and crystallization 14, and 2.4 g of metallic potassium with a purity of 4N were placed. The autoclave 1 was then filled with 15.9 g ammonia (5N), closed, charged to the furnace set 4 and heated to 450 ° C. The pressure inside autoclave 1 was approximately 2 x 10⁸ Pa (2 kbar). After one day, the temperature of the crystallization zone 14 was increased to 500 ° C, and the temperature of the dissolution zone 13 was decreased to 400 ° C, and autoclave 1 was maintained under these conditions for a further 6 days (diagram Fig. 5). The process resulted in partial dissolution of the material in the dissolution zone 13 and an increase in gallium nitride on gallium nitride nuclei in the crystallization zone 14.

Example 5 In a high pressure autoclave 1 with a volume of 35.6 cm³ (Fig. 9) the source material in the form of a sintered gallium nitride pellet weighing 3.0 g was placed in the dissolution zone 13, gallium nitride seeds obtained by the HVPE method were placed in the crystallization zone 14, and 2.4 g of metallic potassium with a purity of 4N were also introduced. . The autoclave 1 was then charged with 15.9 g ammonia (5N) and closed. Autoclave 1 was then introduced into the oven set and heated to a temperature of 480 ° C. The pressure inside the autoclave was approximately 2 x 10⁸ Pa (2 kbar). After one day, the temperature of the crystallization zone 14 was increased to 500 ° C, and the temperature of the dissolution zone 13 was decreased to 420 ° C, and the autoclave was maintained under these conditions for a further 6 days (diagram Fig. 6). As a result of the process, partial dissolution of the source material in the dissolution zone 13 and the increase of gallium nitride on the seeds in the crystallization zone 14 was found.

Example VI. In a high pressure autoclave 1 with a volume of 35.6 cm³ (Fig. 9) 1.6 g of HVPE-produced gallium nitride source material was placed in the dissolution zone 13, 0.8 g of gallium nitride seed also obtained by HVPE was placed in the crystallization zone 14, and 3.56 g of metallic potassium were introduced with a purity of 4N. Autoclave 1 was charged with 14.5 g of ammonia (5N) and closed. Autoclave 1 was then introduced into the oven set 4 and heated to a temperature of 425 ° C. The pressure inside the autoclave was 1.5 x 10⁸ Pa (1.5 kbar). After one day, the temperature of the dissolution zone 13 was reduced to 400 ° C and the temperature of the crystallization zone 14 was increased to 450 ° C, and the autoclave was maintained under these conditions for another 8 days (diagram Fig. 7). After the process, partial dissolution of the source material in the dissolution zone 13 and the increase of gallium nitride on the HVPE GaN seeds in the crystallization zone 14 was found.

PL 207 400 B1

Example VII. In a high pressure autoclave 1 with a volume of 35.6 cm³ (Fig. 9), in the dissolution zone 13, 2 g of the source material in the form of gallium nitride produced by the HVPE method were placed, and 0.47 g of metallic potassium with a purity of 4N were introduced, and 0.7 g of GaN seeds also obtained by the HVPE method were placed in the crystallization zone 14. The autoclave was charged with 16.5 g of ammonia (5N) and closed. Autoclave 1 was then introduced into the oven set 4 and heated to a temperature of 500 ° C. The pressure inside the autoclave was 3 x 10⁸ Pa (3 kbar). After one day, the temperature of the dissolution zone 13 was reduced to 450 ° C and the temperature of the crystallization zone 14 was increased to 550 ° C, and the autoclave was maintained under these conditions for another 8 days (diagram Fig. 8). After the process, partial dissolution of the source material was found in the dissolution zone 13 and an increase in gallium nitride on the seeds in the crystallization zone 14.

The described method is carried out in a device for the preparation of gallium-containing bulk monocrystalline nitride in a supercritical solvent. The basic component of the device is an autoclave 1 for obtaining a supercritical solvent, equipped with an installation 2 for obtaining a chemical transport of the solvent in a supercritical solution inside the autoclave 1. For this purpose, the autoclave 1 is placed in the chamber 3 of a group of two furnaces 4 provided with heating devices 5 and / or cooling 6 and secured in the desired position with respect to the furnaces 3 by means of a screw locking device 7. The furnaces 4 are mounted on the bed 8, and the bed together with the bed 8. with a set of ovens 4 is rotatably mounted in the base 10 and secured in the desired angular position by means of a pin lock 11, thanks to which the speed and type of convection flow in the autoclave 1 are controlled. In the autoclave 1 located in the set of ovens 4, there is a convective flow of the supercritical solution, determined by the installation 2, made in the form of a horizontal partition or horizontal partitions 12 with central and / or circumferential opening, separating in the autoclave 1 dissolution zone 13 and crystallization zone 14, the height of which is the temperatures of the individual zones in the autoclave 1 in the temperature range from 100 to 800 ° C are set in the ovens 4 by means of the control device 15. In the autoclave 1, the dissolution zone 13, coinciding with the low temperature zone of the furnace unit 4, is positioned above the horizontal baffle or baffles 12, and the source material 16 is introduced into this zone 13, while the crystallization zone 14 coincides with the high temperature zone of the furnace 4, which is below it. horizontal divider or horizontal dividers 12. The embryo 17 is embedded in this zone, the embryo 17 placement point being located below the intersection of the convective rising and falling streams.

The obtained monocrystalline volumetric nitride containing gallium, due to its good crystalline quality, can be used as a support material for optoelectronic semiconductor devices based on nitrides, in particular for laser diodes.