EP0946411A1 - Procede de synthese de cristaux de nitrure de groupe iii - Google Patents

Procede de synthese de cristaux de nitrure de groupe iii

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Publication number
EP0946411A1
EP0946411A1 EP97945616A EP97945616A EP0946411A1 EP 0946411 A1 EP0946411 A1 EP 0946411A1 EP 97945616 A EP97945616 A EP 97945616A EP 97945616 A EP97945616 A EP 97945616A EP 0946411 A1 EP0946411 A1 EP 0946411A1
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Prior art keywords
group iii
liquid
nitride crystals
metal
group
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EP97945616A
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German (de)
English (en)
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EP0946411A4 (fr
Inventor
John C. Angus
Alberto Argoitia
Cliff C. Hayman
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Case Western Reserve University
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Case Western Reserve University
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Publication of EP0946411A1 publication Critical patent/EP0946411A1/fr
Publication of EP0946411A4 publication Critical patent/EP0946411A4/fr
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    • C—CHEMISTRY; METALLURGY
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    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
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    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0632—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with gallium, indium or thallium
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    • C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
    • C01B21/0641—Preparation by direct nitridation of elemental boron
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    • C01B21/072—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with aluminium
    • C01B21/0722—Preparation by direct nitridation of aluminium
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/40—AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
    • C30B29/403—AIII-nitrides
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
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    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/40—AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
    • C30B29/403—AIII-nitrides
    • C30B29/406—Gallium nitride
    • H—ELECTRICITY
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02373—Group 14 semiconducting materials
    • H01L21/02378—Silicon carbide
    • H—ELECTRICITY
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    • H01L21/02367—Substrates
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    • H01L21/02367—Substrates
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    • H01L21/02518—Deposited layers
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    • H01L21/02538—Group 13/15 materials
    • H01L21/0254—Nitrides
    • H—ELECTRICITY
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
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    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02614—Transformation of metal, e.g. oxidation, nitridation
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
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    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02617—Deposition types
    • H01L21/02623—Liquid deposition
    • H01L21/02625—Liquid deposition using melted materials
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    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/82—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by IR- or Raman-data
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    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM

Definitions

  • This invention relates to a method for the synthesis of Group III nitride crystals. More particularly, the invention relates to a method for the synthesis of
  • Gallium nitride is a III-IV semiconductor from the same family as gallium arsenide.
  • the band gap in GaN (3.4 eV) is direct and rather wide. Accordingly, GaN has very favorable light emission properties in the ultraviolet and blue.
  • GaN and other Group III nitrides such as cubic boron nitride (c-BN), aluminum nitride (A1N), and indium nitride (InN) include the following: lasers and light emitting diodes; sensors; imagers; millimeter wave devices; cold cathodes for vacuum electronics; and, luminescent displays.
  • c-BN cubic boron nitride
  • A1N aluminum nitride
  • InN indium nitride
  • cubic boron nitride (c-BN) is very hard and can also be used in grinding, cutting and machining applications. It is
  • Crystals of c-BN may also be used as a closely lattice-matched substrate for growing single crystals of diamond by chemical vapor deposition.
  • GaN and other Group III nitrides have been the subject of intense worldwide research and development effort.
  • GaN gallium nitride
  • GaN gallium nitride
  • polycrystalline Group III nitrides may be used as source materials for growing
  • the method comprises the steps of: (a) optionally pretreating a liquid comprising a Group III metal to clean the surface thereof; (b) raising the temperature of the liquid from about the melting point of the Group III metal to a temperature below which net decomposition of the nitride crystals occurs; (c) saturating the liquid Group III metal using active nitrogen to form a melt comprising the Group III metal and nitrogen; (d) crystallizing the Group III nitride crystals; and (e) optionally etching the Group
  • the Group III metal is one selected from the group consisting of gallium, boron, aluminum and indium. Steps (a) and (b) of the method can be interchanged.
  • the liquid may also comprise one or more metals different than the first Group III metal. Suitable examples of these metals would include another Group III metal, tin and bismuth.
  • the method comprises the steps of directing a plasma comprising active nitrogen onto a surface of a liquid comprising a Group III metal; and crystallizing the Group III nitride.
  • the method is carried out at atmospheric pressure or below.
  • gallium nitride crystals is disclosed.
  • the method comprises the steps of saturating a liquid comprising gallium metal using active nitrogen and crystallizing the gallium nitride crystals.
  • the active nitrogen may be generated by a plasma and can be neutral, ionic, electronically excited or a mixture thereof, fhc active nitrogen may also be generated by the decomposition of ammonia (Ni l,).
  • polycrystalline Group III nitride crystals are disclosed, which are characterized by an ability to luminesce at room temperature.
  • the crystals are prepared by a process comprising the steps of saturating a liquid comprising the Group III metal using active nitrogen, followed by crystallizing the Group III nitride crystals.
  • One advantage of the present invention is that Group III nitride crystals can be synthesized in the absence of extremely high pressures.
  • Another advantage of the present invention is that Group III nitrides can be synthesized at pressures of one atmosphere and lower.
  • Still another advantage of the present invention is that Group III nitrides can be simply and economically synthesized. Still another advantage of the present invention is that Group III nitride crystals can be grown in high yields.
  • crystals grown using the new method described herein can be used as substrates for further growth of Group III nitride films using conventional growth methods such as molecular beam epitaxy and metal-organic vapor deposition.
  • Still another advantage of the present invention is the use of c-BN crystals for machining, cutting, and grinding, especially of ferrous metals and alloys.
  • Still another advantage of the present invention is that polycrystalline Group III nitride films grown at temperatures of one atmosphere and lower are of sufficient quality to show luminescence at room temperature and can therefore be used in device applications.
  • Fig. 1 is a schematic drawing of the system used for growing gallium
  • Fig. 2 is a schematic drawing of a portion of the phase diagram of the
  • Fig. 3 is a schematic drawing of a method used for horizontal freezing of GaN from Ga/N melts
  • Fig. 4 is a schematic drawing of a side view of a crucible used for vertical freezing and of the imposed temperature gradient;
  • Fig. 5 is a polycrystalline GaN "dome” completely covering a liquid gallium drop
  • Fig. 6 is a scanning electron micrograph from a portion of the concave surface of the polycrystalline GaN "dome” of Fig. 5;
  • Fig. 7 is a scanning electron micrograph of a portion of the convex surface of the polycrystalline GaN "dome" of Fig. 5;
  • Fig. 8 is a transmission electron micrograph of a portion of a polycrystalline GaN sample of the present invention.
  • Fig. 9 is a transmission electron micrograph of a portion of a polycrystalline GaN sample of the present invention.
  • Fig. 10 is a steady state luminescence spectra taken at a lattice temperature of (a) 10K and (b) 300K;
  • Fig. 1 1 is a room temperature Raman spectra of two different 1 micron spots on the same polycrystalline GaN sample;
  • Figure 12 is a scanning electron micrograph of GaN crystals grown from a liquid melt of Ga, In, and N.
  • Fig. 1 illustrates a schematic drawing of the growth system of the method of the present invention.
  • active nitrogen is used to saturate a pool of liquid gallium with active nitrogen followed by the subsequent crystallization of solid gallium nitride.
  • the solid gallium nitride may be formed in bulk as distinguished from thin films and microcrystalline materials.
  • bulk As used in the present disclosure "bulk"
  • This method was specifically developed for the growth of gallium nitride crystals. But this method is equally applicable to the growth of other Group III nitrides, including aluminum nitride (A1N), cubic boron nitride (c-BN), and indium nitride (InN).
  • Al nitride Al nitride
  • c-BN cubic boron nitride
  • InN indium nitride
  • Fig. 2 shows a schematic of a portion of the phase diagram of the gallium/nitrogen system (not drawn to scale) and the processes believed to take place during the growth of the gallium nitride.
  • Path a to b shows the dissolution of nitrogen into the liquid gallium from the active nitrogen in the gas phase.
  • solid gallium nitride starts to precipitate.
  • Path c to d shows the melting of the solid gallium nitride with increasing temperature. At point d, all of the solid gallium nitride is melted so that only a liquid phase is present.
  • the path d to c can be retraced to recrystallize solid gallium nitride, e.g., as a single crystal.
  • Other temperature/time profiles to induce crystallization can be used with this invention.
  • the melt can be saturated with nitrogen until the solubility limit of GaN is just reached (curved line on Figure 2). At that point the melt is cooled, and solid GaN is precipitated. This crystallization path is indicated by the line d to c. Still other process paths can be used and will be evident to a skilled person.
  • the present invention contemplates saturation of the gallium or Group III metal using active nitrogen as opposed to molecular nitrogen.
  • active nitrogen as opposed to molecular nitrogen.
  • active nitrogen is meant to include neutral atomic nitrogen (N), ionized atomic nitrogen (N + ), excited states of molecular nitrogen (N 2 * ) or mixtures thereof and is conveniently generated by a plasma, e.g., a microwave plasma, a DC plasma or a radio frequency plasma.
  • the active nitrogen can also be generated by thermal decomposition of ammonia (NH 3 ), for example, by simply heating the NH 3 to temperatures of approximately 900°C using a furnace.
  • Other nitrogen containing gases can also be used.
  • Use of this method to generate the active nitrogen is disadvantageous, since ammonia is a toxic material.
  • the ammonia introduces large amounts of hydrogen into the growth process, which can be undesirable.
  • ammonia as opposed to nitrogen gas, is more difficult to obtain in a pure form. Whether generated by a plasma or the decomposition of ammonia, the flow rate of the active nitrogen must be sufficient to replenish the nitrogen in the melt as the solid Group III nitride forms.
  • the equilibrium pressure for molecular nitrogen is very high, e.g., for GaN it is approximately 1500 bar at
  • Thc plasma process of the present invention circumvents these extremely high equilibrium pressures by use of a highly non-equilibrium (super-equilibrium)
  • concentration oi ' active nitrogen species derived by dissociating N 2 in a plasma concentration oi ' active nitrogen species derived by dissociating N 2 in a plasma.
  • Thermodynamic calculations show that the formation of solid GaN is highly favored at atomic nitrogen partial pressures that are easily obtained in conventional microwave, RF, or DC plasmas. Moreover, these calculations have been verified experimentally in the method of the present invention by the formation of GaN from a pool of liquid Ga from an atomic nitrogen beam.
  • thermodynamic calculations are summarized in Table I, which sets out the estimated equilibrium pressures of N 2 and N over solid GaN in equilibrium with liquid Ga/N solutions.
  • the data in Table I may be interpreted as follows: at atomic N pressures above the equilibrium pressure, P N , the formation of solid GaN from liquid Ga/N solutions is favored
  • the equilibrium P N pressures may easily be obtained in microwave ECR, in microwave ball plasmas, in RF plasmas, in DC plasmas, or using hollow cathodes, all of which can produce partial pressures of activated species in the millitorr range, i.e., » 10 "6 bar.
  • Reaction (2) is very favored thermodynamically and is aided by the presence of surfaces. It was previously believed, therefore, that the atomic nitrogen would recombine to form N 2 and not enough nitrogen would remain to form the GaN. By the method of the present invention, this difficulty can be circumvented.
  • the plasma is used to produce a sufficient partial pressure of N and other active nitrogen species.
  • a saturated melt of nitrogen and liquid gallium can be formed using active nitrogen. This melt can be subsequently frozen to form gallium nitride at atmospheric pressure and below.
  • the process of the present invention is not limited to an operating pressure of one atmosphere and below.
  • the process of the present invention could also be performed at pressures exceeding atmospheric pressure.
  • pressures of several atmospheres could be used.
  • the equilibrium N pressure is only 5.5 x 10 '9 millitorr (7 x 10 " ' 5 bar) for this reaction; at 1500°K (1227°C), the equilibrium N pressure increases to 4.5 x 10 " '° millitorr (6 x 10 " 12 bar). In contrast, the equilibrium N 2 pressure at 1200 ° C is approximately 1000 bar.
  • the concentration of nitrogen in the Ga/N melt is not extremely low.
  • the nitrogen solubility is only 0.01 atomic percent; at 1600°C the solubility is approximately 1 %.
  • the solubility is as high as possible, since it is easier to grow crystals at higher solubility.
  • the increased solubilities are achieved ( 1 ) by addition of a second metal to the melt that does not form a stable solid nitride and (2) by increasing the melt temperature.
  • the second metal can be another Group III metal or a metal from another group, e.g., tin.
  • Adding a second metal to the melt permits the lowering of growth temperatures and/or increases the solubility of nitrogen in the melt.
  • the second metal should be soluble in the Group III metal, should not form a stable solid nitride at the growth conditions, and should have a low vapor pressure. Suitable examples of the second metal include other Group III metals, tin and bismuth. In some situations it may be advantageous to use one or more metals for the second metal, for example, grow A1N from Al/Ga/In N melts.
  • melt temperatures as high as feasibly possible are used to maximize the concentration of nitrogen in the Ga/N melt.
  • the maximum melt temperature that can be used is limited by several factors. It can be limited by the vapor pressure Ga over the liquid Ga/N solution. If the vapor pressure of the Ga is too high, it can interfere with the operation of the plasma source. For the ECR source, the Ga vapor pressure should be kept below 1 millitorr. Therefore, the melt temperature is limited to approximately 1200K. For the ball plasma microwave source, higher Ga pressures can be tolerated and the melt temperature can be as high as approximately 1400K. Other sources of activc nitrogen, for example, the inductively coupled plasma, can tolerate even higher Ga pressures and consequently higher melt temperatures.
  • Table II below. This table can be used to estimate allowable melt temperatures depending on the Ga pressure that can be tolerated by the source of active nitrogen. It should be noted that the actual Ga vapor pressures over the Ga/N solution will be somewhat less than the values set out in Table II; however, how much less is not known.
  • the method of the present invention was conducted at a maximum melt temperature of approximately 900°C (1173K) when GaN was synthesized.
  • the melt temperature range over which the Group III nitrides can be synthesized by this method depends on the Group III nitride being synthesized.
  • the feasible melt temperature ranges from a few degrees above the melting point of the Group II I metal, to an upper temperature limited either by
  • the temperature at which the Group III nitride undergoes net decomposition is the temperature at which the decomposition rate is equal to the formation rate.
  • the formation rate of the nitride depends on the flux of active nitrogen, so the decomposition temperature depends on the source of active nitrogen being used.
  • temperatures ranging from approximately 400K to 1300K can be used in the method of the present invention for synthesis of GaN.
  • InN can only be synthesized with the ECR source at temperatures ranging from approximately 500K to 1000K because of the higher melting point of In and because of the greater decomposition rate of InN. Similar considerations hold for the other Group III nitrides and will be apparent to anyone skilled in the art.
  • the average rate of growth of GaN crystals depends on the rate at which nitrogen can be supplied to the gallium surface.
  • the present method using the ECR source yields average linear growth rates of about 8 ⁇ m per hour. The growth rate is significantly greater at the beginning of the synthesis before the liquid gallium is totally covered with the solid GaN crust. It is believed that this rate is limited by the solid GaN crust formed on the liquid gallium. If the melt
  • a solid crust of GaN does not form and the growth rate of the GaN is not limited by the formation of a crust.
  • 0.95 gram of solid GaN was grown in 3 hours.
  • the linear growth rate for this run was approximately 600 microns per hour.
  • the size of the crystals grown using the method of the present invention is on the order of a millimeter.
  • the ability to synthesize GaN depends critically on having a low rate of recombination of N to N 2 , which is strongly favored thermodynamically. A small rate of recombination compared to the rates of liquid phase diffusion in reaction to form GaN enhances the amount of saturated Ga/N solution that can be formed.
  • the characteristic thickness, L, of the nitrogen containing layer is to order of magnitude given by
  • D is the diffusion coefficient of N in liquid Ga and k is the (linearized) rate constant for the recombination of N to N 2 in liquid Ga.
  • the effective value of D is increased by stirring the pool of liquid gallium.
  • the rate constant, k is not known. It is not known why the recombination of N to
  • N 2 does not take place more rapidly than it does.
  • one factor reducing the recombination rate may be the fact that the nitrogen in the Ga melt is coordinated by four or more Ga atoms. These loosely bound Ga atoms, which form a shell surrounding the N atoms, can shield the nitrogen atoms
  • the reactants are pure Ga and N, possible contamination from the use of NH 3 or metal organics is avoided.
  • Crucibles made from pyrolytic boron nitride can be used for the synthesis of the nitrides. No evidence of contamination of the GaN by boron
  • a related issue is contamination of the gallium surface, which might hinder dissolution of N and catalyze recombination of N to N 2 on the surface.
  • Gallium is very reactive and easily oxidized by residual water vapor and oxygen.
  • Non-metallic impurities in the gallium such as arsenic and sulphur, are known to segregate at the surface. Surface concentrations can be large, even for elements present only in trace amounts in the bulk gallium.
  • the gallium surface is pretreated with a beam of atomic argon followed by a beam of atomic hydrogen. This pretreatment step reduces any Ga 2 0 3 and also removes arsenic and sulfur by forming volatile arsines and hydrogen sulfide.
  • argon ion sputtering prior to the formation of the gallium nitride is also helpful in reducing contamination, particularly when an ECR source is used to generate nitrogen plasma.
  • the argon ion sputtering is done
  • Ga and GaN have almost the same density (6.1 g/cm 3 ).
  • GaN floats on Ga at 1000°C because of surface tension.
  • the crystallization of the solid gallium nitride can be accomplished in many various ways, including, with and without a substrate or lattice matched seed crystal.
  • Possible methods include, but are not limited to, directional freezing, remelting and freezing a crust, thermal gradient transport from a crust, Czochralski growth on a seed, inverted submerged Czochralski growth, and growth through a thin liquid gallium film onto a substrate.
  • the preferred method depends on the type of product that is desired and there is some overlap between the methods. However, for growth of large single crystals, directional freezing is preferred; thermal gradient transport from a crust is simpler to implement but will produce smaller crystals. For producing large polycrystalline masses of crystals, remelting and refreezing a crust is preferred. For producing oriented or single crystal films, growth through a thin liquid gallium film is preferred. In each of these cases the advantage of operating below atmospheric pressure is obtained.
  • the molten Ga/N alloy is slowly cooled in a directional manner. This can be done in two general ways: (1) freezing horizontally, or (2) freezing vertically. In the horizontal method, a temperature gradient is imposed across the liquid surface and the average temperature slowly decreased while the atomic nitrogen beam is maintained. A freezing front progresses across the Ga/N liquid melt, yielding a large crystal
  • the interior of the crucible is structured in the shape of a point as shown in Fig. 3. In this
  • the solid GaN grows on a liquid surface so that stress and stress- induced dislocations are minimized.
  • a vertical temperature gradient is imposed across the liquid Ga/N alloy, with the highest temperature at the surface.
  • the level of the Ga/N melt is just above a submerged substrate.
  • the average temperature is reduced until nucleation of GaN occurs on the substrate. It is advantageous to maintain the distance between the liquid surface and the substrate constant as the solid GaN grows. This may be done in several different ways. The preferred way is to raise the liquid level by the slow insertion of a boron nitride piece. Alternatively, the substrate can be slowly lowered or gallium added to the melt.
  • Another mode of operation is to transport nitrogen within the melt from regions of high temperature, where the solid nitrides are more soluble, to regions of low temperature, where the solid nitrides are less soluble.
  • One way of doing this is to grow a polycrystalline crust of the solid nitride on top of the melt. One side of the crust is then maintained at a higher temperature than the other side. The solid nitride crust dissolves at the high temperature side; nitrogen is transported through the melt to the low temperature side, where the solid nitride recrystallizes.
  • This mode of operation can be used to convert small crystals of solid nitride into larger crystals.
  • Another possible mode is to grow a polycrystalline crust and then impose a vertical temperature gradient, in which the entire crust is kept at a higher temperature than the bottom of the melt. The crust dissolves and the nitrogen diffuses down the cooled substrate or seed crystal where the solid nitride crystallizes.
  • the formed GaN can be etched with a mixture of hydrochloric acid and nitric acid to remove excess gallium.
  • the conccntratcd acids are used at room temperature in approximately a 1 to 1 ratio by volume.
  • seed crystals are not necessary in this invention, in some circumstances the use of a seed crystal to initiate nucleation of the nitride may be useful.
  • the seed can be a crystal of the same material, e.g., GaN seed for growing GaN, or it can be a different material, e.g., sapphire seed for growing
  • GaN or diamond seed for growing cubic boron nitride GaN or diamond seed for growing cubic boron nitride.
  • the present invention is easily adapted for growth of doped nitride crystals.
  • Doping can be used to enhance the conductivity or change the optical properties of the nitride crystals.
  • the desired doping agent e.g., magnesium
  • the doping agent will be incorporated into the solid nitride crystal as it grows.
  • the amount of doping agent incorporated into the solid nitride crystal will depend on several factors, including the concentration of doping agent in the melt and the distribution coefficient of the doping agent between the melt and the nitride crystal. The distribution coefficient can not be predicated and is best determined by experiment.
  • the crucible is electrically conducting.
  • the crucible can be made of graphite or thin layers of conducting
  • nitride grown on a metal such as molybdenum or titanium may suppress recombination of N to N 2 on the surface of the melt.
  • the bias voltage that is applied typically several hundred volts, is
  • N 2 molecular nitrogen
  • metal atoms e.g., gallium
  • N 2 molecular nitrogen
  • these metal atoms e.g., gallium
  • the bias is not applied, the nitrogen atoms will not be implanted into the melt, but would be more likely to remain on the melt surface where they may have a greater tendency to recombine. Although this mechanism is speculative, it is in agreement with the experimental observations.
  • the nitrogen containing melts strongly wet other nitrides.
  • gallium/nitrogen melts at temperatures above 900°C form a concave meniscus with the boron nitride crucible.
  • Indium/nitrogen melts actually creep out of nitride crucibles upon heating. This effect can be used in another growth mode that is based on the principles of this invention.
  • a thin film of liquid Group III metal e.g., indium or gallium, flows out on a substrate that it wets well, for example a nitrided substrate.
  • the thin film of Group III metal is then reacted with the active nitrogen from the plasma.
  • the thin layer of Group III metal is converted into the nitride.
  • oriented crystals can be formed by this method.
  • Single crystal layers can also be formed.
  • Partially latticed-matched substrates that can be used are sapphire and silicon carbide.
  • Other possible substrates are derived from the Group Ill/Group V compounds, e.g., gallium arsenide and indium phosphide.
  • One advantage of this approach is that there is a very small distance that the nitrogen in the melt must be transported before it crystallizes to form the solid nitride. Also, large single crystal layers may be formed for device applications.
  • Liquid gallium was held in a pyrolytic boron nitride crucible (6 mm diameter and 4 mm high), which could be independently heated to temperatures up to 1 100°C.
  • the electron cyclotron resonance microwave (ECR) source (Wavemat Model MPDR 610) was mounted directly above the crucible. The ECR plasma discharge was maintained with 200 W of microwave power. An ion flux density of approximately 10 16 cm ⁇ V was obtained. The beam was estimated to be approximately 10%N, 10%N + , with the remainder N 2 * and N 2 .
  • the growth procedure was initiated by evacuating the reaction chamber to 10 "9 torr by a combination of a turbo-pump, sublimation-pump, cryo-pump and diffusion-pump.
  • the liquid gallium was next heated at 600°C and 10 "7 torr for 45
  • the pressure in the chamber was maintained at 0.5 mtorr throughout. A supersaturation of nitrogen was obtained and spontaneous crystallization occurred without cooling. After removal from the chamber, the sample was etched with a 1 to 1 mixture of concentrated hydrochloric acid and nitric acid to remove the excess gallium.
  • Fig. 5 shows the polycrystalline GaN "dome” formed in Example I, which completely covered the liquid gallium at the end of a run.
  • Fig. 5 was approximately 0.10 mm thick, weighed 40 mg, and had a surface area of 70 mm 2 .
  • the average linear growth was approximately 8 micrometers per hour.
  • FIG. 6 A scanning electron micrograph of a portion of the concave surface of the polycrystalline GaN dome of Example I is shown in Fig. 6. A dendritic morphology, typical of uncontrolled freezing from a supersaturated solution was seen. In Fig. 7, a scanning electron micrograph of a portion of the convex surface of the dome is seen. Thin platelets of GaN aligned normal to the surface are evident. X-ray diffraction from this region of the sample indicated a strong ⁇ 1 1 -
  • FIG. 8 A transmission electron micrograph of a portion of a polycrystalline GaN sample of Example I is shown in Fig. 8.
  • a small hexagonal platelet was visible in the center of Fig. 8.
  • the insert was the electron diffraction pattern of the crystal tilted to the [11-20] zone axis. Streaks were noted along the [0001] directions, indicating the presence of planar defects. Diffraction spots from the [1-100], [0001] and [10-11] planes were also obtained and matched well with the w-GaN. Dark bands in the transmission electron micrographs at Fig. 9 are believed to arise from stacking faults and microtwins. Dislocations were not imaged.
  • Typical steady state photoluminescence spectra, taken at room temperature and at approximately 10K, of the convex surface of the GaN dome are shown in Fig. 10.
  • the excitation source was the 325 nm line of a HeCd laser, with 0.5 mW power focused to a 50 ⁇ m diameter spot.
  • the detection system was a photon
  • 3.28 eV are in the energy range for impurity bound excitons and donor- acceptor pair recombination as well as for the first and second LO phonon replicas of the free or neutral donor bound exciton as shown in C.FI. Hong, D. Pavlidis, S.W.
  • the near-band-edge luminescence decreased by a factor of 50, and the yellow band by a factor of 2.
  • the spectrally integrated luminescence intensity decreased by a factor of 10. This decrease was due to
  • Fig. 1 1 Depolarized Raman spectra of two different spots are shown in Fig. 1 1.
  • the spectra were taken in backscattering geometry, with the spot size of approximately 1 micron. It was assumed that the crystallites are randomly oriented.
  • the arrow in the main figure indicated the location of the 144 cm " ' E2 line for wurtzitic GaN, which was clearly observed.
  • the arrows in the inset
  • Example 1 was repeated with the exception that an alloy containing 60 weight percent Ga and 40 weight percent In was used as the starting metal and the melt temperature was 670°C. The total growth period was 1 1 hours and 30 minutes. Upon crystallization, only polycrystalline GaN was obtained; no InN was formed despite the presence of In in the melt. Crystals from this example are shown in Figure 12. The crystals were characterized by photoluminescence spectroscopy. The band-edge luminescence was not shifted from the wavelength for pure GaN, which indicates that the crystals of GaN contained no dissolved InN. The crystallite size was larger, up to 0.1 mm, than in
  • Example 1 where pure Ga was used as the starting metal. This is due to the beneficial effect of the enhanced solubility brought about by the addition of the second metal.
  • EXAMPLE 3 where pure Ga was used as the starting metal. This is due to the beneficial effect of the enhanced solubility brought about by the addition of the second metal.
  • Example 1 was repeated in an identical manner with the exception that
  • the total growth period was 1 1 hours. During the growth period bubbles were observed coming from the melt. Upon crystallization, only polycrystalline InN was obtained.
  • the InN was characterized by electron diffraction and by elemental analysis.
  • Example 1 was repeated in an identical manner with the exception that an alloy containing 6.69 weight percent aluminum and 93.31 weight percent gallium was used as the starting metal. Upon crystallization, only polycrystalline AIN was obtained; no GaN was formed despite the presence of
  • Example 1 was repeated in an identical manner with the exception that during growth a microwave ball plasma source was substituted for the microwave ECR source.
  • the ball plasma source operated at 14 torr pressure.
  • the Astex microwave source operated at 2.45 GHz and a power level of 1 kw.
  • the initial charge of pure Ga was placed in a graphite plate that was machined to have a shallow, concave upper surface.
  • the melt temperature was 950 C.
  • the melt was negatively biased at 200V and 0.02 amperes using a DC power supply.
  • a mass of polycrystalline GaN was obtained that weighed 0.9449 grams.
  • the sample was confirmed to be GaN by photoluminescence spectroscopy and Raman spectroscopy.
  • Example 5 was repeated in an identical manner except that pure In was used as the starting metal, which was held in a pyrolytic boron nitride plate that was machined to have a shallow, concave upper surface.
  • the temperature of the susceptor holding the crucible was held at 200°C.
  • the growth period was 1 hour 30 minutes.
  • the In was partially converted into InN.
  • Example 6 was repeated except that a crucible was made from a piece of molybdenum foil.
  • the foil was nitrided by subjecting it to active nitrogen from a microwave plasma before the growth run. During the growth run, approximately 30 minutes after the nitrogen plasma was started, the melt was visually observed to climb out of the bottom of the crucible and to flow out over the top of the crucible lip. After the growth run was completed, many well-faceted hexagonal plates of indium nitride were observed by optical and scanning electron microscopy. The platelets were in clusters; within each cluster the platelets were well oriented with respect to each other. The platelets were identified as indium nitride by electron diffraction and elemental
  • Example 1 was repeated in an essentially identical manner except that a crystal of single crystal sapphire was added to the crucible. The sapphire floated on the surface of the melt. Upon crystallization, gallium nitride was found to have preferentially crystallized on the sapphire seed crystal. Scanning electron
  • the gallium nitride crystals had a preferential orientation with respect to the sapphire.
  • Example 6 was repeated except that the nitrogen plasma was turned off and on in an intermittent manner.
  • the melt surface observed visually while the plasma cycled from on to off.
  • a polycrystalline crust of InN was observed.
  • this crust disappeared almost instantaneously because of decomposition of the InN.

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Abstract

L'invention concerne un procédé de synthèse de cristaux de nitrure de groupe III, qui consiste à faire réagir une forme liquide du métal nitruré de groupe III avec de l'azote actif. Ledit azote actif peut être généré par un plasma ou la décomposition thermique d'ammoniac et peut être neutre, ionique, excité ou un mélange de cela. Le procédé de l'invention permet de former un nitrure cristallin de groupe III à des pressions d'une atmosphère et moins. Les couches polycristallines formées selon ce procédé présentent une qualité suffisante pour être luminescentes à la température ambiante.
EP97945616A 1996-11-04 1997-11-04 Procede de synthese de cristaux de nitrure de groupe iii Withdrawn EP0946411A4 (fr)

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FR2796657B1 (fr) * 1999-07-20 2001-10-26 Thomson Csf Procede de synthese de materiaux massifs monocristallins en nitrures d'elements de la colonne iii du tableau de la classification periodique
US20080282967A1 (en) * 2004-06-16 2008-11-20 Moshe Einav Crystal Growth Method and Apparatus
US7935382B2 (en) 2005-12-20 2011-05-03 Momentive Performance Materials, Inc. Method for making crystalline composition
EP1977029B1 (fr) * 2005-12-20 2020-07-22 SLT Technologies, Inc. Composition cristalline
US7942970B2 (en) 2005-12-20 2011-05-17 Momentive Performance Materials Inc. Apparatus for making crystalline composition
US8039412B2 (en) 2005-12-20 2011-10-18 Momentive Performance Materials Inc. Crystalline composition, device, and associated method
US20130183225A1 (en) * 2012-01-18 2013-07-18 The Regents Of The University Of California Crystal growth using non-thermal atmospheric pressure plasmas
EP2912214B1 (fr) * 2012-10-25 2023-06-07 Wetling IP CCG Ltd Améliorations apportées à la cristallisation et à la croissance des cristaux
CN116288276B (zh) * 2023-03-22 2024-10-18 中国科学院苏州纳米技术与纳米仿生研究所 单原子层二维氮化物的制备方法

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US3326820A (en) * 1965-04-22 1967-06-20 Ibm Arc process for forming high melting point compounds
US3551312A (en) * 1968-05-20 1970-12-29 Bell Telephone Labor Inc Vacuum evaporation deposition of group iii-a metal nitrides
JPS4875499A (fr) * 1971-12-28 1973-10-11
US3829556A (en) * 1972-03-24 1974-08-13 Bell Telephone Labor Inc Growth of gallium nitride crystals
JPS5150899A (en) * 1974-10-30 1976-05-04 Hitachi Ltd gan noketsushoseichohoho
CA1071068A (fr) * 1975-03-19 1980-02-05 Guy-Michel Jacob Methode de fabrication de cristaux simples par croissance a partir de la phase gazeuse
JPS54119400A (en) * 1978-07-11 1979-09-17 Sumitomo Electric Ind Ltd Production of nitrogen compound of 3b group element
JPH01145309A (ja) * 1987-11-30 1989-06-07 Idemitsu Petrochem Co Ltd 金属窒化物の製造方法およびその装置

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