CRYSTAL GROWTH & DESIGN
Ammonothermal Synthesis of III-Nitride Crystals Buguo Wang*,† and Michael J. Callahan‡ Solid State Scientific Corporation, Hollis, New Hampshire 03049, and Sensor Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts 01731
2006 VOL. 6, NO. 6 1227-1246
ReceiVed June 15, 2005; ReVised Manuscript ReceiVed March 2, 2006
ABSTRACT: Ammonothermal synthesis of nitrides is reviewed, with an emphasis on gallium and aluminum nitrides due to their important applications as direct wide band gap semiconductors. Since the crystallization process of nitrides involves the formation of some intermediate compounds during ammonothermal synthesis where a mineralizer is used, some ternary amides and ammoniates of aluminum and gallium with alkali metals or halides are also reviewed briefly. The ammonothermal crystallization of GaN and AlN bulk crystals, which is analogous to the hydrothermal growth of oxides, is introduced. Retrograde solubility, mineralizers, pressure-temperature-volume-concentration (PTVC) relations, phase relations, and transport growth of GaN in alkaline solutions are discussed in detail. Recent progress of GaN single-crystal growth by the ammonothermal technique is reported. We have grown GaN bulk single crystals up to 10 mm2 by 1-mm thick. Issues such as ammonia breakdown, impurity incorporation, and scale-up of the ammonothermal growth of III-nitrides and perspectives on the method are also discussed. 1. Introduction The synthesis of high-quality single crystals of group III nitrides is important for a variety of reasons. The development of a number of advanced technological devices, such as blue lasers, is based on wide band gap semiconductors. Solid-state blue diode lasers are desirable for a number of applications, particularly for increasing optical storage capacity at higher density in CD-ROMs. The nitrides of group III metals, especially aluminum and gallium, are particularly attractive for these applications.1 They exhibit not only wide, direct band gap, but also are chemically and thermally durable. Single-crystal substrates would allow for the homoepitaxial growth of lowdefect-density GaN and AlN thin films for optoelectronic devices. Devices made from low-defect-density thin films would operate at a higher power for longer periods without failure, and the usable yield of devices per substrate would be higher. However, lack of commercial methods for producing low dislocation GaN substrates inhibits performance of III-nitridesbased devices. The development of new chemically based growth techniques has opened the way to ammonothermal growth.2-4 Since hydrothermal growth (high pressure water solution) of quartz is extremely cost-effective, nitrogen-based solution growth should also produce many nitrogen-based compounds (i.e., nitrides). Ammonia is a closer match to the physical properties of water than any other known solvent; the ammonothermal technique is thus a promising method to obtain bulk crystals of nitrides.3 Crystal growth from solutions at low temperature permits production of very low-defect-density material, and this growth technique can be easily scaled up. This paper reviews the present state of ammonothermal synthesis and crystallization of nitrides with an emphasis on growth of AlN and GaN bulk crystals. Ammonothermal crystallization is a chemically complex phenomenon whose underlying mechanisms of growth are not well understood, thus requiring substantial experimental and theoretical efforts to determine the nature and kinetics of crystallization processes. * To whom correspondence should be addressed. Phone: (781) 3775261; fax: (781) 377-7812; e-mail:
[email protected]. † Solid State Scientific Corporation. ‡ Air Force Research Laboratory.
This article will be confined to the experimental work in our group and work in the published literature that addresses the ammonothermal growth of bulk crystals. Ammonia is an intriguing medium for inorganic synthesis in high-pressure fluids.4 It is less polar and less protic than water but still able to solubilize many inorganic compounds. Ammonia-based solvents provide pathways to synthesize a range of compounds that are unstable in aqueous solvents. Compared to aqueous solutions, synthesis in ammonia is relatively unexplored; it is a unique area for chemists seeking new synthetic venues. Ammonia provides its own set of experimental challenges and is somewhat more difficult to handle than water. It is a gas at room temperature and atmospheric pressure, and under ammonothermal conditions ammonia generates considerably higher pressures than water at comparable temperatures and fill ratios. In contrast, its critical conditions (Tc ) 132 °C, Tp ) 112 bar) are lower than those of water, as seen in Table 1. The original work on ammonothermal inorganic synthesis, by Juza and Jacobs6 in 1966, was the preparation of several amides and Be3N2 at 400 °C and 272 bar (4000 psi, 1 bar ≈ 14.7 psi). This work was subsequently followed up more thoroughly by Jacobs and co-workers. They were able to prepare a wide variety of metal amides, imides, and nitrides in supercritical ammonia, including a wide variety of pure amides of extremely oxophilic metals. These include such fundamentally important compounds as MNH2 (M ) alkali metal) and M′(NH2)2 (M′ ) alkaline earth metal), as well as some more exotic species such as M3[M′(NH2)6] (M ) alkali metal, M′ ) lanthanide metal). They were also able to prepare single crystals of several metal nitrides, including Eu3N2 and Mn3N2, which were unknown by any other route. These reactions were generally performed at fairly extreme conditions (400-600 °C and 6 kbar). While easy to make as powders, metal nitrides, particularly the group III nitrides AlN, GaN, and InN, are notoriously difficult to prepare in single-crystal form; only a few extremely specialized routes are currently available.7 One interesting and potentially important method is ammonothermal growth. Several workers have demonstrated that microcrystalline gallium and aluminum nitride can be prepared in supercritical ammonia.3,8 However, obtaining high-quality bulk crystals even only as large
10.1021/cg050271r CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006
1228 Crystal Growth & Design, Vol. 6, No. 6, 2006
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Table 1. Physical Properties of Ammonia and Water5 property
ammonia
water
boiling point, °C freezing point, °C critical temp, °C critical pressure, bar density, g/mL ionic product heat of vaporization, kcal/mol heat of fusion, kcal/mol viscosity of liquid at 25 °C, cP dielectric constant dipole moment, Debye units polarizability, cm3 (×1024) specific conductance, (Ohm-1cm-1)
-33.4 -77.7 132.5 113 0.68 (-33 °C) ∼10-29 5.58 1.35 0.135 22 (-33 °C) 1.46 2.25 4 × 10-10 (-15 °C)
100 0 374.2 221 0.96 (100 °C) 10-14 9.72 2.0 0.891 80 (0 °C) 1.84 1.49 4 × 10-8
Table 2: Analogous Compoundsa in the Water, Ammonia, and Hydrazine Systems9 water system
ammonia system
NaOH H3O+ ROH R2CdO CO(OH)2
NaNH2 NH4+ RNH2 R2CdNH C(NH)(NH2)2
a
hydrazine system NaNHNH2 NH2NH3+ RNHNH2 R2CdNNH2 C(NNH2)(NHNH2)2
R ) an organic group or hydrogen.
as a few millimeters still meets a lot of challenges. Thus, a systematic review of ammonothermal syntheses of nitride crystals and their intermediates during the past decades is appropriate; our own efforts on the growth of high-quality single crystals of the III-nitrides in supercritical ammonia will be also reported. Before reviewing the work on ammonothermal synthesis of nitride crystals, it is necessary to briefly summarize the physical properties of ammonia (see Table 1) in comparison with water and list analogous compounds in the water, ammonia, and hydrazine systems (Table 2). 2. Ammonothermal Syntheses 2.1. Ammonothermal Reactions. The first investigations of metal-ammonia solutions were carried out by Weyl in 1864 and Seely in 1871.10 It is well-known that alkali and alkaline earth metals (except Be), as well as lanthanides such as europium and ytterbium, dissolve in liquid ammonia under atmospheric pressure. For example, metal sodium dissolves in liquid NH3 according to the reaction
Na + NH3 f Na+ + e ‚ xNH3-
(1)
to yield a dark-blue solution that has been said to contain metal ions and solvated electrons. However, if an iron catalyst is added or the solution is kept at a mild temperature, the following reaction will take place:
2Na + 2NH3 f 2NaNH2 + H2
(2)
Chemical reactions with ammonia have three chief classes:11 (1) Addition reactions, “ammoniation” analogous to hydration (2) Substitution reactions, “ammonolysis” analogous to hydrolysis (3) Oxidation-reduction reactions Ammonia can react as a three-basic acid to form amides, imides, or nitrides with electropositive metals, depending on the temperature and ammonia partial pressure of the system. The reaction under normal pressure usually is slow, and the products are mostly microcrystalline or even amorphous. But these problems can be overcome by employing supercritical
Figure 1. An autoclave used for the ammonothermal growth of GaN crystals at AFRL. 140 cm3 volume, 2.22 cm ID, nickel based, capable of 60 kpsi @ 600 °C.
ammonia as the solvent and reactant at high temperatures and pressures. Specially designed autoclaves and equipment such as an ammonia/vacuum line and glovebox to deal with filling ammonia into autoclaves and handling chemicals sensitive to air and moisture are needed for such purposes. 2.2. Equipment: Autoclaves, Glovebox, and Ammonia Line. Autoclaves for ammonothermal syntheses under supercritical conditions should be of a nickel-based superalloy that can withstand high pressure and resist chemical attack at high temperatures and pressures (over 6 kbar and 600 °C). There are several kinds of commercially available autoclaves such as those made of Rene´ 41 by Tem-Pres division, Leco Company12 and those used by a group of German scientists.4b The closure can be either of Bridgman-type13 or a Tuttle cold-seal.14 The sealing rings in Bridgman-type closings are usually made of nickel or platinum or gold. Small nickel tubes can be used as liners for alkaline and alkaline/acidic mixed solvents, and platinum tubes for acidic solvents, when the solutions are extremely corrosive. For more details on autoclave systems, see the handbook by Byrappa and Yoshimura.10 Figure 1 shows an autoclave for the ammonothermal growth of GaN crystals at Air Force Research Laboratory (AFRL) facilities at Hanscom Air Force Base, MA. Since many nitrogen compounds are reactive in air, an inert atmosphere glovebox (also referred to as a “dry box”) is frequently used for manipulating air- and moisture-sensitive materials (see Jacobs et al.4b). A vacuum/ammonia system is needed for filling an autoclave with ammonia. Figure 2 shows the ammonia filling station used at AFRL. The ammonia filling lines are initially pumped down (to ∼10 mTorr), and then anhydrous ammonia is introduced and condensed (at -78 °C) into autoclaves. To prevent ammonia leakage, the system is constructed of 316 stainless steel, and it should be leak-tested before use. A mass controller can be inserted in the line to control and measure the ammonia transfer. Adequate eye, hand, and face protection are required during the filling process. An ammonia sensor can be used to detect an ammonia leak.
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Crystal Growth & Design, Vol. 6, No. 6, 2006 1229
3.3. Cu3N, γ′-Fe4N, Ni3N. Tetraamine copper (II)-nitrate reacts in liquid ammonia at room temperature with copper to give tri- or diamine copper (I)-nitrate. [Cu(NH3)x]NO3 with 2 e x e 3 decomposes from 350 °C up to 580 °C and p(NH3) ) 6 kbar in a temperature gradient within the autoclaves, yielding Cu3N, N2, and H2O; the nitride crystallizes in the hot zone.20
3[Cu(NH3)x]NO3 f Cu3N + 9H2O + 4N2 + (3x - 6)NH3 (6) It is interesting to note that even in the presence of water and nitric ion, no cupuous oxide is crystallized in the process. This reaction, in which Cu3N is stable under nitrogen, is similar to the preparation of stable CrO2 compound resulting from necessary O2 pressure in hydrothermal solvent.21
CrO3 + Cr2O3 f 3CrO2 H2O/NaOH, 400 °C
Figure 2. Schematic diagram of a vacuum/ammonia line used at AFRL for filling ammonia into the autoclave or releasing ammonia from the autoclave.
3. Nitrides Synthesized Ammonothermally in History: A Brief Review 3.1. Be3N2, Mg3N2 and Mn3N2. Reaction of beryllium with ammonia at 400 °C and 200 atm for about a week gives pure cubic Be3N2.15 Be3N2 was also obtained from beryllium imide, which is formed by thermal degradation of the amide Be(NH2)2 in a vacuum at 230 °C, when it was heated to 250 °C. Be(NH2)2 was prepared from the metal and ammonia at 130-370 °C and pressures of ammonia up to 3500 atm. Mg3N2 was obtained from thermal decomposition of Mg(NH2)2, which was synthesized by reacting magnesium with ammonia under ammonothermal conditions.16 Interestingly, similar to MgO reacting with hot water to form Mg(OH)2, Mg3N2 reacts with NH3 at 360-375 °C and 10 atm to form Mg(NH2)2.
Mg3N2 + NH3 f Mg(NH2)2
(3)
The reaction of manganese with ammonia at 6 kbar and temperatures from 400 to 600 °C leads to Mn3N2.17 [Mn(NH3)6]I2 and M2[Mn(NH2)4] (M ) K, Rb) are initially formed when using iodine and the alkali metals as mineralizers respectively and then convert to Mn3N2. In the first case, the nitride crystals occur in the cold zone; in the second case, the nitride crystallizes in the hot zone. 3.2. EuN and LaN. The reaction of metallic K and Eu in the molar ratios from K:Eu ) 12:1 to 1:40 with NH3 as the solvent and reactant at temperatures from 300 to 500 °C and a pressure of 5000 bar formed EuN.18 The thermal degradation of Eu(II)(NH2)2 gives Eu(III)N directly; the ternary amides give EuN and undecomposed KNH2.
KEu(NH2)3 f KNH2 + NH3 + 0.5H2 + EuN
(4)
K3Eu(NH2)6 f 3KNH2 + 2NH3 + EuN
(5)
Thermal degradation of Na3La(NH2)6, which was prepared from Na/La/NH3 at 3000-5000 atm and 250-500 °C, produces LaN. On the other hand, when K3La(NH2)6 is slowly heated in a melt of KNH2 in a silver or nickel crucible to temperatures up to 650 K within 10 days under 5 kbar (70 kpsi) of NH3, wellcrystallized LaN is obtained. The excess of KNH2 is removed with liquid ammonia at room temperature.19
CrO3 98 3CrO2 + 0.5O2
(7) (8)
Single crystals of γ′-Fe4N22 are obtained in the cold zone by chemical transformation from iron through [Fe(NH3)6]I2 in supercritical ammonia (p(NH3) ) 6-8 kbar, 460-580 °C). Ni3N was obtained from [Ni(NH3)6]Cl2, NaNH2, and NH3 at 250 °C, 2 kbar after one week.23 3.4. Sn3N4 and Si3N4. By reaction of SnI4 with KNH2 in liquid ammonia at 243 K, a white product mixture was obtained. After evaporation of ammonia, the solid residue was annealed in a vacuum for 2-5 days at 573 K. X-ray powder diffraction patterns of the product exhibited reflections of KI and of a new compound, Sn3N4.24 The tin(IV) nitride crystallizes in a spinel type structure. Analogous reactions of SnBr2 and KNH2 led to KBr and dark-brown microcrystalline Sn3N4 with excess metallic tin. NH3, 300 °C
4SnBr2 + 8KNH2 98 8KBr + Sn3N4 + Sn + 4NH3 + 2H2 (9) When treated with supercritical ammonia silicon tetrachloride is ammonolyzed in accordance with eq 10 to an ortho-ammono silicic acid known as silicon tetramide. This compound loses ammonia stepwise and is converted finally into Sn3N425 (see eq 11).
SiCl4(l) + 4NH3 f Si(NH2)4 + 4HCl
(10)
Si(NH2)4 f Si(NH2)2 f Si3N4
(11)
3.5. AlN, GaN, and InN. 3.5.1. Synthetic Approaches to Aluminum and Gallium Nitride via Pyrolysis of a Precursor. Aluminum nitride was prepared via pyrolysis of two precursors: Al(N3)3 and Al(NH2)3.26a-c The preparation of a target compound, Al(NH2)3, was attempted by various means, including the preparation of an intermediate, KAl(NH2)4, by anodic oxidation of aluminum in liquid ammonia. A metathetical reaction of AlBr3 with KNH2 in liquid ammonia led to a solid having the empirical formula corresponding to (Al(NH2)NH)n. A similar stoichiometry was observed in the product of the reaction between AlH3‚Et2O and ammonia in ether. Conversion of the amide-imide into the nitride was studied by TG/DTA and infrared spectroscopy. Aluminum nitride starts to form at 600 °C. The reaction of aluminum vapors and ammonia in a metal-atom reactor yields amorphous AlN at room temperature.
1230 Crystal Growth & Design, Vol. 6, No. 6, 2006
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The synthetic approach involving AlH3 gives better control of purity and possible control of the morphology of the nitride.26b Precursors such as [H2GaNH2]3 or Ga(CH)3NH3, which already have a Ga-N bond, could be pyrolyzed to produce GaN at relatively low temperatures. Grocholl et al.27 reported a solvothermal azide decomposition route to GaN nanocrystallites. The solution reaction between gallium chloride and sodium azide produces an insoluble azide precursor that solvothermally decomposes to GaN at temperatures below 260 °C in toluene and in THF. Purdy26c obtained indium nitride from the thermolysis of In(NH2)3, NaxIn(NH2)3+x, and KxIn(NH2)3+x. In(NH2)3 was synthesized from a reaction between InI3 and 3 equiv of KNH2 in anhydrous liquid ammonia at 25 °C. NaxIn(NH2)3+x, and KxIn(NH2)3+x were prepared through dissolving the white solid In(NH2)3, which is nearly insoluble in ammonia, in NH3 solutions of KNH2. 3.5.2. Reaction of Group III Metals with Ammonia in the Presence of Alkali Metals. Metallic gallium and aluminum can react with ammonia in the presence of alkali metals to produce GaN3b or AlN.28a,b Alkali metals can be also substituted by ammonobases such as KNH2 or NaN3. These reactants work as mineralizers (see Section 5.2.1). Kolis et al.2b grew GaN microcrystals through transport, using GaN powder as nutrient and a KNH2 and KI mixture as mineralizer, under ammonothermal conditions. InN microcrystallites were obtained through ammonothermal reaction of metallic indium with KNH2 at 450 °C.28c The reactions for AlN formation are as follows (also see eqs 26-28):
K + Al + 4NH3 f KAl(NH2)4 + 2H2
(12)
KNH2 + Al + 3NH3 f KAl(NH2)4 + 1.5H2
(13)
KAl(NH2)4 f KNH2 + AlN + 2NH3
(14)
3.5.3. Reaction of Metals Gallium and Aluminum with Ammonia in the Presence of Acids. Chen et al.29 obtained AlN powder through the reaction of aluminum metal with ammonia at 450 °C, using NH4Cl as mineralizer, with a yield of nearly 100%. They assumed a soluble intermediate Al(NH3)Cl3 is formed in the process and the following reactions take place:
2Al + 6NH4Cl + 2NH3 f 2Al(NH3)Cl3 f 2AlN + 6NH4Cl + 3H2 Demazeau30 synthesized GaN using gallium, NH2NH3Cl, and ammonia at 600 °C and 150 MPa. Purdy et al.58a-e obtained ammonothermal GaN crystallites using halides, particularly iodide as mineralizer; interestingly, however, no InN but indium metal was transported58f under acidic ammonothermal conditions from the hot zone of a sealed quartz tube to the cool zone. Yoshikawa et al.66 obtained ammonothermal GaN microcrystals, using NH4Cl as a mineralizer. 3.5.4. Metathetical Reaction of Gallium and Aluminum Chlorides and Lithium Nitride in an Organic Solvent under Solvothermal Conditions. The reaction of Li3N and GaCl3 through a benzene-thermal synthetic route yielded GaN, using benzene as a solvent in an autoclave at 280 °C; the yield was 80%.31 AlN nanocrystals were synthesized at atmospheric pressure in xylene,32a and InN nanocrystals were prepared at 250 °C by a solvothermal method32b also in xylene using the same reaction:
MCl3 + Li3N ) MN + 3LiCl (M ) Al, Ga, In) (15)
Figure 3. The crystal structure of NaGa(NH2)4. Reprinted with permission from ref 38a. Copyright 1993 Wiley-VCH GmbH & Co. KG.
Many methods can be used to synthesize III-nitrides under ammono/solvothermal conditions, but only two known approaches have resulted in well-crystallized III-nitrides at relatively low temperatures:4b,33 (1) reaction of the pure metals of group III with ammonia in the presence of alkali metals or acidic salts; (2) thermal decomposition of ternary amides in melts of alkali metal amides. Both result from the investigation of ternary amides or ammonia adducts of group III halides. Therefore, knowledge of the formation and thermal stability of the ternary amides of III-group with alkali metals and the chemistry of ammonia adducts of group III halides would facilitate a planned crystal growth of III-nitrides, particularly, through an ammonothermal process. 4. Ammonothermal Synthesis of Ternary Amides, and Ammonia Adducts of Aluminum and Gallium 4.1 Ternary Amides of Alkalis and Alkaline Earths with Aluminum and Gallium. It is known that ternary amides can be formed under mild conditions when aluminum or gallium reacts with ammonia in the presence of alkali and alkaline earth metals or with alkali and alkaline earth binary amides. Furthermore, these ternary amides can be converted to aluminum or gallium nitrides at high temperatures. It is believed that these ternary amides are soluble intermediates when GaN and AlN crystals are grown from ammonobasic solutions. Therefore, to understand the ammonothermal crystallization process, we shall devote some attention to the formation of the coordination compounds or complexes of these ternary amides, their crystal structures, and particularly at what temperatures or pressures they decompose. Investigation of the formation and decomposition of the ternary intermediates can provide insight into the conditions necessary for ammonothermal crystallization of AlN and GaN. It was reported35-42 that sodium tetraamidometalates of aluminum and gallium, NaAl(NH2)4 and NaGa(NH2)4, are isotypes; sodium pentaaamidometalates of aluminum and gallium Na2Al(NH2)5 and Na2Ga(NH2)5 can be also formed under some conditions, although with different structures. The crystal structure of NaGa(NH2)4 is shown in Figure 3. So far, only potassium tetraamidometalates have been formed for both
Perspective
Crystal Growth & Design, Vol. 6, No. 6, 2006 1231 Table 3. Structural Comparison of Ternary Amides of Alkali or Alkaline Earth with Aluminum and Gallium
ternary amides
crystal structure
ref
NaAl(NH2)4 LiAl(NH2)4 CsAl(NH2)4 NaGa(NH2)4 R-KAl(NH2)4 β-KAl(NH2)4 Na2Al(NH2)5 Na2Ga(NH2)5 K2[Mg(NH2)4] K2Zn(NH2)4
P21/c, Z ) 4, a ) 7.324 Å, b ) 6.050 Å, c ) 13.180 Å, β ) 94° P21/n (P212121?), Z ) 4, a ) 9.478Å, b ) 7.351 Å, c ) 7.351 Å, β ) 90.26° P4/n, Z ) 2, a ) 7.406(4) Å, b ) 5.386(4) Å P21/c, Z ) 4, a ) 7.4087(8) Å, b ) 6.0917(5) Å, c ) 12.855(2) Å, β ) 92.10(1)° C2221 (β ) 105.6(1)°?), Z ) 4, a ) 10.00(2) Å, b ) 5.80(2) Å, c ) 10.14(9) Å Pnma, Z ) 4, a ) 11.37 Å, b ) 8.85 Å, c ) 6.146 Å Cmma, Z ) 4, a ) 23.56 Å, b ) 19.36 Å, c ) 6.78 Å P1h(?), Z ) 4, a ) 15.29 Å, b ) 6.75 Å, c ) 19.48 Å, R ) 90.10°, β ) 129.5°, γ ) 90.10° P21/c, Z ) 4, a ) 7.455(2) Å, b ) 7.024(2) Å, c ) 13.545(9) Å, β ) 105.6(1)° P1h, Z ) 2, a ) 6.730(1) Å, b ) 7.438(1) Å, c ) 8.019(2) Å, R ) 72.03(2)°, β ) 84.45(2)°, γ ) 63.82(1)°
35 36 37 38 39 40 41 42 43 44
Figure 4. Thermal decomposition curves (a) NaAl(NH2)4 (Reprinted with permission from ref 38a. Copyright 1993 Wiley-VCH Verlag GmbH & Co KG). (b) NaGa(NH2)4 and KGa(NH2)4 (Reprinted with permission from ref 39. Copyright 1969 Wiley-VCH Verlag GmbH & Co KG). (c) MAl(NH2)4, M ) K, Na, Li. (Reprinted with permission from ref 40d. Copyright 1968 Wiley-VCH Verlag GmbH & Co KG).
aluminum and gallium; pentaamidometalates have not formed because K+ has a larger radius than Na+. It is thus expected that only cesium tetraamidometalates can be formed and that both lithium tetra- and pentaamidometalates of aluminum and gallium can be formed. Some ternary amides of alkalis and alkaline earths with aluminum and gallium are listed and structurally compared in Table 3. Thermal behaviors of these compounds have been reported in the literature.35-42 The literature indicates that they all gradually decompose from 50 °C to around 185 °C to yield aluminum or gallium imides and finally decompose to nitrides upon further heating, as shown in Figure 4. For a specific example, thermal decomposition of KGa(NH2)4 under vacuum at 180-300 °C gives KGa(NH)2 plus two molecules of NH3; GaN is obtained at 350 °C, as shown in Figure 4b. To prepare these ternary amides, a group of scientists in France39-42 reacted Ga or Al metal with K or Na in liquid ammonia. Jacobs et al.38a obtained the crystals ammonothermally. Both NaAl(NH2)4 and NaGa(NH2)4 were obtained and crystallized through reacting metallic elements for 7 days at
the molar ratio 1:1 with ammonia at 100 °C and p(NH3) ) 60 bar in stainless steel autoclaves. They also found36a that the crystal LiAl(NH2)4 can only be grown from thoroughly pulverized ternary amide. Since the p-type semiconductors Mg:GaN and Zn:GaN can be made through doping pure GaN with magnesium or zinc, the ternary amides of magnesium or zinc with potassium are of interest. Both K2Mg(NH2)2 and K2Zn(NH2)4 were synthesized through ammonothermal reactions of magnesium and zinc with potassium.43,44 The structures of these two compounds are also listed in Table 3. Interestingly, a liquid ammonia solution of potassium amide dissolves aluminum to form a potassium ammono-aluminate and reacts with zinc to form a sparingly soluble potassium ammono-zincate;34 although there is no acidic salt of magnesium in the water system, K2[Mg(NH2)4] can be made in the ammonia system. The alkali metal amides are much more soluble in ammonia than are aluminum or gallium amides. For successful syntheses of the pure ternary amides, the amount of alkali metal must generally exceed the molar ratio given by the composition of the ternary amides. The solubility of alkali metal amides increases with increasing atomic weight. One has to consider that high reaction temperatures and low pressures (< atmospheric pressure) favor the formation of imides and nitrides. When III-nitride crystals are to be grown under ammonothermal conditions, the crystallization temperatures of the nitride must be higher than the final decomposition temperature of their intermediates (see Section 5.5.2). 4.2 Ammonia Adducts of Aluminum and Gallium with Halogens. Ammoniates of aluminum chloride, AlCl3‚xNH3, can be used as starting materials for the synthesis of aluminum nitride.50 Reacting AlCl3 with NH3 can result in AlCl3‚xNH3 with x ) 1, 3, 5, 6, 7, and 14. Bremm and Meyer51 reported the syntheses and crystal structures of AlCl3‚3NH3 and AlCl3‚3NH3‚ (NH4)Cl, which were obtained during reactions of aluminum or aluminum trichloride, respectively, with ammonium chloride in sealed Monel metal ampules used as mini-autoclaves. Ammoniates of aluminum bromide and iodide, AlBr3‚xNH3 and AlI3‚xNH3 with x ) 1, 3, 5, 6, 7, have also been synthesized. Single crystals of AlBr3‚NH3 and AlI3‚NH3 were obtained by evaporation/sublimation of the respective compound from its melt.45 Colorless crystals of penta-ammoniates of aluminum halides grow in the colder part of a glass ampule when AlX3‚ 5NH3 (X ) Cl, Br, I) is heated for 3-6 days to 330 °C (Cl), 350 °C (Br), and 400 °C (I), respectively. The chloride formed hexagonal prisms, while the bromide and iodide were obtained as a congregation of lancet-like crystals.46 A number of ammonoacid complexes of aluminum and gallium with halogens are listed, and their crystal structures are compared, in Table 4. The reaction of NH4F with AlN and InN in supercritical ammonia at 400 °C leads to the formation of two metal fluoride amine complexes, AlF3(NH3)2 and InF2(NH2)(NH3), respectively.52
1232 Crystal Growth & Design, Vol. 6, No. 6, 2006
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Table 4. Structural Comparison of Some Complexes of Metals (III, II) with Ammonia and Halogens complexes
crystal structure
ref
AlCl3‚5NH3 [Al(NH3)6]I3‚NH3 AlF3(NH3)2 InF2(NH2)(NH3) Mg(NH3)2I2 [Mn(NH3)6]I2
Pnma, Z ) 4, a ) 13.405(1) Å, b ) 10.458(1) Å, c ) 6.740(2) Å Pnma, Z ) 4, a ) 13.334(2) Å, b ) 7.570(1) Å, c ) 15.066(2) Å Immm, Z ) 2, a ) 3.7184(8) Å, b ) 7.001(1) Å, c ) 7.307(2) Å P21/n, Z ) 4, a ) 7.723(3) Å, b ) 5.394(2) Å, c ) 8.638(2) Å, β ) 95.52(2)° Pbam, Z ) 2, a ) 6.285(1) Å, b ) 12.559(3) Å, c ) 4.302(1) Å Fm3m, Z ) 4, a ) 10.984(5) Å
46 47 52 52 48 49
400 °C, 10 kpsi
AlN + NH4F + NH3 98 AlF3(NH3)2 400°C, 33 kpsi
InN + NH4F + NH3 98 InF2(NH3)(NH2)
(16) (17)
The crystal structure of AlF3(NH3)2 is shown in Figure 5. A similar reaction was found for GaN with NH4F in supercritical ammonia. Hexamine aluminum iodide53 was prepared by ammonothermal reaction of Al and NH3 with NH4I or I2 at 120 °C and 90 bar or by reaction of AlI3 with NH3 at 1 bar and -35 °C; see eqs 18-21. 120 °C, 90 bar
Al + 1.5I2 + 6NH3 98 [Al(NH3)6]I3
(18)
I2 + 3NH3 f 2NH4I + 0.5N2 + 0.5H2
(19)
Figure 5. The crystal structure of AlF3(NH3)2. (Reprinted with permission from ref 2b. Copyright 1998 the Materials Research Society.)
Scheme 1
120 °C, 90 bar
Al + 3NH4I + 3NH3 98 [Al(NH3)6]I3 + 1.5H2 (20) -35 °C, 1 bar
AlI3 + 6NH3 98 [Al(NH3)6]I3
(21)
The ammonolysis of hexammine aluminum iodide produces AlN: NH3/N2, 350-390 °C
[Al(NH3)6]I3 (solid) 98[Al(NH3)6]I3 NH3/N2, >500 °C
(gas) 98 AlN + 1.5I2 + 6NH3 (22) In short, reactions of aluminum (III) and gallium halides with ammonia have been widely investigated, and a number of complexes can be formed. These reactions actually form the basis of halide vapor phase epitaxy of III-nitrides. 5. Ammonothermal Crystallization of III-nitrides 5.1. Dissolution-Crystallization Processes in a Solvothermal System. The solvothermal (including hydrothermal and ammonothermal) growth of crystals can be divided into two steps: the nutrient dissolves in the solvent in the presence of mineralizer (also see Section 5.2) and then crystallizes on the seeds or somewhere else. This process is described by the “dissolution-crystallization” or “transport growth” model, as shown by Scheme 1. The dissolution-crystallization process of R-Al2O3 in hydrothermal solvents and AlN in ammonothermal solvents is compared in Table 5. It is noted that intermediates in most cases of crystal growth should not be seen in the final product (see Section 5.5.2). According to Laudise et al.,54,55 transport growth could be easily realized if the solubility of a material in the hydrothermal solvent is greater than 3% and a temperature gradient exists in the system (normally 10-25 °C between growth zone and dissolution zone). In our current research, we found that both GaN powder and polycrystalline pieces have a solubility of greater than 1% in all ammonobase or ammonoacid solutions at temperatures varying from 350 to 600 °C that were evaluated.
The temperature gradient used in the experiments was greater than 10 °C/cm. However, initial microscopic examination of the seeds and their holders, which were put in the cold zone and nutrient was put at the bottom of the autoclave (the hot zone), revealed no transported GaN in the period when this research began a few years ago. Why? Obviously, transport growth was not established in such a configuration, although the dissolving process is not a rate-determining step. Since the temperature difference ∆T is large and various baffle designs were used in our experiments, solute transport should not have been a problem. Therefore, according to Scheme 1, the remaining issue is how to realize the crystallization of GaN from these solutions, or say, to push a phase transformation from intermediates to GaN crystals, since all the dissolved GaN was converted into Ga-containing intermediates. So the temperature at which range is chosen for crystal growth and in which zone the seeds are placed (we use two zones, i.e., cold zone and hot zone for transport growth) is a key issue for realizing the transport growth of III-nitrides. 5.2. Mineralizers. Under hydrothermal growth, the term mineralizer refers to any component added to the water solution without which crystallization would either not occur or would be extremely slow.54 The mineralizer most often acts to increase the solubility of solute by the formation of species complexed with the mineralizer. The increased solubility permits increased supersaturation without spontaneous nucleation and consequently allows increased growth rates. In some cases, the mineralizer may act as a complexing agent to reduce the molecular size of the species in solution, thereby speeding up the crystallization process. An examination of the chemical analogues of hydrothermal mineralizers for use as ammonothermal mineralizers to dissolve gallium and aluminum nitrides in supercritical ammonia was initially undertaken. These compounds can be divided into two types: the one ammono bases and the other ammono acids, as seen in Table 6. 5.2.1. Ammono Base Mineralizers. Amide in ammonia is exactly analogous to hydroxide in water. Just as hydroxides attack R-Al2O3 to form soluble aluminate anions,55 as seen in Table 5, amides attack metal nitride feedstock, leading to soluble
Perspective
Crystal Growth & Design, Vol. 6, No. 6, 2006 1233 Table 5. Comparison of Dissolution-Crystallization Process in Hydrothermal and Ammonothermal Solventsa oxides: hydrothermal O+
nitrides: ammonothermal
OH-
water: 2H2O T H3 + Al2O3 in alkali water solution: Al2O3 (nutrient) + 2OH- + 3H2O f 2Al(OH)4- (dissolved species) mineralizers acids: HNO3, HCl, HI bases: KOH, NaOH, LiOH salts: Na2CO3, KF, NaCl a
ammonia: 2NH3 T NH4+ + NH2AlN in ammonobase solution: AlN (nutrient) + NH2- + 2NH3 f Al(NH2)4- (dissolved species) mineralizers acids: NH4Cl, NH4I, HCl bases: KNH2, NaNH2, LiNH2 salts: KI, NaCl, NH2NH3Cl
Assumes complete ionization of the aluminates formed. Table 6. Mineralizers Used in Ammonothermal Crystal Growth of III-Nitrides bases
halides
amides
pure alkali metals
azides
ammono acids
alkali halides
hydrazine hydrochloride
NaNH2, KNH2
Li, Na, K, etc.
NaN3, KN3, LiN3
NH4I, NH4Cl, NH4F
KI, LiF, etc.
NH2NH3Cl
metal amide anions. These soluble species then deposit as large crystals of the nitrides if the proper conditions are met. According to Section 4.1, for AlN crystal growth and potassium amide used as mineralizer, the eq 23 is valid and the intermediate KAl(NH2)4 is formed; when sodium amide is used, the intermediate Na2Al(NH2)5 is probably formed:
KNH2 + AlN + 2NH3 T KAl(NH2)4
(23)
AlN + 2NaNH2 + 2NH3 T Na2Al(NH2)5
(24)
As shown in Section 3, pure alkaline metals such as potassium and sodium also can be taken as mineralizers. When these metals are used as mineralizers, the metals react with ammonia first to form amides and hydrogen, and then amides subsequently attack the nitride nutrient:
2Na + 2NH3 f 2NaNH2 + H2
(25)
When an azide is used as a mineralizer, the azide will initially decompose at its melting point or slowly break down with increasing autoclave temperature and produce amide and N2/ H2 gases; then amide reacts with ammonia and nutrient, as seen in eqs 26-28. This has been verified by pressure measurements (see Section 5.4).
2NaN3 f 2Na + 3N2
(26)
2NaN3 + 2NH3 f 2NaNH2 + 3N2 + H2
(27)
2NaN3 + 6NH3 + 2GaN f 2NaGa(NH2)4 + 3N2 + H2 (28) When pure alkali metals and azides are used, the system will produce hydrogen and consume more ammonia. In all three cases of alkali metals, amides, and azides, the intermediate ternary amides of aluminum or gallium with alkali metals are formed initially. Therefore, not only polycrystalline GaN and AlN can be used for nutrients; Ga- and Al-containing intermediates, or even Ga and Al metals, which will react with these mineralizers to form intermediates, can also be used as nutrients (although polycrystalline GaN is mainly used as a nutrient in our current research of GaN crystal growth). The thermodynamics of the system is not fully understood, however, we believe that the formed intermediate has a high solubility in ammonia and is very stable under mild conditions. Transformation of IIInitride from these intermediates is very limited and leads to a poor transport growth rate at lower temperatures, which will be discussed in Section 5.5.
5.2.2. Ammono Acid Mineralizers. Although there are some cases where alkaline salts or acids have been used as mineralizers under hydrothermal conditions,56 alkaline hydroxides have been predominantly chosen as mineralizers in the growth of oxides57 such as R-SiO2, ZnO, and R-Al2O3, etc. The reason is that basic mineralizers allow for high solubility, simple chemistry, and less corrosion with the stainless steel autoclaves. In ammonothermal growth, salts or acidic mineralizers can also be used. However, using salts or acidic mineralizers is problematic: (1) Halides corrode the nickel-based autoclave and plunger severely, so a liner such as quartz or platinum must be used. (2) A more complex chemistry may occur in the mineralization stage as compared to ammonobasic mineralizers. This complexity is highlighted by a very unusual observation by Purdy,58 who was able to grow single crystals of the metastable, cubic form of GaN with ammonium halides as the mineralizer. For most experimental conditions, the hexagonal phase is the thermodynamically stable form of GaN. All references in this paper to GaN are about the hexagonal (wurtzite) form unless stated otherwise. Crystals of cubic GaN (c-GaN) were grown ammonothermally using hexagonal gallium nitride (h-GaN) as a nutrient.58b,c h-GaN was sealed in a quartz tube with anhydrous ammonia, an acid (NH4X; X ) Cl, Br, I) and lithium halide (LiX) as a co-mineralizer. The bottom of the tube was heated to 470-510 °C in a vertically oriented pressure vessel containing a hydrostatic pressure 2-3 kbar. h-GaN dissolved in the hot zone and GaN (mostly c-GaN) crystals deposited in the cooler zone near the top of the tube. Purdy58c found that higher contents of NH4I favor the formation of c-GaN. When using an NH4Cl/ LiCl mineralizer system, addition of more NH4Cl has little effect on transport rate once a threshold of about 0.015 M is reached. The LiCl co-mineralizer concentration is very significant, and the transport rate is very low when LiCl is absent. Scale and run time also affect the phase purity of the deposit in the cool zone, with the fraction of hexagonal phase GaN increasing with increasing run time and with the amount of mass transported. The importance of the mineralizer is also emphasized by the fact that the cubic form only occurs in the presence of iodide, as opposed to the ammonium salt of any other halide. Wang et al.29a reported that c-GaN only formed with high concentrations of NH4I in a large-scale direct reaction of Ga with NH3 (see Table 7). However, Purdy58a obtained c-GaN using not only NH4I, but also NH4Cl and NH4Br as mineralizer under somewhat different conditions. Bulk crystalline c-GaN was prepared in an acidic supercritical NH3 medium at temperatures as low as 275 °C. Deposits of c-GaN crystals were prepared from Ga metal and NH3/NH4X (X ) Cl, Br, I), from ammonolysis of GaI3, and from acidic ammonolysis of cyclotrigalla-
1234 Crystal Growth & Design, Vol. 6, No. 6, 2006
Perspective
Table 7. Effect of Acidic Mineralizer on Phase Formation of GaN under Ammonothermal Conditionsa mineralizer
content (mol %)
reaction temp (°C)
reactive time (h)
products
NH4I NH4Cl NH4Cl
50 50 5-30
300 300 350-500
40-50 40-50 72-120
c-GaN c-GaN, h-GaN h-GaN
a
Reprinted with permission from ref 29a. Copyright 2001 Elsevier.
Table 8. Effect of Different Ammonium Halides on Synthesis of BN by High-Pressure Solvent Methoda solvent
melting point of solvent (°C)
phases of the product
conversion rate of c-BN (%)
NH4F NH4Cl NH4Br NH4I
sublimation 340 452 551
c-BN c-BN, h-BN c-BN, h-BN h-BN
100 90 20 0
a Precursor: h-BN, 5.6 GPa, 1500 °C, 30 min. Reprinted with permission from ref 59. Copyright 1979 Elsevier.
zane.58d Pure phase c-GaN was also prepared by ammonothermal conversion of gallium imide, {Ga(NH)3/2}n using NH4I mineralizer; however, with NH4Cl mineralizer, h-GaN nanoparticles formed.58e There are apparently fundamental differences in chemical reactions in the ammonoacidic system when starting from gallium metal and starting with h-GaN or other gallium compounds. There are also some differences in solute transport and thermodynamics in the acidic system when using iodide and using other halides. Therefore, the chemistry and thermodynamics for the formation of c-GaN and for the ammonothermal growth of h-GaN crystals in an acidic medium is not yet well understood. Different phases of GaN have opposite transport directions. Transport direction is from the hot zone to the cold zone for c-GaN when using iodide, and using NH4X/LiX (X ) Cl, Br, I) as mineralizer in the system at 350-480 °C,58 whereas transfer from cold to hot was observed for GaN in the KNH2-GaNH3 system at 500-600 °C. Halide acidic mineralizers also influence the phase formation of BN under ammonothermal conditions.59 Table 8 shows that the conversion rate from hexagonal BN to cubic BN increases as the halogen group ascends in the periodic table. It is difficult, however, to suggest which halide could favor the formation of either c-GaN or h-GaN by the ammonothermal technique; the reaction temperature also plays a key role in the formation of a specific phase, as seen in Table 7. Aluminum reacts with ammonium iodide and ammonia to form hexamine aluminum iodide (see eq 20).53 This compound is the transport intermediate for AlN from the cold (400 °C) to the hot zone (600 °C) of the autoclave. Similarly, GaN was grown between 650 and 750 °C in the presence of NH4F.73b It is also noted that using an ammono acidic mineralizer, AlN is formed as an ultrafine and uniform powder, but using an ammono basic mineralizer it is possible to get single crystals of AlN up to 1 mm, which grows predominantly in the direction of the crystallographic c-axis. It was reported60 that NH4I is also an excellent mineralizer for the crystal growth of SrS and chalcogenides in ammonia above 300 °C. The ammonia solvent will complex to harder di- and trivalent metal centers preferentially rather than to sulfide ligands. To inhibit the complexation of amines to the highervalent metal centers, it appears that acidic solutions and higher temperatures are required.
NH3/400 °C
SrS + NH4I 98 SrS (crystal)
(29)
In general, the use of acidic supercritical ammonia for ammonothermal synthesis of inorganic materials at higher temperatures is a relatively unexplored area and ripe for development. It is worthwhile to mention that the mechanisms involved in the dissolution-crystallization process should be basically the same if the same kinds of mineralizers are used. For example, whether amides, azides, or alkali metals are used as mineralizers in GaN or AlN crystal growth, the dissolution and crystallization mechanisms are all similar to each other. Mineralization mechanisms should be also similar for using various halides as mineralizers where ammonia adducts of aluminum and gallium with halogen might be formed first and then convert to nitride. However, basic mineralizers have different mechanisms compared to acidic mineralizers (halides) in both the dissolution and the “transport” processes, because the formation conditions of the ternary amides and ammonia adducts of aluminum and gallium halides, as well as the transformation conditions of these compounds into the nitrides, are different. Therefore, the dissolution and crystallization of GaN in KNH2-NH3 and in KI-NH3 solutions are certainly different, and we should refrain from using a mixture of basic and halide mineralizer to avoid a more complicated crystallization process. In hydrothermal growth, an efficient mineralizer for growing single crystals in an A-B-H2O system should meet the following conditions61: (a) congruent dissolution of the compound A; (b) a sufficiently sharp change in the solubility of the substance A in the presence of B with temperature; (c) a certain absolute value of solubility of the compound being crystallized; (d) the formation of mobile associates (complexes) in the solution which involves the basic components of compounds A, and which decompose in the growth zone to form single crystals of A; (e) the build-up of the necessary redox potential of medium in the reactor to maintain the ions at a required oxidation state; and (f) fewer impurities from B incorporated into crystal A. Such criteria should also be met in the search for an efficient mineralizer for the ammonothermal growth of nitrides. 5.3. Solubility of GaN in Ammonobasic Solutions. We have found that, without mineralizers, little GaN can dissolve in supercritical ammonia (10 days, 500 °C, 30 kpsi). Even using mineralizers, the degree to which GaN dissolves in ammonia varied with conditions applied. In some runs, the solubility is low while in other runs, the solubility is higher. In his famous monograph, Franklin11 wrote: “Ammonous aluminum nitride dissolves readily in liquid ammonia solutions of ammonium bromide to form aluminum bromide, and of potassium amide to form potassium ammonoaluminate.” (p 60). Therefore, it was thought that polycrystalline gallium nitride or aluminum nitride nutrient should dissolve in basic or acidic ammonothermal solutions. We determined the solubility of GaN in ammonobasic solution by a weight loss method, using Tem-Pres cold-seal autoclaves (5.5 mL) over the range 350-600 °C. Dense polycrystalline GaN pieces of φ 2-4 mm in size that were synthesized by the chemical vapor reaction process (CVRP)62 or powder (commercially purchased high-purity GaN powder or ground from GaN chunks in a drybox to 500 °C). We also used gallium-containing intermediates (ternary amides or imides of gallium and alkali metals) as nutrient, which still was placed at the midpoint of the autoclave chamber to check where GaN will grow, since in this case GaN obtained after the run are all from phase transformation, so that clearly show the transport direction.76b The result showed the GaN was transported from the cold zone to the hot zone. All these experiments confirmed that GaN has the same negative solubility coefficient, as seen in Figure 7, at its crystallization temperature as AlN in ammonobasic solution. Figure 15 shows the setup for the ammonothermal growth of both AlN and GaN. The nutrient was placed at the cooler upper zone, and the seeds were positioned at the bottom hotter zone. For a cold-seal Tuttle-type autoclave and almost all vertical autoclave systems, the lower zone is kept hotter than the upper zone because the temperature gradient and system pressure are easily controllable in such a configuration.
Perspective
Crystal Growth & Design, Vol. 6, No. 6, 2006 1239
Figure 17. SEM photographs of spontaneously nucleated GaN crystals from the wall and the bottom of autoclave.
Figure 16. Phase diagram of GaN in the KNH2-NH3 system (curves based on our experiments between 5 and 35 kpsi, below 5 kpsi the tendency is estimated).
5.5.2. Phase Relations and Crystallization Conditions. The high solubility of GaN and AlN in ammonobasic solutions does not mean that transport growth can be realized easily. On the contrary, Ga- and Al-containing intermediates are easily formed and are stable under certain conditions. Therefore, the phase transformation from intermediates to nitrides is a key issue for realizing the transport growth of polycrystalline nitride to single crystals. The phase transformation of Ga or Al intermediates to GaN or AlN crystals is a slow thermodynamic process in which temperature is the rate-controlling factor. For example, the reaction KGa(NH2)4 f KNH2 + GaN + 2NH3 does not occur within 10 days with temperatures below 400 °C at a pressure of 10-15 kpsi (0.7-1 kbar). Plotted in Figure 16 is the phase diagram of the GaN-KNH2-NH3 system. From this figure, one can see clearly that the behavior of GaN in the KNH2-NH3 system is very similar to R-Al2O3 in the H2O system (for the Al2O3-H2O phase diagram, see ref 69, page 291). Therefore, the only way to increase the rate of transformation from intermediates to nitrides, i.e., the growth rate, is to operate the ammonothermal growth at as high a temperature as possible. Pressure changes within 15-35 kpsi had no significant effect on the growth rate of the nitride crystals. According to the chemical equilibrium of this process (eq 14), lower pressure and higher temperature in the system favor GaN or AlN crystallization (for example >475 °C,
