Ammonothermal Recrystallization of Gallium Nitride with Acidic Mineralizers Andrew P. Purdy,* R. Jason Jouet, and Clifford F. George Chemistry Division, Code 6125, Naval Research Laboratory, 4555 Overlook AV, SE, Washington, DC 20375 Received October 4, 2001;
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 2 141-145
Revised Manuscript Received November 16, 2001
ABSTRACT: Crystals of cubic GaN (c-GaN) were grown ammonothermally using hexagonal gallium nitride as a nutrient. h-GaN was sealed in a quartz tube with anhydrous ammonia, an acid (NH4X; X ) Cl, Br, I), and in some cases a lithium halide (LiX). The bottom of the tube was heated to 470-510 °C in a vertically oriented pressure vessel containing a hydrostatic pressure of 30000-40000 psi. The thermal gradient was approximately -10 °C/cm. h-GaN dissolved in the hot zone and GaN crystals deposited in the cooler zone near the top of the tube. Most (>80%) of the deposit was c-GaN for most of the experimental conditions studied, but the hexagonal form was a persistent contaminant. When LiX was used as a co-mineralizer triangular prisms of c-GaN often grew. Triangular prismatic needles of c-GaN grew in a (11 h 1) direction in experiments containing a low concentration (0.28 mol %) of NH4Cl, a larger concentration (2.0 mol %) of LiCl, and a 55% fill factor. The crystals were tapered, becoming wider as they grew, and crystals of up to 0.1 mm in width were obtained. Introduction Research on crystal growth of GaN is being driven by the currently unmet need for native substrates that can be used for epitaxial growth.1,2 Most means of synthesis and/or crystal growth of GaN afford only the hexagonal phase, or the c/h-mixed nanocrystalline phase.2,3 The metastable cubic (zinc blende) phase has been observed in the reaction products of Ga with N2 in an alkali-metal flux,4 benzenethermal reactions of Li3N with GaCl3,5 and ammonothermal reactions of Ga metal compounds when acidic mineralizers (ammonium halides) are used.6 Ammonothermal processes with alkaline mineralizers (alkali amides) seem to transport only the hexagonal phase, and 0.5-mm sized crystals of h-GaN have been grown from this medium.7 Ammonothermal growth with acidic mineralizers is apparently a very complicated process as the product mix (c-GaN vs h-GaN), product yield, crystal sizes, and crystal morphology are extremely sensitive to reaaction conditions,6,8 and can also be sensitive to reaction scale.9 However, we consider ammonoacidic processes to be the most accessible means for the synthesis and crystal growth of c-GaN. As such, we are systematically investigating the use of acidic mineralizers to synthesize c-GaN with the ultimate goal of growing crystals large enough to be wafered into single-crystal substrates. Our previous papers in this series have provided an initial overview of the synthesis of c-GaN using Ga metal, GaI3, or Ga iodide-imides,6a and cyclotrigallazane6b as the gallium sources. Ammonothermal reaction of all the latter substances involved either redox or ammonolysis reactions, and consequently conditions that changed during the reactions that led to scale-dependent behavior. Normally, for uniform and scalable crystal growth in any kind of solvothermal process one wants the nutrient to be the same chemical composition as the crystals that are grown so that pressures and concen* To whom correspondence should be addressed. Phone: 1-202404-7444. Fax: 1-202-767-0594. E-mail:
[email protected].
10.1021/cg015557k
trations do not change once a steady state is reached.10 Thus, we have done a study using GaN as the feedstock. Previous work with Ga metal and GaI3 feedstocks has shown that LiX and CuX additives can be of value as co-mineralizers for preferentially growing deposits of the cubic phase.9 Thus, the effect of LiX or CuX co-mineralizers were also investigated. Experimental Section General Comments. All handling of solid reactants and Na/K alloy was done in a Vacuum-Atmospheres dri-lab. The anhydrous NH3 (Air Products) was used as received. The Li salts were heated under vacuum at >300 °C to dry, and the ammonium halides were vacuum sublimed before use. All potentially hydroscopic solids were stored in the dri-lab. X-ray powder diffraction patterns were obtained with a Phillips diffractometer with a graphite monochromator using Cu KR radiation. SEM photographs were recorded on a LEO 1550 electron microscope and an Intel QX3 microscope was used for optical photographs. The fractions of the GaN deposits consisting of the cubic and hexagonal phases was determined from the intensities of their X-ray powder diffraction lines.6a A reported procedure (chemical vapor reaction process) was used to synthesize the CVRP-GaN feedstock.11 Proper eye, face, and hand protection must be used when handling NH3 filled tubes. Ammonothermal Reactions in Quartz Tubes. Anhydrous ammonia was condensed (at -196 °C) into a 17-20 cm long, 5-mm OD/3-mm ID or a 8-mm OD/4-mm ID quartz tube containing the reactants and the tube was flame-sealed at a height (interior measure) of about 13-17 cm. The exterior of the tube was pressurized with water inside a MRA-114R or MRA-138R pressure vessel attached to a Leco HR-1B hydrothermal system to about 10000 or 5000 psi, respectively, and the pressure vessel was heated in a vertical orientation resulting in a pressure of 30000-40000 psi at equilibrium temperature. All reaction temperatures were measured in the thermowell near the bottom of the pressure vessel (hot zone). The formula for estimating the temperature in a 5-mm OD quartz tube as a function of thermowell temperature and vertical position is est temp ) Thermowell temp - (24 + 10 × height (cm)) °C based on earlier work,6a and the formula est temp ) Thermowell temp - (14 + 9.8 × height (cm)) °C was determined for the 8-mm OD tubes in a similar manner. After
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Purdy et al.
Table 1. Experimental Data for Recrystallization and Transport of GaNi middle/top of tubee tubea mmol length fill of NH3 (cm) (%)b
T, °C (time)c
mass left on bottom of tubef (mg)
appearance, deposit location,d pdr X-ray analysis
exp
reactants (mg)
42
CVRP-GaN (105), NH4Cl (50), LiCl (50)
25.0
17
66 520 (17 h)
35.7
grn dep, 10.6-14.2, GaN 95% cub
43
CVRP-GaN (121), NH4Cl (50), LiCl (30) CVRP-GaN (205), NH4I (210), LiI (210) CVRP-GaN (105), NH4Br (59)
25.0
16.8
19.0
20.8
16.3
67 516 (15 h), 496 (9 h) 65 480 (25 h)
grn dep, 10.7-14, GaN 100% cub no transport
19.7
14.3
59 518 (17 h)
44.1
CVRP-GaN (100), NH4I (50) CVRP-GaN (66), NH4I (66) above tube above tube
20.8
14.7
61 496 (19 h)
35.7
20.8
14.5
62 441
48
CVRP-GaN (125), NH4I (52), LiI (55)
17.8
14.2
56 515 (17 h)
91.6
49
CVRP-GaN (110), NH4I (55), CuI (75) CVRP-GaN (110), NH4Br (40), LiBr (40)
18.3
14.6
54 491 (17 h)
97.2
18.3
14.5
54 500 (17 h)
56.1
51
CVRP-GaN (60), LiBr (53), NH4Br (53)
12.5
14.5
38 518 (17 h)
38.9
52
Cerac h-GaN (75), NH4I (17) Cerac h-GaNh(120), NH4Cl (16), LiCl (42) Cerac h-GaNh(31), NH4Br (34), LiBr (36) Cerac h-GaNh(40), NH4Cl (20), CuCl (20)
13.4
62 479
20.2
13.6
77.6
16.5
14.1
63 376; 25; 417 (17) 49 480 (?)
20.9
14.2
62 413 (91 h)
13.0
crystal size and morphology (from SEM data)
yieldf (mg)
5-mm OD/3-mm ID tubes
44 45 46 47
50
53 54 55
455 510 (15 h)
197
8.9
3.5
grn dep, 7.6-10.2, glob formation of GaN 89% cub, needles + plates, brd xrd
