Temperature dependent control of the solubility of gallium nitride in supercritical ammonia using mixed mineralizer
Using a mass-loss method, we investigated the solubility change of gallium nitride (GaN) in supercritical ammonia with mixed mineralizers [ammonium chloride (NH4Cl) + ammonium bromide (NH4Br) and NH4Cl + ammonium iodide (NH4I)]. The solubilities were measured over the temperature range 450–550 °C, at 100 MPa. The solubility increased with NH4Cl mole fraction at 450 °C and 100 MPa. The temperature dependence of the solubility curve was then measured at an equal mole ratio of the two mineralizers. The slope of the solubility–temperature relationship in the mixed mineralizer was between those of the individual mineralizers. These results show that the temperature dependence of the solubility of GaN can be controlled by the mineralizer mixture ratio. The results of the van’t Hoff plot suggest that the solubility species were unchanged over the investigated temperature range. Our approach might pave the way to realizing large, high-quality GaN crystals for future gallium-nitride electronic devices, which are increasingly on demand in the information-based age.
Introduction<!>Materials and methods<!><!>Discussion<!><!>Discussion<!><!>Discussion<!>Conclusions
<p>In an increasingly information-based society, high-speed wireless communications systems with massive information-transmission capability are expected as a ubiquitous network technology in the near future. However, to realize such systems, the power and operating frequency of electronic devices need to be increased. Gallium-nitride devices offer a promising solution, as their power and frequency is expected to exceed those of Si-based devices. However, these devices require a large-diameter, high-quality GaN bulk single-crystal substrate, which does not yet exist. Although heteroepitaxial growth can be carried out on sapphire substrate by the hydride vapor phase epitaxy (HVPE) method, the lattice mismatch increases the dislocation density of the growth. For this reason, there has been a race to develop bulk GaN single-crystal substrates using various methods. Single-crystal GaN is mainly grown by the Na flux method [1, 2] or the ammonothermal method [3–11]. The ammonothermal method is promising for its relatively mild crystal growth conditions and the ease of up-scaling the equipment.</p><p>Previously, we reported a GaN crystal growth rate exceeding the minimum requirements of industrial application (100 μm/day) using the ammonothermal method with NH4I as the mineralizer [7]. However, because the GaN solubility rapidly increases around 530 °C, the supersaturation level was difficult to control by this approach [12]. Although supersaturation is a driving force for crystal growth, spontaneous nucleation overcomes crystal growth under excessive supersaturation conditions. In fact, when NH4I is used as the mineralizer, a large number of needle crystals are deposited on the inner wall of the autoclave [13]. Changing the temperature difference between the raw material dissolution region and the crystal growth region, the type of mineralizer, and other factors can control the supersaturation level. Controlling the temperature dependence of GaN solubility by altering the mineralizer-mixing ratio would be very useful for ammonothermal crystal growth, because mineralizer addition is an easily adjustable parameter.</p><p>Several researchers have measured the solubility of GaN in supercritical ammonia with a single mineralizer [12, 14–19]. However, the solubility of GaN in supercritical ammonia with mixed mineralizers has not been reported yet. Thus, the present study investigates the change in the solubility of GaN in supercritical ammonia under addition of a mixed mineralizer, and whether the mixing ratio can control the temperature dependence of the solubility.</p><!><p>GaN crystals were grown by HVPE. The mineralizers NH4Cl (purity 99.5%), NH4Br (purity 99.0%), and NH4I (purity 99.5%) were purchased from Wako Pure Chemical Industries (Japan), and dried at 100 °C for 24 h before use. Ammonia (NH3, purity 99.999%) was obtained from Japan Fine Products Co. Ltd (Japan).</p><p>The solubility was measured by the mass-loss method, as described in our previous paper [18]. The uncertainties in the temperature and pressure values were ± 2 °C and ± 2 MPa, respectively. The composition of the sample mixtures was determined by weighing the chemicals at the desired molar ratio. The estimated measurement uncertainty in the solubility was within ± 10%.</p><!><p>Solubility of GaN in supercritical ammonia with NH4Cl + NH4Br mixed mineralizer</p><p>Solubility of GaN in supercritical ammonia with NH4Cl + NH4I mixed mineralizer</p><p>Mineralizer-composition dependence of GaN solubility in supercritical ammonia (450 °C, 100 MPa, and 3.1 mol% mixed mineralizer): a NH4Cl + NH4Br; b NH4Cl + NH4I</p><p>Temperature dependence of GaN solubility in supercritical ammonia with different mineralizers (100 MPa, 3.1 ± 0.1% mineralizer): NH4Cl, circle (from previous work [18]); NH4Br, square (from previous work [12]); NH4I, triangle (from previous work [12]); NH4Cl + NH4Br (equal mole ratio), rhombus (present study); NH4Cl + NH4I (equal mole ratio), nabla (present study): a NH4Cl, NH4Br, NH4Cl + NH4Br; b NH4Cl, NH4I, NH4Cl + NH4I</p><!><p>According to our results, the slope of the GaN solubility curve can be changed by adding a mixed mineralizer, and can be controlled by the mixing ratio of the mineralizers.</p><p>In our previous studies [12, 18], the solubility of GaN in supercritical ammonia with acidic mineralizers (NH4Cl, NH4Br, and NH4I) was described by the van't Hoff equation. Here we apply this equation to the solubility of GaN in supercritical ammonia with mixed mineralizers (NH4Cl + NH4Br, NH4Cl + NH4I).</p><p>In general, the van't Hoff equation extracts the heat of solution from the temperature dependence of the solubility. The equation is given by1\documentclass[12pt]{minimal}
				sepackage{amsmath}
				sepackage{wasysym} 
				sepackage{amsfonts} 
				sepackage{amssymb} 
				sepackage{amsbsy}
				sepackage{mathrsfs}
				sepackage{upgreek}
				\setlength{\oddsidemargin}{-69pt}
				egin{document}$${ \ln }s = - \Delta H/RT + C,$$\end{document}lns=-ΔH/RT+C,where s is the solubility in mol%, ∆H is the heat of solution in kJ/mol, R is the gas constant in J/(mol K), T is the temperature in K, and C is a constant. The compositions of the solvent and the dissolving species are assumed fixed under all experimental conditions.</p><!><p>Relationship between lns and 103/T for the solubility of GaN in supercritical ammonia with different mineralizer mixtures (100 MPa, and 3.1 mol%): a NH4Cl + NH4Br (equal mole ratio); b NH4Cl + NH4I (equal mole ratio)</p><!><p>The slope of the plot is almost constant in both systems, suggesting that the solubility species were unchanged over the investigated temperature range.</p><!><p>∆H = 42.1 kJ/mol for NH4Cl + NH4Br</p><p>∆H = 39.0 kJ/mol for NH4Cl + NH4I.</p><p>Photographs showing the appearance of sapphire glass a before corrosion resistance test b after corrosion resistance test using NH4F mineralizer (b) after corrosion resistance test using NH4Cl mineralizer: corrosion resistance test conditions were 450 °C, 102 MPa, 2.0 ± 0.1 mol% mineralizer concentration</p><p>Mass of sapphire glass before and after the corrosion resistance test (conditions: 450 °C, 102 MPa, mineralizer concentration 2.0 mol%, and 6 h)</p><p>Mass of platinum (Pt) crucible and polycrystalline gallium nitride (GaN) before and after heating for 100 h under a nitrogen atmosphere</p><p>Mass of platinum (Pt) plate before and after solubility measurements (conditions: 420 °C, 101 MPa, mineralizer concentration 3.0 mol%)</p><!><p>In their experimental procedure, Schimmel et al. released ammonia to adjust the pressure if necessary. However, because NH4Cl easily dissolves in ammonia, NH4Cl would also be released with the ammonia. Therefore, the mineralizer amount-of-substance fraction could not be accurate. They also did not weigh the ammonia, and there is uncertainty as to the amount of ammonia they used. In the experimental section, they describe that ammonia introduced into the autoclave up to fill factor of 60%. But, they do not mention the uncertainty around the amount of ammonia. In summary, it is not clear why our solubility data differ from those of Schimmel et al.</p><!><p>We investigated the change in solubility of gallium nitride (GaN) in supercritical ammonia in the presence of mixed mineralizers. The solubility curve of the NH4Cl + NH4Br system gradually became convex with increasing NH4Cl molar fraction. In contrast, the GaN solubility in the NH4Cl + NH4I system increased almost linearly with NH4Cl molar fraction. The temperature dependence of the solubility was investigated in 1:1 molar ratio mixtures. The slope of the dependence in the NH4Cl + NH4Br (NH4Cl + NH4I) system was intermediate between the slopes of the systems with single NH4Cl mineralizer and single NH4Br (NH4I) mineralizer. These results show that adding a mixed mineralizer to the system changes the slope of the solubility curve. Moreover, the GaN solubility can be controlled by the mixing ratio of the individual mineralizers.</p>