On the solubility of gallium nitride in supercritical ammonia–sodium solutions

Abstract

Due to the disparity between observed gallium nitride (GaN) growth under conditions for which literature reports normal solubility, GaN solubility in supercritical NH 3-Na containing solutions was re-evaluated. Isothermal gravimetric experiments on polycrystalline GaN were performed in the temperature range (T = 415-650 °C) for which retrograde growth of GaN routinely occurs (P 200 MPa, molar NH 3 : Na fill ratio = 20:1). Two previously-unreported error contributions to the gravimetric determination of GaN solubility were identified: Ga-alloying of exposed Nicontaining components, and the presence of a dense, Ga-absorbing Na-rich, second phase under these conditions. Due to the inability to measure Ga-alloying of the exposed autoclave wall for each experiment, considerable scatter was introduced in the refined GaN solubility curve. No clear dependence of GaN solubility on temperature was resolvable, while most solubility values were determined to be within a band of 0.03-0.10 mol. % GaN, normalized by fill NH 3 .

Ammonothermal Synthesis of Nitrides: Recent Developments and Future Perspectives

“…In situ data are available for NaNH 2 ,N H 4 F, and NH 4 Cl so far, [73,74] whereas gravimetrical measurements were reported for NaNH 2 ,K NH 2 , and NH 4 X( X= Cl, Br,I )a sm ineralizers. [68,[75][76][77][78] Higher solubilities of GaNi na cidic solutions were ascertained, according to present data, which coincidew ith the generally higherg rowth rates in acidic systems. Notably,d ifferent experimental parameters, as well as the evolution of Ga sinks and sources during the runs have to be considered, whichc an result in significant deviations of the evaluated solubilities.…”

“…Notably,d ifferent experimental parameters, as well as the evolution of Ga sinks and sources during the runs have to be considered, whichc an result in significant deviations of the evaluated solubilities. [75] Impurities and defectsi nb ulk GaN crystalsr epresent another important aspecti nc urrent research. Suihkonen et al studied and reviewed the formation and concentration of defects in ammonothermal GaN.…”

“…Besides, the dissolution rate can be directly monitored at varying temperatures. In situ data are available for NaNH 2 , NH 4 F, and NH 4 Cl so far, whereas gravimetrical measurements were reported for NaNH 2 , KNH 2 , and NH 4 X (X=Cl, Br, I) as mineralizers . Higher solubilities of GaN in acidic solutions were ascertained, according to present data, which coincide with the generally higher growth rates in acidic systems.…”

“…Higher solubilities of GaN in acidic solutions were ascertained, according to present data, which coincide with the generally higher growth rates in acidic systems. Notably, different experimental parameters, as well as the evolution of Ga sinks and sources during the runs have to be considered, which can result in significant deviations of the evaluated solubilities …”

Defects in Single Crystalline Ammonothermal Gallium Nitride

“…Post-run analysis of powders present in ammonothermal autoclaves would suggest Ga tends to form compounds with ammonia, such as [Ga(NH 3 ) 6-n X n ] (3-n)+ (n = 0, 1, 2, 4; X = F, Cl, Br, I), for acidic solutions [48] and with amides, such as [Ga(NH 2 ) 4 ] − , for basic solutions containing alkali metals. [49,50] While the accurate assessment of GaN solubility in solution during operation is still being explored, solubilities on the order of 0.1% mol Ga /mol NH3 [51] have been measured for basic systems, while higher values have been measured for the acidic systems: 0.1-1% mol Ga /mol NH3 . [52] Furthermore, basic systems exhibit retrograde solubility (increase in temperature leads to a reduction in solubility, Figure 1 b)), while acidic systems typically exhibit normal solubility, though at higher temperatures (>600 °C) they may also exhibit retrograde solubility.…”