Ammonothermal Conversion of Cyclotrigallazane to GaN:  Synthesis of Nanocrystalline and Cubic GaN from [H₂GaNH₂]₃

Abstract

Under ammonothermal conditions, cyclotrigallazane, [H2GaNH2]3, was converted to nanocrystalline GaN. The coherent length of the GaN product could be controlled by manipulating reaction time and temperature. Nanocrystalline GaN powders with average crystallite sizes of 3−17 nm were produced in the temperature range of 150−450 °C. Analysis of the powders by XRD and TEM indicated that the nanocrystals possessed a structure composed of a random arrangement of cubic and hexagonal close packed planes. The addition of NH4I to the reaction mixture resulted in the isolation of bulk samples of phase pure, cubic GaN at temperatures as low as 200 °C. The synthesis of the cubic phase was found to be temperature dependent, with mixtures of cubic and hexagonal GaN being produced at temperatures greater than 350 °C.

Ammonothermal epitaxy of wurtzite GaN using an NH4I mineralizer

“…The previous [5,8,[9][10][11] and present results are consistent with thermodynamics prediction: because the difference in Gibbs' free energy of WZ-GaN and ZB-GaN increases with the increase in temperature, WZ phaseGaN becomes more stable at high temperatures.…”

“…According to the previous reports [5,8,[9][10][11], GaN crystals grown using NH 4 I mineralizer tended to have a mixed structure of WZ-and ZB-phases, and the mixing ratio of ZB-GaN decreased as the growth temperature was increased. The previous [5,8,[9][10][11] and present results are consistent with thermodynamics prediction: because the difference in Gibbs' free energy of WZ-GaN and ZB-GaN increases with the increase in temperature, WZ phaseGaN becomes more stable at high temperatures.…”

“…In order to increase the growth rate, the use of NH 4 I as an alternative mineralizer is preferable, because NH 4 I afforded greater change in GaN solubility in supercritical NH 3 for a given temperature change than the case of NH 4 Cl [8], especially at a particular temperature range between 500 and 550 1C. However, as for the NH 4 I mineralizer, spontaneously nucleated only tiny GaN needle crystals have been grown so far [5,[8][9][10][11][12], and seeded epitaxial growth of GaN has not been reported. In addition, the mixing ratio of zincblende (ZB) cubic phase GaN in the needle crystals has been reported to increase when NH 4 I mineralizer was used.…”

Structure of nanocrystalline ammonothermal GaN in dependence of temperature and type of acidic mineralizer

“…Recently, the ammonothermal method has been employed to grow nanocrystalline GaN. Lan et al [16] synthesized nanocrystalline GaN by ammonothermal reaction, using lithium as the mineralizer under ammonothermal conditions, and derived nanocrystalline cubic GaN from cyclotrigallazane [17]. Purdy [18] and Ehrentraut et al [19] published the synthesis of bulk cubic and hexagonal/cubic GaN by ammonothermal reactions under acidic mineralizers (NH 4 Cl, NH 4 F and NH 4 Br).…”

Crystal growth in supercritical ammonia using high surface area silicon nitride feedstock

“…In contrast, GaNgrowth in liquid gallium requires temperatures of 1273-1873 K and a nitrogen pressure of 1-2 GPa due to the high decomposition pressure of GaN [11]. Crystallization in sc-NH 3 also allows the synthesis of metastable cubic gallium nitride [12,13] and, at low temperatures (473 K), nanocrystalline GaN is accessible [14].…”