Ammonothermal Crystal Growth of GaN Using an NH₄F Mineralizer

Abstract

NH 4 F is demonstrated to be a promising mineralizer for the acidic ammonothermal crystal growth of GaN. In comparison with other acidic mineralizers such as NH 4 Cl, NH 4 Br, and NH 4 I, NH 4 F behaves distinctively different. First, NH 4 F affords a negative temperature gradient for crystal growth of GaN in supercritical NH 3 at a temperature range from 550 to 650 °C. Second, it enables GaN crystal growth in polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH 4 F remarkably increases both the growth rate and quality of the GaN crystal. With the aid of NH 4 F, self-nucleation of GaN and bulk growth of hexagonal GaN crystals from the self-nucleated seed have been realized.

Chemistry of Ammonothermal Synthesis

“…Additionally, there are two examples of GaN manifesting a negative solubility in ammonoacidic milieu reported: A change in the temperature dependance of the solubility of GaN from positive to negative, using NH 4 Cl has been noticed at temperatures above 923 K with pressures of 110 MPa [62]. Also, a negative solubility has been revealed in the temperature range 823-923 K using NH 4 F as mineralizer [46]. Up to now NH 4 F and NH 4 Cl are the only acidic mineralizers known to evoke a negative solubility for GaN and consequently the crystallization in the hot zone.…”

“…Thus, the existence of positively charged complex ions such as [Ga(NH 3 ) 6 ] 3+ and [Ga(NH 3 ) 5 Cl] 2+ may explain a higher growth rate on the negatively charged face (see Figure 6a) [87]. Furthermore, a growth on both c-faces, the negatively charged N-face and the positively charged Ga-face, is observed using NH 4 F as mineralizer [46]. Figure 6b).…”

“…According to the latest research results this problem seems to overcome. Recently, researchers increased growth rates for GaN by ammonothermal method from 24-106 µm/d [45] to 250 (c-plane) and 300 µm/d (m-plane) in the presence of NH 4 F, used as so-called mineralizer [46]. Even m-plane growth rates of up to 40 µm/h (=960 µm/d) with rates of 10-30 µm/h (=240-720 µm/d) for all planes were reported recently [39].…”

Defects in Single Crystalline Ammonothermal Gallium Nitride

“…Reported impurities result from bulk GaN crystals grown in a) basic ammonothermal chemistries in a NiCr superalloy autoclaves without a liner (NiCr), within a silver capsule (Ag), within a molybdenum capsule (Mo) and b) acidic ammonothermal chemistries in a lined Ni-based alloy autoclave (lined) or a TZM autoclave (TZM). [22,26,32,43,58,59,79,[126][127][128] (7 of 18) 1600496…”

Determination of GaN solubility in supercritical ammonia with NH4F and NH4Cl mineralizer by in situ x-ray imaging of crystal dissolution

“…Additionally, all of the reported quantitative data (for ammonoacidic mineralizers: NH 4 Cl [10][11][12], NH 4 Br [12] and NH 4 I [12]) have been obtained by gravimetric investigation of residual reactants after dissolution experiments, i.e., by ex situ methods. No investigations of GaN solubility with NH 4 F as mineralizer have so far been reported although it shows promising properties: NH 4 F offers a negative temperature gradient of solubility observed at temperatures of 550-650 1C [13], growth in polar, semipolar as well as nonpolar directions [13,14], high growth rates and good crystal quality [13,14].…”