An exploratory study of acidic ammonothermal growth in a TZM autoclave at high temperatures

Abstract

Recently, we have reported on the use of the titanium-zirconium-molybdenum alloy (TZM) as an autoclave material for acidic ammonothermal growth of bulk GaN using chloride based mineralizers. Here, we discuss the trends of growths with temperatures ranging from 650 to 900 °C at pressures around 100 MPa in a TZM autoclave. The resulting crystal quality as measured by x-ray diffraction appears to be dependent upon growth thickness. Secondary ion mass spectrometry studies of impurities such as O, H, Cl, and Fe show a tendency to decrease with increasing growth thickness, with concentrations ranging from 1018 to 1020 cm−3 for O and H, and 5 × 1016 to 5 × 1018 cm−3 for Fe. Cl did not have a standard to quantify the concentration. Ti increased in concentration with increasing growth rate with a range of 4 × 1014–1 × 1017 cm−3. C had steady concentrations between 3 × 1016 and 5 × 1017 cm−3 and the Mo concentration was measured in the low 1016 cm−3 or less. Zr did not have a standard and thus could not be quantified but was near the detection limit in all cases. The instability of the ammonia solvent at high temperatures requires very high pressures for ammonothermal growth above 800 °C but short-term growth has been demonstrated up to 900 °C.