MA: Advancement of a highly improved autoclave purification process for the ammonothermal growth of high-purity nitrides

4. Dezember 2025

Yi Wei –

In ammonothermal GaN crystal growth, residual moisture and oxygen species adsorbed on internal autoclave surfaces can severely degrade crystal quality. This thesis presents a high-temperature autoclave purification procedure aimed at eliminating these impurities from the growth environment. We purified the autoclave through inert-gas flushing combined with a multi-stage heating procedure. The heating program consisted of successive temperature intervals from 20 ± 1 °C to 89 ± 1 °C, 89 ± 1 °C to 274 ± 2 °C, 274 ± 2 °C to 488 ± 3 °C, and 488 ± 3 °C to 543 ± 2 °C, followed by an isothermal holding period of 21.5 ± 1 h. After these treatments, we evaluated the desorption of adsorbed H₂O and the getter-like oxygen removal behavior inside the autoclave.
The experiments showed that, once water release started, the majority of the adsorbed moisture was desorbed within approximately ten hours. At higher temperatures, the rate of water release diminished markedly, indicating that the bulk of moisture had already been expelled in earlier stages.
After repeated Ar-flushing and multi-step thermal conditioning, the previously unused auto-claves exhibited a clear oxygen-gettering response. The measured oxygen concentration decreased substantially at elevated temperatures, indicating that a significant fraction of residual oxygen was absorbed by the autoclave walls rather than remaining in the gas phase. The influence of surface pre-treatment on outgassing was significant: rigorous cleaning, an initial bake-out, or deliberate moisture pre-loading of the autoclave strongly affects the overall amount of water released, likely by activating surface sites for more effective moisture re-lease. Detailed analysis of the moisture-decay curves and the calculated amounts of de-sorbed water at each temperature stage confirmed that pronounced water-desorption reactions occurred throughout the applied heating protocol. Crucially, maintaining the autoclave temperature above the dew point of water ensured that all released volatiles remained in the vapor phase, preventing internal re-condensation and loss of moisture to surfaces. Any localized thermal gradients or adsorption/desorption anomalies were minimal and were accounted for in the error analysis to ensure accurate interpretation of the sensor data.
Overall, the optimized multi-stage heating and purging regimen proved highly effective. The autoclave showed a clear capability to absorb oxygen and to release adsorbed water during the successive heating stages, indicating its potential to suppress impurity formation during ammonothermal GaN growth. These findings highlight the practical relevance of the identified water-desorption and oxygen-gettering behaviors, which help establish a cleaner initial environment for ammonothermal GaN growth and offer practical guidance for achieving high-purity nitride crystal synthesis.

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