High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors—A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN

Abstract

For the fundamental understanding and the technological development of the ammonothermal method for the synthesis and crystal growth of nitrides, an in situ monitoring technique for tracking mass transport of the nitride throughout the entire autoclave volume is desirable. The feasibility of using high-energy computed tomography for this purpose was therefore evaluated using ex situ measurements. Acceleration voltages of 600 kV were estimated to yield suitable transparency in a lab-scale ammonothermal setup for GaN crystal growth designed for up to 300 MPa operating pressure. The total scan duration was estimated to be in the order of 20 to 40 min, which was sufficient given the comparatively slow crystal growth speed in ammonothermal growth. Even shorter scan durations or, alternatively, lower acceleration voltages for improved contrast or reduced X-ray shielding requirements, were estimated to be feasible in the case of ammonoacidic growth, as the lower pressure requirements for this process variant allow for thinned autoclave walls in an adapted setup designed for improved X-ray transparency. Promising nickel-base and cobalt-base alloys for applications in ammonothermal reactors with reduced X-ray absorption in relation to the maximum operating pressure were identified. The applicability for the validation of numerical simulations of the growth process of GaN, in addition to the applicability of the technique to further nitride materials, as well as larger reactors and bulk crystals, were evaluated.

Ammonothermal Crystal Growth of Functional Nitrides for Semiconductor Devices: Status and Potential

“…While nickel-base alloys generally exhibit good corrosion resistance, as well as oxidation resistance at high temperature, this is not necessarily the case for ammonothermal conditions, and stress corrosion cracking is a major concern in contact with ammonoacidic solutions [165,166,303]. More recently, cobalt-based alloys have been considered [166,304], as the system Co-Al-W provides the opportunity to harden the alloys by the precipitation of the ordered phase (γ′) in the Co solid solution matrix phase (γ), and the higher melting point of Co results in high liquidus and solidus temperatures [166]. Aiming for higher temperatures at the expense of maximum operating pressure, the molybdenum-based alloy titanium-zirconium-molybdenum (TZM) [164] and the nickel-base alloy Haynes 282 [177,186,301,305,306] have been used for ammonothermal autoclaves.…”

“…Decline of yield strength with increasing temperatures for several alloys used or considered for use as load-bearing material for hydrothermal and ammonothermal reactors, respectively. Data taken from the same references as in [304], while the data for stainless steel (A 286) were taken from [307]. The fit lines model the yield strength as a third degree polynomial and serve as a guide to the eye.…”

“…Windowless autoclaves with improved X-ray transparency: In addition to the above developments, which have been tested experimentally under ammonothermal process conditions, the use of adapted reactor designs in conjunction with computed tomography with high photon energies has been estimated to be a feasible means of enabling in situ monitoring of crystal growth, even in windowless autoclaves [304]. An encouraging factor beyond that published feasibility study is that other application fields of superalloys would likewise benefit from superalloys with lower density [327], suggesting that the availability of suitable alloys with improved X-ray transparency may improve further in the future.…”

Prospective view of nitride material synthesis

“…chemical resolution becoming possible (see Figure 4). 49 With these techniques, internal behaviors such as gallium diffusivity in ammonia and gallium dissolution rates can be measured in near-real time, while crystal etch-back and growth can also be directly measured and tied to process parameters that can be iterated many times in a single run to optimize process parameters.…”

“…For x‐ray techniques, initial experiments were based on sapphire and carbon‐based windows, 45–48 while they were later expanded to include the computed tomography technique with an unprecedented spatial and chemical resolution becoming possible (see Figure 4). 49 With these techniques, internal behaviors such as gallium diffusivity in ammonia and gallium dissolution rates can be measured in near‐real time, while crystal etch‐back and growth can also be directly measured and tied to process parameters that can be iterated many times in a single run to optimize process parameters.…”