Epitaxial Growth of AlN on (0001) Sapphire: Assessment of HVPE Process by a Design of Experiments Approach
This study aims to present the interest of using a design of experiments (DOE) approach for assessing, understanding and improving the hydride vapor phase epitaxy (HVPE) process, a particular class of chemical vapor deposition (CVD) process. The case of the HVPE epitaxial growth of AlN on (0001) sapphire will illustrate this approach. The study proposes the assessment of the influence of 15 process parameters on the quality or desired properties of the grown layers measured by 9 responses. The
... y 9 responses. The general method used is a screening design with the Hadamard matrix of order 16. For the first time in the growth of AlN by CVD, a reliable estimation of errors is proposed on the measured responses. This study demonstrates that uncontrolled release of condensed species from the cold wall is the main drawback of this process, explaining many properties of the grown layers that could be mistakenly attributed to other phenomena without the use of a DOE. It appears also that the size of nucleation islands, and its corollary, the stress state of the layer at room temperature, are key points. They are strongly correlated to the crystal quality. Due to the intrinsic limitations of the screening design, the complete optimization of responses cannot be proposed but general guidelines for hydride (or halogen) vapor phase epitaxy (HVPE) experimentations, in particular with cold wall apparatus, are given. Author Contributions: Raphaël Boichot and Frédéric Mercier conceived and designed the experiments, based on preceding experimental data obtained by Gaël Giusti, conceived the modeling for island size calculation and wrote the article. Danying Chen was the main experimenter. She was trained by Mikhail Chubarov on the CVD apparatus and helped for structural characterizations and data treatment. Francis Baillet was the DOE expert that helped Raphaël Boichot with statistical exploitation of the data. DOE data were cross-validated with Matlab and Design Expert softwares. Gaël Giusti and Thomas Coughlan wrote the numerical code for island size calculation and validated its robustness on pre-existing experimental data with two programming language : Matlab and Python. Michel Pons did the computational fluid dynamics calculations for simulating the kinetics into the CVD reactor. He also helped for broad literature review. The choice of parameters and outputs involved in the DOE study was validated by the whole team. Conflicts of Interest: The authors declare no conflict of interest.