This NSF-MRI grant funded the work required to develop and build a novel high pressure spatial metal-organic chemical vapor deposition tool (HPS-CVD). The novelty of this tool is its ability to operate at high pressures (up to 100 atm), thereby enabling the synthesis of materials not readily achievable using traditional ~1 atm growth systems. The high pressure specifically prevents the decomposition of desired materials, for example indium-containing group-III nitrides, via increasing its thermodynamic stability. This permits growth at higher temperatures, leading to higher quality materials yielding higher performing devices, or new material alloys altogether, for example oxynitride materials. Key to enabling this synthesis platform, is the development of a novel system that overcomes existing, known problems of high-pressure systems due to the increased density of the gases, namely the formation of vortices (leading to contamination or non-uniform crystal growth) and loss of precursors due to premature, gas-phase interactions leading to a significant drop or outright loss of material synthesis on the desired, heated substrate. 

The scope of this grant was to develop the tool using computational studies, and then build and operate the tool to demonstrate growth of group-III nitrides and oxynitrides. This would in turn would permit novel insight into the growth mechanisms for thin film synthesis of electronic, optical, and quantum materials at high pressure. Additionally, it would explore the fundamental physical properties of these materials and their (heterogenous) integration into device structures. 

Development of this synthesis tool was staged to initially include the design of the synthesis chamber via computational study of the tool (using COMSOL, a commercial, multi-physics modeling software package) and the development of associated control systems and facilities. The optimized tool would then been manufacturing and placed in the suitably upgraded facilities to perform initial growth demonstrations of group-III nitrides and oxynitrides. 

A graduate student was hired on this project, was trained on the use of COMSOL, and was educated on a wide variety of topics related to the mechanical, fluid dynamics, thermal, and chemical reactivity of thin film synthesis approaches. Outcome of this work included a publication that was published to demonstrate viability of this tool and its ability to operate at high pressures. Specifically, it was demonstrated that the initially proposed concept of spatially separating the precursors into individual chambers prior to mixing in the boundary layer immediately above the substrate could be successfully implement to result in vortex free flow of the system. Multiple, different approaches were investigated using different configurations of the system, leading to an ideal configuration of optimal fluid flow. 

An industrial collaborator was identified in the later stages of the project and in collaboration with them and a post-doctoral researcher who was hired and trained similarly in the art and science of modeling and all aspects of thin film crystal growth, the identified chamber design was rigorously investigated using COMSOL including coupled fluid dynamics and thermal heat transfer modes. An extensive study was performed to investigate fluid flow patterns and thermal profiles off all components of the system permitting finalization of the design and materials selection of the chamber and identification of suitable process conditions required to achieve vortex-free flow of the system using three different gases simultaneously (hydrogen, nitrogen, ammonia). Systems were modeled up to 30 atm and results of these investigations were submitted in two contributions for publication. 

Due to COVID and other unforeseeable and unexcepted challenges, the tool and associated control system were designed on paper during the duration of this grant, though, the associated facility upgrades required to safely house the tool were not completed in time to install and use the prototype tool as expected. Lehigh University is committed to fulfilling its original obligations under this grant and construction drawings and facility upgrades are on-going with an anticipated construction completion window of Q1 2025. Upon completion, the tool will be installed and tested enabling the scientific and technological research on semiconductor and other functional materials as originally proposed. 

Broader impacts of this work have been achieved via one published and two manuscripts under review, along with four conference presentations/posters, outlining scientific knowledge gained in the realm of design of high-pressure systems and associated fluid dynamic patterns observed. Recruitment of an industrial partner who is interested in further developing and using tool further advances its dissemination at a larger scale with the potential for scaling for industrial adoption. The facility upgrades at Lehigh University are occurring in a shared facility so the tool, once operational, can be used by any interested researcher, internal or external to Lehigh, with suitable support offered via the resident technical staff. This tool and its general accessibility to the research community will have significant impact as new material systems, even those beyond the originally proposed materials, can be explored for a wide array of applications and interests. 

Last Modified: 12/22/2023
Modified by: Siddha Pimputkar