Ammonothermal cubic boron nitride single crystal growth near ambient pressure and temperature

Lehigh University, Bethlehem PA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Boron nitride is a semiconductor material, and in its cubic crystal structure, it is one of the hardest materials known. Unlike diamond, boron nitride can be used in more reactive environments given its superior stability against oxidation and unintended chemical reactions with, for example, iron at elevated temperatures. Synthesis of large diameter single crystals would significantly advance high-pressure experimental configurations when used as anvils, and the power electronics field by achieving high-efficiency electrical power conversion. Availability of large single crystals would be transformative to multiple additional areas by enabling wide-scale research using high-quality material and permitting exploration of novel electronic devices. Current methods used to synthesize this material yield only small crystals at high cost making them impractical for wide-scale adaption. This project investigates growth of single crystal boron nitride using a different, more scalable technique which operates under milder conditions. The PI and co-PI are devoted to education and accordingly, their research results are being incorporated into graduate courses. The PI is hosting wiki articles and videos on a world-accessible server by working closely with student volunteers. Additionally, the PI is visiting regional, underrepresented and rural, high schools to educate the public on the impact of materials on society. The co-PI is an advocate for underrepresented groups and is continuing his participation in Open Houses and similar events. TECHNICAL DETAILS: Cubic boron nitride (c-BN) is a wide bandgap semiconductor and one of the hardest materials known. Synthesis of this materials is currently limited to high-pressure anvil systems limiting their size to ~1 mm in diameter. The ammonothermal method, utilizing supercritical ammonia at pressures below 300 MPa and temperatures below 800 C, is a solution-based, bulk single crystal growth technique which is being explored to enable growth of large single crystal c-BN. Solubility of BN in a variety of solutions utilizing alkali and alkali-earth metals and halides as mineralizers is under investigation. Suitable growth conditions are applied to bulk, single crystal growth of c-BN on c-BN seeds synthesized as part of this research in anvil systems. BN crystals are characterized for their properties and tied to growth conditions. Availability of large, single crystal c-BN would prove transformative for power electronics research in the form of substrates given its highly beneficial physical properties (high thermal conductivity and breakdown voltages) and the ability to be doped both p- and n-type. Similarly, large single crystal material would have a profound impact on the fabrication of super-hard ceramic tools. This research provides in-depth education for students on the topic of single crystal growth methods and associated (novel) equipment development reinvigorating the waning single crystal growth community. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.