GOALI: Thermal Transport by Phonons in Device-Grade Nitride Nanostructures

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

PI: Jonathan A. Malen Proposal Number: 1133394 Nitride-based semiconductors are a centerpiece of light-emitting diodes (LEDs) for solid-state lighting, select high-frequency/power electronics, and future multi-junction photovoltaics. Progress towards commercialization of these technologies demands high current densities that compel improved thermal management to lower operating temperatures. While packaging strategies improve heat dissipation, preliminary data show that the device itself has a large thermal resistance. Thermal conductivity (k) measurements on device-grade nitride films show very low values of k in thin 100nm aluminum nitride (AlN) and 100nm gallium nitride (GaN) layers. Electron microscopy images suggest that these reductions are caused by structural imperfections inherent to industrial growth processes. As yet, a clear scientific explanation is lacking. Intellectual Merit: The objective of this GOALI proposal is to study thermal transport in nitride semiconductor heterostructures composed of GaN, AlN, and indium gallium nitride (InGaN) thin films (~100nm), fabricated by scalable growth processes. An interdisciplinary team of academic investigators at Carnegie Mellon University (CMU), in partnership with Kyma Technologies, will study the nature of thermal transport by phonons in nitride nanostructures. Defective films and interfaces separated by distances commensurate to the bulk phonon mean free paths will be considered. Open scientific questions include: What is the thermal boundary resistance (R) at the device-substrate interface and between nitride layers? Can highly defective crystals transport heat in a manner similar to disordered materials? How do growth techniques impact thin film thermal properties? To answer these questions, the investigation will focus on a common base structure that includes a sapphire, silicon carbide, or GaN substrate with an AlN nucleation layer followed by an InGaN buffer layer. Specific inquiries include: (i) the effect of the substrate on R and k of the AlN nucleation layer, (ii) the effect of growth technique on k and R of the AlN nucleation layer, and (iii) the effect of indium concentration on k and R of the InGaN buffer layer. Controlled growth of nitrides and imaging of defect density and structure (Davis, Paskova) will be used to inform the atomic structure for molecular dynamics simulations (McGaughey). Simulation results will be compared with direct measurements of k and R on these samples, made using a pump-probe optical method called Frequency Domain Thermoreflectance (Malen). Broader Impact: Partnership with Kyma Technologies makes this research directly transferable to industry, where nitride devices have the potential to revolutionize lighting and to outperform silicon-based electronics. Kyma recognizes the critical need for incorporating thermal-management into the nitride device structure. Access to Kyma's growth technologies, which represent the future of nitrides, will make any discoveries far-reaching. Continuing collaborations on nitride science and technology with the Cree Corporation and the Naval Research Laboratory further support the need for this research. The educational activities will expose students to industry-driven academic research through curriculum development, guest lectures and seminars, and summer internships at Kyma. Kyma, in turn, will receive exposure at CMU through integration of the research topics and results within courses taught by the academic PIs. An LED-based outreach activity "Lighting: The Next Generation" will be developed for the Society of Women Engineers High School Day, Pittsburgh public schools, and the YWCA TechGyrls program.