Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering
Ananth Saran Yalamarthy, Hongyun So,* Miguel Muñoz Rojo, Ateeq J. Suria, Xiaoqing Xu, Eric Pop, and Debbie G. Senesky*
Progress in wide bandgap, III-V material systems based on gallium nitride (GaN) has enabled the realization of high-power and high-frequency electronics. Since the highly conductive, 2D electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface is based on built-in polarization fields and is confined to nanoscale thicknesses, its charge carriers exhibit much higher mobilities compared to their doped counterparts. This study shows that such 2DEGs also offer the unique ability to manipulate electrical transport separately from thermal transport, through the examination of fully suspended AlGaN/GaN diaphragms of varied GaN buffer layer thickness. Notably, ≈100 nm thin GaN layers can considerably impede heat flow without electrical transport degradation. These achieve 4× improvement in the thermoelectric figure of merit ($zT$) over externally doped GaN, with state-of-the-art power factors of 4–7 mW m$^{-1}$ K$^{-2}$. The remarkable tuning behavior and thermoelectric enhancement, elucidated here for the first time in a polarization-based heterostructure, are achieved because electrons are at the heterostructured interface, while phonons are within the material system. These results highlight the potential for using 2DEGs in III-V materials for on-chip thermal sensing and energy harvesting.
1. Introduction
Over the past decade, gallium nitride on silicon (GaN-on-Si) substrates have gained widespread use as a materials platform for high-power,[1] high-frequency,[1] and extreme temperature electronics.[2] This technology is enabled by the presence of a 2D electron gas (2DEG) that is formed when a nanometer-thick layer of unintentionally doped aluminum gallium nitride
(AlGaN), indium gallium nitride (InGaN), or aluminum indium nitride (AlInN) is deposited on an underlying GaN buffer layer.[3–5] The 2DEG, created from built-in polarization fields and surface states in the undoped III-V layers, has a high sheet density $n_s \approx 10^{13}$ cm$^{-2}$, high room-temperature mobility $\mu \approx 1500–2000$ cm$^2$ V$^{-1}$ s$^{-1}$, and has been reported to operate at temperatures as high as 1000 °C.[2]
GaN thin films have been utilized in the design of thermoelectric devices because of their high-temperature operation and potential for on-chip energy harvesting and sensing.[6] As a result, the electro-thermal transport in various GaN-based materials is being investigated, including externally n-doped AlInN with a thermoelectric figure of merit $zT \approx 0.1$ at 25 °C,[7] and bulk InGaN with a $zT$ as high as 0.34 at ≈600 °C.[8] Recently, 2DEG heterostructures[9,10] and GaN-based superlattices have attracted great attention in the design of thermoelectric devices, as bulk doped mate-
rials are limited by lower mobilities[11] (200–400 cm$^2$ V$^{-1}$ s$^{-1}$). For example, recent experiments showed ≈10× improvement in power factor ($S^2\sigma$) over bulk doped GaN (as high as $2 \times 10^{-3}$ Wm$^{-1}$ K$^{-2}$ using an AlGaN/GaN superlattice[10]), as well as simultaneous increase in Seebeck coefficient ($S$) and electrical conductivity ($\sigma$) for the 2DEG,[9] contrary to bulk doped materials. The primary contributor in the power factor enhancement is the improved 2DEG mobility,[10] yet, the overall
A. S. Yalamarthy, Dr. A. J. Suria Department of Mechanical Engineering Stanford University Stanford, CA 94305, USA
Prof. H. So Department of Mechanical Engineering Hanyang University Seoul 04763, South Korea E-mail: hyso@hanyang.ac.kr
Dr. M. Muñoz Rojo, Prof. E. Pop, Prof. D. G. Senesky Department of Electrical Engineering Stanford University Stanford, CA 94305, USA E-mail: dsenesky@stanford.edu
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201705823.
Dr. X. Xu Stanford Nanofabrication Facility Stanford University Stanford, CA 94305, USA
E. Pop Department of Materials Science and Engineering Stanford University Stanford, CA 94305, USA
E. Pop, D. G. Senesky Precourt Institute for Energy Stanford University Stanford, CA 94305, USA
D. G. Senesky Department of Aeronautics and Astronautics Stanford University Stanford, CA 94305, USA