William Alan Doolittle (Georgia Tech.)

Date: Mon. October 9th, 2023, 12:45 pm-1:45 pm
Location: Rock 221 (Foldy) & Zoom
Website: https://ece.gatech.edu/directory/william-alan-doolittle

Resurgence in III-Nitride Semiconductor Science and Applications via New Extreme Bandgap Semiconductor Discoveries

W. Alan Doolittle

School of Electrical and Computer Engineering, Georgia Institute of Technology

Zoom recording

Abstract: After revolutionizing lighting technology in the 1990/2000’s and high-power RF technologies in 2000/2010’s III-Nitride semiconductors seemingly took a hiatus as commercial markets matured and research waned. But recently two exciting discoveries have reenergized the III-Nitride community, offering pathways to optical devices with unprecedented photon energies and high voltage, power and temperature electronics.

Recently, the longstanding barrier to doping of AlN has been breached by our research resulting in the demonstration of substantial n and p-type AlN as well as AlN homojunction pn diodes. Aluminum nitride (AlN) is a material of great interest for high performance power electronics, extreme environment semiconductor devices, radio frequency devices, and deep ultraviolet (DUV) optoelectronics due to its excellent electrical and thermal properties. Compared to other commonly used semiconductors (i.e., Si, SiC, GaN, and β-Ga₂O₃), AlN has the highest critical electric field and theoretical breakdown voltage, which lead to the highest Baliga’s and Johnson’s figures-of-merit (BFOM & JFOM). AlN also has the second highest saturation velocity, and thermal conductivity among semiconductors with a commercially available substrate. However, AlN has traditionally only been an insulator without the ability to be converted to a semiconductor via doping. We recently demonstrated substantial bulk doping (carrier concentrations above 10¹⁸ cm^-3) for both p- and n-type AlN using Be and Si dopants, respectively. From these results, we also demonstrated rectification and a 6 V turn-on in AlN PN diodes. The substantial bulk doping of AlN is enabled by the combination of the use of a new dopant element (Be) and control of growth kinetics at low growth temperatures enabled by MME. Using MME we are able to reduce the incorporation of compensating impurities and the reconfiguration of metastable defects such as DX centers. This has led to: 1) the first reported bulk p-type AlN (4.4×10¹⁸ cm^-3), 2) the highest ever reported bulk Si-doped n-type AlN (n=6×10¹⁸ cm^-3, a nearly 6000 times improvement over previous state-of-the-art), and 3) the first AlN homojunction PN diode. Progress toward AlN transistors will also be described.

AlScN is an exotic nitride material that is predicted to enable HEMTs with 2.5x the current capability of current state of the art AlGaN based HEMTs. Doolittle was the first to recognize the ultra-critical catalytic effect of metallic Sc and its temperature dependence on the decomposition of plasma excited N₂. This key knowledge allows metal rich growth of AlScN instead of the previously used N-rich methods, greatly improving the quality of the material and allowing record conductivity in HEMTs. AlScN/GaN sheet charges of ~6×10¹³ cm^-3 (~5x AlGaN HEMT counterparts) and ~150 Ω/□ in HEMT structures are capable of carrying 2.5× the current of a traditional AlGaN/GaN HEMT as realized by Doolittle.  Also discussed are the acoustic applications that include filters operating in excess of 50GHz, non-linear optical figure of merit that exceed capabilities of Lithium Niobate and ferroelectric properties that enable new high temperature memory applications.

 

Host: Walter Lambrecht