arXiv subjects

Nancy Haegel

Publications and source records attributed to Nancy Haegel.

2 recordsLinked to original sources

arXiv2025

Epitaxial growth and semiconductor properties of NiGa2O4 spinel for Ga2O3/NiO interfaces

Unintentionally formed interfacial layers are ubiquitous in semiconductor devices that operate at extreme conditions. However, these layers' structure and properties often remain unknown due to the thinness of these naturally formed interphases. Here, we report on the intentional epitaxial growth and semiconductor properties of NiGa2O4 spinel layers that form at Ga2O3/NiO interfaces used in high-power and high-temperature electronic devices. Cubic spinel NiGa2O4 films of 10-50 nm thicknesses and low surface roughness (~ 2 nm) were grown using pulsed laser deposition at a substrate temperatures in the 300-900 {\deg}C range on {\alpha}-Al2O3 and {\beta}-Ga2O3 substrates of different orientation. The optical absorption onset (3.6-3.9 eV) and thermal conductivity (4-9 W m-1 K-1) vary systematically with substrate temperature, consistent with theoretical predictions of varying Ni and Ga cation ordering on the spinel lattice. The valence band offset between NiGa2O4 and {\beta}-Ga2O3 is determined to be 1.8 eV. The NiGa2O4-based p-n heterojunction devices on Ga2O3 (001) substrates exhibit a rectification ratio of 10^8 (for +/-2V) and a turn-on voltage of 1.4 V, maintaining diode behavior up to 600 {\deg}C. These results highlight the potential of NiGa2O4 as a p-type interlayer in Ga2O3-based devices and shows a new approach to investigate such interfacial layers.

cond-mat.mtrl-sci↗

arXiv2024

From Prediction to Experimental Realization of Ferroelectric Wurtzite Al$_{1-x}$Gd$_{x}$N Alloys

AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors such as Si, SiC and III-Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al$_{1-x}$Gd$_x$N alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction $x$ of $x>$ 0.10$-$0.15. Experimentally, ferroelectric switching is observed at room temperature in Al$_{1-x}$Gd$_x$N films with $x>$ 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously (Lee et al., $\textit{Science Advances}$ 10, eadl0848, 2024). This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials.

cond-mat.mtrl-sci↗