Ultra-wide bandgap materials are pivotal for next-generation electronic and optoelectronic devices, yet their widespread adoption is impeded by challenges in bipolar doping. Rutile germanium dioxide (r-GeO2) is a promising candidate, predicted to enable ambipolar doping and to exhibit high thermal and electronic conductivity. However, critical knowledge gaps remain regarding its lattice dynamics and phonon-related properties. In this study, we use polarization angle-resolved Raman spectroscopy on high-quality, large r-GeO2 single crystals to unambiguously determine the energies and relative Raman tensor elements of all first-order Raman-active phonons. Our experimental findings are complemented by density functional perturbation theory calculations, which reveal a consistent underbinding of phonon energies across various exchange-correlation functionals. This highlights a previously unrecognized limitation in the theoretical modeling of r-GeO2. The comprehensive characterization and accurate assignment of phonon modes provide a solid foundation for quantitative simulations of phonon-assisted processes and pave the way for the design of r-GeO2-based devices.
Single-Crystal AlN Substrates
In their Research Article (10.1002/aelm.202500393), Eungkyun Kim, Debdeep Jena, Huili Grace Xing, and co-workers demonstrate single-crystal high electron mobility transistors (XHEMTs) on bulk AlN substrates for the first time, delivering exceptional RF performance. AlN XHEMTs feature ultra-thin GaN channels fully strained between AlN layers, eliminating epitaxy-substrate thermal boundary resistance. By leveraging AlN's superior thermal conductivity, these XHEMTs promise to overcome thermal performance limitations.
We investigated a TiO2-engineered interfacial strategy to enhance the stability and reliability of hafnia-based ferroelectric field-effect transistors (FeFETs) employing a metal-ferroelectric-metal-insulator-semiconductor (MFMIS) architecture. Although the MFMIS configuration facilitates optimized voltage distribution and suppresses charge injection into the dielectric layer, interfacial defects—particularly oxygen vacancies (VOs)—at the floating gate/ferroelectric interface continue to degrade switching performance. To address this issue, we introduced an ultrathin TiO2 interlayer between the floating gate (FG) and the ferroelectric layer. Acting as an oxygen reservoir, the TiO2 interlayer effectively mitigates VO formation and stabilizes the interfacial structure. X-ray photoelectron spectroscopy and electron energy loss spectroscopy analyses confirm a reduced concentration of VO at the interface. Consequently, TiO2-inserted MFMIS devices exhibit enlarged and more stable memory windows, along with enhanced ferroelectric characteristics. Furthermore, low-frequency noise analysis reveals a significant reduction in defect-related fluctuations, indicating suppressed trap dynamics. Collectively, these results demonstrate that TiO2 interface engineering offers a scalable and complementary metal-oxide-semiconductor-compatible strategy to address reliability challenges in hafnia-based ferroelectric transistors.
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