We report the investigation of negative differential resistance (NDR) in negative capacitance (NC) germanium (Ge) pFETs. The NDR in NC transistors is attributed to the coupling of drain voltage to the internal gate voltage V _int via the gate-to-drain capacitance. It is demonstrated that NDR strongly depends on the matching between the NC induced by ferroelectric capacitance C _FE and the positive capacitance associated with the underlying transistor capacitance C _MOS . For the non-hysteretic devices, NDR gets pronounced with an increased thickness of ferroelectric film t _fe and V _GS . This is attributed to the fact that the drain coupling factor is improved with an increased t _fe and V _GS , leading to the better matching between C _FE and C _MOS . For the hysteretic NC transistors, however, NDR is only obtained at the lower V _GS , but not observed at higher V _GS .

HfO ₂ -ZrO ₂ superlattice (SL) ferroelectric (FE) capacitor is demonstrated to have improved endurance performance and higher fatigue recovery capability compared to the HfZrO _x (HZO) device. During the cycling of polarization ( ${P}$ ) vs. voltage ( ${V}$ ) loops, the SL metal-FE-metal (MFM) capacitor exhibits the higher ${P}$ and the lower leakage current over the HZO device indicating the lower defect density in SL. The SL capacitor achieves an endurance of ${5}\times {10} ^{{12}}$ cycles, which is three orders of magnitude higher than the HZO device. The ${P}$ fatigue of the SL capacitor can be fully recovered through a ~30 s break, and that of HZO is only partially recovered utilizing the higher field cycling. This is because the trapping/detrapping process significantly decreases in HfO ₂ -ZrO ₂ SL over HZO capacitor by the reduced defect density. These results prove that the HfO ₂ -ZrO ₂ SL is a promising technology for endurance unlimited FE random access memory.

Single crystallinity (AlGa)₂O₃ solar-blind photodetectors are epitaxially grown on sapphire. Oxygen pressure during the growth has a great impact on the Al composition in (AlGa)₂O₃, which is investigated utilizing X-ray photoelectron spectroscopy and X-ray diffraction measurements. Measured transmittance spectra and responsivity demonstrate that (AlGa)₂O₃ photodetectors achieve a wider bandgap compared to a Ga₂O₃ device. An (Al_0.12Ga_0.88)₂O₃ device obtains 10 times higher photocurrent I_photo than a Ga₂O₃ photodetector. However, as Al composition increases, significant I_photo degradation is observed in an (Al_0.35Ga_0.65)₂O₃ photodetector. Meanwhile, an (Al_0.35Ga_0.65)₂O₃ device exhibits the stronger persistent photoconductivity compared to the Ga₂O₃ control device. Analysis shows that defect states in a bandgap of (AlGa)₂O₃ might be associated with the performance change in (AlGa)₂O₃ photodetectors.

In this article, a phase-field polarization switching model for ferroelectric HfO ₂ -based thin films considering oxygen vacancies ( ${V}_{o}$ ) has been developed based on the 2-D time-dependent Ginzburg–Landau (TDGL) equation coupling with Poisson’s equation. The impacts of nonuniform ${V}_{o}$ distributions induced by the monolayer grains in ultra-scaled Hf _0.5 Zr _0.5 O ₂ (HZO) films and ${V}_{o}$ concentrations on ferroelectric characteristics are investigated in detail by the developed model which is verified and calibrated by measurement results of HZO. Furthermore, possible mechanisms of wake-up and fatigue are revealed by the simulation with the proposed model. It is clarified that the redistribution of nonuniform ${V}_{o}$ in the in-plane direction leads to the transition from a pinched polarization-voltage curve to a conventional one at the early stage of wake-up, while the generation of ${V}_{o}$ within a ferroelectric film results in ferroelectricity enhancement and reduction in wake-up and fatigue processes, respectively.

Negative capacitance (NC) GeSn pFETs integrated with HfZrO _x (HZO) ferroelectric film is demonstrated with sub-20 mV/decade subthreshold swing (SS) over two orders of magnitude of IDS. The ratio of remnant polarization to coercivity in HZO is significantly improved with the increasing of the annealing temperature from 400°C to 500°C, which contributes to the effective reduction of hysteresis in ferroelectric NC GeSn transistors. Ferroelectric NC GeSn pFET annealed at 500°C achieves a hysteresis of 70 mV while maintaining a steep SS dramatically lower than 60 mV/decade, and an improved IDS over control device without HZO.

We fabricated β-Ga ₂ O ₃ photodetectors on bulk substrate and sapphire. Bulk Ga ₂ O ₃ photodetector demonstrates the improved responsivity compared with the device on sapphire, due to the higher crystal quality in bulk material. Optical gain is achieved in both the devices. For the first time, we report that the Ga ₂ O ₃ photodetector epitaxially grown on sapphire achieves a blueshift of bandgap in comparison with bulk device. Based on the measured responsivity characteristics, the direct and indirect E _G of bulk Ga ₂ O ₃ are 4.78 and 4.59 eV, respectively. The Ga ₂ O ₃ photodetector on sapphire exhibits a maximum cutoff wavelength at 253 nm, corresponding to the direct E _G of 4.90 eV. The increment of E _G in Ga ₂ O ₃ on sapphire over bulk material is attributed to the residual strain in the film. The time-dependent photoresponse of the devices suggests that Ga ₂ O ₃ on sapphire might have more oxygen vacancies than the bulk material.