Vanadium dioxide undergoes a reversible metal-insulator phase change near 68°C. This transition is often recognized for its electronic properties; however, the concomitant change in optical properties is also attractive. One application is to exploit the optical properties within this transition region to control and tune the refractive index between its insulating and metallic states. To achieve this, we used atomic layer deposition to grow high-quality, low-temperature (≤150°C), ultrathin films of vanadium dioxide on sapphire substrates. Measurements of optical transmittance and reflectance as a function of temperature are then used to create wavelength and temperature dependent models of the complex optical refractive index. Our models create a foundation for developing vanadium dioxide as a tunable refractive index material.

A simple room‐temperature process of depositing MXene on a III‐V structure with embedded 2D electron gas (2DEG) is used, which results in a large area, , photodetector (PD) device that greatly outperforms vacuum deposited Ti/Au metal‐semiconductor‐metal (MSM) PD's. By co‐optimizing properties of 2D MXene contacts and the III‐V material heterojunctions, this device sets new operating records with responsivity up to 1.04 A W^‐1 at low optical powers, corresponding to >230% internal quantum efficiency, dark current of 50 , >105.6‐dB dynamic range, and 25–150 ps response time, which improves the previous MXene‐Semiconductor‐MXene responsivity by >2.7 times and is 7 × 10³ –−10⁶ times faster compared to other MXene‐based PDs. This is achieved by enhancing the Schottky barrier height by forming a Van der Waals (vdW) heterojunction between a wide bandgap AlGaAs surface layer and spin coated Ti₃C₂T_z electrodes. A layered architecture transports the optically generated electrons to a 2DEG channel at the GaAs/AlGaAs heterointerface, where they are rapidly collected. The landscaped electric field pushes the slow holes to an underlying low temperature‐grown GaAs (LT‐GaAs) region where they recombine. The proposed Schottky‐2DEG Photoconductor‐Schottky model for device operation shows how this device circumvents the canonical limitations of gain‐bandwidth product, and carrier transit time, while replacing the need for vacuum deposition of gold or other precious metals.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 °C. To grow VO₂ on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (≤150 °C) growth of ultrathin films (100-1000 Å) of VO₂. For this demonstration, the VO₂ films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO₂ films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10^-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO₂ films. The crystallinity, phase, and strain of the VO₂ were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO₂ in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO₂ agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films' optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO₂ as a tunable refractive index material.

We compare how altermagnetic and nonmagnetic order, together with epitaxial strain, influences the electronic, vibrational and optical properties of RuO ₂ , using a combination of experiment and theory.