Yunzhen Cao

Binary and ternary NiTi-based shape memory films deposited by simultaneous sputter deposition from elemental targets

The most challenging requirement for depositing NiTi-based shape memory thin films is the control of film composition because a small deviation can strongly shift the transformation temperatures. This article presents a technique to control film composition via adjustment of the power supplied to the targets during simultaneous sputter deposition from separate Ni, Ti, and X (e.g., Hf) targets. After optimization of sputter parameters such as working gas pressure, target-substrate distance, and target power ratio, binary Ni100−xTix thin films were fabricated and characterized by energy dispersive x-ray spectroscopy in a scanning electron microscope (to measure the film composition and uniformity), in situ x-ray diffraction (to identify the phase structures), and differential scanning calorimetry (to indicate the transformation and crystallization temperatures). To explore the possibility of depositing ternary shape memory NiTi-based thin films with a high temperature transformation >100°C, a Hf target was ...

Vanadium dioxide film protected with an atomic-layer-deposited Al2O3 thin film

A VO2 film exposed to ambient air is prone to oxidation, which will degrade its thermochromic properties. In this work, the authors deposited an ultrathin Al2O3 film with atomic layer deposition (ALD) to protect the underlying VO2 film from degradation, and then studied the morphology and crystalline structure of the films. To assess the protectiveness of the Al2O3 capping layer, the authors performed a heating test and a damp heating test. An ultrathin 5-nm-thick ALD Al2O3 film was sufficient to protect the underlying VO2 film heated at 350 °C. However, in a humid environment at prolonged durations, a thicker ALD Al2O3 film (15 nm) was required to protect the VO2. The authors also deposited and studied a TiO2/Al2O3 bilayer, which significantly improved the protectiveness of the Al2O3 film in a humid environment.

Atomic Layer Deposition of V1–xMoxO2 Thin Films, Largely Enhanced Luminous Transmittance, Solar Modulation

V1-xMoxO2 thin films were fabricated by nanolamination of VO2/MoO3 alternating layers using atomic layer deposition (ALD) process, in which tetrakis-dimethyl-amino vanadium(IV) [V(NMe2)4] and molybdenum hexacarbonyl(VI) [Mo(CO)6] were used as vanadium and molybdenum precursors, respectively. The dopant content of V1-xMoxO2 films was controlled by adjusting MoO3 cycle percentage (PMo) in ALD pulse sequence, which varied from 2 to 10%. Effects of PMo on V1-xMoxO2 crystal structure, morphology, semiconductor-to-metal transition properties, and optical transmittance were studied. A linear reduction of phase transition temperature (Tc) by approximately -11 °C/cycle % Mo was observed for V1-xMoxO2 films within PMo ≤ 5%. Notably, dramatic enhanced luminous transmittance (Tlum = 63.8%) and solar modulation (ΔTsol = 23.5%) were observed for V1-xMoxO2 film with PMo = 7%.