Chunhui Ji,
Zhiming Wu,
Xuefei Wu,
Jun Wang,
Xianchao Liu,
Jun Gou,
Hongxi Zhou,
Wei Yao,
Yadong Jiang,
https://www.sciencedirect.com/science/article/pii/S0169433218313849
In this work, we investigate the terahertz transmittance and metal-insulator phase transition properties of M2 phase VO2 films induced by Cr doping. Firstly, Cr-doped VO2films were successfully produced on silicon substrates using DC reactive magnetron sputtering and then fully characterized by XRD, XPS, Raman shift and SEM tests. These results demonstrate the formation of M2 phase and the effect of Cr doping on composition, crystal structure and surface morphology of VO2 films. Compared with undoped VO2, Cr doping significantly enhances the amplitude modulation in the frequency of phonon-absorption peaks by the suppression of infrared-active phonon modes. More concretely, the amplitude modulation is largely increased from 29.6% to 39.0% at 8.25 THz and from 24.1% to 37.1% at 9.33 THz, respectively. This enhancement strongly contributes to the development of VO2-based devices in high THz range. In addition, Cr doped VO2 films exhibit outstanding metal-insulator phase transition properties with very narrower hysteresis width (8.0 °C) as well as smaller transition sharpness (3.4 °C) than undoped film, which is ascribed to the increase of heterogeneous nucleation site density and the transformation of crystal structure from M1 phase to M2 phase, respectively. This work indicates that VO2 films with suitable Cr doping concentration have great potential for THz modulation applications.
Chunhui Ji, Zhiming Wu, Xuefei Wu, Haoqian Feng, Jun Wang, Zehua Huang, Hongxi Zhou, Wei Yao, Jun Gou and Yadong Jiang
http://pubs.rsc.org/en/content/articlelanding/2018/tc/c7tc05536f#!divAbstract
Vanadium dioxide (VO2), due to its well-known metal-insulator phase transition (MIT), is a promising candidate to realize optical modulation devices operating at terahertz (THz) frequencies. Moreover, the application of VO2 on modulation devices requires a narrow hysteresis width associating with a significant change in optical properties while it is quite challenging for Si-based polycrystalline VO2 films. In this paper, by doping high-valence metal ions (W6+ or Nb5+) into polycrystalline VO2 films, a narrowed hysteresis width and a decreased phase transition temperature are observed. Intriguingly, these doped VO2 films always maintain high THz field modulation depth despite the low phase transition temperature with using either W or Nb dopants. To sum up, the optimized VO2 film with 6.5% Nb doping deposited on high-purity silicon substrates exhibits the best MIT characteristics with a giant field THz modulation depth of 62.5%, a small hysteresis width down to 4.8 °C and a low phase transition temperature around 31.1 °C, which is very excellent for practical application. Furthermore, we synthetically investigate the influences of W and Nb doping on the microstructures and MIT characteristics of polycrystalline VO2 films. Doping Nb5+ and W6+ ions has similar effects through the similar mechanism. In addition, the excellent balance between THz modulation ability and phase transition temperature is related to the high crystallinity degree of doped films. The annealing process may play a key role in this peculiar case. These results show that the excellent MIT properties of polycrystalline VO2 films can be effectively tailored by our distinctive preparation method, which provides a feasible solution to the design and fabrication of VO2 films with suitable MIT properties for THz devices.
- Xing Zheng
- , Zhiming Wu,
- Jun Gou
- , Ziji Liu
- , Jun Wang
- , Jie Zheng
- , Zhenfei Luo
- , Weiqing Chen
- , Dongcheng Que,
http://link.springer.com/article/10.1007%2Fs10762-016-0287-4
A real-time terahertz (THz) imaging system was demonstrated based on a 320 × 240 uncooled microbolometer detector combined with a 2.52 THz far-infrared CO2 laser. On the top of micro-bridge structure (35 × 35 μm2), a 10 nm nickel-chromium (NiCr) thin film was deposited to enhance THz absorption, which was fabricated by a combined process of magnetron sputtering and reactive ion etching (RIE). By mechanical simulation using design of experiment (DOE) method, the minimum deformation was optimized to 0.0385 μm, and a measured deformation of 0.097 μm was achieved in the fabrication. The fabricated micro-bridge pixel was used for THz detection, and a responsivity of 1235 V/W was achieved with a noise equivalent power (NEP) of 87.4 pW/Hz1/2. THz imaging of metal gasket covered by label paper, paper clip in an envelope, and watermark of a banknote was demonstrated by a combination of histogram equalization (HE) and linear enhancement algorithm.
