Thin films of thermochromic VO2, V1−xWxO2 and V1−x−yWxTiyO2 (x=0.014, and y=0.12) were synthesized onto quartz substrates using a reactive pulsed laser deposition technique. The films were then characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The W and Ti dopant effects on the semiconductor-to-metal phase transition of VO2 were investigated by measuring the temperature dependence of their electrical resistivity and their infrared transmittance. Remarkably strong effects of Ti–W codoping were observed on both the optical and electrical properties of V1−x−yWxTiyO2 films. The IR transmittance was improved, while the transition temperature could be varied from 36°C for W-doped VO2 film to 60°C for Ti–W codoped VO2 film. In addition, at room temperature, a higher temperature coefficient of resistance of 5.12%∕°C is achieved. Finally, both optical and electrical hysteresis are completely suppressed by Ti–W codoping the VO2 films.
A detailed characterization of the impurity centers involved in the photoluminescence (PL) of p-type CdTe doped with arsenic (As) and antimony (Sb) has been performed. The PL spectrum has been measured from 1.35 eV up to the band edge and as a function of temperature (4.2 up to 30 K). In addition to the familiar broad PL line centered at 1.45 eV and present in undoped and doped materials, the doped samples exhibit a new band near 1.54 eV showing a fine structure composed of two peaks whose intensities vary with temperature. The observed longitudinal optical (LO) phonon replicas associated with the zero-phonon lines, at 1.45 eV and 1.54 eV, respectively, are characterized by a Huang-Rhys factor S=1.3±0.1 and S=0.30±0.02. The various electron-hole recombination processes are explained by means of a simple analytic model correlating the position of the zero-phonon lines to the relative intensities of the phonon side bands. The model accounts for the chemical shift of the defect centers and describes the effect of the charge carrier LO-phonon interaction in the framework of the adiabatic approximation within the envelope function approach. Comparison between theory and experiment leads to the following values for the effective Bohr radii: aAs=(10.6±0.1) Å, aSb=(10.3±0.1) Å, and ionization energies: EAs=(58±2) meV, ESb=(61±2) meV. It also leads to conclude to the presence of native shallow donors with binding energy ED=(13±2) meV and of deeper native acceptor complexes with effective Bohr radius aA=(6.1±0.1) Å and ionization energy EA=(157±2) meV.
The parameters of reactive pulsed laser deposition were successfully optimized for fabrication of vanadium dioxide thin films. It is observed that the O2 concentration in Ar gas and the total deposition pressure are critical in stabilizing the single VO2 phase. Thermochromic VO2 and V1−xWxO2 (x=0.014) thin films were synthesized on various substrates (silicon, quartz, and sapphire) at 5% of O2/Ar ratio gas and total pressure of 90 mTorr. The structural properties of the deposited films were analyzed by x-ray diffraction, while their semiconductor-to-metal phase transitions were studied by electrical resistivity using the four-point technique and infrared transmittance from room temperature up to 100 °C. The observed transition temperature was about 36 °C for W-doped VO2 compared to 68 °C for VO2 films. This transition temperature was then lowered by about 22.85 °C per 1 at. % of W added. The temperature coefficient of resistance was about 1.78%/°C for VO2 and about 1.90%/°C for W-doped VO2. Using the pump-probe experiment, the application of these thermochromic films as optical switches was demonstrated at the wavelength of 1.55 μm. The transmission switching was about 25 dB for VO2 and 28 dB for W-doped VO2. In addition, application of VO2 on optical fiber components was demonstrated by direct VO2 coating on the end faces of cleaved single mode optical fibers and optical fiber connectors.
We have successfully fabricated two types of 1 × 2 optical switch devices, namely, all-optical switch (VO2/quartz) and electro-optical switch (VO2/TiO2/ITO/glass) based on the semiconductor-to-metallic phase transition characteristic of vanadium dioxide (VO2) smart coatings. The VO2 active layer, the TiO2 buffer layer and the ITO transparent conductive electrode used in these devices were achieved by reactive pulsed laser deposition. The optical switching of the fabricated devices was investigated at λ = 1.55 µm. The semiconductor (on) to metallic (off) phase transition was controlled by photo-excitation of VO2 in the case of the all-optical switch and by an external electric field applied between the ITO and the VO2 layer in the case of the electro-optical switch. The extinction ratio (on/off) is found to be much higher for the all-optical switch than for the electro-optical switch. For the all-optical switch, extinction ratios of about 22 and 12 dB are obtained in the transmission and reflection modes, respectively. In the case of the electro-optical switch, the extinction ratio is about 12 dB in the transmission mode and 5 dB in the reflection mode. Finally, to explain our optical switching results, we propose a simple model based on the energy band diagram of VO2 in which the charge density increases under an external excitation (either photo-excitation or an electrical field), and then induces the semiconductor-to-metallic phase transition in the VO2 active layer.
Thermochromic undoped and metal (Ti and W)-doped VO2 smart coatings were achieved on Kapton HN by reactive pulsed laser deposition. The optimization of the deposition was conducted with Si (100) substrates. The coatings were deposited at relatively low deposition temperatures (250, 300, and 350°C), which are compatible with the characteristics of Kapton. The stoichiometry of the VO2-coated Kapton was confirmed by x-ray photoelectron spectroscopy analysis of the vanadium and oxygen bands. Moreover, the single phase VO2 was confirmed by x-ray diffraction of VO2∕Si synthesized at 300°C. Unlike VO2/Kapton, the VO2∕Si exhibited the well-known semiconductor-to-metallic transition, as shown by the temperature dependence of the infrared transmittance. This coating exhibited a similar transition temperature to that of VO2 single crystal (≈68°C), but a small transmittance switching (about 7%) at 2.5μm. The temperature dependence of the electrical resistivity of all coatings on Kapton was investigated by means of the standard four-point probe technique. The resistivity decreased with increasing temperature. No abrupt semiconductor-to-metallic transition was observed either for undoped or for metal-doped VO2 coatings. It was found that Ti and W dopants have an antagonistic effect on the resistivity. The resistivity was enhanced by the Ti dopant, whereas it was decreased for W-doped VO2 coatings. These results show that the tunability of the resistivity can be tailored either by controlling the deposition temperature or by adjusting the concentration of Ti and W dopants. In addition, at room temperature a much higher temperature coefficient of resistance of −3.29%∕°C was achieved in W(0.5%)-doped VO2/Kapton. Finally, these VO2 smart coatings are promising materials for the IR sensing and sunshield applications.
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