Spatial and Temporal Characteristics of the Semiconductor to Metallic Phase Transition in VO 2 Thin Films
- PDF / 428,800 Bytes
- 7 Pages / 414.72 x 648 pts Page_size
- 30 Downloads / 201 Views
Mark Corbett, Keith L Lewis and Andrew M Pitt, Defence Research Agency St Andrews Road, Malvern, WR14 3PS, UK ABSTRACT
Vanadium dioxide exhibits a semiconductor to metallic phase transition at a temperature of about 68 °C. This can be exploited in the form of optical thin film structures which will exhibit non-linear behaviour when exposed to pulsed infra-red radiation. Since the phase transition is structural in nature, it is of interest to explore the temporal and spatial properties when irradiated with pulsed laser sources. Fast CMT detectors have been used to resolve nanosecond temporal detail on an experimental basis and the spatial charateristics have been explored using a simple adiabatic heating model. The dynamic transmission values measured for V02 devices are complex combinations of the temporally and spatially varying characteristics of the film. INTRODUCTION
Considerable interest has been shown in the understanding of the fundamental processes accompanying the semiconductor to metallic phase transition in various materials, particularly those occuring as a result of pulsed laser irradiation. Of the different materials exhibiting semiconductor to metallic phase transitions (SMPT), some of the most attractive from the point of view of their application as thermochromic materials are those based on the vanadium oxide system. The materials are well known for their multiphase behaviour, with compositions ranging from VO0. 2 to V205. Several of these oxides exhibit semiconductor-metallic phase transitions at temperatures between -263°C and 130C [1-9], the best known being found in the case of vanadium dioxide at 68°C [1,3,10-17]. Studies have addressed the properties of thin films of
V02 using material deposited by a number of different techniques. Several workers have also examined the use of impurities to modify the semiconductor to metallic phase transition in V02. Donor species (eg F, Mo, W, Nb) reduce the phase transition temperature with a corresponding effect on the electrical characteristics, whilst acceptor species (eg trivalent Cr, Fe, Ga, Al) have the opposite effect. The onset of the phase transition can also be controlled by small departures from stoichiometry and such behaviour has been used by the present authors to 'fine-tune' characteristics for given applications. Small amounts of oxygen deficiency result in a reduced phase transition temperature, with the width of the transitional region increasing to result in more diffuse characteristics. At temperatures below 68°C, V02 exhibits semiconductor properties with an indirect absorption edge at about 2.5eV. On heating to temperatures in excess of 68°C, a shear crystallographic structural transformation occurs to enhance the degree of d-d orbital overlap, with a resulting delocalisation of electrons. The plasma edge associated with the electron gas is at energies of about 1.5eV, giving rise to the onset of metallic reflectance. The refractive index of the material shows a significant amount of dispersion, both in the visible and in the infra
Data Loading...
