Several oxides of vanadium undergo a transition from a semiconductor or insulating state to a metal phase at a critical temperature. Vanadium dioxide undergoes this transition near 68-degrees-C, while V2O5 undergoes a similar phase transition near 257-degrees-C. During the transition a change in oxide crystal structure is accompanied by large changes in electrical and optical behavior. Thin films of vanadium oxides are capable of reversibly switching from the semiconductor to the metallic state at high speeds and with high spatial resolution. Therefore, these oxides have potential use, particularly in thin film form, for a wide variety of applications involving thermally activated electronic or optical switching devices. Such films are of considerable technical interest because of applications in chemical sensors, energy-conserving coatings, transparent conductors, and switching materials. The numerous potential electronic, optical, and optoelectronic device applications which have been suggested have stimulated work on the preparation of thin films by a variety of techniques, including chemical vapor deposition, solgel, evaporation, and sputter deposition. This paper reviews the optical properties of vanadium oxide coatings and stresses the dependence of film properties on sample preparation and resultant film microstructure.

Thin-film x-ray diffractometry with Seemann-Bohlin focusing provides enhanced sensitivity to the microstructures of optical thin films. Results on vanadium oxide films are presented.

Ibin films of various vanadium oxides, including the phase transition materials vanadium dioxide and vanadium pentoxide, have been prepared by reactive ion-beam sputtering. Results show that film microstructure is the dominant factor affecting the optical properties of these coatings. Changes in optical reflectance and transmittance data, which accompany the structural changes, are presented and discussed.

The optical properties of a material in the infrared spectral range are predominantly determined by composition and structure. Methods are presented which are used to evaluate the information contained in optical properties of microscopically heterogeneous systems and to obtain parameter values. These methods are based on effective-medium theory and utilize accurate bulk dielectric function values of constituents. Infrared ellipsometric techniques may be used to provide optical data, which can be related to the microstructural information. Theories that establish limits on allowed values of the dielectric response of two-component systems, regardless of their microstructure, provide an indication of the realiability of these parameters. Results are presented for vanadium oxide films.

Oxides of vanadium, 190–310 nm thick, were deposited by ion-beam sputtering of a metallic target. The ion beam consisted of an argon–oxygen mixture where the oxygen percentage was varied from 10% to 50%. Spectrophotometry was used for infrared optical characterization, while x-ray diffractometry and scanning electron microscopy were used to examine coating structure. Results show that as the oxygen fraction of the sputtering gas is increased, the crystallography evolves from a face-centered-cubic vanadium oxide to a mixed oxide phase containing VO2 in which there is a degree of preferential grain orientation. Increase of the oxygen content above 25% leads to a mixed oxide phase containing V2O5 with a structure of long, randomly oriented grains. Changes in optical reflectance and transmittance data, which accompany the structural changes, are presented and discussed.

Thin films of various vanadium oxides, including the phase transition material vanadium dioxide, have been deposited onto sapphire substrates by reactive ion-beam sputtering. In this process, an argon ion beam sputters a vanadium target in the presence of an oxygen background. Coating chemistry, structure, surface morphology and electrical properties, together with the optical response at 3.4 μm wavelength, were studied as a function of the variation of film growth temperature Ts. Films deposited in the range 350 °C

Black molybdenum, a cermet consisting of small molybdenum particles embedded in MoO 2 , was prepared using two separate chemical vapor deposition techniques involving deposition from either Mo(CO) 6 or MoO 2 Cl 2 . Through the application of the theory of bounds placed on the effective dielectric function of two- phase materials, the spectral reflectance of these two types of black molybdenum was modelled over the range of the experimental modifications of the fractional composition. The theory, which does not require detailed microstructural characterization, qualitatively reproduces the differences in the optical properties of the two types of black molybdenum, as well as the changes in these properties through compositional modifications. The spectral selectivity of black molybdenum of either type, which suggests its application for photothermal solar energy conversion, is also reproduced by the theory. Quantitative agreement between experiment and theory is obtained through a consideration of the film surface texture.

Black tungsten thin films, combining significant solar absorptance with a high IR reflectance, were produced by the thermal decomposition of W(CO) 6 onto silicon substrates in the presence of an oxygen bleed. The effect of annealing treatments on the reflectance was investigated for annealing conducted in a reducing atmosphere at temperatures up to 1000°C.

Chemical Vapor Deposition (CVD) of thin films from molybdenum carbonyl (Mo (CO) 6) at 300 °C under atmospheric pressure has resulted in either reflective or black molybdenum, depending on the absence or presence of an oxygen bleed during the deposition. After post-deposition anneal, the reflective film exhibits an infrared reflectance of 98.7 % at 10 microns while the black film exhibits a solar absorptance of a = 0.74 and a thermal (500°C) emittance of en = 0.08, (en referring to the normal emittance). The reflectance of both films, before and after anneal, has been related to their composition, grain size, and crystal structure. In particular this has been done in the infrared for the reflective films and in the visible for the black films. The solar absorptance of both film types can be enhanced by one or two antireflection layers with only slight increases in the emittance. A bilayer of CVD silicon (700 A) and silicon nitride (700 A) on the annealed reflective film results in a = 0.75 and e (500 °C) = 0.05. A layer of CVD silicon nitride (750 A) on the black film results in a = 0.91 and en (500°C) = 0.11. Both configurations have been tested for over 1000 hours at 500 °C in a roughing pump vacuum without measured deterioration.

On the basis of thermodynamical and diffusion analysis, a mechanism is suggested for the growth and the hydrogen reduction of thin films that display the spectral selectivity required for efficient photo-thermal converters. Such Black Molybdenum films consist of a suspension of metallic molybdenum grains in a host matrix of MoO2. A film of the cermet-like material has a solar absorptance and thermal emittance that depends on its microstructure and fractional composition with respect to the two components, Mo and MoO2. Depending on the choice of the deposition parameters, the composition can be changed and the solar performance adjusted accordingly. The films have been deposited by two different chemical vapor deposition (CVD) methods. In the first method, deposition proceeds by the pyrolytic decomposition of molybdenum hexacarbonyl in the presence of oxygen. Annealing of the resulting monoclinic MoO2 film in a hydrogen-bearing atmosphere partially reduces the oxide to metallic Mo, the degree of reduction depending on the temperature and length of the anneal. In a second method, the Black Molybdenum cermet is grown by the hydrogen reduction of molybdenum dioxydichloride. In this anneal-free, one-step process, the substrate temperature and hydrogen concentration determine the resultant structure and fractional composition.

Black molybdenum thin films fabricated by chemical vapor deposition demonstrate a significant solar absorptance coupled with a high IR reflectance. Because antireflected black molybdenum demonstrates such spectral selectivity in a coating of simple design, it is an attractive candidate for use in photothermal solar energy conversion. Films deposited onto Incoloy alloy 800 and stainless steel 316 have survived testing in vacuum at 500 °C and in air at 350 °C. If the process parameters are varied during deposition, this can lead to black molybdenum with a graded composition.

Black molybdenum thin films, combining high infrared reflectance with significant solar absorptance, have been prepared by pyrolyzing Mo(CO)6 under oxidizing conditions and subsequent annealing under a hydrogen atmosphere. X-ray and electron diffraction, electron microprobe, and Auger analysis show that the films, as deposited, are fine grained (300 A), stress-free, monoclinic MoO2 with [010] preferred orientation. As the annealing progresses the reflectance in the infrared rises faster than in the visible. Consequently, the spectral selectivity can be optimized in a partially annealed state characterized by a complex mixture of Mo and MoO2. Like black chrome, black molybdenum is a composite material, consisting of metal particles suspended in an oxide matrix. However, black molybdenum is capable of prolonged operation at temperatures in excess of the decomposition temperature of black chrome. After passivation with Si3N4, very simple photothermal solar converter coatings with a respectable spectral selectivity and significant temperature durability at 500 C in vacuum are obtained.

A high solar absorptance coupled with a high IR reflectance is necessary for efficient photothermal solar conversion. The most common selective blacks operate on the principle of the absorber-reflector tandem, in which a highly reflecting film is overcoated with a layer which absorbs over the solar spectral range. Molybdenum films fabricated by chemical vapor deposition (CVD) under oxidizing conditions and subjected to post-deposition annealing and passivation provide both optical functions—high IR reflectance together with sufficient solar absorptance—in a single layer. Film deposition proceeds under atmospheric pressure and at 300°C through pyrolytic decomposition of molybdenum carbonyl (Mo(CO)6) in the presence of an oxygen bleed. Films thus deposited contain a high concentration of oxygen and exhibit typically a solar absorptance of a = 0.77 and a thermal emittance for 500°C black-body radiation of e = 0.31. In subsequent annealing in a reducing atmosphere at 770°C this high solar absorptance is nearly retained whereas the thermal emittance is lowered to values of the order of e = 0.08. The passivation of these black molybdenum films against deterioration through operation in open air requires an overcoating. In this case Si3N4 deposited by CVD was used. This passivation layer can be used as an antireflection coating if it is of the proper thickness, raising the solar absorptance to values greater than a = 0.91 while only slightly increasing the thermal emittance to e = 0.11. Changes in the optical properties of black molybdenum during annealing are related to alternations in structure and composition. Since annealing proceeds at temperatures in excess of 700°C no thermal deterioration processes are expected in the absence of oxygen at the lower operating temperature of 500°C. This was demonstrated through testing at 500°C in a roughing pump vacuum; no changes in the films' optical characteristics were observed after 1000 h. All depositions and anneals proceed at atmospheric pressure, facilitating a continous flow-through procedure important for large-scale economic fabrication.