Research Highlights

• What is a thin film?
• Deposition of thin films
• Optical properties of thin films
• Thin film gas sensors

What is a thin film?

A thin film is a layer whose thickness is extremely small, usually less than a micron. As the name implies, a thin film has a high surface-to-volume ratio.  Thin films can be deposited on a substrate by thermal growing or by vapor deposition. Thin films behave differently from bulk materials of the same chemical composition in several ways.  For instance, thin films are sensitive to surface properties while bulk materials generally aren't.

Deposition of thin films:

Thin films can be deposited using a variety of techniques. In our lab, we use physical vapor deposition (PVD) and pulsed laser deposition (PLD).

Optical properties of thin films:

Our main interest is the determination of the optical properties of thin films. These are among the most important materials properties. Optical properties include the optical constants (refractive index and extinction coefficient) and the bandgap. Such properties are very important in optoelectronic applications and optical element design and fabrication. In our lab, we are using spectrophotometry (measurement of reflectance and transmittance) to determine the optical properties of thin films. We have developed many techniques for the determination of these quantities for metal and dielectric films.

We have studied the following classes of materials:

o Optical materials: these materials have important optical applications (such as the fabrication of optical elements) or optoelectronic applications (such as the fabrication of LEDs, detectors and transparent conducting materials). In this regard, we have studied the optical properties of SiO, Sc₂O₃, TeO₂, Ga₂O₃, ZnO, CeO₂, Er₂O₃, MnO₂, CuO, V₂O₅, Cr₂O₃, Ta₂O₅.

o Chromogenic materials: these are materials that change their optical properties (become colored) under a source of excitation. The source of excitation can be charge injection (electrochromism), heat (thermochromism), or UV excitation (photochromism). In this regard, we have studied the thermochromic properties of WO₃ and MoO₃.

o Luminescent materials: these are materials that Luminesce (emit light) under a suitable source of excitation. The source of excitation can be an energetic electron beam (cathodoluminescence), chemical reaction (chemoluminescence), or energetic photons (photoluminescence). In this regard, we have studied the luminescent properties of sodium salicylate.

o Energy-efficient coatings: these are coatings made of a metallic layer sandwished between two dielectric layers. The purpose of this structure is to fabricate energy-efficient coatings (windows) that transmit visible light but reflect infrared (heat). We have studied heat mirrors based on TiO₂, HfO₂, ZnS, and WO₃.

o II-VI compounds: In addition to the above applications which are mainly concentrated on metal oxide thin films, we have studied the optical properties of II-VI compounds, such as ZnTe, ZnSe, ZnS and CdTe. These materials find important applications in optoelectronics and solar cells.

Thin film gas sensors:

Thin film gas sensors (also known as chemical sensors) are important to all aspects of modern life. They provide information about chemical substances needed by instruments. They can protect environment, assist in medical diagnosis and life support systems, provide safety in homes, offices and transportations, and help automate laboratory and industrial operations. More information can be found. We have studied the gas sensing properties of SnO₂, SiO, HfO₂, CeO₂, and MoO₃.