Apparatus for accurate measurement of the optical and electronic properties of narrow bandgap semiconductors

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Materials that absorb or emit light in the mid-infrared, with wavelengths in the range of 1.25 to 25 microns, are becoming increasingly important for a number of applications.  The primary absorption of many biomolecular and chemical species is in this range and optical methods can be used to detect them in a sensitive and selective way.  The characteristic temperatures in this wavelength range are 160 to 3200K and so these materials are ideal for thermal imaging at room temperature (300K) and for capturing waste heat from industrial processes using thermophotovoltaic devices (~1500K).  Higher efficiency solar cells can be made by also capturing infrared radiation in this range.  Important materials for this purpose are III-V and II-VI compound semiconductors, some of which have narrow energy gaps and correspondingly long wavelength sensitivity.  Many compound semiconductor materials can be grown in a very controlled way, one atomic layer at a time, to produce high quality single crystal layers.  Their composition can be altered during the growth process to make complex multi-layer structures that can be further processed to make lasers, detectors and photovoltaic devices.  We are developing new methods for growing such materials on substrates such as Silicon to reduce cost and to facilitate their integration with electronic and optical devices that are already commonly made using Silicon.  To further this development, it is crucial to be able to accurately characterize the grown materials to determine their energy gaps and optical properties.  This is particularly challenging for semiconductors in this spectral range because of the strong background from ambient thermal sources and absorption by air and its constituents.  Vacuum-based optics and differential techniques are used to measure small signatures, while eliminating intrinsic and extrinsic background effects, to measure the energy gaps in this spectral range with high accuracy and resolution.  These methods are based on Fourier transform spectroscopy which uses all of the captured light in the most efficient manner to determine an energy spectrum.  These methods have only been demonstrated very recently at long wavelengths and this system will be the first of its kind in Canada.