A prototype instrument for making contactless measurements of free carrier concentration as a function of the surface coordinates has been developed. The instrument requires minimal sample preparation, and the measurements are nondestructive. Measurements are based upon the free carrier absorption of photons at a wavelength of 10.6 μm. The measurement sensitivity is determined by the ability to resolve small variations in the transmitted wave. These variations can be significantly influenced by changes in sample thickness. A scheme has been devised which eliminates thickness variations by rotating the sample through small angles to achieve a transmission maximum at each data point. Preliminary results indicate that system drift limits the sensitivity of the prototype system to measurements of the order of 1015 free carriers per cm3. Data on CdS samples indicate accurate measurements for free electron densities greater that 1016 cm-3. Refinements in the instrument should reduce this level to 1014 cm-3 or less.
J. L. Boone et al., "Free Carrier Density Profiling By Scanning Infrared Absorption," Review of Scientific Instruments, vol. 59, no. 4, pp. 591 - 595, American Institute of Physics, Dec 1988.
The definitive version is available at https://doi.org/10.1063/1.1139838
Electrical and Computer Engineering
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01 Dec 1988