Abstract

Laser optical gas sensors are fabricated by using the crystalline silicon carbide polytype $6H\text{-}\mathrm{Si}\mathrm{C}$, which is a wide-bandgap semiconductor, and tested at high temperatures up to $650°\mathrm{C}$. The sensor operates on the principle of semiconductor optics involving both the semiconductor and optical properties of the material. It is fabricated by doping $6H\text{-}\mathrm{Si}\mathrm{C}$ with an appropriate dopant such that the dopant energy level matches the quantum of energy of the characteristic radiation emitted by the combustion gas of interest. This radiation changes the electron density in the semiconductor by photoexcitation and, thereby, alters the refractive index of the sensor. The variation in the refractive index can be determined from an interference pattern. Such patterns are obtained for the reflected power of a He–Ne laser of wavelength $632.8\mathrm{nm}$ as a function of temperature. SiC sensors have been fabricated by doping two quadrants of a $6H\text{-}\mathrm{Si}\mathrm{C}$ chip with Ga and Al of dopant energy levels $E_V+0.29\mathrm{eV}$ and $E_V+0.23\mathrm{eV}$, respectively. These doped regions exhibit distinct changes in the refractive index of SiC in the presence of carbon dioxide (${\mathrm{CO}}_2$) and nitrogen monoxide (NO) gases respectively. Therefore Ga- and Al-doped $6H\text{-}\mathrm{Si}\mathrm{C}$ can be used for sensing ${\mathrm{CO}}_2$ and NO gases at high temperatures, respectively.

© 2010 Optical Society of America

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