Optical Energy Gap Measurement of Semiconductor Ultrathin Films Using Optical Waveguides

Measurement of the optical energy gap ( E o ) using a slab optical waveguide (SOW) has been studied. With increasing hydrogenated amorphous silicon ( a -Si:H) film thickness, transmittance was found to decrease with periodic oscillation. It has been clarified that the oscillation is caused by the change of the field strength distribution of a guided mode in a -Si:H according to the increase of a -Si:H film thickness, using the four-layer structure SOW simulation. By removing the oscillation due to the dependence of transmittance on a -Si:H film thickness, the influence of the oscillation on the values of E o can be eliminated. As a result, it has been demonstrated that E o of a -Si:H with thickness on the order of 1/40 that of samples used in the conventional method can be measured. It has been clarified that sufficient absorption to determine E o of a -Si:H ultrathin films is obtained because the sensitivity of the sensor is increased by changing the size of the SOW.