ULTRA-LOW VALUES OF THE ABSORPTION-COEFFICIENT FOR BAND-BAND TRANSITIONS IN MODERATELY DOPED SI OBTAINED FROM LUMINESCENCE

The absolute value of the absorption coefficient alpha(bb)(HBAR omega) for band-band transitions near the band edge was determined in modera tely doped silicon by photoluminescence spectra analysis. The major ad vantage of this method in determining alpha(bb)(HBAR omega) is the lac k of interference with free carrier' absorption, in contrast to conven tional methods like transmission or photothermal deflection measuremen ts. We deduce values for alpha(bb)(HBAR omega), which are nearly five orders of magnitude smaller than the absorption coefficient alpha(fc)( HBAR omega) for free carrier absorption. With this method it is possib le to examine in detail the influence of doping on the absorption coef ficient for band-band transitions near the absorption edge. The appear ance of band tails and band-gap narrowing are very well reflected. Wit h conventional methods, which can only detect the overall absorption o f the incident radiation, the determination of alpha(bb)(HBAR omega) i n the vicinity of the band edge is impossible for moderately and heavi ly doped silicon, because it is completely masked by the free carrier absorption. (C) 1996 American Institute of Physics.