Abstract
We present the results of a detailed study of the band edge absorption changes in GaN and InGaN thin films induced by the presence of excess photo-generated free carriers. The 1s A and B free excitonic resonances in GaN are shown to decrease with increasing above-gap nanosecond optical excitation due to screening by free carriers and exciton–exciton scattering. The decrease in excitonic absorption with increasing above-gap excitation is accompanied by a significant increase in the below-gap absorption coefficient. To further explore this behavior, femtosecond nondegenerate optical pump–probe experiments were also performed using an amplified Ti : sapphire laser. Exciton saturation due to screening by free carriers and excitonic phase space filling was again observed, along with a modest amount of below-gap induced absorption attributed to band gap renormalization. Similar experiments were performed on InGaN thin films. The band edge absorption changes observed in InGaN films were found to be significantly different than those observed in GaN.
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