Absorption in ultrathin GaN-based membranes : the role of standing wave effects

A methodology is described to extract the absorption coefficient spectrum and exciton oscillator strength of GaN layers and GaN/AlGaN quantum wells by analyzing microtransmittance experiments in high-quality, free-standing membranes with thicknesses in the 160–230 nm range. The absorbance of a subwavelength GaN membrane is found to be an oscillating function of its thickness, in keeping with the standing wave effect. We analyze our results using two alternative models including interference effects and extract identical absorption coefficient values. The room-temperature absorption coefficient of bulk GaN membranes at the main exciton peak is found to be 9 × 104 cm−1. In the case of GaN/AlGaN quantum wells, the enhancement and blue shift of the excitonic absorption are observed, as a result of quantum confinement.