Sub-bandgap absorption of gallium nitride determined by Photothermal Deflection Spectroscopy

Photothermal Deflection Spectroscopy (PDS) is used to study the sub-bandgap absorption of hexagonal gallium nitride (GaN) in the energy range from 0.6 to 3.8 eV. Auto-, n- and p-doped GaN layers deposited by chemical vapour deposition from organometallic precursors (MOCVD) or electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE) are measured to investigate the optical band edge, the sub-bandgap absorption and the absorption due to free carriers. PDS is also useful to study the long term and thermal stability of GaN. Changes in sub-bandgap absorption by heating the samples to 600 °C for several hours are presented.