Far-Infrared Spectrum of Free Exciton Transitions under Magnetic Fields

We investigate magneto- far-infrared (FIR) transmission on a 100 nm thick Wurtzite GaN film that is grown on a Si/HR-GaN substrate. Under zero magnetic field (B), we observe three absorption peaks at the energy of 30.6 meV (valley I), 34.7 meV (valley II), and 40.0 meV (valley III), respectively. Under perpendicular B, small energy blue shifts of the three absorptions (I, II, III) are all linear to B. Meanwhile, minimum II splits into two distinct valleys, with a distance of about 0.08 meV/T in between. And three distinct splits are observed around valley III with a distance of about 0.11 meV/T between neighboring peaks. We propose that the absorption patterns can be attributed to the transitions between the energy levels of the fine structure of free excitons in GaN, i.e. the direct transitions between n = 1 (1s) and n = 2 (2p) energy levels under magnetic fields up to 14.5 T.