Unusually strong excitonic absorption in molecular-beam-epitaxy-grown, chemically lifted GaAs thin films

Exceptionally strong excitonic absorption has been observed in GaAs thin films. The thin, undoped films, which were grown by molecular-beam epitaxy, were prepared by selectively etching an AlAs release layer and either gluing the film to a quartz substrate or van der Waals bonding to a sapphire substrate. Transmission spectra were taken at a number of temperatures between 20 and 295 K, and simultaneous transmission and reflection spectra were taken at 90 and 295 K. The excitonic absorption observed was much stronger than has been reported previously for GaAs. The heavy-hole exciton (strain split from the light-hole exciton) has an absorption coefficient of (2.3±0.3)×${10}^4$ ${\mathrm{cm}}^{\mathrm{-}1}$ at 90 K. The heavy-hole absorption coefficient at temperatures below ∼100 K is more than twice the value reported by other workers. The absorption coefficient at the exciton is strain independent for strains up to ${10}^{\mathrm{-}3}$.