Unambiguous observation of subband transitions from longitudinal valley and oblique valleys in IV–VI multiple quantum wells

PbSe/PbSrSe multiple-quantum-well (MQW) structures were grown on BaF2(111) substrates by molecular-beam epitaxy and characterized by Fourier transform infrared transmission spectroscopy. To reduce unwanted Fabry–Pérot interference fringes, the top surface of the MQW samples was coated with an anti-interference film, enabling clear observation of subband transitions without superposed interference fringes. Transition energies involving longitudinal and oblique valleys were unambiguously resolved and are in good agreement with calculations made using the envelope wave function approximation.