Piezoreflectance characterization of resonant tunneling and modulation-doped heterostructures
We used piezoreflectance, a stress modulation technique, to optically characterize both double-barrier resonant tunneling structures and modulation-doped heterostructures. This technique easily probed the single quantum wells buried below the ∼0.5-μm-thick n+ GaAs surface layers required for ohmic contacts. In fact, the positions of salient piezoreflectance spectral features denoted subtle variations of well width and barrier height and width; lineshapes indicated interface roughness. Since piezoreflectance spectra of our modulation-doped samples had sharp features associated with quantum confinement, and the corresponding photoreflectence spectra had Franz-Keldysh oscillations that depended on the built-in electric field, we could calculate energy levels that reflected not only growth parameters but experimentally determined potential profiles as well. Moreover, the sensitivity of piezoreflectance to carbon and silicon acceptor states permitted us to assess the material quality.