Different spectral features near the energy bandgaps of normal and inverse heterostructures of In_0.52Al_0.48As/InP

Abstract

The spectroscopy of photoconductivity (PC) and photoluminescence (PL) were used to characterize two heterostructure configurations of InAlAs/InP grown by molecular beam epitaxy (MBE) on the InP (100) substrate. The sample A is the type called normal heterostructure with the In<sub>0.52</sub>Al<sub>0.48</sub>As layer grown on InP whereas the sample B is called the inverse type formed by a InP cap layer on the In<sub>0.52</sub>Al<sub>0.48</sub>As layer. The front excitation was employed in both PC and PL experiments and the measurements were conducted at 77 K. The PC spectrum of sample A shows an abnormal step-like drop when the photon energy is larger than the energy band gap of In<sub>0.52</sub>Al<sub>0.48</sub>As. The phenomenon imply that the conductance of sample is a multilayer effect including the contribution of interfacial two dimensional electron gas (2DEG). Moreover, a conductance peak was observed at 916 nm below the bandgap of InP. Accordingly, an intense luminescent peak at the wavelength manifests in the PL spectrum. The origin of the 916 nm peak is attributed to the recombination of 2DEG electrons with the valence band holes being excited near the interface. However, the spectral feature of the above energy is absent in both PC and PL spectra of sample B. Such a difference may be interpreted with the different interfacial electronic structure of the inverse interface. For the latter case, the band bending effect should be weaker regarding that a graded variation of In-As-P composition is generally associated with an inverse interface of InP/InAlAs. In such a case, the bound energy of 2DEG in the interface potential well rises closer to the conductance band of the bulk. Consequently, the recombination energy of 2DEG at the inverse interface with the holes in the valence band is close to the band-to-band transition of InP bulk and the corresponding luminescence is hard to be distinguished from that of bulk InP. The work also demonstrates that the comparative study with both PC and PL techniques is helpful to provide full insight into the spectral features with the interface electronic property.