In this thesis, we reviewed the modulation mechanisms of the photoreflectance (PR) and investigated the optical properties of (Al,In)GaAs/ GaAs heterojunctions by photoreflectance. We observed the signal by the interfacial structures in PR spectra of InGaAs/GaAs and AlGaAs/GaAs heterostructures. Through the PR measurement of the selectively doped GaAs samples we suggested the hole-ionized acceptor(h-$A^{-}$) pair modulation mechanism for the first time.
There are several modulation mechanisms in PR. Electric field modulation by photoinduced electrons and holes is a basic mechanism, and its PR signal has the third derivative functional form. The reflectivity is also changed by band filling effect of photoinduced carriers. This effect is confirmed by the PR of the heavily Si-doped GaAs samples. In addition, the PR can be affected by the spacial modulations of the critical point, such as exciton modulation, hole-ionized acceptor pair modulation, and impurity state modulation. The critical points, where excitons or hole-acceptor pairs are ionized by the built-in electric field and where an impurity level crosses the Fermi energy level, are changed as the electric field changes. The electric fields necessary to ionize excitons and hole-acceptor pairs are about 4 kV/cm and 300 kV/cm, respectively. We also observed the PR signals due to the transitions between excited states in quantum well structures.
For InGaAs/GaAs heterostructures we observed a PR signal near 1.4 eV coming from the InGaAs/GaAs interfaces. And we suggested the possibility that there are metastable InGaAs clusters at the InGaAs/GaAs interface. While the double crystal x-ray diffraction data show that 15% compressive strain is remained in InGaAs epilayer, on an average, the PR data reveals that there are tensile-strained regions up to 67% in InGaAs layer. The residual tensile strain may be related to the clusters at the interface. The process, in which the strain relaxes, is also investigated; homogeneous relaxation → periodically inhomogeneous relaxation → inhomogeneous relaxation. In quantum well structure we comfirmed that symmetry forbidden transitions can be observed when the built-in electric field is sufficiently strong.
In many reported PR spectra of modulation doped AlGaAs/GaAs heterostructures there are several discrepancies,(i) a broad PR signal or a step-like PR signal near the GaAs band edge, (ii) a single peak shape PR signal or a double peak shape(DPS) PR near the AlGaAs band edge, and (iii) a short period oscillation(SPO) PR signal is observed by a few authors only. By the PR measuerment of the etched sample we could know that the broad PR signal comes from the cap layer, while the steplike PR signal is due to the triangular potential well which has 2 dimensional electron gas, and that the higher energy signal of the DPS PR comes from the cap/n-AlGaAs interface. Through the annealing and the etching experiments we observed that there is an unknown PR signal near 1.5 eV and suggested the possibility for the existence of the interfacial structure at AlGaAs/GaAs interface. The built-in electric field is strengthened as the annealing time increases. This is an aging effect and results from the Si-Si pair diffusion. In PR spectra of the selectively doped GaAs samples the SPO signal is splitted into two distinct signals because of the high built-in electric field. We confirmed that these two signal are originated from the exciton and the hole-ionized acceptor pair modulation, respectively, through the investigation of the temperature dependance of the PR spectra.
