Overlapping Effects of the Optical Transitions of GaNAs Thin Films Grown by Molecular Beam Epitaxy

Photoreflectance and ellipsometry are two very useful optical spectroscopy techniques employed to analyze the electronic band structure of semiconductors. Photoreflectance and ellipsometry spectra of gallium arsenide nitride (GaNAs) thin films on gallium arsenide (GaAs) layers may be composed of transitions from the conduction bands of the alloy and the binary (E-, E+, and E-0, respectively) in conjunction with their interaction spin-orbit split-off valence band (E-+Delta(0) and E-0+Delta(0)). For low concentration of nitrogen (between 0.2 and 0.6 %), the determination of the E+ conduction band becomes difficult to distinguish by the fact that critical points are superimposed in the spectrum of both characterization techniques. In this work, a method to determine the E+ conduction band of GaNAs thin films grown on GaAs by molecular beam epitaxy is proposed when the overlapping of spectral features influences their optical determination. When using spectroscopic characterization techniques, the modulation/excitation region depends on the wavelength employed and sample characteristics, and consequently low nitrogen concentration in the alloy in conjunction with the thickness of the GaNAs layers have been found responsible for the signal overlapping. It is demonstrated that by decreasing the sample temperature in the photoreflectance process the overlapping is avoided, allowing for a correct interpretation of the GaNAs conduction band splitting analysis and discarding the contribution of built-in electric fields. From these results, we achieved a precise experimental determination of the presence/absence of the band splitting predicted by the band anti-crossing model using non-destructive characterization tools.