Raman and AFM studies on nominally undoped, p- and n-type GaAsBi alloys

We study structural properties and surface formation of undoped, n- and p-type doped GaAsBi alloys with various bismuth compositions using Micro-Raman, Fourier Transform (FT) Raman, Photoluminescence (PL) and Atomic Force Microscopy (AFM) techniques. PL is used to determine the suitable excitation source for Raman measurements and evaluate the Raman spectroscopy results. Room temperature PL results reveal that the bandgap energy of GaAsBi decreases with increasing bismuth composition at a rate of 82meV/%Bi. In micro-Raman spectra recorded using 532 nm laser line, a peak at around 185 cm−1 and a broad peak located between 210 and 250 cm−1 are observed. The absence of these peaks in the Raman spectrum of low temperature (LT) growth GaAs indicates that those peaks are bismuth-induced vibrations. Besides, the forbidden transverse optical (TO) mode (269 cm−1) becomes more pronounced for the samples as bismuth composition increases. FT-Raman spectra taken by 1064 nm laser line of the n- and p- type samples exhibit different characteristics; while TO and longitudinal optical (LO) peaks (291 cm−1) are present in n-type sample as sharp vibrational peaks, a broad peak located at slightly lower wavenumber than TO mode appears and LO peak is suppressed. The ionised acceptor and free hole system respond incident electromagnetic field as plasma oscillations. Moreover, the amplitude of the plasma oscillations enhances with higher doping density. Therefore, the appearance of the broad peak in FT-Raman spectrum of p-type GaAsBi is explained with an LO-plasmon coupling (LOPC). As for n-type samples, free electrons are compensated by free holes in GaAsBi that originate from Bi-induced acceptor-like defects. Therefore, we do not observe the effects of plasma oscillations in FT-Raman spectrum. AFM results reveal that all samples have surface droplets, and the size of the droplets is not affected from doping density or doping type, but bismuth composition. The surface droplets are removed by a chemical process to investigate the effect of the droplets on Raman spectrum. Our results reveal that removing surface droplets does not change the characteristic of Raman spectrum, but enhances the Raman intensity due to the reflected light by droplets decreases, leading to more photons penetrate and scattered from inside the sample.