Interband transitions and phonon modes in B_xGa_1−xAs (0 < x < 0.03) and GaN_yAs_1−y (0 < y < 0.037): A comparison

We report on the experimental observation of direct interband-critical-point transitions and phonon modes in B_xGa_1−xAs alloys (0 < x < 0.03) and their evolution with increasing boron concentration using spectroscopic ellipsometry and Raman scattering. Our results are compared to the corresponding values in the GaN_yAs_1−y (0 < y < 0.037) material system. For B_xGa_1−xAs, we obtain only a small bowing coefficient of the E_g(x) dependence in contrast to the giant redshift of the GaN_yAs_1−y band-gap energy with y. The higher lying interband-transition energies of B_xGa_1−xAs (E₁, E₁ + Δ₁, E₀′, E₂, and E₁′) are slightly redshifted with increasing boron concentration. A similar behavior is found for the critical points E₀′, E₂, and E₁′ in GaN_yAs_1−y. In B_xGa_1−xAs, we observe, as in GaN_yAs_1−y, a two-mode phonon behavior using Raman scattering. However, from infrared-ellipsometry or -transmission experiments, we can estimate that the oscillator strength or polarity of the BAs-like phonon is at least one order of magnitude smaller than the oscillator strength of the GaN-like phonon measured at a GaN_yAs_1−y layer with comparable thickness and composition. All results will be explained using a simple model that takes into account the different nature of the chemical bonds in both alloy types.