Density functional theory (DFT) calculations to investigate the effects of substitution of ga with al on the band gap and the optical spectrum of Al (x) Ga (1 -z)N (x=0.125, 0.25) = Studi pengaruh subtitusi Ga dengan Al dalam sifat-sifat optis dan struktur elektronik Al (x) Ga (1 -z)N (x=0.125, 0.25) berbasis density functional theory (DFT)