Electronic band structure of GaN_xP_yAs_1-x-y highly mismatched alloys: Suitability for intermediate-band solar cells

Formation of an intermediate band in GaN_xP_0.4As_0.6-x alloys due to the isovalent doping by nitrogen is studied by photoreflectance and absorption spectroscopy. The fundamental energy gap transition (E₀) observed for an N-free alloy is replaced by two optical transitions (E_ and E₊) in GaNPAs layers. The E_ and E₊ transitions are explained within the band anticrossing model, where the localized level of nitrogen interacts with the conduction band of the GaPAs host, splitting it into two subbands. The valence band (VB) is mostly unaffected by nitrogen incorporation as confirmed by the same spin-orbit splitting for N-free and N-containing alloys. The energy position of the E_- subband and a strong optical absorption between the VB and the E_- subband indicates the GaNPAs alloys have an electronic structure suitable for intermediate-band solar cells. Such an electronic structure is not observed for other III-V alloys like GaInAs, GaInAsP, etc., for which the virtual crystal approximation can be applied to describe the evolution of the electronic structure with the alloy content. Results obtained in this work clearly show that GaNPAs with a few percent of nitrogen is an unusual material system, for which the electronic structure properties differ very significantly from properties of well-known III-V alloys, and the application of virtual crystal approximation in this case is inappropriate or very limited.