A novel method is presented which allows to expose and investigate interior interfaces in the Ga(NAsP)/GaP material system by a combination of highly selective etching and subsequent atomic force microscopy. We demonstrate the selectivity of the chemical etchant and prove that structural information and atomic-scale z-resolution is fully preserved when applying the method. A correlation between metal organic vapour phase epitaxy growth parameters and interior interface morphology is confirmed by subjecting compressively strained and lattice matched Ga(NAsP) material to a growth interruption. The interior interfaces of both materials smoothen during growth interruption, and for the compressively strained Ga(NAsP) we obtain a monolayer-structured morphology. The optimization of this material is very interesting from an application point of view as it has shown electrical injection lasing near room temperature and might be applied for optoelectronic integrated circuits based on Si in the future.