A microcavity enhancement of room temperature photoluminescence (PL) of a hydrogenated amorphous silicon (a-Si:h) was performed. A quantum confinement model was developed to describe the occurrence of the PL in the bulk a-Si:H. According to the model, small a-Si clusters are in a matrix of a-Si:H. The regions with Si-H, having larger energy gaps due to strong Si-H bonds, isolate these clusters, and form barrier regions around them. The PL originates from these a-Si clusters.