Defect Properties and Band Energetics of Atomic Layer Deposited MoO_X on Crystalline Si

Molybdenum oxide (MoO_x) thin films have been extensively investigated for selective hole extraction in solar cells, including as an efficient alternative to the well-studied p-type a-Si:H layers in Si heterojunction cells. Among the various methods for producing MoO_x films, atomic layer deposition (ALD) enables the growth of conformal, thin, and electronically tunable materials. However, while several studies have examined ALD-grown MoO_x, a comprehensive understanding of its electronic structure, density of states, and the influence of growth conditions on its interface and energetic alignment with c-Si is still evolving. In this study, we investigate and comparatively analyze the properties of MoO_x films deposited using different ALD processes, including those based on oxygen plasma-enhanced and ozone-assisted ALD growth. Through analysis of optical absorption spectra, we find that different ALD processes can be used to tune the densities of active defects, including both near-band edge tail states and deep defect states located ∼1.1 eV below the conduction band edge. Our comparative analysis reveals that ozone-assisted growth leads to increased defect densities compared to oxygen plasma-enhanced growth. In addition, all films are characterized by a prominent sub-gap absorption feature that is consistent with the formation of small polarons. Finally, we used a combination of optical and X-ray spectroscopic methods to evaluate the band alignment between ALD MoO_x thin films and c-Si, confirming the presence of a small energetic barrier that can permit selective hole injection across the interface. Overall, ALD enables highly controllable growth of films with variable defect concentrations and film properties that are of key relevance for the development of heterojunction solar cells.