HAXPES Measurements of Heterojunction Band Alignment

Abstract

Heterojunctions, the abrupt change of materials at interfaces, are an integral feature of modern electronic devices. The alignment of electronic energy levels at a heterojunction can be used to tailor charge transfer across the interface, for example to improve carrier injection or to block leakage current. An overview of the understanding of heterojunction energy-level alignment with specific examples of the unique contributions that hard X-ray photoelectron spectroscopy (HAXPES) provides to the understanding of this topic is presented in this chapter. Many theoretical approaches have been applied to heterojunction band alignment, and have had some success in predicting band-alignment values in some but not all cases. Band-alignment measurements have been made using electronic measurements such as internal photoemission, as well as photoelectron spectroscopy either measuring valence bands directly or through the use of core levels. Examples of measurements made by these techniques is presented. HAXPES measurements provide a greater analysis depth, which provides the advantage of measuring “real” heterojunctions fabricated by industrially-relevant techniques. HAXPES has been used to query the fundamental limitations on interlayer thickness for band-offset engineering, and the use of new materials for photovoltaic applications. These and other applications are presented.