Hydrogen passivation of electronic gap states at the interfaces of ultrathin SiO2 layers on crystalline Si

Ultrathin SiO2 layers with a thickness of a few nanometers are proposed to act as functional elements in various high efficiency solar cell concepts: (i) A tunnel oxide layer integrated at the interface of a classical c-Si heterojunction with an a-Si:H emitter is proposed to reduce interface recombination due to excellent passivation properties [1]. Normally, these structures suffer from difficulties in the appropriate wafer pre-treatment as well as from controlling the a-Si:H deposition conditions for very low interface defect densities. (ii) Ultrathin SiO2 layers can be applied as barriers in Si-based multi-junction solar cells that consist of stacked quantum wells of different thickness to provide bandgap tunability and thus efficiency enhancement via utilization of quantum size effects [2]. Essential requirement for the realization of such structures is the preparation of uniform ultrathin oxide layers that have ideally structurally abrupt interfaces with very low concentrations of defect states. Interface roughness increases the density of traps and charges, and thereby increases recombination velocities and lowers charge carrier mobilities. Here, we report on the development of a complete in situ processing cycle that allows for preparation of ultrathin SiO2 layers, treatment with H plasma as well as interface gap state analysis by nearUV photoelectron spectroscopy without breaking ultrahigh vacuum (UHV). The particular advantages of nearly thermal impact energies (Ekin < 1 eV) of (i) neutral O atoms for the formation of chemically abrupt interfaces and (ii) neutral H atoms for significant passivation of dangling bonds at the ultrathin-SiO2/Si interfaces are explored.