Optical Properties of Tantalum Oxide Films on Silicon

Abstract

Tantalum oxide films on silicon were prepared by thermal oxidation of vacuum‐deposited Ta films. The optical absorption of these noncrystalline films resembles closely that of crystalline , indicating a strong similarity in their short‐range order structures. For given oxidation conditions, the refractive index of the oxide films increases from ∼1.93 to 2.34 as the thickness increases from 12.5 to 111.7 nm. For a given tantalum film thickness, higher oxidation temperatures result in thicker oxides of lower refractive index. Additional oxide growth occurs during postoxidation heat‐treatment in oxygen while the refractive index decreases. The refractive index of a given oxide film increases from the Si/oxide interface toward the outer surface, e.g., from 2.08 to about 2.4. These phenomena are attributed to the incorporation of silicon into the Ta oxide during its growth. However, the estimated amount of silicon in the oxide is not sufficient to explain the observed values if it is assumed that the lowering of the refractive index is due simply to mixing with . Thus, it is concluded that the structure of noncrystalline has a great flexibility which is further enhanced by incorporating silicon; the polarizability of the Ta‒O bond is then strongly affected by silicon. This oxide has been applied as antireflection film in recently developed shallow junction silicon solar cells of increased conversion efficiency.