Composition, Temperature, and Size Regulation of Refractive Index in Ternary Group-Ⅲ Nitride Alloys: a Bond Relaxation Investigation

Abstract

Abstract The physical origins of composition-, temperature-, and size-motivated changes in refractive index in crystals have long been a puzzle. Combining the bond-order-length-strength theory, local bond average approach, and core-shell structural model, we investigated the refractive indexes in dependencies of composition, temperature, and size for the ternary wurtzite group-Ⅲ nitride alloys. The theoretical reproduction of the observations disclosed that (ⅰ) the doping of small atoms caused the contraction in bond length, the strengthening in bond energy, and the decrease of refractive index, whereas the doping of large atoms led to an elongation of bond length, a weakening of bond energy, and an increase of refractive index; (ⅱ) the refractive index is inversely proportional to the cohesive energy and the cube of the Debye temperature; and (ⅲ) with the gradual decrease in solid size, the coordination number lowers, the bond length contracts, the bond energy gains, the surface-to-volume ratio rises, and the refractive index decreases. The proposed formulation not only shows an in-depth comprehension of the physical essence of the stimuli impact on the refractive index but also is expected to be conducive to the exploitation, optimization, and operation of the new-type photonic, piezoelectric, and pyroelectric nanometer devices for the ternary wurtzite alloys.