Analysis of second harmonic generation (SHG) in the pyrex glass

초록

In order to form crystal GaAs quantum dots dispersed in SiO2 matrix at room temperature, an alternate RF magnetron sputter deposition is used with GaAs volume fraction fixed, GaAs quantum dots are varied in size changing GaAs nominal thickness during alternate deposition. From images of High Resolution Transmission Electron Microscopy (HRTEM), discrete circular GaAs quantum dots of diameter of 20-30 A are found to form in case of GaAs nominal thickness of 6 A, but percolating network is observed in case of GaAs nominal thickness of 20A. In either case, deposited GaAs is formed crystalline and stoichiometric GaAs compound-like according to HRTEM and XPS analysis respectively.From measurement of Transmittance using a spectrophotometer, Absorption edges of the composite thin films with varying nominal thickness of GaAs are compared to display an absorption edge shift toward a shorter wavelength with decreasing quantum dots size. Band gaps of composite films are determined from Tauc plots which are in close agreement with PL measurment. The results show that the band gaps of composite thin films decrease linearly with increasing GaAs nominal thickness i.e. 2.72 eV, 2.5 eV, 2.17 eV, and 1.89 eV for 6 A, 10 A, 15 A, and 20 A.The band gap energy is controlled to have a specific value by a proper design of composite thin filmes for the given material system.