Nonlinear Optical And Photoluminescence Study Of Indium Nutride Thin Films

The investigations on optical properties of novel semiconductor thin films have recently received intensive research interest both theoretically and experimentally. The nonlinear optical and photoluminescence (PL) properties of semiconductor thin films have large potential applications in the opto-electronic communication field, such as all-optical switches, optical limiters, optical coupling devices and so on. Indium nitride (InN) is a promising semiconductor in the applications of optoelectronic and microelectronic devices and also has possible applications in high-speed/high-frequency electronics and solar cells. Therefore, it is crucial to study the nonlinear optical and photoluminescence properties of InN thin films.The Z-scan technique is the most popular method for studying the nonlinear properties of a large variety of materials due to its high sensitivity, simple experimental setup, easy alignment and high accuracy of the obtained data. In this thesis, the principles of the third- and fifth-order nonlinear optical processes have been reviewed firstly. Then the Z-scan technique has been introduced, together with the description of the instruments and apparatus setup. The main part of this thesis focuses on the study of the nonlinear optical properties in InN thin films. Based on the detailed calculation of the optical transmission spectrum profile, basic optical parameters of InN have been yielded. By utilizing Z-scan technique, the nonlinear optical properties of InN have been well studied.A combined investigation of the reflection and transmission Z-scan (RZ- and TZ-scan) technique by femtosecond laser pulse at the wavelength of 800 nm has been carried out for extracting both the third- (n₂), fifth- (n₄) order nonlinearity refractive indices and the nonlinear absorption coefficient (β) in InN thin films. The observation of the nonlinear refractive index saturation in the intensity-dependent RZ-scan indicates the existence of the fifth-order nonlinear refraction contribution. The obtained small saturation intensity I_s further reveal the saturation behavior of InN. The enlarged cascaded nonlinearity is realized by TZ-scan due to the increasing laser interaction length, which results in the great enhancement of the fifth-order nonlinearity. On the basis of the Gaussian decomposition method, large n₂ of -2.5×10^-11 cm²/W and n4 of 2.1×10^-19 cm⁴/W² have been determined by fitting the valley-peak TZ-scan trace. Intensity-dependent TZ-scan measurement further proves this conclusion.Detailed investigation of growth and depth dependence of photoluminescence is also presented. The cross-sectional PL measurements revealed the blueshift of the PL peak with the laser spot focusing from the epilayer surface toward the interface, while the PL peak was found to redshift with the increasing growth temperature for normal surface incidence. The results have been well explained by the strain in InN.