Second harmonic generation in AlGaN, GaN and AlxGa1–xN/GaN multiple quantum well structures

Second harmonic generation coefficients of GaN and AlxGa1–xN (x=0.08) thin films deposited by MOCVD on a sapphire 0001 substrate were deduced through the standard Maker fringes method. Measurements were performed at λ=1064 nm using a Nd:YAG Q-Switched laser. The measured ratio between the d33 and d31 coefficients allowed one to retrieve information on the degree of crystallinity of the thin film samples that was found to be higher in thicker films. The presence of tiny oscillations in the SHG detected signal as a function of the incidence angle of the fundamental beam on the samples is discussed and related to the reflectance at the sample-air interfaces. Finally, nonlinear coefficients for three AlxGa1–xN/GaN multiple quantum well (MQW) samples (x=0.08 and x=0.15) were determined and compared to the values obtained for GaN crystalline thin films.