Measurement of second order susceptibilities of GaN and AlGaN

Rotational Maker fringes, scaled with respect to chi((2))(11) of crystalline quartz, were used to determine the second order susceptibilities chi((2))(31) and chi((2))(33) for samples of thin AlxGa1-xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064 nm. The AlxGa1-xN samples, ranging in thickness from roughly 0.5 to 4.4 mu m, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x=0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample similar to 70 mu m thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film similar to 226 mu m thick removed from its growth substrate, and a crystal similar to 160 mu m thick grown by high-pressure techniques. For the AlxGa1-xN samples, the magnitudes of chi((2))(31) and chi((2))(33) decrease roughly linearly with increasing x and extrapolate to similar to 0 for x=1. Furthermore, the constraint expected for a perfect wurtzite structure, namely chi((2))(33)=-2 chi((2))(31), was seldom observed, and the samples with x=0.660 and x=0.666 showed chi((2))(31) and chi((2))(33) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample. (C) 2005 American Institute of Physics.