Nonlinear Optical Characterization of GaN Layers Grown by MOCVD on Sapphire
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STRACT
Optical second and third harmonic generation measurements were carried out on GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The measured d33 is 33 times the d11 of quartz. The angular dependence of second-harmonic intensity as well as the measured ratios d33/d15 = -2.02 and d33/d31 = -2.03 confirm the wurzite structure of the studied GaN layers with the optical c-axis oriented perpendicular to the sample surface. Fine oscillations were observed in the measured second and third harmonic angular dependencies. A simple model based on the interference of the fundamental beam in the sample was used to explain these oscillations.
INTRODUCTION
Recently there has been strong interest in studying gallium nitride and alloys because of their promising device applications [1]. In particular, the nonlinear optical properties of GaN films are of interest for optoelectronic and all-optical device applications. Several second-harmonic generation (SHG) and third-harmonic generation (THG) studies of GaN epilayers have been reported recently [2-5]. However, no attention was paid to the differences between front and back excitations nor to the angular step resolution on the nonlinear optical response. In the present work we report on the study of nonlinear optical effects in GaN/sapphire samples as a function of excitation geometry and angular step resolution. Both the second-harmonic (SH) and third-harmonic (TH) signals measured in dependence on the incident angle of the laser beam were found to be modulated due to the interference of the fundamental beam in the sample.
F99W11.52
EXPERIMENT
The GaN layers used in our experiments were grown by low-pressure MOCVD on (0001) c-plane sapphire using trimethylgallium (TMGa) and ammonia (NH3) as source materials. A horizontal growth reactor in a modified EMCORE GS-3200 system was used for this purpose. A buffer layer of about 25 nm thick GaN was first grown at 510 oC. The GaN layers grown on top at a temperature of 1100 oC had thicknesses of about 1 µm. Polarized SHG and THG measurements were carried out in transmission mode for both front (the GaN layer facing the incident pump beam) and back (the sapphire substrate facing the incident pump beam) excitations. As a fundamental beam, the 1064 nm output of a Q-switched Nd-YAG laser (Spectra Physics GCR-170) with 10 Hz repetition rate and 7 ns pulse width was used. To minimize the influence of the laser output fluctuations, the measured SH ( λ2ω = 532 nm ) and TH ( λ3ω = 355 nm ) intensities were normalized by the simultaneously monitored laser intensity in the reference channel. The sample was mounted on a step-motorized rotation stage. The direction of the fundamental beam polarization was changed by rotating a half-wave plate placed in front of the sample. The fundamental wavelength was filtered out from SHG and THG signals by using appropriate colour filters and a grating monochromator.
RESULTS AND DISCUSSION
The wurzite structure of GaN belongs to the 6mm point group symmetry. In t
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