NON-LINEAR OPTICAL CHARACTERIZATION OF GALLIUM NITRIDE AIDS MATERIAL IMPROVEMENT.Gallium nitride and related alloy materials are making enormous economic impact with the realization of semiconductor lasers and light-emitting diodes emitting in the blue for data storage, solid-state lighting, and displays. However, problems with material quality remain, particularly in the case of substrates. Furthermore, the lack of a reliable database for the linear optical properties of the group III-nitrides is hampering development of engineering design tools. A NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. scientist has applied non-linear optics (NLO NLO Next-to-Leading Order NLO Nonlinear Optics NLO Nobody Likes Onions (website) NLO National Liaison Officer NLO Naval Liaison Officer NLO National Labor Office NLO NETg Learning Object NLO No Load Operation ) to the characterization of gallium nitride grown by industry and university collaborators. NLO offers rapid and versatile measurement capabilities that can be used to examine materials at various stages of crystal growth and device processing. Furthermore, material uniformity evaluated using NLO methods has been correlated directly with the information derived from analytical methods that are not conveniently adapted to the manufacturing environment. NLO was used to compare the optical and structural properties of GaN bulk crystals grown by high-pressure processing, and thin films of the material grown by hydride vapor phase epitaxy epitaxy Process of growing a crystal of a particular orientation on top of another crystal. If both crystals are of the same material, the process is known as homoepitaxy; if the materials are different, it is known as heteroepitaxy. , metal-organic chemical vapor deposition, and molecular beam epitaxy A technique that "grows" atomic-sized layers on a chip rather than creating layers by diffusion. . The results show distinct variations in both the index of refraction Index of refraction A constant number for any material for any given color of light that is an indicator of the degree of the bending of the light caused by that material. Mentioned in: Eye Glasses and Contact Lenses and the magnitude of the non-linear optical coefficients, depending on the growth method. The refractive index A property of a material that changes the speed of light, computed as the ratio of the speed of light in a vacuum to the speed of light through the material. When light travels at an angle between two different materials, their refractive indices determine the angle of transmission measurements were verified by a prism-coupling technique. NLO is extremely sensitive to such undesirable structural features as stacking faults, domain reversals, and mixed cubic and hexagonal phases. The presence of stacking faults and other mixed phases was revealed in transmitted second-harmonic generation (SHG SHG Second Harmonic Generation SHG Short-Handed Goals (hockey) SHG Self Help Group SHG State History Guide (formerly State House Girls) SHG Sacred Heart Griffin ) by the appearance of extra SHG polarization components that would not occur if the material had purely a single-phase hexagonal structure. High-resolution x-ray diffraction imaging measurements revealed full-crystal images of stacking faults and domain reversals. However , the x-ray technique is less sensitive to the presence of mixed phases. Thus, a combination of NLO analysis and x-ray imaging methods resulted in reduced ambiguity compared to the case if only one method was employed. The results of both are being used to help gallium nitride suppliers improve material quality for future generations of devices. |
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