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November, 2004 EEEL Measures Optical Properties of III Nitrides |
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Researchers in the Optoelectronics Division of EEEL in collaboration with colleagues in MSEL and collaborators external to NIST have measured optical properties of III-nitride semiconductors critical to the manufacture of efficient lasers and light-emitting diodes (LEDs). Alloys of the III-nitride compound semiconductors, including AlGaN and InGaN, are the only known practical materials for use in the development of LED-based solid state lighting technology and compact blue-emitting semiconductor lasers for high-density data storage. Blue- and UV-emitting devices fabricated in this material system are also finding important applications in medicine, sensor systems for the detection of biological and chemical agents, compact sterilization units, and systems for water purification. In order to facilitate crystal growth and processing of these materials, and to aid in the predictive modeling of LED and laser devices, it is important to have at hand reliable data that correlate the refractive index and birefringence of these materials with alloy composition and structural properties. Norman Sanford of EEEL measured the refractive indices and birefringence on a variety of AlGaN (supplied by the University of California, Santa Barbara, and by TDI, Inc.) and InGaN samples (supplied by Lumileds Lighting) whose alloy compositions spanned a significant range of engineering interest. For example, x ranged from 0 to 0.7 for the AlxGa1-xN alloys, and x ranged from 0 to 0.11 for the InxGa1-xN alloys. The refractive index values, measured by prism coupling to waveguide modes, have a maximum uncertainty of ± 0.005 for AlGaN and ± 0.01 for InGaN. For the set of AlGaN samples, the prism coupling measurements were correlated with spectroscopic reflection and transmission measurements (performed by Lawrence Robins of MSEL.) Additionally, to establish the utility of these refractive index data, the results were correlated to 'a' and 'c' lattice constants (the material is hexagonal) measured with x-ray diffraction analysis (by Albert Davydov of MSEL and by TDI, Inc.), and composition measured by both energy dispersive x-ray microanalysis (Alexander Shapiro, MSEL) and Rutherford backscattering analysis (collaborators at Rutger University). Other analytical techniques included transmission electron microscopy (Igor Levin, MSEL) and field-emission scanning electron microscopy (Alexander Shapiro, MSEL). Taken together, these are the most comprehensive set of such correlated measurements to date for this material system. In a somewhat
more academic study, the nonlinear optical coefficients of the set of
AlGaN samples were also measured. The interest here was to quantify where
in the composition space a critical change in symmetry of the nonlinear
coefficients should occur. Experimentally, the end point members GaN and
AlN are observed to have distinctly different symmetries in their nonlinear
coefficients, which is in agreement with the recent Kohn-Sham local density
approximation but in violation of older semiclassical theories based primarily
on the structural symmetry of the material. It was found that the symmetry
change for the nonlinear coefficients occurs in AlxGa1-xN
alloys for x approximately equal to 0.60. The measurements are also important
for the design of domain-engineered devices for optical sum and difference
frequency generation.
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