Absorption Coefficient and Refractive Index of GaN, AlN and AlGaN Alloys
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INTRODUCTION The III-nitrides, including GaN, InN, and AlN, have proven to be robust materials for the development of blue/green LEDs and violet lasers lasers [1]. The AlGaN alloys also hold great promise for developing ultraviolet photodetectors [2]. In photodetector applications, the wide band gap of the nitrides aids to minimize the dark current, thereby increasing the detector sensitivity. The transparency to visible light will reduce the need for the extensive external filters that are required when detectors sensitive to the visible portion of the spectrum are employed. Multilayered Bragg mirrors and filters fabricated from these materials also have applications for lasers and photodetectors. Currently there has been only a limited amount of information reported [3,4] about the fundamental optical properties of AlGaN alloys which makes design of these devices difficult. The relative immaturity of this materials system also means that there may be some variance in the properties of the materials produced by different workers. In this paper, we report the refractive index and absorption coefficient for AlGaN thin film compositions of up to 38 % obtained by means of reflectance/transmittance spectroscopy. For light below the band gap of the semiconductor, the interference within the thin film modulates both the reflection and transmission spectrum. Above the band gap, the high absorption coefficient causes the film to absorb any multiple reflections of the light. For these materials, unless the film is less than ~ 1 µm thick, there is seldom enough signal for commercial spectrophotometers to obtain an accurate ratio measurement. Due to the short penetration depth of the light above the band gap, reflection measurements become more dependent on the surface
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condition of the semiconductor, but have been used with great success to gather information about band structure. In the spectral region around the band gap the situation is complicated, for in addition to going from a transparent to an absorbing Fabry-Perot cavity, the excitonic structure becomes important. This is especially true in the case of nitride materials, where the binding energy of the exciton is strong (> ~20 meV). The three closely spaced valence bands lead to three strong absorption features, the A, B, and C exciton. The selection rules also make the transmission and reflection polarization dependent. The birefringence of the hexagonal material and the scattering from the columnar grain structure found in most nitride materials further complicates matters. In semiconductor alloys, fluctuations in composition broaden the exciton making the absorption features less distinct. However, the continuum absorption of the exciton still influences the absorption spectra, even when the excitonic features are no longer clearl
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