Absorption Coefficient and Refractive Index of GaN, AIN and AlGaN Alloys
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* Department of Physics, North Carolina State University, Raleigh NC 27695 Department of Electrical and Computer Engineering, North Carolina State University, Raleigh NC 27695
Cite this article as : MRS Internet J. Nitride Semicond. Res. 4S1, G5.2 (1999) ABSTRACT The design of optoelectronic devices fabricated from III-nitride materials is aided by knowledge of the refractive index and absorption coefficient of these materials. The optical properties of GaN, AIN and A1GaN grown by MOVPE on sapphire substrates were investigated by means of transmittance and reflectance measurements. Thin (less than 0.5 Vm) single crystal films were employed to insure that transmission measurements could be obtained well above the optical band gap. The influence of alloy broadening on the absorption edge was investigated by using a series of AIGaN alloy samples with a range of Al compositions. The optical absorption coefficient above the band gap was obtained for AIGaN having up to 38% A] composition. The refractive index below the band gap was determined for the same series of samples. These properties provide information critical to the optimal design of solar blind detectors or other optoelectronic devices. INTRODUCTION The ITI-nitrides, including GaN, InN, and AIN, have proven to be robust materials for the development of blue/green LEDs and violet lasers lasers [1]. The AIGaN 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 Ipm 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
G 5.2
Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Resea
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