Optical Characterization of Radio-Frequency Magnetron-Sputtered Gallium-Arsenide Films under Non-Uniform Thickness Conditions

Measurements were carried out on radio-frequency magnetron-sputtered amorphous and microcrystalline gallium-arsenide (GaAs) films, which were fabricated under various sputtering conditions such as annealing temperature (up to 450˚C), substrate temperature (up to 200˚C), sputtering pressure (up to 20 mTorr), rf sputtering power (up to 800 W), and hydrogen partial pressure (25% H 2 in a mixture with 75% Ar). The experimental goals were to obtain GaAs films exhibiting a low optical absorption coefficient (< 100 cm-1) and a high index of refraction (n > 3.5) for wavelengths greater than 1,000 nm. The key contribution of this work is the introduction of a novel, simple technique to calculate the absorption coefficient under conditions of non-uniform film thickness. The absorption coefficient calculated using the new technique generally decreases as a function of increases in annealing temperature (up to 450˚C), substrate temperature (up to 200˚C), sputtering pressure (up to 20 mTorr), rf sputtering power (up to 800 W), and hydrogen partial pressure (25% H 2 in a mixture with 75% Ar). Index of refraction, on the other hand, generally decreases as a function of the parameters above except for the case of rf sputtering power, where the index of refraction significantly increases. The absorption coefficients obtained in this research are the lowest values ever published using conventional spectrophotometry. INTRODUCTION GaAs thin films find many potential applications in the semiconductor industry. The most prominent is the multiple-layer bandpass thin-film filter for Dense Wavelength-Division Multiplexing (DWDM) used in fiber-optic communications where each signal is transmitted on a