Optical characterization of radio frequency-magnetron sputtered gallium arsenide thin films

Measurements were carried out for radio-frequency-magnetron sputtered amorphous and microcrystalline gallium-arsenide (GaAs) films, which were previously fabricated at various sputtering conditions. The parameters explored were annealing temperature, substrate temperature, sputtering pressure, radio-frequency sputtering power, and hydrogen partial pressure. Over 200 samples were fabricated and optically characterized. The experimental goals were to obtain GaAs thin films exhibiting a low absorption coefficient (α < 100 cm −1) and a high index of refraction (n > 3.5) for wavelengths greater than 1 μm. Coherent transmittance expression was derived from Maxwell's equations, which can be generalized for any number of layers. Coherent transmittance at oblique incidence was also derived for parallel-polarization and perpendicular polarization.; The key contribution of this work is the introduction of a novel, simple technique to calculate the absorption coefficient under conditions of non-uniform thickness. Using the wavelength at which transmittance extrema are found rather than the actual transmittance values enables us to calculate the index of refraction to within 2% precision. Identifying the correct order of the transmittance extrema enables the calculation of the index of refraction provided that the thickness measurement is precise to within 2%. The correct order is obtained by comparing the dispersion between amorphous/microcrystalline GaAs films and single-crystal GaAs. Under non-uniform thickness, the absorption coefficient cannot be obtained directly at all wavelengths; artifacts are especially pronounced at the points of minimum and maximum transmittance. Therefore, a summing average of transmittance is carried out between adjacent minima or maxima. The absorption coefficient is iterated on a Matlab program until the theoretical transmittance agrees with the experimental transmittance within 0.1% precision. The absorption coefficient calculated using the new technique generally decreases as a function of increase 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% H₂ 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 significantly increases. The absorption coefficients obtained in this research are the lowest values ever published using conventional spectrophotometry. The Urbach energy shows a value of 100 meV, which is an order of magnitude higher than that of single-crystal GaAs. The highest optical energy gap obtained is 1.15 eV at sputtering pressure of 20 mTorr.