AbstractFluorescence detection in integrated micro-bioanalytical systems requires the selective exclusion of light from the excitation source (typically blue or UV) and transmission of the longer wavelength fluorescence signal. In the present research, the application of (In,Ga)N alloy thin films as optical filters for absorption of blue (470 nm) and transmission of green (530 nm) has been evaluated. Absorption spectra, photoluminescence (PL) and Rutherford backscattering spectroscopy (RBS) results are presented for 200-400 nm (In,Ga)N thin films grown by MOCVD on sapphire substrates with 2-4 μm GaN buffer layers. A sigmoidal function was used to model the absorption coefficient of (In,Ga)N as a function of energy, Eg, and the broadening (ΔE) associated with the Urbach tail. Experimental data showed that, as expected, the absorption band edge for (In,Ga)N films broadened with increasing InN mole%. An increase in ΔE of 35 meV was observed when the InN mole% was increased from 10 to 16%. The sigmoidal function model provided a good fit to the experimental data, which allowed the experimental data to be extrapolated to higher InN concentrations. Based on this analysis, it is predicted that a 5 μm thick In0.22Ga0.78N film should transmit 50% of green light (530 nm) and only 3.29 x 1 0-4 of blue light (470 nm). The (In,Ga)N films that have been evaluated range in InN mole% from ~2-16%. The higher InN mole% samples (10-16%) can successfully filter lower wavelengths (e.g., 400 nm). For a filter application, it is also important that photoluminescence be effectively suppressed. In MQW structures, high PL intensity is often obtained by means of InN segregation (InN-rich nanoclusters). But for our evaluation with 200-400 nm (In,Ga)N films, relaxation of coherency strain results in large densities of dislocations, and correspondingly low PL intensity.
Synthesis of High-Quality α-Ga2O3 thin films on Sapphire Substrates with Introduction of Buffer Layers
