Application of Iii-V Semiconductor Heterostructures to Optical Chemical Sensing

Changes in the room temperature photoluminescence (PL) intensity from III-V semiconductor heterostructures due to chemisorption at the semiconductor surface have been explored as the basis for a possible chemical sensor device. The interaction of SO_2 with GaAs surfaces has been used as a model system. Etching, photowashing, and sulfide passivation techniques were used to unpin the Fermi level of the GaAs surface in order to observe electronic changes at the surface with adsorption of SO_2. The stability of these surface preparations were characterized using measurements of the surface electric field in UN^+ structures with the photoreflectance (PR) modulation spectroscopy optical technique. The PL intensity from semiconductor heterostructures and its sensitivity to changes in surface charge density and surface recombination velocity have been modeled using analytical and numerical solutions of the photogenerated minority carrier distribution within these structures. The finite element method was used for the numerical solution. The results of these models have been used to design heterostructures to optimize the efficiency of the PL intensity and its sensitivity to electronic surface changes. These structures were grown by metal-organic vapor phase epitaxy (MOVPE) and contained Al_{0.3}Ga _{0.7}As barrier layers and In_{0.2}Ga _{0.8}As quantum wells. The response of the PL intensity to SO_2 adsorption in these structures demonstrated improved chemical sensor signal characteristics. This room temperature PL measurement was also performed in a compact configuration utilizing a light emitting diode (LED) excitation source and silicon photodiode detector to demonstrate the practicality of the measurement for chemical sensor applications.