InGaAs on GaAs extended wavelength linear detector arrays

We report on In(80%)GaAs line scan sensors with 128 pixels on 50μm pitch for use as thermo-electrically-cooled spectroscopic sensor in the short-wave infrared (1-2.5μm). Whereas most of the literature reports on lattice mismatched growth of extended InGaAs on InP, the InGaAs diode material is grown on a 3″ GaAs substrate by means of Molecular Beam Epitaxy. The biggest driving force is reduced cost of the substrate and the availability of up to 6″ GaAs wafers. Despite the large mismatch between GaAs and In(80%)GaAs (∼6%, biaxial compressive strain) the layers show very low dislocation density (below 10⁵cm^-2), indicating excellent heteroepitaxial growth. A bondable linear diode process for top illumination was developed with emphasis on manufacturability. The average R₀A product over a fully processed array was 120Δcm² at 250K. Measurements at 77K revealed R₀A products largely exceeding 10⁵Δcm². The homogeneity of the dark current at 250K was about 5% while the homogeneity of the photocurrent was 7%. The sensor was connected to dedicated commercial CMOS readout circuit (using a capacitive feedback transimpedance amplifier read-out scheme, CTIA). Various layouts of the InGaAs layer stack have been studied. Photocurrent and dark current depent significantly on diode design, e.g. (p+n-n+), (p+p-n-n+) or (p+p+.n-n+).