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GaSb-based, 6.1 Å lattice-constant, infrared photodetector materials were grown on large diameter, 6-in. GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb ternary alloys, were investigated to bridge the 7.8% mismatch gap between the GaAs substrates and the GaSb-based epitaxial layers. Unique surface morphologies and crystal structure properties, as revealed by atomic force microscopy and cross-section transmission electron microscopy, pointed to different relaxation mechanisms for different buffer architectures. GaSb nucleation results in a more island-like surface morphology with a mix of 90° misfit and 60°-type threading dislocations, while the graded ternary buffer results in a cross-hatch surface morphology with effective filtering of the threading dislocations. Low root-mean-square roughness values of 5–20 Å were obtained for this type of metamorphic epilayer growth. A generic InAsSb/AlAsSb nBn photodiode structure, where the first “n” is the contact layer, the “B” is the wide-bandgap barrier layer, and the second “n” is the n-type narrow bandgap absorber layer, with ∼4 μm cutoff wavelength was grown on 6 in. GaAs substrates using the different metamorphic buffers. Cross-wafer optical and structural measurements showed excellent epitaxial layer uniformity, with PL wavelength variation <0.1 μm. Dark currents of 2–3 × 10-6 A/cm2 were measured on devices fabricated from the photodiode material grown on GaAs. This was about 5-times higher than measured on the same structure grown on GaSb substrates. This work demonstrates a promising path to satisfy the increasing demand for even larger area focal plane array detectors in a commercial production environment.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures (Volume:31 , Issue: 3 )
Date of Publication: May 2013