InP1-yAsy epitaxial layers grown lattice-mismatched (LMM) on InP substrates were investigated as a new materials system for multiplication layers in Geiger-mode avalanche photodiodes (GM APDs) for detection of photons in the range 1.6-2.5 mm. LMM InP1-yAsy epilayers were grown on semi-insulating (1 0 0) InP substrates misoriented 0.2 and 2 [1 1 0] by organometallic vapor phase epitaxy at a growth temperature of 580 1C. The growth scheme used for the InP1-yAsy buffer layer was optimized based on surface step structure and X-ray diffraction. It was found that step-flow growth is a minimum criterion for obtaining good material quality. A narrower XRD full-width at half-maximum values were measured for 21-miscut substrates compared to 0.21-miscut substrates. A highquality buffer was obtained by step-grading the InP1-yAsy composition in increments of y = 0.05 over a layer thickness of 0.5 mm to a final y = 0.25. The device performance of LMM GM APDs was compared to that of measured more traditional lattice-matched GaSbbased devices. At 77 K, dark count rates of LMM devices are ~50 kHz at 5V overbias, and are comparable to GaSb-based p-i-n diodes operated in Geiger mode, while reset times of 0.02 ms are approximately 3 orders of magnitude lower than GaSb-based GM APDs.