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The quantum efficiency and frequency response of simple Ni-Si-Ni metal-semiconductor-metal (MSM) photodetectors at long wavelengths are significantly enhanced with a simple, ion-implantation step to create a highly absorbing region /spl sim/1 μm below the Si surface. The internal quantum efficiency is improved by a factor of /spl sim/3 at 860 nm (to 64%) and a full factor of ten at 1.06 μm (to 23%) as compared with otherwise identical unimplanted devices. Dark currents are only slightly affected by the implantation process and are as low as 630 pA for a 4.5-μm gap device at 10-V bias. Dramatic improvement in the impulse response is observed, 100 ps vs, 600 ps, also at 10-V bias and 4.5-μm gap, due to the elimination of carrier diffusion tails in the implanted devices. Due to its planar structure, this device is fully VLSI compatible. Potential applications include optical interconnections for local area networks and multi-chip modules.