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The power conversion efficiency of cryogenically cooled InP-based diode lasers is limited by excess electrical voltage caused by the freeze-out of holes at low temperature. Hall-effect measurements are performed to determine the ionization energy of Zn in bulk InP and In0.90Ga0.10As0.24P0.76 (the values obtained are 18.6 and 11.6 meV, respectively). A laser design with an InGaAsP p-cladding layer shows a large decrease in the 77 K voltage defect relative to a more traditional InP design. Peak conversion efficiency of 73% and >10-W maximum power are reported at 1493 nm from a single 200-μm stripe laser operating at 77 K.