The surface quality of lead halide perovskite crystals can extremely influence their optoelectronic properties and device performance. Here, we report a surface engineering crystallization technique in which we in situ grow a polycrystalline methylammonium lead tribromide (MAPbBr) film on top of bulk mm-sized single crystals. Such MAPbBr crystals with a MAPbBr passivating film display intense green emission under UV light. X-ray photoelectron spectroscopy demonstrates that these crystals with emissive surfaces are compositionally different from typical MAPbBr crystals that show no emission under UV light. Time-resolved photoluminescence and electrical measurements indicate that the MAPbBr film/MAPbBr crystals possess less surface defects compared to the bare MAPbBr crystals. Therefore, X-ray detectors fabricated using the surface-engineered MAPbBr crystals provide an almost 5 times improved sensitivity to X-rays and a more stable baseline drift with respect to the typical MAPbBr crystals.