The effect of Si co-doping on electrical, optical and spectroscopic properties of In-doped CdTe was investigated. The concentration of Si atoms in the charge was 1×1017 cm−3. All Si co-doped samples were n-types, with the resistivity higher than 1×109 Ω cm. The dominant deep level ED=0.67 eV was determined by temperature dependence of the Hall effect measurement and compared with the low-temperature photoluminescence. Cd-rich or Te-rich annealing was used to eliminate this deep level, which strongly affects the charge collection efficiency of samples. The deep level together with a poor charge collection efficiency were found in both as-grown or annealed Si co-doped samples contrary to samples with only In doping, where the detector quality improvement was observed after Te-rich annealing.