Low-temperature characteristics of buried-channel charge-coupled devices

The characteristics of buried-channel CCDs in the temperature range between 40 K and 300 K were studied. It was found that the transfer loss had a peak at about 130 K and increased rapidly below 90 K. In these specific temperature regions, the transfer loss depended on the clock period, the data cycle time and the clock fall time. The characteristics were analyzed based on free-charge transfer mechanisms and the Shockley-Read-Hall theory. The peak at 130 K was caused by a trap at the energy level 0.22 eV below the conduction band, and the increase below 90 K by the carrier freeze-out. Furthermore, the reduction of impurity concentration in the buried channel turned out to be an effective method of improving the transfer efficiency below 90 K.