Effects of temperature on photoconductivity in n-type cubici Bi12SiO20

In the only study to our knowledge of the photorefractive effect in n-type cubic Bi12SiO20 (n-BSO) at low temperatures (300 to 80 K), optical four-wave mixing via the photorefractive effect was found to decline over one order-of-magnitude as the temperature was lowered from ~ 300 K to below ~ 200 K.1 The mixing returned to normal upon warming. To better understand this unexpected result, we have undertaken both pulsed and dc photoconductivity studies at various temperatures on the same n-BSO crystal, called "CT1" (Crystal Technology, Inc., 1984), as well as on other n-BSO crystals. The photocurrent excited by a nanosecond or picosecond optical pulse is known to decay non-exponentially in time, with characteristic times in the nanosecond and microsecond regimes.2 In CT1 we see further long decay tails (~ seconds) at room temperature. However, in another n-BSO crystal called "SU1" (Sumitomo Corp., 1986), this long tail is essentially absent.