Low temperature electron trapping lifetimes and extrinsic photoconductivity in n-type silicon doped with shallow impurities

Abstract

The trapping lifetimes of conduction electrons photoexcited from shallow impurities have been measured in the liquid helium temperature range for samples of n-type silicon whose donor and compensating acceptor concentrations vary from 10 13 to 10 16 impurities/cm 3. A steady state method was employed, in which the conduction electron lifetime t L is equal to n e τ i / N o D . n e is the conduction electron concentration which is determined from low temperature photo-Hall measurements, and N O D τ i is the electron generation rate. The neutral donor concentration No is measured by the Hall effect and T i , the lifetime against photoionization of a neutral donor, is obtained directly by an electron spin resonance technique. The trapping lifetimes t L are generally independent of temperature between 4-2°K and 1–2°K, and are inversely proportional to the compensating acceptor concentration. This latter was determined using a recently developed combined infrared radiation and spin resonance method. The trapping cross-section result for phosphorus donors is about 5 × 10 −12 cm 2 at 3°K. This is about an order of magnitude larger than the value obtained from the giant trap theory of L ax. Also, the concentration independence of the cross-section in the region of temperature independent t L . is not easily accounted for. The mobility of electrons photoexcited with 2 micron radiation considerably exceeds the mobility of electrons photoexcited with 8–25 micron radiation. This suggests the possibility of excitation to another band or minimum by the 2 micron radiation, as is also suggested by the free carrier absorption peak near 2 microns found in n-type silicon by S pitzer and F an.