It is shown that, in pure silicon, the recombination time of photocreated excess carriers depends on the relative spin orientation of the carriers and of the recombination centers. This is evidenced by the observed decrease of the photoconductivity when the magnetization of the recombination centers is reduced. Several experiments show independently that the spin-dependent recombination mechanism involves surface states: (i) Reducing the magnetization of the recombination centers through spin resonance leads to a resonant change of the height of the surface potential barrier. (ii) The size of the effect on photoconductivity increases when surface recombination is favored with respect to bulk recombination. The change in photoconductivity when the magnetization of the surface recombination centers is saturated, provides a means to observe the spin resonance of these centers with a good signal-to-noise ratio, whereas the conventional electromagnetic detection does not yield any observable signal. One thus obtains an order of magnitude for the spin-lattice relaxation time of the centers $T_1\approx {10}^{-6\mathrm{}}$ sec and an upper limit for their density $N_t\lesssim \frac{{10}^{12}}{{\mathrm{cm}}^2}$.

• Received 21 January 1972

DOI:https://doi.org/10.1103/PhysRevB.6.436