Carrier lifetime measurements from transient electrical photoresponses

Abstract

Carrier lifetime measurements from photoconductivity (with constant current and constant voltage polarizations of the sample), diffusion photocurrent and open circuit photovoltage responses to transient carrier excitation are compared. Using analytical expressions, the key expressions, conditions of validity and applications of each method are discussed, assuming uniform carrier injection in an unidimensional sample of homogeneous material; the discussion is done taking into account the sample characteristics (width W, bulk lifetime τv, surface recombination velocities s1,2, minority carrier diffusion coefficient D, equilibrium majority carrier density p0), excess carrier density Δp, light pulse duration T and times t of the response considered. The experimental study especially deals with silicon samples up to W = 1 mm thick, illuminated by T = 2 ns to 30 ns laser beams. The responses to penetrating light (λ = 1.06 μm, α = 40 cm-1 in ordinary silicon, Δp = 10-4 to 102 p0) and strongly absorbed light (λ = 0.69 or 0.53 μm, α = 2 x 103 and 10 4 cm-1, beam intensity E0 = 10-4 to 103 mJ/cm2) are used to show the conditions under which the photoresponses observed lead to the same measured values of carrier lifetime and give bulk lifetime measurement