Precise measurements of spatial density distributions of damages introduced into gap by MeV-electron beam irradiations based on its optical properties (I)

Abstract

GaP crystals were irradiated uniformly on their flat surfaces by 10 MeV-electrons. The “below-gap” absorption coefficient δαb(hv) and the normalized white-light optical density D/d in these samples increased linearly with a dose φ as δαb(2.0) = 3.3 × 10–16 φ and D/d= 1.42 × 10–16φ. The free electron density n in the conduction band estimated from the X1→X3 absorption band decreased with φ, and as the decrease in the free electron density δn is equivalent to the density of introduced defects N it could be expressed that N=δn = Rcφ where the value of the carrier removal rate Rc was 5.8 cm-1 for the S-doped sample. These expressions lead to the basic relation that N is proportional to D/d as expressed in N=4.1 × 1016 D/d.

Two-dimensional distributions of D/d were measured in the samples irradiated by collimated electron beams at 10 and 16 MeV by using a microdensitometer, and they were converted into the two-dimensional distributions of damage density according to the linear relation of N-D/d. Damages gradually spread laterally with increasing depth at first and then shrink. The equi-density profile of damages for the outside-sample is little more slender than that for the inside-sample due to the escape effect of electrons. The equi-density profile of damages for the sample irradiated at 16 MeV is considerably slender and extends deep compared with that irradiated at 10 MeV because 16 MeV-electrons are less scattered and penetrate deeper than 10 MeV-electrons.