NUCLEON IRRADIATION OF SILICON SEMICONDUCTORS

THOMAS ANTHONY LONGO, Purdue University

Abstract

There is great interest in the effects of high energy nucleon irradiation matter. In particular, the physical properties semiconductors have been found to be very sensitive the to disorder introduced by energetic nucleon irradiation. However, most of the previous information had been obtained for germanium. Therefore, the object of this work was to investigate the effects of nucleon irradiation on the electrical and optical properties of silicon. After fast neutron irradiation, the inverse temperature dependence of the logarithms of Hall coefficient and resistivity of both n-type and p-type single crystal silicon samples indicated that intrinsic behavior was approached in each sample" A 1.75fμ peak the optical absorption coefficient was observed. This peak is believed to result from optical excitation rather than ionization of an introduced defect level since it is not accompanied by an increase photoconductivity. Appreciable photoconductivity was observed beyond the fundamental region to about 1.45 microns. Annealing experiments were continued on a polycrystalline p-type silicon sample which had been previously irradiated with fast neutrons. After annealing at temperatures ranging from 150°C to 25°C, the slopes of the logarithms of Hall coefficient and resistivity vs. 103/T decreased, suggesting the rearrangement of introduced defect levels accompanying annealing. During annealing, the absorption peak at 1.75μ gradually disappeared and an absorption tail extending past 30μ increased. Quantitative information was needed in order to establish the nature and numbers of the defects introduced by the irradiations. Therefore, n- and p-type silicon were irradiated with 9.6 Mev deuterons from the Purdue cyclotron. The Hall coefficient and conductivity of degenerate and non-degenerate samples were measured as a function of irradiation. The carrier concentrations of all samples were reduced by irradiation. For n-type degenerate samples, the change in carrier concentration with flux Δn/Φ = .670 electrons cm per deuteron and for p-type degenerate samples Δp/Φ = -750 holes cm per deuteron. These compare favorably with a calculated rate of defect pair introduction ΔN/ΔΦ = 775 defect pairs cm-1 per deuteron, with one carrier removed per pair, This work supports the belief that the introduction of close vacancy interstitial pairs is the predominant effect of irradiation. The changes of carrier concentration with flux decreased rapidly as the Fermi level moved into the forbidden band. The interstitial states appeared to be at 0.025 ev from conduction band and the vacancy states at 0.055 ev from the valence band. For heavy irradiations, the removal rates were lower in the degenerate and higher in the non-degenerate regions than what was observed for smaller irradiations. This suggested clustering of defects and the states associated with these clusters appeared to be introduced deep in the forbidden gap.

Degree

Ph.D.

Subject Area

Physics

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