Evidence of Defect Levels in p and n-Type Electron Irradiated Si
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EVIDENCE OF DEFECT LEVELS IN p AND n-TYPE ELECTRON IRRADIATED Si S.D.KOUITfZI, J.J. Thomson Physical Laboratory.University of Reading,Reading,U.K. versity of Thessaloniki,Thessaloniki,Greece.
and Uni-
ABSTRACT Capacitance bridge measurements at low temperature on irradiated specimens of p and n-type Si are reported. Elec-tronic defect levels are seen at Ev + 0.10 eV , Ev + 0.14 eV and Ec - 0.11 eV. The change of the band structure caused by irradiation is also described. INTRODUCTION In a previous paper [1] we used capacitance bridge measurements to monitor radiation defects in electron irradiated Si from 4.2K. The method is used here to explore defect levels around Ev + 0.10 eV , Ev + 0.14 eV and Ec - 0.11 eV. Experimental procedure 14
Floating zone and pulled specimens having boron concentrations of 5xl0 and 5x10 1 6 atoms cm- 3 respectively were electron irradiated at 1.5MeV at temperatures not higher than 11K. At the end of the irradiations,the samples were warmed up to about 160K to induce mobility of the introduced vacancie 4 [2]. The n-type pulled specimens had a phosphorus concentration. of 3xl0 cm'. The defects introduced into Si specimens are monitored by bridge measurements of capacitance C and loss factor D formed by the evaporation of gold electrodes on opposite faces of a thin slab of the specimen which had a thickness ofZ40um. Following McKeighen and Koehler [3,4] we obtain for reasonably low D the a.c. conductivity oac =6,395 f'Dwhere f'is the frequency of the bridge oscilator. The results of the measurements between 4.2K and ~100'.K or above are attributed in two different temperatur.e regimes to (a) a silicondielectric capacitor with loss arising from a.c. hopping,for T20K. On such a model changes of the effective mass at the band edges are expected to be pictured as changes of the conductivity. Thus at the transition temperature we would expect the conduction to increase not only because the concentration of carriers increases but mainly because their effective mass becomes smaller (Fig,l). As tunnelling to the upper band occurs the rate, at which the energy Vs k vector changes (that is the effective mass),will change sign,the conductivity should decrease and eventually will settle to a low value determined by heavier effective mass. The changes of the effective mass,at the transition temperature,between the two regimes (Fig.2, Curve A) result in the large peak of aac while the capacitance rises to the value determined by the barrier thickness ( Fig, 3, Curve A). The peak aac may be modified in shape and temperature of occurrence by irradiation. The conductivity below the transition temperature has been observed to be frequency dependent [3] while above that temperature,it becomes temperature independent as evidenced by the bulk conduction. As the concentration of the induced defects increases the valence or the conduction band extendes into the band gap [5] ,the rate of change of the derivative of the energy with respect to the k vector decreases and the peak in the curve of the conductance V
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