Electron Irradiated InP: A Positron Annihilation Study
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ELECTRON IRRADIATED InP : A POSITRON ANNIHILATION STUDY. T. BRETAGNON, S. DANNEFAER AND D. KERR Department of Physics, University of Winnipeg R3B 2E9 Winnipeg, MB, Canada. Abstract : Positron lifetime measurements show that electron irradiation produces indium vacancy related defects in InP. Divacancies are also found in semi-insulating and lightly doped p and n-type materials. Temperature investigations show a change in the divacancy charge state. Introduction : The damages generated by electron irradiation in InP have been investigated with different techniques. DLTS measurements showed that in p-type materials electric active defects are produced with a high introduction rate [1]. These defects results from the perturbation of the phosphorus sublattice [2,3]. The phosphorus antisite P,. [4] and the phosphorus vacancy [5] have been characterized in electron irradiated InP by EPR measurements. However, no systematic investigation using positron annihilation technique have been published on the subject. The positron lifetime technique is a powerful technique in the detection of open volumes defects. It has been successfully used in the past few years for semiconductors [6,7]. However, most of the results published are centered around silicon and gallium arsenide. We have previously reported [8] that grown-in monovacancy type defects are present in InP independently of the type and impurities concentration of the samples investigated. In this communication a detailed study of defects generated by electron irradiation as function of doses in semi-insulating and lightly doped p-type materials is presented. In addition n-type and heavily doped materials are also investigated for a dose of 8xl0'7e/cm3 . It was found that divacancy defects are produced during electron irradiation. The concentration of the defect depends on the impurities concentration of the sample. Indium vacancies related defects are also introduced by irradiation. Experimental details: The positron lifetime measurements were performed using 22NaCl source evaporated onto a thin (0.2mg/cm2) aluminium film. The strength of the source ranged from 4 to 10iCi. The time resolution (width at half maximum) of the spectrometer was about 190ps. In order to achieve accurate decomposition of the lifetime spectra each of them contains at least 5x10 6 counts. The analysis were done using the program POSITRONFIT [9]. A source correction of 250ps with 3% intensity has been used for the aluminium foil contribution. The samples investigated are various type of commercial InP. Electron irradiation were performed at room temperature with an energy of 2.5 MeV using a pulsed beam. A He closed-cycle cryogenic system was used to vary the sample temperature between 30 and 325K. Positron trapping: In a perfect crystal positrons annihilate from Bloch states with a single lifetime Mat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society
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the so-called bulk lifetime. In non perfect crystal positrons can also annihilate from localized states formed at defect
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