Special features of recombination processes in silicon doped by palladium

Full text: Silicon photoelectric properties, processes of generation and recombination of charge carriers depend mainly from contaminant and radiation centers parameters which form of the deep levels in the forbidden band. The particularities of generation and recombination of charge carriers in silicon doped with palladium which creates the deep levels E1 = Ec - 0,18 eV, E2Ev+ 0,34 eV of acceptor and E3 = Ev+ 0,32 eV of donor nature in the forbidden band are investigated in the work. The possible mechanism is analyzed of reduction in the photo- and radiation sensitivity depending on Fermi level position in the forbidden band of Si. The single-crystal silicon plates of n and p-type conductivity with 0,3-100 Ohmxcm specific resistance, with ∼104 cm-2 dislocation density and with ∼ 1018 cm-3 oxygen content were used. Doping of the silicon plates by palladium was done in the 1000-1280 C temperature range during 5-10 hours. Irradiation was carried out by the reactor neutrons up to 3x1015 cm-2 fluences. The recombination properties of the doped samples were studied by measuring of the life time (τ) of the minority charge carriers by stationary photoconductivity (SPC) method and by control of the recovery time (τrec) of the diode reverse resistance from the transient characteristics (TC). It is revealed from the values measurements that the selectivity of SPC method in contrast to TC makes possible to study of the special features of recombination of charge carriers at the deep levels having amphoteric nature. It is possible to measure the value over wide limits both for the acceptor and for donor levels by changing the Fermi level position due to the regulation of the compensation degree for silicon. It was proved from the analysis of the dependence of τon Fermi level (EF) position that value is determined mainly by the acceptor level E1 when EF < E1. The level E1 is depleted in proportion to the displacement of Fermi level to middle of the forbidden band and becomes as trapping level having an essential effect on kinetics of photoconductivity. In this case the recombination is determined by the ground levels E2 and E3 and by the value τ-1=τ2-1+τ3-1. It is established from the studies of the capture cross section of the charge carriers of recombination centers in silicon with different degree of compensation, that the presence of the Pd donor and acceptor levels in the lower half of the forbidden band of overcompensated silicon from n- type into p- type (n p-Si) and compensated p-Si lead to considerable reduction in their photo- and radiation sensitivity. Physical mechanism is proposed which explains reduction of the photo- and radiation sensitivity in the indicated samples. This mechanism is the following: n p-Si samples contain E2 acceptor level with concentration, E3 donor level, deep levels of radiation origin with the total concentration Na all in the lower half of the forbidden band, and also shallow donor level with the concentration Nam . The acceptor level is partially ionized at Nam < Na, and the donor levels are neutral. The holes can be trapped by E2 acceptor level at illumination of the doped sample by light with hν≥ Eg. In this case, the rate of the repeated thermal excitation of these holes into the valence band will be equal where, ν- frequency of thermal vibrations, equal to 109 s-1. However, because of the high recombination rate of these holes after the E3 palladium deep donor level and levels of radiation defects, the time t, when a hole is trapped at the level, is comparable with the transit time td = L2/μpV of electron between two resistance contacts of photoconductor, which leads to sensitivity reduction of overcompensated Si. On the basis of the obtained experimental results and analysis of the τ values of silicon doped by palladium the conclusion is done that the photo- and radiation sensitivity of Si is determined by concentration and recombination parameters of Pd acceptor level and of radiation defect levels. This work was supported by STDCC (Uzbekistan) No FA-F2-F066+F072. (authors)