Poster abstract

The co-doping process usually involves high concentrations of donor and acceptor impurities. This situation naturally leads to large random fluctuations of the impurity concentrations from place to place around the average values. These fluctuations in return cause a spatial modulation of the charged impurity potential. These potential fluctuations affect both the band edges and the impurity energy levels. As a result, the occupancy of deep impurity levels fluctuates, with a mean ionisation rate higher than the one obtained without fluctuations. We present a phenomenological analysis of the above effect by considering Gaussian fluctuations. The probability distribution of the potential has a standard deviation which is related to a screening length as formulated by Shklovskii and Efros for compensated semiconductors. The local density of states in the continuum is assumed to take its classical form. The average neutrality equation is then solved self consistently to yield the Fermi energy level. The total number of free carriers in the band is finally calculated. The results are plotted as a function of the parameters of the problem: concentration (Na) and ionisation energy (Ea) of the majority impurity, concentration (Nd) of the compensating impurity. A clear positive effect of co-doping appears in the high concentration range (Na = 10^19, 10^20 /cm3) and for deep impurity level (Ea = 0.25 eV), with a maximum doping enhancement occuring at Nd = Na/2. The increase in free carrier concentration due to co-doping is thus fifteenfold for Na = 1.5x10^20/cm3 and threefold for Na = 10^19/cm3. In conclusion, potential fluctuations at high doping levels could play an important role in the enhancement of dopant activation due to co-doping.