Metal-Insulator Transition Induced by Shallow Donor Impurity in n-ZnSe

Abstract

The resistivity and Hall coefficient of n-ZnSe monocrystals were investigated in the region of the metal±insulator (M±I) transition induced by variation of shallow donor impurity Al concentration from 2 Â 10 16 to 2 Â 10 18 cm À3 . It has been found that the transition from an activated conduction to a metallic one occurs at donor concentration 1X3 Â 10 17 cm À3 and takes place within the impurity band before it merges with the bottom of conduction band. On the dielectric side of the M±I transition the conduction with an activation energy e 1 induced by electron excitation from the impurity states into the conduction band is replaced by the hopping conduction with a constant activation energy e 3 which, in the lowest temperature region, is changed to a hopping conduction with variable activation energy.The principal feature of the theory of metal±insulator (M±I) transitions is the variation of the metallic conductivity as well as other parameters near the transition at zero temperature. Starting from the qualitative analysis of conductivity in the Anderson model, Mott [1] has predicted a sharp drop of metallic conduction to zero at the M±I transition and therefore the existence of a minimum metallic conductivity s mm lim n3n c s0. Cohen [2] advanced an alternative of a continuous attenuation of metallic conduction to zero at the M±I transition starting from the analogy of localization with percolation [3]. To investigate the M±I transition in doped semiconductors it is convenient to use the axial compression or a strong magnetic field which allow us to vary the radius of the electron wave function without changing the real concentration of impurities and the character of their distribution in the crystal [4 to 6].In this paper an attempt is made to obtain a M±I transition in ZnSe, in which the Bohr radius of the impurity is about 2.7 nm due to a certain change of the shallow donor concentration (aluminium impurity). With that in view, the n-ZnSe samples were Al-doped by long-term (100 h), high-temperature (T 950 C) treatment of the originally high-ohmic (r % 10 8 to 10 10 W cm) monocrystals in the Al + Zn alloy with varying aluminium percentage (from 0.05 to 1.25 at.%). The crystals were annealed in sealed ampoules at % 1X33 Â 10 À2 Pa, which were placed in a vertical furnace. In the temperature range from 1.6 to 300 K the temperature dependencies of resistivity, Hall coefficient and electron mobility are investigated in samples with concentration of shallow donor impurity varying within the limits of 2 Â 10 16 and 2 Â 10 18 cm À3 . In Fig. 1, a series of curves of the temperature dependence of resistivity for the investigated n-ZnSe samples is given. It is seen that as the donor concentration increases the slopes of the low-temperature and high-temperature portions of the rT dependence

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