Abstract
Co-doped LEC InP was studied using resistivity and Hall measurements, thermal and optical capacitance spectroscopy, photoluminescence (PL) and PL-excitation (PLE) techniques. The derived activation energy for n-type conduction is about 0.50 eV in semiinsulating InP:Co samples of resistivity 106 to 107 Omega cm. The same level at Ec-0.53 eV is found in high concentration in DLTS measurements. A hole trap at Ev+0.3 eV in similar concentrations is detected in optical DLTS and photocapacitance experiments. Relationships between optical cross sections for transitions to the valence band and to the Gamma and L regions of the conduction band are derived for the 0.53 eV level. Co is shown to substitute for In in InP from the observation of the 4T2(F)4 to 4A2(F) (F) crystal field transition of Co2+ (d7) at 474.0 meV in photoluminescence. Furthermore, PLE is used to show that the Co2+ charge state is presented in n-type and semi-insulating InP:Co from observation of the 4A2(F)-4T1(F) transition of Co2+ at 785.8 meV. M From these results and the electrical and capacitance measurements, the level at Ec-0.53 eV is identified with the transition Co2+ to Co3++e, whereas the level at Ev+0.3 eV is attributed to either Co4 + to Co3++h or to a transition involving a Co-related complex. The trends of the ionisation energies of 2+ charge states of transition metals in III-V semiconductors are discussed and it is concluded that the value for Co2+ in InP (0.53 eV) is anomalously low.