Deep levels in Co-doped InP

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 10 6 to 10 7 Omega cm. The same level at E c -0.53 eV is found in high concentration in DLTS measurements. A hole trap at E v +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 4 T 2 (F) 4 to 4 A 2 (F) (F) crystal field transition of Co 2+ (d 7 ) at 474.0 meV in photoluminescence. Furthermore, PLE is used to show that the Co 2+ charge state is presented in n-type and semi-insulating InP:Co from observation of the 4 A 2 (F)- 4 T 1 (F) transition of Co 2+ at 785.8 meV. M From these results and the electrical and capacitance measurements, the level at E c -0.53 eV is identified with the transition Co 2+ to Co 3+ +e, whereas the level at E v +0.3 eV is attributed to either Co 4 + to Co 3+ +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 Co 2+ in InP (0.53 eV) is anomalously low.