Electronic Structure of Copper Impurities in ZnO

We have measured the near infrared absorption, Zeeman effect, and electron spin resonance of Cu²⁺ ions introduced as a substitutional impurity into single-crystal ZnO. From the g values of the lowest Γ₆ component of the T₂ state (the ground state), gII=0.74 and g_⊥=1.531, and from the g values of the Γ₄Γ₅ component of the E state, gII=1.63 and g_⊥=0, we have determined the wave functions of Cu²⁺ in terms of an LCAO MO model in which overlap only with the first nearest neighbor oxygen ions is considered. These wave functions indicate that the copper $3d$ (t₂) hole spends about 40% of its time in the oxygen orbitals, and that the copper t₂ orbitals are expanded radially with respect to the e orbitals. Corroboration for the radial expansion of the t₂ orbitals is obtained from an analysis of the hyperfine splitting. It is concluded from our model that the large values of the hyperfine constants, |A|=195×10^-4 cm^-1 and |B|=231×10^-4 cm^-1, are due to the contribution from the orbital motion of the t₂ hole.