Realization of Cu-Doped p-Type ZnO Thin Films by Molecular Beam Epitaxy

Cu-doped p-type ZnO films are grown on c-sapphire substrates, by plasma-assisted molecular beam epitaxy. Photoluminescence (PL) experiments reveal a shallow acceptor state at 0.15 eV above the valence band edge. Hall effect results indicate that a growth-condition window is found for the formation of p-type ZnO thin films, and the best. conductivity is achieved with a high hole,Concentration of 1.54 x 10(18) cm(-3), a low resistivity Of 0.6 Omega cm, and a moderate mobility of 6.65 cm(2) V-1 s(-1) at room temperature. Metal oxide semiconductor capacitor devices have been fabricated on the Cu-doped ZnO films, and the characteristics of capacitance-voltage measurements demontrate that the Cu-doped ZnO thin films under proper growth conditions are p-type. Seebeck measurements on these Cu-doped: ZnO samples lead to positive Seebeck coefficients and further:, confirm the p-type conductivity. Other measurements such as X-ray diffraction, X-ray photoelectron., Raman, and absorption spectroscopies are also performed to elucidate the structural:and; optical characteristics of the Cu-doped p-type ZnO films. The p-type conductivity is explained to originate from Cu substitution of Zn with a valency of +1 state. However, all p-type samples are converted to n-type over time, which is Mostly due to the carrier compensation from extrinsic defects of ZnO.