p- and n-type doping of zinc oxide through electrochemical methods

Cu- and Cl-doped ZnO thin films were electrodeposited on conductive glass substrates in a non-aqueous electrolyte containing Cu, Cl and Zn salts. The Cu content of the films was found to be proportional to the [Cu2+]/[Zn2+] precursor ratio in the deposition electrolyte. Electrodeposited ZnO:Cu thin films exhibit hexagonal wurtzite structure with (002) preferred orientation. A transition from n-type to p-type was observed for ZnO:Cu films with a [Cu2+]/[Zn2+] ratio higher than 2% as inferred from the change in the direction of the photocurrent. Further, by the addition of different amounts of chlorine ions in the starting electrolyte results in an effective n-type doping of electrochemically synthesized ZnO thin films. The ratio between chlorine and zinc cations was varied between 0 and 2 while the zinc concentration in the solution was kept constant. When the concentration of chloride in the bath increases an effective n-type doping of ZnO films takes place. n-type doping is evidenced by the rise of donors concentration obtained from Mott-Schottky measurements as well as from the blue shift observed in the optical gap owing to the Burstein-Moss effect. When electrodepositing ZnO:Cu on ZnO:Cl layers a homojunction device is formed so evidencing the p- and n-type respective character of the layers. The rectifying characteristics of the ZnO:Cu/ZnO:Cl homojunction further confirm the p- and n-type behavior of ZnO:Cu and ZnO:Cl layers, respectively.