Paper EN+SS+TF-ThA1
Synthesis of ZnO:F by DC Reactive Magnetron Sputtering

Thursday, October 21, 2010, 2:00 pm, Room Mesilla

Nowadays, numerous applications in the electronics and/or optoelectronics field need transparent thin films presenting a good electrical conductivity. The transparent conductive oxides (TCO) which reveal a large band gap and a good electrical conductivity fulfil these requirements. Recently, due to the significant increase of the demand, the prize of the most employed TCO, namely indium tin oxide (ITO) has strongly increased. Therefore, an alternative to this material becomes necessary. Among all the candidates, ZnO:F and Cd₂SnO₄ present the best performance in term of transparency and electrical conductivity. For obvious environmental reasons the latter cannot be considered. Therefore, ZnO:F is indentified as the best candidate to replace ITO in TCO applications.

The most employed techniques for the synthesis of ZnO:F are Chemical Vapor Deposition and Spray Pyrolisis which both require organometallic precursor and high temperature processing. Another drawback of these technologies is the low chemical purity of the synthesized films because of the presence of the precursor decomposition products. At the contrary, reactive magnetron sputtering is an environmentally friendly technology allowing the synthesis of thin films with very fine control of the chemistry. Therefore, the aim of this works is to study the reactive magnetron sputtering of ZnO:F.

Thin films were prepared by DC reactive sputtering using a zinc target in an Ar/O₂/F₂ mixture. In a first attempts, ZnO films have been synthesized in order to optimize the matrix properties in terms of cristallinity and transparency. The studied parameters were the DC power (P_DC), the total pressure (P_Tot) and the O₂ flow (f_O2). Our data reveal that the ZnO films presenting the best features are prepared for P_DC = 70 W, P_Tot = 30 mTorr and f_O2 = 3 sccm.

The second step was to introduce fluorine in this matrix. Therefore, we have studied the crystallographic, chemical, electrical and optical properties of the deposited films as a function of the fluorine content. In our deposition window, all films present a high transmission in the visible (> 80%). Our XRD data reveal decrease of the crystallite size with the increase of the fluorine content. Above a fluorine concentration of 2-3%, the ZnO:F cristallinity decreases. Our XPS and XRD data suggest that F atoms substitute O atoms in the ZnO structure. Finally, the electrical properties have been investigated by Hall effect measurements. For the optimal synthesis conditions (~ 2% of fluorine in the film), a charge carrier density of ~ 10²⁰ cm^-3, an electrical resistivity of 10^-2 Ω.cm and charge mobility of about 4 cm²/V.sec have been measured.