Deposition of zinc oxide by spray pyrolysis

Abstract

The objective of this work was to grow doped ZnO by spray pyrolysis at low temperature. This was achieved via the initial objective of growing ZnO in unhoped-for over a wide range of conditions, in order to understand the growth behaviour of ZnO from Zn(acac)2.H(_2)O and to establish the optimum growth procedure at high and low temperature. Various techniques were employed to characterize the films and thus determine the optimum growth conditions (i.e measurement of film thickness, resistivity, Hall coefficient. X-ray diffraction and reflection electron diffraction to name but a few).With the growth of undoped films it was found that the film properties varied with temperature and moisture content and that the optimum conditions for low temperature growth in a dry ambient were at 200ºC and for high temperature growth in a wet ambient at 300ºC.An analysis of the growth behaviour of ZnO was carried out and it was suggested that there were at least four mechanisms leading to the decomposition of the precursor used (Zn(acac)2.H(_2)O). They were decomposition by miramolecular, mtermolecular, thermolysis and hydrothermolysis processes. A kinetic analysis demonstrated that evaporation was the dominant process which reduced the efficiency of utilization of Zn(acac)(_2).H(_2)O. The growth of doped ZnO in glass and plastic at low temperature using InChl(_3) as a dopant yielded conducting films. The results also showed that films obtained using solutions with low concentrations of Zn(acac)(_2).H(_2)O and high concentrations of InCl(_3) were even more conducting ( p ≈ 10(^-5)Ωm). The morphology of film growth was dominated by the presence of dopant. High temperature growth of doped ZnO in a wet ambient using InCl(_3) also yielded conducting films and these were compared with indium, aluminium and gallium doped films where alternative dopant matericds such as In(acac)(_3), Al(OPr)(_3), AICL(_3) and Ga(acac)(_3) at a variety of different solution concentrations had been used. This was undertaken to discover whether these materials functioned as well as InCl(_3) in producing low resistivity ZnO. Doped films were characterised using the same techniques as before. Elemental analysis, photoluminescence and optical measurements were also carried out on these films. The main conclusions were that : (1) The growth rate of ZnO from Zn(acac)2 .H2O is heavily influenced by the growth temperature and other conditions. (2) The film resistivity was influenced by growth temperature. A minimum in the film resistivity was observed when a growth temperature of 300ºC was used. (3) Undoped ZnO films grown below 200ºC had a different preferred order to those grown above 200ºC.(4) The best high temperature conditions for the deposition of undoped conducting adherent ZnO lay in the region of 276 - 306ºC. (5) The best low temperature conditions for the growth of undoped conducting ZnO were in the region of 200ºC. (6) The low temperature growth (175-200ºC) of doped ZnO produced films with resistivities of the order of l-5xl0(^-5)Ωm and a visible transmittance of 80%. This compares favourably with ZnO:Al and SnO(_2) which have also been grown at low temperature.