Zinc oxide (ZnO) has been the focus of intense investigation due to the unique properties it possesses. It is a transparent, wide band gap semiconductor (E_g~3.4eV) with high exciton binding energy (~60 meV). Its conductivity, if properly doped, approaches the high value of the widely used n-type indium-tin-oxide (ITO), and if it could also be reproducibly of p-type, ZnO would be the ideal material for many emerging transparent optoelectronic applications.
However, if ZnO is to be used as an active layer in optoelectronic devices, it must have electrical and optical properties within a wide range of values. Thus, it would be of paramount technological importance if ZnO films can be fabricated with tuned properties by adjusting the deposition parameters.
In this investigation the properties of undoped and Al-doped ZnO films fabricated by pulsed laser deposition (PLD) on UV-grade fused silica and Si substrates are presented. The ZnO and ZnO:Al(2%) target were ablated by a 248 excimer laser and the films were deposited in a wide range of PLD parameters, including substrate temperature (up to 500^oC), oxygen pressure (from 10^-2 to 10^-5 mbar) and at different target-substrate distances. Polycrystalline films having the wurtzite structure were fabricated, with carrier concentration between 10¹⁵ and 10²¹ cm^-3, resistivity from 100 to 3x10^-4 Ωcm and E_g spanning from 3.2 to 3.7eV. Sharp emission excitonic peak at 353.6nm was observed for highly conductive (3.3x10^-4 Ωcm) ZnO:Al films.
The properties of the ZnO films were investigated using atomic force microscopy (AFM), Hall-effect measurements and various spectroscopic techniques such as UV-NIR optical measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) as well as low temperature photoluminescence (PL) for identifying excitonic emission peaks and band gap defects. The use of the ZnO films, fabricated in this investigation, in optoelectronic devices like LEDs, TFTs, etc., will also be addressed.