Zinc oxide (ZnO) is a highly applicable and widely used II-IV semiconducting material with several attractive properties such as the wide band gap (3,37eV), high chemical stability and good photoelectric and piezoelectric properties. It is used in various technological domains such as surface acoustic wave devises, gas sensors, optical devices and transparent window layer in CIS, CGS solar cells. Various transparent conducting oxides (TCO) such as ITO.and SnO2 have been reported for their solar cell applications but ZnO has been considered to be the most promising TCO material in view of cost and properties. Many techniques have been used to deposit ZnO films such as sputtering, evaporation, chemical vapour deposition (CVD), sol-gel, molecular beam epitaxy (MBE) and pulsed laser deposition PLD. Among these techniques, PLD is a very efficient method to deposit high quality and well crystallized films at lower temperature than other methods. This is due to the high energy of the ablated particles in the laser-produced plasma plume.ZnO films were deposited on glass substrates employing a typical homemade PLD system. All glass substrates were cleaned in an ultrasonic bath with acetone for 10 min before deposition. A UV pulsed KrF excimer laser operated at 248nm with pulse frequency 10 Hz, pulse duration 10 ns and fluence 2.6 J/cm 2 was used to evaporate the metallic Zn target. The metallic zinc disk (Aldrich 99,9% purity), with diameter 2,5cm and thickness 0,5cm was placed parallel 5 cm away from the substrate. To avoid fast drilling, the target was placed on a vacuum compatible, computer-controlled XY stage synchronized with the pulsed laser. The deposition chamber was initially evacuated to a base pressure 10 -4 Pa, the substrate temperature was determined in situ during growth with a thermocouple placed below the substrate. At a fixed deposition time of 2 hours, the ZnO films were deposited at the substrate temperatures of 100 and 300 o C with oxygen partial pressures (flow) of 5, 10 , 20 , 30 , 40 and 50 Pa.
Results and DiscussionThe increase of substrate temperature is shown to increase the ZnO (002) diffraction peak intensity (figure 1). It is believed that the temperature rise accelerates the migration of adatoms to the energy favorable positions, resulting in the enhancement of crystallinity of the ZnO films.The FWHM value of (002) peak is observed to decrease with increasing oxygen pressure from 5 to 50 Pa (figure 2). ZnO is always deviated from stoichiometry and presents intrinsic defects such as zinc interstitials and oxygen vacancies, especially at Zn-rich or O-deficient atmospheres. The number of these intrinsic defects could be decrease by the increase of partial-oxygen pressure, so the ZnO film becomes better crystalline [1].The surface roughness of ZnO films (figure 3) is seen to increase with increasing oxygen pressure during deposition. This tendency is similar to other reported investigations [2, 3].For the selected experimental conditions all the produced ZnO films show high transmission up to 90...