Semiconductor WO3 thin films deposited by pulsed reactive magnetron sputtering.

A pulsed reactive magnetron sputtering system with a tungsten target and a gas mixture of argon and oxygen was investigated as a source for the deposition of semiconductor WO 3 thin films on soda lime glass substrates and on the glass with transparent conductive SnO 2 :F (FTO) electrode. The reactive sputtering process was performed in HiPIMS mode with low pulse repetition frequency f p ≈ 50–100 Hz and short pulse duration in HiPIMS discharge T on = 100 μs. The second mode investigated was the mid-frequency (MF) magnetron discharge with pulse frequency f p = 40 kHz and pulse length T on = 15 μs. The plasma parameters were investigated for both HiPIMS and MF modes using the planar RF probe operating at the frequency f probe = 350 kHz and the grid QCM with biased collector electrode. Ion density n i and tail electron temperature (T e) were determined in both pulsed reactive magnetron sputtering discharge modes with time resolution. The maximum value n i ≈ 5 · 1017 m−3 was found in the reactive HiPIMS mode, and the maximum value n i ≈ 7 · 1016 m−3 was found in the reactive MF (40 kHz) mode. The degree of ionization of sputtered particles in reactive HiPIMS was determined for different values of (Q O2) and was found to be in the range of r i ≈ 0.1–0.3. The deposition rate determined by QCM in reactive HiPIMS was practically independent on (Q O2), but in the case of reactive MF, the measured deposition rate decreased significantly with increasing (Q O2). The WO 3 films deposited in both modes have a predominantly monoclinic crystal structure. The light and dark conductivity and the light/dark conductivity ratio (L d) were measured under dark conditions and UV light illumination. At higher (Q O2), the maximum value of L d ≈ 300 was found for MF deposited WO 3 and the maximum value of L d ≈ 30 was found for HiPIMS deposited WO 3. The photoelectrochemical measurement of WO 3 deposited on FTO electrodes confirmed the n-type conductivity, and these films functioned as photoanodes in photoelectrochemical cells. MF deposited WO 3 films systematically exhibited slightly higher photocurrents than HiPIMS deposited WO 3. It was shown that these optimum photocurrents for HiPIMS and MF were found at Q O2 ≈ 80 sccm and could not be improved by further increasing of (Q O2). [Display omitted] • HIPIMS and MF reactive pulsed sputtering were used to deposit semiconductor crystalline WO 3 films with photoresponse. • Plasma parameters in HiPIMS and MF sputtering systems were determined by RF probe and g-QCM. • HiPIMS deposited WO 3 films have significantly larger crystallites than MF deposited WO 3 films. • MF deposited WO 3 films have higher photoresponse and lower dark conductivity than HiPIMS deposited WO 3. [ABSTRACT FROM AUTHOR]