Characteristics of Pulse Electrodeposited CuAlSe2 Films

CuAlSe₂ thin films were deposited by the pulse plating technique using non aqueous ethylene glycol solution. 0.4 M Al₂(SO₄)₃, 0.1 M CuSO₄, 0.1 M SeO₂. The films were deposited at 80°C and at different duty cycles at a constant current density of 5 mA cm^-2. Tin oxide coated glass (5 ohms/sq) were used as substrates. Thickness of the films measured using Mitutoyo surface profilometer was in the range of 800 nm – 1400 nm with increase of duty cycle from 6 – 50 %.  The films were characterized by Xpertpanalytical x-ray diffraction unit with Cukα radiation. Composition of the films was estimated by EDS attachment to JOEL SEM.  Electrical measurements were measured by providing gold ohmic contacts at the four corners of film surface. Hall – Vander Pauw geometry was used for the electrical measurements. Photoelectrochemical cell measurements were made with 1 M polysulphide (1 M S, 1 M Na₂S, 1 M NaOH) as the redox electrolyte. 250 W tungsten halogen lamp was used as the light source.

The X-ray diffraction pattern of CuAlSe₂ films formed at different duty cycles is shown in Fig.1. The films were polycrystalline exhibiting the peaks corresponding to the single phase CuAlSe₂. Peaks corresponding to (112), (220), (204), (312) and (116) orientations of the chalcopyrite structure were observed (JCPDS No.44-1269). The evaluated lattice parameters were around a = 5.62 Å and c = 10.99 Å . The crystallite size was calculated from the Full width half maximum of the diffraction profiles using Scherrer’s equation. The crystallite size increased from 15 nm – 40 nm as the duty cycle decreased from 50 % - 6 % .

Composition of the films was determined by the Energy dispersive x-ray analysis (EDAX) attachment of the Scanning Electron Microscope (SEM). At low duty cycles, more concentration of Cu and Se are deposited due to the fact that Cu and Se are more noble compared to Al, also since the ON time is less, low concentration of Al is co-deposited along with Cu and Se. As the duty cycle is increased, due to the longer ON times, more aluminium is deposited along with Cu and Se, hence the aluminium concentration increased. Hence, at lower duty cycle, the Cu/Al ratio is greater than unity. With increase of duty cycle, due to the increase in concentration of Al, Cu/Al ratio approaches unity.

The room temperature transport parameters were measured by Hall Van der Pauw technique by providing gold ohmic contactThe films exhibit p-type conductivity. The EDAX results support the p-type conductivity, since, the non-stochiometry parameter is greater than zero. The magnitude of the resistivity increased from 0.10 ohm cm to 3.67 ohm cm as the duty cycle is increased. The variation in resistivity with duty cycle can be explained in terms of the Cu/Al ratio obtained from EDAX measurements. At low duty cycles, a high Cu/Al ratio is observed. At 50 % duty cycle, Cu/Al ratio is unity due to the decrease of copper concentration and increase of Al coincentration,  hence as the concentration of copper decreases, the resistivity increases. The values of carrier density and mobility are one order lower than single crystalline CuAlSe₂ films.

The transmission spectra of the CuAlSe₂ films deposited at different duty cycles. The spectra exhibit interference fringes and the value of the refractive index was estimated by the envelope method. The value of the refractive index at 350 nm, calculated from the above equations was in the range of 3.10 - 2.65 for the samples deposited at different duty cycle. These values coincide with the values  obtained on CuAlSe₂ single crystals. The films exhibited a high absorption co-efficient of the order of 10⁴ cm^-1. A plot of (αhν)² against hν, exhibits linear behavior near the band edge, the band gap of the deposited films was determined to be in the range of 2.50 – 2.75 eV. At lower duty cycles, viz., 6 % and 15 %, the band gap was lower than the reported value of band gap for CuAlSe₂, due to the fact that the films were Cu rich. As the duty cycle increased to 33 %, the films were nearly stochiometric and the band gap value was 2.65 eV. Further increase of duty cycle to 50 % resulted in increase of band gap to 2.75 eV. This may be due to the films being Al rich. The band gap of CuAlSe₂ thin film prepared by thermal evaporation technique was in the range of 2.66 eV - 2.91 eV.