Sol Gel Dip Coated CdO : Ga Films

Cadmium oxide (CdO) is a n-type semiconductor with nearly metallic conductivity and has a direct band gap of 2.3 eV with two indirect transitions at lower energies. The n-type conductivity is attributed to its native oxygen vacancies and cadmium interstitials; which can be further increased by controlling these native defects or by doping with different metallic ions. In this work, CdO:Ga films were deposited by the sol gel dip coating technique.

CdO films were deposited on clean glass substrates at a temperature of 70°C. Prior to deposition, the glass substrates of size 2.5 x 7.5 cm² were boiled in chromic acid for one hour followed by rinsing in triple distilled water three times and finally cleaned in acetone by ultrasonic agitation. The precursor solution consisted of AR grade cadmium chloride and gallium chloride. The pH of the solution was adjusted to 8 by adding AR grade ammonium hydroxide. To this AR grade acrylamide was added and the mixture was heated to 70°C. N,N bis acrylamide was then added and the solution was stirred for about 10 minutes. A small amount of ammonium persulphate was added slowly to this mixture. Just before formation of the gel, the cleaned glass slides were dipped in the solution and withdrawn at a rate of 1cm/s. The coated glass substrate was then dried in air for about 10 min after removing the tape from the uncoated side. The films were doped with different concentrations of Gallium by adding different concentrations of GaCl₃ in the range 1 – 10 at %. The dried films were heat treated at 450°C for 60 min. Thickness of the films was in the range of 400 – 600 nm.

X-ray diffraction (XRD) patterns of the prepared undoped CdO and CdO: Ga films show polycrystallinity of cubic CdO structure. The lattice constant calculated for a undoped CdO sample was 0.469 nm, which is almost identical with those values reported.

The intensity of [Ga], [O] and [Cd] is in increasing order and the highest peak intensity has been observed for [Cd]. The fact that the [O]/[Cd] ratio is smaller than unity confirms the presence of oxygen deficiency in GCO films, which serves as donors and are responsible for n- type conductivity of GCO thin films. With increase in concentration of Ga in the films, the [O]/[Cd] ratio reduces and causes an increase in the [Ga]/[Cd] ratio.

The average transmittance of 75% in the wavelength range of 550–800nm is observed. The behavior of high light absorption (poor transmittance and reflectance) in respective spectral region of 350–450nm can be assigned to the mechanical stress in the films produced by impurities and structural defects. The direct gap determined by extrapolating the linear portion of Tauc’s plot varies from 2.35 to 2.45 eV. This value agrees with the earlier reports.

The value of resistivity for undoped CdO found in the present work is larger than those values mentioned in some other references because of different method of preparation. The obtained electrical parameters and their variation with doping level can be explained by considering that, the dopant Ga ions added to CdO film can either adsorb in the grain boundaries or very limitedly enter into the crystalline structure of CdO occupying interstitial positions. When the Ga content level was increased to 7%, more Ga³⁺ ions were obligated to enter into the crystalline structure of CdO occupying interstitial positions which leads to reduction of carrier concentration.

The films obtained in this investigation can be used in photovoltaic devices.