Cuprous oxide/cadmium stannate heterojunction diodes obtained by dip-coating method

The fabrication of p-n heterojunction diodes using semiconducting metal-oxides obtained from a solution-based method was investigated. Initially, n-type transparent cadmium stannate (Cd2SnO4) conducting films were deposited by the dip-coating method on glass substrates. The films were sintered in air for 1 h at 550 °C and annealed in vacuum for 10 min at 550 °C. A low resistivity value was obtained (ρ ≈ 2 × 10−3 Ω-cm) for films with average thickness of ∼ 380 nm. For the p-type layer, cupric oxide (CuO) was deposited using the dip-coating method over the transparent conductive oxide. Several layers were deposited and a drying process in air was applied at 250 °C for each coating. CuO thickness (τ) was in the 49-270 nm range as the number of coats varied from 5 to 14. After that, the samples were subjected to a rapid thermal annealing treatment (RTA) in vacuum during 10 min at a temperature TA, within the 300 °C ≤ TA ≤ 450 °C range, depending on τ, to reduce the CuO phase into the cuprous oxide (Cu2O) phase. Cu2O has shown a better crystalline quality and lower resistivity than CuO, obtaining a more efficient charge transport. From the X-ray diffraction patterns obtained from the heterojunction, the presence of Cd2SnO4 is confirmed. The CuO layer after RTA treatment shows three different types of films, depending on both TA and τ: i) CuO (amorphous)+Cu2O, ii) Cu2O and iii) Cu2O + Cu. Specular reflectance measurements were performed to show the presence of amorphous CuO and to confirm the full transformation from CuO to Cu2O. Current-voltage measurements were performed using silver paste on Cd2SnO4 and a graphite probe on the copper oxide layer, as ohmic contacts. All heterojunctions with a copper oxide layer of ∼ 49 nm, show an ohmic behavior. For higher τ, independent of the type of copper oxide film, a rectifying behavior is shown, which improves as τ increases and only the Cu2O phase is obtained. Best rectification is achieved in samples with ∼ 270 nm of Cu2O at TA = 425 °C. In this work, as far as we know, for first time Cu2O/Cd2SnO4 diodes were fabricated using the dip-coating method. The electrical parameters for the best rectification were: ideality factor (n= 4.8), saturation current density (J0 = 3.20 × 10−5 A/cm2) and turn-on voltage (Vto = 1.1 V).