Chemical fabrication of p-type Cu2O transparent thin film using molecular precursor method

Abstract

A transparent p-type Cu2O thin film of 50 nm thickness was successfully fabricated by means of a solution-based process involving the thermal reaction of molecular precursor films spin-coated on a Na-free glass substrate. The precursor solution was prepared by the reaction of an isolated Cu2+ complex of ethylenediamine-N, N, N′, N′-tetraacetic acid with dibutylamine in ethanol. The Cu2O thin films resulting from heat treatment of the precursor film at 450 °C for 10 min in Ar gas at a flow rate of 1.0 L min−1 were characterized by X-ray diffraction which indicated a precise cubic lattice cell parameter of a = 0.4265(2) nm, with a crystallite size of 8(2) nm. X-ray photoelectron spectroscopy peaks, attributable to the O 1s and Cu 2p3/2 level of the Cu2O film were found at 532.6 eV and 932.4 eV, respectively. An average grain size of the deposited Cu2O particles of ca. 200 nm was observed via field-emission scanning electron microscopy. The optical band edge evaluated from the absorption spectrum of the Cu2O transparent thin film was 2.3 eV, assuming a direct-transition semiconductor. Hall Effect measurements of the thin film indicated that the single-phase Cu2O thin film is a typical p-type semiconductor, with a hole concentration of 1.7 × 1016 cm−3 and hole mobility of 4.8 cm2 V−1 s−1 at ambient temperature. The activation energy from the valence band to the acceptor level determined from an Arrhenius plot was 0.34 eV. The adhesion strength of the thin film on the Na-free glass substrate was also determined as a critical load (Lc1) of 2.0 N by means of a scratch test. The method described is the first example of fabrication and characterization of a p-type Cu2O transparent thin film by a wet process.