Phase Transformation and Optoelectronic Properties of p-Type CuAlO[sub 2] Thin Films

Abstract

Annealed Cu-Al-O films showed marked structural changes and differing optoelectronic properties with varying annealing temperature. Results of X-ray diffraction ͑XRD͒ demonstrated that CuO and CuAl 2 O 4 were the intermediate reaction phases. XRD also showed that the phase grown above 800°C annealing temperature was pure CuAlO 2 phase. Cross-sectional high-resolution transmission electron microscopy revealed that the crystallization behavior of the Cu-Al-O films belonged to an outward model. The optimum properties of delafossite structure CuAlO 2 film was attained after annealing at 800°C. The surface morphology of CuAlO 2 had a cell-like surface appearance and the grain sizes were approximately 20-100 nm. The optical direct bandgap of the CuAlO 2 film was estimated to be 3.11 eV. Hall effect measurements revealed that the CuAlO 2 film belonged to the p-type conduction category, with a carrier concentration of 4.81 ϫ 10 16 cm −3 and the conductivity of 3.8 ϫ 10 −2 ͑⍀ cm͒ −1 . The optoelectronic properties of the Cu-Al-O system are dominated by the delafossite CuAlO 2 .The n-type transparent conducting oxides ͑TCOs͒, such as In 2 O 3 , 1 ZnO, 2-4 and SnO 2 , 5,6 have been extensively studied and widely used in flat panel displays, touch panels, and solar cells. p-Type TCOs are neither well studied nor thoroughly explored yet. Recently, a series of p-type TCOs such as CuAlO 2 , 7,8 CuGaO 2 , 9 and SrCu 2 O 2 10 have been successfully produced by various techniques. The p-type CuAlO 2 film of delafossite structure in particular has been attracting an immense research interest because of its attractive properties, which have great potential for practical applications. One of the main applications proposed for CuAlO 2 film is in the transparent p-n junction semiconductor device, which is the structure that combines the two types of TCO materials in the form of a p-n junction. The device is capable of absorbing ultraviolet light but transmitting the visible light. As such, it can be used as a transparent functional window in microelectronic devices. [11][12][13] So far, little research has been devoted to the study of microstructure and associated optoelectronic properties of the CuAlO 2 film. The present paper reports the study on the CuAlO 2 film prepared by the dc reactive sputtering technique and annealed under controlled Ar atmosphere. In particular, properties of the CuAlO 2 thin films characterized by X-ray diffraction ͑XRD͒, field-emission scanning microscope ͑FESEM͒, high-resolution transmission electron microscope ͑HRTEM͒, X-ray photoelectron spectroscopy ͑XPS͒, UV-visible ͑UV-vis͒ spectroscopy, and Hall effect measurement are presented. In addition, the structural, optical, and electrical properties of the p-type CuAlO 2 are discussed. ExperimentalThe Si wafer and quartz substrates used for the film were thoroughly cleaned by ultrasonic vibration in baths of hot degreasing solvent, acid solution, and deionizer water sequentially. Deposition of the CuAlO 2 films was carried out by a dc reactive sputtering sys...

Synthesis of CuAlO2 from chemically precipitated nano-sized precursors

“…Targeting these optoelectronic applications, thin films of CuAlO 2 have been prepared using a variety of techniques such as pulsed laser deposition (PLD) [4,5], sputtering [6,7], chemical vapor deposition [8], and sol-gel [9,10]. It has been demonstrated that the transmittance of CuAlO 2 thin films over the visible spectrum can reach 80% with an optical band-gap of 3.5 eV [11][12][13].…”

Effect of substrate temperature on the physical properties of dc magnetron sputtered CuAlO2 films

“…Combining of all these properties, CuAlO 2 becomes an important material in the last few years. Several deposition techniques such as pulsed laser deposition [3,4], chemical vapor deposition [13,14], dip-coating [15], spin-on technique [16], ion exchange [17], polymer-assisted deposition [18], sputtering [19][20][21], and electron beam evaporation [22] were employed for the preparation of CuAlO 2 films. Among these techniques, direct current (dc) magnetron sputtering is one of the best technique for the preparation of films on large area substrates with required chemical composition at low substrate temperatures.…”

Effect of Sintering Temperature on the Properties of CuAlO₂ Synthesized from Nanosized Precursors for Application in Smart Infrastructure Systems

“…Such high temperatures can be reduced significantly through several techniques. Yu et al performed the synthesis of CAO and proved that the reaction could be completed at 1000 K using thin films of precursors applied onto silicon and quartz wafers; in this research study, as the annealing temperature increased from 1000 K to 1100 K, there was a substantial increase in the grain size [4]. At temperatures below 700 K, CAO is unstable thermodynamically in the air, and, if the temperature goes below 700 K, the instability of CuAlO2 causes the formation of Cu 2+ cations in the CuAl2O4 phase [32].…”

“…The most prominent application for using transparent TE devices is in the construction industry, where UV-absorbing windows are required. CAO is a wide-bandgap material that can absorb UV light and transmit visible light [4]. Zheng et al showed that CAO has the ability of ozone sensing, and it can form transparent p-n junction ozone sensors with existing ntype transparent materials such as In2O3 [5].…”