Influence of Al concentration and annealing temperature on structural, optical, and electrical properties of Al co-doped ZnO thin films

• Published in: Applied surface science 2015-09, Vol.349, p.549-560
• Main Authors: Guerbuez, Osman, Kurt, Ismail, Caliskan, Serkan, Guener, Sadik
• Format: Article
• Language: English
• Subjects: Aluminum; Annealing; AZO; Crystallites; Crystals; Electrical conductivity; Optical and electrical properties; RF magnetron sputtering; Thin films; Zinc oxide
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2015.04.233

•RF magnetron sputtering technique seems to be very efficient method for fabrication of Al doped ZnO (AZO) films.•Long range single crystalline structure improves with annealing process.•Optical properties became much better after annealing process especially for the AZO films that include high Al concentration.•Much greater conductivity with increasing Al concentration and annealing process.•AZO films have potential applicability in spintronic devices. The pure ZnO and Al-doped ZnO (AZO) thin films (thickness: 200nm) were prepared on both side polished silica (SiO2) substrates via RF magnetron sputtering at room temperature by using 2.5 inches high-purity ZnO (99.9%) and Al (99.9%) targets. The samples were annealed at 300°C, 400°C and 500°C for 45min in N2 ambient in quartz annealing furnace system, respectively. We investigated the effects of various Al concentrations and annealing treatment on the structural, electrical, and optical properties of films. The preferred crystallization was observed along c axis (single (002) diffraction peak) from substrate surface assigning the single crystalline Würtzite lattice for pure ZnO and AZO thin films. Although increasing Al concentration decreases the order of crystallization of as-grown films, annealing process increases the long range crystal order. The crystallite sizes vary between minimum 12.98nm and maximum 20.79nm for as-grown and annealed samples. The crystallite sizes decrease with increasing Al concentration but increase with increasing annealing temperature as general trend. The grain size and porosity of films change with annealing treatment. The smaller grains coalesce together to form larger grains for many films. However, a reverse behavior is seen for Al2.23ZnO and Al12.30ZnO samples. That is, Al concentration plays critical role as well as temperature on grain size. Low percent optical transmittance (T%) is observed due to higher Al concentration and worse crystal quality for as-grown AZO films. T% decreases until 34.5% for as-grown Al15.62ZnO film. T% increases by increasing annealing temperature. AZO samples annealed at 500°C have around 80% transparencies in the visible range of spectrum. Optical energy band gap values range between 3.17eV and 3.60eV for as-grown and annealed samples. Band gap increments are attributed to increasing free electron concentration depending on doped Al ratio known as Burstein–Moss effect. Annealing process increases the band gap values, too. The electrical cond