Atomic structures of supersaturated ZnO–Al2O3 solid solutions

Abstract

Supersaturated ZnO-Al 2 O 3 ͑Ͼ20 at. % Al͒ thin films are grown by pulsed laser deposition technique on silica glass substrates at 600°C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures ͑XANESs͒, high resolution transmission electron microscope ͑TEM͒ imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10 at. %. The expansion disappears after annealing the films at above 800°C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of ͑ZnO͒ 3 ͑Al 2 O 3 ͒ as the local environment of Al in the supersaturated solid solution. Simplified substitutional models with Al at the Zn site in wurtzite ZnO cannot explain these experimental results. First principles calculations show that the homologous phase is energetically more favorable than the simplified substitutional models, although decomposition into ZnO and ZnAl 2 O 4 is more favorable than the homologous phase. The local atomic structures of the supersaturated solid solution are therefore concluded to be analogous to the metastable homologous phase.

Carrier Compensation Induced by Thermal Annealing in Al-Doped ZnO Films

“…Conversely, further increases in T g would significantly promote diffusion of the adatoms and precursors, leading to migration of ions in the bulk films, and desorption of Zn at the growth surface. Consequently, excess Al may degrade the film crystallinity by forming local atomic structures similar to a homologous phase ((ZnO) m Al 2 O 3 ) [14,15]. The effect of inserting homologous phases in a ZnO film would be to increase the out-of-plane dimension of the film.…”

Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition

“…At higher Al doping concentration above 3%, a decrease of the intensity of the (100) diffraction peak indicates a degeneration of the crystal quality; as a consequence, an increase of the resistivity was shown in Figure  4. The reason for the increase of the resistivity at high Al concentration is probably related to the formation of Zn vacancy acceptors or the formation of homologous phase like ZnAl x O y or Al 2 O 3 in the AZO films [9,22]. …”

“…As a wide-bandgap semiconductor material, ZnAl 2 O 4 was also used as host of phosphors doping with Mn and rare earth ions [5,6]. AZO and ZnAl 2 O 4 thin films have been deposited by different techniques [7], such as sol–gel coating [8], pulsed laser deposition [9], chemical vapor deposition [10], radio-frequency sputtering [11], and atomic layer deposition (ALD) [12,13]. Recently, ALD technology has been employed to grow transparent conductive AZO films with low resistivity in the order of 10 −3 Ω·cm [14,15].…”

Multi-component transparent conducting oxides: progress in materials modelling

“…The Zn-rich alloy was recently grown by pulsed laser deposition in the work of Yoshioka et al [104], where a superlattice structure was identified, which is consistent with the homologous crystal structure models developed for the In and Ga compounds, as later verified by electronic structure calculations [105]. There is growing evidence for the presence of the homologous phase in typical Aldoped ZnO samples [106,107], which can contribute to lower than expected electron carrier concentrations: excess electrons are compensated by additional oxygen incorporation in to the lattice.…”