Structural, optical, and electrical properties of (Zn,Al)O films over a wide range of compositions

Abstract

͑Zn,Al͒O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0 -50 at. %. The structural, optical, and electrical properties of ͑Zn,Al͒O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region ͑Ͻ10 at. % ͒, Al-doped ZnO ͑AZO͒ thin films could be obtained at 400°C in an Ar-O 2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4 at. %, with low resistivity ϳ10 −4 ⍀ cm, high transmittance ϳ90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52 eV at 4 at. % Al due to the Burstein-Moss shift ͓E. Burstein, Phys. Rev. 93, 632 ͑1954͔͒ modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the residual, compressive stress in film, which may be the reason for the improvement in film stability and the increment in grain size as well. In a medium Al content region ͑10-30 at. % ͒, however, the film quality was degraded, which was presumably due to the formation of clusters or precipitates in the grains and boundaries. Besides the ͑002͒ plane, other diffraction peaks such as ͑100͒ and ͑101͒ planes of ZnO were observed, but the ͑Zn,Al͒O films still exhibited a single-phase wurtzite ZnO structure. An intragrain cluster scattering mechanism was proposed to interpret the reduction of carrier mobility in films with the Al contents in the 7 -20 at. % region. The solubility limit of Al in ZnO film was identified to be in the 20-30 at. % range, much higher than the thermodynamic solubility limit of 2 -3 at. % in ZnO. In a high Al content region ͑ജ30 at. % ͒, there were distinct observations for ͑Zn,Al͒O films. As the Al content was 30 at. %, the film appeared in a two-phase nature with ZnO hexagonal and Al 2 O 3 rhombohedral structures. At the 50 at. % Al content, the matrix of the ͑Zn,Al͒O film was Al 2 O 3 , and no evident trace of wurtzite ZnO was observed. The electrical and optical properties for both cases were also very different from those at the Al contents below 30 at. %.

An enhanced gas ionization sensor from Y-doped vertically aligned conductive ZnO nanorods

“…Thus the resistivity will increase with Y dopant concentration above a certain threshold (0.10 at%), accompanied by the reduction of crystal domain size. Similar observations were reported for Al and Ga doped ZnO [46,47].…”

High electron mobility and low carrier concentration of hydrothermally grown ZnO thin films on seeded a-plane sapphire at low temperature

“…Two strong peaks in the ranges of 34.51°to 34.71°(ICSD 98-004-0986) and 37.5°t o 37.7°(ICSD 98-002-6165) were observed which can be indexed to the ZnO (002) plane and a-plane sapphire, respectively. High intensity of the ZnO (002) peak simply indicates that the preferable growth orientation of ZnO structures is along the c-axis [39]. It is confirmed that the values of full width at half maximum (FWHM) determined from the ZnO (002) diffraction peak of the hydrothermally grown layers are similar as compared to the corresponding CVD ZnO seeds.…”

Zinc-Vacancy–Donor Complex: A Crucial Compensating Acceptor in ZnO

“…Among them, V Zn is probably the most relevant defect, since it has the lowest formation energy among native point defects in n-type ZnO [1] and is commonly found in bulk and nanostructured materials [4][5][6][7][8]. V Zn is also suggested to be the origin of the observed n-type doping limit in ZnO [9,10] by forming complexes with donors leading to their compensation [11][12][13]. Therefore, it is crucial to understand the formation of intrinsic defects, especially V Zn , and their interaction with extrinsically important impurities such as shallow dopants in ZnO, which remains far from complete.…”