High-Hall-Mobility Al-Doped ZnO Films Having Textured Polycrystalline Structure with a Well-Defined (0001) Orientation

Abstract

Five hundred-nanometer-thick ZnO-based textured polycrystalline films consisting of 490-nm-thick Al-doped ZnO (AZO) films deposited on 10-nm-thick Ga-doped ZnO (GZO) films exhibited a high Hall mobility (μH) of 50.1 cm2/Vs with a carrier concentration (N) of 2.55 × 1020 cm−3. Firstly, the GZO films were prepared on glass substrates by ion plating with dc arc discharge, and the AZO films were then deposited on the GZO films by direct current magnetron sputtering (DC-MS). The GZO interface layers with a preferential c-axis orientation play a critical role in producing AZO films with texture development of a well-defined (0001) orientation, whereas 500-nm-thick AZO films deposited by only DC-MS showed a mixture of the c-plane and the other plane orientation, to exhibit a μH of 38.7 cm2/Vs with an N of 2.22 × 1020 cm−3.

Influence of Oxygen on the Resistivity of Co‐Sputtered Transparent AZO Films

“…This also indicates that the lower carrier concentration is influenced by the amount of oxygen incorporated in the AZO film. On the contrary, the Hall mobility is positively influenced by better crystallinity, and the higher degree of the c ‐axis alignment between columnar grains is also essential for enhancing carrier mobility …”

Characteristics of Carrier Transport and Crystallographic Orientation Distribution of Transparent Conductive Al-Doped ZnO Polycrystalline Films Deposited by Radio-Frequency, Direct-Current, and Radio-Frequency-Superimposed Direct-Current Magnetron Sputtering

“…For such polycrystalline films, an approach to investigating the factors limiting carrier transport from a viewpoint different from those of conventional studies of single crystals is required. The study of the relationship between the carrier transport and the distribution of crystallographic orientations of transparent conductive polycrystalline AZO films [ 14 , 15 , 16 , 17 , 18 ] is, thus, of great importance from both viewpoints of fundamental materials science on the structural factors limiting carrier transport and practical technology how to achieve the carrier mobility required by the applications including electrical and optoelectronic devices.…”

“…Carrier transport is limited by two factors: the intrinsic carrier mobility in intragrains and the contribution of grain boundary scattering to the carrier transport governed by the crystallographic orientation distribution [ 14 , 15 , 16 , 17 , 18 ]. The degree of preferential orientation in a polycrystalline film with a columnar structure with a dominant orientation may vary widely from low for a weak fiber texture to high for a strong one.…”

“…For a fiber-textured polycrystalline film [ 14 , 15 , 16 , 17 , 18 ], one crystallographic axis of the film is parallel to the substrate normal, while there is a rotational degree of freedom around the fiber axis. In previous studies [ 14 , 15 , 16 , 17 , 18 ], we reported the difference in orientation distributions between direct-current (DC), radio-frequency (RF), and RF-superimposed DC (RF/DC) magnetron-sputtered AZO films. The RF-magnetron-sputtering technique produced 500-nm-thick AZO films having a strongly fiber-textured polycrystalline structure, which had most of the grains with the c -axis orientation aligned within a fraction of a degree of the film normal.…”

“…%), we obtained the following findings: AZO films deposited by DC magnetron sputtering had high optical mobility ( μ opt ), corresponding to high in-grain carrier mobility, high carrier concentration, and a large contribution of grain boundary scattering to carrier transport compared with AZO films deposited by RF magnetron sputtering; the Hall mobility ( μ H ) was reduced owing to the above-mentioned large contribution of grain boundary scattering to carrier transport for DC-magnetron-sputtered AZO films, whereas AZO films deposited by RF magnetron sputtering exhibited μ H close to μ opt owing to the little contribution of grain boundary scattering to carrier transport. Here, the contribution of grain boundary scattering to carrier transport is defined as a ratio of μ opt to the carrier mobility at grain boundaries ( μ GB ), μ opt / μ GB , on the basis of Matthiessen’s rule [ 14 , 15 , 16 , 17 , 18 ].…”