Annealing to achieve lower resistivity in Ga-doped ZnO epitaxial films grown from low-temperature aqueous solution

Abstract

Various annealing conditions were examined for enhancing the electrical conductivity of galliumdoped ZnO (ZnO:Ga) epitaxial films grown from a low-temperature aqueous solution. The resistivity of the films decreased with increasing annealing temperature, accompanying increases in both the carrier concentration and mobility. However, the improvement obtained from 0.5 h of annealing was limited because the films spalled when the temperature exceeded ~300 °C. The temperature at which spalling of the films occurred depended on the annealing atmosphere, and it was lower in 5%H2-Ar than in air or vacuum. Thermal desorption spectroscopy showed that the cause of spalling was expulsion of H2O vapor from the interior of the film, indicating that water molecules or hydroxides were incorporated into the crystal during film growth. Analysis of the variations in electrical properties with increasing annealing duration revealed that the electrical properties were quickly improved upon heating before the film spalled. Thus, by limiting the annealing duration to ≤20 s, annealing could be performed at a temperature as high as 580 °C without spalling of the film, which yielded the lowest resistivity of 4.7 × 10-4 Ω cm. The annealed film exhibited transmittance as high as >75% in the visible spectrum.

Growth and Electrical Properties of Epitaxial ZnO Films Prepared by Chemical Bath Deposition Using a Flow Reactor

“…It has been reported that ZnO crystals grown from aqueous solutions contain a small amount of hydroxide ions as impurities, which can reduce the mobility of electrons in the crystal. 26,31) The amount of hydroxide ions incorporated in ZnO during the crystal growth is expected to depend on the pH of the reaction solution.…”

Using Modified-Intake Plasma-Enhanced Metal–Organic Chemical Vapor Deposition System to Grow Gallium Doped Zinc Oxide

“…The ionic radii of Ga (0.062 nm) and Zn (0.074 nm) and covalent bond length of Ga-O (1.92 Å) and Zn-O (1.97 Å) are similar, resulting in low deformation of the ZnO lattice even at a high doping concentration [7,[22][23][24]. GZO thin films have been fabricated successfully through several deposition technologies such as molecular beam epitaxy (MBE) [25,26], pulsed laser deposition (PLD) [27,28], metal-organic chemical vapor deposition (MOCVD) [29], atomic layer deposition (ALD) [22], sputtering [30,31], thermal oxidation [32], aqueous solution deposition [33], and the sol-gel method [34]. PLD and MBE grown-GZO thin films exhibit excellent crystalline structures and low resistivity; nevertheless, scaling them up to standard industrial substrate sizes is difficult [22].…”