Effects of H2 ambient annealing in fully 002-textured ZnO:Ga thin films grown on glass substrates using RF magnetron co-sputter deposition

Background

Indium tin oxide (ITO) is the representative transparent conductive oxide (TCO) for displays, solar cells and organic light-emitting diodes, but the cost of indium has driven work on doped zinc oxide (ZnO). Aluminum-doped ZnO shows relatively low thermal stability and degrades during long exposure to air because of the high reactivity of aluminum. Gallium is less reactive, and the Ga–O bond (1.92 Å) is slightly shorter than the Zn–O bond (1.97 Å), which allows higher solubility of Ga in ZnO. Reported properties of polycrystalline ZnO:Ga films vary widely, and the authors attribute these differences to film composition, microstructure, deposition technique and post-deposition processing.

Design and approach

ZnO:Ga films were deposited on glass from separate ZnO (99.99%) and Ga2O3 (99.9%) targets, with the RF power fixed at 150 W and 75 W, respectively, for 20 min; a dual-target system was chosen with control of dopant concentration in mind. The deposition temperature was varied from room temperature (RT) to 623 K, and the as-deposited films were then annealed in H2 ambient at 623 K for one hour. Hydrogen annealing was examined as a post-treatment at relatively low temperature; the authors later attributed its effect to hydrogen acting as a reducing agent that decomposes gallium-containing oxide phases formed during deposition and releases Ga for doping.

How it was evaluated

Films deposited at 298, 373, 473 and 623 K were compared before and after annealing. Thickness and morphology were examined by scanning electron microscopy (SEM), surface roughness by atomic force microscopy (AFM), crystal structure by X-ray diffraction (XRD), Ga content by energy dispersive X-ray spectroscopy (EDS) and chemical state by X-ray photoelectron spectroscopy (XPS). Resistivity, carrier concentration and mobility were measured at room temperature by the van der Pauw method, and transmittance and optical band gap by UV–vis spectrophotometry.

Key results

All films had densely packed columnar grains and a preferred c-axis orientation from self-texturing at every deposition temperature. Film thickness was 420 to 450 nm and did not change after annealing. The root mean square roughness rose from 7.9 to 12.7 nm with deposition temperature in the as-deposited films and was 7.5–11.1 nm after annealing, a change the authors judged small enough to exclude as a factor in the electrical and optical results.

The as-deposited films contained Ga2O3 and ZnGa2O4. EDS gave a Ga concentration of about 3.5 ± 0.3 at%, above the reported solubility limit of about 2.32 at%. After H2 annealing, the Ga2O3 XRD peaks nearly vanished and ZnGa2O4 peaks remained, with substantially reduced intensity, only in the film deposited at 473 K. XPS of the annealed films showed a shoulder at 1114 eV, 2 eV below the Ga 2p3/2 peak, which the authors assigned to elemental Ga released by reduction of the oxide phases; they concluded that hydrogen acted as the reducing agent and that part of the released Ga could not be incorporated because the Ga content already exceeded the solubility limit.

As-deposited films improved electrically up to 473 K but became poorer at 623 K because of decreasing carrier concentration, and their resistivity remained in the order of 10⁻¹ Ω cm. After annealing, the carrier concentration rose from the order of 10¹⁹ to 10²¹ cm⁻³ and the resistivity fell to between the mid 10⁻⁴ and low 10⁻³ Ω cm range, a decrease of 2–3 orders of magnitude; films deposited at lower temperatures gave lower resistivity, and mobility improved slightly only for the RT and 373 K films. The lowest resistivity was 5.2 × 10⁻⁴ Ω cm at a carrier concentration of 2.5 × 10²¹ cm⁻³.

All films transmitted more than 90% of the visible light transmitted by the glass substrate. The optical band gap was about 3.35 eV for all as-deposited films and ranged from 3.57 to 3.42 eV after annealing, depending on the initial deposition temperature; the authors attributed the overall increase to the Burstein–Moss shift of the Fermi level with the higher carrier concentration.

How it connects to other research

The related Electronic Materials Letters study by Myoung, Lee and colleagues reported in its abstract that hydrogen post-annealing improved the electrical properties only of Ga-doped ZnO films deposited at room temperature and recommended controlling the Ga concentration to suppress ZnGa2O4 and Ga2O3; the present study instead reports lower resistivity after annealing across deposition temperatures and traces it to reduction of these phases by hydrogen.