Preparation of Low-Resistivity Ga-Doped ZnO Epitaxial Films from Aqueous Solution Using Flow Reactor

Abstract

An aqueous process based on a unique flow-reactor design was developed for the preparation of gallium-doped ZnO (ZnO:Ga) epitaxial films with a low electrical resistivity. In this process, a ZnO:Ga film was grown on a ZnO-seeded sapphire substrate heated at 80 • C under a constant flow of a reaction solution. The Ga content of the resulting films was found to increase in relation to the concentration of GaCl 3 used-0 to 9 mM GaCl 3 -resulting in epitaxial growth of ZnO containing 0-5% Ga, whereas a polycrystalline ZnO film was produced with 10 mM GaCl 3 . The electrical resistivity of the as-grown ZnO:Ga films varied from 0.2 to 2 cm, but was reduced by two to three orders of magnitude after the films were annealed in air at 300 • C. Thus, the lowest resistivity of 7 × 10 -4 cm was obtained with an annealed film containing 2.5% Ga, whose carrier concentration and mobility were 7 × 10 20 cm -3 and 13 cm 2 V -1 s -1 , respectively. Furthermore, even though the non-doped ZnO film was rendered translucent by annealing, ZnO:Ga films with 1.8-4.0% Ga still exhibited transmittance as high as ∼80% in the visible spectrum. Zinc oxide (ZnO) has quite promising potential for use as a transparent semiconductor in a wide variety of applications such as sensors, light-emitting diodes (LEDs), and solar cells, 1 with a number of different techniques having already been developed for the fabrication of ZnO films. Low-temperature aqueous fabrication methods have attracted particular attention, as they tend to offer greater capability to produce large-area ZnO films at minimal cost and with low environmental impact. Though often used to produce nanowires and polycrystalline films, these aqueous-solution methods can also be used to obtain epitaxial ZnO films with good electrical conductivity and high transparency.2,3 N-type doping with group-III elements (Ga, In, or Al) to further enhance the conductivity is also feasible when using the aqueous methods. 4,5 Although there have been studies on the practical application of aqueous-synthesized ZnO films as currentspreading layers 6 or as the n-type layer in GaN-based light-emitting diodes (LEDs), 7 further improvement in the quality of such films is still needed. For instance, there is much room for improvement in terms of film conductivity, as current values still fall short of the low resistivity (<3 × 10 -4 cm) that can be obtained with more conventional vapor-phase processes. [8][9][10][11][12][13] In aqueous synthesis, thin films are typically grown on a substrate immersed in an aqueous solution by either heating the solution or adding another reactant to induce a chemical reaction. However, using such methods means that the chemical reaction occurs in the bulk solution, resulting in not only film growth on the substrate, but also precipitation in the solution.14,15 Furthermore, any precipitate formed can potentially settle on the growing film, thereby reducing its crystallinity and surface flatness. Any change in the concentration or pH of the solution as the reaction progresses...

Comparison of chemical bath-deposited ZnO films doped with Al, Ga and In

“…This concept was used by several groups to deposit intrinsic [9] and doped ZnO thin films [10][11][12][13][14]. The key to obtain dense materials rather than nanorods was in all approaches the use of citrate as structuredirecting agent, which selectively decelerates crystal growth along the ZnO c-axis [9,15].…”

“…The studies in the literature have dealt either with Al-doped [10,11,13] or with Ga-doped [12,14] ZnO. Due to differences in the used recipes and reactor designs, it is hard to make a conclusive comparison on the effect of the different dopants on the processing and properties of the doped films.…”

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

“…ZnO:Ga films were prepared from an aqueous solution using a flow reactor. The details of the deposition procedure using the flow reactor are described elsewhere [7]. An A-plane single crystal sapphire (5 × 7 × 0.43 mm) was used as the substrate.…”

“…These aqueous-solution methods can produce not only polycrystalline films but also epitaxial ZnO films [3,4]. Preparation of highly conductive (7 × 10 -4 Ω cm) ZnO epitaxial films doped with group III elements has also been demonstrated [5][6][7].…”

“…Post-fabrication thermal annealing is a useful approach to enhance the electrical properties of ZnO-based films. Indeed, the resistivity of films prepared by the aqueous-solution methods can be significantly reduced by annealing [5][6][7], and it is expected that the film properties can be further improved by optimizing the annealing conditions. There is, however, only a limited number of studies on the annealing of aqueous-synthesized ZnO epitaxial films, although annealing for films prepared by vapor-phase processes have been intensively studied [10][11][12][13][14].…”