Properties of In-, Ga-, and Al-doped ZnO films grown by aerosol-assisted MOCVD: Influence of deposition temperature, doping level and annealing

In-, Ga- and Al-doped ZnO films were grown via the aerosol-assisted MOCVD technique using the following airstable metal-organic precursors: Zn(thd)(2), In(thd)(3), Al(thd)(3) and Ga(acac)(3) (thd = 2,2,6,6-tetramethy1-3,5-heptanedione, acac = acetylacetonate). Solution of a precursor mixture in 1,2-dimethoxyethane was transformed into an aerosol via ultrasound (2.56 MHz), and then the aerosol was transported into the deposition zone by N-2 + O-2 gas. The depositions were performed at 350-600 degrees C. The influences of the substrate, the deposition temperature, the doping element and the doping level on the structural, and the electrical and optical properties of the films were investigated using various characterisation techniques: XRD, SEM, AFM, UV-Vis and FT-IR spectroscopies, and Hall measurements. Epitaxial films on sapphire-R substrates exhibited the best electrical properties. Films doped by Ga and Al had lower resistivity compared to In-doped films. The best Ga- and Al-doped ZnO films (similar to 150-200 nm thick) grown on sapphire-R at 400 degrees C exhibited a resistivity of approximate to(7-8) x 10(-4) Omega cm, a carrier concentration of approximate to 1.5 x 10(20) cm(-3) and a relatively high carrier mobility of approximate to 50-60 cm(2) V-1 s(-1). Because the low resistivity of Ga- and Al-doped films was mainly determined by the high carrier mobility, the optical transmittance (T) was high in both the visible (T> 90%) and the mid-IR spectral ranges (T approximate to 90% at lambda = 2.5 mu m). High transmittance in the mid-IR spectral range makes these films suitable for use as transparent electrodes in applications of the IR optoelectronic devices. Annealing of the films for 30 min at 400 degrees C in an Ar + H-2 (10%) mixture markedly increased the carrier concentration, which resulted in the increase in the optical band gap (up to approximate to 3.5 eV) and in the decrease in film resistivity and transparency in the mid-IR spectral range. Annealed Al- and Ga-doped films exhibited a resistivity of approximate to(4 - 6) x 10(-4) Omega cm. (c) 2014 Elsevier B.V. All rights reserved.