Indium-doped-conducting ZnO crystals have been grown by the hydrothermal technique. The hydrothermal growth mechanism, as influenced by impurities, particularly In3+ is investigated. Indium ions or indium coordinated anionic groups such as In(H2O)(2)(OH)(4)(-) in alkaline solutions tend to absorb on both (0 0 0 (1) over bar) and (0 0 0 1) faces of ZnO, resulting in a reduction of growth oil the two polar faces, while facilitating growth on the m faces; the grown crystals exhibited a plate-like crystal morphology. We have used this growth characteristic to increase the diameter of c-plate ZnO seeds along the m-planes for the growth of nominally undoped (lithium-doped) large-size ZnO crystals. Transmission spectra of both nominally undoped and indium-doped ZnO crystals were measured for comparison. Indium doping reduced the transmittance of ZnO crystals; the absorption edge of In:ZnO was red-shifted with respect to nominally undoped ZnO. The temperature dependence of the resistivity and carrier concentration measured at temperatures from 86 to 360 K indicated that the indium-doped ZnO, which contains about 150-175 ppm wt of indium in the crystals, are of high conductivity, with a resistivity lower than 0.015 Omega-cm and Hall carrier concentration of 1.09 x 10(19) electrons/cm(3) at room temperature. This research on hydrothermal growth of ZnO bulk crystals in the presence of In3+ will not only have an impact on the supply of ZnO-conducting substrates, but also contribute to the understanding of ZnO growth mechanisms in the presence of impurities. (C) 2007 Elsevier B.V. All rights reserved.