Atomic absorption photometry of excess Zn in ZnO

Abstract

PACS 61.50. Nw, 61.72.Ji, 64.70.Hz Zn excess in ZnO is built up automatically at high temperatures. Excess Zn in hydrothermally grown ZnO single crystals were investigated by the atomic absorption photometry (AAP) method. To determine the excess zinc in ZnO samples, the AAP of zinc vapour was used in the conditions of solid-vapour equilibrium. Zn AAP allowes to eliminate excess Zn connected differentially in ZnO samples. To fix Zn non-stoichiometry, all the ZnO samples tested were previously heat treated at temperature interval from 850 to 900 0 C and at fixed Zn vapour pressures from 0.1 to 0.9 of saturated zinc vapour pressure at given treatment temperature. The analysis of temperature dependence of zinc vapour pressure indicated that the impurity metals take active role in the determination of non-stoichiometric zinc. The impurities Mn, Fe, Co, Ni and Cu form oxides which will reduce during annealing in Zn vapor up to metals form. During AAP measurement in optical cuvette, these metals react with ZnO and give additional Zn vapor pressure.1 Overview ZnO is used in a wide range of scientific and technological applications, such as optoelectronic devices, varistors, devices for surface acoustic waves and planar optical waveguides, transparent electrodes and transparent windows for a solar cell, gas sensors, flat panel diplays, catalysts, ferroelectric memories and spintronic devices, antibacterial agent and ZnO can satisfy the commercial desire for blue and ultraviolet light emitting devices -it is suitable to grow the high quality ZnO homoepilayer on the ZnO substrate [1][2][3]. For that reason, it is important to produce high quality ZnO bulk crystals for substrates of ZnO-related devices. The hydrothermal method has an advantage to grow large bulk crystals due to the growth under low supersaturation [4][5][6]. Some doubts were raised about the quality of such crystals because they have often exhibited large X-ray rocking curve widths and low photoluminescence yield with large linewidths [7]. But if the surface damage due to mechanical polishing was overcome, hydrothermal ZnO crystals provided a competitive substrate for homoepitaxial growth [7,8]. As the hydrothermal crystals invitably involve alkali metals from solution and small amounts of metallic impurities then one of the problems in hydrothermally grown ZnO crystals is the contamination of the crystal with metallic impurities. The concentration of impurities exceeds often the concentration of intrinsic defects. The colourless crystals were found with a total impurity content of less than 1 ppm [4,9] but it is found that the green colouration of the hyrothermally grown crystals may be due to the presence of iron in concentration up to 60 ppm [9]. The solubilities of metal oxides in ZnO are high. For example the solubility of Fe in ZnO is 7 mole% at 800 0 C [10]. Fe in ZnO appears as a result of the corrosion of the inner wall of the autoclave [11]. The thermal treatment of ZnO crystal results in the formation of associates between metal impurities [12,...

Point defects in ZnO

“…The excess of electron charge carriers is commonly attributed to excess zinc present in typical nonstoichiometric samples of ZnO; it is usually assumed that the excess Zn is accommodated as interstitials. 6 Complex luminescence spectra (including ultraviolet, violet-blue, green, yelloworange and red bands in its visible part) characterise ZnO samples of different origin subject to various treatments, and are usually associated with ubiquitous Cu, Li, Na, Fe and other impurities as well as elementary point defects and their complexes. Furthermore, ZnO is an ionic dielectric with a wurtzite structure in ambient conditions, which can be represented as an AB stacking of hexagonally arranged ZnO sheets in the c direction of the crystallographic axes (see Fig.…”

The effect of excimer laser annealing on ZnO nanowires and their field effect transistors

“…The EDX results indicate that there is an excess of Zn in the spherical tip region of the ZnO nanowire relative to the other nanowire region that was not influenced by laser annealing. The oxygen vapour pressure is much higher than the vapour pressure of zinc in ZnO [18,19], and the ZnO results in a stoichiometric excess of Zn [19]. Therefore, it is expected that the surface melting of ZnO nanowires by laser annealing in a vacuum chamber that lacks oxygen induces Zn-rich ZnO nanowires due to increased oxygen vacancies [20].…”

High temperature electrical conductivity in undoped ceramic ZnO

“…Physical properties of undoped ZnO crystals depend strongly on the concentration of native defects caused by the deviation from the stoichiometric composition . To determine the excess zinc in powder, in ceramic and in single crystal ZnO, the atomic absorption photometry (AAP) of zinc vapour was used in the conditions of solid‐vapour equilibrium . Undoped ZnO exhibits n‐type electrical conductivity, which may be due to an excess of the metal component excess or/and by an impurity caused shallow donor.…”