Defects in virgin hydrothermally grown n-type ZnO studied by temperature dependent Hall effect measurements

摘要

Temperature dependent Hall (TDH) effect measurements have been performed on three virgin and hydrothermally grown ZnO samples with resistivities between ～5 and ～200 Ω cm at room temperature. The electrical conduction observed experimentally in the temperature range of 330-70 K can be accurately described by three donor levels with positions 41-48, 60-66, and ～300 meV below the conduction band edge (E_C) and an acceptor level in the lower part of the energy band gap (E_G). Correlation of the TDH data with results from secondary ion mass spectrometry and admittance spectroscopy on the same samples suggests a rather firm association of the intermediate donor level with complexes involving Al impurities, while the shallowest one is tentatively ascribed to H-related centers. A large fraction of the deep donor remains nonionized in the temperature range studied and contributes substantially to the neutral-impurity-scattering of the conducting electrons. A detailed analysis of the TDH data, using the relaxation time approximation, reveals, however, that ionized-impurity-scattering and optical phonon scattering are the main mechanisms limiting the electron mobility which exhibits a maximum value of ～125 cm~2/V s at ～200 K. The major reason for this modest value is the high concentration of compensating acceptors in the lower part of E_G reaching values of ～3 × 10~(17) cm~(-3) and where Li plays an important role. However, the Li content is not sufficient to account for all the acceptors and additional impurities, excluding group I elements, and/or intrinsic defects have to be considered.