Electrical and optical properties of ZnO bulk crystals with and without lithium grown by the hydrothermal technique

Lithium is usually added into the solution to improve ZnO hydrothermal growth; however, lithium doping affects the properties of the resulting crystals. Optical and electrical properties of hydrothermal ZnO bulk crystals without lithium, have been studied by photoluminescence and Hall-effect measurements. High quality ZnO crystals without lithium were grown in H2O/D2O and in NH3-H2O solutions. The crystals grown from H2O/D2O are conductive with resistivities of 0.6-0.7 Omega cm and mobilities of similar to 100 cm(2)/Vs, while lithium doped ZnO crystals typically have resistivities of similar to 10(3) Omega-cm and mobilities of similar to 200 cm(2)/Vs, but can be varied from dozens to 1010 Omega-cm depending on lithium concentration. Lithium-free but nitrogen doped crystals grown in NH3-H2O solution have resistivities of 1x100 Omega-cm and sometimes show p-type conduction; the resistivity increases to similar to 1x108 Omega-cm after annealing at 600 degrees C in air. Lithium and nitrogen co-doped ZnO crystals have resistivities of 10(8)-10(12) Omega-cm and are semi-insulating after annealling. Electronic irradiation also increases the ZnO resistivity. For lithium-doped samples, a 3.357 eV peak can be seen in the photoluminescence spectra. This is close to the donor-exciton peaks in indium-doped ZnO where 3.3586 eV and 3.357 eV were found on the C+ and C(-)faces, respectively. More studies are needed to identify lithium-related complexes (defects).