Origin of n-type conductivity in ZnO crystal and formation of Zn and ZnO nanoparticles by laser radiation

• Published in: Optics and laser technology Vol. 111; pp. 121 - 128
• Main Authors: Kaupužs, Jevgenijs, Medvids, Arturs, Onufrijevs, Pavels, Mimura, Hidenori
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01-04-2019; Elsevier BV
• Subjects: c-ZnO; Conductivity; Crystal defects; Crystal growth; Crystal structure; Crystal surfaces; Electrical resistivity; Interstitials; Irradiation; Laser processing; Lasers; Nanoparticles; Nd:YAG laser; Neodymium lasers; Optical properties; Point defects; Radiation; Semiconductor lasers; Size distribution; Surface layers; Surface structure; Temperature gradients; YAG lasers; Zinc oxide; Nanoparticles; Nd:YAG laser; Conductivity; c-ZnO; Zn
• ISSN: 0030-3992; 1879-2545
• DOI: 10.1016/j.optlastec.2018.09.037

•Zn interstitials as origin of n-type conductivity in ZnO crystal.•Laser-induced thermal generation and redistribution of point defects.•Controlled formation of Zn and ZnO nanoparticles by laser radiation.
Electrical and optical properties of hydrothermally grown ZnO crystal, as well as structural changes at its surface have been investigated before and after irradiation by pulsed Nd:YAG laser. The spreading resistance measurements have shown a monotonous increase of conductivity by three orders of magnitude when the laser intensity I has been varied from zero to 290 MW/cm2. The PL spectra have revealed an increase of concentration of Zn interstitials at the surface after irradiation by I = 3.5 MW/cm2. Formation of Zn nanoparticles on the crystal surface has been observed at I > 290 MW/cm2. The study of surface structure at I = 315 MW/cm2 has shown that these Zn nanoparticles tend to transform into ZnO nanoparticles after an irradiation by more than 2 laser pulses. A theoretical model of thermal generation and redistribution of point defects has been elaborated to explain the origin of experimentally observed n-type conductivity. According to this model and experimental facts, the n-type conductivity originates from Zn interstitials, which are moved to the crystal surface by large temperature gradient during the laser processing. As a result, Zn-rich surface layer is formed and Zn nanoparticles grow, which are later oxidized into ZnO nanoparticles. We have shown a possibility to control the size distribution of these nanoparticles by choosing appropriate intensity and number of laser pulses.