We investigated the growth mechanism and microstructure of beta-Ga2O3 nanowires (NWs) deposited on amorphous SiN/Si(001), SiOx/Si(001), and glass substrates using radio-frequency powder sputtering. During growth, the beta-Ga2O3 changed from an initial amorphous thin film into NWs. Since the deposition was performed in an Ar gas environment, the initial amorphous thin films were non-stoichiometric Ga oxide (Ga2O3-x). Oxygen-deficient deposition led to the formation of intermediate thin films by the phase separation or Ga2O3-x into metallic Ga clusters and stoichiometric beta-Ga2O3. The Ga clusters acted as catalyst seeds in the growth of the beta-Ga2O3 NWs through the self-catalytic vapor-liquid-solid mechanism. We found that the growth of beta-Ga2O3 NWs is possible at temperatures greater than 450 degrees C, above which phase separation and Ga cluster formation occur. We also observed that NWs inherit the planar defects, such as twin boundaries, of the host Ga seed. (C) 2014 Elsevier B.V. All rights reserved.