Oxygen Deficiency and Sn Doping of Amorphous Ga2O3

The potential of effectively n-type doping Ga₂O₃ considering its large band gap has made it an attractive target for integration into transistors and solar cells. As a result amorphous GaO_x is now attracting interest as an electron transport layer in solar cells despite little information on its opto-electrical properties. Here we present the opto-electronic properties, including optical band gap, electron affinity, and charge carrier density, for amorphous GaO_x thin films deposited by pulsed laser deposition. These properties are strongly dependent on the deposition temperature during the deposition process. The deposition temperature has no significant influence on the general structural properties but produces significant changes in the oxygen stoichiometry of the films. The density of the oxygen vacancies is found to be related to the optical band gap of the GaO_x layer. It is proposed that the oxygen deficiency leads to defect band below the conduction band minimum that increases the electron affinity. These properties facilitate the use of amorphous GaO_x as an electron transport layer in Cu(In,Ga)Se₂ and in Cu₂O solar cells. Further it is shown that at low deposition temperatures, extrinsic doping with Sn is effective at low Sn concentrations.