As designing semiconductor devices, band alignment is one of the most important parameters: various semiconductor devices such as thin-film transistors, solar cell and quantum well structure have to be designed by considering suitable band alignment. We focus on amorphous oxide semiconductor (AOSs) which have a lot of advantages such as relatively high carrier mobility compare with that of amorphous Si, low process temperature, and etc. AOSs can be the ability to tune their properties such as band gap, the energy positions of the conduction band minimum (CBM) and the valence band maximum (VBM) by changing the composition. In this study, we fabricate amorphous Cd–In–Ga–O (a-CIGO) films which were tuned the CBMs and the VBMs, independently, by tuning cations’ ratio.
n-Type a-CIGO films were fabricated by RF magnetron sputtering on silica glass substrates. CdO, In2O3, and Ga2O3 mixed powders were employed as sputtering targets. Carrier type, carrier concentration, and electron mobility were evaluated by Hall measurements using the van der Pauw method. Ionization potentials (Ip) were measured using ultraviolet photoelectron spectroscopy (UPS): UPS measurement chamber is connected with the RF magnetron sputtering chamber in UHV.
Ip which is energy difference from vacuum level (Evac) to the valence band maximum (VBM) of a-CIGO films were increased by increasing Cd concentration. On the other hand, only CBM was shifted by tuning In:Ga ration but keeping Cd concentration. As a result, the a-CIGO system can independently be controlled energy position of VBMs and CBMs by tuning Cd:In:Ga ratios. Carrier concentration of a-CIGO system were from 1020 to 1016 cm-3 by increasing Ga concentration because Ga ion suppresses oxygen vacancy, which might origin of carriers in this system.