Figure 3 Sheet resistance, carrier concentration and Hall mobility of the IHfO:H films prepared with 0 %, 0.5 %, 0.8 %, 1.2 % and 1.5 % H2 concentration after post-annealing at 200 ℃.
The Hall mobility increases at first and then decreases with increasing the H2 concentration, reaching a maximum of 102.92 cm2V-1s-1at a H2 concentration of 0.8 %. Three carrier scattering mechanisms including acoustical phonon scattering, ionized impurity scattering, and grain boundary scattering are generally considered as the main mechanisms to affect the mobility of the TCO films.29 Considering that the electrical properties were measured at the room temperature and no significant crystallinity difference can be determined (as Figure 2 shows), phonon scattering and grain boundary scattering should not be the main factors that cause the mobility difference in the IHfO:H films with different H2 concentrations. Therefore, it can be inferred that the variation of Hall mobility is mainly caused by the difference in the intensity of ionized impurity scattering. The roughly opposite variation tendency of the carrier concentration and the Hall mobility in Figure 3 proves the above analysis, since the carrier density is strongly related to the impurity concentration.29
Oxygen vacancies, substitution high valence metallic ions for indium, and H doping are all the important sources of conducting electrons in hydrogenated indium oxide films.25,30 XPS measurement was carried out to investigate the valence states of Hf and oxygen elements in the IHfO:H films prepared with different H2concentration. The significant Hf 4f XPS line and the element ratio of Hf/In (3.87±0.04 %, calculated from the XPS peaks) similar to the evaporation target indicate effective and similar Hf doping in all the IHfO:H films (Figure S1). Varying the H2 concentration during the IHfO:H film deposition does not seem to change the doping effect of Hf elements. According to literature, hydrogen doping enables the replacement of double-electron donors (oxygen vacancy Vo) for single-electron donors (interstitial Hi+ or substitutional HO+)26,31 and passivation of structural defects,32 which may reduce the scattering intensity and thus improve the Hall mobility of the TCO films. Thereby, the following discussion will focus on the influence of the H2 concentration on the oxygen elements.