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.