The conductivity and Hall effect measurements of undoped 20 nm In2O3 films deposited
at 400◦C in a pure Ar gas together with those coated with ∼ 2nm SiO2−x are presented
inFig. 7.9. SiO2−x were deposited from Si target using different oxygen concentrations
in sputter gas and deposition temperatures of either room temperature or at 400 ◦C. The undoped and uncoated In2O3 samples show a conductivity of 956.6 S/cm, a carrier
concentration of 1.04 × 1020 cm−3 and a mobility 57.5 cm2/Vs. This conductivity is slightly higher than those3 reported in section 6.4. The Hall effect measurements show that SiO2−x deposition induces a moderate changes in carrier concentration and mobility.
The room temperature deposition of SiO2−x changes the conductivites of the In2O3
thin films by up to ± 200 S/cm. Eventually, the conductivity is increased only for films deposited with higher oxygen concentration of 1.7 and 1.8 %. The Hall effect measurements revealed that it is mostly due to the carrier concentrations as it slightly increased for the last three films in the series from 1 × 1020 cm−3 to 1.4 × 1020 cm−3.
On the other hand, the carrier mobility showed a slight reduction with increasing oxygen concentration, having the lowest mobility of 39.5 cm2/Vs for the sample with oxygen
7.4 Electrical Study 1200 1000 800 600 400 200 0 σ [S/cm] RT 400 °C 1.6 1.4 1.2 1.0 0.8 0.6 n [ x 10 20 cm -3 ] 60 40 20 0 2.0 1.5 1.0 0.5 0.0
Oxygen content in Argon [%] 60 40 20 0 µ [ cm 2 /Vs] 5 4 3 2 1 0
Oxygen content in Argon [%]
Figure 7.9: Conductivity and Hall effect characteristics of uncoated 20 nm thick In2O3 deposited at 400 ◦C using pure Ar as a sputtering gas and those coated with sputtered SiO2−x layers from Si target at different oxygen concentration with a sputtering temperature of RT(black diamond) and 400 ◦C (red diamond) as a function of oxygen content in the processing gas during SiO2−x deposition.
concentration of 1.65 % sample, see Fig. 7.9. These variations could also be observed from the In2O3 thin films themselves, as the conductivity of In2O3 deposited in the
SiO2−x deposition at 400◦C causes a slight reduction of the conductivity of the In2O3
films with increasing oxygen content in the processing gas, seeFig. 7.9. The Hall effect measurements revealed that this is mostly due to a reduction of the carrier mobility, which slightly decreases with oxygen concentration and having the lowest mobility of 33.5 cm2/Vs for 4.2 % sample. On the other hand, the carrier concentrations do not
show a noticeable change upon SiO2−x coating with values between ≈9.5 × 1019 cm−3
to 1.22 × 1020 cm−3.
In2O3thin films have sufficiently fast oxygen exchange with the ambient at a temperature
of 300◦C [225]. Thereby, it is expected for oxygen to incorporate into the surfaces of In2O3 during the coverage of SiO2−x layers at 400 ◦C. Partially similar situaltion is
expected for RT SiO2−x depositions, as the In2O3 thickness is only 20 nm. The impact
of oxygen is expected for higher oxygen concentrations in the sputter gas and might result in a reduction of the carrier concentration in accordance with oxygen partial pressure (σ ∼ p−1/6O
2 ) dependence of free electrons in In2O3 [116,117]. This is evidently
not the case for the studied films, as the carrier concentration does not decrease with increasing oxygen content, seeFig. 7.9.
The conductivity and Hall effect results indicate that, under the current deposition conditions, the addition of SiO2−x layer on 20 nm In2O3 thin films does not induce
the desired defect modulation doping. This could be related, on the one hand, to the impinging of oxygen particles on the indium oxide surface, which reduce the carrier concentration. On the other hand, the absence of defect modulation doping might be related to the intrinsic point defect properties of SiO2. Richard and coworkers [260]
performed a first principle study of neutral and charged intrinsic-defects in amorphous SiO2. The defect configurations were generated by adding an atom to the silica model
cell (for the interstitials) or removing one from it (for vacancies). The calculations have been performed for all the possible point defect sites in the cell, that are oxygen vacancy (VO), oxygen interstitial (IO), silicon vacancy (VSi), and silicon interstitials
(ISi). Richard et al. [260], found that oxygen interstitials (IO) are the dominant defect
throughout the whole Fermi level range. The variation of formation energies as a function of the Fermi level at 300 K for the important point defects of SiO2 reported by
Richard et al. [260] is shown in Fig. 7.10. According to the calculation the Fermi level position of SiO2 is only 1.25 eV, which is far low than the mid gap of SiO2 and the band
gap is much lower than that of SiO24. Hence, the data can only be used qualitatively.
In addition the, figure does not include the defects at the higher Fermi energies. At the higher Fermi energy the silicon interstitial-Sii can be a donor with a low formation
energy and can pin the Fermi energy above the mid gap.
Therefore, the formation of interstitial oxygen during deposition of SiO2−x would reduce
the Fermi energy of the layers. This will lead to lowering of the In2O3/SiO2 interface
4The band gap of SiO
7.4 Electrical Study
Figure 7.10: Boltzman average of the formation energies (EF) of each studied self-defect in function of the Fermi level at 300 K. (Redrawn from [260]).
Fermi energy. In order to realize the prospect of SiO2 dopant layer in modulation
doping, further optimization of deposition conditions are required. Furthermore, during deposition, the interstitial oxygen should be kinetically suppressed.