Effective mass of high-mobility ${\mathrm{In}}_2{\mathrm{O}}_3$-based transparent conductive oxides fabricated by solid-phase crystallization

Polycrystalline ${\mathrm{In}}_2{\mathrm{O}}_3$ films, which are solid-phase crystallized (spc) from amorphous films doped with hydrogen (H) and transition metals (TMs), exhibit remarkably high mobilities $(100–160{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1})$; moreover, their specific resistance is equivalent to $2–3\times {10}^{-4}\mathrm{\Omega }\mathrm{cm}$ or less, even when the carrier density is reduced to one-fifth $(1–4\times {10}^{20}{\mathrm{cm}}^{-3})$ that of conventional transparent conductive oxide (TCO) films. The high mobility and low carrier density significantly reduce free-carrier absorption, and the transparent region of the TCO films extends across the visible and near-infrared regions. In this study, we experimentally demonstrate that spc-${\mathrm{In}}_2{\mathrm{O}}_3$:H and ${\mathrm{In}}_2{\mathrm{O}}_3$:TM,H films with an impurity concentration of less than a few at. % exhibit superior mobility compared to that of optimized ${\mathrm{In}}_2{\mathrm{O}}_3$:Sn,H films $(\sim 70{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1})$ with similar Sn and close-carrier concentrations. Photoelectron spectroscopy measurements revealed that the electronic state of the spc films, including the valence of TM impurities, changed with varying In/O ratios of the amorphous films. Furthermore, the In, O, TM, and H compositions in the amorphous films were found to be critical for effective activation of ${\mathrm{TM}}_{\mathrm{In}}^{+}$, ${\mathrm{H}}_{\mathrm{O}}^{+}$, and ${{\mathrm{H}}_{\mathrm{i}}}^{+}$ donors after crystallization. To determine the origin of the high electron mobility, the effective mass and relaxation time of electrons in these films were evaluated by spectroscopic ellipsometry. The results showed that the high mobility could be primarily attributed to the long relaxation time instead of the small effective mass. Additionally, the dispersion of conduction bands near the Fermi energy was found to be almost independent of the type of metallic impurity (Sn, H, Ce, Zr, and W) for the investigated impurity concentrations. Moreover, the increase in relaxation time by H and TM doping was examined.

Figure 1

(1) Plan-view TEM images and (2) electrical properties [resistivity $\left(\rho \right)$, carrier density (N), and mobility $\left(\mu \right)$] of (a) as-deposited and (b) postannealed polycrystalline ${\mathrm{In}}_2{\mathrm{O}}_3$:Me films deposited without intentionally adding water vapor, and (c) as-deposited amorphous and (d) postannealed solid-phase crystallized ${\mathrm{In}}_2{\mathrm{O}}_3$:Me,H films deposited under an optimized water-vapor pressure. In (1), TEM images for metallic-impurity free ${\mathrm{In}}_2{\mathrm{O}}_3$ are shown.

Figure 2

(a) Resistivity $\left(\rho \right)$, carrier density (N), and mobility $\left(\mu \right)$ values of $a$- and spc-${\mathrm{In}}_2{\mathrm{O}}_3$:H films as a function of oxygen flow ratio [$F_{\mathrm{MS}}\left({\mathrm{O}}_2\right)$] during magnetron sputtering (MS). (b) N and μ of ${\mathrm{In}}_2{\mathrm{O}}_3$:H films deposited at different $F_{\mathrm{MS}}\left({\mathrm{O}}_2\right)$ values as a function of temperature. The properties were measured during heating from room temperature to $200{}^{\circ }\mathrm{C}$ and during cooling from 200 to $40{}^{\circ }\mathrm{C}$ in vacuum. During the heating, solid-phase crystallization occurred at temperatures over $150{}^{\circ }\mathrm{C}$. (c) TDS profiles of substrate and $a\text{−}{\mathrm{In}}_2{\mathrm{O}}_3$:H films for In (115) and ${\mathrm{O}}_2$ (32). (d) Refractive index ($n$) and extinction coefficient ($k$) of spc-${\mathrm{In}}_2{\mathrm{O}}_3$:H films obtained by point-by-point fitting.

Figure 3

Spectra near the (1) valence band, (2) valence-band minimum, and (3) Ce $2p_{3/2}$ or W $3d_{5/2}$ core-level spectra of as-deposited amorphous and solid-phase crystallized (a) ${\mathrm{In}}_2{\mathrm{O}}_3$:Ce,H and (b) ${\mathrm{In}}_2{\mathrm{O}}_3$:W,H films prepared by RPD at different $F_{\mathrm{RPD}}\left({\mathrm{O}}_2\right)$ values.

Figure 4

(a) Spectra near the VBM and (b) core-level spectra of Ce $2p_{3/2}$ for spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Ce,H films prepared by RPD and MS with varying $F_{\mathrm{RPD}}\left({\mathrm{O}}_2\right)$ and $F_{\mathrm{MS}}\left({\mathrm{O}}_2\right)$, respectively.

Figure 5

Spectra near the (a) valence band and (b) VBM of spc-${\mathrm{In}}_2{\mathrm{O}}_3$:H, ${\mathrm{In}}_2{\mathrm{O}}_3$:W,H, ${\mathrm{In}}_2{\mathrm{O}}_3$:Ce,H, and ${\mathrm{In}}_2{\mathrm{O}}_3$:Sn,H films with similar carrier densities.

Figure 6

Hall mobility $\left(\mu \right)$ as a function of Hall carrier density (N) for spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Me,H films fabricated by (a) MS and (b) RPD using different ceramic targets and tablets, respectively.

Figure 7

(a) SE spectra obtained from the spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Ce,H (red) and spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Sn,H (black) films on ${\mathrm{SiO}}_2/\mathrm{Si}$ substrates. (b) SE spectra and the result of the SE fitting analysis for the spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Ce,H sample. Optical model for a TCO layer formed on a ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate is also shown in the inset. (c) Extracted point-by-point fitted dielectric functions (open circle) and calculation result obtained from the parametrization assuming the TL and Drude models (solid line) for spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Ce,H and spc-${\mathrm{In}}_2{\mathrm{O}}_3$:Sn,H films with similar electrical carrier density and different mobility. The inset shows the enlarged dielectric function in the infrared region. The open circles and triangles are drawn every ten data values for clarity.

Figure 8

(a) Optical mobility $\left({\mu }_{\mathrm{opt}}\right)$ as a function of Hall mobility $\left(\mu \right)$ and (b) optical carrier density $\left(N_{\mathrm{opt}}\right)$ as a function of Hall carrier density (N) for the spc films deposited by MS and RPD. The plot labels are identical to those shown in Fig. 6.

Figure 9

(a) Optical mobility $\left({\mu }_{\mathrm{opt}}\right)$, (b) optical effective mass $(m_{\mathrm{opt}}^{*}/m_0)$, and (c) carrier relaxation time $\left({\tau }_{\mathrm{opt}}\right)$ of the spc films deposited by MS and RPD as a function of optical carrier density $\left(N_{\mathrm{opt}}\right)$. The plot labels are identical to those shown in Fig. 6.