Thermopower modulation clarification of the intrinsic effective mass in transparent oxide semiconductor $\mathrm{BaSn}{\mathrm{O}}_3$

The exact intrinsic carrier effective mass $m^{*}$ of a well-studied transparent oxide semiconductor $\mathrm{BaSn}{\mathrm{O}}_3$ is unknown because the reported $m^{*}$ values are scattered from $0.06m_0$ to $3.7m_0$. This paper identifies the intrinsic $m^{*}$ of $\mathrm{BaSn}{\mathrm{O}}_3,m^{*}=0.40\pm 0.01m_0$, by the thermopower modulation clarification method and determines the threshold of the degenerate/nondegenerate semiconductor. At the threshold, the thermopower values of both the La-doped $\mathrm{BaSn}{\mathrm{O}}_3$ and $\mathrm{BaSn}{\mathrm{O}}_3$ thin-film transistor structures are $240\mu \mathrm{V}{\mathrm{K}}^{-1}$, the bulk carrier concentration is $1.4\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$, and the two-dimensional sheet carrier concentration is $1.8\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$. When the Fermi energy $E_{\mathrm{F}}$ is located above the parabolic shaped conduction band bottom, the mobility is rather high. In contrast, $E_{\mathrm{F}}$ below the threshold exhibits a very low carrier mobility, most likely because the tail states suppress the carrier mobility. The present results are useful to further develop $\mathrm{BaSn}{\mathrm{O}}_3$-based oxide electronics.

Figure 1

Change in the thermopower $S$ of La-doped $\mathrm{BaSn}{\mathrm{O}}_3$ films grown on (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrates as a function of the carrier concentration $n$ at room temperature. The red line is to guide the eye. Effective mass $m^{*}$ values, which are calculated using Eqs. (1, 2, 3), are shown. The gray line is the $S\text{−}n$ curve calculated using $m^{*}=0.4\pm 0.1m_0$. The dotted line is the threshold of the degenerate/nondegenerate semiconductor around $n=1.4\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. The inset shows a schematic explanation of the energy dependence of the DOS.

Figure 2

$\mathrm{BaSn}{\mathrm{O}}_3$ TFT structure [(a) schematic structure, (b) photograph of the thermopower measurement, (c) the magnified photograph, (d) topographic AFM image of the $\mathrm{BaSn}{\mathrm{O}}_3$ film surface, and (e) cross-sectional HAADF-STEM images]. (a)–(c) Channel length $L$ and channel width $W$ of the TFT are 800 and 400 µm, respectively. Capacitance per unit area $C_{\mathrm{i}}$ of the $a$-C12A7 gate insulator is $39\mathrm{nF}\mathrm{c}{\mathrm{m}}^{-2}$. Two thermocouples (K type, $150\mu \mathrm{m}$ in diameter, SHINNETSU), which are mechanically attached at both edges of the channel, monitor the temperature difference. (d) Stepped and terraced surface. (e) Multilayer structure of Ti/$a$-C12A7/$\mathrm{BaSn}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$. $\mathrm{BaSn}{\mathrm{O}}_3$ is heteroepitaxially grown on the (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. The abrupt heterointerface between $a$-C12A7/$\mathrm{BaSn}{\mathrm{O}}_3$ is clearly shown.

Figure 3

Typical transistor characteristics of the resultant $\mathrm{BaSn}{\mathrm{O}}_3$ TFT at room temperature [(a) transfer characteristics $I_{\mathrm{d}}\text{−}{V}_{\mathrm{g}}$ at $V_{\mathrm{d}}=1\mathrm{V}$, (b) $I_{\mathrm{d}}^{0.5}\text{−}{V}_{\mathrm{g}}$ plot, (c) output characteristics $I_{\mathrm{d}}\text{−}{V}_{\mathrm{d}}$, and (d) field effect mobility ${\mu }_{\mathrm{Hall}}]$. The on-off current ratio is $\sim {10}^3$. The threshold voltage $V_{\mathrm{th}}$ is $+5.5\mathrm{V}$. The maximum field effect mobility ${\mu }_{\mathrm{FE}}$ is $\sim 40\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$.

Figure 4

Electric field modulated $S$ of the resultant $\mathrm{BaSn}{\mathrm{O}}_3$ TFT as a function of the sheet carrier concentration at room temperature. The inflection point occurs around $\left(\right|S|,n_s)=(240\mu \mathrm{V}{K}^{-1},1.8\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2})$. The slope of the $S\text{−}log{n}_{\mathrm{s}}$ plot is $\sim 200\mu \mathrm{V}{\mathrm{K}}^{-1}$/decade (gray line) when $n_{\mathrm{s}}$ exceeds $1.8\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$. These data indicate the threshold of a degenerate/nondegenerate semiconductor is around $n=1.8\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$.