Influence of interband transition on the laser-induced voltage in thermoelectric Ca${}_x$CoO${}_2$ thin films

Laser-induced voltages in incline-oriented thin films of thermoelectric Ca${}_x$CoO${}_2$ have been investigated at four laser wavelengths. Large film in-plane voltage signals, several tens of volts, were identified at each wavelength regardless of the occurrence of interband optical transition. The thermal model based on heat conduction equation demonstrates that this transverse voltage is closely related to heat conduction normal to the film surface and, hence, that it is associated with a phenomenon called the off-diagonal thermoelectric effect. However, distinct discrepancies between the experiment and the thermal model were identified for short laser wavelengths at which interband transition occurred. We discuss the discrepancies in terms of a photovoltaic effect (the photo-Dember effect), showing that the laser-induced voltage may not be purely thermal in origin when it is accompanied by interband transition.

Figure 1

(a) Experimental configuration to measure the ODTE effect. A schematic cross-section view of the tilted Ca${}_x$CoO${}_2$ thin films are shown. The $\mathit{abc}$ axes and the $\mathit{xyz}$ axes represent the crystal axes and the spatial axes, respectively. Dotted lines, which are tilted with respect to the film surface by an angle $\theta$, represent the CoO${}_2$ planes ($\mathit{ab}$ planes) of Ca${}_x$CoO${}_2$.

Figure 2

Absorbance spectrum of a 150-nm-thick Ca${}_x$CoO${}_2$ thin film in the near-infrared to ultraviolet region measured by a UV-vis spectrophotometer. The absorbance measured using a Nd:YAG laser as the light source is also shown. The dotted line is a guide for the eye.

Figure 3

Laser-induced voltage waveforms of a 150-nm-thick Ca${}_x$CoO${}_2$ thin film at various $\lambda$. Solid lines and dashed lines represent the experimental waveforms and the simulated waveforms, respectively. Waveforms at each $\lambda$ are vertically offset for clarity.

Figure 4

Measured voltage waveforms (upper panel) and simulated voltage waveforms (lower panel) of the 400-nm-thick film at (a) $\lambda =1064$ nm, (b) 532 nm, (c) 355 nm, and (d) 266 nm (solid lines). Note that each waveform is normalized by the peak values. Exponential decay fits for each waveform are also shown, by dashed lines.

Figure 5

Film thickness $d$ dependence of the voltage decay time for different $\lambda$. Filled symbols are experimental values, which are characterized by a single-exponential decay. Simulated voltage waveforms, in contrast, are characterized by a double-exponential decay, $A_{\mathrm{s}}\exp (-t/{\tau }_{\mathrm{s}})+A_{\mathrm{f}}\exp (-t/{\tau }_{\mathrm{f}})$, with a slower (${\tau }_{\mathrm{s}}$) and a faster (${\tau }_{\mathrm{f}}$) decay component. Here, only ${\tau }_{\mathrm{s}}$ is shown, for clarity, by dashed lines. Note that ${\tau }_{\mathrm{f}}$ appear only for $d>200$ nm and are nearly $d$ independent in this $d$ range, with values of 6–8 ns. Inset: $d$ dependence of $A_{\mathrm{f}}/(A_{\mathrm{s}}+A_{\mathrm{f}})$, which represents the ratio of the additional fast decay component to the entire voltage signal.

Figure 6

Film thickness dependence of the peak voltage: (a) experimental and (b) simulated by the thermal model. Dashed lines in (a) are a guide for the eye.

Figure 7

Laser fluency $I$ dependence of the peak voltage of a 150-nm-thick Ca${}_x$CoO${}_2$ film measured at different $\lambda$ values. Dashed lines represent linear fits to the data at each $\lambda$ in the low-$I$ range.

Figure 8

(a) Schematic cross section of a tilted Ca${}_x$CoO${}_2$ thin film, which describes the development of the transverse photo-Dember voltage. The left and right panels describe the condition immediately after laser illumination (at $t=0$) and after a certain period of time (at $t=t_1$), respectively. Photoexcited carriers generated near the film surface are expected to diffuse diagonally along the tilted CoO${}_2$ planes. (b) Simulated film thickness dependence of the ODTE voltage $V_{\mathrm{TE}}$ (dotted line), the photo-Dember voltage $V_{\mathrm{pD}}$ (dashed line), and the sum of $V_{\mathrm{TE}}$ and $V_{\mathrm{pD}}$ (solid line). Note that $V_{\mathrm{TE}}$ and $V_{\mathrm{pD}}$ have different polarities. See the polarity of $V_x$ ($=V_{\mathrm{TE}}$) depicted in Fig. 1.