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Transparent oxides forming conductor/insulator/conductor heterojunctions for photodetection

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 215203 (http://iopscience.iop.org/0957-4484/26/21/215203) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 26 (2015) 215203 (4pp)

doi:10.1088/0957-4484/26/21/215203

Transparent oxides forming conductor/insulator/conductor heterojunctions for photodetection Satoshi Ishii1,2, Thang Duy Dao1,2,3, Kai Chen1,2 and Tadaaki Nagao1,2 1

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan 2 CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan 3 Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan E-mail: [email protected] Received 1 February 2015, revised 7 April 2015 Accepted for publication 10 April 2015 Published 7 May 2015 Abstract

Photoexcited hot electrons from conductors can be injected into the conduction bands of widebandgap materials, thus enabling the visible and near-infrared (NIR) photoactivities of lightharvesting devices. While metals have been dominantly used as conductors to excite hot electrons, we demonstrate that transparent conductive oxides (TCOs) can also be used for this purpose. Trilayer structures consisting of a thin dielectric layer sandwiched by TCOs show photoresponsiveness in UV, visible, as well as NIR wavelength range. As these trilayer structures are transparent, they can be used to monitor light without blocking it. Keywords: hot eletrons, photocurrent, thin films, transparent conductive oxide, photodetection (Some figures may appear in colour only in the online journal) Introduction

Among conducting materials, metals have been studied most widely and intensively. In metals, photoexcited hot electrons can be extracted by forming either at metal–semiconductor interfaces [3–6] or metal–insulator–metal (MIM) junctions [7–9]. The former is nothing but a Schottky contact that can rectify current. The most conventional way to excite hot electrons is free space illumination. However, recent studies have shown that evanescent excitation in the form of waveguide geometries is also possible [10–12] and that the excitation of surface plasmons is effective for enhancing the photocurrent; these approaches are advantageous for photovoltaic and photocatalytic applications [13, 14]. In contrast to photocarrier generation at p–n junctions, the excitation of hot carriers at Schottky contacts and MIM structures is an internal photoemission process; therefore, the photon energies can be smaller than the bandgaps of semiconductors and insulators forming heterojunctions with metals. This is a strong advantage relative to photodiodes having p–n junctions. For example, while a silicon photodiode can detect up to ∼1100 nm, a silicon–gold Schottky

Oxides used in optics, such as quartz, sapphire, and glass, are generally transparent. These materials show nearly zero absorption in the visible to near-infrared (NIR) spectral range, and therefore, they are widely used as lenses, transparent substrates, and windows. While those oxides are dielectrics, many other oxides are transparent and also conductive. Some commonly used transparent conductive oxides (TCOs) include indium-tin oxide, aluminum-doped zinc oxide (AZO), and, recently, InGaZnO [1]. Research on TCOs has been one of the major fields in the electronics community, with TCOs being used in flat-panel displays and photovoltaic cell electrodes[2]. Although TCOs are considered transparent, their transparency is dependent on their thicknesses. Specifically, TCO films are transparent only when their thicknesses are of the order of a few hundred nanometers or less, at which their absorptions are negligible. When photon absorption occurs in conductors through nonradiative decay processes, hot electrons are excited. 0957-4484/15/215203+04$33.00

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Figure 1. Complex permittivities of AZO and ZnO films, plotted as solid and dashed lines, respectively.

contact can detect even up to 1550 nm, thus covering telecommunication wavelengths [10]. As the excitation of hot electrons is not limited to metals but is also observed in other conductors, it is natural to consider the excitation of hot electrons in TCOs [15, 16]. Hot electron generation in TCOs has been discussed previously [14]; however, thus far, few studies have focused on it in detail [17]. With regard to transparent photodetectors, oxide photodiodes demonstrated thus far can detect only UV light, that were limited by the bandgaps [18–20]. Another type of transparent photodetector has been demonstrated with selenium nanobelts; however, selenium nanobelts, whose bandgap is only 1.67 eV, cover the transparent substrate partially [21]. In the present work, we adopted MIM structures and experimentally demonstrated that TCO-dielectric-TCO (TDT) structures can detect not only UV but also visible and NIR spectra. The TCO and the dielectric used in our experiment were AZO and zinc oxide (ZnO), respectively; however, other TCOs and dielectrics can also be used. When all the three films are thin, the TDT structures are transparent and are hardly visible. Transparent photodetectors may find applications in windows and wearable glasses in which detecting a broad range of light without blocking the view is desired.

Figure 2. (a) Schematic drawing of sample and measurement setup.

(b) Photograph of TDT, with 20 nm thick ZnO layer. (c) Schematic band structure of TDT under bias where EF denotes Fermi level. (d) Measured (exp) and calculated (sim) transmittance (T), reflectance (R), and absorption (A) of TDT sample. ZnO layer thickness was 20 nm in measurement and calculation.

Al doping. The increase of imaginary part in longer wavelength is due to Drude damping which is similar to metals. Thus, one could expect visible and NIR light absorption at AZO. The TDT structures were also fabricated using the same RF sputtering instrument on quartz substrates. After the deposition of the bottom AZO film, a shadow mask was introduced to form a monolayer area for an electrical contact. Subsequently, the middle ZnO and the top AZO films were sputtered. The TDT area was approximately 1 × 5 mm2. The thicknesses of the bottom and top AZO films were always 30 nm, and the thickness of the ZnO film was either 10 or 20 nm. A schematic drawing of a sample with the experimental setup and a photograph of the fabricated sample are shown in figures 2(a) and (b), respectively. Figure 2(c) shows the simplified band structure of the TDT which is similar to a MIM structure. The transmittance of the TDT and the substrate was measured using a spectrometer (EPP2000, StellarNet), as shown in figure 2(d). The transmittance of the quartz substrate itself is ∼93% (data not shown), where the reflection reduced the transmittance. The formation of the TDT on a quartz substrate further reduced the transmittance to ∼85% in the visible and NIR range. Using the retrieved permittivities plotted in figure 1, we calculated the transmittance, reflectance, and absorption of the TDT with an online tool based on the T-matrix approach [25] developed by one of the authors and plotted them in figure 2(d). Although there is a ∼4% discrepancy between the measured and the calculated transmittances, the overall

Experiment and discussion In order to obtain the complex permittivity of the oxides, first, single films of AZO and ZnO were sputtered on silicon substrates by an RF magnetron sputtering instrument at room temperature (i-Miller CFS-4EP-LL, Shibaura). An AZO target containing 2 wt% of Al2O3 and a ZnO target were used for sputtering. While the ZnO film was not conductive, the resistivity of the AZO film was ∼200 kΩ cm. The complex permittivities were retrieved by spectroscopic ellipsometry measurements (M-2000U, J A Woollam). Figure 1 summarizes the real and imaginary parts of the retrieved permittivities. As the bandgaps of AZO and ZnO are 3.2 eV or higher [22–24], light absorption above the bandgaps caused imaginary parts in the UV spectra. In the visible and NIR spectra, AZO still has a nonzero imaginary part arising from 2

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responsivity in UV light, the responsivity in visible light is roughly two orders of magnitude lower and that in NIR light is even lower. Although the absorptions in visible and NIR light do not show much difference, the photon energy in NIR light is smaller than that in visible light, which results in a lower probability of hot electrons crossing the ZnO barrier to contribute to the photocurrent. Figure 3(b) also includes the responsivities of the TDT sample with a 10 nm thick ZnO layer. The thinner the insulator layer in a TDT structure, the higher is the probability of hot electrons crossing the insulator barrier. Therefore, it is reasonable for the responsivities of the TDT sample with a 10 nm thick ZnO layer to be higher for each spectrum. As shown in figure 3(b), we argue that the responsivities of the TDT samples are a few orders of magnitudes lower than those of photodiodes having p–n junctions. However, as our structure is basically transparent in visible and NIR light having ∼1.4% absorption, it could be placed on windows or glasses as an invisible photodetector, which should not block light. For instance, it could be used as a photomonitor for windows or glasses. In addition, the responsivities of TDT detectors can be improved in several ways. As shown in figure 2(c), nearly 20% of the incident light is reflected. This unwanted reflection can be reduced by adding antireflection coatings on the top. Preventing the charge recombination of hot carriers is another effective means for better performance; this could be achieved by adding a few-nanometers-thick blocking layer such as silica. Another possibility is to pattern nanostructures to induce plasmonic resonances [26]. Recent studies have shown that TCOs can serve as alternative plasmonic materials [27, 28], and therefore, nanopatterning of TCOs [29] can be used to introduce plasmonic resonances similar to those of gold and silver, hence harvests light much more with wavelength selectivity.

Figure 3. (a) Photocurrent of TDT sample with 20 nm thick ZnO

layer. When no filters were inserted, the sample was illuminated by the full spectrum of the solar simulator, indicated by ‘full’ in the legend. (b) Responsivities of TDT sample with 20 nm thick ZnO layer (solid lines) and 10 nm thick ZnO layer (dashed lines).

features show good agreement. With regard to the absorption, strong absorption is present in UV range, which is due to light absorption above the bandgaps of AZO and ZnO. Visible and NIR light show ∼1.4% absorption where light is absorbed by the AZO layers. To evaluate the photoresponse of the samples, the samples were illuminated by an AM 1.5 solar simulator (Otentosan, Bunkoukeiki), and the photoresponse was recorded using a source meter (2635A, Keithley). Glass color filters were used to select UV (350 < λ < 395 nm), visible (417 < λ < 706 nm), and NIR (658 < λ nm) light [20]. The irradiance of the solar simulator was 161 mW cm−2, and upon inserting the filters, the irradiances of UV, visible, and NIR light were 0.65, 54, and 82 mW cm−2, respectively. Figure 3(a) shows the photocurrents of the TDT sample with a 20 nm thick ZnO layer. The results show that photocurrents were generated not only by UV illumination and full solar spectrum illumination, which contained a UV spectrum, but also by visible and NIR illumination. The UV response was primarily due to photon absorption above the ZnO bandgap. As the photon energies of visible and NIR light are below the bandgaps of AZO and ZnO, the photocurrents for visible and NIR light are attributed to be hot electrons excited at the AZO layers. To evaluate the sensitivities of the TDT sample, the responsivity was calculated from the data plotted in figure 3(a) and shown in figure 3(b). Compared to the

Conclusion We have experimentally demonstrated that the TDT structures formed by AZO and ZnO can detect not only UV light but also visible and NIR light. The transmittance of the samples was above 80%, and they are essentially transparent. While the photocurrent in UV light is attributed to photon absorption above the bandgap, the photocurrent in visible and NIR light is attributed to the excitation of hot electrons in the AZO layers. We believe that our structures will find applications where photodetection needs to be performed without blocking the light.

Acknowledgments This work is partially supported by the Japan Prize Foundation. 3

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conductor heterojunctions for photodetection.

Photoexcited hot electrons from conductors can be injected into the conduction bands of wide-bandgap materials, thus enabling the visible and near-inf...
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