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Sub-bandgap photodetection from titanium nitride/germanium heterostructure Satish Laxman Shinde, Satoshi Ishii, and Tadaaki Nagao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Sub-bandgap photodetection from titanium nitride/germanium heterostructure Satish Laxman Shinde,1,* Satoshi Ishii,1 and Tadaaki Nagao1,2,* 1

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan 2

Department of Condensed Matter Physics Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

KEYWORDS: hot electron, sub-bandgap photodetection, titanium nitride, germanium, photoelectric effect.

ABSTRACT: Photo-excited hot carriers through a non-radiative decay offer new opportunities for harnessing longer wavelength light. Here, we have demonstrated a hotcarrier mediated sub-bandgap photodetection in germanium-based planar heterojunction devices. The planar samples that form in-situ germanium/titanium nitride (Ge/TiN) interfaces are fabricated by DC sputtering technique, and the generation of photocurrent by nearinfrared (NIR) light illumination is confirmed up to 2600 nm, well exceeding the absorption limit of Ge. The photocurrent obtained with nickel contacts is three order larger than that obtained without metal contacts or with gold contacts in similar structures. The specific detectivity (D*) value for TiN/Ge photodetector is obtained to be 6.32  105 Jones at the subbandgap excitation wavelength of 2000 nm without applying any bias. The superior performances of our device are attributed to the broad absorption of the TiN, the plasmonic 1 ACS Paragon Plus Environment

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hot carrier transfer from the TiN to Ge and built-in potential of the TiN/Ge non-ohmic junction, which allows efficient separation of photoexcited electron–hole pairs. Our results further support the use of TiN, which is robust and cost-effective, as an alternative to metals for NIR photodetection and photovoltaics when it forms a heterostructure with Ge.

INTRODUCTION Semiconductors are suitable for photodetection when the incident photon energy is larger than the bandgap. In contrast, for sub-bandgap excitations, the absence of absorption results in no photon detection in linear range.1-7 However, using hot carrier excitations from metal nanostructures allow to detect low-energy photons. By placing metallic nanostructures on a semiconductor, extra energy levels for sub-bandgap absorption is created. This allows to overcome the limitation arising from the intrinsic bandgap of the semiconductor.8-12 Since metals do not have bandgaps, photo-excited hot carriers can be injected into the adjacent semiconductor. Hot carriers can be utilized for various applications such as photocatalysis, photovoltaics, and photodetection. 13-17 Hot-carrier based photodetectors widen the measurement range of the detector toward the smaller energy than the semiconductor bandgap, thus paving a way to a new paradigm in photodetection as well as in photovoltaics. The excitation of hot carriers in metal-insulator or metal-semiconductor contacts are wellstudied.8-15 Various metals (gold11, silver12, copper18, and aluminum19), transparent conductive oxides (aluminum-doped zinc oxide20, heavily doped molybdenum trioxide21, and tungsten oxide22, SrRuO323), carbides (titanium carbide24, tantalum carbide25), transition metal nitrides (titanium nitride (TiN)26-28, zirconium nitride29, and tantalum nitride30) have been used to excite hot carriers in the visible (Vis) to NIR region. With regard to transition metal nitrides, because of their metallic band structures and high carrier concentrations (~1022 cm-3), making them suitable candidates for efficiently exciting hot carriers in the Vis-NIR region.26, 29-30 Very recently TiN nanoparticles and thin film have been used to demonstrate 2 ACS Paragon Plus Environment

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Vis-NIR photodetector and photocatalysis, where the photocurrent generations are either due to the enhanced scattering and near-field enhancement of localized surface plasmon resonances (LSPRs) or due to Landau damping followed by direct electron transfer.26, 31-32 In a photodetector, the responsivity is directly proportional to the mobility of free carriers. In this regard, Ge is advantageous because it has a larger carrier mobility and smaller bandgap compared to Si, which enable broader optical absorption extending into the NIR region.33-36 Since Ge is an elemental semiconductor, its thin film can be grown easier compared to compound semiconductors such as InAs and InGaAs which are difficult to control stoichiometry. Also, Ge has a higher refractive index and extinction coefficient than silicon, resulting in stronger light localization at the its interface.33-34 In spite of these remarkable properties, Ge has not been explored intensively as a photodetector beyond the NIR (> 1800 nm) region.35 So far, device-grade crystalline bulk Ge, metal/crystalline Ge heterostructure has been studied mostly in the Vis−NIR region (up to the wavelength of 1800 nm), practically for high performance Ge photodetectors and photoelectric application. 33-35, 3741

Also, amorphous Ge (a-Ge) is known to be highly absorptive and suitable for light

harvesting and also found to be useful for reduction in dark current for photodetection. 42-45 Hence, a-Ge with plasmonic material will be a novel and effective route for NIR absorption and photodetection. Here, we experimentally demonstrated hot-carrier mediated sub-bandgap photoresponse in a-Ge-based planar heterojunction structure. The planar samples that form Ge/TiN interfaces were fabricated in-situ by DC sputtering technique. The fabricated TiN/Ge interface formed non-ohmic contact and showed absorption in the Vis-NIR region. The generation of photocurrent by NIR light illumination was confirmed up to 2600 nm that well exceeds the absorption limit of Ge. The sub-bandgap excitations was attributed to the hot carrier transferred from the TiN to Ge. In contrast, Ge film alone generated two orders of magnitude

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lower photocurrent for sub-bandgap excitations. The photocurrent at the TiN/Ge interface was further enhanced by improving the contacts with Ni interlayer. Our results facilitate the use of TiN to extend the detection limit of photodetector and photovoltaics when it is combined with a-Ge. EXPERIMENTAL SECTION Fabrication: A silicon wafer with 100 nm thick SiO2 layer was used as a substrate. A kapton (polyimide) tape was used as a deposition mask to define a photodetector area of ~ 1 mm by 1 mm. The TiN and Ge films were sputtered on masked substrate. In the first step, 20 nm bottom Ge film was deposited with a Kapton tape mask. In the second step, after adding a mask, (to have the clean interface for TiN/Ge) 10 nm of Ge and TiN films were deposited (shown in the dotted pattern in Figure 1(a)). Removing the first mask, in the third step, 20 nm TiN film was finally deposited. This process results in 30 nm think Ge film and TiN at the interface while the single film regions were 20 nm thick. Two different types of device geometries were fabricated, i.e. top TiN (substrate/Ge/TiN) and top Ge (substrate/TiN/Ge). The total thicknesses of the TiN and Ge films at the hetero junction area were always maintained at 30 nm. The TiN/Ge photodetector was electrically connected either directly or by depositing Au and Ni contacts away from the photosensitive area (Figure 1(a)). The thicknesses of the Au and Ni film contacts was fixed at 30 nm. In the Supporting Information, the detail sputtering deposition parameters are provided as Table S1. Characterization: The scanning electron microscope (SEM) images the samples was recorded by Hitachi FE-SEM SU8000 microscope. The X-ray diffraction (XRD) pattern was measured by a Rigaku Ultima III, Rint 2000 instrument. Raman spectra were recorded by WITec system 300 alpha with 532 nm wavelength Nd:YAG Laser. The reflectance, permittivity and carrier concentration of the films were measured using a UV-Vis spectrometer (V-570, Nihon Bunko), spectroscopic ellipsometer (SE 850, Sentech) and a Hall


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measurement instrument (RegiTest 8400ACRJ, Toyo Co.), respectively.26 The photocurrents of the samples were recorded using a sourcemeter (2635, Keithley) with a solar simulator (OTENTO-SUN IV, Bunkoukeiki) combined with a monochromator for the visible range and a wavelength tunable NIR laser system (Solstice, Spectra-Physics; TOPAS Prime, SpectraPhysics) for the NIR range. The average power in the visible was ~1.2 mW with the 2 mm by 2 mm illumination area. The average laser power in the NIR range was on the order of a few tens of mW, and the repetition rate was fixed at 1 kHz with a diameter of ∼3 mm. was normally incident at the detection area. During the photocurrent measurement, each sample was illuminated at normal incidence without bias. When plotting the photocurrent, the sign of the current was taken to be negative when the electrons flew from the top TiN layer to the bottom Ge layer through an external circuit. As such, the sign of the photocurrent is negative for top TiN sample and positive for top Ge sample as we always connect the top and bottom layers to the positive and negative terminals, respectively, of the source meter. The electrons always flow from TiN to the Ge for sub-bandgap excitations.

RESULTS AND DISCUSSION The SEM images of the individual TiN and Ge films are shown in Figures S1(a) and S1(b). The films are observed to be continuous and smooth. A schematic diagram of the TiN/Ge sub-bandgap detector is shown in Figure 1(a). The SEM image of the top view of TiN/Ge device is shown in Figure 1(b). The top and bottom layers are TiN and Ge, respectively. As the films were deposited layer-by-layer a height difference in the top and bottom layer is clearly seen (as shown in the schematic of Figure 1(a)). The XRD pattern of films is shown in Figure S1(c). The broad peaks are observed, suggesting that the as-sputtered Ge film was amorphous-like. The peaks are indexed to the cubic structure of Ge (JCPDS No. 04-0545).46



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In XRD pattern of TiN film, the strong peaks are observed and indexed to the cubic structure of TiN (JCPDS No. 38-1420).47

Figure 1. (a) Schematic diagram of a TiN/Ge sub-bandgap photodetector, and TiN/Ge films deposition steps from 1 to 3. The region ② is formed in one vacuum chamber without exposing to air. (b) SEM image of TiN/Ge interface. (c) Real and (d) imaginary parts of the permittivities for Ge and TiN films, respectively. The Raman spectra as shown in Figure S1(d), confirm the TiN and Ge phases of the as sputtered films. For Ge film, the intense peak at 300 cm-1, tailing for low energy side, corresponds to optical phonon of Ge with poor cristallinity.48-49 The broad optical phonon peaks are observed for TiN film. The broad peaks centered at 240, 308 and 554 cm–1 are related to the first order Raman vibration in TiN due to nitrogen vacancies defects.50-51 The carrier concentration of TiN film obtained from Hall measurement is 7.92 × 10 22 cm−3.The permittivities of TiN and Ge are plotted in Figures 1(c) and 1(d). The permittivity of TiN was fitted by the Drude-Lorentz model.52 The real part of the permittivity of TiN (Re(TiN)) 6 ACS Paragon Plus Environment

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becomes negative at the wavelength greater than 567 nm. The permittivity of the Ge film was fitted by the Tauc-Lorentz model (Figures 1c and d) and was used in reflection and absorption calculations.

Figure 2. Experimental absorptance (A) spectra for (a) individual TiN and Ge films on the Si/SiO2 substrate. Experimental (solid line) and calculated (dotted line) total reflectivity (R), transmittance (T) and absorptivity (A) of the (b) top Ge and (c) top TiN layer. Figure 2(a) shows the absorptance spectra of individual TiN and Ge film deposited on Si/SiO2 substrate. Both the TiN and Ge films show the broad absorptions over a visible to NIR range. Figure 2(b and c) show the measured and analytically calculated total reflectivity, transmittance and absorptivity spectra of top Ge and top TiN. The reasonable agreement between the calculated and measured absorptance verifies our measurement. Figure 3(a) shows the I-V curve of the bare Ge and TiN/Ge sub-bandgap detector sample under dark. The asymmetric I-V curve indicates a non-ohmic contact at the TiN and Ge



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interface. The bare Ge formed a non-ohmic contact with the probe (Figure 3(a)). From the literature, the Fermi level of TiN is 3.5 – 4.6 eV,26,

53-54

and the electron affinity, work

function and band gap of Ge estimated from absorption and hall measurements are 4.0 eV, 4.17 eV, 0.69 eV, respectively. These values indicate that a non-ohmic contact can be formed at the TiN/Ge interface (Figure 3(b)). Considering the band alignment of TiN and Ge, there could be a barrier height of 0.3 eV for electrons. This limit the detection wavelength of TiN/Ge detector to 0.3 eV (i.e. ~ 4100 nm). The energy band diagram plotted assuming the bulk properties of TiN and Ge, at nanoscale level the band alignment may vary from the present one. Especially, the curvature of the band bending can be more straight compared to the exponential one. This suggests that, for charge carrier transport, both hot electrons and holes can contribute upon optical illumination. The wavelength dependence of the photo-responsivity is shown in Figure 3(c and d). Figures 3(c) and 3(d) show the photoresponse for the range of 400 – 800 nm and for the range of 1600 – 2600 nm, respectively. As the current values are different in significant magnitude for the Vis and NIR range, the photo-responsivity values are plotted in separate graphs. From Figure 2(a), for 30 nm Ge film, the absorption longer than 1770 nm (i.e. below bandgap energy) is very weak. As expected, at sub-bandgap excitations, the photoresponsivity of bare Ge film is very small compared to the excitation above the bandgap energy. For the case of Ge with top TiN sample, the enhancement in photo-responsivity is observed. As TiN has broad absorption in the visible to the NIR region, the improvement in photo-responsivity is expected. For top TiN and top Ge samples, the magnitudes of photoresponse did not vary significantly for the excitation energy higher than the bandgap of Ge. However, for sub-bandgap excitation, in the case of top TiN geometry, the photoresponse is two times higher compared to that of top Ge geometry. For all the devices, the TiN thickness of 30 nm was kept constant. To further investigate the role of TiN thickness on


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TiN/Ge device performance, we fabricated the devices with top TiN thickness of 20 and 50 nm. Figure S2 shows the photo-responsivity for the excitation wavelength of 1600 – 2600 nm for top TiN detector with TiN and Ge film thickness of 20, 30 and 50 nm. For TiN, thinner film (20 nm) absorb less light, and thicker film (50 nm), block the light to reach at the interface, this leads to smaller photocurrent generation than 30 nm thick TiN. We found that the top TiN 30 nm is best among all the samples.

Figure 3. (a) I-V curve of bare Ge and TiN/Ge for top TiN and Ge geometry. (b) Schematic band diagram of TiN/Ge before and after contact. Wavelength-dependent photo-responsivity at zero bias for bare Ge and TiN/Ge sub-bandgap detector (c) for the range of 400 to 800 nm and (d) for the range of 1600 to 2600 nm. Schematics of the carriers transfer pathways for sub-bandgap excitations (>1800 nm) in the TiN/Ge heterojunctions for (e) top TiN and (f) top Ge geometries. 9 ACS Paragon Plus Environment


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Under illumination, the TiN/Ge device shows a significant photoresponse due to the builtin potential from the non-ohmic junction at the TiN/Ge interface (Figure S3). The depleted region by the built-in potential allowed efficient separation of photoexcited electron–hole pairs, which enabled the TiN/Ge photodetector to operate at zero bias. Figure S4(a-c) shows the time-dependent photocurrent response of bare Ge, top TiN and top Ge samples without bias at 500, 1000, 2000, and 2600 nm where the thickness of both TiN and Ge film are 30 nm. In case of both the geometries i.e. top TiN or top Ge, the current flow corresponds to hot electrons transfer from the TiN layer to the Ge layer. The sign of the photocurrent is negative (i < 0) for the top TiN geometry (Figure 3(e)) and positive (i > 0) for the top Ge (Figure 3(f)) geometry in TiN/Ge. The photocurrent magnitude of the top TiN sample is higher than that of the top Ge sample particularly in NIR range, which cannot be simply explained by the relative comparison of the absorption as shown in Figures 2(b) and 2(c). The plasmonic local heating effect of TiN can also contribute in enhancement in hot carrier mediated photo-response. As Wen et. al. reported, the photothermally induced plasmonic local heating effect of Au improves the performance of the plasmonic absorber based Si photodetectors.55-56 For our TiN/Ge detector (Figure S4), we observed only fast photoresponse, which correspond to drift of electrons from TiN to Ge. As the slow photoresponse that corresponds to photothermally generated hot carrier was not observed in TiN/Ge detector, even if plasmonic local heating effect of TiN was present, their contribution to photo-response was minimal or negligible. Next we show how the deposition of Ni or Au interlayer on TiN and Ge film improves the contact to enhance the photocurrent. The Ni or Au film deposited on TiN and Ge film are well separated from the photosensitive area of TiN/Ge (Figure 1a). The I-V curve of TiN/Ge with Ni contact is shown in Figure 4(a). With the Ni contacts the amplitude of the current increased by three order than without Ni for TiN/Ge (Figure 3(a)). The photo-responsivity of


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TiN/Ge with Ni contact is shown in Figure 4(b and c). The photo-responsivity is also observed to be three orders higher magnitude compared to that of without Ni or Au contacts TiN/Ge in the Vis-NIR range. The Ni interlayer is observed to improve the contact conductivity.

Figure 4. (a) I-V curve of bare Ge and TiN/Ge for top TiN and Ge geometry with Ni contacts. Wavelength-dependent photo-responsivity at zero bias for bare Ge and TiN/Ge sub-bandgap detector with Ni contacts, (b for the range of 400 to 800 nm and (c) for the range of 1600 to 2600 nm. Figure 5(a-c) shows the time-dependent photocurrent response of bare Ge, top TiN and top Ge samples with Ni contacts without bias at 500, 1000, 2000, and 2600 nm excitations. The magnitudes of photocurrent for top TiN-Ni and top Ge-Ni at 2600nm are always higher than the sample without TiN contacts, as expected from Figure 4(b and c). The Ni contacts



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facilitate in efficient charge transport across the device and probe, resulting in enhanced photodetection.

Figure 5. Time-dependent photoresponse of (a) Ge, (b) top TiN and (c) top Ge with Ni contacts at the excitation wavelengths of 500, 1000, 2000 and 2600 nm. All the measurements was done at zero bias and the light was turned on and off for every five seconds. Similarly, for the TiN/Ge with Au contact, the Au forms a non-ohmic contact with Ge as the asymmetric I-V curve is observed for Au/Ge (Figure S5(a)). The bias dependent photoresponsivity under different excitation wavelength for the top TiN and top Ge geometries with and without Ni and Au contacts are shown in Figure S6 and S7. We observed clear photoresponse change under different excitation wavelength for all the devices. The photoresponsivity of TiN/Ge with Au contact is improved by two order of magnitude than bare TiN/Ge in Vis-NIR range (Figure S5(b and c)). Figure S8(a-c) shows the time-dependent photocurrent response of bare Ge, top TiN and top Ge samples with Au contacts. With the Au contacts for TiN/Ge, the photocurrent response (Figure S5(b and c)) is three order higher than the bare Ge and nearly three-fold less than of TiN/Ge with Ni contacts (Figure 5(b and c)). Overall, the bare Ge shows very weak or negligible photoresponse for sub-bandgap 12 ACS Paragon Plus Environment

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excitations, however, this further get improved by three order higher by forming TiN/Ge heterostructure with an addition of Ni contacts. The photo-response maxima is observed around 1800 nm for TiN/Ge with Ni (Figure 4c) and Au (Figure S5c) interlayer sample, whereas TiN/Ge without Ni/Au interlayer (Figure 3d) samples did not show the photo-response maxima around 1800 nm. We also found that the TiN/Ge detector did not precisely follow the absorption dependent photo-response. As observed previously for plasmonic metals, reported by Halas et. al., 57 an ohmic contact of Au with Ti-TiO2 shows the absorption-dependent photo-response (photocurrent mostly due to both plasmon decay and interband transition in Au), while Schottky contact (Au-TiO2) shows nearly absorption-independent photo-response (photocurrent due to only plasmon decay in Au). We expect that due to the non-ohmic nature of TiN/Ge heterojunction, practically it does not show strong absorption-dependent photocurrent. Also, the addition of Ni and Au interlayer found to greatly improve photo-excited carrier transport across the contacts/probes. The TiN/Ge heterojunction becomes slightly rectifying and the conductivity of the device increased by three order as ascertained from the I-V curves shown in Figure 3(a) and 4(a). The photo-responsivity peak around 1800 nm is largely related to the band edge absorption of Ge. Overall, due to the band edge absorption in Ge as well as improved conductivity of the device after the addition of Ni and Au, results in maxima in photoresponse around 1800 nm. For TiN/Ge detector, the photoresponse from 1800 to 2600 nm is almost constant. We expect that for wavelengths longer than 2600 nm, the photo-responsivity may decrease up to detection wavelength of ~ 4100 nm. Another possible reason might be that minute current generation takes place via thermoelectric effect across the TiN/Ge junction. Since the TiN layer is absorbing light very efficiently in the broad wavelength region below the band edge, it can uniformly generate temperature gradient in the depth direction throughout the wide spectral region. Therefore optical absorption can takes place irrespective of incident 13 ACS Paragon Plus Environment


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wavelength as we can see in their flat spectral feature (Figures 2(a) and (b)) and also the induced thermoelectric effect at the TiN/Ge interface can exhibit flat wavelength dependence, we assume. The superior performance for photocurrent generation of top TiN with Ni contacts than with Au contact can be explained as follows. Form Figure 3(c and d), Figure 4(b and c) and S5(b and c), it is observed that the photocurrent of TiN/Ge device gets improved by forming contacts with the interlayer of Ni or Au. The Ni and Au form chemical contacts or alloy with Ge, which reduces contact resistance. Ni is well known to diffuse in Ge and forms a metallic NiGe alloy, similar to Si, which has lower electrical resistivity for the smaller concentration of Ni.58 During Ni deposition it is expected that small percentage of Ni can diffuses in to Ge, and this forms NiGe alloy at the interface. The Au forms eutectic alloy with Ge at higher temperature (361 oC), which has lower electrical resistivity only at high temperature. 59 In our case, Au deposition on Ge was carried out at room temperature, hence the chance of formation of AuGe alloy is low. Also, from the I-V curve for TiN/Ge, compared to without and with Ni and Au (Figure 3(a), 4(a) and S5(a)), we observed the three order increased in dark current. Since TiN/Ge with Ni contacts have higher photoresponse than with Au contacts, as ascertained from Figure 4(b, c) and S5(c, d). So it is known and also confirmed here that Ni and Au on Ge improved the conductivity at the contacts. Overall, Ni and Au contacts facilitated efficient charge transport across the device to increase the photocurrent by three orders. Lastly, we discuss on responsivity and detectivity of our device. The responsivity (R) is the minimum number of photons that device can detect, and defined as the photocurrent per unit power. R is calculated using the formula, R  = (Ip - Idark)/(PS) and external quantum efficiency (EQE), by the relationship EQE = hcR/(q), where Ip is the current upon illumination, Idark the dark current, P the illumination intensity, S the effective area under 14 ACS Paragon Plus Environment

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illumination, and  the wavelength of the incident light. 2, 60 The photocurrent responses in the TiN/Ge device with Ni and Au contacts at zero bias are presented in Table S2. For the top TiN sample with Ni contacts, the room-temperature values of R, at zero bias at an excitation wavelength of 2000 nm are found to be 7.17  10-5 A/W. The response time is typically defined as the time it takes for the detector to increase its output from 10% to 90% of final output level. Overall, for all the TiN/Ge devices, the response times were less than 100 ms. Detectivity (D*) is the effectiveness of the detector in the presence of noise and is the standard figure of merit expressed as D* = (Sf)1/2R/In where f is the electrical bandwidth (Hz) and In the noise current.2, 60-61 For a photovoltaic detector, the noise mainly comes from the thermal noise, the shot noise, and the flicker noise. 62-63 At zero bias, no reverse current flows through the TiN/Ge device within the detection limit of the current measurement setup. So, contribution from the shot noise and flicker noise is negligible in our devices, and the thermal noise mostly is responsible for noise current. The detectivity is evaluated by considering the major contribution from the thermal (Johnson) noise. 63 The specific detectivity is expressed as D* = R/(4KB T/Rd S)1/2 where, KB is the Boltzmann constant, T is the room temperature. Rd is the zero-bias differential resistance of detector, which is obtained by Rd = (∂V/∂I)V=0.The room-temperature value of D* without applying any bias for the top TiN-Ni device is obtained to be 6.32  105 Jones. The detectivity values for all the devices are presented in Table S2. The quantum efficiency of the TiN/Ge devices are shown in Table S3. The R , D* and EQE values of our TiN/Ge-Ni devices are relatively smaller than to those reported for other high-performance narrow-bandgap crystalline Ge based Vis-NIR detectors (Table S4). It is to be noted that the performance of devices are reported at zero bias and for amorphous Ge. There are various reasons for lower performance of devices such as amorphous nature of Ge



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film, not optimized the device geometry for maximum absorption, and non-ohmic nature of TiN/Ge interface. While Ge is used as a semiconductor in the present study, the photocurrent amplitude and detection wavelengths of the structure depend on the

semiconductor, as well as the

plasmonic material. Further, the performance of a device is expected to be higher for the crystalline Ge and TiN heterostructure. Recent studies have shown that absorption of TiN is enhanced by fabricating nanostructures including optical metamaterials.32, 64-65 Thus, the hot carrier excitation in TiN can be improved by tailoring the carrier concentration and fabricating plasmonic nanostructures, and the better performance than the presented for TiN/Ge detector is expected. However, this is the first and preliminary study for using TiN for semiconductor/TiN hetrostructures for sub-bandgap photodetection. CONCLUSIONS In summary, we experimentally demonstrated hot-carrier mediated sub-bandgap photoresponse in Ge-based planar structure with a TiN thin film. The planar samples that formed in-situ Ge/TiN interface were fabricated by DC sputtering technique, and the generation of photocurrent by NIR light illumination was confirmed at least up to 2600 nm which is much longer than the absorption limit of Ge. The photocurrent obtained with Ni contacts was much larger than that obtained with Au contacts in the similar structures. Our findings pave the way toward the rational design of optoelectronic devices by combining plasmonic ceramics with semiconductors for NIR photodetection and photovoltaics.

ASSOCIATED CONTENT Supporting Information. Sputtering conditions; SEM, XRD and Raman spectra of TiN and Ge films; TiN thickness dependent photoresponse; TiN/Ge I-V curve; time-dependent photoresponse of Ge and TiN/Ge; bias and excitation wavelength dependent photoresponse, 16 ACS Paragon Plus Environment

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I-V curves, photocurrent, and time-dependent photoresponse of TiN/Ge with Au contacts; table of responsivity and detectivity; table of externa quantum efficiency; summary of the photodetection properties of various Ge based photodetectors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ACKNOWLEDGMENTS This work was partially supported by the JSPS KAKENHI (16K17496, 16F16315,

16H03820, 16H06364, JP16H06364,17H04801, and 17K19045) and CREST "Phase Interface Science for Highly Efficient Energy Utilization" (JPMJCR13C3) from JST, Japan, the JFE 21st Century Foundation, the Kao Foundation for Arts and Science, and the Japan Association for Chemical Innovation.

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