Interband plasmonic nanoresonators for enhanced thermoelectric photodetection

1 Introduction

Photodetection – the conversion of photons into an electrical signal for the purpose of detecting electromagnetic radiation – has a wide range of applications including imaging, optical communication, optical instrumentation, and solar cells. In the visible spectrum, photodiodes based on photoexcitation in semiconductors are commonly used for their high speed and sensitivity [1]. An alternative to photodiodes is thermoelectric detectors with certain advantages, such as broadband sensitivity from ultraviolet to far-infrared, greater resilience to electromagnetic interference and temperature fluctuations, and the ability to operate without external power [2], [3], [4], [5], [6]. The reliability of thermoelectric devices has made them the superior choice for extreme environments and they are, for instance, found aboard space vehicles. These devices exploit the thermoelectric effect, where a photo-induced temperature difference in a material generates an electric potential difference [6]. On the downside, the electrical signal generated by the thermoelectric effect is relatively weak compared to photoexcitation in semiconductors, and thermoelectric detectors are thus limited by their low detectivity [2].

Detectivity can be enhanced by utilizing high zT thermoelectric materials, miniaturizing the device, and increasing optical absorptance. Telluride compounds, such as Sb₂Te₃ and Bi₂Te₃, are well known for their outstanding thermoelectric properties. These materials exhibit high Seebeck coefficients and low thermal conductivity, which are essential for efficient thermoelectric conversion [7]. These properties make them highly effective in generating electrical power from temperature gradients and in applications requiring heat-to-electricity conversion. Because of these advantages, Sb₂Te₃ and Bi₂Te₃ have been used in thermopile photodetectors [5], [8], [9], [10]. However, the photo-thermoelectric performance of these materials is limited by poor optical absorptance of ∼40 % in the visible range, which is essential for generating a strong electrical photoresponse. Enhancing the optical absorptance in thermoelectric devices is thus a key factor that can improve detection efficiency.

Previous efforts to increase optical absorptance have employed indirect approaches, where a highly absorptive blackened material or structure is coated onto the thermoelectric materials. For instance, metallic metasurface absorbers have been added onto thermoelectric materials to enhance detectivity [2], [8], [11]. As these absorbers heat up from light absorption, thermal conduction into adjacent or underlying thermoelectric materials then increases its temperature. A consequence of indirect heating may be a reduced responsivity of the photodetectors. Adding an absorbing material increases the total heat capacity of the device, leading to reduced temperatures and slow response times. Nonetheless, these results indicate that plasmonic properties of noble metals can be exploited to concentrate and trap light within the active material, effectively increasing the absorption cross section and improving device performance [12]. Additionally, the tunability of plasmonic nanoresonators allows for precise control over the absorption spectrum, enabling optimization for specific applications and wavelengths of interest.

Intriguingly, p-block compounds, such as Sb₂Te₃ and Bi₂Te₃ and even p-block elements like Si, have recently received attention because features in their electronic band structure lead to strong interband transitions and plasmonic-like properties [13], [14]. This combination of plasmonic-like and thermoelectric properties in the same materials is relatively unexplored [15] yet it provides an opportunity for novel device designs with enhanced functionality. By fabricating plasmonic nanoresonators directly out of the thermoelectric materials, localized heating and large thermal gradients can be achieved by optical excitation. Due to the Seebeck effect, these thermal gradients lead to a nonuniform distribution of charge carriers and a measurable potential difference. A nanoresonant thermoelectric detector was previously demonstrated with a narrowband absorptance of ∼60 % directly in Bi₂Te₃ and Sb₂Te₃ nanowires [5]. To improve the performance of thermoelectric detectors, it is desirable to further enhance the optical absorptance across the entire visible spectrum and beyond, e.g., for applications in photoelectric energy conversion.

In this work, we exploit the confluence of plasmonic-like and thermoelectric properties in the same materials (Sb₂Te₃/Bi₂Te₃) to design a responsive thermopile detector. By fabricating plasmonic nanoresonators directly out of thermoelectric materials, we enhanced optical absorptance to ∼90 % across the visible spectrum and achieved direct photo-thermoelectric conversion. Furthermore, nanoscale plasmonic effects could enable miniaturized thermopile devices with greatly enhanced responsivities. A cascaded thermopile detector consisting of five thermocouple junctions, featuring nanoresonators composed of Sb₂Te₃/Bi₂Te₃, exhibits a responsivity of 1.5 V/W for illumination at 650 nm.

2 Methods

2.2 Device fabrication

Figure 1(a) illustrates the process of device fabrication. The devices were fabricated through three photolithographic cycles, with a lift-off process performed after the deposition of each layer. To ensure a clean liftoff, a bilayer of positive photoresists (PGMI and MIR 701) was spin coated on the fabricated nanostructures. The device structures were patterned using a Nanyte BEAM direct laser writing system. The patterned structures were developed in AZ726MIF for 60 s. And 50 nm thin films of Sb₂Te₃ and Bi₂Te₃ were deposited through different aligned photolithography and liftoff steps by RF magnetron sputtering using an AJA Orion 5 system. The deposition power and pressure were set to 20  W and 0.5  Pa, respectively. To crystallize the thin film of Sb₂Te₃, the sample was annealed at 150  °C for 5  min. For electrode connections, we utilized the same sputtering system to deposit a 50  nm thick gold (Au) film using the DC gun with a power of 50  W and deposition pressure of 0.5  Pa. The deposition of the films was done in a base vacuum pressure of 2.7 × 10⁻⁵  Pa. The final step involved a 1 h liftoff process in N-methylpyrrolidone (NMP) at 60  °C. After device fabrication, the substrate was wire bonded onto a printed circuit board (PCB), and the device electrodes were wire bonded to the PCB pads.

Figure 1:

Fabrication process, schematic, and fabricated Sb₂Te₃/Bi₂Te₃ photodetector. (a) Fabrication process flow. The HSQ nanoposts are patterned using electron beam lithography. The remaining parts of the device are fabricated using maskless photolithography, magnetron sputtering (RF for tellurides and DC for Au), and liftoff. (b) Schematic of the thermoelectric detector showing temperature and voltages generated from light illumination. A nanostructure absorber array is incorporated at a Sb₂Te₃/Bi₂Te₃ thermocouple junction forming the active area of the detector to enhance its optical absorptance. Strong confinement of visible light directly in the thermoelectric materials provides an efficient conversion of photon energy to heat. A temperature difference builds up in the device and produces a Seebeck voltage difference across the thermocouple. (c) Scanning electron micrograph of fabricated Sb₂Te₃/Bi₂Te₃ nanostructures. (d) Micrograph of a fabricated single-unit device. The dark squares are the nanostructure absorbers, from which Sb₂Te₃ and Bi₂Te₃ legs are extended outward. The gold contact pads are used for electrical measurements.

3 Results and discussion

To exploit the plasmonic properties of the material for enhanced optical absorption, nanoresonators were fabricated by depositing thermoelectric materials on top of dielectric nanoposts. Figure 1(a) and (b) illustrates a schematic of the fabrication process flow and the final device. We fabricated plasmonic nanoresonators composed of Sb₂Te₃ and Bi₂Te₃ by sputter deposition on an array of hydrogen silsesquioxane (HSQ) nanoposts that were defined using electron beam lithography, forming the active area of the thermopile photodetector. The plasmonic nanoresonators are incorporated into a thermopile structure, which consists of two electrodes: a thermoelectric p-type leg made from Sb₂Te₃, and a thermoelectric n-type leg made from Bi₂Te₃.

When the photodetector is exposed to visible light, the nanoresonators enhance the absorption of incident photons due to the plasmonic properties of Sb₂Te₃ and Bi₂Te₃. Photothermal conversion leads to localized heating at the active area of the detector. The localized heating creates a temperature difference between the nanoresonator area and the cool (room temperature) ends of the legs. In the p-type Sb₂Te₃ leg, holes diffuse toward the cooler end, while in the n-type Bi₂Te₃ leg, electrons diffuse toward the cooler end, resulting in the creation of a voltage difference V = (S _p + S _n ) ΔT, where ΔT is temperature difference, and S _p and S _n are Seebeck coefficients of the p and n type thermoelectric materials [5], [11]. The voltage difference is measured through the deposited gold electrodes and contact pads. This voltage signal corresponds to the intensity of the absorbed light, enabling photodetection based on the generated electrical signal.

To investigate the optical properties of thermoelectric nanoresonators, 50 nm of Sb₂Te₃ and 50 nm of Bi₂Te₃ were deposited on the HSQ nanoposts. Figure 1(c) shows the SEM image of nanoposts with a 300 nm pitch after deposition of Sb₂Te₃ and Bi₂Te₃, increasing the width of the nanoposts from 60 nm to approximately 160 nm wide.

Figure 2(a) shows the dielectric functions of Sb₂Te₃ and Bi₂Te₃. Both materials are expected to exhibit plasmonic-like behavior [17], which arises for wavelengths where the real part of the permittivity, ε′(ω), is negative. Despite not exhibiting as large a negative permittivity as other plasmonic materials such as Ag or Au, the permittivities of Sb₂Te₃ and Bi₂Te₃ do become negative in the visible light range for wavelengths between 400 and 800 nm. Therefore, by designing Sb₂Te₃ and Bi₂Te₃ nanostructures in which highly lossy plasmonic modes can be optically excited, photo-thermoelectric conversion can be enhanced for detection of visible light.

Figure 2:

Optical properties of Sb₂Te₃/Bi₂Te₃ nanostructures. (a) Measured dielectric function of Sb₂Te₃ and Bi₂Te₃. The negative real permittivity indicates plasmonic behavior for visible wavelengths. (b–d) Real permittivity, electric field intensity normalized to the incident plane wave, and absorbed power density per unit volume (a.u.) of Sb₂Te₃/Bi₂Te₃ nanostructures at λ = 625 nm. (e) Optical micrographs of fabricated nanostructure arrays with varying pitch and fixed pillar diameters of 60 nm. The lower pitch structures are darker because of their larger absorptance in the visible spectrum. (f) Experimental and simulated absorptance spectra for Sb₂Te₃/Bi₂Te₃ nanostructure arrays with HSQ nanopost diameters of 60 nm and a pitch size of 300 nm.

To gain insights into the near field-interactions between incident light and the nanoresonators, full-wave electromagnetic simulations using finite-difference time-domain (FDTD) were conducted. Periodic unit cells of the bilayer thermoelectric structures were modeled based on the nanostructure geometry from SEM inspection and the measured dielectric functions. The cross-sectional simulation model of the nanoposts made of Sb₂Te₃/Bi₂Te₃ is shown in Figure 2(b), where the color bar corresponds to the real permittivity at 625 nm wavelength. The dark green and light green regions represent 50 nm Sb₂Te₃ and Bi₂Te₃, respectively. Due to the nondirectional nature of sputter deposition, the sidewalls of the nanoposts are also coated, resulting in a mushroom-shaped structure [18]. The E-field intensity of the Sb₂Te₃/Bi₂Te₃ nanoposts is shown in Figure 2(c), and notably, the field is strongly localized around the perimeters of the structures, similar to the behavior of gold or silver ellipsoid nanoparticles [13]. Field localization and enhancement arising from plasmon resonances result in light absorption at specific hot spots on the structure. The observation of strong electric field localization at the edges of the nanostructures suggests a similar behavior in plasmonic semiconductors [19].

Figure 2(d) shows the calculated absorbed power density within the dual-layer structures. Adding a layer of Bi₂Te₃ significantly alters the absorbed power density distribution. The dual-layer structure exhibits enhanced absorption, with a more uniform distribution of absorbed power compared to the single-layer nanoposts (Figure S1(a)–(c)). This change suggests that the combination of materials in a layered structure can improve light absorption efficiency, potentially leading to better performance in photodetection applications. Notably, the absorbed power density is highly localized within the nanostructure, with the highest absorption occurring at the base of the structure. While the absorption is localized, it is reasonable to assume the temperature distribution becomes homogeneous within the nanostructures in tens of nanoseconds (see Supplementary Information), due to the small thermal diffusion length scale [20].

The effects on the optical response for different HSQ nanopost diameters, pitch size, and annealing conditions for single-layer Sb₂Te₃ structures were investigated prior to device fabrication, as shown in Figure S2(a)–(c). The results demonstrate that nanoposts with a diameter of 60  nm and a 300  nm pitch exhibited the strongest light absorption, and these parameters were, therefore, chosen for the dual-layer devices. Additionally, nanoring structures were fabricated, as shown in Figure S3(a)–(c). Similar optical measurements were conducted on these nanorings, revealing lower absorptance compared to the nanopost structures.

To further investigate the effect of dual-layer structures on optical absorptance, 50  nm of Sb₂Te₃ and Bi₂Te₃ were deposited on the nanostructures with a 60  nm diameter and 300  nm pitch. Optical absorptance spectra of the fabricated nanoresonators together with nonpatterned reference spectra of the thermoelectric films are shown in Figure 2(f). The results demonstrate a significant enhancement in optical absorptance when Sb₂Te₃ and Bi₂Te₃ are fabricated as nanostructures compared to as plain films. Both the simulated and experimental spectra of the nanoresonators show a broad resonance peak with its maximum at 625 nm. The broad spectral feature is likely due to the relatively large imaginary permittivity and concomitantly large optical loss. Consequently, a substantial absorptance of 80–90 % is observed across the whole visible spectrum, making the patterned film twice as absorptive compared to unpatterned films. This enhancement indicates efficient light trapping and an increased interaction between incident light and the nanostructured film.

In addition to optical absorption, a thermoelectric photodetector depends on the heat transfer within the device; therefore, multiphysics finite element simulations were performed to aid the thermal design. Experimental absorptance values and illumination intensities were used as inputs for the heat sources in the thermoelectric model, together with measured Seebeck coefficients of 164 and −85 μV/K for Sb₂Te₃ and Bi₂Te₃. Figure 3(a)–(c) shows thermoelectric simulations of the nanostructured thermoelectric device under uniform illumination. The hottest part is in the middle, resulting in a voltage difference (ΔV) between the hot region and the cold electrodes at either end. Heat flows from the central region (where the nanostructures are located) into the Sb₂Te₃ and Bi₂Te₃ pads on the sides of the device, into the substrate and the surrounding medium (air), leading to cooling and resetting of the detector once the illumination is removed. Figure 3(d) presents the simulation results for the responsivity of the proposed thermoelectric photodetector across the visible spectrum. The results show that the device achieves its highest responsivity at a wavelength of 625  nm, indicating that the photodetector is maximally sensitive to light at this wavelength.

Figure 3:

Thermoelectric simulations and photoresponse measurements. (a) A nonuniform temperature distribution is generated in the device due to photothermal heating. The central region gets hotter due to the enhanced absorptance of the nanostructure absorber. (b) The temperature gradient produces a thermoelectric potential along the device due to its n-type/p-type thermocouple configuration. (c) Simulated temperature and voltage response due to photothermal heating. The response time is on the order of 200 µs. (d) Simulated spectral responsivity. The responsivity is wavelength dependent due to the wavelength dependent optical absorptance. (e) Photoresponse measurement as an illuminating laser beam at a wavelength of 650 nm is turned on and off, comparing 300 and 400 nm pitch devices. (f) Photoresponse comparing laser wavelengths of 405 and 650 nm for the device with 300 nm pitch size. (g) High-resolution transient measurement, from which a rise time of 160 µs could be extracted.

Figure 3(e) shows our fabricated photodetector featuring a nanostructured absorber in its central region. To demonstrate the photodetection capability, the device was illuminated with a laser beam focused onto the absorber region. The resulting photovoltage due to the laser being turned on and off is shown in Figure 3(e), and it is apparent that the device is photosensitive. To investigate the influence of nanostructure pitch and, therefore, absorptance on device performance, we compared the photovoltage of two devices with pitches of 300 nm and 400 nm. The results demonstrate a higher voltage for the device with a 300 nm pitch. This observation is consistent with the finding that absorptance is enhanced with decreasing pitch (Figure S2(c)). Further analysis was conducted by measuring the photoresponse of the 300  nm pitch device under different laser illuminations, comparing wavelengths 405 nm and 650 nm, as shown in Figure 3(f). The power output of both lasers was tuned to ∼1 mW, and the detector responsivities for each wavelength were calculated to be 1.8 V/W at 405 nm and 2.5 V/W at 650nm. These results suggest that while the device is more responsive to light at 650 nm, it still maintains considerable sensitivity at 405 nm, matching its broad absorptance spectrum. The higher responsivity at 650 nm is attributed to the higher absorption (∼90 %) of the Sb₂Te₃ and Bi₂Te₃ nanoposts in the 600–650 nm wavelength range, and this trend agrees with the spectral responsivity simulation in Figure 3(d). The slightly higher simulated responsivities could be attributed to variations in material properties from the device fabrication. The response time of the thermoelectric photodetector was measured in Figure 3(g) to be ∼160 µs, which agrees with the time constant of ∼220 µs that can be extracted from the thermal simulation in Figure 3(c). This response time is competitive, agreeing well with other published works in the field [5].

The photoresponse of the detector in Figure 3 can be further enhanced by connecting multiple units in series. The resulting device is commonly referred to as a thermopile. The simulation of multiple cascaded thermoelectric devices (Figure 4(a)–(c)) shows an increase in voltage difference proportional to the number of devices cascaded together (N × ΔV). The cascading of multiple devices results in a cumulative increase in voltage difference, indicating enhanced electrical output. Furthermore, the neighboring absorbers in the cascaded configuration facilitate heat transfer and redistribution across the devices. This redistribution of heat helps to mitigate losses by harvesting the excess heat from neighboring devices, thereby improving the overall efficiency of the thermoelectric system. The findings highlight the importance of device architecture and configuration in optimizing thermoelectric device performance. Cascading multiple thermoelectric elements, with all elements ideally arranged in the central region as shown in Figure 4(d), enables the collective harvesting of heat and enhances electrical output. This configuration is a favorable strategy for improving device efficiency. Efficient heat transfer and utilization across the cascaded devices contribute to minimizing heat losses and maximizing energy conversion efficiency.

Figure 4:

Photoresponse of a five-unit thermopile detector. (a) Simulated thermal distribution of a 5-unit thermopile detector. (b) Thermoelectric potential of the 5-unit device. (c) Simulated temperature and voltage difference of the 5-unit device. (d) Fabricated device with five serially connected thermocouples. The central array of dark squares is the nanostructure absorbers, from which alternating Sb₂Te₃ and Bi₂Te₃ legs are extended outward. The gold contact pads are used for electrical measurements. (e) Photoresponse due to unfocused laser illumination and different laser powers. (f) Power-dependent photoresponse. (g) Photoresponse due to broadband illumination from a halogen lamp with an intensity of ∼18,000 W/m².

The experimental results of our cascaded thermoelectric photodetector, composed of Sb₂Te₃ and Bi₂Te₃ nanoposts with a 300 nm pitch size, offer valuable insights into the device performance and sensitivity. The photoresponse of a 5-unit thermopile device was measured under uniform illumination using a collimated 650 nm laser beam with a diameter 2.4 mm, applied at varying power levels (Figure 4(e)). At an input power of 19.7 mW, the thermopile voltage was 21 µV, corresponding to a responsivity of 1.5 V/W when adjusted for light incident on total active area of the device. While this responsivity is 40 % lower compared to the one measured using focused light – likely due to additional photothermal heating of the unpatterned legs – the result shows that the device also operates well under full area illumination. To evaluate the noise performance, we assume the noise floor to be electrical Johnson noise, which is typically what thermal detectors are limited by. The Johnson-limited specific detectivity D ^* can be calculated as D * = R A 4 k T R , where k is the Boltzmann constant, T is the operating temperature of 300 K, and A is the area of the photosensitive region, which was taken as 120 µm² from the rectangular region encompassing the five absorbers. The electrical resistance, R, was determined to be 25 kΩ, as measured from the IV curves under dark conditions, as shown in Figure S4(a). The detectivity for full-area illumination was found to be 3.2 × 1 0 6 cm H z 1 2 W − 1 for a wavelength of 650 nm. Despite the lack of thermal isolation from the substrate, this detectivity is formidable compared to devices made on thin membrane [6]. A comparison of different thermoelectric photodetectors is listed in Table S1. The thermoelectric nanopost array presented in this work provides a key advantage over previous designs by directly fabricating the nanoresonators from the thermoelectric materials (Bi₂Te₃/Sb₂Te₃), enabling efficient direct absorption of incident light. This design eliminates the need for intermediate layers or additional absorption-enhancing components, which can introduce thermal resistance and slow down heat transfer. This approach allows for immediate photon-to-heat conversion within the active material, minimizing energy loss and reducing the thermal diffusion time, leading to a faster photothermal response.

The responsivity of the photodetector was observed to be 1.5 V/W for all the measured input powers, as indicated by the linear scaling of the photovoltage in Figure 4(f). This direct proportionality between output voltage and input light intensity supports the photodetector sensitivity and potential for scalable signal output based on light intensity. The photoresponses of single and five-unit thermopile detectors under uniform illumination using an unfocused 650 nm laser were measured (Figure S4(b)). The results show larger voltages for the five-unit device compared to the single-unit device. We also measured the photoresponse of a five-unit thermopile detector under laser illumination focused on individual subdevices. As shown in Figure S4(c) and (d), the voltage difference is larger when the middle device is illuminated. In Figure 4(g), we investigated the photoresponse of the cascaded photodetector when exposed to a broadband light source covering the entire visible spectrum with an intensity of ∼18,000 W/m². The total incident power on the device was adjusted according to the power spectral density of the halogen lamp (see Supplementary Information), and the responsivity was calculated to be 1.5 V/W, which is comparable with the response to coherent light. The result indicates that the device is responsive to visible light across a broad range of wavelengths, confirming its applicability in multiwavelength detection.

4 Conclusions

This work offers a new look at the traditional thermoelectric materials Sb₂Te₃ and Bi₂Te₃ and their recently discovered ability to support interband plasmonic resonances – in the context of photodetection. By fabricating nanoresonators directly from the thermoelectric materials, we show that plasmonic field localization enhances the optical absorptance in a thermocouple to 90 percent across the visible spectrum. The direct photo-thermoelectric conversion within the materials eliminates the need for external nanoengineered materials or black-body coatings to collect photons, resulting in reduced device complexity and bulk. Importantly, the reduced heat capacitance improves the speed of the device, which is generally a limitation of thermal-type photodetectors.

In contrast to metallic nanostructures that typically support plasmonic resonances with narrow spectral features, the interband resonances observed here are broad with high absorptance across the visible spectrum. The broad resonances are owed to the large absorptive losses of the thermoelectric materials and could be advantageous for thermoelectric photodetectors as they are commonly used for broadband applications.

As demonstrated here, nanophotonic engineering is a powerful tool to improve the functionality and performance of optoelectronic devices. By applying mature nanofabrication techniques like electron beam lithography to unconventional materials like thermoelectrics, new advancements can be made to align optical properties with application-specific requirements. Against this backdrop, we foresee that thermoelectric nanoresonators, like the one presented here, offer a promising path forward for various optoelectronic technologies, such as imaging, sensing, solar energy harvesting, and integrated spectrometers.