Sub-bandgap Photodetection Research Articles

Silicon (100) surface passivation-driven tuning of Ag film crystallinity and its impact on the performance of Ag/n-Si mid-infrared Schottky photodetector

The utilization of metal/semiconductor Schottky devices for plasmonic harvesting of hot carriers holds immense potential in the field of sub-bandgap photodetection. In this work, we explore a surface passivation scheme using air plasma exposure to modify the Si (100) surface and subsequently the crystal orientation of the deposited Ag film for photon detection in the mid-infrared (MIR) regime. This tailoring was achieved by varying the plasma exposure duration (0, 150, 300, 450, and 600 s). As a result, we could tune the crystal orientation of Ag from the (200) to the (210) crystal plane, with the Ag (111) orientation present in all devices. Furthermore, the photo-response behavior under MIR exposure at λ = 4.2 µm was studied both experimentally and using COMSOL simulations. It was observed that both photoelectric (PE) and photothermal (PT) effects contributed to the photo-response behavior of all devices. The Ag/Si device exposed to air plasma for 300 s exhibited the maximum PE-driven response (2.73 µA/W), while the device exposed to air plasma for 600 s showed a significant PT-driven response (13.01 µA/W). In addition, this strategy helped reduce the reverse leakage current by up to 99.5%. This study demonstrates that MIR detection at longer wavelengths can be optimized by tailoring the crystal orientation of the metal film.

Enhanced hot-electron injection across Au/GaN heterojunction by the piezo-phototronic effect for efficient sub-bandgap photodetection

Performances of plasmon mediated optoelectronic devices are mainly limited by the hot carrier injection efficiency in metal-semiconductor plasmonic heterojunctions. Herein, the piezo-phototronic effect was used to promote hot-electron injection in Au/GaN plasmonic heterojunction, and the dynamics of hot-electron transport were investigated. The lateral and vertical Au/GaN plasmonic heterojunction devices were fabricated based on freestanding GaN film. The comparisons of hot-electron transport behaviors and photoresponse performances between lateral and vertical Au/GaN plasmonic heterojunction devices for sub-bandgap photodetection were systematically investigated. The photoresponse current of vertical heterojunction device is three orders of magnitude higher than lateral heterojunction device owing to the shortened electron transport path. The height of Schottky barrier at the heterojunction interface was tuned by the piezo-phototronic effect to enhance the injection efficiency of hot-electron. Optimized by the piezo-phototronic effect, the photoresponse current increases by nearly 89% under an applied compressive strain of 0.57% and the hot-electron injection efficiency was increased from 37.50% to 90.45%. The manipulating dynamic transport process of hot-electron under the regulation of applied strain is further confirmed by using transient absorption (TA) spectral measurement. This work provides a new architecture for efficient sub-bandgap photodetection based on GaN, and provides insight into the hot-electron dynamics in metal–semiconductor heterostructures.

Highly Efficient Silicon-Based Thin-Film Schottky Barrier Photodetectors

Internal photoemission (IPE) is a promising phenomenon for sub-bandgap photodetection at near-infrared wavelengths using large bandgap semiconductor materials. To improve the photon-to-electron conversion efficiency in silicon-based sub-bandgap Schottky barrier photodetectors (SBPDs), previous studies have mainly focused on subwavelength-scale nanostructures to enhance the electric fields and optical absorption. Here, in a different way from the previous approaches, we theoretically and experimentally demonstrate a rigorous quantum efficiency analysis framework that can quantitatively explain the hot carrier transport processes. Guided by the design principles from the hot carrier loss mechanism analysis, we experimentally demonstrate patternless thin-film SBPDs that can surpass the performance of conventional nanostructured SBPDs, exceeding the external quantum efficiency of 10–3. Our work shows that optical absorption and hot carrier generation are responsible for only a part of the entire IPE process and other transport mechanisms should be carefully considered in a wholistic manner, indicating the importance of quantitative quantum efficiency analysis.

Electromechanically reconfigurable plasmonic photodetector with a distinct shift in resonant wavelength

Plasmonic photodetectors have received increasing attention because their detection properties can be designed by tailoring their metal structures on surfaces without using any additional components. Reconfiguration of the plasmonic resonant state in a photodetector is relevant for various applications, including investigating in situ adaptive detection property changes, depending on the situation, and performing single-pixel spectroscopy in geometrically limited regions. However, the spectral responsivity change with conventional reconfiguration methods is relatively small. Here, we propose a plasmonic photodetector that reconfigures its spectral responsivity with electromechanical deformation instead of bias tuning. The photodetector consists of a gold plasmonic grating formed on an n-type silicon cantilever, and the spectral responsivity is reconfigured by electromechanically scanning at an incident angle to the grating on the cantilever. The photodetector exhibits peak shifts in spectral responsivity in a wavelength range from 1250 to 1310 nm after electromechanical reconfiguration. Finally, for potential future applications, we demonstrate near-infrared spectroscopy using the photodetector. This photodetector has the potential to be adopted as a near-infrared spectrometer in industrial silicon imaging systems because its structure enables subbandgap photodetection on silicon by a Schottky junction.

Photogating Effect-Driven Photodetectors and Their Emerging Applications.

Rather than generating a photocurrent through photo-excited carriers by the photoelectric effect, the photogating effect enables us to detect sub-bandgap rays. The photogating effect is caused by trapped photo-induced charges that modulate the potential energy of the semiconductor/dielectric interface, where these trapped charges contribute an additional electrical gating-field, resulting in a shift in the threshold voltage. This approach clearly separates the drain current in dark versus bright exposures. In this review, we discuss the photogating effect-driven photodetectors with respect to emerging optoelectrical materials, device structures, and mechanisms. Representative examples that reported the photogating effect-based sub-bandgap photodetection are revisited. Furthermore, emerging applications using these photogating effects are highlighted. The potential and challenging aspects of next-generation photodetector devices are presented with an emphasis on the photogating effect.

High-efficiency narrow-band plasmonic hot electron conversion from nanoscale sodium–silicon heterostructures

Plasmonic harvesting of hot electrons has stimulated intensive research activities for applications ranging from sub-bandgap photodetection to photocatalysis. Both high photoelectric conversion efficiency and tunable spectral response are pursued by manipulating resonant metal–semiconductor (M–S) nanostructures. Although noble plasmonic metals have been exclusively employed in hot electron conversion studies, exploring new materials may offer an additional degree of freedom to manipulate the hot electron generation, transport, and emission processes. In this paper, we propose to employ the low-loss alkali metal sodium as an alternate plasmonic material for developing a narrow-band resonant hot electron device. Based on a backside-illumination (BSI) configuration where plasmonic hot electrons generate locally at the M–S interface, the transport loss can be significantly suppressed. Thanks to its ultralow imaginary part of the permittivity, bringing Na into the BSI design allows for efficient shrinking of the resonant linewidth down to sub-20 nm. Another intriguing feature is that Na has more preferred electron density of state distribution for facilitating hot electron emission at the M–S junction. The optimized Na BSI device can yield a photocurrent responsivity up to 50 mA/W at a wavelength of 1400 nm as predicted by our electromagnetic simulation and theoretical model. Our study highlights that the alkali metal could be a promising alternative material for the development of high-Q resonant hot electron devices for near-infrared wavelengths.

Polarization-Insensitive Waveguide Schottky Photodetectors Based on Mode Hybridization Effects in Asymmetric Plasmonic Waveguides.

Two types of configurations are theoretically proposed to achieve high responsivity polarization-insensitive waveguide Schottky photodetectors, i.e., a dual-layer structure for 1.55 µm and a single-layer structure for 2 µm wavelength band. Mode hybridization effects between quasi-TM modes and sab1 modes in plasmonic waveguides are first presented and further investigated under diverse metal types with different thicknesses in this work. By utilizing the mode hybridization effects between quasi-TE mode and aab0 mode, and also quasi-TM and sab1 mode in our proposed hybrid plasmonic waveguide, light absorption enhancement can be achieved under both TE and TM incidence within ultrathin and short metal stripes, thus resulting in a considerable responsivity for Si-based sub-bandgap photodetection. For 1.55 µm wavelength, the Au-6 nm-thick device can achieve absorptance of 99.6%/87.6% and responsivity of 138 mA·W−1/121.2 mA·W−1 under TE/TM incidence. Meanwhile, the Au-5 nm-thick device can achieve absorptance of 98.4%/90.2% and responsivity of 89 mA·W−1/81.7 mA·W−1 under TE/TM incidence in 2 µm wavelength band. The ultra-compact polarization-insensitive waveguide Schottky photodetectors may have promising applications in large scale all-Si photonic integrated circuits for high-speed optical communication.

Reconfigurable cavity-based plasmonic platform for resonantly enhanced sub-bandgap photodetection

Sub-bandgap photodetection based on plasmonic excitations represents a promising route for expanding the spectral range of photodetectors, enabling, for instance, silicon-based devices to be employed at telecom wavelengths. This approach harnesses internal photoemission, where hot carriers are generated via nonradiative plasmonic decay and are subsequently emitted from the metal to a semiconductor, yielding a photocurrent not spectrally limited by the bandgap. However, many schemes based on this approach suffer from low responsivities that hinder their uptake in real-world technologies. Here, we demonstrate a cavity-based platform for both enhancing the generated photocurrent and providing a means for dynamic reconfiguration of the operating wavelength. The proposed device is composed of an optical cavity where one of the mirrors is patterned with a nanoscale grating and interfaced at the other side with a semiconductor. Fabry–Pérot resonances supported by the cavity provide resonant excitation of plasmonic modes at the metal/semiconductor interface, leading to augmented hot-carriers and photocurrent generation compared to the non-resonant case. By employing this cavity-grating geometry, we experimentally demonstrate a fivefold increase in photocurrent due to the presence of cavity resonances. Electromechanical reconfiguration of the photodetector cavity length is also achieved, illustrating dynamic control over the detection wavelength. This cavity-based architecture is compatible with a variety of plasmonic nanostructures, including nanoparticles and nanoantennas, thus providing a flexible means of significantly increasing the photoresponse and hence bringing on-chip plasmonic hot-carrier technologies closer to realization for sub-bandgap photodetection, energy harvesting, and sensing.

IR hot carrier based photodetection in titanium nitride oxide thin film-Si junctions

Abstract:Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency.

Efficient Sub-Bandgap Photodetection via Two-Dimensional Electron Gas in ZnO Based Heterojunction

Sub-bandgap photodetection with two-dimensional electron-confinement in CdZnO-MgZnO heterojunction is realized. The reported photodetection is based on metal-less internal photoemission via optical excitation in 2D electron gas (2DEG) present at the bottom of the conduction band offset in the heterojunction. A physical model is also employed to explain the underlying mechanism of transmission of electrons across the heterojunction. The rib structure of CdZnO waveguide provides strong optical confinement which leads to an improved electro-optic coupling. A high density of 2DEG, uniform momentum distribution across heterojunction and improved electro-optic coupling result in a responsivity of 8.5 mA/W at a wavelength of 532 nm.

Role of surface passivation in integrated sub-bandgap silicon photodetection.

We study experimentally the effect of oxide removal on the sub-bandgap photodetection in silicon waveguides at the telecom wavelength regime. Depassivating the device allows for the enhancement of the quantum efficiency by about 2-3 times. Furthermore, the propagation loss within the device is significantly reduced by the oxide removal. Measuring the device 60 days after the depassivation shows slight differences. We provide a possible explanation for these observations. Clearly, passivation and depassivation play an essential role in the design and the implementation of such sub-bandgap photodetector devices for applications such as on-chip light monitoring.

2D Perovskites with Giant Excitonic Optical Nonlinearities for High-Performance Sub-Bandgap Photodetection.

Two-dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub-bandgap photodetection by employing excitonic effects in 2D Ruddlesden-Popper-type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two-photon absorption (D-2PA) coefficients as giant as 0.2-0.64 cm MW- 1 . 2D RPP-based sub-bandgap photodetectors show excellent detection performance in the near-infrared (NIR): a two-photon-generated current responsivity up to 1.2 × 104 cm2 W-2 s-1 , two orders of magnitude greater than InAsSbP-pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered-RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D-2PA coefficients (β[001] /β[011] = 2.4, 70% larger than the ratios reported for zinc-blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C4 H9 NH3 )2 (CH3 NH3 )Pb2 I7 can be utilized for three-photon photodetection in the NIR region.

Ultra-compact silicon waveguide-integrated Schottky photodetectors using perfect absorption from tapered metal nanobrick arrays.

We propose and numerically analyze an integrated metal-semiconductor Schottky photodetector consisting of a tapered metal nanoblock chain on a silicon ridge waveguide. The metal-semiconductor junctions allow broadband sub-bandgap photodetection through the internal photoemission effects. The tapered array structures with different block widths can gradually tailor the cut-off frequencies and group velocities of the tightly confined plasmonic modes for enhanced light absorption and suppressed reflection of the photonic mode in the silicon waveguide. As a result, according to our simulations, six metal nanobricks with a total device length of 830 nm can almost perfectly absorb the incident sub-bandgap light and subsequently generate photocurrents with a peak responsivity value of 0.125 A/W at 1550 nm. We believe that the proposed design can provide a simple and viable solution for broadband and compact photodetection in the integrated silicon photonics platform.

Sub-Band Gap Photodetection from the Titanium Nitride/Germanium Heterostructure.

Photoexcited hot carriers through nonradiative decay offer new opportunities for harnessing longer wavelength light. Here, we have demonstrated a hot-carrier-mediated sub-band gap photodetection in germanium-based planar heterojunction devices. The planar samples that form in situ germanium/titanium nitride (Ge/TiN) interfaces are fabricated by the dc sputtering technique, and the generation of photocurrent by near-infrared (NIR) light illumination is confirmed up to 2600 nm, well exceeding the absorption limit of Ge. The photocurrent obtained with nickel contacts is 3 orders larger than that obtained without metal contacts or with gold contacts in similar structures. The specific detectivity ( D*) value for the TiN/Ge photodetector is obtained to be 6.32 × 105 Jones at the sub-band gap 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 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.

Omnidirectional sub-bandgap photo-detection using functionalized moulded composite flexible platforms

The substrate is an optically functional component of a thin film organic optoelectronic device. In the well-known context of photovoltaics, the functionality of substrate is relatively simple – that of a broadband transmittance which allows almost all the incoming light into the absorber. A less explored idea is that of incorporating other optical functionalities in the substrate bulk. Such unconventional design can help surpass the performance limitations of commonly available optoelectronic materials to achieve novel functionality. Bulk functionalization of substrates is particularly easy when the substrates are made using moldable transparent materials which allow mixing of optically functional materials into a composite substrate bulk. In this paper, the example of photonic-upconversion is considered as a representative optical function, to make organic photodetectors which detect up-converted infrared light. By incorporating this functionality in a composite substrate two advantages are demonstrated in which the composite substrate architecture surpasses the performance of the conventional thin film device design. First, the composite substrate preserves the electronic performance of the device by spatially isolating the optical and the charge transport functions. Second, the composite architecture enables sub-bandgap photo-detection which is invariant with respect to the angle of incidence of the infrared input light, by placing the optical function before the light can encounter optically reflecting interfaces. In summary, the unconventional design employing a composite optically functional substrate can be extended to multiple optical functionalities and represents a key design finding that can enable considerably more efficient optoelectronic device designs.