The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum, and it drops rapidly beyond 800 nm in near-infrared wavelengths. We have experimentally demonstrated a technique utilizing photon-trapping surface structures to show a prodigious improvement of photoabsorption in 1-μm-thin silicon, surpassing the inherent absorption efficiency of gallium arsenide for a broad spectrum. The photon-trapping structures allow the bending of normally incident light by almost 90 deg to transform into laterally propagating modes along the silicon plane. Consequently, the propagation length of light increases, contributing to more than one order of magnitude improvement in absorption efficiency in photodetectors. This high-absorption phenomenon is explained by finite-difference time-domain analysis, where we show an enhanced photon density of states while substantially reducing the optical group velocity of light compared to silicon without photon-trapping structures, leading to significantly enhanced light–matter interactions. Our simulations also predict an enhanced absorption efficiency of photodetectors designed using 30- and 100-nm silicon thin films that are compatible with CMOS electronics. Despite a very thin absorption layer, such photon-trapping structures can enable high-efficiency and high-speed photodetectors needed in ultrafast computer networks, data communication, and imaging systems, with the potential to revolutionize on-chip logic and optoelectronic integration.
Silicon avalanche photodiodes (Si-APD) are widely explored due to their high sensitivity, rapid response time, high quantum efficiency, intrinsic multiplication gain, and low signal-to-noise ratio. We present an experimental demonstration of a wavelength selective APD stack epitaxially grown in two different doping orders:–1) N-on-P and 2) P-on-N.We present a performance comparison between N-on-P and P-on-N based on the quantum external efficiency (EQE), Ion/Ioff ratio, and the reverse biased dark state leakage current. By reversing the doping from P-on-N to N-on-P, we show a 40% increase in the EQE. By introducing the photon-trapping hole array we show a 60% improvement in the EQE. We have utilized a low temperature (450oC) forming gas (5% H2 and 95% N2) annealing process to passivate the surface states and show a dark state leakage current improvement from sub- 10nA to sub-1nA current range. The proposed devices are complementary metal oxide semiconductor process compatible and can enable ‘detectors-on-chip’ technology for numerous applications such as internet-of-things, data communication, biomedical imaging, high-speed cloud computing, remote sensing, as well as single-photon detection.
Controlling light penetration depth in Avalanche Photodiodes (APDs) and Single Photon Avalanche Diodes (SPADs) play a major role in achieving high multiplication gain by delivering light near the multiplication region where the electric field is the strongest. Such control in the penetration depth for a particular wavelength of light has been previously demonstrated using integrated photon-trapping nanostructures. In this paper, we show that an optimized periodic nanostructure design can control the penetration depth for a wide range of visible and near-infrared wavelengths simultaneously. A conventional silicon APD structure suffers from high photocarrier loss due to recombination for shorter wavelengths as they are absorbed near the surface region, while silicon has low absorption efficiency for longer wavelengths. This optimized nanostructure design allows shorter wavelengths of light to penetrate deeper into the device, circumventing recombination sites while trapping the longer wavelengths in the thin silicon device by bending the vertically propagating light into horizontal modes. This manipulation of penetration depth improves the absorption in the device, increasing light sensitivity while nanostructures reduce the reflectance from the top surface. While delivery of light near the multiplication region reduces the photogenerated carrier loss and shortens transit time, leading to high multiplication gain in APDs and SPADs over a wide spectral range. These high gain APDs and SPADs will find their potential applications in Time-Of-Flight Positron Emission Tomography (TOF-PET), Fluorescence Lifetime Imaging Microscopy (FLIM), and pulse oximetry where high detection efficiency and high gain-bandwidth is required over a multitude of wavelengths.
Optical spectrometers are widely used scientific equipment with many applications involving material characterization, chemical analysis, disease diagnostics, surveillance, etc. Emerging applications in biomedical and communication fields have boosted the research in the miniaturization of spectrometers. Recently, reconstruction-based spectrometers have gained popularity for their compact size, easy maneuverability, and versatile utilities. These devices exploit the superior computational capabilities of recent computers to reconstruct hyperspectral images using detectors with distinct responsivity to different wavelengths. In this paper, we propose a CMOS compatible reconstruction-based on-chip spectrometer pixels capable of spectrally resolving the visible spectrum with 1 nm spectral resolution maintaining high accuracy (<95 %) and low footprint (8 μm × 8 μm), all without the use of any additional filters. A single spectrometer pixel is formed by an array of silicon photodiodes, each having a distinct absorption spectrum due to their integrated nanostructures, this allows us to computationally reconstruct the hyperspectral image. To achieve distinct responsivity, we utilize random photon-trapping nanostructures per photodiode with different dimensions and shapes that modify the coupling of light at different wavelengths. This also reduces the spectrometer pixel footprint (comparable to conventional camera pixels), thus improving spatial resolution. Moreover, deep trench isolation (DTI) reduces the crosstalk between adjacent photodiodes. This miniaturized spectrometer can be utilized for real-time in-situ biomedical applications such as Fluorescence Lifetime Imaging Microscopy (FLIM), pulse oximetry, disease diagnostics, and surgical guidance.
The gain in Avalanche Photodiodes (APDs) and Single Photon Avalanche Diodes (SPADs) is dependent on the probability of photo-generated carriers to trigger an avalanche process, which is correlated to the depth where a photon is absorbed by the photodiode. For silicon photodiodes, most of the photons with wavelengths in the visible spectrum are absorbed near the surface in the highly doped contact regions where the recombination rate is high. Thus, they do not contribute significantly to the avalanche multiplication process. By integrating photon-trapping nanostructures, we facilitate deeper penetration of photons into the devices, enhancing light absorption to generate more carriers that can trigger the avalanche process. This improves the gain-bandwidth of silicon APDs and SPADs significantly. Photon-trapping nanoholes can reduce the thickness of silicon without compromising its quantum efficiency, while a perforated surface reduces the device capacitance improving the bandwidth. Therefore, the manipulation of light penetration depth using photon-trapping nanoholes leads to ultrafast high-gain photodetectors capable of detecting faint light signals particularly useful for low light applications such as fluorescent lifetime imaging microscopy and time-of-flight positron emission tomography.
Current deepfake production methods use auto-encoders augmented by a generative adversarial network (GAN) to create fraudulent but convincing video footage. Developing neural networks to counteract these deepfakes is a highly active area of research—but software-based methods can be immediately used to benchmark even better deepfakes. Thus, there is a need for hardware based solutions to complement existing deepfake detection methods. Here, we present on-chip silicon spectrometer arrays to enhance the number of color channels detected in the imaging system by a factor of 100. These arrays are made up of unique photodiodes engineered to have distinctive spectral responsivities that arise from their photon-trapping, surface based, nanostructures. Videos recorded with this hyperspectral imaging device could complicate the training process for deepfake producers because it collects information that a standard camera cannot. It could also assist novel authentication methods, such as heartbeat monitoring, camera fingerprinting techniques, etc. These spectrometer arrays show a promising direction for continued research in deepfake detection.
Improving the time resolution and sensitivity of Silicon-based Single Photon Avalanche Photodetectors (Si-SPAD) across the entire visible spectrum is critical to improve image quality in biomedical imaging applications such as positron emission tomography or fluorescence lifetime imaging. This work reports on the feasibility of manipulating the penetration depth of photons with 450 nm wavelengths to enhance absorption in Si-SPAD by means of photon trapping structures. Optical-electrical simulations suggest light can be directed towards critical regions of the semiconductor increasing the absorption from 54 to 90% with only 1.2μm of silicon and enhancing the probability of avalanche by electrons that leads to higher multiplication gain and speed of operation.
The performance of mid- and long-wavelength infrared (IR) detectors is still restricted with the dark current characteristics and associated noise behavior. In this work, we propose reducing the dark current and related noise of the IR detectors to elevate high operating temperature and improve the detector quantum efficiency (QE), by using a thin absorption layer of IR absorbing materials like lead selenide (PbSe) and mercury cadmium telluride (HgCdTe). A photon bending and trapping mechanism based on integrated micro/nanoscale holes was employed to ensure high quantum efficiency despite using a thin absorbing layer. Using finite-difference time-domain (FDTD) method, the effect of embedded hole arrays on the optical absorption enhancement of ultra-thin PbSe and HgCdTe has been investigated. The calculated optical absorptions in ultra-thin IR structures without holes were compared with that of similar structures embedded with hole arrays. The optical absorption in 2 μm thick PbSe film without holes is less than 5% for 3-5 μm mid-IR wavelengths. Although applying conventional anti-reflecting (AR) coatings leads to a slightly higher absorption, it can cause higher dark current due to increased surface traps. Integration of hole arrays in 2 μm thick PbSe film has shown a significant optical absorption enhancement, up to 20% at 4.5 μm wavelength. This is equivalent to more than 2- and 4-folds' enhancement compared to a 2 μm thick flat-surface structure with AR and without AR, respectively. In addition, embedding the hole arrays in 1.2 μm thick HgCdTe IR films enhances the optical absorption up to 75% at 4.5 μm, which is more than 4 times higher than that in HgCdTe structure without holes. Additionally, the optical absorption in 1.2 μm thick HgCdTe film with periodic array of photon-trapping micro-holes enhances to 27% at 10 μm, long-IR wavelength. This is more that 3 times higher than that in HgCdTe films without holes. Our work revealed that the embedding hole arrays not only enhances the optical absorption in IR ultra-thin structures but also can reduce that material filling ratio to ~50%, which leads to a lower dark current, ensuring higher temperature operation of the IR detectors.
Development of cost-effective and power-efficient optical interconnects is required to meet high demand of data transfer in the era of Internet of Things (IoT) that is expected to connect billions of sensors with different functionalities. The cost of optical links must be reduced for a wide adoption of optical interconnects in the fast data transmission systems. Monolithic integration of ultra-fast photodetectors (PDs), one of the major components of optical receivers-with CMOS/BiCMOS circuits, can reduce the cost dramatically. However, expensive material systems and non-CMOS-compatible processing utilized in the current high-speed photodetectors do not promise a monolithic integration to the required circuitry in the near future. On the other hand, high speed PDs with CMOS-compatible material systems such as silicon (Si), germanium (Ge) or SiGe alloys have poor responsivity for the wavelengths of interest at data rates 10 Gb/s or higher. Our solution to this problem is to increase the optical absorption properties of the semiconductor by introducing micro-/nanoscale air holes to the material. Such micro/nanoholes support an ensemble of modes that propagate laterally inside in a very thin layer of semiconductor (<2µm) which is required for high speed operations. The recent demonstration of surface-illuminated high-speed (>25Gb/s) and high efficiency (>50%) Si PDs with integrated micro-/nanoholes proved that light bending can enable ultra-fast Si-based PDs for monolithic integration with CMOS/BiCMOS circuits to realize cost-effective all-Si optical receivers. In this talk, a review of state-of-the art ultra-fast Si PDs for short-reach data communication will be presented and high speed and high efficiency PDs with alternative Si-based material systems will be demonstrated for the applications in long-reach optical links. Future opportunities that light-bending phenomenon can offer in high performance PD design for various applications such as single photon detection, LIDAR and high-performance computing will be discussed.
The implementation of ultra-thin and highly efficient photodetectors and photovoltaic devices is crucial to realize flexible and wearable products in the era of Internet of Things (IoT). CMOS-compatible processing and well-established manufacturing makes Silicon (Si) a great material of choice in many applications but thin crystalline-Si is not as efficient as bulk Si in absorbing light. Light bending phenomenon enabled by micro-/nanoscale holes have been recently demonstrated to achieve high speed Si photodiodes and high efficiency thin crystalline-Si solar cells. Such small-scale devices can be released and transferred from mother substrate to various platforms such as the tips of fiber optic cables for realizing fiber receivers and probing applications in vivo studies. In this study, preliminary results of morphological and electrical characterization of transferred devices are demonstrated and details of the transfer techniques are presented. The quantum efficiency of devices transferred to aluminum coated glass were observed to get enhanced compared to the ones on Si substrate.
Micro and nanoscale holes on the surfaces of indirect band gap semiconductors such as silicon can enable perpendicular light bending and trapping of photons to enhance the light material interactions and absorption by orders of magnitude. The ‘bending’ of a vertically oriented light beam at nearly 90 degrees can be visualized as radial waves generated by a pebble dropped into a calm pool of water. Such bending and photon trapping result in an increased optical absorption path enabling very high light absorption coefficients. This observation led to the design of silicon photodetectors with high broadband efficiency above 50% and record ultrafast response contributing to more than 40 billion bits of data per second (Gb/s) communication speed.
Photodetectors (PDs) in datacom and computer networks where the link length is up to 300 m, need to handle higher than typical input power used in other communication links. Also, to reduce power consumption due to equalization at high speed (>25Gb/s), the datacom links will use PAM-4 signaling instead of NRZ with stringent receiver linearity requirements. Si PDs with photon-trapping micro/nanostructures are shown to have high linearity in output current verses input optical power. Though there is less silicon material due to the holes, the micro-/nanostructured holes collectively reradiate the light to an in-plane direction of the PD surface and can avoid current crowding in the PD. Consequently, the photocurrent per unit volume remains at a low level contributing to high linearity in the photocurrent. We present the effect of design and lattice patterns of micro/nanostructures on the linearity of ultra-fast silicon PDs designed for high speed multi gigabit data networks.
Crystalline silicon (c-Si) remains the most commonly used material for photovoltaic (PV) cells in the current commercial solar cells market. However, current technology requires “thick” silicon due to the relative weak absorption of Si in the solar spectrum. We demonstrate several CMOS compatible fabrication techniques including dry etch, wet etch and their combination to create different photon trapping micro/nanostructures on very thin c-silicon surface for light harvesting of PVs. Both, the simulation and experimental results show that these photon trapping structures are responsible for the enhancement of the visible light absorption which leads to improved efficiency of the PVs. Different designs of micro/nanostructures via different fabrication techniques are correlated with the efficiencies of the PVs. Our method can also drastically reduce the thickness of the c-Si PVs, and has great potential to reduce the cost, and lead to highly efficient and flexible PVs.
We present a CMOS compatible fabrication technique to create micro/nanostructures on silicon and germanium surfaces for effective photon trapping and enhanced absorption. We achieved many times of absorption enhancement enabled by these photon trapping micro/nanostructures compared to bulk silicon and germanium counterparts. This method can lead to designing both highly efficient photovoltaics, ultra-fast photodetectors and highly sensitive photon counting devices with dramatically reduced device thickness. We also demonstrate that different fabrication techniques (dry etch, wet etch, and their combination) and different geometries of these micro/nanostructures can affect the ability and extent of the photon trapping and light manipulation in semiconductor.
Nanostructures allow broad spectrum and near-unity optical absorption and contributed to high performance low-cost Si photovoltaic devices. However, the efficiency is only a few percent higher than a conventional Si solar cell with thicker absorption layers. For high speed surface illuminated photodiodes, the thickness of the absorption layer is critical for short transit time and RC time. Recently a CMOS-compatible micro/nanohole silicon (Si) photodiode (PD) with more than 20 Gb/s data rate and with 52 % quantum efficiency (QE) at 850 nm was demonstrated. The achieved QE is over 400% higher than a similar Si PD with the same thickness but without absorption enhancement microstructure holes. The micro/nanoholes increases the QE by photon trapping, slow wave effects and generate a collective assemble of modes that radiate laterally, resulting in absorption enhancement and therefore increase in QE. Such Si PDs can be further designed to enhance the bandwidth (BW) of the PDs by reducing the device capacitance with etched holes in the pin junction. Here we present the BW and QE of Si PDs achievable with micro/nanoholes based on a combination of empirical evidence and device modeling. Higher than 50 Gb/s data rate with greater than 40% QE at 850 nm is conceivable in transceivers designed with such Si PDs that are integrated with photon trapping micro and nanostructures. By monolithic integration with CMOS/BiCMOS integrated circuits such as transimpedance amplifiers, equalizers, limiting amplifiers and other application specific integrated circuits (ASIC), the data rate can be increased to more than 50 Gb/s.
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