Achieving higher photoabsorption than group III-V semiconductors in silicon using photon-trapping surface structures

The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum, and it drops rapidly beyond 800 nm in near-infrared wavelengths. Herein, we have experimentally demonstrated a technique utilizing photon-trapping surface structures to show a prodigious improvement of photoabsorption in one-micrometer-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 ninety degrees to transform into laterally propagating modes along the silicon plane. Consequently, the propagation length of light increases, contributing to more than an order of magnitude improvement in absorption efficiency in photodetectors. This high absorption phenomenon is explained by FDTD 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-nanometer 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 ultra-fast computer networks, data communication, and imaging systems with the potential to revolutionize on-chip logic and optoelectronic integration.Comment: 24 pages, 4 figure

High-Speed and High-Efficiency CMOS-Compatible Photodiodes for Datacom Interconnects

As big data processing and cloud computing continue to grow exponentially, more than 85% of global data traffic has remained within data centers. A significant part of communication in datacenters, including rack-to-rack, rack-to-server, and building-to-building, is based on short-reach (850 nm, up to 500 m) and long haul (1310–1550 nm, up to 10 km) optical communication. High-speed and high-efficiency photodiodes (PDs) play an important role in optical communication links. III-V-based optical transceivers could reach up to 25 Gb/s speed per channel. However, III-V-based photodiodes have not shown promising potential for monolithic optoelectronic integration via CMOS technology. Novel materials and techniques are needed to pave the way for monolithic integration of optical components with signal-processing electronics on a single silicon chip to reduce cost, energy consumption, and improve link efficiency. We designed efficient micro/nanoholes light-trapping structures to overcome the low responsivity of silicon-based photodiodes. Higher optical absorption in an ultrathin active region of a photodiode provides us the opportunity to realize a high-speed and highly efficient photodiode for short-reach and long-haul optical communication. The fabricated Si pin device exhibits an ultrafast impulse response (full-width at half-maximum) of 30 ps and a high efficiency of more than 50% quantum efficiency (responsivity more than 0.35 A/W) at 850 nm. Different passivation methods were applied to improve the surface damages/traps to reach low leakage less than 1 nA. A high-speed photodiode model consisting of an optical generation mechanism and equivalent circuit based on the measured DC/RF characterizations has been developed. The equivalent PD model is used in a comprehensive system simulation of an end-to-end optical link to evaluate the device in terms of bit error rate (BERT) and link power budget. A transimpedance amplifier (TIA) and a 2-Tap Feed-Forward Equalizer (FFE) were designed and simulated based on the photodiode specifications to provide the optimum optoelectronics scheme for hermetic packaging. Comparing the experimental and simulation results, we explored design and fabrication challenges and offered solutions to reach the design targets. A 10 Gb/s CMOS-compatible surface-illuminated Ge/Si photodiode integrated with photon-trapping microhole arrays with broadband, high efficiency up to 1700 nm is designed and fabricated. The Ge/Si photodiode has > 80% and > 73% EQE at 1310 nm and 1550 nm, respectively. The Ge/Si photodiode exhibited acceptable performance close to Ge bandgap, making it a promising candidate for the emerging technologies in the L-band communication window. To move toward CMOS monolithic optoelectronic integration, a novel Metal-Semiconductor-Metal (MSM) PD with photon trapping structure was designed, fabricated, and characterized. The MSM device could reach more than 70% quantum efficiency (7-folds enhancement) at 850 nm while maintaining more than 10 Gb/s bit rate performance. The MSM structure offers practical solutions to most of the challenges experienced by a vertical pin PD. More importantly, the MSM structure has the potential to accommodate the CMOS foundry design rules check (DRC) flow. A photodiode with microhole surface arrays integrated with a high-speed TIA is designed and simulated in SiPh BiCMOS technology platform. The design received the DRC approval from Tower Semiconductor foundry to be fabricated in their facilities

A Natural Language Processing and deep learning based model for automated vehicle diagnostics using free-text customer service reports

Initial fault detection and diagnostics are imperative measures to improve the efficiency, safety, and stability of vehicle operation. In recent years, numerous studies have investigated data-driven approaches to improve the vehicle diagnostics process using available vehicle data. Moreover, data-driven methods are employed to enhance customer-service agent interactions. In this study, we demonstrate a machine learning pipeline to improve automated vehicle diagnostics. First, Natural Language Processing (NLP) is used to automate the extraction of crucial information from free-text failure reports (generated during customers’ calls to the service department). Then, deep learning algorithms are employed to validate service requests and filter vague or misleading claims. Ultimately, different classification algorithms are implemented to classify service requests so that valid service requests can be directed to the relevant service department. The proposed model – Bidirectional Long Short Term Memory (BiLSTM) along with Convolution Neural Network (CNN) – shows more than 18% accuracy improvement in validating service requests compared to technicians’ capabilities. In addition, using domain-based NLP techniques at preprocessing and feature extraction stages along with CNN-BiLSTM based request validation enhanced the accuracy (>25%), sensitivity (>39%), specificity (>11%), and precision (>11%) of Gradient Tree Boosting (GTB) service classification model. The Receiver Operating Characteristic Area Under the Curve (ROC-AUC) reached 0.82

Observation and Modeling of Near-Bistable Dark-Mode Current-Voltage Characteristics in Semi-Insulating Gallium Arsenide With Implications for Photoconductors

In this work, we demonstrate and model the deep-level defect physics of semi-insulating gallium arsenide bulk photoconductive semiconductor switches (PCSS) with gap size of $10 ~\mu \text{m}$ and $25 ~\mu \text{m}$ in dark-mode operation. Experimental measurements up to biasing field of 10 kV/cm show near-bistable characteristics in the dark-mode current-voltage relations for the PCSS, which cannot be reproduced through commercial Technology Computer-Aided Design simulations. Thus, we model the PCSS by solving for homogeneous non-equilibrium steady-state of the PCSS trap dynamics, where we introduce two semi-analytical models both involving two deep levels with impact ionization effects. Both models have an excited deep-level that can capture electrons from or emit electrons to the conduction band. The two models differ, however, by the fact that one has a ground state with capture and emission, whereas the other does not include such mechanisms but instead includes electron excitation and relaxation processes directly between the ground state and the excited state without interactions with the conduction band. We find that the former does not fit with experimental near-bistable features while the latter achieves a good match with the same total number of fitting parameters. Further measurements of bias upto 50 kV/cm on one $10 ~\mu \text{m}$ PCSS confirms the validity of the second model as well. Finally, a brief discussion of the implications on the illuminated operation of the PCSS is also given to illustrate the importance of including defect interactions and defect avalanche effects

Scanning interferometric near-infrared spectroscopy.

In diffuse optics, quantitative assessment of the human brain is confounded by the skull and scalp. To better understand these superficial tissues, we advance interferometric near-infrared spectroscopy (iNIRS) to form images of the human superficial forehead blood flow index (BFI). We present a null source-collector (S-C) polarization splitting approach that enables galvanometer scanning and eliminates unwanted backscattered light. Images show an order-of-magnitude heterogeneity in superficial dynamics, implying an order-of-magnitude heterogeneity in brain specificity, depending on forehead location. Along the time-of-flight dimension, autocorrelation decay rates support a three-layer model with increasing BFI from the skull to the scalp to the brain. By accurately characterizing superficial tissues, this approach can help improve specificity for the human brain

Modeling of nanohole silicon pin/nip photodetectors: Steady state and transient characteristics

Theory is proposed for nanohole siliconpin/nipphotodetector (PD) physics, promising devices in the future data communications and lidar applications. Photons and carriers have wavelengths of 1μm and 5 nm, respectively. We propose vertical nanoholes having 2D periodicity with a feature size of 1μm will produce photons slower than those in bulk silicon, but carriers are unchanged. Close comparison to experiments validates this view. First, we study steady state nanohole PD current as a function of illumination power, and results are attributed to the voltage drop partitions in the PD and electrodes. Nanohole PD voltage drop depends on illumination, but series resistance voltage drop does not, and this explains experiments well. Next, we study transient characteristics for the sudden termination of light illumination. Nanohole PDs are much faster than flat PDs, and this is because the former produces much less slow diffusion minority carriers. In fact, most photons have already been absorbed in thei-layer in nanohole PDs, resulting in much less diffusion minority carriers at the bottom highly doped layer. Why diffusion in PDs is slow and that in bipolar junction transistors is quick is discussed in appendix

Engineering the gain and bandwidth in avalanche photodetectors.

Avalanche and Single-Photon Avalanche photodetectors (APDs and SPADs) rely on the probability of photogenerated carriers to trigger a multiplication process. Photon penetration depth plays a vital role in this process. In silicon APDs, a significant fraction of the short visible wavelengths is absorbed close to the device surface that is typically highly doped to serve as a contact. Most of the photogenerated carriers in this region can be lost by recombination, get slowly transported by diffusion, or multiplied with high excess noise. On the other hand, the extended penetration depth of near-infrared wavelengths requires thick semiconductors for efficient absorption. This diminishes the speed of the devices due to the long transit time in the thick absorption layer that is required for detecting most of these photons. Here, we demonstrate that it is possible to drive photons to a critical depth in a semiconductor film to maximize their gain-bandwidth performance and increase the absorption efficiency. This approach to engineering the penetration depth for different wavelengths in silicon is enabled by integrating photon-trapping nanoholes on the device surface. The penetration depth of short wavelengths such as 450 nm is increased from 0.25 µm to more than 0.62 µm. On the other hand, for a long-wavelength like 850 nm, the penetration depth is reduced from 18.3 µm to only 2.3 µm, decreasing the device transit time considerably. Such capabilities allow increasing the gain in APDs by almost 400× at 450 nm and by almost 9× at 850 nm. This engineering of the penetration depth in APDs would enable device designs requiring higher gain-bandwidth in emerging technologies such as Fluorescence Lifetime Microscopy (FLIM), Time-of-Flight Positron Emission Tomography (TOF-PET), quantum communications systems, and 3D imaging systems

High Speed Surface Illuminated Si Photodiode Using Microstructured Holes for Absorption Enhancements at 900–1000 nm Wavelength

A surface-illuminated silicon photodiode with both high speed and usable external quantum efficiency from 900 to 1000 nm wavelength is highly desirable for intra/inter data center Ethernet communications, high performance computing, and laser radar application. Such Si photodiodes have the potential for monolithic integration to CMOS integrated circuits which can significantly reduce the cost of data transmission per gigabit below one US dollar. To overcome silicon’s intrinsic weakness of absorption in these wavelengths, photon-trapping microstructured hole arrays are etched into the silicon surface, and the operational wavelengths of a high-speed silicon PIN photodiode are extended to 1000 nm. In this paper, the design and fabrication of such photon-trapping structures integrated into all-silicon photodiodes with significantly reduced absorption layer thicknesses to achieve high external quantum efficiency and fast response are presented. Different designs and geometries of the submicron holes on the silicon surface can affect the light trapping and ultimately contribute to different external quantum efficiencies at these wavelengths. Some designs are capable of enhancing the absorption by more than an order of magnitude compared to a photodiode without the submicron hole arrays. With the silicon <i>i</i>-layer thickness less than or equal to 2 μm, the all-silicon photodiode with integrated submicron holes exhibited an external quantum efficiency of more than 40% at 900 nm and greater than 15% at 1000 nm. This thin absorption layer also allows the fast speed of the photodiode with temporal responses of ∼30 ps at these wavelengths