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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2020)

Extremely sensitive optical receivers operating at high bandwidth are critical for optical communications technologies and other emerging applications that require the detection of an extremely low number of photons. Light imaging, detection and ranging (LIDAR), quantum communications, biophotonics, medical imaging systems, and other emerging applications will require a new generation of photodetectors that can detect extremely low power levels and be mass manufacturable. Today, Avalanche Photodetectors (APDs) and Single Photon Avalanche Photodetectors (SPADs) are the only detectors that can meet those requirements, but their bandwidth, sensitivity, and noise limitation must be overcome.

This thesis presents the development of new silicon and germanium-on-silicon-based APDs and SPADs with high speed and absorption efficiency at visible and near-infrared wavelengths. Through the modeling, fabrication, and characterization we show that the implementation of photonic nanostructures in photodetectors enhances their responsivity and gain by guiding the light parallel to its surface, greatly enhancing the interaction with the semiconductor material. These nanostructures also allow enhancing their time response by two methods: (i) use of thinner absorption layers to reduce the transit time of the photogenerated carriers, without sacrificing their absorption capabilities and (ii) reduction of junction capacitance by reducing the effective area of the device through the removal of material. This thesis also develops the concept of penetration depth engineering. We show that it is possible to guide photons to a critical depth in semiconductors and maximize the gain-bandwidth performance and absorption efficiency in avalanche-based photodetectors by integrating photon-trapping nanoholes. Our new Si APDs have shown superior amplification gain and speed compared to their conventional counterparts.

The advantages of photon-trapping are exploited in Germanium (Ge) on Si photodetectors. Our Ge-on-Si PDs show enhanced absorption capabilities at the O (original band: 1260-1360 nm) and C (conventional band: 1530 nm to 1565 nm) optical wavelengths of bands and enable their use at longer wavelengths in the L band (long wavelength: 1565 nm to 1625 nm). These PDs with photon-trapping holes have the potential to be monolithically integrated with CMOS/BiCMOS ASICs and offer a promising solution for waveguide PDs required for Photonic Integrated Circuits (PICs).

A new generation of APDS and SPADs are now designed with the implementation of appropriate doping profiles for high amplification, thin semiconductor layers for high speed, and the accurate design of micro/nanoholes for highly sensitive and ultrafast optical receivers.

    Cover page: High-speed avalanche photodiodes for low light level detection
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2023)
    Since the advent of impact ionization and its application in avalanche photodiodes (APD), numerous application goals have contributed to steady improvements over several decades. The characteristic high operating voltages and the need for thick absorber layers (π-layers) in the Si-APDs pose complicated design and operational challenges in complementary metal oxide semiconductor integration of APDs. In this work, we have designed a sub-10 V operable Si-APD and epitaxially grown the stack on a semiconductor-on-insulator substrate with a submicron thin π-layer, and we fabricated the devices with integrated photon-trapping microholes (PTMH) to enhance photon absorption. The fabricated APDs show a substantially low prebreakdown leakage current density of ∼50 nA/mm2. The devices exhibit a consistent ∼8.0 V breakdown voltage with a multiplication gain of 296.2 under 850 nm illumination wavelength. We report a ∼5× increase in the EQE at 850 nm by introducing the PTMH into the device. The enhancement in the EQE is evenly distributed across the entire wavelength range (640-1100 nm). The EQE of the devices without PTMH (flat devices) undergo a notable oscillation caused by the resonance at specific wavelengths and show a strong dependency on the angle of incidence. This characteristic dependency is significantly circumvented by introducing the PTMH into the APD. The devices exhibit a significantly low off-state power consumption of 0.41 μW/mm2 and stand fairly well against the state-of-the-art literature. Such high efficiency, low leakage, low breakdown voltage, and extremely low-power Si-APD can be easily incorporated into the existing CMOS foundry line and enable on-chip, high-speed, and low-photon count detection on a large scale.
      Cover page: Design and Fabrication of High-Efficiency, Low-Power, and Low-Leakage Si-Avalanche Photodiodes for Low-Light Sensing.
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2021)
      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.
        Cover page: Modeling of nanohole silicon pin/nip photodetectors: Steady state and transient characteristics
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2021)
        Enhancing photon detection efficiency and time resolution in photodetectors in the entire visible range is critical to improve the image quality of time-of-flight (TOF)-based imaging systems and fluorescence lifetime imaging (FLIM). In this work, we evaluate the gain, detection efficiency, and timing performance of avalanche photodiodes (APD) with photon trapping nanostructures for photons with 450 nm and 850 nm wavelengths. At 850 nm wavelength, our photon trapping avalanche photodiodes showed 30 times higher gain, an increase from 16% to >60% enhanced absorption efficiency, and a 50% reduction in the full width at half maximum (FWHM) pulse response time close to the breakdown voltage. At 450 nm wavelength, the external quantum efficiency increased from 54% to 82%, while the gain was enhanced more than 20-fold. Therefore, silicon APDs with photon trapping structures exhibited a dramatic increase in absorption compared to control devices. Results suggest very thin devices with fast timing properties and high absorption between the near-ultraviolet and the near infrared region can be manufactured for high-speed applications in biomedical imaging. This study paves the way towards obtaining single photon detectors with photon trapping structures with gains above 106 for the entire visible range.
          Cover page: Avalanche photodetectors with photon trapping structures for biomedical imaging applications.
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2022)
          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.
            Cover page: Engineering the gain and bandwidth in avalanche photodetectors.
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