This PDF file contains the front matter associated with SPIE Proceedings Volume 10729, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Single photon detection is realized by utilizing semiconductor quantum structures in a wide range of the terahertz (THz) region. In this paper, we review quantum-dot type THz photon detectors and its applications for passive THz imaging.

Semiconductor nanowires are frequently highlighted as promising building blocks for next-generation optoelectronic devices. In this study, we explore infrared photodetectors based on selective-area nanowire arrays, spanning the wavelength spectrum from near-infrared (NIR) to mid-wavelength infrared (MWIR). Examples of these nanowire detectors include: NIR GaAs photodiodes, NIR InGaAs avalanche photodetectors (APDs), NIR InGaAs-GaAs single-photon photodiodes (SPADs), short-wavelength infrared (SWIR) InAs photodiodes, and MWIR InAsSb photodiodes. The small fill factor of nanowire arrays, i.e., the small junction area, is advantageous as it causes significant suppression of dark current, which further decreases the noise level and increases the detectivity. In addition, by utilizing metal nanostructures as 3D plasmonic gratings, we can enhance optical absorption in nanowires through excitation of surface plasmonic waves at metal-nanowire interfaces. Our work shows that, through proper design and fabrication, nanowire-based photodetectors can demonstrate equivalent or better performance compared to their planar device counterparts.

Atomically thin materials like semimetallic graphene and semiconducting transition metal dichalcogenides (TMDs) are an ideal platform for ultra-thin optoelectronic devices due to their direct bandgap (for monolayer thickness) and their considerable light absorption. For devices based on semiconducting TMDs, light detection occurs by optical excitation of charge carriers above the bandgap. For gapless graphene, light absorption causes a large increase in electron temperature, because of its small electronic heat capacity and weak electron-phonon coupling, making it suitable for hot-electron detectors. Here we show that, by nanostructuring graphene into quantum dots, we can exploit quantum confinement to achieve hot-electron bolometric detection. The graphene quantum dots are patterned from epitaxial graphene on SiC, with dot diameter ranging from 30 nm to 700 nm [1]. Nanostructuring greatly increases the temperature dependence of the electrical resistance, yielding detectors with extraordinary performance (responsivities of 1 × 10^(10) V W^(−1) and electrical noise-equivalent power, ∼2 × 10^(−16) W Hz^(−1/2) at 2.5 K). We will discuss how the dynamics of the charge carriers, namely the hot-electron cooling, affects the device operation and its power dependence. These detectors work in a very broad spectral range, from terahertz through telecom to ultraviolet radiation [2], with a design that is easily scalable for detector arrays. [1] El Fatimy, A. et al. , "Epitaxial graphene quantum dots for high-performance terahertz bolometers," Nature Nanotechnology 11, 335-338 (2016). [2] El Fatimy, A. et al. , "Ultra-broadband photodetectors based on epitaxial graphene quantum dots" Nanophotonics (2018).

There are a growing number of applications demanding high sensitivity visible to mid-infrared photodetectors operating at room temperature. Graphene is ideally suitable for optoelectronic photodetectors sensitive from visible to mid-infrared frequencies. Here we report the integration of graphene with thin film high-κ dielectric layers prepared by e-beam thermal evaporation, sputtering deposition and atomic layer deposition methods for the graphene field effect transistor photodetector development. The impact of dielectric layers on graphene properties and the operation of photodetectors varies based on the choice of dielectric and deposition parameters. This work provides a route for use of graphene in the infrared detection at room temperature.

III–V semiconductor nanowires combine the properties of III–V materials with the unique advantages of the nanowire geometry, allowing efficient room temperature photodetection across a wide range of photon energies, from a few eV down to meV. For example, due to their nanoscale size, these show great promise as sub-wavelength terahertz (THz) detectors for near-field imaging or detecting elements within a highly integrated on-chip THz spectrometer. We discuss recent advances in engineering a number of sensitive photonic devices based on III–V nanowires, including InAs nanowires with tunable photoresponse, THz polarisers and THz detectors.

In comparison with planar devices, nanowire photodetectors present advantages in terms of miniaturization, speed, and design flexibility. III nitride nanowires are particularly suitable for spectrally-selective UV photodetection, thanks to their band gap energy and their stability against chemical, mechanical or electrical stress. However, their UV photoresponse scales sublinearly with the optical power, which hinders their introduction in applications that require power quantification. Here, we present GaN nanowires containing a single AlN/GaN/AlN heterostructure, whose response is linear with the optical power when their diameter is small enough to ensure a complete depletion of the wire due to surface states. On the other hand, III-nitride nanowires are also interesting for infrared photodetection using intersubband (ISB) transitions in nanodisks inserted in the wire. We systematically investigated ISB transitions in the near-infrared wavelengths focusing around 1.55 µm in GaN/AlN nanowire heterostructures. Attaining this short wavelength requires small GaN/AlN nanodisks (2 nm / 3 nm). Based on this study, we present the first single-nanowire quantum well infrared photodetector (NW-QWIP), observing photocurrent at 1.55 µm. Finally, we introduce an extension of the study to cover the mid-infrared spectral range, up to around 6 µm, using ISB transitions in GaN/AlGaN nanowires.

Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.

During the past decade, significant advancement has been made on InGaAsP/InP Geiger-mode APDs (GmAPDs) through improvements of material growth, device design and operating circuitry. With the increase in device performance and the growing maturity of device fabrication technology, high performance, large format InGaAsP/InP GmAPD arrays have been successfully designed and manufactured. These arrays have single photon sensitivity in the short wavelength infrared (SWIR) spectral band and can provide 3-D imagery. InGaAsP/InP GmAPD arrays provide an enabling technology for many active optical applications, such as 3-D light detection and ranging (LiDAR) and other photon-starved applications where single photon sensitivity in the SWIR band is critical. InGaAsP/InP-based Geigermode LiDAR has been extensively used on airborne platforms. By using optical wavelengths along with sub-ns laser pulse widths, 3-D Geiger-mode LiDAR techniques provide centimeter-scale range resolution over extremely long distances on the order of tens of kilometers. Through the use of high-performance single photon detectors, Geiger-mode LiDAR systems achieve an order of magnitude improvement in mapping rate over other competing LiDAR technologies. A more recent exciting application of InGaAsP/InP GmAPD-based LiDAR is to enable advanced driver assistance systems (ADAS) and vehicle autonomy on automotive platforms. The single-photon sensitivity of GmAPDs and greater eye-safety of diode lasers at wavelengths beyond 1400 nm provide disruptive automotive LiDAR performance that will be essential to future autonomous vehicle navigation. Single photon sensitivity and simple pixel circuit operation enable the reduction in overall system SWaP, while the scalability of these semiconductor devices enables dramatic reduction in LiDAR cost.

Recently the GaN/AlN multi-quantum-well structure avalanche photodiode (MAPD) has been demonstrated with PMT-like multiplication gain larger than 1E4. In this work, the photocurrent of GaN/AlN MAPD has been investigated and negative differential conductance (NDC) is found in the photocurrent characteristic of MAPD. Through self-consistent calculation, conduction band structure and discrete energy states in each quantum well layer have been obtained for MAPD. The discrete states drop down and align with the conduction band edge of absorption layer around the NDC peak voltage, so the NDC feature is proposed as resonant tunneling of photoelectrons into MQW structure. The proposed resonant tunneling process is confirmed by the observation of resonant tunneling peaks in a specially designed resonant tunneling diode simulating the band profile of MAPD. The finding of NDC feature is beneficial for understanding and increasing the quantum efficiency of MAPD, since the photoelectron blocking at AlN barrier is greatly reduced by the resonant tunneling process.

Quantum light and in particular single photons have become essential resources for a growing number of quantum applications including quantum computing, quantum key distribution and quantum metrology. Solid-state atomlike systems such as semiconductor quantum dots and color defects in crystals have become the hallmark of highly pure single photon emitters in the past two decades. A particular interest has been developed in nanocrystal quantum dots (NQDs) and color centers in diamond as potential compact room-temperature emitters. There are however several challenges that inhibit the use of such sources in current technologies including low photon extraction efficiency, low emission rates and relatively low single photon purities. In this work we will review our efforts in overcoming these technical difficulties using several complementary methods including designing several nanoantenna devices that enhance the directionality and emission rate of the nanoemitter. In addition, we developed several temporal heralding techniques to overcome the hurdle of low single photon purity in NQDs in an effort to produce a highly pure, bright and efficient single photon source on-chip.

Avalanche gain and breakdown voltage in most wide bandgap semiconductor materials are dependent on temperature and most instruments utilizing APDs rely on temperature stabilization or voltage compensation circuitry to maintain a constant avalanche gain. The complexity in operation circuitry can be reduced by incorporating material with inherently superior temperature stability in its avalanche gain and breakdown voltage. In state of the art APDs, the temperature dependence of avalanche breakdown voltage is quantified by the temperature coefficient of avalanche breakdown, C_bd. We report on the temporal and temperature stability of avalanche gain and breakdown voltage of 100 nm thick avalanche layers of Al_0.85Ga_0.15As_0.56Sb_0.44 (AlGaAsSb). The C_bd (1.60 mV/K) is smaller compared to state of art InP and InAlAs APDs for similar avalanche layer thickness. The temporal stability of avalanche gain for the AlGaAsSb APD was also evaluated in temperature ranges of 294 K to 353 K. The APD was biased at room temperature gain of 10 and maximum fluctuation of ±0.7% was recorded at 294 K which increases to ±1.33% when the temperature was increased to 353K. The promising temperature stability of gain indicates the potential of AlGaAsSb lattice matched to InP in achieving higher tolerance to temperature fluctuations and reduction of the operational complexity of circuitry. The dark currents are robust and do not show significant thermal degradation after gain measurements at elevated temperatures.

Mid-infrared (MIR) spectroscopy is a reliable tool for the identification of gaseous and liquid mixtures due to their unique and inherent absorption spectra. Quantum Cascade (QC) Lasers and Interband Cascade Lasers are modern reliable sources to penetrate the MIR spectral range. To increase the functionality of QC devices we designed and optimized a QC material that can be used as a QC laser and as a QC detector for the very same MIR wavelength, respectively. Switching from laser to detector is achieved by biasing the semiconductor (lasing mode) or operate it without any electric field applied (detecting mode), respectively. Due to this functionality increase the on-chip integration of a designable QC light source, an interaction zone and a QC detector is now feasible and has been demonstrated recently. In this talk we present improved bi-functional QC material for the integration and further development of sensor systems, as well as different cavity concepts for gas and liquid sensing scenarios. Proof of concept sensing examples to demonstrate the integrated sensor systems will be given. Multi mode and single mode lasers made from bi-functional materials show comparable performance to regular state of the art QC lasers and no performance drop due to the additional detection functionality. While QC lasers are already accepted within the scientific community, QC detectors still need to be further promoted. Thus, in addition to the improvement of the bi-functional QC material, we demonstrated a single period quantum cascade photo-detector with a responsivity of up to 1.3 A/W.

In this work, we report InGaAs based photodiodes integrating liquid crystal (LC) microcells resonant microcavity on their surface. The LC microcavities monolithically integrated on the photodiodes act as a wavelength selective filter for the device. Photodetection measurements performed with a tunable laser operating in the telecom S and C bands demonstrated a wavelength sweep for the photodiode from 1480 nm to 1560 nm limited by the tuning range of the laser. This spectral window is covered with a LC driving voltage of 7V only, corresponding to extremely low power consumption. The average sensitivity over the whole spectral range is 0.4 A/W, slightly lower than 0.6 A/W for similar photodiodes that do not integrate such a LC tunable filter. The quality of the filter integrated onto the surfaces of the photodiodes is constant over a large tuning range (70 nm), showing a FWHM of 1.5 nm.

A silicon micro-lens is proposed and analyzed when it is integrated into the photodiode for the application of a backside illuminated (BSI) image sensor (Pixel size is around 1 um). Due to the small dimension of the BSI pixel, each pixel of the image sensor receives from its adjacent pixels cross-talk (x-talk) due to large light incident angle and light diffraction, resulting in reduced sensor MTF and possible color artifacts. A silicon ulens formed between the photodiode and RGB color filter works as an inner lens to improve the focus of the light and guide it into its corresponding pixel, thus decreasing optical x-talk and reducing noise. Since the silicon ulens is integrated into the photodiode and could be doped as part of the photodiode, this design would eliminate any internal reflection caused by traditional inner micro lens solutions (made of Si oxide, Si nitride or polymer). ‘By color' anti-reflection coatings (ARC) on top of the silicon ulens can work as a versatile optical filter to compensate the light spectrum and angular mismatch. Our design and analysis provide a solution to improve the quantum efficiency (QE) and x-talk of the BSI image sensor and the QE enhancement for each pixel are discussed in detail.

We present a sensor design based on a Mach-Zehnder interferometer utilizing sub-wavelength gratings (SWGs) that were included in the waveguide to compensate for the short optical path length and to provide phase modulation. According to 2D finite element method simulations, it is possible to achieve 3-fold enhancement in sensitivity and 50% increase in modulation frequency with the inclusion of SWGs in the sensing arm as well as in the reference arm.

The usefulness of avalanche photodiodes (APDs) resides in their ability to produce internal gain via impact ionization without generating excessive noise. This process is stochastic and the gain values fluctuate around a mean value, giving rise to the so-called excess noise. In this work, we evaluate the gain fluctuations in APDs using a multi-channel analyzer (MCA). Two Al_0.85Ga_0.15As_0.56Sb ^0.44 APDs, one p-i-n and one n-i-p were used. Illuminated with a pulsed light source, the APDs were connected to a charge-sensitive amplifier, counting the number of charges created by each avalanche event initiated by the light pulse. The signal was subsequently sent to an MCA, recording the gain values and outputting a gain spectrum. Both APDs were investigated for mean gains up to ~ 9. For a given mean gain, the gain distribution for the n-i-p diode was found to be significantly broader than for the p-i-n diode, as expected from the excess noise values previously measured in those devices. The coefficient of variance (CoV), defined as the ratio of standard deviation to mean value of the gain peaks, was found to be low for the p-i-n APD, consistent with the low excess noise values in this material. For higher mean gain values, the CoV of the n-i-p APD gave higher values than for the p-i-n APD, again corroborating the conventional excess noise measurements.

Publisher’s Note: This paper, originally published on 18 September 2018, was replaced with a corrected/revised version on 28 May 2020. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.