Research

[1/5] Single photon detectors based on Geiger-Mode APD

Background:

Detectors with single photon sensitivity are of great use for many applications, such as time resolved spectroscopy、three-dimensional imaging and quantum communication.

Over the last century, photomultiplier tubes(PMTs) are mainly used for low light level detection. PMTs can achieve a rather high detection efficiency(40%) when operating at the wavelength around 500nm, but as the operating wavelength increases, the detection efficiency drops rapidly(2% at 1550nm). At the same time, PMTs are vrey fragile and sensitive to magnetic field. These sever handicaps forced the search for an alternative to PMTs.

Single-photon avalanche photodiodes, known as SPADs, are now a well-established alternative to PMTs in single photon detection. The SPAD is based on the structure of avalanche photodiode(APD), which incorporates separate absorption、grading、and multiplication layers. When the diode is reverse biased above the breakdown voltage, macroscopic current will be generated in response to single-photons. This avalanche process will not stop until get quenched by an external circuit. The band gap of the absorption layer decide the frequency of the photon can be detected. Typically, when the material is chosen as silicon, then the operating wavelength is around 650nm; when it is InGaAs, then is around 1550nm.

To meet the consequent demand for high-performance SWIR single photon detectors, we will focus on developing high-performance InP-based single photon detectors.

Fig. (a).Geiger-mode SPAD Products (b).Band structure of PN junction and the processing of self-sustaining avalanche (c).Single photon detectors based on SPAD and integrated quenching circuits(Products from PicoQuant)

Research contents:

  1. Design and manufacture of SPAD
  2. Fig. (a).Structure of SPAD (b).Typical dark and illuminated (1-µW) I-V characteristics for a 2.5-µm diameter SPAD at 293K

  3. Control circuitry and signal processing of SPAD
  4. Fig. Repetition frequency,PDE,and afterpulsing probability for SPADs with various types of operating circuitry

    (1).Compensation of parasitics related to fast gating and counting

    (2). Avalanche charge flow reduction by external circuitry

Publications:

[2/5] High Resolution Nanolithography

Background

In past decades, photolithography has been one of the most primary technologies which promote semiconductor industry growth. However, as the node size continuously scaling down, this process becomes more expensive and difficult due to the ever-increasing complexity of projection system and time-consumed mask. Some alternative techniques, such as self-assembly lithography, nano-sphere lithography, plasmonic interference lithography, nano-imprint lithography, and near-field scanning optical lithography (NSOL),are generating a lot of interests. In the contrast of conventional photolithography, NSOL using ridged aperture can achieve sub diffraction-limit feature sizes which are typically comparable to results obtained by electron beam lithography on the monolayer. Besides, through controlling the relative motion between the mask and substrate, an arbitrary pattern can be created directly without any photo-masks, which can greatly reduce the cost and design circle. For high-throughput manufacturing, an array of thousands of ridged apertures, each can be individually controlled but working in parallel, will be used for scaling up the process.

However, this ultra strong light confinement only exits in near-field regime, which is so-called localized surface plasmon (LSP). When light exits from the ridged aperture over tens of nanometers, it’ll decay exponentially and diverge to a large spot. Therefore, it’s crucial to decrease the gap between exit-plane of ridged aperture and photoresist as small as possible. Theoretically, when the mask and photoresist are in contact mode, it can produce the best lithography results —— the highest light intensity and the smallest spot size. Yet, the adhesion and friction in contact mode always cause unfavorable contamination like generating tiny particles or photoresist deformations, which has serious impacts on the roughness of lithography patterns. To solve this problem, a novel flexure orientation stage is designed to hold the mask for maintaining intimate contact while minimizing relative lateral motions. Combination the flexure design and our unique mask fabrication process, we achieve 16 nm (FWHM) line-width lithography results.

Numerical and experimental study of nanolithography using nano-scale C-shaped aperture

Table. Comparison of C and regular apertures

Fig. (a)SEM image of the lithography mask pattern, (b)SEM image of the lithography results, (c)Schematic diagram of the experimental lithography setup, (d)Field intensity profile across the centre of the C aperture and other regular apertures at 24nm behind the exit plane

Resonant Effects in Nanoscale Bowtie Apertures

Fig. Fabrication process of lithography mask and SEM images of the backside milling bowtie aperture and the front side milling bowtie aperture. (a) Schematic of the fabrication process. (b,c) Tilted SEM image on the exit plane of a bowtie aperture fabricated using the new back side milling method and a bowtie aperture milled from the front side(scale bar:100 nm)

Fig. Numerical simulation results of the two kinds of bowtie apertures. (a) Schematic of the electric field intensity (background color), surface current (dotted line) and charges (signs) on the exit plane of the backside milling bowtie aperture made in Cr film.(b,c) Cross-sectional electric field intensity of the backside milling bowtie aperture andthe front side milling bowtie aperture. (d) Modulation transfer function at different resolution for the two apertures at 5 nm and 10 nm distance. (e) Achievable resolution as a function of pattern depth for the two apertures.

A novel flexure orientation stage designed for near-field scanning optical lithography

Fig. (a) The schematic drawing of NOSL system. (b) The magnified drawing of part of system. Every part is spaced by a fixed distance for being easily distinguished. (c) The mask used in NOSL system.

16nm Resolution Lithography Using Ultra-small Gap Bowtie Apertures

Fig. Lithography results of two kinds of bowtie apertures.(a) SEM of the two kinds of bowtie apertures with their lateral gap size indicated for comparison. (b) AFM image of lithography results of the array. (c) Zoomed-in scan of the lower right hole. (d) Cross section profile of the hole in (c) in x direction.

Fig. Lithography results of bowtie apertures with different gap size. (a) SEM of four bowtie apertures with different scan passes and gap size. (b) AFM image of lithography results of the aperture array with the FWHM of each hole indicated. (c) Achievable resolution as a function of patterning depth for bowtie apertures with different gap width g. The solid lines are obtained with the numerical analysis and the color asterisks are experimental results.

Fig. Near-field scanning optical lithography results.(a) AFM image of a line produced by near-field scanning optical lithography. (b) The cross-sectional profile of the line.

Publications:

  1. Liang Wang et al. “16nm resolution lithography using ultra-small gap bowtie apertures” Scientific reports. Accepted for review
  2. Liang Wang et al. “Resonant Effects in Nanoscale Bowtie Apertures”. Scientific reports. Accepted for review
  3. Li Ding, Liang Wang, Numerical and experimental study of nanolithography using nano-scale C-shaped aperture, Applied Physics A-Materials Science & Processing,2015, 10.1007/s00339-015-9

[3/5] Micro/nano Photonic Device Based on Surface Plasmon Polarizaton

Background

New HWTP Waveguide

Fig. Schematic illustration of the nanotube based hybrid plasmonic waveguide(a), hybrid wedge plasmonic waveguide(b) and the proposed HWTP waveguide(c)

Fig. ependence of hybrid plasmonic modal properties on gap distance and radius of the dielectric cylinder for NHP waveguide, HWP waveguide and LRHTWP waveguide: (a) and (c) propagation length; (b) and (d) normalized modal area; the radius of the dielectric cylinder and the outer radius of the tube in these waveguides are the same as 120 nm. The inset pictures are electric field distributions of hybrid modes.

Fig. Schematic diagram of the process flow.

Publications:

  1. Li Ding, Jin Qin, Kai Xu, and Liang Wang, "Long range hybrid tube-wedge plasmonic waveguide with extreme light confinement and good fabrication error tolerance," Opt. Express 24, 3432-3440 (2016)

[4/5] Nanoimprint Lithography

Background

Lithography is the key technique for nanostructure fabrication. One significant application in which lithography has had a significant impact is in the processing of integrated circuits. The semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area. Other areas of development in which lithography has been employed include biotechnology, optical technology, and mechanical systems. Therefore lithography becomes increasingly important

High contrast mark used for UV nanoimprint lithography

Our group designed and fabricated high contrast alignment marks used for UV imprint lithography in situ alignment. Since the imprint resist filled in the imprint pattern will deteriorate the intensity and contrast of the Moiré fringes, it’s hard to perform alignment based on the Moiré image. Simulations based on rigorous coupled-wave analysis (RCWA) are performed to design and optimize the high contrast alignment mark in order to obtain high quality Moiré fringes. Designed high contrast mark is fabricated and tested on imprint alignment system. Experiments demonstrated that the simulation results are correct and feasible.

Fig. Schematic of in-liquid alignment(a),imprint cross-section illustrating the high contrast marks used for in-situ liquid alignment(b) the phase grating pattern in mask(c) and the checkerboard pattern on the wafer.

Fig. Checkerboard structures on the wafer(a) and 1-D grating on the mask(b) and high contrast moiré pattern(c)

Fig. The solid line represents the reflection efficiencies change with different Cr coating thicknesses theoretically and the histogram represents the signal strength received by a PMT.

Nanoimprint lithography on the flexible substrate

Fig. The process flow of the nanoimprint lithography on flexible substrate

High-speed plate-to-roll nanoimprint lithography (P2RNIL)

Fig. Photography of the P2RNIL prototype apparatus

Experiment results:

Fig. a) top-down SEM micrograph and b) cross-sectional SEM micrograph of the template

Fig. a)top-down SEM micrograph and b) cross-sectional SEM micrograph of the flexible substrate

Publications:

  1. Jin Qin, Li Ding, and Liang Wang, "In situ UV nano-imprint lithography alignment using high contrast mark," Opt. Express 23, 18518-18524 (2015)
  2. Li Ding, Jin Qin and Liang Wang, “High contrast mark used for in-situ UV nano-imprint lithography allignment,” IEEE pp. 1-4. doi: 10.1109/CSTIC.2015.7153371
  3. Huiwen Luo, Haosen Tan, Kai Xu, and Liang Wang, “High-speed plate-to roll nanoimprint lithography on flexible substrate”, in preparation for NANOTECHNOLOGY

[5/5] Quantum Dot Single-Photon SourceSingle photon source(SPS) based on quantum dots

Background:

Single Photon Source play an important role in quantum computing and quantum information:

  1. A good carrier of quantum bit: transport in long distance, coherent almost can be ignored and its polarization, path and so on many degrees of freedom can be used to encode information into bits.
  2. Deterministic excitation: resulting in identical single photon sequence, which is particularly important in linear optical quantum computing.

In addition, the quantum dot single-photon source mainly includes the following five major advantages:

  1. Scalable qubit system
  2. Well prepared arbitrary initial state qubit
  3. Long decoherence time
  4. To achieve a common set of quantum logic gate operation
  5. Qubits can be read on

Single Photon Source Based on Single Quantum Dot

Fig. Schematic of single quantum dot

Easily integrated into the DBR microcavities, or embedded into the p-i-n junction in order to produce integrated circuit devices, and the maximum repetition rate, single photon emission quality, source size, etc. have advantages.

Project Plan

Fig. Schematic of project plan

Publications: