In this contribution, we analyze the photogating effect in InP nanowire arrays with embedded InAsP quantum discs by detailed numerical modeling. The model comprises a drift-diffusion current coupled to the nonlinear Poisson equation, solved on a 2-dimensional geometry with rotational symmetry. By comparison to experimental data, surface trap states are identified that explain both the current versus voltage behavior in dark and under illumination. The current versus illumination power is highly nonlinear, and shows measured gain up to 320 for a power of 20 nW at 980 nm for an array of millions of wires.
In this talk, I will give an overview of our research efforts on developing InP/InAsP nanowire (NW) heterostructure photodetectors for NIR-LWIR detection. I will discuss design, modeling, growth, device processing and optoelectronic properties of both single NW devices and large square millimeter devices comprising millions of NWs connected in parallel. The developed device family includes p-i-n and SAM avalanche diodes, as well as state-of-the-art photoconductors with embedded quantum discs. I will discuss a newly developed model that unravels the high, non-linear optical gain observed in photoconductors that stems from complex carrier dynamics involving multiple traps on the surface of the NWs. I will also briefly introduce our recently initiated efforts on realizing bias-tunable detectors with NW arrays embedded in designed photonic crystals for spectral tuning of optical modes.
Geometrically designed III-V nanowire arrays are promising candidates for optoelectronics due to their possibility to excite nanophotonic resonances in absorption spectra. Strong absorption resonances can be obtained by proper tailoring of nanowire diameter, length and pitch. Such enhancement of the light absorption is, however, accompanied by undesired resonance dips at specific wavelengths. In this work, we theoretically show that tilting of the nanowires mitigates the absorption dips by exciting strong Mie resonances. In particular, we derive a theoretical optimum inclination angle of about 30° at which the inclined nanowires gain 8% in absorption efficiency compared to vertically standing nanowires in a spectral region matching the intensity distribution of the sun. The enhancement is due to engineering the excited modes inside the nanowires regarding the symmetry properties of the nanowire/light system without increasing the absorbing material. We expect our results to be important for nanowire-based photovoltaic applications.
In this study, bias mediated tuning of the detection wavelength within the infrared wavelength region is demonstrated for quantum dots-in-a-well (DWELL) infrared photodetectors. In DWELL structures, intersubband transitions in the conduction band occur from a discrete state in the quantum dot to a subband in the quantum well. Compared to "conventional" quantum dot infrared photodetectors, where the transitions take place between different discrete bands in the quantum dots, new possibilities to tune the detection wavelength window are opened up, partly by varying the quantum dot energy levels and partly by adjusting the width and composition of the quantum well. In the DWELL structure used, an asymmetric positioning of the InAs quantum dot layer in a 8 nm wide In0.15Ga0.85As/GaAs QW has been applied which enables tuning of the peak detection wavelength within the long wavelength infrared (LWIR; 8 - 14 µm) region. When the applied bias was reversed, a wavelength shift from 8.5 to 9.5 µm was observed for the peak position in the spectral response. For another DWELL structure, with a well width of 2 nm, the tuning range of the detector could be shifted from the medium wavelength infrared (MWIR; 3-5 µm) region to the LWIR region. With small changes in the applied bias, the peak detection wavelength could be shifted from 5.1 to 8 µm. These tuning properties of DWELL structures could be essential for applications such as modulators and two-colour infrared detection.
In this paper we present a comprehensive calculational model for the noise equivalent temperature difference (NETD) of infrared imaging systems based on uncooled bolometer arrays. The NETD model is validated and benchmarked using published performance data of state-of-the-art uncooled infrared bolometer arrays. The calculational model is used to evaluate possible infrared sensor and system design tradeoffs that allow optimization for low-cost infrared systems with improved reliability and lifetime, while still achieving a NETD of about 150 mK, required for pedestrian injury mitigation systems. We propose an approach in which high performance crystalline semiconductor materials with very low 1/f-noise properties and a temperature coefficient of resistance (TCR) of 3 %/K are used as thermistor material for the bolometers. The resulting increased bolometer performance can be used to operate the infrared imaging arrays in a vacuum atmosphere with increased gas pressure while still achieving useful NETD levels. The proposed calculational model suggests that a NETD on the order of 150 mK can be reached with uncooled infrared bolometer arrays operating in vacuum pressures on the order of 6 mbar. Such specifications for the bolometer vacuum package dramatically simplify wafer-level vacuum packaging and ease long-term reliability issues, contributing to lowering the vacuum packaging and thus, the overall infrared imaging chip costs.
A new low-cost long-wavelength infrared bolometer camera system is under development. It is designed for use with an
automatic vision algorithm system as a sensor to detect vulnerable road users in traffic. Looking 15 m in front of the
vehicle it can in case of an unavoidable impact activate a brake assist system or other deployable protection system. To
achieve our cost target below €100 for the sensor system we evaluate the required performance and can reduce the
sensitivity to 150 mK and pixel resolution to 80 x 30. We address all the main cost drivers as sensor size and production
yield along with vacuum packaging, optical components and large volume manufacturing technologies.
The detector array is based on a new type of high performance thermistor material. Very thin Si/SiGe single crystal
multi-layers are grown epitaxially. Due to the resulting valence barriers a high temperature coefficient of resistance is
achieved (3.3%/K). Simultaneously, the high quality crystalline material provides very low 1/f-noise characteristics and
uniform material properties. The thermistor material is transferred from the original substrate wafer to the read-out
circuit using adhesive wafer bonding and subsequent thinning. Bolometer arrays can then be fabricated using industry
standard MEMS process and materials. The inherently good detector performance allows us to reduce the vacuum
requirement and we can implement wafer level vacuum packaging technology used in established automotive sensor
fabrication. The optical design is reduced to a single lens camera. We develop a low cost molding process using a novel
chalcogenide glass (GASIR®3) and integrate anti-reflective and anti-erosion properties using diamond like carbon
coating.
We report on a quantum dots-in-a-well infrared photodetector (DWELL QDIP) grown by metal organic vapor phase epitaxy. The DWELL QDIP consisted of ten stacked InAs/In0.15Ga0.85As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 1010 cm-2 per QD layer. The energy level structure of the DWELL was revealed by optical measurements of interband transitions, and from a comparison with this energy level scheme the origin of the photocurrent peaks could be identified. The main intersubband transition contributing to the photocurrent was associated with the quantum dot ground state to the quantum well excited state transition. The performance of the DWELL QDIPs was evaluated regarding responsivity and dark current for temperatures between 15 K and 77 K. The photocurrent spectrum was dominated by a LWIR peak, with a peak wavelength at 8.4 μm and a full width at half maximum (FWHM) of 1.1 μm. At an operating temperature of 65 K, the peak responsivity was 30 mA/W at an applied bias of 4 V and the dark current was 1.2×10-5 A/cm2. Wavelength tuning from 8.4 μm to 9.5 μm was demonstrated, by reversing the bias of the detector.
Pedestrian fatalities are around 15% of the traffic fatalities in Europe. A proposed EU regulation requires the automotive industry to develop technologies that will substantially decrease the risk for Vulnerable Road Users when hit by a vehicle. Automatic Brake Assist systems, activated by a suitable sensor, will reduce the speed of the vehicle before the impact, independent of any driver interaction. Long Wavelength Infrared technology is an ideal candidate for such sensors, but requires a significant cost reduction. The target necessary for automotive serial applications are well below the cost of systems available today. Uncooled bolometer arrays are the most mature technology for Long Wave Infrared with low-cost potential. Analyses show that sensor size and production yield along with vacuum packaging and the optical components are the main cost drivers. A project has been started to design a new Long Wave Infrared system with a ten times cost reduction potential, optimized for the pedestrian protection requirement. It will take advantage of the progress in Micro Electro-Mechanical Systems and Long Wave Infrared optics to keep the cost down. Deployable and pre-impact braking systems can become effective alternatives to passive impact protection systems solutions fulfilling the EU pedestrian protection regulation. Low-cost Long Wave Infrared sensors will be an important enabler to make such systems cost competitive, allowing high market penetration.
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