Three types of electroabsorption modulators (EAMs) based on III-V semiconductor multiple quantum wells (MQW) are presented in this work. One is a novel monolithic integration traveling-wave EAM for an analog optical transmitter/transceiver to achieve integrated photonic mm-wave functions for broadband connectivity. Another one is composed of an integrated EAM 1D array in a photonic beam-former as a Ku-band phased array antenna for seamless aeronautical networking through integration of data links, radios, and antennas. The third one addresses the use of MQW EAMs in free space optical links through biological tissue for transcutaneous communication.
We report on the device characterization of In(Ga)Sb/InAs quantum dots (QDs) based photodetectors for long wave IR
detectors. The detection principle of these quantum-dot infrared photodetectors (QDIPs) is based on the spatially indirect
transition between the In(Ga)Sb QDs and the InAs matrix, as a result of the type-II band alignment. Such photodetectors
are expected to have lower dark currents and higher operating temperatures compared to the current state of the art InSb
and mercury cadmium telluride (MCT) technology.
The In(Ga)Sb QD structures were grown using metal-organic vapour-phase epitaxy and explored using structural,
electrical and optical characterization techniques. Material development resulted in obtaining photoluminescence up to
10 μm, which is the longest wavelength reported in this material system. We have fabricated different photovoltaic IR
detectors from the developed material that show absorption up to 8 μm. Photoresponse spectra, showing In(Ga)Sb QD
related absorption edge, were obtained up to 200 K. Detectors with different In(Ga)Sb QDs showing different cut-off
wavelengths were investigated for photoresponse. Photoresponse in these detectors is thermally activated with different
activation energies for devices with different cut-off wavelengths. Devices with longer cut-off wavelength exhibit higher
activation energies. We can interpret this using the energy band diagram of the dots/matrix system for different QD sizes.
Quantum structures base on type-II In(Ga)Sb quantum dots (QDs) embedded in an InAs matrix were used as active
material for achieving long-wavelength infrared (LWIR) photodetectors in this work. Both InAs and In(Ga)Sb are
narrow band semiconductor materials and known to possess a large number of surface states, which apparently play
significant impact for the detector’s electrical and optical performance. These surface states are caused not only by
material or device processing induced defects but also by surface dangling bonds, oxides, roughness and contaminants.
To experimentally analyze the surface states of the QD structures treated by different device fabrication steps, atomic
force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray
photoelectron spectroscopy (XPS) measurements were performed. The results were used to optimize the fabrication
process of the LWIR photodetectors in our ongoing project. The dark current and its temperature dependence of the
fabricated IR photodetectors were characterized in temperature range 10 K to 300 K, and the experiment results were
analyzed by a theoretic modeling obtained using simulation tool MEDICI.
Narrow bandgap semiconductors GaSb, InAs, and InSb are important building blocks for infrared photodetectors based
on type-II InSb quantum dots or an InAs/GaSb strained layer superlattice. Understanding the surface chemical
composition of these materials can provide valuable information that enables optimization of device surface passivation
techniques leading towards surface leakage free IR photodetectors. We report on an investigation into Ga-, In-, Sb-, and
As-oxides and other chemical species on the surface of untreated, dry etched and thermally treated GaSb, InAs and InSb
samples by x-ray photoelectron spectroscopy. The experimental results reveal the presence of Sb- and Ga-oxides on the
surfaces of the untreated and treated GaSb samples. Both Sb- and In-oxides were observed on the surface of all InSb
samples, and especially the dry etched sample had thicker oxide layers. In the case of the InAs samples, not only In- and
As-oxides XPS signals were obtained, but also AsCl species were found on the ICP dry etched sample. These results
helped to analyze the dark current of our fabricated IR detectors.
InSb nanostructures embedded in InAs and InAsSb matrices were grown on InAs (001) and GaAs (001) substrates by
molecular beam epitaxy. The diameter and height of InSb quantum dots (QDs) on InAs with 2ML-InSb coverage grown
by Stranski-Krastanov (S-K) are ~36.8 nm and ~3.1 nm, respectively. The density of QDs is ~2.5×1010 cm-2. The size
distribution of InSb QDs on InAs with 2ML-InSb coverage grown by migration enhanced epitaxy (MEE) was larger than
that of its S-K counterpart. Unique InSb quantum dashes (Q-dashes) on InAsSb elongated along two directions were
found on an AlSb-buffered GaAs substrate. InSb Q-dashes grown by migration enhanced epitaxy (MEE) were ~159 nm
in length, ~63 nm in width, and ~11 nm in height. A large reduction of volume of InSb structures between those in the
matrix and those on the surface was found. Threading disl°Cations resulting from the Q-dash structures were also
observed. This may be attributed to As-Sb exchange.
Qin Wang, David Rihtnesberg, Andreas Bergström, Susanne Almqvist, Andy Zhang, Wlodek Kaplan, Jan Andersson, Abhilash Sugunan, Xuran Yang, Muhammet Toprak
ZnO nanorods (NRs) sensors utilizing hybrid or monolithic integration of the NRs on nanoscale or microscale
interdigitated electrodes (IDEs) were fabricated and characterized. The IDEs with their finger electrode width ranging
from 50 nm to 3 μm were formed on SiO2/Si substrates by nanoimprint lithography or conventional photolithography
and metallization techniques, whereas the ZnO NRs were grown by chemical synthesis method. The average diameter of
the ZnO NRs is about 100 nm, and their length can be varied from 2 to 5 μm by controlling growth time. When sensing
targets, such as molecules or nanoparticles, bind onto the ZnO NRs, the conductance between IDEs will change. As
probing test, II-VI quantum dots (QDs) were attached on the ZnO NRs, and clear responses were obtained by measuring
and comparing current-voltage (I-V) characteristic of the sensor before and after binding the QDs.
Nowadays novel micro-fabrication and wafer-based manufacturing approach allows realizing micro-optics in a way
scientists have dreamt for generations, in particular, utilizing nano-imprint lithography as fabrication tooling enables
greatly accelerating the micro-optics technology to its frontier. In this report, we present wafer-scale fabrication of
various types of micro-optical elements based on photoresist, benzocyclobutene, photocurable imprint resist, and
semiconductor materials by using thermal reflow, reactive ion etching, and imprint techniques. Especially, several
concave or convex 3-dimensional micro-optical structures shaped by imprint method are detailed. These micro-optical
elements can be monolithically or hybrid integrated onto optoelectronics devices, such as photodetectors and emitters as
optical beam focuser, collimator, filter, or anti-reflectance elements. As application examples, polymer microlenses were
integrated directly on the top of UV dual functional devices and quantum dot long wavelength infrared photodetectors,
respectively.
Performance, advantages and drawbacks of GaN- and SiC-based ultraviolet (UV) photodetectors are analyzed and
compared. This includes metal-semiconductor-metal photodetectors, p-i-n photodiodes and avalanche photodiodes.
Design, process and characterization of these devices are described. Fabricated p-i-n InGaN/GaN quantum well
photodetectors are shown to exhibit a quantum efficiency of about 50% at 365 nm with a peak ultraviolet to visible
rejection ratio more than 3 orders of magnitude. A novel SiC avalanche photodiode design is shown to produce stable
avalanche breakdown characteristics for devices up to 2mm in diameter. A significant increase of its photo responsivity
is also demonstrated when the avalanche voltage is applied.
We have demonstrated surface normal detecting/filtering/emitting multiple functional ultraviolet (UV) optoelectronic devices based on InGaN/GaN, InGaN/AlGaN and AlxGa1-xN/AlyGa1-yN multiple quantum well (MQW) structures with operation wavelengths ranging from 270 nm to 450 nm. Utilizing MQW structure as device active layer offers a flexibility to tune its long cut-off wavelength in a wide UV range from solar-blind to visible by adjusting the well width, well composition and barrier height. Similarly, its short cut-off wavelength can be adjusted by using a GaN or AlGaN block layer on a sapphire substrate when the device is illuminated from its backside, which further provides an optical filtering effect. When a current injects into the device under forward bias the device acts as an UV light emitter, whereas the device performs as a typical photodetector under reverse biases. With applying an alternating external bias the device might be used as electroabsorption modulator due to quantum confined Stark effect. In present work fabricated devices have been characterized by transmission/absorption spectra, photoresponsivity, electroluminescence, and photoluminescence measurements under various forward and reverse biases. The piezoelectric effect, alloy broadening and Stokes shift between the emission and absorption spectra in different InGaN- and AlGaN-based QW structures have been investigated and compared. Possibilities of monolithic or hybrid integration using such multiple functional devices for biological warfare agents sensing application have also be discussed.
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.
Retro-communication by the joint use of light modulators and retro-reflecting devices has been proposed and demonstrated as a promising technique to further extend the use of free-space optical communication (FSO) to mobile communication. We have developed and demonstrated high-speed surface-normal GaAs-based and InP-based multiple quantum well electroabsorption modulators and arrays operating at different near-infrared wavelengths for FSO link applications. In this paper we report on the structure and device design issues, and trade-offs when GaAs-based or InP-based modulators are needed in the FSO links. In particular, the modulator structure properties, the device fabrication techniques and the device static and dynamic performance are investigated and compared.
Novel single crystalline high-performance temperature sensing materials (quantum well structures) have been developed for the manufacturing of uncooled infrared bolometers. SiGe/Si and AlGaAs/GaAs quantum wells are grown epitaxially on standard Si and GaAs substrates respectively. The former use holes as charge carriers utilizing the discontinuities in the valence band structure, whereas the latter operate in a similar manner with electrons in the conduction band. By optimizing parameters such as the barrier height (by variation of the germanium/aluminium content respectively) and the fermi level Ef (by variation of the quantum well width and doping level) these materials provide the potential to engineer layer structures with a very high temperature coefficient of resistance, TCR, as compared with conventional thin film materials such as vanadium oxide and amorphous silicon. In addition, the high quality crystalline material promises very low 1/f-noise characteristics promoting an outstanding signal to noise ratio and well defined and uniform material properties, A comparison between the two (SiGe/Si and AlGaAs/GaAs) quantum well structures and their fundamental theoretical limits are discussed and compared to experimental results. A TCR of 2.0%/K and 4.5%/K have been obtained experimentally for SiGe/Si and AlGaAs/GaAs respectively. The noise level for both materials is measured as being several orders of magnitude lower than that of a-Si and VOx. These uncooled thermistor materials can be hybridized with read out circuits by using conventional flip-chip assembly or wafer level adhesion bonding. The increased bolometer performance so obtained can either be exploited for increasing the imaging system performance, i. e. obtaining a low NETD, or to reduce the vacuum packaging requirements for low cost applications (e.g. automotive).
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.
We report on a quantum dot (QD) structure grown on a 4'' GaAs substrate by metal organic vapor phase epitaxy (MOVPE), which consists of five stacked InAs/InGaAs/GaAs QD layers embedded in the center of a typical in-plane waveguide. The density of the QDs is about 2.5 x 1010 cm-2 per QD layer. The photoluminescence (PL) peak wavelength at 1322 nm corresponding to the interband transition of the QD ground states was observed at room temperature with a full width at half-maximum of 49 meV. A good uniformity of the QD structure across the 4'' wafer was verified with a variation of the PL peak wavelength of 0.9 % from the wafer center to the edge. Top p-contacts and a bottom n-contact were processed on the QD structure, and electroluminescence (EL) spectra were measured at different temperatures. An EL peak corresponding to the QD ground states emission was obtained at 1325 nm at room temperature.
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.
We report on the design, fabrication and characterization of 1550 nm electroabsorption modulators based on InGaAs/InAlAs coupled quantum wells grown on InP substrate by MBE. Large and small single modulators and modulator arrays have been fabricated on a wafer scale with an optimized device fabrication technology. The modulator size, shape, contact arrangements, and the array configurations have been varied to achieve suitable device performance for different retro-reflective free-space optical communication links. The device electrical and optical properties have been characterized by I-V, photoluminescence, absorption, transmittance and reflectance measurements. Modulators exhibit contrast ratios of 2:1 at a 3V driving bias and contrast ratios of 2:1 over a 30 nm bandwidth at 6V. A maximum contrast ratio of 4:1 is obtained at a 12 V driving voltage.
We present design and fabrication methods for surface normal monolithic amplitude modulators with an aperture up to 14 x 14 mm2, a contrast ratio of 6:1 and for low driving voltages (≤8 V). The modulators consist of undoped GaAs/AlGaAs quantum wells embedded in a Fabry-Perot (FP) resonance cavity grown by MOVPE. To improve the device performance the FP cavity, the period and thickness of the quantum well and doping concentration were optimised. Also, the dimension of the modulator were varied from 0.5 x 0.5 to 14 x 14 mm2. The results show that the yield of the modulators increases significantly when decreasing the size of the modulators. To remedy the low yield issue for wide aperture modulator, a pixelated approach was used to divide the mono pixel in a monolithic modulator into several pixels, for example from 4 to 48. The modulation speed of the modulators with different dimensions was characterised by electro-optic (EO) response measurements. The temporal optical response of the large modulators was satisfactory up to the order of MHz modulation frequency where the RC constant limited the performance. A few of the modulators with wide apertures are to be assembled into an optical link system for free-space communication.
Acreo in Sweden has been invovled in Quantum Well IR Photodetector (QWIP) reserach and development since 1986. During the first years a small group led by Jan Andersson was dedicated to research on QWIP structures and means of coupling radiation into the quantum-well structure. One of the resarech results is a 2D optical grating couler to create an optimal elecgtric field pattern for highest possible absorption and responsivity. Acreo holds a patent for this grating coupler. Since 1988 FLIR Systems, later FMV, Saab Dynamics, Celsius Tech and NUTEK have sponsoered an R and D project with the goal to develop QWIP detectors and start up production. Soon it became clear that an adapted ROIC deisng and the hybridization of the focal plane array are key issues in order to achieve the highest possible performance and operability of the complete detector device for voluem production. Extended measures where taken in 1996. In 1997 the industrial interest increased further, ClesiusTech and Saab Dynamics, merged in 2000, and now a division of FLIR Systems AB started to sponsor the R and D project. Because of its success during the last years it has now been expanded and scheduled until 2003 and beyond. Volume production of QWIP FPAs started in 2000.
Multiple quantum well spatial light modulators (MQW SLMs) are promising devices for future high-speed applications. We present results obtained with a single-pixel amplitude modulator. We discuss the status of our work on a 128x128-pixel ternary SLM. This SLM will run at 10 kHz and have one low-reflectance level and two high reflectance levels with a phase difference of pi. We also present a study of the relation between the coding domain and the structural design of modulators.
As CMOS technology evolves, more and more functions can be integrated on the infrared focal plane array (FPA). This paper presents a study on the integration of analog to digital (A/D) conversion onto the FPA. A possible application for this is high-resolution (640 X 480 pixels) quantum well infrared photodetector (QWIP) FPAs operating at 70K. Operation at liquid nitrogen temperatures and below gives both advantages and disadvantages. CMOS transistors are performing better at these temperatures: increased transconductance and no leakage currents. On the other hand power dissipation needs to be limited to prevent a high load on the cooling system. The system aspects will ultimately determine the requirements for the A/D converters on the FPA. Some of the most stringent requirements are on: power dissipation, number of bits, die area and throughput. An FPA lends itself very well for the utilization of parallelism, so a trade-off can be made between sample rate per A/D converter and number of converters. With all these parameters in mind, an overview of state-of-the-art A/D converters at room temperature will be given. Trends will be identified and different architectures like delta/sigma and successive approximation will be evaluated. Also different implementation technologies such as switched current and switched capacitor will be reviewed with respect to their applicability in our application.
To obtain the optimal performance of quantum well IR photodetectors (QWIP) focal plane array, the readout circuit (ROIC) has to be tailored to the detector characteristics. Different concepts will be compared with respect to signal to noise performance, linearity and power consumption. In particular an active direct injection (DI) circuit will be compared with a passive direct injection circuit. The active and passive DI circuits give similar temporal NETD but the passive DI is shown to have superior spatial noise performance. Furthermore, the latter circuit type offers higher flexibility with respect to on-chip signal processing. The intended application is in high-resolution QWIP focal plane arrays.
A camera system has been designed using a focal plane array with 320 X 240 pixels. The detector array is based on quantum wells in the GaAs/AlGaAs material system grown onto a GaAs substrate and flipchip mounted to a readout circuit. The camera system uses f-number equals 1.5 optics to create an image of the scene on the FPA. The detector is cooled to approximately 70 K by an integrated Stirling cooler. The system also includes electronics for amplification and analog to digital conversion of the detector signal. The images are either displayed on a monitor or stored in digital format on an integrated hard disk. The short-term temporal noise was measured and the noise equivalent temperature difference was calculated to 16 mK. The spatial noise was found to be comparable to the temporal noise. The properties of the infrared images were valuated with respect to short and long term stability. The stability was found to be very good, giving a high quality image even 1 hour after a calibration. The number of dead pixels was less than 0.1% for several detectors.
Due to the well established GaAs material and processing technology QWIPs are viable candidates for high resolution (greater than 128 by 128 pixels), low cost LWIR (8 - 12 micrometer) focal plane arrays (FPAs). Usually n-doped AlGaAs/GaAs QWIPs are used since, at least to date, these have been shown to provide the highest performance. Fabrication and evaluation of 320 by 240 pixels QWIP arrays have also been done. The fabrication involves hybridizing GaAs chips consisting of detector mesas to specially designed CMOS readout chips. The hybridization is effected by indium bump flip-chip bonding. Optical coupling into the detectors is performed by using optimized, etched, two-dimensional gratings combined with GaAs substrate thinning down. The advantages of substrate removal are: (1) reduction of thermal mismatch between materials and thus permitting large array sizes, (2) enhancement of absorptance, and (3) elimination of optical cross-talk between pixels. The intended operating temperature range is 70 to 73 K, achievable by a miniature Stirling cooler. Excellent wafer uniformities resulting in responsivity uniformities of 3.3% across an array are found, and a temperature resolution NETD (noise equivalent temperature difference) equals 40 mK is achieved. Finally, the presence of fixed-pattern noise and its influence on the image performance are discussed.
The most prominent mechanism of dark current in infrared detectors based on intersubband transitions in quantum wells (QWIP) is due to interaction of electrons with longitudinal optical phonons. Theoretical expressions are derived for the carrier lifetime, and for generation currents due to both photo-excitation as well as thermal excitation in a quantum well. Detector gain is discussed briefly. Calculated values of thermal generation currents and the ratio of photo-current to thermal current are found to accord well with experimental data. Finally the BLIP performance of QWIPs is investigated and the theory gives TBLIP equals 81 K for a 9 micrometers cut-off detector with a 2D grating and optical cavity, for 300 K background temperature, optics f-number equals 1 with 100% transmission, and if a photo- to dark current ratio equals 1 criterion is used.
In this paper we present an experimental investigation of the performance of GaAs/AlGaAs multiple quantum well photodetectors. The purpose of this exercise was to independently evaluate and verify the responsivity of the GaAs/AlGaAs quantum well infrared photodetectors developed at the Industrial Microelectronics Center in Sweden. These devices use 2D gratings to couple radiation into the detectors and a cladding layer to enhance the coupling of radiation. The devices were of two types: those optimized for high detectivity, and those optimized for high quantum efficiency. The tests performed on these devices included measurement of optical responsivity vs. bias, spectral response, Detectivity (D*), and measurement of cross-talk between pixels. Several interesting observations were made during the investigation and will be reported in the paper.
In0.53Ga0.47As/InP infrared detectors with peak absorption at a wavelength of 8.5 micrometers have been fabricated and tested. It is shown that very high current responsivities and high gain are obtained. It is found that gain increases drastically when approaching detector voltages close to -8.5 V, which is explained by carrier impact excitation of electrons from the QW ground state to the excited extended state. The detectivity D* is about 1.2(DOT)1010 cm Hz1/2 W-1 at 80 K for a 45 degree(s) polished edge detector assuming unpolarized radiation. Grating coupling is studied by etching crossed gratings into the upper part of the mesas. The increase in responsivity as compared to a polished edge detector is by a factor of 2.5 to 3, irrespective to mesa sizes 500 X 500 or 150 X 150 micrometers 2. This gives a corresponding detectivity of (3 - 3.5)(DOT)1010 cm Hz1/2 W-1 at 80 K for unpolarized radiation.
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