Search

Emerging applications in sensing, LIDAR, spectroscopy, and SWIR imaging require photodetectors operating at wavelengths beyond the range of silicon technology and that can be produced cost-effectively for very high-volume consumer and commercial markets. In this paper, Array Photonics, Inc. introduces a new generation of photodetectors designed to meet these requirements. We provide an update on PIN photodetectors and detector arrays based on our dilute nitride material (GaInNAsSb) grown epitaxially on and lattice matched to gallium arsenide (GaAs) substrates. We present our latest photodetector results demonstrating broadband operation from 450 nm to 1450 nm with peak responsivity of over 0.85 A/W at 1300 nm and dark current density of 3×10-5 A/cm2 at -1V bias voltage. These dilute nitride-on-GaAs photodetectors are now commercially available as single elements, 1-D arrays and small 2-D arrays and are referred to as eGaAs™ (extended GaAs) photodetectors. We also briefly discuss ongoing work to migrate from 4” (100 mm) to 6” (150 mm) GaAs wafers and the implementation of larger size 2-D arrays for imaging applications in the automotive and industrial markets.

Novel InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are described. These detectors, in which the active region consists of InAs quantum dots embedded in an InGaAs quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a quantum dot infrared photodetector (QDIP). Like QDIPs, the DWELL detectors display normal incidence operation without gratings or optocouplers while demonstrating reproducible “dial-in recipes” for control over the operating wavelength, like QWIPs. We report the first demonstration of a two-color infrared focal plane array based on a voltage-tunable quantum dots-in-well (DWELL) design. The active region consists of multiple layers of InAs quantum dots in an In0.15Ga0.85As quantum well. Spectral response measurements yielded a peak at 5.5 µm for lower biases and at 8–10 µm for higher biases. Using calibrated blackbody measurements, the mid-wavelength and long wavelength specific detectivity (D*) were estimated to be 7.1 × 1010 cmHz1/2 /W (Vb = 1.0 V) and 2.6 × 1010 cmHz1/2 /W (Vb = 2.6 V) at 78 K, respectively. This material was processed into a 320 × 256 array and integrated with an Indigo 9705 readout chip and thermal imaging was achieved at 80 K. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate the electromechanical side instability and the stable travel range of comb-drive actuators. The stable travel range depends on the finger gap spacing, the initial finger overlap, and the spring stiffness ratio of the compliant suspension. Proper design of the suspension structure is the most effective way to stabilize the actuator and therefore to achieve a large deflection. In this paper, we propose an improved suspension design, the so-called tilted folded-beam suspension. Using such suspension, the stability of the comb-drive actuator is improved and the stable travel range is enhanced. We give the expressions for the spring constants of the proposed suspension, both in the stroke direction and perpendicular to it. The suspension designs are also studied numerically using the finite element method (FEM), in which the geometric nonlinearities, such as large deflections and stress stiffening, are considered. Analytical calculations and FEM simulations are compared. The results demonstrate that an enhanced stable travel range, compared to that of a comb-drive actuator with the most commonly used folded-beam flexure, can be achieved by using the proposed suspension design. Comb-drive actuators with various tilted folded-beam suspensions have also been fabricated using the standard surface micromachining technology and their operational performances have been characterized. The experimental results are in good agreement with the theoretical predictions.

Post-growth rapid thermal annealing has been performed with In(Ga)As quantum dots (QDs) at different strain statuses. It is confirmed that the strain-enhanced interdiffusion decreases the inhomogeneous size distribution. The preferential lateral interdiffusion of QDs during annealing was observed. we attribute it to the naturally anisotropic strain distribution in/around the dots and the saturation of strain difference between the base boundary and the top of the dots. There exist strain-enhanced mechanism and vacancy diffusion enhanced mechanism during the annealing. As to which one dominates the QD interdiffusion depends on the thickness of capping layer and the annealing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

In this paper, a modeling approach is introduced that includes micromechanical components into the system level simulation. In our approach, a complicated microsystem is first decomposed into a set of much simpler components. Each component is treated as an n-pole functional block. The behavioral model of a micromechanical component is generated from the finite element (FE) analysis results of its corresponding 2D/3D FE model by using the system identification methods. The behavioral model is then described by an analog hardware description language (AHDL). The global performance of the system is simulated in an analog behavioral simulator by connecting the subdivided behavioral models together. The proposed approach can be implemented by combining the commercially available software ANSYS (for finite element method (FEM) analysis), MATLAB (for system identification) and an analog behavioral simulator such as SPECTRE (for system level simulation) together. As examples of the proposed methodology, simulations and results of a linear lateral resonator, a Q-controlled resonator and a series two-resonator micromechanical bandpass filter are presented.

We report the first two-color 320 x 256 infrared Focal Plane Array (FPA), based on a voltage-tunable InAs/InGaAs/GaAs DWELL structure. The detectors, grown by solid source molecular beam epitaxy (MBE) comprise of a 15-stack asymmetric DWELL structure sandwiched between two highly doped n-GaAs contact layers, grown on a semi-insulating GaAs substrate. The DWELL region consists of a 2.2 monolayer deposition of n-doped InAs quantum dots (QDs) in an In 0.15 GaAs 0.85 As well, itself placed in GaAs. The well widths below and above the dots are 50a and 60a, respectively. The absorption region asymmetry results in a bias dependent spectral response, with the peak wavelength varying from 5.5 to 10 μm. Using calibrated black body measurements, mid-wavelength and long wavelength specific detectivities (D*) of top-illuminated test pixels at 78K were estimated to be 7.1 x 10 10 cmHz 1/2 /W (V b = 1.0V) and 2.8 x 10 10 cmHz 1/2 /W (V b = 2.5V), respectively. Subsequently, a 320 x 256 QDIP FPA array was fabricated on a 30 μm pitch and was hybridized with an Indigo 9705 ROIC. Thermal imaging was successfully carried out at an estimated FPA temperature of 80K, using different optical filters between 3-5 μm, and 8-12 μm, so as to demonstrate two-color operation. The operability of the FPA was greater than 99%, and the noise-equivalent temperature difference was estimated to be less than 100 mK for f#1 (3-5 μm) and f#2 (5-9 μm) optics.

Quantum-dot infrared photodetectors (QDIPs), based on intersubband transitions in nanoscale self-assembleddots, are perceived as a promising technology for mid-infrared-regime sensing since they are based on a matureGaAs technology, are sensitive to normal incidence radiation, exhibit large quantum con“ned stark eect thatcan be exploited for hyperspectral imaging, and have lower dark currents than their quantum-well counter-parts. High detectivity ( D =10 11 cmHz 1 / 2 /Watt at 9 µ m) QDIPs have been recently shown to exhibit broadspectral responses ( 2 µ m FWHM) with a bias-dependent shift in their peak wavelengths. This controllable,bias-dependent spectral diversity, in conjunction with signal-processing strategies, allows us to extend the op-eration of the QDIP sensors to a new modality that enables us to achieve: (1) spectral tunability (single- ormulti-color) in the range 2-12 µm in the presence of the QDIPs dark current; and (2) multispectral matched“ltering in the same range. The spectral tuning is achieved by forming an optimal weighted sum of multiplephotocurrent measurements, taken of the object to be probed, one for each bias in a set of prescribed operationalbiases. For each desired spectral response, the number and values of the prescribed biases and their associatedweights are tailored so that the superposition response is as close as possible, in the mean-square-error sense,to the response of a sensor that is optically tuned to the desired spectrum. The spectral matching is achievedsimilarly but with a dierent criterion for selecting the weights and biases. They are selected, in conjunctionwith orthogonal-subspace-projection principles in hyperspectral classi“cation, to nullify the interfering spectralsignatures and maximize the signal-to-noise ratio of the output. This, in turn, optimizes the classi“cation of theobjects according to their spectral signatures. Experimental results will be presented to demonstrate the QDIPsensors capabilities in these new modalities. The eect of dark-current noise on the spectral-tuning capabilityis particularly investigated. Examples of narrowband and wideband multispectral photocurrent synthesis as wellas matched “ltering are presented.Keywords: quantum dot, self assembled, infrared detectors, spectral tuning, matched “ltering

Spectrally tunable quantum-dot infrared photodetectors (QDIPs) can be used to approximate multiple spectral responses with the same focal-plane array. Hence, they exhibit the potential for real time adaptive detection/classification. In the present study, it is shown that we can perform the detection/classification operation at the adaptive focal-plane array (AFPA) based on QDIPs by fitting the QDIP's response to the correspondent operators. With a new understanding of spectral signature in the sensor space, the best fitting can be achieved. Our simulation results show how well QDIPs perform in different regions of the spectrum in the mid- and long wave infrared. The results indicate that the AFPA performance does not match that of the ideal filtering operators, but reliable measurement can be accomplished.

The realization of single-mode Distributed Feedback (DFB) and Distributed Bragg Reflector (DBR) lasers, based on surface grating structures is of considerable interest. Such devices offer a relatively simple grating fabrication process without complicated multistep-epitaxial growth or regrowth, as required in more conventional devices. This simplified processing could potentially reduce the fabrication cost for these lasers. A key concern for the surface grating lasers is designing the structure to provide sufficient feedback to achieve single mode operation with high yield and high quality as compared to conventional buried-grating DFB and DBR lasers. This paper reports numerical modelling of surface grating DFB and DBR lasers based on 1.55 micrometers wavelength InGaAs/InGaAsP/InP with graded-index separate confinement heterostructure multiple quantum well (GRINSCH-MQW) structures. Bragg wavelength operation of the DFB and DBR lasers may be satisfied by deeply etching fine surface gratings on both side portions and along top of the ridge stripe respectively. Sufficiently strong optical coupling between the corrugated structure and evanescent field can be achieved by controlling various parameters such as ridge width, etch depth, and grating width. The numerical results obtained allow optimum grating geometries to be developed to provide the desired feedback effect. Furthermore, the model has been used to investigate the influence on the device performance of fabrication errors and processing effects on the grating structures, prior to fabrication.

The electromechanical side-instability and the stable travel range of comb-drive actuators are investigated. The stable travel range depends on the finger gap spacing, the initial finger overlap, and the spring stiffness ratio of the compliant suspension. Proper design of the suspension structure is the most effective way to stabilize the actuator and therefore to achieve a large deflection. In this paper, an improved suspension design, so called tilted folded-beam suspension, is proposed. The expressions for the spring constants of the proposed suspension both in and perpendicular to the stroke direction are given. Using such suspension, the stability of the comb-drive actuator is improved and the stable travel range is enhanced. Comb drive actuators with various tilted folded-beam suspensions were fabricated using the standard surface micromachining technology and their operational performances were characterized. The experimental results are in good agreement with the theoretical predictions.

The concept of nondiffracting beams was first introduced by Durnin. The beam spot of nondiffracting beam undergoes diffraction-free spreading over a long propagating distance. Therefore, nondiffracting beams could have potential applications in precision alignment, optical interconnections, and power transport. In this paper, hybrid genetic algorithms that combine genetic algorithms (GAs) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive optical elements (DOE's) for the generation of nondiffracting beams. In the hybrid genetic algorithms, an offspring obtained by genetic operators, such as crossover and mutation, is not included in the next generation directly but used as a seed for the sequent local search. The local search method searches the neighborhood of each offspring, and selects a better point, which is included in the next generation. In such a manner, the efficient exploitation of local information is provided by the incorporated local search procedure and the reliable locating of the global minimum is provided by the use of mechanisms of nature selection. The proposed hybrid methods exploit the global nature of the GAs as well as the local improvement capabilities of the gradient-based local search techniques, and will perform a more improved search while comparing with both of the single ones. The incorporated local search technique we used here is the Davidon-Fletcher-Powell (DFP) method, which is well known for its good convergence property. Numerical results demonstrate that the designed DOE's can successfully produce both zero-order and high-order nondiffracting beams.

In this paper, a modeling approach is introduced that includes the micromechanical components into the system level simulation. The proposed approach can be implemented by combining the commercially available software ANSYS (for FEM analysis), MATLAB (for system identification), and analog behavioral simulator (for system level simulation) together. Modeling and simulation results based on our methodology are presented.

It has recently been reported that by using a spectral-tuning algorithm, the photocurrents of multiple detectors with spectrally overlapping responsivities can be optimally combined to synthesize, within certain limits, the response of a detector with an arbitrary responsivity. However, it is known that the presence of noise in the photocurrent can degrade the performance of this algorithm significantly, depending on the choice of the responsivity spectrum to be synthesized. We generalize this algorithm to accommodate noise. The results are applied to quantum-dot mid-infrared detectors with bias-dependent spectral responses. Simulation and experiment are used to show the ability of the algorithm to reduce the adverse effect of noise on its spectral-tuning capability.

The activation energies of tellurium and beryllium in InxAl1−xSb grown by solid-source molecular beam epitaxy on semi-insulating GaAs substrates are reported. Temperature-dependent Hall measurements of carrier concentration show that while the activation energy of beryllium is relatively low for all compositions (EA 100meV) in the composition range between approximately 0.1

We report the demonstration of a two-color infrared focal plane array based on a voltage-tunable quantum dots-in-well (DWELL) design. The active region consists of multiple layers of InAs quantum dots in an In0.15Ga0.85As quantum well. Spectral response measurements yielded a peak at 5.5μm for lower biases and at 8–10μm for higher biases. Using calibrated blackbody measurements, the midwavelength and long wavelength specific detectivity (D*) were estimated to be 7.1×1010cmHz1∕2∕W(Vb=1.0V) and 2.6×1010cmHz1∕2∕W(Vb=2.6V) at 78 K, respectively. This material was processed into a 320×256 array and integrated with an Indigo 9705 readout chip and thermal imaging was achieved at 80 K.

The modeling of single-mode distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers based on an index-coupled sur- face grating on the InGaAs/InGaAsP multiple-quantum-well (MQW) structures is carried out. Such lasers require relatively simpler grating fabrication processes without multistep-epitaxial growth or regrowth, as required in conventional devices. A key concern for surface grating is designing structure that can provide sufficient feedback to achieve single-mode operation and high laser performance. Bragg wavelength operation of the surface grating (SG) DFB and SG DBR lasers can be satisfied by etching deep and fine gratings on the surface of both side portions and along the top of the ridge stripe, respectively. The numerical results obtained enable optimization of grating and laser geometries to provide the desired feedback effect. The simulated laser design shows that single-frequency operation at a wavelength of 1.55 mm with high a side-mode suppression ratio (SMSR) is attainable. © 2002 Society of Photo

Hybrid approaches that combine genetic algorithms (GA’s) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive phase elements (DPE’s) for laser beam shaping. These hybrid methods exploit the global nature of the GA’s as well as the local improvement capabilities of the gradient-based local search techniques and will perform a more improved search in comparison with each of the individual approaches. The incorporated local search technique that we used here is the Davidon–Fletcher–Powell method. A cost function that can directly control the performance of the final solutions is also used. By performing the DPE design with different desired diffraction efficiencies, we obtain a set of results that approximately reflect the trade-off between the design objectives, namely, signal-to-noise ratio (SNR) and diffraction efficiency. Reasonable solutions can be chosen on the basis of the knowledge of the problem. Simulation computations are detailed for two rotationally symmetric beam-shaping systems, in which an incident Gaussian profile laser beam is converted into a uniform beam and a zero-order Bessel beam. Numerical results demonstrate that the proposed algorithm is highly efficient and robust. DPE’s that have high diffraction efficiency and excellent SNR can be achieved by using the algorithm that we propose.

The theoretical diffraction efficiency upper limit of diffractive phase elements (DPE's) with finite apertures is investigated. A successful numerical method of evaluating the efficiency upper bound of DPE's is proposed. The method includes a hybrid optimization procedure that combines a genetic algorithm with the conjugate gradient method. This efficient global optimization technique can also be used to design DPE's. Simulation computations are detailed for rotationally symmetric beam shaping in which a Gaussian profile laser beam is converted into a uniform beam. Numerical results demonstrate that the estimated diffraction efficiency upper bound is consistent with the design results.

In this thesis, Ar plasma induced intermixing has been developed with various plasma systems, including Inductively Coupled Plasma (ICP) machine. A plasma induced QWI technique has been demonstrated to successfully tune a wide range of III-V compound semiconductor materials for photonic integration applications. DOCTOR OF PHILOSOPHY (EEE)