InAs quantum dots in a GaAs matrix are studied. Those quantum dots are used in applications to enhance the overall
efficiency of multi-junction solar cells beyond 40%. Photoluminescence measurements at 77 K using a 532 nm laser
have been performed on an epitaxially grown structure of self-assembled InAs quantum dots in a GaAs matrix upon a Ge
substrate, where three energy levels are determined at En=0=1.01 eV, En=1=1.07 eV and En=2=1.13 eV. Theoretical
calculations of the energy levels determine the quantum dots to be 7 nm high and have a 37 nm base diameter, which is
close to atomic force microscopy measurements performed on the samples. Intensity dependant photoluminescence
measurements reveal the saturation of the first excited energy level at 5×106 W/m2. A general model for the saturation of
the first quantum dot excited energy level is then developed. This saturation model is applied to the AM1.5D solar
spectrum at 297 K to determine the concentration of solar energy needed to saturate the first excited energy level within
a multi-junction solar cell. Saturation was determined to be at ~1.56×104 suns (where 1 sun = 1000 W/m2). Since current
solar concentrations are between 500-1000 suns concentration, the saturation of such quantum dots will not occur.
Triple-junction AlGaInP/InGaAs/Ge solar cells with embedded InAs quantum dots are presented, where typical
samples obtain efficiencies of > 40% under AM1.5D illumination, over a range of concentrations of 2- to 800-suns
(2 kW/m2 to 800 kW/m2). Quantum efficiency measurements show that the embedded quantum dots improve the
absorption of the middle subcell in the wavelength range of 900-940 nm, which in turn increases the overall
operating current of the solar cell. These results are obtained with 1 cm2 solar cells, and they demonstrate the
solar cells' low series resistance, which and makes them ideal for the current generation in commercial
concentrator systems. The thermal management and reliability of the solar cell and carrier is demonstrated by
testing the experimental samples under flash (up to 1000-suns) solar simulator and continuous (up to 800-suns)
solar simulator. Under continuous solar illumination, the solar cell temperature varies between ~Δ3°C at 260-suns
linearly to ~Δ33°C at 784-suns when the solar cell is mounted with thermal paste, and ~Δ27°C at 264-suns linearly
to ~Δ91°C at 785-suns when no thermal paste is used. The solar cells experience the expected shift in open circuit
voltage and efficiency due to temperature, but otherwise operate normally for extended periods of time.
Site-selective epitaxy and standard electron beam lithography techniques are employed to spatially couple small InAs/InP quantum dot ensembles to 2D photonic crystal membrane cavities. The small InAs quantum dot ensembles, consisting of just a few dots, are localized to areas 100x100nm2 at predetermined positions dictated by a nanotemplate consisting of InP pyramids. The dots are embedded in a 2D membrane using a planarization growth step and single missing-hole defect cavities are fabricated in the membrane with the defect sites centered on the dot ensembles. This spatially couples the ensembles to the χ-dipole mode of the cavities. Emission from the cavities shows the expected mode structure, with quality factors of 2000.
This paper reviews our research on photon upconversion devices for wavelengths from 1.5 μm to 0.87 μm. The 1.5 μm is
chosen for its importance for eye-safe active imaging; whereas 0.87 μm corresponds to the bandgap of GaAs which is the
active region of our high efficiency light emitting diode (LED). The basic idea is to integrate a 1.5 μm detector with a 0.87
µm LED, connected in series. The detected photocurrent drives the LED, thereby achieving the upconversion. Various
approaches of integration methods and device designs have been tested. The upconversion approach provides an alternative
to the standard hybrid integration with readout circuits and may be advantageous for some applications.
Semiconductor nanostructures, such as quantum wells and quantum dots (QD), are well known, and some have been
incorporated in applications. Here will focus on novel structures made of QDs and related devices for terahertz (THz)
generation. Their potential advantages, such as low threshold current density, high characteristic temperature, increased
differential gain, etc., make QDs promising candidates for light emitting applications in the THz region. Our idea of
using resonant tunneling through QDs is presented, and initial results on devices consisting of self assembled InAs QDs
in an undoped GaAs matrix, with a design incorporating GaInNAs/GaAs short period superlattice, are discussed.
Moreover, shallow impurities are also being explored for possible THz emission: the idea is based on the tunneling
through bound states of individual donor or acceptor impurities in the quantum well. Initial results on devices having an
AlGaAs/GaAs double barrier resonant tunneling structure are discussed.
We report on the progress of devices and applications of quantum-well photodetectors (QWIP) for the terahertz (~ 1-10 THz) spectrum region. We discuss device design and show that the device dark current can by effectively reduced by employing wider quantum barriers. We demonstrate several GaAs/AlGaAs QWIPs for different peak wavelength with background limited infrared performance (BLIP). We report experimental results on intersubband absorption spectra, measured using multi-pass waveguide geometry. We show that the experimentally measured intersubband energy levels agree excellently with the theoretical simulations, provided that many-body effects are taken into consideration, including exchange-correlation and depopulation effects. We report the results of QWIP photo-current spectra and detector responsivity. We discuss the high frequency capability of THz-QWIP and present experimental results of device time response measured using microwave rectification technique. We discuss its application in free space terahertz communication in combination with a terahertz quantum cascade laser (QCL). We discuss the terahertz to near infrared (THz-to-NIR) optical upconversion using a monolithic integration of THz GaAs/AlGaAs QWIP and NIR GaAs/AlGaAs LED, and its potential applications in terahertz imaging.
This paper presents our research and development effort in realizing and perfecting photon upconversion devices for
wavelengths from 1.5 μm region to 0.87 μm. The basic idea is to integrate a 1.5-μm detector with a 0.87-μm light
emitting diode (LED), connected in series. The detected photocurrent drives the LED, thereby achieving the
upconversion. Various approaches of integration methods and device designs have been tested.
Terahertz (1 - 10 THz) quantum-well photodetectors and quantum- cascade lasers have been investigated. The design and projected detector performance are presented together with experimental results on several test devices, all working at photon energies below the optical phonons. Background limited infrared performance (BLIP) operations were observed for all samples (three in total) designed for different wavelengths. For lasers, a set of THz quantumcascade
lasers with identical device parameters except for the doping concentration has been studied. The δ-doping density for each period was varied from 3.2 × 1010 to 4.8 × 1010 cm-2. We observed that the lasing threshold current increased monotonically with doping. Moreover, the measured results on devices with different cavity lengths provided evidence that the free carrier absorption caused waveguide loss also increased monotonically. Interestingly however, the observed maximum lasing temperature displayed an optimum at a doping density of 3.6 × 1010 cm-2.
Many applications are expected in the terahertz spectral region and terahertz technology is viewed as one of the most important ones in the coming decade. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1 - 10 THz) or the very-far-infrared region. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance (BLIP). The device dark current characteristics were optimized by employing thick barriers to reduce inter-well tunneling. BLIP operations were observed for all samples (three in total) designed for different wavelengths. BLIP temperatures of 17, 13, and 12 K were achieved for peak detection frequencies at 9.7, 5.4, and 3.2 THz, respectively. Furthermore, we discuss areas of improvement to make these detectors a viable technology.
An InGaAs photodetector array interconnected with a silicon readout IC is the industry standard for 1.2-1.6 μm imaging applications. However, the indium-bump technique it employs for interconnection makes it expensive. An alternative approach is to combine a CCD with a device that upconverts 1.2-1.6 μm radiation to a wavelength
below 1 μm. Here we report the realization of a 1.5 μm to 0.87 μm optical upconversion device using wafer fusion technology. The device consists of an InGaAs/InP PIN photodetector and an AlGaAs/GaAs light emitting diode (LED). Incoming 1.5 μm light is absorbed by the InGaAs photodetector. The resulting photocurrent drives the GaAs LED, which emits at 0.87 μm. The PIN and LED structures are epitaxially grown on an InP and a GaAs substrate, respectively. The two wafers are wafer fused together, the GaAs substrate is removed, and the sample is processed using conventional microfabrication technology. In this paper, we first present the design and fabrication process of the device. We then discuss the approaches to increase device efficiency. We show, both experimentally and theoretically, that the active layer doping affects the LED internal quantum efficiency, especially under low current injection. An optimum doping value is obtained. The LED extraction efficiency is increased using several approaches, including micro-lens and surface scattering. Overall device efficiency is further improved by introducing a gain mechanism into the photodetector. Our results show the potentials of this integrated photodetector-LED device for 1.2-1.6 μm imaging applications.
Imaging devices working in the near infrared (NIR), especially in the so-called eye-safe range, i.e., around 1.5 mm, have become increasingly important in many military and commercial applications; these include night vision, covert surveillance, range finding and semiconductor wafer inspection. We proposed a new approach in which a wafer-fused optical up-converter, combined with a commercially available charged coupled device (CCD), functions as an infrared camera. The optical up-converter converts incoming infrared light into shorter wavelength radiation that can be efficiently detected by the silicon CCD (cutoff wavelength about 1 mm). An optical up-converter with high efficiency at room-temperature is critical for low cost and large-area infrared imaging applications. A prototype 1.5 mm optical up-converter based on wafer fusion technology has been successfully fabricated. The device consists of an InGaAs/InP pin photodetector and a GaAs/AlGaAs light emitting diode. Experimental results show that the end-to-end up-conversion efficiency is 0.0177 W/W at room-temperature, corresponding to an internal quantum up-conversion efficiency of 76%. In this paper, the design, fabrications and characterization of the optical up-conversion devices is presented. Issues related to device optimization, such as improving internal and external up-conversion efficiency, are addressed. Preliminary results demonstrate the room-temperature up-conversion imaging operation of a pixelated wafer-fused device.
Directly modulated lasers (DMLs) have two high performance applications: 1310 nm 10 Gb/s uncooled and 1550 2.5 Gs/s extended reach. Two key elements are gain coupled gratings and buried heterostructures. Gain coupled gratings simultaneously increase the DML's intrinsic relaxation oscillation frequency and damping, while the buried heterostructure reduces thermal chirp and parasitic capacitance. Large relaxation oscillation frequencies and reduced parasitic capacitance allow 85°C operation; large damping and reduced thermal chirp enable extended reach.
We analyze the high-temperature continuous-wave performance of 1.3 micron AlGaInAs/InP laser diodes grown by digital alloy molecular beam epitaxy. Commercial laser software is utilized that self-consistently combines quantum well bandstructure and gain calculations with two-dimensional simulations of carrier transport, wave guiding, and heat flow. Excellent agreement between simulation and measurements is obtained by careful adjustment of material parameters in the model. Joule heating is shown to be the main heat source; quantum well recombination heat is almost compensated for by Thomson cooling. Auger recombination is the main carrier loss mechanism at lower injection current. Vertical electron escape into the p-doped InP cladding dominates at higher current and it causes the thermal power roll-off. Self-heating and optical gain reduction are the triggering mechanisms behind the leakage escalation.
The material physics of digitally grown InAlGaAs quaternary alloy systems are investigated using Molecular Beam Epitaxy (MBE) grown layers. With MBE, arbitrary epitaxial alloy compositions can be achieved, without changing the group III elemental constituents flux rates, by simple sequential shuttering of the relevant fluxes. Monolayer fluctuations create inhomogeneities that lead to a broadening of the photoluminescence (PL) spectra. Multiple PL peaks are also seen in select alloy compositions.
Experimental measurements of threshold current density as a function of temperature have been analyzed in terms of the characteristic temperature, T0, and temperature gradient (Delta) TJth equals (delta) Jth/(delta) T, for a number of semiconductor laser device structures. These include AlInGaAs/InP, InGaAsP/InP, and AlGaAs/GaAs. A theoretical model is used to investigate the possible loss mechanisms in laser diodes that cause the superlinear increase of threshold current with temperature. The characteristic temperature T0 is found to vary with temperature and device length, thus making it somewhat misleading when quoted without qualification. A different approach based on plotting ln((Delta) TJth) vs. ln(Jth) shows a linear relationship that is dependent on device structure only, allowing the use of a new figure of merit for the temperature performance of semiconductor lasers.
The development of photonic devices for the next generation of optical networks is dependent on advances in ultrafast materials and in the success of waveguide devices comprised of these materials. This includes new methods of producing integrated-optical devices by innovative growth techniques or novel hybridization schemes. We describe aspects of the ultra-fast optical communications program at Georgia Tech that involve the development of hybridized and integrated- optical devices and devices for use in ultrafast optical data links. Two major components are under development: (1) a tapered rib electro-absorption modulator that includes an integrated spot-size converter for hybridization with a passive silica-waveguide tapped delay line. This unique hybridized semiconductor/glass waveguide provides the basic building block of the transmitter multiplexer. (2) a quasi- phase matched multilayer AlGaAs waveguide designed for surface-emitted second-harmonic generation. This device provides an all-optical serial-to-parallel converter and thereby demultiplexes ultrafast optical data streams. We describe our recent advances in materials growth and waveguide design and the impact on the performance of these devices.
Time-resolved photoluminescence (PL), steady-state PL, and electro-luminescence techniques have been used to characterize the carrier relaxation processes and carrier escape mechanisms in a self-assembled AlInAs quantum dots (SAQD) p-i-n laser structure under reversed bias. The measurements were performed between 5 K and room temperature on a ring mesa sample as a function of bias. At 100 K, the PL decay time originating from the SAQD decreases with increasing reversed bias from approximately 450 ps under flat band condition to approximately 150 ps for biases of -4 V. The data can be explained by a simple model based on electron recombination in the quantum dots or escape our of the dots. The escape can occur by one of three possible routes: direct tunneling out of the distribution of electron excited state level, thermally assisted tunneling of ground state electrons through the upper electron excited states or thermionic emission to the wetting layer.
This paper describes the development and testing of a 4 channel, 10 Gbps/channel WDM module. This model incorporates four DFB lasers whose wavelengths are spaced 1.6 nm apart within the 1550 nm fiber window. A driver circuit have been developed based on a commercially-available monolithic amplifier which provides high-speed operation at low cost. The performance of the module was measured at 10 Gbps and 2.5 Gbps. Time-domain performance and crosstalk results are presented.
The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of a GaAs/AlGaAs doublebarrier
resonant tunnelling diode have been studied with sub-meV resolution as a function of the applied
bias voltage. For voltages which bias the device in the resonant tunnelling regime, a monotonic blue shift
of the PLE peak is observed, concomitant with a monotonic red shift of the corresponding PL peak. Over
the same range of voltages, the linewidth (FWHM) increases from 4.8 to 6.3 meV in the case of the PL
and from 3.6 to 8.7 meV in the case of the PLE. These results are interpreted as representing the
influence of the resonantly accumulated electron population in the well region on the heavy hole exciton
resonance.
Photoluminescence (PL), PL excitation (PLE), and time-resolved PL are employed to study the tunneling
of photoexcited holes through a GaAs/A1GaAs double-barrier resonant tunneling structure. Lifetime
measurements of the n= 1 heavy-hole (hh) exciton transition from the well were obtained as a function of
the applied voltage. For voltages biasing the structure in the non-resonant tunneling regime, beyond the
region of negative differential resistance (NDR), the exciton decays with two time constants. The fast
component, which was observed at all voltages, is attributed to the decay of the exciton population
originating from holes photoexcited directly in the well. The slower time constant is associated with
excitons that are created from hOles which are photoexcited in the GaAs contact region, and which
subsequently tunnel into the well. This picture for hole tunneling is further supported by the observation
of the n= 1 hh exciton emission using exciting photon energies lower than the quantum well bandgap but
larger than the GaAs bandgap, when the structure is biased beyond the region of NDR.
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