Multiple Quantum Well Materials Research Articles

Materials and device designs for thermophotovoltaic power conversion

AbstractTwo IV–VI semiconductor alloys, Pb0.81Sn0.19Se and Pb0.8Sr0.2Se, are proposed for use in designing multiple quantum well (MQW) materials and devices for thermophotovoltaic (TPV) power conversion. These materials can be epitaxially grown on silicon substrates, so they offer the potential for a low cost TPV device manufacturing technology. MQW materials examples are provided for fabricating triple junction TPV devices for power conversion with a 1400°C radiator. Optimal n‐type and p‐type layer thicknesses for each junction were determined using internal quantum efficiency expressions derived assuming all photogenerated charge collection is by diffusion. Depending on MQW material quality and assumed optical absorption levels, predicted power generation for current matched triple junction devices ranged from 1.7 to 4.6 W/cm2 and power conversion efficiencies ranged from 15.9% to 35.7%. These device performance parameters were used in a levelized cost of energy (LCOE) calculation to predict the cost of energy provided by TPV devices with a molten silicon thermal energy storage system. Results show that even for TPV devices fabricated from low quality materials, such energy storage systems will have a low enough LCOE to be competitive with other forms of energy storage and power generation.

PbSnSe/PbSrSe quantum well materials for thermophotovoltaic devices

Multiple quantum well (MQW) materials composed of Pb0.81Sn0.19Se wells and Pb0.80Sr0.20Se barriers with intersubband energy gaps of 343 meV and 450 meV were modeled for thermophotovoltaic (TPV) device performance. The effect of L-valley degeneracy removal in these (111)-oriented IV-VI semiconductor quantum wells was evaluated. Degeneracy splitting reduces the effective densities of states in both the valence and conduction bands. Thermally generated intrinsic charge carrier concentrations are smaller by a factor of three as compared to bulk materials with the same bandgap energies. A current-matched dual junction TPV cell made from these MQW materials in a generator with a 1215°C radiator is predicted to have a power density of 2.34 W/cm2, 49% better than the power density generated by a cell made from bulk materials with the same bandgap energies.

Generation of High Speed Polarization Modulated Data Using a Monolithically Integrated Device

We report on the generation of high speed polarization modulated data via direct electrical binary data injection to the phase shifter section of a monolithically integrated laser diode integrated with a polarization controller. The device is fabricated on standard InP/AlGaInAs multiple quantum-well material and consists of a semiconductor laser, a passive polarization mode convertor, and an active differential phase-shifter section. We demonstrate the generation of 300 Mb/s polarization shift keyed data.

Tight-binding calculation of optical gain in tensile strained [001]-Ge/SiGe quantumwells

It is known that under a tensile strain of about 2% of the latticeconstant, the energy of the bottom conduction state of bulk Ge at theΓ point falls below the minimum at the L point, leading to a direct gapmaterial. In this paper we investigate how the same condition is realizedin tensile strained Ge quantum wells. By means of a tight-bindingsp3d5s* model, we studytensile strained Ge/Si0.2Ge0.8 multiple quantum well (MQW) heterostructures grown on a relaxed SiGeSn alloy bufferalong the [001] direction. We focus on values of the strain fields at the crossover betweenthe indirect and direct gap regime of the MQWs, and calculate band edge alignments,electronic band structures, and density of states. We also provide a numericalevaluation of the MQW material gain spectra for TE and TM polarization underrealistic carrier injection levels, taking into account the leakages related to theoccupation of the electronic states at the L point. The analysis of the differentorbital contributions to the near-gap states of the complete structure allows usto give a clear interpretation of the numerical results for the strain-dependentTM/TE gain ratio. Our calculations demonstrate the effectiveness of the structures underconsideration for light amplification.

Effect of p–n Junction Location on Characteristics of InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with different p–n junction locations have been investigated by current–voltage (I–V), electroluminescence (EL), and capacitance–voltage (C–V) measurements. At 20 mA injection current, the operating voltage of the LEDs decreases by 0.15 V, the EL intensity increases by 14% and the EL wavelength shifts from 465 to 455 nm owing to the shortening of the distance between n-InGaN/GaN MQW and p-type materials. C–V measurements indicate that only the quantum well immediately adjacent to the p-type region still remains inside the depletion region when LEDs are forward biased. Analysis reveals that the characteristics of the LEDs can be improved by optimizing the p–n junction location through the improvement of the holes' tunneling injection and weakening of the piezoelectric field in InGaN quantum wells.

Optimization of Multiple Quantum Well Electroabsorption Modulators Based on Transmission Performance Simulation

We present the optimization of multiple-quantum-well (MQW) electroabsorption (EA) modulators based on system transmission performance simulation. The absorption spectra of InGaAsP MQW material under different electric fields are worked out using a non-variational numerical method. Transmission simulation is then carried out using the calculated extinction ratios and chirping parameters to evaluate the performance of the EA modulator under a predefined operation condition. Voltage-dependent chirping parameters are adopted to offer a more realistic description of the performance of the EA modulator in actual fiber communication systems. Simulation results show that the barrier bandgap wavelength and the well width have the most significant influence on device performances.

Multispectral long-wavelength quantum-well infrared photodetectors

Multispectral detector arrays for application in the 8–11 μm spectral band have been fabricated from GaAs/AlGaAs multiple quantum-well (MQW) materials. For efficient narrow-band coupling of the infrared radiation, three-dimensional diffractive resonant optical cavities are patterned into the MQW material. The peak wavelengths of the multispectral quantum-well infrared photodetectors are tuned by controlling the dimensions of the resonant structure. The full width at half maximum of the spectral response is as narrow as 0.7 μm. As many as 24 different detector designs were implemented on a single array. The spectrally tuned detectors exhibit peak responsivities as high as 2.6 A/W and quantum efficiency as high as 57%. Total conversion efficiency is as high as 35%. The background limited peak detectivity measured at 9.0 μm and 55 K with a 295 K background and f/2.5 is ⩾1×1011 cm Hz0.5/W.

Comparison of structural and optical properties in strained GaInAsP MQW structures grown by MOVPE and MOMBE

The growth parameter dependence of the transition from 2D to 3D growth of GaInAsP multiple quantum well (MQW) structures up to ε B=0.5% tensile-strained barriers was examined. Identical MQW structures with ε W=1% compressively strained wells were grown by metal organic vapor-phase epitaxy (MOVPE) and metal organic molecular beam epitaxy (MOMBE) and characterized by photoluminescence (PL), X-ray diffraction and transmission electron microscopy. Increasing the tensile barrier strain resulted in deteriorated optical and crystalline properties beyond a critical strain limit, which depends on growth temperature. The deterioration originates from lateral layer thickness and strain modulations. Their density, amplitude and thus their effect on the optical MQW properties are different for both growth methods. High-quality MOMBE-grown MQW structures up to ε W=2% compressive well strain and ε B=0.5–1% tensile barrier strain could be achieved by inserting thin intermediate layers at each internal interface. The composition of these intermediate layers has a significant effect on MQW material properties.

Resonant exciton contributions to quantum-well electroabsorption

We demonstrate that electroabsorption in shallow multiple quantum-well materials can be significantly affected by the presence of excitons in which the electron or hole occupies a three-dimensional resonant state. The calculated and experimental absorption spectra are seen to retain a two-dimensional character even for photon energies above the quantum well threshold. Additional steplike features that appear in the high energy portion of the absorption spectra, especially at high electric field, are further shown to be associated with the resonant exciton states transitions. Our theoretical results are in excellent agreement with experimental measurements on multiple quantum-well systems with small conduction band energy discontinuities.

All-optical switching and pulse routing in a distributed-feedback waveguide device

Results of all-optical switching and pulse-routing experiments with a distributed-feedback multiple-quantum-well (MQW) nonlinear waveguide, operating at 1.55microm , are reported. The MQW material has been engineered, through the controlled introduction of defects, to lower the carrier lifetime from 2.5 ns to 280 ps. The energy required for switching is of the order of 1 pJ or less, the switching time is ~600 ps , and the on-off contrast exceeds 17 dB.

A comprehensive analysis of multilayer channel waveguides

Using a simple approach based on the scalar finite element method, propagation characteristics of multilayer channel waveguides are calculated. The effective index, modal field, confinement factor, far-field intensity pattern, and radiation angle of the far-field pattern (full width at half maximum intensity) for multilayer channel waveguides formed with multiple quantum well (MQW) materials and with the MQW materials replaced by a single homogeneous material with the root mean square value of the refractive indexes are compared. Numerical results confirm that the root-mean-square-value approximation, which has been widely used for planar MQW (two-dimensional) waveguides, is useful also for MQW channel (three-dimensional) waveguides with a large number of layers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Refractive index approximations from linear perturbation theory for planar MQW waveguides

The mode powers and propagation constants for planar multilayer waveguides formed with multiple quantum well (MQW) materials and with the MQW layers replaced by a single uniform layer are compared. By considering linear perturbations of the solutions of the wave equation, the optimal choice of average for the dielectric constant of the substituted single layer is determined. For the case of TE modes, minimal error in the propagation constant is predicted if a weighted average of the dielectric constants of the MQW materials is used. For the case of TM modes, using a weighted average of the reciprocal of the dielectric constants is predicted to yield the minimum error. Numerical results confirm these predictions. >

Modelling the optimum performance of InGaAs/InP-based asymmetric Fabry-Perot modulator devices

The performance of long-wavelength asymmetric Fabry-Perot modulator (AFPM) devices has been modelled to establish whether InGaAs/InP multiple quantum well (MQW) material can yield the high contrast (in-state:off-state) of 100:1 observed at shorter wavelengths using GaAs/AlGaAs. The absorption and electroabsorption spectra for MQWS with 70 AA barriers were predicted using photocurrent data from single 45, 78 or 110 AA InGaAs wells. Using these spectra the thickness of intrinsic MQW material required to obtain the necessary absorbance changes was calculated. The widest wells lead to the best modulation for a given bias voltage. However, the modulation obtained using 78 AA wells is only slightly worse and it occurs at an ideal wavelength of 1.55 mu . Narrow 45 AA wells never yield very good performance. A high contrast can be obtained with a reasonable insertion loss and a low biasing voltage if 1.4 mu m of 78 AA wells or 1.7 mu m of 110 AA quantum wells are used. The spectral performance of these absorbers in an AFPM cavity has been modelled over a 100 nm wavelength range. A contrast of 164:1 (22 dB) for an insertion loss of only 3.95 dB is predicted at 14 V bias. The performance of AFPM devices at very low biasing voltages around 5 V is also detailed.

Low loss InGaAs/InP multiple quantum well waveguides

Double heterostructure planar waveguides with an InGaAs/InP multiple quantum well (MQW) core and InP cladding layers were grown by atmospheric pressure metalorganic chemical vapor deposition. Ridge waveguides had a low propagation loss of 0.8 dB/cm for 1.52 μm input light. The indices of refraction for the guided TE and TM modes have been measured and the bulk dispersion curves of the MQW material for the 1.46–1.55 μm wavelength region were derived.

Mode locking of semiconductor diode lasers using saturable excitonic nonlinearities

Multiple-quantum-well (MQW) structures of GaAs and GaAlAs have been used for passive mode locking of commercial GaAs semiconductor diode lasers. We present an extended discussion of this application of the sensitive room-temperature excitonic absorption saturation in MQW material. We review the criteria for passive mode locking and discuss two methods—carrier diffusion and proton bombardment—for reducing the aborption recovery time without destroying the excitonic nonlinearity. A simple probabilistic theory is derived for the effect of bombardment on the excitonic effects that is in order-of-magnitude agreement with experiment. We have performed experiments using MQW material to mode lock a GaAs laser. A continuous train of pulses as narrow as 1.6 psec has been obtained with a pulse-repetition rate of 2 GHz.