Strain-balanced Multiple Quantum Wells Research Articles

Tailoring strain and lattice relaxation characteristics in InGaAs/GaAsP multiple quantum wells structure with phosphorus doping engineering

Minimization of the internal strain in growing high quality strained multiple quantum wells (MQWs) has attracted enormous attention due to their high potential with optimized crystal quality for next-generation diode lasers technology applications. Among which the alternative multilayer structures with high performance of the strain quantum wells are prerequisite in the quantum cascade lasers and vertical cavity lasers. The periodic structure with an appropriate design strain-balanced MQWs are imperative for high performance devices. In present work, the strain compensated InGaAs/GaAsP multiple quantum well structures are developed. By engineering the composition of phosphorus in the barrier layer, i.e., 0.15 at.%, the best performance was achieved with high crystal quality via modulating the indium and phosphorus. Our systematic theoretical and experimental studies are performed upon the strain-balanced values investigations by complementary atomic force microscope and photoluminescence characterizations. The influence of variables on the measurement results and the internal correlated impact factors are discussed in detail. With tailored strain compensated structure, our developed InGaAs/GaAsP MQW opens new opportunities for innovative applications with optimized integration in next-generation diode lasers field.

Thin-film multiple-quantum-well solar cells fabricated by epitaxial lift-off process

Thin-film solar cells fabricated using the epitaxial lift-off (ELO) process enable considerable cost reduction as well as light-trapping. A polyimide film was used as a support substrate for thin film devices to address the poor temperature tolerance of poly(ethylene terephthalate) films. Samples were stuck to polyimide films by an Au–Au bonding process. It was found that the insertion of a wetting layer enhanced the adhesion between a polyimide film and an Au layer. The achieved bonding was strong enough to carry out the ELO process, and no degradation was observed in cell performance. Thin-film solar cells with strain-balanced multiple quantum wells were also fabricated using the ELO process to benefit from the light-trapping effect. Quantum efficiency enhancement was observed in the long-wavelength range of 850–980 nm compared to the non-ELO-processed devices. The photo-absorption enhancement was due to the Fabry–Perot resonance between the surface and the rear Au electrodes.

GaAsP/Si tandem solar cells: Realistic prediction of efficiency gain by applying strain-balanced multiple quantum wells

The combination of tunable direct bandgap III-V absorbers with active Si substrates promises high-efficiency tandem solar cells. To yield the ideal III-V bandgaps between 1.6 and 1.8eV which is considered to achieve current matching with the Si bottom-cell, however, either a big lattice-mismatch or dilute nitrides had to be considered so far. Here, we propose a new structure for a two-terminal III-V-on-Si solar cell which bases on strain-balanced III-V multi-quantum wells (MQWs) embedded in a metamorphic GaAsP top-cell matrix. Strain-balanced MQWs extend the absorption edge to a longer wavelength and enable a reduction of the As content in the GaAsP metamorphic top-cell matrix. And accumulation of carriers in MQWs favors radiative recombination, which is beneficial for high efficiency while deep quantum wells lower the charge carrier collection efficiency. Here, we predict solar energy conversion efficiencies over 42.6% with an entire MQW as thin as 500nm. The applied model takes into account the drawbacks of MQWs, such as limited light absorption and the bottleneck of charge carrier collection from the confinement.

Strain-Balanced InGaAsP/GaInP Multiple Quantum Well Solar Cells With a Tunable Bandgap (1.65–1.82 eV)

Currently available materials for III–V multijunction solar cells lattice matched to GaAs covering the spectral range from 1.65 to 1.82 eV are composed of either immiscible quaternary alloys or contain aluminum. We report the fabrication of a novel aluminum-free In $_x$ Ga $_{1-x}$ As $_{1-z}$ P $_z$ /Ga $_{1-y}$ In $_y$ P ( x > y ) strain-balanced multiple quantum-well (SBMQW) p-i-n solar cell structure lattice matched to GaAs, grown by metal–organic chemical vapor deposition. SBMQWs consist of alternating layers of In $_x$ Ga $_{1-x}$ As $_{1-z}$ P $_z$ wells and Ga $_{1-y}$ In $_y$ P barriers ( x > y ) under compressive and tensile strain, respectively. When compared with standard GaInP devices, SBMQW structures exhibit longer photoluminescence wavelength (680–780 nm) emission and enhanced light absorption with improved short-circuit current density. In this study, the SBMQW emission and absorption wavelength is controlled by adjusting the layer thickness of InGaAsP wells, while the arsenic and indium compositions are fixed. We show that carriers generated in QWs are extracted via thermionic emission. The proposed SBMQWs allow more flexibility in the design of current multijunction solar cells and future cells with more than four junctions. InGaAsP/GaInP SBMQWs may also be used in applications other than solar cells, such as light-emitting diodes (LEDs) and lasers, with the advantages of tuning the emission and absorption processes.

InGaP-based quantum well solar cells: Growth, structural design, and photovoltaic properties

Raising the efficiency ceiling of multi-junction solar cells (MJSCs) through the use of more optimal band gap configurations of next-generation MJSC is crucial for concentrator and space systems. Towards this goal, we propose two strain balanced multiple quantum well (SBMQW) structures to tune the bandgap of InGaP-based solar cells. These structures are based on InxGa1−xAs1−zPz/InyGa1−yP (x > y) and InxGa1−xP/InyGa1−yP (x > y) well/barrier combinations, lattice matched to GaAs in a p-i-n solar cell device. The bandgap of InxGa1−xAs1−zPz/InyGa1−yP can be tuned from 1.82 to 1.65 eV by adjusting the well composition and thickness, which promotes its use as an efficient subcell for next generation five and six junction photovoltaic devices. The thicknesses of wells and barriers are adjusted using a zero net stress balance model to prevent the formation of defects. Thin layers of InGaAsP wells have been grown thermodynamically stable with compositions within the miscibility gap for the bulk alloy. The growth conditions of the two SBMQWs and the individual layers are reported. The structures are characterized and analyzed by optical microscopy, X-ray diffraction, photoluminescence, current-voltage characteristics, and spectral response (external quantum efficiency). The effect of the well number on the excitonic absorption of InGaAsP/InGaP SBMQWs is discussed and analyzed.

InP-Based Active and Passive Components for Communication Systems at 2 μm

Progress on advanced active and passive photonic components that are required for high-speed optical communications over hollow-core photonic bandgap fiber at wavelengths around 2 μm is described in this paper. Single-frequency lasers capable of operating at 10 Gb/s and covering a wide spectral range are realized. A comparison is made between waveguide and surface normal photodiodes with the latter showing good sensitivity up to 15 Gb/s. Passive waveguides, 90° optical hybrids, and arrayed waveguide grating with 100-GHz channel spacing are demonstrated on a large spot-size waveguide platform. Finally, a strong electro-optic effect using the quantum confined Stark effect in strain-balanced multiple quantum wells is demonstrated and used in a Mach-Zehnder modulator capable of operating at 10 Gb/s.

Strain-balanced InGaN/GaN multiple quantum wells

InGaN/GaN multiple quantum well (MQW) structures suffer from a high amount of compressive strain in the InGaN wells and the accompanied piezoelectric field resulting in both a blue shift in emission and a reduction of emission intensity. We report the growth of InxGa1−xN/GaN “strain-balanced” multiple quantum wells (SBMQWs) grown on thick InyGa1−yN templates for x > y by metal organic chemical vapor deposition. SBMQWs consist of alternating layers of InxGa1−xN wells and GaN barriers under compressive and tensile stress, respectively, which have been lattice matched to a thick InyGa1−yN template. Growth of the InyGa1−yN template is also detailed in order to achieve thick, relaxed InyGa1−yN grown on GaN without the presence of V-grooves. When compared to conventional InxGa1−xN/GaN MQWs grown on GaN, the SBMQW structures exhibit longer wavelength emission and higher emission intensity for the same InN mole fraction due to a reduction in the well strain and piezoelectric field. By matching the average lattice constant of the MQW active region to the lattice constant of the InyGa1−yN template, essentially an infinite number of periods can be grown using the SBMQW growth method without relaxation-related effects. SBMQWs can be utilized to achieve longer wavelength emission in light emitting diodes without the use of excess indium and can be advantageous in addressing the “green gap.”

Modelling of GaAsP/InGaAs/GaAs strain-balanced multiple-quantum well solar cells

A model of strain balanced quantum well solar cells is presented, together with a high efficiency design for a GaAsP/InGaAs/GaAs device. The effect of tensile and compressive strain on bandstructure is considered in order to compute the electron and hole dispersion relation E(k) in conduction and valence bands. The optical transitions in quantum well and barrier are evaluated and the quantum efficiency, dark current and the photocurrent calculated. Experimental data quantum efficiency and dark current are compared with theoretical calculations in the presence of strain, showing a good agreement. The resulting model is initially applied to a GaAsP/InGaAs/GaAs solar cell and the structure optimised to yield the greatest output power. The model is also applied to the problem of determining the highest efficiencies achievable for quantum well solar cells as a function of strain and confirms the high efficiency potential of strained quantum well solar cells.

Impact of Strain Accumulation on InGaAs/GaAsP Multiple-Quantum-Well Solar Cells: Direct Correlation between In situ Strain Measurement and Cell Performances

The effects of accumulating strain inside InGaAs/GaAsP multiple-quantum-well (MQW) solar cells were investigated and their correlation with in situ wafer curvature measurement was examined. The p–i–n GaAs solar cells, containing 20-period InGaAs/GaAsP MQWs in an i-GaAs layer, were fabricated by metalorganic vapor phase epitaxy. The strain inside MQWs was varied by changing In content in an InGaAs well, while maintaining other parameters. As evidenced by curvature transience, the excessive strain led to lattice relaxation, resulting in defects, dislocations, and poor crystal quality. Consequently, short circuit current density and open circuit voltage deteriorated, and solar cell performance degraded. The highest conversion efficiency was obtained in a strain-balanced MQW solar cell. InGaAs/GaAsP MQWs have a great potential for extending the absorption edge of GaAs cells and for enhancing the efficiency of III/V multijunction solar cells by current matching. Hence, the growth of InGaAs/GaAsP MQWs for photovoltaic application requires a strain monitoring system and careful control such that the accumulating strain is minimized.

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3s at 610°C.

Characterization of bulk and surface currents in strain-balanced InGaAs quantum-well mesa diodes

We compare the electrical and optical characteristics of mesa diodes based on In0.62Ga0.38As∕In0.45Ga0.55As strain-balanced multiple-quantum wells (SB-MQW) with lattice-matched (LM) In0.53Ga0.47As diodes. The dark current density of the SB-MQW devices is at least an order of magnitude lower than the LM devices for voltages >0.4V. Sidewall recombination current is only measured on SB-MQW diodes when exposed to a damaging plasma. While radiative recombination current dominates in the SB-MQW diodes, it is less than the diffusive current in the LM diodes for the same applied voltage.

Extended defects in InGaAs/InGaAs strain-balanced multiple quantum wells forphotovoltaic applications

Different strain-balanced InGaAs/InGaAs multiple quantum wells(MQWs) were grown on (001) InP changing the In composition in thewells/barriers in order to extend the absorption edge beyond 2 μm forthermophotovoltaic applications. The strain increase in the structuresresults in the formation of isolated highly defected regions taking theirorigin from lateral layer thickness modulations. Experimental results areconsistent with the existence of a critical elastic energy density for thedevelopment of MQW waviness. An empirical model for predicting themaximum number of layers that can be grown without modulations as afunction of the strain energy stored in the MQW period is presented.

Defect free InGaAs-based strain balanced MQW grown on virtual substrate by metallorganic chemical vapor deposition

AbstractIn this work we describe a novel system for photovoltaic applications which combines InGaAs based strain-balanced multiple quantum wells (MQWs) with a “virtual substrate”, designed to extend the absorption edge of the photovoltaic devices to about 1 eV. The virtual substrate is designed by properly choosing a sequence of InGaAs layers having different In content, in order to obtain the desired lattice parameter at the topmost layer and to confine at the deepest interfaces the misfit dislocations, well away from the QW active region.A series of InGaAs p-i-n junctions, containing a strain balanced MQW in the intrinsic region, were deposited by metallorganic chemical vapor deposition on different virtual substrates. In all the samples the virtual substrates were proved to be successful to grow zero net strain MQW and to confine defects at the buffer/substrate interface. Transmission electron microscopy observation shows that no defects propagate from the strain accommodating layers to the active region. The total density of threading dislocations reaching the surface was found to be less than 1*E5/cm2.The confined misfit dislocation network, however, results in marked cross-hatched morphology that was found to affect the lateral strain distribution in the whole structure. By optimizing the growth condition of the structures, the influence of the surface roughness induced by CH pattern is partially suppressed.

Fabrication of strain-balanced Si/Si1−xGex multiple quantum wells on Si1−yGey virtual substrates and their optical properties

Strain-balanced Si/SiGe multiple quantum wells (MQWs), which are designed to overcome the limitation of the number of wells coming from the strain accumulation, were fabricated, and their optical properties were investigated. X-ray diffraction spectra and cross-sectional transmission-electron-microscope images showed a high-crystalline quality of samples and excellent uniformity of the well width. Well-resolved no-phonon and TO-phonon-assisted transitions from strain-balanced MQWs were observed by low-temperature photoluminescence spectroscopy, and both their temperature and excitation power dependence showed blueshifts due to the delocalization of excitons, the band bending, and/or the band-filling effect.

Quantum confined Stark effect near 1.5 μm wavelength in InAs0.53P0.47/GayIn1−yP strain-balanced quantum wells

We report that high-quality InAsP/GaInP strain-balanced multiple quantum wells (MQWs) at 1.5 μm wavelength can be grown by gas-source molecular beam epitaxy. In order to obtain a large quantum-confined Stark effect, relatively thick (∼100 Å) InAs0.53P0.47 layers (lattice mismatch 1.8%) have been grown without strain relaxation. Our theoretical calculation shows that, while fulfilling the strain balance condition, the GaInP barriers should have a large thickness and small strain to reduce strain relaxation in these layers. Large Stark shift (32 meV at 180 kV/cm field) has been observed in a p-i-n diode structure with 20-period InAs0.53P0.47/Ga0.16In0.84P MQWs as the intrinsic region. The large change of absorption coefficient under electric field (∼3800 cm−1 at 80 kV/cm) indicates that this material is very promising for optical modulators near 1.5 μm.