GaAsP Barriers Research Articles

Comparison of quantum well structures for room temperature continuous wave 980 nm lasers grown on (001) Si by MOCVD

We report room temperature (RT) continuous-wave (CW) lasing of quantum well (QW) lasers grown on (001) Si substrates emitting at 980 nm. Two different QW structures, including conventional compressively strained InGaAs/GaAs QWs and strain-compensated InGaAs/GaAs/GaAsP QWs, were investigated. Photoluminescence properties and device performance of both structures on native GaAs and (001) Si substrates are discussed. By adding GaAsP barriers to the InGaAs/GaAs QWs, the lowest threshold current density of ridge waveguide edge-emitting QW lasers obtained on Si is 550 A/cm2, measured on a 10 μm × 2 mm device at RT. The working temperature of the InGaAs/GaAs/GaAsP QW lasers grown on Si can be over 95 °C in the CW mode. This work suggests a feasible approach to improve the 980 nm laser performance on Si for monolithic optoelectronic integration.

Thermal energy effect on optoelectronic characteristics of InGaAsN/GaAs laser diode under the variation of quantum well number

A laser device based on InGaAsN quantum well active regions emitting at 1.26 mm is reported. The performances of the laser under the effect of thermal energy are investigated in terms of threshold current, Ith, gain parameter, gt, photon energy, hn, and cavity length, Lc. Four structures with one, two, and three quantum wells along with a structure that have three quantum wells with tensile strained barriers are proposed to study the relation between the peak gain, gpeak, and photon energy. The founds show that structures with one and two quantum wells operating at room temperature and under pulse wave condition, exhibit a linear dependence of gpeak on both Lc and photon energy. It is shown that the threshold current density, Jth, increases at any temperature with the cavity length Lc ranging from 250 nm to 1000 nm. Also, the investigation of the proposed structures shows that gt decreases with increasing temperature, while the ratio of the cur-rent density parameter to internal efficiency, Jt/hi, per well increases with the quantum well number. A comparison was carried out for two particular structures with three quantum wells and GaAsP barriers, the results show a decrease in the threshold current per well.

Design of gain region of high-power vertical external cavity surface emitting semiconductor laser and its fabrication

<sec> The vertical external cavity surface emitting laser (VECSEL) is one of the hottest research fields of semiconductor lasers, due to its high power and good beam quality. However, there are few reports about how to systematically design the active region of VECSEL. In this paper, the gain design of quantum wells, which are the most important region within the VECSEL, is carried out. </sec><sec> To achieve low power consumption under high temperature condition, epitaxial structure of the VECSEL is optimized by using the commercial software PICS3D. Firstly, the relationship between the structure of quantum well and the gain is simulated by the <i>k</i>·<i>p</i> method. Then, the gain spectra of quantum wells at different carrier densities and temperatures are compared with each other, and the optimal composition and thickness of quantum well are thus determined. The temperature drift coefficient is 0.36 nm/K, obtained by simulating the drift of the gain peak wavelength at the working temperature. Finally, the gain spectra of quantum wells with five different barriers are compared with each other. The slight blue shift of the gain peak in the quantum well with five different barriers accommodates the different emission thermal drifts of the quantum well at high temperature operation. With the GaAsP barriers on both sides of quantum well the gain characteristics of quantum wells can be improved efficiently. </sec><sec> The designed structure is deposited by the MOCVD system. According to the reflection spectrum of the gain chip, measured by ellipsometer, the stop-band over 100 nm is centered at the about 970 nm wavelength, confirming accurate growth of the VECSEL. The 808 nm pump laser is focused on the surface of VECSEL chip at an incident angle from 30° to 50°. The VECSEL light-light characteristics are tested under the output coupling mirror with different reflectivity. The output power of VECSEL with a 97.7% reflectance output coupling mirror reaches 9.82 W at the pumping power of 35 W, without saturating the power curve. By using the external mirrors with different reflectivity, there appears the wavelength shift with the pumping power changing from 0.216 nm/W to 0.16 nm/W. Thus, the internal heating effects are different for VECSEL with different mirrors. The divergence angles at two orthogonal directions are 9.2° and 9.0°, respectively. And the circle profile of optical field shows good symmetry. </sec>

Interplay of GaAsP barrier and strain compensation in InGaAs quantum well at near-critical thickness

Abstract The effect of GaAs1−yPy tensile-strained barriers on suppressing the partial strain relaxation of InGaAs/GaAs multiple quantum wells (MQWs) is investigated when the thickness of a heavily strained InxGa1−xAs QW is near-critical thickness. The strain relaxations of In0.4Ga0.6As MQWs with and without strain-compensating GaAs1−yPy barriers are characterized using X-ray diffraction reciprocal space mapping (RSM) and micro-photoluminescence (µ-PL) mapping. A significant amount of strain relaxation (~1.53%) is measured when the thickness of each In0.4Ga0.6As QW within a 4-period MQW becomes 9.5 nm in the absence of strain-compensating layers. By adding two ~5 nm GaAs0.67P0.33 tensile-strained barriers sandwiching each QW, the strain relaxation in the In0.4Ga0.6As/GaAs0.67P0.33/GaAs MQWs is reduced to ~0.3% together with decreased surface roughness. Our study shows that tensile barriers with proper elastic energy densities are essential to achieve efficient strain compensation in a heavily-strained InGaAs MQW structure, which provides an important insight into the understanding of how to better achieve the benefits of strain compensation in III-V based QWs and superlattices.

In brief

PAGE 1534 Work from Canada presents the fabrication of a simultaneous multi-channel microwave chirped waveform generator. The waveguide design consists of an arrayed grating interferometer and linearly chirped fibre Bragg grating. Device tunability, flexibility and reconfigurability within the central frequency microwave bandwidth are demonstrated by the generation of two independent chirped waveforms. PAGE 1537 A distributed Bragg reflector-free semiconductor disc laser capable of 10 W continuous wave output at 1007 nm, has been created by researchers in Finland, Germany and the UK. The laser is formed of InGaAs quantum wells and GaAsP barrier layers that are bonded through non-covalent interactions to a SiC intra-cavity. The laser output wavelength beam can be tuned from 995 nm to 1020 nm with low optical loss. Simultaneous multi-channel microwave waveguide with tunable central frequency chirped waveform generation. PAGE 1506 Researchers in China report a wireless ingestible capsule topology for non-invasive gastrointestinal tract disease diagnosis. The proposed MIMO antenna capsule consists of two loop elements placed in an orthogonal orientation to achieve different polarisation directions. Simulated and measured transmission frequencies are shown to be within the industrial, scientific and medical bandwidth. A reflector-free semiconductor laser that can be tuned across a broad range of optical wavelengths. PAGE 1502 A miniaturised hexagonal ring frequency selective surface is proposed in work from China. The reported surface is 2.5-dimensional and achieves ultra-wide bandwidth transmission with good polarisation-independent performance. Simulated results are confirmed by prototype fabrication and the surface is shown to be suitable for both S- and C- band communication systems. A MIMO antenna with two loop elements for medical diagnosis applications. PAGE 1518 Researchers in China report the mechanisms of secondary discharge in electronic devices. The voltage, switch gap material, geometry and humidity are all parameters that are found to influence the secondary discharge current, which occurs when applied voltages are higher than the threshold voltage. The secondary discharge current is also found to significantly vary, leading to unpredictable electronic equipment failure events. S- and C- band communication achieved by a miniaturised hexagonal ring frequency selective surface. Revealing the mechanisms underlying secondary discharge in mobile electronic devices.

Effect of Barrier Thickness on Carrier Transport Inside Multiple Quantum Well Solar Cells Under High-Concentration Illumination

Carrier transport inside InGaAs/GaAs/GaAsP multiple quantum well (MQW) solar cells was discussed under high-concentrated sunlight illumination up to 338 suns. Current–voltage (I–V) characteristic curves for a GaAs reference cell and MQW cells with GaAsP barrier thickness of 2, 4, and 6 nm were investigated under dark and high-concentration illumination. Carrier collection efficiency (CCE) was estimated by net photocurrent, which is the difference between illuminated current and dark current density at each bias voltage normalized by the value at the saturated point. For the 2-nm barrier, CCE exhibited almost no degradation compared with the GaAs reference cell. On the other hand, CCE for the 6-nm barrier degraded with forward biases as the sunlight concentration ratio increased. The degradation in CCE under a high-concentration ratio is shown to be the result of carrier accumulation in quantum wells. Thin barriers seemed to eliminate such accumulation with the help of the carrier tunneling effect through the barriers.

Thickness-modulated InGaAs/GaAsP superlattice solar cells on vicinal substrates

InGaAs/GaAsP superlattice (SL) is a promising narrow-gap material for III–V multi-junction solar cells on Ge. In metal-organic vapor phase epitaxy (MOVPE) of SL on vicinal substrates, the component layers tend to be undulated due to step bunching occurring at high temperature. In this paper, the effects of growth temperature and thickness modulation of the SL-region on the photovoltaic performance were investigated. Lowering the growth temperature successfully enabled epitaxy of an extremely uniform SL, from which a clear step-like absorption spectrum including sharp exciton peaks was obtained due to layer-by-layer deposition of the individual layers. Larger layer undulation at higher temperature led to poorer in-plane coverage of the InGaAs region, resulting in the reduction of both light absorption and short circuit current. The open circuit voltage, on the other hand, was higher for the cells grown at higher temperature owing to suppressed dark current as a result of reduced crystal defects. Moreover, the lateral thickness variation of the GaAsP barriers in the undulated SL allowed efficient tunnel transport through the thinner part of the barrier, and improved the carrier collection and the fill factor. By optimizing the growth temperature for SL on vicinal substrates, an N-on-P cell including 100-period SL with a bandgap of 1.21 eV achieved 1.11 times higher efficiency than a GaAs reference cell with 36% current enhancement as middle cell performance.

Photoluminescence of a high strain InGaAs single quantum well with GaAsP barriers

To investigate the relationship between indium content and optical properties during epitaxial growth of an InGaAs/GaAs single quantum well (SQW), simulation and experiments are demonstrated. The epitaxial growth is demonstrated with low-pressure metal–organic chemical vapor deposition. Photoluminescence (PL) spectroscopy is applied to research the PL properties at room temperature. The In/(In+Ga) varies from 0.24 to 0.36, resulting in an increasing of the full-width half-maximum (FWHM) with the wavelength exhibiting a red-shift. A SQW with an In/(In+Ga) of 0.36 is manufactured, where a FWHM of 23.9 meV is obtained. An InGaAs SQW sandwiched by GaAsP is prepared, which is observed to exhibit a diminished FWHM of 17.0 meV with the wavelength revealing a blue-shift.

InGaAs/GaAsP superlattice solar cells with reduced carbon impurity grown by low-temperature metal-organic vapor phase epitaxy using triethylgallium

In this paper, we investigated the effects of carbon incorporation on photovoltaic performance of InGaAs/GaAsP superlattice (SL) solar cells grown by low-temperature MOVPE (LT-MOVPE), which is required for stable SL growth on vicinal substrates. Using trimethylgallium (TMGa) as the gallium precursor, methyl radicals formed by its pyrolysis tend to be absorbed on the surface at low temperature, causing severe carbon incorporation and p-type background doping. High background carrier concentration flattens the band-lineup of the intrinsic region and blocks the carrier transport across the SLs, and resulted in serious degradation of photocurrent. Intentional sulfur doping to cancel out the background doping and hence to recover the built-in field greatly improved the cell performance, but was found to require very precise control of doping level to achieve an exact compensation doping condition. Use of triethylgallium (TEGa) instead of TMGa much reduced the carbon incorporation at low temperature and significantly enhanced the photocurrent extraction without sulfur doping treatment. By thinning GaAsP barriers to 3 nm to facilitate efficient tunneling transport, a 50-period SL cell with bandgap of 1.22 eV grown on 6°-miscut substrates achieved 1.13 times higher efficiency with 31% current enhancement as middle cell performance than a GaAs reference cell.

Carrier Time-of-Flight Measurement Using a Probe Structure for Direct Evaluation of Carrier Transport in Multiple Quantum Well Solar Cells

Carrier transport across multiple quantum well (MQW) structures inserted in the i-region of a p-i-n diode is an important mechanism that determines the performance of MQW solar cells. We have employed a carrier time-of-flight measurement technique using a quantum-well probe to investigate the electron transport time across MQW structures. Delay of the carrier arrival time, defined as time-of-flight, caused by MQWs shows almost linear increment to the number of wells. Tunneling transport in InGaAs/GaAsP MQW structures is studied by varying the GaAsP barrier thickness. Barrier thickness of 2 nm results in extremely small electron time-of-flight, less than a hundred picoseconds per well, whereas MQW with 8-nm-thick barriers results in approximately 20 times slower transport. Furthermore, the fast time-of-flight in thin barriers shows no degradation after the insertion of GaAs interlayers that form the multistep potential. This suggests that the fast thermally assisted tunneling transport dominates the electron escape dynamics in thin-barrier InGaAs/GaAsP MQWs. The rapid carrier transport results in the significant suppression of current drop at the cell maximum power point of MQW solar cells.

Growth of laser diode structures with emission wavelength beyond 1100 nm for yellow–green emission by frequency conversion

Laser structures for emission wavelengths of 1120nm and 1180nm, suitable for non-linear frequency conversion to yellow–green and yellow–orange, were developed. At 1120nm emission wavelength different active regions and structures were investigated. The introduction of a GaAs spacer layer between GaAsP barriers and InGaAs QWs reduces threshold and transparency current density significantly. Lifetime measurements were done successfully over 1700h for broad area and 10000h for ridge waveguide tapered lasers.Broad area laser diodes with a partly strain-compensated 6nm InGaAs QW, emitting at 1180nm, show lifetimes above 1000h at an output power of 1.5W. The required beam quality was achieved by processing a ridge waveguide laser with an included distributed Bragg reflector. Such a laser emits up to 200mW in single mode output power.

Strain effect for different phosphorus content of InGaAs/GaAsP super-lattice in GaAs p–i–n single junction solar cell

The GaAs p–i–n single junction solar cell with InGaAs/GaAsP super-lattice (SL) in the i-region was fabricated by metal organic vapor phase epitaxy (MOVPE). Using the in situ wafer curvature monitoring, a series of SL solar cell samples with different phosphorus composition in the barrier GaAsP layer was evaluated the accumulated strain during MOVPE growth. The sample with larger phosphorus content in GaAsP barrier layer reduced total strain accumulation, resulted in improved solar cell performance regardless to the higher potential barrier. This result indicated the about 3-nm thick barrier is sufficiently thin for carrier extraction by assisting the tunneling effect. Furthermore, the accumulated strain during MOVPE growth of SL deteriorate solar cell.

GaInP/GaAs Tandem Solar Cells With InGaAs/GaAsP Multiple Quantum Wells

Lattice-matched multiple quantum wells (MQWs) consisting of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As wells with very thin GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> barriers have been incorporated into a GaInP/GaAs tandem solar cell. InGaAs/GaAsP MQWs increase the short-circuit current of the GaAs cell by extending the absorption range, with minimal impact on an open-circuit voltage, thus alleviating current matching restrictions placed by the GaAs cell on multijunction solar cells. MQWs with very thin, tensile strained, high phosphorus content GaAsP barriers allow tunneling to dominate carrier transport across the MQWs and balance the compressive strain of the InGaAs wells such that material quality remains high for subsequent top cell growth. We show that the addition of the QW layers enhances the GaAs cell, does not degrade the performance of the GaInP top cell, and leads to potential efficiency enhancements.

Carrier Escape Time and Temperature-Dependent Carrier Collection Efficiency of Tunneling-Enhanced Multiple Quantum Well Solar Cells

Tunneling enhancement of cell performance in InGaAs/GaAsP multiple quantum well (MQW) solar cells has been studied to investigate the potential in overcoming the carrier collection problem, which hinders the maximum performance of quantum structure solar cells. To accurately investigate the effects of the tunneling effect, the study was carried out in samples with different GaAsP barrier thickness, controlled absorption edge, and constant built-in field. The tunneling effect has been confirmed by evaluating carrier escape times using the time-resolved photoluminescence technique and measuring carrier collection efficiency at various temperatures. The collection efficiencies at low temperature are found to be remarkably improved when barrier thickness was below 3 nm, which can be regarded as the critical thickness for efficiently facilitating tunneling enhancement. It can also be concluded that the carrier transport model based on thermal and tunneling processes is practical enough to describe most of the carrier sweep-out dynamics in MQW solar cells.

100-period, 1.23-eV bandgap InGaAs/GaAsP quantum wells for high-efficiency GaAs solar cells: toward current-matched Ge-based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain-balancing requirement, growing high-quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade-off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e-1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23-eV bandgap quantum wells, a cell with 100-period In0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti-reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut-off using a 665-nm long-pass filter, thus indicating the strong potential of MQW cells in Ge-based 3-J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.

Minority Carrier Transport and Their Lifetime in InGaAs/GaAsP Multiple Quantum Well Structures

Minority carrier transport across InGaAs/GaAsP multiple quantum wells is studied by measuring the response of p-i-n and n-i-p GaAs solar cell structures. It is observed that the spectral response depends critically upon the width of the GaAsP barriers and the device polarity. Electron tunneling is not as efficient as hole tunneling due to a higher conduction band barrier. The spectral response depends on the relative magnitude of the carrier lifetime as compared with the tunneling lifetime. This paper deduces an estimated electron lifetime of 110 ns in In0.14Ga0.86As wells and 25 ns in In0.17Ga0.83As wells, which agree with published results.

Effect of GaAs Step Layer Thickness in InGaAs/GaAsP Stepped Quantum-Well Solar Cell

A multiple-stepped quantum-well (MSQW) solar cell, in which GaAs step layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed, and the improved sub-GaAs-bandgap quantum efficiency has been demonstrated. The optical properties of MSQW solar cells with different GaAs step layer thickness are investigated. The recombination losses inside the QWs have been studied by bias-dependent photoluminescence. The recombination losses decrease with increasing the GaAs step layer thickness. Controlling the GaAs step layer thickness is a feasible way to increase short-circuit current without largely degrading open-circuit voltage.

Growth and Characterization of In x Ga1−x As/GaAs1−y P y Strained-Layer Superlattices with High Values of y (~80%)

Strained-layer superlattice (SLS) structures, such as InGaAs/GaAsP lattice matched to GaAs, have shown great potential in absorption devices such as photodetectors and triple-junction photovoltaic cells. However, until recently they have been somewhat hindered by their usage of low-phosphorus GaAsP barriers. High-P-composition GaAsP was developed as the barrier for InGaAs/GaAsP strained-layer superlattice (SLS) structures, and the merits of using such a high composition of phosphorus are discussed. It is believed that these barriers represent the highest phosphorus content to date in such a structure. By using high-composition GaAsP the carriers are collected via tunneling (for barriers ≤30 A) as opposed to thermionic emission. Thus, by utilizing thin, high-content GaAsP barriers one can increase the percentage of the intrinsic in a p-i-n structure that is composed of InGaAs wells in addition to increasing the number of periods that can be grown for given depletion width. However, standard SLSs of this type inherently possess undesirable compressive strain and quantum size effects (QSEs) that cause the optical absorption of the thin InGaAs SLS wells to shift to higher energies relative to that of bulk InGaAs of the same composition. To circumvent these deleterious QSEs, stress-balanced, pseudomorphic InGaAs/GaAsP staggered SLSs were grown. Staggering was achieved by removing a portion of one well and adding it to an adjacent well. The spectral response obtained from device characterization indicated that staggering resulted in thicker InGaAs films with reduced cutoff energy. Additionally, these data confirm that tunneling is a very effective means for carrier transport in the SLS.

Optimization of Gas-Switching Sequence for InGaAs/GaAsP Superlattice Structures Using In situ Wafer Curvature Monitoring

By high-accuracy in situ curvature measurement during the growth of InGaAs/GaAsP superlattice structures by metal organic vapor phase epitaxy, we have successfully observed the effect of thin GaAs insertion layers between InGaAs wells and GaAsP barriers on strain control. By analyzing curvature transients, we found that an inadequate gas-switching sequence induces the carry over of indium from the InGaAs layer to the overlying GaAs insertion layer. The resulting carry-over layer has an estimated thickness of 0.6 nm and adversely affects the average strain of the structure. Through consideration of the kinetics of surface atoms, it has been revealed that an optimized gas-switching sequence with a 1 s hydrogen purge after the growth of InGaAs wells is effective for preventing the carry over.

Optimized interfacial management for InGaAs/GaAsP strain-compensated superlattice structure

In this paper, the interfacial management for InGaAs/GaAsP strain-compensated superlattice (SL) structures with thin wells and barriers and a large number of hetero-interfaces has been investigated. By means of X-ray diffraction (XRD) measurement and high-accuracy in-situ curvature measurement, we found that the hetero-interfaces with substantial lattice mismatch tend to generate interfacial defects, which can be mitigated by the insertion of ultrathin GaAs interlayers. However, an inadequate gas-switching sequence induces unintended atomic content near the hetero-interfaces and in the GaAs insertion layers, which influences the average strain of the structure. It was found effective to extend the stabilization time for the arsenic and phosphorus mixture before GaAsP barrier growth to 3s and to insert an 1s hydrogen purge after InGaAs well growth. Static photoluminescence (PL) and time-resolved photoluminescence (TRPL) results were also used to evaluate the crystal quality of grown structures.