Active Region Material Research Articles

Dilute-As InGaNAs/GaN Quantum Wells for High-Efficiency Red Emitters

The dilute-As InGaNAs-based quantum well (QW) active region and the corresponding large-overlap design is analyzed by using a self-consistent 6-band <inline-formula> <tex-math notation="LaTeX">$k\cdot p$ </tex-math></inline-formula> method. Our study evaluates the optical transition energy, ground state electron-hole wavefunction overlap, and optical gain characteristics of a dilute-As InGaNAs QW consisting of a 3.3 nm thick In_0.19Ga_0.81N_0.94As_0.06 well and 10 nm thick GaN barriers. The calculation suggests that the In_0.19Ga_0.81N_0.94As_0.06 QW may result in a transition wavelength and optical gain comparable to a conventional 3.3 nm thick In_0.35Ga_0.65N QW, while preserving low In-content (&#x007E;19&#x0025;). In addition, a staggered dilute-As InGaNAs QW design with a modulated As-doping profile in the InGaNAs QW is presented, and the calculation results in a significantly enhanced wavefunction overlap and optical gain. This study reveals the excellent potential of the dilute-As InGaNAs semiconductor for application as a high efficiency active region material for future long wavelength emitters.

Electrically injected GeSn lasers with peak wavelength up to 2.7 μm

GeSn lasers enable the monolithic integration of lasers on the Si platform using all-group-IV direct-bandgap material. The GeSn laser study recently moved from optical pumping into electrical injection. In this work, we present explorative investigations of GeSn heterostructure laser diodes with various layer thicknesses and material compositions. Cap layer material was studied by using Si 0.03 Ge 0.89 Sn 0.08 and Ge 0.95 Sn 0.05 , and cap layer total thickness was also compared. The 190 nm SiGeSn-cap device had threshold of 0.6 kA / cm 2 at 10 K and a maximum operating temperature ( T max ) of 100 K, compared to 1.4 kA / cm 2 and 50 K from 150 nm SiGeSn-cap device, respectively. Furthermore, the 220 nm GeSn-cap device had 10 K threshold at 2.4 kA / cm 2 and T max at 90 K, i.e., higher threshold and lower maximal operation temperature compared to the SiGeSn cap layer, indicating that enhanced electron confinement using SiGeSn can reduce the threshold considerably. The study of the active region material showed that device gain region using Ge 0.87 Sn 0.13 had a higher threshold and lower T max , compared to Ge 0.89 Sn 0.11 . The performance was affected by the metal absorption, free carrier absorption, and possibly defect density level. The maximum peak wavelength was measured as 2682 nm at 90 K by using Ge 0.87 Sn 0.13 in gain regions. The investigations provide directions to the future GeSn laser diode designs toward the full integration of group-IV photonics on a Si platform.

Improved performance of InP-based 2.1 μm InGaAsSb quantum well lasers using Sb as a surfactant

We demonstrate significantly enhanced performance of 2.1 μm InGaAsSb quantum well lasers using Sb as a surfactant. The lasers are grown on an InP substrate by metal-organic vapor-phase epitaxy. Following the introduction of Sb, a double-channel waveguide laser, with uncoated facets, shows a remarkably increased continuous-wave output power of 330 mW, compared with 160 mW for a Sb-free InGaAs quantum well laser, measured at 10 °C. Moreover, the introduction of Sb improves the temperature performance of the device and doubles its wall-plug efficiency. In addition, the active region material and interface quality are investigated by transmission electron microscopy, which help to elucidate the basic physical mechanism of the Sb surfactant effect.

First-principle electronic properties of dilute-P AlNP deep ultraviolet semiconductor

The electronic properties of dilute-P AlN1-xPx alloys are investigated by means of First-Principle Density Functional Theory (DFT) calculations, where the phosphorus (P) content is varied from 0% up to 6.25%. Band structure calculations indicate significant modifications of the electronic properties with the introduction of P-atoms, with the possibility of tuning the energy band gap from 6.19eV down to 4.32eV by inserting 6.25% P-content in the AlN-based system. The carrier effective masses and lattice parameters (a and c) were analyzed, and a large bowing parameter of b = 28.3 ± 0.5 eV was found. Moreover, our findings suggest a potential pathway to engineer the valence band crossover between the crystal-field split-off (CH) band and the heavy hole (HH) band by inserting low amounts of P-content (∼1-2%) into the AlN1-xPx alloy. Thus, the dilute-P AlN1-xPx alloys may serve as potential candidates for implementation as the active region material for dominant transverse electric (TE) polarization for deep-UV emitting devices.

Characteristics of OMVPE grown GaAsBi QW lasers and impact of post-growth thermal annealing

Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast scanning transmission electron microscopic) analysis revealed inhomogeneous Bi distribution within the QW active region.

Proton implantation for electrical insulation of the InGaAs/InAlAs superlattice material used in 8–15 μm-emitting quantum cascade lasers

We demonstrate the conversion of lattice-matched InGaAs/InAlAs quantum-cascade-laser (QCL) active-region material into an effective current-blocking layer via proton implantation. A 35-period active region of an 8.4 μm-emitting QCL structure was implanted with a dose of 5 × 1014 cm−2 protons at 450 keV to produce a vacancy concentration of ∼1019 cm−3. At room temperature, the sheet resistance, extracted from the Hall measurements, increases by a factor of ∼240 with respect to that of an unimplanted material. Over the 160–320 K temperature range, the activation energy of the implanted-material Hall sheet-carrier density is 270 meV. The significant increase in room-temperature sheet resistance indicates that upon implantation deep carrier traps have been formed in the InAlAs layers of the superlattice. Fabricated mesas show effective current blocking, at voltages ≥10 V, up to at least 350 K. Thus, the implanted InGaAs/InAlAs superlattices are highly resistive to at least 350 K heat sink temperature. Such implanted material should prove useful for effective current confinement in 8–15 μm-emitting InP-based single-emitter QCL structures as well as in resonant leaky-wave coupled phase-locked arrays of QCLs.

Large Optical Gain AlInN-Delta-GaN Quantum Well for Deep Ultraviolet Emitters.

The optical gain and spontaneous emission characteristics of low In-content AlInN-delta-GaN quantum wells (QWs) are analyzed for deep ultraviolet (UV) light emitting diodes (LEDs) and lasers. Our analysis shows a large increase in the dominant transverse electric (TE) polarized spontaneous emission rate and optical gain. The remarkable enhancements in TE-polarized optical gain and spontaneous emission characteristics are attributed to the dominant conduction (C)-heavy hole (HH) transitions achieved by the AlInN-delta-GaN QW structure, which could lead to its potential application as the active region material for high performance deep UV emitters. In addition, our findings show that further optimizations of the delta-GaN layer in the active region are required to realize the high performance AlInN-based LEDs and lasers with the desired emission wavelength. This work illuminates the high potential of the low In-content AlInN-delta-GaN QW structure to achieve large dominant TE-polarized spontaneous emission rates and optical gains for high performance AlN-based UV devices.

Dilute-As AlNAs Alloy for Deep-Ultraviolet Emitter.

The band structures of dilute-As AlNAs alloys with As composition ranging from 0% up to 12.5% are studied by using First-Principle Density Functional Theory (DFT) calculation. The energy band gap shows remarkable reduction from 6.19 eV to 3.87 eV with small amount of As content in the AlNAs alloy, which covers the deep ultraviolet (UV) spectral regime. A giant bowing parameter of 30.5 eV ± 0.5 eV for AlNAs alloy is obtained. In addition, our analysis shows that the crossover between crystal field split-off (CH) band and heavy hole (HH) bands occurs in the dilute-As AlNAs alloy with As-content of ~1.5%. This result implies the possibility of dominant transverse electric (TE)-polarized emission by using AlNAs alloy with dilute amount of As-content. Our findings indicate the potential of dilute-As AlNAs alloy as the new active region material for TE-polarized III-Nitride-based deep UV light emitters.

Optical Gain Optimization of Al0.8Ga0.2N-Delta-GaN Quantum Well Laser in Ultraviolet Spectra Using Genetic Algorithm

In this work, AlGaN based quantum well laser has been designed with delta layer and characterized the designed laser structure. After the characterization, Genetic Algorithm process has been performed over optical gain to optimize it. In this design, AlN has been used as barrier material, Al0.8Ga0.2N has been utilized as active region material and GaN layer has been used as delta layer. For optimization, well width, delta layer width and barrier width have been taken as the variable for which genetic algorithm has performed the optimization process. Characterization of the designed laser shows good agreement with previously published works. Optimization of optical gain for both TE and TM gain also exhibits good improved result than previously published works. From characterization, it is found that the designed laser structure gives peak optical gain of 3589.5 cm−1 and peak spontaneous emission rate of 12.94×1028 s-1cm-3eV-1 at 246.89 nm for 5×1019 cm-3 carrier density. From optimization, it is obtained that optimum TE optical gain is found 6479.1 cm-1 for 11 Å of QW along with 7 Å delta layer at 279 nm wavelength and optimum TM gain is found found 116.95 cm-1 for 20 Å of QW along with 3 Å delta layer at 289.8 nm wavelength. In this work, electronic characterization has been performed with 6 band k.p Hamiltonian for valence subband calculation including valence band mixing effect, strain effect due to lattice mismatch, spontaneous and piezoelectric polarization. For conduction subband calculation, single band Hamiltonian has been solved with strain and polarization effect. Figure 1

A hybrid plasmonic waveguide terahertz quantum cascade laser

We present the realization of a quantum cascade laser emitting at around 2.85 THz, based on a hybrid plasmonic waveguide with a low refractive index dielectric cladding. This hybrid waveguide design allows the performance of a double-metal waveguide to be retained, while improving the emission far-field. A set of lasers based on the same active region material were fabricated with different metal layer thicknesses. A detailed characterization of the performance of these lasers revealed that there is an optimal trade-off that yields the best far-field emission and the maximum temperature of operation. By exploiting the pure plasmonic mode of these waveguides, the standard operation conditions of a double-metal quantum cascade laser were retrieved, such that the maximum operating temperature of these devices is not affected by the process. These results pave the way to realizing a class of integrated devices working in the terahertz range which could be further exploited to fabricate terahertz on-chip circuitry.

Experimental characterization and modeling of the improved low frequency response of a current modulated bulk RSOA slow light based microwave phase shifter

The enhancement of the low frequency gain response of a microwave phase shifter based on slow light in a TO-can packaged bulk reflective semiconductor optical amplifier, by using forced coherent population oscillations achieved by simultaneously modulating the input optical power and bias current, is experimentally demonstrated. The beat-signal gain improvement ranges from 35 to 0dB over a frequency range of 0.5–3.5GHz, thereby improving the noise performance of the phase shifter. Tunable phase shifts of up to 55° are possible over this frequency range. Simulations that show good agreement with experiment, based on a wideband dynamic model that includes spontaneous emission and band structure based calculations of the amplifier active region material, are also presented.

Low‐strain, quantum‐cascade‐laser active regions grown on metamorphic buffer layers for emission in the 3.0–4.0 μm wavelength region

We have investigated metamorphic buffer layers (MBLs), as so-called virtual substrates, for accessing a compositional range of In x Ga 1-x As/Al y In 1-y As superlattice (SL) materials which would otherwise be prohibited due to excessive strain when grown on conventional substrates. Such materials have application in the realisation of high-performance Quantum Cascade Lasers (QCLs) of short emission wavelengths (i.e., ≤4.0 μm). Simulation studies suggest that significant enhancement of performance in terms of reduced device temperature sensitivity and reduced thermal resistance is possible over conventional InP-substrate devices by employing MBL-based QCL designs on a GaAs substrate. Furthermore, such devices would exhibit significantly lower strain compared to conventional QCLs on InP emitting within the 3.0-4.0 μm wavelength region. To improve the planarity of MBL top surfaces, we employ chemical mechanical polishing (CMP) prior to the growth of the QCL SL structures. 20-period In x Ga 1-x As (wells)/Al y In 1-y As (barriers) SLs are grown by metalorganic vapour phase epitaxy (MOVPE) on an InGaAs step-graded, hydride vapour phase epitaxy (HVPE)-grown MBL. Employing CMP on the top of the MBL, prior to the SL growth, results in significantly improved X-ray-diffraction SL fringes. Electroluminescent devices, incorporating a single stage of QCL-SL active-region material grown on an MBL subjected to CMP, demonstrate intersubband emission near 3.6 μm.

RETRACTED: A Novel Material for Laser Diodes of Optical Fiber Communication

Retracted paper: The conventional material for the optical fiber communication laser diode is based on InGaAsP, which might be substituted by a potential candidate, InGaAlN. The new active region material, InN, is introduced regarding to its crystal growth and characterization, including the structural, optical and electrical properties. This material is promising for providing good performance of temperature stability of the wavelength. In addition, it is environmentally friendly.

SUB-THz AND Hα ACTIVITY DURING THE PREFLARE AND MAIN PHASES OF AGOESCLASS M2 EVENT

Radio and optical observations of the evolution of flare-associated phenomena have shown an initial and rapid burst at 0.4 THz only followed subsequently by a localized chromospheric heating producing an H{\alpha} brightening with later heating of the whole active region. A major instability occurred several minutes later producing one impulsive burst at microwaves only, associated with an M2.0 GOES X-ray flare that exhibited the main H{\alpha} brightening at the same site as the first flash. The possible association between long-enduring time profiles at soft X-rays, microwaves, H{\alpha} and sub-THz wavelengths is discussed. In the decay phase the H{\alpha} movie shows a disrupting magnetic arch structure ejecting dark, presumably chromospheric, material upwards. The time sequence of events suggests genuine interdependent and possibly non-thermal instabilities triggering phenomena, with concurrent active region plasma heating and material ejection.

Indium incorporation effects on luminescence mechanisms in quaternary AlInGaN layers

The optical and structural properties of quaternary AlInGaN layers grown by a pulsed metalorganic chemical vapor deposition (PMOCVD) have been investigated by means of photoluminescence (PL) and time-resolved PL (TRPL) measurements and X-ray diffraction. The AlInGaN layers show strong blueshift and linewidth broadening of the PL emission band with increasing excitation power. With increasing indium mole fraction, the AlInGaN layer exhibited a stronger PL intensity, a faster PL decay, and a smaller lattice mismatch between the AlInGaN layer and the GaN buffer layer. Based on both the PL and TRPL data, we suggest that the incorporation of indium creates more band-tail states and enhances the luminescence efficiency, indicating that the PMOCVD-grown AlInGaN is better suited as the active region material for ultraviolet light-emitting diodes than conventional MOCVD-grown AlInGaN.

1 W high brightness index guided tapered laser at 980 nm using Al-free active region materials

Index guided tapered laser diodes at 0.98 /spl mu/m have been optimised for high brightness. We demonstrate an index guided tapered laser using an Al-free active region with an optical power of 1 W at 20/spl deg/C under continuous-wave operation. The parallel beam quality parameter M/sup 2/ of these devices has been measured to be less than 3 at 1 W at 980 nm.

The influence of melt purification and structure defects on mid-infrared light emitting diodes

Mid-infrared light emitting diodes which exhibit more than 7 mW (pulsed) and 0.35 mW dc output power at 3.3 μm and at room temperature have been fabricated by liquid phase epitaxy using Pb as a neutral solvent. Using Pb solution an increase in pulsed output power of between two and three times was obtained compared with InAs light emitting diodes (LEDs) made using rare-earth gettering. The performance improvements were attributed to a reduction in residual carrier concentration arising from the removal of un-intentional donors and structure defects in the InAs active region material. These LEDs are well matched to the CH4 absorption spectrum and potentially could form the basis of a practical infrared CH4 gas sensor.

MBE growth of room-temperature InAsSb mid-infrared detectors

Some key growth issues for InAsSb mid-infrared detectors are presented. We investigate the influence of growth temperature and Sb/As flux ratio on InAsSb alloy composition and on carrier lifetime. The Sb mole fraction in InAs 1− x Sb x depends linearly on the Sb flux using either As 2 or As 4 (0.02⩽ x⩽0.16). However, the Sb incorporation rate is significantly lower using As 2 flux. The carrier lifetime in thick InAsSb layers closely lattice-matched to GaSb is measured by time-resolved photoconductivity. The values obtained vary from 15 to 200 ns depending on growth conditions. The main result is that the carrier lifetime increases as the growth temperature decreases and seems to be limited by Shockley–Read recombinations. Finally, an InAs 0.91Sb 0.09 p–i–n photovoltaı̈c detector is grown and operates at room temperature. A detectivity of 4.5×10 9 cm Hz / W at 3.39 μm and 250 K is measured. We show that the quality of the active region material ensures a sufficiently low generation–recombination current.

Room temperature InAsSb photovoltaic midinfrared detector

An InAs0.91Sb0.09 p-i-n photovoltaic midinfrared detector grown by molecular beam epitaxy and operating at room temperature is presented. An R0A of 1.05 Ω cm2 at 250 K and 0.12 Ω cm2 at 295 K has been achieved, resulting in a detectivity of 4.5×109 cmHz/W at 3.39 μm and 250 K. The quality of the active region material ensures a sufficiently low generation-recombination current. Room temperature performances are limited by the diffusion of holes from the active region through the confining barriers.

Comparison of linewidth enhancement factors in midinfrared active region materials

We report calculations of the linewidth enhancement factor for five midinfrared active region materials. The linewidth enhancement factors for two type-I quantum wells based on InAsSb are 2.5 and 5.4, which represent a reduction of up to a factor of 2.6 with respect to bulk InAs0.91Sb0.09. However, active region materials based on the type-II, InAs/GaInSb system have linewidth enhancement factors near 1.0, which is a factor of 2–5 reduction compared to the type-I quantum wells. The reduction of the linewidth enhancement factor is associated with both a reduction of the mismatch between the conduction and valence band densities of states and the presence of conduction band dispersion. We describe an additional optimization that is possible in the type-II materials: Carefully placed intersubband absorption features can be used to further reduce the linewidth enhancement factor. We show that linewidth enhancement values as low as 0.3 can be obtained in the type-II superlattices when fabricated into a distributed feedback structure.