High quality above 3-μm mid-infrared InGaAsSb/AlGaInAsSb multiple-quantum well grown by molecular beam epitaxy

The GaSb-based laser shows its superiority in the 3–4 μm wavelength range. However, for a quantum well(QW) laser structure of InGaAsSb/AlGaInAsSb multiple-quantum well(MQW) grown on GaSb, uniform content and high compressive strain in InGaAsSb/AlGaInAsSb are not easy to control. In this paper, the influences of the growth temperature and compressive strain on the photoluminescence(PL) property of a 3.0-μm InGaAsSb/AlGaInAsSb MQW sample are analyzed to optimize the growth parameters. Comparisons among the PL spectra of the samples indicate that the In0.485GaAs0.184Sb/Al0.3Ga0.45In0.25As0.22Sb0.78MQW with 1.72% compressive strain grown at 460 C posseses the optimum optical property. Moreover, the wavelength range of the MQW structure is extended to 3.83 μm by optimizing the parameters.     

A method to extend wavelength into middle-wavelength infrared based on InAsSb/(Al)GaSb interband transition quantum well infrared photodetector

We present a method to extend the operating wavelength of the interband transition quantum well photodetector from an extended short-wavelength infrared region to a middle-wavelength infrared region. In the modified In As Sb quantum well, Ga Sb is replaced with Al Sb/Al Ga Sb, the valence band of the barrier material is lowered, the first restricted energy level is higher than the valence band of the barrier material, the energy band structure forms type-II structure. The photocurrent spectrum manifest that the fabricated photodetector exhibits a response range from 1.9 μm to 3.2 μm with two peaks at 2.18 μm and 3.03 μm at 78 K.     

Strain Compensated AlInGaAs/InGaAs/InAs Triangular Quantum Wells for Lasing Wavelength beyond 2μm

The subband energy and lasing wavelength of compressively strained triangular Ino.53Ga0.47As/InAs quantum well are calculated and compared with the conventional rectangular ones with the same strain contents. The strain compensation using Al0.33In0.36Ga0.31As barrier is introduced. The results show that lasing wavelength can be extended dramatically to beyond 2.8μm by changing the energy band from the conventional rectangular shape to a triangular one, the realization of such a structure using molecular beam epitaxy technology is also discussed.     

GaAs-Based Metamorphic Long-Wavelength InAs Quantum Dots Grown by Molecular Beam Epitaxy

A bilayer stacked InAs/GaAs quantum dot structure grown by molecular beam epitaxy on an In0.05Ga0.95As metamorphic buffer is investigated. By introducing a InGaAs：Sb cover layer on the upper InAs quantum dots （QDs） layers, the emission wavelength of the QDs is extended successfully to 1.533 μm at room temperature, and the density of the QDs is in the range of 4× 10^9-8 ×10^9cm^-2. Strong photoluminescence （PL） intensity with a full width at half maximum of 28.6meV of the PL spectrum shows good optical quality of the bilayer QDs. The growth of bilayer QDs on metamorphic buffers offers a useful way to extend the wavelengths of GaAs-based materials for potential applications in optoeleetronic and quantum functional devices.     

An optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well(QW) active region design for terahertz(THz) lasing in the wavelength range of 30 μm～ 40 μm and operating at room temperature.The fast longitudinal optical(LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state,and more importantly,the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures.The influences of temperature and pump intensity on gain and electron densities are investigated.Based on our simulations,we predict that with a sufficiently high pump intensity,a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.     

Optically pumped GaN/AlGaN quantum well intersubband terahertz laser

We propose an optically pumped nonpolar GaN/AlGaN quantum well (QW) active region design for terahertz (THz) lasing in the wavelength range of 30 μm～ 40 μm and operating at room temperature. The fast longitudinal optical (LO) phonon scattering in GaN/AlGaN QWs is used to depopulate the lower laser state, and more importantly, the large LO phonon energy is utilized to reduce the thermal population of the lasing states at high temperatures. The influences of temperature and pump intensity on gain and electron densities are investigated. Based on our simulations, we predict that with a sufficiently high pump intensity, a room temperature operated THz laser using a nonpolar GaN/AlGaN structure is realizable.     

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect o~ temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.     

Experimental investigation of loss and gain characteristics of an abnormal In_xGa_(1-x)As/GaAs quantum well structure

In this Letter, the loss and gain characteristics of an unconventional InxGa1-xAs∕Ga As asymmetrical step well structure consisting of variable indium contents of InxGa1-xAs materials are measured and analyzed for the first time, to the best of our knowledge. This special well structure is formed based on the indium-rich effect from the material growth process. The loss and gain are obtained by optical pumping and photoluminescence(PL)spectrum measurement at dual facets of an edge-emitting device. Unlike conventional quasi-rectangle wells, the asymmetrical step well may lead to a hybrid strain configuration containing both compressive and tensile strains and, thus, special loss and gain characteristics. The results will be very helpful in the development of multiple wavelength In Ga As-based semiconductor lasers.     

GaSb-based type-Ⅰ quantum well cascade diode lasers emitting at nearly 2-μm wavelength with digitally grown AlGaAsSb gradient layers

We report a Ga Sb-based type-I quantum well cascade diode laser emitting at nearly 2-μm wavelength. The recycling of carriers is realized by the gradient Al Ga As Sb barrier and chirped Ga Sb/Al Sb/In As electron injector. The growth of quaternary digital alloy with a gradually changed composition by short-period superlattices is introduced in detail in this paper. And the quantum well cascade laser with 100-μm-wide, 2-mm-long ridge generates an about continuous-wave output of 0.8 W at room temperature. The characteristic temperature T0 is estimated at above 60 K.     

Metamorphic growth of 1.55 μm InGaAs/InGaAsP multiple quantum wells laser structures on GaAs substrates

We fabricate a Ga As-based In Ga As/In Ga As P multiple quantum wells(MQWs) laser at 1.55 μm. Using two-step growth method and thermal cyclic annealing, a thin low-temperature In P layer and a thick In P buffer layer are grown on Ga As substrates by low-pressure metal organic chemical vapor deposition technology. Then, highquality MQWs laser structures are grown on the In P buffer layer. Under quasi-continuous wave(QCW) condition, a threshold current of 476 m A and slope efficiency of 0.15 m W/m A are achieved for a broad area device with 50 μm wide strip and 500 μm long cavity at room-temperature. The peak wavelength of emission spectrum is1549.5 nm at 700 m A. The device is operating for more than 2000 h at room-temperature and 600 mA.     

Quantum dot quantum cascade photodetector using a laser structure

We report on a quantum dot quantum cascade detector(QD-QCD), whose structure is derived from a QD cascade laser. In this structure, more ordered In As QD layers formed in the Stranski–Krastanow growth mode on a thin Ga As buffer layer are incorporated into the active region. This QD-QCD can operate up to room temperature with a peak detection wavelength of 5.8 μm. A responsivity of 3.1 mA/W at 160 K and a detectivity of 3.6 × 10~8 Jones at 77 K are obtained. The initial performance of the detector is promising, and, by further optimizing the growth of InA s QDs, integrated QD-quantum cascade laser/QCD applications are expected.     

High quality above 3-μm mid-infrared InGaAsSb/AIGaInAsSb multiple-quantum well grown by molecular beam epitaxy

The GaSb-based laser shows its superiority in the 3-4 ~tm wavelength range. However, for a quantum well （QW） laser structure of InGaAsSb/AIGaInAsSb multiple-quantum well （MQW） grown on GaSb, uniform content and high com- pressive strain in InGaAsSb/A1GaInAsSb are not easy to control. In this paper, the influences of the growth tempera- ture and compressive strain on the photoluminescence （PL） property of a 3.0μm lnGaAsSb/A1GaInAsSb MQW sample are analyzed to optimize the growth parameters. Comparisons among the PL spectra of the samples indicate that the Ino.485GaAso.184Sb/Alo.3Gao.45Ino.25Aso.22Sbo.78 MQW with 1.72% compressive strain grown at 460 ~C posseses the op- timum optical property. Moreover, the wavelength range of the MQW structure is extended to 3.83 μm by optimizing the parameters.     

Generation of a mid-infrared broadband polarized supercontinuum in As2Se3 photonic crystal fibers

A simplified structure of birefringent chalcogenide As 2 Se 3 photonic crystal fiber（PCF） is designed.Properties of birefringence,polarization extinction ratio,chromatic dispersion,nonlinear coefficient,and transmission are studied by using the multipole method,the finite-difference beam propagation method,and the adaptive split-step Fourier method.Considering that the zero dispersion wavelength of our proposed fiber is about 4 μm,we have analysed the mechanism of spectral broadening in PCFs with different pitches in detail,with femtosecond pulses at a wavelength of 4 μm as the pump pulses.Especially,mid-infrared broadband polarized supercontinuums are obtained in a 3-cm PCF with an optimal pitch of 2 μm.Their spectral width at 20 dB reaches up to 12 μm.In the birefringent PCF,we find that the supercontinuum generation changes with the pump alignment angle.Research results show that no coupling between eigenpolarization modes are observed at the maximum average power（i.e.,37 mW）,which indicates that the polarization state is well maintained.     

Generation of a mid-infrared broadband polarized supercontinuum in As₂Se₃ photonic crystal fibers

A simplified structure of birefringent chalcogenide As 2 Se 3 photonic crystal fiber(PCF) is designed.Properties of birefringence,polarization extinction ratio,chromatic dispersion,nonlinear coefficient,and transmission are studied by using the multipole method,the finite-difference beam propagation method,and the adaptive split-step Fourier method.Considering that the zero dispersion wavelength of our proposed fiber is about 4 μm,we have analysed the mechanism of spectral broadening in PCFs with different pitches in detail,with femtosecond pulses at a wavelength of 4 μm as the pump pulses.Especially,mid-infrared broadband polarized supercontinuums are obtained in a 3-cm PCF with an optimal pitch of 2 μm.Their spectral width at 20 dB reaches up to 12 μm.In the birefringent PCF,we find that the supercontinuum generation changes with the pump alignment angle.Research results show that no coupling between eigenpolarization modes are observed at the maximum average power(i.e.,37 mW),which indicates that the polarization state is well maintained.     

Very long wavelength infrared focal plane arrays with 50% cutoff wavelength based on type-Ⅱ In As/GaSb superlattice

A very long wavelength infrared(VLWIR) focal plane array based on In As/Ga Sb type-Ⅱ super-lattices is demonstrated on a Ga Sb substrate. A hetero-structure photodiode was grown with a 50% cut-off wavelength of 15.2 μm, at 77 K.A 320×256 VLWIR focal plane array with this design was fabricated and characterized. The peak quantum efficiency without an antireflective coating was 25.74% at the reverse bias voltage of-20 mV, yielding a peak specific detectivity of 5.89×1010cm·Hz~(1/2)·W~(-1). The operability and the uniformity of response were 89% and 83.17%. The noise-equivalent temperature difference at 65 K exhibited a minimum at 21.4 mK, corresponding to an average value of 56.3 mK.     

Effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition

The effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition was investigated using atomic force microscopy, Raman spectroscopy, and deep ultra-violet photoluminescence(PL) with the excitation wavelength as short as ～ 177 nm. Annealing experiments were carried out in either N2 or vacuum atmosphere with the annealing temperature ranging from 1200℃ to 1600℃. It is found that surface roughness reduced and compressive strain increased with the annealing temperature increasing in both annealing atmospheres. As to optical properties,a band-edge emission peak at 6.036 eV and a very broad emission band peaking at about 4.7 eV were observed in the photoluminescence spectrum of the as-grown sample. After annealing, the intensity of the band-edge emission peak varied with the annealing temperature and atmosphere. It is also found that a much stronger emission band ranging from 2.5 eV to 4.2 eV is superimposed on the original spectra by annealing in either N2 or vacuum atmosphere. We attribute these deep-level emission peaks to the VAL–ONcomplex in the AlN material.     

High-Quality and Strain-Relaxation GaN Epilayer Grown on SiC Substrates Using AIN Buffer and AlGaN Interlayer

We study the effect of the AlGaN interlayer on structural quality and strain engineering of the GaN films grown on SiC substrates with an AIN buffer layer. Improved structural quality and tensile stress releasing are realized in unintentionally doped GaN thin films grown on 6 H—SiC substrates by metal organic chemical vapor deposition.Using the optimized AlGaN interlayer, we find that the full width at half maximum of x-ray diffraction peaks for GaN decreases dramatically, indicating an improved crystalline quality. Meanwhile, it is revealed that the biaxial tensile stress in the GaN film is significantly reduced from the Raman results. Photoluminescence spectra exhibit a shift of the peak position of the near-band-edge emission, as well as the integrated intensity ratio variation of the near-band-edge emission to the yellow luminescence band. Thus by optimizing the AlGaN interlayer,we could acquire the high-quality and strain-relaxation GaN epilayer with large thickness on SiC substrates.     

Effect of high-temperature annealing on AIN thin film grown by metalorganic chemical vapor deposition

The effect of high-temperature annealing on A1N thin film grown by metalorganic chemical vapor deposition was investigated using atomic force microscopy, Raman spectroscopy, and deep ultra-violet photoluminescence （PL） with the excitation wavelength as short as ~ 177 nm. Annealing experiments were carded out in either N2 or vacuum atmosphere with the annealing temperature ranging from 1200 ℃ to 1600 ℃. It is found that surface roughness reduced and compres- sive strain increased with the annealing temperature increasing in both annealing atmospheres. As to optical properties, a band-edge emission peak at 6.036 eV and a very broad emission band peaking at about 4.7 eV were observed in the photoluminescence spectrum of the as-grown sample. After annealing, the intensity of the band-edge emission peak varied with the annealing temperature and atmosphere. It is also found that a much stronger emission band ranging from 2.5 eV to 4.2 eV is superimposed on the original spectra by annealing in either N2 or vacuum atmosphere. We attribute these deep-level emission peaks to the VAL--ON complex in the A1N material.     

Band Structure and Optical Gain of InGaAs/GaAsBi Type-Ⅱ Quantum Wells Modeled by the k·p Model

Optical gains of type-Ⅱ In Ga As/Ga As Bi quantum wells(QWs) with W, N, and M shapes are analyzed theoretically for near-infrared laser applications. The bandgap and wave functions are calculated using the self-consistent k·p Hamiltonian, taking into account valence band mixing and the strain effect. Our calculations show that the M-shaped type-Ⅱ QWs are a promising structure for making 1.3 um lasers at room temperature because they can easily be used to obtain 1.3 um for photoluminescence with a proper thickness and have large wave-function overlap for high optical gain.     

InP-based InGaAs/InAlGaAs digital alloy quantum well laser structure at 2 μm

The structural and optical characteristics of InP-based compressively strained InGaAs quantum wells have been significantly improved by using gas source molecular beam epitaxy grown InAs/In 0.53 Ga 0.47 As digital alloy triangular well layers and tensile In 0.53 Ga 0.47 As/InAlGaAs digital alloy barrier layers.The x-ray diffraction and transmission electron microscope characterisations indicate that the digital alloy structures present favourable lattice quality.Photoluminescence (PL) and electroluminescence (EL) measurements show that the use of digital alloy barriers offers better optical characteristics than that of conventional random alloy barriers.A significantly improved PL signal of around 2.1 μm at 300 K and an EL signal of around 1.95 μm at 100 K have been obtained.