Strain-compensated Multi-quantum Wells Research Articles

Strain-compensated InGaAsSb/InGaAsSb multiquantum-well structure grown on InP (0 0 1) substrate as optical absorber for wavelengths beyond 2 μm

Abstract We report on successful growth on InP substrates of InGaAsSb-based strain-compensated multiquantum-well (SC-MQW) structures with band-gap wavelengths longer than 2 μm. For SC-MQW samples with four quantum wells, the use of InGaAsSb instead of InGaAs as a barrier layer suppressed the thickness modulation significantly, and its use as a well layer increased the photoluminescence peak wavelength by more than 100 nm. The number of wells in the InGaAsSb/InGaAsSb SC-MQW could be increased from 4 to 30 by adjusting the net strain to be 0%. The absorption coefficient of the 30-period SC-MQW sample was approximately 7000 cm-1 at a wavelength of 2.1 μm.

Submilliampare threshold 1.3μm vertical-cavity surface-emitting lasers

We present the design and fabrication of 13 μm vertical-cavity surface-emitting lasers （VCSELs）. This laser exhibits continuous-wave single-mode operation up to 82℃. The theshold current as low as 051 mA and a slop efficiency of 029 W/A have been achieved. This laser is composed of a wafer-bonded GaAs/Al（Ga）As distributed Bragg reflector （DBR） and a dielectric SiO2/TiO2 DBR with InAsP/InGaAsP strain-compensated multi-quantum wells grown in the cavity. The effect of cavity mode-gain offset on device performance is then discussed.

A novel GaAsN/InGaAs strain-compensated multi-quantum wells solar cell

This paper reports on the device characteristics of strain-compensated multi-quantum wells (SC-MQWs) GaAsN/InGaAs single junction solar cell based on this very promising InGaAs material. Limitations of lattice mismatch could be overcome because the compressive strain in the InGaAs layers matches the tensile strain in the GaAsN layers. GaAsN/InGaAs SC-MQWs solar cells, lattice matched to GaAs, are proposed as a means of extending the long-wavelength absorption to make a competitive candidate in a cascade solar cell structure. We demonstrated that this GaAsN/InGaAs SC-MQWs solar cell could achieve a high short-circuit current density of about 20.87 mA cm−2, under AM1.5G illumination, which is higher than that of the InGaNAs cell. The GaAsN/InGaAs SC-MQWs structures show many of the characteristics required to make it a candidate for the next generation of multi-junction solar cells, which means this design can be used as the third junction in future generation ultrahigh-efficiency three- and four-junction devices.

High-speed and uncooled operation of 1.3-/spl mu/m InGaAsP strain-compensated MQW BH lasers fabricated on patterned InP substrates

The fabrication and characteristics of 1.3-/spl mu/m InGaAsP strain-compensated multiquantum well (SCMQW) buried heterostructure (BH) laser diodes (LDs) grown by selective metal-organic chemical vapor epitaxy on a patterned InP substrate are demonstrated. The photoluminescence (PL) of the SCMQW active region grown on the patterned grooves has a narrow full-width at half-maximum of /spl sim/ 47 meV. The BH LDs exhibit a threshold current of 6.8 mA, a slope efficiency of 0.45 mW/mA, and a light output power exceeding 30 mW at 80 mA and 20/spl deg/C. The maximum operating temperature is 120/spl deg/C with a characteristic temperature of 72 K in 20/spl deg/C-80/spl deg/C. The 3-dB modulation bandwidth can be extended as far as 11.4 GHz under a bias level of 40 mA, and the back-to-back tests show a clear and symmetric eye diagram at 10 Gb/s with a PRBS of 2/sup 31/-1 word length and a peak-to-peak voltage of 1.08 V at 85/spl deg/C. The average increase in operational current is smaller than 0.84% after the 1500-h aging test. The mean time of the BH LDs operated at 85/spl deg/C to fail is calculated as 9/spl times/10/sup 4/ h. These BH LDs satisfy the reliability requirement for strict loop environment and 10-Gb Ethernet application without special hermetic packaging.

Growth of thin AlInN∕GaInN quantum wells for applications to high-speed intersubband devices at telecommunication wavelengths

In this article, we report on growth of AlInN∕GaInN multi-quantum-wells (MQWs) with high Al content (93%) by rf-plasma-assisted molecular-beam epitaxy on (0001) GaN/sapphire templates and on bulk GaN crystals. A series of samples with a barrier thickness of 3nm and with different well thicknesses of 1.5–3nm was grown. The wells were doped with Si at a concentration of 5×1019cm−3. Structures grown on (0001) GaN-based substrates are crack-free, as demonstrated by Nomarski contrast and scanning electron microscopy measurements. X-ray diffraction mapping of a and c lattice parameters shows that AlInN∕GaInN MQWs are fully strained and have up to 7% indium in the barriers and up to 10% In in the quantum wells. These structures exhibit intersubband absorptions at room temperature at a wavelength in the range of 2.45–1.52μm. The AlInN∕GaInN strain-compensated MQW structures, having good quality, are very attractive for ultrahigh-bit-rate telecommunication applications at 1.55μm wavelengths. In addition, because of their low average refractive index, they could be used as thick cladding layers for optical waveguides.

Optical characteristics of 1.55μm GaInNAs multiple quantum wells

We report the optical characterization of high-quality 1.55μm GaxIn1−xNyAs1−y multiquantum wells (MQWs), grown on GaAs with Ga(In)N0.01As spacer layers. The transitions between the quantized QW states of the electrons and holes have been identified using photoluminescence excitation spectroscopy. Their energies are consistent with theoretical fitting based on the band anticrossing model. It is also confirmed by detailed spectroscopic measurements that the addition of even a small amount of In to GaN0.01As barriers remarkably improves the optical characteristics of the QWs. The results imply that although strain-compensated GaInNAs MQWs provide a feasible approach to realizing 1.55μm optical emission, the relative lattice mismatch between the wells and barriers is critical to the optical quality of the related QWs.

Design optimization of InGaAsP–InGaAlAs 1.55 µm strain-compensated MQW lasers for direct modulation applications

In this paper, a simulation study of InGaAsP(well)/InGaAlAs(barrier) 1.55 µm strain-compensated multi-quantum well (MQW) lasers is presented. Due to a large conduction band discontinuity in this material system, a higher material gain and differential gain can be obtained from such a quantum well (QW) as compared to a traditional InGaAsP/InGaAsP quantum well. The deeper electron well should also improve elevated temperature operating characteristics and reduce the electron spillover from QWs. For MQWs, a uniform vertical distribution of holes is achieved due to a reduced effective hole confinement energy by optimizing the bandgap and the strain in the barriers. A large number of quantum wells can be uniformly pumped, reducing the carrier density in each individual well. A uniform and low carrier density in all the wells help reduce the total Auger recombination current. High p-doping in the active region is shown to enhance the carrier and gain non-uniformity in the MQWs. A simulated high modulation bandwidth has been demonstrated, promising directly modulated lasers as a low-cost source for short to medium distance (1–10 km) high speed optical links.

MOCVD growth of strain-compensated multi-quantum wells light emitting diode

In the present paper, strain-compensated multi-quantum wells (SCMQW) InGaAs/GaAsP grown by low-pressure metal organic chemical vapour deposition (MOCVD) were used as the active layers of near infrared LED. The single nude LED chip was put down on the copper header p-side down, using a GaAs transparent substrate and AuBe alloy mirror reflector. The forward voltage under a bias current of 20 mA was measured to be 1.2 V while the reverse voltage at 10 μA was 5–6 V. The output power of 10.5 mW and 18% quantum efficiency were obtained under a bias current of 50 mA.

The influence of InGaP barrier layer on the characteristics of 1.3 μm strain-compensated multiquantum-well InAsP/InP/InGaP laser diodes

In this article, we report the influence of InGaP barrier layer on the performance of 1.3 μm InAsP/InP/InGaP strain-compensated multiquantum-well (SCMQW) laser diodes (LDs) grown by metalorganic chemical vapor deposition. Sharp satellite peaks with narrow width in double-crystal X-ray diffraction measurements and abrupt interface in secondary ion mass spectrometry analyses are observed, indicating that a good epitaxial-layer quality can be obtained through the use of strain-compensation coupled by InGaP barrier layers. By increasing InGaP barrier thickness to above 6 nm, a redshift of the photoluminescence peak is observed in the (1 0 0)-oriented strained layers. It is probably attributed to redistribution across the samples of the huge built-in electric field induced by the piezoelectric effect. The threshold current of InAsP/InP/InGaP SCMQW ridge-waveguide LDs decreases from 78.8 to 33.8 mA through the employ of tensile-strained InGaP barrier layer and the lasing wavelength is 1.307 μm under 44 mA. In addition, the experimental data of broad-area LDs for the InAsP/InP strained multiquantum-well and InAsP/InP/InGaP SCMQW structures are compared in detail.

Very high compositional homogeneity of 1.55 μm strain-compensated InGaAsP MQW structures by MOVPE under N 2 atmosphere

In order to improve the compositional homogeneity in metalorganic vapor phase epitaxy (MOVPE), growth under N 2 used as carrier gas has been applied for the first time to strain-compensated InGaAsP multi-quantum-well (MQW) structures on InP. Various InGaAs(P) layers and MQW structures have been grown by low-pressure MOVPE in a horizontal reactor at total gas flows Q tot=3.0–7.6 slm with substrate rotation using standard precursors. By replacing H 2 by N 2 the lateral homogeneities of the gap wavelength, lattice mismatch and layer thickness were considerably improved. The role of the carrier gas is discussed. The standard deviation σ(λ) of the wavelength over 40 mm of a 50 mm wafer of strain-compensated λ=1.55 μm laser structures with 10 QWs was reduced from 6.5 nm (H 2) to 0.55 nm (N 2), which is a factor of about 10! This is one of the best homogeneity values reported so far for InGaAsP grown in any type of MOVPE reactor with any precursors. Details of the growth parameters and results are presented along with the characteristics of distributed feedback (DFB) lasers and laser arrays produced from these structures.

Structural and optical characterization of strained and strain-compensated InGaAsP/InP quantum well laser structures

InGaAsP/InP strained and strain-compensated multi-quantum wells grown by low pressure metalorganic chemical vapor deposition for fabricating 1.3 μm lasers were characterized by double crystal X-ray diffraction, transmission electron microscopy, atomic force microscopy, cathodoluminescence, and photoluminescence. The quantum wells were compressively strained at 1.14% or 2.55%. Strain-compensated structures were obtained by using appropriately tensile-strained barriers so that the net strain of the multi-quantum well region is approximately zero. It is observed that the sample which had a strain of 2.55% in the wells and which was strain-compensated showed the poorest structural and optical quality. Also, the sample which had a strain of 2.55% and uncompensated showed an inhomogeneous layer growth compared to the sample which had a strain of 1.14%. The results also showed that a quantum well strain of 1.14% can be compensated without affecting the structural and optical quality of the layers.

Metalorganic molecular beam epitaxy of strain-compensated InAsP/InGaAsP multi-quantum-well lasers

Strain-compensated InAsP/InGaAsP multi-quantum-wells (MQWs) grown by metalorganic molecular beam epitaxy (MOMBE) are characterized by conventional photoluminescence (PL), micro-PL, transmission electron microscopy, and x-ray diffraction measurements and applied to fabricate 1.3 μm wavelength laser diodes. These methods reveal that there is no deterioration in the optical properties or structure of strain-compensated MQWs having up to 25 wells, which means that the critical thickness of InAsP grown by MOMBE exceeds 1000 Å. The critical conditions of strain and thickness over which misfit dislocations are generated are determined for the MQWs. The MQW lasers with ten wells (Lz=55 Å) have no misfit dislocations and have uniform threshold current densities of 0.9±0.1 kA/cm2. The maximum operating temperature Tmax of the lasers increases with increasing well number, the highest Tmax is 155 °C, which is obtained for MQW lasers with 15 wells. The lasers have no problems in terms of long-term reliability.

Comparison of chemical beam epitaxy and metalorganic chemical vapour deposition for highly strained multiple quantum well InGaAsP/InP 1.5 μm lasers

The performance and reliability of strained layer optoelectronic devices are in general limited by the integrity of metastable heterostructures. Misfit strain relaxation (and concomitant defects) can be avoided if the structural stability is optimised and elevated temperature exposure minimized. Chemical beam epitaxy (CBE) holds great promise in strained layer epitaxy, since by reducing growth temperature the overall thermal budget for epitaxy and processing can be significantly reduced. The design, epitaxial growth, fabrication and reliability issues related to strain and strain-compensated multi-quantum well lasers are first considered in order to determine the upper limits of compressive or tensile strain permissible in such structures. The benefits of strain (both tensile and compressive) on threshold current density are related to the amount of strain in the wells (via the reduction of the Auger recombination coefficient) and the well width (via the optical confinement factor). It is therefore the strain well-width product for the active region which is of key interest. In this survey the practical upper bound to stability is defined theoretically using an energy balance model, where the effect of strain compensation from oppositely strained barrier layers, balances the strain in the quantum wells and renders the multilayer stack “strain neutral”. The susceptibility of strained multilayers to defect injection through epitaxial growth and subsequent device fabrication is determined by growth simulation. Using this model as a design tool we have investigated the structural stability of a compressively strained multiple quantum well (MQW) laser through the concept of “effective stress” for misfit dislocation injection. The upper limits for quantum well strain incorporation with and without strain compensation are quantitatively defined in light of recent laser reliability data. The evolution of the driving force for misfit strain relaxation is mapped out through a typical epitaxial growth sequence highlighting the points in the growth process of highest vulnerability to defect injection. These design concepts were used to optimize structures for highly strained quantum wells (QWs) in strain compensated InGaAs/InP MQW lasers. The stability of strain-compensated MQW laser structures is demonstrated for devices grown by conventional metalorganic chemical vapour deposition.