GaInNAs Quantum Research Articles

Nearest-neighbor distributions inGa1−xInxNyAs1−yandGa1−xInxNyAs1−y−zSbzthin films upon annealing

We examine the distribution of N-In nearest-neighbor bonds in GaInNAs(Sb) quantum wells (QWs) and observe quantitatively the evolution of the distribution during thermal annealing. We use near-edge x-ray absorption fine structure to compare the behavior of compressively strained quantum wells with relaxed thick-film samples, and find no significant effect of strain on the nearest-neighbor bonding. Photoluminescence (PL) and electroreflectance (ER) spectroscopies are used to quantitatively measure the distribution of N-In nearest-neighbor states for a series of variously annealed GaInNAsSb QW samples. We find that increased annealing temperature or time leads to a blueshift of the band gap that saturates after sufficient annealing. This saturation is related to a thermodynamic equilibration of the N-In nearest-neighbor bonding in the material toward highly In-coordinated states, from an as-grown material having a nearly random bonding arrangement dominated by N-Ga bonds. The different N-In nearest-neighbor states form a fine splitting of the band gap of the material. The average spacing between these levels is found to be considerably smaller for GaInNAsSb $(\ensuremath{\sim}18\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ than for GaInNAs $(\ensuremath{\sim}35\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$. Furthermore, we present absorption measurements that reveal an increased optical efficiency of the higher In-coordinated N states that form upon annealing. Additionally, the line shape observed at room temperature in all of the spectroscopic measurements is Gaussian, indicating a strong exciton-phonon coupling in these alloys.

Structural and optical properties of high In and N content GaInNAs quantum wells

We have analysed by Transmission Electron Microscopy and Photoluminescence (PL) the structural and optical properties of Ga 1−xIn xN yAs 1−y quantum wells with Indium and Nitrogen contents in the range 0.20< x<0.35 and 0.013< y<0.023, respectively. Our results have shown that a degradation of the structural quality of the wells when increasing the In content takes part in two distinct steps. In the first one, an undulation of the surface of the wells on raising the In content from 20% to 25% has been observed. This is accompanied by the appearance of a high density of extrinsic dislocation loops, probably due to a high local stress concentration in the material. For these samples the PL spectra show intense and narrow emission peaks at 1.1–1.2 μm, therefore the existence of dislocation loops seems not to affect considerably the optical efficiency of the material at this stage. In a second step, on increasing the In composition to 35%, the PL emission efficiency of the wells is severely degraded and the samples show the appearance of threading dislocations. An unfaulting of the dislocation loops as a consequence of a coalesce reaction with two Shockley partials is proposed as the mechanism for the formation of the observed threading dislocations.

Enhanced optical and structural properties of 1.3μm GaInNAs∕GaAs multiple quantum-well heterostructures with stepped strain-mediating layers

The effects of inserting GaInNAs single or stepped strain-mediating layers between GaInNAs quantum wells and the surrounding matrix on the structural and optical properties of 1.3μm GaInNAs∕GaAs multiple quantum-well (QW) structures have been investigated. Compared to control samples with simple GaInNAs multiple quantum wells, samples with strain-mediating layers exhibit improved optical and structural properties. A further strong enhancement of photoluminescence efficiency at room temperature and a remarkable reduction of emission linewidth of GaInNAs multiple QWs have been observed using a compositionally stepped strain-mediating layer. These results imply that the strain profile between the quantum well and the surrounding matrix has a major effect on the optical and structural qualities of GaInNAs quantum wells.

Dilute nitride absorbers in passive devices for mode locking of solid-state lasers

We report on the growth, material and nonlinear optical properties of GaInNAs quantum wells (QWs) used in semiconductor saturable absorber mirrors (SESAMs) for passive mode locking of solid-state lasers in the telecommunication wavelength range from 1.3 to 1.55 μm. The antiresonant SESAMs were grown by molecular beam epitaxy with a nitrogen concentration of 1.6% and 2.6%, respectively. They were subject to rapid thermal annealing to fine-tune the absorption wavelength by blueshifting the photoluminescence wavelength. The appearance of QW intermixing upon thermal annealing was studied by X-ray rocking curve measurements. The thermal annealing procedure was proved not to alter the GaInNAs SESAM by QW intermixing for temperatures up to 800 °C. Optical characterization was applied to investigate the nonlinear SESAM properties. Degenerate pump-probe experiments revealed similar recovery times for both SESAMs in the tens of picoseconds range. We demonstrated stable self-starting passive cw mode locking with sub-10 ps pulses at 1314 and 1534 nm.

Comparison of Lasing Characteristics of GaInNAs Quantum Dot Lasers and GaInNAs Quantum Well Lasers

ABSTRACTSelf-assembled GaInNAs/GaAsN single layer quantum dot (QD) lasers grown using solid source molecular beam epitaxy have been fabricated and characterized. Temperature-dependent measurements have been carried out on the GaInNAs QD lasers. High-temperature operation up to 65°C was demonstrated from an unbonded GaInNAs QD laser (50 × 1060 μm2), with high characteristic temperature (T0) of 79.4 K in the temperature range of 10-60°C. For comparison, temperature-dependent operation has also been studied on the GaInNAs single quantum well (SQW) lasers. Unlike the relation between the cavity length and T0 in GaInNAs SQW lasers, longer-cavity GaInNAs QD laser (50 × 1700 μm2) showed lower T0 of 65.1 K, which is presumably believed due to the nonuniformity of the GaInNAs QD layer.

Electroluminescent Properties of Heterostructures with GaInNAs Quantum Wells

GaInNAs quantum wells were grown by metal-organic vapor-phase epitaxy. In order to improve the optical properties, the GaNAs barriers were incorporated on both sides of the quantum well; these barriers compensated the elastic stresses. Characteristics of the optical transitions were assessed from the measurements of photoluminescence and photocurrent. In order to fabricate light-emitting diodes, nonalloyed ohmic contacts based on heavily δ-doped layers were used. Electroluminescence was observed at a wavelength of ∼1.2 μm at temperatures of 77 and 300 K; the electroluminescence intensity depended linearly on the injection current if the latter exceeded a certain threshold value.

Polarization Dependent Characteristics of a Resonant Cavity Enbanced Photodetector

In this paper, polarization dependent characteristics of resonant cavity enhanced (RCE) photodetector (PD) with respect to the incident light angle were analyzed in detail. The simulations carry out on a 1.3μm GaInNAs quantum wells RCE PD, which is fabricated in our group. Analysis results show that the influence of polarization can only be neglected at small angle of incidence.

Improved performance of MOVPE-grown GaInNAs quantum wells by control of interfacial strain

A comprehensive study to achieve optically efficient GaInNAs quantum wells (QWs) grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) has been accomplished. In order to extend emission wavelength over 1.3 μm effectively, GaInNAs QW structures with multiple barriers of GaNAs/InGaAs were proposed. Significant improvement of photoluminescence efficiencies and easy controllability of emission wavelengths over the 1.3 μm range were demonstrated with the proposed structure. It is believed that the wavelengths emitted from our employed QW structures are correlated to the amount of strain, determined by the strain competition between InGaAs (compressive) and GaNAs (tensile) layers. As N contents increased in the QWs, the emission wavelength was able to extend up to 1.38 μm without deteriorating spectral width.

GaAs -based 1.3μm microlasers with photonic crystal mirrors

We present results of GaAs-based microlasers with photonic crystal mirrors. The lasers are fabricated from GaAs∕AlGaAs layer structures with a doub1e GaInNAs quantum well emitting at 1.3μm wavelength. The devices are realized as ridge waveguide lasers with two coupled cavities and a total length between 175 and 600μm. Photonic crystals are used to define the front and back mirrors of the lasers. Threshold currents around 40mA and output power levels of 80mW were achieved. Single-mode emission with 30dB side-mode suppression ratio is obtained due to mode interference between the two cavities.

MOVPE growth optimization for optically efficient GaInNAs quantum well structure

A detailed study to achieve optically efficient GaInNAs QWs by low-pressure metal-organic chemical vapor deposition (LP-MOCVD) is reported. It was found that the insertion of GaNAs/InGaAs layers to GaInNAs QW/GaAs barrier is very effective to control the emission wavelength and improve the optical property of QW. Indium rich (as high as 40%) GaInNAs QWs grown at higher V/III ratio are more desirable to extend emission wavelength over 1.3 μm due to reduced incorporation of impurities. The optical efficiencies of the QW structure were confirmed by the lasing performance in edge-emitting laser. The threshold current density and slope efficiency per facet of a 1360 nm laser diode are measured to be 892 A/cm 2 and 0.135 W/A, respectively.

Distribution of nitrogen in GaInNAs/GaAs quantum wells

Quantitative high-resolution transmission electron microscopy was applied to determine the indium and nitrogen distributions in a GaInNAs/GaAs heterostructure. Two almost identical samples grown by gas-source molecular beam epitaxy on GaAs (001) substrates were investigated. The samples contained InGaAs and GaInNAs wells with the same thickness and In concentration. According to high-resolution X-ray diffractometry the average In concentration is 27% in both samples, with an N concentration of 1.1% in the GaInNAs quantum well. The evaluation of the photoluminescence peak position of the InGaAs sample and additional photoreflectance measurements yielded an In concentration of 30%. In-concentration profiles were obtained with the composition evaluation by the lattice fringe analysis (CELFA) technique for the InGaAs well using the chemical sensitivity of the (002) reflection. The averaged In concentration in the InGaAs quantum well was determined to be (30±1)% in good agreement with the other measurement techniques. The N-concentration profiles in the GaInNAs wells were measured by the comparison of the CELFA results in the samples with and without nitrogen. The N distribution is characterised by two maxima close to the GaInNAs/GaAs interfaces. The measured N concentration in the central part of the quantum well is (2.5±1.0)%.

Composition fluctuations in GaInNAs multi-quantum wells

GaInNAs/GaAs(001) multi-quantum wells grown by MBE at temperatures in the range 360–460 °C have been studied by transmission electron microscopy and photoluminescence. The authors have observed the existence of periodic contrasts with 220BF reflection, which appear more pronounced when increasing the growth temperature. These strain contrasts have been associated to composition fluctuations in the wells and, therefore, it is suggested that an enhancement of the phase separation in the GaInNAs quantum wells occurs when varying the growth temperature from 360 °C to 460 °C. The photoluminescence results show a broadening of the emission peak over a similar growth temperature range. Thus, the degradation of the optical properties in the GaInNAs structures is suggested to be linked to the composition fluctuations.

Growth of self-assembled GaInNAs quantum dots by atomic-H assisted RF molecular beam epitaxy

Self-organized Ga x In 1− x N y As 1− y quantum dots (QDs) were fabricated on GaAs (001) substrates by atomic hydrogen-assisted molecular beam epitaxy with a radio frequency nitrogen plasma source (RF-MBE). We have investigated the effect of N 2 flow rate on growth dynamics and optical properties of GaInNAs QDs. With increasing N 2 flow rate, both the QD size and size fluctuation became significantly larger possibly due to phase separation and non-uniformity of N atom concentration, thereby causing linewidth broadening of PL emission of band-to-band transition. We found that N 2 flow rate during the growth of GaInNAs QDs on GaAs (001) in RF-MBE strongly affects the growth kinetics and optical properties. Further, appropriate post-growth annealing was found to be effective in improving PL emission efficiency of both thin GaInNAs films and QDs. Finally, we obtained high-quality 1.3-μm wavelength PL emission with a narrow linewidth of 98.4 meV at room temperature from optimally annealed GaInNAs QDs.

Room-temperature continuous-wave operation of GaInNAs∕GaAs quantum dot laser with GaAsN barrier grown by solid source molecular beam epitaxy

We present the results of GaInNAs∕GaAs quantum dot structures with GaAsN barrier layers grown by solid source molecular beam epitaxy. Extension of the emission wavelength of GaInNAs quantum dots by ∼170nm was observed in samples with GaAsN barriers in place of GaAs. However, optimization of the GaAsN barrier layer thickness is necessary to avoid degradation in luminescence intensity and structural property of the GaInNAs dots. Lasers with GaInNAs quantum dots as active layer were fabricated and room-temperature continuous-wave lasing was observed. Lasing occurs via the ground state at ∼1.2μm, with threshold current density of 2.1kA∕cm2 and maximum output power of 16mW.

Growth temperature dependence and study of multilayer self-assembled GaInNAs/GaAsN quantum dots grown by solid source molecular beam epitaxy

Self-assembled GaInNAs quantum dots (QD) with inserted GaAsN strain-reducing layer were grown on GaAs (001) substrate by solid source molecular beam epitaxy. The highest room temperature photoluminescence (PL) intensity at wavelength of ∼1.57μm was obtained from the sample grown at 500°C, which has the most uniform QD size observed from atomic force microscopy measurement. A number of samples with different spacer thickness and stacked QD layers were grown to investigate the effect of growth condition for GaIn0.5N0.01As QDs. Multilayer structures with 5QD stacks and spacer layer thickness of 2nm were found to show the optimum result in terms of PL intensity.

Quantum-confined Stark effect of GaInNAs(Sb) quantum wells at 1300–1600nm

We report the measurement of electroabsorption spectra from GaInNAs and GaInNAsSb quantum wells grown on GaAs showing quantum-confined Stark effect behavior suitable for optical modulation at 1300 and 1550nm wavelength, respectively. The high quality of our material is evidenced by sharp exciton resonances with a full width at half maximum &amp;lt;25meV at 295K, and peak absorption coefficient of 18 000cm−1 for GaInNAs and 34 800cm−1 for GaInNAsSb. Changes in absorption coefficient 10 000cm−1 with an applied electric field were measured. Device performance from these materials is expected to be comparable to or better than the competing material grown on InP.

Structural and optical properties of GaInNAs/GaAs quantum structures

Structural and optical properties of GaInNAs/GaAs quantum structures grown bymetalorganic vapour phase epitaxy (MOVPE) are studied. The growth of arsenide–nitridesby MOVPE is reviewed and compared to the other major growth technique, molecularbeam epitaxy. Post-growth thermal annealing and laser irradiation are employed and foundto affect GaInNAs quantum wells differently: with laser treatment no undesired blue-shiftof the photoluminescence peak is observed. The critical thickness for misfit dislocationformation of GaAsN on GaAs is found to be about twice as large as the theoreticalprediction. GaAsN epilayers are also found to contain Ga vacancies in defect complexes.The current status of experimental research on GaInNAs quantum dot structures isreviewed. Self-organized and strain-induced GaInNAs quantum dots are grownand their formation and optical properties are studied. The emission wavelengthof InGaAs quantum dots is extended by using InGaAs and GaInNAs barrierlayers.

The microstructural influence of nitrogen incorporation in dilute nitride semiconductors

The influence of nitrogen incorporation on the microstructure of dilute nitride films andquantum wells has been investigated using cross-section scanning transmission electronmicroscopy. The elemental distributions of Ga, In, N and As in GaInNAs andGaAsN films have been mapped by energy dispersive x-ray analysis. Below thesolubility limit for nitrogen, homogeneous GaAsN and GaInNAs compositionsare obtained of good crystalline quality near to the epilayer/GaAs interface. InGaAsN films containing nitrogen above the solubility limit a graded transition fromarsenide-like to nitride-like material occurs after some 50 nm of growth. GaInNAs filmscontaining nitrogen exceeding the solubility limit exhibit a ‘cellular’ microstructureconsisting of a gallium-depleted InGaAs centre with walls of a GaN-like composition.A similar segregation effect is observed in GaInNAs quantum wells containingnitrogen around and above the solubility limit. The formation of In-rich regionsoccurs within the wells, which causes three-dimensional growth at higher nitrogencontents, with the consequent initiation of growth defects. The electronic structure ofGaInNAs quantum wells has been investigated by mapping the variation in theplasmon frequency using electron energy loss spectrometry on a dedicated scanningtransmission electron microscope. Kramers–Kronig analysis of the singly scatteredlow-loss region yields a measure of the local effective valence electron density.

Influence of growth temperature on the structural and optical quality of GaInNAs/GaAs multi-quantum wells

This work focuses on the influence of the growth temperature on the structural and optical quality of GaInNAs/GaAs and GaInAs/GaAs multi-quantum wells with similar lattice mismatch, studied by transmission electron microscopy and photoluminescence. We have found compositional fluctuations in all the GaInNAs samples, which vary from negligible to very strong over the temperature range studied (360–460 °C). In comparison, at the same growth temperature, GaInAs heterostructures appear more homogeneous. It is proposed therefore that the introduction of N in the structure could be responsible for the enhanced phase separation. A broadening of the photoluminescence peak in GaInNAs structures when raising the growth temperature has also been found associated with the increase in composition fluctuations. Moreover, a kinetically limited Stranski–Krastanow growth mode has been observed in both GaInAs and GaInNAs structures, taking place at lower temperature in GaInNAs quantum wells. We suggest that the enhancement of this growth mode is a consequence of the increased phase separation. The influence of the introduction of N into the GaInAs alloy on the compositional fluctuations and 3D growth mode is discussed.

Use of transmission electron microscopy in the characterization of GaInNAs(Sb) quantum well structures grown by molecular beam epitaxy

The quaternary GaInNAs alloy is a very promising material system for lasers in the 1.2–1.6 μm range with application in telecommunication fiber-optic networks. While good quality laser material has been demonstrated at 1.3 μm, pushing the emission beyond 1.5 μm by adding up to 40% In and 2% N has been unsuccessful. Recently, the addition of small amounts of Sb has put this alloy back on track for the 1.5 μm challenge by dramatically improving the luminescence efficiency of the material. In this work, high-resolution transmission electron microscopy (TEM), energy-filtered TEM, dark-field TEM, and energy-dispersive x-ray spectroscopy were used to structurally characterize both GaInNAs and GaInNAsSb quantum well structures. The results provide insight into the role of antimony in improving the optical properties of the material, namely reducing the local compositional fluctuations of In.