Multiple Quantum Well Heterostructures Research Articles

Investigation of micro-strain in ZnO/(Cd,Zn)O multiple quantum well nanowires grown on Si by MBE

The present work is dedicated to ZnO/CdO and ZnO/ZnCdO multiple quantum well (MQW) nanowires grown by molecular beam epitaxy (MBE) on Si substrates. The samples were investigated by means of micro-Raman (μ-Raman) spectroscopy and X-ray diffraction (XRD) for the presence of strain as well as verification of structural properties. For micro-Raman experiments two excitation wavelengths of 514.5 nm and 325 nm were used. The non-resonant Raman spectra show phonon modes originating from the Si substrate as well as ZnO, whereas the resonant Raman measurements exhibit multiphonon modes characteristic only for ZnO. Both non-resonant and resonant Raman spectra of ZnO/ZnCdO and ZnO/CdO MQW heterostructures reveal a red-shift of Raman peaks position with respect to that of bulk ZnO. Moreover, in the case of ZnO/CdO sample a significant broadening of the A1LO ZnO line with comparison to the one observed for pure ZnO has been observed. Both effects, the red-shift and a broadening of ZnO-related phonon modes indicate a tensile type of strain occurring in analyzed samples. The XRD signals having its source in ZnO buffer and MQWs are also significantly broadened. Such an effect has been assigned to the micro-strain in nanowires as it affects the XRD peak profile. Thus, based on μ-Raman and XRD measurements the value of micro-strain has been estimated for the studied samples and obtained results are consistent. Application of both complementary methods, μ-Raman spectroscopy and XRD, allowed to study the nanowire structures as well as MQWs in the analyzed samples.

Tunable Out-of-Plane Excitons in 2D Single-Crystal Perovskites

Hybrid organic-inorganic perovskites have emerged as very promising materials for photonic applications, thanks to the great synthetic versatility that allows to tune their optical properties. In the two-dimensional (2D) crystalline form, these materials behave as multiple quantum-well heterostructures with stable excitonic resonances up to room temperature. In this work strong light-matter coupling in 2D perovskite single-crystal flakes is observed, and the polarization-dependent exciton-polariton response is used to disclose new excitonic features. For the first time, an out-of-plane component of the excitons is observed, unexpected for such 2D systems and completely absent in other layered materials, such as transition-metal dichalcogenides. By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton-photon coupling. Such vertical dipole coupling is particularly sought in those systems, e.g. plasmonic nanocavities, in which the direction of the field is usually orthogonal to the material sheet. Organic interlayers are shown to affect also the strong birefringence associated to the layered structure, which is exploited in this work to completely rotate the linear polarization degree in only few microns of propagation, akin to what happens in metamaterials.

Nanoscale visualization of electronic properties of AlxGa1-xN/AlyGa1-yN multiple quantum-well heterostructure by spreading resistance microscopy

Cross-sectional spreading resistance microscopy has been used to investigate nanoscale variations in electronic properties of an undoped Al0.75Ga0.25N/Al0.95Ga0.05N multiple quantum well (MQW) heterostructure grown by plasma-assisted molecular beam epitaxy on an AlN/c-sapphire template, prepared by metalorganic vapor phase epitaxy. It is found that a current signal from the MQWs can be detected only at a negative sample bias. Moreover, its value changes periodically from one quantum well (QW) to another. Analysis of the current-voltage characteristics of the contacts of a tip with the structure layers showed that periodic contrast of MQWs is the result of fluctuations of the chemical composition of the QWs and the concentration of electrons accumulated in them. Mathematical simulations indicate that this modulation is associated with the periodic fluctuations of an Al-mole fraction in the barrier layers of the structure due to counter gradients of the intensity of Al and Ga molecular fluxes across the surface of a substrate rotating slowly during growth. The nanoscale fluctuations of the current contrast observed along the QW layers are caused, most likely, by the presence of the areas of lateral carrier localization, which originate during the formation of QWs by sub-monolayer digital alloying technique.

Onset of surface stimulated emission at 260 nm from AlGaN multiple quantum wells

We demonstrated onset of deep-ultraviolet (DUV) surface stimulated emission (SE) from c-plane AlGaN multiple-quantum well (MQW) heterostructures grown on a sapphire substrate by optical pumping at room temperature. The onset of SE became observable at a pumping power density of 630 kW/cm2. Spectral deconvolution revealed superposition of a linearly amplified spontaneous emission peak at λ ∼ 257.0 nm with a full width at half maximum (FWHM) of ∼12 nm and a superlinearly amplified SE peak at λ ∼ 260 nm with a narrow FWHM of less than 2 nm. In particular, the wavelength of ∼260 nm is the shortest wavelength of surface SE from III-nitride MQW heterostructures to date. Atomic force microscopy and scanning transmission electron microscopy measurements were employed to investigate the material and structural quality of the AlGaN heterostructures, showing smooth surface and sharp layer interfaces. This study offers promising results for AlGaN heterostructures grown on sapphire substrates for the development of DUV vertical cavity surface emitting lasers (VCSELs).

Time-resolved photoluminescence of Ga(NAsP) multiple quantum wells grown on Si substrate: Effects of rapid thermal annealing

Time-resolved photoluminescence (TR-PL) spectroscopy has been used to study the impact of rapid thermal annealing (RTA) on the optical properties and carrier dynamics in Ga(NAsP) multiple quantum well heterostructures (MQWHs) grown on silicon substrates. TR-PL measurements reveal an enhancement in the PL efficiency when the RTA temperature is increased up to 925°C. Then, the PL intensity dramatically decreases with the annealing temperature. This behavior is explained by the variation of the disorder degree in the studied structures. The analysis of the low-temperature emission-energy-dependent PL decay time enables us to characterize the disorder in the Ga(NAsP) MQWHs. The theoretically extracted energy-scales of disorder confirm the experimental observations.

Effect of interface grading and lateral thickness variation on x-ray diffraction by InGaN/GaN multiple quantum wells

We develop a method for simulating the effects of interface grading and lateral variation in layer thickness on x-ray diffraction by InGaN/GaN multiple quantum wells (MQWs). Using the resulting simulation scheme, we perform detailed fitting of symmetric (0002) ω/2θ scans measured for a selection of typical InGaN/GaN MQW heterostructures. We find that incorporation of the combined effects of interface grading and thickness variation substantially improves the goodness of fit relative to a conventional model that assumes ideal MQW structure. The improved simulations of experiments reveal that the examined InxGa1−xN/GaN MQWs (0.17≤x≤0.24) grown on the basal plane of GaN have graded heterointerface widths, w, in the range 0.5≤w≤1.1nm concomitant with lateral variations in total MQW thickness of 0.7–6.3nm rms. Atomic force microscopy of 10×10μm2 areas of the as-grown MQWs finds surface roughnesses of 1.0–5.6nm rms in agreement with corresponding rms thickness variations found by simulating the XRD measurements. For samples with smaller thickness variations, higher order MQW satellites are observed in high-dynamic-range diffraction experiments and best-fit simulations find evidence for asymmetric MQW interface widths. The lower interfaces are narrower than the upper interfaces in agreement with recent transmission electron microscopy and atom-probe studies of MQW interfaces by other groups. These under-recognized structural features—heterointerface grading and lateral film-thickness variation—will influence not only x-ray diffraction, but also polarization, bandstructure, and carrier localization within InGaN-based MQW heterostructures.

Investigation of nanostructure properties in InGaN/GaN multiple quantum wells by X-ray diffraction analysis

We report a study on the nanostructure properties of InGaN/GaN multiple quantum well (MQW) heterostructures. The temperature dependent photoluminescence (PL) spectra from the InGaN/GaN MQWs with different indium contents were carried out. Based on the band-tail states model, the degree of the localized states dispersion is estimated. The broadening of localized states in the samples is enhanced as the indium content is increasing. To further investigate the results, X-ray diffraction spectra of the samples were measured and analyzed. The nanostructure size distribution of the MQW heterostructure is determined via the calculation of the diffracted intensity of X-ray beam in Fourier space. It is found that the sample with higher indium content exhibits a wider nanostructure size distribution, and is consistent with the experimental results from PL spectra.

Full $C$-Band 10-Gb/s 40-km SMF Transmission of InGaAlAs Electroabsorption Modulator

This paper describes the 10-Gb/s, full C-band (1530 to 1565 nm) operation of 1.55-m electroabsorption modulators (EAMs) designed to eliminate the cost of a back up light source for a 10-Gb/s dense wavelength division multiplexing (DWDM) system. We employ InGaAlAs as the material for the EAM multiple-quantum well (MQW), and design it to achieve a sufficient extinction ratio and a low chirp () parameter over a wide wavelength range. A ridge waveguide buried in benzocyclobutene (BCB) is also introduced for the EAM waveguide. We design the width of the ridge waveguide and the thickness of a core layer that includes an MQW and separate confinement hetero-structure (SCH) layers by calculating the EAM capacitance. 6-well and 12-well based EAMs are fabricated to allow us to compare the wavelength dependence of their static/dynamic extinction ratios (SER/DER), and their small-signal electrical to optical (E/O) responses. We confirm that the 12-well based EAM provides a sufficient DER over the C band. We also determine that the wavelength detuning ( ) between an input light wavelength and an EA absorption peak is 100 nm at an input wavelength of 1530 nm, and evaluate the wavelength dependence of the chirp parameter, and the 40-km SMF transmission characteristics such as eye diagrams and the power penalty (PP). These experiments were conducted using a wavelength tunable laser while maintaining the input power of the EAM at about 13 dBm and changing the input wavelength over the C band. The EAM was also driven with a constant modulation voltage swing (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> ) of 2.0 V while changing only the EAM bias voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> ). We also measured the input power dependence of the chirp parameter. These results show that the 12-well based InGaAlAs EAM is a promising candidate for an optical modulator that can be used over the C band in a low cost 10-Gb/s optical network system.

Anomalous disorder-related phenomena in InGaN/GaN multiple quantum well heterosystems

The influences of InGaN/GaN multiple quantum well (MQW) heterostructures with InGaN/GaN and GaN barriers on carrier confinement were investigated. The degree of disordering over a broad range of temperatures from 20 to 300 K was considered. The optical and electrical properties were strongly influenced by structural and compositional disordering of the InGaN/GaN MQW heterostructures. To compare the degree of disordering we examined the temperature dependence of the luminescence spectra and electrical conductance contingent on the Berthelot-type mechanisms in the InGaN/GaN MQW heterostructures. We further considered carrier transport in the InGaN/GaN disordered systems, probability of carrier tunneling, and activation energy of the transport mechanism for devices with InGaN/GaN and GaN barriers. The optical properties of InGaN/GaN disordered heterosystems can be interpreted from the features of the absorption spectra. The anomalous temperature-dependent characteristics of the disordered InGaN/GaN MQW structures were attributable to the enhancement of the exciton confinement.

Laser threshold and optical gain of blue optically pumped InGaN/GaN multiple quantum wells (MQW) grown on Si

AbstractOptical and laser properties of a series of MQW heterostructures with varying geometry grown on silicon with Al predeposition were investigated. Photoluminescence (PL) band positions covered a spectral range of 430 – 460 nm under Iexc=1 MW/cm2 and 445‐505 nm under Iexc=0.15 W/cm2. Laser action was achieved under transversal optical pumping at room temperature using only cleaved lateral facets of the samples as laser mirrors. The laser threshold rose from 137 kW/cm2 to 300 kW/cm2 with laser wavelength increase from 440 nm to 465 nm. Numerical simulation of the laser conditions shows that the minimal threshold is realized on the fifth order mode. However, the calculated value of material optical gain of InGaN at the laser threshold increases only from 750 cm–1 to 1020 cm–1, mainly due to absorption rise in the substrate with increasing wavelength. Correlation was observed between PL characteristics and laser threshold. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of corrugated MQW heterostructure prepared on patterned (001) GaAs substrate by MOVPE

AbstractWe have studied Si‐doped n‐type quantum well infrared photodetectors (QWIPs) using an Al0.28Ga0.72As/GaAs MQW heterostructure grown on patterned (001) GaAs substrate. Two patterning techniques were applied to reveal stripe patterns. Wet etching with a H3PO4‐based solution using [1‐10]‐oriented stripe mask was used to reveal ridges confined by {111}A quasi‐facets. We also prepared various mesa ridges with sidewall angle varied from 30 to 45 degree, using an additional sacrificial heterostructure based on a GaAs/AlAs system. The patterned substrates were MOVPE‐overgrown to prepare nonplanar Si‐doped AlGaAs/GaAs n‐QWIP. Properties of non‐planar multiple quantum well (MQW) heterostructures were investigated by cros‐sectional transmition electron microscopy (TEM) and low temperature photoluminescence (PL). QWIP with non‐planar MQW heterostructure exhibited wide absorption spectra peaked at ∼8‐9 μm wavelength without additional gratings or corrugations. A relatively large asymmetry was observed in dark I‐V characteristics at 77 K. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical properties of excitons in ZnO-based quantum well heterostructures

Recently the developments in the field of II–VI-oxides have been spectacular. Various epitaxial methods have been used to grow epitaxial ZnO layers. Not only epilayers but also sufficiently good-quality multiple quantum wells (MQWs) have been grown by laser molecular-beam epitaxy (laser-MBE). We mainly discuss the experimental aspect of the optical properties of excitons in ZnO-based MQW heterostructures. Systematic temperature-dependent studies of optical absorption and photoluminescence in these MQWs were used to evaluate the well-width dependence and the composition dependence of the major excitonic properties. Based on these data, the localization of excitons, the influence of exciton–phonon interaction and quantum-confined Stark effects are discussed. The optical spectra of dense excitonic systems are shown to be determined mainly by the interaction process between excitons and biexcitons. The high-density excitonic effects play a role in the observation of room-temperature stimulated emission in the ZnO MQWs. The binding energies of exciton and biexciton are enhanced from the bulk values, as a result of quantum-confinement effects.

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.

Design and characteristics of widely tunable quantum-well laser diodes

Light-current and tuning characteristics of semiconductor injection lasers in an external cavity are analyzed. The active region of the lasers is an asymmetric multiple quantum-well heterostructure under conditions of non-uniform excitation. Such a design of laser diodes results in a wide-range tunable emission spectrum.

GaInN/GaN multiple-quantum-well light-emitting diodes grown by molecular beam epitaxy

GaInN and GaN were grown by molecular beam epitaxy on c-plane sapphire using NH3. 9 K photoluminescence performed on both GaInN thin layers and GaInN/GaN multiple-quantum wells (MQWs) exhibits narrow emission (∼50 meV linewidths). Transmission electron microscopy images show sharp GaInN/GaN interfaces and homogeneous GaInN layers. Strong indium surface segregation is also evidenced. Light-emitting diodes were fabricated from 5×GaInN (25 Å)/GaN (35 Å) MQW heterostructures. The 300 K electroluminescence yields blue light at 440 nm.

GaAs-based VCSEL-structures with strain-compensated (GaIn)As/Ga(PAs)-MQWH active regions grown by using TBAs and TBP

GaAs-based vertical cavity surface emitting laser structures (VCSEL) with strain-compensated (GaIn)As/Ga(PAs) multiple quantum well heterostructures (MQWH) active regions have been deposited using the less hazardous, liquid group-V-sources tertiarybutyl arsine (TBAs) and tertiary butyl phosphine (TBP) in metal organic vapour phase epitaxy (MOVPE). We report on growth optimization, characterization of the VCSEL structures, and ultrafast emission dynamics after femtosecond optical excitation. The improved decomposition characteristics of the alternative compounds yield a small variation in the cavity thickness of only ±0.35% across the 2″ wafer. The high quality of the VCSEL structures having a 2 λ cavity with 4 stacks of 3 (GaIn)As/Ga(PAs)-MQWH and AlAs/GaAs-Bragg mirrors is revealed by the large normal mode splitting of 10.6 meV. Excellent pulse response with a pulse width of 3.2 ps and a peak delay of 4.8 ps at 30 K is obtained after femtosecond optical excitation. The high crystalline perfection with respect to layer homogeneity, optical properties as well as ultrafast emission dynamics demonstrate the advantages of the strain-compensated material system for VCSEL structures grown by MOVPE using TBAs and TBP.

1.3-μm AlGaInAs-AlGaInAs strained multiple-quantum-well lasers with a p-AlInAs electron stopper layer

1.3-/spl mu/m AlGaInAs-AlGaInAs strained multiple-quantum-well (MQW) lasers with a p-AlInAs electron stopper layer have been fabricated. The electron stopper layer was inserted between the MQW and p-side separate confinement heterostructure (SCH) layers to suppress the electron overflow from the MQW to p-SCH. The characteristic temperatures of the threshold currents and slope efficiencies were improved in the lasers with the stopper layers, especially at higher temperatures. As a result, a maximum operating temperature of 155/spl deg/C was achieved, which was 20/spl deg/C higher than that without the stopper layer.

Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {101̄1} planes) and an open hexagonal inverted pyramid which is defined by the six {101̄1} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101̄1} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the following characteristics: (i) its intensity is correlated with the side-wall quantum wells; (ii) the temperature independence of the full width at half maximum strongly supports a localized exciton recombination process.

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity tBuSH, tBu2Se and the adduct Me2Zn:Et3N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm2.

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity tBuSH, tBu2Se and the adduct Me2Zn:Et3N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm2.