MQW Structures Research Articles

Piezoelectric field determination in strained InGaAs quantum wells grown on [111]B GaAs substrates by differential photocurrent

The measurement of the differential photocurrent (DP) generated in PIN GaAs diodes with embedded strained InGaAs quantum wells has allowed us to determine the fields both in the GaAs barriers, as well as in the InGaAs strained quantum wells. Based on an electroreflectance (ER) setup, this technique relies on the generated photocurrent and does not require detection of the reflected light. The field in the GaAs barriers is obtained from DP Franz–Keldysh oscillations at photon energies above the GaAs bandgap, while the piezoelectric field in the wells is deduced from a phase change in the DP spectra under flat-band conditions in the quantum wells (QW). With optical power illuminations of a few nW, the in-well screening in these structures becomes negligible. With just 35mVRMS of modulating signal added to the diode bias and without photovoltaic effects due to the bias control of the sample, uncertainties in field calculations are reduced. The piezoelectric field values obtained at room temperature in MQW structures with 10 wells of 12 and 18% In content are close to those obtained by similar related techniques. For higher In content (20, 25 and 30%), single quantum well structures were used, also allowing the confirmation of pyroelectric behavior of the structures. Besides, direct comparisons of simultaneously grown [100] and [111] samples clearly reveal the earlier relaxation of the first ones when their QW absorption feature disappears.

Dynamical behaviour of photo-excited and voltage biased MQW structures with bistable electro-optical absorption

In this work we have studied the dynamics of switch-on and switch-off processes in biased MQW structures where every well shows optical bistability in a light intensity range (l, h). We have analysed in detail MQW structures with negligible inter-well transport. We have found that the switch-on mechanism consists of a time sequence where every QW jumps into the high-absorption state. Therefore a step-like switching wave propagates through the structure. The switch-off process resembles a reverse wave propagating in the opposite direction and step-like processes in the plasma concentration decay. These effects can be used for conversion of an analogue optical signal to digital (optical and electrical) signal(s).

Reproducibility and Uniformity of MOVPE Planetary Reactors® for the Growth of GaN Based Materials

ABSTRACTProduction scale MOVPE reactors such as the AIXTRON 2000HT Planetary Reactor® offer unique possibilities to fabricate highly efficient GaN based devices at a low cost of ownership. The scope of this investigation is to understand the dependence of wavelength, thickness and doping uniformity on parameters such as total gas flow, temperature distribution in the reactor and purity of the precursors. Wafer to wafer uniformity in the 7×2” wafer configuration as well as run to run reproducibility will be discussed. We obtained a wafer to wafer standard deviation of 2.7% for the sheet resistance of Si-doped GaN/InGaN/GaN double heterostructures. The wafer to wafer standard deviation of the main PL emission wavelength at 412.3 nm is 1.8 nm. The run to run reproducibility of the main emission wavelength is <3 nm. We obtained reproducible resistivities of GaN:Mg layers of less than 1 Ωcm which corresponds to 5−10×1017cm−3. Statistical data of p-type doping taking 20 runs into account gave an average hole concentration of 5.5×1017cm−3. Together with the wafer to wafer thickness uniformity of <1% the most sensitive layer properties are well controlled to allow a cost-effective mass production process. Structures such as SQW and MQW structures were grown to understand the performance of a production system with respect to interface properties.

GaN Homoepitaxy for Device Applications

Epitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec.Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pn- and InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

GaN-BASED LASER DIODES

We discuss optical properties of III-Nitride materials and structures. These properties are critical for the development of III-Nitride-based light-emitting diodes and laser diodes. Minority carrier diffusion length in GaN has been determined to be ~ 0.1 μm. The properties of lasing in GaN have been studied using optical pumping. The red shift of emission peak observed in stimulated emission of GaN has been modeled and attributed to many-body interactions at high excitation. The correlation of photoluminescence and optical pumping has shown that band-to-band, or shallow donor-related bandtail to valence band transition is the necessary mechanism of lasing in GaN. This work showed that the thermal instability of InGaN at growth temperature is of main concern in the fabrication of InGaN-based MQW laser diode structures. Photoluminescence has shown that the InGaN composition is very sensitive to the growth temperature. Therefore InGaN growth temperature should be strictly controlled during InGaN-based MQW growth. This work discovered that proper annealing of Si-doping of InGaN/GaN MQW structures that are properly annealed could reduce the lasing threshold and improve the slope efficiency. Over-annealing of these MQWs can lead to thermal degradation of the active layer. Si-doping in over-annealed MQW structure further degrades its quality. The degradation has been attributed to the increase of defects and/or nonuniform local potential formation. P-type doping on the top of InGaN/GaN could also lead to the formation of compensation layer which also degrades laser diode performances. Optical confinement and carrier confinement in InGaN-based laser diode structures are evaluated for optimum laser diode design. The state-of-the-art and fundamental issues of InGaN-based light-emitting diodes and laser diodes are discussed.

Switching-on and -off dynamics of MQW structures with bistable electro-optical absorption

In this work we have studied the dynamics of switch-on and switch-off processes in biased MQW structures where every well shows optical bistability in a light intensity range (I l ,I h ). We have analyzed in detail MQW structures with negligible inter-well transport. We have found that the switch-on mechanism consists of a time sequence where every QW jumps into the high- absorption state. Therefore a step-like switching wave propagates through the structure. The switch- off process resembles a reverse wave propagating in the opposite direction and step-like processes in the plasma concentration decay. These effects can be used for conversion of an analog optical signal to digital (optical and electrical) signal(s).

Growth of thick GaN layers by hydride vapor‐phase epitaxy

The hydride vapor-phase epitaxy method, useful for growing thick layers, and the epitaxial lateral-overgrowth (ELO) method, useful for reducing the number of dislocations that occur in heteroepitaxy, have been combined to grow thick GaN layers with a low dislocation density (∼ 107 cm−2). A mechanism that reduces the dislocation density was recently found; it is accompanied by facet formation at the beginning of ELO. Using these GaN layers as a substrate, InGaN MQW structures were grown. A substrate with a low dislocation density was found to improve the optical quality of these structures. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(7): 57–63, 1998.

Valence band splitting and luminescence Stokes shift in GaInN/GaN thin films and multiple quantum well structures

Ga1−xInxN/GaN multiple quantum well heterostructures and thin Ga1−xInxN/GaN films (0.03⩽x⩽0.22) are studied by photoreflection and photoluminescence spectroscopy at room temperature. In the thin single films we find strong interaction of heavy hole and light hole valence bands throughout the composition range. In MQW structures a splitting of those bands due to quantization is observed and accompanied by a decreasing interaction for increasing InN-fraction. In parallel the Stokes shift of the photoluminescence vanishes as the coupling of the valence band decreases.

Influence of etching parameters on the defect profile and the depth of damage of AlGaAs induced by ion beam etching

The influence of the dry etching process parameters on the ion induced damage profile was investigated by nitrogen ion beam etching (IBE) of AlGaAs at two different ion energies by spatial resolved confocal photoluminescence (PL) measurements on specially prepared beveled sections of Al 0.35Ga 0.65As/GaAs MQW structures. The results reveal that the extension of the defect profile is not proportional to the energy of the nitrogen ions, i.e. the lower ion energy leads to a higher defect profile deep in the materal. The difference in both defect profiles can be explained by the different etching rates and defect diffusion coefficients. The high-energy etching leads to a higher defect concentration at the surface measured by RBS/channeling and in accordance with a simple model calculation.

Study on GaAs/GaAlAs MQW structure for photovoltaic applications

GaAs/GaAlAs MQW structures with varying well widths having narrowest wells on the top layers and gradually wider wells for the inner layers were studied and confirmed by Auger spectroscopy and photoluminescence measurements. Multiplication of quantum wells gives stronger photoluminescence due to higher density of quantized states in the structure. This MQW structure was experimented in the photoconductivity and photocurrent measurements at room temperature. It is found that at low multiplication of quantum wells in MQW structure, the photoconductivity effect was mainly controlled by bulk material as shown in the spectral response. Photocurrent at low bias voltage shows relatively better spectral sensitivity at shorter wavelength. The photoconductivity and photocurrent measurements indicate that appropriate MQW structures acting as broader absorbers due to graded characters of quantized energy states are needed for photovoltaic application. Integration of MQW structure to solar cell structure was also investigated.

GaN Homoepitaxy for Device Applications

AbstractEpitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec.Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pn- and InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

Photodiffraction in InGaAs/InGaAsP multiple quantum wells enclosed in a microcavity

We report new results on the diffraction properties of photoinduced gratings in InGaAs/InGaAsP MQW structures. The original feature of this device is that the QWs are enclosed in an asymmetric Fabry–Perot microcavity in order to increase the diffraction efficiency. We observe oscillations in the diffraction efficiency due to resonant effects in the microcavity. The experimental spectra are compared with theory. Diffraction efficiency at 1.55 μm attains a maximum value of 2.7% at a write beam fluence of 260 μ J cm−2, and then decreases at higher fluences. We explain this phenomenon by an absorption saturation at high excitation.

High energy ion implantation enhanced intermixed quantum well structures and their absorption characteristics in passive optical waveguides

We have shown that high energy ion implantation enhanced intermixing (HE-IIEI) technology for quantum well (QW) structures is a powerful technique which can be used to blue shift the band gap energy of a QW structure and therefore decrease its band gap absorption. Room temperature (RT) photoluminescence (PL) and guided-wave transmission measurements have been employed to investigate the amount of blue shift of the band gap energy of an intermixed QW structure and the reduction of band gap absorption. Record large blue shifts in PL peaks of 132 nm for a 4-QW InGaAs/InGaAsP/InP structure have been demonstrated in the intermixed regions of the QW wafers, on whose non-intermixed regions, a shift as small as 5 nm is observed. This feature makes this technology very attractive for selective intermixing in selected areas of an MQW structure. The dramatical reduction in band gap absorption for the InP based MQW structure has been investigated experimentally. It is found that the intensity attenuation for the blue shifted structure is decreased by 242.8 dB/cm for the TE mode and 119 dB/cm for the TM mode with respect to the control samples. Electro-absorption characteristics have also been clearly observed in the intermixed structure. Current-Voltage characteristics were employed to investigate the degradation of the p-n junction in the intermixed region. We have achieved a successful fabrication and operation of Y-junction optical switches (JOS) based on MQW semiconductor optical amplifiers using HE-IIEI technology to fabricate the low loss passive waveguide.

Optical Polarization Spectroscopy on Quantum Confined Stark Effect in Resonant-Cavity AlGaAs/GaAs MQW Structures

The effect of an electric field on s-polarized reflectivity and spectroscopic ellipsometry has been studied on planar-cavity PIN photodiode structures based on MOVPE grown (Al, Ga)As/GaAs systems with 20 times MQW absorption layers. In order to tune the cavity resonance and the exciton energy and to shift both modes, the direction of light incidence was varied in a wide range of angles and an electric field was applied perpendicular to the layer planes. For field strengths F > 5 MV/m and exciton energies lower than the cavity mode energy, the quantum confined Stark effect reveals strong anomalies: (i) the cavity mode attracts completely the exciton and (ii) the next red-side positioned cavity mode stops ultimatively the quantum confined Stark shift of the exciton.

Modeling of nonlinear wave propagation in GaAs-AlAs MQW with a perturbation technique

An explicit closed-form solution for the nonlinear wave equation in nonlinear GaAs-AlAs MQW structures is obtained with the use of a perturbation technique. Incidence of a plane wave onto a nonlinear medium is studied. As expected, the reflection coefficient is found to be strongly dependent on the amplitudes of incident waves, and the wave velocity becomes a heavy function of wave amplitude. The computation results demonstrate the efficiency and effectiveness of the analytical approach. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 198–200, 1997.

Photoluminescence of low-temperature multiple quantum wells

The photoluminescence (PL) of low-temperature-grown AlGaAs GaAs multiple quantum wells (LT-MQWs) has been investigated and compared with a normal-temperature-grown AlGaAs GaAs MQW structure (NT-MQWs) implanted with protons. The as-grown LT-MQWs show much more intensive PL than the as-implanted NT-MQWs. Upon anneal, the PL from the LT-MQWs and the NT-MQWs exhibits different behaviors. For the NT-MQWs, the PL intensity increases monotonously with rising annealing temperature due to the decrease of implantation-induced defects. By contrast, the PL from the LT-MQWs is drastically quenched, and the PL intensity drops nearly three orders of magnitude after anneal at 600°C. The annealing behavior of the PL from the LT-MQWs is attributed to the formation of As clusters that act as deep trap centers for the photoexcited carriers and cause the quenching of the PL intensity. After anneal, enhanced interface intermixing and roughening have been observed in the LT-MQWs.

Continuously graded buffers for structures grown on GaAs

Abstract We report on the preparation under optimized conditions and on the study of InGaAs buffers grown on GaAs, intended for MQW structures for 1.3 and 1.5 μm optical operation at 300 K; the buffers have linear, square-root and parabolic composition profiles. They were designed so that MQWs grown atop the buffers are virtually unstrained, unlike those prepared following the conventional approach that are under compressive strain. The results obtained by the concomitant use of TEM, HRXRD, AFM and PL show that, by carefully designing the buffers: (i) the misfit dislocation (MD) profiles and thicknesses of the MD-free regions in the buffers can be predetermined, (ii) active structures atop the buffers are virtually unstrained and have efficient 300 K photoluminescence in the 1.3 and 1.5 μm windows of photonic interest, (iii) the structures have threading dislocation concentrations in the low 10 6 cm −2 range and show smooth and symmetric cross-hatchings.

Influence of substrate dopant type on the optical properties of [formula omitted] multiquantum well structures grown by low pressure MOVPE

A correlation between substrate dopant type and the absorption spectra of MQW structures has been identified. Optical absorption spectra from MQW structures grown on S-doped InP can show poorly resolved excitonic features with respect to those obtained from structures grown on both Sn- and Fe-doped substrates. It is proposed that this degradation is due to enhanced interfacial diffusion on the Group V sublattice for structures grown on substrates incorporating low defect densities.

MOVPE-grown (AlGa)As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence

We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified “double-barrier multiquantum well” (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.92-2.1 kA cm−2 for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.

Growth and characterization of MQW structures

We report for the first time on the growth of high-quality tensile-strained InAlGaAs InGaAsP multi-quantum wells on InP as well as bulk InAlGaAsP and InAlGaAsP InGaAsP multi-quantum wells on InP by LP-MOVPE. These structures show very good X-ray rocking curves and photoluminescence spectra and can be an interesting alternative for commonly used InGaAsP InGaAsP and InAlGaAs InAlGaAs structures.