Growth Of Multiple Quantum Wells Research Articles

Effect of indium incorporation on optical and structural properties of m-plane InGaN/GaN MQW on LiAlO 2 substrates

We report about the optical and structural characterization of m-plane InGaN/GaN multiple quantum well (MQW) structures. All samples were grown on 2-inch LiAlO 2 substrates in AIXTRON MOVPE reactors. By lowering the MQW growth temperature, an increase of an indium fraction from 5% to 30% is achieved. High-resolution X-ray diffraction (HRXRD) reciprocal space mapping reveals fully strained growth of the quantum wells. The photoluminescence (PL) peak full width half maximum (FWHM) values increase from ∼130 to ∼240 meV at room temperature when the indium fraction rises from 10% to 30% . Also, PL peaks show good wavelength stability for indium contents up to 10% when the excitation intensity is varied by two orders of magnitude. However, at 20% indium and above, a clear excitation intensity-induced peak shift of around 50 meV is detected which is likely caused by indium segregation. Cross-section transmission electron microscopy (TEM) hints at indium fluctuation near extended defects. We conclude that an improved growth process for nonpolar InGaN/GaN MQW is necessary to achieve high-performance long-wavelength light-emitting devices.

In situ reflectance monitoring for the MOVPE of strain-balanced InGaAs/GaAsP quantum-wells

A strain-balanced InGaAs/GaAsP multiple quantum-wells (MQWs) structure was grown by metal organic vapor phase epitaxy (MOVPE) on GaAs substrate, aiming at a middle cell that improves current matching in a tandem solar cell. In order to detect the instant of strain relaxation in the course of MOVPE, which deteriorates crystal quality significantly, we employed in situ optical surface reflectivity measurement. When strain balancing was incomplete, surface reflectivity dropped during the growth of MQWs, indicating lattice relaxation. Such drop in surface reflectivity occurred at a smaller number of stacked quantum wells when the absolute value of an average strain per well/barrier pair was larger. The accumulated strain, i.e., the product between the average strain and the total thickness at the moment of reflectivity dropped, was roughly constant for all the MQWs, indicating a possibility that we can use this value as the measure to predict the maximum number of MQWs for a given value of the average strain. The reflectance anisotropy (RA) was also monitored in the course of the growth. The value of RA showed linear periodic behavior before the lattice relaxation, corresponding to the well/barrier stacks, suggesting that anisotropy of surface atoms reflects accumulated strain of the growth surface.

Influence of MOVPE growth temperature on the structural and optical properties of InGaN MQW laser diodes

The morphological and optical properties of InGaN multiple quantum wells (MQWs) emitting at 405 nm are studied with respect to the MQW growth temperature. The latter was varied between 760 and 840 °C in structures grown on c-plane sapphire substrates by metal-organic vapor-phase epitaxy (MOVPE). The indium content in the quantum well was kept constant for all temperatures by adjusting the trimethylindium supply. The MQWs were inserted as active region in both optically pumped laser heterostructures and laser diodes (LDs). We found that low growth temperatures result in a reduced spatial uniformity of the luminescence emission wavelength due to well thickness variations, whereas at higher temperatures it is difficult to obtain a spatially homogeneous indium concentration. A minimum threshold power density for optically pumped lasing was found for growth temperatures of the active region between 780 and 820 °C. LDs with an MQW grown at these conditions showed an onset of lasing at a current density of 6.5 kA/cm 2 with output powers of more than 350 mW.

X‐ray diffraction studies of selective area grown InGaN/GaN multiple quantum wells on multi‐facet GaN ridges

AbstractThe structural properties of InGaN/GaN multiple quantum wells (MQW) were studied using synchrotron based high resolution x‐ray diffraction (HRXRD). MQW structures were grown on the top and sidewall facets of triangular and trapezoidal shaped GaN ridges by metalorganic vapour phase epitaxy (MOVPE) in the regime of selective area growth (SAG). Period and strain variations as a function of oxide mask width were determined for both the sidewall and the top facet growth. Oxide mask widths ranged between 2 and 20 µm with openings between adjacent masks of 4 and 6 µm. Analysis of the x‐ray diffraction curves revealed a sidewall / vertical growth rate ratio of ∼0.3 through a comparison of the top to sidewall facet MQW periods. Masks orientated along the 〈11‐20〉 crystallographic direction showed stronger growth enhancement along with large global strain for MQW growth on the top (0001) plane. Interpreting our results within the framework of vapour phase diffusion revealed that inter‐facet migration of group‐III species needs to be taken into account. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Intrafacet migration effects in InGaN∕GaN structures grown on triangular GaN ridges studied by submicron beam x-ray diffraction

Synchrotron radiation has been utilized for x-ray diffraction and reciprocal space mapping of InGaN∕GaN multiple-quantum-well (MQW) structures grown on the sidewalls of 10-μm-wide triangular GaN ridges with {1−1.1} facets. Samples were produced by lateral overgrowth through a patterned dielectric mask by using metal-organic vapor-phase epitaxy. Global MQW strain, period, and the tilt of the (00.1) crystallographic planes have been measured across the sidewall facets using a 240nm x-ray beam. Results of this study are interpreted in terms of suppressed intrafacet migration of In and Ga precursors during the MQW growth.

A well protection layer as a novel pathway to increase indium composition: a route towardsgreen emission from a blue InGaN/GaN multiple quantum well

We have investigated the effects of a well protection layer (WPL) onthe optical and crystal properties of an InGaN/GaN multiple quantumwell (MQW). The five-pair MQW, consisting of an InGaN well grown at750 °C and a GaNbarrier grown at 850 °C, was simply embedded between GaN cladding layers on a sapphire (0001) substrate. Whilethis dual-temperature MQW growth scheme seemed better suited to the GaN barrierquality, it exposed the volatile InGaN well to a higher temperature ambient during theramping-up process to grow the barrier. In order to prevent damage to the fragile well, athin GaN WPL was subsequently coated on each well layer at the same temperature beforeramping-up the temperature. Consequently, it was found that the WPL directly influencedthe indium composition and optical properties of the MQW. The indium composition wasin fact increased, as was evident from x-ray diffraction experiments. In addition,photoluminescence measurements showed that the emission peak wavelength was increasedfrom 464 to 520 nm. These results provide evidence that the WPL effectively suppressesindium re-evaporation during the ramping-up time. The present study proposesthat the WPL leads to a new way to increase the wavelength of InGaN/GaNMQWs.

In Ga N ∕ Ga N multi-quantum-well structures on (111)-oriented bonded silicon-on-insulator substrates

We report growth of InGaN∕GaN multiple quantum wells (MQWs) on (111)-oriented bonded silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition (MOCVD). Prior to MOCVD growth of MQWs, about a 1.2μm thick GaN layer was deposited on SOI substrate with a high-temperature transitional buffer layer. The growth conditions were tuned to realize blue-green emission peaks centered around 420–495nm from such MQWs on SOI. X-ray diffraction, atomic force microscopy, scanning electronic microscopy, and photoluminescence techniques were used to characterize these MQWs. Such an approach to realize multicolor light-emitting layers on SOI substrates is suitable for the integration of InGaN∕GaN-based optoelectronic structures on SOI-based micro-optoelectromechanical systems and sensors.

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

Long-Wavelength SiGe/Si MQW Resonant-Cavity-Enhanced Photodiodes (RCE-PD)

Si1-xGex/Si optoelectronic devices are promising for the monolithic integration with silicon-based microelectronics. SiGe/Si MQW RCE-PD (Resonant-Cavity-Enhanced photodiodes) with different structures were investigated in this work. Design and fabrication of top- and bottom-incident RCE-PD, such as growth of SiGe MQW (Multiple Quantum Wells) on Si and SOI (Si on insulator) wafers, bonding between SiGe epitaxial wafer and SOR (Surface Optical Reflector) consisting Of SiO2/Si DBR (Distributed Bragg Reflector) films on Si, and performances of RCE-PD, were presented. The responsivity of 44mA/W at 1.314 mum and the FWHM of 6nm were obtained at bias of 10V. The highest external quantum efficiency measured in the investigation is 4.2%.

Effect of high temperature and interface treatments on photoluminescence from InGaN/GaN multiple quantum wells with green light emissions

InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.

Investigation of V-Defects and embedded inclusions in InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor deposition on (0001) sapphire

We have examined the nature of V-defects and inclusions embedded within these defects by atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM)/cathodoluminescence (CL) in InGaN/GaN multiple quantum wells (MQWs). To date, indium distribution nonuniformity in the well or GaN barrier growth temperature have been identified as the main factors responsible for the V-defect occurrence and propagation. Further complicating the matter, inclusions embedded within V-defects originating at the first InGaN-to-GaN interface have been observed under certain growth conditions. Our AFM and high-resolution SEM/CL findings provide evidence that some V-defects occur merely as direct results of barrier temperature growth, and that there are additional V-defects associated with In-rich regions, which act as sinks for further indium segregation during the MQW growth. Both types of V-defects have a tendency of promoting inclusions at low-temperature (800 °C) GaN barrier growth in an H2-free environment. Localized strain-energy variations associated with the apex of V-defects may be responsible for the inclusion occurrence. Adding H2 during the GaN barrier growth reduces V-defect formation and suppresses inclusion propagation entirely, rendering a uniform nanoscale CL signal.

Diffraction response of a low-temperature-grown photorefractive multiple quantum well modulator

The transient diffraction response of a low-temperature-grown (LTG) photorefractive multiple quantum well (MQW) spatial light modulator (SLM) is studied experimentally as a function of the frequency and the on/off ratio of a single-sided applied voltage when a grating is written using a continuous-wave sinusoidal illumination. LTG MQW modulators have high spatial resolution, but their diffraction efficiency and speed are reduced. By varying the on/off ratio of the applied voltage, it is shown that the frequency response of the SLM can be optimized while maintaining its high spatial resolution. The flatness of the frequency response with respect to the peak diffracted signal is thus shown to increase by an order of magnitude. By temporally gating the diffracted signal, low and bandpass filtering capabilities of the SLM are experimentally demonstrated for applications such as speckle elimination and novelty filtering. When the diffracted signal is measured over an adjustable time window, it is shown that high frame rate operations in integrated optoelectronic systems can be achieved. A numerical model that includes the effect of high trap densities on the space-charge dynamics is developed. It is shown that the space-charge field grows nonuniformly along the MQW growth direction with the largest modulation in lateral field occurring only near the entrance face of the incident light, thus accounting partially for the low diffraction efficiency of LTG materials.

High-quality InGaN/GaN multiple quantum wells grown on Ga-polarity GaN by plasma-assisted molecular-beam epitaxy

High-quality InGaN/GaN multiple-quantum wells (MQWs) with different In fractions varying from 0.04 to 0.30 have been grown on Ga-polarity GaN by N2 plasma-assisted molecular-beam epitaxy (rf-MBE). High-resolution x-ray diffraction results have indicated that the high interface quality and good reproducibility of the InGaN QW have been achieved. Photoluminescence spectra reveals the superior and intense luminescence properties of InGaN MQWs from ultraviolet (∼388 nm) to green-yellow (∼528 nm) range. We have shown that the Ga-polarity GaN underneath is the key parameter for the successful growth of InGaN MQWs by rf-MBE.

Molecular-beam epitaxy flux transient monitoring and correction using in situ reflection mass spectrometry

The use of reflection mass spectrometry (REMS) for in situ monitoring of group III flux transients during molecular-beam epitaxial growth is reported. Normal shutter closure caused effusion cell heating which resulted in flux overshoots of up to 30%. The variation of the magnitude and time constant of the overshoot, as a function of group III material and shutter closed time, was determined for our Riber 2300 system. It was verified that the observed REMS transients corresponds to growth transients using photoluminescence and transmission electron microscopy analysis of GaAs/AlGaAs and InGaAs/InAlAs multiple quantum well (MQW) structures. Flux transient correction during GaAs/AlGaAs MQW growth was accomplished by ramping the temperature of the Ga effusion cell, based on our previous analysis of the transients.

In Situ Flux Transient Monitoring and Correction During MBE Growth of Quantum Well Structures

ABSTRACTShutter closure during MBE deposition causes source overheating and results in flux transients. These transients are particularly detrimental to the thickness and compositional accuracy of thin quantum well layers. In this paper, we document the effects of flux transients on growth of multiple quantum well (MQW) and resonant tunneling diode (RTD) structures, and demonstrate rudimentary transient correction by employing real-time flux detection.Reflection mass spectrometry (REMS) provides a convenient in situ method for MBE flux monitoring. The Group III partial pressures can be detected in the presence of Group V overpressure, and REMS is compatible with wafer rotation. We used REMS to characterize In, Al and Ga flux transients as a function of shutter closed time, cell flux and substrate temperature. Overshoot magnitudes up to 30% were observed. We verified the correspondence of REMS signal transients and effusion cell flux transients using GaAs/AlGaAs and InGaAs/lnAlAs MQW and test structures. We also successfully demonstrated flux transient correction by cell temperature ramping during MQW and RTD growth.

MBE growth of multiple quantum wells with room-temperature exciton peaks at 1.3 μm

GaAlInAs AlInAs multiple quantum wells have been grown lattice matched on InP substrates by taking into account the time dependence of the fluxes as the shutters open and close. RHEED oscillations are observed and used to calibrate layer thicknesses and growth quality. The half width of the lowest exciton peak is 12 meV.