Multiquantum-well Structures Research Articles

Characterization and thermal stability of GeSn/Ge multi-quantum wells on Ge (100) substrates

High-quality, high-Sn-content GeSn/Ge multi-quantum well (MQW) structures were grown on Ge (100) substrates by molecular beam epitaxy. The Sn content of the GeSn layers in the MQWs was 7.85 %. High crystalline quality and smooth interfaces were observed by high-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). Clear multistage satellite peaks were identifiable in the HRXRD results and were consistent with those obtained in simulations. No threading or misfit dislocations were observed in the TEM images. The room temperature photoluminescence showed an enhanced emission peak of direct transitions around 2025 nm due to the quantum confinement effect. Rapid thermal annealing was also performed and indicated that the MQW structures remained stable when annealed at 400 °C for 30 s. With increasing temperature, the intermixing of the GeSn and Ge layers increased, and the structure completely degenerated at 600 °C. These results will provide a valuable reference for the processing of GeSn devices.

Wafer-scale controlled exfoliation of metal organic vapor phase epitaxy grown InGaN/GaN multi quantum well structures using low-tack two-dimensional layered h-BN

Recent advances in epitaxial growth have led to the growth of III-nitride devices on 2D layered h-BN. This advance has the potential for wafer-scale transfer to arbitrary substrates, which could improve the thermal management and would allow III-N devices to be used more flexibly in a broader range of applications. We report wafer scale exfoliation of a metal organic vapor phase epitaxy grown InGaN/GaN Multi Quantum Well (MQW) structure from a 5 nm thick h-BN layer that was grown on a 2-inch sapphire substrate. The weak van der Waals bonds between h-BN atomic layers break easily, allowing the MQW structure to be mechanically lifted off from the sapphire substrate using a commercial adhesive tape. This results in the surface roughness of only 1.14 nm on the separated surface. Structural characterizations performed before and after the lift-off confirm the conservation of structural properties after lift-off. Cathodoluminescence at 454 nm was present before lift-off and 458 nm was present after. Electroluminescence near 450 nm from the lifted-off structure has also been observed. These results show that the high crystalline quality ultrathin h-BN serves as an effective sacrificial layer—it maintains performance, while also reducing the GaN buffer thickness and temperature ramps as compared to a conventional two-step growth method. These results support the use of h-BN as a low-tack sacrificial underlying layer for GaN-based device structures and demonstrate the feasibility of large area lift-off and transfer to any template, which is important for industrial scale production.

Strain dependence of In incorporation in m-oriented GaInN/GaN multi quantum well structures

We demonstrate a strong dependence of the indium incorporation efficiency on the strain state in m-oriented GaInN/GaN multi quantum well (MQW) structures. Insertion of a partially relaxed AlInN buffer layer opens up the opportunity to manipulate the strain situation in the MQW grown on top. By lattice-matching this AlInN layer to the c- or a-axis of the underlying GaN, relaxation towards larger a- or smaller c-lattice constants can be induced, respectively. This results in a modified template for the subsequent MQW growth. From X-ray diffraction and photoluminescence measurements, we derive significant effects on the In incorporation efficiency and In concentrations in the quantum well (QW) up to x = 38% without additional accumulation of strain energy in the QW region. This makes strain manipulation a very promising method for growth of high In-containing MQW structures for efficient, long wavelength light-emitting devices.

Correlation of optical and structural properties of GaN/AlN multi-quantum wells—Ab initio and experimental study

The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape o...

The study of light-emitting diode fabricated on c-axis patterned and flat sapphire substrate

GaN thin films and multi-quantum wells (MQWs) were grown on the c-axis patterned sapphire substrates and c-axis flat sapphire substrates by metal organic chemical vapor deposition, respectively. The surface morphology of patterned sapphire substrate and flat sapphire substrate were measured by scanning electron microscopy. The crystal structure of GaN thin films and MQWs were measured by X-ray. The optical performance of MQWs was measured by photoluminescence spectra. The residual stress of GaN thin films was studied. GaN thin films on patterned sapphire substrate has a better crystalline quality in the [102] direction than that on flat sapphire substrate. The residual stress in MQWs on PSS is δ xx = 1.10 GPa, δ yy = 0.13 GPa, while the residual stress of films on FSS is δ xx = 0.83 GPa, δ yy = 0.10 GPa. It is found that the wavelength becomes shorter and the emission intensity becomes stronger on patterned sapphire substrate.

Realization of high-luminous-efficiency InGaN light-emitting diodes in the "green gap" range.

Light-emitting diodes (LEDs) in the wavelength region of 535–570 nm are still inefficient, which is known as the “green gap” problem. Light in this range causes maximum luminous sensation in the human eye and is therefore advantageous for many potential uses. Here, we demonstrate a high-brightness InGaN LED with a normal voltage in the “green gap” range based on hybrid multi-quantum wells (MQWs). A yellow-green LED device is successfully fabricated and has a dominant wavelength, light output power, luminous efficiency and forward voltage of 560 nm, 2.14 mW, 19.58 lm/W and 3.39 V, respectively. To investigate the light emitting mechanism, a comparative analysis of the hybrid MQW LED and a conventional LED is conducted. The results show a 2.4-fold enhancement of the 540-nm light output power at a 20-mA injection current by the new structure due to the stronger localization effect, and such enhancement becomes larger at longer wavelengths. Our experimental data suggest that the hybrid MQW structure can effectively push the efficient InGaN LED emission toward longer wavelengths, connecting to the lower limit of the AlGaInP LEDs’ spectral range, thus enabling completion of the LED product line covering the entire visible spectrum with sufficient luminous efficacy.

Monte Carlo simulation of hot carrier transport in III-N LEDs

Recent measurements aiming to resolve the origin of the efficiency droop in III-Nitride (III-N) light-emitting diodes (LEDs) have given invaluable information on the phenomenon and reinforced the interest for detailed device-level simulations. We have developed a microscopic device-level Monte Carlo model of electronic transport in III-N multi-quantum well (MQW) LEDs to analyze their operation in more detail and to increase the understanding of hot electrons generated by Auger recombination and electron overflow. In this work we apply the model to simulate the LED structure studied experimentally by Lin et al. (IEEE Photonics Technol Lett 24(18):1600, 2012) to compare the results to predictions given by the widely used drift-diffusion model and to investigate the relationship between the efficiency droop, Auger recombination, and the transport of hot electrons. To evaluate the reliability of the results and to enable comparison to other works, we also study how some of the most important uncertainties in the material parameters affect the results. In particular, we study how changing the polarization charges, properties of p-type GaN, and the sidevalley energy offset of the conduction band within the range reported in recent literature affects the results. Based on the results we discuss the difficulty to fit device-level models to measurements of the efficiency droop and the incomplete understanding of current transport in III-N MQW structures that might be limiting the development of next-generation optoelectronic devices.

Electroreflectance spectroscopy of compressively strained InGaN/GaN multi-quantum well structures

The built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) was investigated using the electric field dependent electroreflectance (ER) spectroscopic method. InGaN/GaN MQW structures were prepared on sapphire substrates of different thicknesses. Thinning the sapphire substrate enables control of the compressive stress by changing the curvature of the wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW. The flat band voltage, estimated by measuring the applied reverse bias voltage that induces a 180° phase shift in the ER spectra, was decreased from −11.21 V to −10.51 V by thinning the sapphire substrate thickness from 200 to 60 μm. To calculate the piezoelectric field (Fpz) from the compensation voltage, the depletion width was obtained from the capacitance–voltage measurement. The Fpz estimated from the energy shift in ER peak in a bias range from 0 to −12 V was changed by 110 kV/cm.

GeSn/Ge multiquantum well photodetectors on Si substrates.

Vertical incidence GeSn/Ge multiquantum well (MQW) pin photodetectors on Si substrates were fabricated with a Sn concentration of 7%. The epitaxial structure was grown with a special low temperature molecular beam epitaxy process. The Ge barrier in the GeSn/Ge MQW was kept constant at 10 nm. The well width was varied between 6 and 12 nm. The GeSn/Ge MQW structures were grown pseudomorphically with the in-plane lattice constant of the Ge virtual substrate. The absorption edge shifts to longer wavelengths with thicker QWs in agreement with expectations from smaller quantization energies for the thicker QWs.

Comparison of electrostatic and localized plasmon induced light enhancement in hybrid InGaN/GaN quantum wells

The light enhancement phenomena in InGaN/GaN multi-quantum wells (MQWs) infiltrated with metal nanoparticles (NPs) are studied using resonant and off-resonant localized plasmon interactions. The emission and recombination characteristics of carriers in InGaN/GaN MQW structures with inverted hexagonal pits (IHPs) are modified distinctly depending on the nature of their interaction with the metal NPs and with the pumping and emitted photons. It is observed that the emission intensity of light is significantly enhanced when the emission energy is off-resonant to the localized plasmon frequency of the metal nanoparticles. This results in enhanced emission from MQW due to Au nanoparticles and from IHPs due to Ag nanoparticles. At resonant-plasmon frequency of the Ag NPs, the emission from MQWs is quenched due to the re-absorption of the emitted photons, or due to the drift carriers from c-plane MQWs towards the NPs because of the Coulomb forces induced by the image charge effect.

Experimental and simulation study on ultraviolet light emission from quaternary InAlGaN quantum wells with localized carriers

Different-indium-content quaternary InAlGaN multi-quantum-well (MQW) structures with emission wavelength around 290–310 nm were grown by metalorganic chemical vapor deposition, and their emission characteristics including the indium-segregation effect were investigated by using not only experimental evaluation but also applying simulation technique, on the basis of the weakly localized exciton model. The value of an effective localized level in the quaternary InAlGaN MQWs was estimated to be around 70 meV from the relationship between photoluminescence lifetimes and photon energies. The simulation study, which was conducted by fitting the emission spectra, also derived the value of around 50 meV. The present study also indicated that the quaternary InAlGaN MQW structures with the indium segregation have clear advantages over ternary AlGaN MQW ones especially in the range of dislocation density larger than approximately 1 × 108 cm−2.

Spatially resolved optical emission of cubic GaN/AlN multi-quantum well structures

ABSTRACTIn this contribution we report on the optical properties of cubic AlN/GaN asymmetric multi quantum wells (MQW) structures on 3C-SiC/Si (001) substrates grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) and spatially resolved cathodoluminescence (CL) at room temperature and at low temperature are used to characterize the optical properties of the cubic AlN/GaN MQW structures. An increasing CL emission intensity with increasing film thickness due to the improved crystal quality was observed. This correlation can be directly connected to the reduction of the linewidth of x-ray rocking curves with increasing film thickness of the c-GaN films. Defects like stacking faults (SFs) on the {111} planes, which also can be considered as hexagonal inclusions in the cubic crystal matrix, lead to a decrease of the CL emission intensity. With low temperature CL line scans also monolayer fluctuations of the QWs have been detected and the observed transition energies agree well with solutions calculated using a one-dimensional (1D) Schrödinger-Poisson simulator.

INVESTIGATION OF ULTRA-WIDE REFLECTION BANDS IN UV REGION BY USING ONE-DIMENSIONAL MULTI QUANTUM WELL PHOTONIC CRYSTAL

Enhancement of the reflection bands in ultraviolet region by using one-dimensional multi quantum well (MQW) photonic crystal (PC) structure has been investigated theoretically. The proposed structure is composed of three MgF2/SrTiO3 MQWs. The range of reflection band is investigated from the reflectance spectra of the one-dimensional MQW photonic crystal structure obtained by Transfer Matrix Method (TMM). From the numerical analysis it is observed that a range of reflection band for a single MQW PC is very narrow though it increases as the thickness of layers increases. But when three MQWs of MgF2/SrTiO3 are used we get much enlarged reflection band covering the range 119.8 nm- 311.3 nm (reflectivity > 99%) with bandwidth 191.5 nm, for normal incidence. Further, we see that when the angle of incidence is increased, the width of reflection band increases in case of TE wave with a decrease for TM wave, because this omnidirectional reflection (ODR) band is very much narrow in UV region. We have computed ODR band upto incidence angle 50 ◦ for single as well as combined MQW PC. Analyzing the reflectance curve for incidence angle up to 50 ◦ for both TE and TM polarizations we find that by applying the combined MQW PC, omnidirectional reflection band increases significantly in comparison to single MQW structure. The proposed MQW photonic crystal structure is very useful in designing ultraviolet shielding for drugs, ultraviolet reflector for protecting damage of DNA and in skin diseases especially for skin cancer.

High‐responsivity AlGaN–GaN multi‐quantum well UV photodetector

AbstractIn this paper, we propose a set of AlGaN–GaN multi‐quantum well (MQW) photodetectors based on p‐i‐n heterostructures with 14 AlGaN–GaN MQW structures in i‐region, where GaN quantum well has 6 nm thickness and AlxGa1−xN barrier thickness is 3 nm. In this structure, the peak responsivity of 0.19 A/W at 246 nm is reported. In addition, we investigate effects of various parameters on responsivity, and we show that responsivity of MQW‐based photodetectors strongly depends on proper device design, that is, number of quantum wells, well thickness, barrier thickness, and mole fraction. We also show that increasing number of quantum wells, thickness of wells, and mole fraction as well as decreasing thickness of barriers, increase the responsivity. Using obtained results, we proposed optimal structure. Copyright © 2013 John Wiley & Sons, Ltd.

Microcathodoluminescence spectra evolution for planar and nanopillar multiquantum-well GaN-based structures as a function of electron irradiation dose

Effects of low energy electron beam irradiation (LEEBI) of planar and nanopillar InGaN/GaN multiquantum well light emitting diode structures are discussed. The bands observed in microcathodoluminescence (MCL) spectra were attributed to recombination involving two types of InGaN quantum dots with lower (2.92 eV MCL band) and higher (2.75 eV) indium concentration. During the LEEBI treatment, the intensity of both MCL lines first decreased, presumably due to the introduction of radiation defects, then, after the dose of 0.2 C/cm2 increased, reached a maximum and then again decreased. At the same time, the peak energy showed a red shift at low irradiation doses and a blue shift at high doses. The results are explained by an interplay between the increasing density of nonradiative recombination defects and quantum dots during irradiation. The difference between the nanopillar and planar structures is attributed to a stronger impact of surface defects in nanopillars.

Effect of external tensile stress on blue InGaN/GaN multi-quantum-well light-emitting diodes

The influence of external tensile stress on blue InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) is demonstrated. It was found that applying external tensile stress effectively compensates for the compressive strain developed in the InGaN active layer, thus reducing the quantum-confined Stark effect by attenuating the piezoelectric polarization from the InGaN layer. With 35A/cm2 of current density (∼50mA), the light output power could be improved by ∼40% when the LEDs were subjected to an external tensile stress. The blueshift in electroluminescence (EL) spectra was reduced by applying the external tensile stress. In contrast, when the LEDs were exposed to external compressive stress, the light output power intensity was decreased by ∼12% at a current density of 35A/cm2. The simulation results confirm that the relaxation of compressive strain in the InGaN/GaN MQW structure results in the reduction of the piezoelectric field and improves the overlap of electron and hole wave functions.

Optical and Magnetic Properties of Ten-Period InGaMnAs/GaAs Quantum Wells

Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.

Mechanical and electrical properties of plasma and thermal atomic layer deposited Al2O3 films on GaAs and Si

Mechanical and electrical properties of Al2O3 films are compared for plasma-assisted atomic layer deposition (ALD) and thermal ALD on two substrates, GaAs and Si, of different thermal expansion coefficient. Films with stable chemical structure and mechanical residual stress could be produced by both techniques without inducing any damage to sensitive multiquantum-well structures. However, the as-deposited residual stress in the plasma ALD Al2O3 films is lower and decreases, while that in the thermal ALD films increases with the deposition temperature. Moreover, the stress hysteresis observed upon thermal cycles is much lower for the plasma ALD films compared to that for the thermal ALD films. The biaxial elastic modulus (BEM or stiffness parameter) increases with the deposition temperature for both ALD films, being higher for the plasma ALD than that for the thermal ALD at a given temperature. The higher BEM is reflected in better electrical properties of the films. Thus, the leakage current of metal–oxide–semiconductor capacitors with the plasma ALD-Al2O3 film is three orders of magnitude lower and the breakdown voltage 20% higher than that of the capacitors with the thermal ALD film.

Growth mechanisms in semipolar [formula omitted] and nonpolar m-plane [formula omitted] AlGaN/GaN structures grown by PAMBE under N-rich conditions

The growth of multiquantum well (MQW) AlGaN/GaN structures was carried out by plasma assisted molecular beam epitaxy simultaneously on semipolar (202¯1), m-plane (101¯0) and c-plane (0001) GaN substrates under nitrogen-rich (N-rich) conditions in order to investigate the growth mechanisms on different crystal surfaces. A smooth surface with atomic steps was found only for the semipolar (202¯1) structure as opposite to m-plane and c-plane surfaces which were rough and covered by islands. Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality. At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {10-11} and {10-10} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m-plane structure showed no clear periodicity and very uneven distribution of Al. The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m-plane. The growth rate was slightly smaller for the semipolar structure and much smaller for the m-plane one, as compared to the c-plane MQW structure. This can be attributed to Ga losses from semipolar and m-plane surfaces, in contrast to c-plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed.

Effect of Growth Pressure and Gas-Phase Chemistry on the Optical Quality of InGaN/GaN Multi-Quantum Wells

ABSTRACTBlue light-emitting diodes (LED's), utilizing InGaN-based multi-quantum well (MQW) active regions deposited by organometallic chemical vapor epitaxy (OMVPE), are one of the fundamental building-blocks for current solid-state lighting applications. Studies [1,2] have previously been conducted to explore the optical and physical properties of the active MQW's over a variety of different OMVPE growth conditions. However, the conclusions of these papers have often been contradictory, possibly due to a limited data set or lack of understanding of the fundamental fluid dynamics and gas-phase chemistry that occurs during the deposition process.Multi-quantum well structures grown over a range of pressures from typical low-pressure production processes at 200 Torr, up to near-atmospheric growth conditions at 700 Torr, have been investigated in this study. At all growth pressures, clear trends of gas-phase chemical reactions are observed for increased gas residence times (lower gas speeds from the injector flange and lower rotation rates) and increased V/III ratios (higher NH3 flows).Confocal microscopy, excitation-dependent PL (PLE), and time-resolved photo-luminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Confocal emission images show spatially-separated bright and dark regions. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting microscopic spatial localization of high indium content regions. As the growth pressure and gas residence times are reduced, a larger difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities can be obtained, indicating that higher optical quality material can be realized. Optimized MQW's grown at high pressure exhibit higher PLE slope intensities and IQE characteristics than lower pressure samples. Results on simple LED structures indicate that the improvement in MQW optical quality at high pressures translates to higher output power at a 110 A/cm2 injection current density.