Thick Quantum Wells Research Articles

The role of surface diffusion and wing tilt in the formation of localized stacking faults in high In-content InGaN MQW nanostructures

Yellow and green emitting multiple quantum well structures are grown on nanostripe templates with {10-11} facets. SEM and cathodoluminescence measurements show a correlation between rough surface morphology near the bottom of the stripes and non-radiative recombination centers. Transmission electron microscopy (TEM) analysis shows that these surface instabilities are a result of stacking faults generated from the quantum well (QW) regions near the bottom of the pyramid that propagate to the surface. HRTEM images show that the stacking faults are I1 type which is formed by removal of one half basal plane to relieve the compressive strain in the InGaN QW. Thicker QWs near the bottom as a result of growth rate enhancement due to the surface diffusion of the precursors from the mask regions cause increased strain. Additionally, the compressive strain induced by the bending of the nanostructure towards the growth mask further increases the strain experienced by the QW thereby causing the localized defect generation.

Nanoscopic Insights into InGaN/GaN Core-Shell Nanorods: Structure, Composition, and Luminescence.

Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.

Structural and optical properties of (112̅2) InGaN quantum wells compared to (0001) and (112̅0)

We benchmarked growth, microstructure and photo luminescence (PL) of (112̅2) InGaN quantum wells (QWs) against (0001) and (112̅0). In incorporation, growth rate and the critical thickness of (112̅2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112̅0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112̅0). The slight deviation of (112̅2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112̅2) and (0001) while (112̅0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about lower on (112̅2) and more than lower on (112̅0).

Polarization-induced confinement of continuous hole-states in highly pumped, industrial-grade, green InGaN quantum wells

We investigate industrial-grade InGaN/GaN quantum wells (QWs) emitting in the green spectral region under high, resonant pumping conditions. Consequently, an ubiquitous high energy luminescence is observed that we assign to a polarization field Confined Hole Continuum (CHC). Our finding is supported by a unique combination of experimental techniques, including transmission electron microscopy, (time-resolved) photoluminescence under various excitation conditions, and electroluminescence, which confirm an extended out-of-plane localization of the CHC-states. The larger width of this localization volume surpasses the QW thickness, yielding enhanced non-radiative losses due to point defects and interfaces, whereas the energetic proximity to the bulk valence band states promotes carrier leakage.

Influence of hydrostatic pressure on the built-in electric field in ZnO/ZnMgO quantum wells

We used high hydrostatic pressure to perform photoluminescence measurements on polar ZnO/ZnMgO quantum well structures. Our structure oriented along the c-direction (polar direction) was grown by plasma-assisted molecular beam epitaxy on a-plane sapphire. Due to the intrinsic electric field, which exists in polar wurtzite structure at ambient pressure, we observed a red shift of the emission related to the quantum-confined Stark effect. In the high hydrostatic pressure experiment, we observed a strong decrease of the quantum well pressure coefficients with increased thickness of the quantum wells. Generally, a narrower quantum well gave a higher pressure coefficient, closer to the band-gap pressure coefficient of bulk material 20 meV/GPa for ZnO, while for wider quantum wells it is much lower. We observed a pressure coefficient of 19.4 meV/GPa for a 1.5 nm quantum well, while for an 8 nm quantum well the pressure coefficient was equal to 8.9 meV/GPa only. This is explained by taking into account the pressure-induced increase of the strain in our structure. The strain was calculated taking in to account that in-plane strain is not equal (due to fact that we used a-plane sapphire as a substrate) and the potential distribution in the structure was calculated self-consistently. The pressure induced increase of the built-in electric field is the same for all thicknesses of quantum wells, but becomes more pronounced for thicker quantum wells due to the quantum confined Stark effect lowering the pressure coefficients.

Room-temperature CW operation of a nitride-based vertical-cavity surface-emitting laser using thick GaInN quantum wells

We demonstrated a room-temperature (RT) continuous-wave (CW) operation of a GaN-based vertical-cavity surface-emitting laser (VCSEL) using a thick GaInN quantum well (QW) active region and an AlInN/GaN distributed Bragg reflector. We first investigated the following two characteristics of a 6 nm GaInN 5 QWs active region in light-emitting diode (LED) structures. The light output power at a high current density (∼10 kA/cm2) from the 6 nm GaInN 5 QWs was the same or even higher than that from standard 3 nm 5 QWs. In addition, we found that hole injection into the farthest QW from a p-layer was sufficient. We then demonstrated a GaN-based VCSEL with the 6 nm 5 QWs, resulting in the optical confinement factor of 3.5%. The threshold current density under CW operation at RT was 7.5 kA/cm2 with a narrow (0.4 nm) emission spectrum of 413.5 nm peak wavelength.

Band gaps and built-in electric fields in InAlN/GaN short period superlattices: Comparison with (InAlGa)N quaternary alloys

The general trends in the behavior of the band gaps in short period superlattices (SLs) composed from InAlN and GaN layers and grown along the wurtzite $c$ axis have been analyzed for different alloy compositions and several thicknesses of quantum wells and barriers. The composition dependence of the gaps is compared to that of the quaternary alloys $\mathrm{I}{\mathrm{n}}_{y}\mathrm{A}{\mathrm{l}}_{1\ensuremath{-}x\ensuremath{-}y}\mathrm{G}{\mathrm{a}}_{x}\mathrm{N}$ with the same effective contents of In, Al, and Ga as the SLs. The differences in the gaps are explained mainly by the built-in electric fields in the SLs caused by spontaneous and piezoelectric polarizations. Other factors, such as internal strain caused by lattice mismatch between wells and barriers and wave function hybridization, are taken into account. A simplified approach to determine the built-in electric fields directly from the dielectric properties of the constituent materials has been applied and verified by comparison to ab initio calculations for selected SLs. The calculated band gaps are compared with existing experimental data.

(Invited) Photocurrents in Topological Insulators Based on Hgte

The paper overviews experimental and theoretical studies of photocurrents induced in various Dirac fermions systems by polarized electromagnetic radiation. We consider second order opto-electronic phenomena [1,2] excited by terahertz radiation in topological insulators (TIs). In focus of our discussion are two- (2D) and three- (3D) dimensional HgTe-based topological TIs [3,4]. We present the state-of-the-art of the experiments, discuss the phenomenological and microscopic theory, and show that nonlinear transport excited by laser radiation opens up new opportunities for the probing of Dirac electrons. A particular attention is paid to the helicity sensitive edge photogalvanic currents in 2D TIs [5] and cyclotron resonance assisted spin polarized currents in 3D TIs [6] and HgTe/HgCdTe quantum wells of a critical thickness [7,8]. We demonstrate that cyclotron resonance induced photocurrents in the surface states of 3D TIs provides a sensitive method to probe the cyclotron masses of Dirac fermions, which can be applied even for micrometer sized structures, to measure Fermi velocity, and to study tiny details of the spin orientation in TIs, in particular, to explore the role of bulk and structure inversion asymmetry in the band structure. [1] S.D. Ganichev and W. Prettl, Intense Terahertz Excitation of Semiconductors, Oxford University Press (2006) [2] M.M. Glazov and S.D. Ganichev, Physics Reports 535 101-138 (2014) [3] B.A. Bernevig, et al., Science 314, 1757 (2006) [4] M.S. König, et al., Science 318, 766 (2007) [5] Z.D. Kvon, et al., JETP Letters 99, 290-294 (2014) [6] K.-M. Dantscher, et al., Phys. Rev. B 92, 165314 (2015) [7] P. Olbrich, et al., Phys. Rev. B 87, 235439 (2013) [8] C. Zoth, et al., Phys. Rev. B 90, 205415 (2014)

Exciton recombination in spontaneously formed and artificial quantum wells AlxGa1‐xN/AlyGa1‐yN (x<y∼0.8)

AbstractWe report on photoluminescence (PL) spectroscopy and electron microscopy studies of an AlGaN quantum well (QW) structure grown by molecular beam epitaxy under metal‐rich conditions with substrate rotation. Both techniques reveal unintentional formation within the AlGaN barriers of a quasiperiodic structure of thin Ga‐rich layers, whose period is controlled by both the substrate rotation rate and the AlGaN growth rate. These compositional modulations act as 1‐3 monolayer thick QWs emitting below 250 nm with an internal quantum efficiency (IQE) as high as ∼30% at room temperature under weak excitation. Variational calculations of the QW exciton properties indicate that the observed high IQE can result from strong three‐dimensional localization of the excitons confined in the narrow QWs. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of indium gallium nitride facet-dependent nonpolar growth rates and composition for core–shell light-emitting diodes

Core-shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longi- tudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emit- ting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both f11-20g a-plane and f10-10g m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were inves- tigated by scanning electron microscopy, transmission electron microscopy, and cathodolumi- nescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the f10-10g m-plane facets, even at a reduced growth rate. The sample grown at 750°C and 100 mbar had an InN mole fraction of 25% on the f10-10g facets of the NRs. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the origi-

Polarization field screening in thick (0001) InGaN/GaN single quantum well light-emitting diodes

We demonstrate through simulation that complete screening of polarization-induced electric fields in c-plane InGaN/GaN quantum wells (QWs) is possible by equal n- and p-doping of 10 nm layers immediately adjacent to the QW at a level of 7 × 1019 cm−3. Single QW light-emitting diodes with varying QW thickness are grown using the simulated structure. Biased photoluminescence (PL) measurements show no wavelength shift, indicating complete screening of the polarization field. The behavior of PL peak intensity as a function of bias can be explained as a competition between radiative recombination and carrier escape through tunneling or thermionic emission.

Quantitative parameters for the examination of InGaN QW multilayers by low-loss EELS.

We present a detailed examination of a multiple InxGa1-xN quantum well (QW) structure for optoelectronic applications. The characterization is carried out using scanning transmission electron microscopy (STEM), combining high-angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS). Fluctuations in the QW thickness and composition are observed in atomic resolution images. The impact of these small changes on the electronic properties of the semiconductor material is measured through spatially localized low-loss EELS, obtaining band gap and plasmon energy values. Because of the small size of the InGaN QW layers additional effects hinder the analysis. Hence, additional parameters were explored, which can be assessed using the same EELS data and give further information. For instance, plasmon width was studied using a model-based fit approach to the plasmon peak; observing a broadening of this peak can be related to the chemical and structural inhomogeneity in the InGaN QW layers. Additionally, Kramers-Kronig analysis (KKA) was used to calculate the complex dielectric function (CDF) from the EELS spectrum images (SIs). After this analysis, the electron effective mass and the sample absolute thickness were obtained, and an alternative method for the assessment of plasmon energy was demonstrated. Also after KKA, the normalization of the energy-loss spectrum allows us to analyze the Ga 3d transition, which provides additional chemical information at great spatial resolution. Each one of these methods is presented in this work together with a critical discussion of their advantages and drawbacks.

Two-color detection with charge sensitive infrared phototransistors

Highly sensitive two-color detection is demonstrated at wavelengths of 9 μm and 14.5 μm by using a charge sensitive infrared phototransistor fabricated in a triple GaAs/AlGaAs quantum well (QW) crystal. Two differently thick QWs (7 nm- and 9 nm-thicknesses) serve as photosensitive floating gates for the respective wavelengths via intersubband excitation: The excitation in the QWs is sensed by a third QW, which works as a conducting source-drain channel in the photosensitive transistor. The two spectral bands of detection are shown to be controlled by front-gate biasing, providing a hint for implementing voltage tunable ultra-highly sensitive detectors.

Optical and magnetotransport properties of InGaAs/GaAsSb/GaAs structures doped with a magnetic impurity

InGaAs/GaAsSb/GaAs bilayer quantum-well structures containing a magnetic-impurity δ-layer (Mn) at the GaAs/InGaAs interface are experimentally studied for the first time. The structures are fabricated by metal organic chemical-vapor deposition (MOCVD) and laser deposition on substrates of conducting (n+) and semi-insulating GaAs in a single growth cycle. The InGaAs-layer thickness is varied from 1.5 to 5 nm. The significant effect of a decrease in the InGaAs quantum-well thickness on the optical and magnetotransport properties of the structures under study is detected. Nonlinear magnetic-field dependence of the Hall resistance and negative magnetoresistance at temperatures of ≤30–40 K, circular polarization of the electroluminescence in a magnetic field, opposite behaviors of the photoluminescence and electroluminescence emission intensities in the structures, and an increase in the contribution of indirect transitions with decreasing InGaAs thickness are observed. Simulation shows that these effects can be caused by the influence of the δ-layer of acceptor impurity (Mn) on the band structure and the hole concentration distribution in the bilayer quantum well.

Optical properties of strain-compensated CdSe/ZnSe/(Zn,Mg)Se quantum well microdisks.

Strain-compensated CdSe/ZnSe/(Zn,Mg)Se quantum well structures that were grown on (In,Ga)As allow for efficient room-temperature photoluminescence and spectral tuning over the whole visible range. We fabricated microdisk cavities from these samples by making use of a challenging chemical structuring technique for selective and homogeneous removal of the (In,Ga)As sacrificial layer below the quantum structure. The observed whispering gallery modes in our microdisks are mainly visible up to photon energies of ~ 2.3 eV due to strong self-absorption. As extinction coefficients and effective refractive indices are dominated by the quantum well material CdSe, thick quantum wells (> 3 monolayer) are necessary to observe resonances in the corresponding quantum well emission.

Variation of efficiency droop with quantum well thickness in InGaN/GaN green light-emitting diode**Project supported by the National Natural Science Foundation of China (Grant Nos. 61574135, 61574134, 61474142, 61474110, 61377020, 61376089, 61223005, and 61321063), the One-Hundred Person Project of the Chinese Academy of Sciences, the Basic Research Project of Jiangsu Province, China (Grant No. BK20130362), the Scientific Research Fund of Chongqing

InGaN/GaN multiple quantum well (MQW) green light-emitting diodes (LEDs) with varying InGaN quantum well layer thickness are fabricated and characterized. The investigation of luminescence efficiency versus injection current reveals that several physical mechanisms may jointly influence the efficiency droop, resulting in a non-monotonic variation of droop behavior with increasing quantum well (QW) thickness. When the QW is very thin, the increase of InGaN well layer thickness makes the efficiency droop more serious due to the enhancement of polarization effect. When the QW thickness increases further, however, the droop is alleviated significantly, which is mainly ascribed to the enhanced nonradiative recombination process and the weak delocalization effect.

Nanoscale cathodoluminescene imaging of III‐nitride‐based LEDs with semipolar quantum wells in a scanning transmission electron microscope

The optical and crystalline properties of a c‐plane GaN‐based LED structure with embedded semipolar InGaN quantum wells (QW) were investigated using highly spatially resolved cathodoluminescence spectroscopy (CL) directly performed in a scanning transmission electron microscope (STEM). Direct correlation of the cross‐sectional STEM image with the simultaneously recorded spatially resolved CL mapping at room‐temperature reveals the most intense emission coming from the semipolar InGaN QWs. We observe an inhomogeneous wavelength distribution due to local indium fluctuations and varying QW thickness. In contrast, the donor‐acceptor pair recombination (DAP) becomes the dominating luminescence process at 16 K resulting in a superposition of the DAP luminescence and the InGaN QW emission.

Fabrication of InAs quantum dot stacked structure on InP(311)B substrate by digital embedding method

Self-assembled InAs quantum dots (QDs) grown on an InP(311)B substrate were embedded using lattice-matched InAlAs/InGaAs superlattice with the digital embedding method. The thickness of quantum wells and barriers of the superlattice varied from 2 to 16 monolayers. The six layer stacking structures were successfully grown without any degradation of the QD and superlattice structure. The cross-sections of QDs embedded within the superlattice were visualized by scanning transmission microscope. The emission wavelength of the QDs was measured by photoluminescence and could be changed by changing the thickness of the superlattice.

Structure dependence of oscillation characteristics of resonant-tunneling-diode terahertz oscillators associated with intrinsic and extrinsic delay times

We investigate the effect of intrinsic and extrinsic delay times on the oscillation characteristics of resonant-tunneling-diode (RTD) terahertz oscillators. The intrinsic delay time is composed of the electron dwell time in the resonant tunneling region and the electron transit time in the collector depletion region. We obtain and discuss the structure dependence of these factors in terms of the oscillation frequency and output power measured for RTD oscillators with different quantum-well and collector-spacer thicknesses and different air-bridge widths between the RTD and a slot antenna. The highest oscillation frequency achieved in this experiment is 1.86 THz for the well and spacer thicknesses of 2.5 and 12 nm, respectively, with a 1-µm-wide air bridge. In this structure, the extrinsic delay time (80 fs) estimated from the parasitic elements is more than double the intrinsic delay time (35 fs). It is shown theoretically that an oscillation frequency of over 2 THz is possible upon the reduction in the extrinsic delay time caused by the bulk and spread resistances in RTDs.

Investigation of optical and electrical properties of GaN-based blue light-emitting diodes with various quantum well thicknesses

Optical and electrical properties of gallium nitride (GaN)-based blue light-emitting diodes (LEDs) with various indium gallium nitride (InGaN) quantum well (QW) thicknesses were investigated. As the QW thickness was increased, the light output power of GaN-based LEDs also increased. The increase can be attributed to the increase in the carrier radiative recombination rate in the active region. However, the turn-on voltages of these fabricated LEDs are different. This was attributed to the increase in the polarization field with increasing QW thickness. In regard to the hot/cold factor, LEDs with a thicker QW achieved better performance at a low-injection current owing to the lower defect density. The hot/cold factor at a high-injection current would be mainly influenced by the efficiency droop mechanism.