Nonpolar Quantum Research Articles

Exciton ionization induced by intersubband absorption in nonpolar ZnO-ZnMgO quantum wells at room temperature

We present a systematic investigation of intersubband transitions in nonintentionally doped $m\text{\ensuremath{-}}\mathrm{plane}$ ZnO/ZnMgO quantum wells (QWs). The investigation is performed using photoinduced absorption spectroscopy at room temperature under optical pumping by a UV laser to generate electron-hole pairs. All samples exhibit TM-polarized intersubbandlike absorption resonances. However, the peak transition energy is largely blueshifted (>100 meV) with expectations from electronic quantum confinement simulations. Based on calculations of the exciton binding energies, we attribute the photoinduced absorption at room temperature to the dissociation of $h{h}_{1}\text{\ensuremath{-}}{e}_{1}$ excitons towards free carriers in the ${e}_{2}$ state and not to $h{h}_{1}\text{\ensuremath{-}}{e}_{1}$ to $h{h}_{1}\text{\ensuremath{-}}{e}_{2}$ excitonic transitions induced by the intersubband absorption as previously stated by Olszakier et al. [Phys. Rev. Lett. 62, 2997 (1989)]. This effect is a consequence of the huge binding energy of excitons in the ZnO material system, which is further enhanced in QWs due to the quantum confinement. This may pave the way for a better understanding of semiconductors' excitonic processes as well as for developing intersubband devices with a blueshifted operating range.

Nonpolar Graphene Quantum Dot-Based Hydrophobic Coating from Microwave-Assisted Treatment of Styrofoam Waste

Graphene quantum dots (GQDs), one of the most promising nanoforms of carbon, have shown exceptional physiochemical properties owing to their strong three-dimensional quantum confinement. By altering the band gap or edge functionalization in GQDs, the properties can be tuned to meet challenging demands of modern computational and quantum devices. GQDs have primarily been synthesized at the lab scale so far, by both top-down and bottom-up approaches, using various natural or synthetic carbon-rich precursors. Only a few milligrams of the precursor is typically used in the synthesis, and it also involves hazardous acids, bases, or other catalysts, which impose additional time, labor, and cost for their removal. GQDs made hitherto are also functionalized with polar moieties due to the use of acids or bases in the synthesis, resulting in insolubility of GQDs in nonpolar solvents. Herein, an acid/base-free, microwave-assisted pyrolytic process is developed where GQDs of controlled size without any polar functionalities are synthesized using Styrofoam waste as the precursor. The structure of GQDs is confirmed by transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and other spectroscopic analysis, and it is found to be single layered with an average size of 5.5 ± 1.5 nm. A maximum yield of 15% is obtained by optimizing the pyrolysis temperature and exposure time of the precursor, and GQDs are found to be highly soluble in nonpolar solvents, including toluene and cyclohexane. The obtained GQDs were used as a promising agent to provide durable hydrophobicity and excellent self-cleaning property when coated on cotton fabric. The use of nonbiodegradable polymer waste such as Styrofoam to generate nonpolar GQDs of controlled dimensions provided a complete solution to not only curb deleterious effects of polymeric waste but also enable production of high value-added products for vast applications.

Decreased Fast Time Scale Spectral Diffusion of a Nonpolar InGaN Quantum Dot

Spectral diffusion can lead to considerable broadening of the line width of nitride quantum dots. Here, InGaN quantum dots grown on a nonpolar plane were shown to exhibit a decreased spectral diffusion rate compared to polar nitride dots. A robust intensity correlation method was used to measure the spectral diffusion rate of six quantum dots. A maximum spectral diffusion time of 1170 ± 50 ns was found. An increase of the rate with increasing power was observed. The decreased internal field leads to a lifetime for the nonpolar dots that is shorter than that for polar dots; the important ratio of spectral diffusion time to lifetime is more favorable for nonpolar quantum dots, thereby increasing the chances of generating indistinguishable photons.

Non-polar GaN/AlGaN quantum-well polariton laser at room temperature

We demonstrate room temperature (RT) polariton lasing in an all-dielectric microcavity containing non-polar III-nitride quantum wells (QWs) as active media. The microcavity is fabricated using the photo-electrochemical etching method, by which an optimally grown $m$-plane III-nitride active region is detached from the substrate in the form of a membrane, which is subsequently inserted between two $\mathrm{Si}{\mathrm{O}}_{2}/{\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ distributed Bragg reflectors, with 4 and 10 pairs for the top and bottom mirrors, respectively. The active region consists of 25 $\mathrm{GaN}/{\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{N}$ (5 nm/3 nm) QWs. The produced microcavities exhibit two closely spaced polarization-resolved lower polariton branches at RT, in line with the selection rules of the non-polar orientation, having a Rabi splitting of 62 and 72 meV in the $E\ensuremath{\parallel}a$ and $E\ensuremath{\parallel}c$ polarizations, respectively. In a positively detuned 3\ensuremath{\lambda}/2-thick microcavity, polariton lasing is observed at ambient conditions in the $E\ensuremath{\parallel}a$ polarization, with a threshold \ensuremath{\sim} 3 times lower than previous state of the art, despite the use of a relatively weak top reflector.

Atomistic modeling of InGaN/GaN quantum dots-in-nanowire for graded surface-emitting low-threshold, blue exciton laser

While the development of the quantum photonics field, including emitter–light coupling using photonic system, has given a lot of attention, it offers quantum control of light. However, the main challenge is coupling to strongly photonic mode localization with nanosystem accuracy. This paper highlights the numerical simulations employed by atomically spin tight-binding model of realistically sized InGaN/GaN dots-in nanowire (NW) architectures. The effects of active region size/shapes, crystal growth directions, graded surface/interface properties, strain relaxation distributions, and polarization-induced potentials on the oscillator strengths of InGaN-based lasers have been investigated. The 3D nanofocused photonic modes, which have been deterministically coupled to multiple quantum dots (QDs) through graded surface/interface technique, have been demonstrated. By using a thin graded layers of a site-controlled pyramidal QDs, the photonic nanofocusing on these QDs at the nonpolar pyramid apex has been geometrically accomplished and successfully leads to stronger characteristics in terms of exciton bandgap energy and polarized emission rate compared to its polar counterpart. For optimum coupling, the nonpolar NW, with intrinsically lower built-in field, exhibits an enhancement of the QDs emission rate as high as 0.98, which is 12% greater than that in a recently reported semipolar MQD structure. The atomistic simulated emission rate for the core QDs buried in NW structure is then incorporated into a TCAD simulator to obtain laser device characteristics. Here, the achievement of truncated pyramid–electrically injected graded surface-emitting laser by nanofocusing the photonic modes formed in InGaN NW has been reported. The nonpolar laser device operates at ~ 402 nm and exhibits a threshold current of ~ 391 A/cm2, which is lower nine order of magnitude lower compared to recently reported semipolar green laser diodes. Our model benchmarking has been done against a reported experiment of polar InGaN disks in GaN NW. Significantly, this engineering innovation proves the viability of InGaN nonpolar quantum dots-in-nanowire architecture as low threshold, high polarized, coherent blue nanoscale lasing emitters, and opens future trends toward a next-generation of electrically injected and interfacial grading-emitting nanolasers operating at high-frequency up to a GHz range.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

Reduced radiative emission for wide nonpolar III-nitride quantum wells

The radiative rate of GaInN/GaN quantum well structures on nonpolar substrates is investigated for different quantum well widths, showing a significant decrease of the radiative emission towards larger well widths. This effect can be explained by the strict selection rules that apply for radiative transitions in nonpolar structures without any polarization fields in the direction of quantization. The selection rule $\mathrm{\ensuremath{\Delta}}n=0$ reduces the number of possible radiative transitions that involve higher quantized hole states. These states will get occupied towards room temperature for wider quantum wells due to the decrease in quantization energies. Since the effective masses are strongly different in the conduction and valence bands, the thermal population of higher states is imbalanced between electrons and holes. Applying a simple model in a nondegenerate limit, we can well describe the width dependence of the experimentally determined radiative rates. At room temperature, the decrease amounts to a factor of 2--4 for nonpolar quantum wells of $8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ thickness.

Stimulus-responsive electrochemiluminescence from self-assembled block copolymer and nonpolar carbon quantum dot composite nanospheres

A self-assembled superstructure composed of block copolymers with stimulus-response properties is attractive, but how to achieve electrochemiluminescence (ECL) with good stimulus responsiveness from biocompatible materials is a big challenge. Herein, F127/NPCD nanospheres comprising the block copolymer Pluronic F127 and nonpolar carbon quantum dots (NPCDs) are designed and prepared. The materials with hydrophobic interaction show excellent ECL stimulus response. The ECL mechanism is investigated by analyzing the cyclic voltammetry, ECL, photoluminescence, and electron paramagnetic resonance results. There are two secondary structures in the F127/NPCDs nanospheres, namely a hydrophobic layer and a hydrophilic layer. The hydrophobic layer contains a polyoxypropylene PPO moiety of F127 and NPCDs, and the hydrophilic layer contains polyoxyethylene PEO of F127. The hydrophilic layer as a repository of co-reactants can adsorb a large amount of SO4•– to excite the NPCDs in the nearby hydrophobic layers to produce ECL at a gentle voltage. In nonpolar solvents, separation of the NPCDs from F127 leads to insufficient contact between NPCDs and SO4•–, and hence, the NPCDs require excitation of a larger potential to produce ECL to exhibit the stimulus response via a unique turn-on mechanism. The in vitro cytotoxicity experiments indicate that the F127/NPCDs nanospheres have good cytocompatibility. The stimulus-responsive ECL nanospheres are broadly applied as biological microelectrode materials in biological monitoring, tissue component recognizing, drug distribution, and sustained release.

Surface Nonpolarization of g-C3 N4 by Decoration with Sensitized Quantum Dots for Improved CO2 Photoreduction.

Photocatalytic conversion of CO2 can provide a solution for simultaneously addressing global warming and solar fuel generation. However, its applicability is presently limited by the unsatisfactory photoconversion efficiency of the state-of-art photocatalysts. In this regard, enhancing CO2 adsorption through surface modification could be an efficient way to improve the photoconversion efficiency. Herein, doping of nonpolar carbon quantum dots (CQDs) onto g-C3 N4 is reported for the construction of a metal-free heterojunction photocatalyst (CQDs/g-C3 N4 ). CQDs offer several advantages such as band-gap reduction and electron-withdrawing effect to improve light absorption and photocarrier separation efficiency. However, this study reveals that nonpolar CQDs could also improve CO2 adsorption, photoinduced H2 production, reaction kinetics, and alter CO2 photoreduction pathways to generate CH4 . Consequently, the CQDs/g-C3 N4 could generate six times more CO and CH4 without detectable H2 compared to pristine g-C3 N4 , under similar conditions. Therefore, this study demonstrates a promising strategy for efficient adsorption, activation, and subsequent photoreduction of CO2 by nonpolar surface modification of g-C3 N4 .

Evidence of exciton complexes in non polar ZnO/(Zn,Mg)O A-plane quantum well

Excitons complexes in ZnO/(Zn, Mg)O non-polar quantum well have been studied by using continuous-wave photoluminescence according to temperature and to the density of excitation. The spectrum of photoluminescence of the quantum well consists of three different lines. The ratio between the intensities of these lines depends of the temperature but also of the density of excitation. The lines are attributed to the free exciton, the negatively charged trion and the biexciton. The binding energy of trion and biexciton are found to be respectively 19.5 ± 2 meV and 45 ± 5 meV.

Nonpolar and semipolar ultraviolet multiple quantum wells on GaN/sapphire

The surface morphologies, anisotropic quality and optical properties of nonpolar and semipolar ultraviolet multiple quantum wells (MQWs) structures on GaN/sapphire deposited by metal organic chemical vapor deposition are investigated. The crystal planes of the MQWs are identified as nonpolar (112¯0) plane and semipolar (112¯2) plane by the 2θ−ω scan of high-resolution X-ray diffraction (HRXRD). The results of the atomic force microscope and scanning electron microscope show that the surface morphologies of nonpolar and semipolar MQWs structures have obvious differences. The X-ray rocking curves of nonpolar and semipolar MQWs on-axis diffraction planes by HRXRD demonstrate that the semipolar MQWs structure has a better crystalline quality and a weaker anisotropy of crystal quality than nonpolar MQWs structure. The temperature dependence photoluminescence (PL) spectra reveal that the nonpolar MQWs structure have a stronger yellow band than semipolar MQWs structure. Besides that, the PL spectra of nonpolar MQWs structure also show a blue-shift as temperature increases within a certain range, and then, when the temperature increases continuously, it turns to a red-shift. However, for the semipolar MQWs structure, only the red-shift is observed in PL spectra.

Influence of surface stoichiometry and quantum confinement on the electronic structure of small diameter InxGa1-xAs nanowires

Electronic structures for InxGa1-xAs nanowires with [100], [110], and [111] orientations and critical dimensions of approximately 2 nm are treated within the framework of density functional theory. Explicit band structures are calculated and properties relevant to nanoelectronic design are extracted including band gaps, effective masses, and density of states. The properties of these III-V nanowires are compared to silicon nanowires of comparable dimensions as a reference system. In nonpolar semiconductors, quantum confinement and surface chemistry are known to play a key role in the determination of nanowire electronic structure. InxGa1-xAs nanowires have in addition effects due to alloy stoichiometry on the cation sublattice and due to the polar nature of the cleaved nanowire surfaces. The impact of these additional factors on the electronic structure for these polar semiconductor nanowires is shown to be significant and necessary for accurate treatment of electronic structure properties.

Ultrafast Carrier Capture and Auger Recombination in Single GaN/InGaN Multiple Quantum Well Nanowires

Ultrafast optical microscopy is an important tool for examining fundamental phenomena in semiconductor nanowires with high temporal and spatial resolution. Here, we used this technique to study carrier dynamics in single GaN/InGaN core–shell nonpolar multiple quantum well nanowires. We find that intraband carrier–carrier scattering is the main channel governing carrier capture, while subsequent carrier relaxation is dominated by three-carrier Auger recombination at higher densities and bimolecular recombination at lower densities. The Auger constants in these nanowires are approximately 2 orders of magnitude lower than in planar InGaN multiple quantum wells, highlighting their potential for future light-emitting devices.

Reprint of: Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.

The nature of carrier localisation in polar and nonpolar InGaN/GaN quantum wells

In this paper, we compare and contrast the experimental data and the theoretical predictions of the low temperature optical properties of polar and nonpolar InGaN/GaN quantum well structures. In both types of structure, the optical properties at low temperatures are governed by the effects of carrier localisation. In polar structures, the effect of the in-built electric field leads to electrons being mainly localised at well width fluctuations, whereas holes are localised at regions within the quantum wells, where the random In distribution leads to local minima in potential energy. This leads to a system of independently localised electrons and holes. In nonpolar quantum wells, the nature of the hole localisation is essentially the same as the polar case but the electrons are now coulombically bound to the holes forming localised excitons. These localisation mechanisms are compatible with the large photoluminescence linewidths of the polar and nonpolar quantum wells as well as the different time scales and form of the radiative recombination decay curves.

Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.

Polar, semi- and non-polar nitride-based quantum dots: influence of substrate orientation and material parameter sets on electronic and optical properties

In this work we present a detailed analysis of electrostatic built-in fields, electronic and optical properties of InGaN-based quantum dots grown on different crystallographic planes. The calculations are performed by means of a symmetry adapted \(\mathbf {k}\cdot \mathbf {p}\) model. Special attention is paid to the influence of different effective mass and deformation potential parameter sets on the results. Our analysis reveals that the built-in potential profile is strongly dependent on the growth plane. These changes in the built-in potential affect the electronic structure and therefore the optical properties of semi-polar InGaN quantum dots significantly. For instance, while we observe a clear spatial separation of electron and hole ground state wave functions for quantum dots grown on the \((10\bar{1}3)\)-plane, for the \((20\bar{2}1)\)-plane our results indicate a strong spatial overlap. Furthermore, we show that the calculation of the degree of optical linear polarization in the considered semipolar InGaN quantum dot systems significantly depends on the chosen material parameter set for substrate incline angles of \(0^\circ <\theta <58^\circ\). For instance, for growth on the \((10\bar{1}3)~(\theta =32^\circ)\)-plane, the degree of optical linear polarization changes from 90 to 10 % when changing the input material parameter set.

Radiative and nonradiative recombination mechanisms in nonpolar and semipolar GaInN/GaN quantum wells

AbstractVia temperature‐dependent time‐resolved photoluminescence spectroscopy, we investigate the radiative and nonradiative recombination processes in thin (quantum well widths of about 1.5 nm) a‐plane, m‐plane, () and () GaInN/GaN fivefold quantum well structures of varying indium content grown on low defect density GaN substrates and GaN templates. At room temperature, we observe surprisingly short radiative lifetimes in the range from 100 ps to 1 ns for these structures, being about one to two orders of magnitude shorter than for similar c‐plane quantum wells. This large difference cannot solely be explained by the larger overlap matrix element in non‐ and semipolar wells. Higher exciton binding energies and lower effective density of states masses may contribute to an enhanced radiative probability. The nonradiative recombination exhibits a thermally activated behavior with activation energies of about 10 meV for () and around 25 meV for nonpolar quantum wells. These values are lower than the quantum well barrier height and the exciton binding energy, but in a similar range as the localization energies estimated from the radiative recombination.

Macrocrystals of Colloidal Quantum Dots in Anthracene: Exciton Transfer and Polarized Emission.

In this work, centimeter-scale macrocrystals of nonpolar colloidal quantum dots (QDs) incorporated into anthracene were grown for the first time. The exciton transfer from the anthracene host to acceptor QDs was systematically investigated, and anisotropic emission from the isotropic QDs in the anthracene macrocrystals was discovered. Results showed a decreasing photoluminescence lifetime of the donor anthracene, indicating a strengthening energy transfer with increasing QD concentration in the macrocrystals. With the anisotropy study, QDs inside the anthracene host acquired a polarization ratio of ~1.5 at 0° collection angle, and this increases to ~2.5 at the collection angle of 60°. A proof-of-concept application of these excitonic macrocrystals as tunable color converters on light-emitting diodes was also demonstrated.

Optical properties of plasmonic light-emitting diodes based on flip-chip III-nitride core-shell nanowires.

In this work, we utilize the finite difference time domain (FDTD) method to investigate the Purcell factor, light extraction efficiency (EXE), and cavity quality parameter (Q), and to predict the modulation response of Ag-clad flip-chip GaN/InGaN core-shell nanowire light-emitting diodes (LEDs) with the potential for electrical injection. We consider the need for a pn-junction, the effects of the substrate, and the limitations of nanoscale fabrication techniques in the evaluation. The investigated core-shell nanowire consists of an n-GaN core, surrounded by nonpolar m-plane quantum wells, p-GaN, and silver cladding layers. The core-shell nanowire geometry exhibits a Purcell factor of 57, resulting in a predicted limit of 30 GHz for the 3dB modulation bandwidth.