Sidewall Quantum Wells Research Articles

Control of the 3-Fold Symmetric Shape of Group III-Nitride Quantum Dots: Suppression of Fine-Structure Splitting.

Controlling the in-plane symmetry of wide-bandgap semiconductor quantum dots (QDs) is essential for room temperature quantum photonic applications using polarization entangled photon pairs. Herein, we report the formation of 3-fold symmetric group III-nitride QDs at the apex of a triangular pyramid via a self-limited growth mechanism. We employed the in-plane rotational symmetry of the c-plane of a Wurtzite crystal and the large built-in piezoelectric field to reduce fine-structure splitting. The QDs exhibit emission that is distinguishable from that of sidewall quantum wells, and the biexciton-exciton cascade possesses a single-photon nature. We observed the relatively low optical polarization anisotropy and small fine structure splitting under the measurement limit (270 μeV) with the 3-fold symmetric QD. In contrast with current strategies that consider group III-nitride QDs as strongly polarized single-photon emitters, our approach for controlling the QD symmetry provides a new perspective on such QDs, as polarization-entangled photon pairs.

Controlled bunching approach for achieving high efficiency active region in AlGaN-based deep ultraviolet light-emitting devices with dual-band emission

Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.

Numerical Simulation on Electroluminescent property of GaN Nanorod LED array

The quantum well thickness and In concentration of the multi-section nanorod array structure increase with the sidewall height. At present, a two-section nanorod structure with a 466 nm spectrum at the bottom of the nanorod and a 488 nm spectrum at the top has been prepared. In this paper, according to the theory of yellow-blue light mixing to achieve white light, we have proposed a structure of the two-section nanorod, with a white LED model emitting a blue light spectrum at the bottom of the nanorod and a yellow light spectrum at the top. By varying the In concentration in the structure, the spectra of different In concentrations in different positions of the sidewall quantum wells were simulated to obtain the blue light in the sidewall quantum wells. We observed that the blue light of In cooperation range are 0.15∼0.22, the yellow light of In cooperation are 0.27∼0.34, and the corresponding wavelengths varied from 443nm to 498nm and 542nm to 610 nm. Therefore, the proposed structure will be a potential candidate in phosphor-free white-color emission LED.

Dependencies of the emission behavior and quantum well structure of a regularly-patterned, InGaN/GaN quantum-well nanorod array on growth condition.

To achieve green emission from the sidewall non-polar quantum wells (QWs) of a GaN nanorod (NR) light-emitting diode, regularly patterned InGaN/GaN QW NR arrays are grown under various growth conditions of indium supply rate, QW growth temperature, and QW growth time for comparing their emission wavelength variations of the top-face c-plane and sidewall m-plane QWs based on photoluminescence and cathodoluminescence (CL) measurements. Although the variation trends of QW emission wavelength by changing those growth conditions in the NR structure are similar to those in the planar structure, the emission wavelength range of the QWs on an NR is significantly shorter than that in a planar structure under the same growth conditions. Under the growth conditions for a longer NR QW emission wavelength, the difference of emission wavelength between the top-face and sidewall QWs is smaller. Also, the variation range of the emission wavelength from the sidewall QWs over different heights on the sidewall becomes larger. On the other hand, strain state analysis based on transmission electron microscopy is undertaken to calibrate the average QW widths and average indium contents in the two groups of QW of an NR. The variation trends of the calibrated QW widths and indium contents are consistent with those of the CL emission wavelengths from various portions of NR QWs.

Electroluminescence from the sidewall quantum wells in the V-shaped pits of InGaN light emitting diodes

InGaN/GaN multi-quantum well (MQW) light emitting diodes with heavily Mg doped and unintentionally doped (UID) low-temperature Al0.2Ga0.8N electron blocking layer (EBL) were investigated. Broad short-wavelength electroluminescence peak, which has strong relative intensity to the main emission, was found in the UID-EBL sample at cryogenic temperatures. Study suggests that the broad peak is emitted by the sidewall MQWs. This result indicates that the electroluminescence of sidewall MQWs, in which the carrier density is high enough, can be detected at cryogenic temperatures. The lineshape variation with current density reveals detailed information on the process of carrier injection into the sidewall MQWs.

On the origin of blue‐green emission from heteroepitaxial nonpolar a‐plane InGaN quantum wells

AbstractThe optical and structural properties of nonpolar a‐plane InGaN/GaN multiple quantum wells grown on r‐plane sapphire substrates were investigated as a function of defect density. The photoluminescence spectrum is characterised by a peak at 380 nm (3.24 eV) and a blue‐green emission band from 425‐550 nm (2.25‐3.10 eV). The near‐UV peak is assigned to carrier recombination in the quantum wells lying on the a‐plane. On the basis of microscopy, photoluminescence spectroscopy and cathodoluminescence imaging, the blue‐green emission band is attributed to emission from sidewall quantum wells formed on the various semipolar facets of small (∼100 nm) surface pits which form during quantum well growth at the lower temperatures and high NH3 flow in a nitrogen atmosphere. The density of the surface pits equals that of partial dislocations and threading dislocations in the quantum wells structure. Thus, the 300 K photoluminescence spectrum is dominated by the blue‐green emission band at a high defect density (>1010 cm‐2) and its intensity is reduced with lower defect densities. For a spectrum with a single emission from the nonpolar a‐plane InGaN/GaN quantum wells the defect density needs to be below ∼109 cm‐2. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Effect of Dislocation Density and Surface Morphology on the Optical Properties of InGaN/GaN Quantum Wells Grown on r-Plane Sapphire Substrates

The optical properties of non-polar InGaN/GaN multiple quantum wells grown on r-plane sapphire substrates are investigated as a function of threading dislocation density. The 6 K emission spectrum consists of a peak at 3.25 eV and a broad band centred around 2.64 eV. From microscopy and cathodoluminescence studies, the higher energy peak is assigned to recombination within quantum wells lying on the (1120) plane which are intersected by basal-plane stacking faults. The lower energy band is attributed to emission from sidewall quantum wells of varying width and composition which form on the various semi-polar facets of structural defects that develop during the quantum well growth.

Do Sidewall Quantum Wells Exist in GaInN-Based Light-Emitting Diodes?

Sidewall quantum wells in the vicinity of hexagonal V-shaped pits with a reduced thickness and a higher band gap than the regular c-plane quantum wells were suggested as the origin of high luminescence efficiency in GaInN-based light-emitting diodes. In this Brief Note, by transmission electron microscopy combined with energy dispersive x-ray analysis, it was found that there must be no sidewall quantum wells around the V-shaped pits, which are just interference fringes arising from lattice strain.

Dislocation screening and strongly increased internal quantum efficiency in heteroepitaxialGaN/AlxGa1−xNultraviolet-emitting quantum wells

While today's ${\text{Ga}}_{x}{\text{In}}_{1\ensuremath{-}x}\text{N}$-based blue-light-emitting quantum wells exhibit internal quantum efficiencies in excess of 60%, ultraviolet structures at wavelengths shorter than 360 nm still are limited to efficiencies of a few percent. Recently, it was demonstrated that antilocalization of charge carriers due to energy barriers arising from sidewall quantum wells in V-shaped pits decorating dislocations is a key to understanding the high efficiency of ${\text{Ga}}_{x}{\text{In}}_{1\ensuremath{-}x}\text{N}$-based quantum wells. Here we show that the unexpected presence of V-shaped pits in $\text{GaN}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ quantum wells leads to a dramatically increased internal efficiency of about 30%. In contrast, the efficiency of quantum wells with flat interfaces in the vicinity of dislocations remains very low.

Anti-localization suppresses non-radiative recombination in GaInN/GaN quantum wells

The light emission efficiency of (AlGaIn)N heterostructures and light-emitting diodes is exceptionally high, despite the high density of threading dislocations generally found in such structures. It has become common to attribute the high efficiency to compositional fluctuations or even phase separation in the active GaInN quantum well region. The resulting localization of charge carriers is thought to keep them from recombining non-radiatively at the defects. Here, we show that carriers are mobile at room temperature rather than localized and that under suitable growth conditions hexagonal V-shaped pits decorating the defects exhibit narrow sidewall quantum wells with an effective bandgap significantly larger than that of the regular c-plane quantum wells. Thereby nature provides a unique, hitherto unrecognized mechanism generating a potential landscape which effectively screens the defects themselves by providing an energy barrier around every defect. Thus even for mobile charge carriers non-radiative recombination is effectively suppressed leading to the unexpectedly high emission efficiency.

Suppression of Nonradiative Recombination by V-Shaped Pits inGaInN/GaNQuantum Wells Produces a Large Increase in the Light Emission Efficiency

Despite the high density of threading dislocations generally found in (AlGaIn)N heterostructures, the light emission efficiency of such structures is exceptionally high. It has become common to attribute the high efficiency to compositional fluctuations or even phase separation in the active GaInN quantum well region. The resulting localization of charge carriers is thought to keep them from recombining nonradiatively at the defects. Here, we show that random disorder is not the key but that under suitable growth conditions hexagonal V-shaped pits decorating the defects exhibit narrow sidewall quantum wells with an effective band gap significantly larger than that of the regular c-plane quantum wells. Thereby nature provides a unique, hitherto unrecognized mechanism generating a potential landscape which effectively screens the defects themselves by providing an energy barrier around every defect.

Photoconductive characteristics in a trench-type InGaAs quantum-wire field effect transistor

We describe the photoconductive characteristics of a narrow trench-type InGaAs quantum wire field effect transistor (QWR-FET) with a negative differential resistance (NDR). The photoconductive intensity peak of the trench-type QWR was observed at a drain voltage (VDS) region lower than an onset voltage of NDR (VNDR), and decreased at the VDS larger than the VNDR. This result indicates that the carriers transfer from the QWR level to other quantum levels at the VDS region larger than the VNDR. By numerically solving the two-dimensional single band Schrödinger equation using the finite element method, we confirm that the carrier can transfer from the high mobility QWR layer to the low mobility sidewall quantum wells, which is thought to be a mechanism of the negative differential resistance of the QWR-FETs.

Determination of carrier-transfer length from side-wall quantum well to quantum wire by micro-photoluminescence scanning

Micro-photoluminescence (μ-PL) line scanning across a single V-groove, GaAs/AlGaAs quantum wire (QWR) has been performed at room temperature, revealing a clear spatial-dependence of the PL. After fitting each PL spectrum by multi-Gaussian line shapes, intensity profiles of each PL component from confined structures have been obtained as functions of the scanning position. The PL quenching of a side-wall quantum well (SQWL) has been recognized in a certain area in the vicinity of the QWR and is interpreted by carrier transfer into the QWR within effective transfer length. By simulating the carrier-transfer process from SQWL to QWR as a convolution of a step function for carrier distribution and a Gaussian function for exciting laser irradiance, the effective transfer length of about 1.8±0.3 µm has, therefore, been concluded.

Growth and Characterization of Triangular InGaAs/GaAs Quantum Wire Structures Grown by Low-Pressure Metalorganic Chemical Vapor Deposition

Noble quantum wire structures were grown by low-pressure metalorganic chemical vapor deposition (MOCVD). Triangular InGaAs/GaAs quantum wire structures with sharp tips and smooth sidewalls were grown on SiO2 masked GaAs substrates. To characterize and analyze the selectively grown structures, scanning electron microscopy and temperature-dependent photoluminescence were used. Even though the opening width was different, it was possible to obtain a similar triangular structure. If we use this quantum confined structure of different size, it would be possible to grow quantum wire structures with various thicknesses and fabricate uniquely integrated optical devices which include light-emitting sources of multiple wavelength. The emission peak from quantum wires was observed at 975 nm. With increasing temperature, the emission intensity from side wall quantum wells decreased abruptly. However, the intensity from quantum wires decreased slowly compared to that from side wall quantum wells and it became even stronger above 50 K. The possible carrier capture processes in the quantum wire structures were also discussed.

Electron Transport Properties in a GaAs/AlGaAs Quantum Wire Grown on V-Grooved GaAs Substrate by Metalorganic Vapor Phase Epitaxy

A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (QWR) field effect transistors (FETs) prepared by the flow-rate modulated epitaxy (FME) technique. These are attributed to Coulomb blockade at a subthreshold gate bias, universal conductance steps near the threshold, real space transfer under a forward gate bias and a large source-drain bias condition. Weak carrier coupling between sidewall quantum wells and QWR is responsible for the small step height of the measured universal conductance (Gm≪G0=2e2/h). Shubnikov de Haas oscillation measurements revealed that sidewall quantum wells in the V-groove quantum wire act as additional current paths and are switched or mixed with QWR depending on the gate bias conditions and device geometry. The gate-bias-dependent current path switching is found to be responsible for the large current steps and negative differential resistance (NDR) in the drain current (IDS)-gate bias (VGS) and IDS-drain bias (VDS) relationships, respectively.

Evidence of blocking effect on carrier trapping process by necking region in very narrow AlGaAs/GaAs V-grooved quantum wire structure

Transient band-gap renormalization (BGR) effects are investigated in AlGaAs/GaAs V-grooved quantum structures. The temperature-dependent transient BGR effects in the sidewall quantum well (SQWL) provide direct evidence of the existence of the blocking effect by the necking region barrier on the carrier trapping process. These effects provide a useful method to show the existence of the necking region, particularly for very thin SQWL structures. The temperature-dependent lifetimes of the SQWL and quantum wire (QWR) provide further proof of the carrier trapping process from the SQWL to the QWR.

Origin of multiple peak photoluminescence in InGaN/GaN multiple quantum wells

Optical spectroscopy has been performed for a set of In0.12Ga0.88N/GaN multiple quantum wells (MQW) grown by metalorganic vapor phase epitaxy at 820 °C. Time-resolved, temperature- and power-dependent photoluminescence as well as spatially-resolved cathodoluminescence measurements have been applied to elucidate the nature of the recombination mechanisms responsible for the radiative transitions in the samples. The photoluminescence spectra in this set of samples are dominated by strong multiple peak emissions associating with both confined levels of the MQW system (the higher energy band) and with strongly localized states of energies much lower than the QW band gap. We suggest that the photoluminescence originate from (i) the MQW exciton recombination, (ii) excitons localized in the quasidot regions with indium concentrations higher than in the alloy due to segregation processes, and (iii) from localized states in zero-dimensional quantum islands created by surface defects such as pits and V defects. Buried side-wall quantum wells caused by V defects might also influence the photoluminescence spectra.

THE MICRO-PHOTOLUMINESCENCE OF A SINGLE V-GROOVE GaAs/AlGaAs QUANTUM WIRE

The micro-photoluminescence(μ-PL) scan has been performed on a single V-groove GaAs/AlGaAs quantum wire in the direction perpendicular to the wire.The variatio n of μ-PL spectra from various quantum structures with different spatial positi ons has been observed.In the region of quantum wire (QWR) the PL spectra contain the peaks from QWR,necking quantum well (NQWL) and vertical quantum well (VQWL) ,while in the area about 1μm and farther away from the QWR the PL spectra show the peaks from side-wall quantum well (SQWL) and top quantum well (TQWL).All the scanned PL spectra have been fitted by Gauss line shape.The asymmetric PL peaks of QWR and SQWL were decomposed into two components which were ascribed to the optical transitions between the electron ground state in the conduction band and the heavy hole and the light hole ground states in the valence band.The variati on of PL intensity from luminescent components with the spatial positions direct ly demonstrates the origin of the photoluminescence relating to different quantu m structures,and reflects the carrier transfer from SQWL to QWR,resulting in th e PL quenching of SQWL.

Energy sublevels in an Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire

We report the successful fabrication of a V-grooveAl0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire system and the temperature-dependent photoluminescence (PL) measurement. The PL spectra are dominated by four features at 681, 642, 635 and 621 nm attributed to the luminescences from quantum wire, top, vertical and side-wall well regions by micro-PL measurements. By the calculations of the energy structure, discrete states (localized sublevels) in the quantum wire region and continuum states (extended along the side-wall and vertical quantum wells) in side-wall and vertical quantum wells have been obtained in both the conduction and valence bands. The calculated excitation energies explain very well the peak positions and their temperature dependence in the photoluminescence measurements.

Optical and continuous-wave characteristics of V-grooved quantum well wire lasers confined by a p-n junction array

Role of the side-wall quantum wells in a V-grooved quantum well wire (QWW) is briefly reviewed by temperature-dependent photoluminescence, and then continuous-wave (cw) characteristics of QWW lasers confined by a p-n junction array are reported. In terms of the effectiveness in current confinement, very high power operation (over 11 mW) and a single longitudinal mode operation up to 8mW are achieved. Room-temperature threshold currents are measured to be 32.5 mA (pulsed) and 47.8 mA (cw) for a 200 μm long uncoated cavity. The current- and temperature-tuning rates of the oscillation wavelength are as low as 0.038 nm/mA and 0.17 nm/°C, respectively.