Nonpolar Quantum Wells Research Articles

Inhomogeneous broadening of optical transitions observed in photoluminescence and modulated reflectance of polar and non-polar InGaN quantum wells

In this work, the broadening of interband transitions in InGaN/GaN quantum wells (QWs) resulting from structural inhomogeneities is analyzed. The role of a polarization-induced electric field in the mechanism behind the inhomogeneous broadening observed in photoluminescence (PL) and electromodulated reflectance (ER) spectra of InGaN QWs dedicated to green/blue lasers is explained. Spectra of both polar and nonpolar QWs are simulated within the random QW model distinguishing contributions of individual transitions taking into account QW inhomogeneities (QW width and indium content fluctuations). On this basis, we interpret the ER and PL spectra measured for a polar multiple QW InGaN/GaN structure. The built-in electric field shifts the emission wavelength to red and enhances the broadening of optical transitions. It is clearly shown that for polar QWs the Stokes shift can be easily overestimated if PL spectra are compared with ER spectra since the intensity of the fundamental transition observed in ER spectra significantly decreases with the increase in QW width. In this way, an ER signal related to excited states starts to dominate. This effect is strongly enhanced by QW inhomogeneities.

Role of hole confinement in the recombination properties of InGaN quantum structures

We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions.

Comparison of optical properties of polarization-matched c-plane and lattice-matched a-plane BInGaN/GaN quantum well structures

Light emission characteristics of polarization-matched polar (c-plane) and lattice-matched nonpolar (a-plane) BInGaN/GaN quantum well (QW) structures were investigated as a function of B content and well width. The peak intensity of the lattice-matched a-plane BInGaN/GaN QW structure is shown to be similar to that of the polarization-matched c-plane BInGaN/GaN QW structure, which is about two and half times larger than that of the conventional InGaN/GaN QW structure. The peak intensity is a weak function of the In content. Also, the QW structure with thick well width shows the peak intensity comparable to that for the QW structure with thin well width. Hence, we expect that the lattice-matched a-plane BInGaN/GaN QW structure could be used as a thick BInGaN active layer for a high efficiency and a reduced droop. In addition, nonpolar QW structures show high polarization ratio, which ranges from 0.976 to 0.992 in investigated In content and well width.

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.

Localization and transient emission properties in InGaN/GaN quantum wells of different polarities within core-shell nanorods.

Transient photoluminescence (PL) characteristics and localization phenomena in InGaN/GaN core-shell nanorods (NRs) were investigated from 6 K up to 285 K. The NRs exhibit three well-defined PL bands in the near-UV, blue, and green range ascribed to the emission of quantum well (QW) areas situated at the (1.00) sidewalls, (10.1) top facets, and (00.1) tip, respectively. At low temperature, time-resolved PL shows a fast decay time of about 0.5 ns for the semi- and non-polar QWs, while the polar QWs exhibit at least a twice-longer time. Rapid delocalization of carriers above 50 K indicates shallow potential fluctuations in the QWs. At room temperature, the characteristic fast PL decay time of the three QW bands stabilizes around 300 ps. The slow decaying PL components have different characteristic decay times that are explained by additional localization at basal stacking faults (BSFs), taking into account the quantum confined Stark effect. In addition, narrow excitonic luminescence lines are observed in the BSF-enriched polar QWs, providing direct evidence of the impact of the BSF/QW crossings on the optical properties of the NRs. A PL rise time of about 100 ps does not show any deviation between bands. These findings are suggestive of similar transport mechanisms in temperature equilibrium without inter-facet transport between different QWs. We believe that predictable transient characteristics can play a key role in creating uniform NR ensembles for device applications.

Strain-induced indium clustering in non-polar a-plane InGaN quantum wells

In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.

Understanding GaN/InGaN core–shell growth towards high quality factor whispering gallery modes from non-polar InGaN quantum wells on GaN rods

GaN microrods are used as a basis for subsequent InGaN quantum well (QW) and quantum dot deposition by metal-organic vapor phase epitaxy. The coverage of the shell along the sidewall of rods is dependent on the rod growth time and a complete coverage is obtained for shorter rod growth times. Transmission electron microscopy measurements are performed to reveal the structural properties of the InGaN layer on the sidewall facet and on the top facet. The presence of layers in the microrod and on the microrod surface will be discussed with respect to GaN and InGaN growth. A detailed model will be presented explaining the formation of multiple SiN layers and the partial and full coverage of the shell around the core. Cathodoluminescence measurements are performed to analyze the InGaN emission properties along the microrod and to study the microresonator properties of such hexagonal core–shell structures. High quality factor whispering gallery modes with are reported for the first time in a GaN microrod/InGaN non-polar QW core–shell geometry. The GaN/InGaN core–shell microrods are expected to be promising building blocks for low-threshold laser diodes and ultra-sensitive optical sensors.

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.

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.

S shape in polar GaInN/GaN quantum wells: Piezoelectric-field-induced blue shift driven by onset of nonradiative recombination

In this paper, we critically review the usual explanation of the blue shift in the temperature dependence of the light emission, which is commonly observed for polar GaInN/GaN quantum wells at intermediate temperatures. We demonstrate that this blue shift is not necessarily caused by a thermally induced change of occupation in an inhomogeneously broadened density of states. Instead, different energy dependencies of radiative and nonradiative lifetimes may lead to an energy dependence of the internal quantum efficiency strongly influencing the peak position of the luminescence: the piezoelectric fields within the quantum well induce an approximately exponential relationship between the electron-hole overlap matrix element for the radiative transition between the lowest quantized states and the respective transition energy. Via time-resolved photoluminescence spectroscopy, we observe a corresponding exponential energy dependence of radiative lifetimes at low temperatures and almost energy independent nonradiative lifetimes toward room temperature throughout the emission of polar single quantum wells. An analytical model is demonstrated, predicting a significant blue shift of several 10 meV for polar and a negligible shift for nonpolar GaInN/GaN quantum wells at the transition from high to low internal quantum efficiency due to the energy-dependent competition between radiative and nonradiative recombination processes. This model consistently explains the lack of a blue shift in nonpolar quantum wells grown on $m$-plane bulk GaN as well as a reduced characteristic temperature for the onset of the blue shift after artificial defect generation via argon ion implantation.

Regularly patterned non-polar InGaN/GaN quantum-well nanorod light-emitting diode array.

The growth and process of a regularly patterned nanorod (NR)- light-emitting diode (LED) array with its emission from sidewall non-polar quantum wells (QWs) are demonstrated. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR LED device is similar to that of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence spectral shift under reversed bias confirms that the emission of the fabricated NR LED comes from non-polar QWs.

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.

Optical properties of ZnO/(Zn, Mg)O quantum wells

This paper reviews the optical properties of ZnO/(Zn, Mg)O single quantum wells grown by molecular beam epitaxy. Both heteroepitaxial quantum well growth along the polar c-direction and homoepitaxial quantum well growth on the nonpolar M plane cases are considered. The optical properties of these quantum wells are investigated by using reectance, continuous wave photoluminescence, and time-resolved photoluminescence spectroscopies. The quantum-conned Stark eect dominates the properties of the excitons for polar quantum wells. The magnitude of the internal electric eld that is induced by both spontaneous and piezoelectric polarizations is determined by comparing the experimental results with a variational calculation of excitonic energies and lifetimes. For nonpolar quantum wells, the optical spectra reveal strong in-plane optical anisotropies, as predicted by the group theory. Moreover, the radiative recombination of free excitons is dominating the quantum well photoluminescence even at room temperature.

Three dimensional numerical study on the efficiency of a core-shell InGaN/GaN multiple quantum well nanowire light-emitting diodes

This paper presents the findings of investigating core-shell multiple quantum well nanowire light-emitting diodes (LEDs). A fully self-consistent three dimensional model that solves Poisson and drift-diffusion equations was employed to investigate the current flow and quantum-confined stark effect. The core-shell nanowire LED showed a weaker droop effect than that of conventional planar LEDs because of a larger active area and stronger recombination in nonpolar quantum wells (QWs). The current spreading effect was examined to determine the carrier distribution at the sidewall of core-shell nanowire LEDs. The results revealed that a larger aspect ratio by increasing the nanowire height could increase the nonpolar-active area volume and reduce the droop effect at the same current density. Making the current spreading length exceed a greater nanowire height is critical for using the enhancement of nonpolar QWs effectively, when an appropriate transparent conducting layer might be necessary. In addition, this paper presents a discussion on the influences of the spacing between each nanowire on corresponding nanowire diameters.

Growth control of nonpolar and polar [formula omitted] quantum wells by pulsed-laser deposition

Growth control of nonpolar and polar ZnO/MgxZn1−xO quantum wells (QWs) is demonstrated by in situ RHEED during the pulsed laser deposition process. Nonpolar QWs were grown homoepitaxially on m-plane and on a-plane ZnO single crystals. For m-plane (101¯0) ZnO QWs we report a change of growth mode from a two dimensional layer by layer growth evidenced by RHEED oscillations to the formation of surface nanostripes as observed by atomic force microscopy. The aspect ratio of the self organized nanostripes depends on the oxygen partial pressure. a-lane (112¯0) ZnO QW-structures show a smooth surface with a rms-roughness of 0.3nm. Homoepitaxial nonpolar QWs do not show the quantum-confined Stark effect while polar quantum wells on a-plane sapphire does with an internal electric field of approximately 0.53MV/cm. Furthermore, by implementing a low temperature MgxZn1−xO buffer layer, the interface quality of heteroepitaxially grown polar ZnO/MgxZn1−xO QWs on a-plane sapphire substrates is considerably improved. RHEED oscillations were observed during the whole growth of such QWs.

Indium incorporation and emission properties of nonpolar and semipolar InGaN quantum wells

We report indium incorporation properties on various nonpolar and semipolar free-standing GaN substrates. Electroluminescence characterization and x-ray diffraction (XRD) analysis indicate that the semipolar (202¯1¯) and (112¯2) planes have the highest indium incorporation rate among the studied planes. We also show that both indium composition and polarization-related electric fields impact the emission wavelength of the quantum wells (QWs). The different magnitudes and directions of the polarization-related electric fields for each orientation result in different potential profiles for the various semipolar and nonpolar QWs, leading to different emission wavelengths at a given indium composition.

The influence of inhomogeneities on broadening of fundamental transition in polar and nonpolar InGaN quantum wells dedicated for green emitters

AbstractBroadening of the fundamental transition was calculated for polar and nonpolar InGaN QWs emitting in the green spectral range within the model of a ‘random quantum’ well (QW) [M. Gladysiewicz and R. Kudrawiec J. Phys.: Cond. Mater. 22, 485801 (2010)]. In order to simulate QW inhomogeneities in this model, it was assumed that the QW width, barrier widths and indium concentration vary with a Gaussian distribution where the nominal QW width, barrier widths and indium concentration correspond to the mean value in this distribution and their fluctuations correspond to the deviation from the main value. In this way the influence of the each parameter (QW width, barrier width, and indium concentration) on the broadening of the fundamental transition was considered independently and, finally, all the parameters were taken into account at the same time. It has been found that the same magnitude of QW width fluctuations and indium concentration fluctuations leads to much larger broadening of the fundamental transition for polar QWs than for nonpolar QWs. It means that the reduction of built‐in electric field in InGaN QWs can be a promising way to improve the homogeneity of light emission from this system. For polar InGaN QWs a significant reduction of polarization‐related electric fields can be obtained by a slight intentional doping in QW and/or barriers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep traps and enhanced photoluminescence efficiency in nonpolar a-GaN/InGaN quantum well structures

Nonpolar (11-20) a-GaN/InGaN quantum well (QW) structures were grown by metalorganic chemical vapor deposition on r-plane (1-102) sapphire substrate using a two-stage growth procedure. Our studies demonstrate that, in contrast to polar QWs, these structures show the presence of deep electron traps with activation energy of 0.41 eV in admittance spectra and 1 eV electron traps in capacitance transient spectra. These traps are suspected to be nonradiative recombination centers, decreasing the nonpolar QW photoluminescence (PL) efficiency in our structures compared to polar structures. The PL efficiency of nonpolar QWs is shown to be greatly enhanced by coupling to localized surface plasmons formed by Ag nanoparticles.

Injection efficiency and optical gain characteristics of polar and nonpolar III‐nitride light emitters

AbstractOptical properties of III‐nitride quantum well (QW) structures grown on nonpolar or semipolar crystallographic planes benefit from reduced internal fields in QWs and, as a result, from enhanced overlap between electron and hole states participating in lasing transition. Detailed modelling of active QWs shows, however, that smaller separation of the lasing states from the high‐energy subbands in nonpolar QWs and related thermal carrier redistribution between subbands can reduce the QW differential optical gain thus increasing the laser threshold. Efficiency characteristics of multiple QW emitters are additionally affected by inhomogeneous QW injection and by large residual QW charges. Polar and nonpolar light‐emitting diode structures reveal different mechanisms of the efficiency droop. In polar structures, the droop is dominated by electron leakage and is noticeably affected by the active region ballistic overshoot. Efficiency droop in nonpolar structures is dominated by combined effect of radiative time saturation and nonradiative Auger recombination while the carrier leakage becomes a factor of secondary importance. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

M-Plane Core–Shell InGaN/GaN Multiple-Quantum-Wells on GaN Wires for Electroluminescent Devices

Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {11-00} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1-8 nm) and In-composition in the wells (∼16%). The core-shell MQW light emission characteristics (390-420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.