Nonpolar InGaN Quantum Wells Research Articles

Precise determination of deformation potentials in InGaN alloy material in semipolar and nonpolar InGaN quantum wells

Deformation potentials of InGaN have been precisely determined in order to make a reliable prediction of optical gain characteristics in semipolar and nonpolar InGaN quantum wells (QWs) laser didoes (LDs). Since the optical polarization properties in semipolar and nonpolar InGaN QWs are very sensitive to the values of deformation potentials, all the reported data on the optical polarization properties have been theoretically analyzed based on the k · p perturbation theory in this study, and we have made a precise determination of the deformation potentials’ set. In addition, optical gain characteristics of InGaN QWs on GaN substrates with arbitrary substrate orientations have been theoretically calculated by using the determined deformation potentials’ set. It is found that low-angle semipolar substrate orientation (θ ∼ 45°) is very promising for low-cost and high-performance green LDs with cleaved-facet cavity mirrors.

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.

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.

Recombination dynamics in planar and three-dimensional InGaN/GaN light emitting diode structures

Abstract

The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem

We have used high resolution transmission electron microscopy (HRTEM), aberration-corrected quantitative scanning transmission electron microscopy (Q-STEM), atom probe tomography (APT) and X-ray diffraction (XRD) to study the atomic structure of (0001) polar and (11-20) non-polar InGaN quantum wells (QWs). This paper provides an overview of the results. Polar (0001) InGaN in QWs is a random alloy, with In replacing Ga randomly. The InGaN QWs have atomic height interface steps, resulting in QW width fluctuations. The electrons are localised at the top QW interface by the built-in electric field and the well-width fluctuations, with a localisation energy of typically 20meV. The holes are localised near the bottom QW interface, by indium fluctuations in the random alloy, with a localisation energy of typically 60meV. On the other hand, the non-polar (11-20) InGaN QWs contain nanometre-scale indium-rich clusters which we suggest localise the carriers and produce longer wavelength (lower energy) emission than from random alloy non-polar InGaN QWs of the same average composition. The reason for the indium-rich clusters in non-polar (11-20) InGaN QWs is not yet clear, but may be connected to the lower QW growth temperature for the (11-20) InGaN QWs compared to the (0001) polar InGaN QWs.

Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells

We report on a comparative study of efficiency droop in polar and non-polar InGaN quantum well structures at T = 10 K. To ensure that the experiments were carried out with identical carrier densities for any particular excitation power density, we used laser pulses of duration ∼100 fs at a repetition rate of 400 kHz. For both types of structures, efficiency droop was observed to occur for carrier densities of above 7 × 1011 cm−2 pulse−1 per quantum well; also both structures exhibited similar spectral broadening in the droop regime. These results show that efficiency droop is intrinsic in InGaN quantum wells, whether polar or non-polar, and is a function, specifically, of carrier density.

The microstructure of non-polar a-plane (112¯0) InGaN quantum wells

Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average quantum well composition measured by atom probe tomography and quantitative scanning transmission electron microscopy quantitatively agrees with measurements by X-ray diffraction. Atom probe tomography is further applied to study the distribution of indium atoms in non-polar a-plane (11-20) InGaN quantum wells. An inhomogeneous indium distribution is observed by frequency distribution analysis of the atom probe tomography measurements. The optical properties of non-polar (11-20) InGaN quantum wells with indium compositions varying from 7.9% to 20.6% are studied. In contrast to non-polar m-plane (1-100) InGaN quantum wells, the non-polar a-plane (11-20) InGaN quantum wells emit at longer emission wavelengths at the equivalent indium composition. The non-polar a-plane (11-20) quantum wells also show broader spectral linewidths. The longer emission wavelengths and broader spectral linewidths may be related to the observed inhomogeneous indium distribution.

On optical polarization and charge carrier statistics of nonpolar InGaN quantum wells

AbstractAbstractauthoren Optical polarization is a fundamental property of light emission from m‐plane InGaN quantum wells. It is a result of band structure, anisotropic strain, the degree of polarization depending on thermal occupation, and band‐filling of valence subbands. We analyze the optical‐polarized light of nonpolar samples at low and at room temperature over a wide range of excitation density. We observe that the measured energy separation and linewidth of the orthogonally polarized emission depend on carrier density, too. A consistent explanation of these effects is possible in the context of Fermi–Dirac statistics for a degenerate carrier density and inhomogeneously broadened density of states. In addition, pulsed electroluminescence experiments were performed to validate our conclusions for optoelectronic devices based on nonpolar InGaN quantum wells.

Indium clustering in a-plane InGaN quantum wells as evidenced by atom probe tomography

Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application of the focused ion beam microscope enabled APT needles to be prepared from the low defect density regions of the grown sample. A complementary analysis was also undertaken on QWs having comparable In contents grown on polar c-plane sample pseudo-substrates. Both frequency distribution and modified nearest neighbor analyses indicate a statistically non-randomized In distribution in the a-plane QWs, but a random distribution in the c-plane QWs. This work not only provides insights into the structure of non-polar a-plane QWs but also shows that APT is capable of detecting as-grown nanoscale clustering in InGaN and thus validates the reliability of earlier APT analyses of the In distribution in c-plane InGaN QWs which show no such clustering.

Defect recombination induced by density-activated carrier diffusion in nonpolar InGaN quantum wells

We report on the observation of carrier-diffusion-induced defect emission at high excitation density in a-plane InGaN single quantum wells. When increasing excitation density in a relatively high regime, we observed the emergence of defect-related emission together with a significant efficiency reduction of bandedge emission. The experimental results can be well explained with the density-activated carrier diffusion from localized states to defect states. Such a scenario of density-activated defect recombination, as confirmed by the dependences of photoluminescence on the excitation photon energy and temperature, is a plausible origin of efficiency droop in a-plane InGaN quantum-well light-emitting diodes.

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)

Optical polarization and anisotropic gain characteristics in semipolar and nonpolar InGaN quantum well lasers

AbstractBy using a new simple theoretical approach, the previously‐reported experimental results of the polarization properties in semipolar and nonpolar InGaN quantum wells (QWs) were analyzed. On the basis of the k·p‐perturbation theory, we derived a useful analytical expression for describing the polarization properties of these QWs, and used this expression to analyze experimental data reported from various research groups. All the reported data are successfully fitted by the analytical expression, and realistic prediction of polarization properties is possible for unexplored substrate orientation and In composition. Based on these analyses, it is predicted that the optical gain characteristics favourable for laser diodes with cleaved‐facet cavity mirrors would appear in the green‐InGaN QWs on the lower‐angle semipolar planes (30° ∼ 40° inclined from the c‐plane). (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 0) and on semipolar (1 0 2), (1 1 2), (1 0 1) as well as (2 0 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1) < (1 1 2) = (0 0 0 1) < (2 0 1) < (1 0 0) = (1 0 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 0) and the semipolar (1 0 2) and (2 0 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2) and (1 0 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

Spectral properties of polarized light from semipolar grown InGaN quantum wells at low temperatures

AbstractThe polarization dependent photoluminescence at low temperatures of strained semipolar and nonpolar InGaN quantum wells was studied as a function of the emission wavelength. We found for semipolar QWs that the maximum of the spectral resolved optical polarization is either red‐ or blue‐shifted with respect to the maximum of the emission.In contrast, the nonpolar emission exhibits no clear maximum. We assign all effects to an inhomogeneous broad‐ ening of the emission caused by indium fluctuations and explain this behavior here in the light of the optical polarization switching. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A simple theoretical approach to analyze polarization properties in semipolar and nonpolar InGaN quantum wells

By using a simple theoretical approach, the previously reported experimental results of the polarization properties in semipolar and nonpolar InGaN quantum wells (QWs) were analyzed. On the basis of the k⋅p-perturbation theory, we derived a useful analytical expression for describing the polarization properties of these QWs, and used this expression to analyze experimental data reported from various research groups. Based on these analyses, we predicted that the negative polarization degree, which is favorable for laser diodes with cleaved-facet cavity mirrors, would appear in the blue- or green-InGaN QWs on the lower-angle semipolar planes (30°–40° inclined from the c-plane).

Impact of band structure and transition matrix elements on polarization properties of the photoluminescence of semipolar and nonpolar InGaN quantum wells

AbstractPartial or full linear polarization is characteristic for the spontaneous emission of light from semipolar and nonpolar InGaN quantum wells. This property is an implication of the crystalline anisotropy as a basic property of the wurtzite structure. The influence of this anisotropy on the band structure and the transition matrix elements was calculated by a ${\bf k}\cdot {\bf p}$‐method for arbitrary quantum well orientations with respect to the c‐axis; results are shown here in detail. Optical polarization is a direct consequence of a broken symmetry, mainly affecting the transition matrix elements from the conduction to the valence bands. Furthermore, the strain of the InGaN quantum well strongly depends on the crystal orientation of the substrate, resulting in a valence band mixing. The composition of the eigenfunctions has emerged to be most important for the polarization dependence of strained semipolar and nonpolar InGaN QW. The matrix elements, in combination with the thermal occupation of the bands, determine the polarization of the spontaneously emitted light. Our photoluminescence measurements of nonpolar QW match well with this model. However, in contrast to calculations with standard band parameters, the two topmost subbands show a larger separation in the emitted energy.

Theoretical investigations on anisotropic optical properties in semipolar and nonpolar InGaN quantum wells

AbstractThe anisotropic band structures and spontaneous emission of the non c plane InGaN/GaN quantum wells (QWs) were computed based on the k·p perturbation theory. Spontaneous emission due to the A valence band is strongly polarized perpendicular to the c axis, while that from the B band has 90º‐rotated polarization, for the crystal angle θ larger than 50°. However, it was found that excited carriers can distribute into both the A and B bands, so that the effective polarization Peff defined by the integrated luminescence intensity is much reduced even for nonpolar InGaN/GaN QWs. Thus, Peff was systematically studied for arbitrary planes. We found that the energy separation between the A and B bands becomes significant and carrier population of the B valence band is suppressed as the In content increases, when Peff approaches unity. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)