Semipolar InGaN Quantum Wells Research Articles

Optical anisotropy of (112¯3) semipolar InGaN quantum wells homoepitaxially grown on GaN substrates

InGaN/GaN quantum wells (QWs) are homoepitaxially grown on (112¯3) GaN substrates, and atomically smooth surfaces and abrupt InGaN/GaN interfaces are achieved. The optical anisotropy is examined using photo- and electroluminescence spectroscopies. Both methods consistently show in-plane optical polarization along the direction parallel to the projection of the c-axis on the QW plane. This polarization direction is rotated by 90° from that of unstrained, bulk InGaN, demonstrating the occurrence of polarization switching in strained, semipolar InGaN QWs. Specifically, the polarization direction is [1¯1¯22], which offers an advantage in that the {11¯00} m-planes can be used as cavity mirrors in edge-emitting lasers. The previously proposed analytical equation reasonably reproduces the experimental results.

Kinetics of the radiative and nonradiative recombination in polar and semipolar InGaN quantum wells

We studied mechanisms of recombination in InGaN quantum wells in polar and semipolar structures. Photoluminescence measurements show that the optical emission linewidths for polar and semipolar structures are almost identical suggesting the same level of indium fluctuations in quanutm wells. Their “peak-energy-versus-temperature” relations demonstrate very pronounced “s-shape” effect. Emission linewidth measured by cathodoluminescence does not depend on area from which the light is collected meaning that the fluctuations are smaller that 100 nm. The time scale of recombination process are of the order of 80 ns for polar and 2 ns for semipolar. Energy dispersion of the recombination time is strong in polar structures and very weak in semipolar ones which can be interperted in terms of electric field influence on photoluminescence lifetime energy dispersion. At room temparture emmission is dominated by Schockley-Hall-Read recombination and does not show any dispersion. Rate equation analysis of photoluminescence transients show domination of excitonic recombination in the case of polar samples (low temperature) and bimolecular in the case of semipolar ones. Both types of quantum wells, polar and semipolar look similar from the point of view of localization but differ in their radiative recombination mechanisms.

Spatially resolved luminescence properties of non- and semi-polar InGaN quantum wells on GaN microrods

Spatially resolved emission properties of InGaN/GaN quantum wells on the facets of microrods are analyzed by means of photoluminescence and cathodoluminescence measurements. We observe strongly localized emissions of the non- and semi-polar InGaN quantum wells from the top part of the microrods, suggesting optical emitters of high efficiency. The quantum wells are characterized by transmission electron microscopy measurements with respect to their indium composition and thickness. Those wells oriented in non- and semi-polar directions possess high In concentrations as well as low internal polarization fields which makes these directions excellent candidates for InGaN quantum well emitters. These investigations show that the applied microrod growth concept is an effective method to realize high quality InGaN quantum wells on GaN facets oriented in non- and semi-polar directions which makes optoelectronic emitters in the green gap region feasible.

Influence of well width fluctuations on recombination properties in semipolar InGaN quantum wells studied by time- and spatially-resolved near-field photoluminescence

Scanning near-field photoluminescence spectroscopy has been applied to distinguish the relevance of quantum well (QW) alloy composition and well width fluctuations on emission linewidth and recombination times in semipolar (202¯1) plane InGaN QWs. It has been found that well width fluctuations, compared to variations of InGaN alloy composition, play a negligible role in defining the photoluminescence linewidth. However, the well width strongly affects the recombination times. Prolonged radiative and nonradiative carrier lifetimes in wide QWs have been attributed to electron and hole separation by in-plane electric fields caused by nonplanarity of QW interfaces.

Green emission from semipolar InGaN quantum wells grown on low‐defect () GaN templates fabricated on patterned r‐sapphire

In this article, InGaN/GaN multiple quantum wells (MQWs) grown on low‐defect () GaN templates are investigated by photoluminescence (PL), cathodoluminescence (CL), and transmission electron microscopy (TEM). The emission wavelength is tuned from 450 nm (blue) to 550 nm (green) by varying the TMGa flux in the QWs, while keeping the temperature constant. The In content in the QWs is found to increase with increasing TMGa flux. CL measurements show that the first QW and often the second one emit systematically at wavelengths shorter than the following QWs, while TEM measurements indicate that these first QWs are slightly thinner and display less In content than the rest of the stack. Both observations might be explained by considering that these first QWs grow under larger compressive strain than the subsequent QWs. Furthermore, since TEM shows that misfit dislocations oriented along the [] direction are mainly located at the lower MQW stack interface, i.e., between the first QW and the underlying template, the correlation between TEM and CL points toward a plastic relaxation occurring after the stacking of a sufficiently large number of QWs.

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

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

Demonstration of phosphor-free polarized white light emission from monolithically integrated semipolar InGaN quantum wells

We report a device design that monolithically integrates an electrically injected blue light-emitting diode grown on the (202¯1¯) face of a bulk GaN substrate and yellow emitting optically pumped InGaN quantum wells (QWs) grown on the (202¯1) face. Emission peaks are observed at 440 nm and 560 nm from the electrically injected and optically pumped QWs, respectively. The ratio of blue to yellow light can be optimized to result in polarized white light emission from this phosphor-free device. We demonstrate emission with Commission Internationale de l'Eclairage x- and y-chromaticity coordinates of (0.28, 0.29) and an optical polarization ratio of 0.40.

Energy offset between valence band anti-crossing and optical polarization switching in semipolar InGaN quantum wells

Light emitted from a semipolar InGaN quantum well parallel to the surface normal is partially polarized, in contrast to the unpolarized emission of c-plane quantum wells. Anti-crossing of the two topmost valence bands causes polarization switching between ordinary and extraordinary polarizations for certain semipolar quantum well orientations with increasing indium content. Two properties of the two measured spectra, their polarization resolved but spectrally integrated intensity and their measured energy splitting, are associated with anti-crossing. Here, we show that only the observable energy splitting coincides with band anti-crossing, while the switching point of the polarization can deviate from the anti-crossing point.

Suppressing void defects in long wavelength semipolar (202¯1¯) InGaN quantum wells by growth rate optimization

We report on void defect formation in (202¯1¯) semipolar InGaN quantum wells (QWs) emitting in the green spectral region. Fluorescence and transmission electron microscopy studies indicate that this type of defect is associated with voids with {101¯1}, {101¯0}, and {0001¯} side facets in the QW region. Systematic growth studies show that this defect can be effectively suppressed by reducing the growth rate for the active region. Green light-emitting diodes (LEDs) with reduced active region growth rate showed enhanced power and wavelength performance. The improved LED performance is attributed to the absence of void defects in the active region.

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.

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.

Polarization switching of the optical gain in semipolar InGaN quantum wells

AbstractWe investigate the influence of polarization switching on the optical gain of semipolar InGaN quantum wells (QWs) depending on indium content and charge carrier concentration using self‐consistent 6 × 6 k·p‐band structure calculations. The semipolar planes considered here are the $(11\bar {2}2)$‐ and the $(20\bar {2}1)$‐plane. In contrast to the $(20\bar {2}1)$‐plane, the dominant polarization of the optical gain in a QW on the $(11\bar {2}2)$‐plane can depend on both the indium content and the charge carrier concentration, as reported from experiments. These effects are explained by a detailed analysis of the wave function composition of the topmost valence bands in a semipolar QW.