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Dislocation Related Droop in Ingan/Gan Light Emitting Diodes Investigated Via Cathodoluminescence

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Dislocation Related Droop in Ingan/Gan Light Emitting Diodes Investigated Via Cathodoluminescence Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence Galia Pozina, Rafal Ciechonski, Zhaoxia Bi, Lars Samuelson and Bo Monemar Linköping University Post Print N.B.: When citing this work, cite the original article. Original Publication: Galia Pozina, Rafal Ciechonski, Zhaoxia Bi, Lars Samuelson and Bo Monemar, Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence, 2015, Applied Physics Letters, (107), 25, 251106. http://dx.doi.org/10.1063/1.4938208 Copyright: American Institute of Physics (AIP) http://www.aip.org/ Postprint available at: Linköping University Electronic Press http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-125162 Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence Galia Pozina, Rafal Ciechonski, Zhaoxia Bi, Lars Samuelson, and Bo Monemar Citation: Applied Physics Letters 107, 251106 (2015); doi: 10.1063/1.4938208 View online: http://dx.doi.org/10.1063/1.4938208 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/107/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-power low-droop violet semipolar ( 30 3 ¯ 1 ¯ ) InGaN/GaN light-emitting diodes with thick active layer design Appl. Phys. Lett. 105, 171106 (2014); 10.1063/1.4900793 Influence of dislocation density on carrier injection in InGaN/GaN light-emitting diodes operated with alternating current Appl. Phys. Lett. 102, 011115 (2013); 10.1063/1.4773588 Dependence of radiative efficiency and deep level defect incorporation on threading dislocation density for InGaN/GaN light emitting diodes Appl. Phys. Lett. 101, 162102 (2012); 10.1063/1.4759003 Improvements of external quantum efficiency of InGaN-based blue light-emitting diodes at high current density using GaN substrates J. Appl. Phys. 101, 033104 (2007); 10.1063/1.2432307 Effects of In composition on ultraviolet emission efficiency in quaternary InAlGaN light-emitting diodes on freestanding GaN substrates and sapphire substrates J. Appl. Phys. 98, 113514 (2005); 10.1063/1.2134885 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. IP: 130.236.83.101 On: Tue, 08 Mar 2016 11:19:10 APPLIED PHYSICS LETTERS 107, 251106 (2015) Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence Galia Pozina,1 Rafal Ciechonski,2 Zhaoxia Bi,3 Lars Samuelson,2,3 and Bo Monemar1,3,4 1Department of Physics, Chemistry and Biology, Linkoping€ University, SE-581 83 Linkoping,€ Sweden 2GLO AB, Scheelevagen€ 22, SE-22363 Lund, Sweden 3Solid State Physics, Lund University, Box 118, SE-22100 Lund, Sweden 4TokyoUniversity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan (Received 10 November 2015; accepted 7 December 2015; published online 23 December 2015) Today’s energy saving solutions for general illumination rely on efficient white light emitting diodes (LEDs). However, the output efficiency droop experienced in InGaN based LEDs with increasing current injection is a serious limitation factor for future development of bright white LEDs. We show using cathodoluminescence (CL) spatial mapping at different electron beam currents that threading dislocations are active as nonradiative recombination centers only at high injection conditions. At low current, the dislocations are inactive in carrier recombination due to local potentials, but these potentials are screened by carriers at higher injection levels. In CL images, this corresponds to the increase of the dark contrast around dislocations with the injection (excitation) density and can be linked with droop related to the threading dislocations. Our data indicate that reduction of droop in the future efficient white LED can be achieved via a drastic reduction of the dislocation density by using, for example, bulk native substrates. VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4938208] The reduction of radiative efficiency in InGaN based specific actions to reduce the dislocation density are taken. light emitting diodes (LEDs) for visible light with increasing In this letter, we study an InGaN-based LED structure grown current injection, the so-called droop, is a definite limitation heteroepitaxially on a sapphire substrate and, thus, it is a for future improvement of the light output from these LEDs, suitable object to investigate the role of dislocations for including the related white lamps.1 The physical mechanisms droop. These dislocations are generally known to be impor- for the droop have been discussed intensely during the last tant nonradiative centers at room temperature and above (as decade, as reviewed by several authors.1–3 These models are relevant for LEDs).7–9 At present, there is no published typically based on the assumption of homogeneous material investigation of the spatial dependence of droop across an properties in the active region producing the light, i.e., LED wafer, in order to demonstrate whether the droop is a defects are not discussed as a major reason for droop. Some homogeneous intrinsic effect (as presently assumed in most previous work pointed out that defect recombination could theoretical models). The intrinsic recombination processes be viewed as consistent with the droop behavior in the are expected to be evenly distributed at high excitation con- LEDs, however.4,5 At injection conditions close to the peak ditions (free carrier recombination), and produce a homoge- of the LED efficiency curve (like external quantum effi- neous bright background in a cathodoluminescence (CL) ciency vs current density, often called the LI-curve), the topograph of the emission from a certain area of the LED injected carriers are generally viewed as largely localized wafer. If defects like TDs are involved an inhomogeneous and, thus, immune to the major non-radiative defect recom- emission pattern across the LED wafer is expected, with bination at dislocations. At high injection the localization dark spots at the defect sites.7–9 If this dark contrast increases potentials are screened, and the carriers are free to move to with the injection (excitation) density, we are observing defect sites.4 Other authors have shown using high resolution droop related to the TDs. scanning near-field optical microscopy techniques, which the The LED structure investigated in this work is a conven- dislocations themselves are protected from nonradiative tional violet GaN/InGaN multiple quantum well (MQW) recombination events by local potentials for the violet structure grown with metalorganic chemical vapor deposi- InGaN LEDs, under the conditions of these experiments.6 tion at Lund University on a patterned sapphire substrate, This property leaves them inactive in carrier recombination, including a 5 quantum well active region, an AlGaN electron unless these potentials are screened by carriers at higher blocking layer, and a p-layer. The contact metallization was injection levels.4 In this scenario, dislocations may play an excluded. On-wafer test with the Ni-dot method confirmed important role for droop, because in these violet (or blue) satisfactory emission performance. Room temperature CL LEDs they are essentially active as nonradiative recombina- measurements were performed using a MonoCL4 system tion centers only at high injection conditions. integrated with a field emission gun (FEG) cathode LEO The dominating growth technologies for the nitride 1550 Gemini scanning electron microscope (SEM). The LEDs at present employ foreign substrates like sapphire, SiC acceleration voltage of the electron beam was kept to 10 kV. or silicon, leading to a high threading dislocation (TD) den- With this voltage, the penetration depth is 1 lm in the case sity of 109 cmÀ2 in the active region of the device, unless of GaN material. The threading dislocation density was 0003-6951/2015/107(25)/251106/4/$30.00 107, 251106-1 VC 2015 AIP Publishing LLC Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. IP: 130.236.83.101 On: Tue, 08 Mar 2016 11:19:10 251106-2 Pozina et al. Appl. Phys. Lett. 107, 251106 (2015) FIG. 1. SEM surface pictures (a) and (d) and corresponding panchromatic CL images taken from the same area of the wafer at two different electron beam currents (b) and (e) 0.11 nA and (c) and (f) 3.73 nA. The images in (d)–(f) are similar to those in (a)–(c), but shown with lower magnification to underline a pronounced CL contrast around dislocations of different geom- etry at higher electron beam current. estimated from the dark spot density of 5 Â 107 cmÀ2. The layer, caused by somewhat excessive p-doping. These spots SEM instrument allows six different apertures, which were appear dark in the SEM pictures but bright in the CL. These used to vary the intensity of the electron beam. Additionally, spots have no relation to the dislocation contrast from the the beam current was measured for each aperture separately. active region CL.) CL images were acquired using a Peltier-cooled GaAs pho- We have attempted to follow the evolution of the inte- tomultiplier tube. A diffraction grating with 150 l/mm blazed grated intensity of the MQW emission with increasing elec- at 500 nm and a CCD detector were used for the CL spectral tron beam current, for a small local area of the wafer. As measurements. The spectral resolution under the experimen- noted from the CL images, the signal is not uniform. The tal conditions was 5 nm. The spatial resolution of the CL bright contrast, i.e., a higher signal, corresponds to the areas system was better than 100 nm, which is similar to the SEM with less structural defects (dislocations), while darker resolution for the conditions of the experiment. contrast can be related to the areas with the higher disloca- Panchromatic CL (PCL) images measured at room tem- tion density. As already mentioned, the contrast in CL perature for low and high electron beam current show differ- images is especially obvious for a higher electron current ent distribution of the CL signal, which can be described as a beam.
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