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Correlated Optical and Electrical Analyses of Inhomogeneous Core Correlated optical and electrical analyses of inhomogeneous core/shell InGaN/GaN nanowire light emitting diodes Hezhi Zhang, Valerio Piazza, Vladimir Neplokh, Nan Guan, Fabien Bayle, Stéphane Collin, Ludovic Largeau, Andrey Babichev, Francois Julien, Maria Tchernycheva To cite this version: Hezhi Zhang, Valerio Piazza, Vladimir Neplokh, Nan Guan, Fabien Bayle, et al.. Correlated opti- cal and electrical analyses of inhomogeneous core/shell InGaN/GaN nanowire light emitting diodes. Nanotechnology, Institute of Physics, 2020, 10.1088/1361-6528/abc70e. hal-03031214 HAL Id: hal-03031214 https://hal.archives-ouvertes.fr/hal-03031214 Submitted on 30 Nov 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Page 1 of 12 AUTHOR SUBMITTED MANUSCRIPT - NANO-127052 IOP Publishing Nanotechnology 1 Nanotechnology XX (2020) XXXXXX https://doi.org/XXXX/XXXX 2 3 4 5 6 7 Correlated optical and electrical analyses of 8 9 10 inhomogeneous core/shell InGaN/GaN nanowire 11 12 13 light emitting diodes 14 15 16 H. Zhang1,2*, V Piazza2,3, V. Neplokh2,4, N. Guan2, F. Bayle2, S. Collin2, L. Largeau2, 17 A. Babichev5, F. H. Julien2 and M. Tchernycheva2 18 19 1 School of Microelectronics, Dalian University of Technology, 116024 Dalian, China 20 2 C2N-CNRS, Univ. Paris Saclay, 91120 Palaiseau, France 21 3 Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland 22 4 National Research Academic University of the Russian Academy of Sciences, 194021, Saint 23 Petersburg, Russia 24 5 ITMO University, 197101, Saint Petersburg, Russia 25 26 27 E-mail: [email protected] 28 Received xxxxxx 29 Accepted for publication xxxxxx 30 Published xxxxxx 31 32 Abstract 33 34 The performance of core-shell InGaN/GaN nanowire light emitting diodes can be limited by 35 wire-to-wire electrical inhomogeneities. Here we investigate an array of core-shell InGaN/GaN 36 nanowires which are morphologically identical, but present electrical dissimilarities in order to 37 understand how the nanoscale phenomena observed in individual nanowires affect the working 38 performance of the whole array. The light emitting diode shows a low number of nanowires ( ̴ 39 20%) producing electroluminescence under operating conditions. This is related to a presence 40 of a potential barrier at the interface between the nanowire core and the radially grown n-doped 41 layer, which differently affects the electrical properties of the nanowires although they are 42 morphologically identical. The impact of the potential barrier on the performance of the 43 nanowire array is investigated by correlating multi-scanning techniques, namely electron beam 44 induced current microscopy, electroluminescence mapping and cathodoluminescence analysis. 45 It is found that the main cause of inhomogeneity in the array is related to a non-optimized 46 charge injection into the active region, which can be overcome by changing the contact 47 architecture so that the electrons become injected directly in the n-doped underlayer. The light 48 emitting diode with so-called “front-n-contacting” is developed leading to an increase of the 49 yield of emitting nanowires from 20% to 65%. 50 51 Keywords: core-shell nanowire, LED, nitride, EBIC, cathodoluminescence 52 53 54 55 56 57 58 59 60 xxxx-xxxx/xx/xxxxxx 1 © xxxx IOP Publishing Ltd AUTHOR SUBMITTED MANUSCRIPT - NANO-127052 Page 2 of 12 1 2 3 (CL) and EL maps. Finally, we developed an alternative 4 Introduction processing scheme to overcome this issue by injecting the 5 electrons through n-GaN underlayer thus avoiding the Gallium nitride based Nanowires(NWs) appear to be 6 potential barrier. The final device shows a significant promising to boost the performance of light emitting diodes 7 improvement in the EL homogeneity of the NW LEDs. 8 (LEDs) [1-8], particularly for full-color micro-LEDs [9, 10]. 9 Indeed, three-dimensional nanowire-based LEDs can improve Nanowire growth, device fabrication and preliminary the material quality and offer new opportunities for strain 10 analysis 11 engineering. Moreover, the core-shell architecture is expected 12 to reduce the efficiency droop at high current by increasing the Organized InGaN/GaN core-shell NWs are grown by 13 active area. In nanowires (NWs) having their elongated axis Metalorganic Vapour-Phase Epitaxy (MOVPE) on n-type 14 aligned with the [0001] direction, quantum confined Stark GaN/sapphire substrates coated with a SiNx mask with sub- 15 effect is not present thanks to the growth of the active region micrometer openings. First, n-type GaN core is grown along 16 on non-polar m-planes [11, 12]. Additionally, the use of the c-direction, folowing the lateral overgrowth of n-doped 17 nanowires, rather than 2D layers, opens new possibilities for GaN underlayer, which is overgrown with a single InGaN 18 the development of multifunctional light sources. For QW. The structure is completed with a magnesium doped p- 19 example, flexible nanowire LEDs were demonstrated by type cap layer consisting of an AlGaN electron blocking layer 20 transferring the active nanowires to flexible substrates [13, and a p-GaN shell. Figure 1(a) shows a schematic of the 21 14]. internal structure of an individual nanowire in the LED. The 22 The core/shell LEDs were demonstrated using self- NWs consist of a base part, which has the form of a hexagonal 23 assembled growth [15, 16] and selective area growth [17, 18]. cylinder defined by m-planes of about 1 µm in height and of a 24 Organized growth following the selective area epitaxy pyramidal top part defined by semi-polar facets. The NW 25 procedure provides a better control over the nanowire ensemble presents a high morphological wire-to-wire 26 morphology in comparison with spontaneous growth, since it homogeneity as seen from the SEM image of Figure 1(b). 27 reduces the composition fluctuations between the NWs and However, due to SiN mask imperfections, some of the NWs 28 thus facilitates the control of the emission wavelength and the in the pattern are missing (e.g. an empty place can be seen in 29 broadening of the spectral emission. However, due to the 30 the right part of Figure 1(b)). The quantity of missing wires complexity of the three-dimensional growth it is difficult to 31 amounts to ~5 - 8%, no other morphological defects were control the homogeneity of indium composition inside the 32 observed. These pattern defects are used in the present work quantum well (QW) resulting in compositional gradients with 33 to facilitate correlation between different maps as discussed in the NW and in the formation of indium-rich regions. In 34 the following. 35 particular, the change of the emission color with the injection The NW structure is confirmed by scanning transmission 36 current was observed not only in self-assembled NW LEDs electron microscopy (STEM) as illustrated in Figure 1(c). The 37 [14, 16], but also in organized NW LEDs [19-21]. The growth on the semipolar plane is almost inhibited (cf. Figure 38 transition from a 2D to a 3D geometry also affects the 1(d)), so that the thickness of the InGaN QW on the pyramidal 39 electrical and optical properties of the LEDs. Previously, facets is below 1 nm and it does not patriciate in the EL 40 charge carrier trapping [22] has been observed at emission [23]. However, an indium-rich region is formed at 41 core/underlayer interface of core-shell NWs creating a the junction between the m-plane and the semi-polar plane, as 42 potential barrier, which may lead to an inefficient carrier confirmed by the Energy-dispersive X-ray diffraction (EDX) 43 injection towards QWs region. This kind of electrical measurements shown in Figure 1(d). 44 inhomogeneities can critically affect the LED performance, In order to test the nanowire array, the sample was 45 therefore they deserve a deeper analysis. processed into standard 300×300 µm2 size LED mesas. The 46 In this paper, we investigate core-shell InGaN/GaN NWs Indium tin oxide (ITO)-covered mesas were defined by optical 47 by means of macroscopic and nanoscale-resolved techniques. lithography and a metal frame (Ti(10 nm)/Au(150 nm)) was 48 A sample which shows a good morphological order, but deposited on the top surface. To obtain the bottom contact, 49 exhibits inhomogeneities of the electroluminescent (EL) was Inductively Coupled Plasma Etching (ICP) etching was used 50 deliberately chosen. First, the morphology and the internal to remove partially the nanowires in order to access the n-GaN 51 structure of the NWs is analyzed, together with the EL of the buffer layer from the top where a Ti(10 nm)/Al(30 nm)/Ti(10 52 array. Cross-sectional electron beam induced current nm)/Au(150 nm) metallization was applied. 53 microscopy (EBIC) is performed, revealing a potential barrier Under operating condition, the LED produces 54 induced by charge carrier traps at the core/underlayer luminescence starting from a bias of 3.3 V. An example of the 55 interface. Then, the impact of this potential barrier on the EL spectrum and the corresponding optical image of the LED 56 performance of the InGaN/GaN NW LED is statistically is shown in Figure 1(e)) for electrical power of 1.1 mW. 57 analyzed, by correlating top view EBIC, cathodoluminescent Similarly to our previous report [23], the spectrum presents 58 59 60 Page 3 of 12 AUTHOR SUBMITTED MANUSCRIPT - NANO-127052 Nanotechnology XX (2020) XXXXXX H Zhang et al 1 2 3 two contributions: the high-energy peak at 2.6 eV (477 nm) region at the m-plane/semi-polar plane junction.
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