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Insights into the Silver Reflection Layer of a Vertical LED for Light Emission Optimization Mansoor Ali Ali Khan, Hansheng Chen, Jiangtao Qu, Patrick W. Trimby, Steven Moody, Yin Yao, Simon P. Ringer, and Rongkun Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04854 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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ACS Applied Materials & Interfaces

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Insights into the Silver Reflection Layer of a 8

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Vertical LED for Light Emission Optimization 12 13 14 16

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Mansoor Ali Khan§†ǂ, Hansheng Chen§†ǂ, Jiangtao Qu§†ǂ, Patrick W. Trimbyǂ, Steven Moodyǂ, 17 18

Yin Yaoǁ, Simon P. Ringer‡ǂ, and Rongkun Zheng*§†ǂ 20

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§ School of Physics; † Australian Institute for Nanoscale Science and Technology; ‡ School of 24

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Aerospace, Mechanical and Mechatronic Engineering; ǂ Australian Centre for Microscopy and 25 27

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Microanalysis, The University of Sydney, Sydney, NSW, 2006, Australia 28 30

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ǁ Electron Microscope Unit, University of New South Wales, Sydney, NSW, 2052, Australia 31 32 3 34 35 36 38

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KEYWORDS: Ag, GaN, vertical LED, elemental diffusion, grain morphology, surface 40

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topography, light extraction efficiency. 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59

1 60 ACS Paragon Plus Environment


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ABSTRACT: In this work, Ag as highly reflective mirror layer of GaN-based blue vertical light 5

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emitting diode (VLED) has been systematically investigated by correlating scanning electron 6 8

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microscopy/energy dispersive x-ray spectroscopy/transmission Kikuchi diffraction/electron 10

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backscatter diffraction, aberration-corrected scanning transmission electron microscopy, and 12

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atomic force microscopy techniques. In context of high efficiency lighting, three critical aspects: 13 15

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1) chemical diffusion, 2) grain morphology, and 3) surface topography of Ag layer have been 17

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scrutinized at nanoscale. We found that nanoscale inhomogeneous distribution of In in 18 19

InGaN/GaN quantum wells, interfacial diffusion (In/Ga out-diffusion into Ag layer, the diffusion 20 2

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of Ag into p-GaN and quantum wells), and the Ag agglomeration deteriorate the light 24

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reflectivity, which account for the decreased luminous efficiency in VLED. Meanwhile, surface 25 26

morphology and topographical analyses revealed nano-morphology of Ag layer, where 29

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nanograined size of ~300 nm with special nanotwinned boundaries and an extreme smooth 31

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surface of ~3-4 nm are strongly desired for better reflectivity. Further, based on these 32 34

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microscopy results, suggestions on light extraction optimization are given to improve the 36

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performance of GaN-based blue VLED. Our findings enable fresh and deep understanding of 38

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performance-microstructure correlation of LEDs at nanoscale, providing guidance to design and 39 41

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manufacture high performance LED devices. 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59


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1. INTRODUCTION 5

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With the increasing traditional energy resource crisis and environmental pollution arisen from 6 8

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electrical production, the appealing light-emitting diodes (LEDs) have become a competitive 10

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candidate for next-generation solid-state lighting devices due to their high energy efficiency, 12

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small size, high color rendering index, reliable life time, and wide spectral line width from deep 13 14

ultra-violet-to-yellow.1–3 Currently, LEDs have been considered extensively as industry focused 17

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optoelectronic devices for illumination in high-value applications, such as smartphones, traffic 18 19

lights, automotive headlights, full-color displays, home lighting, backlighting source in liquid 20 2

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crystal displays (LCD), LED displays, and other medical and commercial devices.4–7 In addition, 24

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2014 Noble Prize in Physics to Shuji Nakamura's and Akasaki's groups for the invention of 26

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efficient blue LEDs, which has enabled bright and energy-saving white light sources.8 This 29

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invention recognizes the significance of environment-friendly LEDs, and attained considerable 31

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attention in further optimization. Although nowadays most of the LEDs luminous efficacy 32 34

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(~150-250 lm/W) has surpassed the traditional incandescent, line and compact fluorescent 36

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technologies (~10-110 lm/W),9,10 still, cost-effective design of high-power and high-efficiency is 38

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essential to penetrate in general illumination market. 39 41

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In this regard, Gallium Nitride (GaN)-based vertical light emitting diode (VLED) has been 43

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developed, which possesses superior light emission property compared to the lateral LED 4 45

(LLED) architecture, including large light-emitting area by using one top electrode (n-side up 48

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and p-side down), mitigation of current-crowding problem, small series resistance (vertical 50

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current injection), better heat dissipation via metal-based substrate or bendable substrateless 51 52

techniques, higher light-extraction efficiency by roughening the top n-GaN surface, and better 5

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reflection of downward-absorbing light by placing mirror layer on the top of substrate.11–19 56 57 58 59



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Despite of those progresses, VLED still needs improvement, particularly on the luminous 5

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efficacy and admittedly external quantum efficiency (EQE). The EQE (ƞEQE) is the product of the 6 8

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injection efficiency ƞinj, the internal quantum efficiency (IQE) ƞIQE, and the light extraction 10

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efficiency (LEE) ƞextr, i.e., ƞEQE = ƞinj . ƞIQE . ƞextr.20 The LEE or ƞextr, which is ratio between the 12

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externally emitted photons and the internally generated photons in active region, is mainly 13 15

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determined by the LED architecture (chip design), surface roughening, patterned substrates, 17

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materials absorption and reflection, etc.; while ƞIQE, which is ratio between the electrically 18 19

injected carriers and the internally emitted photons is mainly connected to the quality of the 20 2

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active layer and is evaluated by its growth conditions, impurity incorporation (diffusion), doping 24

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profile, defect density of the material and surface morphology.20,21 Hence, it is great importance 25 26

to correlate LEE and IQE, consequently to enhance overall EQE. It is therefore the EQE has 29

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been taken as the main criteria for the LED performance, and is approaching 60-80% in recent 31

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LED products.21–24 Nevertheless, optimization across these aspects needs to be explored 32 34

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systematically in details for further improvement in EQE. 36

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In context of LEE, extraction of high count of generated photons from the active region, i.e., 38

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multiple quantum wells (MQWs) (Figure 1) is an essential requirement for high-brightness LED 39 41

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devices. In this regard, a highly reflective mirror incorporated with textured or scattered surface 43

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12,25–28

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has shown an effective approach to improve LEE as compared to other techniques.29 It is

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believed that incorporating reflective coating in nitride-based LEDs the light output increased 48

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about 200% as compared to conventional LED devices (without the reflective layer).30 The 50

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concept is to confine the light to the GaN layer by a bottom reflective mirror and to extract light 51 52

via a textured surface at the topside of a LED chip, as illustrated in Figure 1. In this way, 5

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generated photons are reflected from the bottom highly reflective mirror layer, and finally escape 56 57 58 59


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from the top textured surface of LED rather than being absorbed by base substrate or the lead 5

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frame in the LED package. Thus, reflective layers have been widely adopted in LED chip design 6 7

to maximize LEE by major LED vendors.31–33 Among various highly reflective materials, Ag is 10

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an ideal choice for green and blue GaN-based light emitters due to the high figure of merits, 12

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including electrical resistivity (lowest of all metals), high thermal conductivity (highest of all 13 15

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metals, except non-metal diamond), low absorption, and the highest optical reflectivity (reflects 17

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visible wavelengths ~400-650 nm more efficiently than other readily available conductive 19

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metals, e.g., copper, aluminum, and gold).14,34 Besides, the Ag layer also acts as the p-electrode, 20 2

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which not only enhances the LEE but also the output power of LEDs.25,35,36 However, the 24

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concrete effects of the interface between the GaN and Ag, and the grain morphology of the Ag 25 26

layer on the LEE are still blurred, which impedes the improvement of the reflective layer in 29

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LEDs. Therefore, a detailed systematic investigation of Ag layer at atomic scale is essential to 31

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facilitate the high performance LEDs’ designing and engineering. Also, such investigations are 32 34

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crucial for optimizing the Ag reflective layer as they will open new opportunities to further 36

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improve the luminous efficacy and light output power of LEDs. 38

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In this work, various aspects affecting the Ag reflectivity have been scrutinized, including 39 41

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interfacial diffusions, crystallographic structure and defects, texture, grain boundaries (GB), 43

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special boundaries, interface boundaries, and surface roughness, which have substantial effects 4 45

on LEE. Elemental diffusions in and between GaN (semiconductor) and Ag (metal) reflection 48

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layers were determined by energy dispersive x-ray spectroscopy (EDX) by scanning electron 50

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microscopy (SEM) and scanning transmission electron microscopy (STEM). Grain morphology 51 52

of the Ag reflective layer was analyzed by SEM-based transmission Kikuchi diffraction (TKD) 5

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and electron backscatter diffraction (EBSD) techniques. SEM-TKD, a sophisticated 56 57 58 59



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microanalytical technique provided robust collection of grain orientation, special boundaries and 5

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pattern quality maps at a spatial resolution of 2-10 nm, with significant advantage over other 6 8

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techniques. In parallel, SEM-EBSD facilitated the detailed microstructural characterization of 10

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top surface of the bulk Ag reflective layer. Further, surface topography (surface roughness) was 12

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examined by atomic force microscopy (AFM). Finally, based on these correlative microscopy 13 15

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results, suggestions are presented for performance optimization of GaN-based VLED. 16 18

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2. MATERIALS AND METHODS 19 20 21

The process flow of fabricating vertical LEDs involved four steps:15,37,38 (1) metal-organic 24

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chemical vapour deposition (MOCVD) epitaxial layers grown on sapphire substrate, and 26

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deposition of reflective mirror by electron-beam evaporation, diffusion barrier, adhesive and 27 29

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bonding metal layers; (2) wafer bonding to carrier substrate; (3) removal of sapphire substrate by 31

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using laser lift off technique; and (4) surface structuring on the LED die. The schematic cross32 3

section of the final VLED device is illustrated in Figure 1. 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 52

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Figure 1. Cross-sectional schematic of VLED 53 54 5 56 57 58 59


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The GaN-based blue VLED sample was analyzed with a set of the powerful microscopic 5

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techniques as summarized in the Supporting Information (Table S1). The TEM and SEM-TKD 6 8

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require the region of interest (ROI) on sample with thickness around 60-100 nm, and accordingly 10

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lamella specimen was prepared, and mounted on Cu/Mo grid in accordance with standard TEM 12

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foil preparation procedure based on FIB lift-out techniques,39 as shown in Figure 2. Furthermore, 13 15

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the sample chemistry, surface morphology, compositional variations and crystallographic texture 17

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were determined by EDX and Nordlys-nano EBSD detectors, respectively, equipped in a single 19

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Carl Zeiss Ultra Plus SEM.40 For TKD analysis, a standard 70° tilted TKD sample holder was 20 2

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used, where specimen was mounted on and experiment was carried out at 20° stage tilt in order to 24

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align the ROI surface at 90° (70°+20°=90°) towards the EBSD detector as shown in Figure S1. 25 26

Thereafter, TKD patterns were recorded and processed by the Oxford Instruments Aztec 29

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software. In addition, the roughness (surface topography) of the Ag reflection layer and its 31

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interface with GaN were further examined by AFM and FEI Titan Themis Z (probe) aberration32 34

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corrected STEM at 300kV techniques, respectively. 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59



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Figure 2. Sample preparation by FIB. (a)-(g) For STEM and SEM-TKD analysis 28

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3. EXPERIMENTAL RESULTS AND DISCUSSION 31 32

3.1. ELEMENTAL DIFFUSION 3 35

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SEM-EDX spectra collected along the yellow marked line in Figure S2 provide rapid 37

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identification of quantification of the compositional variations across multilayered device 38 40

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structure. It indicates that interface between Ag (metal) and GaN (semiconductor) layers is 42

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somewhat diffused. The complete stack of elemental analysis is detailed in the Supporting 4

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Information (Figure S2). Also, Table S2 provides lattice parameters of the phases (Figure S3) 45 47

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identified by the EDX data. 49

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Robust compositional analysis across multilayered device through SEM-EDX was very 50 51

beneficial to locate elemental inter-diffusion at micron scale for a broad field of view. However, 52 54

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spatial resolution of SEM-based EDX was not high enough to investigate the delicate 56

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nanostructure of samples involving quantum wells (QWs) and elemental inter-diffusion or 57 58 59


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interfacial diffusion (Ag-GaN). Therefore, aberration-corrected high resolution (HR) STEM was 5

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employed to measure the accurate compositions at atomic-scale. In Figure 3, cross-sectional 6 8

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layers enriched with different elements were differentiated using with different colours. In Figure 10

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3a, STEM-EDX mapping shows that Ag reflective layer is followed by epitaxial layers, which 12

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consist of GaN, Al as electron blocking layer (EBL), active region comprising four pairs of 13 15

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InGaN/GaN MQWs, and In-based superlattice with stress release functionality. Given the 17

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importance of elemental diffusion occurring within active region (MQWs) and at Ag-GaN 18 19

interface, Figure 3a-d STEM-EDX results provide three interesting aspects of diffusion: 1) In 20 2

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and Ga migration into Ag reflective layer, 2) Ag (metal) migration into GaN (semiconductor), 24

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especially into active region (MQWs), and 3) Ag-O and Ag-Ga bond formations at Ag and GaN 25 26

interface. 28

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Figure 3. (a) STEM-EDX elemental chemical analysis. (b),(d) High-angle annular dark-field 57

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(HAADF) images. (c),(e) Line spectrum across the MQWs and GaN-Ag interface. 58 59


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1) STEM-EDX line spectrum (Figure 3b,c) reveals that In is quite diffusive and migrates into 5

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the Ag reflection layer. Also, it can be observed from Figure 3c,e that the In distribution in 6 8

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MQW’s is inhomogeneous, i.e., two pairs of MQW’s towards Ag side have slightly less atomic% 10

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in relationship to other two pairs of MQW’s towards superlattice end. This verifies that the 12

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source of In diffusion into Ag layer is from either InGaN/GaN-based MQWs or In-based 13 15

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superlattice. The reason for In diffusion and In content fluctuations along the MQWs is the well17

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known thermal variability phenomenon, which is related to the difference in growth temperature 18 19

between the GaN layers and InGaN wells. The high growth temperature of p-GaN (hole transport 20 2

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layer) and later Ag thermal annealing process at higher temperatures attribute to the In re24

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evaporation or thermal damage to InGaN/GaN MQWs.41 Consequently, out-of-plane In diffusion 25 26

penetrates into Ag mirror layer and the In content in the quantum wells decreases, particularly 29

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for those quantum wells, which are located close to the interface of p-GaN and Ag. This 31

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nanoscale inhomogeneous distribution of In will seriously change the potential profile of 32 34

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InGaN/GaN MQWs due to In segregation at Ag mirror layer. This is because the optoelectronic 36

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properties of LEDs are very sensitive to the In content distribution in the InGaN well layers, and 38

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any alternation in the In composition profile has both a direct and secondary effects on the LED 39 40

device band structure.42 Subsequently, this results in the reduced overlap between the electron 43

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and hole wave functions in the InGaN quantum well layer, and thus decreases the IQE. 4 45

Moreover, In diffusion into Ag mirror layer will also impact optical (reflectivity) and electrical 48

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(resistivity) properties of LED as well. Ag reflectivity gets reduced as of lattice distortion caused 50

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by In diffusion.43 Also, any impurity in an Ag mirror layer degrades its surface quality, hence 51 52

increases its contact resistivity, which leads in lowering the wall-plug efficiency (amount of light 5

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power produced compared to the electrical power applied) of VLED. 56 57 58 59



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2) Also, it is important to note that Ag-GaN interface is not abrupt, and that Ag metal gets 5

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diffused into GaN semiconductor layer, as indicated from STEM-EDX line spectrum (Figure 6 8

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3b,c). Possible reasons involve poor adhesion between Ag reflective (metal) and GaN 10

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(semiconductor) layers, different thermal annealing (temperature cycle) and lattice mismatch 12

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(stress) during fabrication of GaN and Ag layers.44,45 Furthermore, thermal annealing performed 13 15

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in O2 ambient exhibited interdiffusion between Ag and GaN, which disrupts Ag-GaN interface 17

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(Figure 3d).46 In this case, interfacial Ga vacancies will be generated substantially, which can 18 19

degrade ohmic contact (metal-semiconductor junction) or change energy levels (e.g., Schottky 20 2

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barrier height).13,47 In addition, dissimilar properties between Ag metal and GaN semiconductor 24

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materials (Table S3) can potentially result in unwanted diffusion and intermixing. For instance, 25 26

lattice mismatch (strain) and different miscibility between different materials cause Ag atoms 29

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segregation into the upper layer of GaN (as no barrier layer exist), which will lead to 31

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inhomogeneous distribution of carrier concentration or photon transport at Ag/p-GaN interface, 32 34

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ultimately influence the optical and electrical performances of VLED. 36

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Moreover, Figure 4 chemical distribution map reveals that Ag further diffused into quantum 38

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wells (active region), even it contaminated and disrupted In-based superlattice layer. Such 39 41

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excessive Ag diffusion into GaN region will mainly affect the quantum wells crystal quality, as 43

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shown in Figure 4b (yellow marked circles). The degradation of crystal quality of InGaN/GaN 4 45

MQWs will decrease the efficiency of the active region in generating photons by varying the 48

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energy levels (bandgap). This additional reliability issue of the Ag is due to its electromigration 50

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phenomena, and such interfacial reactions between Ag metal and GaN were also addressed by 52

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other groups using SIMS technique,45,46 wherein it diffused into the light-producing active region 5

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(MQWs) thereby creating deep levels in GaN semiconductor material, thus hindered the light 56 57 58 59


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Figure 4. (a) STEM-EDX analysis showing Ag diffusion into InGaN/GaN-based MQWs. (b) HR50 52

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STEM HAADF images indicating the Ag particles contamination (yellow marked) into quantum 54

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wells making their interfaces non-abrupt. 5 56 57 58 59



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output.48 Furthermore, Ag migration inside MQWs serves as a non-radiative recombination 5

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centres that can shift the spectrum (optical wavelength) of particular blue LED device by 6 8

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impairing the carrier confinement and transport mechanism at the active region of LED. 10

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Consequently, diffusion of Ag (metal) into GaN (semiconductor) or MQWs (active region) will 12

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not only reduce the IQE but also overall EQE of the VLED device. 13 15

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3) In Figure 3a, STEM-EDX analysis indicates Ag oxidisation at Ag-GaN interface, which is 17

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consistent with previous reports regarding the Ag thermal instability issues, such as Ag 18 19

oxidisation during post annealing, void formation, and agglomeration due to thermal annealing 20 2

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process (500-600 °C) performed in O2 ambient environment.49,50 Also, in Figure 3c, the long 24

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diffusion tail of Ga in the Ag layer clearly indicates the dissolution of Ga atoms in Ag. Thus, the 25 26

formation of Ag-O and Ag-Ga bonds at Ag-GaN interface indicates the oxidation of Ag, and 29

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existence of Ag-Ga solid solution, respectively. The reason behind is that the oxygen molecules, 31

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which get incorporated during oxidation annealing result in decomposition of GaN-Ag interface 32 34

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to form GaOx rather than GaN, because Ga-O has higher bonding strength compared to Ga-N 36

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bond, thus contributes Ga out-diffusion to Ag metal layers. After the oxidation annealing 38

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process, while the Ag layer was brought in to make a direct contact with the p-GaN layer, the Ga 39 41

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atoms from GaN layer dissolved into Ag layer to form Ag-Ga solid solution because of high 43

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solubility of Ga in Ag.45 In fact, Ag in-diffusion promotes the Ga-out diffusion. Hence, such Ag 4 45

oxidation and agglomeration degrade the Ag surface morphology; subsequently reduce the 48

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reflectivity of the LED device. 50

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In context of optical reflectivity, even though, the Ag is considered as the most common 51 52

reflector for GaN-based flip-chip or vertical LEDs due to its good ohmic characteristic on p5

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GaN, and high reflectivity at ultraviolet-visible regions. However, the 2) and 3) aspects of Ag 56 57 58 59


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diffusion discern that Ag reflector undergoes electromigration and thermal degradation issues, 5

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such as Ag oxidisation and/or agglomeration upon annealing, which lead to the degradation of 6 8

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LED performances. 10

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In summary, In and Ga show out-diffusion into Ag layer, while Ag tends to diffuse into GaN 12

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region during LED manufacturing process. In/Ga diffusion into Ag degrades the reflectivity of 13 15

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the Ag mirror layer. While the composition variations or reactions induced by the diffused Ag 17

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into GaN region can compromise IQE of the LED by forming non-radiative recombination or 18 19

scattering centers in an active layer. Thus, whether semiconductor material diffuses into metal 20 2

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layer or vice versa, in either case it will impact the performance of the finished VLED product. 24

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Overall, these findings from STEM analysis are critical in exploring the origin of decreased 25 26

luminous efficiency, for instance, almost 60% of IQE has been achieved so far for the blue LEDs 29

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operating at wavelength (λ)∼ 450nm,51 and still there is a potential to improve its performance 31

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by optimization. Moreover, these results suggest the different diffusion widths originate from the 32 34

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growth temperature variation (between GaN, InGaN and Ag layers), and non-availability of 36

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diffusion barrier between Ag and GaN interface. To mitigate such in/out-diffusion issues, Ag37 38

GaN interface need to be optimized by incorporating appropriate transparent barrier layers, for 39 41

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instance, Tin-Zinc oxide interlayer.47 Also, the formation of Ag–O and Ag alloy (Ag–Ga) during 43

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thermal annealing process can be supressed by having multilayer stack of Me/Ag/Ru/Ni/Au 45

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(where Me= Ni, Ir, Pt or Ru) contacts or barriers for high-power GaN-based VLEDs.45 Further 48

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suggestions are listed in optimization section. 49 50 51 52 53 54 5 56 57 58 59



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3.2. GRAIN MORPHOLOGY 5

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VERTICAL THIN SAMPLED (SEM-TKD) 6 8

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Another factor that affects the quality of Ag layer and LEE is the grain morphology. Grain 10

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morphology of the Ag mirror layer need to be scrutinized in order to correlate its impact on 1 12

transport related properties, such as electrical resistivity, reflectivity, mass transport and thermal 15

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conductivity.52–54 17

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In this regard, powerful SEM-based TKD with spatial resolution of 2-15 nm provides key 18 20

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nanostructural information, such as orientation mapping (OM), grain size analysis, poles figures 2

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(texture) and Kikuchi diffraction patterns.40 SEM-based TKD and EBSD techniques have 24

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significant advantage over other counterparts because of their automated analysis of grain 25 26

morphology with diffraction system.55,56 Automated analysis of grain morphology is inevitable 29

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because grains and their grain boundaries occupy a considerable volume in the LED structure. 30 31

Figure S4 shows the complete picture of cross-sectional grain morphology, poles figures (PF) 34

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and crystallographic information obtained from TKD mapping data across the multi-layered 36

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structure of VLED. However, for particular ROI, i.e., GaN (semiconductor) and Ag (metal) 37 38

layer/interface is shown in Figure 5. 41

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In grain morphology, OM analysis identifies GaN as mono-crystalline material with no GBs 43

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(Figure 5a). However, beneath GaN layer, the Ag metal reflective layer has special GBs oriented 4 46

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in specific direction, as shown in Figure 5b. Aztec software processed TKD data also reveals that 48

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most of Ag mirror region is dominated by the high-angle grain boundaries (HAGBs) but with 49 50

special type of boundary called sigma 3 (3) twin boundaries are shown in red diagonal-shaped 53

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lines (Figure 5b). TKD analysis discerns that 3 twin boundaries (TBs) were found abundant in 54 5

Ag reflective layer (~80.9%); however, a few were also identified as at barrier/adhesion layers, 56 57 58 59


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Figure 5. Cross-sectional SEM-TKD analysis. (a) GaN. (b) Ag having twin boundaries with 32

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their respective pole figures (PFs) and Kikuchi diffraction patterns. 3 35

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i.e., Pt (~9.36%) and W (0.65%), as marked in red (Figure S4). The tendency of face centered36 37

cubic (FCC) metals to form TBs is related to the TB energy, i.e., those with low TB interfacial 40

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energy, such as Ag easily form twins. Contrary to that, metals with high TB energy interferes 42

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with twins formation, thus twins are quite rare there, e.g., Pt, W. 43 45

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In context of correlating these GB orientations with material’s properties, it has been reported 47

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that in order to enhance material’s strength, HAGBs are required as they are more resistant to 49

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dislocation motions.57 However, it is also noted in microelectronic devices that HAGBs are more 50 52

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electrically resistant than LAGBs (subgrain) boundaries with the exception of special type of 54

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high-angle 3 (TBs).58,59 Special TBs at the Ag mirror layer will impact its electrical resistivity 5 56

and mobility.58 In fact, special TBs with less interfacial energy have low electrical resistance and 58

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less mobility compared to HAGBs.60 Moreover, contrary to HAGB, special 3 TBs are 5

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considered to be poor pathways for electromigration voiding or mass transport.38,60 6 8

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Interestingly, OM also highlights the position of special TBs and it can be observed that the 10

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width of twins (diagonal lines) at the Ag layer is getting sharp while moving towards Ag-GaN 1 12

interface, i.e., narrow twins are slightly towards upper GaN layer (Figure 5b). The measured 15

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average twin spacing in Ag film found to be ~10 nm. Here, identifying such special TBs at cross17

16

sectional layers of VLED is important for following aspects: 1) Such narrow (nano) and dense 18 20

19

twins towards GaN layer may influence the reflectivity of the Ag mirror layer, consequently 2

21

enhances the LEE of LED device, 2) TBs with relatively low mobility contribute in a high 24

23

degree of microstructural stability,61,62 3) TBs improve electrical conductivity because of their 25 26

low resistance, 4) TBs provides better thermal stability,63,64 5) Also, it enables us to associate 29

28

27

TBs with origin of defects (light loss) in a reflective layer of VLED, which may act as barriers 30 31

for other HAGBs during growth and lead to the initiation of points of failure, 6) TBs with less 32 34

3

energy will provide poor diffusivity paths for electromigration mass transport while appearing at 36

35

Ag metal reflective layer of the VLED.65 Nevertheless, TBs have shown the capability to 37 38

increase the strength in other FCC metals (Cu, Al, Ni, etc.) by acting as barriers to the 41

40

39

transmission of dislocations.61,66 But also such special TBs have significant benefit on electrical 43

42

properties, which imperatively contributes towards the lifetime of the indispensable Ag reflective 4 46

45

layer in the VLED. 48

47

In addition, the grain size also influences the Ag reflectivity because the existence of any fine 50

49

or coarse grain size induces certain type of diffusion paths, i.e., their GBs, which might result in 51 53

52

either scattering paths or defective (trapping) points in photons and electrons transportation. Our 5

54

TKD cross-sectional grain size analysis of Ag reflective layer showed that it has average fine 56 57 58 59


Page 19 of 40


1 3

2

grain size with ~ 90-110 nm (including TBs). Also, Figure 5a shows the pole figures (degree of 5

4

preferred orientation) and Kikuchi diffraction patterns from top most GaN layer indicated that it 6 8

7

has wurtzite crystal (HCP) structure with strong {0001} texture in the growth direction (c-axis). 10

9

In Figure 5b, the pole figures (PFs) obtained from cross-sectional layer of Ag hold peculiar 12

1

shaped texture at {100}, {110} and {111} with FCC diffraction patterns due to the special TBs. 13 15

14

Since a twining relationship between the two component grains occurs, i.e., in between any two 17

16

strong component grains there exists a weak component grain, and verse versa. 18 19

Hence, such a variety of grains size and texture across different layers of VLED (Figure S4) 20 2

21

might exhibit unusual properties in light emission operation. For instance, resistivity of a coarse24

23

grained sample is lower than that of a fine-grained one because the former has a smaller number 26

25

of GBs.57 Similarly, in the matter with Ag metal reflective layer, grain size and texture (grain 29

28

27

orientation) impact the electrical and optical properties of thin film LED. This effect is mainly 31

30

attributed to the mechanism of charge or photon trapping and scattering at the GBs. For instance, 32 3

EM lifetime is highly influenced by the grain size distribution and the texture.67 The reason is 36

35

34

that the EM normally occurs via diffusion along GBs, hence grain size needs to be increased in 38

37

order to reduce the grain boundary area for diffusion. This minimizes the metal diffusion and 39 41

40

trapping of charge or photons since there are fewer sources of vacancies or less fast diffusion 43

42

paths will be available for them. Also, point defects (vacancies, impurities) and extended defects 4 45

(grain boundaries) scatter photons and electrons, thus shortening their mean free paths. On 48

47

46

account of the photon and electron scattering at GBs, a polycrystal has a lower thermal and 50

49

electrical conductivity than a single crystal. In case of W or Pt (Figure S4), coarse grains 51 52

contribute in stability of diffusion barriers as they are most effective in filling voids or holes at 5

54

53

HAGBs (reducing sources of vacancies), hence yields in higher median times to failure 56 57 58 59



Page 20 of 40

1 3

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(MTTFs).68 Apart from that, coarse grains would help in minimizing hillocks (short circuit) 5

4

formation, reducing film resistivity (less scattering of charge carriers), thereby better controlling 6 8

7

of Ag mirror reflectivity by protecting it from chemical etching, diffusions or contaminations 10

9

during deposition or photolithography process. 12

1

In summary, SEM-TKD provides valuable pattern quality maps, grain size and orientation 13 15

14

mapping, texture, and Kikuchi diffraction patterns from cross-sectional layers of vertical LED. 17

16

Also, in general coarse-grained materials with low-angle grain boundaries (LAGBs) are 18 19

preferred in semiconductor devices; however, promoting certain grain structure with particular 20 2

21

orientation depends on the functionality of each material (layer or interface) in that specific 24

23

device. For instance, in coarse-grained metals with LAGBs, the increased thermal stability, 25 26

electrical conductivity and ductility merits are accompanied by loss in strength. Contrary to that, 29

28

27

nanocrystalline metals with HAGBs, the increased strength is accompanied by loss in thermal 31

30

stability, electrical conductivity and ductility. However, a special case of nanotwinned or TB 32 34

3

materials (such as in Ag mirror layer) exhibit high tensile strength with good ductility, thermal 36

35

stability, and electrical conductivity.63,64 Therefore, in prospective electronic devices, these 38

37

nanotwinned structures may express distinctive properties in comparison to coarse-grained and 39 41

40

nanocrystalline metals. 42 43 4 45 47

46

PLANAR BULK SAMPLE (SEM-EBSD) 49

48

Nevertheless, SEM-TKD or transmission EBSD (t-EBSD) has better spatial resolution (2-15 50 51

nm) compared with conventional EBSD technique (30-100 nm).40 However, SEM-TKD cannot 54

53

52

be performed on bulk or thick specimen, as it requires electron transparent thin samples. 56

5

Although robust SEM-TKD results reveal interesting special TB’s features (Figures 5b and 6a) at 57 58 59


Page 21 of 40


1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 56

5

Figure 6. SEM-EBSD analysis. (a)-(i) On the top (bulk) surface of the Ag reflection layer. 57 58 59



Page 22 of 40

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the thin Ag mirror layer (in a cross-sectional view), but it is equivalent important to further 5

4

investigate the nature of top bulk surface of Ag mirror layer by applying SEM-EBSD and AFM 6 8

7

techniques (in a planar view). Figure 6b shows SEM-EBSD analysis on the top bulk surface of 10

9

mirror layer, and its spectrum verifies the presence of Ag (Figure 6c). Note that SEM-EBSD was 12

1

performed on the top (bulk) surface of Ag mirror layer, whereas SEM-TKD analysis was applied 13 15

14

on the vertical (thin) sample of Ag mirror layer (Figure 5). 17

16

On top bulk surface of Ag layer, almost 2000 grains were detected by SEM-EBSD in a scan 18 19

area of scan area of 5x5 μm at step size of 25 nm, and average grain size was found to be ~300 20 2

21

nm (Figure S5). Such compact fine grains promote better reflectivity compared to coarse grains 24

23

structure because coarse grains involve larger micro area for more loss of light by absorption 26

25

rather reflection.69 Also, Figure 6d shows SEM-EBSD-based BC and GB analysis of bulk Ag 29

28

27

mirror layer, which verifies enrichment of TBs. Figure 6e reveals in detail microstructural 31

30

analysis, i.e., grain morphology and twinning structures (black marked lines) indicate special 32 34

3

TBs for bulk (planar) surface of Ag reflective layer. Figure 6f map shows grain reference 36

35

orientation deviation (GROD) analysis, which displays the misorientation angle of a point from 38

37

the grain’s average orientation. It can be seen that all misorientation angles are less than 2° so the 39 41

40

visible deformation is really quite small, i.e., majority grains are in a same color. GROD map has 43

42

a significant role in interpretation of the local accommodation of deformation within the 4 45

microstructure. 48

47

46

In Figure 6g, another OM component called texture component (TC) shows a map of 50

49

misorientation relevant to the reference point (here grains with {111} direction), and indicates 51 52

that most of the mapped area is within the ideal orientation (only peak of 10° deviation is found 5

54

53

in TC inset histogram). In the scope of mirror layer reliability, such texture has its advantages, as 56 57 58 59


Page 23 of 40


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2

it indicates higher MTBF (circuit life) and higher density of GB pathways are inactive for EM 5

4

mass transport.60 To further explain that texture, PFs and inverse pole figures provide 6 8

7

considerable evidences of texture in the {111} direction parallel to the Z-axis (axial direction). 10

9

The high exposure densities (red regions) are shown at the centre of the {111} pole figure 12

1

(Figure 6h) and the {111} corner of Z-axis (Figure 6i) indicates a strong fibre texture. Hence, Ag 13 15

14

reflective mirror layer holds strong fiber texture in Z-axis (normal to the bulk surface 17

16

plane). 18 19

In addition, EBSD data analysis further manifested that Ag layer has a specific class of TBs, 20 2

21

i.e., coincident site lattice (CSL) boundaries with dominant 3-value and misorientation angle of 24

23

60° (rotation about ), as evident from Figure S5 graphs. The Σ3 CSL boundaries in 25 27

26

Ag (FCC) metal typically provide lower electrical resistivity, lowest TB energy, higher strength 29

28

and inherently more thermal stability properties. Such combination of properties is ideal for 30 31

reducing the effects of EM, hence promote low-contact and highly reflective p-contact mirrors in 32 34

3

VLED. Furthermore, MQW generated source light once hits such TBs of an Ag layer; light will 36

35

adopt certain reflection path depending on these special TBs as they possess specific mirror 37 38

lattice symmetry operation. Therefore, the role of twin structures at Ag mirror layer in 41

40

39

enhancement of photons scattering (light reflectivity) can be correlated with top textured 43

42

(structured) surface of GaN layer or bottom patterned substrate (PS) layer. Because they 4 46

45

normally improve light extraction of LED by generating a specific escape cones (twin shaped) so 48

47

that major portion of light reflects back or scattered out rather than being absorbed by base 50

49

susbtrates.70–72 Thus, these EBSD results at highly reflective mirror layer are significant in 51 53

52

failure and optimization analysis because GB engineering substantially relies upon CSL analysis. 54 5 56 57 58 59



Page 24 of 40

1 3

2

3.3. SURFACE TOPOGRAPHY 4 5

Apart from GB engineering at Ag mirror layer, surface roughness also impacts the reflectivity 8

7

6

of light, i.e., reflectivity degrades due to increase of surface roughness.69,73 In order to evaluate 10

9

the roughness (micro surface topography index), AFM analysis has been conducted on bulk 1 13

12

surface of Ag-based mirror sample, and measured RMS roughness value of ~3-4 nm (Figure 7). 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 54

53

Figure 7. (a) SEM image. (b) 2D AFM. (c) 3D AFM analysis on the of top bulk surface of Ag mirror layer. 56

5 57 58 59


Page 25 of 40


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2

This value indicates that Ag surface is flat (smooth), which is better for the quality of Ag 5

4

reflectivity. Notably, SEM-EBSD provides grain morphology (grain size, GBs or TBs), whereas 6 8

7

AFM assists in surface topography (roughness). Collective data analysis from EBSD (grain 10

9

morphology) and AFM (surface topography) reveal that the Ag reflective layer possesses fine12

1

grained structure ~300 nm with TBs, and smooth surface of ~3-4 nm. Such fine grains with 13 14

lower roughness results in enhancement of the total optical light output of the VLED device.30,74 17

16

15

Also, in previous research about the structure of reflective metal surfaces stated that metal 18 19

surfaces are highly reflective if their microstructure consists of crystallites smaller than the 20 2

21

wavelength of visible light, i.e., λ < 400nm.69 On that account, our SEM-EBSD and AFM data 24

23

are quite consistent in identifying the nano-morphology of Ag layer, i.e., grains morphology 25 26

(grain size of ~300 nm) and surface topography (roughness of ~3-4nm), respectively. Thus, 29

28

27

finest (smaller) grain size with special TB’s and flat surface are necessary features to maximize 31

30

the reflectivity of Ag layer. 32 34

3

However, in Figure 8, it can also be discerned from AFM, SEM and forward scatter detector 36

35

(FSD) images that protrusions in the form of whiskers (like prills or bubble particles) exist at Ag 38

37

mirror layer. The formation of these whiskers is related to agglomeration phenomena that occur 39 41

40

in Ag films during thermally assisted O2 annealing process, which involves the evolution of 43

42

capillary instabilities.49,50 Such Ag agglomeration and/or Ag oxidisation will increase the 4 45

roughness of micro surface of Ag up to 10-15 nm, leading to a decrease of reflectance over the 48

47

46

whole spectral range. The reason is that a rougher surface not only has larger micro area to 50

49

reflect light but also there is more loss of light by absorption, thus decreasing the reflectance 52

51

which eventually led to drop in luminous efficiency.69 Also, these whiskers area might become a 5

54

53

source of pits for trapping of light. 56 57 58 59



Page 26 of 40

1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 4

43

Figure 8. Images of Ag mirror surface from: (a),(b) AFM. (c),(d) SEM and FSD (detectors for 45 47

46

characterizing material microstructures in the SEM) showing whiskers formation marked in 49

48

yellow dotted circles. 50 51 52 53 5

54

Further, AFM results are also consistent with TEM-EDS results (Figure 3), which verify the 57

56

presence of O at Ag interface that leads to Ag agglomeration or oxidisation. Interestingly, EBSD 26 59

58 60

ACS Paragon Plus Environment

Page 27 of 40


1 3

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and AFM analysis of Ag film identify that these whiskers or agglomeration forms mostly along 5

4

the GB or TBs (defective paths), as shown in Figures 7b and S6. In order to mitigate that, 6 8

7

thermal annealing temperature need to be optimized to obtain smooth Ag microsurface. 10

9

Hence, application of SEM-EBSD and AFM techniques on nanotwinned Ag metal is exciting, 12

1

because this combination provides a better understanding in the physics of failure (PoF) in 13 15

14

methodology of Ag fabrication. Thus, the smaller grain size and smoother the surface, better the 17

16

quality of Ag for reflection purpose. 18 19 20 21 23

2

4. PROSPECTIVE OPTIMIZATION 24 25

In context of optimization for device performance enhancement in terms of LEE and output 28

27

26

power, factors such as nano-structural composition, selection of metallic elements at reflection, 30

29

barrier or adhesion and wafer bonding layers need to be considered in correlation with electrical 31 3

32

resistivity, CTE or mechanical stress, fabrication methodology and grains morphological 35

34

parameters. Based on our analysis and results, possible optimization suggestions are given as 37

36

follows: 38 40

39

1) Grain boundary engineering, i.e., grain structure, SB (TBs) and texture orientation need 42

41

to be well controlled as electrons or photons scattering mechanism and electromigration 43 4

lifetime are highly dependent by the grain size distribution.53,60,75 47

46

45

2) Nano-twinned metals, e.g., Ag should be promoted as these structures exhibit distinctive 48 49

properties (e.g., high tensile strengths, good ductility, thermal stability, and electrical 50 51

conductivity) as compared to the nanocrystalline or ultrafine-grained metals.59,61 54

53

52

3) To mitigate issues of Ag diffusion into GaN or active region and In/Ga out-diffusion into 5 56

Ag mirror layer, high thermal treatments should be avoided on Ag and p-GaN layers to 57 58 59



Page 28 of 40

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2

prevent metal-semiconductor diffusions.41,46 Ni/Ag/TiW metal stack are found to tolerate 5

4

high-temperature annealing.76 6 8

7

4) To reduce thermal stress between GaN and metal-alloy-based interfaces, the tunable 10

9

incorporation of diamond-like carbon (DLC) layers on reflective layer has the distinct 12

1

capability to match its CTE with GaN, hence enhances the thermal diffusion.77,78 13 15

14

5) GaN-Ag interface needs diffusion barrier layers such as Tin-Zinc oxide (TZO)47 and 17

16

nickel-titanium (NiTi) related alloys that can effectively block Ag diffusion.79 18 20

19

Alternatively, nickel-vanadium (NiV) as diffusion barrier can be used to increase the 2

21

mirror reflectivity in high power led chips.80 23 24

6) To supress the Ag oxidisation (Ag-O), a multilayer stack of Me/Ag/Ru/Ni/Au (where 25 27

26

Me= Ni, Ir, Pt or Ru) contacts or barriers are required for high-power GaN-based 29

28

VLEDs.45 To avoid the Ag agglomeration, i.e., whiskers formation which increases 30 31

roughness and resistivity of the mirror layer, thermal annealing process should be 32 34

3

performed at low optimum temperature to attain smooth highly reflective mirror 36

35

contacts.81 Moreover, the Ag replacements or combination with other elemental candidate 37 38

such as Sn (which forms less whisker than Ag does) or Rh as RhZn/Ag (which has high 39 41

40

reflectivity and barrier property), will improve the efficiency of extracting light in 43

42

LEDs.69,82 4 46

45

7) Ag alloys (AgNi/AgCu/AgAl) or SiO2/TiO2 dielectric Bragg reflectors (DBRs) combined 48

47

with highly reflective Ag layer could overcome the limitation of LEE.83,84 49 50

8) In addition, selection of adhesion or barrier materials and their film thickness should also 51 53

52

be considered in terms of fabrication cost. For instance, rather multiple thick layered 5

54

structure of Pt or Pt/W on both device and carrier wafers, single thin layer of Ti or Ti/W 56 57 58 59


Page 29 of 40


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only at device wafer will be substantially less expensive approach.15 In parallel, fine 4 5

grains at adhesion layers will reduce the cost of manufacturing blue VLED. Thus, optimal 6 8

7

agitation parameters should be established for each particular layer in order to achieve the 10

9

best micro-uniformity. 1 12

9) Contrary to metallic barriers, non-metal barriers (e.g., SiOxNy, SiCN or TaN) would 13 15

14

have a significant edge because of their closely matched CTE to dielectric materials (e.g., 17

16

substrates).67,85 18 20

19

10) In VLEDs, the top small surface (n-GaN) and large backside reflective mirror (p2

21

contacts) normally cause current crowding issue, to mitigate this highly transparent 23 24

conducting layer (TCL) or graphene current spreading layers (CSL) are useful to increase 25 26

light output power and wall-plug efficiency.86 However, the trade-off between optical 29

28

27

transmittance and electrical conductivity must be addressed as well. 30 31

11) Selecting suitable position of the Ag reflection layer also affects the LEE of the VLED 32 34

3

because overall LED LEE quite sensitive to the location of mirror layer and base 36

35

substrate height.87 37 38

12) To enhance optical output power of GaN-based light-emitting diodes graded In 39 41

40

composition quantum wells and on-top layers nanoparticle-assembled or nanowires 43

42

(ZnO) can be used.23,88 4 46

45

13) Influence of laser lift-off (LLO) or chemical lift-off (CLO) on optical and structural 48

47

properties of InGaN/GaN vertical blue LED should be minimized with alternative 50

49

approaches, such as natural substrate lift-off (NSLO), and plastic substrates.89,90 The 51 53

52

reason is that CLO and LLO can produce adverse effects involving etching non5

54

uniformity and tensile strain, which will lead to piezoelectric polarization electric fields. 56 57 58 59



Page 30 of 40

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2

Such fields hinder the electrons and holes recombination for photons generation, a 4 5

behavior termed as quantum-confined Stark effect (QCSE) can be produced as similar in 6 8

7

cases of LLED. QCSE reduces recombination efficiency and increases wavelength 10

9

shifts.91 1 12

14) New microscopy technique of atom probe tomography (APT) can assist in better 13 15

14

understanding of key interfaces (e.g., Ag-GaN) of the VLED by providing 3D chemical 17

16

compositions.92 18 19

15) Introduction of new chips design like OSRAM: UX312 and integration of GaN-on Si 20 2

21

technology LEDs with HEMT, and CMOS-MEMS may open emerging paths in 23 24

prospective smart solid-state lighting. 25 26

Hence, optimizing the Ag reflective layer is crucial to further improve the luminous efficacy 29

28

27

and light output power of LEDs. 30 31 32 34

3

5. CONCLUSIONS 36

35

In this work, the compositional variation (elemental diffusion), grain morphology (grain 37 38

structure/orientation) and surface topography (surface roughness) of the Ag reflective mirror 39 41

40

layer of GaN-based blue VLED have been systematically investigated with advanced 43

42

microscopies for performance optimization, and their corresponding results as follow: 4 45 47

46

1) STEM-EDX analysis at key interface of Ag-GaN (metal-semiconductor) determined that 49

48

the In and Ga from GaN (semiconductor) region out-diffuses into Ag (metal) layer, while 50 52

51

Ag (metal) diffuses into GaN (semiconductor) region. Ag migration occurs not only in p54

53

GaN layer but also into InGaN/GaN quantum wells and In-based superlattice, as well. 56

5

Further, formation of Ag-O (Ag oxidation), Ga-O and Ag-Ga bonds lead to non-abrupt 57 58 59


Page 31 of 40


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Ag-GaN interface. Such interfacial diffusion between Ag (metal) and GaN 4 5

(semiconductor) degrades both electrical and optical properties of the LED. 6 8

7

2) SEM-based TKD combined with EDX as an sophisticated technique provided vital 10

9

information for each cross-sectional layers of VLED, i.e., grain structures (size), grain 1 12

boundaries (GB), special boundaries, crystallographic orientation (textures), defects and 13 15

14

phases exist in between semiconductor (epitaxial) layers, and metal (adhesion, barrier, 17

16

reflecting) layers. SEM-TKD results for thin specimen of Ag reflectivity layer discerned 18 20

19

that it has special nano-TBs. 2

21

3) Further, SEM-EBSD analysis on the top bulk surface of an Ag reflective layer depicted 23 24

its surface morphology that consists of fine-grained structures (~300nm), and more 25 27

26

importantly possesses special CSL TBs with strong fibre texture. Such nano-morphology 29

28

of Ag with specific texture is vital for highly reflective material in order to design 30 31

efficient LED light sources. 32 34

3

4) Moreover, surface topography of Ag reflective layer with an average 3-4nm roughness 36

35

was determined by AFM, and shows Ag has smooth (flat) surface. However, results also 37 38

indicated the presence of whiskers induced by thermal annealing process (in O2 ambient 39 41

40

environment), increases the micro surface roughness; consequently, deteriorate the 43

42

reflectivity of the Ag mirror layer. 4 46

45

5) Based on these correlative microscopy analysis and results, feasible suggestions on 48

47

performance optimization are proposed, which can be integrated in prospective VLED 49 50

device scaling and modelling. 51 52 54

53

In summary, quality of the Ag reflective layer is strongly dependent on the control of 56

5

interfacial diffusions, grain morphology and surface topography. Abrupt Ag-GaN interface and 57 58 59



Page 32 of 40

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2

compact smaller (fine) grains with smooth surface (low roughness) are essential criteria for 5

4

better reflectivity of light at Ag mirror layer. Hence, these results are critical in understanding the 6 8

7

origin of diminution in IQE and EQE, which will assist significantly to enhance the overall light 10

9

emission efficiency of the VLED devices. 1 12 13 14 16

15

6. FUTURE OUTLOOK 18

17

We believe that our correlative microscopy results will open new opportunities for 19 21

20

optimization of enhancing LED performances by controlling the grain boundaries; especially by 23

2

controlling the nano-TBs. In fact, these results could have direct effects on the “Application of 24 25

Ag material in micro-optoelectronics industry” because the special nano-TBs at Ag reflective 26 28

27

layer might have peculiar attributes towards enhancement of critical LEE factor. Furthermore, 30

29

these attributes will not only make Ag an attractive model system for the study of TBs formation 31 32

and twin-induced strengthening mechanisms in LEDs but also for other organic solar cells 35

34

3

(OPV) and OLED devices, where Ag serves diverse functionality either as a contact electrode, 37

36

reflection or wafer bonding layer.93–95 38 39

Moreover, SEM-based TKD and EBSD techniques will facilitate us in improvement of grain 42

41

40

alignment by correlating it with grain boundary engineering, and subsequently further structure4

43

properties-process-performance relationship (material design paradigm) can be determined. 45 47

46

Although, SEM-based TKD and EBSD is less beam damaging compared to TEM (30kV vs 49

48

300kV). However, there are certain limitations, such as time consuming FIB-preparation of thin 50 51

samples for TKD or requisite of smooth (flat) surface for EBSD analysis, and somewhat 52 54

53

difficulty in obtaining patterns or textures for a very thin (narrow) film layers (below

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