Design and analysis of novel high-performance III-nitride MQW-based nanowire white-LED using HfO2/SiO2 encapsulation

A novel, white light-emitting diode structure with improved thermal characteristics is designed for providing efficient light which may be used especially in the underground mining environment. It describes a new nano hafnium oxide-silica doped silicone layer as LED encapsulation material that promises enhanced efficiency by 30.1% and reduced efficiency droop of 0.491%. The enhanced power and efficiency of the LED with HfO2/SiO2 doped bi-layer are attributed to the significant minimization of overflow of electrons which is fundamentally responsible for efficiency degradation through p-GaN region. In this article encapsulant material based on nano HfO2/SiO2 not only enhances light extraction but opens a broad new range of encapsulant engineering capabilities composites. Our designed LED is generated from a monolithic composition of blue and yellow spectrum which eventually creates the white light. This minimizes the problems related to multiple numbers of LEDs, green gap and phosphor color rendering.


Introduction
Over the past decades, mining industry has a constructive impact over the economic growth worldwide in terms of employment opportunities, country's income rate, and ample supply of raw materials. However, due to hazardous underground nature, exceptional importance must be given to the safety measures of the miners. The miners mostly depend on visualization during their navigation in order to identify obstacles and overcome disasters. Clear vision is essential to avoid slip, trip and fall (STF) hazards, potential machinery related pinning and striking accidents. The underground environment consists of dust, compact areas, paucity of adequate lighting, lack of reflecting surfaces, glare, heavy machinery equipment which make the path very difficult to be illuminated; as cited by the Illuminating Engineering Society of North America (DiLaura et al. 2018).
To enhance visibility in the path direction, cap lamps are used by miners on their hats to get a clear picture. In comparison to incandescent and fluorescent light bulbs, the white light-emitting diodes (LEDs) are emerging to be a better replacement because of their larger efficiency, long lifespan and energy saving potential (Schubert and Kim 2005;Smet et al. 2011;Sammarco and Lutz 2011). The white light emission from a cap LED with encapsulated layer permits accurate color identification which eventually leads to a strong contrast to dark backgrounds and high insight of luminance. Studies suggest that white LEDs have a maximum of 80 Chromaticity Index (CRI) as compared to incandescent and fluorescent lamps having CRI 95 and 60 respectively (Dmello 2019). Hence white LEDs are preferred in mining industry because of high CRI and small size. Unlike traditional lights the intensity of LED does not fail catastrophically, rather their light power gradually decreases with time; known as lumen maintenance. The LED technology potentially enhances miner's visual capability while being underground, improves communicating signals and warnings, calls for less power, and minimizes exposure to maintenance related hazards (Reyes et al. 2011;Martell et al. 2017).
One of the drawbacks faced by LED is that light is trapped inside a high refractive index (RI) semiconductor (Dai et al. 2010). The light extraction efficiency (LEE) thus severely reduces when the incident light in the semiconductor is trapped due to total internal reflection (TIR) and Fresnel reflection. This happens if the angle of incidence is larger than the critical angle ( C ) for TIR. In order to increase the LEE of LEDs, various methods have been reported such as LEDs with textured plains (Fujii et al. 2004), LEDs with photonic crystals (Matioli et al. 2010;Wierer et al. 2009), LEDs substituted on patterned substrates (Cho et al. 2005), LEDs with upgraded die shapes (Wang et al. 2009), and LEDs coated with graded-RI anti-reflection coatings (Kim et al. 2008;Chiu et al. 2009). In this paper, the LED chip is encapsulated with silicone layer coated with nanoparticles (NP) thus increasing both C and light-escape cone size. The schematic of how light rays travel from an encapsulated LED is explained in Fig. 1. Encapsulation will eventually enhance the LEE of the device. Analytically LEE of LEDs with encapsulation can be calculated as: where, η 1 and η 2 are the LEE of LEDs with and without encapsulation respectively. C,1 and C,2 are the respective critical angles for semiconductor/encapsulant and semiconductor/air interfaces. Also according to Snell's law, the relation between critical angle and RI can be mathematically written aswhere 1 and 2 are the RI of rarer and denser medium respectively.

Design of nanowire white leds
White LEDs on the basis of solid-state technology have achieved large attention due to their enormous energy saving potentials (Schubert and Kim 2005;Tan et al. 2012;D'Andrade and Forrest 2004), approach for liquid crystal displays (Ugustov 2000;Jang et al. 2010;Piprek 2020), lighting technology and many more. From earlier research, there are two ways of producing white light from a LED-by mix matching equal amount of red, blue, green (RBG) lights (Jerome 1976;Azevedo et al. 2009) which will cost more space; thus not a suitable option for miners. At the same time "green gap" limits the usage of this method resulting in low CRI and reducing the efficiency of light (Auf Der Maur et al. 2016;Zhou et al. 2018). Secondly by converting monochromatic light from blue LED to generate broad-spectrum white light when a phosphor coating is used in conjunction with it (Mueller-Mach et al. 2005;Xie et al. 2007;Allen and Steckl 2008;Ye et al. 2010). Phosphor absorbs some blue light and emits them in a wide yellowish color. But due to poor color rendering of phosphor emission, the objects illuminated have slightly more blue tint. In our designed LED, blue and yellow spectrum is generated from a monolithic composition which eventually creates the white light. Thus, the problems related to multiple numbers of LEDs, green gap and phosphor color rendering are minimized. In various researches, thorough study has been done for white LED nanowire structures (Rajan Philip, et al. 2017;Philip et al. 2017;Zhao et al. 2015;Nguyen et al. 2013;Nguyen et al. 2011) which exhibit high quantum efficiency because of less dislocation densities, the resultant polarization fields (Nguyen et al. 2011).
InGaN nanowire LEDs have high electron mobility due to low effective mass of carriers, high saturation velocity, high thermal conductivity and large heat capacity (Guo et al. 2011a). It opposes high radiation doses while preserving its optoelectronic properties. Nanowire white LEDs with Mn +4 doped fluoride nanosheets have improved color quality (Vu, et al. 2021). Thus, InGaN alloys are considered hard materials and used in extensive areas such as underground mining. (1) In this study, the commercially available Victory TCAD is used to systematically study the electrical and optical characteristics of InGaN nanowire white LED (Clara 2004). Figure 2 displays the final three InGaN/GaN/InGaN nanowire LED structures.
The first conventional structure LED 1 in Fig. 2a has 60 µm sapphire substrate, 200 nm n-GaN layer, four 7 nm InGaN quantum wells (QW) stacked by five 3.5 nm GaN quantum barriers (QB), 20 nm p-Al 0.2 Ga 0.8 N electron blocking layer (EBL), 50 nm p-GaN cladding layer and 70 nm p-GaN contact layer. The n-GaN layer has doping concentration of 5 × 10 18 cm −3 , EBL has 3 × 10 19 cm −3 , p-cladding layer has 1 × 10 20 cm −3 and p-contact layer has 1 × 10 19 cm −3 . In particular the thermal conductivity of sapphire is extreme at low temperature making it a strong material in underground routes during mining. The first three QWs, with 42% indium will focus on yellow light emission and the fourth well with 30% indium focuses on blue emission. To obtain a neutral white emission, the barrier between blue and third yellow wells is replaced with InGaN having 1.5% indium content. The doping in this barrier is tuned from 1.5% to 3.6% to control the spectral color temperature. As a result, it maintains confinement in the blue well and increases the flow of carriers towards n-side i.e. yellow well; thus producing strong white emission. The more yellow content is created; warmer white light is obtained (Zhao et al. 2021). The overall area of the LED chip is 250 µm × 250 µm operated under a temperature of 300 K. Other parameters used in this work are the band offset ratio of conduction and valence band, i.e., 0.8/0.3, Auger Coefficient 2.8 × 10 -30 cm 6 s −1 , Shockley-Read-Hall (SRH) lifetime is 10 ns, electron and hole mobility of 100 cm 2 /V.s and 10 cm 2 /V.s, respectively.
At the LED package level, operating efficiency can be increased by using higher-index nanoparticles. For maintaining good optical efficiency and protect the device from environmental hazards, we use hemi-spherical silicone encapsulation layer doped with silica (SiO 2 ) nanoparticles on top of p-GaN layer to improve the LEE as shown is Fig. 2b (LED 2 ). SiO 2 has high RI (1.47) and superior light scattering capability. To further increase the efficiency and reduce internal reflection loss, this mono layer SiO 2 is replaced by bi-layers of hafnium oxide (HfO 2 )-SiO 2 nanoparticles as displayed in Fig. 2c. The fabrication procedure of this type of LEDs includes the following steps. After the nanowire LED is grown by molecular beam epitaxy, the sample is cleaned with a standard cleaning process with acetone, isopropanol, and DI water. Polyimide is spin-coated to fully cover the nanowire for surface planarization. The oxygen plasma dry etching is then applied to etch the polyimide until the top surface of the nanowire is exposed for the top metal contact deposition process. Subsequently, the SiO 2 and HfO 2 /SiO 2 encapsulation processes are performed on the fabricated InGaN/GaN nanowire LEDs. The encapsulation can be performed by spin-coating method. The materials, SiO 2 and HfO 2 in addition to optical clarity and high temperature also have the advantageous property of high RI (Guerette, et al. 2015;Huang et al. 2015;Kukli et al. 2002). HfO 2 has high melting point (~ 2774 °C) and a varying RI of 1.85-2.1 (Kukli et al. 2002;Martínez et al. 2007). Also SiO 2 has higher band gap (7.52-9.6 eV) which shows better transmission in the visible range (Tan et al. 2005). To specify the reproducibility of the assays demonstrated in this work, we follow our standard fabrication procedure which is published in our previous study Nguyen et al. 2013;Nguyen et al. 2011;Nguyen et al. 2012;Philip et al. 2017;Nguyen et al. 2015;Jain et al. 2020;Bui, et al. 2019). Similar fabrication methods have been used by other researchers (Zhao et al. 2015;Bai and Wang 2012;Guo et al. 2010;Guo et al. 2011b). Additionally, the fabrication of encapsulation with SiO 2 and HfO 2 /SiO 2 is standard method which is commonly used. The LED package as a whole should be stable when exposed to heat as well as high radiation intensity and light gets extracted out with negligible loss.

Properties of the fabricated samples
High dielectric constant materials have received proper attention for the past few years, due of their vast range of micro and nanoelectronics applications, such as Organic LED. Due to the impressive properties of HfO 2 such as high dielectric constant, wide band-gap, suitable mechanical, chemical and electrical characteristics, these are used in the manufacturing of electronic appliances. In nano-electronic industry HfO 2 has been identified as the best materials for the replacement of SiO 2 or TiO 2 because of its high dielectric constant and stability in contact with silicon (Wilk and Wallace 2001;Ferrari et al. 2004;Angus et al. 2000). Under normal condition of pressure and temperature HfO 2 exhibits monoclinic structure. It can transform into tetragonal form when heated to temperature above 1700 °C. Further transformation into the nanoparticles polymorphic form takes place at 2700 °C (Villanueva-Ibańez et al. 2003;Tang et al. 2005).

Results and discussion
Initially the indium composition in the barrier is varied from 1.5% to 3.6% which provides warm white light at a fixed bias of 4.5 V as shown in Fig. 3. The spectral density has a wavelength range from 440 to 520 nm signifying a white light emission. The proposed device with In 0.036 Ga 0.964 N barrier achieves numerically higher output of 30.19 W/(cm eV) at 480.23 nm wavelength than with In 0.015 Ga 0.985 N barrier having 25.36 W/(cm eV) at 479.65 nm wavelength indicating a warmer white light. This value is 46.7% higher than the one with GaN barrier delivering a mild and cool white light. Now a comparative study of important parameters has been made among the designed structures LED 1 , LED 2 and LED 3 . The calculated energy band diagrams for LED 1 , LED 2 and LED 3 are shown in Fig. 4a, b and c respectively. The grays lines in the figures represent the MQW of the LED.
The conduction band barrier height (CBBH) in Fig. 4a for LED 1 is 126.6 meV which is progressively lower than LED 2 (451.5 meV) and LED 3 (577.4 meV). This signifies that higher energy is required in LED 3 for electrons to overcome the quantum barriers (QBs) and flow into the p-region. As compared to LED 1 and LED 2 more electrons will be accommodated inside the wells of LED 3 and have poor electron leakage. This justifies the resultant higher electron concentration of LED 3 in Fig. 5a. The measured values of electron Fig. 3 Normalized electroluminescence spectra measured at 300 K showing that white LED with In 0.036 Ga 0.964 N barrier has highest peak emission Fig. 4 a. Energy Band Diagram of active region without any encapsulation layer, b. Energy Band Diagram with SiO 2 encapsulation layer.c. Energy Band Diagram with HfO 2 /SiO 2 encapsulation layer concentration for LED 1 , LED 2 and LED 3 are 1.18 × 10 20 , 1.26 × 10 20 and 1.97 × 10 20 cm −3 respectively.
From Fig. 4b, the valence band barrier heights (VBBH) for holes in LED 2 are 508.1 meV which are higher than LED 3 (264.5 meV). In LED 2 this happens due to lack of lattice alignment between the EBL and QB. Because of superior indium composition in the QB, the polarization fields mitigate at the heterointerfaces in LED 3 (Zhang et al. 2014). Also higher indium composition is responsible for creating positive polarization sheet charges near the QWs. Because of the aforesaid reasons LED 3 exhibits higher hole concentration (2.48 × 10 20 cm −3 ) in the active regions as displayed in Fig. 5b.
Maximum efficient light extracted from the semiconductor chip is our primary goal in designing a nanowire LED. Higher index encapsulants reduce TIR at the interface between the LED chip and the encapsulation material. In LED 1 , light rays travelling from layered semiconductor chip (GaN material with RI 2.38 (Muth, et al. 2015)) escapes outside to air only when it emerges at angles less that the critical angle ( C,2 = 25.3° from Eq. 2) while the remaining portion is lost in TIR. In LED 2 , light rays escape from the chip into epoxy of SiO 2 (RI = 1.47 and C,1 = 38.9°). From Eq. 1, LEE for LED 2 comes out to be 2.31 times that of LED 1 which signifies a rise in the EQE. As displayed in Fig. 6, LED 2 has 79.59% EQE at 300 A/cm 2 which is 29.05% more than the efficiency of LED 1 . The SiO 2 -doped silicone encapsulated layer significantly increases the average RI light scattering ability, thus reducing the reflection loss between the LED chip and the epoxy layer. In our proposed device LED 3 , bi-layers HfO 2 /SiO 2 are used in the epoxy dome. Because of higher dielectric constant of HfO 2 (16.64), more energy is stored and heat dissipation is lowered. Also because of the spherical curvature of epoxy dome, light rays strike the epoxy-air interface at 90 • and emerge out with negligible reflection loss. Due to minimized electron leakage in LED 3 , it has the highest EQE (80.19%) at 300 A/cm 2 as shown in Fig. 6. Moreover the reduction in efficiency droop in LED 3 (0.491%) as compared to the other cases is shown.
The better carrier concentration of LED 3 leads to more desirable output light power as illustrated in Fig. 7. The LED 3 is observed to emit a higher output power providing a warmer beam of white light compared to other two cases. The designed LED 3 exhibits Fig. 6 Normalized external quantum efficiency measured at 300 K for LED 1 , LED 2 and LED 3 . Inset depicts the efficiency droop measured at 300 A/cm 2 Fig. 7 Normalized power-current density curves for LED 1 , LED 2 and LED 3 . Inset: Enhanced ratio defined as the light-output power of the treated sample (LED 2 and LED 3 ) divided by conventional (LED 1 ).
outstanding current-voltage properties with quite little leakage current that is only ∼ 6 µA at 5 V as shown in Fig. 8. The device's series resistance is also minimized in LED 3 indicating a definite level of enhancement in the transportation of holes. The radiative recombination is significantly increased in LED 3 , (4.36 × 10 28 cm −3 ) as depicted in Fig. 9 and Table 1.

Conclusion
The procedure of color mixing and band engineering has been coupled together to construct the white LED nanowire with great control over the color temperature. The LED with 3.6% indium content in the barrier produces a warmer white light with a spectral peak of 30.19 W/(cm eV) (λ = 480.23 nm). The proposed device with HfO 2 /SiO 2 doped silicone encapsulated layer has relatively highest EQE of ~ 80.192% with remarkably high emission of light and virtually negligible efficiency droop till current density 400 A/cm 2 . The CBBH for LED 1 is 126.6 meV which is lower than LED 2 and LED 3 indicating higher energy in the latter. This leads to higher electron concentration of 1.97 × 10 20 cm −3 for LED 3 . Because of superior carrier concentration in LED 3 , a higher white light output power and high radiative recombination rate is observed. Therefore, when the LED is encapsulated with HfO 2 / SiO 2 nanoparticles, the light scattering capability is enhanced. This reduces reflection losses and remarkably stable white light emission is extracted out which is quite useful in the underground mining industry. While this work is preliminary, continued refinement of this technology platform offers exciting new approaches to improved performance and manufacturing of solid-state lighting systems.

Author contributions All authors equally contributed for the preparation of the manuscript.
Funding This work is the outcome of DST-SERB; Govt. of India sponsored MATRICS Project No MTR/2021/000370 which is duly acknowledged for support.

Conflict of interest
The authors declare that they have no conflict of interest.

Consent for publication All authors have given consent for publication.
Ethics approval The manuscript is ethically approved.
Human beings or animals This article does not contain any study on human beings or animals.