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100 , 0.5nm f = =

100 , 100 , 2001 2nm d nm d nm = = =

Middle-East Journal of Scientific Research 22 (2): 193-198, 2014ISSN 1990-9233 IDOSI Publications, 2014DOI: 10.5829/idosi.mejsr.2014.22.02.21845

Corresponding Author: Samah. G. Babiker, Department of Physics, Faculty of Education, University of the Holy Quran and Islamic Sciences, Omdurman, Sudan.

193

Polarized Gan-Based Light-Emitting Diode with a Silver Sub Wavelength Grating and Dielectric Layer

Samah. G. Babiker, Mohamed Osman Sid-Ahmed, Shuai Yong and Xie Ming1 2 3 3

Department of Physics, Faculty of Education,1University of the Holy Quran and Islamic Sciences, Omdurman, Sudan

Department of Physics, Faculty of Sciences,2Sudan University of Science and Technology, Khartoum, Sudan

Department of Engineering Thermophysics,3School of Energy Science and Engineering, Harbin, P.R. China

Abstract: The effect of silver/ dielectric sub-wavelength gratings and silicon dioxide transition layer, whenembedded onto InGaN/GaN -LED system, has been studied for enhancement of the polarized light. The SiO2transition layer is placed between InGaN/GaN -LED and the grating section. The effect of the gratings geometricparameters on the transmission of transverse magnetic (TTM) and electric mode (TTE) and polarizationextinction ratio (ER) were calculated numerically by using the rigorous coupled-wave analysis (RCWA). Theresults show that the proposed structure embedded onto InGaN/GaN-LED system has enhanced thetransmission and extinction ratio by the presence of the SiO transition, due to the interference in the Ag sub-2wavelength gratings and SiO transition layer. The optimum parameters of the proposed structure are: grating2period and . TTM >80% and ER >15dBhave been obtained over the whole range of with theSiO transition layer. The performance of this LED system has proved to be superior to that with single-layer2metallic gratings. The optimum parameters of the proposed structure are and f= 0.5which are results TTM >80% and ER >15dB. The performance of this LED system proved to be superior to thatwith single-layer metallic gratings.

Key words: Sub-wavelength grating Rigorous Coupled-Wave Analysis (RCWA) Light emitting diodes Polarization.

INTRODUCTION LEDs. The EQE can be increased by increasing the IQE

Light-emitting diodes (LEDs) are semiconductor (approaching100%) of IQE have already beensolid-state lighting components which convert electrical demonstrated by the epitaxial growth and deviceenergy into electromagnetic radiation; most of the fabrication technologies [1, 9]. The LEE is very lowradiation is visible to the human eye. LEDs have superior compared to the IQE, because it is limited by the totalcharacters, compared to the traditional light sources, such internal reflection (TIR) at the interface between theas having low-cost, smaller size, long-terms stability, high light-emitting material and the outside of the LED [4, 10].efficiency, strong brightness, long lifetime and Several methods have been proposed to increase theenvironmentally-friendly [1-3]. LEDs have been commonly efficiency of LEDs. It has been shown that light extractionused in various fields such as traffic signals, automobile efficiency LEE can be sufficiently improved, using properlighting, full-color displays, liquid crystals and optical micro/nanostructures, such as hemispherical source LEDsinterconnects [3-7]. The external quantum efficiency of [11], funnel shaped LEDs [12], truncated-inverted-pyramidLEDs (EQE) is mainly dependent on the internal quantum LEDs [13], surface textured LEDs [14], photonic crystal orefficiency (IQE) and light extraction efficiency (LEE). It is grating assisted LEDs [15-16] and metal assisted LEDs an important criterion for estimating the performance of [17].

and/or enlarging the LEE [1, 8]. Very high values

( , ) ( , ) ( )exp( )j xj

x z x z Z jPk= + =

1j =

( , ) exp( )tt tj xj zjj

E x z E ik x ik z=

2j tjT E=

100 , 0.5, 1201wnm f d nm = = = =

Middle-East J. Sci. Res., 22 (2): 193-198, 2014

194

Fig. 1: Schematic of the proposed structure consisting of periodic gratings. RCWA analyzes the diffraction problemAg sub-wavelength gratings and dielectric layer by solving Maxwells equations accurately in each of theSiO three regions (input, grating and output), based on2

Polarization properties of sub-wavelength gratings depends solely upon the number of terms retained inLEDs depend on the wavelength ( ) of incident light, the space harmonic expansion of electromagnetic fields,grating period ( ) and grating width (w) [6]. Zhang et al which corresponds to the diffraction order. The incident[18] used a surface emitting linearly polarized InGaN/GaN wave and the transmitted waves are located in region ILED with a sub-wavelength metallic nano-grating to and V, respectively. The relative dielectric function inobtain a large polarization ratio. The diffraction efficiency grating region II can be expanded as a Fourier series andand transmission are very sensitive to the choice of expressed asgrating structure. For example, if the period of the gratingis close to , diffraction would be influenced by Woods (1)anomaly. The Polarization properties from LEDs such asthe emission are desired for imaging and backlighting of Where is the grating period, is the Fourier componentflat panels. of the grating permittivity, K is the magnitude of the

In this paper, we propose a structure consisting of grating vector and . The normalized total electricAg sub-wavelength grating structures and SiO as a2dielectric layer which is embedded onto LED system. Theinfluence of gratings geometric parameters, of theproposed structure, on the transmission properties (TTE,TTM) and extinction ratio (ER=10log (TTM/TTE)) will beinvestigated. We will also investigate the optimum gratingparameters which would lead to low fabrication ofpolarized LEDs. All numerical results are obtained byusing the rigorous coupled-wave analysis (RCWA)method [19- 20].

Grating Simulations: Figure 1. displays the schematic ofthe proposed structure which is composed of silversub-wavelength grating and SiO dielectric layer,2embedded onto the polarized GaN LED system throughthe methods of vacuum evaporation and electron-beamlithography. The polarized InGaN/GaN LED consists ofGaN buffer layer, grown on sapphire substrate bymetal-organic chemical vapor deposition (MOCVD).The InGaN/GaN is a quantum-well (QW) with 150 nm GaNcap layer. The thickness of the QW with GaN buffer layer

is about of 5nm. A protection layer was deposited on topof the system to avoid grating oxidation. The z axis is thelight propagation direction, the x and y lies in the directionof p-polarization (TM) and s-polarization (TE),respectively. and represents the polar angle and theazimuthal angle, respectively.

MATERIALS AND METHODS

Rigorous coupled-wave analysis (RCWA),formulated in the 1980s by Moharam and Gaylord, is usedfor analyzing the diffraction of electromagnetic waves by

Fourier expansion.The accuracy of the solution computed

p

field in (E ) can be expressed ast

(2)

Where k and k are the j order wave vector in the x andxj zj th

z direction respectively. E is the normalized amplitude oftjthe j order transmitted wave. The transmission (Ti) of theth

j order transmitted wave can be calculated byth

(3)

RCWA accurately describes diffraction from an Agsub-wavelength grating with and without dielectric layer.

RESULTS AND DISCUSSION

Effect of the Dielectric Layer Thickness: TTM,TTE and ER when and different

dielectric layer thickness d (from 0 to 300) nm, at normal2incidence, are shown in Figure 2. The results show that

100 , 0.5nm f = =


195

Fig. 2: The simulated transmission spectra for TM and TE and extinction ratio, with different dielectric layer thickness(a) TTM (b) TTE (c) ER

Fig. 3: The simulated transmission spectra and ER, with different metal grating thickness (a) TTM and TTE (b) ER.TTM1,TTE1 and ER1 refer to the grating with SiO layer, TTM2, TTE2 and ER2 are for grating without SiO layer, d =2 2 20nm

the proposed structure embedded onto LED system has Effect of the Metal Grating Thickness: The effect of thetwo TTM, TTE peaks and two ER troughs at d = 60nm metal grating thickness d on the transmission and2and 200nm. ER is also high at about 23 dB (low TTE) at extinction ratio is studied with different d (from 0 to 300)d = 15nm and 280nm.The oscillatory behavior of TTM, nm, and d = 200nm, at normal incidence.2TTE and ER is caused by the interference effect for a The results in Figure 3 (a) show the proposed structurethree-layer structure. It can be seen that the insertion of embedded onto InGaN/GaN LED system. The TTM > 70%SiO transition layer into the grating lead to enhancement at d 20nm with SiO layer. It can be seen that the values2of the transmission and polarization extinction ratio of TTM are low for grating without the SiO transition\compared to that without SiO transition layer (d = 0nm). layer.TTE is zero at d 50nm with and without the SiO2 2It can be concluded that the enhancement of the layer, which is the region where the polarization ratiotransmission and extinction ratio can be tuned by different (TTM-TTE)/TTE approaches infinity. In our area ofthicknesses of the dielectric layer depending on the interest, we notice a high TTM1(with SiO ) value atspecific requirement of the polarized LEDs system. d = 100nm.The maximum value of the ER is about 50dB for

1

1

2

1 2

2

1 2

2

1


196

Fig. 4: The simulated TTM and ER, with different metal grating period (a) TTM (b) ER. For grating with and without SiO2layer (d = 0nm )2

Fig. 5: The simulated TTM and ER, with different filling ratio (a) TTM (b) ER. TTM1, TTE1 and ER1 refer to the gratingwith SiO layer; TTM2, TTE2 and ER2 are for grating without SiO layer d = 0nm2 2 2

grating with SiO transition layer. It has to be noticed that presence of the SiO layer. TTM1 (with SiO transition2the insertion of SiO layer to the grating has enhanced the layer) is above 80%, TTE1 >20% and 8 < ER < 20dB when2transmission. 0.2 < f < 0.6. Without SiO transition layer, the

Effect of the Metal Grating Period: Figure 4.shows the to that with SiO transition layer. The results show theTTM and ER of the proposed structure embedded dependences of the transmission and extinction ratio ononto GaN LED system with, d = 100nm, d = 200nm the filling ratio. The optimum filling ratio is about f = 0.5.1 2and f =0.5, for different metal grating period ( ) (from 0 to300) nm, at normal incidence angle. The results show Effect of the Wavelength of Light: The polarizedthat the system has a high value of TTM and ER for transmission by varying the wavelength of light is alsograting with SiO transition layer. TTM >80% and ER studied (all the above simulations were done for =4002>15dB when the grating period is in the range 10nm < nm). Figure 6 shows TTM, TTE and ER at (from 400 to110nm . It is clear that without the SiO transition layer, 500) nm with grating parameters of = 100nm, , d =2TTM and ER values are much lower than those with the 100nm and d = 200nm and f = 0.5 for normal incidence. ItSiO transition layer. The optimum metal grating period can be seen that TTE is zero, which results in high values2which is used in our study is = 100nm for high TTM for ER. The results indicating that the transition layer hasand ER value. the largest effects in the UV/blue spectral region.

Effect of the Filling Ratio: The effect of the filling ratio (f) Effect of the Plane of Incidence: The effect of the plane ofon the transmission and extinction ratio of the proposed incidence on the TTM, TTE and ER of the proposedstructure (embedded onto InGaN/GaN LED system) with structure embedded onto InGaN/GaN - LED system isand without SiO transition layer, is studied with different studied with grating parameters of = 100nm, d = 100nm2filling ratio (f) (from 0.1 to 0.9), = 100nm, d = 100nm and d = 200nm and f = 0.5 for different polar angles at and1d = 200nm at normal incidence. It can be noted that, -90 90, = 0. Figure 7 shows that TTM > 80%2Figure 5, the transmission has increased due to the and ER > 15dB over the whole range of with the SiO

2 2

2

transmission and extinction ratio are very low compared2

1

2

1

2

2


197

Fig. 6: The simulated TTM and ER with different wavelength (a) TTM (b) ER. TTM1, TTE1 and ER1 refer to the gratingwith SiO transition layer; TTM2, TTE2 and ER2 are for grating without SiO layer d = 0nm2 2 2

Fig. 7: The simulated TTM, TTE and ER with different , = 0 (a) TTM (b) TTE (c) ER. TTM1, TTE1 and ER1 refer tothe gratings with SiO transition layer; TTM2, TTE2 and ER2 are for grating without SiO transition layer d = 0nm2 2 2

transition layer. It is clear that the TTM has been greatly CONCLUSIONincreased by inserting the sub-wavelength Ag/ dielectricgrating into the system. In order to enhance LED transmission and extinction

The proposed structure which consists of the Ag ratio we embedded Ag sub-wavelength gratings and SiOsub-wavelength gratings and SiO transition dielectric transition dielectric layer onto InGaN/GaN LED system.2layer integrated onto InGaN/GaN-LED system has The influence of gratings geometric parameters on theexhibited enhanced polarization performance. It has transmission and extinction ratio were studied by usingyielded high TTM and ER with the following grating the rigorous coupled-wave analysis (RCWA). The resultsparameters values: = 100nm, d = 100nm d = 200nm and showed that the transmission and extinction ratio are1 2f = 0.5. affected by the gratings geometrical parameters.

2


198

The optimum parameters are = 100nm, d = 100nm 10. Kitagawa, H., M. Fujita, T. Suto, T. Asano and1d = 200nm and f = 0.5. We have shown that TTM and ER S. Noda, 2011. Green Gain photonic-crystal2performance could be significantly enhanced by inserting light-emitting diodes with small surfaceSiO dielectric layer into the grating section. This can be recombination effect, Journal of Applied Physics2explained by the interference in the Ag sub-wavelength Letters, 98: 181104.gratings and SiO transition layer. TTM > 80% and ER 11. Carr, W.N. and G.E. Pittman, 1963. One watt GaAs p-n2> 15dB were obtained over the whole range of with the junction infrared source, Journal of Applied PhysicsSiO transition layer. Letters, 3: 173-175.2

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x 1-x 0.5 0.5

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