We demonstrate novel method for improving light extraction efficiency for n-side-up vertical InGaN/GaN light-emitting diodes (V-LEDs) using MgO nano-pyramids and ZnO refractive-index modulation layer. The MgO nano-pyramids structure is successfully fabricated on n-GaN/ZnO surface using electron-beam evaporation. The light output power of n-GaN/ZnO/MgO V-LEDs is enhanced by 49% compare to that of n-GaN V-LEDs. The angular-dependent far-field emission shows the significant increase of side emission for the n-GaN/ZnO/MgO V-LEDs due to the increase of critical angle for total internal reflection as well as the roughened surface by MgO pyramids structure. These experimental results indicate the critical role of surface texturing in improving the light extraction efficiency of the V-LEDs for solid-state lighting.
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GaN-based light-emitting diodes (LEDs) are attracting great interest as candidates for next-generation solid-state lighting, because of their long lifetime, small size, high efficacy, and low energy consumption [1,2]. However, for general illumination applications, the external quantum efficiency of LEDs, determined by the internal quantum efficiency (IQE) and the light extraction efficiency, must be further increased. The IQE is determined by crystal quality and epitaxial layer structure and high value of IQE more than 70% for blue LEDs have been already reported [3,4]. However, there is much room for improvement of light extraction efficiency because most of the generated photons from active layer remain inside LEDs by total internal reflection (TIR) at the interface of semiconductor with air due to the high refractive index difference between LEDs epilayer (for GaN, n = 2.5) and air (n = 1) . The light confining in LEDs will be reabsorbed by the metal electrode or active layer, reducing the efficacy of LEDs. Therefore, to improve light extraction efficiency, several kinds of methods have been investigated, such as surface roughening by KOH-based wet chemical etching [6,7], photonic crystals [8,9], anti-reflection coatings , patterned sapphire substrates , n-side-up vertical-structure LEDs (V-LEDs) by laser lift-off (LLO) technique [12,13], and nanowire-based LEDs [14,15].
Surface roughening of N-face GaN (000-1) of V-LEDs using wet-chemical etching is known to be very efficient to increase light extraction efficiency [6,7]. However, wet-chemical etching needs additional passivation process to prevent the unintentional etching of n-type ohmic contact and sidewall of LEDs. Although the photonic crystal also could enhance the light extraction efficiency of LEDs, it is difficult to form the submicron pattern using conventional photolithography. Recently, it was found that the light extraction efficiency of LEDs with the planar graded-refractive-index ITO nanorod anti-reflection coating was increased by 24.3% .
Here, we present the first demonstration of enhanced light extraction by forming a MgO nano-pyramids structure on the surface of V-LEDs. The MgO nano-pyramids structure was successfully fabricated at room temperature using conventional electron-beam evaporation. The nano-sized pyramids of MgO are formed during growth due to anisotropic properties of MgO between crystal orientations . The ZnO layer with quarter-wavelength in thickness is inserted between GaN and MgO layers to increase the critical angle for TIR, because the refractive index of ZnO (n = 1.94) could be matched between GaN (n = 2.5) and MgO (n = 1.73). The MgO nano-pyramids structure and ZnO refractive-index modulation layer enhanced the light extraction efficiency of V-LEDs about 49%, comparing with the V-LEDs with a flat n-GaN surface. The angular-dependent emission intensity shows the enhanced light extraction through the side walls of V-LEDs as well as through the top surface of the n-GaN, because of the increase in critical angle for TIR as well as light scattering at the MgO nano-pyramids surface.
InGaN/GaN multiple quantum well (MQW) LED structure was grown by metal-organic chemical vapor deposition on c-plane sapphire substrates and consisted of a 500-nm-thick undoped GaN buffer layer, a 4-um-thick n-type GaN, InGaN/GaN multiple quantum-well active region, a p-type AlGaN electron blocking layer, and a p-type GaN layer. For the fabrication of vertical LEDs, active regions were defined by dry etching into the sapphire substrate. Sample was treated with boiling aqua regia and HCl solutions . The Ag-based reflective p-type ohmic contact was deposited on p-GaN, followed by annealing at 400 °C for 2 min in air ambient. A 50-μm-thick Ni layer was electroplated on the p-contacts as a platform and subsequently the LLO process of the sapphire/MQW LED/Ni structure was performed in air using a Lambda Physik Compex 205 KrF pulsed excimer laser. After the LLO, the Cr/Au n-type ohmic contact was deposited on n-type GaN, forming n-side-up vertical InGaN/GaN LEDs on the Ni metal substrate. The V-LEDs sample was loaded into an e-beam evaporator, followed by the deposition of ZnO (60 nm) layer using high purity ZnO pellet (99.99% with a diameter of 3 mm) as a refractive-index modulation layer. The films were grown at a rate of 0.3 nm s−1, at a substrate temperature of 200 °C. To form the nano-pyramids structured surface of MgO, the V-LEDs sample was loaded again into an e-beam evaporator. MgO (0.7, 2, 4 μm) layer was evaporated using high purity MgO pellet (99.99% with a diameter of 3 mm). The films were grown at a rate of 2 nm s−1 at room temperature.
The light output power of V-LEDs was measured at an unpackaged (on-wafer) configuration using an integrating sphere. The far-field emission intensity of V-LEDs was measured with a resolution of 1°. The refractive indices of ZnO and MgO layer were measured by using the ellipsometry (J. A. Woollam Co., Inc M-44). The SEM is done using a PHILIPS XL30S with an accelerating voltage at 10 kV and a working distance of 5 mm. The HRTEM images were collected using a Cs-corrected JEM 2200FS operated at 200 kV.
3. Results and discussion
The schematic illustrations of light ray trace incident at various angles from V-LEDs with both (a) flat n-GaN and (b) n-GaN/ZnO/MgO nano-pyramids surfaces are shown in Fig. 1 . The refractive indices of n-GaN and air are 2.5 and 1, respectively, and the critical angle (θ crit) can be calculated based on Snell’s Law θ crit = sin−1n2/n1, where n1 is the refractive index of the more optically dense medium, and n2 is the refractive index of the less optically dense medium. Therefore, at the interface of flat n-GaN with air, the critical angle for TIR is only 23.6°. Assuming that the light emitted from sidewalls is neglected, only 4% of the generated light can be extracted from a surface. If an additional two layers with different refractive index of 1.94 for ZnO and 1.73 for MgO are deposited in sequence on n-GaN, the critical angle for TIR increases and thus more lights are able to enter from n-GaN to MgO layer. The MgO layer with the pyramid structure is able to enhance the light extraction at the MgO/air interface because the roughened surface reduces internal light reflection.
The Fig. 2 shows the (a) schematic illustration and (b) SEM image of fabricated n-side-up V-LEDs (1 × 1 mm) with ZnO refractive-index modulation layer and 2-μm-thick MgO layer. The enlarged surface SEM image of V-LEDs clearly shows the MgO nano-pyramids structure formed on the n-GaN/ZnO/MgO surface.
Figure 3 shows the top-view SEM images for the ZnO grown on n-GaN (n-GaN/ZnO), and the MgO layer grown on n-GaN/ZnO (n-GaN/ZnO/MgO) with different thickness of MgO layer. After deposition of 60-nm-thick ZnO on n-GaN, the surface morphology was smooth. However, a nano-pyramids structure was formed when MgO layers were deposited on ZnO surface. As the thickness of MgO layer increases from 0.7 μm to 2 μm, the size of MgO nano-pyramids increases. However, further increase up to 4 μm thickness of MgO causes severely tilted pyramid shape with non-uniform pyramid size distribution.
The tilted pyramid shape at thick MgO film is due to the difference in adatom density between planes of MgO pyramid. At equilibrium state, MgO pyramid has uniform adatom density at the plane 1, 2, and 3 as shown in Fig. 4(a) . The equilibrium state is destroyed by adatom fluctuation during growth and specific plane (plane 3) has more adatom density than other planes. Because the growth rate of facet plane is proportional to the density of adatom , specific plane 3 overgrows, resulting in tilted MgO pyramid shape. This speculation could be confirmed by depositing MgO on tilted substrate. Figure 4(b) shows the top-view SEM images of MgO film with 200 nm thickness deposited on non-tilted substrate and 3° tilted substrates. The MgO nano pyramid deposited on 3° tilted substrate shows the tilted pyramid shape due to disequilibrium density of adatom by substrate tilting.
Figure 5(a) shows the HR-TEM micrograph at the tip of 400 nm thick MgO film. From the HR-TEM image, well aligned termination of MgO (200) plane with lattice spacing of 0.21 nm was clearly shown and these MgO nano pyramids have the crystal structure like Fig. 5(b). The MgO pyramid is formed due to anisotropic properties between (111) and other main planes of MgO; (200) and (220). The (111) orientation of MgO with alternating array of Mg cation and O anion is very unstable because of dipole energy accumulation induced by polarity (Mg2+ plane and O2- plane) [19–21]. So, MgO films tend to grow with surface termination by (200) and their family plane to acquire the most stable atomic arrangement . The clear MgO pyramid shown from the rock-salt MgO crystal structure is the result of randomly distributed hexahedrons enclosed by neutral (200), (020), and (002) planes as shown in Fig. 5(b).
The light transmittance spectra of ZnO refractive-index modulation layer and MgO layers as a function of film thickness is shown in Fig. 6 . The 0.7-μm-thick MgO layer showed high transmittance of 93% at 460 nm wavelength due to the large band-gap (~7.8 eV) of MgO. As the thickness of MgO layer increased up to 4 μm, the light transmittance was decreased up to 84%. Meanwhile, the ZnO layer showed a little low light transmittance of 84% at 460 nm wavelength and this is attributed to the point defect (i.e., VO), and disorder in the ZnO layer, as previously reported .
Figure 7(a) shows the electroluminescence (EL) spectra of V-LEDs with n-GaN, n-GaN/ZnO, n-GaN/MgO and n-GaN/ZnO/MgO measured using an integrating sphere at 350 mA injection current. The light output powers of V-LEDs with n-GaN/ZnO and n-GaN/MgO layer were increased compared to V-LEDs with flat n-GaN surface. Furthermore, for the V-LEDs with n-GaN/ZnO/MgO layers, as the thickness of MgO layer increased up to 2 μm, the EL intensity increased. However, further increase in thickness of MgO layer caused reduction of light output power. The current-voltage (I-V) characteristics of V-LEDs with n-GaN, n-GaN/ZnO, and n-GaN/ZnO/MgO layers are shown in Fig. 7(b). The forward voltages of V-LEDs with the flat n-GaN and n-GaN/ZnO/MgO layers at an injection current of 20 mA are 2.81V and 2.79V, respectively. Furthermore, the reverse leakage currents of V-LEDs are almost same before and after ZnO/MgO deposition. These results showed that the formation of ZnO/MgO nano-pyramids structure on n-GaN does not cause the deterioration of electrical properties.
Figure 8 shows the normalized light output power of V-LEDs with the surface texturing method. The error bars are the ranges of 9 V-LEDs devices. The light output power of n-GaN/ZnO V-LEDs is only 10% higher than that of n-GaN V-LEDs. For the n-GaN/MgO V-LEDs, the increase in light output power was about 14%, although the MgO pyramids structure was formed at surface. The critical angle for TIR at n-GaN/MgO interface is only 43.8° due to the large difference in refractive index between GaN (n = 2.5) and MgO (n = 1.73), Therefore, with the insertion of ZnO layer, more light could be escaped from n-GaN to MgO layer, resulting in the increase of light output power. After employing ZnO/0.7-μm-thick MgO layer, it increases up to about 18%. Furthermore, the n-GaN/ZnO/2-μm-thick MgO V-LEDs showed the highest improvement of 49%, because larger size of MgO nano-pyramids as shown in Fig. 3(c) increases the light scattering at the MgO surface, resulting in the increase of light extraction efficiency. However, the light output power of V-LEDs with n-GaN/ZnO/4-μm-thick MgO shows only 22% increase compared to that of n-GaN V-LEDs. The reduction of light output power in V-LEDs with n-GaN/ZnO/4-μm-thick MgO could be attributed to the decrease in the light transmittance of 4-μm-thick MgO (Fig. 6) as well as the tilted MgO pyramid structure (Fig. 3(d)).
Figure 9(a) shows the angular-dependent far-field emission of both n-GaN and n-GaN/ZnO/MgO (2 μm) V-LEDs. For the n-GaN V-LEDs with flat surface, the emission intensity is maximized at the surface normal and decreased rapidly with the increase of detection angle. This is due to that only light incident within the critical angle of 23.6° to surface normal could be extracted from LEDs. However, in the n-GaN/ZnO/MgO V-LEDs, the emission intensity apparently increased in the direction of side wall as well as in the surface normal. The normalized improvement of EL intensity as a function of detection angle is shown in Fig. 9(b). For the normal direction to surface, the emission intensity is only increased by a factor of 1.2. However, it significantly increased with the increase of detection angle. The maximum enhancement was found at the detection angle of 60° with the factor of 2.75. These results indicate that the MgO nano-pyramids on n-GaN played a role in enlarging the critical angle for TIR to enhance the light extraction efficiency.
The significant enhancement of light output power in V-LEDs with ZnO/MgO nano-pyramids is attributed to the not only increase of critical angle for TIR by refractive-index modulation but also strong light scattering at MgO nano-pyramids surface. Considering the refractive indices of GaN (n = 2.5) and air (n = 1), the critical angle for TIR is about 23°. Therefore, a smooth n-GaN surface results in waveguided modes that cannot escape, as shown in Fig. 1a. If additional ZnO layer with refractive index 1.94, and MgO layer with refractive index 1.73 are formed on n-GaN surface, the ray is refracted, rather than reflected due to increased critical angle for TIR at n-GaN/ZnO and ZnO/MgO interfaces, and may fall within the escape cone and thus could be extracted into air (Fig. 1(b)), leading to enhancement of light extraction efficiency in V-LEDs (Fig. 7(a) and 8). In addition, nano-pyramids structures formed spontaneously at MgO surface during deposition process reduce internal reflection and scatter the light outward. At pyramid shape surface, the incident angle of light is reduced at each trap step, and after all, the incident angle could be smaller than critical angle, resulting in the light escape from LEDs and enhancement of light output power. Therefore, the enhancement of light output power in V-LEDs depends on the thickness of MgO layer due to the size variation of MgO nano-pyramids, as shown in Fig. 3. It is anticipated that the light output power of V-LEDs with ZnO/MgO layer could be further improved by optimizing the size of MgO nano-pyramids.
In conclusion, we present the enhanced light extraction in vertical-structure LEDs using MgO nano-pyramids structure and ZnO refractive-index modulation layer. The light output power of n-GaN/ZnO/2-μm-thick MgO V-LEDs is enhanced by 49% compare to that of n-GaN V-LEDs. The significant increase of side emission for the n-GaN/ZnO/2-μm-thick MgO V-LEDs could be attributed to the increase of critical angle for TIR as well as the roughened surface by MgO nano-pyramids structure, resulting in the increased light-out-coupling from LEDs epilayers. The MgO film with nano-pyramids structure is promising material in optoelectronic devices such as organic light-emitting diodes, solar cells where high surface roughness is required to improve the quantum efficiency of the devices.
J. H. Son and H. K. Yu equally contributed to this work as first authors. This work was supported in part by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0094037), and in part by the Industrial Technology Development Program funded by the Ministry of Knowledge Economy (MKE, Korea).
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