WO2008035932A1

WO2008035932A1 - Iii-nitride based light-emitting diode structure with monolithically integrated sidewall deflectors and method thereof

Info

Publication number: WO2008035932A1
Application number: PCT/KR2007/004590
Authority: WO
Inventors: Heon Su Jeon; Jae Soong Lee; Joon Hee Lee
Original assignee: Seoul National University Foundation
Priority date: 2006-09-20
Filing date: 2007-09-20
Publication date: 2008-03-27
Also published as: KR20080028292A

Abstract

This invention relates to light-enitting diode with enhanced surface extraction efficiency, and, above LED has angled mesa sidewall deflectors along the epi layer. The LDE structure with angled mesa sidewall deflectors according to the invention is manufactured by comprising the follwing steps, step for forming a protecting layer on the upper side of IH -nit ride epi layer grown on support substrate! step for forming above protecting layer to hemisphere shape through reflow process; step for forming angled mesa deflector in the sidewall of UJ -nit ride epi layer to correspond with hemisphere shape of sides of protecting layer passing through ICP-RIEC Inductively Coupled Plasma- Reactive Ion Etching) process; and step for exposing Ill-nitride epilayer by removing above protecting layer. According to the present invention, it can form angled mesa sidewall deflector while the conventional manufacturing process of LED can be used as it stands, and also manufacturing is easy. Especially, surface extraction efficiency is improved by 3 times theoretically.

Description

[DESCRIPTION]

[Invention Title]

IE-nitride based Light-Emitting Diode Structure With Monolithically Integrated Sidewall Deflectors And method thereof

[Technical Field]

This invention relates to light-eni Lt ing diode with enhanced surface extraction efficiency, and, more particularly, to IE-nitride based light- emitting diode structure with angled mesa sidewall deflectors for enhancement of surface extraction efficiency

[Background Art]

The nitride semiconductor are expectatiomal semiconductor materials in direct transition type, but are difficull to be manufactured as bulk single crystal. Therefore, heteroepitaxy technique which grows El-nitride on foreign substrate such as sappire, SiC, etc using MOCVlXmeta1organic chemical-vapor- deposition technique) is widely used. It is already improved that sappire is desirable substrate to grow light emitting device of a nitride semiconductor with high efficiency under high temperatue ammonia surrounding of epitaxy growth procedure.

But, the sappire is insulator with bad thermal conductivity, and so the structure of nitride semiconductor device is limited. For example, as for GaAs or GaP, it is possible to install one electrode on the upper side and the other electrode on the lower side of device, but as for light emitting device(diode) grown on the sappire, two electrodes need to be installed on the upper sideCthe same side). So, sappire insulator substrate has small effective light-emitting area in comparison with other comductor substrate. Also, in using insulator substrate, the number of element(device, chip) available from the same sized wafer is fewer.

The nitrdie semiconductor device using insulator substrate such as sappire has face up type or face down type, but, as these have two electrodes on the same side, and so current density becomes high partially, device degradation is accelerated by generation of heat. And, as respective wire is needed at pn two electrodes in wire bonding procedure to electrode and so chip size is big, the yield of chips is lower. Also, the sappire is hard and hexagonal crystal. Therefore, in using sappire as growing substrate, chip separation by scribing the sappire subsliate is needed and the number of manufacturing procedure is more than other substrate.

Also, recently LED in ultra violet region has been put to practical use. Ultra violet region is below 400nm. Band gap of GaN is 365nm, and surface extraction efficiency became lower notably owing to absorption of GaN of contact layer etc when making short wavelength below 365nm.

Surface extraction efficiency of LED is prescribed as ratio of outer quantum efficiency versus inner quantum efficiency of LED. Generally, surface extraction efficiency of package type LED is less than 1, namely the majority of light created within LED active area can not reach outer environment.

In the ni-nitride based light-emitting diode structure grown by heteroepitaxy method on the traditional sappire substrate, the light created inside device(diode) is emitted to the outside just in the case that the light created inside device enters the surface of device with the angle below total internal reflection critical angle. The refractive index of IH-nitride thin film is very high in comparison with air which is constituted of upper and lower part of thin film or sappire substrate. Therefore, the plenty part of light which can not get to the outside is guided to the side through optical waveguide composed of air/UI-ni tride/sappire. Therefore, surface light extraction efficiency, the ratio extracted to the outside through the surface of light created inside the device is much restricted.

Namely, the low extraction efficiency is due to the light created within GaN material having higher refractive index than that of srroundings. This phenomenon known as total internal reflection(TIR) give rise to the result that the majority of emitted photons is reflected and confined to the inside of GaN material. In addition, the majority of confined photons guided to epilayer of GaN is located between two claddings with low refractive index such as air and sappire substrate, as a result, it is of no use.

There were a few suggestions to solve these problem of GaN LED. Surface-roughening([2]T. Fujii, Y. Gao, R. Sharraa, E. L. Hu, S. P. DenBaars, and S. Nakamura, "Increase in the extraction efficiency of GaN-based light- emitting diodes via surface roughening," Appl. Phys. Lett., vol. 84, no. 6, pp. 855-857, Feb. 2004.), GaN epi-growth on patterned sappire substrate(L3]K. Tadatomo, H. Okagawa, Y. Ohuchi , T. Tsunekawa, Y. Iraada, M. Kato, and T. Taguchi , "High output power InGaN ultraviolet light-emitting diodes fabricated on patterned substrates using metalorganic vapor phase epitaxy," Jpn. J. Appl. Phys., vol.40, no. 6B, pp. L583-L585, Jun. 2001, [4] [1] J. Cho, H. Kim, H. Kim, J. W. Lee, S. Yoon, C. Sone, Y. Park, and E. Yoon, "Simulation and fabrication of highly efficient InGaN-based LEDs with corrugated interface substrate," Phys. Stat. Sol. (c), vol. 2, no. 7, pp. 2874-2877, Mar. 2005) and integration of photon crystal pattern(L5]T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, "Ill-nitride blue and ultraviolet photonic crystal light emitting diodes," Appl. Phys. Lett., vol. 84, no. 4, pp. 466-468, Jan. 2004. L6J J. J. Wierer, M. R. Krames, J. E. Epler, N. F. Gardner, M. G. Craford, J. K. Wendt, J. A. Simmons, and M. M. Sigalas, "InGaNOGaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures," Appl. Phys. Lett., vol. 84, no. 19, pp. 3885-3887, May. 2004 [7]D.-H. Kim, C-O Cho, Y.-G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q-H. Park, "Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns," Appl. Phys. Lett., vol. 87, pp.203508, Nov. 2005.) are those.

Above methods are related to total internal reflection or the methods offering the optimum condition for photon escaping from surface of device. In other wods, with the aid of integrated micro or nano structure causing enhanced photon extraction efficiency, photons can be extracted by scattering or diffraction.

On the other hand, the extraction efficiency can be improved by the shape of LED chip.([8]M. R. Krames, M. Ochiai-HoIcomb, G. E. Hofler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui , J.-W. Huang, S. A. Stockman, F. A. Kish, and M. G. Craford, "High-power truncated-inverted-pyramid (AlGa)InP/GaP light-emitting diodes exhibiting >50% external quantum efficiency," Appl. Phys. Lett., vol. 75, no 16, pp. 2365-2367, Oct 1999. [9] V. Zabelin, D. A.Zakheim, and S. A. Gurevich, "Efficiency improvement of AlGaInN LEDs advanced by ray-tracing analysis," IEEE J. Quantum Electron., vol. 40, no. 12, pp. 1675-1686, Dec 2004.)

[Disclosure]

[Technical Problem]

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide LED structure with enhanced surface extraction officency(enhanced surface emission) by modification of structure and use of the total internal reflection effect on the contrary known as the worsening factor of surface light extraction efficiency.

We suggest light-emitting diode structure with angled mesadnonolithically integrated) sidewall deflectors and the manufacturing method thereof.

[Technical Solution]

The invention, developed to solve above mentioned problems, provides LDE structure with IE-nitride epi layer on the support substrate, wherein angled mesa deflector is formed in the sidewall of EQ-nitride epi layer and sidewall angle is between 20° and 40° .

Also, the invention gives the manufacturing method of LDE structure with angled mesa sidewall deflectors, wherein the method comprise the follwing steps, step for forming a protecting layer on the upper side of El-nitride epi layer grown on support substrate; step for forming above protecting layer to hemisphere shape through reflow process; step for forming angled mesa deflector in the sidewall of EH-nitride epi layer to correspond with hemisphere shape of sides of protecting layer passing through ICP-RIE process: and step for exposing JH-nitride epi layer by removing above protecting layer.

[Advantageous Effects]

According to the present invention, it can form angled mesa sidewall deflector while the conventional manufacturing process of LED can be used as it stands, and also manufacturing is easy. Especially, according to verification by optical pumping method, surface extraction efficiency is improved by 3 times theoretically, by 2 times in practical estimation by current application to LED. [Description of Drawings]

Figure 1 is a schematic representation of process to deflect light ,at the end of sides, guided in side direction along epi layer grown on the upper substrate in the LED with angled mesa sidewall deflector according to the invention.

Figure 2 is a schematic representation of manufacturing process to manufacture the LED with angled mesa sidewall deflector according to the invention.

Figure 3 is a schematic representation of LED, in detail SDI-LED represents LED with angled mesa sidewall deflector according to the invention and the reference is conventional LED. Inserted picture is SEM image of a produced SDI-LED to verify this invention.

Figure 4 is emission spectra from an SDI-LED structure with inclination angle=30° and the reference, SDI represents LED with angled mesa sidewall deflector according to the invention and Ref is the conventional LED without angled mesa sidewall deflector.

Figure 5 is Near-field emission image of the SDI-LDE and the reference, the LED according to the invention improved that the light guided to the side under confinment inside epi layer of upper part joins in angled sidewall at the edge of device and deflects in perpendicular direction. There are bright ring-shaped light at the circumference and light emitted in the center.

Figure 6 are (a) Far-field patterns of SDI-LDE structure and the reference, and (b) computer-simulated far—field patterns of corresponding devices. Figure 7 are the plane figure and the profile to confirm the location of n-electrode and p-electrode respecively when manufacuring the LED with angled mesa sidewall deflector using the concept of figure 3.

Figure 8 is the graph which represents the intensity of light according to the sidewall angles in applying the current to structure of figure 7.

Figure 9 is (a) schematic diagram of SDI-LED, (b) SEM images of a fabricated SDI-LED

Figure 10 is (a) light output and applied voltage versus current(L-I-V) characteristics of SDI-LED for various sidewal] angles, together with the reference LED, (b) light output enhancement factor as a function of the sidewall angle.

Figure 11 is (a) electroluminescence spectra of the θ = 30° SDI-LED and the reference LED at various injection currents, (b) space-resolved emission spectra from the SDI-LED(Θ = 30° ). Shown together for comparison is the far-field spectrum from the same device, which is vertically shifted for clarity.

Figure 12 illustrates Near-field emission intensity patterns of the SDI-LED and the reference LED.

[Best Mode] Detailed description will be explained.

*

The LEDUight-emitting diodes) structure accoring to the invention is charcterized in forming the angled mesa deflector at the sidewall of edge of HI-nitride epi layer(we have named the device structure as sidewal 1-deflector- integrated LED; SDI-LED). Namely, as a LED structure with El-nitride epi layer on the support substrate, there are angled deflectors over epi layer total thickness at the sidewall of edge of HI-nitride epilayer.

The sidewall angle(θ) which is formed between support substrate and the nitride semiconductor layer is desirable between 20° and 40° , but is not restricted in this extent, more desirable between 25° and 35° verified in figure 10.

Support substrates used in this invention can be the sappire, Si, SiC, MgAl₂O₄, ZnO, MgO, etc. In particular, substrate which has lower refractive index as compared with the ITf-nitride such as the sappire is desirable.

Also, not only HI group nitride compounds such as GaN but also the material generally available such as GaP, GaAs, etc can be used as above IE- nitride.

The manufacturing method of LED structure with angled sidewall deflectors is follwing.

In the first place, El-nitride epi layer is grown on the support substrate, and coating the protecting layer is follwed. Support substrates used in this invention can be the sappire, Si, SiC, MgAl₂O₄, ZnO, MgO, etc.

Not only IE group nitride compounds such as GaN but also the material generally available such as GaP, GaAs, etc can be used as above El-nitride epi layer .

Also, photoresist, polyimide, polymer, etc generally availabe can be used as the layer for the protection and the shaping, and the photoresist layer is more desirable because patterning can be performed at the same time.

And then, forming protecting layer to hemisphere shape through reflow process. Generally, polymer material flows easily because the viscosity becomes low at the temperature above specific transition temperature. When performing reflow process, Shaping as hemisphere or a part side of sphere happens instead of wide spread thanks to surface tension effect acted on surface of substrate.

And then, forming angled mesa deflector in the sidewall of Hi-nitride epi layer to correspond with hemisphere shape of sides of protecting layer passing through ICP-RIE process. The sidewall angle(θ) which is formed between support substrate and the nitride semiconductor layer is desirable between 20° and 40° , but is not restricted in this extent, more desirable between 25° and 35° verified in figure 10.

It is desirale that sidewall with angled deflector is formed over total thickness of IE-nitride epi layer but the thickness can be variable case by case.

Also, it is desirale that angled deflectors are formed at four edges of sidewall of device but in necessity, can be formed in some parts.

And then, exposing EI-nitride epi layer by removing remaining above protecting layer. The remaining above protecting layer can be removed by general available method such as wet or dry etching.

Surface extraction efficiency of LED according to the invention has improved effect by 3 times in comparison with the conventional LED structure. Also, according to the invention, because it is sufficient to etch the sidewall of device as the extent within above angle, the conventional manufacturing process of LED can be used as it stands except manufacturing angled sidewall .

Above mentioned, it is possible to manufacture LED structure with angled sidewall deflectors by ordinary method. As a concrete type, p electrode is formed at the upper side of n electrode, and the total thickness of El-nitride thin film including n-electrode and p-electrode is between 1/M and 10/M preferably, and between bμm and 6μm generally. The thickness of p- electrode is preferably between 0.05μm and 0.50μm and between 0.15/ΛΠ and 0.25/im generally.

And, it is preferable that the sidewall etching is performed over the total thickness of IE-nitride film including n-electrode and p-electrode to maximize the effect of angled sidewall.

For reference, as for the conventional LED structure, the depth of etching of sidewall is about O.Sμm for removing p-electrode formed on upperside, and the steepness is not considered generally. Therefore, the structure of device, the depth of etching, amd shape of side according to this invention have much difference with the conventional structure.

When composing n-metal and p-metal respectively using above device, estimated surface extraction efficiency is improved by 3 times theoretically, by 2 times in practical application in comparison with the conventional LED structure. We guess that there are complicated optical actions such as reflection and absorption of some light to the metal owing to metal electrode when exciting the device.

Following embodtment is to describe process of our invention in more detail. But, following embodiment is only one example and this invention is not limited to this embodiment.

Following embodiment use the sappire substrate but this invention is not limited to sappire substrate. Also, as the forming method of nitride semiconductor layer, MOCVD or MOWE is preferable in the following embodiment. But MBE, HalideVPE, LPE, etc can be used and each layer can be formed by different grwoing method. Also, for example, in epitaxial growth of nitride semiconductor to the sappire substrate, it is desirable to form buffer layer for crystalline or correction of lattice mismatching etc.

Further, the forming electrode or detailed shape can be various according to the design of emitting device though we explain the main parts not to confuse the point.

Manufacturing example 1

This manufacturing example is the manufacturing method of GaN LED structure with angled mesa sidewall with reference to figure 1 and figure 2.

1. The inventor coated photoresist and define GaN LED pattern by standard photolithography technique. The photoresist should be sufficiently thick because it is protecting layer in etching the side incline. We use 5μm thickness photoresist.

2. After making pattern by developing photoresist, we made the center of pattern convex lens shape through reflow with raising the temperatue between 110°C and 130°C. The shape can be controlled by temerature and reflow time.

3. Dry etching of GaN thin layer was performed using photoresist protecting layer in the shape of lens as sacrificial mask. To maximize the effect of the invention, we etch total GaN thin layer of 5-6/M thickness until the sappire substrate was exposed. Angled sidewall can be controlled by forming shape of photoresist lens and etching condition.

4. Remaining photoresist is removed bv wet or dry etching.

Comparison example 1

The conventional GaN LED structure was manufactured in order to compare with the device according to the invention. 1. We define the ordinary LED shape by photolithography. Thick photoresist is not needed, so the photoresist of 1/M thickness was used.

2. Without reflow process, we etched upper p-electrode GaN, the depth of etching was about 0.5μm slightly thicker than p-electrode GaN thickness. The incline of etched sidewall is almost vertical without photoresist reflow.

3. Remaining photoresist is removed by wet or dry etching.

SDI-LED(sidewall~detector~integrated LED) through above Manufacturing example 1 and the reference through Comparison example 1 is compared in figure 3. Two devices are patterened as circle as shown in AFM image of figure 3.

Embodiment 1

We examined the efficiencies of LED structure accoding to the invention. We estimate character of device by optical pumping method without metal electrode.

We estimate emission spectra using SDI-LED structure through above Manufacturing example 1 and the reference through Comparison example 1, and the result is shown in figure 4.

In Figure 4, SDI represents LED with angled mesa sidewall deflector according to the invention and Ref is the conventional LED without angled mesa sidewall deflector.

As a reult, surface extraction efficiency is improved by 3 times in figure 4.

<Near-field emission pattern profile>

Near-field emission pattern profiles using SDI-LED structure through above Manufacturing example 1 and the reference through Comparison example 1 are estimated and the result is shown in figure 5.

As shown in Figure 5, there are bright ring-shaped light at the circumference according to the invention, this confirms the theory illustrated in figure 1.

As a reference, Figure 1 is a schematic representation of process to deflect light guided in side direction along epi layer in the LED with angled mesa sidewall deflector according to the invention.

<Far-field patterns with sidewall angles>

Far-field patterns using SDI-LED structure through above Manufacturing example 1 and the reference through Comparison example 1 with sidewall angles are estimated and the result is shown in figure 6.

The efficiency is most enhanced in the sidewall angle of 30° as shown in figure 6(a), this is corresponded with computer-simulated result as shown in the figure 6(b).

Manufacturing example 2- The structure of LED

This manufacturing example is the manufacturing method of LED structure with metal electrode using the principal result from manufacturing example 1, and with reference to figure 7.

1. The shape of GaN LED is in accordance with above manufacturing example 1.

2. Photolithography for making n-eleelrode is performed in the partial area of defined LED, and p-electrode GaN layer is removed within relational area by etching. N-electrode metal layer is formed on the n-electrode GaN exposed in relational area. 3. P-electrode metal layer is formed on the p-electrode GaN.

Comparison example 2

After fabrication of the conventional LED structure without angled sidewall in order to compare the result of the invention in accordance with comparison example 1, n-electrode and p-electrode metal layer were formed by the same method of above manufacturing example 2.

Embodiment 2

We made three kinds of LED with sidewall angles of 20° , 30° , and 40 , and estimated the character, and compared it with the device from Comparison example 2.

The schematic construction of fabricated LED is shown in Figure 7. As shown in figure, as p-metal electrode is formed to have high refractive index, the majority of light is extracted through backside of device, namely the sappire substrate, therefore the estimation was performed in the sappire substrate side.

With applying the current to each device, the intensity of light estimated as a function of current is shown in figure 8.

As shown in the figure, the LED with angled sidewall showed enhanced surface emitting efficiency irrespective of sidewall angle in comparison with the LED without angled sidewa11.

Further, when the sidewall angle is 30° , surface emitting efficiency is increased by over 2 times.

On the other hand, the role and efficiency of angled sidewall deflector over above embodiment in terms of current are followings.

First, new GaN LED structure suggested in the invention uses total internal reflection(TIR) as the extracting method of the photon guided along normal direction to the surface. We have named these device structure as a sidewall-deflector-integrated(SDI) LED since it has angled mesa sidewall that serve as light deflectors vir the TIR process.

The schematic representation of the device as well as the operation mechanism of the SDI-LED is illustrated in Figure 9(a). A conventional GaN multiple-quantum-well (MQW) LED heterostructure, which has a thickness of 5 μm, was grown by a low-pressure metalorganic chemical-vapor-deposition technique. The LED epi layer structure from the substrate side consists of an undoped GaN buffer, a Si-doped bottom clad, five InGaN/GaN MQWs, a 20 nm thick Mg-doped AIc₂Ga₀^N electron blocking layer, and a 0.2 μm thick Mg-doped

GaN top cladding layer.

To avoid the effects of spatial inhomogeneity, samples were taken from the central region of the wafer where the emission efficiency variation was less than 20%, which was confirmed by wafer-scale photoluminescence mapping.

Special care was taken in preparing the angled sidewall deflectors that play a key role in this SDI-LED device.

Using a standard photolithography technique, a 5 μm thick photoresist layer was patterned into an array of LED mesas, each square being 300 300

2 μm in area. Then, the photoresist pattern was subjected to a reflow process at an elevated temperature. The reflow process makes the photoresist sidewalls to reform in a particular angle. The angle of the photoresist sidewalls is a function of several parameters, including the photoresist viscosity/thickness and photoresist reflow temperature/time. The modified photoresist pattern served as the sacrificial mask, while the GaN epi layer was completely etched off in a chlorine-based high density plasma gas until the sapphire substrate was reached. Details of the angled sidewall fabrication procedures can be found above mentioned.

The angle of the mesa sidewalls was determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The AFM measurement also provided information on the roughness of the etched surface, which was i measured to be ~ 2 nm across a lxi /an area.

For comparison, a set of reference devices with the same mesa area was also prepared. Since the reference should represent the conventional GaN LED, its sidewalls were made shallow (~ 600 run, just deep enough to remove the MQWs) and vertical i.e., the mesas were patterned with a thin (~1 /m) photoresist without reflow and etched only slightly.

We employed a highly reflective Ag-based metal contact for the p- electrode while Ti/Al was employed for the n-electrode.([12]J.-0. Song, J. S. Kwak, Y. Park, and T.-Y. Seong. "Ohmic and degradation mechanisms of Ag contacts on p-type GaN," Appl. Phys. Lett., vol. 86, pp. 062104, Feb. 2005. [13]D. L. Hibbard, S. P. Jung, C. Wang, D. Ullery, Y. S. Zhao, H. P. Lee, W. So, and H. Liu, "Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag," Appl. Phys. Lett., vol. 83. no. 2, pp. 311-313, JuI. 2003.) Figure 9(b) shows the SEM images of a fabricated SDI-LED device.

To characterize our fabricated SDI-LEDs, we drove the devices in a DC bias condition at room temperature without any special heatsink. Emission spectra were obtained through the backside-polished sapphire substrate, using a fiber optic spectrometer.

A wide aperture multimode fiber tip was located 10 cm away from the device, facing the LED in a direction perpendicular to the device plane. Figure 10(a) shows the L-I characteristics of the typical SDI-LEDs with various sidewall angles, together with that of the reference LED. The SDI- LEDs exhibited improved light outputs regardless of the sidewall angle θ, while the one with θ= 30° gave the highest enhancement factor of ~2 over the reference device. Considering the simple structural modification made to the mesa sidewalls, this enhancement is substantial.

Figure 10(a) also shows the I-V characteristics of the SDI-LED (θ = 30° ) and the reference LED; both I-V curves are almost identical. This implies that the angled sidewall integration did not cause any adverse effect on the electrical injection property, which is in good contrast with the PC- LEDs where the top contact layer was partially removed for the integration of two-dimensional PC patterns: thus, the I-V characteristics were degraded. ([7]D.-H. Kim, C-O Cho, Y.-G. Koh, H. Jeυn, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q-H. Park, "Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns," Appl. Phys. Lett., vol. 87, pp. 203508, Nov. 2005.)

Figure 10(b) summarizes the light output enhancement factor as a function of the mesa sidewall angle, which is defined as the light output ratio between the SDI-LED and the reference LED.

The enhancement factor, deduced from Fig. 10(a), ranges from 1.2 to 2.1, the maximum occurring at θ~ 30° . For theoretical confirmation, we simulated the SDI-LED structures using a program based on ray optics.

A total of 2X10 rays were launched from across the MQW plane with random positions and directions, and their final trajectories were monitored.

To establish a realistic model structure, we included a thick reflecting p-metal on the mesa top and allowed the absorption of light by the metal layer as well as the active MQW layer. The simulation results are compared with the experimentally determined data in Fig 10(b). The overall agreement was very high, both qualitatively and quantitatively; the simulation correctly reproduced that the strongest enhancement factor of ~2 occurs at the sidewall angle of θ~ 30°

Figure ll(a) compares the electroluminescence spectra of the SDI-LED ( θ = 30° ) and the reference LED. As is already clear from Fig. 10(a), the emission intensity from the SDI-LED is about two times stronger than that from the reference LED throughout the entire driving current range. However, here we note that, despite the significant difference in the emission intensity between the two types of LEDs, the absolute modulation depths of the Fabry-Perot oscillations are very similar at a given current. This means that the amount of photons that are responsible for the Fabry-Perot oscillations are the same, and the extra emission intensity from the SDI-LED originates from photons other than those that are directly emitted from the surface (within the TIR cone). To verify this, we examined the spatially resolved emission spectra of the SDI-LED. We employed an optical fiber whose core diameter was 7 μm and brought it close to the LED device by gently touching the polished sapphire substrate with the cleaved fiber tip. The emission spectra were taken from two regions of the SDI-LED: one was from the central region where only direct surface emission could exist and the other was from the sidewall region where the guided photons should dominate. The results are shown in Fig. 1Kb), where the maximum intensities are normalized for the direct comparison of spectral shapes. The spectrum from the center shows strong Fabry-Perot oscillations with a high modulation depth, whereas there is no noticeable oscillation in the spectrum taken from the sidewall region. The latter indicates that the propagation direction of the photons detected near the sidewalls is not normal to the surface from the beginning. It should be noted that the far-field spectrum, also shown in Fig. 1Kb), has a moderate Fabry-Perot modulation depth, which is consistent with our conjecture that the far-field spectrum should be a result of the addition of both the direct surface-emitting photons and the guided~and-defleeted photons. The above argument was further confirmed by the near-field emission patterns, which are shown in Fig. 12. The near-field images were taken using a high-magnification optical microscope and recorded by a charge-coupled- device camera. While the reference LED shows a uniform emission pattern across the entire active region (which corresponds to the area covered by the p-electrode), the SDI-LED exhibits an additional intensive emission pattern around the device edges that spatially coincide with the angled sidewalls. It should be noted that the additional emission pattern along the mesa sidewalls forms a completely closed square loop around the mesa, including even the n- metal area where the surface emission was completely absent (the lower left corner of fig 12).

References

[I]H. Benisty, H. De Neve, and C. Weisbuch, "Impact of planar microcavity effects on light extraction-Part I÷basic concepts and analytical trends," IEEE J. Quantum Electron., vol. 34, no. 9, pp. 1612-1632, Sep. 1998 "Impact of planar microcavity effects on light extraction-Part II Selected exact simulations and role of photon recycling," IEEE J. Quantum Electron., vol. 34, no. 9, pp. 1632-1643, Sep. 1998.

[2]T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, "Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening," Appl. Phys. Lett., vol. 84, no. 6, pp. 855-857, Feb. 2004.

[3]K. Tadatomo, H. Okagawa, Y. Ohuchi , T. Tsunekawa, Y. Imada, M. Kato, and T. Taguchi , "High output power InGaN ultraviolet light-emitting diodes fabricated on patterned substrates using metalorganic vapor phase epitaxy," Jpn. J. Appl. Phys., vol.40, no. 6B, pp. L583-L585, Jun. 2001. [4]J. Cho, H. Kim, H. Kim, J. W. Lee, S. Yoon, C. Sone, Y. Park, and E. Yoon, "Simulation and fabrication of highly efficient InGaN-based LEDs with corrugated interface substrate," Phys. Stat. Sol. (c), vol. 2, no. 7, pp. 2874-2877, Mar. 2005.

[5]T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, "Ill-nitride blue and ultraviolet photonic crystal light emitting diodes," Appl. Phys. Lett., vol. 84, no. 4, pp.466-468, Jan. 2004.

[6]J. J. Wierer, M. R. Krames, J. E. Epler, N. F. Gardner, M. G. Craford, J. R. Wendt, J. A. Simmons, and M. M. Sigalas, "InGaNOGaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures," Appl. Phys. Lett., vol. 84, no. 19, pp. 3885-3887, May. 2004. [7]D.-H. Kim, C-O Cho, Y.-G. Roh, H. Jeon, Y. S. Park, J. Cho, J. S. Im, C. Sone, Y. Park, W. J. Choi, and Q-H. Park, "Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two- dimensional photonic crystal patterns," Appl. Phys. Lett., vol. 87, pp. 203508, Nov. 2005.

[8]M. R. Krames, M. Ochiai-HoIcomb, G. E. Hofler, C. Carter-Coman, E. I. Chen, L-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui , J.-W. Huang, S. A. Stockman, F. A. Kish, and M. G. Craford, "High-power truncated-inverted- pyramid (AlGa)InP/GaP light-emitting diodes exhibiting >50% external quantum efficiency," Appl. Phys. Lett., vol. 75, no 16, pp.2365-2367, Oct 1999. [9]V. Zabelin, D. A.Zakheim, and S. A. Gurevich, "Efficiency improvement of AlGaInN LEDs advanced by ray-tracing analysis," IEEE J. Quantum Electron., vol.40, no. 12, pp. 1675-1686, Dec 2004.

[10]J.-S. Lee, J. Lee, S. Kim, and H. Jeon, "GaN-based light-emitting diode structure with monolithically integrated sidewall deflectors for enhanced surface emission," IEEE Photon. Techno1. Lett., vol. 18, no. 15, pp. 1588- 1590, Aug. 2006.

[H]S.-H. Park, J. Kim, H. Jeon, T. Sakong, S.-N. Lee, S. Chae, Y. Park, C- H. Jeong, G.-Y. Yeom, and Y.-H. Cho, "Room-temperature GaN vertical-cavity surface-emitting laser operation in an extended cavity scheme," Appl. Phys. Lett., vol.83, no. 11, pp. 2121-2123, Sep. 2003.

[12]J.-0. Song, J. S. Kwak, Y. Park, and T.-Y. Seong, "Ohmic and degradation mechanisms of Ag contacts on p-type GaN," Appl. Phys. Lett., vol. 86, pp. 062104, Feb. 2005.

[13]D. L. Hibbard, S. P. Jung, C. Wang. D. Ullery, Y. S. Zhao, H. P. Lee, W. So, and H. Liu, "Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag," Appl. Phys. Lett., vol. 83. no. 2, pp. 311-313, JuI. 2003.

Claims

[CLAIMS] [Claim 1]

The LDE structure with Hi-nitride epi layer on the support substrate, wherein angled mesa deflector is formed in the sidewall of above IU-nitride epi layer and sidewall angle is between 20° and 40° .

[Claim 2]

The LDE structure of claim 1, wherein said sidewall angle which is formed between support substrate and the nitride semiconductor layer is between 25° and 35° .

[Claim 3]

The LDE structure of claim 1, wherein said support substrate is seleted one among the group of the sappire, Si, SiC, MgA^O₄, ZnO, and MgO.

[Claim 4]

The LDE structure of claim 1, wherein said epi layer is seleted one among the group of GaN, GaP, and GaAs.

[Claim 5]

The LDE structure of claim 1, wherein said epi layer has the structure that p- electrode is formed at the upper side of n- electrode, and the total thickness of IU-nitride thin film including n-electrode and p-electrode is between 1_/im and lO/m).

[Claim 6]

The LDE structure of claim 5, wherein thickness of p-electrode is between 0.05_/ini and 0.50/itu.

[Claim 7]

The manufacturing method of LDE structure with angled mesa sidewall deflectors, wherein the method comprises the follwing steps, step for forming a protecting layer on the upper side of HI-nitride epi layer grown on support substrate; step for forming above protecting layer to hemisphere shape through reflow process; step for forming angled mesa deflector in the sidewall of IU-nitride epi layer to correspond with hemisphere shape of sides of protecting layer passing through ICP-RIE process; and step for exposing Dl-nitride epi layer by removing above protecting layer.

[Claim 8]

The manufacturing method of LDE structure of claim 7, wherein sidewall angle which is formed between support substrate and the nitride semiconductor layer is between 20° and 40° .

[Claim 9]

The manufacturing method of LDE structure of claim 7, wherein said support substrate is seleted one among the group of the sappire, Si, SiC, MgAl₂O₄, ZnO, and MgO.

[Claim 10]

The manufacturing method of LDE structure of claim 7, wherein said epi layer is seleted one among the group of GaN, GaP, and GaAs. [Claim 11]

The manufacturing method of LDE structure of claim 7, wherein said step of removing protecting layer is performed by wet etching or dry etching.