Structural Effects on Highly Directional Far-Field
Emission Patterns of GaN-Based Micro-Cavity
Light-Emitting Diodes With Photonic Crystals
Chun-Feng Lai, Student Member, IEEE, Hao-Chung Kuo, Senior Member, IEEE, Chia-Hsin Chao, Peichen Yu, and
Wen-Yung Yeh
Abstract—This study theoretically and experimentally
in-vestigates the highly directional far-field emission patterns of GaN photonic crystal (PhC) micro-cavity light-emitting diodes (MCLEDs) depending on varying structural parameters. An-gular-spectra-resolved electroluminescence measurements reveals the behavior of guided-mode extraction which is significantly affected by the structural parameters of GaN PhC MCLEDs, where the GaN cavity thickness decides the extracted guided mode numbers, PhC lattice constant influences the distribution of far-field emission, and PhC hole depth affects the interaction with guided modes. The proposed GaN ultrathin MCLED (uMCLED) with PhC lattice constant of 420 nm and deep hole depth of 250 nm exhibited a maximum output light extraction efficiency of 248% under one-watt input power compared to GaN non-PhC uMCLED and produced a directional far-field emission pattern at half intensity near 17 . The present results indicate that highly directional light extraction enhancement could contribute to developments of many applications, especially for etendue-limited applications such as pico-projectors.
Index Terms—GaN, light-emitting diodes (LEDs), micro-cavity,
photonic crystals (PhCs).
I. INTRODUCTION
L
IGHT-EMITTING diodes (LEDs) have recently attracted great concern as a high-saving energy efficiency of lighting technology [1]. Developing next-generation GaN-based LEDs for applications of projecting displays, automobile head-lights, and general lightings requires further improvement in light extraction efficiency (LEE) and directionality of far-field emission distribution. GaN micro-cavity (thin-film) LEDs (MCLEDs) are potentially powerful high-brightness LEDs and enable higher power operation due to their excellent thermalManuscript received May 03, 2010; revised July 08, 2010; accepted July 15, 2010. Date of publication July 29, 2010; date of current version September 22, 2010.
C. F. Lai is with the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C., and also with the Electronics and Opto-electronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, R.O.C.
H. C. Kuo and P. Yu are with the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: [email protected]).
C. H. Chao and W. Y. Yeh are with the Electronics and Opto-electronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, R.O.C.
Color versions of one or more of the figures in this paper are available at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2010.2061836
dissipation and electrical conductivity substrate [2], [3]. To extract the guided light of GaN MCLEDs for optimal LEE enhancement, GaN effective cavity length of GaN MCLEDs should be within the micro-cavity regime to support just a few cavity modes so that some cavity modes lying within the extraction cone are leaky to increase the LEE [4]. Nevertheless, a fraction of light power of GaN MCLEDs is still trapped in the GaN waveguide as guided modes. Using the characteristics of PhCs is another way to improve the LEE and directional far-field emission pattern of GaN MCLEDs [5]–[7]. The depen-dence of the LEE on the micro-cavity and PhC parameters of GaN MCLEDs has been reported by theoretical discussion [8], [9]. However, previous work did not study the structural pa-rameter effects of GaN-based PhC MCLEDs on the directional far-field emission pattern.
This study theoretically and experimentally demonstrates that the LEE and directional far-field emission character-istics of GaN PhC MCLEDs are influenced significantly by several structural parameters, including the GaN cavity thickness , PhC lattice constant , and PhC hole depth . Angular-spectra-resolved electroluminescence (EL) measure-ment under electrical injection reveals guided-mode extraction behavior depending on the different GaN cavity thickness, PhC lattice constant size, and PhC hole depth.
II. EXPERIMENTS
The blue GaN LED wafers used in this study were grown by metal organic chemical vapor deposition (MOCVD) onto -face (0001) 2-in-diameter sapphire substrates. The GaN LED structure consists of a 30-nm GaN nucleation layer, a 2- m un-doped GaN buffer layer, a 2- m Si-un-doped n-GaN layer, a 120 nm InGaN/GaN MQW active region with 12 periods of 2/8
nm well/barrier width (dominant wavelength nm),
a 20-nm Mg-doped p-AlGaN electron blocking layer, a 125-nm Mg-doped p-GaN contact layer. After epilayer wafer bonding to the Si wafer, the sapphire substrate was removed with the laser lift-off (LLO) technique. Fig. 1 shows the fabrication steps of GaN MCLEDs incorporated with square PhC lattice struc-tures. First, the fabrication process of the GaN LEDs on Si began with the deposition of transparent contact Ni layer (5 nm), silver mirror (130 nm), and Ti/W/Ti/W/Ti/Au bonding metal layer on p-GaN. The GaN LED wafers were then bonded onto a Ti/Pt/Au coated p-type conducting Si substrate by commercial SUSS SB6e wafer bonder. Second, the wafer bonded samples were taken to undergo the LLO process to remove the sapphire
Fig. 1. Schematic diagram of fabrication steps of GaN-based MCLEDs com-bined with PhC lattice structures.
substrate. In this process, the beam size of KrF laser was larger than our desired size (1.0 1.0 mm ) of MCLEDs. Therefore, the laser irradiation on the interface of sapphire and GaN was uniform. After the sapphire removed samples were dipped into HCl solution to remove the residual Ga on the un-GaN. Third, the resulting structure was then thinned down by chemical-me-chanical polishing (CMP) to obtain the four GaN micro-cavity
thickness of approximately ( nm), ( nm), 8.
( nm), and ( nm), which were measured
by -step profilometer after fabrication. Then the residue was removed with an oxygen reactive ion etch (RIE); this process was found not to affect the current spreading over n-GaN layer. Fourth, to fabricate PhCs on the n-GaN surface, a 200-nm-thick layer of SiN was deposited on the n-GaN by plasma-enhanced chemical vapor deposition (PECVD) to serve as a hard mask. The PhC with a square lattice of circular holes was then defined by interference lithography on the hard mask. Fifth, PhC holes were then etched into the top n-GaN surface using inductively coupled plasma (ICP) dry etching to a deep hole depth of 250 nm. Sixth, the associated mesas (800 800 m ) were etched to the bonding metal interface to isolate each single chip. Then using a buffer oxidation etchant (BOE) removed the residual SiO layer. Seventh, the SiO was deposited on the sidewall as the passivation layer. Finally, a patterned Cr/Au (50/5000 nm) electrode was deposited on n-GaN to form an n-type contact layer and Cr/Au (5/1000 nm) metal was deposited on back of
Fig. 2. Schematic diagram of the GaN MCLED structures incorporated with PhC structures. The SEM images of PhC structures were designed to the square lattice nanoholes with lattice constant (a)a = 270, (b) a = 370, and (c) a = 420 nm, where the diameter d were fixed to the ratio d = 0:7a.
the Si substrate to form a p-type contact layer. Fig. 2 shows the GaN MCLED structure incorporated with square PhC lat-tice. According to the previous work as in [6], three different PhC lattice constant values are 270, 370, and 420 nm and
the hole diameter are fixed to the ratio for
col-limated far-field patterns, their scanning electron microscopy (SEM) images shown in Fig. 2(a)–(c). After dies fabrication, the dies were mounted on the transistor outline (TO-46) package without epoxy encapsulation.
III. THEORETICALMODEL
After sample preparation, the micro-cavity effect on the LEE and far-field emission pattern of planar (non-PhC) GaN MCLEDs was studied. We performed angular-spectra-resolved EL measurement with current injection to investigate the behavior of guided mode extraction and the distribution of far-field emission [10]. Fig. 3(a) shows the angular spectra at various angles from the thick planar GaN MCLED. It fully char-acterized the far-field emission distribution of thick planar GaN MCLED. The MQW emission of thick planar GaN MCLED is clearly modulated by the Fabry–Perot (FP) effect. The angular spectra of Fig. 3(b) were normalized by the shape of the MQW line, as in [11]. The angular dependent spectra was shown in the
general form of ( ,
where is the cavity mode number, is the refractive
index of GaN material, and is the GaN cavity thickness. The theoretical results showed the GaN cavity thickness of
2.25 m, GaN refractive index , and FP mode number
from 20 (green dashed line) to 26 (light yellow dashed line) above the air cone, as shown in Fig. 3(b). These measured results agreed well with the actual structure of GaN MCLED. The 2.25- m thick planar GaN MCLED has twenty-six reso-nance modes. Fig. 3(b) reveals only seven FP modes outside the extraction cone, and other resonance modes trapped in the GaN waveguide as guided mode. Additionally, the FP effect modulated the MQW emission of planar GaN MCLED and showed near perfect detuning between the emission wavelength
Fig. 3. (a) Un-polarized angular-spectra-resolved EL measurements of the planar GaN MCLED with thicknessT 2:25 m at a driven current of 50 mA. (b) Normalized EL angular spectra for the planar GaN MCLED is the resonant 2.25m cavity device, the figure inset shows the theoretical fitting results. White dashed line is the MQW dominant emission wavelength. (c) The light intensity emitted from the planar GaN MCLED was calculated by the different source position S and S in the planar GaN MCLED with monochromatic light emitting wavelength of 455 nm. Light emitted outside (dashed vertical line) is extracted to air. Inset the TEM image of the planar GaN MCLED with 12x InGaN/GaN MQW. (d) LEE of planar GaN MCLED structure is plotted as a function of the GaN cavity thicknessT . The dipole source was placed in the position of the fifth QW layer about 186 nm from S to Ag mirror.
(white dashed line with arrows) and the cavity length indicated by divergent emission [11].
According to the micro-cavity theory [4], the GaN cavity thickness and position of QW layers in the GaN MCLED with bottom metallic reflector mirror is an important factor that strongly modifies light emitting characteristics such as the LEE, spontaneous emission rate, and far-field emission patterns [12],
[13]. The light emission intensity is controlled by two factors. The airy function that indicated transmittance behavior of the FP cavity was controlled by the cavity thickness of GaN, . The anti-node factor is controlled by the position of QW layers relative to the high-reflectance metal mirror, which governed the coupling efficiency of the light source in guided mode. In this study, the transmission electron microscopy (TEM) image
layer. The LEE was maximal when the source was located at an anti-node of the given mode. Therefore, each of the QW po-sitions in GaN MCLEDs has to be designed at the optimal dis-tance from the Ag mirror, to improve the extraction efficiency with the micro-cavity effects.
The influence of GaN cavity thickness on the LEE has been studied numerically in detail though [8], [9]. This study considers the actual device structure that contains the 120-nm MQW and 145-nm p-GaN. The dipole source was placed in the position of the fifth QW layer. Fig. 3(d) shows the light
extracted out from critical angle of GaN
MCLED structures as a function of GaN cavity thickness . The transmittance curve which is obtained from the formula in [4] depends on the airy function and anti-node factor for a monochromatic light emitting at a wavelength of 455 nm in GaN MCLED structure. The LEE periodically changes with GaN cavity thickness varying from 0.2 to 2.6 m. The periodicity of the GaN cavity thickness is about 93 nm which correspond to . The peak (valley) of Fig. 3(d) is caused by the con-structive (decon-structive) interference of upward emitted light and downward reflected light that will influence the LEE and dis-tribution of far-field emission. The results reveal that LEE of planar GaN MCLED strongly depends on GaN cavity thickness
of the structural parameters.
IV. EXPERIMENTRESULTS ANDDISCUSSION
From the above discussion, the thick planar GaN MCLED still trapped some guided modes in the material. To both in-crease the LEE and obtain a directional far-field emission pat-tern from a non-optimized cavity length of planar GaN MCLED, we used the GaN MCLED incorporated with square PhC lat-tice to enhance the LEE and control the distribution of far-field emission. Many studies have investigated LEE improvement in GaN MCLED with PhC structures [14]–[16]. Furthermore, we will discuss the structural parameters of GaN PhC MCLEDs af-fecting the distribution of far-field emission.
A. GaN Cavity Thickness Effect
In this section, we studied the different GaN cavity thickness of GaN PhC MCLEDs influence on the distribution of di-rectional far-field emission using the same PhC lattice constant and depth of PhC hole . Angular-spectra-resolved EL mea-surement under the driven current of 50 mA indicated the GaN
PhC MCLED with the PhC lattice constant nm, PhC
hole depth nm, and two different GaN cavity
thick-nesses of about and . This is clearly shown in Fig. 4(a) and (b) with light collected along the X (left) and M (right). The shape of the MQW line normalized these angular spectra of GaN PhC MCLED [11]. The light traveling through the wave-guide in the GaN PhC MCLEDs was diffracted by reciprocal
Fig. 4. Un-polarized angular-spectra-resolved EL measurements of the GaN PhC MCLED with PhC lattice constanta = 420 nm, and PhC hole depth t = 250 nm measured the different GaN cavity thickness (a) T 11 and (b) T 3 under the driven current of 50 mA, where the left shown in 0 X direction and the right in0 M direction.
wavevectors associated with the square PhC lattice. The sharp lines could be attributed by extracted wave guided modes prop-agating in the GaN waveguides formed with the GaN cavity thickness between the Ag reflector mirror and the air. Fig. 4(a) shows many guided modes extraction of the thick
GaN PhC MCLED, in contrast to Fig. 4(b), the GaN PhC
ul-trathin MCLED (uMCLED) is extracted only two
guided modes. The extracted guided mode numbers were sig-nificantly affected via the GaN cavity thickness of GaN PhC MCLED. Furthermore, the guided mode dispersion of the thick GaN PhC MCLED is still quite close to the free-pho-tons band structure because the PhC diffraction grating only has a perturbative effect on dispersion (the same as previous liter-atures, these photon bands diagram are not shown) [6], [17].
Nonetheless, GaN PhC uMCLED with extracts
ap-proximately two lower-order guided modes whose dispersion is more strongly affected by PhC hole depth. The photonic disper-sion of thin device combining PhC and micro-cavity effects dif-fers significantly from that of the GaN sapphire-based structures [17].
Due to the PhC diffracting these guided modes into the air, they could be quantitatively analyzed using the Ewald construc-tion of the Bragg diffracconstruc-tion theory in reciprocal space [15]. The extraction of waveguided light into air could be described by the
Fig. 5. The guided mode dispersion curves determined from the data shown in Fig. 4(b) and compared with the fundamental mode dispersion curves for the transverse electric (TE) modes.
effective refractive index guided mode, is the guided mode number, represents the various reciprocal diffraction vectors
(as , and of square PhC lattice [15]), and
is an integer, the order of diffraction. The radius
of the air circle was . Each waveguide mode circle
could be characterized by , where
is internal angle of the guided mode propagation within the GaN waveguide. When the in-plane component of the resul-tant wavevector after coupling to a reciprocal lattice vector fell within the circle of air, the diffracted light escaped into the air at
an angle, . Therefore, the
condition of diffraction depends on the wavelength , PhC lat-tice constant , and guided mode propagation angle . Each of the guided modes propagates in the GaN waveguide at different
angles due to the MQW emission from the GaN MCLED
influenced by the anti-node factor [4], as shown in Fig. 3(c). Therefore, we determined that each guided mode interacts with the PhC differently [5]. To study behavior of these modes more clearly, the spectra shown in Fig. 4(b) were transformed to the guided mode dispersion curves shown in Fig. 5 [17]. The image shows the normalized dispersion curves for each mode line in the X (left) and M (right) directions. To study the observed lines of guided mode, the dispersion of the effective refractive
index of fundamental mode was calculated
by a slab waveguide with the GaN material dispersion formula, a Sellimer equation [18]. The dispersion curves of the funda-mental mode were evaluated by a Bragg diffraction equation as
[15], where was a
wavevector of the fundamental mode, 2 (square, ),
(circle, ), (triangular, ), and 2 (diamond, ) of
four diffraction vectors in X and M directions, respectively, of the square PhC lattice, as shown in Fig. 5. These dispersion curves were used to match the observing guided mode lines in Fig. 5. The fundamental guided mode was clearly visible and could be matched to the corresponding dispersion curves of fun-damental mode. The results for guided mode dispersion were different with thick GaN PhC MCLEDs, where the dispersion was simply folded free-photon bands.
The waveguided light diffracted through PhC exhibited anisotropy in the far-field pattern, both in the direction of the zenith and the azimuth [6], [15]. The far-field angle is an included angle made by two lines with common vertex (the origin) across the half intensities of the far-field patterns. The far-field emission pattern of GaN PhC MCLED was measured in X (solid line) and M (dashed line) directions of square PhC lattice. To investigate how the GaN cavity thickness influences the distribution of far-field emission of GaN PhC MCLED, the 3-D far-field radiation pattern of GaN PhC MCLED wit different GaN cavity thickness, was measured by means of imaging spheres (Radiant imaging IS-LI) under the same current 50 mA for beam shape comparison, normalized with peak intensity. The measured far-field emission patterns
of GaN MCLEDs with different GaN thicknesses of
and were shown in Fig. 6(a) and (b) respectively. From Fig. 6, far-field patterns of non-PhC MCLED plotted in dash-dot lines have far-field angles of about 118.56 and
96.24 in GaN thicknesses of and , respectively,
which are much lower than that of the typical 120 Lambertian far field distribution due to the strong micro-cavity effect [11].
The GaN PhC MCLEDs with PhC lattice constant nm
and PhC hole depth nm in thick ( and thin
( GaN thicknesses cause differing far-field emission patterns plotted in solid (dash) lines for X ( M) PhC lattice orientation, also shown in Fig. 6(a)–(b). The patterns peaked near normal on the surface of the device with far-field angles at half intensity of 98.72 (80.68 ) and 34.23 (41.39 ) in X ( M) respectively. The ultrathin cavity length of GaN PhC MCLEDs based on two lower-order guided modes revealed the strong influence of micro-cavity on a highly-directional far-field emission pattern. Therefore, the GaN cavity thickness of the structural parameters affects the guided modes extraction behavior and significantly modifies the distribution of the directional far-field emission on GaN PhC MCLED.
B. PhC Lattice Constant Effect
Thinner GaN cavity thickness of GaN PhC MCLEDs from the above discussion revealed more highly-directional far-field emission patterns. Further, we discussed the PhC structural pa-rameters including lattice constant and hole depth in GaN PhC MCLEDs affecting the directional far-field emission dis-tributions. In this section, we studied the influence of different PhC lattice constant of GaN PhC MCLEDs on the distribu-tion of direcdistribu-tional far-field emission using the same GaN cavity thickness and PhC hole depth . The far-field emission
distri-butions of thick GaN PhC MCLEDs were
signif-icantly modified by the PhC lattice constant [6]. This study
discussed thicker GaN PhC MCLEDs with various
PhC lattice constants.
Angular-spectra-resolved EL measurement under the driven current of 50 mA for GaN PhC MCLEDs with thick GaN cavity
thickness , PhC hole depth nm, and two
dif-ferent PhC lattice constant and 370 nm are shown in Fig. 7(a) and (b) with light collected along the X (left) and M (right), respectively. In Fig. 7(a)–(b), many guided modes ex-traction due to a stronger effects of thick GaN cavity thickness and lattice constant can be observed, like the Fig. 4(a) with the
Fig. 6. The far-field emission patterns of GaN PhC MCLEDs with various GaN cavity thickness (a)T 11 and (b) T 3. Solid line is 0 X direction, dashed line is0 M direction, and dash-dot line is non-PhC.
Fig. 7. Un-polarized angular-spectra-resolved measurements of the GaN PhC MCLED with11 GaN cavity thickness and PhC hole depth t = 250 nm measured the different PhC lattice constant (a)a = 270 and (b) 370 nm at a driven current 50 mA, where0 X is left and 0 M is right direction.
same cavity thickness . The guided mode dispersion of Fig. 7(a) and 7(b) are close to the free-photons band structures. The FP modes in Fig. 7(a) is more clear than in Fig. 7(b). This is because trapped light was extracted more by the PhC with larger lattice constant 370nm due to more light coupling directions [15], which make FP modes behind the extracted guided modes. To investigate how the PhC lattice constant influences the distribution of far-field emission of GaN PhC MCLED with the
same GaN cavity thickness and PhC hole depth
nm, the far-field radiation patterns of GaN PhC MCLED under the driven current of 50 mA, with the different PhC lattice constant shown in Fig. 8(a)–(b) were measured. Results show the differing far-field emission patterns peaked near normal on
the surface of the device with far-field angles of 74.9 (93.3 ) and 112.6 (88.39 ) in X ( M), respectively. Although exper-imental results did not reveal highly-directional far-field emis-sion patterns of thick GaN PhC MCLEDs, it was also observed significantly modified by the PhC lattice constant .
C. PhC Hole Depth Effect
This section discussed the influence of different PhC hole depth of GaN PhC uMCLED on the distribution of directional far-field emission using the same GaN cavity thickness and PhC lattice constant . Angular-spectra-resolved EL measure-ment under the driven current of 50 mA indicates GaN PhC
uMCLED with a square PhC lattice constant of nm,
and two different PhC hole depths, where the shallow and deep etching of the PhC hole depth were about 100 nm and 250 nm. This is clearly shown in Fig. 9(a)–(b) with light collected along the X (left) and M (right). The light traveling through the waveguided in the GaN PhC uMCLEDs was diffracted by reciprocal wavevectors associated with the PhC structures. Ob-servations of the shallow PhC etching of GaN PhC uMCLEDs revealed only two lower-order guided mode extractions, as shown in Fig. 9(a). In contrast, Fig. 9(b) illustrated the deep PhC etching of GaN PhC uMCLEDs in nearly single guided mode extraction. The photon band structures of Fig. 9(a)–(b) agreed with the dispersion curves of the guided mode effective refractive index. There have been discussed carefully elsewhere as [19]. The photon band structure results in this study differ from those in previous works, which showed shallow PhC etching of thick GaN PhC MCLEDs and dispersion that was simply the results of folded free-photon bands [6], [17].
To investigate how the PhC hole depth influence the distribu-tion of far-field emission of GaN PhC uMCLED, this study mea-sured the far-field radiation pattern under the driven current of 50 mA, as shown in Fig. 10(a)–(b). The shallow ( nm) and
deep ( nm) PhC hole depth of the GaN PhC uMCLEDs
caused differing far-field emission patterns. The patterns peaked near normal on the surface of the device with far-field angles of 120.54 (84.88 ) and 30.75 (34.38 ) in X ( M) respec-tively. The deep PhC hole of GaN PhC MCLEDs based on near single lower-order guided modes and the strong influence of micro-cavity caused a highly-directional far-field emission pat-tern. Therefore, the depth of the PhC hole , PhC lattice constant , and GaN cavity thickness , that affected the behavior of guided mode extraction, had significantly modified the distribu-tion of the direcdistribu-tional far-field emission of GaN PhC uMCLED.
Fig. 8. The far-field emission patterns of GaN PhC MCLED with the cavity thickness ofT 11 shows various PhC lattice constant (a) a = 270 and (b) 370 nm. Solid line is0 X direction and dashed line is 0 M direction.
Fig. 9. Un-polarized angular-spectra-resolved measurements of the GaN PhC uMCLED with3 GaN cavity thickness and PhC lattice constant a = 370 nm measured the different PhC hole depth (a) shallowt = 100 nm and (b) deep t = 250 nm under the driven current 50 mA, where 0 X is left and 0 M is right direction.
D. Light Extraction Efficiency Discussion
After sample fabrication, we measured the light emission characteristics of the brightness and color uniformity using imaging colorimeters and photometers (Radiant imaging PM-1600F). Fig. 11(a)–(b) shows the optical micrography of
GaN uMCLED with and without PhC lattice constant
nm under the lower driven current of 1 mA. They uniformly distributed across the chip area by the blue light distribution. It is noteworthy that the current spread under the low injection current could be not affected by the ultrathin GaN
cavity thickness due to the high electron mobility of n-GaN. In addition, the GaN PhC uMCLEDs exhibited a strong LEE, compared with the GaN non-PhC uMCLEDs with the same cavity thickness.
The experiments in this study the absolute light output
power-current-voltage characteristics were also measured
using an integration sphere with back-thinned charge coupled device (CCD) detector (CAS 140CT—the standard for array spectrometers). The turn-on voltage of the devices was about 2.8 V. This high forward voltage is due to the p-ohmic contact (Ni/Ag) agglomeration caused by high series resistance. The measured external quantum efficiency (EQE) decreased mono-tonically at current density values larger than 3.125 A/cm
% , because the non-optimized chip processes may suffer current crowding at high current. We considered the above poor characteristics of devices; therefore, Fig. 11(c) shows the absolute light output power of GaN MCLEDs with and without PhC as function of GaN cavity thickness under the electrical input power of 1 watt (W). The absolute light output power of the GaN PhC MCLEDs with GaN cavity
thick-ness variation at ( nm), ( nm), (
nm), ( nm), and the PhC lattice constant
nm under the electrical input power of 1 W were enhanced by 248%, 245%, 197%, and 168%, compared with the GaN non-PhC MCLED as shown in Fig. 11(c). This figure clearly shows that the GaN PhC MCLED has a higher light output power than the GaN non-PhC MCLED. The LEE of GaN cavity thickness variation in GaN non-PhC MCLEDs was also influ-enced by micro-cavity effect, as shown in Fig. 11(c). In addition, the LEE of GaN PhC MCLED with fixed PhC hole depth
nm strongly dependeds on the thin GaN cavity thickness and PhC lattice constant on the n-GaN layer. A larger PhC lat-tice constant creates high output power due to more light cou-pling directions [14]–[16]. In summary, the GaN cavity thick-ness , PhC lattice constant and PhC hole depth variation significantly modify the light extraction enhancement and direc-tional far-field emission pattern. For highly direcdirec-tional light ex-traction enhancement, structural parameters including the GaN ultrathin cavity thickness , larger PhC lattice constant
( nm), and deep PhC holes ( approaching MQW
layer) are optimized to achieve the LED with both high LEE and collimating far fields. Therefore, the study make the highly di-rectional LED promising for entendue-limited application, like pico-projectors [20].
Fig. 10. The far-field emission patterns of GaN PhC uMCLED shows the PhC lattice constanta = 370 with different hole depth (a) shallow t = 100 nm and (b) deept = 250 nm. Solid line is 0 X direction and dashed line is 0 M direction.
Fig. 11. The colorimeter of optical micrograph showing the blue light distri-bution across the GaN uMCLED for (a) non-PhC and (b) with PhC lattice con-stanta = 420 nm operated under the low injection current 1 mA, respectively. (c) Light output power characteristics of the GaN MCLED with and without PhC as function of GaN cavity thickness.
V. CONCLUSION
In conclusion, this study conducted experiment and a theo-retical investigation into highly-directional far-field emission patterns of GaN-based PhC MCLEDs with various structure pa-rameters. Angular-spectra-resolved EL measurements showed the guided modes extraction behavior that were strongly af-fected by the structural parameters of GaN PhC MCLED, where GaN cavity thickness influenced the extracted guided mode numbers, PhC lattice constant affected the far-field emission distribution, and PhC hole depth affected the PhC interaction with guided modes. The GaN PhC MCLED with the ultrathin
cavity thickness , PhC lattice constant nm
and deep PhC hole depth nm exhibited a maximum
output LEE of 248% compared to GaN non-PhC MCLEDs, and produced a highly-directional far-field emission pattern at
half intensity near . Highly-directional far-field emission patterns have been demonstrated in an optimized structure. The results indicated that the highly-directional light enhancement of GaN PhC uMCLEDs makes them promising candidates for etendue-limited applications, such as projecting displays.
REFERENCES
[1] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, “Status and future of high-power light-emitting diodes for solid-state lighting,” J. Display Technol., vol. 3, no. 2, pp. 160–175, Jun. 2007.
[2] O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steiger-wald, M. O. Holcomb, P. S. Martin, and M. R. Krames, “High per-formance thin-film flip-chip InGaN-GaN light-emitting diodes,” Appl.
Phys. Lett., vol. 89, 2006, Art. ID 071109.
[3] H. K. Cho, S. K. Kim, D. K. Bae, B. C. Kang, J. S. Lee, and Y. H. Lee, “Laser liftoff GaN thin-film photonic crystal GaN-based light-emitting diodes,” IEEE Photon. Technol. Lett., vol. 20, no. 24, pp. 2096–2098, Dec. 2008.
[4] H. Benisty, H. D. Neve, and C. Weisbuch, “Impact of planar micro-cavity effects on light extraction-Part I: Basic concepts and analytical trends,” IEEE J. Quantum Electron., vol. 34, no. 9, pp. 1612–1631, Sep. 1998.
[5] J. J. Wierer, A. David, and M. M. Megens, “III-nitride photonic-crystal light-emitting diodes with high extraction efficiency,” Nature Photon., vol. 3, pp. 163–169, 2009.
[6] C. F. Lai, C. H. Chao, H. C. Kuo, H. H. Yen, C. E. Lee, and W. Y. Yeh, “Directional light extraction enhancement from GaN-based film-transferred photonic crystal light-emitting diodes,” Appl. Phys. Lett., vol. 94, 2009, Art. ID 123106.
[7] K. Bergenek, C. Wiesmann, H. Zull, C. Rumbolz, R. Wirth, N. Linder, K. Streubel, and T. F. Krauss, “Strong high order diffraction of guided modes in micro-cavity light-emitting diodes with hexagonal photonic crystals,” IEEE J. Quantum Electron., vol. 45, no. 12, pp. 1517–1523, Dec. 2009.
[8] H. Y. Ryu, “Modification of the light extraction efficiency in micro-cavity vertical ingan light-emitting diode structures,” J. Kor. Phys. Soc., vol. 55, p. 1267, 2009.
[9] H. Y. Ryu and J. I. Shim, “Structural parameter dependence of light extraction efficiency in photonic crystal InGaN vertical light-emitting diode structures,” IEEE J. Quantum Electron., vol. 46, no. , pp. 714–720, May 2010.
[10] Y. S. Choi, M. Iza, E. Matioli, G. Koblmuller, J. S. Speck, C. Weis-buch, and E. L. Hu, “Submicron-thick microcavity InGaN light emit-ting diodes,” Proc. SPIE, vol. 6910, p. 69100R, 2008.
[11] P. M. Pattison, A. David, R. Sharma, C. Weisbuch, S. DenBaars, and S. Nakamura, “Gallium nitride based microcavity light emitting diodes with2 effective cavity thickness,” Appl. Phys. Lett., vol. 90, 2007, Art. ID 031111.
[12] D. Ochoa, R. Houdre, R. P. Stanley, C. Dill, U. Oesterle, and M. Ilegems, “Device simultaneous determination of the source and cavity parameters of a microcavity light-emitting diode,” J. Appl. Phys., vol. 85, p. 2994, 1999.
[13] Y. C. Shen, J. J. Wierer, M. R. Krames, M. J. Ludowise, M. S. Misra, F. Ahmed, A. Y. Kim, G. O. Mueller, J. C. Bhat, S. A. Stockman, and P. S. Martin, “Optical cavity effects in InGaN/GaN quantum-well-het-erostructure flip-chip light-emitting diodes,” Appl. Phys. Lett., vol. 82, p. 2221, 2003.
[14] A. David, H. Benisty, and C. Weisbuch, “Optimization of light-diffracting photonic-crystals for high extraction efficiency LEDs,” J. Display Technol., vol. 3, no. 2, pp. 133–148, Jun. 2007. [15] C. F. Lai, J. Y. Chi, H. C. Kuo, H. H. Yen, C. E. Lee, C. H. Chao, W. Y.
Teh, and T. C. Lu, “Far-field and near-field distribution of gan-based photonic crystal leds with guided mode extraction,” IEEE J. Sel. Top.
Quantum Electron., vol. 15, no. 4, pp. 1234–1241, Jul./Aug. 2009.
[16] C. Wiesmann, K. Bergenek, N. Linder, and U. T. Schwarz, “Photonic crystal LEDs—Designing light extraction,” Laser Photon. Rev., pp. 1–25, 2008.
[17] A. David, C. Meier, R. Sharma, F. S. Diana, S. P. DenBaars, E. Hu, S. Nakamura, C. Weisbuch, and H. Benisty, “Photonic bands in two-di-mensionally patterned multimode GaN waveguides for light extrac-tion,” Appl. Phys. Lett., vol. 87, 2005, Art. ID 101107.
[18] M. Bass, Handbook of Optics, vol. 2: Devices, Measurements, and
Properties, 2nd ed. New York: McGraw-Hill, 1995.
[19] C. F. Lai, H. C. Kuo, P. Yu, T. C. Lu, C. H. Chao, H. H. Yen, and W. Y. Yen, “Highly-directional emission pattern based on near single guided mode extraction from GaN-based ultrathin microcavity light-emitting diodes with photonic crystals,” Appl. Phys. Lett., vol. 97, 2010, Art. ID 013108.
[20] F. Fournier and J. Rolland, “Design methodology for high brightness projectors,” J. Display Technol., vol. 4, no. 1, pp. 86–91, Mar. 2008. Chun-Feng Lai (S’10) received the Ph.D. degree in photonics from the Insti-tute of Electro-Optical Engineering, Chiao Tung University, Hsinchu, Taiwan, in 2010.
Since 2010, he has been with Electronic and Opto-Electronics Research Lab-oratories, Industrial Technology Research Institute, Hsinchu, Taiwan. His cur-rent research is focused on high-efficiency GaN-based LEDs and GaN micro-cavity LEDs via photonic crystals.
Hao-Chung Kuo (M’98–SM’06) received the B.S. degree in physics from Na-tional Taiwan University, Taipei, Taiwan, the M.S. degree in electrical and com-puter engineering from Rutgers University, New Brunswick, NJ, in 1995, and the Ph.D. degree from the Electrical and Computer Engineering Department, University of Illinois at Urbana Champaign, Urbana, in 1999.
Since October 2002, he has been with the National Chiao Tung University (NCTU), Hsinchu, Taiwan, as a Faculty Member of the Institute of Electro-Op-tical Engineering. He is now the Associate Dean, Office of International Affair, NCTU. His current research interests include semiconductor lasers, VCSELs, blue and UV LED lasers, quantum-confined optoelectronic structures, optoelec-tronic materials, and solar cells. He has authored and coauthored 140 internal journal papers and six granted and ten pending patents.
Prof. Kuo is an associate editor of the JOURNAL OFLIGHTWAVETECHNOLOGY
and the special issue of the IEEE JOURNAL OFSELECTEDTOPICS INQUANTUM
ELECTRONICSon “Solid-State Lighting.” He was the recipient of the Ta-You Wu Young Scholar Award from the National Science Council and the Young Photonics Researcher Award in 2007.
Chia-Hsin Chao received the B.S.E.E. and Ph.D. degrees from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 1999 and 2006, respectively.
From 2005 to 2006, he was a Visiting Scholar with the Electrical and Com-puter Engineering Department, University of Illinois at Urbana-Champaign, Ur-bana. Since 2006, he has been with the Electronic and Opto-Electronics Re-search Laboratories, Industrial Technology ReRe-search Institute, Hsinchu, Taiwan. He is now a Project Deputy Manager and working on photonic crystal LEDs and LED micro-display. His research interests are in the area of numerical electro-magnetic field analysis, photonic crystals, and surface plasmon.
Peichen Yu received the B.S. degree in electrophysics and M.S. degree in elec-trooptical engineering from National Chiao-Tung University, Hsinchu, Taiwan, in 1996 and 1998, respectively, and the Ph.D. degree in electrical engineering from the University of Michigan, Ann Arbor, in 2004.
She was a Design Engineer with the Advanced Design Group, Intel Corpo-ration, Hillsboro, OR, for two years. Her main responsibilities were OPC-re-lated design rule definition and OPC algorithm development of the diffusion layer for the CMOS 65-nm and 45-nm nodes. She joined the Department of Photonics, National Chiao-Tung University as an Assistant Professor in August 2006. Her current research interests include the design and development of pho-tonic crystal-based and quantum dot-based semiconductor devices.
Wen-Yung Yeh received the Ph.D. degree in materials science and engineering from National Tsing Hua University, Taiwan, in 2001.
He is now the Deputy Director of the Opto-electronic Device and System Application Division, Electronics and Opto-electronics Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan. His current research interests include photonic crystal LED, AC LED, and GaN solar cells. He leads the AC LED research tea.
