CHAPTER 3 EFFECT OF THE SILVER MIRROR LOCATION ON THE LUMINANCE
3.4 Summary
Three kinds of LEDs were used to investigate the effect of the mirror location on the performance of LEDs. PR-LED was LED with roughened p-GaN surface. The PR-LED was utilized to fabricate the DRM-LED and DRSM-LED by wafer-bonding, laser lift-off and surface-roughening technologies. It was found that the light intensity of DRSM-LED was 2 times higher than that of PR-LED. This enhancement is not surprising, since the Ag mirror redirected the downward-traveling light back to the top surface. By changing the mirror location from the
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backside of the sapphire to the u-GaN/sapphire interface, the LED light intensity was further enhanced. This is because, compared with DRM-LED, the photon path inside the DRSM-LED structure has to pass through an extra bonded interface (adhesive layer/sapphire) 2 times. Besides, when the mirror of DRSM-LED redirecting the downward-traveling light, light traveling from sapphire to the adhesive layer will only cross within a critical angle of 61.9°. The light reaching the adhesive layer beyond the critical angle will undergo total internal reflection.
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Figure 3.1 Schematic diagrams of (a) PR-LED, (b) DRSM-LED, and (c) DRM-LED.
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Figure 3.2 Fabrication process of the PR-LED.
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Figure 3.3 Experiment flowchart.
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400 450 500 550 600 650
70 75 80 85 90 95 100
Reflectivity (%)
Wavelength (nm)
Ag as deposited Ti/Ag as deposited
Ti/Ag 300oC 24h annealing in Ar Al as deposited
Figure 3.4 Reflectivity of different mirrors.
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Figure 3.5 SEM image of the u-GaN surface etched by 90oC KOH for 1 min.
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0 50 100 150 200
0 1 2 3 4 5 6 7 8
Voltage (V)
Current (mA)
DRM-LED DRSM-LED PR-LED
Figure 3.6 I-V characteristic of the LEDs.
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0 50 100 150 200
0 200 400 600 800 1000 1200
Luminance Intensity (mcd)
Current (mA)
DRM-LED DRSM-LED PR-LED
Figure 3.7 The effects of injection current on the luminous intensity of the LEDs.
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Figure 3.8 Possible photon paths inside the structures of the (a) DRSM-LED and (b) DRM-LED.
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CHAPTER 4
IMPROVED PERFORMANCE OF INGAN-GAN LIGHT-EMITTING DIODE BY A PERIODIC N-BOWL MIRROR ARRAY
4.1 Introduction
High-brightness GaN-based light-emitting diodes (LEDs) for blue and ultraviolet light sources have attracted intense research for their versatile applications in mobile phones, full-color displays, mini projector and lightings in the past few years.[1] Internal quantum efficiency and light extraction efficiency are two principal parameters for improving the efficiency of LEDs;
both of which can be improved through the improvement of crystal quality and the modification of the LED structure. The internal quantum efficiency of GaN LEDs can exceed 70% now, which is much higher than the 10%-25% efficiency of conventional light sources.[2] However, the light extraction is limited by their inability to emit all of the light that is generated from the active layer.[3] According to Snell’s law, the critical angle for light traveling from GaN (n=2.5) to air (n=1.0) is 23°. Only the light within the critical angle will cross the air, while the other experience total internal reflection until it is absorbed. Many methods have been developed to overcome this problem. These methods were not beyond the scope of changing the shaping and surface morphology of the LED, which included random texturing of the LED’s surface,[4]
sidewall obliquing by dry etching,[5, 6] pattern sapphire application,[7, 8] and etc.. In this study, a periodic n-bowl mirror array was introduced into LEDs to improve the LED performance and adjust the view angle.
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4.2 Experiments
4.2.1 Illustration and Abbreviation of LEDs
Two kinds of LEDs were investigated in this study. Their specifications and structures are schematically illustrated in Fig. 4.1(a) and (b). Samples designated as “PR-LED” were LEDs with roughened p-GaN surface, while samples designated as “NBM-LED” were LEDs composed of roughened p-GaN surface, n-bowl structure, and mirror on n-bowl surface. Three types of NBM-LEDs were investigated. They were denoted as 25-4 (25 μm diameter and 4 μm spacing, as shown in Fig. 4.1(c)), 4-3 and 3-3 NBM-LEDs, as listed in Table 4.1. The cross section scanning electron microscope (SEM) image of 4-3 n-bowl structure was shown in Fig. 4.1(d).
4.2.2 Crystal Growth and Device Process of PR-LED
The basic processes of these LED were the same, and were shown in Fig. 4.2. The InGaN-GaN films were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on a sapphire substrate. The LED structures were consisted of a 5-nm-thick Si-doped n+-InGaN tunnel contact structure, a 0.4-μm-thick p-type Mg-doped GaN layer, an InGaN-GaN multiple quantum well (MQW), a 2-μm-thick Si-doped GaN layer, a 2-μm-thick undoped-GaN (u-GaN) layer film and a buffer layer on a sapphire substrate. The Si-doped n+-InGaN layer was used to form the ohmic contact between ITO and p-GaN.[9] The device mesa with a chip size of 300×300 μm2 was then defined by an inductively coupled plasma (ICP) to remove Mg-doped GaN layer and MQW until the Si-doped GaN layer was exposed. After annealed at 600 °C for 10 min, the indium tin oxide (ITO) layer was deposited to form a p-side contact layer and a current spreading layer. The Cr/Au layer was deposited onto the ITO layer to form the p-side and n-side
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electrodes. The roughened p-type GaN surfaces of PR-LED were formed by lowering the epitaxy growth temperature of the p-type GaN layer.[10, 11]
4.2.3 Fabrication of NBM-LEDs
Fig. 4.3 shows the fabrication processes of the NBM-LEDs. To fabricate the NBM-LED, PR-LED wafer was bonded to a host substrate using a glue layer.[9] The bonding process was performed at 200 °C for 1 hour. After bonding, the sapphire substrate was lifted off by irradiating it with KrF pulsed excimer laser (248 nm) to expose the u-GaN surface. To fabricate the n-bowl structure, photoresist reflow and dry etching method were used.[12] A photoresist layer was spun onto the u-GaN surface and patterned by standard photolithography into cylindrical disks. When baked at the temperature higher than 200 °C, they turned to hemispherical shapes because of the surface tension. The performed shapes were then transferred to u-GaN using ICP process. Fig.
4.4 shows the SEM image of the 4-3 n-bowl structure. A mirror system (Ni/Ag/Ni) was then introduced on the etched surface to reflect the downward photons.[13] Sample was subsequently bonded to Si substrate with an adhesive layer.[9] The host substrate and glue layer were subsequently removed. For baseline comparison the performance of the PR-LED, sapphire was chosen as the substrate of NBM-LED. With the same sapphire substrate, we could investigate the influence of n-bowl structure on the performance of the LED chips. The samples described herein were only cut into chips without encapsulation.
4.3 Results and Discussions
Fig. 4.5 shows the current-voltage (I–V) characteristics of LEDs. It was found that the
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forward voltages of NBM-LEDs were in the range of 3.4-3.6 V (at 20 mA), which were similar to that of PR-LED, indicating that transfer method did not change much of LED performance. The little difference might due to the uniformity of the original wafer. The effects of the injection current on the luminous intensity are depicted in Fig. 4.6. The intensities of three NBM-LEDs were all higher than that of PR-LED. This is because the n-bowl mirror structure in NBM-LEDs not only reflected the downward photons to the front side, but also redirected the photons which were originally emitted out of the escape cone, back into the escape cone.[9]
Fig. 4.6 also indicates that the light intensity of NBM-LEDs increased with the decrease of n-bowl dimension. As shown in Table 4.1, the light intensity of 3-3 NBM-LED was 176.0 mcd,
which was 2.33 times higher than that of the PR-LED, 1.95 times higher than that of the 25-4 NBM-LEDs, and 1.43 times higher than that of the 4-3 NBM-LED. This is because Ni/Ag/Ni n-bowl system acting as a concave mirror. The scattering ability of concave mirror increases with
the curvature of n-bowl. According to the optical lever principle, the variation of the reflection angle of photons is two times than that of the surface angle.[14] It means that the surface of n-bowl with steep slope (large curvature) can cause more changes in the photon paths. As
mentioned by Oder et al.,[12] the bowl structure is a part of the sphere (Fig. 4.7). If the sphere has radius R, then R can be deduced from the measured values of bowl radius r and height h, and gives the relation as
The radius of curvature is listed in Table 4.1.
The light output versus injection current curves for the LEDs were also shown in Fig. 4.8.
LEDs were measured in an integrating sphere and were not encapsulated during the measurements. It was found that the 3-3 NBM-LED achieved an output power of 6.21 mW at 20
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mA, which was 43% larger than the PR-LED, 19% larger than the 25-4 NBM-LED, and 4%
larger than the 4-3 NBM-LED, as shown in Table 4.1. The improvement of the light output was also due to the reflection of the downward photons to the front side and redirection of photons from the roughened u-GaN surface and the n-bowl mirror on sapphire substrate.
The radiation patterns of the LEDs are shown in Fig. 4.9. Their view angles (half-center brightness, which is the angle for 50% of full luminosity) were shown in Table 4.1. It was found that the view angle decreased with the diameter of n-bowl. The viewing angle of PR-LED was 130°. As the diameter of n-bowl decreased from 25 to 3 μm, the view angle decreased from 120°
to 118°. This is because the surface of n-bowl can change the photon path. These changes can be explained by the basic geometric optics, as shown in Fig. 4.10. For a spherical mirror,[15] a simple equation relates the source distance S, the imaging distance S’, the focal length f, and the radius of curvature R is as follow:
1 1 2 1
'
S+S =R = f (4.2)
From the measured values listed in Table 4.1, the focal lengths of the 3-3, 4-3, and 25-4 n-bowls could be calculated as 1.73, 2.25, and 31.12 μm, respectively. The actual source
distances S from active layers to three different n-bowl structures were 3.78, 3.93, and 4.15 μm.
If the source is at the focal point (S=f), the imaging distance will be infinite (S’=∞). In other words, photons were redirected to the vertical direction as shown in Fig. 4.11.
The positions of light sources of 3-3 and 4-3 NBM-LEDs were quite near their focal points.
As a result, photons from the active layers might be redirected toward the vertical direction and benefit to the vertical intensity. On the other hand, the location of the light source of 25-4 NBM-LED was far from the focal point. As a result, the redirect ability of 25-4 NBM-LED is not as good as that of 3-3 and 4-3 NBM-LEDs.
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These improvements were similar to our previous finding of RM-LED (LED with a roughened KOH-etched GaN surface and an Ag mirror on the backside of sapphire substrate).[9]
The output power enhancement of 3-3 NBM-LED was 1.43 times, which was less than that of RM-LED (1.49 times). This observation suggested that scattering ability of the n-bowl structure was not as good as that of KOH-etched GaN surface. On the other hand, the vertical intensity enhancement of 3-3 NBM-LED was 2.33 times, which was higher than that of RM-LED (2 times). This is because the n-bowl structures had good focusing ability.
4.4 Summary
NBM-LEDs with roughened p-GaN surface and n-bowl mirror structure were fabricated by wafer bonding and laser lift-off technology. The forward voltages of these NBM-LEDs were close to PR-LED, indicating that transfer method did not change much of LED structure. The performance of NBM-LEDs was better than that of PR-LED. The luminance intensity of 3-3 NBM-LED at 20 mA was 176.0 mcd, which was 2.33, 1.95, and 1.43 times higher than PR-LED, and 25-4 and 4-3 NBM-LEDs. The 3-3 NBM-LED achieved an output power of 6.21 mW, which was 43% larger than the PR-LED, 19% larger than the 25-4 NBM-LED, and 4% larger than the 4-3 NBM-LED. Besides, the view angle decreased with the diameter of n-bowl. The viewing angle of PR-LED was 130°. As the diameter of n-bowl decreased to 3 μm, the view angle decreased to 118°. This is because the n-bowl mirror structure acting as a concave mirror. It not only reflected the downward photons to the front side, but also redirected the photons which were originally emitted out of the escape cone, back into the escape cone.
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Figure 4.1 Schematic illustration of LEDs: (a) PR-LED, (b) NBM-LED, and (c) the n-bowl mirror array of the 25-4 NBM-LED. (d) is the cross-sectional SEM image of the 4-3 n-bowl structure.
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Table 4.1 The parameters and performances of LEDs.
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Figure 4.2 Fabrication process of the PR-LED.
67
Figure 4.3 Experiment flowchart.
68
Figure 4.4 SEM image of the 4-3 n-bowl structure.
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0 50 100 150 200
0 1 2 3 4 5 6 7 8
Voltage (V)
C urre nt (mA)
3-3 NBM-LED 4-3 NBM-LED 25-4 NBM-LED PR-LED
Figure 4.5 Current-voltage characteristic of the LEDs.
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0 10 20 30 40 50
0 50 100 150 200 250 300 350
In te nsi ty (m cd )
Current (mA)
3-3 DSM-LED 4-3 DSM-LED 25-4 DSM-LED PR-LED
Figure 4.6 The effects of injection current on the luminous intensity of the LEDs.
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Figure 4.7 Illustration of the physical parameters of the bowl. The bowl has radius r, height h, and R is the radius of the complete sphere.
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0 10 20 30 40 50
0 5 10 15
3-3 DSM-LED 4-3 DSM-LED 25-4 DSM-LED PR-LED
Po we r (mW)
Current (mA)
Figure 4.8 The effects of injection current on the light output of the LEDs.
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-100 -80 -60 -40 -20 0 20 40 60 80 100 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
3-3 NBM-LED 4-3 NBM-LED 25-4 NBM-LED PR-LED
Ill umi na nc e (a .u.)
Angle (degree)
Figure 4.9 The radiation patterns of LEDs.
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Figure 4.10 Illustration of the physical parameters of the concave mirror.
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Figure 4.11 Schematic illustration of the n-bowl structure.
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CHAPTER 5
INGAN-GAN LIGHT-EMITTING DIODE PERFORMANCE IMPROVED BY ROUGHENING INDIUM TIN OXIDE WINDOW
LAYER VIA NATURAL LITHOGRAPHY
5.1 Introduction
High-brightness GaN-based light-emitting diodes (LEDs) have attracted intense research for their various applications in mobile phones, full-color displays, mini projector and lightings in the past few years. Internal quantum efficiency and light extraction efficiency are two principal parameters for improving the efficiency of LEDs; both of which can be improved through enhancing crystal quality and modifying the LED structure.[1-3] The internal quantum efficiency of GaN LEDs generally exceeds 70%, which is much higher than the efficiency of conventional light sources such as incandescent lamps and fluorescent lamps.[4] However, the light extraction efficiency of GaN-based LEDs is limited by the large difference in refractive index between the GaN film and the surrounding air. According to Snell’s law, the critical angle for light traveling from GaN (n=2.5) to air (n=1.0) is 23°. Only the light within the critical angle will cross the air, while the other experiences total internal reflection until it is absorbed. Many methods have been developed to solve this problem, which focus mostly on sidewall obliquing,[5] n-GaN morphology changing,[6] and sapphire patterning of the LED.[7] p-Side treatment, however, is still hard to perform because of the small thickness of p-GaN. In addition, Fresnel loss is another problem for light extraction. When photons pass through the GaN and air interface, there will be a reflection loss of about 18.4% caused by the large refractive index difference between the two
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mediums.[8] Inserting an intermediate layer, like indium tin oxide (ITO), can moderate this phenomenon. This ITO layer not only serves as a good window layer, but also makes the p-side treatment possible. In this study, a roughened ITO surface was introduced to LED by natural lithography, in which a photoresist layer was used as a mask for dry etching.
5.2 Experiments
5.2.1 Illustration and Abbreviation of LEDs
Two kinds of LEDs were investigated in this study. As shown in Fig. 5.1, samples designated as “CV-LED” were conventional LEDs without any surface treatment, while samples designated as “IR-LED” were LEDs with ITO roughened surface. Two types of IR-LEDs were investigated. They were denoted as “IR1.6-LED” (ITO roughened surface produced from a 1.6-μm photoresist mask) and “IR1.9-LED” (ITO roughened surface produced from a 1.9-μm photoresist mask).
5.2.2 Fabrication of the CV- and IR-LEDs
InGaN-GaN films were grown by metalorganic chemical vapor deposition (MOCVD) on a sapphire substrate. The LED structure contained a 0.3-μm-thick p-type Mg-doped GaN grown at 950°C, an InGaN-GaN multiquantum well (MQW) with six pairs of InGaN (3 nm)/GaN (9 nm) at 800°C, a 2-μm-thick n-type Si-doped GaN at 1050°C, a 2-μm-thick undoped-GaN layer film at 1050°C, and a buffer layer at 550°C on the sapphire substrate. The Si-doped n+-InGaN layer was employed to form the ohmic contact between ITO and p-GaN.[9] A 2.8-μm ITO film, which was much thicker than that of conventional LED (0.3 μm), was then deposited on the top of the LED
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structure. This extra thickness of the ITO layer was employed to create the roughened surface.
The experiment flowchart is shown in Fig. 5.2. To perform ITO roughening, photoresist layers (AZ-1518) of different thicknesses (1.6 and 1.9 μm) were coated as the dry etching mask.
The inductively coupled plasma (ICP) etching conditions used in this study were Cl2/BCl3/CH4/He (4:6:1:2) mixture gas, 5 mTorr pressure, 400 W ICP power, and 100 W rf power. During etching, the surface morphology of the photoresist layer changed due to the formation of giant folds.[10] This undulation was subsequently transferred to ITO in the same ICP run which etched the entire photoresist and most of the ITO away until only 0.3 μm of ITO remains. The device mesa with a chip size of 300 × 300 μm2 was then defined by ICP to remove Mg-doped GaN layer and MQW until the Si-doped GaN layer was exposed. Samples were then post-annealed at 600°C to reconstruct the ICP damaged layer. Finally, Cr/Au was deposited onto the ITO layer and n-GaN layer as electrodes. For baseline comparison, all samples were prepared from the same epitaxial wafer. The samples described herein were only cut into chips without encapsulation.
5.3 Results and discussions
The dry-etching process is a critical step in the fabrication of nitride-based LEDs. During the ICP etching process, the ICP-damaged layer is a well-known and inevitable problem.[11]
Therefore, the variation in contact between the ICP-etched ITO surface and Cr/Au pad is an important issue. The specific contact resistance (ρc) was examined using the transmission line model (TLM).[12] Five CV-LED samples covered with 2-μm ITO were utilized to study the effect of post-annealing temperature on the ρc (ICP-etched ITO surface). Sample A was untreated, while samples B-E underwent ICP treatment for 10 min. Then, samples C-E were post-annealed
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at 200°C, 400°C and 600°C for 30 min, respectively. Sample B did not undergo any annealing.
TLM gives two relations as follow:
2 SH 2 SK t SH
where RT is the impedance between the two adjacent contact pads, RC is the contact resistance, RSH is the sheet resistance of the semiconductor layer outside the contact region, W is the width of the contact area, L is the distance between two adjacent pads, RSK is the modified sheet resistance under the contact, and Lt is the transfer length. The derived specific contact resistances are shown in Table 5.1. It was found that specific contact resistances of the samples after ICP treatments were smaller than the untreated one (sample A). This was due to the creation of oxygen vacancies during ion bombardment.[13] The ρc increased slightly with post-annealing temperature and reached the maximum value of 7.2 × 10-7 Ω·cm2 at 400°C. This was probably due to the In2O3 aggregation and SnO formation at low temperature, which led to inhomogeneous distribution of Sn, and reduction in carrier concentration.[14] When annealing temperature reached 600°C, crystallites grew and the grain boundary reduced. The tin oxide in the matrix also changed from SnO to SnO2 state.[14] Thus, the ρc reduced to about 1.8 × 10-7 Ω·cm2. From this measurement, it appears that the pad/ITO contact was not deteriorated when the ITO surface was after ICP etching.
The ρc of the interface was even reduced to its minimum when the sample was treated at the post-annealing temperature of the regular device process (600°C).
The surface morphologies of ITO films were examined by scanning electron microscopy (SEM) as shown in Fig. 5.3(a)-(c). The SEM image was taken at an angle of 45° from the perpendicular direction of the surface. A wave-like structure formed on IR-LED was clearly observed. This was caused by the formation of giant folds.[10] During ion bombardment, the
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photoresist layer turned into two distinct layers, which comprised a highly cross-linked polymer top layer modified by the high-energy ions, and a bottom layer of normal resist not affected by
photoresist layer turned into two distinct layers, which comprised a highly cross-linked polymer top layer modified by the high-energy ions, and a bottom layer of normal resist not affected by