Chapter 3 V-shape sapphire facet reflector LEDs
3.3 Characteristics of V-shape sapphire facet reflector LEDs
Fig. 3-7 shows the (a) output power (L-I curve) and (b) current-voltage (I-V curve)
characteristics of flat and V-shape Al-deposited sapphire reflector LEDs as a function of forward driving current. The L-I-V characteristics were measured with an on-wafer testing configuration, consisting of the Si detector mounted directly above the LED and the driving current being applied through the probes. It means that the power measurement is a relative axial output from the top surface of the chip. As can be seen in Fig. 3-7(a), the light output power of both structures increased continuously as the driving current was increased from 0 to 100 mA. The light output power of the V-shape sapphire facet reflector LEDs has higher output power of about 40% compared to the flat Al-deposited sapphire reflector LEDs at an injection current of 20 mA, i.e., a significant improvement attributed to the V-shape sapphire facet reflector to effectively reflect the emission light toward to the chip surface. In Fig.
3-7(b), about 3.3 V of forward voltages was measured on both devices at the injection current of 20 mA and no significant difference of the I-V curves were observed under the measurement condition of the driving current up to 100 mA, indicating that a feasible process for high brightness GaN-based LEDs was achieved without electrical damage.
In order to realize the enhancement mechanism of output power by adopting the V-shape sapphire facet reflector, the top view light-emission of LEDs were observed by charge-coupled device (CCD) and the obtained images are shown in Fig. 3-8 The photograph of the GaN-based V-shape Al-deposited sapphire reflector LED without current driving is shown in Fig. 3-8(a). According to this figure, a V-shape sapphire facet reflector is successfully attached to the LED epitaxy using double transferred technique. Fig. 3-8(b) shows the light-emission image of the enlarged photograph of area A in Fig. 3-8(a) under a driving current of 20 mA. In this figure, the emitting light from the LED mesa edge is redirected toward to the axial direction by the V-shape sapphire facet reflector, thus higher intensity was observed on the individual V-shape pattern than that on other regions, indicating that employing the V-shape sapphire facet reflector has the superior benefit for improving the
light extraction efficiency by effectively redirecting the guided light in side the LED chip toward to the top escape cone.
(a)
thickness index
sapphire 90 μm 1.7
epoxy glue 0.2 μm 1.6
n-GaN 4 μm 2.45
InGaN of MQW 3 nm 2.65
GaN of MQW 3 nm 2.45
p-GaN 0.2 μm 2.45
(b)
reflectance transmission absorptance
Al mirror 90% 0% 10%
ITO 10% 90% 0%
p-pad 50% 0% 50%
n-pad 50% 0% 50%
Table 3-1 (a) shown the material variables of the simulated models. And, (b) shown the surface variables of the models.
(a)
(b)
(C)
Fig. 3-1 (a) shown the structure of the simulated models. (b) shown the models in the TracePro software. (c) is a sketch of the pattern on the sapphire substrate.
(a)
(b)
(c)
0 30 60 90 120
0 10 20 30 40 50
Enhancement (%)
Etching time (sec)
Fig. 3-2 The Monte-Carlo ray-tracing calculated results of radiation patterns of the calculated enhancement on the light extraction efficiency with the increasing of sapphire etching time. (a) and (b) shown the irradiance maps of flat and etching 120sec structures. (c) shown the comparison of overall light extraction efficiency.
0 100 200 300 400 500 600 700 800 0
20 40 60 80 100
Enhancement (%)
Etching time (sec)
Fig. 3-3 show the simulated results with long etching time from 0 to 750 sec.
Fig. 3-4 Schematic of fabrication steps for GaN LEDs with sapphire facet mirror adopting double-transferred technique.
Fig. 3-5 shown the detail size of the V-shape sapphire facet reflector LEDs. The conventional LEDs have the same structure except sapphire substrate.
(a)
Fig. 3-6 SEM images of the wet etching sapphire substrate with R-plane of {1-102}. (a) top view , (b) and (c) cross-section side view images.
(a)
(c)
(b)
(b)
Fig. 3-7 (a)The output power (L-I curve) and (b) current-voltage (I-V curve) characteristics of flat and V-shape Al-deposited sapphire reflector LEDs as a function of forward driving current.
Fig. 3-8 The top view light-emission images. (a) Plan-view photograph of the GaN-based V-shape Al-deposited sapphire reflector LED. (b) The light-emission image of the enlarged photograph of area A in Fig. 3-8(a) under a driving current of 20 mA.
Chapter 4
Chemical wet-etched patterned sapphire substrate (CWE-PSS) LED
4.1 Fabrication of CWE-PSS LED 4.1.1 Process procedure
The GaN-based LEDs used in this study were grown using a low-pressure metal-organic chemical vapor deposition (Aixtron 2600G) system onto the C-face (0001) 2”-diamerter chemical wet-etched patterned sapphire substrates. The LED layer-structure comprised a 30-nm-thick GaN nucleation layer, a 2-µm-thick undoped GaN layer, a 2-µm-thick Si-doped n-type GaN cladding layer, an un-intentionally doped active region of 450-nm emitting wavelength with five periods of InGaN/GaN multiple quantum wells (MQWs), and a 0.2-µm-thick Mg-doped p-type GaN cladding layer. The grown wafer was patterned with square mesas of 350 x 350 µm2 in size by a standard photolithographic process and was partially etched until the exposure of n-GaN to define the emitting area and n-electrode; a 300-nm-thick ITO was deposited as the transparent conductive layer and Cr/Au was then deposited as n and p electrodes and was alloyed at 200 oC in N2 atmosphere for 5 minutes. Fig.
4-1 shows the process chart of the GaN-based LED grown on CWE-PSS.
For fabricating the CWE-PSS, the SiO2 film with hole-patterns of 3-µm-diameter and 3-µm-spacing was deposited onto the sapphire substrate by plasma-enhanced chemical vapor deposition (PECVD) to serve as the wet etching mask. The sapphire substrate was then wet etched using an H3PO4-based solution at an etching temperature of 300 oC. The sapphire wet-etching rate was about 1 µm/minute in this study and can be related to the H3PO4
composition and etching temperature [23-24].
4.1.2 SEM images of patterned sapphire substrates
Fig. 4-2 (a) and (b) show the SEM images of the pattern sapphire substrate of the etching time of 90s and 120s, respectively. In Fig. 4-2(a), the crystallography-etched pattern of an (0001)-oriented sapphire substrate has a flat-surface of {0001} C-plane with triangle-shape in the center. Surrounding the triangle-shape C-plane are three facets of {1-102} R-plane with angle of 57° against the [0001] C-axis. However, due to the relative fast etching rate of C-plane than that of R-plane, the triangle-shape flat-surface of {0001} C-plane in the pattern center finally vanishes with the increasing of the etching time. As shown in Fig. 4-2(b), the {0001} C-plane is absent and only {1-102} R-plane is observed on the CWE-PSS with the etching time of 120s. Fig. 4-2(c) shows the evolution of CWE-PSS with the increasing of sapphire etching time. It should be noted that the diameter of the sapphire pattern also increases with the increasing of etching time due to the side-etching effect; however, the period of the sapphire pattern keeps the same as 6-µm. Additionally, the high slope (57°) crystallography-etched facet of CWE-PSS is hard to fabricate by dry etching and it has been demonstrated in our previous work that this inclined facet has the superior capability for improving light extraction efficiency [15].
4.2 Characteristics of CWE-PSS LED 4.2.1 The HR-XRDs of the CWE-PSS LEDs
In order to investigate the film quality of CWE-PSS LEDs, the epitaxial wafers were analyzed by the high-resolution X-ray rocking curves (Bede D1 HR-XRD). Fig. 4-3 shows the HR-XRDs of the (0002) reflection of the CWE-PSS LEDs.
The measurements over wide range (-4000~3000 arcsec) and narrow range (-100~100 arcsec) are shown in Fig. 4-3(a) and Fig. 4-3(b), respectively. In Fig. 4-3(a), the same location of satellite-peaks over the wide measurement range for the conventional (sapphire etching
time of 0s) and all CWE-PSS LEDs indicates that the LED composition and growth rate were not associated with the CWE-PSS. However, according to Fig. 4-3(b), the full-width at half-maximum (FWHM) of main peak was about 40 arcsec for the conventional LED and was about 30 arcsec for all CWE-PSS LEDs. An obvious broad shoulder was observed near the main peak for the conventional LED. It suggested that the better crystalline quality was achieved on CWE-PSS LEDs and was consistent with the well accepted concept that the growth on the pattern sapphire substrate exhibited a considerable improvement on internal quantum efficiency by reducing the threading dislocation density, no matter on dry etching or chemical wet etching patterned sapphire substrates [16-22].
4.2.2 L-I measurement and external quantum efficiency
For comparing the LED performances with different crystallography-etched facet patterns, the sapphire substrates of etching times of 0s, 30s, 60s, 90s, and 120s, were employed into this report. All of these CWE-PSSs were then grown and processed at the same time, eliminating any artificial issue during LED fabrication. The LED chips were packaged into TO-18 without epoxy resin for the subsequent measurement. The typical light-current-voltage (L-I-V) measurements were performed using a high current measure unit (KEITHLEY 240).
The light output power of the LEDs was measured using an integrated sphere with a calibrated power meter.
Fig. 4-4(a) shows the measurement results of room temperature output power (L-I curve) of conventional and CWE-PSS LEDs as a function of the forward-bias current. In this figure, all the CWE-PSS LEDs demonstrate a significant improvement on output power as comparing to the conventional LED under our measurement condition up to 200 mA. The enhanced factor of output power of CWE-PSS LEDs compared to the conventional LED at a driving current of 20 mA is shown in Fig. 4-4(b). According to this figure, the optimized CWE-PSS condition was achieved on the etching time of 90s, corresponding to an enhanced
factor of output power of 1.4. Fig. 4-4(c) shows the external quantum efficiency (EQE) of the conventional and CWE-PSS LEDs with the forward injection currents up to 100 mA. It was found that the EQE of the CWE-PSS LED with etching time of 90s reached a maximum value of about 25 % at an injection current of 5 mA and then decreased significantly with a further increase in the forward bias current. Nearly the same trend was also obtained for the conventional LED sample except for a lower EQE value of 17.8 %. The degradation at the higher current might be due to reflow of injection carriers and the joule heating effect. It should be pointed out that even though the CWE-PSS LEDs performance in absolute terms of external quantum efficiency does not exceed state-of-the-art devices using other approaches, comparison is being made on the overall intensity enhancement using the CWE-PSS scheme.
4.2.3 Monte-Carlo ray-tracing calculations
In order to investigate the fundamental of enhancement of light output with different etching time of CWE-PSS LEDs, we used a commercial ray-tracing software employing the Monte-Carlo algorithm for forward ray-tracing, various outputs including efficiency value, spatial distributions of radiometric and photometric data. Shape and size of the solid model for the ray-tracing calculation was determined and exactly the same as the SEM images and microscopic measurements of the geometry of CWE-PSS LEDs, as shown in Fig. 4-1 and Fig.
4-2. The solid model was built up as a combination of simple solid objects, each semiconductor layer adjacent to the other. According to the recombination process, light rays were generated in the active layer with a uniform random distribution. Monochromatic radiation representing the peak wavelength of the measured spectral emission (450 nm) was used in the simulation.
Fig. 4-5 shows the calculated radiation patterns of (a) CWE-PSS LEDs with etching time of 30s (b) the conventional. A stronger axial radiation on the CWE-PSS LED than that of the conventional LED was observed in this figure. The same epitaxial models were also built on
the other CWE-PSS LEDs with the etching time of sapphire substrate of 60s, 90s, and 120s.
The comparison of overall light extraction efficiency was plotted and shown in the Fig. 4-5(c).
According to this calculation, the light extraction efficiency is dramatically enhanced with the increasing of sapphire etching time and. even more than twice large of magnitude of light output was observed on the CWE-PSS LED with the etching time of 120s. Therefore, the crystallography-etched patterns that evolving with the increasing of etching time of sapphire substrate affect the light extraction efficiency profoundly. With the increasing of the etching time, the triangle-shape flat-surface of {0001} C-plane in the pattern center finally vanishes, due to its relative fast etching rate than that of {1-102} R-plane. The sustained {1-102}
R-plane has an inclined crystallography-etched facet with a high slope as large as 57o, adding the opportunity of the guided light to meet the escape cone on the top of chip surface.
Fig. 4-6 is a simple schematic ray-tracing of the CWE-PSS LEDs with the increasing of sapphire etching time. In the case of the CWE-PSS LED with the large {0001} C-plane pattern, i.e., a short period of sapphire etching time, the light emitting from the LED active region (MQW) was much easier to be guided inside the LED chip, as compared to that of the longer period of etching time, corresponding to the larger surface of high-slope crystallography-etched facets of {1-102} R-plane. As shown in Fig. 4-6, more guided light can be extracted from the LED top surface, enhancing the total light output power. This is the reason why in Fig. 4-5(b), we can observe the strong illumines in the axial direction on the CWE-PSS LED.
As comparing the ray-tracing calculation and real device measurement, an inconsistent behavior on the sapphire etching time of 120s was observed due to the un-optimization of epitaxial condition by MOCVD. The cross-section side-view SEM images of CWE-PSS LEDs with different etching time of (a) 30s, (b) 60s, (c) 90s, and (d) 120s were shown in Fig.
4-7. The crystallography-etched sapphire patterns can be buried completely by the GaN
epitaxy in all CWE-PSS LEDs, except for the sample of the etching time of 120s. According to the ray-tracing calculation, the light extraction efficiency was significantly improved with the increasing of sapphire etching time and could be contributed to the high-slope crystallography-etched facet of {1-102} R-plane. However, the large inclined crystallography-etched surface also indicates the deep depth of the sapphire pattern, and it also takes more effort for adjusting the growth condition to obtain a high-quality GaN film.
As shown in Fig. 4-7 (d), a void locating inside the sapphire pattern can be observed due to the relative difficult for MOCVD to grow on this deep and inclined crystallography-etched facet pattern. Therefore, in the Fig. 4-4(b), a drop of light extraction efficiency was observed on the CWE-PSS LED of sapphire etching time of 120s. Beside, we do not consider the surface morphology while building the ray-tracing calculation model. Thus, another conflict on the absolute term of enhancement of the light output was observed, as comparing to Fig.
4-4(b) and Fig. 4-5(c). As can be seen in Fig. 4-7, the surface of LED chip is quite rough, and that is not taken into account on the ray-tracing calculation. By ignoring these epitaxial issues as mentioned above, both the ray-tracing calculation and real device measurement depict the same trend on the enhancement of light extraction efficiency, indicating the modeling of LED chip by ray-tracing calculation can be a powerful tool in predicting the efficiency of LED optics designs.
Fig. 4-8(a) and (b) show the simulated results with long etching time from 0 to 750 sec. In this figure, the enhanced factor is increase with the raise of etching time and gradually converges. In the future, if we can improve epitaxial processes, we can employ longer etching time PSS and enhance extraction efficiency further.
4.2.4 Reliability test of CWE-PSS LEDs
In Fig. 4-9, A reliability test was performed on the conventional and CWE-PSS LEDs under a driving current of 55 mA at 55 oC. Referring to previous data, the decay of the output
power at 55 mA, 55 °C after 96 hours is equal to the results at 20mA, 25 °C after 1000 hours.
the EL intensity to the initial EL intensity is shown as a function of the aging time.
According to this figure, all the life-testing samples exhibit the same degradation trend.
However, all the CWE-PSS LEDs present a gradual degradation in the EL intensity under our measurement condition up to 600 hours. In general, the EL intensity of conventional and CWE-PSS LEDs were decayed by about 20% and 10%, respectively; indicating the improvement on the epitaxial quality could be achieved via grown on the CWE-PSS scheme.
Fig. 4-1 The schematic drawing of process charts with chemical wet-etched patterned sapphire substrate
(b)
(C)
Fig. 4-2 (a) and (b) show the SEM images of the CWE-PSS of the etching time of 90s and 120s, respectively.
(c) A top-view drawing depicts the evolution of CWE-PSS with the increasing of etching time.
-4000 -2000 0 2000
Intensity (a.u.)
Arcsec
0s 30s 60s 90s 120s
-150 -100 -50 0 50 100 150
Intensity (a.u.)
Arcsec
0s 30s 60s 90s 120s
Fig. 4-3 The high-resolution X-ray rocking curves (Bede D1 HR-XRD). Measurements over (a) wide range (-4000~3000 arcsec) and (b) narrow range (-100~100 arcsec).
(a)
(b)
(a)
(b)
1.0 1.1 1.2 1.3 1.4 1.5
0 30 60 90 120 150
Etching Time (s)
E nha nc em ent
(c)
Fig. 4-4 (a) The measurement results of room-temperature output power (L-I curves) of the conventional and CWE-PSS LEDs (b) the enhancement factor on output power while comparing the CWE-PSS LEDs to the conventional LED under the driving current of 20 mA, and (c) the external quantum efficiency (EQE) of the conventional and CWE-PSS LEDs with the forward injection currents up to 100 mA.
(a)
(b)
(c)
Fig. 4-5 The Monte-Carlo ray-tracing calculated results of radiation patterns of (a) the conventional (b) CWE-PSS LEDs with etching time of 30s. (c) The calculated enhancement on the light extraction efficiency with the increasing of sapphire etching time.
Fig. 4-6 A schematic ray-tracing of the CWE-PSS LEDs with the increasing of sapphire etching time.
Fig. 4-7 The cross-section side-view SEM images of CWE-PSS LEDs with different etching time of (a) 30s, (b) 60s, (c) 90s, and (d) 120s.
0 100 200 300 400 500 600 700 800
Fig. 4-8 show the simulated results with long etching time from 0 to 750 sec.
0 100 200 300 400 500 600
Fig. 4-9 Reliability test of the conventional and CWE-PSS LEDs under stress condition of 55 oC and 50 mA.
Chapter 5
Conclusions and future work
5.1 Conclusions
In summary, we fabricated two kinds of LEDs with novel structures. In the first structure, high light-extraction-efficiency GaN-based LEDs employing an Al-deposited, R-plane of {1-102} with 57o against C-axis and V-shape sapphire facet reflector were successfully fabricated. A Monte-Carlo ray-tracing calculation was employed to design the geometric patterns of these V-shape sapphire facet reflector LEDs. The output power is increased by approximately 40% on this novel structure compared with the standard one with a flat reflector at an injection current of 20 mA, and with an normal forward voltage of about 3.3 V.
The improvement is attributable to the geometrical shape of sapphire facet reflector that enhances the light extraction efficiency by redirecting the guided light toward to the top exit cone of the LED surface.
And, in the second structure, the characteristics of GaN-based LEDs grown on patterned sapphire substrate fabricated by the chemical wet etching were specifically analyzed. By chemical wet etching, the sapphire substrate exhibited a particular crystallography-etched facet of {1-102} R-plane with an inclined slope as large as 57o, demonstrating a significant enhancement of the light extraction efficiency. A Monte-Carlo ray-tracing calculation was also employed to further investigate and design the geometric patterns of this novel CWE-PSS LEDs, and a consistent agreement was observed between theoretical calculation and the real device measurement. Moreover, an improvement of epitaxial quality was also observed on CWE-PSS LEDs, according to the measurement results of the high-resolution X-ray rocking curves (HR-XRDs) and device reliability testing. Therefore, by using this novel
CWE-PSS scheme, an overall enhancement of 40 % on the external quantum efficiency can be achieved, that is not only due to the improvement of the internal quantum efficiency upon reducing the dislocation density, but also contributed to the geometrical shape of the inclined crystallography-etched facets that can efficiently scatter the guided light to enter the escape cone.
5.2 Future work
In this report，we discovered that the epitaxial layer grown on patterned sapphire substrate was provided with better quality. Besides, PSS structure can redirect the light emitted from active layer toward to the exit cone of the LED surface as well as enhance
In this report，we discovered that the epitaxial layer grown on patterned sapphire substrate was provided with better quality. Besides, PSS structure can redirect the light emitted from active layer toward to the exit cone of the LED surface as well as enhance