Diode Fabrication

Fabrication of patterned sapphire substrates (PSS) was accomplished by process steps as shown in Figure B-5. A SiNx film was first deposited on 2-inch sapphire substrate by PECVD method (plasma-enhanced chemical vapor deposition) as etching mask and was patterned using polymethyl methacrylate (PMMA) by contact aligner with Deep UV lamp. Then, the spacing of the developed structure was reduced using thermal reflow technique. Figure B-6 shows the SEM (Hitachi S4700) images of spacing variation with the thermal reflow time. A spacing of 0.5μ m was achieved while reflow time was up to 2min, of which cross-section SEM image is shown in Figure B-7. After the hemisphere-like profile was transferred to SiNx by reactive ion etcher, the sapphire substrate was then etched utilizing inductively coupled plasma etcher with BCl₃ plasma. By altering the thickness ratio between PMMA and SiNx, a mesa-shape and a hemisphere-shape-patterned sapphire substrate (MPSS and HPSS) were obtained, of which the cross-section SEM images are shown in Figure B-8. The height of each PSS was 1μ m. Figure B-9 shows the top-view SEM image of the patterned sapphire substrates (MPSS and HPSS). The diameter and the closest spacing of the hemisphere on both substrates were 4.3μ m and 0.5μ m, respectively.

The InGaN-based LEDs were grown on c-plane 2inch-diameter patterned sapphire substrate (PSS) and conventional sapphire substrate (CSS) using metal-organic chemical vapor deposition (MOCVD). The epitaxial structure consisted of an undoped GaN buffer layer, a Si-doped n-type GaN layer, an active region with five periods of InGaN/GaN multiple quantum wells, an undoped Al0.05Ga0.95N layer, and an Mg-doped p-type GaN layer. The epi-structure was confirmed using Omega-2Theta scan by high-resolution x-ray diffraction (Bede D1) and relevant lattice architecture simulation, as shown in Figure B-10. The epitaxial wafers were fabricated by the conventional LEDs process flow as described below. The p-GaN layer was partially etched to the n-GaN layer to define the device size of 350×350μ m². 300nm-thick indium tin oxide (ITO) layer was deposited and then patterned on

the p-GaN layer. Finally, a metal stack of Ti/Al/Au was evaporated onto both p-GaN and n-GaN layers as contact electrodes. Figure B-11 illustrates the cross-section of the finished InGaN-based LEDs on PSS.

B.3 Results and Discussion

Figure B-12 shows the cross-section TEM (JEOL 2100) bright field images of GaN buffer layer grown on mesa-shape and hemisphere-shape-patterned sapphire substrate (MPSS and HPSS), respectively. As shown in Figure B-12(a), there are two voids observed clearly near the mesa-shape edge for MPSS. These voids were thought to be the evidence of free standing laterally growth GaN which helps reduce the threading dislocation density [B-7]. For HPSS, however, no void was observed but 90^o bending dislocations appear above the hemisphere shape region, as shown in Figure B-12(b). It suggests HPSS prevent dislocation propagation in c-axis direction and reduce the threading dislocation density. The founding is similar to the GaN grown on cone-shape-patterned sapphire substrate reported by J. H. Lee, et al [B-8].

To further realize the 90^o bending dislocations found in HPSS sample, another two TEM bight field images under multiple-beam and two-beam condition were taken, as show in Figure B-13. By comparing both TEM images, it can be concluded that the 90^o bending dislocations are edge dislocations since the edge dislocations are invisible under the two beam condition of g = (002).

To compare the dislocations density of the GaN buffer layer grown on CSS, MPSS, and HPSS, rocking curve of high-resolution x-ray diffractions were performed with an accuracy of ±7 arcsec and the results are shown in Figure 14. In Figure B-14(a), the full width at half maximum (FWHM) of the (002) plane rocking curves of the GaN films grown on each substrate were 274arcsec, 277arcsec, and 256arcsec, respectively. These similar results could be due to that the rocking curves of the symmetric planes, such as (002) plane, is insensitive to the edge threading dislocations which are the predominant component for the threading dislocations in GaN films grown on sapphire [B-9]. It has been reported that the pure edge threading dislocations distort the asymmetric planes so that the rocking curves of asymmetric planes are required to analyze the pure edge threading dislocations of the GaN films [B-10]. Figure B-14(b) compares the asymmetric (102) plane rocking curves of GaN grown on CSS and PSS. In comparison with GaN grown on CSS, the FWHM of GaN

decreases from 480arcsec to 293arcsec and 262arcsec for MPSS and HPSS, respectively. It indicates that the quality of the GaN film grown on PSS was improved and GaN on HPSS was slightly better than that on MPSS.

Figure B-15 shows the electrical characteristics of the LEDs grown on CSS and HPSS. It can be seen that the leakage current was reduced by more than one order at –30V by using HPSS technique. This can be contributed to the better material quality of the GaN buffer grown on HPSS. Fig B-16 plots the EL spectrum of the LEDs on each substrate. Under the driving current of 20mA, the wavelength of each LED was approximately 460nm. Figure B-17 plots the light-output power as a function of the injection current for non-encapsulated 460nm-LEDs grown on CSS and PSS, where the output power was measured using an integrated sphere detector.

The output powers were 4.05mW, 5.32mW, and 5.86mW for CSS, MPSS, and HPSS, respectively, under the typical driving current of 20mA. As compared with LEDs on CSS, the output power of LEDs on MPSS and HPSS were enhanced by 31% and 44%, respectively. It has been reported that the inclined facets of the PSS can redirect photons back to the device surfaces so that the efficiency of the light extraction can be increased [B-11]. Therefore, enhancement of brightness in this work resulted not only from the improvement of the epitaxial layer quality of the GaN films by PSS technique but also from the increase of the light extraction by the inclined facets of the PSS. It is worth noticing that LEDs on HPSS exhibited higher output power than those on MPSS. In addition to slightly better quality of GaN grown on HPSS, the HPSS could also redirect more photons due to its fully inclined geometry. As a result, there is an additional 13% increase in the output power for the LEDs grown on the MPSS compared to those grown on HPSS.

B.4 Conclusion

For the optoelectronic application, the performances of 460nm InGaN-based LEDs grown on mesa-shape and hemisphere-shape-patterned sapphire substrates (MPSS and HPSS) were presented. From rocking curve measurements of GaN asymmetric (102) plane, the full width at half maximum decreases from 480 arcsec to 293arcsec and 262 arcsec for GaN grown on MPSS and HPSS, respectively. It indicates that lower threading dislocation density can be achieved through such PSSs technique. For light-output power performance, although GaN materials grown on

MPSS and HPSS demonstrated similar dislocation density, a 44% improvement of light-output power for the LEDs grown on HPSS was observed, which is higher than a 31% improvement for the LEDs grown on MPSS. This result can be contributed not only to better quality of LEDs grown on HPSS but also to HPSS‟s fully inclined facets which increases light redirecting and thus increases the LED light extraction efficiency. In addition to the LED application, patterned sapphire substrates presented in this letter can also be used for laser diode applications due to their contributions to the improvement of GaN film quality which is very important for high quality laser diode.

Figure

Figure B-1. Cross section TEM image of GaN epilayer grown on (a)conventional and patterned-sapphire substrate, (b) pattern depth = 0.5 μm, and (c) 1.5 μm.

Lateral grown

region

Figure B-2. A schematic ray-tracing of the LEDs grown on conventional and patterned sapphire substrates.

Sapphire

Conventional Substrate

Patterned Sapphire

Patterned Substrate

Active Layer

Active Layer

Figure B-3. AFM images of cone-shaped-patterned sapphire substrates with different spacing.

Figure B-4. Light output power of LEDs grown on cone-shaped-patterned sapphire substrates with different spacing.

3 μm

m

2 μm

m

1 μm

m

3μm 2μm

1μm

Figure B-5. Process flow of the hemisphere-shape-patterned sapphire substrate with submicron spacing.

Sapphire

SiNx

1 μm

PMMA

Sapphire

S

iNx

Thermally reflowed PMMA

0.5 μm

Sapphire

Inclined SiNx mask

Sapphire

Figure B-6. Top-view SEM images of lithography result with reflow (a) 0 min (b) 1 min.

(a)

3.85 μm

1.14μm

(b)

4.2 μm

0.7μm

Figure B-6. Top-view SEM images of lithography result with reflow (c) 2min

Figure B-7. Cross-section SEM image of lithography result with reflow time of 2min.

4.3 μm

0.5μm

(c)

Figure B-8. Cross-section SEM images of patterned sapphire substrates (a)MPSS (b)HPSS.

SiNx PSS (a)

(b)

Figure B-9. Top-view SEM images of patterned sapphire substrates (a)MPSS (b)HPSS.

(a)

(b)

Figure B-10. Omega-2Theta scan of high-resolution x-ray diffraction (black line) and the lattice architecture simulation(red line).

Figure B-11. Schematic cross-section of the fabricated LED grown on hemisphere-like shape patterned sapphire substrate.

p-GaN p-AlGaN

GaN buffer layer

HPSS

p-Pad

n-Pad

ITO

MQW (InGaN/GaN)

n-GaN

102

103

104

105

106

-4 0 0 0 -3 5 0 0 -3 0 0 0 -2 5 0 0 -2 0 0 0 -1 5 0 0 -1 0 0 0 -5 0 0 0 5 0 0 1 0 0 0

Intensity

Seconds

conventional_02aa001.X01 temp.sim

Al

_0.05

GaN M M ul u lt ti ip pl l e e Q Qu ua an nt tu um m W We el ll l

( ( In I n

_0.0.11

Ga G aN N/ /G Ga aN N)

-5 - 5 -4 - 4 -3 - 3 -2 - 2

-1 - 1 0 0

+1 + 1

GaN

Figure B-12.Cross-section bright-field TEM image of GaN buffer layer grown on (a) mesa-shape- and (b) hemisphere-shape-patterned sapphire substrate. The images were taken on <11-20> zone axis.

.

(a)

(b)

400nm

400nm [002]

[002]

GaN buffer

GaN buffer Sapphire

Sapphire

Figure B-13. Cross-section bright-field TEM image of GaN buffer layer grown on hemisphere-shape-patterned sapphire substrate. The images were taken on (a) <11-20> zone axis and (b) under two beam condition of (002).

Sapphire

GaN buffer

400nm

Sapphire

GaN buffer

400nm

(a)

(b)

[002]

[002]

Figure B-14. Rocking curves of the GaN films grown on CSS, MPSS, and HPSS. (a) (002) plane, (b) (102) plane.

-1000 -500 0 500 1000

CSS

Normalized Intensity

Omega (arcsec)

GaN (002)

HPSS MPSS (a)

-1500 -1000 -500 0 500 1000 1500

CSS

Omega (arcsec)

Normalized Intensity

GaN (102)

HPSS

MPSS

(b)

Figure B-15. Forward and reverse I-V characteristics of the LEDs grown on conventional (CSS) and patterned sapphire substrate (HPSS).

Figure B-16. The EL spectra of the InGaN-based LEDs grown on CSS, MPSS, and HPSS, where the injection current was 20mA.

-30 -25 -20 -15 -10 -5 0 5

10^-8 10^-7 10^-6 10^-5 10^-4 10^-3 10^-2

Current (A)

Voltage (V)

CSS

HPSS

440 460 480

Intensity (a. u.)

Wavelength (nm)

HPSS

MPSS

CSS

Figure B-17. The light-output power characteristics of 460nm InGaN-based LEDs grown on CSS, MPSS, and HPSS.

20 40 60 80 100

5 10 15 20

MPSS HPSS CSS

Current (mA)

Output Power (mW)

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CURRICULUM VITAE

 Name: Chia-Ta Chang

 Gender: Male

 Birth: 20th Dec. 1981

 Personal Information.

Home Address: 1F., No.42, Fuxing St., Banqiao Dist., New Taipei City 220, Taiwan Telephone: + 886-928-149591

E-mail: [email protected]

 Education

2011 PhD., Department of Material Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan

2004 B.S., Department of Material Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan

 Professional Experience

2010/3 ~ 2011/2 Guest scientist in Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany

2010/3 ~ 2011/2 Guest scientist in Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany

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