Optimization of ITO annealing and

Color conversion using yellow phosphor for LED Applications

Freddy G. Setiawan

Nano – Bio Photonics Laboratory

(Prof. Cho Yong-Hoon)

Supervised by :Dr. RodriguezDr. Isnaeni

Introduction• GaN has been used in many optoelectronic devices

• Characteristics : high brightness and thermal conductivity, mechanically stable.

• GaN-based LED structure

• Problem : p-GaN has poor holes mobility.

MotivationsWe have high threshold voltage on the IV characteristic of LED with our previous fabrication

process. We suspect the problem comes from ITO annealing.

We have not found the optimum annealing conditions for ITO.For red emission, InGaN has poor efficiency, therefore we use color conversion using phosphor

(red)

We have not known the combination of yellow phosphor material for high color conversion.

Goals

1. Determine the annealing condition for ITO on p-GaN (time & temperature)

2. Determine the composition of coating material (phosphor, epoxy / PR) for high efficiency and color conversion.

Motivations and Goals

Determine the annealing condition for ITO on p-GaN (time & temperature)

1st topic

Schottky vs Ohmic

Metal

𝜙𝑀𝜙𝐵

𝜙𝑆𝜒𝑆

𝐸𝐹

𝐸𝐶

𝐸𝑉

Vacuum

N-Semiconductor

Schottky ContactOhmic Contact

Indium Tin Oxide (ITO) can improve current spreading and transparent to visible light. These characteristics depend on annealing condition[1]

[1] T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Hansen et al. Indium tin oxide contacts to gallium nitride optoelectronic devices. Appl. Phys. Lett. 74, 3930 (1999); doi: 10.1063/1.124227.

Metal

𝐸𝐹

𝐸𝑉

𝐸𝐶𝜙𝑀

𝜙𝑆

N-Semiconductor

Vacuum

𝜒𝑆

09A1 U5 – SPEM (PRT)

Why ITO annealing make ohmic contact ?

After annealing :• Ga atoms will diffuse to ITO Ga-O formed• There will be Ga vacancy below the contact. • Increase the net concentration of holes bellow the contact

enhance the probability of carrier tunneling.[2][3]

p-GaN

ITO / VGa / GaO

After Annealing

Increasing the annealing temperature of ITO will make the degree of crystallization higher, so it will increase the ITO conductivity. [4]

[2] Kow-Ming Chang, et. al. Investigation of indium–tin-oxide ohmic contact to p-GaN and its application to high-brightness GaN-based light-emitting diodes. Solid-State Electronics 49 (2005) 1381–1386.[3] Kim JK, et. al. Microstructural Investigation of Ni/Au ohmic contact on p-type GaN. JElectrochem Soc 2000;147:4645–51.[4] Soo Young Kim, Ho Won Jang, and Jong-Lam Lee. Transparent Ohmic Contacts on p-GaN Using an Indium Tin Oxide Overlayer. Phys. stat. sol. (c) 0, No. 1, 214–218 (2002)

In2O3 (400) In2O3 (440)

In2O3 (222)

phys. stat. sol. (c) 0, No. 1, 214–218 (2002)

Ga

- -

-

+N +

+

Interdiffusion of gallium and indium atoms could be the main cause that leads to poor contact characteristic.[2]

How to define a good ohmic contact : Transmission Line Method (TLM)

L1 L2 L3 L4

1

2

3

4

0 500 1000 1500 2000 2500 3000 35000

200

400

600

800

1000

1200

1400

1600

f(x) = 0.396943380897219 x + 71.7739424627411R² = 0.998096387280537

Distance Between 2 Pads (µm)

Tota

l Res

istan

ce (O

hm)

contact resistances (2RC)

𝑅𝑇=𝑅𝑆𝐿𝑊

+2𝑅𝐶

L : distance between padsW : width of pads

[5]

[5] Dong-Jin Yun, et. al. Contact resistance between pentacene and indium–tin oxide (ITO) electrode with surface treatment. Organic Electronics 8 (2007) 690–694

RT : Total ResistanceRS : Sheet ResistanceRC : Contact Resistance

W-20 -15 -10 -5 0 5 10 15 20

-100

-80

-60

-40

-20

0

20

40

60

80

100

L 1L 2L 3

Voltage (V)

Curr

ent (

mA)

Gradient = 1/RT

Fabrication for TLM Annealing Patterning Etching

• Temperature Dependent200 – 800oC, 15 min

• Time Dependent1 and 15 min, 400oC

LED Structure

ITO

Photoresist DNR

Pre-Bake 110°C for 30s

UV exposure 5s

LED Structure

ITO

Photoresist DNR

Mask

LED Structure

ITO

Photoresist DNR

Developing DPD-200,30s and rinse water

LED Structure

ITOPhotoresist DNR

Etching by Solution :HCl : HNO3: H2O = 50 : 3 : 50

for 2min30s at 50°C

LED Structure

Photoresist DNRITO ITO

Ultrasonification with acetone, 10min

Nitrogen ambient

Soft Bake110°C, 1min30s

Hard Bake110°C, 10min

Fabrication for TLM Annealing Patterning Etching

LED Structure

ITO ITO 100nm

Cross section view

p-GaN

1cm

1mm

ITO

Top view

Results

Temperature Dependence Results

𝑅𝑇=𝑅𝑆𝐿𝑊

+2𝑅𝐶

-10 -8 -6 -4 -2 0 2 4 6 8 10

-100

-75

-50

-25

0

25

50

75

100

50

100

200

400

800

1600

3200

Voltage (V)

Curr

ent (

mA)

-15 -10 -5 0 5 10 15

-1.5

-1

-0.5

0

0.5

1

1.5

5010020040080016003200

Voltage (V)

Curr

ent (

mA)

100 200 300 400 500 600 7000.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Temperature (0C)

Cont

act R

esis

tanc

e (o

hm)

200 – 6000C 700 - 8000CDistance between2 pads (µm)

Distance between2 pads (µm)

Temperature200 300 400 500 600

(0C)

Contact Resistance352.3 39.2 38.7 207.6 577.1

(ohm)

240 nm ITO thicknessVan der Pauw measurements

Time Dependence Results

-10 -8 -6 -4 -2 0 2 4 6 8 10

-100

-75

-50

-25

0

25

50

75

100

50

100

200

400

800

1600

3200

Voltage (V)

Curr

ent (

mA)

-15 -10 -5 0 5 10 15

-1.5

-1

-0.5

0

0.5

1

1.5

5010020040080016003200

Voltage (V)

Curr

ent (

mA)

Annealing time : 1 min Annealing time : 15 min

Contact resistance : 1.03 x 104 Ω

Contact resistance : 38.7 Ω

So far, we get the optimum annealing condition for ITO is 4000C for 15 minutes.

Distance between2 pads (µm)

Distance between2 pads (µm)

Determine the composition of coating material (phosphor, epoxy / PR) for high

efficiency and color conversion.

2nd topic

Fabrication of yellow thin film

100 µL epoxy / PR

100 mgyellow phosphor

mixGlass substrate

Thin Film

Blue LED

Spin coatingUV Lamp exposure

2 min

Blue LED

Glass substrateHardened Thin Film

Characterization

LIV SystemSEM & Optical Microscope

drop

or or

330 380 430 480 530 580 630 680 730 7800

0.2

0.4

0.6

0.8

1

1.2

Wavelength (nm)

Rela

tive

Inte

nsity

A1

A2 A3

Color Conversion : =

Power Efficiency : =

Yellow phosphor thin film : best materials

Sample PowerEfficiency

Color Color Position Output

Conversion x y luminescent (lm) power (mW)

Epoxy 57.29% 30.96% 0.2296 0.1951 4.698 22.1447DNR 36.31% 23.31% 0.2277 0.1858 3.4477 16.9728A6 59.13% 34.59% 0.2422 0.2199 5.1387 22.0212

350 400 450 500 550 600 650 700 750 800 8500

0.1

0.2

0.3

0.4

0.5

0.6

EpoxyA6DNR-L300-40

Wavelength (nm)

Rela

tive

Inte

nsit

y

541 – 556 nm

456 nm

epoxy DNR A6 Surface :epoxy A6DNR

20 µm 20 µm 20 µm

---- Blue LED

---- Sample

50 100 150 200 250 300 350 400 450 5000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Thickness (um)

Transmission

Fitting TransmissionAbsorption

Fitting Absorption

Yellow phosphor thin film : effective thickness

asymptotic

𝑇=𝐼 (𝑧)𝐼 0

=𝑒−𝛼 𝑧

𝑓 (𝑥 )=exp (−0.05947𝑥 )+0.01534

Penetration depth = 16 µm

0 200 400 600 800 1000 1200

-100

0

100

200

300

400

500

600

700

Wavelength (nm)

Rela

tive

Inte

nsit

y

We choose 100 µm as most effective thickness

0 10 20 30 40 50 60 70 8020.00%

25.00%

30.00%

35.00%

40.00%

45.00%

50.00%

55.00%

60.00%

Current (mA)

Perc

enta

ge

Yellow phosphor on top blue LED

300 400 500 600 700 800 9000

0.1

0.2

0.3

0.4

0.5

0.6

0.72.5 mA5 mA10 mA20 mA40 mA80 mA

Wavelength (nm)

Rela

tive

Inte

nsit

y

456 nm

545 - 560 nm

---- sample 1

---- sample 2

---- sample 3

---- sample 4

Power efficiency

Color Conversion

133.62 µm

500 µm

Blue LED

500 µm

212.81 µm

(1)

500 µm

291.99 µm

(2)

500 µm

321.69 µm

(3)

500 µm150.94 µm

(4)

Conclusions• Experiment on ITO annealing and yellow phosphor color conversion has been conducted.

• So far, the optimum condition for ITO annealing is 4000C for 15 minutes.

• The best combination for phosphor materials are yellow phosphor with epoxy, with 100 µm as the most effective thickness.

0 200 400 600 800 1000 1200-1000

100200300400500600700

Wavelength (nm)

Rela

tive

Inte

nsit

y

100 200 300 400 500 600 7000.0

100.0200.0300.0400.0500.0600.0700.0

Temperature (0C)

Cont

act R

esis

tanc

e (o

hm)

Thank YouFor

Your Attention