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화합물 반도체 ( III-4 ) – p type ohmic contact

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(1)

화합물 반도체 ( III-4 ) – p type ohmic contact

2007 / 가을 학기

(2)

화합물반도체 - GaN 2

Ohmic Contact to p-GaN

From seminar Note of Prof. Tae-Yeon Seong in G-Jist

(3)

Ohmic mechanisms for Pt/p-GaN

- High Work Function Metal (Pt, …) - Surface Treatment

(4)

화합물반도체 - GaN 4

Two-Step Treated Ohmic Contact to p-GaN

From seminar Note of Prof. Tae-Yeon Seong in G-Jist

(5)

I-V characteristics and transmittance of Pt/Ru contacts

- Pt/Ru (20nm/50nm)

- annealed in N2 for 2 min.

rms roughness

=0.9nm

- true ?

(6)

화합물반도체 - GaN 6

Thermal stability of Pt and Pt/Ru ohmic contacts at 600 ºC

(7)

P-Ohmic Contact to GaN with InGaN Contact Layer

- nonalloyed Pd/Au (20nm/130nm) contact

- dipping in in a HCl:H2O (1:1) solution before metallization - high hole concentration in In0.19GaN – 8x1018cm-3

& polarization-induced field @ small contact resistance

Ref.: K. Kumakura, et al., Appl. Phys. Lett., p. 2588, 2001

(8)

화합물반도체 - GaN 8

Role of Current Spreading in Epi-Up LED

Transparent Conducting Oxide ?

(9)

Oxide Properties

Theory

Transition Metal Oxide

- High k Oxide

- Superconductor Oxide - Spintronics

UTOpenCourseWare

“Physics of Transition Metal Oxide”

(10)

화합물반도체 - GaN 10

Transparent Conducting Oxides

„ Characteristics of a transparent conducting oxide (TCO) z High transparency in the visible (Eg > 3.0 eV)

z High electrical conductivity (σ > 103 S/cm)

„ Applications of transparent conductors

z Optoelectronic devices (LED’s, Semiconductor lasers, photovoltaic cells, etc.) z Flat Panel Displays (liquid crystals, electroluminescent displays)

z Heat efficient windows (reflect IR, transparent to visible light)

z Smart windows and displays based on electrochromics ; dark or lightened with voltage (WO3)

z Defrosting windows and antistatic coatings

„ TCO Materials

z In2O3:Sn (ITO) * Amorphous In2O3 crystallize at 150°C and absorbs water.

z SnO2:Sb & SnO2-xFx * SnO2 absorbs water.

z ZnO:F & ZnO:M (M=Al, In, B, Ga Ga) * difficulty for p-doping

z Cd2SnO4 * Cd is poisonous.

z CuAlO2 & CuGaO2 * p-type TCO (Eg ~ 3.5eV)

(11)

Transmittance of TCOs

Transmittance of 100nm ITO deposited by negative sputter ion beam technology

(Ref.: M. H. Sohn, et al., J.V.S.T.A, p.1347,2003) 100nm thick ITO

Transmittance of SnO2 Transparent Conducting Oxide

(Ref. : G. O. Crawford, “flexible flat panel displays,” Chap. 5, Wiley, 2005)

SnO2 (Eg = 3.6 eV)

(12)

화합물반도체 - GaN 12

Conductivity of TCOs

- ITO (Indium Tin Oxide, 90% Indium Oxide/10% Tin Oxide, Tin-doped Indium Oxide) z n-dopant : oxygen vacancy and Sn dopant

- IZO (Indium Zinc Oxide), IO (Indium Oxide, Eg=3.75eV)

(13)

Transparent Flexible Thin-Film Transistor

z Amorphous InGaZnO4 Transparent Oxide Semiconductor

„ Deposited by pulsed laser deposition at RT

„ Bandgap = 3.1eV, μhall = 15cm2/Vs

(Ref.: Kenji Nomura, et al., Nature, p.488, 2004)

* Flexible ZnO TFT was also demonstrated.

(14)

화합물반도체 - GaN 14

Flexible LED based on New p-type TCO

z P-type TCO

- CuAlO2, SrCu2O2, LaCuOS - LaCuOSe (large μhall=8cm2/Vs)

TCO-based pn junction (LED)

Ref.: H. Hiramatsu, et al., Appl. Phys. Lett., 87, 211107, 2005

Ref.: H. Ohta, et al., Materialstoday, p. 42, June 2004

(15)

Transparent p-contact to GaN

(16)

화합물반도체 - GaN 16

p-Contact to GaN with Oxidizing Ni/Au Films ( I )

Ni/Au (5/10nm)

Ref.: J.-K. Ho, et al., Appl. Phys. Lett., p. 1275, 1999

Large sheet resistance to prevent current spreading

(17)

p-Contact to GaN with Oxidizing Ni/Au Films ( II )

Au/Ni/p-GaN

Oxidizing heat treatment

- NiO barrier to prevent outdiffusion of Au (p-type NiO with Eg=4eV)

- Au-Ga complex to make Ga vacancy

(18)

화합물반도체 - GaN 18

Degradation of GaN LED due to p-contact Metal Penetration

I-V characteristics of GaN LED - Au/Ni/p-GaN contact on top

(annealed at 500°C)

- aging in flow N2 for 10 min.

After 900°C aging

* EDS-1 ; metallic bubble (Au, Ni, Ga) Ref.: C.-Y. Hsu, et al., Appl. Phys. Lett, p. 2447, 2003

(19)

Transparent Ni/Au/Indium-Tin-Oxide(ITO) p-Contact to GaN

ITO - high transparency ( > 80%)

- good conductivity (~104 Ω-1 cm-1) - RF Magnetron Sputtered

1 min. annealing O2 ambient

λ=470nm

(20)

화합물반도체 - GaN 20

High Brightness LED with Ni/Au/ITO p-contact

Notation Deposition step

O-annealed contact Ni(20Å)/Au(30Å) + O2 annealing at 500 °C + ITO(600Å) N-annealed contact Ni(20Å)/Au(30Å) + N2 annealing at 500 °C + ITO(600Å) Nonannealed contact Ni(20Å)/Au(30Å) + ITO(600Å)

Ni/Au contact Ni(20Å)/Au(30Å)

Ref.: S. Y. Kim, et al., J.V.S.T.B, p. 1851, 2004 λ ~ 462nm

Current = 20mA Annealing Temp. = 500°C

(21)

Oxidized Ru and Ir Contact to p-GaN

Ru/Ni (5/5nm) Ir/Ni (5/5nm)

Ni/Au (5/5nm) Ru/Ni (5/5nm) Ir/Ni (5/5nm)

Ref.: H. W. Jang, et al., J. Appl.

Phys., p. 5416, 2003 1 min. annealing

O2 ambient

500°C, 1 min. annealing under O2 ambient

- Ru/Ni contact

- 500°C, 1 min. annealing under O2 ambient

(22)

화합물반도체 - GaN 22

Ag/Sb-doped SnO2 Transparent p-Contact to GaN

f E-beam evaporated Ag(3 nm)/Sb-doped SnO2 (ATO, 200 nm) deposited by pulsed laser deposition

f Rapid thermal-annealed at 530°C for 1 min in air

f ρC=8.7x10−5 Ω cm2, 99% light transmittance at 400 nm, and forward-bias voltage less than 3.42 V at 20 mA

Ref.: June-O Song, et al., Appl. Phys. Lett., p. 6374, 2004

Ag-Ga complex formation

z Surface acceptors due to Ga vacancy

z EF shifted by 0.36eV toward valence band by annealing

(23)

High Brightness Flip-Chip GaN LED

Light extraction efficiency is improved in flip chip designs by;

8No attenuation of light by semi-transparent metal electrodes, 8Absorption of wave-guided light is dramatically reduced through

the use of highly reflective metallization schemes, 8No light is obscured by bond pads or wires.

Conventional GaN LED

Ag : high reflectance > 95%

(24)

화합물반도체 - GaN 24

Highly Reflective Ohmic Contact to p-GaN ( I )

- Ni/Ag (5/120nm)

- Ni/Ag/Ru (5/120/50nm)

- Ni/Ag/Ru/Ni/Au (5/120/50/20/50nm) - Ni/Au (20/100nm)

Ref.: H. W. Jang, et al., Appl. Phys. Lett., p. 4421, 2004 f dipping into boiling HCl:HNO3 (3:1)

to remove surface oxides before metal f Annealing for 1 min in O2 ambient

f Ru could act as as a diffusion barrier for Ag outdiffusion to the surface. >

Ag works as an effective reflection mirror

*Ga solubility in Ag is 1.5 times of that in Au.

(25)

Highly Reflective Ohmic Contact to p-GaN ( II )

SIMS depth profiles (a) before and

(b) after annealing at 500 °C for 1 min in O2 ambient

NiO prevents

outdiffusion of Ga and N

(26)

화합물반도체 - GaN 26

Highly Reflective CIO/Ag p-Ohmic Contact

CIO/Ag p-contact

- CIO deposited by e-beam - ρc = 1.28x10−5 Ω cm2

- reflectivity ~90% at 460nm

annealing at 530 °C for 1 min in air

3.0V at 20mA

Ag after alloy CIO/Ag after alloy

Ref.: June-O Song, et al., Appl. void

Phys. Lett., 86, 062103, 2005

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