화합물 반도체 ( III-4 ) – p type ohmic contact
2007 / 가을 학기
화합물반도체 - GaN 2
Ohmic Contact to p-GaN
From seminar Note of Prof. Tae-Yeon Seong in G-Jist
Ohmic mechanisms for Pt/p-GaN
- High Work Function Metal (Pt, …) - Surface Treatment
화합물반도체 - GaN 4
Two-Step Treated Ohmic Contact to p-GaN
From seminar Note of Prof. Tae-Yeon Seong in G-Jist
I-V characteristics and transmittance of Pt/Ru contacts
- Pt/Ru (20nm/50nm)
- annealed in N2 for 2 min.
rms roughness
=0.9nm
- true ?
화합물반도체 - GaN 6
Thermal stability of Pt and Pt/Ru ohmic contacts at 600 ºC
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
화합물반도체 - GaN 8
Role of Current Spreading in Epi-Up LED
Transparent Conducting Oxide ?
Oxide Properties
Theory
Transition Metal Oxide
- High k Oxide
- Superconductor Oxide - Spintronics
UTOpenCourseWare
“Physics of Transition Metal Oxide”
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)
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)
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)
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.
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
Transparent p-contact to GaN
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
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
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
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
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
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
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
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%
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.
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
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