1. ELECTRICAL, CHEMICAL, AND STRUCTURAL CHARACTERIZATION OF THE INTERFACE FORMED BETWEEN Au/Pd CONTACT STRUCTURES AND CLEANED p-TYPE GaN (0001) SURFACES.
North Carolina State University
P.J. Hartlieb, A. Roskowski, and R.F. Davis
Dept. of Materials Science and Engineering Raleigh, NC
B.J. Rodriguez, W. Platow, and R.J. Nemanich
Dept. of Physics Raleigh, NC
February 12, 2002

2. Outline Challenges and approaches for ohmic contacts to p-GaN.
Motivation for Pd-based contact structures.
Chemical vapor cleaning (CVC) of p-GaN.
Schottky barrier formation at the Pd/p-GaN interface.
Structure and morphology of Au/Pd contact structures.
Electrical properties of Au/Pd contact structures.
Conclusions.

3. Challenges for p-type GaN
Large ~6.5eV work function (FS) of p-GaN.
Fmetal does not exceed 5.8eV.
Mg incorporation during growth limited to ~ 1x1020 cm-3.
Large EAcceptor limits ionization to 0.1 – 2.0%.
Tenacious layer of native contamination ~ 2 nm
adds an additional 0.2 eV to Barrier height.

4. Approaches for p-type GaN
contact
contamination
Ex-situ and/or In-situ cleaning
p-GaN
p-GaN
Barrier reduction with intimate contact
As-grown
Post-metallization annealing
p-GaN
p-GaN
Dispersal of contamination.
Interfacial reaction products (VGa).
Schottky barrier reduction (FB).
Metal-gallide phase

5. Ohmic contacts on p-GaN: (Pd)
Pd-Ga
Pd/Au
Anneal
p-GaN
p-GaN
p-GaN
700°C
3HCl:1HNO3, HCl, untreated
rc~ 1x10-3 W•cm2
No reaction
Anneal
Pd/Au
p-GaN
p-GaN
p-GaN
Boiling HCl:1HNO3
700°C, 800°C
rc~ 1.99x10-4W•cm2
D-W. Kim, J.C. Bae, W.J. Kim, H.K. Baik, J-M. Myoung, S-M. Lee, Journal of Electronic Materials, 30(3), 2001.

6. ? Approach to ohmic contacts on p-GaN: (NCSU) Surface Contamination
metallization
In-situ
p-GaN
p-GaN
p-GaN
NH3-based
CVC ?

7. Integrated Surface Analysis and Growth System
Wafer Bonding
XPS/UPS
LEED/
E-beam
Load-Lock
N2 Plasma
High (V) testing
Field Emission
GSMBE
Si-Ge MBE
ARUPS
Diamond Growth
H2/O2 plasma
10’

8. Approach: Chemical Vapor Clean
Ex-Situ Cleaning
1 min rinses in TCE, Acetone, Methanol
10 min HCl rinse
10 sec DI water rinse
N2 blow dry
In-Situ Cleaning
Base Pressure ~ 1E-9 torr
Heat to 500°C (TC)
Introduce NH3 Flux
P=8E-5 to 1E-4 torr
Hold for 15 min at 1000°C (TC)
Cool sample to 500°C (TC)
Shut NH3 Flux
Ammonia
Doser
Tungsten
Heater
Molybdenum
Sample Holder
p-GaN
Sample
Flux
Thermocouple

9. CVC Results: X-ray photoelectron spectroscopy (XPS)
Post – CVC
C < 0.3 at%, O = 2 at%
As-loaded
C = 6 at%, O = 15 at%

10. CVC Results: (XPS)
Post – CVC
Ga:N = 1.0
As – loaded
Ga:N = 1.5

11. CVC Results: Ultra-violet photoelectron spectroscopy (UPS)
Post - CVC
As - loaded

12. Band Structure: Cleaned p-GaN
As - loaded
Post - CVC

13. Schottky Barrier Formation: (XPS)

14. Schottky Barrier Formation: (UPS)

15. Band Structure: Metallized p-GaN
Fb experimental
1.3±0.1 eV
Fb Schottky-Mott
0.9 eV
Interface dipole
0.4±0.1 eV

16. Contact Formation:Palladium Growth (XPS)
Volmer-Weber (VW)
Stranski-Krastanov (SK)
Frank-van der Merwe (FM)

17. Contact Formation: low energy electron diffraction (LEED)
100nm Au
50nm Pd
2nm Pd
50nm Pd
CVC p-GaN
CVC p-GaN
CVC p-GaN

18. Evolution of contact morphology as f(T): (SEM) CVC p-GaN
500°C RMS ~ 6.0nm
600°C RMS ~ 5.1nm
700°C RMS ~ 7.3nm
800°C RMS ~ 80.0nm

19. Evolution of contact morphology as f(T): as-loaded p-GaN
500°C RMS ~ 5.7nm
600°C RMS ~ 11.8nm
700°C RMS ~ 80.2nm
800°C RMS ~ 61.4nm

20. Evolution of contact morphology as f(T): (AFM)
Au 100nm/ Pd 50nm
CVC-GaN
Au 100nm/ Pd 50nm
As-loaded GaN
Bare p-GaN
As-loaded GaN

21. Electrical Properties: (I-V)
Au 100nm
Pd 50nm
CVC-GaN
As-loaded GaN

22. Conclusions
The band bending and electron affinity for the clean p-GaN surface were measured to be 1.4 ± 0.1 and 3.1 ± 0.1 eV respectively.
The Pd grew epitaxially on the clean surface in a layer-by layer mode and formed an abrupt, unreacted metal-semiconductor interface.
The final Schottky barrier height at the Pd/p-GaN interface was 1.3 ± 0.1 eV; the interface dipole contribution was 0.4 ± 0.1 eV.
Contacts on the CVC surface maintain significantly smoother morphology after high temperature annealing compared to identical contact structures on the as-loaded surface.
The least resistive contact structures with uniform metal coverage were obtained for Pd/Au contacts on the CVC surface annealed at 700°C.