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 27695-7907 B.J. Rodriguez, W. Platow, and R.J. Nemanich Dept. of Physics Raleigh, NC 27695-8202 February 12, 2002

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.

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.

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

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 3HCl: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.

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

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’

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

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

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

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

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

Schottky Barrier Formation: (XPS)

Schottky Barrier Formation: (UPS)

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

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

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

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

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

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

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

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.