Schottky barrier heights, carrier density, and negative electron affinity
of hydrogen-terminated diamond
K. Tsugawa,
1,2,
* H. Noda,
1
K. Hirose,
3
and H. Kawarada
1
1
School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
2
Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
3
Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai,
Sagamihara, Kanagawa 229-8510, Japan
Received 21 May 2009; published 8 January 2010
Chemical trends of Schottky barrier heights of ten kinds of metal contacts on hydrogen-terminated diamond
001 surfaces are estimated from the temperature dependence of their current-voltage characteristics. In
addition to the measurements, the interface of the metal/hydrogen-terminated diamond is theoretically modeled
including the carrier density of the surface conductive layer and the electron-affinity variation from the clean
surface of the hydrogen-terminated diamond. Based on the model, a relation among the carrier density, the
electron affinity variation, and the barrier heights are derived. The relation explains well experimental results
of and other than the present work.
DOI: 10.1103/PhysRevB.81.045303 PACS numbers: 73.30.y
I. INTRODUCTION
Surfaces of diamonds prepared by chemical vapor depo-
sition CVD technique are terminated by hydrogen. Despite
undoping, the hydrogen-terminated surface of diamond ex-
hibits p-type surface conduction
1
in a subsurface region of
10 nm in depth.
2,3
The surface conduction disappears by
oxidation and consequently the surface becomes
insulating.
4,5
Furthermore, Schottky and ohmic metal con-
tacts are easily formed on the same surface by selecting
kinds of metals.
6,7
Utilizing this nature, electronic devices
such as, in particular, metal-semiconductor field-effect tran-
sistors MESFETs have been fabricated on hydrogen-
terminated diamond surfaces.
2,5,6,8
Especially in microwave
power electronics, the performance has been improving since
a diamond microwave MESFET reported for the first time in
2001.
9
Recently, cut-off frequencies for the current and
power gains, f
T
and f
max
have reached 45 and 120 GHz,
respectively. At the same time, an output power density of
2.1 W/mm at 1 GHz has been realized.
8
In spite of its potential for applications of the surface
conduction on the hydrogen-terminated diamond, the detail
understanding of its origin and transport properties is not
adequate and remains still controversial at present.
10–12
Schottky barrier heights SBHs of metal/oxygenated-
diamond contacts have been reported to be comparatively
high 1–2 eV and independent of the metal work function or
the metal electronegativity.
6,13–18
The results suggest that the
Fermi level at the interface is pinned in the band gap of
diamond. This agrees with a result of x-ray photoemission
spectroscopy that the Fermi level of the oxygenated surface
is pinned at 1.7 eV above the valence-band maximum
VBM.
19
In contrast to this, the Fermi-level pinning is re-
duced at the interface of the metal/hydrogen-terminated dia-
mond. In our previous work,
6,20
barrier heights of various
metal contacts on the hydrogen-terminated diamond 001
were estimated from their current-voltage I-V characteris-
tics using high-quality point-contact diodes. A strong corre-
lation between the barrier heights and metal electronegativi-
ties was observed. In addition, ohmic characteristics were
obtained in metals with comparably high electronegativities,
such as Pt and Au. The results indicate that the Fermi level at
the interface is not strongly pinned. However, the barrier
heights were obtained by a rough estimation based on as-
sumed contact areas, which were probably overestimated.
The hydrogen-terminated surface of diamond exhibits
negative electron affinity NEARefs. 21–24 while those
of clean and oxygen-terminated surfaces are positive. This
nature indicates vast potentials for electronic applications
such as cold cathode emitters, etc.
The electron affinity of diamond varies with its surface
structure. The variation reaches up to 3 eV from the oxygen-
terminated surface to the hydrogen-terminated surface.
24
The
NEA of the hydrogen-terminated surface is related to its sur-
face C-H dipoles.
23,24
On the other hand, the interfacial di-
poles affect the barrier height of an interface.
25
Hence the
electron-affinity variation should affect barrier heights. How-
ever, very few arguments have appeared up to the present.
In this work, the interfaces of metal/hydrogen-terminated
diamond contacts are modeled including the carrier density
of the surface conductive layer and the electron-affinity
variation from the clean surface. Based on the model and
experimentally obtained barrier heights, the carrier density
and the electron affinity of the hydrogen-terminated diamond
surface are discussed. In addition, the Fermi-level position of
the free surface is also discussed.
II. EXPERIMENT
The starting materials were homoepitaxial diamond films
deposited by microwave plasma-assisted CVD technique on
high-pressure high-temperature synthetic Ib diamond 001
substrates 1.5 2.0 0.3 mm
3
in size. The reaction gases
were CH
4
2.25% / O
2
0.75% diluted with H
2
in a total gas
flow of 100 sccm and under a total pressure of 35 Torr. The
substrate temperature was raised up to 900 ° C by plasma
PHYSICAL REVIEW B 81, 045303 2010
1098-0121/2010/814/04530311 ©2010 The American Physical Society 045303-1