Suku Kim, B. S. Kang, and F. Ren
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611
Y. W. Heo, K. Ip, D. P. Norton, and S. J. Pearton
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
͑Received 18 November 2003; accepted 5 January 2004͒
Ohmic and Schottky contacts to p-type Zn
O are reported. The lowest speciﬁc contact
resistivity of 3 ϫ10
was obtained for Ti/Au annealed at 600 °C for 30 s. Ni/Au was less
thermally stable and showed severe degradation of contact morphology at this annealing
temperature. Both Pt and Ti with Au overlayers showed rectifying characteristics on p-ZnMgO,
with barrier heights of ϳ0.55–0.56 eV and ideality factors of ϳ1.9. Comparison of these results
with the same metals on n-type ZnO indicates that high surface state densities play a signiﬁcant role
in determining the effective barrier height. © 2004 American Institute of Physics.
ZnO shows great promise for applications in UV light
emitters, spin functional devices, gas sensors, transparent
electronics, and surface acoustic wave devices.
ports have demonstrated transparent ZnO thin ﬁlm transistors
on Si substrates.
While n-type conductivity is easily realized
by substituting a trivalent cation ͑Al, Ga, In͒ on the Zn site
or by via oxygen vacancies, Zn interstitials, or hydrogen
there is a pressing need to achieve reliable p-type
doping. Low doping efﬁciency may be due to low solubility
for the dopant or lattice relaxations can drive the energy level
deep within the gap.
There can also be compensation
from native point defects or dopant atoms located on inter-
stitial sites. Valence electron arguments suggest that the
group V elements ͑nitrogen, arsenic, antimony, and phos-
phorus͒ are possible acceptor dopants in ZnO.
initio electronic band structure calculations indicate that the
Madelung energy increases with group V anion substitution,
indicating signiﬁcant localization of these acceptor states.
Studies of N-doped ZnO crystals indicate an acceptor state
due to nitrogen substitution.
P-type doping was reported
for ﬁlms grown by N
O plasma pulsed laser deposition
plasma-assisted molecular beam epitaxy.
The acceptor level was estimated to be 170–200 meV. A hole
mobility of 0.5–3.5 cm
/V s and a carrier density of
upon annealing were reported in phosphorus-
doped ZnO thin ﬁlms deposited by sputter deposition.
Similarly, annealing of P-doped ZnO ﬁlms yielded semi-
insulating behavior, consistent with activation of a deep ac-
Once reliable p-type doping of ZnO is achieved, it is
desirable to develop Ohmic and rectifying contacts to this
material in order to exploit device opportunities. In this let-
ter, we show that both Ti and Ni provide rectifying contacts
to p-type ZnMgO and that Ti/Au and Pt/Au show Ohmic
characteristics after annealing at у500 °C. A speciﬁc contact
resistivity of 3 ϫ10
was achieved for Ti/Au an-
nealed at 600 °C.
The phosphorus-doped Zn
O oriented polycrys-
talline ﬁlms in this study were grown by PLD on top of a
glass substrate, using a ZnO:P
target and a KrF excimer
laser ablation source. A laser repetition rate of 1 Hz was
used, with a target to substrate distance of 4 cm and a laser
pulse energy density of 1–3 J/cm
. The Mg, Zn, and P com-
position were measured and determined to be close to that of
the ablation targets. The 600 nm-thick ﬁlms were grown at
400 °C in an oxygen pressure of 20 mTorr. The samples were
annealed in the PLD chamber in O
ambient ͑100 mTorr͒ for
60 min to reduce the background n-type conductivity. Type
conversion to p-type conductivity was achieved for anneals
at 600 °C, with hole concentrations in these p-type ﬁlms on
the order of 10
. Ohmic contact was made by lift-off
of e-beam evaporated Ti ͑200 Å͒/Au͑800 Å͒ or Pt Au͑800
Å͒. Circular top Schottky contacts with diameter 100
either 200-Å-thick Ti or Ni, topped by 800-Å-thick Au to
reduce sheet resistance were patterned by lift-off. Figure 1
shows a schematic ͑top͒ and photograph ͑bottom͒ of the
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FIG. 1. Schematic of contact layout for p-ZnMgO diodes ͑top͒ and plan
view photograph of devices ͑bottom͒.
APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 11 15 MARCH 2004
19040003-6951/2004/84(11)/1904/3/$22.00 © 2004 American Institute of Physics