Current conduction mechanism of Pt/GaN and Pt/Al 0.35 Ga 0.65 N Schottky diodes
Jong Kyu Kim, Ho Won Jang, and Jong-Lam Lee
Citation: Journal of Applied Physics 94, 7201 (2003); doi: 10.1063/1.1625101
View online: http://dx.doi.org/10.1063/1.1625101
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JOURNAL OF APPLIED PHYSICS
VOLUME 94, NUMBER 11
1 DECEMBER 2003
Current conduction mechanism of PtÕGaN and PtÕAl0.35Ga0.65N
Schottky diodes
Jong Kyu Kim, Ho Won Jang, and Jong-Lam Leea)
Department of Materials Science and Engineering, Pohang University of Science and Technology
(POSTECH), Pohang, Kyungbuk 790-784, Korea
共Received 9 June 2003; accepted 18 September 2003兲
Electrical properties of Pt/Alx Ga1⫺x N Schottky diodes and chemical bonding states of Alx Ga1⫺x N
surface were examined simultaneously to investigate the change in the current transport mechanisms
of the Pt/Alx Ga1⫺x N diodes with increasing Al mole fraction. The Pt/GaN diodes showed electrical
properties given by the thermionic-emission theory, while the Pt/Al0.35Ga0.65N showed a nonideal
Schottky behavior. The oxygen donors were predominantly incorporated at the surface of
Alx Ga1⫺x N with increasing Al mole fraction, causing the surface to be heavily doped n type.
Consequently, the current transport in the Pt/Al0.35Ga0.65N diodes was dominated by the field
emission of electrons through the Schottky barrier, leading to the nonideal Schottky behavior.
© 2003 American Institute of Physics. 关DOI: 10.1063/1.1625101兴
I. INTRODUCTION
Alx Ga1⫺x N-based devices are currently under intense
study for applications in visible-blind ultraviolet photodetectors and high-power high-frequency electronic devices, such
as Alx Ga1⫺x N/GaN heterostructure field effect transistors
共HFETs兲.1,2 Understanding the electrical properties of
Schottky contacts on Alx Ga1⫺x N is one of the key elements
for the successful design and fabrication of both types of
devices.
For the realization of true solar-blind Alx Ga1⫺x N ultraviolet photodetectors with a short cutoff wavelength 共⬍275
nm兲, an Al mole fraction as high as ⬃0.4 is required.3 In
addition, an Al mole fraction larger than ⬃0.2 is required in
order to maximize the two-dimensional electron gas density
in a HFET channel. However, it was reported that the
Schottky contacts to Alx Ga1⫺x N showed nonideal Schottky
behaviors when the Al mole fraction was higher than
⬃0.2.4 –7 The ideality factor 共n兲 of Ni/Alx Ga1⫺x N diodes increased from 1.12 to 1.37 as the Al mole fraction increased
from 0 to 0.23.4 When the Al mole fraction was 0.4, n was as
high as 3.5.7 Similar results were reported for
Ni/Alx Ga1⫺x N (x⫽0, 0.15) 共Ref. 5兲 and Re/Alx Ga1⫺x N (x
⫽0, 0.16, 0.26) 共Ref. 6兲 Schottky diodes. Furthermore, the
Schottky barrier height 关共SBH兲 ␾ b ] largely deviated from the
theoretically predicted value at a high Al mole fraction.4
These experimental results suggest that the current transport
mechanism in metal/Alx Ga1⫺x N Schottky diodes changes
with increasing the Al mole fraction. However, no report on
the origin of the nonideal Schottky behaviors of the metal/
Alx Ga1⫺x N Schottky diodes with a high Al mole fraction has
been made experimentally so far.
In Alx Ga1⫺x N, oxygen atoms occupying nitrogen sites,
ON, act as a shallow donor, but cation vacancies (VGa ,VAl)
act as triple acceptors.8 The O donors compensate with the
cation vacancies to form VGa –ON and/or VAl –ON
a兲
Electronic mail: [email protected]
complexes.9 Because Al has a strong affinity for oxygen,10,11
the concentration of these complexes may increase when Al
mole fraction increases. Therefore, at a high Al mole fraction, both the O donor and the complexes, localized near the
surface of Alx Ga1⫺x N, could play a critical role in inducing
the nonideal behavior of the metal/Alx Ga1⫺x N Schottky diodes.
In the present study, we studied the current transport
mechanism of Pt/Alx Ga1⫺x N Schottky diodes with an Al
mole fraction through investigating the relation between the
electrical properties of the Schottky diodes and the evolution
of chemical bonding states at Alx Ga1⫺x N surface. The
current–voltage (I – V) and capacitance–voltage (C – V)
characteristics were measured in order to examine the electrical properties of Pt/GaN and Pt/Al0.35Ga0.65N Schottky diodes. The evolution of chemical bonding states with an Al
mole fraction was analyzed using synchrotron radiation photoemission spectroscopy 共SRPES兲. From these results, the
origin of the nonideal Schottky behaviors of the Alx Ga1⫺x N
Schottky diodes with a high Al mole fraction is proposed.
II. EXPERIMENTAL PROCEDURE
GaN and Alx Ga1⫺x N films used in this study were
grown on a c-plane sapphire substrate by metalorganic
chemical vapor deposition. GaN films with a thickness of 1.5
␮m were initially grown, and then 1-␮m-thick Alx Ga1⫺x N
layers with a different Al mole fraction were grown on the
GaN films. The Al mole fractions in the Alx Ga1⫺x N were
determined to be x⫽0, 0.12, 0.22, 0.33, and 0.35 using the
high-resolution x-ray diffraction method. The electron concentration was measured to be 5.1⫻1016 cm⫺3 for GaN film
and 1.0⫻1018 cm⫺3 for Al0.35Ga0.65N by the Hall-effect measurements.
Pt Schottky diodes were fabricated on both the GaN and
the Al0.35Ga0.65N films using a photolithographic technique.
At first, Ti/Al/Ni/Au 共300/1200/400/500 Å兲 ohmic contact
metals were deposited in sequence on both photoresist-
0021-8979/2003/94(11)/7201/5/$20.00
7201
© 2003 American Institute of Physics
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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003
patterned samples using electron-beam 共e-beam兲 evaporation, followed by dipping into acetone to remove the metals
deposited on the photoresist. After the lift off of metals on
the photoresist, the ohmic metals were annealed using a
rapid thermal annealing system at 600 °C for 1 min under a
N2 ambient. Then, Pt共100 Å兲 Schottky contact 共50⫻50 ␮m2兲
was deposited using e-beam evaporation. The vacuum condition of the evaporator was maintained lower than 3
⫻10⫺7 Torr during metal depositions. The Pt Schottky contact was surrounded by the Ti/Al/Ni/Au ohmic contact. The
gap spacing between the ohmic and Schottky contacts was
20 ␮m. Prior to the deposition of metals, all of the samples
were dipped in the HCl solution for 2 min, followed by
cleaning in deionized water. Note that no oxygen plasma
cleaning to remove polymeric residue was carried out in this
study because the plasma-induced damages could lead to severe degradation of the Schottky contact. I – V and C – V
characteristics of the Schottky diodes were measured using
HP 4156A semiconductor parameter analyzer and HP 4280 1
MHz C-meter, respectively.
The chemical bonding states and their depth information
near the surface of Alx Ga1⫺x N (x⫽0, 0.12, 0.22, and 0.33,
respectively, were characterized using SRPES in the 4B1
beamline at the Pohang Accelerator Laboratory. The incident
photon energies of 600 eV and 250 eV were used for obtaining core-level spectra, and valence-band spectra, respectively. For angle-resolved scans, the detection angle was varied by rotating the samples. The surface normal of the
samples was set as the detection angle of 90°. The onset of
photoemission was measured at a bias of ⫺20 V on the
sample. The incident photon energy was calibrated with the
Au 4 f core-level spectrum of a clean Au foil. The energy
resolution in the measurements was 0.1 eV.
III. EXPERIMENTAL RESULTS
Figure 1共a兲 shows the forward current density–voltage
(J – V) characteristics, where J is current density, of the Pt/
GaN and the Pt/Al0.35Ga0.65N Schottky diodes. The forward
J – V curves were analyzed using the equation, I
⫽AA * T 2 exp(⫺q␾b /kT)关exp(qV/nkT)⫺1兴, where A is the
device area and A * is the effective Richardson constant. The
values of q ␾ b and n were determined from the intercept and
the slope of the linear region in the plot of ln(J) versus V.
For calculating the A * for Al0.35Ga0.65N, we estimated m *
⫽0.27m 0 for Al0.35Ga0.65N by a linear extrapolation of m *
⫽0.35m 0 for AlN 共Ref. 11兲 and m * ⫽0.22m 0 for GaN. The
q ␾ b was also determined from a linear plot of 1/C 2 versus
V, as shown in Fig. 1共b兲. The values of q ␾ b and n were
summarized in Table I. For the Pt/GaN diode, n is near unity,
1.06⫾0.12 eV. In addition, the q ␾ b value determined from
the I – V characteristics is nearly consistent with that from
the C – V considering the image force lowering for the Pt/
GaN. This indicates that the current transport in the Pt/GaN
diodes is dominated by the thermionic emission of electrons
over the Schottky barrier. In the meanwhile, the n for the
Pt/Al0.35Ga0.65N diode is 2.11⫾0.23 eV, largely deviated
from unity. Furthermore, the ␾ b value obtained from the
I – V characteristics 共1.39⫾0.15 eV兲 is much smaller than
Kim, Jang, and Lee
FIG. 1. 共a兲 J – V curves and 共b兲 1/C 2 vs V relationship of the Pt/GaN and the
Pt/Al0.35Ga0.65N Schottky diodes. The size of the Schottky contact was
50⫻50 ␮m2.
that from the C – V measurement 共2.48⫾0.06 eV兲. Such a
large discrepancy could not be explained in terms of the
image force lowering for the Pt/Al0.35Ga0.65N. These results
suggest that the current transport mechanism in the
Pt/Al0.35Ga0.65N diodes was quite different from that of the
Pt/GaN diodes.
Figure 2 displays Ga 3d, N 1s, Al 2 p, and O 1s corelevel spectra on the surface of Alx Ga1⫺x N (x⫽0, 0.12, 0.22,
and 0.33兲, measured using SRPES. The binding energy of
each element gradually shifted toward a higher binding energy with an increasing Al mole fraction, due to the increase
in the band gap of Alx Ga1⫺x N. In addition, the intensity of
Ga 3d decreased while that of Al 2p and O 1s increased simultaneously as the Al mole fraction increased. To examine
the change in the chemical bonding states at the surface of
Alx Ga1⫺x N, spectral deconvolutions of the Ga 3d and the
Al 2p spectra were performed. The spectral line shape was
simulated with a suitable combination of Gaussian functions.
The Gaussian width and the difference in binding energies of
TABLE I. The ideality factor and the Schottky barrier height of both the
Pt/GaN and the Pt/Al0.35Ga0.65N Schottky diodes. The size of the Schottky
contact was 50⫻50 ␮m2. The data in the table are the average value obtained from measurement of ten diodes.
SBH 共eV兲
Pt/GaN
Pt/Al0.35Ga0.65N
Ideality factor, n
(I – V)
(C – V)
1.06⫾0.12
2.11⫾0.23
1.46⫾0.07
1.39⫾0.15
1.56⫾0.04
2.48⫾0.06
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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003
Kim, Jang, and Lee
7203
FIG. 3. Relative atomic ratio of 共a兲 N/关Ga⫹共Al兲兴 and 共b兲 O/关Ga⫹共Al兲兴 at
the surface of Alx Ga1⫺x N with Al mole fraction as a function of detection
angle. The surface normal was set as ␪⫽90°.
FIG. 2. 共a兲 Ga 3d and N 1s and 共b兲 Al 2p and O 1s core-level spectra at the
surface of Alx Ga1⫺x N (x⫽0, 0.12, 0.22, and 0.33兲, measured using SRPES.
each bond were fixed with constant values.12 When the
Ga 3d and the Al 2p spectra were deconvoluted into two
components, Ga—N and Ga—O, Al—N and Al—O bonds,
respectively, they fitted well with experimentally obtained
spectra. The relative atomic concentrations of each element
in Alx Ga1⫺x N were determined from the integrated intensities of each spectrum considering the atomic sensitivity factors of each element,12 as summarized in Table II. The intensity ratio of 关Ga—O兴/关Ga—N兴 was almost independent of Al
mole fraction. On the other hand, the 关Al—O兴/关Al—N兴 bond
and the intensity of O increased simultaneously with Al mole
fraction. This provides evidence that oxygen atoms are easily
incorporated with Al by occupying a substitutional nitrogen
site. Furthermore, the incorporation of oxygen in Alx Ga1⫺x N
is much more pronounced at a higher Al mole fraction.
Angle-resolved scans of the SRPES were performed to
obtain the depth information of chemical compositions near
the Alx Ga1⫺x N surface. At a smaller detection angle 共␪兲, the
intensity of photoelectrons emitting from the surface becomes dominant due to the inelastic mean-free path of photoelectrons. Figures 3共a兲 and 3共b兲 show the variation of
N/关Ga⫹共Al兲兴 and O/关Ga⫹共Al兲兴 ratios on the Alx Ga1⫺x N surface with ␪. The N/关Ga⫹共Al兲兴 ratio in the Alx Ga1⫺x N (x
⫽0.22 and 0.33兲 slightly increases with decreasing ␪. This
indicates that the surface of the Alx Ga1⫺x N (x⫽0.22 and
0.33兲 maintains a N-rich condition in comparison with the
Alx Ga1⫺x N (x⫽0 and 0.12兲 surface due to the existence VGa
and/or VAl near the surface. In the case of O/关Ga⫹共Al兲兴, the
ratio increases with increasing the Al mole fraction, indicating a strong interaction of O atoms with Al ones. In addition,
the increase in the ratio is much more pronounced as the
detection angle decreases. This clearly shows that the concentration of oxygen is higher near the surface than in the
bulk of Alx Ga1⫺x N.
Figure 4 shows the valence-band spectra and the onset of
secondary electrons. It was found that the energy difference
between the Fermi level (E F ) and the valence-band maximum (E V ) gradually increased, that is, 2 eV for x⫽0.12 and
2.4 eV for x⫽0.22, and 2.8 eV for x⫽0.33. Considering the
increase in the band gap (E g ) of the Alx Ga1⫺x N with x, the
surface Fermi-level position below the conduction-band
maximum 共CBM兲, E g ⫺(E F ⫺E V ), is almost same, 1.6⫾0.1
eV, independent of the Al mole fraction. This is in good
agreement with the previously reported value of 1.65 eV.13
The onset of secondary electron shifts toward a lower kinetic
TABLE II. The relative atomic concentrations 共%兲 of each element and bond in Alx Ga1⫺x N determined from
the integrated intensities of each spectrum considering the atomic sensitivity factors of each element. The
detection angle was 90°.
Ga
GaN
Al0.12Ga0.88N
Al0.22Ga0.78N
Al0.33Ga0.67N
Al
Ga—N
Ga—O
Al—N
Al—O
N
O
关Ga—O兴/
关Ga—N兴
关Al—O兴/
关Al—N兴
39.0
25.3
24.1
16.5
12.4
8.3
7.8
5.3
¯
13.1
10.8
12.8
¯
8.2
12.4
17.8
31.8
24.3
22.5
22.9
16.7
20.8
22.3
24.7
0.32
0.33
0.33
0.32
¯
0.62
1.15
1.39
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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003
FIG. 4. Valence band and onset of secondary electron spectra at the surface
of Alx Ga1⫺x N (x⫽0, 0.12, 0.22, and 0.33兲 measured using SRPES.
energy as the Al mole fraction increased. This supports the
hypothesis that the work function of the Alx Ga1⫺x N surface
decreases mainly due to the decrease of the electron affinity
of the Alx Ga1⫺x N.
IV. DISCUSSION
In GaN and AlN, the formation of point defect complexes between cation vacancies and substitutional O donors
such as VGa –ON is energetically stable.9,14 The O donors and
the acceptorlike cation vacancies simultaneously increased
with the Al mole fraction in Alx Ga1⫺x N 关Figs. 3共a兲 and
3共b兲兴. However, the amount of cation vacancies is smaller
than that of O donors, considering the changes in N/关Ga
⫹共Al兲兴 and O/关Ga⫹共Al兲兴 with ␪ in Figs. 3共a兲 and 3共b兲. The
remaining O donors at the surface of Alx Ga1⫺x N could play
a key role in doping the surface to a n-type condition. Moreover, the surface states composed of both the acceptorlike
VGa共VAl兲–ON complex and the O donor could lead to the
Fermi-level pinning at the fixed surface potential of ⬃1.6 eV
below the CBM.
Based on the experimental results, we propose a current
transport mechanism in Pt/Alx Ga1⫺x N Schottky diodes with
an Al mole fraction, which is explained with the energy band
diagrams in Fig. 5. The current transport in Pt/GaN is well
described by the thermionic emission of electrons over the
Schottky barrier, as shown in Fig. 5共a兲, considering the near
Kim, Jang, and Lee
unity ideality factor and the coincidence of the SBHs obtained from the I – V and the C – V measurements. In
Alx Ga1⫺x N, the surface Fermi-level position was independent of the Al mole fraction 共Fig. 4兲. The oxygen donors
uncompensated with cation vacancies increased with increasing the Al mole fraction 关Fig. 3共b兲兴. This was more pronounced near the surface of Alx Ga1⫺x N. This suggests that a
thin layer of the Alx Ga1⫺x N near the surface became a
heavily doped n-type layer. Because the width of the
Schottky barrier is inversely propotional to the root of doping concentration, the thickness of the barrier containing
oxygen donors became thin enough to allow electrons to
tunnel through the thin surface layer at a high Al mole fraction, as shown in Fig. 5共b兲. This leads to the nonideal
Schottky behavior, the reduction in the SBH and the increase
in ideality factor, of the Pt/Al0.35Ga0.65N Schottky diode.
V. CONCLUSION
In conclusion, the current transport in the Pt/GaN diode
is well described by the thermionic emission. The ideality
factor of the Pt/GaN diode is near unity, 1.06⫾0.12 eV and
the Schottky barrier heights obtained from the I – V and the
C – V measurements are nearly consistent. In the meanwhile,
the ideality factor of the Pt/Al0.35Ga0.65N diode is 2.11⫾0.23
eV, largely deviated from unity. Furthermore, the ␾ b value
obtained from the I – V 共1.39⫾0.15 eV兲 is much smaller than
that from the C – V measurement 共2.48⫾0.06 eV兲. The position of the surface Fermi level was ⬃1.6 eV below the CBM
independent of the Al mole fraction, but oxygen content at
the surface of Alx Ga1⫺x N increased with increasing the Al
mole fraction. The oxygen donor impurities were predominantly incorporated at the surface of Alx Ga1⫺x N, causing the
surface to be heavily doped n type. Therefore, the thickness
of the Schottky barrier became thin enough to allow electrons to tunnel through at a high Al mole fraction. This leads
to the nonideal Schottky behavior of the Pt/Al0.35Ga0.65N
Schottky diode.
ACKNOWLEDGMENTS
The author would like to thank Dr. I. H. Lee, Dr. J. S.
Kwak, and Dr. O. H. Nam from Samsung Advanced Institute
of Technology 共SAIT兲 for material growth. This work was
performed through the project for ‘‘National Research Laboratory’’ sponsored by the Korea Institute of Science and
Technology Evaluation and Planning 共KISTEP兲.
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