Journal of The Electrochemical Society, 153 共11兲 F266-F270 共2006兲
F266
0013-4651/2006/153共11兲/F266/5/$20.00 © The Electrochemical Society
Electrical Characteristics of Postmetallization-Annealed
MOCVD-TiO2 Films on Ammonium Sulfide-Treated GaAs
Ming-Kwei Lee,z Chih-Feng Yen, Jung-Jie Huang, and Shi-Hao Lin
Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan
The electrical characteristics of titanium dioxide films deposited on p-type gallium arsenide substrate by metallorganic chemical
vapor deposition 共MOCVD兲 were studied. For gallium arsenide with ammonium sulfide treatment, the electrical characteristics are
improved due to the reduction of interface state density. The electrical characteristics can be further improved by the postmetallization annealing. Hydrogen from the postmetallization annealing process can passivate the defects and the grain boundary of
polycrystalline titanium dioxide films. The leakage current densities can reach 2.5 ⫻ 10−7 and 5 ⫻ 10−7 A/cm2 under positive and
negative electric fields at 1.5 MV/cm, respectively. The dielectric constant and effective oxide charges are 66 and 1.86
⫻ 1012 C/cm2, respectively. The interface state density is 5.96 ⫻ 1011 cm−2 eV−1 at the energy of 0.7 eV from the edge of the
valence band.
© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2349360兴 All rights reserved.
Manuscript submitted February 8, 2006; revised manuscript received July 20, 2006. Available electronically September 20, 2006.
Due to its high electron mobility and direct energy bandgap compared with Si, much attention has been focused on gallium arsenide
共GaAs兲 high-speed and optoelectronic devices. Usually, the metalsemiconductor field-effect-transistor 共MESFET兲 is the main structure of GaAs high-speed devices due to the lack of high quality of
oxide on it. Compared with metal-oxide-semiconductor field-effecttransistors 共MOSFETs兲, the main disadvantage of MESFETs is the
high gate currents of Schottky contact under the positive bias of
several tenths of a volt, which severely limits the maximum drain
currents, the lower noise margin, and the lower flexibility of the
circuit design.
Usually, silicon dioxide 共SiO2兲 was used as the gate oxide of
GaAs MOSFETs.1-3 Three unit cells of GaAs共100兲 may match well
with four unit cells of TiO2 in the 关110兴 direction.4 It has potential to
use TiO2 as the gate oxide of GaAs MOSFETs. In addition, TiO2 is
a high-dielectric-constant 共high-k兲 material 共␧// = 170, ␧⬜ = 89 for
its rutile phase兲.5,6 The transconductance of MOSFETs is proportional to their gate capacitance. Their gate capacitance is proportional to the dielectric constant of gate material. Therefore, the transconductance of MOSFETs is higher with the higher dielectric
constant material as the gate oxide.7
Polycrystalline TiO2 films were prepared by metallorganic
chemical vapor deposition 共MOCVD兲,8 sol-gel,9 and sputtering.10
MOCVD-TiO2 was used in this study because of its simple process
and higher quality. Usually, the leakage current of MOCVD-TiO2 on
GaAs is high from the polycrystalline grain boundary11,12 and high
interface states.13,14 The high interface states are mainly from the
native oxides on GaAs. The ammonium sulfide 共NH4兲2Sx treatment
can remove the native oxides on GaAs 共S-GaAs兲. It can also passivate the surface dangling bonds of GaAs and prevent it from
oxidizing.15-18
The low-temperature postmetallization annealing 共PMA兲 is an
effective process to reduce the oxide charge density and the interface
state density in SiO2 /Si metal–oxide–semiconductor 共MOS兲
technology.19,20 The mechanism of the PMA process is from the
reaction between the aluminum contact and hydroxyl groups existing on the SiO2 film surface. Then the active hydrogen is produced
to diffuse through the oxide and passivate the oxide traps.19-21 From
our previous study,22 PMA was used to reduce the leakage current
from the defects and grain boundary of polycrystalline TiO2 films
deposited on silicon. In this study, we try to improve the electrical
characteristics of MOCVD-TiO2 film deposited on GaAs by the
combination of 共NH4兲2Sx passivation and PMA treatment 共PMAMOCVD-TiO2 /S-GaAs兲.
z
E-mail: [email protected]
Experimental
Zn-doped p-type GaAs共100兲 with carrier concentration of 5
⫻ 1016 cm−3 was used as the substrate. The GaAs substrate was
prepared by the liquid encapsulation Czockralski 共LEC兲 method.
GaAs substrate was degreased in solvent followed by chemical
etching in a solution 共H2SO4:H2O2:H2O = 5:1:1兲 for 3 min and
then rinsed in deionized water. After cleaning, GaAs substrate
was immediately dipped into 共NH4兲2Sx solution for 40 min at 50°C
and then blown dry with nitrogen gas. After 共NH4兲2Sx treatment,
the GaAs substrate was thermally treated at 220°C in a nitrogen
ambience for 10 min to desorb the excess of weakly bonded sulfur.
The GaAs substrate with 共NH4兲2Sx treatment is ready for
MOCVD-TiO2 growth.
Polycrystalline TiO2 thin films were deposited on GaAs by a
horizontal coldwall MOCVD system. Tetraisopropoxytitanium
关Ti共i-OC3H7兲4兴 was used as a Ti precursor and kept at 24°C. Nitrogen was used as the carrier gas and its flow rate was 10 sccm.
Nitrous oxide gas 共N2O兲 was used as an oxidizing agent and its flow
rate was 100 sccm. Molybdenum was used as the oxidation-resist
susceptor. The reactor pressure was kept at 5 Torr during the
growth. The growth temperature was kept at 400°C for 5 min. The
chemical reaction steps during the deposition of TiO2 on GaAs in
the MOCVD system are as follows: 共i兲 Transport of reactants by
diffusion from the main gas stream through the boundary layer and
adsorbed on wafer surface, 共ii兲 surface processes including chemical
decomposition or reaction and surface migration, and 共iii兲 desorption of by-products from the surface and transport by diffusion
through the boundary layer and back to the main gas stream.
In the PMA procedure, aluminum was deposited upon TiO2 films
as the cap layer. Then, these films were annealed in nitrogen ambience for 10 min at temperatures of 300, 350, and 400°C, respectively. Finally, the aluminum was etched away with an etching solution 共H3PO4 /HNO3 /CH3COOH/H2O = 73:4:3.5:19.5兲.
A MOS structure was used to examine the electrical characteristics. In–Zn alloy 共In 10% and Zn 90%兲 was evaporated on the GaAs
back side for ohmic contact and then thermally annealed at 400°C
for 3 min in nitrogen atmosphere. Then, Al was evaporated on TiO2
film as the top contact via a shadow mask with an area of 7.07
⫻ 10−4 cm2. A HP4145B semiconductor-parameter analyzer was
used for current–voltage 共I–V兲 characterization. A high-frequency
共1 MHz兲 HP4280A capacitance–voltage 共C–V兲 meter was used for
C–V characterization. The interface state densities were derived
from C–V curves by the Terman method,23 which can provide a
good evaluation24 of the interface state density higher than
1010 cm−2 eV−1 with 10% error.25,26
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Journal of The Electrochemical Society, 153 共11兲 F266-F270 共2006兲
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Figure 1. SEM cross section of MOCVD-TiO2 /S–GaAs.
Results and Discussion
Scanning electron microscopy 共SEM兲 was used to examine the
thickness of TiO2 film. The SEM cross section of MOCVD-TiO2
film on GaAs substrate with 共NH4兲2Sx treatment is shown in Fig. 1.
The thickness of MOCVD-TiO2 film deposited at 400°C for 5 min
is 62 nm and is used as the standard sample in this study. The SEM
picture also shows that there is an interfacial layer. The thickness of
the interfacial layer is about 3 nm, from the interdiffusion between
TiO2 and GaAs examined by secondary ion mass spectroscopy
共SIMS兲 depth profile shown in Fig. 2a.
The leakage current densities of MOCVD-TiO2 film deposited
on GaAs substrate with and without 共NH4兲2Sx treatments are
shown in Fig. 3. Figure 3 also shows the leakage current densities
of PMA-MOCVD-TiO2 /S–GaAs treated at PMA temperatures
of 300, 350, and 400°C. The leakage current densities of
MOCVD-TiO2 /GaAs substrate without 共NH4兲2Sx treatment are
9.0 ⫻ 10−2 and 6.2 ⫻ 10−1 A/cm2 under positive and negative electric fields at 1.5 MV/cm, as shown in Fig. 3a. The high leakage
currents are mainly from the high density of defects in the grain
boundary of polycrystalline TiO2 film11,12 and the high interface
states at the MOCVD-TiO2 /GaAs interface due to GaAs native
oxide.13,14
For MOCVD-TiO2 /S–GaAs as shown in Fig. 3b, the leakage
current densities are 4.3 ⫻ 10−6 and 3.2 ⫻ 10−2 A/cm2 under positive and negative electric fields at 1.5 MV/cm, respectively. For
p-type substrate, the leakage currents are mainly from the interface
state density and grain boundary of the TiO2 film at positive electric
field and negative electric field, respectively. After 共NH4兲2Sx treatment, the interface state density is reduced and the leakage current
has 4 orders of magnitude improvement during the positive electric
field. Better TiO2 film can be deposited on the reconstructed GaAs
surface but still with the grain boundary, and only a 1 order of
magnitude improvement in the leakage current is obtained during
the negative electric field.
The leakage current density can be further improved by
PMA treatment as shown in Fig. 3c-e. The lowest leakage current
densities of PMA-MOCVD-TiO2 /S–GaAs can reach 2.5 ⫻ 10−7
and 5 ⫻ 10−7 A/cm2 under positive and negative electric fields at
1.5 MV/cm at the PMA treatment of 350°C, as shown in Fig. 3d.
After the PMA process, the thickness of the interfacial layer is
fixed, which can be examined from SIMS depth profiles as shown
in Fig. 2a and b. Therefore, the improvement of leakage current
is not from the increase of interfacial layer thickness after PMA.
The reduction of leakage current in SiO2 film is from the hydrogen
passivation.19-21 The H may play the same role in TiO2.22 H atoms
are distributed in the whole MOCVD-TiO2 film after PMA treatment, as shown in Fig. 2b. The fast diffusion of H could be along
Figure 2. SIMS depth profiles for 共a兲 MOCVD-TiO2 /S–GaAs and 共b兲 PMA
共350°C兲-MOCVD-TiO2 /S–GaAs.
the grain boundary. The leakage current densities are 7.4 ⫻ 10−7
and 2.3 ⫻ 10−6 A/cm2 under positive and negative electric fields
at 1.5 MV/cm at the PMA treatment of 300°C, as shown in
Fig. 3c. The slight increase of leakage current compared with Fig.
3d could be from the lower PMA temperature, which cannot provide
sufficient energy for H atoms to passivate the defects in the
MOCVD-TiO2 film and the interface. For the PMA temperature
fixed at 400°C, as shown in Fig. 3e, the leakage currents are higher
Figure 3. Leakage current densities of MOCVD-TiO2 /GaAs with and without 共NH4兲2Sx treatments and PMA-MOCVD-TiO2 /S–GaAs at different
PMA temperatures.
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Journal of The Electrochemical Society, 153 共11兲 F266-F270 共2006兲
Figure 4. C–V characteristics of MOCVD-TiO2 /GaAs with and without
共NH4兲2Sx treatments and PMA-MOCVD-TiO2 /S–GaAs at different PMA
temperatures.
than that of 300°C and 350°C. It is thought that the higher PMA
temperature would destroy the H passivation, which can be examined by C–V characteristics.
The C–V characteristics of MOCVD-TiO2 /GaAs, MOCVDTiO2 /S–GaAs, and PMA-MOCVD-TiO2 /S–GaAs are shown in
Fig. 4. The C–V characteristics of MOCVD-TiO2 /GaAs show a
stretch-out phenomenon under negative bias as shown in Fig. 4a. It
is from the high density of interface states due to the existence of
native oxides on the GaAs surface. The breakdown at higher negative bias is from the higher leakage current, resulting in the grain
boundary of the MOCVD-TiO2 polycrystalline structure. Figure 4b
shows the C–V characteristics of MOCVD-TiO2 /S–GaAs. The capacitance in the accumulation region is high due to the improved
interface quality. The capacitance decay at higher negative bias is
due to the high leakage current, which comes from the defects and
the grain boundary of polycrystalline MOCVD-TiO2 film. Sharp
C–V curves PMA-MOCVD-TiO2 /S–GaAs can be obtained after
PMA treatments at 300, 350, and 400°C, as shown in Fig. 4c-e,
respectively. The ideal C–V curve is also shown in the figure as a
reference. It is derived from the neglect of the effective oxide
charges and the interface states, but the work function difference
共⌽ms = −1.31 V兲 of metal 共A1兲 and semiconductor 共GaAs兲 is taken
into account. The optimized PMA temperature is 350°C, as shown
in Fig. 4d, in which the stretch-out phenomenon and the flatband
voltage shift are minimized. The dielectric constants and effective
oxide charges of PMA-MOCVD-TiO2 /S–GaAs films as functions of
PMA temperature are shown in Fig. 5a and b, respectively. The
dielectric constant increases with the PMA temperature due to the
improvements of interface and film qualities, but the value decreases
at PMA temperature higher than 350°C. The higher PMA temperature causes the H bonds to break and loss of the passivation
function,27,28 resulting in the increase of leakage current as shown in
Fig. 3. The dielectric constant and the effective oxide charges can
reach 66 and 1.86 ⫻ 1012 C/cm2 at the PMA temperature of 350°C.
The thickness of MOCVD-TiO2 film is 62 nm. The interfacial layer
is very thin, which has a minor effect during the extraction of dielectric constants.
Moreover, the C–V hysteresis loops as a function of PMA
temperature are shown in Fig. 6. The C–V hysteresis loop of
MOCVD-TiO2 /S–GaAs without PMA treatment is counterclockwise, as shown in curve 共i兲 of Fig. 6a, which is from the high density
of oxide trapped charges22,24 in MOCVD-TiO2 /S–GaAs film without H passivation. The C–V hysteresis loops of PMA-MOCVDTiO2 /S–GaAs film are clockwise at 300 and 350°C, as shown in
curve 共ii兲 of Fig. 6a and curve 共i兲 of Fig. 6b. The mobile ions
are responsible for the C–V clockwise hysteresis loop due to the
decrease of oxide trapped charges from film quality improvement.
The C–V hysteresis loop changes back to counterclockwise
Figure 5. 共a兲 Dielectric constant and 共b兲 effective oxide charges of
MOCVD-TiO2 /S–GaAs and PMA-MOCVD-TiO2 /S–GaAs at different
PMA temperatures.
at 400°C, as shown in curve 共ii兲 of Fig. 6b. It is dominated by
oxide trapped charges due to the break of H bonds and hence, the
loss of the passivation function at higher PMA temperature.27,28 The
sum 共Not兲 of oxide trapped density and mobile ion density can be
derived from the difference of flatband voltage 共⌬VFB兲 of the C–V
hysteresis loops measured at high frequency.24 The formula is as
follows
Not = − ⌬VFBCox /Aq
where Cox is the oxide capacitance, A is the contact area 共7.07
⫻ 10−4 cm2兲, and q is the magnitude of an electron charge. In C–V
measurement, the bias scans first from the accumulation region to
the inversion region 共forward scan兲 and then back to the accumulation region 共backward scan兲. ⌬VFB is defined as the difference of
VFB between the forward scan and the backward scan. So, the polarity of the oxide trapped charge is negative and that of the mobile
ion charge is positive. The Not of PMA-MOCVD-TiO2 /S–GaAs as a
function of PMA temperature is shown in Fig. 7. The lowest mobile
ion density is 5.7 ⫻ 1010 cm−2 at the PMA temperature of 350°C.
The interface state densities 共Dit兲 of MOCVD-TiO2 /S–GaAs and
PMA-MOCVD-TiO2 /S–GaAs at different PMA temperatures are
shown in, Fig. 8. The lowest Dit is 5.96 ⫻ 1011 cm−2 eV−1 at an
energy of 0.7 eV from the edge of the valence band as the PMA
temperature is fixed at 350°C.
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Journal of The Electrochemical Society, 153 共11兲 F266-F270 共2006兲
F269
Figure 8. Interface state density of MOCVD-TiO2 /S–GaAs and PMAMOCVD-TiO2 /S–GaAs at different PMA temperatures.
and polycrystalline films. For GaAs substrate with 共NH4兲2Sx treatment, the interface quality of MOCVD-TiO2 /S–GaAs is much
improved. The PMA treatement can further improve the leakage
current from the defects and the grain boundary of the polycrystalline TiO2 film due to hydrogen passivation. For the PMA 共350°C兲MOCVD-TiO2 /S–GaAs, the leakage current densities are 2.5
⫻ 10−7 and 5 ⫻ 10−7 A/cm2 under positive and negative electric
fields at 1.5 MV/cm. The lowest Dit is 5.96 ⫻ 1011 cm eV−1 at an
energy of 0.7 eV from the edge of the valence band. The dielectric
constant and the effective oxide charges are 66 and 1.86
⫻ 1012 C/cm2.
Acknowledgments
The authors thank the National Science Council of China for
their support under contract no. 93-2215-E110-009.
References
Figure 6. C–V hysteresis loops of 共a兲 MOCVD-TiO2 /S–GaAs 关curve 共i兲兴
and PMA at 300°C 关curve 共ii兲兴; and 共b兲 PMA at 350°C 关curve 共i兲兴 and PMA
at 400°C 关curve 共ii兲兴.
Conclusions
MOCVD-TiO2 films deposited on 共100兲 p-type GaAs substrate
with and without 共NH4兲2Sx treatments are investigated. For GaAs
substrate without 共NH4兲2Sx treatment, the electrical characteristics
of MOCVD-TiO2 /GaAs are poor due to high interface state density
Figure 7. Oxide trap density and mobile ion density as a function of PMA
temperature.
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