Impact of surface chemical treatment on capacitance-voltage characteristics of GaAs
metal-oxide-semiconductor capacitors with Al 2 O 3 gate dielectric
Davood Shahrjerdi, Emanuel Tutuc, and Sanjay K. Banerjee
Citation: Applied Physics Letters 91, 063501 (2007); doi: 10.1063/1.2764438
View online: http://dx.doi.org/10.1063/1.2764438
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/6?ver=pdfcov
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APPLIED PHYSICS LETTERS 91, 063501 共2007兲
Impact of surface chemical treatment on capacitance-voltage
characteristics of GaAs metal-oxide-semiconductor capacitors
with Al2O3 gate dielectric
Davood Shahrjerdi,a兲 Emanuel Tutuc, and Sanjay K. Banerjee
Microelectronics Research Center, The University of Texas at Austin, Austin, Texas 78758
共Received 17 May 2007; accepted 3 July 2007; published online 6 August 2007兲
The authors examine the impact of two different chemical surface treatment methods on
capacitance-voltage characteristics of GaAs metal-oxide-semiconductor 共MOS兲 capacitors using
NH4OH and 共NH4兲2S prior to atomic layer deposition 共ALD兲 of Al2O3. In both cases, x-ray
photoelectron spectroscopy data confirm the removal of As2O3 / As2O6 upon Al2O3 deposition.
However, Ga–O bonds appear to incorporate in the final gate stack at the Al2O3 / GaAs interface.
MOS capacitors exhibit a steep transition from accumulation to depletion as well as very low
leakage current density indicating high quality of ALD-Al2O3. The midgap interface trap density
was evaluated to be 共⬃3 – 5兲 ⫻ 1011 / cm2 eV using the Terman method. In addition, quasistatic
capacitance-voltage 共C-V兲 measurement confirms the formation of true inversion layer in GaAs
using both chemical treatment protocols. However, sulfur-passivated GaAs demonstrates better
frequency dispersion behavior and slightly smaller capacitance equivalent thickness than
hydroxylated GaAs. A statistical study substantiates the reproducibility of these results.
© 2007 American Institute of Physics. 关DOI: 10.1063/1.2764438兴
III-V based structures have attracted a lot of interest to
drive complementary metal-oxide-semiconductor 共CMOS兲
technology beyond the 22 nm node. Superior electron transport properties of these materials make them suitable for
low-power and high-speed applications. However, the lack of
a compatible oxide has been the paramount challenge for
CMOS technology to replace silicon with III-V materials.
Therefore, there has been tremendous ongoing search for an
appropriate gate dielectric which unpins the Fermi level and
also provides a thermodynamically stable interface with the
semiconductor. As a result, several techniques have been
proposed and are being extensively studied to realize GaAsbased enhancement mode metal-oxide-semiconductor field
effect transistor 共MOSFET兲 including the incorporation of a
very thin silicon and germanium interfacial layer,1–3 in situ
molecular beam expitaxy-grown Ga2O3共Gd2O3兲,4,5 and
atomic layer deposition of Al2O3.6,7 Recently, Xuan et al.6
have demonstrated enhancement mode InGaAs MOSFETs
using atomic layer deposition 共ALD兲-Al2O3, albeit with a
relatively low drive current. ALD technique is also gaining
popularity in mainstream silicon-based CMOS technology as
an attempt to replace SiO2 with high-␬ dielectrics. ALDgrown Al2O3 dielectric offers several potential advantages
including a relatively large dielectric constant 共8–9兲 and a
good thermal stability. Moreover, the interfacial self-cleaning
attribute of ALD-Al2O3 on GaAs-based substrates has been
previously reported.7,8 Arsenic oxides are believed to be the
origin of Fermi level pinning and therefore the Fermi level is
expected to be unpinned after oxide deposition. In this work,
we study the impact of chemical surface treatment prior to
the growth of ALD-Al2O3 films on the electrical characteristics of GaAs MOS capacitors.
MOS capacitor fabrication was started with chemical
surface treatment of 共100兲 p-type GaAs substrates with dopa兲
Electronic mail: [email protected]
ing concentration of 0.5– 1 ⫻ 1018 cm−3. In order to examine
the effect of surface treatment, different chemical treatment
schemes were carried out prior to Al2O3 deposition. The
chemical treatment protocols basically include native oxide
removal in HF 共1%兲 solution for 1 min followed by sample
dip in either NH4OH 共10%兲 for 1 min or 共NH4兲2S 共20%兲 for
10 min. The chemical treatments were carried out at room
FIG. 1. XPS Ga 2p3/2 and As 3d spectra of GaAs surface 共a兲 before and 关共b兲
and 共c兲兴 After Al2O3 deposition using NH4OH and 共NH4兲2S, respectively. It
is evident that As2O3 / As2O6 are removed upon ALD-Al2O3 deposition.
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91, 063501-1
© 2007 American Institute of Physics
216.165.95.70 On: Tue, 26 Aug 2014 19:50:43
063501-2
Appl. Phys. Lett. 91, 063501 共2007兲
Shahrjerdi, Tutuc, and Banerjee
FIG. 2. 共a兲 HFCV characteristics of chemically treated p-type GaAs measured at 1 MHz. The inset demonstrates quasi static CV behavior of
共NH4兲2S-treated samples. 共b兲 Leakage current density vs. voltage for both
chemical treatment techniques. The effect of different PDA conditions on
hysteresis is shown in the inset.
FIG. 3. 共a兲 Capacitance equivalent thickness 共CET兲 vs physical thickness.
The 共NH4兲2S-treated capacitors exhibit slightly smaller CETs than the
NH4OH-treated ones. 共b兲 The leakage current density measured on MOS
structures with different Al2O3 thicknesses. The inset illustrates FN plot for
a MOS structure with 65-Å-thick Al2O3.
tors demonstrate a very low leakage current density 关Fig.
temperature. The NH4OH treatment leaves the surface OH
2共b兲兴. The inset of Fig. 2共b兲 shows the impact of PDA treatterminated, which renders the surface properties suitable for
ments in N2 ambient on hysteresis behavior of MOS capaciALD growth. As a result, self-limiting reaction and full surtors obtained from bidirectional HFCV measurements at
face coverage tend to occur at the very beginning of the ALD
1 MHz. The hysteresis of capacitors with as-deposited Al2O3
run.9 In addition, sulfur passivation of III-V materials using
on hydroxylated and sulfur-passivated GaAs was 380 and
共NH4兲2S has been shown to be very effective in order to rule
430 mV, respectively. The midgap interface trap density was
out native oxide regrowth after its removal.10 After chemical
evaluated to be 共⬃3 – 5兲 ⫻ 1011 / cm2 eV using the Terman
treatment, samples were immediately transferred to a commethod.11 The capacitance equivalent thickness 共CET兲 of
mercial Savannah™ 200 ALD reactor where Al2O3 was desamples is shown as a function of oxide physical thickness in
posited onto GaAs surface at 250 ° C by alternating water
Fig. 3共a兲 in which 共NH4兲2S-treated samples exhibit slightly
and trimethylaluminum precursors. Physical thickness of
smaller CETs as compared to NH4OH-treated samples. FigAl2O3 films was measured by ellipsometry. Postdeposition
ure 3共b兲 illustrates the leakage current density 共J兲 at Vfb-1 V
annealing 共PDA兲 was performed at 600 ° C in N2 ambient.
vs CET. The current transport through the insulator film in an
Finally, 2200-Å-thick TaN was deposited as a metal gate
MOS structure can be described by tunneling mechanisms.
using a dc magnetron sputtering system and was patterned
The inset of Fig. 3共b兲 shows the Fowler-Nordheim 共FN兲 plot
using a standard photolithography and reactive ion etching.
for a sulfur-passivated sample with 65-Å-thick Al2O3. The
2
Figure 1 illustrates the XPS spectra of GaAs surface 共a兲 be兲 vs 共1 / Eox兲 indicates FN tunneling
linear relation of ln共J / Eox
fore and 共b兲 and 共c兲 after Al2O3 deposition using NH4OH
through the oxide layer at high electric field where the slope
and 共NH4兲2S, respectively. From XPS spectra, it is evident
is given by the following expression:12
that As2O3 / As2O6 are nearly removed right after Al2O3
2
deposition. Nonetheless, it appears that Ga–O bonds remain
兲兴 4冑2m*
d关ln共J/Eox
S
=
共1兲
=
共⌽兲3/2 ,
relatively intact upon oxide deposition. High-frequency
d共1/Eox兲
3qប
capacitance-voltage 共HFCV兲 characteristics of the MOS cawhere m* is the electron effective mass within the tunneling
pacitors fabricated using different surface preparations are
ALD-Al2O3 and ⌽ is the tunneling barrier height. Assuming
shown in Fig. 2共a兲. The fast transition of CV curve from
m* = 0.23 and with the measured slope of the curve, we
accumulation region to depletion region indicates a relatively
evaluate the tunneling barrier height to be ⬃1.7 eV. This
low interface trap density between Al2O3 and GaAs. The
effective mass is an average based on previous studies.13,14
formation of true inversion layer in sulfur-passivated
Normalized accumulation capacitance and flatband voltage
samples using quasistatic C-V measurement corroborates the
variations were monitored at different frequencies. The typiFermi level unpinning at the Al2O3 / GaAs interface 关the inset
cal frequency dispersion behavior of 共NH 兲2S- and
in Fig. 2共a兲兴. The hydroxylated samples also exhibit similar
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NH4OH-treated samples with 105-Å-thick Al2O3 are shown
quasistatic C-V behavior 共data not shown兲. The MOS capaci216.165.95.70 On: Tue, 26 Aug 2014 19:50:43
063501-3
Appl. Phys. Lett. 91, 063501 共2007兲
Shahrjerdi, Tutuc, and Banerjee
has been previously shown that using ammonia solution, the
GaAs surface will be rich in hydroxyl 共OH兲 group, formed as
Ga共OH兲x and GaOx,15,16 whereas ammonium sulfide tends to
preclude the regrowth of native oxides. In addition, the
Ga 2p3/2 x-ray photodetection spectroscopy 共XPS兲 spectrum
of the sulfide-treated sample implies the presence of thinner
GaOx layer at the Al2O3 / GaAs interface compared to the
hydroxylated sample. We conjecture this attribute of ammonium sulfide reduces the number of interface states, which in
turn translates into better C-V characteristics.
In summary, we have examined the effect of different
chemical surface preparations of GaAs prior to Al2O3 deposition using NH4OH and 共NH4兲2S on capacitance-voltage
behavior of MOS capacitors. The presented electrical characteristics of GaAs MOS capacitors imply the effectiveness
of these chemical surface preparation techniques in producing a high-quality interface between Al2O3 and GaAs substrate. Compared to the hydroxylation method, the sulfur
passivation technique resulted in better frequency dispersion
characteristics and slightly smaller CET for a given Al2O3
thickness. However, both chemical treatment procedures
demonstrated an unpinned Al2O3 / GaAs interface confirmed
by quasistatic CV measurements.
FIG. 4. Frequency dispersion characteristics of 共a兲 共NH4兲2S- and 共b兲
NH4OH-treated samples, where the flatband voltages at different frequencies are depicted as an inset in 共a兲. The statistical study of process variation
on frequency dispersion behavior corroborates the reproducibility of these
chemical processes 关the inset of 共b兲兴.
in Figs. 4共a兲 and 4共b兲, respectively. A relatively small variation of accumulation capacitance 共⬍3 – 4 % 兲 was observed
for both hydroxylated and sulfur-passivated samples. However, 共NH4兲2S-treated samples exhibit smaller flatband voltage shift at different frequencies as opposed to the
NH4OH-treated devices, as shown in the inset of Fig. 4共a兲.
This flatband voltage shift is due to slow interface traps
which could be due to an As-rich surface and/ or a GaOx
interfacial layer. In order to examine the overall process
variations of frequency dispersion characteristics, we have
carried out statistical study on 100 devices for each pretreatment method. The samples were measured from five different runs in order to further confirm the reproducibility of
these processes. As shown in the inset in Fig. 4共b兲, we have
a good reproducibility as well as uniformity of frequency
dispersion behavior of 共NH4兲2S-treated samples. In addition,
the statistical study on NH4OH-treated samples produced
nearly the same results where more than 80% of devices
exhibit a flatband voltage shift of ⬃240 mV due to frequency dispersion 共data not shown兲. Electrical characteristics
of GaAs MOS capacitors indicate that surface treatment using 共NH4兲2S prior to atomic layer deposition of Al2O3 is
more effective than surface hydroxylation using NH4OH. It
This work was supported in part by DARPA and the
MICRON Foundation.
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