Electrochemical and Solid-State Letters, 13 共10兲 H350-H353 共2010兲
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1099-0062/2010/13共10兲/H350/4/$28.00 © The Electrochemical Society
Effective Treatment on AlGaN/GaN MSM-2DEG Varactor
with „NH4…2SÕP2S5 Solution
Y. C. Ferng, L. B. Chang,*,z A. Das, C. Y. Chen, and C. C. Lin
Department of Electronic Engineering, Chang Gung University, Tao-Yuan 333, Taiwan
The effect of surface passivation using 共NH4兲2S and 共NH4兲2S/P2S5 on a AlGaN/GaN-based metal-semiconductor-metal diode
above a two-dimensional electron gas 共MSM-2DEG兲 varactor was investigated. The surface property, capacitance ratio
共Cmax /Cmin兲, and leakage current of the prepared samples were studied before and after treatments using X-ray photoelectron
spectroscopy and capacitance–voltage and current–voltage analyses. It showed that the 共NH4兲2S/P2S5-treated sample had the most
excellent surface state and Cmax /Cmin and the least leakage current because of either reduced native oxide or deposited phosphorus
compounds only provided by 共NH4兲2S/P2S5 and sulfide upon the surface, also validated by having the highest sheet carrier density.
Hence, these promising results promote further potential for varactor applications.
© 2010 The Electrochemical Society. 关DOI: 10.1149/1.3473728兴 All rights reserved.
Manuscript submitted March 8, 2010; revised manuscript received June 28, 2010. Published July 30, 2010.
The metal-semiconductor-metal diode above a two-dimensional
electron gas 共MSM-2DEG兲 has shown its potential as a varactor that
can be easily integrated with high electron mobility transistor devices. In addition, its voltage-dependent capacitance ratio is much
larger than that of conventional varactor diodes and can be tuned by
electrode geometry in contrast to the conventional p–n, Schottky, or
heterostructure diodes where the ratio is only defined by the layer
structure.1-3 After the development of SiO2 /AlGaN/GaN-based
double metal-oxide-semiconductor heterojunction capacitors with
reduced leakage current, the MSM-2DEG based on this layer structure was proposed as a robust radio-frequency switch.4 Most of
these applications seek the large capacitance ratio. However, except
by tuning the electrode geometry, few investigations on the improvement of capacitance ratio obtaining the superior varactor performance are published.5,6 Marso et al.5 reported that metal-oxidesemiconductor heterojunction field effect transistor 共MOSHFET兲
MSM with an oxide layer between the metal and the semiconductor
decreased the Cmax and made the capacitance–voltage 共C-V兲 characteristic asymmetric even though it could reduce the leakage current. Besides, they also indicated that better controllable capacitance
ratio and stable C-V properties could be obtained in the heterostructure field effect transistor 共HFET兲 MSM without an oxide layer between the metal and the semiconductor, but it was with a worse
leakage current. It was reported that sulfur treatment is very effective in reducing the surface states and surface recombination velocity in III-V compound semiconductors.7-12 Of the various sulfur
treatments, only the 共NH4兲2Sx treatment achieved promising results
due to its capacity to etch the native oxide and the GaAs surface and
to tie up the dangling bonds with sulfur on a freshly exposed prismatic GaAs surface. Although increasing immersion time promotes
the performance, longer immersion time causes a higher surface
roughness and a decline in the mobility of electrons by high sulfide
contamination and chemical reaction. Hence, hot 共NH4兲2Sx treatment or 共NH4兲2Sx + UV illumination is employed to reduce immersion time, but these two methods are difficult to get characteristic
reproduced devices from.13,14 Due to the hydrolysis of P2S5 in the
共NH4兲2S solution leading to an exothermic reaction, there is no need
to employ an extra heating or UV illumination during the 共NH4兲2Sx
treatment. Thus, the AlGaN/GaN-based MSM-2DEG varactor prepared by the 共NH4兲2Sx and 共NH4兲2S/P2S5 treatments are studied in
this work.
Experimental
The Al0.17Ga0.83N/GaN episamples were grown by metallorganic
chemical vapor deposition on a sapphire substrate, and the above
* Electrochemical Society Active Member.
z
E-mail: [email protected]
electrodes were made by Ni/Au metallization. For comparison, all
samples were emanated from the same wafer consisting of a sapphire substrate, a buffer layer, a 4 ␮m undoped GaN layer, and a 47
nm undoped Al0.17Ga0.83N layer. All the samples were cleaned by
standard processes. In addition, the Al0.17Ga0.83N/GaN heterostructures were etched in HCl/H2O 共1:1兲 solution for the first removal of
the native oxide before sulfur treatments. Three kinds of samples
were prepared as follows: 共i兲 for untreated samples, without sulfur
treatment; 共ii兲 for 共NH4兲2S-treated samples, dipped into a 50°C
共NH4兲2S solution for 5 min; and 共iii兲 for 共NH4兲2S/P2S5-treated
samples, dipped into a 共NH4兲2S/P2S5 共3:1兲 solution for 5 min. In the
device processing, the Schottky contacts were fabricated to make
C-V measurements and the ohmic contacts with annealing at 900°C
were fabricated to make Hall measurements. Three kinds of
Schottky contact areas were also fabricated with 4000, 5250, and
9750 ␮m2, respectively, to confirm the relationships between the
Schottky contact area and the capacitance swing, as mentioned in a
previous article,5 and the largest Schottky contact with 9750 ␮m2
was used to determine the C-V curve after sulfur treatments. X-ray
photoelectron spectroscopy 共XPS兲 was used to investigate the surface property of the samples after sulfur treatments. The C-V and
current–voltage 共I-V兲 characteristics of the prepared samples were
measured using an HP 4285A and a Keithley 2430, respectively.
Results and Discussion
The 2DEG sheet carrier density and mobility obtained by making
Hall measurements were 3.6 ⫻ 1013 cm−2 and 600 cm2 /共V s兲, respectively. In addition, the calculated 2DEG sheet carrier density
from the C-V curves was 2.64 ⫻ 1012 cm−2.15,16 Both the resulting
sheet charge densities, even though different, agree with Chen et
al.’s article.15 Figure 1a presents the XPS spectra of the Al 2p core
levels of the three kinds of prepared samples. The binding energies
of the pure Al and Al2S3 are 72.9 and 74.6 eV, respectively. The
signal peaks of the three kinds of prepared samples without sulfur
treatment and with the 共NH4兲2S and 共NH4兲2S/P2S5 treatments are
shifted to 73.6, 73.8, and 74.3 eV, respectively. It is observed that
more Al–S bonds are formed by the 共NH4兲2S/P2S5 treatment. The
XPS spectra of the Ga 3d core levels of the three kinds of prepared
samples are shown in Fig. 1b. The binding energy of the pure Ga,
GaS, GaN, and GaP are 18.6, 19.97, 19.5, and 19.3 eV, respectively.
The signal peaks of the three kinds of prepared samples following
without sulfur treatment and the 共NH4兲2S and 共NH4兲2S/P2S5 treatments are shifted to 19.5, 19.8, and 19.3 eV, respectively. The Ga–P
bonds are formed following the 共NH4兲2S/P2S5 treatment. Figure 1c
presents the XPS spectra of the N 1s core levels of the three kinds of
prepared samples. The binding energy of GaN is 397 eV. The signal
peaks of the three kinds of prepared samples without sulfur treatment and with the 共NH4兲2S and 共NH4兲2S/P2S5 treatments are
shifted to 397.2, 397.5, and 398 eV, respectively. The signal peaks of
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Electrochemical and Solid-State Letters, 13 共10兲 H350-H353 共2010兲
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Figure 1. 共Color online兲 The XPS spectra
of 共a兲 Al 2p, 共b兲 Ga 3d, 共c兲 N 1s, 共d兲 S 2p,
and 共e兲 P 2p of the sample without treatment 共no sulfur treatment兲 and the
共NH4兲2S-treated and the 共NH4兲2S/P2S5treated samples.
all the samples shift to the higher bonding energy side. Figure 1d
presents the XPS spectra of the S 2p core levels of the three kinds of
prepared samples. The binding energy of pure S and GaS are 164
and 162.2 eV, respectively. The signal peaks of the three kinds of
prepared samples without sulfur treatment and with the 共NH4兲2S and
共NH4兲2S/P2S5 treatments are shifted to 160.5, 160.5, and 160.9 eV,
respectively. It indicated that more sulfides are formed by the
共NH4兲2S/P2S5 treatment because a signal peak appears at 164 eV.
The XPS spectra of the P 2p core levels of the three kinds of prepared samples are shown in Fig. 1e. The binding energy of pure P is
134 eV. Because a signal peak appears at 134.5 eV, it validated that
there are phosphorus compounds having a better thermal stability
and adsorbing strongly on the surface17 upon the
共NH4兲2S/P2S5-treated sample, which yields a more stable surface
state than the 共NH4兲2S-treated sample.11,14
Figure 2 reveals the relationship of the capacitance and immersion time 共C-T兲 of both sulfur treated samples, and it is indicated
that the C-T relationships of both treated samples have a similar
trend, shown as the fitting curve in Fig. 2. The capacitance with a
further increase in an immersion time of 5 min would not be greater
than the Cmax,5 min anymore even though the capacitance is definitely a function of the immersion time, as presented in Fig. 2. The
increasing capacitance before the immersion time of 5 min was be-
Figure 2. 共Color online兲 The relationship of C-T of both 共NH4兲2S-treated
and 共NH4兲2S/P2S5-treated samples. The C-T relationships of both treated
samples have similar trend, shown as the fitting curve, and the Cmax was
indicated at immersion time of 5 min.
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H352
Electrochemical and Solid-State Letters, 13 共10兲 H350-H353 共2010兲
Figure 3. 共Color online兲 The C-V curves on the sample without treatment
共no sulfur treatment兲 and the 共NH4兲2S-treated and the 共NH4兲2S/P2S5-treated
samples conducted on immersion time of 5 min, and it is also used to deduce
the sheet carrier densities 共Ns兲.
cause of the removal of the native oxide following sulfur
treatments,11,14 then the capacitance reached the maximum at the
immersion time of 5 min. Then the capacitance decreased because
the insulating sulfide and phosphorus compounds started to deposit
on the surface with a further increase in the immersion time of 5
min;14 thus, our investigated samples were conducted on the immersion time of 5 min to obtain larger Cmax /Cmin. As shown in Fig. 3, it
is observed that the capacitance of the 共NH4兲2S/P2S5-treated sample
can be improved most significantly. Table I summarizes the measured capacitances, calculated Cmax /Cmin, and calculated sheet carrier densities 共Ns兲 of the three kinds of prepared samples. It is reasonable to conclude that C is directly proportional to Ns, as shown in
Table I, due to the surface state of the AlGaN layer having noticeable influence on the sheet carrier density 共Ns兲 and the relationship
deduced from Ns = Q/Se,15,16 C = Q/V, and C = ⑀ ⫻ S/d, where
Q is the charge quantity under the Schottky contact area S, e is the
electronic charge, C is the capacitance of prepared samples at applied voltage 共V兲, ⑀ is the dielectric constant, and d is thickness of
the dielectric layer. The larger calculated Ns of the prepared sample,
the better surface state, and the larger capacitance, with a thinner
dielectric layer because of the removal of native oxide, were obtained. The increases in the calculated Cmax /Cmin of both the
共NH4兲2S-treated and 共NH4兲2S/P2S5-treated samples were presented
as 20.9 and 33.7%, respectively, corresponding to the rising Ns of
16.7 and 18.2%, and it is indicated that the prepared sample using
共NH4兲2S/P2S5 had the most excellent performance. It can be concluded that the Cmax /Cmin of the MSM-2DEG varactor not only can
be tuned by the electrode geometry but can also validated by obtaining the largest Cmax /Cmin in the maximum Schottky contact area
9750 ␮m2 共increasing from 10 to 19.6兲 and is consistent with a
previous article,5 and can be further improved by our proposed sulfur treatments 共increasing from 19.6 to 26.2兲.
Figure 4 shows the I-V curve of the prepared samples described
previously. It is observed that the characteristic of the MSM-2DEG
Figure 4. 共Color online兲 The forward and reverse I-V curve of the sample
without treatment 共no sulfur treatment兲 and 共NH4兲2S-treated and
共NH4兲2S/P2S5-treated samples conducted on immersion time of 5 min.
varactor has almost no change after both sulfur treatments. Because
the MSM-2DEG varactor consists of two Schottky diodes connected
back to back above a 2DEG layer structure, the forward current and
the reverse current have a similar trend.5 Because the main current
of the Schottky diode is dominated by a reversed bias, when the
applied voltage 共reversed bias兲 increased, a leakage current path
occurred from the reverse-bias depletion zone to the forward-bias
depletion zone. When the applied voltage further increased above
the threshold voltage 共Vth兲, about ⫾4 V in this article, the reversebias depletion zone penetrated through the 2DEG channel and
caused an additional leakage current path, occurring from the 2DEG
channel to the forward-bias depletion zone. Then the leakage current
increased strongly and saturated even though the applied voltage
increased continuously. In addition, due to the removal of the native
oxide and the deposited insulating sulfide and extra excellent phosphorus compounds,17 having a better thermal stability and adsorbing
strongly on the surface and upon the surface, as discussed previously, the surface state of the 共NH4兲2S/P2S5-treated sample was
improved most efficiently;11,14 therefore, its leakage current was also
blocked most effectively, compared with the untreated and
共NH4兲2S-treated samples with only sulfide upon the surface. Besides, because we treated samples in less time rather than that in
Ref. 14 共5 min only兲 to obtain the thinner insulating layers and
guaranteed higher Cmax /Cmin, it caused a larger leakage current than
that in Ref. 14.
Conclusions
In summary, we have demonstrated the great effectiveness on
surface passivation using 共NH4兲2S/P2S5 in the MSM-2DEG varactor. Due to the 共NH4兲2S/P2S5-treated sample having the highest
sheet carrier density, it is reasonable to conclude that it had the most
excellent surface state by reducing the native oxide and depositing
extra stable phosphorus compounds and sulfide upon the surface,
validated by the XPS measurements, resulting in the largest
Cmax /Cmin and the least leakage current, also validated in the C-V
and I-V measurements. The 共NH4兲2S/P2S5 treatment technology
Table I. A summary of investigated varactor capacitance ratios „Cmax ÕCmin… and sheet carrier densities „Ns… before and after treatments.
Sulfur passivation
Without sulfur treatment
共NH4兲2S
共NH4兲2S/P2S5
Minimum capacitance 共Cmin兲
共pF兲
Maximum capacitance 共Cmax兲
共pF兲
Capacitance ratio
共Cmax /Cmin兲
Sheet carrier density 共Ns兲
共cm−2兲
0.36
0.35
0.32
7.05
8.29
8.39
19.6
23.7
26.2
2.64 ⫻ 1012
3.08 ⫻ 1012
3.12 ⫻ 1012
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Electrochemical and Solid-State Letters, 13 共10兲 H350-H353 共2010兲
proposed in this study not only provides a simple manufacturing
process but also easily obtains all the promising device performances, mentioned above, compared to the MOSHFET MSM and
HFET MSM described previously. This surface passivation using
共NH4兲2S/P2S5, therefore, provides a great opportunity for future
MSM-2DEG varactor applications.
Acknowledgments
The authors thank the Green Technology Research Center of
Chang Gung University and the High Valued Instrument Center of
National Science Council for providing the AlGaN/GaN-based epiwafers and the XPS measurements, respectively.
Chang Gung University assisted in meeting the publication costs of this
article.
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