HAI-DANG TRINH, YUEH-CHIN LIN, CHIEN-I KUO,
EDWARD YI CHANG,1,2HONG-QUAN NGUYEN,1YUEN-YEE WONG,1 CHIH-CHIEH YU,3CHI-MING CHEN,4CHIA-YUAN CHANG,4
JYUN-YI WU,4HAN-CHIN CHIU,4TERRENCE YU,4
HUI-CHENG CHANG,4JOSEPH TSAI,4and DAVID HWANG4
1.—Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. 2.—Department of Electronics Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. 3.—Instrument Technology Research Center, Hsinchu 30010, Taiwan. 4.—Taiwan Semiconductor Manufacturing Company Limited, Hsinchu Science Park, Hsinchu 30010, Taiwan.5.—e-mail: trinhhaidang@gmail.com
The electrical properties of Al2O3/n-InGaAs metal–oxide–semiconductor
capacitors (MOSCAPs) with In content of 0.53, 0.7, and 1 (InAs) have been investigated. Results show small capacitance–voltage (C–V) frequency dis-persion in accumulation (1.70% to 1.85% per decade) for these MOSCAPs, mostly being assigned to border traps in Al2O3. With higher In content,
shorter minority-carrier response time and smaller C–V hysteresis are observed. The reduction of C–V hysteresis might be related to the reduction of Ga-bearing oxides in Al2O3/InGaAs interfaces as indicated by x-ray
photo-electron spectroscopy.
Key words: ALD Al2O3, surface treatment, InGaAs, InAs, MOSCAPs
INTRODUCTION
High-k/III–V structures have been extensively studied recently to realize complementary metal– oxide–semiconductor (MOS) technologies at the 16-nm node and beyond.1 However, regardless of long-term efforts by the community, the high-k/III–V interface trap density (Dit) still remains a challenge.
Passivation of high-k/III–V interfaces is always needed to reduce Dit. Recently, reports have
indi-cated that the quality of high-k/III–V interfaces not only depends on the passivation method but is also influenced by the III–V compounds themselves.2,3 Study of high-k/n-InGaAs structures with In content from 0 to 0.53 showed a significant reduction of capacitance–voltage (C–V) frequency dispersion at the accumulation region as the In content reaches 0.53.3Besides, some recent reports showed that the frequency dispersion in accumulation in high-k/ In0.53Ga0.47As MOS capacitors (MOSCAPs) is closely
related to the traps inside the gate oxide, or so-called border traps.4–6 Simulations have been done on Al2O3/In0.53Ga0.47As MOSCAPs to study the effect of
border traps on the frequency dispersion in the accumulation capacitance.5,6In this work, we intend to study the electrical properties of high-k/n-InGaAs structures in which the In content varies from 0.53 to 1. The purpose of the study is to investigate the electrical properties of atomic layer deposition (ALD) Al2O3/n-InGaAs MOSCAP structures related to
border traps and interface traps, such as the fre-quency dispersion and hysteresis.
EXPERIMENTAL PROCEDURES InGaAs wafers with different In content used in the study were (a) 100 nm In0.53Ga0.47As, (b) 5 nm
In0.7Ga0.3As/10 nm In0.53Ga0.47, and (c) 5 nm InAs/
3 nm In0.7Ga0.3As/10 nm In0.53Ga0.47As epilayer
stacks grown on n+-type InP substrates by molecular beam epitaxy and supplied by IQE Inc. The top layer in each structure was a nominal 2 9 1017cm 3 Si-doped In(Ga)As material. Wafers
(Received December 13, 2012; accepted April 13, 2013; published online May 29, 2013)
were degreased by acetone and isopropanol before dipping in 4% HCl solution for 1 min to remove native oxides. They were then loaded into the ALD system for Al2O3 deposition. In the ALD chamber,
in situ trimethylaluminum (TMA) clean was used by employing 10 TMA/N2pulses before the deposition of
120 cycles of Al2O3 at 300°C using TMA and H2O
as precursors. In situ TMA cleaning is effective for further removal of native oxides via ligand-exchange reactions between TMA and InGaAs native oxides.7–10 Besides, HCl + TMA treatment was reported to give a good-quality Al2O3/n-InAs
inter-face.8,11 After oxide deposition, samples were post-deposition annealed (PDA) at 400°C in H2/N2 (1:1)
gas for 10 min. Finally, Pt/Au gate metal and Au/Ge/ Ni/Au back-side contact metal were deposited, fol-lowed by post-metal-deposition annealing (PMA) at 400°C in N2 gas for 30 s. The MOSCAPs were
char-acterized by multifrequency C–V, conductance–volt-age (G–V), and current–voltconductance–volt-age (I–V) measurements using an Agilent HP 4284A precision LCR meter and a Keithley 4200 semiconductor analyzer, respec-tively. The Al2O3/InGaAs, InAs interfaces were
analyzed by x-ray photoelectron spectroscopy (XPS) measurements and high-resolution transmission electron microscopy (HRTEM).
RESULTS AND DISCUSSION
Figure1 illustrates the As 3d, In 3d5/2, and Ga
2p3/2 XPS spectra of the native-oxide-covered
Fig. 2. HRTEM micrographs of MOSCAP structures: (a) Al2O3/In0.53Ga0.47As, (b) Al2O3/In0.7Ga0.3As, and (c) Al2O3/InAs.
Fig. 1. As 3d, In 3d5/2, and Ga 2p3/2XPS spectra of samples: bare native-oxide-covered (a) In0.53Ga0.47As, (b) In0.7Ga0.3As, and (c) InAs
surfaces; and (d) Al2O3/In0.53Ga0.47As, (e) Al2O3/In0.7Ga0.3As, (f) and Al2O3/InAs interfaces after using HCl + TMA treatment followed by ALD of
1.5 nm Al2O3.
Fig. 3. I–V characteristics of MOSCAP structures.
In0.53Ga0.47As, In0.7Ga0.3As, and InAs surfaces
(Fig.1a, c, e) and Al2O3/In0.53Ga0.47As, Al2O3/In
0.7-Ga0.3As, and Al2O3/InAs interfaces after using
HCl + TMA treatment following deposition of 1.5 nm Al2O3(Fig.1b, d, f). It can be seen that, after
surface treatment and oxide deposition, As2O3and
As2O5oxides reduced to below the detection level of
XPS for all samples, as shown in Fig.1b. In- and Ga-related oxides of these samples were also sig-nificantly reduced, as indicated in Fig.1d, f. The As 3d and In 3d5/2 spectra after normalization show
very similar profiles. On the other hand, the Ga 2p3/ 2 spectra show reduction of the intensity at the
Ga-O sides with increase of the In content, espe-cially for the samples after surface treatment and high-k deposition (Fig.1c, f). This observation indi-cates that the relative ratios of Ga oxides to the bulk Ga-As decreased with increase of the In content. In addition, this reduction of Ga-related oxides is more significant for the high oxidation states, i.e., Ga2O3
and GaAsO4, as shown in Fig.1f.
Figure2 shows HRTEM micrographs of the Al2O3/InGaAs and Al2O3/InAs/InGaAs structures.
Normally, air-exposed Ga(In)As surfaces have native oxide layers with thickness above 2.0 nm to 3.0 nm.12,13 Here, the samples show an abrupt transition from In0.53Ga0.47As, In0.7Ga0.3As, and
InAs layers to Al2O3 without an interface oxide
layer. The highly ordered lattice images at the interface and the smooth surface suggest that the structures exhibit good thermal stability after PDA
at 400°C. The Al2O3films are amorphous as shown
in the figure, and the thickness of the oxide films estimated from TEM micrographs is about 12.0 nm. The current density–voltage (J–V) characteristics of the samples are presented in Fig. 3. All samples exhibit similar, low leakage currents with break-down field of 7.2 MV/cm. This indicates that the ALD Al2O3is a good-quality oxide layer.
The C–V responses of the MOSCAPs at frequency of 1 MHz are shown in Fig.4a. When the In content increases, the C–V responses change from high-frequency to low-high-frequency C–V behavior. For the case of InAs, a strong inversion layer is observed at the high frequency of 1 MHz. This behavior was observed before14 and is more obvious in this experiment. The variation of the C–V curves in the depletion/inversion regions can be explained by the increase of the intrinsic carrier concentration ni in
InGaAs with increase of the In content (from 6.3 9 1012cm 3 for In0.53Ga0.47As to 10159
cm 3for InAs15). This would result in a decrease of the minority-carrier response time sR (or an
increase of the minority-carrier generation rate) according to the relationship sR= sT/ni,16 where sT
is the carrier lifetime.
The multifrequency C–V responses of the MOS-CAPs shown in Fig.4b–d indicate that the fre-quency dispersion in the accumulation region is small and very similar (1.85%, 1.75%, and 1.70% per decade for x = 0.53, 0.7, and 1, respectively) for all the samples. This behavior seems to be different for
high-k/InxGa1 xAs structures with x < 0.53, for
which the frequency dispersion is strongly depen-dent on the In content.3 This result suggests that, for high-k/InGaAs with high In content, interface traps do not seem to contribute to the frequency dispersion in the accumulation regime. Study of Al2O3/n-InAs structures with various surface
treatments also showed that the frequency disper-sion in the accumulation regime is small, even for untreated samples with high Dit values.8,11 In the
equilibrium state, with doping concentration NDof
2 9 1017 cm 3, the Fermi level is located very close to the conduction-band edge (estimated about 0.0013 eV below EC) for In0.53Ga0.47As and
0.022 eV above EC for InAs.15,17 Thus, in the
accumulation region, due to the band bending, the Fermi level at Al2O3/InGaAs and Al2O3/InAs
inter-faces would be located within the conduction band. The interface states inside the conduction band have too short response time, thus, they can follow
Fig. 5. Bidirectional C–V responses of Al2O3/InxGa1 xAs MOSCAP structures with different In contents at frequency of 1 kHz.
Fig. 6. Conductance contours Gp/x(f, V) at different temperatures (77 K, 180 K, and 300 K) for Al2O3/InxGa1 xAs MOSCAP structures.
as indicated by the very similar frequency disper-sion values.
Bidirectional C–V responses show a reduction of hysteresis with increase of the In content (Fig.5). In this experiment, the contribution of the gate oxide to the hysteresis is expected to be the same for all samples, because the thermal processes were per-formed at a relatively low temperature of 400°C to minimize the effect of channel material diffusion into the gate oxide. Therefore, the effect of interface traps on hysteresis will decrease with increasing In content. This result could be related to the reduction of Ga-related oxides with increase of the In content, especially Ga3+oxides as discussed in the analyses of the XPS spectra. Such reduction of Ga oxides leading to the improvement of C–V characteristics has also been seen for HfO2/InxGa1 xAs structures
with x in the range from 0 to 0.53.3,19 First-princi-ples calculations showed that the large amount of Ga-O and As-As bonds at the high-k/Ga(In)As interfaces would cause a high interface state den-sity.20 Experimentally, it was also reported that reduction of Ga2O3 by using a Si interface
passiv-ation layer could result in a significant improve-ment of the electrical properties of Al2O3/GaAs MOS
structures.21The other possible explanation for this phenomenon is a decrease of the InGaAs bandgap with increase of the In content, as proposed and explained by an empirical model.2,22 According to this model, when the In content increases, the bandgap decreases, which would lead to a reduction of the total density of traps in the bandgap as well as their effect on the electrical behavior of high-k/ InGaAs structures.
Temperature-dependent C–V and G–V measure-ments were performed at 77 K, 180 K, and 300 K for all samples. Based on the C–V and G–V data, the parallel conductance was determined and the con-ductance maps of Gp/x were plotted to trace the
movement of the Fermi level.23Figure6a shows the conductance maps Gp/x at different temperatures
for the Al2O3/n-In0.53Ga0.47As structure. Except for
weak inversion C–V responses at frequency below 4 kHz at room temperature, high-frequency curves are observed across the whole range of measured frequencies (1 kHz to 1 MHz) and temperatures (data not shown here). This confirms the accuracy of the extracted conductance values.16 From the con-ductance map, the concon-ductance peak maximum
(here, a value of the capture cross-section of 10 14 cm2 was taken to determine the locations of the traps).
Figure6b and c show the conductance maps for the Al2O3/In0.7Ga0.3As and Al2O3/InAs structures,
respectively. For these two structures, high-fre-quency C–V responses were not observed even when the temperature was decreased to 77 K (data not shown here). Since inversion layers always respond to the full range of measured frequencies and tem-peratures, their contribution to the conductance will affect the accuracy of the conductance method.26As can be seen in the figures, the conductance contours are closed due to the contribution of inversion car-riers. Thus, the extracted value of Ditis an
overes-timate and the traces of the Fermi level could not be observed. To eliminate the contribution of the inversion layer from the extraction of the Ditvalue,
application of the full conductance method is needed in future study.26
CONCLUSIONS
We studied material and electrical characteristics of ALD Al2O3/InxGa1 xAs structures with In content
of 0.53, 0.7, and 1. XPS analysis shows a significant reduction of native oxides after HCl + TMA treat-ment, as supported by HRTEM imaging. Multifre-quency C–V responses show low-frequency dispersion in the accumulation region. This fre-quency dispersion seems to be mostly due to border traps in the oxide rather than due to interface traps. The C–V hysteresis shows a reduction of the inter-face traps with increasing In content, which could be related to the reduction of Ga-related oxides. The conductance method showed a low interface trap density distribution at energy positions from 0.4 eV to 0.70 eV above the valence band of In0.53Ga0.47As.
The conductance contours showed free movement of the Fermi level with the gate bias for the case of the Al2O3/In0.53Ga0.47As structure. However, for the
cases of Al2O3/In0.7Ga0.3As and Al2O3/InAs, traces of
Fermi level movement were not observed due to the inversion layer contribution.
ACKNOWLEDGEMENTS
This work was supported by the NCTU-UCB I-RiCE Program and sponsored by the Taiwan
National Science Council under Grant No. NSC-102-2911-I-009-301.
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