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Structural and electrical properties of sub-1-nm EOT HfO2 grown on InAs by atomic layer deposition and its thermal stability Yu-Seon Kang, Hang-Kyu Kang, Dae-Kyoung Kim, Kwang-Sik Jeong, Min Baik, Youngseo An, Hyoungsub Kim, Jin Dong Song, and Mann-Ho Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10975 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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ACS Applied Materials & Interfaces

Structural and electrical properties of sub-1-nm EOT HfO2 grown on InAs by atomic layer deposition and its thermal stability

Yu-Seon Kang 1, Hang-Kyu Kang 1, Dae-Kyoung Kim 1, Kwang-Sik Jeong 1, Min Baik 1, Youngseo An2, Hyoungsub Kim2, Jin-Dong Song3, Mann-Ho Cho 1,*

1

2

Institute of Physics and Applied Physics, Yonsei University, Seoul, 120-749, Korea

School Department of Material Science and Engineering, Sungkyunkwan University, Suwon440-746, Korea 3

Center of opto-electronic materials, Korea institute of Science and Technology, Seoul 136-791, Korea

KEYWORDS: HfO2, InAs, indium arsenide, band alignment, interfacial defect states, interfacial reactions

ABSTRACT We report on changes in the structural, interfacial, and electrical characteristics of sub1nm equivalent oxide thickness (EOT) HfO2 grown on InAs by atomic layer deposition (ALD). When the HfO2 film was deposited on an InAs substrate at a temperature of 300 °C, the HfO2 was in an amorphous phase with an sharp interface, an EOT of 0.9 nm, and low preexisting interfacial defect states. During post deposition annealing (PDA) at 600 °C, the HfO2 was transformed from an amorphous to a single crystalline orthorhombic phase, which minimizes the interfacial lattice mismatch below 0.8 %. Accordingly, the HfO2 dielectric after the PDA had a dielectric constant of ~24 because of the permitivity of the well-ordered orthorhombic HfO2 structure. Moreover, border traps were reduced by half than the as-grown sample due to a reduction in bulk defects in HfO2 dielectric during the PDA. However, in terms of other electrical properties, the characteristics of the PDA-treated sample were degraded compared to the as-grown sample, with EOT values of 1.0 nm and larger interfacial defect states (Dit) above 1014 cm-2 eV-1. XPS data indicated that the diffusion of In atoms from the InAs substrate into the HfO2 dielectric during the PDA at 600 °C resulted in the development of substantial midgap states. 1

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* Electronic mail: [email protected]

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I. INTRODUCTION As Si-based complementary metal oxide semiconductor (MOS) devices are being aggressively scaled down in the semiconductor industry, more elaborate gate stacks with sub-1 nm equivalent oxide thickness (EOT) gate dielectrics and high mobility channel materials with a low power consumption are required for use in MOS device applications.1-5 In the next generation large scale integrations, Hf based gate dielectrics (high-κ) on III-V compound semiconductors such as GaAs, InGaAs, and InAs are being seriously considered.1-14 Among them, HfO2/InAs (bulk mobility ~ 30,000 cm2V-1s-1) shows high band offsets of over 2.0 eV, a high transconductance, high drive current at low voltages, and high-frequency operation, which are all very promising electrical properties for use in MOS devices.10-14 However, it was reported that the electrical properties of HfO2/InAs are significantly affected by the crystalline structure of the high-κ dielectric layer and the surface orientation of the InAs substrate.14-16 In addition, calculated electronic structure of the HfO2/InAs interface using density functional theory showed that interfacial defect states within InAs band gap are inevitably generated.17,18 In particular, the elemental As states at the HfO2/InAs interface leads to the development of a large amount of mid-gap traps and oxide charge traps. Therefore, it is essential to develop a systematic understanding of the origin of the interfacial defect states in HfO2/InAs and to develop various defect control technique for further development. Although some reports have shown the appropriate electrical properties for the HfO2/InAs, a systematic analysis including the origin of interface traps have not been examined yet. The focus of this study was on the structural, chemical, and electrical properties of a HfO2 dielectric grown on an InAs substrate with sub-1 nm EOT as well as its thermal stability by conducting comprehensive physical and electrical measurements and analyzing the resulting data. In addition, we systematically examined the mechanism responsible for the formation of interfacial traps of the HfO2/InAs by analyzing (i) the crystalline structure, (ii) interfacial reactions, (iii) energy band alignment, and (iv) electrical properties of ALD-HfO2/InAs. InAs 3



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based MOS capacitors with a 0.9 nm EOT show good interface properties without an interfacial layer and with low interfacial defect states (Dit). The dielectric constant can be increased and border trap density decreased by post annealing at 600 °C. Moreover, thermal degradation was observed: i.e., the Dit is substantially increased as the result of the diffusion of elemental In from InAs substrate during the PDA at 600 °C.

II. EXPERIMENTAL HfO2 films were deposited on i-InAs (001) substrates (carrier concentration of ~ 5 × 1016 cm-3) by atomic layer deposition (ALD). Before the deposition of the HfO2 films, the InAs substrates were dipped in a dilute HF solution (~ 1% HF in D.I. water) for 2 min to remove native oxides. The substrate was then rinsed in deionized H2O and dried by blowing N2 over the substrate. The chemically etched substrate was loaded into the ALD chamber within a few minutes. The ALD temperature was 300 °C and Tetrakis (ethylmethylamido) hafnium Hf[N(CH3)(C2H5)]4 (TEMAHf) and H2O vapor was used as the Hf metal precursor and oxygen source, respectively. N2 gas was used as a purge gas during film growth. The growth rate of HfO2 in our ALD system (commercially built lab-scale ALD system) was ~0.78 Å/cycle. We performed 64 cycles and 102 cycles of ALD to deposit a HfO2 film with thicknesses of ~ 5 nm and ~8 nm, respectively. To investigate the thermal stability on the structural and electrical properties of HfO2 on InAs, the films were annealed at 600 °C by a rapid thermal process for 1 min in an environment of N2 (PDA). Structural properties of the HfO2 films on InAs were investigated using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). For band alignment, the band gaps of the HfO2 films were measured by reflection electron energy loss spectroscopy (REELS) with a primary electron beam energy of 1.0 keV. Valence band analysis and the interfacial chemical bonding configuration were examined by high resolution x-ray 4


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photoelectron spectroscopy (XPS) using a monochromatic Al Kα x-ray source (hν = 1486.7 eV) with a pass energy of 20 eV (FWHM value of under 0.43 eV for Ag 3d5/2). C 1s, Hf 4f (In 4d), In 3d (Hf 4p), As 3d (Hf 5p), and O 1s core-level spectra were taken. To deconvolute the XPS core-level spectra, the background was removed by Shirley-type subtraction and the full-widthat-half-maximum (FWHM) values of the constituent peaks were maintained constant. Fitting curves were determined by Gaussian and Lorentzian distributions, in which the Gaussian distribution ratio was higher than 70%. In addition, for the case of the In 3d and As 3d doublets, the intensity ratio of the doublet caused by spin-orbit splitting was determined by the transition probability during photoionization. To examine the electrical characteristics of the films, a metal oxide semiconductor capacitor (MOSCAP) with a sputter-deposited TaN top electrode with an area of 7850 µm2 and a thickness of 600 nm was fabricated via a lift-off technique. Capacitance-voltage (C-V) and leakage current characteristics were measured using an Agilent E4980A LCR meter and an Agilent B1500A semiconductor device analyzer. To obtain CET, EOT, and the dielectric constant (k) of the HfO2 film, we calculated the CET of the HfO2 film using accumulation capacitance in C-V curve using the following equation.

where C/A is the capacitance obtained from C-V measurements and ε0 = 8.85 x 10-12 F/m is the vacuum permittivity. A conductance method was used to compare the interface trap density (Dit) of HfO2/InAs before and after thermal annealing.

Dit was calculated using

classical Nicollian-Brews model without correction for series resistance.19

III. RESULTS AND DISCUSSION Figure 1 shows cross-sectional high-resolution transmission electron microscopy (HRTEM) images of the HfO2 films on the InAs substrate before and after post deposition annealing (PDA) at 600 °C. The thickness of the as-grown HfO2 film is ~ 5 nm with an amorphous phase and an 5



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abrupt interface without an interfacial layer was observed under the TEM detection limit: i.e., a clean interface between a high-dielectric and the III-V substrate can be achieved by wetcleaning and self-cleaning during the ALD process. The self-cleaning involves a process in which the metal organic precursor (TEMAHf) consumes the native oxide effectively during the first few ALD cycles, as reported for the ALD growth for HfO2 on a compound semiconductor.20-23 Kirk et al. recently reported that native arsenic oxides are not produced during the ALD process at a temperature of 300 °C because arsenic oxides such As2O3 and As3O5 are unstable and melt at temperatures below 315 °C.22,23 On the other hand, a drastic changes in the thickness and structure of the HfO2 dielectric is observed after the PDA, as shown in Fig. 1(b). During the PDA at 600ºC, the thickness of the HfO2 dielectric is increased up to about 6.1nm and the film is fully crystallized without any detectable interfacial layer. From TEM images, d-spacing values of 5.05 Å for the film growth direction and 4.63 Å for the in-plane direction parallel to the substrate correspond to orthorhombic (001) and orthorhombic (110), respectively.24 Based on the orientation relationship between o-HfO2/InAs in the HRTEM images, the atomic arrangement is o-HfO2[110]∥InAs[110], o-HfO2[-110]∥InAs[110], and o-HfO2(001)∥InAs(001). Moreover, fourteen atoms of o-HfO2 (110) are matched with fifteen atoms of InAs (110), as shown by the red arrows in Fig. 1(b). The estimated interfacial lattice mismatch between the o-HfO2 dielectric and the InAs substrate is ~ 0.8 % for o-HfO2/InAs {a = 4.63 Å, 4.63 Åｘ14 = 64.8 Å and a = 4.285 Å, 4.285 Åｘ15 = 64.3 Å}. This result indicates that the HfO2 dielectric is transformed from an amorphous phase to a nearly single crystalline orthorhombic phase when sufficient thermal energy is employed and this transformation minimizes the interfacial energy through minimizing the lattice mismatch to 0.8 %. On the other hand, while the exact origins of the increased film thickness after PDA are not known yet, the influence of interfacial strain 6


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between HfO2 and InAs is a likely factor. As discussed above, the HfO2 film is in direct contact with the InAs substrate in the absence of an interfacial layer in the case of an interfacial lattice mismatch of ~0.8 %. In addition, the lattice constant of the o-HfO2 (110) film and the InAs (110) substrate is 4.63 Å and 4.285 Å, respectively. These results indicate that the HfO2 film is under compressive strain, which could lead to increasing film thickness. XRD measurements were carried out to confirm the crystalline phase of the HfO2 dielectric layer, as shown in Fig. 2. The as-grown HfO2 shows no notable diffraction peaks except for a strong peak at 29.6 °, which is assigned to the InAs (002) substrate.25 On the other hand, the HfO2 dielectric after the PDA showed a sharp peak at 35.6 ° assigned to the orthorhombic phase of HfO2 (002)24: i.e., the amorphous HfO2 film in the as-grown state is changed to a well-ordered crystalline structure after PDA. Several previous reports showed that the HfO2 dielectric preferentially crystallized in the monoclinic phase because m-HfO2 has the lowest formation energy compared with any other crystalline structure (heat of formation energy of m-HfO2 is -1239.3 kJ/mol).26 However, in the HfO2/InAs structure after the PDA, the crystallization of the HfO2 dielectric is dominantly affected by the interfacial energy under the lattice constraint conditions between the HfO2 dielectric and the InAs substrate, as shown in previous HRTEM results. Therefore, an orthorhombic phase of HfO2 is induced because the structure minimizes interfacial lattice mismatch. To investigate the detailed chemical state at the interface in HfO2/InAs, XPS measurements were performed. Figure 3 shows As 3d (Hf 5p) and In 3d (Hf 4p) core-level spectra of 5 nm-thick-HfO2/InAs before and after the PDA. Peak fitting of the core-level spectra as shown in Fig. 3(a) and (b) provides an accurate quantitative analysis. Based on the phase diagram of an In-As-O ternary system, the As*, As1+, As3+, and As5+ states in the As 3d spectra can be attributed to elemental As (As or As-As bonding) and multiple oxidation states of AsOx, As2O3 (△G~-137.7 kcal/mol), and As2O5 (△G~-187 kcal/mol), respectively.27,28 The 7



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In3+ states in the In 3d spectra can be attributed to oxidation states of In2O3 (△G~-198.6 kcal/mol).27,28As2O3 and As2O5 states were rarely detected in the as-grown HfO2/InAs, while elemental As and In2O3 states were clearly detected. These are the result of interfacial reactions between inter-diffused oxygen and the InAs substrate. Following two types of chemical reactions are predicted.27,28 1) formation of As2O3 and In2O3 : 3O2 + 2InAs → As2O3 + In2O3 (△G~-310.7 kcal/mol) 2) formation of In2O3 and elemental As : As2O3 + 2InAs → In2O3 + 4As (△G~-35.3 kcal/mol) and 1) formation of As2O5 and In2O3 : 4O2 + 2InAs → As2O5 + In2O3 (△G~-360.0 kcal/mol) 2) formation of In2O3 and elemental As : 3As2O5 + 10InAs → 5In2O3 + 16As (△G~-304.0 kcal/mol) The oxidation states of As2O3, As2O5, and In2O3 were generated during the post annealing at 600 °C. It is noteworthy that these oxidation sates are dominantly formed at the surface region of the HfO2 dielectric, as evidenced by the XPS signal at a grazing (20°) take-off angle with an analysis depth of ~ 0.34 nm in Fig. 3(c). This is due to the out-diffusion of In- and Aselements: i.e., the InAs substrate undergoes decomposition at a temperature of 600 °C, and the decomposed atomic In and atomic As contribute to the diffusion process as reactants.27,28 The following thermodynamically oxidation reactions are predicted to occur.27,28 4In + 3O2 → 2In2O3 (△G~-397.2 kcal/mol) 4As + 3O2 → 2As2O3 (△G~-275.4 kcal/mol) 4As + 5O2 → 2As2O5 (△G~-374.0 kcal/mol) Although the thermodynamic data indicate that As2O5 is more stable than As2O3, more 8


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As2O3 is present than As2O5 in the As 3d XPS spectra of the PDA sample. This phenomenon can be understood by the following chemical reactions, 3As2O5 + 16In → 5In2O3 + 6InAs (△G~-508.8 kcal/mol) and As2O3 + 4In → In2O3 + 2InAs (△G~-86.5 kcal/mol).27,28 According to the above Gibbs energy of the formation equations, it is evident that the chemical reaction that occurs between As2O5 and elemental In is favorable. That is, this process induces the formation of more In2O3 oxidation states, which is in good agreement with the XPS results in Fig 3(c). The InAs that is produced during the chemical reaction at the interface immediately decomposes to In and As atoms during the annealing process at 600 °C, which are also sources for oxidation.27,28 Energy band parameters such as band gap (Eg), conduction band offset (CBO), and valence band offset (VBO) were investigated. In general, the potential barrier at each band must be over 1 eV in order to inhibit conduction by the Schottky emission of electrons or holes in oxide bands. Energy band alignments of as-grown HfO2/InAs can be obtained from the combination of valence band (VB) spectra and reflection electron energy loss spectroscopy (REELS). The VBO for HfO2/InAs was obtained by subtracting the VB spectra of the InAs substrate from that of HfO2/InAs as shown in Fig. 4(a). A VBO value of ~ 2.5 ± 0.2 eV between HfO2 and InAs was obtained. The Eg can be defined as the threshold energy of band-to-band excitation, as shown in the REELS spectra in Fig. 4(b). The Eg value of ~ 5.8 ± 0.2 eV is almost the same as the reported value for HfO2.13 The effective electron barrier height, CBO, is calculated using the following equation: CBO = EgHfO2 – EgInAs – VBOHfO2/InAs.29,30 Finally, the energy band diagram of the as-grown HfO2/InAs is introduced in Fig. 4(c). In the as-grown HfO2/InAs, the effective hole barrier height and the effective electron barrier height was found to be ~ 2.5 ± 0.2 eV and ~ 2.9 ± 0.2 eV, respectively. Unfortunately, the energy band alignments for the PDA sample could not be determined. As shown in previous XPS results, large amounts of oxidation products are formed at free surface of a HfO2 film after post annealing at 600 °C. 9



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As a result, it is difficult to measure the exact values of both Eg and the VBO at the HfO2/InAs interface in the PDA sample. Figure 5 shows the change in leakage current of the as-grown HfO2/InAs for substrate electron injection (positive voltage) and gate electron injection (negative voltage) conditions. The I-V feature is a sensitive indicator of the insulating characteristics of a dielectric layer and an interface between a dielectric and a substrate, because it reflects the defect states and the barrier height between HfO2 and InAs. The leakage current in a 5 nm-thick HfO2/InAs film gradually increased with increasing gate voltage. There are three possible causes for the high leakage current at low applied field. The first cause may be low potential barrier for electrons and holes between the dielectric and the substrate. However, VBO and CBO values of ~ 2.5 ± 0.2 eV and ~2.9 ± 0.2 eV, respectively, were found, which are sufficiently high to inhibit the Schottky emission of electrons or holes into oxide bands as discussed previously. Since direct tunneling occurs at lower electric fields when the thickness of the dielectric is in the order of a few nanometers (< 5 nm), direct tunneling in the thin dielectric film is a second possibility.31 The third cause is intrinsic defects such as oxygen vacancies, oxygen interstitials, and oxygen deficiency defects in the HfO2 layer produced by ALD. An I-V simulation of direct tunneling (D-T) for a 5 nm-thick-HfO2 film was performed using the following equation:

J 2 −(82 )(1 − (1 − /) / =( )( − 1)exp [ ] 8ℎ

where J is the current density in A/cm2, E is the electric field in V/cm2, q is the elementary charge, h is Planck’s constant, mox is the electron mass in the oxide. The data showed that the leakage current level over 0.5 V is higher than D-T curve. This result indicates that the large leakage current in 5 nm-thick-HfO2 film results from both D-T and defect assisted tunneling. A low leakage current level in the order of ~10-9 A/cm2 was maintained in the 8 nm-thick HfO2 sample up to near ~ 1.7 V. In addition, the Fowler-Nordheim tunneling (F-N) current at a high voltage of over ~ 3.1 V was observed in the 8 nm-thick HfO2 film. To determine the effective 10


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barrier height for electrons between the HfO2 and InAs, curve fitting in the F-N tunneling region was performed as shown in Fig 5. F-N tunneling of an electron or hole is given by: J/E2 =

!"# ℏ

%& '(

exp(-

) (%& )*/# / +, /E) ℏ

where J is the current density in A/cm2, E is the electric field in V/cm2, e is the electronic charge, m is the free electron mass, mox is the electron or hole mass in the oxide, 2πħ is Planck’s constant, and φ0 is the effective barrier height. For the HfO2 dielectric, we used an effective electron mass of 0.3 m. An effective barrier height of ~ 3.0 eV was obtained in asgrown 8 nm-thick HfO2/InAs, which is in good agreement with the CBO value as shown in the previous band diagram in Fig. 4. On the other hand, the leakage current in the voltage from ~ 1.7 V to ~ 3.1 V is increased. This can be significantly influenced by Poole-Frenkel (P-F) conduction (trap assisted tunneling) resulting from a high density of intrinsic defects in the HfO2 dielectric. The low leakage current level was maintained up to 3 V in the PDA sample, which would be the result of the decreased density of traps in HfO2 dielectric and the increasing total thickness of HfO2 by post annealing, as shown in the TEM results. Frequency-dependent C-V curves as a function of AC frequency were measured in asgrown and PDA samples to evaluate the dielectric constant and the density of border traps, as shown in Fig. 6. The 5 nm-thick HfO2/InAs film had the maximum accumulation capacitance (Cmax) value of ~ 2.8 µF/cm2 at 100 kHz with high frequency dispersion and large hysteresis characteristics. Although the Cmax in 8 nm-thick HfO2/InAs is smaller than that of 5 nm-thick HfO2/InAs, the C-V curve features of the 8 nm-thick HfO2/InAs are very similar to that of 5 nm-thick HfO2/InAs. To correctly obtain the equivalent oxide thickness (EOT) value of the HfO2 dielectric, an average value was determined from over 20 measurements at a 100 kHz frequency as shown in Fig. 6(c). Figure 6(d) shows the EOT for the as-grown and post annealed sample as a function of dielectric thickness and AC frequency. Based on the linear fit line at the 100 kHz AC frequency, we obtained a dielectric constant (κ) of ~ 22 and an EOT value of ~ 0.9 11



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nm for the as-grown 5 nm-thick HfO2. The EOT value was slightly increased by post annealing to ~ 1.0 nm due to the increase in physical thickness, while the dielectric constant increased to ~ 24. Quasi-static capacitance voltage (QS-CV) measurements were carried out for the asgrown 8nm-thick-HfO2 film to confirm the calculated dielectric constant, as shown in the Fig. 6(b). The dielectric constant of the as-grown HfO2 film was determined to be ~ 21.2, which is almost same value (~22.1) as that obtained using the high-frequency CV (HF-CV) method. The HfO2 dielectric has various crystalline phases (monoclinic, cubic, tetragonal, and orthorhombic). The monoclinic phase is the most stable phase for HfO2 under normal conditions, but it has the lowest κ value. The cubic phase as a meta stable structure has a higher κ value than the monoclinic phase. The tetragonal phase with high polarizability has the highest κ value. In the HfO2/InAs structure, amorphous phase HfO2 was transformed to an orthorhombic phase by post annealing at 600 °C as shown in the TEM and XRD results. In addition, the obtained lattice parameters from the TEM results indicate that the crystalline structure of o-HfO2 after PDA is similar to a tetragonal phase HfO2 structure. Therefore, the enhancement in long range ordering of crystalline HfO2 with an orthorhombic phase affects the high permittivity of the HfO2 dielectric after PDA. However, the dielectric constant of HfO2 after PDA would also be affected by the In2O3 states with a κ value of 8.9, which is mixed into the HfO2 as shown in XPS results. Finally, the HfO2 after PDA has the dielectric constant of ~ 24. Figure 7 shows C-V curves measured at 100 kHz and the border trap density in a 5 nmand 8 nm- thick HfO2/InAs film before and after post annealing. The shift in the C-V curve towards a positive voltage during the reverse sweep indicates that substrate-injected electron trapping with a slow response time into HfO2 film occurs near the interface between HfO2/InAs.32 Because the border traps exchange charge with the semiconductor substrate on the time scale of the measurement, the C-V measurement was performed at a slow sweep rate as low as ~0.1 Vs-1 to encompass most of the border traps.33 The border trap density was 12


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calculated from the capacitance difference during forward and reverse C-V sweeps as [Crf = │Cr - Cf│], where Cr and Cf are the capacitance density during the reverse and forward sweep, respectively. The border trap density in the HfO2/InAs after post annealing was decreased by almost half than that of the as-grown sample. The high border trap density in the as-grown sample could result from intrinsic defects in the HfO2 dielectric, as evidenced by the P-F conduction phenomenon in the previous I-V results. It is known that bulk defects such as oxygen vacancies, the Hf3+ ion (an electron trapped at Hf4+), and superoxy radicals (or oxygen interstitial) are produced in bulk HfO2 during the ALD process. These bulk defects influence the charge trapping and detrapping process, resulting in a large hysteresis. In general, the bulk defect and interfacial defects can be reduced by an additional annealing process using N2 gas, O2 gas, forming gas (N2/H2 mixture), or other nitrogen containing gasses such as ammonia.34 In our previous study, we observed pre-existing O vacancies in HfO2 during the ALD process. These can induce substantial defect states in the band gap of the HfO2, which can cause charge trapping and Fermi level pinning problems.35 In general, the border trap density can be reduced by reducing the oxygen vacancies in HfO2 dielectrics. During post annealing using inert N2 gas at 600 °C, pre-existing interstitial oxygen in HfO2 can bond to oxygen vacancy sites under sufficient thermal energy, resulting in the elimination of oxygen-vacancies in HfO2.35 Consequentially, we conclude that decreasing the leakage current and hysteresis in the PDA film are the result of reduced charge trap states inside the HfO2 film. Figure 8 shows C-V curves measured at room temperature as a function of frequency. In the as-grown sample, a large frequency dispersion in the accumulation and depletion regions was observed, which can be attributed to a higher Dit. In addition, capacitance behavior in the inversion region is dependent on frequency, which is also due to the response of a higher Dit near the midgap. These results can be explained by the interface trap response rather than with the generation of a minority carrier. Figures 8(b) and 8(c) show the normalized parallel 13



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conductance (Gp/Aωq) of the as-grown sample as a function of frequency and gate voltage. The as-grown sample had a high interfacial trap density (Dit) of ~ 3.0ｘ1013 eV-1cm-2. After post annealing at 600°C, the capacitance at a gate voltage of ~ -0.4 V was slightly increased (the C-V slope becomes lower) and the flat band voltage (Vfb) is shifted toward a negative voltage, as shown in Fig 8(d). Moreover, the following conductance map shows evidence for two peaks in the Dit at gate voltage of ~ -0.4 V and ~ 0.1 V. As combined with previous XPS results, we suggest that the generation of both interfacial defects and positive fixed charges causing negative Vfb shifts is due to the diffusion of In into the HfO2 dielectric and the interface of HfO2/InAs during the post annealing process. Figure 9(a) shows the trap energy level obtained by Shockley-Read-Hall statistics for the capture and emission rates. Under conditions of 1ｘ10-16 cm2 for the σ value, a measurement temperature of 295 K, and a measured frequency of 100 kHz ~ 1 MHz, it is possible to calculate the energy level of the defect states from 0.18 eV to 0.25 eV from the valence band edge of InAs. From the conductance data in Fig 8, the value for Dit is shown in Fig. 9(b). The maximum Dit level of ~ 2ｘ1013 eV-1cm-2 in the as-grown sample is drastically increased to ~ 8

ｘ1014 eV-1cm-2 in the post annealed sample. In addition, two Dit peaks are detected near the midgap. In conductance measurements, the trapping and detrapping of the charge carrier occur when the Fermi level in InAs is aligned with interfacial trap states. Therefore, these results show evidence for a newly generated Dit at a gate voltage of ~0.4 V and -1.2 V, corresponding to a different energy level in the band gap of InAs. Previous reports showed that the diffusion of elemental In into a HfO2 dielectric induce the formation of occupied and partially occupied defect states near the mid gap energy level of a semiconductor, which is consistent the conductance results indicated as blue arrows in Fig 9(b).35 An interfacial charge trap density level of 1013 cm-2 eV-1 is high for a MOS capacitor. However, it is possible to further reduce Dit 14


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by an additional treatment. Wu et al. reported that the Dit level depends on the orientation of the InAs substrate, the deposition temperature during ALD, and the post annealing temperature.15 They showed that Al2O3/InAs has various interfacial charge trap density values from ~1012 to ~1013 cm-2 eV-1 depending on the condition of the sample. In addition, a previous report showed that a forming gas (N2/H2 mixture) annealing (FGA) treatment improves frequency dispersion by the interface traps.34 They showed that Dit can be decreased by more than one order of magnitude by an additional FGA treatment. We fabricated a defective HfO2 film on InAs to confirm the FGA effect (See the supporting figure 1). From the C-V curves as shown in the figure below, large hysteresis and hump features were observed in the as-grown sample. This result indicates that the as-grown sample includes a large amount of Dit and bulk traps. On the other hand, the hump feature was largely removed by the FGA treatment, and greatly decreased the C-V hysteresis. These results mean that most of interface traps and fixed charges can be effectively terminated with FGA treatment. The stress induced leakage current (SILC) characteristics of sub-1 nm EOT HfO2/InAs were investigated to evaluate electrical reliability under voltage stress, as shown in Fig. 10. Both forward and backward I-V were measured as a function of ramp voltage. In the 5 nmthick HfO2/InAs, reversible leakage current characteristics were maintained within a ramp voltage range of 1 V - 2.6 V with a 0.1 V ramp step, as shown in Fig. 10(a). An increase in the leakage current of the backward I-V after a ramp stress of over 2.7 V is clearly observed. Since electrical stress can increase the number of bulk defects, likely due to quantum mechanical tunneling, since trap-assisted tunneling through the dielectric, defects can be generated in the HfO2 by repeated high field stress. Thus, the high field stress on thin gate dielectrics produces a leakage current, which limits further oxide scaling in future VLSI technology. Such thickness dependent leakage current characteristics have been reported frequently.36,37 Figure 10(b) shows the SILC characteristics of the 8 nm-thick HfO2 sample. The leakage current level in the 15



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forward I-V scan is drastically reduced due to the absence of direct tunneling. In addition, the SILC is observed at a ramp voltage of 3.1 V and breakdown occurs at 3.4 V, indicating that the HfO2 dielectric can be broken down at slightly higher voltages than that for the 5 nm-thick HfO2 sample. However, the induced SILC level in the backward scan is almost the same in both the 5 nm- and 8 nm- thick HfO2 samples, which increased linearly with increasing trap density and local distribution, suggesting that the same tunneling mechanism is in force. This may have originated from the stress-induced weak breakdown in the HfO2 dielectric. A crystalline film can contain crystal defects such as crystal faults. In our previous report, we observed that the presence of crystalline defects induce a leakage current through the film and facilitate the soft break down of the HfO2 film.38 However, the PDA film showed improved properties in that the SILC level is even maintained at a high electric field region, as shown in Fig 10(c) and (d). Finally, from TEM and SILC results, we conclude that a high quality crystalline HfO2 film was induced by the PDA process.”

IV. CONCLUSIONS In summary, we investigated the structural and electrical characteristics of ultra-thin (sub-1 nm EOT) HfO2 grown on InAs prepared by ALD and its thermal stability. The as-grown HfO2 dielectric on InAs has an amorphous phase structure and an abrupt interface. The sharp interface facilitates the crystallization of the HfO2 to an orthorhombic phase upon thermal annealing. Consequently, a HfO2 dielectric constant of ~ 22 and an EOT value of ~ 0.9 nm were obtained in 5 nm-thick as-grown HfO2 films, while a HfO2 dielectric constant of ~ 24 was observed after post annealing at 600°C. However, the thermal annealing gives rise to some severe problems. Substantial amounts of In2O3 states were generated due to the diffusion of In atoms during the thermal annealing. The diffusion of In into HfO2 induces the formation of mid-gap defect states within the band gap of InAs, which can cause charge trapping and Fermi 16


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level pinning problems. Moreover, our C-V data indicated that interstitial In3+ charged states in HfO2 induce a threshold voltage shift toward a negative voltage. Similar phenomena have frequently been reported in HfO2/InGaAs and HfO2/InP structures. The collective results reported herein suggest that controlling the diffusion of In is key factor in solving Fermi level pinning problems and other degraded electrical properties in HfO2/In-V compound semiconductor gate-stacked structures. It should be noted that the interfacial and electrical properties of HfO2/InAs for use in MOS devices can be improved through additional optimization in post annealing and the surface passivation of InAs.

ASSOCIATED CONTENT Supporting Information Multi-frequency C-V and quasi-static C-V (QS-CV) characteristics of ALD- HfO2/InAs for asgrown and forming gas annealing (FGA) treatment. This information is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGMENTS This work was partially supported by an Industry-Academy joint research program between Samsung Electronics-Yonsei University and by JinDong-Song acknowledges the support from KIST institutional programs of flag-ship.

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Figure Captions

Figure 1. Cross-sectional HR-TEM images of a HfO2 dielectric grown on an InAs substrate: (a) as-grown HfO2 film deposited at 300 °C and (b) post annealed HfO2 film at 600 °C.

Figure 2. XRD patterns of as-grown HfO2 and post annealed HfO2 at 600°C.

Figure 3. XPS (a) As 3d (Hf 5p) spectra and (b) In 3d (Hf 4p) spectra of the as-grown and post annealed HfO2/InAs with normal (90°) and (c) with grazing (20°) take-off angle XPS conditions. (d) Schematic diagram of the interfacial reaction of the HfO2/InAs during post annealing at 600°C.

Figure 4. (a) Valence band spectra of as-grown HfO2/InAs and a cleaned InAs substrate. (b) REELS spectra for energy band gap of HfO2 film. (c) Schematic band diagrams of HfO2/InAs.

Figure 5. Leakage current vs voltage characteristics of the 5 nm-thick- and 8 nm-thick- HfO2 films for as-grown and PDA. I-V simulation of F-N tunneling and was carried out for an as-grown 8 nm-thick-HfO2 film to evaluate the barrier height between the HfO2 film and the InAs semiconductor. I-V simulation of direct tunneling (D-T) for a 5 24


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nm-thick-HfO2 film is indicated by a green dotted line.

Figure 6. Multi-frequency C-V characteristics of (a) the 5 nm-thick- and (b) 8 nm-thickHfO2/InAs for the forward scan (from inversion to accumulation direction) and the reverse scan (from accumulation to inversion direction). Quasi-static C-V (QS-CV) characteristics measured in 8nm-thick-HfO2 film and only the forward scan (inversion to accumulation). CET values are average values measured over at least 20 times, as shown in (c). (d) CET values for as-grown HfO2/InAs and post annealed HfO2/InAs.

Figure 7. C-V characteristics of (a) 5 nm- and (b) 8 nm-thick-HfO2/InAs. Effective border trap density at 100 kHz of as-grown and post annealed (c) 5 nm- and (d) 8 nm-thickHfO2/InAs.

Figure 8. Multi-frequency C-V characteristics of (a) as-grown and (d) post annealed HfO2/InAs. Parallel conductance (Gp/ωqA) vs voltage characteristics of [(b),(c)] as-grown and [(e),(f)] post annealed HfO2/InAs.

Figure 9. (a) Relationship between electron trap energy levels and the working frequency of HfO2/InAs. (b) Interfacial defect states (Dit) from conductance results of as-grown 25



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and post annealed HfO2/InAs.

Figure 10. Stress induced leakage current characteristics of (a) 5 nm and (b) 8 nm-thick-HfO2 for the as-grown sample and (c) 5 nm and (d) 8 nm-thick-HfO2 for the PDA sample with different ramp voltages. The reverse rainbow color represents increasing ramp voltage.

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Structural and Electrical Properties of EOT HfO2 ... - ACS Publications

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