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IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 10, OCTOBER 2016
Improved Data Retention of InSnZnO Nonvolatile Memory by H2O2 Treated Al2O3 Tunneling Layer: A Cost-Effective Method Jayapal Raja, Cam Phu Thi Nguyen, Changmin Lee, Nagarajan Balaji, Somenath Chatterjee, Kyungsoo Jang, Hyoungsub Kim, and Junsin Yi
Abstract— An experiential aspect regarding the improvement of retention characteristics of InSnZnO (ITZO) thin-film transistor-based nonvolatile memory (TFT-NVM) devices with a hydrogen peroxide H2 O2 treated Aluminum oxide (Al2 O3 ) tunneling layer is reported. A better performance in retention of ∼92% (after ten years), a smaller subthreshold swing of 96 mV/decade, and a higher field effect mobility of 31.08 cm2 /V·s were obtained in H2 O2 treated TFT-NVM devices compared with untreated one. Furthermore, employing the H2 O2 treatment in the Al2 O3 layer provided oxygen-rich (O/Al ratio = 1.45) and OH− residuals free Al2 O3 , which effectively minimized the interface states (1.34 × 1011 cm−2 eV−1 ) between the ITZO/(Al2 O3 /SiO x /SiO2 ) stack through strong oxidation. These results suggest that high-quality Al2 O3 dielectric layer can be obtained through cost-effective H2 O2 oxidation techniques for TFT-NVM devices. Index Terms— Charge trap memory, Al2 O3 , peroxo group, low-temperature oxidation, ITZO NVM.
I. I NTRODUCTION ONTINUOUS scaling down (gate length in submicron regime) of complementary metal oxide semiconductor (CMOS) devices is ensured due to the use of transition metal oxides of high dielectric permittivity (κ) to overcome the physical limitations of SiO2 gate dielectric [1]. Recently, an extensive attention using a high-κ materials as dielectric oxide for charge-trap type nonvolatile flash memory (CT-NVM) cells have been noticed to improve the program/erase (P/E) functionality, vertical scaling, and charge retention characteristics as well as achieve low operating voltage.
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Manuscript received July 12, 2016; revised August 4, 2016; accepted August 9, 2016. Date of publication August 11, 2016; date of current version September 23, 2016. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea Funded by the Ministry of Science, ICT and Future Planning under Grant NRF2014R1A1A3053287 and Grant NRF-2014R1A2A2A01006568. The review of this letter was arranged by Editor T. Wang. (Cam Phu Thi Nguyen, Changmin Lee, and Somenath Chatterjee contributed equally to this work.) (Corresponding author: Junsin Yi.) J. Raja, C. P. T. Nguyen, K. Jang, and J. Yi are with the College of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, South Korea (e-mail:
[email protected];
[email protected]). C. Lee and H. Kim are with the School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, South Korea N. Balaji is with the Department of Energy Science, Sungkyunkwan University, Suwon 440-746, South Korea. S. Chatterjee is with the Department of Electronics and Communication Engineering, Sikkim Manipal Institute of Technology, Sikkim Manipal University, Majitar 737-136, India. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2016.2599559
Among various high-κ materials, Al2 O3 has emerged as one of the most promising gate dielectrics because of its excellent thermal stability with Si, a large barrier height, and the better high-κ value (∼9) compared with SiO2 [2], [3]. Although the low temperature processing with high quality of gate dielectric and semiconductors is very crucial for flexible display applications. The main intrinsic defect in low temperature grown ALD (Atomic layer deposition) based Al2 O3 dielectric layer is usually with oxygen deficiency (Vo ) and raises high levels of impurities, such as H, C and OH− residues [4]–[6]. Incorporation of up to 7.5 at. % of H [6], [7] and 0.2 at. % of C [8] has been found in low temperature Al2 O3 film. Due to the presence of such impurities/defects in Al2 O3 film, a shift of energy levels in the band gap may occur, which act as traps for electrons or holes. Therefore, such defect sites could be the origin of an electron pathway in the Al2 O3 (used as a tunneling or blocking layers). Several post-deposition treatments have been proposed to overcome this issue, such as high temperature oxygen/nitrogen annealing (>900 °C) [9]–[11], plasma/ozone treatment [12], [13], ion-beam implantation [14] etc., However, all the above mentioned approaches involve high thermal budget/extra processing steps, ion/plasma damage issues, expensive vacuum techniques, and not compatible for flexible electronics industry. Researchers have proposed a method to decrease intrinsic defects and process temperature by introducing a peroxo group into metal oxide materials. Kwon et al. reported adding H2 O2 in IGZO precursor solution, which can improve the stability of low temperature IGZO TFTs during negative bias stress [15]. Park et al. found the formation of peroxo species in the solution-processed ZrO2 film mixing with 6.67 M H2 O2 , which has a smooth dielectric surface, and less leakage current [16]. Further, Kim et al. observed the deep-level defects, which are effectively removed in H2 O2 treated ZnO surface, leading to high quality Schottky barrier diodes [17]. Thus, H2 O2 treated Al2 O3 layer would be desirable to adopt for a nondestructive low temperature oxidation and surface passivation procedure in flexible electronics industry. In this letter, a low temperature H2 O2 oxidation technique applied on deposited ALD-Al2 O3 dielectrics to prepare better-quality Al2 O3 layer used for tunnel oxide. Experimental results indicate that the potent oxygen radicals presence in the H2 O2 solution strongly oxidizes, effectively eliminates residual impurities in the Al2 O3 and improves its stoichiometry. Thus the proposed H2 O2 treated tunneling oxide is a potential strategy for Al2 O3 /SiOx /SiO2 stacked ITZO CT-NVMs, for better electrical characteristics and better data retention time.
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RAJA et al.: IMPROVED DATA RETENTION OF ITZO NVM BY H2 O2 TREATED Al2 O3 TUNNELING LAYER
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To the best of our knowledge, this is the first attempt to use a cost-effective oxidation technique for fabricating ITZO based CT-NVM devices. II. E XPERIMENTAL P ROCEDURE In this study, the bottom-gate ITZO TFT-NVMs were fabricated on p++ Si (100) substrate. First, a 30-nm SiO2 film as a blocking layer, and a 20-nm Si-rich SiOx film as a storage layer was deposited by Inductive coupled plasmaCVD system as described previously [18]. Subsequently, a 6-nm Al2 O3 film was deposited using thermal ALD system, which served as a tunneling layer. For one cycle of deposition, Al2 O3 films were grown using Al(CH3 )3 for 1 s and H2 O for 1.5 s. N2 was injected for 20 s as the carrier gas in a viscous flow reactor. The deposition was done at 200 °C with a rate of ∼0.11 nm/cycle. After the samples were split into three groups and underwent one of the following treatments: as deposited (untreated), dipped 5 min (5 min-H2 O2 ) and 10 min (10 min-H2 O2 ) into 30% H2 O2 solution at room temperature. Next, the samples were annealed at 100 °C for 30 min to remove moisture. After that, a 50-nm ITZO film as a channel layer was deposited by DC magnetron sputtering system using a ceramic target of In2 O3 :SnO2 :ZnO (30:35:35 at. %). The sputtering conditions has been described elsewhere [19]. Then, the samples were annealed at a temperature of 250 °C for 90 min in air ambient. Finally, 150-nm Al source/drain electrodes were thermally evaporated on top of the ITZO channel layer through a shadow mask. The channel length and width of our NVMs were 200 μm and 200 μm, respectively. The electrical characteristics of the fabricated devices were executed by semiconductor parameter analyzer (EL420C) and impedance analyzer (HP4192A) system at room temperature under a black box. III. R ESULTS AND D ISCUSSION To determine the qualities of Al2 O3 tunneling layer, the C-V characteristics of the three different metal-insulatormetal (Al/Al2 O3 /n-Si) capacitor (MIMCAPs) structure were measured at 1 MHz, as shown in Fig. 1. It is to be noted that the untreated Al2 O3 showed a negative flat band voltage (VFB = −0.96 V) and a lower capacitance (Cox = 93 pF) at accumulation region compared to that of 10 min-H2 O2 treated Al2 O3 (VFB = 0.21 V; Cox = 110 pF) samples. The VFB and Cox are extracted from the forward sweep of C-V curves. Furthermore, the inset of Fig. 1 shows the variation of leakage current density (Jg ) of the corresponding MIMCAPs. We observed that Jg exhibits a two and a half fold lower values from 3.57×10−5 to 5.24×10−8 A/cm2 at same gate bias = 4 V for the untreated sample, as compared with the 10 min-H2 O2 treated Al2 O3 samples, respectively. These results suggest that H2 O2 treated Al2 O3 has fewer defects, which is consistent with the results as described by Fan et al. [20]. Kim et al. [21] and Huang et al. [22] reported Al-Al, Al-OH− and Al-O-H defects like bonds are commonly observed in ALD-Al2 O3 films when H2 O is used as oxygen precursor. These defects are indicated to be the origin of acceptor-like border traps and positive fixed charges in untreated Al2 O3 . Although, Previous work points out that, at lower process temperatures, the chemical reaction between Al(CH3 )3 –H2 O is incomplete and the surface coverage of OH− species is higher on the Al2 O3 surface, which form shallow localized states [4]. The possible reasons for the VFB shift towards a positive direction for 10 min-H2 O2 treated
Fig. 1. C-V plots of the MIMCAPs: (a) untreated (b) 5 min, and (c) 10 min H2 O2 treated Al2 O3 samples, respectively. The inset shows the Jg of the corresponding MIMCAPs, and schematic illustration of Al2 O3 structure.
Fig. 2. XPS spectra of O1s for Al2 O3 films prepared with (a) Untreated, (b) 5 min, (c) 10 min H2 O2 treated samples, and (d) Al 2p core level.
Al2 O3 based device is the neutralization of positive fixed charges (v O filling) and/or the induction of negative charge in the film due to absence of defect bonds. This confirms the better quality of films prepared with H2 O2 , and is believed to be the absence of hydroxyl groups, which was further investigated by XPS (ESCA2000 VG) study. Fig. 2 shows the spectra of O1s and Al 2p peaks of Al2 O3 with and without H2 O2 treatment. The O1s spectrum may be deconvoluted into two-peaks having binding energies of ∼530.8 eV (OI ) and ∼532.5 eV (OII ), which correspond to O bound to the Al lattice and O in the H2 O, OH− impurities [21], [23], respectively. Due to the duration of H2 O2 treatment increases, the O1s peaks have been shifted towards the highest binding energy. Estimated OI /Otot and OII /Otot values (shown in Fig. 2) indicates that stronger Al-O bonding and considerable OH− species are decomposed effectively in the Al2 O3 film followed by H2 O2 treatment. Optimum 10 min-H2 O2 treatment results in a drastic reduction of OH− intensity from 41.1% to 7.4%. Nakamura and Temmyo [24] and Gu et al. [25] proposed that higher binding energy shift and drop of OH− intensity was caused by H2 O2 treatments. Additionally, the increment of O/Al ratio from 1.38 to 1.45 is an excellent agreement with Al2 O3 stoichiometry (O/Al ratio = 1.5) [23] configuration. Although Al 2p core level,
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Fig. 3. P/E and retention characteristics of stacked ITZO CT-NVMs for (a-b) CT-NVM1 [untreated H2 O2 ], (c-d) CT-NVM2 [5 min treated with H2 O2 ], and (e-f) CT-NVM3 [10 min treated with H2 O2 ].
shows a single symmetric peak at 74.25 eV for all samples, corresponding to Al3+ ; which apparently lacks any significant sub-stoichiometric of Al2 Ox or Al-Al clusters [21]. Generally H2 O2 are easily dissociate into OH· and HO·2 radicals even at room temperature. The OH· may be reacted with H+ atom and form H2 O (OH· + H+ + e− → H2 O). The weak acid nature of HO·2 can further split in the presence of metal oxide surface, ·− following the reaction (HO·2 → H+ + O·− 2 ). The formed O2 radical is adsorbed and stabilized on the Al2 O3 surface [26]. Then the OH− group is removed, the Vo are filled, and to form a strong Al2 O3 (schematic shown in the inset of Fig. 1). Moreover, the Al2 O3 layer was strongly oxidized and its defect levels are passivated by OH· radicals, which have a high oxidation potential (2.8 V), than the ozone (2.07 V) and H2 O2 (1.77 V) [27]. This contributes to improve the stoichiometry and quality of Al2 O3 dielectric, which is suitable for tunneling layer of fabricated NVMs. Fig. 3 shows the P/E and retention characteristics (with a drain voltage of 1.25 V) of the fabricated stacked ITZO CT-NVMs for the Al2 O3 tunneling layer with and without the H2 O2 -treatment, untreated (CT-NVM1), 5 min treated (CT-NVM2), and 10 min treated (CT-NVM3) devices, respectively.The field-effect mobility (μFE ), threshold voltage (Vth ), and subthreshold swing (S.S) are extracted from the initial sweep of the devices. It is clearly observed that the μFE , Vth , and S.S of CT-NVM3 (31.08 cm2 /V.s; 0.28 V; and 96 mV/dec) are larger than that of CT-NVM1 (27.6 cm2 /V.s; 1.24 V; 290 mV/dec), respectively. The S.S degradation of CT-NVM1 is related to excitation of defect traps in the Al2 O3 . Meanwhile the maximum areal density of states (Nmax s ) was calculated by the S.S value using equation mentioned in [28]. The Nmax for the CT-NVM1 samples is s 8.4 × 1011 cm−2 eV−1 , which was reduced to the value of 1.34 × 1011 cm−2 eV−1 , as calculated for the CT-NVM3 device, which indicates the excellent interface between ITZO and H2 O2 treated Al2 O3 layer for NVM devices. In order to observe the program characteristics, all CT-NVMs are programmed at 9 to 12 V for the duration of 1 ms from the initial state. When a positive voltage pulse is applied to the gate electrode across the Al2 O3 /SiOx /SiO2 dielectric stack during the programming process, it is expected that most electrons injected from the ITZO channel conduction band through the
IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 10, OCTOBER 2016
Fig. 4. (a) Schematic illustration of the electron trapping model under a programmed state of CT-NVMs. Retention properties extrapolation upto 10 yr of (b) CT-NVM1, (c) CT-NVM2, and (d) CT-NVM3 devices.
Al2 O3 tunneling layer will be trapped (via Fowler-Nordheim tunneling injection) in forbidden gaps of Si-rich SiOx . A lot of shallow and deep traps are existed in Si-rich SiOx matrix [29]. The extracted Vth shift (Vth ) values for the CT-NVM1, CT-NVM2, and CT-NVM3 were obtained 2.88, 2.65, and 2.58 V, respectively. To explain the Vth under programming state, a trapping model was considered as shown in Fig. 4(a). The higher memory window of CT-NVM1 indicates that electron trapping including in deep and shallow traps of SiOx (1) but also in traps located at the interfaces (2) OH− , H+ residues (3), and Vo sites (4) of bulk Al2 O3 layer, respectively. During the erasing process, the injected electrons are detrapped from the programmed state using white light [18] combined with external negative gate bias of −5V for 1 s. Li et al. reported that the detrapping speed are improved effectively under light-assisted erasing process [30]. The retention characteristics of the stacked ITZO NVMs are shown in Fig. 4(b)-4(d). After applying 12 V for 1 ms, the retention time was measured for 104 s and extrapolated upto 10 yr. The results ensure a remaining of ∼92% trapped charges after 10 yr, which corresponds to a memory window of 2.39 V for CT-NVM3 samples. Whereas, the memory window of the CT-NVM1 remains at 1.75 V corresponding to ∼61% stored charges upto 10 yr. Because, stored charges in the SiOx layer can flow along the defect sites in Al2 O3 tunnel layer, and the higher leakage current is the reason which limits its retention performance for CT-NVM1. This H2 O2 oxidation treatment has the advantages of cost-effective, less thermal budget, and ease of fabrication process for high quality Al2 O3 dielectric layers, which shows its applicability for flat-panel display devices. IV. C ONCLUSION In this study, a comparison of the Al2 O3 /SiOx /SiO2 stacked ITZO CT-NVM device with untreated and H2 O2 -treated Al2 O3 as a tunneling layer is discussed here. The optimum 10 min H2 O2 -treated Al2 O3 ITZO CT-NVM exhibited a higher μFE (31.08 cm2 /V.s), steeper S.S (96 mV/dec), smaller charge loss (∼8% after 10 yr), compared with untreated Al2 O3 ITZO CT-NVM (charge loss ∼39% after 10 yr) devices. These results are attributed to the H2 O2 -treated Al2 O3 dielectric with a lower leakage current density (5.24 × 10−8 A/cm2 ) and the hysteresis free C-V, resulting in higher retention probability for tunneling layer.
RAJA et al.: IMPROVED DATA RETENTION OF ITZO NVM BY H2 O2 TREATED Al2 O3 TUNNELING LAYER
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