Huang, XD

Lai, PT

Leung, CH

Shi, RP

Microelectronics Reliability

X.D. Huang

R.P. Shi

C.H. Leung

P.T. Lai

Crossref

Charge-trapping characteristics of BaTiO3 with and without nitridation for nonvolatile memory applications

HKU Scholars Hub

Title Charge-Trapping Characteristics of BaTiO3 with and withoutNitridation for Nonvolatile Memory ApplicationsAuthor(s) Huang, XD; Shi, RP; Leung, CH; Lai, PTCitation Microelectronics Reliability, 2014, v. 54 n. 11, p. 2388-2391Issued Date 2014URL http://hdl.handle.net/10722/217021Rights© 2014. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/; This work is licensed under a Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 InternationalLicense. 1Charge-Trapping Characteristics of BaTiO3 with and without Nitridation for Nonvolatile Memory Applications X. D. Huang  Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, China R. P. Shi, C. H. Leung, and P. T. Lai  Department of Electrical & Electronic Engineering, the University of Hong Kong, Pokfulam Road, Hong Kong   Abstract: The charge-trapping characteristics of BaTiO3 with and without nitrogen incorporation were investigated based on Al/Al2O3/BaTiO3/SiO2/Si (MONOS) capacitors. The physical properties of the high-k films were analyzed by transmission electron microscopy and X-ray photoelectron spectroscopy. Compared with the MONOS capacitor with BaTiO3 as charge-trapping layer, the one with nitrided BaTiO3 showed higher program speed even at lower operating voltage (4.3 V at +8 V for 100 μs), better endurance property and smaller charge loss (charge loss of 10.6 % after 104 s at 85 oC), due to the nitrided BaTiO3 film exhibiting higher charge-trapping efficiency caused by nitrogen incorporation and suppressed leakage induced by nitrogen passivation.       ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ a) Electronic mail: laip@eee.hku.hk   21. Introduction     Metal-oxide-nitride-oxide-silicon (MONOS)-type flash memories with discrete traps in dielectric for charge storage are regarded as a promising candidate for next-generation nonvolatile memories, mainly due to their stronger scaling ability and higher reliability than their floating-gate counterparts. Si3N4 was the first dielectric used as the charge-trapping layer (CTL). The main shortcomings of Si3N4 as CTL lie in its low dielectric constant (k ~ 7) [1], which limits continually down-scaling of cell size; and poor data retention due to the small conduction-band offset (∆Ec = 1.1 eV) between Si3N4 and SiO2 [2]. Recently, extensive researches have been carried out to study high-k dielectrics instead of Si3N4 as CTL mainly due to their higher charge-trapping efficiency and stronger scaling ability, e.g. Gd2O3 (k ~ 14) [3], Y2O3 (k ~ 18) [4], HfON (k ~ 22) [5, 6], and ZrO2 (k ~ 37) [7]. Among various high-k dielectrics, perovskite-type dielectrics (e.g. SrTiO3, BaTiO3) are regarded as a promising material as charge-trapping layer [8, 9], mainly due to their high dielectric constant (k > 100 for SrTiO3) [10, 11], which can enhance the electric field across the SiO2 tunneling layer, thus improving program/erase (P/E) speeds at low operating voltage; as well as large ∆Ec (~ 3.2 eV for SrTiO3) with respect to SiO2 [1], which is desirable for data retention because the tunneling probability of electrons from the CTL to the substrate is inversely exponential to the barrier height under retention mode. Compared with SrTiO3, BaTiO3 even displays a larger ∆Ec (~ 3.6 eV) [1], which is helpful to further improve the retention performance. Moreover, nitrogen incorporation also has a profound impact on the charge-trapping characteristics of dielectrics because it can induce more deep-level traps in the band-gap [5][7, 8]. Also, nitrogen incorporation can improve the thermal stability of dielectrics and suppress the inter-diffusion of elements in the dielectric stacks [1][8]. Therefore, based on MONOS capacitors, this  3work aims to study the charge-trapping characteristics of nitrided BaTiO3 by comparing with pure BaTiO3.   2. Experiment               MONOS capacitors with Al/Al2O3/BaTiO3/SiO2/Si were fabricated on p-type silicon substrate. After the standard RCA (Radio Corporation of America, a standard clean procedure) cleaning, 2-nm SiO2 as tunneling layer (TL) was grown on the wafers by thermal dry oxidation. Then 10-nm BaTiO3 was deposited on the SiO2 by reactive sputtering using a BaTiO3 target in mixed Ar/N2 or Ar/O2 ambient, and the corresponding MONOS capacitors were denoted as the nitrided BaTiO3 sample and the BaTiO3 sample respectively. Next, 15-nm Al2O3 as blocking layer (BL) was deposited by atomic layer deposition using trimethyl-aluminum (Al(CH3)3) and H2O as precursors at 300 oC. Finally Al was evaporated and patterned as gate electrodes by wet etching, followed by forming-gas annealing at 300 oC for 20 min. The electrical characteristics were measured by HP4284A LCR meter and HP4156A semiconductor parameter analyzer. Their flatband voltage (VFB) was extracted from their measured capacitance-voltage curve at the capacitance equal to the calculated flatband capacitance.  3. Results and Discussion   Fig. 1(a) shows the Ti 2p XPS (X-ray photoelectron spectroscopy) spectrum for the samples with and without nitrogen incorporation. Compared with the BaTiO3 sample, the Ti 2p spectrum for the nitrided BaTiO3 sample shifts to lower binding energy by 0.2 eV due to nitrogen incorporation [7]. The nitrogen content is calculated to be 3.3 % from XPS analysis as shown in the inset of Fig. 1(a). Fig. 1(b) shows the  4Si 2p spectrum for the samples with and without nitrogen incorporation, which presents two intense peaks located at 99.7 eV and 103.2 eV, corresponding to the Si substrate and SiO2 tunneling oxide respectively.  Moreover, compared with the nitrided BaTiO3 sample, the BaTiO3 sample displays an obvious shoulder at 101.6 eV which agrees with the value for Ti silicate [12]. Fig. 2 shows the cross-sectional TEM (transmission electron microscopy) images of the samples. The nitrided BaTiO3 sample displays a negligible interlayer at the CTL/SiO2 interface compared with the BaTiO3 sample due to nitrogen passivation, which is consistent with the XPS observation.   Fig. 3(a) shows the program transient characteristics of the MONOS capacitors with and without nitrogen incorporation at various operating voltages. It is noted that the nitrided BaTiO3 sample has a similar equivalent oxide thickness (EOT, 11.6 nm) as the BaTiO3 sample (12.2 nm), which suggests a similar electric field across the SiO2 tunneling layer of both samples under the same operating voltage. As the operating voltage increases from +8 V to +12 V with a fixed pulse-width of 100 μs, the VFB shift (ΔVFB, defined as ΔVFB = VFB - VFB0, where VFB is the flatband voltage under stress, and VFB0 is the neutral flat-band voltage of the fresh device) increases from 2.22 V/4.33V to 4.55 V/7.16 V for the BaTiO3 and nitrided BaTiO3 samples respectively. It is worth mentioning that the nitrided BaTiO3 sample shows a similar ΔVFB (~ 4.33 V) even at a smaller gate voltage of +8 V as the BaTiO3 sample (~ 4.55 V) at +12 V for the same stress time of 100 μs, further supporting its much higher program speed. To gain more insight on this phenomenon, the gate leakage as a function of positive gate voltage (I-VG) is also shown in Fig. 3(b), which corresponds to electron injection from the substrate toward the gate electrode. The injected electrons from the substrate can be divided into two parts: one part flows  5through the dielectrics to the gate electrode, resulting in gate leakage; the other part is trapped in the dielectric, leading to VFB shift of the device. The higher gate leakage of the BaTiO3 sample indicates more electrons flowing from the substrate to the gate electrode rather than being trapped in the charge-trapping layer. This is consistent with its smaller ΔVFB (~ 5.12 V) after VG sweeping than that of the nitrided BaTiO3 sample (~ 7.17 V). Therefore, it is believed that the higher program speed of the nitrided BaTiO3 sample should be mainly due to its higher charge-trapping efficiency resulting from more deep-level traps induced by nitrogen incorporation [5][7, 8]. Moreover, unlike resistive random access memories (RRAM) [13], the I-VG curves in Fig. 3(b) for both samples show no switching phenomenon. This should be due to high quality of the SiO2 tunneling layer and the Al2O3 blocking layer with few traps as shown in the inset of Fig. 3(b), which blocks the formation of conductive filaments between the Al electrode and Si substrate. Fig. 3(c) shows the erase transient characteristics of the samples with and without nitrogen incorporation, where both samples are prepared with the same ΔVFB = +8 V for fair comparison before erasing. Both samples (especially the nitrided BaTiO3 sample) cannot return the fresh state and have no over-erase phenomenon even operated at high gate voltage (-14 V), suggesting that hole trapping is negligible and electron de-trapping is dominant under the erase operation. The nitrided BaTiO3 sample exhibits lower erase speed than the BaTiO3 sample under the same operating conditions, and needs higher operating voltage to achieve a similar ΔVFB (~ -7.3 V at -14 V, 1 s) as the latter (~ -7.2 V at -12 V, 1 s). This should be due to more deep-level traps (thus higher ratio of deep-level traps and shallow-level traps) in the nitrided BaTiO3 film; therefore, electrons trapped in these deep traps are more difficult to de-trap from the CTL to the substrate than  6those trapped in the shallow-level traps [6]. This inference can be further confirmed later by the retention property shown in Fig. 4.  Fig. 4 displays the retention characteristics of the MONOS capacitors with and without nitrogen incorporation measured at 85 oC. For fair comparison, both samples are programmed/erased to achieve a similar initial P/E memory window (5.2 V ~ 5.3 V). The VFB degrades with time during the retention mode, which is mainly due to the loss of charge stored in the charge-trapping layer via tunneling back to the substrate and gate electrode. The decay rate of the nitrided BaTiO3 sample is 117.5 mV/dec and 20.0 mV/dec under the program and erase states respectively. For comparison, the corresponding decay rate of the BaTiO3 sample is 310.0 mV/dec and 60.0 mV/dec. Also, as shown in the inset of Fig. 4, the normalized retained charge after 10 years is evaluated by extrapolation to be 56.3 % and 24.0 % for the samples with and without nitrogen incorporation respectively. The superior data retention of the nitrided BaTiO3 sample should be ascribed to more deep-level traps in the nitrided BaTiO3 film induced by nitrogen incorporation [5][7, 8]. In addition, nitrogen incorporation can block elemental inter-diffusion, thus leading to an excellent CTL/SiO2 interface with negligible formation of non-stoichiometric interlayer (see Fig. 2) [1][8]. This also contributes to better data retention of the nitrided BaTiO3 sample because the charge-loss path via defects located at the CTL/SiO2 interface by trap-assisted tunneling can be suppressed. This inference can be further confirmed by the I-V characteristics shown in Fig. 3(b), where the nitrided BaTiO3 sample displays lower gate leakage than the BaTiO3 one in the low-voltage range corresponding to the retention mode.  Fig. 5 shows the endurance characteristics of the MONOS capacitors with and without nitrogen incorporation, where P/E stressing conditions are chosen to achieve a similar initial P/E memory window for fair comparison. The initial P/E window of the  7nitrided BaTiO3 and BaTiO3 samples is 3.79 V and 3.94 V respectively. After a 105-cycle P/E stressing, the P/E window decreases to 3.65 V and 3.52 V, corresponding to a degradation of 3.7 % and 10.7 % for the nitrided BaTiO3 and BaTiO3 samples respectively. The better endurance of the nitrided BaTiO3 sample should be mainly ascribed to its higher P/E speeds at lower operating voltages resulting from the higher charge-trapping efficiency caused by nitrogen incorporation.    4. Conclusion     In summary, the charge-trapping characteristics of BaTiO3 film with and without nitrogen incorporation are investigated based on MONOS-type capacitors. The MONOS capacitor with nitrided BaTiO3 as CTL shows better electrical characteristics in terms of higher program speed, better endurance and smaller charge loss than the one without nitridation, because the nitrided BaTiO3 film exhibits higher charge-trapping efficiency induced by nitrogen incorporation and suppressed leakage through nitrogen passivation. Therefore, the nitrided BaTiO3 film is a promising charge-trapping layer for high-performance nonvolatile memory applications.            8Reference  [1] Wilk GD, Wallace RM, Anthony JM. High-κ gate dielectrics: Current status and materials  properties considerations. J Appl Phys 2001; 89:5243-75.  [2] Lin SH, Chin A, Yeh FS, McAlister SP. Good 150oC retention and fast erase charge-crapping  engineered memory with scaled Si3N4. In: IEDM Tech Dig; 2008. pp. 843-46. [3] Wang JC, Lin CT, Lai CS, Hsu JL. Nanostructure band engineering of gadolinium oxide        nanocrystal memory by CF4 plasma treatment.  Appl Phys Lett 2010;97:023513. [4] Pan TM, Yeh WW.  High-performance high-k Y2O3 SONOS-type flash memory. IEEE Trans  Electron Devices 2008; 55: 2354-60.  [5] Yang HJ, Cheng CF, Chen WB, Lin SH, Yeh FS, McAlister SP, Chin A. Comparison of        MONOS memory device integrity when using Hf1-x-yNxOy trapping layers with different N        compositions. IEEE. Trans on Electron Devices 2008;55:1417-20. [6] Wu JY, Chen YT, Lin MH, Wu TB. Ultrathin HfON trapping layer for charge-trap memory        made by atomic layer deposition. IEEE Electron Device Lett 2010;31:993-5.   [7] Wu YH, Chen LL, Lin YS, Li MY, Wu HC. Nitrided tetragonal ZrO2 as the charge-trapping        layer for nonvolatile memory application. IEEE Electron Device Lett 2009;30:1290-2. [8] Huang XD, Lai PT, Liu L, Xu JP. Nitrided SrTiO3 as charge-trapping layer for nonvolatile        memory applications. Appl Phys Lett 2011;98:242905. [9] Huang XD, Sin KO, Lai PT. BaTiO3 as charge-trapping layer for nonvolatile memory applications. Solid-State Electron 2013;79:285-9. [10] Chiang KC, Huang CC, Chen GL, Chen WJ, Kao HL, Wu YH, Chin A,  McAlister SP. High  performance SrTiO3 MIM capacitors for analog applications. IEEE Trans on Electron Devices 2006; 53:2312-19. [11] Pontes FM,  Escote MT, Escudeiro CC, Leite ER, Longo E, Chiquito AJ, Pizani PS, Varela JA. Characterization of BaTi1-xZrxO3 thin films obtained by a soft chemical spin-coating technique. J Appl Phys 2004; 96:4386-90. [12] Brassard D, Sarkar DK, El Khahani MA, Compositional effect on the dielectric properties of high-k titanium silicate thin films deposited by means of a cosputtering process, J Vac Sci Technol A 2006; 24:600-605.  9[13] Banerjee W, Maikap S, Chen YY, Yang JR,  D, Sarkar DK, El Khahani MA, Unipolar resistive switching memory characteristics using IrOx/Al2O3/SiO2/p-Si MIS structure, ECS Trans 2012; 45:345-348.                            10 Fig. 1. XPS spectra for the samples with and without nitrogen incorporation: (a) Ti 2p spectrum and (b) Si 2p spectrum. The inset of (a) is the N 1s spectrum.  456 458 460 462 464 466395 396 397 398  IntensityBinding Energy (eV)N 1s  IntensityBinding Energy (eV) Nitrided BaTiO3 BaTiO3Ti 2p (a) 98 100 102 104 106SilicateSi  IntensityBinding Energy (eV) Nitrided BaTiO3 BaTiO3SiO2Si 2p (b)   11 Fig. 2. Cross-sectional TEM images of the MONOS capacitors (a) with and (b) without nitrogen incorporation, where the factual thickness of the charge-trapping layer is 11.5 nm and 10.2 nm for the nitrided BaTiO3 and BaTiO3 samples respectively.       20 nm(a) (b)BaTiO3Si Al Al2O3Nitrided BaTiO3Si Al Al2O3SiO2 SiO2 12 Fig. 3. (a) Program transient characteristics of the MONOS capacitors with and without nitrogen incorporation as a function of pulse width at various gate voltages. (b) Gate leakage current as a function of positive gate voltage corresponding to electron injection from the substrate. The inset shows the capacitance-voltage (Cox-VG) hysteresis loops for the Al/Al2O3/SiO2/Si sample under ±6 sweeping voltage, and no hysteresis memory window indicates few traps in the SiO2 and Al2O3 layers. (c) Erase transient characteristics of the samples with and without nitrogen incorporation. The samples are programmed to be with the same ΔVFB = +8 V firstly before erase test.   10-4 10-3 10-2 10-1 100246810   +12 V  +10 V   +8 V V FB Shift (V)Time (s)Solid symbol:  Nitrided BaTiO3Open symbol: BaTiO3 (a) 0 2 4 6 8 1010-1010-910-810-710-6-6 -4 -2 0 2 4 60.00.20.40.60.81.0 +6 V->-6 V -6 V->+6 V  Normalized  CoxVG (V)  Leakage Current (A/cm2 )VG (V) Nitrided BaTiO3 BaTiO3   (b)  1310-4 10-3 10-2 10-1 100-8-7-6-5-4  -8 V  -12 V  -14 VSolid symbol: Nitrided BaTiO3Open symbol: BaTiO3  V FB Shift (V)Time (s)Fresh state (c)  14 Fig. 4. Retention characteristics of the MONOS capacitors with and without nitrogen incorporation measured at 85 oC. The inset shows normalized retained charges for the P/E states as a function of time.   100 101 102 103 104 105 106-2-10123100 102 104 106 108020406080100  Retained Charge (%)Time (s) Nitrided BaTiO3 BaTiO310 yearsEraseProgram  Nitrided BaTiO3  BaTiO3  V FB (V)Time (s)    15 Fig. 5.  Endurance characteristics of the MONOS capacitors measured at room temperature.  To achieve a similar initial P/E memory window, the P/E stress is +8 V, 100 μs/-8 V, 1 ms for  the nitrided BaTiO3 sample,  and +10 V, 100 μs/-10 V, 100 μs for the BaTiO3 sample  respectively.     100 101 102 103 104 105-2-10123Erase  V FB  (V)Number of P/E Cycles  Nitrided BaTiO3  BaTiO3Program    

Charge-Trapping Characteristics of BaTiO3 with and without Nitridation for Nonvolatile Memory Applications

http://hub.hku.hk/bitstream/10722/217021/1/Content.pdf

Charge-Trapping Characteristics of BaTiO3 with and without Nitridation for Nonvolatile Memory Applications

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub