HfTa-based oxide and oxynitride with or without Ta Ox Ny interlayer are fabricated on Ge substrate to form metal-oxide-semiconductor (MOS) capacitors. Their electrical properties and reliabilities are measured and compared. The results show that the MOS capacitor with a gate stack of HfTa-based oxynitride and thin Ta Ox Ny interlayer exhibits low interface-state/oxide-charge densities, low gate leakage, small hysteresis, small capacitance equivalent thickness (∼0.94 nm), and high dielectric constant (∼24). All these should be attributed to the blocking role of the Ta Ox Ny interlayer against penetration of O into the Ge substrate and interdiffusions of Hf, Ge, and Ta, thus effectively suppressing the formation of unstable low- k Ge Ox and giving a superior Ta Ox Ny Ge interface. Moreover, incorporation of N into both the interlayer and high- k dielectric greatly improves device reliability through the formation of strong N-related bonds. © 2008 American Institute of Physics.published_or_final_versio

Chan, CL

Guan, JG

Lai, PT

Li, CX

Xu, JP

Zhang, XF

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TitleImproved electrical properties of Ge metal-oxide-semiconductorcapacitor with HfTa-based gate dielectric by using TaOxNyinterlayerAuthor(s) Zhang, XF; Xu, JP; Li, CX; Lai, PT; Chan, CL; Guan, JGCitation Applied Physics Letters, 2008, v. 92 n. 26Issued Date 2008URL http://hdl.handle.net/10722/58775Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Improved electrical properties of Ge metal-oxide-semiconductor capacitorwith HfTa-based gate dielectric by using TaOxNy interlayerX. F. Zhang,1 J. P. Xu,1,a C. X. Li,2 P. T. Lai,2,a C. L. Chan,2 and J. G. Guan31Department of Electronic Science and Technology, Huazhong University of Science and Technology,Wuhan 430074, People’s Republic of China2Department of Electrical & Electronic Engineering, The University of Hong Kong, Pokfulam Road,Hong Kong, People’s Republic of China3State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Science and Technology, Wuhan 430070, People’s Republic of ChinaReceived 2 April 2008; accepted 11 June 2008; published online 2 July 2008HfTa-based oxide and oxynitride with or without TaOxNy interlayer are fabricated on Ge substrateto form metal-oxide-semiconductor MOS capacitors. Their electrical properties and reliabilitiesare measured and compared. The results show that the MOS capacitor with a gate stack ofHfTa-based oxynitride and thin TaOxNy interlayer exhibits low interface-state/oxide-chargedensities, low gate leakage, small hysteresis, small capacitance equivalent thickness 0.94 nm,and high dielectric constant 24. All these should be attributed to the blocking role of the TaOxNyinterlayer against penetration of O into the Ge substrate and interdiffusions of Hf, Ge, and Ta, thuseffectively suppressing the formation of unstable low-k GeOx and giving a superior TaOxNy /Geinterface. Moreover, incorporation of N into both the interlayer and high-k dielectric greatlyimproves device reliability through the formation of strong N-related bonds. © 2008 AmericanInstitute of Physics. DOI: 10.1063/1.2954012With the continuous scaling down of the dimensions ofmetal-oxide-semiconductor field-effect transistor MOS-FET, silicon is approaching its fundamental scaling limits.Germanium is an alternative candidate for future channelmaterial because of its high carrier mobility.1–3 However,unlike Si oxide, the thermodynamically unstable and solubleGe oxide hinders the development of high-quality Ge MOS-FET. Recently, Ge-based MOS capacitors and transistorswith high-permittivity high-k stacked gate dielectrics usinga thin germanium oxynitride GeOxNy interlayer haveshown superior electrical properties.4–6 It has been shownthat the GeOxNy interlayer provides a stable and smooth in-terface with improved electrical properties. However, itslower k value limits the scalability of Ge MOS devices, evenas an interlayer. More recently, Lu et al.7 reported that anultrathin tantalum nitride film deposited between high-k di-electric and Si substrate could suppress the formation of theSiOx interlayer during subsequent high-temperature anneal-ing. This is because tantalum nitride can function as a diffu-sion barrier and can be oxidized to form nonconductiveTaOxNy after annealing in O2. Moreover, Sugawara et al.8demonstrated that a thin plasma-synthesized TaON interlayeron Ge could reduce stress-induced flatband-voltage shift.Since TaOxNy has a high k value 26 and high thermalstability,9 it should also be a promising candidate as the in-terlayer between high-k dielectric and Ge substrate. In thiswork, a thin TaNx layer is deposited by reactive sputteringprior to HfTa-based oxide or oxynitride deposition. The thinTaNx is used to suppress the formation of inferior GeOx andinterdiffusions of relevant elements during the annealing ofthe deposited high-k dielectric, and at the same time TaNxitself is converted to TaOxNy during the annealing. Ta isintentionally added to improve the crystallization tempera-ture and dielectric constant of Hf-based oxide andoxynitride.10 As a result, excellent electrical properties suchas low interface-state density, low gate leakage, and highreliability are obtained for the Ge MOS capacitors withHfTaON /TaOxNy and HfTaO /TaOxNy gate stacks.N-type 100 Ge wafers with a resistivity of0.040–0.047  cm were cleaned in organic solvents andrinsed in de-ionized water followed by a 30 s diluted HF1:50 dipping for several cycles to remove the native oxideGeOx. After drying in N2, the wafers were immediatelytransferred into Denton Vacuum Discovery Deposition Sys-tem. First, a 1 nm TaNx interlayer was deposited by reac-tive sputtering of Ta in an Ar+N2 ambient Ar:N2=12:18,followed by the deposition of a 5 nm HfTaO or HfTaN bycosputtering of Ta and Hf in an Ar+O2 Ar:O2=24:3 orAr+N2 Ar:N2=24:6, a smaller ratio used to incorporatemore nitrogen in the dielectric ambient, respectively de-noted as HfTaO /TaON or HfTaON /TaON sample. Forcomparison, a 6 nm HfTa or HfTaN was directly depositedon the cleaned Ge substrate without the TaNx interlayer ascontrol samples denoted as HfTaO or HfTaON sample.A postdeposition annealing was carried out in wet N2500 ml /min at 500 °C for 5 min to transform the films intooxides or oxynitrides i.e., HfTaO, HfTaON, and TaOxNy byusing the oxygen in water vapor.11 The wet-N2 atmospherewas realized by bubbling pure N2 through de-ionized waterat 95 °C with a flow rate of 500 ml /min. Subsequently, Alwas evaporated and patterned by lithography as gate elec-trodes with an area of 7.8510−5 cm−2. Finally, forming-gasannealing was performed at 280 °C for 20 min. Physicalthicknesses of the gate dielectrics for the samples were de-termined by a multiwavelength ellipsometer. All electricalmeasurements were carried out under a light-tight and elec-trically shielded condition using HP4284A precision LCRaAuthors to whom correspondence should be addressed. Electronic ad-dresses: jpxu@mail.hust.edu.cn and laip@eee.hku.hk.APPLIED PHYSICS LETTERS 92, 262902 20080003-6951/2008/9226/262902/3/$23.00 © 2008 American Institute of Physics92, 262902-1Downloaded 11 Feb 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsmeter and HP4156A precision semiconductor parameteranalyzer.Typical HF 1 MHz C-V curves of the samples, sweptin bidirections at 1 MHz, are shown in Fig. 1. The oxidecapacitance Cox obtained by correcting for frequency dis-persion from maximum accumulation capacitance,12 physicalthickness tphys, capacitance effective thickness CET andequivalent k value =Coxtphys /0A for all the samples areextracted and listed in Table I. An obvious distortion is ob-served in the region from depletion to inversion of the C-Vcurves for the two samples without the TaOxNy interlayer,especially for the HfTaO sample, but does not exist for thetwo samples with a thin TaOxNy interlayer. This should beassociated with significant interfacial defects originated fromthe interdiffusion between the gate dielectric and substrate,and the growth of a low-k GeOx interlayer, especially for theHfTaO sample. Therefore, it can be concluded that theTaOxNy interlayer can effectively block the oxidation of Gesurface and thus suppress the formation of unstable GeOx.Another fact is that Cox of the oxynitrides is larger than thatof their oxide counterparts, indicating that larger k valueshave been obtained for the oxynitrides than the oxides foralmost the same physical thickness, with the largest k value24 and the smallest CET 0.94 nm for theTaHfON /TaOxNy sample. Furthermore, the C-V hysteresis,the equivalent oxide-charge density Qox=−CoxVfb−ms /q,where the work-function difference ms between Al gate andn-Ge substrate is calculated to be 0.0706 V and theinterface-state density near midgap Dit estimated from theHF C-V curve by the Terman’s method13 are depicted in Fig.2. As compared to the control samples, Dit is obviously re-duced by inserting a thin TaOxNy interlayer between thehigh-k gate dielectrics and Ge substrate, especially for theHfTaON /TaOxNy sample. Moreover, the hysteresis and Qoxare less for the two samples with the TaOxNy interlayer thanthe control samples, with HfTaON /TaOxNy sample beingthe smallest. The negative Qox might arises from OH−, whichis related to the wet-annealing ambient,14,15 the acceptorlikeinterface states and electron traps in the oxide since the oxidecharge density calculated from flatband voltage is a net effectof all charges in the oxide and at the interface. The smallerQox of HfTaO /TaON and HfTaON /TaON samples shouldbe partly attributed to their smaller Dit hence, less acceptor-like interface-state charges, as shown in Fig. 2. Further ex-amination of the data in Fig. 2 implies the HfTaO /TaON andHfTaON /TaON samples having more positive charges thanthe samples without the TaON interlayer because their Qoxreduction is only a portion of their Dit reduction. These ad-ditional positive charges could be introduced due to theHfTaO /TaON and HfTaON /TaON interfaces.The inset of Fig. 1 shows the gate-leakage properties ofthe samples. The two samples with the TaOxNy interlayerhave lower leakage current than their control samples.Moreover, the oxynitride samples have lower leakage currentthan their oxide counterparts, with the HfTaON /TaOxNysample being the lowest and HfTaO sample being the highest3.310−3 and 8.910−2 A cm−2 at Vg=Vfb+1 V, respec-tively. It was reported that the incorporation of nitrogen intohigh-k dielectrics could reduce their leakage current by in-hibiting species interdiffusion and changing the local coordi-nation of high-k material, thus suppressing the onset ofcrystallization.16 So, the smaller leakage current for the ox-ynitride samples should be ascribed to the N incorporation.In fact, this is closely associated with the Dit and Qox of thesamples shown in Fig. 2.A high-field stress at 10 MV /cm =Vg−Vfb / tphys for3600 s, with the capacitors biased at accumulation, is used toexamine the reliability of the samples. The leakage currentdensity Jg is measured for all the samples before and afterthe stress, as shown in Fig. 3. The Vfb shift after the stress isextracted from the HF C-V curves measured before and afterthe stress, as shown in the inset of Fig. 3. Increase of theleakage current and shift of flatband voltage after the stressare larger for the control samples than the samples with theTaOxNy interlayer because of an unstable GeOx layer grownat the interface of the former samples. Among all thesamples, the HfTaON /TaOxNy sample exhibits the best reli-ability due to the formation of strong N-related bonds byFIG. 1. Typical high-frequency C-V curves of the Ge MOS capacitors underdark condition at room temperature, swept in both directions at a frequencyof 1 MHz. Inset is gate leakage current density of the samples.TABLE I. Physical parameters extracted from the HF C-V curves of the GeMOS capacitors.Sample Cox pF tphys nm CET nm Equivalent kHfTaO /TaOxNy 253 5.86 1.07 21.4HfTaON /TaOxNy 289 5.83 0.94 24.2HfTaO 201 5.96 1.35 17.2HfTaON 240 6.06 1.13 20.9FIG. 2. Hysteresis, interface-state density and equivalent oxide-charge den-sity extracted from the HF C-V curves of the Ge MOS capacitors.262902-2 Zhang et al. Appl. Phys. Lett. 92, 262902 2008Downloaded 11 Feb 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionssignificant incorporation of N into both the interlayer andhigh-k gate dielectric.Therefore, the thin TaOxNy interlayer can give excellentdevice performances due to its strong barrier role againstdiffusions of Hf, Ta, Ge, and O, and its good interface prop-erties with the Ge substrate. Moreover, when the high-k di-electric layer is oxynitride, the electrical properties and reli-ability of the devices are further improved due to theformation of strong N-related bonds.The electrical properties and reliability of HfTa-basedoxide and oxynitride with or without TaOxNy interlayer areinvestigated as the gate dielectric of Ge MOS capacitors. Thethin TaOxNy interlayer can effectively block the interdiffu-sions of Ge, Hf, and Ta and also the penetration of oxygeninto the Ge substrate, thus greatly suppressing the growth ofunstable low-k GeOx during high-temperature annealing, andimproving the interface quality and reliability of the devices.Low interface-state density and low leakage current havebeen achieved for the devices with HfTaON /TaOxNy orHfTaO /TaOxNy gate stack. Particularly, the sample withHfTaON /TaOxNy gate stack exhibits higher dielectric con-stant 24 and better high-field reliability than the othersamples while keeping small CET 0.94 nm, Dit, Qox, andlow leakage current. Therefore, the HfTaON /TaOxNy stackstructure is a promising gate dielectric for making advancedsmall-sized Ge MOSFET with excellent electrical perfor-mances and high reliability.This work is financially supported by the National Natu-ral Science Foundation of China Grant No. 60776016,Small Project Funding 200707176147 of the University ofHong Kong, the University Development Fund Nanotech-nology Research Institute, 00600009 of the University ofHong Kong, and Open Foundation of State Key Laboratoryof Advanced Technology for Materials Synthesis and Pro-cessing Project No. WUT2006M02.1A. Ritenour, S. Yu, M. L. Lee, N. Lu, Wu. Bai, A. Pitera, E. A. Fitzgerald,D. L. Kwong, and D. A. Antoniadis, Tech. Dig. - Int. Electron DevicesMeet. 2003, 18.2.1.2P. Zimmerman, G. Nicholas, B. De Jaeger, B. Kaczer, A. Stesmans, L.-A.Ragnarsson, D. P. Brunco, F. E. Leys, M. Caymax, G. Winderickx, K.Opsomer, M. Meuris, and M. M. Heyns, Tech. Dig. - Int. Electron DevicesMeet. 2006, 655.3S. Joshi, C. Krug, D. 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Electron Devices 46, 1500 1999.13E. H. Nicollian and J. R. Brews, MOS Physics and Technology Wiley,Hoboken, NJ, 1982, p. 325.14J. P. Xu, P. T. Lai, C. X. Li, X. Zou, and C. L. Chan, IEEE Electron DeviceLett. 27, 439 2006.15H. Yano, F. Katafuchi, T. Kimoto, and H. Matsunami, IEEE Trans. Elec-tron Devices 46, 504 1999.16P. S. Lysaght, J. Barnett, G. I. Bersuker, J. C. Woicik, D. A. Fischer, B.Foran, H. H. Tseng, and R. Jammy, J. Appl. Phys. 101, 024105 2007.FIG. 3. Gate-leakage increase and flatband-voltage shift insrt of the GeMOS capacitors after a high-field stressing at 10 MV /cm for 3600 s, withthe samples biased at accumulation.262902-3 Zhang et al. Appl. Phys. Lett. 92, 262902 2008Downloaded 11 Feb 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Improved electrical properties of Ge metal-oxide-semiconductor capacitor with HfTa-based gate dielectric by using TaOxNy interlayer

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Improved electrical properties of Ge metal-oxide-semiconductor capacitor with HfTa-based gate dielectric by using TaOxNy interlayer

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