Amaral, V S

Araùjo, J P

Carvalho, I

Cavalcante, F H M

Correia, J G

Gomez, M R

Gonçalves, J N

Lopes, A M L

Lourenço, A

Mendonça, T M

Oliveira, G N

Pereira, A M

Peres, M B

Redondo, L

Silva, M R

Soares, J C

Sousa, C T

Ventura, J O

CERN Document Server

CERN-INTC-2010-013/INTC-I-08725/01/2010New insights in Metal-Oxide junctions for nano-electronic applications.  A.M.L. Lopes*, M.R. Gomes, F.H.M Cavalcante, G.N. Oliveira, M.R. Silva, L. Redondo, J.C. Soares Universidade de Lisboa, Centro de Física Nuclear, Portugal  J. G. Correia* ITN, Sacavém, Portugal and ISOLDE-CERN  J.O. Ventura, A.M. Pereira, C.T. Sousa, T.M. Mendonça, J.P. Araújo* Universidade do Porto, Faculdade de Ciências, IFIMUP, Portugal  A. Lourenço, M.B. Peres, J.N. Gonçalves, I. Carvalho, V.S. Amaral Universidade de Aveiro, Departamento de Física, CICECO, Portugal  Introduction and Motivation  The development of sustainable technological solutions conformant to the current low cost, high performance and environmentally friendly paradigm is a major challenge in scientific research [1]. Driven by such concerns some fields of research are dedicated to the study of new nanostructure architectures that will allow accomplishing the current technological necessities [1- 4]. One of such examples is the research in metal-oxide nanostructures that have a crucial role in several areas of application ranging from energy conversion and storage (e.g. fuel cells and hydrogen production), thermal barrier coatings, catalysis to microelectronic applications [1-3]. In the latter, metal-oxide junctions arise as a convenient solution to overcome the “dichotomy” between the continued scaling down imposed by the industry and fundamental limitations imposed by physics, as directly evidenced by the recently introduced Metal-high-k Oxide-Semiconductor (MOS) Field Effect Transistors (FETs) [3, 4] and also the promising Resistive Metal-Insulator (MIM) non-volatile Random Access Memories (ReRAMs) [7]. In fact the scaling down on MOS-FETs obliged the replacement of SiO2 oxide gate by high dielectric constant oxides (high-k) and the polycrystalline-Si gate has now to be replaced by metals gates [3, 4]. Thus, chemical stable metal gates with the adequate effective work function (EWF) have to be found [4]. In particular, the gate's EWF should equal the conducting band (CB) or the balance (VB) energy of the Si channel to create n-type (~4.2 eV) or p- type (~5.2 eV) FET, respectively. This band alignment modulates the voltage needed to turn the transistor ON, thus it is crucial for the transistor performance. This, however, is not an easy task because despite the nominal compatibility of the Si band edges with metal WF, band misalignment is being systematically observed when the metal is put in contact with high-k dielectric/Si stack, especially after the common post-deposition annealing treatments [5, 6, 10, 11]. To overcome this problem, deep understanding of the fundamental physics behind band misalignment is necessary. It has been suggested that band misalignment is connected with intrinsic effects such as the pinning of the metal/high-k oxide Schottky Barrier (SB), nevertheless theoretical calculations have shown that interface SB barrier is unpinned [8, 15]. Thus, extrinsic effects such point defects/vacancies should play an important role here. In fact, some authors pointed out that oxygen vacancies created in the oxide layer due to oxygen transfer from the high-k oxide to the metal and, consequently, charge transfer from the vacancy level∗ to the metal can justify band misalignment [10]. An objection to this mechanism is the large formation energy of oxygen                                                         ∗ Spokesperson:  A.M.L.Lopes              Co‐spokesperson: J.P. Araújo     Contact person: J.G. Correia  vacancies. Also, experimental studies in Ru/(HfO2, Al2O3 or SiO2) have shown that although neither reduction nor oxidation of the oxide and metal gates was observed, all studied stacks evidenced a clear shift of the band edges [6]. Other models involve reaction at high-k oxide/Si interface and band bending at the metal oxide interface [9]. This illustrates that despite several studies, the fundamental physics behind band misalignment is still not fully understood, and the fine-tuning of the gate effective work function is still a great challenge [4, 10, 11]. Nevertheless alternative solutions, that include the implantation of dopants in the gate stack [12, 13,14], are being tried. Experimental tests have been performed and, for example, it was found that Te-doped metal/ high-k oxide junction meets the n-MOSFET conditions [13]. Aiming to predict the band shift dependence on the chemical nature of dopant and its lattice location, first principles calculations are being performed to help in the design of metal/high-k stacks [12, 13, 14].  More over, high- k oxides (HfO2, Al2O3…) are also being tested for capacitor dielectrics in MIM Resistive RAMs, where band offset, metal/oxide/metal stack stability, vacancies and point defects are of crucial important. The major difficulty in characterizing the mechanism responsible for band misalignment and resistive switching in metal-oxide nanostructures is that the corresponding active regions are extremely small and hidden under the metallic contact layer. To overcome this problem and achieve a comprehensive understanding of these phenomena an adequate description of the structural and electronic properties down to atomic length-scales, in the different components of the metal-oxide structures, is required. Perturbed angular correlations (PAC) being a local and very sensitive technique, appears as “ideal” to give us new insights in metal-oxide structures relevant for nanoelectronics applications.  Proposal Summary  Here we intend to disclose the reasons behind band misalignment (oxygen vacancies, defects, interface dipoles, etc) and the role of doping on the band shift control. In order to accomplish these objectives the research will focus on the structural and electronic local environment of selected elements existing in the stack. Thus the systems under investigation will be thorough analyzed by PAC spectroscopy and the results will be correlated with band energy studies obtained using x-ray photoemission spectroscopy (XPS) and Electron Energy Loss Spectroscopy (EELS).  More specifically, at this point, the studies to be performed at ISOLDE would complement our PAC studies performed at CFNUL/Lisbon. In the proposed test experiment we intend to study two simple stacks, namely, Ag/HfO2 and Au/HfO2 structures (Ag and Au are, in this context, “n and p-type metals”, respectively), although several other relevant metals (Pt, Ir, Ru, Re among others), for which PAC probes and the necessary ISOLDE beams exist, but would be tried only after successful accomplishment of this test experiment.  The adopted methodology will be the following: HfO2 layers will be studied at CFNUL Lisbon, where the Hf in the oxide layer can be neutron activated (181Hf). Then, to study possible defects or oxidation of the Ag and Au metal layers and the La doping of the oxide layer ISOLDE beams, with appropriate PAC probes, will be used (111Ag, 197mHg and 140La).  To be able to have a successful test experiment each metal (Ag, Au) should be investigated into two different HfO2 film thicknesses previously submitted to different annealing procedures (to intentionally induce vacancies in the oxide film). Remarks that we were already able monitor the EFG of oxygen vacancies in the HfO2 thin film matrix (see fig. 1) To perform the necessary tests we request a total of four shifts (see table 1) in co-sharing with solid-state teams at ISOLDE. All isotopes will be collected in the general-purpose implantation chambers at GLM and the number of implanted atoms per sample will range from 1010 up to 1011. The post-deposition annealing treatments and PAC measurements will performed in the Solid State Lab.      Right) R(t) and Fourier transforms experimental functions and correspondent fits on HfO2 thin film with oxygen vacancies. The typical HfO2 frequency and the frequency corresponding to the O vacancies are observed. Left) Reflectometry spectra from the prepared HfO2 film on silicon substrate. Film roughness: 0.1nm and thickness: 5.9nm (different thickness also produced for this work) [Patent: PPP 40063/09 – Univ. Aveiro].  Table I: Beam time request Required isotope Implanted beam PAC experiment Intensity [at/µC] Target / ion source nº of shifts 111Ag 111Ag γ−γ 5E7 UC2, laser Ag 1 197mHg 197mHg/197Hg e−γ 2E8 Molten Pb/plasma 2 140La 140Cs  (also possible with  140LaF+ or 140LaO+) γ−γ 1E8 UCx/W  (with option CF4) 1  We expect that the results obtained in this way, once complemented with band energy studies, will allow us to prove that relevant information can be extracted using PAC and set strategies for future studies.   References 1 Nanoscale design to enable the revolution in renewable energy, J. Baxter, Z. Bian, et. al, Energy Environ. Sci., 1-53, 2009 3 Is Silicon’s Reign Nearing its end? Robert F. Service, Science, 323, 1000, 2009 4 Integrated nanoelectronics for the future, R. Chau, B. Doyle, S. Datta, J. kavalieros and kevin Zhang, Nature materials 6, 810, 2007  5 Effects of annealing on the valence band offsets between hafnium aluminate silicon, S.Y. Chiam, et. al., J. of Appl. Phys. 104, 063714 (2008)  6 Band offsets of a ruthenium gate on ultrathin high-k oxide films on silicon, S. Rangan, E. Bersch, et. all. Phys. Rev. B 79, 075106 (2009) 7 Redox-Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges, Rainer Waser, Regina Dittmann, Georgi Staikov, and Kristof Szot, Adv. Mater., 21, 2632, 2009 8 Control of Schottky Barrier Heights on High-K Gate Dielectrics for Future Complementary Metal-Oxide Semiconductor Devices, K. Y. Tse and J. Robertson, Phys. Rev. Lett. 99, 086805 (2007) 9 Experimental evidence for the flatband voltage shift of high-k metal-oxide-semiconductor devices due to the dipole formation at the high-k/SiO2 interface, K. Iwamoto, Y. Kamimuta, A. Ogawa, et al., Appl. Phys. Lett. 92, 132907 (2008) 10 Extended Frenkel pairs and band alignment at metal-oxide interfaces, O. Sharia, K. Tse, J. Robertson, et. al, Phys. Rev. B 79, 125305 (2009) 11 Thermodynamic stability and band alignment at a metal–high-k dielectric interface, A.A. Demkov, Phys Rev B. 74, .085310 (2006) 12 Effect of Al-diffusion-induced positive flatband voltage shift on the electrical characteristics of Al-incorporated high-k metal-oxide-semiconductor field-effective transistor, Wenwu Wang, Koji Akiyama, Wataru Mizubayashi, et al., J. of Appl. Phys. 105, 064108 (2009) 13 Te-induced modulation of the Mo / HfO2 interface effective work function Ka Xiong, Pietro Delugas, Jacob C. Hooker, Vincenzo Fiorentini, John Robertson, Dameng Liu, and Geoffrey Pourtois, Appl. Phys. Lett. 92, 113504 (2008) 14 Passivation of oxygen vacancy states and suppression of Fermi pinning in HfO2 by La addition D. Liu and J. Robertsona, Appl. Phys. Lett. 94, 042904 (2009) 15 Fermi level pinning by defects in HfO2-metal gate stacks, J. Robertson, O. Sharia and A. Demkov, Appl. Phys. Lett. 91, 132912 (2007) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0110102103104105106Thickness : 59ÅDensity:      11.33g/cm3Roughness:   1Å I nt en si ty  ( a. u. )Theta!"Degrees)HF1c : HfO2 on Silicon Substrate

New insights in Metal-Oxide junctions for nano-electronic applications

New insights in Metal-Oxide junctions for nano-electronic applications

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