Pasquarello, A

Stoneham, AM

UCL Discovery

INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTERJ. Phys.: Condens. Matter 17 (2005) V1–V5 doi:10.1088/0953-8984/17/21/N01VIEWPOINTAtomically controlled interfaces for futurenanoelectronicsAlfredo Pasquarello1,2 and A Marshall Stoneham31 Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Theoretical Physics,CH-1015 Lausanne, Switzerland2 Institut Romand de Recherche Nume´rique en Physique des Mate´riaux (IRRMA),CH-1015 Lausanne, Switzerland3 Centre for Materials Research, Department of Physics and Astronomy, University CollegeLondon, Gower Street, London WC1E 6BT, UKE-mail: Alfredo.Pasquarello@epfl.ch and A.Stoneham@ucl.ac.ukReceived 19 April 2005Published 13 May 2005Online at stacks.iop.org/JPhysCM/17/V1Device miniaturization, speed of operation, and lower power demands are all responses toconsumer needs. These driving forces are at the origin of the spectacular development of themicroelectronic Si-based technology, characterized by an exponential scaling behaviour whichhas been persisting for over four decades [1].During this extended period of time, this technology has almost exclusively relied onthe extremely convenient physical properties of the Si–SiO2 interface, the interface betweencrystalline silicon and its thermal oxide [2, 3]. In the present generation, gate-oxide layers aregrown with thicknesses around 20 Å and with interfaces which are nearly abrupt at the atomicscale. Furthermore, specific nitrogen concentration profiles are engineered across the thin filmto control the diffusion of dopants [3]. Further scaling of the Si–SiO2 system is now preventedbecause of fundamental limits associated to leakage currents and reliability concerns [1].Overcoming these limitations will require the use of alternative gate-oxide materials, of higherdielectric constant (high-κ materials) than SiO2 [4]. The ability to control both the atomicstructure and composition of these new oxide layers is emerging as one of the major challengesfor the next generation of Si-based electronic devices. Controlling the atomic properties of thindielectric films is currently becoming a relevant issue also for the development of oxide-baseddevices with other functional properties. Recent reports on surprising magnetic propertiesin oxides, including room-temperature ferromagnetism with giant magnetic moments, offernew perspectives for the development of magneto-optic and spin-electronic devices that couldoperate in ambient conditions [5–7]. The fundamental nature of the dielectric response inthin-film ferroelectric perovskite oxides is currently being addressed in view of identifying keymaterials for the development of novel random access memory technology [8]. Other relevantdevelopments include the fabrication of field-effect transistors based on perovskite oxides asactive elements [9] and atomically engineered hetero-oxide interfaces showing unusual chargestates with surprisingly high carrier mobilities [10]. In all these developments, sophisticatedoxide films play the central role.0953-8984/05/210001+05$30.00 © 2005 IOP Publishing Ltd Printed in the UK V1V2 ViewpointThe current gate dielectric in Si-based devices is far from the typical system of condensedmatter physics. It is neither homogeneous nor isotropic, nor is the system in equilibrium,such that most properties can be obtained from an approximation to the state of lowest freeenergy. The processing and operating conditions introduce complexities, including the needto understand electronic excited states, quantum tunnelling, and the role of electron–phononcoupling near interfaces between media as different as silicon and oxides like amorphousSiO2 or crystalline HfO2. Their technological relevance for the microelectronic deviceindustry has ensured that semiconductor–oxide interfaces have physically been characterizedby a wide range of experimental techniques. In the case of the Si–SiO2 interface, theseinclude photoemission spectroscopy [11], ion-scattering [12–14], x-ray scattering [15, 16],electron-energy-loss spectroscopy [17], transmission electron microscopy [18], electron spinresonance [19], photodiffraction spectroscopy [20], optical reflectance spectroscopy [21], andnonlinear optical spectroscopy [22]. The complexity and the wealth of the underlying physicalphenomena set challenges to which the best theory and modelling should provide insight andguidance. At this specific time, the reach of the atomic-scale regime on the one hand andthe necessity of addressing new materials on the other hand are bringing to the forefront avariety of issues which call for a theoretical understanding. The relevance of these issuesis pushing new ideas of structure and atomic and electronic processes in condensed matterphysics and materials science, of which the impact might reach far beyond the research areaof microelectronic devices.In the current regime, the transition region at the interface between the semiconductorsubstrate and the gate oxide constitutes a significant fraction of the device calling for an atomic-scale understanding of the fundamental processes occurring at the interface. Such processesinclude the oxidation mechanism, the role of hydrogen, the defect generation process, the diffu-sion of dopants, and breakdown processes. As far as the modelling of such atomistic processesis concerned, there is still an enormous gap between the actual process going on in a processingstep and the computer simulation. Dynamic simulations are rare and not always helpful, giventhe large spread of relevant timescales. Furthermore, the theoretical treatment generally intro-duces several approximations, such as short times, small model systems, neglect of strain buildup, limited statistics, approximate structural models, equilibrium state, and zero temperature.Nevertheless, first-principles modelling approaches based on density functional theory haveincreasingly been applied to the study of such atomic processes [23–27]. However, the domainof application of such techniques remains limited unless combined with computationally lessexpensive methods. It is anticipated that the development of multiscale approaches spanningseveral time or length scales through the sequential use of increasingly simplified method-ologies will be the only way to address complex atomistic processes at semiconductor–oxideinterfaces [28–31]. For gate dielectrics, there has been strong progress on growth processes,yetthere has been relatively limited analysis of how modelling might lead to oxide with fewer fixedcharges or traps. The modelling of breakdown and routes to minimize degradation has beeneven more limited, with much of the work being phenomenological and statistical in nature.A successful replacement for SiO2 as gate oxide is subject to specific conditions of bandalignment at both the interface with the gate electrode and the interface with the siliconsubstrate. To identify promising materials, it appears imperative to acquire a predictivetheoretical tool for investigating band offsets at interfaces, including interface dipoles. Asstandard density functional calculations underestimate the band gap, their capability ofpredicting band offsets is questionable [32–34]. At this time, GW calculations [35] donot appear to offer a viable solution for carrying out systematic investigations, due to theirexcessively high computational cost. An alternative theoretical tool involving the same level ofcomplexity as ordinary total energy calculations is therefore highly desirable. Such an approachViewpoint V3appears within reach since simplified models based on the alignment of charge neutrality levelsoffer quite successful predictions [36]. The issue of aligning energy levels correctly alsoemerges when defect levels need to be situated with respect to the band extrema [37, 38].High-κ materials such as ZrO2 and HfO2 and their silicates are currently being consideredas promising alternatives to SiO2 [4]. As opposed to SiO2, these materials give rise toimportant concentrations of oxide defects [39–41], which contribute to degrading the electricalperformance of the device. Currently, an important research effort is being devoted to thecharacterization of these defects. Similarly, the presence of defects of unknown origin at SiC–SiO2 interfaces prevents the SiC-based technology from making progress [42]. Furthermore,the role that defects play in affecting properties such as the mobility [43] or the (soft)breakdown [44] represent great open issues. It is therefore clear that the identification ofdefects constitutes a major challenge for modelling approaches. When the defect centre isparamagnetic, electron spin resonance experiments provide indications on its atomic structureand may ultimately lead to its identification [45–47]. By contrast, diamagnetic centres offer nodirect information on their origin. Traditionally, theoretical investigations focus on crystallinesystems as parent structures for defect generation. Only recently disordered model structuresand their full statistical variety have been considered for the study of defects [28–30, 48].Another difficulty in modelling defect centres in amorphous environments originates from thefact that model defects are generally constructed on the basis of defect-free structures. Whilethis appears justified for defects in crystals, this procedure applied to amorphous materialsmight miss relevant defect structures.The search for alternative oxides motivates recent theoretical studies aiming at predictingthe dielectric permittivity [49–52]. While ordinary density functional calculations are verysuccessful in coping with crystalline materials, the study of amorphous systems often exceedspresent computational resources and requires the development of simplified schemes [49].Furthermore, it becomes increasingly evident that future gate stacks will be composed ofseveral atomically thin layers with physical properties differing significantly from their bulkcounterparts. The study of the permittivity of such stacks is driving new theoretical studies inwhich the local evolution of the permittivity is addressed [53, 54].The understanding of transport properties is another theme of paramount importance. Theway the inhomogeneities [43] and soft phonons in the oxide [55] affect the mobility clearlyhas direct implications on the electrical quality of the device. At present, the modelling oftransport properties in nanoelectronic systems is a growing research area in the theoreticalcommunity [56–58]. One might expect that new developments will find early application inthe Si-based research area, given the prominent role of electrical measurements in this field.As required functions of future nanoelectronic devices will diversify, different demandswill need to be satisfied, leading to radical materials challenges at least for some functions.Foreseeable devices integrated in standard Si-based chips include high-speed opticalmodulators [59, 60] and quantum gates for high-temperatureoperation [61]. Future electronicsmight even involve materials other than silicon. For instance, carbon-based materials areunder scrutiny for some functions [62], and germanium-based devices with high-κ gateoxides have been identified as alternatives to meet the severe scaling requirements of the nextgenerations [63, 64]. In perspective, as developments in nanoelectronics address an increasingrange of functions and materials, great opportunities lie ahead for theory and modelling to playa leading role.References[1] Schulz M 1999 Nature 399 729[2] Sofield C J and Stoneham A M 1995 Semicond. Sci. Technol. 10 215V4 Viewpoint[3] Green M L, Degraeve E P and Garfunkel E L 2001 J. Appl. Phys. 90 2057[4] Wilk G D, Wallace R M and Anthony J M 2001 J. Appl. Phys. 89 5243[5] Venkatesan M, Fitzgerald C B and Coey J M D 2004 Nature 430 630[6] Coey J M D, Venkatesan M and Fitzgerald C B 2005 Nat. Mater. 4 173[7] Hoa Hong N, Sakai J, Preiller W and Hassini A 2005 J. Phys.: Condens. Matter 17 1697[8] Saad M M, Baxter P, Bowman R M, Gregg J M, Morrison F D and Scott J F 2004 J. Phys.: Condens. Matter16 L451[9] Ueno K, Inoue I H, Yamada T, Akoh H, Tokura Y and Takagi H 2004 Appl. Phys. Lett. 84 3726[10] Ohtomo A and Hwang H Y 2004 Nature 427 423[11] Himpsel F J, McFeely F R, Taleb-Ibrahimi A, Yarmoff J A and Hollinger G 1988 Phys. Rev. B 38 6084[12] Feldman L C, Silverman P J, Williams J S, Jackman T E and Stensgaard I 1978 Phys. Rev. Lett. 41 1396[13] Gusev E P, Lu H C, Gustafsson T and Garfunkel E 1995 Phys. Rev. B 52 1759[14] Bongiorno A, Pasquarello A, Hybertsen M S and Feldman L C 2003 Phys. Rev. Lett. 90 186101[15] Awaji N, Ohkubo S, Nakanishi T, Sugita Y, Takasaki K and Komiya S 1996 Japan. J. Appl. Phys. 35 L67[16] Kosowsky S D, Pershan P S, Krish K S, Bevk J, Green M L, Brasen D, Feldman L C and Roy P K 1997Appl. Phys. Lett. 70 3119[17] Muller D A, Sorsch T, Moccio S, Baumann F H, Evans-Lutterodt K and Timp G 1999 Nature 399 758[18] Ross F M and Gibson J M 1992 Phys. Rev. Lett. 68 1782[19] Stesmans A and Afanas’ev V V 1998 J. Appl. Phys. 83 2449[20] Dreiner S, Schu¨rmann M and Westphal C 2004 Phys. Rev. Lett. 93 126101[21] Yasuda T, Yamasaki S, Nishizawa M, Miyata N, Shklyaev A, Ichikawa M, Matsudo T and Ohta T 2001Phys. Rev. Lett. 87 037403[22] Lu¨pke G, Bottomley D J and Vandriel H M 1993 Phys. Rev. B 47 10389[23] Pasquarello A, Hybertsen M S and Car R 1998 Nature 396 58[24] Stoneham A M, Szymanski M A and Shluger A L 2001 Phys. Rev. B 63 R241304[25] Kato K, Uda T and Terakura K 1998 Phys. Rev. Lett. 80 2000[26] Rashkeev S N, Fleetwood D M, Schrimpf R D and Pantelides S T 2002 Appl. Phys. Lett. 81 1839[27] Bongiorno A and Pasquarello A 2004 Phys. Rev. Lett. 93 86102[28] Bongiorno A and Pasquarello A 2002 Phys. Rev. Lett. 88 125901[29] Bongiorno A and Pasquarello A 2004 Phys. Rev. B 70 195312[30] Mukhopadhyay S, Sushko P V, Stoneham A M and Shluger A L 2004 Phys. Rev. B 70 195203[31] Watanabe T, Fujiwara H, Noguchi H, Hoshino T and Ohdomari I 1999 Japan. J. Appl. Phys. 38 L366[32] Giustino F, Bongiorno A and Pasquarello A 2004 Japan. J. Appl. Phys. 1 43 7895[33] Tuttle B R 2004 Phys. Rev. B 70 125322[34] Hobbs C C, Fonseca L R C, Knizhnik A, Dhandapani V, Samavedam S B, Taylor W J, Grant J M, Dip L G,Triyoso D H, Hegde R I, Gilmer D C, Garcia R, Roan D, Lovejoy M L, Rai R S, Hebert E A, Tseng H H,Anderson S G H, White B E and Tobin P J 2004 IEEE Trans. Electron Devices 51 971[35] Hybertsen M S and Louie S G 1986 Phys. Rev. B 34 5390[36] Robertson J 2002 Appl. Surf. Sci. 190 2[37] Blo¨chl P E 2000 Phys. Rev. B 62 6158[38] Gavartin J L, Shluger A L, Foster A S and Bersuker G I 2005 J. Appl. Phys. 97 053704[39] Houssa M, Stesmans A, Naili M and Heyns M M 2000 Appl. Phys. Lett. 77 1381[40] Houssa M, Afanas’ev V V, Stesmans A and Heyns M M 2001 Appl. Phys. Lett. 79 3134[41] Afanas’ev V V and Stesmans A 2002 Appl. Phys. Lett. 80 1261[42] McDonald K, Weller R A, Pantelides S T, Feldman L C, Chung G Y, Tin C C and Williams J R 2003 J. Appl.Phys. 93 2719[43] Bersuker G, Barnett J, Moumen N, Foran B, Young C D, Lysaght P, Peterson J, Lee B H, Zeitzoff P M andHuff H R 2004 Japan. J. Appl. Phys. 1 43 7899[44] Blo¨chl P E and Stathis J H 1999 Phys. Rev. Lett. 83 372[45] Cook M and White C T 1987 Phys. Rev. Lett. 59 1741[46] Stirling A, Pasquarello A, Charlier J C and Car R 2000 Phys. Rev. Lett. 85 2773[47] Stirling A and Pasquarello A 2002 Phys. Rev. B 66 245201[48] Martin-Samos L, Limoge Y, Crocombette J P, Roma G, Richard N, Anglada E and Artacho E 2005 Phys. Rev.B 71 014116[49] Rignanese G-M, Detraux F, Gonze X, Bongiorno A and Pasquarello A 2002 Phys. Rev. Lett. 89 117601[50] Rignanese G-M, Gonze X, Jun G C, Cho K J and Pasquarello A 2004 Phys. Rev. B 69 184301[51] Fiorentini V and Gulleri G 2002 Phys. Rev. Lett. 89 266101Viewpoint V5[52] Zhao X, Ceresoli D and Vanderbilt D 2005 Phys. Rev. B 71 085107[53] Giustino F, Umari P and Pasquarello A 2003 Phys. Rev. Lett. 91 267601[54] Giustino F and Pasquarello A 2005 Phys. Rev. B 71 144104[55] Fischetti M V, Neumayer D A and Cartier E A 2001 J. Appl. Phys. 91 546[56] Datta S 2000 Superlatt. Microstruct. 28 253[57] Demkov A A, Zhang X and Drabold D A 2001 Phys. Rev. B 64 125306[58] Di Ventra M and Lang N D 2002 Phys. Rev. B 64 045402[59] Liu A, Jones R, Liao L, Samara-Rubio D, Rubin D, Cohen O, Nicolaescu R and Paniccia M 2004 Nature 427615[60] Almeida V A, Barrios C A, Panepucci R R and Lipson M 2004 Nature 431 1081[61] Stoneham A M, Fisher A J and Greenland P T 2003 J. Phys.: Condens. Matter 15 L447[62] Stoneham A M 2004 Nat. Mater. 3 3[63] Meuris M, Delabie A, Van Elshocht S, Kubicek S, Verheyen P, De Jaeger B, Van Steenbergen J, Winderickx G,Van Moorhem E, Puurunen R L, Brijs B, Caymax M, Conard T, Richard O, Vandervorst W, Zhao C,De Gendt S, Schram T, Chiarella T, Onsia B, Teerlick I, Houssa M, Mertens P W, Raskin G, Mijlemans P,Biesemans S and Heyns M M 2005 Mater. Sci. Semicond. Proc. 8 203[64] Delabie A, Puurunen R L, Brijs B, Caymax M, Conard T, Onsia B, Richard O, Vandervorst W, Zhao C,Heyns M M, Meuris M, Vitanen M M, Brongersma H H, de Ridder M, Goncharova L V, Garfunkel E,Gustafsson T and Tsai W 2005 J. Appl. Phys. 97 064104

Atomically controlled interfaces for future nanoelectronics

http://discovery.ucl.ac.uk/129095/1/0953-8984_17_21_N01.pdf

Atomically controlled interfaces for future nanoelectronics

Abstract

Similar works

Full text

Available Versions

UCL Discovery