A nonlithographic approach to produce self-assembled spatially separated Si structures for nanoelectronic applications was developed, employing the metal-induced silicon growth. Densely packed Si whiskers, 500–800 nm thick and up to 2500 nm long, were obtained by magnetron sputtering of Si on a 25 nm thick Ni prelayer at 575 °C. The nucleation of the NiSi2 compound at the Ni–Si interface followed by the Si heteroepitaxy on the lattice-matched NiSi2 is suggested to be the driving force for the whisker formation

Anderson, Wayne A.

Guliants, Elena A.

Ji, Chunhai

English

University of Dayton

University of DaytoneCommonsElectrical and Computer Engineering FacultyPublicationsDepartment of Electrical and ComputerEngineering2002Self-Assembly of Spatially Separated SiliconStructures by Si Heteroepitaxy on Ni DisilicideElena A. GuliantsUniversity of Dayton, eguliants1@udayton.eduChunhai JiState University of New York at BuffaloWayne A. AndersonState University of New York at BuffaloFollow this and additional works at: https://ecommons.udayton.edu/ece_fac_pubPart of the Computer Engineering Commons, Electrical and Electronics Commons,Electromagnetics and Photonics Commons, Optics Commons, Other Electrical and ComputerEngineering Commons, and the Systems and Communications CommonsThis Article is brought to you for free and open access by the Department of Electrical and Computer Engineering at eCommons. It has been acceptedfor inclusion in Electrical and Computer Engineering Faculty Publications by an authorized administrator of eCommons. For more information, pleasecontact frice1@udayton.edu, mschlangen1@udayton.edu.eCommons CitationGuliants, Elena A.; Ji, Chunhai; and Anderson, Wayne A., "Self-Assembly of Spatially Separated Silicon Structures by Si Heteroepitaxyon Ni Disilicide" (2002). Electrical and Computer Engineering Faculty Publications. 128.https://ecommons.udayton.edu/ece_fac_pub/128Self-assembly of spatially separated silicon structures by Si heteroepitaxyon Ni disilicideElena A. Guliants,a) Chunhai Ji, and Wayne A. AndersonDepartment of Electrical Engineering, State University of New York at Buffalo, 212 Bonner Hall,Amherst, New York 14260~Received 22 August 2001; accepted for publication 21 February 2002!A nonlithographic approach to produce self-assembled spatially separated Si structures fornanoelectronic applications was developed, employing the metal-induced silicon growth. Denselypacked Si whiskers, 500–800 nm thick and up to 2500 nm long, were obtained by magnetronsputtering of Si on a 25 nm thick Ni prelayer at 575 °C. The nucleation of the NiSi2 compound atthe Ni–Si interface followed by the Si heteroepitaxy on the lattice-matched NiSi2 is suggested to bethe driving force for the whisker formation. © 2002 American Institute of Physics.@DOI: 10.1063/1.1469205#I. INTRODUCTIONBecause of the continued scaling of microelectronic de-vices for very large scale integration ~VLSI! applications,low-dimensional silicon systems have been extensively stud-ied in recent years. Fabrication of Si nanostructures on thesize scale below 100 nm generated even greater interest afterthe discovery of their unique electronic and optical proper-ties. However, it poses significant challenges for the entiresemiconductor community from the technological perspec-tive. A number of various methods to synthesize individualSi structures have been reported to date. These methods canbe divided into two main categories depending on whetherthe formation of such structures occurs in a controlled orspontaneous manner. The vast majority of techniques, whichcan be classified as controlled fabrication methods, are basedon the artificial cluster formation in nanolithographically de-fined areas. The well-established approaches are optical1 orelectron-beam2,3 lithography followed by reactive ion1,2 orchemical3 etching. Many reported methods rely on the localoxidation of a Si wafer using microscope tip–substrateinteractions,4–6 among which scanning tunnelingmicroscopy5 and atomic force microscopy6 appeared to bethe most promising techniques. All these methods involvethe formation of nanostructured silicon on prefabricated Silayers. However, direct growth of Si wires through the win-dows in patterned SiO2 layers, or so-called local epitaxy, hasalso been proposed.7The self-assembly methods do not employ lithographictechniques and take advantage of structural disorder at theinterface between the two materials with different physicalproperties. Thus, the vapor–liquid–solid ~VLS! model forspontaneous Si whisker growth explains the function of me-tallic particles as growth catalysts.8,9 The lattice-matchedmetal–silicides were also shown to induce Si wire formationduring the excimer laser ablation of Si powder at hightemperatures.10–12 In other studies, nanometer-scale struc-tures formed spontaneously during Si-on-sapphire epitaxy13and in highly doped Si films caused by the randomly occur-ring fluctuations of the dopant concentration.14The two methodologies described above have certain ad-vantages and drawbacks. The lithographic methods are gen-erally considered to be superior since they can providesmaller variation in the cluster size and, therefore, a higheruniformity of the entire layer. On the other hand, the litho-graphically defined dots frequently reveal a characteristic in-stability when subjected to a subsequent chemical or thermaltreatment ~e.g., microscopic columns tend to coalesce,15etc.!, while self-organized structures initially assemble at en-ergetically favored sites. In addition, self-assembled growthtechniques allow easy fabrication without further processingsteps such as patterning before or after growth and etching,which is essential for reducing the technological cost.At present, there is a critical need for nonlithographictechniques to fabricate ordered nanostructures in a well-controlled way, with improved uniformity of size and higherdot density. These techniques can be developed by under-standing the physical mechanisms underlying the processesof whisker formation, which would establish unique relation-ships between the experimental conditions and the resultingstructural film properties. In previous publications,16,17 wedescribed the metal-induced formation of polycrystalline sili-con by depositing Si films on a thin Ni prelayer at;500– 600 °C. Silicon columnar grains grew heteroepitaxi-ally on the lattice-matched NiSi2 crystals nucleated at theNi–Si interface. This article reports that the modifications tothis technique can be used to produce individual Si struc-tures. A brief discussion will be offered on how the Si-on-NiSi2 heteroepitaxial processes and Si deposition kineticspossibly contribute to the whisker assembly process.II. EXPERIMENTThe starting substrate in our approach was an n-type~100! oriented single crystal Si ~c-Si! wafer, chosen forsmoothness, coated with a 300 nm thick plasma-enhancedchemical vapor deposited SiOx in order to provide a diffu-a!Present address: Taitech, Inc., AMC, P.O. Box 33630; Wright-PattersonAFB, OH 45433-06030; electronic mail: elena.guliants@wpafb.af.milJOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 9 1 MAY 200260770021-8979/2002/91(9)/6077/4/$19.00 © 2002 American Institute of Physics Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  131.238.108.131 On: Thu, 24 Mar2016 14:51:11sion barrier for Ni. The experiments carried out using c-Siwafers with different crystal orientation and resistivity, glassslides, and Mo sheets have led to the conclusion that none ofthe substrate properties except for surface morphology influ-enced the Si nanowire growth, possibly due to the interme-diate silica layer. A 25 nm thick Ni film was thermallyevaporated at a base pressure of ;231026 Torr. Silicon wasthen deposited by direct current ~dc! magnetron sputteringfrom a 0.006 V cm phosphorus-doped Si target at a substratetemperature of 575 °C and a magnetron power of 20 W,which provided a growth rate of 0.2 mm/h. The sputteringprocess took place in a 5%H2 /Ar mixture at a pressure of 1mTorr after an initial vacuum of better than 1027 Torr wasachieved. Ar served as a carrier gas, and H2 was used tosaturate dangling bonds in Si. Both the surface and crosssection of the resulting Si films were examined using a Hi-tachi S-4000 field emission scanning electron microscope~SEM! operated at 20 keV in the secondary electron mode.III. RESULTS AND DISCUSSIONThe surface morphology of the sample grown for 6 husing the above deposition conditions is shown in Fig. 1~a!.The entire surface of the sample is covered with denselypacked and relatively uniformly distributed whiskers. From ahigher magnification SEM image in Fig. 1~b!, the whiskershave a regular wire shape with a 500–800 nm cross-sectionaldiameter and are 500–2500 nm in length.The cross-sectional SEM images shown in Fig. 2 weretaken in order to provide mechanistic insights into thewhisker-growth process. Previously,17 we proposed the sce-nario for the Si epitaxy on NiSi2 in the case of Si depositiononto a Ni prelayer, which is as follows. At the beginning ofdeposition, sputtered Si atoms react with Ni to form Ni sili-cide. When Ni is completely consumed, Si saturates in theNiSi2 compound, the most energetically favorable silicidephase,18 with a cubic lattice structure and a lattice constant of5.416 Å ~0.4% lattice mismatch with Si!. As a result, the Siatoms that continue to arrive at the growth front attach to thealready established NiSi2 crystalline network more stronglythan to each other, completing uniform monolayers. We havealso shown previously that the Si overgrowth is essentiallyfree from Ni Ref. 16 due to the complete Ni consumption bythe stable NiSi2 compound, which leaves no atomic Ni avail-able for diffusion. The Si monolayers reproduce the surfacemorphology of the Ni disilicide crystals due to the heteroepi-taxial phenomenon, and the initial Si film growth follows anatomic layer-by-layer sequence.Although the precise mechanism of the whisker forma-tion is yet to be clarified, it is most likely related to theFIG. 1. SEM images of a Si sample grown on a 25 nm thick Ni prelayer at575 °C and a magnetron power of 20 W ~deposition rate of 0.2 mm/h! for 6h at low ~a! and high ~b! magnification.FIG. 2. Cross-sectional SEM images of the whiskers shown in Fig. 1.6078 J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Guliants, Ji, and Anderson Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  131.238.108.131 On: Thu, 24 Mar2016 14:51:11heteroepitaxial processes in the Si-on-NiSi2 system. Thegrains comprising the Si epilayer have a cubic lattice with alattice constant equal to that of NiSi2 , i.e., the Si film growsmechanically strained. It follows from the classical theory ofheteroepitaxy that the strain energy increases linearly withthe film thickness until the latter reaches some critical valuewhich is a function of the lattice mismatch between a filmand a substrate.19 Above this critical thickness, the strainenergy exceeds the sum of the substrate surface energy andthe interface energy and is released via rapid and spontane-ous structural changes. One of the most common ways forthe relaxation of a strained system is the introduction of mis-fit dislocations which lie at the interface between the sub-strate and epilayer, as was observed in the Stranski–Krastanov growth mode, previously adopted for the lattice-matched systems.20 It has also been established that misfitdislocations at the epilayer–substrate interface represent anideal source for stacking faults that propagate from the inter-face into the bulk of the growing layer.21 This, in turn, im-plies that the inverted pyramids seen in Fig. 2 are in fact thenuclei containing stacking faults, which coalesce with nor-mal nuclei and grow into the bulk of the film. The schematicof this process can be illustrated as in Fig. 3. When suchclusters exceed the surrounding grains in size, they begin togrow faster, extending into the growth ambient transverselyto the base of the substrate, and form spatially separatedindividual wire-shaped structures.On the other hand, no cluster development was observedat high levels of the magnetron power. Increasing the mag-netron power threefold, i.e., to ;60 W ~0.5 mm/h growthrate!, led to the formation of a dense polycrystalline Si filmwith a columnar structure.16,17 Although a high concentrationof stacking faults was introduced at the grain boundaries ~be-tween the columns!, no individual structures were formed onthe film surface. According to the VLS model,8 the morphol-ogy of a growing film is strongly dependent on the adatomsurface mobility. Whisker growth is initiated when the ada-tom diffusion length is sufficiently small to provoke the ada-tom trapping at the sites located near the nuclei. The diffu-sion length of adatoms is determined mainly by their kineticenergy, ambient pressure, and deposition rate. The adatomkinetic energy, in turn, consists of two portions: the initialkinetic energy, which depends on the atom acceleration tothe substrate in the electric or electromagnetic field, and thekinetic energy acquired due to the elevated substrate tem-perature. Reducing the dc power of the magnetron sputteringgun results in a corresponding almost linear decrease in theinitial kinetic energy of the silicon atoms which, in turn,limits the migration of the Si atoms on the growing filmsurface.21 Although a lower magnetron power also results ina lower deposition rate, this effect apparently is less pro-nounced. As follows from the discussion above, most of theincident sputtering flux is deposited on high points on thefilm, with little material reaching the valleys ~shadowing ef-fect!. The larger grains grow at the expense of the smallerones, becoming thicker closer to the surface as seen in Fig. 2.Despite the fact that the density of the synthesized Siwhiskers is relatively high, they are not suitable for devicefabrication in their present form. Such issues as the whiskersize and the pattern uniformity still need to be addressed.Because the quantum effect was demonstrated for the struc-tures with features smaller than 100 nm,22 further decrease inthe whisker cross-sectional diameter is needed. The mostconvenient and readily available technique to reduce the lat-eral dimensions of the Si whiskers is thinning of prefabri-cated structures by oxidation. Since the oxidation rate of sili-con and, respectively, the oxide thickness are easilycontrolled, this method may offer a simple way to producearrays of nanostructures with the desired size.IV. CONCLUSIONSIn summary, a method to fabricate spatially separatedindividual Si structures by Si deposition on a thin Ni prelayerwas developed. The Si whiskers produced are 500–2500 nmlong and 500–800 nm thick with the potential of furtherdecrease their dimensions by oxidation. The strain relaxationin the Si-on-NiSi2 heteroepitaxial system was proposed to bethe driving force for the whisker formation. In addition, thekinetics of Si deposition was suggested to contribute to thefinal structure.1 D. Ali and H. Ahmed, Appl. Phys. Lett. 64, 2119 ~1994!.2 E. Leobanhung, L. Guo, Y. Wang, and S. Y. Chou, Appl. Phys. Lett. 67,938 ~1995!.3 H. Namatsu, K. Kurihara, M. Nagase, and T. Makino, Appl. Phys. Lett.70, 619 ~1997!.4 Y. Takahashi, M. Nagase, H. Namatsu, K. Kurihara, K. Iwadate, Y. Naka-jima, S. Horiguchi, H. Murase, and M. Tabe, Electron. Lett. 31, 136~1995!.5 T. Ono, H. Saitoh, and M. Esashi, Appl. Phys. Lett. 70, 1852 ~1997!.6 B. Legrand and D. Stievenard, Appl. Phys. Lett. 74, 4049 ~1999!.FIG. 3. Progressive growth of a misoriented cluster containing stackingfaults propagating from a misfit dislocation in the Si-on-NiSi2 growth sce-nario.6079J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Guliants, Ji, and Anderson Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  131.238.108.131 On: Thu, 24 Mar2016 14:51:117 M. Shibata, S. S. Stoyanov, and M. Ichikawa, Phys. Rev. B 59, 10 2891999!.8 R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 ~1964!.9 A. M. Morales and C. M. Lieber, Science 279, 208 ~1998!.10 D. P. Yu, C. S. Lee, I. Bello, X. S. Sun, Y. H. Tang, G. W. Zhou, Z. G. Bai,Z. Zhang, and S. Q. Feng, Solid State Commun. 105, 403 ~1998!.11 Y. F. Zhang, Y. H. Tang, N. Wang, D. P. Yu, C. S. Lee, I. Bello, and S. T.Lee, Appl. Phys. Lett. 72, 1835 ~1998!.12 H. Z. Zhang, D. P. Yu, Y. Ding, Z. G. Bai, Q. L. Hang, and S. Q. Feng,Appl. Phys. Lett. 73, 3396 ~1998!.13 S. Yanangiya et al., Appl. Phys. Lett. 71, 1409 ~1997!.14 K. Morimoto, K. Araki, K. Yamashita, K. Morita, and M. Niwa, Appl.Surf. Sci. 117, 652 ~1997!.15 Y. Takeda, Y. Matsuoka, and T. Motohiro, J. Appl. Phys. 85, 6486 ~1999!.16 E. A. Guliants and W. A. Anderson, J. Appl. Phys. 87, 3532 ~2000!.17 E. A. Guliants and W. A. Anderson, J. Appl. Phys. 89, 4648 ~2001!.18 K. N. Tu, G. Ottaviani, U. Go¨sele, and H. Fo¨ll, J. Appl. Phys. 54, 758~1983!.19 J. W. Mathews, J. Vac. Sci. Technol. 12, 126 ~1975!.20 D. J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64, 1943 ~1990!.21 M. Ohring, in The Material Science of Thin Films, edited by M. Ohring~Academic, San Diego, CA 1992!.22 A. P. Alivisatos, Science 271, 933 ~1996!, and references therein.6080 J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Guliants, Ji, and Anderson Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  131.238.108.131 On: Thu, 24 Mar2016 14:51:11

Self-Assembly of Spatially Separated Silicon Structures by Si Heteroepitaxy on Ni Disilicide

https://ecommons.udayton.edu/cgi/viewcontent.cgi?article=1129&context=ece_fac_pub

Self-Assembly of Spatially Separated Silicon Structures by Si Heteroepitaxy on Ni Disilicide

Abstract

Similar works

Full text

Available Versions

University of Dayton