Availableonlineatwww.sciencedirect.com Reports: A Review ournal .. Science Direct ELSEVIER R Materials Science and Engineering R 60(2008)1-51 wwelsevier.com/locate/mser Growth of nanowires N. Wang ,, Y Cai, R.Q. Zhang Department of Physics and the institute of Nano Science and Technology, the Hong Kong University of Science and Technology, Hong Kong, China Center of Super-Diamond and Advanced Films(COSDAF)& Department of Physics and Materials Science City Universiry of Hong Kong, Hong Kong, Chin Available online 5 March 2008 Abstract The tremendous interest in nanoscale structures such as quantum dots(zero-dimension)and wires(quasi-one-dimension )stems from their size- dependent properties. One-dimensional (1D) semiconductor nanostructures are of particular interest because of their potential applications in nanoscale electronic and optoelectronic devices. For ID semiconductor nanomaterials to have wide practical application, however, several areas require further development. In particular, the fabrication of desired ID nanomaterials with tailored atomic structures and their assembly into functional devices are still major challenges for nanotechnologists. In this review, we focus on the status of research on the formation of nanowire structures via highly anisotropic growth of nanocrystals of semiconductor and metal oxide materials with an emphasis on the structural characterization of the nucleation, initial growth, defects and interface structures, as well as on theoretical analyses of nanocrystal formation, reactivity and stability. We review various methods used and mechanisms involved to generate lD nanostructures from different material systems through self-organized growth techniques including vapor-liquid-solid growth, oxide-assisted chemical vapor deposition (without a metal catalyst), laser ablation, thermal evaporation, metal-catalyzed molecular beam epitaxy, chemical beam epitaxy and hydrothermal reaction. ID nanostructures grown by these technologies have been observed to exhibit unusual growth phenomena and unexpected properties, e.g., diameter dependent and temperature-dependent growth directions, structural transformation by enhanced photothermal effects and phase transformation induced by the point contact reaction in ultra-thin semiconductor nanowires. Recent progress in controlling growth directions, defects, interface structures, structural transformation, contacts and hetero- junctions in lD nanostructures is addressed. Also reviewed are the quantitative explorations and predictions of some challenging ID nanostructures and descriptions of the growth mechanisms of ID nanostructures, based on the energetic, dynamic and kinetic behaviors of the building block nanostructures and their surfaces and/or interfaces C 2008 Elsevier B V. All rights reserved Contents Introduction 2 2. Growth technologies for nanowires 2.1. Vapor-liquid-solid(VLS)technique 2. 2. Laser-assisted growth 5 2.3. Thermal evaporation 6 4. Metal-catalyzed molecular-beam epita 2.5. Solution method 3. Growth mechanisms of nanowires 9 3. 2. Vapor-solid growth 14 3.2.1. Internal anisotropic surfaces 3.2.2. Crystal defects 3.2.3. Self-catalytic growth front matter C 2008 Elsevier B V. All rights reserved. 10.1016 j.mser.2008.01.001
Growth of nanowires N. Wang a, *, Y. Cai a , R.Q. Zhang b a Department of Physics and the Institute of Nano Science and Technology, the Hong Kong University of Science and Technology, Hong Kong, China bCenter of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China Available online 5 March 2008 Abstract The tremendous interest in nanoscale structures such as quantum dots (zero-dimension) and wires (quasi-one-dimension) stems from their sizedependent properties. One-dimensional (1D) semiconductor nanostructures are of particular interest because of their potential applications in nanoscale electronic and optoelectronic devices. For 1D semiconductor nanomaterials to have wide practical application, however, several areas require further development. In particular, the fabrication of desired 1D nanomaterials with tailored atomic structures and their assembly into functional devices are still major challenges for nanotechnologists. In this review, we focus on the status of research on the formation of nanowire structures via highly anisotropic growth of nanocrystals of semiconductor and metal oxide materials with an emphasis on the structural characterization of the nucleation, initial growth, defects and interface structures, as well as on theoretical analyses of nanocrystal formation, reactivity and stability. We review various methods used and mechanisms involved to generate 1D nanostructures from different material systems through self-organized growth techniques including vapor–liquid–solid growth, oxide-assisted chemical vapor deposition (without a metal catalyst), laser ablation, thermal evaporation, metal-catalyzed molecular beam epitaxy, chemical beam epitaxy and hydrothermal reaction. 1D nanostructures grown by these technologies have been observed to exhibit unusual growth phenomena and unexpected properties, e.g., diameterdependent and temperature-dependent growth directions, structural transformation by enhanced photothermal effects and phase transformation induced by the point contact reaction in ultra-thin semiconductor nanowires. Recent progress in controlling growth directions, defects, interface structures, structural transformation, contacts and hetero-junctions in 1D nanostructures is addressed. Also reviewed are the quantitative explorations and predictions of some challenging 1D nanostructures and descriptions of the growth mechanisms of 1D nanostructures, based on the energetic, dynamic and kinetic behaviors of the building block nanostructures and their surfaces and/or interfaces. # 2008 Elsevier B.V. All rights reserved. Contents 1. Introduction . .................................................................................. 2 2. Growth technologies for nanowires . . ................................................................. 3 2.1. Vapor–liquid–solid (VLS) technique . ............................................................. 3 2.2. Laser-assisted growth . . . ..................................................................... 5 2.3. Thermal evaporation ......................................................................... 6 2.4. Metal-catalyzed molecular-beam epitaxy . . ......................................................... 7 2.5. Solution methods . .......................................................................... 9 3. Growth mechanisms of nanowires . . . ................................................................. 9 3.1. Metal-catalyzed growth . . ..................................................................... 9 3.2. Vapor–solid growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.1. Internal anisotropic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.2. Crystal defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.3. Self-catalytic growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 www.elsevier.com/locate/mser Available online at www.sciencedirect.com Materials Science and Engineering R 60 (2008) 1–51 * Corresponding author. Tel.: +852 2358 7489; fax: +852 2358 1652. E-mail address: phwang@ust.hk (N. Wang). 0927-796X/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mser.2008.01.001
N. Wang et al /Materials Science and Engineering R 60(2008)1-51 3.3. Oxide-assisted growth 3.3.1. Kinetics and reactivity of silicon oxide in nucleation and growth 3.3.2. Effect of defects in ID growth 3.3.3. Effect of extermal electrical field in lD growth 3. 4. Self-assembly growth from solution 3.4. 1. Solution-liquid-solid (SLS) growth from seeds 9000 3.4.2. Self-assembly oriented attachment growth 3.4.3. Anisotropic growth of crystals by kinetic control 4. Controlled growth of nanowires 4. 1. Control of structures, growth direction and defects in nanowires 4.1.1. Interface structures 4. 1.2. The growth direction of VLS nanowires 4. 1.3. Defects in 4.1.4. From nanowire to nanoribbon 4.2. Structural transformation in nanowires 4.2.1 Surface relaxation and saturation of zinc oxide nanowires 4.2. 2. The stability of Si nar 4.2.3. Optical rapid annealing effect 4.3. Contacts and heterostructures in nanowires 23832245779 4.3.1. Metal-semiconductor contacts 4.3. 2. Heterostructures in nanowires 5. Other challenging nanowire structur 5.1. Non-tetrahedral Si nanowires 5.2. Oxide nanowires 5.2.1. Silicon oxide nanowires 5.2.2. Silicon dioxide tube-like nanowires 5.2. 3. Zinc oxide tube-like nanowire 46 6. Concluding remarks Acknowledgements 1. ntroduction nanostructures formed"naturally"(also called self-organized growth)without the aids of ex situ techniques, such as chemical In the physics structures, quantum effects play etching, are desirable not only in fundamental research but also n increasingly role [1]. Quantum wires have in future nanodevice design and fabrication demonstrated int trical transport properties that are In this paper, various novel technologies for synthesizing not seen in bulk his is because, in quantum wires, nanowires are reviewed. A well-known self-organized growth electrons could be quantum-confined laterally and thus could mechanism for creating nanowires is the vapor-liquid-solid occupy discrete energy levels that are different from the energy (VLS) process(also known as metal catalytic growth [5]). This bands found in bulk materials. Due to low electron density and technique can produce free-standing crystalline nanowires of low effective mass, the quantized conductivity is more easily semiconductor and metal oxide materials with fully controlled observed in semiconductors, e. g, Si and GaAs, than in metals nucleation sites and diameters from pre-formed metal catalysts 2]. In addition to the opportunity to describe the new physics Since the 1960s, semiconductor whiskers grown by this demonstrated by nanowires, much effort has been devoted to technique [5, 6] have been extensively studied. In recent years, fabricating high-quality semiconductor nanowires by employ- various new techniques have been developed to realize ID ing different techniques because of the importance of nanostructures, such as laser-assisted chemical vapor deposi semiconductor materials to the electronics industry. The most tion(CVD)[7-10), oxide-assisted CVD(without a metal pular technique used to fabricate semiconductor artificial catalyst)[11], thermal CVD [12], metal-catalyzed molecular structures with feature sizes in the sub-100 nm range is beam epitaxy (MBE)[13-15] and chemical beam epitaxy lithography (3, 4], which involves tedious processes of ( CBE)[16]. Though the number of various kinds of ID hotoresist removal, chemical or ion-beam etching and surface nanostructures fabricated via different techniques increase passivation, etc. On semiconductor nanostructures, etching dramatically every year, our understanding of the basic process processes always lead to significant surface damage, and thus of ID nanostructure formation has not reached maturity. How to urface states are introduced to the nanostructures. Such fabricate desired id nanomaterials with tailored atomic damage may not be serious for the structures in the micrometer structures and how to integrate functional nanostructures into range. However, structures with dimensions in the nanometer devices are still challenging issues for materials scientists For very sensitive to the surface states or impurities ID semiconductor nanomaterials to have wide practical induced by fabrication processes. One-dimensional(ID) applications, however, many areas require further pursuing
3.3. Oxide-assisted growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.3.1. Kinetics and reactivity of silicon oxide in nucleation and growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.3.2. Effect of defects in 1D growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.3.3. Effect of external electrical field in 1D growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.4. Self-assembly growth from solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.4.1. Solution-liquid-solid (SLS) growth from seeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.4.2. Self-assembly oriented attachment growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.4.3. Anisotropic growth of crystals by kinetic control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4. Controlled growth of nanowires. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.1. Control of structures, growth direction and defects in nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.1.1. Interface structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.1.2. The growth direction of VLS nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.1.3. Defects in nanowires. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.1.4. From nanowire to nanoribbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.2. Structural transformation in nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.2.1. Surface relaxation and saturation of zinc oxide nanowires. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.2.2. The stability of Si nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.2.3. Optical rapid annealing effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4.3. Contacts and heterostructures in nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.3.1. Metal-semiconductor contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.3.2. Heterostructures in nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5. Other challenging nanowire structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.1. Non-tetrahedral Si nanowires. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.2. Oxide nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.2.1. Silicon oxide nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.2.2. Silicon dioxide tube-like nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 5.2.3. Zinc oxide tube-like nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 1. Introduction In the physics of nanoscale structures, quantum effects play an increasingly prominent role [1]. Quantum wires have demonstrated interesting electrical transport properties that are not seen in bulk materials. This is because, in quantum wires, electrons could be quantum-confined laterally and thus could occupy discrete energy levels that are different from the energy bands found in bulk materials. Due to low electron density and low effective mass, the quantized conductivity is more easily observed in semiconductors, e.g., Si and GaAs, than in metals [2]. In addition to the opportunity to describe the new physics demonstrated by nanowires, much effort has been devoted to fabricating high-quality semiconductor nanowires by employing different techniques because of the importance of semiconductor materials to the electronics industry. The most popular technique used to fabricate semiconductor artificial structures with feature sizes in the sub-100 nm range is lithography [3,4], which involves tedious processes of photoresist removal, chemical or ion-beam etching and surface passivation, etc. On semiconductor nanostructures, etching processes always lead to significant surface damage, and thus surface states are introduced to the nanostructures. Such damage may not be serious for the structures in the micrometer range. However, structures with dimensions in the nanometer range are very sensitive to the surface states or impurities induced by fabrication processes. One-dimensional (1D) nanostructures formed ‘‘naturally’’ (also called self-organized growth) without the aids of ex situ techniques, such as chemical etching, are desirable not only in fundamental research but also in future nanodevice design and fabrication. In this paper, various novel technologies for synthesizing nanowires are reviewed. A well-known self-organized growth mechanism for creating nanowires is the vapor–liquid–solid (VLS) process (also known as metal catalytic growth [5]). This technique can produce free-standing crystalline nanowires of semiconductor and metal oxide materials with fully controlled nucleation sites and diameters from pre-formed metal catalysts. Since the 1960s, semiconductor whiskers grown by this technique [5,6] have been extensively studied. In recent years, various new techniques have been developed to realize 1D nanostructures, such as laser-assisted chemical vapor deposition (CVD) [7–10], oxide-assisted CVD (without a metal catalyst) [11], thermal CVD [12], metal-catalyzed molecular beam epitaxy (MBE) [13–15] and chemical beam epitaxy (CBE) [16]. Though the number of various kinds of 1D nanostructures fabricated via different techniques increases dramatically every year, our understanding of the basic process of 1D nanostructure formation has not reached maturity. How to fabricate desired 1D nanomaterials with tailored atomic structures and how to integrate functional nanostructures into devices are still challenging issues for materials scientists. For 1D semiconductor nanomaterials to have wide practical applications, however, many areas require further pursuing. 2 N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51
N. Wang et al. /Materials Science and Engineering R 60 (2008)1-51 This review focuses on describing the status of research on the surface [6]. At a temperature above 363C, Au particles can formation of semiconductor and metal oxide nanowires. It form Si-Au eutectic droplets on Si surfaces, and the reduction consists of four sections. After a brief introduction, the first of Si occurs at the Au-Si droplets due to a catalytic effect. The section introduces the growth technologies currently employed Au-Si dro pets abso orb Si from the vapor pl to synthesize nanowires with an emphasis on advances in the supersaturated state. Since the melting point of Si(1414C)is newly developed techniques of metal-catalyzed MBE and CBe much higher than that of the eutectic alloy, Si atoms precipitate by which high-quality ultra-thin nanowire structures have been from the supersaturated droplets and bond at the liquid-solid fabricated. These techniques allow high levels of control over interface, and the liquid droplet rises from the Si substrate atomic structures, chemical composition, defects, doping surface. The absorption, diffusion and precipitation processes states, junctions, and so forth. We next discuss several novel of Si as schematically shown by the path 1-2-3 in Fig. 1(c) nucleation and growth mechanisms and theoretical analyses of involve vapor, liquid and solid phases. The typical feature of the the formation, reactivity and stability of nanocrystals. The VLS reaction is its low activation energy compared with normal initial alloying process of metal catalysts, growth of nanowire vapor-solid growth. The whiskers grow only in the areas seeded nuclei, changes in nanowire shapes and diameters as well as by metal catalysts, and their diameters are mainly determined deposition of source materials are described in the second by the sizes of the catalysts. The Vls method can result in section. In the third section, we describe the controlled growth unidirectional growth of many materials [6]. It has become a and structures of nanowires. Recent progress in controlling widely used technique for fabricating a variety of ID growth directions, defects, interface structures, structural nanomaterials that include elemental semiconductors [6- transformation, contacts and hetero-junctions is addressed In 8, 17-23], II-VI semiconductors [24-26], Ill-V semiconduc- the last section, we describe some theoretical nanowire tors 27-41, oxides [4247, nitrides[48]and carbides [ 49, 50] structures that have not yet been observed or are challenging The experimental setup of the VLs reaction has been to synthesis eported in previous work [5,6]. In brief, for Si nanowire growth, the sources can be Sih, mixed in H2 at a typical ratio of 2. Growth technologies for nanowires 1: 10. The reaction gases have to be diluted to about 2% in an Ar atmosphere. The pressure for the reaction is about 200 Torr, and 2.1. Vapor-liquid-solid (VLS) technique he flow rate is kept at 1500 sccm. Au nanoparticles can be prepared simply by first depositing an Au thin film on an Si The VLS technique was first described by Wagner and Ellis substrate using sputtering or thermal evaporation and then 5] in 1964. They used Au particles as catalysts to grow annealing the thin film to form droplets. Fig. 2(a) shows crystalline semiconductor whiskers from vapor sources such as uniform Au nanoparticles formed by annealing an Au thin film SiCl4 or SiH4. The principle for Si whisker growth is (thickness =l nm) at 500C. A thick film results in large thematically shown in Fig. 1(a). The Au particles deposited diameters of Au particles. Au particles arrays can be prepared on the surface of an Si substrate react first with Si to form Au-Si by lithography techniques. Fig. 2(b) shows an Au disc array alloy droplets at a certain temperature. As shown in the Au-Si prepared by e-beam lithography. The thickness of the Au phase diagram in Fig. 1(b), the melting temperature of the Au- pattern is critical to the final sizes of the nanoparticles Si alloy at the eutectic point is very low(about 363C at an generated by the subsequent annealing. Au films that are too Au: Si ratio of 4: 1)compared with that of Au or Si. Au and Si thin always result in splitting of the Au pattern(Fig. 2(c)). A can form a solid solution for all Si content(0-100%). In the proper treatment of the substrate surface by chemical etching case of Si deposition from the vapor mixture of SiCl4 and H2, and cleaning can result in the catalyst totally wetting the the reaction between SiCl4 and H2 happens at a temperature substrate surface(see Fig. 3(a)), which is important for later above 800C without the assistance of catalysts. Below this growth of the nanowires epitaxially on the substrate. Because of temperature, almost no deposition of Si occurs on the substrate the oxide layer on the substrate surface or impurities on the Si Whisker Au-Si uSi)1115 Substrate Fig. 1. Schematic illustration of Si whisker growth from vapor phases via Au-Si catalytic droplets. (a) The Au-Si droplet formed on an Si substrate catalyzes the whisker growth: (b)the Au-Si phase diagram. (c)The diffusion path of the source materials through a metal droplet; (d) the whisker growth can be catalyzed with
This review focuses on describing the status of research on the formation of semiconductor and metal oxide nanowires. It consists of four sections. After a brief introduction, the first section introduces the growth technologies currently employed to synthesize nanowires with an emphasis on advances in the newly developed techniques of metal-catalyzed MBE and CBE by which high-quality ultra-thin nanowire structures have been fabricated. These techniques allow high levels of control over atomic structures, chemical composition, defects, doping states, junctions, and so forth. We next discuss several novel nucleation and growth mechanisms and theoretical analyses of the formation, reactivity and stability of nanocrystals. The initial alloying process of metal catalysts, growth of nanowire nuclei, changes in nanowire shapes and diameters as well as deposition of source materials are described in the second section. In the third section, we describe the controlled growth and structures of nanowires. Recent progress in controlling growth directions, defects, interface structures, structural transformation, contacts and hetero-junctions is addressed. In the last section, we describe some theoretical nanowire structures that have not yet been observed or are challenging to synthesis. 2. Growth technologies for nanowires 2.1. Vapor–liquid–solid (VLS) technique The VLS technique was first described by Wagner and Ellis [5] in 1964. They used Au particles as catalysts to grow crystalline semiconductor whiskers from vapor sources such as SiCl4 or SiH4. The principle for Si whisker growth is schematically shown in Fig. 1(a). The Au particles deposited on the surface of an Si substrate react first with Si to form Au–Si alloy droplets at a certain temperature. As shown in the Au–Si phase diagram in Fig. 1(b), the melting temperature of the Au– Si alloy at the eutectic point is very low (about 363 8C at an Au:Si ratio of 4:1) compared with that of Au or Si. Au and Si can form a solid solution for all Si content (0–100%). In the case of Si deposition from the vapor mixture of SiCl4 and H2, the reaction between SiCl4 and H2 happens at a temperature above 800 8C without the assistance of catalysts. Below this temperature, almost no deposition of Si occurs on the substrate surface [6]. At a temperature above 363 8C, Au particles can form Si–Au eutectic droplets on Si surfaces, and the reduction of Si occurs at the Au–Si droplets due to a catalytic effect. The Au–Si droplets absorb Si from the vapor phase resulting in a supersaturated state. Since the melting point of Si (1414 8C) is much higher than that of the eutectic alloy, Si atoms precipitate from the supersaturated droplets and bond at the liquid–solid interface, and the liquid droplet rises from the Si substrate surface. The absorption, diffusion and precipitation processes of Si as schematically shown by the path 1 ! 2 ! 3 in Fig. 1(c) involve vapor, liquid and solid phases. The typical feature of the VLS reaction is its low activation energy compared with normal vapor–solid growth. The whiskers grow only in the areas seeded by metal catalysts, and their diameters are mainly determined by the sizes of the catalysts. The VLS method can result in unidirectional growth of many materials [6]. It has become a widely used technique for fabricating a variety of 1D nanomaterials that include elemental semiconductors [6– 8,17–23], II–VI semiconductors [24–26], III–V semiconductors [27–41], oxides [42–47], nitrides [48] and carbides [49,50]. The experimental setup of the VLS reaction has been reported in previous work [5,6]. In brief, for Si nanowire growth, the sources can be SiH4 mixed in H2 at a typical ratio of 1:10. The reaction gases have to be diluted to about 2% in an Ar atmosphere. The pressure for the reaction is about 200 Torr, and the flow rate is kept at 1500 sccm. Au nanoparticles can be prepared simply by first depositing an Au thin film on an Si substrate using sputtering or thermal evaporation and then annealing the thin film to form droplets. Fig. 2(a) shows uniform Au nanoparticles formed by annealing an Au thin film (thickness = 1 nm) at 500 8C. A thick film results in large diameters of Au particles. Au particles arrays can be prepared by lithography techniques. Fig. 2(b) shows an Au disc array prepared by e-beam lithography. The thickness of the Au pattern is critical to the final sizes of the nanoparticles generated by the subsequent annealing. Au films that are too thin always result in splitting of the Au pattern (Fig. 2(c)). A proper treatment of the substrate surface by chemical etching and cleaning can result in the catalyst totally wetting the substrate surface (see Fig. 3(a)), which is important for later growth of the nanowires epitaxially on the substrate. Because of the oxide layer on the substrate surface or impurities on the Fig. 1. Schematic illustration of Si whisker growth from vapor phases via Au–Si catalytic droplets. (a) The Au–Si droplet formed on an Si substrate catalyzes the whisker growth; (b) the Au–Si phase diagram. (c) The diffusion path of the source materials through a metal droplet; (d) the whisker growth can be catalyzed with a solid catalyst. N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51 3
N. Wang et al/Materials Science and Engineering R 60(2008)1-51 Fig. 2.(a) Au catalysts prepared by annealing a thin Au film.( b)Au prepared by e-beam lithography. (c) Splitting of the Au particles by annealing catalyst surface induced by the lithography technique, Au Under isothermal conditions, the crystalline structures of Si catalysts may not wet the substrate surface. In this case, Si whiskers are generally perfect, though steps and facets occur on nanowires may not have orientation relationship with the the whiskers'surfaces Twinning structures and twin-dendrites substrate and grow along random directions(Fig 3(b)). For Si (or branched whiskers) have been frequently observed in the nanowires with diameters larger than 20 nm, their growth is whiskers. Though the cross-section of most whiskers is round enerally along the(1 11) direction. Thin Si nanowires with (determined by the metal droplets), ribbon-like whiskers with a diameters smaller than 20 nm, however, show interesting rectangular cross-section often coexist and show the(11 1or growth behaviors for example the diameter-dependent and (1 1 2) growth direction [17]. Dislocations or other crystalline temperature-dependent growth direction( see details in Section defects are not essential for the growth of the whiskers via the 4.1) VLS method. In different semiconductor material systems, Before growing Si nanowires, activation of Au nanoparticles whiskers with similar morphologies and structures have been may be needed. An inactivated Au particle will not lead to fabricated by the VLS reaction and a variety of whisker forms nanowire growth. The activation of Au-Si alloy droplets can be have been obtained [6]. Although the VLS technique has been carried out in Ar or H2 atmospheres. We have found that plasma widely used for the fabrication of nanowires in recent years, the treatment is effective for cleaning and activating the surfaces of real absorption, reaction and diffusion processes of source Au catalysts. HCI mixed in the reaction gases can also atoms through the catalyst are complicated and largely depend effectively activate Au particles. However, the activation on the experimental conditions and the material systems [52- temperature largely relies on the diameters of Au catalysts For 54]. Many experiments have shown the deviation of some large Au catalysts(diameter >50 nm), the activation tempera- nanowire growth from the classical VLS mechanism. For tures can be 800C or higher. Large Au catalysts can easily wet example, it has been observed that nanowires of Ge [18, 19], Si a Si substrate at sufficiently high temperatures and thus Si [22], GaAs [27] and InAs [28] can grow even at temperatures nanowires grow epitaxially even on an untreated substrate. In below their eutectic points. There has been a long-standing the growth of thin Si nanowires(diameter <20 nm), the growth debate on whether the metal catalysts in these cases are solid temperatures are about 500C. Too high activation tempera- particles(see Fig. 1(d))or liquid droplets [54]. There are two tures may cause evaporation of the catalysts. The vacuum uncertainties in this debate: (1)because of the nanosize condition is another critical experimental parameter that affects effect, the melting temperatures of nanoparticles are always nanowire growth. Low vacuum conditions may cause lower than those of bulk materials and(2)it is not possible to evaporation of Si from the substrate surface and thus result measure the real temperature at the catalyst tips. In fact, in some in a rough surface cases, nanosized metal droplets are in a partially molten state Au Catalyst Si 50nm 20um Fig. 3. (a)An Au catalyst reacts with the substrate after the activation treatment. (b) Si nanowires grow in different directions
catalyst surface induced by the lithography technique, Au catalysts may not wet the substrate surface. In this case, Si nanowires may not have orientation relationship with the substrate and grow along random directions (Fig. 3(b)). For Si nanowires with diameters larger than 20 nm, their growth is generally along the h111i direction. Thin Si nanowires with diameters smaller than 20 nm, however, show interesting growth behaviors for example the diameter-dependent and temperature-dependent growth direction (see details in Section 4.1). Before growing Si nanowires, activation of Au nanoparticles may be needed. An inactivated Au particle will not lead to nanowire growth. The activation of Au–Si alloy droplets can be carried out in Ar or H2 atmospheres. We have found that plasma treatment is effective for cleaning and activating the surfaces of Au catalysts. HCl mixed in the reaction gases can also effectively activate Au particles. However, the activation temperature largely relies on the diameters of Au catalysts. For large Au catalysts (diameter > 50 nm), the activation temperatures can be 800 8C or higher. Large Au catalysts can easily wet a Si substrate at sufficiently high temperatures and thus Si nanowires grow epitaxially even on an untreated substrate. In the growth of thin Si nanowires (diameter <20 nm), the growth temperatures are about 500 8C. Too high activation temperatures may cause evaporation of the catalysts. The vacuum condition is another critical experimental parameter that affects nanowire growth. Low vacuum conditions may cause evaporation of Si from the substrate surface and thus result in a rough surface. Under isothermal conditions, the crystalline structures of Si whiskers are generally perfect, though steps and facets occur on the whiskers’ surfaces. Twinning structures and twin-dendrites (or branched whiskers) have been frequently observed in the whiskers. Though the cross-section of most whiskers is round (determined by the metal droplets), ribbon-like whiskers with a rectangular cross-section often coexist and show the h111i or h112i growth direction [17]. Dislocations or other crystalline defects are not essential for the growth of the whiskers via the VLS method. In different semiconductor material systems, whiskers with similar morphologies and structures have been fabricated by the VLS reaction and a variety of whisker forms have been obtained [6]. Although the VLS technique has been widely used for the fabrication of nanowires in recent years, the real absorption, reaction and diffusion processes of source atoms through the catalyst are complicated and largely depend on the experimental conditions and the material systems [52– 54]. Many experiments have shown the deviation of some nanowire growth from the classical VLS mechanism. For example, it has been observed that nanowires of Ge [18,19], Si [22], GaAs [27] and InAs [28] can grow even at temperatures below their eutectic points. There has been a long-standing debate on whether the metal catalysts in these cases are solid particles (see Fig. 1(d)) or liquid droplets [54]. There are two main uncertainties in this debate: (1) because of the nanosize effect, the melting temperatures of nanoparticles are always lower than those of bulk materials and (2) it is not possible to measure the real temperature at the catalyst tips. In fact, in some cases, nanosized metal droplets are in a partially molten state Fig. 2. (a) Au catalysts prepared by annealing a thin Au film. (b) Au patterns prepared by e-beam lithography. (c) Splitting of the Au particles by annealing. Fig. 3. (a) An Au catalyst reacts with the substrate after the activation treatment. (b) Si nanowires grow in different directions. 4 N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51
N. Wang et al. /Materials Science and Engineering R 60 (2008)1-51 51]. The surface and interface regions are liquid, while the technique is particularly useful in the synthesis of nanowires with cores of the droplets are solid a high-melting temperature, such as SiC nanowires[56]. It is also The VLS mechanism is very successful in generating large a very effective method in synthesizing nanowires with multi- quantities of ID nanomaterials(single nanowires and hetero- components and doping nanowires during growth. The vaporized ructured nanowires) with uniform crystalline structures not molecules(or clusters)by the high power laser have high kinetic only in semiconductors but also in oxide, nitride and other energy(about 100 eV), and this largely enhances the chemical material systems. However, it seems to be difficult to grow reaction, e.g., the reaction with oxygen or other gases, and thu metal nanowires by the VLS method. The disadvantage of the can largely improve the crystal quality of the nanowires at a low VLS method may be the contamination caused by the necessary substrate temperature. This special technique has many practical use of a metal particle as the catalyst. This may result in the uses for the control of the stoichiometries of nanowires. For change in the nanowires properties. However, by selecting an example, ZnO nanowires grown by thermal CVd always have appropriate catalyst, the affection of the contamination for oxygen vacancies and other defects that cau on- specific properties of the nanowire can be minimized. band edge emission) and electrical (low conductivity compared with bulk Zno crystals) properties. These defects cannot be 2.2. Laser-assisted growth easily eliminated even by annealing in oxygen after nanowire growth. Zno nanowires synthesized by laser ablation, however, Among the various techniques developed to synthesize ultra- generally show better optical properties. Another example is that thin nanowires, of particular interest is the laser ablation of indium oxide nanowires synthesized by laser ablation have a metal-containing solid targets or similar techniques [7-10], by significantly high mobility [57] which bulk-quantity nanowires can be readily obtained directly Fig. 4 is a schematic of the experimental setup of the laser from solid source materials. When using metal catalysts, for ablation technique. The laser used in the experiment can be any example, for the synthesis of Si nanowires, this method is high-power pulsed laser, e. g, a Nd: YAG laser [7], an interfered suggested to rely on the VLS mechanism, whereby the vapor(or femto-second laser[58]or an excimer laser [59]. The synthesis of gaseous clusters) generated by laser ablation dissolves in a Si nanowires by the experiment reported in Refs. [ 10,59, 60] was molten metal catalyst and then crystallizes to form nanowires. carried out using a high-power KrF excimer pulsed laser Ultrasmall nanoparticles of metals or metal silicides in large (248 nm, 10 Hz, 400 mJ/pulse)to ablate a target in an evacuated quantities are rather easy to obtain from the high temperature (500 Torr) quartz tube with Ar (50 sccm) flowing through the induced by laser ablation. Assisted by laser ablation, these tube. Otherinert gases, such as He, H2 and N2, can also be used as nanoparticles act as the critical catalyst for the nucleation and the ambient gases. The use of different ambient gases may growth of nanowires influence the diameters of the nanowires and affects their optical The laser-assisted method has unique advantages over other properties [60]. The temperature around the target materials in growth techniques in synthesizing nanowires containing com- the experiment was about 1200C. The target was highly pure Si plex chemical compositions. This is because no matter how many powder mixed with Fe, Ni, or Co(about 0. 5%o). The laser beam elements are involved, it is not necessary to prepare the target(or (1 mm x 3 mm)was focused on the target surface. Si nanowire the source materials)in a crystalline form. A simple mixture of products( sponge-like, dark yellow in color as shown in Fig. 5(a)) the elements is good enough as the source material. The source formed on the Sisubstrate or the inner wall of the quartz tube near materials are ablated into a vapor phase, which may have the the water-cooled finger after I h of laser ablation. The same composition as the source materials. The vapor phase can temperature of the area around the substrate where the nanowire e easily transferred to the substrate where nanowires nucleate grew was approximately 900-1000'C The growth rate of the Si and grow. A high-energy laser can ablate solid materials in an nanowires was about 10-80 um/h ultra short time and vaporize the materials in a non-thermo By laser ablation, the metal powder is evaporated out of the equilibrium process, also called congruent evaporation [55]. This target to form clusters. They are in a semi-liquid state and serve Window Furnace Cold finger →Ar(He) 500To 1200°C Evaporation target Pumping Fig. 4. Experimental setup for the synthesis of Si nanowires by laser ablation( Courtesy of Prof. I. Bello)
[51]. The surface and interface regions are liquid, while the cores of the droplets are solid. The VLS mechanism is very successful in generating large quantities of 1D nanomaterials (single nanowires and heterostructured nanowires) with uniform crystalline structures not only in semiconductors but also in oxide, nitride and other material systems. However, it seems to be difficult to grow metal nanowires by the VLS method. The disadvantage of the VLS method may be the contamination caused by the necessary use of a metal particle as the catalyst. This may result in the change in the nanowire’s properties. However, by selecting an appropriate catalyst, the affection of the contamination for specific properties of the nanowire can be minimized. 2.2. Laser-assisted growth Among the various techniques developed to synthesize ultrathin nanowires, of particular interest is the laser ablation of metal-containing solid targets or similar techniques [7–10], by which bulk-quantity nanowires can be readily obtained directly from solid source materials. When using metal catalysts, for example, for the synthesis of Si nanowires, this method is suggested to rely on the VLS mechanism, whereby the vapor (or gaseous clusters) generated by laser ablation dissolves in a molten metal catalyst and then crystallizes to form nanowires. Ultrasmall nanoparticles of metals or metal silicides in large quantities are rather easy to obtain from the high temperature induced by laser ablation. Assisted by laser ablation, these nanoparticles act as the critical catalyst for the nucleation and growth of nanowires. The laser-assisted method has unique advantages over other growth techniques in synthesizing nanowires containing complex chemical compositions. This is because no matter how many elements are involved, it is not necessary to prepare the target (or the source materials) in a crystalline form. A simple mixture of the elements is good enough as the source material. The source materials are ablated into a vapor phase, which may have the same composition as the source materials. The vapor phase can be easily transferred to the substrate where nanowires nucleate and grow. A high-energy laser can ablate solid materials in an ultra short time and vaporize the materials in a non-thermoequilibrium process, also called congruent evaporation [55]. This technique is particularly useful in the synthesis of nanowires with a high-melting temperature, such as SiC nanowires[56]. It is also a very effective method in synthesizing nanowires with multicomponents and doping nanowires during growth. The vaporized molecules (or clusters) by the high power laser have high kinetic energy (about 100 eV), and this largely enhances the chemical reaction, e.g., the reaction with oxygen or other gases, and thus can largely improve the crystal quality of the nanowires at a low substrate temperature. This special technique has many practical uses for the control of the stoichiometries of nanowires. For example, ZnO nanowires grown by thermal CVD always have oxygen vacancies and other defects that cause poor optical (nonband edge emission) and electrical (low conductivity compared with bulk ZnO crystals) properties. These defects cannot be easily eliminated even by annealing in oxygen after nanowire growth. ZnO nanowires synthesized by laser ablation, however, generally show better optical properties. Another example is that indium oxide nanowires synthesized by laser ablation have a significantly high mobility [57]. Fig. 4 is a schematic of the experimental setup of the laserablation technique. The laser used in the experiment can be any high-power pulsed laser, e.g., a Nd:YAG laser [7], an interfered femto-second laser[58] or an excimer laser[59]. The synthesis of Si nanowires by the experiment reported in Refs. [10,59,60] was carried out using a high-power KrF excimer pulsed laser (248 nm, 10 Hz, 400 mJ/pulse) to ablate a target in an evacuated (500 Torr) quartz tube with Ar (50 sccm) flowing through the tube. Other inert gases, such as He, H2 and N2, can also be used as the ambient gases. The use of different ambient gases may influence the diameters of the nanowires and affects their optical properties [60]. The temperature around the target materials in the experiment was about 1200 8C. The target was highly pure Si powder mixed with Fe, Ni, or Co (about 0.5%). The laser beam (1 mm 3 mm) was focused on the target surface. Si nanowire products (sponge-like, dark yellow in color as shown in Fig. 5(a)) formed on the Si substrate or the inner wall of the quartz tube near the water-cooled finger after 1 h of laser ablation. The temperature of the area around the substrate where the nanowire grew was approximately 900–1000 8C. The growth rate of the Si nanowires was about 10–80 mm/h. By laser ablation, the metal powder is evaporated out of the target to form clusters. They are in a semi-liquid state and serve Fig. 4. Experimental setup for the synthesis of Si nanowires by laser ablation (Courtesy of Prof. I. Bello). N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51 5