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TEM Study of the Growth Mechanism, Phase Transformation, and Core/shell Structure of Semiconductor Nanowires

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Tai Lun Wong
Abstract:
In this thesis, the fabrication and characterization of one-dimensional nanostructures have been studied systematically to understand the growth mechanism and structure transformation of one-dimensional nanostructures. The growth behavior of the ultrathin ZnSe nanowires with diameter less than 60 nm was found to be different from classical vapor-liquid-solid (VLS) process. The growth rate increases when the diameter of nanowires decreases, in contrast to the classical VLS process in which the growth rate increases with the diameter. The nucleation, initial growth, growth rates, defects, interface structures and growth direction of the nanowires were investigated by high resolution transmission electron microscopy (HRTEM). We found the structure and growth direction of ultra-thin nanowires are highly sensitive to growth temperatures and diameters of nanowires. At a low growth temperature (380°C), the growth direction for most nanowires is along . Planar defects were found throughout the nanowires. At a high growth temperature (530°C), uniform nanowires with diameters around 10nm were grown along and directions, and the nanowires with diameters larger than 20nm were mainly grown along direction. The possible growth mechanism of ultrathin nanowires was proposed by combining the solid catalytic growth with the interface diffusion theory, in order to explain how the growth temperature and the size of the catalysts influent the morphology, growth direction and growth rate of ultrathin nanowires. Structural and phase transformation of a nickel coated Si nanowire to NiSi2/SiC core-shell nanowire heterostructures has been investigated by the in-situ Transmission Electron Microscope (TEM). The phase transformation is a single-site nucleation process and therefore a single crystalline NiSi2 core resulted in the core-shell nanowire heterostructures. The transformation of the Si nanowire to NiSi2/SiC core-shell nanowire heterostructures was extremely fast and completed instantly due to high temperature annealing. Furthermore, the phase transformation preferred to begin at the defect and bending region of the nanowires, as the nickel can easily diffuse through the native oxide on the Si nanowires rather than the other regions. By removing the native oxide on the Si nanowires using HF, the temperature required for the phase transformation was decreased significantly. However, without the native oxide, the phase transformation became a multi-site nucleation process, and the nanowire became a polycrystalline and multiphase nickel silicide nanowires after the reaction. A simple and effective method is developed for fabricating high-quality vertically aligned ZnO nanowire arrays using carbonized photoresists. ZnO nanowires fabricated by this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. We further fabricated vertically aligned ZnO/a-Si core-shell heterojunction nanowire arrays through direct chemical vapor deposition (CVD) of amorphous silicon on ZnO nanowire surfaces. The thickness of the a-Si shells linearly increases with deposition time and the deposition rate was about 5nm/min at 530 °C. Since the Si shell is p-type and the ZnO core is intrinsic n-type semiconductors, the ZnO/ amorphous silicon core-shell nanowires naturally formed hetero p-n junctions. The antireflection property of the ZnO/amorphous silicon core-shell nanowires is dramatically enhanced due to the rough interface between the ZnO and amorphous silicon. Additionally, the intensity of the Photoluminescence spectrum of ZnO/amorphous silicon core-shell structures is decreasing with the thickness of the amorphous silicon shell increases.

v Table of Contents Title pages i

Authorization ii

Signature iii

Table of Contents v

List of Figures vii

List of Tables xii

Abstract xiii

Chapter 1 Introduction 1

1.1 Introduction to nanowire growth mechanisms 2

1.1.1 The classical nanowire growth mechanism 3

1.1.2 The growth mechanisms developed recently 7

1.2 Introduction to Nanowire Heterostructures 9

1.2.1 The axial heterostructures 11

1.2.1 The radial core-shell heterostructure 12

Chapter 2 TEM Technique for Nanowire Investigation 14

2.1 Transmission electron microscopy 14

2.1.1 Structure and operation modes of TEM 15

2.2 Electron Diffraction 17

2.3 TEM imaging 20

2.4 In-situ TEM 21

Chapter 3 Growth and Structural Characterization of ZnSe Ultrathin nanowires 27

3.1 Introduction 27

3.2 Growth and properties of ultra-thin ZnSe nanowi res 28

3.3 Nanowires growth by interface incorporation and diffusion of source atoms 36

Chapter 4 In-situ Observation of Phase Transformati on in Si Nanowires 46

4.1 Introduction 46

4.2 In-situ TEM Experiment 47

vi 4.2.1 Experimental section 48

4.2.2 The In-situ TEM observation of Si nanowires p hase transformation 53

4.3 Structure analysis of the resulted nanowire 56

4.3 The mechanism of the formation of the NiSi 2 /SiC core-shell nanowire 63

4.4 Conclusion 64

Chapter 5 Fabrication and Characterization of ZnO/a -Si Core/shell Nanowire Junctions 65

5.1 Introduction 65

5.2 Fabrication of ZnO/a-Si core-shell heterojuncti on nanowire arrays 66

5.3 Photoluminescence spectroscopy of the ZnO/a-Si core-shell heterojunction nanowire arrays 76

5.3.1 Introduction of luminescence 76

5.3.2 Photoluminescence of the ZnO nanowire and ZnO /a-Si nanowire arrays 78

5.4 Conclusion 83

Chapter 6 Summary and further studies 84

6.1. Summary 84

6.2 Further studies 85

Publication 86

vii List of Figures Figure 1.1 The schematic diagram of Si whisker grow th mechanism from SiCl 4 by Au-Si catalytic droplets. (a) Au-Si droplet and the whisker growth; (b) A whisker tip; (c) Au-Si binary phase diagram. .................................... ................................................... ......................... 4

Figure 1.2 (a) TEM picture of Si nanowires synthesi zed by the oxide assisted method with a uniform diameter of about 20nm and length of 100µm (b) The diagram shows that the presence of SiO 2 in the powder target can enhance the growth of Si nanowire significantly. (c) Schematic description of the oxi de assisted Si nanowire growth process (d)The experimental set-up for the oxide as sisted method[32]. ........................ 8

Figure 1.3 Nanowire heterostructure synthesis. A ch ange in the reactant leads to either ( a ) axial heterostructure growth or ( b ) radial heterostructure growth depending on whethe r the reactant is preferentially incorporated ........... ................................................... ............ 11

Figure 2.1 Schematics of TEM consisting of five sys tems: illumination, specimen stage, imaging, magnification and data recording systems. The enlar ged parts on the right are EDS and EELS for chemical composition analysis. ........... ................................................... ...... 16

Figure 2.2 Ray paths in TEM (a) diffraction mode an d (b) imaging mode................................. ... 17

Figure 2.3 Ray diagrams showing (a) SAED and (b) CB ED pattern formation respectively. ....... 18

Figure 2.4 Diffraction planes for FCC crystals in the [011] zone and its corresponding diffraction pattern. .......................................... ................................................... ............................. 19

Figure 2.5 Geometry for electron diffraction patter n ................................................. .................... 19

Figure 2.6 Comparison of the use of an objective ap erture in TEM to select (a) the direct or (b) the scattered electrons forming BF and DF images, r espectively................................. 20

Figure 2.7 (a) The schematic diagram of the structu re of the window-type environmental cell TEM sample holder, (b) The schematic diagram of the structure of environmental TEM. .............................................. ................................................... ............................ 25

Figure 2.8 The schematic diagram of the experimenta l setup ........................................... ............. 26

Figure 3.1 The EDS spectra taken from (a) a ZnSe na nowire and (b) a Au-catalyst tip. The Cu signal was from the Cu supporting grid. ........... ................................................... ........ 30

Figure 3.2 ZnSe nanowires nanowires grown on the GaAs (001) substrate at (a) 530 °C, v iewed along the [110]zone axis; (b, c) 390 °C, viewed alo ng the [110] and [100] zone axes,

viii respectively. (d,e) ZnSe nanowires grown on the GaA s (110) substrate at 530 °C and 390 °C, respectively, viewed along the [11¯ 1]zone a xis. ............................................. 32

Figure 3.3 Bright field TEM images show the typical morphology of ZnSe nanowires grown at (a) 530 o C and (b) 380 o C, respectively. .................................. ............................................. 34

Figure 3.4 TEM images of a typical [111] direction ZnSe nanowire dispersed on the carbon film. (a)Bright-field TEM image shows a gradually taped n anowire with a Au catalyst at the end. HRTEM images are taken from (b) the tip and (c ) middle of the nanowire, respectively. (d) Corresponding SAED pattern from t he tip area of the nanowire. The pattern is indexed to be ]0112[ zone axis of hexagonal wurtzite structure. (e)-(f)

HRTEM images of ZnSe nanowires with diameters of (e ) 7.5nm and (f) 8.9 nm exhibit the hexagonal wurtzite structure and cubic zinc ble nde structure. ............................... 36

Figure 3.5 (a) The solid dots indicate the growth r ates measured from different diameters of ZnSe nanowires. The dashed line is the fitting curve by dL/dt=Ar-4/3 and the solid line by dL/dt=Ar-4/3+C. (b) Experimental data of the whiske r growth rates reported by Givargizov[36]. ................................... ................................................... ....................... 37

Figure 3.6 Different diffusion models for the sourc e atoms to incorporate into the growth front of the nanowire. (a) The classical VLS. (b) The metal droplet is in partially molten state. Its surface and interface are liquid, while the cor e of the droplet may be solid. (c) The metal catalyst is solid, but the interface is liqui d. ................................................ ......... 37

Figure 3.7 In-situ TEM observation of Ge nanowires growth[67]......................................... ........ 39

Figure 3.8 (a) The schematic concentration profile of the source atoms at the catalytic interface. (b) The flux J(t) of the source atoms flowing into the catalytic int erface. ................... 40

Figure 3.9 The log of different diffusion coefficie nts vs. TM(melting temperature)/T(temperature). The nanowire growth temperature region is indicated by the dot line. ........................ 41

Figure 3.10 Fisher’s model of an isolated grain bou ndary δ is the grain-boundary width............. 42

Figure 3.11 The schematic figure for the growth rat e of nanowires vs diameter ........................ ... 45

Figure 4.1 The schematic figure illustrates the spe cimen of the point contact annealing reaction of different nanowires ............................... ................................................... ..................... 47

Figure 4.2 In situ TEM image sequence of the growth of a NiSi nanowire within a Si nanowire[94] ...................................... ................................................... ........................ 48

Figure 4.3 The silicon nanowire coated with nickel is approaching the heating filament. ............ 4 9

ix Figure 4.4 The processes of mounting Si nanowires o n STM tip. (a) A 5 nm gold thin film is sputtered on the Si substrate. (b) The gold thin fi lm on the Si substrate formed gold nanoparticles by annealing. (c) The Si nanowires wa s fabricated by silane gas and gold nanoparticles as the catalysts. (d) Scratched the Si nanowires off the Si substrate onto another Si substrate (e) Mounting the Si nanowires on the STM tip. (f) The schematic figure of the STM tip with Si nanowires ........... ................................................... ........ 51

Figure 4.5 Calibrated filament surface temperature and applied filament current. ..................... ... 52

Figure 4.6(a)-(d) The Ni coating formed droplets on the native oxide before the transformation. (e)-(h) Formation process of nickel silicide (e-f). The starting point of the reaction is indicated by the circles. (e) The nanowire is compl etely transformed into nickel silicide. (f) The nickel silicide nanowire starts t o evaporate by further increasing the temperature. The numbers at upper right are the vid eo recording time. ...................... 53

Figure 4.7 (a) The Si nanowire treated by HF and th en coated with Ni before the transformation. (b) The reaction started without forming Ni droplet s. ................................................ .. 55

Figure 4.8 (a) The point contact annealing for Ni a nd bending Si nanowires. The circle indicates the reacted regions. (b) Enlarged picture of the na nowires marked by the circle. ....... 55

Figure 4.9 (a) and (b) show the morphology of the h eterojunction of the NiSi 2 nanowire and SiC nanotube. (c) The SAED of the NiSi 2 nanowire viewed along the [314] zone axis. (d) The SAED of the SiC nanotube. The diffraction rings have been indexed by the cubic SiC plus the (002) diffraction of graphite. (e) and (f) The dark field images of the NiSi 2 /SiC heterojunction shown in (b). The NiSi 2 nanowire appeared bright in (e) when using the (-351) diffraction (shown in (c)) fo r imaging, while the SiC nanotube was bright when using the diffraction ring (111) in (d) for imaging. ........................... 56

Figure 4.10 (a)-(e) The simulated electron diffract ion pattern of NiSi 2 (f)-(g) The simulated the electron diffraction pattern of Si.(no double diffr action) ........................................... .. 57

Figure 4.11 The binary phase diagram of Ni-Si alloy . ................................................. .................. 58

Figure 4.12 X-ray diffraction results for 100 nm Ni thin film on Si wafer annealed at a ramp rate of 5°C/min to 410, 500, 600, 756, 811, and to 910°C .................................................. 60

Figure 4.13 The schematic figure of the crystal str ucture (a) NiSi (b) NiSi 2 (c) Si. ...................... 60

Figure 4.14 (a) and (b) are the SAED pattern of the resulted silicide nanowire without native oxide shown in Figure 4.7. The nanowire is polycrys talline Ni 31 Si 12 . ......................... 62

x Figure 4.15 The formation mechanisms of the Ni sili cides nanowires and SiC heterojunction. (a) The catalytic effect of Ni may result in the format ion of SiC in the native oxide shells and this causes cracks or other defects. Ni atoms c an diffuse into the Si nanowire through these defects. (b) After Ni atoms penetrate the native oxide to react with the Si core, the released carbon will react with the oxide shell to form SiC. (c)-(d) Due to the Si nanowire is bended, cracks or other defects are generated in the native oxide shells. Ni atom can diffuse into the Si nanowire through th e cracks.(e)-(f) The reaction mechanism for the Si nanowire without clad oxide. N i atoms can directly react with the Si wire at low temperatures everywhere and thus result in polycrystalline silicide nanowire by the multi-site nucleation. ............ ................................................... .......... 63

Figure 5.1 (a) The schematic diagram of experimenta l setup for synthesis of ZnO nanomaterials. (b) The distribution of temperature in the stove fr om the center. ................................. 67

Figure 5.2 Fabrication process of ZnO nanostructure arrays directly from PR. (a) Si substrates coated by PR. (b) The resulting nanowire arrays. .. ................................................... ... 68

Figure 5.3 The optical (a) and SEM (b) images of Zn O nanowire arrays grown on a PR-coated silicon substrate. (c) ZnO nanowires formed on an A u-coated silicon substrate. (d) XRD data recorded from the samples shown in (b) and (c). ........................................ 69

Figure 5.4 (a) Raman spectra of the photoresists be fore (the bottom curve) and after annealing (the top curve); (b) nucleation and growth mechanis ms of ZnO nanowires on the photoresist patterns. ............................. ................................................... ...................... 70

Figure 5.5 TEM images of tips of ZnO nanowires. (b) -(d) are HRTEM images of tips ................ 72

Figure 5.6 (a) The cross-section TEM image of sampl e of ZnO on PR in initial growth stage. The PR layer is cleaved for stress. (b) The ending of Z nO nanowires on PR. (c) The interface between the root of ZnO nanowires and the PR. (d) The corresponding FFT pattern shows the ZnO structure. .................. ................................................... ............. 72

Figure 5.7 (a) Schematic illustration of the fabric ation process of ZnO/a-Si core-shell nanowire arrays. (b) and (c) are SEM images of the ZnO nanow ire arrays before and after a-Si coating. .......................................... ................................................... ............................ 74

Figure 5.8 (a) The TEM image of a ZnO/a-Si core-she ll nanowire fabricated at 530ºC with an a- Si deposition duration of 5 min. The inset in the r ight shows the apex of the nanowire. (b) The corresponding HRTEM image taken at the inte rface. (c) The SAED pattern

xi recorded along the [1-100] direction. (d) The corre sponding EDS spectrum. (e) The EDS nanoprobe line-scan for the elements Zn, O, and Si, over a horizontal section of the ZnO/a-Si core-shell nanowire as indicated by th e line in (a). (f) The EDS elemental mappings for Zn, O and Si. (g) The change in the a-Si thickness versus the deposition duration. (h) and (i) are TEM images of ZnO/a-Si core-shell nanowires synthesized at 530 ºC and 700 ºC with a-Si depositi on durations of 10 min and 5 min, respectively. ..................................... ................................................... .......................... 75

Figure 5.9 (a) Band diagram of semiconductor. (b) E lectrons are excited from VB to CB (c) Electron transition from CB to VB. ................ ................................................... ........... 77

Figure 5.10 The experimental set-up for PL measurem ents .............................................. ............. 79

Figure 5.11 Photographs of the original ZnO nanowir e arrays and three samples with different deposition durations at 530°C. .................... ................................................... .............. 80

Figure 5.12 (a) The transmission path of light as i t enters in a ZnO nanowire (left) and a ZnO/a-Si core-shell nanowire (right). The red stars mark the place where total reflection occurs. (b) Reflectance spectra of ZnO nanowire arrays and ZnO/a-Si nanowire arrays obtained at a deposition temperature of 530 º C for different deposition durations. (c) The reflectance difference curve follows the shape of the refraction index curve of a- Si.[141] (d) PL spectra of ZnO nanowire arrays and ZnO/a-Si nanowire arrays obtained at a deposition temperature of 530°C for d ifferent deposition durations. ..... 81

Figure 5.13 (a) A typical I-V curve of the FET devi ce made by a ZnO/a-Si core-shell nanowire. The inset shows the SEM image of the FET device. (b ) I ds -V ds plots at different V g . The inset shows the transfer characteristics at V ds = 8 V. ........................................... .. 82

xii List of Tables Table 1.1 Kinetic coefficients, supersaturations an d critical diameters for Si(Au) ................... ....... 6

Table 3.1 The comparison of the Interface diffusion mechanism and the VLS Mechanism ......... 45

xiii TEM Study of the Growth Mechanism, Phase Transforma tion, and Core/shell Structure of Semiconductor Nanowires

By Wong Tai Lun

Nano Science and Technology The Hong Kong University of Science and Technology

Abstract In this thesis, the fabrication and characterizatio n of one-dimensional nanostructures have been studied systematically to understand the growt h mechanism and structure transformation of one-dimensional nanostructures. The growth behavior of the ultrathin ZnSe nanowires with diameter less than 60 nm was found to be different from classical vapor-liquid-solid (VLS) process. The growth rate increases when the diamete r of nanowires decreases, in contrast to the classical VLS process in which the growth rate incr eases with the diameter. The nucleation, initial growth, growth rates, defects, interface st ructures and growth direction of the nanowires were investigated by high resolution transmission e lectron microscopy (HRTEM). We found the structure and growth direction of ultra-thin nanowi res are highly sensitive to growth temperatures and diameters of nanowires. At a low growth tempera ture (380°C), the growth direction for most nanowires is along <111>. Planar defects were found throughout the nanowires. At a high growth temperature (530°C), uniform nanowires with diamete rs around 10nm were grown along <110> and <112> directions, and the nanowires with diamet ers larger than 20nm were mainly grown along <111> direction. The possible growth mechanis m of ultrathin nanowires was proposed by combining the solid catalytic growth with the inter face diffusion theory, in order to explain how the growth temperature and the size of the catalyst s influent the morphology, growth direction and growth rate of ultrathin nanowires. Structural and phase transformation of a nickel coa ted Si nanowire to NiSi2/SiC core-shell

xiv nanowire heterostructures has been investigated by the in-situ Transmission Electron Microscope (TEM). The phase transformation is a single-site nu cleation process and therefore a single crystalline NiSi2 core resulted in the core-shell n anowire heterostructures. The transformation of the Si nanowire to NiSi2/SiC core-shell nanowire he terostructures was extremely fast and completed instantly due to high temperature anneali ng. Furthermore, the phase transformation preferred to begin at the defect and bending region of the nanowires, as the nickel can easily diffuse through the native oxide on the Si nanowire s rather than the other regions. By removing the native oxide on the Si nanowires using HF, the temperature required for the phase transformation was decreased significantly. However , without the native oxide, the phase transformation became a multi-site nucleation proce ss, and the nanowire became a polycrystalline and multiphase nickel silicide nano wires after the reaction. A simple and effective method is developed for fabr icating high-quality vertically aligned ZnO nanowire arrays using carbonized photoresists. ZnO nanowires fabricated by this method show excellent alignment, crystal quality, and opti cal properties that are independent of the substrates. We further fabricated vertically aligne d ZnO/a-Si core-shell heterojunction nanowire arrays through direct chemical vapor deposition (CV D) of amorphous silicon on ZnO nanowire surfaces. The thickness of the a-Si shells linearly increases with deposition time and the deposition rate was about 5nm/min at 530 °C. Since the Si shell is p-type and the ZnO core is intrinsic n-type semiconductors, the ZnO/ amorphous silicon core-shell nanowires naturally formed hetero p-n junctions. The antireflection pro perty of the ZnO/amorphous silicon core-shell nanowires is dramatically enhanced due to the rough interface between the ZnO and amorphous silicon. Additionally, the intensity of the Photolu minescence spectrum of ZnO/amorphous silicon core-shell structures is decreasing with the thickn ess of the amorphous silicon shell increases.

1 Chapter 1

Introduction One-dimensional nanostructures, such as nanotubes, nanowires and nanobelts have attracted a lot of interest since Iijima first reported the car bon nanotube in 1991[1]. The electrical, mechanical and optical properties of one-dimensiona l nanostructures are different from bulk materials due to the size and dimensionality effect . One-dimensional nanostructures are promising in interconnection applications and as fu nctional constituents in the fabrication of nano-scale electronic and optoelectronic devices. A nanowire is defined as a nanostructure with a dia meter of several nanometers to a few hundred nanometers with no constraint in its longit udinal dimension. The quantum mechanical effect plays an important role on the properties of the nanowires. The electronic and optical properties of nanowires are deviated substantially from those of bulk materials. Physically, nanowires possess a high aspect ratio (surface to v olume ratio) compared to thin-films or bulk samples. This property makes nanowires a potential candidate for gas sensors. For instance, in the case of conventional gas sensors (the polycryst alline ceramic or film devices), only a small fraction of the species adsorbed near the grain bou ndaries is active in modifying the electrical transport properties. This causes the low sensitivi ty because of the limited surface-to-volume ratio, which is difficult to overcome. The most fascinating type of nanowires is semicondu ctor nanowires. They are of particularly important because they are emerging as versatile na noscale building blocks for the assembly of photonic devices [2, 3], including polarization sen sitive photodetectors [4], light-emitting diodes [5-8], and lasers [9-12]. They include elementary s emiconductors (such as Si and Ge) and compound semiconductors (such as InP, GaAs, GaN and SiC). For example, Si nanowires have been investigated in the use of ultra-fast transist or circuit [13]; an field emission transistor (FET)

made of a single Ge- nanowire was reported to have a significantly improved power- performance[14]; The InP nanowire leading to 1-D po larization sensitive photodetectors [15] and p – n junction light emitting diodes [16] have alre ady been demonstrated; Recently, the potentialities of GaAs nanowires as a cold electron source material have been demonstrated for various applications, such as a monochromatic elect ron beam for high-resolution microscopy [17] and spin-polarized electron sources for scanning tu nneling microscopy [18]; The n-type GaN

2 nanowires are reported to be an ideal candidate for electrically driven nanowire lasers [19] and SiC nanowires are expected for the reinforcement of various nanocomposite materials or as nanocontacts in harsh environment, due mainly to th eir superior mechanical properties and high electrical conductance [20]. Among all the semiconductor nanowires, Si nanowires have attracted more interest compare to the other semiconductor nanowires. For example, Si becomes a direct-band-gap semiconductor due to quantum confinement effects at nanoscale.[21 ] This property enables Si nanowires to exhibit visible photoluminescence at room temperatu re. The bulk Si does not exhibit any good optoelectronic properties because of the indirect b and gap structure. Moreover, Si nanowires are compatible with existing Si-based technologies, for example, Complementary Metal-Oxide-Semiconductor (CMOS) tech nology. In the mid 1990s, Morales and Lieber synthesized truly nanoscopic Si nanowire s and demonstrated laser ablation as a novel technique to fabricate Si nanowires. Their discover y and advances in microelectronics has triggered new interest in silicon nanowires. The pu blications on Si nanowire have dramatically increased since 1998 and the investigations on Si n anowires gained significant recognition. Si nanowires can be fabricated by an attractive “to p down” approach using lithography and a reactive-ion etching process to produce Si nanowire s. Si nanowires have also been synthesized by “bottom up” approaches, for instance, the solvother mal process [22, 23], laser-assisted catalytic growth [24], oxide-assisted CVD [25], thermal evapo ration [26], chemical vapor deposition (CVD) [27, 28], and metal-catalyzed molecular beam epitaxy (MBE) [29]. 1.1 Introduction to nanowire growth mechanisms Investigation on the growth of wires for semiconduc ting materials already began in the 1960’s [30]. However, the diameters of these wires were in the range of 100nm to several micrometers which were denoted as whiskers. Signifi cant progress has been achieved in recent years by developing various methods for synthesizin g the nanowires with diameters down to 10nm and a length of several micrometers. So far, v arious methods have been employed to grow a large variety of semiconductor nanowires, for ins tance chemical beam epitaxy (CBE) [31], oxide-assisted CVD (without metal catalyst) [32], t hermal CVD [26], laser-assisted chemical vapor deposition (CVD) [3, 24, 33], metal-catalyzed molecular beam epitaxy (MBE) [34, 35], and the like. The widely accepted mechanism behind most of above mentioned synthesis

3 methods is the so called VLS method (also known as the metal catalytic growth) which was first proposed by Wagner and Ellis in 1964[30] and descri bed in more detail by E. I. Givargizov [36] in 1975. VLS uses vapor phases or laser ablated particle vap ors of the growth material, and exposed to catalysts which are generally metal nanoparticles. This method can fabricate diameter-controlled and free-standing single crystalline nanowires for semiconductor and metal oxide materials. Moreover, there are a number of advantages which le ad the VLS mechanism better than other techniques. For example, the position and the diame ter of the nanowires can be well controlled by pre-formed metal catalysts. 1.1.1 The classical nanowire growth mechanism Wagner and Ellis first proposed the VLS mechanism t o explain the growth process of Si whiskers [30]. For the VLS mechanism, metal (for e xample Au) particles are used as the catalysts and vapor source materials (for example s ilane) to grow crystalline whiskers of semiconductor materials. The mechanism for the grow th of the Si whisker is shown schematically in Figure 1.1. Either Au particles or thin film are deposited on a Si substrate, then the substrate with Au is annealed at an appropriate temperature for the Au and Si substrate to react and form Au-Si alloy droplets before the Si w hisker growth. According to the Au-Si binary phase diagram, the temperature for the annealing is chosen at above the eutectic point (only about 363°C). For Si whisker growth from the vapor source of the mixture of SiCl4 and H2, without the assistance of catalysts, the reaction between two g ases requires a temperature of over 800°C. When the temperature is below 800°C, barely any Si deposits appear on the surface of the substrate without the assistance of catalysts [37]. On the other hand, once the temperature goes above 363oC, Au particles can easily form Au-Si eut ectic droplets with the Si substrate and the reduction reaction of the SiCl4 occurs at the Au-Si eutectic droplets due to the catalytic effect. The Au-Si alloy droplets absorb Si by decomposing t he SiCl 4 and a supersaturation of Si in Au-Si alloy droplets is formed. As a result, Si crystal growth starts at the liquid-solid interface between

the Au-Si alloy droplet and Si substrate by precipi tation for the nanowires growth. The procedure of the Si nanowires growth is divided into three st eps, which are absorption, diffusion and precipitation. In Figure 1.1b, the procedure is sho wn following the path 1

2

3 schematically. As vapor, liquid and solid phases are involved in t he process, the process is named as the VLS

4 mechanism.

Figure 1.1 The schematic diagram of Si whisker grow th mechanism from SiCl 4 by Au-Si catalytic droplets. (a) Au-Si droplet and the whisker growth; (b) A whisker tip; (c) Au-Si binary phase

5 diagram.

We now discuss

two main properties of the VLS technique. First is the anisotropic growth of the crystal. Compared to the solid crystal surfaces , the liquid droplet surface is an ideal rough surface with a much larger sticking coefficient. Th at means if the temperature is appropriate, almost all the impinging Si source atoms can be cap tured by the liquid surface, while the solid Si surface rejects almost all Si source atoms. Therefo re only the areas seeded by the Au-Si particles can grow the Si nanowires. The whisker’s diameter a nd length are mainly determined by the droplet size and the time of the whisker’s growth p rocess respectively. By switching sources while the nanowires are still in the growth phase, compound nanowires with super-lattices of alternating materials can be fabricated, for exampl es, the nanowires with superlattices of Si/SiGe [38] and of InP/InAs [39]. The second main point of concern is the relation be tween the growth rate and the whisker diameter. Givargizov [37] pointed out that due to t he Gibbs-Thomson effect, the growth rate of the whiskers increases with the diameter. The super saturation of the metal liquid catalyst droplet is the driving force of the growth. Since the Gibbs -Thomson effect is designated that the decrease of supersaturation is a function of whisker diamete r d which is given as

d T k T k T k B vs B B 1 4 0 α µ µ Ω − ∆ = ∆

(1.1) where k B is the Boltzmann’s constant, T is the absolute temperature, µ ∆ is the effective difference between the chemical potential of nanowi re component materials in the nutrient (vapor or liquid) phase and in the whisker, o µ ∆ is the same difference at a plane interface ( ∞ → d ), vs α

is the specific free energy of the wire surface, Ω is the atomic volume of nanowire materials. From the experiment results, the dependence of the growth rate of whisker V on supersaturation

) ( T k B µ ∆

[36] is

n kT b V ) / ( µ ∆ =

(1.2) where b is a coefficient which is independent of the super saturation. Substituting T k B µ ∆ from Equation (1.1) in equation (1.2), Givargizov first described the growth rate according to the experimental results by the relationship

n B vs B o T rk T k b dt dL V         Ω − ∆ = = α µ 2

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Abstract: In this thesis, the fabrication and characterization of one-dimensional nanostructures have been studied systematically to understand the growth mechanism and structure transformation of one-dimensional nanostructures. The growth behavior of the ultrathin ZnSe nanowires with diameter less than 60 nm was found to be different from classical vapor-liquid-solid (VLS) process. The growth rate increases when the diameter of nanowires decreases, in contrast to the classical VLS process in which the growth rate increases with the diameter. The nucleation, initial growth, growth rates, defects, interface structures and growth direction of the nanowires were investigated by high resolution transmission electron microscopy (HRTEM). We found the structure and growth direction of ultra-thin nanowires are highly sensitive to growth temperatures and diameters of nanowires. At a low growth temperature (380°C), the growth direction for most nanowires is along . Planar defects were found throughout the nanowires. At a high growth temperature (530°C), uniform nanowires with diameters around 10nm were grown along and directions, and the nanowires with diameters larger than 20nm were mainly grown along direction. The possible growth mechanism of ultrathin nanowires was proposed by combining the solid catalytic growth with the interface diffusion theory, in order to explain how the growth temperature and the size of the catalysts influent the morphology, growth direction and growth rate of ultrathin nanowires. Structural and phase transformation of a nickel coated Si nanowire to NiSi2/SiC core-shell nanowire heterostructures has been investigated by the in-situ Transmission Electron Microscope (TEM). The phase transformation is a single-site nucleation process and therefore a single crystalline NiSi2 core resulted in the core-shell nanowire heterostructures. The transformation of the Si nanowire to NiSi2/SiC core-shell nanowire heterostructures was extremely fast and completed instantly due to high temperature annealing. Furthermore, the phase transformation preferred to begin at the defect and bending region of the nanowires, as the nickel can easily diffuse through the native oxide on the Si nanowires rather than the other regions. By removing the native oxide on the Si nanowires using HF, the temperature required for the phase transformation was decreased significantly. However, without the native oxide, the phase transformation became a multi-site nucleation process, and the nanowire became a polycrystalline and multiphase nickel silicide nanowires after the reaction. A simple and effective method is developed for fabricating high-quality vertically aligned ZnO nanowire arrays using carbonized photoresists. ZnO nanowires fabricated by this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. We further fabricated vertically aligned ZnO/a-Si core-shell heterojunction nanowire arrays through direct chemical vapor deposition (CVD) of amorphous silicon on ZnO nanowire surfaces. The thickness of the a-Si shells linearly increases with deposition time and the deposition rate was about 5nm/min at 530 °C. Since the Si shell is p-type and the ZnO core is intrinsic n-type semiconductors, the ZnO/ amorphous silicon core-shell nanowires naturally formed hetero p-n junctions. The antireflection property of the ZnO/amorphous silicon core-shell nanowires is dramatically enhanced due to the rough interface between the ZnO and amorphous silicon. Additionally, the intensity of the Photoluminescence spectrum of ZnO/amorphous silicon core-shell structures is decreasing with the thickness of the amorphous silicon shell increases.