DOI: 10.1021/cg900741y
Defect Formation in Ga-Catalyzed Silicon Nanowires )
S onia Conesa-Boj,*,† Ilaria Zardo,‡ S onia Estrade,† Li Wei,§ Pierre Jean Alet,§ § Pere Roca i Cabarrocas, Joan R. Morante,†, Francesca Peir o,† Anna Fontcuberta i Morral,‡,^ and Jordi Arbiol*,†,#
2010, Vol. 10 1534–1543
)
† Department d’Electr onica, Universitat de Barcelona, Martı´ i Franqu es 1, 08028, Barcelona, CAT, at M€ unchen, Spain, ‡Walter Schottky Institut and Physik Department, Technische Universit€ Am Coulombwall 3, 85748 Garching, Germany, §Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), Ecole Polytechnique, CNRS, 91128 Palaiseau, France, IREC, Catalonia Institute eriaux Semiconducteurs, for Energy Research, Barcelona 08019, Spain, ^Laboratoire des Mat Institut des Mat eriaux, Ecole Polytechnique F ed erale de Lausanne, 1015 Lausanne, Switzerland, and # Instituci o Catalana de Recerca i Estudis Avanc-ats (ICREA) and Institut de Ci encia de Materials de Barcelona, CSIC, 08193 Bellaterra, CAT, Spain
Received July 2, 2009; Revised Manuscript Received February 18, 2010
ABSTRACT: The synthesis of silicon nanowires by Ga-assisted plasma enhanced chemical vapor deposition (PECVD) has been recently demonstrated. In the present work, we study in detail the structural characteristics of the synthesized nanowires. High resolution transmission electron microscopy (HRTEM) analysis reveals the existence of various types of structural defects, which can be classified mainly according to the orientation into axial twins, lateral twins, and transverse twins. We compare our results with previous studies of Si nanowires synthesized with other catalyst metals. Understanding both the origin and the effects of the observed defects is important for technological applications. The presence of twinned domains changes locally the structure of the material. As a consequence, one should find a different local density of states and band gap, which should result in a variation of the carrier transport and optical properties of the nanowires.
1. Introduction One-dimensional (1D) nanostructures, such as nanotubes and nanowires (NWs), have attracted extraordinary attention in the past decade because of their unique physical and chemical properties, which allow them to be used in novel miniaturized electronic,1-5 optoelectronic,6,7 chemical,8 magnetic (e.g., spintronics),9 and energy conversion devices.10,11 Nanowire synthesis has been approached by several methods, most of them catalyst-assisted. In particular, the use of gold as nucleation and growth seeds has been the most successful method for the nanowire synthesis process.12-14 However, gold is an undesired metal for Si-based technology. Gold incorporation in Si leads to the creation of deep level defects in the band gap, leading to recombination traps, which results in a strong suppression of the carrier mobility and a decrease in the minority carrier lifetime.15 As a consequence, the use of other catalysts such as Cu,16,17 In,18-21 Al,18,22 Sn,21 or Bi23,24 or even catalyst-free nanowires growth25-28 has been studied. In the case of Si NWs, the strategy in looking for alternative catalysts is to find metals that are either significantly less soluble in silicon or for which the ionization energies are close to the band edges (thereby inducing a possible doping of the nanowire). Recently, Ga has been successfully implemented as an alternative catalyst for GaAs and Si nanowires growth.29-33 Gallium acts as a p-dopant in silicon. In principle, its incorporation should increase the carrier density of the nanowire, without strongly suppressing mobility or minority carrier lifetime. *Corresponding authors. E-mail addresses:
[email protected] and arbiol@ icrea.cat. pubs.acs.org/crystal
Published on Web 03/08/2010
As is the case every time new catalysts or new growth methods are found, the Si nanowire morphology, growth direction, and crystallization are affected. The formation and resulting morphology of randomly distributed stacking faults and twins in nanowires has been investigated by several authors.17,34-38 The presence of twins is a source of carrier scattering, which may reduce the mobility and coherence in electronic transport experiments. A rotationally twin plane in a zinc-blende (ZB) nanowire changes the atomic stacking locally and can be considered as a monolayer of the Wurtzite (WZ) phase.17,34,35,39 Formation of hexagonal domains in group IV and III-V nanowires has been the subject of intensive study in the past few years due to the dramatic influence of these domains on the electronic, optoelectronic, and thermal conductivity properties of these heterostructures as elements for electronic devices.34,35,40-42 Hexagonal silicon is believed to have a bandgap close to 1.3 μm (∼0.85 eV).43 Therefore, silicon nanowires with hexagonal domain inclusions form quantum heterostructures which could find applications in the optical communications domain.17 This, together with the possibility of achieving extremely high aspect ratios and radial quantum confinement can potentially transform silicon technology into an alternative to III-V and SiGe.12,44,45 In the present work, we analyze in detail the structural characteristics of silicon nanowires synthesized by plasma enhanced chemical vapor deposition (PECVD) using gallium as a catalyst. The use of gallium as a catalyst and PECVD has a critical influence on the nanowire crystallization, sometimes leading to the formation of twins and stacking-fault defects on the cubic structure. The periodicity r 2010 American Chemical Society
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Figure 1. Scanning electron micrographs of silicon nanowires synthesized with 2 nm (a-c) and 5 nm of gallium (d-f). The growth temperature was varied from 500 to 600 C, as indicated on the images.
Figure 2. (a and b) BF and HAADF STEM images of the Si NW, respectively. (c) HAADF STEM micrograph of the Si NW. The red arrow is indicating the scanning profile used for EELS analysis. (d) Relative quantification along the highlighted red arrow in part b.
of these faults, mainly twin defects, and the intersection among them will be shown to create a local hexagonal superstructure. In order to study in detail the various classes of defect arrangements found in our nanowires, a highresolution transmission electron microscopy analysis has been performed. Since our aim is to understand defect formation in Ga-assisted Si nanowires, we compare our results with those corresponding to different catalysts, as reported in the literature. On top of this, to improve the understanding of our results, the semiquantitative geometrical model of ref 46, previously applied to Au-seeded group III-V and group IV nanowires, is now used for Ga-seeded Si
nanowires, and their predictions are confronted with our direct HRTEM measurements. 2. Experimental Details Si nanowires were synthesized by plasma enhanced chemical vapor deposition (PECVD). For the nucleation and growth of the silicon nanowires, gallium was investigated as catalyst. Gallium layers with a thickness of 2 nm and 5 nm were previously deposited in an UHV molecular beam epitaxy (MBE) chamber on epiready (001) oriented GaAs wafers. Prior to growth, the substrates were exposed for 5 min to a 5 W hydrogen plasma. The advantage of the hydrogen plasma was 2-fold: first, the substrate could be cleaned from any contamination,
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Figure 3. Characteristic examples of the three main types of twinning defects which are found in silicon nanowires: (a) transverse twins (TT) in Cu-assisted Si NWs sometimes named as rotational or lamellar twins; (b) axial twins (AT) in Ga-assisted Si NWs; and (c) lateral twins (LT) in In-assisted Si NWs. and second, the surface gallium oxide was reduced in order to ensure a catalytic action of gallium. The PECVD runs were realized at temperatures between 500 and 600 C. More details on nanowire synthesis can be found in previous work.33 The samples were prepared for the TEM observation by mechanically removing the Si nanowires from the substrate with a razor blade and diluting them in a hexane suspension. A drop was then deposited on a holey carbon copper grid. Before introducing the sample into the microscope, it was introduced into the plasma cleaner for 15 s. The morphology and structure of the nanowires with atomic resolution was characterized by means of high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) in bright field (BF) and high angular annular dark field (HAADF) modes in a Jeol 2010F field emission gun microscope with a 0.19 nm point to point resolution.
3. Results and Discussion 3.1. Electron Microscopy Analysis: General Features. After synthesis, the morphology of the nanowires was characte rized by scanning electron microscopy (SEM). As shown in Figure 1, the nanowires appear to be oriented with respect to the GaAs substrate. Depending on the growth direction, the angle between the nanowire and the substrate is different. This is a result of the heteroepitaxy of the Si nanowire and the GaAs substrate. In a first overview of the preferential growth directions, nanowires grown with a 2 nm Ga layer exhibit mainly growth in the Æ112æ and Æ110æ directions, while, for nanowires grown with a 5 nm Ga layer, the orientation depends on the growth temperature. At 600 C the predominant growth direction is Æ111æ, at 500 C, it is Æ112æ, and at 550 C, a mixture between both is found. The bright field (BF) and high-angle annular dark field (HAADF) images shown in Figure 2 a and b, respectively, are representative of a typical Si nanowire obtained with a 2 nm Ga layer at 600 C. We can observe a contrast variation along its length this is due to the fact that these nanowires do not have a uniform diameter along their length and exhibit tapering. The average length of these nanowires is about 0.7-0.9 μm. The HAADF image in Figure 2c shows the Ga droplet which catalyzed the growth. The compositions of nanowires and catalyst particles were analyzed using electron energy loss spectroscopy (EELS). As shown in Figure 2d, the scanning result suggests that the catalyst particle and the nanowires are composed of Si (green line) and Ga (orange line), respectively. The rest of the nanowire is composed of pure Si with no presence of Ga up to the EELS detection limit (around 1 atom %).
After analyzing the sample general morphology, a precision analysis at atomic scale is required. From HRTEM images we have classified the different types of defects which arise in Si nanowires and which will be discussed at length below. We show in Figure 3 the three main types of defects that are typically observed in this type of nanowires: transverse twins (TT) perpendicular to the growth axis, axial tTwins (AT) parallel to the growth axis, and lateral twins (LT) corresponding to twin defects appearing on the tapered NWs lateral sides. These last defects, LTs, occur along the (1,-1,-1) and (-1,-1,1) planes and form 82.28 (LTA) and 97.72 (LTB) angles with respect to the [1,-1,1] growth axis, respectively. 3.2. Defect Types. 3.2.1. Lateral Twins. Now we turn to discuss in detail the HRTEM analysis of the sample of Si nanowires grown at 600 C from 5 nm of gallium (see Figure 4); nanowires grown in the [1,-1,1] direction present lateral twins (LTs) corresponding to twin defects occurring along the (1,-1,-1) and (-1,-1,1) planes and forming 82.28 (LTA) and 97.72 (LTB) angles with respect to the [1,-1,1] growth axis, respectively. Another important characteristic is present in the Ga-Si system, in special for nanowires grown at high temperatures: these nanowires exhibit a slight tapering (conical morphology) due to sidewall deposition. This is a result of the use of PECVD for the synthesis.33 This implies the decomposition of the precursors already in the gas phase, which leads to sidewall deposition during the growth of the nanowires. During PECVD process, silane molecules are dissociated in the gas phase with no need of catalyst. This leads to a nonzero planar rate. Planar growth rates were compared to nanowire growth rates, observing a 9-fold difference.33 By lowering the growth temperatures, the degree of tapering can be strongly reduced. This behavior is also observed in other cases like Al-Si system.22 Such tapering of the nanowires is a relevant feature since it might induce the formation of lateral twins. In most cases, these LT planes are observed at the lateral region of the nanowires and intersect the TT planes perpendicular to the nanowire growth axis. The intersection of a TT with a LT in some cases may lead to interesting structure. The accumulation of twin defects was interpreted to be responsible for the occurrence of the diamond hexagonal phase in silicon.47 In the present work, we will show how the intersection between TT and LT twins may lead to the formation of nanodomains with diamond hexagonal phase.
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Figure 4. (a) HRTEM micrograph of a typical Si NW grown at 600 C from 5 nm of gallium on a GaAs substrate; (b) power spectrum (FFT) of the crystalline structure in the region with a transverse twin (TT) present; (c) a zoom of the selected region marked with a pink square in Figure 4a; (d) power spectrum of a lateral region of the NW; (e) a zoom of the selected region marked with a blue square in Figure 4a, where four green arrows signal the presence of fringes of the hexagonal phase.
In Figure 4e, a region of the nanowire presenting the crossing of three different types of twin planes is presented (TT, LTA, and LTB). We have marked this special region with a blue square in Figure 4a. After a careful analysis, it is possible to observe the appearance of four fringes of diamond hexagonal structure (wurtzite-like (WZ)) with green arrows marking the (0001) planes. In order to better understand the formation of a hexagonal phase from the intersection of twins in different spatial directions, we have modeled the crossing of twins with a simple crystallographic model. In Figure 5 we show a schematic illustration of intersecting twins in a fcc matrix M in [110] projection with primary (1) and secondary (2) twins and a transformed hexagonal region (H). This situation corresponds to the twinning phenomena observed in the lateral regions of our nanowires; see Figure 4e. Notice that when the role of the matrix (M) and twin (1) are interchanged, then twin (2) becomes a secondary twin with respect to M. If local stress conditions lead to a secondary twin being nucleated within the first twin, its propagation into the matrix would lead to development of a hexagonal plate. The phenomena of twin intersections in fcc structures and the subsequent
crystallographic transformations have been studied for bulk materials in detail by Blewitt et al.48 and Mahajan and coworkers.49 The interface (111) represents the usual cubic/ hexagonal transformation in a fcc crystal, which is achieved by the shear of 1/6[112] partial dislocations on every other (111) plane. This mechanism changes the stacking sequence from ABCABC to ABABAB. The resulting orientation is (111)Si I//(0001)Si IV. 3.2.2. Axial Twinning. Figure 6a shows a HRTEM image of a typical Si NW grown at 600 C from 2 nm of gallium. NW with several twin defects crossing its entire length is shown. We have indicated the twin interfaces with dashed lines. In this nanowire with 20 nm in diameter, there are a total of 9 twin interfaces, separated by between 2 and 5 nm. The growth direction is unambiguously {112}, and the zone axis is [1,-1,0], which corresponds to a 90 rotation along the growth axis versus the [-1,-1,1] zone axis, assuming a simple Si cubic phase. An example of a similar NW to that shown in Figure 6a but observed along the [-1,-1,1] zone axis is shown in Figure 7a. It is well-known that {111} twinning frequently occurs in nanowires.32,35,38,50-53 Twins have been mainly observed as TT
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Figure 5. Schematic illustration of intersecting twins in FCC, where M denotes the matrix, 1 and 2 are twins, and H is the hexagonal phase.
Figure 6. (a) Si NW grown at 600 C from 2 nm Ga, growing along the [112] direction with twin defects in several directions. (b) Power spectrum (FFT) of the crystalline structure corresponding to the red square in part a. (c) Twinning in various nonparallel orientations along the (1,1,-1), (-1,1,1), and (1,1,1) directions.
perpendicular to the growth axis for nanowires grown along the [111] axis.16,17 Axial twins parallel to the growth axis have also been observed in nanowires grown along the [112] axis. However, twinning phenomena may occur on the four sets of {111} planes. In particular, in this sample, three different twin families have been identified (see Figure 6c), occurring along the (1,1,-1), (-1,1,1), and (111) planes. The changes in growth direction are summarized in Table 1. The (1,1,-1) twin planes are located in separated regions oriented along the [1,0,1] and [1,-1,0] zone axes, being labeled as the A and B regions, respectively. The (-1,1,1) twin planes separate
regions oriented along the [1,0,1] and [-1,-1,0] zone axes, being labeled the A and D regions, respectively. Finally, the (1,1,1) twin planes separate regions oriented along the [1,-1,0] and [0,-1,1] zone axes, being labeled as the B and E regions, respectively. It has been suggested that axial twins formation probably occurs during nanowire nucleation.46 These types of twins are present along the whole length of the nanowire, up to the interface with the eutectic alloy. This suggests that these twin defects originate during nucleation and that as the nanowires continue to grow, the twins extend down along the length of
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Figure 7. Detailed structural analysis of a 2 nm Ga-catalyzed Si NW grown at 600 C: (a) typical HRTEM micrograph; (b) power spectrum (FFT) of the crystalline structure. The power spectrum can be indexed as a silicon cubic or silicon hexagonal phase.
Table 1. Indexation of the Spots of the Power Spectrum in Figure 6b region A spot no.
indexation
C A1 A2 A3
(1,1,-1) (0,2,0) (-1,1,1) (-2,0,2) zone axis: [1,0,1] growth direction: [-1,2,1] region D spot no.
indexation
A2 (-1,1,1) D (-1,1,-1) zone axis: [-1,-1,0]
region B spot no.
indexation
C (1,1,-1) B1 (0,0,2) B2 (1,1,1) B3 (2,2,0) zone axis: [1,-1,0] growth direction: [1,1,2] region E spot no. B2 E
indexation
(1,1,1) (0,-1,1) zone axis: [0,-1,1]
the nanowire. The {111} twins formed during nucleation provide preferential addition sites that subsequently maintain nanowire growth in the [112] directions as opposed to the [111] direction. It is interesting to point out that a high density of axial twins periodically separated can lead to the formation of a wide variety of Si polytypes, with the diamond hexagonal phase being one of those, as Lopez et al.38 demonstrated recently. Within our sample (Si nanowire grown at 600 C from 2 nm of gallium), we also find the presence of NWs oriented along the [-1,-1,1] zone axis (Figure 7a). These NWs are rotated 90 along the [112] growth direction with respect to those described above (Figure 6a). It is interesting enough to comment that along this orientation the axial twins cannot be observed, as they lay perpendicularly to the zone axis (parallel to the observed plane). For this reason, these nanowires do not show apparent structural defects. As an example, in Figure 7a, we show a HRTEM image of one such nanowire. From the micrograph, the nanowire seems to be single crystalline, with no appreciable defects in the structure. However, contrast variations along its length are observed denoting that some crystallographic phenomena are occurring. In order to analyze in more detail the crystallographic properties of this orientation, we have calculated the power spectrum of the HRTEM micrograph in Figure 7a, as shown in Figure 7b. From fast Fourier transform (FFT), the angles between planes and interplanar
Table 2. Indexation of the Spots of the Power Spectrum in Figure 7b Depending on the Considered Crystalline Structure Si I or Si IV case Si I spot no.
indexation
1 (0,2,2) 2 (2,0,2) 3 (-2,2,0) 1 /3(2,2,4) 4 5 (2,2,4) zone axis: [-1,-1,1]
case Si IV spot no. 1 2 3 4 5
indexation
(2,-1,-1,0) (1,1,-2,0) (1,-2,1,0) (1,0,-1,0) (3,0,-3,0) zone axis: [0,0,0,1]
distances were calculated. This numerical calculation should allow the determination of the crystalline structure of the wire and its growth direction. In order to know what structure this is and to index the diffraction spots, we simulated diffraction patterns for different possible Si structures; using Carine software;and compared them with the power spectrum obtained from the HRTEM measurements. Remarkably, the pattern could be perfectly indexed with both the silicon diamond cubic (Si I) and the silicon diamond hexagonal (Si IV or wurtzite) phase.14 In Table 2 we show the correspondence between the plot labels and the indexation, depending on the considered crystalline structure: silicon diamond cubic (Si I) or silicon diamond hexagonal (Si IV). If we considered the structure of the nanowire to be hexagonal, the observed zone axis would be [0,0,0,1] and the nanowire growth direction [1,0,-1,0]. In this case, all the spots that are present in the power spectrum could be easily indexed (as shown in Table 2). However, if we assumed that the crystal phase was silicon diamond cubic (Si I), the observed zone axis would correspond to {111}-type, and the nanowire would grow along the [112] direction. In this case, some additional weak spots that appear in the power spectrum cannot be easily indexed (e.g., the one labeled with number 4 in Figure 7b). This spot and those at the same radial distances are visible at a distance of 1/3 to the {224} main spots (labeled as 5 in Figure 7b). These spots, according to the literature, appear frequently in certain Si NWs and have been identified as 1/3 [224] reflections, which are forbidden for simple fcc structures. The authors in ref 53 found these reflections in Si nanowires grown along the [112] axis and oriented in the [111]
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Table 3. Indexation of the Spots of the Power Spectrum in Figure 4b region A spot no. C A1 A2
indexation
(1,-1,1) (1,-1,-1) (002) zone axis: [1,1,0] growth direction: [1,-1,1]
region B spot no.
indexation
C B1 B2
(1,-1,1) (2,0,0) (-1,-1,1) zone axis: [0,1,1] growth direction: [1,-1,1]
direction, which were the consequence of [111] rotational twins lying along the growth direction of the nanowires (AT), in good agreement with the results observed by us in those nanowires observed along the [1,-1,0] axis (Figure 6a). In addition,54 they also established that these extra spot reflections in the diffraction patterns could be explained by a normal cubic silicon structure with additional twinning defects, also in good agreement with our observations. Two different cases for the extra spots can be identified: those created by double diffraction between microtwins superposed along the electron beam direction and those created by nanotwins due to streaking effects. If the defect is thin (1 or 2 monolayers), the streaks are elongated and can intersect the Ewald sphere in order to produce “extra” diffraction spots. The above discussion implies that we can attribute the Si I structure to our sample. Moreover, the extra spot reflections observed in our sample, which are forbidden for a simple fcc structure, can be explained in terms of the presence of axial twins, in agreement with previous results in the literature.53,54 Note that this is also consistent with our own observations of these axial twins in nanowires with perpendicular orientation; see Figure 6. 3.2.3. Transverse Twins Perpendicular to the Growth Direction. Figure 4 corresponds to the HRTEM micrograph of a typical Si NW grown at 600 C from 5 nm of gallium. The growth axis of the nanowires is along the [1,-1,1] direction. These nanowires exhibit rotational twins perpendicular to the growth axis (transverse twins (TT) or lamellar twins, as described by Davidson et al.46). This type of defect is very common in III-V and group IV nanowires, as in most of the cases the growth direction is along the {111} axis.17,25,55 For clarity, we have marked the TT planes with red arrows in Figure 4. In Table 3 we show the indexation of the power spectrum of Figure 4b. It is also interesting to note that twins on the nanowire borders (lateral twins (LT)), corresponding to {111} family planes, have also been observed. This is highlighted with the yellow lines in Figure 4. Interestingly, the lateral {111} twins do not seem to propagate to the center of the nanowire and stay close to the facets. Reference 46 proposes the development of a semiquantitative geometric model for the prediction of whether or not {111} transverse twinning in III-V and IV nanowires is expected to occur for the growth axis in the Æ111æ direction. This model can be applied to nanowires grown by VLS, solid-liquid-solid (SLS), and supercritical-liquid-fluidsolid (SFLS) processes. The main outcome from the model is a geometrical equation that relates the surface tensions of the system and several angles: γSi-droplet sin νmax ¼ σ s -σsl cos νmax -σl cos θL ð1Þ where θl is the contact angle between the liquid droplet and the sidewall facet, ν is the angle between the direction of the nanowire sidewall facet and the crystallization interface at
Figure 8. Curves represent the variation of νmax as a function of γSidroplet, calculated from eq 1 using different values of interfacial tension and contact angle from the literature. For Au seeding (red) and Ga seeding (blue). The dotted black line defines the lower value of νmax for which twinning is expected to occur in this model. Table 4. Values of Interfacial Energies and Surface Tensions Used for the Model Calculationsa metal seed particle
metal type
θL (deg)
σS
σl
Au Ga
A C
9.3-19.846 17.74
1.24 J m-2 1.24 J m-2
0.85 J m-2 (0.6-0.8 J m-2)60
a The solid-liquid tension σsl is obtained from the contact angle θL and the solid and liquid energies.46
the tip of the nanowire, and σs, σsl, and σl are the solid, solid-liquid, and liquid interfacial tensions, respectively, and γSi-droplet is the interfacial energy. The interfacial tension at the TPB is related to the contact angle by σ2 þ σ2 - σ2
cos θL ¼ s 2σsl σl sl . For angles νmax greater than 109 (see ref 46 for the geometrical interpretation of this angle), twinning will be thermodynamically possible in the nanowires. In ref 46 the model is in perfect agreement with what it is experimentally observed with Au-seeded group IV nanowires with a Æ111æ growth direction. In this work, we apply this model to silicon nanowires such as grown with Ga as a different catalyst seed, in order to possibly explain the observed twins. To achieve this, the value of the νmax angle has been computed, which determines if the formation of twins in the case of growth along the [111] axis is allowed. In Figure 8 we show the variation of νmax as a function of γSi-droplet, calculated from eq 1, using different values of interfacial tension and contact angle from the literature. In Table 4 we list the relevant parameters for Si nanowires with Au-seeding. Different values have been measured for θL.46 Using θL = 9.3, we obtain the closed symbol blue line, whereas for θL = 19.8, we obtain the open symbol blue line. We can observe that those different θL values give similar νmax to γSi-droplet curves that we can use as a reference. Assuming νmax = 109 (horizontal black line) as the limit for twinning and taking into account that the γSi-Au value is known to be 0.09 J m-2,46 twins are not expected to appear in Au seeded as discussed in ref 46. We can apply a similar estimation for Ga seeding. In our case, the contact angle cos θL has been determined directly from HRTEM measurements of our samples being θL = 17.74. This introduces an uncertainty, as the value measured
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Table 5. Summary of the Main Morphological Features Which Appear in Si NWs as a Function of the Catalyst Used as Well as of the Different Growth Conditions (Such as Temperature) catalyst-seeded gold (Au)
16 nm >20 nm
aluminum (Al)
gallium (Ga)
30 nm ∼ 35 nm
comparison between
400 ∼430-490 (lower than the eutectic temperature of the Al-Si binary phase diagram, 577 C)
500 (from 5 nm of gallium layer) 600 (from 5 nm of gallium layer)
nanowires with the same
600 (from 2 nm of gallium layer)
diameter
indium (In)
In-droplet
