Corrigendum: Low and high density InAs nanowires on Si(001) and their Raman imaging (2013 Semicond. Sci. Technol. 28 015025)

Micro-Raman imaging along with other techniques are applied to study the morphology, structure and crystalline quality of various types of InAs nanowires (NWs). The NWs of low and high densities are formed using metal organic vapor phase epitaxy. Raman mapping is effectively used as a local probe to gain information about the structure and crystalline quality of low-density NWs where the conventional characterization techniques are not very useful. However, for high-density NWs, the image and crystalline quality obtained from the LO phonon strongly corroborate with scanning electron microscopy and x-ray diffraction (XRD) results, respectively. These low-density (10 4 cm − 2 ) and high-density (10 8 cm − 2 ) NWs are grown on Si(0 0 1) under various growth conditions such as catalyst-assisted and catalyst-free growth, growth on native oxide-covered and oxide-cleaned Si, grooved Si surfaces and also varying the V / III ratio and growth temperature. NWs (1 μ m long and 50–100 nm wide) with high density and tapered NWs (50–80 μ m long and 200–500 nm wide at the tip) with low density are formed under different growth conditions. The growth of hillock- and wire-like structures is observed under the same growth condition. Raman, XRD, scanning electron microscopy and atomic force microscopy analyses conﬁrm that the hillocks are grown along the (cid:2) 0 0 1 (cid:3) direction, whereas the wires are grown along [1 1 0] directions in the plane of Si(0 0 1). Furthermore, the Raman analysis of these NWs conﬁrms that the smaller NWs have much better crystalline quality (half-width of LO phonon frequency ∼ 6 cm − 1 ) compared to the larger NWs (half-width of LO phonon frequency ∼ 15 cm − 1 ) although both NWs are oriented with the Si(0 0 1) surface. (Some

Raman study of long InAs NW grown by MOCVD in self catalyst assisted method is shown in figure 3 of the original article. It shows a strong peak at ∼240 cm −1 . We had noted this peak as LO phonon of InAs as per literature survey available at that time. However, our recent study on the similar NWs (Suparna Pal et al 2014 Appl. Phys. Lett. 105 012110) shows that the Raman mode which appears at that frequency (240 cm −1 ) with large intensity at power density >100 kW cm −2 is basically InAs-oxide related peak (InAsO 4 ). In our recent finding we observed that the laser induced oxidation process of InAs occurs on the surface of the nanowire above a particular laser power density. The threshold power itself depends on the diameter of the wire. In the light of our present findings we believe that the 240 cm −1 peak shown in figure 3 should be attributed to InAsO 4 and not the InAs LO phonon. Further, systematic power dependent Raman study carried out on similar NWs with very low laser power density shows TO phonon exclusively from the center of the wire suggesting that growth of these NWs is not epitaxial with the Si(001) substrate.

Introduction
III-V nanowires (NWs) are realized to have a potential role in the advanced technologies such as high performance field effect transistors, photodetectors, chemical/biosensors and thermo-electric devices [1][2][3]. Various III-V NW structures 6 Author to whom any correspondence should be addressed.
based on GaAs, GaN, GaP, InP, InAs and their related materials are found to be technologically important [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. In particular, InAs NWs are promising because of their narrow bandgap with small electron effective mass, high electron mobility, strong quantum confinement effect and unique electro-optical properties enabling an application in infrared photodetectors and high-speed electronics [1,13,14]. The integration of III-V compound semiconductors, which are dominant in applications of all kinds of optoelectronic devices, with main stream silicon technology is a very important goal for the semiconductor industry because of its promise to combine the best performance of different material systems with cost effectiveness. However, this has remained a challenging task for a long time due to the large epitaxial strains and defect densities arising from the large lattice mismatch. Recently, several researchers reported growth of high-quality III-V thin films on the Si substrate using MOVPE [15][16][17]. This integration led to successful fabrication of bright LED [18].
To combine this with the advantages of physical and electronic properties of nanostrctures, presently great efforts are made to grow III-V NWs directly on Si.
The vapor-liquid-solid (VLS) process has now become a widely used method for generating one-dimensional nanostructures of elemental and compound semiconductors . In a VLS process, catalyst-assisted NW growth, an important issue is that the catalyst (gold, for example) diffuses into the wires and introduces deep level traps leading to a change in the electronic properties of NWs. Therefore, the use of metals acting as a catalyst such as gold (Au), copper (Cu) and silver (Ag) should be avoided while integrating with Si because they can diffuse in the devices under high electric field and high operating temperature conditions. Hence, considerable efforts are made to achieve self-catalyst/catalystfree growth of III-V NWs on Si. Self-catalyst e.g. indium droplets and SiO x -Si matrix are used for the NW growth, where it is shown that the reaction between indium and SiO x gives rise to liquid droplets that induce the NW nucleation [23][24][25]. The growth of catalyst-free InP NWs was obtained on Si substrates, where In droplets were formed on a specific crystal orientation after thermal treating of the surface. A surface reconstruction induces the indium droplet to form on the surface and thus act as nucleation sites for the NW growth [12]. The catalyst-free growth of III-V NWs was achieved by selective area (SA) MOVPE and molecular beam epitaxy on patterned substrates using an additional nanolithography step [26][27][28][29]. The SA growth (SAG) has better cite control and uniformity for the growth of NWs but the requirement of a prior nanolithography step makes it complicated. The catalyst-free growth of InAs NWs on Si(1 1 1) by the Volmer-Weber growth mode using MOVPE is also reported [1,12,[26][27][28][29][30][31][32][33]. It is interesting to know that the study of all these catalyst-assisted and catalyst-free growth techniques including the SAG was mostly confined to (1 1 1)-oriented substrates. However, for the industrial application, the integration of III-V NWs to the well-established Si(0 0 1) technology is extremely important. In spite of successful growth of InAs NWs on the Si(1 1 1) surface, these mechanisms are not been extensively explored on the Si(0 0 1) surface, which is preferred for industrial applications and still a challenging task [34]. In this paper, we have addressed the MOVPE growth of InAs NWs on the Si(0 0 1) substrate under various growth conditions such as self-catalyst-assisted and catalyst-free, on oxide-covered and oxide-cleaned, by varying the V/III ratio and growth temperature. Furthermore, the catalyst-free growth on grooved Si(0 0 1) is also explored. It has been observed that depending on the growth conditions, various types of NW structures with low and high densities are formed on Si(0 0 1). To the best of our knowledge, such a systematic and comparative study on the growth of InAs NWs has not been reported earlier on Si(0 0 1). Very few studies reported the growth of InAs NW on Si(0 0 1) with the assistance of catalyst (gold particle or self-catalyst) [22,30]. We report the catalyst-free growth on Si(0 0 1) and our results are very different from the earlier reports. The earlier works reported the InAs NW growth on Si(0 0 1) along the [1 1 1] direction. However, we have observed the in-plane growth of various types of InAs NWs on Si(0 0 1), which are oriented with the substrate under different growth conditions and confirmed that the NWs are not grown along the [1 1 1] direction in our case. Along with the NWs, we have observed island growth along the 0 0 1 direction under the same growth condition. To the best of our knowledge, such a report on the catalyst-free growth of InAs NWs on the (0 0 1) substrate is not available. We also report the growth of very long and tapered NWs on the native oxide-covered Si(0 0 1) surface and present a possible growth process for it. We also compare our results of InAs growth on native oxide-covered Si(0 0 1) with those reported by SAG of InAs NWs where the substrate is patterned with a thick SiO 2 before growth [27,28]. This systematic study of growth led to the formation of different novel nanostructures of InAs on the Si(0 0 1) surface with different lengths (1-100 μm) and densities. SEM, TEM and x-ray diffraction (XRD) are the main techniques which are applied to study the morphology, structure and crystalline quality of various types of NWs. However, for obtaining the structure and crystalline quality of low-density NWs at local regions, none of these techniques are suitable. Under such conditions, the micro-Raman mapping is found to be a suitable technique to obtain structure and crystalline quality at the local region of NWs. Recently, Raman imaging has emerged as an ideally suited technique for structural and compositional identification of nanostructures since it is non-evasive and does not require any sample preparation. This can give information of chemical composition, structural changes, stress and electron-phonon coupling along the length/diameter of an NW, which can be related to the morphology obtained from SEM/TEM. In addition, spatially resolved data at different sites on a bigger nanostructure can give interesting information. Ni et al have shown that Raman imaging can be used as a quick and unambiguous method to determine the number of graphene layers [35]. They have also studied the effect of substrates, top insulator deposition, annealing as well as folding and stacking order on the physical and electronic structure of graphene using Raman imaging and spectroscopy. In addition, we report the micro-Raman imaging for high-density small NWs in which images are generated using LO phonon peaks. To the best of our knowledge, such Raman imaging studies of NWs have not been reported except for few reports on Raman studies (without imaging) of InAs NWs grown on (1 1 1)-oriented substrates [36][37][38][39]. Furthermore, the morphology and the crystalline quality of high-density NWs are also studied using SEM, TEM, AFM and XRD.

Experimental details
The NWs were grown on native oxide-covered (SiO 2 ) and oxide-cleaned Si surface using AIXTRON low pressure MOVPE machine (AIX-200). Trimethylindium (TMIn) and Arsine (AsH 3 ) were used as source materials. Two growth methods were adopted: (i) self-catalyst, i.e. In dropletassisted growth on SiO 2 and Si surfaces; (ii) catalyst-free growth on SiO 2 and Si surfaces. The catalyst-free growth was also explored on a grooved surface of SiO 2 /Si. Native SiO 2 cleaning on an Si wafer was performed following the standard Radio Corporation of America (RCA) cleaning procedure described by Dixit et al [15]. The growth of InAs NWs was carried out mainly in the temperature range of 425-550 • C, which is the preferred temperature zone for the growth of InAs NWs [1,12,[30][31][32][33]. The preference of this temperature zone can be explained considering two main growth parameters: (i) decomposition temperature of TMI and AsH 3 ; (ii) surface diffusion of group-III atoms, i.e. indium and incorporation of the adatoms into the NW crystal lattice. Joyce et al claimed that TMI decomposition is complete at 425 • C, whereas AsH 3 decomposition increases dramatically between 350 and 525 • C. Therefore, as the NW growth temperature is raised above 425 • C, TMI decomposition remains relatively steady, whereas AsH 3 decomposition increases [4]. However, Jacko and Price [40] reported that decomposition of TMI is a multi-step process and complete decomposition of TMI occurs at much higher temperature (>450 • C). Moreover, the surface diffusion of indium atoms increases with temperature. Considering these two parameters, we selected a growth temperature zone between 425 and 550 • C which favors a considerable fraction of decomposition of the source materials and also the surface diffusion to enhance the one-dimensional growth. For all the growth conditions, the TMI flow was kept constant at 3 μmol min −1 , while the group V/III ratio was varied between 100 and 325 by changing the AsH 3 flow. The grown NW structures were characterized using Raman imaging, SEM, TEM, AFM and XRD. SEM was carried out using a Phillips XL 30CP and for TEM measurement, a Phillips CM 200 transmission electron microscope was used. XRD measurements were carried out using the PANalytical X'pert machine and D8 Discover, Bruker, with source of CuKα. The Raman imaging was performed using excitation of He-Cd and Ar-ion laser with 50 × microscope objective (spatial resolution of ∼1 μm) and Acton 2500i monochromator with a CCD detector, a part of SPM_integrated Raman system setup, WiTec (Germany).

Results and discussions
Different naostructures of InAs with low (∼10 4 cm −2 ) and high (10 8 cm −2 ) densities have been observed on Si(0 0 1) by varying the growth conditions, which are described below.

Growth method
3.1.1. Self-catalyst-assisted growth: low-density NWs. In the self-catalyst-assisted growth, both RCA-cleaned and native oxide-covered Si(0 0 1) wafers were introduced simultaneously for the formation of In droplets at 425 • C into the MOVPE reactor. The In droplets were formed by flowing 3 μmol of TMI for 5 s and immediately after this, the InAs growth was carried out by incorporating TMI and AsH 3 simultaneously. Under this condition, the island growth of InAs of size 50 nm diameters was observed in both RCA-cleaned and native oxide-covered Si(0 0 1) wafers. For another set of samples, the RCA-cleaned and native oxidecovered Si(0 0 1) wafers were annealed first at 625 • C under H 2 flow for 5 min and then the reactor was cooled down to the growth temperature, which is ∼425 • C. Thereafter, InAs nanostructures were grown under the same growth conditions as mentioned above. It is observed that this growth condition led to the formation of long (50-80 μm), tapered (tapering factor of 0.02) NWs on the oxide-covered surface (figure 1(a)), while the 3D island growth was observed on the oxidecleaned surface ( figure 1(b)). The growth was carried out at different growth temperatures in the range of 425-550 • C by varying the V/III ratio from 100 to 350 with and without a preannealing step at 625 • C. We observed that without the preannealing step, only the island growth took place on native oxide-covered Si(0 0 1) and this observation remains the same irrespective of the change in the growth temperature (up to ∼500 • C) and the V/III ratio. Only the island size increases with the increase in the V/III ratio. The growth of few NW structures of large dimension (200 nm diameter) takes place only at higher temperature, i.e. 550 • C, which are not tapered (figure 1(c)) as in the case of growth at 425-475 • C with a preannealing step. On the other hand, when the preannealing step at 625 • C was used, long-tapered NWs were grown on the sample. The length, diameter and tapering factor and the density of the NWs did not change with change in the growth temperature or V/III ratio in the range mentioned above. The growth temperature was not increased beyond 500 • C as it was close to the preannealing temperature. Hence, it can be concluded that in the temperature range of 425-500 • C, the oxide-cleaned surface of Si(0 0 1) led to the island formation, irrespective of preannealing of wafer. This indicates that low growth temperature (<500 • C) though may be sufficient for the InAs growth, the surface diffusion at this temperature is not enough which can lead to anisotropic growth giving rise to wire-like structures. Rather, InAs expands isotropically after nucleation leading to island growth [30]. However, the native oxide-covered wafer led to two different morphologies of InAs nanostructures depending on the wafer preannealing condition. The growth of tapered NW on oxide-covered Si(0 0 1) with a preannealing step can be understood in the following way. The preannealing (∼625 • C) of oxide-covered Si wafer can produce irregular Si +2 , Si +4 bonds which might have influenced the nucleation centers [41]. Alternately, fine cracks/craters formed on the thin native oxide layer reaching up to the Si surface might have formed during the preannealing [27][28][29][30]. After nucleation, the wire seems to start growing in the limited space of the nanocrater and then extend beyond the crater size. The wire size might be larger than the crater/crack size and the tapered nature of these long NWs indicates that this is a catalyst-assisted growth (in the present case, selfcatalyst In droplet) which leads to gradually extinguishing shape. The NW growth on the SiO 2 layer is not possible since the surface diffusion is very high on the SiO 2 surface leading to very low sticking probability [27]. From the observation that the dimension of the NWs does not vary with the growth temperature and V/III ratio, it is inferred that the InAs growth on native SiO x is not controlled by the growth parameters but by the surface condition. Using a preannealing step, the growth of NWs on this native oxide-covered surface is possible but it is difficult to control the NW dimension, etc, by varying the growth temperature and V/III ratio. Our study reveals that the growth of NWs on native oxide-covered Si(0 0 1) is mostly governed by the craters formed on the Si surface during the preannealing step. The dimension of the NWs grown under this condition is related to the crater/crack size, which does not vary significantly with growth temperature but is mostly determined by the preannealing step. It should be mentioned that along with long-tapered NWs, the spherical island-like growth has also taken place as can be seen from figure 1(a). These two different nanostructures have been characterized using position-resolved Raman spectroscopy and will be discussed later.

3.1.2.
Catalyst-free growth: high-density NWs. It is to be noted that under similar growth conditions used for the catalyst-assisted growth, we have not observed any InAs growth on oxide-cleaned and native oxide-covered Si(0 0 1) wafers at 425 • C without In droplet formation. This may be attributed to very low decomposition of TMI and AsH 3 at this low temperature. Furthermore, the surface diffusion is As the V/III ratio is increased from 100 to 250, the diameter of the NWs reduces and length increases. At V/III ratio = 250, the trend in the diameter shows a minimum and the length shows a maximum. These trends can be understood by the consumption of arsenic flux by indium with an increase in the V/III ratio. As the length increases due to the enhanced growth rate in the axial direction of the NW, the diameter automatically reduces in the lateral direction and both show extrema at V/III = 250. Increasing the ratio further to 350, the too large flow of arsine suppresses the axial growth rate and the length of the NW falls and the average diameter also starts increasing slightly showing a trend toward the 3D growth [12]. On the other hand, under the same growth condition on native oxide-covered Si(0 0 1) wafers, arbitrarily scattered, large-sized NWs (∼200 nm diameter and ∼1-2 μm long) were observed ( figure 1(d)). Thus, the InAs NWs at 550 • C are formed on the oxide-cleaned Si(0 0 1) wafer even without any catalyst under the optimum V/III ratio of 250. For the catalyst-free growth, the preannealing step has not been explored since the growth temperature is 550 • C, which is closer to the annealing temperature (625 • C). Under all these growth conditions, along with small NWs, some flat-topped nanohillock (NH)-like structures of much larger height were also formed as shown in figure 4. The density of these hillocks varied on the sample surface. A growth mechanism for the formation of two different types of structures (NW and NH) on the Si surface under the same growth condition is discussed. It is known that for the growth of III/V on the Si substrate, two issues can play an important role: (i) lattice mismatch (7% in the case of InAs on Si); (ii) antiphase domain (APD) formation [16]. Due to the large lattice mismatch between InAs and Si, selfassembled InAs islands (nuclei) grow on the oxide-free Si surface with the Volmer-Weber island growth mode under these growth conditions. The InAs growth starts on the Si(0 0 1) in the preferential direction of 0 0 1 with island formation (majority) which gives rise to NH-like structures in our case. It is clearly seen from figure 4 that the growth of NWs is generally initiated from the edge of the NH-like structures and continued in both the directions. In this case, it may be anticipated that after nucleating at the edge of the coalescing islands, APD edge, the wire-like growth takes place in the lateral direction, which is perpendicular to 0 0 1 , i.e. in the (0 0 1) plane. However, the formation of the NW need not be necessarily associated with an NH. It may nucleate through the Volmer-Weber island growth mode. After nucleation, it is the anisotropic diffusion in the (0 0 1) plane that gives rise to a much higher growth rate in the axial direction compared to the lateral direction leading to the wire-like growth. Schmidbauer et al reported that on a (0 0 1) plane, diffusion is anisotropic [42]. This anisotropy in the surface diffusion leads to the growth of NW. This is to be further noted that the growth of the NWs on the native oxide-covered Si surface is found to be random and confined in the limited region with very low density, as seen in figure 1(d). This confirms that this growth mechanism is predominantly masked by the presence of SiO 2 on the Si surface. We therefore conclude that under the catalyst-free condition, the layer growth of InAs is favored on oxide-cleaned Si(0 0 1) at temperatures below 500 • C. As the temperature increases, the surface diffusion of In on the Si surface also increases significantly and ∼550 • C; we observe two types of growths simultaneously, one along 0 0 1 (hillock growth) and another in plane wire-like growth along [1 1 0] directions.

Growth on the grooved surface.
The grooved surface on the oxide-covered Si(0 0 1) wafer was formed using a mechanical scriber. Subsequently, the growth of InAs NWs was carried out on these surfaces under the above-optimized condition (the same as section 3.1.2). We observed from SEM images that under this growth condition, non-tapered InAs NWs were grown on the grooved surface in an upright fashion ( figure 1(e)). Normally, it is difficult to grow standing NWs on (0 0 1) substrates without the assistance of any catalyst. Therefore, (1 1 1) substrates are a natural choice for the vertical growth of NWs. However, in our study we observe that standing NWs can be formed on (0 0 1) substrates even without catalyst by simple surface modification e.g. making v-grooves on the (0 0 1) surface.
Thus, we have three types of InAs NWs formed under different growth conditions, i.e. self-catalyst-assisted growth (NW SC ), catalyst-free growth (NW CF and NH CF ) and NWs grown on grooved surfaces (NW GS ).
Hence, detailed analyses of these NWs have been further carried out, which is described below.

Self-catalyst-assisted grown NWs (NW SC ) (low density).
The SEM image shows that the density of the NW SC on Si is very low, as shown in figure 1(a). Thus, conventional XRD did not give a measurable signal. Spatially resolved (∼1 μm) Raman spectroscopy was employed to obtain information of the orientation of these NW SC , as the length is 80 μm and base to tip width changes from 2 μm to 200 nm. The Raman data in backscattering geometry were taken at base, center and close to tip of the tapered NW SC . Only the LO phonon ( figure 3) is observed in the different positions of NW SC indicating that the growth of NW SC is oriented with Si(0 0 1). The full width at half-maxima (FWHM) of LO phonon mode peak is 15 cm −1 that suggests the NW SC are of reasonably good crystalline quality. The spherical structures grown along with these long NW SC show the presence of TO phonon dominantly, indicating their non-epitaxial nature of growth with Si(0 0 1). This could be due to the fact that these structures have been grown on the Si surface where SiO 2 was present, whereas the oriented NW SC have been grown on the cracks/craters reaching up to the Si surface and were in physical contact with the Si(0 0 1) substrate. In SAG also the growth of NWs takes place on a patterned SiO 2 covered substrate. We compare our results, which is the growth on native SiO x covered substrate, with that of SAG. First, the study of Mandl et al [27] reported that no growth of InAs (layer or NW) was observed on the SiO 2 mask under any growth condition performed by them. But in our case, we observe the growth of two different morphologies of InAs on native SiO x covered Si(0 0 1). When growth occurred directly on Si (through craters in SiO x ), 50-80 μm long-tapered NW SC , oriented with Si(0 0 1), were formed. Whereas, non-epitaxial spherical islands were formed when grown on SiO x as revealed by Raman spectroscopy. This has not been reported by earlier authors. Second, they also used a preannealing step at 625 • C before growing at 540-550 • C. But they did not clarify the role of this preannealing step in the NW growth. In our study, this preannealing step is observed to play a crucial role in the NW formation. Without this step, no NW formation (only island growth) was observed irrespective of the growth temperature and V/III ratio. However, after using this step, the long-tapered NW formation was observed at all growth temperatures ranging from 425 to 500 • C. Hertenberger et al [28] observed the growth of InAs clusters on the SiO 2 mask when the growth temperature was low, i.e. 460 • C. But on increasing the growth temperature to 480 • C, the InAs growth took place only in the predefined holes of mask due to significant increase in the surface diffusion and lower sticking probability on SiO 2 . But the NW length in their case reduced significantly with the increase in growth temperature >480 • C due to enhanced thermal dissociation. In our study, the density of InAs clusters on native SiO x reduces with the increase in growth temperature but unlike their study, the density or dimension of the NWs does not change with growth temperature in the range of 425-500 • C. We do not see any effect of thermal dissociation of grown InAs even up to 550 • C growth temperature (catalyst-free growth).

Catalyst-free NWs (NW CF ) (high density).
The SEM image of NW CF shows relatively large density and smaller size compared to NW SC . It is clear from the SEM picture that the NW CF length and diameter (width) vary from 0.5 to 1 μm and 40-80 nm, respectively. These dimensions are reconfirmed from AFM. The heights of the ∼40 nm and ∼80 nm diameter (width) NW CF are ∼70 nm and ∼160 nm, respectively, as measured by AFM ( figure 4). Furthermore, XRD shows (figure 5) two peaks at 29.45 • and 61.16 • identified due to (0 0 2) and (0 0 4) reflections of InAs. This confirms that the grown InAs nanostructures are oriented with Si(0 0 1). The FWHM of the InAs (0 0 2) reflection is 0.2 • . The particle size computed from this FWHM using the Debye-Scherrer formula [43] is 41 nm, which agrees well with one of the dimensions (width/diameter) of the NH CF /NW CF . XRD and TEM (selected area electron diffraction (SAED)) were also performed to determine the crystal orientation of the NW CF and NH CF . The XRD and SAED results confirm that the horizontal surface of both the structures are oriented with Si(0 0 1). In a few earlier reports of catalyst-assisted growth of III/V NWs on Si(0 0 1), similar observations were reported, where the authors claimed that the growth on Si(0 0 1) took place randomly along four available 1 1 1 directions that can be extracted from the (0 0 1) plane [22,30,31]. The four orientations form a 35.3 • angle with the surface and 90 • angles with each other. However, our SEM (cross-sectional view and with different tilts) and AFM studies (figure 4) clearly show that there is no tilt/angle between the grown nanostructures and the Si(0 0 1) plane surface. We therefore conclude that NH CF is grown predominantly in the 0 0 1 direction and the NW CF is grown perpendicularly to the direction of NH CF , i.e. in the Si(0 0 1) surface either perpendicularly or parallel to each other. The possible growth direction of these NWs could be a set of [1 1 0] directions because mostly, these sets of planes are perpendicular or parallel to each other. Mandl et al also reported the growth of high-density InAs NWs along [1 1 0] planes of InAs substrate when the (0 0 1) surface was masked with thick SiO x . Now, we investigate the structural properties of the NW CF and NH CF using the spatially resolved micro-Raman experiment and analysis. The spatial resolution of this  measurement (∼1 μm) allowed us to image a single/cluster of NW CF along with some NH CF structures. The Raman signal from NW is expected to be weak due to very small scattering volume and therefore the obtained Raman signal might be arising only from the region of highest intensity part of the laser beam profile, leading to a better resolved Raman image compared to the spatial resolution permitted by the instrument. Figures 6(a) and (b) show the Raman images generated from the LO phonon peak and the TO phonon peak, respectively. were observed. The Raman imaging indicates that the NW CF maintain the same crystalline quality throughout the sample; however, the NH CF seems to have some variation leading to difference in morphology, as observed. Raman imaging of NW CF sample was performed. Relatively isolated NW CF (marked 'X' and 'Y' in figure 6(a)) and NH CF and also an ensemble could be separately observed in the image. Raman spectra of all the InAs nanostructures in the sample show two peaks related to TO phonon and LO phonon of InAs. Surprisingly, very strong TO is observed from these nanostructures, although as per the bulk selection rule, the TO phonon in these oriented structures is not allowed in the backscattering geometry. Intensity of LO phonon peak is found to vary extensively depending on the position on the NW CF . At the center of the top surface of NW CF , the LO phonon peak is always stronger and becomes comparable to the TO phonon peak intensity. The intensity of LO phonon peak falls drastically at the edges of the NW CF , where only TO phonon dominates. The intensity of LO phonon peak remains strong along the length of the NW at the center. These observed results are analyzed as follows.
Two mechanisms are proposed as the plausible explanation for the observation of strong forbidden TO phonon. First is that it may have a geometrical origin. To elucidate this aspect, morphologies of these nanostructures were further investigated using AFM. Figure 4 very small. Therefore, the contribution of the backscattered signal from the top region (0 0 1) is much less compared to the signals scattered from the side facets, i.e. (0 1 1). As a result of this, when we focus on the middle of the flat top region of an NH CF or on an NW CF (where the flat top region is maximum), the LO phonon should be observed, whereas when light is focused on side facets, the TO phonon is expected to dominate (figures 7(b) and (d)). Due to lower resolution of Raman image compared to the size of nanostructure, generally both the TO and LO phonon peaks are observed. However, the above point can be easily confirmed by checking the Raman image in the center and on the periphery of these nanostructures, which clearly shows the increase in LO phonon intensity when moved toward the center, as shown for Raman spectra (figures 7(a) and (c)) for both 'X' and 'Y' NWs marked in the Raman image ( figure 6(a)). Furthermore, the Raman image ( figure 6(b)) generated from the TO phonon shows that it is not a representative of NWs alone, whereas the Raman image (figure 6(a)) of LO phonon shows the NW region. The second plausible mechanism for the observation of strong TO mode could be that the nanostructures in the present case are working as dipole antennas due to photon confinement. Experimental and theoretical studies of the polarized first-order TO and LO Raman scattering from InAs and GaP NWs were reported [38,45]. Wu et al have shown that NWs below a certain diameter work as a dipole antenna, where the Raman selection rule for bulk is masked. These Ramanintensity polar patterns were quantitatively explained by a simple theory considering the interplay of photon confinement, the NW growth direction and the orientation of the NW crystallographic axes with respect to the incident electric field [45].
In summary, although the observation of TO phonon may be due to masking of bulk selection rule as NW works as a dipole antenna, the observation of change in intensity of LO phonon peak along the diameter of the NW suggests the oriented growth of InAs NW with the Si(0 0 1) surface.

NWs grown on grooved surfaces (NW GS ).
These structures were grown under the same growth conditions used in the case of NW CF growth by varying the growth temperature and V/III ratio. Under the optimized growth condition, that is growth temperature of 550 • C and V/III ratio of 250, the estimated average growth rate was 0.45 μm min −1 . Diameter and length of the NW GS structures varied between 90 and 250 nm and 1 and 3.5 μm, respectively. It should be noted that the density of NW GS is small compared to NW CF and mainly grown at the grooved regions. The structural quality of NW GS was determined using TEM and XRD. The XRD results (not shown here) of the grooved region show the ZB structure of the NW GS and the exposure of Si(1 1 1) surface on which the NWs are formed in a standing fashion. Our XRD results of these standing NWs are very similar to the XRD data reported by Ihn and Song (figure 4 in [22]). However, the InAs NWs are reported to be grown on the Si(0 0 1) plane with the assistance of Au catalyst, in the case of [22] and the same in our case are clearly grown on the Si(1 1 1) planes exposed in the v-groove region without the assistance of any catalyst. The crystal structure of the standing NW GS is identified using TEM measurement of a single InAs NW after detaching it from Si substrates. Figure 8(a) shows the bright field TEM image of an NW GS of ∼150 nm diameter. Furthermore, figure 8(b) shows the SAED pattern of the same NW, which has been indexed to the [−1 1 2] zone axis of ZB InAs. The corresponding highresolution TEM image (figure 8(c)) shows high-resolution fringes of the (1 −1 1) planes with 0.35 nm interplaner spacing and they are along the axis of the NW. SAED and HRTEM suggest that the growth direction is along the [1 1 1] direction. The NW GS image, shown in figure 8(a), shows bright/dark bands, which do not seem to be from defects as the HRTEM image and the SAED pattern show high crystalline quality. These bands are typical diffraction contrast bands and are more likely to arise from slight waviness in the thickness of the NW.
This study suggests that standing NWs can be grown on the v-grooved Si(0 0 1) surface, using conventional photolithography and chemical etching process [46], which is otherwise very difficult to grow.

Conclusions
Versatile growth of InAs NWs with high and low densities is observed on Si(0 0 1) using MOVPE by varying the growth conditions and the growth mechanisms are discussed. Hillock-and wire-like InAs structures are formed on Si(0 0 1) under the same growth condition and the growth mechanism for this variation is explained. For very low-density NWs, where conventional techniques are not found to be very useful, Raman imaging is successfully used to analyze the structural variation within an individual nanostructure. Raman imaging of high-density small NWs showed a very close resemblance with the SEM image. This study suggests that both horizontal and standing (on the v-groove surface) NWs of different dimensions can be grown on the Si(0 0 1) surface by engineering the substrate surface and the growth conditions. This study can be used for the controlled integration of III-V NW technology on the Si substrate. Furthermore, the mechanism that led to the growth of long-tapered InAs NWs (∼100 μm) may also be used as an interconnection for highspeed devices.