Synthesis and structural characterization of vertical ferromagnetic MnAs/semiconducting InAs heterojunction nanowires

The purpose of this study is to synthesize vertical ferromagnetic/semiconducting heterojunction nanowires by combing the catalyst-free selective-area growth of InAs nanowires and the endotaxial nanoclustering of MnAs and to structurally and magnetically characterize them. MnAs penetrates the InAs nanowires to form nanoclusters. The surface migration length of manganese adatoms on the nanowires, which is estimated to be 600 nm at 580 °C, is a key to the successful fabrication of vertical MnAs/InAs heterojunction nanowires with atomically abrupt heterointerfaces.


Introduction
Vertical free-standing semiconducting nanowires (NW) have recently been demonstrating extraordinary versatility and extremely high possibility for use in potential applications to next-generation electronic, photonic, and bio-chemical sensing devices. [1][2][3][4][5][6][7][8][9][10][11][12] For future electronic industry, for example, practical ways to install vertical surrounding gate field-effect transistors (FET) using NWs to current integrated circuits based on the CMOS technologies were intensively demonstrated. 13) Most of these NWs reported elsewhere have thus far been synthesized and demonstrated by using some of the most popular bottom-up fabrication techniques in the world, i.e., vapor-liquid-solid (VLS) methods that typically use gold nanoparticles as catalysts. Conventional metal catalysts used in the VLS method, however, possibly lead to the deterioration of performance of NW devices mainly due to deep level formation by incorporated metal impurities. In addition, the VLS-grown NWs are randomly distributed on a semiconducting substrate in most cases, which might hinder the practical applications of NWs in devices in the future. We, on the other hand, have demonstrated the catalyst-free formation of vertical free-standing semiconducting NW arrays, e.g., GaAs, InGaAs, InP, InAs, GaAs=AlGaAs core-shell, and GaAs= GaAsP core-shell NWs, by selective-area metal-organic vapor phase epitaxy (SA-MOVPE) using partially SiO 2masked {111}A and B substrates, which has enabled us to control the size, aspect ratio, position, and density of NWs on various semiconducting substrate materials. 14,15) Nanoelectronic devices such as vertical surrounding gate FETs have been fabricated, and their performance has been demonstrated using our semiconducting NWs grown by SA-MOVPE for future device applications. 16,17) The perspective and outlook for possible and attractive magneto-nanoelectronic or nanospintronic device applications in semiconducting NWs have not been fully discussed in one of the most important and latest papers that reviewed future applications of semiconducting NWs. 18) (There was no section dealing with "NW spintronics".) Activities to achieve diluted magnetic semiconducting layers (DMS) 19) and the heteroepitaxy of ferromagnetic layers and III-V compound semiconductors (FM III-V hybrids) have attracted a great deal of attention in recent semiconductor nanospintronics research owing to possible additional functionalities that have been added to current semiconducting devices and integrated circuits. Of these approaches, granular hybrid structures in which ferromagnetic nanoclusters (NC) are embedded into semiconductor layers are one of the most attractive candidate materials for future nanospintronic devices because they have been reported to have huge magnetoresistance (MR) effects and a relatively long spin-relaxation time. [20][21][22] These devices have been fabricated on semiconducting substrates mostly by low-temperature molecular beam epitaxy in combination with conventional top-down fabrication techniques. Various kinds of NWs using FM III-V hybrids or DMS 23) have recently been synthesized to produce NW spintronic devices using several techniques based on conventional VLS methods as bottom-up fabrication approaches, e.g., GaMnAs NWs, 24,25) GaAs=GaMnAs core-shell NWs, 26,27) MnAs nanoparticles on InAs 28) and GaAs 29) NWs, GaAs=MnAs core-shell NWs, 30,31) GaN-related DMS NWs, 23,32,33) heterojunction NWs between MnGe alloys and Ge 34,35) and between MnSi alloys and Si, 36) and vertical multisegment AuGe=Ge NWs. 37) However, it is also crucial to avoid and overcome the possible problems arising in the current nanoelectronic and photonic NW devices fabricated by these conventional VLS methods particularly when fabricating practical NW spintronic devices of the future. We have, therefore, developed hybridization techniques of ferromagnetic MnAs NCs 38) on vertical free-standing semiconducting NW templates grown by our catalyst-free SA-MOVPE technique 14,15) in the research fields of semiconductor nanospintronics, and reported on the formation and characterizations of MnAs NC=GaAs hybrid NWs. 39,40) We developed an SA-MOVPE technique in our previous studies to directly synthesize single-crystal ferromagnetic MnAs NCs on a semiconducting substrate, and we demonstrated that the NCs could be used as promising building blocks for fabricating nanospintronic devices of the future since they demonstrated large angle-dependent MR effects. However, we realized that it is still difficult to form vertical heterojunction NWs between ferromagnetic MnAs and semiconducting layers, which would enable us to inject spin-polarized carriers and currents into semiconducting NW channels. It is also difficult to control the crystal orientations of MnAs NCs in a 〈111〉B-oriented GaAs NW template 39,40) and to obtain atomically abrupt heterointerfaces between MnAs and GaAs even when using our hybridization techniques. It is likely that the difficulty in forming vertical heterojunction NWs was mainly due to the relatively stable crystal facet of the c-plane, i.e., the {0001}-oriented surface, at the surface of hexagonal NiAs-type MnAs NCs grown by SA-MOVPE. 39) We believe that the combination of a magnetic tunnel junction (e.g., CoFeB=MgO 41) and CoFe= MgO 42) ) electrode and vertical ferromagnetic=semiconducting heterojunction NWs with atomically abrupt heterointerfaces 35) could overcome major obstacles of conductance mismatch and poor controllability in the heterointerface formation in the current spin MOSFETs. 43) In addition, the {111}B-oriented semiconducting NW channels possibly have a great advantage for enhancing spin lifetime in the channels of spin MOSFETs. 44) Therefore, the authors of this paper report on the synthesis of high-quality vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs with atomically abrupt heterointerfaces on semiconducting substrates utilizing the so-called endotaxy of MnAs NCs after the SA-MOVPE of InAs NW templates and by adjusting the synthesis conditions. A possible mechanism of the formation of singlecrystal MnAs NCs in and on the InAs NW templates is discussed on the basis of the detailed dependences of the formation of MnAs NCs on synthesis conditions and structural characterizations obtained from scanning and transmission electron microscopies (SEM and TEM, respectively). We believe that our vertical ferromagnetic MnAs NC= semiconducting InAs heterojunction NWs in the current work are definitely exhibiting new possibilities and versatility for creating novel magneto-nanoelectronic or nanospintronic devices using vertical free-standing semiconducting NWs, e.g., spin-NW-MOSFETs and spin-NW-light-emitting diodes, as was reported and discussed in our review paper. 45)

Experimental methods
We used InAs NW arrays as a template, which were fabricated heteroepitaxially on GaAs(111)B substrates by utilizing the SA-MOVPE process, for creating high-quality vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs. First, we prepared the initial circular openings, which were arranged and defined in SiO 2 thin films by electron beam lithography. There were typically two types of the observed diameter d 0 of the initial circular openings: one was approximately 80 to 90 nm and the other was approximately 110 to 140 nm. The distances between the initial circular openings or the periods a were 0.5, 1.0, and 3.0 µm on the substrates. The SiO 2 thin films, whose typical thicknesses were estimated to be approximately 20 to 30 nm, were deposited on GaAs(111)B wafers by plasma sputtering. The growth temperature T g and the growth time t for InAs NWs were 580°C and 30 min, respectively. Conventional organometallic and hydride sources, such as (CH 3 ) 3 In and 20% AsH 3 diluted in H 2 , were used as the group III sources for the former and group V sources for the latter in all the growth experiments. The estimated partial pressures of (CH 3 ) 3 In 46) and 20% AsH 3 diluted in H 2 were 4.9 × 10 −7 and 1.3 × 10 −4 atm, respectively, for the SA-MOVPE of undoped InAs NW templates. We utilized the phenomenon of the "endotaxy" of MnAs in InAs for MnAs NC growth after InAs NW growth. Endotaxy is associated with a diffusion process that leads to the redistribution of substances, i.e., MnAs NCs in host crystals, InAs NWs in the current work, and the formation of new stable phases. This is the key technique for forming ferromagnetic MnAs NCs "into" semiconducting NWs grown by SA-MOVPE. We observed that some singlecrystal MnAs NCs grew in GaAs NW templates in our previous studies even without any supply of AsH 3 source gas as a result of endotaxy. 39,40) During the endotaxial growth of MnAs NCs into the host InAs NW templates, we only supplied the organometallic source of (CH 3 C 5 H 4 ) 2 Mn diluted in H 2 . The growth temperatures T g for the MnAs NCs were changed from 400 to 580°C, and their growth times t were all 1 min. The estimated partial pressure of (CH 3 C 5 H 4 ) 2 Mn was 3.0 × 10 −6 atm. During the decrease in temperature during the purging process after MnAs NC growth, 20% AsH 3 diluted in H 2 was supplied. Structural characterizations in terms of the size and position of MnAs NCs and InAs NW templates were carried out by SEM. TEM was also used for obtaining lattice images of NCs and NWs, and we conducted detailed structural characterizations, such as the analyses of crystal structures and solid compositions of the NCs and NWs, by electron-beam diffraction (ED) and energy dispersive X-ray (EDX) spectroscopy in combination with TEM, using an electron beam with a spot diameter of about 1 nm. For the magnetic characterization of MnAs NCs, we used magnetic force microscopy (MFM; Digital Instruments Nanoscope IIIa), first in the conventional phase detection (PD) mode of the system at room temperature and under zero magnetic field condition after applying the external magnetic fields, B, of approximately 5,700 G. For MFM measurements, vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs were separated from GaAs(111)B substrates mechanically by ultrasonic vibration in isopropanol solution and deposited on SiO 2 =Si substrates. To ensure that magnetic responses of MnAs NCs were observed, we also conducted amplitude detection (AD) mode measurements in the MFM system. In the AD mode measurements, the resonance curve of the MFM cantilever is shifted when it is affected by stray magnetic fields. At the same time, the amplitude change of the MFM cantilever is detected at the point of drive frequency, which is set to be lower or higher than the resonance frequency of a free-vibrating MFM cantilever. The change in drive frequency gives the reversals in contrasts of the magnetized region in MFM images.

Results and discussion
We first grew a template structure of InAs NW arrays by SA-MOVPE to fabricate vertical ferromagnetic MnAs NC= semiconducting InAs heterojunction NWs. Figure 1(a) shows a typical bird's-eye view obtained by SEM of the template structure of InAs NW arrays before the endotaxy of MnAs NCs. The period of InAs NWs, a, in Fig. 1(a) was 1.0 µm. The diameter of the InAs NWs was estimated to be approximately 150 nm and their height was estimated to be approximately 1.5 µm, when we used the initial circular openings with the diameters, d 0 , of 110 to 140 nm in the SiO 2 thin films. We observed InAs NWs typically with a diameter of approximately 120 nm for the initial circular openings with the d 0 values of 80 to 90 nm from rough estimates. The inset in Fig. 1(a) is a highly magnified top view of one of the InAs NWs. We observed that InAs NWs with a hexagonal prismatic shape were surrounded by six f0 11g crystal facets and grew in the 〈111〉B direction on GaAs(111)B substrates. The tops of the NWs revealed that flat {111}B crystal facets surrounded by tilted f 1 10g ones were formed. Figure 1(b) shows typical ferromagnetic MnAs NC=semiconducting InAs hybrid NWs grown at 580°C for 1 min. The NCs were formed on the top {111}B crystal facets of the InAs NWs, and we also observed that some additional NCs were formed around the middle parts of the NWs.
We next conducted structural characterizations by TEM for comparably similar hybrid NWs to carefully investigate structural characteristics of the ferromagnetic MnAs NC= semiconducting InAs hybrid NWs shown in Fig. 1(b). Figures 2(a) and 2(b) show highly magnified cross-sectional bright-field TEM images of MnAs NCs that were formed in the middle and on the top {111}B crystal facets of the host InAs NWs, respectively. The width and height of the NC in Fig. 2(a) were estimated to be approximately 85 nm for the former and 55 nm for the latter, and those in Fig. 2(b) were estimated to be approximately 72 nm for the former and 54 nm for the latter. We confirmed from TEM measurement results that rotational twin defects were randomly formed in the InAs NWs, and that no dislocations or defects, on the other hand, were observed in the MnAs NCs. We conducted EDX spectroscopy of the MnAs NCs formed in the middle and on the top {111}B crystal facets of NWs in addition to the cross-sectional TEM observations, as shown in Figs. 2(c) and 2(d), respectively. The atomic compositions of NCs and NWs in terms of three elements, i.e., arsenic, indium, and manganese, were estimated from the line profiles obtained by EDX spectroscopy. We eliminated possible external contamination by chemicals (or atoms), such as carbon, oxygen, and silicon, which were possibly introduced during the sample preparation processes and from the materials of the sample holders used for the TEM observations to precisely examine the solid compositions of NCs and NWs. The solid compositions (atomic compositions in %) of arsenic, indium, and manganese elements were estimated to correspond to approximately 52, 1.0, and 47% in the regions of the NC in Fig. 2(c). We also concluded that the MnAs NCs that were formed on the top {111}B crystal facets of the host InAs NWs had similar solid compositions, i.e., they corresponded to approximately 51, 0, and 49% of arsenic, indium, and manganese elements, as shown in Fig. 2  stacks.iop.org/JJAP/55/075503/mmedia]. The c-axes, i.e., the 〈0001〉 directions, of the NiAs-type MnAs NCs were approximately parallel to the 〈111〉B directions of the host ZB-type InAs NWs. We confirmed that atomically abrupt heterointerfaces between MnAs NCs and InAs NWs were formed, as shown in Fig. 2(f). It appeared that a small number of the observed MnAs NCs possibly rotated along the c-axes parallel to the 〈111〉B directions of the host InAs NWs, judging from other cross-sectional lattice images of the NCs (not shown here). Similar results in terms of the heterointerfaces were obtained for the NCs formed on the top {111}B crystal facets of NWs. We concluded from these results of structural characterization that the vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs with the atomically abrupt heterointerfaces were successfully formed by utilizing the endotaxy of MnAs NCs at the T g of 580°C.
Subsequently, we conducted MFM observations of the vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs. Figures 3(a) and 3(b) show SEM and corresponding MFM images of one of the comparable heterojunction NWs, respectively. It was quite difficult to examine the same heterojunction NW observed by TEM in Fig. 2 by MFM since a large number of NWs were formed at the same time on the same GaAs(111)B substrate with various types of SiO 2 -mask openings, i.e., different d 0 and a values. However, it was highly possible, as determined from the SEM image in Fig. 3(a) and the TEM observation results shown in Fig. 2, that most of the NCs penetrated the host InAs NWs. It was likely in this case, therefore, that the c-axes of the NCs were parallel to the 〈111〉B direction of the NW (i.e., the magnetic easy axes, i.e., a-axes, of the NCs were perpendicular to the 〈111〉B direction of the NW). Most of MnAs NCs, e.g., NCs "I" and "II", had a marked single magnetic domain, although some of the NCs like NC "III" had two magnetic domains. For the NCs I and II, a dark area was observed at the center of the NC, and the bright and dark areas were aligned perpendicular to the 〈111〉B direction of the NW. It was revealed that the magnetized direction for the NCs I and II was possibly along one of the a-axes, which were presumably parallel to the applied B-direction. To ensure that the detected dark and bright contrasts in the images were due to magnetic responses, we also conducted MFM measurements of the NCs I to III in the AD mode, as shown in Figs. 3(c)-3(f). The drive frequencies were set higher in Figs. 3(c) and 3(e), and lower in Figs. 3(d) and 3(f) than the resonance frequency of a free-vibrating MFM cantilever. It was revealed that changing the drive frequency resulted in reversal contrasts in the regions of NCs I, II, and III, which showed magnetic responses from the NCs. We have not determined the Curie temperature T c of the MnAs NCs in InAs NWs. However, the hysteresis curves 47) and the increased T c of 340 K 48) were observed for the comparable samples of NiAs-type MnAs NCs on InGaAs layers in our previous studies. Some researchers have reported on a single magnetic domain of MnAs that was epitaxially grown on GaAs NWs and observed the hysteresis curve of the MnAs and temperature-dependent curves, which revealed the T c of MnAs on GaAs NWs to be 313 K. 49) We observed the endotaxial phenomenon of MnAs nanoclustering in the case of the MnAs NCs grown at 490°C for 1 min, which was first reported in our previous paper. 50) The detailed TEM observations, ED measurements, and EDX spectroscopy in the current study revealed that the MnAs NCs were not only formed similarly on the top {111}B crystal facets of the host InAs NWs, but also grew partially into the NWs from the f0 11g sidewall crystal facets and=or six ridges between them (i.e., the MnAs NCs did not com- In the case of the NCs grown at 490°C, some of the c-axes of MnAs NCs were not parallel to the 〈111〉B directions of InAs NWs, as shown in Fig. 4(b). We observed that the hexagonal truncated pyramidal shapes of the NCs formed on the top of host InAs NWs were rotated by 30°against the host NW hexagonal prisms [see Fig. S2 in the online supplementary data at http://stacks.iop.org/JJAP/55/075503/mmedia]. This was consistent with the results of our previous study. 50) We found that the MnAs NCs grown at 490°C for 1 min, which were mainly composed of arsenic and manganese elements, had a hexagonal NiAs-type crystal structure, similarly to the NCs in Fig. 2  was roughly estimated from the obtained TEM images to be approximately 140% by changing the T g from 490 to 580°C. In addition, the TEM and SEM measurement results revealed that the host InAs NW diameters in the vicinities of MnAs NCs tended to slightly and gradually decrease after the MnAs NC growth, compared with those around the middle of the NWs away from the MnAs NCs, as shown in Figs. 2(a) and 4(b). Bending of NW sidewalls was observed in the vicinities of NCs. Here, we defined the bending angles of NW sidewalls, θ, and the θ values for the NCs grown at 580 and 490°C were defined as θ 580 and θ 490 , respectively, as shown in Figs. 2(a) and 4(b). We confirmed that θ 580 was much shallower than θ 490 , as estimated in Figs. 2(a) and 4(b). It is reasonable because the desorption rate of indium and arsenic atoms should increase with increasing T g . The indium and arsenic atoms in the host InAs NWs are easily desorbed during the endotaxy of MnAs NCs because only (CH 3 C 5 H 4 ) 2 Mn and H 2 are supplied. When we supplied only H 2 at 490°C for 1 min immediately after the growth of the InAs NW templates grown at 580°C for 30 min, we actually observed the decrease in NW height after the H 2 treatment. The decrease was estimated to be approximately 70%. A slight and negligible decrease in NW diameter was also observed. Therefore, we clearly observed that the indium and arsenic atoms were markedly desorbed from the host InAs NWs under the current synthesis conditions for MnAs NCs. Figure 5(a) shows the dependences of MnAs NC formation on the period a of the host InAs NWs. The MnAs NCs were grown at 580°C for 1 min in all the experiments. The average height of the host InAs NWs and vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs were estimated using 30 randomly chosen NWs observed in the SEM images. The decreases in NW height after the endotaxy of MnAs NCs were estimated to be approximately 30% at least. In addition, the differences in NW height between before and after the endotaxy of MnAs NCs were almost constant (approximately 30% at least) for all the periods a of 3.0, 1.0, and 0.5 µm, i.e., no significant dependence of the decreases on the periods a was observed.
These results clearly indicated that the incorporation of manganese atoms, which reacted with arsenic atoms in the host InAs NWs, suppressed the desorption of arsenic and=or indium atoms from the host NWs. Therefore, it was highly possible that the decreases in the NW height were mainly caused by the desorption of indium and arsenic atoms from the host InAs NWs during the endotaxy of MnAs NCs. The results suggest that the desorption of indium and arsenic atoms from the host InAs NWs was a major trigger of the synthesis of MnAs NCs in the InAs NWs. Some of the desorbed arsenic atoms were incorporated into the solid phase after the chemical reactions with the supplied manganese atoms from the vapor phase. That resulted in the nucleation of MnAs on the NW surface. The synthesis of MnAs NCs proceeded "into" the host InAs NWs from the nuclei because of the following reasons. One is that the arsenic atoms in the host InAs NWs were consumed for the MnAs NC formation. Another reason is that the D of manganese atoms in the {111}B plane was possibly larger than those in other planes since we observed that the crystal facets of the NCs and the MnAs=InAs heterointerfaces parallel to the InAs{111}B plane were quite flat and abrupt compared with the other facets. Moreover, the penetration depth from the NW sides for almost all of the NCs was larger than these height in the InAs〈111〉B direction.
The insets (SEM images) in Fig. 5(a) show that the host InAs NW height itself and the number of MnAs NCs formed in one NW decreased with the decreasing period a of the NWs. The decrease in NW height was possibly caused by the change in the amount of the indium source supplied per NW. The amount of indium source supplied per NW decreased with decreasing a because the surface migration length of indium atoms on the substrates was sufficiently larger than a. This resulted in the decrease in host InAs NW height at a relatively small a. This decrease in NW height led to the decrease in the number of NCs. The supplied manganese atoms were first adsorbed physically on the InAs NW surfaces after the diffusion from the vapor phase and then migrated on the surfaces. The manganese adatoms reached one of the possible chemical adsorption sites, where the indium and arsenic atoms presumably desorbed, and were then adsorbed chemically at vacant sites after indium and arsenic atoms had desorbed from the host InAs NWs. In the case of a small a, more of the manganese adatoms possibly reached one certain site because the surface migration length was sufficiently larger than the NW height. Therefore, the smaller the height of NWs, the fewer NCs were formed.
In the case of a large a, on the other hand, the height of the host InAs NWs increased. A smaller number of manganese adatoms possibly reached one certain site because the surface migration length was not sufficiently larger than the NW height. This resulted in the growth of a large number of MnAs NCs in relatively long NWs. The inset in Fig. 5(b) shows a typical bird's-eye view of a SEM image of the vertical MnAs NC=InAs heterojunction NWs, in which several NCs were formed in the middle of NWs depending on the NW height. The MnAs NCs were grown at a T g of 580°C for 1 min. The NWs had a diameter of approximately 80 nm. We estimated the average heights of NWs as a function of the number of NCs formed in the middle of NWs from SEM images. As shown in Fig. 5(b), the average height of NWs increased with increasing number of NCs. This suggests that the number of NCs was strongly affected by NW height and the surface migration length of manganese adatoms on the NW surface. Finally, we investigated the T g dependences for the endotaxy of MnAs NCs. We measured the size of NCs formed on the top {111}B crystal facets, W ct , and the vertical distance between the NCs, D c , to carefully examine NC formation in and on the NWs for the NWs in which MnAs NCs were grown at 490, 540, and 580°C. Figure 5 Fig. 5(c) is a schematic that illustrates the definitions of W ct and D c . Both W ct and D c markedly increased with increasing T g , as plotted in Fig. 5(c). It was difficult to estimate D c for the MnAs NCs grown at 400°C for 1 min in the host InAs NWs because the NCs formed too closely to one another to identify. However, we observed a tendency consistent with that observed in Fig. 5(c) [see Fig. S3(d) in the online supplementary data at http:// stacks.iop.org/JJAP/55/075503/mmedia]. These experimental results suggest that the surface migration length of manganese adatoms on the sidewalls of the host InAs NWs is one of the key factors for the endotaxial formation of MnAs NCs. The larger the surface migration length became at higher T g values, the larger the number of manganese adatoms that possibly reached one certain site. In addition, the desorption rates of indium and arsenic atoms and the D of manganese atoms increased at higher T g values. These phenomena led to the increase in the size of NCs at a certain site and the decrease in the number of NCs. However, when the surface migration length decreased at lower T g values, a small number of manganese adatoms possibly reached one certain site. The manganese adatoms in this case were possibly incorporated into other different sites within the range of their surface migration lengths. The desorption became less active and D became smaller at lower T g values. As a result, it led to the decrease in the size of NCs and the increase in their number. Thus, the size and number of NCs were changed by T g which affected the surface migration length of manganese adatoms, the desorption rate of indium and arsenic atoms, and the D of manganese atoms. The average height of NWs, at which no NC was formed in the middle, and the D c obtained for the NCs grown at 580°C were both approximately 600 nm, as shown in Figs. 5(b) and 5(c). It was therefore possible for the surface migration length of manganese adatoms at a T g of 580°C to be roughly estimated from these results to be at least 600 nm. We concluded from these results that the period a of the host InAs NWs and T g of MnAs NCs are the main factors for MnAs NC formation. At higher T g values such as 580°C, MnAs NCs penetrated the host InAs NWs with atomically abrupt heterointerfaces, as shown in Fig. 2. Although the possibilities of the surface segregation of MnAs NCs and=or the merging among several NCs, which were reported elsewhere for a different-materials system, 54) are not excluded, they might be low in the current study because no dislocations nor defects were observed in the MnAs NCs and because atomically abrupt heterointerfaces were formed between MnAs and InAs. Since rotational twin defects were randomly and densely formed in the host InAs NWs, there are supposed to be a large number of relatively unstable InAs bonds at the edges of atomic steps owing to the twin defects. These unstable InAs bonds at the edges can be the first sites for the nucleation of MnAs because the indium and arsenic atoms are easily desorbed there. Therefore, the difference between the densities of the twin defects might determine where the NCs are formed in the host NWs, although further experiments are required before we can conclude.

Conclusions
We reported on the synthesis and structural characterizations of vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs by combining the SA-MOVPE of InAs NWs and the endotaxial nanoclustering of MnAs NCs. The c-axes, i.e., the 〈0001〉 directions, of the hexagonal NiAstype MnAs NCs were approximately parallel to the 〈111〉B directions of the host ZB-type InAs NWs in the heterojunction NWs with the atomically abrupt complete heterointerfaces. Some of the MnAs NCs had marked single magnetic domains, and the observed results for magnetization directions were consistent with the structural characterization results. The detailed growth condition dependences revealed that the parameters T g and a are the key factors for the formation of MnAs NCs into InAs NWs. The diffusion coefficients of manganese atoms, D, the surface migration length of manganese adatoms on the host InAs NWs, and the desorption rates of indium and arsenic atoms in the NWs strongly depended on the growth temperature T g , and NW height changed according to the period of the host InAs NWs, a. At a relatively low T g of 490°C, the D, surface migration length, and desorption rate decreased. This led to a relatively large number of small MnAs NCs, which were shallowly formed into the NWs from the f0 11g sidewalls. When T g increased, on the other hand, the D, surface migration length, and desorption rate increased. This resulted in relatively large NCs that formed deeply into the host NWs and the decrease in the number of NCs. At a T g of 580°C, the MnAs NCs, which formed from the f0 11g sidewalls of the NWs, penetrated the host InAs NWs. This phenomenon successfully led to the formation of vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs with atomically abrupt complete heterointerfaces between MnAs and InAs layers. We believe that our vertical ferromagnetic MnAs NC=semiconducting InAs heterojunction NWs in the current work have demonstrated new possibilities and versatility for creating future novel magneto-nanoelectronic or nanospintronic NW devices.