Introduction

Heterostructures in semiconductors enable diverse functions in many planar electronic devices [1]. Meanwhile, semiconductor nanowires are unique building blocks of electronics on a nanometer scale. Akin to planar devices, heterostructures in the nanowires should enable diverse functions that are promising for high-performance nanowire devices [2–4]. Indeed, recent studies of heterostructure semiconductor nanowires for electronics, such as diodes [3], field-effect transistors (FETs) [5], sensors [6], and the solar cell [7], have demonstrated the higher performances that are ascribed to heterostructures.

Fabrication of heterostructure nanowires have been mostly carried out by consecutive chemical vapor deposition, which supplies precursors one by one in the order of layer stacking sequences [8, 9]. Limited studies also showed formation of heterostructure nanowires through a self-organization mode in a one-step process [10, 11]. In this study, we report about an approach on the fabrication of coaxial heterostructure Si1−x Ge x nanowires, which are promising building blocks for high-performance nano-electronic devices. Our approach is based on the O2 gas flow-induced bifurcate Ge deposition or oxidation reaction of Si1−x Ge x nanowires. It enables selectively prepared Ge/Si1−x Ge x or SiO2/Si1−x Ge x coaxial heterostructure nanowires by the kinetics of gas flow.

Experimental Procedure

Figure 1 shows a schematic of our procedure. Compositionally controlled Si1−x Ge x nanowires were synthesized on an Au catalyst deposited Si (1 1 1) substrates at 900–1,000°C in a chemical vapor transport system [12, 13]. Germanium (Ge, Alfa Aesar, 99.9999%) powder and Si tetrachloride (SiCl4, Aldrich, 99.998%) gas were used as source materials for the synthesis of the nanowires [14]. Ge powder and Si substrates placed in inner quartz tube at a distance of 1 inch were inserted into the center of outer quartz tube. The SiCl4 precursor through a H2 bubbler system was introduced into the system at a flow rate of 20 sccm. H2 (100 sccm) and Ar (100 sccm) were used as ambiance gases for the synthesis. After the growth of Si1−x Ge x nanowires for 30 min, the flow of SiCl4 was terminated and then a flow of O2 was introduced at 900°C under vacuum maintained by mechanical pump. The flow rate of O2 was controlled from 50 to 300 sccm for bifurcate reactions.

Figure 1
figure 1

Schematic of the Si1−x Ge x heterostructure nanowires synthesized procedure. After the growth of Si1−x Ge x nanowires, the flow of SiCl4 was terminated and then a flow of O2 was introduced under vacuum maintained by mechanical pump

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We observed as-grown Si1−x Ge x nanowires and modulated coaxial heterostructure nanowires by a scanning electron microscope (SEM) and a high-resolution transmission electron microscopy (HRTEM). High-resolution X-ray diffraction (HR-XRD) measurements were carried out at 3C2 and 11A1 beam line of the Pohang Accelerator Laboratory (PLS).

Results and Discussion

As shown in Fig. 2, the Si1−x Ge x nanowires for x = 0.05, 0.15, and 0.3 were grown and well-aligned on the substrate whose diameter ranged from 50 to 300 nm. The density of nanowires was approximately 108/cm2. The composition of the Si1−x Ge x nanowires could be controlled by the substrate distance from the Ge powder [14]. Among them, Si0.85Ge0.15 nanowires, which showed better electrical transport properties than other compositions, were investigated for the fabrication of coaxial heterostructures. HRTEM images showed the single crystalline nature with a thin layer of native oxides. The energy-dispersive spectroscopy (EDS) analysis profile in the radial direction of the nanowire did not show any evidence of phase inhomogeneity; that is, it showed no Ge segregation within the nanowire, as often found in thin film chemical vapor depositions [15, 16].

Figure 2
figure 2

a SEM image of Si1−x Ge x nanowires with diameter ranged from 50 to 300 nm. b TEM image of individual single crystal Si1−x Ge x nanowires with a thin layer of native oxides. Upper inset is SAED pattern image that shows the nanowire is single crystal and a growth direction is <1 1 0>. Below inset is EDS profile in the radial direction of the nanowire

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After the growth of Si1−x Ge x nanowires, SiCl4 flow was terminated and O2 gas was introduced under vacuum. Under these conditions, Ge vapor from the Ge source together with O2 gas were introduced to the substrate where the nanowires were vertically grown. Our systematic studies showed that Ge/Si1−x Ge x coaxial heterostructure nanowires were the result of the deposition of Ge under the low flow rate of O2 (i.e., <100 sccm that maintains the total pressure of <2 torr) while SiO2/Si1−x Ge x coaxial heterostructure nanowires resulted from the oxidation of Si1−x Ge x nanowires under the high flow rate (i.e., >100 sccm that maintains the total pressure of >2 torr).

Figure 3 shows the Ge/Si1−x Ge x coaxial heterostructure nanowires. The synchrotron XRD patterns were indexed to a diamond structure on the Si1−x Ge x core and Ge deposited on the surface as a shell. The EDS profile in the radial direction of the nanowire clearly shows the uniform thickness of Ge. We investigated the kinetics of the deposition of Ge for different diameters of nanowires by measuring the thickness of the shell as a function of time. As shown in Fig. 3c, the rate of Ge deposition is constant with time. It indicates that Ge deposition on Si1−x Ge x nanowires was dominated by an interfacial reaction, i.e., the deposition of Ge on the surfaces. It also showed that the rate of deposition was not dependent on the diameter of nanowires.

Figure 3
figure 3

a The synchrotron XRD pattern of Ge/Si1−x Ge x coaxial heterostructure nanowires. b TEM image and EDS profile in the radial direction of Ge/Si1−xGe x coaxial heterostructure nanowires. Inset is SEM image of the heterostructure nanowires. c TEM images measuring the thickness of core and shell as a function of time. Inset is schematic of the deposition procedure. d Plot of the thickness of Ge shell versus the diameter of Si1−x Ge x core nanowires

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Figure 4 shows SiO x /Si1−x Ge x coaxial heterostructure nanowires. The synchrotron XRD patterns of the oxidized Si1−x Ge x nanowires were indexed to a diamond structure. The compositional profiles in the radial directions of the oxidized nanowires showed that the composition of the oxide is primarily SiO x . It was also observed that the (1 1 1) Bragg peak shifted to lower angles with oxidation time. It is due to preferential oxidation of Si in Si1−x Ge x nanowires that Ge-rich cores result. It is noted that no evidence of Ge segregation, which had frequently been observed in the oxidation of Si1−x Ge x thin films, was found [17–19]. It is known that Ge segregation in the Si1−x Ge x nanowires during oxidation is dependent on the rate of oxidation. In a fast oxidation rate, redistribution of Ge may not be achievable, and segregation results. The oxidation rate in this study is thus believed to be rather slow, and Ge is efficiently redistributed over the nanowires and maintains a homogeneous Ge-rich Si1−x Ge x alloy composition. The shift of (1 1 1) Bragg peak may also due to the growth stresses associated with the oxidation and the thermal mismatch stresses between the SiO x /Si1−x Ge x on cooling to the room temperature. The volume expansion of Si1−x Ge x during oxidation can induce the lattice expansion near the SiO x /Si1−x Ge x interface and shift of the (1 1 1) Bragg peak to lower angles [20]. The lattice expansion can also be induced by a thermal expansion coefficient mismatch between the grown SiO x sheath and the Si1−x Ge x core [21]. During the cooling to the room temperature, the significant mismatch in the thermal expansion coefficients of SiO x and Si1−x Ge x (~5 × 10−7 K−1 and ~2.9 × 10−6 K−1 where x = 0.1, respectively) can induce tensile stress and lattice expansion in the longitudinal direction of the nanowire and, in turn, shift of the (1 1 1) Bragg peak to lower angles.

Figure 4
figure 4

a The synchrotron XRD patterns of SiO x /Si1−x Ge x coaxial heterostructure nanowires. b TEM image and EDS profile in the radial direction of SiO x /Si1−x Ge x coaxial heterostructure nanowires. Inset is SEM image of the heterostructure nanowires. c TEM images measuring the thickness of core and shell as a function of time. Inset is schematic of the oxidation procedure. d Plot of the thickness of SiO x shell versus the oxidation time of Si1−x Ge x nanowires

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The oxidation kinetics of Si1−x Ge x nanowires was further studied. As shown in Fig. 4, the oxidation thickness follows the typical diffusion-controlled reaction, that is , with self-limiting oxidation [22]. Generally, nanowires show the self-limiting oxidation behavior that can be explained by the evolution of compressive stress normal to the Si/SiO2 interface [23, 24]. As new oxide grows at the interface, the old oxide expands due to the increase in volume of SiO2 compared to the core, resulting in a compressive stress normal to the interface that slows the interfacial reaction between the oxidant and Si at the Si/SiO2 interface. Meanwhile, the magnitude of stresses is inversely proportional to the radius of the curvature of nanowires and thus the oxidation rate depends on the diameters. In fact, the thicker nanowires oxidized faster in this study.

Our results show that Ge/Si1−x Ge x or SiO2/Si1−x Ge x coaxial heterostructure nanowires can be selectively prepared. It is noted that the formation of the shell can be controlled by the flow of O2 gas. In fact, the reactions were very sensitive to the flow of O2 pressure and changed abruptly from Ge deposition to oxidation at 2 torr of total pressure without any transition or intermediate region. Therefore, it could be considered as bifurcate reactions that one of the two reactions is selectively preceded by the initial condition (i.e., O2 pressure in this study).

The reason for bifurcation could be given as follows. Under our experimental conditions, reactions as follow can occur on the surface of nanowires:

  1. 1)

    Ge(vapor) = Ge(solid) shell

  2. 2)

    O2(vapor) + Si(solid) in SiGe nanowires = SiO2(solid) shell

Thermodynamic calculation shows that the equilibrium partial pressure of O2 for reaction is almost 10−24 torr. Therefore, oxidation of Si1−x Ge x nanowires can be progressed in our experimental conditions. However, nanowires were densely aligned on the substrate where the gas flow is limited. Meanwhile, deposition or oxidation reaction on the surface nanowires requires diffusive penetration of Ge vapor or oxygen from the reactor atmosphere into the dense array of nanowires. It will depend on the mass and partial pressure of vapor components in the atmosphere. Accordingly, the penetration of O2 into the nanowire array would be rather difficult for Ge due to its light mass [25, 26]. It may be why a Ge layer is deposited under low O2 pressure of < 2 torr. Meanwhile, at O2 pressure higher than 2 torr, O2 can penetrate into the nanowire array and induce oxidation. The bifurcation is thus a result of compete penetration of Ge vapor and O2 gas into dense nanowire array, and thus can be understood in terms of kinetics of gas diffusion.

Conclusion

In summary, our study demonstrates that bifurcate Ge deposition or oxidation of aligned Si1−x Ge x nanowires can be achieved by the control of O2 gas flow kinetics. The process is simple, however, and efficient to fabricate different coaxial heterostructure nanowires with sharp interfaces, as shown in Figs. 2 and 3. It is also noted that there is no transition between the two reactions and thus a high quality shell of Ge or SiO2 is achieved. Such nanowires would be used as building blocks for the development of high-performance nanowire-based electronics. For example, coaxial heterostructure nanowires would be helpful to improve electrical transporting in nanowire-based transistors [27]. Oxidized nanowires would be helpful in developing advanced nanowire devices such as surround gated transistors [28].