# Fabrication of Coaxial Si_{1−x
}Ge_{
x
} Heterostructure Nanowires by O_{2} Flow-Induced Bifurcate Reactions

- Ilsoo Kim
^{1}, - Ki-Young Lee
^{1}, - Ungkil Kim
^{1}, - Yong-Hee Park
^{1}, - Tae-Eon Park
^{1}and - Heon-Jin Choi
^{1}Email author

**Received: **21 April 2010

**Accepted: **7 June 2010

**Published: **17 June 2010

## Abstract

We report on bifurcate reactions on the surface of well-aligned Si_{1−x
}Ge_{
x
} nanowires that enable fabrication of two different coaxial heterostructure nanowires. The Si_{1−x
}Ge_{
x
} nanowires were grown in a chemical vapor transport process using SiCl_{4} gas and Ge powder as a source. After the growth of nanowires, SiCl_{4} flow was terminated while O_{2} gas flow was introduced under vacuum. On the surface of nanowires was deposited Ge by the vapor from the Ge powder or oxidized into SiO_{2} by the O_{2} gas. The transition from deposition to oxidation occurred abruptly at 2 torr of O_{2} pressure without any intermediate region and enables selectively fabricated Ge/Si_{1−x
}Ge_{
x
} or SiO_{2}/Si_{1−x
}Ge_{
x
} coaxial heterostructure nanowires. The rate of deposition and oxidation was dominated by interfacial reaction and diffusion of oxygen through the oxide layer, respectively.

## Keywords

_{1−x }Ge

_{ x }nanowiresCoaxial heterostructureBifurcate reactionsInterfacial reactionDiffusion-controlled reactionSelf-limiting oxidationKinetics of gas diffusion

## 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 Si_{1−x
}Ge_{
x
} nanowires, which are promising building blocks for high-performance nano-electronic devices. Our approach is based on the O_{2} gas flow-induced bifurcate Ge deposition or oxidation reaction of Si_{1−x
}Ge_{
x
} nanowires. It enables selectively prepared Ge/Si_{1−x
}Ge_{
x
} or SiO_{2}/Si_{1−x
}Ge_{
x
} coaxial heterostructure nanowires by the kinetics of gas flow.

## Experimental Procedure

_{1−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 (SiCl

_{4}, 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 SiCl

_{4}precursor through a H

_{2}bubbler system was introduced into the system at a flow rate of 20 sccm. H

_{2}(100 sccm) and Ar (100 sccm) were used as ambiance gases for the synthesis. After the growth of Si

_{1−x }Ge

_{ x }nanowires for 30 min, the flow of SiCl

_{4}was terminated and then a flow of O

_{2}was introduced at 900°C under vacuum maintained by mechanical pump. The flow rate of O

_{2}was controlled from 50 to 300 sccm for bifurcate reactions.

We observed as-grown Si_{1−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

_{1−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 10

^{8}/cm

^{2}. The composition of the Si

_{1−x }Ge

_{ x }nanowires could be controlled by the substrate distance from the Ge powder [14]. Among them, Si

_{0.85}Ge

_{0.15}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].

After the growth of Si_{1−x
}Ge_{
x
} nanowires, SiCl_{4} flow was terminated and O_{2} gas was introduced under vacuum. Under these conditions, Ge vapor from the Ge source together with O_{2} gas were introduced to the substrate where the nanowires were vertically grown. Our systematic studies showed that Ge/Si_{1−x
}Ge_{
x
} coaxial heterostructure nanowires were the result of the deposition of Ge under the low flow rate of O_{2} (i.e., <100 sccm that maintains the total pressure of <2 torr) while SiO_{2}/Si_{1−x
}Ge_{
x
} coaxial heterostructure nanowires resulted from the oxidation of Si_{1−x
}Ge_{
x
} nanowires under the high flow rate (i.e., >100 sccm that maintains the total pressure of >2 torr).

_{1−x }Ge

_{ x }coaxial heterostructure nanowires. The synchrotron XRD patterns were indexed to a diamond structure on the Si

_{1−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 Si

_{1−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.

_{ x }/Si

_{1−x }Ge

_{ x }coaxial heterostructure nanowires. The synchrotron XRD patterns of the oxidized Si

_{1−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 Si

_{1−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 Si

_{1−x }Ge

_{ x }thin films, was found [17–19]. It is known that Ge segregation in the Si

_{1−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 Si

_{1−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 }/Si

_{1−x }Ge

_{ x }on cooling to the room temperature. The volume expansion of Si

_{1−x }Ge

_{ x }during oxidation can induce the lattice expansion near the SiO

_{ x }/Si

_{1−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 Si

_{1−x }Ge

_{ x }core [21]. During the cooling to the room temperature, the significant mismatch in the thermal expansion coefficients of SiO

_{ x }and Si

_{1−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.

The oxidation kinetics of Si_{1−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/SiO_{2} interface [23, 24]. As new oxide grows at the interface, the old oxide expands due to the increase in volume of SiO_{2} 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/SiO_{2} 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/Si_{1−x
}Ge_{
x
} or SiO_{2}/Si_{1−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 O_{2} gas. In fact, the reactions were very sensitive to the flow of O_{2} 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., O_{2} pressure in this study).

- 1)
Ge(vapor) = Ge(solid) shell

- 2)
O

_{2}(vapor) + Si(solid) in SiGe nanowires = SiO_{2}(solid) shell

Thermodynamic calculation shows that the equilibrium partial pressure of O_{2} for reaction is almost 10^{−24} torr. Therefore, oxidation of Si_{1−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 O_{2} 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 O_{2} pressure of < 2 torr. Meanwhile, at O_{2} pressure higher than 2 torr, O_{2} can penetrate into the nanowire array and induce oxidation. The bifurcation is thus a result of compete penetration of Ge vapor and O_{2} 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 Si_{1−x
}Ge_{
x
} nanowires can be achieved by the control of O_{2} 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 SiO_{2} 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].

## Declarations

### Acknowledgments

This research was supported by a grant from the National Research Laboratory program (R0A-2007-000-20075-0) and Nano R&D (Grant No. 2009-0082724) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science & Technology. This work was also supported by Hi Seoul Science (Humanities) Fellowship from Seoul Scholarship Foundation.

**Open Access**

This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

## Authors’ Affiliations

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