Morphology control and optical properties of SiGe nanostructures grown on glass substrate
© Chang and Lee; licensee Springer. 2012
Received: 30 November 2011
Accepted: 27 February 2012
Published: 27 February 2012
With the rapid progress of nanotechnology, nanostructures with different morphologies have been realized, which may be very promising to enhance the performance of semiconductor devices. In this study, SiGe nanostructures with several kinds of configurations have been synthesized through a chemical vapor deposition process. By controlling growth conditions, different SiGe nanostructures can be easily tuned. Structures and compositions of the nanostructures were determined by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The optical properties of various SiGe nanostructures revealed some dependence with their morphologies, which may be suitable for solar cell applications. The control of the SiGe morphology on nanoscale provides a convenient route to produce diverse SiGe nanostructures and creates new opportunities to realize the integration of future devices.
KeywordsSiGe reflectance nanowire core-shell transformation
Semiconductor nanostructures, such as nanowires, nanotubes, and nanoflowers, have been extensively studied as building blocks for emerging devices [1–3]. Recently, a substantial interest has focused on the synthesis of one-dimensional nanostructures because they are expected to play a critical role as interconnects or functional units in fabricating promising nanodevices [4, 5]. Among various kinds of nanomaterials, SiGe-based nanostructures are of great importance to study because they incorporate desirable characteristics of Si and Ge. With a low power consumption nature, SiGe-based electronic devices may achieve better performance than Si-based ones. Besides, SiGe provides additional flexibility through bandgap engineering, and it is also compatible with standard semiconductor processing.
However, due to different thermodynamics and kinetics of SiH4 and GeH4 [6, 7], it is very challenging to achieve controlled growth of SiGe nanowires and their heterostructures by vapor-liquid-solid (VLS) method . To explore the unique properties of SiGe nanostructures, detailed understanding of their characteristics in different growth conditions is required. Moreover, for the fabrication of versatile nanoscale devices, developing morphology-controlled growth of SiGe nanostructures is also an important issue.
The present work reports the fabrication of SiGe nanostructures using a chemical vapor deposition (CVD) method under different growth conditions. Our motivation is to find the morphology dependence on the nanostructures' preparation parameters. The optical properties of the synthesized SiGe nanostructures were also investigated.
The experiments were carried out in a hot-wall thermal chemical vapor deposition system using GeH4 (10% premixed in N2) and SiH4 (10% premixed in N2) as the precursor gases. Glass substrates (Corning 1737F, Corning Inc., Corning, NY, USA) were first cleaned in piranha solution (3:1 (v/v) H2SO4/H2O2) and sonicated in DI water. Subsequently, poly-L-lysine solution was dripped on several pieces of cleaned glass substrates. After that, commercially available Au nanoparticles were deposited on these poly-L-lysine functionalized glass substrates. The substrates were loaded into the deposition chamber after removing poly-L-lysine. During the heating period, the reaction chamber was flushed with N2 and pumped out with a mechanical pump. The reaction temperature was varied from 405°C to 475°C, which is well controlled by a computer. The total pressure in the reaction chamber was fixed at 30 Torr. The flow rate of SiH4 was maintained at a constant value, but the flow rate of GeH4 was set to 24 or 40 sccm.
The structure and morphology of the as-grown samples were examined by field emission scanning electron microscopy (Gemini LEO 1530, Carl Zeiss Microscopy, Carl-Zeiss-Straße, Oberkochen, Germany) and by high-resolution transmission electron microscopy (300 kV Philips Tecnai F30, FEI Co., Hillsboro, OR, USA) equipped with an energy dispersive X-ray (EDX) detector. The phases and crystal orientation analysis of the synthesized nanowires were identified by X-ray diffraction (XRD; Cu Kα radiation, X'pert, PANalytical B.V., Almelo, The Netherlands) with a Bragg angle ranging from 20° to 80°. In order to explore the optical properties of different SiGe nanostructures, the reflectance spectra were also measured using a UV-1650PC spectrometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) for a wavelength range from 350 to 1,000 nm.
Results and discussion
SiGe nanorod growth
SiGe/Ge core-shell nanowire growth
In order to find the condition for eliminating the bead-like structure, we tried to choose a much higher temperature for the growth, wherein we wish more germane decomposition to cover the wavy sidewall. The reason may be ascribed to the larger activation energy for the decomposition of silane than that of germane . Some group has also observed the increase in Ge deposition  at a higher temperature. In addition, several theoretical studies also indicated that a core-shell structure could be more stable than a homogenous structure [12, 13]. However, at a higher temperature such as 475°C, there might be some oxide deposition on the sidewalls of our nanowires due to the residue oxygen in the reaction chamber. To confirm whether the oxygen will participate in the deposition, we performed EDX analysis to the samples grown at 475°C, and no oxygen content was detected.
Optical properties of SiGe nanostructures
In summary, we have grown SiGe nanostructures under different growth conditions via a simple CVD method. Temperature-dependent morphology changes and their optical properties have been further discussed. The identification of different growth conditions for SiGe nanostructures has great potential for preparing diverse nanostructures, which may be suitable for multifunctional device applications.
chemical vapor deposition
scanning electron microscopy
transmission electron microscopy
The authors would like to thank the National Science Council of the Republic of China for the financial support under Contract No. NSC 100-2221-E-002-054-MY3.
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