Low Temperature Selective Growth and Field Emission Characteristics of Tapered SiGe Nanowires Array
Project No
： 甲
91-E-FA04-1-4
April 1, 2005 to Jan. 10, 2006
H-L Hsiao, C-Y Chen, J-L Chen
Department of Physics, Tunghai University
Abstract
Tapered SiGe nanowires array were successfully synthesized onto gold patterned Si substrates by metal-catalyzed low pressure chemical vapor synthetic approach at 380 o
C and exhibit high degree of vertical alignment and high density. The tip regions of SiGe nanowires are about 100nm in diameter and the base regions are measured to be less than 10nm. It is noted that the synthesized nanowires exhibit hexagonal shape cross section and smooth faceting surface. It is believed that coexistence of two growth mechanisms, vapor-liquid-solid synthetic reaction governing the axial growth and vapor-solid surface diffusion leading to the lateral epitaxial deposition, result in the formation of tapered nanostructures. The tapered SiGe nanowires exhibit a turn-on field of 5.8V
/ μm and a threshold electric field of 8.8V/μm. The excellent field emission characteristics are attributed to the tapered geometry of the crystalline SiGe nanowires.
Introduction
The design and fabrication of vacuum microelectronic devices based on field-emission cathodes of various types have proven to be a fruitful area of research for many groups over the last three decades [1].
Many investigations have shown that various forms of nanostructured graphitic carbon have very good emission properties [2-4]. Due to the multiplicity of carbon nanotubes in the cathodes of these devices, numerous emission sites are present, yielding devices capable of delivering high emission currents at relatively low operating voltages. However, it has been demonstrated that this feature may be the source of significant device-to-device nonuniformity [5]. Devices using mats of carbon nanotubes for the field-emission cathode possess no way to precisely control the location or density of the emission sites.
Nanowires have recently attracted attention as an alternative system to carbon nanotubes. Nanostructures of different materials systems have been characterized for their field-emission properties[6-8]. It has been reported that field emission from sponge-like Si nanowires demonstrates low-threshold and is comparable to that from various one-dimensional materials, including carbon nanotubes [9-11].
Nevertheless, to fabricate a promising nanowires field emission device, the electric field at the tip must be large enough to obtain a high emission current, and selective area growth of the nanowires array must be achieved at lower temperature for large area flat panel display applications. To fulfill these requirements, vertically aligned tapered nanowires should be synthesized onto pre-patterned substrates at temperature lower than melting point of glass or cheaper stainless substrates. In this paper, we report the successful synthesis of tapered SiGe nanowires by using metal-catalyzed low pressure chemical vapor synthetic route onto gold pre-patterned Si wafers at 380 o
C. The morphology, structural properties, elemental compositions and the field emission characteristics of these nanowires are investigated.
Experimental
For catalyst patterning, a transmission electron microscope copper grid was used as the metal mask.
Ultra-thin Au film was sputtered onto the masked p-type
Si(001) substrates. The samples were then transferred into a quartz tube furnace and evacuated to a base pressure less than 1х10 -6 torr. Hydrogen, silane and Ar diluted germane (GeH
4
:Ar=1:9) gases were introduced into the chamber. Subsequent Au-catalyzed vapor phase synthetic reaction was carried out at a substrate temperature of 260-440 o
C and pressure of 20 Torr. After
CVD growth, the surface morphology of sample was inspected by a field-emission scanning electron microscope (JEOL JSM-6500F) operated at an accelerating voltage of 15 kV. A transmission electron microscope (JEOL JEM-2010), operating at 200kV, equipped with an energy-dispersive x-ray spectroscopy
(EDS) was used for microstructure and elemental composition examination. The bonding configurations and the crystallinity of nanowires were characterized by micro-Raman spectroscopy (NanoFinder).
The field-emission measurement was carried out in a homebuilt high-vacuum (HV) system with a parallel plate diode configuration. In which a spherical stainless-steel (5mm in diameter) was used as the anode and the samples with nanowires array was connected to ground. The gap between the anode and emitting surface is fixed at 100μm. All field-emission measurements were performed in vacuum with a base pressure of <2x10−7
Torr.
Results and Discussions
Scanning electron microscopy images of nanowires protruding from the top surface of the substrate are shown in Figure 1. SiGe nanowires were synthesized on a
Au-patterned Si substrate. The growth temperature is optimized at 380 o C and growth time is about 30 min under 10:3 flow rate ratio of silane to germane. It is observed that, the SiGe nanowires exhibit tapered structures with non-uniform diameters along its length.
The nanowires with taper-like morphology and high

degree of vertical alignment can be clearly seen.
(a) (b)
(c) (d)
Fig.1 (a) SEM image of controlled growth of SiGe nanowires onto Au-patterned Si substrate. (b) Magnified image of single pixel as shown in (a). (c) 25 o
tilted and highly magnified image in (b). Tapered SiGe nanowires are clearly seen. (d) A mat of nanowires cut from the sample was laid on the top surface of other nanowires. It indicated that the synthesized product exhibit high degree of vertical alignment and high density. to the favorable sites (vapor-solid) [12]. The diameter of tip regions is determined by the nucleation and catalyst size while the diameter of base regions is determined by the incoming precursors flux, surface diffusivity and the axial growth rate.
Figure 3 is a bright-field TEM image of an individual SiGe nanowire. The inset is the selective area electron diffraction pattern. The dark rounded end is Au catalyst particle at the tip of the nanowire. The synthesized nanowires were found to be single crystal with a <011> growth direction. The atom resolved TEM image depicted in right inset of Fig.3 indicated high quality of crystallinity and free of oxide sheath. Actually, only part of the nanowires exhibits Au catalyst particles tip. Systematically experimental investigations demonstrate that the thickness of catalyst directly influences the probability of finding Au catalyst at the nanowires tip. For sputtered Au catalyst with thickness less than 2nm, no apparent Au catalyst particles can be found at the tip of nanaowires.
(a) (b)
Fig.2 (a) Plane-view SEM image of tip regions in SiGe nanowires. The diameter of tip regions is about 5~10nm.
(b) Cross-sectional SEM image of cracked sample. It is noted that the synthesized nanowires exhibit hexagonal shape cross section as shown in inset. And the diameter of base regions is about 100nm.
The diameters of SiGe nanowires tip and base are measured from cross-sectional SEM images (as shown in
Fig.2) and estimated to be about 10nm and 100nm, respectively. The growth rate of nanowires was evaluated to be about 0.06
5 μm/ minute. Actually, the diameter of base regions enlarged with increasing the growth time while the diameter of tip regions changed with respect to the catalyst thickness. It is observed that, the richness of
Ge content and incorporation of doping gases would also affect the growth rate and the diameters. Therefore, the aspect ratio and the degree of sharpness can be tuned by adjusting the process parameters.
From the inset of Figure 2(b), nanowires with hexagonal shape cross-section and smooth surface are noticed. This observation strongly suggests that lateral epitaxial growth of incoming Si and Ge precursors onto the surface of nanowires progressed even under such a low temperature. That means, constructions of tapered
SiGe nanowires are rely on two different pathways: one way is through vapor-liquid-solid synthetic reaction, and the other is through surface diffusion of reaction species
Fig.3 TEM image, corresponding electron diffraction pattern and atom resolved lattice fringes of individual
SiGe nanowire.
Raman scattering measurement was carried out to clarify the crystalline property. Figure 4 shows the SEM images and corresponding Raman spectra of intrinsic Si,
Ge, and SiGe nanowires. To prevent from the interference of Si wafer signals, nanowires were dispersed onto the corning 7059 glass substrates.The full-width-at-half-maximum (FWHM) in the Raman scattering spectra of Si and Ge nanowires is compatible to that of the Si wafer and demonstrates high crystalline quality. However, the spectrum of SiGe nanowires is more complicated. Except three bands that associated with the Si-Si, Si-Ge, and Ge-Ge stretching modes down-shifted with respect to that of the pure nanowires, we can find additional two broad vibration bands located at around 440cm -1 and 250cm -1 . The 440cm -1 band is identified to be come from the localized Si-Si stretching mode [13]. Second band located around 250cm -1 is a characteristic feature of amorphous Ge phase [14, 15].

Does it mean amorphous alloy phase exist in the synthesized nanowires? We can not rule out this possibility. But from the SEM and TEM examinations, it is believed that the formation of amorphous thin film on the Au coated substrate surface could possibly be responsible for the broad bands.
The characteristics of emission current versus electric field for the typical sponge-like Si nanowires, tapered Ge and SiGe nanowires is shown in Fig. 5. The electron emission turn-on field and threshold field, defined as the macroscopic fields required for generating a current density of 1 0 μA/cm 2 and 10mA/cm 2 , respectively, are estimated to be about 9.5V/μm and 11.9
V/μm for Si nanowires , 7.0V/μm and 9.9V/μm for Ge nanowires, and 5.8V/μm and 8.8V/μm for SiGe nanowires. This is comparable to many other types of emitters such as carbon nanotubes, diamond, amorphous carbon nitride, and silicon carbon.
(a)
(b) (c)
Fig.4 (a) Raman spectrum of Si, Ge and SiGe nanowires.
The surface morphology of sponge-like Si nanowires sample is shown in (b). The surface morphology of tapered Ge nanowires sample is shown in (c). For Raman scattering measurement, the synthesized nanowires were dispersed onto corning 7059 glass substrates to preventing from the interference of Si wafer signals.
In the inset of Fig.5, the exponential dependence relationship between the emission current and the applied electric field is shown. It is tempting to use the FN equation to calculate the electrical and geometric properties (work function, emission area and field enhancement factor) from the J-E data collected.
However, it is difficult to identify. The electron emission is a result of combined effects from both electrical and geometric contributions. These effects can not be separated easily from the emission results. The ln(J/E 2 )-1/E plot did not give a straight line. Therefore, a qualitatively comparison between these three types of nanowires reveals that field enhancement factor of tapered SiGe nanowires is larger than that of tapered Ge nanowires and than that of sponge-like Si nanowires.
This result conforms to the experimental observations on the surface morphology: the tip curvature of tapered SiGe nanowires is smaller than that of tapered Ge nanowires and than that of Si nanowires. These results indicate that the tapered nanowires may have promising applications for vacuum microelectronic devices and field emission display.
Fig.5 Field emission current-electric field characteristics of sponge-like Si, tapered Ge, and SiGe nanowires at measurement distance of 100μm. Inset is the corresponding Fowler-Nordheim plot.
Conclusion
In summary, tapered SiGe nanowires have been successfully synthesized onto Au-patterned Si wafer by metal-catalyzed low pressure chemical vapor deposition at 380 o
C and exhibit high degree of vertical alignment and high density. The high crystalline quality depicted from SEM, TEM and Raman characterizations demonstrate potential applications of this low temperature selective growth techniques to large area flat panel display. The tip regions of SiGe nanowires are about 100nm in diameter and the base regions are measured to be less than 10nm. It is noted that the synthesized nanowires exhibit hexagonal shape cross section and smooth faceting surface. It is believed that coexistence of two growth mechanisms, vapor-liquid-solid synthetic reaction governing the axial growth and vapor-solid surface diffusion leading to the lateral epitaxial deposition, result in the formation of tapered nanostructures. The tapered SiGe nanowires exhibit a turn-on field of 5.8V/μm and a threshold electric field of 8.8V/μm. The excellent field emission characteristics are attributed to the tapered geometry of the crystalline SiGe nanowires.
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