1. Introduction
The ways to develop one-dimensional (1D) nanostructures, such as nanowires, nanorods, nanobelts and nanotubes, are being studied intensively, due to their unique applications in mesoscopic physics and nanoscale electronic devices [1-3]. Structural phase transition between the wurtzite (WZ) and zinc-blend (ZB) GaN induced by the deposition conditions [4], temperature-mediated phase selection during the growth of GaN [5], and substrate control [6] by the crystallographic alignment of GaN have all been observed. It is known that x-ray scattering technique plays an important role in investigating the lattice excitations and structural transformation associated with thermal strain in 1D nanowires [7]. For example, Dahara and co-workers [8] reported a phase transformation from hexagonal to cubic in Ga+ implaned GaN nanowires (GaNWs). The SC16 phase of GaAs appears at high pressure can be transformed to the hexagonal WZ phase by reducing the pressure to the ambient one. WZ GaAs is stable in resisting a transformation to the ZB phase at temperatures up to 473 K at ambient pressure [9]. Currently, most of the studies on the crystalline structure of GaNWs are focused on the stable hexagonal α-GaN and metal-stable cubic β-GaN. In this work, we study the crystalline structure of GaNWs by using
2. Important
I
3. Experimental details
GaN is a direct wide band-gap semiconductor at room temperature. It is a prominent candidate for optoelectronic devices at blue and near ultra-violet wavelengths [11-14]. In addition, it exhibits high thermal conductivity and little radiation damage, suitable for high temperature and high power microelectronic devices[15]. GaN nanowires have been synthesized by several groups using different methods[16-22]. The randomly oriented GaNWs used in this study were synthesized by a low pressure thermal chemical vapor deposition (LPTCVD) technique. The samples were grown at 950 oC on Si [001] substrates precoated with a 5 nm Au catalyst layer by an E-Gun evaporator. Molten gallium was used as the source material and NH3 (30 sccm) as the reactant gas in a horizontal tubular furnace. Details of the growth process may be found elsewhere [23]. A low temperature
4. Results and discussion
4.1. SEM results
The morphology of sample was characterized by a field emission scanning electron microscope (FE-SEM, JEOL JSM-6500F) equipped with an energy dispersive x-ray spectroscope (EDS, Oxford Instrument INCA x-sight 7557). Atomic-resolution transmission electron microscopic (TEM) analysis and high-resolution transmission electron microscopy (HRTEM) images were taken with the CCD-camera of an electron microscope (JEOL JEM-2100) at 200 kV. Analysis software (Digital Micrograph) was employed to digitalize and analyze the obtained images. Figure 1 displays a portion of the SEM image showing the morphology of the GaNWs. The diameters of the GaNWs assembly ranged from 20 to 50 nm, with a length of several tens of microns. The diameter distribution of the GaNWs assembly, as shown in the Fig. 2, is quite asymmetric and can be described using a log-normal distribution function (solid line). The log-normal distribution is defined as follows:
4.2. TEM and HRTEM results
Figure 3 shows the TEM morphology of a typical nanowire. TEM image reveals that most of the nanowires are straight, and the diameter along the growth direction is uniform, with a mean diameter of 40(3) nm. Figure 4 shows the selected area electron diffraction (SAED) pattern taken on a region close to the surface of a single nanowire. It clearly reveals a single crystalline nature for the sample studied. The Bragg spots correspond to the [001] reflection of the wurtzite structure of the GaNW. The pattern of the main spots can easily be seen as hexagonal cells with lattice parameters of
structure of GaNWs, on the surface of [001], each Ga atom has three complete bonds to the underlying nitrogen atomic plane. Details of the description of crystal structure may be found with the earlier finding [24].
4.3. X-ray diffraction
X-ray diffraction patterns are known as the fingerprints of crystalline materials. They reveal details of the crystalline structure and their formation during synthesis, and even the crystalline phase transitions or separation at various temperatures. The x-ray and Rietveld refined diffraction patterns of the GaNWs, taken at 320 K and 80 K, are shown in Fig. 6 and 7, respectively. Diffraction patterns were utilized to characterize the crystalline structure in the prepared samples. The diffraction peaks appeared to be much broader than the instrumental resolution, reflecting the nano-size effects. The analysis was performed using the program package of the General Structure Analysis System (GSAS) [25] following the Rietveld method [10]. Several models with different symmetries were assumed during the preliminary analysis. In our structural analysis we then pay special attention to searching for the possible symmetries that can describe the observed diffraction pattern well. All the structural and lattice parameters were allowed to vary simultaneously, and refining processes were carried out until
A series of new peaks, at scattering angles of 44.08o, 56.22o, 58.2o 68.2o, and 75.3o, becomes visible in the diffraction patterns taken at 80 K, as can be seen in the Fig. 7. These peaks were not observed at 320 K and cannot be associated to the α-GaNW. They, however, may be indexed as the {220}ZB, {311}ZB, {222}ZB, {400}ZB, {331}ZB, and {420}ZB reflections of the ZB phase, shown in Fig.8. All these new peaks may be identified to belong a cubic
4.4. In situ low temperature X-ray diffraction
Figure 9 shows the temperature dependency of the
{420}ZB reflection increases rapidly, which is accompanied by a reduction in the peak width. Clearly, these behaviors signal the development of the ZB-phase GaNWs below 260 K.
It is known that the reduction in the peak width with decreasing temperature indicates the growth of the crystalline domain. The observed peak profiles for the ZB-phase are much broader then the instrument resolution function show that the crystalline domains are finite sized, which can be described by the finite lattice model [28]. It follows the instrumental resolution function, which can be well approximated by a Gaussian function. We propose that the intensity of the Bragg reflection from finite size systems can be described [29] as
where 2θ is the scattering angle,
Here λ is the wavelength of the incident x-ray, θB is the Bragg angle of the {
This critical scattering originates from the short range ordered domains that can be indexed by the ZB-GaNWs, as observed by the
5. Conclusion
In conclusion, we have fabricated GaN nanowires employing the LPTCVD method, which we take the advantage of the reaction of gallium with NH3. The mean diameter of the GaN nanowires fabricated was 40(3) nm, and their crystallized into the known wurtzite GaN structure at ambient temperatures. Profile refining of the diffraction patterns shows that the low temperature patterns cannot be described using the hexagonal α-GaN solely. The ZB-GaN phase was found to develop below 260 K. A new short range ordered ZB-GaN phase was observed. The width of the diffraction profile associated to ZB-GaN is noticeably larger than that of the WZ-GaN phase. Short range ordering effect and the phase distribution of random ZB-GaNWs must be taken into account. A short range modeling was employed to identify the correlation lengths of the temperature dependence to the ordered domains [31]. The short-range ordered domains observed are not only of great interest for understanding the thermal effect of the phase separation in the GaNWs system (e.g., for CuO [32, 33], WO2 [34], MoO2 [35] and Ta2O5 nanowires [36-41]) but also for investigating fundamental physics and mechanisms in the future.
Acknowledgments
We would like to thank the National Science Council of the Republic of China for the financial support through project numbers NSC 97-2112-M-259-004-MY3.
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