Effect of growth temperature on the morphology and phonon properties of InAs nanowires on Si substrates
© Li et al; licensee Springer. 2011
Received: 15 April 2011
Accepted: 21 July 2011
Published: 21 July 2011
Catalyst-free, vertical array of InAs nanowires (NWs) are grown on Si (111) substrate using MOCVD technique. The as-grown InAs NWs show a zinc-blende crystal structure along a < 111 > direction. It is found that both the density and length of InAs NWs decrease with increasing growth temperatures, while the diameter increases with increasing growth temperature, suggesting that the catalyst-free growth of InAs NWs is governed by the nucleation kinetics. The longitudinal optical and transverse optical (TO) mode of InAs NWs present a phonon frequency slightly lower than those of InAs bulk materials, which are speculated to be caused by the defects in the NWs. A surface optical mode is also observed for the InAs NWs, which shifts to lower wave-numbers when the diameter of NWs is decreased, in agreement with the theory prediction. The carrier concentration is extracted to be 2.25 × 1017 cm-3 from the Raman line shape analysis. A splitting of TO modes is also observed.
PACS: 62.23.Hj; 81.07.Gf; 63.22.Gh; 61.46.Km
Semiconductor nanowires (NWs) have been intensively studied in the last decade due to their novel physical properties and potential applications in high-performance devices, such as field-effect transistors, lasers, photodetectors, and photovoltaic devices [1–5]. Such NWs are usually grown through vapor-liquid-solid mode where metal nanoparticles (Au, Ni, or other metals) act as catalysts [6–9]. However, for certain materials the metal catalysts can result in unintentional incorporation into pure crystalline NWs, which causes serious problems for materials doping and limits their device applications. In order to avoid the contamination from Au and other metal atoms, it is highly preferred that NWs can be grown without catalysts.
On the other hand, one of the most attracting features of NWs is that lattice mismatch or strain in NWs can be significantly relaxed due to their high surface/volume ratio and small lateral size. This can be used to realize one of the dreams in semiconductor community--integration of III-V semiconductor on Si platform [10, 11], which presents a big challenge due to the significant lattice mismatch and differences in coefficient of thermal expansion between Si and III-V materials. The integration of III-V semiconductor on Si will allow people to take advantage of both the key features of Si like low cost and mature processing technology and those of III-V semiconductor like direct bandgap and high-quality heterostructure growth. Among the III-V semiconductors, InAs NWs possess excellent electron transport properties such as high bulk mobility, small effective mass, and low ohmic contact resistivity, which can be used for preparing high-performance electronic devices such as high mobility transistor [12, 13].
Though some work has been done on catalyst-assisted InAs NWs [8, 14], little work has been devoted to catalyst-free InAs NWs, especially on Si substrates [5, 15, 16]. In this paper, we present a study on the catalyst-free synthesis and phonon properties of InAs NWs on Si substrates. By varying the growth temperature, InAs NWs with different diameters were grown on Si substrates. The phonon properties of the InAs NWs are investigated using Raman scattering characterization. The effects of growth temperature on the frequency shift of longitudinal optical (LO), transverse optical (TO), and surface optical (SO) modes are analyzed. Furthermore, a splitting of TO modes also is observed and discussed.
Vertical InAs NWs arrays were grown on n-type Si (111) substrates in a close-coupled showerhead metal-organic chemical vapor deposition (MOCVD) system (Thomas Swan Scientific Equipment, Ltd., Cambridge, UK) at a pressure of 100 Torr. Trimethylindium (TMIn) and AsH3 were used as precursors and ultra-high purity H2 as carrier gas. First, Si substrates were cleaned (ultrasonicate in trichloroethylene, acetone, isoproponal, and deionized water sequentially) and etched in buffered oxide etch solution (BOE, six parts 40% NH4F and one part 49% HF) for 30 s to remove the native oxide, and then rinsed in deionized water for 15 s and dried with N2. Then, the substrates were loaded into the MOCVD chamber for growth. The substrates were heated up to the growth temperature ranging from 530°C to 570°C, and after 5-min stabilization time, the growth was initiated by simultaneous introducing TMIn (2 × 10-6 mole/min) and AsH3 (2 × 10-4 mole/min) into the reactor chamber for 7 min. After the growth, InAs NWs were cooled down with the protection of AsH3 flow. To obtain more understanding about the controlled growth of catalyst-free InAs NWs on Si, InAs NWs were grown at various temperatures ranging from 530°C to 570°C, i.e., 530°C for sample A, 550°C for sample B and 570°C for sample C. The morphology of InAs NWs was characterized by field emission scanning electron microscopy (S-4800, Hitachi, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20, 200 keV; FEI, Eindhoven, Netherlands). Raman scattering measurements were performed in backscattering geometry at room temperature with a Jobin Yvon HR800 confocal micro-Raman spectrometer (Horiba Ltd., Longjumeau, France). Scattering configuration ( ) was adopted. The samples were excited by the 514.5 nm line of an Ar-ion laser to a 1 µm spot on the surface with an excitation power of 0.05 mW.
Results and discussion
Growth parameters and morphology statistics of InAs NWs grown in sample A, B, and C.
H2 flow rate
To study the structural properties of InAs NWs, HRTEM measurements were carried out. Figure 1d shows the typical HRTEM image of InAs NWs (sample B). It is observed that the InAs NW is uniform in diameter. It should be noted that alternative dark and bright contrast bands are observed, which can be attributed to the rotation twins and stacking faults. Figure 1e shows the HRTEM image of sample B with its inset showing the fast Fourier transforms (FFTs) image. The HRTEM image combining with FFT image indicates that the InAs NWs has a cubic, zinc blend structure and grows along the < 111 > direction normal to the Si (111) substrate. Such rotation twins and stacking faults are formed by random stacking of the closest-packed planes during crystal growth, which have also been observed in III-V NWs grown along the < 111 > direction [18, 19].
Vice versa, the free carrier concentration can be calculated if the frequency of SO phonon mode and the size of the NWs are known. Here, the free carrier concentration in sample B is estimated to be 2.25 × 1017 cm-3using the measured diameter (42 nm) and SO phonon frequency (230.0 cm-1). This result is close to the value obtained through electrical measurements in . This high free carrier concentration in the InAs NWs might be caused by the unintentional doping due to carbon background incorporation . To get more understanding of this SO phonon mode in InAs NWs, temperature-dependent Raman measurements are also performed on the InAs NWs in sample B, the results are shown in Figure 2c. It is observed that the SO phonon peak shifts to lower frequency with increasing the temperature, which is similar to the temperature behavior of the LO and TO mode of InAs NWs, and can be explained by the lattice expansion in NWs. It should be noted that though the SO feature is not apparent at high temperatures (> 173 K) the free carrier concentration should still be around the value (2.25 × 1017 cm-3) at low temperatures considering the fact that the free carrier concentration induced by unintentionally doping is much higher than that of intrinsic carrier in InAs materials (~1 × 1015 cm-3).
To summarize, the catalyst-free, growth, and phonon properties of InAs NWs on Si (111) substrates are investigated in detail in this paper. Both the density and the length of InAs NWs decrease with increasing growth temperatures, while the diameter of InAs NWs increases with increasing growth temperature, suggesting that the catalyst-free growth of InAs NWs are governed by the nucleation kinetics in the system. The LO and TO mode of InAs NWs both present a phonon frequency smaller lower than those of InAs bulk materials, which is speculated to be mainly caused by the defects in the NWs. Apart from LO and TO phonon modes, a SO mode is also observed for the InAs NWs, the signal feature of which becomes more prominent with reducing the diameter of NWs due to the increased surface/volume ratio. A splitting of transverse optical (TO) modes also is observed.
metal-organic chemical vapor deposition
scanning electron microscopy
high-resolution transmission electron microscopy.
The work was supported by the National Natural Science Foundation of China (No. 60625402 and 60990313), and the 973 program.
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