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Effect of growth temperature on the morphology and phonon properties of InAs nanowires on Si substrates.

Li T, Chen Y, Lei W, Zhou X, Luo S, Hu Y, Wang L, Yang T, Wang Z - Nanoscale Res Lett (2011)

Bottom Line: 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.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.A splitting of TO modes is also observed.PACS: 62.23.Hj; 81.07.Gf; 63.22.Gh; 61.46.Km.

View Article: PubMed Central - HTML - PubMed

Affiliation: Key Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Science, Beijing 100083, People's Republic of China. yhchen@semi.ac.cn.

ABSTRACT
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.

No MeSH data available.


Raman spectra of InAs nanowires and temperature-dependent Raman shift. (a) Micro-Raman spectra of InAs nanowires with an average diameter of 42 nm. The black line is the recorded data while the lighter colored (green) lines are results from a multiple Lorentzian fit; (b) Raman spectrum from bulk (111) InAs; (c) Temperature-dependent Raman shift of the TO, SO, and LO phonon mode of InAs NWs.
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Figure 2: Raman spectra of InAs nanowires and temperature-dependent Raman shift. (a) Micro-Raman spectra of InAs nanowires with an average diameter of 42 nm. The black line is the recorded data while the lighter colored (green) lines are results from a multiple Lorentzian fit; (b) Raman spectrum from bulk (111) InAs; (c) Temperature-dependent Raman shift of the TO, SO, and LO phonon mode of InAs NWs.

Mentions: Figure 2a shows the Raman spectrum of InAs NWs in sample B measured with incident laser beam parallel to the c-axis of NWs. Three main scattering peaks are observed, which are located around 237.9, 230.0, and 216.2 cm-1, respectively. To probe the origin of these three Raman peaks, Raman measurements are also performed on bulk InAs (111) substrate for comparison, the spectrum of which is shown in Figure 2b. For bulk InAs materials, two Raman peaks are clearly observed: one is located around 241.0 cm-1, the other around 218.7 cm-1, which can be attributed to the LO and TO phonon modes of bulk InAs. Therefore, Raman peaks located at 237.9 and 216.2 cm-1 in Figure 2a can be attributed to the LO and TO phonon mode of InAs NWs. Except the downshift of their phonon frequency relative to InAs bulk material, the LO and TO phonon peaks of InAs NWs also show a larger full width at half maximum. To explain such frequency downshift and line-width broadening of LO and TO phonon peaks of InAs NWs, three possible reasons should be taken into account. One is the small lateral size of InAs NWs. According to the "spatial correlation" model proposed by Richter et al. [20] and Tiong et al. [21], and also generalized by Campbell and Faucher et al. [22], the reduction in physical dimension of materials can lead to a downshift of phonon frequency and a broadening of the LO phonon peak due to the strong quantum confinement and the relaxation of q = 0 selection rule. However, the diameter of our InAs NWs is very large (> 20 nm) and shows almost no quantum confinement effect, which cannot explain the observed downshift in phonon frequency of LO and TO phonon peaks. Another one is the thermal anharmonicity effect caused by temperature change. Anharmonicity entails the participation of phonons at frequencies multiple of the fundamental in the scattering events [23]. Such anharmonic effects become prominent at higher temperatures due to the larger extent of lattice vibrations, and are irrespective of the longitudinal or transverse character of the phonon modes. Theoretically, an increase in temperature can induce both line-width broadening and frequency downshift of phonon peaks. However, our Raman spectra are measured under a low laser excitation power of 0.05 mW, where the heating effect can be ignored. The last one is the existence of structural defects in NWs. As indicated by the work on GaAs and InAs NWs grown on SiO2 and GaAs substrates, defects can cause a frequency downshift and line-width broadening to the phonon peaks [8]. As shown by the HRTEM study, defects like rotation twins and stacking faults exist in the samples, which might relax the q = 0 selection rule and lead to the frequency downshift and line-width broadening of the phonon peaks.


Effect of growth temperature on the morphology and phonon properties of InAs nanowires on Si substrates.

Li T, Chen Y, Lei W, Zhou X, Luo S, Hu Y, Wang L, Yang T, Wang Z - Nanoscale Res Lett (2011)

Raman spectra of InAs nanowires and temperature-dependent Raman shift. (a) Micro-Raman spectra of InAs nanowires with an average diameter of 42 nm. The black line is the recorded data while the lighter colored (green) lines are results from a multiple Lorentzian fit; (b) Raman spectrum from bulk (111) InAs; (c) Temperature-dependent Raman shift of the TO, SO, and LO phonon mode of InAs NWs.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211884&req=5

Figure 2: Raman spectra of InAs nanowires and temperature-dependent Raman shift. (a) Micro-Raman spectra of InAs nanowires with an average diameter of 42 nm. The black line is the recorded data while the lighter colored (green) lines are results from a multiple Lorentzian fit; (b) Raman spectrum from bulk (111) InAs; (c) Temperature-dependent Raman shift of the TO, SO, and LO phonon mode of InAs NWs.
Mentions: Figure 2a shows the Raman spectrum of InAs NWs in sample B measured with incident laser beam parallel to the c-axis of NWs. Three main scattering peaks are observed, which are located around 237.9, 230.0, and 216.2 cm-1, respectively. To probe the origin of these three Raman peaks, Raman measurements are also performed on bulk InAs (111) substrate for comparison, the spectrum of which is shown in Figure 2b. For bulk InAs materials, two Raman peaks are clearly observed: one is located around 241.0 cm-1, the other around 218.7 cm-1, which can be attributed to the LO and TO phonon modes of bulk InAs. Therefore, Raman peaks located at 237.9 and 216.2 cm-1 in Figure 2a can be attributed to the LO and TO phonon mode of InAs NWs. Except the downshift of their phonon frequency relative to InAs bulk material, the LO and TO phonon peaks of InAs NWs also show a larger full width at half maximum. To explain such frequency downshift and line-width broadening of LO and TO phonon peaks of InAs NWs, three possible reasons should be taken into account. One is the small lateral size of InAs NWs. According to the "spatial correlation" model proposed by Richter et al. [20] and Tiong et al. [21], and also generalized by Campbell and Faucher et al. [22], the reduction in physical dimension of materials can lead to a downshift of phonon frequency and a broadening of the LO phonon peak due to the strong quantum confinement and the relaxation of q = 0 selection rule. However, the diameter of our InAs NWs is very large (> 20 nm) and shows almost no quantum confinement effect, which cannot explain the observed downshift in phonon frequency of LO and TO phonon peaks. Another one is the thermal anharmonicity effect caused by temperature change. Anharmonicity entails the participation of phonons at frequencies multiple of the fundamental in the scattering events [23]. Such anharmonic effects become prominent at higher temperatures due to the larger extent of lattice vibrations, and are irrespective of the longitudinal or transverse character of the phonon modes. Theoretically, an increase in temperature can induce both line-width broadening and frequency downshift of phonon peaks. However, our Raman spectra are measured under a low laser excitation power of 0.05 mW, where the heating effect can be ignored. The last one is the existence of structural defects in NWs. As indicated by the work on GaAs and InAs NWs grown on SiO2 and GaAs substrates, defects can cause a frequency downshift and line-width broadening to the phonon peaks [8]. As shown by the HRTEM study, defects like rotation twins and stacking faults exist in the samples, which might relax the q = 0 selection rule and lead to the frequency downshift and line-width broadening of the phonon peaks.

Bottom Line: 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.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.A splitting of TO modes is also observed.PACS: 62.23.Hj; 81.07.Gf; 63.22.Gh; 61.46.Km.

View Article: PubMed Central - HTML - PubMed

Affiliation: Key Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Science, Beijing 100083, People's Republic of China. yhchen@semi.ac.cn.

ABSTRACT
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.

No MeSH data available.