Morphology and Structural Change in Ammonia Annealed Core Shell

ARTICLE pubs.acs.org/JPCC

Morphology and Structural Change in Ammonia Annealed Core Shell Silicon Nanowires Bhabani S. Swain,† Bibhu P. Swain,*,‡ Sung S. Lee,† and Nong M. Hwang† †

National Research Laboratory of Charged Nanoparticles, Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea ‡ Research Center for Photovoltaics, National Institute of Advanced Industrial Science of Technology, Umezone, Ibaraki, Japan ABSTRACT:

Oxide shielded core shell silicon nanowires (Si NWs) of diameters ranging from 60 to 80 nm were grown on the p-type Si substrate by an atmospheric pressure chemical vapor deposition. These Si NWs were annealed for 60 min at 900 °C in ammonia atmosphere, and the microstructure evolution was studied by field emission scanning electron microscopy. Annealing caused the formation of silicon oxynitride (SiON) within the outer matrix of the amorphous silicon dioxide. The surface chemistry and chemical bonding of Si NWs have been characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared spectroscopy. The transverse optic vibration of Si
O shifted to lower frequency while longitudinal acoustic vibration of Si
O shifted to higher frequency. The total reflectance of ammonia annealed SiO2/SiNWs decreased from 8% to 2% over the UV
vis range as a result of SiON formation. The composition of the resulting SiON was converted from Si35O61 to Si41O36N17 with the silicon to oxygen ratio ranging from 0.57 to 1.13 at the ammonia flow rate of 400 standard cubic centimeters per minute. Moreover, the XPS results confirmed that the SiON networks were quite random with inhomogeneous compositions, and the valence band structure was tuned from a dumbbell to spherical shape by varying the ammonia flow rate.

1. INTRODUCTION Coaxial amorphous silicon oxide/crystalline silicon nanowires (a-SiO2/c-Si NWs) have attracted much attention in photovoltaic cells,1 nonvolatile flash memory,2 and light emitting diode.3 Especially when the mechanical and optical properties are concerned, a better core shell nanostructure needs to be fabricated for device application. Hence, one can realize the coaxial SiO2/Si NWs covered with silicon nitride (Si3N4/SiO2/Si) or silicon oxynitride (SiON/SiO2/Si) for future optoelectronic devices. When the core silicon is covered with a-SiO2, material properties are degraded due to the chemical instability of the SiO2/Si interface.5 However, Si3N4 and SiON are materials chosen for antireflection coating and a protective layer for both core silicon and SiO2 sheath in the solar cell application. Si3N4 and SiON have a higher dielectric constant, chemical stability, temperature strength, and thermal shock resistance than those of SiO2. Si3N4 can also be used as a diffusion barrier for impurity. Therefore, the SiON/SiO2/Si NWs could be an effective structure for photovoltaic cells and nonvolatile memory devices. r 2011 American Chemical Society

Baraban et al.6 demonstrated the Si/SiO2/Si3N4 core
shell nanostructure and its electroluminescence (EL) properties. The EL spectra displayed the emission bands typical of a silicon oxide layer and an intense band at 2.7 eV characteristic of the radiative relaxation of excited silylene centers. Cao et al.7 reported the SiC/SiO2 core shell NWs using methane and silica with iron as a catalyst. Du et al.8 reported the synthesis of Si3N4/Si nanowires and studied the photoluminescence (PL) properties. On the other hand, coaxial SiON/SiO2/Si nanowires have not been reported elsewhere. In the conventional vapor
liquid
solid growth process, it is difficult to fabricate such a core shell structure. One of the efficient methods to fabricate the one-dimensional core shell structure may be thermal annealing of SiO2/Si NWs in an atmosphere of nitrogen and ammonia mixture. This method efficiently controls the thickness of the oxide sheath of Received: February 27, 2011 Revised: July 28, 2011 Published: July 29, 2011 16745

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The Journal of Physical Chemistry C SiO2/Si NWs, consequently affecting photonic and optoelectronic properties. Khandelwal et al.9 investigated the nitridation of SiO2 by Ar/ N2 remote plasma, which increased the nitridation rate of SiO2 thin films and the saturated nitrogen concentration in the resultant oxynitride. Thermal nitride oxide films exhibited improved electrical and chemical properties compared with thermally oxidized films.10 Thermally treated ammonia modified the pore structure of mesoporous silica and enhanced the radiation hardness of SiO2/Si.11 Gritsenko et al.12 also described the phenomenal interfacial modification of SiO2/SiON. The present report is aimed to fabricate the SiON/SiO2/Si NWs by thermal nitridation of SiO2/Si NWs. The microstructures as well as the structural and chemical bonding of SiON/ SiO2/Si NWs were analyzed by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), UV
vis
NIR spectroscopy, and Fourier transforms infrared (FTIR) spectroscopy. The core orbital spectra of Si(2p), O(1s), and N(1s) were discussed.

2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation. The Si NW samples were prepared on p-type Si (100) wafers by using an atmospheric pressure chemical vapor deposition (APCVD) reactor. The synthesis of Si NWs has already been reported elsewhere.13
16 The Si NWs were synthesized under a typical condition by using SiH4 (4 standard cubic centimeters per second (sccm)), H2 (5 sccm) and N2 (1000 sccm). The as-synthesized Si NWs were subjected to annealing at 900 °C for 60 min in nitrogen and ammonia in a clean quartz tube under atmospheric pressure. To study the effect of nitridation of Si NWs on the chemical and atomic bonding structure of the Si NW surface, we intentionally varied the NH3 flow rate ranging from 0 to 400 sccm. The total flow rate was fixed at 1000 sccm. During the heating and cooling process, ammonia was not supplied but nitrogen was flowed at 1000 sccm to maintain inert atmosphere. The prepared samples were put in a desiccator to avoid moisture and oxidation from the environment for accurate microstructure and interface analysis. 2.2. Characterization. The microstructure of Si NWs was characterized by FESEM (JEOL, JSM-6700F) operating with 5-kV accelerating voltage. The microstructure was also characterized by high angle annular dark field (HAADF) STEM in a high resolution mode (FEI, Tecnai-F20). Chemical composition of the ammonia annealed a-SiO2/c-Si NW surface was analyzed with a XPS (ESCALAB 210) instrument using nonmonochromatic Mg KR X-ray radiation (hυ = 1253.6 eV). Vibrational characteristics were analyzed by FTIR spectroscopy (Nicolet2000) to detect vibration bands present in the Si NWs. The FTIR measurements were performed on the samples in a transmission mode. Prior to the FTIR study of NWs, the FTIR spectrum of the substrate was recorded and used as a reference and finally normalized with the FTIR spectrum of the silicon substrate to the yield of the spectrum of the Si NWs. The optical properties of Si NWs were analyzed by UV
vis
NIR spectrometer (Hitachi, U-4000).

3. RESULTS 3.1. Microstructure. The synthesis and microstructure of assynthesized Si NWs is reported elsewhere in our last report.17

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Figure 1. FESEM micrograph of ammonia annealed coaxial SiO2/Si NWs: (a) 100 sccm, (b) 200 sccm, (c) 300 sccm, and (d) 400 sccm NH3.

The as-synthesized Si NWs were straight with a smooth outer surface. The diameters of the Si NWs were in the ranges of 60
80 nm. The TEM study revealed that the Si NWs consist of core silicon surrounded by an amorphous outer oxide matrix. The interface of SiO2/Si and the surface of an outer oxide layer have a smooth surface and a uniform thickness along the growth direction. The HRTEM images of the SiO2/Si interface confirmed that the core silicon has a high degree of crystalinity and the oxide sheath is amorphous.17 Figure 1 shows the FESEM images of ammonia annealed co axial a-SiO2/c-Si core shell Si NWs. After being annealed in ammonia atmosphere, the morphology of Si NWs was remarkably changed. At the low ammonia flow rate of 100 sccm, the morphology was not changed much, but when the ammonia flow rate increased to 200 sccm, the Si NWs were broken into small pieces. When the ammonia flow rate further increased to 300 sccm, the Si NWs were changed to coiled Si NWs, as shown in Figure 1c. A nanobundle type of morphology was observed at the high ammonia flow rate of 400 sccm (Figure 1d). The unusual morphological change during annealing will be explained in a latter section. 3.2. TEM study. Figure 2 shows the TEM image of Si NWs annealed at the ammonia flow rate of 400 sccm. The diameter of the core silicon is 24 nm, which is not much different from that of the hydrogen annealed sample reported previously.13 In Figure 2a, the core silicon and the outer oxide layer are distinguishable by the contrast. The sample was observed by HAADFSTEM in a high-resolution mode. Since Si (Z = 14) has a large atomic number compared to O (Z = 8) and N (Z = 7), Si would be easily resolved by HAADF-STEM. The dark-field and bright field HAADF images are shown in Figure 2, panels a and b, respectively. The contrast change in the HAADF image revealed that there are two distinguishable phases in Si NWs after being annealed in ammonia atmosphere. No significant contrast change was observed in the outer oxide layer. The reason why it is difficult to identify the SiON layer in the SiON/SiO2/Si NW, especially in an amorphous matrix, might come from two facts: (1) N and O have very close atomic numbers and (2) it is difficult to distinguish between SiO2 and SiON in their amorphous states. 16746

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Figure 2. (a) Dark field HAADF image, (b) bright field HAADF image, (c) TEM image for elemental analyses, (d) EDX analysis of the square region marked in (c), and (e) line elemental profiles of Si, O and N from ‘A’ to ‘B’ in (c) of SiON/SiO2/Si NW.

The energy-dispersive X-ray (EDX) analysis was further carried out for the chemical composition of core and outer sheath of SiON/SiO2/Si NWs as shown in Figure 2c
e. The EDX analysis shows that the NW consists of Si, O, and N. The elemental analysis was scanned from point A to point B in Figure 2c. The EDX analysis in Figure 2, panels d and e, is consistent with the FTIR and XPS data (see section 3.4). Some signals of N and O were also detected in the core region. The main reason for the existence of N and O would be that the electron beam from the electron source first penetrates the SiON and SiO2 shell and then the Si core. The line profile of composition

in Figure 2e shows that the Si peak is strongest, the O peak is moderate, and the N peak is weak in the core of the NW. All three peaks of Si, O, and N decreased from the interface to the surface. These elemental analyses confirmed that nitridation occurred in the outer oxide layer, and as a result, SiON was formed. 3.3. Chemical Composition of Ammonia Treated Si NWs. Ammonia-treated core shell Si NWs were analyzed by XPS to study the elemental analysis and the chemical state change. It is well-known that XPS is sensitive to the surface state as well as the bulk state up to 20 nm. Figure 3a shows the broad scan of XPS spectra (in the range of 0
1200 eV) of ammonia annealed core 16747

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Figure 3. (a) Broad band XPS spectra of ammonia annealed coaxial a-SiO2/c-Si NWs and (b) atomic concentration of constituent elements calculated from broad band spectra.

shell Si NWs to examine the chemical compositions of an outer oxide layer. The core orbital spectra of Si(2p), Si(2s), Au(4f), C(1s), and O(1s) are observed at ∼100, 150, ∼80
95, 285, and 532 eV, respectively. The intensity of the N(1s) peak increased with increasing NH3 flow rate. When the Si NWs were annealed without NH3 (only N2: 1000 sccm), no nitrogen was found by XPS. It seems that N2 did not dissociate at a relatively lower temperature of 900 °C. Thermal nitridation of silicon by pure nitrogen may be possible at much higher temperature around 1250 °C. On the other hand, ammonia appears to cause considerable nitridation of an outer oxide layer at 900 °C. The intensity of the N(1s) peak is significant for ammonia-treated SiO2/Si NWs. Figure 3b shows the oxygen, nitrogen, and silicon content of the SiON/SiO2/Si NW surface. For the NH3 flow rate of 100 sccm, the nitrogen incorporation is limited to ∼1 at.%, but for 400 sccm it increased to 17 at.%. The silicon content increased from 35 to 41 at.%, whereas oxygen content decreased from 61 to 36 at.% as the NH3 flow rate increased from 0 to 400 sccm. The carbon contamination, ∼2
5 at.% seems to come from ethanol washing of the quartz tube or handling of samples. The Au content varied between 0.1 and 0.5 at.%, which would be originated from Au nanoparticles used as a catalyst. 3.4. Electronic Spectra of Si(2p), O(1s), N (1s), and Au(4f). Figure 4 shows the core level XPS spectra of the Si(2p), O(1s), N (1s), and Au (4f) regions of ammonia annealed core-shell Si NWs. Figure 4a shows that the binding energy of Si (2p) core orbital spectra is in the range of 90 to ∼110 eV. The peak energy associated with ∼99.4 eV corresponds to crystalline silicon, which is contributed from core silicon, and the peak energy at ∼103.5 eV is associated with an outer oxide layer of Si NWs.

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Both peaks shifted systematically toward lower binding energy arising from different oxidizing states of silicon and/or backing of different bonding networks connected to foreign atoms (O, N, and Au). The Si (2p) peak energy for the NH3 flow rate of 400 sccm is 102.25 eV, which shifts to lower energy 101.54 eV by adding nitrogen into the SiO2 network. Though the position of Si(2p) shows the peak shift for 200
400 sccm NH3 flow rate, the full width at half maxima (fwhm) of Si (2p) did not change with increasing nitrogen content in the SiON network (fwhm = 3 eV.). It indicates that the nitrogen diffused though SiO2 networks and did not change subatomic electronic states with nitrogen incorporation. Figure 4b shows the binding energy of O(1s) core orbital spectra of SiON with varying NH3 flow rate. The peak position of O(1s) is observed at 531.8 eV for the sample without NH3 annealing, whereas NH3 annealing shifts to lower energies of 531.7 and 531.4 eV due to the removal of oxygen from the oxide surface. The fwhm of O(1s) varied between 1.8 and 2.2 eV, indicating unstable subelectronic states of oxygen modified with nitrogen incorporation. Figure 4c shows the N(1s) core orbital of the surface level of SiON/SiO2/Si NWs layers. No peak is observed for annealing in pure nitrogen atmosphere. However, the associated peaks appear for the nitrogen and ammonia mixture annealed Si NW. It is obvious that the nitrogen dissociation and its reaction with silicon/silicon dioxide require the temperature higher than 1250 °C. The intensity of N(1s) peaks is very weak for a 100 sccm NH3 flow rate, implying that SiON is not formed where as the nitrogen concentration increases from 0.23 to 17 at.% for a 400 sccm NH3 flow rate. The binding energy of N(1s) core orbital peak position shifts from 397.55 to 395.25 eV when a NH3 flow rate varies from 0 to 200 sccm. When the ammonia flow rate is further increased to 400 sccm, the N binding energy is shifted to 398.05 eV. The fwhm of N(1s) increases with increase in the NH3 flow rate from 1.5 to 2.1 eV. Figure 4d shows the core orbital spectra of gold and its electronic environments of SiON/SiO2/Si NWs. The position of Au (4f) peaks depends strongly on the chemical/electronic environment of atoms surrounding it. The peaks located at around 84.2 and 86.8 eV were assigned to the spin
orbit spitted components of Au (4f) spectra in the pure Au metal, corresponding to Au (4f7/2) and Au (4f5/2), respectively.18 The remaining peaks at 92.0 and 77 eV represent the emissions from the Au (4d5/2) and Au (5p1/2), respectively, within a different chemical environment. It is well established that the binding energy shifts are characteristic of physical or chemical changes in the environment of the analyzed species. On the other hand, it has been observed that the electronegativity of the incorporated metal plays a decisive role in the type of interaction between the metal and Si NWs. The fwhm of 4f core orbital decreased with increasing NH3 flow rate. When a higher electronegativity atom is removed, i.e., oxygen from the Si NWs, an electron transfer from the metal to the silicon would take place. In our case, since the electronegativity of Au, which is 2.2 in the Pauling scale, is very high, the gold peak position shifts to lower binding energy. We can observe that the Au (4f5/2) peak position shifted to lower energy with respect to that of pure gold (88.0 eV) for the annealed samples. As the transfer of electrons from the Si NWs matrix to gold particles is equivalent to the reduction process of gold, the lower energy shift of the Au 4f band is expected. The relative intensity of Au (4d5/2) and Au (5p3/2) core orbital shifted to Au (5f5/2,7/2) core orbital with increasing ammonia flow rate. 3.5. Characteristics of Valence Band of Silicon Oxynitride. Figure 5 shows the valence band spectra of ammonia annealed 16748

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Figure 4. Binding energy of core orbital spectra of individual elements of ammonia annealed a-SiO2/c-Si NWs for (a) Si(2p), (b) O(1s), (c) N (1s), and (d) Au (4f).

Figure 5. Valence band spectra of ammonia annealed a-SiO2/c-Si NWs.

core shell Si NWs. The valence band of SiON appears in three distinct regions. The peak energy between 0 and 10 eV is assigned to being p-orbital like, and the binding energy between 20 and 30 eV is assigned to being 2s orbital like. The binding energy between 10 and 20 eV is assigned as being sp orbital like. The s-like valence band appears while 2p and sp overlap each other. In the sample without nitrogen, the peak appears at 6.4 and 12.5 eV, with the dumbbell-like valence band transforming to the spherical valence band by incorporation of 17 at.% nitrogen. The valence band decreases its intensity as well as shifts toward the lower energy by increasing nitrogen incorporation in the SiON network. This

indicates the decrease of the band gap of SiON through the replacement of oxygen by nitrogen. 3.6. Chemical Bonding of SiON/SiO2/Si NWs. Figure 6 show the absorption FTIR spectra of ammonia annealed core shell Si NWs. The broad spectrum was taken in the range of 400
4000 cm
1 with a step size of 2 cm
1. The bonding signatures of without ammonia annealed Si NWs shows four distinguish vibrational bands. The vibrational band at 455 cm
1 is longitudinal optic (LO) Si
Si peak, whereas the signatures of Si
O
Si stretching appeared at 940
1350 cm
1. The signature at 843 cm
1 is rotational Si
H2 stretching. When Si NWs is being annealed by ammonia, more distinct vibrational bands of SiNWs were identified as different bonding configuration. The vibrational signatures appearing at 45519 and 843 cm
1 20 correspond to Si
N stretching in the β-Si3N4. The vibrational band of LO Si
Si and Si
N satisfy under same vibrational range. Therefore, fwhm of the vibrational peak at 455 cm
1 was increased due to nitrogen incorporation. The vibrational signature at ∼667 cm
1 corresponds to Si
H wagging in an outer amorphous oxynitride network. The total number of Si
H bonding increased with increasing of ammonia flow rate indicating an increase of hydrogen content. The Si
N
Si stretching band appears at 941 cm
1 and the N
Si
N stretching band appears at 843 cm
1, well matched with the previously reported IR spectra of amorphous silicon nitride.21,22 The peaks observed between 960 and 1050 cm
1 increased with increases of ammonia flow rate due to increasing of Si
N bonds in the amorphous network. In addition, the spectrum of the nanowires shows the 16749

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after the appreciable SiON layer was formed by increasing the ammonia flow rate to 400 sccm. The total reflectance is only 11% for the as-deposited SiO2/Si NWs in the infrared region. The detailed multireflection process in the SiON/SiO2/SiNWs is complicated due to inhomogeneity in the SiO2 and SiNWs. The total reflectance of SiO2/SiNWs in the previous study26,27 was found to be 5
40%, which is much higher than that of SiON/ SiO2/SiNWs observed in the present study.

Figure 6. FTIR spectra of various bonding signatures of annealed a-SiO2/c-Si NWs as a function of ammonia flow rate.

4. DISCUSSION The morphology as well as the diameter of SiNWs was changed after being annealed in ammonia atmosphere. The effect of nanocoiling is significant during ammonia treatment of SiO2/Si NWs. The unusual morphology changes can be explained by stress-induced phenomena caused by oxygen migration during annealing. However, the exact reason of the microstructure change from straight to coiled nanowires during annealing in the ammonia atmosphere is not clearly understood. The combustion of ammonia to produce nitrogen and water is exothermic and can be written as follows 4NH3 þ 3O2 f 2N2 þ 6H2 OðgÞ ðΔHor ¼
1267:20kJ=molÞ 4NH3 þ 5O2 f 4NO þ 6H2 O

Figure 7. UV
vis
NIR spectra of SiON/SiO2/Si NWs at varying ammonia flow rate.

presence of a Si
O stretching band between 1100 and 1080 cm
1.23 The vibrational signature at 1173 cm
1 is attributed to Si
O
Si.13 The systematic increasing of the LO Si
O stretching vibration at 1250 cm
1 indicates the modification of mechanical behavior of SiON/SiO2/SiNWs due to nitrogen incorporation. The signature of 1500
1550 cm
1 is due to N
H2 bending and Si
O2 bending.24 The vibrational signature at 2347 cm
1 corresponds to Si
O2 bonds present at the nitrite network. The signature at ∼3733 cm
1 is assigned as an O
H stretching mode attached to Si.25 The overall FTIR analysis suggests that the silicon nitride shielding over SiO2/Si NWs was formed during annealing in ammonia atmosphere. 3.7. Optical Properties. Figure 7 shows the total reflectance of SiON/SiO2/Si NWs in the spectral range 300
2000 nm. The total reflectance of SiON/SiO2/Si NWs decreases with the ammonia flow rate. The optical behavior is gradually tuned by changing the effective refractive index resulting from nitridation of the outer oxide layer. The total reflection by SiON/SiO2/Si NWs depends on the thickness of different layers and dielectric properties, and so the change in the total reflectance decreases as the nitridation increases. The sharp decrease of the total reflectance at 300
800 nm is attributed to enhanced light trapping (i.e., increasing the path length of incoming light) in nanowire arrays. The increases in the ammonia flow rate enhanced the nitridation of the outer a-SiO2 network, modifying the dielectric constant. The total reflectance is ∼8% at the ultravisible range for SiO2/Si NWs. The reflectance for the 100 sccm NH3 flow rate is about 8
9%, indicating that the formation of SiON in the SiO2 layers is negligible. However, the reflectance decreased to 2
3%

The combustion of ammonia in air is very difficult in the absence of a metal catalyst,28 as the temperature of the flame is usually lower than the ignition temperature of the ammonia
air mixture. The nitridation of the outer oxide sheath occurred when nitrogen along with NH3 was supplied to the APCVD reactor. Although nitrogen was supplied throughout the experiment, the nitrogen content in the SiON is limited to 0.23 at.%, for pure nitrogen-annealed SiO2/Si NWs, indicating that the dissociation of N2 at such a low temperature of 900 °C is very small. Since N2 has a high dissociation temperature (∼1250 °C), it cannot dissociate to atomic nitrogen under such an annealing condition but, ammonia is easily dissociated to NH2
and H+.29 The reason to supply nitrogen along with ammonia is to maintain constant pressure during annealing. It is well established that the molecular diffusion is slower than the atomic diffusion. Hence, the diffusion of NH2
is limited to few nanometers, causing nitridation in the outer layer, which is confirmed by XPS. At the same time, the reduction of the outer oxide layer would produce a-Si and water vapor. As a result the transformation of silicon oxide to silicon would accompanied by volume shrinkage. So the continuous reduction of the outer layer decreases the thickness of Si NWs. The nitrogen content increased by 17 at.%, and the oxygen content decreased from 61 to 36 at.%. Hence, the reason why the oxygen content is reduced and the nitrogen content is increased is as follows. First, the major part of NH3 decomposes at the outer oxide surface and dissociates into negative NH2 and positive hydrogen ions. The positive hydrogen ion is highly reactive and diffuses through SiO2 sheath, but the diffusion rate of the negatively charged NH2 is slower than that of hydrogen and migrates to just a few nanometers from silicon oxynitride. The diffusion length of NH2
depends upon local amorphous network, ammonia flow rate, temperature, and pressure of the CVD process. The dissociation of ammonia can be written as NH3 T NH2
þ Hþ T NH2
þ 2Hþ 16750

ð1Þ

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Second, the positive hydrogen diffuses to the oxide surface, reacts with oxygen, and produces water vapor. The water vapor diffuses out from the surface of Si NWs and increases the silicon content in the SiON layer. Consequently, the oxygen content is decreased as observed from the outer oxide surface, which is in good agreement with the XPS results. Hence SiO þ 2H ¼ Si þ H2 O

(4)

ð2Þ

Finally, reactive hydrogen could be attached to silicon and increases the Si
H, Si
H2, and SiH3 related compounds within the SiON network. Therefore, the Si
H signature at 667 cm
1 increases with ammonia flow rate, which is supported by the FTIR signature. The Si(2p) core orbital shifts toward lower binding energy while both N(1s) and O(1s) decrease binding energy up to 200 sccm but increase it for a higher ammonia flow rate. This can be explained as follows. Despite nitrogen incorporation by removing oxygen from SiON, the O(1s) peak shifting to higher energy could be due to cross-linking double bonds between oxygen or to stronger nitrogen double bonds attached to it. The nitrogen core orbital peak observed at 400 eV indicates the nitridation on the SiO2/Si NWs. The diameter of Si NWs remains constant, indicating that nitrogen atoms penetrate into the SiO2 network. Finally, the valence band is modified from dumbbell to spherical shapes, which is notable observation in the ammonia treatment. The S orbital shifts toward lower binding energy due to hybridization with nitrogen, hydrogen, oxygen, gold and silicon constituents. This indicates that the overall SiON is amorphous. The optical absorption of SiNWs are due to following, above the band gap (Eg(λ) < E(λ)), where the photon can absorb by band-to-band transition. However, the defect absorption (Eg(λ) > E(λ)), which is the interband transition from between localized states at (a) the shallow states, (b) deep states, (c) localized states between valence or conduction bands, and (d) interface states of SiNWs/SiO2. The SiON has a smaller band gap than SiO2 with the successive absorbance increased due to multireflection between SiON and SiNWs. However, the optical absorption from SiON/SiO2/SiNWs modifies the surface states and regulates surface dangling bonds, creating novel quantum confinement effects in the core SiNWs and involving new surface states in the interface between SiO2/Si NWs. The detailed multireflection process in the SiON/SiO2/SiNWs structure is complicated due to inhomogenity in the SiO2 and SiNWs. The total reflection is ∼8% at the ultravisible range for the SiO2/ SiNWs structure; the reflection for 100 sccm NH3 flow rate is about 8
9%, indicating that the formation of SiON in the SiO2 layers is negligible. However, the reflection decreased to 2
3% after the SiON layer formation by increasing the ammonia flow rate to 400 sccm. For successive improvement in reflection properties of SiON/SiO2/SiNWs, the relation of nSiO2 = (nSiNWnSiON)1/2, is obeyed. The total reflectance is only 11% for the as-deposited SiO2/Si NWs structure in the infrared region. Ammonia-treated SiO2/Si NWs modified the bonding structure, as already discussed, by changing the binding energy and the microstructure with varying NH3 flow rate. Some significant observations from FTIR spectra are summarized as follows: (1) The relative increase of Si
Hn wagging indicates the increase of hydrogen content. (2) Si
O transverse optic (TO) of Si
O stretching band shifts toward a lower wavenumber. This indicates the

(5)

(6)

(7)

change of dielectric properties of an outer oxide layer is due to nitridation of the outer oxide sheath. The intensity of the Si
N stretching mode was observed in the FTIR spectra; however, the peak position did not change at 941 cm
1. This indicates that the net Si
N dipole moment remains constant during nitrogen incorporation. The longitudinal optic (LO) vibration of Si
O increases with the increase of the NH3 flow rate, indicating the change from the ring geometry of Si
O
Si to Si
Si bonding and the increase in the shear stress in the Si NWs, which may be one reason to cause coiling of Si NWs The bending vibration of Si
O
Si, associated with 800 cm
1, shifts to lower energy. This indicates that oxygen atom motion occurs in the Si
O
Si plane along the Si
O
Si angle bisector. The TO and LO of Si
O
Si are observed at 1077 and 1153 cm
1, respectively, for SiO2/Si NWs. However, the TO mode of Si
O
Si shifts to the lower wavenumber of 1064 cm
1 while the LO band of Si
O
Si shifts to the higher wavenumber of 1214 cm
1 with increasing ammonia flow rate. The main reason for the peak shift would be that nitrogen and hydrogen, which have low electronegativity, are incorporated with silicon to replace oxygen.

5. CONCLUSION We have prepared the SiON/SiO2/Si NW structure by annealing SiO2/Si NWs in ammonia. The composition of the silicon oxynitride nanowires was converted from Si35O61 to Si41O36N17 with the silicon to oxygen ratio ranging from 0.57 to 1.13. The total reflectance decreased from 8% to 2% due to the increases in multireflection. The XPS results confirmed that the SiON network is quite random networks with inhomogeneous compositions for tuning the valence band structure from dumbbells to spherical shapes by ammonia treatment. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT The financial support from BK21 program, Republic of Korea, is acknowledged. A part of work also supported by The Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program by the Ministry of Science and Technology (No. M10600000159-06J0000-15910) also supported this work. ’ REFERENCES (1) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885–889. (2) Chen, S. C.; Chang, T. C.; Liu, P. T.; Wu, Y. C.; Chin, J. Y.; Yeh, P. H.; Feng, L. W.; Sze, S. M.; Chang, C. Y.; Lien, C. H. Appl. Phys. Lett. 2007, 91, 193103–193103
3. (3) Hayden, O.; Greytak, A. B.; Bell, D. C. Adv. Mater. 2005, 17, 701–704. (4) Balland, B.; Glachant, A.; Bureau, J. C.; Plossu, C. Thin Solid Films 1990, 190, 103–128. 16751

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dx.doi.org/10.1021/jp201896r |J. Phys. Chem. C 2011, 115, 16745–16752