Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271II. EXPERIMENTSThe material used as the vapor source was pure silicon (99.99%) powder. It was loaded in analumina boat. A p-type silicon wafer (10 x 20 mm) was used as the substrate for the growth of Sinanowires. The substrate was cleaned by a standard treatment in HF/NH4F solution, rinsed withde-ionized water, and dried in air. Subsequently, a Au film with the thickness of 10 - 30 nm weredeposited by DC sputtering technique on the substrate as the catalyst. Then the alumina boat,which containes the pure silicon powder, was placed in the center of a quartz tube at the hottestzone of the horizontal tube furnace. The Si substrate coated with Au layer was placed next to theSi powder and along the downstream side of the flowing Argon. Before hea<s<strong>trong</strong>>tin</s<strong>trong</strong>>g, the quartz tubewas first purged with a high purity Argon gas flow (200 sccm) for 30 min to reduce oxygencontent in the quartz tube. Then, under a constant flow of high purity argon gas (50 sccm), thefurnace was heated to 1200 oC and held at this temperature for 45 min. The samples were thenoxidized under flow of gas mixture of argon and oxygen (50 sccm) for 15 min at 300 oC to makea complex SiOx structure. The furnace was then cooled down to room temperature naturally.Mor<strong>ph</strong>ological properties of samples were examined by using scanning electron microscopy(FESEM - S4800). Raman spectra were taken on LABRAM 1B YVON Raman spectrometer atroom temperature in backscattering configuration with 488 nm argon laser line.Photoluminescence (PL) measurements were carried out on the spectroscopic system consis<s<strong>trong</strong>>tin</s<strong>trong</strong>>gof a Peltier cooled CCD and a He-Cd laser (325 and 442 nm lines) as an excitation source. Thesample was mounted on the cold finger of the variable temperature cryostat. The PLmeasurements were done at the different temperature ranging from 11 to 300 K. All the spectrawere then corrected to the system sensitivity.III. RESULTS AND DISCUSSIONFigure 1 shows the typical electron microscopic image of the SiNWs, where a large quantityof nanowires can be observed. The diameters of the nanowires vary from 30 - 80 nm and thetypical length is 400 nm. Most of the nanowires were smooth and uniform along the wire axis.We can see some particles located on the tip of the nanowires. The particles may be the Auparticles formated in the catalytic reaction processes. From this result, we believe that the growthof nanowires observed here was based on the vapour-liquid-solid (VLS) mechanism [6].Fig. 1. The SEM Image of Si nanowires on Si substrate with Au catalyst.44
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271Figure 2 shows a broad red PL emission band at room temperature centered around 1.9 eV(670 nm). This red spot was clearly visible to the human eye. The origin of this PL emissionband can be attributed to the emission of Si nanocrystals embedded in the complex SiOx matrix.Fig. 2. Room temperature <strong>ph</strong>otoluminescence ofnc-Si embedded in SiO x matrix. The wavelengthof the excitation source is 325 nm.Fig. 3. Raman spectrum of SiNWs taken with488 nm Ar laser line compared with the fit<s<strong>trong</strong>>tin</s<strong>trong</strong>>gspectrum, L = 4.9 nm.According to the calculations of Proof et al. [12] for silicon nanocrystal embedded in SiOx theband-gap expansion of 0.78 eV compared to that of bulk silicon, the average size of Si NC mustbe 4.9 nm. This result is in good agreement with calculation of the size of silicon nanocrystalsbased on quantum confinement effect calculated by Istam et al. [13]. To confirm thisobservation, we have taken a Raman scattering spectrum of the SiNWs removed from thissample. Figure 3 shows Raman scattering spectrum of the SiNWs. Since the quantity of the NWswas very small, the Raman signal was very week, but a peak at 517 cm-1 was clearly observed.The peak at 517 cm-1 corresponds to the optical <strong>ph</strong>onon of crystalline Si embedded in SiOxmatrix. The size of nanocrystallites can be calculated by fit<s<strong>trong</strong>>tin</s<strong>trong</strong>>g Raman scattering spectrum. Aquantitative model was developed by Richter et al. [14] and was later improved by Campbell andFauchet [15] to estimate the average size (or correlation length) L of the nanocrystals from theRaman spectrum. Using this model, the first-order Raman intensity I(ω) is:I ( ω ) ∝exp( −q2L23d q/ 4)2[ ω − ω ( q)]+ ( Γ / 2)Where Γ is the natural line width (Γ = 4 cm-1 for c-Si at room temperature includinginstrumental contributions), q is expressed in units of 2π/a and L is in units of a, with a = 0.54nm being the lattice constant of silicon and ω(q) is the dispersion relation for optical <strong>ph</strong>onons inc-Si. To calculate the line shape, the analytic form ω(q) = A – Bq2, with A = 518.6 cm-1 and B =10 cm-1, which reproduces the dispersion relation for LO <strong>ph</strong>onons along [001] in Si. Since LOand TO <strong>ph</strong>onons are degenerated at (and very near) Brillouin zone center and the <strong>ph</strong>ononscontribu<s<strong>trong</strong>>tin</s<strong>trong</strong>>g to the integral are from and near zone center if L > a, this analytic dispersionrelation should be a good approximation for L > a. In Figure 3, we reproduced their experimentalRS data and fitted this RS data using Eq. (1) and obtained a size of Si NC L = 4.9 nm as shownin Figure 3. This result proved that silicon NCs were present in the wires.2(1)45
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