Superheating of silicon nanocrystals observed by Raman spectroscopy

Physics Letters A 373 (2009) 3779–3782

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Physics Letters A www.elsevier.com/locate/pla

Superheating of silicon nanocrystals observed by Raman spectroscopy Giuseppe Faraci ∗ , Santo Gibilisco, Agata R. Pennisi Dipartimento di Fisica e Astronomia, Universitá di Catania, MATIS - Istituto Nazionale di Fisica della Materia, Via Santa Sofia 64, 95123 Catania, Italy

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Article history: Received 13 July 2009 Received in revised form 23 July 2009 Accepted 27 July 2009 Available online 5 August 2009 Communicated by V.M. Agranovich PACS: 78.67.Hc 78.55.Ap 78.67.Bf 63.22.Kn

a b s t r a c t In the Raman spectra of silicon nanocrystals a new anomalous component was detected. Close to the usual first order Raman peak situated for a bulk crystal at 521 cm−1 at room temperature, two peaks arise shifting towards lower energy and demonstrating a huge temperature increase, as measured by the ratio of the Stokes/anti-Stokes peak intensities. This behavior is dependent on the laser power and on the morphology of the nanocrystals. We can exclude, however, confinement effects, although surface enhanced phonon modes could be responsible of such superheating. Alternative explanations are also suggested and discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor agglomerates and in particular silicon nanocrystals are at present deeply investigated because of their valuable properties. In fact, they exhibit important effects in applied technology, e.g., luminescence emission. As a consequence of their reduced dimensionality, also fundamental physical properties, as thermal behavior and vibrational modes, can undergo large modifications. As a matter of fact, the phonon spectra, with multiple acoustic and optical branches, are very sensitive to the symmetry of the crystal, and to the interatomic forces. Obviously, a limited size particle, with a surface configuration different from the bulk, is expected to show quite distorted phonon energy dispersion curves, as a function of wave vector. Such modifications can be easily detected by Raman spectroscopy. This technique, based on the photon–phonon scattering, usually permits the determination of the optical phonon energy at a wave vector k ≈ 0 because of a well-known selection rule. However, the selection rule is ruled out in nanocrystals by the Heisenberg principle, because of quantum confinement effects. Recently, it was demonstrated both theoretically [1–6] and experimentally [6–16] that quantum confinement determines a first order Raman peak shift and peak broadening. Similar effects are caused by a possible local heating of the nanocrystal ensemble, [17,18] under the laser beam of the Raman apparatus. However, the local temperature can be directly measured by the ratio between the intensities of the Stokes/anti-Stokes peak of the Raman spectrum. For the previous reasons, in Raman spectroscopy it is

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of fundamental importance the detection of both peaks (emitted and absorbed phonon) with the related energy position and peak width. In order to investigate the contributions of quantum confinement and of local heating several studies are at present performed by micro Raman spectroscopy [18]. In this field the present Letter reports some unexpected results of micro Raman measurements performed on silicon nanocrystals (NCs). Whereas for a standard deposition of nanocrystals we observe a single shifted and broadened peak, for a particular morphology of the agglomerates, we find out the splitting of the peak, with a huge temperature for the most shifted feature. 2. Experiment We prepared silicon NCs using the apparatus described in detail in Ref. [11]. A beam of helium, inseminated with silicon vapors, produced clean silicon nanocrystals, expanding in supersonic configuration in a vacuum chamber at a pressure of 10−9 Torr. Being sinthesized in high vacuum, the clean Si agglomerates were deposited in situ, with size in the range 2–12 nm, on a highly oriented pyrolitic graphite (HOPG), quartz, and crystalline Si with native oxide substrates. Several NCs layers were accumulated on these substrates having ascertained that the nanocrystals maintain their individual configuration. Passivation of the surface dangling bonds was obtained by the presence of a low oxygen exposure during cluster evaporation. It is worth noting that this deposition method allows a porous accumulation of many Si nanocrystals of different sizes. For the above samples micro Raman spectroscopy was applied for detecting the transversal optical (TO) vibrational peak situated for bulk Si at 521 cm−1 , when detected at 300 K.

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Fig. 1. (Color online.) Optical micrograph visible in our Raman apparatus. We can observe a very flat deposition and micrograin morphologies.

Micro Raman spectra were taken in backscattering geometry with a HORIBA Jobin–Yvon system, equipped with Olympus BX41 microscope. He–Ne laser radiation at a wavelenght of 632.8 nm is focused to a spot size of about 3 μm by a 100× objective. The maximum laser power on the sample was about 6 mW, and a 550 mm focal lenght spectrometer with 1800 lines/mm grating was used. Our apparatus permitted to collect measurements at two laser powers 6 and 2 mW. Both absorbed and emitted phonon (anti-Stokes and Stokes) peaks were revealed on several spots of the samples. Owing to the large size of the experimental spot, the Raman spectra average over the entire size distribution of our clusters. Nanocrystal characterization was already reported in Ref. [11]. The optical microscope of the micro Raman apparatus shows the morphology displayed in Fig. 1. Two typical shapes are visible in the pictures: a standard flat deposition and irregular micrograin agglomerates. Their Raman spectra look very different. In fact, in Fig. 2 we display typical Raman spectra observed for the silicon nanocrystals of standard morphology (single peak) and of micrograin shape (double peak) at 6 mW. The Raman position is always shifted with respect to the bulk silicon situated at 521 cm−1 at room temperature. The width is also broadened. The ratio of the Stokes/anti-Stokes yields, Y S /Y a− S = exp(¯hω/kT ), being ω the Raman frequency and T the temperature, used for the temperature determination, confirms that a higher shift corresponds to higher temperature. For reference, the same measurements were performed on bulk silicon at two laser powers obtaining always a temperature very close to the ambient one. This confirms that only in reduced size particles an increase of local temperature can be determined by the limited dimensions of the particles, because of symmetry breaking and of shielding effects of the surface. In ambient atmosphere, in fact, the layer surrounding the nanocrystals is very likely silicon oxide with a reduced percentage of not stoichiometric oxide and some adsorbates bonded to dangling bonds. On micrograin agglomerates the Raman peak at 2 mW is a single peak, whereas at high laser power (6 mW) it splits in two peaks as shown in Fig. 2. We indicate them as the low-shifted (L) and high-shifted (H) peak. Of course, the same splitting occurs on the anti-Stokes peak. The check of the yields gives a huge temperature for the H-peak (up to 2000 K), whereas the L-peak maintains the high temperature of single peaks. If the laser power is reduced to 2 mW, the H-peak disappears, being hidden under the L-peak,

Fig. 2. (Color online.) Typical Raman spectra for single peak and double peaks on several spots of the sample. The Stokes and anti-Stokes features are reported. The area under each peak was used for the temperature calculation. Note that the double peaks are obtained on micrograin agglomerates only at 6 mW, whereas at 2 mW the double peak becomes a single peak. On the flat deposition, we get single peaks at either laser power.

and the double peak assumes the shape and characteristics (shift and width) of a single peak. For the splitted features, at the maximum power of our laser (6 mW), we display in Fig. 3 the two peaks position as a function of temperature. As visible in the figure, both peaks are shifted. The H-peak from 519 down to about 510 cm−1 , whereas the L-peak shifts from 510 down to about 500 cm−1 . The correspondent temperature goes respectively from 400 K up to 725 K, and from 875 K up to 2075 K, exceeding the melting point of bulk silicon (1685 K). For a comparison of the behavior of a single peak with that of a splitted peak, we added in the figure also the dependence of single peaks as a function of temperature. We note a perfect matching of single peaks with L-peaks. They are therefore originating from the same vibrational branch. Also the line width of the splitted features, reported in Fig. 4 as a function of temperature, shows a surprising trend. In fact, we observe a large spread and a peak broadening quite different for the two features of the splitted peaks, but again the width of L-peaks is in good agreement with that of single peaks, inserted in the same figure for comparison. It is important to observe that no correlation was found between the two peaks of the splitted features, since no rigid relative position was determined for the two peaks when moving from a spot to another. This observation is a clear symptom of a significant dependence on the specific morphology of each agglomerate, and in particular on the different contribution of possible adsorbates on irregular surfaces of highly dispersed agglomerates.

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Fig. 3. (Color online.) Raman peak position of phonon features in Si NCs as a function of temperature. Data are collected on different spots of the sample. The triangular symbol refer to the less shifted feature (L-peak) of the double peaks, the squared symbols to the most shifted (H-peak). The circles are from single peaks. The uncertainty is lower than ±0.2% for the wave vector shift, ±25 K for the temperature.

Fig. 4. (Color online.) Raman line width of phonon features in Si NCs as a function of temperature. The triangular symbol refer to the less shifted feature (L-peak) of the double peaks, the squared symbols to the most shifted (H-peak). The circles are from single peaks. The uncertainty is lower than ±10% for the line width, ±25 K for the temperature.

3. Discussion We discuss now the present results indicating the possible reasons of these experimental results. 3.1. Single peak Single peaks and L-peaks behave in the same manner. In fact, the shift and the broadening follow the same trend as a function of the temperature. Different spots with the same temperature show very limited change in their Raman shift, demonstrating that at these sizes a marginal contribution can be ascribed to quantum confinement both for the shift and for the peak widening. This conclusion is obviously due to the average over the size distribution which minimizes the influence of smaller sizes favoring larger volumes. We note that the local increase of temperature mea-

sured by the ratio between the two Raman peak yields (Stokes and anti-Stokes) can reach values up to about 800 K. It is obvious to attribute to the specific configuration of each agglomerate ensemble, averaged by the laser spot, the temperature value and consequently the shift and linewidth observed. Note that the layer of (sub)oxides surrounding each cluster makes it possible radiation trapping and therefore the temperature increase. As a matter of fact, the absence of long range order and of thermal exchange, since the oxide is an insulating layer, justifies the high temperature measured. Actually no such effects are observed in bulk silicon. 3.2. Double peak A different explanation should be invoked for the H-peak. Here, the measurements point out a huge increase of temperature accompanied by a wider line width. We can attribute such charac-

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teristics to surface phonon modes, whenever the reduced size amplifies the surface to volume ratio. In this case one should expect a vibrational behavior of interface silicon quite dishomogeneous depending on the peripherical bonds with a modification of the phonon energy and of the local temperature. The non-equilibrium interface situation should cause local disorder with a consequent high spread of the different adsorbates linked to silicon. Substoichiometric oxides are mainly expected on the NCs surface. The variety of possible configurations on the agglomerate surface should give a large spread of peak positions, intensities and widths (i.e. local temperatures). This is really observed. A different hypothesis of a binary size distribution, attributing the H-peak to very small NCs was disproved for two reasons: (i) quantum effects of such entity could be possible only for size lower than 3 nm, but the low size volume is relatively negligible with respect to high size NCs. In contrast, the double peaks are of similar area. (ii) Considering that the local temperature observed is often higher than the melting point of bulk silicon, an amorphization of the NCs should produce the transfer of the crystalline silicon peak towards the correspondent amorphous position at about 480 cm−1 . This is not observed. As shown in Fig. 2, the double peak interdistance is changing for different spots of the sample, and sometimes the peaks are so close that only a shoulder is really evident on the left-hand side of the main peak. This suggests a possible coalescence of the second peak under the first one, when their characteristics are so similar to make the deconvolution impossible. In some cases instead, the peaks are well separated with huge temperature difference. We attribute therefore the highest temperature to surface bonds since the oxide melting point is very high (2100 K). Local annealing could also take place, with a progressive passivation of the silicon dangling bonds and the growth of a SiO2 layer. It could be raised the hypothesis of compressed NCs because of the high temperature observed. However, a tensile or compressed Si bond undergoes a Raman shift of very reduced amount, and therefore this effect cannot be responsible of our remarkable observations. In the literature, surface Raman modes have been calculated for reconstructed Si surfaces. In Ref. [19], surface Si dimers give, e.g., phonon modes at 59 and 64 meV. This last was attributed to dimer-bond stretching mode, and is very close to the 65 meV TO phonon of bulk Si. On the other hand, no theoretical calculations are at present available for simulating the Raman spectra as a function of both size and temperature. Only ambient temperature models have been published neglecting any temperature effect [1–6]. Therefore, we believe that the present Letter is the first experimental observation of surface phonon contribution for nanocrystals, where temperature effects have been detected. Such surface phonon contribution may be produced by several different phonon modes, according to the silicon termination bonds. We point out

in fact the important role of oxygen passivation at the surface, where Si–Si–O groups in a variety of substoichiometric configuration, as already mentioned, can determine the observed Raman shift of H-peaks. An important question to be solved concerns the presence of the double peak only at high power density. The answer is very likely due to the fact that the H- and L-peaks are so similar that stay usually at the same Raman position, except when the huge temperature increase makes them well separated. Another puzzling question is why the apparent temperature of the H-peak can be so high. Here, we assume that the oxide layer surrounding the NCs acts as a radiation trap or mirror, making the local surface layer superheated with respect to the entire nanocrystal. A different point of view could attribute to phonon absorption (anti-Stokes yield) a cross section strongly reduced with respect to the phonon emission. In conclusion, we reported the first observation of a splitted Raman peak temperature dependent in Si NCs. The effect is visible at 6 mW laser power and depends on the nanocrystal morphology. The new feature was attributed to surface phonon modes. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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