Femtosecond and nanosecond laser assistant formation of Si nanoclusters in silicon-rich nitride films

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1 Femtosecond and nanosecond laser assistant formation of nanoclusters in silicon-rich nitride films Vladimir A. Volodin a,b, Taisiya T. Korchagina a, Gennadiy N. Kamaev a, Aleksandr Kh. Antonenko a, Jurgen Koch c, Boris N. Chichkov c. a Institute of Semiconductor Physics SB RAS, pr.lavrentjeva 13, Novosibirsk , Russia, E- mail: volodin@isp.nsc.ru; b Novosibirsk State University, Pirogova street, Novosibirsk , Russia; c Laser Zentrum Hannover, Hollerithallee 8, 3019 Hannover, Germany. ABSTRACT Nanosecond laser treatments (KrF laser 8 nm wavelength, 0 ns pulse duration) and femtosecond laser treatments (Tisapphire laser, 800 nm wavelength, <30 fs pulse duration) were applied for crystallization of amorphous silicon nanoclusters in silicon-rich nitride films. The energy densities for crystallization of the silicon nanoclusters were found for films of various non-stoichiometric parameters (N x :H, 0.6 x<1.33) grown on silicon or glass substrates. The effect of laser assisted formation of amorphous nanoclusters in SRN films with relatively low concentration of additional silicon atoms was observed. The developed approach can be used for the creation of dielectric films with semiconductor nanoclusters on non-refractory substrates. Keywords: silicon nanoclusters, silicon-rich nitride films, pulse laser annealing, Raman spectroscopy, non-refractory substrates 1. INTRODUCTION Amorphous semiconductor nanoclusters [1-3] and semiconductor nanocrystals (NCs) [] embedded in dielectric films attract much interest due to their optical and electronical properties. Because the sizes of nanoclusters can be smaller than de-broil wavelength for electrons and holes, the quantum size effect can modify electron spectra of the heterosystem. The dependence of energy of optical transition on the sizes of silicon amorphous nanoclusters and silicon NCs in silicon nitride films was observed earlier [1-3]. For practical use it is important that the quantum size effect for NCs in dielectric matrix becomes brightly apparent at room temperature. In some experiments a single NC reveals deltafunction-like photoluminescence spectra [], so, the silicon NCs in dielectric matrix can be called as quantum dots. The NCs have shown significant promises for a wide range of nanoelectronic and optoelectronic applications. So, the NCs are interesting for both fundamental and applied points of view. It is interesting that the quantum-size effect was observed not only for NCs but also for amorphous silicon nanoclusters [1-3]. In metal-dielectric-semiconductor structures based on silicon-rich nitride (SRN) films an effective electroluminescence was achieved, and red, green and blue light emitting diodes were demonstrated []. SRN films are non-stoichiometric Nx films (x</3). There are reproducible, not expensive, low temperature technologies for deposition of SRN films. The additional silicon atoms in some SRN films are randomly distributed in a N x matrix. They do not form nanoclusters (random bonding - RB model). Some SRN films contain amorphous nanoclusters, which depends on growth condition. This situation is close to random mixture - RM model. Because amorphous semiconductors are meta-stable, semiconductor NCs are more preferable for device applications. Modern technique of deposition allows producing of SRN films on substrates with temperature of plasticity as low as o C. The problems of formation of amorphous nanoclusters and its crystallization are topics in current research. The laser induced crystallization has several advantages comparing to long time high temperature furnace annealing [6]. This work is devoted to development of laser assisted formation of amorphous nanoclusters in SRN films and its crystallization. The development of low thermal budget technology of crystallization of a- nanoclusters is actual, and is of interest up today, especially for femtosecond laser crystallization and modification. International Conference on Micro- and Nano-Electronics 009, edited by Kamil A. Valiev, Alexander A. Orlikovsky, Proc. of SPIE Vol. 71, 710X 010 SPIE CCC code: X/10/$18 doi: / Proc. of SPIE Vol X-1

2 . EXPERIMENTAL.1 Sample preparation and treatments The initial SRN films were deposited using several techniques on crystalline () and glass substrates. Low frequency ( khz) plasma enhanced chemical vapor deposition (-PECVD) and induction plasma (13.6 MHz) PECVD techniques were used. In last case the PECVD- setup was used (manufactured by PTI RAS, Moscow). The stoichiometric parameter x in the N x :H films was varied from 0.6 to 1.3 using various ratios of H and NH 3 gases in reactor during deposition. The films were grown at low temperatures and contain hydrogen. In the case of PECVD- setup films were deposited from silicon organic and nitrogen mixture and do not contain hydrogen. Some parameters of the films are shown in Table 1. The non-stoichiometric parameter x was estimated from the refractive indices of the films and from the red shift of the optical gap in the films, using approach described in work [7]. Several sets of initial films were studied. The growth temperature was changed from o C up to o C. A Ti-Sapphire laser (FemtoPower Compact Pro, Femtolasers Produktions GmbH) with a central wavelength of 800 nm and pulse duration of <30 fs was used for laser treatments. The distribution of energy density in the laser spot has Gaussian form: () r = E0 exp r r0 E (1) where E 0 is the maximum energy and r 0 was taken as radius of the laser spot. Scanning treatments using x-y sample translation by computer-controlled motors were carried out. The pulse repetition rate was 1 khz. The diameter of laser spot was about 0 micrometers. The overlapping of laser spots was 96%. The laser fluence was changed by varying the E pulse laser pulse energy E pulse : E0 =. For the femtosecond treatments we used laser fluences from 6 to 0 mj/cm. πr 0 For nanosecond laser impacts the excimer KrF laser was used. The parameters of KrF laser were next: wavelength λ=8 nm, pulse duration 0 ns. The pulse repetition rate was also 1 khz. The laser spot in the case of nanosecond excimer laser treatments had rectangular shape with sizes 0-0 micrometers. For the nanosecond treatments we used laser fluences from 130 to 170 mj/cm.. Methods for structural analysis Raman spectroscopy technique was used to identify the structure (amorphous or crystalline) of nanoclusters in the films. Raman spectra were recorded in back-scattering geometry. The 1. nm Ar + laser line was used as a source. Triple spectrometer T6000 Horiba Yobin Yvon with micro-raman setup was used. The backscattering geometry was used. The incident light was always linearly polarised, the polarization geometry was chosen to reduce Raman signal from () substrate. To minimize overheating of the films the laser beam was slightly unfocused (diameter of spot was about 3- micrometers, the laser power reaching the sample was -10 mw). All spectra were measured at room temperature. The effects of heating of the films by laser will be discussed below. For some samples the photoluminescence (PL) and IR-spectroscopy studies were carried out. For PL studies the same triple spectrometer T6000 Horiba Yobin Yvon or double PL-spectrometer SDL-1 (LOMO, Sankt-Petersburg) were used. For IRspectroscopy studies FTIR Bruker Vertex 70 setup was used with spectral resolution cm RESULTS AND DISCUSSION 3.1 Homogeneity of pulse laser scanned treatments The question of homogeneity of laser treatments is very important for practical uses. The optical microscope images for nanosecond and femtosecond laser scanning treatments are shown in figure 1. In the case of excimer nanosecond laser treatments spot overlapping was practically absent. One can see the non- homogeneity of laser fluence the rectangles of laser spots is not homogeneous (figure 1A). Because of non- homogeneity the laser fluence is reduced to upper right corner. So, a special optics should be used in this case for homogeneous treatments. In the case of femtosecond laser treatments the overlapping of laser spots during scanning was about 96%. So, one can see quite homogeneous lines (figure 1B). But to treat whole area the step between lines should be smaller. Anyway, the Proc. of SPIE Vol X-

3 treatments with quite big overlapping are more homogeneous even in the case of not- homogeneous distribution of energy density in the laser spot (Gaussian shape). Sample s-LT s-lt 3s-LT s-lt s-lt 1s s 3s s s W13 W1 W1 PECVD technique (lowfrequency) Substrate T, oc IP(induction plasma) IP(induction plasma) IP(induction plasma) NH3/H flow ratio 10 d, nm Plasma power, Wt Stoichiometry parameter x Table 1. Specifications of the SRN films. A B Fig. 1. The examples of optical microscope images for nanosecond (figure A - left) and femtosecond laser (figure B right) scanning treatments. The size of side of rectangle (left) is 0 micrometers, the step between lines is 0 micrometers. The spot in the center is Ar laser spot (the image was photographed in T6000 spectrometer microscope). Proc. of SPIE Vol X-3

4 3. Nanosecond laser treatments Some results of structural changes of SRN films after nanosecond laser impacts are shown in figures and 3. In the case of SRN films with relatively big concentration of excess silicon (N x, when x lower than 1.-1.) initial films contain amorphous silicon nanoclusters. The Raman spectrum of a- contains two broad peaks at approximately 80 cm -1 and 10 cm -1 appearing as result of effective density of transversal optical (TO) and transversal acoustical (TA) vibration modes correspondingly. Due to scattering on optical phonon modes localized in the nanocrystals, the Raman spectrum of nanocrystals is characterized by narrow peak at position between 00-0 cm -1. The position and the width of the peak strongly depend on size and structure of the nanocrystals according to dispersion of the localized modes [8]. The peak width is mainly determined by size dispersion of the nanocrystals. The intensity of the nanocrystal peak depends on the amount of nanocrystal phase. So, one can determine the volume part of nanocrystal phase: V nano- = I c /(I c + σ a I a ) (1), where σ a is relation between the integral cross sections of crystal and a-. According to literature data, σ a can change from 0.1 for large grain polycrystalline silicon up to 0.88 for nano- [9]. So, one can see in figure (left) that with the growth of laser fluence the volume of crystalline part is grow. One Raman spectrum is shown in anti-stokes area to estimate the temperature of films. This will be discussed bellow. Sample 6 initial 130 mj/cm 10 mj/cm 170 mj/cm Sample 3 initial 170 mj/cm mj/cm Anti-Stokes Fig.. Raman spectra of initial SRN films on glass substrates and treated by nanosecond pulse XeCl laser with various fluences. Sample s-lt initial 130 mj/cm 10 mj/cm 170 mj/cm Sample initial 130 mj/cm 10 mj/cm 170 mj/cm Fig. 3. Raman spectra of initial SRN films on substrates and treated by nanosecond pulse XeCl laser with various fluences. Proc. of SPIE Vol X-

5 As one can see in figure (right), when the SRN film contain relatively low concentration of additional silicon (x is about 1.) the laser treatments lead to formation of new amorphous silicon nanoclusters. There is no nanocrystalline peaks in treated samples, but one can clearly see the growth of amorphous peak. So, as one can see from figure, the beginning of crystallization for XeCl laser nanosecond treatments took place for energy density 130 mj/cm and higher only for SRN films that contain high concentration of excess silicon. The nanosecond laser treatments lead to growth of amorphous nanoclusters in the case of SRN films with stoichiometric parameter x between 1. and 1.. The results of structural modification of the SRN films deposited on substrate is similar to that results for SRN films deposited on glass substrate (see figure 3). Because laser radiation (wavelength 8 nm, photon energy is about ev) is mainly absorbed in films. Only some difference in absorbance and in film and substrate thermo-conductivity can take place in this case. For example, SRN films grown at low temperatures (figure 3, left) have more nanocrystalline part after nanosecond laser treatments than SRN grown at higher temperatures (figure 3, right). Because the films are semi-transparent, one can see in some spectra the narrow peak at 0 cm -1 from substrate. 3.3 Femtosecond pulse laser treatments In the case of femtosecond laser pulse crystallization of amorphous films on glass substrates the threshold for full crystallization of the films with thickness about nm was found to be of about 6 mj/cm [10]. Because absorbtion in the SRN films is lower, the laser treatments with fluences up to 0 mj/cm were applied. Some Raman scattering data for initial and treated films are shown in figures and. General trend in laser treatments of this SRN films is the more additional atoms contains the film the less laser fluences were needed to crystallize or to modify silicon nanoclusters. It is obvious, because the absorbance coefficient grow with growing of silicon amount in the SRN films. Even the femtosecond laser treatments with fluences up to 0 mj/cm was not enough to makes visible structural changes in the sample number 1 (Table 1). In sample number the laser treatments stimulate only creation of small amorphous nanoclusters. In sample number 3 the initial films contains amorphous nanoclusters, and as one can see in figure (right), the laser treatments not only stimulate the growth of amorphous silicon nanocluster (amorphous peak is grow), but also crystallized some nanoclusters ( nanocrystalline peak is appeared). For SRN films with stoichiometric parameter x lesser that 1.0 it was possible to crystallize silicon nanoclusters using laser treatments with fluences 10 mj/cm and higher (figure, left). In the case of the SRN films on silicon substrates in Raman spectra of some semi-transparent films one can see narrow peak at 0 cm -1 from substrate (figure ). It is also remarkable, that femtosecond laser treatments lead not only crystallization of existed amorphous silicon nanoclusters, but to growth or formation of new amorphous nanoclusters in SRN films (one can see growth of amorphous peak in figure ). Let s discuss some effects of laser heating during Raman spectra registration. One can see that the femtosecond laser impact leads to dramatic changes in structure of the sample 3 (figure, right). One can see both growth of amorphous peaks (10 and 80 cm -1 ) and appearing of nanocrystalline peak with maximum at 13 cm -1. As it was mentioned, from position of this peak one can estimate the average size of NCs. But for our conditions of measurements it is important to take into account possible heating of the film and shift of Raman peak due to anharmonic effects. It is well known that Raman scattering intensities in Stokes I A / I S ( ω0 + Ω) = ( ω Ω) 0 e hω kt I S and anti-stokes I A spectral regions depend on temperature: () ω is frequency of initial photon, Ω is frequency of phonon k is Boltzmann constant, where h is Plank constant, 0 T is absolute temperature in Kelvin scale. So, from formula, one can determine temperature as: T ( K ) = 1.36 Ω( cm 1 ) / ln(1.3 I S / I A ) (3) Proc. of SPIE Vol X-

6 The Raman intensity in the same measurement condition was measured for sample 3 and according to formula 3 the temperature was determined as K. In bulk heating at K (according to our experimental data) leads to. cm -1 red shift compared with Raman shift for bulk at room temperature (300 K). One can assume, that anharmonic effects for NCs have the same effects, so taking into account the shift due to heating, the shift due to phonon confinement effects is about 3. cm -1. Using model of folding of vibration modes in NCs [8], one can determine the average size of NCs in film 3 (laser treated) as nm. Sample 6 initial 110 mj/cm 10 mj/cm 130 mj/cm 10 mj/cm Sample 3 initial 00 mj/cm 0 mj/cm Anti-Stokes 0 mj/cm Fig.. Raman spectra of initial SRN films on glass substrates and treated by femtosecond laser pulse treatments with various fluences. Sample S-LT initial 10 mj/cm Sample S initial 10 mj/cm 10 mj/cm 00 mj/cm Fig.. Raman spectra of initial SRN films on substrates and treated by femtosecond laser pulse treatments with various fluences. Structure of the SRN films was analyzed also using IR-spectroscopy technique. Some IR-spectra are shown in figure 6. One can see peaks due to absorbance on vibrations of -N, -H, and N-H bonds. It should be noted that there are no - H and N-H peaks in IR-spectrum of sample W13. It is obvious, because this sample was deposited not from silan (H ) and ammonia (NH 3 ), but from nitrogen and hexametilsilasane (silicon-organic components). Proc. of SPIE Vol X-6

7 Absorbance 0,0 0,3 0,30 0, 0,0 0,1 -N 1s-LT 1s W13 0,10 0,0 -H N-H 0, Wavenumber, cm -1 Fig. 6. IR-spectra of initial SRN films α, cm α- Brodsky Phys.Rev.B, v1, p63 α- 3 N Samle α=α 0 (E-Eg) 3 /E , 1,0 1,,0, 3,0 3,,0,,0, 6,0 Energy, ev Fig. 7. The spectral dependence of absorption coefficient obtained from reflectance and transmission spectra of initial a-nx:h films. Proc. of SPIE Vol X-7

8 To investigate, what part of energy in pulse was absorbed by the films, the next experiments were carried out. The reflectance and transmittance of the light in spectral region were measured. From this data the absorbance coefficient The results are presented in figure 7. As one can see, for not intensive, probe beam, the absorbance even in the film 7 for photons with energy 1. ev (femtosecond laser) is low. So, non-linear effects two or three photon interaction should play role in absorbance of very intensive femtosecond pulse.. SUMMARY The possibility of the nanosecond and femtosecond laser pulses to crystallize amorphous nanoclusters in SRN films on glass and silicon substrates was shown. Non-linear effects may play some role in femtosecond treatments. It should be noted that femtosecond laser treatments were applied for the first time for crystallization of amorphous silicon nanoclusters in SRN films. The laser fluences for crystallization were found for the films of various non-stoichiometries. The effect of laser assistant formation of amorphous nanoclusters in SRN films with relatively low concentration of additional silicon atoms was observed. The developed approach can be used for the creation of dielectric films with semiconductor nanoclusters on non-refractory substrates.. ACKNOWLEDGEMENTS The Russian Fund for Basic Researches supports this studies (projects ). The authors are grateful to Dr. Aleksandr Popov for some experimental samples. Vladimir A. Volodin is grateful to DAAD for stipend. 1 REFERENCES Nae-Man Park, Chel-Jong Choi, Tae-Yeon Seong, Seong-Ju Park. Phys. Rev. Lett., 86, (001). Nae-Man Park, Tae-Soo Kim, Seong-Ju Park. Appl. Phys. Lett., 78, 7-77 (001). 3 T. T. Korchagina, D. V. Marin, V. A. Volodin, A. A. Popov, and M. Vergnat. Semiconductors, 3, (009). Tae-Youb Kim, Nae-Man Park, Kyung-Hyun Kim, Gun Yong Sung, Young-Woo Ok, Tae-Yeon Seong, Chel-Jong Choi. Appl. Phys. Lett., 8, 3-37 (00). I.Sychugov, R.Juhasz, J.Valenta, and J.Linnros, Phys. Rev. Lett., 9, 0870 (1-) (00). 6 V.A.Volodin, M.D.Efremov, V.A.Gritsenko, S.A.Kochubei. Appl. Phys. Lett., 73, (1998). 7 M.D. Efremov, V.A. Volodin, D.V. Marin, S.A. Arzhannikova, G.N. Kamaev, S.A. Kochubey, A.A. Popov. Semiconductors,, 0-07 (008). 8 V. Paillard, P. Puech, J. Appl. Phys., 86, (1999). 9 R.Tsu, J.G.-Hernandes, S.S.Chao, S.C.Lee, K.Tanaka: Appl. Phys. Lett., 0, 3-3 (198). 10 V.A.Volodin, M.D.Efremov, G.A.Kachurin, A.G.Cherkov, M. Deutschmann, N. Baersch, JETP Letters, 86, (007). Proc. of SPIE Vol X-8