ISSN 10637826, Semiconductors, 2010, Vol. 44, No. 12, pp. 1611–1616. © Pleiades Publishing, Ltd., 2010. Original Russian Text © T.T. Korchagina, V.A. Volodin, B.N. Chichkov, 2010, published in Fizika i Tekhnika Poluprovodnikov, 2010, Vol. 44, No. 12, pp. 1660–1665.
FABRICATION, TREATMENT, AND TESTING OF MATERIALS AND STRUCTURES
Formation and Crystallization of Silicon Nanoclusters in SiNx:H Films Using Femtosecond Pulsed Laser Annealings T. T. Korchaginaa^, V. A. Volodina, b^^, and B. N. Chichkovc a
A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, pr. Akademika Lavrent’eva 13, Novosibirsk, 630090 Russia ^email:
[email protected] b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia ^^email:
[email protected] c Laser Zentrum Hannover e.V., Hollerithallee 8, Hannover, D30419 Germany Submitted April 22, 2010; accepted for publication May 7, 2010
Abstract—SiNx:H films of different compositions grown on glass and silicon substrates using plasmachem ical vapor deposition at a temperature of 380°C have been subjected to pulsed laser annealings. The treat ments are performed using titanium–sapphire laser radiation with a wavelength of 800 nm and a pulse dura tion of 30 fs. Structural changes in the films are studied using Raman spectroscopy. Amorphous silicon nan oclusters are detected in asgrown films with molar fractions of excess silicon of ~1/5 and larger. Conditions required for pulsed crystallization of nanoclusters were determined. According to the Raman data, no silicon clusters were detected in asgrown films with a small amount of excess silicon (x > 1.25). Pulsed treatments resulted in the formation of silicon nanoclusters 1–2 nm in size in these films. DOI: 10.1134/S1063782610120146
1. INTRODUCTION Silicon nitride films are widely used in microelec tronics [1]. Interest in nonstoichiometric silicon nitride films (SiNx with x < 4/3) arose due to the pros pects of their applications in optoelectronic devices. Intense photoluminescence (PL) was observed in sili con nitride films containing both amorphous [2–4] and crystalline [5] silicon nanoclusters. Crystalline nanoclusters are more preferred than amorphous ones for the following reasons. First, the amorphous state is metastable, and characteristics of devices based on amorphous clusters can degrade; second, the proba bility of nonradiative transitions in crystalline clusters is lower; therefore, their PL intensity is higher [6], which is important for optoelectronic applications. For practical applications, it is important that largearea substrates on which films are deposited would be inexpensive; however, this limits the use of refractory materials for substrates. It is preferable to use inexpensive glass or plastic types of substrates. There are lowtemperature plasmaenhanced chemi cal vapor deposition methods that allow fabrication of SiNx:H films with amorphous silicon clusters at low growth temperatures (no higher than 100°C). How ever, crystallization of amorphous clusters requires high temperatures. Furnace annealings are unaccept able for many structures since these annealings require high temperatures, from 980 to 1150°C, and durations up to 5 h [6, 7]. For such long times, even refractory glass substrates become deformed. The solution to the
problem is the use of pulsed laser exposures. First, radiation is almost completely absorbed in the film and, hence, does not reach the substrate and does not heat it. Second, for short times of the laser pulse and for the time (typically, tens of nanoseconds) of subse quent cooling of the film, the substrate has no time to become overheated due to heat diffusion from the film. Third, the low thermal cost of pulsed exposures saves time and electric power. Previously, amorphous silicon clusters in SiNx films were crystallized by nanosecond pulsed treatments using excimer XeCl [8] and KrF [9] lasers with wavelengths of 308 and 248 nm, respec tively. The aim of this research is to study the forma tion and crystallization of silicon nanoclusters in SiNx:H films using femtosecond pulsed treatments. 2. EXPERIMENTAL SiNx:H films of different compositions were depos ited from a silane (SiH4) and ammonia (NH3) gas mixture using a plasmaenhanced lowfrequency (55 kHz) discharge. The SiNx:H film composition (0 < x < 4/3) depended on the monosilane and ammonia flow ratio; to obtain silicon nitride close to stoichio metric, a mixture with a significant ammonia excess was used. The growth technology is described in more detail in [4]. The growth temperature was 380°C for all films. The parameters of all films are listed in the table. The film thickness and refractive index were deter mined by ellipsometry using a Mikroskan scanning
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Growth parameters and film composition Sample NHx/SiH4 Thickness, Parameter Parameter flow ratio nm no. x y 1 2 3Si 3 4 5 6 7
10 5 2.6 3 2 1.5 1 0.5
322 280 430 250 254 259 281 404
1.33 1.3 1.25 1.25 1.15 1.0 0.9 0.6
~0 0.07 0.17 0.17 0.32 0.5 0.59 0.79
Note: Samples 1–7 were deposited on glass substrates, sample 3Si was deposited on silicon substrate.
laser ellipsometer (equipped with a He–Ne laser with a wavelength of 632.8 nm) and a spectral ellipsometer (both produced by the Institute of Semiconductor Physics). The spectral ellipsometer range was from 250 to 800 nm, its wavelength resolution was 2 nm, and a xenon lamp was used as a light source. The film com position was estimated from the refractive index at the He–Ne laser wavelength (~633 nm) by the depen dence given in [1] and by the absorption edge shift (the technique is described in [10]). According to infrared spectroscopy data, asgrown films contained hydro gen [4]. Structural properties of asgrown and lasertreated films were studied by Raman scattering (RS). Raman spectra were measured at room temperature in the backscattering geometry; an Ar+ laser line at a wave length of 514.5 nm was used for excitation. Equipment of the Nanosystems and Modern Materials science and education center of the Novosibirsk State Univer sity, i.e., a T64000 (Horiba Jobin Yvon) spectrometer with a triple monochromator, was used. Its spectral resolution was no worse than 1 cm–1. A silicon array of photodetectors cooled by liquid nitrogen was used as a detector. An attachment for RS microscopic studies was used. The laser beam power incident on the sam ple was 2–3 mW. To minimize structure heating under the laser beam, the sample was placed slightly below the focus; the spot size was 4–5 μm. For some samples, PL spectra were measured using the same setup. The spectra were not normalized to the detector sensitivity depending on the wavelength. For femtosecond pulsed treatments, a titanium– sapphire laser (FemtoPower Compact Pro, Femtolas ers Produktions GmbH) with a wavelength of 800 nm and a pulse duration of 50 nm, its position does not differ from the RS peak of singlecrystal silicon, 520.6 cm–1. The peak width is controlled by nanocrystal size vari ance and by the phonon lifetime. The peak intensity is proportional to the nanocrystal phase fraction. The models describing the dependence of the peak posi tion on silicon nanocrystal sizes are well known and tested; the dependence is given in [9] and references therein. In the acoustic region, we can see a peak at ~300 cm–1 caused by twophonon scattering at TA modes in silicon nanocrystals. This peak is also observed for singlecrystal silicon [11]; however, it is less intense in comparison with the peak from optical phonons in this case. We can see that the “nanocrys talline” peak position (~511 cm–1) is almost indepen dent of the energy density in the laser pulse, although its intensity appreciably increases with the density.
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A similar dependence arises for films 6, 5, and 4 as well (Figs. 2, 3). The general feature is that, the smaller
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Raman intensity, arb. units 30
20
25
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15 1
20
2 15
10 1
4 3 3
10
4 5 100
200
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400 500 Raman shift, cm−1
100
200
300
400 500 Raman shift, cm−1
Fig. 3. Raman spectra of films 5 (1, 2) and 4 (3, 4): (1, 3) asgrown film and (2, 4) after femtosecond annealings at energy densities of 150 and 200 mJ/cm2, respectively.
Fig. 4. Raman spectra of films 3 (1, 2) and 2 (3, 4): (1, 3) asgrown film and (2, 4) after femtosecond anneal ings with energy densities of 250 mJ/cm2. The intensity scale is logarithmic.
the excess silicon content, the lower the absorption in the film and the higher the energy densities that should be used to crystallize amorphous silicon nanoclusters. However, at energy densities of >250 mJ/cm2, film ablation is already observed; hence, such treatments are unacceptable. For sample 3 (Fig. 4), peaks corre sponding to RS in amorphous silicon clusters are still observed. Laser treatments resulted in partial crystalli zation of amorphous silicon nanoclusters (Fig. 4, curve 2). In the spectrum of asgrown film 2, no peaks from amorphous silicon are observed. It seems that the silicon cluster content is below the observation thresh old for this method. However, laser annealings initi ated the formation of a weak peak of amorphous sili con (Fig. 4, curve 4). Pulsed exposures resulted in sil icon assembling into clusters; however, their size is obviously too small to form nanocrystals. Thus, fem tosecond pulsed annealings lead not only to crystalli zation of amorphous nanoclusters existing in asgrown films, but also to the formation of new nanoclusters. To confirm the latter observation, a film deposited on the silicon substrate with orientation (001) (Fig. 5) was studied. Since the silicon nitride film is semitranspar ent, the spectra contain a substrate signal. To decrease the background signal from the substrate, an appropri ate RS polarization geometry was used. In the geome try Z(X 'Y ') – Z, longitudinal optical phonons from the silicon substrate with orientation (001) are inactive in RS. Here the incident light polarization direction
(X ' axis) coincides with the (110) crystallographic direction and the scattered light polarization direction (Y ' axis) coincides with the (1 10) crystallographic direction. The Z axis is directed along the (001) crys tallographic direction; Z and –Z are the directions of the wave vector of incident and scattered light (back scattering), respectively. In the chosen geometry, the substrate signal is smaller than in the allowed geometry by a factor of 15–30. Nevertheless, it is impossible to totally suppress it, and this signal is present in the spectra. The spectrum of the asgrown film contains only very weak peaks caused by amorphous silicon clusters. According to estimations, the molar fraction of excess silicon in this film is ~0.17. According to our estimates, if all excess silicon is assembled into nano clusters, the spectrum should contain a detectable sig nal from scattering at Si–Si bond vibrations; i.e., excess silicon in this case is mostly randomly dispersed over the atomic network and the film structure model is closer to the RM model. We note that peaks from amorphous clusters are observed for the film deposited on the glass substrate at a larger concentration ratio of ammonia/monosilane gases (Fig. 4, curve 1). It seems that processes of plasmachemical deposition on insu lating and conductive substrates somewhat differ, which results in different film structures. Femtosec ond pulsed annealings with energy densities as high as 150 mJ/cm2 resulted in silicon assembling into amor SEMICONDUCTORS
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FORMATION AND CRYSTALLIZATION OF SILICON NANOCLUSTERS
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Raman intensity, arb. units 1
2
PL intensity, arb. units
Nanocrystal Si 102
4 aSi 3 101 2 1 Singlecrystal Si substrate 100
200
300
400 500 Raman shift, cm−1
Fig. 5. Raman spectra of film 3Si: (1) asgrown film and after femtosecond annealings at energy densities of (2) 120, (3) 150, and (4) 200 mJ/cm2. The intensity scale is logarithmic.
phous nanoclusters (growth of amorphous silicon peaks in spectra 3 and 4); annealing with an energy density of 200 mJ/cm2 resulted in partial crystalliza tion of silicon clusters. Figure 6 shows the PL spectra of the asgrown and treated films. According to estimations, the molar fraction of excess silicon in films was also ~0.17. If we assume that PL in the asgrown SiNx:H film is caused by excitons localized in amorphous silicon clusters, then the observed shift can be attributed to the quan tumsize effect [2, 3]. Then, according to estimates [2, 3], the cluster size is 2 nm. According to Raman data, clusters are partially crystallized in the film sub jected to pulsed femtosecond annealing and the PL peak is shifted to longer wavelengths (curve 2 in Fig. 6). Apparently, pulsed treatments resulted in an increase in the size of an average silicon cluster; accordingly, the PL peak shifted to longer spectral wavelengths. We note that, according to the peak shift of RS at optical phonons localized in nanoclusters (510–513 cm–1), the average nanocrystal sizes are also 1.5–2 nm. As noted above, nanocrystal sizes are almost independent of laser treatment parameters and weakly depend on film composition. It is likely that the cluster growth is diffusionlimited; during the pulse and film cooling, silicon atoms diffuse the maximum to several interatomic distances. Therefore, nanocrys tal sizes are extremely small. SEMICONDUCTORS
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550
600
650
700
750 800 Wavelength, nm
Fig. 6. Photoluminescence spectra of film 3: (1) asgrown film and (2) after femtosecond pulsed annealing at an energy density of 250 mJ/cm2.
Detailed consideration of the laser annealing mechanism is beyond the scope of this paper. We only note that nonlinear effects play an important role in light absorption in the case of ultrahighpower femto second pulsed irradiations. It is known that the optical gap of stoichiometric amorphous silicon nitride is ~5 eV [1]. The average photon energy of titanium– sapphire laser radiation is 1.5 eV, and it is not high enough for efficient absorption of radiation even in aSi:H films with an optical gap of 1.5–1.9 eV [12]. Presumably, multiphoton absorption occurs. The phase transition mechanism can also be not purely thermal, i.e., can occur not according to the ordinary scheme light absorption–heating–melting–cooling– crystallization. The pulse is much shorter than the electron–phonon interaction time in the semicon ductor (1–2 ps). Therefore, “hot” electron–hole plasma does not excite vibrational modes in silicon during the pulse and has no time to relax. The temper atures of electronic and atomic subsystems differ sig nificantly. According to some theoretical estimations, silicon becomes unstable when the concentration of “hot” (excited from the valence band to the conduc tion band) electrons becomes as high as 9–20% of the Si atom concentration [13]. This metastable state can relax into a more stable crystalline phase without melt ing, but with generation of crystallization latent heat ing. This process is similar to “explosive” crystalliza tion. The effect of excess silicon assembling into clus ters under femtosecond pulsed treatments was observed for the first time. Since the pulse is shorter
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than the atomic vibration period, postpulse laser diffu sion due to film heating seems to be an acceptable explanation of the observed effect. It is known that the aSi film cooling time below the melt temperature after nanosecond laser treatments is ~100 ns [14]. For femtosecond exposures, the time of postpulse cooling is probably the same. This time can be sufficient for excess silicon diffusion (as noted above, by several interatomic distances) and amorphous nanocluster formation, as was shown by experiments. 4. CONCLUSIONS For the first time, femtosecond laser treatments were used for crystallization of amorphous Si nano clusters in SiNx films enriched with silicon. According to the RS data, amorphous clusters existing in as grown films either partially or almost completely crys tallized due to laser exposures with corresponding pulse energy density. The effect of laserenhanced for mation of silicon nanoclusters in SiNx films with a rel atively low concentration of excess silicon atoms was also observed. A PL maximum shift to longer wave lengths, probably caused by an increase in the average silicon nanocluster sizes, was detected. The developed approach can be used to fabricate insulating films with semiconductor nanocrystals or amorphous semicon ductor nanoclusters on nonrefractory substrates. ACKNOWLEDGMENTS We are grateful to A.A. Povov (Yaroslavl Branch, Physicotechnological Institute, Russian Academy of Sciences) for asgrown SiNx films put at our disposal and J. Koch (Laser Zentrum Hannover) for assistance in laser processing. This study was supported by the Novosibirsk City Hall (Grant for Young Scientists no. 1909), the Par ticipants of Youth ScientificInnovation Competition foundation, the Russian Foundation for Basic
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Translated by A. Kazantsev
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