Formation of and Light Emission from Si nanocrystals Embedded in Amorphous Silicon Oxides

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1 / , copyright The Electrochemical Society Formation of and Light Emission from Si nanocrystals Embedded in Amorphous Silicon Oxides D. Comedi a, O. H. Y. Zalloum b, D. E. Blakie b, J. Wojcik b, and P. Mascher b a Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Lab. Física del Sólido, Dep. de Física, Fac. Ciencias Exactas y Tecnología, Universidad Nacional de Tucumán, 4000 Tucumán, Argentina b Centre for Emerging Device Technologies and Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada L8S 4L7 The formation of Si-nc embedded in amorphous Si oxides promoted by thermal annealing of Si y O 1-y films (y= ) fabricated by plasma enhanced chemical vapor deposition is examined by X-ray diffraction and electron microscopy. UV and synchrotron radiation excited photoluminescence from the obtained structures is also studied and its origin elucidated. Introduction The indirect bandgap of Si has been the major obstacle for the fabrication of light sources from Si towards the development of Si-photonics (1). Much work has been directed in recent years towards the development of efficient Si light sources (2), which would allow economical and mass production of monolithically integrated optoelectronic on-a-chip systems. A promising line of research concentrates on Si nanocrystals (Si-nc), where quantum confinement (QC) effects lead to enhanced radiative quasi-direct bandgap transitions (2-6). An interesting approach to the preparation of Si-nc, which is fully CMOS compatible, is taking advantage of the well-known good passivation properties and stability of the Si/SiO 2 interface by dispersing the Si-nc in an amorphous SiO 2 matrix. Such a Si-nc/SiO 2 composite can be in principle easily obtained by promoting decomposition of substoichiometric Si oxides (Si y O 1-y where y>1/3) into the SiO 2 and elemental Si phases through annealing at high temperatures. However, a major problem has been that, despite the simplicity of the preparation technique, the resulting structures show considerable structural complexity that has significant impact on their lightemission characteristics (4,6) In this work, we combine X-ray diffraction (XRD), transmission electron microscopy (TEM), and ultra-violet (UV) and synchrotron radiation excited photoluminescence (PL) in an effort to effectively characterize the formation and light emission from Si-nc/Si oxide structures obtained by thermal annealing of Si y O 1-y films fabricated by plasma enhanced chemical vapor deposition (PECVD). Experimental Details Si y O 1-y oxide thin films where deposited on c-si substrates by PECVD using two distinct deposition systems, namely electron cyclotron resonance (ECR) and inductively coupled (ICP) PECVD, using Ar+30%SiH 4 and Ar+30%O 2 precursor gases. All deposition parameters were kept the same in all depositions except for the Ar+30%O 2 flow rate 3

2 which was varied so as to produce Si y O 1-y films with different Si concentrations y (y= ). Isochronal annealing of the samples was performed for two hours at constant temperatures in the ºC range, in a quartz tube furnace under flowing Ar or (Ar+5%H 2 ). Details on the XRD measurements have been reported in a previous article (5). TEM measurements were performed with a Philips CM12 120keV electron microscope. UV PL measurements were carried out using the 325 nm line of a He-Cd laser and an Ocean Optics spectrometer. X-ray excited PL was measured at the SGM beamline at the Canadian Light Source using a monochromator and a photomultiplier tube. Formation and structure: Results and Discussion The structure of the as-grown Si y O 1-y films is amorphous as can be deduced from the XRD data from these films (not shown here). A crystalline Si phase can be observed after the films have been annealed at a temperature whose exact value is smaller the larger the Si content. Figure 1 shows the results obtained for y=0.42 for two distinct Si y O 1-y films, one prepared by ECR- and the other by ICP-PECVD. After annealing at 900ºC signs of Si-nc appear only barely, but they become much clearer in the samples annealed at 1000 o C, as the Si (111) diffraction peak emerges. The narrowing of these peaks after annealing at 1100ºC is an indication that the mean size of Si-nc is larger in this sample. ECR a-sio 2 Si (111) 1100 o C X-ray intensity (a.u.) 1000 o C ICP 900 o C 1100 o C 1000 o C 900 o C θ (degrees) Figure 1. XRD curves obtained from ECR (top) and ICP-PECVD (bottom) deposited Si y O 1-y films (y=0.42) and annealed for 2h at 900, 1000 and 1100ºC. The broad peak at lower 2θ is due to the amorphous oxide. A careful analysis of this peak as a function of the annealing parameters reveals that increasing the annealing 4

temperature leads to increasing medium range structural and chemical order (5). However, the oxide regions remain amorphous within the studied temperature range.

XRD curves obtained for samples with y=0.45, in turn, clearly show that in this case Sinc readily form after annealing at 900ºC (5).

3 temperature leads to increasing medium range structural and chemical order (5). However, the oxide regions remain amorphous within the studied temperature range. Also appreciated from Figure 1 is that the main features in the XRD curves do not seem to depend strongly on the deposition method (i.e. ECR or ICP) used to obtain the Si oxide films. XRD curves obtained for samples with y=0.45, in turn, clearly show that in this case Sinc readily form after annealing at 900ºC (5). Figure 2 presents TEM pictures obtained from ICP samples annealed at 900ºC and 1000ºC. The Si-nc can be seen at both temperatures, however it is clear that they are more likely to occur close to the Si substrate. The reason for this effect is not presently understood. In addition, the density and mean size of the Si-nc increase significantly as the annealing temperature increases from 900ºC to 1000ºC. The mean size as estimated from these TEM pictures agrees within 20% with values estimated from a recent XRD quantitative analysis, and the size growth with temperature agrees well with recently estimated activation energies of 1.5 ev (5). Figure 2. TEM pictures obtained from ICP-PECVD deposited Si y O 1-y films (y=0.45) annealed for 2h at 900 (left) and 1000ºC (right). The dark spots are due to Si-nc, the line at the bottom of the picture is the oxide film/si substrate interface. UV excited photoluminescence: The samples studied here show no or very small photoluminescence before annealing when excited by UV light at 325 nm. This wavelength corresponds to a photon energy of 3.8 ev, which is larger than the Si-nc but much smaller than the SiO 2 bandgap. After annealing, and in correlation with Si-nc formation, broad PL peaks are observed in the nm region whose peak position, intensity and width depends on the Si concentration and annealing temperature. Figure 3 shows normalized photon flux spectra as deduced from PL curves obtained from ECR samples for various combinations of y (0.34, 0.36 and 0.40) and annealing temperatures (1100 and 1200ºC). Spectra peaked at larger wavelengths are always obtained for larger Si concentrations and/or annealing temperatures, which yield to larger Si-nc. Hence, the behavior of the peak position is in 5

4 qualitative agreement with the quantum confinement mechanism for light emission, supporting conclusions from previous studies (4,6). Normalized Photon Flux Wavelength (nm) Figure 3. Typical normalized emitted photon flux spectra as deduced from PL measurements in various ECR-PECVD deposited Si y O 1-y films (y=0.34, 0.36 and 0.40) annealed for 2 h at 1100ºC and 1200ºC in (Ar+5%H 2 ). Si K-edge Si in SiO 2 TEY, PLY (a.u.) Si in Si PLY TEY X-ray energy (ev) Figure 4. Total emitted electron yield (TEY) and total photoluminescence yield (PLY) as a function of the X-ray excitation energy across the Si K-shell absorption edge from an ICP deposited Si y O 1-y film (y=0.38) annealed at 1100ºC in (Ar+5%H 2 ). The spectra were shifted vertically for clarity. 6

5 X-ray excited photoluminescence: The excitation of PL spectra with X-rays permits the potential benefit of adding up the chemical selectivity of core-level spectroscopy to the optical characterization of Si oxide/si-nc films. Figure 4 shows the total PL yield (PLY) as a function of the X-ray excitation photon energy. The simultaneously measured total electron yield emitted by the sample (TEY), which is proportional to the photon absorbance, is shown as a reference. The strong peak at 1847 ev is due to the K-shell absorption edge of Si bonded to O in the oxide region of the film. At approximately 1840 ev, a peak due to Si bonded to Si is expected (7), however it is only barely seen in the TEY spectrum as a result of the very small volume fraction of elemental Si in this film. The contributions from this region is clearly seen, however, in the PLY spectrum, showing that luminescence from Si-nc is excited one to two orders of magnitude more efficiently than from the oxide matrix. PL (a.u.) ICP (y=0.38) X-ray excited Photon flux (a.u.) UV excited Wavelenght (nm) Wavelength (nm) Figure 5. PL spectra from the same sample as in Figure 4, as excited by 1840 ev (line) and 1847 ev (line+symbol) X-ray photons. The inset shows the corresponding emitted photon flux spectrum from PL measurements in the same sample using laser-excitation at 325nm (3.8 ev). Figure 5 shows the PL spectra obtained from the same ICP sample as in Figure 4 excited by 1840 and 1847 ev X-ray photons, i.e., in resonance with the Si-in-Si and Si-inoxide absorption edges, respectively. Both spectra exhibit a prominent peak centered at about 455 nm. In is interesting to note that this peak is not observed in the PL spectrum when using 3.8 ev excitation (see inset in Figure 5). A peak at nm, whose exact position is somewhat excitation-energy dependent, has been reported in oxygen-deficient oxides (the γ luminescence peak ) (7, 8) and ascribed to defect complexes involving O vacancies. As can be seen in Figure 5, when exciting at the Si-in-Si absorption edge energy, enhanced PL due to Si-nc is observed in the IR region of the spectrum. This is additional evidence that the UV excited luminescence in the IR (inset in Figure 5) is emitted from the Si-nc cores. 7

6 Conclusions The study of Si-nc embedded in amorphous Si oxides formed during thermal annealing of Si y O 1-y PECVD thin films shows that Si-nc form as a result of annealing for 2h at temperatures above 900ºC, the exact temperature depending on the Si concentration. The mean size and density of the Si-ncs increase with increasing Si annealing temperature. Evidence that the density decreases from the oxide film/si substrate interface towards the film surface has been found by TEM. PL measurements using UV and X-ray excitation demonstrate that light emission in the nm spectral range originates from Si-nc embedded in the oxide matrix, in agreement with previous studies (4,6). Emission in the green (455 nm), probably from defect complexes in the oxide matrix, is observed from annealed samples during X-ray excitation but not for UV excitation. Acknowledgments In Canada, this work has been supported by the Ontario Research and Development Challenge Fund under the auspices of the Ontario Photonics Consortium (OPC) and by Ontario Centres of Excellence (OCE) Inc., through their Centres for Photonics and Materials and Manufacturing. Part of the research described in this paper was performed at the Canadian Light Source (CLS), which is supported by NSERC, NRC, CIHR, and the University of Saskatchewan. Professor T.K. Sham from UWO is acknowledged for help in preparing the experiment proposal to the CLS. D.C. gratefully acknowledges the CLS staff for kind hospitality and fruitful discussions during the measurements run at the CLS. References 1. L. Pavesi and D.J. Lockwood (Eds), Silicon Photonics, Springer, Berlin (2004). 2. R. J. Walters, G. I. Bourianoff, H. A. Atwater, Nature Materials 4, 143 (2005). 3. F. Iacona, G. Franzo, and C. Spinella, J. Appl. Phys. 87, 1295 (2000). 4. D. Comedi, O. H. Y. Zalloum, P. Mascher, Appl. Phys. Lett. 87, (2005). 5. D. Comedi, O. H. Y. Zalloum, E. A. Irving, J. Wojcik, T. Roschuk, M.J. Flynn, and P. Mascher, J. Appl. Phys. 99, (2006). 6. D. Comedi, O. H. Y. Zalloum, E. A. Irving, J. Wojcik, and P. Mascher, J. Vac. Sci. Technol. A 24, 817 (2006). 7. T. K. Sham et al., Phys. Rev. B 70, (2004). 8. F. Meinardi and A. Paleari, Phys. Rev. B 58, 3511 (1998). 8

Pelotas, , Pelotas, RS, Brazil 2 Instituto de Física, Universidade Federal do Rio Grande do Sul C.P ,

Pelotas, , Pelotas, RS, Brazil 2 Instituto de Física, Universidade Federal do Rio Grande do Sul C.P , XXVII SIM - South Symposium on Microelectronics 1 Structural and luminescence properties of Ge nanocrystals before and after an ion irradiation process 1 F. Nornberg, 2 F. L. Bregolin, 1,2 U. S. Sias francielefnornberg@gmail.com,