Luminescence properties of SiOxNy irradiated by IR ...

APPLIED PHYSICS LETTERS 100, 042104 (2012)

Luminescence properties of SiOxNy irradiated by IR laser 808 nm: The role of Si quantum dots and Si chemical environment Rosa Ruggeri,1 Fortunato Neri,1 Antonella Sciuto,2 Vittorio Privitera,2 Corrado Spinella,2 and Giovanni Mannino2,a)

1 Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita` di Messina, Salita, Sperone 31, 98166 Messina, Italy 2 CNR-IMM, Strada VIII n 5, Zona Industriale, 95121 Catania, Italy

(Received 23 May 2011; accepted 6 January 2012; published online 25 January 2012) We investigated optical, structural, and chemical properties of SiOxNy layers irradiated by CW IR laser during a time lapse of few milliseconds. We observed tunable photoluminescence signal at room temperature in the range 750–950 nm, without Si/SiO2 phase separation, depending on the IR laser power irradiation. Furthermore, no photoluminescence signal was recorded when the IR laser power density was high enough to promote phase separation forming Si quantum dots. By chemical analysis the source of the luminescence signal has been identified in a change of silicon C 2012 chemical environment induced by IR laser annealing inside the amorphous matrix. V American Institute of Physics. [doi:10.1063/1.3679395]

Research on the fabrication of light-emitting devices based on materials compatible with Si technology is nowadays of great importance, since it would facilitate the integration of microelectronic and optical devices onto a same chip. However, the indirect band gap of Si makes it an inefficient light source. So far this difficulty has precluded the achievement of true large-scale optoelectronic integration at reasonable costs. Initial studies pointed to porous Si,1 but the recognized instability of this material limited its use in luminescent devices. The development of new structures, e.g., Si quantum dots (Si-QDs), has renewed interest in the production of Si-based luminescent devices, and there is currently significant activity in the field of light emission from Si nano-sized structures. Si-rich oxide (SiOx, x < 2) has been obtained by Siþ ion implantation in SiO2,2 plasma enhanced chemical vapor deposition (PECVD),3 sputtering and reactive evaporation.4 Arguably, it is a more robust material that exhibits similar porous Si optical properties but is significantly less susceptible to damage. More generally, Si oxynitride (SiOxNy) thin films, deposited by PECVD technique utilizing nitrous oxide (N2O) and silane (SiH4) as precursor gases, have been proposed as an alternative to SiOx. With adequate adjustment of the deposition parameters, it is possible to obtain layers of varying stoichiometry from Si dioxide to Si-rich oxynitride. The optical properties and dielectric constants vary depending on the nitrogen content and make SiOxNy extremely versatile for waveguide fabrication, SiQD, and other optical and electronic applications. After annealing at high temperature (>1000 C), x-ray diffraction, transmission electron microscopy, Raman scattering, and x-ray photoemission spectroscopy studies have confirmed the presence of Si-QDs both in SiOx and in SiOxNy matrix.5–7 Optical studies have demonstrated that such materials containing Si aggregates of varying dimensions emit broadband light emission from the blue to near infrared.8 a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2012/100(4)/042104/3/$30.00

The smaller the Si-QDs size the larger is the band gap of the materials. This peculiarity is useful for application as tunable wide band gap materials in Si based tandem solar cell.6 Actually, it is still under debate the relation between the nature of the luminescence mechanism, attributed to the quantum size confinement of Si-QDs, the role of the SiO2-SiNSiOxNy matrix, and the role QDs interface with the matrix.9–11 Several models have been proposed explaining the mechanism of light emission: quantum confinement model;12,13 surface recombination models;14 and luminescence of siloxene molecules.15,16 This controversy is due to the difficulty to control the structure and the composition of the Si-QDs and, at the same time, that of the matrix and to separate individual contributions. Understanding the photoluminescence mechanisms in Si nanostructures hence remains the key aspect for the production of Si-based lightemitting devices. In a recent work it was demonstrated that crystalline Si-QDs are synthesized in SiO0.86N0.18 (x þ y 1.05) layers by millisecond IR laser irradiation at 808 nm, provided Si excess concentration is high enough. However, the optical efficiency is gained only by subsequent high temperature annealing in a furnace for several minutes since the Si-QDs contain defects or they are surrounded by an impurity rich matrix.17 In this work, we formed the Si-QD by the same IR laser annealing process but we investigate the role of the amorphous matrix as a source of luminescence signal having access to a timescale in which Si-QD are not yet formed. A 130 nm thick SiO0.49N0.38 (x þ y 0.9) layer was deposited at 280 C by PECVD and encapsulated between two thin SiO2 layers in order to avoid contamination during high temperature annealing by IR laser irradiation in air. The IR laser equipment is an array of GaAs diodes emitting a continuous radiation at 808 nm. The annealing time is maintained fixed at 6 ms, whereas the power was varied in the range between 107 and 143 kW/cm2. Six different conditions were obtained and were analyzed by photoluminescence spectroscopy (PL), energy filtered transmission electron microscopy

100, 042104-1

C 2012 American Institute of Physics V

Downloaded 29 May 2012 to 150.145.42.11. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

042104-2

Ruggeri et al.

(EFTEM), and x-ray photoemission spectroscopy (XPS). Figure 1 shows the PL spectra of as-deposited and annealed samples performed by pumping with the 488 nm line of an Ar laser at room temperature. The PL signal consists of a broad band centered at 700 nm (“as-deposited” sample) that continuously shifts up to 950 nm by increasing the IR laser annealing irradiation power. However, at the highest irradiation power the PL signal suddenly disappears. It is noteworthy that PL is achieved with IR laser irradiation alone, without post-process annealing in furnace, and this emission is tuned by changing the IR laser power (i.e., the sample temperature).18 The inset shows the dependence of the center of each band on the IR laser power: at first, it shifts almost linearly (107–121 kW/cm2), and then it changes faster (121–135 kW/cm2). This systematic shift of the PL peak energy does not depend on any observable structural change. Figure 2 shows the cross-sectional EFTEM micrographs of samples annealed at 135 kW/cm2 (Fig. 2(a)) and at 143 kW/cm2 (Fig. 2(b)) with the respective diffraction patters in the insets. The grey contrast, characteristic of the sample annealed at 135 kW/cm2, is homogeneous, and there are no visible nanostructures; its diffraction pattern confirmed that the sample is still amorphous. The EFTEM analyses (not showed) performed on all the samples annealed up to 135 kW/cm2 are identical. On the contrary, the micrograph of the sample irradiated at 143 kW/cm2, the highest power, shows clearly the presence of white spots, a typical contrast of Si-QDs, and the diffraction pattern confirms that the sample is partially polycrystalline. The size distribution is 5 6 3 nm, measured on a collection of more than one hundred Si-QDs. We concluded that the PL signal, in IR laser irradiated samples, does not depend on the presence of a net phase separation, discussed elsewhere in Ref. 17, but rather on the local arrangements of the chemical components (Si, O, and N) forming the amorphous matrix.

Appl. Phys. Lett. 100, 042104 (2012)

FIG. 2. Energy filtered transmission electron microscopy of irradiated samples still amorphous at 135 kW/cm2 (a) and with Si-QDs at 143 kW/cm2 (b). The insets are the diffraction patterns.

In order to investigate the chemical modifications induced by IR laser irradiation we collected the XPS spectra of each sample in the Si2p, O1s, and N1s regions. The Si2p core level was deconvoluted taking into account all the possible chemical configurations for Si: the metallic state (Si0) and four oxidation states (Si1þ, Si2þ, Si3þ, and Si4þ). Fixed positions on the energy scale with equal FWHM GaussianLorentzian bands were used to carry out the best-fit procedure. We assigned to Si0 (metallic Si) the lowest binding energy 99.7 eV whereas the 100.7, 101.8, and 102.7 eV binding energies were associated with the intermediate oxidation states Si1þ, Si2þ, Si3þ, and finally 103.7 eV was associated to Si4þ, i.e., the SiO2 molecular configuration.19 Table I summarizes the Si2p core levels deconvolution model. Figure 3 shows the concentration percentage of each Si oxidation state in as-deposited and irradiated samples. With increasing the IR laser power, from 107 to 135 kW/cm2, the concentration of all species undergo a monotonic development: Si1þ and Si2þ decrease with respect to the asdeposited sample, whereas the concentration of Si3þ and Si4þ increases. The variation of the concentration of Si0 plays a key role. Compared to the as-deposited, the atomic percentage of metallic Si, after annealing process, decreases progressively confirming that the PL signal is not necessarily due to the formation of Si metallic bonds, possibly the tetrahedral bonds precursors of crystalline Si-QDs. In fact, in agreement with TEM analyses, the concentration of Si0 is the highest in the sample where we observed the formation of Si-QDs. However, the phase separation has a negative impact on the luminescence signal that, as mentioned before, totally disappears. XPS analysis demonstrates that the IR laser irradiation induces a change in the chemical environment of Si in the amorphous matrix, while no phase separation occurs, correlated with the PL signal shift from 750 to 950 nm. TABLE I. XPS Si2p peaks decomposition and relative molecular configurations. Oxidation state

FIG. 1. (Color online) PL signal at room temperature from as-deposited and laser irradiated samples. The insert shows the center of each band as a function of the laser power.

Si0 Si1þ Si2þ Si3þ Si4þ

Binding energy (eV)

Molecular configuration

99.7 100.7 101.8 102.7 103.7

Si4 Si2O/Si6O3N2/Si3N2 SiO/Si6O3N4/Si3N4 Si2O3/Si3O3N2/Si6O9N2 SiO2


042104-3

Ruggeri et al.

Appl. Phys. Lett. 100, 042104 (2012)

tion compatible with the formation of siloxenes since SiOxNy layers initially contain a concentration of H as high as 15% as described elsewhere.20 We conclude that IR laser irradiation in non melting regime offers the possibility to investigate a temperature/time regime, not accessible by conventional furnace annealing, in which the Si in the SiOxNy amorphous layer rearranges itself forming very small and localised molecules/clusters. This is consistent with previously published experiments.17 Depending on the irradiation power, i.e., the sample temperature, we probed the thermal stability of these local atomic arrangements which at lower temperature stabilise into N-rich configurations and, with increasing the temperature, move to higher oxidation degree status, i.e., O-rich configurations. The variation of the local composition within the amorphous matrix implies a variation of the emission spectrum. Finally, when separation phase occurs, it seems that metallic Si in the QD is not the source of luminescence at room temperature in laser-annealed samples. 1

FIG. 3. (Color online) Si oxidation states, expressed in atomic percentage, as a function of the power density.

Summarizing, we observed that in PECVD as-deposited sample Si atoms in excess are preferably bonded with N or with other Si atoms. The IR laser irradiation induces a progressive variation in the amorphous matrix attested by a red shift of the PL band. Correspondingly the Si chemical environment changes, and progressively the Si atoms are bound to more electronegative chemical species, i.e., the concentration of O-rich Si configurations increases. When the thermal budget induced by IR laser irradiation is high enough, the phase separation occurs, and Si-QDs are embedded in the Si oxinitride matrix, but at this time no PL signal is emitted by the sample. Since we cannot observe directly the emitting centers by electron microscopy, it is reasonable to assume that they are very local arrangements on the atomic scale. In the literature, Stutzmann and co-workers have observed that siloxene molecules Si6O3R are characterized by direct band gap.15,16 These authors describe siloxene as a molecule with Si6-ring configuration: each Si atom is bound to one oxygen atom, two Si atoms, and to a monovalent radical ligand of variable nature (it can be H or NH2, OH, etc.). The width of the gap depends on the nature of the ligand: the more electronegative ligand reduces the gap. Si6-rings with different oxidation degrees emit at different wavelengths centered in the range between 600 and 900 nm. The width of the luminescence band depends on the inhomogeneous structure, i.e., different degrees of oxidation. Our samples have a chemical composi-

L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990). T. Komoda, J. Kelly, F. Cristiano, A. Nejim, P. L. F. Hemment, K. P. Homewood, R. Gwilliam, J. E. Mynard, and B. J. Sealy, Nucl. Instrum. Methods B 96, 387 (1995). 3 A. J. Kenyon, P. F. Trwoga, C. W. Pitt, and G. Rehm, J. Appl. Phys. 79, 9291 (1996). 4 S. Zhang, W. Zhang, and J. Yuan, Thin Solid Films 326, 92 (1998). 5 D. Di, I. Perez-Wurfl, G. Conibeer, M. A. Green, Solar Energy Mater. Solar Cells 94, 2238 (2010). 6 G. Conibeer, M. L. Green, R. Corkish, Y. Cho, E. C. Cho, C. W. Jiang, T. Fangsuwannarak, E. Pink, Y. Huang, T. Puzzer, T. Trupke, B. Richards, A. Shalav, and K.l. Lin, Thin Solid Films 511–512, 654 (2006). 7 F. Rochet, G. Dufour, H. Roulet, B. Pelloie, J. Perriere, E. Fogarassy, A. Slaoui, and M. Froment, Phys. Rev. B 37, 6468 (1988). 8 F. Iacona, G. Franzo`, and C. Spinella, J. Appl. Phys. 87, 1295 (2000). 9 T. W. Kim, C. H. Cho, B. H. Kim, and S. J. Park, Appl. Phys. Lett. 88, 123102 (2006). 10 Y. Kanzawa, T. Kageyama, S. Takeoka, M. Fujii, S. Hayashi, and K. Yamamoto, Sol. State Commun. 102, 533 (1997). 11 M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999). 12 L. T. Canham, Phy. Stat. Sol. B 190, 9 (1995). 13 A. G. Cullis and L. T. Canham, Nature 353, 335 (1991). 14 Y. Kanemitsu, T. Ogawa, K. Shiraishi, and K. Takeda, Phys. Rev. B 48, 4883 (1993). 15 M. Stutzmann, M. S. Brandt, M. Rosenbauer, H. D. Fuchs, S. Finkbeiner, J. Weber, and P. Deak, J. Lumin. 57, 321 (1993). 16 M. S. Brandt, H. D. Fuchs, M. Stutzmann, J. Weber, and M. Cardona, Solid State Commun. 81, 307 (1992). 17 G. Mannino, C. Spinella, C. Bongiorno, G. Nicotra, F. Mercorillo, V. Privitera, G. Franzo`, A. M. Piro, M. G. Grimaldi, M. A. Di Stefano, and S. Di Marco, J. Appl. Phys. 107, 023703 (2010). 18 G. Mannino, C. Spinella, R. Ruggeri, A. La Magna, G. Fisicaro, E. Fazio, F. Neri, and V. Privitera, App. Phys. Lett. 97, 022107 (2010). 19 J. Cova, S. Poulin, O. Grenier, and R. A. Masut, Appl. Phys. 97, 073518 (2005). 20 R. Ruggeri, V. Privitera, C. Spinella, E. Fazio, F. Neri, R. De Bastiani, M. G. Grimaldi, M. A. Di Stefano, S. Di Marco, and G. Mannino, J. Electrochem. Soc. 158, H25–H29 (2011). 2
