- Email: [email protected]
Absorption and emission of light in spark-processed silicon
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549 www.elsevier.com/locate/jpcs
Absorption and emission of light in spark-processed silicon$ J.G. Polihronova, M. Hedstro¨mb, H.-P. Chengb, R.E. Hummela, a
Department of Materials Science and Engineering, University of Florida, 216 Rhines Hall, Gainesville FL 32611-6400, USA b Quantum Theory Project, University of Florida, Gainesville FL 32611, USA Received 9 June 2005; received in revised form 31 October 2005; accepted 8 January 2006
Abstract Spark-processed Si (sp-Si) exhibits blue, green and red photoluminescence at around 385, 525 and 650 nm, depending on the wavelength of excitation. Its optical absorption spectrum reveals bands peaked approximately at 245, 277, 325 and 389 nm. The centers where absorption takes place were modeled as Si and silica clusters in an amorphous SiOxNy matrix using various embedding schemes. Geometry optimizations were applied prior to calculations of the absorption spectra of the clusters. The measured absorption spectrum of sp-Si and calculated absorption spectra were compared. Best agreement is achieved for Si particles embedded in amorphous SiOxNy matrix. The importance of the various embedding schemes is discussed and conclusions for the centers of emission are established. r 2006 Elsevier Ltd. All rights reserved. Keywords: A. Amorphous materials; A. Nanostructures; A. Optical materials; A. Oxides; D. Luminescence
Spark-processed Si (sp-Si) is an amorphous solid-state material exhibiting strong photoluminescence (PL) at 385 (blue), 525 (green) and 650 nm (red), depending on the excitation wavelength [1]. The bulk of sp-Si is a random mixture of four phases—mostly amorphous silicon dioxide (a-SiO2), amorphous silicon (a-Si), crystalline silicon (c-Si) and silicon nitrides in the form of SiNx or SiOxNy [2,3]. The porosity of the material can be considered to be a separate phase and is estimated to be in the neighborhood of 43% [4]. The light-emission from sp-Si is characterized by a remarkable stability against thermal annealing up to 1100 1C, etching in hydrofluoric acid, UV irradiation, and aging [2]. Since the emission of light originates from a Si-based material, the understanding of the properties of the lightemitting centers in sp-Si is of relevance to telecommunications technology and opto-electronic applications [5]. So far it has been experimentally shown, that the PL of sp-Si does not originate from hydroxyl groups [6], silanolrelated compounds [6,7], silicon carbides [7], defects in the $ Based on a dissertation submitted by J. Polihronov as a partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Florida. Corresponding author. Tel.: +1 (352) 392 6667; fax: +1 (352) 392 6359. E-mail address: [email protected]fl.edu (R.E. Hummel).
0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.121
a-SiO2 matrix [7], hydrogen-related compounds in the a-Si phase [7] or possible metallic ions, embedded in the material during the growth process [8]. It has been experimentally established as well that the process of light-emission in sp-Si differs substantially from the same phenomenon in porous Si [2]. The latest time-resolved PL measurements with blue and green-emitting sp-Si show PL decay times in the order of a few picoseconds [9,10], while the lifetimes of the PL in porous Si are three to six orders of magnitude longer [11]. Temperature-dependent PL measurements with sp-Si reveal shifts, which are opposite compared to the widening of the band gap in c-Si, a-Si and Si quantum dots [3]. Raman spectra of blue- and greenemitting sp-Si material are virtually identical and do not suggest the presence of Si particles with diameters of less than 15 nm [3]. Such Si dots are rather large and cannot contribute to the visible PL of sp-Si [12]. Besides, when Si is spark-processed in pure oxygen atmospheres the resulting material contains Si nanoparticles ([13], transmission electron micrographs) but does not emit light [9,10]. This paper presents computational models of the emitting centers in sp-Si, based largely on comparison with available experimental data. The experimental facts outlined above were considered when various molecular clusters were studied as possible contributors to the PL
ARTICLE IN PRESS 1544
J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
of sp-Si. Other experimentally established facts as listed below were also of key importance for the completion of the present work. First, it has been shown that the presence of both N and O gases in the spark-processing atmosphere is essential for the light emission process in sp-Si [9,10]. Second, the strongest PL has been found to originate from the a-SiO2–rich surface layers, which contain very limited amounts of N atoms and Si particles [2,3,14]. Third, it has been experimentally shown that for sp-Si grown in N-rich atmospheres, the stability [7] and the intensity [9,10] of the PL decreases. The bulk of the material consists of a dense framework of Si clusters, which does not produce PL [3]. All taken, the experimental observations indicate that the light-emitting centers in sp-Si are positioned in a-SiO2 matrix, containing Si clusters and N atoms. Since the PL in sp-Si is observed at reproducible wavelengths, the emitting centers are expected to have similar physical properties. It has to be noted also that sp-Si is a highly amorphous material and therefore the optically active centers, although similar throughout the bulk, are positioned in different atomic surroundings. They belong to the same family of molecular clusters and have similar physical characteristics, but do not have identical molecular geometries. The light-emitting centers in sp-Si were modeled as molecular clusters. The cluster geometries were based on previously published studies, having sufficient experimental and theoretical support for their existence. The a-SiO2 matrix was modeled as a network of silica rings (Fig. 1) which are formed by a number of interconnected SiO4 tetrahedra. The Si particles in sp-Si were modeled as Si clusters (Fig. 2) embedded in a-SiO2. Each Si cluster was bonded to the silica rings network and the resulting structure was allowed to relax. All dangling bonds were terminated with H atoms. Examples of two Si particles embedded in amorphous matrix are given in Figs. 3a and 4a. To the extent possible, all our structures utilize tetrahedral bonding on Si atoms with no dangling bonds allowed. Depending on the particular bonding situation, certain atoms of the Si particles are not bonded to the surrounding network, only to the Si cluster. Further details about the silica and Si rings as well as the other Si structures used in this work have been discussed in [15,16]. The geometries of all clusters in our work were optimized with the NDDO-based AM1 (Neglect of Diatomic Differential Overlap and the Austin Method 1) [17], followed by an INDO/CI calculation (Intermediate Neglect of Differential Overlap with Configuration Interaction, parameterized for spectroscopy) [18], which provided us with optical absorption spectra. The peaks in the spectra are assumed Gaussian with adopted peak width of DE0.4 eV reflecting a thermal line broadening. An absorption spectrum containing N transitions is then expressed as N 2 1 X f ðEÞ ¼ pffiffiffiffiffiffi pk eðEE ok =2DÞ , D 2p k¼1
(1)
where pk is the probability for a transition with energy Eok between the ground and kth excited state. The calculated spectra were compared to the absorption spectrum of sp-Si [19], obtained by differential reflectometry (DR) technique [20]. Using AM1 for geometry optimization of Si carbide molecules, we were able to predict bond lengths within a few percent of their established values. For a detailed description of the parameterization procedure of AM1 and its capabilities the reader is referred to [18,21]. INDO is based on the Hartree–Fock (HF) approximation for the system of valence electrons in a molecule. The method has successfully been used before for modeling of Si systems [22,23]. In addition, we have parameterized the INDO value of the resonance integral b for Si [15]. In the parameterization, we used 10 different Si-containing molecules and optimized b so that the electronic transitions were accurately reproduced. In the studied 10 molecules, Si atoms participated with both p- and s-bonding. We estimated the collective error of NDDO and INDO/CI for prediction of electronic transition energy to be 2000 cm1 (0.24 eV), in agreement with previously published results [18,24]. For many of the clusters we performed unrestricted HF (UHF) calculations. In general the UHF wave functions are not eigenfunctions to the spin operator S2, although we found by projecting the UHF wave functions onto wave functions representing pure spin multiplicities that the contamination from higher states were small. Therefore, the clusters studied in this work have been considered to have closed-shell singlet ground states and the restricted HF method was used in all of the calculations of spectra. The a-SiO2 phase in sp-Si was modeled [15] with 2-, 3-, 4-, 5- and 6-membered silica ring-shaped clusters. As expected, silica glass clusters do not exhibit absorption bands in the visible. The absorption threshold is located around 200 nm. By replacing an O atom with N atom in certain 2-membered silica rings, we observed an absorption peak at 245 nm, similarly to the experimental spectrum of sp-Si. Analysis of the probability amplitude for these electronic transitions indicated that they are preferentially located on certain N atoms. However, the other absorption bands of sp-Si were not reproduced and no electronic transitions below 320 nm were observed in the calculated spectra. The study of the a-SiO2 phase included nearly 100 clusters containing between 6 and 40 atoms each. The a-Si and c-Si phases were modeled [16] with 3-, 4-, 5and 6-membered OH-terminated Si rings and with OHterminated Si clusters and cages [25–30] with sizes between 3 and 14 atoms each. Generally, three resolved absorption peaks are observed in all spectra of Si clusters, lying in the interval between 200 and 350 nm.The peaks are positioned on average around 200, 260 and 320 nm. Additionally, small absorption bands were found between 400 and 500 nm for a substantial number of clusters. The above-described peaks are seen in the experimental spectrum of sp-Si as shoulders of a broad absorption band, peaked at 245 nm. This fact may be interpreted to suggest
ARTICLE IN PRESS J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
1545
Fig. 1. 3-, 4-, 5- and 6-membered silica clusters. The black atoms represent Si, the small atoms represent H and the gray atoms represent O.
that Si clusters are contributing to the optical properties of sp-Si. Still, the above peaks are resolved almost without exception in our calculations and therefore the small OHterminated Si clusters and rings alone cannot be used to explain the emission from sp-Si. The OH termination thus proved to be insufficient to describe the field of the surrounding matrix, which adds a non-negligible term in the potential that the cluster electrons will experience. As a next step, we improved our embedding scheme by positioning Si particles in a-SiOxNy matrix (one example is shown in Fig. 3a). In this section, we studied nearly 600 Si/a-SiOxNy structures. It should be noted that the x and y values have not been strictly defined as they vary from one cluster to another. x and y are characteristics of the matrix, surrounding the Si clusters and have meaning only when averaged over sufficiently large portion of the matrix volume. In the largest clusters of this study, the matrix was limited to silica rings with total of up to 100–180 atoms to guarantee computational efficiency. The unsatisfied bonds at the cluster borders were H-terminated. H does not have influence on the optical
properties of sp-Si [7] and in all optical calculations the electron transition amplitudes in the interval 200–350 nm were not positioned over H atoms. The calculated absorption spectra in this case remarkably well reproduced the experimental spectrum of sp-Si (Fig. 3b). An examination of the excitations in the 245–325 nm range shows that the transition from the highest occupied to the lowest unoccupied molecular orbitals (HOMO to LUMO) contributes largely. We find that the HOMO to a large extent is positioned on nearby O atoms in the matrix and the LUMO on the Si atoms of the Si particle as illustrated in Fig. 3a. This shows the importance of the Si clusters and the matrix in the absorption process. The agreement between theoretical prediction and experiment was generally achieved for any Si/a-SiOxNy structure and therefore the clusters in the figures represent only an example of this fact. The degree of similarity (that is, the w2 of the fit) between calculated and measured spectra varies for the different modeled structures, although the spectral features are similar. It should be emphasized at this point that PL is observed only if sp-Si is processed in mixtures of O and N gases (for
ARTICLE IN PRESS 1546
J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
Fig. 2. Representative sample of the Si clusters used in this work.
instance, air) [9,10] and excited at 325 nm. This suggests that either sp-Si produced in pure oxygen atmosphere does not have an absorption band at that excitation wavelength or the absorbed energy is lost in non-radiative transitions. For this reason it has been inferred [9,10] that N atoms play a crucial role in the optical properties of light-emitting sp-Si. However, this cannot be readily confirmed in our calculations. The calculated spectrum in Fig. 3 (no N involvement) already provides a fit to the experimental data with a w2 1. To study this point further, we calculated optical spectra of structures with Si2–N–O bonding at the Si/oxide interface [31,32]. Such configurations (Fig. 4) are justified, since a large number of experimental studies consistently confirm N atoms piling up at the dielectric/Si interface in aSiOxNy systems [33–41]. The study of the modified structures included a w2 statistical analysis of the fit between the calculated spectra and experiment. Its purpose was to provide a quantitative criterion for the reliability of each fit. In the w2 analysis, spectra of 9 different Si clusters between 3 and 14 atoms [25–30] were included. Four
different embedding schemes were applied to each cluster, namely: (i) Si cluster, embedded in a-SiOx; (ii) OHterminated Si cluster; (iii) Si cluster, embedded in a-SiOxNy with Si2–N bonding at Si/oxide interface; (iv) Si cluster, embedded in a-SiOxNy with Si2–N–O bonding at Si/oxide interface. Only the last two schemes allow N atoms into the studied structures. The value of w2 was determined by fitting only the portions of the spectra lying between 200 and 350 nm, since the main absorption peaks are positioned in this interval. It was found that the reliability of the fit increases with the size of the Si cluster, since w2 decreases almost within one order of magnitude as the Si cluster size increases from 3 to 14 atoms. The values of w2 are calculated with a certain error, which comes only from the NDO calculations (70.24 eV), while the experimental points are measured with an error of only 0.01 eV [20] and thus, do not influence the value of w2. At larger cluster sizes, the embedding scheme Si/a-SiOxNy seems to provide a slightly better numerical fit (a higher w2), compared to the scheme Si/SiOx. However, due to the error bars in w2 and
ARTICLE IN PRESS J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
1547
Fig. 3. (a) Si/SiOx cluster. The HOMO and LUMO isosurfaces for a transition at 252 nm are shown. (b) Comparison between the normalized measured DR spectrum of sp-Si (absorption) and the normalized calculated spectrum of the cluster. Its dark atoms represent O, the gray atoms represent Si, the small atoms represent H.
the very small differences between its values for all embedding schemes, it is not possible to eliminate any of the fits. Thus, small (within 0.15) differences in w2 of the fits were considered to be inconclusive. It is useful to note that we have built an absorption curve taken as a weighted average of the absorption plots of all clusters described in schemes (i)–(iv). While natural abundances of clusters can normally be used as a weighting criterion in such plots, this criterion may not be valid in our case due to the nature of spark-processing. Growing sp-Si involves fast cooling rates of molten material and non-equilibrium cluster structures can be produced and embedded in the sp-Si matrix. The cluster abundances in material may then not necessarily coincide with their natural abundances and for this reason we assumed all weights in our plot to be equal. The
resulting spectrum did not provide substantially different information than the individual cluster plots as all of them already have very small w2 values. One then needs to conclude that if all of the embedded clusters display spectra with comparable values of w2, they can be considered as potential contributors to the optical properties of sp-Si. Since this approach cannot precisely determine the role of N on the PL, modeling of other properties of the lightemitting material could be worthwhile considering if one would be confident that such properties indeed originate from the centers of emission, and not from the surrounding matrix. For instance, sp-Si exhibits ferromagnetic behavior, but it disappears after annealing at 600 1C, while the optical properties stay constant up to 1100 1C. Thus, modeling of the magnetic properties of Si particles in
ARTICLE IN PRESS 1548
J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
Fig. 4. (a) Si/SiOxNy cluster. (b) Comparison between the normalized measured DR spectrum of sp-Si (absorption) and the normalized calculated spectrum of the cluster. Its dark atoms represent O, the gray atoms represent Si, the small atoms represent H. N atoms are marked ‘N’.
a-SiOxNy would be irrelevant. Similarly, the resistance of the PL against HF etching cannot be ascribed to the emitting centers, but rather to the sp-Si matrix. It is known that the HF etch rate of a-SiOxNy decreases as the N concentration in the film increases [42]. It has always been pointed out that N atoms are essential for sp-Si PL. However, it is quite possible that N plays an important role in the dynamics during sp-Si growth by relieving strain fields and assisting the formation of smooth Si/a-SiOxNy interfaces, which are crucial in the processes of absorption and emission. Another aspect of our computational results for the sp-Si absorption is that together with certain experimental facts they can be used to suggest a model for the sp-Si emission: first, it has been observed that blue and green PL in sp-Si exhibits picosecond lifetimes [45]. Concurrently, picosecond decay times have been reported [46–49] for light emission from Si particles. Second, PL excitation spectra (PLE) of sp-Si have been found to exhibit a large peak in
the interval 245–325 nm [19,43,44], which coincides with the location of the main sp-Si absorption peak. Therefore, energy absorbed in this region is most efficiently converted into emitted light. Thus, according to our model electrons in supramolecular Si/a-SiOxNy clusters are excited with 245–325 nm photons followed by fast emission transitions with amplitudes positioned over the same Si/a-SiOxNy structures (modeled as clusters with diameter E1 nm) rather than depending on energy transfer to another distant locale where emission would take place. In conclusion, we have modeled the centers of lightabsorption in sp-Si utilizing silica and Si clusters in a number of embedding schemes. The spectra of certain nitrogen-substituted SiO2 structures reproduce the main absorption band of sp-Si at 245 nm, while the spectra of OH-terminated Si clusters exhibit three resolved bands between 200 and 350 nm. These two important approximations led us to the study of Si/a-SiOxNy systems. Their
ARTICLE IN PRESS J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
spectra reproduce very closely (w2 1) the observed spectrum of sp-Si and the agreement between calculation and experiment improves with increasing size of the Si clusters. Analysis of the calculated electronic transition amplitudes and of the available PLE data in the interval between 245 and 325 nm suggests a common origin of the absorption and emission in sp-Si, while time-resolved PL measurements indicate picosecond-order times of emission transitions, taking place over the Si particles in sp-Si. References [1] R.E. Hummel, S.-S. Chang, Appl. Phys. Lett. 61 (1992) 1965. [2] M. Ludwig, Crit. Rev. Sol. State Mater. Sci. 21 (1996) 265. [3] M.H. Ludwig, A. Augustin, R.E. Hummel, Th. Gross, J. Appl. Phys. 80 (1996) 5318. [4] J. Polihronov, T. Dubroca, M. Manuel, R.E. Hummel, Mater. Sci. Eng. B 107 (2004) 124. [5] N. Shepherd, R.E. Hummel, Phys. Stat. Sol. A 197 (2003) 222. [6] R.E. Hummel, M.H. Ludwig, J. Hack, S.-S. Chang, Solid State Commun. 96 (1995) 683. [7] R.E. Hummel, M.H. Ludwig, J. Lumin. 68 (1996) 69. [8] R.E. Hummel, N. Shepherd, M.H. Ludwig, M.E. Stora, Thin Solid Films 325 (1998) 1. [9] M. Stora, R.E. Hummel, J. Phys. Chem. Solids 63 (2002) 1867. [10] M. Stora, R.E. Hummel, J. Phys. Chem. Solids 63 (2002) 1655. [11] R.E. Hummel, M.H. Ludwig, S.-S. Chang, P.M. Fauchet, Ju.V. Vandyshev, L. Tsybeskov, Solid State Commun. 95 (1995) 553. [12] S. Rupp, J. Quilty, J. Trodahl, M.H. Ludwig, R.E. Hummel, Appl. Phys. Lett. 70 (1996) 723. [13] N. Shepherd, Doctoral Dissertation, University of Florida, 2002. [14] M.H. Ludwig, R.E. Hummel, A. Augustin, J. Hack, J. Menniger, Appl. Phys. Lett. 67 (1995) 2542. [15] J. Polihronov, M. Hedstro¨m, R.E. Hummel, H.-P. Cheng, J. Lumin. 96 (2002) 119. [16] J. Polihronov, R.E. Hummel, H.-P. Cheng, J. Lumin. 101 (2003) 55. [17] J.J.P. Stewart, MOPAC 2000.00 Manual, Fujitsu Ltd., Tokyo, Japan. [18] M.C. Zerner, in: K.B. Lipkowitz (Ed.), Reviews of Computational Chemistry, vol. 2, VCH Publishers, New York, 1991 (p. 313, Chapter 8). [19] R.E. Hummel, N. Sheperd, D. Burton, Appl. Phys. Lett. 79 (2001) 3218. [20] R.E. Hummel, Phys. Stat. Sol. A 76 (1983) 12. [21] J.J.P. Stewart, J. Computer-Aided Mol. Des. 4 (1990) 1. [22] M.J. Caldas, Phys. Stat. Sol. B 217 (2000) 641. [23] R.J. Baierle, M.J. Caldas, E. Molinari, S. Ossicini, Solid State Commun. 102 (1997) 545.
1549
[24] TedA. O’Brien, P.D. Rack, P.H. Holloway, M.C. Zerner, J. Lumin. 78 (1998) 245. [25] K. Raghavachari, C.M. Rohlfing, J. Chem. Phys. 89 (1998) 2219. [26] R. Fournier, S. Sinnott, A. DePristo, J. Chem. Phys. 97 (1992) 4149. [27] S. Wei, R.N. Barnett, U. Landman, Phys. Rev. B 55 (1997) 7935. [28] X. Jing, N. Troullier, D. Dean, N. Binggeli, J.R. Chelikowsky, Phys. Rev. B 50 (1994) 12234. [29] O. Sankey, D.J. Niklewski, D.A. Drabold, J.D. Dow, Phys. Rev. B 41 (1990) 12750. [30] A. Rubio, J.A. Alonso, X. Blase´, L.C. Balbas, S.G. Louie, Phys. Rev. Lett. 77 (1996) 247. [31] D. Bouvet, P.A. Clivaz, M. Dutoit, C. Coluzza, J. Almeida, G. Margaritondo, F. Pio, J. Appl. Phys. 79 (1996) 7114. [32] A. Teramoto, H. Umeda, H. Tamura, Y. Nishida, H. Sayama, K. Terada, K. Kawase, Y. Ohno, A. Shigetomi, J. Electrochem. Soc. 147 (2000) 1888. [33] R.P. Vasquez, A. Madhukar, F.J. Grunthaner, M.L. Naiman, J. Appl. Phys. 59 (1986) 972. [34] B. Brijs, J. Deleu, T. Conard, H. De Vitte, W. Vandervorst, K. Najajima, K. Kimura, I. Genchev, A. Bergmaier, L. Goergens, P. Neumaier, G. Dollinger, M. Dobeli, Nucl. Instr. Meth. Phys. Res. B 161–163 (2000) 429. [35] M.L. Monaghan, T. Nigam, M. Houssa, S. De Gendt, H.P. Urbach, P.K. De Bokx, Thin Solid Films 359 (2000) 197. [36] L. Itoh, Y. Hayamitzu, T. Abe, Mater. Sci. Eng. B 4 (1989) 309. [37] J.-W. Lee, N.-I. Lee, J.-I. Han, C.-H. Han, IEEE Electr. Dev. Lett. 18 (1997) 172. [38] G.P. Kennedy, O. Buiu, S. Taylor, J. Appl. Phys. 85 (1999) 3319. [39] G. Tallarida, F. Cazzaniga, B. Crivelli, R. Zonca, M. Alessandri, J. Non-Cryst. Sol. 245 (1999) 210. [40] A. Borghesi, E. Bellandi, G. Guizetti, A. Sassella, S. Rojas, L. Zanotti, Appl. Phys. A 56 (1993) 147. [41] D. Landheer, G.C. Aers, G.I. Sproule, P. Khatri, P.J. Simpson, S.C. Gujrathi, J. Appl. Phys. 78 (1995) 1818. [42] P. Cova, R.A. Masut, O. Grenier, S. Poulin, J. Appl. Phys. 92 (2002) 129. [43] N. Shepherd, measurement of PLE spectra of sp-Si (2002), unpublished. [44] J. Polihronov, Doctoral Dissertation, University of Florida, 2002. [45] M. Stora, Doctoral Dissertation, University of Florida, 2000. [46] H. Liu, L.F. Fonseca, S.Z. Weisz, W. Jia, O. Resto, J. Lumin. 83–84 (1999) 37. [47] J. Kudrna, F. Trojanek, I. Pelant, S. Banas, V. Kohlova, P. Maly, Thin Solid Films 276 (1996) 58. [48] Y. Kanemitsu, Phys. Rev. B 49 (1994) 16845. [49] X. Zhao, O. Schoenfeld, S. Komuro, Y. Aoyagi, T. Sugano, Phys. Rev. B 50 (1994) 18654.
Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549 www.elsevier.com/locate/jpcs
Absorption and emission of light in spark-processed silicon$ J.G. Polihronova, M. Hedstro¨mb, H.-P. Chengb, R.E. Hummela, a
Department of Materials Science and Engineering, University of Florida, 216 Rhines Hall, Gainesville FL 32611-6400, USA b Quantum Theory Project, University of Florida, Gainesville FL 32611, USA Received 9 June 2005; received in revised form 31 October 2005; accepted 8 January 2006
Abstract Spark-processed Si (sp-Si) exhibits blue, green and red photoluminescence at around 385, 525 and 650 nm, depending on the wavelength of excitation. Its optical absorption spectrum reveals bands peaked approximately at 245, 277, 325 and 389 nm. The centers where absorption takes place were modeled as Si and silica clusters in an amorphous SiOxNy matrix using various embedding schemes. Geometry optimizations were applied prior to calculations of the absorption spectra of the clusters. The measured absorption spectrum of sp-Si and calculated absorption spectra were compared. Best agreement is achieved for Si particles embedded in amorphous SiOxNy matrix. The importance of the various embedding schemes is discussed and conclusions for the centers of emission are established. r 2006 Elsevier Ltd. All rights reserved. Keywords: A. Amorphous materials; A. Nanostructures; A. Optical materials; A. Oxides; D. Luminescence
Spark-processed Si (sp-Si) is an amorphous solid-state material exhibiting strong photoluminescence (PL) at 385 (blue), 525 (green) and 650 nm (red), depending on the excitation wavelength [1]. The bulk of sp-Si is a random mixture of four phases—mostly amorphous silicon dioxide (a-SiO2), amorphous silicon (a-Si), crystalline silicon (c-Si) and silicon nitrides in the form of SiNx or SiOxNy [2,3]. The porosity of the material can be considered to be a separate phase and is estimated to be in the neighborhood of 43% [4]. The light-emission from sp-Si is characterized by a remarkable stability against thermal annealing up to 1100 1C, etching in hydrofluoric acid, UV irradiation, and aging [2]. Since the emission of light originates from a Si-based material, the understanding of the properties of the lightemitting centers in sp-Si is of relevance to telecommunications technology and opto-electronic applications [5]. So far it has been experimentally shown, that the PL of sp-Si does not originate from hydroxyl groups [6], silanolrelated compounds [6,7], silicon carbides [7], defects in the $ Based on a dissertation submitted by J. Polihronov as a partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Florida. Corresponding author. Tel.: +1 (352) 392 6667; fax: +1 (352) 392 6359. E-mail address: [email protected]fl.edu (R.E. Hummel).
0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.121
a-SiO2 matrix [7], hydrogen-related compounds in the a-Si phase [7] or possible metallic ions, embedded in the material during the growth process [8]. It has been experimentally established as well that the process of light-emission in sp-Si differs substantially from the same phenomenon in porous Si [2]. The latest time-resolved PL measurements with blue and green-emitting sp-Si show PL decay times in the order of a few picoseconds [9,10], while the lifetimes of the PL in porous Si are three to six orders of magnitude longer [11]. Temperature-dependent PL measurements with sp-Si reveal shifts, which are opposite compared to the widening of the band gap in c-Si, a-Si and Si quantum dots [3]. Raman spectra of blue- and greenemitting sp-Si material are virtually identical and do not suggest the presence of Si particles with diameters of less than 15 nm [3]. Such Si dots are rather large and cannot contribute to the visible PL of sp-Si [12]. Besides, when Si is spark-processed in pure oxygen atmospheres the resulting material contains Si nanoparticles ([13], transmission electron micrographs) but does not emit light [9,10]. This paper presents computational models of the emitting centers in sp-Si, based largely on comparison with available experimental data. The experimental facts outlined above were considered when various molecular clusters were studied as possible contributors to the PL
ARTICLE IN PRESS 1544
J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
of sp-Si. Other experimentally established facts as listed below were also of key importance for the completion of the present work. First, it has been shown that the presence of both N and O gases in the spark-processing atmosphere is essential for the light emission process in sp-Si [9,10]. Second, the strongest PL has been found to originate from the a-SiO2–rich surface layers, which contain very limited amounts of N atoms and Si particles [2,3,14]. Third, it has been experimentally shown that for sp-Si grown in N-rich atmospheres, the stability [7] and the intensity [9,10] of the PL decreases. The bulk of the material consists of a dense framework of Si clusters, which does not produce PL [3]. All taken, the experimental observations indicate that the light-emitting centers in sp-Si are positioned in a-SiO2 matrix, containing Si clusters and N atoms. Since the PL in sp-Si is observed at reproducible wavelengths, the emitting centers are expected to have similar physical properties. It has to be noted also that sp-Si is a highly amorphous material and therefore the optically active centers, although similar throughout the bulk, are positioned in different atomic surroundings. They belong to the same family of molecular clusters and have similar physical characteristics, but do not have identical molecular geometries. The light-emitting centers in sp-Si were modeled as molecular clusters. The cluster geometries were based on previously published studies, having sufficient experimental and theoretical support for their existence. The a-SiO2 matrix was modeled as a network of silica rings (Fig. 1) which are formed by a number of interconnected SiO4 tetrahedra. The Si particles in sp-Si were modeled as Si clusters (Fig. 2) embedded in a-SiO2. Each Si cluster was bonded to the silica rings network and the resulting structure was allowed to relax. All dangling bonds were terminated with H atoms. Examples of two Si particles embedded in amorphous matrix are given in Figs. 3a and 4a. To the extent possible, all our structures utilize tetrahedral bonding on Si atoms with no dangling bonds allowed. Depending on the particular bonding situation, certain atoms of the Si particles are not bonded to the surrounding network, only to the Si cluster. Further details about the silica and Si rings as well as the other Si structures used in this work have been discussed in [15,16]. The geometries of all clusters in our work were optimized with the NDDO-based AM1 (Neglect of Diatomic Differential Overlap and the Austin Method 1) [17], followed by an INDO/CI calculation (Intermediate Neglect of Differential Overlap with Configuration Interaction, parameterized for spectroscopy) [18], which provided us with optical absorption spectra. The peaks in the spectra are assumed Gaussian with adopted peak width of DE0.4 eV reflecting a thermal line broadening. An absorption spectrum containing N transitions is then expressed as N 2 1 X f ðEÞ ¼ pffiffiffiffiffiffi pk eðEE ok =2DÞ , D 2p k¼1
(1)
where pk is the probability for a transition with energy Eok between the ground and kth excited state. The calculated spectra were compared to the absorption spectrum of sp-Si [19], obtained by differential reflectometry (DR) technique [20]. Using AM1 for geometry optimization of Si carbide molecules, we were able to predict bond lengths within a few percent of their established values. For a detailed description of the parameterization procedure of AM1 and its capabilities the reader is referred to [18,21]. INDO is based on the Hartree–Fock (HF) approximation for the system of valence electrons in a molecule. The method has successfully been used before for modeling of Si systems [22,23]. In addition, we have parameterized the INDO value of the resonance integral b for Si [15]. In the parameterization, we used 10 different Si-containing molecules and optimized b so that the electronic transitions were accurately reproduced. In the studied 10 molecules, Si atoms participated with both p- and s-bonding. We estimated the collective error of NDDO and INDO/CI for prediction of electronic transition energy to be 2000 cm1 (0.24 eV), in agreement with previously published results [18,24]. For many of the clusters we performed unrestricted HF (UHF) calculations. In general the UHF wave functions are not eigenfunctions to the spin operator S2, although we found by projecting the UHF wave functions onto wave functions representing pure spin multiplicities that the contamination from higher states were small. Therefore, the clusters studied in this work have been considered to have closed-shell singlet ground states and the restricted HF method was used in all of the calculations of spectra. The a-SiO2 phase in sp-Si was modeled [15] with 2-, 3-, 4-, 5- and 6-membered silica ring-shaped clusters. As expected, silica glass clusters do not exhibit absorption bands in the visible. The absorption threshold is located around 200 nm. By replacing an O atom with N atom in certain 2-membered silica rings, we observed an absorption peak at 245 nm, similarly to the experimental spectrum of sp-Si. Analysis of the probability amplitude for these electronic transitions indicated that they are preferentially located on certain N atoms. However, the other absorption bands of sp-Si were not reproduced and no electronic transitions below 320 nm were observed in the calculated spectra. The study of the a-SiO2 phase included nearly 100 clusters containing between 6 and 40 atoms each. The a-Si and c-Si phases were modeled [16] with 3-, 4-, 5and 6-membered OH-terminated Si rings and with OHterminated Si clusters and cages [25–30] with sizes between 3 and 14 atoms each. Generally, three resolved absorption peaks are observed in all spectra of Si clusters, lying in the interval between 200 and 350 nm.The peaks are positioned on average around 200, 260 and 320 nm. Additionally, small absorption bands were found between 400 and 500 nm for a substantial number of clusters. The above-described peaks are seen in the experimental spectrum of sp-Si as shoulders of a broad absorption band, peaked at 245 nm. This fact may be interpreted to suggest
ARTICLE IN PRESS J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
1545
Fig. 1. 3-, 4-, 5- and 6-membered silica clusters. The black atoms represent Si, the small atoms represent H and the gray atoms represent O.
that Si clusters are contributing to the optical properties of sp-Si. Still, the above peaks are resolved almost without exception in our calculations and therefore the small OHterminated Si clusters and rings alone cannot be used to explain the emission from sp-Si. The OH termination thus proved to be insufficient to describe the field of the surrounding matrix, which adds a non-negligible term in the potential that the cluster electrons will experience. As a next step, we improved our embedding scheme by positioning Si particles in a-SiOxNy matrix (one example is shown in Fig. 3a). In this section, we studied nearly 600 Si/a-SiOxNy structures. It should be noted that the x and y values have not been strictly defined as they vary from one cluster to another. x and y are characteristics of the matrix, surrounding the Si clusters and have meaning only when averaged over sufficiently large portion of the matrix volume. In the largest clusters of this study, the matrix was limited to silica rings with total of up to 100–180 atoms to guarantee computational efficiency. The unsatisfied bonds at the cluster borders were H-terminated. H does not have influence on the optical
properties of sp-Si [7] and in all optical calculations the electron transition amplitudes in the interval 200–350 nm were not positioned over H atoms. The calculated absorption spectra in this case remarkably well reproduced the experimental spectrum of sp-Si (Fig. 3b). An examination of the excitations in the 245–325 nm range shows that the transition from the highest occupied to the lowest unoccupied molecular orbitals (HOMO to LUMO) contributes largely. We find that the HOMO to a large extent is positioned on nearby O atoms in the matrix and the LUMO on the Si atoms of the Si particle as illustrated in Fig. 3a. This shows the importance of the Si clusters and the matrix in the absorption process. The agreement between theoretical prediction and experiment was generally achieved for any Si/a-SiOxNy structure and therefore the clusters in the figures represent only an example of this fact. The degree of similarity (that is, the w2 of the fit) between calculated and measured spectra varies for the different modeled structures, although the spectral features are similar. It should be emphasized at this point that PL is observed only if sp-Si is processed in mixtures of O and N gases (for
ARTICLE IN PRESS 1546
J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
Fig. 2. Representative sample of the Si clusters used in this work.
instance, air) [9,10] and excited at 325 nm. This suggests that either sp-Si produced in pure oxygen atmosphere does not have an absorption band at that excitation wavelength or the absorbed energy is lost in non-radiative transitions. For this reason it has been inferred [9,10] that N atoms play a crucial role in the optical properties of light-emitting sp-Si. However, this cannot be readily confirmed in our calculations. The calculated spectrum in Fig. 3 (no N involvement) already provides a fit to the experimental data with a w2 1. To study this point further, we calculated optical spectra of structures with Si2–N–O bonding at the Si/oxide interface [31,32]. Such configurations (Fig. 4) are justified, since a large number of experimental studies consistently confirm N atoms piling up at the dielectric/Si interface in aSiOxNy systems [33–41]. The study of the modified structures included a w2 statistical analysis of the fit between the calculated spectra and experiment. Its purpose was to provide a quantitative criterion for the reliability of each fit. In the w2 analysis, spectra of 9 different Si clusters between 3 and 14 atoms [25–30] were included. Four
different embedding schemes were applied to each cluster, namely: (i) Si cluster, embedded in a-SiOx; (ii) OHterminated Si cluster; (iii) Si cluster, embedded in a-SiOxNy with Si2–N bonding at Si/oxide interface; (iv) Si cluster, embedded in a-SiOxNy with Si2–N–O bonding at Si/oxide interface. Only the last two schemes allow N atoms into the studied structures. The value of w2 was determined by fitting only the portions of the spectra lying between 200 and 350 nm, since the main absorption peaks are positioned in this interval. It was found that the reliability of the fit increases with the size of the Si cluster, since w2 decreases almost within one order of magnitude as the Si cluster size increases from 3 to 14 atoms. The values of w2 are calculated with a certain error, which comes only from the NDO calculations (70.24 eV), while the experimental points are measured with an error of only 0.01 eV [20] and thus, do not influence the value of w2. At larger cluster sizes, the embedding scheme Si/a-SiOxNy seems to provide a slightly better numerical fit (a higher w2), compared to the scheme Si/SiOx. However, due to the error bars in w2 and
ARTICLE IN PRESS J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
1547
Fig. 3. (a) Si/SiOx cluster. The HOMO and LUMO isosurfaces for a transition at 252 nm are shown. (b) Comparison between the normalized measured DR spectrum of sp-Si (absorption) and the normalized calculated spectrum of the cluster. Its dark atoms represent O, the gray atoms represent Si, the small atoms represent H.
the very small differences between its values for all embedding schemes, it is not possible to eliminate any of the fits. Thus, small (within 0.15) differences in w2 of the fits were considered to be inconclusive. It is useful to note that we have built an absorption curve taken as a weighted average of the absorption plots of all clusters described in schemes (i)–(iv). While natural abundances of clusters can normally be used as a weighting criterion in such plots, this criterion may not be valid in our case due to the nature of spark-processing. Growing sp-Si involves fast cooling rates of molten material and non-equilibrium cluster structures can be produced and embedded in the sp-Si matrix. The cluster abundances in material may then not necessarily coincide with their natural abundances and for this reason we assumed all weights in our plot to be equal. The
resulting spectrum did not provide substantially different information than the individual cluster plots as all of them already have very small w2 values. One then needs to conclude that if all of the embedded clusters display spectra with comparable values of w2, they can be considered as potential contributors to the optical properties of sp-Si. Since this approach cannot precisely determine the role of N on the PL, modeling of other properties of the lightemitting material could be worthwhile considering if one would be confident that such properties indeed originate from the centers of emission, and not from the surrounding matrix. For instance, sp-Si exhibits ferromagnetic behavior, but it disappears after annealing at 600 1C, while the optical properties stay constant up to 1100 1C. Thus, modeling of the magnetic properties of Si particles in
ARTICLE IN PRESS 1548
J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
Fig. 4. (a) Si/SiOxNy cluster. (b) Comparison between the normalized measured DR spectrum of sp-Si (absorption) and the normalized calculated spectrum of the cluster. Its dark atoms represent O, the gray atoms represent Si, the small atoms represent H. N atoms are marked ‘N’.
a-SiOxNy would be irrelevant. Similarly, the resistance of the PL against HF etching cannot be ascribed to the emitting centers, but rather to the sp-Si matrix. It is known that the HF etch rate of a-SiOxNy decreases as the N concentration in the film increases [42]. It has always been pointed out that N atoms are essential for sp-Si PL. However, it is quite possible that N plays an important role in the dynamics during sp-Si growth by relieving strain fields and assisting the formation of smooth Si/a-SiOxNy interfaces, which are crucial in the processes of absorption and emission. Another aspect of our computational results for the sp-Si absorption is that together with certain experimental facts they can be used to suggest a model for the sp-Si emission: first, it has been observed that blue and green PL in sp-Si exhibits picosecond lifetimes [45]. Concurrently, picosecond decay times have been reported [46–49] for light emission from Si particles. Second, PL excitation spectra (PLE) of sp-Si have been found to exhibit a large peak in
the interval 245–325 nm [19,43,44], which coincides with the location of the main sp-Si absorption peak. Therefore, energy absorbed in this region is most efficiently converted into emitted light. Thus, according to our model electrons in supramolecular Si/a-SiOxNy clusters are excited with 245–325 nm photons followed by fast emission transitions with amplitudes positioned over the same Si/a-SiOxNy structures (modeled as clusters with diameter E1 nm) rather than depending on energy transfer to another distant locale where emission would take place. In conclusion, we have modeled the centers of lightabsorption in sp-Si utilizing silica and Si clusters in a number of embedding schemes. The spectra of certain nitrogen-substituted SiO2 structures reproduce the main absorption band of sp-Si at 245 nm, while the spectra of OH-terminated Si clusters exhibit three resolved bands between 200 and 350 nm. These two important approximations led us to the study of Si/a-SiOxNy systems. Their
ARTICLE IN PRESS J.G. Polihronov et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1543–1549
spectra reproduce very closely (w2 1) the observed spectrum of sp-Si and the agreement between calculation and experiment improves with increasing size of the Si clusters. Analysis of the calculated electronic transition amplitudes and of the available PLE data in the interval between 245 and 325 nm suggests a common origin of the absorption and emission in sp-Si, while time-resolved PL measurements indicate picosecond-order times of emission transitions, taking place over the Si particles in sp-Si. References [1] R.E. Hummel, S.-S. Chang, Appl. Phys. Lett. 61 (1992) 1965. [2] M. Ludwig, Crit. Rev. Sol. State Mater. Sci. 21 (1996) 265. [3] M.H. Ludwig, A. Augustin, R.E. Hummel, Th. Gross, J. Appl. Phys. 80 (1996) 5318. [4] J. Polihronov, T. Dubroca, M. Manuel, R.E. Hummel, Mater. Sci. Eng. B 107 (2004) 124. [5] N. Shepherd, R.E. Hummel, Phys. Stat. Sol. A 197 (2003) 222. [6] R.E. Hummel, M.H. Ludwig, J. Hack, S.-S. Chang, Solid State Commun. 96 (1995) 683. [7] R.E. Hummel, M.H. Ludwig, J. Lumin. 68 (1996) 69. [8] R.E. Hummel, N. Shepherd, M.H. Ludwig, M.E. Stora, Thin Solid Films 325 (1998) 1. [9] M. Stora, R.E. Hummel, J. Phys. Chem. Solids 63 (2002) 1867. [10] M. Stora, R.E. Hummel, J. Phys. Chem. Solids 63 (2002) 1655. [11] R.E. Hummel, M.H. Ludwig, S.-S. Chang, P.M. Fauchet, Ju.V. Vandyshev, L. Tsybeskov, Solid State Commun. 95 (1995) 553. [12] S. Rupp, J. Quilty, J. Trodahl, M.H. Ludwig, R.E. Hummel, Appl. Phys. Lett. 70 (1996) 723. [13] N. Shepherd, Doctoral Dissertation, University of Florida, 2002. [14] M.H. Ludwig, R.E. Hummel, A. Augustin, J. Hack, J. Menniger, Appl. Phys. Lett. 67 (1995) 2542. [15] J. Polihronov, M. Hedstro¨m, R.E. Hummel, H.-P. Cheng, J. Lumin. 96 (2002) 119. [16] J. Polihronov, R.E. Hummel, H.-P. Cheng, J. Lumin. 101 (2003) 55. [17] J.J.P. Stewart, MOPAC 2000.00 Manual, Fujitsu Ltd., Tokyo, Japan. [18] M.C. Zerner, in: K.B. Lipkowitz (Ed.), Reviews of Computational Chemistry, vol. 2, VCH Publishers, New York, 1991 (p. 313, Chapter 8). [19] R.E. Hummel, N. Sheperd, D. Burton, Appl. Phys. Lett. 79 (2001) 3218. [20] R.E. Hummel, Phys. Stat. Sol. A 76 (1983) 12. [21] J.J.P. Stewart, J. Computer-Aided Mol. Des. 4 (1990) 1. [22] M.J. Caldas, Phys. Stat. Sol. B 217 (2000) 641. [23] R.J. Baierle, M.J. Caldas, E. Molinari, S. Ossicini, Solid State Commun. 102 (1997) 545.
1549
[24] TedA. O’Brien, P.D. Rack, P.H. Holloway, M.C. Zerner, J. Lumin. 78 (1998) 245. [25] K. Raghavachari, C.M. Rohlfing, J. Chem. Phys. 89 (1998) 2219. [26] R. Fournier, S. Sinnott, A. DePristo, J. Chem. Phys. 97 (1992) 4149. [27] S. Wei, R.N. Barnett, U. Landman, Phys. Rev. B 55 (1997) 7935. [28] X. Jing, N. Troullier, D. Dean, N. Binggeli, J.R. Chelikowsky, Phys. Rev. B 50 (1994) 12234. [29] O. Sankey, D.J. Niklewski, D.A. Drabold, J.D. Dow, Phys. Rev. B 41 (1990) 12750. [30] A. Rubio, J.A. Alonso, X. Blase´, L.C. Balbas, S.G. Louie, Phys. Rev. Lett. 77 (1996) 247. [31] D. Bouvet, P.A. Clivaz, M. Dutoit, C. Coluzza, J. Almeida, G. Margaritondo, F. Pio, J. Appl. Phys. 79 (1996) 7114. [32] A. Teramoto, H. Umeda, H. Tamura, Y. Nishida, H. Sayama, K. Terada, K. Kawase, Y. Ohno, A. Shigetomi, J. Electrochem. Soc. 147 (2000) 1888. [33] R.P. Vasquez, A. Madhukar, F.J. Grunthaner, M.L. Naiman, J. Appl. Phys. 59 (1986) 972. [34] B. Brijs, J. Deleu, T. Conard, H. De Vitte, W. Vandervorst, K. Najajima, K. Kimura, I. Genchev, A. Bergmaier, L. Goergens, P. Neumaier, G. Dollinger, M. Dobeli, Nucl. Instr. Meth. Phys. Res. B 161–163 (2000) 429. [35] M.L. Monaghan, T. Nigam, M. Houssa, S. De Gendt, H.P. Urbach, P.K. De Bokx, Thin Solid Films 359 (2000) 197. [36] L. Itoh, Y. Hayamitzu, T. Abe, Mater. Sci. Eng. B 4 (1989) 309. [37] J.-W. Lee, N.-I. Lee, J.-I. Han, C.-H. Han, IEEE Electr. Dev. Lett. 18 (1997) 172. [38] G.P. Kennedy, O. Buiu, S. Taylor, J. Appl. Phys. 85 (1999) 3319. [39] G. Tallarida, F. Cazzaniga, B. Crivelli, R. Zonca, M. Alessandri, J. Non-Cryst. Sol. 245 (1999) 210. [40] A. Borghesi, E. Bellandi, G. Guizetti, A. Sassella, S. Rojas, L. Zanotti, Appl. Phys. A 56 (1993) 147. [41] D. Landheer, G.C. Aers, G.I. Sproule, P. Khatri, P.J. Simpson, S.C. Gujrathi, J. Appl. Phys. 78 (1995) 1818. [42] P. Cova, R.A. Masut, O. Grenier, S. Poulin, J. Appl. Phys. 92 (2002) 129. [43] N. Shepherd, measurement of PLE spectra of sp-Si (2002), unpublished. [44] J. Polihronov, Doctoral Dissertation, University of Florida, 2002. [45] M. Stora, Doctoral Dissertation, University of Florida, 2000. [46] H. Liu, L.F. Fonseca, S.Z. Weisz, W. Jia, O. Resto, J. Lumin. 83–84 (1999) 37. [47] J. Kudrna, F. Trojanek, I. Pelant, S. Banas, V. Kohlova, P. Maly, Thin Solid Films 276 (1996) 58. [48] Y. Kanemitsu, Phys. Rev. B 49 (1994) 16845. [49] X. Zhao, O. Schoenfeld, S. Komuro, Y. Aoyagi, T. Sugano, Phys. Rev. B 50 (1994) 18654.