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Quantum confinement effects in nano-silicon thin films
Solid State Communications, Vol. 108, No. 12, pp. 983–987, 1998 䉷 1998 Published by Elsevier Science Ltd 0038–1098/98 $ - see front matter
Pergamon
PII: S0038–1098(98)00478-5
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS Debajyoti Das Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700 032, India (Received 3 August 1998; accepted 1 September 1998 by C.N.R. Rao) Stacked layer Si:H films deposited by interrupted growth and H-plasma exposure were characterized by optical and IR absorption, Raman scattering, TEM and PL studies. Hydrogenation of the network and its structure could be precisely controlled. Extended H-plasma exposure demonstrated quantum confinement effects and the growth of nanostructures in silicon. 䉷 1998 Published by Elsevier Science Ltd Keywords: A. nanostructures, D. optical properties, D. quantum localization, E. luminescence.
Silicon nanocrystals embedded in amorphous matrix may create quantum dots where zero dimensional quantum confinement effects on electrons and holes reveal nonlinearities in optical properties. These optical nonlinearities are strongly dependent on the size and the size-distribution of the nanocrystals and are observable only when the nanocrystals are dimensionally in the order of the Bohr radius of the exciton. The observed phenomena of the visible photoluminescence in ultrafine Si particles [1–3] and microporous silicon [4, 5], resonant tunneling [6, 7] and optical gap widening [8] from ultrafine silicon structures have been attributed to the three dimensional quantum confinement effects on the photogenerated carriers in the Si nanostructures. It has stimulated to realize the potential of new optoelectronic devices based on Si-technology. Recently, fabrication of nanostructures in Si has drawn considerable attention in the field of ultra large scale integrated circuits [9] and quantum effect devices [10–12] where precise control in the size and position of the nano-Si dots are supposed to be very important. The effect of atomic H in the plasma on the growth and characteristics of Si:H are many fold. Hydrogen can terminate a Si dangling bond, may contribute large surface diffusion coefficient to the precursors by its coverage on the growing surface [13], may stabilize broken Si–Si bonds by passivating the created dangling bonds and permit reforming the rigid Si–Si bonds through its outdiffusion from the network by providing sufficient H-chemical potential to the growing surface [14] or by chemical annealing at the growth zone [15]. A
sufficiently high H 2-dilution at the plasma or a large H-chemical potential or an extended chemical annealing by atomic H at the growth zone in interrupted growth process leads to a transition from amorphous to microcrystalline Si:H structure. Interrupted growth and atomic H exposure on stacking layers has been found to be an effective technique in controlling the hydrogenation of the network and its structure [16–20]. The present report introduces some typical features of quantum confinement effects in Si:H thin films and demonstrates a novel deposition technique for precise and controlled growth of nanostructures in silicon. In a capacitively coupled RF glow discharge system the flow of SiH 4 was terminated periodically under steady H-plasma at regular intervals [19]. The growth of Si:H films was interrupted and H-plasma annealing at the growth zone was performed. Repetition of the cycle of film deposition and H-plasma treatment resulted stacked layer films. At a particular parametric condition of the plasma, the dose of the H-plasma annealing on the growing network could be controlled either by changing the plasma exposure time (t p s) or by regulating the ˚ ). At a substrate temperature stacking layer thickness (L A of 150⬚C, a set of films were prepared by controlling L and t p in steps. Nano-Si structure was obtained by proper optimization of stacking layer thickness (L) and H-plasma exposure time (t p). Introduction of interruption in growth, systematic reduction in stacking layer thickness and then the enhancement in the duration in plasma exposure resulted
983
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QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
Fig. 1. UV-visible absorption spectra for Si:H films prepared by interrupted growth and H-plasma exposure. Curve-1 is for the continuously deposited film, curves-2, 3, ˚ , 12 A ˚ and 5 A ˚ respectively 4 are for tp ¼ 30 s at L ¼ 20 A ˚. and curve-5 is for tp ¼ 50 s at L ¼ 5 A in a systematic lowering in the UV-visible absorption of the films as shown in Fig. 1. A simultaneous shift of the onset of optical absorption towards higher energy indicated gradual widening of the optical gap due to extended H-plasma exposure on stacking layers. Tauc’s plot in the absorption spectrum was found to shift almost parallely and the optical gaps of Si:H films were found to ˚ and increase from 1.74 eV to 2.01 eV for L ¼ 5 A tp ¼ 50 s. Infrared absorption studies were done on the H-plasma treated samples. On introduction of interruption in growth
Vol. 108, No. 12
and H-plasma treatment, the bonded H-content was found to reduce gradually from 17.5 at.% to 5.0 at.% ˚ and tp ¼ 50 s, as calculated from the for L ¼ 5 A wagging mode vibration of Si–H bonds around 630 cm ¹1. Simultaneous enhancement in polyhydride type of bonding was observed in the Si–H stretching ˚ and mode vibration as shown in Fig. 2. For L ¼ 5 A tp ¼ 50 s, stretching mode absorption band appeared exclusively around 2100 cm ¹1. The intensity of both the SiH 2 bending and (SiH 2) n (n ⬎ 1) wagging mode absorption around 880 cm ¹1 and 840 cm ¹1 respectively increased with H-plasma treatment, however, their relative intensity did not change much. Figure 3 shows the Raman scattering spectra of the films. Curve-a is the spectrum for the continuously deposited unlayered film which shows a broad spectrum peaked at about 480 cm ¹1 which corresponds to that for pure amorphous structure. In the film prepared by ˚, growth interruption and H-plasma exposure at L ¼ 12 A tp ¼ 30 s, the Si–Si bond transverse optical (TO) vibration at 512 cm ¹1 was observed along with a component of hump at around 480 cm ¹1 (curve-b), indicating the development of crystallinity embedded in amorphous matrix. However, the degree of crystallinity was enhanced remarkably on further increase in the dose of H-plasma ˚ and tp ¼ 50 s, contributing a lone treatment at L ¼ 5 A sharp Raman peak at 511 cm ¹1 (curve-c). Transmission electron microscope studies were done on the samples. The TEM micrograph was almost featureless and the electron diffraction pattern was a halo for the continuously deposited film. However, introduction of interruption in growth and H-plasma ˚ and tp ¼ 30 s contributed a low exposure at L ¼ 12 A density and homogeneous distribution of nano-grains
Fig. 2. IR transmission spectra for Si:H films prepared by interrupted growth and H-plasma exposure. Curve-1 is for ˚ , tp ¼ 50 s respectively. ˚ , tp ¼ 30 s and L ¼ 5 A the continuously deposited film. Curves-2 & 3 are for L ¼ 5 A
Vol. 108, No. 12
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
985
Fig. 3. Raman scattering spectra for Si:H films prepared by interrupted growth and H-plasma exposure. Curve-a is for the continuously deposited unlayered film. ˚, ˚ , tp ¼ 30 s and L ¼ 5 A Curves-b and c are for L ¼ 12 A tp ¼ 50 s, respectively. (15–20 nm in diameter) embedded in the amorphous matrix as observed in the TEM micrograph as shown in Fig. 4(a). Due to the enhancement in the dose of ˚ , tp ¼ 30 s), mostly high H-plasma exposure (at L ¼ 5 A density homogeneous and smaller size nanocrystallites (5–10 nm in diameter) were found to be distributed on the TEM micrograph [Fig. 4(b)]. A very wide gap (Eg ⬃ 2 eV) Si:H material having very low bonded H-content (CH ⬃ 5 at.%) obtained on the further increase ˚ , tp ¼ 50 s) in the dose of H-plasma treatment (at L ¼ 5 A contained a very dense and homogeneous distribution of nanocrystalline particles as observed in the TEM micrograph [Fig. 4(c)]. The very sharp rings exhibited by the corresponding electron diffraction pattern [Fig. 4(d)] represents (1 1 1), (2 2 0) and (3 1 1) planes of Si crystals. The (4 0 0), (3 3 1) and (4 2 2) planes of crystal Si were also identified by the less prominent electron diffraction rings. The increase in the optical gap with the decrease in the bonded H-content and a simultaneous reduction in the size of the nanocrystals along with their increase in the volume fraction could be explained qualitatively by the quantum size effect. Films were studied by high resolution photoluminescence, using a commercial Fourier transform photoluminescence spectrometer supplied by Midac Corporation, U.S.A. The excitation source was the argon-ion laser emitting 514.5 nm wavelength and a laser power of 50 mW was used for the experiment. For PL studies, films were deposited on single crystal Si wafers. Figure 5 represents the typical PL spectrum of the nano-silicon film prepared by layer-by-layer deposition through interrupted growth and H-plasma exposure. A weak broad PL around 0.9 eV is the typical of PL in a-Si:H matrix and has been ascribed normally
Fig. 4. TEM micrograph of nano-Si films prepared by interrupted growth and H-plasma exposure at (a) L ¼ ˚ , tp ¼ 30 s, (c) L ¼ 5 A ˚, ˚ , tp ¼ 30 s, (b) L ¼ 5 A 12 A tp ¼ 50 s and (d) the electron diffraction pattern of ˚ , tp ¼ 50 s. nano-Si film prepared at L ¼ 5 A
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QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
Fig. 5. A typical photoluminescence spectrum of the stacked layer nano-Si film prepared by interrupted growth and H-plasma exposure. due to the dangling bonds in a-Si. A strong PL in the region 1.3 eV–1.5 eV was observed with a peak centered at around 1.4 eV. The width of the spectrum was about 150 meV and the FWHM was around 70 meV. Fine structure of this PL peak splitting into five satellite peaks with a separation of ⬃22 meV was observed specially at low temperatures e.g. at 4.2 K. The well resolved satellite peaks were approximately at around 1.370, 1.391, 1.411, 1.433 and 1.455 eV. The fine structure observed at low temperature turned to be less and less distinct with rise in temperature and finally disappeared at about 50 K. The individual peaks merged into a broad band centered at around 1.4 eV. In addition to these, another transition was observed at 1.545 eV. The very intense PL peak at 1.093 eV corresponds to the exciton bound to boron impurity in the silicon single crystal substrate. The existence of a fine structure in the PL spectrum at around 1.4 eV at low temperature appears to be significant from the structural aspect of the films. It evidences the development of nano-structures in the layer by layer deposited Si:H films prepared by growth interruption and H-plasma exposure. Rao et al. [21] had observed an almost identical fine structure in 1.4 eV luminescence band at 4.2 K in thin film PECVD layers deposited from highly H 2 diluted SiH 4 where they suggested a strong possibility of the formation of nanocrystalline network. Brodsky et al. [22] suggested that, this luminescence can be due to the quantum confinement of carriers in Si between barriers of Si–H. Widening of optical gap accompanied by an elimination of bonded hydrogen from the network appears to be an unique feature in Si:H. Gradual shift of the onset of optical absorption towards higher energy leading to the widening of optical gap is believed to be the manifestation of nanocrystallization and the associated
Vol. 108, No. 12
three dimensional quantum confinement of carriers in the electron hole system. IR absorption spectra and Raman scattering spectra both revealed the transformation from amorphous to micro/nano network structure and the electron micrographs clearly identified the growth of nanograins which increased in volume fraction and reduced in size, demonstrating the quantum size effects on the optical gap. In addition, the existence of an intense PL spectrum at 1.4 eV with a well resolved fine structure at low temperature evidences the three dimensional quantum confinement effects in Si nano-structures in the layer by layer deposited Si:H films. Thus interrupted growth and H-plasma exposure has been demonstrated as a novel deposition technique for the growth of nano structures in silicon. Acknowledgements—The author is grateful to Professor A.K. Barua for his support and constant encouragement during this work and Dr. K.S.R.K. Rao and Professor A.K. Sreedhar of the Dept. of Physics, I.I.Sc., Bangalore, for providing the photoluminescence measurement facility. REFERENCES 1. Furukawa, S. and Miyasato, T., Superlatt. Microstruct., 5, 1989, 317. 2. Takagi, H., Ogawa, H., Yamazaki, Y., Isnizaki, A. and Nakagiri, T., Appl. Phys. Lett., 56, 1990, 2379. 3. Liu, X.N., Wu, X.W., Bao, X.M. and He, Y.L., Appl. Phys. Lett., 64, 1994, 220. 4. Canham, L.T., Appl. Phys. Lett., 57, 1990, 1046. 5. Lehmann, V. and Gosele, U., Appl. Phys. Lett., 58, 1991, 856. 6. Fortunate, E., Martins, R., Ferreira, I., Santos, M., Marcario, A. and Guimaraes, I., J. Non-Cryst. Solids, 115, 1989, 120. 7. Tue, R., Ye, Q.Y. and Nicollian, E.H., SPIE, 1361, 1990, 232. 8. Furukawa, S. and Miyasato, T., Phys. Rev., B38, 1988, 5726. 9. Ono, M., Saito, M., Yoshitomi, T., Feigna, C., Ohguro, T. and Iwai, H., in IEDM Tech. Dig. (IEEE), Washington DC, 1993, pp. 119. 10. Morimoto, K., Hirai, Y., Inoue, K., Niwa, M. and Yasui, J., in Ext. Abstr. Int. Conf. Solid State Devices & Materials, Makuhari (Japan), 1993, pp. 344. 11. Yano, K., Ishii, T., Hashimoto, T., Kobayashi, T., Murai, F. and Seki, K., in IEDM Tech. Dig. (IEEE), Washington DC, 1993, pp. 541. 12. Muto, S., Jpn. J. Appl. Phys., 34, 1995, L210. 13. Matsuda, A. and Goto, T., Mater. Res. Soc. Symp. Proc., 164, 1990, 3. 14. Street, R.A., Phys. Rev., B44, 1991, 10610. 15. Shimizu, I., J. Non-Cryst. Solids, 114, 1989, 145. 16. Asano, A., Appl. Phys. Letts., 56, 1990, 533. 17. Das, D., Shirai, H., Hanna, J. and Shimizu, I., Jpn. J. Appl. Phys., 30, 1991, L239.
Vol. 108, No. 12
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
18. Shirai, H., Das, D., Hanna, J. and Shimizu, I., Appl. Phys. Lett., 59, 1991, 1096. 19. Das, D., Phys. Rev., B51, 1995, 10729. 20. Das, D., Bull. Mater. Sci., 20, 1997, 9.
987
21. Rao, K.S.R.K., Sreedhar, A.K., Bhat, H.L., Singh, R.A., Dubey, G.C. and Kumar, V., Appl. Phys. Lett., 68, 1996, 1458. 22. Brodsky, M.H., Solid State Commun., 36, 1980, 55.
Pergamon
PII: S0038–1098(98)00478-5
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS Debajyoti Das Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700 032, India (Received 3 August 1998; accepted 1 September 1998 by C.N.R. Rao) Stacked layer Si:H films deposited by interrupted growth and H-plasma exposure were characterized by optical and IR absorption, Raman scattering, TEM and PL studies. Hydrogenation of the network and its structure could be precisely controlled. Extended H-plasma exposure demonstrated quantum confinement effects and the growth of nanostructures in silicon. 䉷 1998 Published by Elsevier Science Ltd Keywords: A. nanostructures, D. optical properties, D. quantum localization, E. luminescence.
Silicon nanocrystals embedded in amorphous matrix may create quantum dots where zero dimensional quantum confinement effects on electrons and holes reveal nonlinearities in optical properties. These optical nonlinearities are strongly dependent on the size and the size-distribution of the nanocrystals and are observable only when the nanocrystals are dimensionally in the order of the Bohr radius of the exciton. The observed phenomena of the visible photoluminescence in ultrafine Si particles [1–3] and microporous silicon [4, 5], resonant tunneling [6, 7] and optical gap widening [8] from ultrafine silicon structures have been attributed to the three dimensional quantum confinement effects on the photogenerated carriers in the Si nanostructures. It has stimulated to realize the potential of new optoelectronic devices based on Si-technology. Recently, fabrication of nanostructures in Si has drawn considerable attention in the field of ultra large scale integrated circuits [9] and quantum effect devices [10–12] where precise control in the size and position of the nano-Si dots are supposed to be very important. The effect of atomic H in the plasma on the growth and characteristics of Si:H are many fold. Hydrogen can terminate a Si dangling bond, may contribute large surface diffusion coefficient to the precursors by its coverage on the growing surface [13], may stabilize broken Si–Si bonds by passivating the created dangling bonds and permit reforming the rigid Si–Si bonds through its outdiffusion from the network by providing sufficient H-chemical potential to the growing surface [14] or by chemical annealing at the growth zone [15]. A
sufficiently high H 2-dilution at the plasma or a large H-chemical potential or an extended chemical annealing by atomic H at the growth zone in interrupted growth process leads to a transition from amorphous to microcrystalline Si:H structure. Interrupted growth and atomic H exposure on stacking layers has been found to be an effective technique in controlling the hydrogenation of the network and its structure [16–20]. The present report introduces some typical features of quantum confinement effects in Si:H thin films and demonstrates a novel deposition technique for precise and controlled growth of nanostructures in silicon. In a capacitively coupled RF glow discharge system the flow of SiH 4 was terminated periodically under steady H-plasma at regular intervals [19]. The growth of Si:H films was interrupted and H-plasma annealing at the growth zone was performed. Repetition of the cycle of film deposition and H-plasma treatment resulted stacked layer films. At a particular parametric condition of the plasma, the dose of the H-plasma annealing on the growing network could be controlled either by changing the plasma exposure time (t p s) or by regulating the ˚ ). At a substrate temperature stacking layer thickness (L A of 150⬚C, a set of films were prepared by controlling L and t p in steps. Nano-Si structure was obtained by proper optimization of stacking layer thickness (L) and H-plasma exposure time (t p). Introduction of interruption in growth, systematic reduction in stacking layer thickness and then the enhancement in the duration in plasma exposure resulted
983
984
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
Fig. 1. UV-visible absorption spectra for Si:H films prepared by interrupted growth and H-plasma exposure. Curve-1 is for the continuously deposited film, curves-2, 3, ˚ , 12 A ˚ and 5 A ˚ respectively 4 are for tp ¼ 30 s at L ¼ 20 A ˚. and curve-5 is for tp ¼ 50 s at L ¼ 5 A in a systematic lowering in the UV-visible absorption of the films as shown in Fig. 1. A simultaneous shift of the onset of optical absorption towards higher energy indicated gradual widening of the optical gap due to extended H-plasma exposure on stacking layers. Tauc’s plot in the absorption spectrum was found to shift almost parallely and the optical gaps of Si:H films were found to ˚ and increase from 1.74 eV to 2.01 eV for L ¼ 5 A tp ¼ 50 s. Infrared absorption studies were done on the H-plasma treated samples. On introduction of interruption in growth
Vol. 108, No. 12
and H-plasma treatment, the bonded H-content was found to reduce gradually from 17.5 at.% to 5.0 at.% ˚ and tp ¼ 50 s, as calculated from the for L ¼ 5 A wagging mode vibration of Si–H bonds around 630 cm ¹1. Simultaneous enhancement in polyhydride type of bonding was observed in the Si–H stretching ˚ and mode vibration as shown in Fig. 2. For L ¼ 5 A tp ¼ 50 s, stretching mode absorption band appeared exclusively around 2100 cm ¹1. The intensity of both the SiH 2 bending and (SiH 2) n (n ⬎ 1) wagging mode absorption around 880 cm ¹1 and 840 cm ¹1 respectively increased with H-plasma treatment, however, their relative intensity did not change much. Figure 3 shows the Raman scattering spectra of the films. Curve-a is the spectrum for the continuously deposited unlayered film which shows a broad spectrum peaked at about 480 cm ¹1 which corresponds to that for pure amorphous structure. In the film prepared by ˚, growth interruption and H-plasma exposure at L ¼ 12 A tp ¼ 30 s, the Si–Si bond transverse optical (TO) vibration at 512 cm ¹1 was observed along with a component of hump at around 480 cm ¹1 (curve-b), indicating the development of crystallinity embedded in amorphous matrix. However, the degree of crystallinity was enhanced remarkably on further increase in the dose of H-plasma ˚ and tp ¼ 50 s, contributing a lone treatment at L ¼ 5 A sharp Raman peak at 511 cm ¹1 (curve-c). Transmission electron microscope studies were done on the samples. The TEM micrograph was almost featureless and the electron diffraction pattern was a halo for the continuously deposited film. However, introduction of interruption in growth and H-plasma ˚ and tp ¼ 30 s contributed a low exposure at L ¼ 12 A density and homogeneous distribution of nano-grains
Fig. 2. IR transmission spectra for Si:H films prepared by interrupted growth and H-plasma exposure. Curve-1 is for ˚ , tp ¼ 50 s respectively. ˚ , tp ¼ 30 s and L ¼ 5 A the continuously deposited film. Curves-2 & 3 are for L ¼ 5 A
Vol. 108, No. 12
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
985
Fig. 3. Raman scattering spectra for Si:H films prepared by interrupted growth and H-plasma exposure. Curve-a is for the continuously deposited unlayered film. ˚, ˚ , tp ¼ 30 s and L ¼ 5 A Curves-b and c are for L ¼ 12 A tp ¼ 50 s, respectively. (15–20 nm in diameter) embedded in the amorphous matrix as observed in the TEM micrograph as shown in Fig. 4(a). Due to the enhancement in the dose of ˚ , tp ¼ 30 s), mostly high H-plasma exposure (at L ¼ 5 A density homogeneous and smaller size nanocrystallites (5–10 nm in diameter) were found to be distributed on the TEM micrograph [Fig. 4(b)]. A very wide gap (Eg ⬃ 2 eV) Si:H material having very low bonded H-content (CH ⬃ 5 at.%) obtained on the further increase ˚ , tp ¼ 50 s) in the dose of H-plasma treatment (at L ¼ 5 A contained a very dense and homogeneous distribution of nanocrystalline particles as observed in the TEM micrograph [Fig. 4(c)]. The very sharp rings exhibited by the corresponding electron diffraction pattern [Fig. 4(d)] represents (1 1 1), (2 2 0) and (3 1 1) planes of Si crystals. The (4 0 0), (3 3 1) and (4 2 2) planes of crystal Si were also identified by the less prominent electron diffraction rings. The increase in the optical gap with the decrease in the bonded H-content and a simultaneous reduction in the size of the nanocrystals along with their increase in the volume fraction could be explained qualitatively by the quantum size effect. Films were studied by high resolution photoluminescence, using a commercial Fourier transform photoluminescence spectrometer supplied by Midac Corporation, U.S.A. The excitation source was the argon-ion laser emitting 514.5 nm wavelength and a laser power of 50 mW was used for the experiment. For PL studies, films were deposited on single crystal Si wafers. Figure 5 represents the typical PL spectrum of the nano-silicon film prepared by layer-by-layer deposition through interrupted growth and H-plasma exposure. A weak broad PL around 0.9 eV is the typical of PL in a-Si:H matrix and has been ascribed normally
Fig. 4. TEM micrograph of nano-Si films prepared by interrupted growth and H-plasma exposure at (a) L ¼ ˚ , tp ¼ 30 s, (c) L ¼ 5 A ˚, ˚ , tp ¼ 30 s, (b) L ¼ 5 A 12 A tp ¼ 50 s and (d) the electron diffraction pattern of ˚ , tp ¼ 50 s. nano-Si film prepared at L ¼ 5 A
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QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
Fig. 5. A typical photoluminescence spectrum of the stacked layer nano-Si film prepared by interrupted growth and H-plasma exposure. due to the dangling bonds in a-Si. A strong PL in the region 1.3 eV–1.5 eV was observed with a peak centered at around 1.4 eV. The width of the spectrum was about 150 meV and the FWHM was around 70 meV. Fine structure of this PL peak splitting into five satellite peaks with a separation of ⬃22 meV was observed specially at low temperatures e.g. at 4.2 K. The well resolved satellite peaks were approximately at around 1.370, 1.391, 1.411, 1.433 and 1.455 eV. The fine structure observed at low temperature turned to be less and less distinct with rise in temperature and finally disappeared at about 50 K. The individual peaks merged into a broad band centered at around 1.4 eV. In addition to these, another transition was observed at 1.545 eV. The very intense PL peak at 1.093 eV corresponds to the exciton bound to boron impurity in the silicon single crystal substrate. The existence of a fine structure in the PL spectrum at around 1.4 eV at low temperature appears to be significant from the structural aspect of the films. It evidences the development of nano-structures in the layer by layer deposited Si:H films prepared by growth interruption and H-plasma exposure. Rao et al. [21] had observed an almost identical fine structure in 1.4 eV luminescence band at 4.2 K in thin film PECVD layers deposited from highly H 2 diluted SiH 4 where they suggested a strong possibility of the formation of nanocrystalline network. Brodsky et al. [22] suggested that, this luminescence can be due to the quantum confinement of carriers in Si between barriers of Si–H. Widening of optical gap accompanied by an elimination of bonded hydrogen from the network appears to be an unique feature in Si:H. Gradual shift of the onset of optical absorption towards higher energy leading to the widening of optical gap is believed to be the manifestation of nanocrystallization and the associated
Vol. 108, No. 12
three dimensional quantum confinement of carriers in the electron hole system. IR absorption spectra and Raman scattering spectra both revealed the transformation from amorphous to micro/nano network structure and the electron micrographs clearly identified the growth of nanograins which increased in volume fraction and reduced in size, demonstrating the quantum size effects on the optical gap. In addition, the existence of an intense PL spectrum at 1.4 eV with a well resolved fine structure at low temperature evidences the three dimensional quantum confinement effects in Si nano-structures in the layer by layer deposited Si:H films. Thus interrupted growth and H-plasma exposure has been demonstrated as a novel deposition technique for the growth of nano structures in silicon. Acknowledgements—The author is grateful to Professor A.K. Barua for his support and constant encouragement during this work and Dr. K.S.R.K. Rao and Professor A.K. Sreedhar of the Dept. of Physics, I.I.Sc., Bangalore, for providing the photoluminescence measurement facility. REFERENCES 1. Furukawa, S. and Miyasato, T., Superlatt. Microstruct., 5, 1989, 317. 2. Takagi, H., Ogawa, H., Yamazaki, Y., Isnizaki, A. and Nakagiri, T., Appl. Phys. Lett., 56, 1990, 2379. 3. Liu, X.N., Wu, X.W., Bao, X.M. and He, Y.L., Appl. Phys. Lett., 64, 1994, 220. 4. Canham, L.T., Appl. Phys. Lett., 57, 1990, 1046. 5. Lehmann, V. and Gosele, U., Appl. Phys. Lett., 58, 1991, 856. 6. Fortunate, E., Martins, R., Ferreira, I., Santos, M., Marcario, A. and Guimaraes, I., J. Non-Cryst. Solids, 115, 1989, 120. 7. Tue, R., Ye, Q.Y. and Nicollian, E.H., SPIE, 1361, 1990, 232. 8. Furukawa, S. and Miyasato, T., Phys. Rev., B38, 1988, 5726. 9. Ono, M., Saito, M., Yoshitomi, T., Feigna, C., Ohguro, T. and Iwai, H., in IEDM Tech. Dig. (IEEE), Washington DC, 1993, pp. 119. 10. Morimoto, K., Hirai, Y., Inoue, K., Niwa, M. and Yasui, J., in Ext. Abstr. Int. Conf. Solid State Devices & Materials, Makuhari (Japan), 1993, pp. 344. 11. Yano, K., Ishii, T., Hashimoto, T., Kobayashi, T., Murai, F. and Seki, K., in IEDM Tech. Dig. (IEEE), Washington DC, 1993, pp. 541. 12. Muto, S., Jpn. J. Appl. Phys., 34, 1995, L210. 13. Matsuda, A. and Goto, T., Mater. Res. Soc. Symp. Proc., 164, 1990, 3. 14. Street, R.A., Phys. Rev., B44, 1991, 10610. 15. Shimizu, I., J. Non-Cryst. Solids, 114, 1989, 145. 16. Asano, A., Appl. Phys. Letts., 56, 1990, 533. 17. Das, D., Shirai, H., Hanna, J. and Shimizu, I., Jpn. J. Appl. Phys., 30, 1991, L239.
Vol. 108, No. 12
QUANTUM CONFINEMENT EFFECTS IN NANO-SILICON THIN FILMS
18. Shirai, H., Das, D., Hanna, J. and Shimizu, I., Appl. Phys. Lett., 59, 1991, 1096. 19. Das, D., Phys. Rev., B51, 1995, 10729. 20. Das, D., Bull. Mater. Sci., 20, 1997, 9.
987
21. Rao, K.S.R.K., Sreedhar, A.K., Bhat, H.L., Singh, R.A., Dubey, G.C. and Kumar, V., Appl. Phys. Lett., 68, 1996, 1458. 22. Brodsky, M.H., Solid State Commun., 36, 1980, 55.