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Room temperature visible photoluminescence from crystallized nano-Si thin films
Journal of Non-Crystalline Solids 227–230 Ž1998. 1045–1048
Room temperature visible photoluminescence from crystallized nano-Si thin films Wei Wu b
a,b,)
, Xinfan Huang a , Kunji Chen a , Jian Bin Xu b, Xiang Gao a , Jun Xu a , Wei Li a
a Department of Physics, Nanjing UniÕersity, Nanjing 210093, China Department of Electronic Engineering, The Chinese UniÕersity of Hong Kong, Shatin, N.T., Hong Kong, China
Abstract We report the observation of visible-light emission from crystallized a-Si:H thin films and a-Si:Hra-SiN x :H multi-quantum well ŽMQW. structures, which were deposited on SiO 2rSi substrates by a plasma enhanced chemical vapor deposition system and subsequently crystallized by a KrF excimer laser. Transmission electron microscopy technology revealed the formation of fine grain-sized nanocrystallites with diameter about 10 nm. X-ray diffraction and Raman scattering spectra showed spectral peaks corresponding to crystalline Si Žc-Si. Ž111. face. Room temperature visible photoluminescence ŽPL. emission was observed both from crystallized a-Si:H films and multi-quantum well structures, with peak emission energy around 2.0 eV. The ability to fabricate visible luminescent Si films by a method compatible with the currently mature Si-based microelectronics technology, can provide a promising means in the realization of optoelectronic devices. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Photoluminescence ŽPL.; a-Si:Hra-SiN x :H; Plasma enhanced chemical vapor deposition ŽPECVD.
1. Introduction Since the observation of efficient room temperature photoluminescence ŽPL. from porous silicon ŽPS. in 1990 w1x, materials with nanometer-sized silicon crystallites have attracted interest. Although the radiative recombination processes involved are still debated, several theoretical investigations show that by confining silicon particles small enough to quantize the electronic states, there will consequently be the probability of a large increase of the effective
)
Corresponding author. Tel.: q852-2609 8257; fax: q8522603 5558; e-mail: [email protected].
band gap and a more efficient radiative recombination w2–4x. Therefore, the luminescent properties will be altered. Most of the previous reports had discussed electrochemically etched Si materials. This kind of electrochemical dissolution method is based on wet synthesis w5x. The prepared PS materials had very complicated structures and poor stability. Efforts have then been directed to the materials prepared by dry methods to improve both the stability and efficiency of light emission. We report the observation of room temperature visible PL from crystallized a-Si:H thin film and a-Si:Hra-SiN x :H MQWs deposited by plasma enhanced chemical vapor deposition ŽPECVD. system on SiO 2rSi substrate, using a crys-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 4 5 - 2
1046
W. Wu et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1045–1048
tallization technique with a KrF excimer laser. The laser-induced crystallization method can provide several advantages such as selected area processing, rapid crystallization, and avoidance of the surface contamination which plays a significant role in the strong visible luminescence from porous Si layers w6x. In addition, since the absorption coefficient of a-Si thin films is quite high in the ultraviolet ŽUV.light region of the excimer laser, it will be possible to crystallize a-Si layers without thermal alterations to the silicon substrate. The ability to manufacture luminescent Si films on the SiO 2rSi substrate by methods compatible with the mature Si microelectronics technology will give a new possibility in optoelectronics devices and allow monolithic integration of Si technology to combine with optical signal processing.
2. Experimental procedures a-Si:H thin film and a-Si:Hra-SiN x :H MQWs were deposited on SiO 2rSi substrate by PECVD at the substrate temperature of 2508C and the deposition rate of 0.1 nmrs under a total reaction pressure of 36 Pa. The SiO 2rSi substrate was prepared by means of thermal oxidation of Ž100. Si wafers with the dioxide layer of about 700 nm. The deposited a-Si:H film was 100 nm thick. In the MQW structures, the a-SiN x :H barrier layer thickness L N was grown to 10 nm, and the a-Si:H well layer L W was 4 nm. During the deposition of the MQW structures, the composition of the reactant gases was alternated periodically between pure silane and a silane–ammonia mixture by a computer without interrupting the plasma. Other details of sample preparation have been reported elsewhere w7x. The crystallization process was performed inside a vacuum chamber by a KrF Ž l s 248 nm. excimer laser at room temperature, with a pulse duration of 30 ns and energy densities ranging from 0.13 Jrcm2 to 1.2 Jrcm2 . The laser-beam size was 11 = 4 mm2 on the film surface. Microstructure and grain size of the Si particles were analyzed by transmission electron microscopy ŽTEM.. X-ray diffraction ŽXRD. and Raman scattering were used to study the crystallinity. The PL spectral measurements were made
via excitation from an Arq laser at a wavelength of 488 nm.
3. Results Fig. 1 shows the XRD spectrum of the sample irradiated at an energy density of 560 mJrcm2 with a single pulse. A diffraction peak appears at 2 u s 28.5158 corresponding to the diffraction of the Ž111. face of c-Si. There are also two weak peaks at 47.4398 and 56.7898, coinciding with the diffraction of Ž220. and Ž311. faces of c-Si, respectively. These results show that the sample has been crystallized and has a preferred direction of ²111:. The PL spectrum of the crystallized film is shown in Fig. 2. It demonstrates a PL peak centered at 610 nm Ž2.03 eV.. The full width at half maximum ŽFWHM. of the PL peak is about 0.4 eV. We could not detect any visible luminescence from a SiO 2rSi substrate and the as-deposited samples. A planar-view TEM micrograph of this crystallized sample is shown in Fig. 3. It reveals a structure composed of a large number of very fine Si crystallites with an average diameter of about 10 nm. After the observation of visible PL from the nano-crystalline Si thin films, we found it is difficult to control the grain size and the size distribution of the crystallized films. To achieve a controllable grain size and distribution is still a technical challenge. So
Fig. 1. X-ray diffraction spectrum of crystallized a-Si:H thin film.
W. Wu et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1045–1048
Fig. 2. Photoluminescence spectrum of the crystallized a-Si:H film at room temperature.
we performed a further experiment to study the possibility of visible luminescence due to one-dimensional quantum confinement of thin a-Si:H well layers between a-SiN x :H barrier layers. To fabricate layers of nanometer-sized Si crystallites, the as-deposited MQW structures were subsequently annealed using an excimer laser. Fig. 4 shows a Raman spectrum of the MQW sample annealed at the energy density of 950 mJrcm2 with a single pulse. Near the TO peak of the crystalline Si Žc-Si., a band positioned at a wave number
Fig. 3. A planar-view TEM micrograph of the crystallized a-Si:H thin film.
1047
Fig. 4. Raman scattering spectrum of the crystallized a-Si:HraSiN x :H MQW structure with L W s 4.0 nm.
of 514 cmy1 appears. This band also indicates that the sample has been crystallized. The PL spectrum of the same crystallized MQW sample at room temperature is depicted in Fig. 5. It shows a PL emission from the crystallized sample with a well-layer thickness of 4.0 nm, where the dominant emission at 640 nm was found. To further identify the origin of the
Fig. 5. Room temperature photoluminescence spectrum of the crystallized MQW structure with a-Si:H well-layer thickness of 4.0 nm.
1048
W. Wu et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1045–1048
visible PL, we measured the PL of the as-deposited samples. No visible PL was found at room temperature.
4. Discussion At present, there is not a unique view of the mechanisms responsible for the PL in nanometersized silicon structures. From the TEM micrographs of the crystallized a-Si:H thin film ŽFig. 3. and the MQW structures w7x, we can not find interconnected pores or voids as formed in porous silicon. It is unlikely that these structures can play a key role in the visible luminescence. After comparing both the experimental results of the distinct PL peaks at different ranges of the emission spectrum and the quantum-confinement models w8–10x, we speculated that such a PL is due to the formation of nanometer-sized silicon crystallites. In the process of high laser energy density irradiation of the a-Si:H thin film, thermal effects to the underlayered SiO 2 film are possible. This effect leads to increasing the roughness at the interface between the crystallized a-Si:H thin film and the SiO 2 layer w11x. Hence, a large number of Si nucleation sites would be formed. Space is not available for the regrowth of these melted Si, and it is difficult to increase sizes of grains, as shown in the TEM micrograph of Fig. 3. The grain boundary would play the role as potential barrier in the crystallized structure. In crystallized MQW structures, nanometer Si crystallites were formed in the well layers due to the constrained crystallization between a-SiN x :H barrier layers w7x. Some theoretical models have been proposed to explain the origin of this visible light emission of Si crystallites at room temperature and the energy gap dependence on crystal size w12,13x. The results show that the obtained highly confined Si structures would exhibit a blue shift and an increase in quantum efficiency of the radiation emission. Therefore, the nanometer-sized structure is expected to generate optical transitions in the visible light region. Currently we are improving the emission efficiency and intensity, and trying to find the reason for the blue shift of the PL peak.
5. Conclusions Compared with the previous reports of visible luminescence from ultrafine Si nanometer films prepared by PECVD methods w14x, exhibiting a PL peak at low temperature Ž27 K. and no distinct peak at room temperature, our method seems to be more reasonable for future applications. The ability to fabricate Si films with visible photoluminescent properties on SiO 2rSi substrate by a method which is compatible with the currently mature Si-based microelectronics technology, can provide a promising means in optoelectronics and allow monolithic integration of silicon technology to combine with optical signal processing.
Acknowledgements We would like to express our thanks to Dr. Xiao Yuan Chen and Prof. Zhiguo Liu of Nanjing University for their technical assistance and useful discussion. This work is supported by the National Nature Science Foundation of China.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x
L.T. Canham, Appl. Phys. Lett. 59 Ž1990. 1046. B. Delley, E.F. Steigeier, Phys. Rev. B 47 Ž1993. 1397. M.J. Estes, G. Moddel, Appl. Phys. Lett. 68 Ž1996. 1814. M. Yamaguchi, K. Morigaki, Phys. Rev. B 55 Ž4. Ž1997. 2368. N. Koshida, H. Koyama, Appl. Phys. Lett. 60 Ž1992. 347. N. Sato, K. Sakaguchi, K. Yamagata, Y. Fuiyama, Electrochem. Soc. 142 Ž1995. 3116. K.J. Chen, X.F. Huang, J. Xu, D. Feng, Appl. Phys. Lett. 61 Ž1992. 2069. T. Toyama, T. Matsui, Appl. Phys. Lett. 69 Ž1996. 1261. G. Cicala, P. Capezzuto, G. Bruno, J. Appl. Phys. 80 Ž1996. 6564. B.T. Sullivan, D.J. Lockwood, H.J. Labbe, Appl. Phys. Lett. 69 Ž1996. 3149. G.K. Giust, T.W. Sigmon, Appl. Phys. Lett. 70 Ž1997. 767. J.B. Khurgin, E.W. Forsythe, G.S. Tompa, Appl. Phys. Lett. 69 Ž1996. 1241. M.J. Estes, G. Moddel, Phys. Rev. B 54 Ž1996. 14633. E. Edelberg, S. Bergh, R. Naone, Appl. Phys. Lett. 68 Ž1996. 1415.
Room temperature visible photoluminescence from crystallized nano-Si thin films Wei Wu b
a,b,)
, Xinfan Huang a , Kunji Chen a , Jian Bin Xu b, Xiang Gao a , Jun Xu a , Wei Li a
a Department of Physics, Nanjing UniÕersity, Nanjing 210093, China Department of Electronic Engineering, The Chinese UniÕersity of Hong Kong, Shatin, N.T., Hong Kong, China
Abstract We report the observation of visible-light emission from crystallized a-Si:H thin films and a-Si:Hra-SiN x :H multi-quantum well ŽMQW. structures, which were deposited on SiO 2rSi substrates by a plasma enhanced chemical vapor deposition system and subsequently crystallized by a KrF excimer laser. Transmission electron microscopy technology revealed the formation of fine grain-sized nanocrystallites with diameter about 10 nm. X-ray diffraction and Raman scattering spectra showed spectral peaks corresponding to crystalline Si Žc-Si. Ž111. face. Room temperature visible photoluminescence ŽPL. emission was observed both from crystallized a-Si:H films and multi-quantum well structures, with peak emission energy around 2.0 eV. The ability to fabricate visible luminescent Si films by a method compatible with the currently mature Si-based microelectronics technology, can provide a promising means in the realization of optoelectronic devices. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Photoluminescence ŽPL.; a-Si:Hra-SiN x :H; Plasma enhanced chemical vapor deposition ŽPECVD.
1. Introduction Since the observation of efficient room temperature photoluminescence ŽPL. from porous silicon ŽPS. in 1990 w1x, materials with nanometer-sized silicon crystallites have attracted interest. Although the radiative recombination processes involved are still debated, several theoretical investigations show that by confining silicon particles small enough to quantize the electronic states, there will consequently be the probability of a large increase of the effective
)
Corresponding author. Tel.: q852-2609 8257; fax: q8522603 5558; e-mail: [email protected].
band gap and a more efficient radiative recombination w2–4x. Therefore, the luminescent properties will be altered. Most of the previous reports had discussed electrochemically etched Si materials. This kind of electrochemical dissolution method is based on wet synthesis w5x. The prepared PS materials had very complicated structures and poor stability. Efforts have then been directed to the materials prepared by dry methods to improve both the stability and efficiency of light emission. We report the observation of room temperature visible PL from crystallized a-Si:H thin film and a-Si:Hra-SiN x :H MQWs deposited by plasma enhanced chemical vapor deposition ŽPECVD. system on SiO 2rSi substrate, using a crys-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 4 5 - 2
1046
W. Wu et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1045–1048
tallization technique with a KrF excimer laser. The laser-induced crystallization method can provide several advantages such as selected area processing, rapid crystallization, and avoidance of the surface contamination which plays a significant role in the strong visible luminescence from porous Si layers w6x. In addition, since the absorption coefficient of a-Si thin films is quite high in the ultraviolet ŽUV.light region of the excimer laser, it will be possible to crystallize a-Si layers without thermal alterations to the silicon substrate. The ability to manufacture luminescent Si films on the SiO 2rSi substrate by methods compatible with the mature Si microelectronics technology will give a new possibility in optoelectronics devices and allow monolithic integration of Si technology to combine with optical signal processing.
2. Experimental procedures a-Si:H thin film and a-Si:Hra-SiN x :H MQWs were deposited on SiO 2rSi substrate by PECVD at the substrate temperature of 2508C and the deposition rate of 0.1 nmrs under a total reaction pressure of 36 Pa. The SiO 2rSi substrate was prepared by means of thermal oxidation of Ž100. Si wafers with the dioxide layer of about 700 nm. The deposited a-Si:H film was 100 nm thick. In the MQW structures, the a-SiN x :H barrier layer thickness L N was grown to 10 nm, and the a-Si:H well layer L W was 4 nm. During the deposition of the MQW structures, the composition of the reactant gases was alternated periodically between pure silane and a silane–ammonia mixture by a computer without interrupting the plasma. Other details of sample preparation have been reported elsewhere w7x. The crystallization process was performed inside a vacuum chamber by a KrF Ž l s 248 nm. excimer laser at room temperature, with a pulse duration of 30 ns and energy densities ranging from 0.13 Jrcm2 to 1.2 Jrcm2 . The laser-beam size was 11 = 4 mm2 on the film surface. Microstructure and grain size of the Si particles were analyzed by transmission electron microscopy ŽTEM.. X-ray diffraction ŽXRD. and Raman scattering were used to study the crystallinity. The PL spectral measurements were made
via excitation from an Arq laser at a wavelength of 488 nm.
3. Results Fig. 1 shows the XRD spectrum of the sample irradiated at an energy density of 560 mJrcm2 with a single pulse. A diffraction peak appears at 2 u s 28.5158 corresponding to the diffraction of the Ž111. face of c-Si. There are also two weak peaks at 47.4398 and 56.7898, coinciding with the diffraction of Ž220. and Ž311. faces of c-Si, respectively. These results show that the sample has been crystallized and has a preferred direction of ²111:. The PL spectrum of the crystallized film is shown in Fig. 2. It demonstrates a PL peak centered at 610 nm Ž2.03 eV.. The full width at half maximum ŽFWHM. of the PL peak is about 0.4 eV. We could not detect any visible luminescence from a SiO 2rSi substrate and the as-deposited samples. A planar-view TEM micrograph of this crystallized sample is shown in Fig. 3. It reveals a structure composed of a large number of very fine Si crystallites with an average diameter of about 10 nm. After the observation of visible PL from the nano-crystalline Si thin films, we found it is difficult to control the grain size and the size distribution of the crystallized films. To achieve a controllable grain size and distribution is still a technical challenge. So
Fig. 1. X-ray diffraction spectrum of crystallized a-Si:H thin film.
W. Wu et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1045–1048
Fig. 2. Photoluminescence spectrum of the crystallized a-Si:H film at room temperature.
we performed a further experiment to study the possibility of visible luminescence due to one-dimensional quantum confinement of thin a-Si:H well layers between a-SiN x :H barrier layers. To fabricate layers of nanometer-sized Si crystallites, the as-deposited MQW structures were subsequently annealed using an excimer laser. Fig. 4 shows a Raman spectrum of the MQW sample annealed at the energy density of 950 mJrcm2 with a single pulse. Near the TO peak of the crystalline Si Žc-Si., a band positioned at a wave number
Fig. 3. A planar-view TEM micrograph of the crystallized a-Si:H thin film.
1047
Fig. 4. Raman scattering spectrum of the crystallized a-Si:HraSiN x :H MQW structure with L W s 4.0 nm.
of 514 cmy1 appears. This band also indicates that the sample has been crystallized. The PL spectrum of the same crystallized MQW sample at room temperature is depicted in Fig. 5. It shows a PL emission from the crystallized sample with a well-layer thickness of 4.0 nm, where the dominant emission at 640 nm was found. To further identify the origin of the
Fig. 5. Room temperature photoluminescence spectrum of the crystallized MQW structure with a-Si:H well-layer thickness of 4.0 nm.
1048
W. Wu et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1045–1048
visible PL, we measured the PL of the as-deposited samples. No visible PL was found at room temperature.
4. Discussion At present, there is not a unique view of the mechanisms responsible for the PL in nanometersized silicon structures. From the TEM micrographs of the crystallized a-Si:H thin film ŽFig. 3. and the MQW structures w7x, we can not find interconnected pores or voids as formed in porous silicon. It is unlikely that these structures can play a key role in the visible luminescence. After comparing both the experimental results of the distinct PL peaks at different ranges of the emission spectrum and the quantum-confinement models w8–10x, we speculated that such a PL is due to the formation of nanometer-sized silicon crystallites. In the process of high laser energy density irradiation of the a-Si:H thin film, thermal effects to the underlayered SiO 2 film are possible. This effect leads to increasing the roughness at the interface between the crystallized a-Si:H thin film and the SiO 2 layer w11x. Hence, a large number of Si nucleation sites would be formed. Space is not available for the regrowth of these melted Si, and it is difficult to increase sizes of grains, as shown in the TEM micrograph of Fig. 3. The grain boundary would play the role as potential barrier in the crystallized structure. In crystallized MQW structures, nanometer Si crystallites were formed in the well layers due to the constrained crystallization between a-SiN x :H barrier layers w7x. Some theoretical models have been proposed to explain the origin of this visible light emission of Si crystallites at room temperature and the energy gap dependence on crystal size w12,13x. The results show that the obtained highly confined Si structures would exhibit a blue shift and an increase in quantum efficiency of the radiation emission. Therefore, the nanometer-sized structure is expected to generate optical transitions in the visible light region. Currently we are improving the emission efficiency and intensity, and trying to find the reason for the blue shift of the PL peak.
5. Conclusions Compared with the previous reports of visible luminescence from ultrafine Si nanometer films prepared by PECVD methods w14x, exhibiting a PL peak at low temperature Ž27 K. and no distinct peak at room temperature, our method seems to be more reasonable for future applications. The ability to fabricate Si films with visible photoluminescent properties on SiO 2rSi substrate by a method which is compatible with the currently mature Si-based microelectronics technology, can provide a promising means in optoelectronics and allow monolithic integration of silicon technology to combine with optical signal processing.
Acknowledgements We would like to express our thanks to Dr. Xiao Yuan Chen and Prof. Zhiguo Liu of Nanjing University for their technical assistance and useful discussion. This work is supported by the National Nature Science Foundation of China.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x
L.T. Canham, Appl. Phys. Lett. 59 Ž1990. 1046. B. Delley, E.F. Steigeier, Phys. Rev. B 47 Ž1993. 1397. M.J. Estes, G. Moddel, Appl. Phys. Lett. 68 Ž1996. 1814. M. Yamaguchi, K. Morigaki, Phys. Rev. B 55 Ž4. Ž1997. 2368. N. Koshida, H. Koyama, Appl. Phys. Lett. 60 Ž1992. 347. N. Sato, K. Sakaguchi, K. Yamagata, Y. Fuiyama, Electrochem. Soc. 142 Ž1995. 3116. K.J. Chen, X.F. Huang, J. Xu, D. Feng, Appl. Phys. Lett. 61 Ž1992. 2069. T. Toyama, T. Matsui, Appl. Phys. Lett. 69 Ž1996. 1261. G. Cicala, P. Capezzuto, G. Bruno, J. Appl. Phys. 80 Ž1996. 6564. B.T. Sullivan, D.J. Lockwood, H.J. Labbe, Appl. Phys. Lett. 69 Ž1996. 3149. G.K. Giust, T.W. Sigmon, Appl. Phys. Lett. 70 Ž1997. 767. J.B. Khurgin, E.W. Forsythe, G.S. Tompa, Appl. Phys. Lett. 69 Ž1996. 1241. M.J. Estes, G. Moddel, Phys. Rev. B 54 Ž1996. 14633. E. Edelberg, S. Bergh, R. Naone, Appl. Phys. Lett. 68 Ž1996. 1415.