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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.28 No.2 pp.124-128
DOI : https://doi.org/10.3740/MRSK.2018.28.2.124

Investigation of Photoluminescence and Annealing Effect of PS Layers

Chang-Suk Han1, Kyoung-Woo Park2, Sang-Wook Kim3
1Department of ICT Automotive Engineering, Hoseo University 201, Sandan7-ro, Seongmun-myeon, Dangjin City, Chungnam 31702, Republic of Korea
2Department of Mechanical Engineering, Hoseo University 20, Hoseo-ro 79beon-gil, Baebang-eup, Asan City, Chungnam 31499, Republic of Korea
3Department of Nanobiotronics, Hoseo University 20, Hoseo-ro 79beon-gil, Baebang-eup, Asan City, Chungnam 31499, Republic of Korea
Corresponding author : hancs@hoseo.edu (C.-S. Han, Hoseo Univ.)
20171107 20180125 20180125

Abstract

N-type porous silicon (PS) layers and thermally oxidized PS layers have been characterized by various measuring techniques such as photoluminescence (PL), Raman spectroscopy, IR, HRSEM and transmittance measurements. The top surface of PS layer shows a stronger photoluminescence peak than its bottom part, and this is ascribed to the difference in number of fine silicon particles of 2~3 nm in diameter. Observed characteristics of PL spectra are explained in terms of microstructures in the n-type PS layers. Common features for both p-type and n-type PS layers are as follows: the parts which can emit visible photoluminescence are not amorphous, but crystalline, and such parts are composed of nanocrystallites of several nm’s whose orientations are slightly different from Si substrate, and such fine silicon particles absorb much hydrogen atoms near the surfaces. Light emission is strongly dependent on such fine silicon particles. Photoluminescence is due to charge carrier confinement in such three dimensional structure (sponge-like structure). Characteristics of visible light emission from ntype PS can be explained in terms of modification of band structure accompanied by bandgap widening and localized levels in bandstructure. It is also shown that hydrogen and oxygen atoms existing on residual silicon parts play an important role on emission stability.


초록


    Ministry of Trade, Industry and Energy
    N0000717

    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1.Introduction

    Recently, we have investigated microstructure and crystallinity of PS layers formed on a highly doped n-type silicon substrates, and also photoluminescence nature of the PS qualitatively.1) The results have revealed that all luminescent PS layers show sponge-like structure near their top surfaces, the crystallinity of n-type PS is inferior to that of p-type PS, and secondary anodization is effective in forming luminescent parts having sponge-like structure, especially near the top surface. In order to know the emission mechanism from PS, however, microstructure of PS layer must be investigated in further detail in relation to photoluminescence nature. Thus, in this study we investigated the photoluminescence natures of n-type PS layers prepared under different conditions through various measurements. The main reason why we use a highly doped n-type PS is that it is easier to form luminescent parts than the p-type PS, and it shows a distinct structure, including size and morphology, which are easy to evaluate. The results are compared with those obtained for p-type PS in previous studies.2-6) The correlation between the microstructure and visible light emission is also discussed on the basis of experimental results.

    2.Experimental Procedure

    CZ-Silicon (100) wafers(As-doped) with a resistivity of 0.02 Ωcm were anodized in an electrolyte solution of HF:H2O:C2H5OH = 1:1:2 under the following conditions: current density of 100 mA/cm2 and reaction time of 15 to 30 min. The wafers were illuminated with a 100W halogen lamp during anodization. Thermal oxidization of PS layers was carried out at 100 to 600 °C under an oxygen pressure of 2/3 atm. Thus prepared n-type PS layers were examined by various measuring techniques such as photoluminescence( PL), Raman spectroscopy, IR, and transmittance measurements at room temperature. The lattice spacing and crystallinity of the PS layers were also measured by means of X-ray multi-crystal diffractometry and the microstructure of the PS layers was observed by a high resolution scanning electron microscope(HRSEM). The luminescence characteristics including color and brightness of visible photoluminescence were confirmed for all the PS layers by He-Cd laser excitation at room temperature. PL spectra were measured using an Ar laser (488 nm) with measuring time of 30 sec in order that the laser irradiation might not affect the stability of visible light emission process.

    3.Results and Discussion

    3.1.Cross-sectional microstructure and photoluminescence

    Fig. 1 show the top (a) and bottom (b) parts of PS layer were investigated in detail by HRSEM. As a result, it proved that the top part exhibits a large number of main pores which construct micro-channel systems on their lateral surfaces, resulting in so-called sponge-like structure, while the bottom part exhibits main pores having relatively smooth surfaces with few micro-channel systems. This result was essentially the same as in the previous study.1)

    The top part(front side) of PS layer shows a higher PL peak around 840 nm than the bottom(back side), as shown in Fig. 2.

    The data were obtained from a self-supporting PS layer. This result is consistent with the result by cathodoluminescence( CL) from n-type PS.7) In general, PS layers formed on the silicon wafer with high resistivity always consist of a large number of fine silicon particles throughout the PS layer,5) and no remarkable difference in their number is observed from the top to the bottom. Therefore, the difference in PL spectra between the top and bottom parts can not be expected. The present result is another case, and suggests that visible light emission comes from only the PS layer that has a characteristic structure such as sponge-like structure consisting of a large number of silicon parts of few nm’s in size. This difference in nature was also confirmed by the broadening of Raman spectra in the reflection mode, as shown in Fig. 3, where the spectra from the front side are slightly broader than that from the back side, indicating that the silicon particles are much smaller at the front side than at the back side. The comparison of these results indicated reasonably that PL intensity is related to the number of such fine silicon particles, which are thought to be emission sources for visible photoluminescence.6,8,9) This is the first time to verify directly that the PL intensity from PS layer is strongly dependent of the number of fine silicon particles of 2~3 nm in diameter.

    3.2.Change in PL spectra with anodization time

    Fig. 4 shows PL spectra from various PS layers formed with different anodization times. These spectra were measured from the front side(top part) of the PS layer formed on the silicon wafer. PS layers, when prepared with anodization times shorter than 15 min, exhibited a broad peak around 710 nm, and its peak intensity increases with increasing anodization time. As shown in Fig. 4, however, just after 20 min anodization an appreciable but broad peak appears around 840 nm, and its intensity increases with increasing anodization time, accompanied with a slight blue-shift in PL spectra. The PL spectra from the present n-type PS exhibit a main peak at a slightly longer wavelength than that from the non-degenerated ptype and n-type PS’s with high resistivity more than 1 Ωcm.9) It is known that long anodization leads to the reduction in size of residual silicon parts.2,3,6)

    In fact, this was also confirmed by the broadening of Raman spectrum, as shown in Fig. 5. The broad PL spectrum is thought to be due to the size distribution of such fine silicon particles in PS. IR measurements revealed that absorption spectra ascribed to Si-Hn (n = 1~3) bondings increased their intensity with increasing anodization time, but did not offer any clear evidence to support the existence of luminescent substances such as Si6H6O3 and its derivatives. Thus, it seems likely that the observed blue-shift is ascribed to the reduction in size of initially formed silicon particles. This is consistent with our previous result showing that the secondary anodization, which is effective in forming luminescent parts having sponge-like structure, become more effective with increasing anodization time.1) The present results of Raman and PL spectra also supported that illumination of 100 W halogen lamp is very effective in forming fine silicon particles near the top surface of PS layer. The finding that secondary anodization shorter than 15 min does not give appreciable PL spectra is probably due to a small number of luminescent parts near the top surface formed during illumination. 15 min anodized PS was less active against laser irradiation than 30 min anodized one. This can be also explained in terms of a small number of initially formed silicon particles, which are thought to be slightly smaller than those formed by a long anodization.

    Finally, it is concluded that visible luminescence is mainly related to the top surface(front side) of PS layers where a sponge-like structure consists of fine silicon particles formed by the secondary anodization, and that the secondary anodization is effective in forming such sponge-like structure and is effectively stimulated by halogen lamp illumination during anodization.

    3.3.PL spectra from thermally oxidized PS layers

    When PS layers, formed with 30 min anodization time, were oxidized in an oxygen atmosphere at 100 to 600 °C for 30 min, PL spectra varied with oxidization temperature, as shown in Fig. 6. Just after 100 °C oxidization, an abrupt peak increase in PL spectra was observed, and then PL spectra become broader and decrease their intensity with increasing oxidization temperature.

    Slight blue-shift was observed below 300 °C, but the shift was almost saturated for a further oxidization. A similar blue-shift and the saturation of the shift has been already reported for p-type PS layers, anodically and/or thermally oxidized in order to stabilize the pore structure at room temperature.3,4) This blue-shift is also ascribed to the reduction of silicon particle size caused by thermal oxidization. In fact, in the present study the peak broadening in Raman spectra due to the reduction of silicon particle size was observed for PS layers oxidized at higher temperatures. In this connection, SEM observations revealed that microstructures of PS layers varied with oxidization temperature, and fine silicon particles covered with SiOx oxides were coalesced with each other. Observed decrease in PL spectra with oxidization temperature suggests that requirements for visible light emission from n-type PS are not easy to meet through only thermal oxidization treatments in comparison with p-type PS. IR spectra were also measured for all the oxidized PS layers, and the results are summarized in terms of oxidization temperature in Fig. 7. IR measurements revealed that the shift was associated with the reduction in Si-Hn (n = 1~3) bondings with increasing oxidization temperature, probably due to the oxidization of surfaces of the fine silicon particles. It is also shown that hydrogen and oxygen atoms existing on residual silicon parts play an important role on visible light emission.

    When 30 min anodized PS was oxidized above 400 °C, a peak around 720 nm shifted to 800 nm(red-shift). In this connection, a spectrum due to the Si-O bonding was observed to increase over this temperature range, and the transmittance measurements showed that absorption curves measured from self-supporting PS layers exhibit a slight shift of the absorption edge to the longer wavelength side, although no appreciable shift to a shorter wavelength side in absorption edge was observed when oxidized below 300 °C. This may be also associated with appreciable structure changes observed through HRSEM.

    It is reported that when PS layers are thermally and/or anodically oxidized a blue-shift in PL spectra can be observed, while the shift is saturated for further oxidization. 3,4) The present results suggest that the visible light emission process from PS is ascribed to not only the effect of band-gap widening, but also another origin including the formation of localized levels in the band structure. The blue-shift in PL spectra may be ascribed to the change in surface state of silicon particles: band-gap widening due to the reduction of silicon particle size and the presence of localized levels in the band-gap, as proposed by Rustamov et al..4)

    4.Conclusion

    The photoluminescence natures of n-type PS, formed under various conditions, were studied in relation to the microstructure. The present results lead to conclusions:

    • 1) Common features for both p-type and n-type PS layers are as follows: the parts which can emit visible photoluminescence are not amorphous, but crystalline, and such parts are composed of nanocrystallites of several nm’s whose orientations are slightly different from Si substrate, and such fine silicon particles absorb much hydrogen atoms near the surfaces.

    • 2) Light emission is strongly dependent on such fine silicon particles.

    • 3) Photoluminescence is due to charge carrier confinement in such three dimensional structure(sponge-like structure).

    • 4) Characteristics of visible light emission from n-type PS can be explained in terms of modification of band structure accompanied by bandgap widening and localized levels in bandstructure.

    Acknowledgements

    This research was supported by the Ministry of Trade, Industry and Energy(MOTIE), KOREA, through the Education program for Creative and Industrial Convergence (Grant Number N0000717).

    Figure

    MRSK-28-124_F1.gif

    High-resolution SEM images of PS layers anodized with 100 mA/cm2 for 900 s: (a) top, (b) bottom.

    MRSK-28-124_F2.gif

    PL spectra from self-supporting PS, formed with 30 min anodization.

    MRSK-28-124_F3.gif

    Laman spectra from self-supporting PS, formed with 30 min anodization.

    MRSK-28-124_F4.gif

    PL spectra from anodized PS layers.

    MRSK-28-124_F5.gif

    Laman spectra from anodized PS layers.

    MRSK-28-124_F6.gif

    Change in PL spectra for oxidized PS’s.

    MRSK-28-124_F7.gif

    IR spectra as a function of temperature.

    Table

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