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Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering
Thin Solid Films 515 (2007) 8664 – 8669 www.elsevier.com/locate/tsf
Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering R. Prabakaran a,⁎, T. Monteiro b , M. Peres b , A.S. Viana c , A.F. da Cunha b , H. Águas a , A. Gonçalves a , E. Fortunato a , R. Martins a , I. Ferreira a a
CENIMAT, Department of Materials Science and CEMOP/UNINOVA, Faculty of Sciences and Technology, New University of Lisbon, Caparica 2829-516, Portugal b Department of Physics, Aveiro University, 3810-193 Aveiro, Portugal c Laboratório de SPM da Faculdade de Ciências, Universidade de Lisboa, Ed. ICAT, Campo Grande, 1749-016 Lisboa, Portugal Available online 31 March 2007
Abstract In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO. © 2007 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Gallium zinc oxide; Photoluminescence; IR; XRD
1. Introduction Porous silicon (PS) consists of a network of Si nanocrystallites with nano/micro-pores. Strong PL in PS appears to be strongly influenced by surface hydrides, oxides and defect states. The remarkable room-temperature visible light emission from PS has stimulated striking effort towards developing silicon-based optoelectronic devices [1–3]. While PS shows significant luminescence over a wide spectrum, the photoluminescence (PL) properties undergo substantial degradation with aging due to atmospheric oxidation. Moreover, the tremendous reactivity and fragility of PS skeleton have hitherto prohibited its integration with conventional silicon processing technology. Thus encapsulation would be required for protecting the PS structure for overwhelming application. Nevertheless, this work illustrates that the optical and chemical stability ⁎ Corresponding author. Tel.: +351 219 48 300x10602; fax: +351 212 957 810. E-mail addresses: [email protected], [email protected] (R. Prabakaran). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.098
of PS can be greatly enhanced by coating gallium-doped zinc oxide (GZO) films. The GZO is a wide direct band gap (∼3.3 eV) semiconductor with such potential applications as transparent conductive films in optoelectronic devices like blue and ultraviolet (UV) light emitting diodes and laser diodes [4,5]. Recently, numerous investigations have been focused on the enhancement of PL and electroluminescence efficiency of PS by different routes [2]. It is important to tune the emission line to get enough efficiency for obtaining good PS devices. Two routes have been mainly focused to enhance emission efficiency: organic or inorganic covering/filling and PS doping. In our earlier investigation, we have improved the PL efficiency of PS and c-Si doping with O+ by implantation [6–8]. This work explores the possibility of the use of inorganic covering (GZO) to study their effects on the PL of PS, especially in the blue and UV region, for optoelectronic applications. In the present work, PS specimens were prepared with 10 and 420 min etching times from (100) oriented n-type c-Si. Parts of these PS were coated with GZO films by rf magnetron sputtering at room temperature (RT) while other PS samples
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were left uncoated. The X-ray diffraction (XRD), Fourier transform infrared studies (FTIR), atomic force microscopy (AFM) and PL measurements were carried out to investigate the optical, structural and microscopic properties of the PS specimens. The present work contributes to the understanding of the origin of blue and UV PL from PS and its enhancement after GZO films deposition. 2. Experimental details (100) Oriented phosphorous doped n-type c-Si having a resistivity of 1–2 Ω cm was used to prepare porous silicon at room temperature in an electrolytic etching bath of 1:2 mixture of HF (48%) and ethanol (98%) under a constant current density of 20 mA/cm2. With platinum wire as the cathode, anodization was carried out under illumination provided by a 50 W halogen lamp, placed 35 cm away from the specimen. The anodization was carried out for 10 and 420 min to obtain different porosities. After anodization the specimens were rinsed in de-ionized water and dried under a vacuum of 0.1 Pa for 10 h at RT. The estimated porosities for 10 and 420 min etched PS specimens from gravimetric method are ∼ 18% and 80%. Within a day after the formation of PS, some pieces were coated with GZO of 400 nm thickness and measured using a DEKTAK 3D surface profilometer from Sloan Tech. The GZO film were deposited by rf (13.56 MHz) magnetron sputtering using a ceramic oxide target ZnO:Ga2O3 (95:5 wt.%; 5 cm diameter) from LTS with a purity of 99.99%. The sputtering was carried out under RT and with: argon flux of 20 sccm; oxygen flux 0.4 sccm and a deposition pressure of 0.15 Pa. The distance between the
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substrate and the target was 10 cm and the rf power was maintained constant at 175 W [9]. XRD pattern of the specimens have been taken on a Rigaku DMAX III-C series diffractometer in θ–2θ geometry using CuKα radiation. The data have been collected in the 2θ range of 10° to 60°. Infrared measurements were carried out using a Thermo-Nicolet 6700 spectrophotometer operating with a resolution of 4 cm− 1, in the normal reflectivity mode. All measurements were recorded in the mid-infrared range (400– 4000 cm− 1) using globar source in combination with KBr beam splitter and a DTGS TEC detector. Tapping mode atomic force microscopy experiments were performed in a multimode AFM microscope coupled to a Nanoscope IIIa Controller. Commercial etched silicon tips with typical resonance frequency of ca. 300 kHz have been used as AFM probes. PL measurements were carried out at RT with a 325 nm cw He-Cd laser with an excitation power density typically less than 0.6 W cm− 2. The light collected in 90° geometry, was dispersed by a SPEX 1704 monochromator with a 1200 mm− 1 grating and detected by cooled Hamamatsu R928 photomultiplier. All the recorded spectra are corrected to spectral responses. 3. Results and discussion 3.1. X-ray diffraction measurements The XRD patterns of 10 min PS, 420 min PS, 10 min PS coated with GZO films and 420 min PS coated with GZO films are shown in Fig. 1(a), (b), (c) and (d) in the 2θ range from 10° to 60° respectively. Fig. 1(a) and (b) shows a peak at around
Fig. 1. The X-ray diffraction pattern of (a) 10 min etched PS, (b) 420 min etched PS, (c) 10 min etched PS coated with GZO and (d) 420 min etched PS coated with GZO.
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33.0° 2θ from (200) plane of c-Si. This forbidden peak is appeared due to the defects present in c-Si [10]. Our previous XRD investigation on c-Si and PS samples shows a strong peak at 69.1° 2θ which corresponds to (400) plane of the c-Si crystal [10]. However, this 2θ range is not recorded in the present work in order to protect the detector from high intense (400) c-Si peak. The strong peaks at 34.259° and 34.417° 2θ in Fig. 1(c) and (d) indicate the (002) plane of the GZO films deposited on 10 min and 420 min etched PS specimens respectively. The GZO deposited is polycrystalline with a hexagonal structure and a preferred orientation along the c-axis, that is, perpendicular to the PS specimens [9]. This peak is fitted with pseudovoigt function to get the peak parameters. The average particle size on the GZO films coated PS specimens, as estimated from FWHM using Scherrer formula, increases from 45 to 55 nm on increasing the etching time from 10 to 420 min. In addition, Fig. 1(c) and (d) shows other orientation peaks at around 31.9° 2θ (100), 36.4° 2θ (101), 47.6° 2θ (102) and 56.2° 2θ (110) [11,12]. The peaks related to (100) and (002) orientation in Fig. 1 (c) are enhanced in comparison with Fig. 1(d). It is to be noted that the major (002) peak and other peaks in Fig. 1(d) show a slight shift of ∼0.15° 2θ towards higher diffraction angle in comparison with Fig. 1(c). The observed peak shift might be originated from the incorporation of GZO films in the different pores of PS indicating the changes in lattice constant. 3.2. Fourier transform infrared studies FTIR spectra of 10 min PS, 10 min PS coated with GZO, 420 min PS and 420 min PS coated with GZO are shown in Fig. 2(a), (b), (c) and (d) respectively. The broad absorbance band around 1000 cm− 1 to 1300 cm− 1 corresponds to TO asymmetric stretching mode of the Si–O bond [13]. Si–H wagging, Si–H2 scissors and Si–H stretching vibration in PS specimens are appeared at around 662, 870, 910 and 2101 cm− 1 [14]. The appearance of bands at 406 and 627 cm− 1 is from Si– O–Si bending and Si–Si stretching mode, 1463 cm− 1 is from CH3 asymmetric deformation, 2197 cm− 1 is from Si–H stretching in SiO2–SiH, 2250 cm− 1 is from O–Si–H stretching, 2856 cm− 1 is from CH stretching in CH and 2927 cm− 1 is from CH stretching in CH2, 2958 cm− 1 is from CH stretching in CH3 [2,14]. The absorbance band at 805 cm− 1 is observed only in Fig. 2(c) which corresponds to bending mode of SiO2 [15]. The origins of 723 cm− 1 band in Fig. 2(c) and (d) and 1371 cm− 1 band in Fig. 2(c) are not clear at present. On exposing the freshly prepared PS specimens to atmospheric oxidation, Si–Hx band would decrease in intensity and Si–O band would grow. However, the PS specimens are not completed in oxidation and hence Si–Hx bands appeared in Fig. 2(a) and (c). Si–O stretching mode around 1068 cm− 1 in Fig. 2(b) is very weak as expected from GZO deposition. Conversely, the absorbance value of this mode is enhanced in the case of 420 min etched PS covered with GZO films. However, CH stretching in CH/CH2 and CH3 asymmetric deformation bands in Fig. 2(a) and (c) are completely disappeared after GZO films deposition [see Fig. 2(b) and (d)].
Fig. 2. The FTIR spectra of (a) 10 min etched PS, (b) 10 min etched PS coated with GZO, (c) 420 min etched PS and (d) 420 min etched PS coated with GZO.
Interestingly, in Fig. 2(d) new absorbance bands are observed at 827 cm− 1 and 2197 cm− 1 which are related to SiO bending in O–Si–O and Si–H stretching in SiO2–SiH [2]. Furthermore, the O–Si–H stretching mode at 2250 cm− 1 is enhanced in Fig. 2(d) after GZO deposition. 3.3. Atomic force microscopy In order to identify the microstructure of the PS specimens, 2D and 3D AFM images were undertaken on 10 min PS and 10 min PS coated with GZO films. Fig. 3(a) and (b) shows 3D and 2D profile obtained in a scanned area of 1 μm × 1 μm in 10 min etched PS specimen. A large number of distinguished Si nanocrystallites with pores in between can be visualized in both images. It is to be noted that Si nanocrystallites appear in spherzical shape, with sizes varying from 2 to 20 nm [see section analysis in Fig. 3(b)]. The root–mean–square (rms) roughness value observed here is 6.419 nm. Fig. 4(a) and (b) shows the 3D and 2D profiles of the AFM images of 10 min etched PS deposited with GZO films. The 2D and 3D surface structures are conspicuously different from Fig. 3. The spherical features and pore sizes are larger in comparison with Fig. 3. The overall estimated sizes of the
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Fig. 3. AFM image of 10 min etched PS. (a) 3D profile obtained in a 1 μm × 1 μm scanned area and data scale of 70 nm, (b) 2D profile obtained in a scanned area of 1 μm × 1 μm with respective section.
Fig. 4. AFM image of 10 min etched PS coated with GZO. (a) 3D profile obtained in a 1 μm × 1 μm scanned area and data scale of 70 nm, (b) 2D profile obtained in a scanned area of 1 μm × 1 μm with respective section.
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projections from the line profile in section analysis varied from 40 to 75 nm [see Fig. 4(b)]. The rms roughness value observed here is 14.136 nm. The arrowheads in Fig. 4(b) indicate GZO films coated PS of approximately 45 nm size which is well corroborated with the XRD Scherrer estimated size. These sizes do not correspond to the Si nanocrystallites in the GZO deposited PS. The GZO films deposition covers the entire Si nanocrystallites which results in overall increase in crystallite size. Moreover, this AFM investigation clearly showed colossal change to topographical characteristics between 10 min PS and 10 min PS coated with GZO films. 3.4. Photoluminescence The photoluminescence spectra of 10 min PS, 10 min PS coated with GZO films, 420 min PS and 420 min PS coated with GZO films were recorded in the energy range of 1.54 to 3.5 eV at RT and shown in Fig. 5(a), (b), (c) and (d) respectively. This PL was fitted with three Gaussian line shapes along with a
Fig. 5. Photoluminescence spectra of (a) 10 min etched PS, (b) 10 min etched PS coated with GZO, (c) 420 min etched PS and (d) 420 min etched PS coated with GZO. The solid lines are the fit to a Gaussian peak with a constant background. The peak positions are marked. The curves are shifted along the y-axis for clarity.
constant background to obtain the peak parameters. As observed with 3.81 eV excitation and at low temperature the main recombination processes are dominated by broad emission bands that cover all the studied spectral range. As the etching time increases from 10 min to 420 min the peak position of red emission band of PS shifted from 1.7917 eV to 1.89 eV due to quantum confinement effect giving a strong evidence of the band gap widening for the PS specimens [16]. The observed Si nanocrystallite sizes of less than 4 nm from AFM investigation, which is smaller than the exciton Bohr radius in Si, also support the quantum confinement effect. The peak position for GZO deposited PS is found to blueshift upon etching, namely 1.816 and 1.923 eV as compared to 1.7917 and 1.89 eV for uncoated PS specimens. H. Elhouichet and M. Oueslati [17] have reported a similar blueshift of PS after ZnO deposition. Liu et al. [18] have also reported that the red emission band of PS after deposition of ZnO shows an obvious higher energy shift that increases with an increase in template porosity. Fascinatingly, 420 min etched PS in Fig. 5(c) shows PL enhancement in blue (∼ 2.78 eV) and UV (∼ 3.2 eV) region in comparison with 10 min PS. Blue emission ∼ 2.78 eV in PS specimens has been frequently attributed to oxygen-related luminescence centers in typical surroundings of SiOx (0 b x b 2) [19]. In the present FTIR investigation [Fig. 2(c)] further confirms that the formation of oxygen related defects are responsible for the enhancement of ∼ 2.78 eV PL emission. The relative intensity of the blue PL emission is seen to increase after GZO film deposited on 420 min etched PS as shown in Fig. 5(d). Besides the blue recombination an UV emission band near 3.2 eV [Fig. 5(c)] has been attributed to intrinsic defects, Si–O related species and luminescence centers located at the interface between the Si nanocrystallites and the SiO2 matrix [20]. This UV PL is also enhanced after GZO deposition [see Fig. 5(d)]. This increase in UV emission might be originated from an overlap of emission bands that also account with the near-band-edge recombination of the wide band gap from GZO deposited films [21]. However, Tomioks and Adachi [22] have established the fact that the surface oxide layer (∼ 1065 cm− 1) acts as a good passivation film and gives rise to an efficient oxide edge (UV) emission at the PS/oxide interface. In the present FTIR investigation, the absorbance value observed around 1000 cm− 1 to 1300 cm− 1 from Si–O bond increases after GZO films deposition. In addition, new absorbance bands are observed at 827 cm− 1 and 2197 cm− 1 which are related to SiO bending in O–Si–O and Si–H stretching in SiO2–SiH. Also, the O–Si–H stretching mode at 2250 cm− 1 is enhanced [Fig. 2(d)] after GZO deposition. Hence, we believe that the PS/ oxide interface structural changes after GZO films deposition on 420 min etched PS might also result in an additional contribution to the PL enhancement in blue and UV regions. Before performing the GZO films coating, the PS samples are kept in high vacuum (10− 6 Torr), which might result in the removal of oxygen adsorption from PS surface. After deposition, GZO films are entirely covered on the PS surface and contribute to the surface passivation. Generally, huge porosity (∼ 80%) sample like 420 min PS might consist of complicated internal network structure. Hence, the total coverage and oxygen adsorption are
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less effective. Furthermore, some defect and recombination centers are kept and partially changed after GZO films coating. However, these structural changes are absent in the case of 10 min etched PS after GZO films deposition [see Fig. 2(b)], which might reduce the recombination centers and consequently, increases the non-radiation recombination and decreases the radiation recombination. Obviously, this results in PL quenching after GZO films coating on 10 min PS. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8]
The PL investigation on 10 min and 420 min etched PS shows a systematic blue shift from 1.7917 eV to 1.89 eV due to quantum confinement effect and this PL peak position is further blue shifted upon GZO films deposition. 420 min etched PS shows PL enhancement in blue (∼ 2.78 eV) and UV (∼3.2 eV) region in comparison with 10 min PS which might be due to luminescence centers of oxygen vacancy in SiO2 and luminescence centers located at the interface between the Si nanocrystallites and the SiO2 matrix. The enhancement of its intensity and wideness for GZO deposited films suggests that an overlap of emission bands occurs in these spectral regions due to the contribution of near-band-edge recombination from GZO. FTIR investigation further confirms the formation of surface oxide layer (1068 cm− 1) and intrinsic related defects on PS as well as GZO coated PS. Thus, we show the possible way to produce the enhanced PL for optoelectronic devices like blue and UV light emitting diodes at room temperature.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Acknowledgments One of the authors (R.P.) thanks the Portuguese Foundation of Science and Technology for the postdoctoral fellowship [SFRH/BPD/20674/2004]. This work is financed by the project POCI/CTM/55945.
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[20] [21] [22]
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Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering R. Prabakaran a,⁎, T. Monteiro b , M. Peres b , A.S. Viana c , A.F. da Cunha b , H. Águas a , A. Gonçalves a , E. Fortunato a , R. Martins a , I. Ferreira a a
CENIMAT, Department of Materials Science and CEMOP/UNINOVA, Faculty of Sciences and Technology, New University of Lisbon, Caparica 2829-516, Portugal b Department of Physics, Aveiro University, 3810-193 Aveiro, Portugal c Laboratório de SPM da Faculdade de Ciências, Universidade de Lisboa, Ed. ICAT, Campo Grande, 1749-016 Lisboa, Portugal Available online 31 March 2007
Abstract In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO. © 2007 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Gallium zinc oxide; Photoluminescence; IR; XRD
1. Introduction Porous silicon (PS) consists of a network of Si nanocrystallites with nano/micro-pores. Strong PL in PS appears to be strongly influenced by surface hydrides, oxides and defect states. The remarkable room-temperature visible light emission from PS has stimulated striking effort towards developing silicon-based optoelectronic devices [1–3]. While PS shows significant luminescence over a wide spectrum, the photoluminescence (PL) properties undergo substantial degradation with aging due to atmospheric oxidation. Moreover, the tremendous reactivity and fragility of PS skeleton have hitherto prohibited its integration with conventional silicon processing technology. Thus encapsulation would be required for protecting the PS structure for overwhelming application. Nevertheless, this work illustrates that the optical and chemical stability ⁎ Corresponding author. Tel.: +351 219 48 300x10602; fax: +351 212 957 810. E-mail addresses: [email protected], [email protected] (R. Prabakaran). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.098
of PS can be greatly enhanced by coating gallium-doped zinc oxide (GZO) films. The GZO is a wide direct band gap (∼3.3 eV) semiconductor with such potential applications as transparent conductive films in optoelectronic devices like blue and ultraviolet (UV) light emitting diodes and laser diodes [4,5]. Recently, numerous investigations have been focused on the enhancement of PL and electroluminescence efficiency of PS by different routes [2]. It is important to tune the emission line to get enough efficiency for obtaining good PS devices. Two routes have been mainly focused to enhance emission efficiency: organic or inorganic covering/filling and PS doping. In our earlier investigation, we have improved the PL efficiency of PS and c-Si doping with O+ by implantation [6–8]. This work explores the possibility of the use of inorganic covering (GZO) to study their effects on the PL of PS, especially in the blue and UV region, for optoelectronic applications. In the present work, PS specimens were prepared with 10 and 420 min etching times from (100) oriented n-type c-Si. Parts of these PS were coated with GZO films by rf magnetron sputtering at room temperature (RT) while other PS samples
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were left uncoated. The X-ray diffraction (XRD), Fourier transform infrared studies (FTIR), atomic force microscopy (AFM) and PL measurements were carried out to investigate the optical, structural and microscopic properties of the PS specimens. The present work contributes to the understanding of the origin of blue and UV PL from PS and its enhancement after GZO films deposition. 2. Experimental details (100) Oriented phosphorous doped n-type c-Si having a resistivity of 1–2 Ω cm was used to prepare porous silicon at room temperature in an electrolytic etching bath of 1:2 mixture of HF (48%) and ethanol (98%) under a constant current density of 20 mA/cm2. With platinum wire as the cathode, anodization was carried out under illumination provided by a 50 W halogen lamp, placed 35 cm away from the specimen. The anodization was carried out for 10 and 420 min to obtain different porosities. After anodization the specimens were rinsed in de-ionized water and dried under a vacuum of 0.1 Pa for 10 h at RT. The estimated porosities for 10 and 420 min etched PS specimens from gravimetric method are ∼ 18% and 80%. Within a day after the formation of PS, some pieces were coated with GZO of 400 nm thickness and measured using a DEKTAK 3D surface profilometer from Sloan Tech. The GZO film were deposited by rf (13.56 MHz) magnetron sputtering using a ceramic oxide target ZnO:Ga2O3 (95:5 wt.%; 5 cm diameter) from LTS with a purity of 99.99%. The sputtering was carried out under RT and with: argon flux of 20 sccm; oxygen flux 0.4 sccm and a deposition pressure of 0.15 Pa. The distance between the
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substrate and the target was 10 cm and the rf power was maintained constant at 175 W [9]. XRD pattern of the specimens have been taken on a Rigaku DMAX III-C series diffractometer in θ–2θ geometry using CuKα radiation. The data have been collected in the 2θ range of 10° to 60°. Infrared measurements were carried out using a Thermo-Nicolet 6700 spectrophotometer operating with a resolution of 4 cm− 1, in the normal reflectivity mode. All measurements were recorded in the mid-infrared range (400– 4000 cm− 1) using globar source in combination with KBr beam splitter and a DTGS TEC detector. Tapping mode atomic force microscopy experiments were performed in a multimode AFM microscope coupled to a Nanoscope IIIa Controller. Commercial etched silicon tips with typical resonance frequency of ca. 300 kHz have been used as AFM probes. PL measurements were carried out at RT with a 325 nm cw He-Cd laser with an excitation power density typically less than 0.6 W cm− 2. The light collected in 90° geometry, was dispersed by a SPEX 1704 monochromator with a 1200 mm− 1 grating and detected by cooled Hamamatsu R928 photomultiplier. All the recorded spectra are corrected to spectral responses. 3. Results and discussion 3.1. X-ray diffraction measurements The XRD patterns of 10 min PS, 420 min PS, 10 min PS coated with GZO films and 420 min PS coated with GZO films are shown in Fig. 1(a), (b), (c) and (d) in the 2θ range from 10° to 60° respectively. Fig. 1(a) and (b) shows a peak at around
Fig. 1. The X-ray diffraction pattern of (a) 10 min etched PS, (b) 420 min etched PS, (c) 10 min etched PS coated with GZO and (d) 420 min etched PS coated with GZO.
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33.0° 2θ from (200) plane of c-Si. This forbidden peak is appeared due to the defects present in c-Si [10]. Our previous XRD investigation on c-Si and PS samples shows a strong peak at 69.1° 2θ which corresponds to (400) plane of the c-Si crystal [10]. However, this 2θ range is not recorded in the present work in order to protect the detector from high intense (400) c-Si peak. The strong peaks at 34.259° and 34.417° 2θ in Fig. 1(c) and (d) indicate the (002) plane of the GZO films deposited on 10 min and 420 min etched PS specimens respectively. The GZO deposited is polycrystalline with a hexagonal structure and a preferred orientation along the c-axis, that is, perpendicular to the PS specimens [9]. This peak is fitted with pseudovoigt function to get the peak parameters. The average particle size on the GZO films coated PS specimens, as estimated from FWHM using Scherrer formula, increases from 45 to 55 nm on increasing the etching time from 10 to 420 min. In addition, Fig. 1(c) and (d) shows other orientation peaks at around 31.9° 2θ (100), 36.4° 2θ (101), 47.6° 2θ (102) and 56.2° 2θ (110) [11,12]. The peaks related to (100) and (002) orientation in Fig. 1 (c) are enhanced in comparison with Fig. 1(d). It is to be noted that the major (002) peak and other peaks in Fig. 1(d) show a slight shift of ∼0.15° 2θ towards higher diffraction angle in comparison with Fig. 1(c). The observed peak shift might be originated from the incorporation of GZO films in the different pores of PS indicating the changes in lattice constant. 3.2. Fourier transform infrared studies FTIR spectra of 10 min PS, 10 min PS coated with GZO, 420 min PS and 420 min PS coated with GZO are shown in Fig. 2(a), (b), (c) and (d) respectively. The broad absorbance band around 1000 cm− 1 to 1300 cm− 1 corresponds to TO asymmetric stretching mode of the Si–O bond [13]. Si–H wagging, Si–H2 scissors and Si–H stretching vibration in PS specimens are appeared at around 662, 870, 910 and 2101 cm− 1 [14]. The appearance of bands at 406 and 627 cm− 1 is from Si– O–Si bending and Si–Si stretching mode, 1463 cm− 1 is from CH3 asymmetric deformation, 2197 cm− 1 is from Si–H stretching in SiO2–SiH, 2250 cm− 1 is from O–Si–H stretching, 2856 cm− 1 is from CH stretching in CH and 2927 cm− 1 is from CH stretching in CH2, 2958 cm− 1 is from CH stretching in CH3 [2,14]. The absorbance band at 805 cm− 1 is observed only in Fig. 2(c) which corresponds to bending mode of SiO2 [15]. The origins of 723 cm− 1 band in Fig. 2(c) and (d) and 1371 cm− 1 band in Fig. 2(c) are not clear at present. On exposing the freshly prepared PS specimens to atmospheric oxidation, Si–Hx band would decrease in intensity and Si–O band would grow. However, the PS specimens are not completed in oxidation and hence Si–Hx bands appeared in Fig. 2(a) and (c). Si–O stretching mode around 1068 cm− 1 in Fig. 2(b) is very weak as expected from GZO deposition. Conversely, the absorbance value of this mode is enhanced in the case of 420 min etched PS covered with GZO films. However, CH stretching in CH/CH2 and CH3 asymmetric deformation bands in Fig. 2(a) and (c) are completely disappeared after GZO films deposition [see Fig. 2(b) and (d)].
Fig. 2. The FTIR spectra of (a) 10 min etched PS, (b) 10 min etched PS coated with GZO, (c) 420 min etched PS and (d) 420 min etched PS coated with GZO.
Interestingly, in Fig. 2(d) new absorbance bands are observed at 827 cm− 1 and 2197 cm− 1 which are related to SiO bending in O–Si–O and Si–H stretching in SiO2–SiH [2]. Furthermore, the O–Si–H stretching mode at 2250 cm− 1 is enhanced in Fig. 2(d) after GZO deposition. 3.3. Atomic force microscopy In order to identify the microstructure of the PS specimens, 2D and 3D AFM images were undertaken on 10 min PS and 10 min PS coated with GZO films. Fig. 3(a) and (b) shows 3D and 2D profile obtained in a scanned area of 1 μm × 1 μm in 10 min etched PS specimen. A large number of distinguished Si nanocrystallites with pores in between can be visualized in both images. It is to be noted that Si nanocrystallites appear in spherzical shape, with sizes varying from 2 to 20 nm [see section analysis in Fig. 3(b)]. The root–mean–square (rms) roughness value observed here is 6.419 nm. Fig. 4(a) and (b) shows the 3D and 2D profiles of the AFM images of 10 min etched PS deposited with GZO films. The 2D and 3D surface structures are conspicuously different from Fig. 3. The spherical features and pore sizes are larger in comparison with Fig. 3. The overall estimated sizes of the
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Fig. 3. AFM image of 10 min etched PS. (a) 3D profile obtained in a 1 μm × 1 μm scanned area and data scale of 70 nm, (b) 2D profile obtained in a scanned area of 1 μm × 1 μm with respective section.
Fig. 4. AFM image of 10 min etched PS coated with GZO. (a) 3D profile obtained in a 1 μm × 1 μm scanned area and data scale of 70 nm, (b) 2D profile obtained in a scanned area of 1 μm × 1 μm with respective section.
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projections from the line profile in section analysis varied from 40 to 75 nm [see Fig. 4(b)]. The rms roughness value observed here is 14.136 nm. The arrowheads in Fig. 4(b) indicate GZO films coated PS of approximately 45 nm size which is well corroborated with the XRD Scherrer estimated size. These sizes do not correspond to the Si nanocrystallites in the GZO deposited PS. The GZO films deposition covers the entire Si nanocrystallites which results in overall increase in crystallite size. Moreover, this AFM investigation clearly showed colossal change to topographical characteristics between 10 min PS and 10 min PS coated with GZO films. 3.4. Photoluminescence The photoluminescence spectra of 10 min PS, 10 min PS coated with GZO films, 420 min PS and 420 min PS coated with GZO films were recorded in the energy range of 1.54 to 3.5 eV at RT and shown in Fig. 5(a), (b), (c) and (d) respectively. This PL was fitted with three Gaussian line shapes along with a
Fig. 5. Photoluminescence spectra of (a) 10 min etched PS, (b) 10 min etched PS coated with GZO, (c) 420 min etched PS and (d) 420 min etched PS coated with GZO. The solid lines are the fit to a Gaussian peak with a constant background. The peak positions are marked. The curves are shifted along the y-axis for clarity.
constant background to obtain the peak parameters. As observed with 3.81 eV excitation and at low temperature the main recombination processes are dominated by broad emission bands that cover all the studied spectral range. As the etching time increases from 10 min to 420 min the peak position of red emission band of PS shifted from 1.7917 eV to 1.89 eV due to quantum confinement effect giving a strong evidence of the band gap widening for the PS specimens [16]. The observed Si nanocrystallite sizes of less than 4 nm from AFM investigation, which is smaller than the exciton Bohr radius in Si, also support the quantum confinement effect. The peak position for GZO deposited PS is found to blueshift upon etching, namely 1.816 and 1.923 eV as compared to 1.7917 and 1.89 eV for uncoated PS specimens. H. Elhouichet and M. Oueslati [17] have reported a similar blueshift of PS after ZnO deposition. Liu et al. [18] have also reported that the red emission band of PS after deposition of ZnO shows an obvious higher energy shift that increases with an increase in template porosity. Fascinatingly, 420 min etched PS in Fig. 5(c) shows PL enhancement in blue (∼ 2.78 eV) and UV (∼ 3.2 eV) region in comparison with 10 min PS. Blue emission ∼ 2.78 eV in PS specimens has been frequently attributed to oxygen-related luminescence centers in typical surroundings of SiOx (0 b x b 2) [19]. In the present FTIR investigation [Fig. 2(c)] further confirms that the formation of oxygen related defects are responsible for the enhancement of ∼ 2.78 eV PL emission. The relative intensity of the blue PL emission is seen to increase after GZO film deposited on 420 min etched PS as shown in Fig. 5(d). Besides the blue recombination an UV emission band near 3.2 eV [Fig. 5(c)] has been attributed to intrinsic defects, Si–O related species and luminescence centers located at the interface between the Si nanocrystallites and the SiO2 matrix [20]. This UV PL is also enhanced after GZO deposition [see Fig. 5(d)]. This increase in UV emission might be originated from an overlap of emission bands that also account with the near-band-edge recombination of the wide band gap from GZO deposited films [21]. However, Tomioks and Adachi [22] have established the fact that the surface oxide layer (∼ 1065 cm− 1) acts as a good passivation film and gives rise to an efficient oxide edge (UV) emission at the PS/oxide interface. In the present FTIR investigation, the absorbance value observed around 1000 cm− 1 to 1300 cm− 1 from Si–O bond increases after GZO films deposition. In addition, new absorbance bands are observed at 827 cm− 1 and 2197 cm− 1 which are related to SiO bending in O–Si–O and Si–H stretching in SiO2–SiH. Also, the O–Si–H stretching mode at 2250 cm− 1 is enhanced [Fig. 2(d)] after GZO deposition. Hence, we believe that the PS/ oxide interface structural changes after GZO films deposition on 420 min etched PS might also result in an additional contribution to the PL enhancement in blue and UV regions. Before performing the GZO films coating, the PS samples are kept in high vacuum (10− 6 Torr), which might result in the removal of oxygen adsorption from PS surface. After deposition, GZO films are entirely covered on the PS surface and contribute to the surface passivation. Generally, huge porosity (∼ 80%) sample like 420 min PS might consist of complicated internal network structure. Hence, the total coverage and oxygen adsorption are
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less effective. Furthermore, some defect and recombination centers are kept and partially changed after GZO films coating. However, these structural changes are absent in the case of 10 min etched PS after GZO films deposition [see Fig. 2(b)], which might reduce the recombination centers and consequently, increases the non-radiation recombination and decreases the radiation recombination. Obviously, this results in PL quenching after GZO films coating on 10 min PS. 4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8]
The PL investigation on 10 min and 420 min etched PS shows a systematic blue shift from 1.7917 eV to 1.89 eV due to quantum confinement effect and this PL peak position is further blue shifted upon GZO films deposition. 420 min etched PS shows PL enhancement in blue (∼ 2.78 eV) and UV (∼3.2 eV) region in comparison with 10 min PS which might be due to luminescence centers of oxygen vacancy in SiO2 and luminescence centers located at the interface between the Si nanocrystallites and the SiO2 matrix. The enhancement of its intensity and wideness for GZO deposited films suggests that an overlap of emission bands occurs in these spectral regions due to the contribution of near-band-edge recombination from GZO. FTIR investigation further confirms the formation of surface oxide layer (1068 cm− 1) and intrinsic related defects on PS as well as GZO coated PS. Thus, we show the possible way to produce the enhanced PL for optoelectronic devices like blue and UV light emitting diodes at room temperature.
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Acknowledgments One of the authors (R.P.) thanks the Portuguese Foundation of Science and Technology for the postdoctoral fellowship [SFRH/BPD/20674/2004]. This work is financed by the project POCI/CTM/55945.
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