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Band structures and optical properties of Al-doped α-Si3N4: theoretical and experimental studies
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 3681–3686 www.elsevier.com/locate/ceramint
Band structures and optical properties of Al-doped α-Si3N4: theoretical and experimental studies Zhifeng Huang1, Zhihao Wang1, Fei Chenn, Qiang Shen, Lianmeng Zhang State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 12 October 2015; received in revised form 6 November 2015; accepted 6 November 2015 Available online 14 November 2015
Abstract The band structures and optical properties for Al-doped α-Si3N4 have been studied combining theoretical calculations and experimental measurements. The geometry and band structures for substitutional and interstitial Al defective α-Si3N4 models have been calculated based on density functional theory. The results indicate that the location and composition of the intermediate energy level in the band gap are affected by the partial atomic environment around the Al atoms and the characteristics of Al–N and Al–Si bonds. The maximum energy gap is observably decreased to 1.99 eV because of the appearance of intermediate energy levels in the band gap for interstitial Al defective α-Si3N4. Moreover, the energy gaps deduced from the measured absorption and photoluminescence spectra of the as-prepared Al-doped α-Si3N4 sample are in good agreement with the calculated results. The significant improvement in the optical properties by doping Al makes silicon nitride a potential visible light emitting material. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Band structure; Optical property; Al-doped α-Si3N4; Density functional theory
1. Introduction Silicon nitride (Si3N4) has potential applications in optoelectronic devices that are used under extreme physical environments, due to its excellent high-temperature properties and resistance to corrosion [1–3]. The optical and electrical properties of Si3N4 are dependent on the band structure [4]. One of the most effective methods to regulate the band structure of Si3N4 is to introduce appropriate doping elements [5–7]. Aluminum has been widely selected as an additive and a dopant to improve the sintering performance and to realize the superior optical properties of Si3N4 [8–12]. Investigations based on first-principles calculations have reported the influence of Al on the band structures of Si3N4 [10–12]. By first-principles calculations, Ding et al. reported that γ-Si3N4 exhibited the dielectric property at a low Al concentration, whereas the metallic property was exhibited at a high Al concentration [10]. Xiao et al. and Mao n
Corresponding author. Tel.: þ86 27 87168606; fax: þ 86 27 87879468. E-mail address: [email protected] (F. Chen). 1 These authors contributed equally to this study. http://dx.doi.org/10.1016/j.ceramint.2015.11.036 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
et al. showed that Al can significantly influence the band structure for α-Si3N4 and that the interstitial Al atom produces some impurity energy levels in the band gap [11,12]. It is widely known that the atomic environment around the doping atom is the main factor affecting the bond characteristics of the doping atom with its adjoining atoms. These bond characteristics determine the composition and location of impurity energy levels in the band gap [13,14]. However, currently, no research has reported the impact of atomic environments and bond characteristics on the band structures of Al-doped Si3N4. Simultaneously, various techniques have been developed to synthesize Al-doped Si3N4. These techniques include nitridation of the Si powders with additives [8], chemical vapor deposition (CVD) [12] and pyrolysis of polymer precursors [15,16]. It is well established that Si3N4 has two stable polymorphs under atmospheric conditions, i.e., α-Si3N4 (P31c) and β-Si3N4 (P63) [17,18]. As β-Si3N4 is thermodynamically stable compared with α-Si3N4, most of the reported Al-doped Si3N4 are β phase [19] or α and β mixed phases [15,16]. The impurity introduced by the additives is hard to eliminate, and this limits the investigations of Al-doped α-Si3N4. Therefore, using an appropriate
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method to prepare α phase Al-doped Si3N4 is a pivotal issue in investigating the electronic and optical properties of Al-doped α-Si3N4. The nitridation of the cryomilled nanocrystalline Si powders is an efficient method of synthesizing single-crystalline α-Si3N4 without any additives [20]. Herein, we select the method of nitriding cryomilled nanocrystalline Al-doped Si powders to prepare single-crystalline Al-doped α-Si3N4. Hence, in this paper, the band structures and optical properties of Al-doped α-Si3N4 are discussed combining density functional theory (DFT) calculation and experimental measurement. First, the geometry structures of Al-doped αSi3N4 models are calculated to study the partial atomic environments adjacent to the Al atom. Second, the band structures of Al-doped α-Si3N4 models are investigated in combination with the partial atomic environments and bond characteristics of Al–N and Al–Si bonds. Finally, the experimental absorption and photoluminescence properties of the asprepared Al-doped α-Si3N4 sample are discussed to confirm the influence of the doping Al atom on the band structures calculated by DFT. 2. Methodology 2.1. Theoretical calculations As there are two types of Si atoms and a large interstice in α-Si3N4, Si1 (0.2563, 0.1712, 0.4726) and Si2 (0.0821, 0.5089, 0.6828) [21,22], two Al substituting to Si defective α-Si3N4 models and one interstitial Al defective α-Si3N4 model were calculated, which were marked as AlSi1-α-Si3N4, AlSi2-α-Si3N4 and Ali-α-Si3N4, respectively. Both the cell parameters and the exact internal atomic coordinates were optimized independently using the accurate total energy calculations based on DFT using the CASTEP code [23,24]. The exchangecorrelation function was Perdew Burke Ernzerhof (PBE) of the generalized gradient approximation (GGA) [25]. The interaction between valence electrons and the ionic core was described by the ultrasoft pseudo potential [26]. To guarantee the calculation accuracy and efficiency, the 2 2 1 Al-doped α-Si3N4 supercells were calculated. A plane-wave cutoff energy of 400 eV and a k-point mesh of 3 3 5 were used for the geometry optimizations. Reference configurations for the valence electrons were Si 3s 3p, N 2s 2p and Al 3s 3p. The threshold for self-consistent field iterations was 5 10 5 eV per atom. The convergence tolerance parameters of the optimized calculation were the energy of 2 10 5 eV per atom, the maximum force of 0.05 eV Å, the maximum inner stress of 0.1 GPa and the maximum displacement of 2 10 4 nm. After completing geometry optimizations, the band structures of Al-doped α-Si3N4 models were calculated.
morphology and structure of the obtained Al-doped α-Si3N4 sample were characterized using X-ray diffractometer (XRD, Rigaku Ultima III, Japan) equipped with a Cu-Kα radiation source, field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) and high resolution transmission electron microscopy (HRTEM, JEM-2100F, Japan) with an energy-dispersive X-ray spectrometer. The absorption and photoluminescence spectra were measured using a UV–vis spectrophotometry (Lambda 750s, Perkin Elmer, US) and a laser micro-Raman spectrometer (inVia-Reflex, Renishaw, UK) under the excitation of a 325 He–Cd laser at room temperature.
3. Results and discussion 3.1. Optimized structures of Al-doped α-Si3N4 The optimized cell parameters and bond lengths for all Aldoped 2 2 1 α-Si3N4 supercells are summarized in Table 1. Compared with the cell parameters for pure 2 2 1 α-Si3N4 supercell (a¼ b ¼ 15.564 Å and c ¼ 5.640 Å) optimized with the same computational condition [22], the tiny changes indicate that the crystal structures for all Al-doped α-Si3N4 supercells are still trigonal. As shown in Table 1, there are four new Al–N bonds formed in the AlSi1-α-Si3N4 and AlSi2-αSi3N4 and one Al–N and two Al–Si bonds in the Ali-α-Si3N4. The average lengths of the Si–N bonds adjacent to the Al atoms are shorter by 1.9% and 2.5% for the AlSi1-α-Si3N4 and AlSi2-α-Si3N4 and larger by 2.1% for the Ali-α-Si3N4 compared with 1.743 Å in the pure α-Si3N4. The generation of the new bonds changes the lengths of the adjacent Si–N bonds in Al-doped α-Si3N4, resulting in the variations of the cell parameters. However, the small changes in the Si–N bonds adjacent to the Al atoms eliminate the lattice destructions.
Table 1 Optimized cell parameters and band lengths for Al-doped α-Si3N4 Dopedα-Si3N4
a
Cell parameter (Å)
Band length (Å)
a
b
c
Si–N
AlSi1
15.615
15.623
5.640
1.710
AlSi2
15.610
15.600
5.643
1.700
Ali
15.591
15.615
5.660
1.780
2.2. Experimental measurements To maintain the similar Al doping concentrations of the calculated Al-doped α-Si3N4 models, A 6.25 at% Al-doped αSi3N4 sample was synthesized using the direct nitriding cryomilled nanocrystalline Al-doped Si powders [20]. The
b
Al–N 1.810 1.833 1.838 1.858 1.819 1.840 1.848 1.860 2.129
Al–Si
2.668 2.735
In pure α-Si3N4: a¼b ¼15.564 Å and c ¼5.640 Å, the average bond length of Si–N is 1.743 Å. b The average length of Si–N bonds adjacent to the Al atom. a
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Fig. 2. Partial optimized geometry structures and corresponding electron density difference plots: (a) Al–N–Si in the AlSi1-α-Si3N4, (b) Al–N–Si in the AlSi2-α-Si3N4 and (c) Al–Si–N in the Ali-α-Si3N4. Table 2 Mulliken populations for Al-doped α-Si3N4. Doped α-Si3N4
Fig. 1. Band structures and partial density of states (PDOS) plots of the Al atom and its adjacent atoms for (a) AlSi1-α-Si3N4, (b) AlSi2-α-Si3N4 and (c) Ali-α-Si3N4.
3.2. Band structures of Al-doped α-Si3N4 Fig. 1 shows the calculated band structures and partial density of states (PDOS) plots of the Al atom and its adjacent atoms for all Al-doped α-Si3N4 models. The dashed lines stand for the Fermi level (EF). As shown in Fig. 1(a) and (b), there is no intermediate energy level in the band gap, and the
a
Average population
Al population N population Si population
Si
N
AlSi1
1.84
1.38
1.78
AlSi2
1.83
-1.38
1.82
Ali
1.82
1.37
Adjacent to Al
0.28
1.44 1.43 1.41 1.35 1.44 1.43(2) 1.41 1.31
Adjacent to Al
b
1.62 1.59
a The average populations of Si and N atoms are 1.84 and 1.38 in pure α-Si3N4. b The numbers in the parentheses show the same Mulliken populations.
compositions of the energy levels in the valence band and the conduction band are similar for the AlSi1-α-Si3N4 and AlSi2-αSi3N4, which means that the effects of Al substitution of the two different Si atoms on the band structures has almost no difference. The band energy gap (Eg) is approximately 4.30 eV
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Fig. 3. (a)XRD pattern, (b) SEM image, (c and d) TEM image and (f) EDX pattern for the as-prepared 6.25 at % Al-doped α-Si3N4 sample.
for the AlSi-α-Si3N4, which is consistent with previous results [16]. Compared with the Eg (4.78e V) for the pure α-Si3N4 under the same computational condition [22], the Eg is significantly reduced by the Al atom substituting the Si atom. It is worth noting that there are two intermediate energy levels appearing in the Ali-α-Si3N4, as shown in Fig. 1(c). The energy level at the middle of the band gap is composed of the hybrid electronic states of N 2p, Si 3p and Al 3s 3p, and the energy level below the conduction band consists of the electronic states of Si 3s and Al 3s. The compositions of the electronic states indicate that the intermediate energy levels in the band gap originate from the atomic interactions of the Al atom and its adjacent N and Si atoms of the Ali-α-Si3N4. The energy gaps are significantly narrowed to 1.99 eV and 1.42 eV on account of the appearance of the two intermediate energy levels in the band gap for Ali-α-Si3N4. To investigate the effects of bond characteristics on electronic properties, the electron density difference plots and Mulliken populations are discussed. Fig. 2 shows the partial
optimized geometry structures and the corresponding electron density difference plots of all Al-doped α-Si3N4 models. The electron density distribution along the Al–N bond is similar to that along the Si–N bond for the substitutional Al defective αSi3N4, as shown in Fig. 2(a) and (b), indicating the similar characteristics of the Al–N and the Si–N bonds. For the interstitial Al defective α-Si3N4, there is a small amount of electron density distribution between the Al and Si atoms, as shown in Fig. 2(c), confirming the electronic interactions of the Al and the two Si atoms. Table 2 shows the Mulliken populations of all Al-doped α-Si3N4 models. For the pure αSi3N4, the average populations of Si and N atoms are 1.84 and 1.38. The various Mulliken populations of the Si and N atoms adjacent to the Al atoms in Al-doped α-Si3N4 models indicate that the electron transfers have changed. For the substitutional Al defective α-Si3N4, the population of the Al atoms is similar to that of the Si atoms, suggesting that the electron transfer from the Al atom to its adjacent N atoms is similar to that from the Si atoms to the N atoms. As the
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Fig. 4. (a) UV–vis absorption spectrum of the Al-doped α-Si3N4 sample; (b) the relationship between (αhν)2 and hv; (c) photoluminescence spectrum for the Aldoped α-Si3N4 sample; and (d) the mechanism of electron transitions for Al-doped α-Si3N4. The unit of the energy gap is eV.
metallic property of Al is stronger than that of Si, the quantity of electrons transferring to the N atoms from the Al atom is more than that from the Si atoms. Hence, the population of N atoms adjacent to the Al atom is slightly reduced compared with the average population of N atoms. The absence of redundant valence electrons states results in no intermediate energy levels appearing in the band gap of AlSi-α-Si3N4, as shown in Fig. 1(a) and (b). Moreover, for the Ali-α-Si3N4, the populations of the two Si atoms adjacent to the interstitial Al atom are smaller than the average population of Si atoms, which means that the electronic interactions between the two Si atoms and the Al atom decrease the electrons transferring from the two Si atoms to its bonding N atoms. Succinctly, the different bond characteristics of the Al atom with its bonding atoms originates from the different partial atomic environments, giving rise to the different influences on the band structures of different Al-doped α-Si3N4 models. 3.3. Optical properties of Al-doped α-Si3N4 Fig. 3(a) shows the XRD pattern for the as-prepared 6.25 at% Al-doped α-Si3N4 sample. The strong and sharp peaks indicate that the sample is completely crystallized, which is in agreement with PDF card 09-0250, which is indexed as α-Si3N4. No residual Al or aluminum-containing compounds are observed,
indicating that Al has been incorporated into lattice of α-Si3N4. The microstructure of the sample has been observed by FESEM and HRTEM, as shown in Fig. 3(b), (c) and (d). The images (Fig. 3(b) and (c)) shows that the sample is composed of wirelike crystals with the uniform size of 30 nm in width and up to several tens of micrometers in length. From Fig. 3(d), the measured average distance between two planes is 0.39 nm, which corresponds to the d-space of the (110) plane (PDF card 09-0250). The typical EDX pattern (Fig. 3(f)) shows that the sample contains only N, Si and Al elements. Furthermore, the optical properties of the as-prepared Al-doped α-Si3N4 sample are investigated by UV–vis absorption and PL spectra. Fig. 4(a) shows the absorption spectrum of the Al-doped α-Si3N4 sample, of which the absorption band edge is located at approximately 300 nm. Fig. 4(b) exhibits the relationship between (αhν)2 and hv according to the following equation [27]: Kðhv ΔEÞ1=2 hv where the α is the absorption coefficient, K is a constant and ΔE is the energy gap. The two intersection points of the two epitaxial lines and the hv axis indicate that there are two energy gaps, namely, 1.52 eV and 4.32 eV in the Al-doped α-Si3N4 sample, which correspond well with the calculated energy gap of 1.42 eV in the band gap of Ali-α-Si3N4 and the band energy
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gap (Eg) of Al substituting to Si defective α-Si3N4. Further, the PL spectrum of the as-prepared Al-doped α-Si3N4 sample is shown in Fig. 4(c), with a strong emission peak at 610 nm, which is consistent with the calculated energy gap of 1.99 eV for the Ali-α-Si3N4, as shown in Fig. 1(c). Both the measured energy gaps by the absorption and PL spectra are in agreement with the calculated results of DFT. Additionally, the experimental results indicate that the substitutional and interstitial Al defects coexist in the as-prepared Al-doped α-Si3N4 sample. Consequently, the mechanism of electron transitions for the optical properties is revealed, as shown in Fig. 4(d). For example, the PL peak at 610 nm is attributed to the electron transitions from the Al–Si states to the hybrid states of Al–N and Si–N states. 4. Conclusions The band structures and optical properties for Al-doped α-Si3N4 have been studied combining the DFT calculations and spectra measurements. The substitutional Al defect mainly affects the relative locations of the valence band and conduction band, whereas the interstitial Al defect gives rise to two intermediate energy levels in the band gap for α-Si3N4. The different impacts on the band structures of different Al-doped α-Si3N4 originate from the various partial atomic environments adjacent to the Al atoms and the characteristics of the Al–N and Al–Si bonds. As a result, the energy gaps are observably decreased to 1.99 eV and 1.42 eV for Ali-α-Si3N4. Additionally, the energy gaps measured from the absorption and PL spectra of the as-prepared Al-doped α-Si3N4 are consistent with the calculated results. The electron transition contributing to the main PL peak is approximately 610 nm from the Al–Si states to the hybrid states of Al–N and Si–N states. The significant improvement of the optical properties by doping Al makes silicon nitride a potential visible light emitting material. Our work provides the theoretical and experimental foundation to investigate the electronic and optical properties of silicon nitride with dopant. Acknowledgment The project is supported by the National Natural Science Foundation of China (Nos. 51202171, 51472188, and 51521001), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120143120004) and the “111” project (No. B13035). References [1] F.L. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2) (2000) 245–265. [2] M. Belmonte, Advanced ceramic materials for high temperature applications, Adv. Eng. Mater. 8 (8) (2006) 693–703. [3] H. Klemm, Silicon nitride for high-temperature applications, J. Am. Ceram. Soc. 93 (6) (2010) 1501–1522. [4] L. Zhang, H. Jin, W. Yang, Z. Xie, H. Miao, L. An, Optical properties of single-crystalline alpha-Si3N4 nanobelts, Appl. Phys. Lett. 86 (6) (2005) 1908. [5] R.J. Xie, Optical properties of (oxy) nitride materials: a review, J. Am. Ceram. Soc. 96 (3) (2013) 665–687.
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 3681–3686 www.elsevier.com/locate/ceramint
Band structures and optical properties of Al-doped α-Si3N4: theoretical and experimental studies Zhifeng Huang1, Zhihao Wang1, Fei Chenn, Qiang Shen, Lianmeng Zhang State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 12 October 2015; received in revised form 6 November 2015; accepted 6 November 2015 Available online 14 November 2015
Abstract The band structures and optical properties for Al-doped α-Si3N4 have been studied combining theoretical calculations and experimental measurements. The geometry and band structures for substitutional and interstitial Al defective α-Si3N4 models have been calculated based on density functional theory. The results indicate that the location and composition of the intermediate energy level in the band gap are affected by the partial atomic environment around the Al atoms and the characteristics of Al–N and Al–Si bonds. The maximum energy gap is observably decreased to 1.99 eV because of the appearance of intermediate energy levels in the band gap for interstitial Al defective α-Si3N4. Moreover, the energy gaps deduced from the measured absorption and photoluminescence spectra of the as-prepared Al-doped α-Si3N4 sample are in good agreement with the calculated results. The significant improvement in the optical properties by doping Al makes silicon nitride a potential visible light emitting material. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Band structure; Optical property; Al-doped α-Si3N4; Density functional theory
1. Introduction Silicon nitride (Si3N4) has potential applications in optoelectronic devices that are used under extreme physical environments, due to its excellent high-temperature properties and resistance to corrosion [1–3]. The optical and electrical properties of Si3N4 are dependent on the band structure [4]. One of the most effective methods to regulate the band structure of Si3N4 is to introduce appropriate doping elements [5–7]. Aluminum has been widely selected as an additive and a dopant to improve the sintering performance and to realize the superior optical properties of Si3N4 [8–12]. Investigations based on first-principles calculations have reported the influence of Al on the band structures of Si3N4 [10–12]. By first-principles calculations, Ding et al. reported that γ-Si3N4 exhibited the dielectric property at a low Al concentration, whereas the metallic property was exhibited at a high Al concentration [10]. Xiao et al. and Mao n
Corresponding author. Tel.: þ86 27 87168606; fax: þ 86 27 87879468. E-mail address: [email protected] (F. Chen). 1 These authors contributed equally to this study. http://dx.doi.org/10.1016/j.ceramint.2015.11.036 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
et al. showed that Al can significantly influence the band structure for α-Si3N4 and that the interstitial Al atom produces some impurity energy levels in the band gap [11,12]. It is widely known that the atomic environment around the doping atom is the main factor affecting the bond characteristics of the doping atom with its adjoining atoms. These bond characteristics determine the composition and location of impurity energy levels in the band gap [13,14]. However, currently, no research has reported the impact of atomic environments and bond characteristics on the band structures of Al-doped Si3N4. Simultaneously, various techniques have been developed to synthesize Al-doped Si3N4. These techniques include nitridation of the Si powders with additives [8], chemical vapor deposition (CVD) [12] and pyrolysis of polymer precursors [15,16]. It is well established that Si3N4 has two stable polymorphs under atmospheric conditions, i.e., α-Si3N4 (P31c) and β-Si3N4 (P63) [17,18]. As β-Si3N4 is thermodynamically stable compared with α-Si3N4, most of the reported Al-doped Si3N4 are β phase [19] or α and β mixed phases [15,16]. The impurity introduced by the additives is hard to eliminate, and this limits the investigations of Al-doped α-Si3N4. Therefore, using an appropriate
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method to prepare α phase Al-doped Si3N4 is a pivotal issue in investigating the electronic and optical properties of Al-doped α-Si3N4. The nitridation of the cryomilled nanocrystalline Si powders is an efficient method of synthesizing single-crystalline α-Si3N4 without any additives [20]. Herein, we select the method of nitriding cryomilled nanocrystalline Al-doped Si powders to prepare single-crystalline Al-doped α-Si3N4. Hence, in this paper, the band structures and optical properties of Al-doped α-Si3N4 are discussed combining density functional theory (DFT) calculation and experimental measurement. First, the geometry structures of Al-doped αSi3N4 models are calculated to study the partial atomic environments adjacent to the Al atom. Second, the band structures of Al-doped α-Si3N4 models are investigated in combination with the partial atomic environments and bond characteristics of Al–N and Al–Si bonds. Finally, the experimental absorption and photoluminescence properties of the asprepared Al-doped α-Si3N4 sample are discussed to confirm the influence of the doping Al atom on the band structures calculated by DFT. 2. Methodology 2.1. Theoretical calculations As there are two types of Si atoms and a large interstice in α-Si3N4, Si1 (0.2563, 0.1712, 0.4726) and Si2 (0.0821, 0.5089, 0.6828) [21,22], two Al substituting to Si defective α-Si3N4 models and one interstitial Al defective α-Si3N4 model were calculated, which were marked as AlSi1-α-Si3N4, AlSi2-α-Si3N4 and Ali-α-Si3N4, respectively. Both the cell parameters and the exact internal atomic coordinates were optimized independently using the accurate total energy calculations based on DFT using the CASTEP code [23,24]. The exchangecorrelation function was Perdew Burke Ernzerhof (PBE) of the generalized gradient approximation (GGA) [25]. The interaction between valence electrons and the ionic core was described by the ultrasoft pseudo potential [26]. To guarantee the calculation accuracy and efficiency, the 2 2 1 Al-doped α-Si3N4 supercells were calculated. A plane-wave cutoff energy of 400 eV and a k-point mesh of 3 3 5 were used for the geometry optimizations. Reference configurations for the valence electrons were Si 3s 3p, N 2s 2p and Al 3s 3p. The threshold for self-consistent field iterations was 5 10 5 eV per atom. The convergence tolerance parameters of the optimized calculation were the energy of 2 10 5 eV per atom, the maximum force of 0.05 eV Å, the maximum inner stress of 0.1 GPa and the maximum displacement of 2 10 4 nm. After completing geometry optimizations, the band structures of Al-doped α-Si3N4 models were calculated.
morphology and structure of the obtained Al-doped α-Si3N4 sample were characterized using X-ray diffractometer (XRD, Rigaku Ultima III, Japan) equipped with a Cu-Kα radiation source, field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) and high resolution transmission electron microscopy (HRTEM, JEM-2100F, Japan) with an energy-dispersive X-ray spectrometer. The absorption and photoluminescence spectra were measured using a UV–vis spectrophotometry (Lambda 750s, Perkin Elmer, US) and a laser micro-Raman spectrometer (inVia-Reflex, Renishaw, UK) under the excitation of a 325 He–Cd laser at room temperature.
3. Results and discussion 3.1. Optimized structures of Al-doped α-Si3N4 The optimized cell parameters and bond lengths for all Aldoped 2 2 1 α-Si3N4 supercells are summarized in Table 1. Compared with the cell parameters for pure 2 2 1 α-Si3N4 supercell (a¼ b ¼ 15.564 Å and c ¼ 5.640 Å) optimized with the same computational condition [22], the tiny changes indicate that the crystal structures for all Al-doped α-Si3N4 supercells are still trigonal. As shown in Table 1, there are four new Al–N bonds formed in the AlSi1-α-Si3N4 and AlSi2-αSi3N4 and one Al–N and two Al–Si bonds in the Ali-α-Si3N4. The average lengths of the Si–N bonds adjacent to the Al atoms are shorter by 1.9% and 2.5% for the AlSi1-α-Si3N4 and AlSi2-α-Si3N4 and larger by 2.1% for the Ali-α-Si3N4 compared with 1.743 Å in the pure α-Si3N4. The generation of the new bonds changes the lengths of the adjacent Si–N bonds in Al-doped α-Si3N4, resulting in the variations of the cell parameters. However, the small changes in the Si–N bonds adjacent to the Al atoms eliminate the lattice destructions.
Table 1 Optimized cell parameters and band lengths for Al-doped α-Si3N4 Dopedα-Si3N4
a
Cell parameter (Å)
Band length (Å)
a
b
c
Si–N
AlSi1
15.615
15.623
5.640
1.710
AlSi2
15.610
15.600
5.643
1.700
Ali
15.591
15.615
5.660
1.780
2.2. Experimental measurements To maintain the similar Al doping concentrations of the calculated Al-doped α-Si3N4 models, A 6.25 at% Al-doped αSi3N4 sample was synthesized using the direct nitriding cryomilled nanocrystalline Al-doped Si powders [20]. The
b
Al–N 1.810 1.833 1.838 1.858 1.819 1.840 1.848 1.860 2.129
Al–Si
2.668 2.735
In pure α-Si3N4: a¼b ¼15.564 Å and c ¼5.640 Å, the average bond length of Si–N is 1.743 Å. b The average length of Si–N bonds adjacent to the Al atom. a
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Fig. 2. Partial optimized geometry structures and corresponding electron density difference plots: (a) Al–N–Si in the AlSi1-α-Si3N4, (b) Al–N–Si in the AlSi2-α-Si3N4 and (c) Al–Si–N in the Ali-α-Si3N4. Table 2 Mulliken populations for Al-doped α-Si3N4. Doped α-Si3N4
Fig. 1. Band structures and partial density of states (PDOS) plots of the Al atom and its adjacent atoms for (a) AlSi1-α-Si3N4, (b) AlSi2-α-Si3N4 and (c) Ali-α-Si3N4.
3.2. Band structures of Al-doped α-Si3N4 Fig. 1 shows the calculated band structures and partial density of states (PDOS) plots of the Al atom and its adjacent atoms for all Al-doped α-Si3N4 models. The dashed lines stand for the Fermi level (EF). As shown in Fig. 1(a) and (b), there is no intermediate energy level in the band gap, and the
a
Average population
Al population N population Si population
Si
N
AlSi1
1.84
1.38
1.78
AlSi2
1.83
-1.38
1.82
Ali
1.82
1.37
Adjacent to Al
0.28
1.44 1.43 1.41 1.35 1.44 1.43(2) 1.41 1.31
Adjacent to Al
b
1.62 1.59
a The average populations of Si and N atoms are 1.84 and 1.38 in pure α-Si3N4. b The numbers in the parentheses show the same Mulliken populations.
compositions of the energy levels in the valence band and the conduction band are similar for the AlSi1-α-Si3N4 and AlSi2-αSi3N4, which means that the effects of Al substitution of the two different Si atoms on the band structures has almost no difference. The band energy gap (Eg) is approximately 4.30 eV
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Fig. 3. (a)XRD pattern, (b) SEM image, (c and d) TEM image and (f) EDX pattern for the as-prepared 6.25 at % Al-doped α-Si3N4 sample.
for the AlSi-α-Si3N4, which is consistent with previous results [16]. Compared with the Eg (4.78e V) for the pure α-Si3N4 under the same computational condition [22], the Eg is significantly reduced by the Al atom substituting the Si atom. It is worth noting that there are two intermediate energy levels appearing in the Ali-α-Si3N4, as shown in Fig. 1(c). The energy level at the middle of the band gap is composed of the hybrid electronic states of N 2p, Si 3p and Al 3s 3p, and the energy level below the conduction band consists of the electronic states of Si 3s and Al 3s. The compositions of the electronic states indicate that the intermediate energy levels in the band gap originate from the atomic interactions of the Al atom and its adjacent N and Si atoms of the Ali-α-Si3N4. The energy gaps are significantly narrowed to 1.99 eV and 1.42 eV on account of the appearance of the two intermediate energy levels in the band gap for Ali-α-Si3N4. To investigate the effects of bond characteristics on electronic properties, the electron density difference plots and Mulliken populations are discussed. Fig. 2 shows the partial
optimized geometry structures and the corresponding electron density difference plots of all Al-doped α-Si3N4 models. The electron density distribution along the Al–N bond is similar to that along the Si–N bond for the substitutional Al defective αSi3N4, as shown in Fig. 2(a) and (b), indicating the similar characteristics of the Al–N and the Si–N bonds. For the interstitial Al defective α-Si3N4, there is a small amount of electron density distribution between the Al and Si atoms, as shown in Fig. 2(c), confirming the electronic interactions of the Al and the two Si atoms. Table 2 shows the Mulliken populations of all Al-doped α-Si3N4 models. For the pure αSi3N4, the average populations of Si and N atoms are 1.84 and 1.38. The various Mulliken populations of the Si and N atoms adjacent to the Al atoms in Al-doped α-Si3N4 models indicate that the electron transfers have changed. For the substitutional Al defective α-Si3N4, the population of the Al atoms is similar to that of the Si atoms, suggesting that the electron transfer from the Al atom to its adjacent N atoms is similar to that from the Si atoms to the N atoms. As the
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Fig. 4. (a) UV–vis absorption spectrum of the Al-doped α-Si3N4 sample; (b) the relationship between (αhν)2 and hv; (c) photoluminescence spectrum for the Aldoped α-Si3N4 sample; and (d) the mechanism of electron transitions for Al-doped α-Si3N4. The unit of the energy gap is eV.
metallic property of Al is stronger than that of Si, the quantity of electrons transferring to the N atoms from the Al atom is more than that from the Si atoms. Hence, the population of N atoms adjacent to the Al atom is slightly reduced compared with the average population of N atoms. The absence of redundant valence electrons states results in no intermediate energy levels appearing in the band gap of AlSi-α-Si3N4, as shown in Fig. 1(a) and (b). Moreover, for the Ali-α-Si3N4, the populations of the two Si atoms adjacent to the interstitial Al atom are smaller than the average population of Si atoms, which means that the electronic interactions between the two Si atoms and the Al atom decrease the electrons transferring from the two Si atoms to its bonding N atoms. Succinctly, the different bond characteristics of the Al atom with its bonding atoms originates from the different partial atomic environments, giving rise to the different influences on the band structures of different Al-doped α-Si3N4 models. 3.3. Optical properties of Al-doped α-Si3N4 Fig. 3(a) shows the XRD pattern for the as-prepared 6.25 at% Al-doped α-Si3N4 sample. The strong and sharp peaks indicate that the sample is completely crystallized, which is in agreement with PDF card 09-0250, which is indexed as α-Si3N4. No residual Al or aluminum-containing compounds are observed,
indicating that Al has been incorporated into lattice of α-Si3N4. The microstructure of the sample has been observed by FESEM and HRTEM, as shown in Fig. 3(b), (c) and (d). The images (Fig. 3(b) and (c)) shows that the sample is composed of wirelike crystals with the uniform size of 30 nm in width and up to several tens of micrometers in length. From Fig. 3(d), the measured average distance between two planes is 0.39 nm, which corresponds to the d-space of the (110) plane (PDF card 09-0250). The typical EDX pattern (Fig. 3(f)) shows that the sample contains only N, Si and Al elements. Furthermore, the optical properties of the as-prepared Al-doped α-Si3N4 sample are investigated by UV–vis absorption and PL spectra. Fig. 4(a) shows the absorption spectrum of the Al-doped α-Si3N4 sample, of which the absorption band edge is located at approximately 300 nm. Fig. 4(b) exhibits the relationship between (αhν)2 and hv according to the following equation [27]: Kðhv ΔEÞ1=2 hv where the α is the absorption coefficient, K is a constant and ΔE is the energy gap. The two intersection points of the two epitaxial lines and the hv axis indicate that there are two energy gaps, namely, 1.52 eV and 4.32 eV in the Al-doped α-Si3N4 sample, which correspond well with the calculated energy gap of 1.42 eV in the band gap of Ali-α-Si3N4 and the band energy
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gap (Eg) of Al substituting to Si defective α-Si3N4. Further, the PL spectrum of the as-prepared Al-doped α-Si3N4 sample is shown in Fig. 4(c), with a strong emission peak at 610 nm, which is consistent with the calculated energy gap of 1.99 eV for the Ali-α-Si3N4, as shown in Fig. 1(c). Both the measured energy gaps by the absorption and PL spectra are in agreement with the calculated results of DFT. Additionally, the experimental results indicate that the substitutional and interstitial Al defects coexist in the as-prepared Al-doped α-Si3N4 sample. Consequently, the mechanism of electron transitions for the optical properties is revealed, as shown in Fig. 4(d). For example, the PL peak at 610 nm is attributed to the electron transitions from the Al–Si states to the hybrid states of Al–N and Si–N states. 4. Conclusions The band structures and optical properties for Al-doped α-Si3N4 have been studied combining the DFT calculations and spectra measurements. The substitutional Al defect mainly affects the relative locations of the valence band and conduction band, whereas the interstitial Al defect gives rise to two intermediate energy levels in the band gap for α-Si3N4. The different impacts on the band structures of different Al-doped α-Si3N4 originate from the various partial atomic environments adjacent to the Al atoms and the characteristics of the Al–N and Al–Si bonds. As a result, the energy gaps are observably decreased to 1.99 eV and 1.42 eV for Ali-α-Si3N4. Additionally, the energy gaps measured from the absorption and PL spectra of the as-prepared Al-doped α-Si3N4 are consistent with the calculated results. The electron transition contributing to the main PL peak is approximately 610 nm from the Al–Si states to the hybrid states of Al–N and Si–N states. The significant improvement of the optical properties by doping Al makes silicon nitride a potential visible light emitting material. Our work provides the theoretical and experimental foundation to investigate the electronic and optical properties of silicon nitride with dopant. Acknowledgment The project is supported by the National Natural Science Foundation of China (Nos. 51202171, 51472188, and 51521001), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120143120004) and the “111” project (No. B13035). References [1] F.L. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2) (2000) 245–265. [2] M. Belmonte, Advanced ceramic materials for high temperature applications, Adv. Eng. Mater. 8 (8) (2006) 693–703. [3] H. Klemm, Silicon nitride for high-temperature applications, J. Am. Ceram. Soc. 93 (6) (2010) 1501–1522. [4] L. Zhang, H. Jin, W. Yang, Z. Xie, H. Miao, L. An, Optical properties of single-crystalline alpha-Si3N4 nanobelts, Appl. Phys. Lett. 86 (6) (2005) 1908. [5] R.J. Xie, Optical properties of (oxy) nitride materials: a review, J. Am. Ceram. Soc. 96 (3) (2013) 665–687.
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