武 松，閆金良，焦淑娟

?

Nb摻雜濃度對(duì)SrTiO3的電子結(jié)構(gòu)和光學(xué)性能的影響

武 松，閆金良，焦淑娟

(魯東大學(xué)物理與光電工程學(xué)院，山東 煙臺(tái) 264025)

用第一性原理計(jì)算不同Nb摻雜濃度的n型Nb摻雜SrTiO3，研究了Nb摻雜濃度對(duì)SrTiO3的形成焓、電子結(jié)構(gòu)和光學(xué)性能的影響。在Nb摻雜SrTiO3中Nb替位Ti原子，與實(shí)驗(yàn)結(jié)果一致。Nb摻雜SrTiO3的費(fèi)米能級(jí)進(jìn)入導(dǎo)帶底部，Nb摻雜SrTiO3呈現(xiàn)n型半導(dǎo)體特征。從微觀角度分析了Nb摻雜濃度對(duì)導(dǎo)電性的影響，1.11at% Nb摻雜SrTiO3在可見(jiàn)光范圍有強(qiáng)吸收，是一種有潛在應(yīng)用的光催化材料；而1.67at%和2.5at% Nb摻雜SrTiO3是潛在的透明導(dǎo)電材料。

半導(dǎo)體摻雜； 電學(xué)性能； 光學(xué)性能； 電子結(jié)構(gòu)

1 Introduction

At room temperature, the strontium titanate (SrTiO3) is a typical perovskite type compound with a band gap of 3.2eV[1]. Because of its wide band gap, its photocatalytic properties are only effective in ultraviolet light region[2]. However, ultraviolet light is less than 5% of the whole solar light on the earth. In addition, SrTiO3is an insulator at room temperature. In order to ameliorate the conductivity and photocatalytic ability, some metal elements (Mg， Mn and Nb) and non-metal elements (S and N) have been introduced into pure SrTiO3[3-6], respectively. Among these studies, the metal Nb has been a focus in the research about the properties of impurity doped SrTiO3. Tomio et al found that electrical conductivities of Nb-doped SrTiO3films exhibit difference by changing the concentration of the Nb atom[7]. Nb doping can make SrTiO3change from an insulator to an n-type semiconductor with increasing Nb concentration[8-9]. Conductive Nb-doped SrTiO3films can be a candidate for electrodes that have stability at high temperature and in oxidized atmosphere[10]. However, few theoretical studies explicitly describe Nb-doping effect on electronic structures and optical properties of SrTiO3, and the origin of high photo-catalytic activity under visible light is not investigated.

In the present work, the formation enthalpy, electronic structure and optical absorption of the Nb-doped SrTiO3were computed to investigate the effect of Nb concentration on the electronic and optical properties of SrTiO3. It is helpful to provide a significant theoretical background for the experimental work in the future.

2 Calculation details

All calculations in this paper were carried out by the CASTEP tool within the MS 6.1 package. The interaction between the ion core and its valence electrons was described by the ultrasoft pseudopotential. Exchange correlation potential of electrons was approximated by PBE solution under the generalized gradient approximation (GGA)[11]. The supercell of pure SrTiO3was a cubic structure with Sr at the corner, Ti at the center and O at the face center, which was presented in Fig.1. To investigate the doping concentration effect, we created four supercells, which were 3×3×3 supercell containing 135 atoms, 3×3×2 supercell containing 90 atoms, 2×2×3 supercell containing 60 atoms, and 2×2×2 supercell containing 40 atoms, respectively. Each supercell was doped with one Nb atom at Sr site or Ti site. In other words, we had eight Nb-doped models to be optimized. They were named as Srn-1Nb1TinO3nor SrnNb1Tin-1O3nwith n=27, 18, 12, 8. Nb concentration in SrTiO3crystal was 0.74at%, 1.11at%, 1.67at% and 2.50at% respectively. The valence electronic configurations were 5s2for Sr, 3d24s2for Ti, 2s22p4for O and 4d45s1for Nb, respectively[9]. Monkhorst-Pack grid parameter was 2×2×2 for the pure SrTiO3supercell with 40 atoms, while it was 2×2×2, 2×2×3, 3×3×2, and 2×2×2 for Srn-1Nb1TinO3nor SrnNb1Tin-1O3nwith n=27, 18, 12, and 8, respectively. For convergence tolerance, the energy was set to be 5×10-5eV/atom, maximum force was 0.1eV/?, the largest stress on atoms was 0.2GPa, the displacement was no more than 5×10-3?.

Fig.1 Supercell of 2×2×2 cubic perovskite SrTiO3. The green, gray and red spheres represent Sr, Ti and O atoms, respectively

3 Results and discussion

3.1 Formation enthalpies and geometry parameters

The formation enthalpies (ΔHf) of Nb-doped SrTiO3were calculated to analysis the stability of Nb-doped SrTiO3crystal with different doping concentrations[12-13]. Table 1 lists the formation enthalpies of Nb-doped SrTiO3crystal from our calculation. The formation enthalpy is -19.83eV, -19.67eV, -19.47eV, and -19.13eV for Srn-1Nb1TinO3nwith n=27, 18, 12 and 8, respectively. Obviously, the supercell with smaller size will possess higher formation enthalpy. For SrnNb1Tin-1O3nmodel, we also find that its formation enthalpy increases by decreasing the size of supercell. Comparing to Srn-1Nb1TinO3n, SrnNb1Tin-1O3nshows lower formation enthalpy. From the viewpoint of formation enthalpy, the lower formation enthalpy suggests a more stable structure. But these values are still smaller than those of other compounds calculated in the same condition. We can thus certify that they are thermodynamically stable[14]. We can also conclude that one Nb atom can exist more stably at Ti site than at Sr site, and the size of Nb-doped supercell has a little effect on the formation enthalpy. These results are consistent with some previous report[9]. Through calculating the formation enthalpy, the following calculation and analysis only consider the case that one Ti atom is substituted by one Nb atom in SrTiO3supercell.

Table 1 Formation enthalpies of Nb-doped SrTiO3 with different concentration

Table 2 lists the lattice parameters, volumes, average Mulliken charges and average bond lengths of SrTiO3at various Nb concentrations. The optimized lattice parameters of pure SrTiO3area=b=c=3.938?. The experimental values and calculated values of lattice parameters obtained previously are also shown for comparison[15-17]. The lattice parameters obtained with the present calculations are closer with the experimental values than those obtained previously. Comparing to the experimental parameters ofa=b=c=3.906?, the deviation of our calculation value in lattice parameter is 0.81% (less than 1%), indicating that our calculation is reasonable and trustworthy. In table 2, the lattice parameters and the volumes of all Nb-doped SrTiO3systems show a slight expansion with the increase of Nb concentration. This phenomenon is due to the radius of Nb5+(0.064nm) is greater than that of Ti4+(0.061nm)[8].When more Nb atoms are introduced into the system, more Ti atoms can be replaced, which causes that the expansion of the lattice parameters and the volumes with Nb-doping concentration.

Mulliken charges of all atoms redistribute by one Nb atom replacing one Ti atom. We can find that average Mulliken charge of Ti declines by 4.5%, while average Mulliken charge of Sr increases by 2.2%—2.96%, and average Mulliken charge of O and Nb changes by 5.4% and 0.98% in the Nb-doping system, respectively. Redistribution of Mulliken charges affects the bond length between each atom. Average bond lengths of O-Nb in Nb-doped SrTiO3models are a little larger than that of O-Ti in the undoped SrTiO3model. This phenomenon can be explained by the fact that the ionic radius of Nb5+(0.064nm) is close to that of Ti4+(0.061nm). No significant lattice distortion has been found due to Nb doping at Ti site in cubic perovskite SrTiO3.

3.2 Electronic structures

Fig.2 shows the band structures of pure SrTiO3and 2.5at% Nb-doped SrTiO3. In Fig.2(a), the minimum of conduction band and the maximum of valence band are at G point and R point respectively, which proves that the band gap of pure SrTiO3is indirect, and this is consistent with Ref. [9, 17]. Our calculated indirect band gap of pure SrTiO3is 1.99eV, the direct band gap is 2.34eV. Benrekia reported that their calculated PBE indirect gap is 1.76eV and direct gap is 2.1eV for cubic SrTiO3[17], Wang also calculated the band gap of pure SrTiO3, which is 1.88eV[16]. Calculated value is smaller than the experimental value of 3.2eV. This is due to the well-known limitation of GGA. However, in the same computing conditions, it would not affect the analysis of electronic structure. From Fig.2(b), we can see that the Fermi level moves into the conduction band, which shows that Nb-doped SrTiO3is n-type semiconductor. The Nb atom substituting for the Ti atom introduces extra electrons into the system[8], which results in this phenomenon.

For further understanding the electronic structure of Nb-doped SrTiO3, the density of states (DOS) for Nb-doped SrTiO3is also calculated. From Fig.3(a), we can clearly see that the top of valence band is dominated by O 2p states, the bottom of conduction band is mainly contributed by Ti 3d and Nb 4d states. As can be seen, only Ti 3d and Nb 4d states possess occupied states of electrons in partial density of states of Sr, Ti, O and Nb. Occupied states of electrons are related to donor concentration. To distinguish them easily, we define the relative electron number (n) be the areas under a curve of the DOS versus energy, between the bottom of the conduction band and the Fermi level. As shown in Fig.3(b), the values of black and red points represent relative electron numbers of Nb 4d and Ti 3d at different Nb concentration, respectively. With the increase of Nb concentration in doped models, the relative electron numbers of Nb 4d states and Ti 3d states will increase. The relative electron number of Nb 4d states is less than that of Ti 3d states. Each supercell is doped with only one Nb atom at Ti site, the amount of Ti atoms is far more than that of Nb atoms in Nb-doped SrTiO3supercells. Therefore, the relative electron number of Nb-doped SrTiO3is mainly attributed to Ti atoms. Because relative electron number can affect on the conductivity of doped systems, the Nb impurity does not play the main role in improving the electrical conductivity. The above analysis reveals the argument from Ref. [18], X-ray absorption near edge region (XANES) indicated that Nb does not play a direct role to improve the electrical conductivity[18].

Fig.3(c) presents occupied states of Nb-doped SrTiO3at various concentrations. The values of four blue areas represent relative electron number. We can see that the relative electron number increases with the concentration of Nb. Therefore, the electronic conductivity of Nb-doped SrTiO3will be enhanced with increasing Nb concentration. In addition, the previous reports show that the conductivity of Nb-doped SrTiO3will be changed by controlling the Nb concentration[7,9]. The results of XANES measurements indicated that an increased amount of Nb in the lattice provided the positive effect of Nb on the electronic conductivity of Nb-doped SrTiO3[18]. This gives a reasonable explanation for experimental phenomena based on our calculation and analysis.

3.3 Optical properties

Optical properties are related to the interaction of photons with electrons in the system. In order to make our calculation results compatible with experimental values, we added a scissor approximation of 1.21eV to calculate the optical properties. Absorption spectra of pure and Nb-doped SrTiO3in visible light region are shown in Fig.4. The pure SrTiO3has no absorption in the visible region and absorption edge is found to be 3.1eV (400nm), which is consistent with the previous calculation results[11]. Obviously, comparing to undoped SrTiO3, all of Nb-doped SrTiO3systems

Fig.3 (a) The density of states for 2.5at% Nb-doped SrTiO3, (b) Relative electron numbers from Nb 4d and Ti 3d at different Nb-doping concentration, (c) Occupied states of Nb-doped SrTiO3 at various concentrations

Fig.4 Absorption of pure and Nb-doped SrTiO3 at different concentration, the inset shows the condensed plots

show absorption in visible light region, which come from the intra-band transition of electrons from Ti 3d states and Nb 4d states to O 2p states. All models have low absorption except for 1.11at% Nb-doped SrTiO3model showing a strong absorption centered at 2.03eV. This peak describes the strongest transitions from Ti 3d or Nb 4d states to O 2p states in the conduction band. The absorption in UV wavelength may come from inter-band transition.

As shown in Fig.3(c), some density of states are equal to zero approximately in the conduction bands, which astrict the transition from Ti 3d states or Nb 4d states to O 2p states in the conduction band. Therefore, these states can be considered to be a barrier for the transition from Ti 3d states or Nb 4d states to O 2p states. The conduction band bottom of Nb-doped SrTiO3is mainly contributed by Ti 3d and Nb 4d states, some Ti 3d states mix with Nb 4d states in the conduction band. The TDOS varies with Nb-doping concentration, and the width of barrier in the conduction band varies with Nb-doping concentrations. For 0.74at% Nb-doped SrTiO3, the width of barrier is 0.32eV, which results in the lowest intra-band absorption. The 1.11at% Nb-doped SrTiO3shows the strongest intra-band absorption, since there is no barrier in the conduction band. The visible light absorption induces the light electron-hole transportation and separation which could improve the absorption in visible regions in 1.11at% Nb-doped SrTiO3. For 1.67at% Nb-doped SrTiO3, the width of the barrier is 0.074eV, which decreases the absorption peak to 1332cm-1at 1.5eV. The barrier width of the 2.5at% Nb-doped SrTiO3is 0.153eV, which is wider than that of 1.67% Nb-doped SrTiO3. The 2.5at% Nb-doped SrTiO3shows weaker absorption than that of 1.67at% Nb-doped SrTiO3. In a word, the 1.11at% Nb-doped SrTiO3displays strong absorption in visible regions, and can be a candidate as photocatalytic material. The 1.67at% and 2.5at% Nb-doped SrTiO3models possess higher relative electron numbers and weaker optical absorption in visible lights, they can be applied as potential transparent conductive oxides (TCOs) in optoelectronic devices such as solar cells, light emitting diodes (LED), flat panel displays, and optical detectors as well as flexible displays.

4 Conclusion

In the present work, we have investigated the structural, electronic and optical properties of Nb-doped SrTiO3from first principles calculation. The formation enthalpy of SrnNb1Tin-1O3nis lower than that of Srn-1Nb1TinO3n, and the Nb atom preferentially substitutes for Ti atom in SrTiO3, which is in consistent with experimental results of Nb-doped SrTiO3films. Due to Nb-doping effect, the Fermi level moves into the conduction band and SrTiO3becomes n-type semiconductor. The relative electron number of Nb-doped SrTiO3is mainly attributed to Ti atoms, and the Nb dopant does not play a direct role to improve the electronic conductivity. The electronic conductivity of Nb-doped SrTiO3will be enhanced with the increasing of Nb concentration. The 1.11at% Nb-doped SrTiO3displays strong absorption in visible regions and may give better photocatalytic activity. The 1.67at% and 2.5at% Nb-doped SrTiO3materials have better conductivity and weaker absorption in visible light range. They can be potential transparent conductive oxides.

Reference

[1] Ryoko K, Tatsuya I, Hideki K, Akihiko K. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3under Visible Light Irradiation[J]. The Journal of Physical Chemistry B, 2004, 108(26): 8992～8995.

[2] Wang J S, Yin S, Komatsu M, Sato T. Lanthanum and Nitrogen Co-doped SrTiO3Powders as Visible Light Sensitive Photocatalyst[J]. Journal of the European Ceramic Society, 2005, 25(13): 3207～3212.

[3] Yang C X, Liu T Y, Cheng Z J, Gan H X, Chen J Y. Study on Mn-doped SrTiO3with First Principle Calculation[J]. Physica B, 2012, 407(5): 844～848.

[4] Paul S, Arvids S. Computational Study of Nb-doped SrTiO3[J]. Materials Letters, 2003, 57(12): 1844～1847.

[5] Zhang C, Jia Y Z, Jing Y, Yao Y, Ma J, Sun J H. Effect of Non-metal Elements (B, C, N, F, P, S) Mono-doping as Anions on Electronic Structure of SrTiO3[J]. Computational Materials Science, 2013, 79: 69～74.

[6] Wang J S, Li H, Li H L, Yin S, Sato T. Preparation and Photocatalytic Activity of Visible Light-active Sulfur and Nitrogen Co-doped SrTiO3[J]. Solid State Sciences, 2009, 11(1), 182～188.

[7] Tomio T, Miki H, Tabata H, Kawai T，Kawai S. Control of Electrical Conductivity in Laser Deposited SrTiO3thin Films with Nb Foping[J].Journal of Applied Physics, 1994, 76(10): 5886～5890.

[8] Zhu Y L, Ma X L, Li D X, Lu H B, Chen Z H, Yang G Z. Microstructural Analyses of a Highly Conductive Nb-doped SrTiO3Film[J]. Acta materialia, 2005, 53(5): 1277～1284.

[9] Zhang C, Wang C L, Li J C, Yang K, Zhang Y F. Substitutional Position and Insulator-to-metal Transition in Nb-doped SrTiO3[J]. Materials Chemistry and Physics, 2008, 107(2): 215～219.

[10] Guo X G, Chen X S, Sun Y L, Sun L Z, Zhou X H. Electronic Band Structure of Nb-doped SrTiO3from First Principles Calculation[J]. Physics Letters A, 2003, 317(5): 501～506.

[11] 趙銀女. Sn摻雜Ga1.375In0.625O3透明導(dǎo)電氧化物的第一性原理計(jì)算[J].材料科學(xué)與工程學(xué)報(bào),2015, 33(1): 46～50.

[12] Niu P J, Yan J L, Meng D L. The effects of N-doping and Oxygen Vacancy on the Electronic Structure and Conductivity of PbTiO3[J].Journal of Semiconductors, 2015, 36(4): 043004.

[13] Meng D L, Yan J L, Niu P J. Electronic Structure and Optical Properties of Ti-doped β-Ga2O3by First Principles Calculations[J]. 材料科學(xué)與工程學(xué)報(bào), 2015, 33(4): 484～489.

[14] 趙銀女. 氧空位對(duì)Ga1.5In0.5O3透明導(dǎo)電氧化物性能的影響[J]. 材料科學(xué)與工程學(xué)報(bào), 2015, 33(3): 396～400.

[15] Okhay O, Wu A Y, Vilarinho P M. The Mg Influence on the Properties of SrTiO3thin Films[J]. Journal of the European Ceramic Society, 2005, 25(12): 3079～3083.

[16] Wang C D, Qiu H, Inoue T, Yao Q. Highly Active SrTiO3for Visible Light Photocatalysis: A First-principles Prediction[J]. Solid State Communications, 2014, 181: 5～8.

[17] Benrekia A R, Benkhettou N, Nassour A, et al. Structural, electronic Electronic and optical Optical properties Properties of cubic Cubic SrTiO3and KTaO3: Ab initio Initio and GW calculationsCalculations[J]. Physica B, 2012, 407(13):2632～2636.

[18] Blennow P, Hagen A, Hansen K K, Wallenberg L R, Mogensen M. Defect and Electrical Transport Properties of Nb-doped SrTiO3[J]. Solid State Ionics, 2008, 179(35), 2047～2058.

YAN Jinliang (1965-), professor. E-mail: yanjinliang8@sina.com.

Electronic Structures and Optical Properties of Nb-doped SrTiO3at Different Concentrations

WU Song， YAN Jinliang， JIAO Shujuan

(School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China)

The n-type Nb-doped SrTiO3with different doping concentrations were studied by first principles calculations. The effects of Nb concentration on the formation enthalpy, electronic structure and optical property were investigated. Results show that Nb preferentially enters the Ti site in SrTiO3, which is in good agreement with experimental observation. The Fermi level of Nb-doped SrTiO3moves into the bottom of conduction band, and the system becomes n-type semiconductor. The effect of Nb-doping concentration on conductivity was discussed from the microscopic point of view. Furthermore, the 1.11at% Nb-doped SrTiO3shows strong absorption in visible light range and becomes a very useful material for photo-catalytic activity. The 1.67at% and 2.5at% Nb-doped models will be potential transparent conductive materials.

semiconductor doping; electric property; optical property; electronic structure

TN304.2 Document code：A

1673-2812(2017)02-0209-06

Foundation item：This work is supported by the National Natural Science Foundation of China (11504155), the Natural Science Foundation of Shandong Province, China (ZR2016FM38)

10.14136/j.cnki.issn 1673-2812.2017.02.008

Received date：2016-01-19；Modified date：2016-04-21

Biography：WU Song (1990-), male, Research field: Optoelectronic materials and devices. E-mail: 969677158@qq.com.