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First-principles calculations of Cd-doped ZnO thin films deposited by pulse laser deposition
Solid State Sciences 14 (2012) 698e704
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First-principles calculations of Cd-doped ZnO thin films deposited by pulse laser deposition L.N. Bai a, b, B.J. Zheng a, J.S. Lian a, *, Q. Jiang a a
The Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130025, China The Key Laboratory of Semiconductor Nanocomposite Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2011 Received in revised form 20 February 2012 Accepted 26 March 2012 Available online 4 April 2012
Zn1xCdxO thin films are deposited on quartz substrate by pulse laser deposition. Their band structure and optical properties are experimentally and theoretically investigated. By varying Cd concentration, the band gap of Zn1xCdxO films can be adjusted in a wide range from 3.219 eV for ZnO to 2.197 eV for Zn0.5Cd0.5O, which produces different emissions from ultraviolet to Kelly light in their photoluminescence spectra. Simultaneity, the electronic structure and band gap of Zn1xCdxO are investigated by the density functional theory (DFT) with a combined generalized gradient approximation (GGA) plus Hubbard U approach, which precisely predicts the band-gaps of ZnO and Zn1xCdxO alloys. Both the experimental results and theoretical simulation reveal that with increasing Cd concentration in Zn1xCdxO alloys, their absorption coefficients in visible light range are evidently enhanced. The adjustable photoluminescence emission and enhanced visible light absorption endow Zn1xCdxO alloys potential applications in optoelectronic and photocatalytic fields. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Cd-doped ZnO First-principles Pulse laser deposited Optical properties Electronic properties
1. Introduction ZnO and doped ZnO have become important materials in technological applications for ultraviolet opt-electronic devices, lightemitting diodes, laser diodes and solar blind UV detectors [1]. ZnO crystal with wurtzite structure (whose space group is P63mc [2]) is a wide band gap semiconductor material with band gap energy of 3.37 eV and exciton binding energy of 60 meV. Doping is an effective way to ameliorate the optical and electrical properties of ZnO. There have been several experimental [3e8] and firstprinciples theoretical [8e15] studies on ZnO and Cd-doped ZnO, which have presented reasonable understanding on the effect of doping on the crystalline and electronic structures of ZnO. However, band gap modulation and related optical properties of Zn1xCdxO ternary alloys are not very clear. Particularly, the calculated band gap of ZnO is in the range of about 0.50e1.51 eV, which is much smaller than the experimental band gap value (3.37 eV). In present works, Cd-doped ZnO thin films on quartz substrate are synthesized by pulse laser deposition (PLD) and its structural and photoluminescence properties are characterized.
* Corresponding author. Tel.: þ86 431 85095875; fax: þ86 431 85095876. E-mail address: [email protected] (J.S. Lian). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.03.018
Simultaneously, the structure, chemical bond and optical characteristics of Zn1xCdxO ternary alloys are simulated by means of the density functional theory, which is implemented within the Cambridge serial total energy package (CASTEP) code [16e18]. In this code, we proposed a systematic and general approach with introducing the on-site Coulomb interaction parameter U, to calculate the band gap energies of ZnO and Zn1xCdxO (0 x 0.5) ternary alloys. The chemical bond and optical properties affected by Cd doping are investigated on the basis of theoretical calculations.
2. Experimental and computational methodology 2.1. Experimental methodology High purity cadmium (99.99%) and zinc (99.99%) powder were mixed with cadmium atom concentrations being Cd/ (Zn þ Cd) ¼ 0.02, 0.04, 0.07 and 0.15, respectively. The mixture was pressed under a hydraulic press of 2.5 107 Pa at room temperature to make disks, and then sintered for 2 h in tube furnace under argon atmosphere at 380 C to make targets. Zn1xCdxO films were prepared on quartz glass substrate by pulse laser ablation of the targets. Pulsed Nd: YAG laser with a wavelength of 1064 nm was used. The repetition rate was 10 Hz and the fluency on target was set at 60 J/cm2 for all samples. The distance between the target and
L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704
the quartz substrate was kept at 2.5 cm. The chamber was evacuated first to a base pressure below 5 104 Pa using a turbo molecular pump and then filled with oxygen to a pressure of 10 Pa. The films were deposited at a fixed temperature 300 C for 30 min, yielding a film thickness about 500 nm. The crystal structure of the films was analyzed by XRD (Rigaku Dymax) with a Cu target and a mono-chronometer at 40 kV and 250 mA. The Cd concentration of the films was determined by a SEM attached energy dispersive spectrometer (EDS). The optical properties of the films were characterized by photoluminescence spectrum excitated by a 325 nm HeeCd laser source and UVeVisible spectrophotometer. The film thickness was measured by Stylus Profiler (Veeco Dekatak 150).
Table 1 The possible Cd position of 2 2 2 32 atoms special configurations in wurtzite Zn1xCdxO (0 x 0.25) alloys. Systema
IDa
Site occupied by Cda
Symmetrya
Energy (eV)a
ZnO Zn0.9375Cd0.0625O Zn0.875Cd0.125O
0e1 1e1 2e1 2e2 2e3 3e1 3e2 3e3 4e1 4e2 4e3 4e4 4e5 4e6
Null 10 10,11 10,14 10,16 5,10,14 1,10,16 10,11,12 6,10,13,14 5,6,11,12 5,6,15,16 1,6,9,13 2,6,10,14 5,6,7,10
P63mc P3m1 Pm Cm P3m1 P1 Cm P3m1 P1 Pm Cm Cc Cmc21 P3m1
34,354.393 33,503.097 33,503.161 33,503.223 33,076.076 33,075.970 33,076.288 33,503.097 32,649.071 32,649.083 32,648.831 32,649.162 32,649.246 32,648.829
Zn0.8125Cd0.1875O
Zn0.75Cd0.25O
2.2. Computational methodology The theoretical calculations were performed on CASTEP code, which were based on density functional theory using ultrasoft pseudopotentials method. Exchange and correlation effects were described by the PerdeweBurkeeErnzerhof (PBE) scheme in the generalized gradient approximation (GGA) [19]. For valence electrons, the outermost s and d of Zn and Cd atoms, s and p of O atoms were taken into account in the model. The description of electronic structure of ternary alloy generally requires using a supercell in order to mimic the distribution of local chemical environments. Therefore, we used the wurtzite structure for Zn1xCdxO alloys, and two configurations of 2 2 2 (32 atoms) and 2 2 3 (48 atoms) were used, as shown in Fig. 1(a), where 1e16 Zn atoms in the ZnO lattice were depicted in a ball and stick representation. In order to enable the alloy configurations to converge to their stable conformation, we determine the preferred site of Cd atoms, and all of the possible structures were listed in Table 1. The results illustrate that the energies of Zn1xCdxO (0 x 0.25) ternary alloys for the same concentration x vary in a small range for different configurations. Therefore, the high-symmetry configurations were used to study the structural and optical properties of wurtzite structure Zn1xCdxO (0 x 0.5). Integrations in the brillouin zone were performed using k-point of 4 4 2 mesh parameter grid for 2 2 3 (48 atoms) configuration and 4 4 4 mesh parameter grid for 2 2 2 (32 atoms) configuration. An energy cutoff of 480 eV was used for the plane-wave basis set expansion. Tests of plane-wave cutoff and k-point sampling showed that our results were numerically converged to within 0.1 eV. The G point band gap was estimated after a self-consistent calculation that can generate high quality charge density with the optimized geometrical structure. In order to examine the relative stability of Cd-doped ZnO, the formation energies Eform(D,q) were calculated for the substitutions
699
a The first column gives the compositions of the supercell, the second column indicates the identity (ID) numbers to be used in the present paper, the third column shows the Zn sites replaced by Cd, the fourth columns presents the symmetry of Zn1xCdxO alloys and the last column is energy of the corresponding structure.
of Cd on the zinc site (CdZn), the oxygen site (CdO) and in the interstitial site (Cdi) in neutral charge state. The formation energy of defect D is expressed [20].
Eform ðD; qÞ ¼ Etot ðD; qÞ nZn mZn nO mO nCd mCd þ qme
(1)
Where Eform(D,q) is the total energy of a supercell containing defect D in the charge state q (q is different from zero in neutral charge state), ni is the number of corresponding species and mi is the chemical potential of Zn, O or Cd, me is the Fermi level with respect to the valence band edge. These chemical potentials are determined from separate calculations on an oxygen molecule, metallic zinc or cadmium. 3. Results and discussion 3.1. Experiment Elements of Cd, Zn and O are detected on the surface of Zn1xCdxO films and the actual Cd concentrations in the films are slightly higher than the concentration in the corresponding targets. So, the actual concentrations in the films are considered instead of the nominal compositions of the targets in the following part of the paper. The XRD patterns of the deposited Zn1xCdxO films with different Cd concentrations ranging from x ¼ 0 to x ¼ 0.151 are shown in Fig. 2. For the cadmium content of x 0.074, two peaks of (002) and (101) planes of the wurtzite structure being observed. At
Fig. 1. The structure of wurtzite bulk material, (a) 2 2 2 32 atoms special configurations; (b) The three doping sites of ternary alloy for 2 2 2 32 atoms configurations. Red smaller spheres represent O atoms, blue spheres represent Cd atoms and gray spheres represent Zn atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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*
cubic blende ZnO hexagonal wurtzite ZnO
x=0.151
x=0.151
x=0.074
Intensity (a. u.)
x=0.074
PL Intensity (a.u.)
(200)
*
+
x=0.044
x=0.024
x=0.044
x=0 400
x=0.024
500
600
700
(002)
+
Wavelength (nm) spectra
of
Zn1xCdxO
films
with
different
Cd
(101)
+
Fig. 3. Photoluminescence concentrations.
20
x=0 40
60
80
2Theta (degree) Fig. 2. The XRD data of the deposited Zn1xCdxO films with different Cd concentrations.
the cadmium content of x ¼ 0.151, a peak located at 39.130 is observed, which is the peak of (200) plane of cubic blende ZnO. So the film with x ¼ 0.151 is a mixture of two crystalline structure phases: hexagonal wurtzite ZnO and cubic blende ZnO. The phase transition should accommodate to increase the solubility of Cd in ZnO lattice. That is, heavily Cd-doped ZnO films can be achieved by the pulse laser deposition (without any detection of Cd or CdO phase). Simultaneously, the peak positions (002) and its full-width at half-maximum (FWHM) shift to lower 2q values, signifying increase for lattice constant, which is also observed in the other experiments [6,7]. The experimental PL spectra of the Zn1xCdxO with different Cd concentrations are shown in Fig. 3. With increasing Cd concentration to equal to and less than x ¼ 0.074, the near band edge (NBE) emission peak shows a red shift from 381 nm to 426 nm or from 3.252 eV to 2.908 eV. With further increasing Cd concentration to 0.151, the PL spectra show very wide visible light emission outspreading from 400 nm to 530 nm. This phenomenon is frequently observed in alloy semiconductors [7,21,22]. 3.2. Computation 3.2.1. Point defects of ternary alloys All the possible point defects of ternary alloys induce by Cd doping, including CdZn, CdO and Cdi, are considered. The calculations are performed for the 2 2 2 (32 atoms) configuration of ternary alloys. The three doping sites are indicated in Fig. 1(b). The lattice parameters, formation energies and bond lengths are given in Table 2. The doping of Cd leads to the increase of lattice spacing. The volume of ZnO is swelled after Cd doping. It is caused by the larger ionic radius of Cd2þ (0.97 Å) than that of Zn2þ (0.74 Å) and the strong repulsion between different cations of Cd and Zn.
However, the formation energies and bond lengths indicate that CdZn is a favored defect in ZnO lattice. The density of states (DOS) are calculated for ZnO and ternary alloys with CdZn, CdO and Cdi defects and shown in Fig. 4. Compared with the DOS of ZnO, the remarkable feature in the DOS for CdO and Cdi structures is that the Fermi level shifts upward into the conduction band, which indicates that the alloy is a n-type semiconductor. ZnO itself is a n-type conductor due to the intrinsic defects [23,24]. Our results show that Cd-doped ZnO is still n-type impurity dopants, CdO and Cdi are donors and they can compensate p-type doping. For CdO and Cdi structures, some unoccupied s states are below the Fermi energy, the electronic intraband transition from the occupied bands to the unoccupied ones would occur under irradiation, which may induce intense absorption in the long wavelength visible region [25]. Fig. 5 shows the chemical bond characteristics revealed by the electron densities of (1 1 0) plane of the ZnO, CdZn, CdO and Cdi structures. The strong ionic bond characteristic between Zn atoms and O atoms of CdZn and CdO structures are owing to almost no localization of charge in the middle of them, whereas a few localizations of charge between Zn atoms and O atoms of pure ZnO and Cdi structure imply a little weaker ionic bond than those of CdZn and CdO structures. In case of CdO structures, the electron clouds between Zn atoms and O atoms are significantly non-localization, it is very likely that CdO defect induces a prominent change of electronic structure in Cd-doped ZnO, which is in good agreement with the results obtained from DOS (in Fig. 4). So, it is deduced that both ZneO and CdeO bonds are mainly ionic bond, as Cd atom and Zn atom have same number of valence electrons.
Table 2 Crystal parameters and calculated defect formation energies of the ZnCdO ternary alloys (Cd concentration at x ¼ 0.0625) with geometry optimization.
c(Å) c/a Eform/2 atoms (eV) Bond length (Å)
ZnO
CdZn
CdO
Cdi
5.268 1.601 3.234 ZneO c axis 1.918
5.334 1.613 3.355 CdeO c axis 2.213
5.435 1.632 2.979 ZneCd c axis 2.557
5.084 1.579 2.894 ZneCd CdeO 2.405 2.165
L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704 50
50
ZnO
Density of states
40
30
20
20
10
10
0
0
-10
-10
-20
-20
-30
-30
-40
-40
-50
s p d
CdZn
40
30
50
701
-50 -20
-15
-10
-5
0
CdO
40
550
-20
30
30
20
20
10
10
0
0
-10
-10
-20
-20
-30
-30
-40
-40
-50
-15
-10
-5
0
5
-15
-10
-5
0
5
Cdi
40
-50 -20
-15
-10
-5
0
5
-20
Energy (eV) Fig. 4. The density of states calculated for ZnO, and the ternary alloy (Cd concentration x ¼ 0.0625) with CdZn, CdO and Cdi structures, respectively.
Fig. 5. The electron density in the (1 1 0) plane of ZnO, and the ternary alloy with CdZn, CdO and Cdi crystal structures, respectively.
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3.2.2. Perfect crystal of Zn1xCdxO ternary alloys The calculated equilibrium lattice constant c is plotted as a function of Cd concentration (0 x 0.5) and shown in Fig. 6. The corresponding experimental data are also shown in Fig. 6. The experimental c constant of pure ZnO lattice is 5.189 Å while the theoretical one is 5.293 Å, the two values agree well with each other within the better than 2%. It is noticed that doping of Cd induces a linear increase of lattice constant from 5.293 Å (x ¼ 0) to 5.624 Å (x ¼ 0.5) based on the theoretical calculation or from 5.189 Å (x ¼ 0) to 5.278 Å (x ¼ 0.151) based on the experiment. The increase of crystal lattice constant implies the substitution of Zn2þ by Cd2þ, as the ionic radius of Cd2þ is larger than that of Zn2þ. The dimensionless internal parameter u is defined as the ratio of the length of the bond parallel to the c axis to the lattice constant c. In ideal wurtzite structure characterized by equal lattice parameters, c/a and u are 1.633 and 0.375, respectively. For Cd concentration x less than or equal to 0.375, the internal parameter u (0.385 0.007) indicates a homogeneous expansion of the hexagonal wurtzite lattice due to Cd doping. However, the evident increase of internal parameter u (0.409) at high Cd concentration (x ¼ 0.5) indicates the abnormal or asymmetry expansion of the lattice along c axis. It is hence expected that further increase of Cd concentration should exceed the tolerance of hexagonal wurtzite lattice and induce the transformation to the cubic lattice of CdO. However, the phase separation is strongly dependent on prepared method and grain size, e.g. Ishihara et al. used remote-plasma-enhanced metal organic chemical vapor deposition to fabricate Cd-doped ZnO and the analyzed the phase separation concentration is Cd content x ¼ 0.7 [3]; Ma et al. predicted that the limit concentration of Cd in Zn1xCdxO alloy is x 0.53, based on the de reactive magnetron sputtering technique [4]. These two results are not far from the theoretical predition. But our experimental result and other results [6] are much lower than the theoretical prediction limit. So the experimental preparation method and technique parameters may greatly affect on Cd doping concentration. The band gap is calculated along the lines connecting the highsymmetry points in the BZ. It is well known that the DFT-GGA underestimates the band gaps of semiconductors and insulators. This deficiency of DFT-GGA has been widely discussed, and it has been attributed largely to the exchange-correlation energy [9]. The underestimate could be improved by a computational expedient post-GGA method (GGA þ U) [15,26e30]. The choice of U parameter is obviously crucial as the GGA þ U approach can lead to evident
improvements over the GGA description for real systems. In a previous description of bulk ZnO system, Coulomb interaction parameter U had been added on Zn-3d orbit [15]. It is not sufficient as the band gap of Zn1xCdxO (0 x 0.5) should be determined by all the Zn-3d, O-2p and Cd-4d orbits. Accordingly, the on-site Coulomb interaction should be introduced in all these orbits and appropriate U parameters should be selected. Initial approach for U parameters has to rely on adjusting the parameter to reproduce reasonable band gap value comparable to pure ZnO experimental results, and the same U values are used for Zn1xCdxO alloys. We have investigated how U parameter affects the position of valence band and conduction band edges separately. The electronic property of ZnO is calculated by GGA þ U with different U values. First, when the on-site Coulomb interaction is not considered in the system, the calculated band gap is 0.745 eV and Fermi energy of the system is 3.744 eV. When the on-site Coulomb interaction is only introduced in the Zn-3d orbits (U ¼ 4.5 eV for Zn), the calculated band gap is 1.010 eV and the corresponding Fermi energy is 3.448 eV. When the on-site Coulomb interaction is introduced into both the Zn-3d and O-2p orbits (U ¼ 8 eV for O), the calculated band gap is 3.219 eV, and corresponding Fermi energy is 1.460 eV. This band gap value agrees well with the experimental value of ZnO. It is found that the þU correction can make the Fermi level move to lower energy level, which is physically reasonable. Specifically, the þU correction acts as to open a gap between orbits of same symmetry, the opening of a gap in a p-d semiconductor is a secondorder effect that results from a rehybridisation of electronic states at the band edges. From the above calculation, it is understood that traditional DFT-GGA methods artificially aggrandize p-d coupling and lift the valence band up to unreasonable level, leading to a narrow of the calculated band gap. Therefore, the present GGA þ U approach with appropriate choice of U parameter can predict the valence band well and hence leads to the evident improvements for the GGA description of band gap of real ZnO system. Accordingly, in the following theoretical study of the structural and electronic properties of Zn1xCdxO alloys (0 x 0.5), the GGA þ U method will be applied with the on-site Coulomb interaction being introduced in the Zn-3d, O-2p and Cd4d orbits (U ¼ 2 eV for Cd). Our results show that Zn1xCdxO alloys hold the band structures characteristics of ZnO that both the top of valence band and the bottom of the conduction band are located at the G point. The 3.4
calculated values 3.2
calculated values experimental values experimental values
5.8
experimental values experimental values
Band gap (eV)
Lattice constant c
6.0
5.6
5.4
3.0
2.8
2.6
2.4 5.2
5.0
2.2
0.0
0.1
0.2
0.3
0.4
0.5
Cd concentration Fig. 6. The lattice constant c of Zn1xCdxO alloys, comparison between the theoretical prediction and experimental data. The data shown in hollow diamond came from Ref. [7].
2.0
0.0
0.1
0.2
0.3
0.4
0.5
Cd concentration Fig. 7. Band gaps of Zn1xCdxO alloys versus Cd concentration x, comparison between the theoretical calculation and experimental results. The data shown in hollow diamond came from Ref. [7].
L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704
very low band gap of Zn0.5Cd0.5O alloy can be explained by the even lower band gap of wurtzite structure CdO, which is 0.91 eV [13]. The PDOS of Zn1xCdxO alloys with different Cd concentrations are shown in Fig. 8. For Zn1xCdxO alloys, the first one is predominantly O 2s state. The remaining up to the top of valence band are Cd-4d, Zn-3d and O-2p states. Cd-4d state is gradually observed with increasing Cd concentrations, and mainly locates at an energy level deeper than Zn-3d state. The conduction band consists of the
60 40
ZnO -15
-10
-5
0
80
5
10
Zn0.875Cd0.125O
40 0 60
-15
40
-10
-5
0
5
10
Zn0.75Cd0.25O
20 0 60
-15
40
-10
-5
0
5
a
10
20000
E//c
Zn0.5Cd0.5O
20 0
-15
-10
-5
0
5
10
Absorption
Density of states
20 0 120
703
Energy (eV) Os Op
Zn s Zn d
Cd s Cd d
x=0.5 x=0.375 x=0.25 x=0.125 x=0
10000
Fig. 8. The partial density of states of Zn1xCdxO alloys with different Cd concentrations.
E⊥c
b
600
E⊥c
x=0.375 x=0.25 x=0.125 x=0
10000
400
500
600
60000
700
Wavelength (nm) 3.5
x=0.151 x=0.074 x=0.044 x=0.024 x=0
2.5
80000
700
x=0.5
3.0
ZnO CdZn CdO Cdi
100000
500
20000
0 300
Absorption
Absorption
120000
400
Wavelength (nm)
c
140000
2.0
1.5
1.0
40000
0.5
20000 0
0 300
Absorption
variation of band gap as a function of Cd concentration is shown in Fig. 7, which agrees well with the experimental bang gaps (hollow triangles) translated from the peak positions of the PL spectra shown in Fig. 3. Simultaneity, the calculated values of band gap are very close to the previous experimental results based on the monitoring the fundamental absorption edges of the room temperature absorption spectra [8]. It is seen that the band gap of Zn1xCdxO alloys decreases linearly with the increase of Cd concentration. The heavy doping of Cd induces an evident decrease of band gap. For example, the theoretical results shows that the alloy with Cd concentration x ¼ 0.167 gives a band gap value of 2.793 eV (444 nm) and that with Cd concentration x ¼ 0.25 gives the band gap value of 2.683 eV (462 nm). These band gaps correspond to blue emission, which are proved by the experimental PL spectra of the Zn1xCdxO alloys with the similar concentrations of x ¼ 0.161 and x ¼ 0.227 [7]. With further increase of Cd concentration, the band gap of Zn0.5Cd0.5O alloy decreases to 2.197 eV (564 nm) which corresponds to Kelly emission. This value is even lower than the band gap of steady rock-salt CdO (2.39 eV) [31]. The
0
5
10
15
20
25
Energy (eV) Fig. 9. The absorption spectrum of ZnO and the ternary alloys with different point defects introduced by Cd doping: CdZn, CdO and Cdi, respectively (GGA).
0.0 300
400
500
Wavelength (nm)
600
700
Fig. 10. The absorption spectra of Zn1xCdxO alloys in UV and visible light spectrum, (a) The absorption polarized parallel to the c axis; (b) The absorption polarized perpendicular to c axis; and (c) The experimental data of Zn1xCdxO thin films.
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hybrid states of Zn 4s, Cd 5s, O 2s and 2p. The emergence of Cd 5s state presents an effect on the conduction band maximum (CBM), since the hybridization of Cd 5s with Zn 4s states results in the falling of CBM. The narrowing of band structure involves the raise of the valence band (derived mostly from the anion p valence states) and the fall of the conduction band (derived mostly from cation s valence states). Fig. 9 shows the absorption spectra for light polarized perpendicular to c axic (Etc) of pure ZnO and ternary alloys with three different point defects induce by Cd doping. All the absorption spectra of ternary alloys exhibit peaks in the lower energy region although the main absorption spectra still locate at deep UV region. It is clear that the defects induce by Cd doping cause an increase of absorption in visible light range and CdO defect structure induces the most evident increase of absorption in visible light range. The increased visible light absorption would become a very important factor to influence the photocatalytic activity of Zn1xCdxO alloys. The experimental observed very wide visible light emission shown in Fig. 3 may be attributed to the introduction of various crystalline defects due to high concentration doping of Cd. To concentrate on the influence of Cd doping induces defects on visible light absorption, Fig. 10(a) and (b) show the calculated results of absorption spectra in wavelength range larger than 300 nm, for light polarized parallel to the c axis (Ekc) and light polarized perpendicular to c axic (Etc) of pure ZnO and Zn1xCdxO alloys, respectively. It is seen that the absorption edge of Zn1xCdxO (0 < x 0.5) alloys shift evidently to the low energy range with increasing Cd concentration compared to that of pure ZnO, although the main absorption part still locates at UV region. Moreover, the absorption coefficient of doped ZnO systems increases rapidly on the wavelength range larger than 300 nm, which is mainly ascribed to the enhancement of Cd-4d impurity states. Fig. 10(c) shows the experimental data of Zn1xCdxO with x 0.151. It is interesting that the theoretical absorption spectra agree well with the experimental spectra in their shape and red shift tendency with increasing Cd concentration. In numerical value, the experimental spectra are slightly red shift than the theoretical ones. This may be explained by the intrinsic defects and other possible defects introduced by Cd doping, as it is well known that the existence of various defects would decrease the actual band gap and hence shift the light absorption to visible direction, while the effects of various other defects are not considered in the present theoretical simulation. So, both the experimental and theoretical results show that doping of Cd is an effective way to red shift the band gap or PL spectrum of ZnO film from UV to visible lights, and for practical application, the bang gap or the main PL emission can adjusted in a wide UV and visible light range. 4. Conclusions The band structure and optical properties of Zn1xCdxO films were experimentally and theoretically investigated. Three possible point defects of Zn1xCdxO alloys introduced by Cd doping, including Cd on Zn site (CdZn), on O site (CdO) or in interstitial site (Cdi), were considered. By systematically calculating the formation energies, electronic structures and chemical bond of the possible point defects, it has been illuminated that Zn1xCdxO is n-type dopant, CdO and Cdi are donors and they can compensate p-type doping. It is found that by using a combined GGA þ U approach of electron states of Zn1xCdxO ternary alloys in the first-principles simulation, it is possible to achieve reasonable description of the
optical and electronic properties and yield a good agreement between the calculated and the experimental band gaps. These results reveal that the band gap of Zn1xCdxO films can be adjusted in a very wide range, to produce different emissions from ultraviolet to Kelly light in photoluminescence spectrum. By adjusting Cd concentration, the light absorption coefficient of Zn1xCdxO alloys in visible light range increases rapidly. The red shift of band gap and the enhanced absorption in visible light range would enable Zn1xCdxO alloys suitable for the applications in optoelectronic and photocatalytic fields.
Acknowledgment This work was supported by National Nature Science Foundation (Grant No. 50871046), the Foundation of National Key Basic Research and Development Program (No.2010CB631001) and the Program for Changjiang Scholars and Innovative Research Team in University. The authors also acknowledge the High Performance Computing Center (HPCC) of Jilin University for supercomputer time.
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First-principles calculations of Cd-doped ZnO thin films deposited by pulse laser deposition L.N. Bai a, b, B.J. Zheng a, J.S. Lian a, *, Q. Jiang a a
The Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130025, China The Key Laboratory of Semiconductor Nanocomposite Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2011 Received in revised form 20 February 2012 Accepted 26 March 2012 Available online 4 April 2012
Zn1xCdxO thin films are deposited on quartz substrate by pulse laser deposition. Their band structure and optical properties are experimentally and theoretically investigated. By varying Cd concentration, the band gap of Zn1xCdxO films can be adjusted in a wide range from 3.219 eV for ZnO to 2.197 eV for Zn0.5Cd0.5O, which produces different emissions from ultraviolet to Kelly light in their photoluminescence spectra. Simultaneity, the electronic structure and band gap of Zn1xCdxO are investigated by the density functional theory (DFT) with a combined generalized gradient approximation (GGA) plus Hubbard U approach, which precisely predicts the band-gaps of ZnO and Zn1xCdxO alloys. Both the experimental results and theoretical simulation reveal that with increasing Cd concentration in Zn1xCdxO alloys, their absorption coefficients in visible light range are evidently enhanced. The adjustable photoluminescence emission and enhanced visible light absorption endow Zn1xCdxO alloys potential applications in optoelectronic and photocatalytic fields. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Cd-doped ZnO First-principles Pulse laser deposited Optical properties Electronic properties
1. Introduction ZnO and doped ZnO have become important materials in technological applications for ultraviolet opt-electronic devices, lightemitting diodes, laser diodes and solar blind UV detectors [1]. ZnO crystal with wurtzite structure (whose space group is P63mc [2]) is a wide band gap semiconductor material with band gap energy of 3.37 eV and exciton binding energy of 60 meV. Doping is an effective way to ameliorate the optical and electrical properties of ZnO. There have been several experimental [3e8] and firstprinciples theoretical [8e15] studies on ZnO and Cd-doped ZnO, which have presented reasonable understanding on the effect of doping on the crystalline and electronic structures of ZnO. However, band gap modulation and related optical properties of Zn1xCdxO ternary alloys are not very clear. Particularly, the calculated band gap of ZnO is in the range of about 0.50e1.51 eV, which is much smaller than the experimental band gap value (3.37 eV). In present works, Cd-doped ZnO thin films on quartz substrate are synthesized by pulse laser deposition (PLD) and its structural and photoluminescence properties are characterized.
* Corresponding author. Tel.: þ86 431 85095875; fax: þ86 431 85095876. E-mail address: [email protected] (J.S. Lian). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.03.018
Simultaneously, the structure, chemical bond and optical characteristics of Zn1xCdxO ternary alloys are simulated by means of the density functional theory, which is implemented within the Cambridge serial total energy package (CASTEP) code [16e18]. In this code, we proposed a systematic and general approach with introducing the on-site Coulomb interaction parameter U, to calculate the band gap energies of ZnO and Zn1xCdxO (0 x 0.5) ternary alloys. The chemical bond and optical properties affected by Cd doping are investigated on the basis of theoretical calculations.
2. Experimental and computational methodology 2.1. Experimental methodology High purity cadmium (99.99%) and zinc (99.99%) powder were mixed with cadmium atom concentrations being Cd/ (Zn þ Cd) ¼ 0.02, 0.04, 0.07 and 0.15, respectively. The mixture was pressed under a hydraulic press of 2.5 107 Pa at room temperature to make disks, and then sintered for 2 h in tube furnace under argon atmosphere at 380 C to make targets. Zn1xCdxO films were prepared on quartz glass substrate by pulse laser ablation of the targets. Pulsed Nd: YAG laser with a wavelength of 1064 nm was used. The repetition rate was 10 Hz and the fluency on target was set at 60 J/cm2 for all samples. The distance between the target and
L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704
the quartz substrate was kept at 2.5 cm. The chamber was evacuated first to a base pressure below 5 104 Pa using a turbo molecular pump and then filled with oxygen to a pressure of 10 Pa. The films were deposited at a fixed temperature 300 C for 30 min, yielding a film thickness about 500 nm. The crystal structure of the films was analyzed by XRD (Rigaku Dymax) with a Cu target and a mono-chronometer at 40 kV and 250 mA. The Cd concentration of the films was determined by a SEM attached energy dispersive spectrometer (EDS). The optical properties of the films were characterized by photoluminescence spectrum excitated by a 325 nm HeeCd laser source and UVeVisible spectrophotometer. The film thickness was measured by Stylus Profiler (Veeco Dekatak 150).
Table 1 The possible Cd position of 2 2 2 32 atoms special configurations in wurtzite Zn1xCdxO (0 x 0.25) alloys. Systema
IDa
Site occupied by Cda
Symmetrya
Energy (eV)a
ZnO Zn0.9375Cd0.0625O Zn0.875Cd0.125O
0e1 1e1 2e1 2e2 2e3 3e1 3e2 3e3 4e1 4e2 4e3 4e4 4e5 4e6
Null 10 10,11 10,14 10,16 5,10,14 1,10,16 10,11,12 6,10,13,14 5,6,11,12 5,6,15,16 1,6,9,13 2,6,10,14 5,6,7,10
P63mc P3m1 Pm Cm P3m1 P1 Cm P3m1 P1 Pm Cm Cc Cmc21 P3m1
34,354.393 33,503.097 33,503.161 33,503.223 33,076.076 33,075.970 33,076.288 33,503.097 32,649.071 32,649.083 32,648.831 32,649.162 32,649.246 32,648.829
Zn0.8125Cd0.1875O
Zn0.75Cd0.25O
2.2. Computational methodology The theoretical calculations were performed on CASTEP code, which were based on density functional theory using ultrasoft pseudopotentials method. Exchange and correlation effects were described by the PerdeweBurkeeErnzerhof (PBE) scheme in the generalized gradient approximation (GGA) [19]. For valence electrons, the outermost s and d of Zn and Cd atoms, s and p of O atoms were taken into account in the model. The description of electronic structure of ternary alloy generally requires using a supercell in order to mimic the distribution of local chemical environments. Therefore, we used the wurtzite structure for Zn1xCdxO alloys, and two configurations of 2 2 2 (32 atoms) and 2 2 3 (48 atoms) were used, as shown in Fig. 1(a), where 1e16 Zn atoms in the ZnO lattice were depicted in a ball and stick representation. In order to enable the alloy configurations to converge to their stable conformation, we determine the preferred site of Cd atoms, and all of the possible structures were listed in Table 1. The results illustrate that the energies of Zn1xCdxO (0 x 0.25) ternary alloys for the same concentration x vary in a small range for different configurations. Therefore, the high-symmetry configurations were used to study the structural and optical properties of wurtzite structure Zn1xCdxO (0 x 0.5). Integrations in the brillouin zone were performed using k-point of 4 4 2 mesh parameter grid for 2 2 3 (48 atoms) configuration and 4 4 4 mesh parameter grid for 2 2 2 (32 atoms) configuration. An energy cutoff of 480 eV was used for the plane-wave basis set expansion. Tests of plane-wave cutoff and k-point sampling showed that our results were numerically converged to within 0.1 eV. The G point band gap was estimated after a self-consistent calculation that can generate high quality charge density with the optimized geometrical structure. In order to examine the relative stability of Cd-doped ZnO, the formation energies Eform(D,q) were calculated for the substitutions
699
a The first column gives the compositions of the supercell, the second column indicates the identity (ID) numbers to be used in the present paper, the third column shows the Zn sites replaced by Cd, the fourth columns presents the symmetry of Zn1xCdxO alloys and the last column is energy of the corresponding structure.
of Cd on the zinc site (CdZn), the oxygen site (CdO) and in the interstitial site (Cdi) in neutral charge state. The formation energy of defect D is expressed [20].
Eform ðD; qÞ ¼ Etot ðD; qÞ nZn mZn nO mO nCd mCd þ qme
(1)
Where Eform(D,q) is the total energy of a supercell containing defect D in the charge state q (q is different from zero in neutral charge state), ni is the number of corresponding species and mi is the chemical potential of Zn, O or Cd, me is the Fermi level with respect to the valence band edge. These chemical potentials are determined from separate calculations on an oxygen molecule, metallic zinc or cadmium. 3. Results and discussion 3.1. Experiment Elements of Cd, Zn and O are detected on the surface of Zn1xCdxO films and the actual Cd concentrations in the films are slightly higher than the concentration in the corresponding targets. So, the actual concentrations in the films are considered instead of the nominal compositions of the targets in the following part of the paper. The XRD patterns of the deposited Zn1xCdxO films with different Cd concentrations ranging from x ¼ 0 to x ¼ 0.151 are shown in Fig. 2. For the cadmium content of x 0.074, two peaks of (002) and (101) planes of the wurtzite structure being observed. At
Fig. 1. The structure of wurtzite bulk material, (a) 2 2 2 32 atoms special configurations; (b) The three doping sites of ternary alloy for 2 2 2 32 atoms configurations. Red smaller spheres represent O atoms, blue spheres represent Cd atoms and gray spheres represent Zn atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
700
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*
cubic blende ZnO hexagonal wurtzite ZnO
x=0.151
x=0.151
x=0.074
Intensity (a. u.)
x=0.074
PL Intensity (a.u.)
(200)
*
+
x=0.044
x=0.024
x=0.044
x=0 400
x=0.024
500
600
700
(002)
+
Wavelength (nm) spectra
of
Zn1xCdxO
films
with
different
Cd
(101)
+
Fig. 3. Photoluminescence concentrations.
20
x=0 40
60
80
2Theta (degree) Fig. 2. The XRD data of the deposited Zn1xCdxO films with different Cd concentrations.
the cadmium content of x ¼ 0.151, a peak located at 39.130 is observed, which is the peak of (200) plane of cubic blende ZnO. So the film with x ¼ 0.151 is a mixture of two crystalline structure phases: hexagonal wurtzite ZnO and cubic blende ZnO. The phase transition should accommodate to increase the solubility of Cd in ZnO lattice. That is, heavily Cd-doped ZnO films can be achieved by the pulse laser deposition (without any detection of Cd or CdO phase). Simultaneously, the peak positions (002) and its full-width at half-maximum (FWHM) shift to lower 2q values, signifying increase for lattice constant, which is also observed in the other experiments [6,7]. The experimental PL spectra of the Zn1xCdxO with different Cd concentrations are shown in Fig. 3. With increasing Cd concentration to equal to and less than x ¼ 0.074, the near band edge (NBE) emission peak shows a red shift from 381 nm to 426 nm or from 3.252 eV to 2.908 eV. With further increasing Cd concentration to 0.151, the PL spectra show very wide visible light emission outspreading from 400 nm to 530 nm. This phenomenon is frequently observed in alloy semiconductors [7,21,22]. 3.2. Computation 3.2.1. Point defects of ternary alloys All the possible point defects of ternary alloys induce by Cd doping, including CdZn, CdO and Cdi, are considered. The calculations are performed for the 2 2 2 (32 atoms) configuration of ternary alloys. The three doping sites are indicated in Fig. 1(b). The lattice parameters, formation energies and bond lengths are given in Table 2. The doping of Cd leads to the increase of lattice spacing. The volume of ZnO is swelled after Cd doping. It is caused by the larger ionic radius of Cd2þ (0.97 Å) than that of Zn2þ (0.74 Å) and the strong repulsion between different cations of Cd and Zn.
However, the formation energies and bond lengths indicate that CdZn is a favored defect in ZnO lattice. The density of states (DOS) are calculated for ZnO and ternary alloys with CdZn, CdO and Cdi defects and shown in Fig. 4. Compared with the DOS of ZnO, the remarkable feature in the DOS for CdO and Cdi structures is that the Fermi level shifts upward into the conduction band, which indicates that the alloy is a n-type semiconductor. ZnO itself is a n-type conductor due to the intrinsic defects [23,24]. Our results show that Cd-doped ZnO is still n-type impurity dopants, CdO and Cdi are donors and they can compensate p-type doping. For CdO and Cdi structures, some unoccupied s states are below the Fermi energy, the electronic intraband transition from the occupied bands to the unoccupied ones would occur under irradiation, which may induce intense absorption in the long wavelength visible region [25]. Fig. 5 shows the chemical bond characteristics revealed by the electron densities of (1 1 0) plane of the ZnO, CdZn, CdO and Cdi structures. The strong ionic bond characteristic between Zn atoms and O atoms of CdZn and CdO structures are owing to almost no localization of charge in the middle of them, whereas a few localizations of charge between Zn atoms and O atoms of pure ZnO and Cdi structure imply a little weaker ionic bond than those of CdZn and CdO structures. In case of CdO structures, the electron clouds between Zn atoms and O atoms are significantly non-localization, it is very likely that CdO defect induces a prominent change of electronic structure in Cd-doped ZnO, which is in good agreement with the results obtained from DOS (in Fig. 4). So, it is deduced that both ZneO and CdeO bonds are mainly ionic bond, as Cd atom and Zn atom have same number of valence electrons.
Table 2 Crystal parameters and calculated defect formation energies of the ZnCdO ternary alloys (Cd concentration at x ¼ 0.0625) with geometry optimization.
c(Å) c/a Eform/2 atoms (eV) Bond length (Å)
ZnO
CdZn
CdO
Cdi
5.268 1.601 3.234 ZneO c axis 1.918
5.334 1.613 3.355 CdeO c axis 2.213
5.435 1.632 2.979 ZneCd c axis 2.557
5.084 1.579 2.894 ZneCd CdeO 2.405 2.165
L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704 50
50
ZnO
Density of states
40
30
20
20
10
10
0
0
-10
-10
-20
-20
-30
-30
-40
-40
-50
s p d
CdZn
40
30
50
701
-50 -20
-15
-10
-5
0
CdO
40
550
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30
30
20
20
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10
0
0
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-10
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0
5
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-10
-5
0
5
Cdi
40
-50 -20
-15
-10
-5
0
5
-20
Energy (eV) Fig. 4. The density of states calculated for ZnO, and the ternary alloy (Cd concentration x ¼ 0.0625) with CdZn, CdO and Cdi structures, respectively.
Fig. 5. The electron density in the (1 1 0) plane of ZnO, and the ternary alloy with CdZn, CdO and Cdi crystal structures, respectively.
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L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704
3.2.2. Perfect crystal of Zn1xCdxO ternary alloys The calculated equilibrium lattice constant c is plotted as a function of Cd concentration (0 x 0.5) and shown in Fig. 6. The corresponding experimental data are also shown in Fig. 6. The experimental c constant of pure ZnO lattice is 5.189 Å while the theoretical one is 5.293 Å, the two values agree well with each other within the better than 2%. It is noticed that doping of Cd induces a linear increase of lattice constant from 5.293 Å (x ¼ 0) to 5.624 Å (x ¼ 0.5) based on the theoretical calculation or from 5.189 Å (x ¼ 0) to 5.278 Å (x ¼ 0.151) based on the experiment. The increase of crystal lattice constant implies the substitution of Zn2þ by Cd2þ, as the ionic radius of Cd2þ is larger than that of Zn2þ. The dimensionless internal parameter u is defined as the ratio of the length of the bond parallel to the c axis to the lattice constant c. In ideal wurtzite structure characterized by equal lattice parameters, c/a and u are 1.633 and 0.375, respectively. For Cd concentration x less than or equal to 0.375, the internal parameter u (0.385 0.007) indicates a homogeneous expansion of the hexagonal wurtzite lattice due to Cd doping. However, the evident increase of internal parameter u (0.409) at high Cd concentration (x ¼ 0.5) indicates the abnormal or asymmetry expansion of the lattice along c axis. It is hence expected that further increase of Cd concentration should exceed the tolerance of hexagonal wurtzite lattice and induce the transformation to the cubic lattice of CdO. However, the phase separation is strongly dependent on prepared method and grain size, e.g. Ishihara et al. used remote-plasma-enhanced metal organic chemical vapor deposition to fabricate Cd-doped ZnO and the analyzed the phase separation concentration is Cd content x ¼ 0.7 [3]; Ma et al. predicted that the limit concentration of Cd in Zn1xCdxO alloy is x 0.53, based on the de reactive magnetron sputtering technique [4]. These two results are not far from the theoretical predition. But our experimental result and other results [6] are much lower than the theoretical prediction limit. So the experimental preparation method and technique parameters may greatly affect on Cd doping concentration. The band gap is calculated along the lines connecting the highsymmetry points in the BZ. It is well known that the DFT-GGA underestimates the band gaps of semiconductors and insulators. This deficiency of DFT-GGA has been widely discussed, and it has been attributed largely to the exchange-correlation energy [9]. The underestimate could be improved by a computational expedient post-GGA method (GGA þ U) [15,26e30]. The choice of U parameter is obviously crucial as the GGA þ U approach can lead to evident
improvements over the GGA description for real systems. In a previous description of bulk ZnO system, Coulomb interaction parameter U had been added on Zn-3d orbit [15]. It is not sufficient as the band gap of Zn1xCdxO (0 x 0.5) should be determined by all the Zn-3d, O-2p and Cd-4d orbits. Accordingly, the on-site Coulomb interaction should be introduced in all these orbits and appropriate U parameters should be selected. Initial approach for U parameters has to rely on adjusting the parameter to reproduce reasonable band gap value comparable to pure ZnO experimental results, and the same U values are used for Zn1xCdxO alloys. We have investigated how U parameter affects the position of valence band and conduction band edges separately. The electronic property of ZnO is calculated by GGA þ U with different U values. First, when the on-site Coulomb interaction is not considered in the system, the calculated band gap is 0.745 eV and Fermi energy of the system is 3.744 eV. When the on-site Coulomb interaction is only introduced in the Zn-3d orbits (U ¼ 4.5 eV for Zn), the calculated band gap is 1.010 eV and the corresponding Fermi energy is 3.448 eV. When the on-site Coulomb interaction is introduced into both the Zn-3d and O-2p orbits (U ¼ 8 eV for O), the calculated band gap is 3.219 eV, and corresponding Fermi energy is 1.460 eV. This band gap value agrees well with the experimental value of ZnO. It is found that the þU correction can make the Fermi level move to lower energy level, which is physically reasonable. Specifically, the þU correction acts as to open a gap between orbits of same symmetry, the opening of a gap in a p-d semiconductor is a secondorder effect that results from a rehybridisation of electronic states at the band edges. From the above calculation, it is understood that traditional DFT-GGA methods artificially aggrandize p-d coupling and lift the valence band up to unreasonable level, leading to a narrow of the calculated band gap. Therefore, the present GGA þ U approach with appropriate choice of U parameter can predict the valence band well and hence leads to the evident improvements for the GGA description of band gap of real ZnO system. Accordingly, in the following theoretical study of the structural and electronic properties of Zn1xCdxO alloys (0 x 0.5), the GGA þ U method will be applied with the on-site Coulomb interaction being introduced in the Zn-3d, O-2p and Cd4d orbits (U ¼ 2 eV for Cd). Our results show that Zn1xCdxO alloys hold the band structures characteristics of ZnO that both the top of valence band and the bottom of the conduction band are located at the G point. The 3.4
calculated values 3.2
calculated values experimental values experimental values
5.8
experimental values experimental values
Band gap (eV)
Lattice constant c
6.0
5.6
5.4
3.0
2.8
2.6
2.4 5.2
5.0
2.2
0.0
0.1
0.2
0.3
0.4
0.5
Cd concentration Fig. 6. The lattice constant c of Zn1xCdxO alloys, comparison between the theoretical prediction and experimental data. The data shown in hollow diamond came from Ref. [7].
2.0
0.0
0.1
0.2
0.3
0.4
0.5
Cd concentration Fig. 7. Band gaps of Zn1xCdxO alloys versus Cd concentration x, comparison between the theoretical calculation and experimental results. The data shown in hollow diamond came from Ref. [7].
L.N. Bai et al. / Solid State Sciences 14 (2012) 698e704
very low band gap of Zn0.5Cd0.5O alloy can be explained by the even lower band gap of wurtzite structure CdO, which is 0.91 eV [13]. The PDOS of Zn1xCdxO alloys with different Cd concentrations are shown in Fig. 8. For Zn1xCdxO alloys, the first one is predominantly O 2s state. The remaining up to the top of valence band are Cd-4d, Zn-3d and O-2p states. Cd-4d state is gradually observed with increasing Cd concentrations, and mainly locates at an energy level deeper than Zn-3d state. The conduction band consists of the
60 40
ZnO -15
-10
-5
0
80
5
10
Zn0.875Cd0.125O
40 0 60
-15
40
-10
-5
0
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10
Zn0.75Cd0.25O
20 0 60
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40
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-5
0
5
a
10
20000
E//c
Zn0.5Cd0.5O
20 0
-15
-10
-5
0
5
10
Absorption
Density of states
20 0 120
703
Energy (eV) Os Op
Zn s Zn d
Cd s Cd d
x=0.5 x=0.375 x=0.25 x=0.125 x=0
10000
Fig. 8. The partial density of states of Zn1xCdxO alloys with different Cd concentrations.
E⊥c
b
600
E⊥c
x=0.375 x=0.25 x=0.125 x=0
10000
400
500
600
60000
700
Wavelength (nm) 3.5
x=0.151 x=0.074 x=0.044 x=0.024 x=0
2.5
80000
700
x=0.5
3.0
ZnO CdZn CdO Cdi
100000
500
20000
0 300
Absorption
Absorption
120000
400
Wavelength (nm)
c
140000
2.0
1.5
1.0
40000
0.5
20000 0
0 300
Absorption
variation of band gap as a function of Cd concentration is shown in Fig. 7, which agrees well with the experimental bang gaps (hollow triangles) translated from the peak positions of the PL spectra shown in Fig. 3. Simultaneity, the calculated values of band gap are very close to the previous experimental results based on the monitoring the fundamental absorption edges of the room temperature absorption spectra [8]. It is seen that the band gap of Zn1xCdxO alloys decreases linearly with the increase of Cd concentration. The heavy doping of Cd induces an evident decrease of band gap. For example, the theoretical results shows that the alloy with Cd concentration x ¼ 0.167 gives a band gap value of 2.793 eV (444 nm) and that with Cd concentration x ¼ 0.25 gives the band gap value of 2.683 eV (462 nm). These band gaps correspond to blue emission, which are proved by the experimental PL spectra of the Zn1xCdxO alloys with the similar concentrations of x ¼ 0.161 and x ¼ 0.227 [7]. With further increase of Cd concentration, the band gap of Zn0.5Cd0.5O alloy decreases to 2.197 eV (564 nm) which corresponds to Kelly emission. This value is even lower than the band gap of steady rock-salt CdO (2.39 eV) [31]. The
0
5
10
15
20
25
Energy (eV) Fig. 9. The absorption spectrum of ZnO and the ternary alloys with different point defects introduced by Cd doping: CdZn, CdO and Cdi, respectively (GGA).
0.0 300
400
500
Wavelength (nm)
600
700
Fig. 10. The absorption spectra of Zn1xCdxO alloys in UV and visible light spectrum, (a) The absorption polarized parallel to the c axis; (b) The absorption polarized perpendicular to c axis; and (c) The experimental data of Zn1xCdxO thin films.
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hybrid states of Zn 4s, Cd 5s, O 2s and 2p. The emergence of Cd 5s state presents an effect on the conduction band maximum (CBM), since the hybridization of Cd 5s with Zn 4s states results in the falling of CBM. The narrowing of band structure involves the raise of the valence band (derived mostly from the anion p valence states) and the fall of the conduction band (derived mostly from cation s valence states). Fig. 9 shows the absorption spectra for light polarized perpendicular to c axic (Etc) of pure ZnO and ternary alloys with three different point defects induce by Cd doping. All the absorption spectra of ternary alloys exhibit peaks in the lower energy region although the main absorption spectra still locate at deep UV region. It is clear that the defects induce by Cd doping cause an increase of absorption in visible light range and CdO defect structure induces the most evident increase of absorption in visible light range. The increased visible light absorption would become a very important factor to influence the photocatalytic activity of Zn1xCdxO alloys. The experimental observed very wide visible light emission shown in Fig. 3 may be attributed to the introduction of various crystalline defects due to high concentration doping of Cd. To concentrate on the influence of Cd doping induces defects on visible light absorption, Fig. 10(a) and (b) show the calculated results of absorption spectra in wavelength range larger than 300 nm, for light polarized parallel to the c axis (Ekc) and light polarized perpendicular to c axic (Etc) of pure ZnO and Zn1xCdxO alloys, respectively. It is seen that the absorption edge of Zn1xCdxO (0 < x 0.5) alloys shift evidently to the low energy range with increasing Cd concentration compared to that of pure ZnO, although the main absorption part still locates at UV region. Moreover, the absorption coefficient of doped ZnO systems increases rapidly on the wavelength range larger than 300 nm, which is mainly ascribed to the enhancement of Cd-4d impurity states. Fig. 10(c) shows the experimental data of Zn1xCdxO with x 0.151. It is interesting that the theoretical absorption spectra agree well with the experimental spectra in their shape and red shift tendency with increasing Cd concentration. In numerical value, the experimental spectra are slightly red shift than the theoretical ones. This may be explained by the intrinsic defects and other possible defects introduced by Cd doping, as it is well known that the existence of various defects would decrease the actual band gap and hence shift the light absorption to visible direction, while the effects of various other defects are not considered in the present theoretical simulation. So, both the experimental and theoretical results show that doping of Cd is an effective way to red shift the band gap or PL spectrum of ZnO film from UV to visible lights, and for practical application, the bang gap or the main PL emission can adjusted in a wide UV and visible light range. 4. Conclusions The band structure and optical properties of Zn1xCdxO films were experimentally and theoretically investigated. Three possible point defects of Zn1xCdxO alloys introduced by Cd doping, including Cd on Zn site (CdZn), on O site (CdO) or in interstitial site (Cdi), were considered. By systematically calculating the formation energies, electronic structures and chemical bond of the possible point defects, it has been illuminated that Zn1xCdxO is n-type dopant, CdO and Cdi are donors and they can compensate p-type doping. It is found that by using a combined GGA þ U approach of electron states of Zn1xCdxO ternary alloys in the first-principles simulation, it is possible to achieve reasonable description of the
optical and electronic properties and yield a good agreement between the calculated and the experimental band gaps. These results reveal that the band gap of Zn1xCdxO films can be adjusted in a very wide range, to produce different emissions from ultraviolet to Kelly light in photoluminescence spectrum. By adjusting Cd concentration, the light absorption coefficient of Zn1xCdxO alloys in visible light range increases rapidly. The red shift of band gap and the enhanced absorption in visible light range would enable Zn1xCdxO alloys suitable for the applications in optoelectronic and photocatalytic fields.
Acknowledgment This work was supported by National Nature Science Foundation (Grant No. 50871046), the Foundation of National Key Basic Research and Development Program (No.2010CB631001) and the Program for Changjiang Scholars and Innovative Research Team in University. The authors also acknowledge the High Performance Computing Center (HPCC) of Jilin University for supercomputer time.
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