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n-ZnO:Eu homojunction
Materials Science and Engineering B 178 (2013) 1032–1039
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Deposition of Na–N dual acceptor doped p-type ZnO thin films and fabrication of p-ZnO:(Na, N)/n-ZnO:Eu homojunction R. Swapna, M.C. Santhosh Kumar ∗ Advanced Materials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 20 November 2012 Received in revised form 8 May 2013 Accepted 10 June 2013 Available online 22 June 2013 Keywords: ZnO Thin films Spray pyrolysis Dual acceptor doping p-Type conductivity Homojunction
a b s t r a c t Sodium and nitrogen dual acceptor doped p-type ZnO (ZnO:(Na, N)) films have been prepared by spray pyrolysis technique at a substrate temperature of 623 K. The ZnO:(Na, N) films are grown at a fixed N doping concentration of 2 at.% and varying the nominal Na doping concentration from 0 to 8 at.%. The XRD results show that all the ZnO:(Na, N) films exhibited (0 0 2) preferential orientation. The EDX and elemental mapping analysis shows the presence and distribution of Zn, O, Na and N in the deposited films. The Hall measurement results demonstrate that the Na–N dual acceptor doped ZnO films show excellent p-type conduction. The p-type ZnO:(Na, N) films with comparatively low resistivity of 5.60 × 10−2 cm and relatively high carrier concentration of 3.15 × 1018 cm−3 are obtained at 6 at.%. ZnO based homojunction is fabricated by depositing n-type layer (Eu doped ZnO) grown over the p-type layer ZnO:(Na, N). The current–voltage (I–V) characteristics measured from the two-layer structure show typical rectifying characteristics of p-n junction with a low turn on voltage of about 1.69 V. The ZnO:(Na, N) films exhibit a high transmittance (about >90%) and the average reflectance is 8.9% in the visible region. PL measurement shows near-band-edge (NBE) emission and deep-level (DL) emission in the ZnO:(Na, N) thin films. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Today ZnO has become one of the hottest research fields in advanced materials and devices. It is emerging as one of the most important electronic and photonic materials. It has great potential applications in information technology, biotechnology, nanoscale science and engineering [1–3]. ZnO has been extensively studied for its promising applications in optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), because of its wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [4,5]. For many advanced applications, the development of ZnO based devices such as p–n homojunctions can be realized by utilizing both n-type and p-type ZnO films. However, the difficulty in achieving p-type ZnO impedes the development of such advanced devices. ZnO occurs naturally as an n-type semiconductor due to a large number of intrinsic defects such as oxygen vacancies (VO ), Zn interstitials (Zni ) and Zn antisite defects (ZnO ). Therefore, it is very difficult to form the shallow acceptor levels because these acceptors can be compensated by numerous ZnO native defects, resulting in the formation of deep donor level traps [6].
∗ Corresponding author. Tel.: +91 431 2503611; fax: +91 431 2500133. E-mail addresses: [email protected] (R. Swapna), [email protected], [email protected] (M.C. Santhosh Kumar). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.06.010
The p-type doping in ZnO can be realized by substituting either group V elements (N, P, As, and Sb) for O sites or group I elements (H, Li, Na, Ag and K) for Zn sites [7–12]. Among these acceptors, nitrogen (0.146 nm) is the most suitable dopant because of its similar ionic radius to oxygen (0.132 nm). Although considerable efforts have been focused to realize N-doped p-type ZnO and ZnO based p-n diodes, it is still difficult to achieve reproducible and good quality p-type conduction in N-doped ZnO [13,14]. One way to achieve a good solubility of N into ZnO is to use a dual acceptor method that uses two acceptors. Recently, Na doping gains more attention and the theoretical studies indicate that it produces shallow acceptor state for NaZn . Moreover, p–type ZnO thin films and ZnO based p–n homojunction fabricated by various methods prove Na as an effective p-type dopant in ZnO [15–17]. There have been several reports on the growth of dual acceptor p-type ZnO films such as dual acceptor doping of Li–N [18], Ag–N [19] and K–N [20] by various techniques. Though, there are a few reports on Na-N dual acceptor doping of ZnO films [21,22], still there is a scope for an extensive investigation of the properties of the ZnO:(Na, N) films fabricated by various deposition techniques. In this paper authors report a study of structural, electrical, optical properties and surface morphology of Na–N dual acceptor doped p-type ZnO films deposited on glass substrate by spray pyrolysis and the fabrication of p–n homojunction. Spray pyrolysis [23] has been developed as a powerful tool to prepare various kinds of thin films such as metal oxides, superconducting materials and nanophase materials. It has the advantages of low cost,
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Fig. 1. Schematic diagram of spray pyrolysis unit. Deposition apparatus: (1) air compressor; (2) pressure controller; (3) temperature controller; (4) nozzle for air; (5) nozzle for solution; (6) solution container; (7) stepper motor; (8) microprocessor controller; (9) temperature sensor; (10) heater setup; and (11) computer.
easy-to-use, safe and can be implemented in a standard laboratory. ZnO based p–n homojunctions are characterized by current–voltage (I–V) measurements. The current–voltage characteristics of the p–n junction are studied at room temperature. The experimental data from the I–V measurements of homojunctions are analyzed to calculate the ideality factor, series resistance and the barrier height. However, electrical characteristics of this diode are often influenced by various non-idealities such as interface state, interfacial oxide layer and series resistance. Therefore, the interface states and series resistance play an important role in determining the barrier height.
2. Experimental In the present work the ZnO films are prepared by, Holmarc spray pyrolysis equipment model HO-TH-04. A schematic diagram of the experimental setup is as shown in Fig. 1. The ZnO:(Na, N) thin films were deposited on glass substrates, with dimensions of 76.2 mm × 25.4 mm × 1.2 mm, at a substrate temperature of 623 K by spray pyrolysis technique. The precursor solution was prepared by dissolving 0.1 M of zinc acetate dihydrate [Zn(CH3 COO)2 2H2 O, Sigma–Aldrich, 99.5%] in a 100 ml mixture of 90 ml deionized water and 10 ml ethanol [CH3 CH2 OH, Merck, 99.9%]. Na and N doping was achieved by dissolving an appropriate quantity of sodium acetate [CH3 COONa, Merck, 98.5%] and ammonium acetate [CH3 COONH4 , Sigma–Aldrich, 98.5%] in the zinc acetate solution. The nominal Na concentration was varied from 0 to 8 at.% and the N concentration was 0 at.% and 2 at.%. A few drops of acetic acid [CH3 COOH, Merck, 99.9%] were added in to the spray solution to avoid the formation of milky precipitate of hydroxides [23]. The glass substrates were cleaned with detergent solution and deionized water. Further, ultrasonic cleaning was carried out for 30 min in an ultrasonic bath and then rinsed in acetone for 10 min. The distance between the spray nozzle and the glass substrate was maintained about 10 cm. Compressed air was used as the carrier gas at the pressure of 2 bar. The prepared solution was sprayed (3 ml/min) onto the clean glass substrates for a deposition time of 3 min. During spray pyrolysis deposition, a precursor solution was sprayed as fine droplets onto a heated substrate. When the droplets reach the heated substrate, they spread out and undergo pyrolytic decomposition. Finally, the solid compounds react to become a new chemical compound.
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The structural properties of the as-grown films were analyzed by a Rigaku X-ray diffractometer (D/Max ULTIMA III, Rigaku, Japan) ˚ at 40 kV and 30 mA over a 2 using CuK␣ radiation ( = 1.5406 A) range of 25–60◦ with a scan speed of 2◦ /min and a step size of 0.02◦ . The thickness was measured using Stylus profile meter. The average thickness of Na and N dual acceptor doped ZnO films is 987 nm. The surface morphology and the elemental composition of the films were assessed using a Hitachi-S3000N scanning electron microscope equipped with energy dispersive X-ray spectroscopy and elemental mapping facilities. Atomic force microscopy (NTMDTNTEGRA, Russia) was used to analyze the surface roughness of the films. The AFM 3D images were recorded over a scan area of 2 m × 2 m of the film surface. The optical measurements of dual acceptor doped ZnO thin films were carried out at room temperature using shimadzu UV-1700 Spectrophotometer in the wavelength range of 300–1000 nm. Photoluminescence spectra were recorded at room temperature using Perkin Elmer LS 55 Luminescence spectrophotometer with an excitation wavelength of 325 nm. Electrical resistivity, carrier concentration and mobility were measured at room temperature using Ecopia Hall measurement system (Model: HMS-5000) in the van der Pauw technique at a constant magnetic field of 0.5 T. As the first step in the fabrication of p-n homojunction, p-type ZnO film has been grown on glass substrate with 6 at.% Na and 2 at.% N doped ZnO. For n-type layer, 2 at.% europium (Eu) doped ZnO film has been grown over the p-type ZnO with masking for contacts. Here, a mask is designed using stainless steel sheet such that the active area of each device is 0.2 mm2 . Ohmic contacts have been made on both p-type and n-type ZnO films with indium by thermal evaporation. The I–V measurement on the fabricated p–n homojunction has been carried out using a Keithley 6517A electrometer. 3. Results and discussion 3.1. Structural studies Fig. 2 shows the XRD pattern of ZnO:(Na, N) films with different doping concentration. The crystallinity of the films has been studied by using XRD. It is seen that all the ZnO:(Na, N) films are preferentially oriented along (0 0 2) plane (c-axis) with hexagonal wurtzite structure and free from the formation of secondary phases. The low intensity peaks corresponding to (1 0 0), (1 0 1) and (1 1 0) plane diffraction have also been observed. A shift in the preferential growth from the (1 0 0) plane to (0 0 2) plane is observed when the dopant concentration increases from 0 to 2 at.%. Generally, the direction with the highest planar density of lattice points corresponds to the lowest surface free energy which facilitates an easier grain growth in that direction [24]. The change in the preferential growth from (1 0 0) plane to (0 0 2) plane observed in all the ZnO:(Na, N) films suggesting that the incorporation of Na and N in to ZnO enhances the grain growth by decreasing the surface free energy in (0 0 2) direction. As a result, the high quality ZnO:(Na, N) films exhibit strong (0 0 2) preferred orientation due to its lowest density of surface free energy [25]. The c-axis orientation along (0 0 2) reflection in ZnO:(Na, N) films can be understood from the “survival of the fastest” model proposed by Van der Drift [18]. According to this model, nucleations with various orientations can be formed at the initial stage of the deposition and each nucleus competes to grow but only nuclei having the fastest growth rate can survive. The average crystallite size of the ZnO films deposited at different Na and N concentrations has been calculated using the Scherrer’s formula [26]: D=
0.9 ˇ cos
(1)
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above the solubility limit leads to the formation of Na and N related defects which in turn degrades the crystallinity [14]. The dislocation density ı which represents the amount of defects in the films is determined using the formula [27]: ı=
1 D2
(2)
The dislocation density shows the decreasing trend with increasing Na content upto 6 at.% (Fig. 3). The dislocation density then increases for further increase of dopant concentration, which leads to the increase in the concentration of lattice imperfections. The larger D and smaller ı values indicate the better crystallization of the 6 at.% ZnO:(Na, N) films. The preferential growth orientation is determined using a texture coefficient TC(hkl). This factor is calculated using the following relation [28]:
TC(hkl) =
Fig. 2. XRD spectra of Na–N dual acceptor doped ZnO thin films with various dopant concentrations.
I(hkl) /Io(hkl) (1/N)
I(hkl) /Io(hkl)
(3)
where I(hkl) is the measured relative intensity of a plane (hkl), Io(hkl) is the standard intensity of the plane (hkl) taken from the JCPDS (75-0576) data, N is the reflection number. Randomly oriented crystallites have TC(hkl) = 1, while the larger value represents the greater abundance of crystallites oriented in that (hkl) direction. The texture coefficient calculated for the two main diffraction peaks, i.e. (0 0 2) and (1 0 1), are presented in Fig. 3. It can be seen that the highest TC is obtained for (0 0 2) plane in all the films. The high value of the texture coefficient indicates the maximum preferred orientation of the films along the diffraction plane. This means that the increase in preferred orientation is associated with increase in the number of grains along that plane. 3.2. Surface morphology and compositional analysis
where D is the average crystallite size (nm), is the wavelength of CuK␣ radiation, ˇ is the broadening of the diffraction line (FWHM) and is the Bragg’s diffraction angle. The instrumental broadening effect has been subtracted from the FWHM using the XRD pattern of a standard silicon sample. The crystallite size as a function of Na and N doping concentration is shown in Fig. 3. The average crystallite size of the films increases from 23.0 nm to 43.1 nm with the increase of dopant concentration from 0 to 6 at.%, thereafter it slightly decreases with further increase of dopant concentration to 8 at.%. It is understood that the crystallinity increases with the increase of doping concentration up to 6 at.%. For 8 at.% Na–N doped ZnO, the degradation in crystallinity is due to the solubility limit of Na and N in ZnO lattice sites. The presence of more Na and N atoms
Fig. 4 shows the surface morphology of ZnO:(Na, N) films with various doping concentration obtained from scanning electron microscopy (SEM). The surface of the ZnO:(Na, N) films is composed of regular dense grains with average diameter in the range of 40–60 nm. The change in grain morphologies is observed in the undoped and dual acceptor doped ZnO films. The EDX spectrum and elemental mapping results show the presence and distribution of Zn, O, Na and N for the ZnO:(Na, N) films with 6 at.% Na and 2 at.% N are shown in Fig. 5. The chemical compositions of the Zn, O, Na and N in the sample are identified as 16.76, 57.56, 24.38 and 1.31 at.%, respectively. It has been found that the concentration of O is greater than that of Zn. This lowering of the zinc content compared to oxygen also supports the incorporation of Na at the Zn site (NaZn ). The three-dimensional (3D) images of AFM micrographs are shown in Fig. 6 (a) and (b) for 2 and 6 at.% ZnO:(Na, N) thin films. The root mean square roughness (RMS) and mean roughness (Ra ) parameters are analyzed to study the surface quality of the ZnO:(Na, N) films. At 2 at.%, the RMS and mean roughness are observed as 14.149 nm and 11.762 nm, respectively. When the dopant concentration is further increases to 6 at.%, the values of RMS and mean roughness increases to 17.744 nm and 14.325 nm. The particle size of the deposited films increases from 52 to 76 nm with an increase of dopant concentration. It seen that the particle sizes observed from AFM pictures are larger than those determined from XRD. This finding indicates that the grains visible in AFM pictures are not single crystalline but consist of several crystallites. 3.3. Electrical studies
Fig. 3. Variation of crystallite size, dislocation density and TC(0 0 2), TC(1 0 1) as a function of (Na, N) concentration.
The electrical properties of both p-type and n-type ZnO films have been studied by Hall measurements in Van der Pauw
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Fig. 4. SEM images of the ZnO:(Na, N) films with (a) 0 at.% (b) 2 at.% (c) 4 at.% (d) 6 at.% and (e) 8 at.%.
Table 1 Variation of electrical resistivity (), carrier concentration (n) and mobility () of the Na–N doped ZnO (p-type) with various dopant concentration and 2 at.% Eu doped ZnO (n-type) thin films. Dopant (at.%) Na:N
Resistivity ( cm)
Carrier concentration (cm−3 )
Mobility (cm2 V−1 s−1 )
Carrier type
0:0 2:2 4:2 6:2 8:2 ZnO:Eu (2 at.%)
(3.33 ± 1.62) × 103 (4.06 ± 0.77) × 10−1 (1.41 ± 0.31) × 10−1 (5.60 ± 0.55) × 10−2 (7.56 ± 1.10) × 10−2 (2.93 ± 0.96) × 10−3
(5.65 ± 0.095) × 1015 (3.61 ± 0.21) × 1017 (8.31 ± 0.808) × 1017 (3.15 ± 1.22) × 1018 (1.85 ± 0.67) × 1018 (5.18 ± 1.42) × 1019
0.334 ± 0.78 41.9 ± 0.084 53.4 ± 2.31 35.7 ± 2.17 44.6 ± 0.49 41.1 ± 0.13
n p p p p n
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Table 2 Carrier concentrations as a function of the preservation period after deposition of ZnO:(Na, N) films with dopant concentration. Dopant (at.%) Na:N
As-prepared n0 (cm−3 )
After 90 days n90 (cm−3 )
After 180 days n180 (cm−3 )
Carrier type
2:2 4:2 6:2 8:2
(3.61 ± 0.21) × 10 (8.31 ± 0.808) × 1017 (3.15 ± 1.22) × 1018 (1.85 ± 0.67) × 1018
(2.96 ± 1.03) × 10 (6.54 ± 0.48) × 1016 (5.69 ± 0.92) × 1017 (1.51 ± 0.35) × 1017
(4.18 ± 0.67) × 1015 (7.43 ± 0.15) × 1015 (9.72 ± 0.53) × 1016 (3.21 ± 0.074) × 1016
p p p p
17
16
Fig. 5. EDX and elemental mapping results showing for 6 at.% ZnO:(Na, N) films.
method. The measured carrier concentration, mobility and resistivity of undoped (n-type), Na–N dual acceptor doped (p-type) and europium doped ZnO (n-type) films have been shown in Table 1. As grown pure ZnO film exhibits n-type conductivity. It shows high resistivity of 3.33 × 103 cm with a carrier concentration of 5.65 × 1015 cm−3 compared to that of ZnO:(Na, N) films. The p-type conduction has been confirmed by repeated Hall measurements. In dual acceptor doping, Na ion occupies the Zn site while N ion replaces the O ion, forming NaZn –NO acceptor complexes, which causes p-type conductivity [19]. The shallow acceptor with energy level of NaZn is 164 meV, which produces stable p-type conduction in ZnO [15]. Table 1 shows that the increase in hole concentration from 1017 to 1018 cm−3 with increase of dopant concentration. The highest carrier concentration of 3.15 × 1018 cm−3 with a low resistivity of 5.60 × 10−2 cm is obtained for the ZnO:(Na, N) films at the doping concentrations of 6 at.% Na and 2 at.% N. The decrease in hole concentration with higher Na concentration (8 at.%) is due to the presence of high concentration of impurities, which introduces uncompensated charged defects. The result of structural studies obtained from XRD also well supports the electrical results. In order to study the stability and reliability of the electrical properties for Na–N dual acceptor doped p-type ZnO films, aging analysis has been performed. The samples have been preserved in ordinary silica-gel desiccators for a period of 180 days. Table 2 shows the aging effect of carrier concentration with respect to the preservation period. Their p-type conductivities are relatively stable and do not convert to n-type even over 180 days, which implies the good stability of the films. However, as observed from Table 2, the carrier concentration decreases with time. In p-type ZnO:(Na,N) films, the adsorbed O tends to occupy its own site which in turn develops a stress in forcing N atoms from substitutional site to interstitial site. With aging for 180 days, adsorbed O creates stress so both carrier concentration and mobility decrease as scattering at the grain boundary and trap enhances. Further, the Zn O bond is more stable compared to
Fig. 6. AFM 3D images of (a) 2 at.% and (b) 6 at.% ZnO:(Na, N) films.
Zn N bond as O is more electronegative than N. The N acceptors slowly migrate from substitutional site to a neighboring interstitial site and instead create donors which gradually convert the good p-type film to less conducting p-type and this results in increase in resitivity and decrease in hole concentration [29]. 3.3.1. Current–voltage characteristics Based on the above study on the properties of the Na N dual acceptor doped (p-type) and Eu doped (n-type) ZnO films, consequently, the I–V characteristics of the two-layer-structured ZnO homojunction consisting of the n-type layer and p-type layer processed with spray pyrolysis technique is investigated, as shown in Fig. 7. The schematic structure of the p-ZnO:(Na, N)/n-ZnO:Eu homojunction is shown in the lower inset of Fig. 7. In Fig. 7, the nominal doping concentrations are 6 at.%, 2 at.% and 2 at.% respectively for Na, N and Eu. The I–V measurement is carried out using a Keithley 6517A electrometer. Indium is used as electrode, because it has
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Fig. 9. Plot of d(V)/d(ln I) versus I for the forward bias. Fig. 7. I–V characteristics of the p-ZnO:(Na, N)/n-ZnO:Eu homojunction.
low metal work function to form ohmic contacts [19]. The indium electrodes for the electrical contacts have been made on both ptype and n-type ZnO layers by thermal evaporation. The ohmic behavior of contacts to the p-ZnO:(Na, N) and n-ZnO:Eu layers are confirmed by the linear I–V dependencies as shown in Fig. 8. The I–V characteristics of the p–n homojunction show the typical rectifying behavior of a diode, which indicates the formation of p-type ZnO films by Na–N dual acceptor doping. The current rectification ratio IF /IR for p–n homojunction diode is ∼12.5 at a bias voltage of ±5 V and a reverse current of ∼1.48 A at −5 V (IF and IR stands for forward and reverse currents, respectively). The turn on voltage appears at 1.69 V under forward-biased voltage. Chu et al. [30] reported a turn on voltage of around 6 V for p-ZnO/n-ZnO homojunction. This value is much higher than that of our study. The turn on voltages found by several authors scatter mostly around 1–3 V. The reason for this variation may be attributed to the high defect concentration in the interface. A low value of the turn on voltage is critically important in device applications. The lower leakage current has been observed under reverse bias. The leakage current is
due to the imbalance between the carrier partial currents which is caused by the difference in carrier concentration and mobilities of p and n layers [31]. According to thermionic emission model [32], the current flowing through a junction is given as
I = Is exp
qV nkT
−1
(4)
where IS is the reverse saturation current, q is the electronic charge, k is the Boltzmann constant, T is the absolute temperature, V is the voltage across the junction, n is the ideality factor. The value of Is has been found as 6.96 A by extrapolating the linear mid bias region of the semilog plot to zero applied voltage. The effect of diode series resistance, interface states and ideality factor (>1), it deviates from the ideal thermionic theory. In order to extract these parameters we used Cheung’s method [33]. According to this method, the current flowing through the forward biased junction having a series resistance R is given by
I = Is exp
q(V −IRs ) nkT
−1
(5)
The value of ideality factor and series resistance have been determined using following relation d(V ) d(ln I)
= IR +
n ˇ
(6)
where ˇ = q/kT, thus, a plot of d(V)/d(lnI) vs. I will give R as the slope and n/ˇ as the y-axis intercept, which is shown in Fig. 9. The values of n and R have been found as 7.63 and 39.1 k, respectively. It can be concluded that the ideality factor is much higher than that of the ideal p-n junctions, which is of the order of 1–2. A value greater than 2 for n confirms that the diode is far from being ideal. Several authors have reported varying ideality factor values for ZnO based devices. Hazra et al. [34] reported the ideality factor in the range of 4.7–10.6 for n-ZnO/p-ZnO:N homojunction. Similarly, Balakrishnan et al. [35] also reported high ideality factor of 11.85 for n-ZnO/p-ZnO:(Al, N) homojunction. The high value of the measured ideality factor in the p–n homojunction may be due to: (i) accelerated recombination of electrons and holes in the depletion region [36], (ii) many leakages originating from the surface imperfections in the p–n interface [37] and (iii) the additional resistance offered at the junction area and metal–semiconductor contact area [38]. To evaluate barrier height (ˇ ), we can define a function H(I) [33]: H(I) = V −
Fig. 8. I–V characteristics of the In contacts to p-type and n-type ZnO thin films.
n ˇ
ln
I SA∗ T 2
= IR + nˇ
(7)
where S is the diode area and A* = 120 (m* /me ) is the Richardson’s constant. Here m* (= 0.24 me for ZnO) is the effective mass
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Fig. 10. Plot of H versus I for the fabricated ZnO homojunction.
Fig. 11. Transmittance and reflectance spectra of ZnO:(Na, N) films with various dopant concentration.
of the electrons in the conduction band then SA* value becomes (28.8 A/K2 ) [35]. Using the n value determined from Eq. (6), a plot of H(I) vs. I is a straight line with its y-axis intercept equal to nˇ as shown in Fig. 10. The calculated potential barrier height value at room temperature is 0.68 eV. The slope of this plot also provides another method of determination of R, which can be used to check the consistency of this approach. The calculated value of R by this method is 41.9 k, which is close to the value obtained earlier. The high series resistance observed may be attributed to low hole mobility and ohmic contact resistance between the interfacial layers [33]. Thus performing two different plots [Eqs. (6) and (7)] of the I–V data obtained from one measurement can determine all the three key diode parameters. Successful demonstration of ptype ZnO and homojunction has been proved that the fabrication of ZnO based optoelectronic devices is possible. 3.4. Optical studies The transmittance and reflectance spectra of as-grown ZnO:(Na, N) thin films in the wavelength range of 300–1000 nm are shown in Fig. 11. The spectra clearly exhibit a red shift in band edge due to the variation of dopant concentration. The undoped ZnO film showed an average transmittance of 52% and an average reflectance of 12.07% in the visible region. While the ZnO:(Na, N) films show an average transmittance of about 90% and an average reflectance of 8.9%. The sharp ultraviolet absorption edge is observed at approximately 380 nm with the absorption edge being shifted to longer wavelength side at higher doping concentration. The PL measurements have been performed to investigate the influence of the doping concentration on the optical properties of Na–N dual acceptor doped ZnO films. Fig. 12 shows the room temperature PL spectra of ZnO:(Na, N) films. In the PL spectra, two obvious emission bands are observed: one is the UV emission and the other is the deep level emission. UV emission shows a strong and predominant near-band-edge emission (NBE) at approximately 395 nm (3.15 eV) that originates from the free exciton recombination [39]. Based on the PL measurement of surface characteristics, it is strongly believed that the observed red shift is due to the presence of Na atoms in the films. The red shift in NBE emission well supports the increase of hole concentration from 1017 to 1018 cm−3 and formation of p-type conduction in dual acceptor doped ZnO films [40] observed by the Hall measurements. The deep level emission consists of violet, blue and green bands.
Fig. 12. Photoluminescence spectra of ZnO:(Na, N) films with different dopant concentration.
The peak at 423 nm (2.94 eV) is violet band (VB) emission, which originates from the defect emission of Zn vacancies (VZn ) related to a deeper acceptor state. It is reported that in nominally undoped ZnO, VZn is commonly considered as the dominant defect responding to the p-type conductivity [41–44]. Accordingly, we attribute this violet band (VB) emission to VZn acceptor responding to the ptype conductivity in ZnO:(Na,N) thin films. In addition, the observed stability in p-type behavior of ZnO:(Na, N) probably implies the fact that the formation of the compensating defects can be reduced by Na–N dual doping. Lan et al. [21] reported that NaZn is mainly responsible for the p-type conduction in ZnO:(Na,N) films. Further it is mentioned that Na–N complex acceptor may also exist and contribute to the p-type conduction in the films [21]. Thus the observed p-type conduction in the prepared ZnO:(Na,N) films can be attributed to the NaZn , Na–N and native VZn acceptors. Sui et al. [45] and Zhang et al. [46] attributed the stability in the p-type conduction to the upward shift of the valance band maximum (VBM) due to the dual doping method of ZnO films. Zhang et al. [46] reported that when ZnO is dual doped with Li and N, the formation of Lii –No complex introduces an impurity band above
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the VBM which decreases the ionization energy of the LiZn acceptor which subsequently favor the stability of p-type ZnO. It can be inferred from the works of Lin et al. [47] (existence of Nai ) and Lan et al. [21] (existence of Na–N complex) that there may exist similar mechanism to suppress the compensation of donors to the acceptors and to the stable behavior of the prepared ZnO:(Na,N) films. However, the detailed mechanism for the stable p-type conductivity of ZnO:(Na,N) films is yet to be established, which provides a greater scope for the research community working on p-type ZnO films. 4. Conclusions Na–N dual acceptor doping approach has been proposed for the fabrication of low resistive with high hole concentration p-type ZnO thin films by spray pyrolysis technique. Hall measurements show that all the films exhibit p-type conductivity. Among the p-type ZnO films, 6 at.% Na–N doped ZnO shows low resistivity with high hole concentration. The presence of Na and N, which is responsible for p-type conduction, is confirmed by EDX analysis. The observed red shift in NBE emission well supports the increase of carrier concentration and p-type conduction observed by the Hall measurements. The fabricated homojunction using best Na–N dual acceptor doped ZnO film shows good rectifying characteristics of a diode with a turn on voltage of 1.69 V. The diode parameters of the fabricated homojunction such as ideality factor and barrier height have been calculated and found to be 7.63 and 0.68 eV, respectively. The current rectification ratio for p-n homojunction diode is ∼12.5. These results enhance the prospects and requirement of further efforts to develop ZnO-based optoelectronic devices. The rectification behavior observed from the fabricated p-n homo-junction also authenticates the formation of p-type ZnO. Acknowledgement Author M.C. Santhosh Kumar is thankful to the Department of Science and Technology (DST), Govt. of India for the financial support through SERB-Fast Track project for young Scientists. References [1] Y.I. Alivov, D.C. Look, B.M. Ataev, M.V. Chukichev, V.V. Mamedov, Y.A. Agafonov, A.N. Pustovit, Solid-State Electronics 48 (2004) 2343–2346. [2] T. Makino, C.H. Chia, N.T. Tuan, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Applied Physics Letters 77 (2000) 1632–1634. [3] D.C. Look, Materials Science and Engineering B 80 (2001) 383–387. [4] O. Lupan, Th. Pauporté, I.M. Tiginyanub, V.V. Ursaki, H. Heinrich, L. Chow, Materials Science and Engineering B 176 (2011) 1277–1284. [5] A. Aravinda, M.K. Jayaraj, M. Kumar, R. Chandra, Materials Science and Engineering B 177 (2012) 1017–1022. [6] C.H. Park, S.B. Zhang, S.-H. Wei, Physical Review B 66 (2002) 073202. [7] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Applied Physics Letters 81 (2002) 1830–1832. [8] S. Golshahi, S.M. Rozati, A.M. Botelho do Rego, J. Wang, E. Elangovan, R. Martins, E. Fortunato, Materials Science and Engineering B 178 (2013) 103–108.
1039
[9] I.A. Palani, D. Nakamur, K. Okazaki, M. Higashihata, T. Okada, Materials Science and Engineering B 176 (2011) 1526–1530. [10] L. Tang, B. Wang, Y. Zhang, Y. Gu, Materials Science and Engineering B 176 (2011) 548–551. [11] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, B.H. Zhao, Applied Physics Letters 88 (2006) 062107–062109. [12] L.Y. Chen, W.H. Chen, J.J. Wang, F. Chau-Nan Hong, Applied Physics Letters 85 (2004) 5628–5630. [13] D. Wang, Y.C. Liu, R. Mu, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Journal of Physics: Condensed Matter 16 (2004) 4635–4642. [14] E.C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Physical Review B 64 (2001) 085120–085124. [15] S.S. Lin, J.G. Lu, Z.Z. Ye, H.P. He, X.Q. Gu, L.X. Chen, J.Y. Huang, B.H. Zhao, Solid State Communications 148 (2008) 25–28. [16] L.L. Yang, Z.Z. Ye, L.P. Zhu, Y.J. Zeng, Y.F. Lu, B.H. Zhao, Journal of Electronic Materials 36 (2007) 498–501. [17] S.S. Lin, Z.Z. Ye, J.G. Lu, H.P. He, L.X. Chen, X.Q. Gu, J.Y. Huang, L.P. Zhu, B.H. Zhao, Journal of Physics D: Applied Physics 41 (2008) 155114–155119. [18] T. Prasada Rao, M.C. Santhosh Kumar, Journal of Alloys and Compound 509 (2011) 8676–8682. [19] Z. Yan, Y. Ma, P. Deng, Z. Yu, C. Liu, Z. Song, Applied Surface Science 256 (2010) 2289–2292. [20] Y. Zhao, M. Zhou, Z. Lv, Z. Li, J. Huang, X. Liang, J. Min, Materials Science in Semiconductor Processing 14 (2011) 257–260. [21] L. Lan, Y.Z. Zhen, G. Li, B. Xun, L.J. Guo, Z.B. Hui, Chinese Journal of Luminescence 31 (2010) 199–203. [22] Y. Zhao, M. Zhou, Z. Lv, Z. Li, J. Huang, X. Liang, J. Min, L. Wang, Superlattices and Microstructures 49 (2011) 549–554. [23] R. Swapna, M.C. Santhosh Kumar, Ceramics International 38 (2012) 3875–3883. [24] D. Ma, Z. Ye, L. Wang, J. Huang, B. Zhao, Material Letters 58 (2003) 128–131. [25] N. Fujimura, T. Nishihara, S. Goto, J.F. Xu, T. Ito, Journal of Crystal Growth 130 (1993) 269–279. [26] I. Stambolovaa, V. Blaskov, M. Shipochka, S. Vassilev, V. Petkova, A. Loukanov, Materials Science and Engineering B 177 (2012) 1029–1037. [27] Y. Caglara, S. Aksoya, S. Ilicana, M. Caglar, Superlattices and Microstructures 46 (2009) 469–475. [28] R. Swapna, M.C. Santhosh Kumar, Ceramics International 39 (2013) 1799–1806. [29] S. Majumdar, S. Bhunia, AIP Conference Proceedings 1447 (2012) 667–668. [30] S. Chu, J.H. Lim, L.J. Mandalapu, Z. Yang, L. Li, J.L. Liu, Applied Physics Letters 92 (2008) 152103–152105. [31] C. Jagadish, S.J. Pearton, Zinc Oxide Bulk, Thin Films and Nanostructures: Processing, Properties, and Applications, Elsevier Science and Technology Publishers, 2006. [32] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [33] S.K. Cheung, N.W. Cheung, Applied Physics Letters 49 (1986) 85–87. [34] S.K. Hazra, S. Basu, Solid State Electronics 49 (2005) 1158–1162. [35] L. Balakrishnan, P. Premchander, T. Balasubramanian, N. Gopalakrishnan, Vacuum 85 (2011) 881–886. [36] R. Singh, A.K. Narula, Applied Physics Letters 71 (1997) 2845–2847. [37] Y.W. Song, K. Kim, J.P. Ahn, G.E. Jang, S.Y. Lee, Nanotechnology 20 (2009) 275606–275609. [38] J.M. Shah, Y.L. Li, Th. Gessmann, E.F. Schubert, Journal of Applied Physics 94 (2003) 2627–2630. [39] J.J. Hopfield, D.G. Thomas, Physical Review 122 (1961) 35–52. [40] Y.R. Ryu, T.S. Lee, H.W. White, Applied Physics Letters 83 (2003) 87–89. [41] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Physical Review B 61 (2000) 15019–15021. [42] S.H. Jeong, B.S. Kim, B.T. Lee, Applied Physics Letters 82 (2003) 2625–2627. [43] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, J.G. Lu, H.P. He, L.P. Zhu, B.H. Zhao, Y. Che, S.B. Zhang, Applied Physics Letters 88 (2006) 262103–262105. [44] W.J. Lee, J. Kang, K.J. Chang, Physical Review B 73 (2006) 024117–024122. [45] Y. Sui, B. Yao, L. Xiao, G. Xing, L. Yang, X. Li, X. Li, J. Lang, S. Lv, J. Cao, J. Yang, Journal of Applied Physics 113 (2013) 133101–133105. [46] B.Y. Zhang, B. Yao, Y.F. Li, Z.Z. Zhang, B.H. Li, C.X. Shan, D.X. Zhao, D.Z. Shen, Applied Physics Letters 97 (2010) 222101–222103. [47] S.S. Lin, H.P. He, Y.F. Lu, Z.Z. Ye, Journal of Applied Physics 106 (2009) 093508–093512.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Deposition of Na–N dual acceptor doped p-type ZnO thin films and fabrication of p-ZnO:(Na, N)/n-ZnO:Eu homojunction R. Swapna, M.C. Santhosh Kumar ∗ Advanced Materials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 20 November 2012 Received in revised form 8 May 2013 Accepted 10 June 2013 Available online 22 June 2013 Keywords: ZnO Thin films Spray pyrolysis Dual acceptor doping p-Type conductivity Homojunction
a b s t r a c t Sodium and nitrogen dual acceptor doped p-type ZnO (ZnO:(Na, N)) films have been prepared by spray pyrolysis technique at a substrate temperature of 623 K. The ZnO:(Na, N) films are grown at a fixed N doping concentration of 2 at.% and varying the nominal Na doping concentration from 0 to 8 at.%. The XRD results show that all the ZnO:(Na, N) films exhibited (0 0 2) preferential orientation. The EDX and elemental mapping analysis shows the presence and distribution of Zn, O, Na and N in the deposited films. The Hall measurement results demonstrate that the Na–N dual acceptor doped ZnO films show excellent p-type conduction. The p-type ZnO:(Na, N) films with comparatively low resistivity of 5.60 × 10−2 cm and relatively high carrier concentration of 3.15 × 1018 cm−3 are obtained at 6 at.%. ZnO based homojunction is fabricated by depositing n-type layer (Eu doped ZnO) grown over the p-type layer ZnO:(Na, N). The current–voltage (I–V) characteristics measured from the two-layer structure show typical rectifying characteristics of p-n junction with a low turn on voltage of about 1.69 V. The ZnO:(Na, N) films exhibit a high transmittance (about >90%) and the average reflectance is 8.9% in the visible region. PL measurement shows near-band-edge (NBE) emission and deep-level (DL) emission in the ZnO:(Na, N) thin films. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Today ZnO has become one of the hottest research fields in advanced materials and devices. It is emerging as one of the most important electronic and photonic materials. It has great potential applications in information technology, biotechnology, nanoscale science and engineering [1–3]. ZnO has been extensively studied for its promising applications in optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), because of its wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [4,5]. For many advanced applications, the development of ZnO based devices such as p–n homojunctions can be realized by utilizing both n-type and p-type ZnO films. However, the difficulty in achieving p-type ZnO impedes the development of such advanced devices. ZnO occurs naturally as an n-type semiconductor due to a large number of intrinsic defects such as oxygen vacancies (VO ), Zn interstitials (Zni ) and Zn antisite defects (ZnO ). Therefore, it is very difficult to form the shallow acceptor levels because these acceptors can be compensated by numerous ZnO native defects, resulting in the formation of deep donor level traps [6].
∗ Corresponding author. Tel.: +91 431 2503611; fax: +91 431 2500133. E-mail addresses: [email protected] (R. Swapna), [email protected], [email protected] (M.C. Santhosh Kumar). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.06.010
The p-type doping in ZnO can be realized by substituting either group V elements (N, P, As, and Sb) for O sites or group I elements (H, Li, Na, Ag and K) for Zn sites [7–12]. Among these acceptors, nitrogen (0.146 nm) is the most suitable dopant because of its similar ionic radius to oxygen (0.132 nm). Although considerable efforts have been focused to realize N-doped p-type ZnO and ZnO based p-n diodes, it is still difficult to achieve reproducible and good quality p-type conduction in N-doped ZnO [13,14]. One way to achieve a good solubility of N into ZnO is to use a dual acceptor method that uses two acceptors. Recently, Na doping gains more attention and the theoretical studies indicate that it produces shallow acceptor state for NaZn . Moreover, p–type ZnO thin films and ZnO based p–n homojunction fabricated by various methods prove Na as an effective p-type dopant in ZnO [15–17]. There have been several reports on the growth of dual acceptor p-type ZnO films such as dual acceptor doping of Li–N [18], Ag–N [19] and K–N [20] by various techniques. Though, there are a few reports on Na-N dual acceptor doping of ZnO films [21,22], still there is a scope for an extensive investigation of the properties of the ZnO:(Na, N) films fabricated by various deposition techniques. In this paper authors report a study of structural, electrical, optical properties and surface morphology of Na–N dual acceptor doped p-type ZnO films deposited on glass substrate by spray pyrolysis and the fabrication of p–n homojunction. Spray pyrolysis [23] has been developed as a powerful tool to prepare various kinds of thin films such as metal oxides, superconducting materials and nanophase materials. It has the advantages of low cost,
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Fig. 1. Schematic diagram of spray pyrolysis unit. Deposition apparatus: (1) air compressor; (2) pressure controller; (3) temperature controller; (4) nozzle for air; (5) nozzle for solution; (6) solution container; (7) stepper motor; (8) microprocessor controller; (9) temperature sensor; (10) heater setup; and (11) computer.
easy-to-use, safe and can be implemented in a standard laboratory. ZnO based p–n homojunctions are characterized by current–voltage (I–V) measurements. The current–voltage characteristics of the p–n junction are studied at room temperature. The experimental data from the I–V measurements of homojunctions are analyzed to calculate the ideality factor, series resistance and the barrier height. However, electrical characteristics of this diode are often influenced by various non-idealities such as interface state, interfacial oxide layer and series resistance. Therefore, the interface states and series resistance play an important role in determining the barrier height.
2. Experimental In the present work the ZnO films are prepared by, Holmarc spray pyrolysis equipment model HO-TH-04. A schematic diagram of the experimental setup is as shown in Fig. 1. The ZnO:(Na, N) thin films were deposited on glass substrates, with dimensions of 76.2 mm × 25.4 mm × 1.2 mm, at a substrate temperature of 623 K by spray pyrolysis technique. The precursor solution was prepared by dissolving 0.1 M of zinc acetate dihydrate [Zn(CH3 COO)2 2H2 O, Sigma–Aldrich, 99.5%] in a 100 ml mixture of 90 ml deionized water and 10 ml ethanol [CH3 CH2 OH, Merck, 99.9%]. Na and N doping was achieved by dissolving an appropriate quantity of sodium acetate [CH3 COONa, Merck, 98.5%] and ammonium acetate [CH3 COONH4 , Sigma–Aldrich, 98.5%] in the zinc acetate solution. The nominal Na concentration was varied from 0 to 8 at.% and the N concentration was 0 at.% and 2 at.%. A few drops of acetic acid [CH3 COOH, Merck, 99.9%] were added in to the spray solution to avoid the formation of milky precipitate of hydroxides [23]. The glass substrates were cleaned with detergent solution and deionized water. Further, ultrasonic cleaning was carried out for 30 min in an ultrasonic bath and then rinsed in acetone for 10 min. The distance between the spray nozzle and the glass substrate was maintained about 10 cm. Compressed air was used as the carrier gas at the pressure of 2 bar. The prepared solution was sprayed (3 ml/min) onto the clean glass substrates for a deposition time of 3 min. During spray pyrolysis deposition, a precursor solution was sprayed as fine droplets onto a heated substrate. When the droplets reach the heated substrate, they spread out and undergo pyrolytic decomposition. Finally, the solid compounds react to become a new chemical compound.
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The structural properties of the as-grown films were analyzed by a Rigaku X-ray diffractometer (D/Max ULTIMA III, Rigaku, Japan) ˚ at 40 kV and 30 mA over a 2 using CuK␣ radiation ( = 1.5406 A) range of 25–60◦ with a scan speed of 2◦ /min and a step size of 0.02◦ . The thickness was measured using Stylus profile meter. The average thickness of Na and N dual acceptor doped ZnO films is 987 nm. The surface morphology and the elemental composition of the films were assessed using a Hitachi-S3000N scanning electron microscope equipped with energy dispersive X-ray spectroscopy and elemental mapping facilities. Atomic force microscopy (NTMDTNTEGRA, Russia) was used to analyze the surface roughness of the films. The AFM 3D images were recorded over a scan area of 2 m × 2 m of the film surface. The optical measurements of dual acceptor doped ZnO thin films were carried out at room temperature using shimadzu UV-1700 Spectrophotometer in the wavelength range of 300–1000 nm. Photoluminescence spectra were recorded at room temperature using Perkin Elmer LS 55 Luminescence spectrophotometer with an excitation wavelength of 325 nm. Electrical resistivity, carrier concentration and mobility were measured at room temperature using Ecopia Hall measurement system (Model: HMS-5000) in the van der Pauw technique at a constant magnetic field of 0.5 T. As the first step in the fabrication of p-n homojunction, p-type ZnO film has been grown on glass substrate with 6 at.% Na and 2 at.% N doped ZnO. For n-type layer, 2 at.% europium (Eu) doped ZnO film has been grown over the p-type ZnO with masking for contacts. Here, a mask is designed using stainless steel sheet such that the active area of each device is 0.2 mm2 . Ohmic contacts have been made on both p-type and n-type ZnO films with indium by thermal evaporation. The I–V measurement on the fabricated p–n homojunction has been carried out using a Keithley 6517A electrometer. 3. Results and discussion 3.1. Structural studies Fig. 2 shows the XRD pattern of ZnO:(Na, N) films with different doping concentration. The crystallinity of the films has been studied by using XRD. It is seen that all the ZnO:(Na, N) films are preferentially oriented along (0 0 2) plane (c-axis) with hexagonal wurtzite structure and free from the formation of secondary phases. The low intensity peaks corresponding to (1 0 0), (1 0 1) and (1 1 0) plane diffraction have also been observed. A shift in the preferential growth from the (1 0 0) plane to (0 0 2) plane is observed when the dopant concentration increases from 0 to 2 at.%. Generally, the direction with the highest planar density of lattice points corresponds to the lowest surface free energy which facilitates an easier grain growth in that direction [24]. The change in the preferential growth from (1 0 0) plane to (0 0 2) plane observed in all the ZnO:(Na, N) films suggesting that the incorporation of Na and N in to ZnO enhances the grain growth by decreasing the surface free energy in (0 0 2) direction. As a result, the high quality ZnO:(Na, N) films exhibit strong (0 0 2) preferred orientation due to its lowest density of surface free energy [25]. The c-axis orientation along (0 0 2) reflection in ZnO:(Na, N) films can be understood from the “survival of the fastest” model proposed by Van der Drift [18]. According to this model, nucleations with various orientations can be formed at the initial stage of the deposition and each nucleus competes to grow but only nuclei having the fastest growth rate can survive. The average crystallite size of the ZnO films deposited at different Na and N concentrations has been calculated using the Scherrer’s formula [26]: D=
0.9 ˇ cos
(1)
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above the solubility limit leads to the formation of Na and N related defects which in turn degrades the crystallinity [14]. The dislocation density ı which represents the amount of defects in the films is determined using the formula [27]: ı=
1 D2
(2)
The dislocation density shows the decreasing trend with increasing Na content upto 6 at.% (Fig. 3). The dislocation density then increases for further increase of dopant concentration, which leads to the increase in the concentration of lattice imperfections. The larger D and smaller ı values indicate the better crystallization of the 6 at.% ZnO:(Na, N) films. The preferential growth orientation is determined using a texture coefficient TC(hkl). This factor is calculated using the following relation [28]:
TC(hkl) =
Fig. 2. XRD spectra of Na–N dual acceptor doped ZnO thin films with various dopant concentrations.
I(hkl) /Io(hkl) (1/N)
I(hkl) /Io(hkl)
(3)
where I(hkl) is the measured relative intensity of a plane (hkl), Io(hkl) is the standard intensity of the plane (hkl) taken from the JCPDS (75-0576) data, N is the reflection number. Randomly oriented crystallites have TC(hkl) = 1, while the larger value represents the greater abundance of crystallites oriented in that (hkl) direction. The texture coefficient calculated for the two main diffraction peaks, i.e. (0 0 2) and (1 0 1), are presented in Fig. 3. It can be seen that the highest TC is obtained for (0 0 2) plane in all the films. The high value of the texture coefficient indicates the maximum preferred orientation of the films along the diffraction plane. This means that the increase in preferred orientation is associated with increase in the number of grains along that plane. 3.2. Surface morphology and compositional analysis
where D is the average crystallite size (nm), is the wavelength of CuK␣ radiation, ˇ is the broadening of the diffraction line (FWHM) and is the Bragg’s diffraction angle. The instrumental broadening effect has been subtracted from the FWHM using the XRD pattern of a standard silicon sample. The crystallite size as a function of Na and N doping concentration is shown in Fig. 3. The average crystallite size of the films increases from 23.0 nm to 43.1 nm with the increase of dopant concentration from 0 to 6 at.%, thereafter it slightly decreases with further increase of dopant concentration to 8 at.%. It is understood that the crystallinity increases with the increase of doping concentration up to 6 at.%. For 8 at.% Na–N doped ZnO, the degradation in crystallinity is due to the solubility limit of Na and N in ZnO lattice sites. The presence of more Na and N atoms
Fig. 4 shows the surface morphology of ZnO:(Na, N) films with various doping concentration obtained from scanning electron microscopy (SEM). The surface of the ZnO:(Na, N) films is composed of regular dense grains with average diameter in the range of 40–60 nm. The change in grain morphologies is observed in the undoped and dual acceptor doped ZnO films. The EDX spectrum and elemental mapping results show the presence and distribution of Zn, O, Na and N for the ZnO:(Na, N) films with 6 at.% Na and 2 at.% N are shown in Fig. 5. The chemical compositions of the Zn, O, Na and N in the sample are identified as 16.76, 57.56, 24.38 and 1.31 at.%, respectively. It has been found that the concentration of O is greater than that of Zn. This lowering of the zinc content compared to oxygen also supports the incorporation of Na at the Zn site (NaZn ). The three-dimensional (3D) images of AFM micrographs are shown in Fig. 6 (a) and (b) for 2 and 6 at.% ZnO:(Na, N) thin films. The root mean square roughness (RMS) and mean roughness (Ra ) parameters are analyzed to study the surface quality of the ZnO:(Na, N) films. At 2 at.%, the RMS and mean roughness are observed as 14.149 nm and 11.762 nm, respectively. When the dopant concentration is further increases to 6 at.%, the values of RMS and mean roughness increases to 17.744 nm and 14.325 nm. The particle size of the deposited films increases from 52 to 76 nm with an increase of dopant concentration. It seen that the particle sizes observed from AFM pictures are larger than those determined from XRD. This finding indicates that the grains visible in AFM pictures are not single crystalline but consist of several crystallites. 3.3. Electrical studies
Fig. 3. Variation of crystallite size, dislocation density and TC(0 0 2), TC(1 0 1) as a function of (Na, N) concentration.
The electrical properties of both p-type and n-type ZnO films have been studied by Hall measurements in Van der Pauw
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Fig. 4. SEM images of the ZnO:(Na, N) films with (a) 0 at.% (b) 2 at.% (c) 4 at.% (d) 6 at.% and (e) 8 at.%.
Table 1 Variation of electrical resistivity (), carrier concentration (n) and mobility () of the Na–N doped ZnO (p-type) with various dopant concentration and 2 at.% Eu doped ZnO (n-type) thin films. Dopant (at.%) Na:N
Resistivity ( cm)
Carrier concentration (cm−3 )
Mobility (cm2 V−1 s−1 )
Carrier type
0:0 2:2 4:2 6:2 8:2 ZnO:Eu (2 at.%)
(3.33 ± 1.62) × 103 (4.06 ± 0.77) × 10−1 (1.41 ± 0.31) × 10−1 (5.60 ± 0.55) × 10−2 (7.56 ± 1.10) × 10−2 (2.93 ± 0.96) × 10−3
(5.65 ± 0.095) × 1015 (3.61 ± 0.21) × 1017 (8.31 ± 0.808) × 1017 (3.15 ± 1.22) × 1018 (1.85 ± 0.67) × 1018 (5.18 ± 1.42) × 1019
0.334 ± 0.78 41.9 ± 0.084 53.4 ± 2.31 35.7 ± 2.17 44.6 ± 0.49 41.1 ± 0.13
n p p p p n
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Table 2 Carrier concentrations as a function of the preservation period after deposition of ZnO:(Na, N) films with dopant concentration. Dopant (at.%) Na:N
As-prepared n0 (cm−3 )
After 90 days n90 (cm−3 )
After 180 days n180 (cm−3 )
Carrier type
2:2 4:2 6:2 8:2
(3.61 ± 0.21) × 10 (8.31 ± 0.808) × 1017 (3.15 ± 1.22) × 1018 (1.85 ± 0.67) × 1018
(2.96 ± 1.03) × 10 (6.54 ± 0.48) × 1016 (5.69 ± 0.92) × 1017 (1.51 ± 0.35) × 1017
(4.18 ± 0.67) × 1015 (7.43 ± 0.15) × 1015 (9.72 ± 0.53) × 1016 (3.21 ± 0.074) × 1016
p p p p
17
16
Fig. 5. EDX and elemental mapping results showing for 6 at.% ZnO:(Na, N) films.
method. The measured carrier concentration, mobility and resistivity of undoped (n-type), Na–N dual acceptor doped (p-type) and europium doped ZnO (n-type) films have been shown in Table 1. As grown pure ZnO film exhibits n-type conductivity. It shows high resistivity of 3.33 × 103 cm with a carrier concentration of 5.65 × 1015 cm−3 compared to that of ZnO:(Na, N) films. The p-type conduction has been confirmed by repeated Hall measurements. In dual acceptor doping, Na ion occupies the Zn site while N ion replaces the O ion, forming NaZn –NO acceptor complexes, which causes p-type conductivity [19]. The shallow acceptor with energy level of NaZn is 164 meV, which produces stable p-type conduction in ZnO [15]. Table 1 shows that the increase in hole concentration from 1017 to 1018 cm−3 with increase of dopant concentration. The highest carrier concentration of 3.15 × 1018 cm−3 with a low resistivity of 5.60 × 10−2 cm is obtained for the ZnO:(Na, N) films at the doping concentrations of 6 at.% Na and 2 at.% N. The decrease in hole concentration with higher Na concentration (8 at.%) is due to the presence of high concentration of impurities, which introduces uncompensated charged defects. The result of structural studies obtained from XRD also well supports the electrical results. In order to study the stability and reliability of the electrical properties for Na–N dual acceptor doped p-type ZnO films, aging analysis has been performed. The samples have been preserved in ordinary silica-gel desiccators for a period of 180 days. Table 2 shows the aging effect of carrier concentration with respect to the preservation period. Their p-type conductivities are relatively stable and do not convert to n-type even over 180 days, which implies the good stability of the films. However, as observed from Table 2, the carrier concentration decreases with time. In p-type ZnO:(Na,N) films, the adsorbed O tends to occupy its own site which in turn develops a stress in forcing N atoms from substitutional site to interstitial site. With aging for 180 days, adsorbed O creates stress so both carrier concentration and mobility decrease as scattering at the grain boundary and trap enhances. Further, the Zn O bond is more stable compared to
Fig. 6. AFM 3D images of (a) 2 at.% and (b) 6 at.% ZnO:(Na, N) films.
Zn N bond as O is more electronegative than N. The N acceptors slowly migrate from substitutional site to a neighboring interstitial site and instead create donors which gradually convert the good p-type film to less conducting p-type and this results in increase in resitivity and decrease in hole concentration [29]. 3.3.1. Current–voltage characteristics Based on the above study on the properties of the Na N dual acceptor doped (p-type) and Eu doped (n-type) ZnO films, consequently, the I–V characteristics of the two-layer-structured ZnO homojunction consisting of the n-type layer and p-type layer processed with spray pyrolysis technique is investigated, as shown in Fig. 7. The schematic structure of the p-ZnO:(Na, N)/n-ZnO:Eu homojunction is shown in the lower inset of Fig. 7. In Fig. 7, the nominal doping concentrations are 6 at.%, 2 at.% and 2 at.% respectively for Na, N and Eu. The I–V measurement is carried out using a Keithley 6517A electrometer. Indium is used as electrode, because it has
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Fig. 9. Plot of d(V)/d(ln I) versus I for the forward bias. Fig. 7. I–V characteristics of the p-ZnO:(Na, N)/n-ZnO:Eu homojunction.
low metal work function to form ohmic contacts [19]. The indium electrodes for the electrical contacts have been made on both ptype and n-type ZnO layers by thermal evaporation. The ohmic behavior of contacts to the p-ZnO:(Na, N) and n-ZnO:Eu layers are confirmed by the linear I–V dependencies as shown in Fig. 8. The I–V characteristics of the p–n homojunction show the typical rectifying behavior of a diode, which indicates the formation of p-type ZnO films by Na–N dual acceptor doping. The current rectification ratio IF /IR for p–n homojunction diode is ∼12.5 at a bias voltage of ±5 V and a reverse current of ∼1.48 A at −5 V (IF and IR stands for forward and reverse currents, respectively). The turn on voltage appears at 1.69 V under forward-biased voltage. Chu et al. [30] reported a turn on voltage of around 6 V for p-ZnO/n-ZnO homojunction. This value is much higher than that of our study. The turn on voltages found by several authors scatter mostly around 1–3 V. The reason for this variation may be attributed to the high defect concentration in the interface. A low value of the turn on voltage is critically important in device applications. The lower leakage current has been observed under reverse bias. The leakage current is
due to the imbalance between the carrier partial currents which is caused by the difference in carrier concentration and mobilities of p and n layers [31]. According to thermionic emission model [32], the current flowing through a junction is given as
I = Is exp
qV nkT
−1
(4)
where IS is the reverse saturation current, q is the electronic charge, k is the Boltzmann constant, T is the absolute temperature, V is the voltage across the junction, n is the ideality factor. The value of Is has been found as 6.96 A by extrapolating the linear mid bias region of the semilog plot to zero applied voltage. The effect of diode series resistance, interface states and ideality factor (>1), it deviates from the ideal thermionic theory. In order to extract these parameters we used Cheung’s method [33]. According to this method, the current flowing through the forward biased junction having a series resistance R is given by
I = Is exp
q(V −IRs ) nkT
−1
(5)
The value of ideality factor and series resistance have been determined using following relation d(V ) d(ln I)
= IR +
n ˇ
(6)
where ˇ = q/kT, thus, a plot of d(V)/d(lnI) vs. I will give R as the slope and n/ˇ as the y-axis intercept, which is shown in Fig. 9. The values of n and R have been found as 7.63 and 39.1 k, respectively. It can be concluded that the ideality factor is much higher than that of the ideal p-n junctions, which is of the order of 1–2. A value greater than 2 for n confirms that the diode is far from being ideal. Several authors have reported varying ideality factor values for ZnO based devices. Hazra et al. [34] reported the ideality factor in the range of 4.7–10.6 for n-ZnO/p-ZnO:N homojunction. Similarly, Balakrishnan et al. [35] also reported high ideality factor of 11.85 for n-ZnO/p-ZnO:(Al, N) homojunction. The high value of the measured ideality factor in the p–n homojunction may be due to: (i) accelerated recombination of electrons and holes in the depletion region [36], (ii) many leakages originating from the surface imperfections in the p–n interface [37] and (iii) the additional resistance offered at the junction area and metal–semiconductor contact area [38]. To evaluate barrier height (ˇ ), we can define a function H(I) [33]: H(I) = V −
Fig. 8. I–V characteristics of the In contacts to p-type and n-type ZnO thin films.
n ˇ
ln
I SA∗ T 2
= IR + nˇ
(7)
where S is the diode area and A* = 120 (m* /me ) is the Richardson’s constant. Here m* (= 0.24 me for ZnO) is the effective mass
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Fig. 10. Plot of H versus I for the fabricated ZnO homojunction.
Fig. 11. Transmittance and reflectance spectra of ZnO:(Na, N) films with various dopant concentration.
of the electrons in the conduction band then SA* value becomes (28.8 A/K2 ) [35]. Using the n value determined from Eq. (6), a plot of H(I) vs. I is a straight line with its y-axis intercept equal to nˇ as shown in Fig. 10. The calculated potential barrier height value at room temperature is 0.68 eV. The slope of this plot also provides another method of determination of R, which can be used to check the consistency of this approach. The calculated value of R by this method is 41.9 k, which is close to the value obtained earlier. The high series resistance observed may be attributed to low hole mobility and ohmic contact resistance between the interfacial layers [33]. Thus performing two different plots [Eqs. (6) and (7)] of the I–V data obtained from one measurement can determine all the three key diode parameters. Successful demonstration of ptype ZnO and homojunction has been proved that the fabrication of ZnO based optoelectronic devices is possible. 3.4. Optical studies The transmittance and reflectance spectra of as-grown ZnO:(Na, N) thin films in the wavelength range of 300–1000 nm are shown in Fig. 11. The spectra clearly exhibit a red shift in band edge due to the variation of dopant concentration. The undoped ZnO film showed an average transmittance of 52% and an average reflectance of 12.07% in the visible region. While the ZnO:(Na, N) films show an average transmittance of about 90% and an average reflectance of 8.9%. The sharp ultraviolet absorption edge is observed at approximately 380 nm with the absorption edge being shifted to longer wavelength side at higher doping concentration. The PL measurements have been performed to investigate the influence of the doping concentration on the optical properties of Na–N dual acceptor doped ZnO films. Fig. 12 shows the room temperature PL spectra of ZnO:(Na, N) films. In the PL spectra, two obvious emission bands are observed: one is the UV emission and the other is the deep level emission. UV emission shows a strong and predominant near-band-edge emission (NBE) at approximately 395 nm (3.15 eV) that originates from the free exciton recombination [39]. Based on the PL measurement of surface characteristics, it is strongly believed that the observed red shift is due to the presence of Na atoms in the films. The red shift in NBE emission well supports the increase of hole concentration from 1017 to 1018 cm−3 and formation of p-type conduction in dual acceptor doped ZnO films [40] observed by the Hall measurements. The deep level emission consists of violet, blue and green bands.
Fig. 12. Photoluminescence spectra of ZnO:(Na, N) films with different dopant concentration.
The peak at 423 nm (2.94 eV) is violet band (VB) emission, which originates from the defect emission of Zn vacancies (VZn ) related to a deeper acceptor state. It is reported that in nominally undoped ZnO, VZn is commonly considered as the dominant defect responding to the p-type conductivity [41–44]. Accordingly, we attribute this violet band (VB) emission to VZn acceptor responding to the ptype conductivity in ZnO:(Na,N) thin films. In addition, the observed stability in p-type behavior of ZnO:(Na, N) probably implies the fact that the formation of the compensating defects can be reduced by Na–N dual doping. Lan et al. [21] reported that NaZn is mainly responsible for the p-type conduction in ZnO:(Na,N) films. Further it is mentioned that Na–N complex acceptor may also exist and contribute to the p-type conduction in the films [21]. Thus the observed p-type conduction in the prepared ZnO:(Na,N) films can be attributed to the NaZn , Na–N and native VZn acceptors. Sui et al. [45] and Zhang et al. [46] attributed the stability in the p-type conduction to the upward shift of the valance band maximum (VBM) due to the dual doping method of ZnO films. Zhang et al. [46] reported that when ZnO is dual doped with Li and N, the formation of Lii –No complex introduces an impurity band above
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the VBM which decreases the ionization energy of the LiZn acceptor which subsequently favor the stability of p-type ZnO. It can be inferred from the works of Lin et al. [47] (existence of Nai ) and Lan et al. [21] (existence of Na–N complex) that there may exist similar mechanism to suppress the compensation of donors to the acceptors and to the stable behavior of the prepared ZnO:(Na,N) films. However, the detailed mechanism for the stable p-type conductivity of ZnO:(Na,N) films is yet to be established, which provides a greater scope for the research community working on p-type ZnO films. 4. Conclusions Na–N dual acceptor doping approach has been proposed for the fabrication of low resistive with high hole concentration p-type ZnO thin films by spray pyrolysis technique. Hall measurements show that all the films exhibit p-type conductivity. Among the p-type ZnO films, 6 at.% Na–N doped ZnO shows low resistivity with high hole concentration. The presence of Na and N, which is responsible for p-type conduction, is confirmed by EDX analysis. The observed red shift in NBE emission well supports the increase of carrier concentration and p-type conduction observed by the Hall measurements. The fabricated homojunction using best Na–N dual acceptor doped ZnO film shows good rectifying characteristics of a diode with a turn on voltage of 1.69 V. The diode parameters of the fabricated homojunction such as ideality factor and barrier height have been calculated and found to be 7.63 and 0.68 eV, respectively. The current rectification ratio for p-n homojunction diode is ∼12.5. These results enhance the prospects and requirement of further efforts to develop ZnO-based optoelectronic devices. The rectification behavior observed from the fabricated p-n homo-junction also authenticates the formation of p-type ZnO. Acknowledgement Author M.C. Santhosh Kumar is thankful to the Department of Science and Technology (DST), Govt. of India for the financial support through SERB-Fast Track project for young Scientists. References [1] Y.I. Alivov, D.C. Look, B.M. Ataev, M.V. Chukichev, V.V. Mamedov, Y.A. Agafonov, A.N. Pustovit, Solid-State Electronics 48 (2004) 2343–2346. [2] T. Makino, C.H. Chia, N.T. Tuan, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Applied Physics Letters 77 (2000) 1632–1634. [3] D.C. Look, Materials Science and Engineering B 80 (2001) 383–387. [4] O. Lupan, Th. Pauporté, I.M. Tiginyanub, V.V. Ursaki, H. Heinrich, L. Chow, Materials Science and Engineering B 176 (2011) 1277–1284. [5] A. Aravinda, M.K. Jayaraj, M. Kumar, R. Chandra, Materials Science and Engineering B 177 (2012) 1017–1022. [6] C.H. Park, S.B. Zhang, S.-H. Wei, Physical Review B 66 (2002) 073202. [7] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Applied Physics Letters 81 (2002) 1830–1832. [8] S. Golshahi, S.M. Rozati, A.M. Botelho do Rego, J. Wang, E. Elangovan, R. Martins, E. Fortunato, Materials Science and Engineering B 178 (2013) 103–108.
1039
[9] I.A. Palani, D. Nakamur, K. Okazaki, M. Higashihata, T. Okada, Materials Science and Engineering B 176 (2011) 1526–1530. [10] L. Tang, B. Wang, Y. Zhang, Y. Gu, Materials Science and Engineering B 176 (2011) 548–551. [11] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, B.H. Zhao, Applied Physics Letters 88 (2006) 062107–062109. [12] L.Y. Chen, W.H. Chen, J.J. Wang, F. Chau-Nan Hong, Applied Physics Letters 85 (2004) 5628–5630. [13] D. Wang, Y.C. Liu, R. Mu, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Journal of Physics: Condensed Matter 16 (2004) 4635–4642. [14] E.C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Physical Review B 64 (2001) 085120–085124. [15] S.S. Lin, J.G. Lu, Z.Z. Ye, H.P. He, X.Q. Gu, L.X. Chen, J.Y. Huang, B.H. Zhao, Solid State Communications 148 (2008) 25–28. [16] L.L. Yang, Z.Z. Ye, L.P. Zhu, Y.J. Zeng, Y.F. Lu, B.H. Zhao, Journal of Electronic Materials 36 (2007) 498–501. [17] S.S. Lin, Z.Z. Ye, J.G. Lu, H.P. He, L.X. Chen, X.Q. Gu, J.Y. Huang, L.P. Zhu, B.H. Zhao, Journal of Physics D: Applied Physics 41 (2008) 155114–155119. [18] T. Prasada Rao, M.C. Santhosh Kumar, Journal of Alloys and Compound 509 (2011) 8676–8682. [19] Z. Yan, Y. Ma, P. Deng, Z. Yu, C. Liu, Z. Song, Applied Surface Science 256 (2010) 2289–2292. [20] Y. Zhao, M. Zhou, Z. Lv, Z. Li, J. Huang, X. Liang, J. Min, Materials Science in Semiconductor Processing 14 (2011) 257–260. [21] L. Lan, Y.Z. Zhen, G. Li, B. Xun, L.J. Guo, Z.B. Hui, Chinese Journal of Luminescence 31 (2010) 199–203. [22] Y. Zhao, M. Zhou, Z. Lv, Z. Li, J. Huang, X. Liang, J. Min, L. Wang, Superlattices and Microstructures 49 (2011) 549–554. [23] R. Swapna, M.C. Santhosh Kumar, Ceramics International 38 (2012) 3875–3883. [24] D. Ma, Z. Ye, L. Wang, J. Huang, B. Zhao, Material Letters 58 (2003) 128–131. [25] N. Fujimura, T. Nishihara, S. Goto, J.F. Xu, T. Ito, Journal of Crystal Growth 130 (1993) 269–279. [26] I. Stambolovaa, V. Blaskov, M. Shipochka, S. Vassilev, V. Petkova, A. Loukanov, Materials Science and Engineering B 177 (2012) 1029–1037. [27] Y. Caglara, S. Aksoya, S. Ilicana, M. Caglar, Superlattices and Microstructures 46 (2009) 469–475. [28] R. Swapna, M.C. Santhosh Kumar, Ceramics International 39 (2013) 1799–1806. [29] S. Majumdar, S. Bhunia, AIP Conference Proceedings 1447 (2012) 667–668. [30] S. Chu, J.H. Lim, L.J. Mandalapu, Z. Yang, L. Li, J.L. Liu, Applied Physics Letters 92 (2008) 152103–152105. [31] C. Jagadish, S.J. Pearton, Zinc Oxide Bulk, Thin Films and Nanostructures: Processing, Properties, and Applications, Elsevier Science and Technology Publishers, 2006. [32] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [33] S.K. Cheung, N.W. Cheung, Applied Physics Letters 49 (1986) 85–87. [34] S.K. Hazra, S. Basu, Solid State Electronics 49 (2005) 1158–1162. [35] L. Balakrishnan, P. Premchander, T. Balasubramanian, N. Gopalakrishnan, Vacuum 85 (2011) 881–886. [36] R. Singh, A.K. Narula, Applied Physics Letters 71 (1997) 2845–2847. [37] Y.W. Song, K. Kim, J.P. Ahn, G.E. Jang, S.Y. Lee, Nanotechnology 20 (2009) 275606–275609. [38] J.M. Shah, Y.L. Li, Th. Gessmann, E.F. Schubert, Journal of Applied Physics 94 (2003) 2627–2630. [39] J.J. Hopfield, D.G. Thomas, Physical Review 122 (1961) 35–52. [40] Y.R. Ryu, T.S. Lee, H.W. White, Applied Physics Letters 83 (2003) 87–89. [41] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Physical Review B 61 (2000) 15019–15021. [42] S.H. Jeong, B.S. Kim, B.T. Lee, Applied Physics Letters 82 (2003) 2625–2627. [43] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, J.G. Lu, H.P. He, L.P. Zhu, B.H. Zhao, Y. Che, S.B. Zhang, Applied Physics Letters 88 (2006) 262103–262105. [44] W.J. Lee, J. Kang, K.J. Chang, Physical Review B 73 (2006) 024117–024122. [45] Y. Sui, B. Yao, L. Xiao, G. Xing, L. Yang, X. Li, X. Li, J. Lang, S. Lv, J. Cao, J. Yang, Journal of Applied Physics 113 (2013) 133101–133105. [46] B.Y. Zhang, B. Yao, Y.F. Li, Z.Z. Zhang, B.H. Li, C.X. Shan, D.X. Zhao, D.Z. Shen, Applied Physics Letters 97 (2010) 222101–222103. [47] S.S. Lin, H.P. He, Y.F. Lu, Z.Z. Ye, Journal of Applied Physics 106 (2009) 093508–093512.