Post annealing effect on optical and electrical properties of
Ga-doped ZnCdO thin films 1-xx
DUAN Libing, ZHAO Xiaoru, LIU Jinming, GENG Wangchang, ZHANG Fuli,
5 SHI Xiaolong, SUN Huinan
(Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education of China and
School of Science, Northwestern Polytechnical University, Xi’an 710072)
Abstract: (Cd,Ga)-codoped ZnO thin films were prepared by sol-gel method. The codoping films retained wurtzite structure of ZnO, and showed preferential c-axis orientation. The effects of post
10 annealing ambient (in vacuum and nitrogen) on the optical and electrical properties of the films were investigated. The transmittances of the films were obviously degraded by vacuum annealing to 60-70%, but enhanced to 80-90% after nitrogen annealing, which were about 10% higher than those of (Cd,Al)-codoped ZnO films. The carrier concentration increased, while resistivity decreased with narrowing band gap of Ga-doped ZnCdO, i.e. the conductivity is also improved by Cd codoping. 1-xx
15 The resistivity of nitrogen annealing films is one order higher than that of vacuum annealing films, i.e. the transmittance and conductivity of the films seem irreconcilable, and the trade-off between transmittance and conductivity could be effectively controlled by post annealing ambient. Both the Cd doping (majority) and Burstein-Moss effect (minority) affect the band gap (Eg) modification. Due to 3+ 2+ 3+the ionic radius of Gais closer to that of Znthan Al, and hence less deformation of ZnO lattice
20 after Ga doping, in view of transmittance and conductivity, Ga might be a more appropriate dopant for our band gap engineering transparent conducting oxide (TCO) films than Al.
Keywords: Inorganic nonmetallic materials; ZnO films-based Transparent conducting; Band gap engineering; post annealing
25 0 Introduction
Transparent conducting oxide (TCO) films, which are characterized by a unique combination
xtensively applied in several of low electrical resistivity and high optical transparency, have been e
optoelectronic devices such as light emitting diodes (LEDs), solar cells, and flat panels etc.[1,2]. Indium tin oxide (ITO) is the most commonly used TCO films for these applications. However, a
30 replacement for ITO is now required due to indium is rare and its supply is limited by the availability of natural resources [3,4]. ZnO has attracted increasing attention because of its abundance and relatively low cost. ZnO is a native n-type semiconductor with a wide band gap (E) g
of 3.36 eV. To improve the conductivity, ZnO is typically doped with trivalent elements such as
-4 Al, Ga, etc. [5-7]. Al-doped ZnO (AZO) film resistivity of less than 2×10Ωcm has been attained,
35 which is comparable to those obtained in ITO films . As an alternative donor, Ga should disrupt 3+ the ZnO lattice less than Al, as the ionic radius of Ga(0.47 Å, coordination number CN=4) is
2+ 3+ smaller than that of Zn(0.60 Å, CN=4) but larger than that of Al(0.39 Å, CN=4) .
3+ 3+ Furthermore, Gadopants are less reactive and more resistive to oxidation than Aldopants .
Ga-doped ZnO (GZO) films prepared by Bhosle et al.  using pulse laser deposition (PLD)
-4 40 displayed lowest resistivity of 1.4 × 10Ω cm at 5% Ga doping. Nayak et al.  also reported
that the resistivity of 2 at. % Ga-doped ZnO film synthesized by sol-gel spin-coating method could
-3 reach 3.3× 10Ω cm.
Furthermore, the band gap of ZnO (E=3.36 eV) can be tailored by alloying MgO or CdO etc. g
While Mg is known to enhance the band gap, Cd substitution leads to reduce in band gap, the
45 resultant (Zn,Cd)O and (Zn,Mg)O alloys have allowed band gap covering a wide range of 2.8-4.5
Foundations: Specialized Research Fund for the Doctoral Program of Higher Education (Grant No.
20106102120051), NPU Foundation for Fundamental Research (NPU-FFR-JC201017), and National Natural Science Foundation of China (Grant No. 50872112, 51172186).
Brief author introduction:DUAN Libing, (1981-), Male, Lecturer, Structure and physical properties of oxides films and nanoparticles. E-mail: email@example.com
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eV in practice [12-15]. A TCO with modified band gap gives rise to many of its scientific and technical applications, such as improving the efficiency of different wavelength light emitting devices when used as a transparent electrode, and the realization of heterojunction and superlattice
2+2+ 2+structures [16-18]. Theoretically, due to the isovalent ions of Mg, Cdand Zn, there is no
50 contribution of extra free charge carriers for GZO from the Mg or Cd substitution. However, the incorporation of Mg or Cd may enhance electron scattering and grain boundary barrier effects and then destroy the conductivity of ZnO-based TCO [13,14]. Several investigators have examined the optical and electrical properties of Ga-doped ZnMgO films [19,20], Mg or Cd doping content 1-xx
ight affect both of the optical and electrical properties of band gap engineered AZO. Meanwhile, m
55 the effect of narrowing band gap by Cd doping on the electrical and optical properties of (Cd,Ga)-codoped ZnO films has been rarely reported.
Researchers have employed numerous deposition techniques to prepare ZnO-based thin films, including PLD, molecular beam epitaxy (MBE), chemical vapor deposition (CVD), magnetron sputtering, etc.[10,19,20] However, the industrial production is limited due to the complex and 60 expensive vacuum technique. Furthermore, the preparation of homogeneous and large-area films is also an upfront challenge. The sol-gel method is a kind of cost-effective process and is helpful to realize the preparation of large-area homogeneous films . More importantly, sol-gel method has the distinct advantages in excellent composition control and the ability to achieve atomic scale mixing of individual components . In addition, post-annealing treatment by various 65 atmospheres, such as air, oxygen, hydrogen, nitrogen, or in vacuum for as-prepared TCO films is usually considered as an essential and effective technique to improve the electrical and optical properties . The effects of atmospheres could be simply divided into two classes, (a) Annealing the TCO films in air, oxygen or nitrogen could improve the crystallinity and transmittance, but also would degrade the electrical properties due to the chemisorptions of Oor 2
70 N; (b) Annealing in hydrogen environment was reported to be significant in improving the 2
electrical conductivity, and the reason was considered to be both the production of additional oxygen vacancies and desorption of the absorbed oxygen at the grain boundaries. Treatment in vacuum, which was qualitatively similar to the effect of annealing in hydrogen, was also usually introduced to improve the conductivity of TCO films by enhancing oxygen vacancies [23,24]. In 75 our previous work, we have taken a research on the effect of annealing ambient on the structural, optical and electrical properties of the band gap modified (Cd,Al)-codoped ZnO thin films . To make a comparison, here, we prepared 2 at.% Ga codoped ZnCdO (with nominal Cd content 1-xx
x=0-8 at. %) thin films by dip-coating sol-gel method. Analogously, the effects of post-annealing in two representative ambient (in vacuum and nitrogen) on the optical and electrical properties of 80 the band gap modified (Cd,Ga)-codoped ZnO thin films are investigated.
1 Experimental procedure
Analytical grade zinc acetate was firstly dissolved in a 2-methoxyethanol and monoethanolamine (MEA) solution at room temperature. The concentration of the sol was 0.75 mol/L and the molar ratio of MEA to zinc acetate was kept at 1.0. The solution was stirred at 60 o85 C for one hour until it became clear and homogeneous. Gallium nitrate and/or cadmium nitrate were added into some of the previous solutions in an appropriate ratio and then stirred vigorously
oat 60 C for another one hour. The final solutions served for coating were aged for 36 hours at room temperature. The following dip-coating process and post annealing conditions (in air, vacuum and nitrogen) are exactly same as the preparation of (Cd,Al)-codoped ZnO thin films, 90 more details could be found in our previous work .
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X-ray diffraction (XRD) patterns were collected from 20? to 80? using PANalytical X’pert
radiation. Optical properties such as transmittance were measured by MPD PRO with Cu Kα UV-VIS spectrophotometer (Hitachi UV-VIS spectrophotometer U3010) in the wavelength of 300-800 nm. The thickness of the samples was carried out by spectroscopic ellipsometer (Spec
95 EI-2000-VIS). The electrical properties such as resistivity, carrier concentration, and Hall mobility were detected by Hall effect measurements in the Van der Pauw configuration using an electrical
transport property measurement system (Beijing Jingcheng, China, ET-9000) at room temperature. 2 Results and discussion Fig. 1 displays the XRD patterns of 2 at.% Ga-doped ZnCdO (x=0-8% with an increment 1-xx
100 of 2 at. %) thin films treated by vacuum and nitrogen annealing, respectively. It is implied that all
the films have a single phase which can be identified as the hexagonal wurtzite structure of ZnO (space group P6mc). No trace of other impurities is found within the detection limit of instrument. 3
All the films show an extremely pronounced (002) texture with dominant peak 2θ?34.4?,
indicating that the preferred orientation is along the crystallographic c-axis and perpendicular to
105 the substrate. It also manifests a slight reduction of c-axis preferred orientation, which is revealed
by the appearance of tiny (10l) (l=1,2,3) peaks . From the better c-axis preferred orientation, especially for the samples after nitrogen annealing, it could be concluded that the disruption of
(Cd,Ga)-codoped ZnO crystallinity is indeed less that of (Cd,Al)-codoped ZnO , due to the 3+ 2+ 3+ ionic radius of Ga(0.47 Å, CN=4) is closer to that of Zn(0.60 Å, CN=4) than Al(0.39 Å,
CN=4) [8,9]. 110
Fig. 1 XRD patterns of 2 at.% Ga-doped ZnCdO (x=0-8% with an increment of 2 at. %) thin films treated 1-xx by vacuum (a) and nitrogen (b) annealing.
115 Actually, all (Cd,Ga)-codoped ZnO thin films were originally processed under air annealing.
The optical transmission spectra exhibit a high transmittance (about 90%) in visible region and a high absorption (near 100%) in ultraviolet (UV) region (not shown here). However, the resistivity is out of the measurement range of our Hall effect system, this degradation of conductivity might
be due to the chemisorptions of O. Therefore, we post-annealed these films in vacuum 2 -2 120 (?5×10Pa) and nitrogen, respectively, as we have done for (Cd,Al)-codoped ZnO thin films
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Fig. 2 Resistivity (a), carrier concentration (b), and Hall mobility (c) of 2 at.% Ga-doped ZnCdO films 1-xx post-annealed in vacuum.
Fig.2 displays the summarized electrical properties of Ga-doped ZnCdO films 1-xx post-annealed in vacuum. The resistivity [Fig.2 (a)] decreases gradually with Cd content on the -3 -19 -3 order of 10Ω cm, while the carrier concentration [Fig.2 (b)] increases on the order of 10cm.
-2 The magnitude of resistivity is one order lower than that of the Al-doped ZnCdO films (?10Ω 1-xx
cm) synthesized under identical conditions in our previous work , which demonstrates that Ga 130
might be a more appropriate dopant than Al to enhance the conductivity of ZnO-based TCO films. After the initial increase from x=0 to x=2%, with higher Cd doping, the Hall mobility [Fig.2 (c)]
drops, due to more defects and/or residual strain might be introduced, i.e. the electron scattering was enhanced . Therefore, the doping of Cd in band gap modified (Cd,Ga)-codoped ZnO films
also has an influence on conductivity. 135
Transmittance spectra of vacuum treated (Cd,Ga)-codoped ZnO films in the wavelength range of 300-800 nm are shown in Fig. 3(a). The transmittances in visible region of vacuum
treated films are obviously decreased to 60-70%, due to more oxygen vacancies are introduced and the degradation of crystallization after vacuum processing [22,23]. However, compared to the
Al-doped ZnCdO films in our previous work , the transmittances of Ga-doped ZnCdO 140 1-xx1-xx
films under vacuum annealing have been improved from 50-60% to 60-70%, which might also
3+ 2+ 3+ due to the ionic radius of Gais closer to that of Znthan Aland less deformation of ZnO
lattice after Ga doping . The optical energy gap Efor the direct electron transition can be g
determined using the following equation: 1 / 2 145 (1) )αhν = C(hν ? E g where C is a constant, α is the absorption coefficient and ν is the photon frequency . As shown in Fig. 3(b), the fundamental absorption, which corresponds to the electron excitation from
valance band to conduction band, is usually used to calculate the value of band gap using Equation 2 1 by plotting (αhν)as a function of the photo energy and by extrapolating the linear region to the
150 energy axis. The determined values are shown in the inset of Fig. 3(b). The linear variation of
band gap by Cd doping in GZO further confirms that the Cd is doped into the matrix, which is
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consistent with our result of XRD study. As a result, Cd works effectively on band gap
engineering, irrespective of the existence of Ga codoping.
155 Fig.3 Transmittance of Ga-doped ZnCdO films annealed in vacuum: (a) Transmission spectra with Cd doping 1-xx content x; (b) Plot of square of the absorption coefficient vs photon energy, inset: the optical band gap energies as a function of Cd content.
Fig. 4 Transmittance of Ga-doped ZnCdO films annealed in nitrogen: (a) Transmission spectra with Cd doping 1-xx160 content x; (b) Plot of square of the absorption coefficient vs photon energy, inset: the optical band gap energies as a function of Cd content. The traditional ITO films commonly have high transmittance in visible region (at least 90%), and the transmittance and resistivity of inexpensive Al-doped ZnO are nearly comparable to those 165
obtained from ITO films [1-5]. Obviously, the low transmittance (60-70%) of our vacuum treated (Cd,Ga)-codoped ZnO films is not fulfilling the minimum requirement for TCO, although the
band gap and resistivity could be tailored by Cd doping. The transmittance and conductivity of our TCO films seem irreconcilable (in air and vacuum annealing). As we mentioned above, oxygen and nitrogen annealing have been proved to be good at improving the crystallinity and 170
transmittance , and the optical transmission spectra of our air-annealed films exhibit high
transmittances (about 90%), but bad conductivity. Alternatively, the air annealed films were
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CdO further post-annealed in nitrogen. Transmittance spectra of nitrogen treated Ga-doped Zn1-xx films are shown in Fig. 4(a), the transmittances in visible region of nitrogen treated films are
distinctly increased to 80-90%, comparing with the films annealed in vacuum (60-70%), which are 175
more close to those of ITO films. Meanwhile, similar to the situation after vacuum annealing, the transmittances of Ga-doped ZnCdO films after nitrogen annealing (80-90%) is about 10% 1-xx higher than that of our previous Al-doped ZnCdO films (70-80%) , due to the less 1-xx deformation of ZnO lattice by alternative Ga doping. The determined Eafter nitrogen annealing is g
shown in the inset of Fig. 4(b). The Eof Ga-doped ZnCdO films also decreases linearly with 180 g 1-xx increasing Cd doping content after nitrogen annealing, indicating that Cd works effectively on band gap engineering, regardless of the annealing ambient. Fig.5 displays the summarized electrical properties of Ga-doped ZnCdO films 1-xx post-annealed in nitrogen. Although the evolution is similar to the vacuum annealing films, the
resistivity of nitrogen annealing films [Fig.5 (a)] decreases with Cd content on the magnitude of 185
-2 ?10Ω cm, which is one order higher than that of the vacuum annealing films, while the carrier concentration and Hall mobility both increases. Due to the improvement of crystallization, the transmittance is enhanced with sacrificing a portion of conductivity after nitrogen annealing . However, the magnitude of resistivity after nitrogen annealing of the Ga-doped ZnCdO films is 1-xx
as same order as that of Al-doped ZnCdO films , while the Ga-doped ZnCdO films have 190 1-xx1-xx
?10% higher transmittance. Therefore, although the transmittance and conductivity of our TCO films seem irreconcilable, the trade-off could be effectively controlled by different post annealing ambient. Fig. 5 Resistivity (a), carrier concentration (b), and Hall mobility (c) of 2 at.% Ga-doped ZnCdO films 195 x1-x post-annealed in nitrogen. Generally, there are several scattering mechanisms in TCO films, such as lattice vibration
scattering, grain boundary scattering, ionized impurity scattering, neutral impurity scattering, etc., depending on the range of carrier concentration and the temperature of the films . As in 200
Al-doped ZnCdO films , all the measurements were collected at room temperature and the 1-xx
effect of lattice scattering which was determined by the variation of temperature could be ruled out.
Within the framework of potential barrier model at grain boundaries , defects located at grain
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boundaries can act as carrier traps and the trapped electrons set up a negative charge, contributing
to rise to a space charge region in the grains. This will produce a potential barrier at the grain 205
boundaries and decrease the Hall mobility of carriers. Zhu et al.  find that when the carrier 18-3 concentration is low (approximately N ?5.0 ×10cm), grain boundary model is valid.
However, the barrier height will decrease with the increase in carrier concentration, when the carrier concentration is high, the carrier tunneling current plays a more important role in the
transportation of electrons and the grain boundary scattering even can be neglected. The carrier 210
19-3 cm), so the grain concentrations of our (Cd,Ga)-codoped ZnO films are higher (>1.0×10 boundary scattering was tiny enough to be ignored. In this case, ionized impurity and natural
scattering should make main contribution to the variation of conductivity. Fig. 6 Comparison of band gap E(left axis) and carrier concentration (right axis) of the films annealed in vacuum 215 g and nitrogen. In single dopant ZnO based-TCO films, the band gap modification is commonly related to Burstein and Moss (B-M) effect . The Fermi level in degenerate semiconductors is above the
conduction band edge (due to partially-filled states in the conduction band), and optical excitations 220
from valence band to the Fermi level require an extra energy. It has been shown that this widening
is a function of the carrier density according to the formula: 2 h3 2 / 3 2 / 3 ΔE = ( ) n(2)g c* 8mπ c * where h is the Planck constant, m is the reduced effective mass, and nis the charge c c
225 carrier concentration [24,30]. For clarity, as summarized in Fig. 6, in our (Cd,Ga)-codoped ZnO
films, the dependence of band gap on carrier concentration is contrast to this dependence, i.e. the carrier concentrations increase with narrowing of band gap E, which manifests that the underline g physical origin might be some different. In thermal equilibrium, the electron density in conduction band (n) and hole density (p) in
valence band has the relationship:230 Eπk 2 g 3 3 / 2 3 np = 4( )(m m)T exp(? )(3)de dh 2 h kT and mis the density-of-state where h is the Planck constant, k is the Boltzmann constant, mde dh
effective mass for electron and hole, respectively . In our Ga-doped ZnCdO thin films, the 1-xx n-type carriers are dominant, as shown in Fig.6, the carrier concentration increases expectedly
with narrowing energy band. The conductivity of GZO is also improved by Cd codoping, along 235
with original motivation of Emodification. In addition, as compared in Fig. 6, the carrier g
concentration in the films annealed in vacuum is expectedly higher than that of the films treated in
nitrogen, and the band gap in the former films is slightly larger than that of the latter ones. This
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phenomenon might indicate that the widening of band gap as increasing carrier concentration due
to B-M effect still exists in our codoping films, even if the narrowing trend in general by Cd 240 modification. doping, i.e. the Cd doping (majority) and B-M effect (minority) both affect the Eg 3 Conclusion (Cd,Ga)-codoped ZnO thin films were prepared on glass substrates by sol-gel method. The codoping thin films retained a single phase which can be identified as the hexagonal wurtzite
structure of ZnO, and showed preferential c-axis orientation. The transmittances of these films are 245
obviously degraded by vacuum annealing to 60-70%, but enhanced to 80-90% after nitrogen annealing, which are ?10% higher than those of Al-doped ZnCdO films. It is revealed that the 1-xx conductivity of GZO is also improved by Cd doping, which is originally introduced for E g
modification. the resistivity of the films annealed in nitrogen is one order higher than that of
vacuum annealing films. The transmittance and conductivity of our TCO films seem irreconcilable. 250
However, the trade-off between transmittance and conductivity could be effectively controlled by post annealing ambient. And the Cd doping (majority) and B-M effect (minority) both affect the
Emodification. The magnitude of resistivity of Ga-doped ZnCdO films after vacuum g 1-xx annealing is one order lower than that of Al-doped ZnCdO films synthesized under identical 1-xx conditions. The differences in optical and electrical properties are attributed to the closer ionic 255 3+ 2+ 3+radius of Gato that of Znthan Al, and hence less deformation of ZnO lattice after Ga doping.
Therefore, in view of transmittance and conductivity, Ga might be a more appropriate dopant for our codoping TCO films than Al. Acknowledgements This work is financially supported by Specialized Research Fund for the Doctoral Program of 260
Higher Education (Grant No. 20106102120051), NPU Foundation for Fundamental Research (NPU-FFR-JC201017), and National Natural Science Foundation of China (Grant No. 50872112, 51172186).
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后退火工艺对 Ga 掺杂 ZnCdO 薄膜光学和电学性 1-xx
能的影响 段利兵？赵小如？刘金铭？耿旺昌？张富利？史小龙？孙慧楠 ；西北工业大学理学院？教育部空间应用物理与化学重点实验室？西安 710072？ 摘
要，本文采用溶胶-凝胶法制备了具有 c 轴择优取向的单相 Ga 掺杂 ZnCdO 薄膜？研究 了1-xx 后退火工艺；在真空和氮气氛中？对薄膜的光学和电学性能的影响。真空退火后薄膜的透 过330 率为 60-70%？而氮气退火后提高为 80-90%？均比 Al 掺杂 ZnCdO 薄膜高出约 10%。 1-xx Cd 掺杂可以有效地调控薄膜的禁带宽度？并对能影响薄膜的电学性能？同时 Burstein-Moss 效应依然对禁带宽度的变化产生影响。真空退火后的电阻率比氮气退火的电阻率低一个数量 级？说明在透过率和导电性之间需要选取一个最佳平衡点以取得最优化性能？而不同后退火 3+3+2+气氛能够对其有效地调节。由于 Ga相对于 Al与 Zn的离子半径更为接近？掺杂后对 ZnO 3+3+335 晶格的影响相对较低？因此？综合电学和透光性能？Ga相对 Al更宜作为掺杂元素以提高 ZnO 基透明导电薄膜的光电性能。 关键词，无机非金属材料！ZnO 基透明导电薄膜！禁带宽度调节！后退火工艺 中图分类号，O472
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