Performance of sol
–gel deposited Zn
1
− x
Mg
x
O films used
as active channel layer for thin-film transistors
Chien-Yie Tsay
a,⁎
, Hua-Chi Cheng
b, Min-Chi Wang
a, Pee-Yew Lee
c,
Chia-Fu Chen
b, d, Chung-Kwei Lin
aaDepartment Mater. Sci. Eng., Feng Chia University, Taichung, Taiwan bDepartment Mater. Sci. Eng., National Chiao Tung University, Hsinchu, Taiwan
cInst. Mater. Eng., National Taiwan Ocean University, Keelung, Taiwan dInst. Mater. Eng., Ming Dao University, Changhua, Taiwan
Available online 11 August 2007
Abstract
ZnO thin-film transistors (TFTs) have attracted considerable R&D interest due to their high transparency and low photosensitivity compared with typical a-Si:H TFTs. The electrical characteristics of ZnO thin films may be controlled by doping with ternary element, for instance Al, Ga, In, Mg, Zr, etc. In this study, Zn1− xMgxO (x = 0 to 0.36) thin films were deposited on glass substrates by spin coating. The as-deposited films were baked at
300 °C for 10 min and then annealed at 500 °C for 1 h in air. The results show that, addition of Mg-species in ZnO films markedly enhanced the uniformity of film thickness and improved optical properties. The Zn0.8Mg0.2O film exhibited the best transparency of 92%, an increase of∼15%
over a pure ZnO film, and the rms roughness value decreased to 1.63. The Zn1− xMgxO TFTs were demonstrated to have n-type enhancement
behavior. The optimum device with Zn0.8Mg0.2O channel layer had a field-effect mobility of 0.1 cm2/V s, a threshold voltage of 6.0 V, and an on/off
ratio more than 107.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Zn1− xMgxO films; Sol–gel method; Thin-film transistors
1. Introduction
Zinc oxide (ZnO) is an n-type II–VI group compound semiconductor material with direct wide bandgap of 3.35 eV. Due to its unique electrical and optical properties it has been widely and popularly used in varistor, gas sensor, UV light emitters and surface acoustic wave devices [1]. Recently, ZnO films also attracted attention for energy and optical–electrical applications, such as window layers of solar cells[2], transparent conductive layers of flat panel displays or touch panels[3]and the active channel layer of thin-film transistors[4]. The TFTs array fabrication process for large-area TFT-LCD has been continu-ously developed for simple processing steps, improving perfor-mance and reducing cost in the process of mass production. The sol–gel method offers a simple and low cost thin-film deposition process as an alternative to the vacuum deposition technique, but
it needs soluble types of materials. The carrier mobility of the intrinsic ZnO materials exceed the field-effect mobility of a-Si:H which is the active channel layer in a typical TFT array. ZnO base TFTs possess low photosensitivity and high transparency, and
have been studied by many groups [4–7], but only a few
reported using chemical solution processes for ZnO film deposition. Norrs et al.[8]reported fabrication of an un-doped ZnO TFT using spin coating. Lee et al. prepared Zn1− xMgxO
thin films[9]and Zn1− xZrxO thin films[10]as active channel
using dip coating and spin coating, respectively. Besides, Cheng et al. [11] also fabricated the transparent ZnO TFT using a combined method of the sol–gel and chemical bath deposition.
It has been demonstrated that the electrical characteristic of ZnO thin films could be controlled by doping with ternary element, for instance Al, Ga, In, Mg, Zr, etc. The incorporation of Mg into ZnO tends to decrease the amount of interstitial oxygen vacancies [12]. Moreover, the study of Lee et al. [9] have illustrated that the depletion region in the grains increased
Surface & Coatings Technology 202 (2007) 1323–1328
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with amount of Mg doping and resulted the almost depleted grains in the active layer of Zn1− xMgxO TFT. The ionic radii of
Mg+2(0.65 Å) and Zn+2(0.74 Å) are similar, and thus the limit of solid solubility of MgO in ZnO can approach 40 at.%[13,14]. Many reports show that the solubility limit of Mg content in Zn1− xMgxO films strongly depend on deposition or growth
technique, e.g. MOVPE [14], PLD[13,15], sputter[16] and sol–gel method[17,18].
In the present study, dense ZnO thin films with or without doping with Mg+2were prepared using the sol–gel method and microstructure, optical properties affected by Mg content were investigated. Moreover, Zn1− xMgxO channel TFTs were
fabricated and electrical characteristics of optimum devices evaluated.
2. Experimental
Zinc acetate dehydrate (Zn(CH3COO)2·2H2O) and
magne-sium acetate tetra-hydrate (Mg(CH3COO)2·4H2O) were
dis-solved in 2-methoxyethanol, and then monoethanolamine (MEA) was added to the solution. The concentration of metal ions in Zn1– xMgxO sols were controlled at 0.75 M and Mg+2
was varied from 0 to 0.36 (for x values). The solution was stirred for 2 h at 60 °C until a clear and homogenous sol was obtained. The Zn1– xMgxO thin film was deposited on
alkali-free glass (Corning 1737) using the spin coating method, using 3000 rpm for 20 s. These as-coated films were baked at 300 °C for 10 min, and then annealed at 500 °C for 1 h in air.
Thermal analysis of the Zn1− xMgxO sols used TG-DSC (TA
Instrument, SDT 2960) to identify the evaporation, decomposi-tion and crystallizadecomposi-tion points of the Zn1− xMgxO solution
ma-terial. The crystal structure and microstructure of these thin films were examined using X-ray diffractometry (XRD, MAC Science MAXP3) and scanning electron microscopy (SEM, HITACHI S-4800), respectively. Surface morphology of the Zn1− xMgxO thin films was observed by scanning probe
micro-scope (SPM, Digital Instrument NS4/D3100CL). Optical trans-mittance spectra in the visible range of these films were examined by a spectrophotometer (Mini-D2T, Ocean Optics Inc.).
The typical bottom-gate structure device of Zn1− xMgxO
TFTs was fabricated by a hybrid method that combined the standard micro-electrical fabricated process and sol–gel meth-od. The MoW film (1000 Å thick) was deposited and patterned on a glass substrate as a bottom-gate electrode. Silicon dioxide, prepared by plasma enhanced chemical vapor deposition (PECVD), served as the gate insulator with thickness 3000 Å. The source and drain select ITO thin-film patterns were defined by standard photolithography process. The channel length and width of the test device were 500 and 60 μm, respectively. Finally, the active layer, Zn1− xMgxO thin film, was deposited
by the spin coating method to finish the Zn1− xMgxO TFTs. The
current–voltage (I–V) characteristics of the transistors with Zn1− xMgxO channel layer were performed by semiconductor
parameter analyzer (HP 4155B). Grain boundaries of poly-crystalline ZnO films generally contain a wide distribution of deep-level traps[19]. This factor can affect the donor density of active layer and mobility of the TFT. In order to minimize this effect, voltage sweep ranged from−100 to 100 V or 0 to 100 V was performed prior to the I–V measurements.
3. Results and discussion
Thermogravimetric analysis (TGA) and differential
scan-ning calorimetry (DSC) of pure ZnO, Zn0.8Mg0.2O and
Zn0.7Mg0.3O sols are shown in Fig. 1. In TGA curves of
dried un-doped ZnO and Zn1− xMgxO precursors, weight
losses were observed with three temperature regions at 40∼ 90, 100∼200 and 210 ∼ 260 °C. The first weight loss region is caused by low temperature solvent evaporation. The second and third weight loss regions are due to evaporation of water and decomposition of organic compounds. The raw source material for the Mg ions, magnesium acetate tetra-hydrate, possesses double the water of crystallization than zinc acetate. For this reason, there are lower solid content in the Mg-doped ZnO sols. Moreover, one large and two small endothermic peaks, and an exothermic peak were found in DSC curves of dried un-doped and Mg-doped ZnO sols. The last peak of the DSC curve, the exothermic peak, results from the crystalliza-tion of Zn1− xMgxO materials. It may be noted that, the effect of
Fig. 1. TGA-DSC curves of the dried of Zn1− xMgxO sols with x = 0, 0.2 or 0.3.
Fig. 2. X-ray diffraction patterns of Zn1− xMgxO thin films (0≦×≦0.36), which
were annealed at 500 °C for 1 h. 1324 C.-Y. Tsay et al. / Surface & Coatings Technology 202 (2007) 1323–1328
Mg-doped ZnO can raise and broad this exothermic peak. It reveals the prepared of Zn1− xMgxO thin films can tolerate a
wider process window.
All the Zn1− xMgxO (x = 0∼0.36) thin films were baked at
300 °C for 10 min and then annealed at 500 °C for 1 h in air. The crystallographic structure of thin films was studied by X-ray diffraction.Fig. 2shows the XRD patterns of these Zn1− xMgxO
thin films, which observed that they had wurzite structure. Crystallized ZnO has a hexagonal crystal structure which was confirmed by the three main diffracted peaks for the (100), (002) and (101) planes. The diffractograph also shows that the intensity of these main peaks reduce with increasing Mg content
and the Zn0.8Mg0.2O film shows a highly c-axis oriented (002)
peak. Besides, a significant shift in the (002) and (001) peaks is
observed after Mg doping. Dhananjay and Krupanidhi [16]
have reported, Mg+2 replaced Zn+2 can increase the a-axis length and decrease c-axis of unit cells, and this transformation may have caused the two main peaks to shift. In the XRD patterns of Zn0.7Mg0.3O and Zn0.64Mg0.36O films, the diffracted
peak of the MgO cubic phase was found at 42.9 degrees theta which is its (200) plane. The results show that at a Mg content more than 30 at.% the MgO phase became segregated. An MgO phase separate from the Zn1− xMgxO film can cause
degener-ation of electrical and optical properties. In previously
Fig. 3. SEM micrographs of cross-sections of Zn1− xMgxO thin films: (a) x = 0; (b) x = 0.2; (c) x = 0.3 and (d) x = 0.36.
mentioned research, many reports show that the solubility limit of Mg in Zn1− xMgxO films strongly depends on deposition or
growth technique. Thus, in order to retain the Zn1− xMgxO
film's pure hexagonal crystal structure, the content of Mg+2 should not be more than x = 0.3 according to our research.
The sol–gel method is a simple oxide thin-film deposition technique. It is possible to control the film thickness by merely adjusting the solution viscosity or coating times. On the top view SEM image of annealed pure ZnO film can observe which surface overspread abnormally erumpent streak (not shown). However, these doped samples cannot observe that appearance. Cross-sectional SEM images of the Zn1− xMgxO thin films are
shown inFig. 3.Fig. 3(a) is an SEM micrograph of the ZnO film that shows its average thickness is about 140 nm. SEM micro-graphs of the nanocrystalline Zn1− xMgxO films (x = 0.2, 0.3 and
Fig. 5. The transmittance spectra of Zn1−xMgxO thin films with x = 0, 0.2, 0.3 or 0.36.
Fig. 6. (a) Output characteristics (ID–VDcurve) and (b) transfer characteristics (ID–VGcurve) of TFT using Zn0.8Mg0.2O thin film as the active channel layer.
0.36) are shown inFig. 3(b)–(d). They show that the thickness of doped film is about 150 nm. They also show that addition of Mg-species to the ZnO films markedly enhanced the uniformity of film thickness. The average grain sizes of the pure ZnO and Mg-doped films were about 20 nm and 10∼15 nm, respective. These SEM images (Fig. 3) reveal that Mg doping of ZnO films can reduce the average grain size and improve the flatness of the Zn1− xMgxfilms. In addition, micrography of the Zn0.64Mg0.36O
film, show in Fig. 3(d) revealed a porous and non-compact microstructure. It exhibit poorer film quality with x value than over 0.3. SPM images of the Zn1− xMgxO thin films are shown
inFig. 4. The surface morphology observed shows the influence of the Mg doping on the ZnO thin film. It is apparent that reduction of surface roughness results from the decrease in average grain size in ZnO film after Mg doping. The rms roughness of the ZnO and Zn0.8Mg0.2O thin films was 16.24
and 1.63 nm, respectively. A significant improvement of surface roughness with Mg doping can be noticed. In addition, Zn0.8Mg0.2O thin film also exhibited the smallest rms
rough-ness among all of the annealed Zn1− xMgxO thin films
inves-tigated in the present study. The phase segregated impurities of x = 0.3 and x = 0.36 samples degenerated the surface texture of the thin films, and their rms values were 1.97 and 3.61, respectively.
Fig. 5shows the optical properties of the Zn1− xMgxO thin
films when examined at room temperature. From this figure, all samples showed sharp absorption edges in the UV region and this absorption edge shifted to shorter wavelengths when ZnO thin film was doped with Mg. The optical transmittance in the visible range for the pure ZnO film was about 80% and exhibited absorption edge at about 362 nm (curve b inFig. 5). The Mg-doped samples show higher transparency compared with the un-doped sample. However, the transmittance spectra of higher Mg content samples, when x≧0.3, showed a slight decrease in the 500 to 800 nm region. This result is in good agreement with the results of XRD, SEM and SPM and relates to Mg phase segregation. In this study, the Zn0.8Mg0.2O sample
exhibited the best transparency among doped samples of 92%, which represents an increase of about 15% over the un-doped ZnO sample.
The electrical characteristics of the devices show that all Zn1− xMgxO TFTs operated in n-type enhancement mode due
to require a positive gate voltage to turn it on. A polycrystalline semiconductor thin film with single phase, defect free and uniform thickness can provide the fine channel layer for carrier propagation from source to drain[20]. The Zn0.8Mg0.2O TFTs
were optimum devices that displayed the best performance in the study. This result is in good agreement with the previous dis-cussions of the current study.Fig. 6(a) shows the representative output characteristics (drain current–drain voltage, ID–VD) of the
Zn0.8Mg0.2O TFT measured at room temperature. The slope of
each ID curve is flat for large VD, and hard saturation was
observed.Fig. 6(b) shows the corresponding transfer character-istics (ID–VG) of the same device, measured at a fixed VDof 30 V.
This ID–VGcurve reveals a drain current on to off ratio of more
than 107. The threshold voltage (Vth) was defined by fitting a
straight line and then intercepting the x-axis of the (ID)1/2–VG
plot. At the same time, the field-effect mobility (μsat) was
determined by the following square equation[21]: IDðsatÞ¼ lsatCOX W 2L VG Vth ð Þ2; for V D≪ Vð G VthÞ
where W is the channel width, L is the channel length, COXis
the unit capacitance of gate insulator, Vth is the threshold
voltage andμsatis mobility in the saturated current region. The
Vth and μsat of the Zn0.8Mg0.2O TFT were calculated to be
6.0 V and 0.1 cm2V− 1s− 1, respectively. 4. Conclusions
Preparation and characterization of thin-film transistors with Mg-doped ZnO films for the active channel layer were investigated. The results show that Mg+2doped ZnO films of the form Zn1− xMgxO had markedly improved surface texture
(reduced roughness), optical properties and finer microstructure than ZnO films. The Zn0.8Mg0.2O film exhibited the best
transparency at 92%, increasing by 15% value for the un-doped ZnO film (80%), and its rms roughness value decreased to 1.63. When the Mg content exceeded 0.3 (× value) MgO phase segregation would occur. This impurity phase caused the film quality to degenerate. In this study, the Zn1− xMgxO TFTs were
fabricated using the sol–gel method and exhibited n-type enhancement mode. The optimum device with Zn0.8Mg0.2O
channel layer has a field-effect mobility of 0.1 cm2/V s, a threshold voltage of 6.0 V, and an on/off ratio more than 107. Acknowledgments
The authors gratefully acknowledge the financial support by the National Science Council of Republic of China under contract no. NSC 95-2221-E-035-006 and Taiwan TFT-LCD Association (TTLA) under contract no. A643TT1000-S11. References
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