Digest Journal of Nanomaterials and Biostructures
Vol. 8, No. 2, April - June 2013, p. 721 - 727
SCANNING TRANSMISSION ELECTRON MICROSCOPY INVESTIGATION
OF ZnO:Al BASED THIN FILM TRANSISTORS
E. VASILEa, S. MIHAIUb, R. PLUGARUc*
S.C. METAV-Research & Development S.A, C.A.Rosetti Str. 31, 020011
Bucharest, Romania
b
Institute of Physical Chemistry "I.G. Murgulescu" Romanian Academy,
Spl.Independentei 202, 060021 Bucharest, Romania
*c
National Institute for R&D in Microtechnology-IMT Bucharest, PO-BOX 38-160,
023573 Bucharest, Romania,
a
High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron
Diffraction (SAED), Scanning Transmission Electron Microscopy with High-Angle
Annular Dark-Field detector (HAADF-STEM), and Energy Dispersive X-ray
Spectroscopy (EDXS) studies were performed in order to investigate the nanostructure,
chemical composition and elemental distribution in depth of ZnO:Al thin films used as
active channel layers of the thin films transistors (TFT) as well as at the interface with the
Ti/Au metallization contacts. Energy Dispersive X-ray spectra (EDXS) and elemental
maps acquired in the cross section of a TFT device evidenced the composition and the
localization of atomic species and revealed the local chemistry at the nanometer scale
rough interfaces.
(Received March 19, 2013; Accepted April 28, 2013)
Keywords: HRTEM, SAED, STEM-EDXS, HAADF-STEM, ZnO based TFT,
Cross section
1. Introduction
The nanometer dimensions of the today devices require the precise control of the films
thickness in relationship with the microstructure and chemical composition, and of their interface
properties, since these characteristics strongly affect the devices functionality [1-3]. ZnO:Al doped
thin films are widely investigated for applications in transparent electronics, sensors,
photodetectors, solar cells, of highest interest being optimizing the structure in relationship with
opto-electrical characteristics [4-6]. Several studies have demonstrated the effect of doping
elements e.g. Al, in improving the ZnO conduction properties, and evidenced the drop of optical
properties related to structural changes induced by these impurities. The improvement of transport
properties occurs for a given concentration of dopant element, while its accumulation in certain
regions of ZnO thin films or at the interface with the substrates leads to deterioration of electrical
characteristics of the devices.
Comprehensive analyses by high resolution microscopy techniques were performed in
order to establish the crystallinity and morphology of the ZnO: Al doped thin films as function of
growth temperature [7-8]. Recently, X-ray electron spectroscopy imaging in high resolution
microscopy was used for characterization of various dopants distribution and defects structures in
doped ZnO systems. Several such systems ranging from powders and nanostructures to thin films
were studied [9, 10]. However, the local chemical composition and elemental mapping at the level
of the ZnO based devices structure are still limited.
In this study we used High Resolution Transmission Electron Microscopy (HRTEM),
Selected Area Electron Diffraction (SAED), Scanning Transmission Electron Microscopy with
*
Corresponding author: rodica.plugaru@imt.ro
722
High-Angle Annular Dark-Field detector (HAADF-STEM) images and STEM-Energy Dispersive
X-ray Spectroscopy (EDXS) techniques for cross sectional nanoscale investigation of local
chemical composition and elemental mapping of ZnO:Al thin films used as channel layers in thin
films transistors (TFT). Furthermore, the interface of ZnO:Al with Ti/Au thin films (used as
metallization contacts) was analysed.
2. Experimental
The TFT devices in a bottom gate configuration were fabricated by sol-gel deposition of
ZnO:0.5 at.% at. Al doped films as channel layer on SiO2 (200 nm)/Si substrates. The source, the
drain, and the bottom gate contacts were prepared by thermal evaporation of Ti/Au contacts in
vacuum (10-6 Torr), through a metallic mask. TFTs were designed with channel width (W) 1100
μm and channel length (L) 280 μm. Details of the sol-gel procedure were presented elsewhere [11].
High resolution structural and chemical analysis were carried out on the cross section
samples from the TFT device by using HRTEM, SAED and STEM microscopy with EDX
spectroscopy. The measurements were performed using a TECNAI F30 microscope operated at
300 kV with EDAX and EELS facilities. The cross section samples for TEM analyses were
prepared using an ion-milling equipment FISCHIONE with Ar ions.
3. Results and discussion
The HRSTEM-EDXS cross sectional study of a TFT device was carried out in order to
determine the in-depth elements distribution in the ZnO:Al film used as channel layer.
Additionally, the interfaces with the Si/SiO2 substrate and the Ti/Au films used for source and
drain metallization contacts were investigated. The ZnO:Al thin film with 0.5 at.% Al
concentration and about 50-60 nm thickness was deposited on a Si/SiO2 substrate by a sol-gel
multilayer process previously described [11]. EDX spectra recorded on different regions of the
ZnO:Al film cross section showed a non-uniform distribution of Al. EDX spectra recorded at the
film surface, in the central region, and at the interface with the Si/SiO2 substrate, reveals that the
Al content is about 0.28 at.%, 1.42 at.%, and 0.52 at.% respectively, and that the accumulation of
Al appears in certain zones along the interface [12]. Further, we aim to investigate the interface
between the ZnO:Al and Ti/Au films. Fig. 1 shows the TEM bright field image of the TFT cross
section in this region. It can be observed that the polycrystalline ZnO:Al film has a rather uniform
thickness with surface roughness leading to an irregular interface with the metallization contact.
TEM and HRTEM images of the TFT cross section are presented in Fig. 2 (a)-(c). Fig. 2
(a) shows the Ti/Au and the interface with the channel layer. The HRTEM image of ZnO:Al thin
film presented in Fig. 2 (b), and the corresponding SAED image (Fig. 2 c), reveal the presence of
5-10 nm size nanocrystals in the polycrystalline film, and the hexagonal wurtzite structure of ZnO.
With the aim of locate the atomic species in the cross section of TFT, we acquired EDX
spectra and elemental maps using HAADF-STEM technique. Figs. 3 (a) and (b) show cross
sectional HAADF-STEM images of the TFT channel in the region of Ti/Au metallization contact.
EDX spectrum presented in Fig. 3 (c) was acquired in the region of the ZnO:Al film, in the area
pointed in the STEM image. The spectrum shows intense peaks corresponding to Zn-L and O-K,
and low intensity peaks corresponding to Al-K, Si-K and Ti-K respectively.
723
SiO
Zn
nO:
Ti/A
Fig. 1 TEM
M bright fieldd image of a crross section zo
one of TFT.
(a)
(b)
(c)
Fig. 2 TE
EM cross sectiion and HRTE
EM images: (a
a) TFT with Zn
nO: Al channeel layer and
Ti/Au mettallization con
ntact, (b) ZnO : Al channel layer
l
and nano
ocrystalls in th
the film, (c)
SAED of the region (b), showing the wurtzite type
ty structure of the film.
Drift corrected spectrum imag
ge Scanning
1
STEM HAADF Detectorr
1
1
(aa)
(b)
ED
DX Drift corrected spectrum image
Zn
Zn
60
Counts
Zn
Zn
Au
40
O
20
Au
Au
Zn
Si
Al
Al Si
S Au
Au
Au
Au
Ti
T
Ti
5
Au
10
Energy (keV)
Au Au
Au
Au
(c)
Au
15
Fig. 3 Crross section off the TFT withh ZnO:0.5% at.
a Al channel layer and mettallization
contact: (a) STEM image,, (b) HAADF--STEM image;; (c) EDX spectrum on the m
marked point
onn the image (b
b).
20
724
Figs. 4 show HAADF-STEM image (a), and EDX maps of Au-M (b), Ti-K (c), Zn-L (d)
and O-K (e) elements. Variation in the colour intensity observed in the elemental maps is
correlated with the number of atoms contained in a particular region, and can give an evaluation of
the elemental composition at the interface between the ZnO:Al channel layer and the metallization
contact.
STEM HAADF Detector
Ti-K
(a)
Zn-L
(c)
50 nm
50
nm
(d)
50 nm
50
nm
50 nm
50
nm
Au-M
O-K
(b)
50 nm
50
nm
(e)
50 nm
50
nm
Fig. 4 Cross section of TFT device: (a) HAADF-STEM image and corresponding EDS maps
of: Au (b), Ti (c), Zn (d) and O elements (e).
Figs. 4 (b) and (c) evidence a nonuniform distribution of Ti and Zn atoms in the region of
Au/Ti interface with ZnO:Al film. The O map, Fig. 4 (e), reveals a lower colour intensity and a
nonuniform distribution in the region of ZnO:Al film and an increased intensity in the region of
the SiO2 substrate, where the distribution is uniform.
725
STEM HAADF Detector
EDX Drift corrected s pectrum im age
Au
Au
Counts
150
100
Zn
Au
50
(a)
Au
Au
Au
Si
Zn Al
O Zn
Ti Ti
5
50 nm
50
nm
EDX Drift corrected s pectrum im age
30
Au
Au
10
Energy (keV)
15
20
EDX Drift corrected s pectrum im age
Zn
Zn
Ti
(b)
Au
Au
Zn
Au
60
O
Zn
Au
Counts
Counts
20
Zn
Zn
10
40
O
Zn
Au
Au
Si Au
Al Au
Ti
5
Zn
Au
(c)
Au
Au
Au
Au
10
Energy (keV)
20
Au
Au
Au
Au
Si
Al
Au
15
20
Ti
5
(d)
Au
Zn
Au
Ti
Au
Au
Au
10
Energy (keV)
Au
Au
15
Fig. 5 HAADF-STEM image of the region presented in Fig.3 (a) and EDX spectra (b)-(d),
recorded along the interface between ZnO:Al film and Ti/Au metallization films.
The HAADF-STEM image (a) and EDX spectra (b)-(d) from Figs. 5 (a)-(d) demonstrated
the in depth nonuniform distribution of Ti, Zn and O atoms at the Au/Ti interface with ZnO:Al
channel layer. The spectrum recorded in the region of Au thin film is presented in Fig. 5 (b). The
spectrum displays that Au is the major element, and that Ti is not present in this region. Intense
peaks associated with Ti and Zn appear in the spectrum recorded in the region of Ti film interface
with ZnO:Al film, Fig. 5 (c), in agreement with the Ti and Zn atoms nonuniform distribution
observed in the cross sectional elemental maps, Figs. 4 (c) and (d). EDX spectrum recorded in the
region of ZnO:Al film is presented in Fig. 5 (d). A low intensity peak corresponding to Ti appears
in the spectrum, revealing the presence of Ti atoms in the ZnO:Al film. Conversely, a low intensity
peak associated with Al is also observed in the spectrum shown in Fig. 5 (d).
Previously it was reported that the nonalloyed Ti/Au contact to ZnO shows very linear
current-voltage behavior with a specific contact resistivity of 2.2x10-5 Ωcm [13]. The Auger
electron spectroscopy (AES) correlated with TEM investigation evidenced the presence of a thin
interfacial Ti-O layer, whose formation was related to outdiffusion of oxygen from ZnO. The high
formation energy of Ti-O phases can explain the interfacial layer development without an
annealing process. Also, Auger electron spectroscopy depth profiles of the Ti/Al/Pt/Au contact on
ZnO evidenced that O appears to diffuse outward and determine Ti-O phase formation after
annealing at 250 oC [14]. Ti-Zn compounds were observed within the Ti layer and the outdiffusion of Zn and O occurred from ZnO to Ti in the case of annealing at 500 oC. These processes
that occurred at the interface led to deterioration of contact properties [15]. In our experiment the
STEM-EDX spectrum presented in Fig. 5 (c) shows an intense maximum of Ti in the region of Ti
film and weak peaks associated with Zn and O. The spectrum presented in Fig. 5 (d) reveal intense
peaks corresponding to Zn and O, and a very weak peak associated with Ti in the ZnO:Al film
region. While it would be difficult to advance a phenomenological model of elements evolution
based on the measurements presented, STEM-EDXS maps and STEM-EDX spectra revealed the
atoms migration in the interfacial layer in the nonalloyed Ti/Au contact to ZnO:Al channel layer.
The current-voltage characteristics of the TFT device presented a linear character, with a high
specific contact resistivity [12]. A small leakage current was observed and attributed to an Al
accumulation at the interface of the ZnO:Al channel layer with the substrate [12].
20
726
The elemental line distribution recorded in the STEM-EDAX mode, along the cross
sectional TFT device, is presented in Fig. 6. TEM image from the Fig. 6 (a) highlights the TFT
cross section and the line along which were recorded the elements distribution showed in Fig. 6 (b).
Distribution of the characteristic X-ray intensities Au-M, Ti-K, Zn-K, Al-K, Si-K are roughly
proportional to elements concentrations along the line. It is observed that Zn concentration
presents a small maximum at the interface with Ti film, then increases and is rather constant in the
region of the ZnO:Al film, with a further small maximum at the interface with the SiO2 substrate.
The Al-K characteristic X-ray intensity contour pursues the Zn-K characteristic X-ray intensity
contour along the line, e.g. the ratio Zn/Al is almost constant in the depth of the ZnO:Al film.
(a)
800
(b)
ZnO:Al
Ti
Au
Intensity [arb.units]
SiO2
600
Zn-K
Si-K
Ti-K
Au-M
Al-K
400
200
0
0
50
100
d [nm]
150
Fig. 6 Characteristic X-ray intensities Au-M, Ti-K, Zn-K, Al-K, Si-K along the line showed in (a)
TEM image of TFT cross section.
It can be seen that the Au-K intensity drops rapidly at the interface with Ti film, which
revealed that Au is not diffussing in this area. At the interface between the Ti film and ZnO:Al
film the characteristic X-ray intensities Ti-K and Zn-K slowly decreases and increases respectively,
revealing an elements interdiffusion in this region.
4. Conclusions
HRTEM, SAED, STEM-EDX, HAADF-STEM techniques were used for investigation of
nanostructure, local chemical composition and elements maps in the cross section of a TFT device
with ZnO:Al thin film as active layer and Ti/Au source and drain metallization contacts. The EDX
spectra acquired in different points of the ZnO:Al film evidenced a variation of Al content from
0.28 at.%., at the surface, to 1.42 at.%, in the film bulk, to 0.52 at.% at the interface with Si/SiO2
substrate. The STEM-EDAX line distribution of the characteristic X-ray intensities Au-M, Ti-K,
Zn-K, Al-K in the cross section depth demonstrated an almost constant ratio Zn/Al in the ZnO:Al
film. Moreover Ti and Zn interdiffusion at the interface of the channel active layer with the
metallization contact was evidenced.
Acknowledgements
E. Vasile acknowledges the support of Project POSDRU/89/1.5/S/63700, 2010-2013.
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References
[1] H.-Q. Huang, F.-J. Liu, J. Sun, J.-W. Zhao, Z.-F. Hu, Z.-J. Li, X.-Q. Zhang, J. Phys. Chem.
Solids 72, 1393 (2011).
[2] J. H. Lee, C. H. Ahn, S. Hwang, C. H. Woo, J.-S. Park, H. K. Cho, J. Y. Lee, Thin Solid
Films 519, 6801 (2011).
[3] J.-L. Wang, P.-Y. Yang, T.-Y. Hsieh, C.-C. Hwang, D.-C. Shye, I-C. Lee, Solid-State
Electron. 77, 72 (2012).
[4] Y.-H. Lin, H.-Y. Lee, C.-T. Lee, C.-H. Chou, Mater. Chem. Phys. 134, 1203 (2012).
[5] Y.P. Wang, J.G. Lu, X. Bie, Z.Z. Ye, X. Li, D. Song, X.Y. Zhao, W.Y. Ye, Applied Surface
Science 257, 5966 (2011).
[6] J. Kim,J.-H. Yun, Y. C. Park, W. A. Anderson, Mater. Lett. 75, 99 (2012).
[7] J.H. Han, Y.S. No, T.W. Kim, J.Y. Lee, J.Y. Kim, W.K. Choi, Appl. Surf. Sci.
256, 1920 (2010).
[8] Z. Zhang, C. Bao, W. Yao, S. Ma, L. Zhang, S. Hou, Superlattices and Microstruct.,
644 (2011).
[9] H. Schmid, E. Okunishi, W. Mader, Ultramicroscopy (2012),
http://dx.doi.org/10.1016/j.ultramic.2012.07.014.
[10] L. C. Ann, S. Mahmud, S. K. M. Bakhori, Appl. Surf. Sci. 265, 137 (2013).
[11] S. Mihaiu, A. Toader, I. Atkinson, M. Anastasescu, M. Vasilescu, M. Zaharescu, R. Plugaru,
Proc. SPIE 7821, Opelectronics, Microelectronics and Nanotechnologies V, 78211D (2010).
[12] E. Vasile, S. Mihaiu, R. Plugaru, presented at 35th International Semiconductor Conference
CAS, October 15-17, Sinaia, Romania, published in Proc. CAS, Vol. 2, 329 (2012).
[13] H.-K. Kim, S.-W. Kim, B. Yang, S.-H. Kim, K. H. Lee, S. H. Ji, Y. S.Yoon, Jpn. J.App. Phys.
45, 1560 (2006).
[14] K. Ip, G.T. Thaler, H. Yanga, S. Y. Hana, Y. Lia, D.P. Nortona, S.J. Pearton, S. Jang, F. Ren,
J. Cryst. Growth 287, 149 (2006).
[15] J.-H. Park, T.-H. Kima, N.-Y. Changa, J.-S. Kim, G.-H. Kim, B.-T. Lee, Mater. Sci. Eng., B
167, 51 (2010).
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