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Characteristics of YBa2Cu3O7 Thin Films Deposited on Substrates Buffered by Various
TiO2 Layers
View the table of contents for this issue, or go to the journal homepage for more 2001 Jpn. J. Appl. Phys. 40 L377
(http://iopscience.iop.org/1347-4065/40/4B/L377)
Jpn. J. Appl. Phys. Vol. 40 (2001) pp.L377–L379 Part 2, No. 4B, 15 April 2001
c
2001 The Japan Society of Applied Physics
Characteristics of YBa
2Cu
3O
7Thin Films Deposited on
Substrates Buffered by Various TiO
2Layers
Po-Iem LIN, Chia-Wen LIU, Chich-Chang HSIEH, Kaung-Hsiung WU, Jenh-Yih JUANG, Tseng-Ming UEN, Jiunn-Yuan LIN1and Yih-Shung GOU
Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, R.O.C.
1Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan, R.O.C.
(Received September 21, 2000; accepted for publication February 16, 2001)
Titanium nitride (TiN) and superconducting YBa2Cu3O7 (YBCO) thin films have been deposited sequentially on SrTiO3(STO)(100) substrates by in situ pulsed laser ablation. The TiN films were originally intended to serve as the lower contact electrode of the c-axis YBCO thin films. It was found that, although high-quality YBCO films could be ob-tained with the YBCO/TiN/STO(100) bilayer structure, the TiN(100) layer was oxidized which changed the structure into YBCO/TiO2/STO(100) during YBCO deposition. Comparative studies of depositing YBCO films directly onto a dc-sputtered TiO2/STO(100) template conventionally used in the selective epitaxial growth (SEG) process have, however, resulted in forma-tion of a nonsuperconducting YBCO top layer. The characteristics of the resultant TiO2layers obtained using various processes were analyzed to delineate the apparent discrepancies.
KEYWORDS: TiN films, oxidation of TiN, pulsed laser deposition, YBCO/TiO2/STO bilayer structure, selective epitaxial growth
process
TiN thin films have been studied and used extensively in recent years due to their superior mechanical, thermal, and electrical properties.1–3)Moreover, it has been suggested that
TiN thin films not only serve as buffer layers in depositing superconducting YBa2Cu3O7 (YBCO) thin films on various
substrates such as Si, Hastelloy, Inconel and stainless steel, but also as the electrode for metallization and integration of superconductor and semiconductor devices.4, 5) In the light of these unique properties exhibited by TiN films, we pre-viously attempted to fabricate the YBCO/TiN/substrate struc-ture by sequential in situ pulse laser deposition (PLD) to in-vestigate the out-of-plate (along c-axis) transport properties of YBCO thin films. Surprisingly, we were unsuccessful be-cause of the oxidation of TiN layer during the deposition of YBCO thin films. The reaction of TiN films with oxygen is thermodynamically favorable.6, 7) Insulating and
transpar-ent TiO2 films with a rutile structure were formed
immedi-ately at a temperature of about 800◦C. Since oxidation is unavoidable during deposition of YBCO films, the originally considered YBCO/TiN/substrate bilayer structure should be an YBCO/TiO2/substrate bilayer structure instead. It seems
reasonable to conclude at this point that, even though our at-tempts at using TiN films as an electrode were unsuccessful, the resultant TiO2films can still serve as an excellent buffer
layer for growing YBCO films on some technologically im-portant substrates.
There, however, exist some apparent discrepancies with this conclusion and the results demonstrated in the recently developed selective epitaxial growth (SEG) process.8, 9)In this
method, a detrimental template material was deposited and prepatterned on a bare substrate. Then the subsequent YBCO films growing on top of this template layer become insulating, while those growing directly on the substrate have supercon-ducting characteristics. Damen et al.8)and Cheng et al.9)have
used the patterned Ti template for selective epitaxial growth of microsized YBCO structures. The YBCO thin film grown on regions covered by the oxidized Ti layer became amor-phous and exhibited insulating characteristics, while those de-posited directly on bare STO substrate regions showed ex-cellent superconductivity. Recently, Chuang et al.10) have
prepared a dc-sputtered TiO2 layer directly as the selective
masked template on a bicrystal SrTiO3 (STO) substrate to in situ fabricate dc superconducting quantum interference
de-vives (SQUID). Again, the TiO2layer exhibited excellent
se-lectivity for growing nonsuperconducting YBCO films. In order to resolve these apparent inconsistencies, efforts have been taken to examine the crystallinity, surface morphology, interface, and electronic structure of these films.
Both the TiN and YBCO films have been prepared by PLD. The detailed description of the PLD system was reported pre-viously.11)Briefly, a KrF excimer laser operating at a
repeti-tion rate of 3–8 Hz with an energy density of 2–5 J/cm2 was
used. In order to in situ fabricate the YBCO/TiO2/substrate
structure, both YBCO and TiN targets (99.9% pure) were installed in the deposition chamber simultaneously. It was found that the optimum deposition conditions for the TiN films were obtained under the background pressure (∼5 × 10−6Torr) and at a substrate temperature Ts= 700◦C. On the
other hand, the optimum deposition conditions for the YBCO films deposited on STO were obtained at an oxygen partial pressure of 0.3 Torr and Ts= 780◦C.11)
The dc-sputtered TiO2films were prepared by a
laboratory-built sputtering system. A 50-mm-diameter (99.9% pure) ti-tanium disk was used as the target. The distance between the target and substrate was about 25 mm. Sputtering was carried out in a 1:29 oxygen/argon mixture at a total pres-sure of 0.2 Torr. Since the substrates were not intentionally heated during deposition, the as-deposited films were amor-phous. TiO2 films of 20–50 nm thickness were deposited at
a typical deposition rate of 0.1 nm/min using a total dc input power of 30 W.
The electrical properties of the TiN and YBCO films was measured using a four-probe method. The crystalline struc-ture of the films was examined by X-ray diffraction (XRD, Rigaku D/max-rc/ru200b) using Cu Kα radiation. The sur-face morphology of the films was observed by atomic force microscopy (AFM, Digital Instruments DI 5000) and scan-ning electron microscopy (SEM).
The as-deposited PLD-TiN films, typically about 30–80 nm thick, were shiny golden yellow in appearance. Curve (A) in
L378 Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 2, No. 4B P.-I. LINet al. 0 50 100 150 200 250 300 0 50 100 150 200 (A) TiN/STO (B) YBCO/TiO 2/STO (a) Re s is ti v it y ( -c m ) Temperature(K) 0 10 20 30 40 50 60 70 80 90 100 110 0 500 1000 1500 2000 2500 (b) Ti N (400) Ti N (200) S T O (300) S T O (200) S T O (100) In te n s ity (C P S ) 2 (deg) 5 10 15 20 25 30 35 40 45 50 55 60 65 0 100 200 300 400 500 600 (c) Ti O2 ru tile (2 2 0 ) Y B C O (00 7) Y B C O (00 6)/ S T O (20 0) Y B C O (00 5) Y B C O (00 4) Ti O2 ru ti le (110) Y B C O (00 3)/ S T O (10 0) Y B C O (00 2) Y B C O (001) In te n s ity (C P S ) 2 (deg)
Fig. 1. (a) Resistivity versus temperature curves of a PLD-TiN film (curve (A)) and a YBCO/TiN bilayer structure (curve (B)) deposited on (100)STO substrates; (b) XRD pattern of the (100)TiN film; (c) XRD pattern of the YBCO/TiN/STO structure.
Fig. 1(a) shows the resistivity versus temperature (R-T) curve of an 80-nm-thick TiN film on STO. It is evident that the as-deposited TiN is an excellent metallic compound with a nearly zero residual resistance below 20 K, indicative of al-most impurity-free crystallinity. Figure 1(b) shows the XRD pattern of the film, indicating that the TiN film has grown with a predominance of (100) texture. The surface morphology as revealed by AFM, shows an average grain size of about 50 nm with a rather smooth surface. The root mean square (RMS) roughness of the surface was estimated to be about 0.2 nm.
The high-quality TiN thin films with an excellent electri-cal property and a smooth surface described above seem very suitable as a conductive buffer layer for growing YBCO thin film. Therefore, we prepared a bilayer structure by deposit-ing TiN and YBCO layers sequentially on STO substrates. The thicknesses of TiN and YBCO layers were approximately
25 30 35 40 45 50 55 60 65 0 100 200 300 400 500 600 (a) S T O (200) Ti O2 ru tile (2 2 0 ) Ti O2 ru ti le (110) In te n s ity (C P S ) 2 (deg)
Fig. 2. (a) XRD pattern and (b) AFM image of PLD-TiN transferred TiO2
films. The scanned area of the AFM image was 2µm × 2 µm and the dark-to-light vertical scale was 5 nm.
50 nm and 200 nm, respectively. Figure 1(c) shows the XRD pattern for the YBCO/TiN bilayer structure formed on the STO(100) substrate. Strong (00l) diffraction peaks of YBCO can be observed, indicating that the YBCO film grew with a c-axis-preferred orientation. The YBCO overlayer shows virtually the same transport properties of typical good-quality single-layer YBCO films with a zero-resistance temperature (Tco) of 88 K (Curve (B) in Fig. 1(a)).
However, despite the success of growing an YBCO/TiN/ STO bilayer structure, the effect of oxidation on TiN films was apparently overlooked. Since the in situ deposition of YBCO films is usually performed in oxygen ambient at a high temperature, one would have wondered whether any degrada-tion of TiN took place in such an environment. To verify the suspicion, we etched off the top YBCO layer and found that the gold-colored conductive TiN layer no longer existed. In-stead, the original TiN layer has turned into a transparent in-sulating layer. In order to identify the resultant product, a 50-nm-thick TiN film was loaded into the experimental chamber and treated at 780◦C for 6 min with an oxygen partial pres-sure of 0.3 Torr to simulate the YBCO film deposition pro-cess. Figure 2(a) shows the XRD result for the oxidized TiN films. The diffraction peaks are identified as that of the ru-tile TiO2with the (110)-preferred orientation. The full-width
at half-maximum (FWHM) of the TiO2(110)θ–2θ diffraction
peak was about 0.21◦. This unexpected result suggests that TiN might not be a good underlayer electrode since oxidation is unavoidable during deposition of YBCO. The AFM image in Fig. 2(b) illustrates that the grain size of TiO2thus obtained
Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 2, No. 4B P.-I. LINet al. L379 25 30 35 40 45 50 55 60 65 0 100 200 300 400 500 600 (a) S T O (200) Ti O2 r u tile (2 2 0 ) Ti O2 r u tile (1 1 0 ) In te n s ity (C P S ) 2 (deg)
Fig. 3. (a) XRD pattern and (b) AFM image of annealed sputtered TiO2
film. The scanned area of the AFM image was 2µm × 2 µm and the dark-to-light vertical scale was 60 nm.
TiN films. A greater variation in roughness (RMS roughness
∼1 nm) can also be observed in the figure.
As was already pointed out, very contrary experimental re-sults were observed for YBCO films grown using the SEG technique. Smooth, black, and good superconducting prop-erties were obtained for YBCO films grown on the STO substrate directly, whereas rough, nearly transparent, insu-lating YBCO films (typical resistivity at room temperature
>105·cm) with large particulates were obtained for YBCO
films grown on TiO2buffer layers. For the latter structure, the
absence of an YBCO diffraction peak in XRD measurement indicates that the material is amorphous. It is then interesting to distinguish the differences between TiO2films prepared by
dc sputtering and those transformed from PLD-TiN films. In order to simulate the change of amorphous TiO2 films
pre-pared by dc sputtering prior to YBCO deposition, a 50-nm-thick sputtered-TiO2 film was loaded into a vacuum
cham-ber maintained at 780◦C with a 0.3 Torr oxygen pressure for 6 min. Figures 3(a) and 3(b) show the XRD and AFM im-ages of the annealed sputtered TiO2 film, respectively. As
shown in Fig. 3(a), the diffraction peak of the (110)TiO2
ru-tile phase indicates that the amorphous sputtered TiO2 film
was transformed into the same rutile phase as that of the PLD TiN-transferred TiO2 under the same annealing conditions.
However, the relatively weak diffraction intensity and broad FWHM (∼0.33◦) suggest that the crystallinity of these an-nealed sputtered TiO2 films may not be as good as that of
TiN-transferred TiO2films. The AFM image in Fig. 3(b)
fur-ther reveals that the film consists of many small crystals with irregular facets. The RMS roughness of this film was about 9 nm as compared to 1 nm for the oxidized TiN film.
It is known that the growth of epitaxial YBCO films is closely related to the initial stages of deposition as nucleation and growth first occur on the substrate, as well as to the for-mation and evolution of dislocation and other defects as de-position proceeds. Therefore, the structural properties and the surface morphology of the substrates can influence the final quality of the deposited films significantly. Comparing the XRD and AFM results shown in Figs. 2 and 3 respectively, it seems likely that the poor crystallinity and much higher RMS roughness of annealed sputtered TiO2films might be
re-sponsible for quenching the superconductivity of YBCO films grown on it. Alternatively, another interface layer formed dur-ing YBCO deposition might also be possible. In this scenario, a thin BaTiO3layer formed immediately after the first
deposi-tion of YBCO can be crucial for subsequent growth. Whether the interfacial layer is crystalline BaTiO3or amorphous Ba–
Ti–O layer8)could make significant differences. Experiments
including Auger electron spectroscopy (AES) depth profile analyses and interficial X-ray absorption spectroscopy for de-termining the possible interactions occurring at the YBCO and TiO2 interface are currently in progress and will be
re-ported separately.
In summary, TiN thin films grown on the SrTiO3(100)
sub-strate by PLD are demonsub-strated to be a suitable template for growing YBCO films. However, although the TiN films orig-inally possessed excellent electrical properties, these failed to serve as the underlayer electrode in an YBCO/TiN/STO bilayer structure since it was readily transformed to ru-tile(100) TiO2films during the deposition of top-layer YBCO
films. In contrast to the good superconductivity obtained in the YBCO/TiN-transferred TiO2/STO structure, YBCO films
grown directly on a TiO2 layer prepared by dc sputtering,
turned out to be insulating. The results suggest that poor crystallinity and a drastic increase in RMS roughness of the annealed sputtered TiO2 films might have direct influences
on growing stoichiometric YBCO films. Experiments aimed at resolving the possible interface layer modifications are in progress and are expected to provide more insight on this mat-ter.
This work was supported by the National Science Coun-cil of Taiwan, R.O.C. under grants: NSC89-2112-M-009-027 and -029.
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