6 R. G. Vadimsky, R. P. Frankenthal, and D. E. Thompson, J. Electrochem. Soc. 126, 2017 1979. 7 I. K. Yoo and S. B. Desu, Phys. Status Solidi A 133, 565 1992.
Epitaxial growth of highly conductive RuO2 thin films on (100) Si Q. X. JiaS. G. SongX. D. Wu, J. H. Cho, S. R. Foltyn, A. T. Findikoglu, and J. L. Smith
Citation: Appl. Phys. Lett. 68, 1069 (1996); doi: 10.1063/1.115715 View online: http://dx.doi.org/10.1063/1.115715 View Table of Contents: http://aip.scitation.org/toc/apl/68/8 Published by the American Institute of Physics
Articles you may be interested in Heteroepitaxial growth of highly conductive metal oxide RuO2 thin films by pulsed laser deposition Appl. Phys. Lett. 67, (1998); 10.1063/1.115054
Epitaxial growth of highly conductive RuO2 thin films on (100) Si Q. X. Jia Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
S. G. Song MST5, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
X. D. Wu, J. H. Cho, S. R. Foltyn, A. T. Findikoglu, and J. L. Smith Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
~Received 24 July 1995; accepted for publication 13 December 1995! Conductive RuO2 thin films have been heteroepitaxially grown by pulsed laser deposition on Si substrates with yttria-stabilized zirconia ~YSZ! buffer layers. The RuO2 thin films deposited under optimized processing conditions are a-axis oriented normal to the Si substrate surface with a high degree of in-plane alignment with the major axes of the ~100! Si substrate. Cross-sectional transmission electron microscopy analysis on the RuO2 /YSZ/Si multilayer shows an atomically sharp interface between the RuO2 and the YSZ. Electrical measurements show that the crystalline RuO2 thin films are metallic over a temperature range from 4.2 to 300 K and are highly conductive with a room-temperature resistivity of 3762 mV cm. The residual resistance ratio (R 300 K /R 4.2 K! above 5 for our RuO2 thin films is the highest ever reported for such films on Si substrates. © 1996 American Institute of Physics. @S0003-6951~96!01308-4#
There has recently been considerable interest in the deposition of highly conductive metallic oxide RuO2 thin films. Various applications of RuO2 thin films in very large scale integrated circuits have been extensively explored,1– 4 and the use of RuO2 thin films to fabricate precision thin-film resistors in the microelectronics industry has also been investigated.5 The RuO2 , which exhibits good adhesion to Si or SiO2 , possesses high resistance to chemical attack, and shows good stability at temperatures as high as 800 °C,6 has also been used as electrodes for ferroelectric or highdielectric constant thin-film capacitors.7–12 Experimental results have shown that the remanent polarization fatigue behavior of PbZrx Ti12x O3 ~PZT!-based thin-film capacitors can be substantially improved with RuO2 electrodes, as opposed to conventional Pt electrodes.7,8,10 Polycrystalline RuO2 thin films have been deposited on a variety of substrates such as Si, SiO2 /Si, quartz, glass, and MgO by reactive sputtering2,3,8 –10 and by metalorganic chemical vapor deposition.1,13 Epitaxial growth of such metallic oxides on Si has long been of interest for the development of novel electronic devices. For many electronic and optical devices, however, crystalline thin films are more attractive than polycrystalline ones because they exhibit greater stability, uniformity, and reproducibility. Nevertheless, there has been no report, to our knowledge, of the epitaxial growth of RuO2 thin films on Si substrates. In this letter we describe the structural and electrical properties of crystalline RuO2 thin films heteroepitaxially grown on Si substrates. To deposit crystalline RuO2 thin films on ~100! Si substrates, yttria-stabilized zirconia ~YSZ! was used as an intermediate layer. Both the YSZ and RuO2 crystalline thin films were grown in situ by pulsed laser deposition ~PLD! with a 308 nm XeCl excimer laser. The PLD operated at repetition rates of 2–10 Hz, producing 20 ns pulses with an energy density of 2 J/cm2. Single-crystal, boron-doped ~100! Si ~ptype! wafers with a resistivity of 1–5 V cm were used as the Appl. Phys. Lett. 68 (8), 19 February 1996
substrates. They were cleaned using a sequence of acetone, methanol, de-ionized water, and a dilute HF ~HF: H2O51:10! solution. The YSZ buffer layer was epitaxially grown on the Si at a substrate temperature of 800 °C. To prevent oxidation of the Si substrate surface during the early stages of YSZ growth, the first 5 nm of the YSZ deposition was carried out at a base pressure of 531025 Torr. An oxygen pressure of 1 mTorr was then maintained for the remaining YSZ layer growth. The detailed experimental setup and growth conditions for producing crystalline YSZ thin films on Si have been previously described by Fork et al.14 Following the YSZ layer deposition, the RuO2 layer was deposited by switching the target without breaking vacuum. The RuO2 target was a pellet made from RuO2•xH2O powder. The details of the target preparation will be published elsewhere.15 The deposition temperature for the RuO2 layer was varied from room temperature to 800 °C. The oxygen pressure was optimized and then maintained at 0.5 mTorr for all successive preparations. The nominal RuO2 film thicknesses were 120–150 nm. The structural properties of the RuO2 films were characterized by x-ray diffraction ~XRD! measurements using a Siemens D5000 four circle diffractometer with Cu Ka radiation. The thin films with optimized deposition conditions was examined by cross-sectional transmission electron microscopy. The resistivity and the residual resistance ratios ~RRR5R 300 K /R 4.2 K! of the RuO2 thin films were measured by means of a four-probe measurement. The XRD u-2u scans of the RuO2 thin films deposited above 400 °C show only ~h00! reflections of the RuO2 , indicating that the films are a-axis oriented normal to the substrate surface. The full width at half-maximum ~FWHM! of the v-rocking curve, measured on the ~200! peak of RuO2 , is 0.7° for a deposition temperature of 700 °C, which is comparable to the 0.6° value obtained on single crystal YSZ substrates.15 The in-plane alignment of each layer with the major axes of Si is confirmed by XRD f scans on the RuO2 ~101!, YSZ ~202!, and Si ~101! reflections
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FIG. 1. X-ray diffraction f scans for a heterostructure of RuO2 on YSZ/Si taken on ~101! reflection of RuO2 ~a!, the ~202! reflection of YSZ ~b!, and the ~101! reflection of Si substrate. The RuO2 was deposited at 700 °C.
as shown in Fig. 1. It is clear that the epitaxial and relationship is (h00) RuO2 i (h00) YSZi (h00) Si ^ 011 & RuO2 i ^ 001 & YSZi ^ 001 & Si . A diagonal-type epitaxial growth of RuO2 on YSZ results in the degeneracy of the RuO2 peaks in Fig. 1. This growth pattern can be easily understood based on the lattice orientation relationship illustrated in Fig. 2. Diagonal-type epitaxy of RuO2 on YSZ will result in the smallest lattice strain as can be seen by considering the lattice parameters of YSZ ~a50.513 90 nm! and RuO2 ~a5b50.449 02 nm, c50.310 59 nm!. Our systematic study of the film structure as a function of the deposition temperature reveals that higher deposition temperatures ~in the range of 400–700 °C! result in better film texture. Figure 3 shows the FWHM values from both v-rocking curves on ~200! and f scans on ~101! RuO2 thin films deposited at different substrate temperatures. Wellaligned RuO2 thin films can be grown on YSZ buffered Si at a deposition temperature in the range of 400–700 °C, but the films tend to be polycrystalline if the deposition is done at a substrate temperature outside of this range. The films deposited at 800 °C show not only ~200! and ~110! diffraction peaks from RuO2 but also ~002!, ~100!, and ~102! peaks from elemental Ru, indicating the decomposition of RuO2 . This has also been found for films deposited directly on singlecrystal YSZ substrates.15 Epitaxial growth of RuO2 thin films on YSZ buffered Si was further examined by high resolution cross-sectional transmission electron microscopy ~HRTEM! analysis on RuO2 /YSZ/Si. Figure 4 shows HRTEM micrographs of ~a! a RuO2 /YSZ/Si multilayer structure and ~b! the interface between RuO2 and YSZ. The electron beam in both cases is parallel to the @011# axis of the Si substrate. Epitaxial growth
FIG. 2. Schematic drawing of the epitaxial growth pattern of RuO2 on YSZ. 1070
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FIG. 3. Dependence of crystallinity of RuO2 thin films on the deposition temperatures. The value of FWHM is from the v-rocking curves ~tilt of a axis! and f scans ~in-plane twist misalignment! on the RuO2 ~200! and ~101! reflections, respectively.
of RuO2 on YSZ-buffered Si is clearly evident not only from the HRTEM analysis but also from selected-area electron diffraction ~not shown!. The interface between RuO2 and YSZ is close to atomically sharp, and there is no obvious interdiffusion between the two layers. However, misfit dislocations are observed in RuO2 regions near the interface. The fairly thin ~4 –5 nm! amorphous SiOx layer between YSZ and Si is also shown in Fig. 4~a!. This is commonly observed in epitaxial growth of YSZ on Si and is believed to be due to oxygen diffusion through the YSZ layer during the later stages of YSZ layer growth.16,17 Electrical measurements show that the RuO2 films deposited with optimized processing conditions are highly conductive, with a room-temperature resistivity of 3762 mV cm. This value is comparable to that of single-crystal bulk RuO2 .18 The reduced grain boundary scattering in these well-ordered films likely increases the conductivity. It has been found that the room-temperature resistivity of polycrystalline RuO2 films can be ten times higher than that of the bulk single-crystal RuO2 due to grain-boundary scattering.19
FIG. 4. Cross-sectional TEM micrograph of the RuO2 thin films on YSZ buffered Si, ~a! bright field micrograph of the RuO2 /YSZ/SiOx /Si, ~b! high resolution micrograph of the interface between YSZ and RuO2. Jia et al.
on Si substrates with YSZ as a buffer layer. The epitaxial relationship is found to be (h00) RuO2 i (h00) YSZi (h00) Si and ^ 011& RuO2i^001&YSZi ^ 001& Si . Crystalline RuO2 thin films show not only relatively lower room-temperature resistivity but also a higher RRR than polycrystalline films. The degree of crystallinity and the structural perfection are the main controlling factors in determining the electrical properties of the RuO2 thin films. Work performed under the auspices of the U.S. Department of Energy. We thank D. E. Peterson for his support.
1
FIG. 5. Residual resistance ratio of RuO2 thin films as a function of deposition temperatures. The inset shows the typical normalized resistance vs temperature characteristic of RuO2 on YSZ buffered Si.
Figure 5 shows the RRR of RuO2 films deposited at various temperatures; the RRR increases monotonically from 1.3 to 5.2 as the deposition temperature is increased from 300 to 700 °C. The inset in Fig. 5 shows typical normalized film resistance (R/R 300 K! versus temperature for two films deposited at 300 and 700 °C. We believe that the RRR value above 5 is the highest ever reported for RuO2 thin films; values in the range of 1–2 have been reported for polycrystalline RuO2 thin films,3,19 and an RRR above 20 has been reported for bulk single-crystal RuO2 .18 The high RRR or low resistivity of the RuO2 thin films deposited at a relatively high substrate temperature is consistent with the crystallinity of the films. The higher the deposition temperature, the higher the degree of crystallinity of the films, as shown in Fig. 3, provided the deposition temperature is less than the decomposition temperature. In summary, high-quality crystalline RuO2 thin films have been heteroepitaxially grown by pulsed laser deposition
Appl. Phys. Lett., Vol. 68, No. 8, 19 February 1996
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