- Email: [email protected]
Combinatorial synthesis and high throughput evaluation of functional oxides-A integrated materials chip approach
Materials Science and Engineering B56 (1998) 246 – 250
Combinatorial synthesis and high throughput evaluation of functional oxides-A integrated materials chip approach X.-D. Xiang * Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Abstract Integrated materials chip approach, in which large collections of different thin film metal oxides are integrated and synthesized on a small chip and screened for a particular functionality, is applied to discover or optimize superconductors, luminescent materials, magnetic materials, ferroelectrics and dielectrics. This technology promises to significantly increase the efficiency of the materials discovery and optimization process and improve our understanding of materials structure-property relationship. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Combinatorial synthesis; Functional oxides; Material chip fabrication
1. Introduction Materials play a key role in the advancement of science and technology, the development of semiconductors being a good example. In particular, metal oxides are found to have many interesting properties with a broad range of applications, including superconductors for the electronics and energy industries, phosphors for the display industry, dielectrics and electrooptical materials for the information and communications industries. Even though our understanding of the electronic, magnetic, optical and chemical properties associated with these materials has increased considerably in recent years, the ability to predict structure-property relationships is only possible in a limited number of cases, and involves considerable theoretical effort. This lack of predictive ability is not surprising in light of the complexity of modern materials as well as the effects of dopants/defects on their properties. As a result materials are typically discovered and optimized by an empirical trial and error process that is both time-consuming and costly. We have recently shown [1] that combinatorial methods, an approach in which large collections of materials are rapidly synthesized, processed and screened for specific * E-mail: [email protected]
properties of interest, can significantly facilitate the materials discovery/optimization process. Since the first applications of the combinatorial approach to the materials discovery process [2], significant progress has been made in developing more effective methods for generating and screening complex libraries of different classes of oxide materials. Method for integrating hundreds to tens of thousands oxides of different compositions on a single chip (e.g. inch2) has been developed. Synthesis method has been developed to ensure high quality epitaxial growth of thin film samples on the chip. High throughput screening systems have also been developed to characterize the electronic, optical, and magnetic properties.
2. Materials chip fabrication The first step in the combinatorial approach involves the synthesis of materials libraries (or chips) designed to explore large segments of phase spaces thought to be of interest based on our understanding of the physical and structural properties of a particular class of materials. This is accomplished by the sequential thin film deposition of precursors at different sites on a substrate using a series of precisely positioned shadow masks. The efficiency with which the search can be carried out is
0921-5107/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5107(98)00221-9
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
dictated by the masking scheme employed. To improve the efficiency and increase the diversity of the search, we devised a quaternary combinatorial masking scheme. This scheme uses a series of n different masks, which successively subdivide the substrate into a series of self-similar patterns of quadrants (Fig. 1). The rth (1 5 r5 n) mask contains 4r −1 windows where each window exposes one quarter of the area deposited using the preceding mask. Within each window there are an array of 4n−r sample sites. Each mask is used in up to four sequential depositions; each time the mask is rotated by 90°. This process produces 4n different compositions with 4n deposition steps and can be used to effectively survey materials consisting of up to n elemental components where each component is selected from a group of (up to four) precursors. This approach represents a significant improvement in our ability to screen large landscapes of diverse compositions in comparison to simple binary [1] or gradient masking techniques and should be applicable to many classes of materials. Increases in the spatial density of distinct compositions that can be synthesized and screened in a given library will also improve the effectiveness of any combinatorial search. The implementation of masking schemes is accomplished using either physical shadow mask [1] or photolithographic lift-off [3]. Photolithography is well suited for generating oxide materials chips containing a high density of sites due to its high spatial resolution and alignment accuracy. In this procedure, before each oxide thin film deposition step, photoresist is deposited and patterned, leaving behind open windows. After thin film deposition, the remaining photoresist, together with the overlying film, are lifted off with acetone, leaving behind films only in the open window regions. Our thin film deposition methods include pulsed laser deposition (PLD) and RF sputtering techniques. PLD can be used to tailor the growth of complex multi-element metal oxides thin films and superlattices. PLD also has a major advantage over other deposition techniques such as evaporation in that it can be used to reproduce the stoichiometric composition of the oxide target material onto the film. We have developed a high vacuum PLD system equipped with an automated twodimensional in-situ shuttering mechanism with very high positioning accuracy. Depositions are performed in high vacuum (B 10 − 8 torr) in order to take advantage of the larger angle expanding plume compared to depositions done at higher gas pressures where the plasma plume of the material has a pointed shape. To further increase the uniformity of the deposition, the laser beam is made to raster on 2’’ diameter targets. This is accomplished together with an auto-focus mechanism of the laser beam to ensure a homogenous laser beam spot at different positions on the target. As a
247
result of these efforts, we have achieved thickness uniformity of better than 3% over a 1’’ square area. After the identification of lead compounds from the broad search, a gradient composition technique is used to fabricate the materials chips. In this approach, a smaller number of compositional parameters are continuously varied. The use of our in-situ automated shutters in the pulsed laser ablation system has been proven to be highly effective in the fabrication of gradient chips with quick turn-around. This approach can also be used to study phase diagrams of interesting material systems in condensed matter physics. For library fabrications, proper selection of substrate materials is critical. Refractory single crystal substrates are usually used to minimize substrate-sample reactions. Lattice matching and thermal expansion coefficients are also taken into account in order to fabricate epitaxial thin film libraries in some cases. Common substrates we have used include MgO, LaAlO3, and sapphire.
3. Synthesis of materials chips A critical scientific issue in our combinatorial approach is the development of effective synthesis methods compatible with combinatorial approach. Traditional solid state synthesis involves mixing solid state precursors and heating the resulting mixture at high temperatures in a controlled environment. The typical precursor grain sizes are on order of microns. Due to the high activation energies required for solid state interdiffusion on this length scale, very high temperatures and extended reaction time (hours to weeks) are needed. Since it is difficult to control interdiffusion and nucleation processes in a mixture composed of micron size multi-component grains, diffusion and nucleation are occurring simultaneously. Consequently, one typically isolates thermodynamically stable rather than kinetically stable products. For example, one might in the synthesis of a ternary compound obtain undesired stable binaries, which nucleate first at grain boundaries. As one increases the complexity of the materials these issues become more critical. The ability to control inter-diffusion and nucleation, and the ability to access metastable phases with kinetic control over products are desirable. In our thin film synthesis approach, the precursors are deposited sequentially as thin film multilayers. There exists a critical thickness (few tens to hundreds of ˚ for oxides) for sequential precursor layers. Below this A thickness it is possible to interdiffuse the precursor layers at a relatively low temperature to form an intermediate amorphous state close in stoichiometry to the desired phase, prior to the nucleation of thermodynamically stable phases [4]. Layering sequence can also
248
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
Fig. 1. The quaternary masking scheme.
dramatically influence solid state reaction pathways and therefore, the final products of solid state reactions. We often use an extended period of low temperature (100–400°C) annealing process for proper interdiffusion of thin film precursors. Various analysis tools are used to characterize the films and to optimize the layering sequences as well as the annealing procedures. We have been probing the interdiffusion of the precursors by Rutherford backscattering (RBS) (in collaboration with University of Maryland) systematically after each step of annealing. The use of RBS ensures that resulting films have the intended and uniformly distributed compositions (Fig. 2a). After the low temperature diffusion process, X-ray dif-
Fig. 2. (a) Rutherford backscattering spectrum of a BaTiO3 film made from BaF2/TiO2 precursors. The fit indicates that the compositional ratio of Ba to Ti is approximately 1 at all depths of the film. (b) f scan of 110 reflections of a BaTiO3 film on an MgO substrate. The film was made from BaF2/TiO2 precursors.
fraction studies indicate that the intermediates are in amorphous state with no crystalline formation. For high temperature phase-formation process, multiple identical libraries are often made to undergo different processing conditions, such as annealing temperatures and atmospheres. Finally, atomic force microscopy and X-ray diffraction studies are used to examine the surface and crystallinity of the samples, respectively. We have demonstrated that the present film growth procedure can be used to obtain predominantly singlephase epitaxial films on lattice matched substrates (Fig. 2b), allowing epitaxial growth of integrated materials chips.
4. High throughput screening of materials chips A key aspect of the combinatorial approach is the ability to rapidly measure a particular materials property of interest. Although detailed physical characterization may require bulk samples, quick screening of few key properties of samples on the chips is crucial. This can be accomplished by high throughput parallel or scanning imaging systems with high spatial resolution and high sensitivity. Ideally, imaging should be nondestructive since a materials chip can be screened for multiple properties. A number of detection systems have been developed to date for screening materials chips. An optical scanning spectrophotometer has been used to evaluate the photon output and chromaticity of each member in phosphor libraries (chips) upon excitation with monochromatic UV light [3]. Several other optical imaging systems have also been developed to measure, e.g. electro-optical coefficient, magneto-optical effect and surface magnetization. Quick and reliable characterization of the complex electric impedance of samples on materials chips presents a very challenging task. For example, in the case of frequency agile materials, three key properties— electric-field tunability, dielectric constant and loss tangent—should all be characterized at microwave frequencies. To rapidly screen for electric properties of the materials chips, we have recently developed a scanning tip microwave near-field microscope Fig. 3 (STMNM) [5,6].
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
249
Fig. 3. Schematic of the scanning-tip microwave near-field microscope measurement.
This microscope is consists of a sharpened metal tip, mounted on the center conductor of a high quality factor (Q) l/4 coaxial resonator, extended beyond an aperture formed in the endwall of the resonator [6]. A sapphire disk with a center hole of size close to the diameter of the tip wire (50 – 100 mm) and a metal layer ( 1 mm) coating on the outside surface was used as the center part of the endwall. This thin metal layer coating keeps the far-field propagating components from leaking out, which increase the background signal and decrease the spatial resolution. To prevent the tip from vibrating against the shielding aperture, the tip is affixed to the sapphire disk with insulating glue. The resonant frequency ( fr) and Q change as functions of the properties (e.g. dielectric constant and loss tangent) of the materials placed in the vicinity of the tip. Images can be obtained by recording the changes in fr and Q, while scanning the sample under the tip. To scan the sample without damaging the surface significantly, a very soft, thin tungsten tape cantilever with a spring force estimated (under a typical operating condition) to be B 20 mN is placed under the sample to provide soft contact of the sample with the tip. With this contact force, tips of radius B 1 mm are free from being crushed during the scan. The microscope in this configuration is therefore not sensitive to tip-sample distance changes caused by topographic features on the sample surfaces. Coupled with a theoretical analysis [6], the dielectric constant (or) and loss tangent (tan d) can be determined with an accuracy of 2 – 5% with a spatial
resolution down to 100 nm (l/106). By applying an ac or dc bias voltage between the resonator and electrodes deposited on the backside of the substrate, the nonlinear dielectric constant (equivalent to tunability) can be determined. We have used STMNM to successfully image periodic dielectric constant modulations and domain structures in LiNbO3 ferroelectric single crystal [7] and micro-domain structures of PbMg1/3Nb2/3O3 thin films [6].
5. Applications Combinatorial approach has been used to identify a class of cobalt oxide magnetoresistive materials of the form (La0.88S0.12)CoO3 [2]. Magnetoresistance was found to increase as the size of the alkaline ion increased, in contrast to Mn-containing compounds, in which the magnetoresistive effect increases as the size of the alkaline earth ion decreases. The technique has also been applied to the optimization and discovery of luminescent materials. Systematic variation of composition and processing conditions resulted in the optimization of several rare earth doped refractory metal oxides phosphors with high quantum efficiencies and good chromaticities [7,8]. More recently, a novel blue emitting luminescent composite material Gd3Ga5O12/ SiOx [3], has been discovered using integrated materials chips. This approach has also been recently applied to ferroelectric materials where the effects of
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
250
transition metal dopants on the dielectric constant and tangent loss of a library of thin films of (Bax Sr1 − x )TiO3 were determined at microwave frequencies [9].
Acknowledgements The author would like to acknowledge the important contributions from Drs Peter G. Schultz, Chen Gao, Ichiro Takeuchi, Kai-An Wang, Tao Wei, Gabriel Briceno, Jingsong Wang, Yulin Lou, R. P. Sharma, T. Venkatesan, also Hauyee Chang, Sung-Wei Chen, W. G. Wallace-Fredman, Fred Duewer and Young Yoo to this project. This work was supported by the Director of Advanced Energy Projects Division, Office of Computational and Technology Research, U.S. Department of Energy under contract DE-AC03-76SF00098 and the Office of Naval Research, Order No. N00014-95-F-0099.
.
References [1] X.-D. Xiang, X. Sun, G. Bricen˜o, Y. Lou, K. Wang, H. Chang, W.G. Wallace-Freedman, S. Chen, P.G. Schultz, Science 258 (1995) 1738. [2] G. Brice, H. Chang, X.-D. Sun, P. G. Schultz, X.-D. Xiang, ibid 270 (1995) 273. [3] J. Wang, Y. Yoo, C. Gao, I. Takeuchi, X.-D. Sun, H. Chang, X.-D. Xiang, P.G. Schultz, Science 279 (1998) 1712. [4] L. Fister, T. Novet, C.A. Grant, D.C. Johnson, Adv. Syn. React. Solids 2 (1994) 155. [5] T. Wei, X.-D. Xiang, W.G. Wallace-Freedman, P.G. Schultz, Appl. Phy. Lett. 68 (1996) 3506. [6] C. Gao, T. Wei, F. Duewer, Y. Lu, X.-D. Xiang, Appl. Phy. Lett. 71 (1997) 1872. [7] X.-D. Sun, C. Gao, J. Wang, X.-D. Xiang, Appl. Phy. Lett. 70 (1997) 3353. [8] X.-D. Sun, X.-D. Xiang, Appl. Phy. Lett. 72 (1998) 525. [9] H. Chang, C. Gao, I. Takeuchi, Y. Yoo, J. Wang, P.G. Schultz, X.-D. Xiang, P.P. Shama, M. Downes, T. Verkatesan, Appl. Phy. Lett. 72 (1998) 2185.
Combinatorial synthesis and high throughput evaluation of functional oxides-A integrated materials chip approach X.-D. Xiang * Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Abstract Integrated materials chip approach, in which large collections of different thin film metal oxides are integrated and synthesized on a small chip and screened for a particular functionality, is applied to discover or optimize superconductors, luminescent materials, magnetic materials, ferroelectrics and dielectrics. This technology promises to significantly increase the efficiency of the materials discovery and optimization process and improve our understanding of materials structure-property relationship. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Combinatorial synthesis; Functional oxides; Material chip fabrication
1. Introduction Materials play a key role in the advancement of science and technology, the development of semiconductors being a good example. In particular, metal oxides are found to have many interesting properties with a broad range of applications, including superconductors for the electronics and energy industries, phosphors for the display industry, dielectrics and electrooptical materials for the information and communications industries. Even though our understanding of the electronic, magnetic, optical and chemical properties associated with these materials has increased considerably in recent years, the ability to predict structure-property relationships is only possible in a limited number of cases, and involves considerable theoretical effort. This lack of predictive ability is not surprising in light of the complexity of modern materials as well as the effects of dopants/defects on their properties. As a result materials are typically discovered and optimized by an empirical trial and error process that is both time-consuming and costly. We have recently shown [1] that combinatorial methods, an approach in which large collections of materials are rapidly synthesized, processed and screened for specific * E-mail: [email protected]
properties of interest, can significantly facilitate the materials discovery/optimization process. Since the first applications of the combinatorial approach to the materials discovery process [2], significant progress has been made in developing more effective methods for generating and screening complex libraries of different classes of oxide materials. Method for integrating hundreds to tens of thousands oxides of different compositions on a single chip (e.g. inch2) has been developed. Synthesis method has been developed to ensure high quality epitaxial growth of thin film samples on the chip. High throughput screening systems have also been developed to characterize the electronic, optical, and magnetic properties.
2. Materials chip fabrication The first step in the combinatorial approach involves the synthesis of materials libraries (or chips) designed to explore large segments of phase spaces thought to be of interest based on our understanding of the physical and structural properties of a particular class of materials. This is accomplished by the sequential thin film deposition of precursors at different sites on a substrate using a series of precisely positioned shadow masks. The efficiency with which the search can be carried out is
0921-5107/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5107(98)00221-9
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
dictated by the masking scheme employed. To improve the efficiency and increase the diversity of the search, we devised a quaternary combinatorial masking scheme. This scheme uses a series of n different masks, which successively subdivide the substrate into a series of self-similar patterns of quadrants (Fig. 1). The rth (1 5 r5 n) mask contains 4r −1 windows where each window exposes one quarter of the area deposited using the preceding mask. Within each window there are an array of 4n−r sample sites. Each mask is used in up to four sequential depositions; each time the mask is rotated by 90°. This process produces 4n different compositions with 4n deposition steps and can be used to effectively survey materials consisting of up to n elemental components where each component is selected from a group of (up to four) precursors. This approach represents a significant improvement in our ability to screen large landscapes of diverse compositions in comparison to simple binary [1] or gradient masking techniques and should be applicable to many classes of materials. Increases in the spatial density of distinct compositions that can be synthesized and screened in a given library will also improve the effectiveness of any combinatorial search. The implementation of masking schemes is accomplished using either physical shadow mask [1] or photolithographic lift-off [3]. Photolithography is well suited for generating oxide materials chips containing a high density of sites due to its high spatial resolution and alignment accuracy. In this procedure, before each oxide thin film deposition step, photoresist is deposited and patterned, leaving behind open windows. After thin film deposition, the remaining photoresist, together with the overlying film, are lifted off with acetone, leaving behind films only in the open window regions. Our thin film deposition methods include pulsed laser deposition (PLD) and RF sputtering techniques. PLD can be used to tailor the growth of complex multi-element metal oxides thin films and superlattices. PLD also has a major advantage over other deposition techniques such as evaporation in that it can be used to reproduce the stoichiometric composition of the oxide target material onto the film. We have developed a high vacuum PLD system equipped with an automated twodimensional in-situ shuttering mechanism with very high positioning accuracy. Depositions are performed in high vacuum (B 10 − 8 torr) in order to take advantage of the larger angle expanding plume compared to depositions done at higher gas pressures where the plasma plume of the material has a pointed shape. To further increase the uniformity of the deposition, the laser beam is made to raster on 2’’ diameter targets. This is accomplished together with an auto-focus mechanism of the laser beam to ensure a homogenous laser beam spot at different positions on the target. As a
247
result of these efforts, we have achieved thickness uniformity of better than 3% over a 1’’ square area. After the identification of lead compounds from the broad search, a gradient composition technique is used to fabricate the materials chips. In this approach, a smaller number of compositional parameters are continuously varied. The use of our in-situ automated shutters in the pulsed laser ablation system has been proven to be highly effective in the fabrication of gradient chips with quick turn-around. This approach can also be used to study phase diagrams of interesting material systems in condensed matter physics. For library fabrications, proper selection of substrate materials is critical. Refractory single crystal substrates are usually used to minimize substrate-sample reactions. Lattice matching and thermal expansion coefficients are also taken into account in order to fabricate epitaxial thin film libraries in some cases. Common substrates we have used include MgO, LaAlO3, and sapphire.
3. Synthesis of materials chips A critical scientific issue in our combinatorial approach is the development of effective synthesis methods compatible with combinatorial approach. Traditional solid state synthesis involves mixing solid state precursors and heating the resulting mixture at high temperatures in a controlled environment. The typical precursor grain sizes are on order of microns. Due to the high activation energies required for solid state interdiffusion on this length scale, very high temperatures and extended reaction time (hours to weeks) are needed. Since it is difficult to control interdiffusion and nucleation processes in a mixture composed of micron size multi-component grains, diffusion and nucleation are occurring simultaneously. Consequently, one typically isolates thermodynamically stable rather than kinetically stable products. For example, one might in the synthesis of a ternary compound obtain undesired stable binaries, which nucleate first at grain boundaries. As one increases the complexity of the materials these issues become more critical. The ability to control inter-diffusion and nucleation, and the ability to access metastable phases with kinetic control over products are desirable. In our thin film synthesis approach, the precursors are deposited sequentially as thin film multilayers. There exists a critical thickness (few tens to hundreds of ˚ for oxides) for sequential precursor layers. Below this A thickness it is possible to interdiffuse the precursor layers at a relatively low temperature to form an intermediate amorphous state close in stoichiometry to the desired phase, prior to the nucleation of thermodynamically stable phases [4]. Layering sequence can also
248
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
Fig. 1. The quaternary masking scheme.
dramatically influence solid state reaction pathways and therefore, the final products of solid state reactions. We often use an extended period of low temperature (100–400°C) annealing process for proper interdiffusion of thin film precursors. Various analysis tools are used to characterize the films and to optimize the layering sequences as well as the annealing procedures. We have been probing the interdiffusion of the precursors by Rutherford backscattering (RBS) (in collaboration with University of Maryland) systematically after each step of annealing. The use of RBS ensures that resulting films have the intended and uniformly distributed compositions (Fig. 2a). After the low temperature diffusion process, X-ray dif-
Fig. 2. (a) Rutherford backscattering spectrum of a BaTiO3 film made from BaF2/TiO2 precursors. The fit indicates that the compositional ratio of Ba to Ti is approximately 1 at all depths of the film. (b) f scan of 110 reflections of a BaTiO3 film on an MgO substrate. The film was made from BaF2/TiO2 precursors.
fraction studies indicate that the intermediates are in amorphous state with no crystalline formation. For high temperature phase-formation process, multiple identical libraries are often made to undergo different processing conditions, such as annealing temperatures and atmospheres. Finally, atomic force microscopy and X-ray diffraction studies are used to examine the surface and crystallinity of the samples, respectively. We have demonstrated that the present film growth procedure can be used to obtain predominantly singlephase epitaxial films on lattice matched substrates (Fig. 2b), allowing epitaxial growth of integrated materials chips.
4. High throughput screening of materials chips A key aspect of the combinatorial approach is the ability to rapidly measure a particular materials property of interest. Although detailed physical characterization may require bulk samples, quick screening of few key properties of samples on the chips is crucial. This can be accomplished by high throughput parallel or scanning imaging systems with high spatial resolution and high sensitivity. Ideally, imaging should be nondestructive since a materials chip can be screened for multiple properties. A number of detection systems have been developed to date for screening materials chips. An optical scanning spectrophotometer has been used to evaluate the photon output and chromaticity of each member in phosphor libraries (chips) upon excitation with monochromatic UV light [3]. Several other optical imaging systems have also been developed to measure, e.g. electro-optical coefficient, magneto-optical effect and surface magnetization. Quick and reliable characterization of the complex electric impedance of samples on materials chips presents a very challenging task. For example, in the case of frequency agile materials, three key properties— electric-field tunability, dielectric constant and loss tangent—should all be characterized at microwave frequencies. To rapidly screen for electric properties of the materials chips, we have recently developed a scanning tip microwave near-field microscope Fig. 3 (STMNM) [5,6].
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
249
Fig. 3. Schematic of the scanning-tip microwave near-field microscope measurement.
This microscope is consists of a sharpened metal tip, mounted on the center conductor of a high quality factor (Q) l/4 coaxial resonator, extended beyond an aperture formed in the endwall of the resonator [6]. A sapphire disk with a center hole of size close to the diameter of the tip wire (50 – 100 mm) and a metal layer ( 1 mm) coating on the outside surface was used as the center part of the endwall. This thin metal layer coating keeps the far-field propagating components from leaking out, which increase the background signal and decrease the spatial resolution. To prevent the tip from vibrating against the shielding aperture, the tip is affixed to the sapphire disk with insulating glue. The resonant frequency ( fr) and Q change as functions of the properties (e.g. dielectric constant and loss tangent) of the materials placed in the vicinity of the tip. Images can be obtained by recording the changes in fr and Q, while scanning the sample under the tip. To scan the sample without damaging the surface significantly, a very soft, thin tungsten tape cantilever with a spring force estimated (under a typical operating condition) to be B 20 mN is placed under the sample to provide soft contact of the sample with the tip. With this contact force, tips of radius B 1 mm are free from being crushed during the scan. The microscope in this configuration is therefore not sensitive to tip-sample distance changes caused by topographic features on the sample surfaces. Coupled with a theoretical analysis [6], the dielectric constant (or) and loss tangent (tan d) can be determined with an accuracy of 2 – 5% with a spatial
resolution down to 100 nm (l/106). By applying an ac or dc bias voltage between the resonator and electrodes deposited on the backside of the substrate, the nonlinear dielectric constant (equivalent to tunability) can be determined. We have used STMNM to successfully image periodic dielectric constant modulations and domain structures in LiNbO3 ferroelectric single crystal [7] and micro-domain structures of PbMg1/3Nb2/3O3 thin films [6].
5. Applications Combinatorial approach has been used to identify a class of cobalt oxide magnetoresistive materials of the form (La0.88S0.12)CoO3 [2]. Magnetoresistance was found to increase as the size of the alkaline ion increased, in contrast to Mn-containing compounds, in which the magnetoresistive effect increases as the size of the alkaline earth ion decreases. The technique has also been applied to the optimization and discovery of luminescent materials. Systematic variation of composition and processing conditions resulted in the optimization of several rare earth doped refractory metal oxides phosphors with high quantum efficiencies and good chromaticities [7,8]. More recently, a novel blue emitting luminescent composite material Gd3Ga5O12/ SiOx [3], has been discovered using integrated materials chips. This approach has also been recently applied to ferroelectric materials where the effects of
X.-D. Xiang / Materials Science and Engineering B56 (1998) 246–250
250
transition metal dopants on the dielectric constant and tangent loss of a library of thin films of (Bax Sr1 − x )TiO3 were determined at microwave frequencies [9].
Acknowledgements The author would like to acknowledge the important contributions from Drs Peter G. Schultz, Chen Gao, Ichiro Takeuchi, Kai-An Wang, Tao Wei, Gabriel Briceno, Jingsong Wang, Yulin Lou, R. P. Sharma, T. Venkatesan, also Hauyee Chang, Sung-Wei Chen, W. G. Wallace-Fredman, Fred Duewer and Young Yoo to this project. This work was supported by the Director of Advanced Energy Projects Division, Office of Computational and Technology Research, U.S. Department of Energy under contract DE-AC03-76SF00098 and the Office of Naval Research, Order No. N00014-95-F-0099.
.
References [1] X.-D. Xiang, X. Sun, G. Bricen˜o, Y. Lou, K. Wang, H. Chang, W.G. Wallace-Freedman, S. Chen, P.G. Schultz, Science 258 (1995) 1738. [2] G. Brice, H. Chang, X.-D. Sun, P. G. Schultz, X.-D. Xiang, ibid 270 (1995) 273. [3] J. Wang, Y. Yoo, C. Gao, I. Takeuchi, X.-D. Sun, H. Chang, X.-D. Xiang, P.G. Schultz, Science 279 (1998) 1712. [4] L. Fister, T. Novet, C.A. Grant, D.C. Johnson, Adv. Syn. React. Solids 2 (1994) 155. [5] T. Wei, X.-D. Xiang, W.G. Wallace-Freedman, P.G. Schultz, Appl. Phy. Lett. 68 (1996) 3506. [6] C. Gao, T. Wei, F. Duewer, Y. Lu, X.-D. Xiang, Appl. Phy. Lett. 71 (1997) 1872. [7] X.-D. Sun, C. Gao, J. Wang, X.-D. Xiang, Appl. Phy. Lett. 70 (1997) 3353. [8] X.-D. Sun, X.-D. Xiang, Appl. Phy. Lett. 72 (1998) 525. [9] H. Chang, C. Gao, I. Takeuchi, Y. Yoo, J. Wang, P.G. Schultz, X.-D. Xiang, P.P. Shama, M. Downes, T. Verkatesan, Appl. Phy. Lett. 72 (1998) 2185.