Introduction
Our research program involves fundamental and applied projects in inorganic and materials chemistry. In each of the projects that follow, students may be involved in a) the synthesis of new ligands and metal complexes for use of precursors to solid state materials, b) the development of new thin film deposition processes and/or c) the characterization of solid state materials using X-ray diffraction, scanning and transmission electron microscopy, ion beam scattering and other methods.
Combinatorial Thin Film Synthesis
Combinatorial strategies of materials synthesis have been used to optimize a wide variety of physical properties (luminescence, magnetic behavior, etc.) and catalytic behaviors. Both solution based and vapor phase processes have been used to create libraries, and during the course of several of these studies, entirely new solid state compounds have been discovered.
Strategies for synthesizing thin film libraries of materials have made use of physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods. One can divide these into processes in which the components are codeposited and processes involving sequential deposition. In the latter case, film composition relates to the relative thicknesses of the components, and complete mixing of the layers is achieved in a post-deposition annealing step. This approach can suffer from unwanted reactions that may occur in the annealing step before the materials are completely mixed. Reducing the layer thickness of each component to monolayer coverage solves this problem.
The codeposition approach has the advantage of achieving intimate mixing of the components during the deposition itself. Co-depositions using combinatorial CVD processes developed in our lab were used to form compositional gradients of up to three metal oxides. The following list of publications are representative of our work in this field.
Publications
Zhong, L.; Zhang, Z.; Campbell, S. A.; Gladfelter, W. L. “Combinatorial CVD of ZrO2 or HfO2 Compositional Spreads with SiO2 for High k Dielectrics”, J. Mater. Chem., 2004, 14, 3203 - 3209.
Xia, B.; Chen, F.; Campbell, S. A.; Roberts, J. T.; Gladfelter, W. L. "Combinatorial CVD of Zirconium, Hafnium and Tin Dioxide Mixtures for Application as High k Materials", Chem. Vap. Deposition, 2004, 10, 195 - 200.
Smith, R. C.; Hoilien, N.; Chien, J.; Campbell, S. A.; Roberts, J. T.; Gladfelter, W. L. "Combinatorial Chemical Vapor Deposition: Achieving Compositional Spreads of Titanium, Tin and Hafnium Oxides by Balancing Reactor Fluid Dynamics and Deposition Kinetics", Chem. Mater., 2003, 15, 292 - 298.
ALD
Atomic layer deposition (ALD) involves the sequential exposure of a substrate to two different precursors. The pulse duration for each precursor is time to allow complete reaction of the surface moieties. After this is complete the system is purged with an inert gas and then exposed to the second precursor. Equations 1 and 2 show two steps involved in the well-studied ALD of ZnO thin films using ZnEt 2 and H 2O.
(1) ZnEt2 + (surface-OH) 
C2H6 + (surface-O)ZnEt
(2) (surface-O)ZnEt + H2O (surface-O)ZnOH + C2H6
For compounds that can act as single source precursors in CVD processes, e. g. Ti(O iPr)4, the temperature of the ALD must be high enough to allow complete consumption of all of the surface sites during the individual steps in a cycle (eq. 3 and 4), but low enough to preclude continuous CVD growth (eq. 5). The ideal range is often referred to as the ALD window.
(3) Ti(O iPr)4 + m (surface-OH) m HOiPr + (surface-O) mTi(O iPr)4-m) (m = 1 -3)
(4) (surface-O) mTi(O iPr) 4-m) + 4-m H2O (surface-O) mTi(OH)4-m) + 4-m HOiPr
(5)Ti(OiPr)4 2 HOiPr + 2 C3H6 + TiO2
The first of two primary characteristics of a true ALD is that the film thickness deposited per cycle reaches a limiting (saturation) value as a function of pulse length of each precursor. Second, by operating under these saturation conditions, a plot of thickness vs. number of cycles is linear. The graphs in Figure 1 demonstrate this behavior for our study of ZrO2/SiO2 nanolaminates deposited using anhydrous zirconium nitrate, [NO2][Zr(NO3)5], and tri-t-butoxysilanol, ( tBuO)3SiOH. This example is somewhat unusual because the thickness/cycle of 1 nm is an order of magnitude larger than is normally observed. Consistent with the work of Gordon and coworkers, ( tBuO)3SiOH deposits many monolayers per cycle, however, as shown in Figure 1 it still exhibits a well defined self limiting behavior. Figure 2 shows a cross-sectional TEM of a HfO2/SiO2 nanolaminate prepared using the above chemistry.
The scope of ALD processes extends from metals to compound semiconductors to insulators. A major need in the field is to develop depositions of multicomponent materials. The advantage of ALD over CVD, and especially PVD, is its ability to deposit a uniform thickness over wide areas with complex structure. A disadvantage is the slow rate of deposition. Thus, it is particularly well suited for applications involving films less than 50 nm in thickness or for filling deep trenches or vias. Recent publications from our group in this area include the following.
Recent publications
Zhong, L.; Daniel, W. L.; Zhang, Z.; Campbell, S. A.; Gladfelter, W. L. "Atomic Layer Depostion, Characterization and Dielectric Properties of HfO2/SiO2 Nanolaminates and Comparisons to Their Homogeneous Mixtures" Chem. Vap. Deposition, 2006, 12, 143 - 150.
Zhong, L.; Chen, F.; Campbell, S. A.; Gladfelter, W. L. "Nanolaminates of Zirconia and Silica Using Atomic Layer Deposition", Chem. Mater., 2004, 16, 1098 – 1103.
Ink Jet Deposition of Photovoltaics
Challenging technological hurdles face the widespread use of alternative carbon-free, renewable energy sources such as solar energy. Photovoltaic devices capable of converting solar radiation into electrical energy are commercially available and find limited use in the United States mainly as a power source for remote locations. Major advances are needed in the methods used to construct solar cells, especially thin film solar cells, before their use can become competitive with other energy sources. In this program, which involves collaboration with the National Renewable Energy Laboratory in Golden, CO, we are focusing on the development of precursors and processes that will facilitate the ink jet deposition of an entire solar cell.
The overall goal of the research program is to design precursors and deposition processes that will form the basis for the low temperature construction of solar cells based on copper indium(gallium) diselenide (CIGS), one of the most promising materials in the field of solar energy conversion. Based on knowledge of the known chemical reactions that occur as the solid state material forms, we hope to devise new reactions and processes that will allow this to occur at lower temperatures. Lowering the minimum processing temperature would decrease the energy required to manufacture the device and allow it to be constructed on polymeric substrates. Success will lead to two important outcomes. First, we will gain fundamental knowledge of the reaction mechanisms involved and its relationship to the composition and microstructure of the thin film. Second, the development of a low temperature process could significantly lower the cost of manufacturing thin film solar cells. This project represents the beginning of a larger, multi-year program focusing on methods to construct photovoltaics at low temperature. As noted below in the description of a CIGS-based solar cell, several materials in addition to the semiconductor itself are part of the device.
While this project is new, we have synthesized several new zinc complexes that will be examined as possible precursors to ZnO. We are also exploring the use of nanoparticles as inks for the ZnO and related layers of the photovoltaic device.
Publication
Luo, B.; Kucera, B. E.; Gladfelter, W. L. "Syntheses and X-ray crystal structures of zinc complexes with an amido-diamine ligand" Polyhedron, 2006, 25, 279 - 285.