0246110 Rta(i) 1 1,2 l,4 1,6 1,l 2 22 Rta(i) Figure 2. Temperature dependence of Zr-FTfor pure orthorhombic zirconia Figure 3 Temperature dependence of Zr-0 shell in tetragonal zirconia solid solution Our study has experimentally demonstrated the intrinsic instability of the fluorite-type Zr cation network. Coherent scattering beween Conclusions central Zr ion and distant cations is weaker and vibration modes of the Zr-cation network are It is found that, for all of zirconia solid softer in tetragonal zirconia than in monoclmic, solutions studied, the dopant-oxygen distances are orthorhombic, or stabilized cubic zirconia. In si@icantly ddferent from the Zr-0 distances. addition, the outer foux oxygens in the 8-fold The dopant-cation distances, on the other hand, coordinated Zr-0 polyhedron are only loosely are usually very close to the Zr-Zr distance, solutions. A bonded and are subject to very large static and confirming the formation of solid dynamic distortions. This effect is illustrated by general structural picture for zirconia solid the expanded portion of the Fourier transform solutions is one that places the dopant cations shown in Figure 3. randomly on tbe Zr sites in the cation network, but with distorted and sometimes very different Dopant Structure dopant-oxygen polyhedra surrounding these dopants. In the case of Ge4+ doping and Y-Nb We have also successfully performed co-doping, short-rangecation ordering has been EXAFS experiments at the dopant absorption suggested from our EXAFS results. edges for doped zirconia solid solutions. Dopants Based on the above structural information studied include the Ce-,Nb-, and Y-K edges as and other EXAFS results obtained from NSLS, well as Ce-LIn edge. In particular, we have we are beginning to obtain a complete picture of obtained Nb EXAFS data with a very high the atomistic origins of phase stabilization in signaYnoise ratio using the SSRL Ge detector zirconia. array. This experiment was essentially impossible using a conventional fluorescnece ion Acknowledgements chamber due to the very strong absorption of Zr at Nb-K edge energy. Analysis of these data is in Supported in part by a grant from the NSF progress. (DMR-8807024). 00232471 107 Proposal 2 17 1M Reconstructions and Structural Transitions of the CaF2/Si(lll) Interface C. A. Lucas, G.C. L. Wong* and D. Loretto Center for Advanced Materials, Materials Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. *and Department of Physics, University of California. Berkeley. CA 94720. Introduction growth rate of -1 CaFz triplc layer (TL) per 30 seconds. At 720°C CaF2 films with a thickness of -7TL's (-22A) CaFgSi (11 1) is a prototypical system for studying were grown. This is blow the critical thickness for atomic and electronic structure at the ionic/covalent strained layer growth (-12TL's). The sample was then interface. As a consequence there has been a wide range cooled to room tcmpcrature where, due to the smaller of experiments aimed at determining the interface lattice mismatch, it is possiblc to grow more CaF2 in a structure and a number of models have been proposed layer-by-layer rnodc wilhoul introducing strain relieving [1,2]. Despite this attention no consistent picture has dislocations. Using this 'template' method emerged. Conflicting results have been attributed to pseudomorphic growth tias been achieved up to a differences in sample preparation and the varying thickness of -60 TL's. Smplcs were capped with 50A sensitivity of experimental techniques [3,4,5]. The of amorphous Si and rcrnovcd from the vacuum for TEM problem relates to more general issues concerning the and x-ray diffracljon mcasurcmcnts. differences between surface and interface structure. In particular, it is questionable whether the structure Experimental Details and Results observed at monolayer coverages, which is accessible to many standard surface probes, is representative of the X-ray reflectivity is a powerful technique for true interface between two materials 161. The latter can determining surfacc struckm and has been employed only be studied with techniques that employ highly with considerable succcss in studies of reconsuucted penetrating beams, such as x-ray diffraction and metal surfaces [8.9]. The technique is analogous to transmission elcctron microscopy (TEM). It is the LEED or RHEED measurements of the (0 0) specular structure of the buried interface that influences the rod, but the weak interaction of x-rays with matter electronic properties. enables use of the kincrnatical approximation to The aim of our program is to combine x-ray scattering theory, in the form of a one-dimensional diffraction and TEM to study the CaFzjSi(ll1) interface. Fourier summation over the layers of the crystal [lo]. The samples are grown by molecular beam epitaxy Figure 1 shows the reflectivity profile for a 7TL thick (MBE), enabling strict control of the growth conditions. CaF2 film. The measurement was performed on beam This combination of state-of-the art technologies offers a line 7-2 at the Stanford Synchrotron Radiation unique opportunity to study the relatively unexplored Laboratory using a 4-circle Huber diffractomeler and a phenomena of phase transitions and reconstructions at focussed lmm (vertical x 2mm (horizontal) incident x- buried interfaces. ray beam (h = 1.2398d ). The sample was mounted at the center of the diffractometer axes, with its surface Sample Preparation normal in the vertical scattering plane. Scattered x-rays were detected by a scintillation counter after passing CaF2 was grown by MBE on well-oriented through 6mm (vertical) x lOmm (horizontal) slits at a (misc~t<0.5~)Si (1 11) substrates in an ISA/Riber CBE distance of -1m from the sample. Each point 32P ultrahigh vacuum (UHV) system. The Shiraki corresponds to a background subtracted integrated etched Si substrates [7] were outgassed thoroughly and intensity obtaincd from 9 scans at the appropriate 29 then heated to 880°C to remove the protective oxide. scattering angle along the specular rod. Thc data has Cooling to the growth tcmpcrature (720°C) routinely bccn corrected for the Lorentz factor (sin 28) and the resulted in a sharp (7x7) reflection high-cncrgy elcctron variation of samplc arca illuminatcd by the x-ray beam diffraction (RHEED) pattern. CaF2 was deposited by (sin 9). No dam was collected for LcO.9 rcciprocal evaporation from an effusion cell at 1150°C. During lattice units duc to scattering from the amorphous Si cap deposition the pressure was typically -1~10-~Otorr at a which affccts thc low angle reflectivity data. The 108 0023248: intensity diverges at the Si (111) Bragg reflection and was not measured over the range G-.98-1.02 Si( 11 1) 0 reciprocal lauice units (Si r.1.u.). F(q) = fsi(q) C exp(iqzj) j=-- ' 100 N + exp(iqd) { fca(q)+2fsos(qc/4))C exp(iqzk) h ._.dvl 10 k=O e 7 fSi(q) , f&(q) and fF(s> are atomic form factors which include the Debye-Waller factor [13], zj are the atomic height coordinates, c is the lattice spacing of the CaF2 film with a thickness of N mple layers, and d is the interface separation. Although the model fits the data around the (1 11) Bragg reflection it is clearly not a good fit to the data over the larger q range. Between Bragg reflections the scattering from atoms in bulk positions 0.01 I I I interferes destructively and is roughly equal in intensity 1 .o 1.5 2.0 2.5 u) the scattering from an isolated monolayer [14]. The (0 0 L) recip. latt. units measurement is thus sensitive to the details of the interface structure projected onto the surface normal Figure 1. Measured x-ray reflectivity of a 7 uiple direction. An isolated monolayer at the interface is layer (TL) CaF2/Si(lll) film. The data points are simple to include in the scattering model with an background subtracted integrated intensities corrected for additional multiplying factor pj representing a possible the Lorentz factor and the variation in illuminated partial layer occupation (i.e. pj varies between 0 and 1). sample area. The dashed line is a calculation with a Swain in t'te interface region can be accounted for by simple interface model (interface spacing d=4.55A). The allowing the bulk layers to move away from their ideal solid line is a fit to the data with a two-layer interface positions. There are a number of constraints which according to the parameters in Table I. must be imposcd on the modelling procedure: we look for a model which has physically reasonable bondlengths In a recent paper we presented similar x-ray scattering and is consistent with previous results. It has been measurements from CaFdSi(l1 l), over a reduced range shown that upon initial adsorption the CaF2 molecule of L (0.8-1.2) around the Si (1 11) Bragg reflection [I 11. dissociates to give a sub-monolayer coverage of CaF The measurement is sensitive LO the "long-range" [ 1.4.51. We can reproduce our data by including a CaF interface separation from the bulk of the substrate to the layer at the interface but this results in a Si-Ca bulk of the film [ 121. Over this L range good fits to the separation of -1.6A which is not compatible with the data were obtained with a lattice separation d = 4.55A. Ca-Si bondlength (3.0-3.2A) [15]. The best fit to the This value indicates that there is a layer in between the data with a two-layer interface (the next simplest model) two lattices. The dashed line in Figure 1 corresponds to is shown by the solid line in Figure 1.