Chapter 2. X-Ray Diffuse Scattering

Chapter 2. X-Ray Diffuse Scattering

Sponsor

Joint Services Electronics Program (Contract DAAL03-86-K-0002 and DAAL03-89-C-0001)

Academic and Research Staff

Professor Robert J. Birgeneau, Dr. Joel Brock

Graduate Students

Kenneth Evans-Lutterodt, Hawoong Hong, Alan Mak, D.-Young Noh

In this research program, modern x-ray scattering techniques are used to study structures andphase transitions in thin films and on surfaces. We have two principal experimental facilities.At MIT, we have four high-resolution computer-controlled x-ray spectrometers with highintensity rotating anode x-ray generators. The angular resolution can be made as fine as 1.8seconds of arc, enabling us to probe the development of order from distances of the order ofthe x-ray wavelength, ~ 1A, up to 30,000 A. The sample temperature may be varied between2 K and 500 K with a relative accuracy of 2x10-3K. We also maintain, in collaboration withIBM, a two-spectrometer system at the National Synchrotron Light Source at BrookhavenNational Laboratory. A third beam line designed to operate at short wavelengths is in the finalstages of construction at the laboratory. These systems make possible high resolution scat-tering experiments with a flux more than three orders of magnitude larger than that from arotating anode x-ray generator. These experiments, in turn, have opened up the possibility for anew generation of experiments in this area.

As part of this JSEP program, we have built an x-ray compatible high-vacuum single-crystalapparatus. This apparatus enables us to use synchrotron radiation to study the structure andtransitions occurring at a single surface. Our current experiments in this program are concen-trated in two areas: 1) the phases and phase transitions of metal and semiconductor surfacesand surface overlayers; and 2) the structure and phase transitions of rare gas multilayers onsimple substrates.

2.1 Metal Surface StudiesWe have carried out detailed studies of thereconstruction of two prototype metallic sur-faces: W (001) and Au (110). Both exhibitinteresting new physical phenomena.

2.1.1 W (001)

The reconstruction of the clean W (001)surface at low temperatures has been widelystudied since its discovery by low-energyelectron diffraction (LEED). Below - 200K a

well defined (-2 x - ) R45 ° phase existswith a basic structure that is no longer seri-ously disputed, although certain details are

still emerging (see figure 1). The nature ofthe high-temperature (1 x 1) phase has beeninvestigated experimentally and described as(a) a disordered version of the low-temperature phase, (b) an ordered state,perhaps with a lower-symmetry (1 x 1)reconstruction, or (c) an incommensuratephase. Several groups have investigated thephase transition with LEED and Hediffraction. Theoretical work on the phasetransition has also been divided betweenorder-order and order-disorder models.

The belief that there is a change of structureassociated with this phase transition restsalmost entirely on the LEED reports of a largereduction in intensity of the half-orderdiffraction beams above 200 K. In this

Chapter 2. X-Ray Diffuse Scattering

Figure 1. Sketch of the positions of tungsten atoms onthe clean W(001) (7x2 x)R45" surface.

research we have clarified this basic exper-imental fact by observing that 1) thekinematical x-ray diffraction intensity isnearly constant throughout the entire tem-perature region 130-360 K, and 2) above thetransition temperature Tc, the peak merelychanges shape to conserve its volume. Sincethere is no change in structure factor, thelocal structure is unaltered through the tran-sition, and, therefore, the high temperaturephase must be disordered. By measuring thepeak widths as a function of T, we find that,as the ordered domains diminish in size, theyretain an anisotropic 2:1 axial ratio. Thepeak widths scale approximately linearly withT-To, consistent with the classification of thephase transition as a 2D XY model withstrong cubic anisotropy. Furthermore, wesee no shift of the peak centroid from thetrue half-order position over 130-360 K, and,thus, no sign of incommensurability. Thiswork, therefore, has elucidated and madequantitative all of the essential features of theW (001) surface reconstruction.

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W(001)(F x F)R45

2.1.2 Au (110)

Many noble metal facets are known to favorreconstructed structures at low temperatures.Furthermore, at some temperature Tc, areconstructed surface will typically undergo areversible "deconstruction" to a high temper-ature structure which is no longer recon-structed. One such transition, which hasbeen studied theoretically as well as withelectron diffraction, is the Au (110) 1 x 2 to1 x 1 deconstruction. While the "missingrow" model of the reconstructed 1 x 2surface is well established, the nature of thedeconstruction transition itself remains con-troversial.

Previous work by our group has demon-strated that x-ray scattering is an effectivemeans.for studying surface phase transitions.Specifically, the lineshapes of bulk-forbiddensurface peaks can provide information aboutthe surface height-height (or step-step) cor-relation functions, and are thus a sensitiveprobe of the surface roughening transition.This transition is characterized by a prolifer-ation of atomic steps which results in loga-rithmically divergent height fluctuations ofthe crystalline surface. We have carried outglancing angle synchrotron x-ray scatteringexperiments as a means of studying the Au(110) 1 x 3 to 1 x 1 deconstruction transi-tion. Observing the temperature dependenceof the superlattice and integral order bulk-forbidden (anti-Bragg) surface peaks, wefind that at T, = 485°C, the Au (110) 1 x 3surface becomes incommensurate with thebulk crystalline lattice. During the commen-surate-incommensurate transition, the surfaceboth roughens and deconstructs. Becausethis picture is fundamentally different fromthat suggested by the current literature,further experimental and theoretical work onnoble metal surface problems is required.

2.2 Rare Gases on GraphiteWe have initiated a long-term researchprogram to use rare gas multilayers ongraphite as model systems for the study ofequilibrium crystal growth and surfaceroughening. These experiments should com-plement our ongoing measurements of noblemetal surfaces and overlayers. In the firstgeneration of these experiments, we have

Chapter 2. X-Ray Diffuse Scattering

studied films of xenon on graphite withthicknesses of 1, 2, 3, 6, 24, and 44 layers.Our emphasis to date has been on the lowtemperature structures, the temperaturedependence of the thicknesses, and theintegrity of the layer stacking.

In our experiments, we have used x-ray scat-tering to monitor the thickness changeaccompanying the wetting of the xenonlayers physisorbed onto the basal plane ofsingle crystal graphite. At the same time, wehave observed the in-plane structure of thelayers, particularly of the first few layers. Themotivation for this was the fact that thewetting behavior will probably depend onthe structural adjustment of the first fewlayers. We have, in fact, succeeded in

observing the in-plane structures of thexenon layers as well as out-of-plane struc-tures in some detail. These observationsreveal a variety of unexpected aspects in thestructure of the physisorbed films. Previouslyit was reported that xenon wets a graphitesurface completely at low temperatures. Bycontrast, we find that xenon exhibits incom-plete wetting of the graphite at low temper-atures. Additionally, the xenon films exhibitpronounced stacking disorder between thelayers. The most unexpected result, however,is that the first xenon layer adjacent to thegraphite is commensurate with the graphitesurface structure at low temperatures, whilethe remaining layers are incommensuratewith a structure close to that of bulk xenon.

Professor John D. Joannopoulos

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