Contains table of contents for Section 4, an introduction and a report on one research project.Joint Services Electronics Program Contract DAAL03-92-C-000

Birgeneau, Robert J.

Greven, Martin

Martin, Debra L.

Michael J. Young, Michael J.

Ramstad, Monte J.

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Section 4Chapter 1Chapter 2Chapter 3Surfaces and InterfacesSynchrotron X-Ray Studies of Surface DisorderingSemiconductor Surface StudiesStep Structures and Epitaxy on SemiconductorSurfaces165166 RLE Progress Report Number 139Chapter 1. Synchrotron X-ray Studies of Surface DisorderingChapter 1. Synchrotron X-ray Studies of Surface DisorderingAcademic and Research StaffProfessor Robert J. Birgeneau, Dr. Martin GrevenGraduate StudentsMonte J. Ramstad, Michael J. YoungTechnical and Support StaffDebra L. Martin1.1 IntroductionSponsorJoint Services Electronics ProgramContract DAAL03-92-C-0001In this research program, we use modern x-rayscattering techniques to study structures and phasetransitions in thin films and on surfaces. We havetwo principal experimental facilities, one at MIT andthe other at the National Synchrotron Light Sourceat Brookhaven National Laboratory. At MIT, wehave four high-resolution computer-controlled x-rayspectrometers. The angular resolution can bemade as fine as 1.8 seconds of arc, which enablesus to probe the development of order from dis-tances of the order of the x-ray wavelength 1 A, upto 30,000 A. The sample temperature can bevaried between 2K and 500K with a relative accu-racy of 2 x 10- 3 . At the National Synchrotron LightSource, in collaboration with IBM Corporation, wehave three fully instrumented beam lines. Two ofthese beam lines allow us to make studies withphotons varying in energy between 3 and 12 keV;the third has a fixed energy of 17 keV. We are cur-rently commissioning an undulator-based beam lineat the Advanced Photon Source at ArgonneNational Laboratory. This facility will make pos-sible a new generation of x-ray experiments withunprecedented precision. In collaboration with Pro-fessor Simon G.J. Mochrie, we have designed,built, and commissioned a second-generation x-raysurface facility which is extremely versatile allowinga wide range of surface scattering experiments.Our basic scientific objective is to understand themorphologies and microscopic structures of simplesemiconductor and metal surfaces at high temper-atures. Possible phase changes include surfaceroughening, surface reconstruction, melting, amor-phization, and dilution.Semiconductor surfaces exhibit rich structural andmorphological behavior in both equilibrium and non-equilibrium, steady state conditions. Equilibriumstructures and phase transitions of high-symmetryfacets are often closely related to important modelsin two-dimensional statistical mechanics. Thephysics of stepped (vicinal) surfaces, even in equi-librium is subtle and poorly understood. Forexample, stepped surfaces should always be roughbecause of the freedom of the steps to wander.Steps structures and spacing should be profoundlyaffected by any nearby high-symmetry surfacereconstruction. Their association bears on issuesof faceting and equilibrium crystal shapes. Thesteps themselves may exhibit phase transitions thatinvolve dislocations and/or bunching in the steplattice as well as step-height changes; changes inthe morphology, especially those involving vacancyaggregates, should be strongly affected by nearbysteps.Semiconductor surface physics involves funda-mental issues in statistical mechanics that areimportant to a broad spectrum of other problems.Furthermore, elucidating the factors that control themorphology and perfection of vicinal semiconductorsurfaces can be expected to lead to improved elec-tronic device processing and ultimately improvesemiconductor device performance and lifetime.1.2 Stability of Vicinal Si(111) SurfacesUnder Sublimation and ElectromigrationWe have carried out an extensive investigation ofthe properties of miscut Si(111) surfaces under theapplication of applied electric fields. Specifically,we have studied electromigration-induced mor-phology changes occuring at the surface of vicinallypolished, clean single crystals of Si(111). Fromthese studies, we have determined the non-equilibrium phase diagram and structures.Chapter 1. Synchrotron X-ray Studies of Surface DisorderingT(K)Stepped surface(single-phase region)7x7 Reconstruction7 x7 ReconstructionFoceted surface(with transverse instability)T7x7-1 xl---------------7x7 Reconstruction(with transverse instability),CurrentDirectionFigure 1. Current-direction versus temperature phase diagram for vicinal Si(111) miscut toward <112> indicating someof the regimes where electromigration-driven surface morphologies exist, and the region where reconstruction-drivenstep bunching is observed. The dashed line indicates the T7X7 - 1x1 reconstruction temperature which is weakly dependenton the magnitude of the miscut. Bars indicate regions studies with x-ray diffraction.A current direction versus temperature phasediagram for Si( 11) vicinally cut toward <112> isshown in figure 1. These surface phases can bedescribed as follows. Below the 7X7-1X1 recon-struction transition, one observes a surface phaseseparation independent of current direction. Thistwo-phase region consists of large 7X7 terracesseparated by step bands. The flat terraces have asurface normal (n) along (111) while the bunchedstep regions have a local surface normal which islarger than the macroscopic miscut. In equilibrium,the relative size of the flat regions compared withthe step regions is then determined by themacroscopic miscut. This is illustrated in the sche-matic of figure 2. In this diagram, positive current(+1) is defined as current flowing toward <112>, i.e.in the direction of the step flow. The positivecurrent direction is sometimes referred to as thestep-down direction. The crystallographic directiontransverse to the miscut is the <110> direction. Itwas found experimentally that the phase separationat positive current is accompanied by a transverseinstability due to a meandering of the steps withinthe step bands. The small dependence of recon-struction transition temperature on sample miscut isnot represented in this phase diagram.168 RLE Progress Report Number 1391520K1370K-I 140KIChapter 1. Synchrotron X-ray Studies of Surface DisorderingIFigure 2. A schematic of (a) a uniformly stepped surface, and (b) a step bunched (or faceted) surface. Positive currentflows in the step-down <112> direction, as indicated by the arrow in (a). A uniformly stepped Si(111) surface will havesteps distributed according to the macroscopic miscut (defined by n) and separated by 1X1 terraces. The angle & willbe a function of the step density within the bunched regions (step bands).Above the reconstruction transition temperature atT-1140 K and at negative current (-I in thediagram) there is a transition to a single-phase uni-formly stepped region which persists up to T-1370K. This single-phase surface has steps separatedby disordered 1xl terraces, typically a few hundredangstroms across. Therefore, the integer orderpeak at (1,0) are not present when scattering x-raysfrom the single-phase stepped surface structure. Atpositive current between these same temperatures,the steady-state surface structure consists ofunreconstructed 1X1 facets, which also with atransverse step meandering. The phase behavioron reversing the current direction in this temper-ature regime is driven by electromigration. It shouldbe noted that these low-temperature phases exhibitvery slow dynamics and were not part of the majorthrust of this research.Above T-1370 K and at negative current, the stepstend to bunch into disordered stepped bands,allowing the slow formation of large 1xl facets. Atthis same temperature and positive current, thesesteps bunches slowly reorganize as the facetsdecrease in extent. The eventual surface structureconsists predominately of distributed steps with a169(a) Uniformly Stepped +1<111> nn <112>(b) Step Bunching (Faceting)Chapter 1. Synchrotron X-ray Studies of Surface Disorderinglarge transverse step meandering. The transversestep modes ultimately lead to a dramatic transversemodulation on the order of several micronsdepending on the size of the miscut. Heatingthrough this faceted regime at negative current oneencounters a transition at T-1525 K. In this case,the steps redistribute themselves uniformly with nosignificant transverse wandering. At positivecurrent above T-1525 K, the surface developsfacets with disordered step bands, although theexcessive sublimation rates at these temperaturesprohibit well-ordered facets after a long time. Thiselectromigration- or current-driven faceting sharesmany of the same characteristics withreconstruction-driven faceting; however, it is obvi-ously governed by fundamentally different physicswhich can lead to dramatic dynamical changes.After completion of the above work, further ultra-high vacuum (UHV) surface x-ray diffraction exper-iments were conducted to study thetime-dependences of the effects of a direct currenton the arrangement of the steps at a Si( 11) crystalsurface miscut toward <112> by up to 8 degrees.These studies revealed spectacular dynamicalchanges in the surface morphology which arereversible and can be controlled by the direction ofcurrent flow through the crystal. The current direc-tion- and time-dependent morphology changes inquestion were found to result from an intricate setof competing interactions, electromigration being acentral component and actually occur at sublimatingtemperatures. Moreover, the time-scales involvedin many of these surface transformations proved tobe readily accessible in the x-ray diffraction exper-iments. The surface morphology changes whichcharacterize this dynamical phenomenon occur onmesoscopic length-scales and exhibit time depend-ences which are ascertained directly from thesurface diffraction experiments. Ex-situ atomicforce microscopy experiments were also conductedon quenched samples following each experiment tocomplement the x-ray studies. Direct imagingproved useful in determining large-scale structurewhich in some cases occurred at distance scalesexceeding the resolution of the x-ray experiments.1.2.1 ThesesRamstad, M.J. Instabilities of Vicinal Silicon (111)Surfaces. Ph.D. diss., Dept. of Physics, MIT,1996.Young, M.J. Equilibrium and Non-Equilibrium Phe-nomena in Two- and Three-Dimensional Corre-lated Systems. Ph.D. diss., Dept. of Physics,MIT, 1996.170 RLE Progress Report Number 139

Synchrotron X-Ray Studies of Surface Disordering

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Synchrotron X-Ray Studies of Surface Disordering

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