Contains description of one research project, reports on two research projects and a list of publications.Joint Services Electronics Program Contract DAAL03-92-C-000

Abernathy, Douglas L.

Mochrie, Simon G. J.

Song, Seungheon

DSpace@MIT

Chapter 5. Semiconductor SurfacesChapter 5. Step Structures and Epitaxy on SemiconductorSurfacesAcademic and Research StaffProfessor Simon G.J. MochrieGraduate StudentsDouglas L. Abernathy, Seungheon Song5.1 Project DescriptionSponsorJoint Services Electronics ProgramContract DAAL03-92-C-0001In this program, our goal is to obtain a detailed,experimental description of the temperature-dependent structure, phase-behavior, and mor-phology of semiconductor, especially silicon,surfaces. The results obtained in this way will beessential to theoretical efforts aimed at under-standing silicon surface energetics. To this end, weemploy high-resolution x-ray scattering, using anultra-high-vacuum (UHV) apparatus for surfacex-ray diffraction,' which was funded in-part byJSEP. The apparatus is located at the NationalSynchrotron Light Source (NSLS) and may beemployed either at the MIT-IBM bending-magnetbeamline, X20, or at the NSLS X25 wigglerbeamline, which provides a five-fold intensityimprovement compared to X20. The MIT UHVapparatus, in combination with either X20 or X25,constitutes the world's premier facility for surfacex-ray scattering. In 1993, we continued to exploitits unique capabilities.Specifically we have completed a comprehensivex-ray scattering study of the structure of the Si(113)surface. Current crystallographic techniques makeit possible to determine the full three-dimensionalatomic structure of a surface, with accuraciesapproaching 0.01 A. Knowledge of the atomicstructure is critical for evaluating calculations ofsurface energetics. It is surprising that the Si(113)surface has received much less attention up to nowthan the Si(001) and (111) surfaces. Nevertheless,the Si(113) surface is particularly interesting for anumber of reasons which involve both possibletechnological applications and the basic physics ofsurfaces.First, although Si(113) is a relatively high-indexsurface, it has been found to be remarkably stablewith a surface energy comparable to that of the(111) or (001) orientation. Specifically, recent mea-surements of the shape of voids, created by He-ionimplantation inside a Si crystal, have establishedthat the surface energy of Si(113) is less than 1percent higher than that of the Si(001) surface.2Understanding the geometric and electronic struc-ture of such a low energy silicon surface is animportant challenge in surface physics. Reports ofsuperior epitaxial growth on Si(113)3 make thestructure of this surface and its temperaturedependence especially worthy of study.The rationale that Si(113) may be particularly suit-able as substrate for epitaxial growth of Ill-V semi-conductor films follows: An unreconstructed (113)surface may be envisaged as a sequence (001) ter-races separated by (111) steps. For Si(113), theterraces are a single atomic row wide. The corre-sponding surface unit cell contains two differenttypes of surface atoms: (a) two-fold coordinatedatoms with two dangling bonds, deriving from the(001) terrace, and (b) three-fold coordinated atomswith a single dangling bond, from the (111) step(see figure l a). Thus, there are two differentadsorption sites. It seems likely that one of thesites will be preferred by one of the epitaxial1 D.L. Abernathy, An X-ray Scattering Study of the Si(113) Surface: Structure and Phase Behavior, Ph.D. diss., Dept. of Phys., MIT,1993.2 D.J. Eaglesham et al., "Equilibrium Shape of Si," Phys. Rev. Lett. 70: 1643 (1993).3 G.H. Olsen, T.J. Zamerowski, and F.Z. Hawrylo, "Vapor Growth of InGaAs and InP on (100), (110), (111), (311), and (511) InPSubstrates," J. Cryst. Growth 59: 654 (1982); S.L. Wright, M. Inada, and H. Kroemer, "Polar-on-nonpolar Epitaxy: Sublattice Orderingin the Nucleation and Growth of GaP on the Si(113) Surface," J. Vac. Sci. Technol. 21: 534 (1982).187Chapter 5. Semiconductor SurfacesFigure 1. (a) Top view of unreconstructed Si(113)surface. The centered, unreconstructed unit cell is out-lined. Silicon atoms in the surface layer are shown assolid circles. Atoms in lower layers are shown as opencircles. Solid lines represent bonds between neighboringatoms while dangling bonds are shown as triangles. (b)Top view of Si(113) (3x1) reconstruction. The large boxoutlines the centered, reconstructed unit cell.species, thus keying the growth film to the substratein a definite manner. This feature may alleviate theproblem of twinning that occurs in the growth ofIll-V semiconductors on flat Si(001), 4 which pres-ently makes necessary the use of stepped Si(001)substrates for epitaxial growth. In addition, the (113)surface is non-polar, which avoids the possibility ofstrong disorder in the growth process as a result ofelectric fields. In fact, the clean Si(113) surfacereconstructs to form a (3xl) structure. Ourcrystallographic measurements to determine theatomic positions at the Si(113) surface aredescribed in section 2.5 Our efforts in this regardare coordinated with those of Professor J.D.Joannopoulos, who is presently carrying out a totalenergy calculation of the Si(113) (3x1) structure.In addition, we have initiated a study of theorientational phase diagram of stepped Si(113) sur-faces between 300 and 1200 K. Stepped (orvicinal) Si(113) surfaces afford the opportunity todeepen our general understanding of the mor-phology of crystal surfaces. As described in detailin section 3, we have found that stepped Si(113)surfaces undergo a so-called facetting transforma-tion. Facetting is accomplished through the rear-rangement of surface steps, so that there areregions of the surface with a high step density,together with step-free Si(113) facets ofmacroscopic extent. We believe that the theoreticalstudies of the possible facetting of stepped Si(001)surfaces carried out by Professors A.N. Berker andJ.D. Joannopoulos may lead to a detailed under-standing of Si(113) facetting. Evidently, surfacemorphology is key to a number of technologicallyimportant processes, including epitaxial growth,etching, electromigration, and corrosion. An essen-tial prerequisite for controlling these processes is anunderstanding of the equilibrium morphology of thesurface. For example, our results demonstrate thatlarge, step-free Si(113) surfaces may be preparedby high-temperature annealing. One may hope thatthe use of such surfaces as substrates for epitaxialgrowth might lead to interfaces of unprecedentedperfection, which in turn may possess particularlyfavorable electronic transport properties.5.2 The Structure of the (3x1)Reconstruction of the Si(113) SurfaceX-ray diffraction measurements to determine the fullthree-dimensional atomic structure of the Si(113)surface at 800 K were performed at X20 using ourUHV apparatus.6 Our initial measurements werecarried out in a glancing incidence geometry (in-plane measurements), so that the informationgained concerned the atomic positions projectedonto the surface plane. Relative structure factors at130 surface Bragg reflections were collected, fromwhich was derived the pair correlation function foratoms within the surface unit cell. The pair corre-4 K. Mizugachi et al., "MOCVD GaAs Growth on Ge(001) and Si(001) Substrates," J. Cryst. Growth 77: 509 (1986).5 D.L. Abernathy, S. Song, K.I. Blum, and S.G.J. Mochrie, Phys. Rev. B, forthcoming.6 D.L. Abernathy, An X-ray Scattering Study of the Si(1 13) Surface: Structure and Phase Behavior, Ph.D. diss., Dept. of Phys., MIT,1993; D.L. Abernathy, S. Song, K.I. Blum, and S.G.J. Mochrie, Phys. Rev. B, forthcoming.188 RLE Progress Report Number 136Chapter 5. Semiconductor Surfaceslation function exhibits peaks corresponding to inter-atomic vectors, and therefore may be used toformulate structural models of the surface. Thus,our data indicate that the atomic structure is asshown in figure 1(b): the (3x1) structure is formedfrom the unreconstructed surface by removingevery third (001)-like atom in the (1 -1 0)-direction,dimerizing the remaining two (001)-like atoms, andfinally rebonding the (111)-like atom, neighboringthe vacancy, to the layer below. Subsequent mea-surements at a large angle of incidence werecarried out to be able to locate the atoms in thedirection perpendicular to the surface. In total,approximately 600 intensities were collected, whichreduce to 310 symmetry-inequivalent structurefactors. Then, a full three-dimensional model of thereconstruction was least-mean-squares fit to thecombined set of measured structure factors in orderto refine the atomic positions. A comparisonbetween the measured (open circles) and model(solid circles) in-plane structure factors is shown infigure 2, demonstrating excellent agreement. Thiswork consitutes the first definitive determination ofthe (3xl) structure of the Si(1 13) surface.5.3 Morphology of Stepped Si(113)SurfacesEquilibrium facetting is a remarkable phenomenonin which a crystal surface increases its area todecrease its free energy. Facetting is accom-plished through the rearrangement of surface steps,so that their distribution across the surface is nolonger uniform. Instead, there are macroscopicregions of high and low step density. Our presentunderstanding of equilibrium facetting is basedupon a description of the surface free energy inwhich the surface orientation is a key thermo-dynamic variable in addition to the temperature. Inthis context, facetting is an example of thermo-dynamic coexistence among different phases withdifferent orientations. Moreover, such a descriptionmotivates the construction of orientational phasediagrams, within which are recorded the phaseboundaries between different surface phases as thetemperature and surface orientation are varied.Experiments carried out at X20A using our UHVapparatus reveal a complex phase diagram forsilicon surfaces oriented between (113) and orien-tations tilted roughly 6 degrees away from (113)towards (001), as illustrated in figure 3.7 In figure 3,solid lines indicate phase boundaries and dashedlines indicate triple points. (The solid circles corre-1.0 2.5H (a*)Figure 2. Comparison for Si(113) of the measured struc-ture factors (open symbols) with calculations using thefull three-dimensional model (solid symbols).spond to surface orientations found experimentallyfor the sample used in this study.) Within the rangeof temperature and tilt angle investigated, we havefound three distinct orientational phases. Below T3= 1130 K, phase separation into the (113) orien-tation and the (114) orientation, which lies 5.7degrees away from (113) towards (001) occurs forsurfaces with mean tilt angle lying between (113)and (114). The scattering lineshapes of the (113)and (114) surfaces indicate that both are atomicallysmooth facets in this temperature range. However,at T3 there occurs a phase transformation at whichthe (114) facet disappears and are replaced by asurface with a tilt angle of 6.5 degrees. Our mea-surements indicate that for this phase the scatteringlineshape becomes increasingly broad withincreasing scattering angle, which is the behaviorexpected for a rough surface. Therefore, webelieve that the 6.5 degree-tilted surface corre-sponds to a rough phase. All three phases, namelythe (113) facet, the (114) facet, and the roughphase, may coexist at T3, which thus is a triplepoint temperature. Above T3, there is coexistencebetween the (113) facet and the rough phase. Fortemperatures increasing above T3, the tilt angle ofthe rough surface decreases with increasing tem-perature while the intensity of the scattering fromthe (113) facet decreases. Eventually, at 1200 K,the tilt angle of the rough surface reaches the valuedetermined by the macroscopic cut of the sample,which for the data shown in figure 3 was 3.87 S. Song and S.G.J. Mochrie, Phys. Rev. B, forthcoming.189Chapter 5. Semiconductor SurfacesFigure 3. Orientational phase diagram of the Si surface near the (113) direction for tilt angles towards (001). Single-phase regions are hatched, and two phase regions are plain (unhatched). Solid lines are boundaries between twophases, and dashed lines represent triple points. Solid circles represent tilt angles measured for a sample with a meantilt angle (miscut) of 3.8 degrees.degrees, and the (113) facet essentially disappears.On further increase in the temperature, the tilt angleremains fixed because the surface lies in a one-phase region of the orientational phase diagramshown hatched in figure 3. We have extrapolatedthe boundary of the (113) facet-rough phase two-phase coexistence region to zero tilt angle at To =1245 K. The character of the transformation from(113) facet to rough phase at this point is presentlyundetermined. Future experiments will study theinteresting possibility that it is a critical point.5.4 PublicationsAbernathy, D.L., D. Gibbs, G. GrObel, K.G. Huang,S.G.J. Mochrie, A.R. Sandy, and D.M. Zehner."Reconstruction of the (111) and (001) Surfacesof Au and Pt: Thermal Behavior." Surf. Sci. 283:260 (1993).Abernathy, D.L., S. Song, K.I. Blum, R.J.Birgeneau, and S.G.J. Mochrie. "Chiral Meltingof the Si(113) Reconstruction." Phys. Rev. B 49:2691 (1994).Abernathy, D.L., R.J. Birgeneau, K.l. Blum, andS.G.J. Mochrie. "Critical Behavior at ChiralMelting: Disordering of the Si(113) (3X1)Reconstruction." Phys. Rev. Lett. 71: 750(1993).GrObel, G., D. Gibbs, D.M. Zehner, D.L. Abernathy,A.R. Sandy, and S.G.J. Mochrie. "PhaseBehavior of Au and Pt Surfaces." Surf. Sci.287/288: 842 (1993).ThesisAbernathy, D.L. An X-ray Scattering Study of theSi(113) Surface: Structure and Phase Behavior.Ph.D. diss., Dept. of Phys., MIT, 1993.190 RLE Progress Report Number 136Qo> future study%CD , 6 rough phase-(114) facet+ rough(113) facet phasecoexistence(113) facet I0 -* *- *-@ -*-00*e666****(0* @"800 900 1000 1100 1200 1300 1400TEMPERATURE (K)

Step Structures and Epitaxy on Semiconductor Surfaces

http://dspace.mit.edu/bitstream/1721.1/57275/1/RLE_PR_136_01_04s_05.pdf

Step Structures and Epitaxy on Semiconductor Surfaces

Abstract

Similar works

Full text

Available Versions

DSpace@MIT