Advances in Solid State Physics 33 PDF
Preview Advances in Solid State Physics 33
FESTKORPERPROBLEME ADVANCES IN SOLID STATE PHYSICS 33 FESTK()RPER PROBLEME ADVANCES IN SOLID STATE PHYSICS 33 Editedby Reinhard Helbig v, eweg Editor: Prof.Dr. Reinhard Helbig Institut ftlr Angewandte Physik Universit~it Erlangen - Ntlrnberg Staudtstr. 7 D-91058 Erlangen All rights reserved © Friedr. Vieweg& Sohn Verlagsgesellschaft mbH, Braunschweig/Wiesbaden, 1994 Viewegisa subsidiary company ofthe Bertelsmann PublishingGroup International. No part ofthis publication may be reproduced, storedin a retrieval system or transmitted, mechanical, by photocopying or otherwise, without prior permission ofthecopyrightholder. Printed andboundby Lengericher Handelsdruckerei, Lengerich Cover design: Barbara Seebohm, Braunschweig Printed on acid-free paper Printed in the Federal Republic of Germany ISSN 0430-3393 ISBN 3-528-08041-8 Foreword Rich in traditiontheseries "Advances in SolidStatePhysics"publishedacollectionof review articles aboutactualresultsinsolid statephysicsevery year.For manyyoung an old scientiststhesereviews were a firstintroductioninto a new field orusefulto get a generaloverview. Normally the publishedarticles were selected from plenary and invited talks ofthespringmeeting onsolid statephysics ofthe GermanPhysical Society.As an exception in 1993thespringmeetingwas organizedtogetherwiththe European Physical Society and the proceedingswill be published somewhere else. Therefore we have invited some colleagues to publish a review article (of course togetherwith own results)in thepresentvolume 33 ofthe "Advances in Solid State Physics". Erlangen, December 1993 R. Helbig Contents C.B.Duke Reconstruction of the Cleavage Faces of Tetrahedrally Coordinated Com- pound Semiconductors ................................................ Arno FOrster ResonantTunnelingDiodes: TheEffectof Structural Properties ontheirPer- formance ............................................................ 37 Bernhard Kramer Reproducible Quantum Conductance Fluctuations in Disordered Systems... 63 G.Schaack Raman Scattering inII-VI Compounds .................................. 83 J.-M. SpaethandK. Krambrock On the Microscopic Structures ofthreeArsenic Antisite-related Defects in Gallium Arsenide studied by Optically Detected ElectronNuclearDouble Resonance.......... ................................................. 111 Wolfgang TheiJ3 The Use of Effective MediumTheories in Optical Spectroscopy ........... 149 Dr.Armin W.Wieder,SiemensAG,Munich Systems onChips: The Microelectronics Challenge oftheNext 20 Years... 177 Reconstruction of the Cleavage Faces of Tetrahedrally Coordinated Compound Semiconductors C. B. Duke XeroxWebster Research counter, 800 Phillips Road, 0114-38D, Webster, New York 14580 USA Summary: Tetrahedrallycoordinatedcompound semiconductorsoccur in two crystallographic allotropes: zincblendeandwurtzite.Zincblendematerialsexhibit a single cleavage face: The (110)surface consisting ofequal numbers ofanions and cations which form zig-zagchains directedalong < 110> directions in the surface. Wurtzitematerialsexhibit two cleavage faces, both consisting of equal numbers of anion and cationspecies. The (10i"0) cleavage surfaces consist of isolatedanion-cationdimersbackbondedtothelayerbeneathwhereas the(1150) surfaces consist ofanion-cationchains,analogous to those on zincblende (I10) but with four ratherthan twoinequivalentatoms persurface unit cell. All three surfaces exhibit reconstructionswhichdo not alterthesymmetry of the surface unit cell but which lead to large (~ 1 ,~,) deviations of the positions of the atomic species in theuppermostlayer(s) fromthose in thetruncated bulk solid. Thesereconstructed surfacegeometries have been determined quantitatively for the (110) surfaces of zincblende structure AIP, AlAs, GaP,GaAs,GaSb, InP, InAs, InSb,ZnS, ZnSe,ZnTeandCdTe;the(10T0)surfaces ofwurtzitestructure ZnO andCdSe; andthe(1150)surfaces ofCdSe.Theoreticalpredictionsofthese reconstructed geometries have been given which are in either quantitative or semiquantitativecorrespondence with theexperimentallydeterminedstructures. Analysis ofthetrendsexhibitedby themembersofeachclassofcleavage surface and comparison thereof with theoretical predictions permit the extraction from these resultsofgeneralizationscharacteristicofnovel types ofsurface chemical bonding. The mostimportant ofthese is the notion that foreachclassof surface the atomicgeometries areapproximately"universal" whentheircoordinates are properly scaled with thebulklatticeconstant.Aquantitativedescriptionof this result is presented which reveals that extensions of the concepts ofinorganic molecular coordination chemistry are required to predict the cleavage-surface atomic geometries and electronicstructures ofbinary tetrahedrally coordinated compound semiconductors. 2 C.B. Duke 1 Introduction A studyofthereconstructedatomic geometries ofthecleavagefaces oftetrahedrally coordinatedcompoundsemiconductorsis ofparticularinterestforthreereasons.First, forsemiconductors crystallizing inthezincblende structure, arathercomplete account of the structures ofthe (110)cleavage surfaces is available for spa-bonded binary compounds, i.e., AlP, AlAs, GaP, GaAs, GaSb, InP, InAs,InSb, ZnS, ZnSe, ZnTe, andCdTe [1-3]. Therefore the systematics ofthevariations ofthesestructures from one materialtoanothercanbe determinedandinterpreted.Second,afterconsiderable controversy over a period of nearly a decade, the atomic geometry of GaAs(110), the benchmark ofcompound semiconductorsurfacestructures,seems to have been determined definitively utilizing a widevariety ofexperimentaltechniques including low-energy electron diffraction (LEED) [4-6], ion scattering spectroscopy [7], ion channelling spectroscopy [8],He atomdiffraction [9], scanning tunnelling microscopy [10],andsecondaryionmass spectroscopy [11].Studiesofthefilledelectronic surface states by photoemission [12] and ofempty surfacestatesby inversephotoemission [13,14] have been utilized in attempts todistinguishbetween structuralmodels, the most recent results obtainedusingboth methods [12,14] being compatible with the accepted~;1 = 29° bond-rotatedmodel oftheatomic geometry of GaAs(110). Third and finally, thestudy ofthesystematics ofthe atomic geometries ofthe zincblende (I10)cleavage surface as well as the wurtzite (10]'0) and (1150) cleavage faces has revealed new and unexpected phenomena. For example, in spite ofvery different small-molecule coordination chemistries theIII-VandII-VIcompounds were found to exhibit approximately "universal" surface structures whenthestructuralparameters are scaledlinearly withthebulk lattice constant forbothzincblende [15]and wurtzite [3,16] materials.This remarkable fact,which stands incontradictionbothwith models baseduponlocalcoordinationchemistry (17)andwithexpectationsthationicity might governtrends in thesurface structures [18,19],canbe understood only interms ofa new type oftopologically dominated surface rehybridization in which the effects of thesurface template dominatethoseofthelocalcoordinationchemistry [3,20,21]. Studies ofthesurfaces oftetrahedrally coordinatedcompoundsemiconductorshave a long and venerable history.Initially motivated byearly work at Bell Laboratories [22], extensive LEED and work function measurements were reported during the period 1964-75. Thesemeasurements,reviewedin 1975by Marketal.[23],revealed thatthenon-polarcleavagesurfaces weremorestablethanthelow-indexpolarsurfaces insofaras their symmetry parallel to the surfaceis identicalto that characteristic of the bulk solid whereas in the case ofpolarsurfaces the symmetry is lowered (i.e., thesurface unit cellis largerthanthat inthe bulk). Non-polarsurfaces contain equal numbers ofanion and cation species whereas polar surfaces contain an excess of onetype ofspecies as illustrated in Figs.l-3 forthenon-polar(110) and polar (100) and (111)surfaces ofzincblende-structure binary semiconductors.The reduction in the surface symmetry ofthe polarsurfaces was ascertainedfrom the symmetries of Reconstruction ofCleavageFaces 3 Figure 1 Figure 2 Schematic indication of the truncated bulk Schematic indication of the truncated geometry of the non-polar (110) cleavage bulk geometry of the polar (100) sur- faces ofzincblendestructurecompound se- faces ofzincblendestructurecompound miconductors. Shaded circles indicate an- semiconductors.Shadedcirclesindicate ions whereas opencircles indicatecations. anionswhereas opencirclesindicateca- (Adapted from Duke[168].) tions. (AdaptedfromDuke[25].) Figure 3 Schematic indication of the truncated bulk geometry of the polar (111) sur- faces ofzincblendestructurecompound semiconductors.Shadedcirclesindicate anionswhereas opencirclesindicateca- tions.(AdaptedfromDuke[25].) 4 C. t3. Duke the LEED beams, typically displayed on a fluorescentscreenas a "LEED pattern" using post-diffraction acceleration electron optics [22,23]. Such an apparatus gives no quantitative information aboutthe actualsurface atomic geometries because an analysis ofthe absolutemagnitudes ofthe diffractedelectron beams is required for thatpurpose.Nevertheless,ifthe symmetries ofthe LEEDpatternsare reduced from that characteristic ofthebulk, itcan inferredthatthesurfaceatoms mustbe displaced from their bulk positions,leading to the nomenclature that such reduced-symmetry patternsare characteristic of "reconstructed" surfaces.It subsequently was found [24] that even for the non-polar surfaces, the surface atomic species are displaced by large (~ 1 ,~) distances relative to the truncated bulk geometry. Therefore these surfacesalso arereconstructedinspite ofthefactthattheir LEEDpatternsexhibitthe symmetry ofthe bulklattice.Adrawing ofthe reconstructed non-polar(110)surfaces ofzincblende-structurecompoundsemiconductorsis shown inFig.4. Sometimes these bulk-symmetry-preserving surfacestructuresare referred toas "relaxed" rather than "reconstructed"although we shall use the latter more generic nomenclature in this article. Figure4 Drawing ofthe reconstructed ("relaxed") non-polar(110) cleavage faces ofzincblende structure binary compound semiconductors. (Adapted DukeandWang[21].) A new era in semiconductorsurfacesciencedawned in 1976when the theory of LEED hadadvancedsufficientlythattheactualsurfaceatomicgeometry ofGaAs(110) couldbedeterminedbycomparingmeasuredLEED intensities withthosecomputed for variousstructuralmodels [24]. Duringtheensuingdecadethe structure-determination methodology was refined, improved, and applied to a wide variety of zincblende- structure binary III-IVandII-VI semiconductors.The situationduringthe middleof this period,when thetechniques were still being refinedand structuralresults were fragmentary (and sometimes controversial), may be ascertainedby examination of reviews by Duke [25] and by Mark et al. [26] which were written at that time. By 1983asignificantbodyofquantitativesurface structures fordifferent semiconductors had been accumulated for comparison with modelpredictions [2,15]. Controversies ReconstructionofCleavage Faces 5 remained, which wereresolved over time [4-6]. By 1988an extensive collection of structural determinations forthe (110)cleavage surfaces ofbinary III-Vand II-VI semiconductors hadbeenestablished[1]. Figure5 Drawing ofthereconstructed ("relaxed") nonpolar (10]-0) cleavage facesof wurtzile structure binary compound semiconductors. (After DueandWang[21.]) As noted earlier, the body of "experimental" structures for zincblende (110)sur- faces provedinconsistentwith theoreticalexpectationsbasedon both localcoordina- tion chemistry [17]andionicity-structure correlations [18],thereby stimulating a re- examination ofthetheoryofchemicalbondingatthesurfacesofspabondedcompound semiconductors,especially II-VImaterials [3,27].This re-examination ledtotheiden- tification ofapseudo-Jahn-Tellersurface-statelowering mechanism, associatedwith anactivationless bond-length-conserving rotationofthesurfacespecies,asthedriving force ofa"universal" reconstructionofthe(110)cleavagesurfacesofbothIII-Vand II-VIsp3-bondedzincblendestructurecompoundsemiconductors [3,21,28]. Sincethe same conceptsshouldbeapplicable towurtzite-structure materials,amajor effort was undertakento extend both the modelcalculations [16,21]and experimental structure determinations [29-32] tothesesystems.Analogous"universal" reconstructions were predictedforboththe (10]'0) (Fig.5)and(1120)(Fig 6)surfacesofwurtzite-structure II-VIcompounds. Experimental evidenceis stillfragmentary,buttheresults available todateonthe (10]'0) surfaces ofZnO(29] andCdSe [30-32] as wellas CdSe(1120) [31,32] confirm the expectations basedon thesepredictions, suggesting that more surprises mightbeuncoveredinstudiesofII-VIsurfacechemistry. Our purposes inthis review aretosurveythescopeofthequantitativeexperimental determinations oftheatomic geometries ofthecleavagefaces ofcompound semicon- ductors,theextenttowhichtheresulting structures havebeenshowntobecompatible with theoreticalpredictionsandwithmeasuredelectronic and atomic excitation spec- tra,andthenewconceptsconcerningsurfacechemicalbondingwhichhaveemanated from theseefforts.Weproceedbyfirst examining theextensively-studied zincblende (110) cleavage surfaces and then considering the available results forthe wurtzite (IOTO) and(1120)surfaces.
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