Strain effects in epitaxial monolayer structures: SiGe and SiSiO2 systems

$latertal35ctence and Engmeermg. B0 (1990) 159-169

159

Strain Effects in Epitaxial Monolayer Structures: Si-Ge and Si-SiO 2 Systems J BEVK L C FELDMAN T P PEARSALL*and G P SCHWARTZ 4 To( T Bell Laboratorw3. lhota v Hill. NJ 07974 (U ~ .t )

A OURMAZD 4 Tc~TBell Lahoratorte~ ttolmdel. NJ 0773~ (U 5 A )

IRecewed February 1 1990)

Abstract Stram due to lattice mismatch at semtconductot mterfaces plays an important role m determining both the thin film growth mechamsm~ and electromc 3mwture and phy~wal propertws of matertal3 The 5 t - G e system with tt~ 4% lattice nusmatch ofJet, an opportunity to study various strata-related mterface phenomena, and to exploit strum effects tn novel optoelecttontc devzces A similar mterplay of science attd technology exists m the ~t-StO_, O,stem. where modern e2~penmental techmque~ continue to provtde new m~tghts into the atonuc 3tructure oJ this technologzcally important interface

T h e purpose of this paper is to illustrate the effects of strata on the semiconductor thin film properties with examples from the S i - G e and S~-S10~ systems Because ot the large lattice mismatch, the magnitude and distribution ol strain play dominant roles in determining the material properties of these systems We briefly discuss a Iew experimental techmqueg for quantitative determinauon of strain and elastic behavior of monolayer structures T h e role of interfaces and strain phenomena Is then Illustrated w~th studies of valence band oftsets and optical properties of SI-Ge heterostructures We present recent results from our study of the S i - S 1 0 2 system, and discuss the structure of this technologically important interface

1. Introduction M o d e r n thin film deposition techniques allow the f a b n c a u o n of multllayered systems with individual layers only a few lattice parameters thick These heterostructures exhibit a variety of interestlng properties attributed to band structure modification due to strain, reduced physical dimensions and artificial periodicity These phenomena are not only of scientific interest, but can also lead to improved thin film device performance and novel device concepts In particular, large lattice mismatch between individual layers and the associated strata are no longer viewed necessarily as shortcomings, but rather as new tools to tailor the electromc structure of the material

*Present address Department of Electrical Engineering Umversltyot Washington. Seattle WA 98195, U S A 0921-5107/90/$3 50

2. Sample preparation T h e S1-Ge and S1-SIO~ thin hhn structures discussed in this paper were grown in a molecular beam epltaxy (MBE) system designed for precise deposition of materials at the level of 1% of a monolayer [1] T h e SI(100) substrates were cleaned by the Shlrakl method, yielding a protective carbon-free oxide that is desorbed m the vacuum system (base pressure less than 2 x 10 1~ torr) at about 750°C In all structures a sdlcon buffer layer 1 0 0 0 - 3 0 0 0 A thick was first deposited at 650"C, followed by the desired Sl-Ge structure grown at 4 5 0 - 5 5 0 °C T h e films were capped with an epitaxlal silicon layer, whose thickness depends on the requirement of the particular experimental technique used to analyze the samples On samples intended for particlechannehng analysis, the silicon cap thickness ranged from 140 to 200 A, for electroreflectance © Elsevier Sequola/Pnnted in The Netherlands

measurements ot S1-Ge monolayer superlattlces, the cap was typmally about 1000 A thick, for core level photoemlssion measurements of St-Ge-S1 heterostructures, the experimental constraints limited the cap thickness to 16 A ( 12 monolayers (ML)) SI-S10, samples were prepared by deposition of about 1000 A ot silicon, followed by a ,varlet) of anneahng procedures to enhance the surface perfection T h e samples were oxidized at room temperature either m air, producing a native oxtde about 15 A thick, or m sltu in dry oxygen for about 5 h, followed by exposure to mr for an additional 8 - 2 4 h

3. Structural analysis of ultrathin strained epitaxial layers Studies of thin film growth and of the physical properties of layered structures reqmre detmled knowledge of structural parameters such as strain, individual layer thmkness, superlattice periodicity etc In systems with a large lattice m~smatch, such as S1-Ge (4 2% at the growth temperature), the maximum (crmcal) tlnckness for pseudomorphm growth is only a few atomic monolayers, necessitating sophasticated analysis techniques 40

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A particularly versatile and Informative technique for measuring a variety of relevant structural parameters is Rutherford backscattenng ORBS/ and particle channehng analysis [2] Measuring the minimum yield X...... (ratio of the channeling to random yleldl provides mlormatlon about the crystalline quality ot the film and can be used to determine the crmcal thickness lot epltaxlal growth An example ol this is shown in Fig 1 v~here values ot Zm~,~are plotted lot a series ot germanium layers deposited on Sl(100~ substrates The sharp lnclease in Zm.,, at about 6 monolayers Is a clear Indication ol the degradation ot crystalhne quality al this coverage T h e increased Zm,n may be due to an Increase in detect density in the germanmm or a change in morphology Nevertheless, this implies that ordered SI-Ge superlattmes can be grown on SI( 100t ~ubstrates with germanium building blocks up to (~ monolayers think [ 1 ] Channeling experiments can be used also to measure the elasnc behavlol ot epltaxlal germanium films at strata levels not accessible in bulk materials {3°/,,-4%)111 Figure 2 show~ angular scans measured In the off-normal channehng direction of 2 and 6 monolayer epitaxlal hlms of germanium e m b e d d e d within St(100) The sample geometry consists ot a SI( 100} substrate a strained layer him ot germanium and a silicon epltaxlal cap 2(1(I A thick T h e scan is caIrled out in the 1001) plane through the oilnormal [1 J 01 and l I 101 axe~ Of pamculm Interesl here is the asymmetlx about the I i 101 direction ~45 0°}m each germanium angulal scan This unusual asymmetry IS associated with the unique strain field of the epltaxlal monolavel hires A pseudomorphlc, strained film has ,in Inplane lattice constant a,, equal to the substrate In conventional descriptions ot epttaxy, the perpendlcular lattice constant a is given b), {R(a~,~ - as, ) + as, } where R = ( 1 + v)/t 1 - v} with l, = 0 273, the Polsson latIo lor germanium, and a~,~ and as, are the germanium and silicon lattice constants respectwely In this model, the nth layer of germanium has a perpendmular displacement X,, trom the off-normal channeling direction given by I n - ~)ta~ - a ~ , ) s l n O 4

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Fig 2 C h a n n e h n g angular scans through the [110] dlrectmn m the (001) plane ot G e - S l structures using 1 8 M e V He + runs (al structure consisting of SI( 100)-2 M L Ge-SI (200 A) (b) structure conststmg ot S1(100)-6 M L G e - S i (200 A) Also shown are the results ol c o m p u t e r simulations tor the g e r m a n m m angular scans u s m g the structural model described m the text

due to the four (100) type layers per unit cell For the [110] direction and n = 6 . X n is calculated to be about 0 39 A, a relatively large d]splacement from a "pseudosubstltUtlonal'" site as detected by channeling (or most other techniques) The origin of the asymmetry in the angular scan is also evident as displacements are not the same from both sides ot the channel Rather, because ot the solid-state structure and laminar growth, the displacements from the atomic row are asymmetric as viewed along the channeling direction The sensitivity of this measurement can be estimated as follows Channeling is sensmve to displacements of the order of 0 1 A, so that combinations of parameters in the above equation, wl~ch yield X. >~0 1 A, will result m a measurable strain In practice, this Is true only for layer thicknesses less than the channeling wavelength (about 200 A) When the film thickness becomes comparable with the channeling length, strain may be observed by enhanced dechanneling m an offnormal orlentanon Using the basic concepts of ion scattermgchannehng, the expected angular scattering patterns can be calculated using the following assumptions The thermal vibration amplitudes of the germamum and silicon atoms are equal to those of thmr respective bulk materials, the perpendicular lattice constant between Ge-Ge layers is 1 46 A, corresponding to the value deduced from the bulk elastic parameters and POlSSOn ratio for germamum, the w-plane lattice constant for germamum is equal to that of s]hcon and the Ge-St layer spacing is 1 41 A For reference, the (100) layer spacing m pure silicon is about 1 36 A Comparison between experiment and theory is shown in Fig 2 (dashed curves) The agreement is satisfactory, considering that the simulation does not include beam divergence and the knowledge of thermal vibrations is uncertain These results demonstrate that epitaxial films of germanium embedded in Sl(100) yield a strain close to that expected from the elasnc properties of the bulk material, even for films only a few monolayers think We reach a similar conclusion from measuring shifts in the optm-mode phonon frequencies between the strained layers and unstrained substrates using Raman scattering [4] Relative to unstrained material, the optm-mode frequency of the strained germanium increases by about 4 cm-1, whereas that of strained silicon decreases by 45 cm -~ (Fig 3) Taking into

162

account quantum confinement effects, the calculated total frequency shift amounts to 3 8 c m and - 4 2 cm -~ for strained germanmm and silicon respectively These values are m good agreement with experimental results [4]

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Although the ion scattering and Raman scatterlng experiments reveal the magnitude and nature of strain in the ultrathm Ge-S1 heterostructures, they do not yield reformation on the composmonal periodicity of the ultrathm Si-Ge superlamces This aspect of the materml ~s demonstrated by hlgh-resotuuon transmission electron microscopy ( H R T E M ) which clearly shows (Fig 4) that penodlclt~ is maintained k)r superlamces consisting of alternating 4 monolayers of sdlcon and 4 monolayers of germanmm [5] T h e contrast seen in Fig 4 reveals that the superlamce structure is indeed pseudomorptuc, ~mplymg that germanium layers are fully strained, and that mdfffuslon across the interfaces ~s hmlted to 1 M L or 2 M L

4. Valence band offsets in highly strained Si-Ge heterojunctions Valence band offsets represent important parameters in device deslgn and applications Most present-day device structures employ lattice-matched materials, but growing Interest m strained-layer structures dictates a need for band offset data In strained hetero}uncUons It is ~mportant to recogmze that the degree ot strain retained in each layer of a hetero}unction will affect the valence and conduction band offsets, t e in strained systems, some degree of band offset tailoring is possible

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valence band (CVB) edge states, whereas the uniaxlal component splits the manifold of valence band states relative to the CVB SI-Ge(100) and G e - S I ( 1 0 0 ) h e t e r o j u n c t l o n s were grown as described earlier Further details of the growth and core level strain corrections can be found in ref 4 For this system with about 4% lattice mismatch, the magnitude of the latter corrections IS about 0 4 - 0 5 eV For germanium grown on unstrained slhcon, a valence band offset oI 0 74 eV was measured, and for slhcon on unstrained germanium a value of 0 17 eV was obtained T h e asymmetry In the valence band o f f set spans 570 meV, permitting considerable latitude in the tailoring of band offsets in this system

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X r a y photoemisslon has been extensively used to measure valence band offsets In an A B heterolunctlon (Fig 5), the measurement requires that the respective core level energies E~, and E B be known relative to the top of their respective valence bands A thin heterojunctlon sample is then grown m which the core level separation A C-= CA-- CB is measured, and the valence band offset A V is given by E B - E , , - A C In general, the core level energies Ea and E~ are available only on unstrained bulk materials These energies must be corrected as shown in Fig 6 for latttce-mismatched pseudomorphic growth which results in blaxlal strain This strain can be decomposed into hydrostatic and uniaxial components T h e hydrostatic component shifts the core level relative to the centrold of the

5. Ge-Si Monolayer superlattices: an example of wavefunction engineering The experimental and theoretical studies of Ge-SI superlattices were stimulated partly by scientific interest and partly by technological needs Although silicon has been a dominant material m electronic device technology, its indirect and relatively large band gap severely llmlts ltS usefulness in optoelectronlcs One possible way to overcome this shortcoming is to manipulate the slhcon band structure, and in turn its optical characteristics, by creating artificial silicon-based superlattlces It is Important to recognize that the effect of the artificial periodicity on the material p r o p erties depends quahtatlvely on the superlattlce period Semiconductor superlattlces consisting of well and barrier regions more than about 20 A thick have properties that can be completely described as a series of quantum wells of threedimensional materials with bulk properties The second regime of Interest consists of periodicity less than about 20 A, but greater than the unit cell dimension or about 5 A T h e r e are also superlattices with periodicities less than the lattice parameter GaAs is a well known example of this kind of superlattice T h e question of superlattices and ordering on an atomic scale has been treated in most studies of alloy formation Braunstem e t a l considered the effect of ordering m G e , S I I , alloys on lattice vibrations [6] T h e ordering was modeled by f i x ing the sites of germanium and silicon atoms within the unit cell A similar study showing the effect of ordering within the unit cell on the band

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structme was carried out by Stroud and Ehrenrelch [7] The results of this calculation are shown in Fig 7 The basic teature of this kind of ordering is that the symmetry of the crystal is unchanged and the band structure is some sort of average of the alloy components This is seen clearly in Fig 7 where the broadened regions show the effect of different mtracell order In fact, the resemblance between the band structures of GaAs and germanium reflects the same basic results In our studies of Ge-SI superlatnces, we have measured the optical propemes of an ordered structure consisting of alternate monolayers of germanium and silicon oriented along the (001) plane [8] Thls apparent one-dimensional superlattice is identical to the zlnc-blende structure of GaAs Not surprisingly, the electroreflectance spectrum of this structure (shown in Fig 8) has the same optical transitions as those for a 50 50 alloy of germanium and sihcon [9] For larger superlattice perxods, where the wells and barriers are at least as thick as the fundamental bas~s for the umt cell, the effect of the superlamce on the electronic energy levels can be quite dffterent In the intermedmte l ange of length scales--large enough for the crystal potenuals of the conStltuent materials to be defined (2-4 monolayers) but short enough for a valence band or conducnon band electron to sense coherently four or more periods--the superlatt~ce structure can be used to redefine the umt cell dimensions and symmetry It is through this redefimtlon that wavefunction engineering--imposing a new basis set of electromc wavefunctions~can be achieved To understand the role of superlattlce periodicity, the Ge-S1 system offers the experl-

Frg 8 Electroreflectance spectrum of a superlattlce consistmg of alternating monolavers ot germamum and sflmon The oscdlatmns occur at energies corresponding to opncal transv non between the valence and conductmn bands The arrow~ indicate the energws of these transltmns

mentahst a distinct advantage over superlamces of duect band-gap materials The electronic and optical properties of ~emlconductors are dominated by band-edge states For ln&rect-gap materials, these states are located along certain crystal directions For example, in slhcon they lie along the (100) dwecnons Superlattlces oriented along these axes can be used to mampulate the symmetry associated with these band-edge states, mnoducing a component ot zone center symmetry, and making the superlattlce look more hke a direct-gap material In direct-gap superlattzces, however, the band-edge states already occur at the zone center The effect ot superlatnce symmetry on electronic states occurs for states above the band gap, and the effect~ are much harder to detect The first Ge-SI supellattlce to show the effects ot wavefuncnon engineering was an extended structure consisting of 4 monolayers of silicon and 4 monolayers of germanmm grown on a $1(100) surface ll0j The electroreflectance spectrum of this sample is shown m Fig 9 and can be compared with that of Fig 8, where the d~fferences can clearly be seen in both the complexity and the appearance of new optical transitions near 1 0 eV Theory of optical transmons tot this has been developed by a number of authors [11-17] The band structure of this superlamce (Fig 10) is taken trom the work of Gell and Churctull [17] One should compare this with the bandstructure shown in Fig 7 The differences are quite clear The Ge-SI (4 4) superlattice structure is not a d~rect band-gap materml, because ~t is grown on a sihcon substrate [11-18] The substrate-lmposed

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unlaxml strain ensures that energy bands lying along the M axes will he lowest m energy in the conduction band However, the results of tins lnltml experiment estabhshed that the superlattlce symmetry selects a d~fferent set of electronic wavefunctions trom those of bulk slhcon or germamum, and these wavefunctlons have characteristic energies that are not a s~mple extension of those seen in bulk slhcon or germanmm This, in a nutshell, Is wavefunction engineering The results of an imtial set of experiments estabhshed the following guldehnes for Ge-S1 superlattices ( 1 ) T h e mimmum thickness for the germanium or slhcon regions is 2 monolayers This means that the nun]mum superlattice period IS 4 atomic monolayers (2) Superlattmes consisting of at least 4 monolayers of silicon and germanium have energy levels that can be calculated accurately using a

(3) At least four complete superlattlce periods are required to create a superlattice region with new, measurable energy levels (4) Because of critical ttnckness limitations, we have not determined how large the period of the superlattlce needs to be before the system would be better described at a series of quantum wells ot bulk semiconductors T h e issue of superlattlce periodicity also Includes symmetry For example, a Ge~S17 superlattice IS non-centrosymmemc, whereas a Ge4S16 superlattlce does have central symmetry Alonso et al have identified the tour symmetry classes of Ge-S1 superlattlces and the effects of this symmetry on optical v~bratlonai spectra [19] T h e experiments suggested by Alonso et al could provide a novel means of measuring the number ot monolayers in a superlattlce structure T h e superlatuce periodmlty defines the new set of electromc wavefunctlons However, ff these wavefuncnons do not form the band-edge states in the energy spectrum of the superlattlce, it may be difficult to measure the effects of superlamce formation In the Ge-SI system, strain from commensurate ep~taxy of the Ge-SI structure on a substrate can be used to place superlattlce states at the band edge This sltuanon occurs when the substrate lattice parameter exceeds 5 4 7 A (corresponding to a Ge02Sl0 s alloy, for example) T h e lattice-mismatch strain on the slhcon portion of the superlattice is large enough to ensure that the superlattice potential will act on the lowestlymg states m the conductmn band for crystal growth along the (001 ) direction T h e conditions necessary to create a direct band gap will depend on the orientation ot the superlattxce relative to the crystal axes This orientation is determmed by the orlentatmn of the substrate used m ep~taxlal crystal growth T h e properties of Ge-S~ superlattices grown on (110) substrates have been treated by Froyen et al [20] In this case, the Ge-S1 superlattlce energy band structure has band-edge states dehned by the superlattme, even tor the case of a pure Sl(110) substrate Tins effect is acineved because all three (100)-oriented conduction band minima have a component oriented along the superlattlce direction, permitting the superlattlce potentml to mix these states with states at the zone center For growth on a (001) substrate, the superlattlce

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potential can interact with only the /,001)oriented conduction-band minimum Certain combinations of Ge-S1 monolayer superlattices grown on substrates that impose sufficient strain may be true direct-gap semiconductors Satpathy et al [15] originally predicted that the band structure of 10-atomm layer Ge-SI superlattices grown on (001 ) substrates with a lattice constant of at least 5 5 A would show direct band-gap behavior That is, such structures have a global minimum m the conduction band energy at the zone center, and an optical matrix element within one order of magnitude of that for GaAs Recent experiments provide considerable support for this prediction [21, 22] For example, in Fig 11 we show the 295 K electroreflectance I

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spectra tor two l()-monola>er period supeilattices and compare them ulth the spectrum for germanium, taken under the same conditions The presence ot the direct band gap can be interred trom the increased complexity ot the superlattlce samples near ()8 eV However, as first pointed out by Hybersten and Schluter [23], the problem of the basic type-II alignment between the substrate and the superlattlce needs to be solved before efficient luminescence can be obtained, providing a conclusive proof ot direct band-gap behavior This problem hat been studied in detail by Gell, who has shown clearly that a series of reasonable conditions needs to be satisfied to produce a direct band-gap Ge-S1 superlattice in a type-I band alignment with Ge~SI~ , alloy cladding lawyers [24] This study solves the final obstacle to the preparation ol Ge-SI heterostructures that may be suitable tor optoelectronlc devices Gell's approach is based on the observation that the conduction band ot Ge-S1 superlattlces grown between cladding layers of pure silicon lies higher m energy than the conductton band m the cladding layer For growth of the same superlattice on Ge(001), howevet, the situation is reversed, with the valence band ot the superlattice lying lower in energy than that ot the cladding, leading again to a type-II alignment Gell has shown that there is a range of cladding layer compositions ~or which the conduction band of the superlattlce lies lower in energy than that of the cladding This dehnes the condmons tor a type-I band alignment, and it is achieved by using the strain imposed by the cladding layers to adjust the energy band offset appropriately This range of cladding compositions is drawn in Fig 12 The shaded regions show the conduction and valence band energies of the cladding whose composition IS shown along the horizontal axis For cladding compositions in the range 0 7 < x < 0 9, the conduction band mimmum CI and the valence band maximum ~'1 of the superlattice lie inside the gap ot the cladding The precise range wtll depend on the superlattlce composition This example was calculated for a Ge-SI (6 2) superlattlce Its direct band gap occurs at 0 77 eV The principles of wavefuncnon engineering are based on the application of superlattlce penod~city and strain to redefine the energy level spectrum ot semiconductors Because of the unlaxlal nature of the superlattlces that can be

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XIn Sl1_ x Gox Fig 12 Relationship of the conduction band edge (.~ and the valence band edge V~ ol a Ge-SI (6 2) superlattlce relatwe to the band edges of the Ge ~Sh ~cladding layer The composition of the cladding layer IS shown along the horizontal axis For ~cnear 0 8, a type-I band alignment between the cladding and superlatuce is obtained (Reproduced lrom Gell [24] wlth permission I

grown by epltaxlal methods, wavefunctIon engineering ~s easiest to observe in indirect band-gap semiconductors in which the energy band mmima are also located along certain axes in momentum space T h e reduced symmetry of the new unit cell imposed by the superlattIce structure will d~splay a different set of energy levels from those of the constituent semiconductors, because the set of basis electron wavefunctions is no longer the same Wavefunction engmeenng has been demonstrated in Ge-S1 superlatuces as a way to create direct band-gap semiconductors from redirect band-gap components, but it may also fred uses m other materials systems, where tailoring of the band-edge states may produce improved propertles 6. Structure of the S i - S i O 2 interface

T h e S l - S 1 0 2 interface is a corner-stone of semiconductor technology This, with its intrinsic scientific mterest, has snmulated sustained activity for over 30 years It is thus now generally, but not umversally, accepted that the S102 is structurally amorphous and chemically Stolchiometric at distances of over 10 A from the Interface No such general consensus exists for Interfaclal structure itself Briefly, the various proposals can be dwided into three general categories (a) T h e c-Si ~ a-SIO2 transition IS pro-

posed to occur wa a stable, bulk phase of c-SIO2 Due to the structural similarity between silicon and cristoballte, this oxide represents the most frequently proposed crystalline interfacml phase However, it has a lattice parameter 40% larger than silicon, and it is difficult to achieve a commensurate sihcon-cristobahte interface (b) A metastable, substolchlometrlc oxide IS thought to affect the c-Si ~ a-S10~ transition It IS then necessary to postulate a metastable phase of remarkable stability, able to exist at the relatively l-ugh temperatures employed in sihcon oxidation It is possible that the special conditions present at the interface may stabilize such a phase (c) It has been shown that the c-S~ --*a-SIO~ transmon could occur abruptly, with no intervemng crystalline or substoichiometric phase at the interface [25] High-resolution transmission electron microscopy has naturally been employed to investigate the structure of th~s interface and, in particular, to detect the presence of any crystalline oxide [26] T h e resultant lattice images show an abrupt transition from c-S~ to "amorphous" material T h e Important question regards the interpretation of such images An examination of the SI(100) surface shows that, although the surface is structurally fourfold symmetric, the dangling bonds reduce this to twofold symmetry (Fig 13) A n epitaxlal oxide would necessanly be tied to the danghng bonds, which rotate through 90 ° on crossing a silicon surface step consisting of an odd number of atomic layers This imphes that, unless an epitaxial oxide were fourfold symmetric, it would be polycrystalhne with a gram size determmed by the spacing between the steps on the silicon surface Lattice ~mages of the SI-StO 2 Interface are obtained from cross-sec-

] 6~

tional samples about 100 A thick Thus, for an interracial oxide phase of less than fourfold symmetry, the image would consist of the superposition of the tmages of a n u m b e r of oxide grams rotated by 90 ° with respect to each other due to the presence of silicon surface steps (Fig 14) Consequently. unless the gram size is c o m p a r a b l e with, or larger than, the sample thickness, the detection of a crystalline oxtde phase of less than fourfold symmetry is most unhkely All that can

,~.., F'----

a-SIOi

Sl

projected potential

Fig 14 Schematic representatmn ot c-SI-c-SIO2-a-SIO 2

sample, viewed in cross-section in the TEM Monolayer steps at the c-Sl-c-S~O2 interface cause 90 ° rotations m the silicon danghng bonds, and hence the orientation of the c-S102 grams Thus, unless the c-SiO2 is fourfold symmetric, the atomic columns m the different c-SlO2 grams do not he on top of each other

be surmtsed f r o m the usual l a m c e images ot the S1-$102 interface is that, tf a crystalline interracial oxide ts present, it is less than fourfold symmetric This discussion also provides guidelines for the further investigation oi thts ~mportant system T h e mterlaclal structure can be uniquely determ m e d by H R T E M only if the spacing between silicon surlace steps is much larger than the sample thickness (about 1(10 A) M o d e r n M B E growth of silicon on silicon can produce samples of sufficient perfection [27] Figure 15 shows lattice images of the I001) SI-S10~ I l l 0 ] and [1 iO] projecnons for an M B E - g r o w n , oxidized silicon sample T h e presence of an mterfaclal oxide layer ts immediately apparent, and the dltlerence between the two lattice images shows the oxide to be indeed less than fourfold symmetric As we have been able to obtain lattice images of this phase m three projections and diffraction patterns in four projections, we have determined a three-dimensional m a p of atom positions at the St-S10, interface (Fig 16) [27] Modeling ol lattice tmages Indicates the lnterl-aclal oxide to be t n d y m l t e - - a stable, bulk phase of S10~ This crystalline phase forms a ~tralned, commensurate, epltaxlal oxide layer on the silicon substrate, and IS thus characterized by a critical layer thickness, beyond which strain relaxation must occur In this way, the small thickness of the

Fig 15 [110] and [~]6] lattice ~mages of an atomically flat S l - S 1 0 2 interface A new phase ~s present everywhere at the interlace In agreement wzth the plan-vmw diffraction data, the phase shows 3 8 A (110) periodicity in both projections However, the [ 110] and [T]0] latUce images are different

169

References

~

[0011~ [11 O]

[11-0]

oSI o Fig 16 Schematic representahon of the S1-S102 atomic structure deduced from the diffraction and lattice image data Only one ot the two possible arrangements m the [110 projecuon ~s shown The other ~s shifted by "(S~[110])=3 8 A, but ~s otherwise ~dent~cal Both are observed m superposmon m F~g 15 The dashed hnes indicate possible d~menzauon of the unsaturated bonds

crystalline oxide (about 5 A) and the production of amorphous SIO~ can be understood as simple consequences of strain relaxation [27] Tins concluston is consistent with a recent investigation of the structure of the SI(001 )-SIO_~ interface using grazing incidence X-ray scattering techniques [28] The X-ray diffraction patterns again reveal a highly-ordered, twofold symmetrtc phase at the interface Among the many models tested, such as a buried dlmer reconstruction and an interfacml crIstobahte layer, only an epltaxml trIdymIte layer about 5 A tback is in agreement with the experimental data The results indicate a decay in lateral coherence witban the final two layers of tins lnterfacial structure from about 500 A to the 50 A scale Tins observation elucidates the way in winch the perfect crystalline order in c-S1 decays Into disordered a-SIO2 via an ratermediate crystalline phase

7. Conclusions We have attempted to demonstrate the tmportant role that Interfaces play in siliconbased multIlayer structures Artificial periodicity on a monolayer scale, combined with large strata and quantum confinement effects, can significantly alter the band structure and in turn the physical properties of these semaconductor heterostructures As our understanding of the various interface-related phenomena improves, sIhconbased heterostructures will likely play a more promment role in optoelectronic device technology

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