Optical anisotropy of SiGe superlatticesJesper Engvall, Janos Olajos, Hermann G. Grimmeiss, Hartmut Presting, and Horst Kibbel Citation: Journal of Applied Physics 80, 4012 (1996); doi: 10.1063/1.363360 View online: http://dx.doi.org/10.1063/1.363360 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/80/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of disorder on luminescence from pseudorandomized strained Si1−x Ge x /Si superlattices Appl. Phys. Lett. 69, 3972 (1996); 10.1063/1.117984 Band structure, deformation potentials, and carrier mobility in strained Si, Ge, and SiGe alloys J. Appl. Phys. 80, 2234 (1996); 10.1063/1.363052 Kinetics of thermal annealing in strained ultrathin Si/Ge superlattices on vicinal Si(100) studied by Ramanscattering J. Appl. Phys. 80, 2211 (1996); 10.1063/1.363051 Optical absorption in alloys of Si, Ge, C, and Sn J. Appl. Phys. 79, 8656 (1996); 10.1063/1.362489 High pressure studies of lattice and electronic structures of Si/Si1−xGex superlattices AIP Conf. Proc. 309, 569 (1994); 10.1063/1.46100

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Optical anisotropy of SiGe superlatticesJesper Engvall, Janos Olajos,a) and Hermann G. GrimmeissDepartment of Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden

Hartmut Presting and Horst KibbelDaimler-Benz Research Centre Ulm, Wilhelm Runge Strasse 11, D-89081 Ulm, Germany

~Received 22 April 1996; accepted for publication 10 June 1996!

Optical and electrical properties of SiGe strain-adjusted superlattices have been studied. Diodestructures were processed into waveguide geometries to investigate the role of optical confinementand the lowering of cubic symmetry with regards to the polarization properties of interbandabsorption and emission. The polarization anisotropy of the absorption coefficient suggests that theheavy-hole band of strain-adjusted Si6Ge4 superlattices is the top valence band. ©1996 AmericanInstitute of Physics.@S0021-8979~96!02018-X#

I. INTRODUCTION

SiGe heterostructures have recently attracted further at-tention due to possible applications as optical emitters anddetectors integrated in bulk Si circuits. It is well known thatbulk silicon has fundamental limitations for these applica-tions in optoelectronics. The band gap of Si makes Sip- i -nphotodiodes insensitive to wavelengths of 1.3 and 1.55mm,the most applied wavelengths for use in optical fiber com-munications. Furthermore, due to the low luminescencequantum efficiency, the indirect-band-gap material Si is notvery useful for light-emitting diodes~LEDs! and other typesof emitters.

Strained layer superlattices~SLSs! of Si and Ge mayovercome several of these obstacles, even if the lattice mis-match of 4.2% between Si and Ge puts some limits on pos-sible heterostructures that can be grown pseudomorphic onSi substrates. Some of these limitations can, however, beminimized by, for example, growing a virtual substrate,1 i.e.,a SiGe alloy buffer layer that acts as a substrate for the SLS.Using this technique, the strain in the Si and Ge layers iscanceled and a SLS can in principle be grown to arbitrarythickness without generation of additional dislocations. Theband gap of such strain-symmetrized superlattices, in whichthe Si layers are under tensile strain, can be tuned to energiesof interest for fiber optics by varying the in-plane latticeconstant and the thickness of the Si and Ge layers.2 Detailedband-structure calculations for perfect structures have shownthat for some of these SLSs a larger oscillator strength of thelowest interband transition is predicted than for bulk Si.3–6

Since the final band lineup is critically dependent on an in-teraction between strain and confinement, it is not necessaryso that the theoretically predicted zone-folded conduction-band minima will be lowest in energy in a superlatticesample. For the valence-band states the ordering of the bandsis not crucial for the magnitude of the transition probability,but it has, nevertheless, some influence on the polarizationdependence of the optical spectra for energies around theband gap.

The purpose of this article is to discuss the polarization-dependent optical and optoelectronic properties of strain-adjusted Si6Ge4 SLSs by using data that have been obtained

with short-circuit photocurrent~ISC!, photoluminescence~PL!, and electroluminescence~EL! spectroscopy. Thesemeasurements show that the highest lying valence band ex-hibits heavy-hole character and that the PL and EL emissionare of slightly different nature. By comparing the PL and ELemissions, the role of disorder was studied. It is demon-strated that higher light detection efficiencies than previouslyachieved in strain-symmetrized structures were obtained dueto an improved coupling of light into diodes with wave guidegeometries.

II. EXPERIMENTAL DETAILS

SLS samples were grown in a conventional molecular-beam-epitaxy~MBE! apparatus described elsewhere.7 Si wassputtered from an e-beam source and Ge, Sb, and B wereevaporated from effusion cells. The Ge effusion cell is madefrom pyrolytic boron nitride implying that the Ge depositionalso resulted in a coevaporation of B in the 1016 cm23 range.A SiGe alloy buffer was used to obtain an appropriate in-plane lattice constant minimizing the number of threadingdislocations extending up to the SLS. The lowest dislocationdensity was achieved by growing a graded buffer followedby a constant composition buffer of the same average com-position as the SLS.8,9

A crucial factor regarding the growth of SLSs is themonolayer-sharp interface between Si and Ge in the SLS.Segregation effects occurring at the surfaces10 and diffusionof Si into the Ge layers11 decrease the interface sharpness.Although these processes increase with temperature, a trade-off is possible between sharp interfaces and morphology bychoosing proper growth temperatures since the optimal MBEgrowth temperature for Si is quite high. One way of reducingthe segregation is to cover the surface with a group-V ele-ment. It has been shown that a monolayer of Sb acts as asurfactant hampering the segregation of Ge,12 giving sharpinterfaces between Si and Ge even at growth temperatures ashigh as 500 °C. The Sb adlayer may also improve the mor-phology of the buffer13 but the surfactant has the disadvan-tage that the spontaneous incorporation of Sb in general re-sults in n-type layers. For achievingp-type SLSs, anovercompensation withp dopant is therefore necessary.

The layer structure of the three different Si6Ge4 samplestructures~A, B, and C! used in this study is shown in Fig. 1.a!Electronic mail: ftfjo@macpost.lu.se

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Sample A was grown on a semi-insulating substrate which isbest suited for photoluminescence investigations. The step-graded buffer was grown in a way so that each step impliedan increase of 3% of Ge grown into a thickness of 50 nm.The total thickness of the step-graded buffer was 650 nmwhich resulted in a Ge concentration of 39% at the top. Aconstant composition Si0.6Ge0.4 alloy of 500 nm thicknesssucceeded the step-graded buffer and on top of the alloy a200-nm-thick Si6Ge4 SLS was grown at 500 °C. During thetotal growth sequence an Sb adlayer was used as a surfactant.The sample was finally covered with a 50 nmp1 Si0.6Ge0.4cap layer.

For p–n junction diodes, similar layer structures weregrown onn1 substrates~sample B!; however, the surfactantmethod did not automatically provide the optimal electricalcharacteristics ofp–n junction diodes. Then doping result-ing from the spontaneous incorporation of Sb decreased rap-idly with increasing Ge fraction. Due to the coevaporation ofB from the Ge source, the resulting net electron concentra-tion depended strongly on the Ge concentration and was thusmuch higher for a SLS than for an alloy of the similar Geconcentration. The undesirable electrical characteristics ob-served forp–n junctions containing SLSs on constant com-position buffers14 are probably due to differences in dopingof the Si and Ge layers, respectively. A small modification ofthe buffer was therefore made in samples A and B by in-creasing the doping at the substrate–buffer interface throughdoping by secondary implantation~DSI!.15 Fromcapacitance–voltage measurements then-doping of the SLSwas determined to be around 131017 cm23.

Sample C is similar to sample B. The only difference isthat thep-type alloy cap of sample C is 250 nm thick anddoped to lower concentration in order to provide improvedwave guiding properties for light traveling parallel to theepilayers.

Samples B and C were processed into normal incidencemesa diodes as well as diodes of waveguide geometries. Themesa diodes were of 200–1500mm diameter with a windowin the Au/Ti metallization for optical experiments. Thewaveguide diodes had most of the top surface metallized and

lithographically defined stripe widths of 100mm. Thelengths of the stripes varied between 2000 and 4000mm,with the end^110& facets mechanically polished.

EL and ISC measurements were performed using adouble grating monocromator and a flow cryostat containingthe sample. TheISCmeasurements were done in steady-statemode. For EL measurements, the current was modulated andthe luminescence was detected with a North Coast Ge detec-tor using lock-in techniques. The PL spectra were measuredwith a BOMEM DA8 Fourier transform~FT! spectrometerby exciting the surface of the sample with the 488 nm linefrom an Ar1 laser. The luminescence from the polished^110& facets was detected with an InGaAs detector. All spec-tra were corrected for the spectral dependence of the experi-mental setup, including the polarization dependence. Thiscorrection is small for FT measurements, but can be large formeasurements using grating monocromators.

A disadvantage of studying the polarization dependenceof absorption by photocurrent spectroscopy is that the lightafter numerous internal scatterings can still contribute to thephotocurrent signal. Hence, incident light which is com-pletely polarized in one direction may change polarizationupon scattering before being absorbed. If the scattering isisotropic, this effect will reduce the difference of the signalbetween two polarizations. This means that a polarizationanisotropy measured with PC can be smaller, but neverlarger, than the anisotropy measured in absorption.

III. RESULTS

For photon energies close to the fundamental band gap,the total absorption in SLSs is rather small due to the com-parable low absorption coefficient and the small thickness ofthe layer. In an earlier study14 we found that absorption isbest studied by short circuit photocurrent~ISC! spectroscopy.It was shown that the absorption onset was little affected byphonon-assisted processes and that uncertainties in the pho-non energies therefore had no major influence on the deter-mination of band-gap energies. Since the SLSs in samples B

FIG. 1. Sample structures of the three Si6Ge4 SLSs~samples A, B, and C! used in the study.

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and C were grown with a different buffer concept, they areexpected to be of superior morphology but with a differentstrain distribution as the samples investigated in Ref. 16.This means that the threshold energies are expected to beslightly different.

ISC spectra of samples B and C are presented in the insetof Fig. 2. The spectra were measured at 80 K on mesa di-odes. Good agreement in both the onset and magnitude of theband-gap absorption is seen. Three conclusions can be drawnfrom these observations, as follows.

~i! A similar magnitude of absorption indicates that theSiGe alloy waveguide layer, which is thep layer of thep–njunction in sample C, introduces negligible absorption at thephoton energies of interest, i.e., near the SLS band gap.

~ii ! The similarity in absorption strength also tells usthat the light absorbed in the SLS is converted into currentby the same ratio mesa and waveguide samples. The differ-ence between ap1–n junction and ap–n junction with amore symmetrical doping has obviously no significant influ-ence.

~iii ! Since the onset of interband transitions is similarfor samples B and C, the growth of the waveguide layer hasnot led to any significant interdiffusion of Si and Ge in theSLS during growth of thep-type alloy. In view of the lowannealing temperatures needed to significantly shift the PLpeaks of sample A,17 this could have been a concern.

EL spectra obtained for sample B at 22 K and varyingcurrent densities are shown in Fig. 2. At low current densi-ties the sample exhibits only a broad band of luminescence

between 0.75 and 0.88 eV. At increased current densities, apeak at around 0.9 eV appears and increases superlinearly.This behavior is consistent with EL studies of SiGe SLS onsurfactant buffers18 which showed that the high-energy peakwas due to interband recombination while the broad bandoriginated from defect-assisted recombination. Interestingly,sample C exhibited EL at all temperatures up to room tem-perature but no EL could be detected up to current densitiesat which latching effects were observed. Above this currentdensity the band-gap EL intensity was almost linear in cur-rent and virtually no defect assisted luminescence was seen.At 20 K, the EL linewidth in a 200mm mesa diode was only31 meV.

Table I summarizes the PL, EL, andISC energies. Thereis a consistent difference in EL and PL energies with thefitted ISC threshold energies falling in between the two. Inprevious studies,16 the PL behavior was found to be consis-tent with a model of localized excitons and the peak in PL istherefore expected to be slightly below the band-gap energyas measured withISC. The EL is believed to be due to free-electron–hole recombination and the EL peak should there-fore be observed slightly above the band-gap energy.

Measurements of EL, PL, andISC were also performedwith samples in edge view in order to determine the nature ofthe band-edge states involved in the optical transitions. Fig-ure 3 showsISC spectra of sample C for different polariza-tions, i.e., with the electric field in the plane of the layer~XYpolarized! as well as in the growth direction~Z polarized! fora wave guide structure. The three spectra are plotted semilog.The unit of they axis is theISC divided by the photon fluxper unit area. The area of the face of the wave guide wasestimated to 100mm strip width times 1.6mm epilayer thick-ness. The mesa spectrum in Fig. 3, which is the same spec-trum as in the inset of Fig. 2, was normalized to coincidewith the waveguide spectrum inXY polarization. Around theinterband onset the spectral dependence of both samples issimilar but the spectra of the waveguide diode deviate fromthe spectrum of the mesa diode at higher photon energies.The difference can be explained by considering the effi-ciency of the wave guide which is displayed on they axis.Since the signal is close to unity, the signal is no longerproportional to the absorption coefficient. The large signal isdue to the fact that the absorbing layer thickness is muchlarger than 1/a in this geometry compared with the mesadiode where only a few percent of the light is absorbed.

Figure 3 also shows that different sensitivities were ob-served for the two different polarizations of light. When the

FIG. 2. EL spectra of sample B measured at 20 K with varying injectioncurrent. The inset showsISC spectra of mesa diodes~samples B and C! at 77K.

TABLE I. Summary of absorption and emission energies obtained from thesamples studied. The fitted energies were obtained by assuming a quadraticenergy dependence above the gap.

Sample

EG fittedfrom ISC at

80 K

EG fittedfrom EL at

roomtemperature

EG fittedfrom EL at

low temperaturePL peak at

10 K

A 0.873B 0.913 0.860 0.910~22 K! 0.880C 0.924 0.867 0.927~30 K! 0.876

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light was XY polarized, an up to three times larger signalwas obtained than with theE field Z polarized. The differ-ence is still observable below the interband absorption edge.The inset of Fig. 3 shows ratio of the photocurrent signalobtained for different polarizations. The photocurrent spec-trum with XY polarization is divided by theZ-polarizedspectrum for waveguide diodes of samples B and C, respec-tively, and exhibited together with the ratio of the EL spectrafor sample C.

In all three cases a peak at about 0.92 eV is observed.The largest ratio is obtained for the EL of sample C whereasthe ISC ratio of sample B is smaller than the one of sample C.It is readily seen that the EL andISC ratios of sample C arevery similar.

Our waveguide samples were less suited for PL mea-surements since most of the top surface was metallized. ForPL studies we therefore used unprocessed pieces with theend facets polished. PL spectra obtained by detectingXY-polarized andZ-polarized luminescence, respectively, fromthe end facet of sample A are shown in the lower part of Fig.4. The luminescence peak at 0.88 eV exhibits a considerablyhigher intensity of XY-polarized luminescence thanz-polarized luminescence while the luminescence features atlower and higher energies, which are believed to originatefrom the alloy buffer, seem to be less dependent on polariza-tion. The ratio of the two curves is plotted in the upper partof the figure giving a maximum value of 1.7 at 0.88 eV.

IV. DISCUSSION

A. Band structure and selection rules of SiGe SLSs

It is well known that the strain in the Si and/or Ge layersintroduces a splitting of the conduction band~CB! as well asthe valence band~VB!. In the CB, the sixfold degeneracy oftheD minima is lifted into a fourfold~D4! and a twofold~D2!set and at the VB maximum the initially degenerate light-hole ~lh! and heavy-hole~hh! bands split. This splitting isinfluenced by the spin-orbit split-hole band~soh!. In order tohave the miniband of theD2 levels in the superlattice lowerin energy than the bulklikeD4 minima, the Si layers have tobe under tensile strain as shown in previous band-structurecalculations for short-period SiGe superlattices.3–5 Such aband ordering is of great interest since the new SL minibandoriginating from theD2 states is expected to have a minimumat the SLG point provided the number of SL monolayers ischosen correctly. It is hoped that the zonefolded states willgive rise to significantly enhanced dipole matrix elements2

due to the superlattice period.Many features of the electronic structure of SiGe SLSs

are readily explained by applying a one-band~Kronig–Penney! model in conjunction with deformation potentialtheory for the band alignment. Figure 5 shows the bandlineup of theD minima and the VB maximum obtained fromsuch a Kronig–Penney calculation of a strain-symmetrizedSi6Ge4 superlattice. The band lineup is obtained using Veg-ard’s law for the strain in the layers as well as the deforma-tion potentials of bulk Si and Ge. In our model, a band gap of0.9 eV is obtained in fair agreement with some of the moreelaborate theoretical models. It is interesting to see that the

FIG. 3. Short-circuit current spectra of a waveguide diode~sample C! due toxy-polarized andz-polarized light, respectively. The spectrum of a mesadiode from the same sample is also shown. The spectrum is shifted invertical direction to coincide with the spectrum due toxy-polarized light.

FIG. 4. The lower curves show PL spectra of sample A measured at 20 Kdue to xy-polarized andz-polarized light, respectively. The upper curveshows thexy/z ratio of the two spectra.

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band minima of theD4 conduction miniband is 60 meVhigher in energy than the twofold minima whereas the lh andhh minibands are almost degenerate. This seems to be rea-sonable since for compressive~tensile! strain the hh~lh!miniband is highest in energy and if the structure is strainsymmetrized the splitting of the hole bands is expected to besmall.

The symmetries of all SiGe SLSs can be found in Ref.19. Restricting the discussion to~001! growth, there are threepossibilities for a SinGem superlattice: Ifn andm are even,the point group isD2h and the structure is of orthorhombicsymmetry with an asymmetry in the growth plane; ifn andm are odd, the structure has tetragonal symmetry and noinversion symmetry implying that the point group isD2d;finally, if one of n or m is even and the other is odd, thesuperlattice is of tetragonal symmetry and the point group isD4h. In this case, the unit cell consists of two periods. Thesevariations in symmetry determine the selection rules for op-tical transitions in the ideal superlattices. The interband-transition selection rules for the different space groups arefound in Refs. 5–6 and 20. As pointed out in Refs. 20 and21, the orthorhombic effect in case of, for instance,~a!Si6Ge4 SLS is expected to be small. The largest anisotropy isexpected in the stacking direction. This statement is sup-ported by calculations of the dielectric response of SiGeSLSs in Ref. 20. This article shows that the dielectric func-tion is virtually independent of the angle in the superlatticeplane. Including spin-orbit interaction, the following selec-tion rules can then be extracted:

~1! hh–D2 transitions are only allowed for light withXYpolarization~E field parallel toX or Y, see Fig. 1!;

~2! lh–D2 transitions are mainly allowed forZ-polarizedlight but are not exactly zero forXY polarization;

~3! the dipole elements of the lowest interband transitionscalculated in Ref. 20 indicate that the two transitions in~1! and ~2! are of similar magnitude.

An interesting question is what happens in an imperfect

superlattice. It is likely that imperfections will weaken theselection rules, a problem which has been treated in Ref. 22.Similar to an alloy, certain local variations of the averagecomposition will give rise to tail states below the bottom~top! of the conduction~valence! band. These tail states aremore localized and have therefore not necessarily the samesymmetry as the ideal SLS. Higher up in the bands the sym-metry rules do not apply either. Due to these reasons, theratio px/pz has a finite value, even at the band edge. Suchconsiderations also imply that the maximum anisotropy ofthe absorption is not necessarily observed at the same wave-length as the peak in luminescence. It should, however, bekept in mind that the polarization anisotropy in a strain sym-metrized SiGe superlattice is a superlattice effect. The aver-age lattice constant is the same in all three directions. Thesituation is therefore qualitatively different from SiGe struc-tures pseudomorphic to a Si substrate, where the averagelattice constants parallel and perpendicular to the growth di-rection are different. In the pseudomorphic case, even a ran-dom alloy is expected to show some polarization anisotropy.

B. Waveguiding properties

For the analysis of polarization dependent spectra, it isessential to know if and how much the waveguiding proper-ties influence the measured absorption and emission ratios.To understand the characteristics of the wave guides, thedielectric functions of SiGe alloys and SLSs are needed. Re-garding the absorption, a polarization anisotropy of the di-electric function is expected for a SLSs. In Ref. 21 a tight-binding calculation of the band structure of a Si6Ge4 SLSwas applied together with the Kramers–Kronig relation,23

a852

pPE

0

` sa9~s!

s22v2 ds, ~1!

for the determination of a wavelength-dependent relative di-electric function for different polarizations, wherea8 is thereal part anda9 the imaginary part of a complex responsefunction. The authors showed that the relative dielectric con-stant forl51.3 mm is about 0.4 larger for TE modes~XYpolarization in our notation! than for TM modes~Z polariza-tion!. The later value is very close to the value for aSi0.6Ge0.4 alloy. Hence, using the wavelength-dependent re-fractive index for alloys from Ref. 24 the necessary param-eters for evaluating the waveguide characteristics are readilyobtained. In principle, it is possible to have guided TE modesin a SLS grown on an alloy with the same Ge concentrationas the SLS since the refractive index for a SLS is larger thanfor the corresponding alloy. This, however, would require aSLS that is thicker than what possibly can be achieved inMBE growth.

Another possibility implies that a SiGe alloy claddinglayer is grown to provide guided modes in the SLS. Whetheror not a superlattice mode is obtained is probably not veryimportant in our case, since the SiGe epitaxial layer has alarger refractive index than the Si substrate. A guided modein the epitaxial layer can therefore still have a significantamplitude in the superlattice.

FIG. 5. Band diagram of a strain-symmetrized Si6Ge4 SLS in a Kronig–Penney model. The SLS is of type II and the miniband due to theD minimain the growth direction is lower in energy than the in-plane minima. Theheavy- and light-hole bands are close in energy due to the symmetricalstrain and similar effective masses.

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We have calculated the mode structure of bound opticalmodes, TE and TM, in the epitaxial layer under the assump-tion that the amount of light that is coupled into the layerdepends on the confinement factorG for each mode in theSLS. The following relations are obtained:

G5*0hS–x dx

*0`S–x dx

, S5 12 Re~S3H* !, ~2!

whereS is the Poynting’s vector which specifies the powerper unit area carried in that mode.

A calculation of the waveguiding properties of the low-est mode for a layer structure similar to sample B and sampleC is shown in Fig. 6, where the confinement factor of theSLS slab is plotted versus photon energy in order to facilitatecomparison with experimental spectra. It is interesting to seethat the thicker cap layer of sample C increases the SLSconfinement factor of both the TE and TM modes. The cal-culation thus accounts for the fact that while the photore-sponse of sample B and sample C is similar in magnitude inmesa geometries, the signal in waveguide geometries is threetimes larger for sample C than for sample B. At higher pho-ton energies this result is not very important since most ofthe light is absorbed regardless of polarization. The primaryinterest lies in the region below and near the interbandthreshold energy. It is believed that the polarization differ-ence of sample C at lower energies shown in the inset of Fig.3 may be attributed to the difference inG for different polar-izations.

Our data also show that in all measurements the sensi-tivity for XY-polarized light is larger than forZ-polarized

light. Furthermore, the peak energy is the same for emission~EL! and for absorption~ISC! as demonstrated in the inset ofFig. 3. We therefore assume that theXY/Z ratio in junctionmeasurements is a measure of the ratio of the dipole ele-mentspy/pz of the SLS, but superimposed on a slowly vary-ing background due to the anisotropy in waveguiding.

Regarding PL measurements, the situation is slightly dif-ferent since regions outside the SLS also contribute to the PLintensity. Thexy/z ratio which is shown in the upper part ofFig. 4 cannot therefore entirely be attributed to the SLS. Thisis demonstrated by the fact that theXY/Z ratio is close tounity for the high-energy peak originating from the buffer.The graded buffer produces an alloy that is completely re-laxed and, hence, no polarization anisotropy is expected forthe band-gap luminescence of the alloy. Since the PL peak at0.88 eV originates from the SLS, a strong polarization an-isotropy is expected and indeed observed. The ratio at thepeak is, however, less than the corresponding ratio for theEL peak position. This can be understood as a result of thedifferent nature of the states participating in the respectiverecombination processes. The quasilocalized states partici-pating in the PL extend only over a comparably small part ofthe SLS periodicity. Higher up in energy, i.e., closer to theband edge, the states are more delocalized and more affectedby the SL potential. This interpretation is supported by theupper curve of Fig. 4 which shows that the polarization ratiohas a maximum at a higher photon energy than the PL peakenergy in contrast to the EL spectra which exhibit a peak ofthe xy/z ratio much closer to the emission peak.

Summarizing, we may say that our data strongly suggestthatpy is much larger thanpz at the band edge. Since at thisenergy either a hh or a lh transition is expected, it is con-cluded that it is a hh–D2 transition that dominates the lowestinterband transitions for a strain-adjusted Si6Ge4 SLS.

V. CONCLUSIONS

We have determined the lowest interband transitions in aSi6Ge4 SLS as hh–D2 transitions by using photocurrent andluminescence spectroscopy. The previous assignment of theEL as band-gap luminescence is supported by our data forsuperlattices on graded buffers. A significant increase inquantum efficiency is obtained by using a SiGe SLS detectorstructure as a waveguide where the light is guided in theepilayer due to the higher refractive index of SiGe alloys andheterostructures. The comparison of EL and PL properties inpolarization displays some features of the electronic statesinvolved in the transitions.

ACKNOWLEDGMENTS

This work has been carried out within the European Ba-sic Research program~ESPRIT! under Contract No. P7128.The authors acknowledge financial support from the SwedishNational Science Council, The Swedish Board for TechnicalDevelopment, as well as the Bank of Sweden TercentenaryFoundation.

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FIG. 6. The optical confinementG in the superlattice as calculated forsamples B and C, using the nominal values of the layer thicknesses. Theconfinement is displayed for the lowest modes ofxy-polarized light~TE!,andz-polarized light~TM!.

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