Short-period Si n Ge m strained-layer superlattices grown from gas sources bysynchrotron-radiation-excited chemical-beam epitaxyHousei Akazawa Citation: Applied Physics Letters 83, 461 (2003); doi: 10.1063/1.1593795 View online: http://dx.doi.org/10.1063/1.1593795 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Interface roughening and defect nucleation during solid phase epitaxy regrowth of doped and intrinsic Si 0.83 Ge0.17 alloys J. Appl. Phys. 101, 103508 (2007); 10.1063/1.2732680 Hydrogen-mediated quenching of strain-induced surface roughening during gas-source molecular beam epitaxyof fully-coherent Si 0.7 Ge 0.3 layers on Si(001) J. Appl. Phys. 91, 3579 (2002); 10.1063/1.1448680 Growth of Si 0.75 Ge 0.25 alloy layers grown on Si(001) substrates using step-graded short-period ( Si m / Ge n )N superlattices J. Appl. Phys. 90, 202 (2001); 10.1063/1.1378057 Roughening transition and solid-state diffusion in short-period InP/In 0.53 Ga 0.47 As superlattices Appl. Phys. Lett. 78, 1370 (2001); 10.1063/1.1353839 Structural characterization of Si0.7Ge0.3 layers grown on Si(001) substrates by molecular beam epitaxy J. Appl. Phys. 81, 199 (1997); 10.1063/1.363841

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Short-period Si nGem strained-layer superlattices grown from gas sourcesby synchrotron-radiation-excited chemical-beam epitaxy

Housei Akazawaa)

NTT Microsystem Integration Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi,Kanagawa 243-0198, Japan

~Received 10 February 2003; accepted 20 May 2003!

Short-period SinGem strained-layer superlattices were successfully grown on Si~100! substrate fromgas sources by synchrotron-radiation-excited chemical-beam epitaxy at 250 °C. Atomically sharpinterfaces between the Si and Ge layers and two-dimensional morphology were obtained. Withthinner Ge layers, the misfit strain is relieved by atomic-scale roughness at the interfaces; as the Gelayers become thicker, misfit dislocations become the main relief mechanism. The thickness of Siand Ge layers as derived from spectroscopic ellipsometry agreed nicely with images of the lattices.The dielectric constants of the growing Si and Ge top layers were much lower than those of Si andGe bulk crystals, which is consistent with the mechanism of photoepitaxy, that is, the formation ofa hydrogenated network as the precursor state followed by photolytic crystallization. ©2003American Institute of Physics.@DOI: 10.1063/1.1593795#

The optical properties of short-period SinGem strained-layer superlattices~SLS! strongly differ from those of bulkcrystals of either individual element because of the band-folding effect in the Brillouin zone. To verify that the band-to-band transition is in fact quasidirect, predicted by theoret-ical calculation, we need to prepare an SLS with layers ofprecisely controlled thickness and abrupt interfaces. In theprocess of doing so, the SinGem SLS can also serve as amodel system for examining the atomic-level controllabilityof various crystal-growth techniques. In reality, however,low-temperature solid-source molecular-beam epitaxy~MBE! has been the only means of growing short-periodSLS; this is used with the monitoring of the oscillating spotintensities of reflection high-energy electron diffraction.1 Theoptical transition properties of SLS have previously been ex-amined by resonant Raman scattering,2 optical reflectance,3

spectroscopic ellipsometry~SE!,4 photoluminescence,5 andelectroluminescence.6 Techniques involving thermally acti-vated growth from chemical sources have rarely been usedbecause the growth rate in such techniques is critically de-pendent on temperature and thus is hard to control.

In this letter, we propose synchrotron-radiation-excitedchemical-beam epitaxy7,8 as an alternative way to obtainshort-period SLS and report on layers thus grown and ourapplication of real-time SE to characterize their structuraland optical parameters. A procedure for the spectroscopicanalysis of layered semiconductor structures after comple-tion of their growth has also been established.9–11The previ-ous real-time SE study during the growth of AlAs/AlGaAsand Si/Si12xGex multiple quantum wells featured relativelythick layers,12,13 and there have been very few real-time SEstudies of short-period SLS.

The synchrotron radiation beam emitted from the bend-ing magnet of the ‘‘Super-ALIS’’ compact electron storagering photolytically excited the source gases as well as thegrowing film.14 The energy distribution of the photons wasbroadband, covering the range from 10 to 1500 eV, and the

maximum photon flux was at 100 eV. The photon flux on thesubstrate was 931015 s21 mm22 for an average storage cur-rent of 400 mA. The oxide layer on the Si~100! substratewas removed by using a 2.5% solution of hydrofluric acid,and the resulting atomically rough and hydrogen-terminatedsurface was smoothed by the Si2H6 gas-source MBE of a70-nm-thick Si buffer layer. A SinGem SLS was then grownon this surface through the alternate introduction of Si2H6

and GeH4 gases with the substrate exposed to a perpendicu-lar synchrotron-radiation beam. Sudden admittance and cut-off of the gases was achieved by opening and closing a fineaperture at a distance of 20 cm from the substrate. The valueof (gas exposure time)3(photon beam flux) was kept con-stant even though the storage current was decaying overtime.

The growth temperature must exceed the critical tem-perature for amorphous/crystal transition~200 °C! and mustbe lower than the temperature at which an undulating mor-phology becomes prevalent on the growing surface. Thethickness control imposes the further requirement that thetemperature must be lower than the threshold temperature ofhydrogen desorption from silicon dihydrides and germaniumdihydrides, so that the contribution of the photolytic growthchannel is dominant. The relative insensitivity of the rate ofthis nonthermal growth channel to temperature enables theprecise control of thickness. The upper limit for such growthis 350 °C for Si and 250 °C for Ge.15 The optimum tempera-ture range in terms of satisfying the above requirements isthus 200–250 °C.

Figure 1~a! shows an image of the lattice of a Si8Ge7

SLS grown at 250 °C. The atomically abrupt Si/Ge hetero-interfaces are indicated by the sharp contrast between theadjacent Si and Ge layers. The degrees of both crystallinityand interface abruptness appear to be similar to those seen inSLS grown by solid-source MBE.16–18 This confirms thatlow-temperature photoepitaxy from gas sources is feasible asan alternative way to grow SinGem SLS. Inspection of wholestructure revealed that the two-dimensional morphology wasmaintained and that the SLS with its overall thickness of 240a!Electronic mail: akazawa@aecl.ntt.co.jp

APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 3 21 JULY 2003

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nm was free of misfit dislocations; that is the lattice wasstrained.

An image of the lattice of a Si/Ge multilayer thermallygrown at 600 °C under the same gas flow conditions is givenfor reference in Fig. 1~b! to highlight the abruptness of theinterface. Here, a transitional fuzzy region between the Siand Ge layers appears and is thicker than 30 Å. This clearlydemonstrates that the sharp interface seen in Fig. 1~a! wasbrought about by photolytically promoted crystallizationwhile the migration of atoms was suppressed by the lowprocess temperature.7 Although the interfaces seen in Fig.1~a! are atomically sharp, the individual layers are not ide-ally uniform in the horizontal direction because of theatomic-scale roughness at the interfaces. The Matthews–Blakeslee theory19 indicates that the large molar fraction of0.47 for Ge in a Si8Ge7 SLS will lead to a critical thicknessof only 60 Å for such a film grown on a Si substrate. How-ever, a low-temperature MBE experiment by Beanet al.20

revealed substantially greater values of the critical thickness~100 Å!. In any case, our dislocation-free SLS with thicknessvalues up to 240 nm suggests that the interfacial roughnessserves as the important strain-relaxation channel. In terms ofstrain modification, the interfacial roughness layer is equiva-lent to a layer of Si12xGex alloy, which acts to accommodatethe misfit strain.21

The local roughness may have some relation to the ki-netics of photoepitaxy from gas sources. If the epitaxy isregulated by the completion of full monolayer coverage ineach atomic layer, that is, if the growth proceeds in a layer-by-layer manner, the interfacial roughness would be limitedto less than one atomic layer. In the photoexcited chemical-

beam epitaxy under discussion, however, hydride-radicalprecursors are produced by the photolysis of Si2H6 andGeH4 and form a hydrogenated network layer on the grow-ing surface, which is then photolytically crystallized.7,8 In thecourse of this crystallization process, production of a fewatomic layers of roughness is inevitable.

Further examples are given in Fig. 2, where we see aSi11Ge43 SLS grown at 250 °C. The interfaces between layersare again atomically abrupt. The two-dimensionality of theSLS is maintained, even across the substantially thicker Gelayer units. The image of the whole SLS in Fig. 2~b! showsthat the misfit strain is primarily relieved by very dense dis-locations that extend from the SLS/substrate interface region.

The evolution of the ellipsometric anglesC andD overtime was recorded during growth of the SLS. Here,C andDare defined from the Fresnel reflection coefficient ratior ofp-polarized (Rp) to s-polarized (Rs) light with respect to theSi ~100! surface; that is, asr5Rp /Rs5tanC•exp(iD). Acommon feature of the time-dependent plots for all monitor-ing energies was the numerous short-period amplitude oscil-lations, which are produced by the alternation of the top-layer material from Si to Ge and vice versa.22 Anotherfeature, seen in the transparent energy range, was the longer-period amplitude modulation, which is produced by opticalinterference at the interface between the Si~100! substrateand the SLS. Simulation of the long-period modulation at 2.3and 1.5 eV with the SLS approximated by a dielectric me-

FIG. 1. ~a! An image of the Si8Ge7 SLS lattice grown by synchrotron-radiation-excited chemical-beam epitaxy at 250 °C.~b! image of the latticeof a Si/Ge multilayer thermally grown at 600 °C. The bright fields are Silayers and the dark fields are Ge layers.

FIG. 2. Images of~a! the lattice and~b! the overall form of a Si11Ge43 SLSgrown by synchrotron-radiation-excited chemical-beam epitaxy at 250 °C.

462 Appl. Phys. Lett., Vol. 83, No. 3, 21 July 2003 Housei Akazawa

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dium optically equivalent to SLS yielded thickness valuesfor each Ge/Si layer pair (dGe1dSi) of 20 Å for the Si8Ge7

SLS and 74 Å for the Si11Ge43 SLS. These results are inexact agreement with the cross-sectional transmission elec-tron microscope~XTEM! images of Figs. 1~a! and 2~a!.

Figure 3~a! shows the changes in the dielectric responseat 3.4 eV during the growth of the Si8Ge7 and Si11Ge43 SLS.Consecutive half-spirals were traced in the initial stages ofgrowth, after which the trajectory settled into a closed loop.The top layer changed from Si to Ge at point S~K!, and fromGe to Si at point G~L!. To simulate trajectories S→G, G→S, K→L, and L→K, we used virtual interfaceapproximation23 with the substance beneath the virtual inter-face replaced by a pseudosubstrate, the dielectric constant ofwhich is given by the~C,D! angle at the starting point foreach trajectory. The dielectric constant of Si and Ge layersgrowing on pseudosubstrates was chosen to give the calcu-lated trajectory a good fit to the experimental trajectory.

Traces S→P, S→Q, and S→R in Fig. 3~b! represent thegrowth of Ge layers with slightly different dielectric con-stants on pseudosubstrate S. Similarly, traces G→X, G→Y, and G→Z represent the growth of a Si top layer onpseudosubstrate G. The thickness of the Ge layer is 9.0, 10.5,and 12.5 Å at points P, Q, and R, respectively. Thus, weestimatedGe as 10.562 Å. Similarly, the thickness of the Silayer is 8.5, 9.5, and 10.5 Å at points X, Y, and Z, respec-tively. Since points Y and Z are the closest to point S, weestimatedSi51061 Å. These values ofdGe and dSi agreewith the thickness in the XTEM image. Similar trajectoryanalysis was performed for traces K→L and L→K, and

yielded results ofdGe562610 Å anddSi51763 Å, whichagree with the TEM image in Fig. 2~a!.

The dielectric constants («11 i«2) of the respective Gelayers as derived from the simulated curves of fit are (8.460.8)1 i (18.960.5) for the Si8Ge7 SLS and (8.060.3)1 i (18.260.3) for the Si11Ge43 SLS. Similarly, the dielectricconstants of the Si layers are (21.260.3)1 i (22.260.3) forthe Si8Ge7 SLS and (20.560.5)1 i (20.860.5) for theSi11Ge43 SLS. On the other hand, the dielectric constant of aGe~100! substrate at 250 °C is (1161)1 i (2261) while thatof a Si~100! substrate is (2361)1 i (2861). Obviously, the«1 and «2 values of the SLS layer are considerably lowerthan the values for the bulk materials; the reason for this isclosely related to the basic mechanism of photoepitaxy. Thegrowing top layer is hydrogenated Si and Ge, which are pro-duced through organization of the hydride network,7 and thisprecursor state is continuously being converted into crystal-line form. The small«1 and«2 values are consistent with thehydrogenated network thus produced, which will include nu-merous vacancies and H atoms.

In summary, SinGem SLS with a two-dimensional mor-phology and atomically sharp interfaces can be grown bysynchrotron-radiation-excited chemical-beam epitaxy.Atomic-scale roughness at the interfaces between layers willrelieve the misfit strain.

Simulation-derived curves of fit to the real-time SE sig-nals reproduced the thickness values of the Si and Ge layers.The dielectric constants of the thin growing top layers weremarkedly smaller than those for the bulk crystal, which isconsistent with the mechanism of photoepitaxy.

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FIG. 3. ~a! C–D trajectories monitored at 3.4 eV during the growth ofSi8Ge7 ~dotted line! and Si11Ge43 ~solid line! SLS. ~b! Simulation-derivedcurves of fit to the closed-loop part of the trajectory for the Si8Ge7 SLSgrowth. The experimentally observed trajectory is shown by the fine dottedline; curves of fit for S→P, S→Q, and S→R are shown as solid lines, andcurves of fit for G→X, G→Y, and G→Z are shown as thick dotted lines.The dielectric constants of the Ge overlayer were 7.431 i18.53 for trace S→P, 8.391 i18.94 for trace S→Q, and 9.411 i19.29 for trace S→R. Thedielectric constants of the Si overlayer were 23.081 i21.62 for trace G→X, 21.491 i22.34 for trace G→Y, and 20.881 i22.05 for trace G→Z.

463Appl. Phys. Lett., Vol. 83, No. 3, 21 July 2003 Housei Akazawa

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