Challenges in epitaxial growth of SiGe buffers on Si (111), (110 ...

M.L. Lee et al. / Th<strong>in</strong> Solid Films 508 (2006) 136–139 139higher defect density. Fig. 5 is an XVTEM <strong>of</strong> Si 0.75 Ge 0.25grown on Si(111) at 650 -C show<strong>in</strong>g a high density <strong>of</strong> planardefects accompanied by numerous partial dislocations. Notably,the planar defects (which occur along <strong>in</strong>cl<strong>in</strong>ed {111}-habitplanes) and their associated dislocation cores, are found topenetrate well <strong>in</strong>to the Si substrate. The facetted surfaceroughness seen <strong>in</strong> Fig. 5 is caused by surface step bunch<strong>in</strong>gthat arises from a slight <strong>of</strong>fcut <strong>of</strong> the wafer towards an <strong>in</strong>-plane direction [12,13].High-resolution TEM and selected area diffraction patterns(Fig. 6) reveal that the planar defects <strong>in</strong> Figs. 4 and 5 aremicrotw<strong>in</strong> lamella. A microtw<strong>in</strong> can be formed by the shear<strong>in</strong>gaction <strong>of</strong> numerous partial dislocations on adjacent glide planes[14,15], and gIb contrast analysis done <strong>in</strong> planar-view demonstratesthat microtw<strong>in</strong>s like the one shown <strong>in</strong> Fig. 6 are composed<strong>of</strong> long, parallel 90- Shockley partial segments. Thus, thepresence <strong>of</strong> dislocations deep with<strong>in</strong> the substrate is caused bymutual repulsion between the numerous like-sign dislocations.The deepest dislocation was probably the 1st dislocationnucleated from the surface, and it was subsequently repelled<strong>in</strong>to the substrate by later-nucleated dislocations. Therefore, <strong>in</strong><strong>SiGe</strong> grown on Si(111) and (110), microtw<strong>in</strong>s are formed byrepeated surface nucleation <strong>of</strong> 90- Shockley partials, and not bythe separation <strong>of</strong> pre-exist<strong>in</strong>g 60- dislocations.4. DiscussionThe primary problem with compressive films grown on(111) and (110) is that the nucleation and glide <strong>of</strong> 90- Shockleypartials is a significant mechanism for stra<strong>in</strong> relief. For theseorientations, the glide planes are oriented such that the 90-partial leads and also has a larger Schmid factor than the 60-total dislocation [0.31 vs. 0.27 for (111) substrates and 0.47 vs.0.41 for (110) substrates]. A 90- Shockley partial dislocationhas lower b 2 energy and a relatively larger stra<strong>in</strong>-reliev<strong>in</strong>gcomponent than a 60- total dislocation, s<strong>in</strong>ce the 90- partialresolves only <strong>in</strong>to tilt and stra<strong>in</strong>-reliev<strong>in</strong>g components, whilethe 60- total dislocation possesses an additional screwcomponent. The higher Schmid factor also means that theactivation energy for nucleation <strong>of</strong> a 90- partial dislocationshould be significantly lower than that <strong>of</strong> a 60- totaldislocation. Another situation <strong>in</strong> mismatched heteroepitaxywhere microtw<strong>in</strong>s are frequently encountered is <strong>in</strong> the <strong>growth</strong><strong>of</strong> tensile-stra<strong>in</strong>ed films on (001), where, aga<strong>in</strong>, the 90- partialleads [14,15]. S<strong>in</strong>ce microtw<strong>in</strong> defects <strong>in</strong> the films studied hereappear to arrest the glide <strong>of</strong> other dislocations, the nucleationand propagation <strong>of</strong> planar defects must be avoided <strong>in</strong> order toatta<strong>in</strong> high-quality <strong>epitaxial</strong> materials.While grad<strong>in</strong>g rates <strong>of</strong> 10–20% Ge/Am can be successfullyused for the <strong>growth</strong> <strong>of</strong> low-TDD (001) <strong>buffers</strong>, it is clear thatthe (111) and (110) substrates cannot tolerate such high stra<strong>in</strong>rates. Still, it rema<strong>in</strong>s possible that high quality films could beatta<strong>in</strong>ed on (111) or (110) wafers by us<strong>in</strong>g extremely slowgrad<strong>in</strong>g rates (e.g. 1% Ge/Am), as has been shown for tensilegraded GaAsP <strong>buffers</strong> on GaAs(001) [16]. The appropriategrad<strong>in</strong>g rate for avoid<strong>in</strong>g high TDD on (112) substratesprobably lies between that <strong>of</strong> (001) and (111)/(110).5. ConclusionsIn general, low mismatch <strong>SiGe</strong> layers grown on Si (111),(110) and (112) exhibit TDDs that are >10 higher than thosegrown on (001). S<strong>in</strong>ce relaxation processes on these substrateorientations tend to <strong>in</strong>volve the nucleation and glide <strong>of</strong> partialdislocations and the formation <strong>of</strong> stack<strong>in</strong>g faults and microtw<strong>in</strong>s,achiev<strong>in</strong>g high quality <strong>SiGe</strong> buffer layers is <strong>in</strong>tr<strong>in</strong>sicallymore challeng<strong>in</strong>g than on (001). Avoid<strong>in</strong>g nucleation <strong>of</strong>stack<strong>in</strong>g faults <strong>in</strong> graded <strong>buffers</strong> on (111), (110) and (112)may require the use <strong>of</strong> stra<strong>in</strong> rates considerably lower thanthose typically used on (001).AcknowledgementsWe gratefully acknowledge fund<strong>in</strong>g from the MARCOMaterials, Structures, and Devices focus center. We also thankC.N. Chlerigh for perform<strong>in</strong>g RBS measurements and S. Guptafor assistance with XRD measurements. This work made use <strong>of</strong>the Shared Experimental Facilities supported by the MRSECProgram <strong>of</strong> the National Science Foundation under awardnumber DMR 02-13282.References[1] S.-i. Takagi, A. Toriumi, M. Iwase, H Tango, IEEE Trans. ElectronDevices 41 (1994) 2363.[2] R. Hull, J.C. Bean, Crit. Rev. Solid State Mater. Sci. 17 (1992) 507.[3] M. Yang, E.P. Gusev, M. Ieong, O. Gluschenkov, D.C. Boyd, K.K. Chan,P.M. Kozlowski, C.P. 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