Abstract. 1. Introduction. IEEE International Conference on Emerging Technologies September 17-18, Islamabad

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1 A. Rehmn Khn*, K Mundboth*, J. Stngl*, G. Buer*, H. von Känel, A. Fedorov, G. Isell, D. Colombo *Institute of Semiconductor Physics, Johnnes Kepler University, Linz, Austri INFM nd L-NESS Diprtimento di Fisic, Politecnico di Milno, Polo Regionle di Como, Itly Diprtimento di Scienz dei Mterili, Università degli Studi di, Milno-Bicocc, Itly Tomsk Stte University, Tomsk, Russi Abstrct We present n investigtion of series of smples with GAs/InGAs lyers grown on Ge/Si pseudo-substrtes. The strin in the GAs nd InGAs lyers were clculted fter mesuring the lttice constnts of the lyers using the x-ry diffrction technique. Both lyers were found to be under low tensile stress resulting from the lttice mismtch nd the difference in therml expnsion coefficients between the djcent lyers. We explore new concept bsed on counterblncing this therml stress. 1. Introduction III-V semiconductors hve shown mny options for dvnced electronic nd optoelectronic pplictions. For instnce, GAs hs n edge over Si due to its properties like direct bnd gp, higher mobility, sturted drift velocity nd high resistivity. In the pst, there hve been mny ttempts to combine device qulity GAs-bsed hetrostructures with Si technology [1-3], with, however, limited success so fr. An importnt reson is the lrge lttice mismtch (4.1%) nd the difference in therml expnsion coefficients (~60%) between GAs nd Si, which leds to high disloction densities (threding disloction densities on top of the GAs lyers re typiclly lrger thn 10 7 cm 2 ) leding to inferior performnce nd poor relibility in GAs/Si devices [4]. Due to the remining strin even in prtilly relxed structures crck formtion cn occur [5, 6]. In ddition to the lttice mismtch, GAs/Si growth is polr on nonpolr growth giving rise to nti-phse domins (APDs) [7-11]. As result, both the electronic nd opticl properties of the structures re ffected nd hence the qulity of GAs devices on Si is degrded. In order to enhnce the flexibility in the strin design, relxed lyers such s SiGe lyers re used s pseudosubstrte for subsequent ctive Si or SiGe lyers in the SiGe system. In mny cses, grded SiGe buffer lyers with constnt composition buffers on top re used s such pseudosubstrtes. Using rther elborte preprtion techniques, low threding disloction densities s smll s cm 2 hve been chieved [12], but buffers fbricted by this pproch suffer from very long growth times due to the lrge thickness required, mechnicl stresses due to therml strin (see below) nd bending of wfers, which is problem in production lines. Furthermore, these lyers exhibit strong so-clled crosshtch pttern, consisting of two perpendiculr systems of ridges due to buried disloction pileups with mplitudes up to 100 nm, depending on the thickness nd composition of the grded lyer. While this cross-htch, which is incomptible with the fbriction of integrted circuits, cn be removed by chemicl-mechnicl polishing step [13-14], it reppers during nneling, so tht the therml budget of such smples during further processing is rther limited. Recently, technique being ble to produce SiGe nd Ge buffer much fster hs been presented. In this lowenergy plsm-enhnced chemicl vpour deposition (LEPECVD), growth rtes up to 50 times lrger thn in conventionl buffer growth cn be chieved [15]. Furthermore, it is possible to grow pure Ge buffers without grded SiGe buffers underneth, which reduces the overll pseudosubstrte thickness to bout 1 µm. In comprison to other CVD techniques, nother dvntge of LEPECVD is the much better exploittion of crrier gses of bout 20% compred to only few % in CVD [16]. Ge buffers hve lso been suggested s templte for GAs growth on Si. As GAs nd Ge hve only very smll lttice mismtch, using Ge buffer s virtul /05/$ IEEE 323

2 substrte will not led to lrge number of misfit disloctions t the interfce. Still, however, the problem of polr on non-polr growth is n importnt issue. Recently, it hs been shown tht very flt Ge lyers cn be grown on Si using ultrhigh vcuum chemicl-vpor-deposition (UHV/CVD) [17] nd the Ge film thus grown hs low root-men-squre roughness vlue of bout 0.5 nm. A very importnt issue is the therml strin introduced by cooling the buffers from growth temperture to mbient temperture. Even lthough those strin vlues re not very lrge, the lyers re comprtively thick, nd hence the totl strin energy cnnot be neglected. GAs hs higher therml expnsion coefficient thn Si, nd if GAs lyer is completely relxed t growth temperture, it will contrct during the cooling process by the sme mount s the Si substrte, which is less thn the intrinsic therml contrction of GAs (the sme is true for Ge nd other lyers, s ll epilyers re much thinner thn the Si substrte). At lower tempertures the introduction of disloctions, their multipliction nd glide is hmpered, hence the resulting strin is not relieved. In this pper, we ddress the issue of this therml strin in series of GAs/Ge/Si smples fbricted using the most recent concepts of buffer growth. All studied Ge/Si pseudosubstrtes hve been grown in INFM nd L-NESS Diprtimento di Fisic, Politecnico di Milno, Polo Regionle di Como by the LEPECVD method. In order to reduce the problem of phse domins, the virtul substrtes re grown on Si [001] substrtes with 6 miscut towrds [110]. These pseudosubstrtes hve been subjected to different therml tretments s described below, to reduce the threding disloction density t the surfce [18]. In order to reduce the therml strin, n ttempt ws mde to compenste it with the strin in n dditionl InGAs lyer: s InGAs hs lrger lttice prmeter thn GAs nd Ge, pseudomorphic InGAs lyer (with rther low In content of bout 1%, depending on the lyer thicknesses) would exhibit compressive strin tht cn compenste the tensile therml strin [5,19]. However, the precise control of such low In contents is rther demnding technologiclly. The im of the presented work is to chrcterize the GAs buffer smples with respect to the residul strin, nd identify the optimum growth conditions. We use x-ry diffrction round symmetricl nd symmetricl Brgg reflections in order to ssess the strin stte of the individul lyers. En route, we lso determine precisely the InAs content in the InGAs lyers nd n eventul residul Si content in the Ge lyers, which is importnt to optimize buffer growth. The XRD results re correlted to tomic force microscopy (AFM) investigtions of the smple surfce, from which we ssess the surfce roughness nd morphology, in prticulr the eventul presence of cross-htch pttern. 2. Smple description Figure 1. Smple structure. Two smples hd 3 µm of GAs cp lyer (vk67 nd vk69) while two others (vk66 nd vk68) did not The smple preprtion strted with the deposition of nominlly pure Ge buffer lyer (1 µm) onto 4-inch Si [001] wfers misoriented by 6 towrds [110]. The Ge buffer lyer ws grown by LEPECVD t rte of bout 4 nm/s nd t substrte temperture of 600 C. The wfers were then exposed to ir nd cleved into smller pieces fter pplying protective photoresist coting. The pieces of pseudosubstrte N7329 nd N7328 hve been used for growth of the smples vk66, vk68, vk69 nd vk67 respectively. Both N7328 nd N7329 hve been grown under the sme growth prmeters. Tble 1. Therml tretment of the investigted smples. Smple Ge/Si substrte Structure of III-V tretment over lyers vk66 thermocycling* 3.5 µm InGAs vk67 nneling** 1.0 µm InGAs/ 2.5 µm GAs vk68 nneling** 3.5 µm InGAs*** vk69 thermocycling* 0.5 µm InGAs/ 3.0 µm GAs * heting from T = 650 o C to 870 o C (dt/dt = 30 /min), nneling t T = 870 o C during 5-6 min, cooling to T = 650 o C with the sme dt/dt. The procedure is repeted twice, nd then T is decresed to growth temperture ** nneling t 800 o C during 15 min *** substrte cooled to 20 o C fter nneling of Ge buffer 324

3 After resist stripping nd brief exposure to oxygen plsm the pieces were loded into n EPI Gen II MBE system nd subjected to vrious het tretments s detiled in Tble 1. After tht, lyer of InGAs ( µm) ws deposited. Two of the four smples were cpped with 3 µm thick GAs lyer, while two were left uncpped. All the III-V lyers were grown t bout 600 C. Fig. 1 shows the smple structure. clculted, s well s the tilt of the epilyers, from the symmetric reflection we ccess dditionlly the lttice prmeter prllel to the smple surfce (note tht we refer to the (004) Brgg point s symmetricl diffrction, lthough strictly this is not true in the zimuth where the smple hs 6 miscut; however, this introduces only negligible difference in the obtined tilt nd lttice prmeter vlues). As result, the strin stte nd the composition of the unit cell in the lyer re obtined. 3. Experiment All x-ry mesurements were performed on highresolution diffrctometer t the University of Linz, Austri. A conventionl Cu node x-ry tube (wvelength λ = Å) operted t 40 kv nd 40 ma hs been used together with four-crystl Ge chnnel cut monochromtor ligned to (220) Brgg reflection. The intensities were recorded using point detector. We recorded reciprocl spce mps (RSMs) in coplnr XRD geometry. In this geometry, the incident nd scttered wve vectors, k i nd k f, nd the surfce norml n, lie in one plne in the reciprocl spce. In order to record RSM, series of ω-2θ scns t vrious ω positions were performed round different reciprocl lttice points [20]. Diffrction occurs when the diffrction vector Q = k f k i is equl to reciprocl lttice vector G. The reltion between Q nd the ngles ω (ngle between k i nd the smple surfce) nd 2θ (scttering ngle, i.e., ngle between k i nd k f ) is given below, s well s the link to the inplne ( ) nd perpendiculr lttice ( ) prmeters (the expressions re vlid for tetrgonlly distorted lyers with surfce norml long the cubic [001] direction, nd the in-plne component of Q long the [110] direction). We recorded mps round the (004) nd (115) Brgg points in two orthogonl [110] zimuths. The (115) reflection ws chosen s the (224) reflection, which would llow for higher in-plne strin resolution of the experiment, is not ccessible due to the high miscut of the smples. Precise mesurements of the vlue of Q corresponding to the individul lyers re mde from the RSMs. We clculte the lttice prmeters of the different mterils in the lyers from the following reltions. 4π 2π Q = θ ( ω θ ) = h 2 + k 2 sin sin λ (1) 4π 2π Q = sinθ cos( ω θ ) = l λ From the symmetric reflection, the lttice prmeter perpendiculr to the (001) diffrcting plnes, cn be Figure 2. Reciprocl spce mps round (115) reflection of smples vk67 (,b), nd vk69 (c,d). The close-up in (b,d) shows the intensities from GAs, Ge nd InGAs lyers. The InGAs nd GAs lyers were deposited on nominlly pure Ge buffer lyers. There being only slight difference in lttice constnts between GAs ( Å) nd pure Ge ( Å), the two peks prtly overlpped for some RSMs. To obtin higher precision in the lttice prmeter vlues of GAs nd InGAs, we mesured dditionl RSMs round (002), (204), nd (226) reflections, which re llowed for InGAs nd GAs, but forbidden for Si nd Ge. The resulting overll error br in the obtined lttice prmeters re bout ± Å. 325

The RSMs (115) of smples vk67 (pnels,b) nd vk69 (pnels c,d) re shown in Fig. 2. Beside the substrte pek lbelled Si, the peks from the individul lyers cn be observed (see close-ups in pnels b,d).

Wheres the mxim of the scttered intensities for the Ge nd InGAs peks pper nerly for the sme Q vlues, the corresponding ones for the GAs peks re shifted to lrger Q s.

determined the InAs composition of the InGAs buffer lyer. To clculte the lttice prmeter of Si 1 x Ge x we use the empiric expression 2 = Si + 0.2 x + 0.027 x.

4 The RSMs (115) of smples vk67 (pnels,b) nd vk69 (pnels c,d) re shown in Fig. 2. Beside the substrte pek lbelled Si, the peks from the individul lyers cn be observed (see close-ups in pnels b,d). While for smple vk67 the three peks re clerly distinguished, for smple vk69 this is not the cse. Wheres the mxim of the scttered intensities for the Ge nd InGAs peks pper nerly for the sme Q vlues, the corresponding ones for the GAs peks re shifted to lrger Q s. Consequently, from the inspection of Fig. 2 (b,d) we deduce tht the InGAs lyers grow pseudomorphiclly on the Ge pseudosubstrtes, wheres the GAs top lyers re t lest prtilly relxed. compressively strined. represents the completely relxed lttice constnt of the considered mteril, depending on its composition: in our nlysis we took into ccount possible intermixing of Si nd Ge for the Ge buffer, nd determined the InAs composition of the InGAs buffer lyer. To clculte the lttice prmeter of Si 1 x Ge x we use the empiric expression 2 = Si x x. (3) From this, the Ge content is determined by numericlly solving the reltion 2C 11 ( x) = ( x) + [ ( x) C ( ) ], (4) 12 x Figure 3. AFM imges of the four smples Atomic force microscopy is used to obtin quntittive, three-dimensionl imges of the smple surfces with high resolution. We used Dimensions 3000 AFM from Digitl Instruments in non-contct mode to obtin the surfce morphology of our smples, since in this mode the probe opertes in ttrctive force region nd tip-smple interction is minimized. Imges were tken with different rnges; Fig. 3 shows typicl imges t rnge of 5 5 µm Results nd discussion The strin with respect to mteril is defined s, ε, =, (2) where ε nd ε denote the inplne nd perpendiculr strin, respectively. ε tkes positive vlues for lyer enduring tensile strin nd negtive vlues if the lyer is where C11 nd C 12 re the elstic constnts of Si1 xge x, linerly interpolted between the vlues of pure Si nd Ge. For InAs nd GAs we use equivlent expressions, but ssuming liner vrition of lttice prmeter with InAs content. Tble 2 summrizes the results obtined for the different smples. The vlues of inplne lttice constnt nd the perpendiculr lttice constnt for ll the lyers bove the substrte hve been mesured nd tbulted. Also incorported re the vlues of relxed lttice constnts of ech lyer. The concentrtion of Ge in the SiGe buffer lyer nd tht of In in the InGAs hs lso been determined nd tbulted. For the Ge buffers, we found only insignificnt devitions of pure Ge from the min peks, lthough some RSMs show wek diffuse intensity extending towrds the Si pek, indicting tht the most prt of the buffer is pure Ge, while some interdiffusion t the interfce between Si nd Ge is present. The obtined InAs contents re significntly lrger thn intended for ll smples, nd lso some peks in the RSMs corresponding to InGAs show wek diffuse scttering extended towrds smller Q, corresponding to higher InAs contents. A min result of the lttice prmeter determintion is tht while ll epilyers in ll smples re lmost completely relxed to their lttice prmeters, still the in-plne lttice constnt is lrger thn the perpendiculr lttice constnt, i.e., ll lyers exhibit tensile strin. While for prtilly relxed epilyers with lttice prmeter lrger thn tht of the substrte compressive strin is expected, the tensile strin cn result from differences in the therml expnsion coefficients of djcent lyers, s explined bove. 326

5 Tble 2. Results of lttice constnts obtined from XRD mesurements. The expected strin (ε e ) hs been clculted nd compred to the experimentl (observed) (ε o ). All of them hve been clculted with respect to the Si substrte SAMPLE vk66 vk67 vk68 vk69 LAYER LATTICE CONSTANTS (Å) EXPERIMENTAL INPLANE STRAIN ε,thermal (%) (%),therml ε SiGe x Ge = 99.6% InGAs x In = 1.3% SiGe x Ge =100% InGAs x In = 2.0% GAs SiGe x Ge = 99.3% InGAs x In = 2.0% SiGe x Ge = 98.8% InGAs x In = 2.2% GAs SURFACE ROUGHNESS (RMS) NM In order to estimte this therml strin, we ssumed tht ll lyers re completely relxed t growth temperture, nd during cooling no chnge in the disloction network occurs. From this considertion, the vlues of in-plne lttice constnts,therml follow s listed in Tble 2, nd from them the in-plne strin vlues ε,therml re derived. In relity, this clculted ε,therml represent rther n upper limit to the observed vlues, s some disloction rerrngement will still occur during the cooling. From the listed vlues, exctly this is observed: for most lyers, the experimentl strin vlues re slightly smller thn those estimted theoreticlly. For some lyers the opposite is true, however, the differences re rther smll nd in the order of our experimentl errors. Hence overll the therml strin does very well explin the observed tensile strin vlues. No systemtic differences due to the therml tretment or the thickness of the InGAs buffers re observed. However, it is noteworthy tht the in-plne lttice prmeters of the InGAs lyers mtch quite closely those of the underlying Ge buffers. Consequently, the concept of introducing non-relxed InGAs lyers is fesible. The in-plne lttice prmeters of the top GAs buffers in smples vk67 nd vk69 does not follow tht of the InGAs buffers, but indicte t lest prtil relxtion of GAs t growth temperture. This result is corroborted by the AFM dt. We observe only fint cross-htch-like pttern, over-shdowed by trils of furrows nd ridges in smple vk67, while in smple vk69 cross-htch is hrdly detectble, nd completely bsent in the smples without GAs lyer. Hence it is cler tht under the pplied growth conditions the GAs lyers do not remin pseudomorphic on top of InGAs, but plsticlly relx. The r.m.s. roughness vlues obtined from AFM nd listed in Tble 2 s well show rther low vlues, indicting tht chemicl-mechnicl polishing process seems not to be required in the presented buffer growth method. 5. Conclusion Studies of series of GAs/InGAs/Ge smples grown on 6 miscut Si (001) wfers hve been crried out. We observe tht fter pproprite therml tretment 1 µm Ge buffer lyers fbricted t high rte by LEPECVD ct s 327

6 high-qulity pseudosubstrtes for further GAs/InGAs growth. It is shown tht InGAs cn be grown pseudomorphiclly, in order to compenste the tensile strin due to different therml expnsion of epilyers nd substrte for the finl GAs lyers. The presented method bsed on x-ry diffrction nlysis is sensitive to mesure even these smll thermlly induced strin vlues. The top GAs lyers re prtilly relxed. With reduced In content of the InGAs lyer one cn envision pseudomorphic GAs growth s well nd hence complete compenstion of the tensile therml strin. Acknowledgements: The work of A. Rehmn Khn nd K. Mundboth ws supported by Higher Eduction Commission (HEC) Pkistn nd FWF Vienn, respectively. *Emil: liy.rehmn@jku.t References [1] J.A. Crlin, et l, Impct of GAs buffer thickness on electronic qulity of GAs grown on grded Ge/GeSi/Si substrtes Appl. Phys. Lett. 76, 1884 (2000). [2] S.M.Ting et l Metl-orgnic chemicl vpor deposition of single domin GAs on Ge/GexSi1-x/Si nd Ge substrtes, J. Appl. Phys. 87, 2618 (2000) (nd references quoted therein). [3] S. Ski, T. Sog, M. Tkeysu, M. Umeno, AlGAs/gAs DH Lsers on Si Substrte Grown Using Super Lttice Buffer Lyers by MOCVD Jpn. J. Appl. Phys. Lett. 24, L666 (1985). [4] D. G. Deppe, N. Holonyk Jr., D. W. Nm, K. C. Hsieh, G. S. Jcson. R. J. Mtyi, H. Shichijo, J. E. Epler, H. F. Chung, Room-Temperture continuous opertion of p-n Alx G 1 x As GAs quntum well hetrostructures lsers grown on Si Appl. Phys. Lett. 51, 637 (1987). [5] Y. Chiqrui, G. Sint-Girons, G. Isell, H. von Känel, S. Bouchoule, nd I. Sgnes, Long wvelength room temperture lser opertion of strined InGAs/GAs quntum well structures monolithiclly grown by metlorgnic chemicl vpour deposition on low energy plsm enhnced chemicl vpour deposition grded misoriented Ge/Si virtul substrtes Opticl mterils, 27, pp (2005) [6] V.K. Yng, M.E Groenert, C.W. Leitz et l Crck formtion in GAs heteroepitxil film on Si nd GeSi virtul substrte J. Appl. Phys (2003) [7] Gutkovsky A K, Ktkov A V, Ktkov M I, Pchelykov O P nd Revenko M A Effect of G predeposition lyer on the growth of GAs on vicinl Ge (001) J. Cryst. Growth 201/ , [8] Agrwl S K, Tygi R, Singh M nd Jin R K Effect of growth prmeters on the MOVPE of GAs/Ge for solr cell pplictions Sol. Energy Mter. Sol. Cells (1999) [9] Y. Hsegw, T. Egw, T. Jimbo, M. Umeno, AlGAs/GAs light-emitting diode on Si substrte with self-formed GAs islnd ctive region grown by droplet epitxy Appl. Phys. Lett. 68, 523 (1996). [10] H. K. Choi, C. A. Wng, N. H. Krm, GAs-bsed diode lsers on Si with incresed lifetime obtined by using strined InGAs ctive lyer Appl. Phys. Lett. 59, 263 (1991). [11] M. Albrecht et l, Surfce ripples, crosshtch pttern, nd disloction formtion: Cooperting mechnism in lttice mismtch relxtion Applied Physics Letters Volume 67, Issue 9, pp [12] M.E. Groenert, C.W. Leitz, A.J.Piter, et l Monolithic integrtion of room-temperture cw GAs/AlGAs lsers on Si substrtes vi relxed grded GeSi buffer lyers J. Appl. Phys (2003) [13] G.Gbett, C. Flores, R. Cmpesto, C. Csle, G. Timo, G. Smekens, J. Vnbegin, H. von Knel, G. Isell SJ nd TJ GAs concentrtor solr cells on Si virtul wfers IEEE Photovoltics Specilist Conf., Brcelon( Spin), 2005 [14] M.E. Groenert, A.J.Piter, R.J. Rm, E.A. Fitzgerld J. Vc.Sci.Technol B21 (3) (2003) 1064 [15] C. Rosenbld, H. von Känel, M. Kummer, A. Dommnn, nd E Müler, A plsm process for ultr fst deposition of SiGe grded buffer lyers Appl. Phys. Lett 76, 427 (2000) [16] US Ptent no. 6,454,855 to von Känel et l. [17] L. Colce, G Msini, F Glluzi, G Assnto, G Cpellini, L. Di Gspre, nd F. Evngelisti, Solid Stte Phenom. 54, 55 (1997) [18] H.C. Lun High-qulity Ge epilyers on Si with low threding- disloction densities Appl. Phys. Lett (1999) [19] Y.Tcno, T.Kukuri, K. Kuwhr, nd S. Fuke Residul strin nd threding disloction density in InGAs lyers grown on Si substrte by metlorgnic vpor-phse epitxy Appl. Phys. Lett. 78 (2001) [20] E. Koppenstensteiner, G. Buer, H. Kibbel, nd E. Ksper Investigtion of strin-symmetrized nd pseudomorphic Si m G n superlttices by x-ry reciprocl spce mpping J. Appl. Phys. 76, 3489 (1994) 328

What is thin film/layer?

What is thin film/layer? High-esolution XD Wht is thin film/lyer? Mteril so thin tht its chrcteristics re dominted primrily by two dimensionl effects nd re mostly different thn its bulk properties Source: semiconductorglossry.com