Experimental study of a surfactant-assisted SiGe graded layer and a symmetrically strained Si,Ge..

Experimental study of a surfactant-assisted SiGe graded layer and a symmetrically strained Si/Ge superlattice for thermoelectric applicationsJ.L.Liu a,*,K.L.Wang a ,C.D.Moore b ,M.S.Goorsky b ,T.Borca-Tasciuc c ,G.Chen caDevice Research Laboratory,Department of Electrical Engineering,University of California at Los Angeles,Los Angeles,CA 90095-1594,USAbMaterials Science and Engineering Department,University of California at Los Angeles,Los Angeles,CA 90095,USA cMechanical and Aerospace Engineering Department,University of California at Los Angeles,Los Angeles,CA 90095-1597,USAAbstractA method to grow high-quality SiGe graded buffer layer is presented.The main concept of the method is to use Sb as a surfactant when growing SiGe graded pared with a Si 0.5Ge 0.5graded sample without Sb surfactant,the Sb-assisted Si 0.5Ge 0.5graded layer has much smoother surface and a signi®cantly lower threading dislocation density.Thermal conductivity of a symmetrically strained Si/Ge superlattice grown on Sb-assisted Si 0.5Ge 0.5graded layer is also presented.q 2000Elsevier Science S.A.All rights reserved.Keywords :Sb surfactant;SiGe graded layers;Symmetrically strained Si/Ge superlattice;Thermal conductivity1.IntroductionSiGe has become an important material for electronic and opto-electronic applications.Recently,much attention has also been paid to study thermoelectric properties of SiGe superlattices [1,2].This attention is obviously stimulated by several predictions.First,Hicks and Dresselhaus [3]predicted that the enhancement in the electronic density of states due to quantum con®nement in quantum wells led to much higher thermoelectric power factors than that of their bulk constituent materials.Then Chen's phonon interface scattering [4]and Balandin and Wang's phonon con®ne-ment engineering [5]predicted much lower thermal conduc-tivity of quantum wells and superlattices than those of their bulk constituent materials.Among all kinds of SiGe super-lattices for thermoelectric applications,mostly recent research seems to indicate that symmetrically strained Si/Ge superlattices may signi®cantly outperform alloy-based SiGe superlattices.A critical issue to grow high-quality,symmetrically strained Si/Ge superlattices is to obtain high-quality,relaxed SiGe buffer layers ®rst.In fact,strain-relaxed SiGe buffers have been successfully realized by at least three methods.One is to grow compositionally graded SiGe layers at high temperatures (typically at 700±9008C)with a typical grading rate of 10%Ge per 1m m [6].The second one is to use a low temperature Si buffer (typi-cally grown at 4008C)on which a SiGe layer of constantcomposition is grown at about 5508C [7].Another one is to introduce impurities,such as carbon,into SiGe layers to adjust the strain [8].For thermoelectric applications,one requires a thin buffer layer in order to minimize its thermo-electric contributions and to optimize thermoelectric prop-erties in the superlattice.However,a relatively thin buffer fabricated by the above methods leads to a high residual strain degree,and/or a large number of threading disloca-tions propagating into the superlattice,which degrade the structural quality of the superlattice.Recently,we have reported the signi®cant improvement of the quality of SiGe graded buffer layers by the introduction of Sb as a surfactant [9].In this paper,we report in detail the method to use Sb as a surfactant to grow a high-quality,relaxed SiGe graded buffer.For comparison,a similar SiGe graded buffer without Sb was also grown.Furthermore,a symme-trically strained Si/Ge superlattice was grown on the Sb-assisted SiGe buffer for thermoelectric applications.2.ExperimentalThe growth was carried out using a solid source molecu-lar beam epitaxy system.Si(100)wafers with a resistivity of 1±30V cm were selected as starting materials.The cleaning procedure included a standard Shiraki cleaning method,followed by an in situ thermal cleaning at 9308C for 15min.The nominal Si growth rate was kept at 1AÊ/s all the time.The Ge growth rate was linearly changed from 0to 1AÊ/s during the graded buffer layer growth while kept at 1A Ê/Thin Solid Films 369(2000)121±1250040-6090/00/$-see front matter q 2000Elsevier Science S.A.All rights reserved.PII:S0040-6090(00)00849-X/locate/tsf*Corresponding author.Tel.:11-310-825-8294;fax:11-310-206-8495.E-mail address:jliu@ (J.L.Liu).s during the superlattice growth.Under these conditions,two graded buffer layers (sample A and sample B)and a symmetrically strained Si/Ge superlattice sample (sample C)were grown.Both sample A and sample B have a 2-m m graded SiGe with a grading rate of 25%Ge per 1m m,followed by a 0.3-m m constant Si 0.5Ge 0.5buffer.Sample A was grown at a temperature of 5108C.Before the graded buffer growth of sample A,one monolayer of Sb was depos-ited on the substrate,which acted as a surfactant during the growth.For comparison,sample B was grown at a tempera-ture of 7008C and no Sb pre-deposition was used.Sample C was grown at 5108C and consisted of a 33-period symme-trically strained Si(70AÊ)/Ge(70A Ê)layer and a buffer,which was very similar to sample A (with Sb mediation)except that the constant Si 0.5Ge 0.5layer was 0.5m m thick.Film relaxation and Ge mole fraction of the buffer samples were measured using X-ray diffraction.The surface morphology and surface roughness of the buffer samples were characterized using atomic force microscopy (AFM)and X-ray re¯ectivity.Threading dislocation densities of the buffer samples were obtained by directly counting etch pits after a Schimmel defect etch [10].Sample C (superlattice)was subject to Raman scattering and X-ray diffraction measurements to assure the high-quality superlattice.The temperature dependence of cross-plane thermal conductiv-ity was measured using a 3v method [11].3.Results3.1.Surfactant-assisted SiGe graded buffer layerThe ability to measure the buffer layer composition and relaxation is essential for the calibration of the growth process and control of the thermoelectric and other beha-viors of the material.In this experiment,the Ge mole frac-tion and the epilayer relaxation have been measured using double axis X-ray diffraction.Fig.1a,b shows the symmetric (004)and asymmetric (224)rocking curves of sample A and sample B,respectively.In the absence of substrate tilt,the position of the layer peak intensity with respect to the substrate peak gives the Ge mole fraction and ®lm relaxa-tion [12].The layer peak position in these scans corresponds to a Ge mole fraction of 0.48and a relaxation of 92%for sample A,and a Ge mole fraction of 0.5and a relaxation of 100%for sample B,respectively.In addition,the intensity of the layer peaks is fairly uniform out to 0.48and 0.5for sample A and sample B,respectively,which indicates a well-controlled linear grading.Surface morphology can be obtained by performing AFM measurements.Fig.2a,b shows the images of sample A and sample B,respectively.As one can see,both samples have a cross hatch pattern,which occurs along k 110l directions and indicates the formation of mis®t dislocations.The difference between sample A and sample B is obvious.The cross hatch lines of sample A are long and straight,while those ofsample B are short and somehow kink,indicating fewer threading dislocations in sample A.Furthermore,the surface root mean square (rms)roughness was estimated from a number of such 20£20m m 2AFM images to beapproximately 20and 161AÊ,for sample A and sample B,respectively.Fig.3shows X-ray re¯ectivity measurement results.The dotted data are experimental data,indicating that the surface of sample A is much smoother than that of sample B.The solid line is a simulation result for sampleA,giving a rms roughness of 20AÊ,in close agreement with that obtained by AFM measurements.Besides surface roughness,threading dislocation density is another parameter for evaluating graded buffer quality.In this experiment,we used the Schimmel defect etch method to determine the threading dislocation densities of the samples.The solution used consisted of one part 75g CrO 3in 1000ml H 2O to two parts 48%HF.Fig.4shows typical Nomarski images of as-etched sample A and sample B.The etching was carried out at room temperature by dipping the samples in the solution for 30s.a -Step measurements indicate that almost 0.55-m m-thick material was etched away,and thus these images were from the upper graded layers of the samples.By counting etch pits in the images,the threading dislocation densities were deter-mined to be 1:5£104and 6£107cm 22for sample A and sample B,respectively.The relatively lower threadingJ.L.Liu et al./Thin Solid Films 369(2000)121±125122Fig.1.(a)(004)and (b)(224)X-ray rocking curves of sample A (with Sb mediation)and sample B (without Sb mediation).dislocation density in the Sb-assisted sample (sample A)may be related to a relatively thinner critical thickness of SiGe on Si due to the use of Sb as surfactant and the low temperature growth process [13].A study of the detailed mechanism is in progress.3.2.Symmetrically strained Si/Ge superlatticeIn this subsection,we focus on the study of sample C,a33-period symmetrically strained Si(70AÊ)/Ge(70A Ê)super-lattice grown on a Sb-assisted graded SiGe buffer with the Ge mole fraction varying from 0to 50%.Fig.5shows a typical X-ray diffraction spectrum of sample C.Two orders of superlattice satellite peaks can be seen in the scanning region.From the peak position,one can conclude that theperiod of the superlattice is about 140AÊ,corresponding to a 70-AÊSi ®lm and a 70-A ÊGe ®lm in each period.The super-lattice peaks are a little bit broad,indicating the Si/Ge inter-faces are somehow diffuse due to a relatively high growth temperature at 5108C.Fig.6shows a typical Raman spectrum of sample C.Thespectrum was collected in a 001(110,110)001Åbackscatter-J.L.Liu et al./Thin Solid Films 369(2000)121±125123Fig.3.X-ray re¯ectivity of sample A (with Sb mediation)and sample B (without Sb mediation).The dotted data are the experimental points,while the solid line is the simulation result for sample A,giving the rms surfaceroughness to be 20AÊ.Fig.4.Nomarski images of (a)etched sample A (with Sb mediation),and (b)etched sample B (without Sb mediation).Both surfaces are from the upper graded layers of the samples.The threading dislocation density is estimated by counting etch pits to be 1:5£104and 7£107cm 22for sample A and sample B,respectively.Fig.2.AFM images of (a)sample A (with Sb mediation),and (b)sample B(without Sb mediation).ing geometry at room temperature.Due to the limited laser light penetration depth of about 0.2m m in the Si/Ge symme-trically strained superlattice,all the signals in the spectrum are believed to come from the superlattice.In the optical phonon region,Si±Si (490.4cm 21),and Ge±Ge (287cm 21)modes originate from the Si and Ge layers,respectively.The phonon positions are mainly affected by the built-in strain [14].The mode at 402cm 21corresponds to Si±Ge vibra-tions at the Si/Ge interfaces.The strength of the Si±Ge inter-face mode is a sensitive probe of interface roughness and/or intermixing [15].In the acoustic region,the arti®cial super-lattice periodicity yields a series of approximately equidi-stant phonon doublets as indicated by the arrows in Fig.6.Since the even-order folded acoustic phonons are forbidden for Si/Ge superlatice with equal Si and Ge layer thicknesses [16],these acoustic phonon peaks (indicated with mid-frequencies)can be designated to the ®rst (84.5cm 21),third (107cm 21),and ®fth (128.5cm 21)longitudinal acous-tic (LA)phonons,respectively.The observation of up to the ®fth order of the acoustic phonons once again proved the high quality of the sample and the strong spatial con®ne-ment of the phonons.In Fig.7,we show a temperature dependence of thermal conductivity of the sample by the 3v method [11].At room temperature,the thermal conductivity of the superlattice is3.1W/mK,in comparison with 145and 11W/mK for Si and Si 0.5Ge 0.5alloys,respectively.A reduction of thermal conductivity in the superlattice was observed compared with that of their bulk constituent materials and their equiva-lent composition alloy layers.As one can see,thermal conductivity changed from 2.7to 3.1W/mK between 77and 300K.The temperature dependence is in good agree-ment with theoretical prediction [4].4.ConclusionSb surfactant-assisted growth of a graded buffer layer has been shown to improve the quality markedly.A Si 0.5Ge 0.5graded layer grown by this method shows a threading dislo-cation density of 1:5£104cm 22and a rms roughness ofapproximately 20AÊ.In contrast,a similar Si 0.5Ge 0.5graded layer without surfactant mediation has a threading disloca-tion density of 6£107cm 22and a rms roughness of 161AÊ.Thermal conductivity measurement on a symmetrically strained Si/Ge superlattice grown on a Sb-assisted Si 0.5Ge 0.5graded layer was performed,showing a reduction in thermal conductivity for the superlattice compared with Si 0.5Ge 0.5bulk alloys.This work indicates that the Sb-assisted relaxed SiGe layer can be used for thermoelectric and other applications.AcknowledgementsThe authors are indebted to Professor M.S.Dresselhaus (MIT)for discussions on material science and low-dimen-sional thermoelectrics.This work was supported by the DoD MURI-ONR program on Thermoelectrics (Dr John Pazik).References[1]S.-M.Lee,D.Cahill,R.Venkatasubramanian,Appl.Phys.Lett.70(1997)2957.[2]G.Chen,S.Q.Zhou,D.-Y.Yao,et al.,17th Int.ThermoelectricsConf.,ICT98,Nagoya,Japan,1998,p.202.J.L.Liu et al./Thin Solid Films 369(2000)121±125124Fig.5.X-ray (004)rocking curve of sample C(superlattice).Fig.6.Typical Raman spectrum of sample C(superlattice).Fig.7.Temperature dependence of thermal conductivity of the superlattice.[3]L.D.Hicks,M.S.Dresselhaus,Phys.Rev.B47(1993)12727.[4]G.Chen,Phys.Rev.B57(1998)14958.[5]A.Balandin,K.L.Wang,Phys.Rev.B58(1998)1544.[6]E.A.Fitzgerald,Y.-H.Xie,D.Monroe,et 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