Surface Science 606 (2012) L64 L68. Contents lists available at SciVerse ScienceDirect. Surface Science

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1 Surfce Science 606 (2012) L64 L68 Contents lists ville t SciVerse ScienceDirect Surfce Science journl homepge: Surfce Science Letters Effect of nitrogen incorportion nd oxygen vcncy on electronic structure nd the sence of gp stte in HfSiO films Moon Hyung Jng,1, Kwng Sik Jeong, Kwun Bum Chung c,jinwoolee d,myeungheelee e, Mnn-Ho Cho,, Atomic-scle Surfce Science Reserch Institute, Yonsei University, Seoul , Repulic of Kore Institute of Physics nd Applied Physics, Yonsei University, Seoul , Repulic of Kore c Deprtment of Physics, Dnkook University, Cheonn , Repulic of Kore d LG Innotek, 379, Gsoo-Dong, Osn, , Repulic of Kore e Deprtment of Physics, Yonsei University, Wonju , Repulic of Kore rticle info strct Article history: Received 18 Octoer 2011 Accepted 9 April 2012 Aville online 20 April 2012 Keywords: HfSiO A initio clcultions Medium energy ion scttering Ner-edge x-ry sorption fine structure Reflection electron energy loss spectroscopy Asence of gp stte The effect of nitrogen (N) incorportion into HfSiO on the electronic structure nd nd lignment of HfSiO films ws investigted. N depth profile dt otined y medium energy ion scttering (MEIS) showed tht the concentrtion of N or the onding or electronic stte of N in the film ws stle when the film ws nneled t 950 C, while the oxygen (O) in HfSiON films ws present in dissocited form, s evidenced y the unoccupied electronic stte of O. The vlence nd offsets of the HfSiO films were strongly ffected y N incorportion due to the presence of N in 2p stte. Moreover, reduction in the conduction nd offset of HfSiO film ws confirmed fter the film ws nneled in n tmosphere of N 2. The unoccupied stte of the O vcncy is responsile for the chnge in the conduction nd offset. The results of -initio clcultions for the density of sttes (DOS) of HfSiO nd HfSiON supercells were in greement with the experimentl results. The incorportion on N into HfSiO prevents the formtion of gp-stte inside the nd gp despite the fct tht n O vcncy is generted in the film Elsevier B.V. All rights reserved. HfSiO hs een investigted s gte dielectric, in n ttempt to improve the moility t the interfce nd the structurl stility of HfO 2. However, survey of the literture indictes tht HfSiO films re seprted into two phses comprised HfO 2 nd SiO 2 t tempertures ove 900 C, due to the high chemicl potentil [1]. One possile solution for eliminting this phse seprtion prolem is the nitridtion of HfSiO, since it would reduce the difference in the chemicl potentil of HfO 2 /SiO 2, pssivte dngling onds t the interfce nd lock the penetrtion of dopnts. In prcticl device friction processes, post nitridtion nneling (PNA) is typiclly used to increse the density of the film nd to reduce lekge current, which origintes from trp sites [2,3]. The conduction nd offset (ΔE c ) nd the vlence nd offset (ΔE v ) etween the dielectric nd chnnel mterils hve recently een exmined, since nd lignment hs criticl ffect on lekge current, which is cused y Schottky emission [4]. Although numer of reserch groups hve investigted the nd lignment of HfSiO s function of N incorportion, there re only few experimentl nd theoreticl results ville concerning the electronic stte nd the Corresponding uthor t: Institute of Physics nd Applied Physics, Yonsei University, Seoul , Repulic of Kore. E-mil ddress: mh.cho@yonsei.c.kr (M.-H. Cho). 1 Current ddress: Deprtment of Mterils Science nd Engineering, University of Pennsylvni, Phildelphi, PA 19104, USA. nd structure of HfSiON films prepred using vrious PNA conditions [5]. The focus of this study ws to clrify the origin of chnges in the electronic stte nd the nd gp (E g ) nd the nd offset of HfSiO using PNA conditions tht re ctully employed in the friction process [6]. HfSiO films were deposited on p-type Si (100) sustrtes in 10 Å-thick SiO 2 lyers y tomic lyer deposition (ALD) t 300 C. Nitridtion nd sequentil post nneling tretments were performed to investigte the effect of PNA on the physicl properties nd the nd lignment of HfSiO s follows: (1) Nitridtion of the s-grown film ws performed in NH 3 tmosphere for 60 s t 750 C () (2). After the process, the film ws sequentilly nneled in N 2 tmosphere for 30 s t 950 C (). The depth profile of the HfSiO films ws determined y MEIS using 100 kev proton em in doule lignment geometry. Ner edge x-ry sorption fine structure (NEXAFS) O/N K-edge spectr were otined t the 7B1 emline t the Pohng ccelertion lortory (PAL). N 1s core level nd vlence spectr were otined y x-ry photoelectron spectroscopy (XPS). Reflection electron energy loss spectroscopy (REELS) spectr were otined using VG ESCALAB 210 pprtus with primry energy of 1000 ev. The -initio clcultions were performed for the following systems: (1) HfSiO nd (2) HfSiON without n O vcncy nd (3) HfSiO nd (4) HfSiON with O vcncies (neutrl, positively chrged nd negtively chrged vcncy: V 0,V +,V 2+,V nd V 2 ) to confirm /$ see front mtter 2012 Elsevier B.V. All rights reserved. doi: /j.susc

2 SURFACE SCIENCE L65 the effect of incorported N nd the O vcncies in the HfSiO nd HfSiON films. It is importnt to determine the extent of the chrged O vcncy ecuse these defects ffect the conduction process tht occurs vi electrons or holes [7]. The supercells contined 24, 48, 95 nd 191 toms in the cses of (1) Hf 4 Si 4 O 16, (2) Hf 8 Si 8 O 28 N 4, (3) Hf 16 Si 16 O 63 nd (4) Hf 32 Si 32 O 111 N 16, respectively. In the cses of N incorportion, N is locted sustitutionlly on O site. It ws necessry to increse the size of the supercells to permit only one O vcncy to e generted in (3) nd (4). Since these supercells hve een verified in numerous previous studies, it would e very useful to nlyze the experimentl ehvior of the HfSiO films using -initio clcultions [8]. The totl energy nd the density of the sttes were clculted using the Vienn A-initio Simultion Pckge (VASP) [9]. To optimize the totl energies nd geometry using pseudopotentils, plne-wve cutoff energy of 450 ev with the Perdew, Burke, Ernzerhof (PBE) exchnge-correltion functionl of the generlized grdient pproximtion (GGA) ws used. The electronic sttes were clculted fter geometricl optimiztion using the sme PBE functionl with plne-wve cutoff energy of 500 ev. Three-dimensionl tomistic visuliztions were crried out using VESTA code [10]. Fig. 1 shows MEIS depth profiles determined y fitting the rw dt. The rw dt with the fitting results re shown in supplementry mteril (Fig. S1) [11]. The Hf to Si rtio ws out 1.0 t the interfcil region, while it ws out 1.3 t the surfce region of n sgrown film. After the process, s shown in Fig. 1(), the totl numer of N toms in the entire film ppered to e 25.9% of the totl numer of O toms in n s-grown film. In the men time, the totl numer of O toms decresed y out 29.2% compred to tht for n s-grown film, indicting tht N redily sustitutes for O in the film, in spite of the fct tht there is 3.3% difference etween them. Previous reports showed tht most of the N rects with SiO 2 in HfSiO, resulting in the formtion of chemicl onds such s N`Si 3 nd Si\O\N. The rection process etween N nd SiO 2 is sed on the exchnge process of O in the SiO 2 y N [12,13]. The tendency for N depth profiling is lmost the sme s the Si profiling in MEIS spectr, which is consistent with the rection process. In films, the depth distriution of N ws similr to tht of Si ecuse N forms 1.2 ) s grown Hf Si O more stle ond with Si. Since Si segregtion occurs on the surfce of the HfSiO film when the nneling is done t temperture of over 900 C, the concentrtion of Hf ecomes incresed y 2 nm 5 nm in the depth region [1]. Although slight segregtion of Si in the surfce direction ws oserved due to the phse seprtion, the chnge in the concentrtion of Si nd N ws miniml fter the, which results in stle stoichiometry in the depth direction. The K edge spectr of O nd N were otined using NEXAFS to verify the unoccupied sttes of O nd N. The chemicl sttes of N in the nd films were lso exmined using XPS N 1s core level spectr. Fig. 2() nd () shows K edge NEXAFS spectr for O nd N for different nneling conditions. These re relted to (Hf 5d+O (N) 2p), (Hf 6sp+O (N) 2p), nd (Si 3sp+O (N) 2p) sttes in the HfSiON films [14]. After nitridtion, the intensity of the O K edge ws decresed ecuse N ws sustituted y O. We lso oserved tht the energy difference etween shoulder nd the min pek is lmost the sme in the NEXAFS spectr for oth the K edge of N nd O ecuse of the sustitutionl exchnge of O y N. Moreover, the pek shpe for O remins unchnged fter the, except for chnge in intensity. This indictes tht nitridtion suppresses chnges in the electronic structure of O y rndomized interction etween π-onding sttes [15]. In ddition, the chnge in the intensity of the O pek indictes tht n O vcncy is generted in the film during the nneling tretment ecuse distorted O Incident c s-grown O K edge N5 N4 N3 N2 N1 N K edge Incident Reltive Concentrtion ) c) N Film Depth (nm) Fig. 1. MEIS depth profiling dt for () s-grown, () nd (c) films Binding Fig. 2. () K edge NEXAFS spectr of O in n s-grown film, n film nd n film. () N K edge NEXAFS spectr of nd films. (c) N 1s XPS core level spectr of nd films. Open circles denote the rw dt nd the solid line superimposed on the rw dt represent the results for the convolution of the fitted components (lines).

3 L66 SURFACE SCIENCE onding in nitrided SiO 2 structure is dissocited t the high nneling temperture. In Fig. 2(), the pek shpe nd intensity of the K edge spectr of N for nd re lmost the sme, indicting tht their electronic structure ws not significntly ltered y the process. The XPS N 1s spectr (open circles) in Fig. 2(c) were deconvoluted with severl N chemicl sttes of N1 to N5. Except for the N1 nd N5 lines, no significnt chnges in intensity were evident. Bsed on the pek position of N 1s for HfSiO x N y (Hf\N_Si 2 ) nd Si 3 N 4 (N`Si 3 ) t nd ev, respectively, the N1 line t out ev is presumly due to N component relted to Si-rich HfSiO x N y [12,13]. In ddition, s discussed ove, the concentrtion of N hs tendency to follow tht of Si. Therefore, this core level stte is estimted to e in Si-rich HfSiO x N y stte. In spite of the N 2 nneling t temperture of 950 C, the N1 intensity of is similr to tht of n film. Comining these dt with nitrogen MEIS profiles, it is ovious tht the N in the film nd interfcil region is stle, even fter n process. According to previous reports, the N2 component t ev is relted to the N component in the SiO 2 mtrix [12]. This indictes tht some of the diffused N hs rected with the interfcil SiO 2 s shown previously in the MEIS profiles. N3 nd N4 components re considered to e N components in nonstoichiometric SiO 2 x N x nd Si 2 ON even though it is difficult to precisely determine the pek energy [16,17]. Becuse the N2, N3 nd N4 components re onded to Si compounds without Hf, they re stle fter nneling. The N5 component t out ev cn e ttriuted to the presence of SiO 2 N sttes [12,16]. REELS spectr nd XPS vlence nd spectr were lso otined to verify the vrition in E g nd the vlence nd offset (ΔE v ). The chnge in E g vlues ws redily oserved, s shown in the REELS dt of Fig. 3(). The vlue of E g for n s-grown film ws 5.34 ev, which is nerly the sme s previously reported experimentl results nd initio clcultions [18 20]. The E g vlues re ruptly decresed fter the ecuse the occupied N 2p stte is closer to the Fermi level thn the O 2p stte in HfSiO film [5]. The slight chnge in E g vlue fter the process indictes tht the electronic structures of the O nd N K edges fter nd re oth mintined, which re consistent with the sorption spectr shown in Fig. 2. Fig. 3() shows XPS spectr for the vlence nd with respect to the vlence nd mximum (VBM) for Si nd extrpolted onset vlues, which correspond to the ΔE v etween HfSiO nd Si. The ΔE v vlue (2.25 ev) of n s-grown film is relted to O 2p sttes in the Hf-sed oxide. As shown in Fig. 3(), fter the nd tretments, ΔE v decreses drmticlly to 1.41 ev nd 1.42 ev, respectively. As discussed ove, these low vlues re originted from the N 2p stte elow E F. Moreover, the sence of ny chnge in the film compred with the film provides evidence tht the electronic structure of N in the nitrided film remins unchnged fter. The evluted vlues of E g, ΔE v nd the clculted conduction nd offsets (ΔE c ) re tulted in Tle 1, where ΔE c =E g ΔE v ΔE g,si.animportntfinding is tht, fter nitridtion (), the mgnitude of the decrese in the vlue of ΔE c is not s lrge s tht of ΔE v [21]. However, the vlues for ΔE c for the film re decresed to 0.21 ev, compred to tht for the film. This decrement cn e ttriuted to the O vcncy in HfSiON which is locted djcent to Hf nd Si sites, ecuse the energy stte for the O vcncy is locted elow the conduction nd. Thus, the chnge in E g fter cn e cused y the formtion of n O vcncy s defect stte. To investigte the role of N incorportion nd O vcncies more precisely, -initio clcultions were crried out. Fig. 4() shows the clculted totl density of sttes (DOS) nd E g for severl HfSiO nd HfSiON systems with nd without n O vcncy. In the cse of the HfSiO system, the vlue for E g is estimted to e 4.67 ev, which is lower thn the experimentlly determined vlue for the s-grown HfSiO film ecuse the locl density pproximtion (LDA) is underestimted. Hence, the discussion is confined, not to the exct vlue of E g Intensity (Ar. Units) Primry Energy = 1.0 kev E g 5.34 ev 4.36 ev 4.16 ev s grown Energy Loss (ev) s grown 2.25 ev 1.41 ev 1.42 ev Energy Reltive to VBM of Si (ev) Fig. 3. () REELS spectr of s-grown, nd films denoted with E g vlues. () XPS vlence spectr s function of the energy reltive to VBM of Si for sgrown, nd films. Vlues of ΔE v re denoted with smple conditions. ut the chnge in E g, which is relted to N incorportion nd the O vcncy in DOS. In the HfSiON system, the E g is reduced to 3.62 ev which corresponds to the lrge reduction in E g in the film. The prtil DOS (PDOS) of the HfSiON system (Fig. 4()) indictes tht the electronic sttes of N in the vlence nd re responsile for the reduction of E g, s discussed ove for the experimentl results. Considering the O vcncy in the film, we lso estimted the vlue for E g in the HfSiON system with n O vcncy. The decrese in the Tle 1 Bnd gp nd nd offset for HfSiO films on n s-grown film, n film nd n film. As-grown E g E v E c E v

4 SURFACE SCIENCE L67 Density of Sttes (r. units) HfSiON V 2- V 2+ V - V + V ev 2.56 ev 2.32 ev 2.57 ev 2.56 ev 3.62 ev Density of Sttes (r. units) c Density of Sttes (r. units) d O PDOS N PDOS Hf PDOS Si PDOS O PDOS N PDOS Hf PDOS Si PDOS HfSiON Totl DOS HfSiON with V 2+ Totl DOS V ev Hf HfSiO 4.67 ev O N Si Fig. 4. () Totl density of sttes (DOS) in HfSiO system nd HfSiON system with/without n O vcncy. The two lower spectr re for the HfSiO system nd others re for the HfSiON system. The nd gp vlue is lso indicted. () Prtil DOS (PDOS) in HfSiON system without O vcncy. (c) PDOS in HfSiON system with positively chrged O vcncy (V 2+ ). (d) Supercell of the HfSiON system with positively chrged O vcncy (V + ). N toms tht re locted next to the O vcncy re indicted s open red circles. vlue of E g is not ffected y the chrge in the O vcncy (neutrl, positively chrged nd negtively chrged), s shown in Fig. 4(). In ddition, the findings indicte tht the origin of this chnge is from the conduction nd of the system. The PDOS in the HfSiON system with positively chrged vcncy (V 2+ )infig. 4(c) shows tht the lowering of the conduction nd offset is relted minly to Hf PDOS s discussed ove. However, no evidence ws found for the existence of defect sttes (O vcncy) within the nd gp in the HfSiON system (Fig. S2 in Ref. [22]) while the HfSiO system with n O vcncy hs gp-stte inside the nd gp, s shown in Fig. 4(). Xiong et l. reported on the formtion of VN 2 complex which consists of two N toms nd n O vcncy in the theoreticl results for n N incorported HfO 2 system [23]. The theoreticl results lso show tht the gp-stte is not generted ecuse of the closed shell properties of the VN 2 complex. In the cse of HfSiON systemwithnovcncy,sshowninfig. 4(d) (HfSiON with V + ), the sme sitution holds. N toms re pired next to the vcncy nd the formed VN 2 complex, s indicted y the open red circles. Therefore, we propose tht the sence of gp-stte in the HfSiON system with n O vcncy cn e explined y the formtion of VN 2 complex. The sence of gp stte cn minimize the electron trp inside the nd gp which could susequently ffect the lekge current with Schottky emission. In summry, the effects of N incorportion nd n O vcncy on the electronic stte of HfSiO films were investigted. The incorportion of N results in reduction in E g nd ΔE v ecuse N 2p sttes re min constituents of VBM. In ddition, the most influentil fctor for determining the vlue for ΔE c is the presence of O vcncies in the film ecuse of the unoccupied Hf electronic stte. The sence of gp-stte within the nd gp origintes from the VN 2 complex in HfSiON system with n O vcncy. Acknowledgment This work ws prtilly supported y the Joint Progrm for Smsung Electronics-Yonsei University nd the IT R&D progrm of MKE/ KEIT ( , Development of novel 3D stcked devices nd core mterils for the next genertion flsh memory ). We grtefully cknowledge the technicl dvice of K. Che, C. C. Hwng, nd H.-N. Hwng t PAL on emline 7B1. Appendix A. Supplementry dt Supplementry dt to this rticle cn e found online t dx.doi.org/ /j.susc

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