Thin Film Scattering: Epitaxial Layers

Transcription

1 Thin Film Scttering: Epitxil yers Arturs Vilionis GAM, Stnford University SIMES, SAC 5th Annul SSR Workshop on Synchrotron X-ry Scttering Techniques in Mterils nd Environmentl Sciences: Theory nd Appliction June 1-3, 010

2 Thin films. Epitxil thin films Wht bsic informtion we cn obtin from x-ry diffrction Reciprocl spce nd epitxil thin films Scn directions reciprocl vs. rel spce scenrios Mismtch, strin, mosicity, thickness How to choose right scns for your mesurements Mosicity vs. lterl correltion length SiGe(001) lyers on Si(001) exmple Why we need chnnel nlyzer Wht cn we lern from reciprocl spce mps SrRuO 3 nd 0.67 Sr 0.33 MnO 3 films exmple Summry

3 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 Mteril which dimension in the out-of-plne direction is much smller thn in the in-plne direction. A thin lyer of something on surfce Source: encrt.msn.com

4 Epitxil yer A single crystl lyer tht hs been deposited or grown on crystlline substrte hving the sme structurl rrngement. Source: photonics.com A crystlline lyer of prticulr orienttion on top of nother crystl, where the orienttion is determined by the underlying crystl. Homoepitxil lyer the lyer nd substrte re the sme mteril nd possess the sme lttice prmeters. Heteroepitxil lyer the lyer mteril is different thn the substrte nd usully hs different lttice prmeters.

5 Thin films structurl types Structure Type Perfect epitxil Nerly perfect epitxil Textured epitxil Textured polycrystlline Perfect polycrystlline Amorphous Definition Single crystl in perfect registry with the substrte tht is lso perfect. Single crystl in nerly perfect registry with the substrte tht is lso nerly perfect. yer orienttion is close to registry with the substrte in both inplne nd out-of-plne directions. yer consists of mosic blocks. Crystlline grins re preferentilly oriented out-of-plne but rndom in-plne. Grin size distribution. Rndomly oriented crystllites similr in size nd shpe. Strong intertomic bonds but no long rnge order. P.F. Fewster X-ry Scttering from Semiconductors

6 Thin films structurl properties Mosic spred Mismtch Curvture Misorienttion Relxtion Disloction content Inhomogeneity

7 Wht we wnt to know bout thin films? Crystlline stte of the lyers: Epitxil (coherent with the substrte, relxed) Polycrystlline (rndom orienttion, preferred orienttion) Amorphous Crystlline qulity Strin stte (fully or prtilly strined, fully relxed) Defect structure Chemicl composition Thickness Surfce nd/or interfce roughness

8 Overview of structurl prmeters tht chrcterize vrious thin films Thickness Composition Relxtion Distortion Crystlline size Orienttion Perfect epitxy Defects Nerly perfect epitxy??? Textured epitxy Textured polycrystlline?? Perfect polycrystlline? Amorphous P.F. Fewster X-ry Scttering from Semiconductors

9 Tetrgonl Distortion ttice mismtch between cubic lttice prmeters: R S S Strined, coherent, pseudomorphic c S φ 0 S R S R Prtilly relxed S S Relxed Before deposition R R φ 0 ttice mismtch induces lttice strin: ε ε zz R R d d d R R After deposition S S

10 Relxed yer (00l) (10l) (0l) (000) (100) (00) Cubic: > S Cubic

11 Strined yer (00l) (10l) (0l) Compressive strin (000) (100) (00) Tetrgonl distortion Tetrgonl: Cubic II > S S

12 Perfect yers: Relxed nd Strined (00l) (00l) Reciprocl Spce (hkl) (hkl) (000) (000) Cubic > S Tetrgonl Cubic Cubic

13 Reciprocl spce Ewld sphere 1 OC sinθ d λ 1 * hkl 1 d hkl λ d hkl sinθ * OB d hkl

14 Reciprocl spce Scttering vector Reciprocl ttice Point s s0 λ θ λ sin * dhkl λ d sinθ hkl 1 d hkl Diffrcted bem s λ Scttering vector θ s s 0 λ θ s 0 λ Incident bem (00l) (00l) scn (00l) (00l) scn (hkl) (-hkl) (h00) scn (hkl) Symmetricl Scn Asymmetricl Scn (000) Relxed yer (h00) (000) Strined yer

15 Scn Directions (00l) (hkl) Symmetricl Scn θ - θ scn Asymmetricl Scn ω - θ scn Smple Surfce θ θ θ ω θ α α θ ω

16 Scn Directions (00l) (hkl) Smple Surfce

17 Scn Directions (00l) (hkl) ω scn ω scn θ scn Symmetricl ω - θ scn Asymmetricl ω - θ scn Smple Surfce

18 Rel RP shpes c > S c < S Finite thickness effect S Compressive strin Tensile strin d-spcing vrition Mosicity

19 Mismtch True lttice mismtch is: m R S S Si(004) For cubic (001) oriented mteril the experimentlly mesured norml component of the mismtch: Si 1-x Ge x (004) m S S d d sinθs sin sin( θs + θ ) ( θ + θ ) S The experimentl mismtch, m, cn be relted to the mismtch through the eqution: R S 1 ν m m 1+ ν S where ν is Poisson rtio. For Si, ν 0.8 ν m 1 3 m* The composition of the A 1-x B x lloy cn be clculted from Vegrd slw: R ( x) ( 1 x) A xb + x m B A A

20 yer Thickness Interference fringes observed in the scttering pttern, due to different opticl pths of the x- rys, re relted to the thickness of the lyer: t ( n1 n ) λ ( sinω sin ) ω 1 Substrte yer Seprtion S-pek: -pek: Seprtion: Omeg( ) Omeg( ) Omeg( ) Thet( ) Thet( ) Thet( ) yer Thickness Men fringe period ( ): Men thickness (um): ± Thet/Omeg ( ) Fringe Period ( ) Thickness (um)

21 (00l) (00l) (hkl) (hkl) (000) Prtilly Relxed + Thin (000) Prtilly Relxed + Mosicity

22 Symmetricl scn ω-θ direction S (00l) ω direction Defined by receiving optics (e.g. slits) Defined by incident optics monochromtor Mosicity (000)

23 Symmetricl Scn nlyzer crystl ω-θ direction (00l) nlyzer crystl ω direction receiving slit receiving slit d-spcing vrition (000) mosicity

24 Triple xis diffrctometry Open detector Open detector Triple xis Ge content: 50% 40% 30% 0% 10% Triple xis

25 ω-θ scn ω-scn (00l) (hkl) l-scn h-scn (00l) (00l) scn (hkl) Symmetricl Scn Asymmetricl Scn (000) (h00) (000) (h00) scn

26 Relxed SiGe on Si(001) Shpe of the RP might provide much more informtion

27 Relxed SiGe on Si(001) (oo4) RM Si(004) SiGe(004)

28 ω-scn ω-θ scn (00l) (hkl) (000) (004) (113)

29 Relxtion The relxtion is defined s: R R S S 100 To seprte the lyer tilt from the true splitting we cn mke grzing incidence nd grzing exit mesurements: The effect of tilt on the pek splitting is reversed if the specimen is rotted by 180 o bout its surfce norml. The splitting due to mismtch will not be ffected by such rottion θ gi θ ge θ + ϕ θ ϕ grzing incidence grzing exit Grzing incidence Grzing exit Symmetricl scn θ gi θ ge θ sym θ ϕ B θ + ϕ B θ B θ B θ B θ B

30 Relxtion Considering (tetrgonl distortion): φ φ φ θ θ θ + + S S φ ngle between reflecting plne nd the surfce c b we obtin cell constnts for the (00l )-oriented lyer: sin cos sin l k h l l c + θ λ φ θ λ ν ν ν ν R c

31 Anlysis of terlly Inhomogeneous yers The Mosic Spred nd terl Correltion ength functionlity derives informtion from the shpe of lyer pek in diffrction spce mp recorded using n symmetricl reflection 3 q x + q z q z ϕ q x ξ q x q z cosξ cos 3 sinϕ cosξ ( ϕ + ξ ) nd 1 ϕ q x tn qz 1 ξ qx tn qz (000) ω terl correltion length Microscopic tilt q x q z

32 q z (000) q x1 q x ω Anlysis of terlly Inhomogeneous yers tn tn x x z x z x q q q q q q ω ω ω If the contributing profiles hve Gussin shpe: ω ω ω x x z z z x x x xn n x xn q q q q q q q q q q q + + Two symmetricl reflections

33 Superlttices nd Multilyers Λ t d hkl Substrte

34 Superlttices nd Multilyers (00l) (00l) (00l) (00l) (000) (000) (000) (000)

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37 Structure of SrRuO 3 Orthorhombic Tetrgonl Cubic Å b Å c Å Å c Å C C Å

38 Smples: SrRuO 3 on SrTiO 3 nd DyScO Sr 0.33 MnO 3 on NdGO 3, SAT, SrTiO 3 nd DyScO 3 [001] [110] yer STO [110] yer NGO [1-10] [100] [1-10] Pseudo-cubic lttice prmeters: 0.67 Sr 0.33 MnO NdGO 3 SAT SrTiO 3 SrRuO 3 γ b c DyScO 3

39 ω θ symmetricl scns 1.E+11 1.E+10 SrTiO 3 (00) 1.E+09 Finite size fringes indicte well ordered films Intensity (.u.) 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 SMO(0) 1.E+03 1.E H-K (rlu) SrRuO 3 (0) SrTiO 3 (00)

40 X-ry diffrction scn types for [110] growth Q scn ω θ scn Reciprocl Spce Mp (- 0 4) (0 0 ) SrTiO 3 ( 0 4) ( 0) SrRuO 3 (6 0) (4 4 4) Orthorhombic SrRuO 3 ( 6 0) (4 4 4) Tetrgonl SrRuO 3 b

41 Twinning in SrRuO 3 /SrTiO 3 (0 0 1) (1-1 0) γ c (0 0 1) + (1-1 0) (1-1 0) + (0 0 1) Untwinned b Twinned

42 High-Resolution Reciprocl Are Mpping Orthorhombic SrRuO 3 b Substrte yer (60) (444) (60) (444) Orthorhombic to Tetrgonl Trnsition

43 O T Structurl Trnsition, (60) & (60) reflections Trnsition Orthorhombic to Tetrgonl ~ 350 C Tetrgonl Orthorhombic Cubic iterture: C Appl. Phys. ett. 91, (007)

44 O T Structurl Trnsition, (1) reflection (1) Pek Orthorhombic Present Tetrgonl Absent Trnsition Orthorhombic to Tetrgonl ~ 310 C Intensity (rb units) Orthorhombic O T Trnsition Temperture ( o C) Tetrgonl Cubic iterture: C Appl. Phys. ett. 91, (007)

45 Appl. Phys. ett. 93, (008)

46 Compressive Stress b Unit cell is orthorhombic γ c b

47 Tensile Stress b Unit cell is tetrgonl γ c b

48 γ [110] γ ngle ccommodtes the stress long [1-10] b [1-10] nd b lttice prmeters (Å) C C T T b γ NGO SAT STO DSO γ ngle (deg) Substrte (Å) b (Å) b (Å) c (Å) yer (Å) b (Å) b (Å) c (Å) γ ( O ) NdGO SMO/NGO SAT SMO/SAT SrTiO SMO/STO DyScO SMO/DSO SMO (O) long b long c -1.0 Strin (%) NdGO 3 SAT SrTiO 3 DyScO 3 long b long c long b 0.8 long c 0.8 long b 1.39 long c 1.48

49 Appl. Phys. ett. 95, (009)

50 Summry Reciprocl spce for epitxil thin films is very rich. Shpe nd positions of reciprocl lttice points with respect to the substrte revel informtion bout: Mismtch Strin stte Relxtion Mosicity Composition Thickness. Diffrctometer instrumentl resolution hs to be understood before mesurements re performed.

51 Single crystl Preferred orienttion Polycrystlline