Thin Film Scattering: Epitaxial Layers

Transcription

1 Thin Film Scattering: Epitaxial Layers 6th Annual SSRL Workshop on Synchrotron X-ray Scattering Techniques in Materials and Environmental Sciences: Theory and Application May 29-31, 2012

2 Thin films. Epitaxial thin films What basic information we can obtain from x-ray diffraction Reciprocal space and epitaxial thin films Scan directions reciprocal vs. real space scenarios Mismatch, strain, mosaicity, thickness How to choose right scans for your measurements Mosaicity vs. lateral correlation length SiGe(001) layers on Si(001) example Why we need channel analyzer What can we learn from reciprocal space maps SrRuO 3 and La 0.67 Sr 0.33 MnO 3 films example Summary

3 What is thin film/layer? Material so thin that its characteristics are dominated primarily by two dimensional effects and are mostly different than its bulk properties Source: semiconductorglossary.com Material which dimension in the out-of-plane direction is much smaller than in the in-plane direction. A thin layer of something on a surface Source: encarta.msn.com

4 Epitaxial Layer A single crystal layer that has been deposited or grown on a crystalline substrate having the same structural arrangement. Source: photonics.com A crystalline layer of a particular orientation on top of another crystal, where the orientation is determined by the underlying crystal. Homoepitaxial layer the layer and substrate are the same material and possess the same lattice parameters. Heteroepitaxial layer the layer material is different than the substrate and usually has different lattice parameters.

5 Thin films structural types Structure Type Perfect epitaxial Nearly perfect epitaxial Textured epitaxial Textured polycrystalline Perfect polycrystalline Amorphous Definition Single crystal in perfect registry with the substrate that is also perfect. Single crystal in nearly perfect registry with the substrate that is also nearly perfect. Layer orientation is close to registry with the substrate in both inplane and out-of-plane directions. Layer consists of mosaic blocks. Crystalline grains are preferentially oriented out-of-plane but random in-plane. Grain size distribution. Randomly oriented crystallites similar in size and shape. Strong interatomic bonds but no long range order. P.F. Fewster X-ray Scattering from Semiconductors

6 Thin films structural properties Mosaic spread Mismatch Curvature Misorientation Relaxation Dislocation content Inhomogeneity

7 What we want to know about thin films? Crystalline state of the layers: Epitaxial (coherent with the substrate, relaxed) Polycrystalline (random orientation, preferred orientation) Amorphous Crystalline quality Strain state (fully or partially strained, fully relaxed) Defect structure Chemical composition Thickness Surface and/or interface roughness

8 Overview of structural parameters that characterize various thin films Thickness Composition Relaxation Distortion Crystalline size Orientation Perfect epitaxy Defects Nearly perfect epitaxy??? Textured epitaxy Textured polycrystalline?? Perfect polycrystalline? Amorphous P.F. Fewster X-ray Scattering from Semiconductors

9 Tetragonal Distortion Lattice mismatch between cubic lattice parameters: a a = a R L a a S S Strained, coherent, pseudomorphic c L a L = a S φ 0 a S R a L a S R a L Partially relaxed a S a S Relaxed Before deposition R a L R a L φ = 0 Lattice mismatch induces lattice strain: ε = ε zz = a L a a R L R L = d L d d R L R L After deposition a S a S

10 Single crystal

11 Polycrystalline Textured

12 Polycrystalline Random

13 Relaxed Layer (00l) (10l) (20l) (000) (100) (200) Cubic: a L > a S Cubic

14 Strained Layer (00l) (10l) (20l) Compressive strain (000) (100) (200) Tetragonal distortion Tetragonal: Cubic a a II L L = a > a S S

15 Perfect Layers: Relaxed and Strained (00l) (00l) Reciprocal Space (hkl) (hkl) (000) (000) Cubic a L > a S Tetragonal Cubic Cubic

16 Reciprocal space Ewald sphere 1 OC = sinθ = d λ 2 1 * hkl = 1 2d hkl λ = 2d hkl sinθ * OB = d hkl

17 Reciprocal space Scattering vector Reciprocal Lattice Point s s0 = λ θ λ 2 sin * = dhkl λ = d sinθ 2 hkl = 1 d hkl Diffracted beam s λ Scattering vector θ s s 0 λ θ s 0 λ Incident beam (00l) (00l) scan (00l) (00l) scan (hkl) (-hkl) (h00) scan (hkl) Symmetrical Scan Asymmetrical Scan (000) Relaxed Layer (h00) (000) Strained Layer

18 Scan Directions (00l) (hkl) Symmetrical Scan θ - 2θ scan Asymmetrical Scan ω - 2θ scan Sample Surface 2θ θ θ ω 2θ α α = θ ω

19 Scan Directions (00l) (hkl) Sample Surface

20 Scan Directions (00l) (hkl) ω scan ω scan 2θ scan Symmetrical ω - 2θ scan Asymmetrical ω - 2θ scan Sample Surface

21 Symmetrical Scan

22 Grazing Incidence Diffraction

23 Real RLP shapes c L > a S c L < a S Finite thickness effect L S Compressive strain Tensile strain d-spacing variation Mosaicity

24 Mismatch True lattice mismatch is: m = a R L a a S S Si(004) For cubic (001) oriented material the experimentally measured normal component of the mismatch: Si 1-x Ge x (004) m = a a a S S a = a d = d = sinθs sin sin( θs + θ ) ( θ + θ ) S The experimental mismatch, m, can be related to the mismatch through the equation: R al as 1 ν m = = m a 1+ ν S where ν is Poisson ratio. For Si, ν = 0.28 ν m 1 3 m* 2 The composition of the A 1-x B x alloy can be calculated from Vegard s law: R L ( x) = ( 1 x) aa xab a + x = m a B aa a A

25 Layer Thickness Interference fringes observed in the scattering pattern, due to different optical paths of the x- rays, are related to the thickness of the layer: t = ( n1 n2 ) λ ( sinω sin ) 2 ω 1 2 Substrate Layer Separation S-peak: L-peak: Separation: Omega( ) Omega( ) Omega( ) Theta( ) Theta( ) Theta( ) Layer Thickness Mean fringe period ( ): Mean thickness (um): ± Theta/Omega ( ) Fringe Period ( ) Thickness (um)

26 (00l) (00l) (hkl) (hkl) (000) Partially Relaxed + Thin (000) Partially Relaxed + Mosaicity

27 Symmetrical scan ω-2θ direction S (00l) ω direction Defined by receiving optics (e.g. slits) Defined by incident optics monochromator L Mosaicity (000)

28 Symmetrical Scan analyzer crystal ω-2θ direction (00l) analyzer crystal ω direction receiving slit receiving slit d-spacing variation (000) mosaicity

29 Triple axis diffractometry Open detector Open detector Triple axis Ge content: 50% 40% 30% 20% 10% Triple axis

30 ω-2θ scan ω-scan (00l) (hkl) l-scan h-scan (00l) (00l) scan (hkl) Symmetrical Scan Asymmetrical Scan (000) (h00) (000) (h00) scan

31 Relaxed SiGe on Si(001) Shape of the RLP might provide much more information

32 Relaxed SiGe on Si(001) (oo4) RLM Si(004) SiGe(004)

33 ω-scan ω-2θ scan (00l) (hkl) (000) (004) (113)

34 Relaxation The relaxation is defined as: R = a a L R L a a S S 100 To separate the layer tilt from the true splitting we can make grazing incidence and grazing exit measurements: The effect of tilt on the peak splitting is reversed if the specimen is rotated by 180 o about its surface normal. The splitting due to mismatch will not be affected by such rotation θ gi θ ge = θ + ϕ = θ ϕ grazing incidence grazing exit Grazing incidence Grazing exit Symmetrical scan θ gi θ ge θ sym θ ϕ B 2 θ + ϕ B θ B 2θ B θ B 2θ B

35 Analysis of Laterally Inhomogeneous Layers The Mosaic Spread and Lateral Correlation Length functionality derives information from the shape of a layer peak in a diffraction space map recorded using an asymmetrical reflection L 3 = q 2 x + q 2 z q z ϕ q x ξ q x q z L 2 L L 1 2 L L L 3 L 1 cosξ = cos 3 = 2 sinϕ cosξ ( ϕ + ξ ) and 1 ϕ = q x tan qz 1 ξ = qx tan qz (000) ω L Lateral correlation length Microscopic tilt = = 1 L 1 L q x q z

36 Superlattices and Multilayers Λ t d hkl Substrate

37 Superlattices and Multilayers (00l) (00l) (00l) (00l) (000) (000) (000) (000)

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40 Structure of SrRuO 3 Orthorhombic Tetragonal Cubic a = Å b = Å c = Å a = Å c = Å C C a = Å

41 Samples: SrRuO 3 on SrTiO 3 and DyScO 3 La 0.67 Sr 0.33 MnO 3 on NdGaO 3, LSAT, SrTiO 3 and DyScO 3 [001] [110] Layer STO [110] Layer NGO [100] [1-10] [1-10] Pseudo-cubic lattice parameters: La 0.67 Sr 0.33 MnO NdGaO 3 LSAT SrTiO 3 SrRuO 3 a γ b c DyScO 3

42 Symmetrical scans along STO[001] direction SrTiO 3 (002) LSMO(220) Intensity (a.u.) q (rlu) 10 nm 20 nm 40 nm 60 nm Finite thickness fringes around the Bragg peak indicate very good structural quality throughout the film

43 X-ray diffraction scan types for [110] growth Q scan ω 2θ scan Reciprocal Space Map (-2 0 4) (0 0 2) SrTiO 3 (2 0 4) (2 2 0) SrRuO 3 (6 2 0) (4 4 4) Orthorhombic SrRuO 3 (2 6 0) (4 4 4) Tetragonal SrRuO 3 b a

44 Compressive Stress a b Unit cell is orthorhombic γ c a b

45 Tensile Stress a = b Unit cell is tetragonal γ c a b

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

47 High-Resolution Reciprocal Area Mapping Orthorhombic SrRuO 3 b a Substrate Layer (260) (444) (620) (444) Orthorhombic to Tetragonal Transition

48 O T Structural Transition, (620) & (260) reflections Transition Orthorhombic to Tetragonal ~ 350 C Tetragonal Orthorhombic Cubic Literature: C Appl. Phys. Lett. 91, (2007)

49 O T Structural Transition, (221) reflection (221) Peak Orthorhombic Present Tetragonal Absent Transition Orthorhombic to Tetragonal ~ 310 C Intensity (arb units) Orthorhombic O T Transition Temperature ( o C) Tetragonal Cubic Literature: C Appl. Phys. Lett. 91, (2007)

50 a γ [110] γ angle accommodates the stress along [1-10] b [1-10] a and b lattice parameters (Å) C C T T a b γ γ angle (deg) 5.45 NGO LSAT STO DSO 89.0 Substrate a (Å) b (Å) ab (Å) c (Å) Layer a (Å) b (Å) ab (Å) c (Å) γ ( O ) NdGaO LSMO/NGO LSAT LSMO/LSAT SrTiO LSMO/STO DyScO LSMO/DSO LSMO (O) along ab = along c = Strain (%) NdGaO 3 LSAT SrTiO 3 DyScO 3 along ab = along c = along ab = 0.28 along c = 0.28 along ab = 1.39 along c = 1.48

51 Appl. Phys. Lett. 95, (2009)

52 Summary Reciprocal space for epitaxial thin films is very rich. Shape and positions of reciprocal lattice points with respect to the substrate reveal information about: Mismatch Strain state Relaxation Mosaicity Composition Thickness. Diffractometer instrumental resolution has to be understood before measurements are performed.

53 Single crystal Preferred orientation Polycrystalline