Thin Film Scattering: Epitaxial Layers

Transcription

1 Thin Film Scattering: Epitaxial Layers Arturas Vailionis First Annual SSRL Workshop on Synchrotron X-ray Scattering Techniques in Materials and Environmental Sciences: Theory and Application Tuesday, May 16 & Wednesday, May 17, 2006

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 sometimes we need channel analyzer What can we learn from reciprocal space maps SrRuO 3 (110) on SrTiO 3 (001) 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 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

7 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

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

9 Tetragonal Distortion a L c L a L a L =a S a S a S a S a S Before deposition After deposition ε = ε zz = L d z d 0 d L z L z 0

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

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

12 Scan Directions Reciprocal Lattice Point s s0 = λ 2 sinθ * = dhkl λ 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

13 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

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

15 Scan directions (00l) (hkl) ω scan ω scan 2θ scan Symmetrical ω -2θ scan Asymmetrical ω -2θ scan Sample Surface

16 Real RLP shapes Finite thickness effect c L < a S L S Homoepitaxy Heteroepitaxy Tensile stress Heteroepitaxy d-spacing variation Heteroepitaxy Mosaicity

17 (00l) (00l) (hkl) (hkl) (000) Partially Relaxed (000) Partially Relaxed + Mosaicity

18 ω-2θ direction (00l) Defined by receiving optics (e.g. slits) ω direction Mosaicity (000)

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

20 counts/s 1M 100K (002) SrTiO 3 With receiving slit With channel analyzer 10K (220) SrRuO 3 1K Theta/Omega ( )

21 Mismatch True lattice mismatch is: m = a L a a S S counts/s 10M 1M Si(004) The peak separation between substrate and layer is related to the change of interplanar spacing normal to the substrate through the equation: 100K 10K 1K SiGe(004) δd d = δθ cotθ 100 If it is (00l) reflection then the experimental x-ray mismatch : δ a δd m * = = a d Theta/Omega ( ) And true mismatch can be obtained through: 1 ν m = m* 1 1+ ν ν 3 m* m where: ν Poisson ratio 2

22 F F F F F F F F F F F F F F F F F F L S Layer Thickness counts/s 10M 1M Interference fringes observed in the scattering pattern, due to different optical paths of the x- rays, are related to the thickness of the layers 100K 10K t = ( n1 n2 ) λ ( sinω sin ) 2 ω 1 2 1K Theta/Om eg a ( ) 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)

23 Relaxed SiGe on Si(001) Omega Theta Phi 0.00 Psi 0.00 X 0.00 Y c_TA.xrdml counts/s 10M 1M 100K 10K 1K 100 Shape of the RLP might provide much more information Theta/Omega ( )

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

25 0 0 4 Omega Theta Phi 0.00 Psi 0.00 X 0.00 Y c_TA.xrdml counts/s 10M 1M 100K 10K 1K Theta/Omega ( )

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

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

28 Mosaic Spread and Lateral Correlation Length 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 The mosaic spread of the layer is calculated from the angle that the layer peak subtends at the origin of reciprocal space measured perpendicular to the reflecting plane normal. The lateral correlation length of the layer is calculated from the reciprocal of the FWHM of the peak measured parallel to the interface. Q Z To Origin MS LC Q X

29 Superlattices and Multilayers Λ t d hkl Substrate

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

31 0 0 4 Omega Theta Phi 0.00 Psi 0.00 X 0.00 Y ssl.xrdml counts/s 10M 1M 100K 10K 1K Theta/Omega ( )

32

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

34 (110) (001) SrRuO 3 (1-10) (001) (010) SrTiO 3 (100)

35 X-ray Diffraction Scan Types 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

36 ω 2θ symmetrical scans counts/s 100K SrRuO 3 (220) SrTiO 3 (002) Thickness 3200 Å 10K Finite size fringes indicate well ordered films 1K Theta/O m eg a ( ) counts/s 100K SrRuO 3 (220) SrTiO 3 (002) Thickness 3100 Å 10K 1K Theta/O m eg a ( )

37 Reciprocal Lattice Map of SrRuO3 (220) and SrTiO3 (002) ω 2θ scan Distorted perovskite structure: Films are slightly distorted from orthorhombic, γ = (0 0 2) (2 2 0) φ angle 0 o 90 o 180 o 270 o SrTiO 3 SrRuO 3 Substrate (110) Layer (100) (010) 5.53 Å γ 5.58 Å (110)

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

39 Tetragonal Orthorhombic Cubic Literature: C Transition Orthorhombic to Tetragonal ~ 350 C

40 Structural Transition, (221) reflection (221) Peak Orthorhombic Present Tetragonal Absent Intensity (arb units) Orthorhombic O T Transition Temperature ( o C) Tetragonal Cubic Literature: C Transition Orthorhombic to Tetragonal ~ 310 C Transition Orthorhombic to Tetragonal ~ 310 C

41 Structural Transition, (211) reflection (211) peak is absent in cubic SrRuO α Calculated Intensity (arb units) Rotation Angle (deg)

42 Structural Transition, (211) reflection O T Transition = 310 o C Tetragonal Intensity (a.u.) Orthorhombic Attempt for T C Transition? Cubic Temperature ( o C)

43 Refined Unit Cells Volume (Å) We used (620), (260), (444), (444), (220) and (440) reflections for refinement PLD MBE a b c a b g V PLD PLD PLD PLD MBE MBE MBE MBE MBE Bulk PLD 1 (3) PLD 2 (3) PLD 3 (4) PLD 4 (5) MBE 1 (10) MBE 2MBE 3MBE 4MBE 5 (18) (26) (40) (60) Sample # (RRR)

44 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.

45 Single crystal Preferred orientation Polycrystalline