Characterization Of Layered Structures By X Ray Diffraction Techniques
1. X-Ray Scattering Methods for Characterization of Advanced Materials Workshop Characterization of layered structures by x-ray diffraction techniques Iuliana Cernatescu PANalytical Inc. Westborough, MA, USA 1
2. Outline Thin films definition and XRD applications Reciprocal Space definition Characterization of Epitaxial Layers Characterization of Polycrystalline layers Overview of typical optics and resolutions by sample types and target analysis 2
3. Thin Film Definition Nearly perfect epitaxy (thin film orientated to substrate parallel and perpendicular) Imperfect epitaxy (thin film partially orientated to substrate parallel and perpendicular) Textured polycrystalline (orientation unrelated to substrate but defined by growth) Non-crystalline layers (no correlation beyond a bond length) 3
4. Epitaxial Layers Mismatch Relaxation Composition In-Plane Epitaxy Mosaic spread Super-lattice period Curvature Off-Cut Thickness Density Roughness } XRR
5. Polycrystalline Layers Phase ID Quantification Unit Cell refinement Residual stress Crystallite size & micro-strain Preferred orientation Depth profiling of stress, phases, microstructure Thickness Density Roughness } XRR
9. Reciprocal Lattice and Scattering Vectors Reciprocal lattice vector d*hkl Length 1/d Direction, normal to hkl planes d*hkl S d*hkl kH k0 Incident beam vector, k0, Length n/ Direction, with respect to sample surface k0 2 000 kH Scattered beam vector, kH, Length n/ (user defined) Direction, 2 with respect to k0 By rotating kH and kothe diffraction vector Scan be made to scan through reciprocal space. When S = d*hklthen Bragg diffraction occurs Diffraction vector, S, S = kH – k0 S 9
12. All planes in the same family have the same length |d*|, but different directions
13. The family members have the same 3 indices (in different orders e.g. 400,040,004 etc)004 224 113 d* | d*| = 1/dhkl -440 440 Just a few points are shown for clarity 11
17. RSM features bulk crystals CTR = sample surface streak (and white radiation streak) M = monochromator (or source) streak, parallel to diffracted beam A = analyser (or detector) streak, parallel to tangent of Ewald sphere S = Mosaic spread, curvature (A) (A) S S M (M) CTR CTR 15
18. surface normal high quality substrate -sharp peak broadening normal to sample surface thin layers d spacing variation broadening parallel to surface mosaic structure variable tilts (curvature or dislocations) Shapes in RS 16
19. layer substrate thick layer with grading and overall curvature thin layer mosaic layer Examples Symmetric Reflections 17
20. 4.8o InGaAs tensile and compressive alternating multilayer on 001 InP substrate. Bent multilayer sample Samples with Bend or Tilt 18
21. 19 Buffer Layer Structures Relaxed Buffer layers as virtual substrates:e.g. Si/Ge on Si InGaAs on GaAs GaN on Sapphire Substrate and surface layer lattice parameter calculations from reciprocal lattice coordinates (Bragg’s Law) d*substrate d*cap d*layer tilt InP capping layer Graded InxGa(1-x)As Buffer layer with dislocations GaAs substrate P. Kidd et al, J. Crystal growth, (1996) 169 649-659
22. layer thickness Tilt, thickness and lateral width symmetric asymmetric Spread due to finite size effects Range of tilts In-plane 20
23. Broadening effects on symmetric reflections Omega broadening due to Size effects Omega broadening due to tilts (s-x,sz) (sx,sz) (s-x,sz) (sx,sz) 1/L 000 000 L 21
24. Strained Layer Q at=aS Layer 006 Substrate L 004 224 -2-24 002 aS S fully strained 220 110 Q|| 22
25. Relaxed Layer Q Layer Substrate 006 at= aL L aL 004 224 -2-24 002 S 220 110 fully relaxed Q|| at 23
35. In-Plane Diffraction In-plane diffraction is a technique for measuring the crystal planes that are oriented perpendicular to the surface | d*| = 1/dhkl 115 -2-24 224 004 113 d* 110 -1-10 220 -2-20 33
46. Polycrystalline random oriented 113 000 hkl 0 0 4 A sufficient number of randomly oriented crystals forms a reciprocal “lattice” of spherical shells 43
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48. Different intensities at different directionsSpherical shell radius 1/dhkl S 2 2 1/dhkl S = 1/dhkl 44
49. Characterization of Polycrystalline thin films Phase ID Phase ID with depth profiling Residual stress Residual stress with depth profiling Texture analysis 45
55. Symmetric scan for thin films In the case of very thin films the scattering volume will become smaller and smaller as the symmetric scan progresses to higher angles. The diffraction pattern of the substrate will dominate the diffractogram and could complicate the pattern analysis. 51
59. Glancing Incidence - Diffraction 2Theta scan Phase ID in thin film polycrystalline samples 55
60. 56 GIXRD - Thin film depth profiling phase analysis , Incident angle ZnO CIGS 0.45 deg Mo 1.00 deg ZnO ZnO CIGS Mo 2.00 deg ZnO ZnO CIGS Mo
61. 57 GIXRD - Thin film depth profiling phase analysis , Incident angle ZnO CIGS 0.45 deg Mo 1.00 deg ZnO ZnO CIGS Mo 2.00 deg ZnO ZnO CIGS Mo
62. 58 GIXRD - Thin film depth profiling phase analysis , Incident angle ZnO =0.45 CIGS 0.45 deg Mo 1.00 deg ZnO =1 ZnO CIGS Mo 2.00 deg ZnO =2 ZnO CIGS Mo
63. GIXRD in Reciprocal Space powder single crystal Sampling only the random component of the studied sample. 59
64. Residual Stress in Polycrystalline thin films Non uniform reciprocal lattice Different d-spacings at different directions Polycrystalline components subjected to external mechanical stresses Spherical shell distorted (not to scale!) S 2 2 1/dhklnot constant S = 1/dhkl One hkl reflection 60
65. “Stress” Measurement A stress measurement determines dhkl at a series of Psi positions The sample is stepped to different positions, 2 scan at each position to obtain peak position Repeated for different positions as required Spherical shell distorted One hkl reflection S 2 2 1/dhklvaries with position 61
67. Calssical Residual stress Measure (very small) peak shifts as a function of the sample tilt angle ‘psi’ Plot d-spacing as a function of sin2(psi) Fit straight line 63
68. Multiple hkl residual stress analysis Analysis Determine peak positions Calculate offsets (w-q)=wfixed- ½ (2q)peak Calculate sin2y values y=(w-q) Full range scan needed Low 2q small sin2y (40 o2q sin2y ~0.11) High 2q large sin2y (140 o2q sin2y ~0.87) hkl hkl hkl 2q w 64
69. Stress depth gradient Very small angle of incidence analyzing stress near surface Coating Substrate 65
70. Stress depth gradient Larger angle of incidence analyzing stress near surface AND deeper Coating Substrate 66
71. Stress depth gradient Largest angle of incidence analyzing average stress whole coating Coating Substrate 67
73. Pole Figure Measurement A Pole figure maps out the intensity over part of the spherical shell 2 stays fixed, the sample is scanned over all at different positions One hkl reflection S 2 2 69
78. The incident beam side is monochromated and the type of monochromator depends on the needed resolution.
79. For the diffracted beam side there are choises of TA, RC/open detector or line detector depending on the resolution needed.73
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81. Powder are often time analyzed with parallel beam, micor-spot beam, depending on the type of analysis required.74
82. Configuration for Texture and stress analysis Texture measurements require a point like source due to the tilting in Psi during the data collection of a pole figure. In this case the tube was rotated to point focus in order to avoid defocusing error and have better intensity. 75
85. Summary [2] source Detector S 2 sample An instrument Provides X-rays Aligns a sample Detects diffraction pattern A Material Reciprocal “Lattice” Structure An Experiment Designed to suit the material Designed to answer the question 78
