X-rays techniques as a powerful tool for characterisation of thin film nanostructures Elżbieta Dynowska Institute of Physics Polish Academy of Sciences, al. Lotników 32/46, Warsaw, Poland Workshop on Semiconductor Processing for Photonic Devices, Sept. 30 – Oct. 2, Warsaw, Poland

1. Introduction 2. Basics  General information about nanostructures  What we want to know about thin layers?  How to get this information? 3. Selected X-ray techniques  X-ray reflectivity  X-ray diffraction 4. Synchrotron radiation – new possibilities 5. Summary OutlineOutline

x Homoepitaxial layer – the layer and substrate are the same material (the same lattice parameters). Heteroepitaxial layer – the layer material is different than the substrate one (different lattice parameters). Thin layer – the dimension in the z-direction is much smaller than in the x and y, respectively. single crystal thin layer having the crystal structure and orientation of single crystal substrate on which it was grown. x y z (A) Epitaxial layer z y 0 0

(B) Polycrystalline layers – orientations of small crystallites are randomly distributed with respect to layer surface (C) Amorphous layers - lack of long-distance ordering of atoms Lattice mismatch – f = (a layer - a subs )/ a subs Critical thickness h c – thickness below which the layer grows pseudomorphically the cubic unit cell of layer material is tetragonally distorted: a lz  a lx = a ly = a s (the layer is fully strained). h c decreasing when f increasing. Layer relaxation - a lxy a lz a l relax = a bulk a layer asas ayay axax azaz

What we want to know about thin layers? What we want to know about thin layers?  Crystalline state of layer/layers (epitaxial?; polycrystalline?; amorphous? …)  crystal quality;  strain state;  defect structure;  chemical composition (in the case of ternary compounds layers);  thickness  surface and interface roughness, and so on…

How to get this information? By means of X-ray techniques Because X-ray techniques are the most important, non-destructive methods of sample characterization Why?

Selected X-ray techniques  X-ray reflectivity Small-angle region Refraction index for X-rays n < 1: Roughness investigation x z n = 1-  + i  Layer thickness determination ii ii t The distance between the adjacent interference maxima can be approximated by:  i  / 2t    kIkI kRkR kTkT cc  i  ~10 -5 in solid materials (~10 -8 in air);  - usually much smaller than . 2i2i Si rough wafer - simulation

Example: Example: superlattice Si/{Fe/Fe 2 N}x28/GaAs(001) 28 times repeated GaAs Fe Fe 2 N Si cap-layer Intensity  i (deg)  c  0.3  (2  ) - superlattice period  (2  ) – cap-layer Experiment Simulation Results of simulation 10.4 nm 4.52nm nm All superlattice

 X-ray diffraction wide-angle region  d’ hkl Bragg’s law: n = 2d’sin  d’/n = d = 2d sin 

Geometry of measurement    Detector Incident beam Diffracted beam 22  /2  coupling  /2  coupling Detector Incident beam Diffracted beam  22 ’’

Crystalline state of layer/phase analysis Possibilities MnTe/Al 2 O 3 FeK  radiation CuK  1 radiation ZnMnTe/MnTe/Al 2 O 3

Crystal quality „Rocking curve” Detector    Lattice parameter fluctuations Mosaic structure ? 21 arcsec 112 arcsec

Strain state & defect structure Cubic unit cell of substrate: Cubic unit cell of layer material Strain  tetragonal deformation of cubic unit cell: Pseudomorphic case Partially relaxed Relaxed asas a layer ayay axax azaz a z  a x = a y = a sub a z  a x = a y  a sub axax ayay azaz a layer a z = a x = a y = a layer

The reciprocal lattice maps S = [100] Reciprocal lattice: pseudomorphic  Origin P = [001] sample The sample orientation can be described by two vectors: P - vector which is the direction normal to the sample surface; S – any other vector which is not parallel to the P vector and lies in the horizontal plane. |H| 102 = 1/d 102 Mosaic structure   relaxed Lattice parameter fluctuations

x d 00l z  Symmetric case d hhl dzdz dxdx   Asymmetric case Examples In 0.50 Al 0.50 As/InP (a)(b) For cubic system : For tetragonal system :

chemical composition Vegard’s rule: If AB and CB compounds having the same crystallographic system and space group create the ternary compound A 1-x C x B then its lattice parameter a ACB depends linearly on x-value between the lattice parameters values of AB and CB, respectively. a AB a CB a bulk x01 a ACB x In the case of thin layers a relaxed must be taken for chemical composition determination from Vegard’s rule: c 12, c 11 – elastic constants of layer material

Heterostructure: ZnMn x Te/ZnMn y Te/ZnMn z Te/ZnTe/GaAs 004 rocking curve ZnTe x y z 004  /2  relaxed pseudomorphic

Ti/TiN/GaN/Al 2 O 3 under annealing Towards an ohmic contacts Secondary Ion Mass Spectrometry (SIMS) XRD

NbN/GaN/Al 2 O 3 XRD (SIMS)

20% N 2 Zn 3 N 2 + Zn25% N 2  polycryst. Zn 3 N 2 50% - 70% N 2  monocryst. GaN, Al 2 O 3, ZnO N 2 >80% polycryst. & amorph. Deposition of Zn 3 N 2 by reactive rf sputtering Zn 3 N 2

polycrystalline ZnO on sapphire and quartz ZnO:N by oxidation of Zn 3 N 2 microstructure highly textured ZnO on GaN and ZnO

ZnO by oxidation of ZnTe/GaAs Te inclusions in ZnO film XRD (SIMS)

Synchrotron radiation

Si substrate (001) Si, 115 nm, 780 C Si, 10 nm, 480 C Si, 24 nm, 450 C Si, 2nm, 250 C Ge, 1nm, 250 C 7 times repeated High resolution electron microscopy (HREM) – JEOL-4000EX (400 keV) Example: superlattice of self-assembled ultra-small Ge quantum dots Experimental diffraction patternSimulated diffraction pattern Si 004 Ge 004  2  = o Si 0.8 Ge 0.2 bottom layer „-1” „-2” C Results: HREM XRD superlattice period C.....  33.5 nm 33 nm, thickness of Ge  1.8 nm 2.0 nm thickness of SiGe x bottom layer  6.7 nm 6.7 nm Compositon x  nm Hasylab (Hamburg), W1.1 beamline: X’Pert Epitaxy and Smoothfit software

Acknowledgements I would like to express my gratitude to my colleagues for their kind help: Eliana Kaminska Jarek Domagala Roman Minikayev Artem Shalimov