1. X-Ray Diffraction Analysis of Ⅲ - Ⅴ Superlattices: Characterization, Simulation and Fitting 1 Xiangyu Wu Enlong Liu Mentor: Clement Merckling EPI Group @ imec Project Work Nanoscience

2. Outline 2  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

3. Outline 3  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

4. Introduction 4 Superlattice (SL) is a periodic structure of layers of two (or more) materials. It can also refer to a lower-dimensional structure such as an array of quantum dots or quantum wires. J.J.Gu, et al. IEDM12-529 http://en.wikipedia.org/wi ki/Superlattice http://mbe.rcast.u-tokyo.ac.jp/index_eng.html M. Cooke. Ⅲ - Ⅴ s Review, 2006 19(6): 22-26 S.Y. Cheng, et al. Solid-State Electronics, 1999, 43(4):755-760. Superlattice is linked to very advanced and complicated heterostructures.

5. Introduction 5 Issue of TEM: Only used to characterize small area on wafer; Need sample preparation, very time-consuming; Limit information, only thickness SL Growth Epitaxy Slow growth rate Interfacial layer control Vapor Phase Epitaxy (VPE) Molecular Beam Epitaxy (MBE) Characterization: Transmission Electron Microscopy (TEM) J.Warga, et al. Physica E, 2009, 41(6): 1040-3. Dark: Er-doped silicon-rich nitride; Bright: Si.

6. Outline 6  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

7. XRD: Principle Bragg`s Law: Two beams with identical wavelength and phase approach a crystalline solid and are scattered off two different atoms within it. The lower beam traverses an extra length of 2dsinθ. Constructive interference occurs when this length is equal to an integer multiple of the wavelength of the radiation.

8. XRD: Principle 8 s   X-ray tube Detector ω: Tune the angle between the emitter and substrate; 2θ: Tune the angle between emitter and detector; Ψ: Vertical rotation of substrate plane; Φ: Horizontal rotation of the substrate plane; x, y, z coordinate: move the substrate plane up, down, left, right, without rotation

9. XRD: Principle 9 Incident beam Omega axis sample Mono- chroma tor detector Omega axis detector Analyzer sample Mono- chroma tor detector 2θ2θ ω Si Si(Ge) 2θ=2ω+offset Rocking curve vs Coupled scan

10. Outline 10  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

11. XRD: Superlattice Diffraction 11 The greatest use of HRXRD in industry is the characterization of epitaxial structures on compound semiconductors. The composition of ternaries, mismatch of quaternaries, mis- orientation, layer thickness, tilt, relaxation, indications of strain, curvature and stress, and area homogeneity have important influence on the performance of Ⅲ - Ⅴ and Ⅱ - Ⅵ semiconductors. MQW laser

12. XRD: Superlattice Diffraction 12 Material parameterEffect on rocking curveDistinguishing features Mismatch Splitting of layer and substrate peak Invariant with sample rotation Mis-orientation Splitting of layer and substrate peak Changes sign with sample rotation ThicknessAffects intensity of peak Integrated intensity increases with layer thickness, up to a limit Thickness Introduces interference fringes Fringe period controlled by thickness Mosaic spreadBroadens peak Broadening may increase with beam size, up to mosaic cell size Dislocation contentBroadens peak Broadening invariant with beam size The effect of substrate and epi-layer parameters upon the rocking curve

13. XRD: Superlattice Diffraction 13 Lattice parameter and composition Superlattice under full strain (e.g. In x Ga 1-x As layer on InP substrate) θ1θ1 θ2θ2 d` hkl d hkl For zinc blende structure, For (004) plane, Vegard’s Law: a1a1 a2a2

14. XRD: Superlattice Diffraction 14 XRD Superlattice period characterization where Λ is the thickness of a SL period, λ CuKα1 = 0.15405nm, is the nth-order peak of the MQW, is the zero-order peak. Λ θnθn Λ By averaging over the positions of satellite peaks of order n, we got: n according J.M. Vandenberg, A.T. Macrander, R.A. Hamm, M.B. Panish, Phys. Rev. B 44 (1991) 3991

15. Outline 15  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

16. Results and Discussions 16 Sample Structure (not to scale) S1: InAlAs Superlattice x5 InP InGaAs InP (001) S2 InAlAs Superlattice x5 InP: thickness ~x2 InGaAs: thickness ~x2 InP (001) S3 InAlAs Superlattice x5 InGaAs: Same thickness as S2 InAlAs: Unknown In x Al 1-x As InP In x Ga 1-x As

17. Outline 17  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

18. Results and Discussions 18 XRD Experiments Results From these curves,we need to know: Thickness of InAlAs buffer layer and the period of SL; Composition of each material. With offset

19. Outline 19  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

20. Results and Discussions 20 Peaks belonging Example to identify peaks from different sources. InP 600μm InAlAs 135nm

21. Results and Discussions 21 Peaks belonging InP substrate InGaAs Inp InGaAs Inp InGaAs Inp InGaAs Inp InGaAs Inp

22. Results and Discussions 22 Peaks belonging InP substrate InGaAs Inp InGaAs Inp InGaAs Inp InGaAs Inp InGaAs Inp Repeat of SL period=N+2 N=3 Repeat=5

23. Results and Discussions 23 Peaks belonging The total curve is the superposition of Layer and SL. InP substrate InGaAs Inp InGaAs Inp InGaAs Inp InGaAs Inp InGaAs Inp InP 600μm InAlAs 135nm

24. Results and Discussions 24 Theoretical calculations Red arrow corresponds to fringes produced by InAlAs layer diffraction. Blue arrow corresponds to nth-order peak by diffraction of SL period Sample InAlAs thickness SL period InGaAs InP thickness compositionthickness S1117nm34.2nm0.57717.1 nm InP (001) In x Al 1-x As InP In x Ga 1-x As

25. Results and Discussions 25 Theoretical calculations For S2, the ideal thickness of SL is twice of that in S1. According to previous data in S1, multiply by 2 directly. Blue arrows refer to SL peaks, leading to average period 70 nm; Red arrows refer to Layer peaks, leading to average thickness 126 nm. Coincidence SampleLayer_InAlAsSL_InGaAsSL_InP S2 thickness compositionthickness 126 nm34.0 nm0.57236.0 nm In x Al 1-x As InP In x Ga 1-x As InP (001)

26. Results and Discussions 26 Theoretical calculations Arrows: peaks for Layer; Bracket: peaks for SL. Average Initial values for simulation SampleLayer_InAlAsSL_InGaAsSL_InAlAs S3 thicknessThicknesscompositionthicknesscomposition 135 nm34.0 nm0.55619.6 nm0.52 All maximums remain in XRD, indicating that the thickness of two layers in SL are different. Only the total period can be calculated, which is 53.6nm. Besides, the thickness of InGaAs layer is the same as S2, we can take 34.0 nm as initial one,which can also give that of InAlAs. In x Al 1-x As InP In x Ga 1-x As InP (001)

27. Results and Discussions 27 Simulation and fitting for S1 SampleLayer_InAlAsSL_InPSL_InGaAs S1 thicknesscompositionthickness composition 135 nm0.592517.9 nm17.0 nm0.6073 InP (001) In x Al 1-x As InP In x Ga 1-x As

28. Results and Discussions 28 Simulation and fitting for S2 SampleLayer_InAlAsSL_InPSL_InGaAs S2 thicknesscompositionthickness composition 132 nm0.5236.0 nm34.5 nm0.6039 InP (001) In x Al 1-x As InP In x Ga 1-x As

29. Results and Discussions 29 Simulation and fitting for S3 SampleLayer_InAlAsSL_InGaAsSL_InAlAs S3 thicknesscompositionthicknesscompositionthicknesscomposition 135 nm0.5229.7nm0.6021.6nm0.52 InP (001) In x Al 1-x As InP In x Ga 1-x As

30. Results and Discussions 30 Summary Sample S1S2S3 Substrate_InP600 um Layer_InAlAs135 nm, In_0.59132 nm, In_0.52135 nm, In_0.52 Superlattice Layer1InP17.9 nmInP36.0 nmInGaAs 29.7 nm In_0.60 Layer2InGaAs 17.0 nm InGaAs 34.5 nm InAlAs 21.6 nm In_0.60 In_0.52

31. Outline 31  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

32. Comparisons with TEM Results 32 137 136 140 S1S2S3 In x Al 1-x As Layer Thickness 137nm136nm140nm

33. Comparisons with TEM Results 33 SLs layers thickness 50 nm 30 21 32 20 32 20 32 20 32 19 32 134 InP InAlAs InGaAs InAlAs InGaAs InAlAs 50 nm 136 33 38 33 38 34 38 33 38 37 33 InP InAlAs InP InGaAs InP 20 nm InAlAs InP InGaAs InP 16.0 18.5 16.5 18.0 16.0 18.5 16.0 18.5 16.0 16.5 S1S2S3 InGaAs 16nm InP 18nm InGaAs 33nm InP 38nm InGaAs 32nm InAlAs 20nm

34. Comparisons with TEM Results 34 Comparison SamplePartMaterialXRDTEMDifference S1 LayerInAlAs135 nm137 nm-2nm SL InP17.9 nm18.0 nm-0.1nm InGaAs 17.0 nm16.0 nm+1nm 0,60NA S2 LayerInAlAs132 nm140 nm-8nm SL InP36.0 nm38.0 nm-2nm InGaAs 34.5 nm33.0 nm+1.5nm 0.60NA S3 LayerInAlAs135 nm136 nm-1nm SL InGaAs 29.7 nm32.0 nm-2.3nm 0.60NA InAlAs 21.6 nm20.0 nm+1.6nm 0.52NA

35. Outline 35  Introduction  XRD Principle Superlattice Diffraction  Results and Discussions Sample Structure XRD Experiments Results Curves Analysis and Simulation Peaks Belonging Theoretical Calculation Simulation Results  Comparison with TEM Results  Conclusion

36. Conclusion 36 XRD studies on superlattice samples with different compositions and periods. Based on the information derived from XRD rocking curves, three models were established and simulated. The fitting results of all three models not only gave information which TEM could not, but also corresponded well with data already given by TEM figures, indicating the reliability and accuracy of XRD measurement in superlattice structures. With its non-destructive property and high efficiency in conducting experiments and results derivation, XRD will be a more suitable method for superlattice researches in many fields.

37. 37 Xiangyu Wu Enlong Liu