From DMS to CMS: a characterization protocol

1. From dilute to condensed magnetic semiconductors: a protocol for systematic characterization Mauro Rovezzi1 1 Institute for Semiconductors and Solid State Physics, Johannes Kepler University, Linz, Austria [W. Stefanowicz et al., arXiv.org 0912.4216 (2010)] [A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010)] M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 1 / 32

2. Co-authors A. Navarro-Quezada, B. Faina, T. Devillers, T. Li, A. rois, T. Winkler, A. Bonanni Institute for Semiconductors and Solid State Physics, Johannes Kepler University, Linz, Austria W. Stefanowicz, D. Sztenkiel, R. Jakiela, M. Sawicki, T. Dietl Institute of Physics, Polish Academy of Sciences, Warsaw, Poland R. T. Lechner, G. Bauer Institute for Semiconductors and Solid State Physics, Johannes Kepler University, Linz, Austria F. d’Acapito Italian National Research Council, IOM-OGG, c/o ESRF GILDA, Grenoble, France M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 2 / 32

3. Outline 1 Introduction 2 Growth characterization protocol 3 Case studies Dilute (Ga,Mn)N Fe segregation in GaN 4 Conclusions M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 3 / 32

4. Introduction Building-blocks for Spintronics devices [A. Bonanni and T. Dietl, Chem. Soc. Rev. (2010)] M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 4 / 32

5. DMS → (Ga,Mn)N With holes → sp-d Zener model x=5% and 3.5×1020 h/cm3 [T. Dietl et al., Science (2000)] Controversial experimental reports ⇒ accurate nanoscale characterization M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 5 / 32

6. DMS → (Ga,Mn)N With holes → sp-d Zener model Without holes → double exchange [K. Sato et al., Phys. Rev. B (2004)] x=5% and 3.5×1020 h/cm3 [T. Dietl et al., Science (2000)] [L. Berqvist et al., Phys. Rev. Lett. (2004)] Controversial experimental reports ⇒ accurate nanoscale characterization M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 5 / 32

7. DMS → (Ga,Mn)N With holes → sp-d Zener model Without holes → double exchange [K. Sato et al., Phys. Rev. B (2004)] x=5% and 3.5×1020 h/cm3 [T. Dietl et al., Science (2000)] [L. Berqvist et al., Phys. Rev. Lett. (2004)] Controversial experimental reports ⇒ accurate nanoscale characterization M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 5 / 32

8. CMS → (Ga,Fe)N TMs solubility in (most) semiconductors at thermal equilibrium is low ⇒ chemical/crystallographic phase separation ⇒ a priori unknown magnetic properties ⇒ mostly high TC FM Detection and control of phase separation ⇒ nano-characterization tools M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 6 / 32

9. CMS → (Ga,Fe)N TMs solubility in (most) semiconductors at thermal equilibrium is low ⇒ chemical/crystallographic phase separation ⇒ a priori unknown magnetic properties ⇒ mostly high TC FM Examples of chemical phase separation (spinodal decomposition in DMS literature) (Ga,Mn)As annealing [M. Moreno et al., J. Appl. Phys. (2002)] Detection and control of phase separation ⇒ nano-characterization tools M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 6 / 32

10. CMS → (Ga,Fe)N TMs solubility in (most) semiconductors at thermal equilibrium is low ⇒ chemical/crystallographic phase separation ⇒ a priori unknown magnetic properties ⇒ mostly high TC FM Examples of chemical phase separation (spinodal decomposition in DMS literature) (Ga,Mn)As annealing (Ga,Mn)N clusters [G. Martinez-Criado et al., Appl. Phys. Lett. (2005)] [M. Moreno et al., J. Appl. Phys. (2002)] Detection and control of phase separation ⇒ nano-characterization tools M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 6 / 32

11. CMS → (Ga,Fe)N TMs solubility in (most) semiconductors at thermal equilibrium is low ⇒ chemical/crystallographic phase separation ⇒ a priori unknown magnetic properties ⇒ mostly high TC FM Examples of chemical phase separation (spinodal decomposition in DMS literature) (Ga,Mn)As annealing (Ga,Mn)N clusters (Ge,Mn) nano-columns [G. Martinez-Criado et al., Appl. Phys. Lett. (2005)] [M. Moreno et al., J. Appl. [M. Jamet et al., Nature Mat. Phys. (2002)] (2006)] Detection and control of phase separation ⇒ nano-characterization tools M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 6 / 32

12. CMS → (Ga,Fe)N TMs solubility in (most) semiconductors at thermal equilibrium is low ⇒ chemical/crystallographic phase separation ⇒ a priori unknown magnetic properties ⇒ mostly high TC FM Examples of chemical phase separation (spinodal decomposition in DMS literature) (Ga,Mn)As annealing (Ga,Mn)N clusters (Ge,Mn) nano-columns [G. Martinez-Criado et al., Appl. Phys. Lett. (2005)] [M. Moreno et al., J. Appl. [M. Jamet et al., Nature Mat. Phys. (2002)] (2006)] Detection and control of phase separation ⇒ nano-characterization tools M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 6 / 32

13. Growth characterization protocol M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 7 / 32

14. Metalorganic Vapour Phase Epitaxy (Ga,Mn)N and (Ga,Fe)N[:Si] AIXTRON 200RF horizontal reactor c-plane sapphire substrates Precursors: TMGa, NH3 , Cp2 Fe, Cp2 Mn, SiH4 Monitoring: ellipsometry Well established growth procedure 1 Substrate nitridation 2 LT (540 ◦ C) GaN nucl. layer 3 Annealing/recrystallization 4 1 µm HT (1050 ◦ C) GaN 5 0.5 – 1 µm GaN:(Fe/Mn)[:Si] 800 – 950 ◦ C 50 – 490 sccm (→ 0.05 – 1 %) M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 8 / 32

15. Limits of conventional high-resolution XRD No evidence of second phases GaN(0002) peak broadening ⇒ Synchrotron radiation M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 9 / 32

16. Limits of conventional high-resolution XRD No evidence of second phases GaN(0002) peak broadening ⇒ Synchrotron radiation M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 9 / 32

17. European Synchrotron Radiation Facility, Grenoble M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 10 / 32

18. Case studies 1 Dilute (Ga,Mn)N [W. Stefanowicz et al., arXiv.org 0912.4216 (2010)] 2 Fe segregation in GaN M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 11 / 32

19. TEM Wide range of Mn concentration up to ≈ 1 at. % Energy dispersive X-ray spectroscopy Low-resolution High-resolution No evidence of crystallographic phase separation No Mn-induced defects Is Mn really dilute? → SXRD M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 12 / 32

20. TEM Wide range of Mn concentration up to ≈ 1 at. % Energy dispersive X-ray spectroscopy Low-resolution High-resolution No evidence of crystallographic phase separation No Mn-induced defects Is Mn really dilute? → SXRD M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 12 / 32

21. SXRD Measurements conducted on BM20 at 10 KeV High crystal quality [from the FWHM of rocking curves around GaN(0002)] No evidence for second phases Increment of the lattice parameters with Mn concentration In situ annealing (under N-rich atmosphere) experiments → samples stable up to 900 ◦ C Where is Mn? → EXAFS M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 13 / 32

22. SXRD Measurements conducted on BM20 at 10 KeV High crystal quality [from the FWHM of rocking curves around GaN(0002)] No evidence for second phases Increment of the lattice parameters with Mn concentration In situ annealing (under N-rich atmosphere) experiments → samples stable up to 900 ◦ C Where is Mn? → EXAFS M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 13 / 32

23. EXAFS Measurements conducted on BM08 at Mn K-edge (a) kmin kmax EXAFS signal, k χ(k) 2 Mn substitutional in GaN at Ga sites (MnGa ) #490A Fit(MnGa) #100A Low local disorder 2 4 6 8 10 12 Photoelectron wavevector, k [Å−1] Mn-Ga ﬁrst shell expansion O ∆R=2.5(5)% MnGa MnI T Mn3GaN (b) Mn3GaN MnI Additional contributions can be ruled Magnitude of the FT, |χ(R)| [Å ] −3 MnI Mn O (c) out in the 5% limit I T Mn interstitial defects and Mn3 GaN 1 2 3 4 5 6 Rmin simulated via ab initio codes Rmax [VASP+FEFF8] What Mn charge state? → XANES 1 2 3 4 5 6 7 8 Distance, R [Å] − without phase correction M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 14 / 32

24. EXAFS Measurements conducted on BM08 at Mn K-edge (a) kmin kmax EXAFS signal, k χ(k) 2 Mn substitutional in GaN at Ga sites (MnGa ) #490A Fit(MnGa) #100A Low local disorder 2 4 6 8 10 12 Photoelectron wavevector, k [Å−1] Mn-Ga ﬁrst shell expansion O ∆R=2.5(5)% MnGa MnI T Mn3GaN (b) Mn3GaN MnI Additional contributions can be ruled Magnitude of the FT, |χ(R)| [Å ] −3 MnI Mn O (c) out in the 5% limit I T Mn interstitial defects and Mn3 GaN 1 2 3 4 5 6 Rmin simulated via ab initio codes Rmax [VASP+FEFF8] What Mn charge state? → XANES 1 2 3 4 5 6 7 8 Distance, R [Å] − without phase correction M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 14 / 32

25. Charge state Role of TMs charge state in DMS Exchange interactions Carriers and their localization Inter-atomic Coulomb interactions Mn in GaN Mn2+ → 3d 5 Mn3+ → 3d 4 M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 15 / 32

26. Charge state Role of TMs charge state in DMS Exchange interactions Carriers and their localization Inter-atomic Coulomb interactions Mn in GaN Mn2+ → 3d 5 Mn3+ → 3d 4 M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 15 / 32

27. XANES Measurements conducted on BM08 at Mn K-edge (a) Absorption edge shifts with the #100A #490A charge state ⇒ Mn3+ ⇒ Norm. µ(E) MnO Mn2O3 MnO2 conﬁrmed by pre-edge peaks 6535 6540 6545 6550 6555 6560 6565 6570 (b) #100A #490A Norm. Absorption Coefficient − µ(E) 4 Mn(3d ) 5 Mn(3d ) (c) A4 A3 A1 A2 Bkg Fit 6537 6540 6543 6546 6549 6530 6540 6550 6560 6570 6580 6590 6600 6610 Fine-structure simulated with Energy [eV] FDMNES ⇒ MnGa M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 16 / 32

28. SQUID Accurate subtraction of the substrate diamagnetic signal and extra m(H,T) Magnetization(T) Magnetization(H) ⇒ Paramagnetism from non-interacting Mn3+ ions M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 17 / 32

29. SQUID Accurate subtraction of the substrate diamagnetic signal and extra m(H,T) Magnetization(T) Magnetization(H) ⇒ Paramagnetism from non-interacting Mn3+ ions M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 17 / 32

30. Summary Dilute (Ga,Mn)N - x ≤ 1% Mn substitutional in GaN Mn3+ charge state Paramagnetic Outlook Increase Mn concentration (in the DMS limit) Co-doping: (Ga,Mn)N:Si → n-type → Mn2+ (Ga,Mn)N:Mg → p-type → itinerant h M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 18 / 32

31. Summary Dilute (Ga,Mn)N - x ≤ 1% Mn substitutional in GaN Mn3+ charge state Paramagnetic Outlook Increase Mn concentration (in the DMS limit) Co-doping: (Ga,Mn)N:Si → n-type → Mn2+ (Ga,Mn)N:Mg → p-type → itinerant h M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 18 / 32

32. Case studies 1 Dilute (Ga,Mn)N 2 Fe segregation in GaN [A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010)] M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 19 / 32

33. (Ga,Fe)N: a high TC magnetic semiconductor? Ferromagnetism persisting above Paramagnetism from isolated Fe3+ room temperature What is the origin of the ferromagnetism? M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 20 / 32

34. (Ga,Fe)N: a high TC magnetic semiconductor? Ferromagnetism persisting above Paramagnetism from isolated Fe3+ room temperature What is the origin of the ferromagnetism? M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 20 / 32

35. Presence of ε-Fe3 N nano-crystals Bainite structure (hexagonal), ≈ 15 nm average size, TC = 575 K ⇒ Solubility limit at ≈ 0.4 % Fe in our growth conditions M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 21 / 32

36. Presence of ε-Fe3 N nano-crystals Bainite structure (hexagonal), ≈ 15 nm average size, TC = 575 K ⇒ Solubility limit at ≈ 0.4 % Fe in our growth conditions M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 21 / 32

37. How to control Fe incorporation in GaN? Growth rate Co-doping Growth temperature M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 22 / 32

38. Control by growth rate ⇒ Solubility limit increased by increasing the growth rate (TMGa) M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 23 / 32

39. Control by growth rate ⇒ Solubility limit increased by increasing the growth rate (TMGa) M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 23 / 32

40. Fermi-level engineering by co-doping with Si Partial charge state reduction, Fe3+ → Fe2+ , demonstrated by XANES Fe 3d states resides in the GaN band gap Co-doping with Si → tuning Fermi level → Fe charge state modiﬁcation Coulomb repulsion between Fe ions hinders aggregation [T. Dietl, Nature Mat. (2006)] Do this really hinders the aggregation? M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 24 / 32

43. Control by co-doping ⇒ Solubility limit increased by co-doping with Si M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 25 / 32

44. Control by co-doping ⇒ Solubility limit increased by co-doping with Si M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 25 / 32

45. Growth temperature SXRD and TEM Additional Fe-rich phases: γ -Fe4 N, ζ-Fe2 N, α-Fe, γ-Fe Nano-crystals size: 10-20 nm M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 26 / 32

46. Growth temperature XANES & EXAFS S691 Norm. Absorption Coefficient − µ(E) [arb. shift] S680 S690 950 °C Fit 3 Fit 2 XANES: linear Fit 1 850 °C combination fits of 800 °C theoretical spectra Fe−Ga (FDMNES) ⇒ identification Fe−Fe and concentration of Rmin Fe−N |χ(R)| Rmax different phases EXAFS: refinement of the 1 2 3 4 5 EF Distance, R [Å] − without phase correction local structure 7110 7120 7130 7140 7150 7160 7170 7180 7190 Energy [eV] SXRD, (HR)TEM and XAS ⇒ phase-diagram of Fex Ny in GaN M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 27 / 32

47. Growth temperature XANES & EXAFS S691 Norm. Absorption Coefficient − µ(E) [arb. shift] S680 S690 950 °C Fit 3 Fit 2 XANES: linear Fit 1 850 °C combination fits of 800 °C theoretical spectra Fe−Ga (FDMNES) ⇒ identification Fe−Fe and concentration of Rmin Fe−N |χ(R)| Rmax different phases EXAFS: refinement of the 1 2 3 4 5 EF Distance, R [Å] − without phase correction local structure 7110 7120 7130 7140 7150 7160 7170 7180 7190 Energy [eV] SXRD, (HR)TEM and XAS ⇒ phase-diagram of Fex Ny in GaN M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 27 / 32

48. Growth temperature SQUID Paramagnetic part ⇒ dilute and non-interacting FeGa Superparamagnetic-like part ⇒ ferromagnetic ε-Fe3 N, γ -Fe4 N α-Fe and γ-Fe Antiferromagnetic component ⇒ ζ-Fe2 N and highly-nitrated Fex N (x≤2) phases M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 28 / 32

49. Summary Segregated (Ga,Fe)N High TC FM → ε-Fe3 N clusters Solubility limit controlled by → growth-rate → co-doping Growth temperature → additional Fex Ny phases → wide range of magnetic behaviors Outlook Control of single-phase Fex Ny in/on GaN M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 29 / 32

50. Summary Segregated (Ga,Fe)N High TC FM → ε-Fe3 N clusters Solubility limit controlled by → growth-rate → co-doping Growth temperature → additional Fex Ny phases → wide range of magnetic behaviors Outlook Control of single-phase Fex Ny in/on GaN M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 29 / 32

51. CMS Second generation spintronics devices Spin battery Electromotive force and huge magnetoresistance observed in magnetic tunnel junctions with zb-MnAs clusters Demonstrated at 30 K [N.H. Pham et al., Nature (2009)] M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 30 / 32

52. Conclusions Growth characterization protocol Self-consistent loop 1 Growth/optimization 2 Structural characterization 3 Magnetic characterization 4 Modeling/simulation Synchrotron radiation as required tool M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 31 / 32

53. Thank you for your attention! Contact/further discussion mauro.rovezzi@jku.at References for this work W. Stefanowicz et al., arXiv.org 0912.4216 (2010) → soon in Phys. Rev. B A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010) M. Rovezzi et al., Phys. Rev. B 79, 195209 (2009) A. Bonanni et al., Phys. Rev. Lett. 101, 135502 (2008) M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 32 / 32

From DMS to CMS: a characterization protocol

Recommended

Recommended

More Related Content

Viewers also liked

Viewers also liked (13)

Similar to From DMS to CMS: a characterization protocol

Similar to From DMS to CMS: a characterization protocol (20)

Recently uploaded

Recently uploaded (20)

From DMS to CMS: a characterization protocol