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MBE growth of a novel chalcopyrite-type ternary compound MnGeP 2

TUAT. MBE growth of a novel chalcopyrite-type ternary compound MnGeP 2. K. Sato, T. Ishibashi, K. Minami, H. Yuasa. J. Jogo, T. Nagatsuka, A. Mizusawa, Y. Kangawa and A. Koukitu Tokyo University of Agriculture and Technology,. View from City Park, Denver. Scope of this talk.

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MBE growth of a novel chalcopyrite-type ternary compound MnGeP 2

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  1. TUAT MBE growth of a novel chalcopyrite-type ternary compound MnGeP2 K. Sato, T. Ishibashi, K. Minami, H. Yuasa. J. Jogo, T. Nagatsuka, A. Mizusawa, Y. Kangawa and A. Koukitu Tokyo University of Agriculture and Technology, View from City Park, Denver ICTMC14, Denver, Oct. 1, 2004

  2. Scope of this talk • Summary of previous studies of chalcopyrite type magnetic semiconductors • Results of in-situ photoelectron spectroscopy • Suggested existence of chalcopyrite MnGeP2 • Thermodynamic analysis • MOMBE growth of MnGeP2 • Magnetic and magneto-optical characterization ICTMC14, Denver, Oct. 1, 2004

  3. Brief summary of previous studies of chalcopyrite type magnetic semiconductors • We have been working with Mn-substituted chalcopyrite type semiconductors CdGeP2 and ZnGeP2, in which we have confirmed ferromagnetic behavior up to 423 K and 350 K, respectively. Magneto-optical effect was also observed. • These samples were obtained by deposition and subsequent diffusion of Mn to bulk single crystals of ternary compounds. • Theoretical calculation suggests that CdGeP2 system with vacancies or non-stoichiometric composition will lead to ferromagnetism although ferromagnetism is not favored in stoichiometric (Cd, Mn)GeP2. ICTMC14, Denver, Oct. 1, 2004

  4. II-IV-V2 chalcopyrites ICTMC14, Denver, Oct. 1, 2004

  5. Previous preparation method • Mn was deposited on single crystals of CdGeP2 and ZnGeP2 at Tsub400C, by which Mn was diffused into the bulk to substitute group II and IV cations. • During growth RHEED pattern of chalcopyrite structure seems sustained • Mn-diffused crystals show ferromagnetism above room temperature. ICTMC14, Denver, Oct. 1, 2004

  6. II-IV-V2 single crystal Mn II-IV-V2 single crystal Mn-diffused layer II-IV-V2 single crystal Preparation of Mn-doped chalcopyrites Host crystal: CdGeP2, ZnGeP2 Mn deposition Tsub=RT to 380-400°C Mn diffusion @T=300-500°C ICTMC14, Denver, Oct. 1, 2004

  7. ZnGeP2 Before deposition Tsub.= R.T. Tsub. = 400℃ During deposition After deposition After annealing 550℃30min. RHEED patterns during Mn deosition Tepmerature of Deposition is important to keep CH structure ICTMC14, Denver, Oct. 1, 2004

  8. XRD of ZnGeP2:Mn by RINT RAPID Ring: Unident ified phase (not MnP) Chalcopyrite spot ICTMC14, Denver, Oct. 1, 2004

  9. 0.0004 (a) 0.0002 0 M (emu) -0.0002 in plane perpendicular -0.0004 -10000 0 10000 H (Oe) 0.0004 (b) 0.0002 0 M (emu) -0.0002 in plane perpendicular -0.0004 -10000 0 10000 H (Oe) Magnetization Curvesat RT • Hc~0.4kOe, Hs~2kOe for in-plane magnetization • Ms=3.510-4emu, if all 30nm Mn atoms are incorporated V=3mm5mm0.03m=4.510-7cm3 0.95610-20 emu/atom • gS=1.03 →1.03B (S=1/2) ICTMC14, Denver, Oct. 1, 2004

  10. S = 1/2 Magnetization (emu) S = 5/2 Temperature (K) Temperature-dependence of magnetization at H=0 Tc=320K ICTMC14, Denver, Oct. 1, 2004

  11. CdGeP2-MnMagnetization (VSM) 77K 287K K.Sato et al.: J.Phys.Chem.Solids 64(2003)1461 423K ICTMC14, Denver, Oct. 1, 2004

  12. ZnGeP2-MnMagnetization (SQUID) 5K K.Sato, G.Medvedkin, T. Ishibashi: J.Cryst. Growth 236 (2002) 609 150K 350K ICTMC14, Denver, Oct. 1, 2004

  13. Origin of Ferromagnetism? • It seems two ferromagnetic components are mixed, one with Tc~320K and the other with Tc higher than 400K. ICTMC14, Denver, Oct. 1, 2004

  14. 0.05 0.00 K, K (deg) -0.05 K -0.10 K K -0.15 1 1.5 2 2.5 3 3.5 4 4.5 Photon Energy [eV] Magneto-Optical Kerr Effect K. Sato et al.: J. Magn. Soc. Jpn. 25 (2001) 283. ICTMC14, Denver, Oct. 1, 2004

  15. Problems • Inhomogeneous depth profile of Mn obtained by the deposition-diffusion technique. • Electrical properties of the surface shows a metallic behavior. • Preparation of homogeneously Mn-doped layer is necessary. Effort to obtain CdGeP2:Mn thin films by MBE ICTMC14, Denver, Oct. 1, 2004

  16. Careful preparation necessary • Synthesis of bulk or powder CdGeP2:Mn from constituent elements was tried. However, it was difficult to prevent formation of second phase compounds. • In bulk ZnGeP2:Mn prepared at elevated temperature (T>550C), room-temperature ferromagnetism is suspected as due to MnP precipitated in the material. • Careful preparation of films with homogeneous distribution of Mn is strongly required. ICTMC14, Denver, Oct. 1, 2004

  17. Magnetic properties of bulk ZnMnGeP2 Cho et al. Phys. Rev. Lett. 88 (2002)257203 • Preparation by solid state reaction of Zn+Ge+Mn+P at max 1130C • Antiferromagnetism below 47K • Ferromagnetism between 47 and 312K MH curveMn3% MT curve MH curve Mn5.6% ICTMC14, Denver, Oct. 1, 2004

  18. NMR studies in ZnMnGeP2 Hwang et al.: Appl. Phys. Lett. 83 (2003) 1809 • Very small amount of MnP phase that cannot be found by XRD was detected by NMR in polycrystalline ZnMnGeP2 material prepared by the same method as did by Cho. ZnMnGeP2 Mn15% Mixture of ZnGeP2 and MnP ICTMC14, Denver, Oct. 1, 2004

  19. Inhomogeneous depth profile of Mn in CdGeP2:Mn Medvedkin et al.:JJAP 39 (2000) L949 ICTMC14, Denver, Oct. 1, 2004

  20. In-situ photoelectron spectroscopy Ishida et al. Phys. Rev. Lett. 91 (2003) 107202 • Photoelectron spectrometer with MBE system With Ar-ion etching device • Synchrotron radiation: Photon Factory BL-18A • Specimen: ZnGeP2single crystal, polished and etched • Deposit Mn and interrupt to measure PES • After deposition of 50nm Mn, sputter-etched by Ar-ion and at each stage PES was measured ICTMC14, Denver, Oct. 1, 2004

  21. Radiation DE~100meV Mg Ka X-ray DE~800meV Cleaning the substrate Sputtering out the surface layer Thermocouple Mn evaporator (Omicron EFM-4) 99.999%Mn Heater 1cm P<5x10-9 Torr (sample growth) P<7x10-10 Torr (PES measurement) Ion gun 1.5kV Ar+ Thickness monitor In-situ Photoemission Apparatusat Photon Factory BL-18A ICTMC14, Denver, Oct. 1, 2004

  22. Mn 2 p Mn 2 p Zn 2 p 1/2 3/2 3/2 T = 400 C(const.) 0 < d < 510Å 1024 1020 650 640 PES during deposition MnGeP2 end Mn signal Zn signal ZnGeP2:Mn start ICTMC14, Denver, Oct. 1, 2004

  23. Mn depo 3.0 2.5 P 2p MnGeP? 2.0 1.5 PE yield (arb.units) Ge 3d No Zinc 1.0 Mn 2p 0.5 Zn 2p3/2 0.0 0 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 1 10 100 Nominal thickness (A) Core signal intensity during deposition ICTMC14, Denver, Oct. 1, 2004

  24. PES during sputter End sputter Intensity (arbitrary units) Start sputter T = 400 C d = 250Å ICTMC14, Denver, Oct. 1, 2004 Mg Ka Binding energy (eV)

  25. Intensity (arbitrary units) Mn satellite Binding energy (eV) Fermi edge Sputtering time dependence of Mn 3p - 3d resonance photoemission structure ICTMC14, Denver, Oct. 1, 2004

  26. Zn:Ge:P ~ same as substrate composition MnGeP2 ? Mn2+compounds (DMS phase) Core-level intensity ratio Total sputtering time (min.) Core signal intensity during sputtering Zn Ge Mn ICTMC14, Denver, Oct. 1, 2004

  27. M-H and M-T curves of Mn-deposited ZnGeP2 crystal at 400℃ • M-H curves at T=10 and 330 Kupper: before sputter-etchinglower: after 200 min sputter • M-T curves in the sputtering series ICTMC14, Denver, Oct. 1, 2004

  28. Suggested existence of chalcopyrite MnGeP2 • Photoemission The surface composition is MnGeP2 • RHEED pattern of initial chalcopyrite structure remained during growth Is chalcopyrite-type MnGeP2 really exist? ICTMC14, Denver, Oct. 1, 2004

  29. Cd1-xMnxGeP2 (0≦x ≦1) CdGeP2 MnGeP2 Substrate (x=0) (x=1) Substrate Substrate ・Confirming of chalcopyrite-type MnGeP2. ・Preparing high-quality single crystal thin film by MBE. Two approaches for obtaining II1-xMnxGeP2 thin film Found very difficult ICTMC14, Denver, Oct. 1, 2004

  30. Thermodynamic analysis for MBE growth of MnGeP2 • To know whether MnGeP2 can be obtained as a stable compound using the MBE technique, thermodynamic analysis is performed. ICTMC14, Denver, Oct. 1, 2004

  31. Driving force for deposition In the thermodynamic analysis, we used parameters of driving force for deposition P, Input partial pressure, P0, and equilibrium partial pressure at vapor-solid interface, P. Here, driving force for deposition Pis the difference between input partial pressure and equilibrium partial pressure : P=P0-P; where P0 is input partial pressure, and P equilibrium partial pressure Using these parameters, we can obtain Input mole ratio, RMn, and solid composition, x, asfollows: ICTMC14, Denver, Oct. 1, 2004

  32. Boundary layer P0 DP=P0-P Partial pressure Substrate surface P Distance Driving force for deposition, P P0: Input partial pressure P:Equilibrium partial pressure Driving force for deposition, P X ICTMC14, Denver, Oct. 1, 2004

  33. MBE • Here, we assume P2 molecule as a group-V source, because more than 80% of TBP is cracked and changed to P2 rather than P4 at 813C. • Mn(g)+1/2 P2(g) = MnP(s) • Ge(g)+1/2 P2(g) = GeP(s) Conservation constraints • PMn+Ge0-PMn+Ge= 2(PP20-PP2) • Pi = PMn+PGe+PP2 Equilibrium equation for reaction using these equations equilibrium partial pressure, which is unknoun parameter, is calculated. ICTMC14, Denver, Oct. 1, 2004

  34. Ab-initio calculation of enthalpy of mixing • Enthalpy of mixingHmHm=EMnGeP-{xEMnP+(1-x)EGeP} • Interaction parameter = Hm/x(1-x) • Solid composition xx= PMn/(PMn+ PGe) vs Input molar ratio of MnRMn=P0Mn/(P0Mn+P0Ge) Ab initio calculations using DMOL3 code ICTMC14, Denver, Oct. 1, 2004

  35. Ge P Mn GeP MnP Mn0.5Ge0.5P Enthalpy of Mixing in (Mn,Ge)P as a Function of Solid Composition • The function,Hm, is estimated from the ab initio total energy calculations for structure models. (ZB) (ZB) ICTMC14, Denver, Oct. 1, 2004

  36. Calculated Enthalpy of Mixing • It is found, in the graph, that enthalpy of mixing has negative value. • This is because the chalcopyrite structure with x=0.5 becomes stable compared with random alloy ICTMC14, Denver, Oct. 1, 2004

  37. Interaction parameter  W=DHm/x(1-x) From the calculated enthalpy of mixing, Hm, we estimated interaction parameter, W, to be 3044x-33726 [cal/mol]. Using this function, we carried out the thermo-dynamic analyses and examined the relationships between input mole ratio and solid composition. ICTMC14, Denver, Oct. 1, 2004

  38. Here, the thermodynamic calculations were performed under the following conditions; PMn0+PGe0=1.0x10-7 torr PP20=2.0x10-7 torr It is found, in the graph, that small input mole ratio is required to make MnGeP2 at higher temperatures. This is because that Mn-P bond is easily formed compared with Ge-P bond. MnGeP2 Vapor/solid distribution relationship ICTMC14, Denver, Oct. 1, 2004

  39. MOMBE growth of MnGeP2 • We applied MOMBE technique to obtain MnGeP2 films on GaAs substrate. • Mn and Ge are supplied from solid state source using K-cells • As P source, TBP (tertiary butyl phosphine) MO source is employed. • TBP is cracked to form P2 using cracking cell at 813 C ICTMC14, Denver, Oct. 1, 2004

  40. MBE apparatus • Mn, Ge: solid source K-cell • P: gas sourcecracking cell at 813C ICTMC14, Denver, Oct. 1, 2004

  41. GV Cold cathode gauge TSP DP trap Load-lock chamber Substrate Ion gauge GV Growth chamber TMP DP RHEED gun RHEED screen Exhaust RP V Mn Ge AV RP MFC TBP K-cells V AV Cracking cell Exhaust Exhaust RP TMP Illustration of MBE apparatus ICTMC14, Denver, Oct. 1, 2004

  42. Ta mesh Ta cylinder PBN cylinder Ta heater TBP Stainless pipe Ta shot Dependence of gas composition on the temperature of the cracking cell Laboratory-made Crackin Cell Cracking of TBP Beam et al.: J. Cryst. Growth 116 (‘92) 436 ICTMC14, Denver, Oct. 1, 2004

  43. Mn flux [Torr] Ge flux [Torr] TBP flow [sccm] Growth Temp. [oC] Growth Time [min] Sample Mn:Ge:P 1.0x10-8 1.0x10-8 1.6 541 90 #1 0.81:1.00:1.11 1.1x10-8 1.1x10-8 2.0 414 240 1.72:1.00:2.99 #2 #3 0.9x10-8 0.9x10-8 2.0 400 180 1.37:1.00:2.59 #4 0.64x10-8 0.64x10-81 2.0 400 180 0.95:1.00:2.14- Growth condition on GaAs (001) ICTMC14, Denver, Oct. 1, 2004

  44. GaAs 004 541C GaAs 002 high Ge,Mn flux rate 414C GeP 004 GeP 002 400C MnP Intensity [a.u.] x x Cu Kb #1 400C low Mn Ge flux x x #2 #3 #4 X: from Al holder MnP 10 20 30 40 50 60 70 80 90 2 [deg] XRD in the Films grown on GaAs ICTMC14, Denver, Oct. 1, 2004

  45. GaAs113 Reciprocal lattice mapping • MnGeP2/GaAs • sample#3 Diffraction spots from the MnGeP2 deviates slightly from that of substrate ICTMC14, Denver, Oct. 1, 2004

  46. Mn flux [Torr] Ge flux [Torr] TBP flow [sccm] Growth Temp. [oC] Growth Time [min] Sample Mn:Ge:P #5 0.9x10-8 0.9x10-8 2.0 435 180 1.92:1.00:- 1.0x10-8 1.0x10-8 2.0 342 180 1.29:1.00:- #6 #7 0.65x10-8 0.9x10-8 2.0 342 100 0.95:1.00:- #8 0.64x10-8 0.64x10-8 2.0 435 180 1.40:1.00:- Growth condition on InP (001) ICTMC14, Denver, Oct. 1, 2004

  47. InP 002 InP 004 Ts=435C MnGeP2 008 Cu Kb MnP MnP Cu Kb Intensity [a.u.] #5 Ts=342C #6 #7 #8 10 20 30 40 50 60 70 80 90 2[deg] XRD in the Films grown on InP ICTMC14, Denver, Oct. 1, 2004

  48. Mn,Ge@9.0x10-9Torr TBP@2.0sccm InP 004 InP 002 TG@435oC Thickness@150nm Intensity [cps] MnGeP2 008 MnP Kb Kb 2 [deg] MnGeP2/InP(001) ICTMC14, Denver, Oct. 1, 2004

  49. <001> <110> Reciprocal lattice mapping MnGeP2/InP sample#8 MnGeP2 1110 MnGeP2 008 MnGeP2 008 InP 115 InP 115 MnGeP2 116 InP 004 InP 004 InP 113 ICTMC14, Denver, Oct. 1, 2004

  50. FLAPW Calculation Bulk Crystal c = 10.716 (Å) c = 11.269 (Å) Ge Ge Mn P Mn P a = 5.673 (Å) a = 5.655 (Å) Lattice constants of MnGeP2 Zhao et al, Phys. Rev. B.63(2001) 201202(R) Cho et al., Sol. St. Commun. 129 （2004） 609 ICTMC14, Denver, Oct. 1, 2004

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