Growth of chalcopyrite type magnetic semiconductors

1. Growth of chalcopyrite type magnetic semiconductors TUAT K. Sato, T. Ishibashi, V. Smirnov,H. Yuasa, J. Jogo, T. Nagatsuka,Y. Kangawa and A. Koukitu

2. Scope of this talk • Brief 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 • Characterization

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. • Ab-initio 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.

4. Si, Ge IV Diamond structure GaP III V Zincblende structure II V CdGeP2 IV V Chalcopyrite structure Ge P Cd Chalcopyrite Structure

5. II-IV-V2 chalcopyrites

6. 77K 287K 423K CdGeP2-MnMagnetization (VSM) K.Sato et al.: J.Phys.Chem.Solids 64(2003)1461

7. ZnGeP2-MnMagnetization (SQUID) 5K K.Sato, G.Medvedkin, T. Ishibashi: J.Cryst. Growth 236 (2002) 609 150K 350K

8. 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.

9. Previous preparation method for chalcopyrite-type magnetic semiconductors • 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 to remain. • Mn-diffused crystals show ferromagnetism above room temperature.

10. CdGeP2{112} Directional freezing of the stoichiometric melt in a quartz ampoule or graphite crusible Rate: 4deg/h for 48h Highly compensated n-type Prepared at Ioffe Inst. ZnGeP2(001) Vertical bridgeman technique Bulk ingot of 28mm and 150mm in length Highly compensated p-type Prepared at Siberian Physico-Technical Inst. II-IV-V2 single crystals

11. 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

12. RHEED patterns during Mn deosition ZnGeP2 During depo. After depo. After annealing 550℃30min. Tsub.= R.T. Tsub. = 400℃

13. 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 is proceeding

14. Inhomogeneous depth profile of Mn in CdGeP2:Mn

15. 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, 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.

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

17. NMR studies in ZnMnGeP2 • 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 Hwang et al.: Appl. Phys. Lett. 83 (2003) 1809

18. In-situ photoelectron spectroscopy • 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

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

20. T = 400 C(const.) 0 < d < 510Å PES during deposition end MnGeP2? Intensity (arbitrary units) Mg Ka ZnGeP2:Mn start 結合エネルギー (eV)

21. 3.0 Mn 堆積 2.5 P 2p 2.0 1.5 内殻電子放出強度 (arb.units) Ge 3d 1.0 Mn 2p 0.5 Zn 2p3/2 0.0 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 0 1 10 100 名目上の Mn層厚 (A) Core signal intensity MnGeP? No Zinc

22. End sputter Intensity (arbitrary units) Start sputter Mg Ka Binding energy (eV) PES during sputter T = 400 C d = 250Å

23. Core signal intensity during sputtering Zn:Ge:P ~ same as substrate composition Mn-rich composition Mn2+compounds (DMS phase) Core-level intensity ratio Total sputtering time (min.)

24. Magnetization (by SQUID magnetometer) MPMS MPMS

25. 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?

26. 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 and P4 using cracking cell at 813 C

27. Thermodynamic analysis for MOMBE growth of MnGeP2 • To know whether MnGeP2 can be obtained as a stable compound using the MOMBE technique, thermodynamic analysis is performed.

28. 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:

29. 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 X Driving force for deposition, P

30. MOMBE • 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.

31. 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)

32. enthalpy of mixing of (Mn,Ge)P as a function of solid composition • The function,Hm, is estimated from the ab initio total energy calculations for structure models.

33. Ge Mn P GeP MnP Mn0.5Ge0.5P Ab-initio calculation • Ab initio calculationsusing CASTEP code • Task: Geometry optimization • Electron correlation: GGA • Energy cutoff: 240eV

34. 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 Stable Formation of MnGeP2

35. 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.

36. 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

37. -8 Base pressure @3.0×10 Torr -8 Mn @2.0×10 Torr -8 Ge @1.3,2.1×10 Torr Crystal Growth Conditions Pump C.C gauge Substrate @SI-GaAs(100) Just (Etched by H20+H2O2+NH3) Growth temperature @360,415℃ Substrate Flux intensity: Flux monitor REED (K-cell temp. @725℃) Screen (K-cell temp. @1035,1060℃) TBP (Gas) Cd TBP flow rate @1.6 sccm Mn Ge (Cracking temp. @813℃) (Solid) Deposition time @20,60 min

38. EDX -8 -8 -8 -8 #1 #2

39. #3

40. SEM Observation 300nm 400nm 400nm Sample#1 Sample#2 Sample#3

41. 800nm 800nm 800nm Sample#1 Sample#2 Sample#3

42. 4 10 3 10 6 10 5 2 10 10 Intensity [cps] 65.6 65.8 66 66.2 66.4 2θ [deg] XRD Patterns Sample#3 Sample#1 Sample#2

43. XRD Measurement GaAs(004) Detailed XRD study is underway MnGeP2? GeP? MnGeP2–narrow scan ① 1.72：1.00：2.99 (1h)

44. Perpendicular Parallel -5 M (10 emu) M (10 emu) -5 H (Oe) H (Oe) ・VSM ・Room Temp. 10 Magnetic Properties 8 Sample#3 6 4 2 0 -2 -4 -6 14 -8 12 10 -10 8 6 4 2 0 -2 -4 -6 -8 Mn;1.03 Ge;1.00 P;1.89 -10 -12 -14

45. Polar Magneto-optical Spectra in MnGeP2

46. Summary • As one of approaches to elucidate the origin of room temperature ferromagnetism in magnetic chalcopyrites, growth of chalcopyrite type MnGeP2, which is not existing in nature is investigated. • Thermodynamic study including ab-initio evaluation of Hm confirms stable formation of MnGeP2 by MBE technique. • MOMBE growth of MnGeP2 films are studied. Nearly stoichiometric compounds are obtained. They show ferromagnetism and weak magneto-optical effect. • Further careful investigation is necessary to discriminate the effect of possible second phase material.