Diffusion of 56 Co in GaAs, ZnO and Si 1-x Ge x systems INTC Meeting

Collaboration J. Räisänen, O. Koskelo, S. Väyrynen, V. Tuboltsev University of Helsinki, Accelerator Laboratory, Finland F. Tuomisto Helsinki University of Technology, Laboratory of Physics, Finland K. Johnston CERN/ISOLDE

3 University of Helsinki: - experiments at ISOLDE - diffusion coefficient determination Helsinki University of Technology: - vacancy determination by positron annihilation spectroscopy - ZnO material Cern/ISOLDE: - experiments at ISOLDE ROLE OF PARTNERS

4 Magnetic and magneto transport properties of ferromagnetic materials embedded in semiconductors. Use of GaAs and ZnO in spintronics Possible use of Co as magnetic impurity Use of Si 1-x Ge x alloy - device technology - impurity doping - magnetic impurities MOTIVATION

5 MATERIALS GaAs grown by low temperature molecular beam epitaxy (LT-MBE) and liquid encapsulated Czochralski (LEC) ZnO thick bulk single crystals grown by chemical vapour transport (CVT) Si 1-x Ge x materials: ●1-μm-thick relaxed Si 1-x Ge x (with x = 0.50, 0.65 and 0.80) epilayers grown by low energy plasma enhanced chemical vapor deposition (LEPECVD) ●1-μm-thick Si 0.85 Ge 0.15 material grown by chemical vapor deposition (CVD) ● intrinsic and p-doped (Ga; 3.5×10 18 and 2.5×10 19 cm -3 ) germanium ● intrinsic dislocation free CZ-grown silicon

6 AIMS OF STUDY Diffusion of Co in GaAs, ZnO and Si 1-x Ge x systems at low impurity concentrations Possible effect of Ga vacancies on Co diffusion in GaAs → two types of materials (growth kinetics) ● Excess of Ga vacancies accelerates diffusion in LT-MBE material compared with diffusion in LEC grown GaAs? ● Vacancy concentration can be varied. ● Effect of GaAs growth technique on diffusion properties considered for the first time in IS432 (Mn diffusion in GaAs).

7 AVAILABLE DIFFUSION DATA A. Cobalt diffusion in GaAs - Three previous measurements, but results scatter badly. Arrhenius plot. Solid and long-dashed lines from [1], dashed line from [2]. [1] M.B. Butt, B.L. Sharma, Diffusion in Semiconductors and Non-Metallic Solids, Springer-Verlag (1998); T.D. Dzhafarov, E.A. Skoryatina, E.S. Guds and I.E. Moronchuk, Phys. Stat. Sol. A51(1979)K221; V.A. Uskov and V.P. Sorvina, Izv. Akad. Nauk SSSR, Neorg. Mater. 8(1972)758. [2] G.S. Kulikov, I.N. Nikulitsa, “Diffusion and Solubility of Co in GaAs”, Soviet Physics-Solid State. 14 (1973) 2335.

8 Temperature ranges in previous studies: [1] (750 – 1050 o C) and [2] (580 – 1160 o C). Solubility of cobalt in GaAs sufficiently high (5 x /cm 3 at 1100 o C) [2] Same order as Mn solubility in GaAs Typical Mn diffusion profile in GaAs after annealing at 800 o C for one hour.

9 B. Cobalt diffusion in ZnO - For ZnO no previous diffusion data is available - Solubility of Co in ZnO is very high: at.% for bulk material

10 C. Cobalt diffusion in Si 1-x Ge x systems - For SiGe alloys no previous data available - For Ge one previous study, but experimental conditions differ from those proposed (not intrinsic and material with dislocations) Solid line: low dislocation density material Dashed line: high dislocation density material. L.Y. Wei, "Diffusion of Silver, Copper, Cobalt and Iron in Germanium", J. Phys. Chem. Solids 18 (1961) 162.

11 - Low dislocation density ~10 3 1/cm 2 - High dislocation density ~10 5 1/cm 2 - For dislocation-free material (< 100 1/cm 2 ) no values - Narrow temperature region 750 – 850 o C Available data for Si

12 The extremely low solubility of cobalt in silicon and germanium (about /cm 3 at 1100 o C) is the only foreseen possible cause of difficulties. EXPERIMENTAL TECHNIQUES Modified radiotracer technique Possible isotopes: 57 Co too long half-life (272 d) for studying diffusion under intrinsic conditions. 56 Co ideal (t 1/2 =77.26 d) Advantages of the tracer technique: - sensitive → studies with low Co concentration - wider temperature range (to lower T)

13 EXPERIMENTS AT ISOLDE Implantation of 56 Co: - MK5 ion source - yield 8.2x10 5 ions/μC - Minimum of about 20 successfully determined D-values → 2 runs of 3 shifts at the GLM beam line, total of 6 shifts

14 Interesting possibility: 56 Ni implantation (t= 6.08 d) decays to 56 Co annealing at ISOLDE immediately after implantation depth profiling as 56 Co afterwards yield for 56 Ni too low??

15 Stability of the ferromagnetic state in GaAs and ZnO predicted by first principles calculations.