1. Tack Acky Uyeda School of Information Science, JAIST, Ishikawa, Japan. Ohmic contacts on diamond semi-conductor devices tueeeda@jaist.ac.jp TTI2013@Italy, 30 th Jul.2013

2. Collaboration with Kenya/Eldoret Group Prof. N.W. Makau, Prof. G.O. Amolo, and Mr. I. Motochi

3. Background New Generation Semi-conductors Power semi-conductors Diamond semi-conductor! (c.f,, Si for processors etc.) AC/DC conversion, High frequency signal processing, etc. (SiC, GaN, Diamond ; wide-gap) Si, GaAs, InP Frequency Power GaN SiC Diamond →High Power, High Freq., →Easy to chilled due to heat conductivity

4. How to fabricate Ohmic Contact Required Properties ・ Ohmic barrier ・ Difficult to be peeled. Motivation  Except Si, the process NOT established. Preventing Schottky junction (bring about rectification) For new semi-conductors,... Try & Error such as scratching surfaces..

5. Which kind of metal used? Surface termination for dangling bonds - Tighten the bonding between Electrode & Slab. What is clarified Au, Ta, Ti,... Diamond metal - Introducing carrier to make N/P semi-conductor

6. Project Here By using DFT/QMC, ・ Robustness against peeling ・ Ohmic property Our Conclusion so far Ta with Oxygen-termination gives deepest cohesion with Ohmic property. → Surface Cohesive energies. → DOS between Gap. DOS of Diamond bulk

7. Cohesive Energy x = Ti, V, Ta, and Au, Pd Oxygen, Hydrogen or Without ＜ Slab ＞ ＜ Parent Surface ＞ Diamond metal ＜ Atom ＞ Diamond

8. Method DFT study QUANTUM ESPRESSO (PW basis) ・ XC functionals ; PBE or LDA ・ Pseudo Potentials; UltraSoft & NormConserving (NC) (Trail-Needs) ・ Structure Optimization; by PBE (both Ultrasoft, NC)

9. ・ DFT ・ PBE ・ ultra-soft pseudo potential ・ variable cell relaxed Results Dotted lines ; Motochi's previous work (predicting V as best) → Present results predict Ta is best.

10. Results Motochi's previous work ; V is best Present results ; Ta is best. cutt off energy 37 [Ry] → 90 [Ry] k-point 4x4x1 → 14x14x1

11. ・ DFT ・ LDA ・ Ultra-soft pseudo potential Results From now, sticking to Ta & V cases. LDA doesn't change the trend.

12. In-gap state Ta with Oxygen termination Valenve bond

13. ・ DFT ・ Norm Conserving pseudo potential Results ・ PBE NC-pp evaluation also predict Ta as best.

14. Results V ; 3s(2) 3p(6) 3d(3)4s(2) Ta ; 5s(2) 5p(6) 5d(3)6s(2) Ta without semi-core elec. Now the evaluation using the same core-size in progress... But we expect Ta will get more cohesive energy NC-pp evaluation also predict Ta as best. We note that... Different core sizes for V and Ta for NC-PP. N.B.) similar elec. config. for V and Ta. ( ≒ not changing the conclusion)

15. Results "CASINO" DMC calculations w/o term'd H-term'd O-term'd 221 -3.6878(4) -3.6499(4) -3.7898(4) 441 -3.8571(4) -3.6124(4) (in_Progress) 221 -0.34532(3) -0.3131(2) -0.36244(6) 441 -0.3808(5) -0.3023(6) (in_Progress) system size Cohesion energies in hartree. w/o term'd H-term'd O-term'd 221 -0.48060 -0.66995 -0.61163 441 -0.58996 -0.85234 -0.81761 221 -0.22537 -0.28249 -0.24647 441 -0.35067 -0.39617 -0.36941 DFT calculation(NC PP)

16. Results "CASINO" DMC calculations w/o term'd H-term'd O-term'd z 221 -3.6878(4) -3.6499(4) -3.7898(4) 221 -0.34532(3) -0.3131(2) -0.36244(6) system size Cohesion energies in hartree. w/o term'd H-term'd O-term'd 221 -0.48060 -0.66995 -0.61163 221 -0.22537 -0.28249 -0.24647 DFT calculation(NC PP)

17. Results Cohesion energies in hartree. coh_E(hta221)= -68.09176 - ( -58.06663 + 2*(-3.18760)) = -3.64993 coh_E(hta221)= -68.16341 - ( -58.04167 + 2*(-4.82057)) = -0.48060 coh_E(ota221)= -98.92987 - ( -88.76489 + 2*(-3.18760)) = -3.78978 coh_E(ota221)= -99.12718 - ( -88.81609 + 2*(-4.82057)) = -0.66995 coh_E(xta221)= -66.82612 - ( -56.76308 + 2*(-3.18760)) = -3.68784 coh_E(xta221)= -67.02145 - ( -56.76868 + 2*(-4.82057)) = -0.61163 Comparing DFT and QMC QMC cohesion data DFT cohesion data