Tack Acky Uyeda School of Information Science, JAIST, Ishikawa, Japan. Ohmic contacts on diamond semi-conductor devices 30 th Jul.2013

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

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

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..

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

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

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

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)

・ 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.

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

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

In-gap state Ta with Oxygen termination Valenve bond

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

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)

Results "CASINO" DMC calculations w/o term'd H-term'd O-term'd (4) (4) (4) (4) (4) (in_Progress) (3) (2) (6) (5) (6) (in_Progress) system size Cohesion energies in hartree. w/o term'd H-term'd O-term'd DFT calculation(NC PP)

Results "CASINO" DMC calculations w/o term'd H-term'd O-term'd z (4) (4) (4) (3) (2) (6) system size Cohesion energies in hartree. w/o term'd H-term'd O-term'd DFT calculation(NC PP)

Results Cohesion energies in hartree. coh_E(hta221)= ( *( )) = coh_E(hta221)= ( *( )) = coh_E(ota221)= ( *( )) = coh_E(ota221)= ( *( )) = coh_E(xta221)= ( *( )) = coh_E(xta221)= ( *( )) = Comparing DFT and QMC QMC cohesion data DFT cohesion data