novel dielectrics for advanced semiconductor devices Cristiano Krug and Gerry Lucovsky Department of Physics North Carolina State University

outline band edge states - nanocrystalline HfO 2 and ZrO 2 theory and experiment inherent limitations engineering solutions band edge states - non-crystalline Zr and Hf silicate alloys theory and experiment inherent limitations engineering solutions novel device structure experimental result proposed device structures research plan

band edge states nanocrystalline HfO 2 and ZrO 2 theory and experiment inherent limitations engineering solutions

theory -- crystal field and Jahn-Teller term-splittings model calculation - ZrO 2 band edge d-states two issues can XAS detect mixtures of tetragonal and monoclinic nano- crystallites? and can mixtures account for range of defect state energies in electrical measurements ? EgEg T 2g J-T - orthorhombic monoclinic C-F cubic

E g (2) T 2g (3) E g (2) T 2g (4+) as-deposited MO-RPECVD films by IR are monoclinic similar result for ZrO 2 multiple features in T 2g region are indicative of mixture of monoclinic and tetragonal by XRD nano-crystallite grains - different for different processing Stefan Zollner’s results at Freescale - XRD and SE

theory -- crystal field and Jahn-Teller term-splittings model calculation - ZrO 2 band edge d-states can XAS detect mixtures of tetragonal and monoclinic nano- crystallites? YES model predicts at least 4 features in T2g band observed for reactive evaporation, but not for MO-RPECVD EgEg T 2g J-T - orthorhombic monoclinic C-F cubic

model calculations indicate band edge defect state is associated with a Jahn-Teller distortion at internal grain boundary and is intrinsic to nano-crystalline thin films  -bonded d*-states/defects at conduction band edge in absorption constant (  2 ) and  conductivity (PC) onset of strong optical absorption - lowest E g state - optical band gap

Z. Yu et al., APL 80, 1975 (2002), in Chap High-K dielectrics, M. Houssa (ed), IOP, localized band edge J-T d*-states inherent asymmetry in transport and trapping including BTI’s trap depth eV, same state PC and band edge abs. trapping/Frenkel- Poole transport tunneling but not F-P x'port

crystal field and Jahn-Teller term-splittings model calculation using Zr and O atomic states can mixtures account for range of defect state energies in electrical measurements ? YES 3x energy scale ~ eV EgEg T 2g EgEg J-T - orthorhombic monoclinic C-F cubic

engineering solutions NEC solution limit applied bias so that injection into band edge defect states is not possible modify band tail states by alloying with divalent (MgO) or trivalent oxides (Y 2 O 3 ) e.g. Y 2 O 3 in cubic zirconia introduces vacancies random distribution gives cubic structure and eliminates J-T term splittings, but evidence for absorption associated with excitations to/from midgap state issue: is this state electrically active ? study has just been undertaken

VUV spectroscopic ellipsometry and UV-VIS transmission term-spitting removed - but new absorption band at ~4.1 eV sub-band-gap absorption - O vacancies Jahn-Teller term-split d-states of nc-ZrO 2 not in Y-Zr-O, but edge broadened ZrO %Y 2 O 3 cubic zirconia 4.1 eV

outline band edge states non-crystalline Zr and Hf silicate alloys theory and experiment inherent limitations engineering solutions

IR results - GB Rayner - PhD thesis, NCSU Si-O -1 group shoulder ~ 950 cm-1 grows with increasing x in as-films deposited changes continuously with annealing in inert ambient, Ar SiO 2 features at 1068, 810 and 450 cm -1 sharpen up with increasing T ann chemical phase separation “non-crystalline” by XRD, but, x=0.23 nano-crystalline by TEM and EXAFS

comparison of extended x-ray absorption fine structure and x-ray diffraction crystallite size difference for x ~ 0.25 and x ~ 0.5 from HRTEM x~0.25, ~3 nm x~0.5, ~10 nm

chemical phase separation (CPS) in Zr silicate and ZrSiON alloys after 900°C annealing doubly degenerate E g feature in non-crystalline Zr silicate alloys independent of alloy composition after 900°C anneal, chemical phase separation and crystallization E g narrowed/shifted 0.5 eV in Zr silicate, asymmetric in ZrSiON  E~0.5 eV n-c CPS

i) metal ions, Na 1+, Ca 2+, Y 3+, Zr 4+, etc.. disrupt network converting bridging Si-O-Si to terminal Si-O 1- group ii) number of terminal groups valence of metal ion, 1 for Na, 2 for Ca, 3 for Y and 4 for Zr iii) connectedness of network defined by shared corners, C s between SiO 4/2 units iv) C s = 4 perfect 3 D network, C s = 1,0 completely disrupted mixture of Si n O m molecular ions and metal ions statistical/mean field disruption of SiO 2 network 1:1 representation of silicate alloys rate of network disruption increases with valence of metal ion when normalized on a per/atom basis for x > x o for C s = 0, silicate is “inverted” and Si n O m are minority species xoxo c s = 0 cscs

(SiO 2 ) 0.4 (Si 3 N 4 ) 0.25 (ZrO 2 ) 0.35 pseudo-ternary (SiO 2 ) 1-x-y (Si 3 N 4 ) y (ZrO 2 ) x alloys remote plasma enhanced chemical vapor deposition as-deposited amorphous alloy – significant Si oxynitride bonding after anneal at 1000°C – chemical phase separation into SiO 2, nanocrystalline ZrO 2 with N-bonding

pseudo-ternary (SiO 2 ) 1-x-y (Si 3 N 4 ) y (ZrO 2 ) x alloys remote plasma enhanced chemical vapor deposition (SiO 2 ) 0.3 (Si 3 N 4 ) 0.4 (ZrO 2 ) 0.3 as-deposited amorphous alloy – significant Si oxynitride bonding after anneal at 1000°C – no chemical phase separation and self-organization encapsulating ZrSiO 4 bonding groups viable engineering solution, k~9-10, EOT to nm

novel device structures (one example) experimental results for Ge-SiO 2 no preoxidation C-V is as good as the best discussed by Saraswat of Stanford Univ. at Workshop on Future Electronics 2005 two approaches i) 15 oxidation followed by plasma nitridation ii) grow 3-5 atomic layers of pseudo-morphic Si on Ge and oxidize surgically to prevent Ge-O bond formation use on-line AES this worked in mid-late 80's, but was not followed-up Ge – direct deposition of SiO 2 with & without pre-oxidation, nm same as RPAO step for GaN pre-oxidation of Ge leads to an increase in D it, but a decrease in negative fixed charge – next step interface nitridation! -Q f D it ~V fb 0.4  -cm n-type - Al

research plans device testing - ZrO 2 -Y 2 O 3 and atomically engineered ZrSiON alloys nitrided Ge interfaces - two approaches nano-scale vertical p-n junctions (~25 nm diameter!) a precursor to vertical MOS devices (SRC patent application in process)