Prof. J. Raynien Kwo 郭瑞年

Advances in Oxide Electronics Research: From High k Gate Dielectrics to Diluted Magnetic Oxides Prof. J. Raynien Kwo 郭瑞年

Part I. High k Gate Dielectrics

The Development of Oxide Electronics in Two Decades Sensors High Tc Superconductors Optoelectronics Transparent Conductors Memory Devices DRAM,FRAM Oxide Electronics Field Effect Devices Gate oxide Epitaxial Oxides Materials Integration Dielectrics High k Ferroelectrics Magnetic Oxides For Spintronics

近來大力推動奈米科技的背景 來自微電子學可能遭遇瓶頸的考慮 Moore‘s Law : 摩爾定律 A 30% decrease in the size of printed dimensions every two years. 矽晶上電子原件數每兩年會增加一倍

High Performance Logic Low Operating Power Logic Intel Transistor Scaling and Research Roadmap

In 1997, a gate oxide was 25 silicon atoms thick. CMOS scaling , When do we stop ? Reliability: 25 22 18 16 Å . . . processing and yield issue Tunneling : 15 Å In 2005, a gate oxide will be 5 silicon atoms thick, if we still use SiO2 Design Issue: chosen for 1A/cm2 leakage Ion/Ioff >> 1 at 12 Å Bonding: and at least 2 of those 5 atoms will be at the interfaces. • Fundamental Issues--- • how many atoms do we need to get • bulk-like properties? • EELS -- Minimal 4 atomic layers !! • Is the interface electronically abrupt? • Can we control roughness?

Basic Characteristics of Binary Oxide Dielectrics Dielectrics SiO2 Dielectric constant

Electrical characterization / optimization -- Good News !! • Low electrical leakage is common . • Have Attained an EOT under 1.0-1.4 nm . • -- Major problems are : • High interfacial state density • Large trapped charge • Low channel mobility • Electrical stability and reliability

Research Programs • Low defect high k ultrathin films --Interface engineering --Electrical characterization and optimization • Identify new material candidates for metal gate --Metal gate/high k integration • Integration of high k, and metal gate with Si- Ge strained layer, or with strained Si • High k dielectrics for high mobility semiconductors like Ge and III-V semiconductors

Multi-chamber MBE Within-situ Analysis System annealing & metal chamber Functional chamber In-situ SPM in-situ XPS metal chamber Wafer in oxide MBE III-V MBE oxide & metal MBE Si-Ge MBE

Major Research Topics Novel MBE template approach for ALD growth High k dielectrics on III-V semiconductors Enhancement of k in the new phase through epitaxy Fundamental study by IETS for detections of phonons and defects in high k dielectrics Diluted magnetic oxide Co-doped HfO2

Novel MBE template for ALD growth Can you make an excellent HfO2 Film With low EOT ? Interface Engineering !

Structural properties of MBE-grown HfO2 HRTEM HRTEM HfO2(3.6nm) No IL !! Deposited by MBE IL (1.4 nm) RHEED Deposited by ALD atomically order Si(100) surface amorphous HfO2 surface

MBE template film growth ALD bulk film growth MBE HfO2 ALD HfO2 MBE Al2O3 ALD Al2O3 Case 1 ALD Al2O3 MBE Al2O3 Case 2 MBE HfO2 ALD HfO2 The MBE Template for ALD Growth MBE and ALD composite film deposition procedure Silicon • Low pressure ( <1x 10-8 torr ) maintained during MBE growth • ALD precursors : TMA, H2O, Tsubstrate : 300℃

AR-XPS ALD Al2O3 MBE Al2O3 Silicon No peak formed at 103.4eV No SiO2 formed at interface for both MBE and MBE+ALD Al2O3 The structure of ALD Al2O3 with a MBE Al2O3 template TEM

ALD Al2O3 MBE Al2O3 Silicon k= 8.9 EOT=1.41nm Vfb= -0.6V ALD Al2O3 (1.9nm) with MBE (1.4nm) Al2O3 template Dit : 2.2 E11 cm-2eV-1 (By conductance method) Consistent with two parallel capacitor plates In series [1]. CVC model provide by NCSU, Dr. John Hauser

High k dielectrics on III-V semiconductors Enhancement of k in the new phase through epitaxy Can you make k even higher ?

Basic Characteristics of Binary Oxide Dielectrics

HRTEM of Low Temp Growth of HfO2 on GaAs Amorphous HfO2 on GaAs (100) HfO2 56 Å A very abrupt transition from GaAs to HfO2 over one atomic layer thickness was observed. GaAs

High Resolution TEM Images of Pure HfO2 on GaAs (001) Coexistence of four monoclinic domains rotated by 90º. An abrupt transition from GaAs to HfO2 and no interfacial layer

L KB HB KA K HA 9.2º H The Structure of Pure HfO2 Grown on GaAs(001) Monoclinic phase a=5.116A, b=5.172A, c=5.295A, b=99.180 Thickness is about 43A Coexistence of 4 domains rotated 90º from each other

Can you make k even higher ? Phase transition engineering !

Crystal structures of HfO2 and the corresponding k Hf O Dielectric constants 29 70 16 * 26.17 20 ** Stable phase temperature >2700oC >1750oC <1750oC The dielectric constant increases when HfO2 structure is changed from monoclinic to other symmetry * Xinyuan Zhao and David Vanderbrit, P.R.B, VOL. 65, 233106, (2002). ** G-M. Rignanese and X. Gonze, P.R.B, VOL. 69, 184301, (2004).

l (004) k (040) (400) h Structure of HfO2 doped with Y2O3Grown on GaAs(001) 1x 1.1x Cubic phase a=5.126A, b=5.126A, c=5.126A α=90,β=90, γ=90 • Find many peaks: • (022)(400)(200)(311)(31-1)(113)(420)(133)(20-2) • All peaks of the film match the JCPDS of cubic • phase HfO2 • Use the d-spacing formula to fit the lattice parameters • HfO2 doped with Y2O3 grown on GaAs(001) is CUBIC

Comparison between Monoclinic phase and Cubic phase of HfO2 D C C B B A D A D B A C B D y x Without doping Y2O3 With doping Y2O3 200 L scan 200 & 220 L scan 220Lscan Monoclinic phase Cubic phase

The Electrical Property of HfO2 doped with Y2O3 Grown on GaAs(001) T=110A k=32 Y2O3+HfO2 GaAs(001)

Electrical properties - MBE-HfO2 (0.8) /Y2O3 (0.2) MBE-HfO2 (0.8) /Y2O3 (0.2) Annealing Temperature effects Increase of k from 15 to over 30 !

Cross Sectional HRTEM Study of The Y-doped HfO2 Films in Cubic Phase 11nm ⊙=[110]YDH YDH [001]YDH [1-10]YDH [001]GaAs GaAs 5nm ⊙=[110]GaAs • Interfaces of YDH(100)/GaAs, and YDH(111)/Si are atomically sharp HRTEM image of yttrium-doped HfO2 films 11 nm thick on GaAs (001). HRTEM image of yitturm-doped HfO2 films 7.5 nm thick on Si (111).

The Enhancement of k through “Phase Transition Engineering” • = (1 + 8 am/ 3Vm) / (1 -4am/3Vm) Clausius-Mossotti Relation Change of molar volume of Y doping HfO2 • The high k materials, such as HfO2, ZrO2, TiO2, or Ta2O5, commonly have a high temperature phase with a high dielectric constant. • We plan to achieve the enhancement through phase transition engineeringby additions of dopants such as lower valence cations, followed by proper post high temperature anneals.

IETS Study to Detect Phonons and Defects in High k Dielectrics But what is inside of high k ?

Degradation of Mobility in High k Gate Stack Phonons may have reduced mobility seriously ! Fechetti et al, JAP 90, 4587, (2001). Correction from the charge trapping effect

Inelastic Electron Tunneling Spectroscopy • Elastic tunnel current increases linearly with the potential difference biased on both electrodes; while, inelastic electron tunneling occurs at the characteristic energy of vibrational levels. • (b) The I-V curve, conductance-V curve, and the IETS spectrum would result from both elastic processes and the inelastictunneling channels.

Trap-Related Signatures in IETS Background I-V I Trap Assisted Tunneling Charge Trapping V Trap Assisted Tunneling Charge Trapping V Wei He and T.P.Ma, APL 83,2605, (2003) ; APL 83, 5461, (2003)

Interactions Detectable by IETS • Substrate Silicon Phonons • Gate Electrode Phonons • Dielectric Phonons • Chemical Bonding (Impurities) • Defects (Trap States)

IETS of Si MOS Devices made of High k Gate Dielectrics • Al/HfO2(40 A)/n-type Si • Electrical stress induced traps in Al/HfO2/Si structure • Determination of trap energy and location in staked HfO2/Y2O3/Si structure

IET spectrum of Al/HfO2/Si MOS structure IET spectra of the MBE-grown HfO2(~25Å) annealed at 600oC in vacuum for 3 minutes

Initial After 200 sec 400 sec 800 sec -1.2 V, DC stress1 Forward-bias (gate positive) The features of trap-assisted tunneling d2I/dV2(Arbitrary Units) 0 25 50 75 100 125 150 175 200 Voltage (mV) Electrical stress induced traps in Al/HfO2/Si MOS structure annealed at 600oC in vacuum • The IETS spectra of HfO2 change slightly after electrical stress. • The trap assisted tunneling are observed in IETS spectra after • 400 sec DC stress at -1.2V.

Determination of Physical Locations and Energy Levels of Trap in Stacked HfO2/Y2O3/Si Structure HfO2 ~2nm dt ● ● ● d0 Charge trapping Y2O3 ~2nm Substrate Si(100) Vf and Vr are the voltages where the charge trapping features occur in forward bias and reverse bias. M. Wang et al, APL. 86, 192113 (2005) APL, 90, 053502 (2007)

Major Accomplishments • First demonstration of atomically abrupt high k HfO2/Si interface by the MBE process, and employed as a MBE template for ALD. • With a novel MBE template for ALD growth, we have achieved excellent electrical properties(ID of 240 mA/mm and gm of 120 mS/mm)in HfO2 MOSFETs,compared very favorably to the results reported by other major groups. • Passivation of the GaAs surface by high k dielectrics HfO2 in both amorphous and epitaxial films with sharp interfaces. • Stablization of the cubic HfO2 phase with Y doping by epitaxy at 600C on GaAs (100) as well as on Si (111) to achieve an enhanced k over 33.

Major Accomplishments • IETShas detected phonons, defects (traps), and interfacial structures in ultra-thin HfO2/Si, and stacked HfO2/Y2O3/Si bilayer. • IETS is capable of monitoring the defect formation with electrical stress applied to dielectrics. • By using a simple mode, we are able to estimate the energy and physical location of traps in dielectrics. • MBE grown GGG on n-Ge(100) demonstrated abrupt interface between oxide and substrate. The GeO2 was notably suppressed in the MBE growth process. • For 600°C annealing, GGG remains good dielectric property, but HfO2 becomes leaky. The better thermal stability of GGG makes it promising for GGG based devices.

Part II: Diluted Magnetic Oxides Can you make HfO2 magnetic ? ---Observation of Room Temperature Ferromagnetic Behavior in Cluster Free, Co doped HfO2 Films

Introduction and motivation • Both injection and transport of spin-polarized carriers are necessary for the spintronic devices. Using diluted magnetic semiconductor (DMS) as the ferromagnetic contact is one way to achieve this goal. • Several models such as Zener’s model, bound magnetic polaron, F-center theory and impurity band exchange modelwere used to describe the ferromagnetism. • The potential usage of HfO2 as alternative high-k gate dielectrics in replacing SiO2 for nano CMOS. • Giant magnetic moment in Co doped HfO2 was reported recently. F-center Fe3+ Fe3+ Oxygen vacancy

HR-TEM Images of the High-T and Low-T Grown Films High-T (700°C) grown film Low-T (100°C) grown film

Characterization of High T (700ºC) Grown Films • EXAFS of the high-T grown samples showed a progressive formation of Co clusters in the film (Co = 1-20 at.%). • Superparramagnetic temperature dependence. • Saturation moment increases with increasing Co doping concentration. 1st O shell + 2nd Co shell 2.04A 2.49A

Co:HfO2 YSZ Structural Characterization of Low-T Grown Films 1000Å 40-100℃ • RHEED pattern shows that Co doped HfO2 film grown at low-T is a poly-crystalline structure. • No sign of Co clusters was detected by EXAFS. • Co is likely to be at the interstitial site with 2+ sate.

Magnetic Property of low-T grown films Para + Ferromagnetic Temp. dependence • At 10K, the low T-grown Hf0.953Co0.047O2 thin film showed a Ms of 0.47 μB /Co and a Hc of 170 Oe. • At 300 K, the Ms and Hc decrease to 0.43 μB /Co and 50 Oe, respectively.

Magnetic Property of low-T grown films • Ferromagnetic behavior was observed at both 10K and 300K. • The magnetic moment decreases with increasing Co doping due to enhanced dopant dopant associations. • The magnetic properties are stable after annealing in O2 at 350oC. • Correlation between saturation magnetization with the concentration of oxygen vacancies

F-center Exchange Mechanism: --An electron orbital created by an oxygen vacancy with trapped electrons is expected to correlate with magnetic spins dispersed inside the oxides. Impurity-band Exchange Model : --The hydrogenic orbital formed by donor defect (like oxygen vacancy) associated with an electron overlaps to create delocalized impurity bands. -- If the donor concentration is large enough and interacts with the magnetic cations with their 3d orbitals to form bound magnetic polarons leading to ferromagnetism. γ3δp ≈ 4.3, where γ = ε(m/m*) δp : polaron percolation threshold Xp : cation percolation threshold  H : hydrogenic radius

Theoretical Analysis Using the Impurity Band Exchange Model δp : polaron percolation threshold Xp : cation percolation threshold  H : hydrogenic orbital radius • δpandxp are two landmarks on the magnetic phase diagram. • Ferromagnetism occurs when δ > δp and x < xp. • The δp of HfO2 based DMO was deduced approximately from 1.27×10-6 to 8.15×10-5 • The appearance of ferromagnetic insulator behavior in the TM doped HfO2 is more likely than the widely studied TM doped ZnO, TiO2 and SnO2 .

Major Accomplishment • Uniformly doped Co in HfO2 thin films without clusters or second phaseswere achieved by low-T MBE growth as confirmed by EXAFS,andare stable up to 350℃ anneals under most ambient. • Ferromagnetic behavior in B-H loops and temperature dependence was observed at both 10K and 300K, and magnetic moment of Mn decreases with increasing Mn concentration (4-10 at.%). • Have ruled out the reported “giant magnetic moment effect”. • Found qualitative correlations between the values of saturation magnetization with the concentration of oxygen vacancies. • Consistent with a recently proposed F-center exchange mechanism, or a impurity band exchange model to account for the apparent ferromagnetism. • The appearance of ferromagnetic insulator behavior in the Co doped HfO2 is more likely than Co doped ZnO, TiO2, and SnO2 systems for doping concentrations under cation percolation threshold.

Prof. J. Raynien Kwo 郭瑞年