Oxide Electronics High T c Superconductors Memory Devices DRAM,FRAM Dielectrics High Ferroelectrics Magnetic Oxides For Spintronics Epitaxial Oxides Materials Integration Field Effect Devices Gate oxide Optoelectronics Transparent Conductors Sensors The Development of Oxide Electronics in Two Decades
近來大力推動奈米科技的背景 Moore‘s Law : 摩爾定律 A 30% decrease in the size of printed dimensions every two years. 來自微電子學可能遭遇瓶頸的考慮 矽晶上電子原件數每兩年會增加一倍
Intel Transistor Scaling and Research Roadmap High Performance Logic Low Operating Power Logic
CMOS scaling, When do we stop ? 25 22 18 16 Å 15 Å Reliability: Tunneling : In 1997, a gate oxide was 25 silicon atoms thick. In 2005, a gate oxide will be 5 silicon atoms thick, if we still use SiO 2...... and at least 2 of those 5 atoms will be at the interfaces. processing and yield issue Design Issue: chosen for 1A/cm 2 leakage I on /I off >> 1 at 12 Å Bonding: 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?
Al 2 O 3 Y2O3Y2O3Y2O3Y2O3 HfO 2 Ta 2 O 5 ZrO 2 La 2 O 3 TiO 2 3.9 3.9 9.0 9.0 18 1820 25 25 27 2730 80 80 Band gap (eV) Band gap (eV) Band offset (eV) Band offset (eV) 9.0 9.0 3.2 3.2 8.8 8.8 2.5 184.108.40.206 5.7 1.5 4.5 4.5 1.0 1.0 7.8 7.8 1.4 220.127.116.11 3.0 3.0 1.2 1.2 Free energy of formation MO x +Si 2 M+ SiO 2 @727C, Kcal/mole of MO x - 63.4 63.4116.8 47.6 -52.5 -52.542.398.57.5 Stability of amorphous phase High High Low Low LowLowHigh High High Silicide formation ? -YesYesYesYes Yes Yes Hydroxide formation ? -SomeYesSomeSomeSomeYesSome Oxygen diffusivity @950C (cm 2 /sec) 2x 10 - 14 5x 10 -25 ? ?? 10 -12 ? 10 -13 Dielectrics Dielectric constant Basic Characteristics of Binary Oxide Dielectrics SiO 2
-- Major problems are : High interfacial state density Large trapped charge Low channel mobility Electrical stability and reliability Electrical characterization / optimization Low electrical leakage is common. Have Attained an EOT under 1.0-1.4 nm. -- Good News !!
Low defect high ultrathin films --Interface engineering --Electrical characterization and optimization Identify new material candidates for metal gate --Metal gate/high integration Integration of high , and metal gate with Si- Ge strained layer, or with strained Si High dielectrics for high mobility semiconductors like Ge and III-V semiconductors Research Programs
oxideMBE in-situXPS oxide & metal MBEIII-VMBESi-GeMBE annealing chamber Wafer in Multi-chamber MBE Within-si t u Analysis System In-situ SPM Functionalchamber metalchamber
Major Research Topics Novel MBE template approach for ALD growth High dielectrics on III-V semiconductors Enhancement of in the new phase through epitaxy Fundamental study by IETS for detections of phonons and defects in high dielectrics Diluted magnetic oxide Co-doped HfO 2
Novel MBE template for ALD growth Can you make an excellent HfO 2 Film With low EOT ? Interface Engineering !
Deposited by MBE Structural properties of MBE-grown HfO 2 HRTEM RHEED HRTEM atomically order Si(100) surface amorphous HfO 2 surface IL (1.4 nm) HfO 2 (3.6nm) Deposited by ALD No IL !!
The MBE Template for ALD Growth MBE and ALD composite film deposition procedure Silicon MBE Al 2 O 3 MBE HfO 2 MBE template film growth ALD Al 2 O 3 ALD HfO 2 ALD bulk film growth Case 1 MBE Al 2 O 3 ALD Al 2 O 3 MBE HfO 2 Case 2 ALD HfO 2 Low pressure ( <1x 10 -8 torr ) maintained during MBE growth ALD precursors : TMA, H 2 O, T substrate : 300 ℃
The structure of ALD Al 2 O 3 with a MBE Al 2 O 3 template Silicon ALD Al 2 O 3 MBE Al 2 O 3 AR-XPSTEM No SiO 2 formed at interface for both MBE and MBE+ALD Al 2 O 3 No peak formed at 103.4eV
ALD Al 2 O 3 (1.9nm) with MBE (1.4nm) Al 2 O 3 template = 8.9 EOT=1.41nm V fb = -0.6V D it : 2.2 E11 cm -2 eV -1 (By conductance method) Silicon ALD Al 2 O 3 MBE Al 2 O 3 . CVC model provide by NCSU, Dr. John Hauser Consistent with two parallel capacitor plates In series
High dielectrics on III-V semiconductors Enhancement of in the new phase through epitaxy Can you make even higher ?
Dielectrics SiO 2 Al 2 O 3 GGG Gd 2 O 3 Sc 2 O 3 HfO 2 Dielectric constant 3.9 9.0 12 14 13 15 - 20 Band gap (eV) 9.0 8.8 5.4 6.5 5.7 Breakdown field (MV/cm) 10 5 - 8 3 - 5 3.5 ? 5 Hydroxide formation Slight Some High Likely Some Recrytallization temperature > 1100C ~ 750C > 850C > 900C ? ~500C Thermal stability in contact with GaAs at high T ? ~700C ~ 850C ? ? WSi 2 gate compatibilty ? ? ?Reacted at 530C ? ? Basic Characteristics of Binary Oxide Dielectrics
HRTEM of Low Temp Growth of HfO 2 on GaAs A very abrupt transition from GaAs to HfO 2 over one atomic layer thickness was observed. Amorphous HfO 2 on GaAs (100) 56 Å HfO 2 GaAs
High Resolution TEM Images of Pure HfO 2 on GaAs (001) An abrupt transition from GaAs to HfO 2 and no interfacial layer Coexistence of four monoclinic domains rotated by 90º.
The Structure of Pure HfO 2 Grown on GaAs(001) Monoclinic phase a=5.116A, b=5.172A, c=5.295A, =99.18 0 Coexistence of 4 domains rotated 90º from each other 9.2 º HAHA KAKA H K L HBHB KBKB Thickness is about 43A
Can you make even higher ? Phase transition engineering !
Hf O Crystal structures of HfO 2 and the corresponding The dielectric constant increases when HfO 2 structure is changed from monoclinic to other symmetry Dielectric constants 29 70 16 * 26.17 20 ** * Xinyuan Zhao and David Vanderbrit, P.R.B, VOL. 65, 233106, (2002). Stable phase temperature > 2700 o C >1750 o C <1750 C ** G-M. Rignanese and X. Gonze, P.R.B, VOL. 69, 184301, (2004).
Structure of HfO 2 doped with Y 2 O 3 Grown on GaAs(001) 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 HfO 2 Use the d-spacing formula to fit the lattice parameters HfO 2 doped with Y 2 O 3 grown on GaAs(001) is CUBIC Cubic phase a=5.126A, b=5.126A, c=5.126A α=90, β =90, γ =90 h l k (004) (400) (040) 1x1.1x
Comparison between Monoclinic phase and Cubic phase of HfO 2 Without doping Y 2 O 3 Monoclinic phase 200 & 220 L scan With doping Y 2 O 3 Cubic phase 200 L scan 220 Lscan x y A A B C D D B A B C D D B C
The Electrical Property of HfO 2 doped with Y 2 O 3 Grown on GaAs(001) =32 T=110A GaAs(001) Y 2 O 3 +HfO 2
MBE-HfO 2 (0.8) /Y 2 O 3 (0.2) Electrical properties - MBE-HfO 2 (0.8) /Y 2 O 3 (0.2) Increase of from 15 to over 30 ! Annealing Temperature effects
HRTEM image of yttrium-doped HfO 2 films 11 nm thick on GaAs (001). HRTEM image of yitturm-doped HfO 2 films 7.5 nm thick on Si (111). Cross Sectional HRTEM Study of The Y-doped HfO 2 Films in Cubic Phase 5nm  YDH [1-10] YDH  GaAs GaAs YDH 11nm ⊙ = YDH ⊙ = GaAs Interfaces of YDH(100)/GaAs, and YDH(111)/Si are atomically sharp
The Enhancement of through “Phase Transition Engineering” Change of molar volume of Y doping HfO 2 m / V m m / V m Clausius-Mossotti Relation The high materials, such as HfO 2, ZrO 2, TiO 2, or Ta 2 O 5, commonly have a high temperature phase with a high dielectric constant. We plan to achieve the enhancement through phase transition engineering by additions of dopants such as lower valence cations, followed by proper post high temperature anneals.
IETS Study to Detect Phonons and Defects in High Dielectrics But what is inside of high ?
Degradation of Mobility in High Gate Stack Phonons may have reduced mobility seriously ! Correction from the charge trapping effect Fechetti et al, JAP 90, 4587, (2001).
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 inelastic tunneling channels. Inelastic Electron Tunneling Spectroscopy
Trap-Related Signatures in IETS I V V Trap Assisted Tunneling Background I-V Charge Trapping Trap Assisted Tunneling Wei He and T.P.Ma, APL 83,2605, (2003) ; APL 83, 5461, (2003)
Al/HfO 2 (40 A)/n-type Si Electrical stress induced traps in Al/HfO 2 /Si structure Determination of trap energy and location in staked HfO 2 /Y 2 O 3 /Si structure IETS of Si MOS Devices made of High Gate Dielectrics
IET spectrum of Al/HfO 2 /Si MOS structure IET spectra of the MBE-grown HfO 2 (~25Å) annealed at 600 o C in vacuum for 3 minutes
d 2 I/dV 2 (Arbitrary Units) Initial After 200 sec 400 sec 800 sec -1.2 V, DC stress1 Forward-bias (gate positive) The features of trap- assisted tunneling 0 25 50 75 100 125 150 175 200 Voltage (mV) Electrical stress induced traps in Al/HfO 2 /Si MOS structure annealed at 600 o C in vacuum The IETS spectra of HfO 2 change slightly after electrical stress. The trap assisted tunneling are observed in IETS spectra after 400 sec DC stress at -1.2V.
Substrate Si(100) Y 2 O 3 ~2nm HfO 2 ~2nm ● ● ● Charge trapping dtdt Determination of Physical Locations and Energy Levels of Trap in Stacked HfO 2 /Y 2 O 3 /Si Structure V f and V r are the voltages where the charge trapping features occur in forward bias and reverse bias. d0d0 M. Wang et al, APL. 86, 192113 (2005) APL, 90, 053502 (2007)
Major Accomplishments First demonstration of atomically abrupt high HfO 2 /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 (I D of 240 mA/mm and g m of 120 mS/mm) in HfO 2 MOSFETs, compared very favorably to the results reported by other major groups. Passivation of the GaAs surface by high dielectrics HfO 2 in both amorphous and epitaxial films with sharp interfaces. Stablization of the cubic HfO 2 phase with Y doping by epitaxy at 600 C on GaAs (100) as well as on Si (111) to achieve an enhanced over 33.
Major Accomplishments IETS has detected phonons, defects (traps), and interfacial structures in ultra-thin HfO 2 /Si, and stacked HfO 2 /Y 2 O 3 /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 GeO 2 was notably suppressed in the MBE growth process. For 600°C annealing, GGG remains good dielectric property, but HfO 2 becomes leaky. The better thermal stability of GGG makes it promising for GGG based devices.
Part II: Diluted Magnetic Oxides Can you make HfO 2 magnetic ? ---Observation of Room Temperature Ferromagnetic Behavior in Cluster Free, Co doped HfO 2 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 model were used to describe the ferromagnetism. The potential usage of HfO 2 as alternative high- gate dielectrics in replacing SiO 2 for nano CMOS. Giant magnetic moment in Co doped HfO 2 was reported recently. Oxygen vacancy Fe 3+ F-center
HR-TEM Images of the High-T and Low-T Grown Films High-T (700°C) grown filmLow-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. 2.04A 2.49A 1 st O shell + 2 nd Co shell
Structural Characterization of Low-T Grown Films YSZ Co:HfO 2 1000Å RHEED pattern shows that Co doped HfO 2 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. 40-100 ℃
Magnetic Property of low-T grown films At 10K, the low T-grown Hf 0.953 Co 0.047 O 2 thin film showed a M s of 0.47 μB /Co and a H c of 170 Oe. At 300 K, the M s and H c decrease to 0.43 μB /Co and 50 Oe, respectively. Para + Ferromagnetic Temp. dependence
Magnetic Property of low-T grown films Substrate Temperature ( ℃ ) 40~100 Doping concentration ( at.% ) 4.75.510 Ms at 10K (μ B /Co) 0.470.360.13 Ms at 300K(μ B /Co) 0.430.290.1 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 O 2 at 350 o C. 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 Materialεm * /mγγ H (nm) δ P (10 -6 ) ZnO40.28140.761500 TiO 2 9190.485900 SnO 2 3.90.24160.861000 HfO 2 150.11507.951.27 Al 2 O 3 90.23392.0772 p and x p are two landmarks on the magnetic phase diagram. Ferromagnetism occurs when δ > δ p and x < x p. The δ p of HfO 2 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 HfO 2 is more likely than the widely studied TM doped ZnO, TiO 2 and SnO 2. δ p : polaron percolation threshold Xp : cation percolation threshold hydrogenic orbital radius
Major Accomplishment Uniformly doped Co in HfO 2 thin films without clusters or second phases were achieved by low-T MBE growth as confirmed by EXAFS, and are 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 HfO 2 is more likely than Co doped ZnO, TiO 2, and SnO 2 systems for doping concentrations under cation percolation threshold.
Acknowledgement NTHU---Prof. Minghwei Hong, Prof. T. B. Wu Prof. Y. L. Soo NSRRC---Dr. Chia-hong Hsu, Dr. H. Y. Lee CCMS---Dr. M. W. Chu, Dr. S. C. Liou, Dr. C. H. Chen NTU---Prof. C. W. Liu NCTU---Prof. A. Chin A.S.---Dr. S. F. Lee And All of Our Group Students
Group Members: 何信佳博士、 J. P. Mannaerts 、吳彥達、李 昆育、 李威 縉、朱其新、李毅君、潘欽寒、徐雅玲、邱亦欣、 黃建中、杭孟琦 、 張宇行、洪宏宜 、游璇龍 、謝政義 Acknowledgement : Thanks to the Support of the NSC National Nano Project UST/CNST Project, and ITRI/ERSO Project.