Transcript enee702spin
Motivation: Embracing Quantum Mechanics 34 Years of History: More, More, Moore http://www.intel.com/research/silicon # transistor per IC High Performance Processor @ 90nm Technology Node • 256 million transistors • 37nm gate length • PNO gate: 10 nm EOT • NiSi2/Poly gate • 8 levels Cu with low-k interlevel dielectric Feature Size Transistor Density Chip Size Transistors/Chip Clock Frequency Power Dissipation Fab Cost WW IC Revenue WW Electronics Revenue 1970 Today Change 6 um 90 nm ~10 mm2 1000 100 kHz ~100 mW ~$10 M $700 M $70 B ~400 mm2 200 M > 1 GHz ~100 W >$1 B $170 B $1.1 T 70x Reduction 5000x Increase 40x Increase 200,000x Increase >10,000x Increase ~1000x Increase >100x Increase 240x Increase 16x Increase • Miniaturization power dissipation short channel effect statistical error in dopant distribution quantum mechanical effects – ballistic – tunneling • Alternative schemes to embrace quantum mechanical effects – low power – fast – new functionality, e.g. spin • Intel and IBM embracing spintronis 1 Successful Examples of Spintronics metal-based ferromagnetic devices spin valve hard drive read head GMR magnetic tunneling junction memory • Rapid transition from discovery to commercialization – 1988 giant magneto resistance (GMR) – 1998 IBM read head extends storage from 1 to 20Gbits ($100B) – 2004 Motorola MRAM with 10ns access time – 2010 10Gbit memory chip projected • Non-volatile, no wearout, fast write time, fast read time, low energy for writing, radiation-hard 2 www.sematech.org/meetings/ past/20021028/17_Hummel_Mram.pdf Objective • Demonstrate the essential elements required for ballistic spin devices – semiconductor spin-FET Essential elements: electrical (rather than optical) 100nm 1. injection of highly spinpolarized electrons 2. manipulation of electron spin orientation during coherent transport spin InAs ballistic spin transistor length 3. spin-sensitive detection • All existing semiconductor devices operate in the diffusive transport regime, where scattering results in heat dissipation and limits frequency response • Spin transport in the ballistic regime offers opportunities which are heretofore unexplored and unexploited 3 Semiconductor spintronics: Spin FET Datta and Das, Applied Physics Letters, 56, 665 (1990); 816 citations • Ferromagnetic source injects spin polarized electrons • Gate controlled spin precession through Rashba spin-orbit coupling in 2D channel • Ferromagnetic drain that provides: low resistance if spins are parallel high resistance if spins are anti-parallel • Not experimentally demonstrated yet • Key processes face challenges 4 Our Technical Approach: Electrical + InAs Key obstacle • to study spin manipulation in InAs, ferromagnetic/InAs needed • to develop FM/InAs hybrid junction, spin detection needed • optical detection is not practical, electrical detection needed • FM/InAs hybrid junction needed for electrical detection Our solution: a novel configuration for electrical spin injection and detection Manipulation Injection Electrical tunability of Rashba SO coupling in InAs Efficiency of spin transport across FM/InAs junction Challenges • determination of R 5 Detection Electrical spin detection Challenges • optimal ferromagnetic • control of R • Interface integrity • optimization of R • etching selectivity Current Status: Ferromagnetic/GaAs Hybrid Junction 1 mA, 2 V 4.5 K Magnetic Metal / Tunnel Barriers spin-LED EL Intensity (NRL) 32% Fe meV 55meV 3T 2 Fe 1.53 1.54 1.55 1.56 Schottky PQW Al2O3 1 0 GaAs 40% GaAs Metal oxide Al2O3 Tunneling PQW to 40% (lower bound, 5 K) Photon Energy (eV) 6 Fe AlGaAs GaAs APL 80, 1240 (2002) (NRL) APL 82, 4092 (2003) (NRL) Benchmark performance at 5K • well-defined system state • detector “purely” excitonic • most well studied temperature Past Spin Research : Optical + GaAs • GaAs band gap is ideal for optical experiments • optically generate spin-polarized electrons • detect spin populations with circularly polarized PL Pump-Probe Studies of Spin Precession D. D. Awschalom, UCSB Spin-Light Emitted Diode Fe/barrier/GaAs NRL patent (1999) Pcirc 7 PQW Rashba Spin-Orbit Coupling Structural Inversion Asymmetry • In nature, spins are controlled with magnetic field. • Thanks to spin-orbit coupling, now spins can also be controlled by using conventional gates. rest frame relativistic effect z z 2DEG in quantum well confinement potential moving frame Bin Bin In the absence of an external B 8 H R αR( x k y y k x ) σ g μB B* Optimal Material for Rashba Spin-Orbit Coupling H R αR (σ k ) z σ g * μB Bin SO αR k F αR : Rashba coefficient 2ħSO (meV) InAs 2 SO Si GaAs BIN (Tesla) Advantage of InAs • Larger g* (15 vs. 2 vs. 0.44) Larger spin splitting (meV) • Smaller m* (0.023 vs. 0.067 vs. 0.19) Larger quantization energy (10meV) • Higher RT mobility (40k vs. 8k vs. 1.4k) Longer mean free path (700nm) 9