How a single defect can affect silicon nano-devices. Ted Thorbeck

Transcription

1 How a single defect can affect silicon nano-devices Ted Thorbeck tedt@nist.gov

2 The Big Idea As MOS-FETs continue to shrink, single atomic scale defects are beginning to affect device performance Gate Source Drain

3 Outline The impact of a single atom on a MOSFET Locating a single atom in a transistor The potential for a single atom

4 Review of MOS-FETs I Gate M etal Heavily n-doped Source and Drain Source Drain e e e- e - - e- - h + h + h + h + h e h + e h + h + e- - - e h + h + h + - e- h + h + h + h + h + h h + + h + h + h + Lightly p-doped channel h + O xide S emiconductor

5 I (na) Review of MOS-FETs II Typical MOS-FET Curv 300 K Gate Positive Gate Negative V Gate (V) S Electrons Invert e - D S Holes Accumulate h + h + h h + + h + h + h + h h + + h + h + h + h + h + h + D Threshold Voltage (V T )

6 Not just shrinking. Planar to 3D Strain Gate Gate Source Drain e - Source High-κ dielectrics Gate Hafnium Oxide Silicon Dioxide Source 3 nm Drain

7 Atomic Scale Defects Gate Leakage to gate Source 3 nm Drain - - e e- e - e- - e- - e - - e - - e- e e e e e- - - e - e- Dopant Trap Random Dopants change V T

8 Threshold Voltage 25 devices studied, ΔV T 1 V I 10 nm ~ 10s of dopants ΔV T M Pierre, et al. Single-donor ionization energies in a nanoscale CMOS channel, Nature Nano, V G

9 Ordered Dopant Dopant Arrays Heavily n-doped Source and Drain Source Drain N Std. Dev. = 0.3 V Shinada, et al. Nature (2005) N Std. Dev. = 0.1 V Random V T Ordered V T

10 Outline The impact of a single atom on a MOSFET Locating a single atom in a transistor The potential for a single atom

11 Looking for a single atom Annular dark-field scanning-tem Need to chop up device to look at it K. Van Benthem, et al. Three-dimensional imaging of individual hafnium atoms inside a semiconductor Applied Physics Letters, (2005)

12 The Basic Idea: Cryogenic Temperatures E E F E C Dip: Quantum Dot Peaks: Tunnel Junction z Source Dopant Drain 12

13 The Basic Idea: Coulomb Blockade E Gate E F E C Source Drain ev SD 1 mev I V G e C G e C CG aF V 0.01V G V G 13

14 Nanowire ~20 nm x 20 nm x 500 nm Surrounded by 20 nm SiO 2 Upper Gate Heavily doped Poly A. Fujiwara, et al. APL 88, (2006) Lower Gates Heavily doped Poly nm long 3 independent gates 14

15 Poly Upper Gate Poly Lower Gate Silicon Dioxide Crystalline Silicon Positive voltage on upper gate inverts wire Upper Gate LGS LGC LGD e - e e- - e- e - e e- - e - Source Drain T = 4.4 K V SD = 2 mv V LGS,C,D = 0

16 g ( S) Poly Upper Gate Poly Lower Gate Silicon Dioxide Crystalline Silicon Peaks correspond to transport through QDs Negative voltages on lower gates form tunnel barriers T = 40 mk V UG = 1 V V LGC,D = 0 LGD V (V) LGD Upper Gate LGS LGC LGD e - e e - - e- e - e - e - Source Drain

17 Measure current while scanning V UG and V LGD Periodic Coulomb blockade oscillations I (na) V UG (V) 0.82 Device 1: T =39 mk V SD = 1 mv V LGS,C = 0 17

18 2 flavors of QDs A: few periods, more strongly coupled to LGD B: many periods, more strongly coupled to UG B A Device 1: T =39 mk V SD = 1 mv V LGS,C = 0 18

19 Measure Gate Capacitances LGD (af) UG (af) Ratio LGD/UG LGC (af) Dev. 1: Dot A ± 0.02 < 0.1 Dev. 1: Dot B 3.2 ± ± ± 0.01 <

20 Locate the Dot UG LGC LGD Si Wire Simulated ½ device in FASTCAP 20

21 Measure gate capacitances Simulate capacitances to 1 nm slices of wire Locate the Dot UG Integrate between z 1 and z 2 z 2 dc C sim = dz dz z 1 For what z 1 and z 2 does C sim = C meas for all gates LGC Si Wire LGD LGD (af) UG (af) Ratio LGD/UG LGC (af) Dev. 1: Dot B 3.2 ± ± ± 0.01 <

22 UG Location of Dots LGC LGD Location in nm Si Wire A B z 1 = -40 ± 3 z 1 = 17 ± 1 z 2 = -19 ± 3 Between LGD and UG z 2 = 87 ± 2 LGD A B 22

23 Dopant Location? UG We see same QDs in multiple devices -The cause appears systematic -Strain from temperature change and oxide growth LGC LGD -Could help make faster finfets A B Si Wire E F E C Dopants? Deduced conduction band modulation 23

24 Finding a Dopant Very similar technique has been used to located individual dopants and interface traps Nathaniel Bishop, et al; Triangulating tunneling resonances in a point contact Arxiv (2011)

25 Outline The impact of a single atom on a MOSFET Locating a single atom in a transistor The potential for a single atom

26 Ultimate Transistor? Gate Source Drain Dopant Similar to: Cheng Cen, et al. Oxide Nanoelectronics on Demand Science 323, 1026 (2009) Martin Fueschsle, et al. Spectroscopy of few-electron single-crystal silicon quantum dots Nature Nano, 5, 502 (2010)

27 Beyond the transistor World looks different on the atomic-scale Quantum regime This is a problem for current transistors Tunneling to the gate Could this quantumness become useful

28 Quantum Search Classical Computer: To search x100 boxes takes x100 as long Quantum Computer: To search x100 boxes takes x10 as long Number of Boxes Old Computer New Computer Quantum Computer μs 10 ns 10 ns μs 1000 ns 100 ns

29 Conclusions MOSFETs are reaching the point where the placement of a single atom can affect device performance New tools allow the location of a single atom to be determined within a MOSFET The quantum nature of a single atom could one day allow for much more powerful devices

30 Collaborators Neil Zimmerman Panu Koppinen Michael Stewart Ted Thorbeck