University of Notre DameCenter for Nano Science and Technology Gregory L. Snider Department of Electrical Engineering University of Notre Dame Nanoelectronic Devices

University of Notre DameCenter for Nano Science and Technology What are Nanoelectronic Devices? A rough definition is a device where: The wave nature of electrons plays a significant (dominant) role. The quantized nature of charge plays a significant role.

University of Notre DameCenter for Nano Science and Technology Examples Quantum point contacts (QPC) Resonant tunneling diodes (RTD) Single-electron devices Quantum-dot Cellular Automata (QCA) Molecular electronics (sometimes not truly nano)

University of Notre DameCenter for Nano Science and Technology References Single Charge Tunneling, H. Grabet and M. Devoret, Plenum Press, New York, 1992 Modern Semiconductor Devices, S.M. Sze, John Wiley and Sons, New York, 1998 Theory of Modern Electronic Semiconductor Devices, K. Brennan and A. Brown, John Wiley and Sons, New York, 2002 Quantum Semiconductor Structures, Fundamentals and Applications, C. Weisbuch and B. Vinter, Academic Press, Inc., San Diego, 1991

University of Notre DameCenter for Nano Science and Technology When does Quantum Mechanics Play a Role? W & V, pg. 12, Fig. 5

University of Notre DameCenter for Nano Science and Technology More Realistic Confinement W & V, pg. 13, Fig.6

University of Notre DameCenter for Nano Science and Technology Quantum Point Contacts One of the earliest nanoelectronic devices QPCs depend on ballistic, wave- like transport of carriers through a constriction. In the first demonstration surface split- gates are used to deplete a 2D electron gas. The confinement in the constriction produces subbands.

University of Notre DameCenter for Nano Science and Technology Quantized Conductance When a bias is applied from source to drain electrons travel ballisticly. Each spin-degenerate subband can provide 2e 2 /h of conductance. Va Wees, PRL 60, p. 848, 1988

University of Notre DameCenter for Nano Science and Technology What About Temperature? Thermal energy is the bane of all nanoelectronic devices. As the temperature increases more subbands become occupied, washing out the quantized conductance. T 2 > T 1 All nanoelectronic devices have a characteristic energy that must be larger than kT

University of Notre DameCenter for Nano Science and Technology Resonant Tunnel Devices In a finite well the wavefunction penetrates into the walls, which is tunneling In the barrier: where Transmission through a single barrier goes as:

University of Notre DameCenter for Nano Science and Technology Two Barriers Semiclassically a particle in the well oscillates with: It can tunnel out giving a lifetime  n and: Now make a particle incident on the double barrier: If E i ≠ E n then T = T 1 T 2 which is small

University of Notre DameCenter for Nano Science and Technology If E i = E n then the wavefunction builds in the well, as in a Fabry-Perot resonator: Which approaches unity for T 1 = T 2 :

University of Notre DameCenter for Nano Science and Technology In Real Life! Things are, of course, more complicated: - No mono-energetic injection - Other degrees of freedom In the well: In the leads: where

University of Notre DameCenter for Nano Science and Technology In k Space No One Can Hear You Scream! For Transmission: To get through the barriers electrons must have E > E c but must also have the correct k z. Only states on the disk meet these criteria.

University of Notre DameCenter for Nano Science and Technology J is proportional to the number of states on the disk, and therefore to the area of the disk: E c L is above E o, so no states have the correct k z Note: we have ignored the transmission probability

University of Notre DameCenter for Nano Science and Technology Scattering Scattering plays an important but harmful role, mixing in-plane and perpendicular states B&B p236

University of Notre DameCenter for Nano Science and Technology Single Electron Devices The most basic single-electron device is a single island connected to a lead through a tunnel junction The energy required to add one more electron to the island is: E C = e2e2 2C This is the Charging Energy If E C > kT then the electron population on the island will be stable. Usually we want Ec > 3-10 times kT. For room temperature operation this means C ~ 1 aF.

University of Notre DameCenter for Nano Science and Technology If the temperature is too high, the electrons can hop on and off the island with just the thermal energy. This is uncontrollable. An additional requirement to quantize the number of electrons on the island is that the electron must choose whether it is on the island or not. This requires R T > R K Where R K = h/e 2 ~ 25.8 kΩ Usually 2-4 times is sufficient

University of Notre DameCenter for Nano Science and Technology What is an Island? Anywhere that an electron wants to sit can be used as an island –Metals –Semiconductors Quantum dots Electrostatic confinement

University of Notre DameCenter for Nano Science and Technology Single Electron Box Assume a metallic island The energy of the configuration with n electrons on the island is : E(n) = (ne - Q) 2 2(C s + C j ) Q = C s U

University of Notre DameCenter for Nano Science and Technology At a charge Q/e of 0.5 one more electron is abruptly added to the island. What does it mean to have a charge of 1/2 and electron?

University of Notre DameCenter for Nano Science and Technology Single - Electron Transistor (SET) Now the gate voltage U can be used to control the island potential. The source - drain Voltage V is small but finite. When U=0, no current flows. Coulomb Blockade When (C G U)/e = 0.5 current flows. Why? One more electron is allowed on the island.

University of Notre DameCenter for Nano Science and Technology These are called Coulomb blockade peaks. Is the peak the current of only one electron flowing through the island? No, but they flow through one at a time!

University of Notre DameCenter for Nano Science and Technology What about Temperature? G&D p181 As the temperature increases the peaks stay about the same, while the valleys no longer go to zero. This is the loss of Coulomb blockade. Finally the peaks smear out entirely. This shows the classical regime, such as for metal dots. In semiconductor dots resonant can cause an increase in the conductance at low temperatures (the peak values increase).

University of Notre DameCenter for Nano Science and Technology SET Stability Diagram You can also break the Coulomb blockade by applying a large drain voltage.

University of Notre DameCenter for Nano Science and Technology Ultra-sensitive electrometers Dot Signal Add an electron Lose an electron Sensitivity can be as high as e/sqrt(Hz)

University of Notre DameCenter for Nano Science and Technology Single Electron Trap G&D p123 This non-reversible device can be used to store information.

University of Notre DameCenter for Nano Science and Technology Single Electron Turnstile G&D p124 This is an extension of the single electron trap that can move electrons one at at time

University of Notre DameCenter for Nano Science and Technology Turnstile Operation G&D page 125 Why does it need to be non-reversible? Can this be used as a current standard? Issues: Co-tunneling Missed transitions Thermally activated events

University of Notre DameCenter for Nano Science and Technology Single Electron Pump G&D p128 Here there are two coupled boxes, and an electron is moved from one to the other in a reversible process. Same Issues: Missed transitions Thermally activated events Co-tunneling

University of Notre DameCenter for Nano Science and Technology e-e- VgVg SET Conductance VgVg Nanometer scaled movements of charge in insulators, located either near or in the device lead to these effects. This offset charge noise (Q 0 ) limits the sensitivity of the electrometer. Background charge effect on single electron devices

University of Notre DameCenter for Nano Science and Technology Background charge insensitive single electron memory  A bit is represented by a few electron charge on a floating gate.  SET electrometer used as a readout device.  Random background charge affects only the phase of the SET oscillations.  The FET amplifier solves the problem of the high output impedance of the SET transistor. K. K. Likharev and A. N. Korotkov, Proc. ISDRS’95

University of Notre DameCenter for Nano Science and Technology Plasma oxide – fabrication technique A To diffusion pump Gas inlet

University of Notre DameCenter for Nano Science and Technology Plasma oxide device Two step e-beam lithography on PMMA/MMA. Oxidation after first step in oxygen plasma formed by glow discharge. Oxide thickness characterized by VASE technique. CG FG Ground BG SET 6 nm of oxide grown after 5 min oxidation in 50 mTorr oxygen plasma at 10 W.

University of Notre DameCenter for Nano Science and Technology Hysteresis Loops SET conductance monitored on the application of a bias on the control gate. A back gate bias cancels the direct effect of the control gate on the SET. The change in the operating point of the SET is due to electrons charging and discharging the floating gate.

University of Notre DameCenter for Nano Science and Technology on=“1” Zuse’s paradigm Konrad Zuse (1938) Z3 machine –Use binary numbers to encode information –Represent binary digits as on/off state of a current switch Telephone relay Z3 Adder The flow through one switch turns another on or off. Electromechanical relay Exponential down-scaling Vacuum tubes Solid-state transistors CMOS IC off=“0”

University of Notre DameCenter for Nano Science and Technology Problems shrinking the current-switch Electromechanical relay Vacuum tubes Solid-state transistors CMOS IC New idea Valve shrinks also – hard to get good on/off Current becomes small - resistance becomes high Hard to turn next switch Charge becomes quantized Power dissipation threatens to melt the chip. Quantum Dots

University of Notre DameCenter for Nano Science and Technology New paradigm: Quantum-dot Cellular Automata Revolutionary, not incremental, approach Beyond transistors – requires rethinking circuits and architectures Represent information with charge configuration. Zuse’s paradigm Binary Current switch Charge configuration

University of Notre DameCenter for Nano Science and Technology Quantum-dot Cellular Automata Represent binary information by charge configuration A cell with 4 dots Tunneling between dots Polarization P = +1 Bit value “1” 2 extra electrons Polarization P = -1 Bit value “0” Bistable, nonlinear cell-cell response Restoration of signal levels Robustness against disorder cell1cell2 cell1cell2 Cell-cell response function Neighboring cells tend to align. Coulombic coupling

University of Notre DameCenter for Nano Science and Technology Variations of QCA cell design 4-dot cell2-dot cell5-dot cell6-dot cell Middle dot acts as variable barrier to tunneling. Indicates path for tunneling

University of Notre DameCenter for Nano Science and Technology Clocking in QCA energy x Clock Small Input Applied Clock Applied Input Removed but Information is preserved! 0 Keyes and Landauer, IBM Journal of Res. Dev. 14, 152, 1970

University of Notre DameCenter for Nano Science and Technology Quasi-Adiabatic Switching Clocking Schemes for Nanoelectronics: Keyes and Landauer, IBM Journal of Res. Dev. 14, 152, 1970 Lent et al., Physics and Computation Conference, Nov Likharev and Korotkov, Science 273, 763, 1996 Requires additional control of cells. Introduce a “null” state with zero polarization which encodes no information, in contrast to “active” state which encodes binary 0 or 1. Clocking achieved by modulating energy of third state directly (as in metallic or molecular case) P= +1P= –1Null State Clocking achieved by modulating barriers between dots (as in semiconductor dot case) Clocking signal should not have to be sent to individual cells, but to sub-arrays of cells.

University of Notre DameCenter for Nano Science and Technology Power Will Be a Limiter 5KW 18KW 1.5KW 500W Pentium® Power (Watts)  Microprocessor power continues to increase exponentially  Power delivery and dissipation will be prohibitive ! P6 Transition from NMOS to CMOS Source: Borkar & De, Intel  Slide author: Mary Jane Irwin, Penn State University

University of Notre DameCenter for Nano Science and Technology Power Density will Increase Pentium® P Power Density (W/cm2) Hot Plate Nuclear Reactor Rocket Nozzle  Power densities too high to keep junctions at low temps Source: Borkar & De, Intel  Sun’s Surface Slide author: Mary Jane Irwin, Penn State University

University of Notre DameCenter for Nano Science and Technology QCA power dissipation QCA architectures can operate at densities above devices/cm 2 without melting the chip. QCA Operation Region

University of Notre DameCenter for Nano Science and Technology A B C Out Binary wire Inverter Majority gate M A B C Programmable 2-input AND or OR gate. QCA devices

University of Notre DameCenter for Nano Science and Technology Metal-dot QCA implementation “dot” = metal island 70 mK electrometers Al/AlO 2 on SiO 2 Metal tunnel junctions 1 µm

University of Notre DameCenter for Nano Science and Technology Tunnel junctions by shadow evaporation First aluminum deposition Oxidation of aluminum Second aluminum deposition Thin Al/AlO x /Al tunnel junction

University of Notre DameCenter for Nano Science and Technology Metal-dot QCA cells and devices Demonstrated 4-dot cell A.O. Orlov, I. Amlani, G.H. Bernstein, C.S. Lent, and G.L. Snider, Science, 277, pp , (1997). Input Double Dot (1,0)(0,1) Switch Point Top Electrometer Bottom Electrometer

University of Notre DameCenter for Nano Science and Technology Switching of 4-Dot Cell

University of Notre DameCenter for Nano Science and Technology Majority Gate M A B C Amlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science 284, pp (1999).

University of Notre DameCenter for Nano Science and Technology QCA Latch Fabrication

University of Notre DameCenter for Nano Science and Technology QCA Clocked Latch (Memory)

University of Notre DameCenter for Nano Science and Technology QCA Shift Register

University of Notre DameCenter for Nano Science and Technology Fan-Out

University of Notre DameCenter for Nano Science and Technology From metal-dot to molecular QCA “dot” = metal island 70 mK Mixed valence compounds “dot” = redox center Metal-dot QCA established proof-of-principle. but …low T, fabrication variations Molecular QCA: room temp, synthetic consistency room temperature+ Metal tunnel junctions

University of Notre DameCenter for Nano Science and Technology Charge configuration represents bit “1” isopotential surface “0” Gaussian 98 UHF/STO-3G HOMO

University of Notre DameCenter for Nano Science and Technology Double molecule Considered as a single cell, bit is represented by quadrupole moment. Alternatively: consider it a dipole driving another dipole.

University of Notre DameCenter for Nano Science and Technology “0” HOMOIsopotential (+) “1” Double molecule

University of Notre DameCenter for Nano Science and Technology Core-cluster molecules Five-dot cell

University of Notre DameCenter for Nano Science and Technology Core-cluster moleculesTheory of molecular QCA bistability Allyl group Variants with “feet” for surface binding and orientation

University of Notre DameCenter for Nano Science and Technology Electron Switching in QCA Metal Dots Measure conductance Molecular Dots Measure capacitance C Voltage

University of Notre DameCenter for Nano Science and Technology Electron Switching Demonstration Capacitance peaks correspond to “click- clack” switching within the molecule JACS 125, , 2003

University of Notre DameCenter for Nano Science and Technology Clocked molecular QCA

University of Notre DameCenter for Nano Science and Technology Summary QCA may offer a promising paradigm for nanoelectronics –binary digits represented by charge configuration –beyond transistors –general-purpose computing –enormous functional densities –solves power issues: gain and dissipation –Scalable to molecular dimensions Single electron memories represent the ultimate scaling