2材料物理课件_(14).ppt

nano-devices,1, nano-scale devices 2, molecular devices 3, devices with nano-materials,nanoscale devices,nano-computers,electronic nanocomputers quantum mechanical tunneling effect chemical nanocomputers stores information in the chemical bonds: dna etc. mechanical nanocomputers moving molecular scale parts: how to assemble ? quantum computer each bit of information as a quantum state: spins etc,nanoelectronic devices,solid state nanoelectronic devices,quantum dots “artificial atoms” (qd): 0-d resonant tunneling devices (rtd, rtt): 1- or 2-d single electron transistor (set): 3-d quantum cellular automata (qca) magnetic random access memory (mram),islands confine electrons,microelectronic transistor: structure, operation, obstacles to miniaturization,function of transistor - two state device or switch - amplification,n- and p-type silicon semiconductor,p-type b doped hole carrier n-type p, as doped electron carrier,b,p,depletion region,- + - + - +,+,-,ef,p n,some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. in so doing they leave behind positive ions at the donor impurity sites.,structure and operation of mosfet,i-v charateristics of n-type mosfet,vgate = 5v 4 v 3 v 2 v,obstacles to further scaling of fet,high electric field due to bias voltage to short distance - avalanche break down due to high kinetic energy electrons - tunneling through insulating layers heat dissipation of transistors due to limited thermodynamic efficiency - molecular scaled device: as much heat as gunpowder vanishing bulk properties and nonuniform dopant concentration - few dopant atoms in nano scale: functioning as transistor? - quantum mechanical effect shrinkage of depletion regions(0.1mm) - current leakage shrinkage and uneveness of the thin oxide layer - tunneling,resonant tunneling device (rtd),single electron transistor - island potential is capacitively controlled by the gate. - coulomb blockade is overcome by changing the gate voltage advantage - ultra low power operation - fast,e,source,drain,gate,island,i,v,vg =1v,vg =-1v,single electron transistor,coulomb blockade effect,coulomb blockade effect quantum tunneling of electron between source and drain can be blocked if the charging energy ec = e2/2c kt de = ev ec r1) - current steps at e/2c2 + n(e/c2),e,source,drain,gate,island,n= 2 1 0,n= 2 1,v,i,v,on,off,r1 r2,c1 c2,r2/r1 =1000,r2/r1 =1,example: set at 100k,s.y. chou et al, appl. phys. lett 67, 938 (1995),si dot with 20nm diameter : energy spacing = 40mev current oscillation due to the interference between different modes of quantum waves in a cavity,example: set operating at room temperature,unclear coulomb staircase due to the symmetric size of the tunnel juncton,coulomb staircase with periods of 150mv,single electron memory,e,source,drain,gate,island,storage dot,example: single electron memory,discrete shift in the threshold voltage staircase relation between the charging voltage and the shift self-limiting charging process,nanotube single electron transistor,the amount of current (isd) flowing through the nanotube channel can be varied by a factor of 100,000 by changing the voltage applied to a gate (vg),quantum dot cellular automata,i. amlani et al, science, 284, 289 (1999),wireless interconnection communicate by the electric fields of electrons,nanowire transistor,y. cui and c.m. lieber, science 291, 851 (2001),common base current gain: ic/ie=0.94 common emitter current gain: ic/ib=16,ef,e b c,n p n,veb,vcb,comparison of nanodevices,comparison of nanodevices,drawbacks and obstacles to solid- state nanoelectronic devices,valley current in rtd - not clear on and off sensitivity to input and current fluctuations - accidental on and off cryogenic operation: 30nm si set at 150 k - reducing size: ec and de increase. materials: si is still preferred - sio2 is still best insulator - gaas is far inferior to electric fields background charge problems extreme sensitivity of the tunneling current to width of potential barriers - soi technology or nonpolarizable organic compounds extreme difficulty of making islands and tunnel barriers precisely and uniformly,single molecular devices,molecular electronic devices,chemically assembled configurations rather than artificially drawn structures,voltage pulse yields high conductivity state - data bit stored,bit is read as high in low voltage region,dynamic random access memory,applied perpendicular field favors zwitterionic structure which is planar better pi overlap, better conductivity.,voltage-driven conductivity switch,devices with nanomaterials,light-emitting devices photovoltaic devices memories,light-emitting diodes with nan