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Internal Thermoelectric Effects and Scanning Probe Techniques for Inorganic and Organic Devices Kevin Pipe Department of Mechanical Engineering University of Michigan Collaborators Rajeev Ram (MIT) Ali Shakouri (UCSC) Li Shi (UT) Max Shtein (UM) cool heat 10 m EC cool EFn p n cool EV cool EFp heat h+ e- Outline Heating in Electronic Devices Thermoelectric Effects in Devices Thermoelectric cooling background Microscale thermoelectric coolers Internal cooling / integrated energy harvesting Scanning Probe Techniques for Energy Transfer Scanning probes with active organic heterostructures OLED probes Exciton injection probes Heating in Electronics Increasing transistor density and increasing clock speed have led to rapidly increasing chip temperature. CMOS chips can have microscale hot spots with heat fluxes greater than 300 W/cm2. Nuclear reactor Heating in power electronics and optoelectronics can be >1000 W/cm2. Hot plate Traditional thermoelectric coolers cool only ~ 10 W/cm2. Intel Pentium® III Processor M. J. Ellsworth, (IBM), ITHERM 2004 Intel Itanium® Processor Hot spots C.-P. Chiu (Intel), “Cooling challenges for silicon integrated circuits”, SRC/SEMATECH Top. Res. Conf. on Reliability, Oct. 2004 Is it possible to generate targeted cooling or harvest waste heat energy? Device-Internal Temperature Gradients Large variation in carrier temperature (DT≈1000K) and lattice temperature (DT≈100K) can arise within active devices during operation. MOSFET channel (carrier temperature) SOI MOSFET (lattice temperature) GaAs/AlGaAs high-power laser (facet temperature) Can energy from hot electrons in transistors or lasers (Auger) be harvested in an analogous manner to techniques in solar cells? P. B. M. Wolbert et al, IEEE Trans. Comp.-Aid. Des. Int. Circ. Sys. 13, 293 (1994) Teng, H.-F. and S.-L. Jang, Solid-State Elect. 47, 815 (2003) S. J. Sweeney et al., IEEE J. Sel. Top. Quantum Elect. 9, 1325 (2003) Device-Internal Temperature Gradients Predicted temperature distribution Transistor Intel 90nm MOSFET 5W/m3 heat source over a radius of 20nm S. Sinha and K. E. Goodson, "Thermal conduction in sub-100 nm transistors," THERMINIC 2004 Bulk heating Semiconductor Laser Can microscale 10 m hot spots be cooled efficiently? SEM Facet temperature cross-section Facet heating P. K. L. Chan et al., Appl. Phys. Lett. 89, 011110 (2006) Cooling Methods for Devices Large heat sinks inefficient at cooling microscale hot spots Integrated thermoelectric cooler Device Device Substrate Substrate Heat sink Heat sink Heat sink Junction-up mounting Junction-down mounting Monolithic integration with TE cooler (difficult to remove heat) (better device performance and lifetime but has practical difficulties with electrical contacts, etc.) (complicated processing) p-i-n diode Electronic structure of device optimized for internal thermoelectric cooling HIT cooler Heat sink Junction-up mounting with device-internal thermoelectric cooling (microscale cooling source with minimal processing impact) C. LaBounty, Ph.D. thesis, UC Santa Barbara (2001). Cooling Methods for Devices The operating current of a device causes thermoelectric heating/cooling at every internal device layer junction Internal thermoelectric effects in active devices can be used for both: Targeted cooling of a critical region of the device, moving heat sources to the edge of the device where they are more easily conducted away Energy harvesting using large gradients in lattice and carrier temperatures to reclaim electrical power Electronic structure of device optimized for internal thermoelectric cooling Heat sink Junction-up mounting with device-internal thermoelectric cooling (microscale cooling source with minimal processing impact) Recent Convergence of Thermoelectric / Device Materials Thermoelectric Coolers 12x larger figure-of-merit GaAs/AlAs Superlattice (m*)3/2 l (bulk thermoelectric figure-of-merit) Active Devices T. Koga et al., J. Comp.-Aid. Mat. Des. 4 (1997) Transistors, lasers 4x larger figure-of-merit HgCdTe Superlattice Detectors, Mid-IR lasers R. Radtke et al., J. Appl. Phys. 86 (1999) 300 W/cm2 cooling at 300K InGaAs/InGaAsP SL High-speed transistors, lasers C. LaBounty et al., J. Appl. Phys. 89 (2002) InGaAs/InGaAsP Barrier A. Shakouri et al., Appl. Phys. Lett 74 (1999) High-speed, highpower transistors 680 W/cm2 at 345K SiGe/Si SL A. Shakouri et al., IPRM (2002) 750 W/cm2 at 300K BiTe/SbTe SL R. Venkatasubramanian et al., Nature 413 (2001) A. Shakouri and C. LaBounty, ICT, Baltimore, 1999. High-performance semiconductors have recently been used to create superior thermoelectric devices Conventional TE Cooler Tcold EF EV Heat absorbed p + Heat released Optimum p,n doping EF EV I Thot Electrons _ Holes + Heat absorbed n I I Heat released EC EF _ Thermoelectric figure-of-merit (sometimes written as ZT) s P2 Z = lT2 1 (Thot-Tcold)max= ZT2 2 • Electrical Conductivity s (maximize current) • Thermal Conductivity l (minimize thermal conduction) • Peltier Coefficient P (maximize energy difference at contacts) EC EF Internal Cooling of Devices cool heat EC cool cool _ EFn EC EF EFp n EV metal p cool EV n-type cool Thermoelectric Cooling heat P-N Diode cool EC p heat heat EFp EFn n heat EV cool heat cool n+ Semiconductor Laser Diode EC n EV collector p cool n EC base emitter Heterojunction Bipolar Transistor EF HFET Channel The operating current of a device causes thermoelectric heating/cooling at every internal device junction. Diode Thermoelectric Effects Conventional TE Cooler P-N Diode Thot p I I Thot n electrons holes Tcold EF heat electrons p I I cool heat n EV n I cool heat cool Thot holes EC p Tcold EC EFn cool p n cool cool EFp EV heat The diode is the fundamental building block of most electronic and optoelectronic devices (transistors, lasers, amplifiers, etc.) K. P. Pipe, R. J. Ram, and A. Shakouri, "Bias-dependent Peltier coefficient and internal cooling in bipolar devices", Phys. Rev. B 66, 125316 (2002). Measurement of Bipolar Thermoelectric Effect Unbiased GaAs diode: ND = 5×1018 cm-3, NA = 1×1019 cm-3 p Energy (eV) 1 0.5 EF 0 EC -0.5 -1 EV Built-in potential -1.5 -2 -150 Carrier Concentration (cm-3) n -100 -50 0 50 Position (nm) 20 10 holes 10 10 P<0 for electrons P>0 for holes 0 20 Measurement Theory 4x bulk value 10 0 10x bulk value -10 Voltage measured using SThEM, an STM-based technique -20 -150 -100 -50 0 50 Position (nm) 10 electrons -10 10 Thermoelectric Voltage (mV) 1.5 -150 -100 -50 0 50 Position (nm) Carrier transport calculated with self-consistent drift-diffusion / Poisson equation software • First observation of enhanced thermoelectric effect due to minority carriers • Most active devices use minority carriers for operation H.-K. Lyeo, A.A. Khajetoorians, L. Shi, K.P. Pipe, R.J. Ram, A. Shakouri, and C.K. Shih. Science 303, 816 (2004) Alloys in Devices Semiconductor Laser EC p electrons EV holes Alloys with different bandgaps are added between the p-type and n-type regions: • One alloy traps electrons and holes so that they overlap and recombine to emit light. • Another alloy provides refractive index contrast so that light is confined. n Quantum well temperature is critical to laser operation Electron leakage PE QW Fp Electron injection radiation Lasers are typically biased to “flat-band” EC N EFn N+ (substrate) EV Hole injection Electron/hole injection current Electron/hole leakage current Hole leakage Thermoelectric heating Thermoelectric cooling Optimizing Thermoelectric Heat Exchange Distribution Injection Current Internally Cooled Light Emitter Conventional Design PE heat Fp EFn heat Thermoelectric heat exchange cool heat QW N EC cool P N+ EFp heat cool cool heat less cool EV x Thermoelectric heat exchange EC heat less cool EFn N N+ EV x Active region cooling K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal cooling in a semiconductor laser diode”, IEEE Phot. Tech. Lett. 14, 453 (2002). Optimizing Thermoelectric Heat Exchange Distribution Injection Current Internally Cooled Light Emitter EC P EFp heat cool QW Temperature o(C) 40 GaInAsSb-based laser simulation 35 Conventional Thermoelectric heat exchange cool heat less cool less cool EFn N N+ EV x Active region cooling 30 Optimized 25 18% reduction in operating temperature 20 0 200 400 600 Current Density 800 1000 1200 (A/cm2) K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal cooling in a semiconductor laser diode”, IEEE Phot. Tech. Lett. 14, 453 (2002). Internal Cooling of Transistors Remove hot electrons by thermionic emission Optimizing for thermoelectric/thermionic cooling could reduce device heating. EC Boltzmann transport simulation of AlGaAs/GaAs HBT EF cool cooling (heatsink at emitter) (heatsink at collector) HFET Channel EC n EV collector p n base emitter Heterojunction Bipolar Transistor W. Y. Zhou, Y. B. Liou and C. Huang, Solid-State Electron. 38, 1118 (1995) E. Pop, S. Sinha, and K. E. Goodson, IMECE 2002 Could energy from microscale device waste heat be harvested? Thermoelectric Power Generation Induced voltage measured from cold to hot end S = “Seebeck coefficient” [V/K] T+DT + n V = SnDT holes A temperature difference applied across a material causes a net motion of charge and hence an open-circuit voltage to develop. electrons T+DT V = SpDT + T T p-type material: holes are majority carriers, Sp > 0 n-type material: electrons are majority carriers, Sn < 0 P = “Peltier coefficient” = TS [V] p Attaching a load to a Tthermoelectric generator causes current to flow. hot THot a = # of n / p pairs Vn + n p + Rn Vp Rp Tcold R Load n Vtot = a×(Vn+Vp) p _ + TCold Rtot = a×(Rn+Rp) RLoad Thermoelectric Power Generator Efficiency For an optimized TE device with a matched load (Rload = RTE), QH hopt = TH I TC TH M-1 M+ TC TH Carnot efficiency where M= I2RLoad h= Q H TH - TC TH + TC 1+Z 2 RLoad S2s Z= k Thermoelectric figure of merit ZT averaged over the operating temperature range Increasing Carnot efficiency ZT = 1 THot - TCold (K) THot - TCold (K) Efficiency Curves ZT = 3 TCold (K) ZT = 2 TCold (K) THot - TCold (K) THot - TCold (K) TCold (K) Efficiency (%) ZT = 4 TCold (K) In order to generate significant power density, device must maintain a large DT (high h) or have a high heat flux. These two effects are linked. Efficiency Increase with Increasing Heat Flux L: Thickness of TE generator Q: Heat source k: Thermal conductivity A: Cross-sectional area 10-1 10-2 Increasing heat flux 10-3 10-4 10-5 10-6 ZT = 2 10-7 10-8 For most devices made from (nanostructured) TE materials with high ZT, L 10-5 to 10-2 cm ≈ 10-2 to 10-1 W/cmK k Q/A (W/cm2) Efficiency h LQ DT = kA Thickness Thermal Conductivity Assuming 1D heat flow, (cm2K/W) As heat flux Q/A increases, DT = Thot -Tcold increases, and therefore the efficiency increases. Increased Efficiency for Energy Conversion from Small Hot Spots Using Small TE Generators Area A1 Same QH Net area reduced to A2 I2RL2 I2RL1 QH Wasted Wasted heat heat RL1 Larger TH-TC QH 3 RL2 (each) TCold TCold One-leg generator Small one-leg generator for each heat source In systems with micro/nanoscale heat sources, efficiency can be improved by employing targeted micro/nanoscale thermoelectric generators which only enclose the individual heat sources, reducing the total cross-sectional area and therefore increasing the heat flux QH/A. What systems have micro/nanoscale heat sources with high heat flux? Intel Itanium® Processor VDevice Device-Level Thermoelectric Generation Methods Device-Internal C. LaBounty, Ph.D. thesis, UC Santa Barbara (2001) RLoad Device QH Thermoelectric Generator Device-External Substrate 0.4 20 15 10 0.3 flat 0.2 5 0.1 (data from P. Bhattacharya) 0 0 25 50 75 Temperature ( C) Devices can have large internal heat fluxes and temperature gradients due to high-power operation, low thermal conductivity regions, etc. Is it possible to perform energy harvesting directly at heat sources by integrating thermoelectric structures into the device design (band structure) itself? Slope Efficiency (W/A) Heat sink Threshold Current (mA) Microscale thermoelectric energy harvester monolithically integrated with device High performance chips typically have strong heat sinking which could maintain a significant temperature gradient across the TE generator. Increase in device temperature could be outweighed by energy savings. +- QD lasers can have small temperature dependence 100 VDevice +- Device RLoad Until now we have examined energy conversion within active devices. Now we will look at scanning probe techniques for energy transfer from an active device to a sample. Leaky mode (Radiation) SPP Cathode ETL + HTL V Waveguided Decay rate (a.u.) Energy Outcoupling from Active 520nm spectrum Organic Devices 2.6 Si Substrate Cathode: 18nm Ag ETL: 60nm Alq3 HTL: 50nm a-NPD Anode: 100nm Al / 13nm Ni Substrate: Silicon w/(2pc) Anode Wave Surface Plasmon guided Radiation kx -3 x 10 Waveguided Leaky mode 2.2 520nm 1.8 Surface plasmonpolariton 1 2 3 4 5 -3 kx / 2p x 10 • The amount of dipole energy that goes to a specific mode can be tailored by changing layer materials and thicknesses • By placing an active device on a scanning probe, we can couple this energy to a sample. OLED on an AFM Cantilever Cathode Tipless Cantilever Active Layers + - Insulator Si Cantilever Anode Light Emission from the OLED 6 m 35 m Light emission from the OLED edge K. H. An et al., Appl. Phys. Lett. 89, 111117 (2006) Summary • Recent advances in thermoelectrics have produced large cooling powers over micron-scale regions. • Every junction in a device has thermoelectric heating or cooling. • The bipolar nature of active devices can lead to enhanced thermoelectric effects. • The optimization of internal thermoelectric effects can lead to targeted cooling inside a device. • Large temperature gradients in devices can potentially be used for thermoelectric conversion of waste heat into electricity. • Active devices placed on cantilevers can be used to couple energy to a sample.