Transcript Slide 1
Nanotechnology on our Desktops Transistor Hard Disk Gate Sensor Source Well 6 nm Medium Drain Gate oxide 4 nm Switching layer 5 nm Magnetic grain 10 nm Electrons in the Conduction Band p-type n-type Holes in the Valence Band Benefits of a quantum well: 1) Trap electrons and holes in the same quantum well and thereby keep them together longer. Alferov Kroemer Nobel Prize in Physics 2000 2) Electrons and holes have well-defined, quantized energies, such that most of them contribute to the same laser line. Quantum Well Laser Optimization 6 nm : Optimum Thickness The (MOSFET) Transistor MOS = Metal-Oxide-Semiconductor FET = Field Effect Transistor Electrons The electron flow from source to drain is controlled by the gate voltage. Transistor DRAM (Dynamic Random Access Memory) Transistor + Capacitor Moore’s Law of Silicon Electronics The performance of silicon devices increases exponentially. Corollaries: The storage density doubles every 2 years, linear dimensions x½ every 4 years. The cost per megabyte decreases exponentially. The cost of a factory (“fab”) increases exponentially (now a few billion $). Power consumption by a leaky gate oxide: A show-stopper for Moore’s Law ? Gate oxide has shrunk to < 2nm, < 10 atom layers. Electrons can tunnel through when applying a gate voltage. Uses up to 1/3 of the power. Semiconductor-Insulator Interfaces : From the MOSFET to Molecular Electronics Mismatch of the bond density at the Si/SiO2 interface The Si/SiO2 Interface Intermediate oxidation states at the interface provide a gradual transition from Si to SiO2 . 0.7 nm Limit Predicted for the Gate Oxide Thickness Intermediate oxides insulate poorly CBM Si2p STEM + electron energy loss measurement across the Si/SiO2 interface (see Lect. 5, Slides 16,17) Intel is already at the limit: 0.8 nm oxide reached in demos Need gate insulator with high dielectric constant : HfO2 Thicker oxide with same capacitance and less leakage Atomic Layer Epitaxy (ALE) for Monolayer Control “Digital Growth” General concept of ALE: Adsorb fairly inert precursor molecules onto a reactive sbstrate, such that only one monolayer sticks. Reactivate the surface by chemical treatment. Repeat this cycle. ALE growth of Al2O3 (alumina) from an organo-metallic precursor with reactivation by H2O. Molecular Field Effect Transistors Molecular Control of Gate Dielectric and its Interface Si-Molecule Interfaces Halik et al., Nature 431, 963 (2004) Review: Malliaras and Friend, Physics Today, May 2005, p. 5 Attaching Alkanes to Silicon via Siloxane Chemistry C-C OTS Alkane C-H Moist Ordered Dry Disordered Cl H O Silicon Peters et al., Langmuir 18, 1250 (2002) In Pursuit of the Ultimate Storage Medium 1 bit = 1 atom Silicon Surface CD-ROM Track spacing 5 atom rows 10 m 10 nm Density x106 Bennewitz et al. Nanotechnology 13, 499 (2002) When will we be down to atoms ? Using Moore's Law ... 250 Terabit/inch2 Year 2038 CCD (Charge Coupled Device) The CCD detectors in digital cameras wiped out photographic film. Physics Nobel Prize 2009: Boyle and Smith Operation of a CCD CCD detectors are based on silicon MOS technology (compare Slides 4, 5). Each pixel consists of a MOS capacitor with positive gate voltage. Contrary to a MOSFET, electrons cannot flow directly into the channel. Only those excited by photons are able to charge the capacitor, building up a charge bucket. Readout: Charge buckets are shifted along a row of pixels (“bucket brigade”) . Gate Channel CB VB