Nanoscience and Nanotechnology Lecture 19

1. PHY5540 Spring 2006 Nanoscience and NanotechnologyLecture 19 Only 8 lectures remaining! (including this one) Nanoelectronics Course notes are at: http://www.phys.appstate.edu/nanotech/NanoCourseLecturePPTs2008/

2. Back to Nanoelectronics We briefly started this earlier

3. IBM: Atomic-sized graphene has use in nanoelectronics Electronic News, 3/6/2008 IBM researchers at the company's Yorktown Heights, NY-based lab today detailed a discovery that could allow graphite to be used as a material for building nanoelectonic circuits smaller than those in today's silicon-based computer chips Single layer: Noisy, as predicted by Double layer: Noise reduced to acceptable level

4. IBM: Atomic-sized graphene has use in nanoelectronics IBM'S ATOMIC 'CHICKENWIRE' FOR NANOELECTRONICS: The image on the left shows a single layer, or sheet of carbon molecules known as Graphene. The noise that occurs from electrical signals bouncing around in the material as a current is passed through it is greater as the device is made smaller and smaller, impeding the performance for nanoscale electronics. In the image on the right, the IBM scientists demonstrated for the first time that adding a second sheet of Graphene reduces the noise significantly, giving promise to this material for potential use in future nanoelectronics.

5. Speed Bumps Less Important Than Potholes For Graphene National Institute of Standards and Technology. "Speed Bumps Less Important Than Potholes For Graphene." ScienceDaily 16 July 2007. 17 March 2008 <http://www.sciencedaily.com /releases/2007/07/070713131450.htm>. For electrical charges racing through an atom-thick sheet of graphene, occasional hills and valleys are no big deal, but the potholes--single-atom defects in the crystal--they're killers. That's one of the conclusions reached by researchers from the National Institute of Standards and Technology (NIST) and the Georgia Institute of Technology who created detailed maps of electron interference patterns in graphene to understand how defects in the two-dimensional carbon crystal affect charge flow through the material.

6. The Nanotube transistor Carbon-atom model of singlewall carbon nanotubes. Singlewall carbon nanotubes exist in a variety of structures corresponding to the many ways a sheet of graphite can be wrapped into a seamless tube. Each structure has a specific diameter and chiral (or wrapping) angle. The armchair structures ( = 30o) have metallic character. The zigzag tubes ( = 0o) can be either semimetallic or semiconducting. Nanotubes with chiral angles of 0to 30o] include both semimetals and semiconductors. From Weisman 2004 [465].

7. Nanotube Chirality metalic metallic Semiconducting* Semiconducting* *or semimetallic

8. The Nanotube transistor Cncept AFM of actual nanotube transistor

10. NANOELECTRONICS • What is the basic Physics of nanoelectronics? • The aim of Nanoelectronics is to process, transmit and store information by taking advantage of properties of matter that are distinctly different from macroscopic properties. • The relevant length scale depends on the phenomena investigated: it is a few nm for molecules that act like transistors or memory devices, and it can be 999 nm (~1 micron) for quantum dots where the spin of the electron is being used to process information. • Microelectronics, even if the gate size of the transistor is 50 nm, is not an implementation of nanoelectronics, as no new qualitative physical property related to reduction in size are being exploited.

11. The need for nano electronics • The last few decades has seen an exponential growth in microchip capabilities due primarily to a decrease in the minimum feature sizes. The resulting doubling of processor speed every 18 months (known as Moores Law) is, however, expected to break down for conventional microelectronics in about 2015 for both fundamental and economic reasons • [Nature 406,1027 (2000)]. • 15 years corresponds to only 3 generations of graduate students (2y MS, 3y Ph.D.)! The search is on, therefore, for new properties, paradigms and architectures to create a novel nanoelectronics.

12. What is Nanoelectronics? • Semiconductor electronics have seen a sustained exponential decrease in size and cost and a similar increase in performance and level of integration over the last thirty years (known as Moore's Law). The Silicon Roadmap is laid out for about the next ten years. After that, either economical or physical barriers will pose a huge challenge.

13. Moore’s Law • Moore first posited his exponentially increasing transistor idea in a 1965 magazine article in the 35th anniversary edition of "Electronic Magazine" when the integrated circuit was still in its teething phase (with only a handful of components per chip).

14. Moore’s Law • Moore's first prediction was based upon the progress of the integrated chip up until that point, which showed that since the introduction of the first planar transistor in 1959, there had been a doubling of components contained on a single chip every year. • Moore's wasn't a particularly rigorous line of scientific enquiry, but then history is full of brilliant ideas derived from the intuitive reasoning of geniuses. • How did Gordon Moore come up with Moore’s Law?

15. Moore’s Law • "I took that first few points, up to 60 components on a chip in 1965 and blindly extrapolated for about 10 years and said okay, in 1975 we’ll have about 60 thousand components on a chip," recalls Moore. • "I had no idea this was going to be an accurate prediction. And one of my friends, Dr. Carver Mead, a Professor at Cal Tech, dubbed this Mooreﾕs Law." • So, originally, Moore's prediction was a doubling of complexity annually. But in 1975, after missing a vital factor contributing to the chip's rate of "remarkable" progress, Moore revised this prediction to the one we currently: every two years.

16. Moore’s Law • Moore's Law

17. Moore’s Law • See the Intel technology timeline at: • http://www.intel.com/technology/timeline.pdf • The Revolution Begins Invented 60 years ago, the transistor is a key building block of today’s digital world. Perhaps the most important invention of the 20th century, transistors are found in many devices and are the building blocks of computer chips. Intel, the largest manufacturer of computer chips, continues to innovate to help PCs and laptops become smaller, faster, sleeker and more energy effcient. Many new applications and inventions powered by transistors have impacted all of our lives over the past 60 years. • But this is not nantechnology

18. What is Nanoelectronics? • Moore's Law is related to the difficulty of making a profit in view of the exorbitant costs of building the necessary manufacturing capabilities if present day technologies are extrapolated. • The Silicon Roadmap is a direct consequence of the shrinking device size, leading to physical phenomena impeding the operation of current devices - relying on fairly straight forward improvements in engineering and technology- not new science! • Solid State Physics, however, falls apart at the nanometer scale when the number of surface (and/or interface states) begins to play a dominate role and hence cannot be ignored. • For nanoelectronics, new science and new technology (Nanoscience and nanotechnology) must come into play! • What are some of the issues we face in moving to nanotechnology?

19. What are the some of the new effects? • Quantum and coherence effects, high electric fields creating avalanche dielectric breakdowns, heat dissipation problems in closely packed structures as well as the non-uniformity of dopant atoms and the relevance of single atom defects are all roadblocks along the current road of miniaturization [Nature 406, 1023 (2000)]. • These phenomena are characteristic for structures a few nanometers in size and, instead of being viewed as an obstacle to future progress, might form the basis of post-silicon information processing technologies.

20. More than just electronics • It is even far from clear that electrons will be the method of choice for signal processing or computation in the long term - quantum computing, spin electronics, optics or even computing based on nanomechanics are actively being discussed. • Nanoelectronics thus needs to be understood as a general field of research aimed at developing an understanding of the phenomena characteristic of nanometer sized objects with the aim of exploiting them for information processing purposes. • Specifically, by electronics we mean the handling of complicated electrical wave forms for communicating information (as in cellular phones), probing (as in radar) and data processing (as in computers) [Landauer, Science (1968)].

21. Concepts at the fundamental research level are being persued world-wide to find nano-solutions to these three characteristic applications of electronics. • One can group these concepts into several main categories: 1. Molecular electronics: Electronic effects (e.g. electrical conductance of C60) 2. Synthesis (DNA computing as a buzz word) 3. Quantum Electronics, Spintronics (e.g. quantum dots, magnetic effects) 4. Quantum computing

22. Active fields related to nanoelectronics • Currently the most active field of research is the fabrication and characterization of individual components that could replace the macroscopic silicon components with nanoscale systems. • Examples are molecular diodes , single atom switches or the increasingly better control and understanding of the transport of electrons in quantum dot structures. • A second field with substantial activity is the investigation of potential interconnects. Here, mostly carbon nanotubes and self-assembled metallic or organic structures are being investigated.

23. Nanoelectronics Architectures • Very little work is being performed on architecture (notable exceptions are HP's Teramac project [Heath et al., Science 280, 1716 (1998)] or IBM's selfhealing Blue Gene project). • Furthermore, modeling with predictive power is in a very early stage of development. This understanding is necessary to develop engineering rules of thumb to design complex systems. One needs to appreciate that currently the best calculations of the conductance of a simple molecule such as C60 are off by a factor of more than 30! • This has to do with the difficult to model, but non-trivial influence of the electronic contact leads.

24. Nanoelectronics Architectures • This has to do with the difficult to model, but non-trivial influence of the electronic contact leads- just the ability to have input and output of signals with nanoelectronic devices is a major technical barrier. • The situation in quantum computing is somewhat different. The main activities are on theoretical development of core concepts and algorithms. Experimental implementations are only starting. • An exception is the field of cryptography (information transportation), where entangled photon states propagating in a conventional optical fiber have been demonstrated experimentally.

25. What needs to be done ? • First, nanoelctronics is a wide open field with vast potential for breakthroughs coming from fundamental research. Some of the major issues that need to be addressed are the following: • 1. Understand nanoscale transport! (closed loop between theory and experiment necessary). • 2. Develop/understand self-assembly techniques to do conventional things cheaper. This has the future potential to displace a large fraction of conventional semiconductor applications. • 3. Find new ways of doing electronics and find ways of implementing them (e.g. quantum computing; electronics modeled after living systems; hybrid Si-biological systems; cellular automata). • Do not try and duplicate a transistor, but instead investigate new electronics paradigms! Do research as a graduate student in this field and lay the foundation for the ‘Intel’ of the New Millenium.

26. What should today’s Physics student do? • 1. Understand nanoscale transport! (closed loop between theory and experiment is necessary). Most experiments and modeling concentrate on DC properties, AC properties at THz frequencies are however expected to be relevant. • 2. Develop/understand self-assembly techniques to do conventional things cheaper. This has the future potential to displace a large fraction of conventional semiconductor applications. One needs to solve the interconnect problem and find a replacement of the transistor. If this can be done by self-assembly, a major cost advantage compared to conventional silicon technology would result. • 3. Find new ways of doing electronics and find ways of implementing them (e.g. quantum computing; electronics modeled after living systems; hybrid Si-biological systems; cellular automata). Do not try and duplicate a transistor, but instead investigate new electronics paradigms! Do research as a graduate student in this field and lay the foundation for the Intel of the New Millenium.

27. http://domino.research.ibm.com/comm/pr.nsf/pages/news.20060324_carbonnanotube.htmlhttp://domino.research.ibm.com/comm/pr.nsf/pages/news.20060324_carbonnanotube.html • IBM Milestone Advances Effort to Enhance Semiconductors Through Nanotechnology

28. IBM Milestone Advances Effort to Enhance Semiconductors Through Nanotechnology • Yorktown Heights, NY, March 24, 2006 -- IBM announced that its researchers have built the first complete electronic integrated circuit around a single carbon nanotube molecule, a new material that shows promise for providing enhanced performance over today’s standard silicon semiconductors. • 

The achievement is significant because the circuit was built using standard semiconductor processes and used a single molecule (C nanotube) as the base for all components in the circuit, rather than linking together individually-constructed components. This can simplify manufacturing and provide the consistency needed to more thoroughly test and adjust the material for use in these applications.

29. http://domino.research.ibm.com/comm/pr.nsf/pages/news.20060324_carbonnanotube.htmlhttp://domino.research.ibm.com/comm/pr.nsf/pages/news.20060324_carbonnanotube.html • IBM Milestone Advances Effort to Enhance Semiconductors Through Nanotechnology SEM image of hair

30. When will nanoelectronics be ‘standard’ • At least a decade away, by most peoples guess • Sometime between 2015 and 2025? • The nano-science is rapidly evolving • The nano-engineering to make actual production scale nanoelectronics, with very high yield and throughput, is not yet in sight - it is very difficult to do the engineering unless we first understand the Physics!

31. Recall the areas of critical need • Energy • Information processing • Clean air and water

32. Review of basic semiconductor Physics • When considering the Physics and Engineering of traditional semiconductors, we are mainly concerned with ‘bulk’ properties; and consider surfaces as defects. • Thus approach is the opposite of the nanotechnology approach, where the goal is to use the special properties of materials and devices at the nm scale, where surface atoms control properties.

33. Recall the Si band structure

34. Simplified 2 dim Si showing 4 bonds per atom

35. Electron excitation across the forbidden gap

36. Thermal carrier (electron-hole pair) generation

37. Concept of hole motion in valence band and annihilation

38. Both electrons and holes can drift in an electric field

40. Mass action law np=ni2 Mass action law np=ni2 = NcNvexp(-Egap/kT) Where Nc is the density of states near conduction band edge Nv is the density of states near valence band edge

41. Adding impurities or dopnats

42. Dopant levels for As impurities (valence V, n-type)) Each as provides an extra loosely bound electron

43. Boron as an example of n type doping

44. Band diagram for b doped (p-type) Si Small ionization energy allows thermal ionization at room temperature

45. Band diagram for b doped (p-type) Si Dopants are far apart Note that the electron from the valance band becomes fixed charge (in energy and position) Small ionization energy allows thermal ionization at room temperature

46. For an n-type semiconductor (e.g. Arsenic doped) only the electrons are mobile. The ionized dopant atoms do not move. Thus we have added both ‘mobile” and ‘fixed’ or ‘localized’ charge!

47. For the Case of an extrinsic (doped) semiconductor: we have both ‘mobile’ and ‘fixed’ or ‘localized’ charge! this is a very Important Concept!!! Fixed charge: the ionized dopant atoms Mobile charge: the extra electron or hole contributed by ionization

48. Example of n-type material versus temperature

49. Temperature dependency of conductivity: critical concept! From this data we can measure: Band gap energy Egap, and Dopant ionization energy DEionization

50. Conductivity of a Semiconductor  = ene + eph • = conductivity, e = electronic charge, n = electron concentration in the CB, e = electron drift mobility, p = hole concentration in the VB, h = hole drift mobility Note: this expression is valid for both intrinsic and extrinsic semiconductors! We now see that n≠p for extrinsic semiconductors.