Posted tagged ‘STM’

At the limit of Moore’s Law


Stop to consider the amazing explosion of technology that has rapidly filled our lives in the last 50 years. Such the computer you’re reading this blog on, or perhaps you are viewing this page on a device smaller than your hand, or even the fact that these words can be read by anyone, from anywhere in the world.

Where did this all begin? What was the ‘seed’ that allowed all this technology to grow so fast and sprout up in every aspect of our lives?

I think a good argument could be made that the answer to this question is: The transistor.

A transistor is, basically, a tiny electrical switch. It stops and starts the flow of current along a circuit in response to series of electrical pulses.  These allows a computer’s to send a control signal to all of its various parts using this off/on flow of current to “speak” the 0/1 language of binary and perform operations by forming Logic Gates. In this way, the transistors form part of computer’s “nervous system” relaying signals from the CPU (its brain) to the various “organs” (the hard drive, the speakers etc.)

However, modern transistors are tiny things and the driving force behind the rapid increasing in computing power (while computers themselves shrink) is that scientists have found way to make smaller and smaller transistors allowing them to pack more and more onto the circuit boards in your PC.  Unfortunately, there is an rather obvious physical limit to how small you can make something: the atom. And it’s seems we have hit that limit with the latest reports in Nature Nano of a functioning Single Atom Transistor.

A common type of transistor is the Field Effect Transistor (FET), where the current flows between two electrodes called a Source and a Drain, and is controlled by a third called the Gate. The names are quite helpful as the operation of the FET can be described by a comparison to water. Imagine the Source to be a tap and the Drain, a plughole, and that the water needs to pass through length of rubber tubing dangling from the end of the tap into the sink. Turn on the tap and set the water going, then start to squeeze and release the tube rhythmically, stopping and releasing the flow. This is your Gate.

Of course in a real FET the water corresponds to the movement of charge carriers such as electrons or holes, the hose is a narrow channel of semiconductor whose size and shape is controlled by applying a voltage across it, restricting the flow of charge.

The single-atom transistor works in the same way. Michelle Simmons’ Group at the University of New South Wales first started with a hydrogen-passivated silicon surface. Using the tip of a Scanning Tunneling Microscope (STM) they carved away the hydrogen atoms in a “+ shaped” to form the Source, Drain and two Gate electrodes leaving a tiny gap in the centre. They then added phosphine (PH3) to the system, which binds to the newly bare silicon and ignores the hydrogen covered parts. This dopes the electrodes to make them more conducting.  In the tiny gap of the centre of the + shape, the author, managed to take three PH3 molecules on the surface and; breaking, swapping and reforming through some impressively simple thermal treatment, replaced a single silicon atom with one of Phosphorus. Measurement of the current between source and drain showed that it did indeed depend on the voltage applied across the gate electrodes, confirming the device was “transisting”.

I won’t go into further detail about the conduction measurements and the nitty gritty of energy levels and electrostatic potential calculations, however, I do hope to impress upon you the importance of transistors and the gravity of the work. Though single atoms have previously been shown to behave like transistors in the right circumstances, this is the first time a single atom transistor has been engineered i.e. the electrodes, the doping and the transistor atom itself have been deliberately and deterministically put in place with atomic precision. This is an amazing feat of nano-fabrication and represents the ultimate size limit of present computer hardware: It doesn’t get smaller than one atom.

As an extra bonus there are some signs that such a device can beyond the current level of computing. At low gate voltages, the conduction measurements along with calculations also show that the phosphorus atom retains its discrete quantum levels allowing for potential applications in Quantum Computing, logical operations and devices that make use of the “spookiness” of the quantum mechanics.

Of course this is a long step from use in an actual computer, let alone your laptops. The device can only be operated blow liquid nitrogen (<77 K) temperatures and the device fabrication makes use of ultra-high vacuum conditions. However, this is how science evolves into technology: someone finds out what is actually possible and then someone tries to make it work at a higher temperature or make the device a little bit more stable, inching closer and closer to practicality.

Not every scientific breakthrough makes it from the lab into your home, just as not every drug gets from the petri dish to your medicine cabinet or every species gets from an amoeba to the zoo. However, where there is enough passion and excitement, the Nerds will find the way.