The creation of nanodevices
21 December 2007 (Volume 2 Issue 12)
Continued reductions in size and integration of devices, such as transistors used in integrated circuits, have improved the performance of personal computers and mobile phones. However, in the near future, this approach to improving performance may no longer be available as the quantum phenomena that govern the microscopic world are expected to dominate, so that when devices are reduced to the microscopic level they will fail to function in the conventional classical way. In contrast, the Advanced Device Laboratory (ADL) is rising to the challenge of making use of these quantum phenomena. The laboratory is making efforts to develop nanodevices based on a new principle of operation, defying the boundaries of conventional electronics. The developing research field of ‘nanoelectronics’ is about to take off.
Phenomena that occur in the microscopic world
“Make things very small and probe the physics.” This was the advice given to Koji Ishibashi, now a chief scientist at RIKEN, by his supervisor in the mid 1980s when Ishibashi was taking a graduate course in electronics. “In those days, the word ‘nanotechnology’ did not exist, and research activities were not as active as today,” Ishibashi recollects. “We had no specific ideas, but I was asked to make things very small in the hope that it might lead to finding new phenomena. I think that my former direct supervisor knew in his bones that something new would happen.”
The single-electron transistor is a high-profile nanodevice based on a new principle of operation. A typical transistor currently deals with a flow of about 100,000 electrons. In contrast, a single-electron transistor deals with only a single electron, thus opening up functions that were not possible previously.
When a voltage is applied to the gate of a single-electron transistor, electrons try to jump from the source to the ‘quantum dot’ (Fig. 1, left). Note that only a single electron can enter the quantum dot. The reason is as follows: a quantum dot confines electrons within a very small space. When one electron jumps into the quantum dot, the electrons in the quantum dot increase the repulsive force between the negatively charged electrons (Coulomb force), and this blocks one or more additional electrons from jumping into the quantum dot. This is known as the ‘Coulomb blockade phenomenon’. Thus, enabling one more electron to jump into the quantum dot requires a voltage to be applied to the gate.
Figure 1: Single-electron transistor.
Tunnel barriers of electrons are fabricated between the quantum dot and the source, and between the quantum dot and the drain. When a voltage is applied to the gate, electrons jump over the barriers (tunnel effect), entering the quantum dot from the source on a single electron basis.
Implementation of the Coulomb blockade phenomenon requires fabrication of an extremely small quantum dot. Furthermore, the Coulomb force between electrons confined within the quantum dot is very small. Thus, devices of the size manufactured by conventional microfabrication techniques can generate the Coulomb blockade phenomenon only at very low temperatures. “In the mid-1990s I launched a study into the single-electron transistor made from a semiconductor,” says Ishibashi. However, the Coulomb blockade phenomenon did not occur until the transistor was cooled to a temperature of 1 K (-272 °C). Under those conditions, there was no hope of finding a practical application. The minimum wire width for semiconductors currently achievable by microfabrication is about 20 nm (1 nm is a billionth of a meter). “Thus,” says Ishibashi, “We need to make further efforts to reduce the size of quantum dots so that the Coulomb blockade phenomenon occurs at higher temperatures, for example, at room temperature.”
In 1996, a research group at Delft University of Technology in Holland successfully made a single-electron transistor using a carbon nanotube 1 nm in diameter as the quantum dot. “I was studying there in those days, working in that research group,” says Ishibashi. At that time, nobody had ever thought of the idea of a single-electron transistor made of a carbon nanotube, but a student in the laboratory just tried out the idea, and acquired data that clearly shows the Coulomb blockade phenomenon. “I thought this was great, and started to conduct research into a single-electron transistor using carbon nanotubes.”
The carbon nanotube is an extremely fine tube made of carbon. “Microfabrication techniques for semiconductors can be used to connect a carbon nanotube to electrodes because, although the diameter is about 1 nm, the tube has a length of more than 1 µm,” says Ishibashi. “In our laboratory, we successfully used a carbon nanotube to generate the Coulomb blockade phenomenon at 20–30 K.” (See Fig. 1, right). Ishibashi adds that one report suggests that the Coulomb blockade phenomenon can occur even at room temperature.
In 2003, Ishibashi and members of ADL were successful in using two carbon-nanotube single-electron transistors to make a single-electron inverter similar to the complementary metal oxide semiconductor (CMOS) inverter, which is currently the mainstream of semiconductor devices. They were the first people in the world to get it to work properly (Fig. 2). “The research brought more attention from around the world than we had ever expected,” he adds.
Figure 2: CMOS-like inverter comprising single-electron transistors.
A CMOS inverter acts as a special switch (inverter), in which one transistor is ‘on’ while the other is ‘off’. This function is achieved using two single-electron transistors (SETs) fabricated on a carbon nanotube.
Then, why did the research draw such keen interest? One reason is that they successfully made a device by integrating carbon-nanotube single-electron transistors for the first time, although the device has only two single-electron transistors.
Devices such as transistors have been reduced in size, and more and more devices are integrated into a single semiconductor chip to achieve further improvements in performance. The application of this technique, however, is said to be approaching the limits of performance improvement.
This is because it is predicted that quantum phenomena will dominate when conventional devices are further reduced into extremely small elements, and thus devices will fail to function under the conventional principles of operation. Besides, further microfabrication of integrated circuits increases the heat consumption per unit area, leading to circuit malfunction. This is another serious problem.
The single-electron transistor can function at temperatures closer to room temperature when its size is reduced. Furthermore, the single-electron transistor, which deals with a single electron, is the ultimate energy-saving device, and so it is capable of solving the problem of heat generation. Ishibashi and other members of the laboratory used carbon-nanotube single-electron transistors to successfully implement the specific function of a CMOS inverter, demonstrating the possibility of defying the boundaries of current electronics.
Using artificial atoms to capture terahertz light
Both an atom and a quantum dot confine electrons within a narrow space. In this sense, a quantum dot can be regarded as an artificial atom. Although the nature of atoms is fixed, artificial atoms can be designed as devices after they have been variously changed and modified.
Light is made up of photons. Atoms absorb the energy of photons, emitting electrons when high-energy light, such as ultraviolet or X-ray, shines on the atoms. In contrast, carbon-nanotube artificial atoms are designed so that the artificial atoms can emit electrons when photons of low-energy terahertz light are absorbed. In 2006, Ishibashi and laboratory members successfully observed terahertz photon absorption using carbon nanotube artificial atoms (Fig. 3).
Figure 3: Terahertz light detector.
The energy of the photons causes electrons to be emitted when terahertz light is shone onto artificial atoms in a carbon nanotube. Terahertz light can be detected by capturing the electrons.
Terahertz light—light that has a frequency of about 1 terahertz or a wavelength of 0.3 mm—is between visible light and radio waves in the electromagnetic spectrum. Because terahertz light has moderate permeability and is readily absorbed by materials, it is anticipated to be applied to a number of methods of examination and diagnostics, including how to distinguish normal cells from cancerous ones, or how to detect illegal drugs hidden in envelopes, Conventional optical techniques or radiowave techniques, however, cannot control terahertz light. In particular, the development of highly sensitive detectors has been delayed. Today, a widely used detector called the ‘bolometer’ can detect a temperature rise in silicon material as a change in electrical resistance when terahertz light is shone on the material. This principle, however, has limitations in sensitivity. In contrast, the device that Ishibashi and other members of ADL have been studying will be able to detect a single photon, that is, it will be able to achieve the ultimate sensitivity.
“After presenting these findings, we received inquiries from researchers in radio astronomy,” says Ishibashi. Terahertz light is an unexplored field of observation even in astronomy. Today, the giant radio telescope ‘ALMA’ is jointly being constructed in Chile, South America, in collaboration with Europe, Japan, and North America. This telescope, which was designed to observe a certain type of light, namely, electromagnetic waves in the millimeter, or submillimeter wavelength range, will be used to probe the galaxy, planetary systems, and the origins of life. In the future, a detector with built-in carbon nanotube artificial atoms might contribute to a great astronomical discovery.
Building a quantum computer
“If a single electron can be completely controlled, a quantum computer can be developed,” states Ishibashi. The quantum computer is expected to be capable of completing a prime-factor-decomposed computation in only tens of seconds, whereas current supercomputers require thousands of years to conduct the same computation. The basic element of the quantum computer is called the ‘quantum bit’. Current computers allocate ‘1’ or ‘0’ to each bit, and these bits are processed for the intended calculation. In contrast, the quantum bit is represented by a two-state system described like this, for example: a 30% probability of ‘0’ and a 70% probability of ‘1’. This linear superposition of states that simultaneously enables the two states of ‘0’ and ‘1’ is called ‘quantum superposition’.
Today, scientists are conducting research on how to achieve a quantum bit using various materials and ideas. Ishibashi and other members of ADL are advancing research on the use of electron spin states of carbon-nanotube artificial atoms.
Electron spin is the angular momentum of an electron, similar to the rotation of the earth, and there are two electron-spin states: clockwise (spin-up) and anticlockwise (spin-down). For example, ‘0’ can be allocated to spin-up, and ‘1’ to spin-down. By applying a magnetic field to carbon-nanotube artificial atoms, Ishibashi and his laboratory members successfully created both independent states, spin-up and spin-down, from a single electron-spin state. “Next we need to create a quantum superposition for any values of probability, for example, a 30% probability of ‘spin-up’ and a 70% probability of ‘spin-down’. This is a really tough proposition. Only a few groups have succeeded in creating this level of quantum superposition.”
One of the few groups that have succeeded in it is the Macroscopic Quantum Coherence Laboratory according to Jaw-Shen Tsai, the laboratory head of the Single Quantum Dynamics Research Group at RIKEN Frontier Research System. This laboratory successfully used a superconductive device (Josephson device) to produce a quantum bit, and leads the world in terms of research into quantum computing. “A calculation based on a quantum computer requires many operations of quantum bits,” says Ishibashi. The state of quantum superposition, however, does not last long enough. Even superconductive elements do not give a performance as long as initially expected. Ishibashi says, “Carbon-nanotube artificial atoms are expected to maintain the state of quantum superposition for a longer time.”
I feel as if it were 1948
Having achieved many research results using carbon nanotubes, Ishibashi says, “Different materials may be used instead of carbon nanotubes in five years time.” He adds that microfabrication of carbon nanotubes is really difficult. For example, only one single-electron transistor out of ten prototypes works properly. “This means our single-electron transistors are created only by chance at the moment.”
Ishibashi continues, “I feel as if it were 1948.” This is the year when the transistor was invented. In those days, the transistor was first predicted to be a substitute for the vacuum tube. The transistor, however, was broken easily, and therefore putting it into practical use was also considered difficult. In the meantime, silicon supplanted germanium as a material for transistors and integrated circuits were invented. Thus nearly 60 years were needed to develop the transistor to the current level. In contrast, the carbon-nanotube single-electron transistor can be compared in performance to the transistor in 1948. “We will also try out other materials such as semiconductor nanowires. Anyhow, we will make things very small using various materials, and see what happens.”
Former direct supervisor, Susumu Namba
In April 2007, Susumu Namba, a pioneer of semiconductor engineering passed away (Fig. 4, RIKEN Honorary Researcher and Professor Emeritus of Osaka University). He was Ishibashi’s former direct supervisor during his graduate course in electronics, and he was the supervisor who advised Ishibashi to “make things very small and probe the physics”.
Figure 4: Susumu Namba (far right).
From left, Gottfried Landwehr (Klitzing’s former supervisor), Koji Ishibashi, Chief Scientist, and Klaus von Klitzing (1985 Nobel Laureate in Physics).
“He really was a man of wide vision. He had a keen perception and looked ahead of the times,” says Ishibashi. The study subjects that Namba quickly launched and explored included light modulation, ruby-laser oscillation, ion implantation into semiconductors, excimer laser lithography, and synchrotron radiation lithography. He used to say to his students, “Study things that I do not understand. Otherwise, nothing new will be created in this laboratory.” He told his students, “Follow this way, but don’t get hung up on the details.” Ishibashi adds, “He was a great lover of sake, and often visited my lab at RIKEN until recently, but ....”
Ishibashi fails to complete the sentence.
It seems Ishibashi will accept the baton of his supervisor’s pioneer spirit and will continue to conduct research into small things, just as he was advised.