Production of revolutionary light-weighted radiation resistant devices, sensors, and integrated circuits.
The ACNT has been evaluating semiconducting nanotube devices and examining the contact effects. Experimental data has been collected and analyzed in order to understand the mechanisms that inhibit good current flow. This will arm us with the information to overcome the technical obstacles in design of a viable contact for nanoscale devices. Right: Energy Band Diagram and Equivalent Circuit
Intuitive modeling must be emphasized in this approach. In semiconductor electronics, energy band diagrams and equivalent circuits have been employed successfully to overcome similar obstacles. These same tools will be applied to this challenge.
Energy band diagram method
Using this method, the device performance under the electrode contact is understood graphically, almost instantaneously, without getting into mathematics or extensive programming. Using a gravity analogy, the complicated device operation is reduced to a motion of balls under the influence of gravity, where balls represent electric-charge carriers in the device. This enables rapid feedback as input for the next experiment.
Equivalent circuit method:
After understanding the device operation phenomenologically with the energy band diagrams, we will use an equivalent circuit to express the device characteristics using fundamental circuit elements such as resistors, capacitors, inductors, and dependent current/voltage sources. In the equivalent circuit model, we do not have to consider the carrier motion in the device. The current-voltage characteristics are expressed faithfully with the fundamental circuit elements, and this will lead to the circuit design even before the detailed physics is clarified.
The ACNT has much experience in these methods in the following nanodevice area:
A semiconducting carbon nanotube field-effect transistor with metallic electrodes (APL00).
A semiconducting carbon nanotube picked up with a metallic scanning tunneling microscope tip (APL01).
A monolithic metal-semiconductor carbon nanotube diode with gate bias modulation (APL02).
A vertical semiconducting nanowire field-effect transistor with a metallic drain electrode (Nano Lett 04).
A junction between a semiconducting carbon nanotube and a metallic electrode in vacuum and in air (PRB04).
It has been shown that:
the contact is extremely sensitive to the gaseous environment and device characteristics drastically change because of the contact.
the contact, which is just a connection to the outside world, quite often determines the entire device performance.
Our goal is to understand and manipulate the contact and engineer it so that the signal extraction will be the most efficient.
For NASA's new vision for Human and Robotics Space Exploration, we need (1) radiation tolerant devices that can reliably work in space and (2) devices which are highly integrated (beyond 1 billion devices per chip) such that mass of electronic systems is minimized.
Technically, the former requires the use of special kinds of semiconductors and the latter requires nanoscale devices or "nanodevices". One of the largest fundamental problems preventing implementation of these devices is the difficulty in extracting a signal from these devices because of the ohmic contact problem.
Any single device must be integrated into an electronic system. A signal must be carried in and out of an electronic device via metallic electrodes. This has been trivial for large conventional devices because large contact area is affordable. However, nanoelectronic elements, such as carbon nanotubes, are quite unfriendly to metals and an unwanted large potential barrier is created between the device and the electrode. This does not allow electric current to pass through the contact easily. Additionally, as the devices are miniaturized, it is impossible to make a large contact area, further inhibiting current flow. The contact to devices has already been a serious issue in semiconductor electronics, but is an even greater challenge for radiation-tolerant nanoelectronics for NASA missions.
Right: Schematics of Nanoelectronic Devices: atomic chain and nanotube transistor.
The Ames Center for Nanotechnology (ACNT) has been developing a systematic approach to develop a fundamental understanding of the contact inhibitors for nanoelectronic devices at the nanoscale. Guiding design principles are required rather than design attempts based on an accumulation of trial and error data. It is essential that we understand the mechanisms well at the beginning. This basic understanding should lead to a practical method to achieve good contacts for integrated, radiation-tolerant nanoelectronic systems.