Carbon Nanotubes with a Memory

“Unlike similar devices that have been made, which use carbon nanotubes but can only operate at very low, very impractical temperatures, our device displays impressive long-term information retention characteristics at room temperature,” said lead researcher Jiyan Dai, a physicist at The Hong Kong Polytechnic University, to PhysOrg.com. “This indicates that mainstream carbon nanotube-based flash memory devices are a real possibility.”

Flash memory devices are currently used to store data in many types of electronic items, including digital cameras, USB memory sticks, and cell phones. Flash memory is considered a “non-volatile” form of memory, meaning it can retain data without a constant supply of power.

A typical flash memory device stores information within a grid of transistors called cells. Each cell consists of three layers: a “control gate” compound and a “floating gate” compound separated by a thin layer of an insulating oxide compound. When a voltage is applied to the cell, electrons build up as negative electric charge in the floating gate. At a certain threshold of charge, the floating gate is considered closed and the cell is thought to have a value of “0.” When the charge drops below that level, the gate is open and the cell has a value of “1.” In this way, each cell is able to hold one bit of information (there are eight bits in one byte).

Dai and co-researcher X.B. Lu created their flash memory device using carbon nanotubes as the charge-storage layer. As described in a paper in the online edition of Applied Physics Letters, they embedded the nanotubes in a compound made of the elements hafnium, aluminum, and oxygen, abbreviated HfAlO, which serves as both the control gate and the oxide layer. This carbon-nanotube “sandwich,” with each layer only several nanometers in thickness, sits on a substrate of silicon.

Dai and Lu determined the charge-retention characteristics of the device by measuring, first, its capacitance (how well it stores electric charge) as a function of the voltage applied across it. They also measured how well the device held onto its charge as time elapsed, from fractions of a second up to nearly three hours. They found that the short-term charge retention wasn’t excellent. During the first couple of minutes, the “memory window” — the voltage range over which the device can retain information — became narrower, a property that is not desirable for flash memory devices. However, over the long term, the memory window remained at a value of about 0.5 V.

“We believe that the excellent long-term charge-retention characteristics of our device are due to the unique structure and electrical properties of carbon nanotubes,” said Dai.

Citation: “Memory effects of carbon nanotubes as charge storage nodes for floating gate memory applications,” Applied Physics Letters 88, 113104 (2006)

By Laura Mgrdichian, Copyright 2006 PhysOrg.com