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March 26, 2010

UCLA team couples quantum dots, silicon for room-temperature Spintronic functionality

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In a study to be published in the April issue of Nature Materials, researchers from UCLA's Henry Samueli School of Engineering and Applied Science describe the creation of a new material incorporating spintronics that could help usher in the next generation of smaller, more affordable and more power-efficient devices.

Nature - Electric-field-controlled ferromagnetism in high-Curie-temperature Mn0.05Ge0.95 quantum dots

Electric-field manipulation of ferromagnetism has the potential for developing a new generation of electric devices to resolve the power consumption and variability issues in today’s microelectronics industry. Among various dilute magnetic semiconductors (DMSs), group IV elements such as Si and Ge are the ideal material candidates because of their excellent compatibility with the conventional complementary metal–oxide–semiconductor (MOS) technology. Here we report, for the first time, the successful synthesis of self-assembled dilute magnetic Mn0.05Ge0.95 quantum dots with ferromagnetic order above room temperature, and the demonstration of electric-field control of ferromagnetism in MOS ferromagnetic capacitors up to 100 K. We found that by applying electric fields to a MOS gate structure, the ferromagnetism of the channel layer can be effectively modulated through the change of hole concentration inside the quantum dots. Our results are fundamentally important in the understanding and to the realization of high-efficiency Ge-based spin field-effect transistors



While conventional complementary metal-oxide semiconductors (CMOS), a technology used today in all types of electronics, rely on electrons' charge to power devices, the emerging field of spintronics exploits another aspect of electrons — their spin, which could be manipulated by electric and magnetic fields.

"With the use of nanoscaled magnetic materials, spintronics or electronic devices, when switched off, will not have a stand-by power dissipation problem. With this advantage, devices with much lower power consumption, known as non-volatile electronics, can become a reality," said the study's corresponding author, Kang L. Wang, Raytheon Professor of Electrical Engineering at UCLA Engineering, whose team carried out the research. "Our approach provides a possible solution to address the critical challenges facing today's microelectronics industry and sheds light on the future of spintronics."

"We've built a new class of material with magnetic properties in a dilute magnetic semiconductor (DMS) system," said Faxian Xiu, a UCLA senior researcher and lead author of the study. "Traditionally, it's been really difficult to enhance the ferromagnetism of this material above room temperature. However in our work, by using a type of quantum structure, we've been able to push the ferromagnetism above room temperature."

Ferromagnetism is the phenomenon by which certain materials form permanent magnets. In the past, the control of magnetic properties has been accomplished by applying an electric current. For example, passing an electric current will generate magnetic fields. Unfortunately, using electric currents poses significant challenges for reducing power consumption and for device miniaturization.

"You can think of a transformer, which passes a current to generate a magnetic field. This will have huge power dissipation (heat)," Xiu said. "In our study, we tried to modulate the magnetic properties of DMS without passing the current."

Ferromagnetic coupling in DMS systems, the researchers say, could lead to a new breed of magneto-electronic devices that alleviate the problems related to electric currents. The electric field–controlled ferromagnetism reported in this study shows that without passing an electric current, electronic devices could be operated and functioning based on the collective spin behavior of the carriers. This holds great promise for building next-generation nanoscaled integrated chips with much lower power consumption.

To achieve the ferromagnetic properties, Kang's group grew germanium dots on a silicon p-type substrate, creating quantum dots on top of the substrate. Silicon and germanium are ideal candidates because of their excellent compatibility and ability to be incorporated within conventional CMOS technology. The quantum dots, which are themselves semiconductors, would then be utilized in building new devices.

"To demonstrate possible applications of these fantastic quantum dots, we fabricated metal-oxide semiconductor devices and used these dots as the channel layer. By applying an electric field, we are able to control the hole concentration inside the dots and thus modulate their ferromagnetism," Xiu said.

"This finding is significant in the sense that it opens up a completely new paradigm for next-generation microelectronics, which takes advantage of the spin properties of carriers, in addition to the existing charge transport as envisaged in the conventional CMOS technology."

The key is to be able to use this material at room temperature.

"The material is not very useful if it doesn't work at room temperature," Wang said. "We want to be able to use it anywhere. In this work, we've achieved success on electric field–controlled ferromagnetism at 100 degrees Kelvin and are moving towards room temperature. We feel strongly that we'll be able to accomplish this. Once we've achieved room-temperature controllability, we'll be able to start building real devices to demonstrate its viability in non-volatile electronic devices."

Intel backed the spintronic research

10 pages of supplemental material

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