The Next Frontier of Memory Devices (AMSEN)

Alan Anderson

Cosmas Muiva, born in Kenya in 1971, is one of several RISE students who have been waiting for many years–more than a decade, in his case—for the support he needs to advance in his academic career past the bachelor’s level. Thanks to his own perseverance, and the good fortune of an advisor who knew of the RISE program, he is now well on his way toward the PhD he has desired for so long.

His birthplace was 200 miles southeast of Nairobi, in the semi-arid county of Makueni, where his father had worked as an agricultural officer until the end of British occupation. When the British left, those steady jobs were gone. By the time Cosmas was a child, the British had departed, along with their jobs. “I never saw my father work full-time anywhere during my life,” he said. “Just occasional employment in the then-Athusi chain of hotels.”

His thirst for education began to grow when he saw some people in his village taking A-level coursework at the local high school. “I saw how this later let their families move from grass houses to iron houses,” he said. “I saw that it was worth doing.” He began to study harder, and after getting his own A-levels he won a government scholarship to Kenyatta University. Part of the scholarship was a loan, which he didn’t have to pay back until he got a job. He did so by teaching science in his community.

‘’I knew I had special talents in science. I used to invent some electronic gadgets that would attract people from far and wide to our village to see. At high school and as a high school teacher, our science congress projects were featured occasionally and won prizes at national competitions.’’

In December 2004, when the great tsunami struck Indonesia, Cosmas dedicated himself to designing a prototype of an early warning system which was featured in a 2005 science congress competition. The project won many awards at all levels of competition and at the national level was judged second-best in the physics exhibits category. “We were invited to exhibit at the county trade fair,’’ he said.

However, he knew he needed more schooling to advance in science, the field he loved. ‘’In my teen years and up to my mid–thirties, I was struggling to keep my head up. The country was so run down by politicians that earning a living wage was a challenge even for a graduate. I had to work for over 10 years because I could not raise money to enroll at a local university for an MSc. Life was a daily struggle.

“With the new millennium,” he continued, “we had a new dawn, a new political dispensation. Everybody was more optimistic, and we began looking into the future.” He had a relative in Gaborone, the capital of Botswana, who told him that if he was admitted to the University of Botswana (UB), he was likely to be given a part-time job, and he could live with the relative to supplement his fees and living expenses. He took this advice, began his work and was admitted to the MSc program in Physics at the University of Botswana in 2006. Even though he soon had to take a leave from the job without pay, he was successful at earning his MSc in physics, specializing in materials science. He did his research on optical materials for solar cells, completing his research in “a record 18 months,” and returned to his country, where his wife was supporting their children on her salary as a teacher. He knew he still had career work to do, and began applying for scholarships.

One day, waiting at home, Cosmas was told that he had received an email at an internet café some miles from where he lived. After rushing to the café he found an application form for AMSEN from Prof. Pushpendra Jain of UB, head of the AMSEN node there. His work experience was a good fit with the AMSEN program, and he was accepted.

He decided to shift from studying materials for solar cells to the field of reversible, non-volatile phase-change memory (PCM). This area involved familiar research challenges, but offered the excitement of developing a new form of electronic memory. He knew that the current memory technology – so-called flash memory – had been tremendously successful in bringing robust storage ability to tiny devices, such as iPods and cell phones. But it is widely understood that flash memory is reaching its limits of performance and reliability, and a faster, safer technology is needed. There is also need to improve on materials used in non-volatile memories such as CDs and DVDs.

The leading candidate for next-generation memory is well known to materials science – but not to the general public. It is a family of chalcogenide “glasses” that includes several “metalloid” elements and non-metals, notably sulfur, selenium, and telluride – members of Group 6 of the periodic table, along with “nearby” elements of arsenic germanium, antimony, indium, and others.

These chalcogenides are already used to record data on CD’s, DVD’s, and other devices. As long ago as 1955, researchers had found that chalcogenide glasses had semiconducting properties, and in the 1960s, researchers began to see the potential for developing them for data storage. A key to their potential is that they are solids with a non-crystalline (i.e., amorphous) structure that behaves much like familiar window glass when heated toward the liquid state. Heat produced by the passage of an electric current can switch this material between the two states, crystalline and amorphous, which can emulate the 1’s and 0’s of digital language. That is, heating or cooling can either (1) allow a current to pass (conducting) when in the crystallized state, or (2) impede a current (non-conducting) when in a non-crystallized, or amorphous, state.

The actual mechanism for doing this is astonishing. Each memory location, or bit, has a tiny heater that can melt the glass and then cool it in one of two ways: either (1) to allow crystals to grow or (2) to prevent crystals from growing. These heaters are so small they can heat or melt the material very rapidly – on the order of nanoseconds – in a given location. This allows for extremely fast data “writes” and prevents disturbance of adjacent data bits. From an optical perspective, these two states have different optical properties such as reflectivity. Localized, high intensity photons in the form of lasers are used to “write” data bits onto these materials.

Over the last decade, certain chalcogenides have been used in this way, primarily in the CD-RW and DVD-RW disks in use since the late 1990s. More recently, several companies have begun to market PCM memory and are trying to bring its costs down.

Because PCM is so new, there is a great deal of basic physics to be accomplished in order to understand the best materials and techniques. This basic work is what Cosmas has been doing – what he describes as sintering (creating objects from powders), melting, quenching, and identifying. He is measuring the physical processes that need to be understood to produce the best products: crystallization rates, cooling rates, thermal stability; optimizing glass forming conditions; measuring fragility indexes; and so on. He uses models to calculate the activation energy for glass crystallization, and examines the crystallization mechanism itself. He studies the chemical bonds formed among elements, and how bond energies affect crystallization and melting.

His own particular specialty is the study of a chalcogenide alloy made of selenium, indium, and antimony (Se-In-Sb), especially its thermal and optical properties when heated. For the time being, he will have to be content with this basic work, rather than trying to produce commercial products for the marketplace. “We don’t have the equipment to do that,” he said. “Only industry has that. But we are trying to characterize the materials that industry will want to use. That will be our contribution, and it can be done at our AMSEN laboratories.”

For his research, AMSEN has allowed him to work not only at several of the UB labs (physics, geology, and the Electron Microscope Unit), but also at the Centre of Excellence in Strong Materials and the School of Chemistry at the University of the Witwatersrand, in Johannesburg, and the University of Nairobi. His studies with his colleagues and advisors have been published in peer-reviewed journals.

For now, says Cosmas, as much as he enjoys his research, his own next step will be to bring his family together -- his three children, along with his wife, who is still teaching in Kenya. “ I’ll probably try to find a teaching position at a research-intensive institute or university in the area,” he said. “I can’t support a family going to school.” At least he won’t have to wait any longer to complete his scientific training – he can now begin planning for his next employment opportunity as a full faculty member. “I have set high goals for my career,” he said. “The sky now is the limit. Keep checking Google scholar!”