A Diamond Semiconductor
Diamonds have recently been attracting great attention in the world of electronics. Takashi Sasaki reports on the pioneering work of Dr. Makoto Kasu, who continues to explore the possibilities for diamond semiconductors.
Dr. Makoto Kasu
Credit: TAKASHI SASAKI
In 2003, a joint team of Japan’s Nippon Telegraph and Telephone Corporation (NTT) and Germany’s University of Ulm successfully prototyped a semiconductor with the world’s highest operating frequency of 81 gigahertz (GHz), astounding researchers across the globe. Later, NTT independently attained 120 GHz, which is still the world record.
The semiconductor was created from a diamond crystal. Diamond semiconductor studies have since been led by Dr. Makoto Kasu, who heads the thin-film material research team at the NTT Basic Research Laboratories.
Society today is dependent on semiconductors. They are found in all kinds of electronic equipment ranging from cell phones and personal computers to satellites. At the moment, semiconductor substrates are produced mainly from silicon. However, silicon has such poor heat resistance that semiconductors will generate heat and fail if they are subjected to any current or voltage exceeding certain limits. “Diamond has a melting point that is 2.3 times higher than that of silicon. It is thirteen times superior to silicon in thermal conductivity. Diamond has distinguished characteristics indispensable to enhancing semiconductor output and energy efficiency,” says Dr. Kasu. “That is why we call it the ‘ultimate’ semiconductor.”
Creating a High-Purity Diamond
Given that diamond is the ultimate semi-conductor, one would think it would already be in use, but this is in fact not the case since not only natural diamond but also man-made industrial diamond contains too many non-carbon impurities. Actually, diamond semiconductors have been explored by businesses and universities in the United States for two to three decades. Yet none were able to develop practical applications.
In 2000, Dr. Kasu won an internal competition for research funds and embarked on the development of diamond semiconductors. He began by exploring the conditions for cultivating high-purity diamond crystals.
The research team led by Dr. Kasu addressed the challenge of growing high-purity diamond thin films, using the microwave plasma chemical vapor deposition (CVD) method common to the manufacture of semiconductor de-vices. The CVD method capitalizes on chemical reactions to deposit thin films of different substances on a substrate. The materials for diamond are hydrocarbon gas (methane) and hydrogen gas. A gaseous mixture of these two materials is decomposed by microwave irradiation to produce carbon atoms. They join together on a substrate under high temperatures and grow into a diamond thin film.
However, high-purity diamond thin films will not grow if any non-carbon atom, such as oxygen and nitrogen, is present, if the material gases are mixed at the wrong ratio, or if the temperature is inappropriate.
The research team worked around the clock to carry out tests that varied the temperature, mixing ratio, and other conditions. After more than six months of efforts, the team finally created a high-purity diamond thin film with a thickness of several micrometers. This breakthrough led to the prototyping of a diamond semiconductor featuring the world’s highest frequency characteristics.
Prototype of the diamond semiconductor device developed in 2003
Credit: NTT BASIC RESEARCH LABORATORIES
A Childhood Dream Becomes Reality
The most eagerly anticipated application for diamond semiconductors is in the area of telecommunications. Vacuum tubes are still used at the heart of communications satellites, terrestrial broadcasting stations and airport radars. While they are resistant to high outputs and frequencies, vacuum tubes are power hungry and have short life spans. The replacement of vacuum tubes with diamond semiconductors would sharply improve the performance of the equipment in terms of energy efficiency, durability and reliability.
“Before achieving practical application, we must first stabilize the quality of the proto-types,” says Dr. Kasu. “That in turn requires developing a new technology for manufacturing semiconductor devices. I believe that we will achieve this in a couple of years.”
Surely the cost issue would be a concern if diamond were to be used for semiconductors? Dr. Kasu answers this question with a smile.
“It sounds costly, doesn’t it? But diamond is simply made of carbon. Once the production technique is established, the cost will be no problem at all.”
As a child, Dr. Kasu was enthralled by the story of the transistor, which made him want to become a scientist. As he explores new possibilities with the diamond semiconductor, this is clearly one childhood dream that has come true.