S.S.VERMA
Department of Physics
Sant Longowal Institute of Engineering and Technology
Longowal, Distt.-Sangrur (Punjab)-148 106
Diamond has long held a special place in the hearts and minds of the public to be used as gems and jewels. It has a special interest to scientific community also due to its exceptional material properties. It is the hardest material known and has the highest thermal conductivity among all known materials. Combined with these important properties, diamond has very low thermal expansion and high electrical resistance. Because of its hardness, diamond is far more effective and efficient than other competing materials used for abrasive, cutting, shaping or finishing tools. Its very high thermal conductivity makes it ideal for spreading and conducting the heat out of compact, high-power, high-speed electronic packages. Presently, the use of this wonderful material is being explored towards the growing electronic industry. The days are not far away when the saturating electronic industry based on Silicon will switch over to diamond based electronics. This article gives a summary about the advantages associated with diamond electronics and the developments in the field so far.
Introduction
A material suitable for an electronic device must not conduct electrical current in its pure state at room temperature. However, it should be possible to tune its conductivity in a controllable manner by introducing trace amounts of impurity atoms (dopants). Such materials are termed “semiconductors” the backbone of semiconductor electronics. Pure carbon naturally forms two different crystalline materials: diamond, in which all bonds between carbon atoms are the same, and graphite, with two different types of bonds between the atoms. Because diamond is the higher energy form of the two, its natural occurrence is rare compared with that of graphite. The lowest energy form of related elements such as silicon (Si) and germanium (Ge), the wonders of present electronics, has the same crystal structure as diamond, but do not occur naturally like graphite. Graphitic carbon conducts electricity at room temperature. The quirk of nature that makes graphite the lowest energy form of carbon is the main reason it has not been used in electronic devices, in stark contrast to its neighbor in the periodic table that is silicon (Si). In contrast, diamond is a semiconductor with physical properties (such as maximum electric field, saturation velocity, thermal conductivity and band-gap) that make it the ideal material for electronic devices. All this provides hope that the time has come for diamond electronics. The major barrier to realizing this potential of diamond to date has been the difficulty in synthesizing it in a form that is pure and perfect enough for electronics.
Diamond and its properties
Diamond has been prized for centuries as a gemstone of exceptional brilliance and luster. But to a scientist diamond is interesting for its range of exceptional and extreme properties. When compared to almost any other material, diamond almost always comes out on top. As well as being the hardest known material, it is also the least compressible, and the stiffest material, the best thermal conductor with an extremely low thermal expansion, chemically inert to most acids and alkalis, transparent from the deep ultra violet to infra red light rays and is one of the few materials known with a negative electron affinity (or work function). Diamond is composed of the single element carbon, and it is the arrangement of the C atoms in the lattice that give diamond its amazing properties. The diamond is an extremely simple material, which is made of carbon only. Because it is superlative among all matters in such properties as hardness, heat conductivity, and translucence, from a viewpoint of industrial material development, the diamond is a material too good to be used as jewels. Scientists have started basic researches on the diamond ten years ago, with the intention of utilizing its hardness for applications in surface acoustic wave communication devices. The devices are presently used in such electronic products as high-speed optical communication equipment.
Diamond Synthesis
Natural diamonds have too many defects and impurities for use as semiconductors, regardless of the cost associated with their rarity. Only manufactured semiconductor materials are of the appropriate quality for electronics. The first artificial synthesis of diamond was reported in 1955. It was achieved by subjecting graphite to high pressure and high temperature (HPHT) in the presence of a transition-metal catalyst. But the impurities and defects in HPHT-synthesized diamonds and their small size precluded their use in electronics. Alternative methods aimed to synthesize diamond from the vapor phase. The first practical method for deposition of diamond from the vapor phase used a hydrocarbon plasma. This study heralded a burst of research activity aimed at exploiting the properties of diamond in electronic devices. A low-pressure technique to produce diamond using chemical vapor deposition (CVD) drew worldwide attention in the mid-1980s. There has been an explosion of interest in CVD diamond, diamond-like, and cubic boron nitride (CBN) films and coatings. These films are expected to be used in a variety of applications, from cutting tools to wear-resistant parts, and from electronics to optical applications. One advantage of CVD diamond technology over high-pressure technology is its low cost and its ability to coat on any shape. In the mid-1990s, several new mass-production technologies for producing diamond and diamond-like films emerged, including the production of diamond-like coatings for razor blades. Another technology used an interactive laser technique to produce diamond and diamond-like carbon (DLC) films. Yet another technique that used fullerenes in an argon microwave plasma produced nanocrystalline diamond. Since the advent of these new technologies, diamond and diamond-like films, and coated products have reached a greater level of activity in their applications.
Therefore, two of the key elements required from a semiconductor material suitable for electronic devices–a high-quality crystal that can be doped–are now achievable in diamond. The latest research results reported could be a watershed for carbon electronics. Scientists have artificially synthesized diamond with electronic properties that surpass those expected from theory/measurements. At present, the controlled change in the conductivity of diamond can only be achieved through increase of the hole concentration through boron doping. The results suggest that hole-conducting (p-type) diamond devices may be a practical and better option than electron-conducting (n-type) SiC or GaN for high-frequency and high-power electronic devices.
Advantages
Electronic devices fabricated from conventional materials, such as Si, suffer considerable damage when exposed to ionising radiation. However, diamonds high ionisation and atomic displacement energies mean that devices formed from this material are much less susceptible to damage. Photo detectors fabricated from diamond will be “blind” to visible light whilst being sensitive to deep UV light. Such a device would be of significant interest for a number of industrial, environmental and military applications. Next generation mobile telecommunication systems will require band pass filters in each handset that operate in the GHz regime. This cannot be achieved with sound acoustic waves devices fabricated from conventional electronic (i.e., Silicon) materials. Acoustic wave velocities in diamond are amongst the highest known. This is a particularly useful attribute when considering substrate materials for the fabrication of surface acoustic wave devices for high frequency filter applications. Diamond can display negative electron affinity implying that electrons will be emitted from diamond surfaces in the presence of extremely low electric fields. This has lead to considerable interest in diamond from, amongst others, manufacturers of flat panel displays with a requirement of n-type doping of diamond. The n-Type doping of diamond has long been impossible but recent research has shown lot of prospects in this direction.
