The demand for slimmer and miniaturised electronic gadgets has been steadily growing. Many of these advanced devices are made up of small micro and nanofabricated components. The efficiency of such minute components relies on a good metallization contact with lowest resistance, to allow seamless movement of electronic charges, reduce device power consumption and heat dissipation. Ideally, the contact resistance should be zero ohms; nonetheless, that is not practically achievable. Basically, the metal-semiconductor contact is either ohmic or non-ohmic depending on the characteristics of the barrier at the interface between the semiconductor and metal. Ohmic contacts are created by annealing a metal deposited over a semiconductor. In such a case, the metal diffuses into the semiconductor and in one way or the other, it eliminates the Schottky barrier. Such impeccable results, supported by various observations where high current densities are observed even at very low applied potential is reported. Over the years, various high purity metals: aluminium, antimony and gold, have been used to fabricate ohmic contacts with n-type silicon. Success has been achieved; however, the approaches proposed involve high temperature annealing – a cost exorbitant exercise that makes the entire process industrially unfeasible and environmentally unfriendly. In addition, metals used such as antimony and gold are rare earth metals that are very expensive and some have been reported to induce deep level defects in silicon.
Therefore, serious re-considerations are necessary to address these shortfalls i.e. find an alternative abundantly available metal and a cost-effective fabrication procedure without thermal treatment that produces a lower carbon footprint to establish ohmic contacts for n-type Si. To address this, a team of researchers from the Emerging Technologies Research Center, De Montfort University: Febin Paul (PhD student), Dr. Krishna Nama Manjunatha, Dr. Sridhar Govindarajan and led by Professor Shashi Paul investigated magnesium as an alternative element to establish ohmic contact with n-Si. Their work is currently published in the research journal, Applied Surface Science.
The research team focused on the metal-semiconductor contact on n-type c-Si wafers and explored the possibility of using magnesium to form electron–selective contacts instead of using the conventional gold or antimony films which require high temperature annealing between 350°C -500°C. Overall, the researchers investigated various electrical characteristics including contact resistivity, effect of exposure on the Mg electrode to ambience and its stability, thickness of the Mg layer, its morphology and the effect of annealing temperature.
The authors established that the current-voltage characteristics demonstrated the linear behaviour associated with ohmic contact with a very low resistance to the flow of charges. For instance, due to the fact that the work function of magnesium was lower than the electron affinity of heavily doped n-type silicon, there was flow of charges without a barrier. Of utmost importance was the fact that the team was able to establish the thickness of the magnesium interlayer that provided the lowest resistance.
In summary, De Montfort University scientists successfully demonstrated the use of magnesium an as interlayer to establish ohmic contact with heavily doped n-type silicon. Remarkably, the study introduced a new approach that circumvents the use of alloys, hence completely eliminating annealing process and enabling the ohmic contact fabrication process to be conducted at room temperatures. Even better, in the presented approach, there was minimal handling of samples hence cross-contamination (a common problem with conventional technique) could be avoided.
“The study highlights the advantages of using Al-Mg as an ohmic contact with n-type C-Si wafers, eliminating the need for energy intensive high-temperature phosphorous diffusion or annealing in inert atmospheres for hours, use of rare-earth metals (like Sb used in Au-Sb alloy) or require various equipment,” says Professor Shashi Paul in a statement to Advances in Engineering. He then adds, “Hence, using Al-Mg with n-type c-Si, the formation of ohmic contact could be accomplished in a single-step process at room temperature without breaking vacuum in a thermal evaporator, thereby offering reductions in semiconductor device manufacturing costs, as well as energy requirements and carbon footprint”. Indeed, the study is proof-of-concept to obtain an ohmic contact for n-type silicon using a rapid, economical and environmentally clean approach.
Febin Paul, Krishna Nama Manjunatha, Sridhar Govindarajan, Shashi Paul. Single step ohmic contact for heavily doped n-type silicon. Applied Surface Science.