Single step ohmic contact for heavily doped n-type silicon

Significance 

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.

Single step ohmic contact for heavily doped n-type silicon - Advances in Engineering
The current- voltage plot for (a) p-type silicon and (b) n-type silicon substrates ranging from moderate to high resistivities exhibiting ohmic behavior for n-Si and rectifying behavior for p-Si using Mg as a contact.

About the author

Febin Paul is currently a PhD student at Emerging Technologies Research Center (EMTERC) and is currently pursuing his doctoral training under the supervision of Professor Shashi Paul in De Montfort University (DMU). He was awarded the Highflyer Scholarship as a part of a fully funded doctoral programme in DMU and is an Associate Fellow of Higher Education Academy (AFHEA).

His research work involves two terminal organic and inorganic electronic memory devices using thin film nanocomposites. His works also include organic ferroelectric memory, magnetic effects on metal oxide and high temperature memory devices. His interests include large scale deposition using PECVD, Conductance Spectroscopy and Emerging non- volatile memory.

About the author

Dr Sridhar Govindarajan completed his First Class Engineering degree in Medical Electronics at Bangalore University, India in 2001. He then moved to Newcastle, UK, for his Masters in Biomedical Nanotechnology and then completed his PhD on Micro Biosensors at Newcastle University in 2008. He then joined Swansea University as a Postdoctoral Researcher working on multiple innovative projects relating to biosensors and point-of-care medical diagnostics. He was also part of the established Welsh Centre for Printing and Coating and the Centre for NanoHealth at Swansea University. He later moved to Leicester as a Senior Scientist within a spin-out company working on novel cell-based nanostructured biosensors in 2015, before joining De Montfort University as a Research Assistant within EMTERC in 2017.

He is currently a Lecturer in Electronics Engineering at De Montfort University (DMU), Leicester, UK. He is part of the Emerging Technologies Research Centre (EMTERC) at DMU, and his current research interests include electrochemical biosensors, nanostructured materials for medical diagnostics, flexible electronics, and nanomaterials for renewable energy. He has also taught various modules at undergraduate and postgraduate levels in Electronics Engineering, and supervised student projects.

About the author

Dr. Krishna Nama Manjunatha is currently employed as a Lecturer in microelectronics and Nanotechnology, at De Montfort University, Leicester. Now he is part of Emerging Technologies Research Centre (EMTERC), where he leads research based on emerging materials for energy related applications.

His research interest lies in the field of materials science and electronic devices. Silicon Nanowires for applications in electronic devices such as solar cells, two terminal memory and Li-Ion batteries are his forte. Apart from inorganic solar cells, he also worked on the organic counterpart. Other research interests include development and investigation of nanoparticles, metal oxides, perovskite materials, metal-organic compounds and thin film electronics. He has obtained PhD degree in Nanomaterials for photovoltaic Applications with an award recognising “Best thesis of the year-2018” at De Montfort University. (Orcid : 0000-0002-6074-1340)

About the author

Prof. Shashi Paul is working in the Emerging Technologies Research Centre (EMTERC), De Montfort University, and Leicester, United Kingdom, as Professor in Nanoscience and Nanotechnology and head of EMTERC. He graduated from Indian Institute of Science (IISc), Bangalore, Indai and previously worked at Cambridge University, Durham University and Rutgers University. He has extensive experience in the field of deposition of nano-sized organic and inorganic materials in the context of their applications to emerging electronic memory devices, thin film transistors, biological & chemical sensors and next generation energy generation and storage devices.

He has invented a new method to grow materials at low temperature. He has also proposed a model, based on internal electric field, for emerging two terminal electronic memory devices; which has been verified in different system of materials. And, it is still valid! Currently, he is looking into immortal and time dependent electronic memory.

Orcid: 0000-0002-7077-8235

Reference

Febin Paul, Krishna Nama Manjunatha, Sridhar Govindarajan, Shashi Paul. Single step ohmic contact for heavily doped n-type silicon. Applied Surface Science.

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