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In this exclusive interview, Dr. Ahmad Echresh, a Postdoctoral Researcher at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), shares his expertise on fabricating junctionless transistors and experiences as part of the RADICAL with Project Officer Abhisweta Bhattacharjee (UCC Academy). With a strong background in nanowire-based optoelectronic devices and a passion for exploring the applications of nanomaterials, Dr. Echresh discusses the intricate processes involved in fabricating junctionless transistors, the challenges and breakthroughs he encountered, and the collaborative efforts that drive the project’s success.

Luftbilder des HZDR: Aerial view of the HZDR campus

Q: Could you tell us about your role in the RADICAL project?

Certainly! My primary role is within Work Package 1, titled “Fabrication of Si JNT Devices.” Alongside my colleague, Sayantan Ghosh, and led by Dr Yordan Georgiev at HZDR, I focus on the design and fabrication of junctionless nanowire transistors (JNTs) and silicon-on-insulator (SOI) samples. We use techniques like ion implantation doping, flash lamp annealing, and electron beam lithography to fabricate these devices. My main responsibility is to help Sayantan to fabricate the JNT devices and check the electrical characterisation of the devices to ensure they functioned as transistors.

Q: What does electrical characterisation entail, and why is it important?

Electrical characterisation is essential because it tells us if the fabricated devices actually work as intended. For example, we perform top-gate and back-gate measurements to determine if the device creates N-type or P-type channels. We also analyse whether the device behaviour is symmetric or asymmetric. These measurements help us identify issues and confirm the functionality of the transistors before passing them on for further functionalisation and gas sensing.

Dr Echresh displaying his poster at the 50th International Micro and Nano Engineering Conference in Montpellier (2024)

Q: What were some of the challenges you faced during the fabrication process?

One of the biggest challenges was managing the many intricate steps involved in fabricating junctionless transistors. Every stage—from doping silicon layers to optimising nanowires with lithography—had to be carefully monitored. If something went wrong at any step, it could create a domino effect. Every step therefore had to be carefully monitored through frequent measurements, like Raman spectroscopy and Hall effect tests, to verify progress at each stage.

Environmental factors were also a challenge. For instance, HSQ resists used in electron beam lithography are sensitive to humidity and temperature, so we had to ensure proper storage and handling. To address this challenge, we had to optimise the exposure parameters of the HSQ pattern every few months.

Q: Did you encounter any unexpected results during the project?

Yes, we did! At one point, we observed similar behavior in devices fabricated on both P-type and N-type substrates. This was unexpected. Upon investigation, we discovered that Schottky contacts were forming at certain interfaces. To address this, we adjusted the fabrication parameters, such as using shorter channels and top gates. These changes resolved the issue and allowed us to achieve the desired asymmetric behavior.

Q: How does your work contribute to the RADICAL project’s broader goals?

My work directly supports RADICAL’s mission to develop advanced trace gas sensing technologies. By fabricating and characterising JNTs and SOI samples, we laid the foundation for creating sensors capable of detecting gases like ammonia and NO2. My previous experience with photodetectors helped me understand sensing mechanisms, which I applied to enhance the sensitivity and selectivity of these devices.

Q: What personal or professional lessons did you gain from working on this project?

This project was a fantastic learning experience. I’d say the biggest lessons were about collaboration and patience. RADICAL was a multidisciplinary effort involving chemists, physicists, and engineers, which required mutual understanding and cooperation. We had occasional misunderstandings, but we worked through them by respecting each other’s expertise. That’s what ultimately made the project successful.

Dr Echresh presenting work package 1 updates at the RADICAL final general Assembly meeting at Cork

Q: As the project concludes, what are your plans moving forward?

I’m excited to take what I’ve learned and apply it to new research. My experience with nanowires, 2D materials, and transistor technology has opened doors to exploring optoelectronics and gas sensing further. I’m brainstorming ideas to combine nanowires and 2D materials to create innovative devices. RADICAL has been transformative, equipping me with the knowledge and tools to lead cutting-edge research.

Q: What’s your key takeaway from your involvement in the RADICAL project?

If I had to summarise, I’d say collaboration and patience are critical. In large, multidisciplinary projects, success depends on mutual understanding and teamwork. I’m proud of what we achieved together, and I’m eager to see how our work will impact gas sensing technologies in the future.

Follow our progress with RADICAL