In this guest blog, PhD researcher Sayantan Ghosh at HZDR describes the process behind creating our nanowire sensor foundations.

Detecting radicals

Atmospheric free radicals, particularly hydroxyl (•OH) and nitrate (•NO₃), are the drivers of chemical processes that determine atmospheric composition and thus influence local and global air quality and climate.

Detecting and understanding the behaviour of radical species in the atmosphere is therefore of paramount importance and hence a major research goal in atmospheric science.

Current techniques for measuring radicals are based on spectroscopic and mass spectrometric methods, which although sensitive and robust, are technically complex, cumbersome, and expensive.

Nanowires as sensors

Silicon nanowire (Si NW) sensors are very promising because of their fast, low-cost, label-free, real time detection of chemical and biological species. In particular, silicon junctionless nanowire transistors (JNTs) have recently detected record low concentrations (down to the zeptomolar range) of the protein streptavidin in liquid phase. However, JNTs have not yet been tested for sensing in gas phase.

The main aim of this project is to fabricate and characterize JNTs for the detection of atmospheric radicals. Our role at HZDR is to fabricate the Si JNT devices and then optimize and further explore different approaches in the process flow in the coming years.

Our main technical goals for creating these JNT sensor foundations include:

1. High doping of the silicon-on-insulator (SOI) structures with ion implantation and flash lamp annealing (FLA).
2. Top-down fabrication of Si JNT devices.
3. Initial electrical characterization of the devices based on back-gated architecture.

In order to achieve these milestones in the very first year, a complete ‘recipe’ (or process flow to use the technical term) was designed which includes processes like electron beam lithography (EBL), ultra-violet (UV) lithography, wet and dry etching, ion implantation, FLA and electron beam evaporation.

Recipe for creating a nanowire sensor foundation:

Step 1. High doping of the silicon-on-insulator structures using ion implantation and flash lamp annealing

To fabricate the JNTs, we used silicon-on-insulator (SOI) substrates. SOI is made of layers of silicon-insulator-silicon stacked on top of each other. The base material is a p-doped silicon bulk with 102 nm of thermally grown SiO₂ on top. The layer on top of the oxide layer is a 20 nm intrinsic silicon. It is referred to as “top silicon” or a “device layer” since the electron devices are fabricated exactly in this layer. We doped the device layer using ion implantation and flash lamp annealing (FLA) . The doping was done by a chain implantation process to form box-like dopant distribution across the top Si layer. The targeted carrier concentration was in the range of 10¹⁹ cm^-3. For p-type doping, boron (B) was used, while for n-type doping we used phosphorus (P). We used millisecond-range FLA to explore the activation of both types of dopants and to heal the defects in the substrates resulting from ion implantation.

The P dopants were successfully activated, giving the active carrier concentration of about 10¹⁹ cm^-3. However, the B activation was unsuccessful due to the lightweight nature of the B atoms. Further investigation is underway for the B activation.

Step 2. Top-down fabrication of Si JNT devices

Si NW can be fabricated by two different methods known as bottom-up and top-down approaches. Here at the HZDR nanofabrication facility, we used the top-down approach. Top-down fabrication produces nanowires that can be placed in an orderly arrangement, making the approach a suitable choice for large-scale production and integration of nanowires in functional devices and circuits. This involves successive subtractive procedures on the SOI substrate, such as lithography and etching to ultimately achieve a nanostructure.

The target devices in this project are Si JNTs, which have silicon nanowire channels. We fabricated these nanowires using the negative resist HSQ and EBL process. This creates the nanowire patterns on the highly-doped SOI substrate. Subsequent dry etching then transferred the EBL patterns into the device layer to create the highly-doped nanowires. After this, we placed metal contacts on both sides of the nanowires to act as source and drain contacts. Currently we use nickel and gold as the contact pad metals. We will continue to further optimize this fabrication process in order to increase the overall performance. The image below shows one such Si JNT device which is fabricated by RADICAL researchers Bilal Khan and Sayantan Ghosh in the HZDR nanofabrication facility.

Step 3. Initial characterization of the devices based on back-gating architecture.

Finally, we have carried out initial electrical characterization of the fabricated devices by back gating. We sweep a voltage through the back gate in a butterfly loop of 30 V, while varying the drain source voltage between 0.25 to 1.0 V. A leakage current analysis is also carried out during the electrical measurements. Our initial results have shown a good performance of the Si JNT devices and their potential suitability for detection of atmospheric free radicals.

These initial promising results based on the first batch of fabricated Si JNT devices pave the way for further optimization of the final devices in the next few years of the project.

Find out more:

Find Sayantan’s recent research results on our RADICAL project repository on Zenodo.

About the author: PhD student Sayantan Ghosh is a PhD student in Transport Phenomena in Nanostructures, and a member of the Nanofabrication and Analysis Group at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). He and his colleagues at HZDR fabricate the nanowire sensors for RADICAL.