Emerging non-volatile memory technologies prompt for storage mechanism alternative to charge and materials different from silicon. However, silicon is always preferred by the electronic industries, as it rules the current fabrication technology of integrated circuits. For this reason, more than ever, today silicon at the nanoscale is asked to engrave new paths to semiconductor industry, mainly due to its extraordinary physical and electronic properties in comparison to bulk ones. In this one-year project, I propose the use of silicon nanowires (SiNWs) as the storing component of two-terminal non-volatile memory devices. In particular, the project aims to perform engineering of a new family of Programmable Read Only Memory (PROM), also called One Time Programmable (OTP) proposed a couple of years ago. The idea starts from the fact that at the Centro per le Nanotecnologie applicate all¿ Ingegneria Sapienza (CNIS) a few months ago it has been demonstrated by one researcher of this group that silicon nanowires can be fabricated at low-temperature with different crystallographic structures, namely a metastable body centered cubic configuration (BC8) featuring low resistance and the conventional stable face centered cubic configuration (FCC) featuring high resistance. The core of this one-year project is to optimize conditions for the switch of the metastable BC8 state into the stable FCC state in an electrical way, forcing a current to flow along the nanowire and causing local heat dissipation by Joule effect which makes the metastable state to switch into the stable one. The challenge is identifying the proper voltage waveforms able to switch the memory, in a controlled and reproducible way. If this was verified, it poses the basis for an innovative memory architecture of Programmable Read Only Memory.
In this project, we propose to assess the possibility of using SiNWs as active medium in programmable read-only memory. The working principle of the SiNWs memory is attributed to the resistance of two specific conformations of the silicon nanostructure. The SiNWs will be grown at the CNIS laboratories in Sapienza by PECVD reactor on crystalline silicon substrate. PECVD is a powerful method capable of growing silicon from a gas phase precursor (SiH4). A microwave field is generated at 2.5 GHz by a magnetron and is injected into the reactor chamber by an antenna. This helps the SiH4 molecules dissociation and reduces the deposition temperature. For growing SiNWs, the Si substrate will be first covered by a thin SiOx film, on which Sn will be deposited as catalyst. Sn coalesces on the substrate creating nanoparticles and the Si nanowires grow at the surface raising the catalyst nanoparticles on the top. The presence of that specific metal catalyst overcome the need of reaching the eutectic temperature. The Sn nanoparticles behave as nano-susceptors absorbing energy that produces high local increase of temperature (up to 700 °C). The process breakthrough is the fact that only the deposited metal nanoparticles are strongly heated by microwave irradiation, whereas the silicon remains at 200°C. The wire distribution obtained at CNIS was demonstrated to be uniform, with an aspect ratio between height and size of the nanowire around 10. At the CNIS, it has been proved for the first time in 2020 that silicon nanowires fabricated at low-temperature exhibit different crystallographic structures depending on the energy delivered by the nano-susceptor and the technique was patented by a researcher involved in this project [Palma, F. US Patent App. 16/648,080 2020]. X-ray diffractometry demonstrated that, at the lowest energies, the BC8-Si configuration can be reproducibly obtained at CNIS. The BC8-Si is a body-centered cube configuration, featuring an energy gap much lower than the conventional diamond face-centered-cube (FCC) configuration. Due to the narrow bandgap, the BC8-Si configuration exhibits a very low resistance in comparison to the diamond FCC one. The concept of the Programmable Read Only Memory proposed here is in the cell resistance rather than in the charge stored in it. The BC8-Si configuration is metastable. The core of this one-year project is to demonstrate that the metastable BC8 configuration can switch into the stable diamond FCC one in a controlled electrical way. If this was demonstrated, we could affirm that BC8 SiNWs represent the starting point for proposing a new Programmable Read Only Memory. The idea is to induce switching between the two configurations by local heating of the nanowire. Using the temperature to induce a memory cell to switch is not novel. In fact, this switching mechanism is already exploited in another two-terminal NonVolatile Memory already on the market, which is the Phase Change Memory (PCM). PCM is a non-volatile memory where the cell resistance is in charge of the memory bit. The active material is a chalcogenide layer sandwiched between a metal heater and a ground electrode. The chalcogenide changes from amorphous (high resistance state) into crystalline phase (low resistance state) and vice-versa, via local Joule effect heating obtained applying proper voltage waveforms to the heater. In conclusion, we propose to grow SiNWs in a metastable low resistance state (LRS), which can be possibly converted into a stable high resistance state (HRS). We should identify the condition of switching from LRS into HRS (RESET), forcing a current to flow along the nanowire and causing local heat dissipation by Joule effect. The current will flow between the bottom electrode (silicon) and the Sn nanoparticle on top of the SiNW. This will be made through c-AFM, although I expect that a percentage of nanowires will break during heating, before identifying the proper heating conditions. The challenge will be engineering the voltage waveforms to RESET the memory, respect to the switch temperature and the voltage amplitude (power consumption). If this goal was achieved, it poses the basis for a new family of Programmable Read Only Memory.