Growing smooth polycrystalline VO₂ film on silicon platform for neuromorphic computing

Abstract

Exploiting the phase transition of $ VO_2 $ close to room temperature (68 ̊C) enables many technologies such as optical switches, smart windows, and terahertz antennas. The insulator-metal transition associated with the crystalline phase-change also provides means to fabricate electronic switches and oscillators. Recently, electrically driven VO2 oscillators have been suggested for new AI-based computing paradigms including pattern recognition, solving optimization tasks, and NP-hard problems [1, 2]. A series of coupled $ VO_2 $ devices exploiting self-oscillations induced through an electric current can be harnessed to build an oscillating neural network (ONN) [1, 3, 4]. Being able to process data locally with ONNs offers new power-efficient solutions to bypass the imbalance between the memory access speed and the computational time needed to process such heavy calculations [5, 4]. For the use of VO2 in industrial computing applications, CMOS compatible circuits require high material quality concerning uniformity and surface roughness after depositing on a Silicon-compatible substrate such as Silicon oxide [6, 7]. Devices in planar and crossbar configurations have been fabricated to this end. The $ VO_2 $ devices sit on a Si/SiO2 substrate ensuring CMOS compatibility. Fabrication of $ VO_2 $ layers on non-crystalline substrates, however, tends to result in polycrystalline films with granular structure and considerable surface roughness [6]. This leads to undesired variability among electrical devices such as our oscillators that needs to be mitigated [7]. In this study, we show how to solve this issue by treating amorphous Vanadium oxide films with an annealing step to recover smooth 50 nm-thick film in its crystalline $ VO_2 $ form. The amorphous films are deposited by atomic layer deposition (ALD) using TEMAV and a water-based reaction. We studied different techniques to find a trade-off between crystallization and oxidation rate and phase. One annealing procedure consists of flashing the film with a high energy beam (FLA-50AS, DTF Technology) for 20 ms under oxygen atmosphere (200 mbar) with the sample preheated between 100 ̊C and 310 ̊C. As a reference, the ALD films were annealed by putting the sample in a low oxygen flow chamber (30-40 mTorr) for 10 minutes at a temperature of 520 ̊C. The impact of the different annealing methods on the grain size, the surface roughness, the electric behavior, and the quality of the resulting $ VO_2 $ film are studied through Raman Spectroscopy, Atomic Force Microscopy (AFM), and by measuring the resistance of the film at different temperatures. The nature of the underlying oxide layer and its effect on the quality of the $ VO_2 $ is also studied to define which configuration leads to the best and most reproducible results. Our network of $ VO_2 $-based oscillators shows the promise of an attractive and scalable computing unit for hardware accelerators [3, 5], thanks to its high performance switching properties and CMOS compatibility. This project has received funding from the EU’s Horizon 2020 program under the project No 871501 (NeurONN) and No 861153 (MANIC).