Thickness dependent phase transformation and resistive switching performance of HfO₂ thin films

Resistive random access memory (RRAM) devices are being investigated extensively for future nonvolatile memories using various transition metal oxides like NiO, ZnO, TiO2, and HfO2 as the active material. Here, we have demonstrated the performance of RRAM devices using HfO2 thin films deposited by the ion beam sputtering technique on p(++)-Si (100) substrate with varying thickness from 10 to 30 nm. The density of these films is calculated to be in the range of 9.1-8.6 g/cm(3). A drastic change in the average grain size from similar to 90 to similar to 2000 nm is noticed when the thickness is increased from 20 to 30 nm. We observe a structural transformation from orthorhombic to a dominant monoclinic phase with an increase in thickness from 20 to 30 nm, although a small fraction of the orthorhombic phase remains present in the later film. The phase transformation is accompanied by a significant increase in the average grain size along with a decrease in the oxygen vacancy, as observed from X-ray photoelectron spectroscopy. Further, a red shift in the absorption peak and a reduced band gap of HfO2 film having 30 nm thickness well corroborates with the above fact. We examine the bipolar forming-free resistive switching performance of the RRAM devices fabricated using these films by measuring 100 I-V cycles. Among all films, the film of 20 nm thickness shows better switching behavior with an ON/OFF ratio of similar to 7 attributed to the appropriate grain size, enhanced crystallization, and oxygen vacancies. The endurance and retention measurements show the excellent reliability of the 20 nm thick device. Schematically, the switching mechanism has been discussed based on the Ohmic and Poole-Frenkel conduction models, which is attributed to the formation and rupture of conductive filaments consisting of oxygen vacancies.