Defect-Induced Phase Transition in Hafnium Oxide Thin Films: Comparing Heavy Ion Irradiation and Oxygen-Engineering Effects

Abstract

Hafnium oxide acts as a functional dielectric in a variety of traditional and emerging non-volatile memory technologies. To investigate the effect of heavy ion irradiation on its crystalline structure, highly textured hafnium oxide films were irradiated with 1.635 GeV Au ions of fluences ranging from 1×10 9 to 7×10 12 ions/cm 2 . For monoclinic hafnium oxide films, a fluencedependent defect-induced phase transition to a defect-stabilized tetragonal phase is identified. In low-temperature tetragonal hafnium oxide films, the XRD patterns show an out-of-plane lattice constant decrease with increasing irradiation fluence. Observed crystalline changes are strikingly similar to trends found for oxygen engineered hafnium oxide films, directly grown at varying oxidation conditions. The correlation of structural changes with in vacuo electron spectroscopy data of oxygen engineered films suggests that the irradiation of hafnium oxide leads to an oxygen loss that increases with fluence. Therefore, the underlying mechanism of the monoclinic to tetragonal phase transition is obviously directly related to oxygen defects. This new information allows predictions of device stability under swift heavy ion irradiation of hafnium oxide-based devices.

Heavy Ion Irradiation Hardening Study on 4kb arrays HfO2-based OxRAM

“…Nonetheless, the bright halo corresponds to very small disordered crystallites. Further analysis with improved resolution could confirm whether a phase transition occurs, as already observed [13][14].…”

Structural and Electrical Response of Emerging Memories Exposed to Heavy Ion Radiation

“…According to earlier irradiations of thin films, the number of oxygen-defects rises with increasing ion fluence, resulting in the creation of HfO x suboxides (oxygen-deficient hafnium oxide). 55 The crystalline phase induced by the ion irradiation was so far assigned to a tetragonal phase. 47 , 55 However, we note that the cubic, tetragonal, and several orthorhombic phases in polymorphic hafnium oxide are difficult to distinguish by XRD.…”

“… 55 The crystalline phase induced by the ion irradiation was so far assigned to a tetragonal phase. 47 , 55 However, we note that the cubic, tetragonal, and several orthorhombic phases in polymorphic hafnium oxide are difficult to distinguish by XRD. We thus complemented XRD by analyzing nanobeam electron diffraction (NBED) patterns obtained from 4D-STEM investigations, which allows us to assign the achieved defect-stabilized phase to the low temperature phase of cubic hafnium oxide (LTP c -HfO 2– x ; 56 space group: distorted Fm 3̅ m , ICDD: 04-011-9018), recently described in detail by Kaiser et al 56 This is validated by matching simulated nanobeam electron diffraction (NBED) patterns for the suggested phase to the experimental NBED patterns acquired at the nanocrystals in the irradiated films ( Figure 3 ), which is a significant extension to the so far reported results on irradiated hafnium oxide.…”

“… 52 − 54 In highly textured hafnium oxide films, an oxygen defect-induced phase transition was reported. 55 In contrast to the radiation-induced phase transition from the monoclinic to the tetragonal structure, we recently identified the oxygen-deficient HfO x phase as a defect-stabilized variant of the monoclinic structure with a slightly rhombohedral distorted cubic symmetry (low-temperature phase of cubic HfO x , named LTP c -HfO 2– x ) in as-grown hafnium oxide films. 56 Regarding such effects occurring on a microstructural scale, especially transmission electron microscopy (TEM) has been proven to be a very powerful tool to investigate structural changes on a micrometer or nanometer scale.…”

“…A prominent example is the phase transition of initially monoclinic HfO 2 grains to the tetragonal phase above an electronic energy loss threshold of about 18 keV/nm as described by A. Benyagoub. − Such phase transitions are often directly related to the formation of tracks and modeled by an inelastic thermal spike. , The phase transition mechanism for HfO 2 is described by a double hit process, where a first ion impact creates oxygen defects and a second ion hit of the defective track zone leads to a macroscopically detectable phase transition. Information how thin hafnium-oxide based films respond to energetic ions is rather scarce and inconclusive. − In highly textured hafnium oxide films, an oxygen defect-induced phase transition was reported . In contrast to the radiation-induced phase transition from the monoclinic to the tetragonal structure, we recently identified the oxygen-deficient HfO x phase as a defect-stabilized variant of the monoclinic structure with a slightly rhombohedral distorted cubic symmetry (low-temperature phase of cubic HfO x , named LTP c -HfO 2– x ) in as-grown hafnium oxide films .…”