Defect-Stabilized Substoichiometric Polymorphs of Hafnium Oxide with Semiconducting Properties

Abstract

Hafnium oxide plays an important role as a dielectric material in various thin-film electronic devices such as transistors and resistive or ferroelectric memory. The crystallographic and electronic structure of the hafnia layer often depends critically on its composition and defect structure. Here, we report two novel defectstabilized polymorphs of substoichiometric HfO 2−x with semiconducting properties that are of particular interest for resistive switching digital or analog memory devices. The thin-film samples are synthesized by molecular beam epitaxy with oxygen engineering that allows us to cover the whole range of metallic Hf with oxygen interstitials to HfO 2 . The crystal and defect structures, in particular of a cubic low-temperature phase c-HfO 1.7 and a hexagonal phase hcp-HfO 0.7 are identified by X-ray diffraction, in vacuo electron spectroscopic, and transmission electron microscopic methods. With the help of UV/Vis transmission data, we propose a consistent band structure model for the whole oxidation range involving oxygen vacancy-induced in-gap defect states. Our comprehensive study of engineered hafnia thin films has an impact on the design of resistive memory devices and can be transferred to chemically similar suboxide systems.

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

“… 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. Figure 2 b presents the XRD patterns of 10 nm thin monoclinic HfO x films grown on TiN/SiO 2 /Si (series B) before and after ion irradiation.…”

“…These phenomena are directly comparable to XRD results obtained from nonirradiated oxygen-deficient HfO x layers in the literature. 56 As a possibility to explain the reduction of the reflection intensity, amorphization or grain fragmentation of crystalline grains has to be considered as well. Here, XRD does not allow us to clarify the precise nature of the induced changes on the nm scale, and possible amorphization effects can only be resolved via a spatially resolved data set.…”

“…Therefore, the electron diffraction information on the scanning precession electron diffraction (SPED) data set is processed in a template matching routine, that assigns probability values for a list of user defined templates for each real space position (for each pixel in the map). Here, motivated by the findings in XRD and earlier work, 56 we applied the m -phase of HfO 2 and the low-temperature c -phase of HfO 2– x as input. Matching the templates during ACOM extracts two types of information from the SPED map.…”

“…For even higher fluences, we expect that the electrical properties further change according to the increased cubic phase in the hafnium oxide layer. According to investigations on as-grown samples with the LTP c -HfO 2 -x phase by Kaiser et al, 56 the device conductivity is expected to increase drastically. If a large amount of vacancies has been introduced and a larger amount of cubic phase has been formed, the memory cells will remain fully conducting (lost memory).…”

Hafnium Oxide Nanostructured Thin Films: Electrophoretic Deposition Process and DUV Photolithography Patterning

“…Materials based on hafnium oxide (HfO 2 ) offer remarkable combinations of ferroelectricity, high dielectric permittivity, high energy barriers and high thermodynamic stability, which are of particular interest for next-generation high-κ gate dielectrics in microelectronics or nonvolatile memories, variable capacitors, biosensors, actuators and energy storage/harvesting devices [ 1 , 2 , 3 , 4 ]. These chemical and physical properties are known to be highly dependent on the presence of atomic defects and the amorphous nature or various crystal structures of HfO 2 , i.e., monoclinic (space group P 2 1 / c , the most stable at low temperature), tetragonal ( P 4 2 / nmc ), orthorhombic ( Pca 2 1 ) and cubic ( Fm 3 m ) [ 5 , 6 , 7 , 8 ]. Recently, metal oxide materials such as ZnO, TiO 2 and HfO 2 have emerged as alternatives to conventional organic photoresists and have been shown to be well suited to deep UV and extreme UV (DUV and EUV) photolithography processes [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ].…”