An In Situ and Real-Time Plasmonic Approach of Seed/Adhesion Layers: Chromium Buffer Effect at the Zinc/Alumina Interface

Abstract

The eect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated lms by a combination of surface science techniques, among which a very exible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al 2 O 3 (0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn lms are analyzed from submonolayer to multilayer thicknesses, during their growth. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. As regards to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the submonolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coecient from zero to nearly one due to a diusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr lm percolates, it forms an adhesion layer on which Zn grows in a nearly 2D mode. In all cases, Cr lms are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces which, in line with atomistic simulations, is assigned to the formation of more favorable Cr-Al 2 O 3 and Cr-Zn than Zn-Al 2 O 3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of lms is of great practical interest. It shows that non-continuous and partially oxidized lms of additives, closer to realistic cases of application, can strongly enhance the sticking of lms. Also, anchoring a functional lm by discrete pre-deposited seeds can keep its properties intact.

Cryogenic Temperature Deposition of High-Performance CoFeB/MgO/CoFeB Magnetic Tunnel Junctions on ϕ300 mm Wafers

“…The standard MTJ structure for STT-MRAM and VC-MRAM is CoFeB/MgO/CoFeB with bottom-pinned and top-free CoFeB layers because this trilayer structure can be fabricated on (111)-textured Co/Pt multilayers with a very high PMA, which gives rise to the pinning force of the bottom-pinned CoFeB layer. ,− However, the top CoFeB layer of this standard structure tends to have low quality because island-like initial growth of CoFeB occurs on the MgO(001) surface, reflecting the poor wettability of CoFeB on MgO(001) caused by the low surface energy of MgO(001). − The island-like initial growth results in a rougher CoFeB layer, which degrades magnetic properties such as the TMR ratio and PMA, particularly when the CoFeB layer is ultrathin. ,, Therefore, developing deposition processes to achieve a high-quality ultrathin CoFeB layer on MgO is of considerable importance for next-generation STT-MRAM and VC-MRAM. The quality of the CoFeB/MgO interface is also very important because PMA, TMR, and VCMA effects are interface-sensitive phenomena. − …”

On the O2 “Surfactant” Effect during Ag/SiO2 Magnetron Sputtering Deposition: The Point of View of In Situ and Real-Time Measurements

“…However, this strategy faces intrinsic limits, in terms of technical feasibility and final grain boundary density which drives the film resistivity and its stress state to a large extent. Engineering solutions based on the use of so-called buffer layers (see ref and all references therein) or “surfactants” have been devised to obtain thinner but continuous films. These buffer layers, most of the time made of transition metals or the corresponding metal parent oxides, improve the metal island wettability on the substrate by replacing relatively weak substrate/noble metal bonds with stronger metal/metal and metal/substrate bonds, increasing the sticking coefficient and changing the growth dynamics .…”

“…Engineering solutions based on the use of so-called buffer layers (see ref and all references therein) or “surfactants” have been devised to obtain thinner but continuous films. These buffer layers, most of the time made of transition metals or the corresponding metal parent oxides, improve the metal island wettability on the substrate by replacing relatively weak substrate/noble metal bonds with stronger metal/metal and metal/substrate bonds, increasing the sticking coefficient and changing the growth dynamics . A limitation in buffer thickness is nonetheless often encountered for the final properties, in particular optical transmittance.…”

Manipulation of thin metal film morphology on weakly interacting substrates via selective deployment of alloying species

“…14 Hence, state-ofthe-art surfactant-based strategies are not unconditionally relevant for manipulating film growth on weakly-interacting substrates. Despite the latter, empirical studies have shown that the use transition-metal and semiconducting alloying agents and seed layers, [23][24][25][26][27][28][29] as well deployment of gaseous species 9,[30][31][32][33][34][35] can effectively reverse the tendency toward 3D morphology in Ag and Cu layers on oxide substrates. Concurrently, potential incorporation of alloying species in the film, as well as modification of the film/substrate 4 interface due to the presence of seed layers may affect other physical (e.g., optoelectronic) properties of the noble-metal layer and the overall performance of the corresponding heterostructure device.…”