Researchers Use Pulsed Laser Deposition for Functional Oxide Integration

By Taha KhanReviewed by Laura ThomsonAug 25 2022

In a preproof in journal Thin Solid Films, researchers have realized the epitaxial TiN films on (100) Si by reactive pulsed laser deposition (PLD).

Functional oxide thin films and associated heterostructures display a broad range of features, including ferroelectric photovoltaic effects, 2D electron gas (2DEG) at their interfaces, metal-insulator transition, colossal magnetoresistance, multiferroicity, ferroelectricity and many more.

Perovskite Oxides

Among the functional oxides, the perovskite oxides with the chemical formula ABO₃ are the most adaptable group. Numerous perovskite oxides-based energy-efficient devices have been demonstrated in the literature, including electro-optic modulators and ferroelectric tunnel junctions on BaTiO₃, polar-vortices at PbTiO₃/SrTiO₃ superlattices, metal to insulator transition SmNiO₃, 2DEG at the LaAlO₃/SrTiO₃ interface, ferro-photovoltaics such as BiMnO₃ and BiFeCrO₃ etc.

Epitaxial Integration of Oxides

The aforementioned demonstrations are on epitaxial oxide thin films formed on single crystalline substrates like DyScO₃, NdScO₃ and SrTiO₃ since the physical attributes are crystal orientation dependent. If functional oxide thin films are epitaxially grown on Si (100), the complementary metal oxide semiconductor (CMOS) chip's performance envelope can be improved. However, epitaxial integration of such oxides is complex since Si already has a native amorphous oxide layer.

Despite the difficulty, epitaxially grown SrO, CeO₂, MgO, yittria stabilized zirconia (YSZ), and other oxides have been employed to integrate functional oxide. However, due to their insulating nature, LaNiO₃ or SrRuO₃ must be used between these oxides to evaluate their electrical properties.

This may be accomplished by using titanium nitride (TiN), a substance often employed as a diffusion barrier and contact metal in advanced CMOS devices. Pulsed laser deposition (PLD) has been used to create TiN thin films on Si (100) substrates.

Pulsed Laser Deposition of TiN on Si (100)

A hot-pressed TiN target is commonly ablated in an N₂ environment to perform such PLD of TiN on Si (100). However, the size, price, purity and density of these TiN targets are constrained. Reactive ablation, also known as reactive PLD (RPLD), is a viable option because it involves ablating Ti metal in an N₂ environment. Additionally, RPLD offers better work function tenability, lattice parameter and stoichiometry options.

Although it is a valuable technique, no studies of the viability of epitaxial TiN deposition on Si by RPLD have been done in the literature. Eclipse-PLD geometry is a good option because it is easy to use, reliable, adaptable, and affordable (it does not require significant chamber alteration like a magnetic lens). Although Ag-Co films have been produced using eclipse PLD, it is necessary to verify the viability of manufacturing TiN using reactive pulsed laser deposition with low particle density.

Research Area

To produce epitaxial TiN films with low particle density by RPLD, an eclipsed off-axis geometry with a 30° tilt is used in this research. In addition, a thorough investigation of how deposition rates, film thickness uniformity, and particle density are affected by deposition geometry, including on-axis, off-axis, and eclipsed off-axis (EO), is provided.

Experimentation

The direct TiN deposition was carried out on Si strips after being heated to the required temperatures of up to 700 °C. The following PLD settings were used to ablate a titanium metal target: 2 and 3 J/cm² laser fluences, 5x1 mm² spot size, 25 ns pulse width and 248 nm excimer laser.

In a chamber with an in-situ reflection high energy electron diffraction working at 30 kV, BaTiO₃ was deposited. The BaTiO₃ target was created by sintering 99.5% pure BaTiO₃ powder in the air for six hours at 1300 °C. Substrate temperature, pressure, film growth rate, repetition rate, and Fluence were 600 °C, 5x10^-4 Pa, 3.3 nm/min, 2 Hz, and 1.5 J/cm^2, respectively. Using ImageJ software, the optical micrographs' particle density was examined. The surface morphology of deposited films was also performed.

Significant Findings of the Study

An efficient strategy for controlling particles at high growth rates in reactive PLD of TiN films is eclipsed off-axis with sample tilt. Reactive pulsed laser deposition is used to create TiN films with low particle density (6x103 cm^-2) and minimal edge-to-edge thickness fluctuation ( 5%) at a growth rate of 0.92 μm/hr. The epitaxial integration of TiN-on-Si (100) substrates by RPLD is feasible, and XRD (X-ray diffraction) and cross-section TEM (transmission electron microscopy) results have shown that TiN develops on Si by cube-on-cube pattern through domain matching epitaxy.

The asymmetric and symmetric ω-FWHM (full width at half maximum) epitaxial TiN films on Si are 1.2° and 1.27°, respectively. Film microstructure has a significant impact on film stoichiometry and lattice parameters. The film's microstructure is governed by the plume chemistry (N/Ti), which is governed by the average and instantaneous deposition flux. TiN films become rougher during RPLD deposition due to low lateral edge velocities along 110 directions, which is constrained by N availability. Ferroelectric BaTiO₃ is integrated epitaxially onto Si (100) substrates using the low particle density and smooth TiN-on-Si.

Reference

Sandeep Vura, Rajeev Kumar Rai, Pavan Nukala, Srinivasan Raghavan (2022) Reactive Pulsed Laser Deposition of Low Particulate Density and Epitaxial TiN films on Si (100) for Functional Oxide Integration. Thin Solid Films. https://www.sciencedirect.com/science/article/pii/S0040609022003704

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