[Paper Review] Epitaxial growth of complex oxides on silicon by enhanced surface diffusion in large area pulsed laser deposition

Rik Groenen, Zhaoliang Liao+12 more|arXiv (Cornell University)|Jul 20, 2016

Semiconductor materials and devices3 citations

TL;DR

This study demonstrates epitaxial growth of La0.67Sr0.33MnO3 (LSMO) on 4" silicon wafers using large-area pulsed laser deposition with enhanced surface diffusion enabled by a SrRuO3 buffer layer. The key result is ferromagnetic LSMO films at just 250 °C—previously unachievable without the buffer—due to improved surface diffusion compensating for low-temperature kinetic limitations and compositional flux inhomogeneity during wafer-scale deposition.

ABSTRACT

Homogeneous highly epitaxial LaSrMnO3 (LSMO) thin films have been grown on Yttria-stabilized-Zirconia (YsZ) / CeO2 buffer layers on technological relevant 4" silicon wafers using a Twente Solid State Technology B.V. (TSST) developed large area Pulsed Laser Deposition (PLD) setup. We study and show the results of the effect of an additional SrRuO3 buffer layer on the growth temperature dependent structural and magnetic properties of LSMO films. With the introduction of a thin SrRuO3 layer on top of the buffer stack, LSMO films show ferromagnetic behaviour for growth temperatures as low as 250C. We suggest that occurrence of epitaxial crystal growth of LSMO at these low growth temperatures can be understood by an improved surface diffusion, which ensures sufficient intermixing of surface species for formation of the correct phase. This intermixing is necessary because the full plume is collected on the 4" wafer resulting in a compositional varying flux of species on the wafer, in contrast to small scale experiments.

Motivation & Objective

• To enable high-quality epitaxial growth of complex oxides on industrial 4" silicon wafers using pulsed laser deposition.
• To overcome the challenge of low-temperature epitaxial oxide growth on silicon, where surface diffusion is typically too low for phase-pure crystalline films.
• To investigate the role of surface diffusion kinetics in enabling phase-pure, ferromagnetic LSMO films at reduced temperatures on large-area substrates.
• To demonstrate that a SrRuO3 buffer layer enhances surface diffusion, allowing crystalline and magnetic LSMO films at 250 °C—well below the 650 °C required without the buffer.
• To address the issue of compositional flux inhomogeneity during scanning PLD on 4" wafers by promoting intermixing via enhanced surface diffusion.

Proposed method

• Used a large-area pulsed laser deposition (PLD) system with beam scanning to deposit films uniformly across 4" silicon wafers.
• Employed a YSZ/CeO2/SrRuO3 buffer stack on Si to enable epitaxial oxide growth by removing native SiO2 and reducing lattice mismatch.
• Applied kinetic models to relate surface diffusion coefficient (DS ∼ exp(−EA / kB T)) to growth temperature and activation energy (EA).
• Utilized in-situ RHEED and ex-situ XRD to monitor surface structure and phase formation during growth.
• Performed magnetic measurements (SQUID) at 100 K to quantify magnetic moment and Curie temperature (TC) as functions of growth temperature and buffer layer presence.
• Used cross-sectional SEM to estimate film thickness for accurate magnetization normalization.

Experimental results

Research questions

• RQ1 Can epitaxial LSMO films with ferromagnetic order be grown on 4" Si wafers at temperatures below 550 °C using large-area PLD?
• RQ2 How does the presence of a SrRuO3 buffer layer affect the surface diffusion kinetics and phase formation of LSMO during low-temperature PLD?
• RQ3 What is the role of surface diffusion in compensating for compositional flux inhomogeneity in large-area PLD on 4" wafers?
• RQ4 At what growth temperature does LSMO transition from amorphous to crystalline and ferromagnetic when grown on SrRuO3-buffered substrates?
• RQ5 How does the Curie temperature (TC) of LSMO depend on growth temperature and buffer layer presence?

Key findings

• Ferromagnetic LSMO films with a magnetic moment of 0.1 μB/Mn were achieved at just 250 °C when grown on a SrRuO3 buffer layer.
• The Curie temperature (TC) of LSMO films grown on SrRuO3 reached 250 °C, while non-buffered films required temperatures above 650 °C to achieve TC onset.
• Without the SrRuO3 buffer, LSMO films remained amorphous and non-magnetic up to 550 °C, indicating a critical role of the buffer in enabling low-temperature crystallization.
• The SrRuO3 buffer layer enhanced surface diffusion, enabling sufficient intermixing of surface species to form the correct (001)-oriented perovskite phase despite spatially varying flux during wafer-scale scanning.
• The improved surface diffusion kinetics suppressed the formation of metastable (110) orientations and other secondary phases, ensuring high-quality (001)-oriented epitaxial films.
• The enhancement in surface diffusion is attributed to a lower activation energy for diffusion on the A-site-terminated SrRuO3 surface, similar to mechanisms observed in SrRuO3 and BiFeO3 growth.