Growth mechanism of epitaxial YSZ on Si by Pulsed Laser Deposition

The epitaxial growth of yttria-stabilized zirconia (YSZ) on silicon with native oxide was investigated in order to gain more insight in the growth mechanism. Specifically, attention was paid to the possibilities to control the chemical interactions between YSZ, silicon and oxygen during initial growth. The sources of oxygen during growth proved to play an important role in the growth process, as shown by individual manipulation of all sources present during Pulsed Laser Deposition. Partial oxidation of the YSZ plasma and sufficient delivery of oxygen to the growing film were necessary to prevent silicide formation and obtain optimal YSZ crystalline qualities. In these conditions, thickness increase of the silicon native oxide before growth just started to occur, while a much faster regrowth of silicon oxide at the YSZ-Si interface occurred during growth. Control of all these contributions to the growth process is necessary to obtain reproducible growth of high quality YSZ.


O 1s Y 3d
Binding energy (eV) Counts (a.u.)  Figure S1. XPS Y3d and O1s spectra of films grown at total pressures of 2*10 −2 mbar at a) different pO 2 or b) with different laser repetition rates. The spectra are from the same samples as shown in Figure 1.
In Supplementary Figure S1, the Y3d and O1s spectra corresponding to the XPS measurements of Figure 1 are shown. No clear influence of deposition conditions on the Y3d spectra was observed. Note that all peaks (Zr3d, O1s, Y3d and Si2p) of samples without silicides were shifted to higher binding energies compared to samples with silicides. The shift can be caused by the insulating character of the SiO 2 and YSZ layers 1 , which are thicker in oxidizing conditions.  Figure S3. AFM images of films grown in pO 2 of a) 1*10 −6 and b) 5*10 −3 mbar, at a total pressure of 2*10 −2 mbar. These images are typical for growth at pO 2 were respectively silicides are formed or formation of silicides is prevented. In conditions were silicide formation was avoided, the surface was smoother compared to conditions were silicide formation occurred (the peak-to-peak roughnesses were respectively 0.8 and 1.5 nm).
Supplementary Figure S3 shows typical AFM images of 6 nm thick films grown in conditions were silicides formed, as well as an image of a film grown in optimized conditions. The surface of the film with silicides was grainy and had an RMS of 0.39 nm (peak-to-peak roughness 1.5 nm). The film grown in optimized conditions was much smoother, and had an RMS of 0.18 nm (peak-to-peak roughness 0.8 nm). A similar observation was made by RHEED (see Supplementary Fig. S4). The film grown in optimized conditions had a streaky pattern, indicating a flat film, while the film grown in oxygen deficient conditions had a spotty pattern, indicating island formation. Although both patterns indicate epitaxial grown films, a big difference was observed when O 2 was added directly after growth. Patterns of films grown in optimized conditions remained streaky. However, rings indicating polycrystallinity appeared in the film grown in the more reducing conditions. In the XPS measurements performed on this film, no features indicating silicides were visible anymore, while the ratio of SiO 2 with respect to silicates/SiO x increased (see Supplementary  Fig. S5).
The crystalline quality of the YSZ was low in conditions where silicide formation was observed. Silicides form at the YSZ-Si interface 2 , and are not necessarely good templates for YSZ crystallization. The roughness observed with AFM could therefore be caused by the formation of dewetted YSZ alternated with regions where zirconium silicide phases are exposed.

Zr 3d
Si 2p Oxide Existance of exposed silicide phases is in agreement with the observations made when those films were exposed to oxygen. XPS showed that silicides were not stable in oxygen environment, and tranformed to SiO 2 and YSZ. Formation of polycrystalline phases was observed with RHEED. Probably, crystallization of YSZ from the silicide phase occured simultaneously with the formation of amorphous SiO 2 , in regions visible with RHEED. The presence of exposed polycrystalline YSZ and amorphous SiO 2 results in a bad template for further YSZ growth. This process can still occur in partial oxygen pressures above which silicide formation was detected with XPS. Small amounts of silicides can be transformed to YSZ and SiO 2 during cool down before being measured with XPS, and have a similar effect on the quality of the template. This explains, for example, why an optimum quality was not reached at a pO 2 of 1*10 −4 in 2*10 −2 mbar Ar yet, while no silicides were measured anymore.