SrTiO3 substrates are frequently used to grow epitaxial perovskite oxide thin films, as the (001) SrTiO3 unit cell displays an extremely low lattice mismatch with several functionally significant oxides. Polished “epi ready” single crystal (001) SrTiO3 substrates are readily available from varied vendors for epitaxial thin film growth. The substrates are expected to be single crystal containing high structural purity, or flaws are developed during the film growth process.
The regular defects that adversely impact the characteristics of the film, specifically oxide growths, include twinning and formation of various domains. It is extremely important to examine the quality of commercial substrates prior to thin film growth. Characterization of single crystal substrates often takes place with the development of rocking curves at an azimuthal angle, and the substrates are considered to be single crystals if Gaussian-shaped rocking curves with peak full width at half maximum (FWHM) very near to the instrumental broadening are acquired. This analysis highlights that this method may not be precise in all cases.
The following instruments were used for the analysis:
D8 DISCOVER (Figure 1)
- Rotary absorber
- Goebel mirror
- Cu long fine focus X-ray tube (40 kV/40 mA)
- (022) 2-bounce Ge monochromator
- Centric Eulerian Cradle (CEC)
- LYNXEYE 1D detector
- Universal detector mount (UDM) with slit
Figure 1. D8 DISCOVER setup
Rocking curves, where a sample is “rocked” about the Bragg angle, θ, while an open 0D detector is placed at 2θ, were carried out around the (200) peak of the SrTiO3 substrate. Reciprocal space maps (RSM) were developed of the same peak from a range of fixed 1D detector snapshots centered at 2θ, and the sample was rocked in the same manner at the same time. Based on compound silicon strip technology, the LYNXEYE detector is flexible enough to be software-switched from 0D mode to 1D mode, enabling it to be extremely flexible for HRXRD applications. Total scan time, compared to the conventional 0D RSM, is reduced by collecting the RSM in 1D mode. The utilization of a 2-bounce monochromator over a 4-bounce monochromator enhances the intensity, while continuing to offer the expected X-ray beam divergence and spectral purity. The measurements were executed on the (001) SrTiO3 substrate at varied azimuthal angles to analyze the quality of the substrates just before the growth. The results are shown in Figure 2. The azimuthal angles selected for the measurements include 0, 45 and 90 degrees, in that order. Measurement times usually taken up for the RSMs are 30 minutes.
Figure 2. 1D Fast RSM and 0D rocking curves collected at different azimuthal angles (0, 45 and 90 from left to right).
The measurements taken at azimuthal angles of 0 and 90 degrees highlight that one bright spot in the (002) rocking curve and the (002) RSM profiles exhibit a conventional Gaussian shape. Measurements carried out at an azimuthal angle of 45 indicate evidence of several spots in the RSM to the right hand side of the primary bright spot, as shown in Figure 3.
Figure 3. Comparison between a sectional profile of the 1D RSM in the rocking curve direction vs. 0D rocking curve.
A sectional profile of the RSM taken across the direction of the rocking curve indicates the same profile as the open detector rocking curve, which provides clear evidence of the existence of a number of domains in the substrate. This proved that the SrTiO3 substrate was not a single crystal.
This article emphasizes the significance of characterizing single crystal substrates beyond a single rocking curve to obtain a comprehensive assessment. A high-speed, flexible 1D detector and a high intensity 2-bounce monochromator are used to minimize the extra time required for these measurements.
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.
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