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Figure 4.2a shows a large-area SEM image of an h-BN film grown on Cu under LPCVD conditions. The closely spaced (ca. 0.025 μm) striations running vertically in the image indicate the step flow direction of the underlying Cu surface. These steps result from the mismatch in thermal expansion coefficients between h-BN and Cu, forming only if the overlayer is planar and has a well-ordered crystalline structure [51]. The faint striations running diagonally from upper right to lower left are attributed to wrinkles in the h-BN overlayer formed during the growth process [15, 17, 28, 30, 52]. Figure 4.2b is a small-area SEM image of another region of the same LPCVD grown h-BN sample. The prominent

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feature that approximately bisects the image vertically is a Cu grain boundary. Here, an h-BN wrinkle crosses the Cu grain boundary; similar behavior has been seen for graphene [53].

When growth is conducted at medium pressures of background gas, the Cu step flow features and h-BN wrinkles are absent, and the surface appears morphologically rough (Figure 4.2c). There is also a high density of charged nanoparticles in the image. Similar results are obtained at a growth pressure of 200 Torr (Figure 4.2d) with a rough surface and the absence of Cu step flow features. Under APCVD conditions, the h-BN film is non-planar and exhibits dendritic surface features in both the large-area (Figure 4.2e) and small-area (Figure 4.2f) SEM images. There are no previous reports of such dendritic surface features for APCVD grown h-BN. Again, there are no obvious Cu step flow features or h-BN wrinkles, suggesting that the APCVD grown film is thicker than those grown under LPCVD conditions.

Figure 4.3 shows further SEM images of LPCVD and APCVD grown BN. The h-BN grown at 1.2 Torr (Figures 4.3a and 4.3b) have similar morphology to the h-h-BN films grown at 2.0 Torr. The SEM images of the APCVD grown h-BN sample in Figures 4.3c and 4.3d are slightly less disordered than the APCVD grown h-BN film shown in Figures 4.2e and 4.2f. The surface still has a more polymeric, disordered structure than h-BN grown at lower pressures.

In Figure 4.4, we present AFM images the h-BN films as a function of the growth pressure. Figure 4.4a shows an h-BN film grown at 1.2 Torr (LPCVD regime) after transfer to SiO2 and lithographic patterning (step indicated by the dashed, blue line). The film has similar root-mean-square (RMS) roughness to the SiO₂ surface (0.58 nm) and a measured step height of 0.8 ± 0.1 nm, indicating that the film consists of one to two h-BN layers [9,

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10]. A slight increase of the growth pressure to 2.0 Torr leads to a film with a step height of 1.0 ± 0.3 nm. After annealing the film in Ar/H₂ at 400 °C, the film is smoother (0.45 nm RMS roughness) than the film from Figure 4.4a. However, the Ar/H2 anneal leaves etch tracks in the film. Due to the H₂ etching, other samples were annealed in air (see Section 4.10.2, page 125).

Figures 4.4c and 4.4d show AFM topographs of two different films also grown under LPCVD conditions at 2.0 Torr, but the flux from the precursor reservoir was higher than normal (HM conditions). For these two growth runs, the temperature of the precursor reservoir was higher (~100 °C) than other growths. At this higher temperature, the precursor evolves larger amounts of H2 and volatile boron and nitrogen containing species. For further discussion of the HM conditions, see Section 4.11, page 128. The resulting h-BN film in Figure 4.4c is 3.2 ± 1.4 nm thick (1.51 nm RMS roughness). Films grown under such HM flow conditions are thicker and have rougher morphology than films grown under LPCVD conditions with normal precursor flux. Like graphene growth on Cu foil [47], CVD growth of h-BN is also quite sensitive to the ratio between the precursor (decomposed H3N–BH3) and H2.

When grown at intermediate pressures of background Ar/H₂ gas (20 Torr), the film thickness increases to 3.4 ± 0.6 nm (3.20 nm RMS roughness). The films feature scattered large protrusions that result from the film transfer (Figure 4.4e). Still, there is a higher concentration of smaller protrusions in a linear feature in the AFM image center. A former annealing twin from the Cu growth substrate could produce this linear feature, which is rougher than the surrounding regions (see Figure 4.4e). Such high-index Cu surfaces are

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known to lead to more defective, thicker graphene growth [44], and therefore enhanced h-BN growth on the twin is not surprising.

Figures 4.4f–h show AFM images for h-BN films grown at 200 Torr and 760 Torr (APCVD) Ar/H₂ background pressures after lithographic patterning (see Section 4.10.3, page 125). At medium growth pressures (200 Torr) in Figure 4.4f and 4.4g, the film has a step height of 10.1 ± 0.9 nm (1.53 nm RMS roughness). Under APCVD conditions (Figure 4.4h), the h-BN films are rougher yet (1.64 nm RMS roughness) and possess inhomogeneous depressions with contours corresponding to the morphology seen in SEM (Figures 4.2e and 4.2f).

The summary of the measured RMS roughness values for the h-BN films shown in Table 4.1 indicates that the roughness of the h-BN increases with growth pressure. While the roughness of the lowest pressure growth (1.2 Torr) is slightly higher than that for the 2.0 Torr growth, this is most likely due to the conformal nature of the 1 to 2 h-BN layer film on the substrate compared to the thicker film grown at 2.0 Torr. The outlier RMS roughness value for the h-BN sample grown at 20 Torr can be explained by a former annealing twin from the Cu growth surface.

The step height contours for different growth pressures shown in Figure 4.5 indicate that the film thickness—and thus the growth rate—increases monotonically with increasing growth pressure. Comparing the h-BN samples grown at 1.2 Torr and 2.0 Torr shows a small height difference of 0.2 nm, suggesting that the sample grown at 1.2 Torr consists of one to two h-BN layers, and that the one grown at 2.0 Torr consists of two to three h-BN layers. The step height for the sample grown at 2.0 Torr under HM conditions of ~3.2 nm is close to that

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for the sample grown at 20 Torr, indicating the importance of the ratio between the precursor byproducts and H₂ in determining the growth rate and the resulting film thickness.

Comparing the measured step height in Figure 4.5 for the APCVD grown h-BN shown in Figure 4.4h with that for the h-BN grown at 200 Torr (Figures 4.4f and 4.4g), however, shows that the APCVD grown film has a smaller step height. This runs counter to the trend for the other growth pressures. The APCVD grown h-BN sample in Figure 4.4h was lithographically patterned (see Section 4.10.3, page 125) and exposed to O₂ plasma concurrently with the sample grown at 200 Torr shown in Figure 4.4f. Despite using the same etching conditions, the remaining disordered pattern in the region exposed to the O₂ plasma shown in AFM data (Figure 4.4h) indicates that the APCVD film did not etch completely. It is possible that the APCVD grown h-BN film is in fact thinner than that grown at 200 Torr; the increased H₂ pressure might alter the dehydrogenation rate of the volatile precursor byproducts. If these byproducts cannot dehydrogenate, then they will not form h-BN [27]. However if the APCVD grown h-h-BN thin film has a different chemical and structural state (see Section 4.4), then the film might etch in the O2 plasma at a different rate than the h-BN film grown at 200 Torr.

4.4 X-Ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass