Carrier Gas Flux

M oT e2 growth: results and discussion

M oT e2 growth: results and discussion

After studying the effect of the hydrogen mixture, I investigated the effect of N2 gas flux on the Raman spectra and surface morphologies. We explored three different N2 fluxes: 30, 60 and 90 sccm. The growth time and temperature are kept unchanged as the Sample 1 introduced in the previous case study. For the sake of simplicity, the experimental conditions are summarized in Table 3.3.

Sample ^Upstream

T(^◦C)

Downstream T(^◦C)

Growth Time (minutes)

Carrier Gas Flux (sccm)

3 650 700 120 100 (Ar/H2) - 30 (N2)

4 650 700 120 100 (Ar/H2) - 60 (N2)

5 650 700 120 100 (Ar/H2) - 90 (N2)

Table 3.3: Process conditions used to grow MoT e2 in Sample 3, Sample 4 and Sample 5.

Morphological Analysis

Figure 3.11 a-c shows three representative AFM measurements corresponding to the experiments with 30, 60, and 90 sccm gas fluxes, respectively. Furthermore, the corresponding height profiles along the blue lines are shown in panel d-f.

From these measurements, it can be realized that at 30 sccm flux, the deposition is not uniform and results in cluster formation. Clusters have lateral size of hundread of nanometers and height in the same vertical range. They coalesced in large structures leaving large part of the substrate exposed, for this reason the RMS roughness of the surface morphology is as high as 19nm.

By increasing the gas flux to 60 and 90 sccm, AFM measurements confirm more uniform deposition and a compact granular morphology in both cases. The RMS roughness is estimated at 5.94 and 7.31nm, respectively.

M oT e2 growth: results and discussion

Figure 3.11: 2µm × 2µm topographical image of the MoT e2 a) Sample 3. b) Sample 4. c) Sample 5 - cross-sectional plot along the blue line d) in a. e) in b. f) in c.

Average Grain Size

The self-correlation filter of the topography images at 60 and 90 sccm experiments is depicted in Figure 3.12 a-b. By measuring the FWHM of the center in these images, we conclude that the amount of gas flux does not affect the average grain size.

The topography of Figure 3.11 a, corresponding to the sample obtained at 30 sccm, cannot be analyzed utilizing the self-correlation function filter because of the not homogeneous deposition.

On the other hand, the samples at 60 and 90 sccm show a symmetric self-correlation function of the AFM topographies in Figure 3.12. The symmetric central peak in the self-correlation function demonstrates that the morphology of the samples is constituted by grains with symmetric round shapes in both cases. Therefore, we can assess the mean grain size by measuring the full-width-half-maximum of the peak along a horizontal line profile at the center of the image. The reported mean grain size values have been obtained as the mean value calculated on different AFM measurements; the uncertainty has been obtained as the standard deviation of the measurements from the mean value. We can conclude that the average grain size is 52.62 ± 8.43 nm for the sample grown at 60 sccm and 55.58 ± 10.34 nm for the sample grown at 90 sccm.

M oT e2 growth: results and discussion

Figure 3.12: 2µm × 2µm Self-correlation image of the MoT e2 a) Sample 4. b) Sample 5 - cross-sectional plot along the green line in c) a. d) b.

M oT e2 growth: results and discussion

Raman scattering investigation

As for the previous studies, I used Raman spectroscopy to gain a deeper under-standing of the material deposition. In Figure 3.13, I show the Raman spectrum of the sample grown with 30 sccm gas flux. I carried out a positional analysis of the sample acquiring the Raman spectra in three positions from the left (closer to the tellurium source) to the right. I observed an high variability of the spectra in term of peak intensity. More in detail, all the spectra exhibit two main peaks at 120 and 140.7cm⁻¹. These two peaks are assigned to A¹ and E² peaks of Te. Other peaks at 162cm⁻¹ and 260cm⁻¹ are assigned to Bg and Ag peaks of 1T’ phase of M oT e₂. The spectrum also exhibits the MoO2 peak at 190cm⁻¹. It is interesting to notice that the high degree of structural disorder identified with the AFM investi-gation reflects into broad Raman peaks, see for instance the A1 mode in the spectra.

Figure 3.14 shows the Raman spectrum of the samples grown with 60 sccm and 90 sccm gas flux, respectively. They exhibit three main peaks of the 1T’

M oT e₂ phase: Au, Ag, and Bg at 109, 127, and 162cm⁻¹ respectively. Apart from a slight difference in the peak intensity, the variability of the Raman spectra is more pronounced in the sample obtained with 60 sccm gas flux. Nevertheless, the study clearly demonstrates the formation of the MoT e2 film in both the cases.

Also in this case, the FWHMs of the Raman peaks are larger than those of the exfoliated flakes but comparable to those obtained in other CVD grown MoT e2

films. I reported the analysis fit of the Raman peaks in Table 3.4.

Figure 3.13: Raman Spectrum with the N2 gas flux 30 sccm.

M oT e2 growth: results and discussion

Figure 3.14: Raman Spectrum with the N2 gas flux 60 scccm (left) and 90 sccm (right).

Figure 3.15: The fitted Raman spectrums of the left side (closer to the tellurium source) of the sample 3, 4, and 5 with Voigt function.

M oT e2 growth: results and discussion

Sample A¹Peak E²Peak AuPeak AgPeak BgPeak M oO2Peak AgPeak

Sample 3 7.08 nm 5.44 nm - - 6.32 nm 12.65 nm 21.43 nm

Sample 4 - - 6.31 nm 5.18 nm 5.19 nm 6.79 nm 48.32 nm

Sample 5 - - 4.65 8.80 nm 5.75 nm 12.82 nm

-Table 3.4: Comparison of the FWHM of the Raman A¹, E², Au, Ag, Bg, MoO2, and Ag peaks between the sample 3, 4, and 5.

Preliminary conclusion

We can draw some preliminary conclusions based on the studies of the role of the carrier gas. Firstly, I demonstrated that a reducing atmosphere, provided by the presence of molecular hydrogen during the high-temperature process, is mandatory for the tellurization reaction to occur.

Secondly, I showed that a low gas flux, i.e., lower than 60 sccm, does not result in a uniform and homogenous MoT e2 formation. Both AFM and Raman spectroscopy pointed out that, in this case, the film is highly disordered, and not-reacted tel-lurium deposition is found. This result can be explained considering that such low gas flux could not be enough to provide sufficient tellurium atoms for the reaction to occur before evaporation of the molybdenum film.

Lastly, by comparing the FWHM and intensities of the Raman Bg peak at 60 sccm and 90 sccm gas fluxes, I found more homogeneity in the growth of MoT e2

is obtained at fluxes higher than 60 sccm. This is reported in Figure 3.16, where I used the analysis on different Raman spectra to calculate the mean value and uncertainty on the measure using the standard deviation from the mean value. The data show that at 90 sccm, the dispersion around the mean value is very low both in the intensity and FWHM value of the peak. In addition, I found that the same condition holds for samples obtained in the flux range 90-150 sccm that I identified as the optimal condition for the growth.

M oT e2 growth: results and discussion

Figure 3.16: Comparison of the Bg peak intensities (left) and FWHM (right) at 60 and 90 sccm gas flux