Effects of Deposition Temperature and Working Pressure on the Thermal and Nanomechanical Performances of Stoichiometric Cu3N: An Adaptable Material for Photovoltaic Applications

Abstract

The pursuit of efficient, profitable, and ecofriendly materials has defined solar cell research from its inception to today. Some materials, such as copper nitride (Cu₃N), show great promise for promoting sustainable solar technologies. This study employed reactive radio-frequency magnetron sputtering using a pure nitrogen environment to fabricate quality Cu₃N thin films to evaluate how both temperature and gas working pressure affect their solar absorption capabilities. Several characterization techniques, including X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), Raman spectroscopy, scanning electron microscopy (SEM), nanoindentation, and photothermal deflection spectroscopy (PDS), were used to determine the main properties of the thin films. The results indicated that, at room temperature, it is possible to obtain a material that is close to stoichiometric Cu₃N material (Cu/N ratio ≈ 3) with (100) preferred orientation, which was lost as the substrate temperature increases, demonstrating a clear influence of this parameter on the film structure attributed to nitrogen re-emission at higher temperatures. Raman microscopy confirmed the formation of Cu-N bonds within the 628-637 cm^-1 range. In addition, the temperature and the working pressure significantly also influence the film hardness and the grain size, affecting the elastic modulus. Finally, the optical properties revealed suitable properties at lower temperatures, including bandgap values, refractive index, and Urbach energy. These findings underscore the potential of Cu₃N thin films in solar energy due to their advantageous properties and resilience against defects. This research paves the way for future advancements in efficient and sustainable solar technologies.

Keywords: Cu3N thin films; nanoindentation; optical parameters; reactive magnetron sputtering.

Figure 1

X-ray diffraction patterns of Cu₃N thin films deposited on glass at different temperatures for the two working pressures of (A) 3.5 Pa and (B) 5.0 Pa.

Figure 2

Typical non-RBS spectrum (in black) and simulated spectrum (in red) of a Cu₃N sample deposited at 3.5 Pa at room temperature.

Figure 3

Ratio of Cu/N determined by RBS as a function of the substrate temperature.

Figure 4

Raman spectra for Cu₃N films at different temperatures. (A) 3.5 Pa and (B) 5.0 Pa.

Figure 5

FE-SEM images of Cu₃N films fabricated at different substrate temperatures and working pressures, deposited on silicon substrates of (A) 3.5 Pa and (B) 5.0 Pa.

Figure 6

Grain-size determination for thin films prepared at different temperatures: (A) RT and (B) 250 °C. Fe-SEM images (a.1,b.1); Segmentation of the input images (a.2,b.2); and quantification of the segmentation of the input images (a.3,b.3).

Figure 7

Absorption coefficient α versus photon energy (E) derived from optical and PDS measurements of the sample deposited at 5 Pa and 200 °C. (A) Experimental data—the blue dots are the absorption coefficients α_optical derived from the UV–Vis–IR spectroscopy measurements and the pink circles are the absorption coefficients α_PDS derived from the PDS measurement. (B) Fitting of α(E) with the Urbach-Tauc/indirect–Tauc/direct model. The blue curve represents the usual fit of the strong absorption front and the red curve is the fit of a second front that appears exceptionally in the weak absorption zone of this sample.

Figure 8

Optical transmission and reflection spectra of the Cu₃N films deposited at different temperatures and the working pressures of (A) 3.5 Pa and (B) 5.0 Pa.

Figure 9

Processing of the optical data of the sample deposited at 5.0 Pa and 200 °C. (A) TMM fitting of transmittance and reflectance to obtain the thickness and the refractive index n_∞ of the Cu₃N thin film. The shaded area represents the optical absorbance (1 − T − R). (B) Plot of (αE)^1/2 versus photon energy E for the determination of the indirect band gap E_gⁱ, and the plot of (αE)² versus E for the determination of the direct band gap E_g^d.

Figure 10

Direct bandgap value E_g^d obtained by fitting α(E) as a function of the corresponding indirect bandgap value E_gⁱ. Green squares represent the samples deposited at 3.5 Pa and pink circles; the samples deposited at 5.0 Pa. The deposition temperature is included as the number next to the symbol. The dashed line is a guide for the eye.

Figure 11

Absorption coefficient spectra of Cu₃N layers deposited at different temperatures and the working pressures of (A) 3.5 Pa and (B) 5.0 Pa.