Relationships among growth mechanism, structure and morphology of PEALD TiO₂films: the influence of O₂plasma power, precursor chemistry and plasma exposure mode

Abstract

Titanium dioxide (TiO2) thin films have generated considerable interest over recent years, because they are functional materials suitable for a wide range of applications. The efficient use of the outstanding functional properties of these films relies strongly on their basic characteristics, such as structure and morphology, which are affected by deposition parameters. Here, we report on the influence of plasma power and precursor chemistry on the growth kinetics, structure and morphology of TiO2 thin films grown on Si(100) by plasma-enhanced atomic layer deposition (PEALD). For this, remote capacitively coupled 13.56 MHz oxygen plasma was used to act as a co-reactant during the ALD process using two different metal precursors: titanium tetrachloride (TiCl4) and titanium tetraisopropoxide (TTIP). Furthermore, we investigate the effect of direct plasma exposure during the co-reactant pulse on the aforementioned material properties. The extensive characterization of TiO2 films using Rutherford backscattering spectroscopy, ellipsometry, x-ray diffraction (XRD), field-emission scanning electron microscopy, and atomic force microscopy (AFM) have revealed how the investigated process parameters affect their growth per cycle (GPC), crystallization and morphology. The GPC tends to increase with plasma power for both precursors, however, for the TTIP precursor, it starts decreasing when the plasma power is greater than 100 W. From XRD analysis, we found a good correlation between film crystallinity and GPC behavior, mainly for the TTIP process. The AFM images indicated the formation of films with grain size higher than film thickness (grain size/film thickness ratio ≈20) for both precursors, and plasma power analysis allows us to infer that this phenomenon can be directly related to the increase of the flux of energetic oxygen species on the substrate/growing film surface. Finally, the effect of direct plasma exposure on film structure and morphology was evidenced showing that the grid removal causes a drastic reduction in the grain size, particularly for TiO2 synthesized using TiCl4.

MOS Capacitance Measurements for PEALD TiO2 Dielectric Films Grown under Different Conditions and the Impact of Al2O3 Partial-Monolayer Insertion

“…On the other hand, if the RF power is increased, the film thickness is reduced for both plasma modes. This behavior for the TTIP precursor was demonstrated in our previous study [45], where it was observed that the growth per cycle (GPC) decreases when RF power is increased. The leading cause is related to the increase of the density of O 2 radicals that fragments the TTIP-ligands, hindering the formation of Ti-O bonds due decreasing of the free path.…”

“…These images show that the grain size slight decreases with RF power for both plasma modes. The effect of the electrode grid was evidenced by the reduction of the action of the plasma ions during the capacitively coupled PEALD process [45]. As can be seen for both power values investigated, the change from remote to direct mode caused a slight reduction in grain size, probably due to the higher action of the species from plasma impinging on the substrate.…”

“…To confirm the Raman results, GIXRD measurements were performed, using the same 0.29 • incident angle for all films. This angle was used to reduce the reflections from the Si substrate [45]. Based on the powder diffraction file (JCPDS: 21-1272) [57], all diffraction peaks are identified for the TiO 2 films studied here, as shown in Figure 5.…”

“…Using the impedance equality of the electrical circuits shown in Figure 10a,b, we derived the equations for the leakage admittance YC and Cin, which are shown below [67]: Table 2 shows the Vfb and Qf values of all samples with the front contact area of 4.3 × 10 −3 cm 2 . As shown in the literature, TiO2 grown on Si generates negative Qf being more appropriate for rear passivation in solar cells, basically due to field-effect passivation [45]. Recently, Liao et al [73] showed that TiOx thin-film (63 nm) growth by ALD is also capable of providing passivation of c-Si substrates at the same level of thermal silicon oxide (SiO2), silicon nitride (SiNx) and aluminum oxide (Al2O3).…”

“…In our previous study [45], the effects of O 2 plasma power, titanium precursor chemistry and plasma exposure mode on the growth mechanism, structure and morphology of PEALD TiO 2 films were investigated. Herein, we extend this study with the focus on the fabrication and characterization of TiO 2 /p-Si MOS capacitors for potential applications in microelectronic and photovoltaic devices.…”