Niobium Pentoxide Films with High Laser-Induced Damage Threshold under High Temperature Environment

Abstract

Laser damage resistance of films is the key to the performance and durability of high-power laser systems. High temperature, however, often leads to a certain degree of decline in film properties. Here, aiming to explore the mechanism of laser damage under high temperatures, Nb₂O₅ films were prepared using sol–gel method with NbCl₅ as precursor and citric acid as additive. The effect of annealing temperatures in optical performance, microstructure, surface morphologies, and chemical composition of films were studied. Further investigation was carried out of the laser-induced damage threshold (LIDT) under different in situ high temperatures of the as-deposited films. The results showed that the films had prominent optical transmittance and high LIDT. Under 293 K, the LIDT of the film was the highest of 24.8 J/cm². The increase of temperature brought down the LIDT of the films. It was attributed to the occurrence of oxygen vacancies, the sharp increase of temperature, and rise of defects and destruction of network structure. In this study, even though the LIDT of the film decreased a lot at 523 K, it was still high up to 14.1 J/cm², representing the great potential for applications in authentic high-temperature environments.

1. Introduction

Niobium pentoxide (Nb₂O₅) has been widely applied in modern technologies with its characteristics of high refractive index, wide band gap, and good thermal stability [1,2,3]. The high refractive index of Nb₂O₅ can be utilized to prepare antireflective films and high reflective films together with materials of low refractive index [4]. Films with high laser-induced damage threshold (LIDT) used in high-power laser systems can be prepared with its features of wide band gap and good thermal stability [5,6]. For example, Xu et al. deposited Nb₂O₅ films using electron beam evaporation method with the LIDT of 8.5 J/cm² (1064 nm, 12 ns) [7]. With niobium ethoxide as precursor, Stephen et al. prepared Nb₂O₅ films through the sol–gel method with the LIDT of 8.3 J/cm² (1064 nm, 3 ns) [8]. Our previous studies, adopting niobium chloride as precursor managed to prepare Nb₂O₅ films through the sol–gel method with the LIDT high to 24.9 J/cm² (1064 nm, 12 ns) [9]. Notably, the above research findings were laser damage under room temperature and ambient pressure. With the continuous exploration of laser applications, environmental factors, especially the effect of high temperature in laser damage, become unignorable. For example, the laser needs to withstand the adverse effects of high temperature and high humidity when it is used for the development and utilization of mineral resources in the deep Earth [10]. In addition, the laser used in outer space is not only subject to high and low temperature, but also radiation, which may lead to its failure [11]. Therefore, it is necessary to conduct novel studies on laser-induced damage and the mechanism of films under such circumstances.

Annealing is the most common high temperature post-treatment for optical films [12]. As for films prepared through physical vapor deposition (PVD), annealing under certain temperatures often increases the LIDT to some extent. Tan et al. reported that the LIDT of MgF₂ films increased from 5.21 J/cm² to 7.17 J/cm² (355 nm, 9.6 ns) after being deposited through electron beam evaporation under the annealing temperature of 473 K [13]. Zhang et al. prepared HfO₂ films through dual ion beam reactive sputtering and the LIDT increased from 3.96 J/cm² to 8.98 J/cm² (1064 nm, 12 ns) after annealing at 473 K [14]. However, apparently, annealing influence on the LIDT of films is unable to reflect the influence of high temperature in authentic environments (in situ high temperature). For instance, Xu et al. found that, to the films of ion beam sputtering, even though annealing could repair defects and attain higher LIDT, the LIDT decreased under in situ high temperature [15,16]. As for films prepared through the sol–gel method, the LIDT often went down even after annealing, contrary to what happened to PVD films [17]. Thus, the influence of authentic high temperature on the LIDT of sol–gel films must be more complicated. Currently, there are few studies and empty reports on such temperature effects on LIDT. Moreover, previous studies showed that the crystallinity of oxide films increased with the rise of temperature by different preparation methods [18,19,20]. The crystal phase transition temperature of Nb₂O₅ films often exceeds 673 K, which is much higher than that of HfO₂, ZrO₂, and TiO₂ films [21]. Once the phase transition occurs, it usually leads to the generation of various structural defects in the films, and then reduces the nanosecond laser damage threshold. Accordingly, the higher crystallization temperature of Nb₂O₅ may facilitate its use as a high LIDT film material in the laser system for high temperature environments.

In this study, Nb₂O₅ films were prepared with NbCl₅ as precursor and citric acid as additive, and the optical performance, microstructure, surface topographies, and chemical composition of the films after annealing under different temperatures were studied. Particularly, the LIDT under different in situ high temperatures of the as-deposited films was investigated and the mechanism of temperature effect on the LIDT was further revealed through both experimental results and theoretical calculations.

2. Experimental

2.1. Preparation of Nb₂O₅ Films

NbCl₅ (99.99%), ethanol (99.8%), and citric acid (99.5%) were purchased from Sigma-Aldrich. Nb₂O₅ sol was synthesized according to the previous method with some modifications [9]. In a typical synthesis, the molar ratio of NbCl₅, ethanol, citric acid, and water was 1:45:4:0.6. First, NbCl₅ was dissolved in ethanol and stirred in Ar atmosphere for 0.5 h. Then, citric acid and water were added to the above solution and stirred for 1 h. Finally, the solution was aged at 298 K for 6 days to form Nb₂O₅ sol. Before preparing the films, BK7 substrates were cleaned carefully by ultrasonic cleaning machine with acetone. Nb₂O₅ films were prepared by the dip-coating method using the above sol. The films were baked in air at 353 K for 8 min after each coating. The annealing occurred in air and the temperatures of as-deposited films increased to 373, 423, 473, and 523 K with heat preservation for 1 h.

2.2. Characterization

The viscosity of Nb₂O₅ sol was tested by a glass capillary viscometer. Nb₂O₅ film transmittance was characterized using a Lambda 900 spectrophotometer. The structural properties were measured by a D8 Advanced X-ray diffractometer (XRD). The surface topographies were analyzed by a Dimension V atomic force microscope (AFM). The chemical composition of the films was evaluated by a thermo ESCALAB 250 X-ray photoelectron spectroscope (XPS). The LIDT of the films was tested following the “1-on-1” regime according to ISO standard 11254-1, using a 1064 nm and 12 ns Nd:YAG laser in single longitudinal mode with up to a 5 Hz repetition rate [22]. The experimental setup of the in situ high temperature LIDT testing platform was shown schematically in [23]. The Q-switched Nd:YAG was focused to provide a far-field circular Gaussian beam with a diameter of 0.306 mm at 1/e² of the maximum intensity. The sample was placed inside a temperature-controlled chamber and was driven by a stepper motor. Before the laser damage test, the sample was heated to the set temperature and kept for 1 h. It is worth mentioning that the craft adopted in the heating and holding process was kept the same as that in annealing treatment in the experiment. The LIDT was defined as the incident pulse energy density when the damage occurred at 0% damage possibility. The damage morphologies after laser irradiation were estimated by a Sirion 200 field emission scanning electron microscope (FESEM).

3. Results and Discussion

3.1. Viscosity of the Sol

Figure 1 shows that the viscosity of Nb₂O₅ sol gradually increases with the aging time. It is due to the continuous hydrolysis and polycondensation reactions conducted inside the colloid, bringing about the transition from sol to gel. Hydrolysis (1) and polycondensation (2) of the colloid in the aging process are as follows:

-Nb-Cl + H₂O = -Nb-OH + HCl

(1)

-Nb-OH + HO-Nb- = -Nb-O-Nb- + H₂O

(2)

In the first 12 days, the viscosity has no apparent change, showing its excellent stability. The viscosity quickly increases when the aging time is longer than 12 days. This is mainly attributed to the fact that the network structure formed with hydrolysis and polycondensation decreases the colloid fluidity and increases its viscosity. Moreover, even after aging of 30 days, the viscosity only increases by 0.71 mm² s⁻¹ over the initial value.

3.2. Optical Properties of the Films

The transmittance of films before and after annealing under different temperatures is shown in Figure 2. As shown in the figure, high transmittance is kept after 373, 423, and 473 K annealing, compared to that of the as-deposited film. However, after annealing at 523 K, the transmittance decreases dramatically.

3.3. Microstructure of the Films

Figure 3 shows that the films annealed under different temperatures have no apparent diffraction peak. This indicates that the films are amorphous. Moreover, it is implied that the annealing has no impact on the crystalline structure of the films. This should be explained by the fact that these films are prepared at 298 K and annealed below 523 K, and yet the phase transition temperature of Nb₂O₅ films is mostly over 673 K [24]. It is well known that the increase of film crystallinity with the temperature rise is a general effect for many oxides, since it has been clearly observed in previous studies by various structure-sensitive methods [25,26]. Here, the amorphous Nb₂O₅ structure is speculated to be favorable for obtaining high LIDT. This can be explained by the often-accompanied grain growth and structural defects in the process of preparing films or post-annealing to obtain crystalline structure. These defects will increase the absorption and decrease the LIDT of the films.

3.4. Surface Topographies of the Films

The AFM patterns of films are shown in Figure 4. The surfaces of the as-deposited film and films after annealing at 373, 423, and 473 K are plain with RMS roughness of 0.42, 0.45, 0.47, and 0.51 nm, respectively. This is because low annealing temperature enables the slow evaporation of internal residual organics, which has little effect on the network structure of the films. When the temperature further goes up to 523 K, as shown in Figure 4e, the film surface begins to protrude and roughness increases with RMS of 0.58 nm. This is attributed to the fact that the rapid evaporation or decomposition of organics under high temperature leads to a certain degree of destruction to internal or even surface structure of films.

3.5. XPS Results of the Films

As shown in the XPS spectra of Nb 3d in Figure 5a, there is a representative bimodal structure, and the binding energies of Nd 3d_5/2 and Nd 3d_3/2 are 207.7 and 210.4 eV, respectively. After annealing at 523 K, positions of these peaks shift to the right slightly. Figure 5b demonstrates that there are peaks of Nb-O at 530.8 eV, -OC at 532.3 eV, and -OH at 534.4 eV, respectively, after the peak split of O 1s. Moreover, after annealing, the proportion of Nb-O-Nb grows, while that of Nb-OH representing organic matter falls. This indicates that the residual organics in the film decrease after high temperature annealing. According to the energy difference, Δ(O-Nb) = BE(O 1s) − BE(Nb 3d_5/2) = 323.1 eV, all of the Nb in the films should be of +5 valence [27].

Table 1. XPS results of the element content on the films.

Samples	Elementary Composition (%)			Atom Ratio
	Nb 3d	O 1s	C 1s	Ototal/Nb	ONb-O/Nb
As-deposited	7.40	41.38	51.22	5.59	2.51
523 K annealing	12.24	42.14	45.62	3.44	2.02

shows the changes of Nb, O, and C contents in the films before and after annealing at 523 K. The O_Nb-O/Nb molar ratio estimated from the XPS peak areas with their relative sensitivity factors decreases to 2.02 from 2.51, indicating the generating of oxygen vacancies after annealing. In fact, such oxygen vacancies are always seen as substoichiometric defects, which are very easy to generate in PVD films due to the high temperature deposition. Our previous results indicated that oxygen vacancies were the most serious defects to decrease the LIDT of oxide films [28].

3.6. LIDT Results of the Films

Figure 6 is the LIDT under different in situ high temperatures of the as-deposited films. The LIDT of the film at 293 K is 24.8 J/cm². At the temperatures of 373, 423, and 473 K, the LIDT of films is 21.8, 20.4 and 19.4 J/cm², respectively. As shown, despite the LIDT gradually decreasing with the increase of temperature, it slowly declines by 12.1%, 17.7%, and 21.8% at 373, 423, and 473 K compared to at 293 K. When the temperature reaches 523 K, the LIDT decreases obviously to 14.1 J/cm², 43.1% lower than at 293 K.

3.7. Damage Morphologies of the Films

Figure 7 shows the typical laser damage morphologies of films at different in situ temperatures, where Figure 7a,c,e present the damage morphologies at 293, 423, and 523 K, respectively. It indicates that the damage of the films at different in situ temperatures is typical defect-induced damage [29]. That is, one or multiple defects are present in the center of the damaged spot, while the damage is centered on the defects and gradually expands into a circular area around. The enlarged images of the centers of damaged areas A, B, and C are shown in Figure 7b,d,f. As is shown in Figure 7d, when the temperature reaches 423 K, the damage is more apparent than that at 293 K in Figure 7b, which is attributed to the volatilization of organics at high temperature. However, when the temperature further increases to 523 K, the damage morphology becomes more interesting, as is shown in Figure 7e,f. Here, it should be pointed out that the damage morphologies in Figure 7a,c show many clear rings, which can be attributed to the explosion of residual organics in the film under laser irradiation. However, Figure 7e shows a small damage area, and it is difficult to see such multiple circular images. This may be due to the fact that at higher temperature, the organics in the film are almost completely evaporated.

4. Discussions

In this study, the effects of high temperature on the performance of sol–gel Nb₂O₅ films were studied. It was found that the temperature below 473 K has little effect on the optical transmittance, microstructure, and surface topographies, indicating that the prepared Nb₂O₅ films have excellent high-temperature resistance. It is worth noting that when the temperature rises to 523 K, the optical transmittance and surface topography of the film decrease, especially the variation of the former is more apparent. This decline in optical transmittance is mainly based on the following factors: (i) the increase of temperature leads to the accelerated evaporation of organic matters such as the residual ethanol and citric acid in the film, resulting in internal defects falling down its transmittance; (ii) the high temperature greatly increases the surface roughness of the film (as shown in Figure 4), which enhances the scattering and reduces the transmission; (iii) the oxidation of organics under high temperature brings about oxygen loss (as shown in Figure 5) or even slight carbonization of films, which also significantly decreases the transmittance.

In addition, the LIDT decreases with the increase of temperature. It is noted that the defect is a crucial factor leading to the nanosecond laser damage of thin films [30]. When irradiated by laser, the energy is more easily absorbed by the defect, which usually has a larger absorption coefficient. Consider a spherical defect with radius a embedded in an infinite uniform film, where heat is generated in the sphere at the constant rate A during the time 0 < t < t_p. The temperature changes of the defect and the film are in accordance with the heating diffusion equation

$$\{\begin{array}{l}\frac{1}{{\kappa }_p}\frac{\partial T_p}{\partial t}=\frac{1}{r^2}\frac{\partial }{\partial r}\left(r^2\frac{\partial T_p}{\partial t}\right)+\frac{A}{K_p}, 0\le r<a,t>0\\ \frac{1}{{\kappa }_m}\frac{\partial T_m}{\partial t}=\frac{1}{r^2}\frac{\partial }{\partial r}\left(r^2\frac{\partial T_m}{\partial t}\right), r>a,t>0\end{array}$$

(3)

when 0 < t < t_p (t is time, and t_p is the pulse length of the laser), A = 3QI/4πa³ and when t >t_p, A = 0. Here, the p and m denote the defect and the film, respectively. T, K, κ, and I refer to the temperature, thermal conductivity, diffusivity, and laser power intensity, respectively. Q is the absorption efficiency factor, which can be calculated according to the Mie scattering theory [31]. The boundary conditions are (i) at t = 0, T_p = T_m = 0; (ii) when r = a, T_p = T_m and $K_p\left(\partial T_p/\partial r\right)=K_m\left(\partial T_m/\partial r\right)$; and (iii) T_p finite as $r\to 0$ and T_m finite as $r\to \infty$. The final temperatures of the defect and the film are shown as [32].

$$\{\begin{array}{l}{T}_p=\frac{a^{2}A}{K_p}\{\frac{1}{3}\frac{K_p}{K_m}+\frac{1}{6}(1-\frac{r^2}{a^2})-\\ \frac{2ab}{r\pi }{\displaystyle {\int }_0^{\infty }e(\frac{-y^2{t}_p}{\gamma })}\frac{(\sin y-y\cos y)(\sin (ry/a)dy}{y^2\left[{(c\sin y-y\cos y)}^2+b^2{y}^2{\sin }^{2}y\right]}\}, 0<r<a\\ T_m=\frac{a^{3}A}{r{K}_p}\{\frac{1}{3}\frac{K_p}{K_m}-\frac{2}{\pi }{\displaystyle {\int }_0^{\infty }e(\frac{-y^2{t}_p}{\gamma }).}\\ \frac{(\sin y-y\cos y)\left[by\sin y\cos \sigma y-(c\sin y-y\cos y)\sin \sigma y)\right]dy}{y^2\left[{(c\sin y-y\cos y)}^2+b^2{y}^2{\sin }^{2}y\right]}\}, r>a\end{array}$$

(4)

where $b=(K_m/K_p)\sqrt{{\kappa }_p/{\kappa }_m}$, $\gamma =a^2/{\kappa }_p,c=1-K_m/K_p$, and $\sigma =(r/a-1)\sqrt{{\kappa }_p/{\kappa }_m}$.

Since it is difficult to obtain accurate parameters of the oxygen vacancy, an extreme circumstance, NbO₂ as the oxygen vacancy defect in the films is adopted. In addition, we have made the following assumptions: (i) the physical parameters only change with the initial temperature and remain unchanged during the damage process; (ii) the defects radiuses under different temperatures are obtained from

$$a=a_0(1+\alpha )$$

(5)

where a₀ is the defect radius (293 K), and α is the linear expansion coefficient; (iii) the thermal conductivity data of Nb₂O₅ and NbO₂ at different temperatures are fitted with an approximate formula [33]:

$$k=c\frac{1}{T}$$

(6)

where c is a constant and can be calculated by the thermal conductivity at 293 K. The physical parameters at different temperatures are shown in

Table 2. Parameters of materials under different temperatures.

Materials	$\mathit{k}$ (W/cm/K)	$\mathit{\rho }$ (g/cm3)	$\mathit{C}$ (J/g/K)	$\mathit{n}$ (at 1064 nm)	$\mathit{\alpha }$ (K−1)
Nb2O5-293 K	1.00 × 10−2	4.6	0.496	2.171	4.8 × 10−6
Nb2O5-373 K	7.86 × 10−3	~4.6	0.544	~2.171	~4.8 × 10−6
Nb2O5-423 K	6.93 × 10−3	~4.6	0.568	~2.171	~4.8 × 10−6
Nb2O5-473 K	6.19 × 10−3	~4.6	0.584	~2.171	~4.8 × 10−6
Nb2O5-523 K	5.60 × 10−3	~4.6	0.597	~2.171	~4.8 × 10−6
NbO2-293 K	3.30 × 10−2	5.9	0.458	2.020	7 × 10−6
NbO2-373 K	2.59 × 10−2	~5.9	0.507	~2.020	~7 × 10−6
NbO2-423 K	2.29 × 10−2	~5.9	0.532	~2.020	~7 × 10−6
NbO2-473 K	2.04 × 10−2	~5.9	0.552	~2.020	~7 × 10−6
NbO2-523 K	1.85 × 10−2	~5.9	0.570	~2.020	~7 × 10−6

[34,35].

Suppose the radius of NbO₂ defect is 5 nm, and the energy of laser irradiation is 25 J/cm². After laser irradiation under different in situ high temperatures, according to Equation (4), the calculated temperature variation is shown in Figure 8. The results indicate that the increase of the maximum temperatures of the defect under different in situ high temperatures is much higher than that of the initial temperatures. For instance, when the defect is irradiated at 293 K, the maximum temperature is 905 K. In contrast, when the defect is at the temperature of 373 K, the maximum temperature is 1152 K. The difference between the two maximum temperatures is 247 K, three times higher than the temperature difference between the two environments of 80 K. When the damage temperature is at 523 K, the maximum temperature reaches up to 1615 K, 710 K higher than that of the defect damaged at 293 K. Therefore, not only the increase of oxygen, but also the sharp growth of defects maximum temperatures from the increase of in situ temperatures is a significant reason for the LIDT decline. Moreover, it is a remarkable fact that the excellent laser damage resistance of sol–gel film also benefits from its regular three-dimensional network structure. Consequently, the LIDT reduction is also due to the destruction of the internal network structure of the film at high temperature.

5. Conclusions

In summary, Nb₂O₅ films were prepared using sol–gel method, and the LIDT under different in situ high temperatures was explored. The results show that the sol is stable and the as-deposited film has high transmittance and smooth surface morphology. With the annealing temperature increases, the film transmittance gradually falls, surface morphologies become rough, and oxygen vacancies appear. At 293 K, the LIDT is the highest up to 24.8 J/cm². The increase of temperature leads to the decrease of the LIDT of films. It is attributed to the following three aspects: (a) oxygen vacancies appearing in the films; (b) the growth of in situ temperatures resulting in the sharp increase of maximum temperatures of defects after laser irradiation; and (c) the regular network structure in the films being destroyed. Additionally, even though the LIDT under 523 K goes down 43.1% compared to that at 293 K, it is still high up to 14.1 J/cm², indicating the potential application for a high-power laser system under high temperatures in the future.