Influence of generation control of the magnetron plasma on structure and properties of copper nitride layers

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Influence Of Generation Control Of The Magnetron Plasma On Structure And Properties Of Copper Nitride Layers

Katarzyna Nowakowska - Langier ConceptualizationMethodologyFormal analysisInvestigationResourcesData Cura Lukasz Skowronski , Rafa l Chodun , Sebastian Okrasa , Grzegorz W. Strzelecki MethodologyInvestigation , Magdalena Wilczopolska MethodologyInvestigationData Curation , Bartosz Wicher , Robert Mirowski , Krzysztof Zdunek PII: DOI: Reference:

S0040-6090(19)30757-6 https://doi.org/10.1016/j.tsf.2019.137731 TSF 137731

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Thin Solid Films

Received date: Revised date: Accepted date:

7 February 2019 28 November 2019 29 November 2019

Please cite this article as: Katarzyna Nowakowska - Langier ConceptualizationMethodologyFormal analysisInvestig Lukasz Skowronski , Rafa l Chodun , Sebastian Okrasa , Grzegorz W. Strzelecki MethodologyInvestigation , Magdalena Wilczopolska MethodologyInvestigationData Curation , Bartosz Wicher , Robert Mirowski , Krzysztof Zdunek , Influence Of Generation Control Of The Magnetron Plasma On Structure And Properties Of Copper Nitride Layers, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137731

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Highlights    

Synthesis process with use different frequency modulation Modulating frequency of power has strong impact on the synthesis process Modulating frequency, allows ability to control of plasma generation Control of structure and properties of the layers

Influence Of Generation Control Of The Magnetron Plasma On Structure And Properties Of Copper Nitride Layers Katarzyna Nowakowska - Langier1, Lukasz Skowronski2 Rafał Chodun3, Sebastian Okrasa3, Grzegorz W. Strzelecki1, Magdalena Wilczopolska1 Bartosz Wicher3 Robert Mirowski1 Krzysztof Zdunek3,

1

2

k.nowakowska - [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]

National Centre for Nuclear Research (NCBJ) , Material Physics Department, Andrzeja Soltana 7, 05 - 400 Otwock - Swierk, Poland

Institute of Mathematics and Physics, UTP University of Science and Technology, Al. prof. S. Kaliskiego 7, 85 - 796 Bydgoszcz, Poland 3

Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141 02 - 507 Warsaw, Poland

Corresponding Author: Katarzyna NOWAKOWSKA - LANGIER Material Physics Department, National Centre for Nuclear Research, A. Soltana 7,05 - 400 Otwock - Swierk, Poland Telephone: +48 22 718 04 46, Fax: +48 22 779 34 81 E - mail: katarzyna.nowakowska - [email protected]

Abstract This paper describes one of the series of works that aimed to investigate the impact of modulation frequency. This parameter is specific to plasma excitation and is associated with the properties of the thin films of copper nitride synthesized by magnetron sputtering. The studies reported in this paper focus on the changes in chemical and phase compositions of the Cu-N layers with respect to their electronic properties. The measurements obtained allowed studying the phenomena that occur during the synthesis of the metastable copper nitride. The results revealed that the synthesis processes are very sensitive to changes in sputtering parameters. It was also found that the modulation of sputtering frequency and the power influence the structure and consequently the properties of the synthesized layers. Keywords: metastable material; copper nitride layer; pulsed magnetron sputtering; frequency modulation; electronic properties 1. Introduction Under the conditions of variable frequency modulation, the process of pulsed magnetron sputtering (PMS) can control the synthesis process, and thus the phase composition of the layers [e.g. 1]. Synthesis conditions are determined by the change in power and the adjustment of power density through controlling a number of plasma pulses over time. Consequently, the assessment of the effect produced and understanding the dependence of such synthesis conditions are the main technological aspects focused in a series of works published by our research team and are also the subject of our current research [1–5]. The obtained results showed that by changing the frequency modulation, one can control the phase composition and the morphology of the layers. Such dependencies were noticed during the synthesis of different materials [1,2,4,6,7]. Various frequency modulation parameters have a great influence on the obtained layer material and are therefore considered additional technological parameters of plasma excitation. Copper nitride layers are the subject of the present work. These layers can be synthesized as materials with specific phases ranging from a stoichiometric Cu3N structure up to a two-phase ―composite‖ structure (e.g. Cu3N Cu3N (Cu) or Cu3N Cu [1]). In this work, which was performed based on the results of phase composition observed in our previous experiment, we examined the electronic properties of copper nitride layers from the point of view of the eventual future applications.

The fundamental idea of our research is gaining a deeper understanding of the phenomena occurring during the synthesis of metastable copper nitride under the condition of PMS synthesis. Therefore, the use of properties for describing the obtained structure of copper nitride layer can be considered an additional tool for tracking the effects of subtle material, which are often not identified using standard methods of material characterization. In this paper, we investigate the optical properties of various forms of Cu-N layer obtained by the PMS method. All the processes were carried out under synthesis conditions that assure that the materials formed possess different chemical and phase compositions. The main aim of our study was to determine the dependence between the electrical properties of Cu-N layers and their chemical and phase composition as well as the stoichiometry of the Cu3N phase.

2. Experimental part The Cu-N layers investigated in this work were synthesized using the PMS method [1,5]. The magnetron was powered by a pulse power supply operating at a frequency of 100 kHz and additionally modulated by a frequency (fmod) = of 10, 250, or 1000 Hz. Power was set to 150, 300, and 500 W during the synthesis processes. The specificity of the process can be explained by the characteristic dependencies that accompany the process of synthesis of the material layers as shown in Figure 1. This figure presents the specificity of the plasma generation process using the PMS method, in which a schematic juxtaposition of voltage characteristics over time for different frequency modulations (fmod) used in the synthesis of the Cu-N layer is shown. The use of a variable modulating frequency affects the lifetime (duration) of a single plasma pulse as well as the frequency of generation of the subsequent pulses. On the other hand, an increase in power directly complicates the additional extension of the duration of a single plasma pulse while maintaining the intervals of the fixed values of repetition frequency. A magnetron target of 50 mm in diameter was made of pure copper (99.99%). Layers were deposited on nonheated silicon substrates that were located parallel to the target at a distance of 100 mm. The substrates were ultrasonically cleaned in acetone before being mounted in the vacuum chamber. The chamber was pumped by a diffusion and rotary pump. All the processes were carried out under a nitrogen atmosphere at a constant pressure of 1 Pa. The time taken for the synthesis of layers was 60 min. More details on the conditions for the synthesis of layers obtained with such sets of process parameters are given in our previous work [1]. Ellipsometric measurements were conducted in the ultraviolet– visible–near-infrared (NIR) spectral range (0.62–6.5 eV; 191–2000 nm) using the V-VASE device obtained from J.A. Woollam Co., Inc. Ψ and Δ azimuths were recorded for three angles of incidence: 65, 70, and 75. Measurements and calculations were performed using a standard procedure [6]. The Cu-N layers were structurally characterized by X-ray diffraction using Cu–K radiation and by Raman spectroscopy; X-ray diffractograms of the Cu-N layers are not presented in this paper. A description of the phase composition can be found in Table 1. In this contribution, we recall the results from our previous article [1] because they are crucial for the relationships presented and described here. Additional analysis of the structure and phase composition was performed by high-resolution transmission electron microscopy using the FEI Titan 80-300 Cubed microscope and the Helios-NanoLab 600 dual-beam scanning electron microscope which was used for preparing the focused ion beam sample. Vibrational spectroscopy investigation was carried out using the Jasco NRS 5100 Raman

spectrometer working in backscattering mode using a visible radiation of 2.33 eV (532 nm). Ar+ laser was used as a source of 532-nm light. The chemical composition (at.%) of the Cu-N layers was analyzed by an energy-dispersive X-ray (EDS) analyzer attached to a scanning electron microscope (Zeiss, 20 kV, working distance 10 cm).

3. Results and discussion Changing the power and the modulating frequency [1,5,7] allowed controlling the synthesis of the Cu-N layers differing in chemical and/or phase composition. As confirmed by our previous work [8], the final product was very sensitive to changes in these parameters. Table 1 summarizes the structural properties of the Cu-N layers obtained using the PMS method with various frequency modulations. The phase composition of the Cu-N layers obtained at a low power (150 W) for each fmod used (10, 250, and 1000 Hz) was studied using the same single-phase structure of polycrystalline copper nitride (Cu3N) with the lattice parameters corresponding to the literature values, despite slight differences in the chemical composition (JCPDS 47-1088). It was believed that the presence of additional Cu atoms in the Cu3N structure (due to the increase in Cu content) leads to an increase in the lattice parameters [9– 11]. This phenomenon was clearly observed in the case of Cu-N layers obtained at the higher power. The increase in copper supersaturation was also observed with a decrease of fmod. In the case of the layer obtained at a modulation frequency of 10 Hz, in addition to the coppersaturated nitride (Cu3N (↑Cu)), we observed pure copper (Cu). Then, a multiphasic structure was obtained, as was observed with the lower power in contrast to the structures obtained at the frequencies of 250 and 1000 Hz (Table 1, Figure 2 and Figure 3). Due to a large amount of copper, the Cu-N layers obtained at a power of 500 W were composed of supersaturated Cu3N (↑Cu) and pure copper phases. This result was also confirmed by the data obtained using Raman spectroscopy and is also shown in Figure 2 which presents the Raman spectroscopy spectra as a function of power and frequency modulation. The band located at 635 cm-1 in the Raman spectra represents the stoichiometric phase of Cu3N [12]. Power and frequency modulation influence the band position shifting it toward lower values. This phenomenon is probably connected with changes in the structure of the material (supersaturation  participation effects) [3,13]. It is worth noticing that the sensitivity of the Raman spectroscopy technique allowed identifying subtle differences in the material, which were undetectable with X-ray measurements. In conclusion, the obtained data suggest that the shift of Raman bands is related to changes in the process parameters which may indicate a progressive disruption of the structure (supersaturation of material in the case of copper addition). Figure 3 shows the influence of the parameters of the synthesis process on changes in the phase composition of the layers, which are expressed by changes in the lattice parameters, in comparison with nitrogen content (extracted from the results of the EDS analysis). In the case of the low power, the nitrogen atoms absorbed on the surface were

sufficient for reacting with Cu atoms resulting in the formation of Cu-N bonds of high density for preferential growth along the (100) direction. Due to the further increase of power and thus more intensive sputtering of copper, the proportion of nitrogen in the chemical composition of the layer decreased. Therefore, in these conditions, the concentration of nitrogen was insufficient for the synthesis of stoichiometric Cu3N. With the decrease of fmod and the increase in power, the lifetime of a single plasma pulse increased. We intensified the spraying of the copper target, which led to an increase in the proportion of copper atoms participating in reactions. Under such conditions, at a constant pressure of the reactive gas (nitrogen), the reactivity of the process decreased (too much copper to react with a given amount of nitrogen). Finally, these conditions increased the proportion of copper in the resulting material of layers. The obtained structure was characterized by supersaturation with copper (Cu3N (↑Cu)) or additionally by the presence of pure copper, at the cost of the Cu3N phase. The use of modulation frequency changes was also important in shaping the morphology and growth of layers [5]. The structural properties of the Cu-N material and the observed differences in phase composition as well as the other features of the synthesis product can be correlated with the electronic properties. The Cu-N layers obtained at 500 W were characterized by a large amount of copper in the form of a pure phase (conductor); therefore, only the layers obtained at the lower power (150 and 300 W) were subjected to the detailed investigation of the optical properties. The optical properties of the synthesized Cu-N layers were predicted from ellipsometric measurements. The complex dielectric function ( ̃) and the complex refractive index ( ̃) were related using the following relation: ̃

̃

.

(2)

where n and k are the refractive index and extinction coefficient, respectively.

To calculate the optical constants and the thickness of the films, a five-medium optical model of the sample was used (ambient/rough layer/Cu-N layer/native SiO2 film/Si-substrate). The optical constants of Si and SiO2 were noted from the database [14]. The complex dielectric function of the Cu-N films was parameterized using the Drude term and a sum of Lorentzianand Gaussian-type oscillators [14,15]. The rough layer was described by Bruggeman Effective Medium Approximation [14,15] with the fractions of Cu-N and ambient equal to 0.5.

An example of Ψ and Δ azimuths (both experimental and calculated) is presented in Figure 4. The

value for the fit (S_150W_10Hz sample) was 2.3, while for the other samples, the

value ranged from 1.7 to 5.9. The refractive index (n) and extinction coefficient (k) of the synthesized Cu-N films can be found in Figure 5. The shape of n and k were directly associated with the phase composition of the copper-nitride layers. For the samples deposited at P=150 W (Figure 5a and Figure 5b), the optical constants exhibited two maxima at 2.6 and 4.3 eV which were characteristic of Cu3N [9,16]. However, for photon energies lower than 2.5 eV, the shape of n and k spectra differed significantly. The behavior of the extinction coefficient (and the refractive index according to the Kramers–Kronig relations) is dependent on the type of material. For the sample S_150W_1000Hz, k values decreased with the decrease of the photon energy. A similar trend was observed for S_150W_250Hz; however, when the photon energies were lower than 0.82 eV, the extinction coefficient was found to increase gently. A diametrically different shape was observed for the sample deposited at the lowest modulation frequency (fmod=10 Hz), in which a strong increase of k value was visible. These three different behaviors of extinction coefficient in the NIR (and partly visible) spectral range were directly associated with three different categories of material: metallic, S_150W_10Hz; intermediate (semiconducting with small, however measurable, absorption caused by free carriers), S_150W_250Hz; and semiconducting, S_150W_1000Hz. All the samples synthesized at the higher effective power (P=300 W) exhibited absorption of free carriers (an increase of k with a decrease of the photon energy in the NIR spectral range; Figure 5d). The optical constants of the layers deposited at the higher modulation frequency (250 and 1000 Hz) exhibited the characteristic maxima at 2.6 and 4.3 eV. The data obtained for sample S_300W_10Hz (Figure 5c and Figure 5d) were significantly different from those observed for the other samples. This can be explained by the differences in the phase composition of this layer. Sample S_300W_10Hz consisted of Cu3N, Cu-rich Cu3N, and Cu phases. Moreover, the coexistence of the last compound with copper-nitride phases led to the different shape of the optical constants of this film. Due to nonstoichiometry and/or the coexistence of different phases (Cu3N, Cu- or N-rich Cu3N, and Cu), free carriers led to NIR absorption in Cu-N. Therefore, the Drude term ( ̃ ) was taken into account as a part of the complex dielectric function ( ̃ ): ̃

.

(3)

where

,

, and E are the plasma energy, the free-carrier damping, and the photon energy,

respectively. Based on the determined Drude parameters ( (

and

), the optical resistivity

) and the relaxation time of free electrons ( ) were calculated using the following

relations: (4) and .

(5)

Plasma energy, free-carrier damping, optical resistivity, and relaxation time of free carriers are summarized in Table. The values of Drude parameters for the produced samples were found to strongly depend on the growth conditions. The values of

and

ranged from ~1

to ~6 eV and from ~0.7 to ~11 eV, respectively. It should be noted that the uncertainties observed in these parameters for the samples synthesized at P=300 W and the modulation frequencies (fmod) of 250 and 1000 Hz (S_300W_250Hz and S_300W_1000Hz) were high, and in the case of

, the uncertainty was even higher than the value of the parameter. This

was (most probably) associated with the strong correlation of

and

as well as the

relatively low contribution of the Drude term to the optical response of the material. Therefore, the values of the plasma energy and the free-carrier damping determined should be considered as approximate. Small uncertainties were found in the parameters

and

for

samples showing metallic-like features (the specimens deposited at fmod=10 Hz; S_150W_10Hz and S_300W_10Hz). During the fit procedure performed for sample S_150W_1000Hz, the values of Drude parameters were found to be zero. This indicated that the layer exhibited pure semiconducting behavior without any absorption caused by the free electrons. Optical resistivity and electrical resistivity are determined using different methods; however, the principle of these parameters is the same—interaction of the electric field with free carriers—and so they can be compared. The lowest values of

were obtained for metallic-

like Cu-N films (S_150W_10Hz: 472 μΩcm, S_300W_10Hz: 272 μΩcm) which were found to be in good agreement with the values estimated earlier for the Cu-rich layers [17,18]. For the other samples (except S_150W_10Hz), the uncertainty of

was relatively high, and

therefore, for these layers, the order of the magnitude of this value should be considered. The optical resistivity recorded for these samples at the level of 10–4–10-5 μΩcm was identified to be typical of Cu-N [17–19]. The calculated value of the mean time between collisions of free

electrons ( ; Table 2) confirmed the metallic-like behavior of the films deposited at fmod=10 Hz and the intermediate state of the other prepared samples (except S_150W_1000Hz). To estimate the optical band-gap energy based on the determined extinction coefficient (Figure 5b and Figure 5d), the absorption coefficient (α) was calculated (Figure 6) and the Tauc plot was constructed (Figure 6b). The metallic/semiconducting behavior of the Cu-N layers was clearly visible in the shape of α in the NIR spectral range. Relatively high values (2–3·105 cm-1) of the absorption coefficient were obtained for the metallic-like samples with low optical resistivity and high mean relaxation time of free electrons (S_150W_10Hz and S_150W_10Hz). For the specimens deposited at fmod=250 Hz, the values of α were in the range of 0.5–1·105 cm-1. For the layers synthesized at fmod=1000 Hz, relatively low values of the absorption coefficient were observed. For the photon energies higher than 2.5 eV, the shape of spectra was found to be dependent on interband transitions. The Tauc plot was constructed by plotting

against photon energy (

, where m is the parameter

associated with the type of transition: m=1/2 for direct allowed transition, m=3/2 for direct forbidden transition, m=2 for indirect allowed transition, and m=3 for indirect forbidden transition [20]. Values of α near the optical band gap at a level of 105 cm-1 suggested direct transition in Cu-N [21]; therefore, m=1/2 was chosen for this analysis. An example of the Tauc plot for sample S_300W_1000Hz is presented in Figure 6b. For the samples produced at P=150 W (S_150W_250Hz and S_150Hz_1000Hz), the optical band-gap energy (Eg) was 2.13 eV, while for the samples deposited at P=300 W, that is, S_300W_1000Hz and S_300W_250Hz, the value of Eg was 2.01 and 2.08 eV, respectively. It should be noted that the values of Eg were higher than those reported earlier by experimental [18,21] and theoretical calculations [9,13]. As for the energies lower than Eg, the absorption coefficient was higher than zero, so the value of Eg should be treated as the one above which a strong absorption occurs. Absorption below the optical band gap was found to be associated with heavily disordered or amorphous regions, indirect optical transitions [20], and/or free carriers with an excess of nitrogen [20]. For samples S_150W_10Hz and S_300W_10Hz, the optical band-gap energy was not determined because of their pronounced metallic-like behavior. The optical properties of the Cu-N layers determined showed the structural subtleties of the material. In the case of layers obtained at lower power, for each of the frequencies, the material was characterized by the same single-phase structure (Cu3N) [1], with slight differences in the chemical composition. In addition, these layers differed in their properties. These differences were most probably associated with the slight variations seen in the chemical composition in the entire volume of the layer and hence the local disturbances of the

Cu3N structure. This was also confirmed by the results of Raman spectroscopy (Figure 2), a sensitive measuring technique that allows registering various responses of a material to the given measurement conditions (experimental). Thus, we can conclude that the observed differences were caused by the subtle structural changes that were occurring at the atomic level and associated with the mechanism of growth of the layer under the given synthesis conditions. These changes were so small that they were not reflected in quantity (phase composition changes) but influenced the optical properties. The observed structural subtleties were most probably associated with the initial stages of structural changes in the material under study. As we mentioned in a previous paper [3], these may be local supersaturations related to copper grouping in the Cu3N matrix, whereas in the case of layers obtained at lower power (150 W), these included the changes at the initial stages as mentioned above. There were also changes and/or structural differences which, due to dimensional range (volume fraction), were not detected by X-ray diffraction. Figure 7 shows the morphology of the Cu-N layers obtained with different frequencies of modulations and their corresponding electron diffraction patterns. The layers consisted of nanometric particles. Additionally, their electron diffraction patterns showed that the material was composed of either nanocrystalline (Figure 7a) or amorphous–nanocrystalline phases depending on the frequency of modulations (Figure 7b). It should be noted that the results obtained earlier did not indicate the existence of an amorphous phase. Nevertheless, in the case of layer produced at the lower frequency of modulations, not only the crystalline phase but also the amorphous phase was found. This proves that the decrease in the frequency of modulations resulted in a lower probability of nucleation and crystallization processes during the growth of layers. Therefore, the optical measurements obtained for the Cu-N layer synthesized at the lowest modulation frequency were most likely due to the occurrence of an amorphous structure as a matrix for the nanocrystalline areas.

4. Conclusion Metastable copper nitride layers were synthesized using the PMS method. By changing the process parameters (power and fmod), we were able to synthesize layers with different chemical and/or phase composition, with the most stable structures obtained at the lowest power. Under these conditions, a single-phase structure of Cu3N was obtained for each frequency of modulation. The obtained results allowed correlating the given chemical and phase composition of layers with the determined electronic properties. Using electrical measurements, the structural subtleties observed in the material of the layers were associated with the changes, or rather the initial stages of changes, in the structure of the studied materials. These results confirmed our assumptions that the differences observed in the properties of the layers were dependent on the structure (nanocrystalline or amorphous– nanocrystalline) and small differences in chemical composition in the entire volume of layers. Thus, our study proved that we can control the structure and properties of layers by controlling the plasma generation using the PMS synthesis method.

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Figure captions Figure 1. Schematic view of the voltage characteristics that are typical of the PMS process showing the specificity of the layer synthesis. Figure 2. Raman spectra of the Cu-N layers prepared by the PMS method as a function of various processes parameters. Figure 3. Atomic percent concentration of nitrogen (EDS analysis) in the material of layers and lattice parameters of the Cu3N phase obtained at different fmod as a function of power. Figure 4. Experimental (markers) and calculated (solid and dashed lines) Ψ and Δ azimuths for sample S_150W_10Hz. To make the results more visible, every 10th measuring point is plotted for the experimental data. Figure 5. Refractive index (n) and extinction coefficient (k) of the Cu-N films produced at P=150 W (a and b) and P=300 W (c and d). Figure 6. (a) Absorption coefficient of the Cu-N layers. (b) Tauc plot for the sample synthesized at P=150 W and fmod=1000 Hz. The circles represent the points taken to extrapolate the linear region of the Tauc plot. Figure 7. High-resolution transmission electron microscopy images showing the corresponding electron diffraction patterns of the Cu-N layers obtained by the PMS method at different frequencies of modulation: fmod=1000 Hz (a) and fmod=10 Hz (b).

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Table 1. Frequency modulation (fmod), power (P), phase composition and thicknesses (d) as well as roughness (Ra) and value of chemical composition of Cu-N layers, at.%, (accuracy of microanalysis measurements ± 0.8 at%) Sample id.

P

fmod

N/Cu

Phase

[W]

[Hz]

[at.%]

composition

by EDS

by XRD in

Ra [nm]

d [nm]

RS [cm-1]

ref [1] S_150W_10Hz

150

10

24.3/75.7

Cu3N

1.3±0.5

450±1

627

S_150W_250Hz

150

250

25.6/74.4

Cu3N

6.2±0.1

627±1

630

S_150W_1000Hz

150

1000

24.7/75.3

Cu3N

2.9±0.2

396±1

635

S_300W_10Hz

300

10

20.7/79.3

Cu3N,

5.0±1.1

2100±34

620

Cu3N(↑Cu), Cu S_300W_250Hz

300

250

22.8/77.2

Cu3N(↑Cu)

6.4±1.4

669±16

620

S_300W_1000Hz

300

1000

24.7/ 75.3

Cu3N (Cu)

12.3±1.2 1026±71

622

S_500W_10Hz

500

10

14.7/85.3

Cu3N(↑Cu),

-

3150±20

618

-

1920±20

610

-

1350±20

618

Cu S_500W_250Hz

500

250

19.8/80.2

Cu3N(↑Cu) , Cu

S_500W_1000Hz

500

1000

18.9/ 81.1

Cu3N (↑Cu), Cu

↑Cu – rich in Cu

Table 2. Frequency modulation (fmod), power (P), Drude parameters (??_p, ??), optical resistivity (?_opt), mean time between collisions of electrons (?) and optical band-gap energy (Eg).

Sample id.

(eV)

(eV)

(

)

-(1)

3.79±0.02 0.913±0.010

S_150W_250Hz

1.06±0.29

1.80±1.01

11900±9300

S_150W_1000Hz

-(2)

-(2)

-(2)

-(2)

2.13±0.02

S_300W_10Hz

4.53±0.08

0.725±0.05

262±11

0.908±0.018

-(1)

S_300W_250Hz

5.90±3.04

9.63±9.91

2100±3000

0.068±0.070 2.08±0.02

11±16

2600±5600

0.062±0.095 2.01±0.02

(1)

Metallic layer

(2)

Semiconducting layer

0.721±0.008

Eg (eV)

S_150W_10Hz

S_300W_1000Hz 5.47±4.14

472±6

(fs)

0.365±0.204 2.13±0.03