Effect of lanthanum substrates on the structural, optical and electrical properties of copper selenide thin films designed for 5G technologies

In this work, copper selenide thin films coated onto glass and transparent lanthanum substrates are studied. The (glass, La)/CuSe thin films which are prepared by the thermal evaporation technique under a vacuum pressure of 10−5 mbar are structurally, morphologically, optically, dielectrically and electrically characterized. Lanthanum substrates improved the crystallinity by increasing the crystallite size and decreasing both of the microstrains and defect density of copper selenide. La substrates redshifts the energy band gap and doubled the dielectric constant values. In addition, employing Drude–Lorentz approaches for optical conduction to fit the dielectric constant provided information about the effects of La substrates on the drift mobility, plasmon frequency, free carrier density and scattering times at femtosecond level. The drift mobility increased and the plasmon frequency range is modified when La substrates are used. Verifying impedance spectroscopy tests in the microwave frequency domain have shown the ability of the La(gate)/CuSe/Ag (source) transistors performing as band pass filters. These filters are suitable for 5G technologies. The microwave cutoff frequency reached ~ 5.0 GHz at a notch frequency of 2.80 GHz of the glass/La/CuSe/Ag highpass filters.


Introduction
Copper selenide thin films have captured wide interest due to their applications as electrocatalysts, counter electrodes in solar cells and self-repairable electrodes (Hussain and Hussain 2020). They show novel thermoelectric properties and mentioned being suitable for flexible electronics (Hussain and Hussain 2020). CuSe films also find their location in other class of applications including photocatalysis and thermal phototherapy (Wang et al. 2017). In addition, studies which care about plasmonic interactions in copper selenide have shown their suitability for use in biomedical applications (Ai et al. 2021).
Various preparation methods including physical vapor deposition techniques have been employed to improve the performance of coper selenide thin films (Hussain and Hussain 2020;Abd-Elnaiem et al. 2021). Each preparation method has it is own properties. As for examples, CuSe thin films prepared by the chemical bath deposition technique was not adherents with the substrate at higher precursor concentration (Hussain and Hussain 2020). Films prepared by the electrodeposition technique provided an adherent and uniform films on the substrates but flexible substrate (polymeric) cannot be used in this method. On the other hand, the substrate used for deposition of CuSe films play an additional role on justifying the physical properties of the films. When polyvinyl chloride, polyvinyl alcohol, and paper is employed as substrates to grow CuSe films, the resulting films are found to be highly conductive. The achieved high conductivities remained for several months. The properties of these films were not affected by stretching, bending and folding (Hussain and Hussain 2020).
The applied preparation methods on various types of substrates for depositing copper selenide film motivated us to prepare copper thin films by the thermal evaporation technique onto conductive transparent rare earth lanthanum substrates. The thermal evaporation technique is employed because of its relative simplicity, low cost of the apparatus, higher deposition rates, and scalability (Abd-Elnaiem et al. 2021). Lanthanum is selected because it is highly transparent material in the spectral range of 4.0-1.0 eV, exhibiting the electronic shell configuration of 5d 1 6s 2 (Polyakov 2013). The orbital states of lanthanum can reach those of CuSe allowing easier electronic conduction and optical transitions. For this reason, here in this work, we will report the structural, morphological, optical, dielectric and electrical properties of copper selenide thin films coated onto glass and transparent lanthanum substrates of thickness of 150 nm. The effects of the La substrates on the lattice parameters, crystallite size, microstrain, stacking faults, optical transmittance, reflectance, absorption coefficient, energy band gap, dielectric constant and on the microwave band pass characteristics are reported.

Experimental details
Transparent lanthanum substrates of thicknesses of 150 nm were coated onto ultrasonically cleaned glass substrates in a NORM VCM-600 physical vapor deposition system. The vacuum pressure was kept at ~ 10 -5 mbar. The source material was lanthanum powders (99.99%, Alpha Aesar). The lanthanum substrates were then covered with a1.0 m thick copper selenide (99.99%, Alpha Aesar) using NORM VCM-600 physical vapor deposition system. Some of the CuSe samples were produced onto glass without lanthanum. Others are coated onto glass/La films of thicknesses of 150 nm and of thicknesses of 300-350 nm. The glass/La (300-350 nm)/CuSe films were masked with rectangular masks of areas of 0.0314 cm 2 to produce Ag metal pads on the top surface of CuSe layer. The thickness of La is increased from 150 to 300-350 nm for the purpose of more accurate electrical measurements. The thickness of the glass/La, glass/CuSe and glass/La/CuSe films were measured with the help of an in situ monitoring Inficon STM-2 thickness monitors. The X-ray diffraction patterns were recorded using MiniFlex 600 X-ray diffraction unit at a scanning speed of 0.5°/min. The optical transmittance and reflectance spectra were measured with the help of Thermoscientific Evolution 300 spectrophotometer. The impedance spectra were recorded with the help of Agilent 4291B 1.0 M-1.8 GHz impedance analyzer.

Results and discussion
Thin films of copper selenide which are coated onto glass and 150 nm thick transparent lanthanum substrates are shown in inset-1 of Fig. 1. It is clear from the optical images that the color of glass/CuSe thin films has changed from dark brown to light brown when the glass substrates are replaced by glass/La substrates. The change in the film color is due to interference of light waves at the glass/La/CuSe interfaces. The change in the color gives evidence of the possible changes in optical properties. Figure 1 also displays the recorded X-ray diffraction (XRD) patterns for copper selenide films in the presence and in the absence of lanthanum substrates. The XRD patterns which are analyzed with the help of "TREOR 92" software packages revealed cubic structure of CuSe. All the observed peaks are indexed for the cubic phase of CuSe (PDF card No: 01-071-4325). When La is inserted between glass and CuSe the calculated lattice constant of the cubic CuSe ( a = 5.697 Å) decreases to a = 5.656 Å. XRD patterns of (glass, La)/CuSe contained a peak centered at 2 = 24.45 • which are related to the orthorhombic phase of CuSe ((PDF card no: 2027-0184). In addition, the XRD patterns of CuSe coated onto lanthanum substrates displayed some peaks of hexagonal lanthanum ( a = 3.772 Å and c = 12.141 Å (Polyakov 2013)  Three-dimensional quantum confinement is capable of altering radically the nonlinear optical properties of semiconductors in the transparency regions (Cotter et al. 1992). On the other hand, as can be seen from inset-2 of Fig. 1, the maximum peak of the observed XRD patterns of CuSe shifts toward larger diffraction angles. The shift is associated with a decrease in the maximum peak intensity. This behavior which should have resulted from the large lattice mismatches between the two stacked layers (glass/La and CuSe) is an indication of lattice modification due to lanthanum incorporation. Particularly, the structural parameters (AlGarni and Qasrawi 2020) presented by the crystallite size ( D = 0.94λ cos ; ∶ maximum peak broadening), strain ( = 4tan ), staking faults ( SF% = 2 2 45 √ 3tan( ) ), dislocation density ( = 15ε aD ) and stress ( S = ;γ = 62 GPa (Namsani et al. 2017)) which are calculated and listed in Table 1 provide evidences about the structural modifications. As can be seen from the table, the crystallite size increased by 14.2% while the microstrain, the stress, the stacking faults and the defect density decreased by 14.0%, 14.0%, 14.8%, 24.9%, respectively. The changes in the structural parameters confirm the enhanced crystallinity due to La incorporation. Physically, the ionic radius ( r La ) of La +3 being 1.16 Å (Lee et al. 2016) cannot substitute copper (Cu +2 ; r Cu = 72 pm (Tian et al. 2021)) vacancy. Instead, it can make atomic interstitial substitutions. Thus, the defect density should have increased (Lee et al. 2016). However, as the La-La bond length is (3.918 Å (Arumugam et al. 2020)) larger than that of La-Se (3.123 Å (Guschlbauer et al. 2021)) by 25.4%, broken bonds of lanthanum prefer bonding with excess selenium atoms. It is reported that nanocrystals also have high surface energy due to the incomplete bonding at the surface. To accommodate the high surface energy, nanocrystals tend to aggregate (Bruck et al. 2016). Reduction of surface energy is believed to be the driving factor for grain (accumulation of crystallites) growth (Janney et al. 1991).
The effect of the lanthanum substrates on the stoichiometric composition and surface morphology of CuSe films can be read from Fig. 2. The figure show large area of glass/ La/CuSe films being selected for energy dispersive X-ray spectroscopic (EDS) analysis. Over many selected regions of different samples being prepared in the same and/ or sequential growth cycles, one may see that glass/CuSe (inset-1 of Fig. 2) and glass/ La/CuSe films exhibit variations in the compositional stoichiometry. Namely, while films coated onto glass substrates have tendencies to form wire like ( 1200 × 125 (nm) 2 ) Cu 0.86 Se grains impeded in an amorphous sea of Cu 1.19 Se, those coated onto glass/ La substrates prefer forming rectangular grains of sizes of 500 nm. Some of the white colored grains exhibit sizes of ~ 1.0µm. The EDS test for the rectangular grains predicts the formation of Cu 1.68 Se. In other words, lanthanum induced the formation of Cu deficient Cu 2 Se. Although films coated onto glass substrates prefer the formation of CuSe, those coated onto lanthanum prefer formation of deficient Cu 2 Se. Recent works which targeted studying the atomic mechanism of ionic confinement in copper selenide have shown that Cu 2 Se tend to retain non-fast ionic conducting phase in the grain boundaries in the presence of bismuth as interstitial doping agent . It is reported that under conditions where the interstitial doping agent blocks the migration of Cu ions, barriers inside the Cu 2 Se grains and interfacial phases in the boundaries of Cu 2 Se effectively split a large Cu 2 Se grain into a number of small domains leading to the loss of Cu . It is also stated that the atomic-scale doping in the core of the defects stabilizes the defects and behave as barriers inside the Cu 2 Se grain. Hence, the atomic-scale doping prompts the formation of the defective interfacial phase in the grain boundaries ) as we also observed from the XRD analyses.
The effect of La substrates on the optical transmittance ( T ), reflectance ( R ) and absorption coefficient ( ) spectra is evident from Fig. 3a-c, respectively. As Fig. 3a illustrates, the sharp transmission is initiated as the incident photon energy ( E ) become lower than 2.60 eV. T spectra of glass/CuSe exhibit maxima of 32% at E = 1.82 eV. The transmittance spectra redshifts exhibiting maxima of 27% at 1.68 eV as La replace glass. In addition, as illustrated in Fig. 3b, the reflectively of glass/CuSe films is relatively low ( R max. < 4% ). Coating of CuSe onto La substrates increase R values in all the studied range of light. The maximum reflectance value being 11% is observed at E = 1.14 eV. On the other hand, the absorption coefficient (Fig. 3c) for films of thicknesses of tis calculated with the help of the equation (Pankove 1975), As can be seen from Fig. 3c, copper selenide thin films which are coated onto glass substrates exhibited strong absorption in the incident photon energy ranges of 4.0-3.15 eV and 2.80-2.10 eV. In the range of 3.15-2.80 eV, spectra of glass/CuSe display absorption saturation. The absorption saturation usually originates from the band filling and the carrier screening by the excited carriers (Qasrawi and Omareya 2019). Replacement of glass by lanthanum substrates extended the absorption saturation range (4.0-2.6 eV) and forces disappearance of strong absorption in the high energy range (4.0-3.15 eV). In the low energy range (1.9-1.1 eV), both of the glass/CuSe and glass/La/CuSe displayed increased values of with decreasing incident photon energy. The increase in the absorption coefficient values with decreasing incident photon energies is ascribed to the free carrier absorption in the films. Free carrier absorption phenomena were also observed for La/Cd 2 S 3 thin films (Qasrawi and Omareya 2019). The phenomena are assigned to the ability of long wavelengths (IR range) to reach the lattice and motivates phonon excitations. Excited phonons set charge carriers free, allowing them to recombine with holes forming an additional polarized electron-hole pairs (Qasrawi and Omareya 2019). The nonstoichiometric composition of copper selenide (our EDS results) which result in high order degeneracy could also account for the free carrier absorption in CuSe (Chakrabarti and Laughlin 1981). To get information about the changes in the optical transitions that resulted from replacement of glass by lanthanum substrates, the energy band gap ( E g ) is calculated with the help of Tauc's equation ( E) 2 ∝ E − E g (Pankove 1975; Qasrawi and Omareya 2019). As illustrated in Fig. 3d, the linear plots of the ( E) 2 − E variations crosses the E-axis at E = E g = 2.30 eV and E = E g = 2.23 eV, for glass/CuSe and glass/La/CuSe films, respectively. The obtained energy band gap of glass/CuSe is consistent with literature data (Barman et al. 2021). The redshift in the energy band gap is ascribed to the orbital over lapping. From the electronic configuration point of view, the electronic distribution of the Cu and Se atoms reach 3d 10 4s 1 and 3d 10 4s 2 4p 4 , respectively. On the other hand, the electronic configuration of La metal reaches 5d 1 6s 2 . The lanthanum's higher states make the overlapping of the La orbitals with that of Cu and Se strongly preferable. In addition, the Schottky nature of glass/La/CuSe interfaces forces the flow of holes from p-type CuSe (verified by hot-probe technique) to La metal causing a band bending due to energy barrier height formation (Sze et al. 2021). Another reason for the band gap shrinkage in the presence of metallic substrates is the image force lowering. The image charges lowers the potential barrier to the flow of holes leading to the up bending of the valance band (Sze et al. 2021).
The effect of Lanthanum substrates on the dielectric properties of copper selenide thin films is readable from Fig. 4a, b. The real ( r ) and imaginary ( im ) parts of the dielectric constant are calculated from the equations (Qasrawi and Omareya 2019), In accordance with Fig. 4a, the real part of the dielectric constant of glass/CuSe display broaden peaks at critical energy values of 2.30 and 1.82 eV. Replacing glass by lanthanum substrates leads to the appearance of peaks at 2.59 and 1.99 eV. In addition, the value of r is increased by about two times. r − E variations also follow faster decaying trends in the IR range as a result of insertion of lanthanum between glass and CuSe. The critical energy values being 2.30 eV observed in r spectra of glass/CuSe is assigned to the direct allowed transitions within the band gap of CuSe. The critical energy value centered at 1.82 eV is assigned to transitions from the valence band maxima centered at R 3,4v of the first Brillouin zone of selenium to the conduction band minima centered at H 6c of the first Brillouin zone of Se (Råsander et al. 2013), respectively. On the other hand, the strong interaction between La and Se seems to be a dominant oscillator in the real part of the dielectric spectra. Namely, the critical energy value being 2.59 eV which appeared in r spectra of glass/ La/CuSe corresponds to the direct allowed transitions energy band gap of La 2 Se 3 (Bagde et al. 2003). The critical energy centered at 1.99 eV is also assigned to optical transitions in selenium (Solieman and Abu-Sehly 2010). Figure 4b display the imaginary part of the dielectric constant. The imaginary part is directly proportional with the optical conductivity ( ̌(w) = im w 4 ;w = 2 c∕ ). As seen from the figure, except for the ultraviolet range of light, no remarkable change in the value of the imaginary part can be detected. In the spectral range of 4.0-3.16 eV, glass/La/CuSe is less conductive than that of glass/CuSe. Slight enhancement in the imaginary part value can be observed in the IR range of light. Modeling the imaginary part of dielectric spectra in accordance with the Drude-Lorentz approach (Pankove 1975;Qasrawi and Omareya 2019) allowed determining the optical conductivity parameters. The dispersion relation of the imaginary part of dielectric constant takes the form, In Eq. (6), w is the radial frequency of light signals, w pe = √ 4 Pe 2 ∕m * is the plasmon frequency, W ei is the charge carrier-plasmon coupled oscillator frequency, i is the average scattering time at femtosecond level, P is the free charge carrier density and is the reduced effective mass of the glass/La/CuSe system. Insert- ing the values of m * CuSe = 0.5 m o (Alharbi and Qasrawi 2019) and m * La = 1.0 m o (Qasrawi and Omareya 2019) for copper selenide and lanthanum, respectively, and executing the series up to k = 5 is sufficient to reproduce the experimental data. The theoretically estimated dielectric constant is plotted by circles in Fig. 4b. The fitting of the im spectra allowed determining the optical conductivity parameters which are shown in Table 2. As seen from the table, for the IR oscillator centered near E e = 1.1 eV, the free hole concentration decreases, the plasmon frequency decreases and the drift mobility increases upon replacing glass substrates with lanthanum. The enhancement in the mobility values is probably due to the completed bonding of dangling bonds of Se with lanthanum at the interface. In addition the easier electronic motion which originated from atomic overlapping could also account for the improved mobility values. The other oscillators which are dominant in the visible range of light (2.68 eV and 2.89 eV) display similar characteristics. Oscillators dominant in the ultraviolet range of light (3.93, 4.14 eV) show no remarkable change in the optical conductivity parameters. The scattering time at femtosecond level which is the inverse of the damping coefficient of the electronic frictional forces (Qasrawi and Omareya 2019) slightly increases for the first and second oscillators ( E e = 1.1 eV and 2.68 eV ). The increase in this value means less resistance to the charge carrier motion. Hence, drift mobility is increased.
One interesting feature which is worth of focusing, here, is the values of plasmon frequency. The plasmon frequency exhibit values in the range of 2.30-6.51 GHz. It indicates that signals of such frequencies can pass through glass/La/CuSe interfaces. Signals exhibiting values less than 2.30 GHz are rejected. To verify this belief we have tested the samples through constructing a La (gate)/CuSe/Ag (drain, source) transistor channels. An ac signal of low amplitude is imposed between the terminals of source (Ag) and gate (La). The resulting ac conductivity ( ), impedance ( Z ) and magnitude of reflection coefficient ( S 11 ;Z La∕Cu2Se = S 11 +1 S 11 −1 Z source (Pozar 2011)) spectra are shown in Fig. 5a-c, respectively. It is clear from Fig. 5 (a) that the higher the signal frequency, the more conductive the channel. The ac conductivity increases from 0.06 to ~ 140 (Ω cm) −1 as the signal frequency increases from 0.01 to 1.80 GHz. It means that the closer the signal frequency to the plasmon range, the easier the pass of the electrical signal. Opposite behavior can be observed from the impedance spectra. The larger the ac signal frequency the less the impedance value. The Z − f variation is linear in most of the studied range. On the other hand, as S 11 spectra show, the larger the frequency, the less the value of S 11 . This parameter provides information about the input-output relationship between ports at terminals of antenna system. S 11 = 0.0 means that all incident signals are fully transmitted with no wave power loss. S 11 = 1.0 indicates that all incident electromagnetic signals are rejected. Hence, S 11 spectra which are illustrated in Fig. 5c represent a band pass filter characteristics with notch frequency predictable from modeling the transistors. This is actualized by assuming resistance (R)-inductance (L)-capacitance (C) (RLC) resonator. In this circuit, Z La∕CuSe∕Ag = R + jwL − jwC (Pozar 2011;Al-Yasir et al. 2019a). The experimental data is reproduced (blue colored circles) by substituting R = 100 Ω , L = 10 nH and C = 0.45 pF. These parameters result in a microwave cutoff frequency of f co = (2 RC) −1 of 3.54 GHz. The practically determined cutoff frequency is within the range of plasmon frequency identified from the optical measurements. The ideal parameters (black circles in Fig. 5c) of the glass/La/CuSe/Ag band filters are R = 50 Ω , L = 5 nH and C = 0.64 0.64 pF and f co = 4.98 GHz. The notch frequency ( f n ) of the filter is 2.8 GHz. The numerical data indicate the possible application of the glass/La/CuSe/Ag transistors from filtering 5G signals. 5G technology require resonators workability in the frequency domain of 3.6-26 GHz for 5G technology (Al-Yasir et al. 2019a, b;Watanabe et al. 2020).

Conclusions
Herein, the effect of transparent lanthanum substrates on the structural, optical, dielectric and electrical properties of copper selenide thin films are investigated. Thin films of copper selenide are observed to exhibit enhanced crystallinity accompanied with redshift in the energy band gap and larger dielectric constant values as a result of replacing glass by lanthanum substrates. In addition, the coating of copper selenide onto lanthanum substrates increased the drift mobility values and decreased the friction to charge transport in the films. Verifying electrical tests has shown the suitability of the glass/La/CuSe/Ag transistors to behave as band filters which allow signals pass in the gigahertz level. Such property makes the currently proposed interface suitable for 5G mobile technology.