Elemental structural and optical properties of nanocrystalline zn1 xcuxse films deposited by close spaced sublimation technique

Journal of Science: Advanced Materials and Devices (2017) 79e85 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Elemental, structural and optical properties of nanocrystalline Zn1ÀxCuxSe films deposited by close spaced sublimation technique Muhammad Arslan a, *, Amir Habib a, Muhammad Zakria b, Arshad Mehmood b, Ghulam Husnain c a b c School of Chemical and Materials Engineering, National University of Sciences and Technology, H-12, Islamabad, Pakistan National Institute of Lasers and Optronics, P.O Nilore, 45650, Islamabad, Pakistan Experimental Physics Labs, National Centre for Physics, Quaid-e-Azam University, Islamabad 45320, Pakistan a r t i c l e i n f o a b s t r a c t Article history: Received 17 October 2016 Received in revised form 17 January 2017 Accepted 18 January 2017 Available online 25 January 2017 The elemental composition, film thickness and concentration depth profiles of as-deposited and annealed Zn1ÀxCuxSe films were studied by the Rutherford backscattering spectrometer (RBS) technique The films were deposited on glass substrates by close spaced sublimation (CSS) technique As-deposited films of about 250e300 nm thickness were then annealed in air at temperatures of 200  C and 400  C for h Structural characterization including crystal structure, crystal orientation, stacking fault energy (ҮSFE) and surface morphology were carried out by using X-ray diffraction (XRD) and atomic force microscopy (AFM) XRD studies revealed that the fabricated films are polycrystalline with a zinc-blende structure and a strong (111) texture plane Surface roughness was observed to be enhanced with annealing temperature with a decrease in stacking fault energy (ҮSFE) Spectroscopic ellipsometry has been utilized for the estimation of band gap energy (Eg) and dielectric constant (ε1) Band gap energy of the film increased with increasing annealing temperature while the dielectric constant decreased © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: X-ray diffraction Morphology Dielectric constant Spectroscopic ellipsometer Energy band gap Introduction Pursuant to the reported literature, ZnSe is the most prominent material in optoelectronics and optical coating applications, particularly in the UV region [1e8] It has a direct band gap of 2.7 eV and its thin films are transparent over a wide range of visible spectrum therefore, it is used as a window layer for the fabrication of thin film solar cells [9] ZnSe based solar cells has an efficiency greater than 11% by transmitting higher energy photons to the absorber layer of the solar cell [10,11] A number of approaches have been applied to tailor the physical properties of ZnSe thin films and extract their peculiar properties [12e15] The properties of a thin film are directly determined by composition, structure and microstructure which can be varied with growth conditions such as growth temperature, layer thickness and composition as well [16] In this work, physical properties of Cu enriched ZnSe films with different concentrations of Cu and annealed at 200  C and 400  C temperature were studied in detail In order to obtain precise * Corresponding author E-mail address: shani_788@yahoo.com (M Arslan) Peer review under responsibility of Vietnam National University, Hanoi knowledge of the structural and optical properties, one has to keep the exact composition and stoichiometry of the fabricated film layers especially for designing modern optoelectronic and optical devices Unfortunately, so far just the elemental composition and the thickness had been investigated for the Cu/ZnSe films [17,18] without any insight description of the stoichiometry and structure of interfaces for the deposited layers Multiple approaches have been employed for the deposition of ZnSe:Cu films like lyothermal method [19] two-sourced thermal evaporation [20,21], spray pyrolysis deposition technique [22], layer-by-layer assembly with anionic and cationic alternating polymer layers [23], chemical synthesis [24], and chemical bath deposition [25] Irrespective of the deposition technique, investigation of the microstructure and morphology evolution in polycrystalline Zn1ÀxCuxSe films is of key importance to develop a deeper understanding of the performance of devices employing these layers To characterize accurately such parameters, very diverse and in some cases very complicated diagnostic methods are needed Rutherford backscattering spectrometer (RBS) is a well established surface analyzing technique which can be used for element analysis and for depth profiles of major and minor constituents of thin films in the near-surface region [26] http://dx.doi.org/10.1016/j.jsamd.2017.01.004 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 80 M Arslan et al / Journal of Science: Advanced Materials and Devices (2017) 79e85 Previously, we have reported the structural and optical analysis of annealed Zn1ÀxCuxSe thin films [27] In the present study, we demonstrate several new results of the RBS analysis of the deposited and annealed films using high-energy MeV Heỵ ion beams from a pelletron tandem-type ion accelerator Qualitative and quantitative information about stoichiometry and structure of interfaces doped semiconductor as a function of depth, Cu concentration and annealing effect has been discussed with further details Results of this investigation have been correlated with the films structural and optical properties Among the structural properties, surface roughness and morphology, and crystalline quality of the films have been obtained by using the XRD and AFM methods Additionally, optical properties such as band gap tune-ability and dielectric constant have been determined by spectroscopic ellipsometer Experimental Zn1À-xCuxSe thin films were deposited on glass substrates by close spaced sublimation technique at room temperature Complete experimental detail for the deposition of the films and annealing procedure is discussed somewhere else [27] In this study a total of 15 samples for as-deposited and annealed films are presented according to Cu concentration as 0.00 x 0.20 as given in Table Film composition and concentration depth profiles were determined through RBS Data for all the samples were recorded by a MeV pelletron tandem accelerator (5UDH-2, NEC) using a 2.023 MeV Heỵ collimated beam (2 mm diameter) The sample was mounted on a five-axis adjustable goniometer with an accuracy of 0.01 in a vacuum chamber The backscattering ions were recorded by a surface barrier detector (energy resolution is 25.8 keV) fixed at a backscattering angle of 170 A beam integral connected with the sample holder was used to receive charge on sample from beam and make sure the experiments are comparable and repeatable (dose 15 mC) The structural properties were examined by PANalytical 3040/60 X, Pert PRO X-ray diffraction unit with CuKa (0.154 nm) radiation Surface morphology of the samples was studied by AFM (Quesant Universal SPM, Ambios Technology, USA) (QScope™ 350) in non contact mode An AFM tip of silicon nitride was used having an approximate radius of curvature 10 nm Both topography and phase images were recorded simultaneously in the scanning areas of 2e5 mm2 All the images were collected in air at scan rate of 1.0 Hz with 600 Â 600 pixels resolution AFM images were analyzed by using Nova Px software (NT-MDT Co.) thus generating root mean square (RMS) surface roughness Spectroscopic ellipsometer (J A Woolam M-200VI) was employed to determine the band gap energy (Eg) and dielectric constant (ε1) The ellipsometer consists of QTH lamp as the light source and a dual grating scanning monochromator (370e1670 nm) Incident light was focused on the sample at room temperature in air to determine the optical constants All the spectra were taken at an angle of incidence of 70 Results and discussion 3.1 Rutherford backscattering spectrometry (RBS) The Rutherford backscattering spectrometry (RBS) experiments were carried out for as-grown and annealed Zn1ÀxCuxSe films of various compositions and thicknesses The collected RBS spectra were then fitted by the code RUMP [28] to find the relative concentrations of various elements in the film Fig 1(aec) shows the RBS spectra of as-grown and annealed films of Zn1ÀxCuxSe (x ¼ 0.10, 0.15 and 0.20) which shows that the simulated spectra of the deposited films are in good agreement with the measured data The energy spectrum of the emitted ion yields information about the concentration depth profiles The composition has been altered and copper was seemingly introduced into the ZnSe matrix as a substitutional metallic participant as seen through RBS spectra and the data collected after RUMP code simulation The composition of the samples was calculated and compared with the initial percentage which shows that composition is nearly stoichiometric and the accurate incorporation of added copper is observed The spectra shows that the deposited films mainly have a Zn(Cu)xSe chemical composition, with x varying from 0.10 to 0.20 at the surface region and then decreasing with depth The maximum concentration of the added Cu is present at the surface (~100e200 nm) layers It consists of three superimposed elemental yields of three atomic species at different channels Moreover, it can be seen that there is no impurities or contamination in the asdeposited films though small percentage of oxygen and silicon is seen in the spectra which are coming from the substrate signal The peaks of the heavy elements (Zn, Se and Cu) can be clearly separated by the Heỵ beam and can thus be used to determine the relative thickness of the films (by using the nominal density of the Table Film thickness and compositional analysis investigated by spectroscopic ellipsometer (SE) and Rutherford backscattering spectroscopy (RBS) of Zn1ÀxCuxSe thin films for various Cu concentrations (x) Cu concentration Nature Film thickness Spectroscopic ellipsometer RBS [27] Zn Se Cu 0.00 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 255.1 239.42 222.17 297.48 310.96 315.62 219.56 230.62 250.7 189.38 197.09 201.01 210.63 211.78 222.64 240 230 220 290 300 300 210 215 255 180 185 190 195 200 210 0.5 0.5 0.5 0.450 0.470 0.470 0.480 0.480 0.450 0.420 0.450 0.400 0.320 0.330 0.380 0.5 0.5 0.5 0.500 0.480 0.480 0.420 0.420 0.450 0.430 0.400 0.450 0.480 0.470 0.420 0 0.050 0.050 0.050 0.100 0.100 0.100 0.150 0.150 0.150 0.200 0.200 0.200 0.05 0.10 0.15 0.20 Composition by RBS M Arslan et al / Journal of Science: Advanced Materials and Devices (2017) 79e85 81 has been observed (shown in Table 1) As the progress in semiconductor thin film technology is advancing, the thickness of device circuits is becoming very thinner and thinner Therefore, an accurate measurement for film thickness is required For thin targets, the scattering is proportional to the target thickness Angular frequency changes as the mass of the constituent's changes within the sample from which we can estimate the thickness accurately by using RBS The broadness observed in the RBS peaks with increasing annealing temperature indicates the increase in thickness of the films [29] It was noticed that a protuberant peak occurs at ~1152 elemental yield for copper content which is only prominent for 0.20 Cu as shown in Fig 1(c) This peak arises due to the excessive enrichment of Cu in the ZnSe matrix and corresponds to the unbound Cu content which subsists on the surface By increasing annealing temperature this peak shows declination and fades at 400  C annealed temperature From this it is certain that Cu diffuses into ZnSe matrix with increasing annealing temperature According to the RUMP simulation, the films annealed at 400  C exhibits the most homogeneous Cu concentration as a function of depth compared with the other studied samples The thickness of the films determined by RBS technique, quartz crystal and spectroscopic ellipsometry were in good agreement Moreover, the deposited films were uniform and well adherent with the substrate The analyzed compositions and thicknesses of the composite films are shown in Table 3.2 Structural studies Fig Rutherford backscattering spectra of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe (a) 0.10, (b) 0.15, (c) 0.20 films bulk materials) as well as their stoichiometry The highest channel number corresponds to the highest backscattered energy of Heỵ ion from the heaviest element Se present in the compound The peak occurring at a channel below 950 is due to the silicon contained in the glass substrate The energy channel correspondence to zinc is 1285; selenium is 1344 while for copper is 1152 After annealing the intensity of the RBS spectra increases and become slightly broader This shows that annealing plays an important part in altering the film thickness and prominent change in elemental ratio with respect to stoichiometry within the layers 3.2.1 XRD results To investigate the crystal structure, composition and phase, asdeposited and annealed Zn1ÀxCuxSe thin films were characterized by using X-ray diffraction (XRD) The X-ray diffraction pattern of pristine and heat treated films are shown in Fig 2(a,b) which shows that the crystal planes are preferentially orientated along the (111) plane with zinc-blende structure Fig 2(a) shows the XRD pattern of the Zn1ÀxCuxSe films annealed at 400  C with two different copper concentrations (0.00 and 0.15) while Fig 2(b) shows XRD pattern of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe thin films for 0.05 Cu concentration XRD graphs indicate that the increase of annealing temperature leads to improvement of the thin films growth in the (111) plane orientation This is due to the decrease in film stresses and grain coarsening as described in detail in our published paper [27] It is observed that the annealing temperature of 200  C and 400  C did not affect the predominant (111) crystallographic texture No secondary phase is observed after annealing, however, the intensity of (111) peaks increases with the annealing temperature This increase of the peak intensity is due to the improvement of clusters, relocation of atoms and elimination of defects formed during the film deposition After annealing, the intensity of (220) and (311) reflection decreases while the preferred orientation in (111) direction increases radically at 400  C Slight modification of the crystal structure is observed by annealing at temperature of 200  C however the crystal structure changes significantly after annealing at 400  C A small decrease in diffracting angle 2q for 0.15 annealed at 400  C is also observed which confirms that after annealing the grains are recrystallized and coalescence is assumed to happen Fig shows the effect of annealing on the FWHM and ҮSFE for (111) orientation FWHM can be increased or decreased depending on coalescence and recrystallization of grains Recrystallization may help (111) orientation and peak growth, making the FHWM smaller It is observed that FWHM and ҮSFE decrease with increasing annealing temperature, which is mainly due to grain growth and improvement in crystallinity ҮSFE for the as-deposited 82 M Arslan et al / Journal of Science: Advanced Materials and Devices (2017) 79e85 Fig XRD pattern of (a) 400  C annealed Zn1ÀxCuxSe (0.00, 0.15) and (b) as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe (0.05) thin films Fig Variations of full width half maxima (FWHM) and Stacking fault energy (ҮSFE) of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe thin films with Cu concentration and annealed films is calculated for (111) plane by using the relation [30] " SF ¼ # 2p2 45ð3 tan qÞ 1=2 b (1) where ‘q’ is the Bragg's angle and ‘b’ is the full width half maxima (FWHM) The stacking fault energies (ҮSFE) were calculated from Fig AFM 2-D and 3-D images of (aeb) as-deposited, (ced) 200  C annealed and (eef) 400  C annealed Zn1ÀxCuxSe (0.00 and 0.10) thin films M Arslan et al / Journal of Science: Advanced Materials and Devices (2017) 79e85 83 Table Peak position (2q), full width half maxima (FWHM), stacking fault energy (SFE), Band gap (Eg) and mean square roughness (RMS) of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe thin films for various Cu concentrations (x) Cu concentration 0.00 0.05 0.10 0.15 0.20 Nature of films As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 As-deposited Annealed 200 Annealed 400 2q deg [27] 27.28 27.23 27.24 27.26 27.22 27.25 27.28 27.27 27.22 27.35 27.33 27.2 27.38 27.14 27.25 FWHM (2q) 0.1968 0.1581 0.1168 0.1574 0.1474 0.1074 0.1378 0.1364 0.0978 0.1978 0.1762 0.1378 0.2362 0.1962 0.1574 the shift of the peaks of the X-ray lines of the films with reference to the 2003 JCPDS database No: 89-7130, using Eq (1) ҮSFE also decreases gradually with increasing copper concentration up to 0.10 and crystal growth becomes sharp while an opposite trend is observed beyond this Cu concentration The minimum values of ҮSFE are obtained at 0.10 and 400  C annealed temperature while the maximum at 0.20 Cu concentration The smaller value of the ҮSFE (0.0377 J/m2) obtained at 0.10 Cu exhibits excellent crystalline quality of CuxZn1ÀxSe films There is no report on the ҮSFE of Zn1ÀxCuxSe thin films deposited by closed space sublimation technique All the structural parameters are summarized in Table 3.2.2 AFM results The morphology of as-deposited and annealed samples have been analyzed with the help of AFM diagnostic tool The topography of the surface in 2D and 3D image is shown in Fig for all samples with a root mean square (RMS) roughness are listed in Table Fig 4(a,b) indicates the surface morphology of as-deposited Zn1ÀxCuxSe (x ¼ 0.00, 0.10) films Fig 4(a) presents a low roughness surface with an RMS value of 1.12 nm for 0.00 Cu concentration film, over a scan size of mm2, which suggests the formation of very smooth surface The roughness increases to 1.39 nm for 0.10 Cu concentration The increase in RMS roughness with Cu is due to the grain growth and improved crystallinity as corroborated from our XRD results These formations are in agreement with those reported by Mazon-Montijo et al for CdS films [31] Fig 4(cef) shows the surface morphology of the annealed Zn1ÀxCuxSe (x ¼ 0.00, 0.10) thin films at 200 and 400  C over a scan size of mm2 A careful comparison between both annealed samples reveals that the micro features on the 400  C annealed film surface are almost similar in shape to 200  C annealed films except size of the particles are reduced after 400  C annealing The islands formed on the surface of the films annealed at 400  C shows more improvement in particle size along with finer micro-asperities The addition of Cu contents and annealing enhance the grain growth and roughness This is useful for solar cell applications as rough surface trap more light Light trapping is widely used to enhance the absorption in the absorber layer of thin film solar cells and therefore to increase the current density The most prevalent light-trapping technology is introducing nano-textured interfaces into the solar cells [32] 3.3 Spectroscopic ellipsometry Spectroscopic ellipsometer has been employed to determine the band gap (Eg) and dielectric constant (ε1) of our target thin films A Stacking fault energy (J/m2) 0.07335 0.06068 0.04431 0.05937 0.0569 0.04075 0.05136 0.05114 0.03776 0.07064 0.06371 0.05381 0.08277 0.07923 0.05972 Band gap (eV) 2.74 2.76 2.78 2.70 2.74 2.77 2.68 2.73 2.75 2.62 2.63 2.67 2.61 2.62 2.65 Roughness (nm) AFM SE 1.11 9.29 e 1.25 e e 1.39 e e 1.21 e e 1.08 e e 12.18 16.51 16.41 7.99 27.67 15.61 19.77 15.61 11.31 33.34 40.67 14.09 6.53 30.69 19.20 beam of polarized light is illuminated onto the sample and polarization change is measured from reflection spectra The polarization change in the reflection signal is measured and then characterized by two quantities, psi (J) and delta (D) parameters for amplitude and phase changes respectively tanjị$eiD ẳ r rp rs (2) where tan (J) is the magnitude of the reflectivity ratio, rp is the reflectivity for p-polarized light and rs is the reflectivity for spolarized light The experimental psi (j) and delta (D) spectra were recorded as a function of wavelength over the range (400e800) nm at an incidence angle of 70 These parameters are correlated with thin film optical properties by the above expression and then make a comparison between the experimental and simulated data by utilizing fitting functions [33] The extinction coefficient (k) and refractive index (n) measured ellipsometry data already reported for the same samples in our published paper [27] However, in this article we determined the band gap energy by using extinction coefficient (k) spectra obtained from Ellipsometer The optical band gap energy (Eg) was calculated using the following relations a ¼ 4pk=l À ahn2 ¼ A Eg À hn Á where a is the absorption coefficient, h is Planck's constant, y is frequency and A is proportionality constant To estimate the band gap of these films, (ahn)2 was plotted against hy using the above equation for as-deposited and annealed films of different compositions Extrapolation of the linear portion to the (ahn)2 ¼ axis gives the value of band gap energy as shown in Fig 5(aec) For all compositions, the band gap energy increases with annealing temperature as shown in Fig 5(aec), while decreases with the increases of Cu concentration in the ZnSe matrix as shown in Fig 5c (inset) Our calculated values of optical band gap by spectroscopic ellipsometer are slightly greater than reported by J Kvietkova et al and Dahmani et al by using spectroscopic ellipsometer [34,35] The band gap energies estimated by SE technique is significantly different than transmission data The reason for this is that the reflectivity obtained by the SE is very different from that of optical probing via a spectrophotometer in terms of the spatial frequencies Moreover, measurement of the optical parameters by the SE requires large-scale approximation for the fitting of the model 84 M Arslan et al / Journal of Science: Advanced Materials and Devices (2017) 79e85 diversity in the fitting software's and the mode of simulating the data Furthermore, dielectric quantity (ε1) of Zn1ÀxCuxSe films obtained from the ellipsometry fit is shown Fig 6(aec) ε1 is the real part of the complex dielectric function, ẳ ỵ which represents how much a material is polarized due to creation of electric dipoles Fig Band gap energy of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe (a) 0.00, (b) 0.05, (c) 0.15 films, determined from k-spectra Inset in Fig 6(c) shows the variations of band gap energy of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe films with copper concentration while transmission is real time measurement Comparing the SE and RBS film thickness, the thickness is approximately 5% greater than those calculated by the RBS as listed in Table This variation in the film thickness by the two different techniques is due to the Fig Dielectric constant (31) of as-deposited and annealed Zn1ÀxCuxSe (a) 0.00 (b) 0.05, (c) 0.20 films, determined by spectroscopic ellipsometer Inset in Fig 6(c) shows the variations of dielectric constant of as-deposited and annealed (200  C and 400  C) Zn1ÀxCuxSe films with copper concentration M Arslan et al / Journal of Science: Advanced Materials and Devices (2017) 79e85 in the material by applying electric field Change in the polarization of any material directly affects the dielectric properties of the material This change in the dielectric constant is measured by the fitting of ellipsometry parameters psi (j) and delta (D) SE measures the dielectric constant (ε1) by the following equation [36] ε1 ¼ n (3) Dielectric studies show that the dielectric constant (ε1) values decrease with increasing annealing temperature as shown in Fig 6(aec), while increases with Cu contents addition as depicted in Fig 6c (inset) Post-annealing treatment of films plays an important role and considerably affects the dielectric properties of the prepared Zn1ÀxCuxSe films The decrease in the dielectric constant with annealing temperature may be due to the lower compactness of annealed ZnSe films than as-deposited ZnSe films Similar results have been reported for annealed ZnSe films by Venkatachalam et al [37] Conclusion The effect of post deposition thermal annealing on the compositional stoichiometry and depth concentration of deposited layers of Zn1ÀxCuxSe thin films was investigated by RBS technique By increasing the annealing temperature of films from 200  C to 400  C the physical properties have improved significantly Variations in stoichiometry found with RBS technique is complemented by micro analysis characterizations performed by XRD and AFM, while the optical results obtained by using SE XRD data predicts the improvement of crystallinity with an increase in FWHM value while stacking fault energy decreases with increasing annealing temperature AFM suggests the formation of very smooth surface; larger grains are replaced by smaller and fine grains (micro-asperities) at 400  C accompanied by an increase in surface roughness Spectroscopic ellipsometry analysis reveals that the bang gap increases while dielectric constant decreases with the 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annealed (200  C and 400  C) Zn1? ?xCuxSe (0.05) thin films Fig Variations of full width half maxima (FWHM) and Stacking fault energy (ҮSFE) of as -deposited and annealed... excellent crystalline quality of CuxZn1ÀxSe films There is no report on the ҮSFE of Zn1? ?xCuxSe thin films deposited by closed space sublimation technique All the structural parameters are summarized