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73FUNCTIONAL NANOSTRUCTURES

A B S T R AC T

K. Kaviyarasu1,2*, L. Kotsedi1,2, Aline Simo1,2, Xolile Fuku1,2, F. T. Thema1,2, K. Anand3, K.G. Moodley3, J. Kennedy1,4, M. Maaza1,2

Quantum phenomena of ZnSe nanocrystals prepared by electron beam evaporation technique

Substrate temperature dependent optical and structural properties of Zinc selenide (ZnSe) thin films were studied. Films were grown onto glass substrate by electron beam evaporation technique in high vacuum (10-6 Torr) at various temperatures in range of room temperature (RT) to 300 °C. Microstrain and dislocation density were also found to vary between 0.69×10-3, 1.12×10-3, 3.59×1010 cm-2 and 9.49×1010 cm-2. The lattice parameter value ‘a’ for these films was calculated and found to increase from 5.577 Å to 5.652 Å respectively. Scanning electron micrographs (SEM) showed that clusters are composed of spherical and needle like nanocrystals distributed uniformly over the surface whose c-axis is parallel to the surface of the film. This confirmed the mixture comprised cubic and hexagonal phases. A comparison of atomic force microscopy (AFM) images showed that the grain size increased from about 139 nm for the RT deposited ZnSe film to about 39 nm for the ZnSe film deposited at 300 °C. The binding energy of 54.4 eV is similar to the values obtained for other selenides. The Auger parameter for zinc was calculated from the experimental binding energies of the zinc (2p3/2) XPS line and the LMM Auger line the reference values are 1042.49 eV - 1020.19 eV for zinc. All the films were characterized optically by UV-vis-NIR spectrophotometer in photon wavelength range (300 nm - 2000 nm). The optical transmittance and reflectance were utilized to compute the absorption coefficient, refractive index and band gap energy of the films. The bandgap value of RT and 100 oC deposited ZnSe films are 2.88 eV and 2.78 eV, which are found to be blue shifted by about 0.08 - 0.18 eV from the bandgap value of 2.70 eV for the standard bulk material.

1UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories, College of Graduate Studies, University of South Africa (UNISA), Muckleneuk Ridge, P O Box 392, Pretoria, South Africa. 2Nanosciences African network (NANOAFNET), Materials Research Department (MRD), iThemba LABS-National Research Foundation (NRF), 1 Old Faure Road, 7129, P O Box 722, Somerset West, Western Cape Province, South Africa. 3Department of Chemistry, Durban University of Technology, P O Box 1334, Durban, South Africa, 4000. 4National Isotope Centre, GNS Science, Lower Hutt, New Zealand. 1*Corresponding author

I. INTRODUCTION of the works on II–VI group were focused on ZnO, ZnS, CdSe and CdS [9]. A few reports based on the ZnSe one-dimensional (1D) nanosized structures such as nanowires, nanorods, nanobelts and nanotubes have been published on the basis of their novel properties and widely potential applications [10]. They also provide models to study the relationship between electrical transport, optical and other properties with dimensionality and size confinement effect. Archana et al reported that mono dispersed ZnSe nano wires (NWs) have been synthesized by a wet chemical method using edamine as a surface capping ligand. Ultraviolet visible absorption spectrum confirms that the absorption edge is located at 371 nm which is blue shifted when compared to the value of bulk ZnSe. The photoluminescence spectrum showed that the emission peak is located at 442 nm which corresponds to near band edge emission. The NWs have a uniform average diameter of 80 nm and a length of about few μm. The possible growth mechanism is also addressed as well [11]. Engelhardt et al., reported that research in the synthesis of ZnSe-GaAs heterojuncton has greatly increased in recent years because of possible applications in a number of high speed optoelectronic

Semiconductor nanocrystallites belong to state of matter in the transition region between molecules and solids. Their physical and chemical properties are found to be strongly size- dependent. The synthesis of binary metal chalcogenide of group AII–BVI semiconductors in a nanocrystalline form has been a rapidly growing area of research. This can be attributed to the importance of properties such as; non-linear optics, photoluminescence electroluminescence properties, effects of crystallite or quantum size [1]. Zinc selenide is n-type semiconducting material (AII–BVI group) with wide band gap (2.7 eV) at room temperature [2]. Zinc selenide is one of the promising materials for the use in optoelectronic devices such as blue laser diodes [3], light-emitting diodes (LEDs) [4], optically controlled switches [5] and tunable mid-IR laser sources for remote-sensing applications [6]. Since the dimension of materials is believed to determine the optical, electronic, thermal and mechanical properties control of the size of the particles is very crucial [7]. In recent years, various types of 1D nanostructure, such as III–V, and II–VI group compounds, have been synthesized [8]. However, most

FUNCTIONAL NANOSTRUCTURES 74

devices [12]. Pushpendra et al., investigated pure ZnSe and ZnSe:La NPs with wide bandgap and luminescence which have been synthesized at a low temperature (100 °C) in a single template-free step. A broad photoluminescence (PL) emission across the visible spectrum has been demonstrated by a systematic blue-shift in emission due to the formation of NPs. The band gap energy Eg of ZnSe and ZnSe:La NPs were found to be 3.59 eV and 3.65 eV, respectively. Results showed that hydrazine hydrate played multiple roles in the formation of ZnSe and ZnSe:La NPs [13]. Reiss et al. reported a new synthesis method of colloidal ZnSe nanocrystals exhibiting size-dependent optical properties. The ZnSe quantum dots were prepared in a non-coordinating solvent (octadecane) via direct reaction of zinc stearate with selenium dissolved in trioctylphosphine, i.e., without the use of pyrophoric reagents. The photoluminescence of the resulting nanocrystals can be tuned in the spectral range 390 nm - 440 nm with constant emission line widths of the order of 15 nm [14]. Although 1D ZnSe nanostructures have also been synthesized using relatively simple chemical vapour deposition using ZnSe powder or using gold as catalyst on the (100) silicon substrate this paper, constitutes the first report of a novel 3D ZnSe structure, synthesized by employing a simple electron beam evaporation technique [15-17]. Furthermore the strategy utilizes Se and Zn powders as sources without any additional catalyst.

II. EXPERIMENTAL PROCEDURE

Typical synthesis of ZnSe films

ZnSe films with different substrate temperature of RT, 100 oC, 200 oC and 300 oC with thickness about 200 nm were deposited on glass substrates by electron beam evaporation technique. ZnSe (12 g) was finely ground and sealed in quartz tubes under vacuum heated at 800 °C for 14 hrs. This procedure of grinding, vacuum sealing, and heating was carried out to obtain high quality and homogeneous materials, which was used to make ZnSe pellets of 5.0 mm diameter and 5.0 mm thickness. The vacuum was maintained at 10-6 Torr, the operating voltage was 10 kV and the beam current was 20 mA, during deposition of the films. The electron beam is directed to fall on the ZnSe pellet sample kept in a Cu crucible. The ZnSe vapors emanating from the source due to electron beam heating condensed on the glass substrates attached to substrate holder cum heater placed at a distance of 15 cm above the copper crucible containing the ZnSe pellet.

Characterization studies

Structural characterization was effected via an X-ray diffraction patterns (XRD) recorded on a RigaKu D/max-RB diffractometer with Ni-filtered graphite monochromatized with CuKα (λ=1.5418 Å) with the range of the diffraction angle 2θ values in the ranges 20° - 80°. The morphology and microstructure were

examined using scanning electron microscopy (SEM) performed on a (Philips CM12) with an acceleration voltage of 20 kV. The energy dispersive spectroscopy (EDS) (IH-300X) analysis was performed at several points in the SEM arrangement. All AFM images were acquired under tapping mode on a Digital Instrument Nanoscope IIIA at ambient conditions. A sharp TESP tip (Veeco) with a radius of end of 8 nm was used. Typical values for the force constant and resonance frequency were 42 N/m and 320 kHz, respectively. High-resolution transmission electron microscopy (HRTEM) measurements were made on a HITACHI H-8100 electron microscopy (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for HRTEM characterization was prepared by placing a drop of colloidal solution on carbon-coated copper grid and dried at room temperature. The elemental composition was determined using the selected area electron diffraction (SAED) (IH-300X) analysis was performed at several points in the HRTEM system respectively. The XPS spectrum was recorded on a ESCALAB 250 photoelectron spectrometer (Thermo-VG Scientific, USA) with AlKα (1486.6 eV) as the X-ray source. A Renishaw micro-Raman spectrometer RM 2000 with IR (632.8 nm) and UV (325 nm) excitation lasers was employed to measure the non-resonant and resonant Raman spectra of ZnSe nanosamples. Optical absorption was measured by means of Varian Cary 5E spectrophotometer in the wavelength range of 350 - 700 nm. Current-voltage characteristics studies of the synthesized nanomaterials were measured using the instrument Keithley 2640 source meter. The samples were pelletized and pellets of uniform dimensions were placed between the two copper electrodes and silver paint was coated on the surface of the samples in order to make firm electrical contact. The photoluminescence measurements were performed on a Jobin Yvon Flurolog-3-1 Spectroflurometer using 3 nm slit width. The emission was collected and sent to a Jobin-yuon Triax monochromator and detected by a Hamamastsu Ra28 photomultiplier tube.

III. RESULTS AND DISCUSSION

XRD Analysis

Zinc selenide thin films can grow with metastable sphalerite cubic (zinc blende type) or stable hexagonal type (wurtzite) structure. Previous reports [18, 19] based on chemically deposited ZnSe thin films showed that the films can form either cubic zinc blende or hexagonal wurtzite type structure. Hence, in order to determine the crystal structure of as-deposited and annealed ZnSe films, the XRD pattern of films were analyzed. Fig. 1 (a-d) shows XRD pattern of as-deposited and annealed ZnSe films onto glass substrate. Fig. 1(a) shows ‘as-deposited’ ZnSe thin film is of poor crystallinity and no well-resolved peaks are observed in the XRD patterns of these films. However it is worth noting that the low intensity reflection peak

FUNCTIONAL NANOSTRUCTURES 75

(111): d = 3.26 Å over a broad hump corresponds to the cubic phase of ZnSe. The low intensity of peak confirmed the nanocrystalline nature of ZnSe film and the broad hump is due to the amorphous glass substrate and also possibly due to some amorphous phase in Zn-Se bonding in ZnSe films. Thus, as-deposited ZnSe films are nanocrystalline with some amorphous phase present in it. The films annealed at 100 oC Fig. 1(b) did not show significant improvement in crystallinity. While the film annealed at 200 oC Fig. 1(c), showed well resolved XRD peaks corresponding to the sphalerite cubic (zinc blende-type) structure with highest intensity reflection peak (111): d=3.265 Å. It clearly indicated the significant improvement in the crystallite size of ZnSe thin films annealed at 200 oC. The ZnSe thin film annealed at 300 oC Fig. 1(d) showed the reflection peak that corresponds to sphalerite cubic (zinc blende type) phases of ZnSe films with nearly equal or slightly increased peak intensities. A tendency for the peaks to shift towards lower 2θ values has been observed when the annealing temperature increases that result into increase in interplanar distances. Crystallite sizes of ZnSe thin films were calculated using Scherrer’s relation. The average crystallite size was calculated by resolving (111) peak for cubic phase for all the films [20].

Figure 1(a-d) XRD patterns for the ZnSe films deposited at different substrate temperatures (a) RT ( b) 100 °C (c) 200 °C (d) 300 °C.

Fig. 2(a-c). The lattice parameter value ‘a’ for these films was calculated and found to increase from 5.577 Å to 5.652 Å with increasing substrate temperature as shown in Fig. 2(a). These values are in good agreement with the standard value of 5.618 Å. Such an increase in

lattice parameter values with deposition temperature indicates that the grains in the films were under strain due to the presence of native imperfections. This leads to either elongation or compression of the lattice. It results in a change of the density of the films and the associated microstructural parameters like stress, strain and dislocation density [21]. The grain size values of the ZnSe films deposited at RT, 100, 200, 300 oC are calculated as 11.4, 18.2, 24.4 and 34.5 nm respectively. The grain size increases with temperature is shown in Fig. 2(b). The internal stress is positive (+ve) for temperatures less than 100 oC, while it is negative (-ve) for the ZnSe films deposited at temperatures greater than 200 oC as seen in Fig. 2(c). This internal stress is arising due to the accumulation of crystallographic defects that are developed during the film formation at elevated temperatures. Such defects have the probability of migrating parallel to the substrate surface with increased mobility so that the grains of the films have a tendency to expand and an internal textile stress is developed. At the same time, stress relaxation is taking place at relatively lower substrate temperatures. A decreasing trend is observed for the microstrain and dislocation density values with increasing substrate temperature from RT to 300 oC, which is the same as observed earlier by Venkatachalam et al., [22] and Kalita et al., [23]. The variations are shown in Fig. 3(a-b) respectively. The reason is attributed to the movement of interstitial Zn atoms from the bulk of the grain interior to the boundary region, which dissipates to larger inter-grain area that leads to a reduction in lattice imperfection. From Fig. 1(d), it is observed that there is only one peak is observed, the (111) plane. It shows that the ZnSe films deposited in the present study are highly oriented along (111) plane only with cubic phase. It is obvious that the intensity of this peak is increasing continuously with temperature, because microstrain, internal stress and dislocation density values are decreasing with temperature as mentioned previously. Such a reduction in imperfections and lattice misfil has been observed by Venkatasubbiah et al., [24] for the ZnSe films deposited by closed space evaporation technique.

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Figure 2(a-c) Variation of Lattice constant ‘a’ (b) Grain size ‘D’ (c) Variation of Stress ‘S’ for the ZnSe films with different substrate.

Figure 3(a-b) Variation of Strain ‘ε’ (a) and Dislocation density ‘δ’ (b) for the ZnSe films with different substrate temperatures.

SEM studies

Fig. 4(a-f) shows the scanning electron micrographs (SEM) of as-deposited ZnSe film at 100 oC, 200 oC and 300 oC. The films were uniform, pinhole free and well covered to the glass substrate surface. From this, one can draw two conclusions. First, it is interesting to note that the films seem to be composed of densely packed spherical micro sized clusters 50 nm - 5 nm in size, consists of number of spherical nanosized grains. Hence, it clearly shows that the growth of ZnSe film takes place via cluster by cluster deposition, i.e. aggregation of colloidal particles formed in the solution rather than ion by ion deposition on the surface of substrate, which agrees well with the results previously reported by many workers [25-27]. It is worth to mention that, in the literature there is no evidence for ion by ion growth of ZnSe [28]. Fig. 4(c-f) shows the SEM of ZnSe thin film annealed at 300 oC at 5000x and 10,000x magnifications. From this it is concluded that, the film showed an improvement in crystallinity. The clusters are not merged together. The clusters are composed of spherical and needle like nanocrystals distributed uniformly over the surface whose c-axis is parallel to the surface of film that confirmed the mixture of cubic and hexagonal phases [29]. The surface roughness was decreased; the ZnSe crystallites and/or grains are not diffused together like other II-VI compounds [30]. Thus, it was confirmed that the self-diffusion coefficient of ZnSe nanocrystals and/or grains is very small. From this microstructure results, it is found that sample thinning took place with increasing surface roughness, which may be due to high partial pressure of selenium at higher temperature and longer annealing time. An appreciable amount of evaporation of Se from the surface of film resulted into thinning of sample from 0.26 μm to 0.21 μm. The change in microstructure or surface morphology have been attributed to the partial transformation of sphalerite cubic ZnSe phase into hexagonal wurtzite phase and/or partial conversion of ZnSe into ZnO. To confirm the stoichiometry of the EB evaporated ZnSe films, EDX analysis was carried out for the ZnSe thin films deposited at 300 oC of thickness about 200 nm. Fig. 4(b) shows the typical EDX spectrum of the ZnSe nanocrystalline film deposited on glass substrate. The elemental analysis of Zn and Se was performed by measuring and comparing the peaks related to Zn and Se alone. Other peaks coming from the glass substrate were eliminated. The average atomic percentage of Zn and Se is 48.2:51.8. This is good agreement with the XPS result reported in section 3.5. Further, ZnSe films with selenium excess have been reported by other researchers [31, 32] and a similar result has been obtained for our film also.

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Figure 4(a-f) SEM images of ZnSe films deposited at 300 °C.

AFM studies

The topology of as-deposited ZnSe film surface is viewed three-dimensional AFM images Fig. 5(a-d), and shows the atomic force microscopic 3D images of the EB evaporated ZnSe films with the deposition temperature as RT, 100 oC, 200 oC and 300 oC. The film surface morphologies are mostly uniform, pin hole free and adhered well to the substrate. The film deposited at RT show tiny spherical grains of coarse nature as seen in Fig. 5(a). The surface roughness (Ra) value is about 139.2 nm as can be seen the line profile of the 2D picture shown in Fig. 5(a). The film deposited at 100 oC shows Fig. 5(b) enlarged grain morphology with grown up grain size. The surface roughness value calculated from the line profile scan of the surface from the two dimensional AFM picture is about 141.8 nm. This reduced value shows a uniform surface with uniform grains distributed all over the surface. Figure 5(c) shows the 3D picture of ZnSe film deposited at 200 oC. It shows vertically grown grains distributed over the surface area of analysis but with larger size. The roughness value is found decreased to about 84.9 nm. This is evident from the 2D picture with its line profile seen in Fig. 5(c). Fig. 5(d) shows the 2D AFM picture of ZnSe film deposited at 300 oC. The surface is seen with somewhat non-uniform morphology. The surface contains lot of grains accumulated as clusters. Roughness value is found increased to about 39.8 nm. However, the grain size measured from the line profile shows increased size. Such a deviating roughness values of these ZnSe films is obvious, because the particles were grown with varying size and were observed as clusters. Such morphology has been also observed from SEM pictures as discussed in section 3.2

here. From a comparison of these AFM images, the grain size is found increasing from about 139.2 nm for the RT deposited ZnSe film to about 39.8 nm for the ZnSe film deposited at 300 oC. This morphology is similar to the morphology of the chemically deposited ZnSe films, where the particle growth is supported by cluster deposition [33-35]. Further, the three dimensional (3D) topographic images show very smooth surfaces with vertically grown, which were formed by the ZnSe clusters coming from the ZnSe target. From the Fig. 5(e-h), all images show globular grains surface topography, which confirm the compact surface growth with a comparatively lower roughness values as revealed from the line profile of the surface. Analysis of both the 2D and 3D AFM images shows increased grain size with increasing deposition temperature. This is also supported by the grain size increase revealed variation predicted by the XRD and SEM studies presented earlier.

TEM studies

TEM is the primary technique to determine crystal structure, crystallite shape (size) and orientation of crystals although the specimen preparation is nontrivial and destructive. Fig. 6(a-j) shows transmission electron micrographs of as-deposited ZnSe thin film with different magnifications. The TEM clearly showed that the ZnSe film formation takes place via cluster by cluster nucleation and growth mechanism. The TEM micrograph revealed a globular structure consisting of clusters of nanocrystals. These clusters have a size of 5 nm, which is in well agreements with the values observed from SEM and AFM morphological studies. It also revealed the nanocrystalline nature of ZnSe particles, which are not distinguishable from each other. The high-resolution TEM Fig. 6(b) of as-deposited ZnSe film showed randomly oriented nanocrystals with diameters of 4-5 nm in amorphous matrix [36]. The amorphous phase acts glue in holding ZnSe nanocrystals together. Fig. 6(c, d) shows electron diffraction pattern and EDX from ZnSe nanoparticles. It confirmed the cubic ZnSe film formation with typical lattice spacing d = 3.27 Å (hkl = 111). The crystallite size observed from HRTEM is in good agreement with the values calculated from XRD (6.5 nm).

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Figure 5(a-h) 2D & 3D topographic AFM images of ZnSe films deposited at 300 °C.

Figure 6(a-j) TEM of the ZnSe film deposited at 300 °C.

XPS studies

X-ray photoelectron spectroscopy (XPS) was used to study the stoichiometry and chemical state of the zinc and selenium in the ZnSe film. Only trace levels of carbon and oxygen were detected in the film. Even these reduced during sputter cleaning of the surface, indicating that they were localized on the surface. A typical XPS spectrum is shown in Fig. 7(a-d), where the low energy tail of the XPS spectrum represents the Fermi level. Using high resolution analysis in the low energy region (not shown in Fig. 7) the Fermi level was located at ~6 eV in the present sample. The position of the Fermi level is more accurately determined from the shift of the (Zn 2p3/2) XPS line from 1055.03 eV (standard

value) to 1054.35 eV a shift of 61.32 eV. Thus, all the XPS lines should be shifted corresponding to this. The important zinc XPS lines are 2p3/2, at 1020.19 eV and 2p1/2 at 1042.49 eV. As shown in Fig. 7(b), these are located at 1027.8 eV and 1047.96 eV respectively. Similarly, the selenium 3d line Fig. 7(c) is shifted to 61.1 eV from 55.3 eV. If the shift in the Fermi level is taken into consideration, then there is a chemical shift of % 1 eV for selenium. The binding energy of 54.4 eV is similar to the values obtained for other selenides [37]. The Auger parameter for zinc was calculated from the experimental binding energies of the zinc (2p3/2) XPS line and the LMM Auger line value of 1020.19 eV. The reference values are 1042.49 eV for zinc and 1042.49 eV- 1020.19 eV for ZnSe [38]. These XPS results indicate the change in the valence states of zinc and selenium, resulting from the transfer of electronic charge from zinc to selenium, resulting in the formation of ZnSe. The spectra in Fig. 7(c) and 7(d) show that the line widths (full width at half-maximum (FWHM)) of the XPS lines are small and comparable with the line widths for the reference samples. This observation shows that the zinc and selenium atoms do not exist in multiple-valence states. This is in good agreement with the XRD data, which also showed the presence of a single phase of ZnSe in the film. The Zn/Se elemental ratio determined from the intensities of the respective zinc and selenium XPS lines shows an excess of selenium in the film with a Zn/Se ratio close to 40:60. This selenium excess observation is in good agreement with the results reported for the electrodeposited ZnSe thin films [39].

Figure 7(a-d) XPS spectrum of ZnSe films deposited at 300 °C.

Raman studies

Raman scattering is a complementary non-destructive technique for structural characterization in certain kinds of material [40]. The positions of the active Raman modes depend on lattice vibration of the sample, which is a function of temperatures. The effect is more significant in the case of reduced-size materials. The temperature dependence of Raman scattering has been successfully measured in the

FUNCTIONAL NANOSTRUCTURES 79

Electrical Resistivity Studies

The electrical resistivity variation by two point probe method has been carried out the temperature range of RT-300 oC. The electrical resistivity of the RT, 100 oC, 200 oC and 300 oC deposited ZnSe thin film was about 9x106, 5x106, 2x105 and 1x105 Ωcm respectively. The value of resistivity for the RT deposited ZnSe thin films is greater than that reported by Pramanik and Biswas [49] for their polycrystalline ZnSe thin films. Such a high value of resistivity could be attributed to the nanocrystalline nature of the RT deposited thin films which possess larger grain boundary discontinuities, surface states and high surface roughness as revealed from the microstructural studies by XRD and topological studies by AFM. The variation of log ρ against 1000/T as given in Fig. 9 shows the semiconducting nature of these films. From Fig. 9, it can be observed that the value of electrical resistivity decreases with increasing temperature of measurement and also increasing deposition temperature. The decrease in resistivity with increasing temperature can be to the increased nanocrystalline size. Consequently, decrease in grain boundary density, grain boundary defects and recrystallization of ZnSe films. The log ρ vs 1000/T curves show two types of resistivity variation when the measurement was carried out between RT (27 oC here) and 300 oC. The resistivity variation is slow in the lower temperature region of about less that about 160 oC with smaller slope. Whereas, in the high temperature range between 160 and 300 oC, the change in steep with larger slope. Such an exponential variation of resistivity with temperature can be fit with the equation,

Where ρo is a constant, Ea is the thermal activation energy, K is the Boltzmann’s constant and T is the absolute temperature. Ea values vary between about 0.094 to 0.072 eV in the low temperature region, whereas these values are high of the order of 0.85 eV - 0.72 eV in the high temperature region. This type of electrical conduction mechanism in our ZnSe nanocrystalline films can be explained on the basis of conduction mechanism model for polycrystalline as well as amorphous (nanocrystalline) films [50, 51].

Optical studies

The transmittance spectra of ZnSe thin films deposited at RT, 100 oC, 200 oC and 300 oC were studied in the wavelength range 350-1600 nm, without connecting for reflection and absorption losses. Fig. 10(a-d) shows the respective transmittance spectra. The sharp fall in these spectra beyond 500 nm confirm the ZnSe monophasic nature of all the deposited films. These sharp fall range in the spectral region of 400 nm - 500 nm with red shift for the ZnSe films in the order deposited at RT, 100 oC, 200 oC and 300 oC. The

studies on Si, diamond and C nano-structures [41]. These results have been interpreted in terms of anharmonic processes, which guide to a better understanding of characterization of these nanostructural materials. Although the temperature dependence of Raman and luminescence spectroscopy of ZnSe bulk crystal had been reported, little attention has been paid to ZnSe nano-structures in this feature. Fig. 8(a) and d is the typical Raman spectra of ZnSe thin films deposited at RT, 100 oC, 200 oC and 300 oC. These spectra were measured in the backscattering geometry in which the 632.8 nm line of laser was used as the excitation light source. As the deposition temperature increases from RT to 300 oC, the peak position of the LO mode shifts to the lower wavelength side from 199.24 to 188.332 cm-1, with a decrease in the full-width at half-maximum (FWHM) of the peaks [42]. The TO mode in the films is found shifted from 171.26 to 156.70 cm-1 and the intensity of all the peaks increases with temperature of deposition. The peak in the vicinity of 143.28 cm-1 is assigned to the 2 TA mode [43]. The Raman spectra of nanomaterials show LO-like phonon bands that have a typical asymmetric shape with a more pronounced low-frequency wind as observed in Fig. 8. The asymmetry in the lower frequency side of the Raman scattering of ZnSe nanoparticles is mainly related to surface phonon modes. Further, surface modes are observed because of their larger surface to volume ratio [44]. Our results show that the deposition temperature effect of the ZnSe nanoparticle is stronger than that of bulk material. Such an effect of shift mainly arises for the nano grained thin films, which is observed from the low defect density and the high thermal conductivity of the bulk materials either crystalline or course powders. Thermal conductivity of nano-structure materials is found decreased sharply because of the presence of lot of impurities, defects, and disorders [45], which induces significant absorption of light energy leading to the temperature effect. It is a well know fact that the property of nanograined materials is different from that of bulk material in many aspects, such as larger grain densities, enhanced surface, large surface area to volume ratio, and more defects. Longitudinal Optical (LO) modes, Transverse Optical (TO) and 2-phonon transverse acoustic (2TA) modes have been observed at about 188.32, 199.24 and 143.28 cm-1 respectively by Anand et al [46] for (111) oriented ZnSe films. Further, the intensity of LO, TO and 2TA peaks of our ZnSe films increases with deposition temperature. As far as EB evaporated ZnSe films are concerned, the LO and TO modes appeared at about 188.32 and 156.70 cm-1 respectively [47, 48]. It is a general observation that only the LO mode is allowed for the films with (111) plane orientation. In the present study, the Raman spectra of the EB evaporated ZnSe films have strong LO and less strong TO modes and weak 2TA modes. It shows that our ZnSe films have (111) predominant orientation and weakly oriented planes along (220) and (311) directions as observed from the TEM results for nanocrystalline ZnSe films.

( )/KTEo

aexpρρ =

FUNCTIONAL NANOSTRUCTURES 80

Figure 8 Raman spectrum of ZnSe films deposited at 300 °C.

Figure 9(a-b) UV absorbance spectrum of ZnSe films deposited at 300 °C.

calculated absorption co-efficient (α) values are about 104 cm-1 in this absorption edge region. The variation of (αhν)2 versus hν as seen in Fig. 10(a-d), is linear at this absorption edge, which confirmed a direct bandgap nature of the ZnSe films deposited in the present study. Extrapolating the linear portion (αhν)2 versus hν plots for zero absorption coefficient value gives the direct bandgap values of 2.88, 2.78, 2.69 and 2.63 eV for the ZnSe thin films deposited at substrate temperature RT, 100, 200 and 300 oC respectively. The bandgap value of RT and 100 oC deposited ZnSe films are 2.88 eV and 2.78 eV, which are found to be blue- shifted by about 0.08 – 0.18 eV from the bandgap value of 2.70 eV for the standard bulk material [52]. This is mainly due to the monophase and nanocrystalline nature of the deposited films in the present study. Further, the bandgap value of 2.88 eV is near to the already reported value of 3.10 eV corresponding to the optical transitions from spin-orbit split-off band states. This is a confirmation of the high quality nature of the ZnSe film with nanostructures [53, 54]. As the substrate temperature is increased, a red-shift of the bandgap values (2.69 and 2.63 eV) has been observed, approaching or nearly equal to the bulk value [55]. This is due to the increased grain size of the nanocrystallites of the ZnSe films from about 12 nm to 52 nm as observed by XRD results. Similar red-shift in bandgap energy, with increasing crystallites size has been reported for chemically deposited ZnSe thin films. Previous researchers [56] have reported two band gaps for chemically deposited ZnSe due to spin-orbit interaction. This red shift in the bandgap values with increasing particle size can also be predicted by the three dimensional quantum confinement model based on effective mass approximation ΔEg = Eg

eff - h2π2/ 2μR2, where, 1/μ = (1/me

* +1/ mh*), where Eg

eff is the effective band gap energy, Eg is the bulk band gap energy, R is the nanoparticle radius, h is the Planck constant, μ is the reduced effective mass and me

* and mh* are the

effective masses of electron and hole respectively [57]. All the transmittance spectra exhibit interference fringes revealing the smooth surface. The refractive index was calculated from the maxima and minima in the transmission spectra by the envelope method [58] using the equations mentioned earlier in reference 57. Figure 10 shows the variation of the refractive index for the films deposited at different substrate temperatures. The refractive index is nearly constant about 2.40 in the visible wavelength range 500-900 and about 900 up to 1600 nm there is a slight increase well below the absorption edge region, there is a sharp rise in the refractive index value. However, a small increase is observed in the average refractive index values for the films deposited at higher temperatures of 200 and 300 oC. This is due to the improved crystallinity of the ZnSe films with larger crystallite size at higher temperatures. The observed values of the refractive index are comparable with the value of 2.457 reported for vacuum evaporated films [59].

FUNCTIONAL NANOSTRUCTURES 81

transport in our system. In order to characterize the optical quality of the ZnSe crystal, the absorption measurement was carried out at room temperature in the optical edge region. The absorption edge is very sharp and is located at about 465 nm (Fig. 7), corresponding to that reported in the literature [63]. This is in good agreement with the energy band gap of 2.67 eV.

Figure 11(a-d) PL emission spectrum of ZnSe films deposited at 300 °C.

IV. CONCLUSION

ZnSe films with nanocrystalline nature have been prepared at different substrate temperatures. Films prepared at RT show highly nano grained surface compared to the other ZnSe films prepared at 100 oC, 200 oC and 300 oC. The nanograin size is found increasing with increased deposition temperature. This is confirmed by the analysis of XRD and AFM studies. Cubic structure has been observed for all ZnSe films with highly oriented along (111) direction. Optical transmittance studies show spectra with interference pattern revealing the surface uniformity and the direct band gap nature of the films. Optical band gap is found increasing with deposition temperature from 2.88 eV to 2.63 eV for RT to 300 oC respectively. Morphological studies by SEM/AFM show high uniform surface of these films and the TEM studies confirm the nano grain formation. Raman studies reveal the formation of monophase ZnSe in our study. Electrical studies show the n-type nature of the films and higher resistivity in the range of 105 - 107 ohm cm. XPS and EDAX study the presence of excess selenium in all the films, which confirm the high resistance of the ZnSe films studied in the present work and also show their stoichiometric condition appearing at 300 oC.

V. ACKNOWLEDGEMENTS

The authors gratefully acknowledge research funding from UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories, College

Figure 10(a-d) Refractive index & log ρ vs 1000/T graph of ZnSe films deposited at 300 °C.

Photoluminescence Studies

The photoluminescence (PL) properties of ZnSe polycrystalline were investigated by fluorescence spectrometer at room temperature. Fig. 11 shows emission and excited spectra of ZnSe polycrystal. As shown in Fig. 11(a), it is obvious that a relative weak emission peak at 487 nm which responds to the band-to-band emission at 2.58 eV for sphalerite ZnSe (cubic ZnS). The reason of weak emission peak at 487 nm is the self-absorption effect and nonradiative recombination of semiconductor which will decrease the outer quantum yield ratio of ZnSe semiconductor. In Fig. 11(a), in the long wavelength region, there are four peaks at 503, 571, 577 and 586 nm. The emission peak at 503 nm arises from the free exciton radiative recombination of an electron and a hole. The emission peaks at 571, 577 and 586 nm are assigned to lattice defect, such as 2ZnV + , 2SeV − , 2SeO − and other impurity energy level. Fig. 11(b) shows the excitation spectrum responding to 487 nm emission peak. The result shows the excitation wavelength responding to emission peak at 487 nm appears at 395 nm. The PL peak located at 439nm (2.8 eV) can be associated with the excitonic emission line [60], which is analogous to the excited state of FX (n = 2). It is the fingerprint of a high-quality ZnSe sample the emission line at low intensity could be due to the noise of the PL system or the instabilities of the exciting laser. The broad emission at 418 nm (3.0 eV) is not known to date. This emission band is analogous to the broad line (3.1 eV) described in the literature [61], but it was regarded as the noise of the measurement system by Fang et al. The study about this emission is being carried out. No self-activated luminescence band was detected in the low-energy side of the PL spectrum. This suggests that the ZnSe crystal does not contain impurities and zinc vacancy [62]. Compared with the PL spectrum with only SA luminescence of synthesized ZnSe polycrystals (Fig. 6 inserter), we can conclude that an as-grown ZnSe single crystal with high purity can be grown by Zn(NH4)3Cl5

FUNCTIONAL NANOSTRUCTURES 82

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of Graduate Studies, University of South Africa (UNISA), Muckleneuk Ridge, Pretoria, South Africa, (Research Grant Fellowship of framework Post-Doctoral Fellowship program under contract number Research Fund: 139000). One of the authors (Dr. K. Kaviyarasu) is grateful for the Prof. M. Maaza, Nanosciences African network (NANOAFNET), Materials Research Department (MSD), iThemba LABS-National Research Foundation (NRF), Somerset West, South Africa. The Support Program and the Basic Science Research Program through the National Research Foundation of South Africa are thanked for their constant support, help and encouragement.

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