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Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition
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Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition
D.D. Hile MethodologyvalidationWriting original draftformal analysisinvestigationresourcesdata curationwriting-revie H.C. Swart Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting-re S.V. Motloung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting T.E. Motaung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting K.O. Egbo Formal analysisinvestigationwriting-reviewers and editing , L.F. Koao Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting-rev PII: DOI: Reference:
S0040-6090(19)30734-5 https://doi.org/10.1016/j.tsf.2019.137707 TSF 137707
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
25 May 2019 6 November 2019 18 November 2019
Please cite this article as: D.D. Hile MethodologyvalidationWriting original draftformal analysisinvestigationresource H.C. Swart Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting-re S.V. Motloung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting T.E. Motaung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting K.O. Egbo Formal analysisinvestigationwriting-reviewers and editing , L.F. Koao Supervisionproject administration Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137707
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Highlights
Hydrazine hydrate was used as the only ligand to deposit Zinc Selenide thin films The films were deposited by chemical bath method by varying hydrazine hydrate volume The deposition was carried out at 80 °C for 2 h and annealed at 300 °C also for 2 h. The x-ray diffractometer shows hexagonal wurtzite structure The morphology changed from grains to nanowalls with change in the ligands volume
1
Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition
D.D. Hilea*, H.C. Swartb, S.V. Motloungc, d, T.E. Motaunge, K.O. Egbof, L.F. Koaoa* a
Department of Physics, University of the Free State, 9866, Phuthaditjhaba, South Africa
b
Department of Physics, University of the Free State, ZA9300, Bloemfontein, South Africa
c
Department of Physics, Nelson Mandela University, 6031, Port Elizabeth, South Africa
d
Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94,
Medunsa, 0204, South Africa. e
Department of Chemistry, University of Zululand, KwaDlangezwa, 3886, South Africa
f
Department of Physics, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong
Kong.
*Corresponding author email: [email protected]; and [email protected] Phone: +27642005389/+27587185300
2
Abstract ZnSe thin films were deposited using hydrazine hydrate (HH) as the only complexing agent. The deposition was carried out on glass substrates in an alkaline medium at low temperature using the chemical bath deposition method. The films were annealed at 300 ℃ for 2 h. The effects of varying the HH volumes on the structural and optical properties of the films were investigated. The investigations were based on glancing incidence X-ray diffraction, Raman spectroscopy, Field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS),
UV-Visible
spectrophotometry,
and
photoluminescence
(PL)
spectroscopy. The results revealed that the ZnSe films were of the wurzite structure. Raman spectra showed longitudinal optical vibrational peaks with intense peak at around 251 cm-1. There were fluctuations in the Raman peaks intensities. However, the intensity of the 20 mL HH sample emerged the highest. The morphology of the thin films changed from spherical grains to nanoflakes as the HH volume was increased from 5 to 35 mL, respectively. EDS of the films confirmed the presence of Zn and Se. The bandgaps of the films decreased with an increase in the HH volume. PL measurements indicated two emission peaks at 530 and 678 nm when excited at 325 nm. These emission peaks are attributed to the intrinsic intrabandgap defects. The intensities of the peaks increased with an increase in the HH volume. The commission Internationale de l’Elcairage color coordinates confirmed that the deposited ZnSe films exhibited green-red emission and the emission color was influenced by varying the HH volumes.
Key words: Zinc Selenide; chemical bath deposition; hydrazine hydrate; thin film.
3
1.0
Introduction Zinc Selenide (ZnSe) is one of the earliest semiconductors that was discovered and it
has prominent applications in optoelectronics, luminescence, solar cells and biomedical devices [1-6]. Since Zn and Se belong, respectively to the II and VI groups of the periodic table, ZnSe is referred to as an II – VI semiconductor material with a bulk bandgap of 2.7 eV [7]. The synthesis of ZnSe and many other semiconductors by wet chemical methods generally requires the use of a ligand in addition to the starting precursors. A ligand complexes and controls the release of ions in the solution making deposition or formation of the required semiconductor possible. In synthesizing ZnSe, one or more ligands are used such as sodium hydroxide (NaOH) and hydrazine hydrate (N2H4.H2O) (hereafter called HH) [6, 8], ethylene glycol and HH [9], sodium borohydrate (NaBH4) and mercaptoacetic acid [5], NaBH4 [10], cetyltrimethyl ammonium bromide and potassium borohydrate [3], ethylenediamine [7], ammonia (NH3) and HH [11, 12], NaOH [13], NH3, NaOH and HH [14]. Literature search revealed that, most of the ZnSe synthesized using wet chemistry used HH in addition to one or more complexing agents which shows that HH plays a vital role in the formation of ZnSe. However, for intended device applications, the use of many complexing agents may not be healthy as most of the elements may end-up as unintentional dopants. Mehta et al. [8] deposited ZnSe on glass substrate using chemical bath deposition (CBD) method at different temperatures. The study used NaOH and HH as complexing agents and the structural analysis showed X-ray diffraction peaks of zinc oxide (ZnO). The ZnO could have probably come from the NaOH. For economic reasons also, using more compounds may add cost and complexity, which may not be good for industrial applications.
4
Since HH plays a special role in the formation of ZnSe, its possible use as the only complexing agent will reduce the chances of unintentional dopants and cost. From the broad literature searched, there is currently no work that has been reported on CBD of ZnSe using HH as the only complexing agent. Wei et al. [11] investigated the effect of HH and zinc sulfate (ZnSO4) concentration on the structure and optical properties of ZnSe thin films using CBD with HH and NH3 as the complexing agents. In this work, zinc acetate [Zn(CH3COO)2] was employed as the source of Zn ions, while HH was used as the only complexing agent. HH was chosen as the complexing agent because it has a higher equilibrium constant for a complex formation than the NH3, therefore it could react with metal ions more easily to form a metal complex [15]. Researchers have adopted many techniques of synthesizing ZnSe both as powdered and thin films, such as the hydrothermal synthesis [2, 6, 16], electron beam evaporation method [17], transfer matrix method [18], vacuum evaporation [19], inert gas condensation [20], electrochemical deposition [4, 21], thermal treatment [7], low temperature solid phase process [3, 4], colloidal method [10], arrested precipitation [5], green synthesis [22], coprecipitation [9] and CBD [8, 11, 12, 14, 23]. The CBD method was used in this work for the deposition of the thin films owing to the numerous advantages it has over the other synthesis methods. These include cost effectiveness; ease of assembly of the deposition equipment, low deposition temperature, little or no hazards associated issues, ease of reproducibility, etc. CBD method however is faced with some drawbacks such as wastage of solution after each deposition and change in growth solution with time [24]. In the present study, ZnSe thin films were deposited using CBD and the analysis of the samples showed that the materials obtained could gain potential applications in optoelectronic devices such as window layer for thin film solar cells, infrared cameras and light emitting diodes. 5
2.0
Experimental Procedure:
2.1
Substrates and deposition bath preparation The substrates used for the deposition of the ZnSe thin films were microscope glass
slides with dimension of about 75 mm × 25 mm × 1 mm, purchased from Laboratory Consumables Company in South Africa. The chemicals were Zn(CH3COO)2 (99.99%), sodium sulfite (Na2SO3) (99.5%), selenium black powder (99+) and HH (N2H4.H2O) (80%), all purchased from Sigma Aldrich, and de-ionized water. All chemicals were used as purchased without further purification. Prior to the deposition, the substrates were degreased in a 70% nitric acid (HNO3) solution for one day in order to remove any unwanted compound that could have possibly coated on the surface. They were then washed in tap water using detergent and rinsed with acetone (C3H6O) (≥99.5%) then ultrasonically cleaned using de-ionized water and finally dried in a microwave oven. Zn(CH3COO)2 was used as the source of the Zn2+ ions and sodiumselenosulfate (Na2SeSO3) as the source of the Se2- ions. Due to the fact that selenium has many oxidation states; it tends to be very unstable in an aqueous solution [25]. For that reason, it was first reacted with anti-oxidant Na2SO3 to form Na2SeSO3 before deposition commenced. In this work, Na2SeSO3 was prepared by refluxing in the ratio of 1 g: 5 g: 50 mL of selenium black powder, Na2SO3 and de-ionized water, respectively at 80 ℃ for 4 h. The insoluble Se powder upon heating under constant stirring dissolved completely giving a clear solution with no sediments. Hence the solution was used without filtration contrary to earlier report by some researchers [8, 14].
6
2.2
Deposition of ZnSe thin films The deposition of ZnSe thin films was carried out using different volumes of HH in a
bath solution containing 20 mL, 0.1 M Zn(CH3COO)2 + x mL HH (x = 5, 10, 15, 20, 25, 30, 35) + 20 mL, 0.59 M Na2SeSO3 added in that order at room temperature upon constant stirring. The x mL (x = 5, 10, 15, 20, 25, 30, 35) volumes of the HH correspond to 0.08, 0.17, 0.25, 0.33, 0.41, 0.49 and 0.58 M, respectively. The deposition parameters are summarized in Table 1. The HH was introduced gradually into the Zn(CH3COO)2 solution during each deposition. The first few drops produced white precipitates as a result of zinc hydroxide formation, which, however disappeared to a clear and colorless solution when the HH volume got up to 5 mL and above. After mixing, the solution was immersed in a water bath kept at 80 ℃ on a magnetic stirrer machine. The already cleaned substrates were suspended vertically on substrates holder and immersed into the chemical solution and the solution was stirred continuously throughout the deposition period to avoid sedimentation at the bottom of the beaker. After the deposition period of 2 h, the coated substrates were removed and washed thoroughly with de-ionized water to remove the loosely hung particles and were dried using a hand held drier. There was no deposition when the HH volume was below 5 mL and that is the main reason why the deposition was started at 5 mL in this study. The thin film deposition rate was observed to increase with an increase in HH volume. However, at HH volumes of 25 mL, the deposition reached its peak and the films deposited at higher HH volumes started peeling off when washed with de-ionized water. After the deposition, the samples were annealed in open air at 300 ℃ for 2 h. The annealing conditions were chosen based on some literature reports about ZnSe thin films [26]. The deposition was made possible when there was just enough volume of HH to facilitate the release of the Zn2+ and Se2- ions according to the following reaction processes. 7
(
)
( (
(
)
)
(
)
) (
)
(
)
(1)
(
)
(2)
(
)
(3)
2.3 Characterization of the deposited samples The structural analysis of the annealed thin films of ZnSe was carried out using glancing incidence X-ray diffraction (GIXRD) with a Rigaku Smartlab glancing diffractometer system at Cu_Kα wavelength of 0.1540 nm and operated at a fixed glancing angle of 1⁰. Raman spectroscopy was carried out on the samples using a WITec Confocal Raman microscopy machine (alpha 300R) with an excitation wavelength of 532 nm. The surface morphology and the chemical composition of the thin films were determined using a Joel JSM-7800F field emission scanning electron microscope (FESEM) with a beam voltage of 10 kV equipped with an Oxford Aztec 350X-Max80 energy dispersive X-ray spectrometer (EDS). UV-Visible spectrophotometry was used for the optical characterization of the deposited ZnSe thin films in the wavelength range between 300 – 1000 nm using a Shimadzu UV-1700 PharmaSpec UV-Vis spectrophotometer. Photoluminescence (PL) analysis was carried out at room temperature using Varian Cary Eclipse Fluorescence Spectrophotometer with an excitation source of a 150 W xenon lamp. 3.0 Results and Discussion 3.1
Glancing incidence X-ray diffraction (GIXRD) study Fig. 1(a) shows the GIXRD patterns of all the samples of ZnSe thin films deposited
on the glass substrates at various x mL HH volumes (x = 5, 10, 15, 20, 25, 30, 35). The structural analysis of the samples was carried out using GIXRD covering a wide range of Bragg’s angle 2θ between 20 – 70°. The GIXRD peaks revealed that the thin films were 8
polycrystalline in nature with the peaks indexed to the primitive hexagonal phase of ZnSe with space group P63mc (186) corresponding to the JCPDS no. 150105. The (002) plane was the most preferred orientation plane for all the samples. It could be observed that the film was almost amorphous or nanoparticulates for the sample synthesizes at x = 5 mL. This may imply that ZnSe may have just started forming, containing some unreacted elements or defects. The presence of small diffraction peaks from the (002) and (110) planes, respectively at around 2θ = 27 and 46° may serve as a clear indication of the ZnSe material starting to emerge.
From x = 10 mL, more peaks sprouted out implying improved crystallinity as the
HH volume was increased. HH is a strong reducing agent and so sufficient amount of it in the solution reduces the selenosulfate to provide sufficient Se2- ions concentration to permit increased nucleation and growth. The average crystallite sizes (D) were estimated from the (002) diffraction peak using Scherrer’s formula and were found to range from 5 to 14 nm. Mehta et al. [8] synthesized ZnSe nanocrystallites using CBD and obtained the average crystallite size in the range of 7 to 18 nm when the films were annealed at different temperatures. Generally, the average crystallite sizes have increased with an increase in the HH volume until at x = 20 mL then there was a decrease thereafter as shown in Table 2. This probably could be due to the sufficient concentration of HH that leads to increased nucleation and crystallite growth. However, too much concentration of HH in the solution then means to too high complexation and that leads to low crystallite size formation. The results agreed with the observation of Wei et al. [11] in a similar study where they used different HH volumes and ZnSO4. The lattice parameters of the films were estimated from the most intense peaks (002) and (110). The d-spacing and lattice constants (a = b and c) were calculated respectively using Bragg’s law [27]. The calculated unit cells were in the range a = b = 3.929 to 3.983 Ǻ and c = 6.48 to 6.51 Ǻ which are comparable with the standard values a = b = 3.996 Ǻ and c 9
= 6.55 Ǻ for the hexagonal wurtzite structure according to the JCPDS no. 150105. The dspacing does not show any sequential trend with increased HH volume. This suggests that dspacing of ZnSe thin films under the conditions used in this work does not depend on HH volume. The calculated values for each x mL of HH are presented in Table 2. A similar behavior concerning different HH volumes was observed by Preetha and Remadevi [28], when they used different HH volume (concentration) to deposit thin films of lead selenide by successive ionic layer absorption and reaction method. They have observed that the lattice parameters, crystallites sizes and strain varied with a change in concentration of HH. The parameters were observed to increase with HH volume up to 80% of the concentration used, and then started falling back. The reason for an increase in lattice parameters up to a certain level where after they reduced is that HH plays an important role in the internal strain of the chalcogenide materials. If its volume is low, there will be no much complexation with the metal ions leading to low release of elemental selenium or sulfide in the solution. This results to defects in the structure leading to internal strain. When HH is high, there will be too much complexity leading to low metal ions which again results to defects in the structure of the material [28]. Mehta and co-workers [8] reported that stresses and strains are always present in thin films deposited on glass. The lattice strain (ε) was estimated following equation 4 [26].
(4) Where β is the full width at half maximum of the diffraction peak and θ is the Bragg’s angle. The dislocation density, δ known as the length of dislocation lines per unit volume of crystal in each sample is estimated using equation 5 [27]. (5)
10
The δ values decreased with increased HH volume, which signifies that higher defects are present in films with lower crystallization (lower HH volumes) [27]. The results of the structural parameters of the ZnSe thin films calculated are presented in Table 2. Fig. 1 (b) presents the GIXRD peaks of the (002) plane. The diffraction patterns show no linear correlation between the peaks intensities and the HH volumes. The fluctuation in the GIXRD peaks intensities of the deposited ZnSe thin films is an indication that there is no direct proportional relationship between the peak intensities and the HH volume. A slight shift in the peaks position was observed from the figure but also fluctuates with an increased HH volume. The peak shift may be due to a change in microstructure parameters (crystallize size and lattice strain) as asserted by Shaaban et al. [29]. In this work, the increase in HH volume provided higher complexation and slowed the release of zinc ions in the deposition process leading to improved crystallinity and the lattice parameters of the films. 3.2
Raman spectroscopy study For further investigation of the structural and nature of the ZnSe thin films
nanoparticles, Raman scattering spectroscopy was used. The ZnSe thin films were excited at a wavelength of 532 nm at room temperature. Fig. 2 (a) depicts the Raman intensities versus wavenumber (
) of the ZnSe thin films. Two Raman peaks (1LO) and 2LO all ascribed
to longitudinal optical phonon modes are observed between 256-262 and 490 cm-1, respectively. The sharp peak around 256-262 cm-1 positions is in agreement with that observed by many other workers [1, 30-35]. The 1LO peak in the vicinity of 256-262 cm-1 is significantly lower for the samples prepared with x = 5 and 35 mL HH as compared to the other samples. This observation is in agreement with the GIXRD analysis and it suggests that 5 mL is too low while 35 mL is too high for the formation of quality ZnSe thin films as explained earlier. The peak of 2LO was equally observed by Li et al. [36] and Hu et al. [37].
11
The broadening of the Raman peaks as seen in Fig. 2 (a) is attributed to small crystallite sizes of the films and the defects in the crystalline structure as observed also in the GIXRD analysis [38]. The good crystallinity of the thin films is also proved by the presence of symmetric peaks and the sharpness of the 1LO phonon mode peaks [30, 33, 34]. The shift in the 1LO phonon peak seen in Fig. 2 (b) is related to changes in bond length of the molecules due to different HH volumes used. It can also be noticed that there were no unexpected peaks detected in the Raman spectra measurements shown in Fig. 2 (a), which agrees very well with the GIXRD results obtained in Fig. 1 (a). In order to correlate the structural characteristics between the GIXRD and Raman spectroscopic analyses, the (002) GIXRD peak and Raman spectra 1LO peak intensities versus the change in HH volume are plotted together and presented in Fig. 3. The figure shows that the peak intensities were low for both GIXRD and Raman at x = 5 mL HH. However, as the HH volume increased both peaks intensities increased and reached the climax at x = 10 mL HH for the GIXRD and 20 mL HH for the Raman analysis. However, further increase in the HH above the respective volumes resulted in the degradation of the ZnSe thin films as characterized by the decrease in the intensities of both the GIXRD and Raman peaks though with some fluctuations. Based on the structural analysis, x = 10 – 20 mL HH is suitable in the deposition of ZnSe thin films using CBD and HH as the only complexing agent. 3.3
Field emission scanning electron microscopy (FESEM)/energy dispersive X-ray
spectroscopy (EDS) To study the morphology of the ZnSe thin films, FESEM imaging was carried out on four samples of the deposited films for x = 5, 10, 15 and 35 mL as shown in Fig. 4. Fig. 4 (a) illustrates the sample deposited at x = 5 mL, which shows spherical grains with fairly uniform
12
sizes. The FESEM images further confirmed the role of HH concentration in the formation and morphology of ZnSe. Increasing the HH volume changed the density of the resulting nanoparticles and their morphology [30]. Fig. 4 (b) shows the transformation stage from the spherical to the nanowalls or nanoflakes. At this stage, the increase HH volume has shown a significant change in the morphology of the deposited ZnSe thin films. This agrees with the GIXRD result, which showed improved crystallinity as the volume of HH was increased from x = 5 to 10 mL showing more peaks of ZnSe. Fig. 4 (c) and (d) show further transformation from nanowalls to a higher degree of nanowalls with a flower-like morphology. Thus, the results clearly suggest that the HH volume influences the morphology of the prepared thin films. This result agrees completely with the report of Wei et al. [11] and Bakiyaraj et al. [14]. The uniform and homogenous morphology indicates the presence of a single phase of ZnSe as confirmed by the GIXRD. The presence of Zn and Se in the films were confirmed by the EDS analysis shown in Fig. 5 (a - d), which corresponds to the FESEM images of Fig. 4 (a - d), respectively. Table 3 shows that at a low HH volume as seen on the sample of x = 5 mL, the atomic percentage of Zn was above 60% whereas that of Se was below 40%. This might imply that the material was Zn rich. This implies that lesser Se was released in the solution bath whereas more Zn was produced, which indicates that there were not enough complexing ions to facilitate the release of Se ions into the solution. This is further motivated by the fact that according to Pingale et al. [38] the release of Se ions in the bath solution during ZnSe thin film deposition is sorely controlled by the HH volume. In their work, to deposit ZnSe using ammonia and HH as ligand, they have observed that the growth of Se ions in the solution improved significantly with the increase in HH volume. They have concluded that the ammonia only provides an alkaline environment for the thin films formation [38]. The effect of HH on the formation of ZnSe was therefore confirmed in this work as can be clearly seen on the 13
morphology and Zn:Se ratio of the deposited thin films. HH is a bidenate ligand and can react with a metal ion to form two different kinds of metal complexes [39]. According to Ghamsari and Araghi [39], when HH is added to the CBD, which has metal ions such as Zn2+, Pb2+ etc., it causes a decrease on the releasing rate of the ions in the bath and this gives rise to an increase in the semi-metal growth. It can be seen that as the volume of HH is added to the solution, there is a great improvement in the atomic percentage of Se over Zn and the ratio of the Zn to Se ions, turning to 1:1 as seen in Table 3. The EDS results did not show the presence of unexpected elements, which agree perfectly with the XRD and Raman results, respectively (see Fig. 1 and 2). The other elements: Ca, Mg, Si, Na, O present are from the glass substrates used [14]. 3.4
Optical absorption spectroscopy UV-Visible spectra is used to obtain information about the optical properties of the
deposited thin films of the ZnSe. Fig. 6 (a) depicts the absorption spectra of the ZnSe thin films on the wavelength range between 300 and 1000 nm. Fig. 6 (a) shows that the set of ZnSe thin films samples does not display similar absorption behavior with absorption curves. It can be observed that by increasing the HH volumes, the absorption edges shifted to higher wavelengths though with fluctuations. The observed shift in absorption edge towards higher wavelength is most probably due to increasing thickness of the films as HH volume enhances ZnSe formation rate [40]. To estimate the energy bandgap (Eg) of the ZnSe thin films, a graph of the square of absorbance (A2) is plotted against photon energy (hν) as shown in Fig. 6 (b) [41 - 45]. This is a very quick method of estimating the Eg without going through the problem of measuring the film thickness and therefore calculating absorption coefficient in order to do a Tauc plot. By extrapolating the linear portion of the curves to the point of intercept on the hν axis where A2
14
= 0, gives the optical Eg. The estimated values of the Eg are shown in Table 4. The Eg of the bulk ZnSe material is 2.7 eV [45]. The variation in the Eg may be due to the crystallite size effect, which makes them differ from the theoretical E g. Koao et al. [46] in their work to determine the effect of octadecylammine molar concentration on the structure, morphology and optical properties of ZnO nanostructure prepared by homogeneous precipitation, reported a decrease in Eg with an increase in crystallite size. The same effect is observed in this work as wider Eg were observed in samples with smaller crystallites sizes. The Eg values are nevertheless close to the ZnSe Eg ranges of 1.9 – 3.1 eV estimated by Yildirim et al. [26]. On the other hand, Al-Kuhaili et al. [47] reported the Eg to be in the range 1.75 – 2.58 eV for the same material synthesized using the thermal evaporation method. It is quite interesting to see that these values are in line with this study results. The percentage transmission spectra for the CBD ZnSe thin films is presented in Fig. 6 (c). The transmittance wavelength covered by this study is within the UV-visible to near infrared (300 – 1000 nm). The transmission spectra as can be seen shifted to higher wavelength as the HH volume increased. The shift in absorption edges to higher wavelength observed in Fig. 6 (a) and (c) and the decrease in band gap observed in Fig. 6 (b) may be due to the increase in estimated crystallite sizes with an increase in the amount of HH [48]. 3.5
Photoluminescence (PL) spectroscopy study Fig. 7 (a) shows the excitation spectra of the ZnSe thin film obtained when
monitoring emission at 530 nm. The spectra of all the samples show that there are two excitation bands peaks. The first band peak shifted from 325 nm to 337 nm corresponding to 3.82, and 3.68 eV, respectively. This excitation band can be attributed to defect absorption level in the material, which is due to fundamental absorption of excitons [7, 46]. The shift in excitation peak position may be due to the change in crystallite sizes and nanostructures
15
morphology discussed earlier. The second excitation band is located at 433 nm (2.86 eV) excitation wavelength and this can be ascribed to near-band transition due to the fact that it is close to the Eg of the ZnSe (2.8 eV) estimated from the optical absorption spectra measurement. The excitation wavelength of 325 nm from the first band was, however adopted in this study because of two reasons. (1) It produces excitation bands with higher emission intensity as seen in Fig. 7 (b). (2) It is chosen based on related literature values [40, 50, 51]. Generally, the excitation intensities were observed to increase with an increase in the HH volume although there is fluctuation as seen in Fig. 7 (a). Room temperature PL emission spectra for the CBD ZnSe thin films are presented in Fig. 7 (b). The samples were excited at 325 nm (3.82 eV) and showed two symmetric emission peaks: a strong green emission peak at 530 nm (2.34 eV) and a weak red emission peak at 678 nm (1.83 eV). The emission peak at 530 nm (2.35 eV) is ascribed to the intrinsic intra-bandgap defects such as Zn vacancy defect states [21, 14, 52]. The weak emission peak located at 678 nm corresponding to the defect absorption can be attributed to growth conditions and presence of non-stoichiometric point defects due to Se vacancies or selfactivated luminescence as a result of donor acceptor pairs that are related to Zn vacancies and interstitial states which could be present on the surface of the nanomaterials during preparation [21, 52-54]. The green-red ZnSe PL emission peaks have been observed by many researchers and attributed to defect levels in the material [37, 55]. The origin of the green-red ZnSe PL peaks according to Bakiyaraj et al. [14] may be attributed to intrinsic point defect. In their study on synthesis and characterization of flower-like ZnSe nanostructured thin films using CBD, the typical deep defect related emission was observed between 552 - 658 nm. Careful observation reveals that there is a slight emission peak shift to the higher wavelength with an increase in the HH volume. The shift could be due to defect levels in the films [14].
16
A change in the relative ratio of the two emission peaks is also visible due to higher selfactivated luminescence.
The dependence of the emission intensity as a function of the HH volume based on the 530 nm peak is shown in Fig. 7 (c). The emission intensity increased with an increase in the HH volume with the sample of x = 20 mL having the optimal intensity. This result agrees partly with the Raman spectra analysis and the crystallite size estimated from the GIXRD result. The proposed PL excitation and emission mechanism of the deposited ZnSe thin films samples is represented by a schematic energy band diagram as shown in Fig. 8. When the samples are excited with light energy with a wavelength of 325 nm (3.82 eV), the electron (shown as red circles) in the valence band (VB) are excited in a broad optical band and trapped into the conduction band (CB) of the ZnSe. The electron trapped in the CB thereafter relaxes to the lowest vibrational level of the excited state by non-radiative relaxation (NRR) and give up excess energy to the surroundings. The electron (with energy = 2.86 eV) may return from the CB by NRR back to the VB or through different channels as indicated by the blue arrows in Fig. 8. The electrons also may be trapped in other defect states levels (with energies = 2.34 and 1.83 eV) from where again it will be de-excited to the VB through radiative decay emitting photons of different energies as depicted by the green and red arrows in Fig. 8. It is clear that both emission peaks at 530 (2.34 eV) and 678 nm (1.83 eV) originated from ZnSe according to literature reports [14, 21, 37, 52, 53, 55]. These emissions are ascribed to the radiative decay due to electrons from the different trap levels recombining with the holes (represented in white circles) in the VB [56]. A decay lifetime profile of ZnSe thin films obtained by exciting the samples with 325 nm wavelength sources and by monitoring the emission at 530 nm is illustrated in Fig. 9 (a). Plot of the natural logarithm of the lifetime intensity as a function of HH volume gives a
17
linear graph as seen in Fig. 9 (b), which suggests that the decay is a mono-exponential decay lifetime [57]. Thus, the lifetime decay curves were fitted to a mono-exponential function according to equation 6. ( )
(
⁄ )
(6)
Where t is the time of measurement, τ1 is the decay lifetime component with intensity I1. The variation of lifetime values at different HH volumes obtained from the mono-exponential decay fitting is indicated in Fig. 9 (c). The lifetime of the material increased when the HH volume increased from x = 5 to 20 mL, thereafter it decreased. This shows that the deposited films have a longer emission lifetime with the sample of x = 20 mL HH. This is consistent with the crystallite sizes and Raman analyses and we can conclude that 20 mL of HH is the most ideal volume for the deposition of ZnSe thin films under the condition used in this work. The international commission on illumination (CIE) chromaticity coordinates for the ZnSe thin films are presented in Fig. 10. CIE calculates the emission color of the material as well as the color coordinates in the chromaticity diagram based on the PL emission results. The CIE coordinates for the ZnSe thin films were estimated from the PL emission spectral presented in Fig. 7 (b). The CIE color chromaticity shows a green color emission with a slight shift to longer wavelengths, which, however fluctuated with the increase in the HH volume just as observed in the emission spectra, shown in Fig. 7 (b). This shows that the emission color of these ZnSe nanoparticles can be tuned by varying the volume or concentration of the HH as the complexing agent.
18
4.0
Conclusion ZnSe thin films of single phase hexagonal wurtzite structure have been deposited on
glass substrates via the CBD route using HH as the only complexing agent. The films were highly polycrystalline with average crystallite sizes ranging between 5 and 14 nm. The highest GIXRD intensity was produced by the sample at x = 10 mL, whereas Raman spectra analysis recorded the highest intensity from the sample at x = 20 mL. The surface morphology was observed to change greatly from spherical grains to walls as the HH volume was increased. The films showed wide absorption band from the UV to the red region of the spectrum although not all samples showed clear absorption edges. The bandgap of the ZnSe films was in the range of 1.5 to 2.8 eV. The best bandgap of 2.6 eV with respect to the bulk bandgap of ZnSe was recorded from the sample at x = 10 mL. Room temperature PL studies with excitation wavelength of 325 nm has shown two emission bands at 530 and 678 nm, which were attributed to intrinsic intra-bandgap defects and non-stoichiometric point defect, respectively. The sample deposited with x = 20 mL showed highest emission intensity and longer decay lifetime. The CIE color coordinates showed that the emission color of the ZnSe nanoparticles can be tuned by varying the HH volume. It can be concluded from the findings that 20 mL of HH is the most ideal for the ZnSe thin films deposition under the conditions employed in this study. The study reveals that the deposited materials could have potential applications in solid state lighting such as light emitting diodes based on the observed emission colors. The wide energy bandgaps obtained also suggest that the films are good candidates for optoelectronics devices such as window materials for solar cells and infrared cameras.
19
Acknowledgements The authors would like to acknowledge the National Research Foundation, UID: 99224, the University of the Free State, the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Fund (NRF) (Grant 84415) for their financial support.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Credit Author Statement D.D. Hile: Methodology, validation, Writing original draft, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization. H.C. Swart: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition. S.V. Motloung: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition. T.E. Motaung: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition. K.O. Egbo: Formal analysis, investigation, writing-reviewers and editing L.F. Koao: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition.
20
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List of Tables Table 1. The precursors parameters used for the ZnSe thin films deposition Table 2. Structural parameters of the deposited ZnSe thin films. Table 3. Change in atomic ratio of Zn and Se with increased HH volume. Table 4. Estimated bandgap values of the ZnSe thin films deposited using different HH volumes.
Table 1. Sample name
Zn(CH3COO)2
HH
Na2SeSO3
5 mL
Volume (mL) 20
Conc. (M) 0.1
Volume (mL) 5
Conc. (M) 0.08
Volume (mL) 20
Conc. (M) 0.59
10 mL
20
0.1
10
0.17
20
0.59
15 mL
20
0.1
15
0.25
20
0.59
20 mL
20
0.1
20
0.33
20
0.59
25 mL
20
0.1
25
0.41
20
0.59
30 mL
20
0.1
30
0.49
20
0.59
35 mL
20
0.1
35
0.58
20
0.59
29
Table 2. δ (lines/c m2)
HH volume 2θ (⁰) β (⁰) (mL)
D (nm)
d-spacing (Å)
a (Å)
c (Å)
5
27.38
1.91
5
3.254
3.949
6.51
0.0342
0.04
10
27.50
1.75
5
3.240
3.966
6.48
0.0312
0.04
15
27.45
1.01
8
3.245
3.938
6.49
0.0115
0.016
20
27.40
0.63
14
3.255
3.965
6.51
0.0182
0.005
25
27.47
0.65
13
3.243
3.929
6.49
0.0113
0.006
30
27.41
0.64
13
3.250
3.975
6.50
0.0114
0.006
35
27.45
0.64
13
3.245
3.983
6.49
0.0114
0.006
30
Table 3. HH volume (mL)
5
10
15
35
Element
Zn Se
Zn Se
Zn Se
Zn Se
Atomic (%)
67
57
56 44
51 49
Ratio Zn : Se
1 : 0.49
1 : 0.78
1 : 0. 96
33
43
1 : 0.75
31
Table 4. HH volume (mL) 5 Eg (eV)
10
15
20
25
30
35
2.8 2.6 2.5 2.0 1.7 1.6 1.5
32
List of Figures
Fig. 1 (a) GIXRD patterns of the ZnSe thin films (b) zoomed 002 diffraction peaks of the thin films presented in Fig. 1 (a).
33
Fig. 2 (a). Raman Spectra of the deposited ZnSe thin films (b) the normalized 1LO peak of Fig 2 (a).
34
Fig. 3. GIXRD preferred orientation peak intensity and Raman spectra 1LO peak intensity against the HH volume.
Fig. 4. FESEM micrographs of the ZnSe thin films grown at (a) x = 5 (b) 10 (c) 15 and (d) 35 mL.
35
Fig. 5. EDS spectra corresponding to SEM images of the ZnSe thin films grown at (a) x = 5 (b) 10 (c) 15 and (d) 35 mL.
36
Fig. 6. (a) Absorption spectra (b) estimated energy bandgap plots (c) transmittance spectra of the ZnSe thin films deposited at various HH volumes.
37
Fig. 7. (a) PL excitation spectra (b) PL emission spectra (c) Dependance of the PL emission intensity of the ZnSe thin films for the various HH volumes.
Fig. 8. The proposed excitation and emission pathways mechanism on the ZnSe thin films.
38
Fig. 9. Lifetime (a) decay curve (b) half-log scale plot (c) the variation of decay lifetime with HH volumes for the ZnSe thin films.
39
Fig. 10. CIE color chromaticity coordinates of the CBD ZnSe thin films deposited using various HH volumes.
40
Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition
D.D. Hile MethodologyvalidationWriting original draftformal analysisinvestigationresourcesdata curationwriting-revie H.C. Swart Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting-re S.V. Motloung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting T.E. Motaung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting K.O. Egbo Formal analysisinvestigationwriting-reviewers and editing , L.F. Koao Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting-rev PII: DOI: Reference:
S0040-6090(19)30734-5 https://doi.org/10.1016/j.tsf.2019.137707 TSF 137707
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
25 May 2019 6 November 2019 18 November 2019
Please cite this article as: D.D. Hile MethodologyvalidationWriting original draftformal analysisinvestigationresource H.C. Swart Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting-re S.V. Motloung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting T.E. Motaung Supervisionproject administrationvalidationformal analysisinvestigationresourcesdata curationwriting K.O. Egbo Formal analysisinvestigationwriting-reviewers and editing , L.F. Koao Supervisionproject administration Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137707
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Highlights
Hydrazine hydrate was used as the only ligand to deposit Zinc Selenide thin films The films were deposited by chemical bath method by varying hydrazine hydrate volume The deposition was carried out at 80 °C for 2 h and annealed at 300 °C also for 2 h. The x-ray diffractometer shows hexagonal wurtzite structure The morphology changed from grains to nanowalls with change in the ligands volume
1
Effect of hydrazine hydrate as complexing agent in the synthesis of zinc selenide thin films by chemical bath deposition
D.D. Hilea*, H.C. Swartb, S.V. Motloungc, d, T.E. Motaunge, K.O. Egbof, L.F. Koaoa* a
Department of Physics, University of the Free State, 9866, Phuthaditjhaba, South Africa
b
Department of Physics, University of the Free State, ZA9300, Bloemfontein, South Africa
c
Department of Physics, Nelson Mandela University, 6031, Port Elizabeth, South Africa
d
Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94,
Medunsa, 0204, South Africa. e
Department of Chemistry, University of Zululand, KwaDlangezwa, 3886, South Africa
f
Department of Physics, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong
Kong.
*Corresponding author email: [email protected]; and [email protected] Phone: +27642005389/+27587185300
2
Abstract ZnSe thin films were deposited using hydrazine hydrate (HH) as the only complexing agent. The deposition was carried out on glass substrates in an alkaline medium at low temperature using the chemical bath deposition method. The films were annealed at 300 ℃ for 2 h. The effects of varying the HH volumes on the structural and optical properties of the films were investigated. The investigations were based on glancing incidence X-ray diffraction, Raman spectroscopy, Field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS),
UV-Visible
spectrophotometry,
and
photoluminescence
(PL)
spectroscopy. The results revealed that the ZnSe films were of the wurzite structure. Raman spectra showed longitudinal optical vibrational peaks with intense peak at around 251 cm-1. There were fluctuations in the Raman peaks intensities. However, the intensity of the 20 mL HH sample emerged the highest. The morphology of the thin films changed from spherical grains to nanoflakes as the HH volume was increased from 5 to 35 mL, respectively. EDS of the films confirmed the presence of Zn and Se. The bandgaps of the films decreased with an increase in the HH volume. PL measurements indicated two emission peaks at 530 and 678 nm when excited at 325 nm. These emission peaks are attributed to the intrinsic intrabandgap defects. The intensities of the peaks increased with an increase in the HH volume. The commission Internationale de l’Elcairage color coordinates confirmed that the deposited ZnSe films exhibited green-red emission and the emission color was influenced by varying the HH volumes.
Key words: Zinc Selenide; chemical bath deposition; hydrazine hydrate; thin film.
3
1.0
Introduction Zinc Selenide (ZnSe) is one of the earliest semiconductors that was discovered and it
has prominent applications in optoelectronics, luminescence, solar cells and biomedical devices [1-6]. Since Zn and Se belong, respectively to the II and VI groups of the periodic table, ZnSe is referred to as an II – VI semiconductor material with a bulk bandgap of 2.7 eV [7]. The synthesis of ZnSe and many other semiconductors by wet chemical methods generally requires the use of a ligand in addition to the starting precursors. A ligand complexes and controls the release of ions in the solution making deposition or formation of the required semiconductor possible. In synthesizing ZnSe, one or more ligands are used such as sodium hydroxide (NaOH) and hydrazine hydrate (N2H4.H2O) (hereafter called HH) [6, 8], ethylene glycol and HH [9], sodium borohydrate (NaBH4) and mercaptoacetic acid [5], NaBH4 [10], cetyltrimethyl ammonium bromide and potassium borohydrate [3], ethylenediamine [7], ammonia (NH3) and HH [11, 12], NaOH [13], NH3, NaOH and HH [14]. Literature search revealed that, most of the ZnSe synthesized using wet chemistry used HH in addition to one or more complexing agents which shows that HH plays a vital role in the formation of ZnSe. However, for intended device applications, the use of many complexing agents may not be healthy as most of the elements may end-up as unintentional dopants. Mehta et al. [8] deposited ZnSe on glass substrate using chemical bath deposition (CBD) method at different temperatures. The study used NaOH and HH as complexing agents and the structural analysis showed X-ray diffraction peaks of zinc oxide (ZnO). The ZnO could have probably come from the NaOH. For economic reasons also, using more compounds may add cost and complexity, which may not be good for industrial applications.
4
Since HH plays a special role in the formation of ZnSe, its possible use as the only complexing agent will reduce the chances of unintentional dopants and cost. From the broad literature searched, there is currently no work that has been reported on CBD of ZnSe using HH as the only complexing agent. Wei et al. [11] investigated the effect of HH and zinc sulfate (ZnSO4) concentration on the structure and optical properties of ZnSe thin films using CBD with HH and NH3 as the complexing agents. In this work, zinc acetate [Zn(CH3COO)2] was employed as the source of Zn ions, while HH was used as the only complexing agent. HH was chosen as the complexing agent because it has a higher equilibrium constant for a complex formation than the NH3, therefore it could react with metal ions more easily to form a metal complex [15]. Researchers have adopted many techniques of synthesizing ZnSe both as powdered and thin films, such as the hydrothermal synthesis [2, 6, 16], electron beam evaporation method [17], transfer matrix method [18], vacuum evaporation [19], inert gas condensation [20], electrochemical deposition [4, 21], thermal treatment [7], low temperature solid phase process [3, 4], colloidal method [10], arrested precipitation [5], green synthesis [22], coprecipitation [9] and CBD [8, 11, 12, 14, 23]. The CBD method was used in this work for the deposition of the thin films owing to the numerous advantages it has over the other synthesis methods. These include cost effectiveness; ease of assembly of the deposition equipment, low deposition temperature, little or no hazards associated issues, ease of reproducibility, etc. CBD method however is faced with some drawbacks such as wastage of solution after each deposition and change in growth solution with time [24]. In the present study, ZnSe thin films were deposited using CBD and the analysis of the samples showed that the materials obtained could gain potential applications in optoelectronic devices such as window layer for thin film solar cells, infrared cameras and light emitting diodes. 5
2.0
Experimental Procedure:
2.1
Substrates and deposition bath preparation The substrates used for the deposition of the ZnSe thin films were microscope glass
slides with dimension of about 75 mm × 25 mm × 1 mm, purchased from Laboratory Consumables Company in South Africa. The chemicals were Zn(CH3COO)2 (99.99%), sodium sulfite (Na2SO3) (99.5%), selenium black powder (99+) and HH (N2H4.H2O) (80%), all purchased from Sigma Aldrich, and de-ionized water. All chemicals were used as purchased without further purification. Prior to the deposition, the substrates were degreased in a 70% nitric acid (HNO3) solution for one day in order to remove any unwanted compound that could have possibly coated on the surface. They were then washed in tap water using detergent and rinsed with acetone (C3H6O) (≥99.5%) then ultrasonically cleaned using de-ionized water and finally dried in a microwave oven. Zn(CH3COO)2 was used as the source of the Zn2+ ions and sodiumselenosulfate (Na2SeSO3) as the source of the Se2- ions. Due to the fact that selenium has many oxidation states; it tends to be very unstable in an aqueous solution [25]. For that reason, it was first reacted with anti-oxidant Na2SO3 to form Na2SeSO3 before deposition commenced. In this work, Na2SeSO3 was prepared by refluxing in the ratio of 1 g: 5 g: 50 mL of selenium black powder, Na2SO3 and de-ionized water, respectively at 80 ℃ for 4 h. The insoluble Se powder upon heating under constant stirring dissolved completely giving a clear solution with no sediments. Hence the solution was used without filtration contrary to earlier report by some researchers [8, 14].
6
2.2
Deposition of ZnSe thin films The deposition of ZnSe thin films was carried out using different volumes of HH in a
bath solution containing 20 mL, 0.1 M Zn(CH3COO)2 + x mL HH (x = 5, 10, 15, 20, 25, 30, 35) + 20 mL, 0.59 M Na2SeSO3 added in that order at room temperature upon constant stirring. The x mL (x = 5, 10, 15, 20, 25, 30, 35) volumes of the HH correspond to 0.08, 0.17, 0.25, 0.33, 0.41, 0.49 and 0.58 M, respectively. The deposition parameters are summarized in Table 1. The HH was introduced gradually into the Zn(CH3COO)2 solution during each deposition. The first few drops produced white precipitates as a result of zinc hydroxide formation, which, however disappeared to a clear and colorless solution when the HH volume got up to 5 mL and above. After mixing, the solution was immersed in a water bath kept at 80 ℃ on a magnetic stirrer machine. The already cleaned substrates were suspended vertically on substrates holder and immersed into the chemical solution and the solution was stirred continuously throughout the deposition period to avoid sedimentation at the bottom of the beaker. After the deposition period of 2 h, the coated substrates were removed and washed thoroughly with de-ionized water to remove the loosely hung particles and were dried using a hand held drier. There was no deposition when the HH volume was below 5 mL and that is the main reason why the deposition was started at 5 mL in this study. The thin film deposition rate was observed to increase with an increase in HH volume. However, at HH volumes of 25 mL, the deposition reached its peak and the films deposited at higher HH volumes started peeling off when washed with de-ionized water. After the deposition, the samples were annealed in open air at 300 ℃ for 2 h. The annealing conditions were chosen based on some literature reports about ZnSe thin films [26]. The deposition was made possible when there was just enough volume of HH to facilitate the release of the Zn2+ and Se2- ions according to the following reaction processes. 7
(
)
( (
(
)
)
(
)
) (
)
(
)
(1)
(
)
(2)
(
)
(3)
2.3 Characterization of the deposited samples The structural analysis of the annealed thin films of ZnSe was carried out using glancing incidence X-ray diffraction (GIXRD) with a Rigaku Smartlab glancing diffractometer system at Cu_Kα wavelength of 0.1540 nm and operated at a fixed glancing angle of 1⁰. Raman spectroscopy was carried out on the samples using a WITec Confocal Raman microscopy machine (alpha 300R) with an excitation wavelength of 532 nm. The surface morphology and the chemical composition of the thin films were determined using a Joel JSM-7800F field emission scanning electron microscope (FESEM) with a beam voltage of 10 kV equipped with an Oxford Aztec 350X-Max80 energy dispersive X-ray spectrometer (EDS). UV-Visible spectrophotometry was used for the optical characterization of the deposited ZnSe thin films in the wavelength range between 300 – 1000 nm using a Shimadzu UV-1700 PharmaSpec UV-Vis spectrophotometer. Photoluminescence (PL) analysis was carried out at room temperature using Varian Cary Eclipse Fluorescence Spectrophotometer with an excitation source of a 150 W xenon lamp. 3.0 Results and Discussion 3.1
Glancing incidence X-ray diffraction (GIXRD) study Fig. 1(a) shows the GIXRD patterns of all the samples of ZnSe thin films deposited
on the glass substrates at various x mL HH volumes (x = 5, 10, 15, 20, 25, 30, 35). The structural analysis of the samples was carried out using GIXRD covering a wide range of Bragg’s angle 2θ between 20 – 70°. The GIXRD peaks revealed that the thin films were 8
polycrystalline in nature with the peaks indexed to the primitive hexagonal phase of ZnSe with space group P63mc (186) corresponding to the JCPDS no. 150105. The (002) plane was the most preferred orientation plane for all the samples. It could be observed that the film was almost amorphous or nanoparticulates for the sample synthesizes at x = 5 mL. This may imply that ZnSe may have just started forming, containing some unreacted elements or defects. The presence of small diffraction peaks from the (002) and (110) planes, respectively at around 2θ = 27 and 46° may serve as a clear indication of the ZnSe material starting to emerge.
From x = 10 mL, more peaks sprouted out implying improved crystallinity as the
HH volume was increased. HH is a strong reducing agent and so sufficient amount of it in the solution reduces the selenosulfate to provide sufficient Se2- ions concentration to permit increased nucleation and growth. The average crystallite sizes (D) were estimated from the (002) diffraction peak using Scherrer’s formula and were found to range from 5 to 14 nm. Mehta et al. [8] synthesized ZnSe nanocrystallites using CBD and obtained the average crystallite size in the range of 7 to 18 nm when the films were annealed at different temperatures. Generally, the average crystallite sizes have increased with an increase in the HH volume until at x = 20 mL then there was a decrease thereafter as shown in Table 2. This probably could be due to the sufficient concentration of HH that leads to increased nucleation and crystallite growth. However, too much concentration of HH in the solution then means to too high complexation and that leads to low crystallite size formation. The results agreed with the observation of Wei et al. [11] in a similar study where they used different HH volumes and ZnSO4. The lattice parameters of the films were estimated from the most intense peaks (002) and (110). The d-spacing and lattice constants (a = b and c) were calculated respectively using Bragg’s law [27]. The calculated unit cells were in the range a = b = 3.929 to 3.983 Ǻ and c = 6.48 to 6.51 Ǻ which are comparable with the standard values a = b = 3.996 Ǻ and c 9
= 6.55 Ǻ for the hexagonal wurtzite structure according to the JCPDS no. 150105. The dspacing does not show any sequential trend with increased HH volume. This suggests that dspacing of ZnSe thin films under the conditions used in this work does not depend on HH volume. The calculated values for each x mL of HH are presented in Table 2. A similar behavior concerning different HH volumes was observed by Preetha and Remadevi [28], when they used different HH volume (concentration) to deposit thin films of lead selenide by successive ionic layer absorption and reaction method. They have observed that the lattice parameters, crystallites sizes and strain varied with a change in concentration of HH. The parameters were observed to increase with HH volume up to 80% of the concentration used, and then started falling back. The reason for an increase in lattice parameters up to a certain level where after they reduced is that HH plays an important role in the internal strain of the chalcogenide materials. If its volume is low, there will be no much complexation with the metal ions leading to low release of elemental selenium or sulfide in the solution. This results to defects in the structure leading to internal strain. When HH is high, there will be too much complexity leading to low metal ions which again results to defects in the structure of the material [28]. Mehta and co-workers [8] reported that stresses and strains are always present in thin films deposited on glass. The lattice strain (ε) was estimated following equation 4 [26].
(4) Where β is the full width at half maximum of the diffraction peak and θ is the Bragg’s angle. The dislocation density, δ known as the length of dislocation lines per unit volume of crystal in each sample is estimated using equation 5 [27]. (5)
10
The δ values decreased with increased HH volume, which signifies that higher defects are present in films with lower crystallization (lower HH volumes) [27]. The results of the structural parameters of the ZnSe thin films calculated are presented in Table 2. Fig. 1 (b) presents the GIXRD peaks of the (002) plane. The diffraction patterns show no linear correlation between the peaks intensities and the HH volumes. The fluctuation in the GIXRD peaks intensities of the deposited ZnSe thin films is an indication that there is no direct proportional relationship between the peak intensities and the HH volume. A slight shift in the peaks position was observed from the figure but also fluctuates with an increased HH volume. The peak shift may be due to a change in microstructure parameters (crystallize size and lattice strain) as asserted by Shaaban et al. [29]. In this work, the increase in HH volume provided higher complexation and slowed the release of zinc ions in the deposition process leading to improved crystallinity and the lattice parameters of the films. 3.2
Raman spectroscopy study For further investigation of the structural and nature of the ZnSe thin films
nanoparticles, Raman scattering spectroscopy was used. The ZnSe thin films were excited at a wavelength of 532 nm at room temperature. Fig. 2 (a) depicts the Raman intensities versus wavenumber (
) of the ZnSe thin films. Two Raman peaks (1LO) and 2LO all ascribed
to longitudinal optical phonon modes are observed between 256-262 and 490 cm-1, respectively. The sharp peak around 256-262 cm-1 positions is in agreement with that observed by many other workers [1, 30-35]. The 1LO peak in the vicinity of 256-262 cm-1 is significantly lower for the samples prepared with x = 5 and 35 mL HH as compared to the other samples. This observation is in agreement with the GIXRD analysis and it suggests that 5 mL is too low while 35 mL is too high for the formation of quality ZnSe thin films as explained earlier. The peak of 2LO was equally observed by Li et al. [36] and Hu et al. [37].
11
The broadening of the Raman peaks as seen in Fig. 2 (a) is attributed to small crystallite sizes of the films and the defects in the crystalline structure as observed also in the GIXRD analysis [38]. The good crystallinity of the thin films is also proved by the presence of symmetric peaks and the sharpness of the 1LO phonon mode peaks [30, 33, 34]. The shift in the 1LO phonon peak seen in Fig. 2 (b) is related to changes in bond length of the molecules due to different HH volumes used. It can also be noticed that there were no unexpected peaks detected in the Raman spectra measurements shown in Fig. 2 (a), which agrees very well with the GIXRD results obtained in Fig. 1 (a). In order to correlate the structural characteristics between the GIXRD and Raman spectroscopic analyses, the (002) GIXRD peak and Raman spectra 1LO peak intensities versus the change in HH volume are plotted together and presented in Fig. 3. The figure shows that the peak intensities were low for both GIXRD and Raman at x = 5 mL HH. However, as the HH volume increased both peaks intensities increased and reached the climax at x = 10 mL HH for the GIXRD and 20 mL HH for the Raman analysis. However, further increase in the HH above the respective volumes resulted in the degradation of the ZnSe thin films as characterized by the decrease in the intensities of both the GIXRD and Raman peaks though with some fluctuations. Based on the structural analysis, x = 10 – 20 mL HH is suitable in the deposition of ZnSe thin films using CBD and HH as the only complexing agent. 3.3
Field emission scanning electron microscopy (FESEM)/energy dispersive X-ray
spectroscopy (EDS) To study the morphology of the ZnSe thin films, FESEM imaging was carried out on four samples of the deposited films for x = 5, 10, 15 and 35 mL as shown in Fig. 4. Fig. 4 (a) illustrates the sample deposited at x = 5 mL, which shows spherical grains with fairly uniform
12
sizes. The FESEM images further confirmed the role of HH concentration in the formation and morphology of ZnSe. Increasing the HH volume changed the density of the resulting nanoparticles and their morphology [30]. Fig. 4 (b) shows the transformation stage from the spherical to the nanowalls or nanoflakes. At this stage, the increase HH volume has shown a significant change in the morphology of the deposited ZnSe thin films. This agrees with the GIXRD result, which showed improved crystallinity as the volume of HH was increased from x = 5 to 10 mL showing more peaks of ZnSe. Fig. 4 (c) and (d) show further transformation from nanowalls to a higher degree of nanowalls with a flower-like morphology. Thus, the results clearly suggest that the HH volume influences the morphology of the prepared thin films. This result agrees completely with the report of Wei et al. [11] and Bakiyaraj et al. [14]. The uniform and homogenous morphology indicates the presence of a single phase of ZnSe as confirmed by the GIXRD. The presence of Zn and Se in the films were confirmed by the EDS analysis shown in Fig. 5 (a - d), which corresponds to the FESEM images of Fig. 4 (a - d), respectively. Table 3 shows that at a low HH volume as seen on the sample of x = 5 mL, the atomic percentage of Zn was above 60% whereas that of Se was below 40%. This might imply that the material was Zn rich. This implies that lesser Se was released in the solution bath whereas more Zn was produced, which indicates that there were not enough complexing ions to facilitate the release of Se ions into the solution. This is further motivated by the fact that according to Pingale et al. [38] the release of Se ions in the bath solution during ZnSe thin film deposition is sorely controlled by the HH volume. In their work, to deposit ZnSe using ammonia and HH as ligand, they have observed that the growth of Se ions in the solution improved significantly with the increase in HH volume. They have concluded that the ammonia only provides an alkaline environment for the thin films formation [38]. The effect of HH on the formation of ZnSe was therefore confirmed in this work as can be clearly seen on the 13
morphology and Zn:Se ratio of the deposited thin films. HH is a bidenate ligand and can react with a metal ion to form two different kinds of metal complexes [39]. According to Ghamsari and Araghi [39], when HH is added to the CBD, which has metal ions such as Zn2+, Pb2+ etc., it causes a decrease on the releasing rate of the ions in the bath and this gives rise to an increase in the semi-metal growth. It can be seen that as the volume of HH is added to the solution, there is a great improvement in the atomic percentage of Se over Zn and the ratio of the Zn to Se ions, turning to 1:1 as seen in Table 3. The EDS results did not show the presence of unexpected elements, which agree perfectly with the XRD and Raman results, respectively (see Fig. 1 and 2). The other elements: Ca, Mg, Si, Na, O present are from the glass substrates used [14]. 3.4
Optical absorption spectroscopy UV-Visible spectra is used to obtain information about the optical properties of the
deposited thin films of the ZnSe. Fig. 6 (a) depicts the absorption spectra of the ZnSe thin films on the wavelength range between 300 and 1000 nm. Fig. 6 (a) shows that the set of ZnSe thin films samples does not display similar absorption behavior with absorption curves. It can be observed that by increasing the HH volumes, the absorption edges shifted to higher wavelengths though with fluctuations. The observed shift in absorption edge towards higher wavelength is most probably due to increasing thickness of the films as HH volume enhances ZnSe formation rate [40]. To estimate the energy bandgap (Eg) of the ZnSe thin films, a graph of the square of absorbance (A2) is plotted against photon energy (hν) as shown in Fig. 6 (b) [41 - 45]. This is a very quick method of estimating the Eg without going through the problem of measuring the film thickness and therefore calculating absorption coefficient in order to do a Tauc plot. By extrapolating the linear portion of the curves to the point of intercept on the hν axis where A2
14
= 0, gives the optical Eg. The estimated values of the Eg are shown in Table 4. The Eg of the bulk ZnSe material is 2.7 eV [45]. The variation in the Eg may be due to the crystallite size effect, which makes them differ from the theoretical E g. Koao et al. [46] in their work to determine the effect of octadecylammine molar concentration on the structure, morphology and optical properties of ZnO nanostructure prepared by homogeneous precipitation, reported a decrease in Eg with an increase in crystallite size. The same effect is observed in this work as wider Eg were observed in samples with smaller crystallites sizes. The Eg values are nevertheless close to the ZnSe Eg ranges of 1.9 – 3.1 eV estimated by Yildirim et al. [26]. On the other hand, Al-Kuhaili et al. [47] reported the Eg to be in the range 1.75 – 2.58 eV for the same material synthesized using the thermal evaporation method. It is quite interesting to see that these values are in line with this study results. The percentage transmission spectra for the CBD ZnSe thin films is presented in Fig. 6 (c). The transmittance wavelength covered by this study is within the UV-visible to near infrared (300 – 1000 nm). The transmission spectra as can be seen shifted to higher wavelength as the HH volume increased. The shift in absorption edges to higher wavelength observed in Fig. 6 (a) and (c) and the decrease in band gap observed in Fig. 6 (b) may be due to the increase in estimated crystallite sizes with an increase in the amount of HH [48]. 3.5
Photoluminescence (PL) spectroscopy study Fig. 7 (a) shows the excitation spectra of the ZnSe thin film obtained when
monitoring emission at 530 nm. The spectra of all the samples show that there are two excitation bands peaks. The first band peak shifted from 325 nm to 337 nm corresponding to 3.82, and 3.68 eV, respectively. This excitation band can be attributed to defect absorption level in the material, which is due to fundamental absorption of excitons [7, 46]. The shift in excitation peak position may be due to the change in crystallite sizes and nanostructures
15
morphology discussed earlier. The second excitation band is located at 433 nm (2.86 eV) excitation wavelength and this can be ascribed to near-band transition due to the fact that it is close to the Eg of the ZnSe (2.8 eV) estimated from the optical absorption spectra measurement. The excitation wavelength of 325 nm from the first band was, however adopted in this study because of two reasons. (1) It produces excitation bands with higher emission intensity as seen in Fig. 7 (b). (2) It is chosen based on related literature values [40, 50, 51]. Generally, the excitation intensities were observed to increase with an increase in the HH volume although there is fluctuation as seen in Fig. 7 (a). Room temperature PL emission spectra for the CBD ZnSe thin films are presented in Fig. 7 (b). The samples were excited at 325 nm (3.82 eV) and showed two symmetric emission peaks: a strong green emission peak at 530 nm (2.34 eV) and a weak red emission peak at 678 nm (1.83 eV). The emission peak at 530 nm (2.35 eV) is ascribed to the intrinsic intra-bandgap defects such as Zn vacancy defect states [21, 14, 52]. The weak emission peak located at 678 nm corresponding to the defect absorption can be attributed to growth conditions and presence of non-stoichiometric point defects due to Se vacancies or selfactivated luminescence as a result of donor acceptor pairs that are related to Zn vacancies and interstitial states which could be present on the surface of the nanomaterials during preparation [21, 52-54]. The green-red ZnSe PL emission peaks have been observed by many researchers and attributed to defect levels in the material [37, 55]. The origin of the green-red ZnSe PL peaks according to Bakiyaraj et al. [14] may be attributed to intrinsic point defect. In their study on synthesis and characterization of flower-like ZnSe nanostructured thin films using CBD, the typical deep defect related emission was observed between 552 - 658 nm. Careful observation reveals that there is a slight emission peak shift to the higher wavelength with an increase in the HH volume. The shift could be due to defect levels in the films [14].
16
A change in the relative ratio of the two emission peaks is also visible due to higher selfactivated luminescence.
The dependence of the emission intensity as a function of the HH volume based on the 530 nm peak is shown in Fig. 7 (c). The emission intensity increased with an increase in the HH volume with the sample of x = 20 mL having the optimal intensity. This result agrees partly with the Raman spectra analysis and the crystallite size estimated from the GIXRD result. The proposed PL excitation and emission mechanism of the deposited ZnSe thin films samples is represented by a schematic energy band diagram as shown in Fig. 8. When the samples are excited with light energy with a wavelength of 325 nm (3.82 eV), the electron (shown as red circles) in the valence band (VB) are excited in a broad optical band and trapped into the conduction band (CB) of the ZnSe. The electron trapped in the CB thereafter relaxes to the lowest vibrational level of the excited state by non-radiative relaxation (NRR) and give up excess energy to the surroundings. The electron (with energy = 2.86 eV) may return from the CB by NRR back to the VB or through different channels as indicated by the blue arrows in Fig. 8. The electrons also may be trapped in other defect states levels (with energies = 2.34 and 1.83 eV) from where again it will be de-excited to the VB through radiative decay emitting photons of different energies as depicted by the green and red arrows in Fig. 8. It is clear that both emission peaks at 530 (2.34 eV) and 678 nm (1.83 eV) originated from ZnSe according to literature reports [14, 21, 37, 52, 53, 55]. These emissions are ascribed to the radiative decay due to electrons from the different trap levels recombining with the holes (represented in white circles) in the VB [56]. A decay lifetime profile of ZnSe thin films obtained by exciting the samples with 325 nm wavelength sources and by monitoring the emission at 530 nm is illustrated in Fig. 9 (a). Plot of the natural logarithm of the lifetime intensity as a function of HH volume gives a
17
linear graph as seen in Fig. 9 (b), which suggests that the decay is a mono-exponential decay lifetime [57]. Thus, the lifetime decay curves were fitted to a mono-exponential function according to equation 6. ( )
(
⁄ )
(6)
Where t is the time of measurement, τ1 is the decay lifetime component with intensity I1. The variation of lifetime values at different HH volumes obtained from the mono-exponential decay fitting is indicated in Fig. 9 (c). The lifetime of the material increased when the HH volume increased from x = 5 to 20 mL, thereafter it decreased. This shows that the deposited films have a longer emission lifetime with the sample of x = 20 mL HH. This is consistent with the crystallite sizes and Raman analyses and we can conclude that 20 mL of HH is the most ideal volume for the deposition of ZnSe thin films under the condition used in this work. The international commission on illumination (CIE) chromaticity coordinates for the ZnSe thin films are presented in Fig. 10. CIE calculates the emission color of the material as well as the color coordinates in the chromaticity diagram based on the PL emission results. The CIE coordinates for the ZnSe thin films were estimated from the PL emission spectral presented in Fig. 7 (b). The CIE color chromaticity shows a green color emission with a slight shift to longer wavelengths, which, however fluctuated with the increase in the HH volume just as observed in the emission spectra, shown in Fig. 7 (b). This shows that the emission color of these ZnSe nanoparticles can be tuned by varying the volume or concentration of the HH as the complexing agent.
18
4.0
Conclusion ZnSe thin films of single phase hexagonal wurtzite structure have been deposited on
glass substrates via the CBD route using HH as the only complexing agent. The films were highly polycrystalline with average crystallite sizes ranging between 5 and 14 nm. The highest GIXRD intensity was produced by the sample at x = 10 mL, whereas Raman spectra analysis recorded the highest intensity from the sample at x = 20 mL. The surface morphology was observed to change greatly from spherical grains to walls as the HH volume was increased. The films showed wide absorption band from the UV to the red region of the spectrum although not all samples showed clear absorption edges. The bandgap of the ZnSe films was in the range of 1.5 to 2.8 eV. The best bandgap of 2.6 eV with respect to the bulk bandgap of ZnSe was recorded from the sample at x = 10 mL. Room temperature PL studies with excitation wavelength of 325 nm has shown two emission bands at 530 and 678 nm, which were attributed to intrinsic intra-bandgap defects and non-stoichiometric point defect, respectively. The sample deposited with x = 20 mL showed highest emission intensity and longer decay lifetime. The CIE color coordinates showed that the emission color of the ZnSe nanoparticles can be tuned by varying the HH volume. It can be concluded from the findings that 20 mL of HH is the most ideal for the ZnSe thin films deposition under the conditions employed in this study. The study reveals that the deposited materials could have potential applications in solid state lighting such as light emitting diodes based on the observed emission colors. The wide energy bandgaps obtained also suggest that the films are good candidates for optoelectronics devices such as window materials for solar cells and infrared cameras.
19
Acknowledgements The authors would like to acknowledge the National Research Foundation, UID: 99224, the University of the Free State, the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Fund (NRF) (Grant 84415) for their financial support.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Credit Author Statement D.D. Hile: Methodology, validation, Writing original draft, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization. H.C. Swart: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition. S.V. Motloung: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition. T.E. Motaung: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition. K.O. Egbo: Formal analysis, investigation, writing-reviewers and editing L.F. Koao: Supervision, project administration, validation, formal analysis, investigation, resources, data curation, writing-reviewers and editing, visualization, funding acquisition.
20
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List of Tables Table 1. The precursors parameters used for the ZnSe thin films deposition Table 2. Structural parameters of the deposited ZnSe thin films. Table 3. Change in atomic ratio of Zn and Se with increased HH volume. Table 4. Estimated bandgap values of the ZnSe thin films deposited using different HH volumes.
Table 1. Sample name
Zn(CH3COO)2
HH
Na2SeSO3
5 mL
Volume (mL) 20
Conc. (M) 0.1
Volume (mL) 5
Conc. (M) 0.08
Volume (mL) 20
Conc. (M) 0.59
10 mL
20
0.1
10
0.17
20
0.59
15 mL
20
0.1
15
0.25
20
0.59
20 mL
20
0.1
20
0.33
20
0.59
25 mL
20
0.1
25
0.41
20
0.59
30 mL
20
0.1
30
0.49
20
0.59
35 mL
20
0.1
35
0.58
20
0.59
29
Table 2. δ (lines/c m2)
HH volume 2θ (⁰) β (⁰) (mL)
D (nm)
d-spacing (Å)
a (Å)
c (Å)
5
27.38
1.91
5
3.254
3.949
6.51
0.0342
0.04
10
27.50
1.75
5
3.240
3.966
6.48
0.0312
0.04
15
27.45
1.01
8
3.245
3.938
6.49
0.0115
0.016
20
27.40
0.63
14
3.255
3.965
6.51
0.0182
0.005
25
27.47
0.65
13
3.243
3.929
6.49
0.0113
0.006
30
27.41
0.64
13
3.250
3.975
6.50
0.0114
0.006
35
27.45
0.64
13
3.245
3.983
6.49
0.0114
0.006
30
Table 3. HH volume (mL)
5
10
15
35
Element
Zn Se
Zn Se
Zn Se
Zn Se
Atomic (%)
67
57
56 44
51 49
Ratio Zn : Se
1 : 0.49
1 : 0.78
1 : 0. 96
33
43
1 : 0.75
31
Table 4. HH volume (mL) 5 Eg (eV)
10
15
20
25
30
35
2.8 2.6 2.5 2.0 1.7 1.6 1.5
32
List of Figures
Fig. 1 (a) GIXRD patterns of the ZnSe thin films (b) zoomed 002 diffraction peaks of the thin films presented in Fig. 1 (a).
33
Fig. 2 (a). Raman Spectra of the deposited ZnSe thin films (b) the normalized 1LO peak of Fig 2 (a).
34
Fig. 3. GIXRD preferred orientation peak intensity and Raman spectra 1LO peak intensity against the HH volume.
Fig. 4. FESEM micrographs of the ZnSe thin films grown at (a) x = 5 (b) 10 (c) 15 and (d) 35 mL.
35
Fig. 5. EDS spectra corresponding to SEM images of the ZnSe thin films grown at (a) x = 5 (b) 10 (c) 15 and (d) 35 mL.
36
Fig. 6. (a) Absorption spectra (b) estimated energy bandgap plots (c) transmittance spectra of the ZnSe thin films deposited at various HH volumes.
37
Fig. 7. (a) PL excitation spectra (b) PL emission spectra (c) Dependance of the PL emission intensity of the ZnSe thin films for the various HH volumes.
Fig. 8. The proposed excitation and emission pathways mechanism on the ZnSe thin films.
38
Fig. 9. Lifetime (a) decay curve (b) half-log scale plot (c) the variation of decay lifetime with HH volumes for the ZnSe thin films.
39
Fig. 10. CIE color chromaticity coordinates of the CBD ZnSe thin films deposited using various HH volumes.
40