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Role of reducing environment in the chemical growth of zinc selenide thin films
Materials Letters 92 (2013) 308–312
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Role of reducing environment in the chemical growth of zinc selenide thin films L.P. Deshmukh a,n, P.C. Pingale a, S.S. Kamble a, S.A. Lendave a, S.T. Mane a, B.R. Pirgonde a, Madhuri Sharon b, M. Sharon b a b
Thin Film & Solar Studies Research Laboratory, Department of Physics, Solapur University, Solapur-413 255, Maharashtra, India Emeritus Professor IIT, Bombay, Mumbai, & Research Director, nsnRc, SICES Degree College, Ambarnath-421 505, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2012 Accepted 25 October 2012 Available online 2 November 2012
Chemical deposition of ZnSe (Zn/Se ratio, 0.57rxr0.99) thin films highlighting influence of reducing environment on the structural properties and surface morphology is presented. Hydrazine hydrate was used as a reducing agent to initiate growth process and to reduce selenosulfate to Se2 ions that permits formation of non-stoichiometric ZnSe film layers. ZnSe films thus obtained are adherent, homogeneous and diffusely reflecting with light brown coloured tinge. These films were characterized through XRD, SEM, EDS and AFM techniques to reveal the structural and morphological informations. As-grown films are polycrystalline wurtzite with (1 0 1) preferred orientation. d-values change considerably whereas I/Imax is more or less constant. The average lattice parameters have similar trend of variation with Zn/Se ratio. Non-uniform distribution of spherical ZnSe crystallites was observed through SEM. The micrographs further indicated marginal agglomeration of crystallites forming globule like overgrowth. AFM studies spotlighted influence of reducing environment on the surface roughness of the films. The bulk ZnSe exhibits LO and TO phonon bands at 252 cm 1 and 205 cm 1 respectively as indicated by Raman studies. The optical band gap is decreased a little and is correlated to variation in the excess metallic Se-phase. & 2012 Elsevier B.V. All rights reserved.
Keywords: ZnSe thin films Polycrystalline wurtzite Hydrazine hydrate LO and TO modes Optical gap
1. Introduction Zinc selenide is a II–VI semiconductor having distinctive properties, viz. direct wide energy gap (2.7 eV), high refractive index and low optical absorbance in the VIS–IR regions. It is therefore a prominent material for red, blue and green light emitters, laser screens, ultrasonic transducers, photovoltaic detectors and converters including window layer in thin film based solar cells [1–4]. At present CdS is most extensively used as a buffer layer for high efficiency values in CIGS-based solar cells. However, due to its toxic nature, attention is focused recently on developing cadmium free eco-friendly buffer layers [4]. ZnSe is one of the substitutes having better lattice parameter conformity, non-toxicity and good conduction band that may transfer the high energy photons to absorber layer of solar cell [4]. This letter presents a novel inexpensive chemical route for deposition of thin ZnSe films. Chemical deposition is known for moderately slow chemical reaction in solution bath that results into a solid product on the immersed substrates [5–8]. Requirements of low temperature, inexpensive equipments and ease of deposition on any size and shape of substrate mould make chemical route as an ideal tool for industrial adaptation. Many investigations show up ZnSe thin film deposition
n
Corresponding author. þ 91 9850 697 974; fax: þ91 217 2744 770/77. E-mail address: [email protected] (L.P. Deshmukh).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.098
by a CBD route, but only a handful of reports are associated on the role of reducing environment in the growth mechanism. Manifesting all these facts, influence of the reducing environment in film growth has been underlined in this communication.
2. Experimental ZnSe thin films with various Zn/Se ratio (0.57 rx r0.99) were obtained from an aqueous alkaline chemical bath at the optimized conditions (70 1C, 210 mins and pH¼10 70.2) onto the glass substrates [9]. AR grade precursors consisting of zinc and selenium ion sources were used (0.5 M zinc sulfate and 0.25 M sodium selenosulfate). 25% aqueous NH3 as a complexing agent and 80% (NH2)2.H2O as a reducing agent were used to obtain better quality and high performance thin films. The quantity of hydrazine hydrate in the bath was varied from 3 ml to 12 ml. The film thickness was measured and the films were then characterized through the structure, composition and surface morphology.
3. Results and discussion The state-of-the-art alkaline bath chemical synthesis is reported to be admirable in the deposition of a range of semiconducting thin films [5–8]. In the deposition process, reaction
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
rate is mediated through the bath parameter variation and bath environment control. A sluggish heterogeneous nucleation offers a gradual thin film deposition on the substrate surface rather than agglomeration of large particles [5]. Recent literature shows that, the reducing agent plays a key role in the growth of chemically deposited thin films [3,4] and therefore we attempted hydrazine hydrate as the reducing atmosphere for growth of the ZnSe thin films.
309
To avoid the fast precipitation of zinc sulfate, Zn2 þ ions are complexed with ammonia. ZnSO4 2Zn2 þ þ SO2 4 Zn2 þ þ4NH3 2 ZnðNH3 Þ24 þ : The Zn(NH3)24 þ complex in turn reduces concentration of free Zn ions which helps to prevent bulk precipitation of desired product. 2þ
ZnðNH3 Þ24 þ þ Se2 - ZnSe þ 4NH3 m 3.1. Growth mechanism and chemical kinetics Growth of ZnSe thin film initiates only when ionic product of Zn2 þ and Se2 ions exceeds the solubility product (Ksp ¼10 27) of ZnSe [3–9]. Initially film growth was carried out in the presence of ammonia as the complexing agent. However, it was noticed that the film necessitates bit larger induction period owing to the high volatility of ammonia [4,9]. Although volatile, a sufficient quantity of ammonia (5 ml, 25%) is essential to improve film adherence and to assist slow release of Se2 ions [3–9]. A strong reducing agent (like hydrazine hydrate) is required to be introduced in the chemical bath so as to achieve quality deposition of ZnSe from selenosulfate bath [3,4,9]. Hydrazine hydrate reduces selenosulfate to an extent to have enough Se2 ions available for ZnSe deposition and thus enhances the film growth, film homogeneity and film adherence [3,4,9]. In turn, addition of hydrazine hydrate also helps in reducing the induction phase period [3,4,9]. Therefore, following reaction mechanism is proposed to understand this typical growth [9]. -
Na2 SeSO3 þ OH-Na2 SO4 þ HSe -
HSe þ OH-H2 Oþ Se2
ZnSO4 2Zn2 þ þ SO2 4 2þ 2SeO2 -2ZnSe:1=2N2 H4 k þ3N2 m þ 6H2 O 3 þ4N2 H4 þ2Zn
D or H þ
ZnSe:1=2N2 H4 ! 1=2N2 H4 þ ZnSe ðthin solid filmÞ: In our case, growth rate and therefore particle size is effectively controlled by addition of hydrazine hydrate into the reaction bath [9]. Further, deposition process required less induction period than what is required for deposition using only ammonia [9]. This growth behaviour can be correlated to the number of Zn2 þ and Se2 ions available in the solution. Hydrazine hydrate initializes growth process and presumably reduces selenosulfate to release enough Se2 ion concentration that permits and controls formation of ZnSe in thin film form. The ZnSe films obtained under the above conditions are homogeneous, tightly adherent and diffusely reflecting with light brown coloured tinge. 3.2. Energy dispersive X-ray spectroscopy An EDS technique was used to analyse the as-grown samples compositionally (Fig. 1a). Variation in the Zn/Se ratio was observed (Table 1) with addition of hydrazine hydrate in the reaction bath. Decrease in Zn content whereas increase in the Se content was perceived with addition of N2H4. As the hydrazine
Fig. 1. (a) The EDS spectra and (b) X-ray diffractograms of the as-deposited ZnSe thin films.
310
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
hydrate controls rate of release of Se2 ions, film composition depends on the hydrazine hydrate content. Hence, enhanced rate of reduction at higher concentration of hydrazine hydrate led to the stoichiometric variation in the deposits.
3.3. X-ray diffraction Structural characterization of the as-deposited samples was done using XRD analysis. Fig. 1(b) shows X-ray diffractograms of
Table 1 The composition and micro-structural parameters of ZnSe thin films highlighting the effect of reducing environment. N2H4 (ml) Zn/Se ratio Composition mass Lattice parameters (%)
3 5 7 9 10
0.993 0.940 0.790 0.770 0.570
Grain size, nm
Zn
Se
˚ a(A)
˚ c(A)
c/a
XRD
SEM
49.82 48.88 44.40 43.63 36.47
50.18 51.12 55.60 56.37 63.53
3.950 3.963 3.974 3.984 3.996
6.470 6.495 6.514 6.530 6.560
1.639 1.639 1.639 1.639 1.642
67 61 59 57 55
761 644 564 479 410
Lattice strain 10-4 Dislocation density 1014 Surface roughness Optical gap
5.80 6.06 6.79 7.23 7.58
(lin. m 2)
(lm)
eV
2.27 2.72 2.90 3.07 3.28
0.067 0.056 0.039 0.038 0.033
2.69 2.68 2.61 2.60 2.55
Fig. 2. The SEM and AFM (2D) images of the as-grown ZnSe thin films.
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
311
five typical ZnSe samples with different Zn/Se ratios. The films are crystalline and exhibited wurtzite structure with preferred o1 0 14 orientation. The d-values and I/Imax have excellent consonance with the JCPD values [10]. The d-value increased significantly with decreased Zn/Se ratio whereas I/Imax remained more or less constant. The average lattice parameters follow the similar trend of variation whereas c/a ratio is almost constant. The values of other micro-structural parameters, viz. grain size, micro-strain and dislocation density are evaluated using Scherre’s and Williamson and Hall relations as [11]; d ¼1/D 2 and e ¼ b cos y/4, where, D is the mean crystallite size, b is the full width at half maximum and y is the diffraction angle. All these structural and microstructural parameters are listed in Table 1. 3.4. Scanning electron and atomic force microscopies Surfaces of the various ZnSe films were viewed through a SEM. Typical SEM micrographs (Fig. 2a) revealed distinguishable spherical crystallites nonuniformly distributed. Thus growth of crystalline ZnSe films took place via agglomeration of the crystallites (i.e. inhomogeneous precipitation of the solution on the glass surface) and the crystallites are of nearly equal in size for higher hydrazine hydrate content. These observations are in accordance with the earlier reports [9,12]. Further, the crystallite size calculations showed that the crystallite size is reduced significantly for higher hydrazine hydrate content highlighting the effect of reducing environment. The decrease in crystallite size can be under˚ and Se2 (1.22 A) ˚ stood from the size difference of Zn2 þ (1.53 A) and that higher sized Zn2 þ lattice sites are decreasing with an increasing amount of hydrazine hydrate in the bath solution [9]. The surface topographic studies were made on these as-deposited ZnSe thin films. 2D AFM images (Fig. 2b) of three typical asdeposited ZnSe thin films revealed variation in the surface roughness. Further, AFM showed formation of spherical ZnSe crystallites (deeply embedded in the globular atmosphere) of nearly equal size (crystallite size reduced with added hydrazine hydrate content). The surface roughness variation clearly fortifies the effect of reducing environment on the chemical synthesis process.
Fig. 3. Raman spectra for three typical ZnSe films.
3.5. Raman studies Bulk ZnSe exhibits the longitudinal optical phonon (LO) band at 252 cm 1 and transverse optical phonon (TO) band at 205 cm 1 [1,13]. In the present case (Fig. 3, ZnSe films) LO band positions are at 242 cm 1–244 cm 1 whereas TO bands are at 203 cm 1. The presence of LO mode signifies crystalline nature of the as-deposited ZnSe thin films. Slight shifting of LO mode indicates impact of reducing atmosphere on formation of the ZnSe films [9]. The peak corresponding to 2 TA is also observed at around 122 cm 1. The absorption bands found in the region E1570 cm 1 can be assigned to the absence of centre of symmetry, which again substantiates polycrystalline nature of the deposited samples. In conclusion, existence of LO and TO modes confirmed formation of the ZnSe thin films under impact of the reducing environment.
Fig. 4. Variation of (ahn)2 versus hn for as-prepared ZnSe thin films.
3.6. Optical studies The optical studies are the effective means of determining the energy gap and hence the band structure. The optical band gaps (Eg) were therefore determined for these films (various Zn/Se ratios) using (ahu)¼ A (hu Eg)m, where m and A are constants, a is the absorption coefficient (cm 1) and hu is the photon energy (eV) [8,9]. For this, variations of (ahu)2 versus hu (Fig. 4) were plotted and the linear part extrapolated towards lower photon energies, i.e., (ahu)2 ¼0. A little decrease in Eg is observed with
increased reducing environment (Zn/Se ratio). Particularly, the drop in band gap is a consequence of localized states formation near the edge of conduction band due to the defects present in the structure [14]. The XRD studies suggested variation in dislocation density which causes modification in the localized states and thus leads to the variation in optical band gap. We attribute the decrease in band gap to the presence of excess metallic Se phase which is revealed by the XRD and compositional analyses.
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4. Conclusions
References
1. The reducing environment (N2H4) strongly influences ZnSe film growth. 2. The films are polycrystalline with a wurtzite hexagonal structure. 3. SEM microscopy revealed spherical crystallites with nonuniform distribution. 4. AFM studies showed drastic effect of reducing environment on film formation. 5. Optical gap variation is correlated to variation in crystal defects (XRD) and excess of metallic Se phase (compositional) analyses.
[1] Balu AR, Nagarethinam VS, Syed Basheer Ahmed MG, Thayumanavan A, Murli KR. Mater Sci Eng B 2010;171:93–8. [2] Huang CW, Weng HM, Jiang YU, Heng HY. Vacuum 2009;83:313–8. [3] Mehta C, Saini GSS, Abbas JM, Tripathi SK. Appl Surf Sci 2009;256:608–14. [4] Chen L, Zhang D, Zhai G, Zhang J. Mater Chem Phys 2010;120:456–60. [5] Hodes G. Phys Chem Chem Phys 2007;9:2181. [6] Mane ST, Kamble SS, Deshmukh LP. Mater Lett 2012;67:373–5. [7] Chattarki AN, Kamble SS, Deshmukh LP. Mater Lett 2012;67:39–41. [8] Deshmukh LP, Holikatti SG. J Phys D Appl Phys 1994;2:1786–9. [9] Pingale PC, Pirgonde BR, Deshmukh LP, Sharon M. Proceedings of the sixth international conference on technological advances of thin films & surface coatings, TF-2012, July 14–17, 2012, Singapore. [10] Korneeva, Sov. Phys. Crystallogr. [Engl. Transl.], JCPD data files PDF# 150105, 1962; 6: 505. [11] Chaliha S, Borah MN, Sarmah PC, Rahman A. J Optoelctron Adv Mater 2008;10(2):427–33. [12] Kumar P, Singh K. J Optoelectron Biomed Mater 2009;1:59–69. [13] Venkatachalam S, Agilan S, Mangalaraj D, Naranyandass SaK. Mater Sci Semicond Process 2007;10:128–32. [14] Mott NF, Davis EA. Electronic processes in non-crystalline materials. Oxford: Mott and Davis Clarendon Press; 1971.
Acknowledgement We thank Prof. N.N. Maldar of our university for useful discussions on the growth mechanism of these films.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Role of reducing environment in the chemical growth of zinc selenide thin films L.P. Deshmukh a,n, P.C. Pingale a, S.S. Kamble a, S.A. Lendave a, S.T. Mane a, B.R. Pirgonde a, Madhuri Sharon b, M. Sharon b a b
Thin Film & Solar Studies Research Laboratory, Department of Physics, Solapur University, Solapur-413 255, Maharashtra, India Emeritus Professor IIT, Bombay, Mumbai, & Research Director, nsnRc, SICES Degree College, Ambarnath-421 505, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2012 Accepted 25 October 2012 Available online 2 November 2012
Chemical deposition of ZnSe (Zn/Se ratio, 0.57rxr0.99) thin films highlighting influence of reducing environment on the structural properties and surface morphology is presented. Hydrazine hydrate was used as a reducing agent to initiate growth process and to reduce selenosulfate to Se2 ions that permits formation of non-stoichiometric ZnSe film layers. ZnSe films thus obtained are adherent, homogeneous and diffusely reflecting with light brown coloured tinge. These films were characterized through XRD, SEM, EDS and AFM techniques to reveal the structural and morphological informations. As-grown films are polycrystalline wurtzite with (1 0 1) preferred orientation. d-values change considerably whereas I/Imax is more or less constant. The average lattice parameters have similar trend of variation with Zn/Se ratio. Non-uniform distribution of spherical ZnSe crystallites was observed through SEM. The micrographs further indicated marginal agglomeration of crystallites forming globule like overgrowth. AFM studies spotlighted influence of reducing environment on the surface roughness of the films. The bulk ZnSe exhibits LO and TO phonon bands at 252 cm 1 and 205 cm 1 respectively as indicated by Raman studies. The optical band gap is decreased a little and is correlated to variation in the excess metallic Se-phase. & 2012 Elsevier B.V. All rights reserved.
Keywords: ZnSe thin films Polycrystalline wurtzite Hydrazine hydrate LO and TO modes Optical gap
1. Introduction Zinc selenide is a II–VI semiconductor having distinctive properties, viz. direct wide energy gap (2.7 eV), high refractive index and low optical absorbance in the VIS–IR regions. It is therefore a prominent material for red, blue and green light emitters, laser screens, ultrasonic transducers, photovoltaic detectors and converters including window layer in thin film based solar cells [1–4]. At present CdS is most extensively used as a buffer layer for high efficiency values in CIGS-based solar cells. However, due to its toxic nature, attention is focused recently on developing cadmium free eco-friendly buffer layers [4]. ZnSe is one of the substitutes having better lattice parameter conformity, non-toxicity and good conduction band that may transfer the high energy photons to absorber layer of solar cell [4]. This letter presents a novel inexpensive chemical route for deposition of thin ZnSe films. Chemical deposition is known for moderately slow chemical reaction in solution bath that results into a solid product on the immersed substrates [5–8]. Requirements of low temperature, inexpensive equipments and ease of deposition on any size and shape of substrate mould make chemical route as an ideal tool for industrial adaptation. Many investigations show up ZnSe thin film deposition
n
Corresponding author. þ 91 9850 697 974; fax: þ91 217 2744 770/77. E-mail address: [email protected] (L.P. Deshmukh).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.098
by a CBD route, but only a handful of reports are associated on the role of reducing environment in the growth mechanism. Manifesting all these facts, influence of the reducing environment in film growth has been underlined in this communication.
2. Experimental ZnSe thin films with various Zn/Se ratio (0.57 rx r0.99) were obtained from an aqueous alkaline chemical bath at the optimized conditions (70 1C, 210 mins and pH¼10 70.2) onto the glass substrates [9]. AR grade precursors consisting of zinc and selenium ion sources were used (0.5 M zinc sulfate and 0.25 M sodium selenosulfate). 25% aqueous NH3 as a complexing agent and 80% (NH2)2.H2O as a reducing agent were used to obtain better quality and high performance thin films. The quantity of hydrazine hydrate in the bath was varied from 3 ml to 12 ml. The film thickness was measured and the films were then characterized through the structure, composition and surface morphology.
3. Results and discussion The state-of-the-art alkaline bath chemical synthesis is reported to be admirable in the deposition of a range of semiconducting thin films [5–8]. In the deposition process, reaction
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
rate is mediated through the bath parameter variation and bath environment control. A sluggish heterogeneous nucleation offers a gradual thin film deposition on the substrate surface rather than agglomeration of large particles [5]. Recent literature shows that, the reducing agent plays a key role in the growth of chemically deposited thin films [3,4] and therefore we attempted hydrazine hydrate as the reducing atmosphere for growth of the ZnSe thin films.
309
To avoid the fast precipitation of zinc sulfate, Zn2 þ ions are complexed with ammonia. ZnSO4 2Zn2 þ þ SO2 4 Zn2 þ þ4NH3 2 ZnðNH3 Þ24 þ : The Zn(NH3)24 þ complex in turn reduces concentration of free Zn ions which helps to prevent bulk precipitation of desired product. 2þ
ZnðNH3 Þ24 þ þ Se2 - ZnSe þ 4NH3 m 3.1. Growth mechanism and chemical kinetics Growth of ZnSe thin film initiates only when ionic product of Zn2 þ and Se2 ions exceeds the solubility product (Ksp ¼10 27) of ZnSe [3–9]. Initially film growth was carried out in the presence of ammonia as the complexing agent. However, it was noticed that the film necessitates bit larger induction period owing to the high volatility of ammonia [4,9]. Although volatile, a sufficient quantity of ammonia (5 ml, 25%) is essential to improve film adherence and to assist slow release of Se2 ions [3–9]. A strong reducing agent (like hydrazine hydrate) is required to be introduced in the chemical bath so as to achieve quality deposition of ZnSe from selenosulfate bath [3,4,9]. Hydrazine hydrate reduces selenosulfate to an extent to have enough Se2 ions available for ZnSe deposition and thus enhances the film growth, film homogeneity and film adherence [3,4,9]. In turn, addition of hydrazine hydrate also helps in reducing the induction phase period [3,4,9]. Therefore, following reaction mechanism is proposed to understand this typical growth [9]. -
Na2 SeSO3 þ OH-Na2 SO4 þ HSe -
HSe þ OH-H2 Oþ Se2
ZnSO4 2Zn2 þ þ SO2 4 2þ 2SeO2 -2ZnSe:1=2N2 H4 k þ3N2 m þ 6H2 O 3 þ4N2 H4 þ2Zn
D or H þ
ZnSe:1=2N2 H4 ! 1=2N2 H4 þ ZnSe ðthin solid filmÞ: In our case, growth rate and therefore particle size is effectively controlled by addition of hydrazine hydrate into the reaction bath [9]. Further, deposition process required less induction period than what is required for deposition using only ammonia [9]. This growth behaviour can be correlated to the number of Zn2 þ and Se2 ions available in the solution. Hydrazine hydrate initializes growth process and presumably reduces selenosulfate to release enough Se2 ion concentration that permits and controls formation of ZnSe in thin film form. The ZnSe films obtained under the above conditions are homogeneous, tightly adherent and diffusely reflecting with light brown coloured tinge. 3.2. Energy dispersive X-ray spectroscopy An EDS technique was used to analyse the as-grown samples compositionally (Fig. 1a). Variation in the Zn/Se ratio was observed (Table 1) with addition of hydrazine hydrate in the reaction bath. Decrease in Zn content whereas increase in the Se content was perceived with addition of N2H4. As the hydrazine
Fig. 1. (a) The EDS spectra and (b) X-ray diffractograms of the as-deposited ZnSe thin films.
310
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
hydrate controls rate of release of Se2 ions, film composition depends on the hydrazine hydrate content. Hence, enhanced rate of reduction at higher concentration of hydrazine hydrate led to the stoichiometric variation in the deposits.
3.3. X-ray diffraction Structural characterization of the as-deposited samples was done using XRD analysis. Fig. 1(b) shows X-ray diffractograms of
Table 1 The composition and micro-structural parameters of ZnSe thin films highlighting the effect of reducing environment. N2H4 (ml) Zn/Se ratio Composition mass Lattice parameters (%)
3 5 7 9 10
0.993 0.940 0.790 0.770 0.570
Grain size, nm
Zn
Se
˚ a(A)
˚ c(A)
c/a
XRD
SEM
49.82 48.88 44.40 43.63 36.47
50.18 51.12 55.60 56.37 63.53
3.950 3.963 3.974 3.984 3.996
6.470 6.495 6.514 6.530 6.560
1.639 1.639 1.639 1.639 1.642
67 61 59 57 55
761 644 564 479 410
Lattice strain 10-4 Dislocation density 1014 Surface roughness Optical gap
5.80 6.06 6.79 7.23 7.58
(lin. m 2)
(lm)
eV
2.27 2.72 2.90 3.07 3.28
0.067 0.056 0.039 0.038 0.033
2.69 2.68 2.61 2.60 2.55
Fig. 2. The SEM and AFM (2D) images of the as-grown ZnSe thin films.
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
311
five typical ZnSe samples with different Zn/Se ratios. The films are crystalline and exhibited wurtzite structure with preferred o1 0 14 orientation. The d-values and I/Imax have excellent consonance with the JCPD values [10]. The d-value increased significantly with decreased Zn/Se ratio whereas I/Imax remained more or less constant. The average lattice parameters follow the similar trend of variation whereas c/a ratio is almost constant. The values of other micro-structural parameters, viz. grain size, micro-strain and dislocation density are evaluated using Scherre’s and Williamson and Hall relations as [11]; d ¼1/D 2 and e ¼ b cos y/4, where, D is the mean crystallite size, b is the full width at half maximum and y is the diffraction angle. All these structural and microstructural parameters are listed in Table 1. 3.4. Scanning electron and atomic force microscopies Surfaces of the various ZnSe films were viewed through a SEM. Typical SEM micrographs (Fig. 2a) revealed distinguishable spherical crystallites nonuniformly distributed. Thus growth of crystalline ZnSe films took place via agglomeration of the crystallites (i.e. inhomogeneous precipitation of the solution on the glass surface) and the crystallites are of nearly equal in size for higher hydrazine hydrate content. These observations are in accordance with the earlier reports [9,12]. Further, the crystallite size calculations showed that the crystallite size is reduced significantly for higher hydrazine hydrate content highlighting the effect of reducing environment. The decrease in crystallite size can be under˚ and Se2 (1.22 A) ˚ stood from the size difference of Zn2 þ (1.53 A) and that higher sized Zn2 þ lattice sites are decreasing with an increasing amount of hydrazine hydrate in the bath solution [9]. The surface topographic studies were made on these as-deposited ZnSe thin films. 2D AFM images (Fig. 2b) of three typical asdeposited ZnSe thin films revealed variation in the surface roughness. Further, AFM showed formation of spherical ZnSe crystallites (deeply embedded in the globular atmosphere) of nearly equal size (crystallite size reduced with added hydrazine hydrate content). The surface roughness variation clearly fortifies the effect of reducing environment on the chemical synthesis process.
Fig. 3. Raman spectra for three typical ZnSe films.
3.5. Raman studies Bulk ZnSe exhibits the longitudinal optical phonon (LO) band at 252 cm 1 and transverse optical phonon (TO) band at 205 cm 1 [1,13]. In the present case (Fig. 3, ZnSe films) LO band positions are at 242 cm 1–244 cm 1 whereas TO bands are at 203 cm 1. The presence of LO mode signifies crystalline nature of the as-deposited ZnSe thin films. Slight shifting of LO mode indicates impact of reducing atmosphere on formation of the ZnSe films [9]. The peak corresponding to 2 TA is also observed at around 122 cm 1. The absorption bands found in the region E1570 cm 1 can be assigned to the absence of centre of symmetry, which again substantiates polycrystalline nature of the deposited samples. In conclusion, existence of LO and TO modes confirmed formation of the ZnSe thin films under impact of the reducing environment.
Fig. 4. Variation of (ahn)2 versus hn for as-prepared ZnSe thin films.
3.6. Optical studies The optical studies are the effective means of determining the energy gap and hence the band structure. The optical band gaps (Eg) were therefore determined for these films (various Zn/Se ratios) using (ahu)¼ A (hu Eg)m, where m and A are constants, a is the absorption coefficient (cm 1) and hu is the photon energy (eV) [8,9]. For this, variations of (ahu)2 versus hu (Fig. 4) were plotted and the linear part extrapolated towards lower photon energies, i.e., (ahu)2 ¼0. A little decrease in Eg is observed with
increased reducing environment (Zn/Se ratio). Particularly, the drop in band gap is a consequence of localized states formation near the edge of conduction band due to the defects present in the structure [14]. The XRD studies suggested variation in dislocation density which causes modification in the localized states and thus leads to the variation in optical band gap. We attribute the decrease in band gap to the presence of excess metallic Se phase which is revealed by the XRD and compositional analyses.
312
L.P. Deshmukh et al. / Materials Letters 92 (2013) 308–312
4. Conclusions
References
1. The reducing environment (N2H4) strongly influences ZnSe film growth. 2. The films are polycrystalline with a wurtzite hexagonal structure. 3. SEM microscopy revealed spherical crystallites with nonuniform distribution. 4. AFM studies showed drastic effect of reducing environment on film formation. 5. Optical gap variation is correlated to variation in crystal defects (XRD) and excess of metallic Se phase (compositional) analyses.
[1] Balu AR, Nagarethinam VS, Syed Basheer Ahmed MG, Thayumanavan A, Murli KR. Mater Sci Eng B 2010;171:93–8. [2] Huang CW, Weng HM, Jiang YU, Heng HY. Vacuum 2009;83:313–8. [3] Mehta C, Saini GSS, Abbas JM, Tripathi SK. Appl Surf Sci 2009;256:608–14. [4] Chen L, Zhang D, Zhai G, Zhang J. Mater Chem Phys 2010;120:456–60. [5] Hodes G. Phys Chem Chem Phys 2007;9:2181. [6] Mane ST, Kamble SS, Deshmukh LP. Mater Lett 2012;67:373–5. [7] Chattarki AN, Kamble SS, Deshmukh LP. Mater Lett 2012;67:39–41. [8] Deshmukh LP, Holikatti SG. J Phys D Appl Phys 1994;2:1786–9. [9] Pingale PC, Pirgonde BR, Deshmukh LP, Sharon M. Proceedings of the sixth international conference on technological advances of thin films & surface coatings, TF-2012, July 14–17, 2012, Singapore. [10] Korneeva, Sov. Phys. Crystallogr. [Engl. Transl.], JCPD data files PDF# 150105, 1962; 6: 505. [11] Chaliha S, Borah MN, Sarmah PC, Rahman A. J Optoelctron Adv Mater 2008;10(2):427–33. [12] Kumar P, Singh K. J Optoelectron Biomed Mater 2009;1:59–69. [13] Venkatachalam S, Agilan S, Mangalaraj D, Naranyandass SaK. Mater Sci Semicond Process 2007;10:128–32. [14] Mott NF, Davis EA. Electronic processes in non-crystalline materials. Oxford: Mott and Davis Clarendon Press; 1971.
Acknowledgement We thank Prof. N.N. Maldar of our university for useful discussions on the growth mechanism of these films.