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Characterization of copper-doped sprayed ZnS thin films
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Physica B 381 (2006) 40–46 www.elsevier.com/locate/physb
Characterization of copper-doped sprayed ZnS thin films Mustafa O¨ztas-, Metin Bedir, A. Necmeddin Yazici, E. Vural Kafadar, Hu¨seyin ToktamısDepartment of Engineering Physics, University of Gaziantep, 27310- Gaziantep, Turkey Received 11 July 2005; received in revised form 22 November 2005; accepted 12 December 2005
Abstract Zinc sulfide (ZnS) thin films were deposited on a glass substrate by the spray pyrolysis technique. The films were doped with copper using the direct method, consisting in addition of a copper salt (CuCl2) to the spray solution of ZnS. The films were characterized by Xray diffraction (XRD), scanning electron microscopy (SEM) and thermoluminescence (TL). XRD as well as SEM techniques have been employed to investigate the structure and surface morphology of as-deposited and doped films. Optical properties, such as transmission and the band gap have been analyzed. A drastic change in the resistivity has been observed due to the incorporation of Cu dopant and the results are discussed in detail. The effect of the spraying time on the electrical and optical properties of the films has been studied. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 71.20.Nr; 73.61.Ga; 78.60.Kn Keywords: ZnS; Thin film; Thermoluminescence
1. Introduction Thin ZnS film is a promising material for the use in various application devices. In opto-electronics, it can be used as light-emitting diode in the blue to ultraviolet spectral region due to its wide band gap at room temperature (RT). Furthermore, ZnS films are used as light source for viewing screens and buffer layer for Cu(In,Ga)(S,Se)2 solar cells [1,2]. There are many studies about different properties of electroluminescent devices based on ZnS–Cu structures [3–5]. ZnS is one of the well-known II–VI compound semiconductors suitable to be used as host matrix for large variety of dopants because of its wide direct energy band gap (E3.7 eV). It is known that ZnS phosphors have a broadband luminescence from the near ultraviolet (UV) to the near infrared (IR). Therefore, it has been often used in the field of opto-electronic devices, such as for lightemitting diodes and flat-panel displays [6]. Especially, when ZnS is doped with a small amount of metallic ions, it emits a light in the visible region which is characteristic of the Corresponding author. Tel.: +99 342 2601200x2228; fax: +99 342 3601100. E-mail address: [email protected] (M. Bedir).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.248
incorporated impurity. Therefore, it forms a very important class of phosphors for the fabrication of thin film electroluminescent devices. For example, ZnS:Cu turned out to be a good CRT phosphor and is applied in, for example, color TV and oscilloscopes. Therefore, the luminescent properties of ZnS phosphors doped with metallic ions have been discussed extensively in the literature and the properties of such phosphors can be found in many books dealing with luminescence [7–9]. In this paper, we have investigated the structural, electrical, optical and thermoluminescence (TL) properties of undoped and copper-doped ZnS thin films deposited by spray pyrolysis. 2. Experimental procedure There are many techniques to prepare ZnS thin films [10]. In the given study, ZnS:Cu thin films were obtained by spray pyrolysis in air atmosphere. The experimental set-up used for the preparation of pyrolytically spray deposited films is described in our previous papers [11,12]. The initial solution is prepared from zinc chloride (ZnCl2) at 0.5 M concentration and 0.5 M thiourea (SC(NH)2)2 in deionized water. The Cu (0.1%) dopant is added to spray solution in
ARTICLE IN PRESS M. O¨ztas- et al. / Physica B 381 (2006) 40–46
the form of CuCl2. The prepared solution is sprayed (5 ml/ min) onto the clean glass substrates, heated between 400 and 500 1C and spraying time ranged between 0.5 and 2.0 h. The structural characterization of deposited films was carried out by X-ray diffraction (XRD) technique on Braker AXS D5005 diffractometer (monochromatic CuKa radiation, l ¼ 1.54056 A). The XRD patterns were recorded in 2y interval from 201 up to 601 with the step 0.051. The surface morphology was characterized by scanning electron microscopy (SEM). Perkin-Elmer Lambda 25 UV–VIS–NIR was used to determine the optical absorbance of the films as function of wavelength at RT. The optical band gap energy Eg was determined by extrapolating the high absorption region of the curve to the energy axis [13]. The electrical studies of ZnS and Cu-doped ZnS thin films were determined by van der Pauw measurements at RT. The glow curves of ZnS thin films were measured using a Harshaw QS 3500 manual-type TL reader at a linear heating rate of 1 1C/s under a continuous flux of nitrogen. All the films were irradiated using a 90Sr/90Y b-source (2.27 MeV) at a dose rate of approximately 0.015 Gy/s at RT. The recorded glow curves were analyzed using computer glow curve deconvolution (CGCD) method [14,15]. The CGCD program uses a linear least-square minimization procedure to determine peak area, activation energy, frequency factor and kinetic order.
(c)
41
(002)
2h
3. Results and discussion 3.1. Structure and morphology properties The XRD pattern of a typical ZnS film deposited at 500 1C substrate temperature in the as-deposited and doped condition is shown in Fig. 1a,b. It is observed that, the XRD of undoped ZnS thin film (Fig. 1a) is found to be polycrystalline with preferential orientation along the (0 0 2) plane, the other secondary peaks visible are (2 2 0) and (1 0 3). All the peaks are associated with hexagonal ZnS and no major zinc or sulfide peaks are found. The lattice parameter a is determined from Bragg’s formula for the hexagonal systems and using the angles 2y ¼ 28.51, 48.131 and 51.851 (Fig. 1). The lattice constant calculated from XRD trace is found to be 5.29 A˚. The small differences in the lattice constants of the ZnS thin films given here and in Ref. [16] can be caused by the different ZnS thin film preparation techniques. Structural analysis of Cu-doped ZnS thin film is shown in Fig. 1b. The XRD shows a hexagonal structure with a preferred orientation along the (0 0 2) plane. No major difference is observed in X-ray patterns of the as-deposited and Cu-doped samples. In doped films, the sharpness of the peak increases which in turn decreases the full-width of the half-maximum (FWHM). Also, the calculated lattice parameter for the ZnS:Cu thin film (a ¼ 5.27 A˚) is smaller than that for the undoped ZnS thin film. It can be said that it is attributed to the increase in grain size, which improves the crystalline
Intensity (Arb. Units)
(002)
1.5 h (002)
(220)
(103)
1h
(002) (220)
20
30
40 2θ (Degree)
(103)
50
0.5 h
60
Fig. 1. X-ray diffractogram of (a) undoped ZnS thin film (b) Cu-doped (0.1%) ZnS thin films deposited at 500 1C substrate temperature (spraying time ¼ 0.5 h). (c) X-ray diffractogram of ZnS thin films with spraying times at 500 1C substrate temperature.
quality of the film. It is possible that the increasing doped level of copper results in a change of preferred growth. Also, at high doping level, film growth can be non-oriented and with poor crystallinity. This type of similar results was observed by Paraguay et al. [17] for ZnO thin films.
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Fig. 1c shows the XRD spectrum for ZnS films with different film thickness for spraying times. As shown in Fig. 1c, only the (0 0 2) peak is observed at 2y ¼ 28.51 for all samples. It is shown that with increasing the film thickness for spraying times, the locations of the measured diffraction peaks do not change significantly but the intensities of the peaks become more intense and sharper. This is due to the crystallinity of the films being improved and grain size becoming larger when elevating the film thickness. A similar behavior was observed by Fortunato et al. [18] in Ga-doped ZnO. SEM micrographs of the ZnS and the Cu-doped ZnS films are shown in Fig. 2. A typical ZnS thin film deposited at 500 1C substrate temperature (Fig. 2a) shows globular surface morphology. Some regions are found to possess brighter contrast than the under layer indicating the occurrence of
loosely adhering surface particles. The tendency of ZnS thin film surface to accumulate loose adhering particles increases for thicker depositing. This is indeed not surprising since ZnS thin film is expected to be highly resistive. The Cu-doped ZnS thin film surface morphology shown in Fig. 2b shows the fiber-like formation of crystallites and better connectivity between grains. This type of similar formation of the crystallites was observed by Amalraj et al. [19]. 3.2. Optical properties The transmittance spectra of ZnS thin film and Cudoped ZnS thin film are shown in Fig. 3. It can be seen that the edge of the spectrum is located at about lE336 nm for ZnS and lE340 nm for ZnS:Cu thin films. The optical band gap was determined from the absorption coefficient using the relation: ðahnÞ2 ¼ Kðhn E g Þ,
(1)
where a is the absorption coefficient, hn is the photon energy, K is a constant which is related to the effective masses associated with the valence and conduction bands, and Eg is the energy gap between the bottom of the conduction band and top of the valence band at the same value of wave vector. An energy band gap of 3.69 eV for ZnS and 3.65 eV for ZnS:Cu was obtained by extrapolating the linear part of the curves (ahn)2 versus (hn) are shown in Fig. 4. This decrease in the band gap of ZnS after Cu doping can be related to the structural modification of ZnS thin films. It can also be supposed that the copper ions from the sprayed deposition can replace either substitutional or interstitial the zinc ions in the ZnS lattice creating the structural deformation. It is hypothesized that Cu introduces some additional energy levels in the ZnS band gap close to the valence band edge, with a consequent reduction of the energy associated with direct transition. Due to doping of Cu in ZnS thin films, the optical transmission is reduced due to free hole absorption. And also the decrease of the band gap of ZnS by doping with Cu can be explained by the influence of near-band levels, arising on introduction of copper and related to copper and copper-based complexes [20].
Fig. 2. SEM micrographs of (a) undoped and (b) Cu-doped (0.1%) ZnS thin films deposited at 500 1C substrate temperature (spraying time0.5 h).
Fig. 3. Transmission spectra of the ZnS and ZnS:Cu(0.1%) thin films deposited at 500 1C substrate temperature (spraying time ¼ 0.5 h).
ARTICLE IN PRESS 10 9 8 ZnS:Cu ZnS 7 6 5 4 3 2 1 0 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 Energy (hν)
Fig. 4. The (ahn)2 versus hn plots of (a) ZnS and (b) ZnS:Cu (0.1%) thin films deposited at 500 1C substrate temperature (spraying time ¼ 0.5 h).
43
1.0E+08 ZnS
ZnS:Cu
1.0E+07 Resistivity (Ohm-cm)
(αhν)2 (eV/cm)2
M. O¨ztas- et al. / Physica B 381 (2006) 40–46
1.0E+06
1.0E+05
1.0E+04
1.0E+03 375
400
425
450
475
500
525
Substrate Temperature,°C
3.3. Electrical properties The electrical studies of ZnS thin films have been performed using a four-probe Van der Pauw technique. ZnS thin films are found to be highly resistive in the range 1.5 104–1.7 106 Ocm when the deposition substrate temperature varied from 400 to 500 1C. As the substrate temperature increases (400–500 1C) the resistivity of the film decreases from 106 to 104 Ocm The electrical behavior of the undoped and Cu-doped ZnS thin films are shown in Fig. 5. Hence, the resistivity of Cu-doped ZnS thin film reduces from 107 to 105 Ocm with increasing substrate temperature from 400 to 500 1C. However, the increase in resistivity for copper-doped films may be due to a decrease in electron concentration. At higher substrate temperatures ZnS thin films with improved crystallinity is obtained, which in turn may reduce grain boundary scattering with a consequent reduction in the resistivity of the films. We assume that the Cu in ZnS—analogous to Cu in CdS [21]— can be incorporated on Zn sites as an acceptor and on interstitial sites as a donor. It is known that the sprayed deposited ZnS thin films are usually n-type [22] and the Cu1+ ions introduced in ZnS lattice play the role of an acceptor-type impurity. The consequence is a decrease in the concentration of electrons, which are majority carriers in the sprayed deposited ZnS thin films and an increase in the resistance. Thus it can be said that further increasing the copper content could lead to an increase in the resistance of ZnS thin films. Electrical resistivity, optical properties and other relevant parameters of the prepared samples are presented in Table 1. The resistivity of Cu-doped ZnS films is high. This may be due to the grain boundary effects since the films are polycrystalline in nature. It can be seen that the film thickness and spraying time affect the resistivity of thin film and other properties. The table demonstrates that, as the film thickness (spraying time) increases, the resistivity decreases. This behavior may be due to increase of crystallite size and carrier concentration. Thus it can be said that the films had poor crystallinity and the resistivity markedly increased, which was primarily due to the decrease in carrier concentration. The films had poor
Fig. 5. Variation of resistance for ZnS and ZnS:Cu(0.1%) thin films deposited with different substrate temperatures (spraying time ¼ 0.5 h).
crystallinity, which indicated that the films consisted of a few atomic layers of disordered atoms [23]. Since the atoms in the poor crystallized area were disordered, there were a large number of defects due to incomplete atomic bonding. After trapping the mobile carriers, the traps became electrically charged, creating a potential energy barrier which impeded the motion of carriers from one crystallite to another, thereby reducing their carrier concentration [24,25]. Not many carriers were probably released from the poor crystallized area [26]. The poor crystallinity resulted in lower carrier concentration in this study. Therefore, the resistivity was affected by carrier concentration and spraying time in this study. Table 1 summarizes the values of resistivity, transmittance and band gap according to the substrate temperatures and spraying time. It is showed that substrate temperature and spraying time are both of the most important factors influencing the film property. The result shows that the resistivity decreases when the substrate temperature increases. This signifies, that the dislocations and density of grain boundaries decrease. Therefore, it could be related to an improvement of the crystallinity leading to a decrease of donor sites trapped at the dislocations and grain boundaries [27]. The decrease in resistivity with increase in substrate temperature can also be explained by the fact that the grain size increases significantly with the increase in deposition temperature, thus reducing grain boundary scattering and transmittance. The transmittance spectra of the films decrease as film thickness increases. This is attributed to the increase in the film thickness, which subsequent increase in absorption. In the films, the onset of absorption edge became less sharp, this is due to the fact that bigger crystalline sizes are deposited; and the scattered radiation became remarkable due to the surface roughness [28,29]. With increasing thickness, which results from the onset of fundamental absorption, was observed to shift towards the shorter wavelength [30,31]. In this study, the films have good quality of crystallinity with film thickness increasing. We
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Table 1 Electrical and optical properties of ZnS and ZnS:Cu films at various substrate temperatures and spraying times Transmittance at 340 nm (%)
150.00 73.00 46.00 27.00
3.69 3.65 3.62 3.60
82.0 75.0 70.0 66.0
0.8 1.3 1.6 1.8
4.50 3.20 2.10 1.30
3.64 3.59 3.55 3.50
86.0 81.0 77.0 72.0
0.5 1.0 1.5 2.0
0.5 0.9 1.4 1.7
1.70 1.20 0.70 0.40
3.60 3.55 3.51 3.47
91.0 87.0 83.0 79.0
400
0.5 1.0 1.5 2.0
0.8 1.2 1.6 1.8
5500.00 2570.00 1750.00 1120.00
3.65 3.60 3.57 3.55
65.0 61.0 57.0 52.0
450
0.5 1.0 1.5 2.0
0.7 1.0 1.4 1.6
96.00 45.00 28.00 11.00
3.61 3.56 3.52 3.49
72.0 68.0 62.0 58.0
500
0.5 1.0 1.5 2.0
0.5 0.9 1.1 1.2
35.00 24.00 18.00 9.90
3.58 3.52 3.49 3.46
79.0 74.0 70.0 67.0
Substrate temperature (1C)
Spray time (h)
Thickness (mm)
ZnS
400
0.5 1.0 1.5 2.0
1.0 1.5 1.9 2.2
450
0.5 1.0 1.5 2.0
500
ZnS:Cu
Resistivity ( 104)
Band gap (eV)
Sample
suggest that the increase of film thickness is caused by optical absorption on the film interior and surface. It is seen that the decrease in resistivity is observed up to a spraying time. The films were grown in the temperature region from 400 up to 500 1C substrate temperature. It was found that the film thickness decreases with increasing the growth temperature despite the fact that the kinetics of the films forming reaction should increase with temperature. The observed dependence can be explained by the diminished mass transport to the substrate at higher temperature due to gas convection from the bath pushing the droplets of the precursor. On the other hand, the decrease in film thickness can be attributed to an increase in the rate of reevaporation at higher temperatures. Similar results were reported by Afifi et al. [32]. From these observations, the influence of both spraying time and substrate temperature on Eg values is clearly evident. As the substrate temperature and spraying time increase, the value of Eg decreases, which indicates that crystallization would cause the Eg narrowing. The results of the optical studies revealed that the films formed with small thickness (or small spraying time) are nearly stoichiometric, while those formed at elevated thicknesses are sub-stoichiometric. This substoichiometric film may result from a formation of zinc ion vacancies in the films act as positive structural defect. Another contributor to the decreasing optical band gap Eg with film thickness can be understood on the quantum size
effect observed in thin films of semiconductors. Additionally, we observed a better crystallinity of the layers with increasing substrate temperature and spraying time. 3.4. TL properties TL is observed when, in the process of irradiating a material, part of the irradiation energy is used to transfer electrons to traps and holes to centers. This energy, stored in the form of the trapped electrons, is released by raising the temperature of the material, and the released energy is converted to luminescence. This trapping process and the subsequent release of the stored energy find important application in ionizing radiation dosimetry and in the operation of long persistence phosphors. Much information about the trapping process and the release of trapped electrons is obtained from the TL spectrum, in which, after turning off the irradiating source. The shape and position of the resultant TL glow curves can be analyzed to extract information on the various parameters of the trapping process—trap depth, trapping and retrapping rates, etc. The Cu impurity penetrates through the lattice during film production and thus results in the formation of complex centers (CuZn, Cui). It can also be supposed that copper ions during the spray deposition can replace either substitutionally or interstitially the zinc ions in the ZnS lattice creating the structural deformation. Another consequence is the formation of the structural defects (the trapping levels in the
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Residue
8 0 -4
500
400
300
Peak 1 Peak 2
200
100
0 50
100
150 200 Temperature (˚C)
250
300
Fig. 7. A typical analyzed glow curve of ZnS:Cu (0.1%) thin films at 500 1C substrate temperature measured after 1 h (E55 Gy) b-irradiation at RT. The glow curve was measured by heating the sample to 300 1C at a rate of 1 1C/s. In the figure, open circles represent the experimental points.
Table 2 The values of the kinetic parameters of TL peaks of ZnS:Cu thin films determined by the CGCD method Peak no.
1 2
Fig. 6. Selected glow curves of (a) ZnS and (b) ZnS:Cu (0.1%) thin films developed via chemical spraying method thin films deposited at 500 1C substrate temperature. In the figure, open circles represent the experimental points.
4
-8
TL Intensity (ar.un.)
forbidden band of semiconductor). Therefore, copper ions occupying in the ZnS thin film sites in the spray deposition process, which can create trapping levels. The primary defects in ZnS thin films are resulted from direct atomic displacements (vacancies and interstitials), which are thermally stable at RT and, most of these traps are inactive in TL process. Therefore, only one TL glow peak (Tm ¼ 11872 1C) is observed due to the recombination of an electron, which is thermally released from VZn–Cltype defect with a hole in an O2 type luminescent center for an undoped sample. The typical glow curves (GCs) obtained from ZnS thin films produced using chemical spraying method are shown in Fig. 6(a) [12]. It can be seen from this figure that the shape and intensity of the GCs are changed after the ZnS thin film is doped with copper (Fig. 6(b)). It is well known that the TL performance of ZnS thin film is very sensitive to the structural changes of defects and the presence of shallow and deep trapping levels within the material. Therefore, the number of trap states and intensity of GCs of many TL materials are strongly affected by the variation of defects, defect clusters, surface states and dislocations in the lattice. It is also obvious that the copper-doped ZnS thin film strongly modifies the aggregation state of impurity in the host lattice along with the existing traps within the material. It is reasonable to assume that the variations in the aggregation state of impurity and its interaction with defects, defect clusters or dislocations create new traps within the material that were absent in undoped ZnS thin film. A typical analyzed TL glow curve from 15 min irradiated (E20 Gy) ZnS:Cu thin film is shown in Fig. 7 along with the components obtained from CGCD. It is obvious that two trapping levels are sufficient to explain the shape of the glow curve of the ZnS:Cu. In this case, it was obtained that the best-fit curve differs slightly from the experimental glow curves. A careful investigation of this figure indicates that glow curve structure of ZnS:Cu thin film is described by a linear combination of two glow peaks between RT
45
CGCD method Tm (1C) (b ¼ 1 1C/s)
Ea (eV)
ln(s) (s1)
b
108 155
0.869 0.369
25.74 8.511
1.373 1.326
and 300 1C when the glow curve is immediately recorded after irradiation and also a best fit was always obtained using the general-order kinetics. The computed trapping parameters are presented in Table 2. As seen from Fig. 7, the more than one type of traps become active in TL process for ZnS:Cu thin film. These results clearly suggest that the ZnS:Cu thin film induces defect migration and clustering that may cause an increase or a decrease (via defect creation or destruction) in the trap densities and alterations in the electrically active states. The glow peaks in Fig. 8 are caused by the defects which are produced during the sample processing. Carriers trapped
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crystallinity of ZnS thin films. Therefore, the substrate temperature influences the band gap shift which has to be related to the spraying time, film thickness and grain size. It can also be concluded that the spraying time and the copper concentration have a strong influence on the resistivity and consequently on the band gap energy of the films. References
Fig. 8. Glow curves of the different ratio Cu-doped ZnS thin films deposited at 500 1C substrate temperature. In the figure, open symbols represent the experimental points.
at the surface states or defect sites may be released by heating to recombine. As the contents of the surface states increase, the particles may provide more accessible carriers (holes and electrons) for the TL recombination proportional to the surface states. As the surface states increase rapidly, the TL efficiency is enhanced. The physical properties of these traps may be reflected from the temperature, shape, or symmetry of the glow peak. It can be seen that the glow peak temperature and shape (or symmetry) of all the samples are similar, indicating that the physical properties of the traps (i.e., surface states and/or defect sites) are not sensitive to the particle size, indicating the trap depth does not change as much upon decreasing size. It can be observed that increasing the copper content in the sprayed deposition increases the concentration of the trapped carriers also. In general, the ZnS:Cu thin films will also have larger number of sulfur (VS) and zinc (VZn) vacancies. These are generated during the production of the samples and the irradiation of the samples with high energetic particles. The copper ions can also form defect complexes with these vacancies in the ZnS:Cu lattice which can create trapping levels within the band gap and they can act as electron and hole traps which are responsible for the glow peaks in Cu-doped ZnS thin films. 4. Conclusion Cu (0.1 at%) doped ZnS thin films were prepared by the spray pyrolysis method. The XRD studies have not revealed any change in the thin film structure. However, the crystallinity of the films has shown some improvements with increasing the spraying time. Optical studies of the Cu-doped thin films show a decrease in the transmission and also show that copper doping of the ZnS thin films results in a significant increase in resistivity and a slight decrease in the band gap of the ZnS thin films. The observed results are attributed to the migration of Cu in polycrystalline ZnS thin films by physical adsorption at the grain boundaries and into the grains. It can be said that further increasing the copper content could lead to an increase in the resistivity and to poor
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Physica B 381 (2006) 40–46 www.elsevier.com/locate/physb
Characterization of copper-doped sprayed ZnS thin films Mustafa O¨ztas-, Metin Bedir, A. Necmeddin Yazici, E. Vural Kafadar, Hu¨seyin ToktamısDepartment of Engineering Physics, University of Gaziantep, 27310- Gaziantep, Turkey Received 11 July 2005; received in revised form 22 November 2005; accepted 12 December 2005
Abstract Zinc sulfide (ZnS) thin films were deposited on a glass substrate by the spray pyrolysis technique. The films were doped with copper using the direct method, consisting in addition of a copper salt (CuCl2) to the spray solution of ZnS. The films were characterized by Xray diffraction (XRD), scanning electron microscopy (SEM) and thermoluminescence (TL). XRD as well as SEM techniques have been employed to investigate the structure and surface morphology of as-deposited and doped films. Optical properties, such as transmission and the band gap have been analyzed. A drastic change in the resistivity has been observed due to the incorporation of Cu dopant and the results are discussed in detail. The effect of the spraying time on the electrical and optical properties of the films has been studied. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 71.20.Nr; 73.61.Ga; 78.60.Kn Keywords: ZnS; Thin film; Thermoluminescence
1. Introduction Thin ZnS film is a promising material for the use in various application devices. In opto-electronics, it can be used as light-emitting diode in the blue to ultraviolet spectral region due to its wide band gap at room temperature (RT). Furthermore, ZnS films are used as light source for viewing screens and buffer layer for Cu(In,Ga)(S,Se)2 solar cells [1,2]. There are many studies about different properties of electroluminescent devices based on ZnS–Cu structures [3–5]. ZnS is one of the well-known II–VI compound semiconductors suitable to be used as host matrix for large variety of dopants because of its wide direct energy band gap (E3.7 eV). It is known that ZnS phosphors have a broadband luminescence from the near ultraviolet (UV) to the near infrared (IR). Therefore, it has been often used in the field of opto-electronic devices, such as for lightemitting diodes and flat-panel displays [6]. Especially, when ZnS is doped with a small amount of metallic ions, it emits a light in the visible region which is characteristic of the Corresponding author. Tel.: +99 342 2601200x2228; fax: +99 342 3601100. E-mail address: [email protected] (M. Bedir).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.248
incorporated impurity. Therefore, it forms a very important class of phosphors for the fabrication of thin film electroluminescent devices. For example, ZnS:Cu turned out to be a good CRT phosphor and is applied in, for example, color TV and oscilloscopes. Therefore, the luminescent properties of ZnS phosphors doped with metallic ions have been discussed extensively in the literature and the properties of such phosphors can be found in many books dealing with luminescence [7–9]. In this paper, we have investigated the structural, electrical, optical and thermoluminescence (TL) properties of undoped and copper-doped ZnS thin films deposited by spray pyrolysis. 2. Experimental procedure There are many techniques to prepare ZnS thin films [10]. In the given study, ZnS:Cu thin films were obtained by spray pyrolysis in air atmosphere. The experimental set-up used for the preparation of pyrolytically spray deposited films is described in our previous papers [11,12]. The initial solution is prepared from zinc chloride (ZnCl2) at 0.5 M concentration and 0.5 M thiourea (SC(NH)2)2 in deionized water. The Cu (0.1%) dopant is added to spray solution in
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the form of CuCl2. The prepared solution is sprayed (5 ml/ min) onto the clean glass substrates, heated between 400 and 500 1C and spraying time ranged between 0.5 and 2.0 h. The structural characterization of deposited films was carried out by X-ray diffraction (XRD) technique on Braker AXS D5005 diffractometer (monochromatic CuKa radiation, l ¼ 1.54056 A). The XRD patterns were recorded in 2y interval from 201 up to 601 with the step 0.051. The surface morphology was characterized by scanning electron microscopy (SEM). Perkin-Elmer Lambda 25 UV–VIS–NIR was used to determine the optical absorbance of the films as function of wavelength at RT. The optical band gap energy Eg was determined by extrapolating the high absorption region of the curve to the energy axis [13]. The electrical studies of ZnS and Cu-doped ZnS thin films were determined by van der Pauw measurements at RT. The glow curves of ZnS thin films were measured using a Harshaw QS 3500 manual-type TL reader at a linear heating rate of 1 1C/s under a continuous flux of nitrogen. All the films were irradiated using a 90Sr/90Y b-source (2.27 MeV) at a dose rate of approximately 0.015 Gy/s at RT. The recorded glow curves were analyzed using computer glow curve deconvolution (CGCD) method [14,15]. The CGCD program uses a linear least-square minimization procedure to determine peak area, activation energy, frequency factor and kinetic order.
(c)
41
(002)
2h
3. Results and discussion 3.1. Structure and morphology properties The XRD pattern of a typical ZnS film deposited at 500 1C substrate temperature in the as-deposited and doped condition is shown in Fig. 1a,b. It is observed that, the XRD of undoped ZnS thin film (Fig. 1a) is found to be polycrystalline with preferential orientation along the (0 0 2) plane, the other secondary peaks visible are (2 2 0) and (1 0 3). All the peaks are associated with hexagonal ZnS and no major zinc or sulfide peaks are found. The lattice parameter a is determined from Bragg’s formula for the hexagonal systems and using the angles 2y ¼ 28.51, 48.131 and 51.851 (Fig. 1). The lattice constant calculated from XRD trace is found to be 5.29 A˚. The small differences in the lattice constants of the ZnS thin films given here and in Ref. [16] can be caused by the different ZnS thin film preparation techniques. Structural analysis of Cu-doped ZnS thin film is shown in Fig. 1b. The XRD shows a hexagonal structure with a preferred orientation along the (0 0 2) plane. No major difference is observed in X-ray patterns of the as-deposited and Cu-doped samples. In doped films, the sharpness of the peak increases which in turn decreases the full-width of the half-maximum (FWHM). Also, the calculated lattice parameter for the ZnS:Cu thin film (a ¼ 5.27 A˚) is smaller than that for the undoped ZnS thin film. It can be said that it is attributed to the increase in grain size, which improves the crystalline
Intensity (Arb. Units)
(002)
1.5 h (002)
(220)
(103)
1h
(002) (220)
20
30
40 2θ (Degree)
(103)
50
0.5 h
60
Fig. 1. X-ray diffractogram of (a) undoped ZnS thin film (b) Cu-doped (0.1%) ZnS thin films deposited at 500 1C substrate temperature (spraying time ¼ 0.5 h). (c) X-ray diffractogram of ZnS thin films with spraying times at 500 1C substrate temperature.
quality of the film. It is possible that the increasing doped level of copper results in a change of preferred growth. Also, at high doping level, film growth can be non-oriented and with poor crystallinity. This type of similar results was observed by Paraguay et al. [17] for ZnO thin films.
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Fig. 1c shows the XRD spectrum for ZnS films with different film thickness for spraying times. As shown in Fig. 1c, only the (0 0 2) peak is observed at 2y ¼ 28.51 for all samples. It is shown that with increasing the film thickness for spraying times, the locations of the measured diffraction peaks do not change significantly but the intensities of the peaks become more intense and sharper. This is due to the crystallinity of the films being improved and grain size becoming larger when elevating the film thickness. A similar behavior was observed by Fortunato et al. [18] in Ga-doped ZnO. SEM micrographs of the ZnS and the Cu-doped ZnS films are shown in Fig. 2. A typical ZnS thin film deposited at 500 1C substrate temperature (Fig. 2a) shows globular surface morphology. Some regions are found to possess brighter contrast than the under layer indicating the occurrence of
loosely adhering surface particles. The tendency of ZnS thin film surface to accumulate loose adhering particles increases for thicker depositing. This is indeed not surprising since ZnS thin film is expected to be highly resistive. The Cu-doped ZnS thin film surface morphology shown in Fig. 2b shows the fiber-like formation of crystallites and better connectivity between grains. This type of similar formation of the crystallites was observed by Amalraj et al. [19]. 3.2. Optical properties The transmittance spectra of ZnS thin film and Cudoped ZnS thin film are shown in Fig. 3. It can be seen that the edge of the spectrum is located at about lE336 nm for ZnS and lE340 nm for ZnS:Cu thin films. The optical band gap was determined from the absorption coefficient using the relation: ðahnÞ2 ¼ Kðhn E g Þ,
(1)
where a is the absorption coefficient, hn is the photon energy, K is a constant which is related to the effective masses associated with the valence and conduction bands, and Eg is the energy gap between the bottom of the conduction band and top of the valence band at the same value of wave vector. An energy band gap of 3.69 eV for ZnS and 3.65 eV for ZnS:Cu was obtained by extrapolating the linear part of the curves (ahn)2 versus (hn) are shown in Fig. 4. This decrease in the band gap of ZnS after Cu doping can be related to the structural modification of ZnS thin films. It can also be supposed that the copper ions from the sprayed deposition can replace either substitutional or interstitial the zinc ions in the ZnS lattice creating the structural deformation. It is hypothesized that Cu introduces some additional energy levels in the ZnS band gap close to the valence band edge, with a consequent reduction of the energy associated with direct transition. Due to doping of Cu in ZnS thin films, the optical transmission is reduced due to free hole absorption. And also the decrease of the band gap of ZnS by doping with Cu can be explained by the influence of near-band levels, arising on introduction of copper and related to copper and copper-based complexes [20].
Fig. 2. SEM micrographs of (a) undoped and (b) Cu-doped (0.1%) ZnS thin films deposited at 500 1C substrate temperature (spraying time0.5 h).
Fig. 3. Transmission spectra of the ZnS and ZnS:Cu(0.1%) thin films deposited at 500 1C substrate temperature (spraying time ¼ 0.5 h).
ARTICLE IN PRESS 10 9 8 ZnS:Cu ZnS 7 6 5 4 3 2 1 0 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 Energy (hν)
Fig. 4. The (ahn)2 versus hn plots of (a) ZnS and (b) ZnS:Cu (0.1%) thin films deposited at 500 1C substrate temperature (spraying time ¼ 0.5 h).
43
1.0E+08 ZnS
ZnS:Cu
1.0E+07 Resistivity (Ohm-cm)
(αhν)2 (eV/cm)2
M. O¨ztas- et al. / Physica B 381 (2006) 40–46
1.0E+06
1.0E+05
1.0E+04
1.0E+03 375
400
425
450
475
500
525
Substrate Temperature,°C
3.3. Electrical properties The electrical studies of ZnS thin films have been performed using a four-probe Van der Pauw technique. ZnS thin films are found to be highly resistive in the range 1.5 104–1.7 106 Ocm when the deposition substrate temperature varied from 400 to 500 1C. As the substrate temperature increases (400–500 1C) the resistivity of the film decreases from 106 to 104 Ocm The electrical behavior of the undoped and Cu-doped ZnS thin films are shown in Fig. 5. Hence, the resistivity of Cu-doped ZnS thin film reduces from 107 to 105 Ocm with increasing substrate temperature from 400 to 500 1C. However, the increase in resistivity for copper-doped films may be due to a decrease in electron concentration. At higher substrate temperatures ZnS thin films with improved crystallinity is obtained, which in turn may reduce grain boundary scattering with a consequent reduction in the resistivity of the films. We assume that the Cu in ZnS—analogous to Cu in CdS [21]— can be incorporated on Zn sites as an acceptor and on interstitial sites as a donor. It is known that the sprayed deposited ZnS thin films are usually n-type [22] and the Cu1+ ions introduced in ZnS lattice play the role of an acceptor-type impurity. The consequence is a decrease in the concentration of electrons, which are majority carriers in the sprayed deposited ZnS thin films and an increase in the resistance. Thus it can be said that further increasing the copper content could lead to an increase in the resistance of ZnS thin films. Electrical resistivity, optical properties and other relevant parameters of the prepared samples are presented in Table 1. The resistivity of Cu-doped ZnS films is high. This may be due to the grain boundary effects since the films are polycrystalline in nature. It can be seen that the film thickness and spraying time affect the resistivity of thin film and other properties. The table demonstrates that, as the film thickness (spraying time) increases, the resistivity decreases. This behavior may be due to increase of crystallite size and carrier concentration. Thus it can be said that the films had poor crystallinity and the resistivity markedly increased, which was primarily due to the decrease in carrier concentration. The films had poor
Fig. 5. Variation of resistance for ZnS and ZnS:Cu(0.1%) thin films deposited with different substrate temperatures (spraying time ¼ 0.5 h).
crystallinity, which indicated that the films consisted of a few atomic layers of disordered atoms [23]. Since the atoms in the poor crystallized area were disordered, there were a large number of defects due to incomplete atomic bonding. After trapping the mobile carriers, the traps became electrically charged, creating a potential energy barrier which impeded the motion of carriers from one crystallite to another, thereby reducing their carrier concentration [24,25]. Not many carriers were probably released from the poor crystallized area [26]. The poor crystallinity resulted in lower carrier concentration in this study. Therefore, the resistivity was affected by carrier concentration and spraying time in this study. Table 1 summarizes the values of resistivity, transmittance and band gap according to the substrate temperatures and spraying time. It is showed that substrate temperature and spraying time are both of the most important factors influencing the film property. The result shows that the resistivity decreases when the substrate temperature increases. This signifies, that the dislocations and density of grain boundaries decrease. Therefore, it could be related to an improvement of the crystallinity leading to a decrease of donor sites trapped at the dislocations and grain boundaries [27]. The decrease in resistivity with increase in substrate temperature can also be explained by the fact that the grain size increases significantly with the increase in deposition temperature, thus reducing grain boundary scattering and transmittance. The transmittance spectra of the films decrease as film thickness increases. This is attributed to the increase in the film thickness, which subsequent increase in absorption. In the films, the onset of absorption edge became less sharp, this is due to the fact that bigger crystalline sizes are deposited; and the scattered radiation became remarkable due to the surface roughness [28,29]. With increasing thickness, which results from the onset of fundamental absorption, was observed to shift towards the shorter wavelength [30,31]. In this study, the films have good quality of crystallinity with film thickness increasing. We
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Table 1 Electrical and optical properties of ZnS and ZnS:Cu films at various substrate temperatures and spraying times Transmittance at 340 nm (%)
150.00 73.00 46.00 27.00
3.69 3.65 3.62 3.60
82.0 75.0 70.0 66.0
0.8 1.3 1.6 1.8
4.50 3.20 2.10 1.30
3.64 3.59 3.55 3.50
86.0 81.0 77.0 72.0
0.5 1.0 1.5 2.0
0.5 0.9 1.4 1.7
1.70 1.20 0.70 0.40
3.60 3.55 3.51 3.47
91.0 87.0 83.0 79.0
400
0.5 1.0 1.5 2.0
0.8 1.2 1.6 1.8
5500.00 2570.00 1750.00 1120.00
3.65 3.60 3.57 3.55
65.0 61.0 57.0 52.0
450
0.5 1.0 1.5 2.0
0.7 1.0 1.4 1.6
96.00 45.00 28.00 11.00
3.61 3.56 3.52 3.49
72.0 68.0 62.0 58.0
500
0.5 1.0 1.5 2.0
0.5 0.9 1.1 1.2
35.00 24.00 18.00 9.90
3.58 3.52 3.49 3.46
79.0 74.0 70.0 67.0
Substrate temperature (1C)
Spray time (h)
Thickness (mm)
ZnS
400
0.5 1.0 1.5 2.0
1.0 1.5 1.9 2.2
450
0.5 1.0 1.5 2.0
500
ZnS:Cu
Resistivity ( 104)
Band gap (eV)
Sample
suggest that the increase of film thickness is caused by optical absorption on the film interior and surface. It is seen that the decrease in resistivity is observed up to a spraying time. The films were grown in the temperature region from 400 up to 500 1C substrate temperature. It was found that the film thickness decreases with increasing the growth temperature despite the fact that the kinetics of the films forming reaction should increase with temperature. The observed dependence can be explained by the diminished mass transport to the substrate at higher temperature due to gas convection from the bath pushing the droplets of the precursor. On the other hand, the decrease in film thickness can be attributed to an increase in the rate of reevaporation at higher temperatures. Similar results were reported by Afifi et al. [32]. From these observations, the influence of both spraying time and substrate temperature on Eg values is clearly evident. As the substrate temperature and spraying time increase, the value of Eg decreases, which indicates that crystallization would cause the Eg narrowing. The results of the optical studies revealed that the films formed with small thickness (or small spraying time) are nearly stoichiometric, while those formed at elevated thicknesses are sub-stoichiometric. This substoichiometric film may result from a formation of zinc ion vacancies in the films act as positive structural defect. Another contributor to the decreasing optical band gap Eg with film thickness can be understood on the quantum size
effect observed in thin films of semiconductors. Additionally, we observed a better crystallinity of the layers with increasing substrate temperature and spraying time. 3.4. TL properties TL is observed when, in the process of irradiating a material, part of the irradiation energy is used to transfer electrons to traps and holes to centers. This energy, stored in the form of the trapped electrons, is released by raising the temperature of the material, and the released energy is converted to luminescence. This trapping process and the subsequent release of the stored energy find important application in ionizing radiation dosimetry and in the operation of long persistence phosphors. Much information about the trapping process and the release of trapped electrons is obtained from the TL spectrum, in which, after turning off the irradiating source. The shape and position of the resultant TL glow curves can be analyzed to extract information on the various parameters of the trapping process—trap depth, trapping and retrapping rates, etc. The Cu impurity penetrates through the lattice during film production and thus results in the formation of complex centers (CuZn, Cui). It can also be supposed that copper ions during the spray deposition can replace either substitutionally or interstitially the zinc ions in the ZnS lattice creating the structural deformation. Another consequence is the formation of the structural defects (the trapping levels in the
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Residue
8 0 -4
500
400
300
Peak 1 Peak 2
200
100
0 50
100
150 200 Temperature (˚C)
250
300
Fig. 7. A typical analyzed glow curve of ZnS:Cu (0.1%) thin films at 500 1C substrate temperature measured after 1 h (E55 Gy) b-irradiation at RT. The glow curve was measured by heating the sample to 300 1C at a rate of 1 1C/s. In the figure, open circles represent the experimental points.
Table 2 The values of the kinetic parameters of TL peaks of ZnS:Cu thin films determined by the CGCD method Peak no.
1 2
Fig. 6. Selected glow curves of (a) ZnS and (b) ZnS:Cu (0.1%) thin films developed via chemical spraying method thin films deposited at 500 1C substrate temperature. In the figure, open circles represent the experimental points.
4
-8
TL Intensity (ar.un.)
forbidden band of semiconductor). Therefore, copper ions occupying in the ZnS thin film sites in the spray deposition process, which can create trapping levels. The primary defects in ZnS thin films are resulted from direct atomic displacements (vacancies and interstitials), which are thermally stable at RT and, most of these traps are inactive in TL process. Therefore, only one TL glow peak (Tm ¼ 11872 1C) is observed due to the recombination of an electron, which is thermally released from VZn–Cltype defect with a hole in an O2 type luminescent center for an undoped sample. The typical glow curves (GCs) obtained from ZnS thin films produced using chemical spraying method are shown in Fig. 6(a) [12]. It can be seen from this figure that the shape and intensity of the GCs are changed after the ZnS thin film is doped with copper (Fig. 6(b)). It is well known that the TL performance of ZnS thin film is very sensitive to the structural changes of defects and the presence of shallow and deep trapping levels within the material. Therefore, the number of trap states and intensity of GCs of many TL materials are strongly affected by the variation of defects, defect clusters, surface states and dislocations in the lattice. It is also obvious that the copper-doped ZnS thin film strongly modifies the aggregation state of impurity in the host lattice along with the existing traps within the material. It is reasonable to assume that the variations in the aggregation state of impurity and its interaction with defects, defect clusters or dislocations create new traps within the material that were absent in undoped ZnS thin film. A typical analyzed TL glow curve from 15 min irradiated (E20 Gy) ZnS:Cu thin film is shown in Fig. 7 along with the components obtained from CGCD. It is obvious that two trapping levels are sufficient to explain the shape of the glow curve of the ZnS:Cu. In this case, it was obtained that the best-fit curve differs slightly from the experimental glow curves. A careful investigation of this figure indicates that glow curve structure of ZnS:Cu thin film is described by a linear combination of two glow peaks between RT
45
CGCD method Tm (1C) (b ¼ 1 1C/s)
Ea (eV)
ln(s) (s1)
b
108 155
0.869 0.369
25.74 8.511
1.373 1.326
and 300 1C when the glow curve is immediately recorded after irradiation and also a best fit was always obtained using the general-order kinetics. The computed trapping parameters are presented in Table 2. As seen from Fig. 7, the more than one type of traps become active in TL process for ZnS:Cu thin film. These results clearly suggest that the ZnS:Cu thin film induces defect migration and clustering that may cause an increase or a decrease (via defect creation or destruction) in the trap densities and alterations in the electrically active states. The glow peaks in Fig. 8 are caused by the defects which are produced during the sample processing. Carriers trapped
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crystallinity of ZnS thin films. Therefore, the substrate temperature influences the band gap shift which has to be related to the spraying time, film thickness and grain size. It can also be concluded that the spraying time and the copper concentration have a strong influence on the resistivity and consequently on the band gap energy of the films. References
Fig. 8. Glow curves of the different ratio Cu-doped ZnS thin films deposited at 500 1C substrate temperature. In the figure, open symbols represent the experimental points.
at the surface states or defect sites may be released by heating to recombine. As the contents of the surface states increase, the particles may provide more accessible carriers (holes and electrons) for the TL recombination proportional to the surface states. As the surface states increase rapidly, the TL efficiency is enhanced. The physical properties of these traps may be reflected from the temperature, shape, or symmetry of the glow peak. It can be seen that the glow peak temperature and shape (or symmetry) of all the samples are similar, indicating that the physical properties of the traps (i.e., surface states and/or defect sites) are not sensitive to the particle size, indicating the trap depth does not change as much upon decreasing size. It can be observed that increasing the copper content in the sprayed deposition increases the concentration of the trapped carriers also. In general, the ZnS:Cu thin films will also have larger number of sulfur (VS) and zinc (VZn) vacancies. These are generated during the production of the samples and the irradiation of the samples with high energetic particles. The copper ions can also form defect complexes with these vacancies in the ZnS:Cu lattice which can create trapping levels within the band gap and they can act as electron and hole traps which are responsible for the glow peaks in Cu-doped ZnS thin films. 4. Conclusion Cu (0.1 at%) doped ZnS thin films were prepared by the spray pyrolysis method. The XRD studies have not revealed any change in the thin film structure. However, the crystallinity of the films has shown some improvements with increasing the spraying time. Optical studies of the Cu-doped thin films show a decrease in the transmission and also show that copper doping of the ZnS thin films results in a significant increase in resistivity and a slight decrease in the band gap of the ZnS thin films. The observed results are attributed to the migration of Cu in polycrystalline ZnS thin films by physical adsorption at the grain boundaries and into the grains. It can be said that further increasing the copper content could lead to an increase in the resistivity and to poor
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