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Improved optical and structural properties of cadmium sulfide nanostructures for optoelectronic applications
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Improved optical and structural properties of cadmium sulfide nanostructures for optoelectronic applications Mehdi Moghaddam, Nima Naderi∗, Mojtaba Hosseinifard, Asghar Kazemzadeh Materials and Energy Research Center, Karaj, Iran
ARTICLE INFO
ABSTRACT
Keywords: Sensors Optical properties Electrical properties Spectroscopy
In this study, the effect of ZnO seed layer on the growth of uniform CdS nanostructures was investigated using chemical bath deposition technique. Besides, the influence of molar concentration of reagents on the surface morphology, structural and optoelectrical properties of the deposited CdS thin films were examined. The CdS nanostructures were grown on bare glass and ZnO/glass substrates with different reagent molar concentrations. The results indicated an improvement in the homogeneity and uniformity of the grown CdS nanostructures on ZnO seed layer which can be due to the low lattice mismatch between ZnO and CdS structures. The CdS/ZnO samples were optimized by changing the molar concentration of reagents. A three–dimensional intersecting vertical nanosheet morphology with hexagonal structure was obtained when modified chemical concentration of 0.5 M was applied. The XRD pattern of CdS nanosheets indicated the hexagonal phase of CdS which were strongly orientated along (002) plane. The elevated intensity of dominant peak related to this sample confirmed the improved crystal quality of this CdS nanostructure comparing to the other samples. The UV–Vis spectrum demonstrated a high absorption coefficient for CdS intersecting nanosheets which might be due to the high specific surface area and light trapping behavior of this sample. The photoluminescence study also showed an improvement in optical properties of optimized CdS nanostructures. In order to study the optoelectrical properties of CdS nanostructures, metal–semiconductor–metal photodetectors were fabricated with different CdS samples and their current–voltage characteristics were analyzed. The results indicated an enhancement in photosensitivity, responsivity, and speed of photodetectors based on optimized CdS nanostructures.
1. Introduction Cadmium sulfide (CdS) is one of the most important II–VI semiconductors with a direct band gap (~2.42 eV for bulk material); its bulk and thin films are capable of absorbing the visible spectrum of light [1]. Due to its outstanding electrical and optical properties such as electron affinity, high absorption coefficient and low resistivity, CdS has shown significant potential in developing solar cells [2], photodetectors [3] and photocatalysts [4]. Several methods have been used to fabricate CdS thin films, among which chemical vapor deposition (CVD) [5], chemical bath deposition (CBD) [6], radio frequency (RF) magnetron sputtering [7] and vacuum evaporation method [8] can be mentioned. CBD is a low–cost technique with no vacuum requirements [9]. In this method, an aqueous solution containing reagent is required for thin film deposition. Therefore, many deposition parameters such as molar concentration, pH, temperature, time of deposition and stirring rate can be easily controlled. Moreover, thickness and particle size of deposited thin films can be controlled by adjusting these parameters [10]. Bulk
∗
CdS has two crystal structure phases namely α and β with wurtzite and zinc blende structures, respectively. The wurtzite structure is hexagonal with lattice constants of α = 4.1348 Å and β = 6.7749 Å. The zinc blende has face center cubic (f.c.c) structure with lattice constant of α = 5.818 Å [11]. To improve the light absorption coefficient of CdS structures, designing of various morphologies with enhanced crystal properties is necessary. Thus, CdS nanostructures with unique electrical and optical properties can be utilized in diverse optoelectronic applications [12]. Owing to their higher surface to volume ratio, nanostructures can enhance light absorption by entrapping the incident light [13]. In the recent reports, various morphologies such as hierarchical nanowires [14], nanotubes [15], nanosheets [16] and nanoflakes [13] of CdS were prepared by CBD method. For instance, CdS nanoflakes exhibited high optoelectronic sensitivity due to their high surface to volume ratio [13]. Although CdS nanostructures possess interesting properties, its deposition on various substrates is a challenge. To improve the epitaxy of CdS nanostructures, different seed layers can be applied between the substrate and top layer. Among the numerous seed
Corresponding author. E-mail address: [email protected] (N. Naderi).
https://doi.org/10.1016/j.ceramint.2019.11.234 Received 21 October 2019; Received in revised form 13 November 2019; Accepted 26 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mehdi Moghaddam, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.234
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layers, zinc oxide (ZnO) thin film can facilitate uniform and homogeneous CdS deposition as its lattice constant is similar to CdS [17,18]. Moreover, in CBD technique, existence of a seed layer will enhance the adhesion of deposited nanostructures to the substrate [19]. Thus, it can improve the optoelectrical properties of electronic devices based on these nanostructures. In this research, the effect of ZnO seed layer on improvement of the crystallinity, adherence and uniformity of CBD–deposited CdS nanostructures was addressed. The structural, morphology, optical and electrical properties of different CdS thin films (on glass substrates and ZnO seed layers) were compared. Moreover, the effect of reagents molar concentration on morphology, optical and electrical properties of CdS nanostructures was examined. In order to study the optoelectrical properties of CdS nanostructures, metal–semiconductor–metal (MSM) photodetectors were fabricated on top of different CdS nanostructures and their current–voltage curves were measured at darkness as well as under ultraviolet radiation.
Table 1 The deposition parameters of synthesized CdS nanostructures.
Cd+2 + 4NH3
NH3 + H2 O
NH4+2 + OH
Cd+2 + 2OH
Cd (OH ) 2
[HS ]+ + OH
S
[Cd (NH3)4]+2 + S
2
2
0.50 0.25 0.50 0.75
The structural properties of the thin films were studied using X–ray diffraction (XRD, Philips PW3710) with Cu Kα radiation (λ = 1.5405 Å) in 2θ range of 10°–70°. The surface morphology of the nanostructured thin films was observed using field emission scanning electron microscopy (FESEM, MIRA3 TESCAN). The energy dispersive X–ray analysis (EDX, TESCAN) was also used to identify the elements of the surface. The optical transmittance was measured using a UV–VIS spectrophotometer (Perkin–Elmer Lambda 25) in the wavelength range of 300–1000 nm while the glass substrate and ZnO/glass were used as the blanks for CdS/glass and CdS/ZnO samples, respectively. The room temperature photoluminescence (PL) was measured using fluorescence spectrophotometer (PL, Cary Eclipse Fluorescence Spectrophotometer) equipped with Xenon flash lamp with the excitation wavelength of 250 nm. 3. Results and discussion 3.1. X–ray diffraction analysis Fig. 1 shows the XRD patterns of CdS nanostructures on different substrates and with different molar concentrations. For CdS/G sample (Fig. 1a), the diffraction peak at 2θ = 26.58° is corresponding to (111) plane reflecting the cubic phase of CdS structures (JCPDS No.89–0440). In case of CdS/Z–0.25 sample (Fig. 1b), a broad and low intensity peak at 2θ = 26.73° can be attributed to (002) plane of hexagonal CdS structure (JCPDS No.80–0006). It indicates the effect of ZnO seed layer on cubic–to–hexagonal variation of the CdS structure. For CdS/Z–0.50 sample, the position of the dominant peak corresponding to (002) plane of the hexagonal phase was shifted to 2θ = 26.61° (Fig. 1c). Concerning the CdS/Z–0.75 (Fig. 1d), this shift in the position of CdS peak was moderated and the dominant peak was found at 2θ = 26.71°. These results showed an outstanding difference in the structural properties of CdS/Z–0.50 sample. The highest peak shift in this sample can be attributed to the higher structural stress which might be due to different morphology of this CdS nanostructure. The most intense CdS peak was shown for CdS/Z–0.50 sample reflecting the highest crystal quality of this nanostructure compared to the other samples. The inter–planar spacing (dhkl) and crystallite size (D) of the CdS nanostructures were estimated through Bragg and Scherrer equations,
+ H2 O
CdS + NH3
Bare glass ZnO/glass ZnO/glass ZnO/glass
2.3. Characterization and measurement techniques
[HS +] + H2 O + CH2 N2
CS (NH2) 2 + 2OH
CdS/G CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
In order to fabricate CdS–based MSM photodetectors, two back–to–back Schottky contacts of platinum were sputtered on top of all samples. To decrease the distance between two electrodes, an interdigitated finger–shaped pattern with the length of 2.8 mm and width of 50 μm and electrode distance of 120 μm was applied using a steel mask. The effective area of fabricated photodetectors was estimated to be 0.1 cm2. The devices were then exposed to illumination with different wavelengths and the current–voltage (I–V) characteristics were measured at darkness and under illumination using a computer–controlled source–meter (Keithley 2400, USA).
Prior to deposition, the glass substrates were ultrasonically cleaned in hydrochloric acid and deionized water (1:4) for 6 min. Then, they were rinsed in deionized water and ultrasonicated for 6 min in order to remove the surface contaminations. This step was followed by drying process at ambient temperature. Zinc oxide thin films were deposited on freshly cleaned glass substrates by RF magnetron sputtering technique using an ultrapure (99.99%) ZnO disk as the target. The substrate–target distance was fixed at 6 cm. Before deposition, the vacuum chamber was evacuated to the base pressure of 6✕10−5 mbar. During the deposition, the RF power was maintained at 135 W and the pressure of the ultrapure (99.999%) argon gas was fixed at 6.9✕10−3 mbar (working pressure). After 44 min, the obtained thickness of deposited ZnO layer was 200 nm. This ZnO thin film was then employed as the seed layer to facilitate the epitaxial growth of subsequent CdS nanostructures. In CBD deposition of CdS samples, the bath solution was included cadmium chloride [CdCl2 . H2 O] as Cd+2 source, thiourea [CS (NH2) 2 ] as S 2 source and ammonia as complexing agent. Firstly, solutions of the cadmium salt and thiourea were prepared with molar concentrations ranging from 0.25 to 0.75 M at room temperature by dissolving the reagents in deionized water. Then, ammonia solution was added to a reactor containing cadmium chloride solution to adjust the pH at 11. Using a magnet stirrer, the thiourea solution was mixed with an alkaline precursor at the temperature of 75 C°. After 60 min of deposition, it was observed that CdS thin films were well–adhered to the substrates. In order to remove the weakly adhered particles, all of the samples were ultrasonicated in deionized water for 2 min. The CdS thin films were synthesized from decomposition of thiourea in an alkaline solution containing a cadmium salt and ammonia by the possible chemical reactions as follow [20];
[Cd (NH3)4]+2
Reagents molar concentration (M)
2.2. Device fabrication
2.1. Synthesis of nanostructures
Cd (NH3)4 Cl2
Substrate
details of deposition parameters are summarized in Table 1.
2. Experimental details
CdCl2 + 4NH3
Sample
(1)
In order to study the effect of seed layer and molar concentration of reagents, CdS nanostructures were deposited on bare glass and ZnO/ glass substrates with molar concentrations of 0.25, 0.5 and 0.75 M. The 2
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Fig. 2. FESEM micrographs of CdS nanostructures grown at different molar concentrations: (a) CdS/glass at 0.5 M, (b) CdS/ZnO at 0.25 M, (c) CdS/ZnO at 0.5 M, (d) CdS/ZnO at 0.75 M. Fig. 1. The XRD patterns of CdS samples deposited on glass substrate and ZnO seed layer at different molar concentrations.
particles agglomeration and formation of CdS macro–spheres. The presence of ZnO seed layer on glass substrate plays a pivotal role in changing morphology of the deposited CdS (its transition from particle shape to thin film). As Fig. 2b suggests, the homogeneity of the deposited ZnO thin film enhanced the uniformity of CBD–deposited CdS nanostructures. Moreover, low lattice mismatch and high adhesion between CdS and ZnO structures prevented the agglomeration of CdS particles and encouraged the growth of CdS thin film on ZnO seed layer. A dense CdS thin film with many pinholes and cracks was formed on the surface of this structure which can be due to low concentration of Cd and S ions. The outstanding influence of molar concentration on the morphology of CdS nanostructures is illustrated in Fig. 2c. Accordingly, higher molar concentrations (0.5 M) resulted in formation of three–dimensional CdS nanosheets with hexagonal structures. In this sample, intersecting vertical nanosheets of CdS led to a hollow–shape structure. Due to the special morphology and higher specific surface area, this sample can be a good candidate for light capturing and reducing the reflection coefficient. Moreover, hexagonal morphology of nanosheets conformed the enhanced structural properties of this sample (Fig. 1c) compared to the other CdS samples. According to Fig. 2d, dense micro–sized CdS spherical particles were grown on ZnO seed layer upon increasing the molar concentration (0.75 M) which resulted in formation of CdS thin films. The morphology of this structure is comparable with the sample produced at molar concentration of 0.25 M (Fig. 2b). However, the grain size was increased when higher molar concentrations were employed in the CBD technique. Therefore, the cracks and pinholes in CdS/Z–0.25 were completely eliminated in CdS/Z–0.75 sample. Fig. 3 compares the cross–sectional FESEM micrographs for CdS/G and CdS/Z–0.50 samples. The molar concentration of both samples is 0.50 M and the only difference is in the presence of ZnO seed layer in CdS/Z–0.50. Here, the effect of seed layer on formation of a dense and uniform CdS structure can be clearly observed. Moreover, the CdS thin film texture is illustrated in Fig. 3b. The special morphology of this sample might enhance the exposed area of CdS nanostructures and
respectively [21,22]: (2)
n = 2dhkl sin D=
K cos
(3)
where n is the order of diffraction, λ shows the wavelength of the incident X–ray, θ denotes the diffraction angle and K and β are the Scherrer constant and the full width at half maximum (FWHM), respectively. The structural parameters of different CdS nanostructures are summarized in Table 2. 3.2. Surface morphology Fig. 2 illustrates the FESEM micrographs of the CdS nanostructures deposited on glass substrate and ZnO seed layer through CBD technique. Fig. 2a shows the morphology of CdS/G sample with the average grain size of 500 nm. Due to particles agglomeration and formation of CdS islands, the substrate surface was not completely covered. Here, the absence of a seed layer and low roughness of glass substrate led to CdS Table 2 The structural properties of the deposited CdS nanostructures based on glass and ZnO substrates with different chemical concentrations. Sample
Peak position (2θ)
d-spacing (Å)
(hkl)
FWHM (2θ)
Crystallite size (nm)
CdS/G CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
26.58 26.73 26.61 26.71
3.352 3.332 3.347 3.334
(111) (002) (002) (002)
0.46 0.35 0.25 0.34
17.7 22.7 32.6 24.0
3
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Fig. 3. The cross–sectional micrographs of CdS nanostructures deposited at molar concentration of 0.5 M on (a) glass substrate and (b) ZnO seed layer.
hence improve its light trapping behavior. 3.3. Energy dispersive X–ray analysis
Fig. 5. The transmission spectra of CdS thin films deposited with different molar concentrations.
Fig. 4 illustrates the elemental composition of the CdS nanostructures deposited on glass substrate and ZnO seed layer. The peaks of Cd and S can be observed in all the samples. The Zn and O peaks are related to ZnO seed layer. The rest of the peaks are related to glass substrate and the presence of impurities in different layers. A comparison between Cd and S peak intensities showed that the Cd/S ratio is more than unity which can be due to the sulfur vacancies in crystal structure of all samples. The sulfur vacancy in deposited CdS thin films acts as donor reflecting that the deposited CdS nanostructures are intrinsically n–type.
3.4. UV–visible analysis Fig. 5 shows the UV–visible transmission spectra of CdS nanostructures deposited on glass substrate and ZnO seed layer at the wavelength range of 300–1000 nm. The transmittance of all samples was lower than 30% in UV–Vis range. For CdS/Z–0.50 sample, the transmission spectrum was lower than the others which can be related to its high absorption coefficient. Such elevated light absorption is in good consistency with the special surface morphology of this sample. It implies that the high surface to volume ratio of this sample resulted in
Fig. 4. EDS spectra of CdS/glass at 0.5 M (a) and CdS/ZnO at 0.25 M (b), 0.5 M (c), and 0.75 M (d). 4
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Table 3 The details of PL band-edge emission peak for the different CdS samples. sample
FWHM (nm)
Peak position (nm)
Band gap (eV)
CdS/glass CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
24 23 21 25
531.5 532.5 534.5 533.5
2.333 2.328 2.319 2.324
sample (based on the XRD results). 3.5. Photoluminescence analysis Fig. 7 shows the room temperature PL emission spectra of CdS nanostructures deposited on glass substrate and ZnO seed layer under an excitation wavelength of 250 nm. The electron–hole pairs were generated upon radiation of excitation photons. Thus, the photogenerated electrons can be trapped into interstitials and vacancy sites which will result in the appearance of different peaks [26]. For all the CdS nanostructures, PL spectra exhibit a sharp peak at 535 nm with a lower one at 488 nm and two humps at 421 and 408 nm. As the band–emission energy is usually slightly lower than the band gap energy of CdS nanoparticles [27], the peak at 535 nm can be assigned to the band–edge emission which is attributed to radiative recombination via bound excitons. The elevated intensity of this peak for the sample with molar concentration of 0.5 M can be attributed to its high specific surface area resulting in higher absorption coefficient and light entrapment behavior. Thus, the rate of electron–hole generation was increased and the PL peak was enhanced in this sample [28]. The FWHM of PL peak for CdS/Z–0.50 sample was decreased due to an improvement in the optical quality and decrease of the structural defects [29,30]. These results are in line with the XRD pattern and show the superior crystal quality of CdS/Z–0.50 sample. The details of PL peaks are listed in Table 3. The peak located at 488 nm has been known as the green band emission [26]. This luminescence emission is assigned to interstitial band regarding sulfur vacancy originated as an electronic transition from donor level to valence band [31]. The peak located at 421 nm can be also regarded as the violet emission due to the quantum size effects [6]. For all the samples, the main defect band was observed in the green region which can be attributed to interstitial sulfur sites [32]. The interstitial bands related to impurities play an important role in the luminescence properties of CdS nanostructures due to presence of electron acceptor state levels (Cd vacancy) and/or donor state levels (sulfur vacancy) within the band gap [33–35].
Fig. 6. Tauc plot curves for estimation of optical band gap related to CdS samples based on glass and ZnO seed layer with different molar concentrations.
light capturing at intersecting vertical nanosheets of this CdS structure. The transmission spectra was used to estimate the optical band gap (Eg) of CdS nanostructures using Tauc plot equation [23] as follows;
( h )=
(h
Eg )n
(4)
where, represents the absorption coefficient, hυ denotes the energy of incident photon, A is a constant and n = 1/2 for the direct band gap semiconductors. The Eg values of the samples were estimated from the plots of (αhυ)2 versus hυ by extrapolating of the tangent line (dotted line) as shown in Fig. 6. The band gap of CdS/G sample is ~2 eV, which is lower than the one for bulk CdS (2.42 eV). Such a decrease in optical band gap can be assigned to the presence of impurities in CdS lattice leading to formation of interbands and electronic transitions with lower energies [24]. For CdS/ZnO samples with molar concentrations of 0.25 M and 0.75 M, the band gap values were estimated at ~2.81 and ~2.69 eV, respectively. The widening of optical band gap in these nanostructures comparing to the bulk material can be related to quantum confinement effect regarding the small crystallite size of CdS samples [25]. For CdS/Z–0.50, the band gap value was decreased to 2.62 eV which can be attributed to the enlargement of crystallite size in this
3.6. Electrical measurements Fig. 8 illustrates the current–voltage (I–V) curves for the photodetectors fabricated based on CdS/glass and CdS/ZnO samples with different molar concentrations at darkness and under UV illumination with the wavelength of 365 nm and incident power of 12 μW. The highest photocurrent under UV illumination was recorded for the CdS/ Z–0.50 sample (Fig. 8a). The special morphology of this CdS nanostructure led to its high specific surface area and enhanced surface absorption coefficient. Therefore, the device based on this sample can capture a high ratio of incident light from UV source and generate more electron–hole pairs leading to elevated photogenerated current. Besides, improved crystal quality and the fewer number of grain boundaries in this sample decreased the rate of electron–hole recombination at the boundaries and enhanced the photocurrent under UV illumination [26]. Moreover, the dark current of CdS/Z–0.50 sample was the lowest compare to the other samples which can be due to the high crystal quality of CdS nanosheets [13] reflecting the improved signal–to–noise factor of this device. Fig. 8b indicates higher photocurrent
Fig. 7. The photoluminescence spectra of CdS/glass substrate and CdS/ZnO seed layer with different molar concentrations. 5
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Fig. 8. The current density-voltage curves of the fabricated ultraviolet detectors based on different CdS nanostructures.
for the UV detector based on CdS/Z–0.75 compared to CdS/Z–0.25 which can be assigned to the compact morphology of this sample as shown in Fig. 2d. Photosensitivity (S) is an important factor in determining the quality of photodetector and is defined as the ratio of photocurrent to dark current using the following equation [36]:
S=
IPh
Id
constant. Therefore, the theoretical value of A** for CdS lattice will be 23 A cm−2 K−2 [36]. The Schottky barrier heights and the ideality factors for different UV detectors were calculated as summarized in Table 4. The decrease in barrier height under UV illumination led to an increase in photocurrent. The highest difference in barrier height values (in the dark and under UV light) was calculated for the CdS/Z–0.50 sample indicating the highest sensitivity of this nanostructure compared to the other samples. Quantum efficiency (QE) is another parameter used to estimate the performance of a photodetector. It is defined as the number of carrier collected to produce the photocurrent (IPh) per the number of incident photons [37]:
(5)
Id
where, IPh and Id represent the photocurrent and the dark current of photodetectors, respectively. The calculated values of sensitivity which are summarized in Table 4 showed an improvement in the optoelectrical properties of CdS/Z–0.50 sample. The sensitivity of this sample was tens of times higher than the other devices. The quasi–nonlinear behavior of the I–V curves indicates the formation of Schottky barriers at the Pt/CdS interface. The Schottky barrier height for the fabricated photodetectors can be calculated from I–V measurements using the thermionic emission theory according to the following equations [37];
I = I (exp(qV / nKT )
=
where V is the applied voltage, n shows the ideality factor, K denotes the Boltzmann constant, q is the quantum of electric charge, T represents the absolute temperature, and Iₒ stands for the saturation current given by following equation;
I = AA T 2 exp( q
(7)
B / KT )
where B is the Schottky barrier height, A denotes the junction area and A** is the modified Richardson constant obtainable by the following equation;
A
R=
(8)
= 4 qm K 2/ h3
Table 4 The calculated Schottky barrier height, ideality factor and quantum efficiency for fabricated photodetectors. Sensitivity (S)
Ideal factor (n)
ϕB1(eV) under UV
ϕB2(eV) at darkness
Δ(ϕB2-ϕB1)
η (%)
CdS/glass CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
20.2 6.8 192.1 2.7
14.0 13.2 13.4 17.6
0.6612 0.6016 0.6166 0.7582
0.8366 0.7626 0.3258 0.7577
0.1754 0.1610 0.3358 0.0005
33.90 3.05 212.89 8.81
(9)
IPh Pinc
(10)
This parameter was recorded under different wavelengths of incident light at a fixed voltage of 15 V. Fig. 9 shows the calculated responsivity versus the wavelength of incident light. The increased responsivity (0.62 A/W) of photodetector based on CdS/Z–0.50 nanostructure is in good agreement with the enhanced photosensitivity of this device which can be attributed to its special morphology and reduced crystal defect. Furthermore, the sharp cut–off at the wavelength of 471 nm (2.63 eV) is in line with the optical band gap of CdS nanostructure derived from optical transmission of this sample (2.62 eV). The responsivity of the devices was negligible in infrared range. However, it was increased in the visible range due to the transition of carriers from the defect states. This phenomenon was also observed in the PL peaks related to defect states and is responsible for generation of low photocurrent in this range. For all the devices, the
where m* shows the effective mass of electron and h is the planck's
sample
h Pinc
where Pinc is the optical power on incident light. The values of QE for different UV detectors are presented in Table 4. The highest QE was observed in the photodetector fabricated based on CdS/Z–0.50 which can be due to the extra photogenerated electrons in this device [38]. For this photodetector, the quantum efficiency is even higher than unity. If the energy of an incident photon is sufficiently greater than that of the semiconductor optical band gap, the quantum efficiency can be higher than 100%. In this case, the excited electron has sufficient kinetic energy for one or more subsequent impact which leads to the higher photogenerated electrons and increases the quantum efficiency to be higher than unity [39]. The responsivity of photodetectors can be calculated using the following equation [36]:
(6)
1)
IPh q
6
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Table 5 A comparison between speed of ultraviolet detection for different CdS photodetectors. sample
Rise time (ms)
Recovery time (ms)
CdS/glass CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
290 270 250 320
290 270 310 300
special morphology and high specific surface area. 4. Conclusion In summary, a three–dimensional nanosheet morphology of CdS thin film was deposited on ZnO seed layer using CBD technique upon application of the optimized reagent molar concentration (0.5 M). The XRD patterns indicated the high crystallinity of CdS nanosheets with a strong orientation at (002) plane of the CdS hexagonal phase. This sample illustrated the low transmittance spectrum which can be attributed to the high absorption coefficient of CdS nanosheets and light trapping behavior of this sample. The optical band gap of this sample was calculated through transmission spectrum (2.62 eV). Its PL spectrum also showed an elevated peak at 535 nm with the lowest FWHM in comparison with the other samples implying the enhanced optical quality of CdS nanosheets. This peak can be assigned to the band–edge emission related to the radiative recombination through bound excitons. In order to study the optoelectrical properties of CdS nanostructures, MSM photodetectors were fabricated based on different samples using deposition of Pt interdigitated finger–shaped patterns. The device based on CdS–nanosheet/ZnO indicated the highest responsivity (0.62 A/W) and photosensitivity (192) to UV illumination at voltage of 15 V which can be due to the special morphology and high specific surface area of this sample. In this device, the short rise time (250 m s) and recovery time (310 m s) under pulsed UV illumination indicated the high–speed performance of the optimized sample.
Fig. 9. The optical responsivity of different CdS photodetectors as a function of the incident light wavelength.
maximum responsivity was observed in the UV region with the energy above the threshold excitation energy of CdS nanostructures implying high sensitivity of fabricated devices in this range. For instance, the elevated responsivity of the photodetector based on CdS/Z–0.50 sample indicates its proper wavelength selectivity behavior. Fig. 10 illustrates the consecutive current spectra of different photodetectors in response to the pulsed UV light. The devices were exposed to illumination for a fixed duration and allowed to stabilize. For all devices, the photocurrent was increased to a saturation value upon illumination followed by a decrease when the light was switched off. The rise time is defined as the time taken for the sensor to reach to 90% of its saturation current; while the recovery time refers to the time taken for the sensor to decrease its current from saturation to 10% of its saturation current value. The photocurrents for all the fabricated photodetectors were highly stable and reproducible under pulsed UV illumination. The rise time and recovery time of fabricated photodetectors are presented in Table 5. The CdS/Z–0.50 sample exhibited the shortest rise time and recovery time reflecting the high–speed performance of this MSM photodetector. Such high performance can be assigned to the enhanced light absorption coefficient of CdS nanosheets due to their
Declaration of competing interestCOI 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.
Fig. 10. The consecutive current spectra of fabricated photodetectors under pulsed UV illumination. 7
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Acknowledgements
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This research was supported by Iran National Science Foundation: INSF [Grant No.: 96011972] and Materials and Energy Research Center, Karaj, Iran [Grant number: 99392008]. References [1] K. Sun, B. Foley, P. Norris, T. Globus, Z. Sun, H. Ryan, A. Slonopas, Growth mechanisms and their effects on the opto-electrical properties of CdS thin films prepared by chemical bath deposition, Mater. Sci. Semicond. Process. 52 (2016) 24–31, https://doi.org/10.1016/j.mssp.2016.05.011. [2] G. Sasikala, P. Thilakan, C. Subramanian, Modification in the chemical bath deposition apparatus, growth and characterization of CdS semiconducting thin films for photovoltaic applications, Sol. Energy Mater. Sol. Cells 62 (2000) 275–293, https://doi.org/10.1016/S0927-0248(99)00170-1. [3] S.U. Shaikh, D.J. Desale, F.Y. Siddiqui, A. Ghosh, R.B. Birajadar, A.V. Ghule, R. Sharma, Effects of air annealing on CdS quantum dots thin film grown at room temperature by CBD technique intended for photosensor applications, Mater. Res. Bull. 47 (2012) 3440–3444, https://doi.org/10.1016/j.materresbull.2012.07.009. [4] Q. Hao, J. Xu, X. Zhuang, Q. Zhang, Q. Wan, H. Pan, X. Zhu, A. Pan, Template-free synthesis and photocatalytic activity of CdS nanorings, Mater. Lett. 100 (2013) 141–144, https://doi.org/10.1016/j.matlet.2013.02.091. [5] W. Zhao, L. Liu, M. Xu, X. Wang, T. Zhang, Y. Wang, Z. Zhang, S. Qin, Z. Liu, Single CdS nanorod for high responsivity UV–visible photodetector, Adv. Opt. Mater. 5 (2017) 1–7, https://doi.org/10.1002/adom.201700159. [6] Z. Makhdoumi-Kakhaki, A. Youzbashi, P. Sangpour, N. Naderi, A. Kazemzadeh, Effects of film thickness and stoichiometric on the electrical, optical and photodetector properties of CdS quantum dots thin films deposited by chemically bath deposition method at different bath temperature, J. Mater. Sci. Mater. Electron. 27 (2016) 12931–12939, https://doi.org/10.1007/s10854-016-5430-4. [7] Y. Li, P.F. Ji, Y.L. Song, F.Q. Zhou, S.Q. Yuan, N. Wen, H.C. Huang, Fabrication and electrical properties of (0 0 2)-oriented grown CdS/Si heterojunctions by radio frequency magnetron sputtering, Mater. Lett. 228 (2018) 463–465, https://doi.org/ 10.1016/j.matlet.2018.06.096. [8] M. Tomakin, M. Altunbaş, E. Bacaksiz, Ş. Çelik, Current transport mechanism in CdS thin films prepared by vacuum evaporation method at substrate temperatures below room temperature, Thin Solid Films 520 (2012) 2532–2536, https://doi.org/ 10.1016/j.tsf.2011.10.160. [9] M. Taherkhani, N. Naderi, P. Fallahazad, M.J. Eshraghi, A. Kolahi, Development and optical properties of ZnO nanoflowers on porous silicon for photovoltaic applications, J. Electron. Mater. 48 (2019) 6647–6653, https://doi.org/10.1007/ s11664-019-07484-0. [10] A. Djelloul, M. Adnane, Y. Larbah, M. Zerdali, C. Zegadi, A. Messaoud, Effect of annealing on the properties of nanocrystalline CdS thin films prepared by CBD method, J. Nano Electron. Phys. 8 (2016) 1–7, https://doi.org/10.21272/jnep.8(2). 02005. [11] F. Boakye, D. Nusenu, The energy band gap of cadmium sulphide, Solid State Commun. 102 (1997) 323–326, https://doi.org/10.1016/S0038-1098(97)00012-4. [12] G. Li, Y. Jiang, Y. Zhang, X. Lan, T. Zhai, G.C. Yi, High-performance photodetectors and enhanced field-emission of CdS nanowire arrays on CdSe single-crystalline sheets, J. Mater. Chem. C. 2 (2014) 8252–8258, https://doi.org/10.1039/ c4tc01503g. [13] J. Li, Y. Zhu, M. Li, H. Cai, H. Ding, N. Pan, X. Wang, One-step fabrication of CdS nanoflake arrays and its application for photodetector, Optik (Stuttg) 169 (2018) 190–195, https://doi.org/10.1016/j.ijleo.2018.05.091. [14] L. Li, Z. Lou, G. Shen, Hierarchical CdS nanowires based rigid and flexible photodetectors with ultrahigh sensitivity, ACS Appl. Mater. Interfaces 7 (2015) 23507–23514, https://doi.org/10.1021/acsami.5b06070. [15] Q. An, X. Meng, G. Liu, L. Hong, Annealing of the superlong CdS nanotubes for enhanced performance in fully nanostructured photodetector, Mater. Lett. 161 (2015) 751–754, https://doi.org/10.1016/j.matlet.2015.09.083. [16] M.A. Mahdi, J.J. Hassan, S.S. Ng, Z. Hassan, N.M. Ahmed, Synthesis and characterization of single-crystal CdS nanosheet for high-speed photodetection, Phys. E Low-dimens. Syst. Nanostruct. 44 (2012) 1716–1721, https://doi.org/10.1016/j. physe.2012.05.003. [17] M.A. Mahdi, A. Ramizy, Z. Hassan, S.S. Ng, J.J. Hassan, S.J. Kasim, CdS nanocrystalline structured grown on porous silicon substrates via chemical bath deposition method, Chalcogenide Lett. 9 (2012) 19–25. [18] S. Velanganni, S. Pravinraj, P. Immanuel, R. Thiruneelakandan, Nanostructure CdS/ ZnO heterojunction configuration for photocatalytic degradation of Methylene blue, Phys. B Condens. Matter 534 (2018) 56–62, https://doi.org/10.1016/j.physb.2018.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Improved optical and structural properties of cadmium sulfide nanostructures for optoelectronic applications Mehdi Moghaddam, Nima Naderi∗, Mojtaba Hosseinifard, Asghar Kazemzadeh Materials and Energy Research Center, Karaj, Iran
ARTICLE INFO
ABSTRACT
Keywords: Sensors Optical properties Electrical properties Spectroscopy
In this study, the effect of ZnO seed layer on the growth of uniform CdS nanostructures was investigated using chemical bath deposition technique. Besides, the influence of molar concentration of reagents on the surface morphology, structural and optoelectrical properties of the deposited CdS thin films were examined. The CdS nanostructures were grown on bare glass and ZnO/glass substrates with different reagent molar concentrations. The results indicated an improvement in the homogeneity and uniformity of the grown CdS nanostructures on ZnO seed layer which can be due to the low lattice mismatch between ZnO and CdS structures. The CdS/ZnO samples were optimized by changing the molar concentration of reagents. A three–dimensional intersecting vertical nanosheet morphology with hexagonal structure was obtained when modified chemical concentration of 0.5 M was applied. The XRD pattern of CdS nanosheets indicated the hexagonal phase of CdS which were strongly orientated along (002) plane. The elevated intensity of dominant peak related to this sample confirmed the improved crystal quality of this CdS nanostructure comparing to the other samples. The UV–Vis spectrum demonstrated a high absorption coefficient for CdS intersecting nanosheets which might be due to the high specific surface area and light trapping behavior of this sample. The photoluminescence study also showed an improvement in optical properties of optimized CdS nanostructures. In order to study the optoelectrical properties of CdS nanostructures, metal–semiconductor–metal photodetectors were fabricated with different CdS samples and their current–voltage characteristics were analyzed. The results indicated an enhancement in photosensitivity, responsivity, and speed of photodetectors based on optimized CdS nanostructures.
1. Introduction Cadmium sulfide (CdS) is one of the most important II–VI semiconductors with a direct band gap (~2.42 eV for bulk material); its bulk and thin films are capable of absorbing the visible spectrum of light [1]. Due to its outstanding electrical and optical properties such as electron affinity, high absorption coefficient and low resistivity, CdS has shown significant potential in developing solar cells [2], photodetectors [3] and photocatalysts [4]. Several methods have been used to fabricate CdS thin films, among which chemical vapor deposition (CVD) [5], chemical bath deposition (CBD) [6], radio frequency (RF) magnetron sputtering [7] and vacuum evaporation method [8] can be mentioned. CBD is a low–cost technique with no vacuum requirements [9]. In this method, an aqueous solution containing reagent is required for thin film deposition. Therefore, many deposition parameters such as molar concentration, pH, temperature, time of deposition and stirring rate can be easily controlled. Moreover, thickness and particle size of deposited thin films can be controlled by adjusting these parameters [10]. Bulk
∗
CdS has two crystal structure phases namely α and β with wurtzite and zinc blende structures, respectively. The wurtzite structure is hexagonal with lattice constants of α = 4.1348 Å and β = 6.7749 Å. The zinc blende has face center cubic (f.c.c) structure with lattice constant of α = 5.818 Å [11]. To improve the light absorption coefficient of CdS structures, designing of various morphologies with enhanced crystal properties is necessary. Thus, CdS nanostructures with unique electrical and optical properties can be utilized in diverse optoelectronic applications [12]. Owing to their higher surface to volume ratio, nanostructures can enhance light absorption by entrapping the incident light [13]. In the recent reports, various morphologies such as hierarchical nanowires [14], nanotubes [15], nanosheets [16] and nanoflakes [13] of CdS were prepared by CBD method. For instance, CdS nanoflakes exhibited high optoelectronic sensitivity due to their high surface to volume ratio [13]. Although CdS nanostructures possess interesting properties, its deposition on various substrates is a challenge. To improve the epitaxy of CdS nanostructures, different seed layers can be applied between the substrate and top layer. Among the numerous seed
Corresponding author. E-mail address: [email protected] (N. Naderi).
https://doi.org/10.1016/j.ceramint.2019.11.234 Received 21 October 2019; Received in revised form 13 November 2019; Accepted 26 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mehdi Moghaddam, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.234
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layers, zinc oxide (ZnO) thin film can facilitate uniform and homogeneous CdS deposition as its lattice constant is similar to CdS [17,18]. Moreover, in CBD technique, existence of a seed layer will enhance the adhesion of deposited nanostructures to the substrate [19]. Thus, it can improve the optoelectrical properties of electronic devices based on these nanostructures. In this research, the effect of ZnO seed layer on improvement of the crystallinity, adherence and uniformity of CBD–deposited CdS nanostructures was addressed. The structural, morphology, optical and electrical properties of different CdS thin films (on glass substrates and ZnO seed layers) were compared. Moreover, the effect of reagents molar concentration on morphology, optical and electrical properties of CdS nanostructures was examined. In order to study the optoelectrical properties of CdS nanostructures, metal–semiconductor–metal (MSM) photodetectors were fabricated on top of different CdS nanostructures and their current–voltage curves were measured at darkness as well as under ultraviolet radiation.
Table 1 The deposition parameters of synthesized CdS nanostructures.
Cd+2 + 4NH3
NH3 + H2 O
NH4+2 + OH
Cd+2 + 2OH
Cd (OH ) 2
[HS ]+ + OH
S
[Cd (NH3)4]+2 + S
2
2
0.50 0.25 0.50 0.75
The structural properties of the thin films were studied using X–ray diffraction (XRD, Philips PW3710) with Cu Kα radiation (λ = 1.5405 Å) in 2θ range of 10°–70°. The surface morphology of the nanostructured thin films was observed using field emission scanning electron microscopy (FESEM, MIRA3 TESCAN). The energy dispersive X–ray analysis (EDX, TESCAN) was also used to identify the elements of the surface. The optical transmittance was measured using a UV–VIS spectrophotometer (Perkin–Elmer Lambda 25) in the wavelength range of 300–1000 nm while the glass substrate and ZnO/glass were used as the blanks for CdS/glass and CdS/ZnO samples, respectively. The room temperature photoluminescence (PL) was measured using fluorescence spectrophotometer (PL, Cary Eclipse Fluorescence Spectrophotometer) equipped with Xenon flash lamp with the excitation wavelength of 250 nm. 3. Results and discussion 3.1. X–ray diffraction analysis Fig. 1 shows the XRD patterns of CdS nanostructures on different substrates and with different molar concentrations. For CdS/G sample (Fig. 1a), the diffraction peak at 2θ = 26.58° is corresponding to (111) plane reflecting the cubic phase of CdS structures (JCPDS No.89–0440). In case of CdS/Z–0.25 sample (Fig. 1b), a broad and low intensity peak at 2θ = 26.73° can be attributed to (002) plane of hexagonal CdS structure (JCPDS No.80–0006). It indicates the effect of ZnO seed layer on cubic–to–hexagonal variation of the CdS structure. For CdS/Z–0.50 sample, the position of the dominant peak corresponding to (002) plane of the hexagonal phase was shifted to 2θ = 26.61° (Fig. 1c). Concerning the CdS/Z–0.75 (Fig. 1d), this shift in the position of CdS peak was moderated and the dominant peak was found at 2θ = 26.71°. These results showed an outstanding difference in the structural properties of CdS/Z–0.50 sample. The highest peak shift in this sample can be attributed to the higher structural stress which might be due to different morphology of this CdS nanostructure. The most intense CdS peak was shown for CdS/Z–0.50 sample reflecting the highest crystal quality of this nanostructure compared to the other samples. The inter–planar spacing (dhkl) and crystallite size (D) of the CdS nanostructures were estimated through Bragg and Scherrer equations,
+ H2 O
CdS + NH3
Bare glass ZnO/glass ZnO/glass ZnO/glass
2.3. Characterization and measurement techniques
[HS +] + H2 O + CH2 N2
CS (NH2) 2 + 2OH
CdS/G CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
In order to fabricate CdS–based MSM photodetectors, two back–to–back Schottky contacts of platinum were sputtered on top of all samples. To decrease the distance between two electrodes, an interdigitated finger–shaped pattern with the length of 2.8 mm and width of 50 μm and electrode distance of 120 μm was applied using a steel mask. The effective area of fabricated photodetectors was estimated to be 0.1 cm2. The devices were then exposed to illumination with different wavelengths and the current–voltage (I–V) characteristics were measured at darkness and under illumination using a computer–controlled source–meter (Keithley 2400, USA).
Prior to deposition, the glass substrates were ultrasonically cleaned in hydrochloric acid and deionized water (1:4) for 6 min. Then, they were rinsed in deionized water and ultrasonicated for 6 min in order to remove the surface contaminations. This step was followed by drying process at ambient temperature. Zinc oxide thin films were deposited on freshly cleaned glass substrates by RF magnetron sputtering technique using an ultrapure (99.99%) ZnO disk as the target. The substrate–target distance was fixed at 6 cm. Before deposition, the vacuum chamber was evacuated to the base pressure of 6✕10−5 mbar. During the deposition, the RF power was maintained at 135 W and the pressure of the ultrapure (99.999%) argon gas was fixed at 6.9✕10−3 mbar (working pressure). After 44 min, the obtained thickness of deposited ZnO layer was 200 nm. This ZnO thin film was then employed as the seed layer to facilitate the epitaxial growth of subsequent CdS nanostructures. In CBD deposition of CdS samples, the bath solution was included cadmium chloride [CdCl2 . H2 O] as Cd+2 source, thiourea [CS (NH2) 2 ] as S 2 source and ammonia as complexing agent. Firstly, solutions of the cadmium salt and thiourea were prepared with molar concentrations ranging from 0.25 to 0.75 M at room temperature by dissolving the reagents in deionized water. Then, ammonia solution was added to a reactor containing cadmium chloride solution to adjust the pH at 11. Using a magnet stirrer, the thiourea solution was mixed with an alkaline precursor at the temperature of 75 C°. After 60 min of deposition, it was observed that CdS thin films were well–adhered to the substrates. In order to remove the weakly adhered particles, all of the samples were ultrasonicated in deionized water for 2 min. The CdS thin films were synthesized from decomposition of thiourea in an alkaline solution containing a cadmium salt and ammonia by the possible chemical reactions as follow [20];
[Cd (NH3)4]+2
Reagents molar concentration (M)
2.2. Device fabrication
2.1. Synthesis of nanostructures
Cd (NH3)4 Cl2
Substrate
details of deposition parameters are summarized in Table 1.
2. Experimental details
CdCl2 + 4NH3
Sample
(1)
In order to study the effect of seed layer and molar concentration of reagents, CdS nanostructures were deposited on bare glass and ZnO/ glass substrates with molar concentrations of 0.25, 0.5 and 0.75 M. The 2
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Fig. 2. FESEM micrographs of CdS nanostructures grown at different molar concentrations: (a) CdS/glass at 0.5 M, (b) CdS/ZnO at 0.25 M, (c) CdS/ZnO at 0.5 M, (d) CdS/ZnO at 0.75 M. Fig. 1. The XRD patterns of CdS samples deposited on glass substrate and ZnO seed layer at different molar concentrations.
particles agglomeration and formation of CdS macro–spheres. The presence of ZnO seed layer on glass substrate plays a pivotal role in changing morphology of the deposited CdS (its transition from particle shape to thin film). As Fig. 2b suggests, the homogeneity of the deposited ZnO thin film enhanced the uniformity of CBD–deposited CdS nanostructures. Moreover, low lattice mismatch and high adhesion between CdS and ZnO structures prevented the agglomeration of CdS particles and encouraged the growth of CdS thin film on ZnO seed layer. A dense CdS thin film with many pinholes and cracks was formed on the surface of this structure which can be due to low concentration of Cd and S ions. The outstanding influence of molar concentration on the morphology of CdS nanostructures is illustrated in Fig. 2c. Accordingly, higher molar concentrations (0.5 M) resulted in formation of three–dimensional CdS nanosheets with hexagonal structures. In this sample, intersecting vertical nanosheets of CdS led to a hollow–shape structure. Due to the special morphology and higher specific surface area, this sample can be a good candidate for light capturing and reducing the reflection coefficient. Moreover, hexagonal morphology of nanosheets conformed the enhanced structural properties of this sample (Fig. 1c) compared to the other CdS samples. According to Fig. 2d, dense micro–sized CdS spherical particles were grown on ZnO seed layer upon increasing the molar concentration (0.75 M) which resulted in formation of CdS thin films. The morphology of this structure is comparable with the sample produced at molar concentration of 0.25 M (Fig. 2b). However, the grain size was increased when higher molar concentrations were employed in the CBD technique. Therefore, the cracks and pinholes in CdS/Z–0.25 were completely eliminated in CdS/Z–0.75 sample. Fig. 3 compares the cross–sectional FESEM micrographs for CdS/G and CdS/Z–0.50 samples. The molar concentration of both samples is 0.50 M and the only difference is in the presence of ZnO seed layer in CdS/Z–0.50. Here, the effect of seed layer on formation of a dense and uniform CdS structure can be clearly observed. Moreover, the CdS thin film texture is illustrated in Fig. 3b. The special morphology of this sample might enhance the exposed area of CdS nanostructures and
respectively [21,22]: (2)
n = 2dhkl sin D=
K cos
(3)
where n is the order of diffraction, λ shows the wavelength of the incident X–ray, θ denotes the diffraction angle and K and β are the Scherrer constant and the full width at half maximum (FWHM), respectively. The structural parameters of different CdS nanostructures are summarized in Table 2. 3.2. Surface morphology Fig. 2 illustrates the FESEM micrographs of the CdS nanostructures deposited on glass substrate and ZnO seed layer through CBD technique. Fig. 2a shows the morphology of CdS/G sample with the average grain size of 500 nm. Due to particles agglomeration and formation of CdS islands, the substrate surface was not completely covered. Here, the absence of a seed layer and low roughness of glass substrate led to CdS Table 2 The structural properties of the deposited CdS nanostructures based on glass and ZnO substrates with different chemical concentrations. Sample
Peak position (2θ)
d-spacing (Å)
(hkl)
FWHM (2θ)
Crystallite size (nm)
CdS/G CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
26.58 26.73 26.61 26.71
3.352 3.332 3.347 3.334
(111) (002) (002) (002)
0.46 0.35 0.25 0.34
17.7 22.7 32.6 24.0
3
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Fig. 3. The cross–sectional micrographs of CdS nanostructures deposited at molar concentration of 0.5 M on (a) glass substrate and (b) ZnO seed layer.
hence improve its light trapping behavior. 3.3. Energy dispersive X–ray analysis
Fig. 5. The transmission spectra of CdS thin films deposited with different molar concentrations.
Fig. 4 illustrates the elemental composition of the CdS nanostructures deposited on glass substrate and ZnO seed layer. The peaks of Cd and S can be observed in all the samples. The Zn and O peaks are related to ZnO seed layer. The rest of the peaks are related to glass substrate and the presence of impurities in different layers. A comparison between Cd and S peak intensities showed that the Cd/S ratio is more than unity which can be due to the sulfur vacancies in crystal structure of all samples. The sulfur vacancy in deposited CdS thin films acts as donor reflecting that the deposited CdS nanostructures are intrinsically n–type.
3.4. UV–visible analysis Fig. 5 shows the UV–visible transmission spectra of CdS nanostructures deposited on glass substrate and ZnO seed layer at the wavelength range of 300–1000 nm. The transmittance of all samples was lower than 30% in UV–Vis range. For CdS/Z–0.50 sample, the transmission spectrum was lower than the others which can be related to its high absorption coefficient. Such elevated light absorption is in good consistency with the special surface morphology of this sample. It implies that the high surface to volume ratio of this sample resulted in
Fig. 4. EDS spectra of CdS/glass at 0.5 M (a) and CdS/ZnO at 0.25 M (b), 0.5 M (c), and 0.75 M (d). 4
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Table 3 The details of PL band-edge emission peak for the different CdS samples. sample
FWHM (nm)
Peak position (nm)
Band gap (eV)
CdS/glass CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
24 23 21 25
531.5 532.5 534.5 533.5
2.333 2.328 2.319 2.324
sample (based on the XRD results). 3.5. Photoluminescence analysis Fig. 7 shows the room temperature PL emission spectra of CdS nanostructures deposited on glass substrate and ZnO seed layer under an excitation wavelength of 250 nm. The electron–hole pairs were generated upon radiation of excitation photons. Thus, the photogenerated electrons can be trapped into interstitials and vacancy sites which will result in the appearance of different peaks [26]. For all the CdS nanostructures, PL spectra exhibit a sharp peak at 535 nm with a lower one at 488 nm and two humps at 421 and 408 nm. As the band–emission energy is usually slightly lower than the band gap energy of CdS nanoparticles [27], the peak at 535 nm can be assigned to the band–edge emission which is attributed to radiative recombination via bound excitons. The elevated intensity of this peak for the sample with molar concentration of 0.5 M can be attributed to its high specific surface area resulting in higher absorption coefficient and light entrapment behavior. Thus, the rate of electron–hole generation was increased and the PL peak was enhanced in this sample [28]. The FWHM of PL peak for CdS/Z–0.50 sample was decreased due to an improvement in the optical quality and decrease of the structural defects [29,30]. These results are in line with the XRD pattern and show the superior crystal quality of CdS/Z–0.50 sample. The details of PL peaks are listed in Table 3. The peak located at 488 nm has been known as the green band emission [26]. This luminescence emission is assigned to interstitial band regarding sulfur vacancy originated as an electronic transition from donor level to valence band [31]. The peak located at 421 nm can be also regarded as the violet emission due to the quantum size effects [6]. For all the samples, the main defect band was observed in the green region which can be attributed to interstitial sulfur sites [32]. The interstitial bands related to impurities play an important role in the luminescence properties of CdS nanostructures due to presence of electron acceptor state levels (Cd vacancy) and/or donor state levels (sulfur vacancy) within the band gap [33–35].
Fig. 6. Tauc plot curves for estimation of optical band gap related to CdS samples based on glass and ZnO seed layer with different molar concentrations.
light capturing at intersecting vertical nanosheets of this CdS structure. The transmission spectra was used to estimate the optical band gap (Eg) of CdS nanostructures using Tauc plot equation [23] as follows;
( h )=
(h
Eg )n
(4)
where, represents the absorption coefficient, hυ denotes the energy of incident photon, A is a constant and n = 1/2 for the direct band gap semiconductors. The Eg values of the samples were estimated from the plots of (αhυ)2 versus hυ by extrapolating of the tangent line (dotted line) as shown in Fig. 6. The band gap of CdS/G sample is ~2 eV, which is lower than the one for bulk CdS (2.42 eV). Such a decrease in optical band gap can be assigned to the presence of impurities in CdS lattice leading to formation of interbands and electronic transitions with lower energies [24]. For CdS/ZnO samples with molar concentrations of 0.25 M and 0.75 M, the band gap values were estimated at ~2.81 and ~2.69 eV, respectively. The widening of optical band gap in these nanostructures comparing to the bulk material can be related to quantum confinement effect regarding the small crystallite size of CdS samples [25]. For CdS/Z–0.50, the band gap value was decreased to 2.62 eV which can be attributed to the enlargement of crystallite size in this
3.6. Electrical measurements Fig. 8 illustrates the current–voltage (I–V) curves for the photodetectors fabricated based on CdS/glass and CdS/ZnO samples with different molar concentrations at darkness and under UV illumination with the wavelength of 365 nm and incident power of 12 μW. The highest photocurrent under UV illumination was recorded for the CdS/ Z–0.50 sample (Fig. 8a). The special morphology of this CdS nanostructure led to its high specific surface area and enhanced surface absorption coefficient. Therefore, the device based on this sample can capture a high ratio of incident light from UV source and generate more electron–hole pairs leading to elevated photogenerated current. Besides, improved crystal quality and the fewer number of grain boundaries in this sample decreased the rate of electron–hole recombination at the boundaries and enhanced the photocurrent under UV illumination [26]. Moreover, the dark current of CdS/Z–0.50 sample was the lowest compare to the other samples which can be due to the high crystal quality of CdS nanosheets [13] reflecting the improved signal–to–noise factor of this device. Fig. 8b indicates higher photocurrent
Fig. 7. The photoluminescence spectra of CdS/glass substrate and CdS/ZnO seed layer with different molar concentrations. 5
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Fig. 8. The current density-voltage curves of the fabricated ultraviolet detectors based on different CdS nanostructures.
for the UV detector based on CdS/Z–0.75 compared to CdS/Z–0.25 which can be assigned to the compact morphology of this sample as shown in Fig. 2d. Photosensitivity (S) is an important factor in determining the quality of photodetector and is defined as the ratio of photocurrent to dark current using the following equation [36]:
S=
IPh
Id
constant. Therefore, the theoretical value of A** for CdS lattice will be 23 A cm−2 K−2 [36]. The Schottky barrier heights and the ideality factors for different UV detectors were calculated as summarized in Table 4. The decrease in barrier height under UV illumination led to an increase in photocurrent. The highest difference in barrier height values (in the dark and under UV light) was calculated for the CdS/Z–0.50 sample indicating the highest sensitivity of this nanostructure compared to the other samples. Quantum efficiency (QE) is another parameter used to estimate the performance of a photodetector. It is defined as the number of carrier collected to produce the photocurrent (IPh) per the number of incident photons [37]:
(5)
Id
where, IPh and Id represent the photocurrent and the dark current of photodetectors, respectively. The calculated values of sensitivity which are summarized in Table 4 showed an improvement in the optoelectrical properties of CdS/Z–0.50 sample. The sensitivity of this sample was tens of times higher than the other devices. The quasi–nonlinear behavior of the I–V curves indicates the formation of Schottky barriers at the Pt/CdS interface. The Schottky barrier height for the fabricated photodetectors can be calculated from I–V measurements using the thermionic emission theory according to the following equations [37];
I = I (exp(qV / nKT )
=
where V is the applied voltage, n shows the ideality factor, K denotes the Boltzmann constant, q is the quantum of electric charge, T represents the absolute temperature, and Iₒ stands for the saturation current given by following equation;
I = AA T 2 exp( q
(7)
B / KT )
where B is the Schottky barrier height, A denotes the junction area and A** is the modified Richardson constant obtainable by the following equation;
A
R=
(8)
= 4 qm K 2/ h3
Table 4 The calculated Schottky barrier height, ideality factor and quantum efficiency for fabricated photodetectors. Sensitivity (S)
Ideal factor (n)
ϕB1(eV) under UV
ϕB2(eV) at darkness
Δ(ϕB2-ϕB1)
η (%)
CdS/glass CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
20.2 6.8 192.1 2.7
14.0 13.2 13.4 17.6
0.6612 0.6016 0.6166 0.7582
0.8366 0.7626 0.3258 0.7577
0.1754 0.1610 0.3358 0.0005
33.90 3.05 212.89 8.81
(9)
IPh Pinc
(10)
This parameter was recorded under different wavelengths of incident light at a fixed voltage of 15 V. Fig. 9 shows the calculated responsivity versus the wavelength of incident light. The increased responsivity (0.62 A/W) of photodetector based on CdS/Z–0.50 nanostructure is in good agreement with the enhanced photosensitivity of this device which can be attributed to its special morphology and reduced crystal defect. Furthermore, the sharp cut–off at the wavelength of 471 nm (2.63 eV) is in line with the optical band gap of CdS nanostructure derived from optical transmission of this sample (2.62 eV). The responsivity of the devices was negligible in infrared range. However, it was increased in the visible range due to the transition of carriers from the defect states. This phenomenon was also observed in the PL peaks related to defect states and is responsible for generation of low photocurrent in this range. For all the devices, the
where m* shows the effective mass of electron and h is the planck's
sample
h Pinc
where Pinc is the optical power on incident light. The values of QE for different UV detectors are presented in Table 4. The highest QE was observed in the photodetector fabricated based on CdS/Z–0.50 which can be due to the extra photogenerated electrons in this device [38]. For this photodetector, the quantum efficiency is even higher than unity. If the energy of an incident photon is sufficiently greater than that of the semiconductor optical band gap, the quantum efficiency can be higher than 100%. In this case, the excited electron has sufficient kinetic energy for one or more subsequent impact which leads to the higher photogenerated electrons and increases the quantum efficiency to be higher than unity [39]. The responsivity of photodetectors can be calculated using the following equation [36]:
(6)
1)
IPh q
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Table 5 A comparison between speed of ultraviolet detection for different CdS photodetectors. sample
Rise time (ms)
Recovery time (ms)
CdS/glass CdS/Z–0.25 CdS/Z–0.50 CdS/Z–0.75
290 270 250 320
290 270 310 300
special morphology and high specific surface area. 4. Conclusion In summary, a three–dimensional nanosheet morphology of CdS thin film was deposited on ZnO seed layer using CBD technique upon application of the optimized reagent molar concentration (0.5 M). The XRD patterns indicated the high crystallinity of CdS nanosheets with a strong orientation at (002) plane of the CdS hexagonal phase. This sample illustrated the low transmittance spectrum which can be attributed to the high absorption coefficient of CdS nanosheets and light trapping behavior of this sample. The optical band gap of this sample was calculated through transmission spectrum (2.62 eV). Its PL spectrum also showed an elevated peak at 535 nm with the lowest FWHM in comparison with the other samples implying the enhanced optical quality of CdS nanosheets. This peak can be assigned to the band–edge emission related to the radiative recombination through bound excitons. In order to study the optoelectrical properties of CdS nanostructures, MSM photodetectors were fabricated based on different samples using deposition of Pt interdigitated finger–shaped patterns. The device based on CdS–nanosheet/ZnO indicated the highest responsivity (0.62 A/W) and photosensitivity (192) to UV illumination at voltage of 15 V which can be due to the special morphology and high specific surface area of this sample. In this device, the short rise time (250 m s) and recovery time (310 m s) under pulsed UV illumination indicated the high–speed performance of the optimized sample.
Fig. 9. The optical responsivity of different CdS photodetectors as a function of the incident light wavelength.
maximum responsivity was observed in the UV region with the energy above the threshold excitation energy of CdS nanostructures implying high sensitivity of fabricated devices in this range. For instance, the elevated responsivity of the photodetector based on CdS/Z–0.50 sample indicates its proper wavelength selectivity behavior. Fig. 10 illustrates the consecutive current spectra of different photodetectors in response to the pulsed UV light. The devices were exposed to illumination for a fixed duration and allowed to stabilize. For all devices, the photocurrent was increased to a saturation value upon illumination followed by a decrease when the light was switched off. The rise time is defined as the time taken for the sensor to reach to 90% of its saturation current; while the recovery time refers to the time taken for the sensor to decrease its current from saturation to 10% of its saturation current value. The photocurrents for all the fabricated photodetectors were highly stable and reproducible under pulsed UV illumination. The rise time and recovery time of fabricated photodetectors are presented in Table 5. The CdS/Z–0.50 sample exhibited the shortest rise time and recovery time reflecting the high–speed performance of this MSM photodetector. Such high performance can be assigned to the enhanced light absorption coefficient of CdS nanosheets due to their
Declaration of competing interestCOI 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.
Fig. 10. The consecutive current spectra of fabricated photodetectors under pulsed UV illumination. 7
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Acknowledgements
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