Fast-Response Metal-Semiconductor-Metal Junction Ultraviolet Photodetector Based on ZnS:Mn Nanorod Networks via a Cost-Effective Method.

In this work, Mn2+-doped ZnS nanorods were synthesized by a facile hydrothermal method. The morphology, structure, and composition of the as-prepared samples were investigated. The temperature-dependent photoluminescence of ZnS:Mn nanorods was analyzed, and the corresponding activation energies were calculated by using a simple two-step rate equation. Mn2+-related orange emission (4T1 → 6A1) demonstrates high stability and is comparatively less affected by the temperature variations than the defect-related emission. A metal-semiconductor-metal junction ultraviolet photodetector based on the nanorod networks has been fabricated by a cost-effective method. The device exhibits visible blindness, superior ultraviolet photodetection with a responsivity of 1.62 A/W, and significantly fast photodetection response with the rise and decay times of 12 and 25 ms, respectively.

Introduction

Ultraviolet (UV) photodetectors with fast response times and high performance are critical to the advancement of various civil and military field technologies such as optical imaging, spatial optical communication, flame sensing, chemical analysis, missile guidance, positioning navigation, and so forth.[1−3] In recent years, photodetectors have made remarkable progress; however, the fabrication of UV cost-effective photodetectors with fast response, higher responsivity, and stability has always been a challenge. Wide-band gap chalcogenides are widely known for their fascinating optical, electrical, and photosensing properties, having vast applications in the fields of optoelectronics, photosensing, light harvesting, and so forth.[4] Chalcogenides at a nanoscale exhibit superior structural and optical properties and offer higher sensitivity compared to their bulk counterparts.[5] Particularly, one-dimensional (1D) nanostructures may provide better photodetection performance due to their large surface-to-volume ratio, low dimensions, and size-dependent properties, such as enhanced photon absorption, efficient separation, and migration of charge carriers.[6]

ZnS is one of the well-known II–VI compound semiconductors with a wide band gap of 3.77 eV, and it possesses high transmission in the visible region, making it suitable for designing UV photosensitive devices. However, in the pure form, ZnS exhibits highly insulating behavior due to its low donor–accepter defect level and high crystal quality, and without doping, it is not suitable for device applications.[7] The intentional incorporation of doped impurities is considered as a fundamental approach for tailoring the physical and chemical properties of a semiconductor material,[45] leading to the design and fabrication of photodetector, electronic, optoelectronic, and spintronic devices of desired features and characteristics.[8−10] 1D fluorescent nanomaterials with large surface-to-volume ratios exhibit enhanced light sensitivity, photoluminescence (PL), and fast response due to quantum confinement effects,[11,59] which can be efficiently improved and tuned with impurity concentration levels. Intentional doping of transition metals can generate midgap states in the II–VI compound semiconductors, which are involved in the band gap tuning and photoionization transitions through radiative or nonradiative decay and can contribute to the improvement of the photoconduction process.[12−14] Particularly, Mn2+-doped ZnS nanomaterials are considered as important luminescent materials due to their ability to modify the energy band and generate luminescent centers of different energy levels, giving rise to various interesting properties and applications.[15,16]

Yang et al. studied the phase transition effect on the luminescence properties of ZnS: Mn2+ quantum dots.[17] Cao et al. reported the maximum enhancement in the PL intensity of ZnS nanowires at 3% Mn2+ dopant ion concentration and making composite by surface passivation with SiO2.[18] Recently, Khan et al. synthesized ZnS:Mn2+ nanoparticles encapsulated in silica films, which demonstrated good emission in a broad range from 550 nm to 700 nm with an intense absorption in the UV region from 300 nm to 400 nm.[19] Aziz et al. studied the anomalous Arrhenius and Berthelot behavior of temperature-dependent PL of Mn-doped ZnS nanostructures.[20]

Kim et al. fabricated an ultraviolet photodetector based on ZnS nanobelts sandwiched between graphene layers, a structured photodetector of graphene/ZnS, which shows a high photocurrent of 0.115 mA at an illumination power density of 1.2 mW·cm–2 in air at a bias of 1.0 V. However, the UV detector demonstrates longer rise and decay times of 2.8 and 7.5 s, respectively.[21] Li et al. investigated a self-powered ultraviolet photodetector based on TiO2/Ag/ZnS nanotubes with high stability and a maximum response with rise and decay times of 0.16 and 0.18 s, respectively.[55] Wang et al. studied the rectifying behavior of UV photodetectors based on Cl-doped ZnS nanoribbons.[22] Kumar et al. reported that the photoresponse of ZnS nanoparticles enhanced by ∼15 to 36% by increasing Ni doping in ZnS nanoparticles from 0 to 2 mol %.[23] Wang et al. investigated the optoelectrical properties of p-type nitrogen-doped ZnS nanowires for utilizing them as a building block in UV detectors. They found that the device exhibits high photocurrent gain with considerable spectral selectivity and UV sensitivity.[24] Le Donne et al. reported the luminescence properties of the Mn-doped ZnS nanoparticles for photovoltaic applications.[25]

Despite extensive research into the investigation of structural, electrical, and optical properties of Mn-doped ZnS nanostructures, little attention has been paid to study the UV photodetection properties of Mn-doped ZnS nanostructures. In this work, we reported the synthesis of high-quality ZnS:Mn nanorods (NRs) by a facile hydrothermal route and investigated their structural, optical, and temperature-dependent PL properties. We fabricated a metal–semiconductor–metal (MSM) junction UV photodetector based on ZnS:Mn NRs networks with high performance and fast photoresponse. It is noteworthy that the doping of Mn2+ ions significantly improved the photosensing performance and time response of the ZnS:Mn NRs system.

Results and Discussion

Figure shows the high-resolution transmission electron microscopy (HRTEM) image of ZnS:Mn NRs. The transmission electron microscopy (TEM) images in Figure a,b reveal that the NRs exhibit uniform shapes and size distribution. One can obviously notice the diameter less than 10 nm and the length in the micron scale for these NRs. Figure c displays the HRTEM image of a NR, as marked by the white dotted rectangle [2] in Figure b. Figure d shows the HRTEM image of a typical NR, as marked by the white dotted rectangle [1] in Figure a, where the inset (d*) shows the absence of any defect or distortion in the nanowire by presenting a masked inverse fast Fourier transform (IFFT) pattern. Moreover, Figure e shows the IFFT image of a selected area marked by the white dotted rectangle [3] in Figure d for the doped ZnS system. The measured lattice spacing from the image is 3.13 Å, which corresponds to a ZnS crystal plane spacing of (002) and indicates a homogeneous and well-crystallized ZnS frame structure. The intensity profiles of lines 1 and 2 as marked in Figure e are shown in Figure f,g. In contrast to the uniform distribution of Mn in the matrix, the presence of the Zn/Mn atomic position is obvious in line 2.

Figure 1

HRTEM analysis of ZnS:Mn NRs; (a) low-resolution TEM image, (b) TEM image of a selected area marked by the white rectangle [2] is shown in the (c,d) inset (d*) (IFFT) pattern. (e) IFFT image of the marked white rectangle [3], (f,g) intensity profiles of the doped system, and (h) crystal structure illustration of the doped system.

An intense diffraction peak corresponding to the plane (002) and the lattice plane distance of 3.1 Å dominates the X-ray diffraction (XRD) pattern, as shown in Figure S2. The remaining diffraction peaks from the planes (100), (101), (102), (110), (103), and (112) can also be attributed to the wurtzite ZnS-PDF#36-1450. The absence of secondary phases in the XRD pattern indicates that Mn2+ was successfully incorporated on zinc lattice sites.

X-ray photon spectroscopy analysis further confirms the existence of Mn atoms in the ZnS lattice, which is depicted in Figure S3.

Figure S4a shows the UV–visible absorption spectrum of ZnS: Mn NRs. The prepared samples exhibit a strong optical response in the UV regime. The approximate band gap energy of samples was determined from the absorption spectra by using Tauc’s plot, as shown in Figure S4b. The Eg value obtained through linear fitting is 3.46 eV, which shows a decrease in the band gap as compared to bulk wurtzite ZnS (3.77 eV). Reduction of the band gap can be attributed to the formation intermediate energy states induced by impurity incorporation.[14,27]

Temperature-dependent PL spectra of ZnS/Mn NRs are presented in Figure . The PL spectra of ZnS:Mn NRs mainly consist of two peaks. The PL at 300 K shows only one prominent peak around 582 nm, attributed to the characteristic orange emission (4T1–6T1) of Mn2+ ions.[28] The defect related to the high-energy blue emission shoulder, which is invisible at room temperature, appears at lower temperatures and shows the highest peak intensity at 8 K around 459 nm. It can be seen that the blue emission undergoes a red shift as the temperature rises. However, this shift does not strictly follow the temperature dependence of ZnS band gap energy. Orange emission also exhibits decreasing and increasing behavior, as well as red shift, but it appears to be less affected by temperature and quenches slower than the corresponding partners at the higher-energy counterparts.[29] The luminescence intensity of blue emission decreases sharply from 35 to 300 K, while the Mn2+ emission intensity shows negligible decreasing and increasing behavior, which becomes constant around 250 K. The Mn2+-related emission persists and is visible, whereas blue emission almost completely quenches at room temperature. It is obvious that Mn2+-related orange emission in nanostructures is weakly dependent on temperature as compared to the blue emission, which is in agreement with previously reported work.[30,31] The variations in integrated intensity versus temperature are shown in Figure S5.

Figure 2

Temperature-dependent PL of ZnS:Mn NRs excited at λ = 325 nm.

There seem to be two quenching processes involved in the defect-related emission and Mn2+-related orange emission; one is the fast quenching and other is slow quenching in a relatively high-temperature region. Therefore, temperature dependence of integrated intensity for the ZnS:Mn emission mechanism can be well understood using the Arrhenius equation based on a two-step quenching model.[32]where I0 represents the emission intensity at 0 K temperature, A and B are the temperature-independent coefficients, and Ea and Eb are the thermal activation energies, respectively, related to the thermal quenching process of PL emission intensities.

The defect activation energies obtained by fitting of emission peaks are Ea = 4.07 meV and Eb = 60 meV. The thermal activation energy of 60 meV for blue emission is the same as the binding energy of ZnS excitons,[33] which is most likely due to thermal dissociation of excitons in the host material. The activation energies for Mn emission were determined to be Ea = 3.66 meV and Eb = 105 meV. The high activation energy Eb for Mn2+ emission indicates the stronger binding energy of excitons,[20] which promotes the persistence of long-lived transitions at room temperature. Temperature-dependent luminescence quenching can be attributed to thermionic emission of charge carriers out of confining potential, with the activation energy corresponding to the localization potential.[34,35]Figure presents the Arrhenius plot of the integrated intensity versus inverse temperature over a temperature range of 8–300 K. Moreover, the PL decay curves also were used to investigate the charge carrier dynamics of ZnS:Mn NRs, as shown in Figure S6. The average PL decay time obtained for ZnS:Mn NRs is ∼1.2 ms.

Figure 3

Arrhenius plot of integrated intensity of individual PL peaks with respect to inverse temperature.

The optoelectrical properties of the as-prepared ZnS:Mn NR MSM UV photodetector were investigated by current–voltage (I–V) characteristic measurements. The current voltage (I–V) characteristics recorded at room temperature under the dark and illumination at different wavelengths of 310, 365, 380, 410, 470, 525, and 630 nm are shown in Figure . The nonlinear response I–V curves of the device indicate the existence of the Schottky barrier at the MSM junction. The photocurrent increases rapidly under the illumination of UV light.

Figure 4

I–V characteristics of the ZnS:Mn NR MSM photodetector under incident light of different wavelengths.

While under dark and visible illumination, the photocurrent drops sharply and shows a very weak response, indicating the visible blindness behavior of the device. It is obvious from the results that the photocurrent under UV illumination is significantly higher than that of nonillumination, implying that UV illumination could remarkably enhance the conductivity of the NRs network device. The responsivity (Rλ) is defined as the ratio of the response current to the incident illumination power density.[36]

Here, Iph and Idark are the photocurrent and the dark current, respectively, while Pin is the incident UV light power density, and Aa is the active sensitive area of the device.[37] The calculated values of responsivity of the device with a UV power density of 0.5 W·cm–2 at 380 and 310 nm at an applied voltage of 3 V are 1.62 A/W and 1.8 A/W, respectively. Photons are absorbed in the semiconductor during the photodetection process, producing electron–hole pairs. These photogenerated carriers are separated by the built-in or applied electric field, producing a current proportional to the incident photon flux.[38] The photoresponse mechanism of NR network photodetectors is commonly attributed to the two possible processes.[39−41] (i) In the presence of air, oxygen molecules absorb on the surface of ZnS:Mn NRs and capture the free electrons from the NR surface [O2 (g) + e– → O2–(ad)], consequently creating a depletion layer which causes the low conductivity under dark conditions. When UV light irradiates on the NRs, electron–hole pairs are generated, which are separated by an electric field. Free holes migrate to the surface and contribute in desorption of O2 ions [O2– (ad) + h+ → O2], and the width of the depletion layer decreases, resulting in an increase in the photocurrent. (ii) There is another conduction mechanism that exists in the NR network-based photodetector device. Besides the intrinsic resistance, the resistance at the NR–NR junction must be considered in the NR-network devices. The resistance at the junction of two crossing NRs creates a junction barrier. The photocurrent density increases under UV light illumination and applied bias, allowing electrons to overcome the junction barrier potential via barrier tunneling from one NR to another.

Moreover, the incorporation of Mn2+ ions in the ZnS lattice induces the change in bond lengths by replacing the Zn–S bond with the Mn–S bond. The bond length of the Mn–S bond (2.431 Å) is larger compared to the Zn–S bond (2.341 Å).[42] Therefore, the adsorbed oxygen molecules can easily draw electrons from the ZnS:Mn NRs, which in turn contribute to increase of the charge-depletion layer depth and improve the photoconductivity. In addition, it has been observed that doped semiconductor nanocrystals exhibit a “self-purification effect,[43,44] implying that the Mn impurities at the optimal concentration tend to be repelled and move to the surface layer of NRs, contributing to increase the oxygen adsorption.[45] The schematic illustration of the possible mechanisms is shown in Figure S7.

The number of charge carriers generated per incident photon under UV illumination can be expressed by the relation[36,46]where η is the external quantum efficiency of the device, Rλ is the responsivity, and λ is the incident light wavelength. From the above relation, the values of η at λ = 380 nm and λ = 310 nm are calculated as 538 and 719%, respectively.

Figure shows the time-dependent photoresponse of a UV photodetector that was investigated by periodically turning UV light (380 nm) on and off at a time interval of 2 s. The power intensity of incident UV light was 0.5 W/cm2, while the applied bias was 0.1 V. The NR photodetector showed two distinct states, a low current state (dark current) and a high current state (photocurrent). Under UV illumination, the current increased rapidly from 140.5 μA and attained an average value of 145.5 μA. When the UV light was turned off, the photocurrent quickly dropped to a low current state. The rise and decay times of a photodetector are critical in determining its sensitivity in response to a rapidly changing optical signal. The rise time is defined as the time required for the photocurrent to reach 90% from the dark current value after the light is turned “on”, while the decay time refers to the time required to reach 10% of the photocurrent after switching “off” the incident light.[47]

Figure 5

Photoresponse of the ZnS:Mn NR MSM photodetector at an applied bias of 0.1 V under 380 nm illumination.

To determine the variations in the photoresponse during rise and decay, the rise and decay time curves are well fitted with a single exponential function;[48] the magnified edges of a single “on–off” cycle are shown in Figure .

Figure 6

Response time of ZnS:Mn NRs photodetector fitted using the exponential function. (a) Rise time and (b) decay time under a 380 nm illumination of 0.5 W/cm2 power density.

Based on the fitting functions and parameters, the time constants for photocurrent rise and fall are estimated to be τr = 12 ms and τd = 25 ms, respectively. A fast photoresponse and recovery time is obtained compared to high-performance ZnS-based photodetectors reported in the literature. Figure S6a shows the photoresponse of the device at 310 nm illumination under fast switching, and fast rise and recovery times of 16 and 1.1 ms are shown in Figure S8a,b, respectively. Moreover, photoresponse analysis of the MSM UV photodetector device based on pure ZnS NRs was carried out under the illumination of UV light (380 nm), as shown in Figure S7a. It shows comparatively small photocurrent and slow rise–decay times of 30.8 and 31.7 ms (Figure S9a,b), respectively.

A brief summary of the performance of UV photodetectors based on ZnS nanostructures from recent literature is presented in Table . When the UV light is irradiated on the active area of the device, a rapid generation of electron–hole pairs occurs, and a significant increase in unpaired electrons could result in the enhancement of carrier injection and transport. Consequently, the photocurrent shows dramatic enhancement upon UV illumination. In this mechanism, the fast separation and efficient migration of photogenerated electron–hole pairs explicate the nature of fast response speed of the photodetector. The rise time depends on an equilibrium condition attained by the electron–hole pair generation and recombination rate. It means that when exposed to UV light, a rapid generation of electron–hole pairs occurred, while the recovery time is only associated with the carrier recombination rates.[49] When the UV light is switched off, photogenerated electron–hole pairs are rapidly recombined and the device goes to the low-current state. Moreover, NRs exhibit a high surface-to-volume ratio, with many dangling bonds existing on the surface that can act as recombination centers for charge carriers and contribute in fast decay times. Literature study shows that transition-metal doping improves the conductance of the host material and thus improves the photoresponse.[50−52] The fast rise and decay times of ZnS:Mn NRs are attributed to the fast generation and recombination of electron–hole pairs.

Table 1

Summary of Device Performance of ZnS Nanostructure-Based Photodetectors

photodetector	light source	applied bias (V)	responsivity	rise time (s)	decay time (s)	refs
Ni-doped ZnS nanoparticles	∼365 nm			44	38	(23)
ZnS NT; Ag nanowires	Xenon lamp	2	16.5 A/W	0.12	0.40	(53)
TiO2/Ag/ZnS nanotubes	365 nm	0		0.16	0.18	(54)
ZnO/ZnS core–shell NR array	365 nm	0	0.056 A/W	0.04	0.04	(55)
ZnS nanotubes; Ag nanowires	Xenon Lamp	0	2.56 A/W	0.09	0.07	(40)
ZnS nanowires	0.03 mW/cm2	10		3.2	3.6	(56)
ZnS–rGO (15%) nanocomposites	200–290 nm			0.31	0.47	(57)
ZnS/InP nanowires	332 nm	5		0.75	0.5	(58)
graphene-integrated ZnS nanowires	365 nm	5		0.12	1.5	(59)
ZnS/Mn NRs	380 nm	0.1	1.62 A/W	0.012	0.025	This work
	310 nm		1.8 A/W	0.016	0.001

Conclusions

We investigated the morphology, structure, and temperature-dependent PL of ZnS:Mn NRs. The PL quenching shows that Mn2+-related orange emission (4T1 → 6A1) was comparatively less affected compared to the defect-related emission with temperature variations. A MSM junction photodetector based on ZnS:Mn NRs was fabricated, and the device showed visible blindness, superior UV photodetection, and significantly fast response with a rise time of τr = 12 ms and a decay time of τd = 25 ms. The results show that the enhanced photodetection performance of the ZnS-based photodetector can be achieved by the incorporation of Mn impurities in the ZnS host lattice and can be used effectively for the fabrication of efficient UV sensing devices and integrated systems.

Experimental Section

ZnS:Mn NRs were fabricated by a facile hydrothermal method with an optimum concentration of 5% Mn ions.[26] In order to fabricate the ZnS:Mn NR photodetector device, Ag electrodes were deposited on a Si/SiO2 substrate using radiofrequency magnetron sputtering in an argon atmosphere. Prior to the deposition, the substrate was precleared ultrasonically for 20 min with deionized (DI) water, ethanol, and acetone in succession. The Ag electrodes had a dimension of about 7000 × 5000 μm2. The spacing between Ag electrodes was ∼29 μm. The ZnS:Mn NRs were dispersed into DI water to make a suspension. The typical concentration was 0.2 mg/ml. A droplet of the ZnS:Mn NRs suspension was drop-cast on Ag electrodes and allowed to dry at room temperature. A schematic diagram and an actual image of the fabricated device are shown in Figure S1a,b.

Characterizations

The morphology and crystalline structure of the prepared ZnS:Mn NRs were analyzed by HRTEM (JEOL 2010). The crystal phases of the NR samples were determined by an X-ray diffractometer (TTR-III) equipped with Cu Ka radiation. The X-ray photoelectron spectra were recorded on a Thermo ESCALAB 250Xi system, equipped with a Al Kα X-ray excitation source (1486.6 eV).The UV–vis absorption spectra were recorded by a Shimadzu DUV-3700 spectrophotometer. Temperature-dependent PL measurements were conducted with a spectrofluorometer in the temperature range from 8 to 290 K. The PL decay curves were detected by using a photomultiplier tube (PMTH-S1-CR131, Zolix) equipped on a spectrometer (Kymera 328i, Andor). The signals were connected to an oscilloscope (DSO-X 3052A, Agilent) in parallel with a 5 kΩ resistor, excited by a pulsed YAG laser (355 nm, 7 ns, Q-smart 450, Quantel) with attenuated intensity for avoiding damage to the sample. A Hitachi (SU8000) FE-SEM device was used to take the optical image of the prepared photodetector device. The photoresponse measurements of the NR photodetector device were conducted by using a probe station connected to a semiconductor characterization system, SCS-Keithley 4200 semiconductor analyzer. The photodetection measurements were done under the dark and illuminated conditions, whereby the light intensity was tuned and calibrated by using an optical power meter. Light-emitting diodes with the wavelengths of λ = 310, 380, 410, 470, 525, and 630 nm were used as the illumination sources for the photoresponse measurements.