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ZnO multilayer structure
Superlattices and Microstructures 134 (2019) 106225
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Enhanced performance of ultraviolet photodetector based on sputtered ZnO/Au/ZnO multilayer structure H. Ferhati a, F. Djeffal a, b, * a b
LEA, Department of Electronics, University Mostefa Benboulaid-Batna 2, Batna, 05000, Algeria LEPCM, University of Batna 1, Batna, 05000, Algeria
A R T I C L E I N F O
A B S T R A C T
Keywords: Multilayer ZnO UV-Photodetector Responsivity Sputtering Light-trapping
This work presents the optimization, elaboration and characterization of a new MSM UV photodetector (PD) design based on an amended active region with ZnO/Au/ZnO (ZAZ) hetero structure. An experimental investigation assisted by Genetic Algorithm (GA) global optimization was conducted for engineering the proposed ZAZ multilayer in order to achieve high-responsivity UV sensor. The RF magnetron sputtering technique was then used to elaborate the optimized trilayered structure yielding the highest photoresponse. The structural and optical properties of the sputtered multilayer design were also investigated via X-ray diffraction (XRD) and UV–Vis absorbance measurements. Interestingly, electrical characterizations show that the optimized design is beneficial in two ways, firstly an ultrahigh responsivity of 1.3A/W was successfully demonstrated and secondly, the dark current was greatly reduced in comparison with that of the conventional design. This is attributed to the global optimization approach, enabling exciting opportunities for promoting strong light-matter interactions by improving the light trapping capability and photocurrent generation. Moreover, the inserted Au layer induces the band bending gradient benefit, leading to an efficient transfer of the photogenerated carrier and hin ders the recombination effects, which paves a new path toward fabricating high-responsivity UV PDs based on low-cost ZnO earth abundant material.
1. Introduction Ultraviolet (UV) photodetectors are becoming an important part of the next-generation optoelectronics owing to their wideranging applications including optical communication, military surveillance, biological sensors, environmental monitoring and so on [1–5]. To date, a variety of wide band-gap material candidates such as GaN, Ga2O3, TiO2, ZnMgO and ZnO have been used to elaborate various high-performance UV PDs [6–11]. Basically, wide band-gap semiconductors are attracting significant research and commercial interest owing to their improved thermal stability, high breakdown voltage, low-cost and favorable sensitivity to UV light [11–13]. Accordingly, the existing and potential applications of these materials go far beyond power electronics and include blue, green and UV emitters, chemical sensing and solar blind UV detection [12–14]. In fact, ZnO earth abundant semiconductors are regarded as sustainable solution to potentially substitute its more mature counterpart based on GaN compound and to contribute significantly to this endeavour, although the latter technology has reached so far better performances as compared to the nowadays emerging UV PD designs [9,15,16]. This is mainly due to its unique merits and outstanding characteristics including solar blindness, * Corresponding author. LEA, Department of Electronics, University Mostefa Benboulaid-Batna 2, Batna, 05000, Algeria. E-mail addresses: [email protected], [email protected] (F. Djeffal). https://doi.org/10.1016/j.spmi.2019.106225 Received 26 June 2019; Received in revised form 12 August 2019; Accepted 14 August 2019 Available online 16 August 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
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earth-abundant naturally n-type conductivity, low temperature growth, simple synthesizing process, non-toxicity, large exciton binding energy of 60 meV and favorable UV absorption capabilities [11,17,18]. Despite these attractive properties, the increasing demands and expectations from users are going beyond the actual stage of maturity of the thin-film ZnO-based UV PDs [17–19]. Basically, the weak photo-absorption and degradation related recombination effects constitute its more common limitations and needs to be suppressed. Motivated by these problems, researches are focused on exploring prospective solution to develop high-performance UV sensors at low production cost in which we can notice the use of ZnO nanowires, ZnO nanorods, introducing metallic nanoparticles, nanostructure array and heterjunction [20–23]. The purpose of these investigations was to shed more light on the underlying mechanisms causing the device degradation and to explore new pathways to deal with these issues. Even though the adopted para digms have allowed considerable improvements in the ZnO-based UV PD performance, the dark to illumination current ratio and the device UV-photodetecting performance are still limited and needs more enhancements to elaborate practical UV PDs appropriate for numerous sensing applications. Therefore, avoiding the recombination losses, ensuring a high Schottky barrier height, achieving near perfect UV absorbance, exploring uncomplicated and low-cost manufacturing process could open up the route for the realization of efficient UV sensors. To do so, new concepts and design methodologies are deemed necessary to follow the recent advancement made in the field of UV sensing. While searching for more plausible and effective pathways, we came across a fascinating paradigm of structural engineering by multilayer structure with metal interlayer [24–26]. Basically, this concept is widely exploited as an efficient strategy to improve TCO characteristics, markedly offering outstanding performances over the conventional thin-film wide band-gap materials. In other words, tri-layered designs have been capturing intensive attention because of their ability for bridging the gap between the high visible transparency and reduced sheet resistance, making them potential alternatives for various optoelectronic applications. To the best of our knowledge, there are no experimental investigations based on the multilayer concept were applied to improve the ZnO-based UV PD performances. Accordingly, we investigate the role of inserting Au intermediate metallic layer in enhancing the ZnO UV photodetecting characteristics. An experimental investigation assisted by GA global optimization approach was carried out for adjusting the geometry of the proposed ZAZ multilayer nanostructure with the aim of enhancing the device responsivity and suppressing the dark noise associated with the UV sensor. The optimized ZAZ tri-layered design was fabricated by means of RF magnetron sputtering technique and its structural properties were analyzed using XRD measurements. It was revealed that the elaborated ZAZ-based UV PD enables reaching higher responsivity and lower dark-current as compared to that provided by the ZnO thin-film simple structure. Therefore, the presented design methodology not only provides a facile and promising means to optimize the performance of ZnO UV PDs by exploiting GA metaheuristic approach but also opens up the way for preparing high-responsivity UV sensors. 2. ZAZ multilayer-based UV PDs optimization and experimental procedure In this section, we provide a systematic investigation of novel ZnO UV PD based on ZAZ tri-layered nanostructure in which we give first the details concerning the proposed methodology adopted for the eventual optimization of the analyzed device with the aim of maximizing the device responsivity. Subsequently, we illustrate the fabrication and characterization frameworks of the optimized ZAZ-
Fig. 1. 2-D schema of the proposed UV PD based on an amended active region with ZAZ multilayer nanostructure. 2
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based UV PDs using RF magnetron sputtering technique. 2.1. Designing of the ZAZ multilayer structure Basically, the cornerstone of the investigated design consists on inserting an ultrathin gold metal layer within the ZnO thin-film in order to not only to enhance the UV absorbance efficiency but also to manipulate carrier transfer behavior though modulating the band banding in the active region. To this extent, the investigated ZnO-based UV sensor design with the suggested structural amendment is illustrated in Fig. 1. As it is shown in this figure, the investigated UV PD design is considered with MSM back-to-back configuration consisting on Al top electrodes and ZAZ multilayer active region. In the proposed design, the thickness of the introduced Au metallic layer (tAu) is assumed to be very thin to avoid strong reflection effects. On the other hand, the position of the gold layer from the device surface (PM) and the bottom ZnO sub-layer thickness (tZnO) can be decisive in the development of efficient UV sensors, allowing suitable light management for raising up the device photogeneration capabilities. Essentially, the assessment of the investigated ZnO-based UV PDs optical behavior passes inevitably through estimating the absorbance and total reflection optical parameters. In this framework, the introduction of an intermediate metallic layer could induce various effects such as plasmonic and interference effects, which can influence the device optical behavior. Therefore, these important effects should be considered in our predictive simulation in order to be very close to the UV photodetector realistic behavior. More importantly, the band bending gradient effect occurred at the Au/ZnO interfaces plays a crucial role in determining the carrier transfer efficiency of the proposed device and should be also taken into account in the developed theoretical investigation. Accordingly, Silvaco software is exploited to carry out accurate and efficient predictive simulations, which is regarded as a powerful tool for investigating diverse nanoelectronic and optoelectronic devices [27]. It is worth mentioning that the details concerning the adopted optical and electrical modeling methodology can be found in our previous published works [28–30]. It is believed that both high responsivity and low dark current are indispensable for a myriad of fundamental and practical ap plications like optical communication and environmental monitoring [18–21]. As a matter of fact, the photodetection properties delivered by the advanced MSM UV PDs are not sufficient and require much more improvements. Either the dark conductivity is not suitably decreased to avoid noise effects, or the exhibited responsivity is not much improved. Recently, this trade-off has been attributed to certain imperfections including optical and recombination losses and has ultimately become the research focus in novel high-performance UV PDs [19]. In addition, the use of low-cost metal contacts in MSM structure could reduce Schottky barrier height, allowing high dark current thus degrading the performance of the UV sensor. This issue is still the subject of intensive research works claiming that both the responsivity and the dark performances of UV PDs strongly depend on the light-matter interaction and the structure of energy band bending. Consequently, these adverse effects and discrepancies bring evidence of the extreme complexity of the treated problem, where several trade-offs should be addressed to improve the photoconducting properties of the investigated UV sensor. Alternatively, we seek to avoid the undesired effects by modulating the electrical and optical behavior of the investigated ZAZ-based UV PDs. In other words, the carful adjustment of the inserted Au metallic layer size and position as well as the geometry of the entire ZAZ multilayer structure can give rise to significant improvements concerning the UV PD performance. To shed light on this topic, global optimization approach based on GA is exploited to simultaneously achieving high-responsivity and low dark conductivity of the proposed design. GA as a subfamily of evolutionary algorithms classified in population-based metaheuristic approach is described as a stochastic global search method that aims to reproduce the evolution of genetic species and the presentation of evolution concept provided by Darwin [31]. Fundamentally, this metaheuristic technique is in fact a naturally inspired technique owing to its underlying mechanism based on reproducing the natural selection of biological systems that allows the successive generations in a population to adapt to their environment. Basically, the principle of survival of the fittest is applied to achieve potential solutions represented by populations. GA has attracted a strongly increasing interest to resolve global optimization challenges, where it was distinctively exploited to highly improve the electrical and optical properties of various optoelectronic and nanoelectronic devices [32–35]. The major advantage of the GA-based technique resides on its capability for surmounting very complex mathematical concerns without the need for internal knowledge of the problem under treatment. Interestingly, GAs are in fact black-box methods that employ only fitness information in which the main goal is to select the optimal solution, with respect to one or even more criteria. Basically, chromosomes represent the key elements of the GA population, constituting from variables described as genes. Moreover, the fitness function is formulated according to the main objective of the optimization procedure and subsequently the rank of potential chromosomes in a population is suitably evaluated. Once the genetic representation and the objective function have been defined, GA proceeds to initialize a random population. Selection, crossover and mutation operators should be then specified to build the complete structure of the GA procedure. These operators are repetitively applied to improve the quality of solutions over numerous generations. Commonly, the GA finishes when the optimization convergence conditions are satisfied, which are mainly related to the considered maximum number of generations, the obtained error is infinitesimal or reaching a suitably desired objective level. The ultimate goal of the present study is to design a new ZnO based UV PD that can provide a high responsivity and reduced dark current using low-cost Aluminum contacts. Importantly, we need to optimize the proposed ZAZ structure in an appropriate manner to record a high current ratio and improved photoresponse, which are regarded as critical factors to realize practical UV PDs. Therefore, the fitness function is given by the following formulation Fitness ðXÞ ¼
1 þ IOFF ðXÞ RðXÞ
(1)
3
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whereXi ¼ ðtAu ; tZnO ; PM ; tGlass Þ of the ith generation represents the design parameter vector, R(X) and IOFF are respectively the responsivity and the dark current calculated numerically as it is abovementioned. In order to get reasonable values regarding the architectural design parameters considered in the analysis of the proposed ZAZ multilayer-based UV sensor, the design parameters should be confined in a given range. Thus, these ranges are taken as a set of constraints given by the following condition. ➢ x 2 ½xmmin ; xmmax �, xi 2 X (each design geometrical parameter of the amended active region with multilayer aspect should be confined within this range). For the optimization procedure, the stall generation and the population size are carefully selected, where the population size is assumed with 20 and the stall generation is considered to be equal to 1000. After carrying out the optimization procedure, the evolution of the fitness function as a function of the GA generations is depicted in Fig. 2, where it can be seen that the GA global optimization has succeeded in minimizing the fitness function defined by Eq. (1) from a generation to another. Moreover, we can also notice that the objective function stabilization is reached for around 800 generations. It is found that a maximum responsivity of 1.44A/W has been achieved for the best ZAZ multilayer geometry of 50nm/12nm/42 nm. Therefore, the elaboration of the optimized design seems of paramount importance in order to show experimentally the strength of the optimized design, which constitutes the main objective of the next sub-section. 2.2. Experimental elaboration of the optimized ZAZ UV PDs design Outstandingly, experiments assisted by GA-based global optimization can offer exciting opportunities to eventually bridge the gap between high responsivity and improved dark characteristics in advanced ZnO-based UV sensors. In this perspective, after identifying the optimized ZAZ tri-layered geometrical configuration yielding a high UV photoresponse by adopting the proposed design meth odology detailed in section (2.1), the next step is devoted to the fabrication and characterization of the optimized ZAZ multilayer-based UV PD design. Before the deposition process of the ZnO and Au ultrathin sub-layers, Glass substrates were ultrasonically cleaned up using acetone, water and ethanol and then dried by means of nitrogen jet procedure. Afterwards, successive ZnO and gold thin layers with the aboveoutlined optimized geometry (50nm/12nm/42 nm) were sequentially deposited on the cleaned glass substrate to shape the amended ZAZ multilayer-based active region of the investigated UV sensor. As a matter of fact, the main techniques of depositing ZnO thin-films are DC and RF magnetron sputtering of metal targets, CVD or sol-gel methods [36]. RF magnetron sputtering experimental facility is considered as the most used technique for producing high-quality heterostructures with ZnO/Metal/ZnO multilayer design [37–39]. Moreover, this deposition method allows developing ultrathin heterostructures in a single technological cycle with high crystallinity, low contamination and highly controllable thickness at nanoscale level [38–40]. In our case, we have chosen RF magnetron sputtering technique because of its outstanding capability for controlling the ZnO and gold layers thickness, which can in turn enable accurately elaborating the optimized ZAZ multilayer design with 50nm/12nm/42 nm geometrical configuration. This advantage further increases the possibility to reach the predicted im provements concerning the ZnO-based photodetector performance. Accordingly, the deposition process was then proceeded via efficient RF magnetron sputtering technique (MOORFIELD MiniLab 060), in which 5 cm of distance between the target and substrate was considered, while, the working pressure and power of RF source were respectively chosen to be 10 5 Pa and 250W. In addition, the deposition rate was 0.8 nm/s for the Au metallic ultrathin layer, whereas it has been 0.3 nm/s for the ZnO sub-layers, which has allowed perfectly managing the optimized thickness values of the suggested ZnO and Au sub-layers. The ZnO thin-films were sputtered in ambient conditions and in Ar and O2 atmosphere (66% oxygen partial pressure) using ZnO targets with 99.99%. Next, ellipsometry
Fig. 2. Fitness function evolution against the number of generations in the GA optimization. 4
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measurement was conducted to estimate and confirm the geometry of the sputtered ZAZ multilayer design. Thereafter, 200 nm thick and 9 mm2 surface of Aluminum electrodes were deposited by Electron-beam physical vapor deposition method with a shadow mask to obtain the MSM configuration. Moreover, the spacing between both realized Al contacts is 1.5 cm. Finally, the main parameters used in our experimental procedure of the optimized ZnO-based UV PD are recapitulated in Table 1. For the material characterization, the structural properties of the prepared UV PDs were checked via X-ray diffraction (XRD) method using a diffractometer (ARL Equinox 3000). The device photoresponse performances under UV illumination were recorded by extracting the absorbance spectra using UV–Vis spectrophotometer (F10-RT-UV) at controlled room temperature. Ultimately, I-V characteristics of the fabricated device with optimized ZAZ multilayer structure were measured by means of semiconductor parameter analyzer (Keithley 4200-SCS) and under illumination; we have exploited a UV lamp. 3. Results and discussion In order to get a deep insight concerning the structural properties of the elaborated sample with ZAZ multilayer structure, Fig. 3 shows the XRD pattern of the optimized structure with Au intermediate layer. Besides the diffraction of the gold intermediate layer, all other diffraction peaks located at 34.4� , 67� and 77� could be respectively assigned as (002), (103) and (202) planes of ZnO thin-film. This observation indicates the hexagonal wurtzite structure of the deposited ZnO sub-layers. The low intensity of the obtained peaks associated with the ZnO material is ascribed to the low thickness of the deposited sub-layers. This observation indicates that the developed ZnO thin-films are in the beginning of the crystallization phase. On the other hand, the obtained sharp peaks merely positioned at 38.1� and 44� are respectively attributed to diffraction along (111) and (200) facets of Au. The observed narrow peaks are in fact indicative of the good crystalline quality of Au metallic sub-layer. The inset in Fig. 3 shows the camera image of the elaborated MSM UV PD based on ZAZ multilayer active region. It seems of paramount importance to assess the effectiveness of the proposed design methodology based on experimental inves tigation supported by AG-based global optimization to enhance the ZnO-based UV PD performances for various sensing applications. Accordingly, to explore the role of the optimized design on the performance of UV PDs, the measurements of I-V characteristics have been carried out on the devices based on simple ZnO thin-film and ZAZ multilayer active regions. In this context, Fig. 4 shows I-V curves of both elaborated devices under dark and UV illumination. The obtained results demonstrates the nonlinear rectifying behavior of both elaborated samples with and without Au interlayer, confirming the Schottky contact nature between the ZnO material and Al electrodes. Noticeably, it is found that the dark current is reduced from 2 μA to 0.38 μA at 2V after inserting the Au intermediate metallic layer. This phenomenon can be explained by the band bending gradient induced by the localized Schottky junctions occurred in the photodetector active region between ZnO thin-films and Au middle layer. The work-function difference between the latter materials results in a built in potential at the ZnO/Au interfaces, depleting the carriers in the ZnO sub-layers. This phenomenon leads to enormously decreasing the dark conductivity of the investigated UV sensor. Obviously, it can be seen from this figure that when exposed to 320 nm UV illumination, both elaborated ZnO-based UV PDs demonstrate an apparent UV photodetecting behavior. However, the photoresponse of the optimized design with ZAZ multilayer paradigm is much larger than that of the conventional device based on simple ZnO thin-film, where the proposed design shows an improved ON current of 0.2 mA yielding a high responsivity of 1.3A/W. This achievement can be explained in the following part. The localized depletion regions induced at the Au/ZnO interfaces as it is above-outlined could contribute to the photocurrent enhancement through promoting strong carrier separation. In other words, the band bending realized in these interfaces generates an electric field in the ZnO sub-layers enabling efficient dissociation and transferring probability of the photo-excited electron/hole pairs. This can in turn open up the route for reducing the recombination losses and thereby increasing the derived current capability under UV radiation. On the other hand, considering the role of the inserted Au intermediate layer in modulating the light-matter interaction properties, it can be concluded that the improvement of the responsivity associated with the prepared device with ZAZ multilayer active region should be attributed to the enhanced light trapping capabilities, outperforming the PD UV absorbance performance. To consolidate this explanation, Fig. 5 depicts the measured absor bance spectra associated with the optimized ZAZ multilayer structure compared to that of the conventional ZnO thin-film. It can be confirmed from this figure that the optimized ZAZ tri-layered design exhibits near-unit absorbance behavior in the UV range, indi cating the effectiveness of the optimization approach based on GA metaheuristic technique for offering plausible pathways for improving the ZnO-based UV PD photoresponse. It is to note that the obtained results through the developed experimental investigation of the optimized UV PD-based on ZAZ trilayered nanostructure are found in good agreement with the performed predictive simulations, highlighting the outstanding capability Table 1 Deposition parameters of the elaborated Z/A/Z-based UV PD. Parameters
Value
Target Target to substrate distance (cm) Gas composition (Ar: O2) Substrate temperature (K) power of RF source (W) Working pressure (Pa) ZnO deposition rate (nm/s) Au deposition rate (nm/s)
ZnO with 99.99% of purity 5 (66%: 33%) 300 250 10–5 0.3 0.8
5
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Fig. 3. X-ray diffraction patterns of the elaborated ZAZ tri-layered active region of the investigated UV sensor. (b) A camera image of the elaborated UV PD based on ZAZ multilayer aspect with Al to electrodes.
Fig. 4. Measured I-V curves of the conventional ZnO thin-film device compared to that of the prepared UV PD based on an optimized ZAZ multilayer geometry.
Fig. 5. Absorbance spectra associated with the conventional ZnO thin-film compared to that of the prepared ZAZ multilayer design.
of the proposed methodology for designing practical and high-performance ZnO-based UV sensor. In order to shed light on the self-powered property of the UV PDs, which is considered so prominent and highly desired to meet the increasing demands for developing high-performance and low power consumption UV sensors, we propose to evaluate the elaborated 6
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Fig. 6. Device FoM parameters comparison between the proposed and conventional designs at 3V applied voltage and at self-powered conditions.
devices regarding this aspect. In this perspective, Fig. 6 compares the device FoMs at an applied voltage of 3V and at self-powered conditions associated with the fabricated UV PDs designs with and without Au intermediate layer. Under the self-powered voltage mode, it can be observed from this figure that an acceptable responsivity value of 0.02A/W and a high current ratio of 68.5 dB are reached by considering the optimized device with an amended active region based on ZAZ multilayer aspect. This has led to achieve a relative improvement of 90% in terms of the device FoMs over the conventional structure based on ZnO thin-film. This achievement is mainly due to the gradient band bending allowed by introducing gold middle layer, leading to distinctively manipulate the transfer behavior of carriers in ZnO sub-layers. More importantly, this figure demonstrates the ability of the elaborated UV PD based on nanostructured ZAZ tri-layered for providing an ultrahigh responsivity of 1.3A/W at a low applied voltage of 3V, which is considered comparable and even better than reported ZnO and other types UV sensors. Ultimately, it seems important to consolidate this comprehensive study by carrying out a performance metric comparison with other published results regarding ZnO-based UV PDs, which enables illustrating the strength of the proposed design with ZAZ multilayer aspect. Accordingly, Table 2 recapitulates the device FoMs obtained by the prepared UV PD based on ZAZ tri-layered active region compared to that offered by recently published works based on various strategies including metallic nanoparticles and nanowires [41–44]. This table indicates that the fabricated device with an optimized ZAZ multilayer geometry demonstrates its capability to be one of the most promising architecture to boosting up the ZnO-based UV PD performance, where merely 1.4A/W of responsivity is successfully recorded, which is much larger than that provided by the experimental investigation reported in Refs. [33–36]. It can be concluded from this table that the elaborated ZnO-based UV sensor outperforms the conventional counterparts in terms of the device FoM, where it yields 80% of relative improvement in the responsivity, 140% in the total current ratio. Therefore, by well tailoring the geometry of the ZAZ active region, we were able not only to achieve efficient light trapping capabilities but also to promote effective transfer of the photo-excited carriers, resulting in potential pathway to eventually enhance the performance of ZnO UV PD for various sensing applications. 4. Conclusion In this work, an optimized ZnO UV PD based on ZAZ multilayer active region is proposed and experimentally investigated. A new systematic approach based on GA metaheuristic technique combined with experimental elaboration was used to eventually fabricate high-responsivity UV sensors with reduced dark conductivity. The optimized structure was prepared by means of RF magnetron sputtering technique. The structural characterization of the elaborated ZAZ structure was also carried out via XRD measurement. It was found that by using an optimized ZAZ multilayer design as active region, the UV PD exhibits an ultrahigh responsivity of 1.3A/W as compared to that provided by the pure ZnO-based sensor. Through adjusting the ZAZ geometry using GA global optimization, the associated near unit UV absorbance has led to an improved photoresponse. Moreover, it was revealed that the elaborated ZnO UV PD based on Au intermediate metallic layer showcases an outstanding capability for reducing the dark current. The underlying mechanism is explored and basically attributed to the built-in band bending induced at the ZnO/Au interfaces. We believe that the presented work could contribute to the in-depth understanding of the role of the ZAZ tri-layered structure in enhancing the performance of various optoelectronic devices and offer a novel systematic and effective strategy to optimize the ZAZ multilayer geometry for designing practical and high-responsivity UV PD using ZnO earth abundant material. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106225.
7
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Table 2 Performance metrics comparison of performance parameters associated with the prepared ZAZ multilayer-based UV PD and that of various recently published ZnO-based MSM UV sensors. ZnO-based MSM UV photodetector designs
Current ratio (dB)
Dark current (μA)
Responsivity (A/W)
Responsivity at self-powered conditions (mA/W)
Ref.
Au NPs/TiO2/ZnO:Y NW/glass UV photodetector Au NPs/p-ZnO:K NSs/n-ZnO UV photodetector CH3NH3PbCl3/ZnO UV photodetectors Nanodiamond enhanced ZnO nanowire based UV PD Elaborated ZAZ multilayer-based MSM UV PD
65.03
0.8
0.1
0.35
[41]
61.1
0.55
0.2
1.2
[42]
44.43 49
0.47 2.8
1.28 0.59
0.92 –
[43] [44]
68.5
0.38
1.38
1.9
32
1.2
0.08
–
This work This work
ZnO thin-film MSM UV PD
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Enhanced performance of ultraviolet photodetector based on sputtered ZnO/Au/ZnO multilayer structure H. Ferhati a, F. Djeffal a, b, * a b
LEA, Department of Electronics, University Mostefa Benboulaid-Batna 2, Batna, 05000, Algeria LEPCM, University of Batna 1, Batna, 05000, Algeria
A R T I C L E I N F O
A B S T R A C T
Keywords: Multilayer ZnO UV-Photodetector Responsivity Sputtering Light-trapping
This work presents the optimization, elaboration and characterization of a new MSM UV photodetector (PD) design based on an amended active region with ZnO/Au/ZnO (ZAZ) hetero structure. An experimental investigation assisted by Genetic Algorithm (GA) global optimization was conducted for engineering the proposed ZAZ multilayer in order to achieve high-responsivity UV sensor. The RF magnetron sputtering technique was then used to elaborate the optimized trilayered structure yielding the highest photoresponse. The structural and optical properties of the sputtered multilayer design were also investigated via X-ray diffraction (XRD) and UV–Vis absorbance measurements. Interestingly, electrical characterizations show that the optimized design is beneficial in two ways, firstly an ultrahigh responsivity of 1.3A/W was successfully demonstrated and secondly, the dark current was greatly reduced in comparison with that of the conventional design. This is attributed to the global optimization approach, enabling exciting opportunities for promoting strong light-matter interactions by improving the light trapping capability and photocurrent generation. Moreover, the inserted Au layer induces the band bending gradient benefit, leading to an efficient transfer of the photogenerated carrier and hin ders the recombination effects, which paves a new path toward fabricating high-responsivity UV PDs based on low-cost ZnO earth abundant material.
1. Introduction Ultraviolet (UV) photodetectors are becoming an important part of the next-generation optoelectronics owing to their wideranging applications including optical communication, military surveillance, biological sensors, environmental monitoring and so on [1–5]. To date, a variety of wide band-gap material candidates such as GaN, Ga2O3, TiO2, ZnMgO and ZnO have been used to elaborate various high-performance UV PDs [6–11]. Basically, wide band-gap semiconductors are attracting significant research and commercial interest owing to their improved thermal stability, high breakdown voltage, low-cost and favorable sensitivity to UV light [11–13]. Accordingly, the existing and potential applications of these materials go far beyond power electronics and include blue, green and UV emitters, chemical sensing and solar blind UV detection [12–14]. In fact, ZnO earth abundant semiconductors are regarded as sustainable solution to potentially substitute its more mature counterpart based on GaN compound and to contribute significantly to this endeavour, although the latter technology has reached so far better performances as compared to the nowadays emerging UV PD designs [9,15,16]. This is mainly due to its unique merits and outstanding characteristics including solar blindness, * Corresponding author. LEA, Department of Electronics, University Mostefa Benboulaid-Batna 2, Batna, 05000, Algeria. E-mail addresses: [email protected], [email protected] (F. Djeffal). https://doi.org/10.1016/j.spmi.2019.106225 Received 26 June 2019; Received in revised form 12 August 2019; Accepted 14 August 2019 Available online 16 August 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
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earth-abundant naturally n-type conductivity, low temperature growth, simple synthesizing process, non-toxicity, large exciton binding energy of 60 meV and favorable UV absorption capabilities [11,17,18]. Despite these attractive properties, the increasing demands and expectations from users are going beyond the actual stage of maturity of the thin-film ZnO-based UV PDs [17–19]. Basically, the weak photo-absorption and degradation related recombination effects constitute its more common limitations and needs to be suppressed. Motivated by these problems, researches are focused on exploring prospective solution to develop high-performance UV sensors at low production cost in which we can notice the use of ZnO nanowires, ZnO nanorods, introducing metallic nanoparticles, nanostructure array and heterjunction [20–23]. The purpose of these investigations was to shed more light on the underlying mechanisms causing the device degradation and to explore new pathways to deal with these issues. Even though the adopted para digms have allowed considerable improvements in the ZnO-based UV PD performance, the dark to illumination current ratio and the device UV-photodetecting performance are still limited and needs more enhancements to elaborate practical UV PDs appropriate for numerous sensing applications. Therefore, avoiding the recombination losses, ensuring a high Schottky barrier height, achieving near perfect UV absorbance, exploring uncomplicated and low-cost manufacturing process could open up the route for the realization of efficient UV sensors. To do so, new concepts and design methodologies are deemed necessary to follow the recent advancement made in the field of UV sensing. While searching for more plausible and effective pathways, we came across a fascinating paradigm of structural engineering by multilayer structure with metal interlayer [24–26]. Basically, this concept is widely exploited as an efficient strategy to improve TCO characteristics, markedly offering outstanding performances over the conventional thin-film wide band-gap materials. In other words, tri-layered designs have been capturing intensive attention because of their ability for bridging the gap between the high visible transparency and reduced sheet resistance, making them potential alternatives for various optoelectronic applications. To the best of our knowledge, there are no experimental investigations based on the multilayer concept were applied to improve the ZnO-based UV PD performances. Accordingly, we investigate the role of inserting Au intermediate metallic layer in enhancing the ZnO UV photodetecting characteristics. An experimental investigation assisted by GA global optimization approach was carried out for adjusting the geometry of the proposed ZAZ multilayer nanostructure with the aim of enhancing the device responsivity and suppressing the dark noise associated with the UV sensor. The optimized ZAZ tri-layered design was fabricated by means of RF magnetron sputtering technique and its structural properties were analyzed using XRD measurements. It was revealed that the elaborated ZAZ-based UV PD enables reaching higher responsivity and lower dark-current as compared to that provided by the ZnO thin-film simple structure. Therefore, the presented design methodology not only provides a facile and promising means to optimize the performance of ZnO UV PDs by exploiting GA metaheuristic approach but also opens up the way for preparing high-responsivity UV sensors. 2. ZAZ multilayer-based UV PDs optimization and experimental procedure In this section, we provide a systematic investigation of novel ZnO UV PD based on ZAZ tri-layered nanostructure in which we give first the details concerning the proposed methodology adopted for the eventual optimization of the analyzed device with the aim of maximizing the device responsivity. Subsequently, we illustrate the fabrication and characterization frameworks of the optimized ZAZ-
Fig. 1. 2-D schema of the proposed UV PD based on an amended active region with ZAZ multilayer nanostructure. 2
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based UV PDs using RF magnetron sputtering technique. 2.1. Designing of the ZAZ multilayer structure Basically, the cornerstone of the investigated design consists on inserting an ultrathin gold metal layer within the ZnO thin-film in order to not only to enhance the UV absorbance efficiency but also to manipulate carrier transfer behavior though modulating the band banding in the active region. To this extent, the investigated ZnO-based UV sensor design with the suggested structural amendment is illustrated in Fig. 1. As it is shown in this figure, the investigated UV PD design is considered with MSM back-to-back configuration consisting on Al top electrodes and ZAZ multilayer active region. In the proposed design, the thickness of the introduced Au metallic layer (tAu) is assumed to be very thin to avoid strong reflection effects. On the other hand, the position of the gold layer from the device surface (PM) and the bottom ZnO sub-layer thickness (tZnO) can be decisive in the development of efficient UV sensors, allowing suitable light management for raising up the device photogeneration capabilities. Essentially, the assessment of the investigated ZnO-based UV PDs optical behavior passes inevitably through estimating the absorbance and total reflection optical parameters. In this framework, the introduction of an intermediate metallic layer could induce various effects such as plasmonic and interference effects, which can influence the device optical behavior. Therefore, these important effects should be considered in our predictive simulation in order to be very close to the UV photodetector realistic behavior. More importantly, the band bending gradient effect occurred at the Au/ZnO interfaces plays a crucial role in determining the carrier transfer efficiency of the proposed device and should be also taken into account in the developed theoretical investigation. Accordingly, Silvaco software is exploited to carry out accurate and efficient predictive simulations, which is regarded as a powerful tool for investigating diverse nanoelectronic and optoelectronic devices [27]. It is worth mentioning that the details concerning the adopted optical and electrical modeling methodology can be found in our previous published works [28–30]. It is believed that both high responsivity and low dark current are indispensable for a myriad of fundamental and practical ap plications like optical communication and environmental monitoring [18–21]. As a matter of fact, the photodetection properties delivered by the advanced MSM UV PDs are not sufficient and require much more improvements. Either the dark conductivity is not suitably decreased to avoid noise effects, or the exhibited responsivity is not much improved. Recently, this trade-off has been attributed to certain imperfections including optical and recombination losses and has ultimately become the research focus in novel high-performance UV PDs [19]. In addition, the use of low-cost metal contacts in MSM structure could reduce Schottky barrier height, allowing high dark current thus degrading the performance of the UV sensor. This issue is still the subject of intensive research works claiming that both the responsivity and the dark performances of UV PDs strongly depend on the light-matter interaction and the structure of energy band bending. Consequently, these adverse effects and discrepancies bring evidence of the extreme complexity of the treated problem, where several trade-offs should be addressed to improve the photoconducting properties of the investigated UV sensor. Alternatively, we seek to avoid the undesired effects by modulating the electrical and optical behavior of the investigated ZAZ-based UV PDs. In other words, the carful adjustment of the inserted Au metallic layer size and position as well as the geometry of the entire ZAZ multilayer structure can give rise to significant improvements concerning the UV PD performance. To shed light on this topic, global optimization approach based on GA is exploited to simultaneously achieving high-responsivity and low dark conductivity of the proposed design. GA as a subfamily of evolutionary algorithms classified in population-based metaheuristic approach is described as a stochastic global search method that aims to reproduce the evolution of genetic species and the presentation of evolution concept provided by Darwin [31]. Fundamentally, this metaheuristic technique is in fact a naturally inspired technique owing to its underlying mechanism based on reproducing the natural selection of biological systems that allows the successive generations in a population to adapt to their environment. Basically, the principle of survival of the fittest is applied to achieve potential solutions represented by populations. GA has attracted a strongly increasing interest to resolve global optimization challenges, where it was distinctively exploited to highly improve the electrical and optical properties of various optoelectronic and nanoelectronic devices [32–35]. The major advantage of the GA-based technique resides on its capability for surmounting very complex mathematical concerns without the need for internal knowledge of the problem under treatment. Interestingly, GAs are in fact black-box methods that employ only fitness information in which the main goal is to select the optimal solution, with respect to one or even more criteria. Basically, chromosomes represent the key elements of the GA population, constituting from variables described as genes. Moreover, the fitness function is formulated according to the main objective of the optimization procedure and subsequently the rank of potential chromosomes in a population is suitably evaluated. Once the genetic representation and the objective function have been defined, GA proceeds to initialize a random population. Selection, crossover and mutation operators should be then specified to build the complete structure of the GA procedure. These operators are repetitively applied to improve the quality of solutions over numerous generations. Commonly, the GA finishes when the optimization convergence conditions are satisfied, which are mainly related to the considered maximum number of generations, the obtained error is infinitesimal or reaching a suitably desired objective level. The ultimate goal of the present study is to design a new ZnO based UV PD that can provide a high responsivity and reduced dark current using low-cost Aluminum contacts. Importantly, we need to optimize the proposed ZAZ structure in an appropriate manner to record a high current ratio and improved photoresponse, which are regarded as critical factors to realize practical UV PDs. Therefore, the fitness function is given by the following formulation Fitness ðXÞ ¼
1 þ IOFF ðXÞ RðXÞ
(1)
3
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whereXi ¼ ðtAu ; tZnO ; PM ; tGlass Þ of the ith generation represents the design parameter vector, R(X) and IOFF are respectively the responsivity and the dark current calculated numerically as it is abovementioned. In order to get reasonable values regarding the architectural design parameters considered in the analysis of the proposed ZAZ multilayer-based UV sensor, the design parameters should be confined in a given range. Thus, these ranges are taken as a set of constraints given by the following condition. ➢ x 2 ½xmmin ; xmmax �, xi 2 X (each design geometrical parameter of the amended active region with multilayer aspect should be confined within this range). For the optimization procedure, the stall generation and the population size are carefully selected, where the population size is assumed with 20 and the stall generation is considered to be equal to 1000. After carrying out the optimization procedure, the evolution of the fitness function as a function of the GA generations is depicted in Fig. 2, where it can be seen that the GA global optimization has succeeded in minimizing the fitness function defined by Eq. (1) from a generation to another. Moreover, we can also notice that the objective function stabilization is reached for around 800 generations. It is found that a maximum responsivity of 1.44A/W has been achieved for the best ZAZ multilayer geometry of 50nm/12nm/42 nm. Therefore, the elaboration of the optimized design seems of paramount importance in order to show experimentally the strength of the optimized design, which constitutes the main objective of the next sub-section. 2.2. Experimental elaboration of the optimized ZAZ UV PDs design Outstandingly, experiments assisted by GA-based global optimization can offer exciting opportunities to eventually bridge the gap between high responsivity and improved dark characteristics in advanced ZnO-based UV sensors. In this perspective, after identifying the optimized ZAZ tri-layered geometrical configuration yielding a high UV photoresponse by adopting the proposed design meth odology detailed in section (2.1), the next step is devoted to the fabrication and characterization of the optimized ZAZ multilayer-based UV PD design. Before the deposition process of the ZnO and Au ultrathin sub-layers, Glass substrates were ultrasonically cleaned up using acetone, water and ethanol and then dried by means of nitrogen jet procedure. Afterwards, successive ZnO and gold thin layers with the aboveoutlined optimized geometry (50nm/12nm/42 nm) were sequentially deposited on the cleaned glass substrate to shape the amended ZAZ multilayer-based active region of the investigated UV sensor. As a matter of fact, the main techniques of depositing ZnO thin-films are DC and RF magnetron sputtering of metal targets, CVD or sol-gel methods [36]. RF magnetron sputtering experimental facility is considered as the most used technique for producing high-quality heterostructures with ZnO/Metal/ZnO multilayer design [37–39]. Moreover, this deposition method allows developing ultrathin heterostructures in a single technological cycle with high crystallinity, low contamination and highly controllable thickness at nanoscale level [38–40]. In our case, we have chosen RF magnetron sputtering technique because of its outstanding capability for controlling the ZnO and gold layers thickness, which can in turn enable accurately elaborating the optimized ZAZ multilayer design with 50nm/12nm/42 nm geometrical configuration. This advantage further increases the possibility to reach the predicted im provements concerning the ZnO-based photodetector performance. Accordingly, the deposition process was then proceeded via efficient RF magnetron sputtering technique (MOORFIELD MiniLab 060), in which 5 cm of distance between the target and substrate was considered, while, the working pressure and power of RF source were respectively chosen to be 10 5 Pa and 250W. In addition, the deposition rate was 0.8 nm/s for the Au metallic ultrathin layer, whereas it has been 0.3 nm/s for the ZnO sub-layers, which has allowed perfectly managing the optimized thickness values of the suggested ZnO and Au sub-layers. The ZnO thin-films were sputtered in ambient conditions and in Ar and O2 atmosphere (66% oxygen partial pressure) using ZnO targets with 99.99%. Next, ellipsometry
Fig. 2. Fitness function evolution against the number of generations in the GA optimization. 4
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measurement was conducted to estimate and confirm the geometry of the sputtered ZAZ multilayer design. Thereafter, 200 nm thick and 9 mm2 surface of Aluminum electrodes were deposited by Electron-beam physical vapor deposition method with a shadow mask to obtain the MSM configuration. Moreover, the spacing between both realized Al contacts is 1.5 cm. Finally, the main parameters used in our experimental procedure of the optimized ZnO-based UV PD are recapitulated in Table 1. For the material characterization, the structural properties of the prepared UV PDs were checked via X-ray diffraction (XRD) method using a diffractometer (ARL Equinox 3000). The device photoresponse performances under UV illumination were recorded by extracting the absorbance spectra using UV–Vis spectrophotometer (F10-RT-UV) at controlled room temperature. Ultimately, I-V characteristics of the fabricated device with optimized ZAZ multilayer structure were measured by means of semiconductor parameter analyzer (Keithley 4200-SCS) and under illumination; we have exploited a UV lamp. 3. Results and discussion In order to get a deep insight concerning the structural properties of the elaborated sample with ZAZ multilayer structure, Fig. 3 shows the XRD pattern of the optimized structure with Au intermediate layer. Besides the diffraction of the gold intermediate layer, all other diffraction peaks located at 34.4� , 67� and 77� could be respectively assigned as (002), (103) and (202) planes of ZnO thin-film. This observation indicates the hexagonal wurtzite structure of the deposited ZnO sub-layers. The low intensity of the obtained peaks associated with the ZnO material is ascribed to the low thickness of the deposited sub-layers. This observation indicates that the developed ZnO thin-films are in the beginning of the crystallization phase. On the other hand, the obtained sharp peaks merely positioned at 38.1� and 44� are respectively attributed to diffraction along (111) and (200) facets of Au. The observed narrow peaks are in fact indicative of the good crystalline quality of Au metallic sub-layer. The inset in Fig. 3 shows the camera image of the elaborated MSM UV PD based on ZAZ multilayer active region. It seems of paramount importance to assess the effectiveness of the proposed design methodology based on experimental inves tigation supported by AG-based global optimization to enhance the ZnO-based UV PD performances for various sensing applications. Accordingly, to explore the role of the optimized design on the performance of UV PDs, the measurements of I-V characteristics have been carried out on the devices based on simple ZnO thin-film and ZAZ multilayer active regions. In this context, Fig. 4 shows I-V curves of both elaborated devices under dark and UV illumination. The obtained results demonstrates the nonlinear rectifying behavior of both elaborated samples with and without Au interlayer, confirming the Schottky contact nature between the ZnO material and Al electrodes. Noticeably, it is found that the dark current is reduced from 2 μA to 0.38 μA at 2V after inserting the Au intermediate metallic layer. This phenomenon can be explained by the band bending gradient induced by the localized Schottky junctions occurred in the photodetector active region between ZnO thin-films and Au middle layer. The work-function difference between the latter materials results in a built in potential at the ZnO/Au interfaces, depleting the carriers in the ZnO sub-layers. This phenomenon leads to enormously decreasing the dark conductivity of the investigated UV sensor. Obviously, it can be seen from this figure that when exposed to 320 nm UV illumination, both elaborated ZnO-based UV PDs demonstrate an apparent UV photodetecting behavior. However, the photoresponse of the optimized design with ZAZ multilayer paradigm is much larger than that of the conventional device based on simple ZnO thin-film, where the proposed design shows an improved ON current of 0.2 mA yielding a high responsivity of 1.3A/W. This achievement can be explained in the following part. The localized depletion regions induced at the Au/ZnO interfaces as it is above-outlined could contribute to the photocurrent enhancement through promoting strong carrier separation. In other words, the band bending realized in these interfaces generates an electric field in the ZnO sub-layers enabling efficient dissociation and transferring probability of the photo-excited electron/hole pairs. This can in turn open up the route for reducing the recombination losses and thereby increasing the derived current capability under UV radiation. On the other hand, considering the role of the inserted Au intermediate layer in modulating the light-matter interaction properties, it can be concluded that the improvement of the responsivity associated with the prepared device with ZAZ multilayer active region should be attributed to the enhanced light trapping capabilities, outperforming the PD UV absorbance performance. To consolidate this explanation, Fig. 5 depicts the measured absor bance spectra associated with the optimized ZAZ multilayer structure compared to that of the conventional ZnO thin-film. It can be confirmed from this figure that the optimized ZAZ tri-layered design exhibits near-unit absorbance behavior in the UV range, indi cating the effectiveness of the optimization approach based on GA metaheuristic technique for offering plausible pathways for improving the ZnO-based UV PD photoresponse. It is to note that the obtained results through the developed experimental investigation of the optimized UV PD-based on ZAZ trilayered nanostructure are found in good agreement with the performed predictive simulations, highlighting the outstanding capability Table 1 Deposition parameters of the elaborated Z/A/Z-based UV PD. Parameters
Value
Target Target to substrate distance (cm) Gas composition (Ar: O2) Substrate temperature (K) power of RF source (W) Working pressure (Pa) ZnO deposition rate (nm/s) Au deposition rate (nm/s)
ZnO with 99.99% of purity 5 (66%: 33%) 300 250 10–5 0.3 0.8
5
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Fig. 3. X-ray diffraction patterns of the elaborated ZAZ tri-layered active region of the investigated UV sensor. (b) A camera image of the elaborated UV PD based on ZAZ multilayer aspect with Al to electrodes.
Fig. 4. Measured I-V curves of the conventional ZnO thin-film device compared to that of the prepared UV PD based on an optimized ZAZ multilayer geometry.
Fig. 5. Absorbance spectra associated with the conventional ZnO thin-film compared to that of the prepared ZAZ multilayer design.
of the proposed methodology for designing practical and high-performance ZnO-based UV sensor. In order to shed light on the self-powered property of the UV PDs, which is considered so prominent and highly desired to meet the increasing demands for developing high-performance and low power consumption UV sensors, we propose to evaluate the elaborated 6
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Fig. 6. Device FoM parameters comparison between the proposed and conventional designs at 3V applied voltage and at self-powered conditions.
devices regarding this aspect. In this perspective, Fig. 6 compares the device FoMs at an applied voltage of 3V and at self-powered conditions associated with the fabricated UV PDs designs with and without Au intermediate layer. Under the self-powered voltage mode, it can be observed from this figure that an acceptable responsivity value of 0.02A/W and a high current ratio of 68.5 dB are reached by considering the optimized device with an amended active region based on ZAZ multilayer aspect. This has led to achieve a relative improvement of 90% in terms of the device FoMs over the conventional structure based on ZnO thin-film. This achievement is mainly due to the gradient band bending allowed by introducing gold middle layer, leading to distinctively manipulate the transfer behavior of carriers in ZnO sub-layers. More importantly, this figure demonstrates the ability of the elaborated UV PD based on nanostructured ZAZ tri-layered for providing an ultrahigh responsivity of 1.3A/W at a low applied voltage of 3V, which is considered comparable and even better than reported ZnO and other types UV sensors. Ultimately, it seems important to consolidate this comprehensive study by carrying out a performance metric comparison with other published results regarding ZnO-based UV PDs, which enables illustrating the strength of the proposed design with ZAZ multilayer aspect. Accordingly, Table 2 recapitulates the device FoMs obtained by the prepared UV PD based on ZAZ tri-layered active region compared to that offered by recently published works based on various strategies including metallic nanoparticles and nanowires [41–44]. This table indicates that the fabricated device with an optimized ZAZ multilayer geometry demonstrates its capability to be one of the most promising architecture to boosting up the ZnO-based UV PD performance, where merely 1.4A/W of responsivity is successfully recorded, which is much larger than that provided by the experimental investigation reported in Refs. [33–36]. It can be concluded from this table that the elaborated ZnO-based UV sensor outperforms the conventional counterparts in terms of the device FoM, where it yields 80% of relative improvement in the responsivity, 140% in the total current ratio. Therefore, by well tailoring the geometry of the ZAZ active region, we were able not only to achieve efficient light trapping capabilities but also to promote effective transfer of the photo-excited carriers, resulting in potential pathway to eventually enhance the performance of ZnO UV PD for various sensing applications. 4. Conclusion In this work, an optimized ZnO UV PD based on ZAZ multilayer active region is proposed and experimentally investigated. A new systematic approach based on GA metaheuristic technique combined with experimental elaboration was used to eventually fabricate high-responsivity UV sensors with reduced dark conductivity. The optimized structure was prepared by means of RF magnetron sputtering technique. The structural characterization of the elaborated ZAZ structure was also carried out via XRD measurement. It was found that by using an optimized ZAZ multilayer design as active region, the UV PD exhibits an ultrahigh responsivity of 1.3A/W as compared to that provided by the pure ZnO-based sensor. Through adjusting the ZAZ geometry using GA global optimization, the associated near unit UV absorbance has led to an improved photoresponse. Moreover, it was revealed that the elaborated ZnO UV PD based on Au intermediate metallic layer showcases an outstanding capability for reducing the dark current. The underlying mechanism is explored and basically attributed to the built-in band bending induced at the ZnO/Au interfaces. We believe that the presented work could contribute to the in-depth understanding of the role of the ZAZ tri-layered structure in enhancing the performance of various optoelectronic devices and offer a novel systematic and effective strategy to optimize the ZAZ multilayer geometry for designing practical and high-responsivity UV PD using ZnO earth abundant material. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106225.
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Table 2 Performance metrics comparison of performance parameters associated with the prepared ZAZ multilayer-based UV PD and that of various recently published ZnO-based MSM UV sensors. ZnO-based MSM UV photodetector designs
Current ratio (dB)
Dark current (μA)
Responsivity (A/W)
Responsivity at self-powered conditions (mA/W)
Ref.
Au NPs/TiO2/ZnO:Y NW/glass UV photodetector Au NPs/p-ZnO:K NSs/n-ZnO UV photodetector CH3NH3PbCl3/ZnO UV photodetectors Nanodiamond enhanced ZnO nanowire based UV PD Elaborated ZAZ multilayer-based MSM UV PD
65.03
0.8
0.1
0.35
[41]
61.1
0.55
0.2
1.2
[42]
44.43 49
0.47 2.8
1.28 0.59
0.92 –
[43] [44]
68.5
0.38
1.38
1.9
32
1.2
0.08
–
This work This work
ZnO thin-film MSM UV PD
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