fi lms as promising ultraviolet photodetectors

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Ceramics International

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Hexagonal YMnO

₃

fi lms as promising ultraviolet photodetectors

S.B. Yang

^a

, C.A. Wang

^a

, Y. Li

^a

, Y. Chen

^a

, A.H. Zhang

^a

, M. Zeng

^a,⁎

, Z. Fan

^a

, X.S. Gao

^a

, X.B. Lu

^a,⁎

, J.-M. Liu

^a,b

aGuangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

bLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 21009, China

A R T I C L E I N F O

Keywords:

Hexagonal YMnO3

UV photodetector Photoresponse Multiferroicity

A B S T R A C T

Hexagonal rare-earth manganites (h-RMnO3,R= Sc, Y, Ho-Lu) exhibit intricate multiferroic properties and may be utilized for various technological applications ranging from non-volatile memories to solar cells and photo- detectors. Here, we report the epitaxial growth of hexagonal YMnO₃(h-YMO)films with different thicknesses deposited on yttria-stabilized zirconia (YSZ) substrates and their photoresponses to ultraviolet (UV) light. The structural characterizations illustrate that all thefilms present high quality epitaxy with out-of-plane h-YMO (0001)//YSZ(111) and in-plane h-YMO[1230]//YSZ[110] relationship. Meanwhile, UV light driven photo- responses of these as-preparedfilms based on the planar photodetector structure with transparent Al-doped ZnO (AZO) as top electrodes are explored. It is revealed that the photocurrent willfirst increase and then decrease with increasingfilm thickness. The highest photoconductivity gain is ~387.4 observed at +4 V bias for the device with 160 nm YMOfilm. These results provide a guideline for hexagonal rare-earth manganites to work for UV photodetector application.

1. Introduction

Ultraviolet (UV) photodetectors have attracted extensive research interest due to great potential in the variousfields, such as space sci- ence, industries, military, biological and environmental applications [1,2]. Currently, photodetectors based on traditional semiconductor materials, such as Si, ZnO, SiC, and GaN, are commercially available, which have been applied in photodetection systems of UV monitoring, lithography, ultrafast image, and optical diagnostics[3–7]. However, the application of these UV instrumentations is limited in high tem- perature and harsh environment. In order to improve their perfor- mances, great efforts have been put into developing new materials, such as, organic-inorganic hybrid perovskites, graphene, carbon nano-tubes and two-dimensional MoS2[8–13]. Although these materials exhibit high responsivity and fast response speed, their stability is weak in harsh environment.

Recently, some researchers paid their attention to inorganic ferro- electric materials that may solve the stability problems [14]. Many well-known ferroelectric materials, such as lead zirconated titanate (PZT), barium titanate (BaTiO3), and bismuth ferrite (BiFeO3) have been investigated for the photodetector applications[14–23]. For ex- amples, Su et al.[15]reported a high performance self-powered UV

photodetector of PZT films with excellent responsivity, fast photo- response time, and good stability. Huang et al. [17]revealed a sig- nificant photoresponse in the strained BiFeO3films with tetragonal- rhombohedral mixed phase. Generally, these ferroelectric materials are needed to tailor due to the wide energy band (Eg) gap, so that some composites have been explored by combining ferroelectric with other materials, such as semiconductor and metal nanodots. Pandey et al.

[19] reported an enhanced photocurrent in BiFeO3/ZnO2 hetero- structure due to the polarization dependent interfacial coupling effect.

Sharma et al.[22], revealed a significantly enhanced photoconductivity gain and responsivity in W/BaTiO3photodetector by introducing the W dots to enhance the absorption of UV light.

It is noted that hexagonal rare-earth manganites (RMnO3,R= Sc, Y, Ho, Lu) have a Egof ~1.2–1.5 eV, which meets the requirement for narrowEg[24]. Hexagonal YMnO3(h-YMO), as one representative class of hexagonalRMnO3family, possesses abundant physical properties, such as multiferroicity [25,26], unique ferroelectric vortex domain [27–29], and resistive switching effects [30–32]. Especially, h-YMO also presents a reasonably large spontaneous polarization (Pr~ 10μC/

cm²), which has potential applications in photovoltaic effects, where the photo-generated electron-hole pairs are separated by polarization [33–35]. Han et al.[34]reported a significant polarization switchable

https://doi.org/10.1016/j.ceramint.2018.10.227

Received 10 October 2018; Received in revised form 27 October 2018; Accepted 27 October 2018

⁎Corresponding authors.

E-mail addresses:zengmin@scnu.edu.cn(M. Zeng),luxubing@m.scnu.edu.cn(X.B. Lu).

Available online 29 October 2018

0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

T

PV effects in h-YMOfilm. Thus the photo-generated electron-hole pairs might be circulated through the sample, in turn exhibiting the photo- current, which has a potential application in photodetector. However, the study on UV photodetector of h-YMOfilm is limited.

Under this motivation, we fabricated h-YMO films with different thicknesses deposited on (111) yttria-stabilized zirconia (YSZ) sub- strates, and the high quality epitaxy with the out-of-plane h-YMO (0001)//YSZ(111) and in-plane h-YMO[1230]//YSZ[110] relationship was characterized. Then, transparent Al doped ZnO (AZO) top elec- trodes were deposited on the h-YMOfilm surface, and UV light derived photoresponse based on the planar h-YMO photodetectors was ex- plored. It was revealed that the photodetector with 160 nm YMOfilm has the highest photoconductivity gain (~387.4) at + 4 V bias. The result means a new possibility for the photodetector application based on h-YMOfilm.

2. Experimental procession

YMOfilms with different thicknesses (25, 50, 100, 160 and 240 nm) were grown on the YSZ(111) substrates by pulsed laser deposition (PLD) using a KrF excimer laser (248 nm wavelength). The deposited temperature was 800 °C with an oxygen pressure of 5 Pa. Next, the deposited films were in-situ sintered at 600 °C for 30 min with high oxygen ambient of 100 Pa. Finally, AZO top electrodes (~20 nm thickness) with planar configuration were deposited on the YMOfilms by PLD through a shadow mask. The deposition was performed at room temperature with a high vacuum of 5 × 10⁻⁴Pa.

The crystal structure and epitaxy of the YMO/YSZ(111) hetero- structures were characterized by combining X-ray diffraction (XRD, PANalytical X′Pert PRO) and high-resolution transmission electron microscopy (HRTEM, JEOL 2011). The cross-sectional images were obtained by scanning electron microscopy (SEM, Zeiss Ultra 55). The surface topographies were examined by atomic force microscopy (AFM, Asylum Cypher). The optical absorption spectra were recorded with a UV/VIS spectrometer (Lambda 950, PerkinElmer). The current-voltage (I-V) characteristics were measured with a Keithley 6430 source meter.

An ultraviolet (UV) light-emitting diode (LED) with power density (52 mW/cm²) was used as a light source in the photocurrent mea- surements.

3. Results and discussion

Fig. 1(a) shows the X-ray diffraction (XRD) θ-2θspectra of YMO films with thickness of 25, 50, 100, 160 and 240 nm grown on YSZ(111) substrates. Except for the YSZ(111) reflection, the only diffraction peaks of the (000l) crystal planes from YMOfilm are observed without

any detectable impurity phase. This means that these YMOfilms are of quite high crystalline quality with YMO(0001)//YSZ(111) out-of-plane texture. To further confirm the in-plane epitaxy, a φ-scan was per- formed for a selected YMOfilm with thickness of 160 nm by keeping the Bragg angles at (1012) for the YMOfilm and (113) for the YSZ sub- strate. As displayed inFig. 1(b), the YMOfilm presents 60° inter-spaced peaks, implying a six-fold symmetry. Although the YSZ substrate only shows 120° inter-spaced peaks, it is also a six-fold symmetry since the YSZ substrate is cut along (111) plane of cubic structure. It should be noted that the in-plane symmetry of the YMOfilm is rotated by ~30°

with respect to the YSZ(111) substrate, implying that the in-plane epitaxial relationship should be YMO[1230]//YSZ[110], as schemati- cally illustrated inFig. 1(c). From the XRD data ofFig. 1(a) and (b), the lattice parameters of YSZ(111) substrate are verified to be a=b ~ 3.62 Å and c = 2.95 Å, corresponding to the cubic YSZ structure (aYSZ(cubic)= 5.13 Å)[36]. The lattice parameters of h-YMO film are evaluated to be a =b~ 6.11 Å andc = 11.69 Å, which is in good agreement with the h-YMO single crystal (a =b= 6.14 Å andc = 11.44 Å)[27,29]. In this arrangement (an in-plane rotating angle of 30°

between the h-YMOfilm and the YSZ(111) substrate), seeFig. 1(c), the distance denoted by L corresponds to 3.ah-YSZ(6.27 Å), matches well with the distanceah-YMO, with a lattice mismatch of ~−2.1%. It is thus concluded that the YMOfilm is subjected a tensile strain due to the crystal lattice mismatch between the YMOfilm and the YSZ(111) sub- strate.

Fig. 2(a) shows the cross-sectional SEM images of a selected YMO film with the thickness of 160 nm. The image reveals a bilayer structure with clear interface. To gain better insight of the lattice quality of the YMO/YSZ heterostructure, a cross-sectional TEM observation was per- formed. As demonstrated inFig. 2(b), the YMO/YSZ interface is iden- tifiable, see the white dot line. The clear lattice image of YMOfilm implies a good crystalline quality. Their epitaxial relationship can be further verified by selected area electron diffraction (SAED), and the results are illustrated in theFig. 2(c) and (d) for YSZ substrate and the YMOfilm, respectively. The two diffraction patterns are definitely in- dexed. Note that the normal directions of YMO(0003) and YSZ(111) planes are in parallel as marked by white dot lines, evidencing an ex- cellent epitaxial property. Moreover, the high resolution TEM image of YMOfilm displayed inFig. 2(e) evaluates the lattice parametercto be 11.2 Å, which is consistent with the result measured by XRD.

Prior to device fabrication, the optical performance of h-YMOfilm deposited on YSZ(111) substrate was investigated, and the results are plotted inFig. 3for a representative h-YMOfilm with the thickness of 160 nm. From the optical reflectivity, seeFig. 3(a), it can be seen that the h-YMOfilm exhibits low reflectivity of ~20% between 300 and 700 nm wavelength. Note that the transmittance presents two valleys in

Fig. 1.(a) XRDθ-2θscans of YMOfilms with thickness of 25, 50, 100, 160 and 240 nm grown on the YSZ(111) substrates. (b) Theφ-scan of the h-YMO(1012) and YSZ(113) for the sample with thickness of 160 nm. (c) Schematic diagram of epitaxial relationship of the h-YMOfilm grown on YSZ(111) substrate.

Fig. 2.Cross-sectional (a) SEM and (b) TEM images of h-YMO(0001)//YSZ(111) heterostructure. The selected area electron diffraction patterns for (c) YSZ substrate and (d) YMOfilm. (e) The high resolution TEM image of YMOfilm.

Fig. 3.(a) Reflection, (b) transmission, and (c) absorption spectra of h-YMOfilm with the thickness of 160 nm. (d) The dependence of (αhυ)²on the photon energy of h-YMOfilm.

365 nm and 720 nm wavelength, see Fig. 3(b). Correspondingly, the absorption coefficient (α), seeFig. 3(c) exhibits two peaks in 365 nm and 720 nm. It is thus concluded that h-YMOfilm has strong absorption on UV light. To illustrate the opticalEgof h-YMOfilm, an evaluation was performed by the formula[7,34]: (αhν)²= A(hν-Eg), where A and hνare the constant and the photon energy, respectively.Fig. 3(d) de- pictes (αhν)²as a function ofhν. The evaluatedEgcan be obtained by extrapolating (αhν)²to zero, see the red reference line, and the value is

~1.54 eV, which is consistent with the previous experimental and theoretical reports[33,34].

Next, to study the UV detector application of h-YMO films, the planer photodetectors were fabricated by depositing the transparent AZO top electrodes on thefilm surface. Fig. 4(a) illustrates the sche- matic diagram of the planer photodetector. The AZO planer electrodes are 5 mm in length and 500 µm in width, and the electrode spacing is 200 µm. The photocurrent measurements were performed under UV illumination with a wavelength of 365 nm and an intensity of 52 mW/

cm². Fig. 4(b) and (c) show the dark and light illumination current- voltage (I-V) curves, respectively, for all the photodetectors within a voltage range of ± 4 V. In dark state,Ikeeps a low value of ~10 pA@

4 V for all the detectors except an abnormal in 240 nm YMOfilm. As for light state,Iis significantly enhanced. It is well-known that h-YMO is a p-type semiconductor (Eg~1.5 eV) [33,35]. Under light illumination with sufficient energy, the electron-hole pairs can be generated, in which the photo-generated holes can be trapped by defects, such as oxygen vacancies[35], so that the photo-generated electrons are left as the free electron and circulated to the electrodes before they recombine with the trapped holes. Thus, a large photocurrent can be achieved. The maximum photocurrent is ~300 pA observed at + 4 V bias for the 160 nm h-YMOfilm.

Fig. 4(d) plots the time-resolved photoresponses for all the photo- detectors by using the applied voltage of + 4 V and the 365 nm UV light illumination. The measured current before the UV illumination is in- dicated asIoff. When the UV light is turned on, the measured current is denoted asIon. Obviously, the photocurrent of all the photodetectors is

increased rapidly once the UV light is turned on, and then maintains a constant value (Ion). It returns abruptly to the initial state (Ioff) once the illumination is switched off. These results indicate that all the photo- detectors show repeatable and stable responses on the light illumina- tion. One main factor of evaluating the performance of a UV photo- detector is the photoconductivity gain (G), which can be defined as: G

=Ion/I_off. The G are 9.9, 12.5, 9.9, 387.4 and 5.8 for the h-YMO devices with the thickness of 25, 50, 100, 160 and 240 nm, respectively.

Clearly, the photoresponse is thickness-dependent. Similar conclusion has been revealed by Han et al. [33] who reported the optimized thickness is ~150 nm in studying the photovoltaic effect of h-YMOfilms with different thicknesses. Moreover, thefilm thickness has an effect on photo-generated carrier lifetime. Huang et al.[17]has pointed out that, when the thickness of afilm exceeds a critical value, the photo-gener- ated carrier recombination increases and the lifetime decreases, re- sulting in photocurrent decrease. Although the optimized G value (~387.4) in h-YMO detectors is low compared with the traditional ferroelectric photodetecors, such as BTOetc. where the G is about 800 [22], it is much larger than some semiconductor devices, such as ZnO, AZO and GZOetc, where the G is 67, 29 and 52[5], confirming that the h-YMOfilm has a potential application in the UV detector.

Note that the thickness of afilm has a significant effect on the light absorption. To reveal their relationship, we carried out the light ab- sorption tests for all the devices with different thickness h-YMOfilms, and the results are displayed inFig. 5. Obviously, the light absorption of h-YMO films is thickness-dependent. Similar to the photoresponsece, theαvalue at UV (~365 nm)first gradually increases and then slightly decreases with increasing thickness, and the optimizedαis obtained in the 160 nm YMOfilm. This means that the thickness has a vital effect on photoresponsece. Except the thickness, actually, the declined α and photocurrent in the relatively thick film can be attributed to many factors. One is the surface roughness of YMOfilms with various thick- nesses is somehow different. Another is the thickness-dependent defects in thefilms might play a role on the circulation of photo-generated carrier along with photocurrent since the thicker of an oxidefilm is, and Fig. 4.(a) Schematic of UV photodetector based on AZO/YMO/YSZ structure. The current-voltage (I-V) characteristics under (b) dark and (c) illumination for all the photodetectors. (d) Time-resolved photoresponse characteristics of all the photodetectors by using the applied voltage of + 4 V and 365 nm UV light illumination.

the more defect it has.

4. Conclusions

In summary, high quality epitaxial h-YMO(000l) films with dif- ferent thicknesses were prepared on YSZ(111) substrates by PLD, and the epitaxy with out-of-plane h-YMO(0001)//YSZ(111) and in-plane h- YMO[1230]//YSZ[110] was characterized by using X-ray dif- fractometers and transmission electron microscopy. Optical measure- ments showed that the h-YMOfilms have the strongest absorption on UV light. Then planar photodetectors based on the AZO/YMO/YSZ structure were fabricated, and the photocurrent measurements revealed that all the devices show repeatable and stable photocurrent responses.

The optimized photodetector with G ~ 387.4 was observed at +4 V bias for the 160 nm thicknessfilm. These results provide a guideline of hexagonal rare-earth manganites to work for UV photodetector appli- cation.

Acknowledgements

This work was supported by the National Key Research Program of China (No. 2016YFA0201004), the National Science Foundation of China (Grant Nos.: 51332007, 51431006, and 11574091), the Natural Science Foundation of Guangdong Province of China (No.

2015A030313375). X. Lu thanks for the support from the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016).

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Fig. 5.The absorption spectra of all the h-YMO photodetectors.