Ultraviolet photodetectors with Ga-doped ZnO nanosheets structure

    Ultraviolet photodetectors with Ga-doped ZnO nanosheets structure Sheng-Joue Young, Yi-Hsing Liu PII: DOI: Reference:

S0167-9317(15)30025-3 doi: 10.1016/j.mee.2015.07.009 MEE 9973

To appear in: Received date: Revised date: Accepted date:

29 May 2015 6 July 2015 25 July 2015

Please cite this article as: Sheng-Joue Young, Yi-Hsing Liu, Ultraviolet photodetectors with Ga-doped ZnO nanosheets structure, (2015), doi: 10.1016/j.mee.2015.07.009

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ACCEPTED MANUSCRIPT Ultraviolet photodetectors with Ga-doped ZnO nanosheets structure

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Sheng-Joue Young*and Yi-Hsing Liu Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan

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Abstract

In this study, Ga-doped ZnO (GZO) nanosheets were grown on a glass substrate by

using

aqueous

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method.

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GZO

nanosheet

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metal-semiconductor-metal (MSM) ultraviolet (UV) photodetector (PD) was

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also fabricated. The average length and diameter of the GZO nanosheets were 2.06 μm and approximately 20 nm, respectively. The EDX spectrum determined

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that the Ga-doped sample contains approximately 1.35% at%. The UV-to-visible

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rejection ratio of the device is approximately 42 when biased at 1 V, and the fabricated UV PD is visible-blind with a sharp cutoff at 370 nm. The

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photo-current and dark-current constant ratio of the fabricated PD was approximately 14193 when biased at 1 V. The transient time constants measured

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during the rise time and fall time were 2.45 s and 4.0 s, respectively.

Keywords: Ga-doped ZnO; photodetector; Metal-Semiconductor-Metal * Corresponding author. Tel.: +886 5 6315560; fax: +886 5 6315643. E-mail address: [email protected] (S. J. Young).

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Introduction

In recent decades, Zinc Oxide (ZnO) is an important II-VI semiconductor. It is considered to be the most prominent semiconductor for potential applications

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such as space research, environmental monitoring, UV radiation calibration and

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monitoring, flame sensing, chemical and biological analysis, missile launching detection, astronomical studies, and optical communications [1]. Therefore, it has attracted much interest of research community. The synthesis of ZnO

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nanostructures have been developed to fabricate, such as thermal chemical vapor deposition [2], hydrothermal method [3], pulsed laser deposition [4], and

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metal-organic chemical vapor deposition [5]. The chemical solution method has become a promising approach for the large scale production of nanomaterials

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because of its simple, fast, economical and low growth temperature

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characteristics [6]. It is known that both the highly optical and electrical

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properties of ZnO nanostructures can be achieved by replacing Zn2+ ions by other ions such as In3+, Al3+ and Ga3+. Among the dopants, owning to the fact that atomic radius of Ga3+ (0.062 nm) is similar to Zn2+ (0.074 nm) [7-9].

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In this work, the aqueous solution method was utilized to synthesize Ga-doped ZnO nanosheets on a glass substrate. Then, UV PDs with metal-semiconductor-metal structure were fabricated. Their optical and photoelectric properties were also studied. II. Experimental Fig. 1 shows the device structure of GZO-based UV PD presented in this work. First, a ~25 nm ZnO film was deposited on the glass substrate as seed layer by radio frequency magnetron sputtering system. Subsequently, a thick Au film with 100 nm was deposited by e-gun evaporation through an interdigitated 2

ACCEPTED MANUSCRIPT shadow mask onto the ZnO seed layer to form contact electrodes. This interdigitated shadow mask was designed to have a finger width of 4.5 mm and a finger length of 5 mm. The spacing between the neighboring fingers was kept at

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0.15 mm. Then, the ZnO seed layer coated on a glass substrate was immersed in

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aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2• 6H2O, 0.1M), sodium

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hydroxide (NaOH, 0.4M) and Gallium nitrate hydrate (Ga(NO3)3•xH2O, 0.001M), for 1 hour. Finally, the substrate with GZO nanosheets structure was washed with deionized water. The growth method was similar to the hydrothermal process.

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The morphologies of the synthesized materials were characterized by using

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high resolution scanning electron microscopy (Hitachi S-4800I), and an X-ray diffractometer (XRD) was used to characterize the crystallographic and structural properties of the as-grown GZO nanosheets. A Keithley 2410 semiconductor

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parameter analyzer was then used to measure the I-V and photodetector characteristic of the fabricated device. The spectral responsivity of the

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photodetector was measured by using a Jobin Yvon-Spex system with a 300 W xenon arc lamp light source (Perkin Elmer PE300BUV) and a standard

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synchronous detection scheme measured at 300 Hz.

Fig. 1 A schematic diagram of the UV PD with GZO nanosheets.

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ACCEPTED MANUSCRIPT III. Result and discussion Fig. 2(a) and 2(b) shows top-view and cross-sectional FESEM images of the as-synthesized GZO nanosheets prepared on a seeded layer glass substrate,

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respectively. It was found that the nanosheets were interwoven with an average

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diameter of approximately 20 nm and an average length of 2.06 μm, respectively,

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and the nanosheets were mostly perpendicular to the substrate surface.

Fig. 2 (a) top-view and (b) cross-section FESEM images of the GZO nanosheets.

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ACCEPTED MANUSCRIPT Fig. 3(a) shows the structural property of GZO nanosheets were measured by X-ray diffraction (XRD). This figure shows that the (002) diffraction peak is

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stronger than other peaks, such as (100), (101), (102), (110), (103), (112), (201),

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and (004), indicating that the ZnO crystals preferentially grow along the c-axis

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direction. The possible formation of Ga2O3 phases is not detected; this implies that Ga atoms may replace Zn atomic sites by being incorporated interstitially or substitutionally in the hexagonal lattices. A sharp XRD peak appearing in the

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nanosheets can be indexed to wurtzite ZnO (JCPDS Card No. 3601451). Fig. 3(b)

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shows the energy dispersive X-ray (EDX) spectrum of the GZO nanosheets, indicating that these nanosheets consist of Zn, O and Ga. The respective Ga

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content of the sample is 1.35%.

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Fig. 3 XRD and EDX spectra measured from the as-grown samples.

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Fig. 4 shows the room-temperature photoluminescence (PL) spectrum of GZO nanosheets. The figure clearly shows peak at about 379 nm and a broad deep

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level emission at 575 nm. The UV emission peak centered at 379 nm and corresponded to the near band edge (NBE) emission and free-excitonic transition recombinations [15], the reported typical peak position is at 380 nm [16, 17]. The

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broad green emission band of the visible region is located at ∼570 nm and can be referred to as the intrinsic defects or oxygen vacancies in the ZnO [18]. Recent

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studies have reported many defects detected in the PL spectra [19–23]. The presence of this peak may be related to the exciton bound to structural defects,

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strain-induced structural defects, incorporation of impurity-induced disorder or surface defects during the growth process.

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Intensity (a.u.)

Excitation:  = 325 nm (He-Cd laser)

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~379 nm UV Emission Peak

Green Emission Band Visible Region 350

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Wavelength(nm)

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Fig. 4 PL spectrum of GZO nanosheets measured at room-temperature.

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ACCEPTED MANUSCRIPT Fig. 5(a) shows the I-V characteristics of the GZO nanosheets sample measured under UV illumination (365 nm). The dark current and photocurrent value at 1V bias changed from 1.44 × 10−8 A to 2.05 × 10−4 A when irradiated UV

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light. Furthermore, the photocurrent to dark current contrast ratio of GZO

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nanosheets PD was approximately 14193. Fig. 5(b) shows the photocurrent rise

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and decay by turning the continuous UV illumination ON and OFF with a 1V applied bias. In this paper, we define the rise time and decay time as the time for the current to rise to 90% of the peak value and the time for the current to decay to

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10% of the peak value, respectively. It can be found that the rise time and fall time

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is around 2.45 and 4.0 s. Fig. 5(c) shows the spectral response of the GZO nanosheets PD measured at applied bias of 1V. The responsivites of the GZO nanosheets PD was measured at 5.75 A/W, Here, we defined the UV-to-visible

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rejection ratio as the responsivity measured at 370 nm divided by the responsivity measured at 450 nm. With this definition, the UV-to-visible rejection ratio of the

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GZO nanosheets PD was 36.1. According to previous report [24-26], nanosheets structure can improve the efficiency of light trapping under light illumination. When the light incident nanorods, it would be increased the photon path length

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and the photocurrent efficiency for improving the light trapping by scattering between free-standing nanosheets. Furthermore, it can facilitate oxygen adsorption and desorption at the nanosheet surfaces. The oxygen molecules can adsorb on the nanosheets surface by capturing free electrons in the dark and desorb from the surface by illustrating UV light which lead to an increase in the free carrier concentration and a reduce of the Schottky barrier height. Thus, from the photodetector performance of this study, it was found that GZO nanosheets PD have better performance than other photodetectors.

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In darkness Under UV illumination (365 nm)

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Current (A)

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

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Voltage (V)

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IV. Conclusion

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Fig. 5 (a) shows the current–voltage (I-V) characteristics of fabricated GZO nanosheets UV PD measured in the dark and in UV illumination (wavelength at 365 nm). (b) The current–time (I-T) characteristic of the fabricated GZO nanosheets UV PD measured in dark and in UV illumination at applied biases of 1 V at RT. (c) Room temperature spectral responses of the GZO nanosheets PD measured at applied bias of 1V.

In summary, the GZO nanosheets UV PDs were fabricated on a glass substrate by using aqueous solution. The average length and diameter were 2.06

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μm and approximately 20 nm, respectively. The doped Ga concentrations was

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1.35 at.% by SEM–EDX analysis. Under 365 nm illumination, the photocurrent-to-dark current contrast ratio of GZO nanosheets PD was

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approximately 14193 with 1V bias. In addition, the proposed GZO nanosheets

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UV PD has a high fast rise/fall time characteristics. The transient time constants measured during the rise time and fall time were 2.45 s and 4.0 s, respectively.

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This method provides a simple and low cost approach for the large scale production of nanomaterials with high photoelectric performance to fabricate UV

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photodetector.

ACKNOWLEDGEMENTS: This work was supported by Ministry of Science and Technology under contract number MOST 103-2221-E-150-034. This work was also supported by National Science Council of Taiwan under contract numbers NSC 102-2221-E-150-046 and NSC 101-2221-E-150-043. Common Laboratory for Micro/Nano Science and Technology of National Formosa University that provided the partial equipment for measurement is acknowledged. The authors would also like to thank the Center for Micro/Nano Science and Technology of National Cheng Kung University for the assistance in device 10

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Graphical Abstract

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ACCEPTED MANUSCRIPT Highlights 1. Ga-doped ZnO nanosheets were grown on a glass substrate.

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2. A GZO nanosheets ultraviolet photodetector was fabricated.

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3. The proposed GZO nanosheets UV photodetector has a high fast rise/fall time characteristics.

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