Elsevier Editorial System(tm) for 43rd International Conference on Metallurgical Coatings and Thin Films Manuscript Draft
Manuscript Number: ICMCTF_2016-D-16-00030
Title: Ultraviolet Photodetectors Based on MgZnO Thin Film Grown by RF Magnetron Sputtering
Article Type: Full Length Article Section/Category: Symposia F and G
Keywords: ZnO, MgZnO, magnetron sputtering, photodetector Corresponding Author: Dr. Chuanpu Liu,
Corresponding Author's Institution:
First Author: Jr-Shiang Shiau, M.D.
Order of Authors: Jr-Shiang Shiau, M.D.; Chuanpu Liu, doctor; Jow-Lay Huang, doctor
Abstract: We demonstrate the growth of high quality, single phase, wurtzite MgxZn1-xO thin films on p-type Si (111) substrate by magnetron sputtering using Mg0.3Zn0.7O as target. The films are highly oriented along the c-axis and have nanorod like morphology and no buffer layer is used for the growth. The Mg content of MgxZn1-xO alloys can be varied in a large range (40.7-51 at.%) by changing the substrate temperature from 25 to 250 ºC. The X-ray diffraction analysis reveals a phase
transformation from hexagonal to cubic phase when substrate temperature is above 150 ºC. The heterostructures of MgZnO/Si are fabricated into metal-semiconductor-metal photodetectors. The sensitivity is as high as 3126 % at 2 V bias under 325 nm laser at relatively low illumination intensity (2.77 mW) and the output photocurrent increased with an increase in the UV illumination intensity at both -10 and +10 V biased voltage. The photocurrents are enhanced with the increase in illumination intensity. The peak responsivity of 4.6 A/W is achieved at 292 nm with a cutoff wavelength of 305 nm and a bias voltage of 9 V.
Suggested Reviewers: Michael Stüber Technology, Karlsruhe
He is the session chair of the Functional oxide and Oxynitride Coatings.
I think he is expert in this field.
Zengxia Mei
Beijing National Laboratory for Condensed Matter Physics, Physics [email protected]
He had published the review paper of semiconductor ultraviolet
photodetectors based on ZnO and MgxZn1−xO. I think he is very familiar with ZnO and MgZnO.
Supab Choopun Physics , Maryland [email protected]
He had published lots of papers about MgZnO which had high number of citations.
Date:
Dr. Chuan-Pu Liu, Professor,
Department of Materials Science and Engineering
National Cheng Kung University No.1, University Road, Tainan City 701, Taiwan (R.O.C.)
Tel: +886-6-2757575 ext 62943 Fax: +886-6-2346290
E-mail: [email protected]
Dear Editor
Please find our enclosed manuscript entitled “Ultraviolet Photodetectors Based on MgZnO Thin Film Grown by RF Magnetron Sputtering” for possible consideration of publication in Thin Solid Films.
In this manuscript, We demonstrate the growth of high quality, single phase, wurtzite MgxZn1-xO thin films on p-type Si (111) substrate by magnetron sputtering using Mg0.3Zn0.7O as target. The films are highly oriented along the c-axis and have nanorod like morphology and no buffer layer is used for the growth. The heterostructures of MgZnO/Si are fabricated into metal-semiconductor-metal photodetectors. The peak responsivity of 4.6 A/W is achieved at 292 nm with a cutoff wavelength of 305 nm and a bias voltage of 9 V.
We believe that this is our original work and all the authors have read this paper and agree to this statement of originality. Any advice assisting the publication of report is highly appreciated. Looking forward to your response and thanking you.
Sincerely Yours,
Chuan-Pu Liu
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Ultraviolet Photodetectors Based on MgZnO Thin Film Grown by RF Magnetron Sputtering Jr-Shiang Shiau
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan.
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Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan.
Chuan-Pu Liu
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1. We have deposited the nanorod like MgZnO thin film with high Mg content maintain Wurtzite phase and no buffer layer.
2. The Mg content of MgxZn1-xO alloys can be varied in a large range by changing the substrate temperature.
3. The photoresponsivity result reveal that the photodetector can detect the UV-B region.
*Research Highlights
Ultraviolet photodetectors based on MgZnO thin film grown by RF Magnetron
sputtering
Jr-Shiang Shiau1, Chuan-Pu Liu1, Jow-Lay Huang1, 2, 3
1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan
701, Taiwan.
2 Department of Chemical and Materials Engineering, National University of Kaohsiung,
Kaohsiung 81148, Taiwan.
3 Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan
701, Taiwan
We demonstrate the growth of high quality, single phase, wurtzite MgxZn1-xO thin films on p-type Si (111) substrate by magnetron sputtering using Mg0.3Zn0.7O as target. The films are highly oriented along the c-axis and have nanorod like morphology and no buffer layer is used for the growth. The Mg content of MgxZn1-xO alloys can be varied in a large range (40.7-51 at.%) by changing the substrate temperature from 25 to 250 ºC. The X-ray diffraction analysis reveals a phase transformation from hexagonal to cubic phase when substrate temperature is above 150 ºC. The heterostructures of MgZnO/Si are fabricated into metal-semiconductor- metal photodetectors. The sensitivity is as high as 3126 % at 2 V bias under 325 nm laser at
*Manuscript
relatively low illumination intensity (2.77 mW) and the output photocurrent increased with an increase in the UV illumination intensity at both -10 and +10 V biased voltage. The photocurrents are enhanced with the increase in illumination intensity. The peak responsivity of 4.6 A/W is achieved at 292 nm with a cutoff wavelength of 305 nm and a bias voltage of 9 V.
Keyword: ZnO, MgZnO, magnetron sputtering, photodetector
Introduction
Ultraviolet (UV) photodetectors (PDs) have attracted enormous attention in different fields of industrial, biological and commercial applications, such as engine control, water sterilization, pollution monitoring, flame sensing and early detection of missile [1]. In the last three decades, the new generation of wide-band-gap (WBG) semiconductors (such as, ZnO, AlN and GaN) are under serious consideration, and more solid state UV PDs have been devised based on high- quality WBG semiconductors for applications in UV detection areas [2].
ZnO has been regarded as one of the most promising candidates for UV photodetectors because of its strong radiation resistance, wide band-gap energy (~3.3 eV) and high chemical stability. This is an important semiconductor material that is widely utilized for optoelectronic devices such as photodetectors, nanogenerators, photovoltaic cells, and light emitting diodes [3]. Mollow et al [4], in the 1940 have observed the UV photoresponse in ZnO films for the first time. The ZnO based photodetectors have been researched and gradually prospered since
the 1980 [5]. In the early, the devices with a lot of defect causing poor properties because the devices’ process technology are not mature. With the development of the technology, many
ZnO based photodetectors with intricacy structure (such as MSM, Schottky junction and p-n junction) and excellent properties have been reported [6]. Recently, solar-blind (200-280 nm) photodetectors have received much attention because of their multiple applications [7], which are typically based on wide-bandgap semiconductors such as diamond, AlGaN, Ga2O3, LaAlO3
and MgZnO[8]-[9].
MgZnO is an ideal material for solar-blind photodetectors because of its unique properties such as high radiation hardness, large tunable band gap range (3.3-7.8 eV), intrinsic visible blindness, availability of lattice-matched single-crystal substrates, environmental friendly and relatively low film growth temperatures [10]. The saturated vapor pressure of Mg (Zn) is 377 ºC (292 ºC) at 10-3 torr that facilitate for the formation of good quality MgZnO films. The MgZnO alloy band gap can be tuned by increasing the Mg content from MgxZn1-xO (x=0, 3.3eV) to MgxZn1-xO (x=1, 7.7eV). However, according to the binary phase diagram of ZnO- MgO system, the stable wurtzite MgxZn1-xO alloy with limitation x of 0.04 due to the thermodynamic solubility of the phase diagram is ~4 mol% [11]. In spite of the solubility limitation, an enhancement in the deposition techniques has realized the wurtzite MgZnO alloy with high Mg content up to 36% [12]. The Mg content should be at least over 40 at.% for the solar-blind photodetectors based on MgZnO (4.42eV), but there have been a lot of studies that shows that the maximum band gap of Mg composition (x=0.36) is 4.28 eV due to the large difference of structure between ZnO (wurzite, a = 3.25 Å , c = 5.2Å ) and MgO (cubic, a= 4.25 Å ). However, when Mg content is higher than 36%, cubic MgO phase segregate and cubic MgZnO is obtained, when Mg content exceeds 62% with tunable band gap from 5.4 eV (x=0.62) to 7.8eV (x=1, MgO) [13].
In this paper, the metal-semiconductor-metal (MSM)-structured photodetectors is fabricated based on MgZnO thin film grown on p-type Si(111) by precision etching-coating system (PECS). The electrodes are gold with interdigital design. The high quality, single
crystalline, MgxZn1-xO nanostructured thin films are deposited by magnetron sputtering without using any buffer layer. The UV response of the photodetector demonstrates the photosensing performances by applying 325 nm laser with various intensities. The optoelectronic results reveal that the photodetector have good performance with high photo-response at 292 nm and high sensitivity with Mg content is 43.7 at.%.
Experimental methods
The MgZnO alloys were grown on a p-type silicon (111) substrates under various substrates’ temperature (25, 100, 150, 200, 250 ºC) to modify the Mg content and thin film’s
crystallinity by rf magnetron sputter deposition technique. In the sputtering system, the target was Mg0.3Zn0.7O (99.99% purity, 2 in. in diameter) in the RF mode. The pure ZnO (99.99%
purity, 2 in. in diameter) also grown with the same conditions. The silicon (111) substrates (3×1 cm2) were cleaned consecutively in acetone, ethanol and deionized water for 10 minutes. The substrate to target distance was about 5.0 cm and the target was pre-sputtered for 3 min to remove surface contaminations (if any). The chamber was evacuated to a base pressure less than 10−5 tor and then sputtering gas (argon) was allowed to enter into the chamber (argon flow rate is kept at 30 sccm). The substrate rotation speed is control at 15 rpm. All the films were prepared with a sputtering power of 100 W, and the film thickness was controlled at 500~800 nm by controlling the deposition time.
X-ray diffractometer (Bruker D8 DISCOVER) and scanning electron microscopy (SEM, Hitachi SU-8000) equipped with energy dispersive X-ray spectroscopy (EDS) was used to examine the morphology and phase composition of the MgZnO thin films. Room temperature transmission and absorption spectrum were recorded using a UV-visible spectrophotometer (JASCO V-600).
A MSM Schottky type photodetector was fabricated on the ZnO and MgZnO films (Fig.
1). Interdigital (IDT) Au electrodes were fabricated by precision etching & coating system
(PECS) followed by the mask screened on the thin film. The finger is 2 mm in length and 250 μm in width, and the spacing between the fingers is 150 μm. The thickness of the gold layer
was about 20 nm. The devices were packaged by polydimethylsiloxane (PDMS) a kind of polymers to prevent the corrosion or oxidation by the vapor from environment. The thickness of the PDMS is thinner than the thickness of the silicon substrate. The UV response of photodetector demonstrate the photosensing performances by applying different illumination intensities 325 nm laser. The photo-responsivity spectra were measured by a monochromator and an Xe lamp source.
Result and discussion
Figure 2 shows the θ-2θ X-ray diffraction (XRD) spectra of the ZnO and MgxZn1-xO (x = 40.7, 41.6, 43.7, 47.6, 51 at.%) nanostructured thin films grown on silicon substrates. The films are characterized as hexagonal wurtzite phase with strong c-axis-preferred orientation for both
ZnO and MgZnO films. As Mg content in ZnO lattice increases, the (002) diffraction peak shifts gradually to the higher bragg angle from 34.29° to 34.68° and that is ascribed to the lower
ionic radius of Mg2+ (0.057nm) as that with Zn2+ (0.06nm) [14]. With the increase in the the substrate temperature the Mg concentration increases which is attributed to the lower vapor pressure of cubic phase MgO as that with ZnO[15]. When the substrate temperature increases, the ZnO will re-evaporation led the Mg content increase. As the Mg content is higher than 43.7 at. %, there is a sharp decrease in the intensity of (002) peak, and the (002) diffraction peak shifts from 34.68º to 34.54º. This may be due to the formation of second phase of cubic MgZnO [16] with the appearance of (200) peak (42 º ) of cubic phase MgZnO (200) [17].
The high crystal quality and the strong texture of MgZnO (40.7 at.%) and MgZnO (43.7 at.%) films have also been confirmed by the cross-section and top-view SEM image (Fig. 3).
The MgZnO film grows with perfect nanorod-like crystal structure (Fig. 3a, c) which can also prove the film has strong preferred orientation with c-axis perpendicular to the substrate. The MgZnO with Mg content (40.7 at.%, 43.7 at.%) shows nanorod-like morphology with a uniform thickness (600 nm), but the grain size decreased with incrase in Mg concentration. The nanorod- like structure is expected to show high photosensitivity because of the large surface-to-volume ratio and the presence of deep level surface trap states that can prolong the photo-carrier lifetime.
In nanorod-like structure devices, reducing the dimensional of the active area cut down the carrier transporting time[18]. According to many reports, it was found that a high deposition rate for PVD processes would be ensured for the deposition of 1D nanostructures[19]. The
deposition rate is generally related to the working pressure due to the kinetic energy of the collisions between sputtered atoms and ambient gas. The deposition rate can be reduced by decreasing the working pressure[20]. The top-view (Fig. 3b, d) shows that the film exhibits dense grains and smooth surface, which is also confirmed by the atomic force microscopy.
Figure 4 shows the atomic force microscopy image shows the dense and smooth surface of MgZnO (40.7 at.%) and MgZnO (43.7 at.%) films deposited on silicon substrate. The average surface roughness of the pure ZnO films was around 4 nm, which decreased to 2 nm with increasing substrate temperature (T<200oC). The atoms will have enough energy for rearrangement to form smooth surface when substrate temperature increases. However, when temperature increase to 200 ºC there exist two phases of MgZnO films appear that cause the lattice distortion and increase the average surface roughness to 15nm. The average surface roughness will affect the electrodes covering and contact.
The typical current-voltage (I-V) characteristic of the MgZnO (40.7 at.%) and MgZnO (43.7 at.%) IDT-Schottky photodetector in the dark and under 325 nm laser illumination at various intensities were measured and shown in Figure 5(a, c). The output photocurrent
increased with an increase in the UV illumination intensity at both -10 and +10 V bias voltage.
Within the limit of hν>Eg, the higher intensity influenced increment in the generation rate (G)
of electron-hole pairs and defined by G =αF(1-R)e-αx where, F is the incident photon flux, R is the surface reflectivity, and α is the absorption coefficient [21]. Under fixed wavelength illumination, the generation rate is only determined by the incident photon flux density (F),
where an increase in F will improve the electron-hole pair generation rate. For example, for the MgZnO (40.7 at.%) at +2V, the current increases from 0.112 μA (dark) to 37.3 μA (2.77 mA cm-2), and further to 154 μA (38.7 mA cm-2). For the MgZnO (43.7 at.%) at +2V, the current increase from 0.913 μA (dark) to 29.4 μA (2.77 mA cm-2), and further to 395 μA (38.7 mA cm-
2). The MgZnO (40.7 at.%) have the lower dark current and lower photocurrent as that with
MgZnO (43.7 at.%). The photocurrents (Iph=Ilight-Idark) dependence of illumination intensity were calculated and plotted in Figure 5(b, d). When the illumination intensity is increased the photocurrents also enhaced without saturation. The sensitivity (Iph/Idark) of MgZnO (43.7 at.%) at +2V was calculated to be 3126 % under 2.77 mA cm-2 and 325nm laser illumination. The MgZnO (43.7 at.%) have a better performance than MgZnO (40.7 at.%) photodetector.
The spectral response of IDT-Schottky photodetector under different bias range of 4-9 V is plotted in figure 6. The response spectrums maintain stable characteristics without any blue shift or red shift when the bias is increased. The maximum photo responsivity of IDT-Schottky photodetector is 4.6 A/W at 292 nm and the cutoff wavelength is about 305 nm with 9 V bias.
The UV (292 nm)-to-Visible (400 nm) rejection ratios of the photodetector is more than two orders, indicating a good performance of photodetector that will not influence by visible light.
The MgZnO (43.7 at.%) IDT-Schottky photodetector exist very high photo responsivity than previously published reports [22]. Besides the photon-generated carriers’ contribution, the photodetector have a high response because the nanorod like structure have large surface-to- volume ratio. In the dark environment, oxygen will adsorb on MgZnO nanorod surface with the
oxygen ionization reaction [O2 + e− → O2−][23]. When this adsorption occur free electrons are depleted, the depletion layer will be created that decrease the conductivity near the film surface.
As UV light illuminated on the device, the electron-hole pairs will be generated. The holes will move toward nanorod surface discharging the adsorbed oxygen ions [h++ O 2−→O2 (g)] and the unpaired electrons be collected by the electrodes and leading to the increase in surface conductivity. At the same time, with the accumulation of excess electrons, oxygen will be readsorbed and reached a new equilibrium. Turning off the UV light would facilitate further recombination of the electrons and holes. This process can be slow due to the hole trapping effects at the surface, mitigating the re-adsorption of oxygen. Oxygen adsorbed and desorbed in MgZnO nanorod like structure cause the hole-trapping mechanism that augments the high density of trap states usually found in NWs due to the dangling bonds at the surface and thus enhances the NW photoresponse[24].
Conclusions
In summary, a IDT-Schottky photodetector and high quality MgxZn1-xO thin films without buffer layer exist the preferred orientation (002) direction have been grown on silicon substrate
by RF-magnetron sputtering. The Mg content x (40.7–51 at.%) can be controlled by changing substrate temperature (25-250℃). The phase transformation from hexagonal to cubic phase
happen when substrate temperature is increased to 200 ºC (Mg 47.6 at.%). Optoelectronic analysis indicates that the MgZnO (43.7 at.%) have the best crystallinity and dense surface. A solar-blind photodetector had been fabricated with IDT-Schottky type photodetector and the
MgZnO (43.7 at.%) have the best performance of sensitivity and photo-response. The sensitivity was as high as 3126 % at 2V bias under 325nm laser relatively low illumination intensity (2.77mW). A peak responsivity of 4.6 A/W at 292nm with a cutoff wavelength of 305nm under 9 V. Based on these results, we predict that the MgZnO thin film photodetector with high Mg content can apply in deep UV region.
Acknowledgements
This project was financially supported by the Ministry of Science and Technology of the ROC under contract No. MOST 104-2221-E-006-032-MY3.
References
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Figure caption
Figure 1. Schematic illustration of MSM structure MgZnO photodetector.
Figure 2.θ-2θ X-ray diffraction pattern of ZnO and MgZnO films on p-type silicon(111) substrates.
Figure 3. SEM images of MgZnO (40.7 at.%) and MgZnO (43.7 at.%) films cross-section (a) (c) and top-
view images(b) (d).
Figure 4. AFM micrographs of MgZnO (40.7 at.%) and MgZnO (43.7 at.%) thin films.
Figure 5. I-V characteristics of the MgZnO (40.7 at.%) and MgZnO (43.7 at.%) IDT-Schottky photodetector
at dark and under various 325nm laser illumination intensities (a) (c). Photocurrent (Iph=Ilight-Idark) dependence of illumination intensity at 2V bias (b) (d).
Figure 6. The spectral responsivity of the MgZnO (43.7 at.%) IDT-Schottky photodetector at different bias
voltage.
Table caption
Table 1. The Mg concentration and substrate temperature of ZnO and MgZnO thin films on p-type silicon substrate.
Target Substrate temperature Mg content
ZnO RT 0 at. %
MgZnO RT 40.7 at. %
MgZnO 100℃ 41.6 at. %
MgZnO 150℃ 43.7 at. %
MgZnO 200℃ 47.6 at. %
MgZnO 250℃ 51 at. %
Table. 1
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