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p-GaN heterojunction
Accepted Manuscript Title: A Self-powered, Visible-blind Ultraviolet Photodetector based on n-Ga:ZnO Nanorods/p-GaN Heterojunction Authors: Lu Yang, Hai Zhou, Mengni Xue, Zehao Song, Hao Wang PII: DOI: Reference:
S0924-4247(16)31231-6 http://dx.doi.org/doi:10.1016/j.sna.2017.08.007 SNA 10261
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
28-12-2016 29-7-2017 2-8-2017
Please cite this article as: Lu Yang, Hai Zhou, Mengni Xue, Zehao Song, Hao Wang, A Self-powered, Visible-blind Ultraviolet Photodetector based on n-Ga:ZnO Nanorods/p-GaN Heterojunction, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.08.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Self-powered, Visible-blind Ultraviolet Photodetector based on n-Ga:ZnO Nanorods/p-GaN Heterojunction Lu Yang, Hai Zhou,* Cong Ye, Mengni Xue, Zehao Song and Hao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferroelectric and Dielectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, China *Corresponding author: [email protected] (Hai Zhou); [email protected] (Hao Wang) HIGHTLIGHTS
1. An n-Ga:ZnO nanorods/p-GaN heterojunction for UV photodetector was first reported. 2. The device with Ga:ZnO nanorods showed more than 3.2×105 of the Iph/Idark ratio at zero biases and its responsivity reached to 0.23 A/W. 3. The reason for better photoelectric response performance of n-GZO NRs/p-GaN was explored.
Abstract A self-powered visible-blind ultraviolet (UV) photodetector based on n-Ga:ZnO nanorods (GZO NRs)/p-GaN heterojunction was reported, in which the n-type GZO NRs were prepared by the water bath method. In this study, we found n-GZO NRs/p-GaN heterojunction devices showed better performance than those of n-ZnO NRs/p-GaN heterojunction devices in terms of the ratio Iph/Idark, responsivity and detectivity.
Under UV illumination with the light intensity of 1.31 mW/cm2, the
ratio Iph/Idark for the n-GZO NRs/p-GaN heterojunction was as high as 3.2×105 at zero bias, which is about 75 times greater than that of an un-doped device (only 4.2×103).
Also its responsivity reached 0.23 A/W which was three times larger than
that of n-ZnO NRs/p-GaN (~0.08 A/W).
The reason n-GZO NRs/p-GaN
heterojunction displayed lager light/dark current ratio, higher responsivity and detectivity, more reliable self-powered performance at zero bias may be that in GZO 1
NRs, the Ga-doping reduced the defect states in the nanorods, enhanced the conductivity and electron mobility of the ZnO material and benefited the photo-generated carriers transmission.
Keywords—self-powered, visible-blind, photodetector, GaZnO nanorod
Ⅰ.
INTRODUCTION In recent years, self-powered ultraviolet (UV) photodetectors based on
nano-devices and nano-systems have attracted more attention because they can operate without an external bias [1-5]. This self-powered system can be operated wirelessly, independently and continuously.
As a wide band gap semiconductor
material, ZnO is widely used in short wavelength optoelectronic devices due to its direct wide band gap of 3.37eV and large exciton binding energy of 60 meV [6-11]. One-dimensional nanostructures based on ZnO, especially nanowires and nanorods (NRs), have higher crystallinity and larger surface-to-volume ratio than that of the film, which can provide more effective carrier transport and improve sensitivity to light.
Thus, ZnO nanostructures are a popular choice for ultraviolet photodetectors
[11-14].
It is thought, that ZnO is the best choice for the pn homojunction
photodetector.
However, p-type ZnO is difficult to obtain.
Therefore, p-GaN as the
p-type material in ZnO based pn junction photodetectors is a very good alternative material due to its similar crystal structure and band gap energy. Many researchers have reported that doping the ZnO nanostructures can modify device performance [11,15,16].
In n-ZnO NRs/p-Si diodes, with increasing Ga
content, the average diameter of the ZnO nanorods increased while the amount of oxygen vacancies was reduced. In addition, the Ga doped n-ZnO NRs/p-Si diodes showed well-defined rectifying behavior in their I-V characteristics and an improvement in the electrical conductivity (diode performance) [15].
Phan et al [16] 2
reported that the CO sensing properties of ZnO NRs are effectively improved by Ga doping, and the optimal 2% Ga-doped ZnO NRs based CO sensors showed a response factor of 25%, which was a 5-fold increase in sensitivity compared with the undoped devices at 150 °C.
Based on this report, we expect that the Ga-doping in ZnO (GZO)
will reduce defect states in the nanorods, enhance the conductivity of thin film and improve the rectifying behavior of ZnO NRs based heterojunction devices.
Herein,
GZO NRs are prepared through the water bath method and the n-GZO NRs/p-GaN heterojunction was formed.
Ⅱ. EXPERIMENT
Synthesis of GZO nanorod arrays: First, the FTO glass (with area of 20 mm×20 mm, thickness of 2 mm, resistance of 14 Ω, light transmittance of 90%) was rinsed in ultrasonic cleaners for 20min with deionized water, acetone and ethanol, respectively. The glass was then dried nitrogen before removing organics by with a UV cleaning treatment.
Next, a 90nm thick ZnO seed layer was deposited by spin coating on the
FTO glass substrate at a speed of 5000 rpm, followed by annealing at 350℃ for 2 hr in air.
As for the preparation of the GZO NRs array, the FTO glass with the deposited
ZnO seed layer was immersed in an aqueous solution of 0.05M zinc nitrate also containing, gallium nitrate with a molar ratio of 5% and held, at a temperature of 89℃ for 20min. Device assembly: After the GZO NRs were prepared, the sample was covered with p-type GaN firstly.
Secondly two clips were used to fixed the sample.
At last, the
sample was sealed with epoxy resin to prevent the movement of two parts (FTO glass and GaN).
And by optimizing our process, the epoxy resin did not enter between
these two parts. contacts.
Finally, an In electrode was applied on the GaN to form Ohmic
Herein, the p-type GaN was the commercially available p-type Mg-doped
GaN with a thickness of 4.9 μm, a mobility of 10 cm2 V−1 s−1 and a carrier concentration of 6×1016 cm-3. mm2.
The effective area of the p-n heterojunction is ~4× 2
The fabrication process of the GZO NRs based heterojunction is 3
schematically illustrated in Fig. 1. The morphology and crystallinity of the n-GZO NRs and n-ZnO NRs were characterized by field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F) and X - ray diffraction (XRD, D8 FOCUSX - ray diffraction) with Cu Ka radiation at 40kv and 40mA, respectively.
The doping content of Ga in GZO
NRs was analyzed by energy dispersive spectrometer (EDS) and scanning electron microscopy.
The absorbance spectrum of the samples were measured by
UV-VIS-NIR spectrophotometer (MPC-3100 SHIMADZU).
The current-voltage
(I-V) and current-time (I-T) properties were measured by a Keithley 4200 electrometer. In this study, discrete wavelengths of monochromatic light ranging from 254 to 800 nm were obtained by using a 66984 Xe arc source (300W Oriel) in combination with an Oriel CornerstoneTM 260 1/4m Monchromator. To determine the I-V and I-T characteristics, the FTO glass side of the device was illuminated and the light intensity was measured by a UV-enhanced Si detector.
Ⅲ. RESULTS AND DISCUSSION
Figure 2 (a) and (b) display the SEM images of ZnO NRs and GZO NRs grown on FTO substrates, respectively.
The average diameter of the ZnO NRs is about 70nm.
When the ZnO is doped with Ga, on the other hand, the diameter of the nanorods increases to ~ 100 nm.
Figure 2 (c) shows the XRD of the ZnO NRs confirming a
wurtzite crystal structure with a c-axis orientation.
Due to the incorporation of Ga,
the diffraction peak intensity of the GZO NRs decreased [17].
Using the (002) peak
width, we can also calculate the grain size of the ZnO according to the Scherrer formula D K
Bcos
[18-20].
Based on this equation, we calculated that the grain
size of the ZnO NRs was about 55.5 nm, while the GZO NRs have a smaller grain size of about 45.7 nm, which may be due to the different atomic radius of Ga ions [14, 17, 21].
The absorption curves (shown in Fig. 2 (d)) show that the two kinds of
nanorods have a good absorption ability in the UV region.
Also the absorption curve 4
of GZO NRs nanorods shows a slight blue shift of the absorption edge from 380 to 373nm, which may be caused by a small amount of Ga doped into the ZnO lattice that results in an in-crease of the band gap [22, 23]. Herein, the Ga doping in ZnO was confirmed by EDS and from the results we calculated that the doping concentration was about 1.8%. For a photodetector, a lower dark current means a greater ability to detect weak light signals in the background noise.
Fig. 3 (a) shows logarithmic I-V
characteristics of n-GZO NRs/p-GaN and n-ZnO NRs/p-GaN in the dark.
From the
graph, the current of Ga-doped device was lower than that of the device without Ga doping at reverse bias, which may result from a decrease of oxygen vacancies in the Ga-doped nanorods [15]. In addition, the rectification ratio of n-GZO NRs/p-GaN heterojunction is 95.1, which is much higher than that of the n-ZnO NRs/p-GaN heterojunction (~ 8.4).
These characteristics may all be due to Ga doping in ZnO
NRs, enhancing the unidirectional transmission properties of the diode.
Moreover,
the Ga-doped devices show excellent photoresponse characteristics under UV illumination with the light density of 1.31 mW/cm2, as shown in Fig. 3 (b).
As can
be see from the figure, our devices show good photovoltaic performance, based on which we can conclude that our devices can work as self-powered systems.
Also,
the GZO NRs based devices display a higher current than that of the ZnO NRs based devices.
The ratio of light to dark current (Iph/Idark) for both photodetectors is
pictured in Fig. 3 (c). The Iph/Idark ratio increases gradually as the applied reverse bias decreases, reaching a maximum value for the n-GZO NRs/p-GaN heterojunction of ~ 3.2×105 at zero bias which is about 75 times greater than that for the un-doped devices (~ 4.2×103).
This discrepancy makes it clear that Ga-doping can improve
the photoelectric response of the NR devices.
Furthermore, in order to identify the
stability and photoresponse rate of the device, we measured current as a function of time without applied bias under UV radiation in a periodic on-off cycle (T=20 s). From Fig. 3 (d), it can be seen that both devices have good stability and response rates, and also that the GZO NRs based devices have a lager response ratio. 5
The spectral responsivity and detectivity are two of the key parameters used to characterize photodetectors and are shown in Fig. 4 (a) and (b), respectively.
As
seen from the curves in Fig. 4 (a), which are measured at zero bias, the responsivity of the n-GZO NRs/p-GaN heterojunction is clearly larger than that of n-ZnO NRs/p-GaN heterojunction.
Also the peak responsivity value of the Ga-doped
device is at ~ 365 nm, which is attributed to GZO, is ~ 0.23 A/W [24-27].
This is
comparable to our previous results and larger than the value reported by others for ZnO-based self-powered UV PDs (see table 1) [28-32].
The peak value of the
undoped device is ~ 0.08 A/W or about one third the value of the doped device. Moreover, the ratio R350nm/R500nm was calculated to further characterize the visible-blind ultraviolet detection properties of the photodetectors.
The R350nm/R500nm
ratio of the n-GZO NRs/p-GaN heterojunction device reaches 8.4, which is 4.4 times greater than that of the un-doped device.
Fig. 4 (b) shows the spectral detectivity
curves of the two diodes measured at zero bias and the detectivity is determined based on the formula below:
D
R 2qJ dark
where Dλ is spectral detectivity of photodetector, q is electron charge, and Jdark is dark current density.
As seen from the graph, the detectivity of the n-GZO NRs/p-GaN
heterojunction achieves the maximum value of 2.32×1012 Jones (1 Jone=1 cm Hz/W) under UV illumination at 365 nm while the n-ZnO NRs/p-GaN heterojunction device reaches only 0.97×1012 Jones.
On the basis of the above analysis, it appears that the
Ga doping enhances the photoresponse characteristics of the visible-blind UV photodetector. According to the results discussed above, the n-GZO NRs/p-GaN heterojunction device has better performance characteristics than those of the n-ZnO NRs/p-GaN heterojunction in terms of the ratio Iph/Idark, responsivity and detectivity.
The reasons
for this may be attributed to the Ga doping, which may result in the increased carrier conductivity and a decrease in the defect-based carrier concentration in ZnO [33]. When a beam of light irradiates the p-n heterojunction, the electron-hole pairs are 6
generated in the depletion layer if the energy of the photon is larger than the band gap of the semiconductor.
The generated electron-hole pairs and separated by the
built-in electric field, i.e., the elections (or holes) arrive at the FTO electrode (or In electrode) through ZnO NRs (or GaN), resulting in the photocurrent. This is the working mechanism of the self-powered photodetector as it transforms the light signal into an electrical signal.
When a small amount of Ga is doped into ZnO, the
conductivity and the mobility of the nanorods will increase due to the saturation of intrinsic defects, such as O vacancies [15, 34,35]. Thus, the GZO NRs will enable carriers to transport rapidly and reduce the recombination of photo-generated carriers, resulting in a larger photocurrent.
In addition, the defect states in the nanorods will
be reduced by incorporating Ga, which will improve the electrical properties of the nanorods [15].
So through the Ga doping, the n-GZO NRs/p-GaN heterojunction
devices show improved photoresponse performance.
. CONCLUSION
In conclusion, self-powered visible-blind ultraviolet photodetectors based on n-GZO NRs/p-GaN heterojunction were fabricated.
In this study, we found that
n-GZO NRs/p-GaN heterojunction devices showed better performance than those of n-ZnO NRs/p-GaN heterojunction devices in terms of the ratio Iph /Idark, responsivity and detectivity, and also displayed more reliable self-powered performance at zero bias.
The reason is that in GZO NRs, Ga-doping reduced the defect-based carrier
concentration, improved both the conductivity of the ZnO material and the photo-generated carrier transmission.
ACKNOWLEDGEMENTS This work is supported in part by the National Nature Science Foundation of China (No. 51372075).
7
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1. Lu Yang is currently working toward the M.S. degree in the Faculty of Physics and Electronic Science, Hubei University, Wuhan,China.
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2. Hai Zhou received the M.S. and Ph.D. degree in Microelectronics and Solid State Electronics from Wuhan University, Wuhan, China. He is currently an Associate Professor of Faculty of Physics and Electronic Science, Hubei University, Wuhan, China. His main research areas include self-power photodetectors, ultraviolet LEDs devices, and photoelectric devices based on perovskite materials.
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3. Mengni Xue is currently working toward the M.S. degree in the Faculty of Physics and Electronic Science, Hubei University, Wuhan,China.
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4. Zehao Song is currently working toward the M.S. degree in the Faculty of Physics and Electronic Science, Hubei University, Wuhan,China.
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5. Hao Wang is Chair Professor of the Faculty of Physics and Electronic Science and Dean of Graduate School of Hubei University. He received 12
his Ph. D from Huazhong University of Science and Technology in 1994 and worked as a postdoc at Peking University and the Chinese University of Hong Kong till 2002. Before his coming to Hubei University, he was appointed as professor of Shanghai Jiaotong University from 2002. He is a visiting professor of University of Cambridge and Aalto University. He has published over 150 refereed journal papers (such as Adv. Energy Mater., ACS Nano, Nano Energy), held 14 patents, and authored 2 books. His current research interests involve energy and information applications of nanostructured materials including solar cells, fuel cells, non-volatile memory and optoelectronic devices, and magnetic nanostructures.
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Figure captions Fig. 1 Schematic illustration of the fabrication process of n-GZO NRs/p-GaN heterojunction. Fig. 2 Top views of the SEM photograph of ZnO NRs (a) and GZO NRs (b) on FTO glass substrate. substrate.
(c) XRD patterns of ZnO NRs and GZO NRs arrays on FTO
(d) Optical absorption spectrum of the ZnO NRs and GZO NRs arrays on
FTO substrate. Fig. 3 (a) logarithmic I-V characteristics of n-GZO NRs/p-GaN and n-ZnO NRs/p-GaN at dark. (b) I-V curves of the devics under dark and UV illumination. (c) Curves of the ratio Iph/Idark versus reverse bias.
(d) The dependence of
photocurrent on operating time at zero bias. Fig. 4 (a) The spectral responsivity curves obtained under zero bias; (b) The spectral detectivity curves under zero bias.
13
Fig. 1
14
Fig. 2
15
Fig. 3
16
Fig. 4
17
Table captions Table 1 The performance parameters of ZnO-based self-powered UV PDs in this and previously reported work Structure
References
Responsivity
Detectivity
(mA/W)
(Jones)
n-Ga:ZnO/p-GaN
230
2.32× 1012
This work
n-ZnO/i-MgO/p-GaN
320
8.0× 1012
21
ZnO/GaN Nanoscale p-n Junctions n-ZnO/p-CuSCN
~132
-
22
7.5
-
23
n-ZnO/p-NiO
0.5
-
24
Au#1–ZnO–Au#2
20
-
25
18
S0924-4247(16)31231-6 http://dx.doi.org/doi:10.1016/j.sna.2017.08.007 SNA 10261
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
28-12-2016 29-7-2017 2-8-2017
Please cite this article as: Lu Yang, Hai Zhou, Mengni Xue, Zehao Song, Hao Wang, A Self-powered, Visible-blind Ultraviolet Photodetector based on n-Ga:ZnO Nanorods/p-GaN Heterojunction, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.08.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Self-powered, Visible-blind Ultraviolet Photodetector based on n-Ga:ZnO Nanorods/p-GaN Heterojunction Lu Yang, Hai Zhou,* Cong Ye, Mengni Xue, Zehao Song and Hao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferroelectric and Dielectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, China *Corresponding author: [email protected] (Hai Zhou); [email protected] (Hao Wang) HIGHTLIGHTS
1. An n-Ga:ZnO nanorods/p-GaN heterojunction for UV photodetector was first reported. 2. The device with Ga:ZnO nanorods showed more than 3.2×105 of the Iph/Idark ratio at zero biases and its responsivity reached to 0.23 A/W. 3. The reason for better photoelectric response performance of n-GZO NRs/p-GaN was explored.
Abstract A self-powered visible-blind ultraviolet (UV) photodetector based on n-Ga:ZnO nanorods (GZO NRs)/p-GaN heterojunction was reported, in which the n-type GZO NRs were prepared by the water bath method. In this study, we found n-GZO NRs/p-GaN heterojunction devices showed better performance than those of n-ZnO NRs/p-GaN heterojunction devices in terms of the ratio Iph/Idark, responsivity and detectivity.
Under UV illumination with the light intensity of 1.31 mW/cm2, the
ratio Iph/Idark for the n-GZO NRs/p-GaN heterojunction was as high as 3.2×105 at zero bias, which is about 75 times greater than that of an un-doped device (only 4.2×103).
Also its responsivity reached 0.23 A/W which was three times larger than
that of n-ZnO NRs/p-GaN (~0.08 A/W).
The reason n-GZO NRs/p-GaN
heterojunction displayed lager light/dark current ratio, higher responsivity and detectivity, more reliable self-powered performance at zero bias may be that in GZO 1
NRs, the Ga-doping reduced the defect states in the nanorods, enhanced the conductivity and electron mobility of the ZnO material and benefited the photo-generated carriers transmission.
Keywords—self-powered, visible-blind, photodetector, GaZnO nanorod
Ⅰ.
INTRODUCTION In recent years, self-powered ultraviolet (UV) photodetectors based on
nano-devices and nano-systems have attracted more attention because they can operate without an external bias [1-5]. This self-powered system can be operated wirelessly, independently and continuously.
As a wide band gap semiconductor
material, ZnO is widely used in short wavelength optoelectronic devices due to its direct wide band gap of 3.37eV and large exciton binding energy of 60 meV [6-11]. One-dimensional nanostructures based on ZnO, especially nanowires and nanorods (NRs), have higher crystallinity and larger surface-to-volume ratio than that of the film, which can provide more effective carrier transport and improve sensitivity to light.
Thus, ZnO nanostructures are a popular choice for ultraviolet photodetectors
[11-14].
It is thought, that ZnO is the best choice for the pn homojunction
photodetector.
However, p-type ZnO is difficult to obtain.
Therefore, p-GaN as the
p-type material in ZnO based pn junction photodetectors is a very good alternative material due to its similar crystal structure and band gap energy. Many researchers have reported that doping the ZnO nanostructures can modify device performance [11,15,16].
In n-ZnO NRs/p-Si diodes, with increasing Ga
content, the average diameter of the ZnO nanorods increased while the amount of oxygen vacancies was reduced. In addition, the Ga doped n-ZnO NRs/p-Si diodes showed well-defined rectifying behavior in their I-V characteristics and an improvement in the electrical conductivity (diode performance) [15].
Phan et al [16] 2
reported that the CO sensing properties of ZnO NRs are effectively improved by Ga doping, and the optimal 2% Ga-doped ZnO NRs based CO sensors showed a response factor of 25%, which was a 5-fold increase in sensitivity compared with the undoped devices at 150 °C.
Based on this report, we expect that the Ga-doping in ZnO (GZO)
will reduce defect states in the nanorods, enhance the conductivity of thin film and improve the rectifying behavior of ZnO NRs based heterojunction devices.
Herein,
GZO NRs are prepared through the water bath method and the n-GZO NRs/p-GaN heterojunction was formed.
Ⅱ. EXPERIMENT
Synthesis of GZO nanorod arrays: First, the FTO glass (with area of 20 mm×20 mm, thickness of 2 mm, resistance of 14 Ω, light transmittance of 90%) was rinsed in ultrasonic cleaners for 20min with deionized water, acetone and ethanol, respectively. The glass was then dried nitrogen before removing organics by with a UV cleaning treatment.
Next, a 90nm thick ZnO seed layer was deposited by spin coating on the
FTO glass substrate at a speed of 5000 rpm, followed by annealing at 350℃ for 2 hr in air.
As for the preparation of the GZO NRs array, the FTO glass with the deposited
ZnO seed layer was immersed in an aqueous solution of 0.05M zinc nitrate also containing, gallium nitrate with a molar ratio of 5% and held, at a temperature of 89℃ for 20min. Device assembly: After the GZO NRs were prepared, the sample was covered with p-type GaN firstly.
Secondly two clips were used to fixed the sample.
At last, the
sample was sealed with epoxy resin to prevent the movement of two parts (FTO glass and GaN).
And by optimizing our process, the epoxy resin did not enter between
these two parts. contacts.
Finally, an In electrode was applied on the GaN to form Ohmic
Herein, the p-type GaN was the commercially available p-type Mg-doped
GaN with a thickness of 4.9 μm, a mobility of 10 cm2 V−1 s−1 and a carrier concentration of 6×1016 cm-3. mm2.
The effective area of the p-n heterojunction is ~4× 2
The fabrication process of the GZO NRs based heterojunction is 3
schematically illustrated in Fig. 1. The morphology and crystallinity of the n-GZO NRs and n-ZnO NRs were characterized by field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F) and X - ray diffraction (XRD, D8 FOCUSX - ray diffraction) with Cu Ka radiation at 40kv and 40mA, respectively.
The doping content of Ga in GZO
NRs was analyzed by energy dispersive spectrometer (EDS) and scanning electron microscopy.
The absorbance spectrum of the samples were measured by
UV-VIS-NIR spectrophotometer (MPC-3100 SHIMADZU).
The current-voltage
(I-V) and current-time (I-T) properties were measured by a Keithley 4200 electrometer. In this study, discrete wavelengths of monochromatic light ranging from 254 to 800 nm were obtained by using a 66984 Xe arc source (300W Oriel) in combination with an Oriel CornerstoneTM 260 1/4m Monchromator. To determine the I-V and I-T characteristics, the FTO glass side of the device was illuminated and the light intensity was measured by a UV-enhanced Si detector.
Ⅲ. RESULTS AND DISCUSSION
Figure 2 (a) and (b) display the SEM images of ZnO NRs and GZO NRs grown on FTO substrates, respectively.
The average diameter of the ZnO NRs is about 70nm.
When the ZnO is doped with Ga, on the other hand, the diameter of the nanorods increases to ~ 100 nm.
Figure 2 (c) shows the XRD of the ZnO NRs confirming a
wurtzite crystal structure with a c-axis orientation.
Due to the incorporation of Ga,
the diffraction peak intensity of the GZO NRs decreased [17].
Using the (002) peak
width, we can also calculate the grain size of the ZnO according to the Scherrer formula D K
Bcos
[18-20].
Based on this equation, we calculated that the grain
size of the ZnO NRs was about 55.5 nm, while the GZO NRs have a smaller grain size of about 45.7 nm, which may be due to the different atomic radius of Ga ions [14, 17, 21].
The absorption curves (shown in Fig. 2 (d)) show that the two kinds of
nanorods have a good absorption ability in the UV region.
Also the absorption curve 4
of GZO NRs nanorods shows a slight blue shift of the absorption edge from 380 to 373nm, which may be caused by a small amount of Ga doped into the ZnO lattice that results in an in-crease of the band gap [22, 23]. Herein, the Ga doping in ZnO was confirmed by EDS and from the results we calculated that the doping concentration was about 1.8%. For a photodetector, a lower dark current means a greater ability to detect weak light signals in the background noise.
Fig. 3 (a) shows logarithmic I-V
characteristics of n-GZO NRs/p-GaN and n-ZnO NRs/p-GaN in the dark.
From the
graph, the current of Ga-doped device was lower than that of the device without Ga doping at reverse bias, which may result from a decrease of oxygen vacancies in the Ga-doped nanorods [15]. In addition, the rectification ratio of n-GZO NRs/p-GaN heterojunction is 95.1, which is much higher than that of the n-ZnO NRs/p-GaN heterojunction (~ 8.4).
These characteristics may all be due to Ga doping in ZnO
NRs, enhancing the unidirectional transmission properties of the diode.
Moreover,
the Ga-doped devices show excellent photoresponse characteristics under UV illumination with the light density of 1.31 mW/cm2, as shown in Fig. 3 (b).
As can
be see from the figure, our devices show good photovoltaic performance, based on which we can conclude that our devices can work as self-powered systems.
Also,
the GZO NRs based devices display a higher current than that of the ZnO NRs based devices.
The ratio of light to dark current (Iph/Idark) for both photodetectors is
pictured in Fig. 3 (c). The Iph/Idark ratio increases gradually as the applied reverse bias decreases, reaching a maximum value for the n-GZO NRs/p-GaN heterojunction of ~ 3.2×105 at zero bias which is about 75 times greater than that for the un-doped devices (~ 4.2×103).
This discrepancy makes it clear that Ga-doping can improve
the photoelectric response of the NR devices.
Furthermore, in order to identify the
stability and photoresponse rate of the device, we measured current as a function of time without applied bias under UV radiation in a periodic on-off cycle (T=20 s). From Fig. 3 (d), it can be seen that both devices have good stability and response rates, and also that the GZO NRs based devices have a lager response ratio. 5
The spectral responsivity and detectivity are two of the key parameters used to characterize photodetectors and are shown in Fig. 4 (a) and (b), respectively.
As
seen from the curves in Fig. 4 (a), which are measured at zero bias, the responsivity of the n-GZO NRs/p-GaN heterojunction is clearly larger than that of n-ZnO NRs/p-GaN heterojunction.
Also the peak responsivity value of the Ga-doped
device is at ~ 365 nm, which is attributed to GZO, is ~ 0.23 A/W [24-27].
This is
comparable to our previous results and larger than the value reported by others for ZnO-based self-powered UV PDs (see table 1) [28-32].
The peak value of the
undoped device is ~ 0.08 A/W or about one third the value of the doped device. Moreover, the ratio R350nm/R500nm was calculated to further characterize the visible-blind ultraviolet detection properties of the photodetectors.
The R350nm/R500nm
ratio of the n-GZO NRs/p-GaN heterojunction device reaches 8.4, which is 4.4 times greater than that of the un-doped device.
Fig. 4 (b) shows the spectral detectivity
curves of the two diodes measured at zero bias and the detectivity is determined based on the formula below:
D
R 2qJ dark
where Dλ is spectral detectivity of photodetector, q is electron charge, and Jdark is dark current density.
As seen from the graph, the detectivity of the n-GZO NRs/p-GaN
heterojunction achieves the maximum value of 2.32×1012 Jones (1 Jone=1 cm Hz/W) under UV illumination at 365 nm while the n-ZnO NRs/p-GaN heterojunction device reaches only 0.97×1012 Jones.
On the basis of the above analysis, it appears that the
Ga doping enhances the photoresponse characteristics of the visible-blind UV photodetector. According to the results discussed above, the n-GZO NRs/p-GaN heterojunction device has better performance characteristics than those of the n-ZnO NRs/p-GaN heterojunction in terms of the ratio Iph/Idark, responsivity and detectivity.
The reasons
for this may be attributed to the Ga doping, which may result in the increased carrier conductivity and a decrease in the defect-based carrier concentration in ZnO [33]. When a beam of light irradiates the p-n heterojunction, the electron-hole pairs are 6
generated in the depletion layer if the energy of the photon is larger than the band gap of the semiconductor.
The generated electron-hole pairs and separated by the
built-in electric field, i.e., the elections (or holes) arrive at the FTO electrode (or In electrode) through ZnO NRs (or GaN), resulting in the photocurrent. This is the working mechanism of the self-powered photodetector as it transforms the light signal into an electrical signal.
When a small amount of Ga is doped into ZnO, the
conductivity and the mobility of the nanorods will increase due to the saturation of intrinsic defects, such as O vacancies [15, 34,35]. Thus, the GZO NRs will enable carriers to transport rapidly and reduce the recombination of photo-generated carriers, resulting in a larger photocurrent.
In addition, the defect states in the nanorods will
be reduced by incorporating Ga, which will improve the electrical properties of the nanorods [15].
So through the Ga doping, the n-GZO NRs/p-GaN heterojunction
devices show improved photoresponse performance.
. CONCLUSION
In conclusion, self-powered visible-blind ultraviolet photodetectors based on n-GZO NRs/p-GaN heterojunction were fabricated.
In this study, we found that
n-GZO NRs/p-GaN heterojunction devices showed better performance than those of n-ZnO NRs/p-GaN heterojunction devices in terms of the ratio Iph /Idark, responsivity and detectivity, and also displayed more reliable self-powered performance at zero bias.
The reason is that in GZO NRs, Ga-doping reduced the defect-based carrier
concentration, improved both the conductivity of the ZnO material and the photo-generated carrier transmission.
ACKNOWLEDGEMENTS This work is supported in part by the National Nature Science Foundation of China (No. 51372075).
7
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1. Lu Yang is currently working toward the M.S. degree in the Faculty of Physics and Electronic Science, Hubei University, Wuhan,China.
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2. Hai Zhou received the M.S. and Ph.D. degree in Microelectronics and Solid State Electronics from Wuhan University, Wuhan, China. He is currently an Associate Professor of Faculty of Physics and Electronic Science, Hubei University, Wuhan, China. His main research areas include self-power photodetectors, ultraviolet LEDs devices, and photoelectric devices based on perovskite materials.
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3. Mengni Xue is currently working toward the M.S. degree in the Faculty of Physics and Electronic Science, Hubei University, Wuhan,China.
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4. Zehao Song is currently working toward the M.S. degree in the Faculty of Physics and Electronic Science, Hubei University, Wuhan,China.
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5. Hao Wang is Chair Professor of the Faculty of Physics and Electronic Science and Dean of Graduate School of Hubei University. He received 12
his Ph. D from Huazhong University of Science and Technology in 1994 and worked as a postdoc at Peking University and the Chinese University of Hong Kong till 2002. Before his coming to Hubei University, he was appointed as professor of Shanghai Jiaotong University from 2002. He is a visiting professor of University of Cambridge and Aalto University. He has published over 150 refereed journal papers (such as Adv. Energy Mater., ACS Nano, Nano Energy), held 14 patents, and authored 2 books. His current research interests involve energy and information applications of nanostructured materials including solar cells, fuel cells, non-volatile memory and optoelectronic devices, and magnetic nanostructures.
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Figure captions Fig. 1 Schematic illustration of the fabrication process of n-GZO NRs/p-GaN heterojunction. Fig. 2 Top views of the SEM photograph of ZnO NRs (a) and GZO NRs (b) on FTO glass substrate. substrate.
(c) XRD patterns of ZnO NRs and GZO NRs arrays on FTO
(d) Optical absorption spectrum of the ZnO NRs and GZO NRs arrays on
FTO substrate. Fig. 3 (a) logarithmic I-V characteristics of n-GZO NRs/p-GaN and n-ZnO NRs/p-GaN at dark. (b) I-V curves of the devics under dark and UV illumination. (c) Curves of the ratio Iph/Idark versus reverse bias.
(d) The dependence of
photocurrent on operating time at zero bias. Fig. 4 (a) The spectral responsivity curves obtained under zero bias; (b) The spectral detectivity curves under zero bias.
13
Fig. 1
14
Fig. 2
15
Fig. 3
16
Fig. 4
17
Table captions Table 1 The performance parameters of ZnO-based self-powered UV PDs in this and previously reported work Structure
References
Responsivity
Detectivity
(mA/W)
(Jones)
n-Ga:ZnO/p-GaN
230
2.32× 1012
This work
n-ZnO/i-MgO/p-GaN
320
8.0× 1012
21
ZnO/GaN Nanoscale p-n Junctions n-ZnO/p-CuSCN
~132
-
22
7.5
-
23
n-ZnO/p-NiO
0.5
-
24
Au#1–ZnO–Au#2
20
-
25
18