GaN p-n heterojunction with porous structure on GaN

GaN p-n heterojunction with porous structure on GaN

Journal Pre-proofs Research paper High switch ratio, Self-Powered Ultraviolet Photodetector Based on a ZnOEP/GaN p-n Heterojunction with porous struct...

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Journal Pre-proofs Research paper High switch ratio, Self-Powered Ultraviolet Photodetector Based on a ZnOEP/GaN p-n Heterojunction with porous structure on GaN Yan Xiao, Wei-Guan Zhang, Zi-Ting Tan, Ge-Bo Pan, Zhengchun Peng PII: DOI: Reference:

S0009-2614(19)30962-5 CPLETT 136981

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

28 September 2019 20 November 2019 22 November 2019

Please cite this article as: Y. Xiao, W-G. Zhang, Z-T. Tan, G-B. Pan, Z. Peng, High switch ratio, Self-Powered Ultraviolet Photodetector Based on a ZnOEP/GaN p-n Heterojunction with porous structure on GaN, Chemical Physics Letters (2019), doi:

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High switch ratio, Self-Powered Ultraviolet Photodetector Based on a ZnOEP/GaN p-n Heterojunction with porous structure on GaN

Yan Xiao,a Wei-Guan Zhang, c Zi-Ting Tan, b Ge-Bo Pan*b, Zhengchun Peng*a

[a] Key Laboratory of Optoelectronic Device and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, 518060 Shenzhen, P. R. China [b] Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, 215123 Suzhou, P. R. China [c] Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, College of Mechatronics and Control Engineering, Shenzhen University 518060 Shenzhen, P. R. China

Abstract In this study, a novel high performance self-powered p-n heterojunction Ultraviolet (UV) photodetector (PD) based on Znic octaethylphorphyrin (ZnOEP) /porousGallium nitride (GaN) was fabricated. The porous structure of GaN has a significant effect on the performance of PD. Compared with flat-GaN, the porous GaN based PD exhibit much lower dark current (~ 34 pA) with a much higher switch ratio (Ip/Id) of 1

106, specific detectivity (D*) of 1.6 × 1012 Jones. The switch ratio reaches the highest among the reported GaN based PD. The high performance combined with self-powered capability of our PD has potential application in many areas.

Keywords: Ultraviolet photodetectors; Electronic materials; GaN; Microstructure; High switch ratio;

___________________ * Corresponding author. Fax: +86-512-62872663. E-mail address: [email protected], [email protected]

1. Introduction 2

Gallium nitride (GaN) has excellent properties, such as direct wide bandgap (3.4 eV), high electron mobility, large saturation velocity, and good thermal stability, and has been widely used for fabricating Ultraviolet (UV) photodetectors (PDs) [1-4]. To date, various GaN based UV PDs with different device structures have been successfully developed, such as Schottky junctions [5], metal-semiconductor-metal sandwich structures [6], p-n junctions [7,8] etc. Among these structures, p-n heterojunction is a promising device structure, because it can operate without external power sources [9]. Although many GaN-based self-powered PDs have successfully prepared, the performance of switch ratio (Ip/Id) and specific detectivity (D*) are still need to improve due to its large dark current. Nanostructured GaN has been proven to improve the UV PD performance by increasing effective light trapping and reducing reflection loss [10-12]. Among various nanostructures, porous structure has exceptional properties such as large surface-tovolume ratio, tunable bandgap and refractive index [13]. Besides, the unique porous structure can also serve as photo traps to increase light absorption and reduce dark current by reducing the background carrier density [12,14]. Thus, fabricating porous GaN is a promising approach to improve the performance of UV PD. In this paper, a novel high performance self-powered of organic/inorganic p-n heterojunction UV PD was fabricated based on porous GaN. Specific organic small conjugated molecule Znic octaethylphorphyrin (ZnOEP, chemical structure displays in Fig. 1a) which has high light harvesting and converting into electron motion ability was used as p-type material [15], and GaN with nano pores was employed as n-type material. A p-n heterojunction was formed at the interface of porous-GaN/ZnOEP and provides self-powered ability. The unique porous structure not only improve light absorption, but also serve as photo traps for its large specific surface area. The proposed device 3

exhibited higher switch ratio, D*, and higher responsivity (R) at 0 V bias to UV light compared to flat GaN based UV PD.

2. Experimental section 2.1 Materials and Chemicals Si-doped n-type f- (0001) films were grown on sapphire (0001) substrates by hydride vapor phase epitaxy. The f-GaN layer was 5 µm thick and with a carrier concentration of 4.8×1018 cm-3. ZnOEP powder was purchased from Aldrich. Ionic liquid of 1-butyl-3-methylimidazolium perchlorate ([BMIM]ClO4) was purchased from Shanghai Cheng Jie Chemical Co., Ltd. Sulfuric acid, acetone and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Before each experiment, the fGaN film was cleaned with ultrasonication sequentially in acetone, ethanol, and deionized water (>18 MX) for 20 min. 2.2 Preparation of nano porous-GaN The porous-GaN was fabricated by photo-assisted electrochemical etching of fGaN, which has reported in previously [16]. Briefly, the UV light-assisted electrochemical etching method was carried out in a three-electrode cell combine with CHI660D potentiostat/galvanostat and etchant. The working electrode, working counter electrode, reference electrode and etchant were flat-GaN substrate, Pt wires, Pt wires, and [BMIM]ClO4 respectively. The applied voltage was 3V and etching time was 20 min. The porous GaN was obtained by applying 3 V constant voltage for 20 min and then immersed in aqua regia and deionized water respectively. 2.3 Device fabrication To construct ZnOE /GaN p-n vertical heterojunction devices, 30 nm ZnOEP thin film was deposited on GaN substrate (flat and porous GaN) by conventional thermal 4

evaporation method [17]. In brief, thin ZnOEP films were deposited from a quartz crucible source charged which was filled with ZnOEP and heated to sublimation temperature at a vacuum of 10−4 Pa. The deposition rate was kept at 7 Ås−1 and the substrate temperature was maintained at room temperature. The ZnOEP/GaN based heterojunction device was completed by thermal evaporating 80 nm Au electrodes on the surface of CoPc thin film and GaN respectively. Schematic of device based on ZnOEP/GaN p-n vertical heterojunction was shown in Fig. 1b.

Fig.1. (a) The chemical structure of ZnOEP; (b) Schematic of device based on ZnOEP/GaN p-n vertical heterojunction

3. Results and Discussion


Fig. 2. SEM images of (a) pure porous-GaN (Inset is flat-GaN) and (b) ZnOEP/porous-GaN film (inset is the cross section); (c) EDS spectra of porous-GaN and ZnOEP /porous-GaN film; (d) Optical absorption spectra of ZnOEP/flat-GaN and ZnOEP/porous-GaN.

Fig. 2a shows the SEM image of the porous-GaN. Unlike flat-GaN which has smooth surface (Fig. 2b), 3D porous structures are formed on the entire surface. Each hole has a hexagonal cross-section with different size. Compared with pure porousGaN, a compact ZnOEP film was formed on GaN surface and fully penetrating into the pores (Fig. 2c). The EDS spectra (Fig. 2d) shows that C, N, Zn and Ga four peaks exist after ZnOEP depositing on GaN surface, which is different from pure GaN that existing only Ga and N. This indicates that ZnOEP has been successfully deposited on porous surface of GaN. The crystal structure of the film is examined by the X-ray diffraction (XRD) pattern (Fig. 2e). In comparison with pure GaN, only one characteristic peak of GaN (0002) plane existed, and two characteristic peaks of 6.80 and 7.50 of ZnOEP were observed, which agreed with (010) and (01-1) plane of ZnOEP in the ZnOEP/GaN film. It further proves that ZnOEP has been successfully deposited on the surface of GaN with high crystal structure. The absorption spectra (Fig. 2f) displays that both ZnOEP/flat-GaN and ZnOEP/porous-GaN film exhibit high absorptivity in UV region. The absorptivity of ZnOEP/porous-GaN film is higher than ZnOEP/flat-GaN film, which is mainly caused by reducing the reflectivity of nano porous structure GaN [12].


Fig. 3. (a) I-V curves of ZnOEP/porous-GaN and ZnOEP/flat-GaN based device. (b) Curves of switch ratio versus bias. (c) Time-dependent on/off switching at 0 V bias with. (d) Power density dependent specific detectivity. All the illumination light was 365 nm with 1.4 mWcm-2 power density.

For comparison, both ZnOEP/flat-GaN and ZnOEP/porous -GaN device were fabricated by depositing 80 nm Au electrodes on the surface of CoPc and flat-GaN or nano porous GaN film, respectively. 2 Fig. 3a shows the I-V curves of both two PDs under dark and 1.4 mWcm-2 365 nm light illumination. Clearly, both devices show a typical rectification characteristic, indicating that a well-behaved diode structure owing to the formed p-n vertical heterojunction between p-type ZnOEP and n-type GaN. Moreover, the photocurrent is much higher than dark current even at 0 V bias, indicating that both devices are sensitivity to the 365 nm UV light and can serve as a self-powered device. In addition, the photocurrent for porous-GaN based device (at 0 V) is ~970 nA and is much higher than that for flat-GaN based device (191 nA), which 7

is caused by the larger contact area between ZnOEP film and porous-GaN. The dark current for porous-GaN based device (at 0 V) is only ~34 pA which is about 87 times lower than that flat-GaN based device (2.93 nA). It is due to the reduction of the background carrier density following the conversion of n+-GaN into a porous structure [13]. The lower dark current represents a better ability to detect weak signals in the background noise. Higher photocurrent combines with much lower dark current of porous-GaN based device induce a much higher switch ratio than flat-GaN based device. The switch ratio increases gradually as decrease the applied reverse bias, and decreases gradually as the applied reverse bias increase, reaching a maximum of ~2.6 × 106 at zero bias for porous-GaN based device (Fig. 3b). This value is about 4 × 104 times greater than flat-GaN based device (~65) and is the highest among the listed works (see table 1). Fig. 3c displays the time-dependent on/off switching of the device. The dark current almost reaches the detectable limit of our instrument, and the photocurrent rapidly increased from “OFF” state to the “ON” state after light illumination, indicating that both devices have high repeatability. The spikes of photocurrent are mainly attributed to the well-known photo-induced pyroelectric potential [18]. R and D* are two key parameters to evaluate the sensitivity of PD and can be expressed as: 𝐼𝑝


R = 𝐴𝑃 D*=




Where Ip, P, A and Id are the photocurrent, power density, device area and dark current of the PD respectively. Especially, D* is an important parameter to represent the detection capability of PD. Fig. 3d shows the D* versus power density under zero 8

bias for both devices. This value is about two-times lower than our previous work, however, it is still better than or similar to the previously reported work (see Table 1). The relatively low value of D* mainly resulted from the low photocurrent of the ZnOEP/p-GaN based device due to relatively low carrier mobility of ZnOEP. The R of porous-GaN based device is 0.4 mA/W at zero bias under 1.4 mW cm-2 light illumination, which is 4 times higher than that of the flat-GaN based device. The above results indicate that our porous-GaN based PD exhibited good performance in terms of detectivity and switch ratio, and the porous structure has a significantly effect on enhancing photo response performance. Table 1 Performance comparison between different self-powered GaN based UV PD Devices



switch ratio








2.6 × 106

1.6 × 1012

This work











5.36 × 1010






2.34 × 1013






4.8 × 1012


n-GZO NRs/p-GaN



3.2 × 105

2.32 × 1012


The mechanism of performance enhancement was studied by the relationship between photocurrent and power density (Fig. 4). The photocurrent increases nonlinearly with increasing P. The non-linear relationship can also be fitted with the power law as Ip ~ Pθ, where θ determines the response of photocurrent to power density. By fitting the curve, θ was determined as 0.83 and 0.55 for flat-GaN and porous-GaN based device respectively, indicating that both devices have carrier trap between the Fermi 9

level and the conduction band edge. It also indicates that flat-GaN has less trapping, because θ is also a parameter related to the trapping of the photo-generated carries, and larger θ corresponds to a weaker trapping [13]. Compared with flat-GaN, porous-GaN has a larger specific surface area which can server as photo traps and decease its reflectivity [12], inducing a larger photocurrent. The larger p-n heterojunction contacting area in porous-GaN film leads to less diminishing the recombination of photogenerated electron-hole pairs also increase its photocurrent. Meanwhile, the conversion of n+-GaN into porous structure induces a low dark current of porous-GaN based device [14]. Combine with the low dark current and large photocurrent, a higher switch ratio and D* were obtained.

Fig. 4. Power density dependent photocurrent and its corresponding fitting curve of (a) ZnOEP /flat-GaN and (b) ZnOEP /porous-GaN based device

4. Conclusion In conclusion, self-powered UV PDs based on ZnOEP/GaN p-n vertical heterojunction was fabricated in this work. The PD performance of both flat and porous structures of the GaN film were studied. Compared with flat-GaN based PD, the porous10

GaN based PD exhibits better performance in terms of the switch ratio, responsivity and specific detectivity to UV light. In particular, the switch ratio of our porous-GaN based PD reaches to 106, which is the highest among the reported GaN based PD. With the large surface-to-volume ratio and the effective light trapping of the porous-GaN, the porous GaN approach have great potential in developing efficient III-nitride based PD in large scale.

Acknowledgements This work was financially supported by the Science and Technology Innovation Commission of Shenzhen (KQTD20170810105439418, JCYJ20170818091233245, JCYJ20170302153341980), the Department of Education of Guangdong Province (2016KZDXM005), the Postdoctoral Science Foundation of China (2019M650209), and the National Natural Science Foundation of China (U1613212, 61671308).

Reference: [1] S. Strite, H. Morkoc, GaN, AlN, and InN: a review, J. Vac. Sci. Technol. B 10 (1992) 1237-1266. [2] Y. Q. Bie, Z. M. Liao, H. Z. Zhang, G. R. Li, Y. Ye, Y. B. Zhou, J. Xu, Z. X. Qin, L. Dai, D. P. Yu, Self‐powered, ultrafast, visible‐blind UV detection and optical logical operation based on ZnO/GaN nanoscale p‐n junctions, Adv. Mater., 23(2011), 649-653. [3] R. Dixit, P. Tyagi, S.S. Kushvaha, S. Chocklingam, B.S. Yadav, N.D. Sharma, M.S. Kumar, Influence of growth temperature on laser molecular beam epitaxy and properties of GaN layers grown on c-plane sapphire, Opt. Mater. 66 (2017) 142-148. [4] W. Zheng, R. Lin, J. X. Ran, Z. Z. Zhang, X. Ji, F. Huang, Vacuum-ultraviolet photovoltaic detector, ACS Nano, 12 (2018), 425−431. 11

[5] J. H. Lee, W. W. Lee, D. W. Yang, W. J. Chang, S. S. Kwon, W. I. Park, Anomalous Photovoltaic Response of Graphene-on-GaN Schottky Photodiodes, ACS Appl. Mater. Interfaces. 10 (2018) 14170-14174. [6] G. Kalita, M. Kobayashi, M. D. Shaarin, R. D. Mahyavanshi, M. Tanemura, Schottky Barrier Diode Characteristics of Graphene‐GaN Heterojunction with Hexagonal Boron Nitride Interfacial Layer, Phys. Status Solidi A. 215 (2018) 1800089. [7] R. R. Zhuo, Y. G. Wang, D. Wu, Z. H. Lou, Z. F. Shi, T. T. Xu, J. M. Xu, Y. T. Tian, X. J. Li, High-performance self-powered deep ultraviolet photodetector based on MoS 2/GaN p–n heterojunction, J. Mater. Chem. C 6 (2018) 299-303. [8] M. Ding, D. X. Zhao, B. Yao, Z. P. Li, X. J. Xu, Ultraviolet photodetector based on heterojunction of n-ZnO microwire/p-GaN film, Rsc. Adv., 5(2015),908-912. [9] D. Guo, H. Liu, P. Li, Z. Wu, S. Wang, C. Cui, C. Li, W. Tang, Zero-PowerConsumption Solar-Blind Photodetector Based on β-Ga2O3/NSTO Heterojunction, Acs. Appl. Mater. Interfaces, 2 (2017) 1619-1628. [10] V. Babichev, H. Zhang, P. Lavenus, F.H. Julien, A. Yu. Egorov, Y. T. Lin, L.W. Tu, M. Tchernycheva, GaN nanowire ultraviolet photodetector with a graphene transparent contact, Appl. Phys. Lett. 103 (2013), 201103. [11] E. Petronijevic, G. Leahu, A. Belardini, M. CentiniR. Li Voti, T. Hakkarainen, E. Koivusalo, M. Rizzo Piton, S. Suomalainen, M. Guina, C. Sibilia Photo-Acoustic Spectroscopy Reveals Extrinsic Optical Chirality in GaAs-Based Nanowires Partially Covered with Gold. International Journal of Thermophysics, 2018,39(4), 46. [12] C. Ramesh, P. Tyagi, B. Bhattacharyya, S. Husale, K.K. Maurya, M. Senthil Kumar, S. S. Kushvaha, Laser molecular beam epitaxy growth of porous GaN nanocolumn and nanowall network on sapphire (0001) for high responsivity ultraviolet photodetectors, J. Alloys. Compd. 770 (2019) 572-581. [13] L. Liu, C. Yang, A. Patane, Z. Yu, F. Yan, K. Wang, H. Lu, J. Li, L. Zhao, Highdetectivity ultraviolet photodetectors based on laterally mesoporous GaN, Nanoscale 9 (2017) 8142-8148. [14] C. Zhang, S. H. Park, D. Chen, D. W. Lin, W. Xiong, H. C. Kuo, C. F. Lin, H. Cao, J. Han, Mesoporous GaN for Photonic Engineering Highly Reflective GaN 12

Mirrors as an Example, ACS Photonics, 2 (2015) 980-986. [15] F. X. Wang, Y. Q. Liu, H. D. Wu, Y. Xiao, G. B. Pan, One-step fabrication of an ultralong zinc octaethylporphyrin nanowire network with high-performance photoresponse, J. Mater. Chem. C,1 (2013) 422-425. [16] M. R. Zhang, S. J. Qin, H. D. Peng, G. B. Pan, Porous GaN photoelectrode fabricated by photo-assisted electrochemical etching using ionic liquid as etchant, Mater. Lett. 182 (2016) 363-366. [17] M. M. El-Nahass, K. F. Abd-El-Rahman, A. A. M. Farag, A. A. A. Darwish, Structural and transport properties of thermally evaporated nickel phthalocyanine thin films, Phys. Scr. 73 (2006) 40-47. [18] W. B. Peng, X. F. Wang, R. M. Yu, Y. J. Dai, H. Y. Zou, A. C. Wang, Y. N He, Z. L. Wang, Enhanced Performance of a Self‐Powered Organic/Inorganic Photodetector by Pyro‐Phototronic and Piezo‐Phototronic Effects, Adv. Mater. 29 (2017) 1606698. [19] X.-X. Chen, X.-H. Xiao, Z.-F. Shi, R. Du, X.-J. Li, High-Performance SelfPowered Ultraviolet Photodetector Based on Nano-Porous GaN and CoPc p–n Vertical Heterojunction, J. Alloy. Copmd. 767 (2018) 368-373. [20] G. Kalita, M. Kobayashi, M. D. Shaarin, R. D. Mahyavanshi, M. Tanemura, Schottky Barrier Diode Characteristics of Graphene‐GaN Heterojunction with Hexagonal Boron Nitride Interfacial Layer, Phys. Status Solidi A 215 (2018) 1800089. [21] R. R. Zhuo, Y. G. Wang, D. Wu, Z. H. Lou, Z. F. Shi, T. T. Xu, J. M. Xu, Y. T. Tian, X. J. Li, High-performance self-powered deep ultraviolet photodetector based on MoS2/GaN p–n heterojunction, J. Mater. Chem. C, 6 (2018) 299-303. [22] Y. Xiao, L. Liu, Z. H. Ma, B. Meng, S. J. Qin, G. B. Pan, High-Performance SelfPowered Ultraviolet Photodetector Based on Nano-Porous GaN and CoPc p-n Vertical Heterojunction. Nanomaterials, 9(2019), 1198. [23] L. Yang, H. Zhou, M. N. Xue, Z. H. Song, H. Wang, A self-powered, visible-blind ultraviolet photodetector based on n-Ga: ZnO nanorods/p-GaN heterojunction. Sens. Actuators. A. 267 (2017) 76-81. 13


The authors declare no conflict of interest.

Author Contributions: conceptualization, G. B. P and Z. C. P; methodology, Y. X; formal analysis, W. G. Z; data curation, Z. T. T; writing—original draft preparation, Y. X and W. G. Zhang; writing—review and editing, G. B. P and Z. C. P;

Graphical Abstract (Synopsis)


1. 2. 3. 4. 5.

High performance ZnOEP/porous-GaN based UV PD was fabricated. The structure of GaN has a significant effect on the PD performance. The PD displays high switch ratio, specific detectivity and self-power property. The switch ratio of PD reaches to 106. The swithch ratio is the highest among the reported GaN based PD.