Realization of in-plane GaN microwire arrays based ultraviolet

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Realization of in-plane GaN microwire arrays based ultraviolet photodetector with high responsivity on Si (100) substrate Dexiao Guo, Xingfu Wang, Hu Wang, Weidong Song, Hang Chen, Mingyue Qi, Xingjun Luo, Xiao Luo, Guang Li, Guogang Qin, and Shuti Li ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00918 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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Realization of in-plane GaN microwire arrays based ultraviolet photodetector with high responsivity on Si (100) substrate Dexiao Guo,†‡ Xingfu Wang,†‡ Hu Wang†‡, Weidong Song,† Hang Chen,† Mingyue Qi,† Xinjun Luo,† Xiao Luo,† Guang Li,† Guogang Qin§ and Shuti Li†* †Guangdong

Engineering Research Center of Optoelectronic Functional Materials and Devices, South

China Normal University, Guangzhou 510631, People’s Republic of China. §State

Key Laboratory for Microscopic Physics, School of Physics, Peking University, Beijing 100871,

People’s Republic of China. *Corresponding author: Shuti Li E-mail address: [email protected] KEYWORDES: in-plane, GaN microwire array, catalyst-free heteroepitaxy, ultraviolet photodetector,

strong responsivity, high quantum efficiency, fast response

ABSTRACT Recent years have witnessed one-dimensional materials and structures delivering their great superiority in various (opto-)electronic devices. However, highly controllable morphology, distribution, structure of one dimensional materials are hard to synthesize, which hindered their progress and applications. In this paper, ultraviolet photodetectors were fabricated on planar, parallel, uniform GaN microwire arrays synthesized via catalyst-free heteroepitaxy by metal-organic chemical vapor deposition. Under a illumination of 325 nm incident light at a bias of 5.0 V, the Schottky-type ultraviolet photodetector exhibits a strong responsivity of 1.17×105 A/W, as well as a high quantum efficiency of 4.47×105, which are much better than the most reported GaN nano/microwire based ultraviolet photodetector and it's thin film based detector. Furthermore, the time dependent photoresponse manifests their excellent repeatability and fast response (36.3 ms for rise time and 75.2 ms for decay time). Our work provide a high repeatability and efficient method to fabricate high performance GaN microwire arrays ultraviolet photodetector for future photoelectronic applications even the on-chip optoelectronic integrated systems.

1. INTRODUCTION Artificial one-dimensional structure has become one of the most important building blocks for the application in microelectronics or optoelectronics. Thanks to the new properties that are different from microscopic particles and bulk materials, various nano/microwires based devices have been fabricated,

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such as high-electron-mobility transistor (HEMT),1 light emitting diode (LED),2 laser3 and photodetctor4-10. Up to now, most of the semiconductor nano/microwires are synthesized through a bottom-up process, like ZnO,11 GaN,12 GaAs,13 etc. Accordingly, different synthesis techniques, including chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) have been developed within this category.11-14 The nano/microwires synthesized by bottom-up method are normally vertically grown on substrate with the help of seed layers or catalysts.1-4 However, the random distribution of the nano/microwires by this method, even the uneven diameter or curving form along the growth direction increase the complexity as well as a poor repeatability of the micro/nanowires based devices preparation. Recently, GaN based nano/microwire has drawn extensive attention for their excellent optical and electrical properties and been widely applied in the field of photoelectric conversion. Various device structures of GaN single micro/nanowire based ultraviolet (UV) photodetectors have been reported, such as Pt nanoparticle coupling GaN nanowire photodetector,14 m-axial GaN nanowire based photodetector,15 piezopotential induced GaN nanowire photodetector,16 a-axial GaN nanowire based photodetector17 and core/shell nanowire based detector,18 etc. In order to boost the application of nanowire based photodetector, some GaN nanowire arrays based photodetectors have also been reported.19,20 Highly controllable morphology, distribution, component and structure are crucial for the realization of the high performance nano/microwire based devices. Recently, we have developed a position-controlled growth method for in-plane GaN nano/microwire arrays and their morphology and structure, as well as component, density and space position can be precisely controlled.21 Through this method, a superior performance visible-blind UV photodetector based on a single microwire and its piezo(photo-)tronic effects have been demonstrated before.22 Most of the reported GaN nanowire photodetector involve additional technologies in their fabrication processes, such as lift off and transfer of GaN nanowire from original substrate, adding to complexity and cost of the devices and preventing them from further practical applications. In this paper, in-plane GaN microwire arrays based UV photodetectors were fabricated with metal-semiconductor-metal (MSM) structure on silicon substrate. By directly depositing metal electrodes on GaN microwire array on original grown substrate without any extra processing, the obtained photodetector showed a high responsivity of 1.17×105 A/W as well as a high external quantum efficiency of 4.47×105, which are much better than most reported nano/microwire based UV photodetectors and the


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GaN thin film based detector. The realization of in-plane GaN microwire arrays based UV photodetectors offer a possibility for future photoelectronic applications even the on-chip optoelectronic integrated systems.

2. EXPERIMENTAL The fabrication process was started by depositing a 300 nm SiO2 thin film on 2 inches high resistance (>105Ω·cm) Si (100) substrate by plasma-enhanced chemical vapour deposition (PECVD). Traditional photolithography and wet etching technique were used to "draw" parallel ~7.0 μm Si stripe separated by ~3.0 μm SiO2 stripe. Then, the treated substrates were anisotropy etched by KOH (40 wt%) solution for 30 min at 40℃ and parallel-arranged trapezoidal channels with two opposite Si(111) facets and a bottom Si(100) facet were exposed. After that, the fabricated substrates were immersed in HF (5%) for 15 s to remove native oxide layer. GaN microwire arrays were synthesized via MOCVD process under a low pressure environment. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH3) were used as the Ga, Al and N precursors, respectively. At the beginning, the reactor pressure was lowered to 100 mbar and the chamber temperature was raised to 800 ℃ to deposit a 300 nm thick AlN buffer layer to improves the crystal quality of GaN microwires and insulates the device against the substrate. Unintentionally doped GaN microwire arrays were synthesized on two opposite Si(111) facets of the channel in high pressure (400 mbar) at 1030 ℃. The synthesis of GaN microwire arrays on patterned Si substrate is schematically indicated in Fig. 1a. The device fabrication is compatible with the planar device processing. Firstly, fabricated wafers were cleaned by acid solution and DI water to keep surface clean without contamination or native oxide layer, which were considered as a barrier for electron transport within the GaN/metal interface.23 Then the electrode patterns were aligned to GaN microwire arrays with the help of photolithography. And Ni/Au (30/200 nm) electrodes were deposited on the two ends of the microwire arrays with a separation of 20.0 μm via thermal evaporation to form schottky contact. The structure of GaN microwires were characterized by scanning electron microscope (SEM, ZEISS ULTRA 55) and high-resolution transmission electron microscopy (HRTEM, EM-2100HR). The optical characteristics of the GaN microwires were investigated by micro-photoluminescence (micro-PL) using a 325 nm He-Cd laser at room temperature. The morphology of the photodetector was observed under an optical microscope. The current versus voltage (I-V) properties under different light excitations as well as the temporal response were measured by using a He-Cd laser equipped with a precision source/measure


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unit (Model B2902, Keysight Technologies, Inc.) at room temperature.

3. RESULTS AND DISCUSSION The GaN microwires were grown successfully which can be confirmed by the energy dispersive spectrometer (EDS) data (Fig. S1†). Fig. 1b shows the cross section SEM image of the GaN microwires on silicon with 300 nm AlN as an interlayer. The trapezoidal grooves and the GaN microwires with triangular cross section can be recognized clearly. The position and length of microwires can be decided precisely by adjusting the position and length of the grooves, which can provide a great convenience and repeatability for the manufacture of microwires and its devices. The HRTEM image taking from the cross-section of GaN microwire is shown in Fig. 1c. The inter-planar spacing is 0.52 nm and the atoms neatly arranged themselves into the highly single-crystalline with no discernible defects, which reveals the good crystalline quality of the microwire. Fig. 1d presents the fabricated on-chip devices, where the photodetectors are orderly arranged in controllable position over a whole substrate. One cell unit contains 12 parallel-aligned individual microwire shown in Fig. 1e. These microwires are of the similar dimension and parallel to each other owing to the same direction of grooves, which provides a strategy to prepare wafer-scale nano/microwire based devices. The optical properties of the GaN microwires were explored by micro-PL at room temperature (Fig. 2a). A sharp and high intensity near band-edge emission (NBE) peak centered at 364.5 nm (~3.4 eV) was observed. Furthermore, the normally occurred yellow luminescence (YL) band emission located at around 2.2-2.3 eV which originate from impurities or defects, such as Ga vacancy (VGa),24 was not observed in our microwires. The strong and narrow NBE without YL band emission indicates the satisfied crystal quality and good optical properties of the GaN microwires. Current-voltage characteristics and time response both in dark and light environment were studied at room temperature by using precision source/measure unit and a He-Cd laser (325 nm). Fig. 2b elucidates the I-V curves measured in the dark and under a UV illumination (2500 μW/cm2). Clearly, the I-V curve under illumination shows a symmetric back-to-back M-S-M transport properties, suggesting that there are two schottky barriers formed at both metal/GaN contact sides. At 5.0 V bias, the measured dark current of UV photodetector is 1.3 μA, and a light current is 2.71 mA. Correspondingly, a high sensitivity about 2.08×105 % is achieved. In order to further examine the light response capability, we illuminated the device with different illumination intensity from 30 to 2500 μW/cm2, and the results are given in Fig. 2c.


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It can be seen that the light current is increased with the increasing of illumination power. Fig. 2d shows the relationship between photocurrent and incident illumination power. Normally, the function between photocurrent and incident power obeys the power law:25 Ilight ∝ P x , where the P is the optical power and x is a constant. The value of x reflects the effect of trap states existing in semiconductor material and the x is much closer to 1 means the more insufficient density of trap states.23 Based our experimental data, the fitting value is 0.995 for 5.0 V from the logarithmic plot as shown in Fig. 2d, which suggests a limited density of traps in our GaN microwires for photo-induced carriers and the high crystal quality of the grown GaN microwires. In general, responsivity (Rλ) and external quantum efficiency (EQE) are two important parameters to testify the performance of a photodetector. Rλ can be defined as the ratio of photocurrent generated by a certain wavelength of light to the intensity of the incident optical power. EQE is defined as the number of electron-hole pairs excited by one absorbed photon. The two parameters can be estimated as following relationship.26 Rλ =

Iλ hc ΔI and EQE = Pλ A eλ Pλ A

where I λ means the photocurrent, P λ is the light intensity, A is the effective device area of the photodetector, ΔI is the difference value between the photocurrent and dark current, λ is the exciting wavelength, h is Planck's constant, c is the velocity of the light and e is the electronic charge. Based on the equations and our experiment date, the values of Rλ and EQE are calculated to be 1.17×105 A/W and 4.47×105, respectively. The responsivity and the external quantum efficiency (EQE) as a functional of the light wavelength have also been explored, corresponding experimental result is presented in Fig.2e. Specific detectivity (D*) is another important factor of photodetector devices. D* is calculated as: 𝐷 ∗ = 𝑅 2𝑞𝐼dark/𝑆

by considering the dark current as the major noise, where q is the electronic charge, and Idark is

the dark current. The calculated maximum D* is in the order of 1016 Jones, which indicates that our microwire-based photodetector presents an excellent performance in detecting weak light signals. The LDR means that within a certain range the photocurrent has a linear response as the incident light intensity changes and it can be calculated by: LDR(dB)=20log(Iph/Idark), the calculated light wavelength dependent LDR is shown in Fig. 2f. Photocurrent of the GaN microwire based photodetector dependent time was collected by periodically tuning on and off the laser for 50 cycles, as shown in Fig. S4, which indicates the photodetector present an excellent stability and repeatability.


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Table 1 summarized the performances of GaN based nano/microwire UV photodetector in previous reports and in this work. It is found that our in-plane GaN microwire arrays based UV photodetector owns a high UV sensitivity, high responsivity, and high EQE, which are much better than the most reported single GaN nano/microwire photodetectors, and nanowire arrays based photodetectors. The good photodetector properties are attributed to the high UV light utilization ratio and the good crystal quality of GaN microwires. Table 1 Summary of the photoconductivity performance of GaN based nano/microwire UV photodetector in this work and in previous reports. Materials and Structure

Light of detection (nm)

Sensitivity

Rλ (A/W)

EQE

Ref.

Single GaN nanowire

380

971%

773

2.71×103

14

Single Pt-GaN nanowire

380

9975%

6.39×104

2.24×105

14

Single m-axia nanowire

3.2eV (387.5)

-

-

1.9×105

15

325

-

5.0×104(no strain)

-

16

325

Two orders

2.2×104

3.2×105

17

357 325

140 -

25 1.1×105

-

19 20 27

325

2.08×105 %

1.17×105

4.47×105

This work

GaN

Single GaN nanowire Single a-axial GaN nanowire GaN nanowire arrays GaN nanowire arrays GaN GaN nanobridges GaN microwire arrays

Temporal response of the photodetector was measured by tuning the light source ON and OFF at an applied bias of 5.0 V under an illumination of 2500 μW/cm2. Fig. 3a clearly shows the repeatable and reversible light sensing behavior of the fabricated device. Reproducible on/off switching curve of the device under 325 nm UV light illumination with different light density at a fixed bias of 5.0 V is also presented in Fig.S3. It is evident that the photocurrent increase rapidly and approach a stable value when turn on the laser. Decay process is also observable after turning off the laser and followed by a relaxation process. In general, the dynamic photoresponse can be described by the following two exponential equations respectively:28

I = I 0 (1 - exp(

-t t t )) and I = I 0 [Aexp( ) + Bexp( )] τ on τ off -fast τ off -slow

where I 0 represents the steady state photocurrent, τ on means the rise time, τ off-fast and τ off-slow are decay time and relaxation time severally to describe recovery process. By fitting the experimental data


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with the two equations, the detailed rise and decay response can be estimated, as shown in Fig. 3b. It can be seen that the in-plane GaN microwire arrays based photodetector exhibits a transient rise/decay time. In the case of illumination, a short rise time about 36.3 ms is fitted, which is much faster than the most reported GaN nano/microwire based photodetectors.,14,20,29 Such a high photoresponse speed is conceivable due to the high crystalline GaN microwires. After the UV illumination was switched off, a biexponential function was adopted to describe the recovery because two temporally distinct processes were detected. Firstly, the photocurrent decreased fleetly with τoff-fast= 75.2 ms, which represent the direct recombination of electrons and holes in GaN. Then followed by a tardy process with a relatively long relaxation time of τoff-slow=9.66 s. It is worth mention that the time constant for the fall time is slower than the rise time which suggests that traps or other defect states probably involved in this process,30 which is the origin of persistent photoconductivity (PPC) process. PPC is a widely reported phenomenon and the main drawback of GaN based photodetectors. The reason of PPC is the photo-induced holes are traped by the deep level defects and reduce the recombination rate with electrons after the light is removed.31,32 As a result, the photocurrent persists for a long time. Fig. 4 is the energy band diagram of the junction of Ni/Au and GaN to explain the electrons transfer mechanism of the device. In Fig. 4a, the different work functions between Ni (5.15 eV) and GaN (4.10

Φ ns( eV) leads to a schottky barrier

) at the interface as well as a depletion region (DR). 33 A high

schottky barrier can impede the transfer of electrons from the neutral region (NR) to the interface and present low dark current. During illuminated the device, electrons-holes pairs are generated and separated quickly due to the strong local build-in electric field in the DR. Meanwhile, holes can be trapped by the semiconductor-metal interface states and schottky barrier are reduced,34 as the green line showed in Fig. 4 b . A s a c o n s e q u e n c e , a l o w e r b a r r i e r h e i g h t ( Φ 'ns) m a k e t h e p h o t o g e n e r a t e d e l e c t r o n s transport between GaN and metal electrodes more easily, which can lead to a larger photocurrent on a macro level and produce gain in the UV photoresponse. More interestingly, based on Katz O’s research,35 the reduce of Schotty barrier height in the GaN based UV photo detectors can also lead to PPC phenomenon due to the trap repopulation. On the other hand, molecular-sensitive on the surface of microwires in air plays a critical role for the observed large photoresponse because the surface states of GaN microwires always function as the adsorption sites.33 In dark circumstance, oxygen molecules can be adsorbed onto the surface through capturing the free electrons to form negative ions [ O 2 (g) + e - → O -2 (ad) ] , which can bend the energy band near the surface and generates a DR with low conductivity as depicted in


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Fig. 4c. Upon illumination, photo-induced holes can move to the surface due to band bending and recombinant with anion to release oxygen molecules [ O -2 (ad) + h + → O 2 (g) ]. Namely, billion of unpaired electrons are “detained” in the body and attributed to the high photocurrent gain of the photodetector, as shown in Fig. 4d schematically. Hence, photocurrent increases dramatically upon the UV. Hence, photocurrent increases dramatically upon the UV illumination. Once the UV illumination is switched off, no electron-hole pair is photo-generated and oxygen molecules are reabsorbed onto the surface again to trap the free electrons, consequently, the current would descend to initial state. As a comparison experiment, GaN film based photodetectors with the same size were fabricated and studied. The epitaxial layer was grown on sapphire substrate with a thickness of about 3.0 μm by MOCVD. The double crystal X-ray diffraction scan of the GaN epilayer shows that the full width at half maximum (FWHM) of (002) and (102) planes were all lower than 300 arcsec (Fig. S2†), which indicated a good crystalline quality. The GaN film was etched and separated into small independent platforms by conventional Cl2 based inductive-coupled-plasma (ICP) dry etching process. Schottky contacts with Ni (30nm)/Au (200nm) bilayer were evaporated onto both ends of the GaN platform by using photolithography and thermal evaporation processes. It is noteworthy that the same photomask and same electrode fabrication process was applied between GaN microwire based photodetector and GaN film photodetector. Fig. 5a shows the room temperature I-V characteristics of the GaN film based photodetector in dark and under UV illumination with a power density of 2500 μW/cm2. The calculated sensitivity, Rλ and EQE are 2.77×104 %, 0.21 A/W and 0.80 respectively, which were comparable to other planar GaN based UV detector,36-38 but much lower than our fabricated GaN microwire arrays based photodetector. Chen et al once made a comparison of properties between the one-dimensional nanostructure and traditional film based photodetector.17 The GaN one-dimensional based detectors have displayed good photoelectric characteres in the UV range, which is unmatched by their thin film counterparts due to the large surface-to-volume ratio of GaN nano/microwires that can increase the illuminated surface area and more UV light photon can be collected to increase the sensitivity and responsivity. In addition, large surface-to-volume can also provide more surface states for the surface adsorption processes which can lead to the photo-carrier multiplication and enlarge the sensitivity and responsivity as well. The on-off switching property test method of the device is same with the in-plane GaN microwire arrays based ultraviolet photodetector’s. (under a bias of 5.0 V and illumination intensity of 2500 μW/cm2). Consequently, the device exhibits a sharp on/off transition in photocurrent with good


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stability upon illumination (Fig. S5†). Fig. 5b presents a magnified view of one of the photocurrent switching cycle, the photo-response time for the detector was found to be 60 ms and the recovery time was found to be 100 ms, respectively, which can be comparable with the microwire arrays based UV detector. Considering the good characters and the facile preparative technology, the in-plane GaN microwire arrays based UV photodetector is hopeful for practical applications and have a promising future in photosensitive applications as well as the optoelectronics on silicon.

4. CONCLUSIONS The in-plane GaN microwire arrays Schottky MSM based UV photodetector have been successfully fabricated on Si substrate. The photodetector shows super UV light detection performances, such as high responsivity of 1.17×105 A/W and a high quantum efficiency of 4.47×105 , which are much better than most reported GaN micro/nanowire based photodetectors and the thin film based detector. Meanwhile, time-dependent photoresponse results exhibit a good repeatability and stability with rapid responding speed (36.3 ms for rise time and 75.2 ms for decay time). Considering the excellent performance of the device and the facile process, the in-plane GaN microwire arrays based UV photodetectors may have potential applications in on-chip based photoelectron devices and silicon photonic devices.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos.11474105, 61404156 and 11804103), the Science and Technology Program of Guangdong, Province of China(Grant Nos. 2015B010105011 and 2015B090903078), Guangdong Natural Science Foundation for Distinguished Young Scholars (2018B030306048), the Science and Technology Project of Guangzhou City (No. 201607010246), Education Department project Foundation Program of Guangdong, Province of China (Grant Nos.2017KDDXM002)

Supporting Infornation: S1. S2. S3. S4.

Energy Dispersive Spectrometer (EDS) data. Double crystal X-ray diffraction scan. Temporal response with different power densities at fixed applied bias The Stability of the photodetector at 50 cycles.


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S5.

The on-off switching property test under a bias of 5.0 V.

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Chen, .R S.; Chen, H. Y.; Lu, C. Y.; Chen, K. H.; Chen, C. P.; Chen, L. C.; Yang, Y. J. Ultrahigh photocurrent gain in m-axial GaN nanowires. Appl. Phys. Lett. 2007, 91, 223106.

(16) Tsai, C. Y.; Gupta, K.; Wang, C. H.; Liu, C. P. Ultrahigh UV Responsivity of Single Nonpolar a-axial GaN Nanowire with Asymmetric Piezopotential via Piezo-phototronic Effect: Dependence of Carrier Screening Effect on Strain. Nano Energy, 2017, 34, 367-374. (17) Wang, X.; Zhang, Y.; Chen, X.; He, M.; Liu, C.; Yin, Y.; Zou, X.; Li, S. Ultrafast, superhigh gain visible-blind UV detector and optical logic gates based on nonpolar a-axial GaN nanowire. Nanoscale. 2014, 6, 12009-12017. (18) Zhang, H.; Messanvi, A.; Durand, C.; Lavenus, J. P.; Babichev, A.; Julien F. H.; Tchernycheva, M. InGaN/GaN core/shell nanowires for visible to ultraviolet range photodetection. Phys. Stat. Sol. 2016, 213, 936-940. (19) Babichev, A. V.; Zhang, H.; Lavenus, P.; Julien, F. H.; Egorov, A. Yu.; Lin, Y. T.; Tu, L. W.; Tchernycheva, M. GaN nanowire ultraviolet photodetector with a graphene transparent contact. Appl. Phys. Lett. 2013, 103, 1431-1436. (20) Liu, Y.; Sun, P.; Meng, X. A GaN Nanowire-Based Photodetector With Ag Nanowires as Transparent Electrodes. IEEE Photo. Tech. Lett. 2016, 28, 23-26. (21) Wang, X.; Tong, J.; Chen, X.; Zhao, B.; Ren, Z.; Li, D.; Zhuo, X.; Zhang, J.; Yi, H.; Liu, C.; Fang, F.; Li, S.; Highly ordered GaN-based nanowire arrays grown on patterned (100) silicon and their optical properties. Chem. Commun. 2014, 50, 682-684. (22) Song, W.; Wang, X.; Xia, C.; Wang, R.; Zhao, L.; Guo, D.; Chen, H.; Xiao, J.; Su S.; Li, S. Improved photoresponse of a -axis GaN microwire/p-polymer hybrid photosensor by the piezo-phototronic effect. Nano Energy. 2017, 33, 272-279. (23) Ishikawa, H.; Kobayashi, S.; Koide, Y.; Yamasaki, S.; Nagai, S.; Umezaki, J.; Koike, M.; Murakami, M. Effects of surface treatments and metal work functions on electrical properties at p-GaN/metal interfaces. J.


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Appl. Phys. 1997, 81, 1315-1322. (24) Neugebauer, J. and Van, d. W. C. G. Gallium vacancies and the yellow luminescence in GaN. Appl. Phys. Lett. 1996, 69, 503-505. (25) Gao, T.; Zhang, Q.; Chen, J.; Xiong, X.; Zhai, T. Performance-Enhancing Broadband and Flexible Photodetectors Based on Perovskite/ZnO-Nanowire Hybrid Structures. Adv. Opt. Mater. 2017, 5, 1700206. (26) Lan, C.; Li, C.; Yin, Y.; Guo, H.; Wang, S. Single-crystalline GeS nanoribbons for high sensitivity visible-light photodetectors. J. Mater. Chem. C, 2015, 3, 8074-8079. (27) Kang, J. H.; Johar, M. A.; Alshehri, B.; Dogheche, E.; Ryu, S. W. Facile growth of density-and diameter-controlled GaN nanobridges and their photodetector application. J. Mater. Chem. C, 2017, 5, 11879-11884. (28) Kung, S. C.; Van, d. V. W. E.; Yang, F.; Donavanet, K. C.; Penner, R. M. 20 μs photocurrent response from lithographically patterned nanocrystalline cadmium selenide nanowires. Nano Lett. 2010, 10, 1481-1485. (29) Chen, H. Y.; Chen, R. S.; Rajan, N. K.; Chang, F. C.; Chen, L. C.; Chen, K. H.; Yang, Y. J.; Reed, M. A. Size-dependent persistent photocurrent and surface band bending in m-axial GaN nanowires. Phys. Rev. B: Condens. Matter Mater.Phys. 2011, 84, 205443. (30) Huang, K.; Zhang, Q.; Yang, F.; He, D. Y. Ultraviolet photoconductance of a single hexagonal WO3, nanowire. Nano Res. 2010, 3, 281-287. (31) Qiu, C. H. and Pankove, J. I. Deep levels and persistent photoconductivity in GaN thin film. Appl. Phys. Lett. 1997, 70, 1983. (32) Chen, H. M.; Chen, Y. F.; Lee, M. C.; Feng, M. S. Persistent photoconductivity in n-type GaN. J. Appl. Phys.1997, 82, 899-901. (33) Guo, J. D.; Pan, F. M.; Feng, M. S.; Guo, R. J.; Chou, P. F.; Chang, C. Y. Schottky contact and the thermal stability of Ni on n-type GaN. J. Appl. Phys. 1996, 80, 1623-1627. (34) Katz, O.; Garber, V.; Meyler, B.; Bahir, G.; Salzman, J. Gain mechanism in GaN Schottky ultraviolet detectors. Appl. Phys. Lett. 2001, 79, 1417. (35) Alsultany, F. H.; Hassan, Z.; Ahmed, N. M.; A high-sensitivity, fast-response, rapid-recovery UV photodetector fabricated based on catalyst-free growth of ZnO nanowire networks on glass substrate. Opt. Mater. 2016, 60, 30-37. (36) Chang, S. J.; Lee, K. H.; Chang, P. C.; Wang, Y. C.; Yu, C. L.; Kuo C. H.; Wu, S. L. GaN-Based Schottky Barrier Photodetectors With a 12-Pair MgxNy–GaN Buffer Layer. IEEE J. Quantum Elect. 2008, 44, 916-921.


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(37) Ravikiran, L.; Radhakrishnan, K.; Dharmarasu, N.; Agrawal, M.; Wang, Z. L.; Bruno, A.; Soci, C.; Lihuang, T.; Ang, K. S. GaN Schottky Metal-Semiconductor-Metal UV Photodetectors on Si(111) grown by Ammonia-MBE. IEEE Sensors J. 2016, 17, 72-77. (38) Chen, Q.; Yang, J. W.; Osinsky, A.; Gangopadhyay, S.; Lim, B.; Anwar, M. Z.; Khan, M. A.; Kuksenkov, D.; Temkin, H. Schottky barrier detectors on GaN for visible-blind ultraviolet detection. Appl. Phys. Lett. 1998, 70, 2277-2279.


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Figure 1. (a) Schematic illustration of GaN-based microwire arrays on patterned Si substrate. (b) A typical cross section SEM image. (c) A HRTEM image of the GaN microwire. (d) Optical microscope image of the fabricated orderly arranged detectors, the scale bar is 100 μm. (e) the enlarged image of one detector, the scale bar is 20 μm.

Figure 2. (a) Room-temperature micro-photoluminescence spectrum of the GaN microwire;

(b) Current-voltage characteristics of the fabricated GaN microwire arrays based photodetector both in the dark (black curve) and upon 325 nm UV light illumination (red curves); inset, the testing schematic diagram of the photodetector; (c)The light density dependent current-voltage curves of the GaN microwire arrays based photodetector; (d) Current variation from photodetector as a function of light intensity. (e) The responsibility


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and EQE dependent wavelength curves of the GaN microwire arrays based photodetector; (f) The specific detectivity dependent power density curves of the GaN microwire arrays based photodetector.

Figure 3. (a) Time dependent current response measured at a fixed bias of 5.0 V under on/off UV illumination. (b)Time-fitting photoresponse for rise and decay processes of the photodetector.

Figure 4. The energy-band diagram of metal semiconductor interface in darkness (a) and under UV light (b); (c) Oxygen molecular adsorption on the surface of GaN microwire in


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darkness. (d) Oxygen molecular desorption and photon-generated carrier separating near surface under UV light.

Figure 5. (a) I-V characteristics of the fabricated GaN thin film based photodetector both in the dark (black curve) and light (red curves); inset, the testing process of the detector. (b) Time-dependent photogenerated current under the illumination of 325 nm.

For Table of Contents Use Only (TOC)


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Figure 1. (a) Schematic illustration of GaN-based microwire arrays on patterned Si substrate. (b) A typical cross section SEM image. (c) A HRTEM image of the GaN microwire. (d) Optical microscope image of the fabricated orderly arranged detectors, the scale bar is 100 μm. (e) the enlarged image of one detector, the scale bar is 20 μm.


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) Room-temperature micro-photoluminescence spectrum of the GaN microwire;

(b) Current-voltage characteristics of the fabricated GaN microwire arrays based photodetector both in the dark (black curve) and upon 325 nm UV light illumination (red curves); inset, the testing schematic diagram of the photodetector; (c)The light density dependent current-voltage curves of the GaN microwire arrays based photodetector; (d) Current variation from photodetector as a function of light intensity. (e) The responsibility and EQE dependent wavelength curves of the GaN microwire arrays based photodetector; (f) The specific detectivity dependent power density curves of the GaN microwire arrays based photodetector.


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Figure 3. (a) Time dependent current response measured at a fixed bias of 5.0 V under on/off UV illumination. (b)Time-fitting photoresponse for rise and decay processes of the photodetector.


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Figure 4. The energy-band diagram of metal semiconductor interface in darkness (a) and under UV light (b); (c) Oxygen molecular adsorption on the surface of GaN microwire in darkness. (d) Oxygen molecular desorption and photon-generated carrier separating near surface under UV light.


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Figure 5. (a) I-V characteristics of the fabricated GaN thin film based photodetector both in the dark (black curve) and light (red curves); inset, the testing process of the detector. (b) Time-dependent photogenerated current under the illumination of 325 nm.


Realization of in-plane GaN microwire arrays based ultraviolet

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