Research Article www.acsami.org
Ultrasensitive and Highly Selective Photodetections of UV‑A Rays Based on Individual Bicrystalline GaN Nanowire Xinglai Zhang,† Baodan Liu,*,† Qingyun Liu,† Wenjin Yang,† Changmin Xiong,*,‡ Jing Li,† and Xin Jiang*,† †
Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72 Wenhua Road, Shenyang 110016 China ‡ Department of Physics, Beijing Normal University, 100875, Beijing, P. R. China S Supporting Information *
ABSTRACT: The detection of UV-A rays (wavelength of 320−400 nm) using functional semiconductor nanostructures is of great importance in either fundamental research or technological applications. In this work, we report the catalytic synthesis of peculiar bicrystalline GaN nanowires and their utilization for building high-performance optoelectronic nanodevices. The asprepared UV-A photodetector based on individual bicrystalline GaN nanowire demonstrates a fast photoresponse time (144 ms), a high wavelength selectivity (UV-A light response only), an ultrahigh photoresponsivity of 1.74 × 107 A/W and EQE of 6.08 × 109%, a sensitivity of 2 × 104%, and a very large on/off ratio of more than two orders, as well as robust photocurrent stability (photocurrent fluctuation of less than 7% among 4000 s), showing predominant advantages in comparison with other peer semiconductor photodetectors. The outstanding optoelectronic performance of the bicrystalline GaN nanowire UV-A photodetector is further analyzed based on a detailed high-resolution transmission electron microscope (HRTEM) study, and the two separated crystal domains within the GaN nanowires are believed to provide separated and rapid carrier transfer channels. This work paves a solid way toward the integration of high-performance optoelectronic nanodevices based on bicrystalline or horizontally aligned one-dimensional semiconductor nanostructures. KEYWORDS: GaN, bicrystalline nanowires, HRTEM, UV-A ray, photodetectors
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UV-A (400−320 nm), UV-B (320−280 nm), and UV-C (280− 200 nm).15 Most UV-B light and all the UV-C rays with shorter wavelength and higher energies are predominantly absorbed by the stratospheric ozone layer and molecules in sunscreens and cannot reach the ground, holding less possibility to harm the human skin. However, the UV-A ray with the longest wavelength (320−400 nm) can easily pass through the ozonosphere and arrive at the earth surface. The overexposure to UV-A rays may lead to various health problems from premature aging to skin cancer.16 Therefore, it is essential and technologically important not only to establish effective strategies to avoid UV-A ray damage, but also to fabricate
ompared with their bulk peers, one-dimensional (1D) semiconductor nanostructures with smaller size, huge surface-to-volume ratio, decent crystal quality and high light extraction/absorption efficiency are considered as promising building blocks for constructing high-performance optoelectronic nanodevices ranging from field-effect transistors,1,2 solar cells,3,4 field emitters,5 lasers diodes,6 piezo nanogenerators,7 and photodetectors.8−13 Among these nanodevices, UV photodetectors received intensive research interest and attentions for their wide and technologically important applications in missile plume detection, binary switches in imaging techniques, secure communication, flame alarms, environmental pollution monitoring, future memory storage, and optoelectronic circuits.14 According to the different wavelengths, the electromagnetic rays covering in the UV range can be further divided into three typical spectral regions: © 2016 American Chemical Society
Received: November 21, 2016 Accepted: December 28, 2016 Published: December 28, 2016 2669
DOI: 10.1021/acsami.6b14907 ACS Appl. Mater. Interfaces 2017, 9, 2669−2677
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a,b) Low and high magnification top view SEM images of as-prepared bicrystalline GaN nanowires; inset of panel a shows the photograph of bicrystalline GaN nanowires grown on a sapphire substrate; (c) cross section SEM image of as-prepared GaN nanowires terminated with Au catalysts, which were marked by dotted circles; (d) XRD pattern of primary bicrystalline GaN nanowires.
conductor nanostructures are strongly dependent on their size, morphology, composition, and crystal structure.32 Especially, the crystalline structure including some structural defects such as bicrystal may have a significant influence on the device performance.33,34 It is speculated that the one-dimensional bicrystalline structure (mostly bicrystalline nanowire, nanorods, or nanoribbons) can be regarded as two separated transfer channels for photogenerated carriers, showing incomparable advantage than individual nanostructure in building highperformance optoelectronic devices. However, the rational synthesis of bicrystalline GaN nanowires and their utilization for ultrasensitive UV detection still remain challenging even though extensive efforts have been made in the past years. Herein, we reported the synthesis of bicrystalline GaN nanowires using a catalyst-assisted vapor−liquid−solid (VLS) method and their utilization for the building of ultrasensitive UV-A photodetector. The GaN nanowires show an asymmetrical nature of bicrystal, superior crystallinity and Auterminated catalytic growth. The UV photodetector based on individual bicrystalline GaN nanowire shows remarkable sensitivity and superior wavelength selectivity with respect to UV-A electromagnetic rays. Furthermore, the GaN photodetector also exhibits ultrafast response, high discrimination ratio, robust stability, and a strong dependence of photocurrent on light intensity. These outstanding optoelectronic properties make GaN UV-A sensor more competitive than its peers in UV ray detection, which will open up more opportunities in nextgeneration optoelectronic nanodevices. To conduct the growth of bicrystalline GaN nanowires, wellpolished sapphire wafer and a thin Au layer above it are utilized as the substrate and the catalyst for the nucleation and fast growth of GaN nanowires during the chemical vapor deposition (CVD) process.35 Figure 1a−c shows the representative scanning electron microscopy (SEM) images of GaN nanowires array grown at 1100 °C for 30 min. A low magnification SEM
various optoelectronic devices with decent sensitivity and wavelength selectivity to monitor it. For the purpose of boosting sensitivity and selectivity of photodetectors, extensive efforts have been made up to now. For instance, Shen et al.17 synthesized the hierarchical CdSe nanowires flexible visiblelight photodetectors with ultrahigh photo/dark current ratio, high specific detectivity, fast response speed, and excellent environmental stability. Li et al.18 first designed the novel pervoskite-based self-powered photodetectors, which demonstrated broad spectra response. Peng et al.19 designed ZnO UV sensors with enhanced UV detection performance through the addition of an absorbing antireflection layer and humectant encapsulation. Additionally, Shen et al.20 designed TiO2− ZnTiO3 heterojunction nanowire-based photodetectors to detect solar-blind UV light for the purpose of enhancing the wavelength selectivity. Nevertheless, the development of highly sensitive and selective photodetection based on semiconductor nanostructures is still urgently required for specific applications such as UV-A ray monitoring. Among various semiconductor materials, GaN as an important III−V semiconductor with a wide direct band gap (3.4 eV)21−23 has received tremendous research interest in recent years because of its high melting point, outstanding chemical stability, and high carrier mobility, as well as high electrical break down voltage.24 Such excellent optical and electrical properties enable GaN crystal to find broad applications in the field of light-emitting diodes (LEDs),25 field-effect transistors (FETs),23 or even nanowire lasers.26 GaN also shows a very promising application potential in ultraviolet ray detection. So far, most works mainly focused on film-based GaN UV detectors for the facile growth of highly crystallized GaN layers,27−29 whereas the nanowire-base UV detectors are rarely reported for the reason for the inavailability of high-quality GaN nanowires.30,31 On the other hand, it has been reported that the optoelectronic properties of semi2670
DOI: 10.1021/acsami.6b14907 ACS Appl. Mater. Interfaces 2017, 9, 2669−2677
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a,b) Low and high magnification TEM images of a single bicrystalline GaN nanowire with Au catalyst terminated at the tip end; scale bar in panels a and b is 500 and 200 nm, respectively; (c−e) HRTEM images of the three selected areas in bicrystalline GaN nanowire in panel b; (f−h) their corresponding FFT patterns.
investigate the microstructure and crystallinity, atomically resolved high resolution transmission electron microscopy (HRTEM) is used to further characterize the as-prepared GaN nanowires. Figure 2 panels a and b show typical TEM images of the GaN nanowire and one can see that the GaN nanowire indeed comprises two crystal domains with a long and narrow crystal boundary along the middle of the GaN nanowire, implying the as-synthesized GaN nanowires are bicrystalline structure. Such bicrystalline GaN nanowires are frequently observed in our as-prepared samples during TEM analysis. In addition, the catalyst particle terminated at the tip-end further verifies the catalytic growth of GaN nanowires via a vapor− liquid−solid process. To examine and disclose the crystallization and crystallographic correlativity of GaN bicrystalline nanowires, three representative areas in the bicrystalline GaN nanowire are selected for HRTEM analysis, as marked in Figure 2b. The corresponding atomically resolved HRTEM images shown in Figure 2 panels c and e confirm that the two crystal domains are well crystallized and no structural defect such as stacking fault and cubic GaN phase is observed. The marked distance of 0.26 and 0.27 nm corresponds well to the d-spacing of (0002) and (101̅0) planes of wurtzite GaN, respectively. The succinct diffraction spots in the fast Fourier transform (FFT) pattern shown in Figure 2f,h further demonstrate the singlecrystal nature of each crystal domain. The structure correlativity of these two crystals can be further verified from the HRTEM image and FFT pattern in the crystal boundary. Figure 2d
image in Figure 1a reveals that the GaN nanowires are about 8 μm in length, and uniformly cover the whole surface of sapphire substrate (the inset of Figure 1a). The corresponding high magnification SEM images in Figure 1 panels b and c confirm that each nanowire has a uniform diameter of 150 nm along its entire length. The Au nanoparticles were clearly visible at the tip of GaN nanowires, and the average radial dimensions of the nanowires are in good agreement with the size of the gold particles, which suggests that the growth of the GaN nanowires is dominated by an Au-catalyzed VLS mechanism, as reported previously.36 An energy-dispersive X-ray spectrum (EDS, in Figure S2) acquired from a nanowire array exhibits strong Ga and N peaks, and the atomic ratio of Ga and N is close to the stoichiometric value of 1:1, suggesting the chemical purity of GaN nanowires. The X-ray diffraction (XRD) pattern (Figure 1d) shows that all the diffraction peaks can be indexed to a hexagonal GaN phase (JCPDS Card, No. 50-0792), a cubic Au phase (JCPDS 04-0784) from catalyst and a hexagonal Al2O3 phase (JCPDS 46-1212) from substrate. Figure S3 depicts a room-temperature micro-Raman spectrum of the GaN nanowires. The peaks, located at 534.8, 557.3, 567.7, and 730.4 cm−1, can be assigned to the Raman signature of GaN.37 GaN exists in the form of structurally stable wurtzite-type hexagonal symmetry and metastable zinc-blende-type cubic symmetry. The catalytic growth of GaN nanostructures occasionally involves the formation of mixed phases and structural defects such as stacking faults and bicrystals.38 To 2671
DOI: 10.1021/acsami.6b14907 ACS Appl. Mater. Interfaces 2017, 9, 2669−2677
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) Schematic diagram of a bicrystalline GaN nanowire UV-A photodetector; (b) I−V curves of the GaN photodetector illuminated with different wavelengths and under dark condition; inset are the logarithmic plots of the I−V curves; (c,d) photocurrent and photoresponse curves of bicrystalline GaN nanowire UV-A photodetector as a dependence of different wavelengths ranging from 200 to 700 nm, which is measured at a bias of 3 V. Inset of panel d is the typical SEM image of a GaN UV-A photodetector.
exhibits a clear HRTEM image of crystal domain boundary, which is separated by an unambiguous interface in the middle. The corresponding FFT pattern shown in Figure 2g reflects that two sets of separated diffraction patterns coexist and constitute the peculiar bicrystalline GaN nanowires. Unlike the mirror-like symmetrical GaN and ZnS bicrystalline nanostructures in our previous work,35,39 the GaN bicrystalline nanowires synthesized in the current work exhibit an asymmetrical characteristic, as observed in the HRTEM image and FFT pattern (Figure 2d,g). This unique bicrystalline GaN nanowire with two single crystal domains provides two ideally segregated carrier transfer channels and thus can be used for the building of a high-performance UV photodetector with sufficient photocurrent collection. Previous studies have shown the potential of GaN nanostructures in constructing sensitive UV-photodetectors.40 To further utilize the peculiar bicrystalline structure (two parallel carrier channels for enhanced photocurrent) of GaN nanowires, a photodetector based on individual GaN nanowire is built and its optoelectronic performance is evaluated. Figure 3a shows a schematic configuration of the GaN nanowire photodetector fabricated on a p+-Si substrate with a 100 nm SiO2 layer and coated with Ti/Au (40 nm/60 nm) electrodes on two ends. The GaN nanodevice is first illuminated under different light wavelengths of 360, 370, 400, and 500 nm as well as under dark condition to study the wavelength selectivity. From the I−V curves presented in Figure 3b, it can be clearly seen that the GaN photodetector shows an equivalent photocurrent of 1.3 nA under 500 nm wavelength irradiation and dark under an applied voltage of 5 V, as evidenced from the logarithmic plot in the inset. Shifting the wavelength to 400 nm can only lead to a slight increase of the photocurrent (Figure 3b). However, further reduction of the irradiation wavelength to 370 nm causes a sharp increase of the photocurrent as
compared to that in 400 nm, and the maximum photocurrent of 246 nA can be obtained from the GaN nanodevice when it is illuminated with a 360 nm wavelength, showing 2 orders of magnitude larger than the photocurrent in 500 nm excitation. The drastic increase of photocurrent at 360 nm reflects the high wavelength selectivity of the GaN photodetector. It is speculated that the generation of vast excited electron−hole pairs are responsible for the significant increase of photocurrent when the bicrystalline GaN nanowire is subjected to a higher energy excitation than its band gap (3.42 eV, 362 nm, in Figure S4). In addition, all I−V plots exhibit approximately linear behavior, indicating a good Ohmic contact between the GaN and Ti/Au electrodes. To obtain the wavelength-dependent photocurrent of the bicrystalline GaN nanowire UV-A detector, the photocurrent of the GaN nanodevice is recorded with a light wavelength scanning from 200 to 700 nm under a bias voltage of 5 V, as presented in Figure 3c. It is found that the photocurrent exhibits the maximum value with the peak wavelength located at 360 nm, in good agreement with the I−V result under 360 nm illumination. The applied light intensity decreases sharply in the UV region with the decreasing of wavelength owing to the spectral output characteristic of the xenon lamp. Therefore, the photocurrent exhibits a drastic decrease at the UV region when it reaches the maximum at 360 nm (see Figure 3c). Coincidentally, the photoresponse spectrum also shows similar wavelength selectivity in the investigated range of 200−700 nm (Figure 3d). The high wavelength selectivity of bicrystalline GaN nanowire photodetector can be explained as follows: the wide band gap of GaN (3.4 eV) can only allow for an absorption of UV-rays and the electron−hole pairs cannot be excited by the incident light with energy less than its band gap, and thus contribute little to the photocurrent. Meanwhile, the weak photoresponse to the wavelength shorter than 350 nm is 2672
DOI: 10.1021/acsami.6b14907 ACS Appl. Mater. Interfaces 2017, 9, 2669−2677
Research Article
ACS Applied Materials & Interfaces
Figure 4. Reproducible on/off switching curves of the bicrystalline GaN nanowire UV-A photodetector (a) under a light intensity of 2.38, 3.84, and 6.41 μW/cm2, respectively, and a 5 V bias voltage and 360 nm light illumination; (b) under a bias voltage of 1, 3, and 5 V, respectively, and a fixed 360 nm light illumination with a power of 6.41 μW/cm2; (c) under different irradiation wavelengths from 360 to 400 nm and a 5 V bias voltage and a power density of 6.41 μW/cm2; (d) the enlarged portion of 360 nm curve in (c).
Figure 5. (a) I−V characteristics of the bicrystalline GaN nanowire photodetector under 360 nm wavelength illumination with different power intensities; (b) the light intensity-dependent photocurrent at a bias of 5 V upon 360 nm light illumination and the corresponding fitting curve using the power law; (c) light intensity-dependent responsivity and external quantum efficiency at an excitation wavelength of 360 nm under a bias of 5 V; (d) a photocurrent−time (Iph−T) plot acquired under an applied voltage of 5 V and 360 nm light illumination.
UV-A ray and is intrinsically blind to UV-B, UV-C, and visible light. Apart from the device sensitivity and selectivity, the response speed and repeatability of the GaN photodetector are also considered as two key factors to comprehensively evaluate the device performance. Figure 4a−c depicts the time-dependent photocurrent of the bicrystalline GaN nanowire UV-A
observed in the GaN photodetector due to the predominant absorption of high-energy photons at or near the surface of the semiconductor, leading to a shorter lifetime.41,42 As a result, the photogenerated carriers produced under shorter light wavelength irradiation will contribute less to the photoresponse. Consequently, the bicrystalline GaN nanowire UV detector shows a very high wavelength sensitivity and selectivity with 2673
DOI: 10.1021/acsami.6b14907 ACS Appl. Mater. Interfaces 2017, 9, 2669−2677
Research Article
ACS Applied Materials & Interfaces Table 1. Lists of Different 1D Semiconductor Photodetector Parameters photodetectors
light of detection
GaN nanowire GaN nanowire GaN nanowire GaN nanowire GaN nanowire ZnO microrod ZnO nanowire ZnO nanowire ZnSe nanobelt K2Nb8O21 nanowire Zn2GeO4 Nb2O5 nanobelt SnO2 nanowire bicrystalline GaN nanowire
360 nm
on/off ratio
Rλ (A/W)
EQE (%)
70.4
>104
D* (Jones)
rise time
decay time >120 s
365 nm 325 nm 325 254 365 390 320 260 320 320 360
nm nm nm nm nm nm nm nm nm
103 102
105 11.3 ∼10 3.3 5 203
0.003 2.2 × 104 4.0 × 104 1.3 × 103 7.5 × 106 0.12 2.53 5.11 × 103 15.2 1.74 × 107
1.5 3.2 × 107 1.9 × 107 1.5 × 105 2.6 × 107 37.2 9.82 2.45 × 106 60.7 1.32 × 109 6.08 × 109
9.91 × 1014 3.3 × 1017
0.3 s
