Investigation on the photoconductive behaviors of an individual AlN nanowire under different excited lights
© Liu et al.; licensee Springer. 2012
Received: 12 July 2012
Accepted: 27 July 2012
Published: 11 August 2012
Ultra-long AlN nanowire arrays are prepared by chemical vapor deposition, and the photoconductive performances of individual nanowires are investigated in our self-built measurement system. Individual ultra-long AlN nanowire (UAN) exhibits a clear photoconductive effect under different excited lights. We attribute the positive photocurrent response of individual UAN to the dominant molecular sensitization effect. It is found that they have a much faster response speed (a rise and decay time of about 1 ms), higher photocurrent response (2.7×106), and more reproductive working performance (the photocurrent fluctuation is lower than 2%) in the air environment. Their better photoconductive performances are comparable to many nanostructures, which are suggested to be a candidate for building promising photosensitive nanodevices in the future.
Nanomaterials are very suitable for photosensitive device applications because their large surface-to-volume ratio and nanometer size are more sensitive to light illumination than in bulk materials. In recent years, many semiconductor nanostructures, such as In2Se3 nanobelt, ZnSe nanowire, CdS nanobelt, and ZnO nanowire[1–4], have been used to fabricate nanoscale photosensitive devices. However, most of the nanostructures are observed to have a low photocurrent response of <102 and a slow response time of >1 s, which cannot meet the basic requirement for practical application in photosensitive devices. Therefore, researchers have been looking for new nanomaterials as candidates.
As one of the important III-V semiconductors, AlN nanostructures have been paid much attention in recent years. It is known that AlN nanostructures have the same high thermal conductivity (K ~ W/m·k), high melting point over 2,300°C, and large direct bandgap (6.28 eV)[5–8] as those of their bulk materials, which are suggested that they should be the promising fundamental blocks in building nanoscale optoelectronic devices. Although there have been many research groups focusing on the synthesis methods of AlN nanostructures, a few reports are devoted to investigate their photoconductive performances. There are a series of difficulties existing in the research of the photosensitive properties of AlN nanostructures, which can be described as the following: (1) AlN nanostructures with uniform morphology are hard to synthesize in large scale; (2) AlN nanostructures have poor conductivity of about 10−8 to 10−6 Ω−1·cm−1 in most reports[9–11], which is too low to be applied on optoelectronic nanodevices in general; and (3) AlN nanostructures have relatively lower lengths (<2 μm) in many experiments, which is too difficult to manipulate for nanodevices. To promote the rapid development of AlN nanomaterials in photosensitive applications, the researchers must solve these difficulties in advance.
Here, we report the synthesis and characterization of ultra-long AlN nanowire (UAN) arrays by chemical vapor deposition (CVD). The photoconductive behaviors of individual UAN under different excited lights are compared together. Moreover, their photoconductive mechanism is illustrated by a combination of two existing models.
Transmission electron microscopy (TEM) (Tecnai-20, Philips Tecnai, Amsterdam, The Netherlands) and X-ray diffraction (XRD) (RINT 2400, Rigaku Corporation, Tokyo, Japan) techniques were used to research on the crystalline structures of UANs. Their optical property measurements were performed on a HITACHI F-4500 type spectrophotometer (Hitachi High-Tech, Minato-ku, Tokyo, Japan) with a 325-nm He-Cd laser as the excitation source. In addition, the working performance of a single UAN under different excited lights was investigated in our self-built measurement system.
Results and discussion
where E is the photon energy, h is the Planck constant (6.63 × 10−34 J·s), γ is the frequency of the emission light, c is the light velocity (3×108 m/s), and λ is the wavelength of the emission light. Hence, the peak at 371.8 nm corresponds to the energy level of 3.34 eV in the bandgap; the peak at 413.8 nm comes from the energy level of 3.01 eV, and the peak at 450.6 nm is from the energy level of 2.76 eV, based on this calculation method. According to the reports of other researchers[16–18], the weak ultraviolet light peak centered at 371.8 nm and the blue light emissions at 413. 8 nm should be attributed to the existence of oxygen impurities in the nanowire. In general, the oxygen atoms substitute for nitrogen sites in the lattice, and they usually form the oxygen point defects,, nitrogen vacancies (VN), and complexes[19–21]. Their corresponding energy levels should be located in the mid gap of AlN nanowires, which result in the formation of the two PL peaks. The oxygen contents are suggested to have two possible sources: one is that the oxygen atoms can be released from the quartz tube used in the high-temperature growth process, and the other possibility is that the oxygen atoms are absorbed on the surface of the nanowire after the sample is removed from the preparation vacuum chamber. In our experiment, the 450.6-nm peak is related to the N vacancy due to the nonstoichiometric composition in the reaction[18–21]. The broadening of the PL spectrum of the AlN nanowire ascribes to the oxygen-related defects and the surface adsorbents. Because the deficiency of oxygen or nitrogen leads to the formation of the deep donor or acceptor levels in the AlN bandgap, the photo-generated electrons and holes simultaneously occur in the conduction band and the valence band under illumination of a 325-nm light. The deep donor levels can provide the recombination centers for these carriers in the energy gap of AlN, and radiative luminescence will be found in the PL measurement, as observed in Figure3c.
The work function of the Cr electrode is about 4.6 eV, which is higher than that of the AlN (3.8 eV). Thus, the Schottky barrier should occur when they are built in nanometer devices, which induces that their I V curves exhibit nonlinear relationships. Based on the improved metal-insulator-vacuum (MIV) model in our recent work, the I V curves at high voltage can reflect the native conductive behaviors of the nanowire because the Schottky barrier at high applied voltage will be tunneled through and will decrease much lower than the intrinsic resistance of the nanowire. In our measurement, the photocurrent at high voltage is almost proportional to the applied voltage, which proves that the photosensitive process of individual UAN under illumination should obey the improved MIV model. In other words, the photosensitive performance to several wavelengths of light resulted from single AlN nanowire rather than the Schottky barrier.
Comparison of the room-temperature photoconductive performance of single UANs with other nanostructures
Operation voltage (V)
The ratio of photocurrent to dark current (IP/ID)
Detector current responsivity (R λ ) (A/W)
Device stability fluctuation
Approximately 20 μs
Approximately 50 s
Approximately 1 s
Approximately 1 ms
In this research
UANs with length over tens of micrometers have been successfully fabricated in a large area by CVD method, and they are integrated into nanometer photoconductors by a simple ultraviolet photolithography technique. Individual UANs are found to have good photoconductive behaviors under some given excited lights, such as a faster response time of about 1 ms, higher photosensitivity (20), and more stable working performance. Their photosensitive mechanism is attributed to the combination of the molecular sensitization effect and the two-center recombination effect. UANs should be a very good candidate in photosensitive applications if their optical properties can be further controlled by element doping.
The authors are thankful for the support from the National Basic Research Program of China (973 Program, grant nos. 2007CB935501, 2010CB327703; the National Natural Science foundation of China, grant nos. 50802117, 51072237), the Fundamental Research Funds for the Central Universities (2009-30000-3161452), China Scholarship Council Fund for Young Backbone Teacher, the Science and Technology Department of Guangdong Province, the Education Department of Guangdong Province, and the Science and Technology Department of Guangzhou City.
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