n-ZnO nanorods/p+-Si (111) heterojunction light emitting diodes
© Tsai et al.; licensee Springer. 2012
Received: 10 July 2012
Accepted: 17 October 2012
Published: 6 December 2012
In this study, we report the effects of thermal annealing in nitrogen ambient on the optical and electrical properties of zinc oxide (ZnO) nanorod (NR) arrays for the application in light emission diodes (LED). The single-crystalline ZnO NR array was synthesized on p+-Si (111) substrate without seed layer using simple, low-cost, and low-temperature hydrothermal method. The substrate surface was functionalized by hydrofluoric acid and self-assembled monolayer of octadecyltrimethoxysilane ((CH3 (CH2)17Si(OCH3)3). ZnO NRs were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and micro-photoluminescence (micro-PL). The results of FESEM and XRD indicate that single crystalline ZnO NRs with (002) preferred orientation along the substrate surface is successfully grown on functionalized p+-Si (111) substrate. The current–voltage and electroluminescence (EL) characteristics of the LED show that the most suitable annealing temperature ranges from 400°C to 600°C. Both PL and EL spectra show broadband emissions, ultraviolet and visible (green-yellow) light. The white-like light emission is able to be observed by naked eyes.
KeywordsZnO Nanorods Hydrothernal method Heterojunction LED Seed layer free
Zinc oxide (ZnO) is a II-VI compound semiconductor that exhibits n-type conduction due to oxygen deficiency or zinc excess. It has a direct wide bandgap of 3.37 eV, large exciton binding energy of 60 meV at room temperature, which is much higher than that of GaN of 25 meV, and defect emissions that cover the whole visible range. Among them, one-dimensional ZnO nanostructures have attracted considerable attention for the possible integration with existing technology and easy controllability compared to other nanostructures, by virtue of their advantages in photoelectric properties. In contrast to ZnO and GaN, ZnO has large single crystals that can be grown easily. It has a very high melting point of 1,977°C, making it difficult to achieve melt to growth. Therefore, the excitons in ZnO are more thermally stable at room temperature. The toxicity and environmental impact are very low than most of the other semiconductors; ZnO is actually used as a UV-blocker in sun lotions or as an additive to human and animal food. In this regard, ZnO structures such as nanowires, nanorods, and nanoflower have been synthesized using different methods under different conditions. Thus, ZnO is a promising material for optoelectronic applications as light emitting diodes, laser diodes, field-effect transistor, solar cells, light detector, and so on. In the present study, the simple, low-cost, and low-temperature hydrothermal method was used to synthesize ZnO nanorod (NR) arrays on the p+-type silicon substrate without seed layer. After annealing in nitrogen, ZnO emits both green and yellow emissions. In recent years, Leung et al. reported that annealing at temperature as low as 200°C can result in significant enhancement of the UV emission, while the yellow defect emission can be almost entirely suppressed by annealing in reducing atmosphere at 600°C. Recently, Lee et al. have reported the preparation of ZnO nanorod/p+-GaN heterojunction light emission diode (LED) by hydrothermal method. In this paper, we investigated on improving the optical properties of the ZnO NRs by annealing the samples in nitrogen ambient under different temperatures. Thermal annealing of ZnO NRs under different temperatures also shows changes in the emission wavelength.
The single crystalline ZnO NR arrays were synthesized on p+-Si (111) substrate without seed layer using the simple, low-cost, and low-temperature hydrothermal method. The p+-Si (111) substrate was employed due to its lattice constant (3.84 Å) which is close to that of ZnO (001) (3.25 Å). Also, it possesses a hexagonal lattice structure. In this study, the substrate resistivity is below 0.01 to 0.05 Ω cm, and the doped density is about 1018 cm−3. The substrate surface was light etching by hydrofluoric (HF) acid for 10 min at room temperature after standard cleaned process to prepare -H terminals on the substrate. Water contact angle increased from about 41° to 63° measured by a contact angle meter showing that the substrate surface was covered by -H terminals. Subsequently, it was then rinsed with deionized (DI) water and dried in N2 gas flow. The Si substrate was placed together into a teflon pressure cooker with a cup of 0.2 ml octadecyltrimethoxysilane (ODS) and sealed with a cap. The cooker was placed in an oven at 150°C for 1 h to form ODS self-assembled monolayer (SAM) on the Si substrate surface. Water contact angle of ODS SAM surface increased to 81°, indicating that almost the entire substrate was covered by ODS. ZnO NRs were grown on the SAM/p+-Si substrate surface in a mixture solution consisting of DI water, 0.08 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O), and 0.08 M hexamethylenetetramine (C6H12N4, (HMT)) for 1 h at 95°C. Subsequently, each sample was thoroughly rinsed with DI water to remove any residual chemicals and salts, and dried at 45°C for many hours in the cooker. The crystalline grains were formed on the SAM, firstly. Zn2+ was continuously supplied from Zn(NO3)2·6H2O solution. There is almost no gradient of concentration mean that the concentration of zinc precursor is uniform from the roots to the top of rods, which results in the formation of ZnO nanorods with flat tops. After the reaction is finished, the highly oriented ZnO rods have been successfully prepared on seed-free Si substrate. Finally, n-ZnO NRs/p+-Si heterojunction was obtained. ZnO NRs grown on Si substrates were annealed in nitrogen gas with 1 × 102 Torr for 20 min at various temperatures (200°C to 800°C) in a rapid thermal annealing furnace. The photoresist (PR) was used as an insulating layer to fill up the space between the ZnO NRs by spin coating. The extra PR was remove by the oxygen plasma treatment in plasma striper; the etch power is 450 W acting for 40 min. ITO film was sputtered onto the top of ZnO as anticathode. The working pressure was 1.5 × 10−2 Torr with a sputtering power of 30 W and the sputtering time was 1 h. The thickness of ITO layers was about 30 nm.
The morphologies and structural properties of ZnO NRs were examined by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The optical properties and crystal defects were determined by room-temperature (RT) micrometer photoluminescence (micro-PL) using a He-Cd laser (λ = 325 nm) as the excitation source. The electroluminescence (EL) spectra were obtained by monochromator equipped in micro-PL system and using a current source (Keithley 2400, Keithley Instruments, Inc., OH, USA). The current–voltage (I-V) characteristics of the devices were measured by a source meter (Keithley 2400, Keithley Instruments, Inc., OH, USA).
ODS (molecular formula, C21H46O3Si; appearance, colorless liquid; molar mass, 374.67 g/mol; density, 0.883 g/cm3; melting point, 16°C to 17°C) was purchased from ACROS (Thermo Fisher Scientific Inc., MA, USA).
Zinc nitrate hexahydrate (molecular formula, Zn(NO3)2·6H2O; appearance, colorless and deliquescent crystals; molar mass, 297.49 g/mol; density, 2.065 g/cm3; melting point, 36.4°C) was purchased from Hayashi Pure Chemical (Osaka, Japan).
HMT (molecular formula, (CH2)6 N4; appearance, white powder; molar mass, 140.186 g/mol; density, 1.33 g/cm3; boiling point, 280°C) was also purchased from Hayashi Pure Chemical (Osaka, Japan).
Results and discussion
In the study, we have successfully synthesized n-ZnO NR arrays on p+-Si (111) substrates without seed layer by hydrothermal method growth technique for the application in heterojunction LEDs. SEM images show that the ZnO NRs are hexagonal crystallographic pillars standing vertically with respect to the substrate surface. XRD spectra results also indicate that the ZnO NRs have the wurtzite structure and grown vertically on the Si substrates along (002) direction. From the results of PL, EL, and I-V measurement, the crystallization was improved after annealing in N2 ambient at a temperature around 400°C to 600°C. The stable white-like light emission was observed by the naked eyes in the dark room at room temperature. For the future work, we will increase the density of NRs to enhance the emission intensity of n-ZnO NRs/p+-Si LED.
JKT designed this work and wrote this manuscript. JHS carried out the preparation of the samples, XRD, micro-PL, EL, and I-V measurements. TCW and THM helped in carrying out the FESEM measurements. All authors read and approved the final manuscript.
JKT obtained his Ph.D. in Physics in 2004 from the National Sun Yet-Sen University (NSYSU). He has been working at the Department of Physics, National Sun Yat-Sen University as a post doc and at the Department of Electronics Engineering and Computer Science, Tung-Fang Institute of Technology as an assistant professor. Currently, he is an assistant professor with the Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan. His research interest includes the synthesis of wide band gap material, GaN base, ZnO, and TiO2 and their application in photoelectron, light emitting diode, sensor, and solar cell. JHS obtained his masters degree in 2012 from the Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan. Now, he is waiting for wok. TCW received his Ph.D. from the Department of Electrophysics, National Chiao Tung University (NCTU), Hsinchu, Taiwan in 2007. Currently, he is an assistant professor in the Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan. His current research interests include semiconductor physics, superconducting thin films, and nanotechnology. THM was born in Tainan, Taiwan on August 1, 1967. He received his BS degree from the Department of Electrical Engineering, National Cheng Kung University (NCKU), Tainan, Taiwan in 1989, and his MSc and Ph.D. degrees from the Institute of Electrical Engineering, National Sun Yat-Sen University (), Kaohsiung, Taiwan in 1991 and 1994, respectively. Currently, he is a professor in the Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan. His current research interests include semiconductor physics, optoelectronic devices, and nanotechnology.
Field emission scanning electron microscopy
Light emission diode
The authors are grateful to WY Pang, YC Hsu, YC Wang, YC Lin, and CD Tsai for their assistance. This work was partially supported by the National Science Council of Taiwan, the Republic of China, and the Core Facilities Laboratory in Kaohsiung-Pingtung area.
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