Anomalous luminescence phenomena of indium-doped ZnO nanostructures grown on Si substrates by the hydrothermal method
© Wang et al.; licensee Springer. 2012
Received: 18 August 2011
Accepted: 30 May 2012
Published: 30 May 2012
In recent years, zinc oxide (ZnO) has become one of the most popular research materials due to its unique properties and various applications. ZnO is an intrinsic semiconductor, with a wide bandgap (3.37 eV) and large exciton binding energy (60 meV) making it suitable for many optical applications. In this experiment, the simple hydrothermal method is used to grow indium-doped ZnO nanostructures on a silicon wafer, which are then annealed at different temperatures (400°C to 1,000°C) in an abundant oxygen atmosphere. This study discusses the surface structure and optical characteristic of ZnO nanomaterials. The structure of the ZnO nanostructures is analyzed by X-ray diffraction, the superficial state by scanning electron microscopy, and the optical measurements which are carried out using the temperature-dependent photoluminescence (PL) spectra. In this study, we discuss the broad peak energy of the yellow-orange emission which shows tendency towards a blueshift with the temperature increase in the PL spectra. This differs from other common semiconductors which have an increase in their peak energy of deep-level emission along with measurement temperature.
KeywordsZinc oxide (ZnO) Nanostructure Hydrothermal method
Zinc oxide (ZnO) is an important II-VI compound semiconductor which belongs to the group with hexagonal close packing and has a wurtzite structure with P63mc symmetry. The lattice constants of ZnO are a = 3.2539 Å and c = 5.2098 Å with a perfect ratio of c/a close to 1.633. This structure has hexagonal symmetry but not a center which displays excellent piezoelectric properties. Moreover, ZnO is a semiconductor with a wide bandgap (3.3 eV) and high exciton binding energy (60 meV)  that is a marvelous property for optics. It is widely recommended for optoelectronics , sensors , and transistors  and has wide applications in transparent conducting films , rheostats , and photocatalysis devices .
Recently, many special ZnO nanostructures such as nanocastles, nanocombs, and nanocages have been synthesized through various methods. Photoluminescence (PL) studies of ZnO have been conducted for several decades. Although the ultraviolet , violet, green , orange , and near IR emissions  of this material have been researched systematically by many groups, their proper emission mechanisms are still under dispute, especially the defect-related emissions. Green emissions of ZnO were first proposed by Vanheusden  who used the V0* single ionized oxygen defect and band-bending interrelation to explain the phenomenon. A series of annealing processes are carried out, and variations in the intensity of the green emissions with the temperature dependence and oxygen vacancies are observed. The results prove that intensity of the green emission is related to the content of V0*.
In this study, the ZnO microrods are fabricated by the hydrothermal method [13–15], but the samples (ZnO) produced by this process have many defects. The PL measurements of the samples show yellow-orange emissions caused by annealing in an oxygen environment; however this may cause the occurrence of many oxygen vacancies or interstitials in the ZnO. The intrinsic emission of ZnO is also called the near band edge with a wavelength of about 378 nm (UV emission)  and has strong defect emissions which are called deep-level emissions with a wavelength of about 617 nm (yellow-orange emission) . The strong defect emissions also occasionally include visual violet and infrared light. The violet emissions can be attributed to the recombination of electrons and holes between the interstitial zinc (Zni) shallow donor levels and the valence band [16, 17]. Normally, the n-type dopants for ZnO comprise the three grouped elements such as indium , aluminum and gallium. Silver and lithium have been used for p-type doping . In this work, the ZnO nanostructure is doped with indium. This technique and doping method can change the carrier concentration leading to more research applications. Additionally, In-doped ZnO nanostructures have been grown by the hydrothermal method. They are easy to grow because of the low pressure and temperature.
The intrinsic emission is a result of the combination of free electrons or exciton-exciton oscillation. In addition, defect emission refers to the oxygen ionization vacancies above the surface and the single ionization charge of ZnO which combines with the holes. The phenomenon arising from strong defect emissions compared to weak intrinsic emissions is due to free-exciton annihilation caused by collision or recombination with each other. The result also reduces the luminescent excitons, the luminescence efficiency, and peak intensity.
This hydrothermal method , which involves heterogeneous nucleation in supersaturated solutions to grow nanocrystals on the surface, has many advantages including being an easy procedure to use, requiring a low temperature and pressure. The surface roughness obtained using a buffer layer to synthesize ZnO offers heterogeneous nucleation points. After the cleaning process, the wafer is placed in acetone, DI water, and isopropyl alcohol (IPA). Zinc acetate which has been dissolved in the alcohol is uniformly distributed onto the silicon substrate in order to increase the density. Subsequently, ZnO nanostructures grow in the combination of zinc aqueous solution blended with 28% ammonia (NH3). Finally, the samples are coated with indium acetate which has been dissolved in alcohol and then annealed in a high-temperature furnace. The heat allows the indium ions to penetrate the ZnO nanostructures.
In this study, we analyze the structure and superficial state of ZnO by high-resolution X-ray diffraction (HR-XRD), and field-emission scanning electron microscopy (FE-SEM). ZnO optical measurements are utilized to survey the temperature-dependent PL spectra. The eight samples include non-annealing and annealing at temperatures from 400°C to 1,000°C. The temperature measurement range is from 20 to 300K, with a measuring point every 20K. We discuss the yellow-orange emission caused by excess oxygen vacancies in ZnO. For realizing the temperature dependence of PL peak energy of the samples, we use the Gaussian distribution to fit the data. Consequently, we prove that the dependence of the peak energy tends to vary with the crystal size as shown by the roughly calculated values of the full width at half maximum (FWHM).
Results and discussion
It is demonstrated that there are two paths for ZnO vacancy emission resulting in strengthened peak energy when the measurement-temperature increases (blue-shift). The first path occurs as low temperature free electrons transit from the conduction band to the Zni level, then combine with a hole in the oxygen vacancy. Another path is that once free electrons attain enough energy to transit from the conduction band to the center of the vacancies under high temperature and also combine with a hole, resulting in green emission. Therefore, free electrons under high temperatures have more energy to transit the bandgap than under low temperatures which induces the blue-shift action. The experimental results reveal that the carrier-transport process is essentially attributed to the both emissions which are the luminescence of vacancy.
It is demonstrated that there are two paths for ZnO vacancy emission resulting in strengthened peak energy when the measurement temperature increases (blueshift). The first path occurs as low-temperature free-electron transit from the conduction band to the Zni level and then combines with a hole in the oxygen vacancy. Another path is that once free electrons attain enough energy to transit from the conduction band to the center of the vacancies under high temperature and also combine with a hole, resulting in green emission. Therefore, free electrons under high temperatures have more energy to transit the bandgap than under low temperatures which induces the blueshift action. The experimental results reveal that the carrier-transport process is essentially attributed to both emissions which are the luminescence of vacancy.
Field-emission scanning electron microscopy
High-resolution X-ray diffraction
Joint Committee on Powder Diffraction Standards
The authors would like to acknowledge the support from the staff of GALOIS (Group of Abel and Lie Operations In Sciences) and the QUEST Laboratory (Quantum Electro-optical Science and Technology Laboratory), Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, Chang Gung University, Taiwan. This study was financially supported by the National Science Council of the Republic of China under contract numbers NSC 97-2112-M-182-002-MY3 and NSC 100-2112-M-182-003-MY2.
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