1.1 Basic property of ZnO and significance of ZnO related photonic devices
Zinc oxide (ZnO) is a promising material for a variety of practical applications [1]
such as piezoelectric transducers, optical waveguides, surface acoustic wave devices, varistors, phosphors, solar cells, chemical and gas sensors, transparent electrodes, spin functional devices, UV-light emitters [2] and ultraviolet laser diodes. The basic properties of ZnO can be obtained by examining Table 1-1, which compares the material properties of other relevant semiconductors [3].
Table 1-1. Comparison of properties of ZnO with those of other wide band gap semiconductors [3].
ZnO has attracted significant scientific and technological attention due to its wide direct band gap (3.37 eV) that is suitable for photonic applications in the ultraviolet (UV) or blue spectrum range [4]. In this regard, a large exciton binding energy of ZnO is 60 meV, which is significantly larger than that of GaN (25 meV) and
ZnS (39 meV) and other wide-gap semiconductors. The large exciton binding energy allows stable existence of excitons and efficient excitonic emission at room temperature (thermal energy 26 meV). The basic material parameters of ZnO are also shown in Table 1-2. To realize any type of device technology, these parameters are important to have control over the concentration of intentionally introduced impurities (dopants), which are responsible for the electrical properties of ZnO. The dopants determine whether the current (and, ultimately, the information processed by the device) is carried by electrons or holes.
Table 1-2. Properties of wurtzite ZnO.
The structure of ZnO crystal is shown in Fig 1-1 which has a hexagonal wurtzite structure (space group C6mc) with lattice parameters a = 0.3249 nm and c = 0.5207 nm. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternately along the c-axis, in which a1, a2, and c are the unit vectors in a unit cell, the large and small circles denote the anion and cation atoms, respectively. The tetrahedral coordination in ZnO results in non-central symmetric structure and consequently the development of piezoelectricity and pyroelectricity.
Fig. 1-1 The wurtzite structure model of ZnO.
1.2 General review of ZnO nanostructures
ZnO nanocrystals have recently attracted broad attention in fundamental studies and technical applications [5] because of their distinguished performance in electronics, optics and photonics. Therefore, in the last few decades, a variety of ZnO nanostructure morphologies, such as nanowires [6], nanorods [7, 8], tetrapods [9], nanoribbons/belts [9], and nanoparticles [10, 11] have been reported. Recently, novel morphologies such as hierarchical nanostructures [12], bridge-/nail-like nanostructures [13], tubular nanostructures [14], nanosheets [15], nanopropeller arrays [16], nanohelixes [17], and nanorings [17] have, amongst others, been demonstrated. These diverse ZnO nanostructures have been fabricated by various methods, such as thermal evaporation [9], metal–organic vapor phase epitaxy (MOVPE) [8], laser ablation, hydrothermal synthesis [7], sol-gel method [10, 11] and template-based synthesis [6]. Several recent review articles have summarized progress in the growth and applications of ZnO nanostructures [4, 18]. Some of the possible ZnO nanostructure morphologies are shown in Fig. 1.2.
Additionally, when the dimension of semiconductors are reduced from three (bulk material) to the quasi-zero dimensional semiconductor structures such as quantum dots (QDs), the optical properties of QDs are much different from the bulk materials. There are two physical mechanisms in modifying the energy band
structure of nanostructures, i.e., the quantum confinement effect (QCE) [19] and surface states [20]. These two mechanisms compete with each other to influence PL spectra. For nanodots or nanostructures in ZnO system with diameters less than 10 nm, the QCE plays a dominant role as has been much reported [21]. On the other hand, the surface-to-volume ratio also brings much influence on the system’s Hamiltonian when the material size is reduced to the nanometer scale [22]. The predominance of surface states is responsible for many novel physical features of nanomaterials. In the past decade, various groups have devoted to produce ZnO QDs and study the properties. For instance, Guo et al. [23] exhibited significantly enhanced UV luminescence, diminished visible luminescence and excellent third-order nonlinear optical response with poly vinyl pyrrolidone (PVP) modified surface of ZnO nanoparticles. Pan et al. [24] predicted a significant increase in the intensity ratio of the deep level to the near band edge emission is observed with ever-increasing nanorod surface-aspect ratio. Fonoberov et al. [25] have theoretically investigated that, depending on the fabrication technique and ZnO QD surface quality, the origin of UV photoluminescence (PL) in ZnO QDs is either recombination of confined exciton or surface-bound ionized acceptor-exciton complexs. Although there were many experiments to describe the behavior of ZnO nanostructure, more and more unique behaviors are still continuously being explored.
Fig. 1.2 Representative scanning electron microscopy images of various ZnO nanostructure morphologies [4].
1.3 Motivations
Recently, the power-dependent photoluminescence (PL) of ZnO bulk associated with biexciton recombination has been investigated by several research groups [26-29]. Zhang et al. [26], who grew ZnO rods by metalorganic chemical vapor deposition, indicated that the biexciton intensity is proportional to the 1.7th power of the excitation density. Besides, we have observed the intensity of biexciton emission in ZnO powder is proportional to the 1.86th power of the excitation power at T = 80 K, but it is close to unity exponent or even sub-linear when it is measured at the lower temperature [30]. Acoustic and optical phonon scatterings playing key roles in efficient exciton relaxation are responsible for bounding two cooled excitons to form
biexciton at various temperatures. At low temperature the acoustic phonon scattering is the dominant mechanism for exciton thermalization while the optical phonon scattering will participate in when the exciton kinetic energy approaches to the energy of the lowest optical phonon about T = 80 K. The efficient cooling of exciton with the assistance of optical phonon scattering allows effectively bounding exciton pairs to form biexcitons.
However, Kim et al. [31] have reported the spectra of power-dependent PL in ZnO nanorods synthesized by standard Schlenk techniques remain nearly unchanged spectral profile as increasing the excitation intensity as compared with 40 meV red shift for bulk crystal with their maximum excitation intensity. They attributed this finding to the quantum confinement effects that can alter the properties of exciton states and claimed in this study the exciton states of the nanorods are stable even when the excitation intensity reaches the Mott density of the bulk crystal due to a smaller exciton size and an enhanced exciton binding energy [32]. On the other hand, Bagnall et al. [33] have observed the red-shifted PL peak of ~50 meV in ZnO powder that may be attributed to the exciton-exciton scattering (or P band) rather than electron-hole plasma (EHP or N band) with > 100 meV red shift.
The characterizations of ZnO QDs are complicated problems to be investigated.
And, it is imperative to understand the optical properties of ZnO QDs since they play
important roles in stimulated emission and gain process in real photonic device structures.
1.4 Organization of the thesis
This thesis is organized as follows. Chapter 2 covers the theoretical background of experiments such as sol-gel method, X-ray diffraction (XRD), photoluminescence (PL) characterization, and a general concept of quantum effect, fundamental optical transitions and ZnO excitons-related emissions. In Chapter 3, we describe the experimental details including the measurement apparatus and processes. By means of the XRD and PL spectroscopy, the crystal structures and the optical emission properties of ZnO QDs grown by the sol-gel method will be investigated and discussed in Chapter 4. Finally, in Chapter 5, we conclude the studies on the ZnO QDs and propose several topics of the future works.