O RGANIZATION OF THE T HESIS

In this thesis, we will discuss and understand the evolution of ZnO quantum dots.

Chapter 2 covers the theoretical background of quantum effect, and fundamental optical transitions of semiconductors, such as absorption and excitons-relaxed emissions. In chapter 3, we describe the experimental details including the process of fabrication and analysis methods after measured. Two of methods of ZnO quantum dots embedded in SiO2 by magnetron sputtering will be investigated and discussed in chapter 4 and 5. Finally, in chapter 6, we conclude the studies.

Table 1-1. Comparison of properties of ZnO with other wide bandgap semiconductors [17].

Figure 1-1. Splitting of energy levels for small quantum dots owing to the quantum confinement effect and aB is Bohr radius.

Figure 1-2. High efficiency of 50.3% for Si-based IBSC. (a) Schematic structure. (b) The device energy band diagram. (c) The efficiency contour plot depends on the

Si-nanodisk quantum structure with maximum efficiency of 50.3%. [12]

Figure 1-3. (a) Varietical ZnO nanostructures. (b) ZnO quantum dot. (c) and(d) are ZnO nanorods. [20]

Figure 1-4. Schematic band diagram of all emissions of ZnO. [19]

Figure 1-5. Schematic diagram of quantum effect due to the size-dependence.

Figure 1-6. The simulation of energy varied with different dot radius.

Chapter 2

Theoretical Background

2.1 Quantum Effect

During the last decade, quantum effect of semiconductor structure has been a subject of extensive studies. Quantum-size effect is one of that. A wide variety of materials has been growth as nanostructure with low-dimensional from three (bulk material) to the quasi-zero dimension. The quasi-zero dimension structures are usually called quantum dots (QDs). Quantum effect is most directly detected as the energy shift of the optical absorption and luminescence peak in the materials from few tens to few hundred in meV. The theoretical calculation of quantum confinement of interacting electron-hole in semiconductor was introduced in the last 1980s.

2.1.1 Quantum Effect

The model derived from the physical nature of cluster molecular orbital (MO) to solve Schrodinger equation using Hamiltonian [5-7]. The general wave function form of a Bloch theory is

_kre ^⃑∙ _kr

kr + nµkr

The _kis periodic function of unit cell with a relatively weak k dependence, and e ^⃑∙ factor is continual plane wave-like with a wavelength /k which is longer

than a unit cell. The quantum wave number k is infinitely delocalized in the limit of band structure, and ignore the weak k dependence of . The ir⃑is only dominated by k state from the conduction band as the cluster is not too small. And the expectation value of electron energy of size-dependence quantum effect is

Ei = | < i Hi > | ≈ E + ∑ _{, ,} [ ]

Here only keep the lowest order nonzero term in Taylor’s series expansion for energy and the cluster is considered as a spherical symmetry to ignore the later terms.

The effective mass tensor is shown as

Mi,j = h 4π

Where h is Planck constant. For the most semiconductor, the effect mass m_e near k=0 of conduction band is taken due to the isotropic tensor with average diagonal element. Here get the energy form as

E ≈ E + h 8m R

The quantum localized energy of an extra electron in a very small crystallite

contributed to the second term on the right hand. It is like a particle in a box. There is only considered about a single electron on the bottom of conduction band above.

However, it is more complex to consider the hole on the top of valence band, spin, spin-orbit interaction, and the perturbation. The later results in the energy band degenerate. Considering the shielded Coulomb interaction, the model of Hamiltonian for the cluster’s lowest excited state is corrected as

H = ∇ ∇ −

| |+ polarization terms

An analytical approximation for the first excited state is

E^∗_{( )} ≅ E _{( )}+ + – ^. + smaller terms

Where E^∗_{( )} and E _{( )} are the energy bandgap of the quantum dots and bulk materials. R is the radius of clusters (quantum dots), m_e and m_h are the effective mass of electrons and holes, and ϵ is dielectric constant.

This formula is based on weak Coulomb interaction and showed an expression of the energy of first excited state (i.e. the lowest eigenstate) in the limit of the strong confinement. It resulted in a lower energy as the Coulomb term R^-1 dominated, a higher energy as the quantum localization term R^-2. As long as the quantum size is small enough, the energy bandgap will increase apparently.

2.2 Optical Principles

2.2.1 Direct Bandgap Semiconductor

In solid state physics, the bandgap of semiconductor is including direct and indirect [4]. The difference of them is depended on the relative position of the conduction band minimum and the valence band maximum on E-K diagram in the Brillouin Zone (B.Z.), as shown in Figure 2-1. Figure 2-1(a) illustrates the direct gap, both are k=0 or the same value of k at the B.Z. center, while Figure 2-1(b) shows the indirect gap with the conduction band maximum does not occur at k=0, but rather at some other value of k. It means that the transition of indirect gap material must involve a phonon to conserve momentum. In other words, the transition of a direct gap material occurs without any phonons being involved.

2.2.2 Optical Absorption

Optical absorption occurs possibly as the photon energy is larger than the bandgap Eg between the lower band and the final state of the upper band [4]. Those bands must satisfy the selection rules and Pauli exclusion principle demands during the interband transition. Figure2-2 illustrates the transition, and the relative law of conservation of energy shows as followed,

E_f = E_i+ hν

where Ei is the energy of electron in the lower band, Ef is the energy of the final state in the upper band, and hνis the photon energy. This is implies that the absorption shows a threshold behavior: interband transitions will not be possible unless hν > E .

The optical absorption is quantified by its absorption coefficient , and assuming a direct transition for semiconductor with parabolic bands. The absorption coefficient can be described with the different powers of the absorption coefficient against photo energy for direct and indirect bandgap material.  is related to photon energy according to the following formula,

α ∗ (hν) ∝ (hν − E ) ^/ , for direct bandgap α∗ (hν) ∝ (hν − E ) , for indirect bandgap

This is a common and simple method to distinguish whether a bandgap is direct or indirect by using absorption spectroscopy.

2.2.3 Photoluminescence (PL)

In solids, the reverse process of absorption of radiative emission in which electrons in an excited state drop down to the lower level by emitting photons is called luminescence. Interband luminescence occurs in a semiconductor when a excited electron at conduction band drop back to the valence band by the emission of a photon.

This simultaneously reduces the number of electrons in the conduction band and holes in valence band by one. It corresponds to the annihilation of an electron-hole pair, and is known as radiative electron-hole recombination. Direct bandgap material has a superiority to emitting a photon owing to that the electron and hole recombine must have the same k-vector. The luminescence spectrum usually consists of a narrow emission line close to the bandgap energy. As for indirect bandgap material, a phonon must be emitted or absorbed when the photon is emitted. Therefore indirect bandgap materials are rarely applied as light emitters.

Photoluminescence is one of all the mechanisms of luminescence. The light re-emits after absorbing a photon of higher energy than bandgap Eg. Each step corresponds to the emission of a photon with the correct energy and momentum to satisfy the conservation laws. Generally excited electrons do not remain in these state high up in conduction band, because they can loss its energy very quickly by emitting phonons. The same conditions apply to the relaxation of the holes in the valence band.

They have enough time to relax to the bottom of their band before emitting photons, as indicated in Figure 2-3. The process occurring during photoluminescence in a direct gap semiconductor after excitation at frequency v_L.

Photoluminescence spectra can be recorded with an experimental arrangement such as the one shown in Figure2-4. The sample is mounted in a variable temperature cryostat and is illuminated with a laser or bright lamp with photon energy greater than E_g. If a liquid helium cryostat is used, sample temperatures from 2K upwards are easily obtained. The luminescence is emitted at lower frequencies and in all directions. A portion is collected with a lens and focused onto the entrance slit of a spectrometer. The spectrum is recorded by scanning the spectrometer and measuring the intensity at each wavelength with a sensitive detector such as a photomultiplier tube. Alternatively, the whole spectrum is recorded at once using an array of detectors such as a charge coupled device (CCD).

Figure 2-1. Interband transitions in solids: (a) direct bandgap, (b) indirect bandgap.

Figure 2-2. Interband optical absorption between an initial state of energy E_i in an occupied lower band and a final state at energy E_f in an empty upper band.

Figure 2-3. Schematic diagram of the process occurring during photoluminescence.

Figure 2-4. Experimental arrangement used for the observation of photoluminescence spectra. .

Chapter 3

Experimental Details of Fabrication and Analysis Systems

3.1 Experimental process

In this thesis, we fabricates the sample of ZnO Quantum dots embedded in SiO₂ using the magnetron sputter. Two of structures to fabricate are ZnO QDs-SiO2

nanocomposite films by co-sputtering and ZnO-SiO₂ multilayer structures, as shown in Figure 3-1 and Figure 3-2. Figure 3-1 illustrates ZnO and SiO2 targets are sputtered simultaneously and the schematic diagram of the structure with ZnO QDs-SiO₂ nanocomposite films. The later with forty alternating layers is that sputtering ZnO and SiO₂ films with ultra-thin thickness separately and showing schematic diagram of the structure in Figure 3-2.

The experimental process of this study is shown in Figure 3-3, fabrication and measurement are described as follows. Before depositing, processes of substrates cleaning are described as the chart in Figure 3-4. The sample I structure is shown in Fig.3-5. First, a SiO2 bottom layer of 20 nm thickness is deposited onto the (100) silicon and quartz substrates to eliminate any influence of the substrates on growth of the quantum dots. It is formed by plasma-enhanced chemical vapor deposition (PECVD, OXFORD INSTRUMENTS, Plasmalab80Plus) at 300℃. The sample with ZnO QDs-SiO2 nanocomposite film is excluded. The bottom layer is followed by sputtering separately. The (100) silicon substrate is used for high resolution

transmission electron microscopy (HRTEM), and 100 nm SiO₂ passivation layer is deposited by PECVD for reducing the damage from focus ion beam (FIB).

In measurement of electrical property, the samples are fabrication into a sandwiched structure (Sample II) on quartz with a top and bottom electrode of Al. It is without 20nm SiO₂ bottom layer. The Al electrode is deposited by the thermal evaporation coater with a thickness of 350 nm. Figure 3-6 illustrates the schematic of the sample II in the sandwiched for the purpose of the electrical measurement. sputter atoms and secondary electrons eject. As secondary electrons have sufficient energy to ionize the gas atom/molecules, the probability of oscillation will increase, resulting in the greater deposit rate. [30-34]

As the affect of magnetic field, which parallel to the target surface, the motion of secondary electron is constrained to the region upon target, increasing the probability of gas atomic ionization by secondary electrons, and intensifying the ion bombardment, as shown in Figure 3-8. Magnetron sputtering overcame the other limitation of basic sputter, e.g. low deposition, and high substrate heating effect.

In addition, for ensure the atom of material can move freely towards the substrate, the low pressure of vacuum condition is necessary. Low pressure leads to the long mean-free-path (MFP). As MFP is more, the probability of atom travel without colliding with another gas atom is more, i.e. it avoids that too many atom-gas collisions after ejection from the target.

Here, both two structures of ZnO QDs-SiO2 nanocomposite film and ZnO QDs-SiO₂ multilayer are deposited on (100) silicon and quartz substrates by sputtering ZnO and SiO2 target. The base pressure of the deposited is 8×10^-7 Torr in a high vacuum. ZnO target is sputtered using DC power supply, and SiO₂ is an insulator resulting in it must be sputtered using RF power supply. The entire deposition process is carried out at room-temperature without annealing of substrates.

3.3 Scanning Electron Microscopy (SEM)

Scanning electron Microscopy (SEM, Hitachi S-4700I) is used for examining the thickness of ZnO QDs-SiO₂ nanocomposite films. SEM is one of electron microscopes, it used a focused electron beam that interact with the sample to produce an image [35-36]. The signals depend on the atomic structure, shape, and conductivity of materials. The main interacting electrons are collected to reveal the morphology of samples, which is including the secondary electrons, and backscattered electrons.

Those produces as focused electron beam hits atoms on the surface, those reflected are called backscattered electrons. The atoms must give off another electron (secondary electron) or emit light for conversation, and becoming stable. The interaction between incident electron beam and sample surface is illustrated as Figure3-9.

A SEM column consists of an electron gun, one or two condenser lenses, an objective aperture, and an objective lens. The electron gun produces a source of electron and accelerates the electrons to energy of 0.5~30 keV. This occurs in a vacuum environment ranging from 10^-4 to 10^-10 Torr. The electron lenses in the column are used to demagnify the image of the gun crossover and focus a final spot on the specimen on the order of 1nm ~ 1m with a beam current in the range of 1pA ~ 1A. The condenser lens controls the amount of demagnification and the probe forming or objective lens focuses the final probe on the specimen. A schematic of a typical SEM is shown in Figure 3-10.

The lens and aperture system in the column provide control of the beam through manipulation of the probe diameter, probe current, and convergence angle. These three parameters can be controlled and used to achieve high depth-of-field, high-resolution, or high beam current for x-ray microanalysis. A small convergence angle is needed for high depth-of-field imaging and can be obtained with a small objective aperture and a long working distance. High resolution imaging requires a small probe size which can be obtained with a strong condenser lens, an objective aperture, and a short working distance. Finally, x-ray microanalysis may require higher beam currents which can be obtained by weakening the condenser lens and removing the objective aperture.

3.4 Transmission Electron Microscopy (TEM)

The transmission electron microscopy (TEM) is a microscopy technique using tunneling electron beam directly through an ultra-thin specimen and image forming from electrons scattering into discrete diffracted beams [37-38]. The diffracted electron beams are then focus in the back focal plane of objective lens. Generally, a TEM is composed of several components, includes a vacuum system in which the electrons travel, an electron emission source for generation of the electron stream, a series of electromagnetic lenses, as well as electrostatic plates. The latter two allow the operator to guide and manipulate the beam as required. Also required is a device to allow the insertion into, motion within, and removal of specimens from the beam path. Imaging devices are subsequently used to create an image from the electrons that exit the system. The detail components are shown in Figure 3-11.

Diffraction mode and image mode are two modes of TEM, as shown in Figure 3-12. When operated in diffraction mode, the diffraction lens is focused on the back focal plane to produce a diffraction pattern. For the imaging mode, the diffraction lens is focused on the first image plane to produce a magnified image. In addition, the beam may be allowed to pass through the sample to obtain a bright-field image however the diffracted beams produce a dark-field image.

The interaction of the electron beam with crystalline material tends to be by diffraction. The orientation of the planes of atoms in the crystal to the electron beam changes the intensity of diffraction. TEM equipment often uses a goniometer to allow the sample to be tilted to a range of angles to obtain specific diffraction conditions.

Diffracted electrons are also selected using different apertures.

The intensity of diffraction is a maximum at the Bragg angle, although a variation of diffraction intensity occurs with deviation from the Bragg 37 angle. This also depends on the thickness of the specimen. The thinner the crystal sample, the further the crystal may deviate from the Bragg condition.

When crystal planes are almost parallel to the electron beam they are close to fulfilling Bragg’s Law. The majority of electrons are diffracted when the electron beam strikes one set of lattice planes exactly at their Bragg angle and only a few will pass through the sample undeviated. If the planes are exactly at the Bragg condition, strong diffraction will occur and the bright field image will appear dark. This variation with diffraction is shown with bend contours which are a feature of bending of the crystal planes. Dark contour images correspond to regions at the Bragg angle, while light contours result in the regions not strongly diffracting.

In this study, TEM system (JEOL, JEM-2100F) is used to image the ZnO quantum dots, which with 0.23 nm of point image, 0.14 nm of lattice image for resolution.

3.5 X-ray Diffractometer (XRD)

X-ray diffraction (XRD) is a powerful tool with non-destructive for investigating the crystalline structure, chemical composition, and physical properties of materials, which those cause a beam of X-rays to diffract into many specific directions. The first XRD patterns of rock salt were obtained in 1911 [39]. For semiconductor, XRD is

mainly use to evaluate the quality of the film, determine the mole fraction of alloys, and investigate the thickness and fine structure of materials with superlattice structures.

This analytics technique of XRD is based on observing the diffraction intensity of an X-ray beam hitting a sample as a function of incident and diffracted angle, wavelength, energy, and polarization. Crystals are regular arrays of atoms, and X-ray can be considered as waves of electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atom’s electrons, like that an ocean wave striking a lighthouse produces secondary circular wave emanating from the lighthouse. Resulting in elastic scattering, an X-ray striking an electron produces secondary spherical waves emanating from the electron. [36] A regular array of scatters produces a regular array of spherical waves. Although these waves cancel one another out in most directions through destructive interference, they add constructively in a few specific directions.

It follows the Bragg’s law,

2d sin= n 

Here d is the spacing between diffracting plane,  is the incident angle between the incidence and the reflect X-ray, n is any integer, and is the wavelength of the incident X-ray beam. Figure 3-13 shows the schematic diagram of Bragg diffraction.

X-ray diffraction results from an electromagnetic wave (the X-ray) impinging on a regular array of scatters (the repeating arrangement of atoms within the crystal).

In our measurement system the X-ray diffraction (Bede, D1) was characterized using -2 mode to identification the structures of ZnO quantum dots embedded in SiO_2.

3.6 UV/VIS/NIR Spectrophotometer

Measurements of the optical transmittance and reflectivity in the visible and ultra-violet are performed to characterize the optical absorption. Optical absorption is calculated using the formula as followed [40].

α =1

dlnT (1 − R ) T

Here,  is absorption coefficient, d is the thickness of film, TQ is transmittance of quartz substrate, and TS, TR are transmittance and reflectivity of sample. Figure 3-14 illustrated the optical path of the UV/VIS/NIR spectrophotometer system (Hitachi U-4100) with beam size is 5×5 mm².

3.7 Photoluminescence System (PL)

Photoluminescence (PL) spectroscopy has been used as a measurement method to detect the optical properties of materials because of its nondestructive spot at sample surface had a Gaussian intensity profile with 1/e² diameter of 50 μm, verified by a knife-edge measurement. The photoluminescence (PL) spectrum was

collected by the same UV objective and coupled into an optical fiber connected to the

collected by the same UV objective and coupled into an optical fiber connected to the