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 input of a spectrometer (Jobin Yvon IHR320)

3.8 Electrical Measurement

A semiconductor parameter analyzer of model Kethley 4200 is utilized to measure the current-voltage (J-V) characteristics of sample II. The resulted current from an increasing voltage is examined, and the limit current is 0.1A.

Figure 3-1. (a) Co-sputtering system. (b) Structure of ZnO QDs-SiO2 nanocomposite film.

Figure 3-2. (a)System of sputtering ZnO and SiO₂, respectively. (b) Structure of ZnO -SiO₂ multilayers.

Figure 3-3. Experimental Process.

Figure 3-4. Clean process chart.

Figure 3-5. Schematic structures of ZnO QDs embedded in SiO₂. (a) ZnO QDs-SiO₂ nanocomposite film. (b) ZnO -SiO₂ multilayers.

Figure 3-6. Schematic diagram of sample II for electrical measurement.

Figure 3-7. The schematic representation of the sputtering mechanism

Figure 3-8. The principle of magnetron sputtering. Electrons are trapped by the Lorentz force K= e (v × B) in an inhomogeneous magnetic field, resulting in an

enhanced ionization of gas atoms.[32]

Figure 3-9. Schematic illustration of the origin of two sources of secondary electron generation in the sample.

Figure 3-10. Schematic of SEM [35]

Figure 3-11. Schematic diagram of optical components in a basic TEM

Figure 3-12. Ray diagram showing two basic operations of TEM. (a) Imaging projecting a diffraction pattern and (b) projection of an image onto a viewing screen.

[37]

Figure 3.13. Diagram of Bragg’s diffraction.

Figure 3-14.The schematic diagram of 3.6.1 UV/VIS/NIR spectrophotometer.

Chapter 4

ZnO _x QDs-SiO ₂ Nanocomposite Film by Co- sputtering

4.1 Fabrication of ZnO

QDs-SiO

NanocompositeFilm

ZnO_x QDs- SiO₂ nanocomposite films with dot diameter from 3 nm to 7 nm were prepared via co-sputtering method. The schematic diagram of ZnOx QDs-SiO2

nanocomposite films is shown in Figure 4-1. First, quartz substrates are cleaned before the sputtering. High purity ZnO target and SiO2 target are placed on the deposited chamber with base pressure of the deposited is 8×10^-7Torr in a high vacuum.

The entire deposition process is carried out at room-temperature without annealing of substrates. ZnO and SiO₂ are deposited simultaneously using the co-sputtered system to produce ZnO quantum dots in an amorphous silicon oxide. In order to form the smaller ZnO quantum dots and expand the relative deposit rate, we set the smallest DC power of 40W for ZnO and largest RF power of 200W for SiO2 in the limit of sputtering system as possible. During sputtering process, the flow rate of argon is controlled as 30 SCCM, pressure is kept at 5, 10, 20mTorr for obtaining the various ZnO contents, and the detail deposited conditions are shown in Table 4-1. The relative deposit rate is defined as

R = D D

Here, R is relative deposit rate, D_ZnO, D_SiO2 are represented as the deposit rate of

The microstructures are examined by tunneling electron microscopy (TEM). Figure 4-3 presents the TEM micrographs of conditions with depositing pressure of 5mTorr, and shows a better result compared to other conditions. It can be clearly seen that ZnO forms QDs with diameter form 3nm to 7nm embedded in amorphous silicon oxide.

Figure 4-3(a) reveals the all-area of ZnOx QDs-SiO2 nanocomposite film, and the image zooming out is shown in Figure 4-3(b). Bright-field and dark-field of same region from TEM sample are used to identify the formation of ZnO nanocrystals, as illustrated in Figure 4-3(c) and Figure 4-3(d). According to the TEM images, we find out that ZnO quantum dots are formed in random diameter no matter how we vary the working pressure from 5mTorr to 20mTorr to decrease the relative deposit rate. It cans not be accurately controlling in the ZnO quantum dots size on ZnOx QDs-SiO2

nanocomposite films. But isolated ZnO nanocrystals are well confined in amorphous SiO2 surrounding.

In addition, with the measurement of the X-ray diffraction, we find that no signal distribution in special direction of ZnO nanocrystal, as shows in Figure 4-4.

Compared to the ZnO film with 100nm of thickness, it is obvious that exhibiting of

two peaks around 34.4° and 62.8, related to (002) and (103) planes, respectively [41]. c-axis growth, and few content of ZnO material, those result the negligible diffraction signal of ZnOx QDs-SiO2 nanocomposite film.

4.3 Characteristics of ZnO

QDs-SiO

Nanocomposite Film

To investigate the optical properties of ZnO_x QDs-SiO₂, the measurements of the optical transmittance in the visible to UV are performed on the transparent quartz substrates. Figure 4-5 shows (hv)² as a function of photon enegy hv, calculated from the optical transmittance spectrum, where  denotes the absorption coefficient. In this absorption spectrum, we could not define the energy bandgap by the intercept of absorption curve as the result of the broad absorption starting from 2.6eV to near 5eV.

Absorbing from ZnO defects (under 3.26eV of bangap of ZnO film in this study), absorbing by ZnO quantum dots (above 3.26eV of energy values), and absorbing of amorphous SiO₂ defects (near 5-6eV) which are those causes the weak absorption [16, 42-43]. The broad absorption band around 5-6eV is associated with the peroxyl radical, for the first time. Another absorption band of SiO₂ is 7.6eV, which is not shown in the spectrum. Even if the energy above 3.26eV is corresponded to the absorbing of ZnO quantum dots, we still could not define the energy bandgap with the size-dependent quantum effect.

Figure 4-6 shows a typical room-temperature photoluminescence (PL) spectrum recorded from 5mTorr to 20mTorr, which is normalized. The differences between those samples are only slight at the intensity of PL signal, and those distributions are almost same, no energy change on the maximum by varying the relative deposit rate.

It is perhaps that random size of ZnO QDs results the similar results in different process conditions. At the same time, the full width at half maximum (FWHM) of 100nm ZnO film has the minimum value, it appears the quality of ZnO materials.

FWHM increasing follows the 10mtorr, 5mtorr and 20mtorr. In addition, compared to the absorption spectra (Figure 4-5), the emission only occurs around 2.5-3.5eV. It demonstrates that defect absorption and emission of ZnO are near 3eV of energy, and near band edge absorption and emission are around 3.2-3.5eV, resulting in the direct bandgap of zinc oxide property. However, the minimum near 3.5eV in PL spectrum illustrates few photons emit with photon energy above 3.5eV, but the strong

FWHM increasing follows the 10mtorr, 5mtorr and 20mtorr. In addition, compared to the absorption spectra (Figure 4-5), the emission only occurs around 2.5-3.5eV. It demonstrates that defect absorption and emission of ZnO are near 3eV of energy, and near band edge absorption and emission are around 3.2-3.5eV, resulting in the direct bandgap of zinc oxide property. However, the minimum near 3.5eV in PL spectrum illustrates few photons emit with photon energy above 3.5eV, but the strong