Motivation

For InGaAsN system, the behavior of the single quantum well structure with different Nitrogen contents is not understood well. And the quality of self-assembled QDs, including the uniformity, shape, and size, remains to be improved by trying and tuning the different parameters or growth methods to optimize. In this thesis, we investigated the structural and

optical properties for different samples including InGaAsN single quantum wells and InGaN quantum dots with different sizes. In order to understand the special behavior in dilute nitride materials and in QDs structure, we carried out temperature dependent photoluminescence and low temperature photoluminescence excitation to analysis the behavior of those nanostructures in various materials. However, the theoretical calculation of band offset values in InGaN system is simply performed and compared with experimental results. Besides, for InGaN QD samples, we also investigate the effect of rapid thermal annealing with different annealing temperatures.

The organization of this thesis is as following: Chapter II discusses the different method to form the QD samples and theoretical background. The experimental principle and setup are described in chapter III. Detailed behaviors expected for InGaAsN single quantum well structures are obtained in chapter IV. The experimental results of InGaN QD for different sizes and the effect of rapid thermal annealing are discussed in chapter V and VI, respectively.

Finally, the summary of this study is presented in chapter VII.

D(E) D(E)

E E

3

3--DD 22--DD

Fig. 1.1.1: The relation between dimension and density of states

Fig. 1.2.1: A brief history of research on QD lasers particularly at the initial stage.

Chapter II Characteristic of III- Nitride nanostructure

2.1 The localized or quantum-dot like state in quantum well structure

The behaviors of localization or quantum-dot like state were investigated well in InGaN quantum well with high Indium content [1]. In that study, the results showed that the origin of strong emission from InGaN/GaN quantum wells can be attributed to exciton localization in the quantum dot-like region.

In InGaAs system, therefore, the origin of quantum-dot like states in the quantum well is not well understood, and there are relatively few reports on InGaAsN/GaAs QDs [7-11]. The present results show the experimental evidence of the 3D growth mode in the sample with high nitrogen content. This phenomenon most likely occurs due to the influence of various nitrogen contents on growth model transition, i.e., 2D to 3D growth to minimize the free energy while increasing nitrogen contents. Moreover, theoretical studies have shown that island formation is results from the competition between strain relaxation and surface energy [12]. A crucial factor in the control of 3D island growth is the lattice mismatch among the island material, underlying substrate and the surrounding matrix. The lattice mismatch between the InGaAs and GaAs substrate is known to be able to be varied by adjusting the nitrogen contents, and therefore offers a range of compressive strains [13, 14]. The introduction of nitrogen into a highly compressive strain system can reduce the average strain;

on the other hand, the local strain around N atom may increase owing to the smaller nitrogen atom radius compared to arsenic. The effect of this local strain may enhance the 3D growth mode. Therefore, the mechanism is not confirmed by strong evidences. Furthermore, Herrera et al. proposed that the introduction of N could be responsible for the enhanced phase separation [10]. The presence of phase separation could also locally reduce the energy barrier for the transformation into stable 3D islands.

2.2 The formation of quantum dot structure

There are three main methods to form the quantum dot structure including Stranski–Krastanow (S–K) growth mode, antisurfactant method, and selective method. We summarize the growth methods in recent years and introduce below:

(i) Stranski–Krastanow (S–K) growth mode (layer-then-island)

The evolution of an initially two-dimensional growth in to a three-dimensional growth front is a well-known phenomenon and has been frequently observed in various systems.

This growth mode is used by various materials grown under compressive stress on heterostructure by strain-driven quantum dot formation during heteroepitaxy as a bottom-up

approach. After deposition of a few two-dimension monolayers (MLs), island structures are self-formed on a 2D wetting layer as a result of the transition of the growth mode and the three-dimension occurs because lattices mismatch between interfaces and the stress needs to be released, as shown in Fig. 2.2.1. The strain relaxation mechanisms would be first published by Stranski and Krastanow and we called the growth mode as S-K growth mode. Especially, S-K growth has been successfully demonstrated to form self-assembled quantum dot (SAQD) on large area substrate with a good throughput and compatibility to current microelectronic technology [15].

(ii) Antisurfactant method

Using this growth way, the surfactant is believed to play an important role in changing the surface free energy of the samples. We can use the unequal equation to show the surface free energy in three- dimension growth mode in a system,

δs<δf+δi, Eq. (2.1.1)

where δs is the surface free energies of substrate, δf shows the surface free energies of film, and δi represents the surface free energies of interface between substrate and film. As a third element like the surfactant is added, the sign of the unequal equation would be change by altering the substrate surface free energy [16].Using this growth method, the surfactant is believed to play an important role in changing the surface free energy of the material.

Some reports have shown the similar reversed effect occurs in GaN-based system [16, 17].

The antisurfactant is believed to inhibit the film growth and intentionally modify the two-dimension mode into three-dimension mode. However, the role of the antisurfactant is still unknown in the enhancement of island growth, Therefore, carrying out the basic mechanism in growth way would be important to us for further detailed studies. In this thesis, the chapter V would be focused on the growth way of antisurfactant method using the SiNx as an antisurfactant. This is different from many other groups.

(iii) Selective method

Self-assembling growth is a convenient method to get QDs but the position of the QDs is difficult to be controlled. Using selective method to obtain QDs, in the contrast, the shape, size, and position of the QDs could be artificially designed and controlled.

In general, the selective method can control the position of the QDs by the methods of focused ion-beam irradiation and photo-assisted wet chemical etching. S. Sakai, et al. [18]

grew the QDs on Si-patterned GaN/sapphire substrates. We could use different patterns to grow QDs in different shapes. Here, we simply introduce the InGaN QDs formation by selective method published by Y. Arakawa, et al. [5]. In this paper, after depositing three

periods of InGaN MQWs on the grid-like SiO2-GaN-sapphire patterned substrates, they believe that InGaN QD structures are formed at the tops of the hexagonal pyramids.

Figure 2.2.2 show a SEM bird’s-eye-view and cross-sectional view of InGaN QDs structures. Shown as Fig. 2.2.2(b), no material was deposited on the SiO2 mask and the positions of the QDs were controlled very well.

2.3 Quantum confinement effects in nanostructure

Regarding nanostructures such as quantum dot-like nanostructure or quantum dot structure, their unique optical and electronic characteristics due to atomic-like discrete states with a δ–function density of states, higher luminescence efficiency, and modified carrier recombination lifetime are investigated by many groups. Within the last decade, there has been a lot of research devoted to realizing the predicted behavior of zero-dimensional (0-D) quantum-confined structures.

For low-dimensional structures, in spite of the fact that the size of the confinement is larger than the exciton Bohr radius, it is known that a compression of the exciton, and thus a change in the exciton binding energy occurs.

In a quantum dot, the electronic states are quantized and the energy levels become discrete.

In Figure 2.3.1, we could see the carriers are localized in the quantum dot structure for three dimension confinement. Because of the localization of carriers trapped at QDs structure where the energy separation between the ground and the first higher exciton state is larger than the thermal energy kBT, the lifetime of the 0-D exciton strongly is almost independent of temperature [19].

Knowing the quantum dot size and the ratio of the dot radius to the exciton Bohr radius in the bulk material, we could calculate the difference of electron and hole sub-bands between two materials to determine the energy gap. The quantum dots which we consider in general have the small ratio of thickness to diameter. Therefore, the energy shift in those structures is due to quantization in the longitudinal direction. The k․p model is used to calculate the interaction matrices of our nanostructures under stain.

Shown as the Fig. 2.3.2, in the strong confinement regime, i.e., in small size QDs, where the sub-band energy separations are much larger than the electron-hole Coulomb interaction of the exciton binding energy, the exciton ground state is mainly composed of the lowest-energy sub-band [15]. As the quantum dot size is increased, the energy levels of the quantized sub-band become smaller. Finally, in the week-confinement regime where the quantum dot size is much larger than the exciton Bohr radius, the quantized sub-bands are

distributed almost continuously and the binding energy is nearly the same as in the bulk material.

Compared to the calculation results, the shift in the energy level due to the three-dimensional quantization could be found from photoluminescence measurements. The detail analysis would be discussed below.

Fig. 2.2.1: The diagram of strain relaxation for S-K growth mode

Fig. 2.2.2: a SEM bird’s-eye-view and cross-sectional view of InGaN QD structures

e2

e1

hh1 hh2

lh1 hh3 e1-hh1

e1

hh1 lh1 lh2

e1-lh1

e1-hh1

e1-lh1

Large QD Small QD

Fig. 2.3.2: The scheme of the calculated transition energy for QDs with different sizes.

Fig. 2.3.1: The carrier behavior in three dimension confinement structure.

Chapter III Experimental Instrument and Setup 3.1 Optical Characterization

Optical characterization is contactless, nondestructive method of probing the initial behavior of materials and structures with minimal sample preparation. Specifically, light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties.

3.1.1 Photoluminescence (PL)

Photoluminescence is the emission of light from a material under optical excitation. The energy of the laser light should be large than the band gap energy of the semiconductor in order to get all information. The exciting light is absorbed in the semiconductor and will produce electrons in the conduction band and a hole in the valence band. When an excited electron in an excited state returns to the initial state forming a photon whose energy is the difference between the excited state and the initial state energies, then we detect the PL signal.

In order to restore the equilibrium, the electrons and holes will combine in many ways. Those processes can be direct or indirect depending on the gap energy of the material. The PL spectra are obtained by analyzing the spectral elements of the emitted light.

At low temperature, there are different possibilities of recombination. An electron and a hole can pair together and form a Free Exciton (FE), normally referred to a Wannier-Mott exciton [20]. This exciton is characterized by a high mobility and comparably low binding energy. The FE may move freely in the lattice until it recombines radiatively or non-radiatively or until it encounters a defect. At the defect, FE exciton forms a Bound Exciton (BE) because of loosing some of energy.

Some applications of photoluminescence are bandgap determination, impurity levels and defect detection, recombination mechanisms and material quality, showed as below:

• Band gap determination. The most common radiative transition in semiconductors is known as the band gap which is the difference between the conduction and valence bands. Band gap determination is a particularly useful and basic process when working with new compound semiconductors.

• Material quality. Non-radiative processes are usually associated with localized defect, whose existence will decrease material quality and device performance. Therefore, material quality can be detected by analyzing the amount of radiative recombination.

• Recombination mechanisms. Recombination known the return to equilibrium can involve both radiative and non-radiative processes. The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. According to analyzing of photoluminescence, we could understand the physical mechanism of the recombination.

• Impurity levels and defect detection. Radiative transitions in semiconductors are also related to localized defect states. The photoluminescence energy associated with these different levels could identify specific defects in materials, and the intensity of photoluminescence can be used to determine their qualities of structures.

The article below discusses how photoluminescence can be used to analyze the optical properties of III-V nitride QD structure materials.

3.1.2 Photoluminescence Excitation

In PL measurement, which is performed at fixed excitation energy, the luminescence properties are generally investigated. While PL excitation (PLE) spectroscopy, which is carried out at fixed detection energy, provides mainly information about the absorption properties. Apart from absorption and PL experiments, the PLE measurement is a widely used spectroscopic tool for the characterization of optical transitions in semiconductors.

It is also very important to note that the PLE also depends strongly on the different carrier relaxation processes that connect the absorbing state to the luminescent state. For example, it is possible to recognize the absorption in a quantum well (QW) from that of the substrate if they have different emission energies, in which because it can be assumed that carrier transfer between substrate and QW is negligible. Nevertheless, in many cases it is difficult to separate the influence of relaxation from that of absorption. The PLE spectrum is strongly influenced by the relaxation depending on different samples.

3.2 Structure Characterization 3.2.1 Atomic Force Microscopy

Atomic force microscopy images the surface of a sample by scanning a sharp tip over it

and measuring the deflection of the tip. The working principle is illustrated in Figure 3.2.1.

A piezoelectric scanner moves the sample in the x-y direction under the tip. The position of the tip is measured by reflecting a laser from the backside of the cantilever to a split photodiode. Depending on the distance between the tip and the sample so that the force acting on the tip is repulsive, the AFM work in contact mode. In non-contact or tapping mode the tip is further away from the sample and in the regime of an attractive force [21]. The cantilever is set to vibrations close to its resonant frequency and changes in the surface morphology lead to changes in the frequency, which can be measured sensitively. The change in the frequency is used as a feedback. The AFM used in our experiments was called Large Sample Scanning Probe Microscope named Digital Instruments DI 5000.

3.2.2 Transmission Electron Microscopy (TEM)

A TEM works much like a slide projector. A projector shines a beam of light through (transmits) the slide, as the light passes through it is affected by the structures and objects on the slide. These effects result in only certain parts of the light beam being transmitted through certain parts of the slide. This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide. TEMs work the same way except that they shine a beam of electrons (like the light) through the specimen (like the slide). Whatever part is transmitted is projected onto a phosphor screen for the user to see. A technical explanation of the typical TEMs is showed as figure 3.2.2[22].

High-resolution TEM (HRTEM) images could provide structural information at better than 0.2 nm spatial resolution. In most crystalline inorganic materials, including ceramics, semiconductors and metals, the positions of individual atomic columns can be resolved, at least in low-index zones. When recorded under optimum conditions, electron micrographs can be directly interpreted in terms of the projected crystal potential.

For highest resolution, specimens must be <10nm thick. In general, specimens prepared by chemical thinning, crushing, or ion beam milling will contain suitable regions [23].

3.3 Post-grown Rapid Thermal Annealer

The RTA system we used here to investigate the post-grown thermal annealing effect is heatpulse 610i rapid thermal processing system. The RTA temperature range is from 400^oC~1250^oC and the duration of maximum time up to 300 seconds.

3.4 Experiment setup

3.4.1 Long wavelength µ-PL setup

The PL of long wavelength samples was excited by the 532 nm line of an Ar⁺-ion laser.

Before the luminescence light excited by laser in focused on the input slit of the monochromator, a long pass filter was set to filter out the stray laser. Adding gray filters enabled the light to be attenuated by various factors. Using the 100x objective here, the light from the lasers can be focused on a spot with a minimum diameter of 3 µm on the sample. All samples were placed in a closed-cycle cryostat with a temperature controller ranging from 10 K to room temperature. Furthermore, the luminescence was dispersed with a 320 mm monochromator (Jobin-Yvon Triax 320) and detected by a thermo-electrically (TE) cooled InGaAs detector. The signal of luminescence was recorded by the lock-in amplifier using phase-loch technique, which was introduced for amplified the modulated signal amplitude.

The wavelength resolution was about 1nm by using 300 grooves/mm grating and the slit of 0.1 mm.

On the other hand, we also set up a system to scam the confocal image, shown as Fig.

3.4.1. Such a confocal optical system enables the highly spatial resolution beyond the diffraction limit of a light wave. We could detect the surface inhomogeneous and/or PL surface emission of our samples.

3.4.2 PL setup

The schematic system setup of PL system is shown in Fig. 3.4.2. The pumping light source was a multi-mode and non-polarized Helium-Cadmium laser operated on 325 nm with 20 mW. After reflecting by three mirrors, the laser light was focused by a lens, which focal length was 5 cm, to 0.3 mm in diameter and the luminescence signal was collected by the same lens. The probed light was dispersed by 0.32 m monochromator ( Jobin-Yvon Triax-320) equipped with 1800, 1200, and 300 grooves/mm grating and which maximum width of the entrance slits was 1 mm. For my case, I chose to use the 300 grooves/mm grating and the slit of 0.1 mm. Under these conditions, the wavelength resolution was approximately 1 nm. In order to prevent the stray laser light from the sample surface passing through the detector, I also used long pass filter with a cut-off wavelength of 360 nm in front of the entrance slit to get real spectra without combining the scattering pumping laser. Eventually, for GaN-based materials, the probed light was detected by the charge couple device (CCD). All samples were

The schematic system setup of PL system is shown in Fig. 3.4.2. The pumping light source was a multi-mode and non-polarized Helium-Cadmium laser operated on 325 nm with 20 mW. After reflecting by three mirrors, the laser light was focused by a lens, which focal length was 5 cm, to 0.3 mm in diameter and the luminescence signal was collected by the same lens. The probed light was dispersed by 0.32 m monochromator ( Jobin-Yvon Triax-320) equipped with 1800, 1200, and 300 grooves/mm grating and which maximum width of the entrance slits was 1 mm. For my case, I chose to use the 300 grooves/mm grating and the slit of 0.1 mm. Under these conditions, the wavelength resolution was approximately 1 nm. In order to prevent the stray laser light from the sample surface passing through the detector, I also used long pass filter with a cut-off wavelength of 360 nm in front of the entrance slit to get real spectra without combining the scattering pumping laser. Eventually, for GaN-based materials, the probed light was detected by the charge couple device (CCD). All samples were