Recently, there has been much interest in the study of nitride semiconductors [1–4].
So far, the efforts were focused on the understanding of the basic material properties of the nitride family as well as the realization of various optoelectronic device fabrications.
Nitride semiconductors have wide range of band gaps which allow the emission wavelengths cover the whole visible spectrum and also ultra violet (UV) light.
Nakamura and co-workers [5–14] have pioneered the development of GaN-based light-emitting diodes (LEDs) which emit in a wide range of wavelengths from yellow to UV. These light sources have applications in full color display technology. Blue and UV laser diodes (LDs) are also key devices for the applications in high-density optical storage. On the other hand, nitride semiconductors are also good candidates for the high power and high temperature electronic devices due to the excellent thermal and chemical stability. Khan et al. [15, 16] have fabricated GaN-based field-effect transistors (FETs) and an AlGaN/GaN modulation-doped FET in which the existence of interfacial two-dimensional electron gas was shown. Furthermore, UV detectors, which have application in national defense, were fabricated based on GaN wide-band-gap semiconductor [17].
Low dimensional heterostructures have the advantage of strong carrier quantum confinement which gives rise to a robust electron and hole Coulomb interaction. The strong Coulomb interaction results in a stable excitonic emission at high temperature. It is essential for the light emitting devices of high brightness operated at high temperature.
In addition, low dimensional heterostructures have unique density of state. Theoretical prediction shows that the light-emitting diodes (LEDs) or laser diodes (LDs) made of quantum dots (QDs) in the active layer could lead to low threshold current and weak temperature dependence for the threshold current [18]. Moreover, the self-organized
quantum dot is expected to be defect-free. As a result, carriers confined inside the QDs could avoid the nonradiative recombination centers formed by dislocations outside [19].
Usually, the practical observation of zero dimensional (0 D) confinement effect in semiconductors requires a size of less than 10 nm in all three dimensions. The first pioneering work in this field used the lithographic patterning to fabricate QDs. However, this approach is practically abandoned today because of the damage caused by the lithographic processing on the lateral walls of QD. The damage seriously degrades the optical quality of the QD devices. By contrast, self-organization resulting from the Stranski-Krastanov growth mode was proven to be very successful in achieving nanostructures with excellent 0D optical properties. In this growth mode, deposition of a strained 2D wetting layer is followed by the strain relaxation through a 3D island formation which results in the growth of defect-free QDs [20]. It has been observed for various material systems grown under compressive stress, such as GaN on Al Gax 1-xN [21], InAs on GaAs [22], InP on GaxIn1-xP [23]. The successful QD fabrication opens the way to achieve highly efficient lasers based on self-organized In Gax 1-xAs/GaAs [24, 25] and InAs/GaAs QDs [26, 27].
Among the QD systems, GaN quantum dots (QDs) attracted significant attention due to its promising application in UV-LEDs or LDs. Progress in GaN technology leads great accomplishment in the fabrication and characterization of different kinds of GaN QDs [28–38]. Molecular beam epitaxial growth in the Stranski–Krastanow mode of wurtzite (WZ) GaN/AlN [28-30] and GaN/Al Gax 1-xN [31] QDs has been reported. On the other hand, MOCVD growth was also proved to be an alternative for the growth of WZ GaN/AlN [32] and GaN/Al Gax 1-xN [21, 33, and 34]. Other types of WZ GaN QDs have been fabricated by pulsed laser ablation of pure Ga metal in flowing N2 gas [35], and by sequential ion implantation of Ga+ and N+ ions into dielectrics [36]. Also, self-organized growth of zincblende (ZB) GaN/AlN QDs has been reported [37, 38]. In
addition to the fabrication and optical characterization of WZ GaN/AlN and GaN/Al Gax 1-xN as well as ZB GaN/AlN QDs, there were intensive theoretical investigations of electronic states and excitonic properties of GaN QDs [39-41].
Base on the important application and interesting physical properties of GaN QDs, we have challenged the fabrication difficulty of GaN QD and successfully grown self-assembled GaN dots on Al0.11Ga0.89N/sapphire substrate using flow-rate modulation epitaxy (FME) growth technique in MOCVD system. The growth and structural properties of our self-assembled dots have been reported elsewhere [21]. In this thesis, we will discuss the structural and optical properties of self-assembled GaN dots by means of atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy.
AFM studies show the disk-like shapes of GaN dots. Size-dependent PL spectra of GaN dots were studied. The emission peak energy of GaN dots was observed to shift to higher energy with decreasing quantum dot size. The blue shift is about 36 to 62 meV at room temperature in comparison with GaN epilayers. In addition, the temperature dependent PL spectra were investigated to extract the activation energy from the Arrhenius plot for the understanding of thermal stability and exciton localization.
There are total of five chapters in this thesis. In Chapter 1, we introduced the studies of GaN. Theoretical background was briefly described in Chapter 2. In Chapter 3, sample structure, growth method, experimental setup as well as operation procedures were presented. In Chapter 4, we discussed the results of morphology investigation, size- and temperature-dependent micro-PL spectra of GaN dots and proposed a reasonable explanation. Finally, in Chapter 5, we summarized the significant points obtained from this study.