Abstract
This dissertation discusses on a thick and crack-free AlGaN film that is deposited on a sapphire substrate by using metal-organic chemical vapor deposition (MOCVD) technology. A violet LED is used that InGaN/GaN multiple quantum wells (MQWs) are deposited on this AlGaN template. The properties of the violet LED are investigated on the basis of material properties and by carrying out measurements such as electrical and optical measurements. Another object, the GaN:Si and GaN:Mg films grown on different pairs, is studied. In addition, the properties of the GaN/AlN buffer layer are presented in this paper. High-quality AlGaN is fabricated on deep ultraviolet (DUV) materials for general illuminance. However, a nitride-based material is usually used with sapphire as the substrate which lack high quality nitride films cause by large lattice mismatch generates high density of dislocation from interface in this system. Especially, an AlN film as nucleation layer (NL) deposited on sapphire following GaN film usually contents dislocation density as high as order to 108~1010 cm-3. Consider, for example, an AlGaN film on GaN template whole Al content increases with decreasing ternary film thickness. The thick Al0.1Ga0.9N film grown on the sapphire substrate may generate a crack when the thickness of the film is greater than 0.1 □m. Such a crack in the film will degrade the device performance. For solving this problem, we investigate AlGaN grown directly on GaN nucleation layer (NL) by using a two-step growth method. A thick and crack free of AlGaN film whose exceeds 1.7 □m is obtained. By using this method, we introduced an effective buffer for growing the white LEDs. Furthermore, the high absorption of GaN at 365 nm can be suppressed by the thick AlGaN buffer. In order to investigate the quality of the AlGaN film, we compare the GaN NL, AlN NL, and HT-GaN templates, and insert an LT-AlN film for the growth of a thick AlGaN film on sapphire. The crystalline quality of AlGaN templates and device characteristics of violet LEDs fabricated from the AlGaN templates are investigated. XRD patterns of GaN (0002), (0004), and (0006), and of asymmetrical GaN (10-15) are used for studying the violet LEDs grown on the NL and templates and for indentifying the crystallite quality. The photo-luminescence indicated MQWs at different growth temperatures with different Indium content. With respect to the dislocation density, the images of cathode- luminescence (CL) showed that AlGaN on a GaN NL had a low dislocation density. Finally, high-resolution transmission electron microscopy (HR-TEM) showed that the thick AlGaN film with a GaN NL had better crystallite quality. In the case of the devices used of violet LEDs measurements, the ideal factor and series resistance of the diodes are estimated on the basis of the temperature dependence of the current-voltage measurement. In order to design MQWs, the capacitance-voltage (C-V) characteristics of all LED structures were measured by using an Agilent 4294A precision impedance analyzer at 100 kHz. Knowledge of the C-V characteristics in the zero-bias depletion region can be of further assistance in the estimation of the doping concentration of barrier and the thickness of the undepleted p-GaN layer. This in turn would lead to the optimization of the thickness of the MQWs and p-GaN layer. Furthermore, power dependent electroluminescence is examined for the LEDs in order to observe emission peaks that exhibit a little blue shift because of the screening effect of the piezoelectric field in the QWs. The temperature dependence of electroluminescence indicated a redshift caused by the Joule heating. GaN can be grown on foreign substrates, for example, GaAs and Si. But, the primary considertion how to growth a lattice mismatch material up to 16 % between GaN and Si substrate, is the requirement for the growth of a high quality film. In general, the lattice between aSi(111) = 0.3840 nm and aGaN = 0.3189 nm is under strongly tensile strain in the case of a GaN film deposited on a Si substrate. In addition, cracks are formed when the growth temperature cools down to the room temperature when the thermal expansion coefficient for GaN = 5.59 × 10–6 K–1, and that of Si = 2.61 × 10–6K–1. AlN is the most commonly used material to avoid such cracks. This method can efficiently suppress cracks and facilitate the growth of a high quality GaN film. However, for commercial application, the first requirement is how to improve the difference in the internal quantum efficient (IQE) of GaN base LEDs on silicon substrate than on sapphire substrate. However, the GaN/AlN relaxation mechanism remains unknown. We investigated GaN:Si and GaN:Mg grown on multiple pair (MP) GaN/AlN buffer layers and verified the residual strain at the top of the GaN with a Si or Mg dopant. By using this method, we examined the strain relaxation in a GaN film on a Si system. First, two sets of samples are investigated by using high resolution X-ray diffraction patterns of the □/2θ scans of the symmetrical GaN (0002), (0004), and (0006), and the asymmetrical (10-12), and (10-15) planes. The increase in the number of pairs led to an increase in AlN lattice c more than the GaN. The cross sectional of SEM and TEM images also indicated that an increase in the MPs resulted in a decrease in the top GaN dislocation density. Futher, the energy band gap toward the narrow band gap due to the biaxial strain increased because of the temperature dependence of the photoluminescence peak and the increase in the MPs. The Raman measurement indicates that the GaN strain decreases with an increase in the MPs, which in turn induces GaN under a strain relaxation.