Abstract
This dissertation uses two kinds of growth technology including hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) to investigate the zinc-blende and wurtzite GaN epitaxial films, respectively. To study zinc-blende, cubic epitaxial films are grown on a 2°miscut GaAs(001) substrate with a low temperature GaN buffer layer before growing the epitaxial film. Experimental results indicate that the crystalline coherence length is 30nm and the rocking curve width is 4.8°. Photoluminescence measurements confirm that a cubic GaN edge emission peak appears at 388nm, as well as a strong yellow emission in the 500nm region. Additionally, the study of wurtzite structure focuses on the investigation of growth mechanism with different reactor types and the novel buffer structure to reduce the defects of GaN epitaxial films. Finally, high-performance homo-junction light emitting diodes (LEDs) were successfully grown on the low-defect substrate consisting of multiple-pair buffer layer.Reactor type is a crucial point in the growth of wurtzite GaN epitaxial films. According to our results, the separate-flow MOCVD horizontal reactor exhibits a better GaN-film quality than the conventional horizontal reactor. Additionally, three types of ceiling were designed to change the gas flow spacing between the susceptor and ceiling, and the three types of flow spacing are compared on different velocities of the upper stream H2 (FH2, up). This spacing affects the reactant gas flow pattern near the substrate surface and thus influences the quality of the epitaxial layers. It is also found that a mirror-like surface can only be grown for both 10 and 18mm spacing designs, and only if the H2 flow rate is adjusted to around 5000cc/min.Furthermore, new growth techniques are developed to further reduce the defects and dislocations embedded in the grown GaN epitaxial films. Some novel approaches are available for the GaN buffer layer to improve the quality of GaN epitaxial layer. One structure consists of a GaN nucleation layer / 6μm GaN-bulk layer on the sapphire substrate. This buffer structure has a wide growth window for different nucleation-layer thicknesses. Another structure is multiple-pair buffer layer (MBL), which consists of a 300A thick GaN nucleation layer grown at a low temperature of 525℃ and a 1-4μm thick GaN epitaxial layer grown at a high temperature of 1000℃. The condition optimizing the quality of GaN epitaxial layers is to grow the four-pair buffer layer with a pair thickness of 4μm on the sapphire substrate.In the device fabrication the homo-junction blue LEDs with MBL on sapphire substrates were demonstrated. The LEDs with three-pair buffer layer exhibit a lower turn-on voltage, a stronger EL intensity, and a higher light output due to more radiative recombinations occurring in the junction. A consistent behavior also appears in both the I-V and L-I, in which the diffusion recombination current can be observed in the low-bias regime. It is inferred that the reduction of dislocations in the epitaxial layers will result in the reduction of impurity diffusion into the junction via these dislocations. Therefore, the light output power of the LED with three-pair buffer layer will be improved by reducing impurities. The growth of MBL in the LED structure is a feasible means to fabricate high-performance LEDs and laser devices.