| dc.description.abstract | This dissertation includes the growth, fabrication, and characterization of Mg-doped GaN and InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) by metalorganic chemical vapor deposition (MOCVD).
First, we utilized Hall measurement, photoluminescence (PL), and second ion mass spectrum (SIMS) to characterize p-type GaN and Mg distribution in GaN films. Although the Mg flow is constant during growth, the measurement data shows that the Mg concentration increases with growth time since the Mg concentration is higher when closer to the surface. Even when the thickness of p-type GaN reaches 2.5 μm, the increasing hasn’t slow down yet. The effect of different growth condition, such as growth temperature, III/V ratio and carrier gas, upon Mg incorporation in GaN is studied also. We found Mg diffusion not only in bulk p-type GaN but also in InGaN/GaN MQW LEDs.
To improve the characteristics of InGaN/GaN MQW LED, we must understand its behavior first. So we tried to identify its emission mechanism by changing the operation temperature and current, and found the major cause of blue shift, which means peak emission wavelength shifts to shorter wavelength with higher injection current, is band-filling effect but not free carrier screen effect. In addition, we can identify whether the emission comes from well states or localized states from the way it shifts.
When analyzing the emission mechanism of InGaN/GaN MQW LED, we also found the emission generated by Mg-doped GaN. Therefore, we designed another experiment to study the effect of Mg doping upon InGaN/GaN MQW LED. By the electroluminescence (EL) spectra measured under different operation temperatures, the peak emission wavelength of quantum wells and Mg-doped GaN is 3.1 eV and 2.7 eV respectively. When analyzing this emission behavior with rate equation and the relative intensity variation of these two peaks under different temperature, we can calculate and obtain the activation energy of Mg doping in GaN being 126 meV, which agrees with the results from other academic organization utilizing different otherwise method.
We utilized selective activation technique in InGaN/GaN MQW LED process to improve its external quantum efficiency. Using this technique to activate Mg-doped GaN except the area underneath P-type bonding pad made us able to maintain its semi-insulating property, and further, it became a semi-insulating current blocking layer during device operation. This layer is utilized to prevent the injection current flowing through the area below P-pad since P-pad blocks the emission light from below and thus reduces the emission efficiency. From the luminescence intensity versus injection current chart, we did succeed in our effort to improve the external quantum efficiency when comparing to normal InGaN/GaN MQW LED.
Since p-type GaN shows poor conductivity, normal InGaN/GaN MQW LED process utilizes a transparent conductive layer for current spreading. However, despite its name, this layer isn’t 100 % transparent, and thus would absorb/reflect certain portion of emission light. To prevent this loss, we changed the normal InGaN/GaN MQW LED structure, which is p-type above and n-type under MQW, by switching the p-type and n-type layers’ positions and adding a n+/p+ tunnel junction. By this change, there’s no need of this transparent conductive layer for current spreading, and the device external quantum efficiency become almost twice as high as before. | en_US |