Design, fabrication and characterization of gallium nitride-based blue light-emitting diodes.

摘要

Group III nitride semiconductors have attracted great interest due to their applications in optoelectronics and power devices. GaN-based light-emitting diodes (LEDs) have important applications in full color displays and color printing. The goal of this dissertation work is to design, fabricate and characterize GaN-based blue LEDs. The research efforts encompass material growth, device fabrication and their characterization.; The epitaxial growth of GaN and InGaN was conducted in a high speed rotating disk metal organic chemical vapor deposition (MOCVD) reactor. Material properties measured by X-ray, Hall effect, and Photoluminescence (PL) indicate that the films are of high quality. The bandgap energy of GaN was studied by thermomodulation spectroscopy. A theoretical model has been established to explain the spectrum results. Varshni coefficients of the energy gap were obtained. The broadening parameter and its temperature dependence were determined for the first time.; The thermal annealing effects on the hydrogen passivation of MOCVD grown Mg-doped p-type GaN have been studied using room and low temperature (5K) PL and secondary ion mass spectroscopy (SIMS). It was found that the hydrogen concentration increases with increasing Mg doping level in the as-deposited films. Thermal annealing releases hydrogen from the film, thus activating the Mg as an acceptor. Annealing temperatures between 700{dollar}spcirc{dollar}C and 800{dollar}circ{dollar}C give the best surface morphology, and relatively high PL intensity and electrical conductivity. Low annealing temperature ohmic contacts with low contact resistivity were developed on n-type GaN using Ti/Al metallization. It was found that Ti/Al can form ohmic contact on n-type GaN film with carrier concentration as low as 3 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}.{dollar} Ni/Au metallization scheme was developed to form ohmic contact on p-type GaN. Contact resistivity as low as 7 {dollar}times{dollar} 10{dollar}sp{lcub}-3{rcub} Omega{dollar}cm{dollar}sp2{dollar} was achieved. Ni/Au metallization was also optimized as the transparent contact with low sheet resistance and high transmission.; GaN homojunction and InGaN double heterostructure (DH) LEDs were designed, fabricated, and tested. The InGaN DH LED has a typical operating voltage of 3.5 V at 20 mA. The external quantum efficiency of the InGaN/GaN multiple quantum well blue LED is 0.5% at a forward current of 20 mA.