This thesis presents studies on GaN based laser diode(LD) structures. The crystal quality of epitaxially grown layers were characterized by cathodoluminescence(CL) and Epitaxially Lateral Over Growth(ELOG) technique was introduced to improve the GaN crytallinity in selected experiments. In addition to conventional ridge type structures, investigation of novel device structures was carried out. Experimental results were presented in conjunction with theoretical calculations.
The crystalline characterization of epitaxially grown LD structure was performed using CL. The CL characteristics of undoped GaN, Si-doped GaN, Mg-doped GaN, and multi quantum wells(MQW) have been studied to idetify the defect-related emission lines and the spatial distribution of emission lines. It has been shown that there are two main emission peaks at 2.8eV and 3.2eV in Mg-doped GaN. In CL images, the 2.8eV emission was distributed over the entire area except for localized regions where 3.2eV emission was detected. The 3.2eV emission is attributed to crystal defects within Mg-doped GaN layer and it could not be completely quenched out even in highly Mg-doped GaN samples. In the case of MQW structures, it has been confirmed that CL emissions were seriously affected by crystal defects within the film. $SiO_2$-removed ELOG substrates were characterized by double X-ray diffraction and CL measurements.
$SiO_2$-removed ELOG substrates have been shown superior substrate qualities, in terms of crystallinity and crystal tilting, to those prepared by normal ELOG techniques.
Thermal treatments were carried out to enhance the activation efficiency of Mg in p-GaN and p-AlGaN layers in laser diode structures. The activation temperature was varied in the range 750℃~1000℃, and the gas ambient constituted of either pure nitrogen or mixed gas of nitrogen and oxygen. P-type contact resistance was reduced to as low as $2.2×10^{-6} Ω-㎠$ using mixed gas for activation (3% oxygen) and p-electrode annealing (20% oxygen). By improving the p-contact resistance, the operating voltage of LD was reduced from 8.7V to 5.9V at an applied current of 100mA. For a better understanding of the GaN LD performances, several simulations were carried out on ridge-type LDs, tacking into account of the ridge geometry, internal losses, and Al composition and thickness of the electron blocking layer. The ridge geometry that consists of its width, length, and etched depth(the distance from active region) strongly effects the core device characteristics such as threshold currents, optical modes, differential quantum efficiencies, and far field patterns. In real device structures, the threshold current can be reduced by as much as 40% from 225mA to 135mA by refining the ridge dimension, as expected from theoretical calculations. Moreover, dependence of device characteristics on operating temperature indicate the hole carrier concentration in p-AlGaN layers within LD structure should be increased to prevent electron overflow into p-cladding layers.
Selective growth was attempted for the first time to realize a buried ridge(SGBR) type LD based on a III-nitride semiconductor. In this novel structure, the laterally grown regions have better crystallinity than the vertically grown regions, as confirmed by CL measurements. It could be expected from electroluminescence images that the current path is well defined by the current blocking layers in SGBR LD structures. Even though lasing has not yet been realized as because insufficient structural optimization, CL and I-V characteristics are greatly improved.
Using the advantages of lateral growth, we ahve also tried other novel structures that demonstrateed stimulated emission. The new approach utilized a patterned n-GaN grown over a Pendeo substrate. The I-L characteristics of LDs grown on patterned substrates were improved, compared to those grown on bare Pendeo substrates. This improvement is attributed to superior crystal quality on laterally overgrown region on the patterned substrate.
