| Summary: | 碩士 === 國立彰化師範大學 === 光電科技研究所 === 93 === AlGaInP laser diodes have been developed for more than two decades. The maximum operation temperature of the AlGaInP laser diodes (LD) has been increased from 70°C in the past to the more recent 100°C. However, some inherent disadvantages, such as small conduction band offset, low p-doping concentration and high thermal resistivity, inhibit AlGaInP LD from obtaining high operation temperature owing to more leakage currents over the p-cladding layer at elevated operation temperature. Therefore, how to reduce the leakage current and enhance the operation temperature have been the major study topics for AlGaInP LDs. In the beginning of this thesis, the material characteristics and laser diode development history of the AlGaInP will be reviewed.
The main purpose of this thesis is to optimize the active layer structure and improve the performance of the AlGaInP LD so that it is suitable for outdoor DVD application. The key parameters will be optimized including quantum well number, barrier and spacer compositions in the traditional strained active layer, and barrier compositions in the strain-compensated active layer. The theoretical analysis is done utilizing LASTIP simulation software by assuming the AlGaInP LD with a constant emission wavelength and a fixed far-field pattern.
First, different quantum well (QW) numbers and barrier compositions in the 0.5% compressive-strained QW active layer are used to analyze the leakage current. It is found that with an increase in quantum well number, the leakage current can be reduced. However, the threshold current is increased accordingly. The simulation results suggest that five quantum wells are good enough to inhibit the electron overflow. Moreover, a (AlxGa1-x)0.5In0.5P barrier with x=0.5 forms a deep well. More carriers and higher stimulated emission rates can be obtained. With a quantum well number of five and a (AlxGa1-x)0.5In0.5P barrier with x=0.5, the leakage current can be greatly reduced and the characteristic temperature is improved without significant increase in operation currents. This simulated result is in good agreement with the experimental result.
Furthermore, the effect of the barrier height for a strain-compensated MQW AlGaInP LDs is theoretically analyzed to improve the LD performance. The strain-compensation technique is used to prevent the formation of dislocations by alternating layers with compressive- and tensile-strain of equal amounts, resulting in zero-net strain. The simulation results suggest that there is a moderate range for the choice of the Al composition in the tensile-strained barrier layer. The performance of the LD is degraded if a higher Al composition is utilized due to the non-uniform carrier distribution inside wells. However, if the Al composition is too small, the LD characteristic is also degraded. It is found that the Al composition from 0.1 to 0.2 should be a better choice for this strain-compensated MQW AlGaInP LDs. The characteristic temperature for this strain-compensated LD is comparable with the traditional strained MQW LD. However, the reliability is expected to be greatly improved due to lower Al composition and strain-compensated structure.
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