| Summary: | 博士 === 國立中央大學 === 電機工程研究所 === 89 === This thesis covers the growth and characterization of In(Ga)As quantum dot (QD) heterostructures and lasers.
QD lasers with different doping schemes in the In0.5Ga0.5As QD active region are investigated. Their lasing wavelength, characteristic temperature, quantum efficiency, and internal loss are characterized and correlated with the size, uniformity, and density of the QDs revealed by atomic force microscopy. Room temperature continuous-wave operation of Be-doped quantum-dot lasers has been achieved. Undoped In0.5Ga0.5As quantum-dot lasers with a characteristic temperature as high as 125 K above room temperature have also been demonstrated.
It is also found that the performance of In0.5Ga0.5As/GaAs multi-stack QD lasers is sensitive to the GaAs spacer thickness between the dots. Reducing the spacer thickness from 30 nm to 10 nm leads to narrow photoluminescence linewidth, low threshold current, high characteristic temperature and high internal quantum efficiency. This behavior is attributed to inhomogeneous broadening caused by dot size fluctuation related to spacer thickness.
We then study the matrix-dependent strain effect in self-assembled InAs QD heterostructures using photoluminescence measurements. A series of samples is prepared to examine the effect of QD position with respect to the so-called strain-reducing layer (SRL). Since the SRL reduces the residual hydrostatic strain in the QDs, long emission wavelength of 1.34 mm is observed for the InAs QDs with an In0.16Ga0.84As SRL. The dependence of the emission wavelength on the thickness of the cap layer on SRL also indicates the critical role of the matrix that plays in the strain relaxation process of the dots. This is further confirmed by using In0.16Al0.84As instead of In0.16Ga0.84As as the SRL. A blue shift in wavelength is observed because the elastic stiffness of In0.16Al0.84As is higher than that of In0.16Ga0.84As and less strain is removed from the dots with In0.16Al0.84As SRL.
The effects of the arrangement between InAs QDs and InGaAs SRL on the optical properties of QD light emitting diodes are also investigated. Electroluminescence wavelength longer than 1.3 mm is obtained as InAs QDs are covered with a thin InGaAs SRL. For the same sample, the full width at half maximum of the ground state emission peak is as narrow as 19 meV at low injection current, and less than 40 meV even at saturation condition. It is also found that the slope efficiency of the diode is higher than that of other samples in the linear region and its light output saturation level is higher because of its higher density of QD.
Finally, we present the lasing properties of InAs/GaAs QD lasers with InGaP cladding layers grown by solid-source molecular beam epitaxy. These Al-free lasers exhibit a threshold current density of 138 A/cm2, an internal loss of 1.35 cm-1 and an internal quantum efficiency of 31 % at room temperature. At low temperature, a very high characteristic temperature of 425 K and very low threshold current density of 30 A/cm2 are measured.
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