| Quantum cascade (QC) lasers are unipolar semiconductor lasers based on one type of carrier (electrons) making transitions between energy levels created by quantum confinement. They have been regarded as a milestone in both semiconductor materials and in semiconductor lasers. In this thesis, we have focused ourselves on the QC lasers made of InGaAs/InAlAs/InP material system, working at the mid- and far-infrared atmospheric window, to carry out the research on physics, material, and device of QC lasers.By expanding the wave-functions as Taylor series, we have proposed a method to calculate the electron subband energy levels and wave-functions in biased, asymmetrically coupled multiple quantum wells. By means of analyzing the population inversion condition in lasers, with the combination of the calculation of transition matrix element strength, we have pointed out the possible range of electric field, well width and barrier thickness in the active region of QC lasers. Also, we have pointed out the way of design and optimization of a QC laser.The dispersion of semiconductor refractive indices has been successfully transplanted into the design of QC lasers. We have considered both the interband and the intrakand absorption of free carriers. Appropriate doping concentration has been adopted in the contact layer to suppress the refractive index, as a result, efficient optical confinement and free carrier confinement have been achievement.The coupling coefficients of coupled modes in index-coupled distributed-feedback QC lasers have been calculated with the perturbation theory for the first time. The coupling coefficients are given as functions of grating depth, and of laser wavelength quantitatively. Moreover, we have compared the coupling coefficients for rectangular grating, sinusoidal grating, and sawtooth grating, with the grating depth fixed as 250 nm.The spontaneous emission spectra and gain spectra of QC lasers have been calculated. We have discussed the dependence of the spectra on temperature and on subband lifespans. The theoretical results are then compared with the experimental ones. The gain spectra for different population-inversion parameter ~= 0, 0.25, 0.5, 1.0, 1.5 are given.A double-color quantum cascade laser structure operating at both X? pin and X? pm has been proposed for the first time. The structure contains 32 stages of X? p.m active regions and injectors, and 35 stages of X? pm active regions and injectors. A well-designed conjunction graded-gap superlattice connects these two kinds of active regions and injectors. The whole structure is proposed to work under an electric field of 65 kV/cm and a threshold current density of2 kAIcm2.we have proposed and demonstrated for the first time that a randomly chosen æ‚?shape modulated superlattice can serve as a better energy filter than an un-modulated superlattice. Theoscillation of transmission probability in the minibands of the æ‚?shape modulated superlattice is shown to be greatly suppressed. We also demonstrate that the precise control of the potential shape is not crucial for energy filters.An overall investigation of x-ray diffraction, photolunainescence, and electroluminescence have been carried out on the materials of InAIAs, JnGaAs single epilayers and 1n0.53Ga0.47As/ 1n0.52Al0 48As quantum wells. The following results have been achieved for the structural and optical properties of the InGaAs/[nAlAsfInP materials for type-I QC lasers:A method of x-ray diffraction with simple geometly was developed to quantitatively analyze the structure of strained InGaAs, InAlAs epilayer with a tilting angle to the In? substrate. Results showed an excellent consistency between data measured and calculated from the model. It was pointed out that the conventional 1 800-rotation twice-measurement method usually got a value smaller than the real value of the tilting angle.The difference between the x-ray-diffraction Bragg peak width under the grazing incidence geometry and under the grazing emergence geometry for InGaAs/L...
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