| Summary: | 博士 === 國立中央大學 === 光電科學研究所 === 87 === This dissertation focuses on the growth, characterization, and fabrication of self-assembled In0.5Ga0.5As quantum dot lasers grown by molecular beam epitaxy.
The detailed growth procedures and structural properties of self-assembled In0.5Ga0.5As quantum dots are systematically studied. Parameters that affect the configuration of the quantum dots, such as the As overpressure, substrate misorientation, the strain in the buffer layer, and matrix materials, have been investigated using atomic force microscopy. It is found that a high dot density corresponds to a high As overpressure and large misorientation, and vice versa. The associated surface strain energy and the free energy for the adatoms are key factors in the formation of the quantum dots. Better morphological characteristics for dots nucleated on a In0.1Ga0.9As matrix are also found.
The optical and electrical characteristics of heterodot structures are investigated, and the intimate relationship between these properties and their morphological characteristics is developed. The photoluminescence (PL) spectra indicate that In0.5Ga0.5As quantum dots on a 4o-off substrate exhibit a higher intensity, compared to those on 15o-off and exact (100) substrates. The activation energy derived from the temperature-dependent PL of In0.5Ga0.5As quantum dots on a 4o-off substrate is higher than that of dots on 0o and 15o-off substrates. These properties are consistent with the results of the laser performance, as shown in the last chapter. As to the matrix material effects, the dots in the In0.1Ga0.9As matrix exhibit higher PL and electroluminescence (EL) intensities, as compared to those in the GaAs and In0.1Al0.9As matrices. The activation energy of dots in a GaAs matrix is higher than that of dots in a In0.1Ga0.9As matrix. The effects of state filling effect may be observed in the power dependent EL spectra for dots in the In0.1Ga0.9As and GaAs matrices. The I-V characteristics show that a carrier tunneling-recombination is the dominant process in the low bias and low temperature region for dots in the In0.1Ga0.9As and GaAs matrices. Comparing the I-V characteristics of three samples, the larger tunneling-recombination current in the In0.1Ga0.9As matrix sample can be attributed to its smaller *Ec, its higher dot density and its larger lateral size. From our experimental results, including the crystallographic qualities, and luminescent and electrical properties, a homogeneous deposition is preferable to a heterogeneous deposition in quantum-effect devices.
Finally, the potential of three-dimensional quantum dot heterostructures to device applications is demonstrated by means of room-temperature operating laser diodes. We have systematically studied the characteristics of In0.5Ga0.5As quantum dot lasers grown on three types of GaAs substrates. Due to its better quantum confinement, the 4o-off sample exhibits a lower threshold current of 47 mA at room temperature, compared to 73 and 65 mA for the 0o and 15o-off samples, respectively. It also has a higher characteristic temperature, 177 K. Furthermore, improvements in the laser''s performance are demonstrated using As dimers. A characteristic temperature as high as 125 K has been measured for quantum dot lasers in the temperature range between 20 and 70 oC. Which is the highest value at such a high operating temperature for self-assembled InGaAs quantum dot lasers. It is also found that the slope efficiency of these lasers does not degrade very much as the temperature is increased. This implies that the quantum dot lasers have an excellent carrier confinement and a low leakage current.
The continuous-wave operation of Be-doped In0.5Ga0.5As quantum dot lasers is fabricated and characterized between 20 and 70 oC. This laser exhibits a threshold current per dot layer of 32 mA and a slope efficiency of 0.12 W/A at 15 oC, and a characteristic temperature of 56 K.
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