Modeling of Exciton and Optical Absorption in the Ge Quantum Well Systems

• Main Authors: Yin-Shun Li, 李銀順
• Other Authors: 郭宇軒
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/04563896546185741423

碩士 === 國立臺灣大學 === 電子工程學研究所 === 97 === The optical modulator is one of the most important devices for optical communication, and the strongest modulation mechanism, the quantum-confined Stark effect (QCSE), had been demonstrated in Ge/SiGe quantum wells on Si substrate. It is fully compatible with silicon electronics and can achieve the GHz-region operation for the long wavelength and C-band (1530–1565 nm) operation. The discovery can highly improve high-speed, low power, and small optical devices. In this thesis, the variational methods based on the 2-dimensional (2-D), 3-D and anisotropic 3-D exciton models are used to study the direct-band-edge ground-level electron-heavy-hole (e1-hh1) exciton behaviors in the infinite Ge quantum well and the finite Ge/Si0.15Ge0.85 quantum well structures. For the finite case, the effect of the conduction-band nonparabolicity effect on the exciton behavior is also investigated. The exciton radius, transition energy, binding energy, and optical oscillator strength are calculated for various quantum well thicknesses and “bias voltage” (or “vertical electric fields”). The exciton becomes weaker with thicker well and stronger bias voltage/electric field for both the infinite and finite well cases. The three exciton models are compared, indicating that the 3-D model is suitable for the infinite well modeling while the exciton in the finite well considered with the nonparabolicity effect case is nearly 2-D for thinner well and becomes anisotropic-3-D for wider well. The exciton radius minimum and oscillator strength maximum occur at 1.6 nm for the finite well case; therefore, this Ge/SiGe quantum well system shows a strong quantum-confinement even with a thin thickness. For the finite case without considering the nonparabolicity effect, the 3-D model is used to investigate the exciton effect. The relative direct-gap-to-indirect-gap absorption ratios are compared, indicating a broad thickness range of ~5–15 nm can provide moderate excitonic absorption and contrast ratio for long wavelength operation. Finally, the nonparabolicity effect can enhance the exciton effect, especially in a thinner well. Also, our work agrees well with the experimental result and other calculation for the 10-nm Ge/SiGe quantum well case.