| Summary: | 博士 === 國立臺灣大學 === 電機工程學研究所 === 92 === In this thesis, the novel metal-oxide-semiconductor (MOS) tunneling diodes with high leakage current were utilized as photodetectors. The leakage of inversion carrier through ultrathin oxide makes the device to operate in the deep depletion region. The dark current is limited by the thermal generation process and can be reduced by the high growth temperature of oxide. For the PMOS detectors, the direct tunneling electron current from Al electrode to n-type silicon is the main component of the photocurrent, which is one order of magnitude larger than minority generation current in the deep depletion region.
The mechanisms of gate inversion tunneling current in MOS tunneling diode are investigated. The inversion tunneling current model is composed of Shockley- Read-Hall (SRH) generation model, band-to-band tunneling model, and band-to-traps tunneling model. The oxide roughness effect on tunneling current in MOS diodes is also studied. Due to the 2-D electrical effect, the increasing roughness height with fixed roughness period can significantly increase the gate tunneling current, and for a given roughness height, the current increases first and drops a little as the period parameter increases.
To increase the cutoff wavelength and efficiency of the MOS detector, Ge and Ge/Si quantum dots are used as absorption layers. The oxide is directly grown on Ge substrate by liquid phase deposition to reduce thermal budget. This Ge photodetector can operate at 1.3 and 1.5 μm lightwave and can be applied to the fiber-optic communications. The maximum external quantum efficiency is estimated approximately 50 %, and responsivity can reach 0.5 A/W at 1.5 μm. The five-period Ge quantum dot MOS device can detect the wavelengths of 820 nm, 1300 nm, and 1550 nm with the responsivity of 130, 0.16, and 0.08 mA/W, respectively. The responsivity at 850 nm reaches 600 mA/W using a 20-period Ge quantum dot absorption layer.
On the other hand, the strain field on the Si cap of self-assembled quantum dots can have preferential oxide deposition during liquid phase deposition process. The oxide dots are formed on the Si cap with tensile strain, and are aligned vertically with Ge dots embedded in the Si caps.
In order to increase the speed of the MOS tunneling photodetectors, the novel fully-depleted silicon-on-insulator (SOI) MOS photodetector is proposed. For devices with 1020 cm-3 buffer layer doping, the device can reach high bandwidth (22 GHz) and are fully compatible with ultra-large scale integration (ULSI) technology. For thin devices, the transit time can be determined by the drift mechanism. For thick devices, however, the diffusion mechanism is needed to describe the device behavior.
Finally, the MOS Ge/Si quantum dot infrared photodetectors (QDIPs) for 2 ~ 10 μm using hole inter-valance subband transitions are demonstrated. The maximum operating temperature is 140 K for 3 ~ 10 μm and is up to 200 K for 2 ~ 3 μm detection with LPD oxynitride.
These simple and high performance Si-based photodetectors together with other devices can be used as building blocks for the future optical signal process and the optoelectronic applications on Si chips.
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