DC and AC Characteristics of Two Dimensional Electron Gases in Undoped Silicon/Silicon Germanium Heterostructures

• Main Authors: Kuan-Yu Chou, 周冠宇
• Other Authors: 李峻霣
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/k28w8p

碩士 === 國立臺灣大學 === 電子工程學研究所 === 105 === As the sizes of the CMOS transistors are scaled down to the atomic scale, new channel materials, alternative structures, or novel computing devices to continue the scaling rule become important. Quantum computing by spin manipulation is a promising candidate to outperform the conventional logic units by charge manipulation. For solid state quantum computing, the most common baseline structure is a two-dimensional electron gas (2DEG) formed in semiconductor heterostructure. Si/SiGe heterostructures attracts tremendous attentions due to the reduced nuclear spin decoherence and compatibility of Si VLSI technology. Thus, we focus on the transport properties of 2DEG in Si/SiGe heterostructures in this thesis. Hall measurement technique is commonly used for the characterisitcs 2DEG transport. We established a home-made Hall system of varying temperature (4 K ~ 300 K) to investigate the DC and AC characteristics of 2DEGs. We fabricated Hall-bar devices with top gate to modulate the 2DEG density. For the first time, a negative differential resistance in DC characteristics is observed. This is due to the crossover of non-equilibrium and equilibrium in the 2DEG system. At low temperatures, electrons cannot be populated at the surface due to the channel freeze-out. In order to conserve the charges, electrons would be accumulated in the underlying 2DEG channel and surpass the equilibrium density. However, as the gate voltage increases further, the increased 2DEG density enhances the electric-field and the tunneling probability of 2DEG electrons to the surface is also enhanced. The mobility of tunneled electrons at the surface is much lower than that in the 2DEG channel, leading to a lower total conductance and current. By varying the temperature, we found at ~50 K, the threshold voltage of Hall devices increases abruptly, which cannot be explained by the theory of MOSFET. Here we propose a model which involves the defect freeze-out at the surface to explain this phenomenon. Last, we investigated the relationship of capacitance vs. gate voltage for 2DEGs at different depths. An interesting depth dependence of frequency dispersion was observed and possible mechanisms are also discussed in this thesis.