Studies of 6H-SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H-SiC were systematically studied. The main works are summarized as follows:The homoepitaxial growth on the commercially available single-crystal 6H-SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p(+)n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K.Using thermal evaporation, Ti/6H-SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V.n-Type 6H-SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of polysilicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to gamma rays. (C) 2002 Published by Elsevier Science B.V.

东北农田生态系统排放CH_4和N_2O的研究

本文首次对我国东北地区稻田和旱田（大豆田）中甲烷（CH_4）和氧化亚氮(N_2O)的排放通量进行观测,研究了环境因素对这两种温室效应气体排放的影响。观测结果表明：稻田在作物生长季节是CH_4的排放源(source)，作物被收割之后则成为甲烷的汇(sink)。在作物生长季，稻田的CH_4排放通量变化在2.41-26.1mgCH_4/m~2·h之间，平均通量为14.82mgCH_4/m~2·h。稻田N_2O通量在-116.89-100.69μgN_2O/m~2·h间变化，平均通量为-6.36 μg/m~2·h。旱田完全是N_2O的排放源，通量在3.99-332.3 μg N_2O/m~2·h之间变化，平均通量是88.54 μgN_2O/m~2·h。旱田主要表现为甲烷的汇。稻田中甲烷主要在0-5cm土层中产生。水稻和大豆分别对CH_4和N_2O的排放起着重要作用。稻田甲烷排放通量与温度(特别是气温)有极显著的正相关性，也与土壤中硝态氮含量呈显著正相关／旱田甲烷排放通量与土壤中铵态氮浓度呈显著负相关，而N_2O通量则与铵态氮浓度呈正相关，但有3-4天的时间滞后现象。旱田甲烷和氧化亚氮的排放受土壤含水量的影响。本文还讨论了实验室条件下施肥对稻田土壤CH_4产生作用的影响。厩肥的施入影响较弱，而稻杆的影响则十分强烈。尿素也促进CH_4的产生。100和200μg/g(土)的尿素加入量增大了产甲烷速率和甲烷产量；300μg/g(土)的尿素加入量没有增大产甲烷速率，却延长产甲烷时间，使甲烷产量增加更多。土壤中CH_4和N_2O的产生过程之间似乎呈相互消长关系。