Abstract
本論文研究電子在氫化非晶矽超晶格結構中傳輸之特性，所研究的樣品是氫化非晶矽╱氫化碳化非晶矽”組成超晶格”．樣品由輝光放電及光激發化學氣相沈積法製作．由Ｘ光繞射及歐傑電子譜儀的分析，樣品結構具有陡峭的介面及良好的週期性，光學能隙及光激光譜峰值均呈現藍遷移的效應．光導電度量測顯示，超晶格結構具有較緩慢的光導電度衰退特性．在等效電子質量為0.1 至0.6 倍自由電子質量之假設下，分析電子在非晶矽雙位壘量子位阱結構中之非彈性散射時間及共振穿透時間，結果顯示非彈性射時間小於共振穿透時間10的5 次方至12次方倍．本質上電子在非晶矽量子位阱結構之傳輸屬於完全非彈性碰撞過程，電子穿透過量子能階之傳輸特性無法存在．在88K 溫度下，對兩個三位壘結構及一個20週期超晶格結構夾於2 個摻磷n 型接面層（n-QW-n）之電性量測顯示，電流對電壓曲線具有”電流突起”之特性，經由穿透率對電壓在電子能量為平均熱能條件下之計算，所觀察到的電流突起不是由於電子穿透過量子能階所造成的．進一步的實驗係以量子位阱及超晶格結構置於於p-i-n結構之i層中進行照光電性量測．對於雙位壘及不同週期超晶格結構進行量測所觀察到之電流突起，嘗試以電子穿透過量子能階解釋．以一系列不同位阱厚度之雙位壘結構進行室溫與77K 溫度下量測，所觀察到的電流突起與位阱層厚度無關，且於77K 時消失．比較一系列單位壘及雙位壘結構置放於i 層中不同位置之電性，兩者之電流電壓特性相似，證實所觀察到之電流突起係由於電子穿透過位壘結構所造成．利用不同的單頻光照射在i 層中分別含有單位壘，雙位壘及超晶格之p-i-n 結構來分析電流電壓特性，所觀察到的電流突起係由於在i 層中兩個區域內由光激發所產生之載子被內電場吸引收集所造成，而區域是由置入i 層中之結構所劃分．由以上之分析及系列實驗證明，電子在氫化非晶矽量子位阱及超晶格結構中之傳輸是穿透過具有大阻值之位壘結構，位阱中之量子能階對電子傳輸沒有影響．電流電壓特性是由i 層中兩個區域光激發載子被收集所造成．///////This thesis studies the electron transport properties in amorphoussilicon superlattices. The samples were prepared by photo-CVD and GD-CVDtechniques. The heterojunction interface and periodicity of theａ－Ｓｉ：Ｈ╱ａ－Ｓｉ1-xＣx:H superlattices were studied by X-raydiffraction and Auger depth profile, and good periodicity and sharpinterface superlattices were obtained. The optical gap and PL signal ofthe different well thickness superlattices were measured. Thephotoconductivity degradation of the superlattices is less than that ofthe bulk a-Si:H films at least one order of magnitude.From the analysis of the resonant tunneling time and inelastic scatteringtime of the amorphous double barrier structure, the inelastic scatteringtime is shorter than the resonant tunneling time for about 5 to 12 orderof magnitude under the assumption that the electron effective mass rangefrom 0.1 to 0.6 free electron mass. The intrinsic tunneling process inamorphous silicon superlattice is in the completely incoherent tunnelingregion. The peak-to-valley tunneling current ratio and the density ofstates in the well go to zero.Two triple barrier structures with different well width and a 20periods superlattice sandwiched between two phosphorus doped n contactlayers (n-QW-n structure) were prepared to study the carrier transportproperties. Current bumps are observed in the 88 K I-V characteristics ofthese samples. From the analysis of the transmissivity versus appliedvoltage at a specific electron energy, the observed current bumps can notbe explained by carrier transport through the bound states in the well.In order to further check these results and identify the real transportproperties, the ａ－Ｓｉ：Ｈ╱ａ－Ｓｉ1-xＣx:H quantum well structures andsuperlattices imbedded at different locations of the i layer of a p-i-nstructure were used to study photogenerated carriers transport throughthese imbedded structures. The room temperature I-V characteristics of thedouble barrier structures imbedded at different locations of the i layerand the different periods superlattices imbedded in the center region ofthe i layer show current dips and current bumps. We tried to interpretthese I-V characteristics to be caused by carrier tunneling through thebound states in the well. However, from the measurement of the I-Vcharacteristics of a series of double barrier structures with differentwell thickness, the current bumps do not shift as the well width variesand they disappear at low temperature. The observed I-V characteristicsare not due to carrier tunneling through the bound states in the well. TheI-V characteristics of the single barrier and double barrier structureswith the same barrier thickness imbedded at the same locations of the ilayer are compared. The current bumps are observed at the same positionsof the I-V curves. These results show taht carriers just tunnel through alarge barrier structure. From the I-V characteristics of these samplesunder the different monochromatic light illumination, the observed currentbumps and current dips are due to collection of photogenerated carriers inthe two regions, hich are separated by the imbedded structure. The I-Vcurves of the different periods superlattices under the differentmonochromatic light illumination also show this characteristics.From the above systematic studies, we conclude that the bound states inamorphous silicon quantum wells or superlattices do not play a role incarrier tunneling. The observed I-V characteristics of the single barrier,double barrier (or triple barrier) structures and superlattices are due tocollection of photogenerated carriers in the two regions of the i layer.