無序Co/SiO2系統研究與其SIF穿隧接面製作

Abstract

本論文旨為研究三維顆粒結構Co/SiO2樣品之電性傳輸性質與探討其SIF穿隧接面製作技術與其穿隧傳輸結果。 無序Co/SiO2樣品是以共同濺鍍的方式製作；樣品涵蓋從弱無序到強無序區域；無序性落於過渡地帶的樣品，其低溫時電阻率與溫度關係具有 形式，符合Beloborodov等人考量電子電子交互作用力修正結果在較大能量時的電導與溫度關係形式。該理論預測顆粒與顆粒之間的耦合強度(穿隧電導)與電子電子庫侖作用力強度的競爭結果決定了樣品的電子傳輸性質gT；使得當顆粒與顆粒之間的穿隧電導接近關鍵穿隧電導gCT時，樣品落於過渡地帶，而當gT > gCT 時樣品為金屬態至弱無序地帶；gT < gCT時，樣品偏向強無序地帶至絕緣態。理論預期過渡地帶之 形式，乃是顆粒結構樣品在金屬絕緣相變地帶的特殊行為，我們的研究結果顯示顆粒結構Co/SiO2樣品的確具有一較寬的過渡地帶 區域，此特性並沒有發生在較均勻融合如CuxGe100-x的無序系統。 穿隧接面樣品製作採用熱蒸鍍製作Pb或Pb0.7Bi0.3底電極，接續熱蒸鍍Al以進行氧氣的輝光放電生成AlOx氧化層，最後共同濺鍍Co/SiO2頂電極。接面製作原目的為使用Tedrow等人提出的平行電容板形式電子穿隧技術來分析磁性無序鈷樣品的自旋極化率。我們成功製作Pb-Al-oxide-Co/SiO2穿隧接面並量測穿隧電導電壓關係隨外加磁場之變化趨勢，發現第一類超導體Pb的臨界磁場HC太小，以至於低磁場下因Zeeman splitting造成的峰值分裂能差太小，能量解析度不足。因此我們嘗試結合第二類超導體Pb0.7Bi0.3高臨界磁場HC2的特性，預期在費米能面附近達到更大的解析空間。結果由所製作Pb0.7Bi0.3-Al-oxide-Co/SiO2樣品穿隧電導電壓關係，我們認為目前所製成這一系列接面，可能更傾向形成其他形式如N-I-disordered N與S-N-I-disordered N結構接面。

The goal of this research is to study the transport properties of the 3D disordered granular Co/SiO2 system and to investigate the fabrication and tunneling transport property of its SIF tunneling junction. The Co/SiO2 samples were prepared using a magnetic co- sputtering system. They span from weakly to strongly disordered regimes. Samples in the crossover regime show behavior at low temperatures in coincidence with the theoretical correction taken account of the e-e interaction by Beloborodov et al. in the large energy scales. The competition between coupling strength (tunneling probability) and e-e Coulomb interaction among granules determines the transport property of the system, gT. When gT gCT (critical tunneling conductance) the system is in the crossover regime from weak to strong disorder. While gT > gCT the system is weakly disordered and on the other hand, gT < gCT the system is in strongly disordered regime. The behavior is the characteristic of the granular system in the metal-insulator transition. For granular Co/SiO2 system, it has been found that there is indeed a lnT regime corresponding to a broader crossover regime from weak to strong disorder. For more homogeneously disordered Cu/Ge system, the crossover regime is rather small. The tunneling junction was fabricated as follows. Firstly, Pb or Pb0.7Bi0.3 was evaporated onto glasses as bottom layer, Al was then evaporated onto the bottom layer and proceeded with O2 glow discharge to grow oxide layer, and finally the Co/SiO2 film were co-sputtered to form the top layer. The technique of spin-polarized tunneling in a planar junction geometry provided pioneerly by Tedrow et al. was adopted to obtain the spin polarization of disordered Co/SiO2 samples. This is our original motivation. We have successfully produced the Pb-Al-oxide-Co/SiO2 tunneling junction and have measured its voltage dependent tunneling conductance in various of magnetic fields. Due to the low critical field of Pb, the Zeeman splitting energy shift between conductance peaks in low magnetic field is not big enough and hence, our IV measurement can not resolve. Therefore the Pb0.7Bi0.3 alloy with high critical field HC2 were chosen to replace Pb as bottom layer of Pb0.7Bi0.3-Al-oxide-Co/SiO2 junction. However, the tunneling conductance reveals that the N-I-disordered N and S-N-I-disordered N rather than S-I-N are more likely to be the realistic junction structure.