鋁/鍺接面特性與鍺錫約瑟夫森場效電晶體

Abstract

超導體/半導體異質結構在拓樸量子計算中扮演重要角色。拓樸量子位元可由馬約拉那零模態組成，理論預測當一個半導體奈米線與超導體相接時，在奈米線中會形成馬約拉那零模態。而要形成馬約拉那零模態，超導體/半導體介面必須要有極高的穿透率，如此才能藉由鄰近效應將超導特性引入半導體中，而半導體材料本身也需要具有強Rashba自旋軌域耦合並配合外加磁場來改變奈米線的能帶結構，進而讓奈米線擁有拓樸特性。鍺基半導體是有潛力的材料選擇，因為超導體/p型鍺介面上不存在蕭特基位能障，使得介面穿透率高；此外，鍺錫/鍺異質結構中的二維電洞氣體具有強Rashba自旋軌域耦合效應。目前已經有研究團隊觀察到鍺約瑟夫森場效電晶體中由臨近效應導致的超導電流，因此本論文著重在研究鋁/鍺介面特性以及由鍺錫/鍺異質結構製作的約瑟夫森場效電晶體。

本論文呈現了鋁薄膜的超導特性與鋁/p+型鍺接面的電性量測結果。鋁薄膜的臨界溫度為1.4 K，代表在溫度為0 K時的超導能隙(Δ)是213 μeV。鋁薄膜的變磁場量測結果則顯示其臨界磁場在溫度0.05 K下高達0.047 T。鋁/p+型鍺接面從室溫一路降溫至0.3 K都是歐姆接觸。一般來說，當接面偏壓小時，超導能隙會阻止半導體中的載子穿隧進入超導體中，造成接面的微分電導在電壓為零附近下降。然而鋁/p+型鍺接面的微分電導不隨偏壓改變，兩個可能的原因分別是極高的超導體/半導體介面穿透率以及接面邊緣的漏電流。

本實驗還製作了兩種基於鍺錫/鍺異質結構的約瑟夫森場效電晶體，其中一種電晶體使用了自行開發的SF6電漿蝕刻製程來去除源極/汲極區域的鍺間隔層，讓鋁能夠直接接觸鍺錫層以增加鋁/鍺錫二維電洞氣體的介面穿透率。低溫量測結果顯示在0.05 K下約瑟夫森場效電晶體的開關比達4×105，代表電晶體在低溫時漏電流很小；然而本實驗並沒有在約瑟夫森場效電晶體中觀察到超導電流，推測是因為蝕刻製程導致鋁/鍺(錫)介面不佳、鍺間隔層太厚、或電晶體通道太長等原因所造成。

A superconductor/semiconductor heterostructure is crucial for topological quantum computing. A semiconductor nanowire in contact with a superconductor is predicted to host Majorana zero modes, which are the building blocks of a topological qubit. To observe Majorana zero modes, a superconductor/semiconductor interface needs to be highly transparent to ensure that superconductivity can be induced in the semiconductor by proximity effect. The semiconductor materials should also possess strong Rashba spin-orbit coupling, which enables topological properties in a nanowire by tuning its band structure via magnetic fields. Ge-based semiconductor materials are promising due to the absence of Schottky barriers at a superconductor/p-Ge interface, leading to the transparency of the interface. Furthermore, two-dimensional hole gas in GeSn/Ge heterostructure shows strong Rashba spin-orbit coupling. Supercurrent induced by proximity effect has been observed in Ge Josephson field-effect transistors (FETs). In this work, the Al/Ge interface and Josephson FETs based on a GeSn/Ge heterostructure are studied.

In this work, the superconductivity of Al thin films and ohmic nature of Al/p+-Ge contact are demonstrated. The Al thin film has a critical temperature as high as 1.4 K, and the corresponding superconducting energy gap (Δ) is 213 μeV at 0 K. The Al thin film is also characterized under magnetic fields, and the results show that it has critical field as high as 0.047 T at 0.05 K. The Al/p+-Ge contact remains ohmic from room temperature down to 0.3 K. Typically, superconducting gap will prevent carriers in semiconductor from tunneling into superconductor at small voltage bias, and thus decrease the differential conductance of superconductor/semiconductor contact around zero voltage. However, the differential conductance of Al/p+-Ge contact remains constant no matter how applied voltage changes. There are two possible reasons including high transparency of superconductor/semiconductor interface and leakage current at the edge of contact.

Two kinds of Josephson FETs based on a GeSn/Ge heterostructure are fabricated. A SF6 plasma etching technique is developed to remove the Ge spacer at the source and drain in order to make Al directly contact with the GeSn layer, to enhance the transparency between the superconductor and two-dimensional hole gas. Electrical measurements of Josephson FETs are carried out in a dilution fridge. The transistors show an on/off ratio as high as 4×105 at 0.05 K, which indicates a low leakage current at low temperatures. However, no supercurrent is observed in all transistors. Possible reasons are poor Al/Ge(Sn) interfaces formed during the etching process, the thick Ge spacer, or the long channel length.