鎳/N型鍺錫接面特性分析和接面電阻率之研究

Abstract

鍺錫材料由於其極高的電洞載子遷移率，因而十分被看好能取代矽成為下一代之電晶體通道材料。然而現今並無太多鍺錫N型電晶體(n-FET)被發表，且效能皆不盡理想，其主因是受限於由費米能階釘扎(Fermi level pinning)所致之較大的N型鍺錫與金屬接面電阻。因此如何減少N型鍺錫與金屬接面電阻率便成為鍺錫能否成為下一代電晶體通道材料之關鍵。 為了研究N型鍺錫與金屬接面的特性，我們首先透過內摻雜與離子佈植兩種方法對鍺錫薄膜進行摻雜，並研究不同退火條件對鍺錫薄膜片電阻以及材料晶格的影響。結果發現較高之退火溫度會使鍺錫出現錫偏析或鬆弛的現象，而較高的離子佈植劑量更會加強上述的現象。透過電流-電壓及電流-溫度方式量測N形鍺錫與鎳接面之蕭基位障高度後，發現蕭基位障高度會隨鍺錫中錫比例上升而下降，顯示出費米能階釘扎會成為N型鍺錫與金屬接面電阻之限制因素。隨後我們分別利用CTLM與RTLM結構量測N形鍺錫與鎳鍺錫合金之接面電阻率，結果顯示接面電阻率會隨鍺錫中載子濃度或錫比例上升而下降，可歸因於介面穿隧能障減少所致。 最後我們提出了一種新穎的立體式CTLM改良結構，藉由將注入電流探針與電流導出探針錯開至兩不同平面以解決傳統CTLM結構因金屬電極電阻所造成電位分佈不均的問題，進而增進量測接面電阻率的精確度。TCAD模擬結果顯示此立體式改良結構可測得較傳統CTLM或RTLM結構更低之接面電阻率，且在接面電阻率小於10-8 Ω-cm2時，此結構量測接面電阻率之精確度較CTLM與RTLM結構為佳。

Germanium-tin (GeSn) is a promising channel material to replace silicon due its high hole mobility. However, only few work on GeSn n-FETs were reported and the device performance was poor. This might be attributed to a large resistance in the metal/n-GeSn contact by Fermi level pinning phenomenon. Therefore, reducing the contact resistivity of the metal/nGeSn interface becomes an essential key for GeSn as the future channel material. To investigate the characteristics of the metal/nGeSn interface, we doped the GeSn films by the technique of in-situ doping and ion implantation. We found that GeSn films were relaxed and Sn atoms would segregate toward surface at higher annealing temperatures. With a higher implantation dose, the effects became even stronger. Through both I-V and I-T methods, the Schoottky barrier heights of NiGeSn/nGeSn contacts were measured. The barrier height decreases with Sn fraction, which suggest the Fermi level pinning would be the limiting factor of contact resistivity for the metal/nGeSn interface. Contact resistivity of the NiGeSn/nGeSn interface was also investigated by CTLM and RTLM methods. As the carrier concentration or Sn fraction increases, the contact resistivity decreases, which could be attributed to the reduction of tunneling barrier at the metal/nGeSn interface. Finally, we proposed a novel three dimensional(3D) CTLM structure to reduce the non-uniformity of potential distribution. By separation of the injecting and collecting current probes in two different planes, the measurement accuracy of contact resistivity was improved. The simulation results by TCAD show that a lower contact resistivity could be probed by the 3D-CTLM structure than the conventional CTLM or RTLM structures. As the contact resistivity is lower than 10-8 Ω-cm2, the accuracy by this structure is higher than that by CTLM or RTLM structure.