利用通道內電場改變二硫化鉬場效電晶體效能

Abstract

由於二硫化鉬之電流開關比與電流穩定性高，本實驗以機械剝離法將二硫化鉬分離成少數層，利用電子束微影、熱蒸鍍等標準製程方法將其製成場效電晶體元件，並在汲極與源極之間製作電極，由此電極在元件中注入電場，改變通道內電場影響汲極與源極間的電流，觀察通道內電場造成電流開關比與載子遷移率的變化，藉此提升元件之效能。 二硫化鉬為n型半導體材料，量測元件之電流開關比約為106。在元件中注入106 V/m以上的高電場，若電場方向與電流相同，則汲極與源極間電流會快速上升，若電場方向相反，則電流將會受其抑制，而電場值越大造成的影響也越劇烈。若在元件中注入雙電場，結果顯示其與單電場的電性趨勢相同，且發現造成電流變化主要是由靠近汲極端之電場影響較大，故僅需施加一電場即可改變汲極與源極間的電流。 為了提升元件效能，在元件中注入與電流方向相同之高電場，隨著電場提升，電流開關比可達到107以上，但在開啟電流趨近最大值後電流開關比開始遞減。然而隨著關閉電流不斷上升，導致元件關閉越來越困難，使得關閉元件之臨界電壓逐漸往負電壓方向移動。此外在載子遷移率方面，當電場增強時載子遷移率持續增加，從原本未加電場時的25.4 cm2/Vs，最大值可以達到約900 cm2/Vs。因此藉由改變元件的通道內電場達到理想的載子遷移率及電流開關比，即可使元件效能提升。

In this study, MoS2 is chosen as channel materials for making field-effect transistors (FETs), because of its high on-off ratio and stability. We utilize mechanical exfoliation to make MoS2 nanometer thin flakes on the silicon substrate. The source and drain FET electrodes are deposited on MoS2 flakes by standard electron-beam lithography followed by thermal evaporation. In this experiment, we made another ohmic-contact electrode between the source and drain electrodes so as to modulate the channel current. In such a design, the addition electrode provide high electric field to raise the efficiency of MoS2 FET devices. An electric field higher than 106 V/m is applied as channel electric field by the additional ohmic-contact electrode. When the additional electric field is in the same direction of the source-drain electric field, the source-drain current increases rapidly. On the other hand, if the directions are reversed, the source-drain current is highly suppressed. To investigate the effect of high electric field in the channel, we made two additional electrodes between the source and drain electrodes. The enhancement of the FET efficiency is similar to that structure of only one ohmic-contact electrode between the source and drain electrodes for the electrode more close to the drain electrode. The high channel electric field help to increase the on-off ratio of FET up to 107. When the channel electric field is higher and higher, the off-current increases continuously and the threshold voltage in the gating dependent source-drain currents shifts to lower and negative voltage. More specially, the mobility in the channel is increased up to 900 cm2/Vs with increasing electric fields in the channel. From our experiments, it is proposed that the high channel electric field can be used to raise the efficiency of MoS2 FETs.