材料組成對二維載子傳導機制之影響

Abstract

本研究與美國空軍研究實驗室合作，他們提供的樣品上有條列式分布的MoO_3、non-crystalline MoO_3、MoO_2、2H-MoS_2、a-MoS_2以及Mo_4 O_11。我們的研究主要是針對這些二維材料進行電性的量測以及光響應的探討。我們設計了一種蒸鍍罩，讓我們得以在每一條材料上進行兩點和四點探針的測量。由於這些條列分布的材料數量眾多，我們利用LabVIEW程式進行自動化的大規模測量，並稱這個方法為auto probe。我們只需輸入一些設定，即可讓其開始自動測量。而利用auto probe的i-v結果，我們計算出電阻並用MATLAB畫出圖形幫助我們比較各材料電阻間之差異。除此之外，我們利用顯微鏡的燈以及不同功率的雷射進行i-v和i-t的測量，並計算響應度、電流的上升時間和衰減時間來研究材料的光響應。在i-v的結果中顯示有光照的情形下，材料的電阻會比無光照時的電阻低。在更高的雷射功率下，i-t 測量結果顯示電流不穩定的情形有顯著改善。在i-t測量時，若有施加電壓，電流會隨著光源被打開而上升、關閉而下降。當無施加電壓時，我們只觀察到MoO_3、non-crystalline MoO_3和Mo_4 O_11隨光源開關有產生光電流。然而，此時的電流在光源被關閉時上升、打開時下降。基於這些結果，我們討論了造成這些材料具如此光響應的機制為何。對於MoO_3、non-crystalline MoO_3和Mo_4 O_11而言，它們產生光電流的機制為光閘效應。至於MoO_2、2H-MoS_2和a-MoS_2，它們的光電流產生機制為光電導效應。另外，當施加電壓上升時，各個材料的響應度、電流的上升時間和衰減時間皆上升。材料響應度上升是因為在更高的施加電壓之下，電場的強度增加使得電子電洞對的分離被加強，形成更多光電流讓響應度上升。電流的上升時間和衰減時間增加則是因為載子在材料的價帶和導帶之間跳躍時，受到途中因材料缺陷而生的陷阱態影響，並且困在其中，進而導致電流的上升時間和衰減時間被延長。當光照功率增加時，各個材料的響應度、電流的上升時間和衰減時間皆下降。在更高的光照功率下，雖然產生更多的電子電洞對，但是光照功率的增加仍然大於光電流增加的幅度，因此響應度下降。電流的上升時間和衰減時間下降是因為在更高的光照下，陷阱態逐漸被載子填滿，導致更多載子可以不受拘束地在導帶和價帶中穿梭，因此電流的上升時間和衰減時間縮短。

In collaboration with the Air Force Research Laboratory (AFRL), we have the sample with MoO_3, non-crystalline MoO_3, MoO_2, 2H-MoS_2, a-MoS_2 and Mo_4 O_11 strips on it. Our work focuses on the electrical measurement and study the photoresponse of these 2D materials. We design a shadow mask that allows us to do the 2-probe and the 4-probe measurement on each strip of material. Given that there are several sample stripes to measure, we use a LabVIEW program to do the large-scale measurement automatically which is called “auto probe”. We just need to put in some simple settings, and the measurement is ready to go. We use the i-v results of auto probe to calculate and compare the resistances of the materials, and they are shown in a heat map generated by a MATLAB code. Besides, we use the microscope light and the 532nm laser with different power of illumination to do the i-v and i-t measurements, and we study the photoresponse of the materials by calculating the responsivity, rise time and decay time. The i-v results show that the resistances of the materials are lower under illumination than with no illumination. With much higher laser power, the i-t results indicate that the situation of the current fluctuation improves a lot. When there is applied bias voltage during the i-t measurement, the current increases as the light source is turned on and decreases as it is turned off. When there is no applied bias voltage, we can only see the photocurrent in MoO_3, non-crystalline MoO_3 and Mo_4 O_11 as the laser is turned on and off continuously. However, their current increases when the light source is turned off and decreases when it is turned on. Based on the results, we discuss the mechanisms of the photoresponse for the materials. For MoO_3, non-crystalline MoO_3 and Mo_4 O_11, the photocurrent generation mechanism is the photogating effect. As for MoO_2, 2H-MoS_2, and a-MoS_2, the photocurrent generation mechanism is the photoconductive effect. Moreover, as the applied voltage increases, the responsivity, rise time and decay time of all six materials increase. The responsivity increases because the separation of the electron-hole pairs are enhanced by the stronger electric field as the voltage increases, resulting in more photocurrent and higher responsivity. The rise time and decay time increase because the carriers are trapped in trap states due to the defects in materials as they transit between the valence band and the conduction band. When the power of illumination increases, the responsivity, rise time and decay time of all the materials decrease. Although there are more photogenerated electron-hole pairs as the power increases, the increase of the power is still much larger than that of the photocurrent. Therefore, the responsivity decreases as the power increases. The rise time and decay time decrease because the trap states between the valence band and the conduction band are gradually filled by the carriers as the power increases, allowing more carriers to transit between two bands freely. As a result, the rise time and the decay time are shortened.