微奈米結構應用於二維材料與熱電子行為之研究

Abstract

光激發熱電子於金屬內部產生時，其在動量空間中是等向分布的，也就是不具有任何的方向性，且只有少部分移動方向垂直於金屬與半導體(蕭特基)接面之熱電子能夠跨越能障，注入半導體，這就是Fowler理論之核心假設。因此，若能夠更清楚地了解光激發熱電子之方向性，將會對各式以熱電子為基礎之光電元件發展相當有利。在本論文中，我們利用一系列實驗及微奈米結構，來探討光激發熱電子於此蕭特基二極體中之方向性與傳輸行為，並發現光激發熱電子之方向性會和入射光之偏振方向有高度關聯性。除此之外，我們也利用磁場來調製矽基光偵測器於光通訊波段之響應。而與一般矽基光偵測器不同的是，我們利用鐵磁性金屬薄膜取代貴金屬薄膜，來建構近紅外光矽基光偵測器。因此，當我們施加磁場於元件時，勞倫茲力與異向性磁阻此兩效應將會影響熱電子於金屬中之傳輸行為。此外，在施加磁場於元件後，此元件將會由具有偏振不敏感之特性，轉換為對偏振敏感之特性。這是第一個結合了熱電子之電、磁、光效應，進而調制、改變紅外光矽基光偵測器於光通訊波段效率之研究。我們認為，當此結果應用於其他基於熱電子的系統和元件時，也能展現極大的效用。 二維材料被廣泛應用於各式元件與領域。因此，二維材料的檢測技術為非常重要的議題。然而，二維材料的性質非常容易受到基材所影響。於本論文中，我們開發了一種空隙共振腔結構，利用此結構能取得各種二維材料之本質特性。而此技術主要是利用了空隙共振腔結構能使二維材料懸空以及大幅地提高二維材料內部電場之優勢。當二維材料被轉置於空隙共振腔結構上時，除了能避免其性質受到基材干擾，其位於各個不同波長的拉曼振動模態幾乎都能被大幅度地等比例增益，例如石墨稀的2D與G振動模態，以及二硫化鉬的E2g與A1g振動模態，都能被有效地增益至相同倍率。此外，為了觀察本質二維材料與其周圍的材料之載子轉移行為，我們紀錄並分析懸空二維材料於不同位置之拉曼散射光譜以及螢光光譜。我們也發現於某個位置，懸空單層石墨稀之殘餘電洞將會被周圍的鋁金屬的摻雜給中和。因此，利用此空隙共振腔結構，我們可以輕易地判斷出懸空二維材料之電荷中性點。另外，我們也利用相同的空隙共振腔結構大幅度地增益二硫化鉬之螢光強度。大幅提高的螢光強度使我們能夠更進一步地探討二硫化鉬處於懸空狀態與接觸不同金屬時，其螢光光譜中激子與三激子之比例變化。因此，使用此種空隙共振腔結構，將使我們能夠精準地分析各種懸空二維材料的本質特性。 除了二維材料之本質特性之外，針對螢光光譜，我們發展了一種複合結構能夠有效地增強二維材料之螢光強度。我們將單層二硫化鉬轉置於淺溝槽薄金屬膜上，由於電場強度的提升，相較於矽基板，二硫化鉬之螢光強度能被大幅地增益超過200倍。經過分析後，我們認為此螢光強度的增益主要是來自二硫化鉬自發輻射率以及光學吸收同時被大幅提升。接著，我們發現另外加入的白金奈米粒子於整個系統之後，除了能對二硫化鉬造成P型摻雜之外，白金奈米粒子還會與表面電漿波耦合產生共振，產生更強的電場強度。我們也於實驗中發現將單層二硫化鉬轉置於此複合結構上，其螢光強度能夠被增益超過2000倍。我們進一步利用勞倫茲函數擬合來分析螢光光譜中激子與三激子之比例變化，發現三激子再結合效率的提升是來自於電場大幅提升而產生Purcell效應。最後，由於二硫化鉬轉置於此複合結構之後，其螢光強度大幅地提高，因此我們利用此優勢，將二硫化鉬應用於酒精之偵測。藉由入射強度經過衰減之雷射，可以降低酒精之揮發與脫附之速率，達到提高偵測效率之效果。因此，本論文同時利用強烈的電場增益以及摻雜效應，大幅度地提高二硫化鉬之螢光強度，將使以過渡金屬二硫化物為主之發光、偵測器等元件之效率能大大地被提升。

The core assumption of Fowler theory is that excited hot electrons are distributed isotropically in momentum space inside of a metal, and only a small fraction of the hot electrons within the emission cone perpendicular to the Schottky junction can possibly be injected into semiconductor contact. A clearer understanding of the direction and momentum space of hot electrons should aid the development of various hot electron–based devices. In this study, in contrast to the assumption of Fowler theory, we first proved experimentally that the direction of hot electrons is highly related to the polarization state of the incident light. Furthermore, the anisotropic movement of hot electrons can be exploited, through the application of a magnetic field, to modulate the optoelectronic response to silicon (Si)-based photodetectors that operate at optical telecommunication wavelengths. Instead of using a noble metal, we applied nickel (Ni), a ferromagnetic material, to construct a hot electron–based photodetector for the detection of infrared (IR) radiation with photon energy well below the band edge of Si. We designed a series of experiments to verify the directionality and transport of hot electrons within the Ni–Si Schottky diode. When a magnetic field was applied to the device, the effects of Lorentz force and anisotropic magnetoresistance (AMR) greatly affected the transport of hot electrons. Furthermore, taking advantage of the anisotropic movement of hot electrons and magnetic effects, we could selectively enhance or weaken the IR radiation–induced signals. In addition, our device could be switched from polarization-insensitive to polarization-sensitive by applying a magnetic field. To the best of our knowledge, this study, combining the electrical, magnetic, and optical effects of hot electrons and Si-based devices, is the first to modulate the efficiency of Si-based photodetectors working at optical telecommunication wavelengths. This strategy would, presumably, also be potentially very useful when applied to other hot electron–based systems and devices. A large variety of two-dimensional (2D) materials attract the attention of the scientific community, and are widely used in various types of devices. Therefore, the detection technology of two-dimensional materials is a very important issue. However, the properties of 2D materials are readily affected by their surroundings. Therefore, the underlying substrates and surrounding materials always disturb the pristine properties of 2D materials. Herein, we describe how the pristine properties of suspended 2D materials can be precisely extracted from Raman and photoluminescence (PL) spectra with great signal enhancements by taking advantage of both air gap suspension and nanocavity enhancement effects. The modes of the Raman emission lines were enhanced to almost the same degree when the 2D materials were positioned over the nanocavity: the 2D/G peaks of suspended single-layer graphene (SLG) and the E12g/A1g peaks of MoS2 were significantly enhanced almost equally. Moreover, recording Raman and PL spectra at different positions of the suspended 2D materials was a very powerful tool for observing charge transfer between the pristine 2D materials and the surrounding materials. We also found that the residual holes of the suspended SLG could be neutralized by aluminum (Al) at certain positions. By employing the air cavity structure, we could readily locate the charge neutrality point of the suspended 2D materials. In addition, the PL intensity of MoS2 could be greatly enhanced when using the same nanocavity. The great enhancements in the PL signals from the suspended 2D materials allowed us to further investigate the spectral weights of both the A0 exciton and A- trion peaks when MoS2 was suspended or supported upon various metal films. This approach may open up new doors for techniques allowing precise characterization of abundant information from pristine and suspended 2D materials. In addition to extract the pristine properties of 2D materials, we developed a composite structure to significantly enhance the PL intensity of monolayer MoS2. We find that the PL intensity from monolayer MoS2 on STTM structure can be enhanced largely by more than 200 times relative to that on barely Si substrate. By systematical investigation, we suggest that the PL enhancement results from both the increase in spontaneous emission rate and the absorption of MoS2. Next, we find that the addition of Pt NPs not only p-dopes MoS2, but also greatly enhance the E-field intensity within MoS2 through the coupling between SPP wave and LSPR effect. Furthermore, we demonstrate experimentally that by using the optimal condition, the PL intensity of monolayer MoS2 is enhanced by more than 2000 times relative to those on a 40 nm barely Si substrate. We further analyze the spectra weight of exciton and trion by Lorentzian peak fitting and determine that the enhancement of radiative recombination efficiency of trion originate from Purcell effect with the intense E-field. At last, taking advantage of intense E-field generated by the composite structure, we can use the attenuated laser to excite the PL of MoS2, and that can effectively prevent the volatilization and desorption of EtOH molecule caused by high temperature generated by laser during the measurement. In this thesis, we use MoS2 to develop a highly efficient alcohol sensor. Therefire, our investigation indicates that the PL intensity from monolayer MoS2 can be enhanced by both intense E-field and chemical doping simultaneously, providing the opportunity to achieve highly efficient TMDCs based-light emitting devices and sensors.