在嚴苛輻射與高溫環境的前瞻光偵測器之物理與奈米材料設計

Abstract

本論文中我們將分別有四個主題來討論利用低維度的奈米材料與奈米薄膜來設計所需要的光偵測器。 在第二章中，我們利用氧化鋅奈米柱陣列(ZnO NRAs)做為抗反射層來提升Si MSM 光偵測器的響應，利用氧化鋅對UV光的強吸收來補足Si光偵測器再UV波段的底響應、並且也因氧化鋅奈米柱的結構對可見光與近紅外光來說可以形成漸變式折射率降低可見光與紅外光的反射，進而提升Si MSM光偵測器在長波長區域的響應。 在第三章中，藉由在p-Si/n-ZnO中間成長LAO奈米層達成具有無視可見光的UV光偵測器。由於LAO層可以減少Si與ZnO的晶格不匹配，同時具有寬能帶、高介電性。故p-Si/LAO/n-ZnO光偵測器比p-Si/n-ZnO光偵測器有更好的整流特性。 加上LAO與p-Si會形成位能障，在小電壓下可以阻擋由光激發產生的光電子從Si流向ZnO，進而形成無視可見光的效果。此UV光偵測器將來可以更便利應用在一般日光環境中，也不受可見光與紅外光影響。 在第四章，我們利用新穎的MoS2二維奈米材料製作超薄的MSM光偵測器，由於MoS2具有很高的光吸收能力(厚度2 nm的MoS2在可以見光區域吸收可以達到~10%)和良好的熱穩定性，加上我們設計成MSM結構的光偵測器。在元件表現上，我們首先發現其具有光增益效果造成光響應比以往研究高約3個數量級，且可以在200 °C的高溫下操作。同時其光響應速度也比過去別人所報導的來的快約2個數量級 (上升時間~70 μs，回復時間~110 μs)。這些元件特性證明了MoS2光偵測器將來可以在高溫環境下進行光偵測、影像、通訊等應用。 在第五章，我們將AlN直接成長在Si上並以此做成深UV MSM光偵測器。由於AlN是良好的輻射阻抗、高熱導性、好的化學穩定性與寬能隙的材料，故再經由質子( 能量: 2 MeV、劑量: 1013 cm-2)轟擊之後操作在5 V偏壓下其PDCR(光暗電流比)值還有0.7。在高溫測試方面，AlN元件可以穩定的操作在300 °C的環境下。其光響應速度也可以達到 (上升時間~110 ms，回復時間~80 ms)。以上的元件操作特性實足以作為太空或是高溫等嚴酷環境下的應用。

This thesis consists of an introduction, four chapters and a conclusion, with each chapter covering a different topic. In the introduction, we introduce the various limitations of the conventional photodetectors that we study in the later chapters. In chapter 2, we demonstrated Si MSM PDs with ZnO nanorod arrays (NRAs) as a top layer, which absorbs the UV photons (photon energy > band gap of ZnO) effectively and serves as an ARC layer, providing an effective refractive-index gradient between Si and air in the long-wavelength region (photon energy < band gap of ZnO), enabling broadband detection with greatly enhanced responsivity. The responsivity of Si MSM PDs is increased by up to 3 orders of magnitude in the UV region and by 2 orders of magnitude in the visible/NIR regions due to ZnO NRA layers. The huge enhancement of broadband detection by Si MSM PDs with ZnO NRAs could allow the low-cost production of photonic devices and extend the application potential for Si-based optoelectronic devices. In chapter 3, the visible-blind UV PDs employing n-ZnO/LaAlO3 (LAO)/p-Si double heterojunction using pulse laser deposition (PLD) are presented. The n-ZnO/LAO/p-Si PDs exhibit visible-blind UV responsivity with the cutoff wavelength of responsivity at 380 nm, corresponding to the near band edge (NBE) absorption of ZnO. Inserted 10-nm-thick LAO layers effectively eliminate visible light responses via blocking the electrons excited by visible photons in p-Si near the interface owing to the high potential barrier between p-Si and LAO layers (~2.0 eV). This study paves the way for visible-blind UV photosensing applications under outdoor lighting. In chapter 4, we report few-layer MoS2 Schottky PDs with back-to-back MSM geometry, capable of broadband photodetection from visible to UV regions with working temperatures up to 200 °C for use in harsh environments. Until few-layer MoS2 is demonstrated here, the broadband responsivity feature is not previously achievable for harsh environment use since all of photodetection materials for harsh environments are wide-bandgap semiconductors. As a new record, the responsivity of 5.7 A/W, has never been obtained in 2D nanomaterial-based PDs due to very high optical absorption of ~10% (very high absorption coefficient of up to 7.5×105 cm-1) of the few-layer MoS2 and a high photogain of ~13.3. In addition, temporal measurements reveal fast response times (~70 μs) and recovery times (~110 μs). The excellent optical properties of few-layer MoS2 promise a new generation of fast, broadband PDs based on 2D nanomaterials for the applications in harsh environments, such as sensing, imaging, and intrachip optical interconnects at the high temperatures. In chapter 5, we demonstrate the Schottky PDs with back-to-back metal-semiconductor-metal (MSM) geometry by employing AlN thin films on Si(100) substrates from reactive sputtering deposition with working temperature up to 300 °C for use in solar-blind UV detection and harsh environments. For 2 MeV proton irradiation, the PDCR value of AlN MSM PDs is 0.7 under a 5 V bias at proton fluences up to 1013 cm-2, indicating that the PDs are well suited for space applications. The AlN MSM PDs show a fast and stable photoresponse, i.e., ~110 ms of the rise time and ~80 ms of the fall time at 5 V bias. This study paves the way for fast and solar-blind photosensing in the space environment and high-temperature conditions.