在此論文中我們探討砷化鎵/砷化鋁鎵異質結構(GaAs/AlGaAs heterostructure), 石墨烯(Graphene), 石墨烯與砷化鎵/砷化鋁鎵半導體複合材料(Graphene-GaAs/AlGaAs composite material) 等二維材料的光電特性。 新穎二維材料有許多特殊的物理特性, 並且可以應用在混成元件(hybrid device)的發展上。 砷化鎵/砷化鋁鎵的拋物線能帶結構展現具有質量的電子。在其異質接面上形成的二維電子氣(two-dimensional electron gas, 2DEG), 展現相當高的載子遷移率(mobility)。 石墨烯是碳原子在二維平面上以六角晶格延伸的天然材料, 其線性能帶結構展現無質量的電子, 由於其行為可由狄拉克方程式描述, 因此又稱為狄拉克費米子(Dirac fermions)。
我們整合了石墨烯與傳統三五族半導體異質結構成為新穎雙層二維系統, 並且在這個材料的基礎之上, 透過研究不同功能性的電子/光子元件(electronic/photonic devices) 來說明其應用潛力。
首先, 我們呈現一雙功能場效電晶體(dual-function field-effect transistor), 其組成包含一石墨烯電晶體(GFET)與一高載子遷移率電晶體(HEMT)。 透過不同的操作模式, 石墨烯可作為HEMT的閘極也可做為通道材料, 其載子濃度可由2DEG背向閘極調控。 我們發現GFET的操作範圍受限於閘極電壓調控的異質接面能帶彎曲(band bending)範圍, 閘電極尺寸則由砷化鎵表面起伏(surface morphology)來決定。 我們證明了這樣雙功能電晶體, 具有兩種場效特性, 並且其特徵表現並不亞於個別單電晶體。
第二, 我們在石墨烯與砷化鎵/鋁砷化鎵複合材料上發展了量子霍爾遠紅外線偵測器。 利用石墨烯作為透明正向閘極(top-gate electrode), 我們可以大範圍並且有效率的調控偵測器的偵測頻率, 其調控頻率範圍遠大於先前已發表的結果。 另外我們發現當偵測器覆蓋上石墨烯後, 其光反應有增強的效果, 我們猜測可能是由於內建電場在石墨烯與二維電子氣之間被增強了。
第三, 我們在大面積石墨烯上發展了微小且靈敏的霍爾探針(Hall probe) 用來作磁場感應器。 傳統霍爾探針的空間解析度受限於砷化鎵空乏層(spacer layer)的厚度。 由於石墨烯是單原子層, 其載子濃度(carrier density)可以被調控, 因此在發展霍爾探針上是相當有淺力的材料。 我們的研究結果發現石墨烯霍爾探針相當靈敏, 其在室溫的表現並不亞於傳統砷化鎵異質結構霍爾探針, 我們同時也指出了其磁場靈敏度與解析度(field sensitivity and resolution) 分別受限於外在雜質(extrinsic impurity)和內在缺陷(intrinsic defect)。
第四, 我們建構了掃描式霍爾探測顯微鏡(scanning Hall probe microscope)作為微米尺度磁場分佈的顯影。 其特徵包含(次)微米的空間解析度, 次釐米(sub-millimeter)的掃瞄範圍, 室溫到約4.2K的操作溫區。 其獨特的設計包含微米空間解析度且公分移動範圍的XY移動平台, 樣品與偵測器直接接觸的操作方式, 不需要複雜的Z方向回饋控制系統(feedback control system)。 我們同時也發展了一套新式的製程用來製備霍爾探針, 此製程方式可以大大降低探針製作的複雜性, 並且有效縮短樣品與探針距離。 最後我們提供三種磁掃描的結果來證明該系統的功能性, 包含室溫下鎳(Ni)薄膜上的人造磁結構, 77 K下鑭鈣錳氧(La_(2/3)Ca_(1/3)MnO_3)的磁區分佈, 4.2 K 下在鈮(Nb)薄膜上的超導漩渦(superconducting vortices)分佈。
最後, 我們提供詳細的化學氣象沉積(chemical vapor deposition)石墨烯的製備流程與製備問題。 我們發現鹽酸清潔後的銅箔可以提升石墨烯薄膜的品質, 有效降低樣品上污染物的數量。我們同時也提供元件製備與材料的基本特性。

We explore the electronic and photonic properties of two-dimensional (2D) materials including GaAs/AlGaAs heterostructure, graphene, and graphene-GaAs/AlGaAs composite material. Novel 2D materials have allowed us to study a plenty of physics phenomena and to develop versatile hybrid devices. GaAs/AlGaAs heterostructure with massive 2D electron gas (2DEG) embedded in the interface exhibits a parabolic energy dispersion and yields high carrier mobility. Graphene, a single sheet of carbon atoms arranged in a honeycomb lattice, is a natural 2D material with a linear low-energy dispersion, displaying massless Dirac fermions.
This thesis elucidates the integration of graphene to conventional III-V semiconductor heterostructure as a novel bilayer 2D system. We investigate three types of devices with different functionalities to study the unique properties of this system and to demonstrate its potential applications.

First, we present the realization of a dual-function field-effect transistor (DFET), consisting of a graphene FET (GFET) and a high electron mobility transistor (HEMT). Depending on the operation scheme, graphene can be used either as a gate electrode for HMET or as a channel material gated by 2DEG formed in the interface of heterojunction. The performance of GFET is limited by the interface band bending of the heterojunction associated with the gating voltages and the intrinsic surface morphology of GaAs substrate. The performance of this hybrid device is demonstrated to be comparable with that of GFET or HMET reported earlier, which bodes a way for the development of integrated bi-FET device for further applications and physical investigations.

Second, we develop a quantum Hall far-infrared (QHFIR) photon detector based on graphene-GaAs/AlGaAs composite material. Graphene is employed as a transparent top-gate electrode to tune the response (cyclotron) frequency of the QHFIR. As covered with graphene, the photoresponse of the QHFIR detector is found enhanced. The enhancement in the photosignal is referred to the built-up electric field in between graphene and 2DEG.

Third, we implement a sensitive micron-sized Hall probe on large-scale graphene for magnetic imaging at room temperature. Conventional Hall probe based on GaAs 2DEG suffers restricted spatial resolution due to a finite spacer layer. Graphene Hall probe (GHP) can overcome this issue and supposedly provide superior field sensitivity in the vicinity of charge neutral regime. Our studies indicate that the fundamental limitation of the field sensitivity and resolution are respectively restricted by extrinsic and intrinsic defects. Our result paves a way for the use of CVD GHPs for scanning Hall probe microscopy with high field sensitivity. We also suggest a scheme based on a stacked double-Hall junction in a graphene-GaAs/AlGaAs composite material to further extend the functionalities of Hall probes.

Fourth, we construct a scanning Hall probe microscope (SHPM) for micron- magnetic profile imaging. The SHPM combines (sub) micron-sized spatial resolution, a large scanning range, and a wide range of operating temperatures (4.2 to 300 K). The unique designs of the SHPM system includes a simple positioning (sub)-micron XY stage, a direct contact scheme without a sophisticated feedback control system for the Z-axis, and a innovative lithography process to fabricate the scanning Hall probe. Detailed experimental procedures of fabricating a scanning Hall probe are given. To demonstrate the capabilities of this system, we present magnetic images of the nickel grid pattern at room temperature, of the surface magnetic domain structure of a La_(2/3)Ca_(1/3)MnO_3 thin film at 77 K, and of the superconducting vortex patterns on striped niobium film at 4.2 K.

Last, we present detailed experimental procedures of preparing chemical-vapor deposited graphene. A hydrochloric acid assisted clean to the copper foil before graphene growth is demonstrated very effective in reducing contaminants gathered on the graphene surface. Summaries of the basic properties of CVD graphene and GaAs/AlGaAs and device fabrication procedures are also given.

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