石墨烯作為深紫外發光二極體透明電流擴散層之應用—石墨烯/氧化鎳/氮化鎵磊晶介面研究

Abstract

石墨烯(Graphene)為一種全為碳原子所組成之層狀結構體，並且每一個碳原子皆為sp2混成(sp2 Hybridization)，最終形成六角形蜂巢狀晶格結構。其諸多特性皆使學界與業界引起了極大興趣，如單原子層之厚度、零能隙、高熱力穩定性、高機械性能、極高之載子遷移率，以及於各光波長下皆有高穿透度，使得其做為深紫外光(UVC)發光二極體之透明電流擴散層(Transparent Conductive Film)眾多材料中之佼佼者。然而，石墨烯作為透明電流擴散層仍有幾項缺陷尚須克服，首先其功函數值遠小於p摻雜之氮化鎵(p-GaN)，使得兩材料無法僅靠熱退火擴散製程消除電洞傳輸時之能障。再者，較難以大面積直接生長石墨烯，過去常以轉印之方式製備，但此方法除無法量產外，轉印期間也容易使石墨烯產生污染與皺摺。為解決上述問題，本研究使用原子層氣相沉積法(Atomic Layer Deposition ; ALD)，於石墨烯與p摻雜氮化鎵層之間，生長功函數介於兩材料之間之氧化鎳作為緩衝層，以改善內建電位(Vbi)之大小，進而使得蕭特基能障(Schottky Barrier)降低，使元件特性由整流特徵轉變為電阻特徵。此外，本實驗使用鎳金屬作為催化劑，並藉由電漿輔助式化學氣相沉積(Plasma Enhanced Chemical Vapor Deposition ; PECVD)直接於深紫外光磊晶基板上生長大面積石墨烯薄膜，該工藝可有效降低製程溫度，亦將轉印之製程消除，使量產目標之可能性增加。透過紫外光電子能譜（UPS）確認功函數之改善。並使用穿透式電子顯微鏡進行選區電子衍射（Selected Area Electron Diffraction ; SAED）測得石墨烯之層距為0.334 nm，其數值與理論值相符。經NiO緩衝層參數改善後介面電性由蕭特基接觸轉變為歐姆接觸，且其特徵接觸電阻ρ_c亦降低至2.6×10^(-5) (Ω-cm2)，因此將石墨烯及緩衝層作為深紫外光發光二極體之透明電流層具相當潛力。Graphene is a layered structure composed entirely of carbon atoms, and each carbon atom is sp2 Hybridization, which then forms a hexagonal honeycomb lattice structure. Its many properties have attracted great interest in academia and industry, such as the thickness of a single atomic layer, zero energy gap, high thermal stability, high mechanical properties, extremely high electron mobility, and high transmittance of UV light. The transmittance makes it the leader among the many materials of the Transparent Conductive Film of the deep ultraviolet (UVC) light-emitting diode.However, as a transparent conductive film, graphene still has several defects that need to be overcome. First, its work function is much smaller than that of p-doped gallium nitride (p-GaN), Making it impossible to eliminate the energy barrier between the two materials only by the thermal annealing process. Furthermore, it is difficult to directly grow graphene on a large area. In the past, Graphene was prepared by a transfer process. However, this method is not only impossible for mass production, but also causes contamination and wrinkling of graphene during transfer.To solve the above problems, this study uses Atomic Layer Deposition (ALD), Growth of NiO Buffer Layer Between Graphene and p-GaN, to reduce the built-in potential (Vbi), thereby reducing the Schottky Barrier, and changing the element characteristics from rectification characteristics to resistance characteristics.In addition, nickel-metal was used as a catalyst in this experiment, and large-area graphene films were directly grown on UVC-LED epitaxy substrates by PECVD (Plasma Enhanced Chemical Vapor Deposition). This process can effectively Lower the process temperature and also eliminate the transfer process, increasing the possibility of mass production targets.Work function improvement was confirmed by UPS (Ultraviolet Photoelectron Spectroscopy). And us transmission electron microscope to carry out SAED (Selected Area Electron Diffraction), the interlayer distance of graphene was measured to be 0.334 nm, which is consistent with the theoretical value. After adding the NiO buffer layer to improve the work function, the electrical characteristic of the interface is changed from Schottky contact to Ohmic contact, and the characteristic specific contact resistance ρ_c is also reduced to 2.6×10^(-5) (Ω-cm2). Therefore, graphene and buffer layers have considerable potential as the transparent conductive films of deep ultraviolet light-emitting diodes.