超高增益石墨烯光感測元件

Abstract

本論文中，我們分別使用低能隙高分子、鈣鈦礦、以及碘化鉍三種半導體吸光材料作為主動層並沉積於石墨烯場效電晶體上，以實現具有超高光導增益之石墨烯/半導體異質接面感光元件。 首先，我們研究經自組裝分子薄膜修飾之二氧化矽基板對於石墨烯/高分子光偵測元件之影響，我們發現此修飾可降低二氧化矽基板與石墨烯間之陷阱密度以及在石墨烯轉印過程中殘留之帶電雜質，並同時提升元件之通道導電率、光響應率、以及偵測率。以外，自組裝分子薄膜修飾技術亦可抑制長時間被捕捉於石墨烯/基板間之光載子所造成的緩慢下降之殘餘光電流。 接著，我們採用蒸鍍製程成功地將緻密且均勻之鈣鈦礦薄膜成長於石墨烯上，並有效減少石墨烯上鈣鈦礦薄膜之空隙及孔洞，在光致螢光的量測中，我們於整個石墨烯/鈣鈦礦複合薄膜之量測範圍內發現顯著且均勻的螢光淬熄，代表光產生之激子可於石墨烯/鈣鈦礦介面有效的分離，並實現具有可閘極調控之超高光響應率達~10^7 A/W以及偵測率達~10^15 Jones之光感測電晶體。 雖然鈣鈦礦具有優異之光電特性，然而，因其含有具毒性之鉛，使得其於光電領域之應用大為受限。我們嘗試將無鉛低毒性之碘化鉍薄膜沉積於石墨烯上作為吸光層，並發現碘化鉍於石墨烯上展現出緻密且極為平整的外觀以及優異之結晶性，其歸因於具層狀晶格結構之碘化鉍與石墨烯間之作用力為極弱之凡德瓦力，其已被光電子能譜術證實。此外，藉由光電子能譜量測獲得之碘化鉍以及石墨烯介面之能帶結構可證實光載子於該介面之轉移行為。最後，使用石墨烯/碘化鉍異質結構之光偵測器其光響應率以及偵測率可達到~10^6 A/W以及~10^14 Jones。 本論文中之石墨烯光感測器於極低光強度之環境下皆展現出優異的光靈敏度，在低可見光之影像感測應用上頗具潛力。此種石墨烯/半導體複合元件亦適合用於研究因照光造成之石墨烯與半導體間之電荷轉移行為。

In this dissertation, low-bandgap polymer, methylammonium lead iodide perovskite, and BiI3 are utilized as light-harvesting materials and deposited on graphene field-effect transistors to achieve ultrahigh-gain photodetectors (PDs) with graphene/semiconductor vertical heterostructures. First, we investigate the influence of self-assembled monolayer (SAM)- functionalized SiO2 substrates on graphene/polymer hybrid PDs. It is observed that SAM modification is capable of removing the trap sites and charged impurities from the graphene transfer process at graphene/SiO2 interfaces, resulting in the simultaneous improvement of channel conductivities, responsivities, and detectivities of the device. Moreover, SAM treatments can reduce the long-lived trapping of photocarriers between graphene and substrates, which depresses the slow decay residual photocurrents in the transient photocurrent relaxation of graphene/polymer hybrid PDs. Subsequently, we carry out sequential vapor deposition to successfully form compact and uniform methylammonium lead iodide perovskite films on graphene and to reduce the density of voids or pin-holes in the perovskite active layers. Both uniform and significant photoluminescence intensity quenching has been observed over the entire measured regions of graphene/perovskite hybrid films, indicating excellent exciton separation at the graphene/perovskite interfaces. Ultrasensitive graphene/perovskite hybrid phototransistors with gate-tunable photosensitivities, an extremely high responsivity of ~10^7 A/W, and detectivity of ~10^15 Jones are realized. Although methylammonium lead iodide perovskite has promising optoelectronic properties, the toxicity of Pb is a main concern for its further applications in optoelectronics. We deposit BiI3, a lead-free alternative absorber material, on graphene sheets via van der Waals epitaxy. The BiI3 deposited on graphene reveals extremely flat and compact morphologies and excellent crystallinities, mainly ascribed to the weak van der Waals interactions between graphene and layered BiI3, which are verified using photoemission spectroscopy. The photoinduced charge transfer occurring at the graphene/BiI3 heterointerfaces can also be consistently illustrated using the results of spectroscopic analysis. Finally, the PD with graphene/BiI3 heterostructures demonstrates an extremely high responsivity of ~10^6 A/W and detectivity of ~10^14 Jones. All the graphene-based hybrid optical detectors developed in this study exhibit outstanding photosensitivities under weak ambient light condition, and thus, they are highly promising for low-visible-light imaging applications. In addition, such graphene/semiconductor hybrid devices are appropriate for the investigation of the photoinduced charge transfer behaviors occurring at graphene/semiconductor interfaces.