臺大光電所所訊

Ultrahigh Responsivity and Superior Sensitivity Graphene-semiconducting Light Absorber Hybrid Phototransistors

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所吳志毅教授

Graphene−semiconducting light absorber hybrid photodetectors have attracted increasing attention because of their ultrahigh photoconductive gain and superior sensitivity. However, most graphene based hybrid photodetectors reported previously have shown a relatively long response time caused by numerous long-lived traps in these hybrid systems, which greatly restricts device speed. In our studies, we found graphene/light-harvesting materials (such as thieno[3,4-b]thiophene/ benzodithiophene polymer (PTB7) and Bismuth(III) iodide (BiI₃)) hybrid photodetectors have high responsivity and short photocurrent response time. Figure 1 shows the graphene-PTB7 hybrid photodetectors fabricated on self-assembled-monolayer functionalized SiO₂ substrates with a maximum responsivity of ~ 1.8 ×10⁵ A/W and a relatively short photocurrent response time of ~ 7.8 ms.

Besides, the graphene/BiI₃ vertical heterostructure phototransistors also have a maximum responsivity of about 6×10⁶ A/W , EQEs of up to 10⁶ % in the visible light region, relatively short photocurrent response times of ~ 8 ms, and detectivity of 7×10¹⁴ Jones under the SNL condition. Compared to BiI₃ on bare SiO₂ substrates, the thermally evaporated BiI₃ films on graphene sheets possessed nearly flatter morphologies and remarkably better crystallinities because of the weak van der Waals interactions between graphene and BiI₃. In Figure 2, the photoemission spectra of graphene/BiI₃ heterojunctions also manifest the evidence that no chemical interactions occur between graphene and BiI₃, resulting in the van der Waals epitaxial growth. The measured band bending consistently illustrates that photo-induced charge transfer occurs at the graphene/BiI₃ interface. The high-quality crystal growth of the layered metal halide BiI₃ could make graphene/BiI₃ heterostructures a promising building block for high-performance optoelectronic applications.

Figure 1. (a) Schematic illustration of a graphene−PTB7 hybrid photodetector (b) Rise time and (c) fall time of the device

Figure 2. SEM images of 50 nm thermally evaporated BiI₃ film on (a) SiO₂ and (b) graphene/SiO₂. Photoemission spectra of graphene with incrementally increased BiI₃ film thickness. (c) XPS spectra of I_3d5/2 and Bi_4f. (d) UPS spectra for SEC (left) and VB (right) regions, respectively. (e) C_1speak-fitting XPS spectra. (f) The onset values of the SEC spectra (black dot) and the corresponding WFs (blue dot) as a function of the BiI₃ thickness. The inset shows the VB maximum position of bulk BiI₃. (g) Individual electronic structures of pristine graphene and BiI₃. The energy level alignment and band bending at the graphene/BiI₃ heterojunction are depicted in the bottom panel.

Reference:

1. Chang P. H., Tsai Y.C., Shen S. W., Liu S.Y., Huang K. Y., Li C. S. Chang, H. P., and Wu C. I., Highly Sensitive Graphene−Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time, ACS Photonics 2017, 4, 2335−2344

2. Chang P. H., Li C. S., Fu F. Y, Huang K. Y, Chou A. S., and Wu C. I., Ultrasensitive Photoresponsive Devices Based on Graphene/BiI3 van der Waals Epitaxial Heterostructures, Advanced Functional Materials 2018, 28, 1800179