Figure 1. Schematic of a photodetector integrated in a waveguide with a Van der Waals PN heterojunction. Credit: Light Publishing Center, Changchun Institute of Optics, Fine Mechanics and Physics, CAS

Photonic integrated circuits (PICs) use photons as data carriers and are characterized by ultra-high transmission speeds, low latency and anti-electromagnetic intersections. These advantages are expected to solve the problems with the bottlenecks of microelectronic chips in terms of speed, power consumption and integration density. This is key to promoting breakthroughs in microelectronics, quantum information and microsensor technology in the post-Moore era.

Currently, driven by the application of information technology, photon integrated chips have made great strides. For example, the silicone PIC is compatible with mature CMOS technology for cheap and large-scale production; PIC of silicon nitride can withstand moderately high optical power and large manufacturing errors; and PIC of lithium niobate can achieve perfect electro-optical modulations with low drive voltage and high linearity.

However, one of the disadvantages of these PICs is the monolithic integration of waveguides and photodetectors with one material. In order to support the transmission of light in the waveguide, PIC materials cannot absorb the optical signal, which makes it impossible to implement the integrated photodetector of one material. To solve this, hetero-integrations of absorbent bulk materials (such as Ge, composite semiconductors III-V, etc.) of PIC have been applied. Although it still poses open challenges such as high costs, complex production processes and problems with the material interface.

Align the BP / MoTe2 PN heterojunction band

Figure 2. Alignment of the BP / MoTe2 PN heterojunction band in thermal equilibrium (left panel); Optical microscope image of the manufactured device (right panel). Credit: Light Publishing Center, Changchun Institute of Optics, Fine Mechanics and Physics, CAS

Recently, two-dimensional (2D) materials have emerged as an attractive photon-absorbing material for chip-integrated photodetectors. 2D materials do not have surface hanging connections, which eliminates the constraints of lattice mismatch to heterointegrate them with the PIC. The 2D family of materials has a wide variety of electronic and optical properties, including semi-metallic[{” attribute=””>graphene, insulating boron nitride, semiconducting transition metal dichalcogenides, and black phosphorus. As a consequence, chip-integrated photodetectors operating at various spectral ranges could be constructed by choosing appropriate 2D materials.

In a new paper published in the journal Light Science & Application on April 20, 2022, a research team, led by Professor Xuetao Gan from Key Laboratory of Light Field Manipulation and Information Acquisition, Ministry of Industry and Information Technology, and Shaanxi Key Laboratory of Optical Information Technology, School of Physical Science and Technology, Northwestern Polytechnical University, China have reported that integrating van der Waals PN heterojunctions of 2D materials on optical waveguides can provide a promising strategy to realize chip-integrated photodetectors with low dark current, high responsivity, and fast speed.

With the 2D layered structure and no dangling bonds, researchers can stack 2D materials with different properties in different orders by “stacking wood” to form van der Waals heterostructures with atomically flat interfaces. The “arbitrary combination” of van der Waals heterojunctions can not only give the advantages properties of a single material, but also generate novel properties, achieving a leap of 1+1>2, as shown in Figure 1.

In this research, the researchers made full use of natural p-doped BP and n-doped MoTe2 for hetero-stacking, and successfully fabricated an efficient van der Waals PN heterojunction.

Second, since there are no dangling bonds on the surface of 2D materials, compared with traditional semiconductors, 2D materials do not need to consider lattice mismatch when integrating with various photonic integration platforms.

Finally, the preparation of source-drain electrodes can also be integrated on the photonic platform through the “stacking wood” technology and placed on both sides of the material, without the cumbersome processes such as photolithography.

This also greatly simplifies the fabrication process of the device, avoiding the contamination of the device interface in processes such as photolithography, which greatly improves the performance of the device.

Reference: “Chip-integrated van der Waals PN heterojunction photodetector with low dark current and high responsivity” by Ruijuan Tian, Xuetao Gan, Chen Li, Xiaoqing Chen, Siqi Hu, Linpeng Gu, Dries Van Thourhout, Andres Castellanos-Gomez, Zhipei Sun and Jianlin Zhao, 20 April 2022, Light: Science & Applications.
DOI: 10.1038/s41377-022-00784-x

On-Chip Photodetection: 2D Material Heterojunctions for “Post-Moore Era” Microelectronics

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