Nitride material has shown its potential for the ultraviolet light detection, which are valuable for both military and civil applications. However, the response ability is restricted by the low conductivity and limited carrier migration distance for that with high Al content. Benefiting from the unique characteristics and compatibility, the combination of 2D graphene with 3D nitride for the construction of van der Waals junction enlightens the advances of ultraviolet photodetectors. It is known that the device could be existed in lateral or vertical configuration, but the later one suffers from high fabrication and technique difficulties according to the dielectric inserting layer deposition or "Mesa" etching. Generally, the response behavior between these two types of devices is different, while none research has been focused on the distinction of device working mechanism and features, making their real application demands still not clear.
To address this problem, Research Fellow Dabing Li from Changchun Institute of Optics, Fine Mechanics and Physics has cooperated with Dr. Yuanyuan Yue from Jilin University of Finance and Economics and Professor Haiyu Wang from Jilin University. They proposed a circular electrode strategy, and thus the lateral or vertical device configuration can be easily converted by regulating the relative position between the circular electrode and graphene, none dielectric inserting layer is required. This work has revealed the difference of response characteristic and working mechanism between these two ultraviolet photodetectors, which achieve the high-sensitivity (3.88×1012 Jones, 2 V) and self-powered (1.34×1011 Jones, 0 V) detection capability. Besides, the photodetector array imaging successfully predicts their specific application demands, which could be satisfied by the design of graphene-AlGaN van der Waals junctions. The relevant research results were published in Photonics Research, Volume 12, Issue 9, 2024. [Yuanyuan Yue, Yang Chen, Jianhua Jiang, Lin Yao, Haiyu Wang, Shanli Zhang, Yuping Jia, Ke Jiang, Xiaojuan Sun, Dabing Li, "Configuration design of a 2D graphene/3D AlGaN van der Waals junction for high-sensitivity and self-powered ultraviolet detection and imaging," Photonics Res. 12, 1858 (2024).]
The circular electrodes are divided into the inter one and outer one, this design enlarges their effective interaction length and optimizes the electric field distribution came from the bias voltage (Fig. 1a). As for the lateral photodetector, all the electrodes were deposited onto the graphene layer. On the other hand, by simply patterning the graphene and making the outer electrode contact with the AlGaN, the photodetector converts into the vertical configuration. Since that the 3D AlGaN possesses much higher ultraviolet light absorption efficiency in contrast to that of graphene, the photon-induced carrier transition mainly occurs in the AlGaN epilayer, and the graphene forms a junction electric field with the AlGaN and serves as a channel for photocarrier transportation.
According to the response behaviors of lateral and vertical photodetectors based on graphene-AlGaN van der Waals junctions, their working mechanism could be much different. As shown in Fig. 1d, the photo-generated carriers in the AlGaN are separated, and electrons drift into the graphene channel by the junction electric field. Then, the recombination of electron with original hole of graphene results in the reduction of electrical conductivity and the negative photoconductivity (NPC) of lateral photodetector. The lifetime of photo-generated carrier is prolonged due to the trap states in AlGaN, as revealed by the response/recovery time in a few seconds, which achieves a high gain. The responsivity and detectivity reach to 1.27×104 A/W and 3.88×1012 Jones at the bias of 2 V. The lateral photodetector array obtains clear imaging of "+" symbol, and the sharpness could be increased by 31% within 25 s (Fig. 1e). As a result, it is suitable for the detection of steady object with low-light emission by prolonging the acquisition time and signal accumulation number. Moreover, the response behavior is alike to the recognition and memory processes of human brain, showing great potential for the biomimetic visual learning.
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Fig. 1 (a) Schematic diagrams of ultraviolet photodetectors with lateral and vertical graphene-AlGaN van der Waals junctions. Response mechanisms of (b) the vertical and (d) lateral device working at zero and 2 V bias, respectively. (c) Stable imaging by an ultraviolet photodetector array in the vertical device configuration. (e) Gradually enhanced imaging sharpness by the lateral ultraviolet photodetector array. The incident light target is a "+" symbol.
For the vertical photodetector in Fig. 1b, it is featured with the self-powered detection capability, and the responsivity of 2.61×10-2 A/W and detectivity of 1.34×1011 Jones are achieved at zero bias, in which the response/recovery speed becomes much faster. This response mechanism relies on the photoelectric potential (0.21 V) induced by the nonequilibrium carrier accumulation at the two sides of junction, and the photocurrent could be spontaneously formed when the external circuit is closed. These characteristics make the application of vertical photodetector array in some specialized environments, such as deep space or other untraversed regions.
