Effect of Doping on the Characteristics of Infrared Photodetectors Based on van der Waals Heterostructures with Multiple Graphene Layers
We study the operation of infrared photodetectors based on van der Waals heterostructures with multiple graphene layers (GLs) and n-type emitter and collector contacts. The operation of such GL infrared photodetectors (GLIPs) is associated with the photoassisted escape of electrons from the GLs into the continuum states in the conduction band of the barrier layers due to the interband photon absorption, the propagation of these electrons, and the electrons injected from the emitter across the heterostructure and their collection by the collector contact. The space charge of the holes trapped in the GLs provides a relatively strong injection and large photoelectric gain. We calculate the GLIP responsivity and dark current detectivity as functions of the energy of incident infrared photons and the structural parameters. It is shown that both the periodic selective doping of the inter-GL barrier layers and the GL doping lead to a pronounced variation of the GLIP spectral characteristics, particularly near the interband absorption threshold, while the doping of GLs solely results in a substantial increase in the GLIP detectivity. The doping"engineering" opens wide opportunities for the optimization of GLIPs for operation in different parts of the radiation spectrum from near infrared to terahertz.
The gapless energy spectrum of graphene layers (GLs) enables their use in the interband detectors of infrared radiation (see, for example, Refs. [1-5]). The incorporation of the GLs into the van der Waals (vdW) heterostructures based on such materials as hBN, WS2, InSe, GaSe, and similar materials [6-15] can enable the creation of novel GL infrared photo detectors (GLIPs) with improved characteristics. As discussed in the recent review , mixed-dimensional van der Waals heterostructures have already shown considerable promise with expectations for more improvements in material quality and reproducibility in the near future. In particular, the feasibility of the exfoliation/transfer/sampling technique including doping techniques in vdW heterostructures was demonstrated [11-19]. Recently, we proposed and evaluated the IR detectors using the vdW heterostructures with the GLs clad by the widegap barrier layers—GL infrared photodetectors (GLIPs) [20, 21]. The GLs serve as photosensitive elements, in which electron-hole pairs are generated due to the interband absorption of IR radiation. The photogenerated electrons tunnel from the GLs through the barrier top to the continuum states in the barrier layers and support the terminal current. The photogenerated holes, which are confined in the GLs, form the space charge. The space charge is determined by the balance of the photogeneration and capture of the electrons propagating above the barriers. Figures 50.1 and 50.2 show the GLIP schematic view and the fragment of the device band diagram with the indicated main electron processes (the photo excitation and the tunneling from and capture into the GLs). The space charge affects the electric field at the device emitter and, therefore, controls the injected electron current. In the devices based on the heterostructures with a low efficiency of the electron capture into the GLs, the injected current can markedly exceed the current created by the photoexcited electrons. This provides a relatively high photoelectric gain and detector responsivity. The rates of the escape of the photoexcited and thermalized electrons from the GLs and the capture of the electrons propagating across the barrier layers strongly depend of the potential profile near the GLs. The doping of the barrier layers, in particular, the selective doping using the delta layers of donors and acceptors as shown in Figs. 50.1 and
50.2 (which is called the "dipole” doping ) can markedly modify this profile resulting in the appearance of the "tooth” adjacent to each inner GL at the donor sheet side. The barrier doping was also effectively used in the unitravelling-carrier (UTC) photodiodes to reinforce the injection of the electrons photogenerated in the emitter of these devices . The doping of the GLs can also lead to shift of the Fermi level in the GLs with respect to the Dirac level. The latter affects the spectrum of the electron photoexcitation, the escape rate of the thermalized electrons, and the capture processes.
The characteristics of the GLIPs considered previously [20, 21] are primarily predetermined by the electron affinities of GLs and barrier materials. In this chapter, we show that the proper doping of the barrier layers and GLs by acceptors and donors can pronouncedly modify the GLIP characteristics and result in an increase in the GLIP responsivity and detectivity, particularly, in the low-energy part of the infrared spectrum.
Figure 50.1 Schematic view of the GUP heterostructure. Horizontal arrows correspond to electron flow from the emitter along the emitter GL and along the collector GL to the collector contact. Vertical arrows indicate flow of the electrons injected from the emitter GL and photoexcited from the inner GLs across one, two, or more inter-GL barriers before being captured.