Time response and circuit model of optical detector based on the structure of graphene layers, graphene nanoribbon, graphene layers

Master's thesis in the field of electrical engineering, electronics orientation

Abstract

Time response and circuit model of the detector based on the structure of graphene-nano ribbon layers Graphene - graphene layers The need to increase the range of wavelengths covered by the detector and the development of applications, along with reducing the cost in the production process, is the main motivation for the development of small-sized structures. Due to its gapless energy spectrum, graphene absorbs electromagnetic radiation from the terahertz spectral range to the ultraviolet. Relatively high quantum efficiency in graphene interband transitions and especially in multilayer graphene structure, has expanded the invention of suitable and new terahertz and infrared optical detectors. Several models of IR/THz detectors have been proposed, evaluated and experimentally studied using single and multi-layer structures along with graphene nanoribbon structures. But despite the efforts that have been made, the final speed range of these devices is still not known. In this thesis, we investigate the graphene photodetector, which consists of two regions of graphene layers (GLs) without an energy gap that is not doped (type i). These absorption regions are fed by side junctions and connected by a graphene nanoribbon (GNR). In this device, absorption is done by graphene layers, which increases the density of electrons and holes in these areas. This phenomenon leads to the flow of electrons and holes in thermionic form on both sides of the potential barrier formed in the graphene nanoribbon, and optical current is created. The presence of graphene nanoribbon and its related potential barrier reduces the dark current. In irradiated graphene layers, most of the absorbed light energy is transferred to electron-hole energy, which usually leads to heating of the electron-hole system. As a result, optical phonons emitted by electrons and holes produced by light accumulate in graphene layers. Therefore, it seems that taking into account the heat of optical phonons cannot be ignored. Therefore, the rate of production and erosion of optical phonons should also be considered in the rate equations. In this treatise, the time response of the optical detector based on the GL-GNR-GL structure is presented by obtaining the rate equation, and the results show that this optical detector has a faster response at high frequencies and a more favorable optical response at low frequencies.

Optical devices, including lasers, modulators, couplers, detectors, switches and other devices, are the heart of the evolution of telecommunications, which have occupied a huge part of electronic engineering. Meanwhile, optical detectors have other applications in addition to being used in telecommunications to convert optical energy into electrical energy in the optical fiber receiver. From simple applications such as automatic doors to imaging, space, military and medical applications.

Among optical detectors, in most infrared detectors and imaging devices, semiconductor structures with a narrow energy gap such as HgCdTe and InSb are used. The need to increase the range of wavelengths covered by the detector and to develop applications along with cost reduction in the production process is the main motivation for the development of small-sized structures [1] such as quantum wells [2], quantum dots [3] and quantum wires [4] in optical detectors [1]. Recently, the study of devices based on carbon compounds such as nanotubes, graphene layers and graphene nanoribbons is rapidly increasing due to their unique properties.In this chapter, as an introduction, a brief explanation will be given about the opto-electronic properties of graphene, then the general description of the optical detector based on GL-GNR[5]-GL[6] will be discussed, and the configuration of the thesis will be presented in the last part. 1-1- Graphene properties Today. The use of silicon devices is evident in all aspects of our lives, so that it is said that we are in the "age of silicon". One of the applications of silicon devices is in light detection, but recent advances in nanotechnology show us bright horizons in the direction of designing sub-micrometer optical devices. The reason for this high capacity for progress is the ability to work with unique molecules or the chemical growth of materials [2].

Considering that silicon-based technology has reached its own limits, a new material has attracted the attention of researchers to replace the role of silicon in semiconductor technology. Many alternatives to silicon have been eliminated, and silicon is still the most reliable semiconductor material available for use in this field. However, as the size of electronic devices becomes smaller, silicon reaches the scale limits in the device, and scientists believe that in the near future, silicon will no longer be able to meet the needs of high-frequency electronic applications in the market. According to Moore's law[7], the number of transistors in integrated circuits doubles every 18 months. With silicon-based semiconductor devices getting smaller and smaller, silicon technology is currently on the limits of fundamental limitations, and some problems such as tunneling in MOSFETs may occur. Therefore, a thorough research is needed to develop alternatives for solid-state device technology without any size problem [3]. Among the various molecules, carbon compounds seem to be the most promising materials, which are chemically very similar to silicon, abundant in nature, and easy to work with. Especially graphene and carbon nanotubes, which have unique electronic and optical properties. The properties of this material have attracted the attention of researchers to such an extent that many claim that we have entered the era after silicon or the "carbon era" [2].

Graphene is an excellent conductor, extremely strong and versatile. Graphene sheets can be in the form of multi-walled and single-walled nanotubes and nanoribbons. Depending on their size and geometrical properties, they can behave like metals, insulators, or semiconductors [4].

Graphene is a layer of carbon atoms arranged in sp2 arrangement in a honeycomb crystal lattice. Due to its unique carrier transport [8] and its optical properties, it has received much attention. As shown in Figure 1-1, the conduction band and valence band of graphene form a symmetric cone at the K and ?K points, which connect to each other at these points, leading to special properties similar to massless relativistic fermions [9] with ultrafast transport without backscattering [10] [5]. Graphene [5].

Intrinsic carrier mobility of more than 200,000 cm2V-1s-1 is predicted for graphene-based devices [3], which is theoretically 200 times the mobility in silicon. Also, the charge storage in graphene is less, so it has a high operating frequency. Despite the nanometer thickness of graphene, it absorbs 2.3% of light with a wavelength between 300 nm and 6 micrometers, which can be absorbed by increasing graphene layers and thus increase the optical response in a wide range of wavelengths. Graphene has one of the highest levels of carrier transfer between the conduction and capacitance bands among known materials. The speed of movement of carriers in graphene is more than 108 cm/s [6].

The electronic properties of graphene, the very high mobility of electrons and holes without mass (due to the linear dispersion of their band structure) and the real two-dimensional electron and hole systems (due to the thin monolayer structure) are superior to the advantages of other semiconductor materials. Due to the linear dispersion relation, the density of states [11] in graphene is proportional to energy, which creates an extremely high saturation density of electrons and holes.