**1. Introduction**

Photodetectors are devices that sense the light and convert it into an electric current. Photodetectors are essential components of many devices that are a part of our day to day life [1–5]. Primarily, silicon (Si) has been a material of choice for photodetector applications. Such photodetectors are readily integrated with complementary metal oxide semiconductor (CMOS) technology. The aggressive scaling has reduced the cost of Si-based devices and expanded their range of applications. Though Si photodetectors have evolved and developed over the years. But their performance is limited by the indirect nature of the bandgap of Si. The absorption of Si is limited to the visible and near-infrared parts of the electromagnetic spectrum. Also, the indirect nature of Si′s bandgap leads to phonon generation to conserve the momentum during the light assisted transition of carriers from lower energy to higher energy. These phonons lead to scattering of the carriers and thereby reduce the efficiency of Si photodetectors. Also, Si as a material is not a good absorber of light in bulk form, further degrading Si photodetectors' efficiency. These limitations of Si photodetectors have prompted a quest in the research community for alternate materials. Two dimensional (2D) materials, among the class of novel materials for optoelectronic applications, have shown favorable characteristics. Features like direct nature and wide range of bandgap, atomically thin nature, efficient light-matter interaction, and heterostructures forming are interesting. The class of 2D materials encompasses materials like graphene,

transition metal-di-chalcogenides (TMDCs), Xenes etc. 2d materials are artificially derived materials. These materials are derived from layered van der Waals solids. In van der Waals solids, the atomic arrangement is such that the constituent atoms are held together by covalent or ionic bonds giving rise to atomic layers, whereas these atomic layers are held together by van der Waals interactions. The weak nature of van der Waals forces makes it possible to cleave individual layers from these materials. It is possible to obtain a free-standing single atomic or few atomic layers via mechanical exfoliation [6, 7] or liquid phase exfoliation [8, 9]. Graphene, which is a single layer of carbon atoms arranged in a hexagonal manner, is regarded as the original 2D material. Over the years, it has been revealed that graphene possesses many appealing electronic, mechanical, optical and thermal properties. [10–12]. Interaction of light with graphene occurs over a broad bandwidth range (terahertz to ultraviolet wavelengths) because of semi-metallic/gapless nature. This makes graphene a candidate for wide spectral range photodetectors. The atomically thin nature of graphene limits its absorption coefficient [13–15]. Graphene absorbs only 2.3% of incident light (visible and ultraviolet), making this a primary limitation of graphene for photodetector applications. A high absorption coefficient is desirable for an optimum magnitude of photocurrent [16–18]. For the efficient operation of a photodetector, a longer lifetime of the photo exited carriers is desired. Graphene's gapless nature results in a shortened lifetime of photo-excited carriers, which further limits graphene photodetectors' performance. Beyond graphene, TMDCs have also attracted a lot of attention for optoelectronic applications over the past decade. One advantage of TMDCs over graphene is their semiconducting nature. TMDCs possess varied bandgaps, thus making them applicable for broadband photodetection. TMDCs can be represented by the general formula of MX2, where M represents a transition metal and X represents a chalcogenide atom. The arrangement of atoms in MX2 is such that the metal atom is sandwiched between the two chalcogenide atoms, as shown in **Figure 1**. TMDCs detect light at different wavelengths because of layer dependent bandgap [19–21]. Most of the TMDCs have a direct

#### **Figure 1.**

*Structural arrangement of TMDCs (MoS2). (a) Top view and (b) side view. Cyan and Yellow balls are Molybdenum and Sulfur atoms respectively.*

**111**

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

**2. Photodetection mechanisms in 2D materials**

applications are considered.

applications.

nature of the bandgap, limiting the phonon scattering in TMDCs photodetectors, which leads to better efficiency [22]. 2D materials have localized electronic bands, leading to sharp peaks in the density of states (DOS) called Van Hove singularities at specific energies [22]. Generally, in 2D materials like TMDCs, these singularities reside near conduction and valence bands. This leads to an increased probability of electron–hole pair generation upon excitation with light [22, 23]. TMDCs photodetectors show excellent light to current conversion with high responsivity [22]. Although TMDCs based photodetectors have shown an appealing development in their performance over the years, these devices are limited by slow response speed. Furthermore, TMDCs photodetectors are still behind the absorption efficiency of bulk Si photodetectors. Apart from these 2D materials, materials like silicene, phosphorene etc., have shown promising theoretical results as far as optoelectronic

Though the field of 2D materials is still developing, the early results of optoelectronic devices based on these materials are very promising. The unique properties of 2D materials have ushered in a lot of theoretical and experimental research for optoelectronic applications over the past decade or so. This has led to the proposal of numerous photodetectors based on 2D materials both theoretically and experimentally. This chapter aims at presenting an insight into the novel photodetectors based on 2D materials. Section 2 offers a discussion on photodetectionchanisms in 2D materials. Section 3 presents a discussion on photodetectors based on 2D materials and their heterostructures; Section 4 presents a brief summary of the chapter and future scope of 2D materials for photodetector

Generally, photocurrent generation mechanisms are divided into three categories, viz. photovoltaic effect, photo-thermoelectric effect, and photo-bolometric effect. In the photovoltaic effect, a built-in electric field results in the separation of the electrons and holes. This built-in electric field may be generated due to a Schottky barrier at the metal–semiconductor interface. Photodetectors working under this mechanism are called photodiodes. In the photo-thermoelectric effect, a non-uniform light source is used. This light source leads to non-uniform heating of the channel, resulting in a temperature gradient within the channel. Due to this temperature gradient, carriers move from the high-temperature region to the low-temperature region. The migration of the carriers leads to their accumulation in the low-temperature region, which results in a potential. The photo-bolometric effect is based on uniform heating of the material under illumination. This uniform heating results in a change in the resistivity of the material. This effect is directly proportional to the variation of the material's conductivity and the increment in temperature caused by light irradiation. In contrast to the photo-thermoelectric effect, the photo-bolometric effect does not drive the current but only changes the intensity of the current under external bias and illumination.Another unique mechanism observed in optoelectronic devices like photodetectors is internal photoemission (IPE). IPE involves photoinjection of electrons from an emitter/source (metal or semiconductor) into the conduction band of a collector/drain (semiconductor or insulator) in a BJT/FET. The holes are photoinjected into the valence band of the collector/drain and is called as hole photoemission [24]. In IPE, an optical excitation of electrons in the metal to an energy above the Schottky barrier is involved. These excited electrons are then transported to the conduction band of the semiconductor. The Initial theory of IPE was proposed by Fowler [25, 26]. However, this theory does not take the thickness of the Schottky

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

transition metal-di-chalcogenides (TMDCs), Xenes etc. 2d materials are artificially derived materials. These materials are derived from layered van der Waals solids. In van der Waals solids, the atomic arrangement is such that the constituent atoms are held together by covalent or ionic bonds giving rise to atomic layers, whereas these atomic layers are held together by van der Waals interactions. The weak nature of van der Waals forces makes it possible to cleave individual layers from these materials. It is possible to obtain a free-standing single atomic or few atomic layers via mechanical exfoliation [6, 7] or liquid phase exfoliation [8, 9]. Graphene, which is a single layer of carbon atoms arranged in a hexagonal manner, is regarded as the original 2D material. Over the years, it has been revealed that graphene possesses many appealing electronic, mechanical, optical and thermal properties. [10–12]. Interaction of light with graphene occurs over a broad bandwidth range (terahertz to ultraviolet wavelengths) because of semi-metallic/gapless nature. This makes graphene a candidate for wide spectral range photodetectors. The atomically thin nature of graphene limits its absorption coefficient [13–15]. Graphene absorbs only 2.3% of incident light (visible and ultraviolet), making this a primary limitation of graphene for photodetector applications. A high absorption coefficient is desirable for an optimum magnitude of photocurrent [16–18]. For the efficient operation of a photodetector, a longer lifetime of the photo exited carriers is desired. Graphene's gapless nature results in a shortened lifetime of photo-excited carriers, which further limits graphene photodetectors' performance. Beyond graphene, TMDCs have also attracted a lot of attention for optoelectronic applications over the past decade. One advantage of TMDCs over graphene is their semiconducting nature. TMDCs possess varied bandgaps, thus making them applicable for broadband photodetection. TMDCs can be represented by the general formula of MX2, where M represents a transition metal and X represents a chalcogenide atom. The arrangement of atoms in MX2 is such that the metal atom is sandwiched between the two chalcogenide atoms, as shown in **Figure 1**. TMDCs detect light at different wavelengths because of layer dependent bandgap [19–21]. Most of the TMDCs have a direct

*Structural arrangement of TMDCs (MoS2). (a) Top view and (b) side view. Cyan and Yellow balls are* 

**110**

**Figure 1.**

*Molybdenum and Sulfur atoms respectively.*

nature of the bandgap, limiting the phonon scattering in TMDCs photodetectors, which leads to better efficiency [22]. 2D materials have localized electronic bands, leading to sharp peaks in the density of states (DOS) called Van Hove singularities at specific energies [22]. Generally, in 2D materials like TMDCs, these singularities reside near conduction and valence bands. This leads to an increased probability of electron–hole pair generation upon excitation with light [22, 23]. TMDCs photodetectors show excellent light to current conversion with high responsivity [22]. Although TMDCs based photodetectors have shown an appealing development in their performance over the years, these devices are limited by slow response speed. Furthermore, TMDCs photodetectors are still behind the absorption efficiency of bulk Si photodetectors. Apart from these 2D materials, materials like silicene, phosphorene etc., have shown promising theoretical results as far as optoelectronic applications are considered.

Though the field of 2D materials is still developing, the early results of optoelectronic devices based on these materials are very promising. The unique properties of 2D materials have ushered in a lot of theoretical and experimental research for optoelectronic applications over the past decade or so. This has led to the proposal of numerous photodetectors based on 2D materials both theoretically and experimentally. This chapter aims at presenting an insight into the novel photodetectors based on 2D materials. Section 2 offers a discussion on photodetectionchanisms in 2D materials. Section 3 presents a discussion on photodetectors based on 2D materials and their heterostructures; Section 4 presents a brief summary of the chapter and future scope of 2D materials for photodetector applications.
