Graphene-Based Detectors

This chapter gives an overview of the performance of emerging graphene-based detectors, comparing them with traditional and commercially available ones in different applications under high- operating-temperature conditions. Generally, 2D-material detectors can be divided into two categories, either photon or thermal detectors. Photon detectors are related to the excitation of free carriers as a result of an optical transition, such as the photoconductive or photovoltaic effects. Thermal detectors operate according to thermal effects, such as the bolo- metric effect or the photothermoelectric (PTE) effect.

Types of Detectors

Photoconductors

Schematic operations of the two most popular photodetectors are shown in Fig. 6.1. The photoconductive detector is essentially a radiation-sensitive resistor with two metal contacts. A photon of energy greater than the bandgap energy is absorbed to produce electron-hole (e-h) pairs, thereby changing the electrical conductivity of the material. The e-h pairs produced are separated by the external electric field, generating a photocurrent.

Assuming that the signal photon flux density S(2) is incident on the detector area A = wl (w: width, /: length), the basic expression describing photoconductivity in semiconductors under equilibrium excitation (i.e., steady state) is

Schematic of the (a) photoconductive and (b) photovoltaic effects

FIGURE 6.1 Schematic of the (a) photoconductive and (b) photovoltaic effects.

where q is the electron charge, Iph is the short-circuit photocurrent at zero frequency, that is, the increase in current above the dark current accompanying irradiation. The quantum efficiency, //, can be defined as the number of electron-hole pairs generated per incident photon, and describes how well the detector is coupled to the incoming radiation. The second parameter, the photoconductive gain,#, is determined by the properties of the detector (i.e., by which detection effect is used, and the material and configuration of the detector) and can be defined as the number of carriers passing contacts per generated pair. The value of g describes how well the generated charge carriers are used to generate the current response of a photodetector.

In general, photoconductivity is a two-carrier phenomenon and the total photocurrent of electrons and holes is given by:

where pe is the electron mobility, ph is the hole mobility; Vb is the bias voltage, and

where n0 and p0 are the average thermal equilibrium carrier densities, and An and Ap are the excess carrier concentrations.

Taking the conductivity to be dominated by electrons (in all known high-sensitivity photoconductors, this is found to be the case) and assuming uniform and complete absorption of the light in the detector, it can be shown [1] that

So, the photoconductive gain can be defined as

where t, is the transit time of electrons between ohmic contacts. This means that the photoconductive gain is given by the ratio of free carrier lifetime, t, to transit time, t,, between the sample electrodes. The photoconductive gain can be less than or greater than unity, depending upon whether the drift length, Ld=vdr, is less than or greater than the interelectrode spacing, l. The value of Ld> 1 implies that a free charge carrier swept out at one electrode is immediately replaced by injection of an equivalent free charge carrier at the opposite electrode. Thus, a free charge carrier will continue to circulate until recombination takes place.

Taking into account Eqs. (6.1) and (6.4), the photocurrent

is linearly dependent on the photon flux density (i.e., excitation power), the photogenerated carrier lifetime, the electron mobility, and the applied bias.

The current responsivity of the photodetector is equal to

where X is the wavelength, h is the Planck constant, and c is the velocity of light.

Photogating Effect

A particular example of the photoconductive effect is photogating. The photogating effect can be realized in two ways by:

  • • Generation of e-h pairs, when one type of carrier is trapped by the localized states (nanoparticles and defects), or
  • • Generation of e-h pairs in trap-states, and one type of carrier is transferred to 2D materials, whereas the other resides at the same place to modulate the layered materials.

In both cases, due to the long carrier lifetime, the enhancement of sensitivity is at the cost of photoresponse speed.

If holes/electrons are trapped in localized states (Fig. 6.2), they act as a local gate, effectively modulating the resistance of active materials. In this case, the photocarriers are limited only by the recombination lifetime of the localized trap states, leading to a large photoconductive gain, g. The trap states, where carriers can reside for long periods, are usually located at defects or at the surface of the semiconducting material. This effect is of

Band alignment under illumination with photons of energy higher than the bandgap generating e-h pairs

FIGURE 6.2 Band alignment under illumination with photons of energy higher than the bandgap generating e-h pairs. Holes are trapped at the band edge and act as a local gate. In consequence, the field-effect induces more electrons in the channel, generating a photocurrent which adds to the dark current. If the electron lifetime exceeds the time it takes for the electron to transit device, then the long time that the holes are trapped ensures that the electrons can circulate through an external circuit many times, resulting in gain.

particular importance for nanostructured materials, like colloidal quantum dots, nanowires, and 2D-semiconductors, where the large surface and reduced screening play a major role in the electrical properties.

In the case of the photodiode, the photoelectric effect is usually equal to 1, due to separation of minority carriers by the electrical field of the depletion region. However, in a hybrid combination of 2D-material photodetectors, photosensitization and carrier transport take place in separately optimized regions: one for efficient light absorption, and the second, to provide fast charge reticulation. In this way, ultrahigh gain up to 10s electrons per photon, and exceptional responsivities for short-wavelength infrared photodetectors have been demonstrated [2,3].

The simple architecture of a hybrid phototransistor, very popular in the design of 2D-material photodetectors, with the fast transfer channel for charge carriers, is shown in Fig. 6.3. Since, e.g., the graphene in these devices is not responsible for light absorption but only for the sensing of charge, absorber choice determines the spectral response. The graphenes large ambipolar mobility (~103-105 cm2/Vs) acts as a built-in photogain (i.e., amplifier) mechanism, enhancing the detector response.

2D materials with thickness down to the atomic layer are more susceptible to local electric fields than are conventional bulk materials, and the photogating effect can strongly modulate the channel conductivity by

Photogating effect in 2D-material photodetectors

FIGURE 6.3 Photogating effect in 2D-material photodetectors: (a) the operation of hybrid phototransistor, (b) closed channel under illumination, (c) photocon- ductive gain, and (d) I-VG trace under illumination.

external gate voltage, VG. Improving the optical gain is particularly important since the quantum efficiency is limited because of the weak absorption in 2D materials. This effect is especially seen in longer-wavelength IR spectral regions, where the light absorption is weak. In the case of the hybrid detector shown in Fig. 6.3(a), the holes are injected into the transporting channel, whereas the electrons remain in the photoactive layer. The injected charges can reticulate several thousand times before recombination, giving contribution in photogain under illumination. The photocarrier lifetime is enhanced through both the bandgap structure and defect engineering, and, at the same time, the trapping mechanisms limit the response time of the photodetector, to as much as several seconds. There is a negative trade-off between the enhancement of sensitivity and photoresponse speed.

The photocurrent change by photogating effect can be written as [4,5]:

where gm is the transconductance and AVG is the equivalent photoinduced voltage. Figure 6.3(d) indicates a shift of the fDS(VG) trace after the illumination. Generally, both positive and negative photoconductance behaviors are observed in hybrid 2D structures, and working points A and B, related to g,„ and A Vc, respectively, perform in opposite directions.

 
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