Related 2D-Material Detectors

Graphene is one of a large number of possible 2D crystals. The discovery of new 2D materials, with direct energy gaps in the IR to the visible spectral regions, has opened up a new perspective on photodetector fabrication. There are hundreds of layered materials that retain their stability down to constituent monolayers, the properties of which are complementary to those of graphene, and which have been covered in review papers [1-9]. More information on this topic is included in Chapter 5.

General Overview

Detection mechanisms in 2D-material photodetectors, as for graphene- based detectors (Chapter 6), have been reported, including the photocon- ductive, photovoltaic, photothermoelectric, and field-effect transistors (FET) effects. For example, Fig. 7.1 shows two types of detectors: the MoSe2 photoconductor and the black phosphorus (bP)-based FET. Whereas 2D transition metal dichalcogenides (TMDs) are limited to UV-NIR, due to their bandgaps, bP can be tuned to below 0.3 eV by doping with As. Black phosphorus-arsenic (bPAs) has been demonstrated to achieve light detection from UV to THz.

Despite the very high responsivity reported in hybrid graphene-based phototransistors, their power consumption, electronic readout circuits, and noise are all determined by the zero-bandgap of graphene, which leads to a large dark current. Alternative 2D materials, especially TMDs,

Cross-section view of 2D material photodetectors

FIGURE 7.1 Cross-section view of 2D material photodetectors: (a) MoSe2 photoconductor and (b) bP-based FET.

have been considered as potential replacements for graphene for transistor channels. The use of 2D TMD channels with a bandgap of 1-2 eV is of particular promise, offering lower leakage current during the operation of the transistor in the depletion mode.

As with graphene-based photodetectors, ultrahigh responsivity and ultrashort time response cannot be obtained at the same time in practice in 2D-related materials. In 2D materials and their hybrid systems, the photogating effect can be realized in two ways, as is described in Section 6.1.2, namely by carriers trapping in the localized states or (in hybrid photodetectors) by carriers trapping and transferring one type of carrier to the 2D materials.

In estimation of detectivity in layered detectors, the shot noise from the dark current is often considered to be the main source of noise. However, for photodetectors with a high photogain and operating at room temperature, the other two noise sources, thermal noise and generation-recombination (g-r) noise, cannot be totally neglected. In the case of devices with a high photogain and a low response speed induced by a long carrier lifetime, the light-induced g-r noise must be considered. In addition, low- frequency noise (1 If) should be investigated for various 2D photodetectors, because it is considered to be an important metric for evaluating the performance of these detectors.

Examples of the performance of infrared photodetectors, based on 2D materials and their hybrid structures, including photodetectors with cutoff wavelength above 1-pm, are collated in Table 7.1 [5,10-15]. Topological insulators can be promising candidate materials for broadband photodetection. Metallic surface state can result in a strong optical absorption, which has been demonstrated for the Bi2Te3/Si heterojunction. The spectral responsivity of this device covers a broadband response from 370.6 nm to 118 pm [15].

TABLE 7.1 Infrared Photoresponsivity of 2D Materials at Room Temperature

Material

Wavelength

(i‘m)

Responsivity

(AAV)

Bias

(V)

Gain

(%)

Time

Peak Detectivity (Jones)

Ref.

MoS2

0.5-1.1

0.1

-10

25%

< 15 ms

[5]

WS2 (CVD)

0.5-0.9

3.5x10s

2

IX 10s

23 ms

10u

[5]

In2Se,

0.4-0.94

9.8X104

0.05

9s

3.3 XlO13

[5]

GeAs

1.6

6

3s

[5]

MoS,/PbS

0.4-1.5

6x10s

1

0.35 s

7x 1014

[5]

MoS2/Si

0.4-1.0

0.9082

-2

56 ns; 825 ns

1.889 XlO13

[5]

MoS,/bP

0.5-1.6

22.3

3

50

15 ps; 70 ps

3.1 X1013

[5]

MoS2/G/WS2

0.4-2.4

IX 104

1

106

53.6 ps; 30.3 ps

1X1015

[5]

bP (gated-photocon.)

3.5

10

0.5

270

6ХЮ10

[10]

bP/MoS, (p-n hetero)

4.3

0.5

~1 ms

2 XlO9

[11]

bAsP (phototransistor)

2-8

(зо-ю)хю-3

0

3X108

[11]

PtSe2 (phototransistor)

0.6-10

4.5

1.1, 1.2 ms

7 XlO8

[12]

PdSe2 (phototransistor)

1-10.6

~ 45

1

103-49

74.5, 93.1 ms

1X109

[13]

PdSe,/MoS2 (p-n hetero)

1-10.6

~ 4

1

65.3,62.4 ps

8 XlO9

[13]

Gr/Ti,03

10

300

0.1

1.2, 2.6 ms

7 XlO8

[14]

Bi2Te3/Si

UV-THz

1

-5

0.1 s

2.5X10" (635 nm)

[15]

Comparison of (a) spectral responsivity and (b) detectivity of 2-D material photodetectors in SWIR spectral range with silicon and InGaAs photodiodes operating at room temperature

FIGURE 7.2 Comparison of (a) spectral responsivity and (b) detectivity of 2-D material photodetectors in SWIR spectral range with silicon and InGaAs photodiodes operating at room temperature.

Figure 7.2 compares the spectral responsivity and detectivity of representative 2D-material photodetectors in the SWIR spectral range with silicon and InGaAs photodiodes operated at room temperature. Generally, the responsivity and detectivity of layered material photodetectors are lower in comparison with the most popular commercial InGaAs photodiodes with a cutoff wavelength range below 3-pm.

Middle- and Long-Wavelength Infrared Detectors

As shown in Table 7.1, in the longer wavelength infrared spectral region above 3-pm, black phosphorus (bP), black phosphorus-arsenic AsxP,.x (bPAs) alloys, and layered semiconductors with narrow bandgaps and high mobilities among the transition metal dichalcogenides (TMDs) are of great significance toward the realization of high-performance 2D infrared detectors.

 
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