Black Phosphorus Photodetectors

The attention paid to 2D bP, rediscovered since 2014, is as described in Section 5.2.2. bPAs as a monolayer or as a few-layer material, has shown attractive properties, such as high mobility, anisotropic optical properties, and a highly tunable thickness-dependent direct bandgap, spanning from 0.3 eV in the bulk case to 1.7 eV in the monolayer case. In this context, black phosphorus is evaluated as a natural compromise between graphene and TMDs.

The first bP paper of great significance, which revealed the potency of bP for practical infrared applications, was published by Guo et al. in 2016 [16]. The authors described a bP photoconductor with room-temperature responsivity of 82 A/W at a wavelength of 3.39 pm. This high responsiv- ity is attributed to the photogating effect and the shallow long trap-level- induced carrier lifetime. Usually, a high photogain limits the bandwidth. For a light chopping frequency greater than 10 kHz, the responsivity value of 60 mA/W results from the intrinsic photovoltaic effect.

Bullock et al. [17] have demonstrated a two-terminal bP/MoS₂ MWIR heterojunction photodiode (Fig. 7.3) that achieved external quantum efficiencies of 35% and detectivities as high as 1.1 X 10¹⁰ cmHz^1/2/W at 2 = 3.8 pm at room temperature. These values are similar to those achieved by the current commercially available state-of-the-art room-temperature photodiodes (Figs. 3.15 and 3.16). Furthermore, by leveraging the anisotropic optical properties of bP, the first bias-selectable polarization-resolved photodetector was demonstrated.

The heterojunction photodiode consists of a bP/MoS₂ heterojunction in which a thin (~10-20 nm) n-type MoS₂ layer acts as an electron contact and an MWIR window. Holes are contacted via a rear back-reflector Au pad. As is shown in the band diagram of Fig. 7.3(b), the MoS₂ heterojunction provides asymmetric band offsets with the bP, allowing the flow of electrons to the MoS₂ contact, while blocking the flow of holes.

The polarized-resolved device uses two bP/MoS₂ heterojunctions, with orthogonal bP crystal orientations separated by a common MoS₂ electron contact. Each bP layer has an isolated hole contact. This configuration is similar to the two-color back-to-back photodiodes that had previously

FIGURE 7.3 Two-terminal bP/MoS₂ MWIR heterojunction photodiodes: (a) schematic of device configuration, (b) energy band diagram of the device (a) under equilibrium, and (c) schematic of polarization-resolved bP/MoS₂ heterojunction photodiode (after Ref. [17]).

been demonstrated in two-color HgCdTe and III-V semiconductor photodiodes [18]. The top MoO_x/Pd stack is an effective hole contact because of its large work function.

By varying the composition of arsenic, x, in the As_xP,__x, the bandgap correspondingly changes from 0.3 to 0.15 eV. This change in the energy gap suggests that b-AsP may interact with light, where the wavelength is up to 8.3 pm. Long et al. [11] reported bAs_{0 83}P₀₁₇ long-wavelength IR photodetectors, with room-temperature operation to 8.2 pm. The design of the phototransistor is similar to that shown in Fig. 7.1(b). The b-AsP thin film flakes of b-AsP, ranging from 5- to 20-nm thick, were mechanically exfoliated from bulk b-AsP samples onto a highly doped silicon substrate covered by 300-nm Si0₂ in a glove box. After device fabrication by standard electron-beam lithography, metallization, and a lift-off process, the spin-coated thin layer of polymethyl methacrylate (PMMA) was deposited to protect the samples from oxidation in the air.

Figure 7.4 schematically shows the photovoltaic response of b-AsP devices in which photogenerated electron-hole pairs are separated at the b-AsP/metal junctions. The photocurrent is mainly generated at the reverse-biased b-AsP/drain junction [Fig. 7.4(a)] if the channel is p-type doped. In the case of a slightly n-type doped b-AsP, the photocurrent is mainly generated at the reverse-biased b-AsP/source junction [Fig 7.4(b)]. The photocurrent has opposite polarity at the two contacts, due to the opposite junction bias direction. Detailed studies of the photoresponse mechanism revealed a more complex phenomenon, with contributions

FIGURE 7.4 Schematic diagrams of energy band structure for MWIR b-AsP photovoltaic detector with different doping types under a bias voltage © (a) the sample of b-AsP working at the p-type region and (b) the device working at the n-type region. The black horizontal arrows indicate the direction of the photocurrent (after Ref. [11]).

from the photogating and PFE effects. The photocurrent is negative, relative to V_rfs, and shows very weak gate-dependence in the highly doped regime, with dominant contributions of photothermoelectric (PTE) and bolometric effects.

A well-known strategy to suppress dark current involves using heterojunctions, in this case, using 2D van der Waals (vdW) heterojunctions. Integrability is an inherent trait of 2D materials, by which different 2D flakes can be sequentially stacked into vdW heterojunctions. Following this idea, Long et al. [11] fabricated photodetectors, based on a b-AsP/ MoS₂ heterostructure, with room-temperature detectivity of about 5 X 10⁹ cmHz^1/2/W at 4.3 pm. Figure 3.15 compares spectral detectivity curves of two types of bAs_{0 83}P₀₁₇ photodetectors (namely, phototransistor and vdW heterojunction) which were comparable with commercially available ones. More recently, Tan et al. [19] have demonstrated that doping bP with carbon, to form bPC, can extend the detectable wavelength to 8 pm, and a peak responsivity of 2163 A/W was achieved from a bPC-based phototransistor at 2 pm.

Amani et al. [10] studied the response spectrum and responsivity of bP and bPAs alloys (composition range 0-91% of As) as the active regions of phototransistors. Figure 3.15 shows the spectral dependence of detectivity for b-PAs gated-photoconductors (phototransistors), with the cutoff wavelength tuned from 3.9 pm to 4.6 pm. It is shown that, for optimized devices with a thickness of ~28 nm, the peak detectivity is 6x 10¹⁰ cmHz^1/2/W for bP and 2.4 X 10¹⁰ cmHz^1/2/W for bP₀₉₁As₀₀₉ at room temperature, with a bandwidth of 117 kPIz [10]. This detectivity is a record achievement for bPAs 2D-material photodetectors in the MWIR spectral range, and is almost one order of magnitude greater than that from commercially available HgCdTe photodiodes.

Figure 7.5 compares the peak detectivity of HgCdTe photodiodes [20] and InAs/GaSb type-II superlattice IB QCIPs [21], operating at room temperature with bPAs and noble metal dichalcogenide photodetectors. We can see that, in the MWIR spectral range, the performance of bPAs detectors outperformed commercially available uncooled HgCdTe photodiodes; in LWIR, however, the detectivity of noble transition metal dichalcogenide photodetectors is the highest. Due to strong covalent bonding of III-V semiconductors, IB QCIPs can be operated at temperatures of up to 400°C, which is not capable of being achieved by their HgCdTe counterpart, especially b-PAs, due to their fatal problem of structural instability. Furthermore, the response time of HgCdTe and IB QCIPs detectors

FIGURE 7.5 Peak detectivity comparison between HgCdTe photodiodes, InAs/ GaSb type-II superlattice IB QCIPs, and representative 2D-material photodetectors operating at room temperatures. Experimental data for HgCdTe photodiodes are taken according to the Vigo catalogue [20]. Data for cascade infrared detectors are taken from many papers, including Ref. [21].

operated at room temperature, typically of the order of nanoseconds, is considerably shorter than that of b-PAs photodetectors.