**2. Graphene-enhanced MWIR photodetectors**

#### **2.1 Motives and objectives of device concept**

HgCdTe, or MCT, the most widely used infrared (IR) detector material in military applications, is a direct energy bandgap semiconductor having a bandgap that is tunable from near-infrared (NIR) and SWIR to VLWIR bands through varying the Cd composition [15]. Typically, 0.3:0.7 Cd:Hg ratio results in a detectivity window over the SWIR to MWIR wavelength range.

Additionally, HgCdTe layer growth is highly controllable with certain deposition techniques. Notably among these is molecular-beam epitaxy (MBE), which yields high precision in the deposition of detector material structures leading to excellent control over optical excitation evidenced by the high quantum efficiencies (QE) demonstrated by HgCdTe-based detectors and sensors over the IR.

While adding considerable cost and bulk, cryogenic cooling is commonly utilized for IR detection to minimize thermally generated dark current. Since dark current increases with cutoff wavelength longevity, this requirement becomes even more important for MWIR and long-wave infrared (LWIR) sensors. IR band detector technologies that can operate at or near room temperature and substantially avoid costly and bulky cooling requirements, therefore, offer great practical benefits for many types of applications.

The incorporation of a high mobility graphene channel in HgCdTe-based detectors is a newly discovered means to offer further performance improvements and operational capabilities for MWIR detection. The intrinsic interfacial barrier between the HgCdTe-based absorber and the graphene layers thereby may be designed to effectively reduce the recombination of photogenerated carriers in the detector. The graphene thus functions as a high mobility channel that whisks away carriers before they can recombine, further contributing to the MWIR detection performance compared to in photodetectors only utilizing HgCdTe absorption layers [16].

*Doping and Transfer of High Mobility Graphene Bilayers for Room Temperature Mid-Wave… DOI: http://dx.doi.org/10.5772/intechopen.101851*

#### **2.2 Physical graphene-enhanced detector structure**

The graphene-enhanced HgCdTe MWIR detector structure fabricated on a silicon substrate comprises three principal layers. First, a layer of CdTe is grown to act as a buffer layer functioning as the gate terminal (1). This layer provides an electrical field in the "vertical" direction into the detector heterostructure that aids in carrier transport in that direction. **Figure 2** shows a schematic of this detector structure as shown in a study by Srivastava et al. [12].

The HgCdTe absorber layer (2) is grown above the silicon substrate and the CdTe buffer layer acts as the active optical layer where photogeneration of carriers takes place. The HgCdTe absorber material and its physical properties, such as bandgap, determine the sensitivity of the absorber layer to the detection wavelength window. In addition, the absorber material governs the photogeneration rate, quantum efficiency, and carrier lifetime, which collectively contribute to overall detection performance.

Finally, the graphene layer (3) incorporates the role of high mobility, low noise channel that quickly whisks away the photogenerated carriers in the absorber into the contacts, and subsequently into the readout integrated circuit (ROIC) for electrical readout. This layer, therefore, is directly contacted to the ROIC.

#### **2.3 Graphene-HgCdTe detector operating principle**

The general operating principle of the graphene-HgCdTe MWIR photodetector may be described in terms of the life cycle of the photogenerated carriers [17]. Incident IR photons transmitted through the Si substrate and CdTe layers into the HgCdTe region are absorbed and produce electron-hole pairs, or excitons (**Figure 3**(**a**)). The vertical electric field in the absorber applied through modulation of the gate voltage effectively separates the electron-hole pairs due to the consequent opposing forces on electrons and holes. This separation of the carriers physically isolates the two photogenerated carrier species and suppresses the Auger recombination within the absorber, minimizing the loss of photogenerated carriers and is thus critical to the ultimate performance of the detector.

After separation, the carriers are transported through the absorber film toward the graphene interface and then injected into it (**Figure 3**(**b**)). Modulation of the gate voltage bias to preferentially inject only one of the photogenerated species into the graphene in a rectifier-like action enables dynamic control of the interface properties. As this process involves the injection of both species, it further prevents any Auger recombinations from taking place in the graphene.

**Figure 3.**

*(a) Generation of excitons from incident photons separation of electrons and holes due to applied gate electric field. (b) Photogenerated carrier transport and injection into graphene. (c) Horizontal transport of the photogenerated carriers in the graphene.*

The carriers injected from the absorber into the graphene are transported laterally to the ROIC terminal and subsequently collected into it (**Figure 3**(**c**)). The establishment of a separate high mobility channel enabled by the graphene allows faster modulation frequencies with reduced **1/***f* noise and consequently higher performance metrics. Dynamic gating is additionally provided through the electrical control of carriers injected into the graphene.
