**Abstract**

High-performance graphene-HgCdTe detector technology has been developed combining the best properties of both materials for mid-wave infrared (MWIR) detection and imaging. The graphene functions as a high mobility channel that whisks away carriers before they can recombine, further contributing to detection performance. Comprehensive modeling on the HgCdTe, graphene, and the HgCdTe-graphene interface has aided the design and development of this MWIR detector technology. Chemical doping of the bilayer graphene lattice has enabled *p*-type doping levels in graphene for high mobility implementation in high-performance MWIR HgCdTe detectors. Characterization techniques, including SIMS and XPS, confirm high boron doping concentrations. A spin-on doping (SOD) procedure is outlined that has provided a means of doping layers of graphene on native substrates, while subsequently allowing integration of the doped graphene layers with HgCdTe for final implementation in the MWIR photodetection devices. Successful integration of graphene into HgCdTe photodetectors can thus provide higher MWIR detector efficiency and performance compared to HgCdTe-only detectors. New earth observation measurement capabilities are further enabled by the room temperature operational capability of the graphene-enhanced HgCdTe detectors and arrays to benefit and advance space and terrestrial applications.

**Keywords:** graphene, HgCdTe, photodetectors, MWIR, mobility, doping, transfer

## **1. Introduction**

#### **1.1 Graphene overview**

The term *graphene* is a combination of two words—*graphite* and *alkene*. Graphene denotes a two-dimensional (2-D) sheet of graphite of atomic-scale thickness resulting due to intercalation of graphite compounds [1, 2]. Graphene can be divided into three different classifications based on the extent of its layer structure, which are monolayer, bilayer, and multilayer graphene, with the latter designating graphene structures consisting of three or more layers [3].

*Graphite*, another well-known compound of carbon, basically consists of stacked sheets of graphene held in place by van der Waals forces. A third relatively recently discovered carbon compound that has likewise been envisioned as a catalyst essentially comprises rolled-up sheets of graphene commonly known as *carbon nanotubes*. As thus comprising the basic building block of these key carbon materials of varying dimensionalities, graphene has come to be considered the "mother" of graphitic compounds [4].

Chemically speaking, graphene comprises a two-dimensional hexagonal benzene ring-like structure consisting of sp2 -bonded carbon, packed into honeycomb lattice as shown in **Figure 1**, where the C▬C bond length is 0.142 nm. It is one of the first 2-D materials known to be stable at room temperature, and in ambient conditions is crystalline and chemically inert. Although stronger than diamond, graphene yet remains as flexible as rubber [6].

From an electrical standpoint, the valence and conduction bands in the band structure of graphene meet at the corners of the Brillouin zone or Dirac points. The consequence of this is that apart from the influence of thermal excitations, the intrinsic charge carrier concentration is zero, and graphene is consequently characterized as a zero-bandgap semiconductor. Notwithstanding the theoretical implications of this, practical and functioning graphene-based devices still require the existence of charge carriers, as well as control over the quantification of the concentrations and types of charge carriers (i.e., for *n*- or *p*-doping) [7, 8].

Intensive research performed over multiple decades into graphene material has further uncovered the remarkable chemical and material properties of this unique and somewhat extraordinary form of carbon. Perhaps most notably are the extremely high charge carrier mobilities in the range of 2000–5000 cm2 /V s, making graphene a choice material for implementation in high-speed electronics, such as flexible ultrafast microelectronics. Graphene is likewise one of the most highly conductive materials known with thermal conductivities reaching 5000 W/m K, facilitating its use for applications such as light-emitting diodes (LEDs).

Furthermore, having Young's modulus reportedly as high as 1 TPa has led to the use of graphene for strength reinforcement in various aerospace and structural/ concrete material applications. In offering one of the largest specific surface areas (2630 m<sup>2</sup> /g) combined with nearly full optical transparency of 97.7%, graphene has likewise been employed for the advancement of numerous optical and

**Figure 1.** *(a) Graphene geometry; (b) bonding diagram; and (c) associated band diagram [5].*

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

optoelectronic applications [9, 10]. The combined benefits of its high mechanical strength, optical transparency, and mobility of charge carriers have made graphene a choice material for a diverse array of electrical and/or optical applications.

During past decades, starting with the disclosure of the Hummers method in the 1950s followed by the chemical reduction of graphene oxides in 1962, there have been numerous research and studies on graphite oxide synthesis [8]. However, it took the work of A. Geim and K. Novoselov to fully isolate and subsequently characterize pristine graphene by a process of mechanical exfoliation that came to be known as the "Scotch tape" method for the properties of graphene to be more adequately understood, upon which it soon became a primary material of interest for many diverse ongoing research efforts. Later in 2010 Geim and Novoselov were awarded the Nobel Prize in Physics for this research encompassing graphene as a 2-D atomic structure [9].

After the initial success of isolating this graphitic material, many in the scientific community commenced to explore different processes and techniques for the large-scale synthesis and fabrication of graphene. As reported contemporarily in literature, the more common graphene synthesis processes included the oxidationreduction growth process, chemical vapor deposition (CVD), liquid phase stripping, and epitaxial growth on silicon carbide (SiC) [10]. Among these various methods, the CVD process soon became the most established technique for producing graphene films with the highest quality crystalline and structural integrity, primarily on Cu substrates [11].

However, Cu and other metal substrates are not practical for most applications, as many optoelectronic-, sensor-, and microelectronics-based applications require the placement of graphene films directly on metal oxide or semiconductors. There, therefore, has been and remains a need for more optimized increasingly effective processes through which graphene films can be directly and effectively transferred onto any desired substrate of choice, while also avoiding cracks, wrinkles, and forms of contamination [12].

#### **1.2 Graphene-based technological device development**

Infrared detector and focal plane array (FPA) technologies are at the heart of many space-based instruments for NASA and defense missions that provide remote sensing and long-range imaging capabilities [13]. While often considered exotic in comparison to more established detector materials such as HgCdTe, on account of its gapless band structure, strong light-matter interaction, and the relative ease by which heterostructures may be fabricated, graphene can provide numerous capabilities from diverse means for effective broad spectra photodetection.

Graphene detector implementation can likewise further facilitate reduced size, weight, power, and cost (SWaP-C) mid-wave infrared (MWIR) sensors on smaller platforms, a high priority for providing improved measurement and mission capabilities in space. Use of process techniques such as post-growth thermal cycle annealing (TCA) has additionally been reported to enable up to an order of magnitude reduction in the dislocation density down to the saturation limit (~106 cm−2) for improved high-temperature operability of HgCdTe-on-Si-based MWIR detectors and FPAs [13].

The overall functionality and applicability of a detector device or system are governed primarily by its wavelength range, that is, band, of operation. Of the different infrared (IR) bands spanning the short-wave infrared (SWIR) to very long-wave infrared (VLWIR), the MWIR region is considered of the highest beneficial for long-range imaging and early threat detection [14]. Specifically, the 2–5 μm MWIR spectral band is crucial for NASA Earth Science applications, especially in

satellite-based LIDAR systems that require measuring a wide variety of natural features, including cloud aerosol properties, sea surface temperatures, and natural phenomena such as volcano and forest fires.

Though prior scientific reporting of associated experimental results has served to illuminate the key 2-D nanomaterial properties of graphene for enhancement of sensing performance particularly for IR band detection, certain challenges still must be addressed, which are as follows:

