Can graphene make better HgCdTe infrared detectors?
© Xu et al; licensee Springer. 2011
Received: 14 August 2010
Accepted: 23 March 2011
Published: 23 March 2011
We develop a simple and low-cost technique based on chemical vapor deposition from which large-size graphene films with 5-10 graphene layers can be produced reliably and the graphene films can be transferred easily onto HgCdTe (MCT) thin wafers at room temperature. The proposed technique does not cause any thermal and mechanical damages to the MCT wafers. It is found that the averaged light transmittance of the graphene film on MCT thin wafer is about 80% in the mid-infrared bandwidth at room temperature and 77 K. Moreover, we find that the electrical conductance of the graphene film on the MCT substrate is about 25 times larger than that of the MCT substrate at room temperature and 77 K. These experimental findings suggest that, from a physics point of view, graphene can be utilized as transparent electrodes as a replacement for metal electrodes while producing better and cheaper MCT infrared detectors.
As an ideal two-dimensional (2D) electronic system (2DES), graphene (single or a few layers of carbon atoms arranged in a hexagonal lattice)  has excellent electronic, electrical transport, and optical properties and interesting physical features . Electronically, the carrier density in graphene  can be as high as 1013 cm-2. It is much larger than that in conventional III-V and SiGe-based 2DESs. More importantly, the carrier density in graphene can be turned easily and efficiently through applying the gate voltages . From an electrical transport point of view, graphene has very high carrier mobility  which can reach up to 20 m2/Vs at room temperature. This value of carrier mobility is about 100 times larger than that in conventional Si-based materials. Furthermore and optically, graphene has a very high light transmittance across the spectrum from the UV to the infrared. The light transmission coefficient for monolayer or bilayer graphene on SiO2 or Si substrates is about 98 or 96%, respectively, in the visible regime . To utilize all these excellent properties and important features for device applications, one of the most significant and practical applications for graphene is in the area of transparent conducting material for optoelectronic devices such as photodetectors and optical displays. Recently, graphene has been proposed as a replacement for the conventional indium tin oxide (ITO) transparent electrodes in producing better and cheaper LCD devices . Such an important application of graphene is based mainly on its excellent electrical, transport, and optical properties in the visible bandwidth. In this study, we would like to explore the possibility to apply graphene in the infrared optoelectronic devices such as infrared photodetectors and light sources.
Principle of designing graphene transparent electrodes
where ϵ 1= 1 for air, ϵ2 is the high-frequency dielectric constant of the substrate material, and σ(ω) is the optical conductance of graphene. This equation suggests that the light transmittance of the graphene layer on a substrate decreases with the increasing dielectric constant of the wafer material at a fixed σ(ω). Moreover, it was found, both experimentally  and theoretically [12, 13], that in the short wavelength or visible regime σ(ω) = Ne 2 / 4ħ is a universal optical conductance with N being the number of layers in graphene film, whereas in the mid-infrared (MIR) bandwidth, there is an optical absorption window existing in graphene. The MIR absorption window in graphene is induced by inter- and intra-band optical absorption channels required for different transition energies [12, 13] and, therefore, the width and depth of the absorption window depend sensitively on carrier density (or gate voltage)  and temperature [12, 13]. The presence of the optical absorption window in the MIR bandwidth indicates that graphene has an even better light transmittance in the infrared regime. Hence, graphene can be applied for MIR optical and optoelectronic devices, especially for infrared transparent conducting material for various applications.
MCT infrared detectors
On the other hand, HgCdTe (MCT)-based infrared detectors are popularly used as high-quality night-vision devices for MIR detection . The MCT infrared detectors are made mainly from photoconductors, photodiodes, and avalanche photodiodes  in which electrodes are required to be made. Because the conventional ITO materials have relatively poor light transmittance in the MIR bandwidth, metal electrodes are often used for making MCT infrared detectors . Normally, the metal electrodes cover about 20-30% area in the active regime of the MCT chips (see, e.g., Figs. 13 to 15 in ). If the metal electrodes in the MCT infrared detectors are replaced by the transparent ones, then the radiation area becomes enlarged and, hence, we are able to enhance the efficiency of the MIR detection and to improve the quality of the infrared images. In this study, we demonstrate that graphene is a good candidate for transparent electrodes to be applied for the production of MCT infrared detectors.
HgCdTe or MCT is a material with relatively high dielectric constant (κ ~ 14). This is one of the reasons why relatively high electric conductance can be achieved for graphene film on the MCT substrate. For 5-10 layers of graphene film on the MCT wafer, the light transmittance in the MIR bandwidth is about 80%. This is slightly lower than that on the SiO2/Si substrate in the visible regime. We find that the graphene films can be placed nicely on the MCT thin wafers with smooth surface. No crack or folding of the graphene film is found in our samples.
In this study, we have developed a simple and low-cost technique to grow graphene films reliably and to transfer the graphene films easily onto the thin HgCdTe wafers at room temperature. This technique can produce large-size graphene films and does not cause thermal and mechanical damages to the MCT thin wafer. We have found that multi-layer (e.g., 5-10 layers) graphene films on MCT thin wafer can have high light transmittance in the MIR bandwidth and relatively high electrical conductance at room temperature and 77 K. The most important conclusion that we drew from this study is that the light transmittance (about 80% in the MIR bandwidth) and the electrical conductance (about 25 times larger than that in the wafer itself at room temperature and 77 K) of the graphene film on the MCT thin wafer can meet nicely the requirements for the infrared transparent electrodes. These interesting findings allow us to propose that graphene can be used as a replacement for metal electrodes to produce better and cheaper MCT infrared detectors. Graphene has been proposed as a replacement for the ITO as transparent electrodes for optical devices such as LCD and LED . The results and analyses presented in this article indicate that graphene has even better features to be utilized as infrared transparent-conducting materials. This study can be considered as a stimulus for future applications of graphene in infrared optoelectronics and infrared optical devices.
2D electronic system
chemical vapor deposition
transmission electron microscopy.
This study was supported by the Chinese Academy of Sciences, National Natural Science Foundation of China (NSFC Grant Numbers 10974206 and 10974141) and by the Department of Science and Technology of Yunnan Province.
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