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We report on the efficient photodetection (PD) properties of graphene based p-i-n photodetector, where all the three layers are either single or multilayer graphene sheets. We report the bandwidth and responsivity performance of the device. This simple structure paves the way for the next generation flexible wireless communication systems. A theoretical model is used to study the carrier distribution and current in a graphene based p-i-n photodetector system.

Photodetectors (PD) play key role in high performance of optoelectronic and photonic systems. P-i-N photodetector is one type of photodetectors which converts optical signal into electrical response. This type of PD usually consists of heavily doped P and N regions, separated by an intrinsic layer. Usual P-i-N PD systems consist of semiconductor materials like, Germanium, Indium gallium arsenide, Lead sulphide and Silicon. The next generation flexible and wearable communication systems require efficient photodetector material which is compatible with the flexible fabrication process. In recent years, copious amount of research is going on to fabricate flexible PD systems using various types of flexible materials, like, Tin Monosulfide, CsPbBr_{3} microcrystals, two-dimensional (2D) layered materials, organic semiconductors and perovskite materials [

The model is based on the previously reported study on photodetector devices [_{3} and H_{2}SO_{4}) and some organic compounds [

Here, the light is incident on the P side. The structure consists of a single layer of N^{+} graphene layer, an undoped multi-layer graphene with thickness l and finally a single layer graphene P^{+} layer. The nominal N^{+} and P^{+} region doping is taken of the order of 1 × 10^{12} cm^{−}^{2}, which is practically reliable value [^{ }

In the theoretical model, we have to consider the effect of photogenerated carriers in the intrinsic layer as because the widths of the three regions are comparable to each other. Thus, the current continuity equations in the depletion layer is given by [

∂ n ( x , t ) ∂ t − v n { ∂ n ( x , t ) ∂ x } = g (1)

∂ p ( x , t ) ∂ t + v p { ∂ p ( x , t ) ∂ x } = g (2)

where, g is the photo carrier generation rate, v is the velocity, n and p denote the electron and hole, respectively. Incident optical powers, absorption coefficient of graphene at the operating wavelength, reflectivity of the graphene surface are some of the important parameters which control the generation rate of the photo carriers.

Carrier distribution N(x,jω) for electrons and P(x,jω) for holes in the depletion region (in frequency domain) are calculated by solving the above current continuity equations simultaneously. In general, all the uppercase variables are used to indicate the Laplace transform of the corresponding lowercase variables. The entire depletion region is subdivided into equal energy spacing (Δx) for calculation. Along its path of motion, each carrier represents a specific position and energy state in the depletion region of the device. For this reason, each carrier is specifically represented as a function of two indices: one position index (i) and one energy index (j). So, we substitute N(x, jω) by N(i, j, jω) and P(x, jω) by P(i, j, jω).

To obtain the photo-current density, the carrier distribution N(i, j, jω) and P(i, j, jω) are multiplied by equal energy spacing (∆x) and then summed for all i and j. The current density J of the device is obtained using Equation (3)

J = q L ∑ i [ ∑ j { N ( i , j , j ω ) v n ( i ) + P ( i , j , j ω ) v p ( i ) } ] Δ x ( i ) (3)

where, L, q and v are the length of the PD, electronic charge and the carrier velocity respectively. We consider here that the carrier velocity is only function of the position. Suffix n and p is used for electrons and for holes respectively.

The material parameters for the graphene layer have been taken from the literature [

^{+} and N^{+} layer thicknesses are varied from 1 to 5 nm and area of the device is also varied, as seen from ^{+} and N^{+} layer thicknesses are changed, which concludes that the

bandwidth of the device is dependent on the intrinsic layer thickness. This conclusion facilitates the applicability of the model in real fabrication of the device, since producing multilayer graphene sheets is comparatively easier than single layer graphene sheets.

^{2}, which gave rise to the maximum 3-dB bandwidth as observed from

Keeping i-layer thickness as 5 nm and device area as 50 µm^{2}, the responsivity of the system has been investigated, where p and n layer thicknesses are kept fixed at 1 nm.

Area (µm^{2}) | Bandwidth (GHz) | Responsivity (A/W) | Rise Time (Sec) | EQE (%) |
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50 | 5.28 | 0.34 | 0.066 × 10^{−9 } | 67.23 |

wavelength, which shows a linear response of the device for different applied bias voltages. We also investigate the responsivity of the PD at 633 nm wavelength, as a function of applied bias which is comparable with the reported values in the literature [

We have designed an effective graphene based flexible PD system whose performance matches well with the experimental values in the literature. The PD system can be effectively used in the next generation communication systems. Further investigations are under progress to fabricate graphene based PD devices on flexible polymer substrates.

The authors would like to thank Prof. S. Chaudhury, Director CEERI-Pilani and Prof. D. Bhattacharya, Director AOT for their support.

Majumder, K., Barshilia, D. and Majee, S. (2018) Study on Graphene Based Next Generation Flexible Photodetector for Optical Communication. Graphene, 7, 9-16. https://doi.org/10.4236/graphene.2018.72002