CHARACTERIZATION AND APPLICATIONS OF CVD-GROWN GRAPHENE FOR PASSIVE MICROWAVE DEVICES

Abstract

In this thesis work, the graphene layer transferred on two substrates, glass and quartz and bilayer graphene deposited on copper substrates were characterized using the multilayer microstrip line technique. Graphene transferred on both glass and quartz substrates of the dimension 1×1 were procured from Graphene Industries, USA. Bilayer graphene was grown by using Hot Filament Chemical Vapor Deposition (HFCVD) system on Cu sheet at two different temperatures 850C and 950 C. A new de-embedding method was adopted and verified with standard ABCD de-embedding method by comparing obtained results such as effective relative permittivity, phase velocity and group velocity in 100 MHz - 10 GHz range.We found occurrence of anomalous dispersion in single layer graphene in terms of higher group velocity than phase velocity. Then, the intrinsic properties, complex relative permittivity and refractive index of single layer graphene (SLG) and bilayer graphene (BLG) have been obtained by using expressions of conformal mapping after S-parameter measurements in 10 MHz to 26.5 GHz range. In SLG, the real part ( ) of relative permittivity was found ~ 6.71 to 7.62 whereas the imaginary part ( ) is decreased from  9.58 @ 10 MHz to 8.47 @ 1.92 GHz. For both BLG samples, same values of and are obtained in the range of 5.58 – 5.67 and 6.97 – 7.025, respectively. The value of the refractive index (n) and extinction coefficient (  ) for SLG are in the range of 2.95 - 3.15 and 1.43 - 1.52, respectively in the frequency range of study. In both BLG samples, same values of n and  were obtained and these are ~ 2.69 and ~ 1.28, respectively. To observe the behaviour of waves in SLG and BLG, phase velocity, group velocity and optical effective mass have been evaluated. In BLG, high phase velocity

and group velocity represent faster propagation of energy than in SLG. The mass ratio ( ⁄ )was found from 10-10 @10 MHz to  0.2  10-8 @ 26.5 GHz and  0.16  10-8 @ 26.5 GHz,respectively for SLG and BLG, where the free electron mass, kg. In addition, the lower effective mass of photons in BLG has resulted due to smaller refractive index, which in turn suggests minor interaction between photon-charge carriers. Theoretically, potential application of graphene as an efficient transparent conducting electrode was investigated in silicon heterojunction cells in n/p and p/n configurations and cell parameters were analyzed using AFORS-HET (Automat FOR Simulation of HETero-structures) software under air mass 1.5 (AM1.5) illuminations with power density of 100 mW/cm2.Independent effects of the layer’s parameters on the performance of cell structure and insights behind the cell responses have been discussed. After optimizing the parameters of both layers of graphene and silicon, an efficiency of 9.812 % was achieved and an optimum efficiency of 11.47 % has been achieved for 100 µm thick commercial silicon layer in p-graphene/n-cSi cells, whereas simulation efficiency of 13.66 % was achieved and an optimum efficiency of 8.491 % has been achieved for commercial silicon of same thickness in n-graphene/p-cSi cells.Finally, we demonstrate that p-type and n-type multilayer graphene can act as an efficient transparent conducting electrode in graphene/silicon heterojunction cell.