Two-Dimensional materials: from large-area growth to the performance in radio-frequency range

This thesis presents a collection of experimental results about the large-area growth of single-layer graphene and MoS2 films. In addition, we present the results of electrical characterisation of two-dimensional (2D) materials in direct current (DC) and radio-frequency (RF) range. The thesis is divided into four parts. In the first part we describe the recipe for the large-area growth of single-layer graphene on Cu substrate with the chemical vapor deposition (CVD) technique. In addition, we present two different methods for the transfer of the grown films onto arbitrary substrates. In order to investigate the intrinsic properties of the grown film we perform the electrical characterization in DC and RF range. The second part is devoted to the large-area growth of MoS2 single-layer films. We describe two different recipes for the growth: two-step thermolysis process of ammonium tetrathiomolybdate (NH4)2MoS4 in S atmosphere and CVD reaction ofMoO3 in S atmosphere. Additionally, we perform the electrical characterisation of the fabricated FETs based on grown MoS2 films. Apart from the grown materials, we also analysed the electrical properties of exfoliated MoS2. In the third part, we investigate the behaviour of 240 nm length top-gated MoS2 transistors operating in RF range. We fabricated MoS2 RF devices based on 1L-MoS2, 2L-MoS2, 3L-MoS2 and as well as multilayer MoS2 film with the thickness of » 5 nm. They exhibit current, power and voltage gain. The best DC and RF characteristics were obtained for 3L-MoS2 device showing a cutoff frequency fT = 6 GHz and a maximum oscillation frequency fmax = 8.2 GHz after the deembedding procedure. In order to boost the RF performance of MoS2 devices, firstly, we fabricated top-gated FETs with the gate length as short as 40 nm, secondly, we introduced the "edge-contacted" injection technique to our devices. Thus, we were able to obtain the record cutoff frequency for MoS2 devices of 6 GHz before deembedding and fT = 25 GHz after the deembedding procedure.


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