Transmission Electron Microscopy and Nanoindentation Studies of Ultrathin Transition Metal Dichalcogenide Semiconductor Membranes

Until now, only two intrinsically two-dimensional materials have been isolated: graphene and boron nitride (BN). As a consequence, the knowledge of two-dimensional systems is still limited and other classes of two-dimensional materials are needed to enrich our understanding of these systems. In this thesis, we studied the structural and mechanical properties of two-dimensional transition metal dichalcogenide (TMD) crystals. Apart from fundamental interest, transition metal dichalcogenides have great appeal due to their possible applications in electronic. Here, the focus is on MoS2, while the main employed techniques are transmission electron microscopy and nano-indentation. The thesis is divided into three parts. The first part is devoted to summarize the general properties of transition metal dichalcogenide materials, to illustrate the instrumentation, and to describe the deposition techniques used in sample preparation. In particular, the transfer technique used to transfer selected flakes among different substrates is described. In the second part, we investigate single-layer MoS2 flakes obtained by mechanical exfoliation. We describe two methods to distinguish single-layer MoS2 flakes from thicker ones by electron diffraction. Ripples are observed over the flake surface. Thanks to diffraction pattern interpretation and high resolution transmission electron microscopy, their amplitude is estimated to be 0.6 - 1 nm in height. Nano-indentation is used to determine the Young’s modulus and the breaking strength of single-layer MoS2 membranes. The Young’s modulus is EY =270±100GPa while the breaking strength is σmax =23±5 GPa. In order to fully take advantage of the promising electronic properties of transition metal dichalcogenide, it is necessary to grow them over large scale. In this way, the pla- nar technology developed for silicon can be implemented and a large number of devices with uniform electrical properties can be realized. When comparing different growths, methods to quantify the quality of the films are mandatory. In this context, we show that transmission electron microscopy is a valuable method to characterize large area MoS2 and MoSe2 films. We are able to estimate the grain size and to have a direct view of the grain boundaries. In fact, grain size and boundary quality constitute the important parameters to qualify two-dimensional poly-crystalline films. Finally, we describe a novel in-situ experiment which allows us to simultaneously perform transmission electron microscopy and electrical characterization on a device based on a suspended MoS2 flakes. The effect of the current flow on the MoS2 flake are considered. These two topic constitute the last part of the thesis.

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

Daria Krasnozhon

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.

EPFL2016

Texture mechanisms and microstructure of biaxial thin films grown by oblique angle deposition

Alireza Karimi, Akshath Raghu Shetty

In order to understand the texture formation mechanism in thin films grown under oblique angle deposition (OAD), TiAlN films were deposited at room temperature (RT) under various incident angles. We show that both in-plane and out-of-plane crystallographic orientations respond strongly to the deposition angle. For a?=?0 degrees, the pole figures show a (111) and (200) mixed out-of-plane orientation with random in-plane alignment. In contrast, under OAD, inclined textures are observed with the (111) direction moving toward the incident flux direction and the (200) moving away, showing substantial in-plane alignment. This observation suggests that TiAlN crystals prefer to grow with the (200) direction perpendicular to the substrate while maintaining the minimization of the surface free energy by maximizing the (111) surface area toward the incident flux. The in-plane texture, which is randomly oriented at normal incidence, gives rise to two preferred orientations under oblique angles one along the direction of flux and other away from the deposition source. The biaxial texture results from a competition among texture mechanism related to surface mobilities of adatoms, geometrical and directional effects. The surface and cross-section of the films were observed by scanning electron microscopy (SEM). OAD films develop a kind of smooth tiles of a roof structure, with no faceted crystallites. The columns of these films were tilted toward the direction of incident flux. The dependence of (111) texture tilt angle and column angle beta on the incidence flux angle a is evaluated using four well-known models. Transmission electron microscopy (TEM) study reveals a voided, intercolumnar structure with oblique growth toward the flux direction. The selected area diffraction pattern (SAED) pattern supports the pole figure observations. Measurements of the nanoindentation test were performed in order to discuss the change of mechanical properties as a function of incident flux angle.

Wiley-V C H Verlag Gmbh2012

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

Daria Krasnozhon

EPFL2016

Texture mechanisms and microstructure of biaxial thin films grown by oblique angle deposition

Alireza Karimi, Akshath Raghu Shetty

Wiley-V C H Verlag Gmbh2012