Effects of impurities on charge transport in graphene field-effect transistors

Licentiate thesis, 2017

In order to push the upper frequency limit of high speed electronics further, thereby extending the range of applications, new device technologies and materials are continuously investigated. The 2D material graphene, with its intrinsically extremely high room temperature charge carrier velocity, is regarded as a promising candidate to push the frequency limit even further. However, so far most fabrication processes unintentionally introduce impurities at the interface between graphene and adjacent materials, which affect the performance. Additionally, due to the lack of a band gap, the important power gain parameter, the maximum frequency of oscillation ($f_\text{max}$), is not impressively high. In this thesis, results of the studies of the effect of impurities on charge transport in a graphene field effect transistor (GFETs) are presented. This study was performed was done by, firstly, setting up a semi-empirical model describing the influence of impurities, i.e., interface states on capacitance and transfer characteristics at low electric fields and, secondly, by developing a method for studying the limiting mechanisms of the charge carrier velocity in the graphene channel at high electric fields. It was found that uncertainties in the material parameters of graphene, such as the Fermi velocity, hamper the possibility to find the correct mobility value by direct measurements on a GFET. Furthermore, it was shown that remote optical phonons limit the saturation velocity and charge carriers emitted from interface states at high fields are preventing the current to saturate and, hence, restricting $f_\text{max}$. By studying the effects and the limitations set by impurities and other parasitic effects in the GFET it is possible to clarify strategies for further development of GFETs towards reliable performance and higher $f_\text{max}$. As is shown in this work, it is necessary to develop a fabrication process which results in clean interfaces and adjacent materials with higher optical phonon energies than today.