arXiv subjects

Davood Momeni Pakdehi

Publications and source records attributed to Davood Momeni Pakdehi.

9 recordsLinked to original sources

arXiv2021

Optimization of epitaxial graphene growth for quantum metrology

(See the complete abstract within the thesis in both English and German versions) In this thesis, the process conditions of the epitaxial graphene growth through a socalled polymer-assisted sublimation growth method are minutely investigated. Atomic force microscopy (AFM) is used to show that the previously neglected flow-rate of the argon process gas has a significant influence on the morphology of the SiC substrate and atop carbon layers. The results can be well explained using a simple model for the thermodynamic conditions at the layer adjacent to the surface. The resulting control option of step-bunching on the sub-nanometer scales is used to produce the ultra-flat, monolayer graphene layers without the bilayer inclusions that exhibit the vanishing of the resistance anisotropy. The comparison of four-point and scanning tunneling potentiometry measurements shows that the remaining small anisotropy represents the ultimate limit, which is given solely by the remaining resistances at the SiC terrace steps. ... The precise control of step-bunching using the Ar flow also enables the preparation of periodic non-identical SiC surfaces under the graphene layer. Based on the work function measurements by Kelvin-Probe force microscopy and X-ray photoemission electron microscopy, it is shown for the first time that there is a doping variation in graphene, induced by a proximity effect of the different near-surface SiC stacks. The comparison of the AFM and low-energy electron microscopy measurements have enabled the exact assignment of the SiC stacks, and the examinations have led to an improved understanding of the surface restructuring in the framework of a step-flow mode. ...

cond-mat.mtrl-sci↗

arXiv2020

Direct Access to Auger recombination in Graphene

Auger scattering channels are of fundamental importance to describe and understand the non-equilibrium charge carrier dynamics in graphene. While impact excitation increases the number of carriers in the conduction band and has been observed experimentally, direct access to its inverse process, Auger recombination, has so far been elusive. Here, we tackle this problem by applying our novel setup for ultrafast time-resolved photoelectron momentum microscopy. Our approach gives simultaneous access to charge carrier dynamics at all energies and in-plane momenta within the linearly dispersive Dirac cones. We thus provide direct evidence for Auger recombination on a sub-10~fs timescale by identifying transient energy- and momentum-dependent populations far above the excitation energy. We compare our results with model calculations of scattering processes in the Dirac cone to support our experimental findings.

cond-mat.other↗

arXiv2020

Traceably Calibrated Scanning Hall Probe Microscopy at Room Temperature

Fabrication, characterization and comparison of gold and graphene micro- and nano-size Hall sensors for room temperature scanning magnetic field microscopy applications is presented. The Hall sensors with active areas from 5 $μ$m down to 50 nm were fabricated by electron-beam lithography. The calibration of the Hall sensors in an external magnetic field revealed a sensitivity of 3.2 mV/(AT) $\pm$ 0.3 % for gold and 1615 V/(AT) $\pm$ 0.5 % for graphene at room temperature. The gold sensors were fabricated on silicon nitride cantilever chips suitable for integration into commercial scanning probe microscopes, allowing scanning Hall microscopy (SHM) under ambient conditions and controlled sensor-sample distance. The height dependent stray field distribution of a magnetic scale was characterized using a 5 $μ$m gold Hall sensor. The uncertainty of the entire Hall sensor based scanning and data acquisition process was analyzed allowing traceably calibrated SHM measurements. The measurement results show good agreement with numerical simulations within the uncertainty budget.

physics.ins-det↗

arXiv2020

Silicon carbide stacking-order-induced doping variation in epitaxial graphene

Generally, it is supposed that the Fermi level in epitaxial graphene is controlled by two effects: p-type polarization doping induced by the bulk of the hexagonal SiC(0001) substrate and overcompensation by donor-like states related to the buffer layer. In this work, we evidence that this effect is also related to the specific underlying SiC terrace. We fabricated a periodic sequence of non-identical SiC terraces, which are unambiguously attributed to specific SiC surface terminations. A clear correlation between the SiC termination and the electronic graphene properties is experimentally observed and confirmed by various complementary surface-sensitive methods. We attribute this correlation to a proximity effect of the SiC termination-dependent polarization doping on the overlying graphene layer. Our findings open a new approach for a nano-scale doping-engineering by self-patterning of epitaxial graphene and other 2D layers on dielectric polar substrates.

cond-mat.mtrl-sci↗

arXiv2020

Substrate induced nanoscale resistance variation in epitaxial graphene

Graphene, the first true two-dimensional material still reveals the most remarkable transport properties among the growing class of two-dimensional materials. Although many studies have investigated fundamental scattering processes, the surprisingly large variation in the experimentally determined resistances associated with a localized defect is still an open issue. Here, we quantitatively investigate the local transport properties of graphene prepared by polymer assisted sublimation growth (PASG) using scanning tunneling potentiometry. PASG graphene is characterized by a spatially homogeneous current density, which allows to analyze variations in the local electrochemical potential with high precision. We utilize this possibility by examining the local sheet resistance finding a significant variation of up to 270% at low temperatures. We identify a correlation of the sheet resistance with the stacking sequence of the 6H-SiC substrate as well as with the distance between the graphene sheet and the substrate. Our results experimentally quantify the strong impact of the graphene-substrate interaction on the local transport properties of graphene.

cond-mat.mes-hall↗

arXiv2018

Homogeneous Large-area Quasi-freestanding Monolayer and Bilayer Graphene on SiC

In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the SiC surface. The proper choice of a set of growth parameters allows the growth of defect-free, ultra-smooth and coherent graphene-free buffer layer and bilayer-free monolayer graphene sheets which can be transformed into large-area high-quality quasi-freestanding monolayer and bilayer graphene (QFMLG and QFBLG) by hydrogen intercalation. AFM, scanning tunneling microscopy (STM), Raman spectroscopy and electronic transport measurements underline the excellent homogeneity of the resulting quasi-freestanding layers. Electronic transport measurements in four-point probe configuration reveal a homogeneous low resistance anisotropy on both μm- and mm scales.

cond-mat.mtrl-sci↗

arXiv2017

Tailoring the SiC surface - a morphology study on the epitaxial growth of graphene and its buffer layer

We investigate the growth of the graphene buffer layer and the involved step bunching behavior of the silicon carbide substrate surface using atomic force microscopy. The formation of local buffer layer domains are identified to be the origin of undesirably high step edges in excellent agreement with the predictions of a general model of step dynamics. The applied polymer-assisted sublimation growth method demonstrates that the key principle to suppress this behavior is the uniform nucleation of the buffer layer. In this way, the silicon carbide surface is stabilized such that ultra-flat surfaces can be conserved during graphene growth on a large variety of silicon carbide substrate surfaces. The analysis of the experimental results describes different growth modes which extend the current understanding of epitaxial graphene growth by emphasizing the importance of buffer layer nucleation and critical mass transport processes.

cond-mat.mtrl-sci↗

arXiv2016

Comeback of epitaxial graphene for electronics: large-area growth of bilayer-free graphene on SiC

We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultra-smooth defect- and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of polymer adsorbates which act as a carbon source. With most of the steps well below 0.75 nm pure monolayer graphene without bilayer inclusions is formed with lateral dimensions only limited by the size of the substrate. This makes the polymer assisted sublimation growth technique the most promising method for commercial wafer scale epitaxial graphene fabrication. The extraordinary electronic quality is evidenced by quantum resistance metrology at 4.2 K with until now unreached precision and high electron mobilities on mm scale devices.

cond-mat.mtrl-sci↗

arXiv2014

Design and Realization of an S-Band Microwave Low-Noise Amplifier for Wireless RF Subsystems

This study undertakes the theoretical design, CAD modeling, realization, and performance analysis of a microwave low-noise amplifier (LNA) which has been accurately developed for operation at 3.0 GHz (S-band). The objective of this research is to thoroughly analyze and develop a reliable microstrip LNA intended for a potential employment in wireless communication systems, and satellite applications. The S-band microwave LNA demonstrates the appropriateness to develop a high-performance and well-established device realization for wireless RF systems. The microwave amplifier simulations have been conducted using the latest version of the AWR Design Environment software.

cs.OH↗