Logotyp: till Uppsala universitets webbplats

The integration of single-layer graphene with diamond substrates offers a promising platform for highperformance electronic devices by utilizing the exceptional properties of both materials. This study describes a fabrication process and transport measurements of single-layer graphene devices on diamond substrates featuring two surface terminations: hydrogen (H-terminated, thermal process) and oxygen (O-terminated, plasma treatment). The carrier transport properties were investigated using Hall effect measurements over a broad temperature range (80-400 K) under high-vacuum conditions (1 x 10^-4 mbar). Our findings reveal that thermal annealing significantly improves the graphene-diamond interface quality, causing a notable increase in carrier mobility for devices on both H- and O-terminated from 1439 to 1644 cm²/Vs and from 1238 to 1340 cm²/Vs, respectively. We also found that the effect of remote interfacial phonon scattering on high-temperature mobility is affected by the termination type. These findings highlight the importance of substrate surface engineering and offer a pathway for optimizing graphene-diamond heterostructures for advanced electronic applications.

Understanding the charge carrier properties and the effects of local scattering mechanisms in semiconductor materials is essential to realize reliable electronic devices. In this study, Monte Carlo simulations on hole transport in diamond were performed including multiple scattering mechanisms and the results are compared with experimental data observed by hole time-of-flight (ToF) measurements. By incorporating interband scattering, the deformation potential for acoustic phonon scattering was extracted. Furthermore, the redistribution of the heavy and light hole populations was recorded, and their dynamic behavior was analyzed. Moreover, detailed analysis uncovered distinct transport behaviors under complex scattering mechanisms, predominantly driven by optical phonon interactions, consistent with experimental observations.

Understanding the electrically active defects and impurities in semiconductors, especially in intrinsic or unintentionally doped wide bandgap materials, still remains a challenge. Here, time-of-flight (ToF) measurement using a solid state light source (355 and 213 nm) was performed on intrinsic silicon carbide and single-crystalline diamond. The charge transient spectroscopy (QTS) and the inverse Laplace (IL) QTS methods were applied to analyze the ToF results. Using these methods, we were able to trace the existing impurities in both materials. However, ILQTS proved to be more sensitive, with higher resolution for detection of existing multiple defects. The results suggest that this system can successfully be employed to investigate electrically active impurities at different energy states in highly resistive and undoped materials.

The high current-carrying capacity of graphene is essential for its use as an interconnect in electronic and spintronic circuits. At the same time, knowing the breakdown limits and mechanism under high fields can enable new device design strategies. In this work, we push the current carrying capacity of the scalable form of chemical vapor deposited (CVD) graphene employing a high-thermal conducting single crystalline diamond substrate. Our experiments on CVD graphene reveal extremely high current densities > 10⁹ A/cm² in graphene on the diamond with both ohmic (low-resistive) and tunneling tunnel (high-resistive) contacts. Measurements on ferromagnetic (TiO_x/Co) and metallic (Ti/Au) contacts demonstrate current densities of ∼1.16×10⁹ A/cm² and ∼1.7×10⁹ A/cm², respectively. The tunnel (high-resistive) contacts exhibit a shunting of graphene under high currents via the bottom graphitized diamond, resulting in dielectric breakdown and via alternative conducting paths. Electrical measurements show a distinct threshold for conducting paths of graphitized diamond, in tune accordance with Middleton-Wingreen's theory. Our results of high current densities achieved in CVD graphene, with distinct dependence on ohmic and tunneling, contact resistance, and the observed breakdown mechanism, provide new insights for enabling high-current all carbon circuits.

Graphene on diamond has emerged as a promising platform for various electronic applications. This brief review article explores the recent advancements and the potential of graphene on diamond for electronic applications with a focus on single-crystal (SC) chemically vapor-deposited and high-pressure and high-temperature diamond. Device fabrication techniques, properties, and performance of single-layer graphene on diamond in various electronic devices are discussed. This hybrid system's challenges and prospects are also analyzed. A particular emphasis is placed on the unique benefits of diamond as a substrate for graphene and its growth, including its high thermal conductivity, mechanical strength, high optical phonon energy, and the importance of achieving high-quality single-layer graphene on SC diamond.

Spiral hollow cathodes represent interesting options for local PVD applications. Radio frequency powered smalldiameter spiral hollow cathodes made from 0.45 mm diameter Ta wire rolled around 0.5 mm diameter rod weretested in PVD regimes on silicon substrates at the gas pressure of 400 Pa (3 Torr). The PVD of Ta and reactivePVD of Ta-N resulted in deposition rates of about 130 nm/min with maximum thickness in the center of thecoating spots. However, part of the coating spots can be heavily eroded. At higher RF powers droplets from themelted Ta tip of the spiral can damage the coating and melt the Si substrate. The PVD rates of Ta in argon weresimilar as those for TaN. However, lower number of droplets of the melted Ta were formed in argon. The heatingof the spiral outlet and its effect on the coating was also more intense in nitrogen than in argon. The temperatureof the Si substrate table reached about 500 ◦C in 20 min in the nitrogen plasma and up to 400 ◦C in argon. Thisheating was higher on electrically grounded substrates than on the floating substrates. The effect of sharp outleton possible eroding of the sample was confirmed by a sharp ended 1 mm diameter stainless steel medical needleused as a hollow cathode.

The outstanding electronic properties of graphene make this material a candidate for many applications, for instance, ultra-fast transistors. However, self-heating and especially the detrimental influence of available supporting substrates have impeded progress in this field. In this study, we fabricate graphene-diamond heterostructures by transferring graphene to an ultra-pure single-crystalline diamond substrate. Hall-effect measurements were conducted at 80 to 300 K on graphene Hall bars to investigate the charge transport properties in these devices. Enhanced hole mobility of 2750 cm(2) V-1 s(-1) could be observed at room-temperature when using diamond with reduced nitrogen (N-s(0)) impurity concentration. In addition, by electrostatically varying the carrier concentration, an upper limit for mobility is determined in the devices. The results are promising for enabling carbon-carbon (C-C) devices for room-temperature applications.

Using the state of valley-polarization of electrons in solids is a promising new paradigm for information storage and processing. The central challenge in utilizing valley-polarization for this purpose is to develop methods for manipulating and reading out the final valley state. Here, we demonstrate optical detection of valley-polarized electrons in diamond. It is achieved by capturing images of electroluminescence from nitrogen-vacancy centers at the surface of a diamond sample that are excited by electrons drifting and diffusing through the sample. Monte Carlo simulations are performed to interpret the resulting experimental diffusion patterns. Our results give insight into the drift-diffusion of valley-polarized electrons in diamond and yield a way of analyzing the valley-polarization of ensembles of electrons.

Although theoretical investigations indicate that the successful combination of graphene and diamond would give interesting properties, only a limited number of reports dealing with the subject have been published. Here, we present a rapid thermal process (RTP) which involves nickel (Ni) as metal catalyst for a direct growth of graphene on diamond at a temperature of 1073 K for 60 s. This process operates with a combination of a lower temperature and for a shorter duration than what has previously been reported. Thin Ni films of different thicknesses were deposited on top of (100) single-crystalline diamond. After RTP, the coverage of monolayer graphene was found to be around 20% shown by the intensity ratio between the 2D- and G-peak using Raman spectroscopy on 50 nm thick Ni films. In addition, x-ray photoelectron spectroscopy and atomic force microscopy analysis were conducted. For electrical characterization, Hall-effect measurements were performed at temperatures between 80 and 360 K.

The interaction between acoustic phonons and electrons in diamond has been investigated by comparing state-of-the-art time-of-flight drift velocity measurements with Monte Carlo simulations. We use a multivariable anisotropic description of acoustic deformation potential scattering. The phonon-electron interaction is the limiting factor for the carrier mobility in ultrapure single crystal diamond. Hence, having a correct description is necessary for both device simulations and for predicting the maximum device performance. The experiments were performed at low temperature and using ultrapure diamond to minimize the influence of other scattering sources. The electronic valley polarization in diamond at low temperatures enables determination of both uniaxial and dilatation deformation potentials in the same experiment. The uniaxial and dilatation deformation potentials are found to be 18.5±0.2 and −5.7±0.3 eV, respectively.