First-principles study of the impact of chemical doping and functional groups on the absorption spectra of graphene

The rational design of the electronic band structures and the associated properties (e.g., optical) of advanced materials has remained challenging for crucial applications in optoelectronics, solar desalination, advanced manufacturing technologies, etc. In this work, using first-principles calculations, we studied the prospects of tuning the absorption spectra of graphene via defect engineering, i.e., chemical doping and oxidation. Our computational analysis shows that graphene functionalization with single hydroxyl and carboxylic acid fails to open a band gap in graphene. While single epoxide functionalization successfully opens a bandgap in graphene and increases absorptivity, however, other optical properties such as reflection, transmission, and dielectric constants are significantly altered. Boron and nitrogen dopants lead to p- and n-type doping, respectively, while fluorine dopants or a single-carbon atomic vacancy cannot create a significant bandgap in graphene. By rigorously considering the spin-polarization effect, we find that titanium, zirconium, and hafnium dopants can create a bandgap in graphene via an induced flat band around the Fermi level as well as the collapse of the Dirac cone. In addition, silicon, germanium, and tin dopants are also effective in improving the optical characteristics. Our work is important for future experimental work on graphene for laser and optical processing applications.

Suppressing Diffraction-Related Intensity Losses in Transmissive Single-Crystal X-ray Optics

The highest-quality X-ray optics can be made of single-crystal materials such as silicon, germanium, or, even better, diamond. Unfortunately, such X-ray optics have one drawback: diffraction losses or the “glitch effect”. This effect manifests itself as follows: at some energies of X-rays, the intensity of the transmitted beam drops due to the fact that some crystalline planes have satisfied the diffraction condition. Diffraction losses are usually observed in spectroscopic experiments when the energy of the X-rays changes in a certain range. However, this effect might also influence any experiment using X-rays, especially at higher energies. In this paper, we propose a method to overcome the glitch problem in transmissive optics. This is achieved using small rotations of the optical element. We describe the algorithm for “glitch-free” measurements in detail and the theory behind it.

Investigation on the performance dependence of proton radiated SiGe HBTs with emitter area and temperature

The DC and AC performances of proton radiated Silicon-Germanium (SiGe) Heterojunction Bipolar Transistors (HBTs) with different emitter areas at liquid nitrogen temperature (77 K), room temperature and heating hotplate (393 K) were presented in this work. Performance dependence on the emitter area and temperature was investigated. Results showed that SiGe HBTs with a large emitter area had more damage by proton radiation. Furthermore, the SiGe HBTs showed better tolerance to proton radiation at extreme temperatures than at room temperature. To reveal the underlying mechanism, the radiated SiGe HBTs were modeled based on the device structure and parameters. The electron density, Shockley–Read–Hall (SRH) recombination and carrier mobility were extracted from the device model and demonstrated to have major impacts on the performance dependence of the radiated SiGe HBTs. The results provide useful guidance for the application of SiGe HBTs at extreme environments.

A 20–44 GHz Wideband LNA Design Using the SiGe Technology for 5G Millimeter-Wave Applications

This paper presents the design and implementation of a low-noise amplifier (LNA) for millimeter-wave (mm-Wave) 5G wireless applications. The LNA was based on a common-emitter configuration with cascode amplifier topology using an IHP’s 0.13 μm Silicon Germanium (SiGe) heterojunction bipolar transistor (HBT) whose f_T/f_MAX/gate-delay is 360/450 GHz/2.0 ps, utilizing transmission lines for simultaneous noise and input matching. A noise figure of 3.02–3.4 dB was obtained for the entire wide bandwidth from 20 to 44 GHz. The designed LNA exhibited a gain (S_21) greater than 20 dB across the 20–44 GHz frequency range and dissipated 9.6 mW power from a 1.2 V supply. The input reflection coefficient (S_11) and output reflection coefficient (S_22) were below −10 dB, and reverse isolation (S_12) was below −55 dB for the 20–44 GHz frequency band. The input 1 dB (P1dB) compression point of −18 dBm at 34.5 GHz was obtained. The proposed LNA occupies only a 0.715 mm2 area, with input and output RF (Radio Frequency) bond pads. To the authors’ knowledge, this work evidences the lowest noise figure, lowest power consumption with reasonable highest gain, and highest bandwidth attained so far at this frequency band in any silicon-based technology.

Optical Switch Performance for Electronic Applications

In this paper, various optically controlled switch performances are studied and analysed. Different types of optically controlled materials are considered in this paper to control the electrical frequency with a modified and simpler structure. Roger and Fr-4 materials are used as substrate material for the basic system design with the dielectric constant of 3.0 and 4.4 respectively. Materials like silicon, germanium, graphene, and polymers are considered for analysis in the proposed system and the semiconductor metals are etched in the middle of the copper strip. The thickness of the copper is 0.008 mm with the standard conductivity of 5.814e7 S/m and the length and width of the copper strip are 2.54x2.54mm. A Copper strip is printed on the substrate to test the performance of the switches. The operating frequency of the given optical signal of silicon is around 1GHz to 25GHz, graphene is around 1GHz to 30GHz, germanium is around1GHz to 30GHz and polymer is around 1GHz to25GHz. The S11 parameter of all the proposed systems is analysed by sonnet simulation software.

Annealing Effect on Seebeck Coefficient of SiGe Thin Films Deposited on Quartz Substrate

Over the past few years, thermoelectrics have gained interest with regard to thermoelectricity interconversion. The improvement in the efficiency of the thermoelectric material at an ambient temperature is the main problem of research. In this work, silicon–germanium (SiGe) thin films, owing to superior properties such as nontoxicity, high stability, and their integrability with silicon technologies, were studied for thermoelectric applications. P-type SiGe thin films were deposited on quartz substrates by DC/RF magnetron sputtering and annealed at three different temperatures for 1 hour. Significant enhancement in the Seebeck coefficient was achieved for the sample annealed at 670 °C. A high power factor of 4.1 μWcm−1K−2 was obtained at room temperature.

Integrated structural reconstruction of unit structures of the meta-stable grain boundaries in diamond-structured materials presents first-order like phase transition.

One of the important issues of studying grain boundaries (GBs) which has recently attracted increasing interests is to investigate the phase behavior of GBs that one GB with determined disorientation and plane orientation (known as macroscopic parameters) can exist as distinct phases and perform phase transition. While such an issue has been investigated in fcc and bcc metals, GB phases in other elemental materials have not been reported. This work by applying molecular dynamics (MD) simulation explored totally around 7000 meta-stable GB phases of the <110>∑9(22‾1‾) symmetric tilt GB of silicon, germanium and diamond carbon as diamond-structured elemental materials. Meta-stable phases commonly exist in different elements were discovered and some of them were successfully verified to be reasonable by first-principle simulation. The verified meta-stable GBs were subsequently proved to have different capability to transform to the ground-stable GB at elevated temperature under MD simulation and to perform different pre-melting behaviors. We discovered a bi-directional structural reconstruction mechanism of the unit structure belonging to one of the verified meta-stable phases, by which the unit structures can transform to identical unit structures of the ground-stable GB which can present ‘opposite orientation’. Through computing the kinetic barriers by nudged-elastic-band and annealing simulation using MD, the integral behavior of the unit structures’ reconstruction is found to be a first-order like phase transition. Our work extended the research on GB phases from metals to diamond-structured materials and discovered a new GB phase transition mechanism which has not been reported before.