Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

인공고관절의 세라믹 볼 헤드의 기계적 안정성 평가를 위한 3 차원 유한요소 해석

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Stimulated Raman scattering of picosecond pulses in barium nitrate crystals

Hall effect, photoluminescence, and Schottky diode measurements were made on the Zn-polar and O-polar faces of undoped, bulk, single crystal, c-axis ZnO wafers. Significant polarity related differences were observed in the PL and Schottky diode characteristics of low carrier concentration, hydrothermally grown wafers. Increased emission from free exciton recombinations and from recombinations between 3.3725 and 3.3750eV was observed on the Zn-polar face. Conversely, emission between 3.3640 and 3.3680eV was more intense on the O-polar face. The barrier heights of silver oxide Schottky diodes were approximately 130meV larger on the Zn-polar face compared to the O-polar face.

Influence of spontaneous polarization on the electrical and optical properties of bulk, single crystal ZnO

Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage (CV) analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices.

Buried p-type layers in Mg-doped InN

Zinc thin films were deposited onto glass substrates using magnetron sputtering in argon or argon and oxygen mixed atmospheres. These films were then oxidized in air at 600 °C for 1 h to form thin oxide films. ZnO films formed by oxidation of Zn thin films deposited in Ar normally had a dense structure with oxide whiskers on the surface. These films had low optical transmittance in visible light region, but exhibited strong UV and weak defect-related emissions. Addition of oxygen into the sputtering gas led to the partial formation of ZnO in the film, further resulting in the formation of porous and highly transparent oxide films, which showed relatively strong defect-related emission with laser excitation at room temperature. The relations between structural and optical properties were then discussed briefly.

Zinc oxide films by thermal oxidation of zinc thin films

We present the structural and photoluminescence properties of 30 keV gadolinium implanted and subsequently annealed zinc oxide (ZnO) single crystals. Rutherford backscattering and channeling results reveal a low surface region defect density which was reduced further upon annealing. For low implantation fluence, around 85% of the Gd atoms are estimated to be in sites aligned with the ZnO lattice, while for higher fluences the Gd is largely disordered and likely forms precipitates. The Raman spectra of the implanted samples show defect-induced modes, which match the one-phonon density of states for the most heavily implanted samples. Annealing eliminates these features implying the removal of Gd-associated lattice disorder. Low temperature photoluminescence spectra revealed a red-shift in the defect emission, from green to orange/yellow, indicating the suppression of a deep level, which is thought to be due to oxygen vacancies. It is suggested that the orange/yellow emission is unmasked when the green emis...

Roger J. Reeves

Papers

Stimulated Raman scattering of picosecond pulses in barium nitrate crystals

Influence of spontaneous polarization on the electrical and optical properties of bulk, single crystal ZnO

Buried p-type layers in Mg-doped InN

Zinc oxide films by thermal oxidation of zinc thin films

Structural and photoluminescence properties of Gd implanted ZnO single crystals