Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

We report the observation of optically pumped lasing in ZnO at room temperature. Thin films of ZnO were grown by plasma-enhanced molecular beam epitaxy on (0001) sapphire substrates. Laser cavities formed by cleaving were found to lase at a threshold excitation intensity of 240 kW cm−2. We believe these results demonstrate the high quality of ZnO epilayers grown by molecular beam epitaxy while clearly demonstrating the viability of ZnO based light emitting devices.

Optically pumped lasing of ZnO at room temperature

InGaN multi-quantum-well (MQW) structure laser diodes (LDs) fabricated from III-V nitride materials were grown by metalorganic chemical vapor deposition on sapphire substrates. The mirror facet for a laser cavity was formed by etching of III-V nitride films without cleaving. As an active layer, the InGaN MQW structure was used. The InGaN MQW LDs produced 215 mW at a forward current of 2.3 A, with a sharp peak of light output at 417 nm that had a full width at half-maximum of 1.6 nm under the pulsed current injection at room temperature. The laser threshold current density was 4 kA/cm2. The emission wavelength is the shortest one ever generated by a semiconductor laser diode.

https://iopscience.iop.org/article/10.1143/JJAP.35.L74/pdf

InGaN-Based Multi-Quantum-Well-Structure Laser Diodes.

Point defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects. The theoretical modeling of point defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of point defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional ($\mathrm{DFT}+U$, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of point defects in real materials.

First-principles calculations for point defects in solids

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.

Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

GaN films were grown on the c‐plane of sapphire substrates by plasma‐enhanced molecular beam epitaxy equipped with an electron‐cyclotron resonance (ECR) source. Sapphire substrates were cleaned by a hydrogen plasma treatment, obviating the need for perilous elevated temperatures. ECR sources are plagued with high energetic ions, particularly at high microwave power levels, where they cause damage in the growing films. To circumvent this problem, we systematically optimized the growth conditions and other pertinent parameters for optimum layer quality. Among the parameters optimized were the magnetic field strength, microwave power, nitrogen over‐pressure, and growth temperature. The quality of the GaN layers were evaluated by electrical and structural measurements as well as observing the surface morphology.

M. E. Lin

Papers

Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

Growth and characterization of GaN on c-plane (0001) sapphire substrates by plasma-enhanced molecular beam epitaxy