The demand for compact ultraviolet laser devices is increasing, as they are essential in applications such as optical storage, photocatalysis, sterilization, ophthalmic surgery and nanosurgery. Many researchers are devoting considerable effort to finding materials with larger bandgaps than that of GaN. Here we show that hexagonal boron nitride (hBN) is a promising material for such laser devices because it has a direct bandgap in the ultraviolet region. We obtained a pure hBN single crystal under high-pressure and high-temperature conditions, which shows a dominant luminescence peak and a series of s-like exciton absorption bands around 215 nm, proving it to be a direct-bandgap material. Evidence for room-temperature ultraviolet lasing at 215 nm by accelerated electron excitation is provided by the enhancement and narrowing of the longitudinal mode, threshold behaviour of the excitation current dependence of the emission intensity, and a far-field pattern of the transverse mode.

Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal.

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Diamond may be grown at low pressures where it is the metastable form of carbon. Recent advances in a wide variety of plasma and electrical discharge methods have led to dramatic increases in growth rates. All of these methods have certain aspects in common, namely, the presence of atomic hydrogen and the production of energetic carbon-containing fragments under conditions that support high mobilities on the diamond surface. Some understanding of the processes taking place during nucleation and growth of diamond has been achieved, but detailed molecular mechanisms are not yet known. Related research has led to the discovery of a new class of materials, the "diamondlike" phases. Vapor-grown diamond and diamondlike materials may have eventual applications in abrasives, tool coatings, bearing surfaces, electronics, optics, tribological surfaces, and corrosion protection.

https://science.sciencemag.org/content/sci/241/4868/913.full.pdf

Low-Pressure, Metastable Growth of Diamond and "Diamondlike" Phases

http://nathan.instras.com/documentDB/paper-69.pdf

Porous silicon: a quantum sponge structure for silicon based optoelectronics

Aqueous HF etching of silicon surfaces results in the removal of the surface oxide and leaves behind silicon surfaces terminated by atomic hydrogen. The effect of varying the solution pH on the surface structure is studied by measuring the SiH stretch vibrations with infrared absorption spectroscopy. Basic solutions ( pH=9–10) produce ideally terminated Si(111) surfaces with silicon monohydride ( 3/4 SiH) oriented normal to the surface. The surface is found to be very homogeneous with low defect density (<0.5%) and narrow vibrational linewidth (0.95 cm−1 ).

Ideal hydrogen termination of the Si (111) surface

Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Formation of SiH bonds on the surface of microcrystalline silicon covered with SiOx by HF treatment

Large area chemical vapour deposition of diamond has been obtained using magneto-microwave plasma. The important point of the developed system is to set the electron cyclotron resonance condition (875 G), where the highest plasma density is expected, at the deposition area by controlling the distribution of an applied magnetic field. Even in 10 Torr where complete electron gyrations cannot be expected, the size of the discharge area controlled by the magnetic field is 70-80 mm in diameter. This value is the largest of all plasma deposition systems of diamond. The crystallinity obtained by the above-mentioned plasma compares favourably with those of the best ones by previous methods.

Large area chemical vapour deposition of diamond particles and films using magneto-microwave plasma

The correlation between free-exciton and bound-exciton recombination has been investigated by cathodoluminescence in high-quality and impurity-controlled chemical-vapor-deposited (CVD) diamond films. The films are formed by [CO(5%)]/[${\mathrm{H}}_{2}$] using microwave-plasma CVD and are doped with ${\mathrm{B}}_{2}$${\mathrm{H}}_{6}$ during deposition. In moderately doped (below 3\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) semiconducting diamonds, the intensity ratio between bound-exciton and free-exciton recombination reflects the boron concentration in the films. It indicates the possibility for a measure of the acceptor concentration in semiconducting diamond. The temperature dependence of the recombination of the excitons bound to neutral acceptors from 80 to 160 K shows the activation energy of 50--60 meV which is comparable to the binding energy of the excitons bound to acceptors. In the same temperature range, the intensity of free-exciton recombination is almost constant due to the high exciton binding energy of 80 meV. As the boron concentration increases, the effective dissociation of the bound excitons becomes low and the emission is stable above 200 K. In heavily doped samples (above ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), the direct recombination without phonon emission from the conduction band to the acceptor level takes place.

Akio Hiraki

Papers

Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Formation of SiH bonds on the surface of microcrystalline silicon covered with SiOx by HF treatment

Large area chemical vapour deposition of diamond particles and films using magneto-microwave plasma

Excitonic recombination radiation in undoped and boron-doped chemical-vapor-deposited diamonds.

Presence of critical Au-Film thickness for room temperature interfacial reaction between Au(film) and Si(crystal substrate)