First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

First-principles calculations for defects and impurities: Applications to III-nitrides

The development of a compact, solid-state light-emitting diode (LED) that emits at 210 nanometres — the shortest wavelength yet achieved for any type of LED — represents an important step towards achieving exciton-related light-emitting devices and replacing inefficient gas light sources with solid-state light sources. Compact high-efficiency ultraviolet solid-state light sources1—such as light-emitting diodes (LEDs) and laser diodes—are of considerable technological interest as alternatives to large, toxic, low-efficiency gas lasers and mercury lamps. Microelectronic fabrication technologies and the environmental sciences both require light sources with shorter emission wavelengths: the former for improved resolution in photolithography and the latter for sensors that can detect minute hazardous particles. In addition, ultraviolet solid-state light sources are also attracting attention for potential applications in high-density optical data storage, biomedical research, water and air purification, and sterilization. Wide-bandgap materials, such as diamond2 and III–V nitride semiconductors (GaN, AlGaN and AlN; refs 3–10), are potential materials for ultraviolet LEDs and laser diodes, but suffer from difficulties in controlling electrical conduction. Here we report the successful control of both n-type and p-type doping in aluminium nitride (AlN), which has a very wide direct bandgap11 of 6 eV. This doping strategy allows us to develop an AlN PIN (p-type/intrinsic/n-type) homojunction LED with an emission wavelength of 210 nm, which is the shortest reported to date for any kind of LED. The emission is attributed to an exciton transition, and represents an important step towards achieving exciton-related light-emitting devices as well as replacing gas light sources with solid-state light sources.

An aluminium nitride light-emitting diode with a wavelength of 210 nanometres

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Materials emitting light in the deep ultraviolet region around 200 nanometers are essential in a wide-range of applications, such as information storage technology, environmental protection, and medical treatment Hexagonal boron nitride (hBN), which was recently found to be a promising deep ultraviolet light emitter, has traditionally been synthesized under high pressure and at high temperature We successfully synthesized high-purity hBN crystals at atmospheric pressure by using a nickel-molybdenum solvent The obtained hBN crystals emitted intense 215-nanometer luminescence at room temperature This study demonstrates an easier way to grow high-quality hBN crystals, through their liquid-phase deposition on a substrate at atmospheric pressure

http://science.sciencemag.org/content/sci/317/5840/932.full.pdf

Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure

Light-emitting diodes with emission wavelengths less than 400 nm have been developed using the AlInGaN material system. For devices operating at shorter wavelengths, alloy compositions with a greater aluminium content are required. The material properties of these materials lie on the border between conventional semiconductors and insulators, which adds a degree of complexity to the development of efficient light-emitting devices. A number of technical developments have enabled the fabrication of LEDs based on group three nitrides (III-nitrides) that emit in the UV part of the spectrum, providing useful tools for a wealth of applications in optoelectronic systems.

Ultraviolet light-emitting diodes based on group three nitrides

We have obtained n-type conductive Si-doped AlN and AlXGa1−XN with high Al content (0.42⩽x<1) in metalorganic vapor phase epitaxy by intentionally controlling the Si dopant density, [Si]. Si-doped AlN showed the n-type conduction when [Si] was less than 3×1019 cm−3. When [Si] was more than 3×1019 cm−3, it became highly resistive due to the self-compensation of Si donors. This indicates that the self-compensation plays an important role at higher [Si] and determines the upper doping limit of Si for the AlN and AlXGa1−XN. For x⩾0.49, the ionization energy of Si donors increased sharply with increasing Al content. These resulted in a sharp decrease in the highest obtainable electron concentration with increasing Al content for the Si-doped AlXGa1−XN.

Intentional control of n-type conduction for Si-doped AlN and AlXGa1−XN(0.42⩽x<1)

AlN/GaN short-period superlattices (SLs) is experimentally shown to have a different polarization property from AlGaN. As the GaN well thickness decreases from 2.5 to 0.9 monolayers, the emission wavelength decreases from 275.8 to 236.9 nm due to a quantum size effect. Because the quantized energy level for holes originates from the heavy hole band of GaN, the emission is polarized for electric field perpendicular to the c-axis (E⊥c). Consequently, the SLs show intense C-plane emission compared with AlGaN, whose emission is inherently polarized for electric field parallel to the c-axis (E||c). Using the SLs, we demonstrate a E⊥c-polarized deep-ultraviolet (UV) light-emitting diode (LED).

Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices

For n-type Si-doped AlN, we have obtained an electron mobility and concentration of 125cm2V−1s−1 and 1.75×1015cm−3 at 300K, respectively. At 250K, the mobility reached the maximum of 141cm2V−1s−1. To explain the temperature dependence of the mobility, we calculated mobilities limited by specific scattering mechanisms. We found that the mobility is limited by neutral impurity scattering rather than ionized impurity scattering or lattice scattering because of a large donor ionization energy (∼250meV).

Electrical conduction properties of n-type Si-doped AlN with high electron mobility (>100cm2V−1s−1)

(112¯0) A-plane AlN p–n junction light-emitting diode (LED) with a wavelength of 210 nm is demonstrated. The electroluminescence from the A-plane LED is inherently polarized for the electric field parallel to the [0001] c-axis due to a negative crystal-field splitting energy. The polarization ratio (electric-field component ratio of parallel and perpendicular to c-axis) is as high as 0.9. The radiation pattern of the A-plane LED shows higher emission intensity along the surface normal, while that of a conventional (0001) C-plane LED shows lower emission intensity along the surface normal. The different radiation patterns can be explained by the polarization property.

Yoshitaka Taniyasu

Papers

An aluminium nitride light-emitting diode with a wavelength of 210 nanometres

Intentional control of n-type conduction for Si-doped AlN and AlXGa1−XN(0.42⩽x<1)

Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices

Electrical conduction properties of n-type Si-doped AlN with high electron mobility (>100cm2V−1s−1)

Surface 210 nm light emission from an AlN p–n junction light-emitting diode enhanced by A-plane growth orientation