The use of nuclear fission to generate power results in waste materials that can be hazardous to people and to other life forms, particularly in the first few hundred years after the wastes are created. There is a worldwide consensus that the management of these wastes must not place undue burdens on future generations. In some countries, the opinion is openly stated that those who enjoyed the benefits of the power that produced the wastes must be responsible for taking care of the wastes. Opinions diverge when discussion turns to the question of whether the wastes will always be a liability or eventually become an asset.

Another point of consensus is that at least some of the radioactive wastes should be disposed of by a method that securely isolates them from the accessible environment of plant, animal, and human life until the radioactivity levels have decayed to levels similar to those found in natural environments. Different countries take different positions on the details of what the activity level might be that constitutes the end of moral responsibility for the wastes, but the general position is basically the same worldwide. I explore some concerns about how disposal containers and waste repositories might be marked with suitable identification. In particular, I examine whether the message to be conveyed by the markings needs to change with time and with the entity examining the objects, and whether we can design markers that will convey this potentially variable message.

The addition of GaN, A1N, InN, and related alloys to the family of device-quality semiconductors has opened up new opportunities in short-wavelength (visible and ultraviolet [uv]) photonic devices for display and data-storage applications, solar-blind uv detectors, and high-temperature/high-power electronics. Silicon will of course continue to dominate in microelectronics applications, and InP and GaAs and their related alloys will be the mainstays of long-wavelength lightwave communication systems and red, orange, and yellow light-emitting-diode (LED) technology, respectively. There are however many existing and emerging uses for wide-bandgap semiconductors with good electrical and optical characteristics. The purpose of this issue of MRS Bulletin is to furnish a background and summary on the exciting new developments involving GaN and related materials.

Strong efforts on the synthesis and device aspects of GaN took place in the 1960s and 1970s because of the potential for realization of blue lasers and LEDs that would extend the existing wavelength range of photonic devices. Progress was hampered because of several severe materials problems. First there was no bulk crystal growth technology for producing substrates, and epitaxial material was grown on highly lattice-mismatched substrates such as sapphire. This heteroepitaxial material was invariably highly conducting because of residual shallow donor defects or impurities. These high n-type backgrounds, combined with the relatively deep ionization levels of all of the common p-type dopant impurities, prevented the achievement of p-type doping and therefore of bipolar or injection devices.

A1N, GaN, and InN are very promising materials for use in optoelectronic and high-temperature electronic devices. These materials and their ternary and quaternary alloys cover an energy bandgap range of 1.9–6.2 eV, suitable for band-to-band light generation with colors ranging from red to ultraviolet (uv), with wavelengths ranging from 650 to 200 nm. On the device front, they are suitable for example for negative electron-affinity cold cathodes, electronic devices, surface acoustic wave devices, uv detectors, Bragg reflectors and waveguides, uv and visible light-emitting diodes (LEDs), and laser diodes (LDs) for digital data read-write applications. Stifled by the absence of native substrates, growth and doping of high-quality III-V-nitride thin films, particularly p-type, have been major obstacles for developing GaN-based devices. Development of electronic devices such as modulation-doped field-effect transistors (MODFETs) and opto-electronic devices such as LEDs and LDs has also proven challenging.

Short-wavelength-emitting devices, such as blue laser diodes (LDs) and light-emitting diodes (LEDs), are currently sought for a number of applications, including full-color electroluminescent displays, laser printers, read-write laser sources for high-density information storage on magnetic and optical media, and sources for undersea optical communications. For these purposes, II–VI materials such as ZnSe and SiC, and III–V-nitride semiconductors such as GaN have been investigated intensively for a long time. However it was impossible to obtain high-brightness (over 1 cd) blue LEDs and reliable LDs. Much progress has been achieved recently on green LEDs and LDs using II–VI-based materials. The short lifetimes prevent II–VI-based devices from commercialization at present. The short lifetime of these II-VI-based devices may be caused by the crystal defects at a density of 103/cm2 because one crystal defect would cause the propagation of other defects leading to failure of the devices. Another wide-bandgap material for blue LEDs is SiC. The brightness of SiC blue LEDs is only between 10 mcd and 20 mcd because of the indirect bandgap of this material.

On green LEDs, the external quantum efficiency of conventional, green GaP LEDs is only 0.1% due to the indirect bandgap of this material. The peak wavelength is 555 nm (yellowish green). As another material for green emission devices, AlInGaP has been used. The present performance of green AlInGaP LEDs is an emission wavelength of 570 nm (yellowish green) and maximum external quantum efficiency of 1%.

The recent advances in the material quality of the group-III-nitride semiconductors (GaN, A1N, and InN) have led to the demonstration of high-brightness light-emitting diodes, blue laser diodes, and high-frequency transistors, much of which is documented in this issue of MRS Bulletin. While further improvements in the material properties can be expected to enhance device operation, further device advances will also require improved processing technology. In this article, we review developments in two critical processing technologies for photonic and electronic devices: ion implantation and plasma etching. Ion implantation is a technology whereby impurity atoms are introduced into the semiconductor with precise control of concentration and profile. It is widely used in mature semiconductor materials systems such as silicon or gallium arsenide for selective area doping or isolation. Plasma etching is employed to define device features in the semiconductor material with controlled profiles and etch depths. Plasma etching is particularly necessary in the III-nitride materials systems due to the lack of suitable wet-etch chemistries, as will be discussed later.

Figure 1 shows a laser-diode structure (after Nakamura) where plasma etching is required to form the laser facets that ideally should be vertical with smooth surfaces. The first III-nitride-based laser diode was fabricated using reactive ion etching (RIE) to form the laser facets but suffered from rough mirror facet surfaces that contributed to scattering loss and a high lasing threshold. This is a prime example of how improved material quality alone will not yield optimum device performance.

In this article, we review recent progress in GaN-based photodetectors and field-effect transistors (FETs), including optoelectronic FETs, and discuss materials parameters and fabrication technologies that determine the device characteristics of these two device families. Many types of visible-blind photodetectors and nearly all types of FETs have been demonstrated in GaN-based materials systems. However many challenges remain, both in improving the existing devices—the performance of which is still quite far from reaching its full potential—and in developing entirely new devices, which use unique properties of this wide-bandgap materials system.

The recent developments in III-V-nitride thin-film technology has produced significant advances in high-performance devices operating in the blue and green range of the visible spectrum. These materials are grown by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. Highly specular surfaces are possible by use of low-temperature buffer layers following the method developed by Akasaki et al. The thin films thus grown have an interesting microstructure, quite different from other known semiconductors. In particular, epilayers with high optoelectronic performance are characterized by high dislocation densities, several orders of magnitude above those found in other optoelectronic semiconductor films. The lattice mismatch between sapphire and GaN is ∼14%, and the thermal-expansion difference is close to 80%. In spite of these large differences, little thermal strain is measurable at room temperature in epilayers grown at temperatures above 1000°C. Epitaxy on other systems, like SiC, with much better similarity in lattice parameter and thermal-expansion characteristics, has failed to produce better performance than films grown on sapphire. The origin of these puzzling properties of nitrides on sapphire rests in its microstructure. This article presents a survey of the microstructure associated with epitaxy of nitrides by MOCVD.