Through my interdisciplinary graduate work, I have attended conferences organized by various professional societies such as the Materials Research Society (MRS), American Physical Society (APS), Optical Society of America (OSA), and American Chemical Society (ACS). I have also participated in some of their employment workshops and have searched their websites for employment resources. This article elaborates on information and services that professional societies and organizations can provide to ease employment search for their student membership.

Career Workshops. These workshops provide excellent guidance, especially for students initiating their employment search. Obviously, the best ones are conducted by people who recruit PhD graduates. I attended an employment workshop organized by the American Institute of Physics (AIP) during the 1995 MRS Fall Meeting. An extremely dynamic hightechnology industrial recruiter conducted the workshop. The mock interviews, mininetworking groups, and resume and cover letter critique involved audience participation and were highly effective. I strongly recommend such a workshop. On the other hand, I have also been frustrated with workshops in which speakers did not have a technical PhD background since I often found their suggestions unsuitable.

Employment Database. A website with current job postings mainly for PhD graduates is extremely useful. The National Academy of Science (http://www2.nas.edu) is among the few sources for this. The critical issue in this case is to have a well-maintained site with frequent updates and a search engine. Another alternative, which we have been using at the University of Rochester, is to circulate job postings via electronic mail. Again, timeliness is the key issue.

The science of materials as a specific discipline is a relatively modern development. Within this rather modern field, the study of semiconductor materials is an even more recent development. Modern textbooks on materials science focus primarily on the properties of metals and second on the properties of ceramics, reflecting the commercial importance of these materials 50 years ago when the transistor was demonstrated using germanium. Today the commercial importance of semiconductor materials is comparable to that of metals and ceramics, and the advances in semiconductormaterials technology are driving rapidly growing business sectors in communications, software, and biotechnology.

For many years, it was believed that when two isovalent semiconductors are mixed, they will phase-separate (like oil and water) at low temperature, they will form a solid solution (like gin and tonic) at high temperatures, but they will never produce ordered atomic arrangements. This view was based on the analysis of the solid-liquid equilibria at high temperatures and on empirical observation of phase separation at low temperatures. These observations were further rationalized and legitimized by applying the classic (Hildebrand) solution models, which predicted just this type of behavior. These models showed that the observed behavior of the AxB1 − x alloys implied a positive excess enthalpy ΔH(x) = E(x) − xE(A) − (1 − x)E(B) (where E is the total energy) and that this positiveness (“repulsive A-B interactions”) resulted from the strain energy attendant upon packing two solids with dissimilar lattice constants. The larger the lattice mismatch, the more difficult it was to form the alloy. Common to these approaches (“regular solution theory,” “quasiregular solution theory,” “delta lattice-parameter model,” etc.) was the assumption that the enthalpy ΔH(x) of an alloy depends on its global composition x but not on the microscopic arrangement of atoms (e.g., ordered versus disordered). Thus, ordered and disordered configurations at the same compo sition x were tacitly assumed to have the same excess enthalpy ΔH(x). Clearly the option for ordering was eliminated at the outset. While these theories served to produce very useful depictions of the immiscibility of many semiconductor alloys (and continue to guide strategies of crystal growth), they also cemented the paradigm that semiconductor alloys don't order, they just phase-separate. This was true, at the time.

Semiconductor alloys have become increasingly useful during the last four decades because, through the use of alloys, the properties of semiconductors can be tailored by varying the composition to precisely match the requirements for specific electronic and photonic devices. In addition the use of alloys allows the production of special structures, such as quantum wells, that require rapid changes in bandgap energy during growth. This has led to so-called “bandgap engineering,” in which device designers and epitaxial growers are working together to produce structures having virtually atomic-scale dimensions.

In 1916 a group of Russian chemists—Kurnakov and his colleagues—discovered that slowly cooled CuxAu1 − xmetal alloys had anomalously low electrical resistivities at simple compositions of CuAu and Cu3Au. Nine years later in 1925, Swedish physicists Johansson and Linde found by x-ray-diffraction experiments that the alloys had ordered structures on the face-centered-cubic lattice, now called CuAu I-type and AuCu3-type. Two years later, Johansson and Linde discovered CuPt-type ordering in Cu0.5Pt0.5 alloy by noticing a similar anomaly in their electrical-resistivity measurements for CuxPt1 − x. These were among the events at the dawn of physical metallurgy and study on order-disorder transformations. Many years later in 1985, the first observation of ordering in semiconductor alloys was made in AlGaAs grown on GaAs(110); the type was CuAu I. CuPt-type ordering was observed in SiGe grown on Si(001). Since the (110) orientation of the substrate for AlGaAs was clumsy for practical purposes and the degree of ordering in SiGe was extremely low, these phenomena seemed to have little relevance in practical applications. Other observations were made such as famatinite-type ordering in GalnAs and chalcopyrite-type ordering in GaAsSb. These observations however were quite rare, with only isolated cases reported.

The application of III-V semiconductor alloys in device structures is of importance for high-speed microelectronics and optoelectronics. These alloys have allowed the device engineer to tailor material parameters such as the bandgap and carrier mobility to the need of the device by altering the alloy composition. When using ternary or quaternary materials, the device designer presumes that the alloy is completely disordered, without any correlation between the atoms on the cation (anion) sublattice. However the thermodynamics of the alloy system often produce material that has some degree of macroscopic or microscopic ordering. Short-range ordering occurs when atoms adopt correlated neighboring positions over distances of the order of a few lattice spacings. This can be manifested as the preferential association of like atoms, as in clustering, or of unlike atoms, as in chemical ordering (e.g., CuPt ordering). Long-range ordering occurs over many tens of lattice spacings, as in the case of phase separation. In either short-range or long-range ordering, the band structure and the crystal symmetry are greatly altered. Therefore it is absolutely critical that the mechanisms be fully understood to prevent ordering when necessary or to exploit it when possible.

The group-III-N materials (GaN and its related alloys, InGaN and AlGaN) are a rapidly emerging family of compound semiconductors that have great potential for a wide range of optoelectronics and electronics applications.

GaN-based blue light-emitting diodes (LEDs) are commercially available and can already be found in full-color, billboard-size displays in major cities in Japan. Achievement of continuous-wave (cw) laser operation at blue wavelengths will lead to the commercialization of these coherent sources probably within the next few years. Like other semiconductor lasers, these devices can be manufactured relatively cheaply (eventually for a few dollars), and they are expected to replace current ($1,000 and up) ultraviolet and blue-laser sources in medical spectroscopy, high-density information storage, and in projection displays.

This article reviews the key role of the crystal structure of GaN grown on sapphire, which is currently the most widely used substrate. We trace the origin of this crystal structure from nucleation and show how it evolves as growth proceeds. We show that the final GaN film is an ordered polycrystal (rather than a single crystal), in which the individual grains are of high crystalline quality but are “tilted” and “twisted” with respect to one another. We conclude by speculating on how this microstructure may explain the unusual optical and electrical properties of GaN.