Whoever said that the steel industry is dead? It is alive and flourishing, this time with an efficiency and environmental cleanliness that will make the semiconductor industry proud! The renaissance is not local, limited to a few developed economies, but global, based on innovations in iron and steel technology and on technology spillover from other areas. The appetite for steel has not decreased: Developing economies such as China and India need millions of tons to build their inadequate and overstretched infrastructure, and the developed countries demand new steels with superior properties for applications in areas previously reserved for other materials. Today the global production of steel is assessed not by mere tonnages but by their quality.

A major attraction of the iron and steel industry is its compatibility with the materials cycle: Iron and steel scrap has become the new raw material, curtailing the global hunger for newly mined iron ores. In a sense, this recycling is an atonement of the excesses of the earlier decades when mining and metallurgical industries paid little attention to energy efficiency and environmental protection. Experts now agree that these and increasing competition from other materials drove the industry to its near extinction. How real then is this renaissance and what are the technology drivers for the growth? Will steel regain its pride of place among materials used by society? In the following article, Alan Cramb reviews the spectacular growth of this industry and predicts an exciting future for it. Silicon, it seems, is not eliminating steel from our materials consciousness!

Chalcopyrite nonlinear optical (NLO) semiconductors are presently enjoying a major renaissance. This rebirth of interest is due primarily to the success of recent materials research-and-development (R&D) programs that have dramatically improved the availability of large crackfree high-quality crystals. This overview provides a general review of chalcopyrites, of their application in laser systems that exploit second-harmonic generation (SHG) or optical parametric oscillation (OPO), and of the materials-selection criteria for laser crystals to assist in focusing R&D efforts. It also suggests broader application areas. The overview concludes with a number of specific recommendations for further R&D efforts to advance this materials technology.

The archetype infrared NLO chalcopyrites are AgGaSe2 (a I-III-VI2 semiconductor) and ZnGeP2 (a II-IV-V2 semiconductor). Using samples of naturally occurring pyrites, Pauling correctly established the chalcopyrite's crystal structure (diamondlike where Zn and Ge cations are ordered) in 1932 after two previous false starts by others. Levine, who has extensively studied the nonlinear susceptibilities of a number of bond types, stated in 1973 that the chalcopyrite structure is so favorable for NLO properties that it will be difficult to ever find materials with larger nonlinearities in the infrared spectral region. That statement has proved to be prophetic.

Goodman of Great Britain first reported that chalcopyrites were semiconductors. However the first observation that these materials were semiconductors is generally attributed to A.F. Ioffe and N. A. Goryunova of the A.F. Ioffe Physico-Technical Institute (IPT) in St Petersburg, Russia.

ZnGeP2 and CdGeAs2 have long been recognized as promising crystals for infrared frequency generation. They exhibit the highest nonlinear optical coefficients (d36 equals 75 pm/V and 236 pm/V for ZnGeP2 and CdGeAs2, respectively) among all known compounds that possess adequate birefringence for phase matching. ZnGeP2's transparency range (0.62−13 μm) makes it the optimum material for shifting the wavelength of 2-μm pump lasers into the 3–5-μm range via optical parametric oscillation (OPO), whereas that of CdGeAs2 (2.3–18 μm) is better suited for doubling the frequency of CO2 lasers (9–11 μm) into the same range via second-harmonic generation. In both cases however, the application of these materials has been hindered by great difficulty in achieving crack-free single crystals, and by large defect-related absorption losses.

The horizontal-gradient-freeze (HGF) growth technique has been instrumental in overcoming these difficulties. “Ultralow” axial gradients (1–3°C/cm) have been used to control stoichiometry by minimizing vapor transport as well as to eliminate cracking due to anisotropic thermal expansion. (The a-axis and c-axis thermal-expansion coefficients of ZnGeP2 differ by a factor of two, whereas those of CdGeAs2 differ by a factor of 15.) In addition, oriented seeds were used to ensure monocrystalline nucleation (because even a small degree of polycrystallinity can lead to cracking even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process.

Silver gallium sulfide (AgGaS2), silver gallium selenide (AgGaSe2), and silver gallium indium selenide (AgGaxIn(1 − x)Se2) are unique nonlinear crystals suitable for a wide range of three-wave mixing applications. They combine strong nonlinear coupling with phase matching across a wide (0.5 − 12.5 μm) transmission range. These properties provide the basis for CO2 laser harmonic-generator (HG), visible (VIS), and near-infrared (NIR) pumped opticalparametric-oscillator (OPO) and sumand difference-frequency-generator (SFG/DFG) systems for producing tunable laser radiation from approximately 0.65 to 12 μm. A wide range of pump lasers is now available, including diode lasers capable of direct pumping of AgGaS2 and efficient diode-pumped lasers for pumping any of these crystals. Significant progress in material characteristics and nonlinear optical (NLO) performance has been accomplished. Bulk absorption for production-quality AgGaS2 is ~0.01 − 0.02 cm across the 0.8−9-μm region. Exceptional crystals of AgGaS2 have absorption <0.0005 cm−1 at 1.064 μm, and even at 0.633 μm, absorption is ~0.015 cm−1. Figure 1 illustrates the late 1980s vintage and present transmission of AgGaS2. For AgGaSe2, absorption for production material ranges from 0.010 cm to 0.018 cm−1 from 1 μm to 11 μm. Certain crystals can have 0.007−0.008 cm−1 absorption at 10.6 μm. The NIR transmission curves for AgGaSe2 and AgGaxIn(1 − x)Se2 appear in Figure 2. This combination of optical properties, along with their good physical characteristics, make these two crystals versatile for NLO applications spanning the VIS to the infrared (ir).

Ternary-chalcopyrite crystals contain a variety of point defects—the most common of which are vacancies, antisite ions, and impurities. Usually these defects are isolated, but they can also appear as complexes involving two or more of the simple defects. Depending on the material, the concentrations of these defects may vary from a few hundred parts per billion to a few hundred parts per million. Many of the point defects in the ternary chalcopyrites have associated optical-absorption bands with significant oscillator strengths. It is these absorption features that become important when the crystals are exposed to intense laser beams during device operation. Even a small amount of absorption will seriously degrade the performance of the device if any of the wavelengths of the various propagating beams happen to overlap an absorption band. This phenomenon can be a problem for both second-harmonic-generator and optical-parametric-oscillator applications. In general the absorption leads to heating of the crystal and results in-thermal lensing (due to temperature dependence of the index of refraction) and dephasing of the beams, and it can ultimately lead to thermal fracturing of the crystal. Thus it is important to develop a fundamental understanding of the defect structure of the ternary-chalcopyrite crystals if they are to serve as the critical component in midinfrared frequency-conversion devices. Once the nature and behavior of the point defects are established, processes can be developed to remove the defects from the crystals either during the growth itself or during post-growth treatments.

Ternary-chalcopyrite structure ZnGeP2, CdGeAs2 (II-IV-V2) and AgGaS2, AgGaSe2 (I-III-VI2) compounds are currently of technological interest. They show the most promise for practical nonlinear optical applications in the areas of high-efficiency optical parametric oscillators and frequency up-converters for the infrared (ir) range as well as for widespectral-range optoelectronic devices. (See also the article by Schunemann, Schepler, and Budni in this issue.) However extensive realization of their potential has still not been achieved. One of the principal difficulties in the way to obtaining high-device-quality ZnGeP2, CdGeAs2, AgGaS2, and AgGaSe2 single crystals is undesired optical absorption in their transparency range near the fundamental band edge induced by lattice-related defects. This article summarizes selected aspects of dopant-incorporation techniques of these crystals including dopant choice of dopant material and monitoring of dopant incorporation as done in our laboratory.

In general for the ternary chalcopyrite compounds, doping-incorporation processes are more complicated in comparison to those of binary zinc-blende III-V compounds. The most common sources of dominant incorporation of acceptors and donors in as-grown chalcopyrites are believed to appear from (1) nonstoichiometric melts as well as by doping with different elements during the growth process and (2) incomplete removal of disorder on the cation sublattice during subsequent cooling. Furthermore the chalcopyrite structure II-IV-V2 undergoes a disorder-order phase transition upon cooling through approximately 1220 K for ZnGeP2 and 900 K for CdGeAs2. At these transition temperatures, solidification can be complicated also by supercooling phenomena, and the crystals transform from the cubic zinc-blende structure (where Zn atoms randomly fill cation sites) to the ordered chalcopyrite structure (e.g., when Zn and Ge occupy alternating cation sites in ZnGeP2).

In recent years, chalcopyrites have distinguished themselves as the nonlinear optical materials of choice for mid- to far-infrared (ir) laser applications. In particular, AgGaSe2, ZnGeP2, and CdGeAs2 have demonstrated the highest conversion efficiencies and output powers in the wavelength range beyond 4 μm. The superior performance of these crystals arises from their high nonlinear optical coefficients (39 pm/V, 75 pm/V, and 236 pm/V, respectively), their relatively large birefringence (sufficient for phase matching), and advances in crystal growth and processing that have improved transparency and eliminated cracking.

The two most direct approaches to generating laser output in the mid-ir (in particular the 3–5-μm atmospheric transmission window) are (1) shifting the output of a solid-state laser to longer wavelengths via optical parametric oscillation (OPO), or (2) doubling the frequency of a CO2 laser (9–11 μm) via second-harmonic generation (SHG). The OPO approach offers the advantage of tunability combined with potentially more compact and efficient solid-state lasers, whereas the SHG approach benefits from the greater maturity of high-power CO2 laser technology. A third process, difference-frequency generation (DFG), is also a 3-wave interaction similar to OPO that can be used to mix two photons (from two lasers or from an OPO) to produce longer wavelength photons (corresponding to the small difference frequency) over a large spectral range. The optimum chalcopyrite crystal (AgGaSe2, ZnGeP2, or CdGeAs2) for a given approach depends on a complex combination of material parameters described in the sections that follow.