The use of epitaxial structures for the generation, detection, and modulation of light has had as its strongest driving force the development of fiber optic communications systems. Partly for that reason, the semiconductor materials systems that can be most readily optimized for wavelengths (800-1600nm) transmittable through state-of-the-art optical fibers, have been the most well developed. It also appears that the communications community has been most fortunate in that the semiconductor systems required, although far more difficult to deal with than Si, are relatively benign compared to those that operate at significantly shorter or longer wavelengths. Two developments that are now more than 15 years old initiated the extensive studies of compound semiconductor epitaxial structures that are now so important. These were the introduction of the heterostructure concept in the study of injection lasers and light emitting diodes (1), and the realization that lattice matched isoelectronic epitaxial layers, consisting in part of solid solutions between the binary compounds, would permit such structures to be grown.

We report on the defects present in doped InP and GaInAs grown by organometallic vapor phase epitaxy (OMVPE). The material was grown in an atmospheric pressure system using group III trimethyl sources, arsine and phosphine. Bis(cyclopentadienyl) magnesium (Cp2Mg) was present as a p-type source of magnesium. Defects in as-grown material were characterized using photoluminescence (PL), Hall-effect, and deep level transient spectroscopy (DLTS). Various levels of Mg doping were investigated, ranging from 5 × 1015 to 1 × 1019 cm−3. Radiative defects were observed at 77 K corresponding to PL emission from conduction band/shallow donor to acceptor levels including emission at 1.37 eV identified as the shallow hydrogenic acceptor, and emission lines at 1.3 eV and 1.0 eV in heavily doped material. Corresponding hole traps in InP:Mg were observed by DLTS having thermal activation energies of 0.20 and 0.40 eV, the 0.40 eV trap being the dominant defect in p-type InP. In GaInAs grown near lattice-matched to InP, radiative emission is also observed from deep centers 100 meV from band edge emission. This emission is observed to be related to lattice-mismatch of the ternary with the InP, and is found to be accentuated and broadened in GaInAs doped with Mg.

The requirements of infrared systems have increased significantly over the years, from the simple linear, low resolution, photoconductive array to the present-day, large area, high-density, photodiode (MIS and metalurgical) arrays, with on-focal-plane signal processing of considerable complexity. The success that has been achieved in meeting the performance goals appropriate for these systems has been due, to a large degree, to significant advances in the relevant materials technologies. The technologies of importance over the last twenty years are briefly reviewed, and the current state of the art, with its dominance by intrinsic alloy materials, is addressed in detail. The limitations of current bulk and epitaxial intrinsic materials technologies are considerable, both from a performance and a producibility point of view, when compared to the quality and quantity of material required by future infrared systems. These limitations are considered together with possible ways to overcome them.

This paper provides a historical perspective on the emergency of HgCdTe as the material of choice for long wavelength infrared (LWIR) imagers. The need for devices which see room temperature objects through the atmospheric window actually drove the development of this material. The lack of elemental or compound semiconductors having the desired wavelength response forced the choice of the alloy semiconductor, HgCdTe. The development of this material in several countries and companies beginning in the late 1950's is traced. The crystal growth methods used to grow HgCdTe have included melt growth techniques such as Bridgman, zone-melting, quench-anneal and slushgrowth. The solution growth techniques include growth from HgTe-rich, Te-rich and Hg-rich solutions. Vapor phase growth has included evaporation, sputtering, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). No perfect method has yet been developed, but several have provided material for the large area arrays needed for modern imagers.

A model for generating the composition and doping profiles from growth and diffusion parameters was developed for heterostructure devices. Poisson's equation was applied to these structures to predict barriers in the conduction band to minority carrier flow for long wavelength HgCdTe infrared detectors prepared by LPE techniques. Spectral response and quantum efficiency measurements illustrate the presence of these barriers and support the use of this model in predicting barrier formation.

Lead chalcogenide diode lasers are useful for spectroscopic and fiber optics applications in the mid-infrared (2.5-30 μm) wavelength range. These devices have previously required cryogenic cooling (<100 K) for CW operation. This limitation has been overcome through the use of a new, lattice-matched alloy system, Pb1-xEuxSeyTe1-y as well as the introduction of advanced, quantum well active region device structures grown by molecular beam epitaxy (MBE). Operating temperatures have been increased to 175 K CW (at 4.4 μm) and to 270 K pulsed (at 3.9 μm). Thermal leakage currents out of the device active region appear to be limiting device performance. This has led to the study of band offsets in PbEuSeTe/PbTe heterojunctions as well as to exploration of alternative high energy band gap alloys of PbTe with Ge, Yb, Ca, Sr, and Ba. The status of this work and examples of ultrahigh resolution studies done with these tunable laser sources will be included.

Epitaxial films of IV-VI semiconductors and their alloys form the basis of an infrared detector technology that offers advantages in material stability as well as spectral versatility. These films are prepared by epitaxial hot-wall techniques and their material properties are essentially the same as those of bulk crystals. Because of their stability, multilayer growths of the materials can be achieved in a straight-forward manner. To date, multi-color detectors and small scale two-color detector arrays have been demonstrated successfully. A brief review of the growth method and the growth characteristics is given. Recent advances in superlattice research, especially those of interest to electro-optical devices, will be discussed. These include persistent photoconductivity and sub-bandgap optical transition.

Selected issues in the growth of bulk single crystals for applications in infrared optoelectronics are reviewed including an overview over materials choices, bulk III-V crystal growth, and the growth of II-VI, IV-VI and I-III-VI2 compounds and alloys. The most important issues are the control of purity, perfection, stoichiometry, and uniformity during crystal growth and the control of the surface properties in wafer fabrication. Specific examples are given to illustrate problems related to these issues and to discuss approaches to their solution.

High gap Hg1−xCdxTe (MCT) crystal is grown by a solvent method using a travelling heater zone. The use of the solvent zone permits a low temperature and low mercury pressure growth of MCT in a large composition range. In the Cadmium rich alloy range the MCT material exhibits a large spin orbit coupling leading to a resonance at Eg = ΔO. Due to this particular resonance effect the ionization coefficient of hole is higher than that of the electron, resulting in a low exess noise factor in the avalanche photodiodes whose bandgap energy is close to the spin orbit splitting ΔO.

We have demonstrated that the atomic distribution of constituents in semiconductor alloys is never truly random. There are always interactions causing correlations; the degree and nature of the correlations depend on which interactions dominate and on the growth conditions. While we have identified most of the interactions which are expected to cause correlations, not all of them have been treated completely to date. Therefore, some details remain unclear, but the principal effects can now be appreciated in broad terms.

A low stress modified horizontal Bridgman technique has been developed and used to grow low defect, large area, subgrain free CdTe crystals for use as substrates in the epitaxial growth of HgCdTe and related IR detector materials. CdTe wafers cut from horizontal Bridgman grown boules exhibit, resistivities in the 107ohm-cm range. Etch pit counts are in the 104cm−2 range. Etch pit patterns as well as x-ray topographs indicate the absence of low-angle grain boundaries. Double crystal x-ray rocking curves are single peaked and very narrow with FWHM(333) as low as 9 arc-sec. Rocking curves of FWHM(333) = 9 to 15 arc-sec, measured at several different laboratories, have been obtained for CdTe wafers cut from several boules. This is in contrast to standard vertical Bridgman grown CdTe samples, which generally show broader x-ray rocking curves sometimes with multiple peaks as a result of subrgrain structure. Low temperature (1.6–4.5 K) photoluminescence (PL) measurements on these low defect samples reveal bright edge emission lines which are the main feature of the spectrum. Additional bound exciton lines and other sharp features associated with donor and acceptor impurities are also present. The very weak defect band luminescence (1.40–1.46 eV) provides additional evidence of sample quality.

The vertical Bridgman method, a gradient freeze technique, is feasible for the growth of high quality, large CdTe crystals. The equi-composition contour of Zn in doped crystals has been used to reveal the solid-liquid interface shape in the growth process. A slow cooling rate is necessary to obtain a convex interface shape. A low temperature gradient at the solid-liquid interface, down to 2 °C/cm, and Zn doping are found to be effective to reduce the etch pit density to 5×103/cm2 (minimum) to 104/cm2 (average). The crystallographic quality has been evaluated by means of X-ray diffraction, X-ray topography, etch pit delineation with the Nakagawa etchant, infrared measurements, photoluminescence and Hall effect measurements. CdTe crystals are found to be free from subgrain structure, Te precipitates and deep levels, and have high electron mobility.

Good quality single crystals of Ti3PSe4 were grown from the melt by the Bridgman technique following improvements in the method of purifying the parent components, and optimization of growth parameters. Crack-free crystals 8 cm in length and 17 mm in diameter were produced.

The quality of the crystals was evaluated by optical transmittance and metallographic techniques. In the range 0.7 to 14 μm the optical transmittance shows elimination of absorption bands exhibited in crystals grown without special purification steps. Etchpit studies showed that the crystals were free from inclusions and lamellar twins and that they show a uniform cross sectional etch pit density.

The alloy variation of the band gap and the electron and hole effective masses have been calculated for HgCdTe and HgZnTe. Band-gap bowing is larger in HgZnTe than in HgCdTe because of the larger bond length mismatch of HgTe and ZnTe; electron and hole effective masses are found to be comparable for the two alloys for a given band gap. We have calculated the electron mobility in both alloys with contributions from phonon, impurity, and alloy scattering. Contributions to the E1 line width due to alloy and impurity scattering in Hg0.7Cd0.3Te have been calculated. Results of calculations of the vacancy formation energies in HgTe, ZnTe, and CdTe are discussed.

Hydrogenated amorphous carbon thin films are well known for their mechanical hardness and optical properties which make them useful for applications in infrared device coatings. In this work films have been prepared by plasma. decomposition of methane using an RF diode reactor operating under conditions of high self bias potential (Vb = 1 KeV). The resulting ion bombardment during film growth leads to the formation of hard, insulating carbon coatings which have a band gap of ≃1.1 eV and are transparent in the IR. Nuclear reaction analysis has been used to quantify the atomic concentration of hydrogen incorporated in the films and extended x-ray absorption fine structure (EXAFS) has been used to determine local site geometry. Only first and second nearest neighbor bond lengths are observed with no evidence of further long range order or microcrystallinity. A model for atomic structure is proposed which includes both sp2 and sp3 bond configurations and direct comparisons are made with data obtained from sputtered carbon films, graphite and diamond.

The implications for recent reports of large valence band offsets the HgTe-CdTe heterojunction are examined. The variation of the band gap and effective mass for transport normal to the layers in the superlattice is examined in detail.

Mechanisms for the low temperature photo-dissociation of alkyl precursors for the epitaxial growth of CdxHg1-xTe (CMT) are discussed. The roles of vapour and surface nucleation are considered in the light of the free radical model which also can provide methods for controlling the vapour phase photochemistry. Higher quality CMT has been grown at 250 °C by using dimethyl mercury as a source of free methyl radicals. Problems encountered in reducing growth temperature for multilayer epitaxy are considered for CdTe and HgTe and conditions established for epitaxial growth at 200°C.The roles of alkyl concentration, substrate temperature, UV intensity and free radical concentration are explored. Results on the first reported growth of CdTe deposition using a cw UV laser source are compared with the arc lamp grown layers.

We report the successful substitutional doping of CdTe epilayers grown by a new technique: photoassisted molecular beam epitaxy, in which the substrate is illuminated during the film deposition process. This new technique was found to produce dramatic changes in the electrical transport properties of the epilayers. In particular, highly conducting n-type and p-type CdTe films have been grown using In and Sb as n-type and p-type dopants,respectively. Photoassisted MBE has also recently been employed to produce for the first time highly conducting CdMnTe epilayers and Cd 1-xMnxTe-CdTe superlattices.

Using the effective medium approximation[1–4] and the theory of photoemission from small particles,[5] we look into the design-rules of infrared materials based upon the metalsemiconductor random heterostructures with Ag particles embedded in the semiconductor. It is found that semiconductor host matrices with higher dielectric constants show better optical absorption in the infrared and that optical properties are closely related to microstructural parameters such as volume fraction of metal particles, percentage of aggregation and particle size. Cu, Ag and Au particles all show similar characteristics. We synthesized materials with small silver particles embedded in Si. Their microstructures are analysized using X-ray diffraction, electron microscopy and sputter Auger profiling. Finally, we present the optical properties of these materials and make comparisons with our theoretical results.

We have used a cw CO2 laser to study the effects of rapidly annealing HgTe-based alloys. Both as-grown and thermally annealed samples of HgCdTe, HgMnTe, HgZnTe, and HgMgTe have been examined for mercury loss and surface damage using energy-dispersive x-ray analysis and optical reflectivity measurements. Small, but systematic differences were found between the asgrown and the thermally annealed samples and among the various materials studied. No degradation of the material at all was observed when the samples were cooled to 77 K and exposed to the laser.