Melvin C. Ohmer

Emergence of Chalcopyrites as Nonlinear Optical Materials

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 AgGaSe 2 (a I-III-VI 2 semiconductor) and ZnGeP 2 (a II-IV-V 2 semiconductor). Using samples of naturally occurring pyrites, Pauling correctly established the chalcopyrites 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.

A theoretical study of native acceptors in

Native acceptor centres in are studied using atomistic simulation techniques for which a new set of interatomic potential parameters consisting of two- and three-body terms is developed. Crystal lattice constants, elastic and low- and high-frequency dielectric constants are well reproduced in this atomistic model. The calculated formation energies for vacancies, interstitials and antisites in this material suggest that the intrinsic disorder is dominated by antisites in the cation sublattice followed by the Schottky and Frenkel defects. The acceptor centre identified by Hall-effect measurements and EPR is found to be related to the delocalized hole shared by the four As neighbours bound to CdGe. For this centre, calculations yield a binding energy of 0.13 eV in an agreement with the experimental value of 0.15 eV obtained by the Hall-effect measurements. Furthermore calculations provide the magnitude of the lattice distortion introduced by this acceptor centre in which can be used for the analysis of ENDOR experiments.

Density functional study of the structure, thermodynamics and electronic properties of CdGeAs2

Structural, thermodynamic and electronic properties of CdGeAs2 with chalcopyrite structure are investigated in the framework of density functional theory. We employ the linear combination of atomic orbitals method with the Gaussian basis sets and present the results for the equation of state, the Grconstant, the electronic band structure and the pressure coefficients of the valence and conduction levels in CdGeAs2.

Nonlinear optical crystal development for laser wavelength shifting at AFRL Materials Directorate

Our objective is to develop crystals that shift the light from well developed laser sources to application specific wavelengths which may be tunable. Military applications extend across the entire spectrum from ultraviolet through the far-infrared but our greatest interest has been in materials for the mid-IR (3-5 μm) and far-IR (8-12 μm) atmospheric windows. Our primary applications that drive crystal development are infrared countermeasures and remote sensing of chemical and biological warfare agents. To achieve these results we have pursued two tracks: birefringent bulk materials and quasi-phase-matched structures. Birefringent studies include the grey track problem in KTiOPO4 (KTP) plus growth of its isomorphs, KTIOAsO4 (KTA), RbTiOAsO4 (RTA), CsTiOAsO4 (CTA), and KxRb1-xTiOPO4 (KRTA); chalcopyrites: ZnGeP2, CdGeAs2, AgGaS2, AgGaSe2, AgGa(1-x)InxSe2, AgGaTe2; plus GaSE, and HgGa2S4. A small portion of the effort is pursuing UV materials, the foremost being the borates MM(B3O5)3 where M=Sr, Ba, Pb; M=Li,Na. Previously, work was done on LiB3O5 (LBO), β-bonding (GaAs), by periodic poling (LiNbO3, LiTaO3, KTP, BaTiO3) and by periodic doping. Th cover the 4.5-5.0 μm band, work is being done on RTA and Pb{MgxNbyTi1-x-y}O3 (PMNT). For periodic poling in the 8-12 μm region studies are being made on CGC (CsGeCl3), CGB (CsGeBr3), Tl3PbBr5, Tl4PbI6 and Tl4HgI6. QPM can also be obtained using total internal reflection devices (GaAs, ZnSe).

Doping of ternary compounds CdGeAs2 and CdSnAs2 by impurities of I, II and III groups

Abstract Research in doping processes of ternary chalcopyrites A 2 B 4 C 2 5 is of primary interest for several reasons. First of all, a study of the dependencies of ‘the properties of crystal vs. the concentration of impurity in melt’ decides a fundamental problem. It is a check on the theory of ordering of a crystal structure for some specifically defined values of dopant concentration leading to a possible phase transformation caused by the self-organization of large-scale fluctuations in a melt. On the other hand, it is necessary also for practical tasks—for example, obtaining high-resistance CdGeAs 2 -crystals for use in nonlinear optics. In the present work data on electrical activity and on an effect in the electrophysical properties of ternary semiconductors CdGeAs 2 and CdSnAs 2 are obtained for dopants of Au, Cu, Zn, In, Sc and Gd in CdGeAs 2 and for Au, Cu and In in CdSnAs 2 , added to a melt during synthesis or recrystallization of a material.

Growth and characterization of epitaxial films of ZnGeP 2 .

Thermodynamic analysis of the vapor phase over ZnGeP 2 in Zn-Ge-P-Cl system has been carried out. The analysis showed that this system can be used for the vapor growth of ZnGeP 2 . Homoepitaxial layers of ZnGeP 2 were grown in a closed system using chemical vapor transport. Electrical and photoluminescence properties of the layers were studied, and crystal lattice parameters were measured. Comparison of properties for bulk and vapor grown ZnGeP 2 crystals were carried out. It was found that the vapor grown crystals have more perfect structure than the bulk ones, particularly, they have significantly lower vacancy concentration.

Frequency Doubling of CW And Pulsed CO 2 Lasers Using Diffusionbonded, Quasi-Phase-Matched Gaas Stacks

We describe the fabrication, characterization and frequency doubling properties of first order stacks of diffusion bonded 2″ diameter GaAs wafers. Near IR imaging through the stacks indicated that excellent bonding was obtained over 40%–70% of the central area of the wafers. A power spectrum analysis of the spectral noise (due to interface reflections) appearing in the transmission data is shown to be a quantitative diagnostic tool useful for determining interface quality and for accurately estimating the thickness of the bonded layers in a stack. The conversion efficiency for a four layer stack at 10.6 microns was found to be 0.03% or 3 mJ for a 10 mJ pulse with a 100 nanosecond pulse length. The corresponding efficiency for cw SHG was found to be 0.0002%.