Jonathan T. Goldstein

Infrared properties of AgGaTe2, a nonlinear optical chalcopyrite semiconductor

The fundamental optical properties of AgGaTe2, a nonlinear optical semiconductor are reported. These properties include birefringence, indices of refraction, infrared transmission, and the temperature dependence of the band gap. The average index for wavelengths greater than several microns was found to be 3.0. The birefringence was found to be rather large and to range from a near band edge value of 0.038 at 1.3 μm to a value of 0.017 at 15 μm. Additionally, native defect related sub-bandgap absorption, photoluminescence, and electrical transport have been studied in these nominally undoped p-type crystals. An activation energy associated with these defects was determined to be 0.3 eV and the corresponding photoluminescence and absorption data showed, respectively, a broad asymmetric emission band centered at 0.8 eV and two bands at 0.95 and 1.01 eV, the absorption band at 0.95 eV being the most intense. The measured properties were utilized to assess the potential of AgGaTe2 for the wavelength conversio...

Fermi level control and deep levels in semi-insulating 4H-SiC

Temperature dependent Hall effect, optical admittance spectroscopy, and optical absorption measurements of semi-insulating bulk 4H–SiC are reported. Both intentionally vanadium doped material and commercial grade semi-insulating material were investigated. The carrier concentration versus inverse temperature results from Hall effect measurements up to 1000 K indicated the samples were dominated by one of two deep levels near midgap. In addition to the deep donor level of substitutional vanadium, Ec−1.6u200aeV, we observed another level at Ec−1.1u200aeV in some samples, indicating that levels other than the vanadium donor can pin the Fermi level in semi-insulating SiC. Optical admittance measurements on the semi-insulating material indicate the presence of levels at Ec−1.73 and 1.18 eV that were previously observed in conducting samples with this technique and we attribute these levels to the same defects producing the 1.1 and 1.6 eV levels seen by Hall effect. Secondary ion mass spectroscopy measurements of dopan...

Anisotropy of optical absorption in GaSe studied by midinfrared spectroscopy

Slab samples of e-GaSe have been prepared with the optical axis (the c axis) in the plane of the face. Previously, most optical studies were made on slabs cleaved from crystals, which have the c axis perpendicular to the face. The infrared absorption spectra of e-GaSe were measured at room temperature in the spectral range of 4000–450 cm−1 for the e-ray (polarized parallel to the optical axis) and the o-ray (polarized perpendicular to the optical axis). The maximum value of the optical absorption coefficient, α, for both geometries, does not exceed 2.5 cm−1 (E∥c). We find anisotropy in the transmission in the spectral range below ∼2250 cm−1, due to the free carrier absorption, only in the e ray. At T=300u200aK, acoustic lattice vibrations are the dominant scattering mechanism for carriers moving parallel to the c axis. Absorption bands were observed in e-ray spectra at 891, 945, 1093, and 1270 cm−1. These bands may be due to multiphonon absorption, or to localized vibrational modes stemming from unintentional...

Temperature and pulse-duration dependence of second-harmonic generation in CdGeAs 2

A detailed investigation of frequency doubling of a transversely excited atmospheric CO2 laser operating at 9.55 microm with a CdGeAs2 crystal was carried out. The temperature of the crystal was varied between 80 and 295 K to maximize the frequency-doubled energy. The temporal shape of the generated beam at 4.775 microm was monitored to calculate its peak power. High values of midwave infrared pulse energy (16.65 mJ) and peak power (92 kW) were obtained, which can be of potential use in lidar systems.

Photoluminescence study of AgGaSe2, AgGa0.9In0.1Se2, and AgGa0.8In0.2Se2 crystals grown by the horizontal Bridgman technique

AgGaSe2, AgGa0.9In0.1Se2, and AgGa0.8In0.2Se2 single crystals grown by the horizontal Bridgman technique were investigated using photoluminescence (PL) at temperatures varied from 8 to 300 K. For the AgGaSe2 crystals, free exciton (FE), exciton bound to neutral donor (D0, X), and edge emissions including donor-acceptor pair (DAP) and free electron to neutral acceptor (e, A0) transitions were observed. Two donor levels with binding energies of 18 and 39 meV and two acceptor levels with 61 and 117 meV were observed. The FE peak positions of the AgGaSe2 were found to be blueshifted when the samples were illuminated with higher laser intensity. This behavior was more pronounced at higher temperature when the peaks were also significantly broadened. For the AgGa0.9In0.1Se2 crystals, three DAP emission peaks at 1.673, 1.570, and 1.545 eV were observed at 8 K. The excitonic peaks were not observed below 100 K because they were overshadowed by the 1.673 eV DAP emission. For the AgGa0.8In0.2Se2 crystal, the excito...

Corrected infrared Sellmeier coefficients for gallium selenide

We have measured the room-temperature refractive indices of GaSe throughout the 0.7–1.4 and 2.4–5μm ranges using the minimum-deviation method of light through a prism with polarization either parallel (extraordinary refractive index ne) or perpendicular to the crystal optical c axis (ordinary refractive index no). The birefringence (Δn) at room temperature has been measured directly using polarized light interference fringes obtained in the transmittance from 1.73to4.97μm (three samples with thicknesses of 1108±1, 2480±1, and 2660±1μm) and in the range from 11.85–16.37μm (sample thickness of 6000±1μm). The no, ne, and Δn values determined from the positions of fringe maxima were compared to those obtained from prism measurements in the mid-IR and with previously published results which were mainly obtained by indirect methods. It was found that the prism method and interference fringe method resulted in values of no, ne, and Δn accurate to ±0.003 and ±0.006, respectively.

Micro-Raman and photoluminescence spectroscopies of horizontal Bridgman-grown AgGaSe2

AgGaSe2 crystals grown by the horizontal Bridgman technique were studied by room-temperature micro-Raman scattering and low-temperature photoluminescence (PL) spectroscopies. The most intense Raman line observed had the frequency of 177cm−1, corresponding to the Γ1(W1) mode. The measurements were performed along the direction of growth of the boule and the line frequency was found to be almost constant within the experimental accuracy. The average full width at half maximum of the Γ1(W1) mode was found to be 4.6cm−1 and its insignificant variation along the ingot suggests its structural and compositional uniformities. At low temperature (8K), the main PL peak at 1.772eV is due to donor-acceptor-pair recombination. A doublet in the excitonic peak, observed at that temperature, suggests a reduced inhomogeneous broadening and the better crystallinity of the sample, when compared with the previous literature. The dispersion of the various PL peaks along the length of the ingot was also found negligible, which...

Temperature dependence of the birefringence of SiC

Measurements of the birefringence of 6H-SiC, as well as its temperature dependence, are presented. The results are used to explore the possibilities this material presents as a mid-infrared non-linear optical parametric oscillator.

Characterization of Vanadium-Doped 4H-SiC Using Optical Admittance Spectroscopy

Vanadium is an important dopant in SiC because it gives rise to donor levels near the middle of the bandgap which can be used to make the material semi-insulating, and semi-insulating material has many applications as a substrate material for high-power electronics. However, conventional means of characterizing electronic levels in the bandgap of the material require very high temperatures, in the neighborhood of 650--800 C, in order to move the Fermi level to midgap and cause ionization of the V donors. The technique of Optical Admittance Spectroscopy permits the ionization of the midgap donors using light of the appropriate energy, and thus avoids the need for high temperatures. Using this technique, the authors have examined several specimens of V-doped and high-resistivity 4H-SiC. They have identified levels previously associated with V, and new levels they attribute to Ti. Pinning of the Fermi level in some specimens was verified by high-temperature Hall effect measurements. SIMS measurements were used to determine impurity concentrations. IR absorption measurements were correlated with the Ti, V, and Cr concentrations determined by SIMS.