P. M. Bridenbaugh

Nonstoichiometry and Crystal Growth of Lithium Niobate

Deviations from stoichiometry occurring during the crystal growth of lithium niobate have been studied by 93Nb NMR and by shifts in the ferroelectric transition temperature. The high‐temperature phase equilibria of nonstoichiometric lithium niobate were accurately determined by these techniques together with DTA and crystal‐growth experiments. The congruently melting composition is located at 48.6 mole% Li2O and the solid‐solution region at room temperature extends from 48% to 50% Li2O. The implications of growth at noncongruently melting compositions on the compositional uniformity of lithium niobate are discussed in detail.

The crystal structure of the high-temperature polymorph of α–hexathienyl (α–6T/HT)

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported. 1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult. 2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Huckel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET) 2 Cu(NCS) 2 .

Photorefractive properties of doped cadmium telluride

The first study of the photorefractive properties of doped CdTe has demonstrated high sensitivity for optical processing applications. Of the binary II‐VI and III‐V semiconductors, CdTe has the highest electro‐optic coefficient r41 in the infrared, some three times larger than that of GaAs and InP. Deep levels introduced into CdTe exhibit appropriate absorption and photoconductivity at 1.06 μm by doping with V and Ti impurities. Photorefractive beam coupling experiments in CdTe:V gave small signal gains of 0.7 cm−1, and diffraction efficiencies with no applied electrical field of 0.7%. Thus, CdTe appears to be superior to previously studied III‐V semiconductors, in the near‐infrared spectrum. Optimization of doping and trap densities is expected to result in gain which exceeds the absorption loss, thereby allowing phase conjugation with infrared injection lasers.

Multicomponent tetrahedral compounds for solar cells

Abstract Preparation and properties are reviewed of solar cells based on multicomponent compounds. Cells made with CuInSe 2 , CuInS 2 , and CuGaSe 2 are discussed in detail. New results are presented on single crystal p - CuInS 2 / n - CdS and p - Cu 2 CdSnS 4 / n - CdS heterodiodes.

Effect of Optical Inhomogeneities on Phase Matching in Nonlinear Crystals

Experimentally, we have observed variations in the phase‐matching temperature (Tpm) for second harmonic generation (SHG) along the pull axis of lithium niobate (LiNbO3) crystals. We have related these to variations in the extraordinary index of refraction caused by changes in crystal stoichiometry occurring in the growth process. We have established a technique, for filtering pathological crystals, based upon a point‐by‐point mapping of the birefringence variations along the direction of interest. This provides information about the details of the large and small‐scale spatial variations. Empirically we can get toleration limits on the birefringence variations for application to parametric oscillator design. We have studied several physical models (single step, multiple step, and linear ramp) for the manner in which monotonic changes in index, appropriate for LiNbO3, are introduced as a function of position. We have found that the usual quality criterion based upon the width of the central peak in an SHG ...

Influence of Ga‐As‐Te interfacial phases on the orientation of epitaxial CdTe on GaAs

When CdTe is grown by molecular beam or organometallic vapor phase epitaxy on (100) GaAs, the layer can grow with either a (100) or (111) orientation. Reflection high‐energy electron diffraction and Auger studies are presented here which show that adsorption of different submonolayer amounts of Te on a GaAs surface can change the surface symmetry and the resulting CdTe orientation. A precursor surface to (111) growth results from the formation of a relatively Te‐poor Ga‐As‐Te surface phase. A relatively Te‐rich structure yields a surface with (100) symmetry and lead to (100) growth.

Effects of Zn to Te ratio on the molecular‐beam epitaxial growth of ZnTe on GaAs

ZnTe films have been grown with Zn:Te flux ratios ranging from 1 to 3.2. The highest quality films have been grown with flux ratios between 2 and 3, substrate temperatures between 300 and 325 °C, and a surface reconstruction that is a combination of c(2×2) and (2×1). Films grown under these conditions have x‐ray rocking curve half‐widths between 125 and 225 arcsec. Photoluminescence spectra show that the relative intensity of emission related to Zn vacancies decreases with increasing Zn:Te ratio. Picosecond photoconductivity measurements show an initial decay rate for photoexcited carriers that correlates well with other material parameters. After several hundred picoseconds, the decay rates for different samples show exponential behavior with a lifetime of approximately 675 ps.

Photovoltaic properties and junction formation in CuInSe2

Studies of diffusion and photovoltaic effects in CuInSe2 p‐n junctions are reported. Junctions were formed by annealing Zn‐, Cd‐, and Cu‐plated p‐type samples at temperatures from 200 to 450 °C. The most efficient photodetectors are formed by 5–10‐min anneals at 200 °C with a calculated interdiffusion coefficient of ∼5×10−10 cm2/sec.

Semileaky thin‐film optical isolator

Two interesting effects have been experimentally demonstrated for the first time: (1) simultaneous reciprocal and nonreciprocal mode conversion to achieve an isolation effect and (2) magneto‐optic switching between guided and radiation modes. These effects were observed in connection with the construction of a previously proposed thin‐film optical isolator. The isolator consists of a piece of LiNbO3 placed on top of a thin film of yttrium ion garnet (YIG) with a selenium layer to avoid optical contact problems. The isolator, which is 1 cm long, exhibited 10 dB of isolation at λ = 1.15 μm. The observed isolation was better than theoretical predictions and a mysterious isolation direction dependence on mode order was observed. Although the device had 10 dB of insertion loss and required a magnetic field of 40 Oe, with a slight change in wavelength and film composition, it should be possible to reduce the insertion loss and field required to under 1 dB and 0.1 Oe, respectively. These characteristics combined...

Dependence of Second‐Harmonic‐Generation Coefficients of LiNbO3 on Melt Composition

The three nonlinear coefficients d31, d22, and d33 that describe optical second‐harmonic generation with LiNbO3 have been determined for crystals pulled from melts with Li/Nb atom ratios, (Li/Nb)m, of 0. 852, 0. 946, and 1. 083. The coefficients d22 and d33 were found to be insensitive to the melt composition whereas d31 increased by about 50% as (Li/Nb)m was increased from 0. 852 to 1. 083. These results suggest a marked Curie temperature dependence of the difference between the two Nb–O bond nonlinearities that give rise to the ds for LiNbO3.