John E. Jensen

Excimer laser‐assisted metalorganic vapor phase epitaxy of CdTe on GaAs

We have successfully grown CdTe (111) on GaAs (100) at 165 °C using a 248 nm excimer laser to photodissociate dimethylcadmium and diethyltellurium in the gas phase. Good crystalline quality of the layers is confirmed by x‐ray diffractometry. Growth rates up to 2 μm/h have been recorded in real time using time‐resolved reflectivity. Auger analysis reveals that the films are stoichiometric throughout the thickness of the layer, and that carbon and oxygen contaminants are below the level of detectability. We have used laser‐induced fluorescence spectroscopy to examine the photodissociation mechanism of diethyltellurium and have observed a linear dependence of Te atom production on excimer laser power.

Effects of Molecular Length on Nematic Mixtures

The properties of nematic mixtures of 4-alkoxyphenyl 4-alkylbenzoates are studied as a function of their average molecular length (L). The Ls of the mixtures vary between 20.46 and 27.14 A, but they all have clearpoints in the 51 to 58° C range. The flow viscosity increases linearly with L. The dielectric constant, refractive index, birefringence, and density decrease linearly with L, and the dielectric anisotropy becomes more negative. With tetrabutylammonium tetraphenylboride added as a salt dopant, the conductivity anisotropy (σ∥/στ) decreases with increasing L, and the dynamic-scattering (DS) threshold voltage increases correspondingly. The effect of L on the DS decay time (τD) is highly dependent on the surface alignment. In surface ⊥ cells, τD increases strongly with L, while in surface ∥ cells, τD decreases slightly with increasing L. The temperature dependence of σ∥/στ indicates that the longer L mixtures, with about ten or more total alkyl carbons from both end groups, have cybotactic n...

Effects of Molecular Length on Nematic Mixtures: II Anisotropic and Dynamic Scattering Properties of 4-Alkoxyphenyl 4-Alkylcyclohexanecarboxylate Mixtures

The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...Abstract The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...

Two-color HgCdTe infrared staring focal plane arrays

Raytheon Vision Systems (RVS) in collaboration with HRL Laboratories is contributing to the maturation and manufacturing readiness of third-generation two-color HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256x256 30μm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral regions. FPAs configured for MWIR/MWIR, MWIR/LWIR and LWIR/LWIR detection are used for target identification, signature recognition and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer-heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all two-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single mesa, single indium bump, and sequential mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.

Effects of Cyclohexyl Groups and Ortho-Substituents on the Viscosity of Ester Liquid Crystal Components

Abstract Structural effects on the relative viscosity of liquid crystal ester components are studied by capillary flow viscosity measurements of each component as an additive (10 to 25%) in a standard nematic mixture of short length 4-aIkoxyphenyl 4-alkylbenzoates. An approximate class viscosity at 25°C is calculated and assigned to each additive component, indicative of its viscosity contribution in the environment of the phenyl benzoate mixture. Studies are made on the viscosity effects of replacing benzene rings with cyclohexane rings, and of using various ortho-substituents on phenyl groups. All of the additive components are esters or thioesters, including several newly synthesized compounds. The effect of a cyclohexyl group is strongly dependent on its position in the structure. The class viscosity values show a large decrease when cyclohexane-carboxylate replaces benzoate, only a modest decrease when cyclohexylbenzoate replaces biphenylcarboxylate, and a slight increase when a cyclohexyl ester repl...

Advances in linear and area HgCdTe APD arrays for eyesafe LADAR sensors

HgCdTe APDs and APD arrays offer unique advantages for high-performance eyesafe LADAR sensors. These include: operation at room temperature, low-excess noise, high gain, high-quantum efficiency at eyesafe wavelengths, GHz bandwidth, and high-packing density. The utility of these benefits for systems are being demonstrated for both linear and area array sensors. Raytheon has fabricated 32 element linear APD arrays utilizing liquid phase epitaxy (LPE), and packaged and integrating these arrays with low-noise amplifiers. Typical better APDs configured as 50-micron square pixels and fabricated utilizing RIE, have demonstrated high fill factors, low crosstalk, excellent uniformity, low dark currents, and noise equivalent power (NEP) from 1-2 nW. Two units have been delivered to NVESD, assembled with range extraction electronics, and integrated into the CELRAP laser radar system. Tests on these sensors in July and October 2000 have demonstrated excellent functionality, detection of 1-cm wires, and range imaging. Work is presently underway under DARPAs 3-D imaging Sensor Program to extend this excellent performance to area arrays. High-density arrays have been fabricated using LPE and molecular beam epitaxy (MBE). HgCdTe APD arrays have been made in 5 X 5, 10 X 10 and larger formats. Initial data shows excellent typical better APD performance with unmultiplied dark current < 10 nA; and NEP < 2.0 nW at a gain of 10.

Formulation and Predicted Properties of Nematic Eutectic Mixtures of Esters

Abstract Techniques are described both for formulating new nematic liquid crystal (LC) eutectic mixtures and for calculating several of their predicted properties. Ester LC components from various structure classes are used. Emphasis is placed on obtaining mixtures which have relatively short average molecular length, and which are suitable for dynamic scattering electro-optical applications. Examples are given of binary eutectic mixtures made with LC esters of essentially the same molecular length. Selection rules are described for mixing short length components, both from homologous series and from different classes of esters. Approximate class values of birefringence, dielectric anisotropy, and viscosity are assigned to each type of ester structure, based on experimental results in selected mixtures of short molecular length. Properties of multi-component eutectic mixtures are calculated, and comparisons are shown between the predicted and the observed values of melting point, clearpoint, viscosity, di...

Epitaxial growth of HgCdTe 1.55-μm avalanche photodiodes by molecular beam epitaxy

Separate absorption and multiplication avalanche photodiode (SAM-APD) device structures, operating in the 1.1 - 1.6 micrometer spectral range, have been fabricated in the HgCdTe material system by molecular-beam epitaxy. These HgCdTe device structures, which offer an alternative technology to existing III-V APD detectors, were grown on CdZnTe(211)B substrates using CdTe, Te, and Hg sources with in situ In and As doping. The alloy composition of the HgCdTe layers was adjusted to achieve both efficient absorption of IR radiation in the 1.1 - 1.6 micrometer spectral range and low excess-noise avalanche multiplication. To achieve resonant enhancement of hole impact ionization from the split-off valence band, the Hg1-xCdxTe alloy composition in the gain region of the device, x equals 0.73, was chosen to achieve equality between the bandgap energy and spin-orbit splitting. The appropriate value of this alloy composition was determined from analysis of the 300 K bandgap and spin-orbit splitting energies of a set of calibration layers, using a combination of IR transmission and spectroscopic ellipsometry measurements. MBE-grown APD epitaxial wafers were processed into passivated mesa-type discrete device structures and diode mini-arrays using conventional HgCdTe process technology. Device spectral response, dark current density, and avalanche gain measurements were performed on discrete diodes and diode mini- arrays on the processed wafers. Avalanche gains in the range of 30 - 40 at reverse bias of 85 - 90 V and array-median dark current density below 2 X 10-4 A/cm2 at 40 V reverse bias have been demonstrated.

Effects of Molecular Length on Nematic Mixtures. III. Anisotropic Properties of 4-Alkylphenyl 4-Alkoxybenzoate Mixtures

Abstract Properties of multicomponent nematic mixtures of 4-alkylphenyl 4-alkoxybenzoates are studied as a function of temperature and their average molecular length (L). The Ls vary between 21.47 and 27.67 A for mixtures with clearpoints in the 43 to 60°C range. The refractive indices, density, and the e1 component of dielectric permitivity all decrease approximately linearly as L increases. There are much larger changes in flow viscosity (η), conductivity anisotropy ([sgrave]|/[sgrave]|), and dielectric anisotropy (Δe). At 25°C as L increases: The is relatively high and increases sharply (from 47 to 78 cP), the [sgrave]|/[sgrave]| decreases and drops below unity (from 1.35 to 0.58), and the Δe changes from positive to negative (from +0.27 to -0.26). The temperature dependence of [sgrave]|/[sgrave]|, with tetrabutyl-ammonium tetraphenylboride as a dopant, indicates cybotactic nematic characteristics are present for the longer L mixtures in which there are an average of about nine or more alkyl carbons f...

Status of HgCdTe/Si Technology for Large Format Infrared Focal Plane Arrays

HgCdTe offers significant advantages over other semiconductors which has made it the most widely utilized variable-gap material in infrared focal plane array (FPA) technology. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially-grown HgCdTe, which are 30 cm2 in size for production and have historically been difficult and expensive to scale in size. This limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to and beyond 2048 x 2048 and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be more readily scaled to large wafer sizes. Most of the effort in the IR community in the last 10 years has focused on growing HgCdTe directly on (112)Si substrates by MBE. At Raytheon we have scaled the MBE (112)HgCdTe/Si process originally developed at HRL for 3-in wafers, first to 4-in wafers and more recently to 6 in wafers. We have demonstrated a wide range of MWIR FPA formats up to 2560 x 512 in size and have found that their performance is comparable to arrays grown on bulk CdZnTe substrates by either MBE or LPE techniques. More recent work is focused on extending HgCdTe/Si technology to LWIR wavelengths. The goal of this paper is to review the current status of HgCdTe/Si technology both at Raytheon and the published work available from other organizations.