David J. Knuteson

Crystal growth, fabrication, and design of mercurous bromide acousto-optic tunable filters

Device-quality single crystals of mercurous bromide were grown by the physical vapor transport method. Crystals transmitted light wavelengths up to 30 µm and did not show any absorption bands. Detailed x-ray Laue and x-ray diffraction studies were used to characterize and orient the crystals. Optical evaluation was performed by fabricating slabs of crystals. A design was developed to fabricate acousto-optic tunable filters with 10-deg off-axis orientation operating in the mid- and long-wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 16-mm optical aperture for the 10-deg design. A theoretical tuning curve for a mercurous bromide crystal-based AOTF using this design was also computed for the first time. Experimentally measured data on frequency matching agreed well with the theoretical predictions, and the transducer thickness was suitable for filtering 7.58 µm with the fabricated AOTF.

Operational characteristics of a long-wavelength IR multispectral imager based on an acousto-optic tunable filter

We have experimentally demonstrated and report on the results of crystal growth, fabrication, design, development, and performance for the long-wavelength infrared (LWIR) hyperspectral imager based on an acousto-optic tunable filter (AOTF) utilizing an efficient crystal, thallium arsenic selenide (Ti3AsSe3 TAS). Results on the growth of 40-mm-diameter, 15-cm-long crystal boules, to fabricate 4.0-cm-long AOTF devices, and on the system design and performance are presented. To achieve an 8-cm−1-resolution AOTF, we developed a design utilizing growth at 10.6 deg off from the c axis of the crystal and achieved >37% efficiency. A system concept was developed with high efficiency, resolution, and throughput utilizing this TAS AOTF. The test setup consisted of an LWIR camera (microbolometer), the AOTF, and a blackbody radiative source (hot filament), and represents the first time AOTF imaging has been achieved with a microbolometer camera. The filament was placed 25 cm in front of the AOTF, and the camera was aligned to the first-order diffracted beam of the AOTF. The AOTF was tuned to 10.6-µm wavelength by applying a 13.9-MHz rf signal to the transducer. Preliminary experimental results obtained for SF6 gas utilizing this system are reported.

Design and fabrication of mercurous bromide acousto-optic tunable filters

Mercurous bromide crystals with very good optical transparency were grown by the physical vapor transport method. A design was developed to fabricate 10-degree orientation acousto-optic tunable filters operating in the mid and long wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 13-15 mm diameter. A theoretical tuning curve for a mercurous bromide crystal based AOTF using this design was also computed for the first time.

Nanowire-based photodetectors: growth and development of chalcogenide nanostructured detectors

This work showcases developments in growth and performance of nanowire (NW) based photodetectors. Specifically the ability to transition from single NW devices to device arrays will be discussed. We have demonstrated the growth of semiconducting nanowires (NWs) using the physical vapor transport (PVT) method. CdSe and ZnSe NWs were grown and showed promising optical properties, including high transparency and a high ratio of band edge/deep level defect emission in photoluminescence (PL) measurements. Metal-semiconductor-metal (MSM) structures were fabricated from an array of ZnSe NWs, which showed an average increase of 10x in photocurrent and up to 720x for an individual device.

Novel Quaternary Semiconductor Materials: Growth and Characterization

Quaternary materials have a large flexibility to design bandgap and effective performance for detectors. In the ternary and quaternary chalcogenides, the semiconducting to seminsulating properties are exhibited with bandgaps from narrow (<1 eV) to large (>3 eV) and have the potential and flexibility to produce large numbers of electron hole pairs and hence increase detector sensitivity. In this paper we describe the Ag-Ga-Ge-Se class of materials, an extension of the In-GaSe system, and the challenges related to the material. The important material in this class is the AgGaGe3Se8 stoichiometry. A comprehensive solution to produce its derivatives and experimental results are presented. These are compared with the In-GaSe class of materials for growth and fabrication which we have developed over many years. We have developed processes for purification, synthesis of large batches of materials, and crystal growth by vertical Bridgman method. These crystals are grown by both capillary seeding and oriented seeding. The details of the growth process, X-ray orientation, cutting, polishing, fabrication and stability of the large size crystals will be presented.

Nanomorphology of ZnSe on patterned substrate

We have grown high quality oriented nano particles of zinc selenide (ZnSe) on patterned gallium arsenide (GaAs) substrates. We have developed and used silver and gold based templates with domains of 35 μm. We observed that the films grew epitaxially on the non-patterned portion of GaAs wafers with 4° miscut from (001). Several samples of thickness ranging from 5 μm to 1 mm thickness of ZnSe were grown in a vertical closed tube using the temperature gradient as the driving force. The quality of the samples was analyzed by X-ray and morphology was studied by SEM, FIB, and AFM and by etching the films. The rocking curve showed that the FWHM values for different films were in the range of 300-350 arcs second. We observed that film on (001) portion of the template grew with smooth morphology but morphology was slightly different on the templates. The grown film had strong (111) peak also on the patterned substrate in addition to the (001) peak observed for the film on unpatterned substrate.

Growth of CZT using additionally zone-refined raw materials

Results will be presented for the growth of CdZnTe by the low pressure Bridgman growth technique. To decrease deeplevel trapping and improve detector performance, high purity commercial raw materials will be further zone refined to reduce impurities. The purified materials will then be compounded into a charge for crystal growth. The crystals will be grown in the programmable multi-zone furnace (PMZF), which was designed and built at Northrop Grumman‟s Bethpage facility to grow CZT on Space Shuttle missions. Results of the purification and crystal growth will be presented as well as characterization of crystal quality and detector performance.

Low Temperature Epitaxy of 3C SiC Using Hexamethyldisilane Precursor on Si Substrates

The CVD growth of SiC thin films using hexamethyldisilane (HMDS) as the singular precursor on Si substrates with an AlN nucleation layer was explored in this study. A statistically designed experiment was used to conclude that growth temperature has the largest impact on crystal quality and surface microstructure. In addition to crystal quality, wafer bow was studied. Crystal quality and growth rate are loosely correlated to wafer bow in our study. SEM surface microstructural analysis of the SiC films shows a changing microstructure with growth temperature consistent changes in measured crystal quality. TEM studies reveal that the films are the 3C polytype having a high density of planar faults.

Properties of layered crystals for radiation detectors: TlGaSe2 system

In this paper we report the thermal and electronic characteristics of the ternary compound TlGaSe2, grown from the melt. The phase-diagram, congruency of material and phase transition were studied by differential thermal analysis. The material melted congruently at 812.9C and did not show any other phases between room temperature and the melting point. Results of crystal growth, and effect of alloying and its effect on crystal growth and properties are also discussed. In high purity material it is easier to cleave the crystal since layers of successive rows of tetrahedrons are turned away from each other by 900. However, by suitable alloying, the angle of the tetrahedrons can be changed and the tendency to cleave can be decreased. This effect will be evaluated by comparing the microstructural properties of pure and alloyed crystals. By measuring the optical transmission band edge we studied details absorption characteristics to evaluate the effect of impurities on recombination characteristics. Results indicated that there were no donor-acceptor recombinations at or above room temperature.

High efficiency LWIR AOTF

We have developed and fabricated a Tl3AsSe3 (TAS) crystal based acousto-optic tunable filter (AOTF) for operation between the 8 to 12.0 μm wavelength regions. We have demonstrated peak efficiency greater than 60% with a 10.6 μm source and 2 watts of RF input power. This high efficiency should enable high resolution and large throuput for AOTF based imaging and spectroscopic systems.