Antimonide Based Compound Semiconductors (ABCS) and a new family of advanced analogue and digital silicon read-out integrated circuits form the basis of the SCD 3rd generation detector program, which builds on the firm platform of SCDs existing InSb-FPA technology. We have devised a staged roadmap at SCD which begins with epitaxial InSb mesa diodes and gradually increases in technological sophistication. In the initial stages we have focused in particular on In1-zAlzSb alloys grown on InSb by Molecular Beam Epitaxy (MBE). Some of our achievements with these materials are presented in this paper. For epitaxial InSb (z = 0), we demonstrate the performance of Focal Plane Arrays (FPAs) with a format of 320x256 pixels, at focal plane temperatures between 77K and 110K. An operability has been achieved which is in excess of 99.5%, with a Residual Non-Uniformity (RNU) at 95K of less than 0.03% (standard deviation/dynamic range) between 15 and 80% well fill. Moreover, after a two point Non-Uniformity Correction (NUC) has been applied at 95K, the RNU remains below ~0.1% at all focal plane temperatures down to 85K and up to 100K without the need to apply any further correction. This is a major improvement in both the temperature of operation and the temperature stability compared with implanted diodes made from bulk material. We also demonstrate rapid progress in the development of epitaxial InAlSb FPAs with comparable operability and RNU to the InSb FPAs but which exhibit lower dark current and offer a range of cut-off wavelengths shorter than in InSb. These FPAs are intended for temperatures of operation in excess of 100K.
Antimonide Based Compound Semiconductors (ABCS) and a new family of advanced analogue and digital silicon read-out integrated circuits form the basis of the SCD 3rd generation detector program, which builds on the firm platform of SCDs existing InSb-FPA technology. In order to cover the MWIR atmospheric window, we recently proposed the epitaxial alloys: InAs1-y Sby on GaSb with 0.07 < y < 0.11 and In1-zAlz Sb on InSb with 0 < z < 0.03. In this paper we focus on the results of some of our recent work on epitaxial In1-zAlz Sb grown on InSb by Molecular Beam Epitxay (MBE). In epitaxial InSb (z = 0), we demonstrate the performance of Focal Plane Arrays (FPAs) with a format of 320x256 pixels, at focal plane temperatures between 77K and 100K. An operability has been achieved which is in excess of 99.5%, with a Residual Non-Uniformity (RNU) at 95K of less than 0.03% (standard deviation/dynamic range). Moreover, after a two point Non-Uniformity Correction (NUC) has been applied at 95K, the RNU remains below ~0.1% at all focal plane temperatures down to 85K and up to 100K without the need to apply any further correction. This is a major improvement in both the temperature of operation and the temperature stability compared with implanted diodes made from bulk material. We also demonstrate rapid progress in the development of low current epitaxial InAlSb photodiodes with high uniformity and low dark current that offer a range of cut-off wavelengths shorter than in InSb. Preliminary results are presented on FPAs with a cut-off wavelength in the range λC~5μ.
Selective absorbers utilize in many cases the enhanced solar absorptance of small metal particles embedded in a layered ceramic medium. A crucial factor in determining the performance of such an absorber is the degradation rate of this layer. The oxidation of ultrafine gas evaporated metal particles is examined here. Several metal develop a stable oxide film around the surface of small particles. It is shown that there is a linear dependence between the size of the metal core and the particle radius for Al, Mg, Be and Sn in room temperature and for Si at high temperatures. Charge quantization is proposed as a possible mechanism responsible for this phenomenon. Thermogravimetric measurements on Ni particles show a rapid oxidation for intermediate temperatures.
We have studied the degradation of cermet selectivity solar absorbing coatings produced by magnetron sputtering. The samples consisted of a reflector layer of molybdenum, a graded Mo-Al2O3 cermet layer, and an antireflection layer of aluminum oxide. The coatings were exposed to temperatures between 673 K and 1123 K in an atmosphere where the pressure was kept below 10-4 Torr. The degradation of the solar absorptance was determined as a function of time and temperature. A remarkable stability at high temperatures in vacuum is reported. The optical properties could be interpreted within a multilayer optical model employing the Bruggeman effective medium theory for the cermet layer. It was found that the metal content of the cermet layer continuously decreased as the degradation proceeded. We assume that oxygen ions move through the amorphous aluminum oxide in microchannels and subsequently oxidize the molybdenum particles in the cermet layer. Theories for oxidation by anion movement are reviewed and used in order to interpret the experimental degradation kinetics.
There are already a large and varied number of existing selective coatings which are used for photothermal energy conversion, but lately coatings based on thin films produced by sputtering have begun to be used in industry. A coating of this type is based on a multilayer stack consisting of both dielectric and absorbing layers. Included in the absorbing layers are layers which have a dispersion of metal particles in a ceramic matrix, and in general the metal concentration in these layers is not uniform but graded. Programs for the calculation of optical parameters of such a multilayer cannot currently be purchased commercially. Therefore it was decided to develop a program which would calculate the reflectance, solar absorbtivity, thermal emissivity, and photothermal efficiency of a selective coating. Throughout the use of this program, it is possible to calculate the highest photothermal efficiency for any temperature, to evaluate the influence of each individual layer on the resultant behaviour of the entire stack, and to calculate the angular dependence of the solar absorbtivity.
Conference Committee Involvement (2)
Optics and Photonics in Global Homeland Security V
14 April 2009 | Orlando, Florida, United States
Optics and Photonics in Global Homeland Security IV
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