M. Nurul Abedin

Backscatter 2-

A 2-μm backscatter lidar system has been developed by utilizing tunable pulsed laser and infrared phototransistor for the transmitter and the receiver, respectively. To validate the system, the 2-μm atmospheric backscatter profiles were compared to profiles obtained at 1 and 0.5 μm using avalanche photodiode and photomultiplier tube, respectively. Consequently, a methodology is proposed to compare the performance of different lidar systems operating at different wavelengths through various detection technologies. The methodology is based on extracting the system equivalent detectivity and comparing it to that of the detectors, as well as the ideal background detectivity. Besides, the 2-μm system capability for atmospheric CO2 temporal profiling using the differential absorption lidar (DIAL) technique was demonstrated. This was achieved by tuning the laser at slightly different wavelengths around the CO2 R22 absorption line in the 2.05-μm band. CO2 temporal profiles were also compared to in situ measurements. Preliminary results indicated average mixing ratios close to 390 ppm in the atmospheric boundary layer with 3.0% precision. The development of this system is an initial step for developing a high-resolution, high-precision direct-detection atmospheric CO2 DIAL system. A successful development of this system would be a valuable tool in obtaining and validating global atmospheric CO2 measurements.

\mu\hbox{m}

A model of the spectral responsivity of In1–GaSb p-n junction infrared photodetectors is developed. This model is based on calculations of the photogenerated and diffusion currents in the device. Expressions for the carrier mobilities, absorption coefficient, and normal-incidence reflectivity as a function of temperature are derived from extensions made to Adachi and Caughey-Thomas models. Contributions from the Auger recombination mechanism, which increase with a rise in temperature, are also considered. The responsivity is evaluated for different doping levels, diffusion depths, operating temperatures, and photon energies. Parameters calculated from the model are compared with available experimental data, and good agreement is obtained. These theoretical calculations help us to better understand the electro-optical behavior of In1–GaSb photodetectors, and can be utilized for performance enhancement through optimization of the device structure.

Lidar Validation for Atmospheric

Detectors operating in the 2 mm regime are critical for several applications such as atmospheric remote sensing of CO2 and optical communication systems. Current state-ofthe-art detectors based on InGaAs and HgCdTe materials epitaxially grown on binary substrates suffer fabrication complexity and performance deterioration when tuned to the 2 mm wavelength. Beside the existing technology, InGaSb ternary alloy systems show a promising performance for near-infrared detectors. By varying the indium composition, one can tune the wavelength of detection from 1.7 to 5 mm, and specifically to 2 mm by growing In1˛xGaxSb with x value of 0.8. Epitaxial growth of InGaSb layers on different binary substrates has been reported using different techniques. 1‐3 Beside the complexity of these techniques, tuning to the 2 mm regime usually involves performance deterioration mainly due to the lattice mismatch problems. Lattice mismatch in the grown layers increases dark current and noise, limiting both dynamic range and sensitivity of a detector. The availability of bulk ternary substrates may significantly simplify the fabrication process by using simpler and lower cost techniques. In this letter the fabrication and characterization of InGaSb p‐n photodetectors using InGaSb substrates are presented. The device fabrication was carried out at Rensselaer Polytechnic Institute, while the characterization was carried out at NASA Langley Research Center. Tellurium doped n-type In0.17Ga0.83Sb substrates were grown from a high temperature melt using the vertical Bridgman technique. 4 The synthesis of InGaSb was carried out in silica crucible under argon ambient using pre-synthesized GaSb and InSb polycrystals. The compositionally graded InGaSb crystal was grown using a ,100. GaSb seed. The growth rate varied in the range of 0.1‐ 0.5 mm/hr during the course of the experiment. The temperature gradient of the furnace near the melt-solid interface was in the range of 10‐ 15° C/cm. No melt stirring was employed during the growth. After the entire melt solidified, the furnace was cooled down gradually to room temperature (at a rate of 15‐ 20° C/hr ) to avoid thermal cracking of the crystal. Wafers were sliced using a diamond wheel saw (Southbay Technology). The wafers were then polished to mirror shining using a three-step optimized lapping and polishing process. Lapping was done using boron carbide 14 micron size abrasives on a PanW pad and the two step polishing was done using alumina 1 and 0.3 micron slurries on nylon and velvet pads, respectively. To form the p-type layers, Zn diffusion was carried out using the leaky box technique. Zn pellets were used as the source and the diffusion was done at 450° C for 2 h under flowing nitrogen gas. A schematic for the photodiode structure is shown in Fig. 1. Metallization was carried out using electron-beam evaporator by depositing 200 A tin followed by 1000 A gold for the back side contact and 400 A tin followed by 800 A gold for the front side metal pads. The p-layer thickness is about 0.4 mm with 10 19 cm ˛3 doping concentration, while the bulk substrate is 750 mm thick with about 10 17 cm ˛3 doping concentration as determined by C‐V measurements. No antireflective coating or surface passivation was applied. Several diodes and photodiodes with different areas were fabricated. The device characterization was

\hbox{CO}_{2}

Two-micron detectors are critical for atmospheric CO2 profiling using the lidar technique. InGaAs and HgCdTe detectors are commercially available for this wavelength but they lack sufficient gain, which limits their detectivity. The characterization results of a novel AlGaAsSb/InGaAsSb phototransistor for 2-μm application are reported. The device was developed by AstroPower, Inc. for NASA Langley Research Center. Spectral response measurements showed the highest responsivity in a 1.9- to 2.1-μm region with a maximum value of 2650 A/W at 2 μm. A 2-μm detectivity of 3.9×10 11 cm Hz 1/2 /W was obtained, which corresponds to noise equivalent power of 4.6×10 –14 W/Hz 1/2 .

Differential Absorption Lidar Applications

A detailed analysis is presented on the temperature and alloy composition dependence of the optical properties of III-V alloys AlxGa1−xAsySb1−y and GaxIn1−xAsySb1−y in the energy range 0.5–6 eV. Expressions for the complex dielectric function are based on a semiempirical phenomenological model, which takes under consideration indirect and direct transitions below and above the fundamental absorption edge. Dielectric function and absorption coefficient calculations are in satisfactory agreement with available experimental data. Other dielectric related optical data, such as the refractive index, extinction, and reflection coefficients, can also be obtained from the model.

Modeling of the temperature-dependent spectral response of In1−χGaχSb infrared photodetectors

The University of Hawaii and NASA Langley Research Center are developing small, compact, and portable remote Raman systems with pulsed lasers for planetary exploration under the Mars Instrument Development Program. The remote Raman instruments developed previously utilized small telescopes with clear apertures of 125 mm and 100 mm diameters and were able to detect water, ice, water bearing minerals, carbon in carbonate form in calcite, magnesite, dolomite, and siderite from a distance of 10 to 50 m under daytime and nighttime conditions. Recently, we significantly reduced the size of our time-resolved (TR) remote Raman system in order to build a compact system suitable for future space missions. This compact time-resolved Raman system was developed by utilizing (i) a regular 85 mm Nikon (F/1.8) lens with a clear aperture of 50 mm as a collection optic, and (ii) a miniature Raman spectrograph that is 1/14th in volume in comparison to the commercial spectrograph used in our previous work. In this paper, we present the TR remote Raman spectra obtained during daytime from various hydrous and anhydrous minerals, water, water-ice, and CO2-ice using this new compact remote Raman system to 50 m radial distance.

InGaSb photodetectors using an InGaSb substrate for 2μm applications

Knowledge of the spatial and temporal distribution of atmospheric species such as CO2, O3, H2O, and CH4 is important for understanding the chemistry and physical cycles involving Earths atmosphere. Although several remote sensing techniques are suitable for such measurements they are considered high cost techniques involving complicated instrumentation. Therefore, simultaneous measurement of atmospheric species using a single remote sensing instrument is significant for minimizing cost, size and complexity. While maintaining the instrument sensitivity and range, quality of multicolor detector, in terms of high quantum efficiency and low noise are vital for these missions. As the first step for developing multicolor focal plan array, the structure of a single element multicolor detector is presented in this paper. The detector consists of three p-n junction layers of Si, GaSb and InAs wafer bonded to cover the spectral range UV to 900 nm, 800 nm to 1.7 micron, and 1.5 micron to 3.4 micron, respectively. Modeling of the absorption coefficient for each material was carried out for optimizing the layers thicknesses for maximum absorption. The resulted quantum efficiency of each layer has been determined except InAs layer. The optical and electrical characterization of each layer structure is reported including dark current and spectral response measurements of Si pin structure and of GaSb and InAs p-n junctions. The effect of the material processing is discussed.

AlGaAsSb/InGaAsSb phototransistors for 2-μm remote sensing applications

An InGaAsSb/AlGaAsSb phototransistor has been validated for lidar atmospheric remote sensing. The validation was performed using the Raman-shifted eye-safe aerosol lidar (REAL) at the National Center for Atmospheric Research. Although the device is optimized for detection around the 2-µm wavelength, the validation was performed at 1.543 µm, where mature commercial detectors are available. Simultaneous measurement of the atmospheric backscatter signals using the custom-built phototransistor and commercial InGaAs avalanche photodiode indicated good agreement between both devices. The validation included detecting 11-km-range hard targets, 5-km atmospheric structure consisting of cirrus clouds, and a near-field boundary layer. Far-field low intensity and spatially narrow atmospheric features were also detectable with the new phototransistor. Preliminary results related to systematic effects are discussed in the first attempt of incorporating a phototransistor in a lidar system.

Calculations of the temperature and alloy composition effects on the optical properties of AlxGa1−xAsySb1−y and GaxIn1−xAsySb1−y in the spectral range 0.5–6 eV

A multispectral instrument based on Raman, laser-induced fluorescence (LIF), laser-induced breakdown spectroscopy (LIBS), and a lidar system provides high-fidelity scientific investigations, scientific input, and science operation constraints in the context of planetary field campaigns with the Jupiter Europa Robotic Lander and Mars Sample Return mission opportunities. This instrument conducts scientific investigations analogous to investigations anticipated for missions to Mars and Jupiters icy moons. This combined multispectral instrument is capable of performing Raman and fluorescence spectroscopy out to a >100  m target distance from the rover system and provides single-wavelength atmospheric profiling over long ranges (>20  km). In this article, we will reveal integrated remote Raman, LIF, and lidar technologies for use in robotic and lander-based planetary remote sensing applications. Discussions are focused on recently developed Raman, LIF, and lidar systems in addition to emphasizing surface water ice, surface and subsurface minerals, organics, biogenic, biomarker identification, atmospheric aerosols and clouds distributions, i.e., near-field atmospheric thin layers detection for next robotic-lander based instruments to measure all the above-mentioned parameters.

Compact time-resolved remote Raman system for detection of anhydrous and hydrous minerals and ices for planetary exploration

Profiling of atmospheric CO2 at 2 μm wavelength using the LIDAR technique, has recently gained interest. Although several detectors might be suitable for this application, an ideal device would have high gain, low noise and narrow spectral response peaking around the wavelength of interest. This increases the detector signal-to-noise ratio and minimizes the background signal, thereby increasing the device sensitivity and dynamic range. Detectors meeting the above idealized criteria are commercially unavailable for this particular wavelength. In this paper, the characterization and analysis of Sb-based detectors for 2 μm lidar applications are presented. The detectors were manufactured by AstroPower, Inc., with an InGaAsSb absorbing layer and AlGaAsSb passivating layer. The characterization experiments included spectral response, current versus voltage and noise measurements. The effect of the detectors bias voltage and temperature on its performance, have been investigated as well. The detectors peak responsivity is located at the 2 μm wavelength. Comparing three detector samples, an optimization of the spectral response around the 2 μm wavelength, through a narrower spectral period was observed. Increasing the detector bias voltage enhances the device gain at the narrow spectral range, while cooling the device reduces the cut-off wavelength and lowers its noise. Noise-equivalent-power analysis results in a value as low as 4x10-12 W/Hz1/2 corresponding to D* of 1x1010 cmHz1/2/W, at -1 V and 20°C. Discussions also include device operational physics and optimization guidelines, taking into account peculiarity of the Type II heterointerface and transport mechanisms under these conditions.