Hiroyuki Fujisada

Overview of ASTER instrument on EOS-AM1 platform

ASTER is an advanced multispectral imager with a high spatial, spectral and radiometric resolutions for EOS-AM1 platform which will be launched in June 1998. ASTER covers a wide spectral region from visible to thermal infrared by 14 spectral bands. Moreover, ASTER has a stereoscopic viewing capability by a near infrared band. Excellent observational performance can be expected by a pushbroom type visible and near infrared radiometer (VNIR subsystem) with a high spatial resolution of 15 m, a pushbroom type short wave infrared radiometer (SWIR subsystem) with a high spectral resolution and a whiskbroom type thermal infrared radiometer (TIR subsystem) with high spatial, spectral and radiometric resolutions. Long life mechanical cryocoolers are developed to enhance the performances of the SWIR and the TIR subsystems.

High Frequency Transport Properties in InSb under a Magnetic Field

Theoretical calculation has been made on high-frequency transport properties of InSb under a d.c. magnetic field, leading to the conclusion that the relative increase of microwave loss in InSb under a magnetic field, γ l , is given by γ 0 {1+ω(τ e +τ h )}, where γ 0 is the relative resistivity increase Δ ρ/ρ 0 under a d.c. magnetic field with d.c. current, and τ e and τ h are the constant relaxation times for electrons and holes respectively. Measurements were made on the microwave power loss at 34 Gc/s at room temperature under magnetic fields up to 1.5 weber/m 2 . The experimental results show an appreciable influence of ωτ on the loss even at room temperature as predicted by the theory.

Planar Type p-Channel InSb-MAOSFET

Planar type p-channel InSb-MAOSFET is fabricated by anodic oxidation and alumina sputtering for MAOS gate structure, and Be ion implantation for source and drain. An excellent on-off ratio of the order of 105 is obtained in the drain current. Results also indicate that threshold voltage depends on various conditions of the application of gate and drain voltages and that a time dependent response is observed for the drain current.

Temperature Dependence of Reverse Current in Be Ion Implanted InSb p+n Junctions

Temperature dependence of the reverse current in Be ion implanted InSb p+n junctions is studied. Results indicate that the generation-recombination is a dominant mechanism for the reverse current in metallurgical junctions. A combination of tunneling and thermal excitation via surface states is proposed as a surface conduction mechanism in field induced junctions.

InSb-MAOS Structure Fabricated by Anodization and Sputtering

A InSb-MAOS structure is fabricated by anodic oxidation and alumina sputtering. Effects of the thickness of the anodic oxide and thermal annealing on interface properties are described. The anodic oxide is not only an excellent gate insulator but also plays a buffer role in alumina sputtering. A thickness of 34 nm is minimum to avoid the sputtering damage. Any thermal annealing gives rise to a deterioration of interface properties, even at a low temperature of around 100°C. The InSb-MAOS structure fabricated with a low temperature process condition below 100°C shows stable C-V characteristics with small flat band voltage shift and low interface state densities.

Galvanomagnetic Properties in Inhomogeneous Indium Antimonide at 77°K

The effective magnetoresistance and Hall effects in inhomogeneous semiconductors with high carrier mobility have been calculated by using a stratified medium model. The theory shows that the effective magnetoresistance consists of two contributions, one proportional to sin 2 ϕ and the other with sharp dependence on ϕ, ϕ being the angle between the magnetic field and the current, and that the Hall effect is the same as in a homogeneous medium. The sin 2 ϕ dependence is mainly due to the physical magnetoresistance and the sharp one due to the stratified inhomogeneities in electron concentration. Experiments were made with high purity n -type InSb at 77°K under magnetic fields up to 1.5 weber/m 2 , showing a good agreement with the theory. It is concluded that the measured values of the magnetoresistance in high purity n -type InSb specimens at 77°K can be explained by considering the influence of stratified inhomogeneities in electron concentration, in addition to the physical magnetoresistance .

Deep Levels in n-Type Undoped and Te-doped InSb Crystals

Deep levels in InSb crystals used for photodiodes have been investigated. An electron trap was found at 0.10 and 0.12 eV below the conduction band in n-type undoped and Te-doped InSb, respectively. A hole trap was observed in both specimens and its level in a Te-doped specimen was estimated to be 0.032 eV above the valence band. The distribution profile and electron capture cross section of the electron traps were also measured.

Performance evaluation of ASTER cryocooler in orbit

The advanced spaceborne thermal emission and reflection radiometer (ASTER) was developed by the MInistry of Economy, Trade and Industry (METI) for installation in the EOS-AM1 spacecraft. The ASTER consists of a visible and near-infrared radiometer (VNIR), a short-wave infrared radiometer (SWIR) and a thermal infrared radiometer (TIR). Two cryocoolers are required to cool the infrared detectors for the SWIR and TIR subsystems. Two cryocoolers have been operating in orbit for over 22000 hours. The temperature of each detector was stabilized in the allowable temperature range. Long-term data have been acquired on the cooling performance and power consumption under normal operation for each cryocooler, the following are described; outline of ground test results and performance of the ASTER cryocooler in orbit for over 22000 hours.

Verification test and evaluation of vibration characteristics for the ASTER cryocoolers

The advanced spaceborne thermal emission and reflection radiometer (ASTER) is an instrument which was selected by NASA to fly on the EOS-AM1 platform in 1998. Two cryocoolers are required to cool infrared detectors for the short-wave infrared radiometer (SWIR) and thermal infrared radiometer (TIR). The mission lifetime of the EOS-AM1 platform is expected to be 5 years, and accordingly, an operation lifetime more than 5 years is required for ASTER cryocoolers. The goals in the development of the ASTER cryocoolers are realization of a operation lifetime of over 50,000 hours and mechanical vibration forces below 0.1 N in the frequency range from 40 Hz to 135 Hz in the driection of all three axes. A split- Stirling cycle cryocoolers with clearance seals and linear motors is employed for this purpose. The compressor design adopts a piston driving mechanism which has a twin-opposed piston configuration in one compression space. The mechanical vibration caused by a displacer in the expander unit is reduced by an active balancer. Cryocoolers for SWIR and TIR have cooling capacity of 1.2 W at 70 K with power consumption lower than 55 W without control electronics. Several engineering models (EM) have been fabricated and are presently undergoing performance and life tests. Results of cryocooler verification tests and effects of jitter of mechanical vibration on the ASTER instrument are described.