Peter K. Shu

Optical quenching of photoconductivity in GaN photoconductors

The observation of optical quenching of photoconductivity in GaN photoconductors at room temperature is reported on. Three prominent quenching bands were found at Ev+1.44, 1.58, and 2.20 eV, respectively. These levels are related to three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He–Ne laser shining on the photoconductor.

Direct observation of transferred‐electron effect in GaN

We report the direct observation of transferred‐electron effect in unintentionally doped GaN epilayers grown by metalorganic chemical vapor deposition. The negative differential resistivity (NDR) was observed from the current‐electric field characteristics in GaN using a metal‐semiconductor‐metal (M‐S‐M) system. The threshold field for the onset of NDR was independent of the spacing of M‐S‐M fingers, and was measured to be 1.91×105 V/cm for GaN with an n‐type carrier concentration of 1014 cm−3. This value is very close to the value obtained from theoretical simulation. This observation is an experimental evidence of transferred‐electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn‐effect devices using GaN materials.

Signal generation in CdZnTe strip detectors

The energy resolution of CdTe and CdZnTe detectors is usually limited by the poor transport properties of holes. Devices segmented into small pixels have been observed to exhibit improved energy resolutions. Simulations have shown that this small pixel effect is due to the fact that small pixels are mostly sensitive to carriers moving close to the pixel, within a distance of the order of the pixel size. In this paper, signals are calculated for CdZnTe strip detectors in order to determine to what extent a similar small electrode effect is produced by strips. The free carrier density distributions following the absorption of a /spl gamma/-ray are calculated by solving the continuity equations. Combined with the strip weighting field, this provides the signal induced in the strip. Simulations are made for various detector geometries and for both the anode and cathode strips. Simulated signals are compared with actual signals measured on CdZnTe detectors.

Radiation damage and activation of CdZnTe by intermediate energy neutrons

We exposed a CdZnTe detector to MeV neutrons from a 252Cf source and found no performance degradation for fluences below 1010 neutrons cm-2. Detector resolution did show significant degradation at higher neutron fluences. There is evidence of room temperature annealing of the radiation effects over time. Activation lines were observed and the responsible isotopes were identified by the energy and half-life of the lines. These radiation damage studies allow evaluation of the robustness of CdZnTe detectors in high neutron and radiation environments.

CdZnTe strip detector for arcsecond imaging and spectroscopy

A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy is being developed for a space flight gamma-ray burst instrument. Two dimensional strip detectors with 100 micrometers pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 micrometers. In addition, good spectral resolution has been achieved. The uniformity of response and relative efficiency of the strip detector will be discussed. Results form electrical characterization which include strip leakage current and strip capacitance will be presented.

CdZnTe strip detectors for astrophysical arc second imaging and spectroscopy: detector performance and radiation effects

CdZnTe strip detectors have been fabricated and tested to show the ability for arc second imaging and spectroscopy. Two dimensional CdZnTe strip detectors with 100 micron pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 microns. Good spectral resolution has also been achieved. The uniformity and relative efficiency of the strip detector are discussed. Radiation damage effects by intermediate energy neutrons and low energy protons on the surface and bulk performance of CdZnTe devices have been investigated and are presented. Activation and annealing of radiation effects have been seen and are discussed.

Microshutter array system for James Webb Space Telescope

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

Fabrication of high-performance CdZnTe strip detector arrays

Cadmium zinc telluride (CsZnTe) has been applied as a X-ray and gamma-ray radiation detector due to its large bandgap and high atomic number which gives low leakage current and good absorption efficiency. In addition, the CdZnTe detectors can be operated at room temperature with very good energy resolution and excellent spatial resolution. The main challenges in fabricating a large area CdZnTe strip detector array are the requirements of low leakage current in the individual strips of each detector and the good adhesion of the pad metal to the CdZnTe surface for wire bonding. The Detector Systems Branch at Goddard Space Flight CEnter has successfully fabricated the double sized CdZnTe strip detectors for a 6 X 6 array. The strip detector array has 36 double sided detectors with each detector having 127 strips and a guard ring on each side with a spatial resolution < 100 micrometers . The total area of the array is 60 cm2 with 762 X 762 strips to give 580,000 pixels - a factor of 100 better than other CdZnTe or scintillator arrays. In this paper, we will present our novel processing for fabricating such high performance strip detectors. The leakage current, interstrip resistance and energy resolution were studied as a function of different etchants/time and post-annealing temperature. The effects of chemical etching and post annealing on the CdZnTe surface and interface property were also discussed.

Laboratory and sky testing results for the TIS H4RG-10 4k x 4k 10-micron visible CMOS-hybrid detector

We present both laboratory and telescope testing results describing the performance of the H4RG-10 CMOS-Hybrid detector. The H4RG-10 is the largest visible hybrid array currently in existence and shows great potential for use in future space missions. We report read noise, dark current, pixel connectivity, persistence, and inter-pixel capacitance measurements for the temperature range 110-240 K. We report on quantitative astrometric and qualitative photometric performance of the instrument based on observations made at USNOs Flagstaff Station observatory and establish an upper limit to the astrometric performance of the detector. We discuss additional testing and future work associated with improving detector performance.

Room temperature semiconductor detectors for hard x-ray astrophysics

Room temperature cadmium zinc telluride (CdZnTe) and mercuric iodide (HgI2) semiconductor hard X-ray detectors are currently being evaluated at NASA Goddard Space Flight Center for use in future balloon and satellite applications. PoRTIA, a small engineering prototype hard X-ray (20 - 150 keV) balloon instrument will contain both a CdZnTe and a HgI2 detector, each 6.5 cm2 x .15 - .2 cm and sharing the same 5 degree(s) field-of-view. PoRTIA will be launched from Alice Springs, Australia in the Spring of 1995 as a piggyback instrument aboard the GRIS balloon payload. PoRTIA will provide valuable information about detector efficiency, durability and material dependent detector background components at balloon altitudes as it observes the Crab Nebula. In addition, a CdZnTe research and development program has been initiated to develop the capability to produce improved CdZnTe detectors for astrophysics applications. The work at Goddard continues in an effort to develop CdZnTe detectors with improvements in electronics, contacts and packaging methods.