Topographic mapping lidar instruments must be able to detect extremely weak laser return signals from high altitudes
including orbital distance. The signals have a wide dynamic range caused by the variability in atmospheric transmission
and surface reflectance under a fast moving spacecraft. Ideally, lidar detectors should be able to detect laser signal return
pulses at the single photon level and produce linear output for multiple photon events. Silicon avalanche photodiode
(APD) detectors have been used in most space lidar receivers to date. Their sensitivity is typically hundreds of photons
per pulse, and is limited by the quantum efficiency, APD gain noise, dark current, and preamplifier noise. NASA is
pursuing three approaches for a 16-channel laser photoreceiver for use on the next generation direct-detection airborne
and spaceborne lidars. We present our measurement results and a comparison of their performance.
An overview of the Intensified Photodiode (IPD) is presented with an emphasis on IPDs optimized for use in the 950nm to 1350nm spectral range for single photon detection applications. The theory of operation of the IPD, two different electron optics designs, and device performance for a multichannel, 4x4 pixel array, low jitter IPD optimized for operation at 1060nm are presented in this paper. Key results include greater than 15% quantum efficiency, large active area, and less than 550ps impulse response.
The burgeoning number of mobile consumer electronics has created a demand for lightweight, low-cost, portable displays. The development of a polycrystalline-silicon thin film transistor (TFT) technology compatible with plastic substrates will enable displays and large-area electronics that are low power, rugged and flexible. Significant challenges exist in the development of a polysilicon TFT fabrication process that is compatible with plastic substrates, since plastic has a much lower thermal budget than glass substrates. In general, superior polysilicon TFT performance is achieved with higher temperature fabrication processes because the quality of the polysilicon and gate- dielectric films are very sensitive to process temperature. In this work, an ultra-low-temperature process for fabricating high-quality self-aligned polysilicon TFTs on flexible plastic substrates is described. All fabrication steps are performed at or below 100 degrees C. Polysilicon is formed by crystallizing sputtered amorphous Si films using a XeCl excimer laser with a pulse duration of approximately 35 ns. Gate oxide deposition is formed using high-density plasma CVD, and metal films are deposited by sputtering.
Electron Bombarded Charge Coupled Devices (EBCCD) which utilize a high performance Gallium Arsenide (GaAs) photocathode have been fabricated and characterized for performance and tube operating life. The EBCCD utilized an 11 mm diagonal, backside illuminated, frame transfer CCD compatible with RS170 video output. The CCD incorporated lateral anti-blooming structures optimized for backside operation. The EBCCD tube was proximity focused and operated with high gain (greater than 150) at low electron landing voltages (less than 2 kV). The EBCCD was integrated in a gated camera system with fast rise and fall times (less than 50 ns). Predicted operating life in a gated camera system as determined by accelerated tests is in excess of 12,000 hours, limited by photocathode degradation.
The energy spectra of electrons emitted from transmission-mode negative electron affinity photocathodes have been measured at high resolution using a parllel-plate retarding technique. The spectra from GaAs photocathodes have a basic structure that varies with temperature, activation layer qualitites, cathode thickness, and illuminating wavelength. A FWHM energy spread of approximately 50meV at room temperature has been achieved. Spectra from a GaAsP cathode show a markedly different structure and a much wider energy spread.
This paper details the status of a program at Intevac ATD to develop high sensitivity transmission photocathodes which function in the 0.95-1.7 micron wavelength range. The goal of the program is to develop this technology for use with both imaging and nonimaging detectors. Sealed tube results are presented. Measured performance characteristics include: cathode spectral response, dark current, linearity, and the effects of cooling. A brief discussion of planned development, potential applications, and simple modeling illustrating the advantages of the proposed detectors are included.
Brightness, energy spread and emission area are key parameters of electron sources for instruments such as electron microscopes and electron beam lithography tools. In developing transmission-mode NEA photocathodes as sources for these applications, these characteristics have been measured in specialized sealed tubes. Average lateral energies were measured at 63 meV for 1.5 micron thick photocathode, and 83 meV for a 0.5 micron thick photocathode, which was known to be emitting `hot' electrons. A current density of 841 A/cm2 was obtained from a 1.7 micron diameter emission area. This high current density can be explained in terms of lateral drift and diffusion of surface trapped electrons. Combined angular and current density data indicate a brightness of approximately 108 A/cm2-sr at 3 kV.
A new class of video rate imagers based on back-illuminated and thinned CCDs is available that shows promise to replace conventional image intensifiers for most military, industrial, and scientific applications. Thinned, back-illuminated CCDs (BCCDs) and electron-bombardment CCDs (EBCCDs) offer low light level performance superior to conventional image intensifier coupled CCD (ICCD) approaches. These new, high performance devices promise to expand the fields of science, provide high contrast, high resolution, low light level surveillance imaging, and make nighttime pilotage safer for military aviators. This paper presents experimental data which illustrates how responsivity, gain, and modulation transfer function (MTF) determine the low light imaging capability, the 'target of interest' signal to noise ratio (SNR) of each of these types of sensors. High SNR and MTF make BCCDs the imager of choice under moderately low light levels and EBCCDs the imager of choice under extremely low light level conditions.
Low light level surveillance cameras with significantly higher performance and reduced form factor, than present state of the art are critical for many commercial and military applications. Towards this end, a new approach to low light level cameras was successfully demonstrated. In a cooperative research and development effort between Scientific Imaging Technologies, Inc. of Beaverton, Ore., and Intevac EO Sensors of Palo Alto, Calif., back-illuminated, electron-bombarded CCD (EBCCD) sensors were designed and fabricated. Experiments demonstrated the EBCCD's sensitivity and contrast resolution superior to conventional intensified CCD (ICCD) approaches. Low light level signal to noise (STN) and contrast transfer function (CTF) data are presented. A model is derived that describes the performance of the EBCCD and the back-illuminated CCD relative to conventional approaches to nighttime imaging. A design and simulated performance of a video rate 2/3 inch, back-illuminated, electron-bombarded CCD currently under development for low light imaging applications is also described.
KEYWORDS: Quantum efficiency, Picosecond phenomena, Indium gallium arsenide, Semiconductors, Photodetectors, Power meters, Information operations, Absorption, Signal to noise ratio, Diodes
This paper reports on the status of a program to develop a streak tube compatible photocathode optimized for 1300-nm operation. Performance characteristics will be presented for a Transferred Electron photocathode with greater than 1% quantum efficiency at 1300-nm. Photocathode performance results will also be presented over the 950 - 1700 nm spectral range.
A novel high performance intensified photodiode (IPD) intended for general use in most applications requiring photomultiplication is described. The IPD has high quantum efficiency and fast time response. The detector is stable and requires a single high voltage power supply to operate. This paper describes the device physics as well as preliminary measurements of D.C. gain, quantum efficiency, and impulse response.
We have fabricated 18mm format vacuum photodiodes incorporating GaAsP/A1GaAsP photocathodes (Eg 8eV) with P-20 phosphor screens. The photocathode response peaks at approximately 550nm. The quantum efficiency at 550nm is in excess of 55 (electrons per incident photon). The photocathode dark current for these tubes is less than i014 Amps/cm2 at room temperature. We have compared this cathode with the GaAs/AlGaAs photocathode.
This paper details the status of a program at Varian EOSP to develop high-sensitivity transmission photocathodes which function in the 0.95 - 1.65 micron wavelength range. One goal of the program is to develop a streak tube compatible cathode with greater than 1% quantum efficiency at 1.3 micrometers . Sealed tube results are presented. Measured performance characteristics include: cathode spectral response, room temperature cathode photoresponse stability, dark current, emitted electron energy distributions, photodiode resolution/MTF, and preliminary time response data. Finally, the paper includes a brief review of transferred electron photocathode physics and potential applications.
KEYWORDS: Quantum efficiency, Diodes, Gallium arsenide, Image intensifiers, Modulation transfer functions, Photons, Signal to noise ratio, Absorption, Interfaces, Electron transport
This paper will describe a high quantum efficiency imaging phosphor diode optimized for 500-700 nm sensitivity. Potential applications for this tube include undersea imaging and detection of 530-nm laser light. The tube is designed to function as a low noise factor optical amplifier. The tube consists of an 18-mm CsO activated GaAs/AlGaAs photocathode and a high resolution P46 phosphor screen enclosed in a Kovar/ceramic vacuum envelope. Measured results for quantum efficiency (QE), MTF, dark current, noise factor, operating life, response time and gain are presented. Finally, the paper discusses the engineering tradeoffs associated with fabricating a GaAs/AlGaAs cathode with high short wavelength QE.
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