SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
New instrument gives quantitive information on "excess noise" in semiconductor-diode laser (SDL's). By proper selection of detector, instrument tests any SDL from visible wavelengths through thermal infrared. Lasers determine excess noise in SKL source by measuring photocurrent generated in photodetector exposed first to reference laser then to SKL under test.
The basic mechanism associated with the photovoltaic phenomena observed in the R.F. sputtered BaTiO3/silicon system is presented. Series of measurements of short circuit photocurrents and open circuit photovoltage were made. The composition depth profiles and the interface characteristics of the BaTiO3/silicon system were investigated for a better understanding of the electronic properties. A Scanning Auger Microprobe combined with ion in depth profiling were used.
A review of the major techniques for measuring minority carrier diffusion lengths in solar cells is given. Limits of applicability are indicated for each method, as relevant to indirect bandgap materials such as silicon, and direct bandgap materials like gallium arsenide. A discussion and bibliography is presented for the following techniques: uniform generation, electron beam induced current, Schottky barrier (and other steady-state) photocurrent, surface photovoltage, transient decay, and luminescence.
The development of an organic semiconductor solar cell and the effects of the recrystallization of metal free phthalocyanine (H2PC) on the characteristics of NESA/H2PC-PVK/Au sandwich cells were investigated. Alfa-H2PC sandwich cells showed photovoltage and photocurrent in a two direction opposite to that shown y as supplied H2PC cells, which consists mainly of beta-H2PC. Some difference was observed in the response times of the two cells. It is suggested that photocharacteristics change with the specific resistance of the H2PC, which is related to its crystal forms. In the cells with low resistance H2PC carriers are generated in H2PC by illumination, while in high resistance H2PC cells, carriers are generated in PVK which is sensitized with H2PC.
High-speed crystal growth techniques demand high-speed characterization techniques to allow a timely feed-back of information to the crystal growers. The unique properties of the Si electrolyte-contact (SEC) provide for an extremely fast and simple measurement of the light-induced photo-current for any piece of Si without lengthy preparation of the specimen. Electropolishing at high anodic current densities allows for insitu generation of fresh surfaces whereas preferential etching of defects in various modes is possible at low current densities. In n-type Si a simple estimation of the minority-carrier diffusion length is possible in many cases. Laser-scanning enables local probing of the photocurrent and provides data about the homogeneity of a sample. The experimental realization of the method is described in detail and examples are given and discussed.
It is shown how a simple integrating amplifier based on commercially available JFET and MOSFET switches can be used to measure photocurrents from detectors with noise levels as low as 1.6 x 10 to the -18th A/root Hz (10 electrons/sec). A figure shows the basic circuit, along with the waveform at the output. The readout is completely nondestructive; the reset noise does not contribute since sampling of the accumulated charge occurs between resets which are required only when the stored charge has reached a very high level. The storage capacity ranges from 10 to the 6th to 10 to the 9th electrons, depending on detector parameters and linearity requirements. Data taken with an Si:Sb detector operated at 24 microns are presented. The responsivity agrees well with the value obtained by Young et al. (1981) in the transimpedance amplifier circuit. The data are seen as indicating that extremely low values of NEP can be obtained for integration times of 1 sec and that longer integrations continue to improve the SNR at a rate faster than the square root of time when background noise is not present.
PIN photodiodes were bombarded with 40- and 85-MeV protons to a fluence of 1.5 x 10 to the 11th power p/sq cm, and the resulting change in spectral response in the near infrared was determined. The photocurrent, dark current and pulse amplitude were measured as a function of proton fluence. Changes in these three measured properties are discussed in terms of changes in the diode's spectral response, minority carrier diffusion length and depletion width. A simple model of induced radiation effects is presented which is in good agreement with the experimental results. The model assumes that incident protons produce charged defects within the depletion region simulating donor type impurities.
The MINP solar cell concept refers to a cell structure designed to be a base region dominated device. Thus, it is desirable that recombination losses are reduced to the point that they occur only in the base region. The most unique feature of the MINP cell design is that a tunneling contact is utilized for the metallic contact on the front surface. The areas under the collector grid and bus bar are passivated by a thin oxide of tunneling thickness. Efforts must also be taken to minimize recombination at the surface between grid lines, at the junction periphery and within the emitter. Results of both theoretical and experimental studies of silicon MINP cells are given. Performance calculations are described which give expected efficiencies as a function of base resistivity and junction depth. Fabrication and characterization of cells are discussed which are based on 0.2 ohm-cm substrates, diffused emitters on the order of 0.15 to 0.20 microns deep, and with Mg MIS collector grids. A total area AM 1 efficiency of 16.8% was achieved. Detailed analyses of photocurrent and current loss mechanisms are presented and utilized to discuss future directions of research. Finally, results reported by other workers are discussed.
PIN photodiodes were bombarded with 40- and 85-MeV protons to a fluence of 1.5 x 10 to the 11th p/sq cm, and the resulting change in spectral response in the near infrared was determined. The photocurrent, dark current, and pulse amplitude were measured as a function of proton fluence. Changes in these three measured properties are discussed in terms of changes in the diode's spectral response, minority carrier diffusion length, and depletion width. A simple model of induced radiation effects is presented which is in good agreement with the experimental results. The model assumes that incident protons produce charged defects within the depletion region simulating donor type impurities.
Dc photoconductivity characteristics of HgI2 single crystals have been measured in the range of 1.75-2.75 eV, using CuI as a transparent electrical contact. Hole and electron photocurrents could be separately measured by applying different voltage polarities to the illuminated electrode. It is shown that charge carrier generation near the surface is highly extrinsic, as displayed by differences in the hole and electron photogeneration spectra. Analysis of the photoconductivity versus voltage characteristics indicates that the hole and electron bulk trapping times satisfy greater than 70 and 3 microsec, respectively. The hole and electron surface recombination velocities were 1.0 x 10 to the 4th cm/s and 8 x 10 to the 5th cm/s, respectively.
Data from the Pioneer Venus Langmuir probe, collected since 1979 (and thus, including the period between solar maximum in 1979-1980 and solar minimum in 1986-1987) are examined. Calculations show that about 51 percent of the solar emission at Venus is due to Lyman alpha (1216 A), 46 percent is produced by wavelengths between 550 and 1100 A, and less than 3 percent is due to wavelengths longer than Lyman alpha. The photocurrents were found to exhibit variations related to the solar cycle and solar rotation, as well as a major 7.2-month periodicity. Three different indices of solar EUV behavior at Venus were derived, which include the photoemission current itself, the total EUV flux, and an F(10.7)-like solar index, and are compared with related measurements made simultaneously at earth.
Surfaces to be bonded tested for cleanliness. Equipment includes commercial photoelectron-emission sensor. Inspected surface irradiated with ultraviolet photons from source in sensor head while anode in sensor head collects photoelectrons ejected from surface by absorbed photons. If intensity and spectrum of ultraviolet light and distance between sensor and surface held constant, then photocurrent varies according to amount and type of contamination.
The power spectrum of the relative intensity noise (RIN) of single-frequency diode-pumped Nd:YAG lasers is observed to be shot-noise limited at frequencies above 20 MHz for a photocurrent of up to 4.4 mA. Relaxation oscillations result in noise 60-70 dB above shot noise at a few hundred kHz. These relaxation oscillations have been suppressed using electronic feedback.
The observation of photon-assisted resonant tunneling in a multiple quantum well structure composed of doped quantum wells separated by variably space superlattice energy filters is reported. Electrons confined in the quantum wells are excited to the second quantized state by intersubband absorption of incident infrared radiation and are subsequently emitted through the filters under appropriate bias conditions. This is manifested by a distinct peak, with an associated negative differential photoconductance, in the photocurrent versus bias voltage characteristic at low temperatures.
Proposed npn AlxGa1-xAs phototransistor incorporates multiple-quantum-well (MQW) infrared photodetector. Has n-doped contacts and is embedded between p-doped base region and n-doped collector region of transistor. Photocurrent amplified, and dark current suppressed.
Device-quality a-Si:H and a-SiC:H films have been deposited using electron cyclotron resonance (ECR) microwave plasmas of SiH4, CH4, and H2 mixtures. Typical material properties of ECR-deposited, photosensitive a-Si:H films are: (1) high photosensitivity up to 2 x 106 with a photoconductivity of 10 exp -5 to 10 exp -4/(Ohm-cm), (2) a Tauc gap of 1.75 to 1.85 eV, (3) an Urbach slope of 50-60 meV determined by the constant photocurrent method, and (4) an integrated defect density of 1-2 x 10 exp 16/cu cm determined by junction capacitance measurements. Highly conductive, p-type a-SiC:H films have been produced by ECR plasmas with a conductivity of 0.2/(Ohm-cm).
A highly reproducible optically stimulated electron emission (OSEE) response steel was achieved in laboratory conditions through suppression of surface photochemistry by replacing air with argon. This permitted the characterization of the OSEE process on D6AC steel as a photodiode. With a low-pressure mercury light source, most of the OSEE current is attributed to photoelectrons released by the 185-nm mercury line with only about 5 percent of the photocurrent associated with the 254-nm line. The forward current-voltage characteristic of the OSEE diode is linear for small fields changing to a square root response at larger fields. These results are all consistent with established theories.
A systematic study was carried out of the properties of Ge:Ga photoconductive infrared detectors for wavelengths approx. 100 micron. The detectors studied were made from Ge:Ga with acceptor concentration N sub A approx. 2 x 10 exp 14/cu cm with both low compensation (10 exp -2) and ultralow compensation (10 exp -4). Noise measurements have been made as a function of background photon rate, bias voltage, and chopping frequency. Detective quantum efficiencies approaching unity have been observed over a side range of experimental parameters. Photocurrent has been measured as a function of voltage and temperature. Hall mobility and lifetime have been measured to determine their effect on detector properties. A small potential drop has been observed in nominally ohmic contacts produced by implantation of B ions.