Effect of signal-modulated optical radiation on the characteristics of a Modfet

The effect of signal-modulated optical radiation on the characteristics of a GaAlAs/GaAs MODFET has been studied analytically. It is found that the offset voltage increases with modulation frequency and the effect of frequency is negligible above 5 MHz. The drain-source current decreases with increase in signal frequency at a constant radiation flux density, doping concentration and drain-source voltage. Studies on sheet concentration and transconductance also show that the signal frequency has a significant effect upto a certain modulation frequency (5 MHz) above which the effect of frequency is insignificant.     

Photoconductive response times of Si-on-sapphire damaged with Si28 ions

We present autocorrelation measurements showing the high-speed sampling and switching capabilities of SOS photoconducting elements which have been ion implanted with 60 and 160 keV Si²⁸ ions. An analysis of the circuit shows the intrinsic photoconductive decay to be very fast (3.5 ps) and that the measured response is primarily limited by the gap capacitance and the associated R-C time.     

The influence of tunneling effects on the efficiency of heterojunction solar cells

A theoretical model is developed which presents the transport properties through the space charge region of ap ⁺ n heterojunction solar cell, whereby not only recombination through interface states but also tunneling through potential barriers is taken into account. It is investigated whether tunneling can give rise to optimum heterojunction structures which have better efficiencies that without tunneling. It is found that only if the strongly doped semiconductor has an optimum bandgap and the weakly doped semiconductor a larger bandgap, tunneling can make the structure optimum. In all other cases of optimum structures, tunneling deteriorates the efficiency. Work supported by the Energy R. D. programmes of the Commission of European Communities and the Belgian Ministry of Science.     

Sulphur-doped silicon background-limited infrared photodetectors near 77 K

The photoconductive properties of sulphur-doped extrinsic silicon infrared detectors prepared by closed-tube diffusion techniques have been investigated. Spectral response data show that this material would be suitable for thermal imaging of 3–5 μm radiation, whilst detectivity measurements as a function of temperature indicate that background-limited operation is achievable near liquid nitrogen temperature (77 K).     

Negative affinity 3–5 photocathodes: Their physics and technology

Negative electron affinity (NEA) photocathodes are defined by the relationship between the potential barrier at the surface and the bottom of the conduction band in the bulk of the material. If the bottom of the conduction band liesabove the potential barrier at the surface, the device is said to have a negative electron affinity. In practice this condition is obtained by heavyp-doping of the semiconductor (to encourage downward band bending at the surface) and by adding a thin film (several atomic layers) of cesium richcesium oxide on the clean semiconductor surface. The physics, development, fabrication, and applications of the NEA cathode are reviewed. The threshold of response of a NEA photocathode is set by the semiconductor bandgap. By alloying to form ternary or quaternary 3–5 compounds (3–5 compounds are formed from elements of the 3rd and 5th columns of the periodic table), the bandgap (and thus the threshold) can be placed at any desired photon energy within certain limits. The most important limit is that at about 1.1 eV which is the lowest limit achieved for NEA cathodes. This limit is set by the point at which the bandgap of the 3–5 material becomes less than the surface potential barrier. Fundamental work aimed at understanding the 3–5: cesium oxide “interfacial” barrier which sets this limitation is briefly discussed. Because of the “interfacial” barrier, the quantum yield of NEA cathodes decreases as the threshold of response moves to lower photon energy. Field assisted photocathodes provide a means of extending the threshold of response beyond 1.1 eV. Two different approaches to field assisted photocathodes and recent achievements are discussed. A major advancement has been the achievement of semi-transparent NEA photocathodes by sealing GaAs to glass. This makes possible practical NEA image tubes. The thermionic emission from 3-5 NEA cathodes can be orders of magnitude lower than that from conventional photocathodes. The reasons for this are discussed. Yield and dark current data are given on 3-5 NEA cathodes in operating photomultipliers. Work supported in part by the Advanced Research Projects Agency of the Department of Defense and monitored by Night Vision Laboratory, U.S Army Electronics Command, under Contract No. DAAK 02-74-C-0069.     

The lateral photovoltaic effect in CdS-Cu2S heterojunction solar cell

The lateral photovoltaic effect has been observed in CdS-Cu₂S thin-film solar cells. The effect is more pronounced on the CdS side than on the Cu₂S side of the cells. On the CdS side, where the contacts were formed by soldering Cu wire by indium and then applying Ag paint, the photovoltage developed were found to increase as the point of illumination was moved towards the contact. The spectral response of photovoltage for coevaporated cells shows a peak at=0.5m (2.45 eV). But for topotaxial cells two peaks, one at=0.5m and the other at=0.65m (1.89eV) were observed. A band model has been proposed for the heat-treated optimized cells.     

On the thermodynamic limit of photovoltaic energy conversion

An infinite stack ofp—n junctions with smoothly varying bandgap from ∞ to 0 is considered. AnI —V characteristic is derived, which is more correct than the classical exponential characteristic. It is shown that open-circuit operation is a reversible process and leads to the Carnot efficiency, if one defines the efficiency in the way that is usual in the theory of thermodynamic engines. If instead one uses the definition of efficiency usual in photovoltaics, open-circuit mode gives rise to zero efficiency. Then operation at maximum efficiency equals operation at maximum power and is not reversible.     

Light trapping effect in silicon wafers with anodically etched surfaces

Received: 10 May 1996/Accepted: 19 November 1996     

Pb2CrO5 photovoltaic device for the detecting light-beam position

A light position detector operating through a photovoltaic effect of a Pb₂CrO₅ ceramic disk with a pair of Au planar electrodes is investigated. A fabrication technique and the basic characteristics of the photovoltaic device for position detection are described. A peak photovoltage is obtained around the edge of the electrode for an incident light beam. The incident light beam shape and the electrode pattern are important factors for obtaining a linear relation between the light-beam position and the output signal from the device. A device fabricated for detecting one-dimensional light position has a high position resolution of 0.5 m and a good linearity of ±2 m or less. A two-dimensional device can be fabricated in the same way as the one-dimensional device, except for the electrode pattern. A method for two-dimensional light position detection using a Pb₂CrO₅ photovoltaic cell is demonstrated for a green LED as a light source.     

Carrier frequency-dependent intrinsic parameters in an ion-implanted silicon photo-MESFET

Carrier frequency-dependent intrinsic parameters of an ion-implanted silicon photo-MESFET have been analysed theoretically. The internal gate source capacitanceC _gs is found to increase with increasing carrier frequency under the normally OFF condition and the change is small under the normally ON condition. Also, the internal drain-source resistanceR _ds increases with frequency at a fixed flux density and wavelength of operation. The ion-implanted photo-MESFET could become useful as optically controlled switching device in digital circuits.     

The performance of Hg1−x Zn x Te photodiodes

This paper considers the Hg_1–x Zn _x Te alloy system as a potential material for the fabrication of infrared photodiodes. The influence of different junction current components (diffusion, tunneling and depletion layer currents) on the R ₀ A product of n⁺-pHg_1–x Zn _x Te photodiodes is analysed. The upper theoretical limits of the R ₀ A product and detectivity are determined. Results of calculations are compared with experimental data reported by other authors and those measured in our laboratory. Preliminary results on related technology and the properties of Hg_1–x Zn _x Te prepared by the ion-etching technique are presented.     

Theoretical characterisation of a superlattice avalanche photodiode

A superlattice avalanche photodiode using III–V materials is expected to be used in long-distance fiberoptic communication systems in the 1.3 to 1.55 m wavelength range. Theoretical studies have been made on the effective ionization rates of electrons and holes of the device, I–V characteristic and the frequency response characteristic of the Al _x Ga_1–x As/GaAs superlattice p⁺-i-n⁺ structure. It is observed that the/ ratio of the device increases with the field and the bandwidth of the response curve increases with decrease in the dc multiplication gain.     

Photoconductivity in a photocrosslinkable second-order nonlinear optical polymer

Photoconductivity measurements on a stable photocrosslinkable nonlinear optical polymer — PVCN (polyvinylcinnamate)/CNNB-R (3-cinnamoyloxy-4-[4-N,N-diethylamino)-2-cinnamoyloxy phenylazo] nitrobenzene) are reported. Photoconductivity in this polymer system, in both crosslinked and uncrosslinked states, was observed without introducing any sensitizer or carrier-transport agents. Absorption of CNNB-R (_peak=518 nm) results in photocarrier generation and it is conjectured that CNNB-R also plays a role in the carrier transport. Such polymers are candidate materials for photorefraction.     

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchatel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Optical gain in photoconductors made of compensated and partially compensated GaAs

The dc gain behaviour of GaAs photoconductors realized utilizing a partially compensated buffer layer of an epitaxial MESFET structure as well as a Cr-doped semiinsulating substrate is studied. The light-power dependence of the gain hints to the dominant role of the bimolecular recombination process and trapmediated gain, and only a minor role of the surface photovoltaic effect. The possible correlation between dark current and gain mechanism in the MESFET-like device is pointed out.     

Digital optical computing

The motivation for a digital optical computer is based on the shortcomings inherent in of electronic computers. Optics has solutions to offer especially in interconnects. Devices based on nonlinear optical effects are still in the early stages of their development. Hence at an intermediate stage hybrid opto-electronic computers might emerge. Their architectures should make use of the special attributes of optics. A specific approach called symbolic substitution logic is outlined.     

Fiber optical communications devices

Both low attenuation silica optical fibers with peak transmission in the wavelength regions of 0.85 μm, and 1.05 μm, and improved lasers at both wavelengths are now available. In this review paper, the principal components for emission, modulation and detection are described. The characteristics of both semiconductor lasers, made of GaAs and related compounds, emitting at 0.85 μm or 1.05 μm and high neodymium-content lasers are discussed. For modulation, current modulation of GaAs lasers and external electro-optic modulation are considered. Concerning detection, the realisation of Si photodetectors suitable at 0.85 μm and the new photodetectors at 1.05 μm from Ga_1−x In _x As are reviewed.     

Imaging properties and applications of scanning optical microscopes

This review paper is concerned with the imaging properties and major uses of scanning optical microscopes. It is shown that the confocal scanning microscope exhibits a form of super-resolution and that the instrument in general has great application in nonlinear microscopy and the inspection of electronic devices.     

Optoelectronic characteristics of close-packed HgTe nanoparticles in the infrared range

Absorption, photoluminescence (PL), photoresponse, and I-V measurements were made for a close-packed HgTe nanoparticle film without organic capping materials to investigate its optoelectronic characteristics in the infrared (IR) range. In the absorption and PL spectra taken for the close-packed nanoparticle film, the wavelength of exciton peak was red-shifted, compared with 1-thioglycerol capped HgTe nanoparticles dispersed in solution. For the HgTe nanoparticle film, dark current was below several pA level, current was increased by about three orders of magnitude at a biased voltage of 3 V under the illumination, and photoresponse was very rapid compared with 1-thioglycerol capped HgTe nanoparticles. These optoelectronic characteristics illustrate that HgTe nanoparticles are one of promising materials for the photodetector in the IR range. Finally, the origin for the increase of photocurrent with increasing temperature observed in this study will be discussed.     

An analysis of optical waveguide tapers

The characteristics of slab waveguide tapers with finite-thickness cladding are investigated by approximating the tapered section as a series of uniform five-layer waveguides with successive changes in the core and cladding thickness. The behaviour of multimode coupling and transmission characteristics for typical tapers are studied in detail by computer simulation, employing parameter values appropriate for practical systems. Strong coupling and efficient conversion between core and cladding modes are demonstrated. In addition, it is shown that for tapers of moderate slopes, the nearest-neighbour coupling approximation is quite adequate for the calculation of the individual mode amplitudes. For steeper tapers, however, additional neighbouring modes have to be taken into account, and the modal energy tends to spread further towards the higher-order modes. It is also demonstrated that relatively large step-size can be used in the iterative numerical calculations. This, in conjunction with the choice of an appropriate coupling scheme (e.g., the nearest-neighbour approximation for mild tapers), makes it possible to reduce the cost of computer simulation. The relevance of the present analysis to fiber tapers is also discussed. Supported in part by the Department of Communications of Canada