The design and fabrication of large-scale two-dimensional (2-D) arrays of visible light emitting diodes (LED's) is described along with their implementation as image sources for prototype virtual displays. The LED pixels were fabricated in the InGaAIP material system using a double mesa etch process. Pixel characteristics are presented and used to predict display luminance properties. The 10-mu m square LED's produced external quantum efficiencies of 0.5% with an emission spectrum peaked near 650 nm (red), This performance level allowed the target display luminance of 10 fL to be attained with just 2.9 mW of array power consumption. The LED arrays consisted of 240 columns x 144 rows (similar to VGA/8) with pixels on a 20-mu m pitch and were driven in a column major matrix addressing mode at 60 frames per second for image display. Pixels were driven at a constant current with pulse width modulation to achieve sixteen levels of gray. An analysis of array luminance uniformity is presented. Sample images of text, graphics, and gray scale images demonstrate the capabilities of the LED arrays as monochrome image sources for virtual displays.

Organically modified silicate (ORMOSIL) sol-gel matrices doped with oxygen-sensitive fluorescence indicators have shown great potential for optical sensing applications. In this work, the luminescence decay behavior of ruthenium(II)-tris-4,7-diphenyl-1,10-phenanthroline perchlorate dissolved in different ORMOSIL matrices was studied. This was done in order to investigate the effect of organic modification on the oxygen sensing properties of the doped sol-gel materials. Bulk xerogels were synthesized from an organically modified precursor, methyltrimethoxysilane (MTMS), tetraethyl orthosilicate (TEOS), and an equimolar mixture of the two. Systematic changes in composition were conducted to examine the structural properties of sol-gel silicates for possible oxygen supports. Luminescence quenching behavior was analyzed as a function of varying sol-gel composition and oxygen partial pressure. The Stern-Volmer quenching ratio was found to increase with increasing MTMS content. In addition, phase fluorimetric analysis was conducted on all doped sol-gel samples to examine the accuracy of the luminescence decay times.

A fibre-optic sensor for oxygen gas and dissolved oxygen is reported. The sensor employs the quenching by oxygen of the fluorescence from a ruthenium complex trapped in a cage-like structure of a sol-gel derived porous thin film coated on a multimode optical fibre. The light from a blue LED is modulated and used as the excitation source. The phase difference between the extremes of the measurement range (0 and 100% O-2 in gas and similar to 0ppm and water saturated with O-2 (9ppm) in liquid) is measured as a function of modulation frequency. The optimum frequency was experimentally found to be 75kHz using a standard lock-in amplifier for phase detection. Using the optimum pulse frequency, the sensor calibration curves in gas and liquid toward oxygen were determined. The sensor exhibited excellent sensitivity and reversibility.

Low-temperature photoluminescence was studied in a large number of pseudomorphic modulation-doped transistor structures having an ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As (x0.3) quantum well with sheet densities up to 2.5\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The Fermi edge appears clearly in the spectra, and its separation from the n=1 transition peak varies linearly with measured sheet densities of the two-dimensional electron gas. Appearance of a strong high-energy peak marks the occupation of the second electron subband if the electron density or well width is large, and a feature due to phonon interactions is present if the Fermi energy exceeds the optical-phonon energy. No enhancement is seen at the Fermi edge except for a slight increase when it lies less than about 10 meV below the second subband.

Photoluminescence and photoluminescence excitation spectroscopy have been used to study excitons in undoped InGaAs pseudomorphic quantum wells with GaAs and AlGaAs barrier layers. Luminescence features from samples incorporating a top AlGaAs barrier layer were strikingly different from those with bottom AlGaAs barriers. The 2-K luminescence from the latter structures showed free and bound exciton contributions with a linewidth ≤1.5 meV. In contrast, the luminescence from samples with top AlGaAs barriers was much broader, presumably the result of statistical variations in well width due to island formation at the top interface. Fits to the temperature dependence of the PL linewidth show that while the homogeneous broadening is similar in all the structures, the inhomogeneous broadening introduced by the interfaces is fundamentally different. The results could have important implications for the optimization of heterostructure devices in this pseudomorphic system.

Measurements of the current/voltage characteristics and electroluminescence spectra of nipi LEDs with selectively diffused contacts have been performed over the temperature range 3-300 K. Good correlation has been observed between the forward characteristics of the diodes and the tuning of the electroluminescence. Analysis of the I/V characteristics indicates that the recombination occurs by electron tunnelling through the parabolic potential barriers.

Pseudomorphic InGaAs quantum well (QW) structures have been the subject of considerable interest for electronic and optoelectronic devices. In order to optimize pseudomorphic QW structures for advanced devices it is necessary to understand the properties of the interfaces with the barrier layers. To this end we have utilized photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy to investigate the optical properties of InGaAs strained single quantum wells (QWs) grown by molecular beam epitaxy with different barrier layers. Samples with 100A In.15Ga.85As layers bounded on top and bottom by either GaAs or Al .15 Ga .85 As were studied over the temperature range 2-300 K. All four possible barrier combinations were included. The GaAs/InGaAs/GaAs samples exhibited linewidths as small as 1.3 meV at 2 K, which are the narrowest yet observed for these structures grown by conventional techniques. Similar results were observed at 2 K for QWs with AlGaAs barriers below, but the linewidths of samples with AlGaAs on top were substantially broadened. Measurements of the excitation spectra of these samples showed a substantial free exciton component to the luminescence from samples without top AlGaAs barriers. Samples with top AlGaAs barriers, however, showed little free exciton contribution. Investigation of the temperature behavior of the luminescence suggest that the homogeneous broadening in all the samples is similar, but that the inhomogeneous contributions are different for the various structures. In addition, the temperature dependent measurements showed an additional bound exciton component of the samples with top barriers that was ionized at temperatures above 15 K. Samples with lower AlGaAs barriers showed an anomalous increase in linewidth with increasing temperature which is still under investigation.

GaAs/AlGaAs light-emitting diodes (LED’s) with nipi active regions have been successfully fabricated using sequential n and p diffusions to selectively contact the doping superlattice. By doing sequential patterned sulfur and zinc diffusions, a lateral injection LED can be readily fabricated. Excellent current-voltage characteristics were achieved with reverse breakdown voltages in excess of 9.5 V, indicating that the selective contacts were nearly optimum. cw outputs of 500 μW at 50 mA drive current have been observed. The LED output spectrum was seen to tune with applied bias at a rate of about 650 meV/V at low temperatures.

AlGaAs doping superlattices (‘‘nipi structures’’) have been successfully grown in 30% AlGaAs by molecular beam epitaxy. Tunable photoluminescence (PL) as a function of incident laser intensity has been observed in samples with a wide range of intrinsic layer thicknesses over a temperature range from 2 to 120 K. Luminescence shifts as large as 230 meV were observed for a range of incident intensities of about 500. Low-temperature PL spectra showed a weaker dependence of the peak energy on incident intensity for thicker spacer layers. This decrease in tuning rate can be associated with the reduced probability for tunneling transitions with increasing spacer thickness.

Devices were fabricated in MBE (molecular-beam epitaxy)-grown wafers with a ten period doping superlattice consisting of 300-Å layers with n=p=3×1018 cm-3, using sequential patterned sulfur and zinc diffusions. The typical contact spacing was 5 μm, and the devices were generally 400 μm long. Excellent I-V characteristics were achieved with reverse breakdown voltages in excess of 9.5 V, indicating that the selective contacts were nearly optimum. CW outputs of 500 μW at 50-mA drive current have been observed from the surface of the devices at room temperature. Measurements of the current-voltage characteristics and electroluminescence spectra have been performed from 3 to 300 K. The LED output spectrum tuned at a rate of about 700 meV/V of applied bias at 120 K. Good correlation was observed between the forward characteristics of the diodes and the tuning of the electroluminescence. The tunneling component of the diode current has been identified and remains significant to temperatures of ≈140 K. As the tunneling component is reduced, the tunable luminescence is quenched due to the competition from thermally injected carriers recombining across the bandgap