Visible Emitters Research Articles

High-brightness AlGaInP light emitting diodes

First commercially introduced in 1990, AlGaInP light emitting diodes (LEDs) currently are the highest (luminous) efficiency visible solid-state emitters produced to date in the red through yellow spectral regime. The attainment of this high-efficiency performance is a result of the development of advanced metalorganic chemical vapor deposition crystal growth techniques, which have facilitated the high-quality growth of this quaternary alloy as well as the implementation of complex device designs. Furthermore, the highest efficiency family of AlGaInP devices (based upon a transparent-substrate platform and commercially introduced in 1994) have been realized as result of the development and implementation of direct compound semiconductor wafer bonding technology. As a result, the luminous efficiency of AlGaInP LEDs exceeds or rivals that of unfiltered incandescent lamps and other conventional lighting sources. Further improvements in these techniques (and the realization of efficient, high-power LEDs) are expected to make AlGaInP LEDs even more competitive with conventional lamp technology, thus enhancing the position of LED's in many applications as a preferred lighting source.

Double Modulation and Selective Excitation Photoreflectance for Characterizing Highly Luminescent Semiconductor Structures and Samples with Poor Surface Morphology

AbstractPhotoreflectance (PR) is a powerful, contactless, and nondestructive technique capable of probing interband electronic transitions and built-in electric-fields at the surface and in interface regions in semiconductor materials and microstructures. It has been widely used as a characterization tool. However, its application to highly luminescent systems, such as quantum-well (QW) lasers and samples with poor surface morphology has been limited because of the difficulty in minimizing the interference from the luminescence and the pump beam that is scattered from the surface. We present a double modulation procedure where both the probe and pump beams are modulated which allows the PR component to be completely separated from the luminescence and scattered contributions. The separation is achieved through the appropriate choice of modulation frequencies and specially designed tuned amplifiers. A complete PR system, along with the necessary circuits, is presented. In addition, we have also exploited the freedom provided by the system to choose any pump wavelength to selectively modulate specific regions of the multilayer structure (QW, barrier, and cladding layer) and extract detailed information regarding the properties of each layer. The applicability of the procedure is demonstrated by fully characterizing a GaInP based QW laser structure used for visible emitters and by measurements on a saw-cut and chemically etched back surface of a commercial GaAs wafer. Finally, procedures attempted in the past to overcome difficulties with the luminescence and surface scattering are discussed and compared to the double modulation PR technique.

(In,Ga)P buffer layers for ZnSe-based visible emitters

The growth of ZnSe on pseudomorphic and partially relaxed (In,Ga)P epitaxial buffer layers has been investigated. The (In,Ga)P and ZnSe layers were grown in separate gas source molecular beam epitaxy systems using elemental sources for the cation species and cracked PH 3 and H 2Se for the anion species. Surface morphology studies using scanning electron microscopy showed that the ZnSe layers were featureless at a magnification of 1.2x10 4. Four crystal (400) X-ray rocking curves indicated that the ZnSe full width at half maximum (FWHM) was 130″, while the pseudomorphic (In,Ga)P buffer layer FWHM was 18″. The (511) reflections of X-ray rocking curves were used to measure the residual strain as well as the composition in the (In,Ga)P buffer layers. The low temperature photoluminescence spectra from the ZnSe films grown on partially relaxed (In,Ga)P exhibited intensities of the donor-bound and free exciton transitions of nearly equal magnitude, as well as transitions due to extended defects, suggesting highly pure material. The luminescence from the (In,Ga)P buffer layers was also detected.

INTERACTION OF HEATED HIGH PURITY METALS WITH METASTABLE OXYGEN

Visible emission is observed when metastable oxygen O2 ([MATH]) produced in a microwave discharge interacts with various heated high purity metal wires containing few ppm of copper. Spectra in the visible and the near infrared are presented. The visible spectra are identical with those obtained in subsequent copper wire experiments. These results tend to prove that CuCl2 is not the visible emitter.

III–V alloys and their potential for visible emitter applications

Atmospheric pressure organometallic vapor phase epitaxy is used for the growth of GaInP-AlGaInP lattice matched to GaAs substrate. The investigation of the GaInP/GaAs heterostructure has been carried out and mobility as high as 75 000 cm 2 V -1s -1 for a sheet carrier concentration of 7.5 10 11 cm -2 has been measured by Shubnikov-de Haas at 4 K. Red and yellow LED's have been obtained with emission wavelength of 670 nm and 576 nm respectively. Oxide stripe laser emitting at 660 nm in pulsed mode have been performed and present a threshold current density of 5 kA cm -2. Three ridge waveguide laser diode arrays with maximum output power of 108 mW have also been achieved.