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Solar tracking system

Description: This report describes an automated solar tracking system which is adaptable to most solar collectors which require alignment with the sun's radiation. The system uses partially shaded photovoltaic solar cells to produce error signals for both elevation and azimuth tracking with a high degree of accuracy. An electrical circuit processes the error signals and controls gearmotors to direct the solar collector toward the sun.
Date: June 1, 1977
Creator: Hammons, B.E.
Partner: UNT Libraries Government Documents Department

Data-reduction facility for digitizing and manipulating data from oscilloscope photographs (users guide)

Description: It is often necessary to analyze analog data from oscilloscope photographs, strip charts, or other graphical forms. Analysis of the data may involve conversion from analog to digital information. The digital information may be mathematically processed to extract the parameters needed for proper analysis of the original graph. In the past, a data reduction facility was operated on a part-time basis by one individual. The facility has been modified and upgraded to make it suitable for more general use by staff personnel.
Date: November 1, 1981
Creator: Hammons, B.E.; Bloomquist, D.D. & Dugan, D.W.
Partner: UNT Libraries Government Documents Department

Anomalous normal mode oscillations in semiconductor microcavities

Description: Semiconductor microcavities as a composite exciton-cavity system can be characterized by two normal modes. Under an impulsive excitation by a short laser pulse, optical polarizations associated with the two normal modes have a {pi} phase difference. The total induced optical polarization is then expected to exhibit a sin{sup 2}({Omega}t)-like oscillation where 2{Omega} is the normal mode splitting, reflecting a coherent energy exchange between the exciton and cavity. In this paper the authors present experimental studies of normal mode oscillations using three-pulse transient four wave mixing (FWM). The result reveals surprisingly that when the cavity is tuned far below the exciton resonance, normal mode oscillation in the polarization is cos{sup 2}({Omega}t)-like, in contrast to what is expected form the simple normal mode model. This anomalous normal mode oscillation reflects the important role of virtual excitation of electronic states in semiconductor microcavities.
Date: April 1, 1997
Creator: Wang, H.; Hou, H.Q. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

Fabrication issues of oxide-confined VCSELs

Description: To insert high-performance oxide-confined vertical-cavity surface- emitting lasers (VCSELs) into the manufacturing arena, we have examined the critical parameters that must be controlled to establish a repeatable and uniform wet thermal oxidation process for AlGaAs. These parameters include the AlAs mole fraction, sample temperature, carrier gas flow, and bubbler water temperature. Knowledge of these parameters has enable the compilation of oxidation rate data for AlGaAs which exhibits an Arrhenius rate dependence. The compositionally dependent activation energies for Al{sub x}Ga{sub 1-x}As layers of x=1.00, 0.98, and 0.92 are found to be 1.24, 1.75, and 1.88 eV, respectively. 7 figs, 1 tab, 14 refs.
Date: April 1, 1997
Creator: Geib, K.M.; Choquette, K.D.; Hou, H.Q. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

Stimulated emission from semiconductor microcavities

Description: Laser-like emissions from semiconductor microcavities at low temperature have attracted considerable attention recently because of the possibility of realizing a non-equilibrium condensate by using cavity-polaritons. In this paper the authors present experimental studies of optical properties of a microcavity near the lasing threshold. They show that the minimum lasing threshold is achieved when the cavity is tuned significantly below the exciton line center. By comparing emission spectra with reflectivity spectra, they also show that well-resolved doublet in the emission spectra near the lasing threshold are not associated with cavity-polaritons. These results suggest that laser-like emissions form the microcavity are due to conventional stimulated emission processes with exciton localization playing a significant role.
Date: April 1, 1997
Creator: Fan, X.; Wang, H.; Hou, H.Q. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

In situ reflectance and virtual interface analysis for compound semiconductor process control

Description: The authors review the use of in-situ normal incidence reflectance, combined with a virtual interface model, to monitor and control the growth of complex compound semiconductor devices. The technique is being used routinely on both commercial and research metal-organic chemical vapor deposition (MOCVD) reactors and in molecular beam epitaxy (MBE) to measure growth rates and high temperature optical constants of compound semiconductor alloys. The virtual interface approach allows one to extract the calibration information in an automated way without having to estimate the thickness or optical constants of the alloy, and without having to model underlying thin film layers. The method has been used in a variety of data analysis applications collectively referred to as ADVISOR (Analysis of Deposition using Virtual Interfaces and Spectroscopic Optical Reflectance). This very simple and robust monitor and ADVISOR method provides one with the equivalent of a real-time reflection high energy electron reflectance (RHEED) tool for both MBE and MOCVD applications.
Date: May 1, 1998
Creator: Breiland, W.G.; Hou, H.Q.; Hammons, B.E. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

In situ pre-growth calibration using reflectance as a control strategy for MOCVD fabrication of device structures

Description: In situ normal incidence reflectance, combined with a virtual interface model, is being used routinely on a commercial MOCVD reactor to measure growth rates of compound semiconductor films. The technique serves as a pre-growth calibration tool analogous to the use of RHEED in MBE as well as a real-time monitor throughout the run. An application of the method to the growth of a vertical cavity surface emitting laser (VCSEL) device structure is presented. All necessary calibration information can be obtained using a single run lasting less than one hour. Working VCSEL devices are obtained on the first try after calibration. Repeated runs have yielded {+-} 0.3% reproducibility of the Fabry-Perot cavity wavelength over the course of more than 100 runs.
Date: August 1, 1996
Creator: Breiland, W.G.; Hou, H.Q.; Chui, H.C. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

In situ growth rate measurements by normal-incidence reflectance during MOVPE growth

Description: We present an in situ technique for monitoring metal-organic vapor phase epitaxy growth by normal-incidence reflectance. This technique is used to calibrate the growth rate periodically and to monitor the growth process routinely. It is not only a precise tool to measure the growth rate, but also very useful in identifying unusal problems during a growth run, such as depletion of source material, deterioration of surface morphology, and problems associated with an improper growht procedure. We will also present an excellent reproducibility ({+-}0.3% over a course of more than 100 runs) of the cavity wavelength of vertical-cavity surface emitting laser structures with periodic calibration by this in situ technique.
Date: May 1, 1996
Creator: Hou, H.Q.; Breiland, W.G.; Hammons, B.E. & Chui, H.C.
Partner: UNT Libraries Government Documents Department

Spectrally and temporally resolved laser emission from vertical cavity surface emitting lasers

Description: We have measured the laser emission spectra of several vertical cavity surface emitting lasers following pulsed laser excitation, with a time resolution of < 1 ps. Correlations between the observed pulse widths and cavity lifetimes were observed.
Date: December 31, 1992
Creator: Sinclair, M.; Gourley, P. L.; Brennan, T. M. & Hammons, B. E.
Partner: UNT Libraries Government Documents Department

Reflection mass spectrometry technique for monitoring and controlling composition during molecular beam epitaxy

Description: This invention is comprised of a method for on-line accurate monitoring and precise control of molecular beam epitaxial growth of Groups III-III-V or Groups III-V-V layers in an advanced semiconductor device incorporates reflection mass spectrometry. The reflection mass spectrometry is responsive to intentional perturbations in molecular fluxes incident on a substrate by accurately measuring the molecular fluxes reflected from the substrate. The reflected flux is extremely sensitive to the state of the growing surface and the measurements obtained enable control of newly forming surfaces that are dynamically changing as a result of growth.
Date: August 15, 1990
Creator: Brennan, T. M.; Hammons, B. E. & Tsao, J. Y.
Partner: UNT Libraries Government Documents Department

Origin of the Time-Dependence of Wet Oxidation of AlGaAs

Description: The time-dependence of the wet oxidation of high-Al-content AlGaAs can be either linear, indicating reaction-rate limitation, or parabolic, indicating diffusion-limited rates. The transition from linear to parabolic time dependence can be explained by the increased rate of the formation of intermediate As{sub 2}O{sub 3} vs. its reduction to elemental As. A steadily increasing thickness of the As{sub 2}O{sub 3}-containing region at the oxidation front will shift the process from the linear to the parabolic regime. This shift from reaction-rate-limited (linear) to diffusion-limited (parabolic) time dependence is favored by increasing temperature or increasing Al mole fraction.
Date: February 15, 1999
Creator: Allerman, A.A.; Ashby, C.I.H.; Bridges, M.M.; Hammons, B.E. & Hou, H.Q.
Partner: UNT Libraries Government Documents Department

Growth of vertical-cavity surface emitting lasers by metalorganic vapor phase epitaxy

Description: We present growth and characterization of visible and near-infrared vertical-cavity surface emitting lasers (VCSELs) grown by metalorganic vapor phase epitaxy. Discussions on the growth issue of VCSEL materials include growth rate and composition control using an {ital in}{ital situ} normal-incidence reflectometer, comprehensive p- and n-type doping study in AlGaAs by CCl{sub 4} and Si{sub 2}H{sub 6} over the entire composition range, and optimization of ultra-high material uniformity. We also demonstrate our recent achievements of all-AlGaAs VCSELs which include the first room-temperature continuous- wave demonstration of 700-nm red VCSELs and high-efficiency and low- threshold voltage 850-nm VCSELs.
Date: October 1, 1996
Creator: Hou, H.Q.; Hammons, B.E.; Crawford, M.H.; Lear, K.L. & Choquette, K.D.
Partner: UNT Libraries Government Documents Department

Selectively oxidized vertical-cavity laser performance and technology

Description: The authors discuss revolutionary performance advances in selectively oxidized vertical-cavity surface emitting lasers (VCSELs), which have enabled low operating power laser diodes appropriate for aerospace applications. Incorporating buried oxide layers converted from AIGaAs layers within the laser cavity produces enhanced optical and electrical confinement enabling superior laser performance, such as high efficiency and modulation bandwidth. VCSELs also shown to be viable over varied environmental conditions such as ambient temperature and ionized radiation. The development of novel VCSEL technologies for advanced system applications is also described. Two dimensional individually addressable VCSEL arrays exhibit uniform threshold and operating characteristics. Bottom emitting 850 nm VCSEL arrays fabricated using wafer fusion are also reported.
Date: February 1, 1998
Creator: Choquette, K.D.; Hou, H.Q.; Geib, K.M. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

Uniformity and performance of selectively oxidized VCSEL arrays

Description: The authors report the uniformity characteristics of low threshold 1,060 nm and high power 850 nm 8 x 8 individually addressable oxide-confined VCSEL arrays. Uniformity of lasing thresholds and operating characteristics are described, as well as thermal issues for 2-dimensional laser arrays.
Date: January 1, 1998
Creator: Geib, K.M.; Choquette, K.D.; Hou, H.Q. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

In-situ optical photoreflectance during MOCVD

Description: This report summarizes the development of in situ optical photoreflectance as a tool for measuring impurity concentrations in compound semiconductors. The authors have successfully explored the use of photoreflectance as an in situ tool for measuring n-type doping levels in metal-organic chemical vapor deposition (MOCVD) grown GaAs materials. The technique measures phase and frequency shifts in Franz-Keldysh oscillations measured on uniformly doped thin films. Doping concentrations from 5 {times} 10{sup 16} to 1 {times} 10{sup 18} can be measured at temperatures below 130 C. A method has been developed to include photoreflectance as the last step in the pre-growth in situ calibration procedure for MOCVD thin film structures. This combined capability now enables one to rapidly and accurately determine growth rates, chemical composition, and doping levels necessary to generate a recipe to fabricate complex optoelectronic compound semiconductor devices.
Date: January 1, 1998
Creator: Breiland, W.G.; Hammons, B.E.; Hou, H.Q. & Mei, X.B.
Partner: UNT Libraries Government Documents Department

Surface-emitting semiconductor laser for intracavity spectroscopy and microscopy

Description: The authors demonstrate lasing action in a novel microcavity laser which can be utilized for intracavity spectroscopy as well as high contrast imaging of small ({approximately} 10{mu}m) structures. The system can be easily visualized as a Fabry-Perot cavity containing a gain media and an object for study. Since the primary constraint on the object is transparency at the lasing wavelength, investigation of lasing in objects such as microspheres, liquid droplets, and biological cells is possible. The resonator consists of an epitaxial NME grown mirror and gain region on a GaAs wafer. This is essentially an open-cavity vertical cavity surface emitting laser (VCSEL). The object to be studied is placed on the wafer and covered with a glass dielectric mirror which acts as the output coupler. When the semiconductor gain region is photo-pumped, the object within the cavity provides lateral optical confinement through its index difference with the surrounding media, increases the cavity Q, and thus encourages lasing action. The emitted laser light can be spectrally and spatially resolved. The narrow lasing lines can provide information about the lasing modes supported and the size of the object. The spatially resolved laser light provides high contrast microscopic images of the electromagnetic modes oscillating in the resonator. The authors present an investigation of stable lasing modes in polystyrene spheres. This device could prove useful in biomedical diagnostics. It covers the correct spatial dimensions as well as wavelength region. In fact, an integrated system of these devices may provide a high speed, compact method of performing cell diagnostics.
Date: March 1, 1995
Creator: Meissner, K.E.; Gourley, P.L.; Brennan, T.M.; Hammons, B.E. & McDonald, A.E.
Partner: UNT Libraries Government Documents Department

Mirror reflectivity and doping considerations for high performance oxide-confined vertical cavity lasers

Description: We report the effects of mirror doping and reflectivity in 850 and 780 nm oxide-confined vertical cavity surface emitting lasers. Decreased doping throughout the n-type mirror produces significantly higher quantum efficiency, while the optimum reflectivity is dependent upon the gain material.
Date: January 1, 1996
Creator: Geib, K.M.; Choquette, K.D.; Chui, H.C.; Hou, H.Q. & Hammons, B.E.
Partner: UNT Libraries Government Documents Department

Ultraviolet photosulfidation of III-V compound semiconductors for electronic passivation

Description: A new vacuum-compatible passivation technique for III-V compound semiconductors has been developed. Sulfur passivation of GaAs(100) is produced by ultraviolet photolytic deposition of a sulfide species from vapor phase elemental sulfur. Photoluminescence studies of the photosulfided GaAs reveal a degree of passivation greater than or equal to that produced by conventional (NH{sub 4}{sub 2}S) solution treatment. X-ray Photoelectron Spectroscopy has shown that the sulfur resides on the surface as a single reduced sulfur species, either as sulfide of disulfide, indicating complete fragmentation of the S{sub 8} ring by UV light in proximity to the surface. The degree of photosulfidation depends strongly on surface preparation as demonstrated by the described surface oxide removal studies.
Date: October 1, 1993
Creator: Zavadil, K. R.; Ashby, C. I. H.; Howard, A. J. & Hammons, B. E.
Partner: UNT Libraries Government Documents Department

Surface-emitting superconductor laser spectroscopy for characterizing normal and sickled red blood cells

Description: We have developed a new intracavity laser technique that uses a living or a fixed cell as an integral component of the laser. The cells are placed on an AlGaAs/GaAs surface-emitting semiconductor wafer and covered with a glass dielectric mirror to form a laser resonator. In this arrangement, the cells serve as optical waveguides (or lens elements) to confine (or focus) light generated in the resonator by the semiconductor. Because of the high transparency, the cells aid the lasing process to generate laser light. This ultra sensitive laser provides a novel imaging/spectroscopic technique for histologic examination which we demonstrate with normal and sickled human red blood cells. Extremely high contrast microscopic images of the cells are observed near 830-850 nm. These images correspond to electromagnetic modes of cell structures and are sensitive to shape of the cell. Using a high resolution spectrometer, we resolve the light emitted from these images into very narrow spectral peaks associated with the lasing modes. Analysis of the spectra reveals that the distribution of peaks is quite different for normal and sickled red blood cells. This technique, in a more developed form, may be useful for the rapid analysis of other kinds of normal and abnormal cells.
Date: February 1, 1995
Creator: Gourley, P. L.; Meissner, K. E.; Brennan, T. M.; Hammons, B. E. & Gourley, M. F.
Partner: UNT Libraries Government Documents Department

Optical generation of radio-frequency power

Description: An optical technique for high-power radio-frequency (RF) signal generation is described. The technique uses a unique photodetector based on a traveling-wave design driven by an appropriately modulated light source. The traveling-wave photodetector (TWPD) exhibits simultaneously a theoretical quantum efficiency approaching 100 % and a very large electrical bandwidth. Additionally, it is capable of dissipating the high-power levels required for the RF generation technique. The modulated light source is formed by either the beating together of two lasers or by the direct modulation of a light source. A system example is given which predicts RF power levels of 100`s of mW`s at millimeter wave frequencies with a theoretical ``wall-plug`` efficiency approaching 34%.
Date: November 1, 1994
Creator: Hietala, V. M.; Vawter, G. A.; Brennan, T. M.; Hammons, B. E. & Meyer, W. J.
Partner: UNT Libraries Government Documents Department

In-situ spectral reflectance for improving molecular beam epitaxy device growth

Description: This report summarizes the development of in situ spectral reflectance as a tool for improving the quality, reproducibility, and yield of device structures grown from compound semiconductors. Although initially targeted at MBE (Molecular Beam Epitaxy) machines, equipment difficulties forced the authors to test most of their ideas on a MOCVD (Metal Organic Chemical Vapor Deposition) reactor. A pre-growth control strategy using in situ reflectance has led to an unprecedented demonstration of process control on one of the most difficult device structures that can be grown with compound semiconductor materials. Hundreds of vertical cavity surface emitting lasers (VCSEL`s) were grown with only {+-} 0.3% deviations in the Fabry-Perot cavity wavelength--a nearly ten-fold improvement over current calibration methods. The success of the ADVISOR (Analysis of Deposition using Virtual Interfaces and Spectroscopic Optical Reflectance) method has led to a great deal of interest from the commercial sector, including use by Hewlett Packard and Honeywell. The algorithms, software and reflectance design are being evaluated for patents and/or license agreements. A small company, Filmetrics, Inc., is incorporating the ADVISOR analysis method in its reflectometer product.
Date: May 1, 1997
Creator: Breiland, W.G.; Hammons, B.E.; Hou, H.Q.; Killeen, K.P.; Klem, J.F.; Reno, J.L. et al.
Partner: UNT Libraries Government Documents Department

Highly uniform and reproducible visible to near-infrared vertical-cavity surface-emitting lasers grown by MOVPE

Description: The authors present the growth and characterization of vertical-cavity surface emitting lasers (VCSELs) from visible to near-infrared wavelength grown by metalorganic vapor phase epitaxy. Discussions on the growth issue of VCSEL materials include the control on growth rate and composition using an in situ normal-incidence reflectometer, optimization of ultra-high material uniformity, and comprehensive p- and n-type doping study in AlGaAs by CCl{sub 4} and Si{sub 2}H{sub 6} over the entire Al composition range. They will also demonstrate the recent achievements of selectively-oxidized VCSELs which include the first room-temperature continuous-wave demonstration of all-AlGaAs 700-nm red VCSELs, high-performance n-side up 850-nm VCSELs, and low threshold current and low-threshold voltage 1.06 {micro}m VCSELs using InGaAs/GaAsP strain-compensated quantum wells.
Date: May 1, 1997
Creator: Hou, H.Q.; Choquette, K.D.; Hammons, B.E.; Breiland, W.G.; Crawford, M.H. & Lear, K.L.
Partner: UNT Libraries Government Documents Department

Composite fermions in 2 x 10{sup 6} cm{sup 2}/Vs mobility A1GaAs/GaAs heterostructures grown by MOCVD

Description: Recent growth by MOCVD (metalorganic chemical vapor deposition) of 2.0x10{sup 6} cm{sup 2}/Vs mobility heterostructures are reported. These mobilities, the highest reported to date, are attributed to use of tertiarybutylarsine as the arsenic precursor. Measurements in tilted magnetic fields of the fractional quantum Hall effect states near filling factor 3/2 are consistent with a spin-split composite fermion (CF) model proposed earlier. Extracted values of the product of the CF g-factor and CF effective mass agree with values previously obtained for MBE samples.
Date: December 31, 1996
Creator: Simmons, J.A., Chui, H.C., Harff, N.E., Hammons, B.E. & Du, R.R., Zudov, M.A.
Partner: UNT Libraries Government Documents Department