960 Matching Results

Search Results

Advanced search parameters have been applied.


Description: "Nuclear Science Series" of the National Research Council. Committee on Nuclear Science. A review of the nuclear and chemical features of these elements which are of particular interest to the radiochemist is presented. A discussion is also included on aspects of sample dissolution and counting techniques followed by a collection of radiochemical procedures for the elements as found in the literature. (J.R.D.)
Date: January 1, 1961
Creator: Lewis, J.E.
Partner: UNT Libraries Government Documents Department

Nucleation and Growth of GaN on GaAs (001) Substrates

Description: The nucleation of GaN thin films on GaAs is investigated for growth at 620 "C. An rf plasma cell is used to generate chemically active nitrogen from N<SUB>2</SUB>. An arsenic flux is used in the first eight monolayer of nitride growth to enhance nucleation of the cubic phase. Subsequent growth does not require an As flux to preserve the cubic phase. The nucleation of smooth interfaces and GaN films with low stacking fault densities is dependent upon relative concentrations of active nitrogen species in the plasma and on the nitrogen to gallium flux ratio.
Date: May 3, 1999
Creator: Drummond, Timothy J.; Hafich, Michael J.; Heller, Edwin J.; Lee, Stephen R.; Liliental-Weber, Zuzanna; Ruvimov, Sergei et al.
Partner: UNT Libraries Government Documents Department

Effect of Inert Gas Additive Species on Cl(2) High Density Plasma Etching of Compound Semiconductors: Part 1. GaAs and GaSb

Description: The role of the inert gas additive (He, Ar, Xe) to C12 Inductively Coupled Plasmas for dry etching of GaAs and GaSb was examined through the effect on etch rate, surface roughness and near-surface stoichiometry. The etch rates for both materials go through a maximum with Clz 0/0 in each type of discharge (C12/'He, C12/Ar, C12/Xc), reflecting the need to have efficient ion-assisted resorption of the etch products. Etch yields initially increase strongly with source power as the chlorine neutral density increases, but decrease again at high powers as the etching becomes reactant-limited. The etched surfaces are generally smoother with Ax or Xe addition, and maintain their stoichiometry.
Date: December 23, 1998
Creator: Abernathy, C.R.; Cho, H.; Hahn, Y.B.; Hays, D.C.; Jung, K.B.; Pearton, S.J. et al.
Partner: UNT Libraries Government Documents Department

Gallium self-diffusion in gallium arsenide: A study using isotope heterostructures

Description: Ga self-diffusion was studied with secondary-ion mass spectroscopy in {sup 69}GaAs/{sup 71}GaAs isotope heterostructures grown by molecular beam epitaxy on GaAs substrates. Results show that the Ga self- diffusion coefficient in intrinsic GaAs can be described accurately with D = (43{+-}25 cm{sup 2}s{sup -1})exp(-4.24{+-}0.06 eV/k{sub B}T) over 6 orders of magnitude between 800 and 1225 C under As-rich condition. Experimental results combined with theoretical calculations strongly suggest Ga vacancy being the dominant native defect controlling the diffusion. No significant doping effects were observed in samples where the substrates were doped with Te up to 4x10{sup 17}cm{sup -3} or Zn up to 1x10{sup 19}cm{sup -3}.
Date: September 1, 1996
Creator: Wang, Lei; Hsu, L.; Haller, E.E.; Erickson, J.W.; Fischer, A.; Eberl, K. et al.
Partner: UNT Libraries Government Documents Department

Reactive Ion Beam Etching of GaAs and Related Compounds in an Inductively Coupled Plasma of Cl(2)-Ar Mixture

Description: Reactive ion beam etching (RD3E) of GaAs, GaP, AIGaAs and GaSb was performed in a Cl2-Ar mixture using an Inductively Coupled Plasma (ICP) source. `The etch rates and yields were strongly affected by ion energy and substrate temperature. The RJBE was dominated by ion-assisted etching at <600 eV and by physical sputtering beyond 600 eV. The temperature dependence of the etch rates revealed three different regimes, depending on the substrate temperature: 1) sputtering-etch limited, 2) products-resorption limited, and 3) mass-transfer limited regions. GaSb showed the overall highest etch rates, while GaAs and AIGaAs were etched at the same rates. The etched features showed extremely smooth morphologies with anisotropic sidewalls.
Date: November 23, 1998
Creator: Abernathy, C.R.; Hahn, Y.B.; Hays, D.; Lambers, E.S.; Lee, J.W.; Pearton, S.J. et al.
Partner: UNT Libraries Government Documents Department

Ohmic Contacts to n-type GaSb and n-type GaInAsSb

Description: An investigation with the objective of improving n-type ohmic contacts to GaSb-based devices is described. This study involves a series of n-GaInAsSb and n-GaSb samples with varying doping, grown on both n-GaSb and semi-insulating GaAs substrates. These samples were fabricated into mesa-etched TLM structures, and the specific contact resistivity and sheet resistance of these layers as a function of majority electron concentration were measured. Extremely low specific contact resistivities of about 2 x 10{sup -6} {Omega}-cm{sup 2} and sheet resistances of about 4 {Omega}/{open_square} are found for n-type GaInAsSb doped at about 3 x 10{sup 18} cm{sup -3}.
Date: June 16, 2003
Creator: Huang, R.K.; Wang, C.A.; Harris, C.T.; Connors, M.K. & Shiau, D.A.
Partner: UNT Libraries Government Documents Department

Outdoor Performance of a Thin-Film Gallium-Arsenide Photovoltaic Module

Description: We deployed a 855 cm2 thin-film, single-junction gallium arsenide (GaAs) photovoltaic (PV) module outdoors. Due to its fundamentally different cell technology compared to silicon (Si), the module responds differently to outdoor conditions. On average during the test, the GaAs module produced more power when its temperature was higher. We show that its maximum-power temperature coefficient, while actually negative, is several times smaller in magnitude than that of a Si module used for comparison. The positive correlation of power with temperature in GaAs is due to temperature-correlated changes in the incident spectrum. We show that a simple correction based on precipitable water vapor (PWV) brings the photocurrent temperature coefficient into agreement with that measured by other methods and predicted by theory. The low operating temperature and small temperature coefficient of GaAs give it an energy production advantage in warm weather.
Date: June 1, 2013
Creator: Silverman, T. J.; Deceglie, M. G.; Marion, B.; Cowley, S.; Kayes, B. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

GaN Metal Oxide Semiconductor Field Effect Transistors

Description: A GaN based depletion mode metal oxide semiconductor field effect transistor (MOSFET) was demonstrated using Ga{sub 2}O{sub 3}(Gd{sub 2}O{sub 3}) as the gate dielectric. The MOS gate reverse breakdown voltage was &gt; 35V which was significantly improved from 17V of Pt Schottky gate on the same material. A maximum extrinsic transconductance of 15 mS/mm was obtained at V{sub ds} = 30 V and device performance was limited by the contact resistance. A unity current gain cut-off frequency, f{sub {tau}}, and maximum frequency of oscillation, f{sub max} of 3.1 and 10.3 GHz, respectively, were measured at V{sub ds} = 25 V and V{sub gs} = {minus}20 V.
Date: March 2, 1999
Creator: Ren, F.; Pearton, S.J.; Abernathy, C.R.; Baca, A.; Cheng, P.; Shul, R.J. et al.
Partner: UNT Libraries Government Documents Department

Upconversion of near GaAs bandgap photons to GaInP{sub 2} emission at the GaAs/(ordered) GaInP{sub 2} heterojunction

Description: The authors have observed upconversion of photoluminescence in several partially ordered GaInP{sub 2} epilayers grown on [100] oriented GaAs substrates. They found that this upconversion occurs even when the excitation photon energy is below the bandgap of GaAs but near the electron-acceptor transitions at {approximately} 1.49 eV. A two-step two-photon absorption model in which the conduction band alignment at the GaAs/GaInP{sub 2} is of type 2 is proposed to explain the results.
Date: September 1, 1996
Creator: Teo, K.L.; Su, Z.P.; Yu, P.Y. & Uchida, K.
Partner: UNT Libraries Government Documents Department

Effect of stoichiometry on defect distribution in cubic GaN grown on GaAs by plasma-assisted MBE

Description: High resolution electron microscopy was used to study the structure of {beta}-GaN epilayers grown on (001) GaAs substrates by plasma- assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N{sub 2} flow for fixed rf power). The best quality of GaN layers was achieved by ``stoichiometric`` growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the ``major flat`` of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN- GaAs interfaces.
Date: December 31, 1996
Creator: Ruvimov, S.; Liliental-Weber, Z.; Washburn, J.; Drummond, T.J.; Hafish, M. & Lee, S.R.
Partner: UNT Libraries Government Documents Department

Optical Properties of InGaAsN: A New 1eV Bandgap Material System

Description: InGaAsN is a new semiconductor alloy system with the remarkable property that the inclusion of only 2% nitrogen reduces the bandgap by more than 30%. In order to help understand the physical origin of this extreme deviation from the typically observed nearly linear dependence of alloy properties on concentration, we have investigated the pressure dependence of the excited state energies using both experimental and theoretical methods. We report measurements of the low temperature photohnninescence energy of the material for pressures between ambient and 110 kbar. We describe a simple, density-functional-theory-based approach to calculating the pressure dependence of low lying excitation energies for low concentration alloys. The theoretically predicted pressure dependence of the bandgap is in excellent agreement with the experimental data. Based on the results of our calculations, we suggest an explanation for the strongly non-linear pressure dependence of the bandgap that, surprisingly, does not involve a nitrogen impurity band. Addhionally, conduction-band mass measurements, measured by three different techniques, will be described and finally, the magnetoluminescence determined pressure coefficient for the conduction-band mass is measured.
Date: January 25, 1999
Creator: Allerman, A.A.; Fritz, I.J.; Jones, E.D.; Kurtz, S.R.; Modine, N.A.; Tozer, S.T. et al.
Partner: UNT Libraries Government Documents Department

Spectroscopic study of partially-ordered semiconductor heterojunction under high pressure and high magnetic field

Description: Photoluminescence upconversion (PLU) is a phenomenon in which a sample emits photons with energy higher than that of the excitation photon. This effect has been observed in many materials including rare earth ions doped in insulating hosts and semiconductor heterostructures without using high power lasers as the excitation source. Recently, this effect has been observed also in partially CuPt-ordered GaInP{sub 2} epilayers grown on GaAs substrates. As a spectroscopic technique photoluminescence upconversion is particularly well suited for studying band alignment at heterojunction interface. The value of band-offset has been determined with meV precision using magneto-photoluminescence. Using the fact that the pressure coefficient of electrons in GaAs is higher than those in GaInP{sub 2} they have been able to manipulate the band-offset at the GaInP/GaAs interface. By converting the band-offset from Type I to Type II they were able to demonstrate that the efficiency of the upconversion process is greatly enhanced by a Type II band-offset.
Date: December 31, 2000
Creator: Yu, P.Y.; Martinez, G.; Zeman, J. & Uchida, K.
Partner: UNT Libraries Government Documents Department

Antimonide-Based Long-Wavelength Lasers on GaAs Substrates

Description: We have investigated the use of GaAsSb in edge-emitting laser active regions, in order to obtain lasing near 1.3 {micro}m. Single quantum well GaAsSb devices display electroluminescence at wavelengths as long as 1.34 {micro}m, but substantial blueshifts occur under high injection conditions. GaAsSb single quantum well edge emitters have been obtained which lase at 1.275 {micro}m with a room-temperature threshold current density as low as 535 A/cm{sup 2}. Modification of the basic GaAsSb/GaAs structure with the addition of InGaAs layers results in a strongly type-II band alignment which can be used to further extend the emission wavelength of these devices. Using GaAsSb/InGaAs active regions, lasers emitting at 1.17 {micro}m have been obtained with room-temperature threshold current densities of 120 A/cm{sup 2}, and devices operating at 1.29 {micro}m have displayed thresholds as low as 375 A/cm{sup 2}. Characteristic temperatures for devices employing various GaAsSb-based active regions have been measured to be 60-73 K.
Date: August 17, 2000
Creator: KLEM,JOHN F. & Blum, O.
Partner: UNT Libraries Government Documents Department

GaN CVD Reactions: Hydrogen and Ammonia Decomposition and the Desorption of Gallium

Description: Isotopic labeling experiments have revealed correlations between hydrogen reactions, Ga desorption, and ammonia decomposition in GaN CVD. Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) were used to demonstrate that hydrogen atoms are available on the surface for reaction after exposing GaN(0001) to deuterium at elevated temperatures. Hydrogen reactions also lowered the temperature for Ga desorption significantly. Ammonia did not decompose on the surface before hydrogen exposure. However, after hydrogen reactions altered the surface, N<sup>15</sup>H<sub>3</sub> did undergo both reversible and irreversible decomposition. This also resulted in the desorption of N<sub>2</sub> of mixed isotopes below the onset of GaN sublimation, This suggests that the driving force of the high nitrogen-nitrogen bond strength (226 kcal/mol) can lead to the removal of nitrogen from the substrate when the surface is nitrogen rich. Overall, these findings indicate that hydrogen can influence G-aN CVD significantly, being a common factor in the reactivity of the surface, the desorption of Ga, and the decomposition of ammonia.
Date: May 26, 1999
Creator: Bartram, Michael E. & Creighton, J. Randall
Partner: UNT Libraries Government Documents Department