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The unusual conduction band minimum formation of Ga(As{sub 0.5{minus}y}P{sub 0.5{minus}y}N{sub 2y}) alloys

Description: The conduction band minimum formation of GaAs{sub 0.5{minus}y}P{sub 0.5{minus}y}N{sub 2y} is investigated for small nitrogen compositions (0.1% < 2y < 1.0%), by using a pseudopotential technique. This formation is caused by two unusual processes both involving the deep-gap impurity level existing in the dilute alloy limit y {r_arrow} 0. The first process is an anticrossing with the {Gamma}{sub Ic}-like extended state of GaAs{sub 0.5}P{sub 0.5}. The second process is an interaction with other impurity levels forming a subband. These two processes are expected to occur in any alloys exhibiting a deep-gap impurity level at one of its dilute limit.
Date: May 11, 2000
Creator: BELLAICHE,L.; MODINE,NORMAND A. & JONES,ERIC D.
Partner: UNT Libraries Government Documents Department

GaInNAs laser gain

Description: The optical gain spectra for GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of the lasing threshold current density of GaInNAs/GaAs quantum well structures.
Date: May 23, 2000
Creator: CHOW,WENG W.; JONES,ERIC D.; MODINE,NORMAND A.; KURTZ,STEVEN R. & ALLERMAN,ANDREW A.
Partner: UNT Libraries Government Documents Department

Minority carrier diffusion, defects, and localization in InGaAsN with 2% nitrogen

Description: Electron and hole transport in compensated, InGaAsN ({approx} 2% N) are examined through Hall mobility, photoconductivity, and solar cell photoresponse measurements. Short minority carrier diffusion lengths, photoconductive-response spectra, and doping dependent, thermally activated Hall mobilities reveal a broad distribution of localized states. At this stage of development, lateral carrier transport appears to be limited by large scale (>> mean free path) material inhomogeneities, not a random alloy-induced mobility edge.
Date: May 3, 2000
Creator: Kurtz, Steven R.; Allerman, Andrew A.; Seager, Carleton H.; Sieg, Robert M. & Jones, Eric D.
Partner: UNT Libraries Government Documents Department

The Growth of InGaAsN for High Efficiency Solar Cells by Metalorganic Chemical Vapor Deposition

Description: InGaAsN alloys are a promising material for increasing the efficiency of multi-junction solar cells now used for satellite power systems. However, the growth of these dilute N containing alloys has been challenging with further improvements in material quality needed before the solar cell higher efficiencies are realized. Nitrogen/V ratios exceeding 0.981 resulted in lower N incorporation and poor surface morphologies. The growth rate was found to depend on not only the total group III transport for a fixed N/V ratio but also on the N/V ratio. Carbon tetrachloride and dimethylzinc were effective for p-type doping. Disilane was not an effective n-type dopant while SiCl4 did result in n-type material but only a narrow range of electron concentrations (2-5e17cm{sup -3}) were achieved.
Date: September 16, 1999
Creator: ALLERMAN,ANDREW A.; BANKS,JAMES C.; GEE,JAMES M.; JONES,ERIC D. & KURTZ,STEVEN R.
Partner: UNT Libraries Government Documents Department

Double Barrier Resonant Tunneling Transistor with a Fully Two Dimensional Emitter

Description: A novel planar resonant tunneling transistor is demonstrated. The growth structure is similar to that of a double-barrier resonant tunneling diode (RTD), except for a fully two-dimensional (2D) emitter formed by a quantum well. Current is fed laterally into the emitter, and the 2D--2D resonant tunneling current is controlled by a surface gate. This unique device structure achieves figures-of-merit, i.e. peak current densities and peak voltages, approaching that of state-of-the-art RTDs. Most importantly, sensitive control of the peak current and voltage is achieved by gating of the emitter quantum well subband energy. This quantum tunneling transistor shows exceptional promise for ultra-high speed and multifunctional operation at room temperature.
Date: July 13, 2000
Creator: MOON,J.S.; SIMMONS,JERRY A.; RENO,JOHN L.; BACA,WES E.; BLOUNT,MARK A.; HIETALA,VINCENT M. et al.
Partner: UNT Libraries Government Documents Department

Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys

Description: The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.
Date: January 27, 2000
Creator: Jones, Eric D.; Allerman, Andrew A.; Kurtz, Steven R.; Modine, Normand A.; Bajaj, K. K.; Tozer, S. T. et al.
Partner: UNT Libraries Government Documents Department

Time-resolved photoluminescence studies of In{sub x}Ga{sub 1{minus}x}As{sub 1{minus}y}N{sub y}

Description: Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a In{sub x}Ga{sub 1{minus}x}As{sub 1{minus}y}N{sub y} (x {approximately} 0.03, y {approximately} 0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07--0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, the authors observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations.
Date: January 27, 2000
Creator: MAIR,R.A.; LIN,J.Y.; JIANG,H.X.; JONES,ERIC D.; ALLERMAN,ANDREW A. & KURTZ,STEVEN R.
Partner: UNT Libraries Government Documents Department

Optical properties of spontaneous lateral composition modulations in AlAs/InAs short-period superlattices

Description: The effect of lateral composition modulation, spontaneously generated during the epitaxial growth of a AlAs/InAs short-period superlattice, on the electronic band structure is investigated using photo-transmission and photoluminescence spectroscopy. Compared with uniform layers of similar average composition, the presence of the composition modulation considerably reduces the band gap energy and produces strongly polarized emission and absorption spectra. The authors demonstrate that the dominant polarization can selectively be aligned along the [{bar 1}10] or [010] crystallographic directions. In compressively strained samples, the use of (001) InP substrates slightly miscut toward [111]A or [101] resulted in modulation directions along [110] or [100], respectively, and dominant polarizations along a direction orthogonal to the respective composition modulation. Band gap reduction as high as 350 meV and 310 meV are obtained for samples with composition modulation along [110] and [100], respectively. Polarization ratios up to 26 are observed in transmission spectra.
Date: May 11, 2000
Creator: FRANCOEUR,S.; ALSINA,F.; ZHANG,YONG; NORMAN,A.G.; MASCARENHAS,A.; JONES,ERIC D. et al.
Partner: UNT Libraries Government Documents Department

Effect of surface steps on the microstructure of lateral composition modulation

Description: Growth of InAs/AlAs short-period superlattices on appropriately miscut (001) InP substrates is shown to alter the microstructure of composition modulation from a 2D organization of short compositionally enriched wires to a single dominant modulation direction and wire lengths up to {approximately}1 {micro}m. The effects of miscut are interpreted in terms of surface step orientation and character. The material is strongly modulated and exhibits intense optical emission. The 1D modulations appear potentially useful for new devices that take advantage of the preferred direction formed in the growth plane.
Date: March 23, 2000
Creator: FOLLSTAEDT,DAVID M.; NORMAN,A.G.; RENO,JOHN L.; JONES,ERIC D.; TWESTEN,R.D.; LEE,STEPHEN R. et al.
Partner: UNT Libraries Government Documents Department

Deep Levels in p- and n-type InGaAsN for High Efficiency Multi-Junction III-V Solar Cells

Description: Red Teaming is an advanced form of assessment that can be used to identify weaknesses in a variety of cyber systems. it is especially beneficial when the target system is still in development when designers can readily affect improvements. This paper discusses the red team analysis process and the author's experiences applying this process to five selected Information Technology Office (ITO) projects. Some detail of the overall methodology, summary results from the five projects, and lessons learned are contained within this paper.
Date: November 11, 1999
Creator: ALLERMAN,ANDREW A.; JONES,ERIC D.; KAPLAR,ROBERT J.; KURTZ,STEVEN R.; KWON,DAEWON & RINGEL,STEVEN A.
Partner: UNT Libraries Government Documents Department

InGaAsN: A Novel Material for High-Efficiency Solar Cells and Advanced Photonic Devices

Description: This report represents the completion of a 6 month Laboratory-Directed Research and Development (LDRD) program that focused on research and development of novel compound semiconductor, InGaAsN. This project seeks to rapidly assess the potential of InGaAsN for improved high-efficiency photovoltaic. Due to the short time scale, the project focused on quickly investigating the range of attainable compositions and bandgaps while identifying possible material limitations for photovoltaic devices. 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 photoluminescence 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. Additionally, 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. The design, growth by metal-organic chemical vapor deposition, and processing of an In{sub 0.07}Ga{sub 0.93}As{sub 0.98}N{sub 0.02} solar cell, with 1.0 eV bandgap, lattice matched to GaAs is described. The hole diffusion length in annealed, n-type InGaAsN is 0.6-0.8 pm, and solar cell internal quantum efficiencies >70% are obtained. Optical studies indicate that defects or impurities, from doping ...
Date: July 1, 1999
Creator: Allerman, Andrew A.; Follstaedt, David M.; Gee, James M.; Jones, Eric D.; Kurtz, Steven R. & Modine, Norman A.
Partner: UNT Libraries Government Documents Department

Effective masses for small nitrogen concentrations in InGaAsN alloys on GaAs

Description: The variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1% and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy is studied at 4K. The excitonic linewidth increases as a function of pressure until about 100 kbar after which it tends to saturate. This pressure dependent excitonic linewidth is used to derive the pressure variation of the exciton reduced mass using a theoretical formalism based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The linewidth derived ambient pressure masses are compared and found to be in agreement with other mass measurements. The variation of this derived mass is compared with the results from a nearly first-principles approach in which calculations based on the local density approximation to the Kohn-Sham density functional theory are corrected using a small amount of experimental input.
Date: May 11, 2000
Creator: JONES,ERIC D.; ALLERMAN,ANDREW A.; KURTZ,STEVEN R.; FRITZ,IAN J.; MODINE,NORMAND A.; SIEG,ROBERT M. et al.
Partner: UNT Libraries Government Documents Department

Reciprocal-space and real-space analyses of compositional modulation in InAs/AlAs short-period superlattices

Description: The microstructure of lateral composition modulation in InAs/AlAs superlattices grown by MBE on InP is examined. The use of x-ray diffraction, TEM, AFM, and STEM to characterize the modulations is discussed. Combining the information from these techniques gives increased insight into the phenomenon and how to manipulate it. Diffraction measures the intensity of modulation and its wavelength, and is used to identify growth conditions giving strong modulation. The TEM and STEM analyses indicate that local compositions are modulated by as much as 0.38 InAs mole fraction. Plan-view images show that modulated structures consists of short ({approx_lt}0.2 {micro}m) In-rich wires with a 2D organization in a (001) growth plane. However, growth on miscut substrates can produce a single modulation along the miscut direction with much longer wires ({approx_gt}0.4 {micro}m), as desired for potential applications. Photoluminescence studies demonstrate that the modulation has large effects on the bandgap energy of the superlattice.
Date: January 25, 2000
Creator: FOLLSTAEDT,DAVID M.; LEE,STEPHEN R.; RENO,JOHN L.; JONES,ERIC D.; TWESTEN,R.D.; NORMAN,A.G. et al.
Partner: UNT Libraries Government Documents Department

The nature and origin of lateral composition modulations in short-period strained-layer superlattices

Description: The nature and origin of lateral composition modulations in (AlAs){sub m}(InAs){sub n} SPSs grown by MBE on InP substrates have been investigated by XRD, AFM, and TEM. Strong modulations were observed for growth temperatures between {approx} 540 and 560 C. The maximum strength of modulations was found for SPS samples with InAs mole fraction x (=n/(n+m)) close to {approx} 0.50 and when n {approx} m {approx} 2. The modulations were suppressed at both high and low values of x. For x >0.52 (global compression) the modulations were along the <100> directions in the (001) growth plane. For x < 0.52 (global tension) the modulations were along the two <310> directions rotated {approx} {+-} 27{degree} from [110] in the growth plane. The remarkably constant wavelength of the modulations, between {approx} 20--30 nm, and the different modulation directions observed, suggest that the origin of the modulations is due to surface roughening associated with the high misfit between the individual SPS layers and the InP substrate. Highly uniform unidirectional modulations have been grown, by control of the InAs mole fraction and growth on suitably offcut substrates, which show great promise for application in device structures.
Date: January 27, 2000
Creator: NORMAN,A.G.; AHRENKIEL,S.P.; MOUTINHO,H.R.; BALLIF,C.; ALJASSIM,M.M.; MASCARENHAS,A. et al.
Partner: UNT Libraries Government Documents Department

Photonic Band Gap Structures as a Gateway to Nano-Photonics

Description: This LDRD project explored the fundamental physics of a new class of photonic materials, photonic bandgap structures (PBG), and examine its unique properties for the design and implementation of photonic devices on a nano-meter length scale for the control and confinement of light. The low loss, highly reflective and quantum interference nature of a PBG material makes it one of the most promising candidates for realizing an extremely high-Q resonant cavity, >10,000, for optoelectronic applications and for the exploration of novel photonic physics, such as photonic localization, tunneling and modification of spontaneous emission rate. Moreover, the photonic bandgap concept affords us with a new opportunity to design and tailor photonic properties in very much the same way we manipulate, or bandgap engineer, electronic properties through modern epitaxy.
Date: August 1, 1999
Creator: FRITZ, IAN J.; GOURLEY, PAUL L.; HAMMONS, G.; HIETALA, VINCENT M.; JONES, ERIC D.; KLEM, JOHN F. et al.
Partner: UNT Libraries Government Documents Department