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Uniaxially stressed Ge:Ga and Ge:Be

Description: The application of a large uniaxial stress to p-type Ge single crystals changes the character of both the valence band and the energy levels associated with the acceptors. Changes include the splitting of the fourfold degeneracy of the valence band top and the reduction of the ionization energy of shallow acceptors. In order to study the effect of uniaxial stress on transport properties of photoexcited holes, a variable temperature photo-Hall effect system was built in which stressed Ge:Ga and Ge:Be could be characterized. Results indicate that stress increases the lifetime and Hall mobility of photoexcited holes. These observations may help further the understanding of fundamental physical processes that affect the performance of stressed Ge photoconductors including the capture of holes by shallow acceptors.
Date: December 1, 1992
Creator: Dubon, O.D. Jr.
Partner: UNT Libraries Government Documents Department

Valence band anticrossing in GaBixAs1-x

Description: The optical properties of GaBixAs1-x (0.04< x< 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms.
Date: July 11, 2007
Creator: Alberi, K.; Dubon, O. D.; Walukiewicz, W.; Yu, K. M.; Bertulis, K. & Krotkus, A.
Partner: UNT Libraries Government Documents Department

Mn l3,2 x-ray absorption spectroscopy and magnetic circulardichroism in ferromagnetic ga1-xmnxp

Description: We have measured the X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) at the Mn L{sub 3,2} edges in ferromagnetic Ga{sub 1-x}Mn{sub x}P films for 0.018 {le} x {le} 0.042. Large XMCD asymmetries at the L{sub 3} edge indicate significant spin-polarization of the density of states at the Fermi energy. The spectral shapes of the XAS and XMCD are nearly identical with those for Ga{sub 1-x}Mn{sub x}As indicating that the hybridization of Mn d states and anion p states is similar in the two materials. Finally, compensation with sulfur donors not only lowers the ferromagnetic Curie temperature but also reduces the spin polarization of the hole states.
Date: July 26, 2007
Creator: Stone, P.R.; Scarpulla, M.A.; Farshchi, R.; Sharp, I.D.; Beeman,J.W.; Yu, K.M. et al.
Partner: UNT Libraries Government Documents Department

Sculpting the shape of semiconductor heteroepitaxial islands: fromdots to rods

Description: In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.
Date: June 20, 2006
Creator: Robinson, J.T.; Walko, D.A.; Arms, D.A.; Tinberg, D.S.; Evans,P.G.; Cao, Y. et al.
Partner: UNT Libraries Government Documents Department

Metal-insulator transition by isovalent anion substitution in Ga1-xMnxAs: Implications to ferromagnetism

Description: We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4percent of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4 percent As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature TC from 113 K to 60 K (100 K to 65 K) upon the introduction of 3.1 percent P (1percent N) into the As sublattice.
Date: February 7, 2008
Creator: Stone, P. R.; Alberi, K.; Tardif, S. K. Z.; Beeman, J. W.; Yu, K. M.; Walukiewicz, W. et al.
Partner: UNT Libraries Government Documents Department

Non-magnetic compensation in ferromagnetic Ga1-xMnxAs and Ga1-xMnxP synthesized by ion implantation and pulsed-laser melting

Description: The electronic and magnetic effects of intentional compensation with non-magnetic donors are investigated in the ferromagnetic semiconductors Ga1-xMnxAs and Ga1-xMnxP synthesized using ion implantation and pulsed-laser melting (II-PLM). It is demonstrated that compensation with non-magnetic donors and MnI have similarqualitative effects on materials properties. With compensation TC decreases, resistivity increases, and stronger magnetoresistance and anomalous Hall effect attributed to skew scattering are observed. Ga1-xMnxAs can be controllably compensated with Te through a metal-insulator transition through which the magnetic and electrical properties vary continuously. The resistivity of insulating Ga1-xMnxAs:Te can be described by thermal activation to the mobility edge and simply-activated hopping transport. Ga1-xMnxP doped with S is insulating at all compositions but shows decreasing TC with compensation. The existence of a ferromagnetic insulating state in Ga1-xMnxAs:Te and Ga1-xMnxP:S having TCs of the same order as the uncompensated materials demonstrates that localized holes are effective at mediating ferromagnetism in ferromagnetic semiconductors through the percolation of ferromagnetic 'puddles' which at low temperatures.
Date: February 5, 2008
Creator: Scarpulla, M. A.; Stone, P. R.; Sharp, I. D.; Haller, E. E.; Dubon, O. D.; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

A Chemical Approach to 3-D Lithographic Patterning of Si and GeNanocrystals

Description: Ion implantation into silica followed by thermal annealingis an established growth method for Si and Ge nanocrystals. Wedemonstrate that growth of Group IV semiconductor nanocrystals can besuppressed by co-implantation of oxygen prior to annealing. For Sinanocrystals, at low Si/O dose ratios, oxygen co-implantation leads to areduction of the average nanocrystal size and a blue-shift of thephotoluminescence emission energy. For both Si and Ge nanocrystals, atlarger Si/O or Ge/O dose ratios, the implanted specie is oxidized andnanocrystals do not form. This chemical deactivation was utilized toachieve patterned growth of Si and Ge nanocrystals. Si was implanted intoa thin SiO2 film on a Si substrate followed by oxygen implantationthrough an electron beam lithographically defined stencil mask. Thermalannealing of the co-implanted structure yields two-dimensionallypatterned growth of Si nanocrystals under the masked regions. We applieda previously developed process to obtain exposed nanocrystals byselective HF etching of the silica matrix to these patterned structures.Atomic force microscopy (AFM) of etched structures revealed that exposednanocrystals are not laterally displaced from their original positionsduring the etching process. Therefore, this process provides a means ofachieving patterned structures of exposed nanocrystals. The possibilitiesfor scaling this chemical-based lithography process to smaller featuresand for extending it to 3-D patterning is discussed.
Date: December 12, 2005
Creator: Sharp, I. D.; Xu, Q.; Yi, D. O.; Liao, C. Y.; Ager, J. W., III; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

Metal-inducd assembly of a semiconductor-island lattice: Getruncated pyramids on Au-patterned Si

Description: We report the two-dimensional alignment of semiconductor islands using rudimentary metal patterning to control nucleation and growth. In the Ge on Si system, a square array of sub-micron Au dots on the Si (001) surface induces the assembly of deposited Ge adatoms into an extensive island lattice. Remarkably, these highly ordered Ge islands form between the patterned Au dots and are characterized by a unique truncated pyramidal shape. A model based on patterned diffusion barriers explains the observed ordering and establishes general criteria for the broader applicability of such a directed assembly process to quantum dot ordering.
Date: August 28, 2005
Creator: Robinson, J.T.; Liddle, J.A.; Minor, A.; Radmilovic, V.; Yi,D.O.; Greaney, P.A. et al.
Partner: UNT Libraries Government Documents Department

Highly Mismatched Alloys for Intermediate Band Solar Cells

Description: It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a ''stepping stone'' for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band.
Date: March 21, 2005
Creator: Walukiewicz, W.; Yu, K.M.; Wu, J.; Ager III, J.W.; Shan, W.; Scrapulla, M.A. et al.
Partner: UNT Libraries Government Documents Department

Mutual Passivation in Dilulte GaNxAs1-x Alloys

Description: The dilute GaN{sub x}As{sub 1-x} alloys (with x up to 0.05) have exhibited many unusual properties as compared to the conventional binary and ternary semiconductor alloys. We report on a new effect in the GaN{sub x}As{sub 1-x} alloy system in which electrically active substitutional group IV donors and isoelectronic N atoms passivate each other's activity. This mutual passivation occurs in dilute GaN{sub x}As{sub 1-x} doped with group IV donors through the formation of nearest neighbor IV{sub Ga-}N{sub As} pairs when the samples are annealed under conditions such that the diffusion length of the donors is greater than or equal to the average distance between donor and N atoms. The passivation of the shallow donors and the N{sub As} atoms is manifested in a drastic reduction in the free electron concentration and, simultaneously, an increase in the fundamental band gap. This mutual passivation effect is demonstrated in both Si and Ge doped GaN{sub x}As{sub 1-x} alloys. Analytical calculations of the passivation process based on Ga vacancies mediated diffusion show good agreement with the experimental results.
Date: March 21, 2005
Creator: Yu, K.M.; Walukiewicz, W.; Wu, J.; Mars, D.E.; Scarpulla, M.A.; Dubon, O.D. et al.
Partner: UNT Libraries Government Documents Department

Compositional tuning of ferromagnetism in Ga1-xMnxP

Description: We report the magnetic and transport properties of Ga{sub 1-x}Mn{sub x}P synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga{sub 1-x}Mn{sub x}As, Ga{sub 1-x}Mn{sub x}P displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga{sub 1-x}Mn{sub x}As, Ga{sub 1-x}Mn{sub x}P is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.
Date: May 23, 2006
Creator: Farshchi, R.; Scarpulla, M.A.; Stone, P.R.; Yu, K.M.; Sharp,I.D.; Beeman, J.W. et al.
Partner: UNT Libraries Government Documents Department

Synthesis of GaNxAs1-x thin films by pulsed laser melting andrapid thermal annealing (PLM-RTA) of N+-implanted GaAs

Description: We present a systematic investigation on the formation of the highly mismatched alloy GaN{sub x}As{sub 1-x} using N{sup +}-implantation followed by a combination of pulsed laser melting and rapid thermal annealing. Thin films of GaN{sub x}As{sub 1-x} with x as high as 0.016 and an activation efficiency of the implanted N up to 50% have been synthesized with structural and optical properties comparable to films grown by epitaxial deposition techniques with similar substitutional N content. The effects of N{sup +} implantation dose, laser energy fluence and rapid thermal annealing temperature on the N incorporation as well as optical and structural properties of the GaN{sub x}As{sub 1-x} films are discussed.
Date: March 11, 2003
Creator: Yu, K.M.; Walukiewicz, W.; Scarpulla, M.A.; Dubon, O.D.; Wu, J.; Jasinski, J. et al.
Partner: UNT Libraries Government Documents Department

Multiband GaNAsP Quaternary Alloys

Description: We have synthesized GaN{sub x}As{sub 1-y}P{sub y} alloys (x {approx} 0.3-1% and y = 0-0.4) using nitrogen N ion implantation into GaAsP epilayers followed by pulsed laser melting and rapid thermal annealing techniques. As predicted by the band anticrossing model, the incorporation of N splits the conduction band (E{sub M}) of the GaAs{sub 1-y}P{sub y} substrate, and strong optical transitions from the valence band to the lower (E{sub -}) and upper (E{sub +}) conduction subbands are observed. The relative strengths of the E{sub -} and E{sub +} transition change as the localized N level E{sub N} emerges from the conduction band forming narrow intermediate band for y > 0.3. The results show that GaN{sub x}As{sub 1-x-y}P{sub y} alloys with y > 0.3 is a three band semiconductor alloy with potential applications for high-efficiency intermediate band solar cells.
Date: December 8, 2005
Creator: Yu, K. M.; Walukiewicz, W.; Ager, J. W., III; Bour, D.; Farshchi, R.; Dubon, O. D. et al.
Partner: UNT Libraries Government Documents Department

Mn L3,2 X-ray absorption and magnetic circular dichroism inferromagnetic Ga1-xMnxP

Description: We have measured the X-ray absorption and X-ray magnetic circular dichroism (XMCD) at the Mn L{sub 3,2} edges in ferromagnetic Ga{sub 1-x}Mn{sub x}P for 0.018 {le} x {le} 0.042. Large XMCD asymmetries at the L{sub 3} edge indicate significant spin-polarization of the density of states at the Fermi energy. The temperature dependence of the XMCD and moment per Mn of 2.67 {+-} 0.45 {mu}{sub B} calculated using sum rules are consistent with magnetometry values. The spectral shapes of the X-ray absorption and XMCD are nearly identical with those for Ga{sub 1-x}Mn{sub x}As indicating that the hybridization of Mn d and anion p states is similar in the two materials.
Date: March 25, 2006
Creator: Stone, P.R.; Scarpulla, M.A.; Farshchi, R.; Sharp, I.D.; Haller,E.E.; Dubon, O.D. et al.
Partner: UNT Libraries Government Documents Department

Ferromagnetism in Ga1-xMnxP: evidence for inter-Mn exchangemediated bylocalized holes within a detached impurity band

Description: We report an energy gap for hole photoexcitation in ferromagnetic Ga{sub 1-x}Mn{sub x}P that is tunable by Mn concentration (x {le} 0.06) and by compensation with Te donors. For x{approx}0.06, electrical transport is dominated by excitation across this gap above the Curie temperature (T{sub c}) of 60 K and by thermally-activated hopping below T{sub c}. Magnetization measurements reveal a moment of 3.9 {+-} 0.4 {micro}{sub B} per substitutional Mn while the large anomalous Hall signal unambiguously demonstrates that the ferromagnetism is carrier-mediated. In aggregate these data indicate that ferromagnetic exchange is mediated by holes localized in a Mn-derived band that is detached from the valence band.
Date: January 11, 2005
Creator: Scarpulla, M.A.; Cardozo, B.L.; Farshchi, R.; Hlaing Oo, W.M.; McCluskey, M.D.; Yu, K.M. et al.
Partner: UNT Libraries Government Documents Department

Correlation between structure and electrical transport in ion-irradiated graphene grown on Cu foils

Description: Graphene grown by chemical vapor deposition and supported on SiO2 and sapphire substrates was studied following controlled introduction of defects induced by 35 keV carbon ion irradiation. Changes in Raman spectra following fluences ranging from 1012 cm-2 to 1015 cm-2 indicate that the structure of graphene evolves from a highly-ordered layer, to a patchwork of disordered domains, to an essentially amorphous film. These structural changes result in a dramatic decrease in the Hall mobility by orders of magnitude while, remarkably, the Hall concentration remains almost unchanged, suggesting that the Fermi level is pinned at a hole concentration near 1x1013 cm-2. A model for scattering by resonant scatterers is in good agreement with mobility measurements up to an ion fluence of 1x1014 cm-2.
Date: November 4, 2010
Creator: Buchowicz, G.; Stone, P.R.; Robinson, J.T.; Cress, C.D.; Beeman, J.W. & Dubon, O.D.
Partner: UNT Libraries Government Documents Department

Doping of GaN{sub 1-x}As{sub x} with high As content

Description: Recent work has shown that GaN{sub 1-x}As{sub x} can be grown across the entire composition range by low temperature molecular beam epitaxy with intermediate compositions being amorphous, but control of the electrical properties through doping is critical for functionalizing this material. Here we report the bipolar doping of GaN{sub 1-x}As{sub x} with high As content to conductivities above 4 S/cm at room temperature using Mg or Te. The carrier type was confirmed by thermopower measurements. Doping requires an increase in Ga flux during growth resulting in a mixed phase material of polycrystalline GaAs:N embedded in amorphous GaN{sub 1-x}As{sub x}.
Date: September 22, 2011
Creator: Levander, A.X.; Novikov, S.V.; Liliental-Weber, Z.; dos Reis, R.; Dubon, O.D.; Wu, J. et al.
Partner: UNT Libraries Government Documents Department

Synthesis and optical properties of II-O-VI highly mismatched alloys

Description: We have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdO{sub x}Te{sub 1-x} thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O{sup +}-implantation in CdTe followed by pulsed laser melting. Quaternary Cd{sub 0.6}Mn{sub 0.4}O{sub x}Te{sub 1-x} and Zn{sub 0.88}Mn{sub 0.12}O{sub x}Te{sub 1-x} with mole fraction of incorporated O as high as 0.03 were also formed. The enhanced O incorporation in Mn-containing alloys is believed to be due to the formation of relatively strong Mn-O bonds. Optical transitions associated with the lower (E{sub -}) and upper (E{sub +}) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of the host are clearly observed in these quaternary diluted II-VI oxides. These alloys fulfill the criteria for a multiband semiconductor that has been proposed as a material for making high efficiency, single-junction solar cells.
Date: January 20, 2004
Creator: Yu, K.M.; Walukiewicz, W.; Shan, W.; Wu, J.; Beeman, J.W.; Scarpulla, M.A. et al.
Partner: UNT Libraries Government Documents Department

Diluted magnetic semiconductors formed by an ion implantation and pulsed-laser melting

Description: Using ion implantation followed by pulsed-laser melting (PLM), we have synthesized ferromagnetic films of Ga{sub 1-x}Mn{sub x}As. Ion-channeling experiments reveal that these films are single crystalline and have high Mn substitutionality while variable temperature resistivity measurements reveal the strong Mn-hole interactions characteristic of carrier-mediated ferromagnetism in homogeneous DMS's. We have observed Curie temperatures (T{sub C}'s) of approximately 80 K for films with substitutional Mn concentrations of x=0.04. The use of n-type counter doping as a means of increasing Mn substitutionality and T{sub c} is explored by co-implantation of Mn and Te into GaAs. In Ga{sub 1-x}Mn{sub x}P samples synthesized using our technique, the implanted layer regrows as an epitaxial single crystal capped by a highly defective surface layer. These samples display ferromagnetism with T{sub c} {approx} 23 K.
Date: July 23, 2003
Creator: Scarpulla, M. A.; Daud, U.; Yu, K. M.; Monteiro, O.; Liliental-Weber; Zakharov, D. et al.
Partner: UNT Libraries Government Documents Department