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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

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

Germanium Nanocrystals Embedded in Sapphire

Description: {sup 74}Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by damage. Embedded nanocrystals experience large compressive stress relative to bulk, as embedded in sapphire melt very close to the bulk melting point (Tm = 936 C) whereas experience considerably lower stresses. Also, in situ TEM reveals that nanocrystals ion-beam-synthesized nanocrystals embedded in silica are observed to be spherical and measured by Raman spectroscopy of the zone center optical phonon. In contrast, reveals that the nanocrystals are faceted and have a bi-modal size distribution. Notably, the matrix remains crystalline despite the large implantation dose and corresponding thermal annealing. Transmission electron microscopy (TEM) of as-grown samples those embedded in silica exhibit a significant melting point hysteresis around T{sub m}.
Date: April 15, 2005
Creator: Xu, Q.; Sharp, I. D.; Liao, C. Y.; Yi, D. O.; Ager, J. W., III; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

Theory of Nanocluster Size Distributions from Ion Beam Synthesis

Description: Ion beam synthesis of nanoclusters is studied via both kinetic Monte Carlo simulations and the self-consistent mean-field solution to a set of coupled rate equations. Both approaches predict the existence of a steady state shape for the cluster size distribution that depends only on a characteristic length determined by the ratio of the effective diffusion coefficient to the ion flux. The average cluster size in the steady state regime is determined by the implanted species/matrix interface energy.
Date: June 13, 2008
Creator: Yuan, C.W.; Yi, D.O.; Sharp, I.D.; Shin, S.J.; Liao, C.Y.; Guzman, 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

Photo-oxidation of Ge Nanocrystals: Kinetic Measurements by InSitu Raman Spectroscopy

Description: Ge nanocrystals are formed in silica by ion beam synthesis and are subsequently exposed by selective HF etching of the silica. Under ambient conditions, the exposed nanocrystals are stable after formation of a protective native oxide shell of no more than a few monolayers. However, under visible laser illumination at room temperature and in the presence of O{sub 2}, the nanocrystals rapidly oxidize. The oxidation rate was monitored by measuring the Raman spectra of the Ge nanocrystals in-situ. The intensity ratio of the anti-Stokes to the Stokes line indicated that no significant laser-induced heating of illuminated nanocrystals occurs. Therefore, the oxidation reaction rate enhancement is due to a photo-chemical process. The oxidation rate varies nearly linearly with the logarithm of the laser intensity, and at constant laser intensity the rate increases with increasing photon energy. These kinetic measurements, along with the power dependencies, are described quantitatively by an electron active oxidation mechanism involving tunneling of optically excited electrons through the forming oxide skin and subsequent transport of oxygen ions to the Ge nanocrystal surface.
Date: November 22, 2006
Creator: Sharp, I.D.; Xu, Q.; Yuan, C.W.; Beeman, J.W.; Ager III, J.W.; Chrzan, D.C. 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

Mechanism of stress relaxation in Ge nanocrystals embedded in SiO2

Description: Ion-beam-synthesized {sup 74}Ge nanocrystals embedded in an amorphous silica matrix exhibit large compressive stresses in the as-grown state. The compressive stress is determined quantitatively by evaluating the Raman line shift referenced to the line position of free-standing nanocrystals. Post-growth thermal treatments lead to stress reduction. The stress relief process is shown to be governed by the diffusive flux of matrix atoms away from the local nanocrystal growth region. A theoretical model that quantitatively describes this process is presented.
Date: August 30, 2004
Creator: Sharp, I.D.; Yi, D.O.; Xu, Q.; Liao, C.Y.; Beeman, J.W.; Liliental-Weber, Z. et al.
Partner: UNT Libraries Government Documents Department

High-purity, isotopically enriched bulk silicon

Description: The synthesis and characterization of dislocation-free, undoped, single crystals of Si enriched in all 3 stable isotopes is reported: {sup 28}Si (99.92%), {sup 29}Si (91.37%), and {sup 30}Si (89.8%). A silane-based process compatible with the relatively small amounts of isotopically enriched precursors that are practically available was used. The silane is decomposed to silicon on a graphite starter rod heated to 700-750 C in a recirculating flow reactor. A typical run produces 35 gm of polycrystalline Si at a growth rates of 5 {micro}m/min and conversion efficiency >95%. Single crystals are grown by the floating zone method and characterized by electrical and optical measurements. Concentrations of shallow dopants (P and B) are as low as mid-10{sup 13} cm{sup -3}. Concentrations of C and O lie below 10{sup 16} and 10{sup 15} cm{sup -3}, respectively.
Date: November 17, 2004
Creator: Ager III, J.W.; Beeman, J.W.; Hansen, W.L.; Haller, E.E.; Sharp, I.D.; Liao, C. 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

Kinetics of visible light photo-oxidation of Ge nanocrystals:Theory and in situ measurement

Description: Photo-oxidation of Ge nanocrystals illuminated with visible laser light under ambient conditions was investigated. The photo-oxidation kinetics were monitored by in situ measurement of the crystalline Ge volume fraction by Raman spectroscopy. The effects of laser power and energy on the extent of oxidation were measured using both in situ and ex situ Raman scattering techniques. A mechanistic model in which the tunneling of photo-excited carriers to the oxide surface for electron activated molecular oxygen dissociation is proposed. This quantitative model successfully describes all experimental photo-oxidation observations using physical parameters.
Date: November 14, 2006
Creator: Sharp, I.D.; Xu, Q.; Yuan, C.W.; Beeman, J.W.; Ager III, J.W.; Chrzan, D.C. 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

Superheating and supercooling of Ge nanocrystals embedded inSiO2

Description: Free-standing nanocrystals exhibit a size-dependant thermodynamic melting point reduction relative to the bulk melting point that is governed by the surface free energy. The presence of an encapsulating matrix, however, alters the interface free energy of nanocrystals and their thermodynamic melting point can either increase or decrease relative to bulk. Furthermore, kinetic contributions can significantly alter the melting behaviors of embedded nanoscale materials. To study the effect of an encapsulating matrix on the melting behavior of nanocrystals, we performed in situ electron diffraction measurements on Ge nanocrystals embedded in a silicon dioxide matrix. Ge nanocrystals were formed by multi-energy ion implantation into a 500 nm thick silica thin film on a silicon substrate followed by thermal annealing at 900 C for 1 h. We present results demonstrating that Ge nanocrystals embedded in SiO{sub 2} exhibit a 470 K melting/solidification hysteresis that is approximately symmetric about the bulk melting point. This unique behavior, which is thought to be impossible for bulk materials, is well described using a classical thermodynamic model that predicts both kinetic supercooling and kinetic superheating. The presence of the silica matrix suppresses surface pre-melting of nanocrystals. Therefore, heterogeneous nucleation of both the liquid phase and the solid phase are required during the heating and cooling cycle. The magnitude of melting hysteresis is governed primarily by the value of the liquid Ge/solid Ge interface free energy, whereas the relative values of the solid Ge/matrix and liquid Ge/matrix interface free energies govern the position of the hysteresis loop in absolute temperature.
Date: August 21, 2006
Creator: Xu, Q.; Sharp, I.D.; Yuan, C.W.; Yi, D.O.; Liao, C.Y.; Glaeser,A.M. et al.
Partner: UNT Libraries Government Documents Department

Stable, free-standing Ge nanocrystals

Description: Free-standing Ge nanocrystals that are stable under ambient conditions have been synthesized in a two-step process. First, nanocrystals with a mean diameter of 5 nm are grown in amorphous SiO{sub 2} by ion implantation followed by thermal annealing. The oxide matrix is then removed by selective etching in diluted HF to obtain free-standing nanocrystals on a Si wafer. After etching, nanocrystals are retained on the surface and the size distribution is not significantly altered. Free-standing nanocrystals are stable under ambient atmospheric conditions, suggesting formation of a self-limiting native oxide layer. For free-standing as opposed to embedded Ge nanocrystals, an additional amorphous-like contribution to the Raman spectrum is observed and is assigned to surface reconstruction-induced disordering of near-surface atoms.
Date: January 28, 2005
Creator: Sharp, I.D.; Xu, Q.; Liao, C.Y.; Yi, D.O.; Beeman, J.W.; Liliental-Weber, Z. et al.
Partner: UNT Libraries Government Documents Department

Large melting point hysteresis of Ge nanocrystals embedded inSiO2

Description: The melting behavior of Ge nanocrystals embedded within SiO{sub 2} is evaluated using in situ transmission electron microscopy. The observed melting point hysteresis is large ({+-} 17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.
Date: May 4, 2006
Creator: Xu, Q.; Sharp, I.D.; Yuan, C.W.; Yi, D.O.; Liao, C.Y.; Glaeser,A.M. et al.
Partner: UNT Libraries Government Documents Department