J. M. Perez

University of North Texas, Denton, TX, United States

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Publications (32)51.68 Total impact

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    ABSTRACT: The semiconducting silicides offer significant potential for use in optoelectronic devices. Full implementation of the materials, however, requires the ability to tailor the energy gap and band structure to permit the synthesis of heterojunctions. One promising approach is to alloy the silicides with Ge. As part of an investigation into the synthesis of semiconducting silicide heterostructures, a series of β- Fe ( Si <sub>1-x</sub> Ge <sub>x</sub>)<sub>2</sub> epilayer samples, with nominal alloy content in the range 0≤x≤0.15 , have been prepared by molecular beam epitaxy on Si(100). We present results of the epitaxial and crystalline quality of the films, as determined by reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, and double crystal x-ray diffraction, and of the band gap dependence on the alloy composition, as determined by Fourier transform infrared spectroscopy. We observe a reduction in band gap with increasing Ge content, in agreement with previous theoretical predictions [Tani etal, J. Solid State Chem. 169, 19 (2002)]. However we also observe Ge segregation in β- Fe ( Si <sub>1-x</sub> Ge <sub>x</sub>)<sub>2</sub> epilayers when x≥0.04 .
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/2005; · 1.27 Impact Factor
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    ABSTRACT: We report the preferential nucleation and synthesis of β‐FeSi2 nanostructures at pinned step bunches on the Si(111) surface. The nanostructures are synthesized by depositing Fe on Si at room temperature and subsequent annealing. The surface topography is studied using scanning tunneling microscopy and atomic force microscopy. The size, shape and orientation of the nanostructures indicate that the phase is the semiconducting β‐FeSi2 phase.
    Applied Physics Letters 05/2005; 86(22):223102-223102-3. · 3.79 Impact Factor
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    ABSTRACT: FeSi2 is a direct band gap semiconductor with a band gap of approximately 0.8 eV having great potential for the realization of Si optoelectronic devices. We report atomic resolution ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) studies of FeSi2 films grown on Si (111) substrates using reactive deposition epitaxy. Half a monolayer of Fe was deposited on Si (111) and then the sample was annealed for 5 minutes at a temperature of 500 C in UHV. UHV STM showed FeSi2 islands having a 2x2 hexagonal structure. Some surface roughening as a result of annealing was observed. Atomic force microscopy studies of the films carried out in air confirmed the island formation observed using UHV STM. This work was supported by the Texas Advanced Technology Program.
    10/2003;
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    ABSTRACT: We compare the field emission (FE) properties of different types of nanostructured carbon films including carbon sheets, single and multi-walled carbon nanotubes, and spiraled and wavy multi-walled carbon nanotubes. The effects of residual gasses such as oxygen, hydrogen, and nitrogen on the FE properties of the materials will be examined in detail. We find that oxygen exposure generally degrades the FE properties, while hydrogen and nitrogen do not significantly affect the FE properties.
    10/2003;
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    ABSTRACT: We compare the effects of residual gases on the field emission (FE) properties of sinle-walled and multi-walled carbon nanotubes, polycrystalline diamond films, and amorphous carbon films. The different films were exposed to oxygen, hydrogen and nitrogen residual gases at a pressure of 10-6 Torr for varying lengths of time under operating FE conditions. We observe that hydrogen and nitrogen exposure do not significantly affect the FE properties of all the samples. Oxygen exposure degrades the turn-on voltage of all the samples. The carbon nanotubes and diamond films are similarly affected by oxygen exposure. However, since nanotubes field emitt at lower voltages, they would be a better material for FE applications.
    10/2003;
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    A. Wadhawan, D. Garrett, J. M. Perez
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    ABSTRACT: We report the effects of microwave irradiation on both unpurified and purified iron-catalyzed high-pressure disproportionation (HiPco)-grown single-walled carbon nanotubes (SWNTs) in ultrahigh vacuum. Under microwave irradiation, we observe that unpurified HiPco SWNTs quickly reach temperatures of approximately 1850 °C. As a result, H2, H2O, CO, CO2, and CH4 gases are observed, and the Fe catalyst nanoparticles melt and coalesce into larger crystallites approximately four times their original diameter. In contrast, carbon black and purified HiPco SWNTs heat up to temperatures of 500–650 °C. We propose that the significant heating of unpurified HiPco SWNTs is due to the Fe catalysts. © 2003 American Institute of Physics.
    Applied Physics Letters 09/2003; 83(13):2683-2685. · 3.79 Impact Factor
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    Chemistry of Materials - CHEM MATER. 09/2003; 15(21).
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    ABSTRACT: We compare the effects of oxygen, argon and hydrogen gases on the field-emission (FE) properties of nanostructured carbon films including single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), amorphous carbon, and polycrystalline diamond. Both unpurified and purifed SWNTs are investigated. We find that argon and hydrogen do not significantly affect the FE properties of carbon films. Oxygen temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after operation in UHV. An oxygen environment causes a permanent decrease of the FE current and an increase in the turn-on field of MWNTs, amorphous carbon and polycrystalline diamond films.
    03/2003;
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    ABSTRACT: beta-FeSi2 shows promise as a Si based light emitter operating in ˜ 1.5mum wavelength range. However, there exists a number of concerns related to the nature of the bandstructure and the ability to dope the material. In order to determine the effect of Cr and Co dopants, temperature-dependent Hall effect measurements were taken on a series of beta-(Fe_1-xCr_x)Si2 , beta-(Fe_1-xCo_x)Si2 doped films, grown by MBE. The results show that Cr is a p-type dopant and Co is an n-type dopant in this material system. It is shown that at low temperature, conduction is dominated by defect band conduction, while at higher temperatures valence band conduction prevails. These samples show good conduction properties with typical resistivity of 1Omega-cm at 30K and .05 Omega-cm at 300K.
    03/2003;
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    ABSTRACT: We describe the growth and structural and optical propeties of FeSi2 films grown on Si (100). The films are grown using a molecular beam epitaxy (MBE) system attached to an ultrahigh vacuum (UHV) scanning tunneling microscopy system (STM). Si and Fe are codeposited at a substrate temperature of 500 ^oC and then annealed at 700 ^oC in UHV. Then, a Si overlayer approximately 0.3 μm thick is grown over the FeSi2 film and the sample is annealed at 900 oC for 15 minutes. The structural properties of the films were analyzed using UHV STM, atomic force microscopy, magnetic force microscopy, and Raman spectroscopy. The photoluminece and magneto-optical properties were also investigated. The results of these experiments will be compared.
    03/2003;
  • R. E. Stallcup, J. M. Perez
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    ABSTRACT: We report ultrahigh vacuum scanning tunneling microscopy studies of the clean nonhydrogen-terminated diamond (100)-2×1 surface showing single- and double-layer steps that are rebonded. The main defects observed are single, multiple, and row dimer vacancies, and antiphase boundaries. Buckling of dimers is not observed, consistent with symmetric dimers. © 2002 American Institute of Physics.
    Applied Physics Letters 12/2002; 81(24):4538-4540. · 3.79 Impact Factor
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    ABSTRACT: Iron disilicide shows great promise as a silicon based light emitter operating in the 1.3 to 1.5 μm wavelength range. However, there exists a number of questions related to the band structure and the ability to alloy and controllably dope the material both n and p type. In this paper we present Raman and magneto transport studies on β-FeSi2, β-(Fe1-xCrx)Si2, and β-(Fe1-xCox)Si2 grown by MBE. By comparing the spectra obtained for undoped and doped samples we provide a general overview of the effects of doping on the crystallinity of the material. The temperature dependent (4K<T<300K) magneto transport illustrates that Cr is a p-type dopant and Co is an n-type dopant in β-FeSi2.The temperature dependence of the resistivity indicates that the transport properties at higher temperatures are determined by free carriers whereas at lower temperatures impurity band conduction prevails.
    MRS Proceedings. 12/2001; 744.
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    ABSTRACT: We compare the effects of O <sub>2</sub>, Ar, and H <sub>2</sub> gases on the field-emission (FE) properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). We find that H <sub>2</sub> and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O <sub>2</sub> temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after operation in UHV. The higher operating voltages in an O <sub>2</sub> environment cause a permanent decrease of the FE current and an increase in the turn-on field of MWNTs. The ratios of the slopes before and after O <sub>2</sub> exposure are approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively. © 2001 American Institute of Physics.
    Applied Physics Letters 10/2001; · 3.79 Impact Factor
  • A. Wadhawan, K. F. Stephens, J. M. Perez
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    ABSTRACT: We compare the effects of oxygen, argon and hydrogen gases on the field-emission (FE) properties of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). We find that hydrogen and argon gases do not significantly affect the FE properties of SWNTs or MWNTs. Oxygen temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after opertation in UHV. The higher operating voltages in an oxygen enviroment cause a permanent decrease of FE current and increase in turn-on field of MWNTs. The ratios of the slopes before and after oxygen exposure are approximnately 1.04 and 0.82 for SWNTs and MWNTs, respectively.
    10/2001;
  • K. F. Stephens, A. Wadhawan, J. M. Perez
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    ABSTRACT: We compare the effects of oxygen, argon and hydrogen gases on the field emission (FE) properties of diamond films and carbon nanotubes. We find that hydrogen does not affect the FE properties of either diamond films or single- and multi-walled carbon nanotubes. Argon slightly degrades the FE properties of diamond films and has no effect of the FE properties of carbon nanotubes. Oxygen degrades the FE properties of both diamond films and carbon nanotubes. The degradation is more significant for diamond films. In addition, we find that diamond films do not field emit after they are heated to 1000 degrees C in ultrahigh vacuum. We propose that this is due to the desorbtion of hydrogen from the surface.
    10/2001;
  • R E Stallcup, J M Perez
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    ABSTRACT: We present a technique for obtaining atomic resolution ultrahigh vacuum scanning tunneling microscopy images of diamond (100) films, and use this technique to study the temperature dependence of the etching of epitaxial diamond (100) films by atomic hydrogen. We find that etching by atomic hydrogen is highly temperature dependent, resulting in a rough and pitted surface at T approximately 200 and 500 degrees C, respectively. At T approximately 1000 degrees C etching results in a smooth surface and is highly anisotropic, occurring predominantly in the direction of dimer rows. This observation supports recent theoretical models that propose anisotropic etching as the mechanism for the growth of smooth diamond (100) films.
    Physical Review Letters 05/2001; 86(15):3368-71. · 7.73 Impact Factor
  • A. Wadhawan, R. E. Stallcup, J. M. Perez
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    ABSTRACT: We report the effects of Cs deposition on the field-emission (FE) properties of single-walled carbon-nanotube bundles. We observe that Cs deposition decreases the turn-on field for FE by a factor of 2.1–2.8 and increases the FE current by six orders of magnitude. After Cs deposition, the FE current versus voltage (I–V) curves show non-Fowler–Nordheim behavior at large currents, consistent with tunneling from adsorbate states. At lower currents, the ratio of the slope of the FE I–V curves before and after Cs deposition is approximately 2.1. Exposure to N2 does not decrease the FE current, while exposure to O2 decreases the FE current. © 2001 American Institute of Physics.
    Applied Physics Letters 12/2000; 78(1):108-110. · 3.79 Impact Factor
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    ABSTRACT: We report the effects of Cs deposition on the field emission (FE) properties of single-walled carbon nanotube bundles. Cs deposition lowers the turn-on field for field emission by a factor of 2.1 - 2.8, and increases the field emission current by 6 orders of magnitude. At large currents, the FE current versus voltage (I-V) curves show non Fowler-Nordheim behavior consistent with tunneling from adsorbate states. At lower currents, the slope of the I-V curves decreases by a factor of approximately 2.1 after Cs deposition. The FE current does not decrease in ultrahigh vacuum and during exposure to nitrogen, but decreases during exposure to oxygen.
    10/2000;
  • R. E. Stallcup II, J. M. Perez
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    ABSTRACT: We describe a technique for obtaining atomic resolution ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) of epitaxial diamond (100) films. We report UHV STM studies of the temperature dependence of the etching of epitaxial diamond (100) films by atomic hydrogen. We find that etching by atomic hydrogen is highly temperature dependent resulting in a rough and pitted surface at T = 200 C and T = 500 C, respectively. At T = 1000 C, etching results in a smooth surface and is highly anisotropic ocurring predominantly in the direction of dimer rows. The anisotropic etching provides an explanation for the growth of smooth diamond (100) films at T = 1000 C.
    10/2000;
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    ABSTRACT: We report the effects of O2, H2, and N2 residual gases on the field emission properties of uncoated and diamond-coated individual Mo microtips. The microtips are made using electrochemical etching techniques and positioned 5 μm from the anode using a scanning tunneling microscopy system. We observe that the field emission (FE) current and turn-on voltage of diamond-coated microtips are significantly less degraded by O2 exposure than those of uncoated Mo microtips. H2 exposure enhances the FE properties of both uncoated and diamond-coated microtips, while N2 exposure does not have any significant effect. © 1999 American Institute of Physics.
    Applied Physics Letters 08/1999; 75(8):1179-1181. · 3.79 Impact Factor