L. Faraone

University of Auckland, Окленд, Auckland, New Zealand

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Publications (350)376.58 Total impact

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    ABSTRACT: Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, dark current levels can be reduced and collection efficiency substantially improved. It is shown that p-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trap-assisted Shockley–Read–Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/Hg0.05Cd0.95Te superlattices have, potentially, substantially better valence band alignment without the need for p-type doping.
    Journal of Electronic Materials 12/2014; · 1.64 Impact Factor
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    ABSTRACT: Crystallization of germanium (Ge) thin films was investigated by depositing Ge on glass and silicon substrates using electron beam (e-Beam) evaporation. The hole carrier concentration in Ge thin films deposited on glass substrates was found to decrease slightly with increasing annealing temperature, whereas its Hall mobility was found to increase monotonically. For all crystallized Ge thin films herein reported, the conductivity was found to be dominated by holes (p-type). This characteristic was then exploited to fabricate p-Ge/n-Si heterojunction diodes on n-type silicon substrates, which were then investigated by analysis of their current–voltage and capacitance–voltage characteristics. It was found that increasing annealing temperatures lead to significant improvements in on/off ratio and ideality factor, as well as increased built-in voltage. After crystallization of the top p-Ge layer through annealing at 600 °C, the devices indicated an on/off ratio of 106, an ideality factor of 1.25 and a built-in potential of 0.58 eV. The improvement in device performance is correlated with the crystallization of the Ge thin films, as confirmed by Hall effect and X-ray diffraction measurement, which indicated increases in hole mobility and improved solid-phase crystallization with increases in annealing temperature. The photoresponse of the devices was characterized employing a two-dimensional mapping approach, which provides insight into the optical generation and recombination processes at the Ge/Si heterojunction. The results herein presented indicate that thermally crystallized Ge thin films may be of significant promise for electronic and optoelectronic device applications.
    Materials Science in Semiconductor Processing 12/2014; 30:413. · 1.34 Impact Factor
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    ABSTRACT: Modern devices consisting of multiple semiconductor layers often result in the population of numerous distinct carrier species. Conventional Hall measurements at a single-magnetic-field strength provide only a weighted average of the electron mobility and carrier concentration of a semiconductor structure and, therefore, are of limited use for the extraction of carrier transport information. In recent years, mobility spectrum analysis techniques, which have been developed to extract a mobility spectrum from magnetic field-dependent conductivity-tensor measurements, have been applied in the analysis of carrier conductivity mechanisms of numerous semiconductor structures and devices. Currently there is a severe lack of reported studies on theoretical calculations of the mobility distribution of semiconductor structures or devices. In addition, the majority of reports on experimental mobility spectrum analysis are of complex, multi layered structures such as type-II superlattices, and the interpretation of the mobility spectra has been difficult. Therefore, a good understanding of the mobility spectrum has yet to be developed. For example, it is often assumed that distinct peaks of a mobility spectrum result from fundamentally different conduction mechanisms such as the bulk and surface conduction of narrow-band-gap semiconductors. In this article, we present calculations of the electron mobility distribution of bulk GaAs, which predict the existence of multiple mobility spectrum peaks that result from electron conductivity in the Γ conduction band. This report serves as an important and simple test case upon which experimentally measured mobility spectra can be compared. It also presents insight into the general nature of electron mobility distributions.
    New Journal of Physics 11/2014; 16(11):113033. · 4.06 Impact Factor
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    ABSTRACT: Modulation of the sub-band electron population in the inversion channel of 10-nm planar fully depleted silicon-on-insulator MOSFETs is evidenced by the bias dependence of inversion layer transport parameters. Two distinct inversion-layer electron species were detected by magnetic-field-dependent magnetoresistance measurements and high-resolution mobility spectrum analysis. According to self-consistent Poisson–Schrödinger calculations, these species correspond to carriers in distinct sub-bands within the Si channel region. The mobility peak of the carrier with the highest sheet density occurs under gate bias conditions that result in a minimum perpendicular effective electric field.
    IEEE Electron Device Letters 09/2014; 35(11). · 2.79 Impact Factor
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    ABSTRACT: A method is described where the valence band discontinuity in HgCdTe-based nBn detectors will be eliminated. The method relies on doping modulation technique, where grading the material composition and doping concentration of the barrier layer at the same time will lead to elimination of the valence band discontinuity in HgCdTe-based nBn detectors. The method is not limited to the nBn structure and can be applied to any barrier detector structure with xBx (with x = n, p) to eliminate the energy band discontinuity in the valence band or conduction band.
    Applied Physics Letters 09/2014; 105(12):121110. · 3.52 Impact Factor
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    ABSTRACT: Three-dimensional (3D) topological insulators (TI) are a new state of quantum matter in which surface states reside in the bulk insulating energy bandgap and are protected by time-reversal symmetry. It is possible to create an energy bandgap as a consequence of the interaction between the conduction band and valence band surface states from the opposite surfaces of a TI thin film, and the width of the bandgap can be controlled by the thin film thickness. The formation of an energy bandgap raises the possibility of thin-film TI-based metal-oxide-semiconductor field-effect-transistors (MOSFETs). In this paper, we explore the performance of MOSFETs based on thin film 3D-TI structures by employing quantum ballistic transport simulations using the effective continuous Hamiltonian with fitting parameters extracted from ab-initio calculations. We demonstrate that thin film transistors based on a 3D-TI structure provide similar electrical characteristics compared to a Si-MOSFET for gate lengths down to 10 nm. Thus, such a device can be a potential candidate to replace Si-based MOSFETs in the sub-10 nm regime.
    Journal of Applied Physics 08/2014; 116(8):084508-084508-8. · 2.21 Impact Factor
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    ABSTRACT: Photostriction-based all-optical actuation of silicon micro-cantilevers has been investigated through experimental characterization of structures fabricated on silicon-on-insulator substrates, and through numerical modeling and analysis of their semiconductor device and micromechanical characteristics. Since the pressure coefficient of the bandgap is negative in Si, photostriction-induced photoactuation in Si-based cantilevers was evident as upward mechanical defections (away from the substrate) in response to pulsed laser illumination on the cantilevers’ top surface; which is in contrast to the downward deflections typical of photothermal effects. For the numerical modeling of photostriction induced effects, carrier lifetime and excess-carrier concentrations were determined from transient photoconductance measurements. The experimentally determined parameters were then employed to simulate carrier-density profiles across the modeled structure. The modeled cantilever deflections were found to be in excellent agreement with experimentally determined deflections. It is also shown that 100-um-long Si cantilevers were deflected by up to 10 nm, and generated a force of 0.14 nN, when optically actuated by a 405-nm laser power density of 400 W/cm2.
    Journal of Microelectromechanical Systems 06/2014; · 2.13 Impact Factor
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    ABSTRACT: In this work, GaSb is proposed as a new alternative substrate for the growth of HgCdTe via molecular beam epitaxy (MBE). Due to the smaller mismatch in both lattice constant and coefficient of thermal expansion between GaSb and HgCdTe, GaSb presents a better alternative substrate for the epitaxial growth of HgCdTe, in comparison to alternative substrates such as Si, Ge, and GaAs. In our recent efforts, a CdTe buffer layer technology has been developed on GaSb substrates via MBE. By optimizing the growth conditions (mainly growth temperature and VI/II flux ratio), CdTe buffer layers have been grown on GaSb substrates with material quality comparable to, and slightly better than, CdTe buffer layers grown on GaAs substrates, which is one of the state-of-the-art alternative substrates used in growing HgCdTe for the fabrication of mid-wave infrared detectors. The results presented in this paper indicate the great potential of GaSb to become the next generation alternative substrate for HgCdTe infrared detectors, demonstrating MBE-grown CdTe buffer layers with rocking curve (double crystal x-ray diffraction) full width at half maximum of ̃60 arcsec and etch pit density of ̃106 cm-2.
    01/2014;
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    ABSTRACT: In this paper, we present the results of a numerical study on the influence of discrete dopant atom distribution and crystal orientation on the electrical characteristics of $p$-channel silicon nanowire-based transistors using 3-D quantum simulations. The valence band was modeled employing a three-band $k.p$ Hamiltonian with optimized Lüttinger parameters, while the device characteristics were obtained by modeling hole carrier transport through a self-consistent solution of Poisson's equation and the nonequilibrium Green's function formalism. Simulation of various discrete impurity configurations show that impurities located near the center of the channel region have the greatest impact on threshold voltage. It is shown that the effect of donor-like impurities on device threshold voltage is virtually independent of the nanowire's crystallographic orientation, whereas discrete acceptor-like impurities induce smaller threshold voltage shifts in nanowire transistors oriented along the [110] and [111] directions; thus suggesting that the devices oriented along these directions would be relatively less sensitive to the effects of unintentional impurities on threshold voltage.
    IEEE Transactions on Electron Devices 01/2014; 61(2):386-393. · 2.06 Impact Factor
  • S. Safa, A. Asgari, L. Faraone
    Journal of Applied Physics 01/2014; 115(14):146102-146102-2. · 2.21 Impact Factor
  • MRS Online Proceeding Library 01/2014; 1633:87-92.
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    ABSTRACT: A miniature Fabry-Perot etalon was designed and fabricated to provide spectral filtering capability at the resonance wavelength of 10 μm. A high transmission peak of 85% and a relatively broad bandwidth of 500 nm are expected based on optical modeling. Optimal deposition conditions for process durable thin film materials were developed and optical constants of these materials were characterized. Fabrication of devices was accomplished using standard surface micromachining technique. Released mirrors exhibited a deflection of 400 nm over a length of 150 μm.
    11/2013;
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    ABSTRACT: An electron transfer quantum well infrared photodetector (QWIP) consisting of repeating units of two coupled quantum wells (QWs) is capable of exhibiting a two color voltage dependent spectral response. However, significant electron transfer between the coupled QWs is required for spectral tuning, which may require the application of relatively high electric fields. Also, the band structure of coupled quantum wells is more complicated in comparison to a regular quantum well and, therefore, it is not always obvious if an electron transfer QWIP can be designed such that it meets specific performance characteristics. This paper presents a feasibility study of the electron transfer QWIP and its suitability for spectral tuning. Self consistent calculations have been performed of the bandstructure and the electric field that results from electron population within the quantum wells, from which the optical characteristics have been obtained. The band structure, spectral response, and the resonant final state energy locations have been compared with standard QWIPs. It is shown that spectral tuning in the long-wave infrared band can be achieved over a wide wavelength range of several microns while maintaining a relatively narrow spectral response FWHM. However, the total absorption strength is more limited in comparison to a standard QWIP, since the higher QW doping densities require much higher electric fields for electron transfer.
    Journal of Applied Physics 11/2013; 114:194501. · 2.21 Impact Factor
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    ABSTRACT: Spin-on dopants offer an expedient, straight forward, and low cost method for doping semiconductors. These sol-gel dopant films contain sufficient impurity content to obtain surface concentrations well above the solubility limit of most semiconductors (e.g., Si, Ge). While much has been published about spin-on dopants for degenerate doping of Si, there is to the authors' knowledge no report in the literature of such films being used to achieve degenerate p-type doping of Ge, as there are a number of technical challenges associated with this technique. In this work, we present a novel technique for degenerate p-type doping of Ge using Ga spin-on dopant films in a regular horizontal tube furnace. This technique gives both excellent uniformity, higher doping concentrations and better potential for ultra-shallow junctions than diffusion from solid sources, is much preferred to rapid melt/alloyed junctions (especially for optoelectronic applications), and is readily applicable to rapid thermal processing. In this preliminary investigation, we report doping concentrations exceeding 1020cm-3 which are shown to be fully electrically activated. We report on the use of a traditional "pre-dep"/"drive-in" approach, which could be used to give shallow dopant profiles, or tailored dopant concentrations, for a wide variety of electronic and optoelectronic applications, and to form back surface fields in photovoltaic and thermo-photovoltaic devices. Values of 3.4 ± 0.2 eV and 3.2 ± 0.4 eV are extracted for the activation enthalpies of the diffusion processes into p-type 1 0 0 and n-type 2 1 1 Ge substrates, respectively, which are shown to be in good agreement with previously published data [1,2]. We conclude that Ga is a more suitable dopant for reliable, low-cost, degenerate p-type doping of Ge, rather than B (also investigated in this work), which is known to be problematic.
    Solid-State Electronics 11/2013; 89:146. · 1.48 Impact Factor
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    ABSTRACT: We report the first planar waveguides made from mercury-cadmium-telluride (MCT) - a material to date exclusively used for mid-infrared (MIR) detector elements - serving as on-chip MIR evanescent field transducer in combination with tunable quantum cascade lasers (tQCLs) emitting in the spectral regime of 5.78-6.35 µm. This novel MIR sensing approach utilizes structured MCT chips fabricated via molecular beam epitaxy (MBE) as waveguide enabling sensing via evanescent field absorption spectroscopy, as demonstrated by the detection of 1 nL of acetone. Complementary finite difference time domain (FDTD) simulations fit well with the experimentally obtained data and predict an improvement of the limit of detection by at least 2 orders of magnitude upon implementation of thinner MCT waveguides. With the first demonstration of chemical sensing using on-chip MCT waveguides, monolithically fabricated IR sensing systems directly interfacing the waveguide with the MCT detector element may be envisaged.
    Analytical Chemistry 10/2013; · 5.82 Impact Factor
  • S. Safa, A. Asgari, L. Faraone
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    ABSTRACT: In this paper, we present a study of the effects of different superlattice structural parameters on the bandgap and on both the vertical and in-plane mobility of electrons in InAs/GaSb type-II superlattices using a fully numerical finite difference method. The analysis of our results clearly indicates the significance of interface roughness scattering and, in particular, that the influence of interface roughness correlation length and height is considerable. A comparison of our calculated results with published experimental data is shown to be in good agreement.
    Journal of Applied Physics 08/2013; 114(5). · 2.21 Impact Factor
  • A. Asgari, L. Faraone
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    ABSTRACT: In this paper we present a detailed study of the effects of temperature on the two-dimensional electron mobility distribution, using a fully numerical calculation in unintentionally doped AlxGa1−xN/AlN/GaN heterostructures. The analysis of our results clearly indicates that the effect of partial sub-band occupancy is considerable, especially at higher operating temperatures when more than one sub-band is occupied. By correlating with published experimental data, our results are consistent with the observed broadening of the mobility distribution with increasing temperature, which is shown to be a direct consequence of the temperature dependence of polar optical phonons. Although other scattering mechanisms also affect broadening of the mobility distribution, the contribution from these other individual scattering mechanisms is found to be independent of temperature. In particular, a comparison of our calculated results with published experimental data is shown to be in excellent agreement across the temperature range from 100 K to 300 K. By fitting our model to experimental results at low temperatures (95 K–125 K), we are able to extract detailed information related to surface roughness scattering, which is dominant in this temperature range. In particular, the average height of interface roughness is found to be 18 A.
    Journal of Applied Physics 08/2013; 114(5). · 2.21 Impact Factor
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    ABSTRACT: We report results of a detailed study of electronic transport in n-on-p junctions formed by 150-keV boron-ion implantation in vacancy-doped p-type Hg0.769Cd0.231Te without postimplantation thermal annealing. A mobility spectrum analysis methodology in conjunction with a wet chemical etching-based surface removal approach has been employed to depth profile the transport characteristics of the samples. In the as-implanted samples, three distinct electron species were detected which are shown to be associated with (a) low-mobility electrons in the top 220-nm surface-damaged layer (E1: μ80K = 2940 cm2/Vs), (b) the B-ion implantation region in the top 500-nm region (E2: μ80K = 7490 cm2/Vs), and (c) high-mobility electrons in the n-to-p transition region at a depth of 600 nm to 700 nm (E3: μ80K = 25,640 cm2/Vs). Due to the maximum magnetic field employed (2 T), hole carriers from the underlying vacancy-doped p-type region were detected only after the removal of the top 220 nm of the profiled sample (μ80K = 126 cm2/Vs), revealing fully p-type character 800 nm below the original sample surface. A comparison of the extracted E2 electron concentration and calculated B-impurity profile suggests that the n-type region is due primarily to near-surface implantation-induced lattice damage. © 2013 TMS.
    Journal of Electronic Materials 06/2013; · 1.64 Impact Factor
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    ABSTRACT: This paper reports on the fabrication of cantilever silicon-on-insulator (SOI) optical waveguides and presents solutions to the challenges of using a very thin 260-nm active silicon layer in the SOI structure to enable single-transverse-mode operation of the waveguide with minimal optical transmission losses. In particular, to ameliorate the anchor effect caused by the mean stress difference between the active silicon layer and buried oxide layer, a cantilever flattening process based on Ar plasma treatment is developed and presented. Vertical deflections of 0.5 $muhbox{m}$ for 70-$ muhbox{m}$-long cantilevers are mitigated to within few nanometers. Experimental investigations of cantilever mechanical resonance characteristics confirm the absence of significant detrimental side effects. Optical and mechanical modeling is extensively used to supplement experimental observations. This approach can satisfy the requirements for on-chip simultaneous readout of many integrated cantilever sensors in which the displacement or resonant frequency changes induced by analyte absorption are measured using an optical-waveguide-based division multiplexed system.$hfill$[2012-0180]
    Journal of Microelectromechanical Systems 06/2013; 22(3):569-579. · 2.13 Impact Factor

Publication Stats

1k Citations
376.58 Total Impact Points

Institutions

  • 2012
    • University of Auckland
      Окленд, Auckland, New Zealand
  • 1900–2012
    • University of Western Australia
      • School of Electrical, Electronic and Computer Engineering
      Perth, Western Australia, Australia
  • 2010
    • University of Southampton
      Southampton, England, United Kingdom
  • 2009
    • Military University of Technology
      • Division of Applied Physics
      Warsaw, Masovian Voivodeship, Poland
  • 2008
    • Università degli studi di Parma
      • Department of Information Engineering
      Parma, Emilia-Romagna, Italy
  • 2005
    • University of Tabriz
      • Research Institute for Applied Physics and Astronomy
      Tebriz, East Azarbaijan, Iran
  • 2004
    • University of California, Santa Barbara
      • Department of Electrical and Computer Engineering
      Santa Barbara, California, United States
  • 1999
    • Vigo System S.A.
      Ożarów-Franciszków, Masovian Voivodeship, Poland
    • Korea Advanced Institute of Science and Technology
      • Department of Electrical Engineering
      Seoul, Seoul, South Korea
  • 1984
    • University of Bradford
      Bradford, England, United Kingdom
  • 1981–1982
    • Lehigh University
      Bethlehem, Pennsylvania, United States
    • University of Toronto
      • Department of Electrical and Computer Engineering
      Toronto, Ontario, Canada