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Review Article: Rare-earth monosulfides as durable and efficient cold cathodesa)
a)This work is dedicated to the memory of Walter Friz.
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10.1116/1.3653275
/content/avs/journal/jvstb/29/6/10.1116/1.3653275
http://aip.metastore.ingenta.com/content/avs/journal/jvstb/29/6/10.1116/1.3653275

Figures

Image of FIG. 1.
FIG. 1.

Cross-section of the Friz cold cathode, which consists of an electron emitter (Au, Ag, or heavily doped InP substrate), a tens of nanometers thick wide-bandgap (2.4 eV) CdS layer, and a LaS thin film on the surface. The NEA at the CdS/LaS interface facilitates the injection of electrons into vacuum. The top emitting surface is biased with an array of emitter fingers. [Reprinted with permission from P. D. Mumford and M. Cahay, Journal of Applied Physics 81, 3707 (1997).]

Image of FIG. 2.
FIG. 2.

Energy band diagram across the Friz cathode from the substrate into vacuum. The substrate is held at ground potential and a positive bias is applied on the LaS thin film using the array of emitter fingers shown in Fig. 1

Image of FIG. 3.
FIG. 3.

(Color online) (a) Pellet of the cubic phase of LaS about 1.5 cm long, 4 mm wide and 2 mm thick prepared by the carbon reduction process. (b) Optical micrograph showing a large golden platelet (about 30 μm2) of the cubic phase of LaS with a staircaselike growth pattern. [Reprinted with permission from M. Cahay, K. Garre, X. Wu, D. Poitras, D. J. Lockwood, and S. Fairchild, Journal of Applied Physics 99, 123502 (2006).]

Image of FIG. 4.
FIG. 4.

Comparison of XRD scans of a LaS bulk sample (a) before, and (b) after the carbon reduction process. In the latter, the Bragg reflection peaks corresponding to the oxysulfide (La2O2S) impurity phase have disappeared.

Image of FIG. 5.
FIG. 5.

(Color online) Thin film of LaS deposited by PLD on a square inch Si substrate. The reflection of the finger shows the metallic character of the thin film. [Reprinted with permission fromM. Cahay, K. Garre, X. Wu, D. Poitras, D. J. Lockwood, and S. Fairchild, Journal of Applied Physics 99, 123502 (2006).])

Image of FIG. 6.
FIG. 6.

(Color online) AFM scans over a 1 × 1 μm2 area of a LaS thin film (about one micron thick) grown on a (001) Si substrate The root mean square of the surface roughness is 1.74 nm over that area. [Reprinted with permission from M. Cahay, K. Garre, X. Wu, D. Poitras, D. J. Lockwood, and S. Fairchild, Journal of Applied Physics 99, 123502 (2006).]

Image of FIG. 7.
FIG. 7.

X-ray diffraction pattern at three different grazing angles of incidence from a LaS thin film deposited on a (100) Si substrate. Miller indices (hkl) of the reflections from the cubic rocksalt phase of LaS are identified. The units along the horizontal axis are in degrees. The peak located around 56o is due to the (311) Bragg reflection observed for a bare Si substrate (Joint Committee on Powder Diffraction Standard card No. 77-211). [Reprinted with permission from S. Fairchild, J. Jones, M. Cahay, K. Garre, P. Draviam, P. Boolchand, X. Wu, and D. J. Lockwood, Journal of Vacuum Science and Technology B 23, 318 (2005).]

Image of FIG. 8.
FIG. 8.

High-resolution TEM image of the LaS/Si interface for a 1 μm thick LaS film grown on a (100) Si substrate. Despite the large lattice mistmatch between the two materials (8%), the interface is rather at the length scale shown in this TEM picture. [Reprinted with permission from S. Fairchild, J. Jones, M. Cahay, K. Garre, P. Draviam, P. Boolchand, X. Wu, and D. J.Lockwood, Journal of Vacuum Science and Technology B 23, 318 (2005).]

Image of FIG. 9.
FIG. 9.

(Color online) AFM scans over a 2 × 2 μm2 area of (a) a LaS thin film grown on a (001) MgO substrate, and (b) a LaS thin film grown on a (100) Si substrate. The PLD parameters for both films were identical (substrate temperature: 400 C, 24 mTorr of H2S background pressure, laser repetition rate of 4 Hz, deposition time: 30 min, substrate to target separation: 10 cm), except that the Si substrate was held at room temperature and no H2S background gas was used during the growth. [Reprinted with permission from S. Fairchild, M. Cahay, L. Grazulis, K. Garre, J. W. Fraser, D. J. Lockwood, V. Semet, V. T. Binh, S. Bandyopadhyay, and B. Kanchibotla, Journal of Vacuum Science and Technology B 26, 891 (2008).]

Image of FIG. 10.
FIG. 10.

UPS spectra of a LaS film deposited by pulsed laser ablation. The inset shows a magnified region at the Fermi energy (EF).

Image of FIG. 11.
FIG. 11.

(Color online) I-V characteristics for LaS thin film and nanoprotrusion cathodes. Vfilm and Vnano were the actual applied potentials to obtain field emission. The factor of 3.6 for nanoprotrusion means that one needs a potential that is 3.6 less than for a flat film to obtain the same field emission current. In other words, the field enhancement factor related to the nanodome geometry is at least 3.6.

Image of FIG. 12.
FIG. 12.

(Color online) I-V characteristics for LaS deposited on MgO and Si. The two sets of data were obtained at the same probe distances, d, for the same field emission fields. [Reprinted with permission from S. Fairchild, M. Cahay, L. Grazulis, K. Garre, J. W. Fraser, D. J. Lockwood, V. Semet, V. T. Binh, S. Bandyopadhyay, and B. Kanchibotla, Journal of Vacuum Science and Technology B 26, 891 (2008).]

Image of FIG. 13.
FIG. 13.

(Top) HRTEM image of the deposited LaS film showing the flat surface and the nanocrystalline nature of the layer. (Bottom) Schematic representation of a patchwork FE through nanocrystallites of a LaS thin film in close proximity of the probe ball (anode with a spherical shape) used in SAFEM experiments. The nanocrystallites (a) have a low work function (crystallites (a) and (b) have the same orientations), and the nanocrystallites (b) are not field emitting because they are embedded in the layer; the lines surrounding the nanocrystallites (a) schematically represent the current lines collecting the emitted electrons. [Reprint with permission from V. Semet, M. Cahay, V. T. Binh, S. Fairchild, X. Wu, and D. J. Lockwood, J. Vac. Sci. Technol. B 24, 2412 (2006).]

Image of FIG. 14.
FIG. 14.

(Color online) (a) Schematic of a similar patch area with a low work function, Φ1, and with a few nm diameter surrounded by a larger work function, Φ2, amorphous phase. The separation between the two regions is not abrupt and the work function linearly increases from Φ1 to Φ2 over a connecting layer of width, LC, around 0.5 nm. [Reprint with permission from V. T. Binh, R. Mouton, Ch. Adessi, V. Semet, M. Cahay, and S. Fairchild, J. Appl. Phys. 108, 044311 (2010).] (b) Schematic of potential energy profile at equilibrium at the interface between the central patch with low work function, Φ1, and the surrounding amorphous material with a larger work function, Φ2.

Image of FIG. 15.
FIG. 15.

(Color online) Evolution of the potential distribution over a 2 nm diameter patch showing the progressive opening over the patch foran increasing applied field, Fapp. The patch has a WF of 1 eV and the surrounding medium has a WF of 2.8 eV. The data in the figures represent the lowest surface barrier height of the opening. In figure (d), for example, for an Fapp of 1 V/nm the surface barrier height at the center of the patch is 1.32 eV. [Reprint with permission from V. T. Binh, R. Mouton, Ch. Adessi, V. Semet, M. Cahay, and S. Fairchild, J. Appl. Phys. 108, 044311 (2010).]

Image of FIG. 16.
FIG. 16.

(Color online) Potential opening over the patch for two patch dimensions (1 and 4 nm) and for the same applied fields (0.1 and 0.5 V/m).

Image of FIG. 17.
FIG. 17.

Plots of I-V characteristics, respectively, for a patch cathode and for two uniform surface cathodes.

Image of FIG. 18.
FIG. 18.

X-ray diffraction pattern at three different grazing angles of incidence from a LaS thin film deposited on a (100) InP substrate. The units along the horizontal axis are in degrees. Miller indices (hkl) of the reflections from the cubic rocksalt phase of LaS are identified.

Image of FIG. 19.
FIG. 19.

(a) and (b) Cross-section TEM images of a LaS thin film on an InP substrate showing that there are two LaS layers on the InP substrate, and the thicknesses of LaS(1) and LaS(2) are 52 and 121 nm, respectively. (c) Selected area diffraction pattern from the LaS layers reveals that the LaS film has a poly-crystalline nature.

Image of FIG. 20.
FIG. 20.

(a) and (b) Ellipsometric spectra, Ψ(E) and Δ(E), measured from LaS on InP, respectively. (c) The extracted complex permittivity dispersion curves, ɛ 1(E) and ɛ 2(E). (d) The estimated penetration depth of the light into the LaS film based on the permittivity dispersion.

Image of FIG. 21.
FIG. 21.

Room temperature Raman spectrum of (a) bulk LaS (from a target similar to that used in the growth chamber) compared with (b) a LaS film grown on (100) InP. Here, A and O refer to acoustic and optical phonons involved in first-order and second-order Raman scattering.

Image of FIG. 22.
FIG. 22.

FE-SEM images of (a) a single, and (b) a cluster of LaS nanoballs grown on a Si substrate using the following PLA parameters: 248 nm excimer laser, laser pulse duration = 25 ns, 700 mJ/pulse, 10 Hz, 5 min deposition, 1 Torr Ar background pressure, target to substrate separation, 5 cm.

Image of FIG. 23.
FIG. 23.

XRD Θ-2Θ scan of LaS nanoclusters deposited by PLA on a Si substrate.

Image of FIG. 24.
FIG. 24.

TEM micrographs of single LaS nanoparticle deposited on a TEM grid. The PLA parameters were: 248 nm excimer laser, laser pulse duration 25 ns, 700 mJ/pulse, 4 pulses, 1 Torr Ar background pressure, target to substrate separation, 5 cm.

Tables

Generic image for table
TABLE I.

Materials parameters of some sulfides of rare-earth metals (cubic form): a (lattice constant in Å), WF (work function at room temperature), Tm (melting point in °C), and ρ, electrical resistivity (in μΩ cm)5. [Reprinted with permission from O. Eriksson, M. Cahay, and J. Wills, Phys. Rev. B 65, 033304 (2002).]

Generic image for table
TABLE II.

Model used for fitting the ellipsometric data for a film of LaS on InP.

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2011-10-28
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Review Article: Rare-earth monosulfides as durable and efficient cold cathodesa)
http://aip.metastore.ingenta.com/content/avs/journal/jvstb/29/6/10.1116/1.3653275
10.1116/1.3653275
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