Escape Depth Of Secondary Electrons Research Articles

Secondary electron emission and photoelectric emission with account of excited electrons’ moving backward emission surface

From existing old photoelectric emission PEE and secondary electron emission SEE models and the process that 50 % excited electrons are treated as as those moving backward emission surface, this study develops new SEE model for negative electron affinity semiconductor NEAS with large lSEE and new PEE model, respectively; lSEE denotes mean escape depth of secondary electrons. From our investigations of PEE from metals and NEAS with large lPEE and SEE from NEAS with large lSEE, it can be concluded that the new SEE model for NEAS with large lSEE and new PEE model developed here are correct; lPEE denotes mean escape depth of photo-emitted electrons. The theories of SEE from insulators with 120 Å≤ lSEE ≤ 1.0 × 104 Å are also developed and discussed.

Secondary electron emission and photoemission from a negative electron affinity semiconductor with large mean escape depth of excited electrons

The formulae for parameters of a negative electron affinity semiconductor (NEAS) with large mean escape depth of secondary electrons λ (NEASLD) are deduced. The methods for obtaining parameters such as λ, B, E pom and the maximum δ and δ at 100.0 keV ≥ E po ≥ 1.0 keV of a NEASLD with the deduced formulae are presented (B is the probability that an internal secondary electron escapes into the vacuum upon reaching the emission surface of the emitter, δ is the secondary electron yield, E po is the incident energy of primary electrons and E pom is the E po corresponding to the maximum δ). The parameters obtained here are analyzed, and it can be concluded that several parameters of NEASLDs obtained by the methods presented here agree with those obtained by other authors. The relation between the secondary electron emission and photoemission from a NEAS with large mean escape depth of excited electrons is investigated, and it is concluded that the presented method of obtaining λ is more accurate than that of obtaining the corresponding parameter for a NEAS with large λ ph (λ ph being the mean escape depth of photoelectrons), and that the presented method of calculating B at E po > 10.0 keV is more widely applicable for obtaining the corresponding parameters for a NEAS with large λ ph.

Secondary electron emission from insulators and negative electron affinity semiconductors

Based on expression of primary range R and the R at Epo ≥ 10.0 keV and L at Epo ≥ 1.0 keV calculated by ESTAR program [1], the method of obtaining formula for R at Epo ≥ 1.0 keV is presented; where Epo is incident energy of primary electron, and L is the energy loss of primary electron per unit path length at the incident surface of the materials. Based on the obtained formula for R, experimental δ, relationships among parameters of δ and the processes and characteristics of secondary electron emission SEE, the universal formula for δ is deduced and the method of calculating δ at Epo ≥ 1.0 keV of insulators and negative electron affinity NEA semiconductors is presented and experimentally proved, where δ is secondary electron yield. The methods of calculating mean escape depth of secondary electrons 1/α and B of insulators and NEA semiconductors are presented, respectively; where B is the probability that an internal secondary electron escapes into vacuum upon reaching the surface of emitter. From the relationships among parameters of SEE and the comparison between the values of 1/α and B obtained in this study and those obtained in former work, it concludes that the methods of calculating 1/α and B are correct.

Electron-lattice and electron–electron interactions and Debye temperature studied by temperature dependence of secondary electron emission from insulators

The formulas for temperature dependence of λ(χreal, Eg, T), λ(χreal, Eg, T)p, ẟ and σ of insulators with less impurities were deduced and proved to be true; λ(χreal, Eg, T) is the mean escape depth of secondary electrons emitted from insulators with efficient electron affinity χreal and band gap Eg at Kelvin temperature T, λ(χreal, Eg, T)p is the λ(χreal, Eg, T) due to electron-lattice scatterring in insulators, ẟ is secondary electron yield, σ is total ẟ. Analyses of data and the deduced formulas indicate that the temperature dependence of ẟ and σ of insulators with less impurities increase with the increasing [λ(χreal, Eg, T)/λ(χreal, Eg, T)p], and that λ(χreal, Eg, T), λ(χreal, Eg, T)p, ẟ and σ of insulators with less impurities decrease nearly linearly with increasing T in the range T > ΘD but decrease nonlinearly with increasing T in the range mΘD < T < ΘD, ΘD is Debye temperature, m of a given insulator is a constant which is < 0.4. The method of obtaining λ(χreal, Eg, T)Mee and corresponding χreal by secondary electron emission (SEE) was presented, λ(χreal, Eg, T)Mee is the λ(χreal, Eg, T) due to electron–electron scatterring in insulator. It concludes that the method presented here to obtain special formula for λ(χreal, Eg, T)p and corresponding χreal is a good method to research electron-lattice interaction by SEE, and that the method presented here to obtain values of λ(χreal, Eg)Mee and corresponding χreal is a good method to research electron–electron interaction by SEE. According to the values of ΘD, experimental σ, characteristics of temperature dependence of ẟ and σ and deduced formulas, it concludes that the method presented here to determine ΘD by SEE is a better method to determine ΘD of insulator with less impurities.

Secondary electron emission properties of double-layer B-doped diamond films

B-doped double-layer diamond films were prepared by microwave plasma chemical vapor deposition. The effects of different B-doping concentrations on the crystal quality, surface morphology, surface composition and the conductivity, secondary electron emission performance of the films were investigated. It is found that the increased conductivity with increased B-doping is beneficial for secondary electron emission. However, the degraded crystal quality of diamond, increased surface sp2 carbon and the segregation of B element at surface due to increased B-doping could degrade the secondary electron emission performance of the film. So it can be considered as a competition between improved vertical conductance that tends to increase secondary electron escape depth and finally degrading surface properties as a function of B-doping. The double-layer film with a light B-doping concentration has good crystal quality and enough conductivity, which helps to obtain the best secondary electron emission performance.

Study of secondary electron emission from semiconductors induced by electrons from 20 eV to 800 eV and surface state of semiconductors

Based on processes and characteristics of secondary electron emission (SEE), experimental data and the relations among parameters of secondary electron yield from semiconductors δ, the formula for δ in the range of 20 eV ≤ Epo ≤ 800 eV of semiconductors with Epom ≤ 800 eV as a function of Epo, Epom, atomic number Z and maximum δ (δm) was deduced and proved experimentally, where Epo and Epom are incident energy of primary electron and the Epo of δm, respectively. From the existing formula and the relation among Z, mean escape depth of secondary electrons emitted from semiconductors and primary range R at Epom deduced in this study, the formula for R in the range of 150 eV ≤ Epo ≤ 800 eV in monocrystalline semiconductors with Epom ≤ 800 eV, χ ≥ 0.6 eV and Eg ≥ 0.6 eV was successfully deduced, where χ is original electron affinity, Eg is the width of forbidden band. The method of calculating B(χreal, Eg) of semiconductor with Epom ≤ 800 eV, χreal ≥ 0.5 eV and Eg ≥ 0.6 eV and that of calculating B of semiconductor with Epom ≤ 800 eV were presented, respectively; Where B is the probability that an internal secondary electron escapes into vacuum upon reaching the surface of semiconductor, and the B(χreal, Eg) is the B relative to semiconductor with Eg and real electron affinity χreal. It can be concluded that the two methods are correct. From the analysis of calculated B, experimental Epom and relations among parameters of SEE, it can be concluded that this paper can research the surface state of semiconductor with Epom ≤ 800 eV by SEE.

A study of secondary electron emission from semiconductors and insulators

No rude approximations were made in the course of deducing formula for B(χ, Eg) as a function of parameters of secondary electron emission SEE; where B(χ, Eg) is the probability that an internal secondary electron escapes into vacuum upon reaching the surface of semiconductors and insulators SI with width of forbidden band Eg and original electron affinity χ. It can be concluded that the formula for B(χ, Eg) as a function of parameters of SEE can be used to calculate B(χ, Eg). The B(χ, Eg) calculated with the formula for B(χ, Eg) as a function of parameters of SEE and characteristics of B(χ, Eg) and SEE from SI were studied, the formula for B(χ, Eg) of SI with Eg≥0.6 eV and χ≥0.5 eV as a function of χ and Eg was determined. It concludes that the determined formula can be used to calculate B(χ, Eg) of SI with Eg≥0.6 eV and χ≥0.5 eV. The processes and characteristics of SEE from SI, energy loss of primary electron and parameters of SEE were studied, the formulae for δm(Epom, χreal, Eg) in the general case and 1/α(χreal, Eg) of SI with Eg≥0.6 eV and χ≥0.5 eV were deduced and experimentally proved, respectively; where δm(Epom, χreal, Eg) is the maximum secondary electron yield δm of SI with Eg and χreal, Epom is primary incident energy corresponding to δm, χreal is real electron affinity,1/α(χreal, Eg) is mean escape depth of secondary electrons emitted from SI with Eg and χreal.

Formulae for maximum yield and mean escape depth of secondary electrons emitted from metals

Based on the characteristics of secondary electron emission, the former formulae for the maximum yield of metals [Formula: see text] and primary range in the energy range of [Formula: see text] eV, relation [Formula: see text] among secondary electron escape probability [Formula: see text], average energy required to produce an internal secondary electron [Formula: see text], [Formula: see text], primary energy of [Formula: see text] ([Formula: see text] and parameter [Formula: see text] was deduced. On the basis of [Formula: see text] and the former formula for [Formula: see text], relation [Formula: see text] among mean escape depth of secondary electrons emitted from metals [Formula: see text], atomic number [Formula: see text], back-scattering coefficient [Formula: see text], material density [Formula: see text], [Formula: see text], atomic weight [Formula: see text] and [Formula: see text] was deduced. According to the deduced [Formula: see text], the formula for the ratio of [Formula: see text] to [Formula: see text] and experimental ratio of [Formula: see text] to [Formula: see text], relation [Formula: see text] among [Formula: see text], original work function [Formula: see text] and [Formula: see text] were empirically obtained. According to the characteristics of metal surface, [Formula: see text], [Formula: see text] and [Formula: see text], the formula for [Formula: see text] as a function of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] and that for [Formula: see text] as a function of [Formula: see text], [Formula: see text] and [Formula: see text] were deduced, respectively. The influences of surface change on [Formula: see text] and [Formula: see text] were analyzed. Calculated [Formula: see text] and [Formula: see text] were compared with the corresponding experimental ones. It was concluded that the deduced formulae for [Formula: see text] and [Formula: see text] can be used to estimate [Formula: see text] and [Formula: see text], respectively.

FORMULA FOR MAXIMUM SECONDARY ELECTRON YIELD FROM METALS

Based on free-electron model, the calculated inelastic mean escape depth of secondary electrons, experimental one, the energy band of metal, the characteristics and processes of secondary electron emission, maximum number of secondary electrons released per primary electron δ(Φ,EF)PEm as a function of parameter Km, work function Φ and Fermi energy EF was deduced, where Km is a constant for a given metal in the energy range 100–800 eV. According to the relationship between maximum secondary electron yield from metal δ(Φ,EF)m and δ(Φ,EF)PEm, the formula for δ(Φ,EF)m as a function of atomic number Z, parameter Km, Φ and EF was deduced. Using the deduced formula for δ(Φ,EF)m, Z, experimental δ(Φ,EF)m, Φ and EF, Km relative to alkali metals, Km relative to earth-alkali metals and the mean value of Km were computed, respectively. And the formulae for maximum secondary electron yield from alkali metals, earth-alkali metals and metals were obtained and proved to be true, respectively. On the basis of the deduced formula for δ(Φ,EF)m and the empirical relation that high Φ are connected with high EF, it can be concluded that high δ(Φ,EF)m are connected with high Φ and vice versa.

Backscattered helium spectroscopy in the helium ion microscope: Principles, resolution and applications

We demonstrate the possibilities and limitations for microstructure characterization using backscattered particles from a sharply focused helium ion beam. The interaction of helium ions with matter enables the imaging, spectroscopic characterization, as well as the nanometer scale modification of samples. The contrast that is seen in helium ion microscopy (HIM) images differs from that in scanning electron microscopy (SEM) and is generally a result of the higher surface sensitivity of the method. It allows, for instance, a much better visualization of low-Z materials as a result of the small secondary electron escape depth. However, the same differences in beam interaction that give HIM an edge over other imaging techniques, also impose limitations for spectroscopic applications using backscattered particles. Here we quantify those limitations and discuss opportunities to further improve the technique.

Mean escape depth of secondary electrons emitted from semiconductors and insulators

Based on dominant physical processes and characteristics of secondary electron emission, formula for maximum yield was deduced. On the basis of relation among secondary electron escape probability, maximum yield from semiconductors and insulators, primary energy of maximum yield and average energy required to produce a secondary electron, mean escape depth of secondary electrons emitted from semiconductors and insulators as a function of atomic number, average atomic number, back-scattering coefficient, material density, atomic weight and primary energy of maximum yield were deduced. Calculated mean escape depth of secondary electrons were compared with the values measured experimentally and the scattering of mean escape depth of secondary electrons from same material was analyzed. It was concluded that the formulae presented were found to be universal for the mean escape depth of secondary electrons emitted from semiconductors and insulators.

Y2O3–Lu2O3 co-doped molybdenum secondary emission material

A new kind of RE2O3–Mo cathodes co-doped with Y2O3 and Lu2O3 were prepared by a solid–liquid method combined with the spark plasma sintering technique. The secondary electron emission property and secondary emission mechanism of the cathode have been studied. The results show that Y2O3–Lu2O3 co-doped Mo cathode exhibits better secondary emission property than single Y2O3 doped molybdenum cathode, and the maximum secondary emission yield (δmax) can reach 4.616 after being activated at 900°Cb. The Y2O3–Lu2O3 co-doped Mo cathode has high penetration depth of primary electrons and moderate escape depth of secondary electrons. Furthermore, the work function of Y2O3–Lu2O3–Mo cathode (2.41eV) is lower than that of Y2O3–Mo cathode (3.59eV). These phenomena are responsible for its excellent secondary emission property.

Emission property of Y 2O 3-doped Mo secondary emitter

Y 2O 3-doped Mo secondary emitters were prepared by liquid-liquid doping and solid-solid doping, respectively. The back-scattered scanning observation result indicates that the emitter prepared by liquid-liquid doping has fine microstructure whereas that prepared by solid-solid doping has large grain size. Y 2O 3-doped Mo emitter with small grain size prepared by liquid-liquid doping exhibits high emission property, i.e., the secondary electron yield can get to 5.24, about 1.7 times that prepared by solid-solid doping. Moreover, Y 2O 3-doped Mo emitter exhibits the best emission performance among La 2O 3-doped Mo, Y 2O 3-doped Mo, Gd 2O 3-doped Mo and Ce 2O 3-doped Mo emitters due to the largest penetration depth of primary electrons and escape depth of secondary electrons in this emitter. The secondary emission of the emitter with small grain size can be explained by reflection emission model and transmission emission model, whereas only transmission emission exists in the emitter with large grain size.

Secondary electron doping contrast: Theory based on scanning electron microscope and Kelvin probe force microscopy measurements

The secondary electron emission flux in a scanning electron microscope is a powerful tool for delineation of electrically active dopant concentration, built-in potentials, and surface electric fields in semiconductor junctions. In all the secondary electron images of p-n junctions, the p-doped regions appear brighter than n-doped regions. We present a theory for the doping contrast in p-n junctions that is based on the secondary electron emission yield and surface band bending extracted from Kelvin probe force microscopy measurements. We show that the contrast is governed by the secondary electron escape depth, and their escape probability which is related to the secondary electron energy distribution and the effective electron affinity. It is found that the escape depth is the main factor determining the dopant contrast, and the escape probability has a smaller effect. In addition, our theory explains the logarithmic dependence of the measured contrast on the acceptor concentration in silicon reported by many groups.

Preparation and characterization of single RE 2O 3–Mo secondary emission materials

La 2O 3–Mo, Y 2O 3–Mo, Gd 2O 3–Mo and Ce 2O 3–Mo were prepared by liquid–liquid doping combined with spark plasma sintering. The microstructure and surface behaviour of rare earth oxide of the cathode have been studied by microscope, Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) methods. Among these four kinds of cathode, Y 2O 3–Mo cathode exhibits the best secondary emission property, i.e., the maximum secondary emission yield could reach 5.24. The penetration depth of primary electrons and escape depth of secondary electrons have been calculated and the energy distribution of primary electrons in the cathode has been simulated by Monte Carlo method. Y 2O 3–Mo cathode has the largest penetration depth and escape depth, which could be attributed to the highest secondary electron emission yield of the cathode.

Mechanisms of charging of insulators under irradiation with medium-energy electron beams

The electron emission and charge characteristics of a large number of massive insulators are investigated qualitatively and quantitatively. It is demonstrated that irradiation of insulators with a medium-energy continuous electron beam leads to a decrease in the equilibrium energy of incident electrons (the second critical energy) as compared to the theoretical value. The equilibrium state of charging to saturation is attained for times from several seconds to several hundred seconds depending on the irradiation current density, the electron energy, and the insulator material. The mechanisms of charging are explained in the framework of the model according to which irradiation is accompanied by the formation of a charged double layer that consists of a positively charged layer (with the thickness equal to the escape depth of secondary electrons) and a negatively charged layer (with the thickness equal to the penetration depth of primary electrons).

High resolution quantitative two-dimensional dopant mapping using energy-filtered secondary electron imaging

We have quantified two-dimensional dopant profiles in Si with high accuracy using a commercial field emission gun scanning electron microscope with added secondary electron energy-filtering capabilities. Quantification was achieved for dopant concentrations of 2.8×1017–7.5×1019cm−3 with an accuracy of ±8.5%. Using energy filtering, we have measured the surface potential difference ΔΦs across a 3nm wide p-doped Si layer with respect to the n-doped substrate of a test structure to be 0.72±0.15V. Spatial resolution limits are discussed and are shown to be limited by the secondary electron escape depth. A technique for the rapid collection of calibrated energy-filtered images is also proposed.

Contribution of dynamic charging effects into dopant contrast mechanisms in silicon

In this work, SEM capability for imaging of both p- and n-doped regions in Si was demonstrated. The best dopant contrast was found when the primary electron range (R) is comparable or larger than the maximum escape depth of secondary electrons (∼5λ) (λ stands for mean free path). Beyond this scale (R > 5λ) the contrast between p-, n-doped and intrinsic regions gradually disappears. The dopant profiles obtained by SEM were judged using scanning capacitance microscopy (SCM), dopant selective etch (DSE) and secondary ion mass spectrometry (SIMS) measurements, and excellent matching was demonstrated. A novel dopant contrast mechanism incorporating dynamic charging effects that take place during e-beam/specimen interaction is suggested. Under threshold steady-state imaging conditions, an E bi field in Si near the surface region is formed. This field, governed by secondary electron (SE) emission and trapping of some incident and generated SE, accelerates electrons towards the surface in p-type regions and decelerates them in n-type regions, compared with the intrinsic material. This results in the observed dopant contrast: C(n) < C(i) < C(p). Use of the SEM for 2D-dopant imaging provides many advantages; giving fast results, covering a wide range of dopant concentrations, applicable to real devices, and does not require sample preparation needed by SCM and DSE. In addition, SEM-dopant contrast data quantification is possible using SIMS standards which needs to be defined with more details.

Electron beam induced deposition of pure, nanoscale Ge

Pure, nanoscale Ge dots were deposited on Si3N4 substrates by decomposing digermane (Ge2H6) using a focused electron beam. Deposited feature diameters are larger than that of the electron beam used for deposition by an amount comparable to the secondary electron escape depth. This result suggests that axial secondary electron emission through the surface of a growing feature limits the minimum attainable feature size. In situ, electron energy-loss spectroscopy shows that the dots are pure Ge with C contents below the carbon detection limit of less than 18%. Analyzing the bright field image in the thin film, single scattering approximation yields the height of the Ge dots allowing the average Ge deposition efficiency to be estimated.

Nanoscale electron stimulated chemical vapor deposition of Au in an environmental transmission electron microscope

Nanoscale Au rich dots were deposited on SiO2 and Si3N4 substrates by decomposing an adsorbed organometallic precursor using a focused electron beam. The precursor, dimethyl acetylacetonate gold ((CH3)2(C5H7O2)Au), does not react with either substrate without electron irradiation. Deposited feature diameters are larger than that of the electron beam used for deposition by an amount comparable to the secondary electron escape depth. This result suggests that axial secondary electron emission through the surface of a growing feature limits the minimum attainable feature size. Real-time composition analysis during growth using electron energy-loss spectroscopy indicates that mature features entrain significant carbon.