Negative Electron Affinity Surface Research Articles

Materials for electron emitter.

Various materials have been examined to use for electron emitters. This paper reviews the results with regard to the materials for several practical electron emitters such as oxide cathodes, impregnated cathodes, LaB6 and semiconductors with negative electron affinity surface. Operation mechanisms, surface structures and stabilities are also described.

Construction of GaAs Spin-Polarized Electron Source and Measurements of Electron Polarization

A simple and compact system for generation and detection of polarized electrons is constructed. Polarized electrons are extracted from negative electron affinity (NEA) surface of GaAs by optical pumping with circulary polarized light from GaAlAs laser diode. The maximum degree of the electron polarization measured by 90 keV Mott analyzer is about ±10%.

Proposal for a semiconductor high resolution tracking detector

A “new” concept for detection and tracking of charged particles in high energy physics experiments is proposed. It combines a well known high purity semiconductor diode detector (HPSDD) with a heterojunction structure (HJ) and a negative electron affinity (NEA) surface. The detector should be capable of providing a two dimensional view (few cm 2) of multi-track events with the following properties: a) position resolution down to a few μm (10 8 position elements); b) high density of information (10 2−10 3 dots per mm of minimum ionizing track); c) high rate capabilities (few MHz); d) live operation with options to be triggered and/or the information from the detector can be used as an input for the decision to record an event.

Narrow beam emission of slow positrons from negative affinity surfaces

We have measured the angular distribution of the slow positrons (e +·) from negative work function (φ +) surfaces of Cu and Al bombarded by keV e + in ultrahigh vacuum. In analogy with √-point electrons emitted from negative electron affinity surfaces, the majority (>50%) of the slow e + leave the surface with energy ≈ φ + and velocity within ∼20° of the surface normal. This agrees qualitatively with the predictions of a simple 1D model.

New activation methods for long-life and highly stable GaP-GaAlP heterojunction cold cathodes

By new activation methods, namely, activation by Cs, Sb, and O2 (our former new method), or Cs, Ag, and O2 (our present new method), stabilized negative electron affinity (NEA) cold cathodes can be obtained, and continuous operation over 6000 h has been achieved with GaP-GaAlP cold cathodes at emission current density of about 10−1 A/cm2. It is found that a major source of rapid degradation of the cold cathode is the loss of the activation material from NEA surface because of nonthermal equilibrium for the activation material. By the new activation methods, the efficiency of the cold cathode showed an increase of 20 ∼60% over that of the conventional method.

The GaAs spin polarized electron source

The design, construction, operation, and performance of a spin polarized electron source utilizing photoemission from negative electron affinity (NEA) GaAs are presented in detail. A polarization of 43±2% is produced using NEA GaAs (100). The polarization can be easily modulated without affecting other characteristics of the electron beam. The electron beam intensity depends on the intensity of the exciting radiation at 1.6 eV; beam currents of 20 μA/mW are obtained. The source is electron optically bright; the emittance phase space (energy-area-solid angle product) is 0.043 eV mm2 sr. The light optics, electron optics, and cathode preparation including the GaAs cleaning and activation to NEA are discussed in depth. The origin of the spin polarization in the photoexcitation process is reviewed and new equations describing the depolarization of photoelectrons in the emission process are derived. Quantum yield and polarization measurements for both NEA and positive electron affinity surfaces are reported. The important considerations for interfacing he polarized electron source to an experiment are illustrated by its application to polarized low energy electron diffraction (PLEED). The advantages of this spin polarization modulated electron gun for PLEED are clearly demonstrated by sample PLEED results for W(100) and ferromagnetic Ni(110). A comparison with other polarized electron sources shows that the GaAs spin polarized electron source offers many advantages for a wide range of applications.

Electron emission from Si(100) negative electron affinity surfaces

A one-dimensional model for describing the transmission and reflection of X electrons at Si(100) negative electron affinity surfaces is justified in the light of a more accurate calculation.

Effect of overlayer thickness on electron emission from Si:Cs-O NEA surfaces

Experimental and theoretical studies of the variation in photoresponse of the Si:Cs-O negative electron affinity (NEA) photocathode with thickness of the Cs-O overlayer and acceptor concentration in the Si bulk, are reported. Resonance transmission peaks in the variation of photoyield with increasing overlayer thickness are predicted and observed. From a comparison of theoretical and experimental curves, the electron attenuation length in the overlayer is estimated to be about 50 AA. The theoretical analysis indicates that the optimum overlayer is metallic with a carrier concentration of about 2*1021 cm-3, varying slightly with bulk acceptor concentration.

Electron Emission Devices with Negative Electron Affinity Surface

Electron Emission Devices with Negative Electron Affinity Surface

The application of heterojunction structures to optical devices

Basic factors are reviewed that enter into the successful operation of single crystal heterojunction devices, with an emphasis on the role of defects. The devices discussed include laser diodes, solar cells and electron emitters using negative electron affinity surfaces.

AES and LEED study of the activation of GaAsCsO negative electron affinity surfaces

Gallium arsenide epitaxial layers have been activated to negative electron affinity by a two-stage process. This consisted of a heat clean in vacuum and activation with caesium and oxygen, followed by a second heating, to partially desorb the caesium, and re-activation with caesium and oxygen. AES measurements during the first activation indicated that the oxygen/caesium ratio increased as the photoemission increased to a maximum. This ratio was always less than the nearly constant ratio during the second activation which gave greater photoemission. After both the first and second activations desorption studies showed that, whereas the caesium Auger peak height decreased monotonically over a wide range of temperature, the oxygen peak height passed through a maximum as the temperature was increased, before decreasing rapidly to zero as the caesium desorption was completed. This temperature of desorption was slightly higher after the second activation. LEED showed that the caesium-oxygen layer was amorphous.

Thermionic emission from negative electron affinity silicon

Thermionic emission from negative electron affinity surfaces on silicon has been studied. The value is dependent on the state of activation of the surface, and dark currents in the range 10−11−10−13 A/cm2 may be obtained. These values agree with those measured on sealed−off tubes. The dark current is thought to originate from surface states acting as traps for the generation of electrons. The density of surface states is estimated from the measurement of the variation of dark current during oxygenation. Field enhancement of dark current and photoemission has been observed.

LEED-Auger characterization of GaAs during activation to negative electron affinity by the adsorption of Cs and O

A combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been used to study the formation of the negative electron affinity (NEA) condition on surfaces of p-type, degenerate, (100) and (111) GaAs. Activation to NEA is achieved by adsorbing Cs and O onto atomically clean GaAs in repetitive cycles of first Cs and then O. Before activation, the clean GaAs surfaces exhibit their characteristic LEED patterns. However, once obtained, there is no significant correlation between the quality of these LEED patterns and the final activation. The adsorption of both Cs and O during activation to NEA is amorphous. Auger measurements have shown that the first photoemission maximum occurs after the adsorption of about a half monolayer of Cs. The initial O adsorption occurs on the GaAs surface between the Cs atoms. The adsorbed O interacts strongly with Cs at any stage during the activation. Peak photosensitivities, after completion of the Cs and O adsorptions, were in the range 400 to 1100 μA lumen . The final activation does not correlate with the quantity of Cs and O on the surface. The temperature dependence of the photosensitivity of NEA GaAs (100) activated at −170°C has a broad maximum at about −50°C and a subsidiary maximum at about 160°C. In addition, the photoemission at −170°C can be either increased or decreased by having heated the sample up to 200°C, even though no Cs or O desorption has taken place. These results can be traced to changes in work function rather than to changes in bulk properties. While the LEED patterns from clean GaAs show no structural changes with temperature, such changes are observed when Cs is on the surface. It is suggested that changes both in photoemission and in LEED patterns are due to the temperature-induced mobility of Cs on GaAs. An atomic model for the NEA surface is discussed in terms of a layer of Cs and O atoms about 10 Å thick on the GaAs.

The application of semiconductors with negative electron affinity surfaces to electron emission devices

Semiconductors with negative electron affinity (NEA) surfaces are used as photoemitters, secondary emitters, and cold-cathode emitters. A comprehensive review of the characteristics and applications of these materials is presented, the concept of NEA is described, and a comparison is made between NEA and conventional emitters. Electron generation, transport, and emission processes of NEA emitters are discussed. NEA III-V compound photocathodes, especially GaAs, are described with respect to their fabrication, performance, and applications to photomultipliers and image intensifier tubes. The structure and performance of NEA secondary emitters are presented. NEA GaP secondary-emission dynodes represent the most important device application. NEA cold cathodes, using GaAs, Ga(As, P), or Si, have been investigated, and their performance characteristics are summarized. NEA Si cold cathodes have been incorporated in developmental TV camera tubes. The characteristics of these tubes are reviewed.

Effects of Cathode Bumpiness on the Spatial Resolution of Proximity Focused Image Tubes

In proximity focused electrostatic image tube; spatial resolution is a strong function of the cathode flatness.Topographic irregularities of the cathode surface establish transverse electric fields near the cathode.The fields impart a component of transverse momentum to the emitted electron, which, in turn,degrades the spatial resolution characteristic of the system. Calculations based on a simple model of a corrugated cathode show that, under a typical set of operating conditions, the point at which loss of resolution becomes considerable is 60 cycles/mm for a bump height of 0.2 microm and a bump period of 1 microm. For the resolution to be comparable to that predicted for a perfectly flat GaAs negative electron affinity (NEA) cathode, which is 160 cycles/mm, the bump period must be 0.1 microm, and its height must be 0.02 microm. Such small bumps represent a relatively smooth surface. Observations of surface irregularities on related NEA surfaces indicate that the bump period and height are about 1.0 micro and 0.2-0.5 microm, respectively, so that the spatial resolution in these cases is limited by the deviation from flatness as opposed to being limited by the finite random velocities of the emitted photoelectrons.

Negative electron affinity surfaces on silicon using a rubidium/oxygen dipole layer

Rubidium/oxygen dipole layers have been used to generate negative electron affinity (NEA) surfaces on Si (100). Rb/O layers behave exactly as Cs/O layers with respect to the activation schedule and to the electron emission characteristics. Rb/O layers show the same crystallographic anisotropy as Cs/O layers in that only the Si (100) surface activates to NEA. The spectral photoresponse curves for each layer are identical. The characteristics of the Si/Cs/O and Si/Rb/O surfaces are compared to III-V NEA emitters and to conventional emitters for which an Rb/O surface layer produces a less efficient emitting surface. These results are explained by the fact that the dipole layer on NEA Si (100) is very well ordered with specific sites for the anions and the cations. The geometrical arrangement of the dipole layers most strongly determines their characteristics, as opposed to the chemical nature of the Cs/O or Rb/O compounds. Among other possible substitutes for Cs and O, neither Cs/I, Cs/F, and Cs/S layers nor Na/O and K/O layers produced NEA on Si (100).

LEED, Auger and plasmon studies of negative electron affinity on Si produced by the adsorption of Cs and O

We have made LEED, Auger, and Plasmon measurements to study how Cs and O adsorb onto the (100) surface of p-type degenerate Si to produce negative electron affinity (NEA). A key factor to producing NEA was found to be a highly ordered Si surface as reflected by very high quality 2×2 LEED patterns. When NEA is produced, both the adsorbed Cs and O give the same LEED pattern as the original Si surface, but with a general enhancement of the half-order spot intensity. The adsorption of both Cs and O is strongly self-limiting, apparently controlled by the number of available appropriate sites on the surface. If Cs and O adsorb amorphously, NEA is not achieved. The thermal desorption of Cs occurs over a fairly broad temperature range centered at about 550°C. After Cs desorbs, the remaining O reverts spontaneously from an ordered layer to an amorphous layer, and then desorbs at about 800°C with an activation energy of 3.3 eV. Measurements of backscattered electron energy losses due to plasmons have shown that the Si surface plasmon is reduced in energy from 12.5 V to 7.0 V by the Cs-O layer. From this, an effective dielectric constant ε = 5.3 for this layer can be deduced which, in turn, enables us to characterize completely the Cs-O dipole layer. The geometrical model described by Levine for the NEA surface is consistent with our experimental results.

AN OPTOELECTRONIC COLD CATHODE USING AN AlxGa1−xAs HETEROJUNCTION STRUCTURE

An efficient optoelectronic cold cathode has been made which includes a Si-compensated AlxGa1−xAs electroluminescent diode covered with an absorbing p-type GaAs layer having a negative electron affinity surface. This structure is designed to minimize current crowding in the vicinity of the Ohmic contact. An over-all efficiency of 1.1×10−3 (current emitted into vacuum/diode current) has been achieved. This represents a factor of 102–103 improvement over previous p-n junction or optically coupled cold cathode structures.