Negative Electron Affinity Photocathodes Research Articles

Spin-resolved electron spectroscopy with highly polarized sources: Inverse photoemission from ferromagnets

We report on the use of recently developed spin-polarized electron sources with very high polarization for electron spectroscopy. In particular we present data of spin-resolved inverse photoemission from Fe(100) films excited by polarized electrons produced by a strained GaAsP negative electron affinity photocathode. This highly polarized source (beam polarization P=65%) allows a direct and almost complete decoupling of the majority and minority empty states in Fe(100), much better than the standard GaAs sources (P=20%–30%). The preparation and characterization of the strained photocathode is also discussed.

Semiconductor on glass photocathodes as high-performance sources for parallel electron beam lithography

The throughput of electron beam lithography has historically been limited by electron–electron interactions that cause blurring at high currents. We present a system configuration for maskless parallel electron beam lithography using a new multiple primary source technology that, by employing widely spaced beams, significantly reduces this problem. The proposed source technology, a negative electron affinity (NEA) photocathode, allows us to generate an array of high brightness, low energy spread, independently modulated beams over a large area. In order to assess the effects of electron–electron interactions in this system, Monte Carlo simulations have been performed. The results of these calculations indicate that this configuration enjoys significant advantages over existing maskless systems. By restricting the area of emission for the individual beamlets to submicron dimensions, the blurring due to statistical electron–electron interactions can be significantly reduced for a given current at the wafer. For example, at 50 kV a total current of more than 2.5 μA can be obtained with less than 10 nm beam blurring. Preliminary experimental results suggest that high brightness emission can be maintained from a NEA photocathode in a demountable vacuum system.

Low energy Mott polarimetry of electrons from negative electron affinity photocathodes

We present data on the spin polarization P and quantum yield Y of electrons photoemitted from negative electron affinity semiconductors, including GaAs(100), GaAsP(100) alloy, and strained GaAs layer epitaxially grown on a GaAsP(100) buffer. Near photothreshold the following values for P(Y) are, respectively, obtained: 26% (2.5×10−2), 40% (1×10−3), and 60% (1.5×10−4). We describe in detail the apparatus used containing a low energy (10–25 keV) Mott polarimeter. The system, completely fitted in a small volume (∼104 cm3) ultrahigh vacuum chamber, is intended as a test facility for characterizing candidate photocathode materials for spin polarized electron sources.

A photomultiplier tube incorporating an avalanche photodiode

A photomultiplier tube incorporating an avalanche photodiode operated in the electron bombardment mode has been developed and tested. It has a large linear dynamic range, can operate with gains up to 10 6, is unaffected by axial magnetic fields and has been measured to have excellent timing properties. Different versions of the tube were made with bialkali, multi-alkali, and negative electron affinity photocathodes, in this paper we will report on results obtained with these different types of photocathodes. This type of PMT will be useful for applications where large dynamic range and high quantum efficiencies are required. The extension of this concept to a multichannel device with a segmented APD array will provide an economical readout for both fiber tracking and calorimetry applications. A tube equipped with such an array is being developed currently as a readout option for the SDC shower maximum detector.

Cs and NF3 coadsorption on GaAs(100) and GaAs0.62P0.38(100) for photocathodes preparation

Negative electron affinity (NEA) photocathodes have been prepared by Cs and NF3 coadsorption on heat-cleaned GaAs(100) and GaAs0.62P0.38(100) surfaces. The use of NF3 as oxidizing gas leads to a simple and reliable activation procedure. The NEA photocathodes are studied by means of quantum yield and photocurrent lifetime measurements. Studies performed at low temperature (120 K) suggest that Cs release from the activated surface is the main reason of photocathode degradation.

Interface reactions in LPE grown InGaAsP/InP ridge waveguide laser diodes during aging : G. Beister, J. Maege, G. Richter and H. Treptow. Solid-St. Electron.34(8), 883 (1991)

This chapter focuses on negative electron affinity (NEA) photocathodes that represent a significant new class of semiconductor electron emission devices. Their original conception and subsequent development has derived from the surface physics of electron emission, widespread availability of ultrahigh vacuum equipment, and parallel development of semiconductor materials technology. A great deal of information on the potential and limitations of the devices has been gained. The engineering difficulties of developing viable tube fabrication techniques, adequate shelf, and operating life stability are being met. Gallium phosphide (Gap), gallium arsenide phosphide (GaAsP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), and Si devices are commercially available. GaAs is the most important NEA surface from both an applications standpoint and in terms of basic surface physics studies to date. Their superior performance is, however, at a higher cost and more restricted operating range when compared with conventional cathodes. The proven potential coupled with a diversity of applications assures a firm future for the NEA semiconductor photocathode.

Comparative study of the preparation of negative electron affinity GaAs photocathodes with O2 and with NF3

We discuss the possibility of replacing oxygen with nitrogen trifluoride in the preparation of stable GaAs negative electron affinity (NEA) photocathodes. The different role of O2 versus NF3 exposure during the activation is examined by means of quantum yield and photocurrent lifetime measurements. Quantum yield results clearly demonstrate that NEA conditions can be attained also with NF3. Measurements performed on the same samples and in the same experimental conditions, show a remarkably different time evolution of the activation process in the two cases. However, as a result of a huge number of activations, no evidence of significantly larger lifetimes for NF3 prepared photocathodes can be deduced.

Electron emission properties of laser pulsed GaAs negative electron affinity photocathodes

The emitted electron energy distribution of a laser pulsed GaAs negative electron affinity (NEA) photocathode was measured with a retarding grid analyzer. The total electron energy distribution was found to depend strongly on laser intensity and cathode thickness. A model based on the separation of the excess photogenerated carriers within the cathode structure has been developed and tested to explain this behavior. Pulsed electron emission patterns were obtained by directing the emitted electrons to a phosphorous screen. The patterns indicate that uniform emission occurs and that the emission develops uniformly as the cathode is activated to a NEA condition. An extrapolated plot of the angular intensity FWHM versus V1/2B indicates that the pulsed cathode possesses a natural semicone angle of approximately 10°. The angular distribution is narrow because of the electron refraction which occurs upon emission due to differences in the effective mass and vacuum rest mass of the electrons.

The adsorption of Cs and O2 on a clean GaAs(110) surface under light illumination

The activation process of the GaAs negative electron affinity (NEA) photocathode is now well known. Activation supplemented by light illumination has not yet been reported. In this work we have investigated the effects of while light illumination on the adsorption and desorption of Cs and O2 on the GaAs(110) surface during various stages of activation of the GaAs NEA photocathode. Its effects on photoemission sensitivity, emission stability, and recovery of impaired photoemission have also been observed. It is suggested that the incident photons would produce electron–hole pairs on the GaAs surface. Phonons would be produced in recombination of the electrons and holes thus produced. Based upon the above supposition, all the investigated effects are explained and discussed.

Influences of MBE growth conditions on quantum efficiency of negative electron affinity GaAs photocathode.

The MBE growth conditions for Be doped GaAs layers are evaluated from quantum efficiency of the negative electron affinity photocathodes made of these layers. The quantum efficiency of the GaAs layers grown by molecular beam epitaxy method at various growth conditions was measured in reflection mode operation. High quantum efficiencies were obtained with Be doping levels between 1018 and 1019 cm-3 at high As/Ga beam flux ratios and high substrate temperatures.

Electron optical characteristics of negative electron affinity cathodes

Stable continuous electron emission has been obtained from GaAs electron cathodes which have been activated to a negative electron affinity condition. Negative electron affinity (NEA) photocathodes offer one means of producing very short electron pulses with a high peak brightness necessary for time-resolved metrology. Despite concerns regarding the stability of such cathodes, we have demonstrated a high degree of short-term stability through the ability to take scanning electron microscope photographs and Auger spectra using a NEA cathode. We have also demonstrated an effective brightness measured at the sample of 1×105 A/cm2 sr at 3 kV using our prototype electron gun. Room-temperature operation and laser excitation of this cathode make it particularly well-suited for low-voltage time-resolved electron probe and electron spectroscopy applications.

The role of oxygen and fluorine in the electron emission of some kinds of cathodes

Experiments have proved that lowering of the work function of a cathode by adsorbing both oxygen and electropositive metallic elements on its surface may be more effective than by adsorbing simply the metallic element. However, the role played by oxygen is not yet completely clear. In this paper we first discuss briefly the situation of the thermionic cathode. Then the role of oxygen or fluorine in the activation of the negative electron affinity GaAs photocathode is discussed. Emphasis will be placed on the atomic core potential field of the surface atoms of the cathode, the bonding state of the surface adsorption layer (Cs or O), and the transfer of electron charges in the surface atoms. The conclusion is drawn that an appropriate quantity of oxygen adsorbed on the cathode surface would be essential to result in a better dispenser cathode and also high quantum yield negative electron affinity photocathodes, while fluorine would play an important part in the electron emission stability.

Preparation of epitaxial GaAs layers for negative electron affinity photocathodes by using the electrical current-induced liquid phase epitaxy

The article reports on a new method of electrical current-induced liquid phase epitaxy used to prepare epitaxial GaAs layers doped with zinc. Measurement results have proved the LPEE to be a suitable method for producing NEA photocathodes.

Investigation of the mechanism of the activation of GaAs negative electron affinity photocathodes

This paper has discussed the mechanism of the formation of negative electron affinity (NEA) GaAs photocathodes which are treated with cesium and oxygen. The formation is mainly due to the drop of the electron affinity when the GaAs surface is covered with cesium atoms. This drop is not due to the formation of surface dipoles as usually supposed. The essence is that in cesium processing the surface potential barrier of GaAs is caused by the considerably weak nuclear force field of the surface cesium layer rather than by that of GaAs. The further decrease of the electron affinity by oxygen is due to the formation of O−2 ions below the cesium layer. These O−2 ions would let the excited photoelectrons in the conduction band pass easily into the weak nuclear force field of the cesium layer and get into the vacuum. A surface structure model and the energy diagram are suggested.

Negative electron affinity gallium arsenide photocathode grown by molecular beam epitaxy

The molecular-beam-epitaxial technique has been used to grow a high-quality GaAs negative electron affinity photocathode structure with a p-type beryllium doping concentration of 3×1019 cm−3. The reflection mode photoemission characteristics such as the white light sensitivity, the quantum efficiency, the minority carrier (electron) diffusion length, and the electron surface escape probability measured on such a cathode are described. The growth technique followed, offers a precise control over the layer thickness and doping level.

The influence of temperature on the oxidation of cesiated GaAs

We report on investigations of the oxidation process of cesiated (100) GaAs surfaces by means of photoelectron spectroscopy (UPS; hν=21.2 eV). Discrete steps of the reaction are found as a function of temperature and interpreted in terms of molecular adsorption layers at 90 K, cesium suboxides at 170 K, and As–O bonds at 280 K. Cesium suboxides are discussed as essential surface compounds in negative electron affinity (NEA) photocathodes because of their low work function. According to our results the existence of these compounds would be limited to a very narrow range of oxidation.

Comparative studies of oxygen adsorption on GaAs(110) surfaces with ultrathin aluminum and cesium overlayers

The oxidation of GaAs(110) surfaces is found to be profoundly influenced by the deposition of very thin metal overlayers. However, the Fermi energy pinning at the interfaces is only slightly influenced by the oxidation of the metal overlayer and semiconductor. Aluminum and cesium covered surfaces are investigated using soft x rays (SXPS). Oxygen uptake by the Al and Cs overlayers is much more rapid than by the bare GaAs surface. Oxidation of the GaAs through the Al overlayer occurs more slowly than for the bare surface. In strong contrast to the aluminized surface, oxygen was found to bond to surface As atoms on the cesiated surface with 106 times faster uptake compared to the bare surface. These results are important in the context of GaAs passivation, Schottky barriers, negative electron affinity (NEA) photocathodes, and metal oxidation processes.

Negative affinity 3–5 photocathodes: Their physics and technology

Negative electron affinity (NEA) photocathodes are defined by the relationship between the potential barrier at the surface and the bottom of the conduction band in the bulk of the material. If the bottom of the conduction band liesabove the potential barrier at the surface, the device is said to have a negative electron affinity. In practice this condition is obtained by heavyp-doping of the semiconductor (to encourage downward band bending at the surface) and by adding a thin film (several atomic layers) of cesium richcesium oxide on the clean semiconductor surface. The physics, development, fabrication, and applications of the NEA cathode are reviewed.

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.

Photoemission characteristics of transmission-mode negative electron affinity GaAs and (ln,Ga)As vapor-grown structures

Several types of transmission-mode negative electron affinity (NEA) photocathodes were investigated. The first group consisted of GaAs cathodes of various thicknesses grown on a composite structure composed of a GaP substrate and a Ga(As,P) buffer layer. These cathodes were of two types, one having an abrupt Ga(As,P)/GaAs interface and the other having a compositionally graded interface. The latter type exhibited the highest transmission-mode quantum efficiency, 0.11 electron per incident photon at 0.85 μm. It is assumed that the electron diffusion length L in the GaAs layer is limited by misfit dislocations arising from the lattice mismatch between the GaAs and the Ga(As,P) buffer layer. L increased with cathode layer thickness more rapidly for the graded structure, suggesting that misfit dislocation propagation into the GaAs layer is less when the dislocations are generated gradually (graded structure) than when they are introduced abruptly (ungraded structure). The second group of samples consisted of (In, Ga)As alloy cathodes of various compositions grown on both GaAs and GaP substrates with lattice-mismatch-reducing buffer layers of (In, Ga)As, (In, Ga)P, and Ga(As,P). It was found that photosensitivity was improved significantly by reducing the amount of lattice mismatch between the (In, Ga)As cathode layer and the substrate or buffer layer. Using an (In, Ga)As cathode with an (In,Ga)P buffer layer grown on a GaP substrate, transmission quantum efficiencies in excess of 0.01 were obtained over the relatively broad wavelength range of 0.7 to 1.04 µm.