The electronic properties of the Cs/GaAs(100) interface and the formation of metastable Cs clusters

The method of threshold photoemission spectroscopy is used to investigate the electronic properties of the ultrafine gallium-enriched Cs/GaAs(100) interface. The rearrangement of the spectrum of surface photo-emission as a function of Cs coating, as well as the temperature dependence of the spectrum, enable one to identify two phases of adsorption with strong (Cs-Ga) and weak (Cs-Cs) bonds. In the first phase of adsorption with the coating of approximately 0.3 monolayers, two surface bands are detected which are due to the local interaction of cesium adatoms with gallium dimers. It is found that the transition from the first to the second phase of adsorption occurs with the Cs coating of approximately 0.7 monolayers, which corresponds to the saturation of all dangling bonds of gallium on the gallium-enriched GaAs(100) surface. In the second phase of adsorption with the coating of more than 0.7 monolayers, a number of additional photoemission singularities are observed in the spectra, whose emergence is associated with the formation of metastable Cs formations. Photoemission peaks at 1.9 and 2.17 eV may be associated with the excitation of quasi-two-and/or quasi-three-dimensional Cs clusters, and the peaks at 2.05, 2.4, and 2.78 eV may be associated with the excitation of an interface plasmon and of surface and bulk Cs plasmons, respectively.     

Charge accumulation nanolayer: A 2D electronic channel in Cs/n-InGaN ultrathin interfaces

This paper reports on the formation of a 2D electronic channel, i.e., a charge accumulation layer on the n-InGaN(0001) surface observed under adsorption of submonolayer Cs coatings. It is found that photoemission from the accumulation layer is excited by light in the InGaN transparency region. It is established that the potential well depth and the density of electronic states in the accumulation layer can be controlled by properly varying the Cs coverage. It is shown that the accumulation layer exhibits quantum-well effects. The photoemission matrix element is calculated, and the energy parameters of the accumulation layer are obtained. An oscillatory structure originating from the Fabry-Perot interference is revealed in the spectra of photoemission from the accumulation layer.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Evidence of substrate metallization by Li adsorption on the Si(001) surface

We report an evidence of substrate metallization induced by Li adsorption on the Si(001) surface, based on the combined results of electron energy-loss (EEL) and angle-resolved photoemission (ARP) measurements. The metallic surface at a low dose of Li manifests itself as a loss peak due to an intraband surface plasmon in EEL spectra and a metallic peak in ARP spectra. These peaks are coherently understood in terms of substrate metallization, where electrons from Li adatoms partially occupy the empty substrate surface bands. Furthermore, the unique negative dispersion of the plasmon reveals that local field effects may cause such an anomalous dispersion.     

Bulk and surface plasmon excitation in X-ray photoemission

A quantitative theory of bulk and surface plasmon excitation by X-ray photoelectrons in thin metallic films is presented. The shape and strength of inelastic peaks in energy spectra are calculated. Their dependence on the point of primary excitation, energy and direction of the outgoing electron is discussed in the case of X-ray photoemission from aluminium.     

Observation of surface photoeffect for an adsorbate: Oxygen on W(1 0 0) and W(1 1 1)

Two strong peaks are observed, one above and one below the tungsten plasmon energy, hv_p′ in the spectral dependence of the photoemission from one 2p-like orbitals of oxygen chemisorbed on W(1 1 1) and W(1 0 0) surfaces. The peaks are interpreted in terms of a surface photoeffect arising from the dielectric response of the surface region. The occurrence of the resonance above hv_p correlates with oxygen being located below the W surface plane. Local dielectric model calculations qualitatively reproduce the observations.     

Electronic properties of alkali metal/silicon interfaces: A new picture

Adsorption of Na and Cs on the Si(100)2 × 1 surface in the monolayer range is investigated by core level and valence band photoemission spectroscopy using synchrotron radiation. The alkali metals are found to induce an electronic interface state near the Fermi level while hybridization between alkali adsorbate “s” and silicon substrate “3p” valence electrons occurs. These results provide evidence that the alkali metal/silicon bonding is covalent. This covalent bond is weak and polarized while plasmon at the alkali metal core level indicates adsorbate rather than substrate metallization.     

Core-level photoemission spectra from Cs monolayers on Si(1 1 1)2 × 1

Band shape in the Cs 4d core level of cesium atom monolayers adsorbed on Si(1 1 1)2 × 1 has been studied by photoemission spectroscopy using synchrotron radiation. Asymmetry appeared on the low kinetic energy side at high coverages of cesium atoms. This is caused by the energy loss due to the overlayer plasmon in the cesium atom monolayer of photoemitted Cs 4d electrons.     

Variation of the electronic and adsorption properties of GaAs(100) in the transition from an As-to Ga-rich surface

This paper reports on a threshold photoemission study of the variation of electronic properties occurring as Cs is adsorbed on GaAs(100) and the surface transfers gradually from the As-to Ga-rich state. The ionization energy and integrated photoemission current are studied as functions of the Cs coverage. The minimum of ionization energy and the corresponding Cs dose are established to differ substantially for the As-and Ga-rich GaAs(100) surfaces. The first observation is reported of anomalous curves with two ionization-energy minima, which are characteristics of surfaces in an intermediate state, with Ga and As dimers present. The sticking coefficient of Cs to the surface enriched in As is found to be several times smaller than that for the Ga-rich surface.     

Surface states and accumulation nanolayer induced by Ba and Cs adsorption on the n-GaN(0 0 0 1) surface

The Ba and Cs adsorption on the n-GaN(0 0 0 1) surface has been studied in situ by the threshold photoemission spectroscopy using s- and p-polarized light excitation. Two surface bands induced by Ba (Cs) adsorption are revealed in surface photoemission spectra below the Fermi level. The surface-Fermi level position is found to be changed from significantly below the conduction band minimum (CBM) at clean n-GaN surface to high above the CBM at Ba, Cs/n-GaN interfaces, with the transition from depletion to electron accumulation occurring at low coverages. Photoemission from the accumulation nanolayer is found to excite by visible light in the transparency region of GaN. Appearance of an oscillation structure in threshold photoemission spectra of the Ba, Cs/n-GaN interfaces with existing the accumulation layer is found to originate from Fabry–Perot interference in the transparency region of GaN.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Charge accumulation layer in Cs,Ba/<Emphasis Type="Italic">n</Emphasis>-GaN(0001) ultrathin interfaces: Electronic and photoemission properties

Experimental studies and theoretical calculations of the photoemission from Cs/n-GaN(0001) and Ba/n-GaN(0001) ultrathin interfaces were carried out. The electronic properties of the interfaces were studied in situ using threshold photoemission spectroscopy under vacuum at a residual pressure of P ～ 5 × 10^?11 Torr. A new effect was revealed, namely, photoemission with a high quantum yield under excitation with light in the transparency region of GaN. It was shown that adsorption of Cs or Ba on n-GaN brings about the formation of a quasi-two-dimensional electron channel, i.e., a charge accumulation layer directly near the surface. The dependences of the photoemission spectra and work function on the thickness of Cs and Ba coatings were investigated. It was established that adsorption of Cs and Ba leads to a sharp decrease in the work function by ～1.45 and ～1.95 eV, respectively. The photoemission spectra were calculated, and parameters of the accumulation layer, such as the energy position of the layer below the Fermi level for different Cs and Ba coverages, were determined. It was demonstrated that the energy parameters of the accumulation layer on the n-GaN(0001) surface can be controlled by properly varying the Cs or Ba coverage. The layer thickness was found to reach a maximum for a cesium coverage of ～0.5 monolayer.     

Oscillations in the threshold photoemission spectra of GaN(0001) with submonolayer Cs coverages

It is found that Cs adsorption on the n-type GaN(0001) surface generates an unusual change in the electronic properties of the surface and the near-surface space-charge layer, which leads to the appearance of photoelectron emission upon excitation in the transparent region of GaN. It is established that the photoemission is due to the formation of quasimetallic states induced by Cs adsorption in the band-bending region near the surface. The behavior of the photoemission threshold upon excitation by s-polarized light is studied as a function of the Cs coverage. It is found that the minimum value of the threshold corresponds to ～1.4 eV at a concentration of Cs atoms of ～4.5×10¹⁴ atom/cm² in the submonolayer coverage. A new effect is revealed, namely, the appearance of oscillations in the spectral curves of threshold photoemission. A model is proposed for photocurrent oscillations that takes into account the formation of quasimetallic states in the near-surface layer of GaN band bending and the occurrence of interference in the GaN slab upon light irradiation in the transparent region.     

Characteristic energy loss structure of solids from x-ray photoemission spectra

Energy loss peaks in x-ray photoemission spectra of nine metals are presented. No strong evidence for intrinsic plasmon structure was observed. Spectra from the free electron-like metals, Al, Li, and Na under ultra-high vacuum (10^?11 torr) conditions show intense bulk plasmons and surface plasmons. Systematic variations in the characteristic energy losses are reported for the series Ag to Te. In addition to losses that may be attributed to plasma oscillations of the 5s5p bands, with N = 1 and 2 electrons, respectively, both Ag and Cd show additional high-energy losses that may arise through 4d-shell participation, with N = 11 and 12.     

Crossover between monopole and multipole plasmon of Cs monolayers on Si(111) individually resolved in energy and momentum

The evolution of the plasmon spectrum of the Si(111) (7 x 7)-Cs surface has been studied by energy loss spectroscopy individually resolved in energy and momentum during the transition from substrate to Cs overlayer metallization. The multipole plasmon is identified by an extremely narrow angular distribution of the inelastic electron scattering, unaccounted for by standard dipole scattering theory. A crossover between multipole and monopole surface plasmon is observed at finite surface wave vectors , depending on Cs coverage, and reveals a high sensitivity of the short-wavelength multipole components on surface morphology.     

Plasmon excitation in core-level photoemission

The surface and bulk plasmon satellites in photoemission from a core level are calculated, treating the photoelectron quantum mechanically and including plasmon dispersion. The long wavelength plasmon excitations are suppressed by interference between intrinsic and extrinsic processes, giving featureless satellites at low electron kinetic energy. Structure develops as the electron kinetic energy increases.     

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 $\text{μA}\text{lumen}$. 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.     

NEA光电阴极的(Cs,O)激活工艺研究

在自行研制的负电子亲和势光电阴极性能评估实验系统上,首次利用动态光谱响应技术和变角X射线光电子能谱(XPS)表面分析技术研究了GaAs光电阴极的(Cs,O)激活工艺.获得了首次导Cs、(Cs,O)导入以及(Cs,O)循环的优化激活条件.XPS分析给出GaAs(Cs,O)的最佳激活层厚度为0.82 nm,首次导Cs达到峰值光电发射时的Cs覆盖率为0.71个单层.在优化激活条件下,可以在国产反射式GaAs上获得1025 μA/lm的积分灵敏度.     

NEA GaN光电阴极表面模型研究

针对目前NEAGaN光电阴极研究中Cs激活或Cs/O激活后表面状态的形成过程还不清楚的问题,围绕NEAGaN光电阴极的光电发射机理,结合GaN光电阴极激活过程中出现的现象及成功激活的最终效果,给出了GaN光电阴极铯氧激活后的表面模型[GaN(Mg):Cs]:O-Cs。利用该模型可很好地解释单独用Cs激活时约-1.0eV的有效电子亲和势和Cs/O共同激活时-1.2eV的有效电子亲和势的成因,也较好地解释了表面吸附原子的组合形式,即Cs/O激活后激活层的化学结构由Cs2O2和CsO2构成。