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.     

Charge accumulation layers and surface states in ultrathin Cs,Ba/n-GaN(0001) interfaces

It is found that ultrathin cesium and barium coatings radically change the electronic properties of the surface and the near-surface region of epitaxial n-GaN(0001) layers. A charge accumulation layer serving as a quasi-two-dimensional electronic channel is first formed by adsorption on the surface of a semiconductor. It is revealed that photoemission from the accumulation layer is excited by visible light from the transparency region of GaN and is characterized by a high quantum yield. It is found that the photoemission thresholds hν _s and hν _p for s-and p-polarized excitation are equal to each other and correspond to the work function. The lowest work function for Cs,Ba/n-GaN interfaces is observed at Cs or Ba coverages close to 0.5 monolayer. Two bands induced by the local interaction of cesium (barium) adatoms with gallium dangling bonds are detected in the electronic spectrum of surface states of Cs,Ba/n-GaN interfaces. An oscillation structure is observed in spectral dependences of the photoyield. This effect is new for photoemission. A model of the effect is proposed. It is found that electronic and photoemission properties of the interfaces correlate with the structural perfectness of the epitaxial n-GaN(0001) layers.     

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.     

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.     

Charge accumulation in ultrathin Cs / n - GaN and Cs / n - InGaN interfaces

We report on the observation of new phenomena that arise under Cs adsorption on n-GaN(0001) and n-InGaN(0001) surfaces. First, an extremely highly quantum efficient photoemission has been found by excitation with visible light in the transparency region of GaN and InGaN. The photoemission is revealed to appear due to the formation of an electron accumulation layer in the vicinity of the surfaces. Second, a large variety of band bending and potential wells are provided by the Cs coverages. The accumulated charge density at the n-InGaN surface is much stronger than that at the n-GaN surface. Third, a new effect is revealed, namely, the appearance of an oscillation structure in the spectral dependences of the threshold photoemission. A model concept is proposed for photocurrent oscillations that takes into account the formation of an accumulation layer and the multiple-beam interference in parallel-sided GaN or InGaN samples.     

Electronic structure and adsorption properties of the system Cs/O/W(110)

Comparative studies are carried out of the Cs/O/W(110) and Cs/W(110) adsorption systems. The method of threshold photoemission spectroscopy is used to study the work function and electronic structure in the energy region near the Fermi level as functions of the sub-monolayer cesium coverage. A significant increase of the saturation cesium coverage is observed on the O/W(110) surface. A new adsorption-induced surface band is observed in the electronic spectrum of the system Cs/O/W(110) with a binding energy ∼0.7 eV. For coverages of about one monolayer metallization of the adsorbed layer is observed. It is shown that the electronic structures of the systems Cs/O/W(110) and Cs/W(110) are similar for low coverages. A difference in the adsorption properties for these two systems occurs for coverages close to one monolayer, which is explained by the creation of new interaction centers of the Cs adatoms on the W(110) surface in the presence of oxygen. Fiz. Tverd. Tela (St. Petersburg) 39, 1683–1686 (September 1997)     

Charge accumulation layer on the <Emphasis Type="Italic">n</Emphasis>-GaN (0001) surface with ultrathin Ba coatings

The adsorption of Ba on the n-type GaN(0001) surface is studied. It is found that submonolayer Ba coatings induce cardinal changes in the electronic properties of the surface with the formation of a charge accumulation layer in the region of the near-surface band bending. The excitation of the Ba/n-GaN system by light from the region of GaN transparency results in photoemission. The lowest value of the work function corresponds to ～1.90 eV at a Ba coverage of ～0.4 ML. Two surface bands induced by Ba adsorption are found in the surface photoemission spectra.     

Core level binding energy shifts of K and Cs adlayers on Ru(001)

Using synchrotron radiation from the VUV ring at NSLS and vacuum ultraviolet radiation from a HeI resonance lamp, we have recorded high resolution photoemission spectra of K and Cs overlayers on Ru(001). It is found that for “thin” multilayer coverages ( 3 ML) the K3p and Cs5p core levels exhibit three sets of core levels which can be assigned to interface, “bulkrd and surface emission in increasing binding energy. The results are discussed in terms of the nature of electronic interaction and a thermodynamic model. The K3p core level spin-orbit splitting is also resolved in these measurements for K in the condensed phase, for the first time with photoemission spectroscopy.     

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.     

Excitation of 2D plasmons in a Cs/W(110) system

The evolution of surface photoemission spectra was investigated for a Cs/W(110) system with metastable Cs coatings larger than a monolayer. It is demonstrated that 2D plasmons can be detected by threshold photoemission spectroscopy. Three photoemission peaks were observed, whose dependence on the Cs adsorption dose showed a complicated behavior. The peaks may be due to the photoinduced excitation of a plasmon in quasi-2D Cs clusters, a surface Cs plasmon, or an interface Cs-W plasmon.     

Photoemission studies of ultrathin Cs,Ba/InN interfaces

Cs/InN and Ba/InN interfaces were studied by UV photoelectron spectroscopy in the submonolayer coverage range for the first time. Normal photoemission spectra from the valence band and spectra from In 4d, Ba 5p, Ba 4d, and Cs 5p core levels were investigated in the excitation energy range of 60–800 eV. It was found that metallization of the interface and narrowing of the valence band is observed upon increasing coverage.     

Accumulation nanolayer and surface states of ultrathin Cs,Ba/<Emphasis Type="Italic">n</Emphasis>-GaN interfaces

It is found that Cs and Ba ultrathin coatings induce formation of a charge accumulation layer (2D electron channel) on the n-GaN(0001) surface. Photoemission of conduction electrons from the accumulation layer under excitation by light in the range of GaN transparency is revealed. It is established that the depth of the potential well of the accumulation layer can be deliberately controlled.     

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.     

Interaction of cesium with charged oxide under UV irradiation imaged by photoemission electron microscopy

We report the first observation of electron transfer from charged SiO₂/Si(1 0 0) by ion-implantation via internal photoemission from Si by photoemission electron microscopy (PEEM) for the purpose of the microscopic control of promotion of catalyst by electron transfer from oxide support. The contrast of the PEEM image varies with the amount and kind of the implanted ion and the deposition of Cs through the formation of electrical double layer consisting of Cs⁺ and trapped electrons at trapping centers created by the implantation. It is then firmly established that oxide charging can be microscopically tuned by ion-implantation.     

Growth and electronic properties of ultra-thin Ag films on Ni(1 1 1)

We studied the growth mode and electronic properties of ultra-thin silver films deposited on Ni(1 1 1) surface by means of scanning tunnelling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES). The formation of the 4d-quantum well states (QWS) was analysed within the phase accumulation model (PAM). The electronic structure of the 1 ML film is consistent with the silver layer which very weakly interacts with the supporting surface. The line-shape analysis of Ag-4d_xz,yz QWS spectrum support the notion of strong localization of these states within the silver layer. The asymmetry of the photoemission peaks implies that the decay of the photo-hole appears to be influenced by the dynamics of the electrons in the supporting surface.     

Atomic Rearrangements and Photoemission Processes at a <Emphasis Type="Italic">p</Emphasis>-GaN(Cs)–Vacuum Interface

Spontaneous changes in photoemission properties of a р-GaN(Cs)–vacuum interface with effective negative electronic affinity induced by rearrangements of its atomic structure have been studied for the first time. The optimum Сs coating that ensures both the maximum photoelectron escape probability and its stability has been found. A thermodynamic model has been proposed to escape the relation of the photoemission properties of the р-GaN(Cs)–vacuum interface to its free energy and entropy.     

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.     

Photoemission from p-GaAs(001) with nonequilibrium cesium overlayers

The dependences of the photoemission current and effective electron affinity on the submonolayer cesium coverage at the adsorption of Cs on a GaAs(001) surface, as well as the kinetics of the photocurrent and affinity after the termination of Cs deposition, which is caused by the relaxation of the structure of a nonequilibrium adsorption layer, have been experimentally studied. The revealed features in the dependence of the photocurrent on the Cs coverage are attributed to a nonmonotonic behavior of the surface band bending in the Cs/GaAs(001) system. It has been established that a relaxation decrease in the photocurrent in the case of coverages smaller than half a monolayer is due to the relaxation of the band bending, whereas an increase in the photocurrent at larger coverages is caused by the relaxation of the electron affinity.