Self-organization of nanostructures on the n-GaN(0001) surface in the Cs and Ba adsorption

A regular self-organized 2D nanostructure of a new type of nanocombs has been created in situ in an ultrahigh vacuum on the n-GaN(0001) surface. The nanostructure is formed as a result of multilayer adsorptions of Cs and Ba. The structure is highly regular in the microrange, arranged in the form of combs 60–70 nm in diameter with a wall height of about 7 nm. It has been revealed that the nanostructure has a quasi-metallic conduction, a low work function of about 1.4 eV, and a high quantum yield of photoemission under light excitation in the GaN transparency region. A self-organization model is proposed, which implies the formation of a surface 2D longperiod incommensurate phase interacting with the superstructure of the Cs⁺ and Ba²⁺ ion clusters with allowance for the polaron compensation on the GaN surface.     

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.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

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.     

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.     

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.     

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.     

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.     

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.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Electron states at Zn(0001) surface

The electronic structure of the Zn(0001) surface is analyzed through a realistic slab calculation with an accurate tight-binding sp-d model hamiltonian. The results are compared with a recent photoemission investigation of this surface. The comparison with noble and near-noble metal surfaces, with reference to the surface states in the s-p gap at the centre of the surface Brillouin zone, is also discussed.     

Surface states of Sc(0001) and Ti(0001)

Calculated electronic properties are compared for 11-layer Sc(0001) and Ti(0001) films. Prominent surface states are found whose locations conform to expectations based on the respective bulk band structures establishing a roughly rigid band picture of the surface bands. Surface core-level shift and work function results are qualitatively explained.     

Silicon interaction with the (0001) surface of La and Gd layers

A study is reported on a system consisting of a Si layer on the surface of rare-earth metals (REMs), which is the reverse of a rare-earth metal on silicon, the system of current widespread interest. Interaction of silicon with the (0001) surface of trivalent La and Gd single-crystal layers grown on a W(110) surface is studied by Auger spectroscopy combined with layer-by-layer argon-ion etching of the system and photoelectron spectroscopy. It is found that silicon interacts with the La(0001) and Gd(0001) surfaces even at room temperature with the formation of silicide, but no mutual mixing of the silicon and substrate atoms occurs. When the Si/La(0001) and Si/Gd(0001) systems are heated at 400°C, silicon does not diffuse into the bulk of the metal substrate or to the REM/W(110) interface.     

Interfacial potential approach for Ag/ZnO （0001） interfaces

Systematic approaches are presented to extract the interfacial potentials from the ab initio adhesive energy of the interface system by using the Chen–M ¨obius inversion method. We focus on the interface structure of the metal（111）/Zn O（0001）in this work. The interfacial potentials of Ag–Zn and Ag–O are obtained. These potentials can be used to solve some problems about Ag/Zn O interfacial structure. Three metastable interfacial structures are investigated in order to check these potentials. Using the interfacial potentials we study the procedure of interface fracture in the Ag/Zn O（0001） interface and discuss the change of the energy, stress, and atomic structures in tensile process. The result indicates that the exact misfit dislocation reduces the total energy and softens the fracture process. Meanwhile, the formation and mobility of the vacancy near the interface are observed.     

Atomic reconstructions and electronic states on the GaAs(001) surface with the adsorbed Sb and Cs layers

In experiments on the adsorption and thermal desorption of Cs on GaAs(001) surfaces with various atomic reconstructions and compositions including those enriched in the cation (gallium) and in the anions (arsenic and antimony), the correlation in the behavior of the atomic structure and the surface electronic states, which determine the band bending, has been established. The cesium adsorption on the anion-rich surfaces results in both the similar disordering of the atomic structure and in the close dose dependences of the band bending, while the adsorption on the Ga-rich surface is ordered and results in the qualitatively different dose dependence, which has several maxima and minima. In the Cs desorption and the subsequent adsorption-desorption cycles, the stabilizing effect of Sb on the atomic structure and the electronic states of the Cs/Sb/GaAs(001) surface has been revealed.     

Band structure of Au layers on the Ru(0001) and graphene/Ru(0001) surfaces

The electronic structures of Au monolayers on the Ru(0001) and graphene-coated Ru(0001) surfaces have been calculated by DFT method using the supercell (repeated-slab) approach. The local densities of states (LDOS) and band structures of the monolayer and bilayer Au films adsorbed on the graphene/Ru(0001) and those of free hexagonal Au layers are found to be very similar. This result indicates that the monolayer graphene almost completely screens the Au layers from the Ru(0001) substrate surface, so that electronic properties of Au films adsorbed on graphene are determined predominantly by the electronic structure of the Au adlayers, essentially independent on the electronic structure of the substrate surface.