Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level E_F and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C₆₀) and fullerene (C₆₀) films on the surface of the real germanium oxide ((GeO₂)Ge) have been presented. The content of the oxide on the (GeO₂)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C₆₀ and C₆₀ coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C₆₀ and APP-C₆₀ films has made it possible to reveal the energy range (6–10 eV above the Fermi level E_F) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C₆₀ and unsubstituted C₆₀ films. The formation of the interfacial potential barrier upon deposition of APP-C₆₀ and C₆₀ on the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface E_vac–E_F by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C₆₀ and C₆₀, the work function of the surface changes only slightly.     

The photovoltaic effect in poly(alkylthiophene) films on a silicon substrate

A study of photovoltage was made for a series of sandwich structures on the basis of poly(3-dode-cylthiophene) films having characteristic thicknesses 100 and 500 nm and being deposited on n-Si and p-Si substrates from a solution. Semitransparent Al and Au electrodes were obtained on the surfaces of these films by thermal evaporation. A clear photoresponse was obtained in films on an n-Si substrate. Two distinct spectral components of the photovoltage were observed in the 1.3-to 3.6-eV (900–300 nm) energy range for incident quanta. The first component corresponds to the absorption edge of the Si substrate (1.4–1.6 eV). The other corresponds to the π-π* absorption of the polythiophene films (1.7–2.1 eV). The dependences of the photovoltage upon radiation intensity are different for these two spectral components. The relaxation time of the photoresponse for the second component, corresponding to the absorption in the film, is 10–20 min. This is 3–4 orders of magnitude higher than the relaxation time for the first component. A model of the potential barrier at the polythiophene/n-Si interface, allowing one to explain the main experimental results, is proposed. This barrier is formed as a result of the chemical interaction of the polythiophene molecules with the substrate.     

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E_F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO₂)Ge). The concentration of oxide on the (GeO₂)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO₂)Ge surface to 6 nm was studied. At energies below 9 eV above E_F, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface, E_vac–E_F, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.     

Investigation of the electronic properties of Au nanoclusters in SiO<Subscript>2</Subscript> by combined scanning tunneling/atomic force microscopy

The tunneling of electrons through Au nanoc lusters formed by pulsed laser deposition in a SiO₂ thin film on a Si substrate has been investigated by combined scanning/atomic force microscopy (STM/AFM). Conducting Pt-coated Si cantilevers were used. The feedback was maintained via the AFM channel, and the current-voltage (I-V) characteristics of the tunnel contact between the AFM probe and the n ⁺-Si substrate through a =4-nm-thick SiO₂ film with Au nanoclusters =2 nm in diameter were measured simultaneously. The current image of the structure contained areas of increased current (tunnel-current channels) 2–15 nm in size, related to tunneling of electrons through Au nanoclusters in SiO₂. The I-V characteristics recorded in the tunnel-current channels exhibit specific features related to the Coulomb blockade of electron tunneling through Au nanoclusters.     

Au/(SiO2/Si/SiO2)纳米双势垒/n+-Si结构的电致发光研究

利用射频磁控溅射方法,在n⁺-Si衬底上淀积SiO₂/Si/SiO_{2纳米双势垒单势阱结构,其中Si层厚度为2至4nm,间隔为0.2nm,邻近n⁺-S i衬底的SiO2层厚度固定为1.5nm,另一SiO2层厚度固定为3nm.为了 对比研究,还制备了Si层厚度为零的结构,即SiO2(4.5nm)/n⁺-Si 结构.在经过600℃氮气下退火30min,正面蒸上半透明Au膜,背面也蒸Au作欧姆接触后,所 有样品都在反向偏置(n^＋-Si的电压高于Au电极的电压)下发光,而在正向偏压 下不发光.在一定的反向偏置下,电流和电致发光强度都随Si层厚度的增加而同步振荡,位 相相同.所有样品的电致发光谱都可分解为相对高度不等的中心位于2.26eV(550nm)和1.85eV (670nm)两个高斯型发光峰.分析指出该结构电致发光的机制是：反向偏压下的强电场使Au/( SiO2/Si/SiO2)纳米双势垒/n⁺-Si结构发生了雪崩击穿 ,产生大量的电子-空穴对,它们在纳米SiO2层中的发光中心(缺陷或杂质)上复 合而发光. 关键词： 电致发光 纳米双势垒 高斯型发光峰 雪崩击穿}     

The effects of deposition temperature and ambient on the physical and electrical performance of DC-sputtered n-ZnO/p-Si heterojunction

The effects of deposition conditions on the physical and electrical performance of the n-ZnO/p-Si heterojunction were systematically investigated. ZnO films were deposited on the Si and glass substrates using direct current (DC) magnetron sputtering with various ambients and substrate temperatures. The results showed that increasing the O₂ content and substrate temperature during the deposition process could improve the crystallinity and stoichiometry of the ZnO film, resulting in a lower carrier concentration and higher resistivity. The electrical properties of the n-ZnO/p-Si heterojunctions were also affected by the deposition parameters. For the junctions fabricated in the pure Ar ambient, the sample deposited at room temperature (RT) showed Ohmic behavior, while the one deposited at 300?°C exhibited poor rectifying behavior. On the other hand, the junctions fabricated in the O₂/Ar ambient possessed ideal rectifying behaviors. The different carrier transport mechanisms for the heterojunctions under forward and reverse bias were systematically studied using a high temperature current–voltage (I-V) measurement. The recombination-tunneling current showed temperature insensitive performance while the space-charge limited current (SCLC) changed with the measurement temperature.     

Structure, morphology and optical properties of SiO2−x thin films prepared by plasma-assisted pulsed laser deposition

The amorphous silicon oxide SiO_2−x thin films were prepared by the plasma-assisted pulsed laser deposition (PLD) method. X-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-VIS-NIR scanning spectrophotometry and ellipsometry were used to characterize the crystallinity, microscopic morphology and optical properties of obtained thin films. The influences of substrate temperatures, oxygen partial pressures and oxygen plasma assistance on the compositions of silicon oxide (SiO_2−x) thin films were investigated. Results show that the deposited thin films are amorphous and have high surface quality. Stoichiometric silicon dioxide (SiO₂) thin film can be obtained at elevated temperature of 200 °C in an oxygen plasma-assisted atmosphere. Using normal incidence transmittance, a novel and simple method has been proposed to evaluate the value of x in transparent SiO_2−x thin films on a non-absorbing flat substrate.     

Effects of oxygen plasma pre-treatments on the characteristics of n-ZnO/p-Si heterojunction diodes

We examine the effects of the oxygen plasma pre-treatments on the material properties of n-ZnO grown on p-Si and characterize the electrical properties of n-ZnO/p-Si heterojunction diodes. The lattice spacing of ZnO becomes larger when the ZnO thin film is grown on the oxygen plasma pre-treated Si substrate. This might be relevant to the growth of (101) ZnO onto the ultra-thin SiO₂ interfacial layer, which is formed during the oxygen plasma pre-treatment onto the Si substrate. The formation of SiO₂ gives rise to the increase in the donor-like defect Zn interstitial, and the increased grain size improves the carrier mobility. Because of all the above, the differential conductance at the on-state is increased for the n-ZnO/p-Si heterojunction diode.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C₈) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level E_F. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C₈ and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level E_F, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level E_F for the PTCDI-C₈ and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C₈ and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, E_vac–E_F, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.     

Study of resonant tunneling in Au nanoclusters on the surface of SiO2/Si thin films using the combined scanning tunneling microscopy and atomic-force microscopy technique

The morphology and electronic properties of Au nanoclusters on the surface of SiO₂ thin films on n ⁺-Si substrates are studied using the combined scanning tunneling microscopy (STM) and atomic-force microscopy (AFM) technique. The peaks associated with the resonant tunneling of electrons from the states of the valence band of the probe material to the states of the conduction band of the substrate material through Au nanoclusters are observed on the current-voltage characteristics for the contact of a p ⁺-Si AFM probe with Au nanoclusters. Experimental results are interpreted by calculating the tunnel transparency of the SiO₂/Au/SiO₂ double barrier structure in a strong electric field.     

The electrical, optical properties of AlSb polycrystalline thin films deposited by magnetron co-sputtering without annealing

In order to fabricate AlSb polycrystalline thin films without post annealing, this paper studies a technology of magnetron co-sputtering onto intentionally heated substrate. It compares the structural characteristics and electrical properties of AlSb films which are deposited at different substrate temperatures. It finds that the films prepared at a substrate temperature of 450 oC exhibit an enhanced grain growth with an average grain size of 21 nm and the lattice constant is 0.61562 nm that goes well with unstained lattice constant (0.61355 nm). The ln(σ_dark) ～1/T curves show that the conductivity activation energy is about 0.38 eV when the film is deposited at 450 oC without an annealing. The transmittance and reflectance spectra show that the film deposited at 450 oC has an optical band gap of 1.6 eV. These results indicate that we have prepared AlSb polycrystalline films which do not need a post annealing.     

Study of well adherent DLC film deposited on piezoelectric LiTaO3 substrate

A well adherent diamond-like carbon (DLC) film was deposited on piezoelectric LiTaO₃ substrate using PECVD by inserting SiO₂ interlayer. DLC film was characterized using Raman spectroscopy and AFM. Physical and mechanical properties were measured using XRR, ellipsometry, scratch test and nano-indentation. The DLC film exhibits the characteristics of hydrogenated amorphous carbon and a very smooth surface with a 0.25 nm RMS. Scratch test shows that critical load (L_c) is 18 N, which is good enough for applying DLC film to SAW device. The measured mass density, refractive index, hardness and Young’s modulus of DLC film deposited on LiTaO₃ are comparable to the reported values for hydrogenated amorphous carbon film, irrespective of substrates on which the films were deposited.     

Morphology, composition and thermal stability of thin SiO2/HfO2 layers grown on silicon by electron-beam evaporation in vacuum

The results of integrated studies of thin-film structures based on silicon and hafnium dioxides on silicon grown by electron-beam evaporation in vacuum are presented. The surface morphology, structural and phase composition of these films depending on the annealing temperature within 500–1100°C are studied. Special consideration is given to the change in the state of the interfaces after annealing. It is determined that annealing in a flow of nitrogen with the addition of oxygen (～10 vol %) at 700°C does not lead to structural and phase changes in the films, but the intensity of the electron paramagnetic resonance (EPR) spectra of uncompensated bonds on the HfO₂-Si interface decreased. Annealing at higher temperatures stimulates crystallization of the HfO₂ films and hafnium silicate is formed on the SiO₂-HfO₂ interface and suboxide SiO _x appears on the HfO₂-Si interface.     

Room temperature deposition of amorphous SiC thin films using low energy ion bombardment

Silicon carbide (SiC) films are prepared by single- and dual-ion beam sputtering deposition at room temperature, respectively. An assisting argon ion beam (ion energy Ei=150 eV) bombards directly the substrate surface to modify the SiC film surface. The thin films are characterized by the Fourier transform infrared spectroscopy (FTIR) and the Raman spectra. With assisting ion beam bombardment, the density of the Si–C bond in the film increases. Meanwhile, the excess carbon or the size of the sp² bonded clusters and the amorphous Si (a-Si) phase decrease. These results indicate that the composition of the films is mainly Si–C bond. UV-vis transmission shows that the E_opt increases steadily from 1.85 eV for the amorphous SiC (a-SiC) films without bombardment to about 2.29 eV for those with assisting ion beam bombardment.     

Unoccupied electronic states and the interface formation between oligo(phenylene-vinylene) films and a Ge(111) surface

Thin films of tri-oligo(phenylene-vinylene) end-terminated by di-butyl-thiole (tOPV) were thermally deposited in UHV on Ge(111) substrates. The surface potential and the structure of unoccupied electron states (DOUS) located 5–20 eV above the Fermi level (E _F) were monitored during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The electronic work function of the surface changed during the film deposition until it reached a stable value of 4.3±0.1 eV at a tOPV film thickness of 8–10 nm. Deposition of the tOPV under 3 nm led to the formation of intermediate DOUS structures that were replaced by another DOUS structure along with an increase in the tOPV deposit thickness up to 8–10 nm. The occurrence of the intermediate DOUS structure is indicative of a substantial reconfiguration of the electronic structure of the tOPV molecules due to the interaction with the Ge(111) surface. Analysis of the TCS data allowed us to assign the unoccupied electronic bands in tOPV located at 5.5–6.5 and 7.5–9.5 eV above the E _F as π* bands and at 11–14 and 16–19 eV above E _F as σ* bands.     

Photoemission investigations on nanostructured TiO2 grown by cluster assembling

Nanostructured titanium dioxide (ns-TiO₂) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO₂ nanocrystals embedded in an amorphous TiO₂ phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO₂ films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.     

The relationship between refractive index-energy gap and the film thickness effect on the characteristic parameters of CdSe thin films

CdSe thin films were deposited on glass substrates using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature and ambient pressure. The relationship between refractive index and energy bandgap was investigated. The film thickness effect on the structural, morphological, optical and electrical properties of CdSe thin films was investigated. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that all the films exhibit polycrystalline nature with hexagonal structure and are covered well with glass substrates. The crystalline and surface properties of the films improved with increasing film thickness. The optical absorption studies revealed that the films are found to be a direct allowed transition. The energy bandgap values were changed from 1.93 to 1.87 eV depending on the film thickness. The electron effective mass (m_e^?/m_o), refractive index (n), optical static and high frequency dielectric constant (ε_o, ε_∞) values were calculated by using the energy bandgap values as a function of the film thickness. The resistivity of the films changed between 10⁶ and 10² Ω-cm with increasing film thickness at room temperature.     

Polycrystalline GaSb films prepared by the coevaporation technique

Gallium antimonide (GaSb) films were deposited onto fused silica and n-Si (100) substrates by coevaporating Ga and Sb from appropriate evaporation sources. The films were polycrystalline in nature. The size and the shape of the grains varied with the change in the substrate temperature during deposition. The average surface roughness of the films was estimated to be 10 nm. Grain boundary trap states varied between 2×10¹² and 2.2×10¹² cm^?2 while barrier height at the grain boundaries varied between 0.09 eV and 0.10 eV for films deposited at higher temperatures. Stress in the films decreased for films deposited at higher temperatures. XPS studies indicated two strong peaks located at ～543 eV and ～1121 eV for Sb 3d_3/2 and Ga 2p_3/2 core-level spectra, respectively. The PL spectra measured at 300 K was dominated by a strong peak located ～0.55 eV followed by two low intensity peaks ～0.63 eV and 0.67 eV. A typical n-Si/GaSb photovoltaic cell fabricated here indicated V _oc～311 mV and J～29.45 mA/cm², the density of donors (N _d)～3.87×10¹⁵ cm^?3, built in potential (V _bi)～0.48 V and carrier life time (τ)～28.5 ms. Impedance spectroscopy measurements indicated a dielectric relaxation time ～100 μs.     

Laser synthesis of nanostructures based on transition metal oxides

Nanostructures based on iron oxides in the form of thin films were synthesized while laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors (Fe(CO)₅) under the action of Ar⁺ laser radiation (λ_L = 488 nm) on the Si substrate surface with power density about 10² W/cm² and vapor pressure 666 Pa. Analysis of surface morphology and relief of the deposited films was carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM). This analysis demonstrated their cluster structure with average size no more than 100 nm. It was found out that the thicker the deposited film, the larger sizes of clusters with more oxides of higher oxidized phases were formed. The film thickness (d) was 10 and 28 nm. The deposited films exhibited semiconductor properties in the range 170-340 K which were stipulated by oxide content with different oxidized phases. The width of the band gap E_g depends on oxide content in the deposited film and was varied in the range 0.30-0.64 eV at an electrical field of 1.6 × 10³ V/m. The band gap E_g was varied in the range 0.46-0.58 eV at an electrical field of 45 V/m. The band gap which is stipulated by impurities in iron oxides E_i was varied in the range 0.009-0.026 eV at an electrical field of 1.6 × 10³ V/m and was varied in the range 0-0.16 eV at an electrical field 45 V/m. These narrow band gap semiconductor thin films displayed of the quantum dimensional effect.