Tellurium Films Research Articles

Reactivity towards oxygen of surfaces investigated by ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy and low energy electron diffraction spectroscopy

Very thin films of tellurium, with mean thickness up to 100 A, were deposited in ultra high vacuum at room temperature on a Si(100) 2 × 1 surface, and were exposed to different amounts of O 2 and to the atmosphere. Ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy and low energy electron diffraction spectroscopy were performed in order to investigate the reactivity of the Te Si (100) system towards oxygen. The Si(100) 2 × 1 surface is found to chemisorb O 2 and to saturate with ≈ 500 Langmuir. In contrast, the tellurium films are not active either towards oxygen or towards the atmosphere for a few minutes: no changes are detectable in the UPS and XPS spectra and LEED patterns of samples with the Si(100) surface completely covered by Te. With deposition of 1 A of tellurium the silicon substrate is not completely covered, and the sample can chemisorb a lower amount of oxygen with respect to the pure Si(100) 2 × 1 surface. The tellurium film thus acts as a protective barrier against the oxidation of silicon.

Electron spectroscopy investigation of Te thin films deposited at room temperature on Si(100) 2 × 1

Ultraviolet and X-ray photoelectron spectroscopies, Auger electron spectroscopy and low energy electron diffraction were used in order to investigate the electronic properties and the growth mode of very thin films of tellurium with mean thickness between 0.5 and 1000 Å deposited at room temperature on a Si(100)2 × 1 surface. The adsorbate-substrate interaction is found to be weak. In the initial stage of the growth the tellurium atoms are chemisorbed on the silicon surface and form a continuous monolayer. The Te 5p valence electrons are involved in order to saturate the Si(100) dangling bonds inducing a 1 × 1 reconstruction of the surface. Tellurium deposited after the formation of the first continuous layer and up to 10 Å of average thickness, forms 1 × 1 ordered islands, that is Te deposited at room temperature on Si(100) follows a Stranski-Krastanov growth mode (one layer plus islands). Increasing the amount of Te the islands coalesce and the deposited atoms show a bulk-like behaviour.

Growth of Te thin films deposited at room temperature on the Si(100)2 × 1 surface

The electronic properties and the growth mode of very thin films of tellurium deposited in ultra-high vacuum, at room temperature, on the Si(100)2 × 1 surface have been studied by means of XPS, AES and LEED spectroscopy. Additionally the measured AES weighted peak to peak intensity ratio between Te and Si was compared with calculated layer by layer and Stransky-Krastanov growth mode curves. Our results indicate that the interaction between tellurium and silicon is very weak like the characteristic behaviour of simple metals near Si in the Periodic Table. Tellurium grows on large 1 × 1 domains reconstructed on Si(100)2 × 1 until a layer is completed, at ≈ 2 Å of nominal coverage. Once the substrate is completely covered by the first Te layer, and up to ≈ 10 Å, the deposited tellurium grows, forming islands which have a 1 × 1 order on Si(100), and covers ≈ 40% of the underlying continuous layer. With increasing thickness the islands begin to coalesce and the Te atoms do not follow the lattice order of the Si(100) substrate any more. The behaviour of Te on Si(100) is found to be very similar to that of antimony because of the similar electronic core level structure and atomic radius. However, the higher mobility of Te atoms with respect to Sb atoms allows the formation of a continuous layer of Te on Si(100)2 × 1 also at room temperature, whereas Sb needs higher temperatures. The observed weak absorbate-substrate interaction, together with a strong adsorbate-adsorbate interaction, leads to the saturation of the dangling bonds of the silicon substrate, which shows a bulk-like behaviour, and are responsible for the growth mode of the tellurium deposited on Si(100)2 × 1.

UPS, XPS, AES STUDIES OF Te THIN FILMS DEPOSITED ON Si(100) 2×1

This work represents the first complete study of the interaction between Te and Si in very thin films of tellurium grown at room temperature on Si(100) 2×1 surfaces. In particular, the electronic properties have been investigated by UPS and XPS measurements and information on growth of tellurium on silicon have been obtained by AES measurements. Our results indicate that the interaction between tellurium and silicon is weak, taking the characteristic behavior of simple metals. Increasing the film thickness, the growth mode is one layer, completed with about 2 Å of Te nominal coverage, plus islanding, until the thickness reaches the value of about 10 Å, followed by coalescence of islands. When tellurium has completed a layer, the valence band surface states of silicon are quenched.

Inhomogeneity in semiconductor films: an analysis from optical studies

A method for determining the volume fraction of crystalline phase in a thin film with a mixed phase (amorphous and crystalline) from reflectance measurements is presented here. The theory has been successfully applied to find out the relative amounts of amorphous and crystalline phases present in tellurium films deposited at various substrate temperatures.

Conduction mechanisms in amorphous tellurium films

In situ dc measurements are made on UHV deposited 100 nm thick tellurium films in sandwich configurations between two copper electrodes. Space-charge-limited conduction through amorphous films held at 125 K is believed to be a dominant mechanism, partially perturbed by the effects due to the velocity saturation of carriers and the migration of electrode material into the Te film. The high value of 5.3×1028 eV−1 m−3 for the density of states, N(Ef), in the energy gap is indicative of the existence of degeneracy in the band structure of the film. On the basis of the temperature-conductivity characteristics, three temperature regimes are assumed to exist, each attributed to a characteristic conduction mechanism: (1) variable range hopping over an optimum distance of 0.55 nm at T⩽225 K, (2) hopping between states localised at Te ions 225 K < T < 350 K and (3) band-to-band conduction with an activation energy of 0.32±0.01 eV at temperatures > 350 K. The high temperature activation energy of 0.32 eV compares reasonably well with a value of 0.33 eV for the optical band gap of polycrystalline tellurium films.

Trap distribution and density in ultrahigh-vacuum-deposited tellurium films

At high fields corresponding to the bias voltageV≫ 1 V, space-charge-limited conduction is believed to be a dominant mechanism in 50 nm thick ultrahigh-vacuum- (UHV) deposited polycrystalline tellurium films in a sandwich configuration between two indium electrodes. From Arrhenius plots, it was found that discrete traps were located at 0.007 eV above the valence band of the film.

Improvement of efficiency of ITO/Se thin film solar cells using tellurium stimulator

AbstractIndium Tin Oxide ITO/Se thin film solar cells have been fabricated by vacuum deposition. Prior to deposition of selenium film, ultrathin tellurium film has been deposited on ITO thin film. Tellurium film acts as a stimulator for bringing improvement in crystallite size, adhesion to ITO and electrical behaviour of selenium thin films. The maximum efficiency of the ITO/Se thin film solar cells without tellurium stimulator and with tellurium stimulator under illumination AM1 (100 mW cm−2) are found to be 1.68% and 2.27% while under fluorescent tubelight (0.15 mW cm−2) are found to be 5.85% and 7.05% respectively. The ITO/Se thin film solar cells exhibits high spectral response at short wavelength. The barrier height of the ITO/Se thin film solar cells was determined.

Novel materials for non‐ablative optical recording

Novel materials for optical data storage based on a new recording method are described. Instead of pits or refractive index changes being produced in the recording medium, marks are the result of the generation of near ‐IR absorbing dyes, which leads to a change in the absorption of light from the read laser. The new materials, thin films of tellurium (IV) dihalide bound in a glass together with a sensitizing dye, undergo thermal reductive elimination of halogen on expousure to laser radiation. This technique could ind applications in electrophotographic mastering.

Holograms obtained by laser-assisted oxidation of thin tellurium films

Holograms are recorded by laser-assisted oxidation of thin tellurium films in air. The process of recording is characterized in situ by measuring the evolution of a self-diffracted beam of a cw Ar+ laser. Stationary diffraction efficiency is shown to be independent both on the recording beam power and on the grating period. Amplitude holograms are obtained with an efficiency of 3.2% at a spatial period of 3 μm.

Characterisation of Polycrystalline thin Films of Tellurium

ABSTRACTStructural, electrical and optical characterisations were performed on tellurium film deposited under high vacuum conditions on ultrasonically cleaned Corning glass 7059 substrates. SEM and TEM studies reveal the polycrystalline nature of deposited tellurium films. At steady state high field conditions, space charge limited conduction is believed to have occurred due to the existence of a fast decaying density of trapping states in the energy gap. The DC conductivity is believed to exhibit a T-1/ law dependence below room temperature, giving a value of 2×10-l6eV/m3 for conduction states close to the Fermi level. From the power law dependence on frequency of AC conductivity, the barrier for bipolaron hopping is estimated to 0.33eV.Optical absorption in 9nm thick tellurium samples is due to direct transitions both allowed and forbidden producing two edges, the first one at 4eV and the other at about 0.5eV, respectively.

Optical absorption in thin tellurium films

Optical absorption spectra over the range of photon energy between 1.37 and 4.60 eV are obtained for tellurium films, 9 nm thick, evaporated on Corning glass 7059 at 125 and 300 K, respectively under ultra high vacuum of the order of 10−7 Pa. Spectroscopic data are analysed in terms of the well-known power law and it is found that there exist two absorption peaks in two different photon energy ranges arising from direct transitions in multi-band structured tellurium films. On mesure les spectra d'absorption pour les photons d'energie entre 1.37 eV et 4.60 eV des couches minces (9 nm) de tellure, preparees par vaporisation dans un vide a peu pres 10−7 Pa, sur les echantillons de verre Corning 7059, a 125 et a 300 K. Les resultats sont analyses avec l'emploi d'une loi bien connue et on trouve qu'il y a deux pics d'absorption dans les deux regions d'energies des photons, qui sont associes avec les transitions directes dans les couches minces de tellure qui possedent une structure multibande electronique.

Optical recording characteristics of implanted tellurium films

Tellurium films were vapour deposited on glass substrates held at room temperature and subsequently implanted with different ions. Optical recording parameters such as threshold writing energy and contrast ratio were evaluated. The experimental results indicate that the threshold writing energy is dependent on the type of implanted ion.

Epitaxial growth of tellurium by electrodeposition

Thin films of tellurium with a thickness up to 4 μm have been grown on monocrystalline tellurium substrates by an electro-deposition method. X-ray diffraction showed that the films deposited at 90°C with a current density of 0.5 mA/cm 2 or less were monocrystalline. Films with a blade-like structure were produced as the deposition current density was increased to 1 mA/cm 2.

Maximum lateral dimensions of laser-induced oxidation of thin tellurium films: A negative feedback effect

Thin tellurium films are deposited on glass and irradiated by a continuous Ar+ laser beam. The maximum dimension of the laser-oxidized zone d is measured as a function of the laser beam power P. The temperature T at the end of 180 s of irradiation is measured by an interferometric method. It is found that T is independent of P between 1.3 and 2.4 W. This correlates with the variation of d with P, at constant T, due to negative feedback between optical absorption and oxidation.

Improved Tellurium Films by Partially Ionized Vapor Deposition as the Semiconductor Layer of a TFT and a Hydrogen Sensor

Semiconducting properties of tellurium films prepared on glass substrate by a partially ionized vapor deposition (PIVD) technique were studied and compared with those of Te films formed by a conventional vacuum evaporation technique. It was found that the PIVD technique effectively increased the Hall mobility and decreased the carrier concentration of Te films. These improved films could be utilized as the semiconductor layer of a thin film transistor(TFT) with smaller pinch-off voltage. A Te TFT with 10 nm-thick paladium film as the gate electrode was fabricated and its sensitivity to hydrogen gas was examined.

LiTe and CaTe thin-film junctions as humidity sensors

Relative humidity ( H)-electrical resistance ( R) characteristics of thin-film LiTe and CaTe junction humidity-sensing devices were investigated. These devices have fairly good and stable sensitivity and reproducibility to relative humidities ranging from 30 to 80 %. The electrical resistance of these devices increases as the relative humidity increases, in contrast to the humistor described in many publications. It was found that the CaTe junction device was more sensitive than the LiTe junction device, and was more suitable for measuring lower humidities. It is thought that the electrical resistance of the boundary junction formed between lithium and tellurium films or calcium and tellurium films may be sensitive to humidity.

Electrical studies of tellurium films

In situ DC measurements made on 50nm-thick tellurium films sandwiched between copper electrodes show an existence of ohmic contacts at the metal/semiconductor interface. Conduction in the polycrystalline film at higher fields is believed to be due to space-charge-limited current flow.

Transport properties of tellurium films prepared by hot wall epitaxy

The electron transport properties of tellurium films deposited by hot wall epitaxy onto glass substrates are reported. It has been observed that the electrical resistivity is less than those of films obtained by conventional vacuum evaporation techniques. A negative temperature coefficient of resistivity is exhibited by tellurium films. The Hall coefficient and mobility show a minimum and a maximum, respectively around 300–330 K. The tellurium films are found to be p type.

Raman scattering by tellurium films on CdTe single crystals

Raman scattering by tellurium films on CdTe single crystals