Properties of evaporated titanium thin films and their possible application in single electron devices

The influence of the deposition rate of titanium thin films on their microscopic structure has been investigated by transmission electron microscopy, X-ray diffraction analyses and atomic force microscopy. Furthermore, the resistivity of the films has been characterized by van der Pauw measurements in a temperature range of 5-300 K. Titanium films with a thickness of 10 nm evaporated with a rate of 1 Å/s showed the typical and well known temperature dependency of the resistivity (i.e. decreasing resistivity with falling temperature). In contrast, the resistivity of thin films deposited with a rate of 0.2 Å/s rises with decreasing temperature. Additionally, the resistivity of the films evaporated with the lower rate rises significantly when reducing their film thickness down to 3 nm. Due to this increase of the resistivity we are able to present an alternative and relatively simple approach for the fabrication of single electron transistors (SETs). The presented SET design is based on the evaporated thin titanium films with a deposition rate of 0.2 Å/s onto well defined trenches previously etched into a dielectric layer of thermally grown silicon dioxide. The tunnel junctions originate from a local increase of the resistance of the metallic wire at the edges of the trenches. The devices fabricated in this manner with lateral dimensions in the 50-100 nm range show clear SET features at an operating temperature of up to 77 K. Additionally, the influence of background charges on the Coulomb oscillations in this devices are demonstrated and discussed in comparison with simulated data.     

Slipping-Free Halide Perovskite Supercrystals from Supramolecularly-Assembled Nanocrystals

Supramolecularly assembled high-order supercrystals (SCs) help control the dielectric, electronic, and excitonic properties of semiconductor nanocrystals (NCs) and quantum dots (QDs). Ligand-engineered perovskite NCs (PNCs) assemble into SCs showing shorter excitonic lifetimes than strongly dielectric PNC films showing long photoluminescence (PL) lifetimes and long-range carrier diffusion. Monodentate to bidentate ligand exchange on ≈ 8 nm halide perovskite (APbX₃; A:Cs/MA, X:Br/I) PNCs generates mechanically stable SCs with close-packed lattices, overlapping electronic wave functions, and higher dielectric constant, providing distinct excitonic properties from single PNCs or PNC films. From Fast Fourier Transform (FFT) images, time-resolved PL, and small-angle X-ray scattering, structurally and excitonically ordered large SCs are identified. An Sc shows a smaller spectral shift (<35 meV) than a PNC film (>100 meV), a microcrystal (>100 meV), or a bulk crystal (>100 meV). Also, the exciton lifetime (<10 ns) of an SC is excitation power-independent in the single exciton regime 〈N〉<1, comparable to an isolated PNC. Therefore, bidentate-ligand-assisted SCs help overcome delayed exciton or carrier recombination in halide perovskite nanocrystal assemblies or films.     

Cathodoluminescence of TiO2 nanotubes prepared by low-temperature anodization of Ti foils

We show that anodization of Ti sheets in an ethylene glycol and HF containing electrolyte at temperatures under 0 °C results in the formation of a self-arranged ordered porous structure at the top surface of the sample. This perforated surface structure initiates the growth of an ordered array of titania nanotubes. The inner diameter of nanotubes can be modified in a controlled fashion in the range from 10 nm to more than 250 nm through the change of the electrolyte temperature from −20 °C to + 50 °C. The spectral distribution of cathodoluminescence from a cluster of nanotubes clearly demonstrates the formation of resonator modes which are separated from each other by around 200 meV.     

Doping effects on the optical properties of evaporated a-Si:H films

Thin films of a-Si:H are deposited on substrates at 300°C by a conventional thermal evaporation technique. The electrical conductivity of these films is modified by the addition of antimony giving n-type films. The optical properties of the films are investigated using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 200–3000 nm. Both the refractive index n and the absorption coefficient increase when the Sb content is increased. The absorption edge shifts to lower energies for doped films. The optical gap E_g is evaluated using three different plots for comparison, namely; (hν)^1/2, (/hν)^1/2 and (hν)^1/3. The value of E_g decreases with doping for the three expressions. The Urbach parameter E₀ is calculated and found to increase with doping from 74 meV for the undoped film to 183 meV for concentrations of 9.4 at.% Sb.     

Influence of thermal annealing on photoluminescence and structural properties of N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) organic thin films

Photoluminescence (PL) spectra and intensities of thin N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) films have been measured at room temperature, during sample heating with various rates and annealing times at constant temperatures, and after annealing. It was found that the temperature of T = 80 °C being considerably lower than the glass transition temperature of α-NPD is sufficient to cause substantial irreversible changes in PL and PL excitation characteristics. A PL efficiency increase up to 10 times, an emission spectrum short-wavelength shift up to 130 meV and a spectral narrowing from 69 to 39 nm are reached using annealing. The surface roughness of the films annealed at the moderate temperature of 80 °C does not undergo observable changes contrary to films annealed at higher temperatures.     

Electron field emission properties of electro-deposited diamond-like carbon coatings

Field emission studies were carried out on diamond-like carbon films deposited by an electro-deposition technique onto SnO₂-coated glass substrates. A mixture of acetic acid and water was the electrolyte. The films are compact with surface roughness 10 nm. Work function (φ) values obtained from the Fowler–Nordheim model varied between 15 and 214 meV, while the field factor (β) varied between 4 and 700. The critical field was found to vary between 2 and 28 V/μm.     

Photo-induced changes of the short wavelength absorption edge in some Ge–As–S amorphous thin films

Amorphous films Ge₂₇As₁₃S₆₀ (FII) and Ge₁₄As₂₇S₅₉ (FIII) were prepared by thermal evaporation. Thermal and photo-induced bleaching were observed in the virgin films. Reversible photo-darkening was studied induced by various light sources (white light, monochromatic light with wavelengths 417 nm, 442 nm and 493 nm, respectively) in thermally well relaxed films. The role of actual conditions of illumination on the magnitude of photo-darkening (the incident photon flux and the penetration depth of absorbed photons) is examined. Considerable photo-darkening (dE_g) observed in the studied films (dE_g ≈ − 200 meV for Ge₁₄As₂₇S₅₉ film) is tentatively attributed to possible disorder associated with nano-scale phase separation. Actually it is attributed to the photo-enhanced interaction between excited lone-pair orbitals of atoms on the cluster surfaces and/or to covalent bonds reconstruction/interaction of the cluster surfaces where existence of stressed bonds, which are supposed to be more susceptible to photo-induced excitation, breaking and reconstruction, is expected.     

Decay of photo-excited conductivity of Er-doped SnO<Subscript>2</Subscript> thin films

Er-doped SnO₂ thin films, obtained by sol-gel-dip-coating technique, were submitted to excitation with the 4th harmonic of a Nd:YAG laser (266 nm), at low temperature, and a conductivity decay is observed when the illumination is removed. This decay is modeled by considering a thermally activated cross section of an Er-related trapping center. Besides, grain boundary scattering is considered as dominant for electronic mobility. X-ray diffraction data show a characteristic profile of nanoscopic crystallite material (grain average size ≈5 nm) in agreement with this model. Temperature dependent and concentration dependent decays are measured and the capture barrier is evaluated from the model, yielding 100 meV for SnO₂:0.1% Er and 148 meV for SnO₂:4% Er.     

Annealing effects in indium oxide films prepared by reactive evaporation

The annealing effects of reactively evaporated indium oxide films in various ambients at temperatures of up to approximately 350°C are reported. It is concluded that the changes in the electrical parameters of the films are due to the chemisorption or desorption of oxygen atoms from the grain boundaries. From these studies the grain boundary barrier heights are found to be 26 meV and 43 meV for as-deposited and air-annealed films respectively.     

Anisotropic behaviors in polycrystalline Cd-doped GaSe thin films

GaSe thin films were deposited by thermal evaporation technique with Cd doping. X-ray diffraction analysis showed that Cd-doped films have polycrystalline structure with the preferred orientation along (008) direction. Temperature dependent electrical conductivity measurements were carried out in the temperature range of 100–400 K along perpendicular and parallel directions to the growth direction for the films exhibiting p-type conduction determined by hot probe technique. The room temperature conductivity values of the films were found to be as 1.5 × 10⁻⁸ and 4.9 × 10⁻¹² (Ω cm)⁻¹ due to the measurements along both perpendicular and parallel directions, respectively. The difference in the conductivity values is the indication of electrical anisotropy in the samples. Carrier conduction in the films was provided by the thermionic emission in the high temperature region (310–400 K) with almost the same activation energies in both directions. Space charge limited current analysis at different temperatures reveals the existence of two discrete sets of trap levels for both perpendicular and parallel directions. Calculated trap levels and trap concentrations are 99 meV, 3.5 × 10¹² cm⁻³ and 418 meV, 2.2 × 10⁵ cm⁻³ for perpendicular direction, 58 meV, 2.1 × 10¹⁸ cm⁻³ and 486 meV, 1.4 × 10¹² cm⁻³ for parallel direction. The differences in the values of the trap levels and concentrations for both directions confirm the existence of electrical anisotropy in Cd-doped GaSe thin films, because of the structural anisotropy between and inside the crystallites.     

Structural and electrical characterization of TiO2 films grown by spray pyrolysis

The sol-gel spray pyrolysis method was used to grow TiO₂ thin films onto silicon wafers at substrate temperatures between 315 and 500 °C using pulsed spray solution feed followed by annealing in the temperature interval from 500 to 800 °C in air. According to FTIR, XRD, and Raman, the anatase/rutile phase transformation temperature was found to depend on the film deposition temperature. Film thickness and refractive index were determined by Ellipsometry, giving the refractive indexes of 2.1-2.3 and 2.2-2.6 for anatase and rutile, respectively. According to AFM, film roughness increases with annealing temperature from 700 to 800 °C from 0.60 to 1.10 nm and from 0.35 to 0.70 nm for the films deposited at 375 and 435 °C, respectively. The effective dielectric constant values were in the range of 36 to 46 for anatase and 53 to 70 for rutile at 10 kHz. The conductivity activation energy for TiO₂ films with anatase and rutile structure was found to be 100 and 60 meV, respectively.     

Optical properties of HgTe colloidal quantum dots

Room temperature photodetection with HgTe colloidal quantum films is reported between 2 and 5 μm for particles of sizes between ~5 and ~12 nm diameter, and photodetection extends to 7 μm at 80 K. The size-tuning of the absorption of HgTe colloidal quantum dots, their optical cross section and the infrared absorption depth of films are measured. The tuning with radius is empirically given by [see formula in text] where R is in nm. The optical cross section of the colloidal dots at 415 nm is approximately proportional to their volume and given by σ(Hg)(415) = 2.6 ± 0.4 10(-17) cm(2)/mercury atom. The size-dependent optical cross section at the band edge ~1.5 10(-15) cm(2) is consistent with the expected oscillator strength of the quantum dots. The absorption depth of HgTe colloidal dot films is short, about 1-2 μm, which is an advantage for thin film devices. These properties agree rather well with the expectation from the k · p model. HgTe colloidal quantum dot thin films show a strong tuning with temperature with a large positive thermal shift between 0.4 and 0.2 meV K(-1), decreasing with decreasing size within the size range studied and this is attributed primarily to electron-phonon effects.     

Properties of nanocrystalline zinc oxide thin films prepared by thermal decomposition of electrodeposited zinc peroxide

Zinc peroxide thin films were electrodeposited from aqueous solution at room temperature using H₂O₂ as the oxidation agent. Nanocrystalline zinc oxide thin films were then obtained from thermal decomposition of zinc peroxide thin films. The grain sizes of ZnO through thermal decomposition of ZnO₂ at 200 °C, 300 °C and 400 °C were estimated from the peak width of ZnO(110) obtained from X-ray diffraction and were 6.3 nm, 9.1 nm and 12.9 nm, respectively. The optical properties of zinc oxide thin films have been studied. The photoluminescence results indicate that ZnO thin films have low Stokes blue shift (about 110 meV) and low oxygen vacancies.     

纳米金属颗粒－绝缘体膜制备的研究

本文介绍了一台利用磁控溅射产生金属团簇，同时蒸发绝缘介质得到金属颗粒－绝缘体包埋团簇的设备，并利用该设备成功地制备出了Ｃｕ：ＣａＦ２和Ｔｉ：ＣａＦ２等金属颗粒膜。该设备适应性广，可产生几乎所有固体金属和半导体团簇。从而可以得到众多组合的功能膜。包埋在绝缘体中的团簇大小在１０ｎｍ－７０ｎｍ之间，为多晶结构；基质ＣａＦ２也为多晶     

Microstructure and magnetic properties of Ti/CoCrPt/Ti pseudo-sandwich nanogranular films

We prepared Ti/CoCrPt/Ti pseudo-sandwich granular films by radio-frequency and dc magnetron sputtering onto glass substrates and subsequent in situ annealing. We investigated the microstructure and magnetic properties of the films as a function of Ti overlayer thickness (x). X-ray diffraction profiles show that the CoCrPt magnetic layers are formed as the hexagonal close-packed (HCP) structure. Vibrating sample magnetometer measurements indicate that the out-of-plane coercivity reaches the maximum 1675.5 Oe when x=5 nm. Atomic force microscopy images show the minimum average grain size D=7.2 nm and the average roughness R/sub a/=1.0 nm. Magnetic force microscopy images show that the minimum average magnetic cluster size is about 6.4 nm at x=5 nm.     

Tin oxide nanocluster hydrogen and ammonia sensors

We have prepared sensitive hydrogen and ammonia sensors from thin films of tin nanoclusters with diameters between 3 and 10?nm. By baking the samples at 200?°C in ambient air the clusters were oxidized, resulting in very stable films of tin oxide clusters with similar diameters to the original Sn clusters. By monitoring the electrical resistance, it is shown that the cluster films are highly responsive to hydrogen and ammonia at relatively low temperatures, thereby making them attractive for commercial applications in which low power consumption is required. Doping of the films by depositing Pd on top of the clusters resulted in much improved sensor response and response times. It is shown that optimal sensor properties are achieved for very thin cluster films (a few monolayers of clusters).     

Enhanced control over selective-area intermixing of In0.15Ga0.85As/GaAs quantum dots through post-growth exposure to radio-frequency plasma

Post-growth treatment with a low pressure, CF₄-plasma is demonstrated to reliably inhibit the interdiffusion of In and Ga atoms in In_0.15Ga_0.85As/GaAs self-assembled quantum dot wafer structures subjected to rapid thermal annealing temperatures between 700 °C and 800 °C for a duration of 20 s. Comparative studies of the effects of rapid thermal annealing were made on plasma treated samples and samples that were capped with either 200 nm of plasma enhanced chemical vapor deposited SiO₂ or 220 nm of thermally deposited TiO₂ prior to plasma exposure, as well as to uncapped, untreated control samples. Room-temperature photoluminescence spectra were acquired using a Ti-Sapphire laser operating at 742 nm as the excitation source. A bandgap differential of 84 meV (94 nm) was measured across a wafer sample annealed at 775 °C, when contrasting sections that were uncapped and treated with the CF₄-plasma versus sections that were annealed without any treatment to the surface. This was comparable to a sample that was capped with the TiO₂ film, which produced a 73.5 meV (82 nm) variance from the raw, annealed-only sample.     

Lateral features of Cu(In0.7Ga0.3)Se2-heterodiodes in the μm-scale by confocal luminescence and focused light beam induced currents

Polycrystalline Cu(In,Ga)Se₂ films (CIGSe) show substantial local variations of properties not only in regime of 10-100 nm but also in the scale length of few microns. We have analyzed optoelectronic properties of CIGSe heterodiodes by confocal luminescence and focused light beam induced currents (LBIC) versus temperature and excitation level with < 1 μm lateral resolution and we observe a strong dependence of the size of local patterns on excitation flux and a considerable dependence of the yield and the spectral shape of luminescence and of microscopic LBI currents on temperature. From experiments we derive activation energies for rates of non-radiative recombination of (2-7) meV and for minority carrier mobilities of about (60-70) meV. These energies are compared with local variations of band edges resulting from potential fluctuations which are formulated after an approach from literature and which has been fitted to experimental shifts of PL peaks and squeezing of PL spectra versus excitation flux. We estimate tunnel barriers for radiative transitions of trapped electrons of about (30-70) meV. Correlating our different results we attribute the activation energy for minority transport in CIGSe reflecting local variations of the conduction band edge mainly to spatial fluctuations of the optical band gap as a consequence of spatially varying elemental composition, and to variations of splitting of the quasi-Fermi levels introduced by spatially varying defect densities.     

Temperature and excitation power dependencies of the photoluminescence of planar and vertically self-organized Si0.70Ge0.30/Si strained superlattices

We have studied the correlation between the morphological characteristics and the photoluminescence (PL) behavior of nominally 20-nm Si/7-nm Si_0.70Ge_0.30 superlattices (SLs) showing planar SiGe layers (#1) and vertical self-aligned undulations (#2). At 8 K and low PL excitation density, no-phonon (NP) peaks were centered at 919 and 934 meV, for SLs #1 and #2, respectively. Increasing excitation power resulted in a high-energy side broadening and a strong blue-shift of sample #2 peaks, which were interpreted by in-plane localization and spreading of the hole wave function within the thickness undulations. Up to intermediate temperature values (120 K), carrier diffusion and localization at the crest (11 nm) of the undulations enhanced PL efficiency. Room temperature SiGe PL was observed for both samples with an intensity exponential decay above 150 K. Activation energies of 260 meV (#1) and 172 meV (#2) were derived from Arrhenius plots. The energy difference can be explained by the lateral channel formed by the troughs (2 nm) in the long scale (120 nm) waviness of sample #2 that assists hole thermoionic emission in the Si barriers.     

Algorithm for the determination of intrinsic optical constants of metal films: application to aluminum

Optical and electron-energy-loss data for evaporated-aluminum films have been critically analyzed and used in an iterative, self-consistent algorithm that represents a combination of the Kramers-Kronig analysis and the semiquantum-model application. The novel values of the intrinsic optical functions of aluminum have been determined in a wide spectral range from 200 μm (6.2 meV) to 0.12 nm (10 keV). These functions are in accordance with recent calculations by Lee and Chang [Phys. Rev. B 49, 2362 (1994)], with dc conductivity measurements, and are in good agreement with both peak positions and line widths obtained from electron-energy-loss experiments. The results are examined for internal consistency by inertial and f-sum rules.