Lattice controlled transport in quantum wires at low temperatures

Theory of acoustic scattering rate of the carriers in a quantum wire has been developed under the condition of low temperature when the approximations of the traditional theory are hardly valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics in a narrow channel GaAs–GaAlAs quantum wire. On comparison with other available results it is revealed that the finite energy of the acoustic phonons and the complete phonon distribution without any truncation lead to significantly different transport characteristics at low temperatures.     

Electron and phonon mechanisms of friction in crystalline solids in atomically close contact at low temperatures

Recent experimental and theoretical studies have shown that an atomically close contact between two crystalline solids can be free from friction of rest in the event that their lattice constants are incommensurate and the interaction at their interface does not exceed a certain threshold value. In this case, the sole mechanisms of friction are the phonon emission and the excitation of conduction electrons. Theoretical estimations of both phonon and electron contributions to the frictional force have been carried out (the latter contribution has been considered both in the normal and superconducting states of metal).     

Magnetization process of single molecule magnets at low temperatures

We show that correlations established before quenching to very low temperatures later drive the magnetization process of systems of single molecule magnets, after a magnetic field is applied at t=0. We also show that in simple cubic lattices m proportional, variant sqrt[t], as observed in Fe8, but only for 1+2log((10)(h(d)/h(w)) time decades, where h(d) is a nearest neighbor dipolar magnetic field and a spin reversal can occur only if the field on it is within (-h(w),h(w)). However, the sqrt[t] behavior is not universal. For bcc and fcc lattices, m proportional, variant t(p), but p approximately 0.7. The value to which m finally levels off is also given.     

Structure of silicon oxide on Si(001) grown at low temperatures

Structure of amorphous oxide thin-films grown on Si(001) wafers under an atmosphere of O₃, and also under an atmosphere of afterglow of microwave plasma of O₂ was investigated by X-ray diffraction. Crystallites were found in both oxidation samples as well as in the usual O₂ oxidation [I. Takahashi et al., J. Phys: Condensed Matter 5 (1993) 6525]. Peak intensity and profile in X-ray diffraction responsible for existence and distribution of the crystallites in the oxide film, depend on the oxidation conditions. Least-squares fitting of the data indicates that crystallite growth is less pronounced in O₃ oxidation compared with O₂ oxidation, and the crystallites are located at the interface only in the afterglow of microwave plasma oxidation. Parameters describing the interface morphology indicate that the interface produced by either oxidation procedure is not as smooth as conventional O₂ oxidized samples.     

Photoluminescence of Si/Ge nanostructures grown by molecular-beam epitaxy at low temperatures

Multilayer Si/Ge nanostructures grown by molecular-beam epitaxy at low temperatures (250–300°C) of germanium deposition are studied using photoluminescence and atomic-force microscopy (AFM). It is assumed that, upon low-temperature epitaxy, the wetting layer is formed through the intergrowth of two-dimensional (2D) and three-dimensional (3D) nanoislands.     

Optical characteristics of light-emitting diodes with high transparent conductive film at low temperatures

In this paper, we report the synthesis and transmittance of a titanium–indium–tin oxide (TITO) film, fabricated through a low-temperature process. The TITO film was fabricated by incorporating a 2-nm-thick titanium barrier at the bottom of an ITO film. The transmittance characteristics of the TITO film were examined for light-emitting diodes (LEDs) of various wavelengths at different post-annealing temperatures. A saturated high transmittance was observed at a temperature of 550 °C, which is relatively low when compared to that in the case of a conventional ITO film. Photoluminescence studies demonstrated that a 450-nm-thick TITO film, fabricated at 550 °C, was highly effective in improving the performance of the LED, when compared to conventional ITO films. The X-ray diffraction peaks, scanning electron microscopy images, and transmittance electron microscopy images confirmed that titanium atoms could improve the crystallization of ITO. It was found that non-crystallization in ITO was effectively activated by the titanium barrier. Furthermore, the optical bandgap (3.77 eV for the conventional ITO film) was improved to 3.92 eV in the TITO film. An infrared LED fabricated with a TITO film displayed 70% higher light output power than that with a conventional ITO film. These results suggest that using a titanium barrier is essential to effectively improve inactive nucleation sites in ITO films grown at low temperatures.     

Growth of TiO2 coating on wood surface using controlled hydrothermal method at low temperatures

This paper examines the growth of anatase TiO₂ coating on a wood surface through the hydrolysis of tetrabutyl orthotitanate (TBOT) in different conditions, using a controlled hydrothermal method at low temperatures. Energy disperse X-ray analysis and Fourier transform infrared spectroscopy analysis confirm that the growth of TiO₂ coating on a wood surface is bonded to hydrocarbon chains. Several reaction factors that influence the morphologies and amount of TiO₂ present on wood surface were also investigated. As observed from the scanning electron microscopy images, the morphology and content of TiO₂ grown on a wood surface could be controlled under appropriate reaction conditions. Approximately 32.6% TiO₂ content on a wood surface could be obtained when specific conditions are applied.     

Amorphous SiC film formation on Si(100) using electron beam excitation

The effect of electron impact on methylsilane (CH₃SiH₃) conversion to amorphous-Si_0.5C_0.5:H (a-Si_0.5C_0.5:H) films on Si(100) has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). It is found that electron impact greatly enhances CH₃SiH₃ decomposition on Si(100) at both 90 K and 300 K, resulting in a-Si_0.5C_0.5:H thin film formation. Thermal annealing of the film causes hydrogen desorption and amorphous silicon carbide (a-SiC) formation. Upon annealing to temperatures above 1200 K, the a-SiC film became covered by a thin silicon layer as indicated by AES studies. Ordered structures are not produced by annealing the a-SiC up to 1300 K.     

Study on SiN and SiCN film production using PE-ALD process with high-density multi-ICP source at low temperature

SiN and SiCN film production using plasma-enhanced atomic layer deposition (PE-ALD) is investigated in this study. A developed high-power and high-density multiple inductively coupled plasma (multi-ICP) source is used for a low temperature PE-ALD process. High plasma density and good uniformity are obtained by high power N₂ plasma discharge. Silicon nitride films are deposited on a 300-mm wafer using the PE-ALD method at low temperature. To analyze the quality of the SiN and SiCN films, the wet etch rate, refractive index, and growth rate of the thin films are measured. Experiments are performed by changing the applied power and the process temperature (300–500 °C).     

Conductivity of ultrathin Pb films during growth on Si(111) at low temperatures

The dipole modes of non-parabolic quantum dots are studied by means of their current and density patterns as well as with their local absorption distribution. The anticrossing of the so-called Bernstein modes originates from the coupling with electron-hole excitations of the two Landau bands which are occupied at the corresponding magnetic fields. Non-quadratic terms in the potential cause an energy separation between bulk and edge current modes in the anticrossing region. On a local scale the fragmented peaks absorb energy in complementary spatial regions which evolve with the magnetic field. Received 3 December 2001 / Received in final form 5 April 2002 Published online 9 July 2002     

Enhancement of the electron mobility with infrared photoexcitation in Si: Te at low temperatures

Hall measurements on Te-doped silicon (N _Te 10¹⁶ cm^–3) have been performed in the temperature range between 10 K and 300 K with infrared photoexcitation of electrons into the conduction band. The samples exhibit electron Hall mobilities which are increased by approximately 50% compared to measurements in the dark. The increased electron mobility can be correlated with an increased electron population of shallow donor levels by photoexcitation. Coulomb scattering due to charged shallow donor centers is converted into less efficient dipole scattering (Te-acceptor pairs) by the light-induced redistribution of electrons.     

Infrared absorption in a high density electron-hole plasma in germanium at low temperatures

Measurements of the absorption in the temperature range 1.8–9 K have been made for wavelengths between 3.1 and 4.2μm, in germanium highly excited by a 1.06μm laser. The shape of the absorption curve is noted to be similar to that observed by Pokrovskii and Svistunova at lower excitation levels, and somewhat broader than that calculated by Smith, Chen and McGill for exciton absorption. A cross section of 4.5 (±1.1) × 10^-17 cm² at 3.39μm is deduced from normalization with the theoretical curves of Smith et al.     

Observation of non-Gaussian conductance fluctuations at low temperatures in Si:P(b) at the metal-insulator transition

We report investigations of conductance fluctuations (with 1/f(alpha) power spectra) in doped silicon at low temperatures (T<20 K) as it is tuned through the metal-insulator transition (MIT) by changing the carrier concentration n. The scaled magnitude of noise, gamma(H), increases with decreasing T following an approximate power law gamma(H) approximately T-beta. At low T, gamma(H) diverges as n decreases through the critical concentration n(c), accompanied by a growth of low-frequency spectral weight. The second spectrum and probability density of the fluctuations show strong non-Gaussian behavior below 20 K as n/n(c) decreases through 1. This is interpreted as the onset of a glassy freezing of the electronic system across the MIT.     

Current transport in Pd2Si/n-Si(100) Schottky barrier diodes at low temperatures

The forward current-voltage (I–V) characteristics of Pd₂Si/n-Si(100) Schottky barrier diodes are shown to follow the Thermionic Emission-Diffusion (TED) mechanism in the temperature range of 52-295 K. The evaluation of the experimentalI–V data reveals a decrease of the zero-bias barrier height ( _b0) and an increase of the ideality factor () with decreasing temperature. Further, the changes in _b0 and become quite significant below 148 K. It is demonstrated that the findings cannot be explained on the basis of tunneling, generation-recombination and/or image force lowering. Also, the concepts of flat band barrier height and T ₀-effect fail to account for the temperature dependence of the barrier parameters. The 1n(I _s /T ²) vs 1/T plot exhibits nonlinearity below 185 K with the linear portion corresponding to an activat ion energy of 0.64 eV, a value smaller than the zero-bias barrier height energy (0.735 eV) of Pd₂Si/n-Si Schottky diodes. Similarly, the value of the effective Richardson constant A** turns out to be 1.17 × 10⁴ A m^–2 K^–2 against the theoretical value of 1.12 × 10⁶ A m^–2 K^–2. Finally, it is demonstrated that the observed trends result due to barrier height inhomogeneities prevailing at the interface which, in turn, cause extra current such that theI–V characteristics continue to remain consistent with the TED process even at low temperatures. The inhomogeneities are believed to have a Gaussian distribution with a mean barrier height of 0.80 V and a standard deviation of 0.05 V at zero-bias. Also, the effect of bias is shown to homogenize barrier heights at a slightly higher mean value.     

Phosphorus distribution profiles in 100-silicon using a spin-on source at low temperatures

 Incorporation of phosphorus into silicon from a spin-on dopant layer (SOD) at 400 ^°C is described. Annealing experiments were carried out with SOD films deposited on (100) silicon substrates by using the spin-on technique. Conventional heating on a hotplate in normal atmosphere and a temperature range up to 400 ^°C was used to study the dopant incorporation. After removing the SOD-films one part of the silicon substrates was annealed at higher temperatures. Investigations were carried out by SIMS, SAM, XPS, HTEM, stripping Hall and Van der Pauw measurements before and after the high temperature annealing. Chemical phosphorus concentration profiles obtained from low temperature annealed samples showed diffusion depths of 60–80 nm (extrapolated to a substrate doping level of 10¹⁶ cm^-3) and surface concentrations of 10¹⁹–10²⁰ cm^-3. Electron concentration profiles exhibiting maximum values around 2⋅10¹⁹ cm^-3 could be measured on high temperature annealed samples only. Received: 28 March 1996/Accepted: 19 August 1996     

An apparatus for the measurement of thermal expansion of solids at low temperatures

A dilatometer, using the three terminal capacitance technique, suitable for measurement of linear thermal expansion of solids in the temperature range 1.3–300 K is described. The dialtometer is designed such that the mounting system for the specimen does not undergo any significant changes in dimensions when the specimen is heated. The apparatus, therefore, yields in principle absolute values of α, the coefficient of linear thermal expansion. The performance of the apparatus has been checked by measurements on copper in the temperature range of 77–300 K. Some preliminary results on the behaviour of α for Y₁Ba₂Cu₃O_6.9 compound in the vicinity of superconducting transition temperature,T _c are also described. The system can detect relative changes in length Δl/l ₀ of about 10⁻⁸. Attempts are being made to improve the sensitivity.     

An experimental study of the mechanical behavior of frozen arteries at low temperatures

An experimental study of the mechanical response of frozen arteries to tensile stresses at low temperatures is presented. The Dynamic Mechanical Analyzer was used to perform the mechanical experiments. It was found that the frozen artery shows a kind of elastic-plasticity when the temperature is between -20 C and -40 C. And with the decrease of the temperature, the plasticity deformation decreases. Thus at the temperature of -120 C no plasticity deformation is observed before the artery's fracture and the tissue shows quite perfect elastic brittleness, both peripherally and axially. These kinds of mechanical characteristics help explain the fracture phenomena occurring during cryopreservation of the arteries. The mechanical properties, including elastic modulus and fracture strength, are also given. It is known that Cryoprotectant (CPA) used in cryopreservation is necessary in maintaining the tissue's biological functions. Our investigation of its effect on the artery's mechanical properties found that the existence of CPA can soften the tissue at low temperatures, thus may decrease the possibility of fractures during the cryopreservation.