Preroughening transitions in a model for Si and Ge (001) type crystal surfaces

The uniaxial structure of Si and Ge (001) facets leads to nontrivial topological properties of steps and hence to interesting equilibrium phase transitions. The disordered flat phase and the preroughening transition can be stabilized without the need for step-step interactions. A model describing this is studied numerically by transfer matrix type finite-size-scaling of interface free energies. Its phase diagram contains a flat, rough, and disordered flat phase, separated by roughening and preroughening transition lines. Our estimate for the location of the multicritical point where the preroughening line merges with the roughening line, predicts that Si and Ge (001) undergo preroughening induced simultaneous deconstruction transitions.Comment: 13 pages, RevTex, 7 Postscript Figures, submitted to J. Phys.

The Ti/c-Si solid state reaction : III. The low-temperature reaction kinetics

Thin Ti layers (≈10nm) are grown on top of a clean Si(111) substrate. Heating these layers initiates a solid state reaction, yielding a monosilicide phase at ≈350°C and a C49 disilicide at ≈450°C. The present study concerns the growth kinetics of both phases by means of ellipsometry. A diffusion-limited growth kinetics is found for the monosilicide formation. However, two growth rates are observed, a fast initial one and a slow terminal growth rate. An enhanced Si diffusion in atomically disordered regions as compared to well ordered regions (grains or clusters) could be an explanation. From the measurements we have found a value of 2×10-15 cm2/s for the diffusion coefficient at ≈370°C and an activation energy of 0.62 ± 0.1 eV. Both values correspond to the fast process. Subsequently increasing the temperature to ≈450°C permits the growth of the homogeneous C49 TiSi2 phase. For this process, both planar layer growth and intermixing are observed, however, quantitative results could not be derived from the present study

Systematic and random errors in rotating-analyzer, ellipsometry

Errors and error sources occurring in rotating-analyzer ellipsometry are discussed. From general considerations it is shown that a rotating-analyzer ellipsometer is inaccurate if applied at P = 0° and in cases when π = 0° or where Δ is near 0° or 180°. Window errors, component imperfections, azimuth errors and all other errors may, to first order, be treated independently and can subsequently be added. Explicit first-order expressions for the errors δΔ and δπ caused by windows, component imperfections, and azimuth errors are derived, showing that all of them, except the window errors, are eliminated in a two-zone measurement. Higher-order errors that are due to azimuth errors are studied numerically, revealing that they are in general less than 0.1°. Statistical errors are also discussed. Errors caused by noise and by correlated perturbations, i.e., periodic fluctuations of the light source, are also considered. Such periodic perturbations do cause random errors, especially when they have frequencies near 2ωA and 4ωA

A novel derivative ellipsometric method for the study of the growth of thin films: Titanium

The growth of a titanium film at room temperature from an evaporation source on a silicon substrate covered by its native oxide layer is continuously monitored with an ellipsometer at three wavelengths. The momentary positions and the derivatives of the trajectories thus obtained in the (Δ, Ψ) plane can be used for uniquely determining the momentary thickness and the momentary dielectric constants of the layer at each of the wavelengths. The optical properties of the titanium, which reflect the film structure and defect rate, strongly depend upon the growth conditions; the top region of a film approximately 40 nm thick appears to contain more voids and lattice defects than the region near the substrate

The Ti/c-Si solid state reaction : I. An ellipsometrical study

This paper is the first of a series of three articles in which we present the results and analyses of an extended study of the c-Si/Ti solid state reaction. In this paper we will discuss the spectroscopic ellipsometric investigation. Thin (≈10nm) Ti films are grown on clean Si(111) surfaces and are subsequently heated. The Si indiffusion and the Si-Ti intermixing are continuously registered by three-wavelengths ellipsometry. Two metastable intermediate phases are observed to form before the final state is obtained Spectroscopic ellipsometry (E = 2−4.5 eV) is used to characterize the as-deposited layer, the metastable intermediate phase and the final state. Analysis of these spectra shows that: (1) Si and Ti intermix during the initial Ti deposition, (2) a fast reordering of the Ti atoms occurs when the system is slightly heated (≈175°C), (3) a metastable, probably monosilicide phase with a large Si concentration gradient is obtained at ≈350°C, (4) a homogeneous metastable TiSi2 forms at ≈450°C, at ≈700°C a roughened TiSi2 layer with a surplus of c-Si is formed

The kinetics of titanium monosilicide growth studied by three-wavelength ellipsometry

Thin titanium layers (approximately 10 nm) have been grown on top of a clean Si(111) substrate. Heating these layers initiates a solid state reaction, yielding an amorphous monosilicide phase at about 350 °C. The kinetics of the solid state reaction has been followed using three-wavelength ellipsometry (340, 450 and 550 nm). A very coarse two-layer model has been applied in the analyses of the measured data: a top layer of pure titanium is consumed by a second layer of TiSi. The dielectric constants of titanium and TiSi are known and the layer thicknesses d1 and d2 have been fitted to the six ellipsometrical angles measured. These analyses reveal a diffusion-limited growth mechanism exhibiting two growth rates: a rapid initial rate followed by a slower final rate. The diffusion coefficient D of the rapid process and its activation energy Ea could be obtained: D = 2 × 10−15cm2s−1atT 370 °CandEa = 0.62 eV The two growth rates have been attributed to silicon diffusion along the grains and diffusion into the grains.\ud \u

Reduced pressure pain thresholds in response to exercise in chronic fatigue syndrome but not in chronic low back pain: an experimental study

Objective The aims of this study were to examine (i) base line pressure pain thresholds in patients with chronic fatigue syndrome and those with chronic low back pain compared with healthy subjects, (ii) the change in mean pain threshold in response to exercise, and (iii) associations with exercise induced increase in nitric oxide Participants Twenty six patients with chronic fatigue syndrome suffering of chronic pain, 21 patients with chronic low back pain and 31 healthy subjects Methods Participants underwent a submaximal aerobic exercise protocol on a bicycle ergometer, preceded and followed by venous blood sampling (nitric oxide) and algometry (hand arm calf low back) Results Patients with chronic fatigue syndrome presented overall lower pain thresholds compared with healthy sub jects and patients with chronic low back pain (p<0 05) No significant differences were found between healthy subjects and patients with chronic low back pain After submaximal aerobic exercise, mean pain thresholds decreased in patients with chronic fatigue syndrome and increased in the others (p<0 01) At baseline nitric oxide levels were significantly higher in the chronic low back pain group After controlling for body mass index no significant differences were seen be tween the groups at baseline or in response to exercise Nitric oxide was not related to pain thresholds in either group Conclusion The results suggest hyperalgesia and abnormal central pain processing during submaximal aerobic exercise in chronic fatigue syndrome, but not in chronic low back pain Nitric oxide appeared to be unrelated to pain processin

Ramsey-type microwave spectroscopy on CO (a3Πa^3\Pi)

Using a Ramsey-type setup, the lambda-doublet transition in the $J=1,\, \Omega=1$ level of the $a^3\Pi$ state of CO was measured to be 394 064 870(10) Hz. In our molecular beam apparatus, a beam of metastable CO is prepared in a single quantum level by expanding CO into vacuum and exciting the molecules using a narrow-band UV laser system. After passing two microwave zones that are separated by 50 cm, the molecules are state-selectively deflected and detected 1 meter downstream on a position sensitive detector. In order to keep the molecules in a single $m_J^B$ level, a magnetic bias field is applied. We find the field-free transition frequency by taking the average of the $m_J^B = +1 \rightarrow m_J^B = +1$ and $m_J^B = -1 \rightarrow m_J^B = -1$ transitions, which have an almost equal but opposite Zeeman shift. The accuracy of this proof-of-principle experiment is a factor of 100 more accurate than the previous best value obtained for this transition