Solid State Phenomena Vols. 205-206

Abstract: The effect of ultrasonic treatment (UST) on the defect structure of the Si–SiO₂ system is characterised by means of electron spin resonance (ESR), metallography, MOS capacitance measurements and secondary ion mass spectroscopy (SIMS). A non-monotonous dependence of the defect densities on the ultrasonic wave intensity has been observed. The influence of the UST frequency on the ESR signal intensity of the defect centres depended on the defect’s type and structure and may be caused by vibrational energy dissipation which is a function of the defect centre’s type. The influence of the UST on the Si–SiO₂ interface properties depends on the oxide thickness and crystallographic orientation. The density of point defects and absorbed impurities at the Si–SiO₂ interface can be reduced and its electrical parameters improved by an appropriate choice of UST and oxidation conditions.

Abstract: This paper investigates the influence of different number of laser pulses on contact behavior and conductivity of the surface layer of femtosecond laser microstructured, sulfur-doped silicon. Single shot laser processed silicon (Pink Silicon) is characterized by low surface roughness, whereas five shot laser processed silicon (Grey Silicon) has an elevated sulfur content with a surface roughness low enough to maintain good contacting. To laterally confine the laser induced pn-junction part of the Grey Silicon sample surface is etched off. The etching depth is confirmed to be sufficient to completely remove the active n-type sulfur layer. While Pink Silicon shows little or no lateral conductivity within the laser processed layer, Grey Silicon offers acceptable conductivity, just as expected by the fact of having incorporated a higher sulfur dopant content. Recombination dominates the irradiated regions of Pink Silicon and suppresses excess charge carrier collection. Grey Silicon, while showing sufficient lateral conductivity, still shows regions of lower conductivity, most likely dominated by the laser irradiation-induced formation of dislocations. According to our results, the optimum laser pulse number for electrical and structural properties is expected to be in the range between one and five laser pulses.

Abstract: Atomic-scale mechanisms of thermal activated self-diffusion on crystal surfaces are investigated through AFM images. Surface evolution is studied by means of the Power Spectral Density (PSD) function over a large spatial bandwidth. We propose a parametric model based on the Mullins-Herring (MH) diffusion equation by adding two stochastic terms. Then, surface evolution during high temperature annealing in reducing ambient can be predicted. Very good agreement between experimental and theoretical roughness and diffusion parameters was observed. Origin and evolution of the stochastic terms, describing conservative and non-conservative noises, are discussed.

Abstract: The orientation dependence of porous Si formation rate has been studied by local anodization of p-Si wafers with a boron concentration N_B = 3·10¹⁵ - 2·10¹⁹ cm^-3 at various currents with the help of a wagon wheel technique. It is demonstrated that the etch rate diagram is drastically transformed as the dopant concentration in p-Si is changed. The highest etch rate is observed in the <100> direction for heavily doped p⁺⁺-Si, whereas for p⁺-Si with N_B < 2·10¹⁸ cm^-3, <111> axis becomes the fastest direction. Further decrease in the doping level makes the anisotropy weaker, and at N_B <2·10¹⁶cm^-3, the anodization rate becomes nearly independent of the crystallographic orientation. For all the impurity concentrations in the substrate, the anisotropy is the strongest in the case of anodization at low currents and gradually decreases with increasing current density.

Abstract: Queue time (QT) between consecutive process-steps is one of the factors affecting the quality of current products in semiconductors industry. Identification of process steps with queue time sensitivity is not an easy and quick analysis, and it requires a high number of lots. An effective tool for a quick identification of sensitivity of a specific operation to a queue time was developed at Micron Israel and it is applied on production material. The algorithm calculates the average absolute delta of responses between consecutive step QTs, where a minimal value is accepted in steps with a linear correlation between the QT and the response. This calculation results in a snap-shot of the impact of QT for specific responses, or failures modes, for the entire production line, where the step with minimal value of this parameter is easily detected. This algorithm was used on production material and indeed helped to detect a new QT dependency in specific process steps. The Queue Time Sensitivity Analysis (QTSA) algorithm is a novel methodology to detect correlation of QT in a single process step to a certain electrical EOL failure. It can be easily implemented in a fabrication site by the IT team. QTSA methodology is very efficient in detecting QT related issues and it can be used both in routine mode on baseline material and during significant elevation in EOL or in-line failures.

Abstract: We present an overview on generation of direct gap photo- and electroluminescence in Ge bulk wafers, Ge thin films deposited on Si, and Ge p-i-n diodes prepared on Si substrates. We analyzed the emission in a spectral range from 0.45 eV to 0.95 eV, covering the radiation caused by direct gap transitions, the indirect one, and also the luminescence related to transition on dislocations. The temperature and excitation level strongly influence the intensities of direct and indirect photoluminescence in bulk samples. As it could be expected, high temperature and excitation favour the generation of direct gap luminescence. Intrinsic bulk Ge shows a quadratic dependence of the direct gap luminescence on the excitation and a sub-quadratic one for the indirect. The photoluminescence spectra taken from intrinsic Ge on Si layers show features related to dislocations. There are two spectral regions associated with dislocation recombination. At room temperature one is at around 0.45 eV and the other at 0.72 eV. We found strong direct gap radiation from the Ge p-i-n diodes with intrinsic, highly dislocated active area (dislocation density of about 10⁸-10¹⁰ cm^-2). There is a threshold current density of 8 kA/cm², at which the direct band luminescence becomes a super-quadratic. The dependence of the radiation intensity on the excitation is governed by a power law with exponent of 1.7 before reaching that threshold and 4.5 after exceeding it. Above the threshold the dislocation radiation shows similar dependence on the excitation as the direct band luminescence.

Abstract: We review two ex-situ doping methods to achieve high n-type doping up to mid-10¹⁹ cm^-3 in Ge-on-Si thin films. For both, delta doping and ion implantation, rapid thermal annealing is used to diffuse phosphorus from a diffusion source into the single crystal Ge layer. The diffusion mechanism is studied and we find that dopant enhanced diffusion in in-situ doped Ge attributes to the high doping level. A band gap narrowing effect is observed in highly doped n-type Ge through photoluminescence measurements by determining the photoluminescence peak shift. An empirical linear expression of the direct band gap narrowing shift with carrier concentration is proposed.

Abstract: One way to further increase performance and/or functionality of Si micro-and nanoelectronics is the integration of alternative semiconductors on silicon (Si). We studied the Ge/Si heterosystem with the aim to realize a Ge deposition free of misfit dislocations and with low content of other structural defects. Ge nanostructures were selectively grown by chemical vapor deposition on periodic Si nanoislands (dots and lines) on SOI substrate either directly or with a thin (about 10 nm) SiGe buffer layer. The strain state of the structures was measured by different laboratory-based x-ray diffraction techniques. It was found that a suited SiGe buffer improves the compliance of the Si compared to direct Ge deposition; plastic relaxation during growth can be prevented, and fully elastic relaxation of the structure can be achieved. Transmission electron microscopy confirms that the epitaxial growth of Ge on nanostructured Si is free of misfit dislocations.

Abstract: Motivated by the importance of the oxidized silicon layers, we have studied the surface photovoltage (SPV) transients in nanoislands of Ge_xSi_1x on silicon and oxidized Si surfaces. It is shown that the SPV decays can be approximated by the stretched-exponential form, with the β values ranging from 0.3 to 0.6 for the islands grown on oxide-covered Si substrates and from 0.5 to 1 for the ones placed on bare Si. On this basis, a simple qualitative model is proposed that takes into account a donor-and acceptor-like interface states at the GeSi/SiO₂ and Si/SiO₂ interface, which act as recombination centers with densities dependent on the GeSi coverage. These results can be used to improve the functionality of photoelectric devices based on Ge/Si.

Abstract: It has been found that isolated V₂⁰ and V₂⁰ localized near tin atoms are formed in Ge doped with tin. Simultaneously with V₂⁰ annealing, the appearance of absorption spectra consisting of sharp lines was observed. The defect to which the spectra found corresponds has hydrogen-like properties. The distances between the lines in spectrum are in good agreement with those predicted by effective-mass theory. The formation of the defect found does not depend on oxygen concentration. An appearance of Fano resonance in the region of continuum was detected in addition to intracenter transitions of the defect. The defect found was identified as SnV₂⁰Ga.