Growth of germanium nanoislands and nanowires on singular and vicinal Si(111) surfaces prior to the formation of a wetting layer

Physics of the Solid State - The growth of germanium nanoislands and nanowires on singular and vicinal Si(111) surfaces is investigated by scanning tunneling microscopy (STM). It is shown that the...     

Results of the liquid scintillation detector of the Mont Blanc Laboratory

Summary The Liquid Scintillation Detector (LSD experiment) is running since October 1984 in the Mont Blanc Laboratory, with 90 tons of active mass. We discuss here some of the research aims of the experiment, and report the preliminary results on searching for neutrino bursts from collapsing stars and on cosmic-ray muons.

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Riassunto Nell'ottobre 1984 è entrato in misura l'esperimento LSD (Liquid Scintillation Detector) del Laboratorio del Monte Bianco, costituito di 90 tonnellate di rivelatore. In questo lavoro si discutono alcuni degli argomenti di ricerca dell'esperimento e si riportano i risultati preliminari sui burst di neutrini da collassi stellari e sui muoni della radiazione cosmica.

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Резюме Зидкий сцинтилляционный детектор работает с октября 1984 года в лаборатории на Мон Блане. Детектор имеет 90 тонн активной массы. Мы обсуздаем некоторые цели этого эксперимента. Сообщаются предварительные результаты по поиску нейтринных вспышек от коллапсирующих звезд и мюонов в космических лучах.

     

The 90 ton liquid scintillation detector in the Mont Blanc laboratory

Summary We discuss here the results from calibrating liquid seintillation counters (1.5 m³ each) used in the large-volume neutrino experiment in the Mont Blanc Laboratory. The electronic and recording systems from the 72 counters of the detector are described, and the method to detect low-energy γ pulse from (n, p) reaction by using²⁵²Cf as a neutron source is discussed. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Folded acoustic phonons in Si/Ge superlattices with Ge quantum dots

The spectra of Raman scattering by folded acoustic phonons in Si/Ge superlattices with pseudomorphic layers of Ge quantum dots (QDs) grown by low-temperature (T = 250°C) molecular beam epitaxy are studied. New features of the folded phonon lines related to the resonant enhancement and unusual intensity ratio of the doublet lines that cannot be explained by the existing theory have been observed. The observed modes are shown to be related to the vibrations localized to the QDs and induced by the folded phonons of the Si spacer layers. The calculations performed in the model of a one-dimensional chain of atoms have allowed the nature of the localization of acoustic phonons attributable to a modification of the phonon spectrum of a thin QD layer to be explained. The observed intensity ratio of the folded phonon doublet lines is caused by asymmetry of the relief of the QD layers.     

Electric and photoelectric properties of GaAs/ZnSe-Ge/ZnSe/Al structures with Ge quantum dots

Current-voltage characteristics and spectral dependences of photovoltage are investigated at T=4.2 and 300 K in stress-free structures with germanium quantum dots (QDs) in the GaAs/ZnSe/QD-Ge/ZnSe/Al system. The “Coulomb staircase” type features in the current-voltage characteristic observed at room temperature without illumination are due to the Coulomb interaction of electrons in resonant tunneling through intrinsic levels in QDs. The features in the photovoltage spectra are related to the absorption of radiation in the system of discrete levels of QDs. An energy band diagram of the structure is constructed based on the experimental data.     

Optical transitions in Ge nanocrystals formed by high-pressure annealing of Ge ion implanted SiO2 films

Optical transitions in Ge nanocrystals formed by high-pressure annealing of the Ge⁺ ion implanted SiO₂ films have been studied by Raman and photoluminescence spectroscopy. It has been found that the E₁,E₁+Δ₁ Raman resonance shift observed from the unstrained and hydrostatically compressed nanocrystals corresponds to the quantization of the electron-hole state spectrum of the Ge band. It has also been established that the appearance of a green photoluminescence band centered at 420-520 nm correlates with the formation of strained nanocrystals. Comparisons of the PL data with HRTEM results have been made, which suggest that the green PL arises from strained Ge nanocrystals of a radius of less than 5 nm. The direct electron-hole recombination at Γ is discussed as a possible origin of the observed photoluminescence band.     

Raman E 1 and E 1 + Δ1 resonances in a system of unstrained germanium quantum dots

The positions and shapes of the Raman E ₁ and E ₁ + Δ₁ resonances of optical phonons are studied as functions of the size of unstrained germanium quantum dots. The quantum dots are grown by molecular-beam epitaxy in GaAs/ZnSe/Ge/ZnSe structures on GaAs(111) wafers. The positions of the E ₁ and E ₁ + Δ₁ resonances are found to shift by at most 0.3 eV. This shift is shown to be well described in terms of a cylindrical model using the quantization of the spectrum of bulk electron-hole states in germanium that form an exciton in a two-dimensional critical point. The fact that the peaks of the E ₁ and E ₁ + Δ₁ resonances appear separately has been detected for the first time, and it is related to the transformation of the interband density of states into a delta function because of spectrum quantization. An increase in the resonance amplitudes in quantum dots as compared to the bulk case is related to the degeneracy multiplicity of the exciton state in the (111) direction.     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Relaxation of mechanical stresses in an array of Ge quantum dots obtained in Si

Raman scattering on optical phonons in Si/Ge/Si structures with Ge quantum dots grown by molecular beam epitaxy at low temperatures 200–300°C has been investigated. A pseudomorphic state of an array of Ge quantum dots to a Si matrix with an ideally sharp interface has been obtained. Features associated with the inelastic relaxation of mechanical stresses have been revealed in the Raman spectrum. Two mechanisms of stress relaxation are separated. It has been shown that the spectrum of the electronic states of the array differs significantly from the set of the discrete levels of a single quantum dot, because the relaxation is inhomogeneous.     

Low-energy neutral-particle detection in the Mont Blanc LSD experiment

Summary In this work we analyse the sources of background that can imitate a real antineutrino interaction in the Mont Blanc LSD detector. The sources are: fast neutrons from nuclear showers produced by muons in the rock, spontaneous fission of²³⁸U and the decay of products of thermal neutron capture by nuclei in the apparatus or in the surrounding rock.

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Riassunto In questo lavoro si analizzano le sorgenti di rumore che possono simulare le interazioni antineutrino attese nell'apparato LSD nel Monte Bianco. Le sorgenti sono: neutroni veloci da sciami nucleari prodotti da muoni nella roccia, fissione spontanea²³⁸U e il decadimento dei prodotti di catture di neutroni termici da parte di nuclei dell'apparato o della roccia circostante.

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Резюме В этой работе анализируются источники фона, которые могут имитировать реальные антинейтринные взаимодействия в детекторе на Мон Блане. Эти источники преставляют: Быстрые нейтроны из ливней, образованных мюонами в скалах, спонтанное деление²³⁸U и распад продуктов, образованных при захвате тепловых нейтронов ядрами в аппаратуре или в окружающих скалах.