The formation of iron silicide nanocrystals (NCs) and their embedding into monocrystalline silicon was studied. Solid phase epitaxy of 0.4 nm Fe at 630 °C resulted in formation of NCs consisted of β-FeSi2 and ε-FeSi phases. Annealing of NCs at 750 °C for 90 min led to transformation of β-FeSi2 and ε-FeSi into α-FeSi2. On the other hand, silicon layer growth over as-formed NCs, at the same temperature, resulted in formation of single phase NCs consisted of β-FeSi2. Silicon deposition rate proved to be the crucial point for a full embedding of NCs. The rate of 1 nm/min resulted in emersion of NCs to the surface during silicon overgrowth irrespective of Si cap layer thickness, while the rate of 8 nm/min led to the full embedding of β-FeSi2 NCs. Both incompletely and fully embedded β-FeSi2 NCs have epitaxial relationship and stress favorable for an indirect to direct band-gap transition at Y point.
Germanium nanocrystals in GeO2 films have been obtained with the use of two methods and have been studied. The first method of Ge nanocrystal formation is a film deposition from supersaturated GeO vapor with subsequent dissociation of metastable GeO on heterophase system Ge:GeO2. The second method is growth of anomalous thick native germanium oxide layers with chemical composition GeOx(H2O) during catalytically enhanced Ge oxidation, x is close to 1. The obtained films were studied with the use of photoluminescence, Raman scattering spectroscopy, high-resolution electron microscopy. Strong photoluminescence signals were detected in GeO2 films with Ge nanocrystals at room temperature. "Blue-shift" of the photoluminescence maximum was observed with reducing of Ge nanocrystal size in anomalous thick native germanium oxide films. So, the correlation between reducing of the Ge nanocrystal sizes (estimated from position of Raman peaks) and photoluminescence "blue-shift" was observed. The Ge nanocrystals presence was confirmed by high-resolution electron microscopy data. The optical gap in Ge nanocrystals was calculated with taking into account quantum size effects and compared with the position of the experimental photoluminescence peaks. It can be concluded that a Ge nanocrystal in GeO2 matrix is a quantum dot of type I. It was shown, that "band gap engineering" approaches can lead to creation of Ge:GeO2 heterostructures with required properties. This heterostructures can be perspective for using in opto-electronics, for creation of elements of quasi-nonvolatile MOS memory using Ge nanocrystals as traps for electrons or holes, etc.
KEYWORDS: Germanium, Ion beams, Chemical analysis, Atomic force microscopy, Silicon, Semiconducting wafers, Ions, Spectroscopy, Electron microscopy, Oxidation
Ge islands less then 10 nm in base diameter and with a number density of about 8×1011 cm2 were created on Si02 films by low-energy ion-beam assisted deposition in high vacuum. The structures obtained were analyzed by Electron Spectroscopy for Chemical Analysis, Atomic Force Microscopy and High Resolution Electron Microscopy. It was found that due to desorption at 300-375 °C less than 50% of Ge deposited remains at the surface. Only pulse regime of ion-beam action results in formation of nanoclusters. It is suggested that the simultaneous nucleation of Ge islands at pulse ion-beam action is the main reason of high homogeneity of size distribution of Ge nanoislands.
Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge+ ion action during molecular beam epitaxy (MBE) of Ge on Si(100). The continuous and pulsed ion beams were used. These studies have shown that ion-beam action during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density and narrowing of island size distribution, as compared with conventional MBE experiments. Moreover, it was found that ion beam assists the transition from hut to dome shaped Ge islands on Si(100).
Raman spectroscopy which provides valuable information on the structural parameters of QDs was used for monitoring of the lateral oxidation of InAs/AlAs QD structures and study of the phonon properties of InAs QDs in aluminium oxide matrix. Optical phonons of InAs QD's were found to be affected by both strain and confinement. Raman spectra measured from non-oxidized area reveal asymmetric lineshape of LO phonons in InAs QDs and demonstrate its low-frequency shift with increasing excitation energy that is explained by QD size distribution and phonon confinement in smaller-size dots. Raman spectra taken from oxidized area show an increase of the LO peak intensity and the shift of the phonon line position towards higher frequency. The first effect is explained by formation of wide bandgap aluminium oxide matrix that leads to the shift of confined electronic states in InAs QDs closer to the resonance with the laser excitation energy. The latter is caused by increasing mechanical strain in InAs QDs due to a shrinkage of the aluminium oxide layers. At the boundaries of oxidized/non-oxidized areas the presence of amorphous and crystalline As clusters is evident.
New generation of ultra high vacuum set, ultra-fast ellipsometer of high accuracy and automatic system for control of technological processes was produced for reproducibility growth of mercury cadmium telluride (MCT) solid solution heterostructures (Hs's) by molecular beam epitaxy (MBE). This system allows to grow MCT HS's on substrate up to 4" in diameter and used for future development technology of growth on Si substrate. The development of industrially oriented growth MCT HS's MBE on GaAs 2" in diameter is presented. The electrical characteristics of n-type and p-type MCT HS's MBE and uniformity of MCT composition over the surface area is excellent and satisfied for fabricating multielements arrays of high quality infrared devices.
With the use of Raman scattering spectroscopy and electron microscopy techniques it was observed that nanosecond pulse excimer laser radiation impacts lead to formation into a- Si:H films on not-orienting glass substrates nanocrystals with preferred (110) orientation and sizes from 2 nm and bigger. The dependence of average size and concentration of nanocrystals on parameters of laser impacts (energy density and number of shots) was studied. Polarization anisotropy of the Raman scattering was observed in the system of mutual- oriented silicon nanocrystals. The analysis of polarization dependence of Raman scattering intensity makes possible to determine the part of the oriented nanocrystals. It is proposed, that preferred orientation is due to both elastic stress in the films and local deformations appearing around the nanocrystals. Features of explosive crystallization during excimer laser impact were observed. This effect can be result of significant mechanical stresses in a-Si:H films on glass substrates.
Transformation of defects in hydrogen implanted silicon and silicon-on-insulator structures caused by external pressure of argon ambient at the stage of defect removal in implanted material and high temperature annealing SOI structures is reported. The results are compared to these for crystals annealed at argon atmosphere of ambient pressure. Formation of the new phase crystallites was found in SOI structures annealed at high temperature in conditions of high pressure. Small insulations were also observed in hydrogen implanted silicon, which can be patterns of the new phase. Two reasons can cause phase transformation in the top silicon layer of as-bonded SOI structures: high hydrogen concentration and high local strain.
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